3 060713 English Compendium (1).pdf

Chemistry and Treatment of Cyanidation Wastes SECOND EDITION

Terry I. Mudder, Ph.D. Michael M. Botz, M.S., P.E. and Adrian Smith, Ph.D.

MINING JOURNAL BOOKS LTD LONDON

Dedication

Dedicated to the memory of Dr. Adrian Smith and to those who still believe in science and have the courage to apply it.

Acknowledgements There are many people without whose efforts the first and now the second edition of this book would have not been completed. With respect to the first edition, we thank Dr. Ian Hutchison of TRC, Inc., Mr. Jim Whitlock of Whitlock Associates, Homestake Mining Company and Dr. Terry Chatwin from the University of Utah for their substantial contribution of thoughts and ideas to various parts of the book. Furthermore, we thank James Scott formally of Environment Canada and the first recipient of the Adrian Smith International Environmental Mining Award. It is the dedication of Mr. Scott and his co-workers within Environment Canada that has contributed much to our understanding of the environmental implications of the cyanidation process. The cooperation exhibited between Environment Canada and the mining industry worldwide has provided over the years an excellent example worthy of copy. We would like to thank SRK Consulting Inc. for their part in the production of the first edition of the book and Homestake Mining Company for providing important srcinal research on the subject of biological treatment. We would like to also thank Clint Strachan and Rick Frechette for their insights and comments on water balances and management at mining operations. Roger Schulz from the Chemistry Centre in Perth, Western Australia, provided an in-depth review of the analytical chapter with his considerable experience and expertise in cyanide measurement. Dr. Karen Hagelstein provided important knowledge on the ecotoxicological properties of cyanide and its related compounds. We would again like to thank Dr. Adrian Smith, who is no longer physically with us, but whose inspiration and spirit will remain forever real in our hearts. Finally, a special thanks to The Mining Journal and Mining Journal Books for believing this book was worth publishing.

TerryI.Mudder,Ph.D. Sheridan,WyomingUSA

MichaelM.Botz,M.S.,P.E. Joliet,MontanaUSA June 2001

Preface The increased awareness of the environmental implications associated with cyanidation has led to a tremendous increase in the knowledge of chemistry, analysis, toxicity and treatment of its process solutions. These four aspects of the cyanidation process are intimately related, with the treatment of cyanidation solutions forming the visible bridge between the mining operation and the receiving system or environments. There is sufficient detail and depth of expertise, experience and information to begin to quantify the impacts of cyanidation and to provide permanent, reliable and environmentally acceptable solutions to long-standing concerns. The chemistry of the cyanidation process is well defined, but its impacts on the characteristics of mine waters and the costs of water treatment have often been underestimated during the initial stages of mine development. Today, however, the mining industry is well aware of the consequences of inadequate environmental management and has invested hundreds of millions of dollars in the development of state-of-the-art tailings disposal and water treatment systems. The analysis of cyanidation process waters has evolved from the free cyanide titration with silver nitrate to the speciation of individual metal cyanide complexes in the part per billion range using specialized ion chromatography methods. Although many analytical methods are available, only a few areexisting capablemethodology, of producing while reliable and accurate data.analytical It is necessary focus upon and improve appropriately using methodstocurrently in wide usage. The toxicity of cyanidation solutions is complex, as it involves not only individual compounds, but combinations of compounds which exhibit characteristics much different than any of the individual components. There is now sufficient toxicological information concerning cyanidation process solutions to quantify and calculate acceptable discharge limitations on a siteby-site basis. This approach requires the practical application of both laboratory and field data to provide acceptable levels of environmental protection. The chemistry, analysis and toxicity of cyanidation process solutions are combined for the development and design of suitable and viable water treatment alternatives. A crucial aspect of this is proper evaluation of mine site water balances and implementation of appropriate water management systems. There are many water treatment alternatives that can be used at mining operations, none of which are applicable in all situations. It is hoped that the second edition of this book will aid those involved in the permitting, design and operation facets of the mining industry to ask the appropriate questions and to make justifiable and practical decisions.

Unit Conversions Length 1 inch = 2.540 cm 1 m = 3.2809 feet = 39.37 inches 1 mile = 5,280 feet = 0.6214 km 1 yard = 3 feet = 36 inches Area 2 1 acre = 43,560 ft 1 hectare (ha) = 2.471 acres 2 2 1 m = 10.764 ft Volume 3 3 1 m = 35.3145 ft = 264.17 gallons 3 1 ft = 7.4805 gallons 1 gallon = 3.7854 liters 3 3 1 acre-foot = 43,560 ft = 1,233.5 m Mass 1 tonne (t) = 1,000 kg = 2,204.62 lbs 1 ton = 2,000 lbs = 907.19 kg 1 tonne (t) = 1.102 tons 1 kg = 2.20462 lbs 1 lb = 453.593 g 1 Troy ounce (ozTR) = 31.10 g Pressure 5 1 atm = 14.696 psi = 1.01325 bars = 760 mm Hg = 1.01325 × 10 Pa 1 kPa = 0.145 psi Flow 3 1 gpm = 0.227 m /hr 3 1 m /hr = 4.403 gpm 1 L/min = 0.264 gpm 3 1 ft /sec (CFS) = 448.8 gpm 3 3 Concentration 1 mg/L = 1 g/m = 0.062 lbs/ft 1 g/tonne (g/t) = 0.0292 ozTR/ton

Temperature o o F = [1.8 × C] + 32 o o C = [ F – 32] ÷ 1.8

Table of Contents

Table of Contents CHAPTER 1

Chemistry Of Free And Complexed Cyanide

1.1 1.2 1.3 1.4

Introduction ................................................................................................................... 1 Gold Dissolution ............................................................................................................ 1 Gold Recovery From Solution ....................................................................................... 5 Solution Chemistry of Cyanide and its Metal Complexes.............................................. 6 1.4.1 Introduction........................................................................................................... 6 1.4.2 Cyanide Bonding .................................................................................................. 6 1.4.3 Free Cyanide ......................................................................................................... 7 1.4.4 Simple Cyanide Compounds................................................................................. 8 1.4.5 Weak and Moderately Strong Cyanide Complexes ............................................. 12 1.5 Solution Chemistry of the Iron Cyanide....................................................................... 16 1.6 Cyanide Related Compounds....................................................................................... 22 1.6.1 Introduction......................................................................................................... 22 1.6.2 Thiocyanate......................................................................................................... 22 1.6.3 Cyanate ............................................................................................................... 22 1.6.4 Ammonia ............................................................................................................ 23 1.6.5 Nitrate ................................................................................................................. 23 1.7 1.7.1 Process Solution Chemistry ......................................................................................... 23 23 Introduction......................................................................................................... 1.7.2 Iron Cyanides...................................................................................................... 25 1.7.3 Copper Bearing Ores........................................................................................... 25 1.7.4 Silver Ores .......................................................................................................... 26 1.8 References ................................................................................................................... 26 1.9 Bibliography ................................................................................................................ 27

CHAPTER 2 2.1 2.2

Analysis of Cyanide

Introduction ................................................................................................................. 29 Analytical Procedures for Cyanides............................................................................. 30 2.2.1 Introduction......................................................................................................... 30 2.2.2 Total Cyanide by Distillation .............................................................................. 32 2.2.3 Cyanide Amenable to Chlorination ..................................................................... 39 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 2.2.9 2.2.10

WAD Cyanide by Distillation ............................................................................. 40 WAD Cyanide by the Picric Acid Method .......................................................... 40 WAD Cyanide by the Zinc Dust and Ammonia Method..................................... 42 Free Cyanide by Titration with Silver Nitrate ..................................................... 42 Free Cyanide by Ion Selective Electrode............................................................. 43 Cyanide by Ion Chromatograph .......................................................................... 44 Cyanide by Automated Ligand Exchange ........................................................... 44

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Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) 2.2.11 Analysis Of Cyanide In Solids ............................................................................ 47 2.2.12 On-Line Process Monitoring For Cyanide .......................................................... 47 2.2.13 Cyanide Detection And Quantitation Limits ....................................................... 47 2.2.14 Thiocyanate Analysis.......................................................................................... 49 2.2.15 Cyanate Analysis................................................................................................. 49 2.2.16 Ammonia, Nitrite and Nitrate Analyses .............................................................. 49 2.3 Analytical Interferences ............................................................................................... 50 2.3.1 Introduction......................................................................................................... 50 2.3.2 Oxidizing Agents ................................................................................................ 50 2.3.3 Sulphides............................................................................................................. 51 2.3.4 Thiocyanate......................................................................................................... 51 2.3.5 Nitrite and Nitrate ............................................................................................... 52 2.3.6 Carbonates .......................................................................................................... 52 2.3.7 Thiosulphates, Sulphites and Other Sulphur Compounds.................................... 52 2.3.8 Metals ................................................................................................................. 53 2.3.9 Effects of Interferences on Cyanide Analyses..................................................... 53 2.4 Preservation and Storage of Samples ........................................................................... 54 2.5 Calculation of a Cyanide Mass Balance....................................................................... 59 2.6 2.7

Comparative ............................................................................. 65 61 Low Levels ofAnalyses CyanideofinSolutions the Environment................................................................. 2.7.1 Natural Sources of Cyanide................................................................................. 66 2.7.2 Anthropogenic Sources of Cyanide..................................................................... 67 2.7.3 Monitoring of Surface and Ground Waters ......................................................... 68 2.8 References ................................................................................................................... 68 2.9 Bibliography ................................................................................................................ 71

CHAPTER 3

Environmental Geo chemistry and Fa te of C yanide

3.1 3.2

Introduction ................................................................................................................. 73 Environmental Geochemistry of Cyanide .................................................................... 74 3.2.1 Environmental Fate: The Cyanide Cycle............................................................. 74 3.2.2 Cyanide Geochemistry ........................................................................................ 74 3.3 Attenuation Mechanisms of Cyanide ........................................................................... 77 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7

Complexation (Chelation)................................................................................... 77 Cyanide Complex Precipitation........................................................................... 78 Adsorption .......................................................................................................... 78 Oxidation to Cyanate .......................................................................................... 79 Volatilisation....................................................................................................... 80 Bioattenuation..................................................................................................... 82 Formation of Thiocyanate ................................................................................... 82

Mudder, Botz & Smith

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Table of Contents

Table of Contents (Continued) 3.3.8 Hydrolysis/Saponification of Free Cyanide......................................................... 83 3.3.9 Summary............................................................................................................. 83 3.4 Natural Cyanide Attenuation in Surface Ponds ............................................................ 83 3.5 Cyanide Geochemistry in Tailings............................................................................... 92 3.5.1 Introduction......................................................................................................... 92 3.5.2 Surface Effects in Tailings Ponds........................................................................ 94 3.5.3 Reactions in the Tailings Mass............................................................................ 94 3.5.4 Overall Effects in Tailings .................................................................................. 99 3.6 Cyanide Geochemistry of the Heap Leach................................................................... 99 3.6.1 Introduction......................................................................................................... 99 3.6.2 Hydrolysis and Volatilisation............................................................................ 102 3.6.3 Oxidation of Free Cyanide ................................................................................ 103 3.6.4 Hydrolysis/Saponification of Free Cyanide....................................................... 103 3.6.5 Aerobic Bioattenuation of Cyanide ................................................................... 104 3.6.6 Simple Cyanide Compounds............................................................................. 104 3.6.7 Metal Cyanide Complexes ................................................................................ 104 3.6.8 Anaerobic Bioattenuation of Cyanide ............................................................... 106 3.6.9 Field Data.......................................................................................................... 106 3.7 3.7.1 Cyanide Geochemistry in Soils .................................................................................. 110 Introduction....................................................................................................... 110 3.7.2 Cyanide Volatilisation from Soils ..................................................................... 110 3.7.3 Biological Attenuation in Soils ......................................................................... 114 3.7.4 Adsorption and Precipitation in Soils................................................................ 114 3.7.5 Hydrolysis/Saponification in Soils .................................................................... 115 3.7.6 Field Data.......................................................................................................... 115 3.7.7 Applications...................................................................................................... 119 3.8 Cyanide Geochemistry in Groundwater..................................................................... 121 3.8.1 Attenuation Mechanisms................................................................................... 121 3.8.2 Field Data.......................................................................................................... 122 3.9 Cyanide in the Atmosphere........................................................................................ 123 3.10 References ................................................................................................................. 124 3.11 Bibliography .............................................................................................................. 129

CHAPTER 4 4.1 4.2 4.3 4.4

Toxicity of Cy anide and Related Compounds

Introduction ............................................................................................................... 133 General Aspects of Toxicology.................................................................................. 133 Sources and Uses of Cyanide..................................................................................... 135 Toxicology of Cyanide .............................................................................................. 136 4.4.1 Physiology of Cyanide Toxicity........................................................................ 136

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Table of Contents (Continued) 4.4.2 Detoxification Mechanisms............................................................................... 137 4.4.3 Cyanide Antidotes............................................................................................. 139 4.5 Cyanide Toxicity to Aquatic Organisms .................................................................... 139 4.5.1 Toxicity of Free Cyanide to Aquatic Organisms ............................................... 139 4.5.2 Toxicity of WAD Cyanide to Aquatic Organisms............................................. 142 4.5.3 Toxicity of Iron Cyanides to Aquatic Organisms .............................................. 146 4.6 Cyanide Toxicity to Birds .......................................................................................... 155 4.6.1 The Most Sensitive Species to Oral Exposure................................................... 155 4.6.2 Summary of Bird Mortalities at U.S. Mines...................................................... 158 4.7 Cyanide Toxicity to Mammals................................................................................... 164 4.8 Toxicity of Cyanide Related Compounds .................................................................. 168 4.8.1 Introduction....................................................................................................... 168 4.8.2 Thiocyanate....................................................................................................... 168 4.8.3 Cyanate ............................................................................................................. 173 4.8.4 Ammonia .......................................................................................................... 178 4.8.5 Nitrate ............................................................................................................... 182 4.9 Toxicity of Actual Mine Waters................................................................................. 183 4.10 References ................................................................................................................. 187 4.11 Bibliography .............................................................................................................. 193 CHAPTER 5

Water Management an d Discharge Strategies

5.1 5.2 5.3

Introduction ............................................................................................................... 199 Water Management Plans .......................................................................................... 199 Water Balances .......................................................................................................... 204 5.3.1 Water Balance Components .............................................................................. 206 5.3.2 Water Balance Assessment ............................................................................... 206 5.3.3 Water Quality Assessment ................................................................................ 215 5.4 Cyanide Management Plan and Mass Balance........................................................... 218 5.5 Development of Effluent Design Goals ..................................................................... 223 5.5.1 Beneficial Use Classifications........................................................................... 223 5.5.2 The Use Attainability Analysis or Risk Assessment ......................................... 224 5.5.3 Effluent Design Goals....................................................................................... 226 5.6 5.7 5.8 5.9 5.10

Screening and Selection of Water Treatment Processes............................................. 227 Effluent Discharge Strategies..................................................................................... 229 Example Water Management Approaches ................................................................. 233 References ................................................................................................................. 236 Bibliography .............................................................................................................. 236


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Table of Contents

Table of Contents (Continued) CHAPTER 6

Treatment and Recovery of Cyanide

6.1 6.2 6.3 6.4

Introduction ............................................................................................................... 239 Laboratory and Pilot Plant Investigations .................................................................. 240 Activated Carbon Adsorption .................................................................................... 242 Alkaline Chlorination................................................................................................. 246 6.4.1 Introduction....................................................................................................... 246 6.4.2 Process Chemistry............................................................................................. 246 6.4.3 Performance ...................................................................................................... 248 6.4.4 Operating Costs................................................................................................. 258 6.4.5 Toxicity of Alkaline Chlorination Effluents...................................................... 260 6.5 Active and Passive Biological Treatment................................................................... 263 6.5.1 Introduction....................................................................................................... 263 6.5.2 Process Chemistry............................................................................................. 265 6.5.3 Homestake Mine Biological Treatment Process................................................ 267 6.5.4 Nickel Plate Mine Biological Treatment Process .............................................. 273 6.5.5 Santa Fe Mine Passive Biological Treatment Process ....................................... 275 6.5.6 Summary........................................................................................................... 279 6.6 6.6.1 Caro’sIntroduction....................................................................................................... Acid................................................................................................................ 282 282 6.6.2 Process Chemistry............................................................................................. 282 6.6.3 Performance ...................................................................................................... 283 6.7 Cyanide Recovery by Tailings Washing .................................................................... 287 6.7.1 Introduction....................................................................................................... 287 6.7.2 Performance ...................................................................................................... 287 6.8 Cyanide Recovery by Stripping and Absorption ........................................................ 292 6.8.1 Introduction....................................................................................................... 292 6.8.2 Process Chemistry............................................................................................. 294 6.8.3 Performance ...................................................................................................... 298 6.8.4 Economics of Cyanide Recovery ...................................................................... 304 6.8.5 The Future of Cyanide Recovery....................................................................... 305 6.9 Hydrogen Peroxide .................................................................................................... 307 6.9.1 Introduction....................................................................................................... 307 6.9.2 Process Chemistry............................................................................................. 307 6.9.3 Performance ...................................................................................................... 308 6.10 Natural Attenuation.................................................................................................... 315 6.10.1 Introduction....................................................................................................... 315 6.10.2 Process Chemistry............................................................................................. 317 6.10.3 Performance ...................................................................................................... 320 6.11 Precipitation of Cyanide............................................................................................. 323 6.11.1 Introduction....................................................................................................... 323

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Table of Contents (Continued) 6.11.2 Performance ...................................................................................................... 323 6.12 Sulphur Dioxide and Air ............................................................................................ 327 6.12.1 Introduction....................................................................................................... 327 6.12.2 Process Chemistry............................................................................................. 327 6.12.3 Performance ...................................................................................................... 329 6.13 Other Treatment Processes......................................................................................... 334 6.13.1 Introduction....................................................................................................... 334 6.13.2 Ion Exchange .................................................................................................... 334 6.13.3 Ozone Oxidation ............................................................................................... 335 6.13.4 Reverse Osmosis ............................................................................................... 335 6.14 Removal of Metals and Cyanide Related Compounds ............................................... 336 6.14.1 Introduction....................................................................................................... 336 6.14.2 Metals Removal ................................................................................................ 337 6.14.3 Removal of Cyanide Related Compounds......................................................... 350 6.14.3.1 Cyanate Removal .................................................................................. 350 6.14.3.2 Thiocyanate Removal............................................................................ 351 6.14.3.3 Ammonia Removal ............................................................................... 352 6.14.3.4 Nitrate Removal .................................................................................... 354 6.15 References Summary.................................................................................................................... 355 6.16 ................................................................................................................. 356 6.17 Bibliography .............................................................................................................. 363 Index................................................................................................................................... 369

LIST OF TABLES 1.1

Minerals Associated with Gold in Sulphide Ores.............................................................. 3

1.2

Solubility of Metal Sulphide Minerals in Cyanide Solutions ............................................ 4

1.3

Classification of Cyanide and Cyanide Compounds in Cyanidation Solutions on the Basis of Stability .................................................................................... 9

1.4

Relative Concentrations of Hydrocyanic Acid and Cyanide Ion in a 10 Molar Solution as a Function of pH ................................................................................ 11

1.5

Stability Constants of Metal Cyanide Complexes ........................................................... 14

1.6

Free Cyanide Concentration Released at Various Metal-Cyanide Complex Concentrations ................................................................................................................ 15

1.7

Solubilities of Ferrocyanides and Ferricyanides.............................................................. 18

1.8

Solubilities of Complex Ferrocyanide and Ferricyanide Salts......................................... 19

-4


vi

Table of Contents

Table of Contents (Continued) LIST OF TABLES CONTINUED 1.9

Effect of Initial pH on Ferrocyanide Solubility............................................................... 19

1.10 1.11

Solubility of Hexacyanoferrates (II) and (III) Separately and in Mixtures ...................... 21 Range of Cyanidation Solution Chemistries ................................................................... 24

1.12

Listing of Several Iron-Cyanide Complexes ................................................................... 24

2.1

Comparison of Total Cyanide Analyses by Autoanalyser (USEPA Method 9012) and Manual (USEPA Method 9010) Methods ...................................................... 35

2.2

Comparison of Autoanalyser and Non-Autoanalyser Analyses for Total Cyanide in a Mining Effluent.......................................................................................... 36

2.3

Comparative Evaluation of Total Cyanide Methods Using Different Acids with a Synthetic Metal Cyanide Solution Containing Thiocyanate ................................. 37

2.4

Thiocyanate Additions to Cyanide Standards Analysed by WAD and Total Cyanide Distillations Using Sulphuric Acid ................................................................... 38

2.5

Comparison of Leach and Rinse Solution WAD Cyanide Analyses ............................... 41

2.6

Comparison of WAD, CAC and OIA-1677 Analytical Methods .................................... 46

2.7

Summary of Method Detection Limits and Practical Quantitation Limits for Cyanide Analyses...................................................................................................... 48

2.8

Comparison of WAD Cyanide Values from Preserved and Unpreserved Samples........................................................................................................................... 57

2.9

Effects of the Use of Preservatives for Oxidants and Sulphides on Cyanide Analyses.......................................................................................................................... 58

2.10

Example Cyanide Mass Balance Calculation.................................................................. 60

2.11

Summary of International Comparative Study of Cyanide and Thiocyanate Analyses.......................................................................................................................... 62

2.12

Summary of Cyanide Analytical Results for a USA Mine Site ....................................... 64

3.1

Cyanide Profile in Interstitial Water in a Decommissioned Tailings Impoundment in South Africa......................................................................................... 95

3.2

Chemistry of Oxidised, Intermediate and Reduced Zones in Tailings Impoundments (Mean Values) in South Africa............................................................... 95

3.3

Cyanide Chemistry of "Mixed" Tailings and Flotation Tailings Solution in North America ................................................................................................................ 96

3.4

Reaction Products Chemistry of "Mixed" Tailings in North America............................. 97

3.5

Time/Cyanide Concentration Dependency of Copper and Cobalt Cyanide in Tailings Interstitial Pore Solution in North America................................................... 97

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Table of Contents (Continued) LIST OF TABLES CONTINUED 3.6 3.7

Cyanide Decay in Tailings Decant Solution in New Zealand ......................................... 98 Cyanide Decay in Tailings Pore Solution in New Zealand ............................................. 98

3.8

Typical Cyanide Reactions in a Heap Leach Environment ........................................... 101

3.9

Relative Stability of Metal Cyanide Compounds in Water............................................ 105

3.10

First Pore Volume Effluent pH and Cyanide Concentration from Leach Pad Neutralisation Testing ............................................................................................ 109

3.11

Comparison of Total Cyanide Levels with Groundwater Composition in South Africa.................................................................................................................. 116

3.12

Attenuation Calculation Summary ................................................................................ 118

3.13

Partition Coefficient Values for Tailings Attenuation Tests.......................................... 120

4.1

Effects of Free Cyanide on Aquatic Organisms ............................................................ 140

4.2

Relative Toxicities to Fish of Metal Cyanide Compounds............................................ 147

4.3 4.4

Ferrocyanide Toxicity Test Summary and 96-Hour LC 50 Values Obtained .................. 152 Ferricyanide Toxicity Test Summary and 96-Hour LC50 Values Obtained ................... 153

4.5

Summary of Iron Complexed Cyanide Chronic Toxicity Test Conditions.................... 154

4.6

Effects of Free Cyanide on Selected Species of Birds................................................... 156

4.7

Wildlife Mortalities Reported by Nevada Mine Operators............................................ 162

4.8

WAD Cyanide in Tailings Ponds.................................................................................. 163

4.9

Cyanide Toxicity to Mammals...................................................................................... 165

4.10

Background Cyanide Concentrations in Selected Plants ............................................... 167

4.11

Toxicological Data for Cyanide Related Compounds ................................................... 169

4.12

Toxicity of Thiocyanate to Fish .................................................................................... 170

4.13

Toxicity of Thiocyanate to Fish, Effect of pH, Hardness and Temperature .................. 171

4.14

Toxicity of Thiocyanate to Fish, Effect of Hardness at pH 8.0 and

4.15

Temperature 12°C......................................................................................................... 172 Chronic Thiocyanate Toxicity Studies for Rainbow Trout............................................ 174

4.16

Chronic Thiocyanate Toxicity Studies for Fathead Minnows ....................................... 174

4.17

Toxicity of Cyanate to Fish, Effect of pH and Temperature ......................................... 175


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Toxicity of Cyanate to Fish, Effect of Hardness at pH 8.0 and Temperature 12°C.............................................................................................................................. 176

4.19

Effect of Hardness on Cyanate Toxicity ....................................................................... 176

4.20

Toxicity of Cyanate and Thiocyanate Mixtures ............................................................ 177

4.21

Percent NH3 in Aqueous Ammonia Solutions for 0°C to 30°C and pH 6 to 10.............................................................................................................................. 180

4.22

Toxicity of Un-Ionised Ammonia to Fish Species ........................................................ 181

4.23

Toxicity of Various Gold Mill Effluents to Rainbow Trout Fry.................................... 184

4.24

Water Quality Associated with the Homestake Lead, South Dakota, USA Mine Water Treatment Plant......................................................................................... 185

4.25

Water Quality Associated with the Golden Cross, New Zealand Mine Water Treatment Plant .................................................................................................. 186

5.1

Options for Water Management.................................................................................... 202

5.2

Potential Water Sources at a Mining Operation ............................................................ 210

5.3

Approaches to Estimating Water Volumes at a Mining Operation................................ 212

5.4

Approximate Chemical Composition Ranges in Cyanidation Solutions ....................... 217

5.5

Cyanide Solutions and Methods for Cyanide Management........................................... 221

5.6

Components of a Cyanide Balance ............................................................................... 222

6.1

Advantages and Disadvantages of Activated Carbon Water Treatment ........................ 245

6.2

Operating Parameters for Full-Scale Alkaline Chlorination Operations........................ 255

6.3

Performance Data for Full-Scale Alkaline Chlorination of Gold Mill Effluents ....................................................................................................................... 256

6.4

Giant Yellowknife Alkaline Chlorination Performance Data........................................ 257

6.5

Typical Analyses at Grey Eagle Treatment Plant.......................................................... 259

6.6

Summary of Alkaline Chlorination Reagent Consumptions.......................................... 259

6.7

Summary of Alkaline Chlorination Reagents Costs ...................................................... 261

6.8

Advantages and Disadvantages of the Alkaline Chlorination Process........................... 262

6.9

Homestake Mine Water Treatment Plant Performance ................................................. 272

6.10

Performance of the Nickel Plate Mine Biological Water Treatment Plant .................... 276

6.11

Reagent Usages in the Nickel Plate Mine Biological Water Treatment Plant in 1999................................................................................................................. 277

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Passive Biological Treatment of Leach Pad Draindown Solution in Laboratory Columns ..................................................................................................... 280

6.13

Advantages and Disadvantages of Biological Water Treatment.................................... 281

6.14

Slurry Treatment Results Using Caro’s Acid............. ................................................... 285

6.15

Clear Solution Treatment Results Using Caro’s Acid ................................................... 285

6.16

Advantages and Disadvantages of the Caro’s Acid Process............. ............................. 286

6.17

Advantages and Disadvantages of Cyanide Recovery by Tailings Washing................. 291

6.18

Performance of Cyanide Recovery in Treating Tailings Impoundment Decant Solution ............................................................................................................ 299

6.19

Results of CANMET Laboratory Cyanide Recovery Studies ....................................... 300

6.20

Pilot Cyanide Recovery Testwork Using Stripping Towers.......................................... 301

6.21

Advantages and Disadvantages of Cyanide Recovery by Stripping and Absorption .................................................................................................................... 306

6.22

Detoxification Results from Three Plants Using the Hydrogen Peroxide Process to Treat Gold Mill Effluents............................................................................. 312

6.23

Advantages and Disadvantages of the Hydrogen Peroxide Process .............................. 314

6.24

Examples of Natural Cyanide Attenuation in Tailings Impoundments in Australia ....................................................................................................................... 316

6.25

Effect of Temperature on the Rate Constants for the Attenuation of Metal Cyanide Complexes at pH 7.0....................................................................................... 319

6.26

Effluent Quality of Canadian Gold Mines Applying Batch Natural Cyanide Attenuation Systems ....................................................................................... 319

6.27

Chemistry of Tailings Impoundment Decant Solution Before and After Natural Attenuation....................................................................................................... 322

6.28

Precipitation of Free Cyanide Using Ferrous Sulphide at the Con Mine....................... 325

6.29

Laboratory Test Results of Cyanide Precipitation Using Ferrous Sulphate................... 326

6.30

Oxidation of Cyanide in Tailings Slurry Using the INCO SO 2/Air Process.................. 332

6.31 6.32

Oxidation of Cyanide in Solutions Using the INCO SO2/Air Process........................... 332 Oxidation of Cyanide in Electroplating Wastes Using the INCO SO2/Air Process.......................................................................................................................... 332

6.33

Advantages and Disadvantages of the INCO SO2/Air Process...................................... 333


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Waste Treatment Options and Performance Data Summary for Antimony

6.35

and Arsenic Removal.................................................................................................... 341 Waste Treatment Options and Performance Data Summary for Beryllium and Cadmium Removal................................................................................................. 342

6.36

Water Treatment Options and Performance Data Summary for Copper Removal........................................................................................................................ 342

6.37

Water Treatment Options and Performance Data Summary for Chromium Removal........................................................................................................................ 343

6.38

Water Treatment Options and Performance Data Summary for Lead Removal........................................................................................................................ 344

6.39

Water Treatment Options and Performance Data Summary for Mercury Removal........................................................................................................................ 345

6.40

Water Treatment Options and Performance Data Summary for Nickel Removal........................................................................................................................ 345

6.41

Water Treatment Options and Performance Data Summary for Selenium and Thallium Removal.................................................................................................. 346

6.42

Water Treatment Options and Performance Data Summary for Zinc Removal........................................................................................................................ 347

6.43

Long-Term Effluent Concentrations Achievable with Several Water Treatment Processes...................................................................................................... 348

6.44

Summary of the Performance of Acid Drainage Treatment Systems ............................ 349

LIST OF FIGURES -

1.1

Relationship Between HCH and CN with pH (25°C)..................................................... 10

2.1

Compounds Included in Total, WAD and Free Cyanide Analyses.................................. 31

2.2

Typical Cyanide Distillation Apparatus.......................................................................... 33

3.1

The Cyanide Cycle.......................................................................................................... 75

3.2

Interrelationships in Cyanide Chemistry ......................................................................... 76

3.3

Eh-pH Diagram for the Free Cyanide-Water System at 25°C ......................................... 81

3.4

Natural Attenuation of Cyanide, Cyanate and Thiocyanate ............................................ 85

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Table of Contents (Continued) LIST OF FIGURES CONTINUED 3.5

Observed and Predicted Attenuation of Cyanide in a Shallow Pond

3.6

Operated in a Batch Mode in Canada.............................................................................. 87 Observed and Predicted Attenuation of Cyanide in a Deep Pond Operated in a Batch Mode in Canada ............................................................................................. 88

3.7

Observed and Predicted Attenuation of Cyanide in a Pilot Scale System Operated in a Continuous-Fill and Batch-Discharge Mode in Canada ............................ 89

3.8

Predicted and Actual Cyanide Attenuation in a USA Tailings Impoundment.................................................................................................................. 91

3.9

Flow Schematic of Tailings Disposal at a Gold/Uranium Mine and Possible Types of Chemical Reactions in the System ..................................................... 93

3.10

Prevailing Geochemical Conditions and Typical Cyanide Reactions in the Abandoned Heap Leach Environment .......................................................................... 100

3.11

Cyanide Reduction Data Illustrating Delayed Release of Cyanide from Rinsing a Heap Leach Pad ............................................................................................ 108

3.12

Unsaturated Soil Model Showing Soil Particles, Water, Gas and Mass Transport/Retardation Mechanisms .............................................................................. 111

3.13

Summary of Head Space Analyses for Cyanide in Saturated and Unsaturated Soil Test Columns..................................................................................... 113

4.1

Spectral Energy Distribution of Natural Sunlight and Spectralite Bulbs....................... 151

4.2

WAD Cyanide Concentration – Mortality Relationship for the Mallard Duck ............................................................................................................................. 159

4.3

Summary of Cyanide Toxicity to Mallard Ducks.......................................................... 160

4.4

Percentage of Total Ammonia as NH3 as a Function of pH and Temperature.................................................................................................................. 179

5.1

Components of a Mine Site Water Management Plan................................................... 201

5.2

Water Balance Schematic Showing Most of the Water Balance Components.................................................................................................................. 205

5.3 5.4

Example Water Balance Schematic for a Gold Mine in a Wet Climate ........................ 207 Example Water Balance Schematic for a Gold Heap Leaching Operation in a Wet Climate ........................................................................................................... 208

5.5

Heap Leaching Operations Pond Design Considerations .............................................. 209

5.6

Example Stream Flow Hydrograph............................................................................... 231

5.7

Schematic of an Effluent Discharge Diffuser................................................................ 232


xii

Table of Contents

Table of Contents (Continued) LIST OF FIGURES CONTINUED 5.8

Example Water Management Approach for a Milling Operation.................................. 234

5.9

Flowsheet Illustrating the Life-Cycle Management of Water at a Heap Leaching Operation....................................................................................................... 235

6.1

Example Schematic of an Activated Carbon Treatment Plant....................................... 244

6.2

Redox and TRC Response Curve During Titration of Gold Mill Barren Solution with Standard Hypochlorite Solution.............................................................. 249

6.3

Mosquito Creek Mine - Alkaline Chlorination Flowsheet............................................. 250

6.4

Baker Mine - Alkaline Chlorination Flowsheet............................................................. 251

6.5

Carolin Mine - Alkaline Chlorination of Tailings Pond Reclaim Water........................ 252

6.6

Giant Yellowknife - Alkaline Chlorination and Arsenic Removal Flowsheet...................................................................................................................... 253

6.7

A Schematic of an Alkaline Chlorination Process ........................................................ 254

6.8

The Homestake Mine Biological Water Treatment Process .......................................... 269

6.9

Rotating Biological Contractor Plant for the Removal of Cyanide and Thiocyanate at the Homestake Mine ............................................................................. 270

6.10

Nickel Plate Mine Biological Water Treatment Plant ................................................... 274

6.11

Schematic of the Santa Fe Mine Passive Biological Treatment System ........................ 278

6.12

Basic Flow Diagram for a Caro’s Acid Treatment System ........................................... 284

6.13

Flow Diagram of Cyanide Recovery by Tailings Washing ........................................... 289

6.14

Performance of Cyanide Recovery by CCD Tailings Washing..................................... 290

6.15

General Schematic of Cyanide Recovery by Stripping and Absorption ........................ 296

6.16

Flow Diagram of the Golden Cross Cyanide Recovery Plant ....................................... 302

6.17

Flow Diagram of the DeLamar Cyanide Recovery Plant .............................................. 303

6.18

Flowsheet for the Hydrogen Peroxide Treatment Process at the Ok Tedi Mine.............................................................................................................................. 310

6.19 6.20

Teck-Corona Hydrogen Peroxide Treatment System.................................................... 311 Simulation of Natural Cyanide Attenuation .................................................................. 322

6.21

Flowsheet for a Two-Stage INCO Cyanide Destruction Process .................................. 330

6.22

The Effects of Copper Concentration and pH on the Performance of the INCO Cyanide Destruction Process.............................................................................. 331

xiii

Table of Contents

CHAPTER ONE

Chemistry o f Free and Complexed Cyanide 1.1

INTRODUCTION

The cyanidation process for the extraction of gold and silver from ore has been employed since 1898 when it was first used in New Zealand and Africa. It is a very efficient process capable of extracting gold in amounts as small as 0.25% of an ounce from a ton of rock with over 90% efficiency. The recovery of gold once in solution is equally efficient. The chemistry of cyanide solutions is quite complex and it is this complexity which is responsible for its ability to dissolve gold and silver. However, cyanide forms complexes with other metals, such as mercury, zinc, copper, iron and nickel, which partially accounts for the consumption of cyanide in gold extraction circuits, generates waters that may be difficult to treat and complicates the analysis of cyanide solutions. The principal reasons for the prominent place of cyanide in gold ore processing include its wide availability, its efficiency at extracting gold and silver, its relatively rapid extraction kinetics and the strength and solubility of its gold cyanide complex. This chapter provides background information regarding the cyanidation process and the chemical reactions associated with gold dissolution and recovery. The purpose of this chapter is to establish a basis for the nature and type of cyanide-bearing solutions which may require treatment prior to discharge from a mining operation.

1.2

GOLD D ISSOLUTION

Gold dissolution by cyanide (i.e. cyanidation) is believed to be a two-step process in which hydrogen peroxide is formed as an intermediate (Marsden and House, 1993): (1.1)

2Au + 4NaCN + 2H 2O + O2 → 2NaAu(CN)2 + 2NaOH + H2O2

(1.2)

2Au + 4NaCN + H 2O2 → 2NaAu(CN)2 + 2NaOH

The overall reaction known as Elsner's equation is as follows: (1.3)

4Au + 8NaCN + O 2 + 2H2O → 4NaAu(CN)2 + 4NaOH

1


Relatively weak cyanide solutions can be used because of the strong complex formed between cyanide and gold. In the absence of other metal cyanide complexes, a 100 mg/L solution of NaCN (i.e. about 50 mg/L free cyanide) can provide the maximum rate and extent of gold dissolution. The reactions with metallic silver are analogous to the aforementioned reactions of cyanide with gold. However, the silver cyanide complex is weaker than the gold cyanide complex and stronger cyanide solutions and/or longer reaction times must be employed for its dissolution. Other conditions being equal, dissolution of an equivalent silver content requires about 10-fold the cyanide level needed for gold extraction. Under these conditions, gold can dissolve twice as rapidly as silver. Alloys of these two precious metals, known as electrum, dissolve at intermediate rates. The need for more aggressive conditions in the presence of silver can increase the attack on other minerals present in the ore, leading to increased cyanide consumption, decreased selectivity towards gold and silver and higher concentrations of other metals in solution. Elevated cyanide and metal-cyanide concentrations in solution can significantly increase the complexity and cost of recovering gold and silver. An economic evaluation of increased cyanide usage and silver recovery on the eventual water characteristics and treatment requirements should be considered during the preliminary design phase of a metallurgical flowsheet. In spite of the relative specificity of the gold-cyanide reaction, other metals and inorganic constituents react both with cyanide and to a certain extent with hydroxide, which must be present to maintain cyanide in its ionic form. Table 1.1 lists several of these constituents. Table 1.2 shows the extent to which zinc and copper, two primary ore constituents, are dissolved from sulphide minerals during cyanidation. The reactions with the iron minerals, pyrite and pyrrhotite, are of greater importance in many cases since these minerals are among the highest cyanide-consuming minerals in a gold ore. When pyrite and pyrrhotite-bearing ores are treated directly with cyanide solution, formation of thiocyanate occurs and is accelerated under conditions which combine partial or inadequate aeration with high alkalinity. The production of thiocyanate is highest for the free sulphur or pyrrhotite containing minerals. -4

The formation of the ferrocyanide ion, Fe(CN) 6 , is often of greater concern from operational and environmental viewpoints. As is noted later in this chapter, conditions for its formation are more limited. It is formed relatively rapidly under conditions of low alkalinity and reduced aeration, particularly if pre-oxidation of the ore has led to the presence of ferrous ion. This is partially due to a drop in pH caused by incomplete precipitation of ferrous hydroxide. It should be noted that the conditions for formation of thiocyanate and ferrocyanide from iron sulphide minerals are to some extent mutually restrictive, since conditions chosen to minimize one may promote formation of the other. This is of particular importance from a water treatment viewpoint, since thiocyanate createspotentially an oxidant demand, is potentially down forming ammonia and nitrate, causes of further concern. toxic itself and breaks


2

Chemistry of Free and Complexed Cyanide

TABLE 1.1

Element Iron

Minerals Associated with Gold i n Sulphide Ores

Sulphides FeS Pyrrhotite FeS2 Pyrite Marcasite

Cobalt

Silver

Au Native Gold Au, Ag Electrum Ag Native Silver Ag, Au Electrum

Cu Native Copper

Lead Zinc Carbon

Tellurides

AuSb2 Aurosibite

Ag2S Argentite (Pb,Ag)S Argentiferrous Galena HgS Cinnabar Cu2S

Ag3AsS3 Proustite (Cu,Fe,Ag)As4S13 Argentiferrous Tennantite

Ag3SbS3 Pyragyrite (Cu,Fe,Ag)Sb4S13 Argentiferrous Tetrahedrite

Cu3AsS4

(Cu,Fe)Sb4S13

Chalcocite CuS Covellite Cu5FeS4 Bornite CuFeS2 Chalcopyrite PbS Galena ZnS Sphalerite

Enargite (Cu,Fe)As4S13 Tennantite

Tetrahedrite

AuTe2 Krennerite Calaverite Ag2Se Naumannite

Ag2Te Hessite

C Graphite C Amorphous C

Arsenic

Antimony Bismuth

Selenides

(Fe,Ni)9S8 Pentlandite

Mercury Copper

Antimonides

CoAsS Cobalite

Nickel Gold

Arsenides FeAsS Arsenopyrite

Bi Native Bismuth

AsS Realgar As2S3 Orpiment Sb2S3 Stibnite Bi2S3 Bismuthinite

Bi2Te2S Tetradymite

Source: Scott and Ingles, 1987

3

Chapter One


TABLE 1.2

Solubility of Metal Sulphide Minerals in Cyanide Solutions

Mineral Sphalerite Chalcocite Chalcopyrite Bornite Enargite Tetrahedrite Metallic Copper

Formula ZnS Cu2S CuFeS2 FeS – 2Cu2S-CuS 3CuS As 2S5 4Cu 2S - Sb2S3 Cu


Notes: (1) 2.0 g/L NaCN solution at 45°C (2) 1.0 g/L NaCN solution at 23°C


4

PercentExtractionofMetal (1) 18.4 (2) 90.2 (2) 5.6 (Cu) (2) 70.0(Cu) (2) 65.8 (Cu) (2) 21.9 (Cu) (2) 9.0


Iron sulphides are not the only minerals capable of generating thiocyanate in cyanide solutions. All sulphide minerals, except lead sulphide (galena), have this potential. The primary requirement is the formation of compounds containing labile sulphur atoms in solution. These include elemental sulphur micelles (S8) and alkali or alkaline earth polysulphides, thiosulphates and polythionates. Cyanide does not react directly with the sulphide ion, but the compounds mentioned above can all yield sulphide ions which arise from alkaline attack on the metal sulphide minerals. One method of minimizing thiocyanate formation is by promoting complete oxidation of sulphides to sulphates, which can be accomplished to some extent by aeration. Better results are possible if the addition of cyanide is postponed until the sulphide oxidation reactions are complete. If, at the same time, the pH of the pulp is maintained strongly basic, some reduction of ferrocyanide formation will occur. However, generation of elevated sulphates may pose additional treatment and environmental problems (e.g. the precipitation of gypsum).

1.3

GOLD RECOVERY FROM SOLUTION

There are two basic methods employed for the recovery of gold from solution; cementation (Merrill-Crowe process) and granular activated carbon adsorption. The zinc cementation (Precipitation) process is based upon an electrochemical reaction between metallic zinc powder and the gold-cyanide complex, in the absence of oxygen, to reduce the univalent gold ions to its free metal: (1.4)

Zn + 2NaAu(CN) 2 → 2Au + Na 2Zn(CN)4

The presence of nickel ions in solution promotes hydrogen evolution by lowering the hydrogen overvoltage of zinc, causing poor gold recovery and high zinc consumption. Lead ions yield metallic lead which increases the hydrogen overvoltage and provides surfaces for gold deposition. Any ferricyanide ions present are reduced to ferrocyanide. Zinc precipitates gold and silver nearly completely, along with a portion of the copper. Other components of the solution, with the exception of mercury, arsenic and antimony, are relatively unaffected. The granular activated carbon (GAC) adsorption process introduces no additional reagents in the gold and silver recovery stage as does the Zinc Cementation process. With the activated carbon process, gold and silver cyanide complexes are adsorbed onto active carbon sites, thereby removing gold and silver from solution. Stripping gold and silver from the loaded carbon is typically accomplished using a solution containing 0.1% NaCN and 1% NaOH at elevated temperatures (Zadra Process). The gold is usually recovered from the strip solution by electrowinning, so that a portion of the cyanide is recycled. The addition of zinc and the characteristics of the zinc precipitation solutions are eliminated using the granular activated carbon method. of activated carbon and can increase decrease the the efficiency concentration undesirable metals in solutionThe (e.g.use mercury and copper) and of ease of water treatment.

5

Chapter One


In addition to cyanidation, cyanide is also used as a depressant in the flotation of base metal sulphide ores. During the flotation process, metal cyanide complexes may be generated (e.g. copper and iron), which require removal prior to discharge of these solutions into the environment. Although the levels of cyanide used are significantly lower than those used in cyanidation, the potential environmental issues relating to this secondary use of cyanide should be considered. The above discussion provides an introduction into the chemistry of cyanide usage and cyanidation in the mining industry. The information is useful in understanding the effect cyanide usage and ore geochemistry have on the chemistry of process solutions generated during metals recovery. The next section includes a discussion of the chemistry of the various forms of free and metal complexed cyanide.

1.4

SOLUTION CHEMISTRY OF CYANIDE AND ITS METAL COMPLEXES

1.4.1

Introduction

The cyanide compounds present in gold mine or cyanidation solutions and effluents include free cyanide, the alkali earth salts and the metal cyanide complexes formed with gold, mercury, zinc, cadmium, silver, copper, nickel, iron and cobalt. These may be classified into five general categories, as shown in Table 1.3. The discussion of cyanide chemistry involves consideration of the following issues: • • • •

The nature of bonding in the cyanide radical The behaviour of hydrogen cyanide and "free" cyanide ions So-called simple cyanide compounds Cyanide complexation

The preceding elements of cyanide chemistry, together with cyanide reactions involving oxygen, sulphur species and biological processes, produce a variety of cyano-species and derivatives which are addressed in the following section. 1.4.2

Cyanide Bo nding

The complex nature of cyanide behaviour can be explained, at least in part, by the bonding in the cyanide radical. Baker (1984) has produced a discourse that examines the nature of cyanide and its chemistry, using these chemical factors as a basis for the general lack of adverse environmental impacts from cyanide.


6


The cyanide ion is an anion which comprises one atom of carbon and one of nitrogen. It has one sigma bond, two pi bonds and two empty bonding orbitals. The first two orbitals in its structure are filled with the maximum number of electrons, while the other orbitals are empty. Because the "s" and "p" (1 + 2) orbitals are filled with electrons it behaves like a halogen (i.e. fluorine, chlorine, bromine and iodine). This means some of the properties of, say, NaCl will not be radically different from NaCN (sodium cyanide). However, its pseudo-halogen behaviour cannot explain the formation of cyanide metal complexes with transition series metals (i.e., Fe, Co, Ni, Cu and Zn). The empty anti-bonding orbitals on the cyanide ion can form bonds with the "d" orbitals (partially or wholly filled) of transition series metals. The contribution of an electron pair (either from the cyanide ion to the metal or vice versa) is known as "back bonding" and explains the stability of the cyanide-metal complexes. The cyanide ion also has a triple bond and triple bonds of this type can be broken easily and, hence, can be considered to be quite reactive. In summary, these factors taken together explain the complex behaviour of cyanide: •

Pseudo-halogen behaviour of the cyanide anion

•

Back bonding utilizing empty anti-bonding orbitals to explain stable complex formations

•

A triple bond which can be easily broken to explain its relatively rapid natural degradation

1.4.3

Free C yanide

Free cyanides are defined as the forms of molecular and ionic cyanide released into aqueous solution by the dissolution and dissociation (or ionisation) of cyanide compounds and complexes. Therefore, the term free cyanide is confined to two species, the cyanide ion (CN ) and hydrocyanic acid or hydrogen cyanide (HCN). The relative proportion of these two forms depends upon the pH of the system. Hydrocyanic acid is a relatively weak acid and its dissociation in aqueous solution into the cyanide anion is expressed by the following equation: (1.5)

+

-

HCN → H + CN

At any particular pH and temperature, the relative amounts of each of these two free cyanide compounds present at equilibrium can be determined from the following expression: [H + ][CN − ] (1.6)

K=

[HCN]

−10

= 2.03 x 10

o

, pKa = 9.31 (20 C)

In Equation 1.6, the compounds in brackets are molar concentrations and K is the equilibrium constant. Figure 1.1 and Table 1.4 present this relationship in graphical and mathematical form. In natural waters with a pH below about 8.5, cyanide is present predominantly as the free acid (i.e., HCN).

7

Chapter One


The pH behaviour of cyanide is important in gold-cyanide extraction processes because at pH 10.5 or greater most of the free cyanide in process slurry water or heap leach interstitial fluid will be as the cyanide anion (CN ). Since the HCN content is low at this pH value, the capacity for cyanide loss by volatilisation is limited. In natural aqueous systems that have pH values in the range of about 5.0 to 8.5, the majority of free cyanide will be in the form of HCN and can be lost by volatilisation. Molecular HCN has a low boiling point and a high vapour pressure and can be lost from solution, particularly where there is a water/air interface. From a practical, economic, or safety point of view, the solution pH must be in excess of about 10.0 to limit HCN formation and loss from aqueous systems. 1.4.4

Simple Cyanide Compounds

The simple cyanides can be defined as the salts of hydrocyanic acid (e.g. KCN and NaCN), which dissolve completely in aqueous solution producing free alkali earth cations and cyanide anions: +

-

(1.7)

NaCN → Na + CN

(1.8)

Ca(CN)2 → Ca

+2

+ 2CN

-

-

The CN then reacts with water to form HCN in an alternative form of Equation (1.5): (1.9)

-

CN + H2O → HCN + OH

-

The simple cyanides are electrically neutral (the positive charges of the metal ion balance exactly with the negative charges of the cyanide ions) and are capable of existing in solid form. The simple cyanide compounds are water soluble and dissociate or ionise readily and completely to yield free cyanide (as defined above) and the metal ion. The most common industrially used cyanide solid is NaCN, though lesser quantities of KCN and Ca(CN)2 are also used.


8


TABLE 1.3

Classification of Cyanide and Cyanide Compounds in Cyanidation Solutions on the Basis of Stability

Classification 1. Free Cyanide 2. Simple Compounds Readily soluble neutral Insoluble salts 3.WeakComplexes 4. Moderately Strong Complexes

CN

Compound , HCN

NaCN, KCN, Ca(CN)2, Hg(CN)2 Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN -2 -2 Zn(CN) 4 , Cd(CN)3 , Cd(CN)4 -2 -2 Cu(CN) 2 , Cu(CN)3 , Ni(CN)2 , Ag(CN)2 -4

5.StrongComplexes

Fe(CN)

6

-4

-

-3

, Co(CN)6 , Au(CN)2 , Fe(CN)6


9

Chapter One


FIGURE 1.1

Relationship Between HCN and CN

-

o

with pH (25 C)

100% 90% 80% N C H s a t n e s e r P e id

70%

n a y C e re F

40%

60% CN-

HCN

50%

30% 20% 10% 0% 6.0

7.0

8.0

9.0 pH


10

10.0

11.0

12.0


TABLE 1.4

Relative Concentrations of Hydrocyanic Acid and Cyanide Ion -4 in a 10 Molar Solution as a function of pH

pH 6.0 6.6 7.0 7.5 7.8 8.0

Cyanide (as CN, mg/L) 0.002 0.007 0.02 0.06 0.11 0.17

Hydrocyanic Acid (as CN, mg/L) 2.598 2.593 2.58 2.54 2.49 2.43

9.0 10.0 11.0

1.09 2.28 2.56

1.51 0.32 0.04

11

Chapter One


1.4.5

Weak and Moderately Strong Cyanide Complexes

The chemistry of the cyano-metal complex is more involved than that of the free cyanide compounds examined in the previous section. Huiatt et al. (1982) gives a description of alkalimetal cyanide complexes and their behaviour. However, not all complex cyanides are alkalimetal cyanide complexes and subsequent discussion must be seen in that context. The following description addresses the transition metal cyanide complexes as an illustration of cyanide-metal complex chemistry. Alkali-metal cyanides complexes have the general formula: AaM(CN)b where: A: M: a: b:

+

+

is the alkali metal cation (e.g. Na or K ) is usually a transition series metal (i.e. Fe, Cu, Co, Ni, Cr and Zn,) is the number of cations is the number of cyanide groups

For example, the formula of potassium ferricyanide would be K3Fe(CN)6: the ferric ion is trivalent, the potassium monovalent. Hence the number of potassium atoms is 3 and the number of cyanide groups is 6. When an alkali-metal cyanide dissolves in water, instead of release of the cyanide ion, as described with the free cyanide or the simple alkali cyanide compounds, the transition metal and cyanide remain combined as a complex radical anion. To illustrate using the compound K3Fe(CN)6: +

-3

(1.10) K3Fe(CN)6 → 3K + Fe(CN)6

The metal cyanide complex anion stability is dependent upon the metal cation with which it is associated, the pH and the redox potential of its associated environment. Another aspect of metal cyanide complex chemistry is the formation of insoluble double-metal cyanide precipitates. This reaction has been used very successfully to remove free cyanide from solutions by the formation of ferric ferrocyanide, Fe 4(Fe(CN)6)3 or other transition metal ferrocyanide precipitates. This cyanide removal mechanism is discussed in greater detail in Chapter 6. The weak and moderately strong cyanide complexes primarily include cyanide complexed with cadmium, copper, nickel, silver and zinc. These complexes form in a step-wise manner, with successively higher cyanide contents as the cyanide concentration of the solution is increased.


12


For example, the step-wise formation of the copper-cyanide complexes occurs as follows: -

-

(1.11) CuCN + CN → Cu(CN)2 -

-

-2

(1.12) Cu(CN)2 + CN → Cu(CN)3 -2

-

-3

(1.13) Cu(CN)3 + CN → Cu(CN)4

Depending on the concentration of cyanide, metals and the solution pH, the metal-cyanide complexes are present in solution in varying proportions. The stability of these cyanide compounds varies according to the metal ion involved, with zinc and cadmium forming the weakest complexes and iron and cobalt forming the strongest complexes. However, even these complex anions in some cases can dissociate to release the cyanide ion in the presence of UV radiation or very strong acids. The dissociation constants in order of metal complex stability are presented in Table 1.5. The rates of metal-cyanide complex dissociation and release of free cyanide are affected by several factors including the intensity of light, water temperature, pH, total dissolved solids and complex concentration. The liberation of free cyanide through photolysis is most pronounced for the iron-cyanide complexes. The dissociation constants can be utilized to calculate the concentration of free cyanide released when these complexes are present in solution. Table 1.6 presents the equilibrium concentrations of free cyanide produced at various complex concentrations dissolved in water at pH 7.0 and 25°C. The very low concentrations of free cyanide indicated in Table 1.6 demonstrate the stability of complexes under ambient conditions. In general, a decrease in pH and complex concentration increases the percentage of free cyanide produced. As a result, the breakdown of each metal cyanide complex increases with decreasing concentration. It is the stability of the specific complex which dictates its ease of treatment and relative toxicity. The most important parameter in determining the stability or extent of dissociation of these metal cyanide complexes is the solution pH. A thorough discussion of the chemistry of metal cyanide complexes can be found in Flynn and McGill (1995). The next section examines the chemistry of the iron cyanide complexes in more detail.

13

Chapter One


TABLE 1.5

Stability Constants of Metal Cyanide Complexes

ComplexIon

StabilityConstant

-

16.0

Zn(CN)3 -2 Cd(CN)4 -2 Zn(CN)4 -1 Ag(CN)2 -2 Ag(CN)3 -1 Cu(CN)2 -2 Cu(CN)3 -2 Cu(CN)4

10 17.9 10 19.6 10 20.5 10 21.4 10 21.7 10 27.0 10 28.5 10

-2

30.2

Ni(CN)4-3 Cr(CN)6 -4 Fe(CN)6 -l Au(CN)2 -2 Hg(CN)4 -3 Fe(CN)6 -3 Co(CN)6

1033.0 10 35.4 10 37.0 10 39.0 10 42.3 10 73.0 10

Source: Flynn and McGill, 1995


14

(1)


TABLE 1.6

(1,2)

Complex -2 [Hg(CN)4] -1 [Ag(CN)2] -2 [Cu(CN)3] -3 [Fe(CN)6] -4 [Fe(CN)6] -2 [Ni(CN)4] -2 [Cd(CN)4] -2 [Zn(CN)4]

Free Cyanide Concentration Released at Various Metal-Cyanide Complex Concentrations

1mg/L 0.00002 0.00009 0.0003 0.0002 0.0012 0.135 (3) (3)

10mg/L 0.00003 0.0002 0.00054 0.0032 0.0016 0.215 2.30 2.26

100mg/L 0.000045 0.0004 0.00097 0.0004 0.0022 0.340 3.64 3.59

1,000mg/L 0.00007 0.0009 0.0017 0.0006 0.0031 0.539 5.77 5.68

100,000mg/L 0.00018 0.0041 0.0054 0.0012 0.0061 1.324 14.49 14.28

Source: Caruso, 1975

Notes: (1) All values in mg/L. o (2) Free cyanide levels calculated at pH 7.0 and 25 C. (3) Calculations indicate that at this dilution the two complexes are completely ionised.

15

Chapter One


1.5

SOLUTION CHEMISTRY O F TH E IRON CYANIDES

From an environmental viewpoint the iron cyanides (i.e. hexacyanoferrates) require special attention due to their stability in the absence of light and their tendency to dissociate in its presence. Considerable controversy has evolved concerning the relative toxicity of the iron cyanides due to photolysis. Although these complexes resist natural degradation, they are nonetheless capable of releasing toxic levels of hydrogen cyanide when exposed to intense ultraviolet radiation. The hexacyanoferrates undergo a much broader range of reactions than do the other metal cyanide complexes and their solution chemistry has been studied more thoroughly. Ferrocyanide and ferricyanide both form stable salts with other metals without undergoing exchange of the cyanide ligand. Similarly, ferrocyanide is readily and reversibly oxidized to ferricyanide although the cyanide content remains unaffected. Since most methods of cyanide removal depend on oxidation processes, the removal of hexacyanoferrates from an effluent requires consideration of other aspects of its chemical behaviour (e.g. chemical precipitation). Cyanide reacts with iron to form stable octahedral complexes including hexacyanoferrate (II) or ferrocyanide, in which the iron is in the reduced state with a valence of +2. Ferrocyanide, which is the usual form in solution at ambient redox potentials, is readily oxidized to ferricyanide or hexacyanoferrate (III). In this case iron is present in the oxidized ferric form with a valance of +3. Hexacyanoferrate (II) can be formed by addition of a soluble ferrous salt or freshly prepared ferrous hydroxide to a solution containing free cyanide. In practice, the reaction appears to be limited to a pH below about 9.0. There is evidence that dissociation of the complex occurs rapidly above this pH. It is much slower than the reactions with copper and nickel, and approximately the same as that of zinc. Only the free cyanide precipitates with ferrous addition, a process which requires 15 to 60 minutes, depending upon solution pH and the cyanide and ferrous levels. The addition of excess ferrous iron increases the amount of complex formed. There is some evidence that a large excess of ferrous, coupled with a pH below 4.0, would precipitate other metal cyanide complexes as well. In spite of its larger stability constant, ferrous iron will not displace zinc, copper or nickel from their cyanide complexes. Hexacyanoferrate (III) cannot be formed directly in solution from ferric iron and cyanide, probably due to the greater insolubility of ferric hydroxide. Its formation is primarily the result of the oxidation of hexacyanoferrate (II). The hexacyanoferrates are classified as "inert" complexes, in that their chemical stability results from extremely slow rates of dissociation and wastes relatively toxicity. theand/or precipitated iron cyanides present in mining solutions and are low mainly in the Although mixed ferroferriforms, other relatively insoluble metal iron cyanide compounds do exist. A compilation of the solubilities of various iron cyanide complexes is shown in Tables 1.7 and 1.8.


16


In the presence of ultraviolet light, photolysis and hydrolysis of iron-cyanides occurs, in which a water molecule displaces one of the cyanide moieties in the complex. On prolonged exposure, hexacyanoferrate (II) and (III) have been shown to release up to 85% and 49% percent of their cyanide content, respectively (Broderius and Smith, 1980). However, the experiments involved closed systems and very high levels of ultraviolet radiation when compared to natural systems. Boiling with phosphoric acid and/or certain catalysts (mercuric or magnesium compounds), or with strong complexing agents (e.g. EDTA or tartaric acid), results in the decomposition of the hexacyanoferrates and liberation of hydrocyanic acid. The hexacyanoferric (II) and (III) acids are relatively strong, (i.e. their hydrogen ions are easily dissociated to liberate the anions), a property which is attributed to the coordination of the cyanide group with a resultant weakening of the hydrogen linkage. The dissociation constants of the acids must be taken into account in calculating solubilities and concentrations of complex ions since they affect the concentration of the anion available for these reactions. As a result, changes in pH affect the solubility and stability relationships of the metal ferrocyanide compounds. The hexacyanoferrate (II) and (III) salts are formed by the reactions of the hexacyanoferrate ions and the corresponding cation. In the case of ferrocyanide, if alkali earth metals ions are present, the resulting precipitate will usually contain the alkali as well, often as a double salt. Ferricyanides are less subject to this phenomenon. The alkali and alkaline earth hexacyanoferrates are all soluble in water, except for barium hexacyanoferrate (II) which is only moderately soluble. All the alkali and alkaline earth salts of hexacyanoferrate (II) are insoluble in alcohol. The heavy metal salts of hexacyanoferrate (II) are insoluble and precipitate in water. Because the corresponding acid is fairly highly dissociated, the solubility of these precipitates, in the absence of metal-complexing ligands, is not greatly affected by changes in pH over the range pH 2 to pH 11 as shown in Table 1.9. In the case of those metals that form strong cyanide or amine complexes such as cadmium, copper, nickel and silver, the precipitates either dissolve or fail to form in solutions that contain excess cyanide ions or free ammonia. However, the complexes can again precipitate if the pH is lowered to a point where the concentrations of these ligands are insufficient to maintain the metal complex.

17

Chapter One


TABLE 1.7

Name Ammonium Ferricyanide Ammonium Ferrocyanide BariumFerrocyanide Cadmium Ferrocyanide Calcium Ferrocyanide CobaltFerrocyanide Copper (I) Ferricyanide Copper (II) Ferricyanide Copper (II) Ferrocyanide Iron (II) Ferricyanide Iron (III) Ferricyanide Iron (II) Ferrocyanide Iron (III) Ferrocyanide LeadFerricyanide

Solubilities of Fe rrocyanides and Fer ricyanides

Formula (NH 4)3Fe(CN)4 (NH 3)5Fe(CN)6 • 3 H2O Ba 2Fe(CN)6 • 6 H2O Cd 2Fe(CN)6 • X H2O Ca 2Fe(CN)6 • 12 H2O Co 2Fe(CN)6 • X H2O Cu 3Fe(CN)6 Cu 3(Fe(CN)6)2 • 14 H2O Cu 2Fe(CN)6 • X H2O Fe 3(Fe(CN)6)2 FeFe(CN) 6 Fe 2Fe(CN)6 Fe 4(Fe(CN)6)3 Pb 3(Fe(CN)6)2 • 5 H2O

Magnesium Ferrocyanide Mg 2Fe(CN)6 • 12 H2O Manganese (II) Ferrocyanide Mn 2Fe(CN)6 • 7 H2O NickelFerrocyanide Ni 2Fe(CN)6 • X H2O Potassium Ferricyanide K 3Fe(CN)6 Potassium Ferrocyanide K 4Fe(CN)6 • 3 H2O SilverFerricyanide Ag 3Fe(CN)6 SilverFerrocyanide Ag 4Fe(CN)6 • H2O SodiumFerricyanide Na 3Fe(CN)6 • H2O Sodium Ferrocyanide Na 4Fe(CN)6 • 10 H2O Strontium Ferrocyanide Sr 2Fe(CN)6 • 15 H2O Thallium Ferrocyanide Th 4Fe(CN)6 • 2 H2O Tin (II) Ferrocyanide Sn 2Fe(CN)6 Tin (IV) Ferrocyanide SnFe(CN) 6 ZincFerrocyanide Zn 2Fe(CN)6 Source: Huiatt et al., 1982


18

Solubility g/L(°C) very soluble soluble 1.7g(15 °) insoluble 868g(25 °) insoluble insoluble insoluble insoluble insoluble -insoluble insoluble slightlysoluble 330g insoluble insoluble 330g (4+°) 278g (12 °) 0.00066 (20°) insoluble 189g (0 °) 318.5g(20 °) 500g 3.7g(18 °) insoluble insoluble insoluble


TABLE 1.8

Solubilities of Complex Ferrocyanide and Ferricyanide Salts

Compound

Solubility

Fe4 (Fe(CN)6)3

25 x 10

-5

6

Zn2 Fe(CN)

260 x 10

Zn3 (Fe(CN)6)2

2.2 x 10

-5

-5

Source: Huiatt et al., 1982

TABLE 1.9

Effect of Initial pH on Fe rrocyanide Solubility

-4

InitialpH 2.0 6.0 8.0 11.0

Cadmium 3.0 3.1 3.1 2.4

Solubility of Fe(CN)6 (mg/L) Copper Iron Manganese (Ferrous) 2.4 4.5 3.7 2.5 3.5 3.7 1.3 3.0 3.2 1.3 2.3 2.8

Zinc 3.3 1.5 1.8 1.9

Source: Hendrickson and Daignault, 1973

19

Chapter One


The heavy metal salts of hexacyanoferrate (III) are sparingly soluble but, nevertheless, are considerably more so than their ferrocyanide analogues. Table 1.10 compares their solubilities as well as those of mixtures of the two. As a cautionary note, while the data in Tables 1.9 and 1.10 are useful for comparative purposes, they should not be considered as absolute values, since the authors provided limited information on the test method or the analytical procedure used and indicated that there could have been a small positive interference during analysis. Hexacyanoferrate (II) and (III) form an oxidation-reduction couple: -3

-

-4

(1.14) Fe(CN)6 + e → Fe(CN)6

Although the reaction does not itself involve the hydrogen ion, it is nevertheless pH dependent due to the difference in dissociation of the corresponding acids and in the relative proportion of the two free ions present at pH values below 7. As a result, hexacyanoferrate (II) is more easily oxidized in neutral than in acid solutions. Hexacyanoferrate (III) is reported to be reduced to hexacyanoferrate (II) by cyanide and would be readily reduced during the Merrill-Crowe precipitation step used for gold recovery (Williams, 1915). Hexacyanoferrate (II) is not oxidized by air in neutral or alkaline solutions in the absence of light or catalysts. It is oxidized by hydrogen peroxide in acid solution but not in alkaline solution. Chlorine, hypochlorite and ozone all yield hexcyanoferrate (III). Where these reagents have been employed for cyanide destruction, the addition of a suitable reducing agent would promote its reduction and removal. The pH must be adjusted into the range of about 3.0 to 7.5 for the reaction. Suitable reducing agents include Na2SO3, Na2S2O3, Na2S2O5, hydroxylamine hydrochloride and hydrazine. Ferrocyanide binds readily to both weak and strong base anion exchange resins because of its high negative charge. The reaction is highly selective for ferrocyanide over other anions including cyanide. Weak base resins can be regenerated using 1% to 10% sodium hydroxide solutions. The bond between ferrocyanide and the usual strong base resins (e.g. IRA-400) is so strong that regeneration is very difficult. However, one resin (IRA-958) has been specifically designed for this purpose and is regenerated with sodium chloride solution.


20


TABLE 1.10 Solubility of Hexacyanoferrates (II) and (III) Separately and in Mixtures

Hexacyanoferrate Compound Ferrocyanide Ferricyanide Ferrocyanide Ferricyanide Ferrocyanide Ferricyanide Ferrocyanide Ferricyanide Ferrocyanide Ferricyanide

Solubility of Metal Iron-Cyanide Complexes (mg/L) Ferrous Ratio Fe(CN)6 Cadmium Copper Manganese Iron 100 750 2.9 4.5 2.9 2.6 0 0 ----75 562.5 2.0 3.6 2.0 7.0 25 187.5 3.2 0.8 1.0 53 50 375 2.2 10.2 2.6 12.8 50 25 75 0 100

375 187.5 562.5 0 750

5.0 2.0 2.8 -327

7.0 13.4 5.7 -307

0.5 3.3 1.2 -74

194 31.5 190 -300

Zinc 2.3 -4.8 45 10.2 55 4.1 47 -256

Source: Hendrickson and Daignault, 1973

21

Chapter One


1.6

CYANIDE RELATED COMPOUNDS

1.6.1

Introduction

As a result of cyanidation, natural attenuation or water treatment, a variety of cyanide related compounds are formed in solution including thiocyanate, cyanate, ammonia and nitrate. These compounds are important from both toxicological and treatment standpoints. More laboratory and field evidence is accumulating indicating that these compounds, once thought relatively unimportant, must be considered during the design of effective and efficient water treatment facilities. 1.6.2

Thiocyanate -

The presence of thiocyanate (i.e. SCN ) in process solutions and effluents results from the reaction of cyanide with labile sulphur atoms, either during pre-aeration or during leaching. The labile sulphur may srcinate directly from the attack of lime or cyanide on pyrrhotite, or be formed by the air-oxidation of sulphide ions released by dissolution of the readily soluble metal sulphide minerals. The dissociation constant for thiocyanic acid (i.e. HSCN) is near zero and exists predominantly as the anion at pH values above about 2.0. Thiocyanate is chemically quite different from cyanide, exhibiting a lower toxicity and a somewhat lower tendency to form soluble metal complexes. The thiocyanate ion and cyanide are classified chemically as pseudohalogens (i.e., they have properties similar to chloride, bromide and iodide) and can form insoluble ionic salts with silver, mercury, lead, copper and zinc. Thiocyanate is chemically and biologically degradable, with the metabolic by-products being ammonia, carbonate and sulphate. Since thiocyanate is more readily oxidisable than cyanide in certain instances, there are concerns regarding its removal during treatment. The primary environmental concerns associated with thiocyanate include its toxicity and its breakdown products (cyanate, ammonia and nitrate), which may be toxic if present in sufficient levels. 1.6.3

Cyanate

Many oxidants, including chlorine, ozone, oxygen and hydrogen peroxide, convert cyanide to cyanate (i.e. OCN ). The mechanism for the dissolution of gold involves formation of hydrogen peroxide as part of the initial step and it is possible that the cyanate present in the cyanide leach solutions arises as a result of peroxide attack on cyanide. Cyanate is normally present in cyanidation solutions, but does not tend to accumulate due to its hydrolysis to ammonia. Cyanate is also chemically quite metal different from cyanide, lower toxicity a much lower tendency to form soluble complexes. The exhibiting toxicity ofa cyanate and itsand breakdown products are important and are discussed in Chapter 5.


22


1.6.4

Ammonia

The two sources at mining operations that are responsible for the majority of ammonia present in waters are from residual blasting agents (i.e. ammonium nitrate fuel oil mixture) and from the hydrolysis of cyanate. Ammonia may also result from the stripping of loaded carbon where relatively hot cyanide solutions partially oxidize to generate ammonia. The hydrolysis of cyanate to ammonia occurs at a relatively slow rate in alkaline solution, but may proceed rapidly at a pH of less than about 6.0 or at elevated temperatures. Free ammonia forms soluble amine complexes with many heavy metals, including copper, nickel, silver and zinc. The hydrolysis constant for the free ammonia - ammonium ion -10 equilibrium is 1.86 x 10 at 10°C (pKa = 9.73). As a result, the presence of ammonia in effluents can inhibit the precipitation of these metals at pH values above 9.0, the pH range generally chosen for precipitation of metal hydroxides. The toxicity of ammonia is discussed in detail in Chapter 5. 1.6.5

Nitrate -

Nitrate (i.e. NO3 ) is the predominant end-product resulting from the oxidation of cyanide and is formed from the oxidation of ammonia through biological and chemical reactions. Nitrate is a relatively non-toxic compound and is stable in the environment under a wide variety of natural conditions. The primary concern with nitrate is with drinking waters where elevated concentrations can be harmful to humans, particularly young children and infants. In addition, nitrate is a biological nutrient and in some cases can lead to accelerated algae growth in waters, thereby consuming dissolved oxygen and potentially impairing the ability of fish to survive. Therefore, it is important that nitrate concentrations in untreated and treated waters be considered, though this topic is left to the large number of texts dealing specifically with nitrogen control.

1.7

PROCESS SOLUTION CHEMISTRY

1.7.1

Introduction

Process solutions resulting from the cyanidation of gold and silver ores can be quite complex, containing various levels of a number of cyanide and related compounds. Formation of these compounds primarily takes place in cyanidation circuits, though their formation and build-up can be complicated by the recirculation of process solution which typically takes place at mining operations. An evaluation of projected long-term cyanidation solution chemistry is critical during upfront metallurgical particularly laboratory testwork often cannot mimic the solution recirculation thattestwork, will take place on the since full-scale. The range of leach solution chemistries possible in cyanidation circuits is reflected in data presented by IEC (1979) as summarized in Table 1.11.

23

Chapter One


TABLE 1.11 Range of Cyanidation Solution Chemistries

Compound

ConcentrationRange (mg/L) 27to650 275 to3 0.1 10 to 36 0.1 to 237 to1 16to510

TotalCyanide Copper Nickel Iron Zinc Thiocyanate Source: IEC, 1979

TABLE 1.12 Listing of Several Iron-Cyanide Complexes

Iron-Cyanide Complex -3

Fe(CN)6

-4

Fe(CN)6

-3

Fe(CN)5

-2

Fe(CN)5

[Fe(CN)5 • H2O]

-3

[Fe(CN)5 • H2O]

-2

Fe2(CN)6 (aq) -2

H2Fe(CN)6

-3

HFe(CN)6


24


In addition to compounds listed in Table 1.11, other cyanide related compounds are frequently encountered in leach solutions, including cyanate, ammonia and nitrate. It is critical that persons involved in the analysis of cyanide be familiar with the interactions of cyanide with metals in solution, particularly for environmental monitoring samples. The following sections provide brief descriptions of a few process solution issues frequently encountered at mine sites. 1.7.2

Iron Cyanides

While there is generally a good understanding of the chemistry of the iron-cyanide complexes, iron-cyanide chemistry is nonetheless complicated. The most commonly encountered iron-4 -4 cyanide complexes are the two formed with ferrous and ferric iron, Fe(CN) 6 and Fe(CN)6 . However, as indicated in Table 1.12 there are a number of other complexes that may be present, though usually these are present at low levels. The presence of these other complexes becomes important when cyanide solutions are exposed to UV irradiation (e.g. sunlight) where the complexes undergo photocatalytic degradation, resulting in the formation of several intermediate iron-cyanide complexes. These complexes vary in stability and some may be detected in analytical procedures not intended for their detection. This is important with environmental monitoring where low levels of cyanide would be significantly influenced by the presence of iron-cyanide complexes or where these complexes may interfere with an analytical procedure. 1.7.3

Copper Bearing Ores

The presence of nuisance copper in cyanidation circuits is of major concern because of its impact on cyanide consumption, gold recovery and tailings treatment. Cyanide-soluble copper is present in many gold and silver ores, and resultant concentrations of copper in process solutions can range from only a few mg/L up to several hundred mg/L. When present in cyanide leach solutions, copper is predominantly present as a mix of three -2 -3 forms: Cu(CN)2 , Cu(CN)3 and Cu(CN)4 . As the cyanide to copper molar ratio (CN:Cu) in solution drops to less than about 3:1, there begins to be less free cyanide in solution to leach gold and silver. This can lead to reduced gold and silver recovery, which is oftentimes countered with an increase in the leach solution cyanide concentration. This in turn, may result in increased leaching of copper and a further consumption of cyanide. In some cases, this circular response leads to very high levels of cyanide in solution, most of which is bound as a copper-cyanide complex and unavailable for gold and silver leaching. The implications of this situation on tailings treatment processes must be considered during the laboratory metallurgical testwork stage. In some cases, full-scale plants may require a treatment system for copper removal, either from the plant tailings or from a leach solution bleed stream.

25

Chapter One


1.7.4

Silver Ores

Silver ores require relatively high levels of cyanide for leaching, commonly at levels two to ten times that required for an equivalent gold ore. However, with higher levels of cyanide, other metals present in an ore that may not normally leach could begin to leach along with silver. Aside from complications this poses to silver recovery processes, this can lead to complicated and expensive downstream water treatment processes. Waters resulting from silver leaching can contain elevated concentrations of both cyanide and metals that are not characteristic of gold leaching solutions. These other metals may include cadmium, cobalt, chromium, copper, mercury, silver, thallium and selenium, all of which are known to form complexes with cyanide. In the design of a silver cyanidation circuit, the cost of water treatment must be considered along with the economics of silver recovery versus leach solution cyanide levels.

1.8

REFERENCES

Baker, D.J., "Cyanide in the Environment", Unpublished Data, 1984. Broderius, S. and Smith, L., USEPA, Grant No. R805291, 1980. Caruso, S.C., "The Chemistry of Cyanide Compounds and Their Behavior in the Aquatic Environment", Carnegie Mellon Institute of Research, June, 1975. Flynn, C.M. and McGill, S.L., Cyanide Chemistry – Precious Metals Processing and Waste Treatment, U.S. Bureau of Mines, NTIS Publication PB96-117841, 1995. Hendrickson T.N. and Daignault, L.G., "Treatment of Complex Cyanides for Reuse and Disposal", Report No. EPA-R2-73-269, USEPA, 1973. Huiatt, J., Kerrigan, J., Olson, F., and Potter, G., Proceedings of a Cyanide Workshop, Cyanide from Mineral Processing, U.S. Bureau of Mines, Salt Lake City, Utah, February 2-3, 1982. IEC, “Factors Affecting Natural Degradation of Free and Metal-Complexed Cyanides from Gold Milling Effluents”, International Environmental Consultants Ltd., Toronto, Ontario, 1979. Marsden, J. and House, I., The Chemistry of Gold Extraction, Ellis Horwood Limited, Hertfordshire, United Kingdom, 1993. Scott, J. and Ingles, J., State-of-the-Art Processes for the Treatment of Gold Mill Effluents, Mining, Mineral, Metallurgical Processes Environment Canada,and Ottawa, Ontario, March, 1987. Division, Industrial Programs Branch, Williams, H.E., Cyanide Compounds, E. Arnold, London, 1915.


26


1.9

BIBLIOGRAPHY

American Cyanamid Co., The Chemistry of the Ferrocyanides, Vol. VII, Cyanamid's Nitrogen Chemicals Digest, American Cyanamid Co., New York, New York, 1953. Adamson, R.J., "The Chemistry of the Extraction of Gold from its Ores", Gold Metallurgy in South Africa, Cape and Transvaal Printers, Ltd., Cape Town, South Africa, 1972. Brickell, R.H., C-I-L Inc., "Chemistry of Cyanide Solutions", January, 1981. Dorr, J. and Bosqui, F., Cyanidation and Concentration of Gold and Silver Ores, McGraw Hill Book Co., 2nd Edition, 1950. Ecological Analysts Inc., "Cyanide: an Overview and Analysis of the Literature on the Chemistry, Rate, Toxicity, and Detection in Surface Waters", prepared for the Inter-Industry Cyanide Group by Ecological Analysts Inc., Towson, Maryland 2124, 1979. Engineering Science Inc., 'Background Information on Cyanide in the Mining Environment", Amax of Canada Ltd., Vancouver, British Columbia, 1980. Finklestein, N.P., "The Chemistry of the Extraction of Gold from its Ores", in: Gold Metallurgy in South Africa, Adamson R.J. (ed.), Chamber of Mines of South Africa, Johannesburg, South Africa, Chapter 10, 1972. Hedley, N. and Tabachnik, H., "Chemistry of Cyanidation", Mineral Dressing Note No. 23, American Cyanamid Co., New York, New York, 1958. Heinen, H.J., Peterson, D.G., and Lindstrom, R.E., "Processing Gold Ores Using Heap LeachCarbon Adsorption Methods", U.S. Bureau of Mines, Information Circular 8770, 1978. Moggi, L., Bolleta, F., Balzani, V., and Scandola, F., "Photochemistry of Coordination Compounds - XIV, Cyanide Complexes", J. Inorganic Nucl. Chemistry, 28, pp.2597,1966. Sillen, L.G., Stability Constants of Metal-Ion Complexes, The Chemical Society, London, England, 1964. Tchobanoglous, G. and Burton, F.L., Water Engineering, Treatment, Disposal, and Reuse, rd Metcalf & Eddy, Inc., 3 Edition, McGraw-Hill, Inc., New York, 1991.

27

Chapter One

28

CHAPTER TWO

Analysis of Cyanides 2.1

INTRODUCTION

No book on cyanide chemistry and treatment would be complete without a chapter on analysis, since it is the chemical analysis that is critical in the control and monitoring of the treatment of cyanide-containing wastewater discharges. Yet, despite its critical importance, the reliable analysis of cyanide in mining-related solutions is frequently a source of concern and confusion to both operators and regulators around the world. The concern and confusion stems from the regulation of the various forms of cyanide using multiple analytical methods of varying reliability for different industries such as mining and electroplating. In the United States, the current status and applicability of methods of cyanide analysis to mining solutions remains in somewhat of a state of flux. Effluent discharge standards, which are based upon free cyanide, are often included in permits using the total cyanide analysis for compliance purposes. The magnitude of an effluent limitation is frequently below the practical quantitation limit (PQL) for the cyanide analysis in mining effluents, making monitoring and compliance difficult at best. This is despite acknowledgement by many professionals and a considerable database to indicate that there are problems with the accurate determination of cyanide when the analyses are not carefully conducted, particularly at levels below 1.0 mg/L. The United States Environmental Protection Agency (USEPA) has been re-evaluating all cyanide analytical methods by examining the fundamentals of these procedures. The agency is considering aspects such as interferences and applicability, with the objective of recommending appropriate and defensible procedures. These may include analysis of specific metal-complexes rather than the present procedures, which place cyanides into groups based on their overall similarities in behaviour. The process of re-evaluation to such a fundamental level will be time consuming and the present state of uncertainty will continue for some time. It is in this context that cyanide analytical methods are discussed in this chapter. Rather than being merely a reiteration of various excellent studies on the analysis of cyanide, for example Conn (1981), the chapter is focused on those methods currently considered either most applicable or most appropriate for mining effluents. This does not mean that the methods described are neither without their problems and interferences, nor are they universally applicable, but that these methods have found successful application in cases in which a cyanide containing solution is being treated and potentially discharged into the environment.

29


Around the world, the trend in the mining industry has turned away from the traditional approach of regulating free cyanide using the overly conservative total cyanide analysis, in favour of employing the weak acid dissociable (WAD) cyanide analysis, which measures the “toxicologically important” forms.

2.2

ANALYTICAL PROCEDURES FOR CYANIDES

2.2.1

Introduction

There are three main categories of cyanide as determined from analytical methods. These three categories are total, weak acid dissociable (WAD) and free cyanide. As indicated in Figure 2.1, total cyanide includes all the cyanide compounds present in solution, including cyanide complexed with metals, except for cobalt. In Australia, a number of mines have significant levels of cobalt in the ore with several mg/L of cobalt cyanide complexes in the process solutions, mainly adsorbed to particulates. Total cyanide analyses in these cases should include cyanide associated with cobalt. WAD cyanide includes the free and complexed forms of cyanide that can be liberated in a weakly acidic environment, including the weak to moderately strong metal-cyanide complexes. Free cyanide includes only the uncomplexed forms of cyanide, which are the cyanide anion and molecular hydrogen cyanide. For a given sample, the level of total cyanide is always greater than or equal to the WAD cyanide level, and the WAD cyanide level is always greater than or equal to the free cyanide level. Selection of an appropriate cyanide analytical method for a particular situation should involve the following considerations: Complete characterization of the solution to be analysed with particular emphasis on the species of cyanide present and potential interfering components. •

Knowledge of the basic chemistry of cyanides.

•

Awareness of the strengths and weaknesses of a specific analytical method for a given set of conditions and chemical matrix.

•

Understanding of the capabilities of equipment and operator expertise and experience.

•

Knowledge of the potential treatments to obviate or reduce the effect of interferences.

•

Recognition of the fact that treatments to eliminate interferences may introduce additional interferences.

•

Understanding of the particular regulatory requirements being applied in a given situation.

In the context of the above considerations, descriptions of several common analytical methods for cyanide and its related compounds are provided in the following sections.


30

Analysis of Cyanide

FIGURE 2.1 Compounds Included in Total, WAD and Free Cyanide Analyses

Strong Metal-Cyanide Complexes of Fe Weak and Moderately Strong Metal-CyanideComplexesof

Total Cyanide

WAD Cyanide

Ag,Cd,Cu,Hg,NiandZn Free Cyanide

31

-

CN HCN

Chapter Two


2.2.2

Total Cyanide by Distillation

Procedures for total cyanide analyses are presented in Standard Methods (APHA, 1998), in USEPA Methods 9010 and 9012 (USEPA, 1986), in ASTM D2036 Method A (ASTM, 1991) and in ASTM D4374 (ASTM, 1997). The most commonly employed total cyanide analysis for compliance and monitoring purposes involves manual distillation under strongly acidic conditions and a slight vacuum, with the addition of magnesium chloride as a catalyst (APHA, 1998). Sulphuric acid is used to adjust the pH of the sample to less than 2.0 at an elevated temperature to facilitate dissociation of the strong iron-cyanide complexes. During distillation, the free and metal complexed forms of cyanide are converted to the volatile molecular HCN, which is then swept from the flask along with the air and recaptured in a separate caustic scrubber. The caustic solution containing the captured cyanide is then removed from the scrubber and subjected to a colourimetric analysis using pyridine, barbituric acid and chloramine-T as reagents. A reddish-blue colour is formed in the presence of cyanide, the intensity of which is related to cyanide concentration at a specific wavelength of visible light using a spectrophotometer. The entire procedure provides an indirect measurement of cyanide, unlike the direct analysis of a metal using atomic absorption spectrometry. Constituents other than cyanide can react with the reagents used in the procedure producing both positive and negative interferences, leading to observed cyanide levels in a sample that may be either higher or lower than the actual concentration. A schematic of a typical distillation apparatus used in the determination of total cyanide is shown on Figure 2.2. A similar apparatus is used in other cyanide analytical procedures, as discussed in subsequent sections of this chapter. The total cyanide method is subject to a number of common interferences, notably thiocyanates and sulphides. However, these interferences are treatable to a certain extent and the detection limits under favourable conditions are good when the analysis is performed carefully. The total cyanide method will recover all cyanide species with the exception of extremely stable metal cyanides like those of cobalt. It should be noted that thiocyanate and cyanate, although related to cyanide, are not cyanide compounds. Therefore, the total cyanide procedure, or any other cyanide analytical procedure, will not (i.e., should not) measure cyanate or thiocyanate. Separate analytical procedures are available for these compounds, as discussed later in this chapter. The traditional detection limit for total and WAD cyanide has been 0.02 mg/L, although some commercial laboratories are employing lower limits. In practice, the detection limit and practical quantitiation limit are often several fold higher due to matrix interferences.


32

Analysis of Cyanide

FIGURE 2.2 Typical Cyanide Distillation Apparatus

33

Chapter Two


From experience, if performed by skilled technicians, the manual distillation method for total cyanide (USEPA Method 9010) is more reliable than the automated USEPA 9012 procedure (USEPA, 1986). While the reliability of the autoanalyser version of the process is accepted by the USEPA, the autoanalyser method has been shown to give erroneous results on samples verified using the manual distillation. The USEPA did not establish a Best Achievable Technology (BAT) limitation for total cyanide, in part due to the problems associated with its analytical measurement, which the agency indicated was unreliable below 0.20 mg/L (USEPA, 1982). Table 2.1 gives comparative values for total cyanide determinations on effluents from a gold mining project in the United States over a six-month period. With anticipated values of total cyanide being in the 0.005 to 0.02 mg/L range, the manual distillation data are seen to be far more representative than the equivalent autoanalyser results. A second example is from a comparative study of three laboratories, two of which used a manual distillation method on samples of effluent from a heap leach project in the United States (Table 2.2). Here the cyanide values are two orders of magnitude higher than the first example, yet the autoanalyser data are still in error, being about three times the values seen in the manually determined samples. These two examples suggest that if it is proposed to use the autoanalyser technique for routine determination of total cyanide, a comparative study should be made with manual distillation data to ensure that the autoanalyser method is appropriate for the sample matrix being evaluated. In several Australian studies (AMIRA, 1991, 1997, 2000 and Schulz, 1992 and 1997) it was shown that the autoanalyser method based on ASTM D4374 but utilising the McLeod microstill produces results comparable to the manual methods (with skilled operators) and is the preferred method if instrumental costs is not a deterrent. Although removing operator variables, increasing sample turnaround time and the ability to conduct total and WAD cyanide analyses simultaneously provide a commercial advantage, it is essential to utilize appropriate quality control samples and to have a skilled operators familiar with cyanide chemistry and the instrumentation. In recent method evaluations at the Chemistry Centre (Schulz Unpublished Data, 2000) it was found that diffusion membranes used in place of the microstill may give unsatisfactory results due to poor reproducibility. In research conducted in the Homestake Mine analytical laboratory during the early 1980’s, it was noted that the total cyanide method using sulphuric acid in the presence of thiocyanate resulted in a positive error, probably due to the breakdown of thiocyanate promoted by the acid (Whitlock, Sharp and Mudder, 1981). As the thiocyanate concentration was increased in a synthetically prepared sample of cyanide, the magnitude of the error increased. These observations are shown in Tables 2.3 and 2.4. It was found that substitution of phosphoric acid for acid basis. minimized the interferences, although this practice for hasuse notwith yet been accepted on asulphuric standardized Phosphoric acid has also been recommended the automated total cyanide method. It should also be noted that the WAD cyanide method was found less susceptible to the thiocyanate interference than the total cyanide method, due largely to the strong acid and oxidizing conditions associated with the total cyanide analysis.


34

Analysis of Cyanide

TABLE 2.1 Comparison of Total Cyanide Analyses by Autoanalyser (USEPA Method 9012) and Manual (USEPA Method 9010) Methods

SampleNumber 1 2 3 4 5 6

TotalCyanide (Autoanalyser) 90 80 1,610 2,780 780 1,200

Total Cyanide (Manual) 14 <5 <5 13 <5 5

7 8 9 10 11 12 13

560 2,520 1,900 1,090 620 1,550 1,770

<5 20 15 <5 40 16 <5

Source: Smith, 1991

Notes: (1) All concentrations in µg/L.

35

Chapter Two


TABLE 2.2

Comparison of Autoanalyser and Non-Autoanalyser Analyses for (1) Total Cyanide in a Mining Effluent

Sample 1 2 3

Unpreserved (2) Preserved Unpreserved (2) Preserved Unpreserved (2)

Preserved

Laboratory A (Manual) 1.80 1.80 2.20 2.00 2.10

Laboratory B (Manual) 2.30 2.20 2.10 2.20 2.50

Laboratory C (Autoanalyser) 7.60 7.50 6.40 6.60 6.80

2.30

2.70

6.50

Source: Smith, 1991

Notes: (1) The anticipated cyanide levels based on previous cyanide analyses of this effluent was about 2 mg/L. (2) Preserved to remove sulphide from the sample. (3) All concentrations in mg/L.


36

Analysis of Cyanide

TABLE 2.3 Comparative Evaluation of Total Cyanide Methods Using Different Acids with a Synthetic Metal Cyanide Solution Containing Thiocyanate

Standard Cyanide (1) Solution (mg/LCN) 0.25 0.25 0.25 0.25 0.25 0.25

Thiocyanate Concentration (mg/LSCN) 100.0 100.0 100.0 100.0 100.0 100.0 Average

Total Cyanide With H2SO4 (mg/LCN) 0.288 0.488 0.572 0.360 0.724 0.604 0.506

Total Cyanide With H3PO4 (mg/LCN) 0.268 0.255 0.252 0.258 0.257 0.266 0.259

Source: Whitlock, Sharp and Mudder, 1981

Notes: (1) Standard 0.25 mg/L cyanide solution prepared from a 25 mg/L solution containing the following complexes: • • • • • •

Sodium Cyanocuprate (I) NaCu(CN)2 (5.0 mg/L as CN) Potassium Cyanozincate (II) K2Zn(CN)4 (5.0 mg/L as CN) Potassium Cyanonickelate (II) K2(Ni(CN)4 (5.0 mg/L as CN) Potassium Cyanoferrate (II) K4 Fe(CN)6 - 3H2O (5.0 mg/L as CN) Potassium Cyanoferrate (III) K3 Fe(CN)6 (5.0 mg/L as CN) Sodium Thiocyanate 100.0 mg/L as SCN (in distillation flask)

37

Chapter Two


TABLE 2.4 Thiocyanate Additions to Cyanide Standards Analysed By WAD and Total Cyanide Distillations Using Sulphuric Acid

Free Cyanide Thiocyanate Concentration Concentration (mg/L CN) (mg/L SCN) 0.2 0 0.2 101 0.2 101 0.2 101 Average(0.2mg/LCN,101mg/LSCN)

Analysed Total Cyanide (mg/L CN) 0.197 0.255 0.240 0.189 0.228

Analysed WAD Cyanide (mg/L CN) 0.202 0.197 0.196 0.205 0.199

0.2 202 0.2 202 0.2 202 Average(0.2mg/LCN,202mg/LSCN) 0.0 200

0.388 0.317 0.237 0.314 0.051

0.207 0.203 0.198 0.203 0.013

Source: Whitlock, Sharp and Mudder, 1981


38

Analysis of Cyanide

The positive interference resulting from thiocyanate is important when considering low and environmentally important levels of cyanide and monitoring of an effluent for compliance purposes. Although the presence of cyanide may be noted through analysis, resulting in an apparent permit violation, it may not be present in reality. For example, in Table 2.4, the prepared sample containing no cyanide and 200 mg/L thiocyanate, resulted in false positive values for total and WAD cyanide of 0.051 mg/L and 0.013 mg/L, respectively. The Chemistry Centre also found that the WAD cyanide method based on ASTM D7374 (modified using McLeod microstill) has negligible interference from thiocyanate. 2.2.3

Cyanide Amenable to Chlorination

The cyanide amenable to chlorination (CAC) method is intended to provide a measure of the free and weakly complexed forms of cyanide (USEPA Methods 9010 and 9012, Standard Methods and ASTM D2036 Method B). The CAC procedure involves two total cyanide analyses and one chlorine oxidation step. With this procedure, a portion of the aqueous sample is first subjected to a total cyanide analysis to establish the total cyanide concentration in the srcinal sample. Another portion of the srcinal sample is then chlorinated for a specified period of time, after which another total cyanide value is obtained. The difference between the first and second total cyanide analyses is reported as the CAC value. This lengthy and involved procedure is subject to multiple sources of error and is inherently less accurate than the corresponding WAD cyanide analysis, which measures in theory the same forms of free and metal complexed cyanide. The CAC method was widely used in the past, prior to the development of the WAD cyanide procedure. Both the WAD and CAC methods should in theory measure essentially the same cyanide compounds, although the CAC method is more time consuming since it involves two total cyanide determinations and a chlorination step. The level of inaccuracy of the CAC analyses is due to the three steps involved in the procedure and the interferences involved in each of these three steps. As discussed in the previous section, the total cyanide procedure is subject to several interferences, primarily as a result of the harsh reaction conditions required with the o total cyanide procedure (i.e. temperature >100 C and pH <2.0). In addition to these two steps, an intermediate chlorination step is required, leading to further interferences. When these three steps, and their associated interferences, are taken in whole to generate a single CAC cyanide concentration, the resulting inaccuracy in the result can be considerable. With the CAC method, the recoveries of cyanide have been reported as incomplete for a number of species and appear to be concentration-dependent (Ingersoll, 1981). Ecological Analysts (1979) reported poor accuracy and precision with the method, with susceptibility to a wide range of interferences. The Chemistry Centre found the interference to be sufficiently severe for some samples as to produce a higher CAC value than the corresponding total cyanide result. In the WAD cyanide method has become widely accepted by the mining industry andrecent manyyears, regulatory agencies for both monitoring and compliance purposes, due to recognition of the problems with the CAC method. Although the mining industry has opted to rely upon the WAD cyanide analysis, the CAC method is still widely used in the electroplating industry in the United States.

39

Chapter Two


2.2.4

WAD Cyanide by D istillation

The two most popular versions of the WAD cyanide by distillation method are from Standard Methods (APHA, 1998) and ASTM Method D2036 (ASTM, 1998). The methods involve the evolution and collection of hydrogen cyanide by reflux distillation for one hour with the sample buffered at pH 4.5 using a zinc acetate/acetic acid buffer. This is followed by quantitation of the cyanide liberated in the distillation using titrimetric, colourimetric or ion-specific electrode techniques. The distillation apparatus used with the WAD cyanide procedures is the same as described for the total cyanide analysis, as shown in Figure 2.2. The WAD cyanide method recovers all free cyanide and weakly complexed cyanides, for example copper, zinc, cadmium and nickel cyanides, but will not dissociate and recover cyanide from strongly complexed forms such as iron and cobalt cyanide. The WAD cyanide method is generally less susceptible to interference by either thiocyanate or sulphides than other cyanide analytical methods, particularly the CAC cyanide method. Analytically, the "toxicologically significant" or "ecotoxicologically important" forms of cyanide are differentiated most accurately from all others by the WAD cyanide procedure. The overall lower limits of detection for the WAD cyanide methods are similar to total cyanide methods, although the procedure is more reliable at concentrations of cyanide encountered in environmental samples. Precision is listed as linear, expressed as 0.085 x cyanide concentration + 0.0032 for reagent water. Conn (1981) reported a 7.5% relative standard deviation at the 0.08 mg/L as CN level. 2.2.5

WAD Cyanide by the Picric Acid Method

The picric acid colourimetric method for WAD cyanide is included in this section as it has been shown to be a relatively reliable and accurate procedure down to about 0.5 mg/L of cyanide in solution. The method involves developing colour with picric acid in the presence of nickel followed by heating over a water bath for 20 minutes prior to measurement using a visible range spectrophotometer. As an example of the use of the picric acid method for WAD cyanide, Brohm Mining Corporation located near Lead, South Dakota (USA) evaluated the applicability of the picric acid to solution from their heap leach pads. The data were developed in a study designed for the State of South Dakota to investigate the chemistry of cyanide in leach pads (ASCI/SRK, 1990). The data in Table 2.5 show that there is generally little difference between WAD cyanide values determined by the picric acid and the ASTM Method C WAD cyanide analytical protocols for these solutions. This is consistent with previous experience which has indicated that the picric acid a reliable analytical method,The capable of providing of WADprocedure cyanide is down to about 0.50 mg/L. method is not asquantitative rigorous asevaluations the standard distillation procedure but can be set up more conveniently and less expensively in a mine laboratory. It can provide a good method for use in estimating the WAD cyanide concentrations during a field pilot plant evaluation.


40

Analysis of Cyanide

TABLE 2.5

Sample Date 11 Apr 90 11 Apr 90 11Apr90 11Apr90 11Apr90 31May90 31May90 31May90 31May May 90 31 90 28Jun90 28Jun90 28Jun90 28 Jun 90 28Jun90

Comparison of Leach and Rinse Solution WAD Cyanide Analyses

Sample ID Barren Solution Cell #5 Effluent ColumnDEffluent ColumnCEffluent ColumnBEffluent ColumnAEffluent ColumnBEffluent ColumnCEffluent Column DEffluent Cell#5Effluent ColumnBEffluent ColumnCEffluent ColumnDEffluent Cell #5 Effluent NeutralizationPondEffluent

Picric Acid WAD Cyanide (mg/L) 220 193 169 172 177 11.8 11.2 11.5

ASTM Method C WAD Cyanide (mg/L) 224 199 170 171 173 11.7 11.2 11.3

12.4 16.5 0.25 0.11 0.21 2.7 0.13

12.2 16.5 0.17 0.07 0.13 2.8 0.11

Source: ASCI/SRK, 1990

Notes: (1) April data: leach cycle. (2) May and June data: rinse cycle.

41

Chapter Two


Prior to implementing the picric acid method for compliance purposes, its accuracy and reproducibility should be confirmed independently on a site-specific basis using an outside commercial laboratory. Quantification below 0.50 mg/L down into the range of 0.10 to 0.20 mg/L is possible with certain samples. It was also found in the study that filtration of the samples prior to analysis provided accurate comparisons of WAD cyanide values below 0.20 mg/L using either the picric acid or standard distillation methods. South African work at MINTEK (AMIRA, 2000) showed a lack of precision and accuracy with the picric acid method below 0.5 mg/L WAD cyanide. Hence the method has in Australia and South Africa not been recommended for compliance analysis since many water discharge limits have been set below 0.5 mg/L WAD cyanide. 2.2.6

WAD Cyanide by the Zinc Dust and Ammonia Method

Another rapid field method for the estimation of WAD cyanide was developed by DeVries and Mathre (1984). This method involves the addition of zinc dust to the sample to precipitate gold, silver and copper in elemental form. Ammonia addition is required to complete the precipitation of copper, and these reactions are analogous to those of the Merrill-Crowe process (see Chapter 1). Following the precipitation of gold, silver and copper, the solution is filtered and analysed for free cyanide using either a colourimetric titration or an ion-specific electrode. DeVries and Mathre compared the results of this method to those obtained from a traditional WAD cyanide distillation and found the results agreed to within about ±10% at cyanide concentrations of about 0.1 mg/L. 2.2.7

Free Cyanide by Tit ration with Silver Nitrate

The silver nitrate titration method is used for the determination of free cyanide or as a finish to the WAD or total cyanide distillation methods. The process involves titrating a known volume of sample with a standard silver nitrate solution, forming silver cyanide in the process. The end point of the titration occurs when all the free cyanide has reacted with silver and is estimated using a p-dimethylaminobenzalrhodanine or potassium iodide (KI) indicator. The solution must be maintained at a high pH with sodium hydroxide to prevent hydrogen cyanide volatilisation. There are four key aspects of the silver nitrate titration that must be understood: •

-

Silver nitrate used in the titration will react with the free cyanide anion (CN ) to form the silver-cyanide complex. In addition, silver nitrate will react with nearly all of the cyanide associated with zinc, provided the solution pH is above about 11.0. Cyanide associated with copper will be measured by the silver nitrate titration, but the recovery is not complete and results can be confusing and sporadic when copper is present in relatively high concentrations. So while this titration is termed the “free cyanide titration”, the actual results will include cyanide associated with zinc and a portion of the cyanide associated with copper. If complexing metals are known to be present, results from the silver nitrate procedure should appropriately be termed “titratable cyanide”.


42

Analysis of Cyanide •

Only if a solution is known to not contain metal-cyanide complexes (except iron-cyanide complexes) will the silver nitrate titration provide a true measurement of the free cyanide concentration. This is the reason for its appropriate use as a finishing method in the total and WAD cyanide distillation procedures.

•

The pH of solutions titrated with silver nitrate must be above 11.0, and preferably above 12.0. In this pH range, the measurement of free, or titratable, cyanide will be fairly complete and all of the titratable cyanide measured. For pH values of less than about 11.0, the amount of free (or titratable) cyanide measured begins to drop, and at a pH of less than about 10.0 an appreciable amount is not measured. It is a good practice to add a small amount of reagent grade NaOH along with the colour indicating reagent when titrating with silver nitrate to ensure the pH is above about 11.0.

•

The silver nitrate titration relies on a colourimetric endpoint which is visually detected by the person conducting the analysis. It is not uncommon for different persons to perceive this colourimetric endpoint at different levels of colour development or intensity. Because of this, different persons will often report differing analytical results for the same solution sample. The variation in analyses from person to person cannot be generally quantified, but steps should be taken to ensure this is not the cause of process and/or environmental monitoring problems. The p-dimethylaminobenzalrhodanine indicator (commonly termed “rhodanine”) develops a salmon pink colour at its endpoint and is generally easier to distinguish relative to the potassium iodide indicator. To alleviate variations imparted by individual perceptions of a titration endpoint, analysts should be well-trained and as few analysts as practical should be used to conduct titrations. As an alternative, automatic titrators can also be used to alleviate variations due to varying perceptions of the colourimetric endpoint.

Taken as a whole, the silver nitrate titration is an acceptable cyanide measurement procedure for process solutions, but its shortcomings must be understood. The method is generally not reliable for the measurement of free cyanide levels below about 1.0 mg/L in process solutions. 2.2.8

Free Cyanide by Ion Selective Electrode

The cyanide ion-specific electrode can be used to determine free cyanide in solution or as a finish for the distillation methods for total or WAD cyanide. The method involves the direct measurement, using an expanded scale pH or voltmeter, of the response of the electrode to the cyanide in solution against a reference electrode. The solution requires stirring throughout the test and is best used with a free cyanide concentration greater than 0.5 mg/L. A calibration curve is prepared using standard additions of a known cyanide solution. It is important to match the matrix of the standard solutions with that of the test solution where the presence of potential interfering species is suspected, a common issue with mining-related effluents. The cyanide ion specific electrode itself often becomes "poisoned" in effluent solution bearing sulphide species, with coatings forming on the electrode junction. These coatings require removal in order to attain credible results.

43

Chapter Two


2.2.9

Cyanide by I on C hromatograph

Ion chromatography has been used by many researchers for the separation of several metal cyanide complexes. This method is in common use at the Chemistry Centre in Western Australia where individual metal-cyanide complexes can be determined (Schulz, 1996; Schulz, 1997 and Black et al., 1997). With this method, characteristic spectra of individual metal cyanide complexes are obtained and the intensity of the spectra is related to the concentration of the specific metal cyanide complex. This speciation method allows determination of the metal cyanide complexes of gold, silver, copper, cobalt, iron (II) and (III), chromium and nickel (Schulz, 2001). The reporting limit for this analysis can be as low as 0.001 mg/L for nearly pure solution, but for typical mining derived solutions, the reporting limit ranges from about 0.05 to 0.5 mg/L depending on the solution salinity. This method has found particular application at the Chemistry Centre in the status determination of active and decommissioned tailings storage facilities and to establish potential environmental implications for closure. Varying methods and conditions give good to poor recovery of the loosely complexed metal cyanides. Indirect determination of cyanide by ion chromatography has been successfully demonstrated by several researchers (Fagan et al., 1996). A method developed by Koch (1983) claims detection of trace quantities of free cyanide by electrochemical means at 0.001 to 1.0 mg/L, and a standard deviation better than 1%. Rocklin and Johnson (1983) have also developed a method using electrochemical detection of free cyanide from easily dissociated metal cyanide complexes. Simultaneous determination of cyanide and sulphide was successfully demonstrated. Pohlandt (1983, 1984, 1985) has shown good recovery of the easily dissociated metal cyanide complexes as well as the individual complexes of nickel, cobalt, iron and gold conductometrically. Copper cyanide was detected photometrically along with nickel cyanide. The method distinguishes between the two cobalt and iron forms which is significant in industrial applications. UV irradiation of more refractory metals is also utilized. A great deal of research and generation of data has been directed toward development of individual methods which, in general terms, measure simple cyanides or complex cyanides. Considerable research in the area of cyanide specification using ion chromatography has been undertaken (Pohlandt, 1983, 1984, 1985). Ms. Pohlandt has developed techniques for the determination of the various cyanide species including free, copper, nickel, zinc and iron. The techniques are useful in measuring low levels of cyanide in complex mining wastewaters. However, the techniques and instrumentation as of yet have not achieved widespread application mainly due to the required expertise of the analyst and the cost of the instrumentation. 2.2.10 Cyanide by Automated Ligand Exchange In the USEPA approved Method OIA-1677 as an additional method oftomonitoring cyanide for1999, compliance purposes (USEPA, 1999). This method is an alternative the CAC method, which is subject to several interferences and generally does not provide suitable results at low cyanide concentrations. Method OIA-1677 measures WAD cyanide by acidifying the samples with hydrochloric acid to convert cyanides to molecular hydrogen cyanide.


44

Analysis of Cyanide

Prior to acidification, the samples are pre-treated with ligand exchange reagents which form stable complexes with metals in solution, thereby releasing cyanide from metals such as cadmium, copper, mercury, nickel and silver. The pre-treated and acidified sample is then passed under a gas diffusion membrane where HCN diffuses through the membrane and into an alkaline receiving solution. The concentration of cyanide in the receiving solution is then monitored amperometrically with a silver/silver chloride electrode system. A comparison of the recovery of cyanide from several metal complexes using the WAD, CAC and OIA-1677 methods is provided in Table 2.6. As indicated, the OIA-1677 method results are comparable to the WAD results except for the case of mercury. The OIA-1677 method provides relatively rapid results, but the analytical equipment is more expensive than that required for a distillation procedure. This method has reportedly overcome many of the interferences of the CAC method, particularly that of thiocyanate. The method is subject to interference by sulphide and samples should be pre-treated for sulphide removal. The developer of this method, OI Analytical, has also developed automated analytical methods for free and total cyanide, and the USEPA is reviewing the total cyanide method for potential listing as a USEPA approved test method. Experience in Australia with this method and instrument on actual mine water samples for WAD and total cyanide has shown it to be variable in terms of reproducibility and obtaining acceptable recoveries from spiked solutions. The Chemistry Centre evaluated two instruments; one located at an Australian university after service and repair by the agent, and the other at a remote mine site laboratory.

45

Chapter Two


TABLE 2.6

Metal Complex Cd Zn Ag Ni Cu Hg Au Co (II)Fe (III) Fe

Comparison of WAD, CAC and OIA-1677 Analytical Methods

WAD Method (% CN Recovery) 101.6% 100.9% 99.0% 97.3% 96.0% 58.9% 0% 0%

CAC Method (% CN Recovery) 102.9% 104.4% 54.8% 72.0% 98.0% 97.6% 0% 1.9%

0% 0%

0% 0%

Source: Solujic et al., 1996

Notes: (1) Analyses using 2.00 mg/L standard CN solutions.


46

OIA-1766 Method (% CN Recovery) 100.0% 98.5% 100.0% 103.0% 99.0% 99.0% 0% 2.0% 0% 0%

Analysis of Cyanide

2.2.11 Analysis of Cyanide in Solids Occasionally, the cyanide content in a solid sample is required, and in this case modified forms of the total and WAD distillation procedures are followed. The method involves placing a small but known weight of the solid sample in the distillation flask and distilling according to either the total or WAD cyanide procedure. Slurry samples should be pressure and not vacuum filtered prior to analysis to minimize loss of volatile forms of cyanide and to yield separate solution and solid samples for analysis. With respect to damp solid samples, allowing them to air-dry without additional heating will minimize loss of cyanide prior to analysis. Alternatively a moisture determination on a separate sub-sample may be applied. The Chemistry Centre has developed and validated an alkaline desorption procedure that allows all forms of cyanide and its related compounds (cyanate and thiocyanate) to be determined. Comparison with manual distillation procedures for WAD and total cyanide was excellent for most matrices (Schulz, 2001). 2.2.12 On-Line Process Monitoring for Cyan ide Automated on-line analysis of process solutions for cyanide is becoming more widespread and several manufacturers provide packaged analytical units. Advantages to on-line monitoring include more steady control of make-up NaCN additions and elimination of analytical variations due to differing operator techniques (Caron et al., 1999). The most common manual route of process solution monitoring for cyanide is via the silver nitrate titration with a colourimetric indicator. On-line cyanide monitors also may use the silver nitrate titration with a potentiometric endpoint, though ion-specific cyanide probes and automated distillation units are also available. The on-line titration and distillation analysers do not provide continuous feedback, as batches of process solution are titrated or distilled on about a ten to sixty-minute basis. For monitoring of cyanide in leach slurries, a pre-filtration step is used to remove the slurry solids prior to titration, which has been found to be troublesome in many cases. The on-line cyanide monitors rely on commonly used analytical techniques (i.e. silver nitrate titration, ion-specific electrode and distillation) and are subject to the same interferences as previously noted for these techniques. 2.2.13 Cyanide Detection and Quantitation Limits Table 2.7 lists the approximate Method Detection Limits (MDL) and Practical Quantitation Limits (PQL) for the total, CAC, WAD and free cyanide analytical methods. MDL is the minimum level at which a particular compound can be reliably and repeatedly determined to be present in a sample. Above the MDL, results can indicate that a particular compound is present, but the concentration may not be sufficiently high as to allow reliable and accurate quantification. Concentrations below the are reported as not detected. The PQL is important because this is the minimum levelMDL at which the concentration of a compound can be reliably and accurately quantified. Only when the level of a particular compound is equal to or above the PQL should a number be assigned to the concentration. When results of an analysis fall between the MDL and PQL, results are reported as detected but not quantifiable.

47

Chapter Two


TABLE 2.7 Summary of Method Detection Limits and Practical Quantitation Limits for Cyanide Analyses

Analytical Method

Recommended Method Detection Limit (mg/L)

Recommended Practical Quantitation Limit (mg/L)

Total by Distillation

0.02

0.10

WAD by Distillation

0.02

0.05

WAD by Picric Acid

0.10

CACthroughDistillations

0.50

0.10

FreebySilverNitrateTitration

>1

FreebyIon-SelectiveProbe

0.10

Total by SFAA

(1)

WAD by SFAA

(2)

0.50 >1 0.50 --

0.01

--

0.01

Notes: (1) Segmented flow auto analyser (SFAA) by the automated ASTM method; includes in-line UV digestion and McLeod microstill reflux. (2) Segmented flow auto analyser (SFAA) by the automated ASTM method; includes McLeod microstill reflux.


48

Analysis of Cyanide

There may be circumstances with long-term repetitive monitoring where lowering of the MDL and PQL levels listed in Table 2.7 may be justified, but this cannot be done prior to full characterization of each sample type. These levels are based on experience with mining process solutions and results of analytical studies as earlier discussed. 2.2.14 Thiocyanate Analysis Thiocyanate is appropriately termed a “cyanide-related compound” since it is not a cyanide compound. The measurement of thiocyanate is presented in Standard Methods (APHA, 1998). The analytical procedure involves the reaction of thiocyanate at a low pH with ferric iron, which forms a deep red colour proportional to the concentration of thiocyanate. A spectrophotometer is used to measure the thiocyanate concentration at a wavelength of 460 nm after being calibrated with suitable thiocyanate standards. The PQL for thiocyanate using this procedure is about 0.5 mg/L. Thiocyanate is also conveniently determined by ion chromatography (IC). The results are directly comparable to the APHA method (APHA, 1998). The interference from salinity is similar for both methods, but the IC method does not suffer the interference from metals (iron) that the APHA method suffers. 2.2.15 Cyanate Anal ysis As with thiocyanate, cyanate is also not a cyanide compound, but a cyanide-related compound. An analytical procedure to measure cyanate is presented in Standard Methods (APHA, 1998). With this procedure, cyanate is converted to ammonia at a low pH and elevated temperature through a hydrolysis reaction. To measure cyanate, the ammonia concentration in a portion of the srcinal sample is measured. Then following hydrolysis of cyanate to ammonia, the ammonia concentration in the treated sample is measured again and compared to the srcinal ammonia level. The precision of this method is related to the precision of the ammonia analyses. The PQL for cyanate using this procedure is about 1.0 mg/L measured as an N equivalent. For mining samples containing elevated ammonia levels, the APHA method lacks accuracy since the difference of two high (possibly nearly the same) numbers is correlated to determine the cyanate concentration. An IC method developed at the Chemistry Centre provides more accurate data for ammonia analyses (Black and Schulz, 1999). 2.2.16 Ammonia, Nit rite and Nit rate Analyses Ammonia, nitrite and nitrate are all cyanide-related compounds, as they oftentimes are derived from the breakdown of cyanide, thiocyanate and cyanate. foranthese compounds are presented in Standard Methods (APHA, 1998). Analytical In the caseprocedures of ammonia, ionselective electrode or phenate colourimetric procedure can be used, both of which have a PQL of about 0.1 mg/L measured as an N equivalent. The direct Nesslerization colourimetric method for ammonia is a simple, accurate and reliable procedure, but was removed from Standard Methods since it involved the use of mercury as a reagent.

49

Chapter Two


Nitrite can be determined colourimetrically, and nitrate can be reduced to nitrite using cadmium and then analysed as nitrite. The PQLs for the colourimetric determination of nitrite and nitrate are both about 0.1 mg/L expressed as an N equivalent. There is also an ion selective electrode method for nitrate, also with a PQL of about 0.1 mg/L as an N equivalent. Automated methods for nitrate and nitrite have been shown to have PQLs in the range of about 0.02 mg/L and 0.001mg/L.

2.3

ANALYTICAL IN TERFERENCES

2.3.1

Introduction

Associated with any analytical procedure are interferences, both positive and negative, which affect the precision and accuracy of that method. All of the methods previously discussed are affected by interferences to varying degrees. This section contains a brief discussion the of various interferences encountered in cyanide analyses, as well a discussion of the research directed towards quantification of various interferences. In summary, the WAD cyanide procedure which measures the weakly complexed forms of cyanide is least affected by interferences, while the total cyanide method, the ion-selective electrode and the titrimetric method for free cyanide are susceptible to many interferences. The principal compounds which cause interference in cyanide analyses are: • • • • • • •

Oxidizing agents Sulphides Thiocyanate Nitrite and nitrate Carbonates Thiosulphates, sulphates and other related sulphur compounds Metals

The action of these interferences in the analysis of cyanide solutions is discussed in the following sections. 2.3.2

Oxidizing Agents

Oxidizing agents, as a general rule, are a negative interference in cyanide analyses as they continue to decompose cyanide in the sample during storage, handling or other manipulation. Common oxidizers are chlorine, oxygen and hydrogen peroxide in mining solutions.


50

Analysis of Cyanide

The only valid means of control is immediate removal of the oxidizers. The common test for oxidizers ASTM (1985) is to use potassium iodide-starch paper moistened with acetate buffer. If oxidizers are present, the most widely accepted means of treatment is to add 0.1 g/L increments of sodium arsenite with re-testing. Other reagents for removal of oxidizers are mentioned in the literature, but all of these may cause problems in analysis: sodium thiosulphate, sodium hydrogen sulphite, ascorbic acid, oxalic acid and stannous chloride. Kelada et al. (1984) preferred sodium arsenite and oxalic acid. Noller and Schulz (1995) reported that sodium arsenite gave the least interference with subsequent analytical procedures. As a general rule, a positive test for sulphide would establish that oxidising agents are not present in the solution 2.3.3

Sulphides

Along with thiocyanate, sulphides are one of the most common interferences found in mining process or discharge waters. Free sulphides are normally only present in solutions with oxygen deficiency Brickell (1981). Oxidized products of sulphide convert cyanide to thiocyanate rapidly at elevated pH. Csikai and Barnard (1983) and Yoshida et al. (1983) have demonstrated that sulphide evolves through the cyanide analysis as hydrogen sulphide. This interference is normally removed by precipitation in the formation of insoluble sulphides. Luthy et al. (1978) reported on precipitation studies. Precipitants commonly used are lead and cadmium carbonates, lead acetate, cadmium nitrate, bismuth citrate and bismuth nitrate. In addition, oxidants such as permanganate are utilized to destroy residual sulphides. There is evidence that cyanidecontaining precipitates may re-solubilise and cause variable results if not removed by pressure filtration prior to preservation and analysis. 2.3.4

Thiocyanate

Thiocyanate is a common interference to many of the methods. Thiocyanate is generated in solution by the reaction of sulphides in ores or process solutions with cyanide added to the solutions. Thiocyanate under acidified conditions, in the presence of a strong oxidant will convert to free cyanide with oxidation of the sulphur to oxysulphur anions. Under these conditions, cyanide values reported from analysis will be higher than appropriate with the rise in value difficult if not impossible to quantify. The phenomena of elevated cyanide values has been documented by Barton et al. (1978), Csikai and Barnard (1983), Conn (1981) and Whitlock et al. (1981), as well as many others. As little as 0.2 mg/L of thiocyanate can contribute as much as 0.01 mg/L cyanide as an interference (Barton, 1978). Higher concentrations of thiocyanate can negate any meaningful attempt at accurate analysis of trace or environmental levels of cyanide.

51

Chapter Two


Under strong acid reflux distillation procedures the conversion of thiocyanate to sulphide is not rapid but proceeds throughout distillation with hydrogen sulphide transferred to the absorber. This occurrence is a major problem to the total cyanide and CAC methods. In less acidic distillations (i.e. the WAD cyanide method), thiocyanate interference is at a minimum. It has also been postulated that thiocyanate or its decomposition products will react with chloramine-T in the colour development step to produce a positive interference. It has been shown that the reaction is stoichiometric and quantitative; that is 1.0 mg/L of thiocyanate will produce a colour equivalent to that produced by 0.45 mg/L of cyanide. Several researchers have included a cadmium compound trap to precipitate sulphide products which are passing to the absorber. Barton et al. (1978) suggested the use of zinc acetate in mildly acidic solutions to inhibit the decomposition of thiocyanate. Whitlock et al. (1981) demonstrated that when using the USEPA total cyanide method for cyanidation process solutions of high thiocyanate concentration, the substitution of phosphoric acid for sulphuric acid lessened the concentration of sulphur products in the absorber and led to more accurate cyanide values. With cyanide standards in deionised water, the phosphoric acid substitution yielded improved reliability when compared with the sulphuric acid method. 2.3.5

Nitrite and Nitrate

Both nitrite and nitrate may be considered as potential interferences to many of the methods at levels as low as 25 mg/L as nitrate. Nitrites may form HCN during distillation and affect the total and CAC methods, as well as the method for cyanates. Although the interferences are usually positive, addition of nitrate to samples containing total cyanide has resulted in negative interferences as well. Kelada et al. (1984) and ASTM (1985) recommend adding 2 g/L of sulphamic acid to samples prior to distillation to obviate the interference of nitrate and/or nitrite. 2.3.6

Carbonates

Under strongly acidic conditions of distillation, carbonates in high concentration cause excessive gassing of carbon dioxide and may reduce the NaOH content of the HCN absorber. Pre-analysis of sample alkalinity will produce information concerning this potential problem. 2.3.7

Thiosulphates, Sulphites and Other Sulphur Compounds

Many different types of sulphur compounds are formed by decomposition during distillation including hydrogen sulphide, sulphur dioxide and sodium sulphite. Thiosulphate breaks down to form elemental sulphur and sulphurous acid with liberation of SO2 during distillation. This decomposition bleaches the colourimetric finish and reduces cyanide recovery Conn (1981). Weak acid distillations are less subject to this interference, and some researchers add as much as 50 g of lead carbonate to the absorber, followed by filtration of the absorber solution. This filtration may also lower cyanide recoveries. Other analyses have used potassium iodide-starch paper to test for an excess of chloramine-T, given that SO 2 forms Na2SO3 which consumes the chloramine-T used in the colourimetric step.


52

Analysis of Cyanide

Samples can be pre-treated with hydrogen peroxide as a treatment for sulphites. However, an excess of hydrogen peroxide will rapidly oxidize cyanide and reduce the recovery values. Makhija and Hitchen (1979) have presented methods for titrimetric determinations of sulphite, thiosulphate and polythionates in mining effluents. Ion chromatographic methods have been developed to determine sulphur species, including the polythionates in mining effluents (Black and Schulz, 2001). 2.3.8

Metals

Certain metals at high concentration may interfere with cyanide analysis in a number of the methods. Mercuric and copper chlorides added as catalysts may inhibit analysis, particularly if thiocyanate is present. Mercury, due to its high volatility, may diffuse to the absorber and cause a negative interference (Kelada, 1984 and Barton, 1978). Metals such as iron, cobalt and mercury may interfere with simple cyanide methods by forming more stable complexes which cannot be quantified by the method, thus reducing the reported cyanide value. 2.3.9

Effects of Int erferences on C yanide Analyses

The research of Ingersoll (1983) included the organization of data expressing the effects of various interferences on selected methods. This work is a significant reference guide to the methods and includes a number of tables summarizing the various interferences. Data from 1981 unpublished from the Homestake Mining Company, are an example of evaluating interferences by spiking a complex mining process solution. One significant concept when dealing with an interference was to limit the sample size to the lowest concentration of cyanide acceptable for analysis in order to dilute the concentration of potential interfering components of the sample. The following general conclusions were drawn: •

Ascorbic acid treatments to reduce oxidizing agents should not exceed 30 mg/L excess ascorbic acid.

•

Thiosulphate acts as a negative interference to cyanide analysis (total) at concentrations as low as 25 mg/L.

•

Sulphide which gains access to the absorption solution may cause positive or negative interference with colourimetric procedures at very low concentrations.

•

Thiocyanate is a major interference to the total cyanide distillation method, usually resulting in high cyanide values; however, decomposition is not predictable and the mechanism is uncertain.

•

Bisulphite appears to be a primary negative interference with the potential for some degree of cyanide destruction. Residual bisulphite is an interference.

53

Chapter Two

Chemistry and Treatment of Cyanidation Wastes •

2.4

Sulphur species interference may be minimized by the following treatments: -

Sulphide precipitation from the sample

-

Distillation of smallest possible sample

-

Sulphide precipitation from the absorption solution

-

Increased levels of chloramine-T in the colourimetric procedure

PRESERVATION AND STORAGE OF SAMPLES

The preservation and storage of samples is crucial to the success of any analytical programme and is a concern when dealing with samples for cyanide analysis. Methods of preservation have been developed for most commonly analysed parameters. However, little information is available to define the effect of preservation on sample integrity, or of any interfering effect the preservative may have on a particular analysis. Noller and Schulz (1995) reported on some of these effects. Immediate analysis of the sample is the most desirable mode, followed by refrigeration at 4°C in a zero headspace container and analysis as soon as possible. All other means are somewhat less appropriate or accurate but, nonetheless, are commonly employed. The best approach is to utilize chemical preservatives only when necessary, only when they are known to be compatible with the method of analysis and only when the preservative has been shown acceptable in the particular application under consideration. Due to the large number of process variations, solution environments, mine locations and treatment technologies, the sample will be site-specific. The sample may also be a solid or a liquid/solid slurry, which offers its own special handling problems. In gold mining operations, there exist three main sample categories: •

Process control for ore treatment and cyanide destruction processes.

•

Monitoring of tailings ponds, effluents and receiving streams.

•

Regulatory or discharge requirements, including bio-monitoring.

Each category produces a different sample type and each maintains a different set of handling and analysis requirements. Process control samples contain a wide variety of cyanide concentrations, as well as metals and introduced chemicals. Chemicals used may act as an analytical interference or contribute to sampling errors. Monitoring samples and regulatory samples are likely to be more stable, with lower cyanide and reaction product (e.g. thiocyanate) concentrations. Problems are created, when cyanide is present in the microgram per litre range, which demands highly skilled analysis.


54

Analysis of Cyanide

General conditions that may be applicable to the preservation and analysis of samples for cyanide include: •

Cyanide species may exist as the cyanide ion, molecular hydrogen cyanide and/or metalcyanide complexes. In addition, thiocyanate, cyanate, ammonia and nitrate may be present.

•

It is well documented that iron-cyanide complexes are subject to photolytic degradation.

•

All metal cyanide complexes differ in stability and solubility, ranging from the highly stable complex ferricyanide to the weak complexes, such as zinc and cadmium cyanide.

•

Thiocyanate may act as an interference to the analytical method.

•

Sulphides and reduced sulphur compounds may act as an analytical interference.

•

Oxidizers, such as residual chlorine or hydrogen peroxide, are known interferences.

Sample stability prior to analysis is the key to acceptable analytical data. The following generalizations and precautions improve the prospects for preservation of the sample while maintaining sample integrity. •

Cyanide standards prepared in deionised water, preserved by the addition of sodium hydroxide to pH > 12 and refrigerated at 4°C the dark, will remainrenewal stable for weeks. Low level calibration standards may in require more frequent to several ensure their integrity.

•

The complex solutions associated with gold milling barren and decant waters will most likely not be stable or representative if preserved in the above manner only.

•

Sulphide or reduced sulphur compounds must be removed prior to pH elevation in order to block thiocyanate formation.

•

Oxidizing compounds must be removed.

•

Solids in the sample container will adsorb cyanide and also lower reported values.

•

Filtration alone may reduce the cyanide value.

•

Volatilisation at pH of less than 10.5 is a major source of free cyanide loss.

•

Preservative reagents, such as sodium hydroxide, will become contaminated if exposed to a cyanide atmosphere.

•

Air contamination in the laboratory may contaminate a sample prior to and during reflux analysis, requiring pre-scrubbing of the air entering the distillation flask. 55

Chapter Two


Some of the issues and relationships with respect to sample preservation and cyanide can be illustrated by reference to two recent studies from the United States. In the first example, the results were obtained from a comparative evaluation of WAD cyanide analyses on refrigerated, non-preserved samples and on sodium hydroxide stabilized (pH 12 or greater) samples (Table 2.8). Essentially, the data are the same over a concentration range of up to about 2 mg/L. These data show that non-preservation was successful in this case in retaining the WAD cyanide components in the sample if the sample was refrigerated and determined in a little over 24 hours. The second example is from an evaluation of the effects of the use of preservatives on total, WAD and free cyanide analyses employing two separate laboratories for quality assurance purposes. The sampling at this mine was repeated over a three-day period and the results for one of these days are given in Table 2.9. The samples were unlikely to have contained any residual oxidants and were shown not to contain any detectable levels of sulphides. The data show considerable variability, the WAD cyanide samples being most affected by the addition of preservatives. The cumulative affect of adding both the preservatives on the WAD cyanide values is particularly striking. In the case of free cyanide values, it is difficult to assess whether the preservatives skew the analysis, as the replication of the analytical method at this level is poor. It should be noted that there was considerable variability between results obtained at different laboratories. The lesson for the mine operator is that if the mine laboratory facilities cannot handle the analysis of its own samples rapidly or that regulatory demands impose the use of an external laboratory, the operator should assess carefully the effect of any and all preservatives on the particular samples prior to entering into long-term analytical programme for cyanide. In addition, the mine operator should check the quality of results produced by external laboratories by submitting Certified Reference Samples disguised as routine samples.


56

Analysis of Cyanide

TABLE 2.8

SampleNo.

Comparison of WAD Cyanide Values from Preserved and Unpreserved Samples

1 2 3 4 5 6 7 8

WADCyanide (1) (Unpreserved & Held 24 hrs) (mg/L) 0.04 0.04 0.09 0.11 0.03 0.03 0.02 0.02

WAD Cyanide (1) (Preserved & Held 6 days) (mg/L) 0.05 0.08 0.09 0.08 0.08 0.05 0.04 0.02

9 10 11 12 13 14 15 16 17

0.08 0.48 1.52 1.92 2.30 2.22 1.97 1.80 1.77

0.09 0.41 1.40 1.81 2.12 2.06 1.41 1.68 1.72

Source: Damon, Smith and Mudder, 1991

Notes: (1) Refrigerated at 4°C.

57

Chapter Two


TABLE 2.9

Effects of the Use of Preservatives for Oxidants and Sulphides on Cyanide Analyses

SampleDescription Total Cyanide Unpreserved Preserved for Sulphide Preserved for Oxidants PreservedforOxidantsandSulphides WAD Cyanide Unpreserved Preserved for Sulphide Preserved for Oxidants Preserved for OxidantsandSulphides Free Cyanide Unpreserved Preserved for Sulphide Preserved for Oxidants PreservedforOxidantsandSulphides Source: Damon, Smith and Mudder, 1991

Notes: (1) All values in mg/L.


58

LaboratoryA

LaboratoryB

1.80 1.80 1.30 2.00

2.30 2.20 2.30 2.10

0.10 0.11

0.18 0.22

0.35 0.75

0.45 0.72

0.03 0.04 0.11 0.02

0.07 0.04 0.08 0.10

Analysis of Cyanide

2.5

CALCULATION OF A CYANIDE MASS BALANCE

It is possible to check the validity of a series of cyanide analyses by completing a mass balance, comparing the various cyanide and metals levels obtained from the analysis of the same solution. The mass balance calculations provide a reasonable check on the validity of process solution analyses for cyanide which also contain metals such as copper, iron, nickel and zinc. The mass calculations are prepared assuming that iron (ferrous or ferric) is bound to six cyanide molecules -3/-4 -2 (Fe(CN)6 ), copper is bound to three cyanide molecules (Cu(CN)3 ), nickel is bound to four -2

-2

cyanide molecules (Ni(CN)4 ), and zinc is bound to four cyanide molecules (Zn(CN) 4 ). These are the four predominant metal-cyanide complexes in cyanidation process solutions, although other metals could be added to the calculations (i.e. lead, silver or mercury) if present in elevated concentrations. Based upon the number of cyanide molecules bound to each metal, the weight ratio of each metal to cyanide is as follows: • • • •

Copper Iron Nickel Zinc

0.82 mg Cu/mg CN 0.36mgFe/mgCN 0.56 mg Ni/mg CN 0.63mgZn/mgCN

-

-

(1.23 mg CN /mg Cu) (2.80 mg CN /mg Fe) (1.77 mg CN /mg Ni) (1.59 mg CN /mg Zn)

-

As an example, assume a total, WAD and free cyanide analysis of the same solution produced concentrations of 225 mg/L, 200 mg/L and 75 mg/L, respectively. It should be noted that the free cyanide analysis is often too high, due to interferences associated with the titration method in mining solutions. In some cases, the free cyanide level is higher than the WAD cyanide level, which is not possible. Assume also that the analysis for copper was 25 mg/L, iron 10 mg/L, nickel 1 mg/L and zinc 55 mg/L. The difference between the total and WAD cyanide values is 25 mg/L. The only metal cyanide complex of the four mentioned that does not report in the WAD procedure is that of iron, which only reports in the total cyanide analysis. Therefore, the corresponding iron concentration present in the solution should be at about 9 mg/L (i.e. 0.36 x 25 mg/L), and this checks well with the analysed iron concentration of 10 mg/L. A convenient calculation check between total cyanide, WAD cyanide and iron analyses in alkaline cyanide solutions is the following: (2.1)

CNTOTAL (mg/L) = CNWAD (mg/L) + Fe (mg/L) x 2.8

The contributions of the other metals to the total and WAD analyses can be calculated using the above metal to cyanide weight ratios. Note that of these metals, only the copper, nickel and zinccyanide complexes will report in the WAD analysis, while all the metal-cyanide complexes will report in the total analysis. Table 2.10 shows the results of these calculations and the corresponding calculated total and WAD cyanide concentrations. As indicated, the calculated total cyanide concentration is 224 mg/L versus the analysed total cyanide concentration of 225 mg/L. Similarly, the calculated WAD cyanide concentration is 196 mg/L versus the analysed WAD cyanide concentration of 200 mg/L. As a result, it appears the metals and cyanide analyses are self-consistent and the analyses are reliable. Differences between calculated and analysed cyanide levels of 10% to 20% are common with metallurgical solutions.

59

Chapter Two


TABLE 2.10 Example Cyanide Mass Balance Calculation

Metal Cu Fe Ni Zn

Analysis Cyanide to Metal (mg/L) Weight Ratio 25 1.23 10 2.80 1 1.77 55 1.59 Analysed Free Cyanide (1) Calculated WAD Cyanide (2) Calculated Total Cyanide

Cyanide Equivalent (mg/L) 31 28 2 88 75 196 224

Notes: (1) WAD cyanide includes free cyanide plus cyanide complexed with copper, nickel and zinc. (2) Total cyanide includes free cyanide plus cyanide complexed with all metals.


60

Analysis of Cyanide

2.6

COMPARATIVE ANALYSES OF SOLUTIONS

A number of published studies have been presented which compare the analyses of cyanide and its related compounds among different analytical laboratories. The purpose of conducting these studies is to quantify the variation in analyses observed among different laboratories, to investigate the analytical properties of a particular mining solution and as a quality control check. Two such studies are presented in this section to illustrate the variability of analyses that can be encountered from laboratory to laboratory. In 1996 the Chemistry Centre in Western Australia conducted an international interlaboratory comparative study of cyanide and thiocyanate analyses (Schulz, 1997). A total of 44 laboratories participated in the study and submitted analyses for total, WAD and free cyanides and thiocyanate. Most laboratories were located in Australia, with the remainder in China, Papua New Guinea and Indonesia. The solutions distributed for analyses were actual gold mining solutions (preserved with NaOH) collected in Australia that were submitted either unchanged, diluted or spiked with metal-cyanide complexes. Analytical methods were left to the discretion of the laboratories. In total, seven solutions were prepared ranging in total cyanide concentration from about 0.25 to 60 mg/L, and in thiocyanate concentration from about 2 to 20 mg/L. Results from the analyses are summarized in Table 2.11. While Table 2.11 may be overwhelming in its data presentation, the results are quite significant, but not unexpected. In general, the variation in total, WAD and free cyanide analyses varied by about one order of magnitude among the different laboratories. In the case of Sample 7, which represented the lowest cyanide levels, the reported total cyanide level ranged from 0.11 to 0.50 mg/L, differing by a factor of about five. In addition, the reported WAD cyanide level for Sample 7 ranged from <0.005 to 0.152 mg/L, differing by a factor of more than thirty. The thiocyanate analyses were more consistent than the cyanide analyses, but still varied by a factor of about two. The solutions used in this analytical study were derived from actual mine solutions, and therefore directly represented the type of solutions of interest in environmental monitoring. The laboratories were aware that the samples were part of an analytical study, and therefore had the opportunity to ensure rigid analytical procedures were followed. All participating Australian laboratories were accredited for cyanide analyses and routinely serviced the mining industry with data that is reported to regulatory agencies. The performance of the Asian laboratories was similar to the Australian. One possible explanation for the difference observed among the various analyses discussed above may be that some laboratories participating in the study were not fully qualified or experienced in conducting cyanide and thiocyanate analyses. This could be demonstrated from the fact that some laboratories reported higher free cyanide than WAD cyanide on samples that were This situation is often encountered, particularly in remote regions of thespiked world,with andthiocyanate. reinforces the recommendation that analytical laboratories must be selected with care. However, even among highly reputable laboratories experienced with cyanide and thiocyanate analyses, wide ranges in the analytical results are often encountered.

61

Chapter Two


TABLE 2.11 Summary of Intern ational Comparative Study of Cyanide and Thiocyanate Analyses

Total Cyanide

WAD Cyanide

Free Cyanide

Thiocyanate

Sample 1 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 2 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 3 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 4 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 5 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 6 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum (mg/L) Sample 7 Number of Analyses Median (mg/L) Maximum (mg/L) Minimum(mg/L)

76 19.4 48.9 7.0

56 17.3 36.9 4.1

34 11.3 26.6 <0.01

9

78 2.02 8.56 0.61

56 1.91 4.13 <0.05

36 1.62 3.61 0.22

8

77 60.8 81.2 24.8

60 19.6 36.9 0.14

35 12.2 55.5 2.43

9

78 6.73 14.4 3.1

59 2.20 3.96 <0.05

36 1.57 6.79 0.13

8

78 13.8 27.4 6.0

58 12.6 28.2 <0.05

34

9

77 23.0 38.5 3.5

57

34

9.5 19.9 <0.05

6.6 23.7 <0.01

78 0.25 0.50 0.11

40 0.019 0.152 <0.005

14 0.012 0.146 <0.001

Source: Schulz, 1997


62

16.2 20.6 15.6

1.5 3.4 0.9

18.0 27.0 14.8

1.9 4.2 1.5

9.3 24.1 <0.01

13.5 16.4 11.6 9 12.0 14.2 11.0 N/A N/A N/A N/A

Analysis of Cyanide

In a study conducted at a USA mine site, several process and environmental water samples were collected and analysed for total, WAD and CAC cyanide (Mudder and Botz, 1998). The study was part of an investigation of cyanide compliance monitoring difficulties at the mine site where cyanide was being detected at locations where it was unlikely to be present. Duplicate samples from several locations at the site were distributed to five USA laboratories, all of which are well known, highly reputable, experienced in conducting these types of analyses and accredited by the USEPA. Further, these laboratories currently conduct cyanide compliance analyses for several USA mining companies, the results of which are submitted to government regulators as required in permits for mine operation. All laboratories were instructed to analyse for total, WAD and CAC cyanides down to their respective detection limits using USEPA approved procedures. The laboratories all used a manual distillation method, with the exception of Laboratory D, which used approved automated methods for cyanide. Average results for four of the samples, which were analysed in duplicate by each laboratory, are presented in Table 2.12. Unlike the Australian study discussed above, laboratories participating in this work were not aware that such a study was taking place. The advantage of this situation is that the laboratories conducted the analyses according to their routine quality control/assurance protocols. Results obtained in this study are representative of what would be obtained with process and environmental monitoring samples from commercial laboratories qualified to conduct cyanide analyses. As indicated in Table 2.12, there was a significant spread in data presented by the various laboratories. At the extreme, total cyanide results for Sample 2 varied by as much as three orders of magnitude. Even for samples with similar analytical results, for example WAD cyanide analyses of Sample 2, the results varied by about a factor of two. Another issue of note is the variation in cyanide detection limits reported by each laboratory. For example, the total cyanide detection limits reported by Sample 1 ranged from a high of 0.01 mg/L down to a low of 0.005 mg/L. Results of this study are similar to results obtained from other studies of this type conducted around the world. Data from these two studies are not presented to indict any particular laboratory or express doubts about the certainty of cyanide analytical procedures. When samples are properly collected and analyses are properly conducted by trained and experienced personnel, cyanide can be analysed at levels of environmental significance. The intent of this presentation is to reinforce the following concepts which must be considered when conducting cyanide analyses: Caution must be taken when selecting an analytical laboratory for cyanide analyses. Prior to selecting a laboratory, duplicate samples of solutions should be sent to several laboratories as part of a blind comparison study. Samples of known concentration, such as Certified Reference Samples should also be included to show the relative accuracy of each prospective laboratory.

63

Chapter Two


TABLE 2.12 Summary of Cyanide Analytical Results for a USA Mine Site

Total Cyanide (mg/L) Sample 1 2 3 4

Laboratory A <0.02 0.05 <0.02 0.02

Laboratory B Laboratory C <0.01 <0.005 0.05 <0.005 0.01 <0.005 1.26 1.27 WAD Cyanide (mg/L)

Laboratory D 0.636 9.0 3.70 3.33

Laboratory E 0.010 0.025 <0.006 1.46

Sample 1 2 3 4

Laboratory A <0.02 0.03 <0.02 <0.02

Laboratory B Laboratory C <0.01 <0.005 0.02 0.033 <0.01 <0.005 0.036 0.158 CAC Cyanide (mg/L)

Laboratory D <0.005 0.029 0.005 0.860

Laboratory E <0.006 0.019 <0.006 0.062

Sample 1 2 3 4

Laboratory A <0.02 0.04 <0.02 <0.02

Laboratory B <0.01 0.05 0.01 1.26

Laboratory D <0.005 0.097 <0.005 1.40

Laboratory E 0.010 <0.006 <0.006 1.25

Laboratory C <0.01 <0.005 <0.005 1.27

Source: Mudder and Botz, 1998


64

Analysis of Cyanide

The analysis of mining solutions for cyanide and thiocyanate is subject to variations imposed by different analytical laboratories. It is important that the magnitude of this variation be understood for each particular solution sample at a particular mine site. Each particular sample should be fully characterized as to its potential interferences, preservation requirements and level at which cyanide can be reliably quantified. Persons responsible for overseeing and/or conducting sampling and analyses of solutions for cyanide and thiocyanate must be familiar with the basis aspects of cyanide chemistry. Specifically, this includes aspects of sample collection, preservation, analysis and interpretation of analytical results.

2.7

LOW LEVELS OF C YANIDE IN THE ENVIRONMENT

The environmental significance of low levels of cyanide in surface waters has been a concern to industry, commercial laboratories and regulatory agencies for nearly two decades. The concerns arise from the inability to confirm the presence or absence of low levels of reported cyanide in its various forms, to verify the sources of the cyanide being either manmade and/or natural, and to determine the extent, if any, of adverse ecological impacts. For purposes of this discussion, low levels of total cyanide are those below about 0.10 mg/1, and for WAD cyanide levels below about 0.05 mg/1. Although these particular levels are lower than those typically associated with adverse impacts to either humans or aquatic life, there remains an ongoing concern with regulatory agencies and communities as to the meaning of these levels of cyanide and their potential sources. To protect the environment and human health, scientific efforts should be directed toward establishing reasonable levels of safety through promulgation of standards that not only protect designated beneficial uses, but can also be achieved through treatment and analysed accurately using approved methods. There are three possible scientific explanations for the appearance of low levels of cyanide in surface waters or treated effluents. The first is analytical error in the measurement of cyanide. The second is the detection of naturally produced cyanide excreted by plants, microorganisms and insects. The third explanation is the release and detection of manufactured cyanide through anthropogenic activities. Each of these potential reasons for the presence and/or improper reporting of cyanide are discussed in the following sections. It is imperative that the interpretation of cyanide analytical data be tempered with an understanding of the reality of the problems associated with its analysis.

65

Chapter Two


An additional issue relates to the levels at which cyanide can be reliably detected and quantified in natural waters and mine process solutions. Over the years, commercial analytical laboratories have lowered their reporting and detection limits for cyanide from the srcinal or traditional level of 0.02 mg/L to lower than 0.005 mg/L in some instances. This is in spite of the fact that the analytical procedure, the analytical reagents and instrumentation have remained essentially unchanged, as has the inherent error in the accuracy and precision of the analytical procedures. In large part, the lowering of detection limits has resulted from regulatory mandates and competitive marketing without the necessary scientific justification or rationale. This trend has contributed to the frequent misinterpretation of cyanide data generated by commercial laboratories and reported to government agencies for monitoring and compliance purposes. 2.7.1

Natural Sources of Cyanide

It is commonly believed that the natural production of low levels of cyanide in its various forms does not occur to any great extent in the global environment. In fact, the natural production of cyanide is ubiquitous throughout the world. Cyanide and chemically related compounds are formed, excreted and degraded in nature by hundreds of species of bacteria, algae, fungi, plants and insects (Knowles, 1976 and U.S. Fish and Wildlife Service, 1991). As a result, it would not be uncommon for low levels of cyanide to periodically or routinely appear in background or ambient surface or groundwater samples which are not expected to contain this constituent. Cyanide is naturally present in the biosphere in both simple and complexed forms. Interestingly, for many decades cyanide and cyanide related compounds have been postulated as the first organic compounds on earth, from which the remaining building blocks of life were derived (Lehninger, 1970 and Oparin, 1938). Research has demonstrated that various amino acids can form from hydrogen cyanide precursors under the environmental conditions thought to exist during the early stages of earth's history and development (Chemical and Engineering News, 1997). Many biological systems, including higher plants, bacteria, fungi, algae and animals produce cyanogenic glycosides which liberate free cyanide upon hydrolysis (U.S. Fish and Wildlife Service, 1991). At least 1,000 species of plants and microorganisms from 90 families have been shown to contain one or more of nearly twenty compounds capable of producing cyanide (Seigler, 1976). About 800 species of higher plants from 70 to 80 families, including agriculturally important species such as the cassava, flax, sorghum, alfalfa, bamboo, peach, pear, cherry, plum, corn, potato, cotton, almond, and beans, are known to be cyanogenic (Eyjolfsson, 1970). The concentration of cyanide produced upon hydrolysis or digestion of cyanogenic plants can range from a few up to several hundred mg/L (U.S. Fish and Wildlife Service, 1991). Cyanide poisoning of livestock by forage sorghums and other cyanogenic plants is well documented. of acyanogenic plantsthan to both domestic andfederal wild ungulates hasissued been recognized for The moretoxicity than half century. More 60 years ago, the government a pamphlet regarding the potential for poisoning of livestock with naturally produced cyanide from cyanogenic plants (U.S. Department of Agriculture, 1932).


66

Analysis of Cyanide

Fungi and bacteria are prevalent producers of cyanide. About 300 species from 52 genera of fungi have been shown to be cyanogenic (Bach, 1956 and Locquin, 1944). One fungus, Fomes scutellatus, can generate a free cyanide concentration in solution of about 65 mg/L or 65,000 µg/L. It appears that cyanide production is widespread in fungi and is a normal metabolite (Knowles, 1976). Two species of bacteria are known to be cyanogenic, including Chromobacterium violaceum, a common soil and fresh water organism which has been shown to produce cyanide in concentrations as high as 21 mg/L (Michaels and Corpe, 1965). In addition, 74 out of 110 strains of Pseudomonas aeruginosa found commonly in soil and fresh water habitats have been shown to produce cyanide (Castric, 1975). In addition to plants and microorganisms, insects have been shown to produce cyanide in its molecular form HCN. Seven species of centipedes, 46 species of millipedes, three species of beetles, four species of moths and three species of butterflies have been shown to synthesize and excrete cyanide (Duffey, 1981). These insects and arthropods excrete cyanide for defensive purposes. Many of the cyanogenic plants, fungi and bacteria are ubiquitous in nature and can survive and thrive under a wide range of environmental conditions. Although no definitive studies are often taken during background investigations related to the permitting of mining operations or projects, it is not unlikely that one or more of these cyanogenic species are or have been residents in a specific area under investigation. It should also be noted that cyanogenic organisms can also produce chemically and structurally related natural organic compounds, which could report as cyanide in its analysis, thereby yielding yet another positive interference. Many of the cyanogenic microorganisms reported in the scientific literature are commonly found in a wide variety of chemical and physical environments, like those associated with mining projects. Consequently, it is possible that naturally produced cyanide has been noted either periodically or randomly during the ongoing monitoring of surface and groundwaters in the vicinity of a mining project. 2.7.2

Anthropogenic Sources of Cyanide

Other than analytical variation or the natural production of cyanide resulting in the appearance of cyanide in aqueous samples, there is the potential for release or discharge of manufactured cyanide from manmade sources. Many proposed mining projects are located in areas of substantial historic mining activity, and cyanide has been used since the turn of the century at many of those defunct mining operations. Older tailings deposited at historical operations are a possible source of low levels of particularly total cyanide, as it is slowly degraded and released to surface or groundwaters in the vicinity of a newly operating or proposed mining project. Cyanide is used at mining operations in generally one of two primary processes. The first process involves the extraction and recovery of gold and silver using elevated cyanide levels in the of about to 400 mg/1. The second selective flotation, utilizes veryrange low levels of 100 cyanide in the recovery of baseprocess, metals,known such asaslead and zinc. In addition, road salt can contribute to elevated levels of total cyanide in adjacent surface waters (Ohno, 1989). Cyanide also has been or is currently used in fire retardants, predator (coyote) poisoning devices and for fishing in Southeast Asia.

67

Chapter Two


Iron cyanide is often used as an anti-caking agent in both table and road salts, and can be present in the salt at concentrations of up to several mg/kg. In the USEPA exposure and risk assessment document for cyanide, it was estimated that about 940 metric tons of iron cyanide used as an anti-caking agent in road salt enters U.S. surface waters each year (USEPA, 1981). The quantity of cyanide used in salting roads annually for each of the cities of Chicago, Detroit, New York and Milwaukee ranged from 12 to 27 metric tons. In contrast, the USEPA estimated that about 2 to 20 metric tons of cyanide are released into U.S. surface waters by the entire ore mining and dressing industry each year. 2.7.3 Monitoring of Surface and Ground Waters Due to the many issues related to the validity and importance of low levels of cyanide, a monitoring program should be established with a specific procedure to deal with cyanide levels reported below the detection and quantitation limits. Typically, any total or WAD cyanide value reported below the method detection limit is recorded but should be reported as “less than” the detection limit. A cyanide value reported above its detection but below its respective quantitation limit may indicate the presence of cyanide but at an unknown level. In this particular situation and at a minimum a second sample should be taken to confirm the presence or absence of cyanide. It is advantageous in this type of situation to utilize more than one laboratory to perform the confirming analyses if available. If the presence of cyanide is confirmed, a systematic procedure should be in place to initiate an investigation into potential sources and causes of the reported values. Depending upon the international, national and/or local regulatory requirements associated with a particular mining operation in a particular country, a well-defined procedure may be in place for the reporting of cyanide values that exceed a promulgated standard or criterion.

2.8

REFERENCES

ASCI/SRK, "An Evaluation of the Effectiveness of Rinsing Procedures on Cyanide Removal from Spent Heap Leach Ore; Brohm Mining Corporation, Lead, South Dakota, USA", for State of South Dakota, September 1990. American Public Health Association (APHA), Standard Methods for the Examination of Water th and Wastewater, 20 Edition, Edited by Lenore S. Cleseri, Arnold E. Greenberg and Andrew D. Eaton, 1998. Chemistry Centre (WA), “Fate of Cyanide in the Environment near Mine Tailings”, AMIRA Project Report P277, 1991. AMIRA, “Cyanide Waste Management: Minimising Environmental and Economic Impacts, Comprehensive Literature Compilation and Critical Review of Cyanide Behaviour and Control”, AMIRA Project Report P497, 1997. AMIRA, “Cyanide Waste Management: Minimising Environmental and Economic Impacts”, AMIRA Project Report P497A, 2000.


68

Analysis of Cyanide

ANZECC, Australian Water Quality Guidelines for Fresh and Marine Waters, 1992. ASTM, Section II, Water and Environmental Technology, Volume 11.02, 1985. Barton, P., Hammer, C.A., and Kennedy, D.C., "Analysis of Cyanides in Coke Plant Wastewater Effluents", Journal Water Pollution Control Federation, pp. 234-239, 1978. Black, S., Schulz, R.S. and McGuire, J.C., “Photo Diode Array Detection Used as a Fingerprinting Tool for Metal Cyanide Complexes in Gold Processing Liquors”, Proceedings of th the 27 International Symposium of E nvironmental and Analytical Chemistry, 1997. Black, S. and Schulz, R.S., “Sulfur Speciation in the Mining Environment”, Accepted for Presentation at World Chemical Congress, Brisbane, Australia, 2001. Brickell, R.H., "Chemistry of Cyanide Solutions", Presented at Cyanide and the Gold Mining Industry: A Technical Seminar, sponsored by Environment Canada and Canadian Mineral Processors, 1981. Caron, S., Chateauneuf, J., Tardif, L. and Bourassa, M., “Automatic Control of Free Cyanide at Yvan-Vezina Plant”, CIM Bulletin, pp. 55-61, November/December, 1999. Conn, K, "Cyanide Analysis in Mine Effluents", Cyanide and the Gold Mining Industry: A Technical Seminar, Ottawa, Ontario, 1981. Csikai, N.J. and Barnard, A.A., Jr., "Determination of Total Cyanide in Thiocyanate-Containing Wastewaters", Analytical Chemistry, Vol. 55, p. 1677,1983. Damon, L., Smith, A. and Mudder, T., "Geochemical Study of Leach Pad Cyanide Neutralization", Brohm Mining Company, Lead, South Dakota, 1991. DeVries, F. and Mathre, O., "Modified Free Cyanide Field Test Without Distillation", Proceedings of the Cyanide and the Environment Conference, Tucson, Arizona, Volume I, p. 189, 1984. Ecological Analysts, Inc., "Cyanide: An Overview and Analysis of the Literature on Chemistry, Fate, Toxicity and Detection in Surface Waters", Prepared for the Inter-Industry Cyanide Group, 1979. Fagan, P., Haddad, P., Dunne, R. and Mitchell, I., “Advances in Ion Chromatography for Monitoring the Gold Cyanidation Process”, Proceedings from Randol Gold Forum, pp. 321-327, 1996. Ingersoll, D., Harris, W., Bomberger, D., and Coulson, D., "Development and Evaluation of Procedures for the Analysis of Simple Cyanides, Total Cyanide, and Thiocyanate in Water and Wastewater", USEPA, EPA-600/u-83-054, October, 1981.

69

Chapter Two


Kelada, N.P., Lue-Hing, C. and Chavich, J.A., "Cyanide Thiocyanate Speciation and Removal of Thiocyanate and Other Interferences", Cyanide and the Environment Conference, Tucson, Arizona, 1984. Koch, WM.F., "The Determination of Trace Levels of Cyanide by Ion Chromatography with Electrochemical Detection", National Bureau of Standards Journal of Research, 1983. Luthy, R.G., Bruce, S.G., Walters, R.W. and Nakles, D.V., "Identification and Reactions of Cyanide and Thiocyanate in Coal Gasification Wastewaters", Sixth Quarterly Report, U.S. Department of Energy, FE-2496-23, 1978. Makhija, L. and Hitchen, A., "The Titrimetric Determination of Sulfate, Thiosulfate and Polythionates in Mining Effluents", Anal. Chim. Acta., Vol. 105, pp. 375-382, 1979. Mudder, T.I. and Botz, M.M., Unpublished Data, 1998. Noller, N.B. and Schulz, R.S., “The Effect of Cyanide Preservation on Analytical Results”, The th 4 RACI Environmental Chemistry Conference, Darwin, NT, Australia, 1995. Pohlandt, C., Jones, E., and Lee, A., "A Critical Evaluation of Methods Applicable to the Determination of Cyanides", Journal of the South African Institute of Mining and Metallurgy, pp. 11-19, January, 1983. Pohlandt, C., "The Determination of Cyanide in Hydrometallurgical Process Solutions and Effluents by Ion Chromatography", Report No. M128, Council for Mineral Technology, Randburg, South Africa, 1984. Pohlandt, C., "Chromatographic Separation and Determination of Stable Metal Cyanide Complexes in Gold Processing Solution", South African Journal of Chemistry, 38 (3), pp. 110114, 1985. Pohlandt, C., "The Determination of Cyanides In the Hydrometallurgical Processing of Gold", in Proceedings of Cyanide and the Environment Conference, Tucson, Arizona, December 11-14, 1984. Rocklin, R. and Johnson, E., "Determination of Cyanide, Sulfide, Iodide and Bromide by Ion Chromatography with Electrochemical Detection", Anal. Chem., 55, pp. 4-7, 1983. Schulz, R.S., “Investigation of Chemical and Cyanide Properties in Saline Tailings of Gold Mines”, Australian Journal of Mineralogy, October, 1996. Schulz, R.S., Cyanide Trial 96, Chemistry Centre, Western Australia, February, 1997. Schulz, R.S., Chemistry Centre, Western Australia, Private Communication, 2001.


70

Analysis of Cyanide

Schulz, R.S., “Cyanide Analysis In Waters– An Interlaboratory Comparative Study”, Chemistry Centre (WA), 1992. Solujic, L., Milosavljevic, E.B. and Hendrix, J.L., “Cyanide Determination Methods: Distillation vs. Flow Injection Analysis”, Proceedings of the Randol Gold Forum, pp. 167-173, 1996. Standard Methods For The Examination of Water and Wastewater, 16th Edition, APHAAWWA-WPCF, 1985. Standard Methods For The Examination of Water and Wastewater, 17th Edition, APHAAWWA-WPCF, 1989. USEPA Environmental Monitoring and Support Laboratory, Methods For Chemical Analysis of Water and Wastes, Cincinnati, Ohio, 1986. USEPA, Test Methods for Evaluating Solid Waste, SW-846, Third Edition, Volume IC, Chapter 7, Section 7.3, 1986. USEPA, Method OIA-1677 Available Cyanide by Flow Injection, Ligand Exchange and Amperometry, EPA-821-R-99-013, August, 1999. Whitlock, J., Sharp, C., and Mudder, T., "Interferences In Cyanide Analysis: Reflux Methods", Homestake Mining Co., unpublished results, 1981. Whitlock, J., Sharp, C., and Mudder, T., "Summary of the Toxicological Significant and Analytical Methodology For Free and Complexed Cyanide," Homestake Mining Co., unpublished results, 1981. Yoshida, T., Tamamura, Y., and Katsura, T., "Practical Method For Determination of Total Cyanide in Metal-Containing Wastewaters", Environ. Sci. Technol., 43, No. 9, pp.441, 1983.

2.9

BIBLIOGRAPHY

Federal Register, Approved Inorganic Test Procedures. Vol. 51, No. 125, Monday, June 30, 1986. Gottfried, G.J., "Application and Evaluation of Analytical Procedures for Trace Metals, Total Cyanide and Phenolics", USEPA Project Summary, EPA-600/S4-82-001, August, 1982. Grigorova, B. and Lenahan, W., "The Determination of Cyanide in Waters", Mintek Symposium on the Analysis of Waters, Randburg, South Africa, 1983. Hendrix, J.L, Nelson, J.H., and Ahmadiantehrani, M., "Fate of Cyanide in Tailings-An Update", Cyanide and the Environment Conference, Tucson, Arizona, December, 1984.

71

Chapter Two


Ray, W.M.R., "A Simple Leach Procedure For Determining the Total Cyanide Content of Heap Leach Tailings", Cyanide and the Environment Conference, Tucson, Arizona, December, 1984. Schaeffer, D. and Kerster, H., "Quality Control Approach to NPDES Compliance Determination", J. Water Pollut. Control Fed., 60, p. 1436, 1988. Sharp, C. and Baker, D., "Accuracy and Precision of the EPA Reflux Method for Cyanide in Gold-Processing Waste Streams", 1981 Society of Mining Engineers Annual Conference, Chicago, Illinois, 1981. Umana, M., Beach, J., and Sheldon, L., Restrictions Rule Support Revision to Method 9010, Total and Amenable Cyanide, Office of Solid Waste, USEPA, Washington, D.C., 1987. Youden, W.J. and Steiner, E., Statistical Manual of the Association of Official Analytical Chemists, Washington, D.C., 1975.


72

CHAPTER THREE

Environmental Geochemistry and Fate of Cyanide* 3.1

INTRODUCTION

The status of cyanide as a social and environmental pariah is not founded in the technical literature but is an emotionally driven issue, consequent perhaps from the past intentional use of this chemical as a toxic agent against humans. However, a technically based evaluation of the nature and behaviour of cyanide in the environment leads to a more sympathetic view of the potential impact of this widely used industrial and mining chemical (Mudder and Smith, 1998). This chapter addresses the environmental geochemistry and fate of cyanide in mining applications, including solution ponds, tailing impoundments, heap leach pads and subsurface fluids. This chapter begins with a discussion of the environmental chemistry and geochemistry of cyanide, followed by an evaluation of mechanisms of cyanide attenuation in natural systems. Basically, the presentation is an evaluation of five cyanide-containing systems that are affected by the geochemistry of their solid phases, such as ore, tailings, soils and subsurface aquifer materials. Each of these systems is discussed separately, namely: • • • • •

Process solution ponds Tailings impoundment supernatant solution, pore fluid and tailing solids Heap leach solution, pore fluid and ore Tailings impoundment seepage and subsurface geologic materials Groundwater and geologic materials

Each of these systems represents a complex geochemical situation, but it is possible to produce a chemical explanation of the behaviour of cyanide in each system. In Chapter One the basic chemistry of cyanide was presented, and now in Chapter Three a solid phase is taken into consideration relative to the chemistry and reactions of cyanide. In order to address how each system affects the cyanide chemistry on a general basis, it is necessary to construct a working hypothesis of chemical behaviour of cyanide in the five geochemical systems. The chapter concludes with a brief discussion of the sources and reactions of cyanide in the atmosphere.

*

Revised and reprinted with the permission of The Society of Economic Geologists, Inc., Littleton, Colorado 80120 (USA)

73


3.2

ENVIRONMENTAL GE OCHEMISTRY OF C YANIDE

3.2.1

Environmental Fate: The Cyanide Cycle

A basic appreciation of the fundamental building blocks of cyanide chemistry was presented in Chapter One. To summarize and expand the range of reactions discussed, one can refer to a compilation of general cyanide reactions produced by Mudder (1984) giving what has been termed the "Cyanide Cycle". This cycle is shown on Figure 3.1 and represents a cyanide system consisting of solution, solids and the atmosphere. This figure gives an appreciation of the large number of chemical, biochemical and geochemical reactions that involve cyanide. These reactions are considered in more detail throughout this chapter. 3.2.2

Cyanide Geochemistry

The physical and chemical properties of natural solid materials are related to geochemical reactions observed with cyanide in the environment. Ores, rocks and soils contain to a greater or less extent silicates, aluminosilicates, clay minerals, sulphides, carbonates and oxides as mineral phases, plus organic matter, water and gases in some cases. These are the potential reactants with the cyanide solutions present in slurry leaching systems, in tailings impoundments, in heap leaching pads and in the subsurface soils and aquifer materials. The solid components can control the solution chemistry with regard to alkalinity or acidity, buffering capacity and the degree of oxidation or reduction, resulting in ion exchange adsorption or chemical reactions in the cyanide solution. In order to use the knowledge of cyanide chemistry and relate it to observed field data, a basic concept of their inter-relationship is required. Figure 3.2, modified by Smith (1988b) from Simovic et al. (1985), takes a geochemical system, such as is found in a tailings/heap leach system with both "strong" and "weak" metal complexes and relates these complexes to cyanide hydrolysis and HCN volatilisation. The contribution of each to the free cyanide, WAD cyanide and total cyanide (TCN) analyses is shown. Figure 3.2 can be expanded to include oxidation of CN /HCN to cyanates or thiocyanates, or one of many other compounds. In addition, the mass balance of cyanide can be calculated using the equations given in Figure 3.2. Clearly, this expression of cyanide inter-relationships is rather simple, but it is an effective tool for combining aspects of theoretical cyanide chemistry with the phenomena observed in industrial settings.


74

FIGURE 3.1 The Cyanide Cycle

7 5

C ha tep r T hr ee


FIGURE 3.2 Interrelationships in Cyanid e Chemistry

Kuv

Kv

Kb M1 CN

Ka

CN

HCN Kc

Kd

1

Ke

2

M2 CN

3

Where: M1 M2 kuv

= “strong” complexing transition metal (TM) e.g., iron = other “weak” TM e.g., copper, zinc, etc. = influence of (Kuv) ultraviolet radiation

ka, kb, kc, ke, kf kv

= rate constants of transformation = volatilisation mass transfer coefficient (loss to atmosphere)

One can add, if necessary, cyanide to cyanate reactions, for example: 2CN- + O2 → 2OCNA cyanide mass balance can be obtained by: 1.

d

( MCN ) dt

= − Ka [ MCN ] − K UV [ MCN ] when inf luenced by ultraviolet radiation

or -Ka[MCN] when not influenced byultraviolet radiation

2.

d

( FCN ) dt

= − Ka [ MCN ] − K UV [ MCN ] −

Kv Z

[ HCN ]

where Z is related to volume and area for volatilisation of HCN

3.

d

(TCN ) dt

=−

KV Z

[ HCN ] when there is no effect of ultraviolet radiation

Source: Smith, 1988b


76

Z =

V (m 3 ) A (m 2 )

1

Free CN

2

WAD CN

3

Total CN

Environmental Geochemistry and Fate of Cyanide

3.3

ATTENUATION MECHANISMS OF CYANIDE

There are a number of natural attenuation mechanisms of cyanide that are often observed in more than one of the geochemical systems addressed in this chapter (Smith and Mudder, 1998). In this section, prior studies of these natural attenuation reactions are presented and the environmental significance of these reactions and the factors affecting them are also described. 3.3.1

Complexation (Chelation)

Ford-Smith (1964) reported that 28 elements are capable of forming complexes with cyanide, with 72 metal cyanide complexes possible. The solubilities of these complexes range from very soluble to insoluble. Numerous transition metals chelate or complex with cyanide and the resulting metal-cyanide compounds are generally less toxic than is the uncomplexed form of free cyanide. However, some of these compounds are not very stable and will decompose and release free cyanide under specific environmental conditions. The toxicity of metal cyanide complexes is generally due to the dissociation of the complex to yield free cyanide rather than due to the metal cyanide complex itself, although the toxicity of the metal must also be considered. Data on the stability constants of metal cyanide complex ions were presented in Table 1.5 in Chapter One. At a pH of 4.5, some of the metal-cyanide complexes typically found in mineral processing waters, such as zinc and copper, may dissociate into free cyanide to varying degrees and at varying rates. These are the so-called weak acid dissociable (WAD) metal cyanide complexes and are of central importance in regard to the environmental chemistry of cyanide. The tightly bound iron cyanide complexes produce free cyanide slowly due to photochemical decomposition in conditions of relatively intense sunlight (i.e., ultraviolet radiation). There is some question as to the significance of this decomposition in surface waters relative to the resultant toxicity under realistic conditions (Doudoroff, 1980). While the formation of metal cyanide complexes does not completely eliminate the toxicity of cyanide, it does effect a substantial reduction in such toxicity. Metal cyanide complexes are also intermediates involved in the formation of more stable compounds that remove free cyanide from the environment. For example, Milne (1950) suggested the use of nickel to form complexes as a method for the disposal of free and complex cyanides in electroplating wastes. Kunz et al. (1979) described a process where copper was complexed with cyanide to enhance the adsorption and oxidation of cyanide onto activated carbon. Bishop and Wright (1977) received a patent for an electrochemical cyanide oxidation process using nickel to enhance the process. The environmental significance of complexation/chelation of cyanide with transition metals is that it is relatively rapid andItitreduces occurs wherever soluble species of these metals areintermediary present, such as found in soils and ores. the toxicity of free cyanide and acts as an to reduce cyanide mobility via adsorption onto organic and inorganic surfaces or precipitation of metal cyanide salts. Based on the relative abundance of iron in soils, it is expected that many of the metal cyanide reactions occurring in soils will involve iron.

77

Chapter Three


The complexation of cyanide can have two very diverse effects on cyanide mobility in soils. If cyanide complexes with iron it is less able than free cyanide to adsorb onto surfaces, hence its mobility can be increased as demonstrated by Fuller (1985). However, if free transition metals such as iron, copper or nickel are present, a ferrocyanide precipitate will cause the cyanide species to be tied-up with the soil. On the other hand, if cyanide chelates as a weak metal complex, it will enhance adsorption of cyanide onto organic carbon, metals oxides, feldspar and clay surfaces. 3.3.2

Cyanide Complex Precipitation -4

-3

The ferrocyanide ion (Fe(CN)6 ) and the ferricyanide ion (Fe(CN)6 ) form insoluble salts with iron, copper, nickel, manganese, lead, zinc, cadmium, tin and silver as stated by Weast (1969). Typically within the soil environment, the oxidation potential would result in the formation of ferrocyanide precipitates. In addition, the iron-cyanide complexes can react with thiocyanate to form even more stable complexes, if sulphur is present in the soil. Hendrickson and Daignault (1973) demonstrated that both ferro- and ferricyanide complexes will precipitate with iron, copper, magnesium, cadmium and zinc through a broad range of pH values ranging from 2 to 11. Stoichiometric additions of transition materials were observed to remove greater than 90% of the ferro- or mixed ferro-ferric complexes. However, at high oxidation potentials where only ferricyanide complexes were present, precipitation only removed about 60% to 90% of the ferricyanide. This indicates that over a broad range of pH and oxidation potentials, iron cyanide metal complexes will precipitate if sufficient free transition metals are available. 3.3.3

Adsorption

Adsorption is another mechanism that attenuates cyanide in soils. Alesii and Fuller (1976) conducted tests with free cyanide in water, potassium ferricyanide in water and free cyanide in landfill leachate to determine cyanide mobility in various types of soils. The test results indicated that soils having high concentrations of hydrous oxides of iron and manganese retained cyanide best. Studies conducted for the USEPA in the mid-1970's on the leachate pollutant attenuation abilities of soils indicated that soils containing aluminium minerals attenuated cyanide better than other soils (Alesii and Fuller, 1976; Fuller, 1977, 1978, 1980). However, Towill et al. (1978) in reviewing the environmental effects of cyanide found that free cyanide ions are not strongly adsorbed or retained by soils irrespective of the presence of aluminosilicate minerals (Murrmann and Koutz, 1972).


78


Based on their tests, Alesii and Fuller (1976) concluded that soils with a high anion exchange capacity would be likely to attenuate cyanide. Those soils with a high anion exchange capacity typically contain kaolin clay, chlorite, gibbsite clay and/or iron and aluminium oxides. Conversely, soils containing predominantly strong cation-exchanging materials, such as montmorillonite, were predicted to have a lesser effect on cyanide adsorption. Bessent et al. (1979) noted that ion exchange resins do not remove free cyanide effectively. However, the indications are that all metal cyanide complexes are readily adsorbed by anion exchange resins. The same may be true for cyanide adsorption in soils. As is well known in precious metals leaching, organic materials will adsorb or react with cyanide. Carbonaceous materials in ore slurries will adsorb free and metal complexed cyanides to limit precious metal recovery. Activated carbon has been used for cyanide removal for approximately 25 years in the mining industry. During this time it has been found that cyanide is first adsorbed and then catalytically oxidized to cyanate (Bernardin, 1973). The presence of copper, cadmium, zinc or nickel ions in solution results in the formation of metal cyanide complexes, which enhances the adsorptive capacity of carbon. In addition, Hoecker and Muir (1987) found that carbon adsorbs almost eight times more cyanate than cyanide as determined from the limited testing performed. Chatwin and Trepanowski (1987) found that the magnitude of cyanide adsorption in sub-soils was correlated with the organic carbon content of the soil. It appears that cyanide adsorbs onto soil organic matter and becomes bound or is subsequently oxidized to cyanate. Free cyanide has also been found to adsorb on the surface of clays and feldspar (Chatwin, 1988). Test results indicate that this mechanism is dependent not only on the mineralogy of the soil but the solution chemistry as well. It was found that metals that form weak metal cyanide complexes, such as copper and nickel, enhanced the adsorption process. It also appeared that cyanide could oxidize to cyanate, apparently utilizing inorganic surfaces as an oxidation catalyst. 3.3.4

Oxidation to Cyanate

Cyanide can be converted to cyanate according to the following simplified reaction: (3.1)

-

-

CN + ½O2 → OCN

Cyanate is significantly less toxic than is free cyanide or molecular hydrogen cyanide. Cyanide conversion to cyanate has been demonstrated when cyanide is in the presence of strong oxidizers, such as ozone, sulphur dioxide/air, hydrogen peroxide and hypochlorite. Ultraviolet light, in conjunction with catalysts such as titanium dioxide, cadmium sulphide or zinc oxide, has been shown to convert cyanide to cyanate (Frank and Bard, 1977). It is theorized that the ultraviolet light causes catalyst(Miles, to convert dissolved solution to ozone, which promptly reacts with thethecyanide 1981). Strong oxygen oxidizersin are typically not found in natural soils. However, in recent work it has been found that cyanide can be converted to cyanate in the soil on the surface of organic and inorganic materials (Chatwin, 1990). This reaction appears to be enhanced by the addition of copper or nickel to the solution.

79

Chapter Three


The thermodynamics of the cyanide-cyanate reaction, as seen on Figure 3.3, indicate that cyanate should be the predominant species under natural conditions. However, under natural ambient conditions the oxidation of cyanide to form cyanate is relatively slow at neutral or alkaline pH conditions. A strong oxidant such as ozone, hydrogen peroxide or chlorine is required to drive this reaction. Bacterial enzymes or catalytic surfaces of titanium dioxide, zinc sulphide and carbon have been found capable of promoting this oxidation as well. 3.3.5

Volatilisation

Hydrogen cyanide, also known as hydrocyanic acid, is a colourless gas or liquid with a boiling point of 25.7°C and a vapour pressure of 100 kPa at 26°C (Huiatt et al., 1982). The equilibrium hydrolysis reaction between cyanide ion and water occurs according to the following reaction: (3.2)

-


-

-

At a pH of 9.36, the molar concentrations of HCN and CN ion are equal, as shown on Figure 1.1 in Chapter One. At lower pH values, the majority of cyanide exists as HCN. Thus, at neutral pH conditions most of the free cyanide will be in the form HCN. Hence, in soils or other natural geochemical systems, where the pH of cyanide solutions are buffered to near-neutral pH values, HCN would be the predominant free cyanide species present. For example, in tests performed by Chatwin and Hendrix (1988) on a suite of arid subsoil samples, the soil pH ranged from 4.5 to 8.9. Should free cyanide be present in contact with these soils, it would result in solutions where the HCN content would be the predominant form of free cyanide present. HCN in solution also maintains equilibrium with HCN in the gas phase. The concentration of gaseous HCN approximates that calculated using Henry's Law. Published values for the Henry's constant for hydrogen cyanide have ranged from 4368 to 4810 mm Hg/mole fraction HCN. Dodge and Zabbon (1952) and Chester Engineers (1977) identified pH, temperature, interfacial surface area, pressure, concentration and degree of agitation as factors affecting cyanide volatilisation. Palaty and Horokova-Jakubu (1959) cited in Simovic et al. (1985) also studied volatilisation of cyanide from simple cyanide solutions. They identified the same variables that affected cyanide volatilisation as Dodge and Zabbon (1952). They also determined solution depth to be an important variable. Their general conclusions were: •

HCN removal rate increases with decreasing pH down to 5.0.

•

An order of magnitude difference in the rates of HCN removal exists between aerated and non-aerated solutions.

•

The HCN removal is indirectly proportional to the solution depth and almost directly proportional to the rate of aeration.

•

A temperature increase of 10°C (from 0.8 to 11°C) causes the HCN removal rate to increase by greater than 40%.


80


FIGURE 3.3 Eh-pH Diagram for the Free Cyanide-Water System at 25°C

Source: Smith, 1988b

81

Chapter Three


While hydrogen cyanide volatilisation from surface waters is quite well understood, cyanide volatilisation from soils is a more complex process. However, in work involving unsaturated soil columns Chatwin and Hendrix (1988) measured HCN volatilising from soil surfaces. 3.3.6

Bioattenuation

Towill et al. (1978) reported that cyanides move only a short distance through soil before being biologically converted under aerobic conditions to nitrates via microbial attenuation to ammonia and then to nitrate. It was also found that cyanides are immobilized after reacting with trace metals through chelation processes. Strobel (1967) tested both sterile and non-sterile soils to determine their effect on cyanide attenuation. The non-sterile soil degraded cyanide, while the sterile soil did not appreciably alter cyanide. Fuller (1985) reported that cyanide up to 200 mg/L was readily converted to fertilizer nitrogen in the soil. In fact, plants responded to cyanide applications nearly identically as they did to sodium nitrate or ammonium nitrate, both common components of fertilizers. Cyanide bioattenuation is currently being utilized to treat industrial wastewater in two separate processes. Homestake Mining Company in Lead, South Dakota utilizes bacteria to treat its wastewater prior to discharge. Another company, ICI Bioproducts, detoxifies cyanide with the strain of the fungus Fusarium lateritium. Bioattenuation under anaerobic conditions is not nearly as prolific as under aerobic conditions. The limit for effective anaerobic attenuation of cyanide was found by Coburn (1949) to be about 2 mg/L cyanide. Above this concentration, the cyanide was found to be toxic to the anaerobic micro-organisms. Below 2 mg/L of cyanide, there is evidence that denitrification occurs in certain soluble cyanides, yielding nitrogen gas (Huiatt et al., 1982). 3.3.7

Formation of Thiocyanate

Free cyanide has been shown to react with various forms of sulphur in the environment to form thiocyanate. Thiocyanate is relatively non-toxic when compared with cyanide. The two forms -2 -2 of sulphur most likely to react with cyanide are polysulphides (S x ) and thiosulphate (S 2O3 ) (IEC, 1979). They react according to the following equations: -2

-

-2

-

-2

-

(3.3)

Sx + CN → [S(x-1)] + SCN

(3.4)

S2O3 + CN → SO3 + SCN

-2

-

In neutral to basic solutions, both polysulphides and thiosulphate are oxidation products of sulphides, the latter being abundant in minerals. As such, these products could possibly be present in oxidizing environments, such as the vadose zone in soils. The concentrations of polysulphides and thiosulphate in a soil are strongly dependent on the sulphur content and the Eh-pH conditions in that soil.


82


McGill et al. (1985) conducted bench tests on various sulphur-bearing ore minerals to determine if they caused the formation of thiocyanate at a pH of 10.0. The laboratory results indicated that chalcopyrite, chalcocite, pyrrhotite and free sulphur contributed a significant portion of their sulphur content to thiocyanate production, while "pure" iron sulphides contributed a lesser yet significant portion. The sulphur contained in sphalerite and pyrite was relatively non-reactive towards cyanide. 3.3.8

Hydrolysis/Saponification of Free Cy anide

At a neutral or acidic solution pH, HCN can be hydrolysed to yield formate, as either formic acid or ammonium formate according to the following reactions: (3.5)

HCN + 2H 2O → NH4COOH (ammonium formate)

(3.6)

HCN + 2H 2O → NH3 + HCOOH (formic acid)

The system pH will dictate the extent of formation of each compound, a lower pH favouring formic acid formation. Hoecker and Muir (1987) have tested high temperature hydrolysis of cyanide in autoclaves and have developed kinetic data for this reaction. A rate for cyanide hydrolysis to ammonium formate of 4% per month is estimated by extrapolating this data to room temperature. This rate is similar to that obtained by DuPont of 2% per month (Longe and DeVries, 1988). Hence, this is not a rapid cyanide attenuation mechanism, but should be considered in systems with relatively long residence times. 3.3.9

Summary

The foregoing discussion of the eight categories of processes and mechanisms of cyanide attenuation gives some idea of the breadth and relative complexity of the chemical behaviour of cyanide and its potential stability in complex forms in the environment. These data are now applied to mill process facilities and natural systems in the subsequent sections.

3.4

NATURAL CYANIDE ATTENUATION IN SURFACE PONDS

This section describes the processes of natural cyanide attenuation that occur in surface ponds. The dominant mechanism in natural cyanide attenuation in most surface ponds is volatilisation of HCN from solution. Over time the pH of ponds is lowered by the natural uptake of carbon dioxide from the air and by the addition of acidic rainwater. In many cyanide solutions from mining operations, the equilibrium pH resulting from carbon dioxide uptake is in the range of about 7.0 to 9.0. This drop in pH induces a change in the cyanide/hydrogen cyanide equilibrium (Figure 1.1), favouring the formation of HCN and its subsequent volatilisation.

83

Chapter Three


Data by Schmidt et al. (1981) illustrate cyanide attenuation that occurs in relation to water depth, time of year and temperature for two ponds at a mine in northern Canada. Figure 3.4 illustrates the following: •

Cyanide decay occurs in both the shallow and deep pond sections, although there is a slight time lag for the deeper pond section.

•

Cyanide transformation to cyanate occurs early on, following cyanate loss from the pond water.

•

Thiocyanate decay was slight early on, followed by rapid decay, particularly in the shallow pond.

At first glance this work appears to disagree with the prior studies by Dodge and Zabbon (1952) who found that cyanide volatilisation was dependant upon the ratio of solution area to depth in stagnant solutions. However, in ponds there is a natural mixing caused by the convective currents due to differences in pond temperature, with temperature differences between the air and water and due to wind action.


84


FIGURE 3.4 Natural Attenuation of Cyanide, Cyanate and Thiocyanate

Source: Schmidt et al., 1981

85

Chapter Three


Simovic et al. (1985) and Zaidi et al. (1987) have attempted to quantify cyanide losses in process ponds in Canada. They have developed an empirical model that predicts cyanide removal and it has been validated against actual pond data. The volatilisation of HCN from surface waters is the dominant mechanism for natural cyanide attenuation. Ninety percent of the free cyanide was removed by volatilisation and an additional 10% was removed by chemical oxidation (Simovic et al., 1985). Cyanide attenuation was found to follow a first order reaction with respect to free cyanide and cyano-metal complexes of zinc, nickel, copper and iron. Temperature and aeration have the most significant effect on the volatilisation rate of free cyanide and most metal cyanide complexes studied. Simovic et al. (1985) found that UV radiation has an effect on the stability and attenuation of iron-cyanide complexes in the surface ponds. Bench-scale laboratory data were fitted to a mathematical model which considered volatilisation of free cyanide and dissociation of the metal cyanide complexes. The model fit the experimental data of single metal cyanide complexes quite well with correlation coefficients of 0.93 to 0.99 (Simovic et al., 1985). Individual first order decay constants were derived from the mathematical model for copper, iron, nickel and zinc cyanide complexes as a function of temperature and solution pH. Empirical calibration of the natural attenuation model developed by Simovic et al. (1985) for predicting the attenuation of cyanide in impoundments has been performed by Environment Canada. The model was used to predict the attenuation of cyanide in pilot and full-scale batch and dynamic systems after calibrating the models for specific conditions at two sites in Canada. The results are reported by Zaidi et al. (1987). Figures 3.5 and 3.6 show a comparison of the model predictions for a deep and a shallow pond in Canada treating barren bleed in a batch treatment mode. As can be seen, the correlation is excellent. Figure 3.7 presents the observed and predicted cyanide attenuation in a pilot system operated in a continuous-fill and batch discharge mode in Canada. Again the agreement between the predicted and observed value is very good.


86


FIGURE 3.5 Observed and Predicted Attenuation of Cyanide in a Shallow Pond Operated in a Batch Mode in Canada

Source: Simovic et al., 1985

87

Chapter Three


FIGURE 3.6 Observed and Predicted Attenuation of Cyanide in a Deep Pond Operated in a Batch Mode in Canada



88


FIGURE 3.7 Observed and Predicted Attenuation of Cyanide in a Pilot Scale System Operated in a Continuous-Fill and Batch-Discharge Mode in Canada


89

Chapter Three


Another approach to modeling natural cyanide attenuation in surface ponds and tailings impoundment supernatant solution was recently developed by Botz and Mudder (2000). Their approach is based on detailed chemical equilibria and kinetic calculations for cyanide compounds in solution and the resultant effect on volatilisation as well as other mechanisms of cyanide removal. The approach is similar to that of Simovic et al. (1985) but rather than being more empirical in nature, the model attempts to be based on first principles of the interaction of cyanide with metals and the associated attenuation reactions. This model was used to predict cyanide attenuation in a North American tailings impoundment which primarily contained iron cyanide complexes. In this case, iron cyanide photolysis and volatilisation were the primary cyanide loss mechanisms, though other loss routes including precipitation were taken into consideration. The results are shown in Figure 3.8 and as indicated there was good agreement between predicted and actual cyanide concentrations in the supernatant solution. The incremental steps observed in the cyanide decay in Figure 3.8 were due to ice coverage of the supernatant in the winter which prevented photolysis reactions from occurring for several months each year. As the cyanide attenuation models are further validated by comparison with measurements from actual field observations, they will become increasingly useful in predicting the rate of natural treatment of process solutions in ponds and tailings impoundments. They will also be helpful in assessing the potential risk of cyanide release and the monitoring time required for decommissioned and inactive process facilities.


90


FIGURE 3.8 Predicted and Actual Cyanide Attenuation in a USA Tailings Impoundment

N C L / g m , n o it a tr n e c n o C e d i n a y lC a t o T

200 180 160 Calculated

140

Actual

120 100 80 60 40 20 0 0

10

20

30

40

50

60

70

Time, months

Source: Botz and Mudder, 2000

91

Chapter Three


3.5

CYANIDE GEOCHEMISTRY IN TAILINGS

3.5.1

Introduction

The objective of this section is to trace the pathways of cyanide through a mill tailings disposal system and evaluate the cyanide behaviour in the system. The evaluation of cyanide in mill tailings systems requires a somewhat different approach to that of a heap leach. In the latter case, the heap itself can be almost the entire focus of the assessment. However, for mill tailings, the total system from thickener to return water facility (where appropriate) bears consideration. It is possible to track theoretically the behaviour of cyanide and its geochemical reactions through a mill with its decant pond and tailings impoundments, or through a heap leach with its subsystems. By understanding and quantifying the cyanide mechanisms from Figure 3.9 that are significant in each geochemical system, the fate of cyanide can be recognized in each of the subsystems. Figure 3.9 is divided into ten sub-systems where geochemical changes can occur, the last of which represents subsurface environmental release. The remaining nine systems include the following where reactions such as dilution, concentration, oxidation, reduction, precipitation, co-precipitation and dissolution can occur to varying extents: •

Metallurgical extraction processes and slurry thickeners.

•

Chemical treatment plants

•

Tailings pumping systems.

•

Discharges into tailings impoundments.

•

Tailings beaches and decant ponds.

•

Oxidized tailings zones.

•

Reduced tailings zones.

•

Tailings drains, penstocks and decants.

•

Water reclaim facilities.

A fundamental question, which is still the subject of debate, is where in such a system are the cyanide "losses" occurring and is there any loss of cyanide within the tailings or heap leach mass? The term "loss" is somewhat of a misnomer in itself. Transformation of cyanide into cyanate or thiocyanate is termed a "loss of cyanide" but large values of cyanate or thiocyanate can be of significance in themselves. Also, the formation of iron cyanide complexes significantly reduces the toxicity of the solutions and they are still considered in the total cyanide mass balance.


92


FIGURE 3.9 Flow Schematic of Tailings Disposal at a Gold/Uranium Mine and Possible Types of Chemical Reactions in the System

93

Chapter Three


It is worth examining certain practical test and field data dealing with actual tailings and tailings facilities where the behaviour of cyanide has been evaluated in various climatic regimes and parts of the world. 3.5.2

Surface Effects in Tailings Ponds

Cyanide attenuation mechanisms described for surface ponds are applicable to the decant waters of a tailings impoundment. Typically the pH of these solutions as they come out of the mill are 10.0 or above. However, as these solutions "age" in the impoundment, the pH is reduced due to rainfall and carbon dioxide uptake, with the pH generally dropping to within a range of about 7.5 to 9.0. As the pH is lowered, the fraction of free cyanide present as HCN increases and volatilisation occurs. Surface effects have been shown to contribute to a large proportion of cyanide loss in tailings systems in South Africa (Smith et al., 1985a). Their data show cyanide being decreased from discharge concentrations from a conventional (non-CIP) mill of about 20 mg/L cyanide in the tailings supernatant to less than 2 mg/L in the tailings porewater. Such values are common. 3.5.3

Reactions in the Tailings Mass

Residual cyanide values in tailings are a source of substantial (though, perhaps, often somewhat unwarranted) concern to regulatory agencies. They cite lack of data on cyanide "loss" or transformation within the tailings mass itself and there is, in many cases, data which does show the presence of residual cyanide levels after significant time intervals (Hendrix et al., 1987). The extent to which these levels are a real risk to the environment is questionable, however. The following work on tailings and tailings impoundments illustrates some of the issues in this respect. The work of Smith et al. (1985b), which is illustrated in Tables 3.1 and 3.2, shows profiles of pore water quality in tailings systems. Table 3.1 shows the variation with depth of total cyanide within a tailings facility. Table 3.2 relates concentration to overall chemistry within the tailings for the oxidized, intermediate and reduced zones in the tailings mass. It is interesting to note the low values of cyanide in the reduced zone (<1 mg/L total cyanide) and the large number of samples examined (n=149). Caldwell and Smith (1985) and Smith et al. (1985b) developed their data based on test work from gold tailings in North America. The difference in cyanide values for the initial flotation tailings and the mixed tailings, after the concentrate tailings had been recombined with the flotation tailings (>300 mg/L total cyanide) is shown in Table 3.3. The geochemical effects of hydrogeochemical and rainwater of the mixed tailings are shown Table 3.4. In the chemically reduction reduced case, cyanide leaching levels decreased to <0.05 mg/L due to inthiocyanate formation. Rainwater leaching reduced the total cyanide from >300 mg/L to about 57 mg/L as total cyanide, of which about 2.5 mg/L was free cyanide.


94


TABLE 3.1

Borehole A

Cyanide Profile in Interstitial Water in a Decommissioned Tailings Impoundment in South Africa

PiezometerNo. 1 2

B

C

D

Depth,m 11 17

3 1 2 3 1 2 3 1 2 3

(2)

Total Cyanide, mg/L 0.16 0.21

25 27 30 47 18 22 26 19 24 28

(1)

0.57 1.54 0.16 (4) <0.01 0.22 2.92 0.35 (3) N/S 1.09 1.86

Source: Smith et al., 1985a Notes: (1) Cyanide discharge concentration varied between 8 and 19 mg/L during operation. (2) of tailings impoundment 32 m (104 ft). (3) Total N/S =height no sample. (4) Samples taken below 32 m are in the sediments below the impoundment.

TABLE 3.2 Chemistry of Oxidised, Intermediate and Reduced Zones in Tailings Impoundments (Mean Values) in South Africa

OxidizedZone,0-3m IntermediateZone,3-15m ReducedZone,15-35m

pH T DS SO4 6.6 3,350 1,850 6.6 3,600 2,210 6.6 2,850 1,600

Fe 47 21 8

Mn 4 11 20

TotalCN 0.6 2.0 0.9

Source: Smith et al., 1985a

Notes: (1) Concentrations in mg/L. (2) Total cyanide in near-surface groundwater below impoundment was 0.8 mg/L (average). (3) Two impoundments ±20 years old. (4) Number of samples = 149. 95

Chapter Three


TABLE 3.3 Cyanide Chemistry of “Mixed” Tailings and Flotation Tailings Solution in North America

Constituent pH Total DissolvedSolids Total Cyanide Free Cyanide Iron Cobalt Copper Mercury Silver

Mixed Tailings Supernatant 7.17 4,320 284 0.35 10

Flotation Tailings Supernatant 7.30 440 <0.05 <0.05 <0.05

0.33 0.03 0.0024 <0.01

<0.01 <0.01 <0.0003 <0.01

Source: Smith et al., 1985b

Notes: (1) All concentrations in mg/L.


96


TABLE 3.4

Reaction Products Chemistry of “Mixed” Tailings in North America

Constituent

MixedTailingsReduced with Na2S2O3 5.17 N/A <0.05 <0.05 1,100 <0.01 0.55 <0.01 0.0062 5.4 <0.01

pH Total DissolvedSolids Total Cyanide Free Cyanide Iron Arsenic Cobalt Copper Mercury Nickel Silver

Mixed Tailings Leached with Rainwater 4.80 2,750 57 2.58 18 0.08 0.08 <0.01 0.0011 <0.05 <0.01

Source: Smith et al., 1985b Notes: (1) All concentrations in mg/L.

TABLE 3.5 Time/Cyanide Concentration Dependency of Copper and Cobalt Cyanide in Tailings Interstitial Pore Solution in North America

Constituent TotalCyanide FreeCyanide Cobalt

Original Tailings Pore Water 170 408 170 408 0.73 2.21

Leached Sample 1

Leached Sample 2

2.66 1.9 0.92 1.4 <0.1 0.05

0.90 0.09 0.04 0.09 <0.1 0.05

1.81 6.34

0.04 0.04

<0.01 <0.01

Copper Source: Smith et al., 1985b Notes: (1) All concentrations in mg/L.

97

Chapter Three


TABLE 3.6

Timed , ays 0 1 3 7 14 28

Cyanide Decay in Tailings Decant Solution in New Zealand

pH 10.2 8.0 8.2 8.3 8.0 7.3

TotalCyanide 29.4 29.4 30.0 30.0 31.9 32.0

WADCyanide 6.8 ----0.33

Source: Mudder and Goldstone, 1989 Notes: (1) All concentrations in mg/L.

TABLE 3.7

Cyanide Decay in Tailings Pore Solution in New Zealand

Constituent pH SO4 Ca Total Hardness (CaCO3) TotalCyanide WAD Cyanide Cd Co Cu Fe Zn Pb Mn Ag

Week0 10.2 2,525 858 2,140 29.4 6.8 0.1 0.33 5 8.7 0.01 <0.1 0.02 0.04

Week1 9.7 2,525 858 2,140 29.4 6.6 <0.01 0.25 4.3 8.9 0.03 <0.1 0.02 0.01

Cr Hg

0.02 0.0002

-<0.0001

Source: Mudder and Goldstone, 1989 Notes: (1) All concentrations in mg/L.


98

Week2 9.1 2,530 858 2,140 31.3 5.5 <0.01 0.2 2.9 9 0.01 <0.1 <0.01 0.01 -<0.0001

Week4 Week8 Week12 9.8 9.8 9.0 2,370 2,370 2,390 858 808 805 2,140 2,013 2,008 25.5 27 25 3.8 2.1 1.51 <0.01 <0.01 <0.01 ---2.1 0.53 0.16 9.3 9.3 9.5 <0.01 0.01 0.01 <0.1 --0.01 0.02 0.02 0.02 0.02 0.02 <0.02 <0.0001

---

---


Smith and Brown (1986) developed data from test work on mill tailings in North America. These data show the loss of cyanide levels over time and during leaching with rainwater. The data in Table 3.5 show that copper and cobalt are lost from the aqueous phase of the system at an enhanced rate as compared with their leach rate. This demonstrates attenuation of the copper cyanide and cobalt cyanide complexes in the system. Both metal species are insoluble at high pH values and hence are precipitated as soon as the complex decays. Subsequent monitoring data have tended to confirm these predicted decay rates. 3.5.4

Overall Effects in T ailings

Kidd (1988) and Burden and Kidd (1987) have developed data on a project in New Zealand that show overall cyanide attenuation, both in the tailings pool and in the tailings mass. Mudder and Goldstone (1989) have described and compared cyanide attenuation both in tailings decant water and in the entrained tailings porewater from test data at a mine in New Zealand. In the case of the tailings testing, tailings samples were sealed in canisters and opened sequentially over time. The test results for the decant are given in Table 3.6 and for the tailings porewater in Table 3.7. While the total cyanide levels remained stable, WAD cyanide levels decreased rapidly, particularly in the decant water (6.8 ppm to 0.33 ppm in 28 days), presumably by precipitation and volatilisation. Similar data are given by Smith et al. (1985a) for tailings facilities from South Africa, for gold tailings projects worldwide (Smith, 1988b) and for projects in the United States (Caldwell and Smith, 1985 and Smith et al., 1985b). In the context of cyanide decay in overall tailings system, the stability of iron cyanide complexes, which often occur in tailings solids, has been debated at length. Some recent, yet unpublished, data from a tailings test programme showed the concentrations of free cyanide which could be formed by the induced attenuation of iron cyanides (cyclic, high intensity UV light exposure of tailings with inert gas/deionised water flush in column tests) were low (approximately 3 mg/L as free cyanide), despite the very harsh test conditions (Smith, 1990).

3.6

CYANIDE G EOCHEMISTRY OF TH E HEAP LE ACH

3.6.1

Introduction

Potential geochemical conditions and cyanide reactions in an abandoned cyanide heap leach operation are presented on Figure 3.10. As with the tailings example, the total system is a continuum and the separation between the fate of cyanide in the heap itself and the environment becomes somewhat indistinct. The upper portions of requirement Figure 3.10 are in an oxidizing environment. Thea reasonable "high" permeability of the heap itself, a basic so that it will actually leach, ensures flow of air. Also oxygen dissolved in leach solution and precipitation which infiltrates into the leach pad maintains the geochemical environment in an oxidized state.

99

Chapter Three


FIGURE 3.10 Prevailing Geochemical Conditions and Typical Cyanide Reactions in the Abandoned Heap Leach Environment

Source: Smith and Struhsacker, 1988


100


Table 3.8

Reaction Equation 1 2 &3

4 5& 6

7 8&9

10 11 12 & 13

Typical Cyanide Reactions in a Heap Leach Environment

Reaction Description HydrolysisofCyanide CN + H2O → HCN + OH Oxidation of Cyanide 2HCN + O2 → 2HOCN 2CN + O2 → 2OCN HydrolysisofCyanate HOCN + H2O → NH3 + CO2 Hydrolysis/Saponification of Hydrogen Cyanide HCN + 2H2O → NH4-COOH HCN + 2H2O → NH3 + HCOOH Aerobic Biodegradation of Cyanide 2CN- + O2 + enzymes → 2OCNThiocyanate Formation -2 -2 Sx + CN → [S(x-1)] + SCN -2 -2 S2O3 + CN → SO3 + SCN Simple Cyanide Salt Dissociation + NaCN → Na + CN Metal-Cyanide Complexation +2 -2 Zn + 4CN → Zn(CN)4 Anaerobic Biodegradation of Cyanide + CN + H2S → HSCN + H + HCN + HS → HSCN + H

Source: Smith and Struhsacker, 1988

101

Chapter Three


Cyanide reactions or potential reactions are listed on the left hand side of Figure 3.10. These reactions and their equations (in a somewhat simplified form) are re-listed in Table 3.8. These reactions are discussed in more detail by Smith and Struhsacker (1988). 3.6.2

Hydrolysis and Volatilisation

Reaction between water and the cyanide ion (hydrolysis) results in the formation of molecular hydrogen cyanide as shown in the following equation: (3.7)

-


-

This reaction is strongly dependent on pH, with pH values of less than about 9.5 favouring the formation of HCN in preference to the cyanide anion (CN ). Most heap leach solutions have an operational pH of about 10.5, corresponding to relatively minor concentration of HCN in solution. However, following decommissioning, there will be a gradual decrease in leach solution pH with time. This is due to neutralisation of the alkaline environment during rinsing procedures and because of infiltration of rainwater and carbon dioxide uptake from the atmosphere. Molecular hydrogen cyanide has a relatively high vapour pressure and volatilises as a gas. This volatilisation process is moderately temperature-sensitive. The HCN produced by hydrolysis of CN is thus volatilised as a gas and is continuously evolved from the heap environment through dissipation or is destroyed by oxidation, as discussed below. The net result is a loss of cyanide from the system due to hydrogen cyanide volatilisation. This mechanism of cyanide loss from heap leach pads occurs during both the operational and decommissioning phases of heap leach operations. As shown on Figure 3.10, hydrolysis of ionic cyanide is most likely to occur within the upper portions of the heap, under oxidized and unsaturated conditions. However, this reaction can also occur in the reduced and saturated portions of the heap environment. In saturated portions of the heap where there is no gas-phase transport available, migration of the HCN out of the heap can occur by bubble formation and migration. It should be noted that if the heap contains agglomerated ore, the cement agglomerating buffers the hydrolysis reaction by keeping the pH elevated above 9.5. The buffering capacity of the cement impedes the production of HCN and retards its removal of cyanide from the system. With time, infiltration of water and carbon dioxide into the heap will eventually neutralize the cement-agglomerating agents. The heap is then no longer buffered at an elevated pH and the hydrolysis of CN to HCN will take place. Natural gases (e.g., CO2) in soils can change the pH of infiltrating solutions. The carbon dioxide concentration in soils is about 100 times higher than in the ambient atmosphere. Interstitial soil water will have a pH value of about 5 to 9 depending on the geologic materials with which it is in contact.


102


As high pH process solutions bearing cyanide react with a soil, the pH of the solution decreases. Along with this pH decrease, the equilibrium condition of the cyanide/hydrogen cyanide reaction changes, with a shift favouring the formation of molecular HCN. Its relatively high vapour pressure causes the HCN to volatilise from the solution into the soil gas phase, thereby reducing the solution cyanide concentration. The HCN can react with soil components or be volatilised to the atmosphere. However, if the soil is saturated with water and no gas phase is available, the HCN will diffuse in the liquid phase at a much slower rate. 3.6.3

Oxidation of Free Cyanide -

The oxidation of either HCN or CN is generally restricted to the upper, oxidized portions of the heap environment as shown on Figure 3.10. The oxidation of HCN produces hydrogen cyanate (HOCN) as shown in Equation 3.8 (Owenback, 1978): (3.8)

2HCN + O 2 → 2HOCN

The direct oxidation of CN- requires a mineralogical, bacteriological or photochemical (sunlight) catalyst, and produces the cyanate ion (OCN ): (3.9)

-

-

2CN + O2 + catalyst → 2OCN

Hydrogen cyanate and the cyanate ion are much less toxic than HCN. Within the heap environment, HOCN and OCN gradually hydrolyse to form ammonia and carbon dioxide which are evolved from the system as gases, as illustrated by Equation 3.10: (3.10) HOCN + H 2O → NH3(g) + CO2(g) +

The ammonia generated in Equation 3.10 can remain in solution as NH 4 and either form ammonium complexes or be oxidized to form nitrates, depending upon the pH and biological activity (Hendrickson and Diagnault, 1973). In this manner, oxidation of HCN or CN lowers the overall cyanide content of the system, but may increase the cyanate, ammonia and/or nitrate concentrations. 3.6.4

Hydrolysis/Saponification of Fre e Cyanide

As a solution pH enters the neutral or acid range, HCN can be hydrolysed by a different route to yield formate, as either formic acid or ammonium formate by: (3.11) HCN + 2H 2O → NH4COOH (ammonium formate) (3.12) HCN + 2H2O → NH3 + HCOOH (formic acid) The system pH will determine the extent of formation of each compound, a lower pH favouring formic acid.

103

Chapter Three


3.6.5

Aerobic Bioattenuation of Cyanide

Under aerobic conditions within the upper, oxidized portions of a heap, biological process may consume hydrogen cyanide and generate hydrogen cyanate as shown in Equation 3.13 (Towill et al., 1978). The hydrogen cyanate is in turn hydrolysed into ammonia and carbon dioxide: (3.13) 2HCN + O 2 + enzymes → 2HOCN (3.14) HOCN + H 2O → NH3(2) + CO2(2) As indicated in Equation 3.13, this process requires an enzyme, but is otherwise similar to the oxidation reaction shown in equation 3.8. 3.6.6

Simple Cyanide Compounds

Cyanide forms simple compounds (i.e., simple salts) with single ions of certain metals. In solution, these simple cyanide compounds oxidize and produce free metal cations and cyanide. An example of this type of reaction is shown in Equation 3.15: +

-

(3.15) NaCN → Na + CN

The solubility of simple cyanide compounds ranges from readily soluble to fairly insoluble in water, however in cyanide solutions they are all soluble. The soluble simple cyanide compounds such as NaCN, KCN, Ca(CN)2 and Mg(CN)2 dissociate readily in solution and produce cyanide ions. At pH values lower than about 9.5, this ionic cyanide will hydrolyse to form hydrogen cyanide. 3.6.7

Metal Cyanide Complexes

Cyanide can also react with metals to form metal cyanide complexes. Metal-cyanide complex ions form as the products of the reaction between the insoluble cyanide compounds and excess cyanide ions. Equation 3.16 is an example of this type of reaction: -

-2

(3.16) Zn(CN)2 + 2CN → Zn(CN)4

As indicated in Table 3.9, some of the metal cyanide complexes are rather stable, whereas others will readily ionise to form CN , which in turn may hydrolyse to form HCN. As with the simple cyanide compounds, the toxicity of metal cyanide complexes is due to production of HCN as a dissociated and hydrolysis product. However, copper and silver cyanide complexes appear to be toxic in their undissociated forms to fish (Gannon, 1981). Although many of the iron cyanide complexes are quite insoluble, they may be decomposed through photolysis (reaction with ultraviolet light), thereby releasing ionic cyanide (CN ) under limited conditions encountered in the natural environment.


104


TABLE 3.9

Cyanide Compound 1. Free Cyanide

Relative Stability of Metal Cyanide Compounds in Water

(listed in approximate order of stability) Examples Present in Heap Leach Environments CN , HCN

2.

SimpleCyanides

NaCN,KCN,Ca(CN)2,Hg(CN) 2 Zn(CN)2, CuCN, Ni(CN)2, AgCN -2 -2 Zn(CN) 4 , Cd(CN)3 , Cd(CN)4

3.

WeakMetal-Cyanides

4.

Moderately Strong Metal-

Cu(CN)2 , Cu(CN)3 , Ni(CN)4 , Ag(CN)2

5.

Cyanides Strong Metal-Cyanide Cyanides

Fe(CN) 6 , Fe(CN)6 , Co(CN)6 , Au(CN)2

-

-2

-2

-

-3

-4

-4

-

Adapted from Huiatt et al., 1982 and Brickell, 1981

105

Chapter Three


3.6.8

Anaerobic Bioattenuation of Cyanide

Anaerobic bioattenuation of cyanide and hydrogen cyanide is restricted to the moderately strongly reduced portions of a heap environment and can only occur if HS or H 2S are present in solution. The sulphur species present will depend on the solution pH. At a pH value greater than about 7.0, HS is the dominant species, while at a lower pH, H2S will be dominant. Equations 3.17 and 3.18 illustrate the anaerobic bioattenuation of cyanide: -

+

(3.17) CN + H2S(aq) → HSCN + H -

(3.18) HCN + HS → HSCN + H

+

The HSCN will then hydrolyse to form NH 3, H2S and CO2 (Schmidt et al., 1981). In comparison with the aerobic bioattenuation of cyanide, anaerobic bioattenuation is much slower and anaerobic bacteria have a cyanide toxicity threshold of only about 2 mg/L compared to about 200 mg/L for aerobic bacteria (Fuller, 1985). Consequently anaerobic bioattenuation is a less effective cyanide removal mechanism. 3.6.9

Field Data

There is now a considerable body of data on the overall cyanide chemistry of effluent or pregnant solutions from leach operations. Principally, the data are concerned with cyanide losses (consumptive) within the system. This is in terms of additional cyanide makeup requirements during operations or meeting residual cyanide regulatory levels in a heap either for unloading or for final decommissioning. Aspects of detoxification to meet regulatory requirements are discussed by Smith and Struhsacker (1988) and Smith (1988a), while Stotts (1985) gives a useful case history on a North American operation, which includes a discussion of their detoxification practices. Work completed at a North American mine indicates the general decline of cyanide levels both in terms of consumptive losses in the system and with water/rainfall recharge or decommissioning (Smith and Brown, 1986). Figure 3.11 gives cyanide levels versus pore volumes over time for a rinse test of the spent ore in comparison with a theoretical first order decay curve for cyanide. From an initial cyanide level of >250 mg/L in the leach solution, the rinsate has a cyanide level of about 130 mg/L. This latter value is decreased to 57 mg/L at one pore volume and then follows the "classic" cyanide first order decay, to eventually reach levels below detection.


106


A second cyanide "spike", common to many rinse and drainage cycles, appears on this graph. This is due to back diffusion into the active, flowing part of the heap void system after the first flush of cyanide is removed. It appears to be both a time and concentration gradient-related phenomenon. This latter effect can be used to benefit the operator. The second "spike", or a subsequent water wash solution, applied to a dormant heap may contain values of gold. Data from column rinse tests for a heap leach project in the United States compare the efficiency of the various chemical treatment methods for cyanide destruction, demonstrating the dependence of cyanide chemistry on pH values (Struhsacker and Smith, 1990). The data also illustrate the analytical problems associated with free cyanide determinations. Table 3.10 shows the cyanide neutralisation levels and pH values achieved in the first pore volume by fresh water rinsing, acidification using sulphuric acid, and acidification coupled with other chemical treatment methods. The cyanide concentrations in the effluent from this test were determined using the total cyanide, the WAD cyanide and the free cyanide analytical methods. As revealed in Table 3.10, acidification to a target pH of 7.5 in the effluent was the most effective method for reducing cyanide levels, with most of the remaining cyanide existing as free or weak acid dissociable cyanide. However, the free cyanide measurements for this case and the fresh water rinse sample are clearly specious because they exceed the total cyanide and WAD cyanide values. This is impossible because both total and WAD cyanide measurements include free cyanide. Moreover, this discrepancy is not due to random laboratory error; the free cyanide values were determined by independent analyses at three laboratories, all producing essentially the same values. This is strong evidence for analytical interference, most likely due to the presence of sulphides and/or thiocyanate species. The interference effect is particularly acute in the case of the pH 7.5 sample where total cyanide and WAD cyanide values are low and almost 50 mg/L of the free cyanide is due to interference. The analytical problems associated with free cyanide analyses have implications for projects in which the cyanide neutralisation compliance criterion is expressed in terms of free cyanide. Using the pH 7.5 case as an example (Table 3.10), an operator could be deemed out of compliance if free cyanide were the species designated as the neutralisation criterion. However, the same effluent would be deemed closer to compliance based upon either a total or a WAD cyanide neutralisation standard.

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FIGURE 3.11 Cyanide Reduction Data Illustrating Delayed Release of Cyanide from Rinsing a Heap Leach Pad

Source: Smith and Brown, 1986


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TABLE 3.10 First Pore Volume Effluent pH and Cyanide Concentration from Leach Pad Neutralisation Testing

NeutralisationMethod Fresh Water Rinsing Acidification AcidificationplusHydrogenPeroxide AcidificationplusFerrousSulphate

pH 11.2 9.1 9.5 8.4

Total Cyanide 110 77 72 42

WAD Cyanide 110 57 50 28

Free Cyanide 120 55 37 38

Source: Struhsacker and Smith, 1990


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3.7

CYANIDE GEOCHEMISTRY IN SOILS

3.7.1

Introduction

The objective of this section is to trace cyanide migration through the soil, evaluate cyanide interactions with soil constituents and assess the cyanide attenuation mechanism in soil. In this assessment, Figure 3.10 will be utilized from the discussion on heap leach cyanide geochemistry, since the figure extends below the heap and into the subsoil system. As is portrayed on Figure 3.10, there is a continuous increase of the moisture content and a continuous decrease of oxygen potential with soil depth. As indicated from the earlier review of cyanide attenuation mechanisms, these two factors have a significant influence on cyanide attenuation in the soil system. Two cyanide attenuation mechanisms are particularly susceptible to changes in soil moisture and oxygen content. They are volatilisation and bioattenuation. This is significant because these mechanisms have been shown to be the two most effective cyanide attenuation mechanisms occurring in the vadose or unsaturated soil (Chatwin and Hendrix, 1988). Hence, the rate of cyanide attenuation in the unsaturated or vadose zone is seen to be more rapid than that found in the saturated zone. In the following paragraphs, cyanide attenuation mechanisms effective in soils and the impacts of various soil parameters on these mechanisms are discussed. 3.7.2

Cyanide Volatilisation from Soils

Two major factors control hydrogen cyanide volatilisation in soils. First is the pH of the interstitial solution, which controls cyanide hydrolysis to HCN and has been discussed previously. The second factor is the availability of continuous vapour path by which HCN vapour can migrate from the cyanide solution to the atmosphere. Figure 3.12 shows an illustration of an unsaturated zone where continuous vapour phase transport is available (Chatwin, 1990). Other properties upon which cyanide volatilisation in soils would be dependent are the cyanide solubility in water, HCN vapour pressure and cyanide concentration in the solution. Soil properties that affect the HCN volatilisation rate include: the soil water content, adsorptive and diffusion characteristics of the soil, and bulk properties of the soil such as organic matter content, porosity, density, mineralogical composition and clay content. Meteorological parameters that would affect HCN volatilisation are airflow rate over the soil, humidity and temperature. The above factors have been grouped by Chatwin (1990) into three categories: (1) those which affect movement of HCN away from the surface into the atmosphere; (2) those which affect the vapour density (concentration of the HCN in soil air); and (3) those which control the rate of movement of HCN to the soil surface.


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FIGURE 3.12 Unsaturated Soil Model Showing Soil Particles, Water, Gas and Mass Transport/Retardation Mechanisms

Source: Chatwin, 1990

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Cyanide in the soil may be partitioned between the solid, liquid and gas phases (i.e., soil, water and air). Considered as a whole, the soil represents all three phases of matter rather than one. The atmosphere constitutes another air compartment which is distinct from the interstitial soil air. The rate of volatilisation of an HCN molecule from a sorption site on the solid phase in the soil (or in solution in the soil water) to the vapour phase in soil air and then to the atmosphere is dependent on the physical and chemical properties of both HCN and the soil, and on the process of moving from one phase to another. It follows that the three main distribution or transport processes involved are: • • •

HCN in soil HCN in solution HCN in vapour phase in soil air

Partitioning of HCN among the three phases can be estimated from either vapour-phase or solution-phase desorption isotherms, as described by Chatwin (1990). Typically, the vapour phase diffusion coefficient is approximately 100 times larger than the 4 solution phase diffusion coefficient. Therefore, a partition coefficient of 10 (g/cc in soil water per g/cc in soil air) is considered a transition for determining whether vapour or liquid diffusion 4 is dominant. Chemicals with partition coefficients much smaller than 10 will diffuse mainly in 4 the vapour phase, and those with coefficients much greater than 10 will diffuse primarily in the solution phase (Thomas, 1982). Based on Henry's Law constant, a ratio of HCN concentration in the solution to vapour phase of about 200 is calculated. Hence, the vapour phase diffusion of hydrogen cyanide is dominant, if there is a vapour phase present as is found in the unsaturated or vadose zone. In saturated column tests at both the University of Utah and the University of Nevada-Reno, no HCN in the head space above the columns was found (Chatwin and Hendrix, 1988). However, in an unsaturated soil column, HCN was found in the air space above the soil column, as shown on Figure 3.13. Besides diffusion, there are other methods by which HCN may exit the soil and porewater. As HCN has a vapour pressure of 100 kPa (14.5 psi) at 26°C, bubbles of HCN can form where the shallow unsaturated zone is less than approximately 32 feet. The HCN bubbles must then find a way out of the zone or else the HCN must migrate via diffusion. These bubbles are often trapped by soil grains and rocks in the soil.


112


FIGURE 3.13 Summary of Head Space Analyses for Cyanide in Saturated and Unsaturated Soil Test Columns

Source: Chatwin and Hendrix, 1988

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3.7.3

Biological Attenuation in Soils

As in the heap leach pile, the upper and oxidized portions of soil where the conditions are aerobic, biological process may consume cyanide and generate cyanate as shown in Table 3.8. The hydrogen cyanate is in turn hydrolysed into ammonia and carbon dioxide as shown in the following equations: (3.19) 2HCN + O 2 + enzyme → 2HOCN (3.20) HOCN + H 2O → NH3(2) + CO2(g) From testing of saturated and unsaturated soils, it was found that the oxidation of cyanide is much more effective in the unsaturated zone; almost 25 times more effective in unsaturated soils. 9 Chatwin and Hendrix (1988) showed that unsaturated soils had substantial bacterial growth (10 counts per gram) versus none in saturated soils. This is consistent with the studies by Fuller (1985) in his comparison of aerobic and anaerobic cyanide removal in soil. Coburn (1949) found the toxic limit for effective anaerobic attenuation of cyanide to be about 2 mg/L in a waste stream. This may explain the data of Chatwin and Hendrix (1988) who used much higher concentrations in their test programmes. 3.7.4

Adsorption and Precipitation in Soils

Two cyanide attenuation mechanisms that appear equally effective under both saturated and unsaturated conditions are precipitation and adsorption. These two mechanisms are placed together because it is difficult to separate them. In measuring the attenuation mechanism it is difficult to differentiate between these two mechanisms unless the cyanide reactant species can be determined. However, in a soil column/cyanide solution system where the cyanide concentration is hundreds of mg/L it is difficult to determine if the cyanide is being removed as a precipitate or as cyanide adsorbing on material surfaces in the soil. In tests with free cyanide solution in soils, it has been found that cyanide attached to soil particles correlates well with soil organic carbon content. Western U.S. subsoils tested showed organic carbon contents ranging from 0.1 to 1.2 percent with a mean of 0.6 percent (Chatwin and Trepanowski, 1987). They showed that the organic carbon in the soil could adsorb about 12 ppm cyanide from a 130 ppm solution as it travelled one foot through the soil. This adsorption results in about 9.2% removal per foot or about 0.5 grams of free cyanide absorbed per gram of contained organic carbon. Additional studies with pure mineral components (Chatwin and Hendrix, 1988) indicated that free cyanide will also adsorb onto inorganic surfaces such as orthoclase, plagioclase, kaolin and montmorillonite. In the subsoils tested, clays and feldspars combined removed cyanide in a range from about 20% to 80% of the input concentration of cyanide.


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3.7.5

Hydrolysis/Saponification in Soils

HCN can be hydrolysed by different mechanisms to give formate, either as formic acid or as ammonium formate by: (3.21) HCN + 2H 2O → NH4COOH (ammonium formate) (3.22) HCN + 2H 2O → NH3 + HCOOH (formic acid) The system pH will determine the extent of formation of each compound, a lower pH favouring formic acid formation. 3.7.6

Field Data

It is a fact that, in many natural soil environments, it is found that cyanide reacts and is "lost" from the solution phase of the system in both field observations and laboratory data. For example, data from South Africa given by Smith et al. (1985a) shows a distinct lack of impact from tailings seepage in terms of cyanide, while the evidence of seepage impact from other species is present in groundwater around and below gold tailings facilities. Table 3.11 shows the total cyanide values, regardless of the age or operational stage of the facilities. Glynn (1983), who researched cases of cyanide contamination of groundwater from cyanide bearing waste facilities could find few examples of impact. Of the cases located, the effects of subsite hydrogeochemical attenuation were very pronounced. These case histories, while not all from mining projects, serve as some of the most practical and persuasive illustrations of the fate of cyanide in groundwater. The salient points of this selection of case histories are noted below: Byron, Illinois (Gilkerson et al., 1977) •

Source: 1,500, 30 to 55-gallon drums of industrial wastes containing metals and cyanide. Many drums were punctured.

•

Concentration of cyanide up to 10,300 mg/L.

•

Cyanide levels detected in groundwater at 3 km were at a maximum concentration of 0.02 mg/L, about 10% of the drinking water cyanide criterion.

•

No samples in any groundwater sample exceeded the drinking water criterion.

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TABLE 3.11 Comparison of Total Cyanide Levels with Groundwater Composition in South Africa

DescriptionoD f am

Total TDS SO4 Fe Mn pH Cyanide <0.01 712 317 0.1 15 6.8 1. Operational dam ±5 years age 2. Decommissioneddam>20yearsage 0.04 500 400 34 69 2.9 0.08 300 70 4 0.8 5.8 3. Recently decommissioned dam ±20 years age <0.01 500 ---7.5 4. Operational dam ±10 years age 5. Backgroundwaterquality <0.01 250 50 0.1 0.1 7.8 Source: Smith et al., 1985a Notes: (1) All concentrations in mg/L.


116


Cologne Bight (Effenberger, 1964) 3

•

Source: gravel pit containing 30,000 m of cyanide-bearing waste from chemical industry.

•

Concentration at 600 metres up to 0.2 mg/L.

•

No hazard as cyanide form was as a hexacyanoferrate.

Flat River Tungsten, Northwest Territories, Canada (Sigma Consultants, 1981) •

Source: cyanide-bearing tungsten mill tailings.

•

Seepage from tailings into a sand and gravel aquifer and then to river.

•

Concentration in groundwater less than 0.1 mg/L and no impact on river.

•

Cyanide removed due to surface complexing and precipitation

Ladybank, Fife, Scotland (Parker and Mather, 1979) •

Source: cyanide-bearing waste from gasworks.

•

Cyanide concentration in aquifer immediately below site down to 0.2 mg/L and 0.004 mg/L 200 metres from site.

Taken together, these examples indicate the fate of cyanide, not only in the groundwater itself, but particularly as a result of reactions occurring also in the unsaturated zone between the source of cyanide and the groundwater itself. Quantification of the effects of hydrogeochemical attenuation has been made in overall studies. A detailed study of cyanide "loss" by dilution, attenuation (decay) and geochemical attenuation was given by Smith and Brown (1986) for a proposed spent ore disposal site in North America. Tables 3.12 (a) and (b) illustrate partition coefficients for cyanide attenuation in a variety of subsite materials and retardation factors calculated from these partition coefficients. There is clearly variation in cyanide reactions with differing types of material, with the weathered materials (e.g., clay and slate) being better cyanide attenuators than the basal limestone.

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TABLE 3.12 Attenuation Calculation Summary (a) Distribution Coefficients (mL/g) and (b) Retardation Factors

Material Clay

(a) Distribution Coefficients (mL/g) Sample Pore Total Volume Cyanide 1.1 1 1.95

Free Cyanide 3.42

3 1 3 1 4 1 4 1 3 1 3 1 3 1 3

8.12 1.48 33.31 0.69 0.84 0.93 1.04 1.93 1.33 2.41 1.18 0.03 0.04 0.03 0.07

1.2 Shale

2.1 2.2

Rubble

3.1 3.2

Limestone

4.1 4.2

Material Clay

Shale

Rubble

Limestone

(b) Retardation Factors Sample Pore Total Volume Cyanide 1.1 1 6 3 13 1.2 1 5 3 82 2.1 1 3 4 5 2.2 1 3 4 4 3.1 1 6 3 5 3.2 1 8 3 5 4.1 1 1.2 3 1.3 4.2 1 1.2 3 1.0

Source: Smith and Brown, 1986 Mudder, Botz & Smith

4.00 1.30 10.81 0.66 1.93 0.65 1.26 1.25 0.93 1.60 0.83 9.06 0.09 0.06 0.07

118

Free Cyanide 7 84 5 94 3 5 4 4 9 6 11 6 1.1 1.1 1.1 1.1


A study conducted for a North American project evaluated cyanide attenuation in oxidized, reduced and alluvial subsite materials, as well as other species attenuation properties (Smith, 1987). Table 3.13 gives these partition coefficient values, including total cyanide values. These data show the enhanced ability of the chemically reduced material to remove cyanide over that of the oxidized materials, presumably by thiocyanate formation. In addition, there is little WAD cyanide metal complexing or removal WAD cyanide metal complexes, as the copper partition coefficient values were universally high. It should be stressed that there needs to be a distinction made between overall cyanide attenuation or decay in soils/rocks and geochemical/hydrogeochemical decay of cyanide. If the decay component of volatilisation is removed, some geological material may have little or no capacity for cyanide attenuation. For example, geochemical studies on a cemented conglomerate material below a tailings facility in the United States showed that when tested in an inert (argonrich) environment with no available pathway for gas losses there was no appreciable cyanide decay in the system (Smith, 1990). 3.7.7

Applications

The commercial areas, where the impact of cyanide soil geochemistry is important, relate particularly to the design of liners for pads and ponds. Presently, these designs are becoming more and more stringent as the concern relating to cyanide release and toxicity becomes greater. Present environmental regulatory design criteria ignore the effect of natural cyanide attenuation. Consequently, if efforts are made to select sites where underlying soils have significant cyanide attenuation and attenuation capacity, this trend towards more stringent regulations might be lessened. In addition, if the capacity of clay liners to degrade and attenuate cyanide were measured and possibly enhanced, this characteristic would be useful in containing cyanide. Hence, clay layers that would degrade and attenuate cyanide during the solution seepage through the clay would minimize any cyanide release. Finally, there are situations where the application of treated cyanide solutions to the land is one of the few options available to dispose of this waste. Hence, selection of sites with appropriate underlying soils would minimize the potential for release from this type of treatment-disposal facility. Examples where this disposal option has been considered are at mines in North America (Smith et al., 1993 and Schafer and Hudson, 1990).

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TABLE 3.13

Constituent Sulphate TDS TotalCyanide Sodium Cobalt Copper

Partition Coefficient Values for Tailings Attenuation Tests

Oxidized 1 3.95 3.12 8.2 3.66 0.97 >10.7

Retardation Factors Oxidized Reduced Reduced 2 1 2 4.24 4.05 3.57 1.44 3.07 3.26 6.66 14.1 11.5 3.66 3.96 3.96 1.45 1.45 0.97 >10.7 >10.7 >10.7

Source: Smith et al., 1985b Notes: (1) Cadmium, lead, nickel, selenium and silver were not present. (2) Iron, free cyanide and mercury showed no attenuation. (3) 7.4 oxidized, 12.8 reduced, 6.3 alluvium


120

Alluvium 3.43 2.52 6.3 2.97 1.45 >10.7

Mean 4.0 3.2 Variable 3.8 1.24 >10.7

(3)


3.8

CYANIDE G EOCHEMISTRY IN GROUNDWATER

3.8.1

Attenuation Mechanisms

The groundwater cyanide geochemical system is similar to that which may be found in saturated soils. Since they have been discussed in detail previously, the cyanide attenuation mechanisms will be briefly summarized and only factors specifically affecting the groundwater system will be discussed. The cyanide attenuation and attenuation mechanisms that appear to be effective in groundwater are adsorption, chelation, precipitation, bacterial attenuation and hydrolysis to formate. Volatilisation may also occur if there is a pathway by which the HCN can escape from the groundwater (Chatwin, 1990). These cyanide attenuation mechanisms are affected by the following parameters: the dissolved metal and oxygen content of the groundwater, the groundwater pH and the aquifer mineral content including clay, feldspar, organic carbon, free metals and/or metal oxides (Chatwin and Hendrix, 1988 and Chatwin, 1990). In unconfined aquifers, a capillary fringe would be located above the groundwater table. The groundwater fringe would be saturated, particularly near the aquifer interface, and gaseous pathways for HCN migration would be limited or non-existent. In confined aquifers, the available pathways would be more limited and gaseous migration of HCN from the aquifer would be quite small. However, because the vapour pressure of HCN at 26°C is 100 pKa, it is possible for bubbles of cyanide to form if the hydraulic head is less than 32 feet. The limitation of this pathway is the difficulty of individual bubbles to ascend through a saturated zone without being trapped by soil particles. As discussed earlier, liquid diffusion of HCN through the groundwater is relatively slow and would probably not be a significant pathway. Hydrogen cyanide in soil water can volatilise through the action of a process called the wick effect or wick evaporation. The HCN is transported from the soil body to the surface by capillary action. Its rate of volatilisation is enhanced by the evaporation of the water causing the capillary action. The soil column acts as a wick, as the water in the soil moves up the capillaries of the soil to replenish that lost at the top by evaporation. The escape of HCN in the vapour phase causes an actual mass flow in the wick. This mechanism may be quite effective in climates where evaporation is much higher than precipitation. While the migration mechanisms for HCN to escape from the groundwater are slow, it is not necessary for the HCN to leave the soil, but just to leave the aquifer to decrease groundwater concentrations of cyanide. Hence, substantial amounts of HCN can be trapped or isolated in the capillary fringe or soil above the aquifer and not report in an analysis of the groundwater. As long as a storm event or precipitation does not drive the isolated cyanide back down into the groundwater, it can be slowly removed by biological attenuation or hydrolysis to formate in the unsaturated zone.

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From bench-scale testing of saturated soil columns, Chatwin and Hendrix (1988) found that chelation followed by iron cyanide precipitation is one of the more effective methods of removing free cyanide and this can occur in saturated conditions in groundwater. This mechanism requires the formation of a ferrocyanide complex combined with available iron or other transition metals to precipitate the cyanide. The precipitation of insoluble ferrocyanide compound occurs over a pH range of 2 to 11 as demonstrated by Hendrickson and Daignault (1973). Bacterial attenuation is usually anaerobic due to the low oxygen content of the groundwater. However, even in very low oxygen environments (<1 mg/L dissolved oxygen), aerobic attenuation will occur. In general, groundwater cyanide bacterial attenuation is slower than that found in the vadose or unsaturated soil, due to the lack of nutrients, inadequate mixing and minimal oxygen. The process of hydrolysis of cyanide to formate in groundwater is slow. The rate of cyanide hydrolysis to formate is about 2% to 4% per month. However, this rate increases with increasing temperature. The activation energy found from an Arrhenius plot was 87±5 kJ/mole (Hoecker and Muir, 1987). At this activation energy the hydrolysis rate would increase by 2.5 times for every 10°C increase in temperature. Hence, geothermal effects in groundwater would enhance cyanide attenuation. 3.8.2

Field Data

There are documented examples of cyanide in groundwater systems due to waste disposal activities. Braids (1977), Goyal et al. (1981) and Kahar and Bhatragar (1981) have all described case histories of cyanide in groundwater due to industrial effluent discharge or by seepage from industrial waste disposal sites. The latter two publications cite examples from India only, while all three studies are focused as non-mining waste materials. In each case, the apparent source ("major leaks") appear to have overwhelmed the cyanide attenuation capacity of the subsite materials. There are cases of cyanide impact on groundwater from mine waste facilities reported in agency files. For example, those of the Nevada Department of Environmental Protection. However, such reports are relatively infrequent and normally indicate limited water quality changes. The general absence of impact on groundwater quality from mining waste facilities has already been cited from field case studies in earlier parts of this chapter. For example, the data from South Africa (Smith et al., 1985a) shows a distinct lack of impact from gold mine tailings seepage in terms of cyanide, while there is evidence of impact from other tailings-related species. Glynn notes a similar absence of groundwater quality and impact from of tailings northern Canada,(1983) as well as from two cases of industrial waste disposal disposal wastesinfrom coal gas production, the examples coming from Europe and the United States. The evaluations by Smith and Brown (1986) and of Smith et al. (1985b) for North American operations confirm the field observations.


122


In addition, there are many mine tailings and heap leach facilities in North America known to the authors where routine groundwater monitoring programmes during operations demonstrate no impact of cyanide on groundwater quality. This lack of effect on groundwater quality may well be a function of the low source concentrations of cyanide (typically in the range of 100 to 300 ppm cyanide) and the relatively low mass loading from the mine waste facilities (facilities are lined). In many cases there are substantial unsaturated zones below the facilities themselves which have demonstrated abilities to attenuate cyanide.

3.9

CYANIDE IN THE ATMOSPHERE

The previous sections of this chapter have dealt with natural reactions of cyanide in solutions and the effects of geologic materials on these reactions. As noted, a primary mechanism for cyanide loss from solutions and solid/solution systems is through the volatilisation of hydrogen cyanide. This results in the transfer of cyanide from solution to the ambient atmosphere in the form of hydrogen cyanide gas. In this section a discussion of hydrogen cyanide in atmospheric air is presented. Hydrogen cyanide may enter the atmosphere from a variety of natural and manmade sources, including natural and man-caused fires, automobile exhaust, natural excretion from plants and organisms, and from industrial activities such as plastics manufacturing and mining (ASTDR, 1997 and Towill et al., 1978). Mudder and Botz (2001) estimated the total amount of hydrogen cyanide produced and subsequently converted to sodium cyanide for use in the mining industry is about 180,000 tonnes per year worldwide. This production rate of hydrogen cyanide compares to about 200,000 to 600,000 tonnes per year of hydrogen cyanide emitted each year worldwide as a result of burning biomass such as grasses and trees (Holzinger et al., 1999). Further, ASTDR (1997) estimated that automobile exhaust accounted for about 20,000 tonnes per year of hydrogen cyanide emitted to the atmosphere. For the year 1998, the USEPA (2001) estimated that metal mining activities in the USA resulted in a hydrogen cyanide emission to the atmosphere of about 280 tonnes. It has been estimated that the worldwide average ambient hydrogen cyanide concentration in the upper atmosphere is about 0.2 parts per billion (ppb) by volume (Cicerone and Zellner, 1983). Fluctuations in this concentration are primarily the result of major fires which result in the release of large quantities of hydrogen cyanide. It was also found that removal of hydrogen cyanide from the atmosphere by uptake in rainwater was a very minor mechanism (Cicerone and Zellner, 1983). Monitoring of hydrogen cyanide in the atmosphere is primarily done as part of monitoring biomass burning since hydrogen cyanide serves as a convenient tracer.

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The half-life of hydrogen cyanide in the atmosphere is about two years, with its attenuation being the result of reaction with the hydroxyl radical in the upper atmosphere (ASTDR, 1997). The reaction products from this reaction are primarily carbon monoxide (CO) and nitric oxide (NO). The reaction rate is an order of magnitude greater at lower altitudes (0 to 8 km) than at upper altitudes (10 to 12 km). Hydrogen cyanide is not photolytically active as it requires wavelengths less than about 190 nm for reaction, and light of less than about 300 nm does not penetrate the earth’s atmosphere. The concern regarding hydrogen cyanide in the atmosphere is focused on the occupational settings at operations utilizing cyanide for metals extraction. In these isolated settings, hydrogen cyanide may be present in worker environments at concentrations higher than observed for the atmosphere as a whole. A discussion of the toxicity of hydrogen cyanide to humans is presented in Chapter Four.

3.10

REFERENCES

Alesii, B.A., and Fuller, W.H., "The Mobility of Three Cyanide Forms in Soil", EPA-600/9-76015, USEPA, Cincinnati, Ohio, 1976. ASTDR, Agency for Toxic Substances and Disease Registry, Toxicological Profile for Cyanide (Update), September, 1997. Bernardin, F.E., "Cyanide Detoxification Using Adsorption and Catalytic Oxidation on Granular Activated Carbon", Journal of Water Pollution Control Federation, 45, p. 221, 1973. Bessent, R.A., Luther, P.A., and Eklund, C.W. "Removal of Cyanides from Coke-Plant Wastewaters by Selective Ion Exchange - Results of Pilot Testing Program", Proc. 34th Ind. Waste Conf., Purdue University, West Lafayette, Indiana, 1979. Bishop, E. and Wright, D.T., "Oxidation of Cyanides", U.S. Patent 4,024,037, Chem. Abs. 87:31152b, 1987. Braids, O.C., "The Prevalence of Subsurface Migration of Hazardous Chemical Substance at Selected Industrial Waste Land Disposal Sites", EPA/530/SW-634, 1977. Botz, M.M. and Mudder, T.I., “Modeling of Natural Cyanide Attenuation in Tailings Impoundments”, Minerals and Metallurgical Processing, Vol. 17, No. 4, pp. 228-233, November, 2000. Brickell, R.H., "Chemistry of Cyanide Solutions," Proceedings from Cyanide and the Gold Mining Industry Seminar, Environment Canada, Ottawa, Ontario, Jan. 22-23, 1981. Burden, R.J. and Kidd, C.H., "The Environmental Fate of Cyanide in Gold Mine Process Tailings", Proc. Ann. Conf. AIMM, Nelson, New Zealand, 1987.


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Caldwell, J. and Smith, A., "Material Considerations in the Design of Downstream Embankments for Tailings Impoundments", Mining Sci. Tech, No. 3, Elsevier Sci. Publ., Amsterdam, Holland, 1985. Chatwin, T.D., Cyanide Attenuation in Soil, Final Report; Resource Recovery and Conservation Company, Salt Lake City, Utah, October, 1990. Chatwin, T.D. and Hendrix, J., "The Fate of Cyanide in Soils", Randol Gold Forum 88, Scottsdale, Arizona, February 23-24, pg. 343, 1988. Chatwin, T.D. and Trepanowski, J.J., "Utilization of Soils to Mitigate Cyanide Releases", Proc. Third Western Reg. Conf. on Precious Metals, Coal and Environment, Rapid City, South Dakota, Sept. 23-26, p. 151, 1987. Chester Engineers, Report on the Lower Monongahela River Study - Water Quality Conditions, Point and Nonpoint Source Waste Loads Allocations, Vols. 1 and 2, Prepared for the U.S. Steel Corp., Coraopolis, Pennsylvania, 1977. Cicerone, R.J. and Zellner, R., “The Atmospheric Chemistry of Hydrogen Cyanide (HCN)”, Journal of Geophysical Research, Vol. 88, No. C15, pp. 10,689-10,696, December 20, 1983. Coburn, S.E., "Limits of Toxic Waste in Sewage Treatment", Sewage Works J., 2:522, 1949. Dodge, B.F., Zabbon, W., "Disposal of Plating Room Wastes IV. Batch Volatilization of Hydrogen Cyanide from Aqueous Solutions of Cyanides", Plating, 39, 1952. Doudoroff, P., "A Critical Review of Recent Literature on the Toxicity of Cyanides to Fish", American Petroleum Institute, Washington, D.C., 1980. Effenberger, E., "Verunreinigungen eives Grund-wassers durch cyanide", Arch. Hyg. Bakteriol, Vol. 148, No. 4-5, pp.271-287, Munich, 1964. Ford-Smith, M., "The Chemistry of Complex Cyanides: A Literature Survey", Her Majesty's Stationery Office, London, 1964. Frank, S.N. and Bard, A.J., "Heterogeneous Photocatalytic Oxidation of Cyanide Ion in Aqueous Solutions with TiO2 Powder", Journal of the American Chemical Society, Vol. 99, Jan 5, pp. 303-304, 1977. Fuller, W., "Movement of EPA-600/1-77-020, Selected Metals, Asbestos Cyanide Soils: Waste Disposal Problems", USEPA, and Cincinnati, p. in 257, 1977.Applications to Fuller, W. "Investigation of Landfill Leachate Pollutant Attenuation by Soils", EPA- 600/2-78158, USEPA, Cincinnati, Ohio, p. 239, 1978.

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Fuller, W., "Soil Modification to Minimize Movement of Pollutants from Solid Waste Operations", CRC Critical Reviews in Environmental-Control, March, pp. 213-270, 1980. Fuller, W., "Cyanides in the Environment with Particular Attention to the Soil", Cyanide and the Environment, Vol. I, Colorado State University, Fort Collins, Colorado, pp. 19-46, 1985. Gannon, D.G., "Toxicity of Cyanide Solutions", Proceeding from Cyanide and the Gold Mining Industry Seminar, Environment Canada, Ottawa, Ontario, Jan. 22-23, 1981. Gilkerson, R.H., Cartwright, K, Folmer, L.R., and Johnson, R.M., "Contribution of Surficial Deposit, Bedrock, and Industrial Wastes to Certain Trace Elements in Groundwater", Proc. Ann. Eng. Geol. Soils, Eng. Symp., Vol. 15, pp, 17-38, 1977. Goyal, M.R., Abrol, O.P., and Vihra, V.K. "Pollution of Upper Aquifer in Punjab, India", Quality of Groundwater Proceedings of an International Symposium. Noordvijkerhout, Netherlands, Studies in Environmental Science. Vol. 17, 1981. Glynn, P., "Cyanide Behaviour in Groundwater Environments", B.Sc. Dissertation, Groundwater Research Institute, University of Waterloo, Canada, 1983. Hendrickson, T. and Diagnault, L., "Treatment of Complex Cyanide Compounds for Reuse or Disposal", USEPA, Report No. EPA R2-73-269, p. 151, 1973. Hendrix, J., Nelson, J., and Ahmadiantehrani, M., "Cyanide in Precious Metals Mill Tailings Impoundments", AIME Annual Meeting, 1987. Hoecker, W. and Muir, D., "Attenuation of Cyanide", The AusIMM Adelaide Branch, Research and Development in Extractive Metallurgy, May, 1987. Holzinger, R., Warneke, C., Armin, H., Jordon, A. and Lindinger, W., “Biomass Burning as a Source of Formaldehyde, Acetaldehyde, Methanol, Acetone, Acetonitrile and Hydrogen Cyanide”, Geophysical Research Letters, Vol. 26, No. 8, pp. 1161-1164, April 15, 1999. Huiatt, J.L, Kerrigan, J.E., Olson, F.A., and Potter, G.L, Cyanide from Mineral Processing, Salt Lake City, Utah, U.S. Bureau of Mines and Utah Mining and Mineral Resources Research Institute, February 2-3, 1982. IEC, Ltd., "Polysulphides for Conversion of Cyanides to Thiocyanate in Gold Mining Effluents", Fisheries and Environment Canada, Burlington, Ontario, 1979. Kahar, P. and Quality Bhatragar, C., "Groundwater Pollution to Industrial Symposium, Effluents in Ludiane,V. India", of N. Groundwater Proceedings of Due an International Noordvijkerhout, Netherlands, Studies in Environmental Science, Vol. 17, 1981. Kidd, C.H., "Prediction of Solute Transport from Gold Mine Tailings, Coromandel, New Zealand", Proc. 3rd Intl. Mine Water Conf., Melbourne, Australia, 1988.


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Kunz, R.G., Casey, G., and Huff, J., "Refinery Cyanides, A Regulatory Dilemma", Hydrocarbon Processing, 57 (10), pp. 98-106, 1979. Longe, G.K. and DeVries, F.W., "Some Recent Considerations on the Natural Disappearance of Cyanide", Economics and Practice of Heap Leaching in Gold Mining, Cairns Queensland, Australia, August, 1988. McGill, S.L., Hendrix, J. and Nelson, J., "Cyanide/Thiocyanate Reactions in Tailings", Cyanide and the Environment, Vol. 1, Colorado State University, Fort Collins, Colorado, pp. 143-159, 1985. Milne, D., "Equilibria in Dilute Cyanide Waste Solutions", Sewage and Industrial Wastes, Vol. 22, No. 7, pp. 904-911, July 1950. Miles, A.M., "A Study of the Photocatalytic Effects of Aqueous Suspensions of Platinized Semiconductor Materials on the Reactions Rates of Candidate Redox Reactions", LAR-13171, National Aeronautics and Space Administration, Hampton, Virginia, 1981. Mudder, T.I., "Conference Summary", Proceedings Cyanide and the Environment, Tucson, Arizona, December 11-14, 1984. Mudder, T.I. and Goldstone, A., "The Recovery of Cyanide from Slurries", Proc. Randol Conference Gold and Silver Recovery Innovations Phase IV Workshop, Sacramento, California, November, 1989. Mudder, T.I. and Smith, A., “An Environmental Perspective on Cyanide”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Mudder, T.I. and Botz, M.M., “A Global Perspective of Cyanide”, Presented at 2001 Annual SME Conference, Denver, Colorado, February, 2001. Murrmann, R.P. and Koutz, F.R., "Role of Soil Chemical Processes in Reclamation of Wastewater Applied to Land", Wastewater Management by Disposal on the Land, Special Report 171, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, pp. 48-76, 1972. Owenback, D., "The Instability of Free Cyanide in Photographic Process Effluents", J. App. Pht. Eng., Vol. 4, pp 72-76, 1978. Palaty, J., and Horokova-Jakubu, M., of "The Course and Removal Cyanides Water Under Natural Conditions", Faculty Technology ofRate Fuelofand Water,of Vol. 3, Partfrom 1, Prague, Czech., 1959. Parker, A. and Mather, J.D., "An Investigation into Pollution from a Disused Gasworks Site Near Ladybank, Fife", AERE-R 9213, p. 19, Harwell, 1979.

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Schafer, W.M. and Hudson, T., “Land Application of Cyanide-Containing Mining Process Solutions”, Proc. Planning, Rehabilitation, and Treatment of Disturbed Lands, Billings, Montana, USA, 1990. Schmidt, J.W., Simovic, L. and Shannon, E., "Natural Attenuation of Cyanides in Gold Milling Effluents", Proc. Sem. Cyanide and Gold Mining Industry Seminar, Environment Canada, Ottawa, Ontario, Jan. 22-23, 1981. Sigma Resources Consultants Ltd and Golder Associates Ltd, Summary Report: Waste Management System, Tungsten, NWT, SRCL 3258, for the Canada Tungsten Mining Corporation, 1981. Simovic, L., Snodgrass, W., Murphy, K., and Schmidt, J. "Development of a Model to Describe the Natural Attenuation of Cyanide in Gold Mill Effluents", Cyanide and the Environment Vol. II, Colorado State University, Fort Collins, Colorado, pp. 413-432, 1985. Smith, A., "Applied Hydrogeochemistry in Mining Wastes Disposal", Proc. of Dunham-Durham Symposium, Durham, United Kingdom, Pub. Special Vol., IMM, London, 1985. Smith, A., "Testimony to Department of Health and Environmental Control", South Carolina, Permit No. SC 0041378 Appeal Hearing, Columbia, South Carolina, December, 1987. Smith, A., “Cyanide Attenuation and Detoxification", Introduction to Evaluation, Design and Operation of Precious Metal Leaching Projects, Van Zyl, Hutchison and Kiel (eds.), Chapter 14, Soc. Mining Engs/Am. Inst. Min. Met. Eng., 1988a. Smith, A., "The Management of Cyanide in the Gold Mining Industry", Seminar Proceedings, AGC, Australia, November, 1988b. Smith, A., Unpublished Test Data, 1990. Smith, A. and Brown, A., "The Potential for Cyanide Migration from the Annie Creek Processing Facility", Rept. No. 1086/2 to Wharf Resources (USA), Inc., SRK, Lakewood, Colorado, November, 1986. Smith, A. and Mudder, T., “The Environmental Geochemistry and Fate of Cyanide”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Smith, A. and Struhsacker, D.W., "Cyanide Geochemistry in an Abandoned Heap Leach System and Regulations forHeap Cyanide Detoxification", Introduction to Evaluation, Designand andKiel Operation of Precious Metal Leaching Projects, Chapter 12, Van Zyl, Hutchinson, (Eds), Soc. of Mining Eng./Am. Inst. of Mining and Metallurgical Eng., 1988. Smith, A., Dehrmann, A., and Pullen, R., "The Effects of Cyanide-Bearing, Gold Tailings Disposal on Water Quality in Witwatersrand, South Africa", Cyanide and the Environment, pp. 221-229, Van Zyl (Ed.), Tucson, Arizona, Pub. CSU, Fort Collins, Colorado, 1985a.


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Smith, A., Moore, D., and Caldwell, J., "Prediction of Groundwater Impact of Tailings Disposal", Proc. 2nd Annual Can/Am Conf. on Hydrogeology, Baniff, Alberta, Canada, 1985b. Smith, S.C., Hudson, T.J. and Schafer, W.M., “Field Evaluation of Land Application Performance: Metals Removal from Barren Leach Solution”, Proc. Planning, Rehabilitation, and Treatment of Disturbed Lands, Billings, Montana, USA, 1993. Stotts, W.G., "Handling Cyanide at Superior Mining Company's Stibnite Heap Leaching Operation", Cyanide and the Environment, Tucson, Arizona, pp. 231-247, 1985. Strobel, G.A. "Cyanide Utilization in Soil", Soil Science, 103(4):299-302, 1967. Struhsacker, D.W. and Smith, A., "Cyanide Neutralization and Reclamation of Heap Leach Projects", Proc. Planning, Rehabilitation and Treatment of Disturbed Lands, Symposium, Billings, Montana, 1990. Thomas, R.G., "Volatilization From Soil", Handbook of Chemical Property Estimation Methods, Ed. by Lyman, W.J. et al., McGraw Hill Book Co., New York, New York, 1982. Towill, L., Drury, J., Whitefield, B., Lewis, E., Galyan, E., and Hammons, A "Reviews of the Environmental Effects of Pollutants: V. Cyanides", EPA-600/1-78-027, USEPA, Cincinnati, Ohio, 1978. USEPA, United States Environmental Protection Agency, 1998 Toxic Release Inventory (TRI) Report for Metal Mining, Available through www.epa.gov/tri/, 2001. Weast, R.C. (ed.), Handbook of Chemistry and Physics, 50th ed, Chemical Rubber Publishing Company, Cleveland, Ohio, 1969. Zaidi, A., Schmidt, J.W., Simovic, L., and Scott, J., "The Art and Science of Treating Wastewaters from Gold Mines", Annual Operator's Conference, Ottawa, Ontario, January 20-22, 1987.

3.10

BIBLIOGRAPHY

Alesii, B.A., Cyanide Mobility in Soils, M. Sc. Thesis, Dept. of Soils, Water and Engineering, University of Arizona, USA, 1976. American Cyanimid Co., Chemistry of Cyanidation, Stamford, Connecticut, p. 54, 1963. ASTM, "Proposed Revision of Standard Methods of Test for Cyanide in Water. Standard D20360-81, American Society for Testing and Materials, 1981.

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Clifton, A. and Smith, A., "Stability of Natural Barriers Exposed to Uranium Mill Wastes." Proc. Geotech. Soc. Conf., Saskatoon, Canada, Sept., 1988. Drever, J., The Geochemistry of Natural Waters, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1982. Dragun, J., The Soil Chemistry of Hazardous Material, Hazardous Materials Control Research Institute, Silver Spring, Maryland, 1988. Driscoll, Fletcher G., Groundwater and Wells, Second Edition, Published by Johnson Division, St. Paul, Minnesota, 1986. Ecological Analysts, Inc., "Cyanide, an Overview and Analysis of the Literature on Chemistry, Fate, Toxicity, and Detection in Surface Waters", Prepared for the Inter-Industry Cyanide Group, 1979. Englehardt, P., "Long-term Attenuation of Cyanide in an Inactive Leach Heap", Cyanide and the Environment, Van Zyl (Ed), Colorado State University, Fort Collins, Colorado, Vol. 2, pp. 539547, 1985. IEC, Ltd., "Factors Affecting Natural Attenuation of Free and Metal-Complexed Cyanides from Gold Mining Effluents", Fisheries and Environment Canada, Burlington, Ontario, p. 38, 1979. Houle, M.J. and Long, D.E., "Interpreting Results from Serial Batch Extraction Tests of Wastes and Soils", EPA-600/9-80-010, pp. 60-81, 1980. Hyatt, D.E., "The Chemical Basis of Techniques for the Decomposition and Removal of Cyanides", AIME, Soc. Min. Eng., Trans., pp. 204-220, 1976. Lyman, W.J., Reehl, W.F., and Rosenblatt, D.H., Handbook of Chemical Property Estimation Methods, Environmental Behaviour of Organic Compounds, McGraw Hill, 1982. Nichol, M.J., Fleming, C.A. and Paul, R.L., "The Chemistry of the Extraction of Gold", In the Extraction Metallurgy of Gold in South Africa, Monograph M7, SAIMM, Vol. 2, Chap. 15 (Stanley G. C., Ed.), 1987. Osseo Asare, K, Xue, T. and Ciminelli, V.S.T., "Solution Chemistry of Cyanide Leaching Systems", Precious Metals: Mining Extraction and Processing, (Ed. V. Kudryk, D. A. Corrigan and W. W. Liang) A.I.M.E. Warrendale, Pennsylvania, pp. 173-197, 1984. Sharpe, A.G., The Chemistry of Cyano Complexes of the Transition Metals, Academic Press, 1976. Smith, A, "Hydrogeochemical Aspects of Waste Embankment Design", Geotechnical News, V. 2, No. 3. p. 26-28, 1984.


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Sonora Mining Company, "Process Chemicals and Reagents: Potential Impact on Water Quality, Research and Review - Jamestown Mine, California", Submission of State of California WQCB, Sacramento, March, 1986.

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CHAPTER FOUR

Toxicity of Cyanide and Related Compounds 4.1

INTRODUCTION

The focus of this chapter is the toxicity of cyanide in mining waters as it relates to cyanide exposure to humans, aquatic life and terrestrial life. A general understanding of this information is necessary when establishing effluent water quality goals and when evaluating water treatment alternatives. As discussed in previous chapters, the chemistry of cyanide is complex and many forms of cyanide and its derivatives may exist in mining solutions. There are four major categories of cyanide and its related compounds which are important from a toxicity viewpoint: • •

Free cyanide Weak acid dissociable (WAD) cyanides

• •

Iron cyanides Cyanide related compounds

The regulation of these forms of cyanide and its related compounds is ultimately tied to the toxicity of these compounds towards specific organisms, which in turn dictates the level to which compounds must be removed in treated waters. The development and/or selection of water quality criteria are crucial to the overall process of water treatment plant design.

4.2

GENERAL ASPECTS OF TOXICOLOGY

Every chemical has an effect on living organisms exposed to it, whether the effect is positive or adverse. An example of both positive and adverse effects exhibited by the same chemical is that of water. For humans, water is a chemical required to sustain life, and for most people water is ingested on a daily basis without any toxic effects. However, if a small quantity of liquid water were inhaled, the normal function of the human lungs would be interrupted and drowning and death may occur.

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In this regard, four components of toxicology must be considered: •

The type of chemical under consideration may have different effects on different organisms; a chemical acutely toxic to one organism may be harmless to another.

•

The duration of exposure of an organism to a chemical may determine the extent of adverse effects. Generally, short-term exposures result in a lower level of effects in comparison to long-term exposures.

•

The route of exposure may also determine the extent of adverse effects due to a chemical, with the routes generally being inhalation (breathing), ingestion (eating), absorption (through the skin) and injection (directly injected into an organism). As noted above, water may be acutely toxic to humans if inhaled, but it is generally not toxic to humans through ingestion, absorption or injection.

•

The dosage of a chemical may also determine the extent of adverse effects, with higher dosages generally corresponding to higher levels of effects.

Therefore, when evaluating the toxicity of a specific chemical, the duration of exposure, the route of exposure and the dosage must be considered in whole. Two commonly employed measures of toxicity are the Lethal Dosage (LD) and Lethal Concentration (LC). The lethal dosage is measured in milligrams of the chemical administered to an organism per kilogram of the organism’s body weight (mg/kg), generally through ingestion or injection. The level of toxicity is then typically expressed as the dosage resulting in the death of 50% of the test organisms (LD50) in a specified time period. Lethal concentrations generally refer to exposure through inhalation or absorption and are 3 measured using both the concentration (e.g., mg/m ) and duration of exposure. A common measure of toxicity to fish is the 96-hour LC50, which refers to the concentration of a chemical in water that results in the death of 50% of the test organisms after a 96-hour exposure period. With respect to humans, the exposure periods are the 8-hour working day or the 15-minute short term exposure. This chapter deals with cyanide toxicology and the adverse effects this chemical has on living organisms, including humans, aquatic life, waterfowl and wildlife. The following definitions are provided in reference to information presented in the following sections (AWWA, 1990): Toxic:

Causing a deleterious response in a biologic system, seriously injuring function or producing death. These effects may result from acute conditions (short-term, high-dose exposure), chronic (long-term, low-dose exposure), or sub-chronic (intermediate-term and dose exposure).

Neurotoxic:

Exerting a destructive or poisonous effect on n ervous tissue .


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Toxicity of Cyanide and Related Compounds

Carcinogenic:

Causing or inducing uncontrolled growth of aberrant cells into malignant tumours.

Mutagenic:

Causing heritable alteration of the genetic material within living cells.

Teratogenic:

Causing non-hereditary congenital malformations (birth defects) in o ffspring.

The toxicity of free cyanide or metal cyanide complexes can manifest itself in either an acute or chronic manner. Acute toxicity is normally associated with the immediate lethal effects observed with organisms exposed to elevated concentrations of a particular compound. The period of acute toxicity ranges from a few minutes to several days.

4.3

SOURCES AND USES OF CYANIDE

The chemistry and toxicology of cyanide have been well studied in the laboratory. Field studies have provided empirical cyanide toxicity on wildlife, and epidemiological studies have evaluated acute and chronic human health effects. The toxicity of free cyanide has been evaluated at various trophic levels including: algae, macroinvertebrates, amphibians, fish, birds, and mammals. The principal toxicological hazard is acute exposure to free cyanide, such as rapid ingestion of cyanide-containing forage or solutions. Chronic exposures to cyanide at sublethal concentrations, in food or water, generally have no bioaccumulative effects due to the rapid detoxification process and excretion of by-products. Populations at risk for cyanide inhalation or ingestion exposures include suicide victims, exterminators, fire fighters, cigarette smokers, cyanide production workers, workers in cyanidation operations and nutritionally susceptible populations ingesting cyanogenic plants. Exposures to cyanide in mining solutions have been generally controlled with physical exclusion methods and with chemical treatment or recovery of cyanide (Mudder, 1998b). Anthropogenic cyanide uses and environmental sources result from the manufacturing of plastics and synthetic fabrics, metal mining operations, electroplating baths, pesticidal agents, intermediates in agricultural chemical production, cyanogenic drugs, and predator control devices. Laetrile (an anticancer preparation made from apricot kernels), sodium nitroprusside (a drug used to reduce high blood pressure), and simple aliphatic nitriles release cyanide upon metabolism (ATSDR, 1995). Sodium cyanide production and consumption in North America has been about 130,000 tons per year; about 90% being used in gold mining (Gold Institute, 1996). About 84% of domestic HCN production is used to produce organic cyanides, also known as nitriles (Towill et al., 1978). Hydrogen vapour, becauseto of exterminate its rapid lethal toxicity, has been to fumigate buildings, cyanide ships and warehouses; rabbits, rodents and used largewidely predators; and in horticultural practice to control insect pests that have developed resistance to other pesticides (Ballantyne, 1988). In the U.S., federal agencies have been using sodium cyanide for over 50 years in an in-ground device, like the M-44, to control coyote populations in an attempt to protect livestock. Tens of thousands of coyotes have been eradicated, but in the process hundreds of other wildlife species have been killed.

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Cyanide has been used to collect various species of freshwater fish in the Missouri and Mississippi River drainages and has been used illegally to obtain Atlantic salmon and brown trout from rivers, leaving no visible damage to the fish (Holden and Marsden, 1964). Cyanide is also used to stun and capture rare coral reef fish in Southeast Asia to supply the growing market for aquarium fish around the world (World Resources Institute, 1998). Natural sources of cyanide include various species of arthropods, insects, bacteria, algae, fungi and higher plants (2,650 plant species) that form and excrete cyanide (Way, 1984). The most widely distributed major food crop with a high content of cyanogenic glycosides is cassava. Ingestion of improperly prepared cassava can lead to chronic cyanide exposure and the onset of Konzo, a disease causing permanent paralysis of the limbs primarily in young children. Cyanogenesis has an important role in plant defence against predatory herbivores. Wildlife and livestock exposure to high concentrations cyanogenic plants can be hazardous if consumed rapidly. Various arthropod species such as centipedes and millipedes, and insects such as beetles, moths and butterflies secrete cyanide for defensive purposes in repelling predators such as amphibians and birds (Duffy, 1981). Some species of soil bacteria suppress plant diseases such as tobacco root rot caused by soilborne fungi by producing metabolites with antibiotic activity. A wide variety of bacteria and fungi can degrade cyanide compounds (Towill et al., 1978). Dried fungal mycelia are now sold commercially to detoxify cyanide in industrial wastes (Knowles, 1988). An estimate of the amount of cyanide released to the environment from natural biogenic processes is not available.

4.4

TOXICOLOGY O F C YANIDE

4.4.1

Physiology of Cyanide Toxicity

The most common forms of cyanide in the environment are free cyanide, metallocyanide complexes and synthetic nitriles. Free cyanide, specifically HCN, is the primary toxic agent in the aquatic environment. The toxicity of complex cyanides is related to their ability to release free cyanide in solution; relatively small fluctuations in pH significantly affect their biocidal properties. Total cyanide refers to all cyanide-containing compounds, including simple and complex cyanides, cyanoglycosides, and free cyanide. The relation between total cyanide and free cyanide in natural waters varies with receiving-water conditions, degree of aeration, type of cyanide compounds present, exposure to daylight and the presence of other compounds. All available evidence suggests that cyanides are neither mutagenic, teratogenic nor carcinogenic (Ballantyne, 1987). Moreover, no detoxification. reports of cyanide biomagnification or cycling in living organisms, probably owingthere to itsare rapid Cyanide seldom persists in surface waters and soils owing to complexation, sedimentation, microbial metabolism and loss from volatilisation as previously discussed. Cyanide has beneficial pharmacological activity and possibly has antineoplastic activity, as judged by a low therapeutic success against rat sarcomas (USEPA, 1985a).


136


Cyanides are readily absorbed through inhalation, ingestion or skin contact and are readily distributed throughout the body via blood (Towill et al., 1978). Inhalation and skin absorption are the primary hazardous routes in cyanide toxicity in occupation exposure. Skin absorption is most rapid when the skin is cut, abraded or moist. Inhalation of cyanide salts is potentially hazardous because the cyanide dissolves on contact with moist mucous membranes. Cyanide is a potent and rapid-acting asphyxiant, inducing tissue anoxia through inactivation of the cytochrome oxidase enzyme in the mitochondria. Tissue hypoxia causes a shift from aerobic to anaerobic metabolism, resulting in the depletion of energy-rich compounds such as glycogen, phosphocreatine and adenosine triphosphate, and the accumulation of lactate with decreased blood pH. The combination of cytotoxic hypoxia with lactate acidosis depresses the central nervous system, resulting in respiratory arrest and death. If the absorption rate is significantly greater than the detoxification rate, there will be a rapid accumulation of free cyanide in the tissues and body fluids, resulting in the prompt onset of signs of acute cyanide poisoning. Clinical symptoms in the first stage of acute cyanide poisoning include headache, vertigo, weak and rapid pulse, nausea and vomiting. In the second stage, there are convulsions, falling, dilated pupils, clammy skin, and a weaker and more rapid pulse. In the final stage, the heart beat becomes irregular and slow, body temperature falls, there is cyanosis of the lips, face, and extremities, coma, frothy bloody saliva flow from the mouth and death (Way, 1981). Chronic cyanide poisoning may develop after ingestion of significant quantities of cyanide or cyanide precursors in the diet as well as by chronic inhalation of tobacco smoke. Effects are exacerbated by dietary deficiencies in vitamin B12 and other water soluble vitamins, iodine, and sulphur amino acids, as well as by low levels and insufficient distribution of detoxifying enzymes such as rhodanese (Solomonson, 1981). Cyanide levels in mainstream cigarette smoke have been reported to range from 0.28 to 0.55 mg per cigarette of hydrogen cyanide, with the ratio of cyanide concentration in sidestream smoke to mainstream smoke ranging from 0.006 to 0.27 (Fiksel and Cooper, 1981). The serum of smokers contains about three times higher cyanide and thiocyanate concentrations than the serum of non-smokers (ATSDR, 1995). 4.4.2

Detoxification Me chanisms

The majority of the absorbed cyanide reacts with thiosulphate in the presence of enzymes to produce thiocyanate, which is excreted in the urine over a period of several days. Owing to this rapid detoxification, animals can ingest high sublethal doses of cyanide over extended periods without harm (Eisler, 1991). Species vary considerably in both the extent to which thiocyanate is formed and the rate at which it is eliminated from the body. Thiocyanate metabolites resulting from the transulphuration process are about 120 times less toxic than the parent cyanide compound. may accumulate abnormalitiesThiocyanate and other adverse effects. in tissues and has been associated with developmental

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The two enzyme systems associated with the transulphuration process are the rhodanese and beta-mercaptopyruvate cyanide sulphurtransferase which are limited by the availability of sulphur. Minor detoxification pathways for cyanide include exhalation in breath as HCN and as CO2 from oxidative metabolism of formic acid, conjugation with cystine, combining with vitamin B12 to form cyanocobalamin, which is excreted in the urine and bile, and binding with methemoglobin in the blood (Ballantyne, 1987). Cyanide concentrates in the erythrocytes through binding with methemoglobin (USEPA, 1985a) and free cyanide concentrations in plasma are an indicator of cytotoxicity. Cyanide accumulates in mammalian blood, spleen, brain, and cerebrospinal fluid which are the biological samples taken for analysis in cyanide poisonings. The measured rates of detoxification of cyanide by a guinea pig and by a human are 0.076 and <0.001 mg/kg/minute, respectively (Ballantyne, 1987). In addition to rapid metabolic detoxification, cyanide antidotes, taken orally and by injection, can be and have been administered successfully to both humans and other animals. These antidotes include amyl nitrite, sodium nitrite and sodium thiosulphate. Different species of migratory birds have been found dead within and adjacent to unnetted or uncovered tailings impoundments in which the total cyanide levels approached the strength typically observed in process solutions. However, injured birds still alive after 60 minutes of exposure to levels as high as 200 mg/L often survived, due to very rapid metabolism and detoxification of cyanide to thiocyanate (Clark and Hothem, 1991 and Wiemeyer et al., 1986). Tissue specific biochemical effects have been studied (Pritsos and Ma, 1996) with the mallard duck exposed perorally to sublethal doses of free cyanide ranging from 0.25 mg KCN/kg body weight (10 ppm) up to 80 ppm KCN. Two hours after exposure, the ATP levels in the brain, heart, and liver were measured. The ATP depletions in the brain and liver were dose dependent. Using 1 mg/kg body weight of KCN (40 ppm), an initial decrease in ATP levels in all tissue occurred with a subsequent increase approaching control levels within 24 hours. In the brain tissue, the recovery was the most dramatic with increases in ATP levels above control levels at 12 hours and a return to control levels within 24 hours. Further research conducted with mallard ducks (Pritsos and Ma, 1996) monitored the detoxification systems in the heart, liver, and brain tissues after cyanide exposure by measuring the enzymatic activities of rhodanese and 3-MPS (3-mercaptopyruvate sulphurtransferase). After 1 mg/kg body weight of KCN exposure, rhodanese and 3-MPS activities were found to be unchanged in heart and liver tissues. The brain showed a 64% increase in rhodanese and 3-MPS activates, which also had the fastest recovery of ATP levels. The conclusion was that different metabolic rates and detoxification enzymes activities exist in different duck tissues. These studies effects with mallard exposed oxidase to 10 to system 80 ppmby KCN, demonstrate thedependent expected toxicological on theducks cytochrome measurable, dose reductions in liver and brain ATP levels. Detoxification mechanisms were not overwhelmed by cyanide, since the ATP levels returned to control levels after 24 hours exposure. Measurements of the detoxification enzyme activities validated that the mallard duck exposure to 40 ppm free cyanide did not impair the organism's physiological detoxification mechanisms, based upon enzyme levels returning to control levels within 24 hours.


138


4.4.3

Cyanide Antidotes

The classic nitrite-thiosulphate treatment of cyanide poisoning, developed about 60 years ago, in combination with excess oxygen is one of the antidotal combinations still employed. Livestock treatment in cases of suspected cyanide intoxication consists of intravenous administration of sodium nitrite at 10 to 20 mg/kg body weight followed by sodium thiosulphate at 30 to 40 mg/kg body weight. However, a sodium thiosulphate dose of 500 mg/kg body weight or more may be more effective (Biehl, 1984). Cobalt compounds, such as hydroxycobalamin and its derivatives have been used to treat cyanide poisoning for more than 100 years after confirmation of their efficacy with pigeons and rabbits (Way, 1981).

4.5

CYANIDE TO XICITY TO AQUATIC O RGANISMS

4.5.1

Toxicity of Free Cyanide to Aquatic Organisms

The observed toxicity to aquatic life of simple and complex cyanides is attributed almost entirely to molecular, undissociated HCN, derived from ionisation, dissociation and photodecomposition of cyanide-containing compounds. The acute toxicity of stable silver cyanide and cuprocyanide complex anions is much less than that of molecular HCN. Toxicity of aquatic organisms to organic, nitrile compounds is similar to that of inorganic cyanides because they usually undergo rapid hydrolysis in water to free cyanide (Towill et al., 1978). There is general agreement that total cyanide concentrations in solution can overestimate the actual cyanide toxicity to aquatic organisms, because the total cyanide analysis can measure non-toxic forms of the compound and is subject to analytical interferences. The other forms of cyanide that must be considered in developing criteria include iron cyanide and weakly bound metal cyanides measured with free cyanide as the WAD cyanide analysis, the most analytically reliable index of toxicity in aquatic pollution. The cyanide effects on selected aquatic organisms are presented in Table 4.1 (Hagelstein and Mudder, 1998b). Fish were generally the most sensitive organisms tested under controlled conditions. Algae and macrophytes were comparatively tolerant; adverse effects were reported at greater than 160 µg free cyanide per litre. Temperature and activity modify the lethality of HCN to juvenile rainbow trout (Leduc and Pierce, 1982); higher resistance to cyanide correlated with higher activity and temperatures suggesting a faster detoxification rate or higher oxidative metabolism. Low levels of cyanide that were harmful when applied constantly, may be harmless under seasonal or other variations that allow the organism to recover and detoxify (Leduc, 1984). Additionally, acclimation by certain species of fish to low sublethal levels of cyanide through continuous exposure might enhance their resistance to potentially lethal concentrations (Leduc, 1984).

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TABLE 4.1

Effects of Free Cyanide on Aq uatic Organisms

AquatSicpecies Algae and Macrophytes ALGAE, Chlorella sp. 7,000 µg/litre 20,000 µg/litre ALGAE, Scenedesmus sp. 160 µg/litre ALGAE, Microcystis aeruginosa 7990 µg/litre WATER HYACINTH, Eichhornia crassipes 300,000 µg/litre

FRESHWATER AQUATIC PLANTS (9 SPECIES) 65,000 µg/litre Invertebrates COPEPOD, Acartia clausi 30 µg/litre ISOPOD, Asellus communis 29-40 µg/litre 1834 µg/litre OYSTER, Crassostrea sp. 150 µg/litre DAPHNID, Daphnia pulex 83 µg/litre AMPHIPOD, Gammarus pseudolimnaeus 58 µg/litre 184 µg/litre MUSSEL, Mytilus edulis 18 µg/litre 100 µg/litre MYSID SHRIMP, Mysidopsis bahia 11-20 µg/litre 43 µg/litre 93-113 µg/litre SNAIL, Physa heterostropha 432 µg/litre


Effects

References

Inhibition of photosynthesis Enzyme inhibition

USEPA, 1985a

Toxic

Towill et al., 1978

Nonphytotoxic in 72 hours Bioconcentration Factor = 22 Plants contained 6.7 g/kg dryweight

USEPA, 1985a Low and Lee, 1981

30 minutes exposure No effect on respiratory oxygen uptake

Azcon-Bieto et al., 1987

LC50 (96 hour)

USEPA, 1985a

Max. acceptable toxicant conc. (MATC) LD50 (11 days)

USEPA, 1985a

Motor activity suppressed after 10 min.

USEPA, 1985a

LC50 (96 hour)

USEPA, 1985a o

LC50 (96 hour) at 20 C o LC50 (96 hour) at 5.2 C

Smith et al., 1978

14 days exposure Growth reduced, glycine uptake inhibited LC20 (14 days)

Thompson, 1983

No measurable effects Adverse effects on reproduction LC50 (96 hour)

EPA, 1985a

LC50 (96 hour)

Leduc et al., 1982

140


TABLE 4.1

AquatS icpecies Fish BLUEGILL, Lepomis macrochirus 50 µg/litre 56-227 µg/litre 109-218 µg/litre 232-365 µg/litre 535-690 µg/litre LARGEMOUTH BASS, Micropterus salmoides 101 µg/litre CUTTHROAT TROUT, Oncorhynchus clarki 1,000 µg/litre COHO SALMON, Oncorhynchus kisutch 7.0 µg/litre RAINBOW TROUT, Oncorhynchus mykiss 10 µg/litre

15 µg/litre 10-20 µg/litre 28 µg/litre 42 µg/litre 46-75 µg/litre 60 µg/litre 68 µg/litre CHINOOK SALMON, Oncorhynchus tshawytscha 10 µg/litre 20 µg/litre YELLOW PERCH, Perca flavescens 76-108 µg/litre 288->389µg/litre FATHEAD MINNOW, Pimephales promelas 82-113 µg/litre 121-202 µg/litre 352 µg/litre 230 µg/litre 650 µg/litre 170-180 µg/litre BLACK CRAPPIE, Pomoxis nigromaculatus 101 µg/litre ATLANTIC SALMON, Salmo salar 10 µg/litre 24 µg/litre 73 µg/litre BROWN TROUT, Salmo trutta 90 µg/litre BROOK TROUT, Salvelinus fontinalis 56-112 µg/litre >212 µg/litre

Continued

Effects

References

Tolerated at higher temps butno reprod. LC50 (96 hour) for juveniles LC50 (96 hour) for fry LC50 (96 hour) for eggs LC50 (96 hour) for eggs at hatching

Smith et al., 1979

LC50 (96 hour) for juveniles

Smith et al., 1979

Exp. 20 min., recovered within 12 min. Wiley, 1984 Growth normal next 6 months Exposure 8 days Reduced swimming performance 50%

Leduc, 1984

Exposure 20 days o No effect on growth at 6 C Exposure 20 days o No effect on growth at 12 C Exposure 20 days, reduced # of eggs, abnormal oocytes, delayed development o LC50 (96 hour) at 6oC LC50 (96 hour) at 12 C LC50 (96 hour) for juveniles o No deaths at 96 hours at 18 C o LC50 (96 hour) at 18 C

Leduc, 1984

Exp. 64 days, increased growth rate Exp. 64 days, reduced growth 27%

Leduc, 1984 USEPA, 1985a

LC50 (96 hour) for juveniles LC50 (96 hour) for eggs and fry

Smith et al., 1978,1979 Smith et al., 1978,1979

o

Leduc, 1984 Lesniak & Ruby, 1982 Leduc, 1984 Leduc, 1984 Leduc, 1984 Leduc, 1984 Leduc, 1984

LC50 (96 hour) for fry at 25o C LC50 (96 hour) for eggs at 25 C o LC50 (96 hour) for eggs at 15 C LC50 (96 hour) for adults LC50 (96 hour) for adults, mixture of NaCN plus NiSO4 LC50 (96 hour) for adults, mixtures of NaCN plus CdSO4 and ZnSO4

Smith et al., 1979 Smith et al., 1979 Smith et al., 1979 Smith et al., 1979

LC50 (96 hour) for juveniles

Smith et al., 1979

Smith et al., 1979 Smith et al., 1979

Exp. 58 days, abnorm. embryonic devel. USEPA, 1985a LC50 (24 hr) at dissolved oxy.=3.5 mg/ L Alabaster et al., 1983 LC50 (24 hr) at dissolved oxy.=10 mg/ L Alabaster et al., 1983 LC50 (96 hour)

Leduc et al., 1982

LC50 (96 hr) for swimup fry & juvenile Smith et al., 1978,1979 LC50 (96 hr) for eggs Smith et al., 1978,1979

141

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Cyanide in aquatic environments, does not persist for extended periods, and is highly species selective. The target sites for cyanide toxicity in freshwater organisms include the gills, egg capsules, and other sites where gaseous exchange and osmoregulatory processes occur. Signs of acute distress include increased ventilation, gulping for air at the surface, erratic swimming movements, muscular incoordination convulsions, tremors, sinking to the bottom, and widely extended gill covers. Thiosulphate administered in the water with cyanide reduced the toxicity of cyanide to fish, presumably by increasing the detoxification rate of cyanide to thiocyanate (Towill et al., 1978). Fish reproduction is affected by cyanide concentrations in the range of 10 to 100 µg HCN/litre, in the reduced number of spawned eggs and the viability of the eggs by delaying the process of secondary yolk deposition in the ovary (Eisler, 1991). Developmental abnormalities were reported in embryos of Atlantic salmon after extended exposure to cyanide (Leduc, 1984). Other adverse effects of free cyanide concentrations of 10 µg/litre to fish include delayed mortality, pathology, impaired swimming ability and relative performance, susceptibility to predation, disrupted respiration, osmoregulatory disturbances and altered growth patterns. Stimulatory effects of cyanide include increased growth of fish during exposure to low sublethal levels; also known as a hermetic effect, applicable to many other chemicals (Leduc, 1984). Sodium cyanide has a stimulatory effect on oxygen-sensitive receptors in lungfish, amphibians, reptiles, birds, and mammals (Smatresk, 1986), which internally assist respiratory activity. Factors increasing the toxicity of free cyanide in aquatic ecosystems include low dissolved oxygen, low pH, but not hardness other than the effect hardness or alkalinity has on pH. Additive and synergistic toxicity of free cyanide to aquatic fauna (Atlantic salmon smelt) has been reported in combination with ammonia and low dissolved oxygen (Alabaster et al., 1983). The acute toxicity of cyanide on fish in solutions containing only free cyanide decreases with increasing pH, indicating the molecular form (HCN) is more toxic (2.3 times) than the anionic form (CN ). 4.5.2

Toxicity of WAD Cyanide to Aquatic Organisms

The weak acid dissociable (WAD) cyanide analysis reports not only free cyanide but also cyanide weakly bound to other metals, most notably copper, nickel and zinc. These three metal complexes include those predominantly associated with mining effluents. These compounds exhibit varying degrees of stability and tendencies for breakdown and release of free cyanide when compared to the iron cyanides. In addition, these compounds exhibit varying degrees of inherent toxicity. From a treatment standpoint, these forms of cyanide are the most readily degradable. As mentioned previously, the WAD cyanide analysis is normally accurate, reliable and measures those forms of cyanide considered to be toxicologically significant.


142


Of the forms reporting as WAD cyanide, free cyanide is the most readily removable or degradable from a treatment viewpoint. Following treatment, the effluent often contains nondetectable levels of free cyanide (i.e., <0.02 mg/L). As a result, the primary concerns in regard to aquatic or mammalian toxicity are the other forms of WAD cyanide. As a rule, metalcomplexed cyanide is less toxic than free cyanide. In addition, the toxicity of the complexed metal is lower than an equal level of the free ionic form of the metal. There is no evidence to support the existence of a hardness relationship with the toxicity of WAD cyanide other than the effect hardness has on pH. The concentration of metals in cyanidation wastewaters varies over several orders of magnitude. In general, there is sufficient cyanide present in mining wastewaters to complex the available metals. The metal cyanide complexes of major and minor importance include those of mercury, cadmium, zinc, nickel, iron, copper, silver and gold. In early work that examined the toxicity of metal-complexed cyanides and certain fish species, the toxic concentrations for zinc cyanide, cadmium cyanide, copper cyanide and nickel cyanide were found to be 0.10 mg/L, 0.30 mg/L, 1.0 mg/L and 30 mg/L as cyanide, respectively (Doudoroff, 1976). Considerable reduction in the toxicity of the four metals and cyanide was noted, but limited information concerning the test conditions and fish species used in the tests was included in the publication. In subsequent experiments, the pH of the test solutions was controlled in the range of 6.5 to 8.0. The 96-hour median tolerance limits for nickel cyanide and fathead minnows at pH 6.6 and 8.0 were 0.42 mg/L and 7.3 mg/L as cyanide, respectively (Doudoroff, 1976). The tolerance limits decreased 10 to 13 fold when the pH was decreased from 7.8 to 7.5. As a result, the acute toxicity of dilute nickel cyanide solutions is determined primarily by the free molecular HCN concentration which results from dissociation. The observation includes the fact that free nickel ion is relatively non-toxic to aquatic life, even in low hardness waters. Although there is evidence to support direct toxicity of the complex itself, other data suggests the opposite hypothesis is valid. As noted by Doudoroff, the practical implications of the nickel cyanide toxicity data indicates nickel is very effective in protecting fish from lethal effects of cyanide in alkaline waters (i.e., pH >7.5), but is not effective in acidic waters (i.e., pH <6.5). Since natural waters and mining effluents are generally basic, complexation with nickel is effective in reducing the relative toxicity of cyanide (Doudoroff, 1976 and Doudoroff, 1979). In addition, free cyanide formation through dissociation at acidic pH values is slow and once formed will be rapidly lost from the water column through natural attenuation (e.g., volatilisation, hydrolysis and oxidation). It is interesting to note that the complexing ability of nickel forms the basis for a cyanide analytical procedure.

143

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In regard to nickel cyanide, the toxicity varies in accordance with pH and is quite pronounced at acidic pH levels. The detailed investigations on the influence of pH on nickel cyanide toxicity have supported the assumption that the toxicity of the complex is attributable to the free cyanide formed through dissociation. In general, the toxicity of waters containing nickel cyanide can be estimated by calculating the free cyanide concentration resulting from dissociation of the complex. The toxicity effects of nickel cyanide have been studied more extensively than any other metal cyanide complex. In early experiments conducted by Doudoroff (1976) using fathead minnows, about 20% to 40% of the organisms died within 168 hours at a total cyanide concentration of 0.50 mg/L. The cyanide present was complexed prior to the experiments by mixing of solutions of nickel sulphate and sodium cyanide until nickel precipitated. It was determined that the toxicity of various solutions increased with time after introduction of the test organisms. The increase in toxicity was attributed to a decrease in pH due to production of carbon dioxide from the respiring fish (Doudoroff, 1976). An increase of 0.50 unit in pH of a test solution with sodium hydroxide was sufficient to induce recovery of fish already affected by the nickel cyanide present in the solution. The molar ratio of cyanide to nickel was about 4.0 in the various tests. In terms of silver cyanide, the metal cyanide complex formed is very effective in preventing the occurrence of free cyanide in solution. For example, at a pH of 7.0 and 8.0, greater than 80% and 99% of the cyanide still remains bound to silver in dilute solutions of the complex. In addition, very low levels of silver are noted in discharges from mining operations due to recovery processes. Cyanide and silver occur in an approximate 2:1 molar ratio. Although free silver ion is very toxic to aquatic life, there is generally sufficient cyanide to promote complete complexation. Once the complex is formed it is quite stable, and there is no tendency for dissociation at the pH associated with mining effluents or natural waters. Concerns with silver cyanide toxicity and criteria development should be focused upon the free metal and cyanide concentrations existing in solution. Work conducted by Broderius (1973) indicated the extent of dissociation of the silver cyanide complex with pH was minimal, as were its toxic effects on bluegills. For example, the mean survival times for bluegills at a silver cyanide concentration of 10 mg/L as cyanide and pH values of 7.5 and 8.5 were 29 and 31 hours, respectively (Doudoroff, 1976). The molar ratio of cyanide to silver is again 2:1. The 96-hour median tolerance limit of silver cyanide was estimated from additional research to be somewhat below 7.0 mg/L as cyanide. Numerous studies have been conducted concerning the toxicity of free cyanide and the metal cyanide complexes of zinc and cadmium. These complexes are weak in nature and dissociate nearly completely at low in natural Theindicates results of comparing free cyanide andconcentrations these two metal cyanidewaters. complexes thattoxicity there istests no definitive relationship with regards to synergism or antagonism between the metal and cyanide. The importance of the stability of either zinc or cadmium cyanide can be ignored when establishing effluent guidelines and criteria for the metals or cyanide. In this instance, criteria should be derived or selected assuming the metal and cyanide act independently due to the weakness of these complexes.


144


On the contrary, both cyanide and copper are less toxic when they are complexed. For example, Doudoroff (1976) found the 24-hour median tolerance limit for copper cyanide, copper, and free cyanide with fathead minnows to be about 2.2 mg/L as cyanide, <0.10 mg/L and 0.25 mg/L, respectively. These results have been confirmed in testing conducted at Homestake Mining Company. Dissociation of the complex produces both copper and cyanide which are quite toxic to aquatic organisms, especially trout. The 48-hour median tolerance limit for copper cyanide and bluegills in slightly alkaline solutions was estimated (Broderius, 1973) at 9.0 mg/L as the complex and 4.0 mg/L as cyanide. Data was obtained indicating penetration of the copper cyanide complex into the fish tissue. The toxicity of the copper cyanide complex was very similar to that of the free metal, resulting in coagulated mucus on the gills and body surfaces of the fish. However, at the pH of mining effluents and natural waters and the levels of the complex in treated effluents, there is little tendency for dissociation and a distinct reduction in toxicity of both copper and cyanide occurs upon complexation. Research conducted at Homestake Mining Company utilized series of acute and chronic bioassays over a three to four-year period. The trout were the most sensitive aquatic species of the organisms tested, with regards to WAD cyanide. The organisms included Daphnia magna , mayflies, caddisflies, stoneflies and midges. One such test examined the long-term or chronic effects (i.e., 45 days) of WAD cyanide on juvenile trout. The bioassay involved a series of covered, flow-through aquaria into which a constant background matrix of 56% treated effluent and 44% receiving stream flow. Various concentrations of iron cyanide were introduced into this matrix. The aquaria were subjected to an 18-hour daylight period. No deaths were reported in 45 days at WAD concentrations ranging from 0.03 to 0.15 mg/L, with maximum values reaching 0.20 mg/L WAD cyanide. The solution contained all complexed metal cyanides in varying concentrations (i.e. cadmium, copper, nickel, zinc and silver). In addition to no mortality, there was no indication of bioaccumulation of metals or cyanide, no difference in growth factors or rates and no evidence of external or internal physiological damage. The tests were conducted using USEPA approved testing procedures. Very strict control of flow rates and testing conditions was practised. Particular attention was paid to the analytical procedures to ensure accurate results.

145

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The elevated pH of mining waters is an important factor in reducing the toxicity of free metals and metal complexed cyanides for two reasons. First, the elevated pH favours the formation of the less toxic cyanide anion. Second, the stability of metal cyanide complexes increases with increasing pH, which also enhances the precipitation of metals. With nickel cyanide, the primary concern is free cyanide, since nickel is a relatively non-toxic metal to aquatic organisms. At ambient pH values associated with natural waters, the toxicity of nickel cyanide is very low. The concentration of silver cyanide is very low due to the recovery process itself. The primary concern with regards to WAD cyanide is the copper complex. However, it is well established that the toxicity of both copper and cyanide are reduced when combined. A summary of additional metal cyanide toxicity data and factors affecting metal cyanide toxicity is presented in Table 4.2 (Hagelstein and Mudder, 1998b). 4.5.3

Toxicity of Iron Cyanides to Aquatic Organisms

The metal cyanide complexes of the ferrous and ferric ions are essentially non-toxic at the levels commonly present in cyanidation wastewaters (i.e., 2 to 40 mg/L). In preliminary 96-hour flowthrough bioassays conducted in varying light conditions for Homestake Mining Company at the Montana State Fisheries Bioassay Laboratory, no deaths were noted when fingerling rainbow trout were exposed to concentrations of ferrous and ferric cyanide of 45.5 mg/L as total cyanide and 53.0 mg/L as total cyanide, respectively (Thurston, 1982). However, toxicity tests conducted in direct or diffused sunlight in poorly ventilated and covered test chambers have shown levels of iron complexed cyanides of <1.0 mg/L as cyanide to be lethal in a few hours. The substantial differences in reported LC50 values for the iron complexed cyanides is due to the susceptibility of these compounds to decompose photolytically, producing acutely toxic levels of free cyanide. The variety of light sources and degree of intensities used in the toxicity tests account for the observed differences when comparing similar species. As a result, mortality due to exposure to iron complexed cyanide is related to the degree of photolytical breakdown and the level of free cyanide remaining in solution, not the compound directly. The chemical stability of ferrous and ferric cyanide is well documented and has been discussed previously. After 31 days a 0.01 M potassium hexacyanoferrate solution (i.e., 3,293 mg/L) kept in the dark at pH values of 6.9 and above contained <0.01 mg/L of free cyanide. Broderius (1973) observed that the molecular HCN concentration of a potassium ferrous cyanide solution of 5.0 mg/L as cyanide kept in the dark increased to about 0.06 mg/L after two months, then decreased and stabilized at 0.01 mg/L for 10 months (Doudoroff, 1976). The pH of the solution was 6.8. Both these experiments yielded non-quantifiable levels of free cyanide based upon the total or WAD cyanide analytical procedures employed in commercial laboratories.


146


TABLE 4.2

Compound

Relative Toxicities to Fish of Metal Cyanide Compounds

TestSpecies

pH

Temperature

Concentration

Toxic Effect

(mg/L)

Nickel Cyanide Complex CN/Ni=-3 CN/Ni=-3.95

Zinc Cyanide Complex CN/Zn 4= CN/Zn 0.17 =

Gold Fish FatheadMinnow

8.1 6.5-8.0 6.5-8.0

104 1.35to1,300 0.42to730

Fathead Minnow Bluegill

Cadmium Cyanide Complex CN/Cd 4= Fathead Minnow

Copper Cyanide Complex CN/Cu=4.0 CN/Cu=3.75 CN/Cu=3.0

WesternBlacknoseDace WesternBlacknoseDace WesternBlacknoseDace

Silver Cyanide Complex Ag(CN)2-1

ThreespineStickleback Bluegill

Hydrogen Cyanide HCN

Sodium & Potassium Cyanide NaCN

KCN

0.18 0.26

LC LC

0.18

LC

7.6-8.0 7.6-8.0 7.6-8.0

0.38 0.47 0.71

7.7 7.7 7.5-8.5 7.5 8.0

Rainbow Trout RainbowTrout RainbowTrout

2-4

°C

12-14°C

50 50

0.028 0.042 0.068

0.28 0.43 0.66 0.02 0.08

- 96 hrs - 96 hrs

50 - 96 hrs LC50 - 72 to 96 hrs

LC LC LC

10.0 6.0 10.0 10.0 7.0

6.3 °C 12.3 °C 18.0 °C

Bluegill Fathead Minnow Fathead Minnow Rainbow Trout

No effect in 24 hrs LC 50 - 24 hr LC 50 - 96 hr

- 24 hrs - 24 hrs 50 - 24 hrs 50 50

LC 50 - 13 hrs LC50 - 24 hrs LC50 - 29 - 31 hrs LC50 - 14 hrs LC50 - 87 hrs

LC50 - 96 hrs LC50 – 96 hrs LC50 – 96 hrs

LC50-96 hrs soft water LC 50-96 hrs soft water LC 50-96 hrs hard water LC50 - 96 hrs LC50 - 96 hrs


147

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The inherent stability of iron-complexed cyanides is exemplified by the conditions associated with their analysis. In order to decompose the complex and release free cyanide, the pH of the sample is reduced to <1.0 with concentrated sulphuric acid and refluxed at elevated temperature for one hour. As a result, under normal conditions in natural waters, (i.e., ambient temperatures and near neutral or slightly basic pH values) iron cyanides are subject to minimal chemical breakdown. These compounds are resistant to catalytic oxidation using ozone, a very powerful oxidant. Biological breakdown of iron cyanide is limited and usually oxidative, not releasing free cyanide. The concern with iron complexed cyanides relates to the degree of photochemical breakdown and the residual free cyanide concentration remaining in solution. The driving force for breakdown is the energy of ultraviolet (UV) radiation, although visible radiation contributes to the photolysis of iron cyanides. The percentage of UV radiation reaching a water surface in which iron complexed cyanides are present is limited and is attenuated rapidly as the light passes through the earth's atmosphere. The percentage of UV light within the total electromagnetic spectrum reaching the earth's surface is about 6%. Additional attenuation occurs as the result of many factors including scattering, reflection and absorption. Scattering alone reduces the percentage of available UV radiation by another 25%. Reflection which increases as the angle of incidence increases at the water surface is particularly important in a turbulent river resulting in an additional 20% attenuation or more (Wetzel, 1975). Once the light penetrates a water surface it is attenuated rapidly in the water column and scattered due to the presence of suspended substances. Ultraviolet radiation is rapidly absorbed by oxygen, humic and fulvic acids and other naturally occurring organic and inorganic substances. As a result, only a very small percentage of UV radiation is available for photochemical breakdown of iron-complexed cyanides. Several researchers have demonstrated that in closed systems under high intensity UV radiation, nearly complete breakdown of iron cyanide forming free cyanide occurs. Unfortunately, the experiments do not generally represent realistic natural conditions and do not consider attenuation of free cyanide by hydrolysis, oxidation, adsorption and volatilisation. In the event that free cyanide does form through photochemical breakdown of iron cyanides, it is converted in part to its molecular form (HCN) under the natural conditions existing in surface water environments. According to research conducted by the Wastewater Technology Centre of Environment Canada, approximately 90% of the free cyanide formed through photochemical decomposition is removed from surface waters (i.e., impoundments, rivers and streams) through volatilisation, while about 10% is removed through chemical oxidation or hydrolysis (Schmidt et al., 1981).


148


The Environment Canada research also concluded that the rate of photochemical degradation is slow when compared to the rate of cyanide volatilisation. The free cyanide formed through photochemical decomposition of iron cyanide is rapidly removed from the water column through volatilisation or by chemical degradation mechanisms, limiting the accumulation of free cyanide within the water body. Examination of total and WAD cyanide analyses in decant ponds indicates the ratio of their concentrations remains relatively constant within experimental error. From experience, an accumulation or increase in the pond's free cyanide concentration is not noted over time. Free cyanide analyses often fluctuate due to the inaccuracy of the analysis. In a study conducted by Homestake Mining Company, the ratio of total to WAD cyanide was monitored during the summer at several stations downstream of a treated water discharge. The study was conducted to examine the effects of low flow, shallow water depth and light intensity on the rate of photochemical breakdown of iron cyanides. The ratio remained constant although the total and WAD cyanide concentrations decreased, indicating dilution and attenuation were occurring, but the free cyanide formed through photochemical breakdown of total cyanide was rapidly removed from the stream. As quoted from the work of Peter Doudoroff: "From the information presented above (i.e., in his own text), one can conclude that, were it not for the phenomenon of photolysis, surface waters receiving moderate amounts of ferrocyanide and ferricyanide complexes probably would never be rendered acutely toxic to fish. Some of the HCN produced by dissociation of the complexes would be constantly escaping to the atmosphere or be otherwise lost, and the ferrocyanide complex would be gradually converted into the more stable ferricyanide complex. Therefore, and because of the low rate of dissociation of the complexes, the low equilibrium levels of HCN would never be attained, and the attainment of acutely toxic HCN levels even in waters contaminated with the ferrocyanide ion would be most improbable" (Doudoroff, 1976). "Water receiving cyanide-bearing, wastes usually are not perfectly clear and very shallow, and often are quite turbid and deep. Because the penetration of sunlight, and especially ultraviolet light, is limited, photolysis of the iron-cyanide complexes in most of the receiving waters exposed to sunlight probably is not nearly as rapid as that observed in aquarium tests. Free cyanide produced by the photolysis is continuously lost to the atmosphere or otherwise eliminated. Therefore, concentrations of free cyanide lethal to fish may not be often attained even in surface waters receiving large amounts of the iron-cyanide complexes" (Doudoroff, 1976). The conclusions of Dr. Doudoroff are supported by the scientific evidence that the discharge of low levels of iron cyanide into surface waters, on the order of a few mg/L or less, has not resulted measurable impacts to aquatic life within the receiving streams (Mudder and Smith, 1998 andinMudder, 1998b).

149

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Experimentally problematic are attempts to simulate natural conditions in the laboratory. To investigate the toxicity of iron complexed cyanides a series of experiments were conducted by Homestake Mining Company to simulate the rate and degree of photochemical breakdown of iron cyanides. The first set of experiments was conducted at the Fisheries Bioassay Laboratory at Montana State University (Meyn et al., 1984). In these 96-hour, flow-through toxicity tests, juvenile rainbow trout were exposed to varying concentrations of ferrous and ferric cyanide in covered aquaria using 18-hour photoperiods and Spectralite tubes as the light source. A comparison of the energy distribution of sunlight and the Spectralite tubes is presented in Figure 4.1. In the dark, the LC 50 values for ferrous and ferric cyanide ranged from 867 to 939 mg/L and from 869 to 1,210 mg/L, respectively. At a light intensity of 680 lux using an 18-hour light period in covered aquaria, the LC50 values for ferrous and ferric cyanide averaged 35 mg/L and 31.2 mg/L, respectively. The corresponding LC50 values for WAD cyanide under the same light conditions ranged from 0.15 to 0.17 mg/L and from 0.31 to 0.42 mg/L, respectively. At the lower LC50 values for iron cyanide (i.e., 10.8 to 44.2 mg/L), the percentage breakdown of iron cyanide ranged from about 0.5% to 2.9%. The results of the 96-hour bioassays are presented in Tables 4.3 and 4.4. To investigate the toxicity of iron cyanides and mining effluents in greater detail, a series of chronic 45-day bioassays were conducted in the Homestake Mine Toxicological Testing Facility (Marshall and Fox, 1985). In these tests juvenile rainbow trout were exposed to a constant background matrix of 56% treated mine effluent and 44% receiving stream water. A solution of potassium ferricyanide was spiked into each set of aquaria on a continuous basis. The ferric iron cyanide concentrations ranged from 0.47 to 22.6 mg/L as total cyanide. The corresponding WAD cyanide concentrations ranged from 0.03 to 0.19 mg/L. The aquaria were covered and a 16-hour light period using Spectralite tubes as the source of UV radiation. The conditions for these tests are presented in Table 4.5. No deaths were noted in 45 days of continuous testing. Neither abnormal growth factors nor bioaccumulation above the control organisms were noted. Physiological examination of all organisms indicated no adverse effects either internally or externally. In these tests, the WAD cyanide formed through breakdown of iron cyanide was allowed to accumulate in covered aquaria. The results indicated iron cyanide was essentially non-toxic as well as the resulting complexed WAD cyanide, when compared to the toxicity of free cyanide. In addition, the breakdown of iron cyanide was monitored through the corresponding WAD cyanide analysis. The test was valuable since it utilized an actual treated mine effluent as a background chemical matrix, a long-term testing period and a sensitive aquatic species. The percentage breakdown of iron cyanide observed during this testwork increased as the concentration of iron cyanide decreased. The percentage breakdown ranged from 0.8% to 2.5%, which is in accordance with the previous discussion on light intensities and with the results of other researchers.


150


FIGURE 4.1 Spectral Energy Distribution of Natural Sunlight and Spectralite Bulbs

Source: Solar Corporation, 1983

151

Chapter Four


TABLE 4.3 Ferrocyanide Toxicity Test Summary and 96-Hour LC50 Values Obtained

Test No.

Light Regime (hours of light/dark)

Mean Light Intensity (lux)

1074 1077

0/24 0/24

0 0

1078

0/24

0

1088

18/6

330

1090

18/6

330

1080

18/6

680

1083

18/6

680

--7.95 (7.89-8.01) 7.77

Mean Temperature °C (range) 9.4 (9.4-9.5) -9.4 (9.4-9.5) 15.7

Mean DO mg/L (range) 8.44 (7.03-8.85) -8.83 (8.76-8.97) 8.22

--1.10 (0.99-1.23) 0.79

Total CN 96-hr LC50 mg/L (95% C.I.) 752 (474-1190) >867 939 (828-1064) 220

(7.66-7.94) 7.83 (7.75-7.95) 7.33 (7.30-7.34) 7.55 (7.50-7.57)

(15.4-16.1) 15.2 (15.2-15.4) 9.6 (9.6-9.7) 9.5 (9.5-9.6)

(8.01-8.49) 7.85 (7.64-8.11) 7.93 (7.77-8.17) 8.37 (8.16-8.58)

(0.63-1.0) 0.71 (0.51-0.98) 0.15 (0.13-0.17) 0.17 (1) (-)

(184-261) 232 (178-302) 33.0 (28.7-37.9) 37.4 (1) (-)

Mean pH (range)

Source: Meyn et al., 1984

Notes: (1) Confidence internal not calculable by method used.


152

WAD CN 96-hr LC50 mg/L (95% C.I.)


TABLE 4.4 Ferricyanide Toxicity Test Summary and 96-Hour LC50 Values Obtained

Test No.

Light Regime hours of light/dark

Mean Light Intensity (lux)

1075

0/24

0

1076 1079

0/24 0/24

0 0

1081

18/6

330

1082 1084

18/6 18/6

330 680

1085

18/6

680

1087

18/6

680

1089

18/6

680

Mean pH (range) 7.87 (7.46-8.23) 7.96 (7.75-8.17) --

Mean Temperature °C (range) 9.3 (9.46-9.4) 9.3 (9.2-9.4) --

Mean DO mg/L (range) 8.18 (7.03-8.85) 8.88 (8.70-9.01) --

7.33 (7.30-7.34) 7.56 (7.53-7.58) -7.78 (7.76-7.80) 7.96 (7.89-8.06) 7.89 (7.79-7.10)

9.6 (9.5-9.7) 9.6 (9.5-9.7) -10.3 (10.1-10.7) 16.0 (15.8-16.1) 16.0 (15.5-16.4)

7.91 (7.61-8.11) 8.28 (8.09-8.43) -8.26 (8.13-8.34) 8.69 (8.58-8.83) 8.16 (8.05-8.37)

WAD CN 96-hr LC50 mg/L (95% C.I.)

->1.14

Total CN 96-hr LC50 mg/L (95%) C.I.) 1,210 (1,060-1,380) 869 (860-1,110) >877

0.24 (0.23-0.26)

69.6 (54.7-88.5)

>0.50 >0.61 0.42 (0.36-8.34) 0.40 (0.36-0.43) 0.31 (0.30-0.32)

>541 >731 44.2 (38.2-51.0) 38.8 (25.0-60.2) 10.8 (9.96-13.1)

--

Source: Meyn et al., 1984

153

Chapter Four

M ud de r, B tzo & Sm tih

1 5 4

TABLE 4.5

Summary of Iron Complexed Cyanide Chronic Toxicity Test Conditions


The percentage breakdown of iron cyanide varied as follows: • • • •

0.04 ppm WAD cyanide/1.62 ppm total cyanide x 100 = 2.5% 0.11 ppm WAD cyanide/5.22 ppm total cyanide x 100 = 2.1% 0.15 ppm WAD cyanide/11.0 ppm total cyanide x 100 = 1.0% 0.19 ppm WAD cyanide/22.6 ppm total cyanide x 100 = 0.8% 2

A regression analysis of these values provides a coefficient of correlation (r ) of 0.98 and an equation useful in estimating the extent of photochemical breakdown of iron cyanides. The percentage breakdown although minimal, is about one to two orders of magnitude greater than the free cyanide produced through the chemical dissociation noted above. As shown through various toxicity tests, both ferrous and ferric cyanide are relatively non-toxic to sensitive fish species at concentrations of about 25 mg/L as cyanide, as long as the corresponding WAD cyanide concentration does not exceed toxic levels. As a result, the level of iron cyanides present in mining waters before and following treatment are generally not toxic. The toxicity of iron cyanides relates to the potential breakdown and release of free cyanide, not the compound directly. There is no evidence of accumulation of free cyanide in mining effluents due to the breakdown of iron cyanides. If free cyanide is formed it is rapidly removed through volatilisation, oxidation or complexation to the less toxic WAD cyanide form. Natural attenuation is a viable process for the removal of free and complexed cyanides from mining wastewaters. Since the total cyanide analysis is plagued with interferences at low levels, the appropriate approach is to utilize the WAD cyanide analysis as the method of choice for monitoring and permitting of the toxicologically important forms of cyanide. Ferric cyanide is used routinely as an anti-caking agent in table and road salt. The FDA limit in table salt for sodium ferrocyanide is 13 mg/kg. Iron cyanide is also used in fire retardants applied by aircraft to suppress forest fires.

4.6

CYANIDE T OXICITY TO BIRDS

4.6.1

The Most Sensitive Species to Oral Cyanide Exposure

The sensitivity to cyanide toxicity of different species of birds is not related to body size, but appears to be associated more with their diet (Wiemeyer et al., 1986). Birds that feed predominantly on flesh, such as owls and kestrels, are more sensitive to cyanide than species that feed mainly on plant material. The exception to this generalisation is the mallard duck, as judged by comparisons of its oral lethal dosages or LD50 (lethal dosage for 50% of the population) values with those of other birds (Eisler, 1991). A summary of the effects of cyanide on selected species of birds is presented in Table 4.6 (Hagelstein and Mudder, 1998b). The toxicity data indicated that the mallard duck, due to its high sensitivity and importance as a migratory waterfowl species, was the best model organism for use in the laboratory evaluation of the toxic effects of cyanide and in the establishment of safe exposure levels (Hagelstein and Mudder, 1998a).

155

Chapter Four


TABLE 4.6 Species Mallard, Anas platyrhynchos

Effects of Free Cyanide on Selected Species of Birds

DoseandOtherVariables Effects Single oral dose of NaCN 0.53 mg CN/kg body weight(BW), No deaths equivalent to 1 mg NaCN/kg BW 1.1 mg CN/kg BW(2.0 mg NaCN/kg BW) About 6% dead

1.27 mg CN/kg BW(2.4 mg NaCN/kg BW) About 33% dead 1.43 mg CN/kg BW(2.7 mg NaCN/kg BW) LD 50; 95% confidence interval (C.I.) of 2.2 and 3.2 mg NaCN/kg BW Single oral dose of 19.1 mg CN/kg BW, equivalent to 36 mg NaCN/kg BW

Up to 80% of the cyanide in blood was present as free cyanide and the remainder as bound cyanide

Single oral dose of 19.1 mg CN/kg BW, equivalent to 36 mg NaCN/kg BW

Average time to death was about 19 min and ranged between 8 and 41 min; cyanide residues postmortem, in mg CN/kg fresh weight (FW), were 6.7 in blood (Max. 21) and 0.6 in liver (Max. 2.8)

Rock dove, Columba livia

1.6 mg CN/kg BW, equivalent to 4.0 mg KCN/kg BW

Minimum lethal dose when administered intravenously or intramuscularly

Black vulture, Coragyps atratus

Single oral dose, as NaCN 1.6 mg CN/kg BW

No deaths in60 min. Mean and maximum blood CN concentrations, in mg/kg FW, were 0.7 and 0.9, respectively

Turkey vulture

Japanese quail, Coturnix japonica

2.4 mg CN/kg BW

Some deaths within 30 min. Mean blood CN residues in mg/kg FW, were 0.7 in dead birds vs. 1.2 in those surviving 60 min

2.54 mg CN/kg BW

Acute oral LD50; 95% C.I. of 2.3 and 2.8 mg CN/kg BW (4.4-5.3 mg NaCN/kg BW)

3.7 and 19.1 mg CN/kg BW

All dead within 16 min.; maximum blood CN levels postmortem were 2.1 mg/kg FW in the low dose group and 4.2 in the high dose group

Single oral dose, as NaCN 4.5 mg CN/kg BW

5.5 mg CN/kg BW


Acute oral LD50 for adult females; 95% C.I. of 3.1 and 6.5 mg CN/kg BW Acute oral LD 50 for adult males; 95% C.I. of 4.0 and 7.5 mg CN/kg BW

156


TABLE 4.6

Continued

Species

DoseandOtherVariables

Effects

American kestrel, Falco sparverius

2.12 mg CN/kg BW, as NaCN

Acute oral LD50; 95% C.I. of 1.6 and 2.8 mg CN/kg BW

Domestic chicken, Gallus domesticus

0.6 mg CN/kg BW, equivalent to 1.5 mg KCN/kg BW 0.78 mg CN/kg BW, as KCN

Lethal

1.3 mg CN/kg BW, as KCN Single oral dose, as NaCN 3.2 mg CN/kg BW, equivalent to 6.0 mg NaCN/kg BW 6.4 mg CN/kg BW 11.1 mg CN/kg BW 25.4 CN/kg BW

Dietary route Fed cassava diets containing 4, 37, 70, or 103 mg total cyanide per kilogram ration to day-old chicks for 8 weeks

Fed diets containing 135 mg HCN/kg Chicks, 20-day exposure Adults, 14-day exposure Eastern screech-owl, Otus asio

4.6 mg CN/kg BW, equivalent to 8.6 mg NaCN/kg BW

European starling, Sturnus vulgaris

9.0 mg CN/kg BW, as NaCN

Andean condor, Vultur gryphus

Single oral dose of 19.1 mg CN/kg BW (36 mg NaCN/kg BW)

Sublethal; thiocyanate excretion increased 10 times after 10 min and returned to normal levels after 3.5 h; the total thiocyanate collected was equivalent to 85% of the administered dose Lethal No deaths in 30 min; maximum CN levels, in mg/kg FW, were 1.1 in blood and 0.06 in liver Some deaths in 30 min; maximum CN levels, in mg/kg FW were 1.6 in blood and 0.12 in liver Acute oral LD50; 95% C.I. of 6.4 and 19.1 mg CN/kg BW Advanced signs of acutepoisoning; death probable within 30 min; maximum CN levels, in mg/kg FW, were 1.5 in blood and 0.6 in liver At all dietary levels, there was no significant effect on survival, growth, histology, haemoglobin, hematocrit, or lymphocyte number; however, serum thiocyanate levels increased in a dose-dependent manner Growth and food intake significantly depressed; plasma thiocyanate concentration increased Urinary excretion of thiocyanate increased 5 times in laying hens Acute oral LD50; 95% C.I. of 3.8 and 5.4 mg CN/kg BW Acute oral LD50; 95% C.I. of 4.8 and 17 mg CN/kg BW Blood sampled immediately after death contained 1.2 mg free CN per litre and 0.5 mg bound CN per litre

Source: Eisler, 1991

157

Chapter Four


The acute oral toxicity of free cyanide in tap water or in the actual mining solution, when expressed in terms of mg/kg for the mallard duck, was consistent with those literature values reported (Wiemeyer, 1986) for sodium cyanide. Figure 4.2 is the cyanide dose versus mortality relationship in the mallard duck illustrating an LD50 (lethal dosage resulting in 50% mortality) of about 150 to 200 mg/litre (Fletcher, 1986 and 1987). Assuming mallard ducks weigh 1.0 kg and the acute ingested dose is about 10 millilitres, the LD50 values were 2.5 and 3.2 mg/kg in tap water and in mine effluent, respectively. Reported acute oral toxicity values for sodium cyanide range from 4.0 mg/kg for the American kestrel to 36 mg/kg of body weight for the turkey vulture (Table 4.6). The data demonstrates that the mallard duck is one of the most sensitive species and provides an excellent model organism for toxicity evaluations due to its importance as a migratory waterfowl species. The absorption of cyanide from solution requires some time, as does the onset of its toxic effects. During this same time period, the bird's metabolism and detoxification of cyanide are initiated. It is interesting to note that if the total cyanide level exceeds 250 mg/kg in a solid material, as measured by the USEPA proposed Reactive Cyanide Method, that material is considered a hazardous waste under RCRA (USEPA, 1985c). As a result, several of these plant species could be classified as hazardous substances using such criteria. In contrast, the same 250 kg animal must rapidly ingest about 20 litres (5.2 gallons) of a solution containing 50 mg/l WAD cyanide to reach a lethal dose. Cattle, sheep, horses, and swine have a daily water intake ranging from about 40 to 65 litres. Therefore, exposure to properly contained and monitored manmade cyanide solutions may be of minor importance compared with the exposure of animals to natural sources of cyanide in plants. Summarized in Figure 4.3 are the laboratory and field data reflecting cyanide toxicity values in mallard ducks (Hagelstein and Mudder, 1998a). In general, the mallard duck tolerates slightly higher concentrations of cyanide when it is in solution with other effluent or process solution ions. Using this data, the LC1 (lethal concentration resulting in 1% mortality for free cyanide was estimated to be about 50 mg/L and is used as the level below which no mortality would be expected (Table 4.6)). The LC1 estimate is conservative, since it assumes a low no effect level, as well as instantaneous and complete absorption of oral cyanide without consideration of metabolic detoxification. 4.6.2

Summary of Bird Mortalities at U.S. Mines

In the U.S., the federal Fish and Wildlife Service has jurisdiction regarding the protection of birds under the 1991 Migratory Bird Treaty Act and the Endangered Species Act. The requirement and goal of these acts is "zero mortality" at mining operations. During the later part of the 1980's, as the number of mining operations in the southwestern U.S. increased sharply, the number of birdand mortalities also increased.at The problem related towhich the establishment of term new solution ponds tailings impoundments the mining operations, served as short resting areas for migratory birds and waterfowl in arid regions of the country.


158


FIGURE 4.2 WAD Cyanide Concentration - Mortality Relationship for the Mallard Duck

Source: Fletcher, 1986 and 1987

159

Chapter Four


1 6 0

FIGURE 4.3 Summary of Cyani de Toxicity to Mallard Ducks


Since the solution ponds and tailings impoundments were not covered and the cyanide levels were at process strength, mortalities resulted. During the period of 1984 through 1989, about 9,000 wildlife deaths were reported at 59 mining operations located in the states of Arizona, California, and Nevada. Nearly 70% of the total wildlife mortalities were reported at 53 mining operations in Nevada. A summary of the wildlife mortalities, by year and number of mining operations, is presented in Table 4.7 (Clark and Hothem, 1991). It shows that the number of mining operations increased from 1986-1990, while the number of mortalities per operation fell from 143 to 17 mortalities per mine as control strategies were developed. Based upon data collected from 1990-1995, bird mortalities declined from nearly 1,300 birds to 221, or about an 80% reduction (The Gold Institute, 1996). The magnitude of the mining-related bird deaths can be placed in perspective with other sources of bird mortality statistics. Research conducted by Dr. William Jackson, professor emeritus of biology, Bowling Green State University, revealed that ten billion birds are estimated to die annually in the U.S., with 80 million birds dying from flying into windows and one million per year colliding with radio and TV towers. About 12 million ducks are killed per year by hunters (The Gold Institute, 1996). The nearly 9,000 deaths in five years at 59 mining operations were less than one tenth of a percent (<0.10%) of the total number of birds taken by hunters in those three states in one year, which translated into about fifty-four million (54,000,000) birds over the same six year (19841989) period. The primary issue was that the hunters could legally shoot the birds with a license, while the mining operations were in violation of the Migratory Bird Treaty Act (GAO, 1991). Most of the other animal mortality at the mining operations included rodents, rabbits, amphibians and reptiles (Henny et al., 1994). In conjunction with the self-monitoring conducted by the various mining operations, the U.S. Fish and Wildlife Service conducted a "spot" tour of 17 mines in Nevada. A summary of their observations is presented in Table 4.8 (Henny et al., 1994). The pond or impoundment sizes examined ranged from 1 to 159 ha, while the WAD cyanide levels ranged from 8.4 to 216 mg/L. Although mortality was noted in the ponds containing 81 and 62 mg/L WAD cyanide, none were noted at the lower WAD cyanide levels. Live birds were observed at all of the operations except at the mines exhibiting the three highest WAD cyanide levels in their ponds. Some of the solutions had been detoxified with hydrogen peroxide or by other means prior to their disposal in the ponds. No mortalities were noted at WAD cyanide levels below 59 mg/L. With the implementation of proven control strategies of hazing, stretch nets, and floating plastic balls over impoundments, as well as treatment technologies, the issue of bird mortality has been nearly eliminated and is not generally considered a major environmental problem at mining operations.

161

Chapter Four


TABLE 4.7

Wildlife Mortalities Reported by Nevada Mine Operators

Year

Total Mortalities

Number of Mines Reporting

Mortalities Per Mine

1986

1,426

10

143

1987

1,488

32

47

1988

2,346

50

47

1989

1,955

57

34

1990

1,643

95

17

Source: Clark and Hothem, 1991


162


TABLE 4.8

WAD Cyanide in Tailings Ponds

Discharge Pipe Mine No.

Pond Size (ha)

Dead Birds

9 4 2 10 13 12 5

100 1 5 15 80 100 80

14 8 15 3 7 16 1 11 6 17

10 150 70 1 5 70 40 20 40 70

WAD CN (mg/L)

pH

Detoxified

No No No Yes Yes No No

216 197 138 81 62 59 53

7.6 10.6 11.3 10.3 10.6 6.0 10.1

Yes No No No No Yes No

No No No No No No No No No No

52 48 34 30 26 23 20 18 18 8.4

10.7 10.7 10.8 NA 11.4 10.5 9.3 10.6 9.4 10.9

No No ? Yes No ? Yes Yes Yes Yes

Source: Henny et al., 1994

163

Chapter Four


4.7

CYANIDE TOXICITY TO MAMMALS

The toxicity data for selected mammals is presented in Table 4.9. Exposure to cyanide experimentally via single oral LD50 doses ranging from about 4 to 24 mg HCN/kg body weight reflected a wide range of mammalian species including rats, mice, rabbits, dogs, and coyote. Livestock found dead near a cyanide disposal site had been drinking surface water runoff from the area that contained up to 365 mg/L free cyanide. No human cases of illness or death in the U.S. due to cyanide ingestion from water supplies are known (USEPA, 1985a). Only three possible accidental th deaths due to cyanide exposure in the mining industry in North America were noted during the 20 century. In Table 4.9 are the various physiological systems and mammalian animals tested with similar toxic target organ responses to inhaled cyanide (Hagelstein and Mudder, 1998a). The acute health effects of inhaling cyanide are respiratory failure and death at concentrations ranging from 100 to 1,000 ppm. Intermediate and chronic effects at lowest observable effect levels ranged from 1 to 100 ppm HCN. The major target organs include the respiratory, neurological, cardiovascular and thyroid functions. The airborne presence of cyanide can be detected by most humans at concentrations around 1.0 ppm HCN, as a bitter almond-like sensation to the olfactory o system and has a vapour pressure of 630 mm Hg at 20 C (Eisler, 1991). Some humans (20 to 40%) are genetically insensitive to the odor of cyanide. The concentration of HCN immediately hazardous to human health is 50 ppm, the occupational short-term (15-minute) exposure limit recommended is 5.0 ppm (NIOSH, 1994). As a chemical asphyxiant, cyanide's oxygen depleting effects are most critical to the brain and heart. A combination of rapid breathing, convulsions, and lactate acidosis leading to unconsciousness are symptoms of acute cyanide poisoning. Skin surfaces, especially abraded sites, wounds, and mucous membranes are penetrable, permeable surfaces for cyanide uptake and its acutely toxic effects. In one case, liquid HCN ran over the hand of a worker wearing a respirator. He collapsed into unconsciousness in five minutes, but ultimately recovered (USEPA, 1985a). Mammalian deaths were recorded at dermal applications between 2.3 mg/ HCN/kg body weight for abraded skin and 100 mg HCN/kg body weight for intact skin (Eisler, 1991). Chronic exposures to low levels of cyanide in the air, diet, or water are generally metabolised, excreted and not accumulated. The detoxification rate for humans is <0.001 mg/kg body weight per minute (Ballantyne, 1987) which is affected by the individual's aerobic metabolic rate and capacity, by adequate nutritional intakes, and by concurrent exposures or health problems. Chronic high cyanide levels from dietary or tobacco smoke sources, increases blood plasma of thiocyanate mammals; by and dietary deficiencies in vitamin B12, iodine, and and goiters sulphurinamino acids,effects as wellare as exacerbated by low levels insufficient distribution of detoxifying enzymes such as rhodanese (Solomonson, 1981).


164


TABLE 4.9

Species&Dose Livestock >200 mg HCN/kg BW 15-20 kg Sudan grass per animal daily Dog 2 mg NaCN/kg BW

24 mg NaCN/kg BW 150 mg NaCN/kg BW

Coyote 4.1 mg NaCN/kg BW 8 mg NaCN/kg BW African Giant Rat 1,000 mg KCN/kg BW

Mouse 8.5 mg KCN/kg BW

3.4 mgmg CN/kg BWdrinking water 1,000 KCN/L Rabbit 2.5 mg HCN/kg BW 5.1 mg NaCN/kg BW 5.8 mg KCN/kg BW 12.8 mg NaCN/kg BW 20 mg of acrylonitrile Lab White Rat 3.6-4.2 mg HCN/kg BW 5.1-5.7 mg NaCN/kg BW 8 mg CN/kg BW 21 mg CN/kg BW 750 mg CN/kg BW

3,778 mg HCN/m3 151-173 mg HCN/m3 Domestic Pig 96 mg CN/kg ration

Cyanide To xicity to Ma mmals

ExposureVariables&Effects

References

Lethal Lethal to 13 cattle, 21 sick

Bapat and Abhyanker, 1984

Recovery in 30 min., no adverse effects in 15 months Lethal, single oral dose Exposure 30 days in diet, no measurable effect on food consumption, blood chemistry, behaviour or organ histology

USEPA, 1985a

LD50, single oral dose Immobilized in 9 min., death within 41 min.

Sterner, 1979 Sterner, 1979

Christel et al., 1977

USEPA, 1985a

Exposure 12 weeks to weanlings, adverse effects Tewe, 1988 on growth when cassava peel exceeds 7.8% of the ration LD50, single oral dose

USEPA, 1989

LD50, single oral dose Exposure 40 days, inhibition of cytochrome oxidase activity

USEPA, Buzaleh, 1989 1989

LD50, single oral dose LD50, single oral dose LD50, single oral dose Lethal in 22 mins. Ballantyne, 1987 Inhalation HCN from combustion, lethal in 12-16 Yamamoto et al., 1979 mins. LD50, single oral dose LD50, single oral dose Exposure 21 days, liver normal Exposure 21 days, increased liver weight Exposure 8 weeks, diet KCN, no measurable effects on growth Inhalation, 10 sec., LC50 Inhalation, 30-60 min., LC50

Exposure 72 days, cassava peel, no effect on food consumption or protein metabolism

165

Ballantyne, 1987 USEPA, 1989 USEPA, 1989 Tewe and Maner, 1985 Ballantyne, 1987 Ballantyne, 1987

Tewe and Pessu, 1982

Chapter Four


Of primary concern for wildlife and stock animals is their exposure through ingestion of cyanogenic or cyanide-producing plants, such as bamboo, sorghum, certain fruits, lima beans and cassava. Neuropathies associated with cyanide intoxication such as due to cassava ingestion, in humans and domestic animals include nerve deafness, optic atrophy, and ataxia as well as inflammations of mucous membranes. The detoxification product thiocyanate can cause goiter by inhibiting the absorption of iodine (Way, 1981). A summary of the potential cyanide production from different plant species is presented in Table 4.10. The recommended limits of cyanogenic plants are less than 200 mg/kg HCN in the diet of livestock animals (Table 4.9). Sorghum, cassava, bamboo and lima beans can produce up to 2,000 mg/kg of total cyanide following ingestion of the plant material (Eisler, 1991). Assuming consumption of 4.0 mg of HCN per kilogram of body weight by a range animal could be fatal (to the most sensitive animals), ingestion of only 1.0 kg of plant material capable of producing 1,000 mg HCN/kg would be necessary to cause death in a 250 kg animal. Poisoning of herbivorous ungulates is more prevalent under drought conditions, when these mammals become less selective in their choice of forage and dry growing conditions also enhance cyanogenic glycoside accumulations in certain plants (Towill et al., 1978). In general, cattle, sheep, horses and pigs, in that order, are most vulnerable to cyanogenic plants. Deer and elk have been observed to graze on forages that contain a high content of cyanogenic glycosides. However, cyanide poisoning has not been reported in these species (Towill et al., 1978). Management of cyanide-containing plants may effectively control potentially adverse mammalian exposures to dietary cyanide. Hay and silage should be properly cured in order to minimize cyanide content before feeding to livestock (Egekeze and Oehme, 1980). Selective breeding of plants with low cyanide content will help reduce livestock and wildlife poisoning, but the most advisable prevention method at present is to prohibit grazing on field where cyanogenic plants are present (Egekeze and Oehme, 1980) or eradicate the cyanogenic plants.


166


TABLE 4.10 Background Cyanide Concentrations in Selected Plants

Concentration Cyanogenic Plant Species Bamboo (Bambusa, Arundinaria, Dendrocalamus) Tip Stem Stargrass, Cynodon plectostachyus, whole Rose Family, Malus spp., Pyrus spp. Cassava, Manihot esculenta Bitter varieties Leaves Roots Dried roots Stem Mash Bark Total cyanide Free cyanide Peel Total cyanide Free cyanide

(mg/kg)

Max. 8,000 Max. 3,000 180 Max. 200

347-1,000 327-550 95-2,450 1,130 162 1,351 102 1,390 255

Pulp Total cyanide Free cyanide Sweet varieties Leaves Roots Dried roots Mash Lima Bean, Phaseolus lunatus United States Burma Puerto Rico Java Almond, Prunus amygdalus, nut Bitter Spicy Sweet Seeds, 4 species, Nigeria, whole, frequently consumed by humans Phaseolus sp. Vigna sp. Cajanus sp. Canavalia sp. Sorghum, Sorghum spp., young plant, whole

810 53 377-500 138 46-<100 81 100-170 2,100 3,000 3,120 280-2,500 86-98 22-54 381-1,093 285-1,223 208-953 285-953 Max. 2,500

Source: Eisler, 1991

167

Chapter Four


4.8

TOXICITY O F C YANIDE R ELATED COMPOUNDS

4.8.1

Introduction

As was noted previously, reactions of cyanide with sulphur compounds can yield thiocyanate (SCN ) and those with oxygen and water can result in the production of cyanate (OCN ), ammonia (NH3) and nitrate (NO3 ). The reaction of cyanide with chlorine and hypochlorite proceeds via cyanogen chloride (CNCl). Thus any, or all, of these compounds can be present in a water containing cyanide. The presence of cyanogen chloride is possible only if adequate pH control is not maintained during the destruction of cyanide by alkaline chlorination. Ammonia and nitrate in varying concentrations (<0.10 to 50 mg/L) are often present in cyanidation process waters as a result of the chemical decomposition of cyanide, cyanate and/or thiocyanate. Bacterial degradation of cyanide and thiocyanate yields ammonia as one of the byproducts, as does the hydrolysis of cyanate. The toxicity of the products from cyanide degradation in the environment is presented in Table 4.11. The lethal dosages to mammals and amphibians, via the oral and subcutaneous routes are tabulated for ammonia, ammonium carbonate, ammonium sulphate, ammonium thiosulphate, potassium thiocyanate, disodium thiosulphate, magnesium thiosulphate and sodium nitrate. The most sensitive organism to ingestion of these products is the frog whose LD 50 is 300 mg potassium thiocyanate/kg body weight. The most sensitive organism to subcutaneous exposure to these products is the guinea pig whose lowest published lethal dose was 150 mg potassium thiocyanate/kg body weight. In general, free cyanide exhibits greater toxicity than the products from cyanide degradation processes. Therefore, criteria which protect against cyanide toxicity should also be protective of these degradation products. The following sections provide additional toxicological information for the various cyanide related compounds. 4.8.2

Thiocyanate -

The 96-hour LC50 values for thiocyanate to fish ranges from about 50 to 200 mg/L as SCN (Speyer, 1981; Speyer and Raymond, 1984 and Parker and Doe, 1983). There are several unusual aspects of thiocyanate toxicity reported in the literature. Some investigators have observed a phenomenon that could be referred to as "sudden death syndrome" in which fish which had been exposed to and survived relatively low levels of thiocyanate for several days, quickly died when subjected to a short (15 to 30 second) period of stress (forced swimming) (Heming et al., 1983). Two investigators reported a pH effect on toxicity but an opposing effect for The observations ondecrease which these conclusions are based are summarized Tables 4.12hardness. through 4.14. The dramatic in toxicity as hardness increases above 150inmg/L as CaCO3 is particularly noteworthy, due to the elevated hardness of many mining waters. The variability in response of fish exposed to thiocyanate results from environmental conditions such as temperature, dissolved oxygen, pH and water hardness.


168


TABLE 4.11 Toxicological Data for Cyanide Related Compounds

Organism Rat Frog

Domestic Animal GuineaPig Mouse GuineaPig Frog Rabbit Human GuineaPig Pigeon Rabbit Frog Mouse Rat Rabbit

Compound ammonia ammonium carbonate ammonium sulphate ammonium thiocyanate ammonium thiocyanate ammonium thiosulphate potassium thiocyanate potassium thiocyanate potassium thiocyanate

Dosage 350 mg/kg 250 mg/kg

potassium thiocyanate potassium thiocyanate disodium thiosulphate disodium thiosulphate magnesium thiosulphate sodiumnitrate Sodiumnitrate

Route oral sub-cutaneous

Effect LD50 Lowest published lethal dose

3,500mg/kg

oral

Lowestpublishedlethaldose

600mg/kg

oral


330mg/kg

oral


1,098mg/kg

oral

LD50

300mg/kg

oral


500mg/kg

oral


428mg/kg

oral


150 mg/kg

sub-cutaneous

Lowest published lethal dose

500 mg/kg

sub-cutaneous

Lowest published lethal dose

4,000mg/kg

sub-cutaneous

LD50

6,000mg/kg

sub-cutaneous

LD50

850mg/kg

sub-cutaneous

LD50

3,236mg/kg 2,680mg/kg

oral oral

LD50 LD50

Source: NIOSH, 1986

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TABLE 4.12 Toxicity of Thiocyanate to Fish

Com pound

TestOr ganism

NH4SCN (1)

MosquitoFish

KSCN (1)

MosquitoFish

Thiocyanate (mg/L) 43 87 320 694 116 77

pH

770 NaSCN (2) SCN- (3)

Rainbow Trout Rainbow Trout -Unstressed -Stressed for 30 seconds (forced swimming) after 96-hour exposure

360 <10

---

---

Exposure (hours) 144 ---48 144

--

--

60

7.4to7.9

--

--

--

Effect

All fish died LC50 96 hrs LC50 48 hrs LC50 24 hrs Fish survived but died within 2 to 7 days on return to clean water All fish died LC 50 96 hrs No acutely toxic effect

--

--

96

>100

--

--

96

All fish died

>8

--

--

--

100% mortality within 45 minutes

Notes: Source: Doudoroff, 1976 Source: Speyer, 1981 Source: Heming et al., 1983


Temp. (oC) 16 to23

170


TABLE 4.13 Toxicity of Thiocyanate to Fish Effect of pH, Hardness and Temperature

Compound

TestSpecies

pH

- (3)

Rainbow Trout

5.0 7.5 8.5

- (3)

Rainbow Trout

SCN

SCN

- (4)

SCN

Rainbow Trout

Hardness mg/L Ca CO 3

Temp. °C

22 22 22

96hourLC 50 mg/LSCN (1) Unstressed Stressed

15 15 15

144 233 203

-144 145

to 78 8to7

20 50

15 15

238 227

-164

8 8to to7 7

100 250

15 15

192 132

-97

6.0 6.0 8.0 8.0

80 80 80 80

5 12 5 12

252 179 217 263

(2)

196 (2) 174 -(2) 240

Notes: (1) Fish were stressed by a 15-second chase with a dip net. (2) Fish were also scooped up in the dip net, then returned to the tank. (3) Source: Parker and Doe, 1983 (4) Source: Speyer and Raymond, 1984; fish weights 0.36 to 0.73 grams; 10 fish per replicate; average of 6 replicates.

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TABLE 4.14 Toxicity of Thiocyanate to Fish Effect of Hardness at pH 8.0 and Temperature 12°C

Thiocyanate Concentration mg/L SCN 275 275 275 275 275 275 275

Hardness mg/L as CaCO3 25 50 75 150 250 275 350

Notes: (1) Estimated from Figure 3 of Speyer and Raymond, 1984 (2) Test species were fingerling rainbow trout.


172

(1,2)

Mortality After 96 Hours 65% 35% 20% 10% 8% 7% 5%


The chronic toxicity of thiocyanate to rainbow trout and the fathead minnow were investigated by Lanno (1990). In this work, these organisms were exposed to varying concentrations of both thiocyanate and cyanide to investigate mortality, thyroid function, reproduction, growth and spawning habits. The rainbow trout were exposed to thiocyanate levels ranging from 0 to 153 mg/L for a period of 16 weeks. Results for the rainbow trout study are shown in Table 4.15. As indicated, 100% mortality occurred after 16 weeks with a thiocyanate concentration of 153 mg/L, about 50% mortality occurred at 115 mg/L thiocyanate and mortalities at 35 to 77 mg/L thiocyanate were minimal. Lanno (1990) noted the occurrence of “sudden death syndrome” at the higher levels of thiocyanate following periods of stress, similar to that noted by Heming et al. (1983). With the fathead minnows, thiocyanate concentrations ranging from 0 to 36 mg/L were used over a period of about 225 days. As shown in Table 4.16, the LC50 value for thiocyanate to fathead minnows was in the range of about 18 to 36 mg/L after 225 days of exposure. From this testwork, the estimated 144-hour LC 50 value for thiocyanate to fathead minnows was 170 mg/L. No evidence of “sudden death syndrome” was noted with the fathead minnow testing. Thiocyanate is also toxic to the water flea ( Daphnia magna) with 48-hour LC50 values ranging from 0.55 to 33 mg/L. The highest level of toxicity was observed at low pH and high temperature (Watson and Maly, 1987). 4.8.3

Cyanate

Recent studies have shown that cyanate is more toxic to fish than thiocyanate. Toxicity decreased with increasing pH over the range 5.0 to 8.0 and with increasing temperature over the range 5°C to 12°C (Table 4.17). Speyer and Raymond (1984) reported a dramatic decrease in toxicity at hardness levels of 100 mg/L and higher (Table 4.18), an observation that was later confirmed (Lee, 1985). On the other hand, others attempting to verify the effect found instead a modest increase in toxicity in proceeding from a hardness of 20 mg/L to a hardness of 100 mg/L or above (Table 4.19). The possibility of a protective effect arising from the hardness of the receiving water is obviously a matter of concern both to the gold mining industry and to regulatory agencies. This relationship is also important since most of the existing cyanide removal processes result in an increase in cyanate concentration. It should be noted that in both cases where a hardness effect was observed, the elevated-hardness water used in preparing the test solutions also contained significant amounts of chloride. It has been noted by some researchers that cyanate toxicity increases as the concentration of chloride increases. When cyanate and thiocyanate were combined in ratios based upon their respective LC 50 concentrations, the resulting mortality was found to be relatively constant. the proportion of thiocyanate increases, so did the susceptibility to stress. The data are shown As in Table 4.20.

173

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TABLE 4.15 Chronic Thiocyanate Toxicity Studies for Rainbow Trout

Thiocyanate Concentration (mg/L) 0 35 77 115 153

Mortality After 16 Weeks of Exposure 5% to 1% 2% 16% 53% 100%

Source: Lanno, 1990

TABLE 4.16 Chronic Thiocyanate Toxicity Studies for Fathead Minnows Thiocyanate Concentration (mg/L) 0 1 7 17 33

Mortality After 225 Days of Exposure 0% 0% 3% 30% 63%

Source: Lanno, 1990


174


TABLE 4.17 Toxicity of Cyanate to Fish Effect of pH and Temperature

Compound - (3) OCN OCN

- (4)

TestSpecies RainbowTrout Rainbow Trout

pH 6.7to8.7 6.7 to 7.4 8 8 6 6

Hardness mg/L Temp. Ca CO 3 °C 20 15 20 15 80 12 80 5 80 12 80 5

Dissolved Oxygen mg/L 10 10 10.6 10.6 10.6 10.6

96hour LC50 mg/LOCN (1) 36 (1) 54 (1,2) 82 (1,2) 44 (1,2) 33 (1,2) 13

-

Notes: (1) Unchanged by stress; stress consisted of a 15-second chase with a dip net at the end of the test. (2) Fish were also scooped up in the dip net then returned to the tank. (3) Source: Parker and Doe, 1983. (4) Source: Speyer and Raymond, 1984; fish weights 0.4 to 1.3 grams; 30 fish per pool; average of 2 to 3 pools.

175

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TABLE 4.18 Toxicity of Cyanate to Fish o Effect of Hardness at pH 8.0 and Temperature 12 C

Cyanate mg/L as OCN 85

Hardness mg/L as CaCO3 25

85 85 85 85 85 85 85 85

50 75 100 125 150 200 275 350

Mortalityat96 Hours 100% 100% 60% 20% 10% 5% 0% 0% 0%

Source: Speyer and Raymond, 1984

TABLE 4.19 Effect of Hardness on Cyanate Toxicity

Reference (1) (2) (3) (4)

pH 6.7 8.7 to 6 8 7.3 7.3 5 7 8.5 8 to 7 8 to 7 8 to 7 8 to 7

Temperature °C 15 5 12 --15 15 15 15 15 15 15

20 50 100 250

Notes: (1) Source: Parker and Doe, 1983 (2) Source: Speyer and Raymond, 1984 (3) Source: Lee, 1985 (4) Source: Vaughan et al., 1985


Hardness mg/LCaCO 20 75 75 131 200 20 20 20

176

96hourLC 50 mg/L OCN

3

36 28 to 15.2 80.6 18.9 94.9 12.5 to 8.3 38.9 41.1 30.7 26.5 24.4 24.4


TABLE 4.20 Toxicity of Cyanate and Thioc yanate Mixtures

Cyanate in Mixture mg/L OCN % of LC50 75 56.2 37.5 18.7 0 0

100% 75% 50% 25% 0% 0%

Thiocyanate in Mixture mg/L SCN % of LC50 0 68.7 137.5 206.2 275 0

0% 25% 50% 75% 100% 0%

Mortality at Mortality at 96hours 96hrs Unstressed Stressed 45% 45% 44% 44% 41% 56% 48% 52% 0% 43% 0% 0%

Source: Speyer and Raymond, 1984

Notes: (1) Stress condition was 15-second chase with a dip net, then fish scooped up in the net and immediately returned to the tank. (2) Test conditions included a pH of 8.0, a temperature of 12°C and a hardness of 75 mg/L as CaCO3.

177

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4.8.4

Ammonia

The acute toxicity of ammonia is primarily due to the un-ionised free ammonia molecule (NH 3). -10 -10 The equilibrium constant for ammonia ionisation is 5.8 10 moles/L at 25°C and 1.14 x 10 moles/L at 5°C, so that the toxicity, criteria and concentration of NH 3 are all pH and temperature-dependent. The magnitude of the dependence is illustrated in Figure 4.4. At a pH of 9.24, NH3 comprises 50% of the total ammonia concentration and this proportion increases with increasing pH. A breakdown of the percent un-ionised ammonia (i.e., NH3) with respect to pH and temperature is presented in Table 4.21. Table 4.22 summarizes the data on the toxic effect of ammonia on a number of fish species. The LC50 values reported here are for the calculated concentration of un-ionised ammonia. As a result, ammonia criteria are based upon the total ammonia concentration allowable (at a specific pH and temperature) which will not yield a toxic level of free ammonia. The level of allowable free ammonia generally ranges from 0.02 to 0.04 mg/L. Factors which affect ammonia toxicity include dissolved oxygen concentration, temperature, pH, previous acclimation to ammonia, intermittent exposure, carbon dioxide concentration, salinity and the presence of other toxicants (USEPA, 1985b). For example, ammonia and cyanide have been reported to be more toxic in combination than when present individually (Scott and Ingles, 1987 and Wuhrmann and Woker, 1988). Acutely toxic concentrations of ammonia to fishes may cause loss of equilibrium, hyperexcitability, increased breathing, cardiac output, oxygen uptake, and in extreme cases, convulsions, coma and death. At lower concentrations, ammonia has various reported effects on fishes including a reduction in hatching success, reduction in growth rate and development, and pathologic changes in tissues of gills, livers, and kidneys. Among 29 fish species, the 96-hour LC50 values ranged from 0.083 to 1.09 mg/L for salmonids and from 0.14 to 4.60 mg/L for nonsalmonids. Chronic bioassays with nine freshwater fishes showed effects ranging from 0.0017 to 0.612 mg/L ammonia (USEPA, 1985b). Ammonia was reported to be acutely toxic to freshwater organisms at concentrations (uncorrected for pH) ranging from 0.53 to 22.8 mg/L NH 3 for 19 invertebrate species. Chronic bioassays with daphnids showed effects at concentrations ranging from 0.304 to 1.2 mg/L ammonia (USEPA, 1985b). Examination of available ammonia toxicity values among freshwater organisms showed that invertebrates are generally more tolerant than fishes, a notable exception being the fingernail clam (USEPA, 1985b). Data for freshwater phytoplankton and vascular plant toxicity to ammonia indicate that freshwater plant species are appreciably more tolerant to ammonia than are invertebrates or fishes. The protective ammonia criterion appropriate the protection of aquatic animals will therefore be sufficiently of plant life (USEPA,for 1985b).


178


FIGURE 4.4 Percentage of Total Ammonia as NH 3 as a Function of pH and Temperature


179

Chapter Four


TABLE 4.21 Percent NH 3 in Aqueous Ammonia Solutions for 0°C to 30°C and pH 6 to 10

Temp. (°C) 0 1 2 3 4 5 6 7 8 9 10 11 12

6.0 0.0083 0.0090 0.0098 0.0106 0.0115 0.0125 0.0136 0.0147 0.0159 0.0172 0.0186 0.0201 0.0218

6.5 0.0261 0.0284 0.0309 0.0336 0.0364 0.0395 0.0429 0.0464 0.0503 0.0544 0.0589 0.0637 0.0688

7.0 0.0826 0.0898 0.0977 0.106 0.115 0.125 0.134 0.147 0.159 0.172 0.186 0.201 0.217

7.5 0.261 0.284 0.308 0.335 0.363 0.394 0.427 0.462 0.501 0.542 0.586 0.633 0.684

pH 8.0 0.820 0.891 0.968 1.05 1.14 1.23 1-35 1.45 1.57 1.69 1.83 1.97 2.13

8.5 2.55 2.77 3.00 3.25 3.52 3.80 4.11 4.44 4.79 5.16 5.56 5.99 6.44

9.0 7.64 8.25 8.90 9.60 10.3 11.1 11.9 12.8 13.7 14.7 15.7 16.8 17.9

9.5 20.7 22.1 23.6 25.1 26.7 28.3 30.0 31.7 33.5 35.3 37.1 38.9 40.8

10.0 45.3 47.3 49.4 51.5 53.5 55.6 57.6 59.5 61.4 63.3 65.1 66.8 68.5

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

0.0235 0.0254 0.0274 0.0295 0.0318 0.0343 0.0369 0.0397 0.0427 0.0459 0.0493 0.0530 0.0569 0.0610 0.0654 0.0701 0.0752 0.0805

0.0743 0.0802 0.0865 0.0933 0.101 0.108 0.117 0.125 0.135 0.145 0.156 0.167 0.180 0.193 0.207 0.221 0.237 0.254

0.235 0.253 0.273 0.294 0.317 0.342 0.368 0.396 0.425 0.457 0.491 0.527 0.566 0.607 0.651 0.697 0.747 0.799

0.738 0.796 0.859 0.925 0.996 1.07 1.15 1.24 1.33 1.43 1.54 1.65 1.77 1.89 2.03 2.17 2.32 2.48

2.30 2.48 2.67 2.87 3.08 3.31 3.56 3.82 4.10 4.39 4.70 5.03 5.38 5.75 6.15 6.56 7.00 7.46

6.92 7.43 7.97 8.54 9.14 9.78 10.5 11.2 11.9 12.7 13.5 14.4 15.3 16.2 17.2 18.2 19.2 20.3

19.0 20.2 21.5 22.8 24.1 25.5 27.0 28.4 29.9 31.5 33.0 34.6 36.3 37.9 39.6 41.2 42.9 44.6

42.6 44.5 46.4 48.3 50.2 52.0 53.9 55.7 57.5 59.2 60.9 62.6 64.3 65.9 67.4 68.9 70.4 71.8

70.2 71.7 73.3 74.7 76.1 77.4 78.7 79.9 81.0 82.1 83.2 84.1 85.1 85.9 86.8 87.5 88.3 89.0

Source: USEPA, 1985b


180


TABLE 4.22 Toxicity of Un-Ionised Ammonia to Fish Spec ies

Species Rainbow Trout:

Adult Fry

Bluegill Bass Coho Salmon Channel Catfish

ExposurT e ime (hours) 24 96 24 96 96 9625 96

LC50 mg/L NH3-N 0.41 to 0.60 0.32 2.29 0.40 to 1.30 1.20 0.72 to 0.452.92 1.50 to 310


181

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It will be noted that effect of pH on ammonia toxicity is opposite to that of cyanide, so that a solution with equivalent levels of the two compounds at the extremes of pH would retain an intermediate toxicity. However, there are some data to indicate that the combined effect of ammonia and cyanide is greater that would be assumed on the basis of their individual toxicities (Scott and Ingles, 1987). A more complete summary of ammonia toxicity is available in the USEPA water quality criteria document (USEPA, 1985b). 4.8.5

Nitrate -

Nitrate (NO3 ) is a relatively non-toxic compound at the concentrations typically observed in mining solutions and is usually not of concern relative to wildlife, waterfowl or aquatic organism toxicity. The primary concern with nitrate is generally related to drinking waters where elevated nitrate concentrations can be harmful to humans, particularly young children and infants. In addition, nitrate is a biological nutrient and in some cases can lead to accelerated algae growth in waters, thereby consuming dissolved oxygen and impairing the ability of fish to survive. Nitrate is a relatively stable compound in surface waters and because of this, its removal from waters discharged to the environment is often required. In quantities normally found in food or feed, nitrates become toxic only under conditions in which they are reduced to nitrite, which reacts with haemoglobin in infants under three months of age. Under certain circumstances, nitrate can be reduced to nitrite in the gastrointestinal tract, which then reaches the bloodstream and reacts directly with haemoglobin to product methemoglobin, with consequent impairment of oxygen transport. •

The USEPA water quality criteria for various nitrogen compounds concluded the following (USEPA, 1976):

•

Levels of nitrate at or below 90 mg/L would have no adverse effects on warm water fish.

•

Nitrite at or below 5 mg/L should be protective of most warm water fishes.

•

Nitrite at or below 0.06 mg/L (which is ten times lower than the 96-hour LC50 values for cutthroat trout) should be protective of salmonid fishes.


182


4.9

TOXICITY OF ACTUAL MINE WATERS

The actual toxicity of waters associated with mining operations is complicated by the fact that these waters have widely varying concentrations of cyanide, thiocyanate, ammonia and nitrate. And as previously discussed, the water temperature, pH, hardness and salinity can further affect the toxicity of these compounds. Table 4.23 presents LT50 toxicity values for various gold mine effluents. The tests were conducted using actual effluents and represent end-of-pipe results. The tests do not take into account the assimilative capacity of the receiving system. In one or two cases the toxicity does not appear to be due to any of the analysed components, indicating that an additional toxic component may be present. In contrast, there are a few cases where the analyses indicate toxic levels of cyanide, but the effluent nevertheless did not exhibit significant acute toxicity. These data illustrate the complexity of cyanidation process solutions and the need for careful numerical and biological evaluation of effluents srcinating from water treatment processes rather than reliance of laboratory studies based on tests with single-solution matrices. To illustrate the success of utilizing site-specific water quality criteria at protecting aquatic ecosystems, three case studies will be considered. The first of these is the Homestake mine in Lead, South Dakota, USA where effluent from a biological treatment plant is discharged into an adjacent trout fishery (Whitlock and Mudder, 1998 and Mudder, 1998a). Effluent from this plant comprises approximately one-half of the total stream flow and site-specific water quality criteria were developed to protect aquatic organisms in this stream. As shown in Table 4.24, the sitespecific criteria included limits for cyanide, ammonia and several metals. Also shown in Table 4.24 are the quality of effluent from the treatment plant, the in-stream water quality in the stream mixing zone and the in-stream water quality downstream of the mixing zone. The second example is from the Golden Cross Mine in New Zealand where treated mine water is discharged into a local trout fishery (Russell et al., 2000). Site-specific in-stream standards were developed for this stream prior to mine development, and comprehensive in-stream biomonitoring was conducted throughout the mine operation and following closure of the mine in 1998. Table 4.25 shows the in-stream chronic water quality standards developed specifically for this site along with actual in-stream water quality. Results from the bio-monitoring programme have indicated no measurable effects on the fish or other aquatic organisms in the river as a result of the mine water discharge. The third example is from the Martha Hill Mine in New Zealand which is located adjacent to the city of Waihi on the North Island. Due to the high amount of precipitation in this area (>2 meters per year) the mine must treat and discharge decant water from the tailings impoundment. An in-stream standard of 0.09 mg/L for WAD cyanide was considered at the site for protection of the receiving stream, which in this case is the Ohinemuri River. The average WAD cyanide concentration treated water the mine wasstudies, 0.04 mg/L 1989tothrough 1996 (Environment in Australia, 1998).discharged Through by biomonitoring trout from continue be recorded adjacent to the mine, both within and outside of the mixing zone where treated water is discharged.

183

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TABLE 4.23 Toxicity of Various Gold Mill Effluents to Rainbow Trout Fry

WAD Cyanide mg/L 0.02 0.01 0.42 1.35 3.1

Cu mg/L

Zn mg/L

As mg/L

Pb mg/L

1 2 3 4 5

Total Cyanide mg/L 0.12 0.14 0.87 -3.3

0.03 0.12 0.1 2.8 3.65

0.005 40.0 0.06 0.34 3.04

1.8 0.005 0.001 0.05 0.001

--0.10 0.02 --

> 96 hrs <15 hrs >96hrs >0.5 hrs 24 hrs

6 7 8

21.0 25 61

16.2 18.5 42.0

0.32 8.15 16.5

2.54 2.64 15.6

0.012 0.002 0.01

0.02 -0.04

<10min <15 min min 5 <

Effluent

Toxicity (1) LT50


Notes: (1) LT50 - The length of exposure time in the effluent by which 50% of the test fish had died. (2) Not acutely lethal.


184

(2)

(2)


TABLE 4.24 Water Quality Associated with the Homestake Lead, South Dakota, USA Mine Water Treatment Plant Constituent

Total Cyanide WAD Cyanide Hardness Ammonia Arsenic Cadmium Copper Iron Lead Mercury Nickel Silver Zinc

Treatment Plant Discharge 0.19 0.02 960 0.10 0.04 0.001 0.032 0.28 0.05 -0.02 0.001 0.009

In-Stream In-Stream Water Quality Water Quality in Mixing Zone Downstream of Mixing Zone 0.06 0.04 0.02 0.02 510 470 --0.025 0.025 0.003 0.003 0.027 0.025 1.19 1.01 0.025 0.0002 0.02 0.003 0.019

0.028 0.0001 0.03 0.003 0.019

Site-Specific In-Stream Water Quality Standard -0.08 -0.23 0.30 to -0.010 0.080 -0.070 0.0008 -0.020 --

Source: Whitlock and Mudder, 1998, Mudder, 1998a and SDDENR, 1999

Notes: (1) All concentrations (total) in mg/L.

185

Chapter Four


TABLE 4.25 Water Quality Associated with the Golden Cross, New Zealand Mine Water Treatment Plant Constituent

Treatment Plant Discharge (2) Limitation

WAD Cyanide Hardness Ammonia Arsenic Cadmium Chromium Copper Iron

Treatment Plant Discharge

0.47 -11 0.25 0.015 0.25 0.081 1

0.07 433 4 0.06 0.003 0.01 0.038 0.17

0.029 0.5 0.0043 1.04 0.014 0.05 0.355

0.004 0.06 0.0021 0.015 0.004 0.012 0.010

Lead Manganese Mercury Nickel Silver Selenium Zinc Source: Russell et al., 2000

Notes: (1) All concentrations in mg/L. (2) Four-day average limitation.


186

Chronic In-Stream Water Quality Standard 0.1 -2.2 0.25 0.002 0.13 0.013 1

Actual In-Stream Water Quality

0.005 1 0.00086 0.183 0.002 0.023 0.060

0.003 0.10 0.0001 0.009 0.001 0.002 0.009

0.01 84 0.6 0.07 0.002 0.01 0.004 0.18

Water Management and Discharge Strategies

The purpose of presenting these three case examples is to demonstrate how site-specific standards for WAD cyanide have been established toprotect sensitive aquatic ecosystems. In all of these cases, various studies were conducted to determine appropriate levels of protection for the receiving streams, and these studies were followed by biomonitoring of the receiving streams to assess adverse impacts due to the discharges. In all cases, no adverse impacts on theaquatic ecosystems were observed. The in-stream WAD cyanide standards in these cases ranged from 0.04 to 0.10 mg/L, which were found to be adequate for the protection of the receiving streams. Another important aspect of establishing site-specific standards in these cases was the ability of these sites to consistently meet the regulated levels of WAD cyanide using well designed and operated water treatment plants. These levels of WAD cyanide could also be accurately analysed to ensure water discharged to the environment was protective of the ecosystems.

4.10

REFERENCES

Alabaster, J.S., Shurben, D.G. and Mallett, M.J, “The Acute Lethal Toxicity of Mixtures of Cyanide and Ammonia to Smelts of Salmon, Salmo salar at Low Concentrations of Dissolved Oxygen”, J. Fish Biol., 22:215-222, 1983. ATSDR, Agency for Toxic Substances and Disease Registry, “Toxicological Profile for Cyanide”, Prepared by Research Triangle Institute for the US Department of Health and Human Services, Atlanta, Georgia, August, 1995. AWWA, American Water Works Association, Water Quality and Treatment, Fourth Edition, McGraw-Hill, New York, 1990. Azcon-Bieto, Murillo, J.J. and Penuelas, J., "Cyanide-Resistant Respiration in Photosynthetic Organs of Freshwater Aquatic Plants", Plant Physiol., 84:701-706, 1987. Ballantyne, B., "Toxicology and Hazard Evaluation of Cyanide Fumigation Powders", Clin. Toxicol., 26:325-335, 1988. Ballantyne, B., “Toxicology of Cyanides”, Chapter 3 in Clinical and Experimental Toxicology of Cyanides, Edited by B. Ballantyne and T.C. Marrs, Wright Publishers, Bristol, England, 1987. Bapat, J.A. and Abhyankar, Y.N., "Cyanide Poisoning in Cattle Due to Feeding of Sorghum", Indian J. Anim. Sci., 54:577-578, 1984. Beyer, W.N., "Evaluating SoilContamination", U.S. Fish Wild. Service, Biol. Rep. 90(2), 1990. Biehl, M. “Cyanide Toxicosis”, Veterinary Professional Topics, University of Illinois at Urbana, Cooperative Extension Service, 10(3):5-6, 1984. Broderius, S.J., "Determination of Molecular Hydrocyanic Acid in Water and Studies on the Chemistry and Toxicity to Fish of Metal-Cyanide Complexes", Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 187 pp., 1973. Buzaleh, A.M., "An Oral Chronic Animal Model", Gen. Pharmacol., 20:323-327, 1989.

187

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Department of

Agriculture and Fisheries of Scotland, Freshwater Salmon Fish. Res., Ser. 33, 1964. Knowles, C.J., "Cyanide Utilization andDegradation by Microorganisms". In D. Evered and S. Harnett, Eds., Cyanide Compounds In Biology, Ciba Found. Symp., 140, John Wiley, Chichester, England, 1988. Lanno, R.P., “The Chronic Toxicity of Thiocyanate to Rainbow Trout and Fathead Minnow ”, Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1990. Leduc, G. and Pierce, R.C., “The Effects of Cyanides on Aquatic Organisms with Emphasis Upon Freshwater Fishes”, Natl. Res. Council Canada, Publication NRCC 19246, 1982. Leduc, G., “Cyanides in Water: Toxicological Significance”, In Aquatic Toxicology, Edited by L.J. Weber, Raven Press, New York, 1984. Lee, John, '"The Acute Toxicity of Cyanate to Trout", Memo Report, Aquatic Toxicity Unit, Waters Branch, Ontario Ministry of the Environment, July 2, 1985. Lesniak, J.A. and Ruby, S.M., "Histological and Quantitative Effects of Sublethal Cyanide Exposure on Oocyte Development in Rainbow Trout",

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“Tissue

Specific Bioenergetic Effects and Increased Enzymatic

Activities Following Acute Sublethal Peroral Exposure to Cyanide in the Mallard Duck ”, Toxic. Appl. Pharmacol., 1996. Russell, L., Botz, M., Miller, S. and Mudder, T.,“Life-Cycle Management of Waste at Golden Cross”, Mining Environmental Management, pp. 7-10, May, 2000. Schmidt, J., Simovic, L., and Shannon, E., "Natural Degradation of Cyanides in Gold Milling Effluents", Presented at the Cyanide and the Gold Mining Industry Seminar, January 22-23, 1981. Scott, J. and Ingles, J., State-of-the-Art of Processes for the Treatment of Gold Mill Effluents, Mining, Mineral, and Metallurgical Processes Division, Industrial Programs Branch, Environment Canada, Ottawa, Ontario, March, 1987. SDDENR, South Dakota Department of Environment and Natural Resources, Total Maximum Daily Load for Whitewood Creek Near Lead, South Dakota, 1999. Smatresk, N.J., "Chemoreflexive Responses to Hypoxia and NaCN in Longnose Gar: Evidence for Two Chemoreceptor Loci", Am J. Physiol., 251(1, Part 2):R116-R125, 1986. Smith, L.L. and Broderius, S.J., "Acute and Chronic Toxicity of HCN to Fish and Invertebrates", U.S. Environmental Protection Agency Report 600/3-79-009, 129 pages, 1979. Smith, L.L. and Broderius, S.J., "Acute Toxicity of Hydrogen Cyanide to Freshwater Fishes", Arch. Environ. Contam. Toxicol., 7:325-337, 1978. Smith, A. and Mudder, T., “An Environmental Perspective on Cyanide”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Solar Corporation, Warren, Pennsylvania, November, 1983.


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“The

Homestake Wastewater Treatment Process: Biological

Removal of Toxic Parameters from Cyanidation Wastewaters and Bioassay Effluent Evaluation”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, England, 1998. Wiemeyer, S., Hill, E., Carpenter, J., and Krynitsky, A., "Acute Oral Toxicity of Sodium Cyanide in Birds", Journal of Wildlife Diseases, Volume 22, Number 4, pp. 538-546, 1986. Wiley, R.W., "A Review of Sodium Cyanide for Use in Sampling Stream Fishes", N. Am. J. Fish. Manage., 4:249-256, 1984. World Resources Institute, “Cyanide Fishing: A Poison Tide on the Reef”, Washington D.C., 1998. Wuhrmann, K and Woker, H.,

“Contributions

to Fish Toxicology. II. Experimental

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192


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BIBLIOGRAPHY

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Gannon, D.J., "Toxicity of Cyanide Solution", in Cyanide and Gold Mining Industry, A technical Seminar, Environment Canada and Canadian Mineral Processors, Environment Canada, Ottawa, 1981. Gosselin, R.E., Hodge, H.G., Smith. R.P., and Gleason, M.H., Clinical Toxicology of Commercial Products -Acute Poisoning, 4th Edition, The Williams and Wilkins Co., Baltimore, Maryland, pp. 105-112, 1976. Guatelli, M.A., "The Toxicology of Cyanides", In: Methods of Forensic Science, Vol. 3, New York, Interscience., pp. 233-265, 1964. Heming, T.A., Thurston, R.V., Meyn, E.L., and Zajdel, R.K., "Acute Toxicity of Thiocyanate to Trout", Trans. Am. Fisheries Soc., 114, pp. 895-905, 1985. Hendrickson, T.N. and L.G. Drignaul, "Treatment of Complex Cyanide Compounds for Reuse and Disposal", Report No. EPA-R2-73-269, USEPA, 1973. Huiatt, J., Kerrigan, J., Olson, F., and Potter, G., Workshop of Cyanide from Mineral Processing, Utah Mining and Mineral Resources Research Institute, College of Mines and Mineral Industries, Salt Lake City, Utah, February, 1982. Kay, F. and McQuivey, R., "Migratory Birds and Toxic Mine Ponds: The Nevada Experience", Gold and Silver Recovery Innovations Phase IV, Randol International, Ltd., Sacramento, California, November, 1989. Kay, F., “Nevada Department of Wildlife’s Role: Past, Present, and Future”, Proceedings of the Nevada Wildlife Mining Workshop, Reno, Nevada, March, 1990. Kimball, G.L., L.L. Smith Jr., and S.J. Broderius, "Chronic Toxicity of Hydrogen Cyanide to the Bluegill", Trans. Am. Fish. Soc., 197: pp. 341-345, 1978. Knowles, C.J., "Microorganisms and Cyanide", Bacterial. Rev., 40: 652-680, 1976. Koenst, W.M., L.L. Smith Jr., and S.J. Broderius, "Effect of Chronic Exposure of Brook Trout to Sublethal Concentration of Hydrogen Cyanide", Environ. Sci. Technol. 11: pp. 883-887, 1977. Kovacs, T.G. and G. Leduc, "The Effects of Temperature on the Acute Toxicity of Cyanide to Rainbow Trout", Can. J. Fish. Aq. Sc., 1982. Kovacs, T.G. and G. Leduc, "The Effects of Temperature on the Chronic Toxicity of Cyanide to Rainbow Trout", Can. J. Fish Aq. Sc., 1982. Kovacs, T.G., "The Effect of Temperature on Cyanide Toxicity to Rainbow Trout (Salmo gairdneri) I. Acute Effects, Il. Chronic Effects", M.Sc. Thesis, Dept. Biol. Sci., Concordia University, Montreal, Quebec, 60 pp, 1979.

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Leduc, G. and K.S. Chan, "The Effects of Chronic Cyanide Poisoning on the Tolerance of Rainbow Trout to Varying Salinity", Proc. 10th Canadian Symp. Water Pollution Res. Canada 19L 110-125, 1975. Leduc, G., "Some Physiological and Biochemical Responses of Fish to Chronic Poisoning by Cyanide", Ph.D. Thesis, Department of Fish and Wildlife, Oregon State University, Corvallis, Oregon, 146 pp, 1966. Leduc, G., "The Role of Cyanide as an Ecological Stressing Factor to Fish", Recent Advances in Fish Toxicology, A Symposium, Tobb, R. (Editor), USEPA, Corvallis, Oregon. EPA-600/3-085, pp. 152-182, 1977. Leduc, G., Gravel, G., Seguin, L.R., Vincent, B., and Guibert, F., "The Use of Sodium Cyanide as a Fish Eradicant in Some Quebec Lakes", Nat Canada, 100, pp. 1-10, 1973. Leduc, G., Pierce, R.C., and McCracken, I.R., "The Effects of Cyanide on Aquatic Organisms With Emphasis Upon Freshwater Fishes", NRCC Associate Committee on Scientific Criteria for Environmental Quality, Report No. NRCC 19246, National Research Council of Canada, Ottawa, 1983. Lind, D.T., L.L. Smith Jr., and S.J. Broderius, "Chronic Effects of Hydrogen Cyanide on the Fathead Minnow", J. Water Pollut., Control Fed. 49: pp. 262-268, 1977. McCracken, I.R. and G. Leduc, "Allometric Growth Response of Exercised Rainbow Trout to Cyanide Poisoning", in Aquatic Toxicology, ASTM STP 707, Edited by J.G. Eaton, P.R. Parrish, and A.C. Hendricks, Am. Soc. Testing Material, Philadelphia, pp. 303-320, 1980. McKee, J.E. and Wolfe, H.W., "Water Quality Criteria", 2nd edition, Publ. 3A, The Resource Agency of California, State Water Quality Board, 1971. McKin, J.M., "Evaluation of Tests with Early Life Stages of Fish for Predicting Long-term Toxicity", J. Fish. Res. Board Canada, 34: 1148-1154, 1977. Monekosso, G.L. and J. Wilson, "Plasma Thiocyanate and Vitamin B12 in Nigerian Patients with Degenerative Neurological Disease", Lancet 1(7446): pp. 1062-1064, 1966. NMA, Nevada Mining Association, Nevada Department of Minerals and the Nevada Department of Wildlife, Proceedings of the Nevada Wildlife Mining Workshop, Reno, Nevada, March, 1990. Patrick, R., J. Cairns, and A. Scheier, "The Relative Sensitivity of Diatoms, Snails, and Fish to Twenty Common Constituents of Industrial Wastes", Prog. Fish Cult., Vol. 30, pp. 137-140, 1968. Ruffier, P.J., Boyle, W.C., and Kleinschmidt, J., "Short-Term Acute Bioassay to Evaluate Ammonia Toxicity and Effluent Standards", J. WPCF 53, No. 3, 1981.


196


Smith, L.L.Jr., Broderius, S.J., Oseid, D.M., Kimball, G.L., Koenst, W.N., and Lind, D.T., "Acute and Chronic Toxicity of HCH to Fish and Invertebrates", USEPA, EPA-600/3-70-009, 130 pp., 1979. Smith, L.L. Jr., S.J. Broderius, D.M. Oseid, G.L. Kimball, and W.M. Koenst, "Acute Toxicity of Hydrogen Cyanide to Freshwater Fishes", Arch. Environ. Contam. Toxicol., 7: pp. 325-337. 1978. Spear, P.A. and Pierce, P.A., "Copper in the Aquatic Environment", NRCC No. 16454, National Research Council of Canada, Ottawa., 1979. Speyer, M.R., "Some Effects of Chronic Combined Arsenic and Cyanide Poisoning on the Physiology of Rainbow Trout", M.Sc. Thesis, Dept. Biol. Sci., Concordia University Montreal, Quebec, 76 pp, 1975. Subcommittee on Heavy Metals, "Effects of Arsenic in the Canadian Environment", NRCC No. 15391, National Research Council of Canada, Ottawa, 1978. Subcommittee on Heavy Metals, "Effects of Nickel in the Canadian Environment", NRCC No. 18568, National Research Council of Canada, Ottawa, 1981. Szumski, D.S., Barton, D.A., Putnam, H.D. and Polta, R.C., "Evaluation of EPA Unionized Ammonia Toxicity Criteria", Journal WPCF, 54, No. 2, pp. 281-291, March 1982. Thurston, R.V., Russo, R.C., Fetterolf, C.M. Jr., Edsall, T.A. and Barber, Y.M. Jr., "A Review of the EPA Red Book: Quality Criteria for Water", Water Quality Section, Am. Fish. Soc., Bethesda, Maryland, pp. 258-261, 1979. Towill, L.E., Drury, J.S., Whitfield, G.L. Lewis, E.B., Galyan, E.L. and Hammons, A.S., "Reviews of Environmental Effects of Pollutants: Cyanide", Oak Ridge Natl. Lab. ORNL/EIS81, EPA-600/1-78-027, pp. 190, 1978. USFWS, U.S. Fish and Wildlife Service,“Cyanide Hazards to Fish, Wildlife, and Invertebrates: A Synoptic Review”, US Department of Interior, Biological Report 85(1.23), Containment Hazards Reviews Report 23, December, 1991. Van Zyl, D.J.A., Editor, Proceedings of the Cyanide and the Environment Conference, Volumes 1-2, Tucson, Arizona, December 11-14, 1984. Vennesland, B., Conn, E.E., Knowles, C.J., Westley, J., and Wissing, F., (Eds.), Cyanide in Biology, Academic Press, London, 548 pp., 1981. Westly, J., "Cyanide and Sulphane Sulphur", in: Cyanides in Biology, B. Vennesland, E.E. Conn, C.J. Knowles, J. Westley, and F. Wissing, eds. Academic Press, London, 548 pp, 1981.

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WHO, "Anticaking Agents, Calcium Potassium, Sodium Ferrocyanide", Geneva, World Health Organization W14H(5): pp. 15-18, 1974. Yake, W.E. and James, R.K., "Setting Effluent Ammonia Limits to Meet In-Stream Toxicity Criteria", Jour. WPCF, 55, No. 3, pp. 303-309, 1983.


198


CHAPTER FIVE

Water Management and Discharge Strategies 5.1

INTRODUCTION

A primary focus of this book is the discussion of cyanide treatment processes presented in Chapter 6 and their ability to achieve varying levels of cyanide removal. Critical to evaluating water treatment processes is the initial establishment of a comprehensive water management plan, even if water treatment is ultimately found to be unnecessary at a particular mining operation. It is the lack of attention given to water management that has caused many of the environmental impacts arising at mining operations. The management of water and cyanide should be simultaneously considered when developing a watervary management A water management has many components, details of of which will from site plan. to site. In general, water plan management plans include the descriptions the following: • • • • •

Site water balance Cyanide management plan Water management practices Water treatment and discharge facilities Approach to maintaining the water management plan

The relationship between each of these components is illustrated in Figure 5.1, and this chapter briefly addresses each of these components as they apply to mine sites.

5.2

WATER MANAGEMENT PLANS

It is essential before reviewing water treatment alternatives that a detailed water management plan be developed to describe how site waters are to be handled. The water management planis closely related to the water balance and the two should be developed concurrently. The difference between the water management plan and the water balance can be described as follows:

199

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The water management plan identifies which site waters must be considered and specifically how they are to be segregated, diverted, collected, stored, treated and/or discharged.

•

The water balance is an accounting process where flow and chemistry values are assigned to each of the individual water sources identified inthe water management plan. A key outcome of the water balance is the determination of whether excess water will need to be treated and discharged. The chemical mass balance is eventually superimposed upon the water balance to form the framework of the comprehensive water management plan.

The water balance is a sub-component of the water management plan, but the two are intimately linked and cannot be independently developed. Selection of various water management approaches will affect the site water balance, so an iterative approach is required in order to formulate an optimal approach to managing water at a site. During development of a water management plan, the following objectives should be employed as a guide to identifying and minimizing environmental impacts: •

Maximize the diversion of runoff water from undisturbed catchments around the mine site in order to minimize the volumes of water requiring handling and treatment.

•

Maximize the recycle and reuse of water and reagents within the mining operation to minimize the need for treatment and the need for fresh make-up water.

•

Minimize theofgeneration of metals-containing waters through the segregation and encapsulation potential leachate forming materials. •

Assess the maximum potential concentrations of constituents in site waters to ensure no potential water source requiring treatment is overlooked.

•

Extreme wet or dry climatic conditions significantly influence the type of water management approaches selected, therefore these conditions must be considered equally with average climatic conditions.

The overall objective of a water management plan is to keep“clean waters clean” and to establish the least costly and most practical approach to controlling water at a mine site, subject to applicable regulations. In arid climates this means minimizing the need for makeup water by maximizing use and recycle of water sources generated within the project so that the storage and treatment requirements can be optimised. In wet climates, this means minimizing the volumeof water to be treated and discharged and the associated capital and operating costs. Table 5.1 outlines a series of water management options that are available for various facilities present at mining operations.


200


FIGURE 5.1 Components of a Mine Site Wat er Management Plan

Water Management Plan

SiteWaterBalance 1.Precipitation&Evaporation 2.SiteRunoff 3. Mine Dewatering (open pit & underground) 4. Groundwater Infiltration/Exfiltration 5. Make-Up Water Supply 6. Potable Water & Sanitary Sewage 7. Process Water Consumption

CyanideManagementPlan 1.CyanideBalance(ornitrogenbalance) 2.CyanideRecycle&Recovery 3. Cyanide Treatment 4. Wildlife Protection Review

Water Management Practices 1. Water Recycle 2. Water Segregation & Diversion 3. Water Collection & Storage 4. Make-Up Water Minimization 5. Emergency Water Discharge Excess Site Water ?

Yes

No

Water Treatment Facilities 1. Water Quality Goals & Discharge Permitting 2. Untreated Water Quality 3. Treatment Process Section & Operation 4. Discharge Strategy Evaluation 5. Effluent Monitoring

Water Management Plan Maintenance 1. Annual Plan Review & Modification 2. Site Water Quality & Quantity Monitoring 3. Weather Monitoring

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TABLE 5.1

Water Source Domestic Sewage MineWater

Ore Moisture Ore Stockpile Runoff

Ore Stockpile Leachate

Waste Rock Storage Area Runoff

Waste Rock Storage Area Leachate


Options for Water Management

Description Package treatment plant or a septic tank system Dust suppression Irrigation Re-use in mill Discharge with or without treatment Separate different quality water sources to reduce the need for treatment Accept in mill Discharge with or without treatment Settling in a pond and discharge Dust suppression Treatment and re-use in the mill or discharge Reduce the volume of runoff by minimizing areal extent and maximizing height of stockpiles Dust suppression Re-use in mill with or without treatment Treatment and re-use in the mill or discharge

Leachate be reduced by grading to promote runoff Dischargequantity with or can without treatment Settling in a pond and discharge with or without additional treatment Dust suppression Re-use in the mill with or without treatment Diversion of runoff into adjacent natural catchments Discharge with or without treatment Dust suppression Re-use in the mill with or without treatment Reduce infiltration by grading and compacting surface Water quality improvement with surfactants (temporary) and/or codisposal with alkaline material (such as limestone)

202


TABLE 5.1

Water Source Mill Site Runoff

Tailings Slurry Water Entrained in Tailings Solids

Seepage Through Tailings Embankment (including infiltration and runoff) Evaporation from Tailings Impoundment Tailings Impoundment Leachate

Continued

Description Discharge with or without treatment Discharge into holding pond and re-use in the mill or for dust

suppression Route into tailings impoundment Discharge into tailings impoundment Under-water discharge results in minimum tailings density and maximises entrained water. Above-water discharge fromvarying locations promotes maximum density and minimum entrained water. Providing underdrains helpsreduce entrained water. Producing "dry" tailings in the mill using filter presses can also be used to reduce entrained water and maximize fluid recovery. Seepage can be reduced by installing a barrier over or within the embankment. Infiltration through downstream shell of embankment can be reduced by vegetating and/or by installing runoff diversion ditches. Runoff into seepage collectionpond can be minimized by use of flow diversion ditches and berms. Evaporation can be reduced by maintaining a small pond volume. Evaporation can be increased by discharge over a large area and use of spray systems. Seepage can be reduced by lining the impoundment: - reworking and compacting foundation - clay liner - synthetic liner Placement of an underdrain system over liner further reduces seepage as it reduces the head on the liner.

Source: Hutchison, 1989

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5.3

WATER BALANCES

This section describes approaches to determining the overall water balance for a mining project and selecting options to deal with water management issues. The management issues usually involve having to cope with too little water, which requires supplemental supplies, or too much water, which requires storage and discharge with or without treatment. The development of a comprehensive site water balance and management plan should begin during the feasibility analysis and permitting of a mining operation with equal emphasis on start-up, the fully developed mine and closure. The proposed methodology emphasizes the use of a systems approach, which includes examining the entire mining operation, developing a complete flow network describing the various sources, sinks and flow pathways associated with bothimpacted and clean water. The natural rainfall/runoff cycle is superimposed on the mine flowsheet to determine the total quantities of water that must be dealt with on an annual basis, as well as during extreme dry and wet periods. Finally, value engineering is employed to determine the most cost effective measures for providing make-up water supplies, storage of excess water that accumulates seasonally, reuse of water, and systems for treatment and discharge of water. Excess runoff and process water is stored in a storm water surge pond or flow equalization basin from which water is recycled in the processing circuit and/or discharged to the treatment facility in a manner to minimize flow fluctuations. The traditional storage area is the tailings impoundment or decant pond, but heap leach facilities have also been used for this purpose. With an understanding of the site water balance and an identification of potential water sources and characteristics, attention can be focused upon selection of an appropriate water treatment option. A typical mine site water balance schematic is shownon Figure 5.2. It can be clearly seen how the various project components are interconnected with respect to the water balance. Water withdrawn from the pit (or underground mine) is used for dust suppression and as make-up water in the mill. During extreme storm events, excess pit water can either be temporarily stored in the pit, or pumped into a surge pond. Runoff and leachate from a low-grade ore stockpile area is also collected for possible reuseor treatment as well. In this example, it is assumed the quality of this water is impaired due to acid generation in the ore and the pond acts as a zero discharge holding facility. The water is ultimately pumped for reuse in the mill. Provision is made for an emergency overflow for extreme precipitation events which exceed the design storm event (where permitted). The emergency overflow option may be needed to maintain the geotechnical stability and integrity of slopes or embankments.


204

FIGURE 5.2

2 0 5

C ha pt er F vei

Water B alance Schematic Showing Most of t he Water Balance Components


Runoff from the mill site is directed either into the tailings pond or into a holding pond for treatment, reuse or discharge. Care is taken to ensure that all low pH solutions drain into this pond and not into a process pond in which hydrogen cyanide gas generation could occur. Process solutions can be combined with acidic drainage after it has been neutralized to a basic pH. Makeup water for the mill is derived from the pit, site runoff water, the tailings pond and a fresh water reservoir. A portion of the makeup water used as boiler feed and gland seal water must be of a very high quality. In this case, it is drawn exclusively from the fresh water reservoir. Tailings slurry is discharged from the mill into the tailings impoundment. In the impoundment, water is lost by permanent entrainment in the tailings solids and by evaporation. Seepage through the embankment or dam is collected in a pond and pumped back into the impoundment, reused in the metallurgical circuit or is sent directly to treatment. Figures 5.3 and 5.4 show water balance schematics for a typical gold mine in a wet climate which requires excess water treatment and discharge, and a heap leach facility operating in a relatively dry climate. 5.3.1

Water Balance Components

Typical water balance components associated with a conventional mine and milling operation are outlined in Table 5.2. Table 5.3 discusses how these components can be quantified and how the quality of these sources can be evaluated. Typical water balance components associated with a heap leach operation are shown on Figure 5.5. For a more detailed discussion on the various components andhow they are evaluated for heap leach facilities, the reader is referred to the publication by Van Zyl et al. (1988). 5.3.2

Water Balance Assessment

The water balance assessment basically involves a water accounting process that includes all water sources, surge storages and uses, the potential flow linkages between these and the shortterm (i.e., days to months) and long-term (i.e., years) surpluses anddeficits. Surpluses must be stored and/or discharged, while deficits must be made up by providing makeup groundwater or surface water supplies. It must also be recognized that short-term surpluses mustbe discharged directly or following treatment. Local or national water quality and waste disposal regulations may prohibit the discharge of any impacted waters without treatment and a discharge permit. Water discharged must meet the effluent standards contained ni ning the facilities water discharge permit for the mine. Some regulations may require that mi operate as zero developed discharge facilities. Regulations may also prohibit discharge of process water unless the mill and tailings disposal area have a net water gain (i.e., are located in wet climates). In practice, this means that no discharge may be allowed from tailings impoundments locatedin dry climates. In wetter climates, discharge may be allowed provided the effluent water quality meets specified standards or guidelines.


206

FIGURE 5.3

2 0 7

C ha pt er F vei

Example Water Balance Schematic for a Gold Mine in a Wet Climate


FIGURE 5.4

Example Water Balance Schematic for a Gold Heap Leaching Operation in a Wet Climate

2 0 8

yl et al., 1988

FIGURE 5 .5

2 0 9

C ha pt er F vei

Heap Leaching O peration Pond D esign Considerations


TABLE 5.2

Water Source Domestic Sewage Mine Water

Ore

Ore Stockpile Runoff


Potential Water Sources at a Mi ning Operation

Description Raw sewage generated by mine personnel. Mine water can be generated from the following sources:

- Groundwater inflow to underground mines - Groundwater inflow to surface mines - Precipitation accumulation in surface mines and pit wall runoff - Water from dewatering boreholes Groundwater inflow can vary in quality from good to poor depending on the nature of the rock through which itflows. In sulphide bearing rock, acid drainage is a possibility. Precipitation accumulation isusually turbid due to entrainment of soil particles and may contain other dissolved contaminants depending on the nature of the exposed rock over which the water flows. Mine water inflow in usually composed of several different sources which frequently have different water quality. It is important to recognize this, as prudent mine water management may demand these sources be dealt with separately. Groundwater inflow can be fairly steady or it can vary by an order of magnitude or more on a seasonal basis. Precipitation accumulation occurs sporadically in response to storm events. The ore that is mined contains water which must be taken into account when performing process plant water balance assessments. This represents a relatively small and constant supply to the plant. This water will contain elevated levels of suspended solids and may also contain other dissolved constituents depending on the chemical characteristics of the ore. The volume of water is generated only during storm or snowmelt events. This is generated by precipitation or snowmelt that percolates through the piles and emerges at itsbase. Its quality is a function of the rock type and can vary from good to poor. Elevated dissolved salts are common and acid conditions with accompanying dissolved metal levels can occur. This drainage occurs more commonly during wet periods. It is, however, more steady than the runoff component, occurs at a lower rate and for a longer duration. Leachate is not usuallya problem in arid climates.

Waste Rock Stockpile The properties of these water sources are similar to those derived from Runoff & Leachate the ore stockpile. Mine Site Runoff This source consists of runoff generated during storm events. It contains elevated suspended solids and may contain dissolved contaminants derived from reagents, process fluid, or fuel spills on the site.


210


TABLE 5.2

Continued

Water S ource Tailings Slurry

Description This consists of process water which is used to convey tailings to the impoundment. This water source usually contains cyanide, metals, ammonia, thiocyanate, cyanate and other dissolved constituents.

Water Entrained in Tailings or Pore Water

This is water entrained in the void space, i.e. between the solid grains of the tailings. When the tailings aresrcinally deposited, they are at a relatively low density. With time and with additional load of new tailings, the material consolidates and squeezes out a portion of the excess water. This component includes both tailings liquid that seeps through the embankment, runoff or infiltration that collects in the seepage collection pond, or water collected in the embankment internal drainsystem. The quality of the embankment seepage is similar to the tailings interstitial pore water. Frequently, the concentrations ofdissolved constituents are reduced by attenuation as the seepage passes through the embankment material. The infiltration through theembankment fill is usually clean and dilutes the embankment seepage flow. Runoff from the embankment face can contain elevated levels of suspended material. This constitutes water lost by evaporation from open water and wet tailings surrounding the water pond.

Seepage from Tailings Embankment

Evaporation from Tailings Impoundment Tailings Impoundment Leachate Mill Consumptive Use Mill Fresh Water Requirements

Evaporation from Various Project Facilities

This is the vertical seepage loss beneath the impoundment footprint that occurs from the deposited tailings. Mill consumptive use includes the water that is lost from the system by uses such as boiler feed water and pump gland seal water. These flows are generally constant as long as the mill is operating. These are the mill water demands that require water with a high degree of purity. They may include, amongst others: - boiler feed water - carbon regeneration water - gland seal water - domestic supply - domestic facilities Evaporation losses occur from the various locations at which water is or sprayed. These include: -stored pit area - water storage ponds & impoundments - heap leach spray systems


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TABLE 5.3

Approaches to Estimating Water Volumes at a Mining Operation

Water S ource Domestic Sewage

Description Use an estimation of the permanent project staff multiplied by a value of between 35 and 70 gallons per day. If personnel live on-site the range is between 70 and 100 gallons per day per person.

Mine Water

Groundwater field and desk studies are required to determine the amount of groundwater inflow to underground openpit surface mines. water also includes water from boreholeand dewatering operations. TheMine investigations typically include geologic mapping, borehole installation, pump testing, geophysics to further refine geology and groundwater flow modeling. Field water quality sampling provides some indication of water quality, however, this may have to be supplemented using laboratory column testing on oreand waste rock samples. It is important to recognize that dewatering of the mine introduces oxygen to geologic formations previously under anaerobic conditions, and changes groundwater quality as the mineis dewatered. Storm runoff into an open pit is usually calculated from rainfalldata. Annual or monthly volumes are usually calculated using a runoff coefficient of approximately 50 percent, while storm period extreme events are based on the design storm, i.e., the 24-hour, 100-year event multiplied by a runoff coefficient of say 80%. This water usually contains suspended solids inthe hundreds or thousands of parts per million. Dissolved concentrations can beinferred

Ore

Ore Stockpile Runoff


from field data and/or laboratory column tests using pit wall rock and rain or distilled water. The quantity of water contained in the ore generally varies from 2 to 10% by weight. A good preliminary design number is 5%. More accurate values can be determined by running standard moisture content laboratory tests on rock samples. The quantity of runoff can be calculated for short-term events using the Rational formula and design storm information. Monthly or annual runoff can be estimated using the appropriate precipitation and runoff factors of 30% to 60%. Where more accurate estimates are required it maybe possible to use stream gauge information from small disturbed catchments in the vicinity of the project. The quality of the runoff can be dealt with on a similar basis to pit precipitation as discussed above. Pile infiltration rates can be estimated from field tests and/or published literature. Rates vary from several tenths of an inch per hour up to an inch or more. Maximum leachate rates fora given time period can be determined by considering the range of possible precipitation or infiltration that could occur. There are complete models available which can be based to developed time sequences of leachate using climatic data and information in the pile. Water quality information can be derived using laboratory column or batchtests. In some cases, it may be possible to obtain field data from ore piles created during bulk exploration programmes.


212


TABLE 5.3

Continued

Water S ource Description Waste Rock The same information applies as discussed under Ore Stockpile. Stockpile Runoff & Leachate Tailings Slurry Values are calculated from the slurry density of the tailings. The quality of the liquid is usually determined from samples collected from pilot scale tests. Water Entrained The final amount of liquid that is entrained in the consolidated tailings In Tailings Solids varies from 70 to 140 gallons per ton dry weight for current gold projects. The exact value is usually determined from consolidation testing conducted in the laboratory. Seepage from The quantity of seepage is estimated from the known hydraulic head on Tailings the tailings embankment and the hydraulic conductivity of the Embankment construction materials. It is usually quite constant and gradually increases with time as the hydraulic head increases with filling of the impoundment. Infiltration and runoff from the embankment can be calculated by the techniques discussed underore stockpiles. Where large embankments are located in wet climates, this flow component can be significant. Evaporation from The amount of evaporation can be calculated from the known water Tailings surface areas, moist tailings areas and available gross evaporation and Impoundment precipitation data. Evaporation loss from moist tailings is difficultto

Tailings Impoundment Leachate

Mill Consumptive Use and Mill Fresh Water Requirements

estimate anyofaccuracy and itwater. is frequently that thenature loss rate is 50% orwith higher that of open Becauseassumed of the shallow of the water pond, evaporation loss rate from it is probably closer to the“A” pan evaporation rate than lake evaporation. This can be calculated from known or assumed moisture conditions in the deposited tailings and the permeability of the underlying geologic formations or materialssuch as clay or synthetic liners. Frequently a drainage sand layer is placed over the impoundment base to collect the seepage from the tailings and convey it to the seepage collection pond. This considerably reduces the amount of seepage that could occur into the geologic formations immediately below the tailings. These loss rates are calculated as part of the process design.


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A useful method of approaching a water balance evaluation is to complete it in two stages. The first stage is a mean annual assessment of the long-term surplus/deficit situation. This involves developing mean annual values for each component, taking into account the leakages and those water sources that can be discharged, followed by determination of the overall water balance. The second stage involves performing short-term water balance assessments over periods of a few hours to a few months. These are required to determine the volumes of surplus water that can be generated over a short period during extreme wet conditions and storm events. For projects that have a water deficit, these are required to determine the amount of storage that must be provided or possibly some short-term water discharge requirements. For projects that have a water surplus, these calculations are used to determine the maximum discharge rates or the amount of storage required to maintain aconstant feed to the treatmentplant. There is a tradeoff between the amount of storage provided and the capacity of a plant required to treat the water before it is discharged. Value engineering should be carried out to determine the best combination of storage capacity and treatment capacity. When evaluating the short-term surplus it is useful to use a statistical basis (i.e., select an extreme event of specified probability). Commonly used events include the 1 in 100-year and the 1 in 10-year storms, although some regulatory agencies are now requiring more extreme events up to the Probable Maximum Precipitation (PMP) eve nt. It is important to note that each project will have its own unique critical duration which results in maximum storage and treatment requirements. As a general rule, the criticalduration is short, typically on the order of a few days or weeks. In wet climates the critical duration mayextend over several months. The critical duration can be determined by selecting one of several time periods such as 24-hour, 7-day, 30-day and/or 90-dayperiods and performingadditional water balance computations. The critical duration is the one which requires the maximum storage volume and/or the maximum discharge rate. The potential water sources are identified through development of a detailed and time-dependent site water balance. These sources include domestic sewage, acidic mine drainage, runoff from ore stockpiles and waste rock storage areas, leachate from disposal areas and tailings impoundments and turbid water srcinating as runoff from disturbed catchments within the mine site. Two sources of water are anticipated from an underground mine. The first source includes groundwater intercepted prior to exposure to the mine workings. The chemistry of the water is derived from the combined characteristics of the various groundwater sources entering the mine. The second potential source of underground mine water includes groundwater impacted by drainage from the mine operation. The chemical of thewater waterdraining are derived from combination of thebackfill groundwater chemistry with thecharacteristics chemistry of pore from the backfilled solids. The constituents of concern in contaminated mine water include trace metals and total suspended solids.


214


The characteristics of the runoff from rehabilitated waste rock storage areas can be derived from the results of batch extraction and long-term column leachate studies conducted under natural conditions. The constituents of concern in the runoff waters from rehabilitated surfaces include trace metals and total suspended solids. The characteristics of leachate from ore stockpiles are generally derived from long-term field leachate studies. The constituents of concern in the leachate include elevated levels of trace metals and total suspended solids. Turbid waters, srcinating as runoff from disturbed areas, may require treatment for the removal of total suspended solids. The concentration of total suspended solidsin the runoff varies with the intensity and frequency of precipitation impacting the catchments. Estimates of the total suspended solids concentrations in the turbid water are derived from either field or laboratory studies. 5.3.3

Water Quality Assessment

A detailed understanding of water sources and chemical characteristics is essential in the selection and development of efficient and reliable water management and treatment systems. This knowledge coupled with the effluent design criteria is utilized in establishing the level, type and capacity of water treatment required to meet environmental goals. The chemical characteristics of a water are dictated by the nature and concentrations of constituents present in the water. These characteristics result from the complex interactions occurring between metallurgical processes and reagents, ore geochemistry and site hydrology. The selection of a treatment process is determined by the water chemistry and the level of treatment required. For example, the use of the zinc precipitation gold recovery process can increase the concentration of zinc in site waters. The recovery of silver from ores often requires relatively high cyanide levels, which increases by many fold the levels of cyanide requiring treatment. Higher levels of cyanide can also increase the solubilisation of other undesirable metals including cadmium, copper, mercury, nickel and/or selenium. During the course of a mine life, site water characteristics may vary considerably due to changes in ore geochemistry, the types of metallurgical processes utilized, the annual and daily precipitation, the size and type of mining activities, the method of tailings disposal and the amounts of reagents utilized. A detailed assessment of water quality is required under different flow conditions to assess when treatment is required and when direct discharge is appropriate. The fluctuation in water affects the(i.e., capacity of the treatment facility and need for onsite storage of waters in flows holding ponds flow equalization basins). The netthe precipitation affects the flow of groundwater and generation of poor quality seepage from tailings impoundments, waste rock storage areas and ore storage piles.

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The ore geochemistry is important in assessing present and future water treatment options. For example, during the early stages of mining the processing of oxidized near-surface ores yields a water chemistry much different than the water produced when reduced ores of high sulphide content are mined and processed later in the mine life. A treatment system capable of treating the process water during the first few years of mine life may be unsuitable during the latter years of mine operation due to changes in both the constituents and their concentrations in the water. A change in ore chemistry or the tonnage of ore mined may alter the type of metallurgical processing, which usually affects the chemistry of the barren solutions or water. An elevated sulphide content in the ore may result in significant acid mine drainage following improper disposal of waste rock or tailings, producing a low pH solution containing elevated concentrations of metals. Certain ores (e.g., silver selenite complexes)pose additional treatment problems, due to the solubilisation of metals which require specialized processes for their removal. As a result of these interactions, the constituents of importance and their respective concentrations vary considerably from site to site. A listing of the constituents of concern and representative ranges of concentrations found in mine waters are presented in Table 5.4. In addition to the chemical characteristics, the two most important physical characteristics of the water include temperature and flow. The water temperature affects treatment system performance and is influenced by local climaticconditions. The variation in the flowof water is affected by the size of the mining operation and the intensity and frequency of precipitation impacting the site. The capacity of a water treatment system is dependent upon the minimum temperature, maximum flow and maximum reaction periods necessary to produce an effluent of acceptable quality. During the initial phases of development of a treatment system it is essential to identify the constituents of concern and quantify their possible concentration ranges as a function of flow rate and time (seasonally and as afunction of mine life). In the case of an existing mining operation, extensive sampling is necessary to account for seasonal variation and variations in mining activity and precipitation. A water-quality monitoring programme should be initiated on a periodic (e.g., monthly or quarterly) basis and include all major site waters. All solutions of importance should be thoroughly analysed and their flows quantified, including the waters of the potential receiving systems, if dataare not already available. It is recommended that an on-site analytical laboratory be utilized or established for these purposes. The use of commercial laboratories can be time-consuming due to shipping and turn-around periods. The rapid turnaround of data, particularly during laboratory or pilot plant evaluations of processes, is essential to making timely water quality decisions. Often daily turn-around of data is required, and the mine may be located in a remote region not accessible to a commercial analytical laboratory.


216


TABLE 5.4

Approximate Chemical Composition Ranges in Cyanidation Solutions

Constituent

RangeofConcentration (mg/L) <0.02 – 10.0

Arsenic Cadmium Chromium Copper Iron Lead Manganese Mercury Nickel Selenium Silver Zinc Total Cyanide WAD Cyanide Free Cyanide Ammonia-N

<0.005 – 0.02 <0.02 – 0.1 0.1 – 400 0.50 – 40 <0.01 – 0.1 0.1 – 20 <0.0001 – 0.5 0.02 – 10 <0.02 – 5 <0.005 – 2 100 0.05 0.5 – 1,000 0.5 – 750 <0.01 – 200 <0.1 – 50

Nitrate-N Cyanate Thiocyanate pH Hardness (as CaCO3) Sulphate Temperature (°C) Flow (gpm)

10 – 200 10 – 200 <1.0 – 2,000 11.5- 2.0 200 – 1,500 50 – 20,000 0 – 35 0 – 6,000

Note: The chemistry of mine waters and solutions frequently changes withtime in response to ore geochemistry changes, etc.

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In the conceptual or design phase of a mining operation, estimates of constituent concentrations must be made based upon experience and laboratory studies. It is important during the initial metallurgical testing to completely characterize the barren water produced and to analyse the solutions for all the constituents indicated in Table 5.4 plus others that may be suspected of being present. These data are required for an initial assessment of water treatment requirements. Based upon a review of the chemical characteristics of the potential water sources, they are combined into one or more principal water sources for treatment. The approach is to minimize the number of discrete flows and to treat waters of similar chemistry. There are usually three principal water sources: the cyanide and metals-containing sources, the metals-containing sources and the turbid water sources. Treatment of the primary cyanide and metals water sources must provide an effluent compatible with and protective of the beneficial uses of the receiving system in the event there is a discharge. An examination of both the chemical and physical characteristics of the water source is required in order to design a suitable treatment system. The maximum constituent concentrations and minimum water flows requiring treatment occur during extended dry periods. The difference between these concentrations and the effluent design criteria controls ht e type and level of treatment required. The characteristics are usually derived from the combined chemistries of the decant water, tailings impoundment leachate and waste rock storage area leachate. These three sources contribute the highest proportion of cyanide and metals entering the primary water source. In addition to the chemical characteristics, information is required regarding the physical characteristics of the water, including temperature and flow. Although increased precipitation may reduce constituent concentrations, the increased runoff and groundwater base flows may increase water volumes. The impact of increased precipitation on constituent concentrations should be addressed during development of a site water management plan.

5.4

CYANIDE MANAGEMENT PLAN AND MASS BALANCE

The cyanide management plan is a key component of the site water management plan and is closely tied to the water balance. The cyanide management plandeals with the management of free, WAD and total cyanides in mine waters, but not cyanide purchased as a reagent (e.g., solid NaCN or concentrated NaCN solutions). The aspects of handling reagent cyanide, including its transport to the site and handling on site, are more appropriately addressed in other mine site management plans. The cyanide management plan describes in detail how cyanide-containing waters be stored, contained, conveyed, treated and it is beneficial to conductare a to nitrogen balance to account for cyanide andmonitored. its relatedOften compounds of cyanate, thiocyanate, ammonia and nitrate.


218


As stated in the introduction, the management of water and cyanide should be simultaneously considered when developing a water management plan. The approach is to develop a water balance and management plan, onto which water chemistries are superimposed to yield a mass balance of cyanide and other dissolved constituents. The overall objective of the cyanide management plan is to ensure the following: •

Wildlife, particularly waterfowl and aquatic life, are protected from exposure to potentially harmful levels of cyanide.

•

Cyanide is re-used to the maximum practical extent within a metallurgical circuit.

•

Adequate treatment facilities are available to deal with excess cyanide-containing solutions on a regular basis or in the event of extreme precipitation events.

•

Contingency response procedures are established in the event cyanide solutions are inadvertently released to the environment.

Table 5.5 lists several cyanide management approaches that can be considered when evaluating alternatives for the various cyanide-containing solutions present at mining operations. Overall, the objective is the minimize the amount of cyanide in mining solutions, re-use cyanide to the maximum practical extent while ensuring that cyanide-containing solutions accessible by wildlife have a safe WAD cyanide content. A useful component of the cyanide management plan is the determination of the site cyanide balance. This is similar to the water balance, in that all cyanide-containing sources are identified along with their associated flow and composition. This is preferably conducted during the design phase of a mine as part of evaluating the need to treat cyanide-containing solutions. The components which are accounted for in the cyanide balance are listed in Table 5.6, and include cyanide inputs, outputs, storage and recycled cyanide. When establishing cyanide management approaches, the following should be considered: •

Minimizing the concentration of cyanide in leach solutions through careful metallurgical process control.

•

Minimizing the long-term volume of cyanide-containing solutions present at the site, including solution in ponds and impoundments.

•

Providing for contingency storage of excess cyanide-containing solutions.

•

Maximizing the recycle of cyanide-containing solutions, including leach solution, ponded solutions, and impoundment seepage and decant solution.

•

Preventing wildlife access to cyanide-containing solutions where practical.

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Chapter Five


Recovering cyanide from leach solutions to allow its reuse in the leaching circuit.

•

Providing adequate treatment facilities to remove cyanide from excess process solutions in the event a discharge to the environment becomes necessary.

There are several situations when solutions may need to be treated for cyanide destruction or recovery, including the following: •

At any mine site using cyanide, protecting wildlife from exposure to potentially harmful levels of cyanide must be considered. In some cases, cyanide-containing solutions or slurries may require treatment to maintain acceptably low levels of WAD cyanide.

•

The site water balance may indicate that excess solution will need to be treated and discharged, either on a regular basis or intermittently. In this case, low levels of WAD cyanide would be required for the protection of aquatic life.

•

In some metallurgical circuits cyanide is detrimental to gold and silver recovery systems, including some flotation circuits and biological sulphur oxidation circuits. In these cases, cyanide solutions may require treatment to maintain acceptably low cyanide concentrations.

In summary, the philosophy of the cyanide management plan is to establish in writing how cyanide solutions are to be stored, contained, conveyed, treated and monitored during mine operation and afterthe mine operations have ceased. The waterfowl ultimate and objective of organisms. the plan is to proactively ensure protection of wildlife, particularly aquatic


220


TABLE 5.5

Cyanide Solutions and Methods for Cyanide Management

Cyanide So lution Ponds and Impoundments

Description It is difficult to prevent wildlife access to larger ponds and impoundments. In these, the cyanide level should be controlled to a level protective of

Tailings Slurry

indigenous species of animals. Ponds and impoundments can be designed to promote natural cyanide attenuation. Ponds and impoundments can be fenced, netted, hazed and/or covered to prevent access by wildlife. The volume of water should be kept to a practical minimum. Excess water from impoundments may require treatment before discharge. Cyanide in tailings slurry should be minimized, recycled, recovered and/or treated to a level below about 50 mg/L WAD for wildlife protection. Tailings thickeners can be used for water recovery. Tailings thickening circuits can be used to wash cyanide from tailings slurry using fresh water make-up. Impoundments can be designed to promote natural cyanide attenuation. Water recycle from the tailings impoundment should be maximized. Tailings pipelines can be contained to prevent discharge of slurry in the

Impoundment Seepage Heap Leach Solution

Process Solution

event of a pipeseepage failure. can be returned to the impoundment or recycled to Impoundment the mill. In-heap storage of leach solutions can be considered to prevent wildlife exposure. See descriptions for smaller and larger ponds above. Excess heap leach solution may require treatment before discharge. Process solution should be recycled to the maximum extent within the leaching circuit to minimize make-up water volumes and to maximize cyanide recycle.

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Chapter Five


TABLE 5.6

Component Cyanide Inputs Cyanide Outputs

Internal Cyanide Recycle

Cyanide Storage


Components of a Cyanide Balance

Description Make-up reagent NaCN Loss through natural attenuation Consumption in leaching circuit to form metal complexes and related compounds Loss to cyanide destruction treatment process Cyanide in discharged water Solutions recycled in the leaching circuit Cyanide recovered from solution for re-use in leaching

Reclaim water from in ponds and circuit impoundments Inventory of cyanide leaching and ponds Cyanide entrained in solution contained in tailings solids or in leach pads

222


5.5

DEVELOPMENT OF EFFLUENT DESIGN GOALS

5.5.1

Beneficial Use Classifications

Over time, excess process solution or other mine site waters can accumulate to the extent that treatment and discharge tothe environment are needed. The discharge of treated effluent can be directed into one of several receiving systems, including: • • • • • • •

Process reuse and recycle Land application Streams Rivers Impoundments (e.g., ponds, lakes and reservoirs) Groundwater and aquifers Marine environments (e.g., oceans, bays and estuaries)

Associated with these receiving systems are one or more beneficial use classifications including: • • • • •

Potable water source Industrial water source Agriculture or irrigation Full or semi-body contact (e.g., swimming and fishing) Aquatic life protection (i.e. fisheries)

Associated with the beneficial use classifications are numerical and/or narrative criteria derived for specific chemical, physical and biological constituents (Mudder, 1998 and Mudder et al., 1998). These criteria must not be violated in order to maintain the water quality associated with a specific receiving system to protect its designated uses. In conjunction with the beneficial use classifications, receiving systems or waters may be classified into general categories, which encompass or include criteria from several beneficial use classifications. The criteria may be numerical or narrative in nature. For example, a numerical value could be specified for copper in drinking water, while a narrative criterion may state discharges of toxic substances or materials into a surface water are prohibited. Regulations for water quality criteria are based upon federal or national guidelines within a particular country of interest. Local or regional criteria may be more stringent than the national or federal criteria. Specific criteria vary considerably from location to location, as much as an order of magnitude in certain cases. The criteria and regulations are periodically reviewed and modified. It is essential in the initial stages of developing an effluent discharge strategy to contact the key personnel in the appropriate government agencies and obtain a current set of water quality regulations. From a communication and legal standpoint, it is important to establish which government agency is responsible for development of permits for the discharge under consideration.

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Since most treatment facilities produce chemical and/or biological sludge, appropriate disposal of the sludge is required. In certain instances the sludge may be classified as hazardous under federal or local regulations, and explicit disposal practices or toxicity tests may be required to classify the waste. The cost and approach associated with solid waste or sludge disposal are very important issues and may affect or dictate the selection of treatment processes in certain instances. The most stringent criteria for a particular constituent may not be associated with the most sensitive beneficial use. For example, the criterion of a particular constituent for the protection of aquatic life, which is a very sensitive classification, may be more stringent than for irrigation or stock watering. The criteria for all uses associated with thepreferred receiving systems must be assembled and reviewed. Consideration must also be given to the potential changes which may occur in beneficial use classifications and criteria. Although a treatment process may produce an effluent of acceptable quality currently, the future application of a more stringent criterion may render the treatment process obsolete or inadequate. In addition to specific federal or local water quality criteria, some governments have adopted general effluent guidelines for proposed and existing industries including mining. These guidelines provide effluent limitations for several constituents based upon the treatment performance achievable through application of the best available technology or BAT. Other terms used in conjunction with BAT include BPT (Best Practical Technology) and BATEA (Best Available Technology Economically Achievable). No USEPA BAT limitation exists forcyanide in mining, although standards do exist for various inorganic constituents and metals. In some cases, alternative site-specific water quality criteria can be developed by conducting bioassays, a recalculation and/ora risk assessment. This process results in the implementation of site-specific water quality criteria that are intended to protect designated uses and sensitive species in the receiving water system. The following section discusses this approach for developing site-specific criteria for cyanide and metals. 5.5.2

The Use Attainability Analysis or Risk Assessment

At locations where no receiving water classifications or criteria exist, or when alternative sitespecific criteria are being considered, these criteria are usually developed by conducting a Use Attainability Analysis (Mudder, 1998). The purpose of a Use Attainability Analysis is to characterize the chemical, physical and biological components of a receiving system and to define current and future beneficial uses(Mudder et al., 1998). The approach is to conduct field surveys to define the use of the waters, the physical habitat of the receiving system and the present and future consumption uses of water from the receiving system. Once the uses have been established, the appropriate criteria can be developed or and selected. The usage identification of potable water usage, stock and wildlife watering, irrigation industrial is straightforward, when compared to aquatic life protection issues.


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Identification of existing biological or aquatic communities within a receiving system requires a scientific approach. The approach involves a bioassessment in which thechemical, physical and biological characteristics of the receiving system are identified. The chemical characteristics include primarily the surface water quality in addition to the quality of water entering the receiving system. The physical characteristics of criticalimportance include hydrology, habitat and temperature, while the principal biological characteristics of importance include animal life and aquatic communities. The detail of a bioassessment or Use Attainability Analysis is dependant upon many factors, including the quantity and reliability of available data and the effect of altering use classifications relative to the level and type of treatment required. The information gathered during a bioassessment is employed in the derivation or modification of beneficial use classifications and establishment of constituents of concern and associated criteria. The data are also used to quantify the impacts of existing natural and humanactivities on the receiving system. There are several steps to conducting a bioassessment or a Use Attainability Analysis. First, the components of the receiving system requiring in-depth examination must be identified. These components include current and potential uses and chemical, physical and biological characteristics. The chemical characteristics include constituents such as metals, organic constituents and inorganics. A detailed analysis of organic constituents in the receiving system may not be required in the case of many mining operations. With regards to the receiving system, analyses of organic contributions are required if there is a suspicion of toxicological effects or complications occurring from the upstream discharge of organic constituents. The purpose of the chemical characterization of potential receiving systems is to gather and screen the historical data for accuracy and reliability. Problems associated with ni terpretation or poor quality analytical data are frequent. Included with the data should be information concerning the time, date and people involved in sampling, as well as the sampling and analytical methods and statistical analyses. At a minimum, analytical data should be collected over a period sufficient to encompass varying seasonal weather patterns. Where surface waters are the receiving systems, complications associated with sampling are usually minimal. If groundwater is the receiving system, care must be takento install the proper type of well and employ the correct sampling procedures. Considerable care should begiven to the selection of analytical laboratories based upon quality of analyses, reliability, cost and turnaround time. The selection of sites for water quality monitoring generally coincides with sites at which biological samples or physical measurements are being taken. In addition, any isolated inputs should be sampled, including discharges, seepages or runoff, if they are from suspected or known contaminated areas.

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There are several physical characteristics of importance, including habitat, hydrology and temperature. The effects of increased temperature are particularly important with regards to discharges to surface waters in which sensitive fish andinsect populations reside. An increase in temperature is tolerableup to the point of inducing stress and/or death. Rapid increases in water temperature (e.g., several degrees in a few hours) may also result in acute toxicity effects and may increase the possibility of early emergence of insects. In addition to temperature, knowledge of site habitat and hydrology are very important in determining receiving system beneficial uses. Selection of sampling sites is critical to development of an accurate representation of the existing physical environment. The biological characteristics of the receiving system must also be delineated. The most important include the resident aquatic communities. The biological species usually identified include the fishes, the aquatic insects (e.g., stone flies, mayflies, caddisflies), benthic organisms (e.g., midges and worms), shell fish and algae (e.g., green, blue-greenand diatoms). In marine environments there are many additional species not commonly associated with fresh water environments. There are many specific procedures available for collection and identification of aquatic organisms. Most procedures require modification in the field to suit a particular situation. It is therefore important to record in detail the procedures utilized in the biological characterization, since the protocol used may affect the type and number of species encountered, as well as the interpretation of data. Coordination of water quality monitoring stations with biological sampling stations where appropriate is preferred. 5.5.3

Effluent Design Goals

Once the beneficial use classifications, constituents of concern and water quality criteria have been identified, an effluent strategy and effluent design goals for the proposed treatment facility can be prepared. The effluent limitations or design goals are then employed in the preliminary screening of potential treatment alternatives. Once the receiving system has been characterized and the appropriate constituents and criteria identified, the preliminary effluent design criteria or goals can be determined. The design criteria translate into numerical values which must be met on a continuous basis to maintain the necessary receiving system water quality and to protect its designated beneficial uses. The numerical effluent design criteria dictate the level and type of treatment required and are used in the screening andselection of viable treatment alternatives. To aid in the development of effluent design goals, the overall effluent strategy identifies the most cost effective and environmentally acceptable discharge pointsthrough orreceiving systems.toFora example, it may be system more beneficial to transport treated effluent a pipeline larger receiving downstream of the mine site for a variety of reasons. The reasons may involve a desire for additional dilution and reduction in the degree of treatment, or to avoid discharge to a particularly sensitive receiving system. It may be desirable to hold and discharge treated water during runoff periods to avoid low flow conditions in the receiving system.


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An integral step in the determination of effluent designcriteria is the wasteload allocation. This process involves identification of the most critical time periods within the receiving system in regard to maintaining the beneficial uses and criteria. The details of this approach are presented in documents prepared by the USEPA (USEPA, 1983, 1985a and 1985b). Some regulatory agencies now require incorporation of a biomonitoring programme into each proposed and renewed discharge permit. The general programme requires periodic testing ofthe effluent for both acute and chronic effects using various species of indigenous or surrogate organisms. In general, the receiving system is used as the dilution water source. A general description of the biomonitoring programme is provided in the USEPA documents previously referenced. Since the specific biomonitoring programmewill vary from discharge to discharge and region to region, it is necessary to contact the regulatory agency in charge to determine the effluent toxicity testing requirements.

5.6

SCREENING AND SELECTION OF WATER TREATMENT PROCESSES

Consideration of water treatment options should be initiated in the early stages of mine development. Often, water treatment considerations are addressedonly after the design of the mine and metallurgical operations arewell underway. This approach is not desirable, since all facets of the mining operation affect the volume and chemistry of the water requiring treatment. Using the expected quality of waters to be treated, the various treatment processes are screened for their ability to meet and maintain the established effluent design goals. For example, treatment in an arctic climate in sub-zero temperatures may eliminate biological treatment as a viable option, unless a source of warm water is available or treatment during warmer months is contemplated. Depending upon the level of cyanide allowed in the effluent, certain processes will be either suitable or not. In addition to cyanide treatment, processes must be evaluated for their effectiveness in treating and removing trace metals and cyanide related constituents present in the cyanidation solutions, acidic minedrainage or seepage. A decision must be made to treat these waters separately or combine them prior to treatment in a single plant. Although similar metallurgical processes are employed in many mining operations, the water characteristics vary widely, and no single approach is available for their treatment. The selection of treatment processes, as in selection of effluent criteria, is a site-specific and iterative process and experience is essential.

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Once the various treatment processes have been screened and assembled into a preliminary process flow sheet, the next step in the selection process involves a comparison of expected capital and operating costs,system reliability and ease of operation. In the event two processes will provide equal performance, the more economical process is chosen, as long as the process is reliable, can be operated on a continuous basis and will produce effluent of suitable quality. As a result of the screening process, usually one to three treatment options are selected for further consideration. At this point, a series of laboratory and/or pilot plant investigations are undertaken to develop the information necessary for the final process design and to verify the effectiveness of the process. In conjunction with these evaluations, bioassays or toxicological testing is useful to verify the environmental compatibility of the effluent with the receiving system. These bioassays may include static or flow-through tests and a variety of organisms (e.g., fish and aquatic insects). The results of the bioassays are used in the selection of viable treatment alternatives and to establish in-stream standards and effluent design criteria. In conjunction with the bioassays, a bioassessment of the receiving system may be required to establish beneficial uses and identify sensitive and important life forms. To meet and maintain water treatment design objectives, there are several concepts which must be considered in conjunction with the design of a water treatment facility. The design of the facility must be sophisticated enough to produce an acceptable effluent quality, but simple enough to allow continuous and reliable operation and maintenance. The treatment facility may need to be designed for continuous 24-hour per day operation for the production of high quality effluent. In addition, the facility design must be flexible in nature to allowprocess modifications as water chemistry and flow fluctuate during the life of the mine. Control of the process and monitoring of process efficiency involve frequent sampling and chemical analyses of the untreated water source and at various stages of treatment. In conjunction with chemical analyses, effluent and in-stream biological monitoring upstream, within and downstream of the water discharge is employed to ensure protection of the aquatic environment and to track any changes which may occur in the receiving system. The ultimate goal of treatment is to maintain the water quality criteria and to protect the beneficial uses and life forms associated with the receiving system. A trained staff of technicians is required for the operation and monitoring of the treatment system. The control and monitoring of the treatment process is accomplished through a combination of computerized systems and a modern on-site analytical laboratory. A water treatment design and operations manual should be prepared and potential technicians subjected to a training course before operating a treatment facility. Periodic training updates should be implemented as part of the overall treatment facility operation. In addition to process design, control, monitoring and operational considerations, contingency plans are formulated to respond to undesirable situations. These situations could involve chemical spills either on-site or in transit, short-term failure of treatment plant components, production of inadequately treated effluent and unpredictable precipitation events. Contingency plans provide practical and detailed responses to these situations in order to circumvent adverse environmental impacts.


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A water treatment facility is oftenan integral part of the mining operation. The current trend is toward minimization of water generation and development of processes for the recovery and reuse of both process solutions and reagents.

5.7

EFFLUENT DISCHARGE STRATEGIES

Using a detailed and dynamic site water balance model, it is possible to compare the flow rates of mine water discharges to that of the receiving system. Often, it is desired to maintain a flow ratio of discharged water to that in the receiving system to ensure site specific criteria are satisfied after the two streams are blended. By varying the rate of water discharged throughout the year, a specific minimum dilution can be maintained in the receiving system at all times. This may require the use of surge ponds for the collection and storage of mine water during periods of low flow in the receiving system. Then during periods of higher flows in the receiving water, stored water can be discharged to maintain a prescribed dilution ratio in the receiving system. This approach is termed a hydrograph controlled release or HCR, a water management practice employed worldwide. The HCR is also referred to as a paced release. An example stream hydrograph is shown for reference in Figure 5.6. This graph illustrates the flow rate of water in a receiving stream as a function of time of year, with higher stream flows associated with spring runoff periods. Utilizing a hydrograph controlled release in this case would include higher releases of mine water during spring runoff to take advantage of the large amount of dilution available duringthis time of year. For the remainder of the year, mine water would be discharged at a lower rate, potentially requiring the storage of some mine water until the next spring runoff allowed a higher discharge. Careful evaluation of stream flows, average and extreme precipitation conditions and appropriate in-stream water quality standards are required to successfully utilize water management strategy. In some situations, this approach may provide a route to minimizing water treatment costs while ensuring established in-stream water quality criteria are satisfied year round. Once the appropriate standards and/or aquatic life criteria have been selected and the corresponding effluent dilution has been derived, proposed discharged permit values can be compared with the anticipated performance of the various treatment processes evaluated. The comparison determines if an acceptable level of treatment is available and whether or not continuous protection of the receiving ecosystem can be achieved. In determining the point of discharge, the selection of the actual outfall location is critical. It may be necessary to move a proposed outfall location due to a sensitive habitat or to increase the available dilution. Relocation of an outfall, even though an expense isrealized in installation of aachievable pipeline, and mayenvironmentally well be the most cost effective only alternative for potential obtaining acceptable effluentand limitations. In order available to minimize impacts and long-term liabilities, it is recommended that a minimum number of outfalls be utilized, with a single outfall being preferred.

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An approach to offsetting acute toxicity concerns in the receiving system is to install an effluent diffuser system to promote rapid and complete mixing within the receiving system. This approach has been successfully applied in the mining industry in the United States and has been accepted by both state and federal regulatory agencies. An example diagram of an effluent diffuser for placement intoa river is shown in Figure 5.7. This diffuser consists of a buried pipe placed perpendicular to the river flow with several smaller discharge pipes extending upwards into the main flow of river water. The multiple discharge ports across allor part of the river accelerates the mixing of effluentwith river water. In addition to mixing water across the width of the river, the discharge ports may also be distributed with depth.


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FIGURE 5.6 Example Stream Flow Hydrograph

Source: USGS, 1984

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FIGURE 5.7 Schematic of an Effl uent Discharge Diffuser

Source: Tchobanoglous and Burton, 1991


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5.8

EXAMPLE WATER MANAGEMENT APPROACHES

In this section, two example water management approaches are considered relative to the preceding discussions of water management, cyanide management and water discharge strategies. These examples are not specific recommendations, but illustrate several general principles of water management that can be considered for gold/silver milling and heap leaching operations. The first example is illustrated in Figure 5.8 relative to an ore milling circuit. In this system, several water management concepts are indicated, including: •

The entire mine site is considered as an overall unit relative to the sources and management of site waters. No single facility at the site operates independent of the others with respect to water management.

•

The tailings impoundment is used for surge storage of site waters, and several water outlets from the impoundment are considered, including enhanced evaporation, land application, water for dust control on site roads, and water treatment and discharge.

•

A water treatment plant is provided for site waters or tailings impoundment water, and treated water can be managed in one of several ways, including its use for dust control or discharge from the site.

The second example is shown in Figure 5.9 and relates to the management of water at a heap leach operation from conception through closure. As indicated, the water balance and management plan are considered along with options for pad decommissioning before the heap leach is placed into operation. Options for water and cyanide removal are considered, including natural attenuation and enhanced evaporation, along with potential options for treatment of pad waters either during operation or atclosure. An important part of the review of water treatment options is the evaluation of potential discharge locations, including land application, discharge to a surface water or pumping excess water to an open pit, or in some cases to a tailings impoundment (Mudder andMiller, 1998). The important concept to note relative to this figure is that water management planning is not a one-time occurrence, rather water management evolves from the planning stage before the operation begins, to active management during operation and finally to determining how waters are to be handled when the operation closes.

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FIGURE 5.8 Example Water Management Approach for a Milling Operation

Land Application

Acidic Water and Other Mine Waters

Enhanced Evaporation

Excess

Cyanide Treatment or Recovery

Ta ilin s

Tailings Impoundment

Excess

Water Treatment

Reuse Excess

Mill and Metal Recovery

Dust Control

Surge Pond

Water Dischar e

Fresh Water


Receiving Water

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FIGURE 5.9 Flowsheet Illustrating the Life-Cycle Management of Water at a Heap Leaching Operation

Development of a Site Water Balance

Natural Attenuation

Derivation of a Water Management Plan

Passive Treatment

Review of Decommissioning Options

Enhanced Evaporation

Fresh Water Rinsing

Recycle

Selection of Water Treatment Process

Land Application

Review of Water Discharge Options

Surface Water Discharge

Open Pit

Transfer of Solutions

Tailings Impoundment

Adapted From: Mudder and Smith, 1998

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5.9

REFERENCES

Hutchison, I., Environmental Solutions, Private Communication, 1989. Mudder, T., Fox, F., Whitlock, J. and Marshall, S.,“Development of Site Specific Discharge Criteria Through Toxicological Testing”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Mudder, T. and Miller, S., “Solution Management and Closure Alternatives for Precious Metals Heap Leach Facilities”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Mudder, T. and Smith, A., “Solution Management During Decommissioning of Heap Leach Operations”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Mudder, T., “Derivation of Aquatic Life Criteria for Total and Iron Cyanide ”, In The Cyanide Monograph, Edited by T. Mudder, Mining Journal Books Limited, London, 1998. Tchobanoglous, F. and Burton, F.L., Wastewater Engineering Treatment, Disposal, and Reuse, Third Edition, McGraw-Hill, New York, 1991. USEPA, Guidance for State Water Monitoring and Wasteload Allocation Programs, ICAO440/4-85-01, Office of Water Regulations and Standards, Washington, D.C., October, 1985a. USEPA, Technical Support Document for Water Quality Based Toxics Control, ICAO-440/4-85032, Office of Water, Washington, D.C., September, 1985b. USEPA, Water Quality Standards Handbook, Office of Water Regulations and Standards, Washington, D.C., December, 1983. USGS, United States Geological Survey, Stream Gauging Station Number 06208400, Elbow Creek Near Joliet MT, 1984. Van Zyl, D., Hutchison, I. and Kiel, J., (Editors), Introduction to Evaluation, Design and Operation of Precious Metal Heap Leaching Projects, Society of Mining Engineers, Littleton, Colorado, 1988.

5.10

BIBLIOGRAPHY

Environment Australia, Best Practice Environmental Management in Mining: Water Management, May, 1999. Environment Australia, Best Practice Environmental Management in Mining: Cyanide Management, June, 1998.


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Minerals Council of Australia (MCA), Minesite Water Management Handbook, 1997. Minerals Council of Australia (MCA), Tailings Storage Facilities at Australian Gold Mines, February, 1996. Mudder, T.I. and Miller, S.,“Solution Management and Closure Alternatives for Precious Metals Heap Leach Facilities”, In The Cyanide Monograph, Edited by T.I. Mudder, Mining Journal Books Limited, 1998. Mudder, T.I. and Smith, A., “Solution Management During Decommissioning of Heap Leach Operations”, In The Cyanide Monograph, Edited by T.I. Mudder, Mining Journal Books Limited, 1998. Standard Methods for the Examination of Water and Water, APHA-WPCF-AWWA, 17th Edition, 1989. Stephan, C., "Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses", USEPA, NTIS No. PB85-227049, Corvallis, Oregon, 1985.

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CHAPTER SIX

Treatment and Recovery of Cyanide 6.1

INTRODUCTION

The increase in global mining activity coupled with greater environmental awareness has generated renewed interest in developing processes for the treatment of metallurgical solutions and other mine waters. Presently, there are a variety of proven and reliable chemical, physical and biological treatment processes available for removal and recovery of cyanides and related compounds which are present in mine waters. The goal of water management and treatment is to develop the most economical process or combination of processes which will produce effluents compatible with the criteria, beneficial uses and life forms associated with a receiving system (e.g., stream, lake, groundwater, etc.). Development of a viable water treatment process requires thorough knowledge of the metallurgical processes, ore geochemistry, site hydrology and potential water sources and characteristics. Every facet of the mining operation affects the quantity and quality of water produced and therefore the subsequent selection of treatment alternatives. The process or processes selected must be reliable and flexible enough to maintain a consistently high quality effluent throughout the life of the mine, and in some cases well beyond mining through closure. In development of new mining operations, early consideration of water management and treatment is essential. Significant increases incost and other environmental problems oftenarise when consideration of water treatment is left until the latter stages of mine operation. In order to minimize treatment costs, every attempt should be made to minimize water consumption and maximize water recycle and reuse. In addition, reagent consumption (e.g., sodium cyanide) should beheld to a practical minimum. Furthermore, in the event thata tailings impoundment is employed, the approach should be to enhance natural attenuation processes for the removal of metals and cyanide. In addition to cyanide and metals, mine waters may contain a variety of other compounds of secondary concern, including thiocyanate, cyanate, ammonia and nitrate. These compounds arise as either products of metallurgical processing or as by-products of natural or treatment processes. These compounds are important from both a toxicity and water treatment standpoint.

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Several different water sources may srcinate at a mining site, resulting from runoff, acidic mine water generation or metallurgical processing. These wastewaters exhibit different characteristics and may require different treatment approaches. For example, the treatment processes utilized in removal of free metals in acidic mine water may not be equally effective in removing metals present in process wastewaters containing cyanide. The complexes formed between cyanide and metals alter their chemical characteristics and the manner in which they respond to treatment. The key to treatment of cyanide and metal containing solutions is to remove cyanide first, thereby placing the metals in forms amenable to removal through conventional treatment processes. The degree and type of treatment selected is governed by economics, efficiency and environmental concerns. The environmental concernsare translated into effluent design goals as previously discussed. The wastewater treatment facility forms the bridge between the mining operation and the receiving system and is often the area of greatest concern from public and political viewpoints. The purpose of this chapter is to discuss various water treatment processes, their common areas of application and treatment performances that can be achieved for various compounds present in mine waters. Emphasis is placed upon treatment processes with proven fieldsuccess, as well as those processes exhibiting significantpotential for future application. The approach is to provide information which is useful in the initial stages of selection of treatment processes and to provide a basic knowledge of cyanide and metals removal from metallurgical solutions and other mine waters.

6.2

LABORATORY AND PILOT PLANT INVESTIGATIONS

Regardless of the preferred water treatment system for a site, a certain degree of testing must be conducted in order to verify the viability of the processes selected and to obtain the necessary design information for a full-scale facility. The level of testing required is dependent upon the chemical matrix of the water source, the degree of treatment required to meet effluent design goals and the experience and expertise of the engineers and scientists investigating the problem. The development of suitable water treatment alternatives requires an integration of chemical knowledge and engineering skills. The chemistry of the water sources may be complexand the constituents of concern must be identified initially to define the type of treatment necessary. The level of testing ranges from laboratory bench-scale studies to continuous flow pilot plant investigations. In addition to numerical evaluations of treatment performance, toxicity testing may be conducted in conjunction with these tests to verify the environmental acceptability of the effluent.


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In the event a chemical treatment process is chosen, the bench-scale tests require about 4 to 6 weeks to complete. It is very important that the sampleutilized in the tests is characteristic of the actual solution to be treated, and in some cases this may necessitate conducting tests on-site. The water sample may be obtained from either preliminary metallurgical testwork or from an actual operation. As a result, initial metallurgical testing hs ould be conducted in a manner to produce sufficient sample for subsequent treatment studies, and testwork should be conducted with solution representative of that anticipated during operations. Critical to the success of a testing programme is to ensure the appropriate process questions are addressed (e.g., effluent qualities, reaction rates and chemical dosages) and the appropriate analytical testing is conducted. About 40% to 60% of the cost of these studies is generally associated with analytical work. A major problem associated with completion ofthese studies is turn around of analytical data, which may require up to several weeks. It is not generally possible to conduct further tests when the data on which further decisions will be made has not been received. The cost of the bench-scale or laboratory investigations may range from about $10,000 to $50,000. Toxicity testing will increase these costs depending upon thenumber and type of tests conducted. During the laboratory evaluations usually either static or fl ow through 48 or 96-hour bioassays are utilized. These costs for a single test with a single organism range from about $1,500 to $2,500, including analytical costs. It should be noted that laboratory certification does not necessarily ensure a toxicity testing or analytical laboratory will yield good results. It is important to choose a laboratory not only on the basis of cost, but also on the basis of quality, reliability, experience and turn around time. Following the initial laboratory studies, it may be necessary to operate the process on a small scale through the use of a pilot plant. Pilot plant testing is conducted to gather additional information on process performance and reliability, to obtain further data on process operation, or to test a novel treatment process on a continuous basis. Where and when possible, theuse of pilot plant testing is recommended. Pilot plant systems can be built or rented in certain cases from various vendors or manufacturers. A pilot plant investigation may require 3 to 18 months to complete and cost from $50,000 to $250,000 including capital, operational, analytical and labour expenditures. The entire design process from conception through construction, and start-up of a full-scale facility usually requires about two to three years. The entire process can be conducted in about twelve months under special circumstances (e.g., ideal weather, simple process design and dedication of an entire engineering staff).

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The operation of the treatment facility and water management system should be integrated with the operation of the mine itself. As a result, the treatment plant operators should work in close contact with mill, metallurgical and mining personnel to ensure efficient operation of the treatment plant. Often problems arise due to the lack of communication between these key personnel. A detailed training programmeshould be developed for the operators, as well as an operations/maintenance (O/M) manual for the treatment facility. This approach aids the operator in making decisions and identifying potential problems before they occur.

6.3

ACTIVATED CARBON ADSORPTION

The use of activated carbon adsorption for cyanide removal srcinated from attempts to employ it as a catalyst in the oxidation of cyanide with oxygen. It was noted that the cyanide was first adsorbed, then catalyticallyoxidized. The presence of metal ions, particularly copper, enhanced the removal of cyanide through formation ofthe copper cyanide complex. The best application of granular activated carbon is as a polishing step for the removal of low levels of WAD cyanide and its complexed metals. Subsequently, the removal of cyanide was investigated using granular activated carbon without oxidation (Reed, 1971). It was found that addition of copper or nickel was necessary for removal of cyanide. The process involves addition of metal ions, followed by a complexation period, followed by adsorption onto granular activated carbon. The study was quite extensive and investigated the regeneration and reuse of activated carbon in conjunction with removal. Regeneration of the carbon was accomplished through an acid stripping cycle using sulphuric acid. Effluent WAD cyanide concentrations of less than 0.5 mg/L were obtained using adsorption columns and influent copper cyanide levels of 75 mg/L as cyanide. However, the cost associated with purchasing fresh carbon and its regeneration render the process impractical in most cases involving elevated WAD cyanide levels. On the other hand, the use of activated carbon adsorption is a viable process for the removal of trace levels of complexed cyanide (<2.0 mg/L) and metals from decant water following reduction through natural attenuation or treatment. Fresh carbon can be employed and once exhausted, recycled into a metallurgical circuit utilizing activated carbon. This approach may find application in a specific instance where low flows of excess process water must be polished prior to discharge. The reduction in WAD cyanide and metals levels has been noted in many instances where activated carbon is used to recover residual gold from decant solutions. In one study of the authors, using a continuous upflow activated carbon column and actual decant water, the initial total and WAD cyanide levels of 1.7 mg/L and 1.2 mg/L were reduced to <0.15 mg/Lwere overalso a 72-hour testing period.levels In addition, excellent removal of copper,mg/L silverand and0.02 mercury achieved. The initial of copper, silver and mercury of 0.39 mg/L, 0.90 mg/L and 0.014 mg/L were reduced to 0.02 mg/L, 0.05 mg/L and <0.005 mg/L, respectively, during a 48-hour adsorption period.


242


In addition to granular activated carbon, studies have been undertaken to investigate the removal of cyanide using powdered activated carbon in a complete mix batch or flow through process (Huff and Bigger, 1980). The process was first examined for use in the petroleum industry. Oxygen and copper were needed to promote free cyanide removal as a metal complex. The best application involves waters in which the metal cyanide complexes exist initially, and low concentrations must be treated. Another potential use is in the treatment of diluted rinse solutions from heap leach operations. A review of the performance of several pilot and full-scale activated carbon systems used to treat mine waters was conducted by Botz and Mudder (1998a). In this work, six case studies were discussed relative to mine waters containing various levels of cyanide and metals. One case study involved the use of an activated carbon treatment plant to polish water exiting a hydrogen peroxide treatment plant. This activated carbon treatment plant was operated for several years primarily for the removal of WAD cyanide and copper. The metallurgical processing facility included a carbon-in-leach (CIL) circuit, and the approach was to add new carbon to the activated carbon water treatment plant. When this carbon was loaded it was then forwarded for continued use in the CIL circuit without any loss in activity relative to gold and silver recovery. In this manner, no additional carbon was consumed at the site and operating costs for the activated carbon water treatment plant were minimal. Over a period of several years, the copper level in the water was reduced from 0.15 mg/L to 0.02 mg/L, and the WAD cyanide level was reduced from 0.12 mg/L to 0.04 mg/L. Laboratory testwork with activated carbon can involve shake flask testing and/or continuous throughput column testing. With the flask testing, varying amounts of carbon and untreated water are placed into shake flasks and allowed to react for a few hours. From this work the cyanide and metals concentrations in the solution are examined as a function of time and activated carbon content. Results are then used to design continuous throughput carbon column tests in which carbon loading and compound removals are assessed on a dynamic system. Normally three or more columns are placed in series and operated in an upflow configuration. Key data gathered from this work includes cyanide removal as a function of carbon depth and the loading capability of the cyanide and metals onto carbon. In some cases it is recommended that loaded carbon be regenerated and re-run in the columns to gather data regarding carbon regeneration techniques and the efficiency of carbon following one or more regenerations. A schematic illustrating the configuration of an activated carbon water treatment plant is presented in Figure 6.1. In this system, three gravity upflow columns are utilized, however in some cases it may be preferable to usepressure carbon columns. Loaded carbon from the system could either be directed to a carbon-in-pulp (CIP), carbon-in-leach (CIL) or carbon-in-column (CIC) circuit for use in recovering precious metals. Another alternative is to utilize a carbon regeneration system and re-use the regenerated carbon in the treatment plant. The advantages of using activated carbon for mine water treatment are presented in Table 6.1.

243

Chapter Six


FIGURE 6.1 Example Schematic of an Activated Carbon Treatment Plant

Carbon Regeneration System

Make-Up Carbon

Regenerated Carbon

CIP, CIL or CIC Circuit e a o L

n o b r a C

Water to be Treated Carbon Column 1


Treated Carbon Column 2

244

Carbon Column 3

Effluent


TABLE 6.1

Advantages and Disadvantages of Activated Carbon Water Treatment

Advantages 1 Process is simple in design and operation. 2 Low levels of cyanide and metals can be achieved. 3 Process can be integrated with existing CIP, CIL or CIC circuits. 4 Treatment chemicals are not introduced into the water. 5 Operating costs are low if carbon does not require separate regeneration. 6 Minimal operator control and process monitoring are required. 7 Equipment is widely available and familiar to mining personnel. Disadvantages

1 2 3

Process not suitable for treatment of elevated levels or metals. Separateiscarbon regeneration facilities, if required, canofbecyanide expensive. Not suitable for the removal of thiocyanate, cyanate, ammonia or nitrate.

245

Chapter Six


6.4

ALKALINE CHLORINATION

6.4.1

Introduction

Alkaline chlorination is a chemical treatment process involving the oxidation of free and WAD forms of cyanide under alkaline conditions. This process is the oldest and most widely recognized of the cyanide destruction processes, with the first industrial applications being in the treatment of metal plating and finishing wastewaters. The use of alkaline chlorination in mine water treatment resulted from the chemical similarity noted between metal plating and mine waters. A few mining operations still use thealkaline chlorination process, although the trend is toward other oxidation processes for the treatment of excess metallurgical and tailings impoundment wastewaters. The alkaline chlorination process is best applied on clear solutions when WAD cyanide, thiocyanate and/or ammonia require removal. 6.4.2

Process Chemistry

In the first stage of the alkaline chlorination process, free and WAD forms of cyanide are converted to cyanogen chloride (CNCl) using either a source of chlorine (Cl 2) or hypochlorite (OCl ): (6.1)

Cl2 + CN- → CNCl + Cl-

(6.2)

OCl- + CN- → CNCl + Cl-

These reactions are rapid andoccur under a wide range of pH values (White, 1972). The source of chlorine for the above reactions is typically either liquefied chlorine or a concentrated solution of sodium or calcium hypochlorite (NaOCl or Ca(OCl) 2). Calcium hypochlorite is also available in solid form, and chlorine or hypochlorite can be generated on-site electrolytically if a large chlorine demand is expected. Cyanogen chloride is an intermediate and can volatilize from solution if the pH is less than about 8.0, therefore this reaction istypically conducted at apH in the range of 10.5 to 11.5. In this pH range, cyanogen chloride hydrolyses to yield cyanate: (6.3)

CNCl + H 2O → OCN- + Cl- + 2H+

These reactions are complete in approximately 15 minutes at a pH of 10.5. The oxidation of cyanide to cyanate requires approximately 2.75 parts of chlorine per part of cyanide, although in practice the chlorine consumption is much higher. The second stage of the oxidation involves hydrolysis of the cyanate in the presence of chlorine or hypochlorite to yield ammonia and carbonate according to the following reaction: (6.4)

OCN- + OH- + H2O → NH3 + CO3-2


246


The hydrolysis requires approximately 1.0 to 1.5 hours to complete, although reaction periods of several hours have been employed. If excess chlorine or hypochlorite is added, ammonia will react further through the process of breakpoint chlorination to yield nitrogen gas as shown in the following equation. (6.5)

3Cl2 + 2NH3 → N2 + 6Cl- + 6H+

The process is not typically conducted to the breakpoint stage due to increased chlorine demand and the excessive reaction period required at the elevated pH utilized in the process. Thiocyanate (SCN-) contributes to the overall chlorine demand and is found in varying quantities in many mine waters. Thiocyanate is oxidized in preference to cyanide and in theoryrequires 4.9 parts of chlorine for each part of thiocyanate according to the following reaction: (6.6)

-

-2

-

-

4Cl2 + SCN + 5H2O → SO4 + OCN + 8Cl + 10H

+

Cyanide is present in several forms in mine and tailings impoundment wastewaters including free cyanide and metal complexed cyanides. The metal-complexed cyanides are presentin two forms, including the stable iron and cobalt cyanide complexes and the weak metal complexes of cadmium, nickel, zinc and copper. Alkaline chlorination will remove, underambient conditions, all forms of cyanide except the stable iron and cobalt cyanide complexes. The concentration of iron complexed cyanides can be reduced by alkaline chlorination through employment of elevated temperatures or the addition of ultraviolet light, both of which are relatively impractical at a large scale and increase the cost of the process significantly. The applications of alkaline chlorination are often limited to those waters containing low levels of the iron complexed cyanides. Since the process is operated at elevated pH values precipitation of metals as their hydroxides also occurs. Design of the process is straightforward with process design information usually obtained through bench-scale and/or a pilot plant testing programme. The chemical feed equipment and monitoring systems are commercially available from many vendors. In the event that liquid chlorine is used, there is a need for storage and handling of either bulk chlorine or one-tonne chlorine cylinders. The storage facility must have scales and a feed system to dispense and monitor consumption. The feed system consists of an evaporator to convert liquid chlorine to a gas and a chlorinator to draw, dissolve and distribute chlorine into the water. The feed system also includes the necessary flow metres, pressure regulators and control valves. Reliable automated chlorine feed systems are available from several vendors. For pH control in the reactions, slaked lime is usually supplied from the existing mill storage and slaking facility. Alternately, a separate lime or caustic storage and feed system can be utilized. The reaction vessel or vessels are constructed of corrosion resistant material and are usually baffled to minimize short circuiting. If required, a dechlorination system utilizing sulphur dioxide or sodium sulphite is employed using either a complete mix reactor or a conventional serpentine dechlorination basin.

247

Chapter Six


In addition to maintaining a pH of 10.5 or greater in the reactor tank, control of the process depends on the measurement of the "redox" potential (or ORP). This potential is maintained at a value high enough to ensure complete conversion of cyanide to cyanate by manual or automatic adjustment of the chlorine addition or by theflow of untreated water. However, this strategy is not always effective. An alternative control strategy involves measurements of "total residual chlorine" (TRC) in the treated effluent. The presence of residual chlorine is an indication that the oxidation of cyanide to cyanate is complete. Periodic TRC and potentiometric titrations of the water against a standard sodium hypochlorite solution are employed to establish the most effective control potentials, as shown on Figure 6.2. 6.4.3

Performance

During the period October 1981 to March 1983, Environment Canada with the assistance of a consultant, completed a study of the performance of alkaline chlorination plants at three mills in British Columbia (Mosquito Creek, Baker and Carolin) and one in the Northwest Territories (Giant Yellowknife) (Brodie, 1983 and Zaidi and Brodie, 1983). Figures 6.3 to 6.6 present the respective flow sheets for these alkaline chlorination processes. A generalized schematic of the process is presented on Figure 6.7. Table 6.2 lists the operating parameters of these plants, while Table 6.3 illustrates the process efficiency with regard to cyanide and metals removal. Although there are wide variations in the compositions of both the treated and untreated waters, all systems were effective in removing WAD cyanide, with removals ranging from 98.7% to 99.9%. Some plants were less effective in removing total cyanide due to the presence of significant levels of iron in solution. This confirms the ineffectiveness of alkaline chlorination for theremoval of iron cyanide. Copper and zinc on the other hand were effectively removed (94.7% to 99.6%). It should be pointed out that the studies which provided this information included performance evaluations over only a few weeks. In addition, all but Giant Yellowknife were new operations and the alkaline chlorination systems had not been optimized. The data of Table 6.4, consisting of averagesof analyses for the entire 1984 operating period at Giant Yellowknife, provide a better indication of the reliability of alkaline chlorination. The excellent quality of the tailings pond decant, except for ammonia, is noteworthy. Except in the case of Giant Yellowknife, WAD cyanide was reduced to <1.0 mg/L in all plant effluents. At Giant Yellowknife, WAD cyanide dropped to0.10 mg/L following retention in a polishing pond. Where thiocyanate levels in the feed were high there was a corresponding increase in the cyanate level in the effluent. The levels of copper and zinc varied considerably and mayhave been influenced cyanate. by the metal-complexing action of the ammonia produced as a result of hydrolysis of


248

FIGURE 6.2 Redox and TRC Response Curve During Titration of Gold Mill Barren Solution with Standard Hypochlorite Solution

2 4 9

C ha pt er ixS


FIGURE 6.3 Mosquito Creek Mine – Alkaline Chlorination Flowsheet



250


FIGURE 6.4 Baker Mine

–

Alkaline Chlorination Flowsheet


251

Chapter Six


2 5 2

FIGURE 6.5

Carolin Mine – Alkaline Chlorination of Tailings Pond Reclaim Water

FIGURE 6.6

2 5 3

C ha pt er ixS

Giant Yellowknife – Alkaline Chlorination and Arsenic Removal Flowsheet


2 5 4

FIGURE 6.7

A Schematic of an Alkaline Chlorination Process


TABLE 6.2

Operating Parameters for Fu ll-Scale Alkaline Chlorination Operations

Mosquito Creek Mine

Mill capacity (tpd)(1) Solidscyanided Solids (tpd) Modeoftreatment Solutiontreated

100 Ore 100 Batch Barren

Formofchlorine

1,250 Concentrate 75 Continuous Barren

14.4 m3 per day

Gaseous 1

216 m3 per day

Calcium hypochlorite 2

Gaseous 1

6

14

11 Manual Manual

11.5 Manual Manual

Giant Yellowknife Mine 1,200 Roastercalcine 140 Continuous Tailingspond overflow 6,545 m3 per day

Carolin Mine

100 Ore 100 Continuous Barren

3-5.5m 3 batches/day

Solutionrate

No. reactor of tanks Retention time (hrs) pH pH control Chlorinecontrol

Baker Mine

Gaseous 1

8 11 Auto Manual

0.5 11.5 Auto Manual


Notes: (1) tpd = metric tons (tonnes) per day.

255

Chapter Six

TABLE 6.3


Constituent, mg/L (1) Mine

Baker

Carolin

2 5 6

Performance Data for Full-Scale Alkaline Chlorination of Gold Mill Effluents

Mosquito Creek

Giant Yellowknife

CNT(2)

CNW(3)

SCN

Cu

Fe

Ni

Zn

As

NH3

TRC (4)

Influent

2,000

1,900

--

290

2.4

--

740

--

--

--

Effluent

8.3

0.7

--

5.0

2.8

--

3.9

--

--

2,800 (5)

% Removal

99.6

99.9

--

98.3

--

--

99.5

--

--

--

Influent

1,000

710

--

97

150

--

110

--

--

--

Effluent

170

0.95

--

0.38

53

--

5.8

--

--

190

% Removal

83

99.9

--

99.6

64.7

--

94.7

--

--

--

Influent

310

226

--

10.0

9.4

--

93

--

--

--

Effluent

25

0.49

--

0.33

8.0

--

1.4

--

--

320

% Removal

91.9

98.8

--

96.7

14.9

--

98.5

--

--

--

Influent

7.5

7.1

6.3

6.7

<0.1

1.2

0.1

12.1

--

--

Effluent

1.3

1.2

1.0

0.09

<0.1

0.7

0.1

--

--

--

% Removal

82.7

85.1

84.1

98.7

--

41.7

--

--

--

--

Polishing Pond O/F

0.15

0.09

--

0.03

<0.1

--

<0.1

0.14

9.4

1.1

98

98.7

--

99.6

--

--

--

99.7

--

--

% Removal Source:

Scott and Ingles, 1987

Notes:

Unfiltered samples. CNT = total cyanide by distillation. CNW = WAD cyanide by ASTM Method C. Total Residual Chlorine. Additional chlorine added with a view of destroying cyanide contained in tailings slurry.


TABLE 6.4

Wastewater Influent Effluent % Removal (2) Pond Effluent % Removal

Giant Yellowknife Alkaline Chlorination Performance Data (1) (Average for 1984)

CNT 7.8 0.05 99.4 0.02 99.75

Cu ---

-0.04 --

Constituent, mg/L Ni Zn Pb ---------0.05 0.05 0.02 ----

As 13.7 0.40 97.1 0.38 97.2

NH3 ---5.5 --

Source: Scott and Ingles, 1987 Notes: (1) Plant only operates in the summer. (2) Following dilution and natural attenuation.

257

Chapter Six


Chlorine consumption in most cases was dependent upon the levels of other oxidizable compounds, varying from 5 to 12.5 mg Cl2 per mg of total cyanide. Residual chlorine levels in the treated barren solutions werehigh in some cases. In the case of the barren solution from the Baker mill, it was deliberately overdosed with chlorine, with the intention of destroying the residual cyanide contained in the tailings slurry when the two streams were combined in the tailings pond. At Giant Yellowknife, chlorine consumption inthe cyanide destruction stagewas increased due to a requirement to oxidize trivalent arsenic to pentavalent arsenic prior to its precipitation and removal. The alkaline chlorination process has been used at the 500 ton per day Grey Eagle open pit mine near Eureka, California to treat about 400 gpm of tailings pond overflow. As well as receiving process effluents, the tailings pond also receives acidic mine drainage containing elevated iron levels, srcinating from an adjacent abandoned underground mine. The treatment plant contains three chlorination reactor tanks in series followed by two clarifiers in parallel. The clarifier underflows are returned to the tailings pond, while the overflows are pumped to a designated land application site. Gaseous chlorine is used to provide a chlorine/total cyanide ratioof about 7.5/1. Retention time in the oxidation reactors is 45 minutes at pH 10 to 11. Table 6.5 shows typical analyses of water to and from the treatment plant. The alkaline chlorination process has been used for treating slurries at mining operations, but with the advent of other slurry treatment processes over the past twenty years, alkaline chlorination has been graduallyreplaced for slurry treatment. The drawback with the alkaline chlorination process with slurry is the high consumption of chlorine resulting from the oxidation of cyanide plus the oxidation of several other soluble and solid compounds. Alkaline chlorination can be considered for temporary treatment of slurry under emergency conditions or when the treatment isanticipated to be intermittent and/or short-term. In these cases, a relatively inexpensive liquid sodium hypochlorite injection system may be practical, but a careful engineering assessment should be conducted before implanting a system of this sort. The treated effluent is usually directed to a polishing pond to permit the settling of metal hydroxides, and the dissipation ofchlorine. There are no data on the stability of the resulting sludges. Limited evaluations conducted by Homestake Mining Company indicated the metal hydroxides sludges to be stable once disposed of in the tailings impoundment, as long as the pH remained elevated above 10.0. 6.4.4

Operating Costs

Table 6.6 is a summary of the reagent consumptions at three mines utilizing the alkaline chlorination process. In the case of two of these, Mosquito Creek andBaker Mines, the data are from a survey conducted forand Environment Canada consisting of five one-week campaigns at Mosquito Creek three one-week campaigns at Baker (Brodie, 1983).sampling These were new operations and the alkalinechlorination systems werenot optimized. The information on Giant Yellowknife Mines consists of the company's own averages of data covering entire annual operating periods (i.e., 181 days in 1983 and 173 days in 1984) for a system that has been in operation since 1981 (Halverston and Cross, 1983).


258


TABLE 6.5

Typical Analyses at Grey Eagle Treatment Plant

Tailing Pond Decant 400 250

Total Cyanide, mg/L (1) Free Cyanide, mg/L mg/L Cu,

Treated Effluent <5.0 0.2

300

0.5

Source: Scott and Ingles, 1987 Notes: (1) Free cyanide was measured using a cyanide specific electrode.

TABLE 6.6

Operating Parameters

Mill Throughput, tonnes per day Volume of Water, m3/day CNT Concentration, mg/L Reagent Consumption 1. Lime (CaO): kg/m3 kg/kg CNT kg/tonne ore 2. Chlorine (Cl2): kg/m3 kg/kg CNT kg/tonne ore

Summary of Alkaline Chlorination Reagent Consumptions

Mosquito Creek (1) 1983

Baker (1,2) 1983

Giant Yellowknife (3) 1983

Giant Yellowknife (4) 1984

100

100

1200

1200

16.5

14.4

7970

8890

310

2000

9

8

3.64 11.73 0.60

4.17 2.08 0.60

5.45 17.6 0.90

13.0(3) (3) 6.5 1.88 (3)

0.36 40.6 2.39

0.30 38.2 2.21

0.12 13.5 0.80

0.13 16.4 0.95

Source: Scott and Ingles, 1987 Notes: (1) Canviro, 1985. (2) Baker Mine used Ca(OCl)2 converted to Cl2 equivalent. (3) Halverston and Cross, 1983. (4) Includes chlorine and lime consumed in arsenic removal.

259

Chapter Six


At Mosquito Creek and Baker Mines, the effluent treated consisted of barren solution, while at Giant Yellowknife it was the overflow from a secondary polishing pond and contained arsenic in 3 addition to cyanide. Reagent consumption is calculated in kg/m , kg/kg total cyanide and kg/metric ton of ore. The differences are due to the relative proportions of WADcyanide, iron cyanides and thiocyanate. In the case of the Giant Yellowknife plant, additional chlorine is consumed in converting trivalent arsenic to pentavalent arsenic and in providing excess chlorine to convert the reagent ferrous sulphate to ferric u s lphate for the precipitation of arsenic. In Table 6.7 these reagent consumptions are converted to reagent costs using a Canadian price of $0.07 2). These costs are F.O.B. Vancouver in per kg for lime (CaO) and $0.66 per kg for chlorine (Cl Canadian dollars and do notinclude transportation to the respective mine sites. In the case of the Giant Yellowknife operation, actual costs, including transportation, are also given (Canviro, 1985 and Halverston and Cross, 1983).

6.4.5

Toxicity of Alk aline Chlorination Eff luents

The primary concern associated with effluent from an alkaline chlorination process, aside from cyanide and metals, is the residual chlorine and chloramines which form through reaction of the oxidant with ammonia in solution. These compounds are toxic to aquatic life and must be removed through dechlorination prior to discharge of treated effluent. Very stringent effluent guidelines have been established for total residual chlorine in many countries. Bioassays have indicated that environmentally acceptable effluents could be produced following dechlorination using sodium sulphite. In the event the removal of iron cyanides is not required and the initial thiocyanate concentrations are low, the alkaline chlorination process provides a viable treatment alternative for the removal of WAD cyanide and metals from solutions. The various advantages and disadvantages of the alkaline chlorination process are summarized in Table 6.8.


260


TABLE 6.7

Cost Basis

Calculated (1) $ per m3 $ per kg CNT per $tonne ore

Summary of Alkaline Ch1orination Reagents Costs

Mosquito Creek Mine, 1983

Baker Mine 1983

3.94 12.70

(2)

Giant Yellowknife (3) Mine, 1983

9.07 4.54

0.65

1.31

0.106 11.95 0.354

(2)

Actual $ per m3 $ per kg CNT per tonne $ ore

----

----

0.41 46.49 1.38


Notes: (1) Costs in Canadian dollars. (2) Delivered at Yellowknife (Halverston and Cross, 1983). (3) Includes chlorine and time consumed in arsenic removal.

261

Chapter Six


TABLE 6.8

Advantages and Disadvantages of the Alkaline Chlorination Process

Advantages 1 Well established process with considerable operational experience and engineering expertise. 2 Process equipment and control are reliable. 3 Heavy metals are precipitated as hydroxides. 4 Thiocyanate is oxidized. 5 Reactions are relatively complete and mechanisms understood. 6 Chlorine is available worldwide in several forms. 7 Free and WAD forms of cyanide are oxidized producing low residual effluent concentrations. 8 Process is adaptable to continuous or batch operation. 9 The process can be employed in the treatment of pulps and clarified metallurgical solutions. 10 Cyanate and ammonia can be removed through breakpoint chlorination. Disadvantages 1 Reagent consumption and costs can be excessive if high concentrations of cyanide and

2 3 4 5 6 7 8 9

thiocyanate are present. Process pH must be controlled carefully to avoid release of cyanogen chloride. Iron complexed cyanides are not removed under ambient conditions. The end products of alkaline chlorination, including residual free chlorine and chloramines, must be removed, as these compounds are toxic to aquatic life. Additional treatment processes may be required to remove ammonia, iron complexed cyanides and metals if very low limits are promulgated. High concentrations of cyanate can be produced, potentially increasing the ammonia concentration in the solution. Large quantities of treatment chemicals may be introduced into the water. Cyanide is not recovered, but destroyed. Cyanide in tailings slurries may not be effectively destroyed and/or reagent consumption may be excessive if the solid phase contains significant concentrations of reduced sulphides or iron.


262


6.5

ACTIVE A ND PA SSIVE BI OLOGICAL TR EATMENT

6.5.1

Introduction

Biological processes have been proven effective in the treatment of elevated concentrations of cyanide, but it is only in the last two decades that they have been utilized in the treatment of mine waters containing cyanide and related compounds. Biological treatment facilities may include chemical and/or physical processes, but the primary treatment takes place through one or more biologically mediated reactions. These reactions occur as the result of enhanced biological activity in the water with the objective of converting potentially harmful compounds into less toxic compounds. Biological treatment occurs under carefully controlled conditions with populations of micro-organisms selected for removing one or more constituents. There are several fundamental aspects of biological water treatment that must be understood before proceeding with a discussion of mine water treatment. In this regard, the following definitions of the general type of biological processes are provided (Tchobanoglous and Burton, 1991). Aerobic:

Aerobic biological reactions occur in the presence of dissolved oxygen and result in the oxidation of chemical compounds. Examples include the oxidation of cyanide (CN-) to yield cyanate (OCN-) and the oxidation of ammonia (NH4+) to yield nitrate (NO3 ).

Anaerobic:

Anaerobic biological reactions occur in the absence of dissolved oxygen and result in the reduction of chemical compounds. An example is the reduction of sulphate (SO4-2) to yield sulphide (S-2).

Anoxic:

Anoxic biological reactions occur through an aerobic pathway, but do not include the use of dissolved oxygen. These reactions take place at lowdissolved oxygen levels or in the absence of dissolved oxygen. An example is denitrification in which microorganisms utilize nitrate (NO3 ) for growth, thereby reducing nitrate to nitrogen gas (N2).

There are a wide variety of equipment and process configurations that have been utilized to promote the growth and propagation of aerobic, anaerobic and anoxic organisms. These include suspended growth systems in which the microorganisms are maintained in suspension within the solution in slurry form. The other major configuration is the attached growth systemin which the microorganisms are attached to someinert medium suchas rocks or plastic media. Each of these systems has been employedin the treatment of mine waters. The choice of one system versus another is based several and aspects of treatment, including the chemistry of untreated water, economics, space upon requirements required treatment efficiency.

263

Chapter Six


In general, attached growth systems are less costly and easier to operate than conventional suspended growth activated sludge systems. Although effluent recycle may be utilized in attached growth systems, recycle of biomass is not usually required as with suspended growth systems. Since the attached growth processes involve single pass treatment systems, the potential for buildup of toxic and refractory components is minimized in comparison with suspended growth systems. In addition, the attached growth processes are well suited to low organic loadings and limited bacterial reproduction. Attached growth systems produce significantly lower volumes of sludge than suspended growth systems, thereby minimizing sludge handling and disposal requirements. Sludge production becomes an important consideration in the event the sludge contains undesirable levels of potentially toxicsubstances. Several critical reaction environment conditions apply to biological treatment processes in regard to providing a suitable environmentfor the growth and reproduction of the organisms. These include the following: • • • •

Temperature pH Oxygen level Nutrient availability

The optimal temperature range for aerobic and anaerobic treatment is about 10 to 30 °C and 25 to 45°C, respectively. The optimal pH range for aerobic and anaerobic treatment is 6.0 to 9.0 and 6.5 to 7.5, respectively. Oxygen must be absent in anaerobic systems, while in aerobic systems the dissolved oxygen level should maintained above to 1 2 mg/L. It is common mine waters to contain insufficient levels of thebe required biological nutrients, including nitrogenfor and phosphorous. Depending upon the bacteria utilized, nitrogen can be added as ammonia or nitrate, while phosphorous can be supplied through addition of phosphoric acid regardless of the bacterial process employed. The lack of phosphate in a process solution inhibits biological activity and usually limits the extent of biological degradation. Even though the proper bacterial species and reaction environment have been provided, biological activity may not occur due to several interrelated factors, including: • • • •

Elevated influent concentration of atoxic compound Inadequate reaction time The presence of toxic and/or inhibitory substances Insufficient organism acclimation periods

In certain cases the compounds targeted for treatment, although known to be degradable, are present in sufficient concentration to induce bacterial inhibition or toxicity. Lowering of the influent concentrations through dilution may be required to initiate treatment.


264


Due to the varied chemical and physical characteristics of mine waters, it is necessary to evaluate the potential for microbial treatment through laboratory studies and/or field pilot plant studies. The information obtained from these investigations is utilized in the development of a final treatment facility design. 6.5.2

Process Chemistry

The success of biological treatment of waters is dependent upon isolating and identifying bacterial species which possess the specific adaptive and constitutive enzymes capable of breaking down the target compounds present inthe water (Mudder, 1998). The microorganisms primarily responsible for biological degradation include a diverse group of anaerobic and aerobic bacteria. These single-celled microorganisms exhibit a wide range of metabolic functions andare capable of degrading a wide range of chemical substances (Mudder andWhitlock, 1984). There are several biological reactions which are of interest in mine water treatment applications and these are briefly discussed below. In any of these reactions, trace metals presentin solution may be removed through uptake into the biological mass or through precipitation as insoluble hydroxides, carbonates or sulphides. Cyanide is oxidized in aerobic biological treatment processes by a number of organisms of the genera of Pseudomonas, Alcaligenes, Achromobacter and others to form cyanate: (6.7)

CN- + ½O2 → OCN-

WAD cyanide can be removed by metalcomplex absorption onto biomass andsubsequent internal decomposition of the cyanide to form cyanate. Strongly complexed cyanides such as iron cyanide are not readily treatable biologically, but can be removed to a limited extent in the process through precipitation and adsorption. Mine waters can contain appreciable concentrations of cyanate due to the oxidation of cyanide. Cyanate is biologically converted to ammonia and bicarbonate according to: (6.8)

OCN- + H+ + 2H2O → HCO3- + NH4+

Thiocyanate is aerobically converted to ammonia, sulphate and bicarbonate: (6.9)

-

-2

+

-

+

SCN + 3H2O + 2O2 → SO4 + NH4 + HCO3 + H

Biological oxidation of thiocyanate is carried out by a number of aerobic, autotrophic organisms which can use inorganic carbon sources, includingThiobacillus thiocyanoxidans, Thiobacillus thioparus and Thiobacillus denitrificans. Thiocyanate can beoxidized by several species ofthe Pseudomonas genera, which are heterotrophic (require an organic carbon source).

265

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Ammonia is oxidized by nitrifying organisms in two steps; first to nitrite and then to nitrate according to the following equations: +

3

+

-

(6.10) NH4 + /2O2 → 2H + H2O + NO2 (6.11) NO2- + ½O2 → NO3-

The overall reaction for the nitrification process is: +

-

+

(6.12) NH4 + 2O2 → NO3 + 2H + H2O The oxidation of ammonia to nitrite and then to nitrate is carried out by two genera of bacteria, Nitrosomonas and Nitrobacter. These organisms are autotrophic and therefore use inorganic carbon sources. In most nitrification processes, the oxidation of ammonia to nitrite is the rate limiting step, therefore nitrite does not build up ot relatively high levels. The combined nitrification reaction consumes alkalinity and releases acidity, therefore it is necessary to control pH and maintain alkalinity to optimize the process. During the process, 7.2 kg of alkalinity (as CaCO3) is destroyed per kg of ammonia nitrogen oxidized. Both Nitrosomonas and Nitrobacter have long generation times and low cell mass yields. Sludge age, therefore, is a critical factor in the nitrification process. Long sludge age can be achieved by operating with a high biomass inventory and by maintaining long retention times. The nitrification °

process istoalso markedly sensitive to temperature. Operatingthetemperatures 15 C are to desirable achieve complete nitrification although potentially system can above be acclimatized operate at lower temperatures. The nitrifying bacteria are also sensitive to low dissolved oxygen; dissolved oxygen concentrations at or above 2.0 mg/L are desirable. Oxygen consumption ratios are 3.22 mg O2/mg NH4+-N for Nitrosomonas and 1.22 mg O2/mg NH4+-N for Nitrobacter. A pH in the range of 7.8 to 8.5 is theoretically optimal, with significant losses of efficiency below 7.0 (USEPA, 1975). The biological process of denitrification involves the conversion of nitrate and nitrite to gaseous nitrogen. The denitrification process is carried out by facultative bacteria of the genera Pseudomonas, Micrococcus, Achromobacter and Bacillus under anaerobic conditions. These organisms are heterotrophic and thereby require an organic carbon source. Methanol has generally been used for this purpose due to its low cost and ease of use, but other organic carbon sources may be used. The conversion of nitrate to nitrogen gas is a two step process, however both steps are carried out by the same organisms. The reactions as written for the use of methanol (CH3OH) as the organic carbon source are: (6.13) 6NO3- + 2CH3OH + 2OH- → 6NO2- + 2HCO3- + 4H2O (6.14) 6NO2- + 3CH3OH → 3N2 + 3HCO3- + 3H2O + 3OH-


266


The overall reaction for denitrification can be described by: (6.15) 6NO3- + 5CH3OH → 3N2 + 5HCO3- + 7H2O + OHOn an empirical basis, the methanol requirement for both energy and synthesis by the denitrifying organisms is 2.47 g/g of NO3--N removed. However, additional methanol may be required to consume any dissolved oxygen present in the feed water. Experience has indicated that the methanol requirement can vary from 1 to 3 g/g of NO 3 -N removed. The endogenous breakdown of biomass from the activated sludge system upstream could supply some of the required organic carbon for nitrate removal. At low methanol dosages, the denitrification rate will vary with the dosage, while changes at higher dosages should not affect the rate. Denitrification produces in the order of 3.5 g of alkalinity as CaCO 3 per g of NO3 -N removed, potentially requiring the addition of sulphuric acid to control pH in the optimum range of 6.5 to 7.5. The denitrification rate is very sensitive to temperature with the rate doubling with each °4C increase. The process will operate at temperatures as low as 5°C but generally requires temperatures greater than 10°C. The biomass yield of the denitrification process is small, therefore maintaining high biomass inventories and sludge ages will reduce the potential for upsets and loss of the process.

The following sections examine three mine sites where biological treatment processes were implemented on a full-scale to treat a variety of compounds. Several other examples of biological treatment processes can be found throughout the world and in published literature. 6.5.3

Homestake Mi ne Bio logical Tre atment Pr ocess

A full-scale biological treatment facility was designed and placed into operation at the Homestake Mine in Lead, South Dakota in late 1984 (Mudder et al., 1998b and Whitlock and Mudder, 1998). The facility has been in operation for over fifteen years without an interruption or a significant decrease in performance. All forms of metal complexed cyanides are removed ni this plant, including WAD cyanides and the stable iron complexed cyanides, which are removed through a combination of oxidation and sorption into the biofilm. The metals present in the wastewater are removed through a combination of coagulant addition and adsorption onto the biofilm. Ammonia is converted to nitrate via a two-stage biological nitrification process. The suspended solids in the water resulting from sloughed biofilm and precipitated metals are removed from the water through clarification. The clarified effluent is passed through a pressure sand filtration system prior to discharge into a stream serving as a permanent trout fishery. Through extensive toxicological testing the effluent was shown to be environmentally acceptable. An inorganic carbon source is periodically added to aid nitrification (soda ash) and phosphorus is added as a trace nutrient (H3PO4).

267

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Process flow sheets for the facility are presented on Figures 6.8 and 6.9. Integral to the overall design and operation of the facility is the flow equalization system, which allows precise control of the flows of mine water and decant water. The mine water flow, which srcinates from the underground operation, varies according tothe sequential operation of two pumps. With one 3 pump in operation, a flow rate of approximately 6,540 m /day reaches the surface and flows continuously to the treatment plant by gravity. Periodically during a 24-hour period,both pumps 3 /day. The excess are operational, thereby increasing the total flow to the surface to 13,100 m flow is stored in the mine reservoir and gravity fed via a separate line to the treatment facility at 3

a controlled flow rate of approximately 4,360 m/day. Decant water from the tailings impoundment is pumped to a standpipe adjacent to the mill reservoir. The decant flow rate to the treatment plant is controlled byan automated valve located at the bottom of the standpipe. Excess decant water flows into the mill reservoir to be used as recycled process water. The automated valve allows precise control of the desired decant flow rate to the treatment facility. The flow is varied seasonally to achieve desired compound concentrations and loading criteria in the combined untreated water. As the first step in the treatment process, the blended water passes through a Parshall flume and drops into an influent rapid mix tank. Concentrated phosphoric acid is added as anutrient to this water, and soda ash is fed as a 3% to 6% solution via a chemical feed system located adjacent to the chemical feed building. The soda ash requirement is determined by the anticipated alkalinity necessary to complete nitrification. Next, the water flows by gravity through an open channel to the first stage of rotating biological contactors (RBC's). The RBC design incorporates 48 biodiscs arranged in five consecutive stages, with 24 shafts distributed symmetrically oneither side of the influent channel. The first and second stage shafts are placed in twelve basins, with six basins adjacent to either side of the influent channel. The flow is distributed to the twelve first stage shafts via manuallyoperated Vnotch slide gates. The water then passes through an influent well which distributes the flow evenly across the entire lengthof the first stage shaft with the use of V-notch weirs. The flow distribution system minimizes short-circuiting and contributes to even biogrowth on the biodiscs. The first and second stages are housed in a common basin which at present is operated unbaffled to promote a complete mix configuration. The complete mix operation aids in distribution of biomass between the first and second stages and minimizes excessive growth on first stage shafts. At any one time, approximately eight of these basins are in operation. The biological species located on the first and second RBC stages are responsible for oxidation of cyanide and thiocyanate and for the adsorption of trace metals. Water leaving the second stage enters an intermediate collection channel prior to redistribution into third,and fourth fifth stages are of the processinresponsible for nitrification. The twenty-four third,the fourth fifthand stage biodiscs arranged eight common basins. Fibreglass removable baffles are located between consecutive stages to promote a plug flow configuration and greater treatment efficiency. The flow into the third stage is regulated in an identical manner asthe first stages.


268


FIGURE 6.8 The Homestake Mine Biological Water Treatment Process

Source: Mudder et al., 1998b

269

Chapter Six


FIGURE 6.9 Rotating Biological Contactor Plant for the Removal of Cyanide and Thiocyanate at the Homestake Mine

2 7 0

Source: Mudder and Whitlock, 1984


Following treatment, the water passes through a second rapid mix tank located in the chemical feed building. At this point, either a coagulant or anorganic polymer can be introduced to aid in the settling of the sloughed biomass. A circular clarifier is used to remove suspended solids from the treated water. The clarifier is fittedwith a fibreglass cover to minimize wind action and temperature loss during winter operation. Settled solids are fed by gravity to a pump house, combined with tailings and pumped to a tailings impoundment for final disposal. Two existing concrete reservoirs were retrofitted with influent flow distribution and effluent weirs and are used as emergency clarifiers in the event of a clarifier failure, thus eliminating the need for a second clarifier. The clarified water is filtered with pressure sand filters retrofitted with media consisting of anthracite coal and garnet. The filtered water passes through a second Parshall flume and down a natural cascade allowing for reaeration and cooling prior to contact with the receiving stream. The entire treatment facility is monitored and controlled via a computer control system. This system allows the operator on duty to monitor flows, chemical feed rates, pump operations, biodisc speed and weight, clarifier operation and various influent and effluent water characteristics. In addition to the monitoring capabilities, the operator can control mine and decant water flows, operate air compressors, adjust air flows, operate sand filter pumps and vary the speeds of the biodiscs. The treatment facility maintains an analytical laboratory for process monitoring and control. Once per eight hour shift, influent and effluent samples are collected for analysis of temperature, pH, copper, phosphorus, alkalinity, ammonia, nitrite, thiocyanate and turbidity. An additional 24-hour composite effluent sample and influent grab sample are collected and analyzed each day for all these compounds plus total cyanide, WAD cyanide, hardness and a number of trace metals. The compounds of interest within the combined untreated water and their respective concentrations are presented in Table 6.9. The concentration ranges were established through a water monitoring programme, encompassing a four-year period with analyses performed in the Homestake analytical laboratory. The mine water chemical characteristics indicate the concentrations of the parameters of interest are normally low, except for periodic elevated concentrations of ammonia and thiocyanate. Since the mine water temperature remains inherently warm throughout the year, the combined influent water temperature is maintained above 15°C, enabling the biological processes to proceed efficiently and effectively. Performance data for the treatment facility are presented in Table 6.9. As indicated, the removals of copper, ammonia, total cyanide and WAD cyanide are all very good. Of concern relative effluent toxicity are the concentrations copper, WAD are cyanide The average to concentrations of these compounds in theoftreated effluent 0.05, and 0.03ammonia. and 0.12 mg/L, respectively, which was found to be protective of the stream into which this treated water is discharged. Also of note is the removal of total cyanide, which is reducedfrom an average of 3.4 mg/L down to 0.31 mg/L.

271

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TABLE 6.9 Constituent Copper maximum minimum average

Homestake Mine Water Treatment Plant Performance

(1)

Untreated Water

Treated Water

0.73 0.17 0.49 0.92 159

0.13 0.01 0.05 0.02 85

6.4 0.8 3.4 1.7 165

0.67 0.06 0.31 0.12 318

5.2 0.31 2.3 1.5 165

0.14 0.01 0.03 0.18 200

------

16.0 1.0 3.0 2.3 318

10.4 1.0 5.3 1.2 165

0.35 0.10 0.12 0.06 165

9.0 7.5 ----

8.35 7.13 ----

std. dev. N Total Cyanide maximum minimum average std. dev. N WAD Cyanide maximum minimum average std. dev. N Total Suspended Solids maximum minimum average std. dev. N Ammonia (as N) Maximum Minimum Average Std. dev. N pH Maximum Minimum Average Std. dev. N Source: Mudder et al., 1998b

Notes: (1) All concentrations total in mg/L.


272


6.5.4

Nickel Plate Mine Biological Treatment Process

During closure of the Homestake Nickel Plate Mine in British Columbia, Canada it was determined that tailings impoundment solution and seepage collected from the tailings impoundment would need to be treated and discharged into a nearby trout stream (Given et al., 1998). Several water treatment alternatives were considered including land application, biological treatment, breakpoint chlorination, hydrogen peroxide and the SO 2/air process, which was already in use at the mine. The constituents of concern included not only cyanide and metals, but also thiocyanate, ammonia andnitrate. Through extensive laboratory and pilot plant evaluations, the decision was made to use a combined aerobic and anaerobic biological treatment system for removal of residual cyanide, thiocyanate, cyanate, ammonia, and nitrate coupled with a new high density sludge (HDS) process for the removal of trace metals. Biological treatment was chosen because the chemical oxidation processes involving hydrogen peroxide and sulphur dioxide do not remove thiocyanate, ammonia or nitrate. Although breakpoint chlorination does remove these compounds, it is expensive and yields elevated residual chloride concentrations. The application at the Nickel Plate Mine was unique in that the existing metallurgical plant used for the extraction of gold and silver was retrofitted into the full scale biological treatment facility with only minor modifications, resulting in substantial cost savings. The full scale combined biological and chemical treatment facility now in operation at the site consists of the following three separate steps: •

A two-stage aerobic activated sludge treatment step for the conversion of cyanide, thiocyanate and cyanate to ammonia and for the oxidation of the ammonia formed to nitrate.

•

An anaerobic denitrification step to reduce nitrate to nitrogen gas.

•

An HDS step utilizing lime and ferric sulphate to precipitate arsenic and other residual metals.

A basic flow diagram of the overall plant is shown on Figure 6.10. The treatment system also includes reagent make-up, storage, metering and control equipment for lime, soda ash, methanol, phosphoric acid, sulphuric acid and flocculant. Air compressors provide air required for the aerobic biological processes. A propane-fired submerged combustion water heater is used to raise the feed water temperature to approximately 20 °C to allow the biological reactions to proceed at acceptable rates.

273

Chapter Six


FIGURE 6.10

Nickel Plate M ine Biological W ater T reatment Plant

Tailings Impoundment Seepage and Decant Waters

Air Phosphoric Acid Soda Ash

Aerobic Biological Treatment Section (converted leach tanks and clarifier) (cyanide, cyanate, thiocyanate & ammonia removal)

Methanol Phosphoric Acid Sulphuric Acid

Anaerobic Biological Denitrification Section (converted leach tanks and clarifier) (nitrate removal)

Ferric Sulphate Lime Flocculant

High Density Sludge Treatment Section (converted tanks and clarifier) (arsenic and metals removal)

Treated Effluent

Adapted From: Given et al., 1998


274


The biological treatment plant was started up in 1997 and initially treated about 300 to 350 gpm of water, however since that timethe water flow has gradually decreased to about 150 gpm. The performance of the plant in 1999 is presented in Table 6.10 and the reagent consumptions are presented in Table 6.11. In addition to the permit limitations listed in Table 6.10, the effluent must not be toxic to rainbow trout using a 96-hour toxicity test. 6.5.5

Santa Fe Mine Passive Biological Treatment Process

In 1996, Homestake Mining Company pursued installation of a passive anaerobic biological treatment system at their closed Santa Fe heap leach facility in Nevada, USA (Mudder et al., 1998a and 1998c). The treatment system was installed to treat draindown collected from the heap leach pad to provide an effluent suitable for discharge into the groundwater system. The system consists of an existing lined leach solution pond into which a layer of organic matter was placed upon a set of evenly spaced perforated pipes. Draindown from the pad is directed by gravity to the perforated pipes and distributed across the cross section of organic matter. The organic matter, consisting of composted manure and straw, was covered with soil and a vegetative cover to enclose the entire system. Treated solution from the system is collected in a gravel layer and that contains perforated piping. The effluent is then routed to a drainfield where it is aerobically treated as it infiltrates into the ground. The peak flow of solution entering the treatment system was predicted to be about 10.5 gpm, corresponding to aminimum retention timeof about 38 days in the system. A schematic of this system is shown in Figure 6.11. The primary biochemical processes at work in the treatment system include anaerobic sulphate reduction, denitrification and cyanide attenuation, coupled with chemical precipitation andphysical sorption of trace metals. All of these processes have been examined in bench scale, continuous flow pilot plant evaluations, and full scale natural and manmade wetlands. In this system, only partial sulphate reduction is necessary to form sulphide and carbonate, which in turn will precipitate the residual metals asinsoluble compounds. The sulphate entering the organic layer is converted to sulphide by several anaerobic and chemoheterotrophic bacteria. The basic sulphate reduction reaction is as follows (Lapakko and Eger, 1981; Dvorak and McIntine, 1992 and Maree and Strydom, 1985): (6.16) SO4-2 + 2C3H4O3- (lactate) → 2C2H4O2- (acetate) + 2CO3-2 + S-2 Carbonate produced in the reaction buffers the solution pH and neutralizes acidityif present. The organic compoundsneeded as a source of carbon are provided by the organic material. In addition to sulphate reduction, the same throughtoathe similar biochemical process et reduce the oxidized forms of selenium (i.e., organisms selenite andcan selenate) elemental metal (Baldwin al., 1985 and Levine, 1924). The sulphide and carbonate produced can combine with free and complexed metals, such as cadmium, lead, nickel, mercury, silver andzinc, to form insoluble compounds.

275

Chapter Six


TABLE 6.10 Performance of the Nickel Plate Mine Biological Water Treatment Plant

Constituent

(1)

pH TotalDissolvedSolids TotalSuspendedSolids Total Cyanide WAD Cyanide Thiocyanate Ammonia Nitrate Sulphate Arsenic (3)

Influent

7.70 2,853 11.5 1.04 0.33 379 25.4 2.83 1,316 0.21

Aerobic Circuit Effluent ---0.50 0.12 0.06 0.01 98.3 1,908 --

Anaerobic Circuit Effluent ---0.51 0.10 0.07 0.01 0.04 1,948 --

HDS Circuit Effluent 8.26 3,176 8.2 0.44 0.04 0.08 0.15 0.13 2,000 0.001

0.82 0.006 0.02

0.76 0.004 0.03

0.72 0.005 0.02

Permit Limitation

10to 7 7,500 3.0 (2) 0.20 3.0 (2) 1.0 10.0 3,500 0.07 (4)

(3)

Cobalt (3) Copper (3) Iron

0.97 0.02 0.06

Source: Given, 2001

Notes: (1) All concentrations in mg/L. (2) Total cyanide plus thiocyanate limitation. (3) Dissolved concentrations. (4) Limitation for total concentration.


276

-0.04 (4) 0.50


TABLE 6.11 Reagent Usages in the Nickel Plate Mine Biological Water Treatment Plant in 1999

Usage (kg/million gallons) 3,274 2,227 441 68

Reagent

Methanol Ferric Sulphate Sulphuric Acid Phosphoric Acid Ash Soda Lime Flocculant

139 3,893 45

Source: Given, 2001

277

Chapter Six


FIGURE 6.11

Schematic of the Santa Fe Mine Passive Biological Treatment System

2 7 8

Adapted From: Mudder et al., 1998


In conjunction with the bacterial reduction of sulphate, nitrate is converted to nitrogen gas or is taken up as a nutrient through the microbially mediated process of denitrification. Although methanol is the preferred carbon source for the engineered processes used inmunicipal wastewater treatment facilities, other organic compounds found in composted manure, wastewater sludge, brewery wastes and food processing wastes canalso be utilized by the microorganisms. The other bacterially mediated reaction of importance is the anaerobic degradation of residual cyanide. Although the aerobic oxidation of cyanideis well documented and forms the basis of full scale heap leach decommissioning and mine water treatment facilities, the degradation of cyanide under anoxic or anaerobic conditions is less common (Canby, 1993 and Altringer et al., 1991). Anaerobic degradation of free and complexed cyanide occurs according to th e following reactions: (6.17) HCN + 3H 2 → CH4 (methane) + NH3 (6.18) HCN + 2H 2O → NH4COOH (ammonium formate) The passive treatment system for the Santa Fe Mine was first evaluated in the laboratory using samples of process solution from the site(Mudder et al., 1998c). A summary of test results is presented in Table 6.12. Excellent removals of cyanide, nitrate and metals was observed in this work, along with varying levels of sulphate removal. Thiocyanate was not reduced in the passive system since its degradation does notoccur readily under anaerobic conditions. 6.5.6

Summary

The various advantages and disadvantages associated with biological water treatment are presented in Table 6.13.

279

Chapter Six


TABLE 6.12 Passive Biological Treatment of Leach Pad Draindown Solution in Laboratory Columns

Constituent

(1)

Eh (mV) pH Alkalinity Sulphate WADCyanide Thiocyanate Nitrate Nitrite Arsenic Copper Mercury Molybdenum Selenium Silver

Influent

-9.17 143 1,850 14 84 55.6 12.2 0.45 10.3 0.164 1.38 0.41 0.31

Effluent Column 1 79 -84 1,600 0.26 88 48.2 <0.4 0.18 4.44 0.011 0.53 <0.02 <0.05

Effluent Column 2 -319 7.93 7,544 500 <0.1 99 0.5 0.3 0.05 <0.09 <0.001 <0.09 <0.006 <0.05

Effluent Column 3 -330 7.94 7,060 700 <0.2 120 0.8 0.2 0.06 0.09 <0.0002 <0.09 0.02 <0.05

Effluent Column 4 -367 7.91 3,205 <100 <0.1 111 0.8 0.3 0.04 <0.09 0.002 <0.09 <0.006 <0.05

Effluent Column 5 -442 8.04 4,778 -<0.1 102 0.4 0.5 0.02 <0.09 0.001 <0.09 <0.006 <0.05

Effluent Column 6 -402 7.53 3,360 1,600 <0.2 67 0.6 1.4 0.02 0.14 0.002 <0.09 0.006 <0.05

Source: Cellan et al., 1998

Notes: (1) All concentrations in mg/L. Column Descriptions: 1: 100% Straw (control), Seeded, Target Retention Time 30 days 2: Composted cow manure/straw (5% by wt.), Unseeded, Target Retention Time 30 days 3: Composted cow manure/straw (5% by wt.), Seeded, Target Retention Time 15 days 4: Composted cow manure/straw (5% by wt.), Seeded, Target Retention Time 30 days 5: Mushroom compost/straw (5% by wt.), Seeded, Target Retention Time 30 days 6: 50% composted cow manure, 50% Peat/straw (5% by wt.), Seeded, Target Retention Time 30

days


280


TABLE 6.13 Advantages and Disadvantages of Biological Water Treatment

Advantages 1 The process is simple in design and process control is minimal. 2 Reagent costs are low in comparison to other treatment processes. 3 All forms of cyanide are treatable including a portion of the stable iron complexed cyanides. 4 Heavy metals are removed through a combination of absorption and precipitation as sulphides. 5 Cyanide, thiocyanate, cyanate, ammonia, nitrate and sulphate can be removed. 6 Can be configured as an active or passive system. 7 Effluent from the biological treatment process has been shown to be environmentally acceptable. Disadvantages 1 Additional treatment may be required if residual effluent metals concentrations exceed environmentally acceptable levels. 2 Cyanide is not recovered. 3 In the below ground passive system described, the organic food source may require periodic replacement.

281

Chapter Six


6.6

CARO’S ACID

6.6.1

Introduction

In the early 1990’s, Caro’s acid was investigated as a reagent for possible use in oxidizing cyanide in cyanidation solutions. Caro’s acid, also known as peroxymonosulphuric acid (H2SO5), is a strong oxidizing agent and has been used for many years in industrial chemical formulations and bleaching agents. It has recently been applied at a few mining operations for tailings detoxification, particularly for tailings slurry. Caro’s acid is produced from concentrated hydrogen peroxide and sulphuric acid in an exothermic reaction (Norcross, 1996): (6.19) H2O2 + H2SO4 → H2SO5 + H2O Due to its instability, Caro’s acid is produced on-site and used immediately for cyanide detoxification with only minimal intermediate storage. At room temperature, Caro’s acid is stable for several hours, however at elevated temperature it is stable only for several minutes, decomposing to liberate oxygen, water and sulphur trioxide (SO 3). Production of Caro’s acid is typically conducted with 1.5 to 3.0 moles of H2SO4 per mole of H2O2 to yield a product of up to 80% purity. Normally, 70% hydrogen peroxide solution and 93% sulphuric acid solution are used to generate Caro’s acid. 6.6.2

Process Chemistry

The reaction of Caro’s acid with cyanide is rapid, typically complete within only a few minutes. Caro’s acid reacts with both free and WAD cyanides to yield cyanate and sulphuric acid according to the following simplified reaction: (6.20) H2SO5 + CN

-

→

-

-2

+

OCN + SO4 + 2H

This reaction does not require a soluble catalyst such as copper, as is required with the hydrogen peroxide and sulphur dioxide/air processes. This is one of the primary benefits of this process, along with the rapid reaction kinetics for cyanide oxidation and the ability to treat slurries. Theoretically, 1.0 mole of H2SO5 is required for each mole of CN- oxidized to cyanate, however in practice 1.5 to 3.0 moles of H2SO5 are used when treating slurry to low WAD cyanide levels (Norcross, 1996 and Castrantas et al., 1995a). One of the products from this reaction is sulphuric acid, which along with the sulphuric acid remaining from the reaction used to generate the Caro’s acid, acts to lower the solution pH. Depending on the Caro’s acid dosage, the solution pH may decrease from an initial level of about 10.5 to 11.5 down to the range of about 6.5 to 9.5 (Castrantas et al., 1995b). In some cases, lime or caustic addition may be needed for pH control when using Caro’s acid.


282


Caro’s acid will also react with thiocyanate to some extent (Castrantas et al., 1995a) to yield cyanate and sulphate. -

-

-2

(6.21) 4H2SO5 + SCN + H2O → OCN + 5SO4 + 10H

+

WAD cyanides are preferentially oxidized by Caro ’s acid, but if thiocyanate is present, the Caro’s acid consumption may be increased. 6.6.3

Performance

The primary area of application of the Caro’s acid cyanide destruction process has been with tailings slurry when initial WAD cyanide levels are less than about 150 mg/L and when treated WAD cyanide levels of about 5 to 20 mg/L are required (Castrantas et al., 1995a and Castrantas et al., 1993). WAD cyanide levels lower than about 5 to 20 mg/L require relatively large amounts of Caro’s acid (Castrantas et al., 1993). A simplified flow diagram illustrating the Caro ’s acid process for cyanide destruction is shown on Figure 6.12. As indicated, hydrogen peroxide and sulphuric acid are stored on-site, pumped to a reactor to generate Caro’s acid and then immediately added to the tailings slurry to oxidize free and WAD cyanides. If a surge vessel for Caro’s acid is required prior to addition to the reaction tank, then the Caro’s acid should be cooled to room temperature, though storage for only a few hours is possible due to its rapid decomposition (Norcross, 1996). The required reaction time of Caro’s acid with cyanide is typically less than about one to five minutes (FMC, 2001). Therefore, sizing of the reaction tank is primarily based on accommodating physical requirements for piping, pumps, instrumentation and the agitator. Full-scale results for treatment of mill tailings slurry with Caro ’s acid were presented by Castrantas et al. (1993, 1995a and 1995b) and are summarized in Table 6.14. In this work, the mole ratio of Caro’s acid to WAD cyanide varied from 0.57:1 to 2.9:1, resulting in treated slurry WAD cyanide levels in the range of 4.0 to 27.1 mg/L. The untreated slurry WAD cyanide concentrations in this work varied from 37.5 to 115 mg/L. Caro’s acid can also be used to oxidize cyanide in clear solutions, but with clear solutions other treatment technologies may be preferred based on economics (Castrantas and Manganaro, 1994). Treatment results using Caro’s acid with synthetic cyanide solutions were presented by Griffiths et al. (1990). These results are summarized in Table 6.15 and illustrate the effectiveness of Caro’s acid in reducing the WAD cyanide concentration in clear solution to less than about 0.5 mg/L. The advantages and disadvantages of the Caro’s acid process are shown in Table 6.16.

283

Chapter Six


FIGURE 6.12

Basic Flow Diagram for a Caro ’s Acid Treatment System

Hydrogen Peroxide

Lime

Caro's Acid Reactor

pH M

Tailings Slurry

Treated Slurry

Sulfuric Acid

Agitated Reaction Tank


284


TABLE 6.14 Slurry Treatment Results Using Caro ’s Acid

Test No.

Mole Ratio Caro’s Acid to WAD Cyanide

Untreated Slurry WAD Cyanide (mg/L)

Treated Slurry WAD Cyanide (mg/L)

1 2 3 4 5 6 7

1.12 2.90 0.57 2.80 0.66 1.07 0.78

44.5 37.5 46.0 39.8 115.0 113.1 101.5

8.5 4.2 14.0 4.0 27.1 16.3 18.7

Source: Castrantas et al., 1993, 1995a and 1995b

TABLE 6.15 Clear Solution Treatment Results Using Caro ’s Acid

Test No.

1 2 3 4 5 6 7 8 9 10 11

Moles H2O2 per

Moles H2SO4 per

Moles NaOH per

Treated Solution

Mole WAD Cyanide 3.50 2.59 2.06 2.02 1.99 1.89 2.02 2.22 1.97 1.57 1.39

Mole WAD Cyanide 0.00 0.07 0.07 0.10 0.13 0.19 0.51 1.11 1.48 1.57 2.09

Mole WAD (1) Cyanide 0.00 0.17 0.19 0.25 0.35 0.49 1.31 2.90 3.80 4.10 5.40

WAD Cyanide (2) (mg/L) 0.50 0.48 0.50 0.48 0.50 0.45 0.42 0.40 0.50 0.30 0.40

Source: Griffiths et al., 1990

Notes: (1) NaOH added to maintain the pH at 11.0 to 11.5. (2) Untreated solution contained 320 mg/L WAD cyanide and 100 mg/L copper.

285

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TABLE 6.16 Advantages and Disadvantages of the C aro ’s Acid Process

Advantages 1 Reaction kinetics are rapid and low reactor residence times are required. 2 Reagents are widely available worldwide. 3 Capital costs may be lower compared to other chemical treatment processes. 4 The process is relatively simple in design and operation. 5 The process can be used to treat tailings slurry. Disadvantages 1 Reagent operating costs can be excessive. 2 Primarily applicable to slurry treatment rather than solution treatment.

3 4 5 6 7 8

Additional treatment may required if residual effluent concentrations of ammonia, thiocyanate or be metals exceed environmentally acceptable levels. Reaction of sulfuric acid and hydrogen peroxide to make Caro ’s acid can be dangerous. To achieve low levels of WAD cyanide, excessive dosages of Caro ’s acid may be required. High concentrations of cyanate can be produced, potentially increasing the ammonia concentration in the solution. Ammonia or thiocyanate are not removed. Cyanide is not recovered.


286


6.7

CYANIDE RE COVERY BY TAILINGS WASHING

6.7.1

Introduction

Tailings slurry thickeners and filters are often used in the mining industry for solid-liquid separations, either as part of the metallurgical extraction process or for water conservation purposes. When water is recovered from tailings slurry using a thickener orfilter, cyanide that is contained within the water is also recovered. This approach can be used to specifically target cyanide recovery from tailings slurry to allow its reuse in the metallurgical circuit (Jagger and Arbuthnot, 1991 and Arbuthnot and Jagger, 1992). Cyanide recovery affected inthis manner is purely a physical process, with the recovery of cyanide accompanying the recovery of water from the slurry tailings. In some cases, thickeners or filters can be used in conjunction with a chemical-based cyanide recovery process (Botz and Mudder, 1998b). In reference to Figure 6.13, the above concept can be extended to include multiple stages of washing and thickening to affectincreasingly higher levels ofcyanide recovery. In this type of countercurrent decantation (CCD) system, the tailings slurry flows countercurrent to a stream of wash solution to successively recovery both water and cyanide from the tailings slurry. The use of CCD circuits is common in metallurgical circuits to recover gold and silver from slurry, and the concept is identical when extended to recover cyanide. As an alternative to using thickeners for solid-liquid separation, filters can also be used in a countercurrent filtration (CCF) system (Crane, 1982 and Scott, 1982). 6.7.2

Performance

The percentage of cyanide that can be recovered from a tailings slurry with a wash circuit depends primarily on three items: The type of solid-liquid separation equipment employed. Filters are capable of achieving a relatively low moisture content in the tailings solids, thereby affecting a relatively high level of cyanide recovery for each filtrationstage employed. As filters can be expensive to purchase and operate, thickeners are often used as a lower cost alternative while sacrificing the degree of separation possible in each stage. The number of wash stages employed. A higher number of wash stages will affect a higherlevel of cyanide recovery, but at the cost of installing additional equipment. An example of the cyanide recovery achievable with upto five CCD wash stages is shown in Figure 6.14. As can be seen in this example, there is a diminishing benefit to adding each successive wash stage, with only a minor difference observed with four versus five CCDwash stages. Typically two to five wash stages are employed, which can affect cyanide recoveries of up to 90% or higher.

287

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The amount of wash solution available. Typically, fresh water make-up, siterunoff water and/or tailings impoundment reclaim water are used for tailings washing for cyanide recovery. A higher flow of wash solution will increase the cyanide recovery, however the site water balance must be carefully evaluated to determine the amount of wash water that can be used in the wash circuit. The evaluation of wash solution flow rate and the number of wash stages is typically an iterative evaluation, conducted until a satisfactory approach is developed. CCD and CCF wash circuits are capable of achieving 90% cyanide recovery, but their implementation must be preceded by a careful examination of the site water balance. The advantages and disadvantages of recovering cyanide through tailings washing are shown in Table 6.17.


288


g n is h a W s g in il a T D C C y b y re v o ce R e d i n a y fC o m a r g ia D w o l F

3 1 . 6 E R U G I F

289

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FIGURE 6.14

Performance of Cyanide Recovery by CCD Tailings Washing

100%

re v o ce R e d i n a y C t en cr e P

90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 012345 Number of CCD Wash Stages

Notes: Based on 45% tailings solids, thickened to 60% solids in each CCD stage and washed with 1 tonne of solution per tonne of solids.


290


TABLE 6.17 Advantages and Disadvantages of Cyanide Recovery by Tailings Washing

Advantages 1 Process is simple in design and operation. 2 High cyanide recoveries can be achieved. 3 Can be implemented at sites where CCD or CCF circuits are being considered or are in operation. 4 Treatment chemicals are not introduced into the solution, and cyanate is not produced. 5 Process can be integrated with gold/silver recovery processes. 6 Requirements for tailings detoxification can be reduced. 7 Depending on the composition of the wash water, all compounds can be removed from the tailings. 8 Mining personnel are familiar with the equipment operation and control. Disadvantages 1 Capital cost of thickeners or filters can be high, and filters can be labour and maintenance intensive. 2 Careful examination of the site water balance is required, and the process may not be feasible for some sites. 3 Thickeners require a large area for installation.

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6.8

CYANIDE RECOVERY BY STRIPPING AND ABSORPTION

6.8.1

Introduction

The recovery of cyanide from mining solutions through acidification, air stripping and reabsorption was srcinally known as the Mills Crowe Process (Lawr, 1929). The process involved acidifying clarified barren solution then stripping the hydrogen cyanide as a gas and reabsorbing it into acaustic solution or milk of lime spray. The historical process was known as the Acidification, Volatilisation and Reneutralisation (AVR) process, with several variants having been recently developed to modernize the process. The AVR process was employed at the Flin Flon Mine operated by Hudson Bay Smelting and Mining Company in Canada from about 1930 to 1975 (Davies et al., 1946). The mining operation involved cyanidation of gold bearing tailings srcinating from the flotation of a copper-zinc ore. To condition the barren solution for hydrogen cyanide stripping, the pH was first adjusted downward to2.5 with sulphuric acid supplied from the zinc electrolytic plant. The acidified solution was thenpassed through four strippingtowers packed with wooden grids. The towers were constructed of concrete and were operated in a series configuration, each connected to one of four concrete absorption towers by means of closed loop recycled air streams. The air 3 and liquid volumes in each stripping loop were about 56,000 m /hour and 107m3/hour, respectively, resulting in an air to liquid ratio of about 525:1. Hydrogen cyanide entrained in the air stream was absorbed by contacting theair stream with lime slurry sprays. In this application, a reduction in total cyanide of 92% was achieved by lowering the cyanide concentration from 560 mg/L to 44 mg/L. The major problem associated with this system was the buildup of gypsum and copper thiocyanate precipitates on the packing internals, which required manually cleaning every two to four months. The problem arose due tothe very low pH and high sulphate concentrations in the solution. The process was utilized for economic, not environmental reasons, and was limited to the stripping of cyanide from clarified barren solution. During the 1930's there were about seven cyanide recovery plants in operation utilizing various designs, but only limited information is available concerning these facilities (Dorr,1936). The plants were primarily utilized in conjunction with silver recovery operations in remote regions of the world. From the mid-1930's into the 1950's, a cyanide recovery process was utilized by the Real del Monte Company in Pachuca, Hidalgo, Mexico (Seeton, 1950). At this site, the stripping of free cyanide was conducted at an acidic pH, employing sulphuric acid generated on-site through the burning of sulphur. The volatilisation stage andabsorption stages were combined alternately and operated in series. The system was enclosed and the air recycled at 100 percent humidity to minimize evaporation. A total of about 93% was with about a 60% removal realized through eachcyanide of the recovery three stripper stages. Theachieved operation was srcinally designed to treat about 350 tonnes of solution per day and was later expanded to accommodate about 1,100 tonnes of solution per day. The free cyanide was concentrated until a 5% solution was obtained and then recycled to the metallurgical circuit. However, the process was restricted to recovery of cyanide from clarified barren solution only.


292


In the 1970's the process was reinvestigated and modernized by CANMET in Canada (McNamara, 1978). The renewed interest in the process was due to the increasing costs of both cyanide and its treatment. In the CANMET study, a laboratory apparatus was employed to investigate the recovery of cyanide from six barren solutions collected from mining operations. The study summarized for the first time the various chemical reactions associated with the process. The process was referred to as the AVR process, which is the current terminology used to describe cyanide recovery processes utilizing a volatilisation approach. The CANMET study advocated the use of single complete mix aeration basins instead of stripping towers to minimize the mechanical problems associated with scaling and plugging of media or internals. It was recommended that aeration ina full-scale operation be accomplished using multiple rows of fine bubble diffusers. The AVR process has been evaluated as a primary water treatment alternative, utilizing pH values below 2.0 to facilitate the removal of WAD and total cyanides through precipitation and volatilisation. Greater than 99% removal of total cyanide was achieved in the studies, with treated effluents containingtotal cyanide levels ranging from 0.10 to 4.0 mg/L. In addition, the investigation of metals precipitation was undertaken, demonstrating that excellent removal efficiencies were achievable following neutralisation with lime. However, the process was confined to solution treatment as it was considered not feasible for treatment of tailings slurries. The modern cyanide recovery process was further investigated as a primary alternative for treatment of mine decant water from 1985 to 1987 by Mudder. The tests employed a smallpilot plant operated in batch mode using barren waters obtained from a silver mining operation. The pilot plant design involved two columns connected in series. One column served as the stripping chamber, while the other served as the absorption chamber. Total cyanide levels were routinely reduced from an average of 330 mg/L to less than 2.0 mg/L. Also in the mid-1980's, investigations were conducted at the Noranda Research Centre aimed at designing a cyanide recovery process employing packed towers and plastic media (Avedesian et al., 1983). Stripping towers significantly reduced the surface area required for the AVR process, as well as the horsepower requirements due to the reduced air pressure losses through the system. Employing information derived in a bench-scale study, a continuous flow pilot plant system was designed and operated. The data obtained in the pilot study was utilized in the development of a 3 design for a full-scale stripping system to treat about 2,000 m /day of barren solution utilizing an air to liquid ratio of 330:1. The pilot plant evaluations indicated thetotal cyanide levels could be reduced from about 4,000 mg/L to about 120 mg/L for an overall removal efficiency of 97%. The stripped cyanide was reabsorbed in a second packed tower using a counter current caustic solution. The process employed a pH of 2.5 and considerable precipitation of copper thiocyanate and calcium sulphateefficiency was observed. It was noted that the precipitated solidsbewould eventually reduce the stripping and mechanical cleaning of the media would necessary. The study demonstrated that use of modern mass transfer technology and materials could be employed in the removal and recovery of cyanide from clarified barren solutions.

293

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A full-scale packed tower cyanide recovery system was designed and operated from 1985 to 1987 by Golconda Engineering and Mining Services at the Beaconsfield Gold Mine in Tasmania (Kitney, 1988). Initially, a continuous flow pilot plant operationwas conducted to establish the appropriate air to liquidratio, stripping time and caustic requirements for the absorber. The fullscale facility was designed to treat 1,200 tonnes per day of clarified barren solution using a system of multiple packed aeration towers. The system was designed for maximum safety incorporating an enclosed negative pressure system. Fan exhausts were monitored routinely for HCN with typical values of <1.0 mg/m3. Cyanide recoveries of nearly 95% were reported in the full-scale system. The various laboratory, pilot plant and full-scale cyanide recovery facilities described have demonstrated that application of mass transfer technology coupled with state-of-the-art monitoring equipment and engineering design approaches can be employed in the construction and operation of full-scale cyanide recoverysystems. However, it had been widely believed that the cyanide recovery process could not be extended to treat slurries. Since 1985, an intensive research effort has been undertaken to expand the capabilities of the process to slurries, while minimising the chemical and mechanical problems associated with past systems. The work involved both detailed laboratoryand continuous flow pilot plantevaluations conducted for Cyprus Gold of New Zealand to develop and design the first full-scale commercial cyanide recovery system for tailings slurries at the proposed Golden Cross Mine in New Zealand. Based on the test results, a full-scale cyanide recovery system utilizing packedtowers was built to treat 200 m 3/hour of a 35% solids tailings slurry from a WAD cyanide level of 200 mg/L to less than 20 mg/L. This approach to recovering cyanide from a slurrywas patented and commercialized by Cyprus, and later Coeur d’Alene Mines Corporaiton, and is known as the Cyanisorb process. Since the 1980’s, several additional cyanide recovery plants have been constructed and operated in Australia, Brazil, Argentina and the USA. The following sections focus on the chemistry, applications, performance and economics of cyanide recovery from solutions and slurries, with a focus on the recently operated plants. 6.8.2

Process Chemistry

Cyanide recovery through volatilisation utilizes the volatility of hydrogen cyanide at a lowered pH to strip free cyanide from solution or slurry and recover it in usable form (i.e., as NaCN). The volatility of hydrogen cyanide is related to its Henry's Law constant, which is affected by several factors including viscosity, hydrogen bonding, pH and temperature (Avedesian et al., 1983). The transfer of hydrogen cyanide from asolution or slurry to air is liquid film controlled and large volumes of air are required to create the necessary air to liquid contact to facilitate its removal.


294


Several factors affect the rate and extent of hydrogen cyanide removal from slurries or solutions through air stripping. These factors include: • • • • • • • •

Form of cyanide and the presence of complexing metals Concentration of cyanide Temperature and pH Pressure maintained within the recovery system Air-to-liquid ratio Type of mechanical dispersion equipment Viscosity of the solution or slurry Air-to-liquid contact period

Considerable information is available regarding the effects of these design parameters on the recovery of cyanide from clarified solutions, but limited data is available concerning the treatment of tailings slurries. In reference to the schematic presented on Figure 6.15, the process of cyanide recovery through volatilisation is conducted in four stages (Stevenson et al., 1998). The first stage, known as acidification, involves lowering of the solution pH to less than 8.5 with concentrated mineral acid. Free cyanide and a portion of the metal-complexed cyanides are converted to aqueous hydrogen cyanide in this step. (6.22) 2CN- + H2SO4 → 2HCN + SO4-2 (6.23) M(CN)4-2 + 2H2SO4 → 4HCN + M+2 + SO4-2 The acid employed most commonly is sulphuric acid due to its relatively low cost and wide availability. The handling, storage and feeding of sulphuric acid is quite common in industry. The potential problems associated with sulphuric acid include an increase in sulphate and total dissolved solids concentrations in the treated solutions and the potential for precipitation of calcium sulphate (gypsum) in oversaturated solutions. The pH of the solution during acidification is monitored in-situ continuously. The acidification step must be enclosed to prevent the escape of HCN gas and requires about 10 to 20 minutes to complete. The reduction of HCN during the acidification step is about 10% to 15% of the WAD cyanide concentration due to volatilization in the reaction tank. The pH of a solution, slurry or sludge is lowered in accordance with the stability of the particular metal-cyanide complexes from which the cyanide will be recovered. For the recovery of free cyanide, a pH in the range of 7.0 to 8.5 is employed. For recovery of WAD cyanides, the pH is lowered further. The best approach is to utilise the highest pH possible tominimize precipitation problems and minimise the use of sulphuric acid. The presence of base metals, particularly copper, can limit the achievable cyanide recovery. Removal of copper from these solutions may be considered as an alternative, either through low pH precipitation of copper cyanide (CuCN) or by precipitation of copper sulfide (Cu 2S) following addition of sodium sulfide to the solution.

295

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FIGURE 6.15

General Schematic of Cyanide Recovery by Stripping and Absorption

Tailings Slurry or Solution

Acidification Stage

Air

Volatilisation Stage

Reneutralisation Stage for Metals Removal

Treated Slurry or Solution


296

Sulphuric Acid Addition

HCN

Reabsorption of HCN for Process Reuse Using NaOH

Lime Addition to pH 10.0 to 10.5

Treatment and Recovery of Cyanide The process chemistry indicates that the initial phase of recovery involves conversion of CN to HCN and the initiation of breakdown and of precipitation of neutral metal cyanide complexes. The extent of the conversion is dependent on the pH chosen for recovery and the forms of cyanide being recovered. At very low pH values, precipitation ofiron complexed cyanide occurs as the copper-iron cyanide complex, which accounts for the removal of this portion of the total cyanide.

From the acidification stage, the acidified solution or slurry passes into the hydrogen cyanide stripping or volatilisation stage, which consists of either open or packed towers, a series of complete mix reactors ora shallow aeration basin. In the volatilisation stage, hydrogen cyanide in the aqueous solution phase is transferred to the gas phase: (6.24) HCN(aq) → HCN(g) The volatilisation system is sealed and operated under a slight vacuum to prevent escape of HCN. A stripping efficiency in the range of 90% to 99% percent is possible depending on the type of metal-cyanide complexes present and the design of the stripping unit. In the case of complete mix reactors or aeration basins, coarse bubble diffusers are employed in the bottom of the reactors and the reactors are sealed to eliminate release of HCN gas to the atmosphere. The use of diffusers provides air for agitation of the solution as well as stripping of hydrogen cyanide. Hydrogen cyanide produced through volatilisation is entrained into an air stream, passed upflow through a packed tower and reabsorbed into a caustic solution moving counter-current to the air. (6.25) HCN + NaOH

→

NaCN + H2O

The caustic solution is then returned to the metallurgical circuit for reuse as a solution of NaCN. The pH of the caustic solution is maintained above about 11.5. Lime slurry has also been used for this application, but careful engineering design is required to prevent excessive scaling within the absorption unit. Once the solution or slurry is free of recoverable hydrogen cyanide, it enters the reneutralisation stage of the process. In this stage, the pH is adjusted to 10.0 to 10.5 with lime to precipitate residual metals and to add buffering capacity. With cyanide removed, the metals are released in free form into solution, allowing precipitation to low levels as their stable and insoluble carbonate and hydroxide complexes. +2

-

(6.26) M + 2OH

→

M(OH)2 (solution neutralization and metals precipitation)

The addition of buffering capacity is important in the event that a slurry being treated exhibits the potential for acid generation through sulphide oxidation.

297

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6.8.3

Performance

As discussed previously, excellent overall stripping and recovery efficiencies of cyanide from solutions are achievable by anyof the recovery processes mentioned. In the case of the Flin Flon Mine, the total cyanide level was reduced from 560 mg/L to 44 mg/L, representing a cyanide removal of 92%. The initial total and free cyanide levels of 200 mg/L and 10 to 30 mg/L, respectively, were reduced to <5.0 mg/L and <0.50 mg/L, respectively. The residual cyanide levels were reduced further once the treated effluent was passed through a carbon adsorption column prior to discharge. In addition, the copper, nickel and zinc concentrations were all reduced to <1.0 mg/L in the treated effluent. The level of treatment obtained was consistent with that achievable by several chemical or biological treatment processes currently used in the mining industry. Similar results were obtained by the authors in pilot scale batch studies evaluating the removal and recover of WAD and total cyanide from impoundment decant water. A summary of solution chemistries before and after cyanide recovery are presented in Table 6.18. Greater than 99% removal of total and WAD forms of cyanide was realized, as well as excellent removals of various metals following pH adjustment and precipitation. The results compared favourably with those obtained in CANMET laboratory evaluations, which are presented ni Table 6.19. The removal efficiencies for metals in the CANMET studies were excellent and ranged from about 80% to 99%. Although recovery of cyanide from solutions is desirable, recovery of cyanide from slurries may be preferred in some situations for a variety of reasons. The first commercial full-scale plant utilized to recovery cyanide from tailings slurry was installed at the Golden Cross Mine in New Zealand in 1991 (Goldstone and Mudder, 1998). Cyanide recovery from slurry in this application was desired in order to avoid relatively expensive solid liquid separations equipment that would be required to produce a clear solution. Recovery of cyanide from the tailings slurry was a more direct route and was therefore selected as the preferred option at this site. Prior to construction of the plant, extensive laboratory and pilot testing was conducted to investigate cyanide recovery fromslurry. Results from the continuous throughput pilot testsare shown in Table 6.20 and a schematic of the full-scale facility constructed for this project is presented on Figure 6.16. This plant was operated from 1990 untilthe mine was closed in 1998. During this time, the tailings slurry WAD cyanide level of 150 to 250 mg/L was consistently reduced to less than about15 to 25 mg/L. By recovering cyanide, the overallpurchases of NaCN were reduced by 50% to 60% when compared to operation of a cyanide destruction circuit at the site. In 1992, for a cyanide recovery plant wason installed theDeLamar silver mine in solution the USA.from A flow diagram this facility is presented Figure at 6.17. In this system, barren the Merrill Crowe circuit was directed to a cyanide recovery plant where the WAD cyanide level in the solution was reduced from 400 mg/L to less than 20 mg/L. The treated solution was then used as wash solution in a five-stage CCD circuit to remove cyanide, silver and gold from the final tailings. This plant was operated until 1999 when the mine was shut down and consistently achieved greater than 90% cyanide recovery from the barren solution.


298


TABLE 6.18 Performance of Cyanide Recovery in Treating Tailings Impoundment Decant Solution

Constituent Arsenic Cadmium Chromium Cobalt Copper Iron Lead Mercury Nickel Silver Zinc Thiocyanate Total Cyanide Method-C Cyanide Ammonia (as N) Nitrate N) (as Sulphate pH

Before Cyanide Recovery Average Concentration 0.36 0.01 0.01 0.21 2.6 16.0 0.14 0.016 0.20 2.0 93.3 33.6 330 294 19.3 --10.4

After Cyanide Recovery Average Concentration 0.01 0.004 <0.02 0.16 0.39 0.07 0.10 0.014 0.09 0.9 0.09 31.3 1.7 1.2 18.6 25.4 1,450 --

Source: Mudder and Goldstone, 1989


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TABLE 6.19 Results of CANMET Laboratory Cyanide Recovery Studies

Constituent (mg/L) Mine Site

CN

(2)

SCN

Cu

Zn

Ni

Fe

Kerr Addison - Barren Feed - AVR Treated Effluent

60 <0.10

47 <1

14 0.3

10 0.01

6 0.05

0.8 <0.1

Dome - Barren Feed - AVR Treated Effluent

23 <0.10

-14

12 0.09

8 0.2

2 0.2

-0.2

260

50

77

98

6

11

Giant Yellowknife - Barren Feed

- AVR Treated Effluent Hollinger Ross - Barren Feed - AVR Treated Effluent Pamour - Barren Feed - AVR Treated Effluent

<0.10

57

0.3

0.1

<0.03

<0.03

610 4.16

540 300-370

310 4

82 <0.01-8.0

1 0.2

11 <0.030.08

990 <0.1-3

1530 1,180-1,320

310 0.3-2

86 <0.01

230 0.1-2

1 <0.03-.01

Source: McNamara, 1978

Notes: (1) Measured by the picric acid colourimetric method.


300

TABLE 6.20

Pilot Cyanide Recovery Testwork Using Stripping Towers

Test Number Variable 1

2

3

4

5

6

7

2.37

2.37

1.94

2.17

5.54

2.10

2.21

2.33

2.39

2.36

2.45

4.10

4.00

4.17

4.00

3.90

3.70

5.00

4.60

845

845

839

839

839

2126

2126

1484

1400

1615

1615

2137

2137

2581

2581

1364

1364

1682

1682

No. of Stripping Tower Passes

1

1

1

1

1

1

1

1

2

1

2

1

2

1

2

1

2

1

2

Air:Liquid Ratio

357

357

432

387

330

1012

962

637

586

684

659

571

534

619

645

349

369

336

365

Influent WAD Cyanide

182

182

156

166

166

192

192

198

19

224

22

174

25

193

22

179

35

184

35

Effluent WAD Cyanide

37

25

45

24

23

15

14

18

6

24

6

29

7

26

7

43

13

40

14

Slurry Flow 3

8

9

10

11

12

13

(m /hour) Air Flow (m3/hour) 3 0 1

Source: Mudder and Goldstone, 1989

C ha pt er ixS


FIGURE 6.16

Flow Diagram of the Golden Cross Cyanide Recovery Plant

Clean Gas HCN Rich Gas

NaOH Feed T

wo S T

Two

tripping

Absorption

owers

Towers

Sulfuric Acid pH CIL Tailings

Air Recovered NaCN

Slurry One Fan Per Stripping Tower

One Acidification Tank

Stripped Slurry Lime pH

Treated Slurry To T ailings Impoundment One Neutralisation Tank


302


FIGURE 6.17

Flow Diagram of the DeLamar Cyanide Recovery Plant

Cyanidation Circuit

Recovered NaCN

Leach Tailings

PregnantSolution

5-Stage CCD Circuit

Merrill-Crowe Circuit

WashSolution

BarrenSolution

Cyanide Recovery

LowCyanideSolution

Final Tailings Tailings Impoundment

Decant Solution Fresh Water Make-Up

303

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Rio Tinto installed a cyanide recovery plant in 1997 at their Morro do Ouro operation in Paracatu, Brazil to recoverycyanide from filtered CILtailings solution. This plant operated until 2000 when it was shut down due to difficulties encountered in operating the CIL slurry filtration system using vacuum drum filters. Since this time, the plant has implemented an enhanced oxygen leaching system andhas reduced the level of cyanide required for goldrecovery. During operation of the cyanide recovery plant, the WAD cyanide recovery ranged from about 70% to 90% depending on the copper concentration in the tailings solution. This was the first modern cyanide recovery plant to utilize milk of lime for the absorption of stripped hydrogen cyanide. In 1998, AngloGold installed a cyanide recovery plant at their Cerro Vanguardia gold and silver operation in Argentina. This plant utilises a two-stage CCD circuit to wash CIL tailings slurry, and the recovered wash solution is directed to the cyanide recoveryplant. WAD cyanide levels in this solution are reduced from about 600 mg/L to about 50 to 150 mg/L, affecting a cyanide recovery in the range of 75% to 90%. Treated solution from the plant isused along with fresh water make-up in the two-stage CCD circuit. The above examples serve to illustrate the utility of cyanide recovery in detoxifying tailings solution while providing for the recycle of NaCN within a leaching circuit. The following section examines the general factors which impact the economics of cyanide recovery. 6.8.4

Economics of Cyanide Recovery

There are several advantages of cyanide recovery which translate into potential economic savings at a mine site, including: •

The cost of detoxifying tailings solution or slurry through cyanide recovery is partially or wholly offset by the value of recovered NaCN. When considered relative to the overall mine life, cyanide recovery is often less expensive than cyanide destruction.

•

Payback periods of less than two years have been realized with cyanide recovery plants.

•

Cyanide recovery does not generate cyanate, which will hydrolyse to yield ammonia and nitrate in a tailings impoundment. If downstream water treatment is required,this may result in savings in water treatment capital and operating costs.

•

At sites where cyanide recovery is utilized, higher levels of cyanide could potentially be used in the leaching circuit without significantly increasing the overall usage of NaCN. This may improve the overall recovery of gold or silver.


304


Items to consider when evaluating the economics of cyanide recovery for a project include: •

Capital investment of the cyanide recovery plant.

•

Sulphuric acid consumption required to adjust the slurry or solution pH to the level required for efficient conversion of WAD cyanides to hydrogen cyanide.

•

Electrical power to supply air to the stripping towers, pump the slurry or solution, and to agitate reaction tanks.

•

Sodium hydroxide or milk of lime required to absorb stripped hydrogen cyanide gas.

•

Lime required to neutralise the slurry after stripping.

•

Manpower to operate and maintain the plant.

These items should be factored into an overall economic assessment of cyanide recovery versus other options for tailings cyanide control. The overall cost of the different options should then be determined for the expected life of the mine to determine the optimal economic and environmental approach to control tailings cyanide. The economics of cyanide recovery will vary from site to site, but should be considered when the tailings slurry or solution contains greater than about 150 to 300 mg/L of WAD cyanide. The economics of cyanide recoveryfor several mining situations have been examined by Botz et al. (1995a and 1995b) and Botz and Mudder (1998b). 6.8.5

The Future of Cyanide Recovery

The need for development and implementation of processes for recovery of cyanide is essential to the success of current and future mining operations. The requirement stems from concerns over metal prices and the realization that more stringent environmental regulations will be developed restricting theconcentrations of cyanide discharged intotailings impoundments. With increasing concern over groundwater and wildlife issues, there will be increasing pressure to regulate cyanide entering tailings impoundments more closely. The advantage of cyanide recovery includes the lowering of cyanide and metals levels entering a tailings impoundment, thereby minimizing downstream treatment requirements and the potential for surface or groundwater contamination. The best application of cyanide recovery is for the reduction of WAD cyanide to levels of <20 mg/L. Cyanide recovery is not well suited as a primary treatment process when very low levels of residual cyanide are required (i.e. <1.0 mg/L), although in some instances these levels may be achievable. A summary of the advantage and disadvantages of cyanide recovery through volatilisation are presented in Table 6.21.

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TABLE 6.21 Advantages and Disadvantages of Cyanide Recovery by Stripping and Absorption

Advantages 1 The WAD cyanide concentration in solutions or slurries can be reduced to <20 mg/L. 2 Cyanide is recovered for re-use as NaCN. 3 Trace metals are removed through precipitation. 4 Lime, caustic and sulphuric acid reagents are widely available. 5 Iron cyanide complexes can be removed through precipitation. 6 Downstream treatment requirements for cyanate, ammonia or nitrate can be reduced or eliminated because process does not produce these compounds. 7 Impoundment liners may be eliminated in some cases due to the high quality of seepage. 8 Operating costs may provide a payback period for the plant capital investment due to the value of recovered NaCN. Disadvantages 1 Hydrogen cyanide vapour is hazardous and good safety practices must be employed. 2 Relatively high capital outlays may be necessary when compared to chemical treatment processes. 3 Additional treatment for cyanide removal may be required for cyanide or metals in the event a discharge to the environment is needed. 4 The process does not remove thiocyanate.


306


6.9

HYDROGEN PEROXIDE

6.9.1

Introduction

In the 1980’s hydrogen peroxide was investigated as a chemical for oxidizing cyanide in mine waters. Prior to this, hydrogen peroxide had been widely used in the detoxification of cyanide wastes at steel hardening and plating operations. A variety of approaches were investigated by DuPont (DuPont, 1974 and Mathre and DeVries, 1981) and Degussa (Knorr and Griffiths, 1984), and several versions of the process have been patented (Mathre, 1969; Zumbrunn, 1970; Harrison, 1983; Griffiths et al., 1989 and Castrantas and Fagan, 1992). The process was first pilot tested on a continuous scale at the Homestake Mining Company Lead, South Dakota operation in the USA in the early 1980’s (Trautman and Ommen, 1981). The first full scale hydrogen peroxide treatment facility in the mining industry was built and operated by Degussa at the Ok Tedi Mine in Papua New Guinea (Knorre and Griffiths, 1984). There are currently many hydrogen peroxide treatment processes in operation worldwide, and the process has found successful application in the treatment of solutions tolow levels of free and WAD cyanide. Use of the process to treat slurries is limited due to the high consumption of hydrogen peroxide resulting from its reaction with solids in the slurry. 6.9.2

Process Chemistry

The chemistry of the hydrogen peroxide cyanide destruction process can be described by the following reactions for the oxidation of free and weakly complexed metal cyanides (i.e., cadmium, copper, nickel and zinc cyanides): (6.27) CN- + H2O2 → OCN- + H2O -2

-

(6.28) M(CN)4 + 4H2O2 + 2OH

→

-

4OCN + 4H2O + M(OH)2 (solid)

In these reactions, a soluble copper catalyst is used to increase the reaction rate. The copper catalyst can be copper present in solution as a result of gold/silver cyanidation activities or can be added as a reagent copper solution. Copper sulphate pentahydrate (CuSO4-5H2O) is usually used to produce an initial copper concentration of about 10% to 20% of the initial WAD cyanide concentration. In the absence of the copper catalyst, the oxidation reactions occur but at a relatively slow rate. As indicated in the above reaction, metals initially bound to cyanide (i.e., cadmium, copper, nickel and zinc) are precipitated as metal hydroxides to a final concentration dependent upon the pH of the solution. The theoretical usage of hydrogenperoxide for the oxidation of free cyanides is 1.31 grams per gram of cyanide oxidized. The stable iron cyanide complexes are not converted to cyanate by hydrogen peroxide, but are removed from solution through precipitation as an insoluble copper-iron-cyanide complex: (6.29) 2Cu+2 + Fe(CN)6-4 → Cu2Fe(CN)6 (solid)

307

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The process does not preferentially attack thiocyanate, although about 10% to 15% is oxidized during treatment. The optimal pH for metals removal after cyanide destruction is about 9.0 to 9.5, although the process operates over a wide range of pH values. A pH below about 9.0 is optimal for precipitation of iron cyanides. Since metals removal is usually of more importance than iron cyanide removal, the higher process pH may be preferred. Cyanate produced in the reactions will hydrolyse to form ammonia and bicarbonate, with the rate of hydrolysis being more rapid atlower pH values. Normally, about 10% to 20%of the cyanide is converted to ammonia during treatment. (6.30) OCN- + H+ + 2H2O → HCO3- + NH4+ Excess hydrogen peroxide added for cyanide oxidation will decompose to yield oxygen and water, an advantage when the concentration of dissolved solids is of concern in the treated water. 2H2O2 → 2H2O + O2 The dosage of hydrogen peroxide required for cyanide oxidation varies according to the initial WAD cyanide level in the untreated water and the possible presence of other oxidisable compounds. Typically, a hydrogen peroxide excess of200% to 450% of theoretical is utilized in full-scale applications. Hydrogen peroxide is most commonly available at 35%, 50% and 70% strength, however 70% hydrogen peroxide is not normally used due to safety concerns. The reaction period required varies according to the initial WAD cyanide concentration and the copper and hydrogen peroxide dosages. Reaction periods typically range fromabout 30 minutes to 3 hours depending upon the copper to cyanide ratio, the untreated and treated cyanide levels, and the quantity of hydrogen peroxide used. The reaction rate is increased sharply as the copper concentration is increased. However, increasing the copper concentration may also increase downstream problems associated with removing the metal from solution. The preferred reaction period, copper concentration and hydrogen peroxide dosage are determined through a series of laboratory and/or pilot plant investigations. One operational parameter which must be evaluated during testing involves the effluent pH. Since the optimal pH for copper removal is about 9.0 to 9.5 and for iron cyanide removal is less than 9.0, a decision must be made initially to optimise removal of one or the other of these. Since the toxicity of iron cyanide is very low, the choice is usually made to optimise copper removal. It should be recognized that copper and cyanide areintimately related and removal of copper is dependent upon removal of WAD cyanide and not iron cyanide. 6.9.3

Performance

The hydrogen peroxide treatment process is employed for cyanide oxidation at several mines worldwide. Two of the srcinal hydrogen peroxide treatmentfacilities were located at the Ok Tedi and Teck-Corona mines. The flowsheet for the Ok Tedi facility is shown on Figure 6.18.


308


In this plant, hydrogen peroxide was metered into the tailings slurry at a point where turbulence ensured rapid and complete mixing. The slurry was then directed to a single complete mix reactor with a two-hour retention time. Since a tailings slurry was being treated, a reliable system for the feeding and monitoring of hydrogen peroxide was necessary to meet the stringent effluent requirements imposed by the New Guinea government. The control system employed a continuous redoxtitration of a sample drawn from the main wastewater stream using a solution of peroxymonosulphate. The signal resulting from the titration was used to control the feed rate of hydrogen peroxide to ensure an adequate dosage. A tailings flow of 1,100 m3/hr containing 110 to 300 mg/L of total cyanide was treated to 1 to 10 mg/L, with the WAD cyanide level being <0.50 mg/L. The hydrogen peroxide treatment flowsheet for the Teck-Corona facility in Canada is shown on Figure 6.19. In this application, the CIP tailings slurry was directed to a tailings impoundment where the cyanide level was reduced through a combination of dilution and natural attenuation. The system to treat decant solution from the impoundment consisted of three reactors followed by a clarifier. Decant from the tailings impoundment was pumped, alongwith copper sulphate, to the first reactor into which hydrogen peroxide was added. The solution then flowed into the second reactor into which ferric sulphate and lime were added to control the pH between 7.0 and 8.0. Cyanide oxidation occurred in the first tworeactors, while the third reactor was usedfor the precipitation of metals. A portion of the clarifier underflow was recycled into the second tank to aid floc formation, while the remainder was pumped into the tailings impoundment. Total retention time in the three tanks was about 65 minutes. Sulphuric acid was added to the final effluent to reduce the pH into the range of 7.5 to 8.5 to minimize the toxic effects of ammonia. The facility treated 2,400 to 3,600 m 3/day (440 to 660 gpm) of decant solution using a hydrogen peroxide-to-cyanide weight ratio of 4.6:1. The degree of cyanide and metals removal achieved is directly related to the initial WAD cyanide and metals concentrations in the untreated water. For example, in a series of laboratory experiments examining the treatment of a cyanidation water with hydrogen peroxide, the best effluent WAD cyanide levels achieved starting with initial WAD cyanide levels of 250, 100, 50 and 10 mg/L were 2.0, 0.50, 0.30 and 0.04 mg/L, respectively. The initial copper concentrations ranged from 6 to 40 mg/L, while the peroxide dosages studied varied from 150% to 250% of theoretical with reaction periods ranging from 1 to 2 hours. The residual copper concentrations achieved were similar in magnitude to the residual WAD cyanide levels since the two compounds are complexed in a nearly 1:1 weight ratio. A summary of additional cyanide destruction performance data is presented in Table 6.22 for three operations utilizing the hydrogen peroxide process.

309

Chapter Six


FIGURE 6.18

Flowsheet for the Hydrogen Peroxide Treatment Process at the Ok Tedi Mine

Source: Knorre and Griffiths, 1984


310

FIGURE 6.19

Teck-Corona Hydrogen Peroxide Treatment System

3 1 1


C ha pt er ixS


TABLE 6.22 Detoxification Results from Three Plants Using the Hydrogen Peroxide Process to Treat Gold Mill Effluents

Before Treatment (mg/L) Total WAD Cu Fe Cyanide Cyanide

Case Study #1 Pond Overflow (1) Case Study #2 Barren Bleed (2) Case Study #3 Heap Leach Solution (3)

After Treatment (mg/L) Total WAD Cu Cyanide Cyanide

19

19

20

<0.1

0.7

0.7

0.4

<0.1

1,350

850

478

178

<5

<1

<5

<2

353

322

102

11

0.36

0.36

0.4 (4)

<0.1

Source: Griffiths, 1989

Notes: (1) Preliminary plant results from pre-operational test runs. (2) Typical results during first six months of operation. (3) Average of 25 measurements made over ten days of plant operation. (4) Value dropped from 1.0 to 0.4 over four days due to coagulation and settling.


Fe

312


A considerable degree of testing was conducted several years ago in the Homestake Toxicological Testing Facility in Lead, South Dakota utilizing the effluents from a pilot plant facility and a variety of test organisms including trout, Daphnia and several aquatic insects. Through extensive toxicity, the hydrogen peroxide process was shown to yield environmentally acceptable effluent. Another application of the hydrogen peroxide process involves the treatment of rinse solutions generated during decommissioning of heap leach pads. Several facilities worldwide have applied or have investigated this process for cyanide destruction due to its simplicity and ease of operation. The process has been shown effective in treating WAD cyanide levels to <0.50 mg/L on a routine basis in both full-scale and bench-scale investigations. Often the existing barren and pregnant ponds can be modified as chemical reactors in which cyanide oxidation takes place (Castrantas et al., 1988). It is important to note that each detoxification problem is unique and that testwork is needed to verify process performance and to estimate treatment costs. Process evaluations are conducted through laboratory and/or pilot plant investigations to determine the efficiency, chemical dosages and reaction rates for a particular water. It is very difficult to compare the relative costs of various treatment options by extrapolating data from solutions exhibiting different chemical characteristics. There is some indication that the rate of the reaction slows at basic pH in the presence of elevated concentrations of thiocyanate. Therefore, it may be necessary to examine the effect of pH and thiocyanate levels more closely in certain instances. The advantages and disadvantages of the hydrogen peroxide process are summarized in Table 6.23.

313

Chapter Six


TABLE 6.23 Advantages and Disadvantages of the Hydrogen Peroxide Process

Advantages 1 Capital costs are less than or equal to those associated with other chemical treatment processes. 2 The process is relatively simple in design and operation. 3 All forms of cyanide including the iron complexed forms can be removed through a combination of oxidation and precipitation. 4 Heavy metals are significantly reduced through precipitation. 5 The process is adaptable for batch and continuous type treatment operations. 6 Close pH control is not required. 7 The process does not produce high quantities of waste sludge and does not introduce significant quantities of dissolved solids. 8 There are no license fees required. 9 The process has been shown effective in bench, pilot plant and full-scale operations. 10 Automation is not necessary, but available if required. Disadvantages 1 Reagent costs and consumption for copper sulphate and hydrogen peroxide can be high. 2 High concentrations of cyanate can be produced, potentially increasing the ammonia concentration in the solution. 3 The process does not remove ammonia or thiocyanate. 4 Additional treatment may be required if residual effluent concentrations of ammonia, thiocyanate or metals exceed environmentally acceptable levels. 5 Cyanide is not recovered. 6 The process is not generally suitable for treatment of tailings slurries.


314


6.10

NATURAL AT TENUATION

6.10.1 Introduction

The typical water management system at a gold mine includes a tailings impoundment area, which receives the mill tailings slurry and frequently site runoff waters. Impoundment sizes are usually determined by the requirement to remove and retain settleable solids, buy not typically to facilitate the removal of cyanide. Over the years, it has been observed thatthe concentration of cyanide in the decant solution in these impoundments was generally lower than the influent concentration. The difference was especially pronounced during warm months, particularly if the decant pond had a long retention time (i.e., several months). This process or phenomenon has been termed "natural attenuation". Examples of natural cyanide attenuation in tailings impoundments are presented in Table 6.24 as observed by several mines in Australia. These data indicate WAD cyanide reductions ranging from about 55% to 99%, reflective of the varying tailings chemistries, climatic conditions and tailings impoundment geometries at these sites. The purpose of investigating natural cyanide attenuation in more detail was to define the dominant attenuation mechanisms and how impoundments may be designed to enhance natural attenuation. In the late 1970's, attention was directed toward developing this process for the primary treatment of cyanide containing tailings slurries and barren solutions. Advantages of this approach included lower capital and operating costs when compared to chemical oxidation processes. However, there were virtually no data available at that timeeither for process design or for the prediction of treatment efficiency attainable through natural attenuation. Collaborating with the gold mining industry, Environment Canada's Wastewater Technology Centre (WTC) began studies in 1979 to develop a better understanding of natural attenuation and to obtain information that could beemployed in process designs. Field and laboratory work was undertaken and research iscontinuing because of the great interest in cyanide treatment. Three Canadian mines (Dome, Cullaton Lake and Lupin) have designed tailings areas or effluentholding ponds to enhance this process as the primary treatment for cyanide. One other mine (Giant Yellowknife) is using the method for partial treatment of its cyanide-containing wastewater, while others are considering it as a pretreatment process to be followed by secondary or advanced treatment systems.

315

Chapter Six


TABLE 6.24 Examples of Nat ural Cyanide Attenuation in Tailings Impoundments in Australia

WAD Cyanide in Tailings Discharge

WAD Cyanide in Tailings Impoundment Decant Solution

Percent WAD Cyanide Reduction

210

94

55%

48

10

79%

57

0.5

99%

150

20

87%

125

22

82%

186

20

89%

82

12

85%

99

9

91%

Source: MCA, 1996


316


6.10.2 Process Chem istry

Several chemistry aspects of natural cyanide attenuation in ponds and impoundments were discussed in Chapter 3. This section builds on this information relative to the use of natural attenuation as a method of controlling cyanide levels in process solutions. It appears that natural attenuation of cyanide in tailings ponds occurs as a result of the interaction of several processes such as volatilization, hydrolysis, photo-degradation, dissociation, chemical and bacteriological oxidation and precipitation (Schmidt et al., 1981 and Leduc et al., 1982). Extensive laboratory experiments carried out at the WTC on synthetic cyanide solutions have shown that the main mechanisms controlling natural attenuation are volatilization of hydrogen cyanide and the dissociation of the metal-cyanide complexes (Simovic, 1984a). In an aqueous solution, the cyanide ion exists in equilibrium with molecular HCN, with the relative amount of each form being pH dependent. As the pH approaches 8.0, most of the free cyanide exists in the volatile HCN form. Molecular HCN has a relatively high vapour pressure and therefore can readily volatilize into the atmosphere. In a tailings pond, the usually high pH of tailings solutions will gradually decrease as a result of natural uptake of carbon dioxide from the air. As a result, the cyanide ion converts to HCN, which isreadily removed from the pond by volatilization. The volatilization of hydrogen cyanide from an agitated solution occurs quickly, but from a stagnant solution it is much slower. Although previous researchers (Dodge and Zabban, 1952 and Lur'e and Panova, 1962) have examined the rate of loss of cyanide from aqueous solutions, little of this information was applicable to gold mill effluents. It was found that several factors affect the natural attenuation process including (Palaty and Horokova-Jakubu, 1959): • • • • • • •

pH Temperature Ultraviolet irradiation Aeration Initial cyanide concentration Metal content Surface area:depth ratio

However, laboratory experiments conducted at the WTC narrowed the principal factors down to pH, temperature, ultraviolet irradiation (UV) and aeration (Simovic, 1984a). Simulated mill effluents were used in the experiments consisting of NaCN and four single and two mixed metal cyanide solutions. The experiments were carried out at pH 7.0 and 10.5, with and without UV, °C and 20 °C. Aeration was found to have a aerated and non-aerated, and at temperatures of 4 significant effect upon the rate of cyanide attenuation only during the first 48 to 72 hours of reaction time. The attenuation rate coefficient, Kv, determined forthe experiments with aeration was 1.14 ±0.63 cm/hour and without aeration 0.71 ±0.39 cm/hour. To extend the data for comparison with field observations, these Kv values were calculated as averages from the results obtained in a number of experiments carried out on synthetic metal cyanide solutions.

317

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Laboratory experiments at 20°C with UV and aeration were undertaken on settled tailings pond water from Cullaton Lake Gold Mine (Simovic, 1984b). The results showed that aeration had a significant effect on the initial rate of volatilization of HCN, but no significant effect on the decay process of the metal cyanides complexes present. In terms of practical applications, the objective to be achieved is to encourage the volatilization of HCN. This can be considered as a two-step process. The first step is to promote a gradual reduction in the pH to less than 8.0 to promote conversion of CN to HCN. The effect can be 2, accomplished by bringing about intimate contact between the solution and atmospheric CO primarily through aerationand/or vigorous mixing. The second step is to promote volatilization of HCN from the solution. This is done by constructing a pond with large surface area and shallow depth (where possible) to maximize exposure of the solution to the atmosphere.

If free cyanide were the only soluble cyanide constituent of concern in mill effluents, the technology for its control and removal would be relatively straightforward. However, the cyanide ion forms stable complexes with a variety of transition metals. Therefore, the chemical properties of the metal cyanide complexes become very important when evaluating natural attenuation processes. The metals most often found in mill effluents include copper, iron,nickel and zinc. During the process of natural attenuation, thedissociation of metal cyanide complexes can be a slower process than HCN volatilization and therefore limit the overall rate of attenuation. The stability of the metal cyanide complexes varies from one element to another and from one oxidation state to another. Tests carried out on metal cyanide complexes by the WTC revealed that regardless of stability, they can be considered as a delayed source of free cyanide that is released at varying rates under certain conditions (Doudoroff et al., 1966). Once the metal cyanide complexes are broken down, the released free cyanide is subject to removal through natural mechanisms. From the results obtained at pH 7.0 in experiments carried out at WTC, temperature was found to have the greatest effect on the overall attenuation process (Simovic 1984a). In this study the metal cyanide attenuation constants (K) were determined at °4C and 20°C and are presented in Table 6.25. These rate constants can be applied to actual mill effluents, but the limitations associated with their usage must be recognized, and corrections for actual temperature and pH conditions must be considered.


318


TABLE 6.25 Effect of Temperature on the Rate Constants for the Attenuation of Metal Cyanide Complexes at pH 7.0

CyanideComplex Na2Cu(CN)3 Na2Zn(CN)4 Na2Ni(CN)4 K3Fe(CN)6

-1

-1

Kat4 °C (hr ) 0.00183 0.023 0.00042 0.000031

K at 20°C (hr ) 0.00550 0.049 0.00097 0.000090

Source: Simovic, 1984a

TABLE 6.26 Effluent Quality of Canadian Gold Mines Applying Batch Natural Cyanide Attenuation Systems

Barren Bleed mg/L Total WAD Cyanide Cyanide 98.6 100 (1983)

Mine

Location

Dome Mines (1)

Porcupine, Ontario

Lupin Mines (2)

Contwoyto,N.W.T.

223

186

Keewatin District

800

140 (1982)

Cullaton Lake (two ponds)

(3)

Final Effluent mg/L Total WAD Cyanide Cyanide 0.02 0.04 (1983) 0.02 0.2 (Sept. 1984) <0.1 -(Sept. 1984)

Notes: (1) Obtained from Simovic, 1985 (2) Taken from Reid, Crowther and Partners, Ltd., 1985 (3) Taken from Robinson, 1984

319

Chapter Six


6.10.3 Performance

Natural attenuation of cyanide in tailings impoundments and ponds has been observed at mines worldwide for many years. However, impoundments and ponds were not necessarily designed specifically to achieve low residual levels of cyanide. Three systems in Canada have been constructed for the purpose of achieving low cyanide residuals. Performance data for these systems are presented in Table 6.26. To exploit the potential of the natural attenuation process, Dome Mines constructed an impoundment area to treat a portion of their barren bleed separately. In 1979 and 1980, the WTC carried out an extensive sampling programme at Dome Mines to closely monitor the performance of this system(Schmidt et al., 1981). At that time, the mill employed the processes of cyanidation and amalgamation to recover gold, with the mill daily capacity being 2,000 tonnes. Barren bleed containing 60 to 100 mg/L of total cyanide was normally discharged with the tailings into the pond, except during the winter when 75% of it was conveyed to a holding pond. The holding pond consisted of two interconnected sections, a shallow section (1 to 1.75 2 m) with a surface area of about 42,000 m and a deep section (6 to 9 m) with about 10% of the surface area of the shallow section. Barren bleed was retained until latesummer (retention time of 5.5 months) and discharged to a stream when the total cyanide concentration reached 0.05 mg/L. The Lupin Mine uses the Merrill-Crowe process forthe extraction of gold. Natural attenuation is used for the removal of cyanide and ferric sulphate isadded to precipitate arsenic. Tailings are discharged to an impoundment constructed around the Esker Lakes basin with a surface area of 3,000,000 m2 and an average depth of 5 m. This tailings area was placed into operation in 1982 with the first overflowoccurring in the summerof 1985. The tailings pond is decanted only over a 3 to 4 week period late each summer. During the 1986 release, the arsenic, copper and total cyanide concentrations in the decant averaged 0.29, 0.15 and 0.22 mg/L, respectively. Another mine practising natural attenuation of cyanide, Cullaton Lake Gold Mines, employed a standard cyanide leaching method for extracting gold and the carbon-in-pulp process for recovery of gold from solution. The mine employed a two-tailings pond system. Tailings were 2 discharged to the first pond, which had an area of about 242,000 mand an average depth of 2 m. The second pond had an area of about 313,000 m2 and an average depth of 1 m. A sampling programme was conducted during the summer of 1984 (Robinson, 1984). The average influent total cyanide concentration measured in themill tailings was about 320 mg/L. In the first pond (from May to September), total cyanide was reduced to an average concentration of 153 mg/L. In the second pond, total cyanide was reduced by September to <0.1 mg/L. At this concentration, the mine is able to discharge the effluent. The allowable effluent quality for Cullaton Lake Gold Mines for total cyanide is 2 mg/L.


320


The studies conducted at the WTC led to the development of a mathematical model for predicting the attenuation of cyanide in batch systems (Simovic, 1984a). The model derived from synthetic solutions was tested on data from tailings pond water (field experiment) from the Cullaton Lake Gold Mine (Simovic et al., 1984). Data describing the characteristics of this solution at the beginning and at the end of the experimental period are given in Table 6.27. Figure 6.26 shows the results predicted by the WTC model as well as the results from the actual batch experimental data which compared quite well. The final concentration of total cyanide obtained in the field experiment was 0.1 mg/L, whereas the model prediction was 0.05 mg/L. The pH over this period remained nearly constant at 8.1. In the mid-1990’s, a modified approach to modeling natural cyanide attenuation processes was undertaken by Botz and Mudder (2000). The approach involved development of a detailed chemical calculation model that predicted chemical kinetic and equilibria reactions in cyanide solutions. Mechanisms of attenuation incorporated into the model included hydrogen cyanide volatilisation, metal-cyanide dissociation and precipitation, iron-cyanide photolysis, hydrolysis and oxidation reactions, and metal hydroxide/carbonate/sulfide precipitation. The chemical calculations are coupled with a pond or impoundment water balance model to account for dilution through water input, solution retention in tailings solids, impoundment seepage and evaporative concentration. The approach eliminated much of the empirical approaches used in prior models and attempts to be based on first principles of the interaction of cyanide with metals and the associated attenuation reactions. This model has been successfully used to evaluate tailings impoundments in North America and Australia on both a batch and continuous throughput basis. In summary, the use of natural degradation may offer the following advantages: •

A relatively inexpensive method of treating mill effluents.

•

Residual total and WAD cyanide levels of <5.0 mg/L are achievable.

•

Iron complexed cyanides can be decomposed if sufficient sunlight is available.

•

The process is suitable for batch or continuous operations.

•

The concentrations of trace metals can also be reduced.

•

The process is suitable as a primary or pre-treatment measure.

Although tailings ponds have been used for many years to treat gold mill effluents, only in recent years have definitive studies beencompleted on the process. Mathematical models forbatch and continuous throughput systems have been developed, which can be used within certain limitations to design full-scale systems or evaluate existing systems.

321

Chapter Six


TABLE 6.27 Chemistry of Tailings Impoundment Decant Solution Before and After Natural Attenuation

Constituent

Influent Concentration (mg/L)

Total Cyanide Copper Zinc Nickel Iron

4.3

Effluent Concentration (mg/L)

0.1 0.7 0.1 0.2 1.0

0.3 <0.1 <0.1 0.6


FIGURE 6 .20

Simulation o f N atural C yanide A ttenuation



322


6.11

PRECIPITATION OF CY ANIDE

6.11.1 Introduction

When an excess of ferrous iron is introduced into a mining solution containing elevated free -4 cyanide levels, the soluble ferrous cyanide complex (Fe(CN) As previously 6 ) is formed. discussed, the complex is very stable and only detectable by the total cyanide analysis. The -3 addition of ferrous iron to solutions already containing ferric cyanide complexes (Fe(CN) 6 ) yields an insoluble blue precipitate known as Turnbull's blue. Ferric iron on the other hand reacts with the ferrous cyanide complex to form the familiar insoluble Prussian blue precipitate. Depending on the concentrations of other metals in solution (e.g., copper, mercury, nickel, silver or zinc), a variety of other iron-containing metal-cyanide complexes may also be precipitated. An example is the precipitation of copper-iron-cyanide complexes observed with operation of the copper catalysed hydrogen peroxide and sulfur dioxide/air cyanide destruction processes. The process has found limited application in the treatment of cyanidation solutions, primarily for the precipitation of the stable iron-cyanide complexes. The use of ferrous sulphate to convert free and WAD cyanides to the stable iron cyanide complexes has been practised to a limited extent for tailings slurry treatment. This approach involves converting free and WAD cyanides to stable iron cyanides, both soluble and insoluble, which are relatively non-toxic. In some situations this can lead to elevated concentrations of soluble iron cyanides in treated solutions. Due to their slower reactivity innatural environments, particularly if not exposed directly to sunlight, the iron cyanide complexes may remain in tailings impoundment decant solution and pore water for relatively long periods of time. If subsequent removal of the soluble iron cyanides to low levels is found to be necessary, this can require long periods of natural attenuation or the use of a chemical treatment process. 6.11.2 Performance

Free cyanide precipitation using ferrous iron was investigated by Kuit and Babcock (1981) and led to a patented tailings treatment process. The process was first examined ona full scale at the Con Mine located in the Northwest Territories of Canada. Before adopting the hydrogen peroxide process for cyanide destruction, the mine employed a cyanide removal system based upon the adsorption of cyanide and metal cyanides on freshly prepared ferrous sulphide. Finely divided insoluble ferrous sulphide was prepared in a mill by mixing ferrous sulphate with sodium sulphide, then the resulting slurry was added to the barren bleed solution. The reaction which was pH dependent was conducted at a pH of 7.5 and with a reaction period of about 15 to 30 minutes. The weight ratio of ferrous sulphide to cyanide was maintained near 3:1.

323

Chapter Six


The treated effluent from the process was discharged to the mill tailings launder. Ferrous sulphate was also added to the launder to remove arsenic through precipitation. The removal of cyanide was not intended to achieve an effluent guideline. Rather, it was to enhance the process of natural attenuation in the tailings impoundment by lowering the total cyanide content in the feed stream. Typical analyses of influent to the treatment process andeffluent from the tailings impoundment are presented in Table6.28. A similar process was patented by Neville(1982) that relies on the use of ferrous bisulphite to precipitate cyanides as Prussian blue. The precipitation of cyanides using ferrous sulphate was further investigated by Adams (1992). In this work, several process variables were investigated relative to their effect on the efficiency of cyanide removal from clear solutions and slurries. The variables investigated included ferrous sulphate dosage, pH, dissolved oxygen concentration and reaction time. Results from several of these tests are presented inTable 6.29. Results from this work indicated that the optimal pH for iron cyanide precipitation was in the range of 5.5 to 6.5 with a ferrous sulphate dosage yielding a molar ratio of 0.5:1 of Fe:CN. Reaction times of five minutes were found to be sufficient. Higher required dosages of ferrous sulphate have been reported by others (Dzombak et al., 1996). Cyanides precipitated with ferrous sulphate must be removed from solution before the solution pH is adjusted upwards ordownwards. It was found by Adams (1992) that raisingor lowering of the pH after cyanide precipitation may result in the redissolution of soluble iron-cyanide complexes. Additional consideration must begiven to disposal of the iron cyanide precipitates.


324


TABLE 6.28 Precipitation of Free Cyanide Using Ferrous Sulphide at the Con Mine

Constituent

Influent to Treatment Process

Tailings Pond Decant Solution

Effluent Limitation

Total Cyanide

750

0.7

1.5

WAD Cyanide

--

0.35

0.40

Fe

140

--

--

Cu

50

0.3

0.30

Zn

155

0.04

0.30

As

--

0.3

0.50



325

Chapter Six


TABLE 6.29 Laboratory Test Results of Cyanide Precipitation Using Ferrous Sulphate

Total Cyanide (mg/L) Untreated Treated

Solution or Slurry

Fe:CN Molar Ratio

Dissolved Oxygen (mg/L)

pH

Solution

0.1:1

6.5

10.4

265

243

Solution

0.25:1

6.5

6.4

265

235

Solution

0.5:1

6.5

6.1

265

0.8

Solution

0.5:1

6.5

13.0

265

88 7

Solution

0.5:1

6.5

6.8

265

Solution

0.5:1

6.5

6.1

265

0.8

Solution

0.5:1

6.5

4.2

265

46 58

Solution

0.5:1

6.5

4.2

265

Solution

0.5:1

6.5

4.7

82

Solution

0.5:1

6.5

3.9

265 265

Solution

0.5:1

>20

4.2

265

21 0.8

85

Solution

0.5:1

6.5

6.1

265

Solution

0.5:1

2.0

5.2

265

6.1

Slurry

--

--

8.5

765

12

Slurry

--

--

6.0

765

5

Source: Adams, 1992


326


6.12

SULPHUR DIOXIDE AND AIR

6.12.1 Introduction

There are two patented versions of the sulphur dioxide cyanide destruction process. The first patented process and mostwidely applied is marketed by INCO Ltd. The INCO process is based upon conversion of WAD cyanides to cyanate using a mixture of SO 2 and air in the presence of a soluble copper catalyst at a controlled pH. In the INCO process, the forms of cyanide are removed by different processes. One process involves the conversion of WAD cyanides to cyanate. Iron complexed cyanides are reduced to the ferrous state and precipitated as insoluble copper-iron-cyanide complexes. Residual metals liberated from the WAD cyanide complexes are precipitated as their hydroxides. The second sulphur dioxide process was developed at Heath Steel Mines Ltd. and the patent assigned to Noranda Incorporated (Ferguson and Walker, 1985). In the Noranda process, pure sulphur dioxide is fed into a solution orslurry to lower the pH into the rangeof 7.0 to 9.0. A copper sulphate solution is then added at such a rate to yield an effluent containing the desired cyanide concentration. The INCO process has been used at over 80 mining operations worldwide and is the process addressed in this section. A primary application of the sulfur dioxide and air process is in treatment of tailings slurries, but it is also effective for the treatment of solutions for the oxidation of free and WAD cyanides. 6.12.2 Process Chem istry

Free and weakly complexed metal cyanides (i.e., WAD cyanides) are oxidized to cyanate by sulfur dioxide and air in the presence of a soluble copper catalyst. (6.32) CN- + SO2 + O2 + H2O -2

Cu +2 Catalyst

   →

(6.33) M(CN)4 + 4SO2 + 4O2 + 4H2O

OCN- + SO4-2 + 2H+

Cu +2 Catalyst

   →

-

+

-2

+2

4OCN + 8H + 4SO4 + M

The reaction is normally carried out at a pH of about 8.0 to 9.0, and due to the formation of acid in the reactions, lime is normallyrequired for pH control. Decreases in process performance can occur if the pH fluctuates outside this optimal range. The optimal pH must be determined experimentally, since maximum cyanide and metals removals occur at different pH values. Temperature has little effect on process performance between °5C and 60°C. The theoretical usage of SO2 in the process is 2.46 grams SO2 per gram of WAD cyanide oxidized, but in practice the actual usage ranges from about 3.0 to 5.0 grams SO 2 per gram of WAD cyanide oxidized. The SO2 required in the reaction can be supplied either as liquid sulphur dioxide, sodium sulphite (Na2SO3) or as sodium metabisulphite (Na2S2O5). Solutions of ammonium bisulphite (NH4HSO3) have also been used in the process, but this requires a careful examination regarding the impact ammonia addition will have on the treated effluent. The choice of one reagent versus another is primarily associated with cost and availability.

327

Chapter Six


The approximate lime requirement can be calculated from the above reactions according to the anticipated acid production resulting from SO2 addition. Oxygen (O 2) is also required in the reaction and this is generally supplied by sparging atmospheric air into the reaction vessels. Required reaction times vary from about 30 minutes to 2 hours. The soluble copper catalyst is normally added as a solution of copper sulphate pentahydrate (CuSO4-5H2O) to a level of about 10% to 20% of the initial WAD cyanide level. However, in cases where dissolved copper is already present in the tailings solution or slurry, the need for copper sulphate addition may be eliminated. Iron cyanide removal is initiated by reduction of iron from the ferric to the ferrous state according to the following reaction: -3

-4

+

(6.34) 2Fe(CN)6 + SO2 + 2H2O → 2Fe(CN)6 + 4H + SO4

-2

The ferrous cyanide complex is then removed through precipitation with either copper, nickel or zinc according to the following generalized reaction: (6.35) 2M+2 + Fe(CN)6-4 → M2Fe(CN)6 (solid) Trace metals remaining in solution following oxidation of the weakly complexed metal cyanides are precipitated as their hydroxides according to the following generalized reaction: +2

(6.36) M + 2OH

-

→

M(OH)2 (solid)

The oxidation of thiocyanate, which is usually limited to 10% to 20% in the process, and the hydrolysis of cyanate occur according to the following reactions: (6.37) SCN- + 4SO2 + 4O2 + 5H2O → OCN- + 10H+ + 5SO4-2 (6.38) OCN- + H+ + 2H2O → HCO3- + NH4+ Generally, the best application of this process is with slurries containing low to moderately high initial levels of cyanide when treated cyanide levels ofless than about 5 mg/L are required. In some cases, solutions treated with this process may be of suitable quality to permit their discharge. With regard to oxidant supply, sodium sulphite (Na 2SO3) or sodium meta-bisulphite (Na 2S2O5) are supplied as powders and must be dissolved in concentrated form prior to use. As a result, a dissolution tank and a chemical storage and feed system are necessary. In the case sulphur dioxide is used, it is usually delivered as a bulk liquid or supplied in one-ton cylinders. Sulphur dioxide gas can also be generated on-site by burning pure sulphur and collecting the combustion products into an acidic solution using a scrubber tower. Exhaust gases can also be directly injected into the slurry or solution without intermediate scrubbing.


328


In a typical two-stage process configuration, the sulphur dioxide, lime and copper sulphate are introduced into the first stage to complete the oxidation of cyanide. Additional lime or other chemicals (e.g., ferric chloride) are added to the second reactor to maximize metals precipitation. This approach is needed in some instances since the complete oxidation of WAD cyanide requires a lower pH than does the precipitation of metals from the solution or slurry. The flowsheet for a typical two-stage process is shown on Figure 6.21. The primary process variables include retention time, air feed rate, copper dosage, pH and sulphur dioxide feed rate. The quantity of sulphur dioxide or other reagent used is adjusted based on the WAD cyanide concentration in treated solution, and determined through laboratory and/or pilot plant evaluations. The copper requirement for the process is also determined experimentally. Generally, the copper dosage can be held to <50 mg/L, unless elevated iron concentrations are present which demand additional copper for iron cyanide precipitation. Laboratory evaluations of the process are generally conducted in one or two reaction vessels placed in series. Sulphur dioxide or another sourceof oxidant is added, either pre-mixed in air or separately as a sulphite solution. The tests are completed at various reagent dosages and pH values to determine the optimal reaction conditions and achievable level of treatment. 6.12.3 Performance

The performance of the INCO process at varying levels of copper addition and pH values is shown on Figure 6.22. As indicated, the process was found to be most effective with a copper concentration above about 10 mg/L and at a pH in the range of about 6.0 to 10.0. Summaries of the cyanide destruction performance and reagent usages achieved for the treatment of several tailings slurries using the N I CO process are presented in Table6.30. A summary of the cyanide destruction performance and reagent usages for the treatment of several barren solutions and decant waters are presented in Table 6.31, and process performances for treatment of three plating solutions are presented in Table 6.32. Capital costs for the process depend upon whether a slurry or solution is being treated, the level of WAD cyanide, iron cyanide and the concentration of copper in the untreated material. The primary capital items include the reactor(s), agitator(s), an air compressor and piping, a feed system for the sulphur dioxide source (i.e., for either sulphur dioxide, sodium sulphite, or sodium metabisulphite), a copper sulphate storage and feed system, and a slaked lime preparation, storage and feed system. Operating costs include labour, reagents, electrical power and maintenance. Lower treatment costs are associated with treatment of solutions containing low cyanide levels, while the higher costs are associated with treatment oftailings slurries and higher levelsof cyanide. The INCO process is patented technology and does require a license and user fee. The various advantages and disadvantages of the INCO sulphur dioxide process are presented in Table 6.33.

329

Chapter Six


3 3 0

FIGURE 6.21

Flowsheet for a Two-Stage INCO Cyanide Destruction Process


FIGURE 6.22 The Effects of Copper Concentration and pH on the Performance of the INCO Cyanide Destruction Process

Source: Robbins, 1996

331

Chapter Six


TABLE 6.30 Oxidation of Cyani de in Taili ngs Slurry Using the INCO SO 2/Air Process CNTOT Assay (mg/l) Before After 364 0.4

Mine Colosseum

Ketza River Equity Casa Berardi Westmin Premier Golden Bear

150 175 150 150 205

5.0 2.3 1.0 <0.2 0.3

Reagent Usage (g/g CH SO2 Lime 4.6 0.12

6.0 3.4 4.5 5.8 2.8

TOT) +2

Cu 0.04

0

0.30 0.03 0.10 0.12 --

0 ----

Source: Devuyst et al., 1989a, 1989b and 1991

TABLE 6.31 Oxidation of Cyanide in Sol utions Using the INCO SO 2/Air Process CNTOT A ssay (mg/l) Before After

Mine

McBean (barren) Lynngold (pond) MineralHill(barren) Lac Short (pond) Citadel (barren) St. Andrew (pond)

370 106 350 10 350 15

0.2 0.6 0.5 0.5 5.0 1

Reagent Usage (g/g CH SO2 Lime

4.0 7.0 6.0 5.0 4.0 5.0

4.0 9.0 9.0 ----

TOT)

Cu

+2

0 0.12 0 0 0 0.10


TABLE 6.32 Oxidation of Cyanide in Electroplating Wastes Using the INCO SO2/Air Process CNTOT A ssay (mg/l)

Reagent Usage (g/g CH

TOT) +2

Mine Kuntz Precious Plate Superfinish

Before 150 30,300 640

After 0.2 60 1.3



332

SO2

6.0 2.7 3.4

NaOH -0.6 --

Cu -0 0.02


TABLE 6.33 Advantages and Disadvantages of the IN CO SO 2/Air Process

Advantages 1 The process has been proven in numerous full-scale applications to yield low effluent cyanide and metals concentrations. 2 The process is effective in treating slurries as well as solutions. 3 The process is suitable for batch or continuous treatment. 4 All forms of cyanide are removed from solution, including the stable iron cyanide complexes. 5 Capital and operating costs are comparable with other chemical treatment processes. Disadvantages 1 If treating high levels of cyanide, the costs for reagents and electrical power can be

2 3 4

high. Cyanide is not recovered. Undesirable levels of sulphate in the treated solution can result. Additional treatment may be necessary for the removal of iron cyanide, thiocyanate, cyanate, ammonia, nitrate and/or metals for solutions to be discharged to the environment.

333

Chapter Six


6.13

OTHER TREATMENT PR OCESSES

6.13.1 Introduction

There are a number of cyanide treatment processes that may be considered in some situations but which have not found wide application. This section presents a brief description of four treatment processes that may be suitable in some situations. 6.13.2 Ion Exch ange

The use of anion exchange resins for removal of metal complexed cyanides was srcinally investigated in detail for the purposes of cyanide recovery (Goldblatt, 1959). Since that time, two ion exchange based cyanide recovery processes have been investigated (Whittle, 1992 and Fleming et al., 1995) as metallurgical metal and cyanide recovery processes, but these either remain in the development phase or have limited utility. The basic approach behind these cyanide recovery processes is to remove cyanide compounds from tailings solution using ion exchange resins. Cyanide collected on the resins is then stripped to produce a smallervolume of solution with a higher concentration of cyanide compounds. The overall effect is to concentrate the tailings cyanide intoa smaller volume of solution. The concentrated cyanide solution is then processed in a conventional volatilisation type of cyanide recovery process, similar to that previously described in this chapter. The difficulties encountered with ion exchange processes for cyanide recovery are primarily associated with the following: Ion exchange resins will adsorb other anions in addition to cyanide, particularly sulphate and thiocyanate. This competition with other anions reduces the selectivity of cyanide recoveryand may result in the need to handle largequantities of resin to affect suitable cyanide recovery. Ion exchange resins may irreversibly adsorb some anions, thereby reducing their subsequent ability to adsorb cyanide anions. Difficulties have been encountered in completely regenerating ion exchange resins, or very harsh chemical environments may berequired for their regeneration. This can lead either to a loss of resin activity over time or the damaging of resins due to exposure to aggressive chemicals. Ion exchange resins experience attrition in slurry systems, potentially resulting in a significant loss of resin in the final tailings. Ion exchange has been used for effluent treatment to remove cyanide and other undesirable compounds, and many laboratory and pilot plant studies have been undertaken. Most of the work employed the strong base quaternary resins of the Duolite and IRA series (Carlson, 1982; Halbe et al., 1979 and Vachon, 1985). A weak base resin has also been employed (Avery and Fries, 1974).


334


A detailed testing programme was undertaken at Homestake Mining Company from about 1980 through 1983 to investigate ion exchange for the treatment of tailings impoundment decant water. A variety of resin types and manufacturers wereevaluated, and the results indicated that the strong base anion exchange resins were most effective in removing metal complexed cyanides. However, problems were encountered with regeneration of the resinsand with loss of adsorptive capacity over time. In addition, thiocyanate competed strongly forresin adsorption sites and was difficult to remove during the resin regeneration step. However, ion exchange was considered as a good polishing process for treatment of metal complexed cyanides when they are present at low concentrations. Careful consideration must begiven to the disposal of the resin regeneration brines with this process, as these brines contain elevated dissolved solids concentrations and exhibit varying levels of toxicity. 6.13.3 Ozone Oxidation

Ozone is a strong oxidant and capable of oxidizing free and WAD cyanides to cyanate, ammonia and/or nitrate (Rowleyand Otto, 1980). In addition, ozone willoxidize thiocyanate to cyanate or cyanide depending on the solution pH (Botz et al., 2000). The reaction rate is rapid and generally only limited by therate at which ozone can be absorbed into the solution. Low effluent cyanide concentrations can be achieved with ozone, but treatment may result in the formation of cyanate, ammonia and/or nitrate. Ozone is relatively expensive to produce and thishas limited its use for cyanide destruction, particularly for large water flows. Iron cyanides are also oxidized by ozone, but the reaction rate is too slow at ambient temperature for practical application (Mauk et al., 1976). At elevated temperature and in the presence of ultraviolet radiation, iron cyanides are converted to cyanate by ozone. 6.13.4 Reverse Osmosis

Reverse osmosis is a desalination process in that it removes a majority of dissolved ions, including cyanide ions, from water using a semi-permeable membrane. Under very high pressure, water is forced through the membrane, but dissolved ions are rejected by the membrane with up to greater than 95% effectiveness. Since most cyanide compounds are present inwater as ions, reverse osmosis can be used to separate clean water from cyanide containing water. If the water pH is less than about 10.0, a portion of free cyanide will be present as HCN and this compound is not rejected by reverse osmosis membranes. Therefore, treatment of cyanidation waters requires an elevated pH in the reverse osmosis feed stream or cyanides must be present in metal complexed form. In one case, excess heap leach solution at451 mg/L WAD cyanide was treated by reverse osmosis to yield water with 15 mg/L WAD cyanide (Irish and Mukhapodhyay, 1992). This water was then further treated with chlorine to produce a final treated water with <0.02 mg/L WAD cyanide.

335

Chapter Six


The reverse osmosis process produces two water streams; a relatively clean permeate and a brine containing a majority of the feed dissolved solids. The permeate may be suitable for discharge, with or without polishing treatment. However, the concentrate must be further treated, evaporated to produce a waste salt or disposed as a liquid waste. At operating mines, the concentrate could potentially be recycled into the leaching circuit to allow for the re-use of its contained cyanide. Reverse osmosis cannot be used to treat tailings slurries, but has beenused on an emergency or intermittent basis to treat solutions. Reverse osmosis has found only limited application for the treatment of cyanidation waters, primarily because of the following drawbacks to the process: •

Extensive pre-treatment of water fed to a reverse osmosis system is required to prevent solids accumulation on the membrane surfaces. Solids accumulation on the membranes must be periodically removed using chemical cleaning solutions, though in some cases the accumulation is irreversible. A typical pre-treatment train mayrequire several steps, including chlorine disinfection, water clarification, sand filtration, ion exchange or lime softening, heating, cartridge filtration and antiscalant feed.

•

The concentrate brine produced by reverse osmosis may have a volume of 10% to 50% of the feed water volume. This brine can be expensive to dispose or further treat, and in some cases the brine may be considered a hazardous waste.

•

Capital and operating costs of reverse osmosis can be excessive. Capital costs include equipment required for the reverse osmosis system plus equipment required for water pre-treatment, which are often more expensive than for the reverse osmosis system. Operating costs include those associated with water pre-treatment, high pressure pumping, membrane replacement, and disposal or further treatment of the waste concentrate stream.

6.14

REMOVAL OF METALS AND CYANIDE RELATED COMPOUNDS

6.14.1 Introduction

The primary constituents of concern in cyanidation solutions include not only the various forms of cyanide, but the cyanide related compounds thiocyanate, cyanate, ammonia and nitrate. In addition, varying concentrations of trace metals are often associated with cyanidation solutions. In many cases, these constituents are important from both a water quality and toxicity standpoint and low levels must be achieved in treated waters. The following sections provide background information on the removal of metals and the cyanide related compounds.


336


6.14.2 Metals Removal

The key to efficient metals removal is to convert the metals, if initially in a complexed form, to a form amenable to removal by conventional treatment. Several trace metals may be present in cyanidation solutions in varying concentrations. The following provides some background information regarding several metals commonly found in cyanidation solutions and some insight into their chemistry and treatment. Arsenic and antimony are classed as metalloids and tend to exhibit similar chemical properties. They can occur in gold ores as the free elements, as the simple sulphides (orpiment, As 2S 3; realgar, As2S2; stibnite, Sb2S3) and as arsenides or sulpharsenides of silver, cobalt, nickel, copper, lead and iron. The principal minerals ofconcern in cyanidation are thesimple sulphides and the sulpharsenide of iron, arsenopyrite (FeAsS). Orpiment, realgar and stibnite dissolve in alkaline solution to form thioarsenite and thioantimonite initially, which convert to the arsenite and antimonite with time. Arsenopyrite itself is almost completely insoluble in cyanide leach solutions, but where the ore has been roasted the arsenic is converted to the highly soluble arsenious oxide and converts to arsenite ion on dissolution. Neither arsenite nor antimonite undergo oxidation in leaching and hence these are the principal dissolved forms present in a barren bleed or tailings slurry. None of the forms resulting from reactions between arsenite ion and metallic ions are sufficiently insoluble for use in meeting environmental criteria for mining effluents. In the past, an attempt has been made to control arsenic in effluents by additions of large excesses of lime, but this practice has not proved suitable, due both to the high solubility of calcium arsenite and, in the case of calcium arsenate, the increase in its solubility as pH decreases in solution (Robins and Tozawa, 1982). Arsenate compounds provide more suitable precipitates; those formed with copper, lead, nickel and zinc being particularly insoluble. Stoichiometric ferric arsenate is relatively soluble but the solubility decreases as the iron to arsenic ratio is increased and as the pH approaches the neutral to slightly acidic range. It has been shown that basic ferric arsenates with molar ratios of 4 or more (weight ratios of 3 or more) give 100 to 1,000 times lower solubilities of arsenic over the pH range 3 to 7 and provide environmentally stable forms(Krause and Ettel, 1985). The current practice is to treat cyanide and then add ferric iron for arsenic precipitation. Antimony chemistry resemblesthat of bismuth more than that of arsenic. Solutions of both +3 and +5 antimony readily hydrolyse when diluted or partly neutralized, and precipitate either as the oxides or basic salts. The salts of both of these, with the exception of their sodium salts, are only sparingly soluble. et al.As (1979) limeprecipitation precipitationinwill remove antimony to levels belowParker 1.0 mg/L. in thereported case of that arsenic, the not presence of ferric hydroxide appears to be more effective, but whether iron antimonite or antimonate compounds are formed or whether the iron hydroxide precipitate simply adsorbs the hydrolysed antimony compounds, has not been established.

337

Chapter Six


In the case of cobalt, its concentration is generally below levels considered an environmental concern and, if present, is bound to cyanide in a complex stronger than iron. As a result, it is rarely an issue from a treatment or toxicity viewpoint and is not discussed in greater detail. Cadmium is very important from an aquatic life and toxicity viewpoint and very low cadmium levels may be imposed on a treated discharge. Conventional treatment processes (e.g., ime l precipitation and addition of ferric salts and sulphides) will remove cadmium effectively, yielding effluent levels in the range of 0.01 to 0.10 mg/L. However, the achievable level is commonly above aquatic life criteria for cadmium and dilution of discharged effluents becomes critically important in order to minimize adverse water quality impacts. A metal of concern in many mining waters is copper, since it is toxic to aquatic life and is difficult to remove due to its strong cyanide complex. The key to effective copper removal is to convert the metal to the free form or a complexed form amenable to conventional treatment. In addition to cyanide, copper can form complexes with ammonia, thiocyanate, flotation reagents and natural organic compounds present in the water (e.g., humic and fulvic acids). The stability of these complexes varies and often accounts for the increased solubility of copper in decant ponds even though cyanide has been removed through natural attenuation. In these cases, flocculants may be helpful in reducing the residual copper level. The approach to copper removal involves removal of cyanide first, followed by removal of the metal. The residual copper levels achievable are on the same order as the cyanide levels, since the molar ratio of cyanide to copper typically ranges from about 2:1 to 3:1. Conventional treatment processes including lime precipitation, sulphide addition and ferric chloride addition can produce effluent copper levels of less than 0.05 to 0.15 mg/L, depending upon the initial concentration. Ferric chloride addition is sometimes effective in removing the copper cyanide complex as well. Activated carbon adsorption willalso reduce residual copper cyanide to low levels (<0.10 mg/L) if the influent concentration is in the range of 1.0 to 2.0 mg/L or less. Chromium is usually present in low concentrations in cyanidation waters. The two forms of chromium of importance arechromium III and chromium VI. Effluent limitations for chromium can be achieved through conventional treatment processes, which often requires reduction of chromium VI to chromium III and precipitation of the chromium III. Complexation of chromium with cyanide is not an important consideration in most mining waters. Iron and manganese may appear in elevated concentrations in mining waters due to acidic mine drainage or due to formation ofthe cyanide complex in the case ofiron. Neither of these metals are a major water quality issue and relatively high aquatic life and human health criteria exist for both of them. Removal of the free metals is accomplished through a combination of aeration and precipitation, with very low effluent concentrations being achievable. Lead is another metal which is usually present in low concentrations in mining waters (<0.10 mg/L). As with cadmium, lead is important from a water quality standpoint and very low effluent lead limitations may be mandated. Since conventional treatment often doesnot reduce lead to acceptable levels, dilution and source control become critically important in minimizing the water quality impacts associated with the metal.


338


Mercury is a metal of environmental concern for a variety of reasons and as a result, very low effluent limitations are normally applied. Mercury is bound in a relatively weak complex with cyanide and has a great affinity for sulphide and activated carbon, both of which form the basis of primary removal processes. Effluent levels of <0.005 mg/L are achievable in the case of activated carbon. With regards to sulphide treatment, one approach isto utilize a commercially available reagent (Degussa's TMT), which reduces effluent mercury levels into the range of <0.001 to 0.002 mg/L, while eliminating the problems associated with handling and feeding of sodium sulphide. Molybdenum exists primarily as the molybdenate anion in metallurgical solutions and does not associate itself with cyanideas do other metals such as copper or nickel. There are a variety of processes which have been examined for the removal of molybdenum from solutions including ion exchange, reverse osmosis, electrodialysis, biological treatment and precipitation at acidic pH with ferric ion. The two demonstrated processes include ion-exchange and precipitation with ferric iron. Based upon actual operating experience in the United States, an effluent molybdenum concentration of 0.50 mg/L is achievable. The primary problem associated with ion exchange other than its sophistication and cost relates to the disposal of brine solutions generated during backwash of the resin beds. These solutions will contain very high levels of dissolved solids and metals and could be classified as hazardous waste. It is well known that addition of ferric ion to solutions at an acidic pH will remove molybdenum through a combination of co-precipitation with and adsorption on the resulting ferric hydroxide floc. A similar process is utilized in the removal of arsenic and selenium from solution. The theoretical minimum solubility of the iron molybdate compound is about 0.20 mg/L. The optimal pH range for removal is 3.5 to 4.5 with an optimal Fe:Mo molar ratio of about 10:1 to 15:l, although higher ratios may be required in practice. Nickel can be present in elevated concentrations in cyanidation waters and forms a relatively stable complex with cyanide. However, conventional treatment forremoval of WAD cyanide and metals will reduce nickel to levels below aquatic life criteria. As a result, nickel is not a metal of primary concern in cyanidation waters. Selenium may appear in cyanidation waters at levels ranging from about0.02 to 5.0 mg/L. The two primary forms of selenium are selenite and selenate. Although selenite is readily removed through conventional precipitation and ferric salt addition, selenate is very difficult to remove. In order to promote selenate removal it must first be reduced to the selenite form, which may be difficult to accomplish. There are several processes available including reductionwith sulphur dioxide, metallic zinc and iron and micro-organisms. The use of biological systems has been studied intensively by the U.S. Bureau of Mines and has been shown effective in removing selenium. In addition, some has beenfrom demonstrated in the re moval of selenium through ion exchange. Selenium is promise very important a water quality standpoint and its removal requires a detailed and thorough investigation on a site-specific basis.

339

Chapter Six


Silver is another metal of environmental concern. It usually is present in cyanidation waters in varying concentrations, depending upon the extent of its recovery. Although silver forms a relatively strong complex with cyanide, as with mercury it has a great affinity for sulphide. Again, sulphide reagent have been shown effective in removing silver to concentrations in the range of <0.01 to 0.02 mg/L in cyanidation waters. Although the water quality criteria for silver are quite low, this degree of treatment and subsequent dilution can reduce the silver concentration in the treated effluent to acceptable levels. Zinc is present in cyanidation waters in concentrations ranging from <0.10 to 100 mg/L, depending upon the gold recovery process being employed. Its complex with cyanide is very weak and conventional removal processes (e.g., precipitation) can often reduce zinc levels to <0.50 mg/L. Zinc removal does not generally pose a major problem in meeting necessary water quality criteria. A summary of various treatment processes and their removal efficiencies for several metals are presented in Tables 6.34 to6.43. In addition, a summary of actual effluent data from five mines treating moderate to high strength acidic mine drainage (AMD) srcinating from various base metal recovery processes are presented in Table 6.44. The results indicate that very good removal is achievable using lime precipitation when the metals are present in their free forms.


340


TABLE 6.34 Water Treatment Options and Performance Data Summary for Antimony and Arsenic Removal

Initial Final Concentration Concentration Treatment Technology Antimony L ime/Filter FerricChloride/Filter Alum/Filter Arsenic Lime Softening Sulphide/Filter Lime(260mg/L)/Filter Lime(600mg/L)/Filter FerricSulphate Ferric Sulphate Lime/FerricChloride/Filter ActivatedAlumina(2mg/L) ActivatedCarbon(3mg/L)

Ferric Chloride Ferric Chloride

pH

(mg/L)

(mg/L)

Removal

11.5 6.2 6.4

0.6 0.5 0.6

0.4 0.2 0.2

28% 65% 62%

-7-6 10.0 11.5 7.5 56.0 10.3 6.8 3.1-3.6

0.2 --5.0 5.0 0.05 5.0 3.0 0.4-10 0.4-10

---

0.3 0.6 0.9 -

0.03 0.05 1.0 1.4 0.005 0.5 0.05 <0.4 <4.0 0.05 <0.13

85% -80% 72% 90% 90% 98 96%-99% 63%-97% 98% --

Source: USEPA, 1984

341

Chapter Six


TABLE 6.35 Water Treatment Options and Performance Data Summary for Beryllium and Cadmium Removal

TreatmentTechnology Beryllium Lime/Filter Cadmium Lime(260mg/L)/Filter Lime(600mg/L)/Filter LimeSoftening Lime/Sulphide FerrousSulphide(Sulphex) FerriteCoprecipitation/Filter

Initial Concentration

Final Concentration

(mg/L)

(mg/L)

Removal

11.5

0.1

0.006

99.4%

10.0 11.5 5-6.5 8.5-11.3 8.5-9.0 neutral

5.0 5.0 0.44-1.0 0.3-10 4.0 240

pH

0.25 95% 0.10 98% 0.008 92% – 98% 0.006 98% <0.01 99% 0.008 99%

Source: USEPA, 1984

TABLE 6.36 Water Treatment Options and Performance Data Summary for Copper Removal

Treatment Technology Lime/Filter Lime(260mg/L)/Filter Lime(600mg/L)/Filter FerricSulphate/Filter Lime Lime Alum

pH 8.5-9.0 10.0 11.5 6.0 >8.5 9.5 6.5-7.0

Lime/Sulphide FerrousSulphide(Sulphex) FerrousSulphide(Sulphex) FerriteCoprecipitation/Filter

5.0-6.5 8.5-9.0 8.5-9.0 --

Initial Final Concentration Concentration (mg/L) (mg/L) 3.2 0.07 5.0 0.4 5.0 0.5 5.0 0.3 10-20 1-2 3.0 0.2 3.0 0.2

50-130 3.2 4.0 --

Source: USEPA, 1984


342

<0.5 0.02 0.01 0.01

Removal 98% 92% 91% 95% 90% 93% 93%

-99% 99% 99%


TABLE 6.37 Water Treatment Options and Performance Data Summary for Chromium Removal

TreatmentTechnology Chromium III

pH


Final Concentration

(mg/L)

(mg/L)

Lime(260mg/L)/Filter

10.0

5.0

0.1


11.5

5.0

0.1

Reduction/Lime

7-8

Reduction/Lime Lime Softening Lime/Filter

7-8 10.6-11.3 7-9

Lime Lime FerriteCoprecipitation/Filter Ferric Sulphate

140(asCrVI) 1,300(asCrVI) ---

9.5 9.5 ---

15 3.2 25

1.0

Removal

98% 98% --

0.06(asCrIII)

--

0.15 0.05

98% --

0.1 <0.1

---

0.01

--

6.5-9.3

---

--

98%

--

5.0

0.05

99%

Chromium VI ActivatedCarbon(powdered) ActivatedCarbon(powdered) ActivateCarbon(granular) FerriteCoprecipitation

3.0 2.0 6.0 --

10 10 3 0.5

1.5 0.4 0.05 notdetectable

SulphurDioxideReduction

--

FerricSulphate/Filter

BisulphiteReduction

--

--

--

0.01-0.1 0.05-1.0

85% 96% 98% ----

Source: USEPA, 1984

343

Chapter Six


TABLE 6.38 Water Treatment Options and Performance Data Summary for Lead Removal

Initial Final Concentration Concentration TreatmentTechnology Lime (260 mg/L)

Lime/Filter

pH

(mg/L) 5.0

10.0 8.5-9.0

(mg/L) 0.25

189

Removal 95%

0.1

99.9%


10.0

5.0

0.075

98.5%


11.5

5.0

0.10

98%

FerrousSulphate/Filter

6.0

5.0

0.075

98.5%

SodiumHydroxide(1hoursettling)

5.5

--

1.6

--

SodiumHydroxide(24hoursettling)

7.0

--

0.04

--

SodiumHydroxide/Filter

10.5

1,700

0.60

99%

SodiumCarbonate/Filter

10.1

1,260

0.60

99%


6.4-8.7

10.2-70.0

0.2-3.6

82%-99%


9.0-9.5

5.0

0.01-0.03

99%

FerrousSulphide(Sulphex) FerriteCoprecipitation/Filter

8.5-9.0

189

--

480

Source: USEPA, 1984


344

0.1 0.01-0.05

99.9% 99.9%


TABLE 6.39 Water Treatment Options and Performance Data Summary for Mercury Removal


Final Concentration

TreatmentTechnology Sulphide

pH --

(mg/L) 0.3-50.0

(mg/L) 0.01-0.12

Removal --

Sulphide

10.0

10.0

1.8

96.4%

Sulphide/Filter

5.5

16.0

0.04

99%

Sulphide/Filter

4.0

36.0

0.06

99.8%

Sulphide/Filter

5.8-8.0

0.3-6.0

0.01-0.125

87%-99.2%

--

6.0-7.4

0.001-0.005

99.9%

FerriteCoprecipitation/Filter ActivatedCarbon

--

0.01-0.05

<0.0005

--

ActivatedCarbon/Alum

--

0.02-0.03

0.009

--

ActivatedCarbon

--

0.06-0.09

0.006

--

Source: USEPA, 1984

TABLE 6.40 Water Treatment Options and Performance Data Summary for Nickel Removal

TreatmentTechnology Lime

pH

Initial Concentration (mg/L)

Final Concentration (mg/L)

Removal

8.5-9.0

75

1.5

98%


10.0

5.0

0.3

94%


11.5

5.0

0.15

97%

Caustic Soda/Filter FerrousSulphide(Sulphex) FerriteCoprecipitation

11.0

--

0.3

--

8.5-9.0

75

0.05

99.9%

--

1,000

0.20

99.9%

Source: USEPA, 1984

345

Chapter Six


TABLE 6.41 Water Treatment Options and Performance Data Summary for Selenium and Thallium Removal

TreatmentTechnology Selenium


Final Concentration

(mg/L)

(mg/L)

pH

Removal

FerricChloride/Filter

6.2

0.1

0.03

75%

FerricChloride/Filter

6.2

0.05

0.01

80%

Alum/Filter

6.4

0.5

0.26

48%

Ferric Sulphate

5.5

0.10

0.02

82%

Ferric Sulphate

7.0

0.10

0.03

75%

Lime/Filter

11.5

0.5

0.3

35%

Lime/Filter

11.5

0.06

0.04

38%

Thallium Lime/Filter FerricChloride/Filter Alum/Filter

11.5

0.5

6.2

0.6

6.4

0.6

Source: USEPA, 1984


346

0.2 0.4 0.4

60% 30% 31%


TABLE 6.42 Water Treatment Options and Performance Data Summary for Zinc Removal

Initial Final Concentration Concentration TreatmentTechnology Lime/Filter

Lime (260 mg/L) Lime(260mg/L)/Filter Lime (600 mg/L) Lime(600mg/L)/Filter

pH 8.5-9.0

10.0 10.0

FerriteCoprecipitation

5.0

0.80

9.0

33

--

42

84% 93%

1.2 16

Removal 93%

83%

0.35

5.0 --

(mg/L) 0.25

0.85

5.0

11.5

Sulphide FerrousSulphide(Sulphex)

5.0

11.5

Lime/Filter Sodium Hydroxide

(mg/L) 3.6

77%

0.02-0.23 1.0 1.2

-97% 97%

8.5-9.0

3.6

0.02

99%

--

18

0.02

99%

Source: USEPA, 1984

347

Chapter Six


TABLE 6.43 Long-Term Effluent Concentrations Achievable with Several Water Treatment Processes

Ferrite Lime & Settling

Lime & Filter

Antimony

0.8-1.5

0.4-0.8

Arsenic

0.5-1.0

0.5-1.0

Beryllium

0.1-0.5

0.01-0.1

Cadmium

0.1-0.5

0.05-0.1

0.01-0.1

0.4-0.7

0.05-0.5

Copper ChromiumIII Lead

0.5-1.0 0.1-0.5

Sulphide & Filter

0.3-1.6

0.05-0.6

0.20 <0.01

0.05-0.5

Silver Selenium

0.4-0.8 0.2-1.0

0.2-0.4 0.1-0.5

0.05-0.2

Thallium

0.2-1.0

0.1-0.5

Zinc

0.5-1.5

0.4-1.2

0.2-0.5 0.02-1.2

0.02-0.5

Ferric Chloride

Activated Carbon

0.05-0.5

0.3 0.1

0.05-0.1 0.05-0.1

Thallium

0.7

Source: USEPA, 1984


Bisulphite Reduction

348

Lime/FeCl2 & Filter

0.02-0.1 0.05-05

0.01

Selenium

0.1-0.6

<0.05

0.05-0.4

0.1-0.5

Silver

0.4-0.8

Alum

<0.05

0.01-0.05 0.2-1.5

Mercury

Soda Ash & Filter

0.01

Nickel

Chromium VI

Soda Ash & Settling

0.05-0.1

0.05-0.5

Mercury

Arsenic

Coprecipitation & Filter


TABLE 6.44 Summary of the Performance of Acid Drainage Treatment Systems

th

th

50 Percentile

Mine A

Mine B

Mine C

Mine D

Mine E

90 Percentile

Mine A

Mine B

Mine C

Mine D

Mine E

of Data TSS

16.2 9.1 0.001 0.001 0.024 0.059 0.050 1.62 0.016 0.20 0.87 1.05 0.010 0.015 0.010 0.025

18.4 9.3 1890 0.0025 0.0072 0.0090 0.063 0.030 0.426 0.005 0.048 0.430 1.14 0.0005 0.0005 -

10.8 0.004 0.015 0.010 0.17 -

9.4 0.045 0.68 -

9.8 0.04 0.22 0.31 -

of Data TSS

pH SO4 D.As T.As D.Cu T.Cu D.Fe T.Fe D.Zn T.Zn D.Al T.Al D.Cd T.Cd D.Cr T.Cr

pH SO4 D.As T.As D.Cu T.Cu D.Fe T.Fe D.Zn T.Zn D.Al T.Al D.Cd T.Cd D.Cr T.Cr

34 9.3 0.250 0.250 0.092 0.20 0.110 3.63 0.060 0.36 14.1 1.22 0.010 0.040 0.025 0.070

112 11.2 2250 0.0055 0.050 0.120 0.32 0.080 3.48 0.120 0.680 3.31 4.33 0.0023 0.0050 -

11.4 0.012 0.040 0.050 0.50 -

11.2 0.27 16.7 -

10.2 0.08 1.49 1.21 -

D.Pb T.Pb

0.025 0.082

-

-

0.12

0.24

D.Pb T.Pb

0.082 0.170

-

-

0.161

Source: Ferguson, 1991

Notes: (1) All concentrations in mg/L. (2) D indicates dissolved and T indicates total concentrations.

349

Chapter Six

0.43


6.14.3 Removal of Cyanid e Related Compounds

This section provides a brief overview of treatment technologies applicable to removing the cyanide related compoundscyanate, thiocyanate, ammonia and nitrate rom f mine waters. These compounds are commonly encountered in cyanidation circuits, and in some cases their removal from discharged effluent may be required to minimize impacts on aquatic ecosystems. Treatment technologies for the removal of these compounds have been employed at many sites worldwide and are, either alone in or in combination, capable of lowering their concentrations to levels that are environmentally acceptable. Treatment technologies includedin this section are limited to those proven applicable to full-scale treatment. 6.14.3.1

Cyanate Removal

The compound cyanate is related to cyanide and is often found in waters that contain cyanide. Cyanate srcinates from the oxidation of cyanide but exhibits different chemical, analytical, treatment and toxicity characteristics. Cyanate is the primary by-product of cyanide treatment when chemical oxidation processes are employed. The primary approach to elimination of cyanate is to select water treatment processes which do not form the compound as a by-product. Treatment of solutions for cyanate removal is uncommon because it is much less toxic than cyanide, is generally present in metallurgical solutions in low concentrations and does not remain in the environment for long periods of time. In some cases, cyanate concentrations in mine waters may be sufficiently high as to warrant implementation of a removal process prior to discharge of the solution. This would be limited to situations where waters would be discharged to the environment and the concentration of cyanate in untreated solution would be toxic to aquatic organisms. There are full-scale water treatment plants that incidentally remove cyanate along with their intended purpose of removing cyanide or other related compounds. Most notable are several biological treatment plants where processes to remove cyanide, thiocyanate and ammonia also result in removal of cyanate. An example is a mine in North America where decant solution is biologically treated for cyanide, thiocyanate and ammonia removal(Given et al., 1998). These compounds are removed to low levels in the aerobic treatment system, although the cyanate level is also reduced. The biological mechanism of cyanate removal is first the oxidation of cyanate to ammonia, and then ammonia removal proceeds through the biological nitrification process.


350


Cyanate may also be removed from solution using chemical oxidation or hydrolysis processes. Chemical oxidation with chlorine at an alkaline pH will convert cyanate to ammonia, and the chlorination process can also be configured to complete the oxidation of ammonia into nitrogen gas through breakpoint chlorination. Ozone at an alkaline pH is capable of converting cyanate directly to nitrate, thereby avoiding the intermediate formation of ammonia. The advantage of the ozone process is that the concentration of dissolved species in treated solution is not increased significantly since ozone dissociates into oxygen or water. Cyanate also can be hydrolysed to ammonia at an acidic pH, though the reaction is relatively slow at low temperatures, which in some cases may require solution heating. With the oxidation or hydrolysis reactions, cyanate is converted either into ammonia or nitrate and subsequent removal of these compounds may be required depending upon their resultant concentrations. 6.14.3.2

Thiocyanate Removal

Thiocyanate is formed through the interaction of cyanide with sulphur-containing compounds, particularly sulphide minerals such as pyrite, pyrrhotite, chalcopyrite or arsenopyrite. As with cyanate, thiocyanate is related to cyanide and usually only found in solutions that also contain cyanide. Thiocyanate is a potential problem for several reasons including tis consumption of cyanide, its consumption of chemicals in water treatment processes, its toxicity and its ability to break down to form ammonia. Removal of thiocyanate should start in ore processing through minimisation of its formation. Thiocyanate is much less toxic than cyanide and exhibits unique chemical, analytical and treatment characteristics. Thiocyanate removal frommine waters is not routinely practiced in the mining industry, but there are full-scale treatment plants that remove thiocyanate. Removal of thiocyanate from mine waters can be accomplished with one of several available chemical processes (Mudder and Botz, 1998). The chemical destruction methods utilize an oxidant such as chlorine to convert thiocyanate to cyanide at an alkaline pH, and then cyanide oxidation rapidly continues to yieldcyanate, ammonia and nitrate. Several oxidants are capable of oxidizing thiocyanate, butonly chlorine and ozone yield suitably rapid reaction kinetics. It is possible to chemically regenerate cyanide from thiocyanate, however this process has not been implemented on a full-scale basis (Botz et al., 2000). Alkaline chlorination and ozone efficiently destroy thiocyanate and are capable of lowering thiocyanate concentrations to a few mg/L. If the residual chloride or chlorine content in treated solution is of concern, then the ozone process may be preferable since ozone dissociates into oxygen or water. Both chlorine and ozone can be used to simultaneously oxidize cyanide, cyanate and thiocyanate, and often the choice of the appropriate chemical oxidant is based on considerations of cost, by-product generation and process efficiency.

351

Chapter Six


A lower cost alternative in many cases for thiocyanate destruction is biological treatment. Microorganisms in an aerobic environment readily oxidize thiocyanate and the reactions are o o rapid at temperatures above about 10C to 15 C. Biological thiocyanate treatment processes can readily be configured to simultaneously remove cyanide, cyanate and ammonia along with thiocyanate. At a North American site, a biological treatment plant is used to lower cyanide, cyanate, thiocyanate and ammonia from initial concentrations of about 0.4 mg/L, 300 mg/L, 500 mg/L and 40 mg/L, respectively, to final concentrations of about 0.08 mg/L, <5 mg/L, 0.3 mg/L and 0.5 mg/L, respectively (Given et al., 1998). The advantages of biological processes over chemical processes for thiocyanate removal are that capital and operating costs are relatively low as is the concentration of reaction by-products. However, biological processes arekinetically slow at colder temperatures and do not respond well to rapid fluctuations in solution flow or chemistry. 6.14.3.3

Ammonia Removal

Ammonia may be a water quality problem for several reasons, including its own toxicity, its ability to consume oxygen in streams during nitrification and the toxicity of its breakdown products nitrite and nitrate. The primary ammonia control mechanism is to choose water treatment processes which do not generate ammonia or its precursor cyanate. Sources of ammonia in mine waters may include the following: •

A mixture of ammonium nitrate and fuel oil (ANFO) is often used as a blasting agent at mining operations. A small percentage of ANFO used n i blasting will remain unreacted and report as ammonia and nitrate in metallurgical circuits or waste rock. The concentration of ammonia srcinating from this source is generally low, though in some circumstances ammonia removal from mine waters is required as a direct result of ANFO usage.

•

Ammonia is a breakdown product of cyanide and forms through the hydrolysis of cyanate. If cyanate is present in mine waters at an elevated concentration, then often there will be a correspondingly elevated concentration of ammonia.

Through a combination of these two sources, ammonia removal from mine waters is occasionally required. The concern is with toxicity to aquatic organisms since ammonia is generally not present in decant solution at concentrations that would be toxic to wildlife or waterfowl. Treatment options considered typically include biological, chlorination and ion exchange processes, though in some cases air stripping may be considered.


352


Ammonia is readily converted to nitrate in an aerobic biological treatment system, and this process is practiced at many municipal wastewater treatment plants throughout theworld. The reaction product from this process is nitrate, which is less toxic than ammonia but may also require removal depending upon its concentration. It is not uncommon for biological treatment plants to reduce ammonia to concentrations below 1 mg/L, however applications are generally limited to situations where the solution flow and chemistry do not rapidly fluctuate. Biological conversion of ammonia to nitrate can be conducted at relatively low temperatures, but reaction rates are higher at temperatures above about 10oC. Ammonia removal through breakpoint chlorination converts ammonia directly into nitrogen gas, thereby avoiding nitrate formation. The chlorination processis efficient at removing ammonia to low levels and can also be configured to affect the simultaneous removal of cyanide, cyanate and thiocyanate as well as ammonia. Ammonia concentrations above about 50 mg/L may require large quantities of chlorine depending upon the solution flow rate. Chlorine added to solution will ultimately convert to chloride and increase the dissolved solids concentration in treated solution. If the concentration of either chloride ortotal dissolved solids is of concern in treated water, then alternatives to chlorination may be considered. Chlorine is also toxic to aquatic organisms and a dechlorination process must follow chlorination prior to discharging water to the environment. Ion exchange can be used to reduce ammonia concentrations without causing a significant increase in the dissolved solids concentration. Ion exchange is also not as prone to process upsets as a result of flow and chemistry fluctuations in comparison to biological treatment processes. Consideration of ion exchange is appropriate when concentrations of interfering species such as sodium, calcium and magnesium are relatively low and when the solution pH is less than about 9.0. Under these conditions, ion exchange resins can be selective towards ammonia removal and the process may be economical for full-scale implementation. A disadvantage of ion exchange is that resins must be periodically regenerated using concentrated solutions of sodium chloride or sulphuric acid. These solutions along with all ammonia emoved r from solution will be present in the waste regenerant solution, and disposal of this waste solution is often difficult and expensive. In addition, ion exchange resins can become fouleddue to the presence of certain dissolved metals in solution, and resin fouling can lead to high costs for purchasing new resin and for disposing fouled resin. Natural zeolite resins can also be used to remove ammonia from solution. Air stripping of ammonia from solution at a pH above about 11.0 is effective at reducing its concentration and in some applications this process may be economical. For example, if the solution to be treated has a pH near 11.0, then stripping of ammonia can be conducted with little or no initial pH adjustment. The disadvantage arises when the initial pH is below11.0 and must be adjusted the alkaline region usingof ilme or sodium hydroxide. Thissolution. adds to the cost of the process and to increases the concentration dissolved solids in the treated In addition, if the pH of stripped solution must be lowered to less than 9.0 before being discharged into the environment, then sulphuric acid additionmay be required. This also adds both to the cost of the process and to the concentration of dissolved solids in treated solution. Scale formation in process equipment may also be problematic due to carbon dioxide absorption from atmospheric air that will occur at elevated solution pH values.

353

Chapter Six


6.14.3.4

Nitrate Removal

Nitrate may be present in mine waters either through the oxidation of ammonia or as a result of using ammonium nitrate compounds as explosive materials in mining operations. There are relatively few treatment technologies that can be implemented on a full-scale basis to reliably lower nitrate levels, though the few that are available are effective and economic in many cases (Kindt et al., 1998). The most widely applied nitrate treatment technology is biological denitrification which proceeds under anoxic conditions. In this process, nitrate is converted to nitrogen gas, which is then vented to the atmosphere. Denitrification is best suited forsituations where the solution flow and chemistry do not fluctuate rapidly and where the solution temperature is above about 10oC to 15oC. The process does not significantly increase the concentration of dissolved solids, but does require the addition of a supplemental food source such as methanol, ethanol or molasses. A key advantage of biological denitrification is that ti can be coupled with an aerobic biological process to affect the removal of cyanide, cyanate, thiocyanate, ammonia and nitrate. Under many circumstances, biological treatment systems are inexpensive to construct, operate and maintain and will provide high-quality effluent. Ion exchange can also be used to remove nitrate to low levels, but as described for ammonia removal, the disadvantages of waste brine disposal and resin fouling have limited its application in the mining industry. Under conditions where theconcentration of interfering compounds such as chloride and sulphate are relatively low, which is usually not the case with mining solutions, ion exchange may be an economical approach.


354


6.15

SUMMARY

The purpose of this book is to provide the reader with the fundamentals and a working knowledge of the various processes available fortreatment of cyanidation waters. The focus has been on processes which are well established or indicate significant potential in the near future. The approach has been to provide a practical review of the strengths and weaknesses of the various chemical, physical and biological processes available and the situations in which each may be applicable. Water treatment and management are integral and essential components within an overall mining operation. Consideration of these components should occurduring initial phases ofmine design. The design and selection of mining and metallurgical processes should be integrated with the water management system to provide the most economical and environmentally acceptable system possible. None of the treatment processes examined aresuitable in all situations. Treatment may require a combination of processes to achieve the desired environmental goals. The trend in developing treatment systems is to minimize generation of large volumes of poor quality water which require treatment. The selection of treatment processes requires a knowledge of present and future ore geochemistry, the metallurgical processes utilized, mine site hydrology and a detailed understanding of the chemistry and engineering aspects of the various treatment options. Each cyanidation water is different as is each mining operation, and this mandates the selection of treatment alternatives on a site-specific basis. The steps involved in development of a treatment system include assessment of the potential receiving systems, development of an effluent discharge strategy and effluent design criteria, followed by an investigation phase to verifythe suitability of preferred treatment processes. This sequence is then followed by detailed design, construction and startup of the treatment facility. Emphasis should be placed on simplicity and reliability, as well as performance. Additional time should be given to training of operators and laboratory technicians to assure the consistent operation of the full-scale facility. The determination of treatment performance must be derived through either laboratory and/or pilot plant investigations, with experience and testing as the primary guides. There are approaches to minimizing the level of data required which are related to the experience of the individual directing the investigations. The development and design of a treatment system requires time and planning, two commodities that seem always in short supply. This book presents the fundamentals and the approaches utilized in the development of the most efficient, economical and environmentally acceptable water management system possible for a particular mining the used cyanidation process. The water management and oftreatment processesoperation discussedutilizing have been alone and in combination to produce effluents sufficient quality to allow discharge into the environment.

355

Chapter Six


6.16

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358

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Chemistry and Treatment of Cyanidation Wastes Krause, E. and Ettel, V.A., "Ferric Arsenate Compounds-Are They Environmentally Safe? Solubilities of Basic Ferric Arsenates", Paper Presented at the 15th Annual Hydrometallurgical Meeting, CIM, Vancouver, Canada, 1985. Kuit, W.J.M. and Babcock, A.R., "Process for the Removal of Cyanides from Effluents", U.S. Patent 4,250,030, 1981. Lapakko, K. and Eger, P., “Trace Metal Removal from Mining Stockpile Runoff Using Peat, Wook Chips, Tailings, Till and Zeolite”, Proceedings Symposium of Surface Mining Hydrology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, December 7-11, 1981. Lawr, C.W., "Cyanide Regeneration or Recovery as Practiced by the Compania Beneficiadore de Pachuca", Technical Report No. 208-B-20, American Institute of Min. Metall. and Pet. Engineers, 1929. Leduc, G., Pierce, R. C. and McCracken, I. R., "The Effects of Cyanides on Aquatic Organisms with Emphasis upon Freshwater Fishes", National Research Council of Canada, Ottawa, Ontario, 1982. Levine, V., “The Reducing Properties of Microorganisms with Special Reference to Selenium Journal of Bacteriology, Vol. X, No. 3, pp. 217-263, 1924. Lur'e, Y. and Panova, R.C., "The Behavior of Cyanide Compounds in Water Ponds", Gidrokhim. Material, Vol. 37, pp. 133-143, 1962. Maree, J. and Strydom, W., “Biological Sulphate Removal in an Upflow Packed Bed Reactor”, The South African Institute of Mining and Metallurgy, MINTEK, Randburg, South Africa, 1985. Mathre, O.B. and DeVries, F.W., "Destruction of Cyanide in Gold and Silver Mine Process Water", Paper Presented at the Annual Meeting of the Metallurgical Society of the AIME, Chicago, Illinois, February 23, 1981. Mathre, O.B., “Destruction of Cyanides in Aqueous Solutions”, U.S. Patent 3,617,567, 1969. Mauk, C.E., Prengle, H.W. and Legan, R.W., “Chemical Oxidation of Cyanide Species by Ozone with Irradiation from Ultraviolet Light”, Transactions Society of Mining Engineers, Vol. 260, pp. 297-300, December, 1976. MCA, Minerals Council of Australia, “Tailings Storage Facilities at Australian Gold Mines”, February, 1996. McNamara,

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Chemistry and Treatment of Cyanidation Wastes Brehm Laboratory, Wright State University, Dayton, Ohio, Final Technical Report: "Demonstration Project. Man-Made Wetland for Acid Mine Drainage Abatement at Lake Hope State Part, Vinton County, Ohio", Accomplished Under Agreements of October, 1985 between Ohio Environmental Protection Agency/Ohio Department of Natural Resources and Wright State University, May 22, 1987. Brenner, Fred, "Evaluation and Mitigation of Wetland Habitats on Mined Lands", National Symposium on Mining, Hydrology, Sedimentology, and Reclamation, University of Kentucky, Lexington, Kentucky, December 8-11, 1986. Broderius, S.J., "Determination of Molecular Hydrocyanic Acid in Water and Studies of the Chemistry and Toxicity of Fish to Metal Cyanide Complexes", Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 1973. Chironis, N., Editor, "Mine-Built Ponds Economically Clear Acid Mine Waters", Coal Age, January, 1987. Clark, D., Poulter, L., Wilson, O. and Christensen, W., "The Treatment and Analysis of Cyanide Wastewater", NTIS Document Number AD/A006-394, Thiokol Corporation, Brigham City, Utah, 1975. Cole, C.A, "Morphometry, Hydrology, and Some Associated Water Quality Fluctuations in a Surface Mine Wetland Complex in Southern Illinois", National Symposium on Mining, hydrology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, December 8-11, 1986. Conard, B. and Tessier, M., "Inco Develops Cyanide Removal Process", The Northern Miner April 21, 1983. Dayton, S., "Equity Silver on Line With Leach Plant", Eng. and Min. J., 1983, No. 1, pp. 78-83, January, 1982. Devuyst, E.A. et al., "A New Process for the Treatment of Waste Waters Containing Cyanide and Related Species", Paper Presented at 21st Annual Conf. of Metallurgists, Toronto, Ontario, August 29-September 1, 1982. Devuyst, E.A., Conard, B.R. and Ettel, V.A., "Pilot Plant Operation of the Inco SO2/Air Cyanide Removal Process", Paper Presented at 29th Ontario Industrial Waste Conference, Toronto, June 13, 1982. Devuyst, E.A., Conard, B.R. and Hudson, W., "Commercial Operation of Inco's SO2/Air Cyanide Removal Process", Proceedings of the Conference on Cyanide and the Environment, Tucson, Arizona, Dec. 11-14, 1984.


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Chemistry and Treatment of Cyanidation Wastes Holm, J.D. and Jones, S., "Passive Mine Drainage Treatment: An Effective Low-Cost Alternative", National Symposium on Surface Mining, Hydroxology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, December 9-13, 1985. Huff, J. and Bigger, J., Cyanide Removal from Refine[y Wastewater Using Powdered Activated Carbon, USEPA, EPA-600/2-80-125, May, 1980. Huntsman, B.E., "Design and Construction of Manmade Wetlands for Acid Mine Drainage Abatement", Sixth Annual West Virginia Surface Mine Drainne Task Force Symposium Proceedings, Surface Mining and Water Quality, Morgantown, West Virginia, April 24-25, 1985. Ingles, J., "The Treatment of Mine-Mill Effluents for the Removal of Iron Cyanides", Mining and Metallurgical Division, WPCD, EPS, Environment Canada, June, 1981. Jackson, D., "Programming New Growth in the Coeur d'Alene", Eng. and Min. J., Vol. 811, No. 4, pp. 62-71, April, 1980. Kleinmann, R.L., Tiernan, T.O. and Solch, J.G., "A Low-Cost, Low-Maintenance Treatment System for Acid Mine Drainage Using Sphagnum Moss and Limestone", Symposium on Surface Mining, Hydrology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, November 27 - December 2, 1983. Knorre, H., Fischer, J., and Stuetzel, K., "Process for the Treatment of Continuous Waste Water Streams Having Changing Contents of Different Oxidizable Materials with Hydrogen Peroxide", U.S. Patent 4, 416, 786, November 22, 1983. Knowles, C., "Microorganisms and Cyanide", Microbiological Reviews, Vol. 40, No. 3, pp. 652680, September, 1976. Lapakko, K. and Eger, P., "Trace Metal Removal from Mining Stockpile Runoff Using Peat, Wood Chips, Tailings, Till and Zeolite", Symposium on Surface Mining, Hydrology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, December 7-11, 1981. Lewis, A., "New Inco Tech Process Attacks Toxic Cyanides", Eng. and Min. Journal, 198(7), pg. 52, 1984. Matsuga, G.M. and Robertson, D.R., "Chlorination and Alkali Feed Equipment", Paper Presented at the Cyanide and the Gold Mining Industry Seminar, Ottawa, 1981. Mine Environment Neutral Drainage (MEND) Program, Assessment of Existing Wetlands Affected by Low pH Metal Contaminated Seepage (AMD), CANMET, May, 1990.


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Treatment and Recovery of Cyanide Mudder, T.I. and Whitlock, J.L, "Biological Treatment of Cyanidation Wastewaters," Paper Presented at the 38th Industrial Wastewater Conference, Purdue University, Lafayette, Indiana, August, 1983. Nutt, S.G. and Zaidi, S.A., "Treatment of Cyanide-Containing Wastewaters by the CopperCatalyzed SO2/Air Oxidation Process", Paper Presented at the 38th Annual Purdue Industrial Waste Conference, May 10-12, 1983. Olem, H. and Unz, R.F., "Rotating Disc Biological Treatment of Acid Mine Drainage", Industrial Environmental Research Laboratory, Office of Research and Development, USEPA, Cincinnati, Ohio, Grant No. R-805132, EPA-600/7-80-006, January, 1980. Petrasek, A. and Esmond, S., "Trace Metal Removals, Industrial Water Engineering, Vol. 11, No. 3, pp. 14-17, May/June, 1974. Petrasek, A. and Kugelman I., "Metals Removals and Partitioning in Conventional Wastewater Treatment Plants", Journal of the Water Pollution Control Federation, Vol. 55, pp. 1183-1190, September, 1983. Rael, S., "Fate of Cyanide and Related Compounds in Aerated Microbial Systems", Parts I and II, Water Research, Vol. 2, pp. 477-492, 1977. Robey, H.L., “Cyanide Destruction in a Commercial-Scale Hydrolysis Reactor”, Plating and Surface Finishing, Vol. 70, No. 6, pp. 80-82, June, 1983. Robinson, A. and Sum, J., Sulfide Precipitation of Heavy Metals, USEPA and Boeing Commercial Aircraft Company, EPA-600/2-80-139, June, 1980. Robuck, S.J. and Luthy, R.G., “Destruction of Iron-Complexed Cyanide by Alkaline Wat. Sci. Tech., Vol. 21, pp. 547-558, 1989. Scott, J. and Ingles, J., "An Overview of Cyanide Treatment Methods", Paper Presented at the th

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Chemistry and Treatment of Cyanidation Wastes Tarleton, A.L., Lang, G.E. and Kelman Wieder, R., "Removal of Iron from Acid Mine Drainage by Sphagnum Peat: Results from Experimental Laboratory Microcosms", Symposium on Surface Mining, Hydrology, Sedimentology and Reclamation, University of Kentucky, Lexington, Kentucky, December 2-7, 1984. Trautman, L.L. and Ommen, R., "Cyanide Removal Testwork at Homestake Gold Mine", Paper Presented at the Cyanide in Gold Mining Seminar, Sponsored by Environment Canada, Ottawa, January, 1981. USEPA, Treatability Manuals, Office of Research and Development, Volumes I and III, EPA600/2-82-001a and EPA-600/2-82-001c, September, 1981. Water

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368

Index Activated Carbon cyanide and metals removal, 242 gold recovery, 5 Air Stripping ammonia, 353 hydrogen cyanide, 297 Alkaline Chlorination advantages and disadvantages, 262 chemistry, 246 chlorine sources, 246 operating costs, 258 performance, 248 toxicity of effluent, 260 Ammonia analysis, 49 biological treatment, 266 ionisation, 179 removal by breakpoint chlorination, 247, 353 removal by ion exchange, 353 removal by stripping, 353 removal by zeolite, 353 sources in mining, 23, 352 toxicity, 178 water treatment, 352 Ammonium Formate, 83 Antimony chemistry, 337 minerals, 3, 337 water treatment, 337, 341, 348 Arsenic chemistry, 337 minerals, 3, 337 water treatment, 337, 341, 348 Beneficial Use Classifications, 223 Beryllium water treatment, 342, 348 Biological Water Treatment advantages and disadvantages, 279 aerobic processes, 263 anaerobic processes, 263

chemistry, 265 denitrification, 266 factors affecting, 264 Homestake process, 267 Nickel Plate process, 273 nitrification, 266, 353 passive, 275 Santa Fe process, 275 sulphate reduction, 275 Bismuth minerals, 3 Cadmium water treatment, 338, 342, 348 Caro's Acid advantages and disadvantages, 286 chemistry, 282 performance, 283 production, 282 Chromium water treatment, 338, 343, 348 Cobalt minerals, 3 water treatment, 338 Copper catalyst in cyanide destruction, 307, 327 concentration in leach solutions, 24 cyanide complexes, 13 in ores, 25 minerals, 3 precipitation, 295 solubility in cyanide solutions, 4 toxicity, 338 water treatment, 338, 342, 348 Cyanate analysis, 49 attenuation, 85 biological treatment, 265, 350 chemistry, 22 formation, 22 hydrolysis, 49, 103, 265, 328, 351

369

Chemistry and Treatment of Cyanidation Wastes oxidation of cyanide, 22, 50, 79, 246, 265, 307, 327 removal by alkaline chlorination, 246 toxicity, 173 water treatment, 350 Cyanidation Elsner's equation, 1 gold leaching, 1 history, 1

interferences, 50, 53 ion chromatograph, 44 ion selective electrode, 43 laboratory selection, 63 ligand exchange, 44 metals interference, 53 monitoring programme, 68 on-line process monitoring, 47 practical quantitation limits, 47

solution compositions, 24, 217 sulphide minerals, 3 Cyanide adsorption in soils, 78 analysis. See Cyanide Analysis attenuation. See Cyanide Attenuation biological treatment, 265 bonding, 6 chelation, 77 classifications, 9, 30 complex precipitation, 78 complex stability, 105 Eh-pH diagram, 81 geochemistry, 73 hydrolysis, 83, 103

precautions, 30 preservation of samples, 54, 57, 58 solids analysis, 47 thiocyanate interference, 38 titration with silver nitrate, 42 total cyanide by distillation, 32 variability in analyses, 61 WAD cyanide by distillation, 40 WAD cyanide by picric acid, 40 WAD cyanide by zinc dust/ammonia, 42 Cyanide Antidotes, 139 Cyanide Attenuation adsorption in soils, 114 attenuation coefficients in soils, 118 bioattenuation, 82, 104, 106, 114

in groundwater, 121 low levels in environment, 65 manmade sources, 67, 135 mass balance. See Cyanide Mass Balance natural sources, 66, 136, 166 oxidation in heap leach, 103 oxidation to cyanate, 79 plants, presence in, 166 precipitation with iron, 323 saponification, 83, 103 simple cyanides, 8 Cyanide Amenable to Chlorination. See Cyanide Analysis Cyanide Analysis autoanalyser, 34, 35, 36 comparison of methods, 61 cyanide amenable to chlorination, 39 detection limits, 47 distillation apparatus, 33 free cyanide by ion chromatograph, 44 free cyanide by titration, 43 general classifications of cyanide, 31

chemistry, 317 cyanide complexes, 318 examples, 315 factors affecting, 86, 317 heap leach pads, 99 modeling, 86, 321 performance, 320 ponds, 83, 94 precipitation in soils, 114 reactions in tailings, 94, 99 retardation in soils, 118 tailings geochemistry, 92 Cyanide Complexes stability, 14, 318 Cyanide Cycle, The, 74 Cyanide Management Plan,218 objectives, 219 Cyanide Mass Balance, 59, 60, 218 components, 222 Cyanide Recovery advantages and disadvantages of stripping and absorption, 306


370

Index analysis. See Cyanide Analysis chemistry, 7 pH equilibria, 10 release from metal complexes, 15 Gold leaching, 1 minerals, 3 recovery by carbon adsorption, 5 recovery by Merrill-Crowe process, 5

advantages and disadvantages of tailings washing, 291 AVR, 293 CCD and CCF circuits, 288 Cerro Vanguardia, 304 chemistry, 294 copper removal, 295 Cyanisorb, 294 DeLamar mine, 298 economics, 304 factors affecting, 295 Golden Cross mine, 298 history, 292 metals precipitation, 297 Morro do Ouro, 304 performance, 298 tailings washing, 287 Cyanide Toxicity alkaline chlorination effluents, 260 aquatic organisms, 139, 142, 146 bioassessment, 225 birds, 138, 155, 158 chronic exposure, 164 definitions, 134

Heap Leach Pads geochemistry of cyanide, 99 reactions of cyanide, 101 rinsing, 107 water management, 208, 235 Hydrogen Cyanide atmospheric, 123 toxicity, 135 Hydrogen Peroxide Process advantages and disadvantages, 314 chemistry, 307 copper catalyst, 307 performance, 308 Ion Chromatograph for Cyanide. See Cyanide Analysis

detoxification mechanisms, 137 humans, 164 hydrogen cyanide, 135 iron cyanides, 146 lethal concentration (LC), 134 lethal dosage (LD), 134 mammals, 164 mine effluents, 183 toxicology, 133, 136 Cyanide Treatment, 239 Cyanogen Chloride, 246 Detection Limits. See Cyanide Analysis Effluent Design Goals, 223, 226 Effluent Discharge controlled release, 229 diffuser, 230 discharge point selection, 229 strategies, 229 stream hydrograph, 229 Elsner's Equation, 1 Formic Acid, 83 Free Cyanide

Ion Exchange, 334 ammonia, 353 cyanide, 334 metals, 334 nitrate, 354 Ion Selective Electrode for Cyanide. See Cyanide Analysis Iron concentration in leach solutions, 24 minerals, 3 solubility in cyanide solutions, 4 water treatment, 338 Iron Cyanide breakdown to from free cyanide, 155 chemistry, 16, 25 complexes, 9, 24 formation, 2 oxidation/reduction, 20 photolysis, 17, 148 precipitation, 323 precipitation with copper, 307, 328 solubility, 18, 19

371

Index

Chemistry and Treatment of Cyanidation Wastes solubility with other metals, 21 toxicity, 146, 149 Iron Cyanide Precipitation chemistry, 323 hydrogen peroxide process, 307 performance, 323 precipitate redissolution, 324 sulphur dioxide/air process, 328 Lead

Nitrite analysis, 49 interference in cyanide analysis, 52 toxicity, 182 On-Line Cyanide Monitors. See Cyanide Analysis Ozone Oxidation, 335 Peroxymonosulphuric Acid. See Caro's Acid Picric Acid. See Cyanide Analysis

minerals, 3 water treatment, 338, 344, 348 Ligand Exchange for Cyanide.See Cyanide Analysis Manganese water treatment, 338 Mercury minerals, 3 water treatment, 339, 345, 348 Metals Removal, 337 acid drainage, 349 antimony, 341 arsenic, 341 beryllium, 342 cadmium, 342

Practical Quantitation Limits.See Cyanide Analysis Reverse Osmosis, 335 Risk Assessment, 224 Selenium chemistry, 339 water treatment, 339, 346, 348 Silver in ores, 26 minerals, 3 water treatment, 340, 348 Silver Nitrate Titration.See Cyanide Analysis Sulfur Dioxide and Air Process advantages and disadvantages, 333

chromium, 343 copper, 342 lead, 344 mercury, 345 nickel, 345 selenium, 346 thallium, 346 zinc, 347 Molybdenum water treatment, 339 Nickel concentration in leach solutions, 24 minerals, 3 water treatment, 339, 345, 348 Nitrate analysis, 49 interference in cyanide analysis, 52 ion exchange, 354 removal by denitrification, 266 sources in mining, 23 toxicity, 182 water treatment, 354

chemistry, 327 copper catalyst, 327 metals precipitation, 327 performance, 329 Sulphide Minerals reaction with cyanide, 5 types of ores, 3 Sulphides interference in cyanide analyses, 51, 52 Thallium water treatment, 346, 348 Thicyanate biological treatment, 352 Thiocyanate analysis, 49 attenuation, 85 biological treatment, 265 chemistry, 22 formation, 22, 82 interference in cyanide analyses, 51 leach solution concentration, 24 regeneration of cyanide, 351


372

Index removal by chlorination, 247, 351 removal by sulphur dioxide/air process, 328 removal in hydrogen peroxide process, 308 toxicity, 168 water treatment, 351 Titration for Cyanide. See Cyanide Analysis Total Cyanide

water management plan components, 201 Water Quality assessment, 215 beneficial use classifications, 223 effluent design goals, 223 receiving system characteristics, 226 Water Treatment acid drainage, 349 activated carbon, 242

analysis. See Cyanide Analysis calculation from WAD and iron analyses, 59 thiocyanate interference, 34 Toxicity. See Cyanide Toxicity Use Attainability Analysis, 224 Volatilisation of Cyanide cyanide recovery, 297 factors affecting, 80 from soils, 80, 102, 110 WAD Cyanide analysis. See Cyanide Analysis chemistry, 12 complexes, 9 concentration in ponds, 163

alkaline chlorination, 246 ammonia, 352 biological, 263 Caro's Acid, 282 cyanate, 350 cyanide recovery, 292 design, 228 hydrogen peroxide, 307 ion exchange, 334 iron cyanide precipitation, 323 monitoring, 228 natural attenuation, 315 nitrate, 354 operations, 228 ozone oxidation, 335

toxicity. See Cyanide Toxicity Water Balance, 200, 204 assessment, 206 components, 206 relation to water management plan, 201 seasonal variation, 216 water volumes, estimating, 212 Water Management, 199 examples, 233 options, 202 sources of water, 210 water balance. See Water Balance water management plan, 199

process performances, 348 reverse osmosis, 335 screening and selection, 227 sulphur dioxide and air, 327 testing, 240 thiocyanate, 351 Zinc cementation, 5 concentration in leach solutions, 24 cyanide complex solubility, 19 minerals, 3 solubility in cyanide solutions, 4 water treatment, 340, 347, 348

373

Index

3 060713 English Compendium (1).pdf

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