Continuous vat leaching™
COPPER LEACHING THE LATEST TECHNOLOGY BY
INNOVAT Limited 760 Brant Street, Suite 405c Burlington, Ontario Canada L7R 4B8 Phone: 905-333-7133
Email
[email protected] Website www.innovat.ca
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LEACHING OF COPPER ORES OVERVIEW
The use of continuous vat leaching is a preferred method of leaching copper ore over both agitated tank leaching and heap leaching. Due to the high capital and operating costs, agitated tank leaching is limited to high-grade, finely ground ores or concentrates. Current practice is to heap leach crushed ore for production of cathode copper.
Heap Leaching, Zaldivar, Chile 1999 It is rare for conventional vats to be used for leaching due to the high cost of loading and unloading the batches of ore. Usually this compromise is done for medium grade ores that will respond to leaching by crushing material to sizes that are too fine for heaps and too coarse to be agitated in a tank.
Conventional Vat Leaching at Mantos Blancos, Chile 1999 Agitated tank leaching is becoming even more rare due to the high cost of countercurrent washing.
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Continuous vat leaching as offered by INNOVAT Limited, avoids this high cost by flowing the crushed or ground ore through the vat with patented intermittent fluidization techniques. Where four vats are required for conventional vat leaching, only one is required for continuous vat leaching. Countercurrent washing is an integral part of the design.
Continuous Vat Leach Pilot Plant (Gold), Costa Rica 1996 Continuous vat leaching has the same kinetics as agitated tank leaching, except that the method is capable of handling crushed ore as well as ground ore. What this means is that crushed ore can be leached in a very short time: hours instead of days, months or years; and ground ore can be leached as quickly as expected in agitated tanks but at much lower capital and operating costs. Control of the leach process is practically instantaneous, meaning that process control changes can be made on-the-run with results analyzed as quickly as the laboratory can process them. Main minerals encountered in heap leaching, listed in order of amenability to leaching are:
Oxide Malachite Azurite Cuprite Chrysocolla Copper 3/7
CuO Cu2CO3(OH)2 2CuCO3.Cu(OH)2 Cu2O CuSiO3.2H2O
Chalcocite Covellite Bornite Arsenopyrite Pyrite Chalcopyrite
Cu2S CuS Cu5FeS4 FeAsS FeS2 CuFeS2
Reactions commonly encountered in sulfuric acid heap leaching are: Fast Reactions, High Dissolutions: Oxides Malachite Chrysocolla
CuO + H2SO4 -> CuSO4 + H2O Cu2CO3(OH)2 + 2H2SO4 -> 2CuSO4 + 3H2O + CO2 CuSiO3.2H2O + H2SO4 -> CuSO4 + 3H2O + SiO2
Medium Reactions, Medium Dissolution Chalcocite Bornite
Cu2S + H2SO4 + 0.5O2 -> CuSO4 + H2O + CuS (CuS does not leach) Cu5FeS4 + 2H2SO4 + 9O2 -> 5CuSO4 + FeSO4 + 2H2O
Slow Reactions, Low Recovery (Requiring Oxidation of Sulfide) Chalcopyrite
CuFeS2 + Fe2(SO4)3 -> CuSO4 + 2FeSO4 + FeS + S0 (With bacteria as catalyst) S0 + H2O + 1.5O2 -> H2SO4 2FeSO4 + H2SO4 + 0.5O2 -> Fe2(SO4)3 + H2O
Since most heap leach operations include both Chalcocite and Chalcopyrite, the requirement of oxygen is emphasized, the Chalcocite requiring the oxygen for the basic reaction and the Chalcopyrite requiring the oxygen to convert ferrous sulfate to ferric sulfate in the presence of sulfuric acid. Commercial methods now in use have extremely long cycles (years) and dependence on bacterial action for oxidation, either by natural methods or induced methods. The actual leaching of the copper in Chalcopyrite takes place as a result of the reaction with ferric sulfate. Bacteria, which requires oxygen to survive, produces ferrous sulfate from the sulfides and breaks down the matrices for access to the metal. Conversion from ferrous sulfate to ferric sulfate can be accomplished by the addition of air. Common problems in heap leaching occur as a result of: • •
Clay in the ore, resulting in poor percolation High generation of fines in crushing, resulting in poor percolation
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• • • • • • •
Disintegration of the ore from dissolution of copper and other elements, resulting in poor percolation Formation of jarosites, which precipitate out in the heap due to the rising pH, resulting in poor percolation High solution losses due to evaporation Slow reactions due to cold temperatures Slow reactions due to oxygen depletion early in the heap Slow reactions due to oxygen depletion early in the heap, resulting in bacteria inactivity Slow reactions due to ferric sulfate depletion
Attempts by operators to fix the above problems, while stubbornly continuing with heap leaching include: • • • • • • •
Separation and removal of clays or fines; treating only the coarse ore Thin-layer leaching, sometimes in lifts of only 2 – 3 meters Heating of solutions Covering the heaps with insulation blankets!!!! Rest periods to allow the heaps to recover oxygen High acid pre-curing with agglomeration, using polymers Alternately draining and blowing air or oxygen into the heap
A fundamental processing problem with heap leaching of copper ore is that control of what happens within the heap is very limited. Once the solution is applied, the time lag required in detecting difficulties and taking corrective action is long, indeed—often years. One can only hope that there is enough basic product being harvested to make up for deficiencies encountered in the learning process. All of the above problems can be addressed with continuous vat leaching at comparable capital costs, and lower operating costs. More important is the time-value of money. Heap leaching, particularly where sulfides are involved, require hectares of leach pads or expensive on/off loading, similar to conventional vat leaching. Often forgotten are the tons of solution in circulation, not to mention evaporative losses. On a typical oxide heap leach with plant with a placement rate of 10,000 tons per day, for example, the amount of copper tied up in the heap and solutions can easily exceed $25,000,000! The equivalent value tied up in vat leaching is a mere $75,000 or so. Continuous vat leaching measures leach rates in hours, not days, weeks, months or years as is common in heap leaching. Control of the process is relatively simple, with monitoring on an hour-by-hour basis. There are no uncontrollable pH excursions, no surprises. Countercurrent washing is integral, minimizing copper losses.
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In some cases, there is an argument for combining heap leaching with continuous vat leaching. And economies can be earned by putting our system together with an existing heap leaching operation. Talk to us about it.
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To Extraction Plant
4
Leach Slimes Paste Thickener 3
2
Main Head Tank
W ash Head T ank 6
7
From Comminution
5 1 1
9
Leach Vat
H 2SO 4
8
10
Fresh W ater Makeup
To Disposal 11
From Extraction Plant
Circulating Pump
Raffinate Pond
INNOVAT Limited Oakville, Canada
Continuous Vat Leach Flow Sheet 0 REV
For Discussion
SIZE
DESCRIPT IO N
DAT E REVISIO NS
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FSCM NO
DW G NO
REV
0
APPRO VED
EcoVat
SCALE
1:1
SHEET
1 OF 1
