75 Gold Recovery Methods

Download 75 GOLD RECOVERY METHODS

chemical methods of recovering gold: - mercury – amalgamation of gold. - cyanide leaching – dissolving gold. - chlorine leaching – dissolving gold. - Haber Gold Process leaching – 2000s research in New Jersey.

- iGoli chlorine leaching – 2000s research in South Africa. - iodine leaching – dissolving gold. - tincture of iodine leaching – 2000s research in Japan. - bromine leaching – dissolving gold. - bromine leaching – 1990s research in Indiana. - thiocyanate leaching – dissolving gold. - thiosulphate leaching - dissolving gold. - thiourea leaching – dissolving gold. - nitric acid attack – liberating gold from sulphides. - aqua regia – dissolving gold. - borax – smelting gold.

biochemical methods of recovering gold: - bioleaching – 1980s research in Wales and California. - biooxidation – 1980s research in British Columbia and California. - gold-binding proteins – 2000s research in Washington State. - phytomining – 2000s research in New Zealand.

oleophilic methods of recovering gold: - agglomeration – 1980s research in Australia and China. - oleophilic adhesion – 1980s research in Alberta. - froth floatation – 1930s research in Idaho and USSR. - gold-paraffin floatation – 1990s research in Brazil using candle wax.

magnetic methods of recovering gold: - magnetic coated gold – 1980s research in Colorado with iron carbonyl.

gravitational methods of recovering gold: gold sluices - some are able to catch very fine gold - riffled sluices – 1960s-1970s research in China and USSR. - flat bar riffles – 1980s research in Yukon, 1990s in Mongolia. - angle-iron riffles – 1980s research in Canada. - expanded metal grating riffles – 1980s research in Canada. - expanded metal mesh riffles – 1980s research in Canada. - McCann’s small sluice – 1980s research in California.

- Damn Fine Sluice – 1990s research in New Mexico. - Popandson sluice – 2000s research in USA. - Loewen electrostatic sluice – 2000s research in Alberta. - Cleangold® sluice – 1990s research in Oregon. - hydraulic riffles – 1980s research in New Zealand and Canada.

gold jigs - recover nuggets as well as some fine gold - simple jigs – 1970s research in China. - Pan-American (PAN-AM) duplex jig – Alaska tests. - Cleaveland/IHC jig – 1980s research in USA and Holland. - Gekko in-line pressure jig (IPJ) – 1990s research in Australia. - Kelsey centrifugal jig – 1980s research in Australia.

gold centrifuges - rather good at catching very fine gold - Knudsen bowl – Alaska tests. - Gilkey bowl – Alaska tests. - Neffco bowl – 1970s research in Utah. - Yunxi bowl – 1960s-90s research in Yunnan. - Knelson bowl – 1980s research in British Columbia.

Knelson concentrator

- Falcon C bowl – 1980s research in British Columbia.

Falcon concentrator

- Falcon SB bowl – 1990s research in British Columbia. - Itomak bowl – 1990s research in Novosibirsk.

Falcon Superbowl

Itomak concentrator

- Younge horizontal centrifuge – 1980s research in British Columbia. - Axzia-Mozley MGS centrifuge – 1980s research in Cornwall.

gold helix - thanks to Archimedes screw - helix wheel (gold wheel) – 1900s research in Colorado. - helix cylinder – 1980s research in British Columbia. - helix belt – 2000s research in Canada and USA.

gold tables - old devices, getting better - Wilfley shaking tables – 1890s research in Colorado. - shaking tables – 1960s-1970s research in China. - shaking tables – 1960s research in USSR. - BGS shaking table – 1990s research in UK. - Gemeni table – 1980s research in Colorado.

Multi-gravity sep.

- U-Tech reverse polarity table – 1990s research in Arizona. - Goltron machine – 1990s research in Utah. - Bartles-Mozley orbital tables – 1970s research in Cornwall.

pinched sluices - very useful, especially as a cone - pinched sluice – historical usage. - Reichert cone – 1960s research in Australia.

gold spirals - quite good at catching fine gold - Humphrey spirals – 1940s research in Colorado. - Mark-7 Reichert spirals – 1980s research in USA.

elutriated sludge tanks - not jigs, and maybe better! - Duke E-tank – 1970s research in Georgia. - Graefe E-tank (Keene Hydromatic Jig) – 1980s research in California. - Pyramid E-tank (Pyramid Jig) – 1990s research in California. elutriation towers - remarkable at catching fine gold! - Lashley ASAT Elutriation tower – 1980s research in New Mexico. - Osterberg E-tower (Quick Gold Separator) – 1980s research in California. - reflux classifier – 2000s research in Australia. - Ecologic E-tower (Ecologic Concentrator) – 2000s research in New Zealand.

compound water cyclones - dewater or catch fine gold! - Visman Compound Water Cyclone (CWC) – 1970s research in Yukon.

vibrating belts (vanners) - may catch fine gold rather well! - Bartles crossbelt – 1970s research in Cornwall. - Lemmon vanner – 1980s research in the Yukon.

Where to find out more... -

join the ALASKA GOLD FORUM, read the posts and post a question! send us an EMAIL and we will be pleased to hear from you. have a look at the references on gold recovery listed below. IF YOU KNOW A METHOD TO ADD, PLEASE EMAIL US!.

World Placer Journal – 2007, Volume 7, pages 66-161. 1:

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Mercury – amalgamation of gold

Operation Mercury is sourced from on-site recycling of waste plus mercury from traders from Hg mines (e.g. China and Kyrgyzstan) and Hg waste exporters (e.g. Spain and USA) [20]. In Mongolia some is sold by panners recovering mercury [29,30,31]. Mercury is added in four situations: ²

²

²

²

Figure 13.

Mercury amalgamation typically recovers in excess of 90% of the gold content of a placer gravel or placer concentrate. Mercury amalgamation is effective only for gold particles larger than 60-70μ [12]. For mercury amalgamation to be effective, preconditions apply:

MERCURY-GOLD AMALGAM

Soft bead of HgAu amalgam ready for firing off the mercury to leave gold. (photo: Peter Appel of GEUS)

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Until 50 years ago, mercury (Hg) was the method-ofchoice for industrial-scale recovery of hardrock gold, and to a lesser degree for recovery of placer gold also. Since then, with the increased recognition of the harmful impact of mercury on human health and ecosystems, mercury use by companies and recreational miners has become strictly controlled and in some regions banned and eliminated. Mercury has been virtually eliminated in industrial placer gold mines in the USA, Canada, New Zealand, Australia, Russian Federation, Kazakhstan, Kyrgyzstan and Mongolia. Yet mercury is prevalent in large placer gold mines in South/Central America, Africa and China. Companies shun mercury for six interlocking reasons: ² ² ² ² ² ²

² ² ² ²

the gold mercury mercury the gold the gold

particle must have a clean surface available; must be put in direct contact with the gold particle; must be clean enough; must be already liberated from the matrix, OR has its surface exposed to adhere to the mercury.

After amalgamation, the resulting lumps of amalgam are retrieved by squeezing out excess mercury through a fine fabric or chamois leather. The amalgam paste is retrieved by hand and the mercury driven off by heating to leave a residue of impure gold containing traces of mercury.

Adoption by placer gold miners Mercury amalgamation is entrenched as the ‘global norm’ for gold recovery from concentrates by artisanal placer gold miners. Mercury has been eliminated amongst placer mining companies in the former Soviet Union and is highly restricted, strictly controlled and virtually eliminated in industrial and recreational placer mining in the west.

human health of employees and local people; environmental protection; insurance risks and liabilities; legal prohibition; viable alternatives to mercury now exist; and mercury is ineffective at recovering gold <70μ [12].

Figure 14.

adding Hg to a milling device (e.g. Muller mill = edge mill) to capture gold and leave amalgam in the milled residue; adding Hg to a pan, sluice, centrifuge, Hg-centrifuge (forced amalgamator) or other device to assist the capture of gold; adding Hg to concentrate to recover gold without the time and trouble of upgrading the concentrate further; and adding Hg to devices to recover gold lost to the tailings.

GOLD RECOVERY BY MERCURY AMALGAMATION

Mercury is ineffective at amalgamating with gold smaller than 70μ or larger than 1.5mm [12]. (compiler: Robin Grayson)

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Cyanide – chemical leaching of gold

Operation The sodium cyanide (NaCN) is either in a dry solid or liquid form, sourced from specialised manufacturers. 1st stage – leaching gold into solution A weak cyanide solution is prepared, usually 0.020.05%, and must be kept strongly alkaline (pH 10-11). Cyanide leaching can be by different methods: ²

Figure 15.

²

CYANIDE LEACHING

A Chinese operation near Zuunkharaa in north Mongolia, leaching fine gold from mercury-laden tailings of edge mills. (photo: Jőrgen Hartwig of Projekt-Consult)

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Cyanide leaching has been used to recover gold since the 1890s but only since the 1960s with the advent of heap leaching has cyanide become the method-of-choice for leaching gold from milled hardrock, and has potential for leaching gold from placer concentrates. Cyanidation uses a very weak cyanide solution to dissolve (‘leach’) fine gold into solution, and then precipitates it as easy-to-recover gold. It leaches gold that – due to flatness, small size or attached quartz – is lost by simple gravitational devices or mercury. Cyanide has risks if handled carelessly or gains access to streams or wells. The cyanide solution must be kept strongly alkaline to prevent the generation of highly toxic hydrogen cyanide gas. A problem is cyanide vapour rising from ponds. Such concerns have triggered the gold industry to seek cost-effective alternatives [32] Coarse gold typical of most placers takes too long to leach. Marcello Veiga noted that for a 0.21mm gold particle to dissolve in cyanide took over 60 hours!

Figure 16.

percolation leaching – very weak cyanide solution percolates down through a vat of concentrate. Commonly used for ground hardrock ore, but also successful in tests on placer gold concentrates in Alaska by Cleland Conwell [33]; agitated leaching – very weak cyanide solution is added to vats that are agitated by paddles or by blowing in compressed air to keep the material in suspension; and heap leaching – very weak cyanide solution percolates down through crushed/milled ore heaped in a ‘heap basin’ lined with leach-proof materials e.g. clay, asphalt or tarpaulin.

2nd stage – recovering gold from solution Having leached the gold and dissolved it into solution as gold cyanide complexaqueous, the solid gold is recovered by a choice of methods, such as traditional methods tested on placer gold concentrates in Alaska [33]: ² ² ²

absorption by activated carbon – 99.85% Au recovery; absorption by ion-exchange resin – 96.31% Au recovery; or precipitation by zinc dust – 99% recovery.

The zinc dust method, as typified by the Merill-Crowe process, first removes oxygen from the cyanide solution and then mixes in a fine zinc powder and recovers the fine gold precipitate (<50μ) on a precoat filter.

Adoption by placer gold miners The author is unaware of the cyanide leaching being used at large-scale placer gold mines. Yet for small-scale and artisanal mining, cyanide leaching is now widespread for hardrock ores and ‘difficult’ lateritic ores in Peru, Ecuador, Columbia, Brazil, Philippines and China.

GOLD RECOVERY BY CYANIDE LEACHING

Cyanide can dissolve (leach) >90% of very fine gold, but is too slow for leaching larger gold. [22] (compiler: Robin Grayson)

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Chlorine – chemical leaching of gold Frothing and foaming may push off the lid causing spills, especially if the concentrate has calcite or dolomite as vein material, rocks (e.g. limestone) or coatings. The foaming is carbon dioxide CO2 gas liberated when HCl dissolves carbonate. When dissolution seems over, the lidded bucket is stirred again and left overnight to allow the process to terminate. Next day the acidity is checked by pH paper (litmus test). If not slightly acid then the acid was completely used, so more 15% HCl is added until the process is complete. Then the bucket is stood for several hours to settle, and surplus water gently tipped away.

Chlorine had been used to recover gold for a couple of hundred years, notably to leach gold from residues of Hg amalgamation. Later chlorination found favour in large gold mining operations in Australia and the USA but has now largely disappeared [34]. Chlorination is safer and simpler than cyanide, and can challenge mercury. By the 1970s, chorination had been largely forgotten. Yet, as noted by WWF-Guianas (www.wwfguianas.org), a basic version of chlorination could replace amalgamation and cyanide, and the following account is based largely on their report [35]. Two common chemicals are required: ²

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bleach – 14-16% sodium hypochlorite NaOCl (household bleach) but products often have additives that may interfere with the process. Training and precautions are ESSENTIAL. hydrochloric acid – 30% HCl strength, as for cleaning swimming pools. Training and precautions are ESSENTIAL.

1st stage – leaching gold into solution A plastic pole is used to make a hollow in the concentrate, and the bleach gently stirred in. Shortly after, the 30% HCl is poured in, taking MAXIMUM precautions, stirring the concentrate with the pole. Gold is dissolved (‘leached’) by attack by nascent chorine to form gold chloride (AuCl2) in solution: goldsolid + sodium hypochloriteaqueous + hydrochloric acidaqueous = gold chlorideaqueous + sodium chlorideaqueous + waterliquid

However bleach (sodium hypochorite solution) forms comparatively stable trihalomethanes (THMs) and haloacetic acids (HAAs) claimed to be carcinogenic and posing other health risks [36]. Only in 2001 was this addressed by Mintex’s ‘iGoli method’, as discussed in a later section [37-40].

2nd stage – recovering gold from solution The liquid is decanted from the bucket and filtered. The filtrate includes leached gold as gold chlorideaqueous. To precipitate the gold from the pregnant solution, several methods are available: sodium metabisulphate, zinc metal (chunks, bars or powder), oxalic acid, ferrosulphate or sodium nitrate. Sodium metabisulphate was used by WWF-Guianas [35]. Zinc often has cadmium (Cd) as impurity that is toxic if released.

Operation The floor is sealed, and a tap plus plastic garden hose kept ready to flush spillages of bleach and acid. The operators must be trained and clad in rubber gloves, rubber apron, laboratory eye-protectors and rubber boots. Preparation – removal of carbonates The first task is to dissolve carbonates as they can interfere with leaching gold. A stock of STRONG 30% HCl is added to water to produce 15% dilute HCl. The concentrate is put in a lidded plastic bucket and 15% HCl added to submerge the concentrate, using a plastic rod to stir. A long handle cut from a plastic sweeping brush is ideal.

Figure 17.

Adoption by placer gold miners Chlorination is a neglected method for recovering gold from placer concentrates and the author is unaware of any companies using this method, although some artisanal placer miners may be doing so.

GOLD RECOVERY BY CHLORINATION

Chlorination can dissolve (leach) about 90% of gold <300μ but is too slow for leaching larger gold. [35] (compiler: Robin Grayson)

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HGP leaching – 2000s research in New Jersey

Operation

Figure 138.

The Haber gold process (HGP) is, in many respects, similar to the cyanide method in the recovery of hardrock gold, but acting faster and without raising any significant environmental issues. Both methods rely on chemicallybased gold extraction technology that makes use of a lixiviant (extracting solution) to leach gold into solution from the ore. Having rapidly and efficiently leached the gold into solution, the HGP uses essentially the same gold-capturing method as the cyanide method in order to recover the gold by precipitation, such as the well-known MerrillCrowe and carbon methods.

HABER GOLD PROCESS

A mobile test unit for the Haber Gold Process (HGP). (photo: courtesy of Haber Inc – www.habercorp.com)

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The Haber gold process (HGP) was developed by Norman Haber of New Jersey for hardrock gold ores, but it has potential for placer gold recovery. The HGP is a chemical leaching process using a non-toxic lixiviant (extracting solution) of proprietary composition. Haber Inc (www.habercorp.com) says the chemicals used are “readily available”. About 100 tons of different types of gold ore have been processed in small-scale HGP pilot plants which are claimed to extract gold in bulk in “significantly less time” than with cyanide. The tested ores responded much faster with the HGP. Reagent costs are similar to cyanide but the costs of the HGP are claimed to be “an order of magnitude lower” than for a cyanide system. Haber Inc. claims the following advantages of HGP leaching over cyanide leaching: ² ² ² ²

²

Adoption by placer gold miners The Haber gold process (HGP) is undergoing commercialisation trials in Ghana for applications to combat and replace mercury usage by thousands of artisanal and small-scale gold miners (ASM). Initial progress has been very encouraging and publication of detailed results is awaited with interest.

routinely recovers more gold; processing rates are significantly faster; effective with more types of gold ore; and overall cost of processing is same or less.

Figure 139.

hardrock ore – the rock is first crushed and milled to 80-120 mesh (130-200μ) – an advantage over using cyanide that typically requires milling to at least 200 mesh (74μ). Therefore it seems highly likely that HGP will prove effective at recovering gold less than 200μ in size. placer ore – with placers there is no requirement to mill the ore, as the commercial gold content is ‘free’ gold. The Haber Gold Process should be effective for placer concentrates screened at 1mm or even 5mm, for a spherical gold particle of 200μ diameter has hydraulic equivalence to a spherical magnetite particle twice this diameter and a quartz particle several times this diameter. The Haber Gold Process should be effective at recovering fine gold in the tailings of conventional wash-plants.

GOLD RECOVERY BY HGP LEACHING – generalised

Recovery of placer gold by the Haber gold process (HGP) based on information from Haber Inc. (compiler: Robin Grayson)

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iGoli chlorine leaching – 2000s research in South Africa

The ‘iGoli Mercury-free Gold Extraction Process’ was invented during the lat few years by the Small Scale Mining and Beneficiation Division (SSMB) of Mintek in South Africa (www.mintex.co.za). The iGoli process is designed to leach gold from >0.1% gold concentrate to produce 99.9% gold product. The iGoli process is a modern version of chlorination, and uses a mixture of pool acid (dilute hydrochloric acid), bleach (sodium hypochlorite) and sodium metabisulphate to leach and recover metallic gold [38,39,187].

Operation 1st stage – leaching gold into solution The process equipment is made of PVC and other plastics, as chlorine in the leach would attack mild steel and other metals. Use of plastics allows transparent vessels allowing the operator to witness the progress. The feed is of finely screened concentrate with a grade of >0.1% gold. A batch is added to the reaction vessel where the gold is dissolved (leached) by a mixture of pool acid (dilute hydrochloric acid) and bleach (sodium hypochlorite). Gold is leached by attack by nascent chorine to form gold chloride (AuCl2) in solution: goldsolid + sodium hypochloriteaqueous + hydrochloric acidaqueous = gold chlorideaqueous + sodium chlorideaqueous + waterliquid

Assessment of concentrate Tests by Mintex show that concentrates from different areas require slightly different recipes for the iGoli process. Mintek SSMB requires a 2-kilo sample of concentrate containing at least 1gram/ton to determine the optimum recipe [187]. In some placers and a few hardrock ores, carbonates are present in such abundance that they interfere with leaching gold and have to be first removed [35].

Leaching the gold usually takes a number of hours. Any fumes that may form are scrubbed in the attached vessel using sodium hydroxide solution. 2nd stage – precipitating gold from solution Once the gold has dissolved, the contents of the reaction chamber are filtered to remove the solids. The pregnant solution is mixed with sodium metabisulphite in the precipitation vessel and manually stirred. If properly done, a gold powder of up to 99% purity is obtained that may be turned into a gold disc just by hitting it with a hammer [38,39,187]. 3rd stage – waste treatment The solid waste from the iGoli process is neutralised using lime or limestone with apatite added if necessary to destroy or precipitate base metals and ferrometals.

Adoption by placer gold miners The iGoli process is new, but is gaining interest in South Africa and elsewhere among artisanal and smallscale miners as an alternative to mercury amalgamation.

Figure 140.

GOLD RECOVERY BY IGOLI CHORINE LEACHING

Recovery of placer gold by the iGoli mercury-free gold extraction process [38,39,187]. (compiler: Robin Grayson)

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Iodine – chemical leaching of gold

Iodine leaching had been widely used to recover gold in the late 1800s and early 1900s, then dwindled with the rise in popularity of cyanide leaching and mercury amalgamation, and the high cost of iodine. Unlike chlorination, recycling of leachate is of paramount importance for commercial viability of the method. High rates of recycling of iodine can be achieved although requiring additional plant layout. Fortunately iodine is a good lixiviant for gold so only very low concentrations of iodine are required. Three methods are noted below. The Prichard method of iodine leaching was invented by Loius M. Pritchard of Idaho and patented in 1907 (US #861,535) and uses an excess of iodine dissolved in potassium iodide in aqueous solution. The gold so dissolved is recovered by adding mercury which reduces the gold in solution to a metallic state whereupon it forms an amalgam that is then washed free. But the Prichard method is unsatisfactory in failing to precipitate colloidal gold, and gold recovery is sometimes only 25%. The Harrison method of iodine leaching was invented by George D. Harrison of Detroit and patented in 1942 (US #2,304,823). It was said to be effective with placer concentrates and difficult ores such as platinum ores and gold telluride ores. The lixiviant is an aqueous solution of iodine and potassium iodide, plus nitric acid to prevent the formation of insoluble gold salts. An in-situ method of iodine leaching of gold ore was invented by Kent McGrew and Jack Murphy of Wyoming and patented in 1985 (US #4,557,759) as a safer alternative to in-situ cyanide leaching. The gold leached by the iodine is recovered by activated charcoal, and the iodine regenerated for re-use by electrochemical oxidation. E-goldprospecting (www.e-goldprospecting.com) has a good account of the pros and cons of iodine leaching. An updated tincture method of iodine leaching was invented in 2006 [40] and is dealt with in a later section.

Figure 18.

Operation The Harrison method in outline is as follows. First a test batch of 0.43 kilos of ore is reduced to a <50-75μ powder and leached without any preliminary roasting step. 1st stage – leaching gold into solution To prepare the leachate, 3.8 kilos of solid potassium iodide are added to four litres of water and then 1.9 kilos of iodine crystals added. The water is agitated until all the crystals dissolve. Then 0.45 litres of concentrated nitric acid is added in small steps with agitation. The leachate consists of water, potassium iodide, hydroiodic acid, free iodine and potassium nitrate able to “completely dissolve all tellurides and selenides, and sulphides of gold present”. The ore is added to the leachate and agitated for an hour to form a pregnant solution. This is filtered and the residue washed in a concentrated solution of potassium iodide to dissolve insoluble silver iodide, until no yellow precipitate remains. The wash water is thoroughly mixed with the filtered pregnant solution. 2nd stage – recovering gold from solution Mercury is added to the pregnant solution to form amalgams of gold, silver and base metals. This residue is filtered off and to it is added a hot solution of four parts concentrated nitric acid and one part water to prevent the gold becoming colloidal. After cooling for 30 minutes, the gold is recovered in a furnace. Some gold and other precious metals remain in the filtrate. This is treated with nitric acid (see patent for details) and the gold precipitated is recovered by filtration.

Adoption by placer gold miners Iodine leaching is a neglected method for recovering placer gold and the author is unaware of any companies, recreational miners or artisanal miners doing so.

GOLD RECOVERY BY IODINE LEACHING

Iodine can dissolve (leach) >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Bromine – chemical leaching of gold

Bromine was fairly popular for recovering gold in the late 1800s and early 1900s but declined in the face of competition from cyanide and mercury amalgamation. Some bromine leaching methods are outlined below. The Schaeffer method of bromine leaching was invented by Charles A. Schaeffer of New York State and patented in 1882 (US #267,723). Bromine in aqueous solution is used to leach the gold as gold bromide in solution over 24 hours. Silver bromide forms as sludge. The gold is recovered by precipitation by adding oxalic acid or iron sulphate to the pregnant solution. By putting sludge into solution with sodium or calcium hyposulphite, silver can be precipitated by adding calcium sulphide. Bromine is a highly corrosive fuming liquid generating a suffocating vapour, making this method hazardous. The Engelhardt method of bromine leaching was invented by Ernest C. Engelhardt of South Dakota and patented in 1893 (US #509,368) and uses bromine in dilute hydrochloric acid to leach the gold as gold bromide in solution. Adding acid increases the solubility of bromine from 2-3%vol to 13-15%vol and the process shortened. The MacArthur method of bromine leaching was invented by John S. MacArthur of Scotland and patented in 1889 (US #411,047) and uses “perbromide of iron” in aqueous solution to leach the gold as gold bromide in solution in a vat heated close to boiling point. Silver, copper, lead and zinc must be removed first by ferric salts. The leach is reused until exhausted; then the “perbromide of iron” is regenerated using bromine. Gold is recovered by filtering the pregnant solution through coke or charcoal. The Fink and Putnam method of bromine leaching was invented by Colin G. Fink and Garth Louis Putnam of New York and patented in 1942 (US #2,283,198). They discovered leaching gold in aqueous bromine solutions is accelerated by chloride and bromide ions.

Figure 19.

Operation The Fink and Putnam method in outline is as follows. First the ore is finely powdered and preferably roasted. Carbonates need not be removed as bromine solvents can dissolve gold in the presence of carbonates. 1st stage – leaching gold into solution Bromine and sodium bromide are sourced from specialised suppliers. Elemental bromine is particularly hazardous and extremely difficult to store or transport safely. Sodium bromide is more stable and cheaper. The powered ore is added to water in a leach tank. The leachate is prepared by adding sodium chloride @ 100 grams/litre, sodium bromide @ 1gram/litre and finally chlorine @ 1.4 grams/litre. Throughout the first 15 minutes, sulphuric acid is added @ 1.25 grams/litre to maintain acidity. The reactive mixture is allowed to stand for a further ten minutes for leaching to finish. In tests 23carat gold leaf dissolved in barely four minutes, and only 11.1% was lost to the tailings. 2nd stage – recovering gold from solution To recover the gold from the pregnant solution, several methods are available: sodium metabisulphate, zinc metal (chunks, bars or powder), oxalic acid, ferrosulphate or sodium nitrate. The solvent power of the barren liquid is restored by either adding chlorine or a hypochlorite and a mineral acid. According to the patent, “Except for losses due to

vaporisation and solution left in the tailings, practically all of the free and combined bromine is recovered.”

Adoption by placer gold miners Bromine leaching is a neglected method for recovering placer gold [41], and the author is unaware of any companies, recreational miners or artisanal miners doing so.

GOLD RECOVERY BY BROMINE LEACHING

Bromine can dissolve (leach) >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Bromine leaching – 1990s research in Indiana An innovative alternative invented by Ahmad Dadgar and Charles C. Shin of Great Lakes Chemical Corporation in Indiana and patented in 1990 (US #4,936,910). This recovers gold by passing the pregnant gold solution through acid-washed Sphagnum peat moss in a suitable contacting apparatus. The sphagnum peat moss (live and dead) is chopped and screened to retain the 75μ to 1.5mm fraction. This enables water to pass through. The screened moss is washed with a protic acid such as 0.5-1.0M HCl (10 grams of dried sieved moss per 100mL HCl) and then rinsed with deionised water until the filtrate reaches pH4. The acid-treated sphagnum moss is packed in a column and the pregnant gold solution passed through in the manner of a typical ion-exchange column. Experiments prove acid-treated sphagnum peat moss is able to recover about 32 milligrams of gold per gram of moss dry-weight when it is then at maximum capacity. The process is very rapid (10-20 minutes) and is indifferent to temperature variation in the range of 20 to 50°C. At least 99.9% of the gold contained in the leachate can be recovered by the sphagnum moss.

The Dadgar method of bromine leaching was invented by Ahmad Dadgar and co-workers of Great Lakes Chemical Corporation in Indiana and patented in 1997 (US #5,620,585). It may yet help to revive interest in bromide leaching, using perbromides with desirable characteristics such as high bromine levels, low bromine vapour pressure and stability even in freezing conditions.

Operation The Dadgar method in outline is as follows. The ore is reduced to a fine powder and leached without any preliminary roasting step. 1st stage – leaching gold into solution Bromine is sourced from specialised suppliers as sodium bromide, as elemental bromine is particularly hazardous and extremely difficult to store or transport safely. Sodium bromide is more stable and cheaper. The powdered ore is delivered as a continuous feed to the first of two cascade agitated leach tanks where it is mixed with an aqueous bromide solution. The resulting slurry overflows from the first leach tank to the second leach tank and overflows again to a thickener. Solids produce a sludge that passes through a countercurrent washing system of several thickeners, the final thickener being fed with an aqueous washing medium. Solids collect in the bottom of the final thickener as tailings, while the liquid fraction is a pregnant gold solution.

3rd stage – final recovery of gold for sale The sphagnum moss is incinerated in a muffle furnace at 750°C to recover the metallic gold.

Adoption by placer gold miners The Dadgar method has potential for recovering fine placer gold from concentrates but awaits field tests and promotion to placer companies, recreational miners and artisanal miners.

2nd stage – recovering gold from solution The pregnant gold solution can be passed through ion exchange columns, as is usual, to recover the gold.

Figure 118.

GOLD RECOVERY BY DADGAR METHOD OF BROMINE LEACHING

Bromine can dissolve (leach) >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Thiocyanate – chemical leaching of gold

Thiocyanate leaching was invented in the 1890s but was largely ignored due to the popularity of iodine and bromine leaching, and the rise of cyanide leaching. Thiocyanate leaching is particularly effective on difficult sulphide-rich refractory hardrock gold ore. This is because thiocyanate leaching requires an extremely acidic environment and breaking down the sulphides in the ore generates highly acidic solutions. The Kendall method of thiocyanate leaching was invented by Edward E. Kendall of New York State and patented in 1899 and 1901 (US #625,564 and #671,704). Ammonium or potassium thiocyanate is used to leach silver and gold, and the pregnant solution directed to an agitated trough where potassium sulphide is added to precipitate silver sulphide that is recovered by settling and filtering. The supernatant liquid is directed to a reaction tank for gold recovery using comminuted zinc metal. The Barrick method of thiocyanate leaching was invented by Kenneth T. Thomas, Christopher Fleming, Andrew R. Marchbank and David Dreisinger of Canada and patented in 1988 (US #5,785,736), the patent assigned to Barrick Gold Corporation of Toronto. The Wan-LeVier method of thiocyanate leaching was invented by Rong Yu Wan and K. Marc LeVier of Colorado and a patent was applied for in 2004 (US #0197249ki). Acid thiocyanate solution is used to leach precious metals as a precious metal-thiocyanate complex. The leach solution may contain a large molar ratio of ferric iron to thiocyanate. The precious metal is removed from the pregnant thiocyanate solution by transferring the precious metals to precious metal-cyanide complex and then loading this onto absorbent material. The residual cyanide in the thiocyanate leach solution is converted to thiocyanate for further leaching. E-goldprospecting (www.e-goldprospecting.com) has a good account of thiocyanate leaching.

Figure 20.

Operation The Wan-LeVier method is summarised as follows. The thiocyanate is sourced from specialised suppliers. Preparation – oxidation of sulphides The ore is finely powdered by milling and added to a heap leach facility. Bio-oxidation of sulphides is accomplished by Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfobocillus thermosulfidooxidans, Sedula, Metallospheara or Acidianus brierley as in US patents #5,246,486, #5,332,559, #5,834,294, #5,127,942 and #5,244,493. Bio-oxidation takes about 90 days – much faster than with cyanide leaching as acidic products are not a problem. 1st stage – leaching gold into solution The leach solution is first conditioned to adjust the concentration and molar ratio of dissolved thiocyanate and dissolved ferric iron, acidity and temperature. The leachate is extremely acidic, ranging from pH 1 to 3. The leach solution is introduced to the milled ore and leaching takes from days to months in a heaped leach facility, or hours in a heated pressurised autoclave. 2nd stage – recovering gold from solution The pregnant thiocyanate leach solution contains dissolved gold in the form of gold-thiocyanate complex, and the pregnant solution is removed for further processing to recover the dissolved gold. Residual solids depleted in gold may be subjected to additional metal recovery operations or to further treatment for disposal in an appropriate manner.

Adoption by placer gold miners Thiocyanate leaching is a neglected method for recovering placer gold and the author is unaware of any companies, recreational miners or artisanal miners doing so.

GOLD RECOVERY BY THIOCYANATE LEACHING

Thiocyanate can dissolve (leach) >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Thiosulphate leaching – 1970s research in Canada

Thiosulphate leaching has the potential to replace cyanide leaching, being relatively cheap, environmentally less hazardous and capable of leaching gold from difficult refractory hardrock ores such as carbonaceous or Carlin types. In contrast, cyanide cannot leach gold from refractory ores without a preliminary step [69]. Thiosulphate leaching is by sodium thiosulphate Na2S2O3, an essentially non-toxic colourless crystalline compound that is more familiar as the pentahydrate, Na2S2O3.5H2O known as sodium hyposulphite or ‘hypo’ as used with silver in traditional photography. Although hypo had been known since the early 1900s to be useful in leaching gold, it was only in the 1970s that it was subjected to detailed study, leading to a patent being awarded to Roman N. Genik-Sas-Berezowsky, Verner Sefton and Lynton Gormely of Canada in 1978 (US #4,070,182) assigned to Sherritt Gordon Mines Ltd. For three decades thiosulphate leaching has been heralded as being close to challenging cyanide leaching [69]. It has been the subject of four to five US patents every year since the late 1970s and the focus of hundreds of research papers. In spite of this, thiosulphate leaching has not yet been properly commercialised. A breakthrough seems tantalisingly close as shown by the patent awarded to Jinxing Ji, Christopher Fleming, Paul West-Sells and Ralph Hackl of Canada and patented in 2006 (US #7,066,983) assigned to Placer Dome Inc. The thiosulphate method uses a solution of hypo Na2S2O3.5H2O in the presence of an oxidising agent to dissolve (‘leach’) fine gold as a strong complex [Au(S2O3)2]3-, and then to precipitate easy-to-recover gold. The preferred oxidising agent is copper ions. In contrast, thiourea leaching uses ferric iron (Fe3+) whereas cyanide leaching uses oxygen direct from the air. Adding more oxidising agent is unnecessary for a Cubearing ore.

Figure 54.

Operation 1st stage – leaching gold into solution Sodium thiosulphate pentahydrate Na2S2O3.5H2O (‘hypo’) is usually supplied in powder form sourced from specialised manufacturers. A solution of hypo is prepared, and then ammonia added to make the leachate strongly alkaline (10 to 10.5pH). Copper ions must also be present as essential oxidising agent. The ammonia and copper are catalyst-like for they are neither produced nor consumed but recycled. In practice, copper may precipitate cupric sulphide that inhibits leaching, while ammonia may escape Hypo tends to be unstable, avoidable by adding sulphite ions to regenerate the thiosulphate and prevent silver precipitating too early as insoluble silver sulphide. The milled ore is added to the thiosulphate leach with a pulp density of 40 to 45% solids. Hypo consumption may be as much as 30 kilos per ton or ore, but can be cut to 13 kilos/ton by adding reducing agents as chelates. 2nd stage – recovering gold from solution The pregnant thiosulphate leach solution contains dissolved gold in the form of gold-thiosulphate complex, and the pregnant solution is removed for further processing to recover the dissolved gold. Activated carbon or resins are ineffective for recovering the gold from the pregnant leach solution. Instead gold is recovered by cementation method using zinc, iron or copper. Under controlled conditions, gold recovery from suitable ores can exceed 90%.

Adoption by placer gold miners The author is unaware of thiosulphate leaching being used at large-scale placer gold mines, large or small. The main deterrent is the uncertain technology, variable results and difficulty of controlling the process efficiently.

GOLD RECOVERY BY THIOSULPHATE LEACHING

Thiosulphate can leach >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Thiourea – chemical leaching of gold

Thiourea has been heralded for decades as an alternative to cyanide, but as yet few if any industrial operations have proved to be a commercial success. In theory, thiourea can be used to recover gold from milled hardrock, and has potential for leaching gold from placer concentrates. Thiourea leaching can proceed four or five times faster than cyanide leaching, making thiourea more effective at dissolving large gold particles, such as those typical of placer gold. [42] Thiourea CS(NH2)2 is an organic compound that is classed by INCHEM/WHO as, “toxic. Known animal

Operation Thiourea is usually supplied in powder form sourced from specialised manufacturers. A weak solution of thiourea is prepared, and the first stage is the oxidation of thiourea to form formamidine disulphide: 2CS(NH2)2 + 2Fe3+ = C2S2(NH)2(NH2)2+ 2Fe2++ 2H+ The role of the formamidine is to oxidise the gold to form a gold-thiourea complex: + 2+ 2Au + C2S2(NH)2(NH2)2 + 2CS(NH2)2 + 2H = 2Au(CS(NH2)2)

carcinogen and probable human carcinogen. May cause irreversible effects. May affect fertility. May be fatal if swallowed. May cause allergic skin reaction. May cause skin ulcers, liver damage. Handle as a carcinogen. Gloves, safety glasses, good ventilation. Protect against spills and the spread of dust.” An end product is cyanamide that

Importantly, “formamidine acts as an oxidiser as well as a complexing agent, supplying about 50% of the ligands to the complexation” and due to this thiourea leaching of gold is faster than cyanide leaching [43]. The overall equation for thiourea leaching is: 2Au + 4CS(NH2)2 + 2Fe3+ = 2Au(CS(NH2)2++ 2Fe2+

contains the cyanide radical and reacts with acids to form a highly toxic gas. Cyanamide is toxic if swallowed, harmful to the skin and is an eye irritant. The thiourea method uses a weak solution of thiourea in the presence of an oxidising agent to dissolve (‘leach’) fine gold into solution, and then precipitate it as easy-to-recover gold. In thiourea leaching of gold, ferric iron (Fe3+) is used as an oxidising agent, it being the most effective compared to alternatives such as hydrogen peroxide, sodium peroxide, ozone, potassium permanganate and formamidine disulphide. In contrast, cyanide leaching uses oxygen as an oxidising agent direct from the air. Sufficient ferric iron (Fe3+) should already be liberated and available to make the addition of more oxidising agent either limited or unnecessary for a highly oxidised hardrock ore, or in a typical placer ore.

Figure 21.

To drive the equation to the right, thiourea must be present in excess, and “the ratio of complexing and oxidising agents must be carefully adjusted’ to avoid excessive oxidation of the thiourea solution and consequent excessive use of reagents [43]. In a final step, the formamidine breaks down irreversibly to cyanamide and elemental sulphur. The sulphur is a potential problem to the success of the thiourea method, for it forms a fine grained sticky coating which can inhibit the leaching of gold.

Adoption by placer gold miners The author is unaware of thiourea leaching being used at large-scale placer gold mines, large or small. The main deterrent is the uncertain technology, variable results and difficulty of controlling the process efficiently.

GOLD RECOVERY BY THIOUREA LEACHING

Thiourea can dissolve (leach) >90% of gold smaller than about 150μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Nitric acid – chemical cleaning of gold

Operation

Figure 22.

The concentrate must be dry and as free of magnetite as possible. The concentrate is dried by placing it in a heat-resistant metal pan on a stove. After being allowed to cool, a magnet removes the magnetite (Fe3O4). The operator must have special training and wear protective clothing and eye-protection in accordance with local regulations and international norms. The ‘acid site’ must be out-of-doors in a well-lit fenced off area away from other people. All non-essential personnel must be excluded to minimise exposure to risk. Only one operator is needed, but a second operative should be within 10 metres to respond to any emergency. It should not be attempted if raining, snowing or in high wind. The concentrate is put in a heat-resistant, acidresistant, pan on a small stove inside the ‘acid site’ and warmed up. Then the operator uses a long-handled pot to pour hot, concentrated nitric acid into the pan of dry concentrate. The operator refrains from leaning forward and must wear protective clothing and eye-protectors. Immediately reaction starts, the operator steps back and vacates the area BEFORE heavy brown fumes appear. The brown fumes are of nitrogen oxides and are EXTREMELY TOXIC and even trace amounts cause severe lung problems. The process is exceedingly dangerous. However, if the process is carried out outdoors in an open place then the brown fumes are blown away after a few minutes. After a short time in the atmosphere the brown fumes disintegrate into harmless nitrogen and oxygen.

NITRIC ACID CLEANING

Extremely toxic fumes being generated by hot concentrated nitric acid poured onto dry concentrate. After a few seconds the brown fumes are completely broken down to harmless nitrogen. Artisanal miners in Kyrgyzstan (photo: Peter Appel of GEUS)

Hot concentrated nitric acid (HNO3) helps to recover fine gold from concentrates. Peter Appel of the DenmarkGreenland Geological Survey noted the method being used by artisanal gold miners in Kyrgyzstan to liberate gold from sulphide ores [18]. It appears over 90% of gold of 100μ to 300μ is recoverable, but tests are needed to confirm what percentage of <100μ can be recovered. However if mercury is present, from previous processing, from contamination or from native mercury or cinnabar, then potentially explosive chemicals may form. Nitric acid has been used in gold recovery for at least 150 years as a minor process step. Paul B. Queneau and John D. Prater of Utah invented a nitric acid method of recovering base metals and gold, patented in 1974 (US #3,793,429) assigned to Kennecott Copper. This method adds nitric acid continuously to decompose pyrite and arsenopyrite to liberate copper, gold etc. but achieves only 80% gold recovery, and requires the feed to be <53μ. These limitations were overcome by Rein Raudsepp, Ernest Peters and Morris J.V. Beattie of Vancouver whose nitric acid method was patented in 1987 (US #4,647,307), achieving 99.3% gold recovery in laboratory conditions. However the patented process is complex and it does not seem to have become commercialised.

Figure 23.

Adoption by placer gold miners The nitric acid method does not appear to be being used by placer mining companies but is being used by artisanal placer gold miners in parts of South America and elsewhere [45].

GOLD RECOVERY BY NITRIC ACID CLEANING

Nitric acid cleans gold and aids recovery of >90% of 100-300μ, gold but its effect on <100μ gold needs study. (compiler: Robin Grayson)

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Aqua regia - chemical leaching of gold

Operation

Aqua regia was invented by Iranian alchemist Abu Musa Jabir ebn Hayyan about 800 A.D. This followed from his discovery of hydrochloric acid upon mixing common salt with sulphuric acid. Aqua regia is a mixture of three to four volumes of concentrated hydrochloric acid to one volume of concentrated nitric acid. It is a corrosive, fuming, aggressive liquid and must only be used by a trained chemist following strict precautions in a properly equipped laboratory or outside space, and only after a hazard analysis has been prepared. Neither of the acids in aqua regia can dissolve gold, but in combination are very aggressive in dissolving gold. The fuming and yellow colour of aqua regia are due to the reaction of nitric acid HNO3 with hydrogen chloride form water H20 plus two chemicals that are yellowish and volatile - nitrosyl chloride NOCl and chlorine Cl2. The newly-formed nitrosyl chloride decomposes to nitric oxide NO and chlorine. Nitric acid HNO3 is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). Ausolid + 3NO3-aq + 6H+aq = Au3+aq + 3NO2 gas + 3H2Oliquid

This text is based on recovering gold from scrap by Shor International www.shorinternational.com. The two acids mix quietly – avoid splashes, protect eyes and work in the open or under a fume hood. Both acids emit acrid fumes. No heat is evolved when mixing but the aqua regia at once starts to emit chlorine gas slowly for several days. Never stopper an aqua regia bottle for chlorine may build up and explode it. The aqua regia is used immediately, or days or weeks later. Typically 1-2 kilos of scrap are put in an empty 6-litre Ehrlenmeyer flask under a fully ventilated fume hood, or outside. The aqua-regia is added slowly, such as drop-bydrop from a bottle set on a shelf above the reaction flask. Dangerous fumes of nitrogen oxides are generated; being heavier than air they require either a very good fume hood, or for the process to be done outside. When bubbling ceases and no more brown fumes are produced then a little hydrochloric acid is added. A further spurt of activity may occur if the original hydrochloric acid has been exhausted. When reaction has ceased, the pregnant solution is poured off into a glass or plastic container, leaving the solid residue in the reaction vessel. The process is repeated with more aqua regia until no bubbling or brown fumes are seen, even if the reaction vessel is gently warmed and gently agitated. All the gold has now been dissolved. The contents of the reaction vessel are vacuum filtered, and the gold precipitated from the pregnant solution as described in www.shorinternational.com.

Hydrochloric acid HCl supplies chloride ions (Cl-) in large amounts which attack the gold to produce chloraurate anions AuCl4- in solution: 3+ Au aq + 4Cl-aq = AuCl4-aq This is an equilibrium reaction favouring formation of chloraurate anions. It results in a removal of gold ions from solution and allows further oxidation of gold by the nitric acid, and so more gold is dissolved. In addition, gold may be oxidized by free chlorine in the aqua regia.

Figure 24.

Adoption by placer gold miners Aqua regia is rarely, if ever, used by placer gold miners. A few recreational miners do use aqua regia but it has not become popular [44].

GOLD RECOVERY BY AQUA REGIA LEACHING

Aqua regia can dissolve (leach) 90-100% of gold even as large as 1mm, but is too aggressive for mining use. (compiler: Robin Grayson)

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Borax – early smelting of gold

Operation

Figure 25.

The artisanal method of ‘early smelting’ is reported by Peter Appel from the Philippines, and outlined below. Hardrock ore is first crushed and milled. The milled ore is then subjected to gravitational separation to produce a black sand concentrate with visible gold. The concentrate (one part) is dried carefully and then mixed in a very small plastic bag only a few centimetres in size with borax (three parts). After mixing, a few drops of water are added. The plastic bag in put in a pottery bowl serving as a crucible, and positioned tilted on a few pieces of charcoal. The mixture of borax and heavy mineral is heated by a blow torch. The blow torch is gasoline fuelled, is very cheap and already used by the artisanal miners for burning amalgam. The heating continues until first the borax melts and later the gold melts. Borax depresses the melting point of gold. The molten gold is heavier than the other heavy minerals which float off as slag to leave a nice gold pellet. The entire process only takes a few minutes.

BORAX SMELTING

Smelting concentrate to recover gold in the Philippines. (photo: Peter Appel of GEUS, Geological Survey of Denmark and Greenland)

‘Early smelting’ is possible on unclean concentrate to recover fine gold that might be lost if upgrading were to be attempted by amalgamation or gravitational means. The ‘early smelting’ method of recreational miners is described at www.nuggethunters.org [46,47]. The ore is screened or milled at 2mm and the black sand soaked in acidic acid for couple of days to help break down sulphides, then carefully dried. The flux is of anhydrous borax Na2B4O7 (5 parts), #70 silica sand SiO2 (40 parts), soda ash Na2CO3 (10 parts) and sodium nitrate NaNO3 (20 parts) as oxidizer. The ingredients are mixed and stored in a container and kept dry. Flux (1-2 parts) is added to the concentrate (1 part) and mixed well. The mixture is spooned into the crucible and dampened with rubbing alcohol. The mixture is fired by the acetylene torch up to 1,100°C and the heat kept until the mixture is molten and a bright yellow white. The molten material is poured into a cast-iron mould, requiring special clothes and care [46,47,48].

Figure 26.

Adoption by placer gold miners ‘Early smelting’ assisted by borax has been used for many years by small-scale gold miners in the Benguet area north of Manila in the Philippines as an alternative to mercury or cyanide. Elsewhere borax-assisted smelting is sometimes used by artisanal miners, but only after mercury amalgamation or cyanide leaching. Smelting assisted by borax is common among recreational and industrial gold miners in North America, Russia and Mongolia upon concentrates that are clean. But ‘early smelting’ is advocated for recreational miners by www.nuggethunters.org [46,47] for recovering fine gold from concentrates that are not particularly clean.

GOLD RECOVERY BY SMELTING WITH BORAX

Smelting with borax can recover fine gold, but usually only from a clean concentrate. (compiler: Robin Grayson)

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Bioleaching – 1980s research in Wales and California

Operation

Bioleaching is the extraction of metal from its ore by means of microbes. An introductory account can be read at: http://en.wikipedia.org/wiki/Bioleaching. The Pooley method of bioleaching was invented by Frederick D. Pooley of Wales and patented in 1987 (US #4,497,778) with limited success. The Geobiotics method of bioleaching was invented by Dennis Kleid, William Kohr and Francis Thiobodeau of California who applied for a patent in 1992, awarded in 1995 (US #5,378,437). It met with greater success. The method is suitable for ores of grades as low as 0.02 Troy ounces per ton. Advantages are: ² ²

This text is based on the Geobiotics method of bioleaching as presented in US patent #5,378,437. Mass cultivation of the chosen micro-organisms is undertaken in outdoor ponds in a sunlit site close to the ore body. The culture pond is lined with plastic or concrete and the water body is 10-30m wide and 20-50m deep. The culture pond is fitted with a pumping system to permit harvesting and recirculation of the growth media. Algae are the most convenient. Each species has particular tolerance of physical and chemical conditions, and nutritional requirements including phosphorous, nitrogen, sulphur, iron, manganese, trace elements and ions. The pond is kept strongly alkaline, pH7-10, by adding lime or phosphate buffer. Following guidance of the patent, annual yields of 40 dry tons per hectare are possible, and the microbes are sprayed onto the ore from the pond at the time of maximal cyanide production. Bioleaching may be done in three settings:

economic: simple and cheap, and few technicians required; environmental: the microbes are natural and easy to cultivate.

Disadvantages are:

² ²

economic: bacterial leaching process is very slow; environmental: Sulphuric acid and H+ ions can leak and turn surface water and groundwater acidic, and heavy metals such as iron, zinc and arsenic be leached by acid mine drainage.

Suitable microbes include:

²

² ²

²

algae – Chlorella vulgaris, Cyanopora paradoxa and Cyanidium caldarium; blue-green cyanobacteria – Anacystis nidulans; bacteria – Chromobacterium violatum, Chromobacterium flavum, certain Bacillus species (pyocyaneus, flourescens, violaceous, mesentericus, nitrificans), certain Pseudomonas species (aeruginosa, fluorescens, aureofaciens, cyanogena, liquifaciens, cepacia); and fungi –notably Marasmius oreades (‘Fairy Rings’), the Snow Mould basidiomycete and some Fusarium species.

² ² ²

Adoption by placer gold miners

An interesting approach is to co-culture microbes:

² ² ²

bacteria producing glycine in bulk; same bacteria able of absorbing gold-cyanide ion complexes; micro-organisms liberating methionine in bulk; and algae able to produce cyanide from a glycine substrate.

Figure 66.

tank bioleaching – milled ore in a tank; heap bioleaching – milled ore in a heap; or in-situ bioleaching – blasted ore in situ.

Oxidation of gold to gold-cyanide complexes commences once the microbes contact the ore, often with the gold (I) ion – [Au+][CN-]2. Biosorption is “automatic and immediate” even if the microbes are dead. The fluid with microbes and biosorbed gold is pumped into a settling pond or tank at least 3m deep and allowed to settle, aided by flocculants. The sludge of living and dead microbes is harvested and the biosorbed gold recovered.

Cyanogenesis is the same in all these microbes, by the oxidative decarboxylation of the glycine in a process stimulated by methionine or other methyl-group donors: NH2CH2COOH J HCN + CO2 + 4[H] ²

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Bioleaching has not been adopted by placer gold miners as it poses too many challenges. Eventually a simplified version may be acceptable.

GOLD RECOVERY BY BIOLEACHING

Bioleaching can dissolve (leach) >90% of gold smaller than about 75μ, but is too slow for leaching larger gold. (compiler: Robin Grayson)

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Biooxidation – 1980s research in British Columbia and California

Biooxidation oxidises both iron and sulphur under acidic conditions, causing the solubilisation of iron as ferric (III) ion and sulphide as sulphate ion. This liberates the encapsulated gold making it accessible to leaching. During the 1980s, biooxidation became the focus of intense research effort, offering a low-cost means of preparing refractory hardrock ores to make them responsive to leaching such as cyanide leaching. Biooxidation research continues unabated and a few milestone patents are outlined below. The Hackl biooxidation method was invented by Ralph P. Hackl, Frank W. Wright and Albert Bruynsteyn of British Columbia, patented in 1991 (US #4,987,081) and assigned to GB Biotech Inc of British Columbia. The method cultures of at least three species of bacteria Thiobacillus thiooxidans, Thiobacillus ferrooxidans and Leptospirillium ferrooxidans. The cultures are subjected to increasing concentrations of dissolved arsenic and low pH to raise their tolerance. The Kohr biooxidation method was invented by William J. Kohr of California, patented in 1995 (US #5,573,575) and assigned to Biotech Inc of California. Refractory sulphide ore is crushed and separated into a fine and coarse fraction. The coarse fraction is stacked in a heap, and a concentrate produced from the fine fraction. Alternatively biooxidation can be assisted by forming particulates that are then heaped (US #5,246,486) and polymer agglomeration may be beneficial (US #5,332,559). Biooxidation of carbonaceous and carbonaceoussulphidic ores is difficult, and requires a specific carbondeactivating microbial assemblage (US #5,244,493). The Oxidor column reactor for testing and evaluating refractory ores was invented by Andrew Carter of Texas and patented in 2002 (US #6,498,031), assigned to Oxidor Corporation.

Figure 67.

Operation The ore is first batch tested to determine if biooxidation is effective. Batch testing may require six months due to the time required for the bacteria to adapt to the substrate and the time gap between inoculation of the ore and its oxidation. The testing and evaluation can be accelerated using a device such as the Oxidor column reactor. Suitable cultures include the following species of bacteria, either alone or in combination: Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Sulfobacillus thermosulfidooxidans, Metallosphera sedula and Leptospirillium ferrooxidans. A bacterial culture is developed that can grow in high acidity and high metal content. The bacteria suspension is used to inoculate ore stacked in the open air resting on a pad system. Biooxidation has a choice of settings: ² ²

tank biooxidation - for refractory ores of relatively high grade heap biooxidation - for refractory ores of relatively low grade.

Typically 180 to 600 days is required to oxidise the iron and sulphur in the ore. This puts pressure on cashflow, increased the mine footprint and adds to production costs. Care is needed in the heap design to ensure fine materials do not plug the voids essential for aeration and liquid flow. Plugging results in starvation of nutrients, carbon dioxide and oxygen and uneven distribution of the bacteria. Adequate air flow is essential to cool the heap from the exothermic effects of biooxidation. After biooxidation the resulting oxidised ore is highly acidic and, for leaching by cyanide must first be treated with lime to raise the pH substantially.

Adoption by placer gold miners Biooxidation is inappropriate to placer gold ores as sulphides are rare and gold is in the form of free particles.

GOLD RECOVERY BY BIOOXIDATION

Biooxidation can oxidise sulphide ores sufficient for leaching. (compiler: Robin Grayson)

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Gold-binding proteins – 2000s in research in Washington

Gold-binding proteins (GBPs) have recently been recognised as having potential applications in recovering fine gold and in gold exploration. GBPs are proteins that have a high specificity and affinity for gold [188]. A research team at the University of Washington led by Clement Furlong with funding from Placer Dome Inc investigated gold-binding proteins and registered a patent in 2005 (US #6,875,254). The patent is highly innovative but broad-brush in character and the methods it proposes are not yet commercial. However the speed of advance in parallel subjects such as immunology and microbiology may lead to sudden breakthroughs. The patent stretches the meaning of GBPs to embrace all “gold-specific non-naturally occurring binding ligand to gold” in a protein, polypeptide, peptide, protein fragment, oligonucleotide, carbohydrate, antibody, chelating agent, magnetic agent, hydrophobic agent or any combination of these. The patent envisages many types of gold recovery mechanisms involving GBP, or rather “binding ligands to gold”, such as: ²

²

²

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Operation GBP-enabled magnetic separation of extremely fine gold particles from magnetite-rich slurry. A “magnetic mineral binding agent including a gold-specific protein” is added to a sample to form a complex of magnetic mineral and gold. A magnetic field is applied and the complex removed from the liquid. In a lab test, 3μ gold beads were coated with GBP antibodies and bound to magnetic beads to form a complex. Being magnetic, the complex was pulled to the wall of a microcentrifuge in a magnetic field while the other material settled to the bottom of the tube. In a very different test, gold was bound to natural magnetite by means of a reagent with both gold-andmagnetite-binding-domains to form a complex that could be separated by magnetic methods. GBP-assisted floatation – a GBP is modified to form a hydrophobic reagent by reacting with valeric anhydride to form a GBP with C5 hydrophobic tails (C5-GBP). A laboratory experiment showed C5-GBP bound to extremely fine gold (“micron gold”) could reside at the oil-water interface. However in the author’s opinion this result should be treated with caution for gold itself is hydrophobic and the merit of adding C5 hydrophobic tails is not demonstrated. GBP-assisted microbial extraction and transport is envisaged for recovering sub-micron gold (<1μ). The patent draws attention to strains of Escherichia coli cells that express an extra-cellular GBP domain will bind small particles of gold, and then the Escherichia coli can be induced to follow a chemical gradient of attractants such as ribose sugar to lead them to a recovery destination.

GBP-enabled magnetic separation to recovering extremely fine gold particles from magnetite-rich slurry. GBP-assisted floatation using GBPs modified to form a hydrophobic reagent. GBP-assisted microbial recovery of sub-micron gold (<1μ).

Adoption by placer gold miners Gold-binding proteins (GBPs) have demonstrated as being commercially viable.

Figure 142.

GOLD RECOVERY BY GOLD-BINDING PROTEINS (GBPs)

Recovery of gold by gold-binding proteins is not yet commercial but may eventually become so. (compiler: Robin Grayson)

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Phytomining – 2000s research in New Zealand

Operation

Figure 143.

The gold-bearing soil, such as a natural placer or more likely an expanse of gold-rich tailings, is first planted with a plant capable of absorbing gold in solution and storing it (‘bioaccumulation’). The plant species needs to be a fast-growing and high-biomass species. When the crop reaches maturity, lixiviant chemicals capable of dissolving gold are applied to the soil that make some of the gold (plus any mercury) and other toxic metals soluble. The plants absorb the solutions with the metals and bioaccumulate the metals in their roots, shoots and leaves. The lixiviant chemical may be toxic to animals and man but harmless to the crop, such as cyanide that may be broken down in the soil. After a few days or weeks, the crop is harvested and processed by incineration to recover gold, mercury and other metals in the ash. Researchers at Massey University in New Zealand are testing the use of common crops such as rapeseed to soak up toxic contaminants from soil at abandoned gold mining sites, and to return the land to safe agricultural use. The idea is that the gold harvested during the operation covers the cost of clean-up and provides revenue for the education and training of the communities to create sustainable incomes by farming the land.

PHYTOMINING

Small-scale field trials of gold phytomining trial in Brazil. (photo: courtesy of Dr. Chris Anderson of Massey University http://ite.massey.ac.nz/staff/rhaverka/Phytomining.htm)

Phytomining is a still largely experimental. Research is investigating plants able to grow on toxic soils polluted by mine waste or from natural high toxic metal anomalies. Not only is a ‘ground cover’ of plants produced, but also some plants absorbed such large amounts of toxic metals that ‘bioremediation’ is sometimes possible by cropping the plants to remove the metals. The next step has been very recent – to investigate if valuable metals can be mined by cropping such plants, the plants absorbing the valuable metals front the soil – and so a new scientific line of investigation emerged for which term ‘phytomining’ has been coined. The first experimental success was as commercial production of nickel metal from plants grown on soils with abnormally high concentrations of nickel. Recently there has been some success by New Zealand researchers with gold recovery from crops of plants grown on soils with high gold content [189-192] see: http://ite.massey.ac.nz/staff/rhaverka/Phytomining.htm.

Figure 144.

Adoption by placer gold miners Phytomining is making rapid progress. The author suggests several routes to commercialisation may emerge: ² ²

GOLD RECOVERY BY ECOLOGIC E-TOWER

phyto-reclamation as an incentive to cleaning up tailings; and gold recovery from difficult placers, e.g. fine gold in laterites.

Recovery of placer gold by the Ecologic ‘gold concentrator’ based on manufacturer’s information. (compiler: Robin Grayson)

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Agglomeration – 1980s research in Australia and China

Operation

Figure 68.

The slurry is piped into a special reactor. Here the hydrophobic-oleophilic properties of gold induce the fine gold to agglomerate into oil-saturated activated carbon particles. In theory gold particles will agglomerate with oil, but in practice gold grades are so low that there is not enough gold to form oil-gold agglomerates. So for CGA to work it is first necessary to use another hydrophobic material (in this case, comminuted coal dust) to either agglomerate with the gold or to act as a carrier of the gold particles. The first step is to create coal-oil agglomerates about 5mm in diameter using coal dust bound by kerosene, light gas oil or fuel oil. Agglomerates smaller than 500μ seem to be more effective in recovering gold. The second step is to add the coal agglomerate to the gold-bearing slurry in the special reactor. The gold particles, due to their oleophilic nature, continue to enter the agglomerate particles until the operator considers the target gold concentration has been reached. Then the gold enriched oil-saturated activated carbon particles are agitated and the agglomerated gold is mechanically separated. Finally gold is recovered from the agglomerates by burning them, the coal and oil incinerating to leave gold and ash. The gold is then separated from the ash.

COAL-GOLD AGGLOMERATION

Flowsheet for placer gold recovery by CGA. (drawing: Robin Grayson)

Agglomeration ‘snowballs’ extremely fine gold with coaly or oily material to produce large particles that can then be recovered easily by floatation. Coal-gold agglomeration (CGA) makes use of gold being hydrophobic (resists water-wetting) and oleophilic (easily wetted by oils). CGA only works with fine gold, making CGA appropriate for placer tailings with fine gold that would otherwise be lost, and for milled hardrock ore. CGA began with BP plc in Australia who built a pilot plant processing 1 ton/hour of placer gold concentrate. A patent was awarded to Mark Cadzow, Graham Elkes, Gavin Ewin and David Mainwaring in 1986 (US #4,585,548) and assigned to BP Australia. The team then tried CGA for low-grade hardrock ores with <1 gram/ton of gold, patented in 1990 (US #4,976,781). In China, Zhao Bing and colleagues found CGA had “many advantages” over cyanide and claimed 88% recovery of gold in amalgamation tailings. In Africa, lab experiments by Kotze and Petersen achieved 85% gold recovery from artificial gold-slurry mixtures [77-82]. A CGA pilot by Envi-Tech Inc under the CanadaAlberta MDA project in 1993/94 indicated gold recovery from agglomeration-adsorption technology may be 9599% but no results seem to have been published.

Figure 69.

Adoption by placer gold miners Early success in recovering placer gold with CGA did not lead to its adoption by placer gold miners. Yet it might allow high % gold recovery from ‘difficult’ placers, such as fine gold in laterites (as in much of South America, Africa, Australia and parts of Kazakhstan, Mongolia and China), without the use of mercury or cyanide. Recent work in Turkey shows high % recovery is possible for gold between 53μ and 300μ [83].

GOLD RECOVERY BY AGGLOMERATION - generalised

Coal-gold agglomeration (CGA) can recover 90% of gold in the range 53-300μ [83]. (compiler: Robin Grayson)

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Oleophilic adhesion – 1980s research in Alberta

Oleophilic adhesion is the selective adhesion of a mineral to a surface coated in oil, grease or wax. Gold is ideal being oleophilic, and proven to be naturally hydrophobic [84,85]. Conversely magnetite and quartz are oleophobic and hydrophilic. Yet gold recovery by oleophilic adhesion failed to challenge froth floatation, in spite of requiring less water and less grinding, ease of regenerating oils/greases/waxes and general simplicity. In contrast, oleophilic adhesion became the standard means of recovering diamonds on grease tables and grease belts. Oleophilic adhesion was formerly known as the ‘contact method of ore concentration’, for which Royer Luckenbach of New York was awarded two patents in 1923 and a third in 1931 (US #1,448,928, US #1,478,237 and US 1,792,544). The patents propose a sticky coating of oil, grease or wax being smeared on an endless belt (e.g. a Frue vanner) to which gold particles would be attracted and remain attached even when the belt inverts over an end roller where black sand and quartz are shed as tailings. The gold is removed from the moving inverted belt by a scraper, and a roller reapplies a sticky smear of oil, grease or wax. Luckenbach added sodium silicate to the smear as a wetting agent to deter settling of magnetite and other gangue minerals, and his patents mention an extraordinary range of suitable oils, greases and waxes – including candle wax, candle tar, coal tar, horse grease, bitumen and lard. The Lurgi method was invented by Ernst Bierbrauer of Germany and patented in 1940 and 1942 (US #2,189,698 and US #2,291,447) but not for gold recovery. The Kruyer method was invented by Jan Kruyer of Alberta and patented in 1983 (US #4,511,461) and rather than using a sticky solid belt uses a sticky mesh belt. Rather than scraping the belt to collect the adhering oleophilic particles, the belt is squeezed between rollers or alternatively be blown or shaken off.

Figure 70.

Operation The text is adapted from the account of the greasy belt described by Royer Luckenbach in his patents. Hardrock ore is milled to 2.5mm – a major advantage over froth floatation that requires much finer milling. Placer ore is screened at 2.5mm. Gold in the oversize is separated in a simple device such as sluice. The <2.5mm fraction is subjected to oleophilic adhesion. Water is added to the <2.5mm feed to create a slurry of about 25% solids by volume. Hydrophilic particles are wetted by adding a trace of wetting agent such as sodium silicate (see patents) or liquid non-frothing detergent. This weakens surface tension and sinks ‘float gold’. The ‘greasy belt’ is an endless rubber belt moving between rollers, one of which is a drive roller. The belt is coated in a thin sticky (‘tacky’) coating of oil, grease or wax but not so liquid that it might drip free when the moving belt inverts on passing over the end roller. Luckenbach suggests a flexible resin binder such as rubber is added to the coating to make it waterproof. The slurry issues as a thin stream onto the moving endless belt and the gold adheres to the sticky coating by oleophilic adhesion. At the end, the water and gangue minerals are shed as tailings whereas the gold and other oleophilic particles remain stuck to the inverted belt from which they are removed by a scraper. The scraper also removes some or all of the sticky coating. The inverted belt passes across a roller that applies a fresh sticky coating and then turns ‘right-way-up’ over its end roller to again capture oleophilic particles from slurry.

Adoption by placer gold miners Oleophilic adhesion does not appear to be being used by placer gold mining companies, artisanal miners or recreational miners in spite of its apparent simplicity.

GOLD RECOVERY BY OLEOPHILIC ADHESION - generalised

Recovery of placer gold by the oleophilic adhesion is unclear and the graph is highly conjectural. (compiler: Robin Grayson)

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Froth floatation – 1930s research in Idaho and USSR

Froth floatation of gold is possible due to gold’s surface hydrophobicity – “the antithesis of gravity concentration" [27] and therefore froth floatation can recover up to 100% of very fine to moderately fine gold (<150μ), much the same as cyanide leaching. The first paper on froth floatation of gold appeared as late as 1914 [49]. Today froth floatation is a widely used method for recovering many types of minerals including gold, platinum, chromite, fluorspar and coal. Regarding placer gold, deterrents are the cost of reagents, the need to recycle effluent and the challenge of floating placer gold due its depressed hydrophobicity, buoyancy and floatability due to impurities and coatings. But placer gold is often irregular in shape (due to natural leaching) – even porous – and is often remarkably flat, and paradoxically it is these factors that make gravitational settling difficult yet froth floatation easier.

Adoption by placer gold miners In 1916 Lang suggested that the platy shape of placer gold in black sand of the Pacific Coast of North America should make it amenable to recovery by floatation [50]. The first research report on placer gold floatation was by Arthur Fahrenwald in 1933 [51] and 1937 saw publication of floatation recovery of placer gold on Idaho gold dredges [3,52]. Reagents were amyl xanthate and Aerofloat-15 plus either pine oil or cresylic acid frother. Gold recoveries were 47-76% – the less slimes then the better the floatation [3]. Contemporary tests in the Soviet Union with similar reagents on clean-up tailings and <150μ fraction of placer ores yielded 75-90% recovery at concentration ratios of 25:1 to 42:1 [3,53,54]. Research resumed in the 1970s in China [27] where 78-99% recovery was attained with rougher concentration ratios of several hundred [3]. Floatation tests of Soviet Union coastal marine sands achieved 70-100% recovery of 75-125μ gold [2,27]. A graph by Wang and Poling shows >95% recovery was possible for “coastal type gold” <150μ with a retention time of 15 minutes, and 100% recovery possible for <120μ with a retention time of only 5 minutes. The Soviet Union appears to have operated the world’s only full-scale floatation circuit for placer gold, a six-cell floatation circuit in the 1930s that scavenged both ‘fine’ and ‘minute’ gold from gravity tailings aboard a bucket-line dredge [2,27]. It processed 300 tons of solids per day and although the gold recovery was “satisfactory” the floatation circuit was deemed uneconomic at the then prevailing low gold price. Today, froth floatation is a neglected method for recovering placer gold and the author is unaware of any commercial placer operations using this method.

Operation For placer ore, the process is most appropriate to gold recovery from fine tailings or from concentrate. The feed consists of finely milled hardrock ore, or else fine tailings or placer concentrate. Slurry is made by adding water, and fed continuously into a floatation tank. In the floatation tank, the slurry is agitated and air bubbles injected. The gold particles attach themselves to the meniscus of the rising bubbles. This is due to gold being hydrophobic – and by selectively enhancing this tendency by adding chemicals known as ‘collectors’; by controlling the collectors using ‘conditioners’; by stimulating wetting by ‘wetting agents’; by stimulating frothing by ‘frothing agents’ and by controlling pH. The resulting froth is then removed and the gold recovered by either gravity settling or chemical means.

Figure 27.

GOLD RECOVERY BY FROTH FLOATATION

Froth floatation can separate >90% of gold <150μ, but is too slow for leaching >300μ gold. (compiler: Robin Grayson)

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Gold-paraffin wax floatation – 1990s research in Brazil

The gold-paraffin wax process was developed in Brazil as a clean non-polluting method for recovering gold as an alternative to mercury amalgamation [167], and is currently being assessed by Eco-Minex in Mongolia. The gold-paraffin wax process as developed by Christina Hamelmann and Fernando Lins of CETEM, Brazil, exploits the preference for gold particles to adhere to hydrophobic materials and thereby can be separated from slurry. The selected hydrophobic material is paraffin-wax which is non-toxic, low-cost and widely available. The concentrate used in the tests was from Minas Gerais with a gold grade of 11 grams/ton. The CETEM researchers found that the greater the volume of paraffin wax compared to the volume of the sample then the greater the gold recovery by the goldparaffin wax method. Attempts were made to obtain an electrostatic attraction between the gold particles and the paraffin globules by controlling the acidity at pH 3, as the isoelectric point for gold is pH 2 and pH 3-5 for paraffin [168]. In theory at pH 3 the gold particles should be negative and paraffin globules positive, and some improvement in gold recovery was achieved at pH 3 suggesting that this mechanism was occurring. The researchers also found xanthate as a gold collector enhanced gold recovery. The CETEM researchers succeeded in achieving gold recoveries of 40-50% by the gold-paraffin wax method [167]. These results are encouraging considering the number of untried variables that await investigation that may be capable of greatly improving the gold recovery sufficient to challenge mercury amalgamation. The gold-paraffin wax method is described at www.egoldprospecting.com/html/gold-paraffin_process.html.

Figure 119.

Operation This account is based on the bench experiments of Hamelmann and Lins in Brazil [167]. The concentrate is finely divided and made into aqueous slurry of 25-30% solids. The slurry is heated in a container to 70°C, just 2 degrees centigrade above the melting point of the paraffin-wax used (about 68°C). The temperature of the slurry is maintained at 70°C, and pieces of paraffin wax are added and allowed to melt. A mechanical stirrer disperses the melted paraffin to encourage it to contact the particles in the slurry. By this simple means, the gold particles being hydrophobic adhere to the globules (droplets) of paraffin wax whereas black sand and quartz do not. When the stirring ceases, the paraffin globules rise to the surface to form a low-density paraffin phase above the water phase. The gold is locked in the paraffin when it solidifies as a floating solid. The solidified gold-containing paraffin is removed from the surface of the aqueous phase and further processed to remove the gold.

Adoption by placer gold miners The gold-paraffin wax method requires further experimentation to improve % gold recovery sufficient to justify interest by placer gold miners.

GOLD RECOVERY BY GOLD-PARAFFIN WAX FLOATATION – results of CETEM experiments

Recovery by the experimental gold-paraffin wax method that merits further investigation. (compiler: Robin Grayson)

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Magnetic coated gold – 1980s research in Colorado

A rather unexpected way to recover placer gold is, having first removed magnetite and other magnetic minerals to then selectively make the gold particles magnetic and remove them by magnetic means. James K. Kindig and Ronald L. Turner of Golden, Colorado gained patents in clean coal technology having discovered that by warming coal with iron carbonyl vapour the pyrite became magnetic and could then be removed more easily (US #3,938,966 and #4,175,924). Kindig and Turner then adapted the method for placer gold with good results and they were awarded a patent in 1980 (US #4,229,209) and assigned it to Hazen Research Inc. First they removed the magnetite and suchlike using magnetic separators. Next they put the non-magnetic placer into a rotating kiln with iron carbonyl vapour in an inert nitrogen atmosphere. Important variables include the temperature, pressure, type of carbonyl used, gas composition etc. The iron carbonyl selectively attaches itself to the gold particles and decomposes to make a magnetic coat of iron for the gold particles.

Operation This text is based on the Kindig and Turner method presented in US patent #4,229,209. Iron carbonyl liquid is sourced from specialist suppliers and stringent precautions are applied during transport, storage and use regarding risk of fire and risk of acute poisoning. The placer ore is either dry-screened or else wetscreened and then dried. Magnetic minerals are removed as fully as possible. The dry non-magnetic fraction is fed to a rotating kiln serving as a reaction vessel to bring the material into direct contact with iron carbonyl vapours in the presence of a gas such as nitrogen that is inert to the reaction. The objective is to cause the decomposition of the iron carbonyl to form a magnetic skin on the gold particles but not on the other particles present. Typically about 0.5 to 4 kilos of carbonyl are added per ton of feed. Generally a reaction time of from half an hour to an hour is adequate, at 110-130°C. The higher the temperature the more complete is the gold recovery, but at higher temperatures the iron carbonyl is liable to coat other minerals beside gold particles. After the treatment, the magnetic coated gold is removed by dry magnetic separators such as a low to medium separator with a magnetic drum having field strengths of up to about 2,500 gauss, electromagnetic drum separators up to about 7,000 gauss or induced roll separators up to about 11,000 gauss.

Fe(CO)5 Fe + 5CO Iron carbonyl Fe(CO)5 is a straw-yellow liquid that is cheap, but unfortunately it requires special precautions as it is not only flammable but is also toxic if inhaled. The inventors recovered 93.3% of placer gold from Clear Creek, Colorado; then 76.5-93.3% of placer gold from the Vulture placer in Arizona, and later on their tests were getting 99.98% gold recovery. Of interest is that the Kindig and Turner method requires no water at all, making it of potential value in enabling large-scale placer gold recovery in arid regions. However, the method has not been tested on gold <100μ and therefore further research is warranted.

Figure 71.

Adoption by placer gold miners This 'making gold magnetic' process unsuitable for artisanal miners, but seems to have merit for large-scale placer mining operations – particularly in dry deserts as the process requires no water.

GOLD RECOVERY BY MAGNETIC COATING GOLD PARTICLES

Recovery of placer gold by the iron carbonyl method according to the original patent. (compiler: Robin Grayson)

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Riffled sluices – 1960s-70s research in China and Soviet Union Some early scientific tests on gravitational recovery of gold by simple sluices were in China and the Soviet Union. In China, placer scientists of the Minerals Processing Laboratory of the Kunming Institute of Metallurgy in Yunnan Province in the 1970s determined the percentage gold recovery of conventional riffles [27]. It is unclear what type of riffles were tested or the size of the feed, but the results showed gold recovery starts to falter at 2mm, is only 90% by 0.6mm, and collapsed to 60% at 0.2mm. In the Soviet Union, placer scientists in the 1970s achieved similar results with expanded metal riffles [2]. Again it is unclear what type of expanded metal riffles were tested or the size of the feed. The results were encouraging compared to the dismal performance of flat bar riffles traditional in the Soviet Union in the 1970s and that are still favoured in the Russian Federation and Mongolia. But the results were poor compared to the Yukon tests on expanded metal riffles a decade later.

Figure 28.

RUSSIAN EXPANDED METAL RIFFLES

Figure 29.

GOLD RECOVERY BY SIMPLE RIFFLED SLUICES – China tests

Figure 30.

GOLD RECOVERY BY SIMPLE RIFFLED SLUICES – Soviet Union tests

Sluice-boxes with expanded metal mesh riffles freshly installed in a Soviet bucket-line dredge in Mongolia. (photo: Robin Grayson)

Poor performance of simple riffled sluices [27]. (compiler: Robin Grayson)

Poor performance of simple riffled sluices [2]. (compiler: Robin Grayson)

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Flat bar riffles – 1980s research in Yukon, 1990s research in Mongolia

Operation

Figure 72.

First, black rubber mats are laid on the floor of the sluice box, with the ends of the mats either butted together or slightly overlapping, imbricated down-slope. About 6-20 riffles are welded to side bars to create a ‘riffle set’. In Mongolia the flat bar riffles are severely slanted at 30-45° to the sluice-box floor. Each set of riffles is slotted into the sluice-box and bedded down on the black rubber mat. The riffle sets are secured by metal or wooden chocks. Generally the riffle sets are orientated with the riffles slanted down-sluice. This helps to stimulate vortices and shields metalwork from damage and abrasion from stones. For clean sands, a 30° slant is preferred. For clay-rich sands the slant may reach 45°. Sometimes the riffles are slanted upstream to act as a nugget catcher. Flat bar riffles are commonly used in conjunction with a Siberian-style PgSh wash-plant typified by violent surging and wide fluctuation in flow and density of slurry. Lacking a lip, a flat bar riffle is less able to guide sand into a vortex, and sand exits its vortex instead of being led into the next vortex. This “severely reduces the opportunity

FLAT BAR RIFFLES

Slanted flat bar riffles on ribbed rubber matting at a placer mine in the Zaamar Goldfield of Mongolia. (photo: Jeanie Barnett of GSA)

Flat bar riffles consist of flat metal bars inclined across a sluice box to trap black sand and gold. They have been popular for at least 70 years. 1980s tests in the Yukon, Canada Flat bar riffles on unbacked NomadTM matting were slanted at various angles to the sluice-box floor in tests by Randy Clarkson and Owen Peer [8]. When slanting 15° upstream, flat bar riffles choked with sand faster than if slanting 15° downstream. Slanting downstream produces a slower vortex with its eye closer to the centre, and the vortex launches material at a lower angle. Performance was inferior to Hungarian riffles.

for gravels and gold to enter the riffles” and “the turbulence… …destroys effective vertical segregation.” [8] Vortices cease after a few hours choked with sediment, yet washing continues for an 8-hour shift. Flat bar riffles are less strong than angle iron (Hungarian riffles) and more prone to bending.

1990s tests in Mongolia Slanted flat bar riffles on ribbed rubber matting were tested by a Soviet team led by Ms. Tsevel Delgertsoo in 1991-95 at four placer gold mines – about 25 tests in all. Each test consisted of panning to estimate the headgrade, measuring the volume washed in an 8-hour shift (1-2,000m3) and sampling tails every 15 minutes across the sluice. Careful panning was done in a gold room. Recovery was 60-70% for medium to large gold. Fine gold was not fully tested – it was often present but lost.

Figure 73.

Adoption by placer gold miners Slanted bar riffles on square-ribbed black rubber mats are the norm for placer gold mines in the former Soviet Union and Mongolia.

GOLD RECOVERY BY SLANTED FLAT BAR RIFFLED SLUICE – Mongolia tests

Recovery of placer gold by slanted flat bar riffled sluice on ribbed rubber matting, tested by Mrs. Tsevel Delgertsoo. (compiler: Robin Grayson)

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Angle-iron (Hungarian) riffles – 1980s research in Canada

Operation

Figure 74.

To comply with the Yukon tests, the sluice-box is tilted at 1-m fall per 4-m length of sluice to ensure the angle iron riffles can generate large distinct vortices. First a roll of unbacked NomadTM matting is cut to size and unrolled on the floor of the sluice box. If several pieces of matting are used then their ends are closely butted together to avoid a ‘step’. Instinctively the NomadTM matting is laid with its smooth side downwards, but there is recent anecdotal evidence that putting the smooth side uppermost either makes no difference or is slightly better (source – Zooka of Alaska Gold Forum). About 6-20 riffles are welded to side bars to create a ‘riffle set’. For best results, the angle-iron riffles are 1-inch high with a ½ to 1-inch wide lip, tilted 15° upstream, and spaced 2 inches apart. Each riffle set is slotted in the sluice-box and pressed down on the NomadTM matting. The riffle sets are secured by metal or wooden chocks. The angle iron is positioned with one of its flat sides uppermost to act as a short slick plate and splitter to guide the bottom flow into the vortex. Its other flat side obstructs the flow to retain the vortex and trap heavies. Slurry is fed at 48.8m3/hour per metre width. A very large concentration ratio is possible (i.e. vast amounts of black sand are shed to produce a gold concentrate). The Yukon tests showed angle iron riffles maintain the captured black sand in a loose state for a long time, so continuing to be able to recover gold. This enables clean-ups to be needed only once every 24 hours.

ANGLE-IRON RIFFLES

A set of 1-inch riffled sluices being made ‘on the spot’ at the Sharin Gol mine of Polymet Potala Ltd in Mongolia The welder has ensured each riffle has a 15° tilt. (photo: Robin Grayson)

The origin of the term ‘Hungarian riffles’ is unclear and predates World War II. By the time of the Yukon tests [8] the term had become synonymous with ‘angle iron riffles’ set across the width of a sluice-box. 1980s tests in British Columbia and Yukon, Canada Lab tests using gold tracer in the University of British Columbia by James Hamilton and George Poling [7] showed angle-iron riffles if on unbacked NomadTM matting can recover >90% of >0.3mm gold, and 85% of 150μ gold, subject to control of the feed and the riffle size, angle and spacing. Tests in the Yukon by Randy Clarkson and Owen Peer [8] confirmed the findings, and included gold tracers, gold radiotracers, flume observation tanks and testing performance of sluice-boxes of placer mines [86-90].

Figure 75.

Adoption by placer gold miners Angle-iron riffles are the norm in North America and common in most of the world but not everywhere. In Siberia and Mongolia, inclined flat bar riffles is the norm. Many artisanal miners use basic ‘Hungarian’ riffles of wood.

GOLD RECOVERY BY ANGLE-IRON RIFFLES ON UNBACKED NOMAD MATTING – British Columbia tests

Recovery of placer gold in lab tests by James Hamilton and George Poling [7] (compiler: Robin Grayson)

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36 continued: Angle-iron (Hungarian) riffles – 1980s research in Canada

Figure 76.

ANGLE-IRON RIFFLES ON BACKED NOMAD MATTING – Yukon field tests

Figure 77.


Figure 78.


Recovery of placer gold by 2x2-inch angle-iron riffles @ 4-inch spacing, on backed NomadTM matting [86]. (compiler: Robin Grayson)

Recovery of placer gold by 3 x3-inch angle-iron riffles @ 6-inch spacing, on backed NomadTM matting [86]. (compiler: Robin Grayson)

Recovery of placer gold by 1½x2-inch angle-iron riffles @ 4-inch spacing on backed NomadTM matting [86]. (compiler: Robin Grayson)

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Expanded metal grating riffles – 1980s research in Canada

Operation

Figure 79.

To comply with the Yukon tests, the sluice is tilted at 50 to 106cm per 4-m length of sluice to ensure the grating can generated many small and distinct vortices. First a roll of unbacked NomadTM matting is cut to size and unrolled on the floor of the sluice box. If several pieces of matting are used then their ends are closely butted together to avoid a ‘step’. Instinctively the NomadTM matting is laid with its smooth side downwards, but there is recent anecdotal evidence that putting the smooth side uppermost either makes no difference or is slightly better (source – Zooka of Alaska Gold Forum). A sheet of expanded metal grating is cut to fit snugly in the sluice box, and secured by metal or wooden chocks. Several sections may be fitted into a sluice box, butted together with no overlap. Each equates to a ‘riffle set’. The grating is inserted with the raised lips facing upstream to serve as riffles. The riffles are “coarse” 4lbs/ft2 raised expanded metal grating identical to 4.0# grating of the Expanded Metal Manufacturers Association (EMMA) 'standards', downloadable: www.naamm.org/emma/literature.php. Expanded metal riffles achieve a very large concentration ratio (i.e. shed vast amounts of black sand to achieve a gold-rich concentrate), as do flat bar riffles and angle-iron (Hungarian) riffles. Expanded metal grating riffles can maintain the captured black sand in a loose state for a long time, so continuing to be able to recover gold. This enables cleanups to be needed only once every 24 hours.

EXPANDED METAL GRATING RIFFLES

Raised expanded metal grating suitable for using as large expanded metal riffles. (photo: Robin Grayson)

Expanded metal grating was invented in the early 1880s. The first innovative use of expanded metal as riffles in a sluice was by Robert Hodgson Postlethwaite, a British subject at the Risdon Iron and Locomotive Works in San Francisco. He applied for a patent in 1897, awarded 1900 (US #652,900). It was only in the 1980s that the effectiveness as riffles was proved by scientific tests. 1980s tests in Yukon, Canada Randy Clarkson and Owen Peer [8] tested relatively “coarse” 4lbs/ft2 expanded metal grating and finer 1-10H expanded metal mesh. In flume tests, both displayed, “similar deposition and vortex patterns” and the mesh developed “smaller and more numerous vortices”. They observed that the grating remained firmly in place whereas the mesh warped off the NomadTM matting causing “excessive scour”. The grating has to be ‘Raised (R) = Standard (S)’ and not ‘Flattened (F)’. According to Vincent Ruth of Continental Wire Cloth, “the applications that this product

Adoption by placer gold miners Raised expanded metal riffles of grating type are used worldwide by placer miners.

dominates would be used when designing something that requires a walking surface.”

Figure 80.

GOLD RECOVERY BY RAISED EXPANDED METAL GRATING ON BACKED NOMAD MATTING – Yukon tests

Recovery of placer gold by expanded metal grating, type 4lbs/ft2 (4.0 grating) on backed NomadTM matting [86]. (compiler: Robin Grayson)

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Expanded metal mesh riffles – 1980s research in Canada

Operation

Figure 81.

To comply with the University of British Columbia tests, the sluice is tilted at 50 to 106cm per 4-m length to ensure the mesh can generate many small and distinct vortices. First, a roll of unbacked NomadTM matting is cut to size and unrolled on the floor of the sluice box. If several pieces of matting are used then their ends are closely butted together to avoid a ‘step’. Instinctively the NomadTM matting is laid with its smooth side downwards, but there is recent anecdotal evidence that putting the smooth side uppermost either makes no difference or is slightly better (source – Zooka of Alaska Gold Forum). A roll of raised expanded metal mesh type 1-10H is unrolled and cut to fit the sluice box, and secured by metal or wooden chocks. Several sections may be butted together with no overlap. Each equates to a ‘riffle set’. The mesh is inserted with the raised lips facing upstream to serve as riffles. When unrolling the mesh, flatten it. Keep the sluice narrow to reduce warping. Clamping too tight may compress the NomadTM matting and warp the mesh. Tying the mesh to the floor of the sluice-box inhibits warping but prolongs cleanups; quick release bolts are better. Expanded metal riffles achieve a very large concentration ratio (i.e. shed vast amounts of black sand to achieve a gold-rich concentrate), as do flat bar riffles and angle iron (Hungarian) riffles. Expanded metal 1-10H mesh can maintain the captured black sand in a loose state for a long time, so continuing to be able to recover gold. This enables cleanups to be needed only once every 24 hours.

EXPANDED METAL MESH RIFFLES

Raised expanded metal mesh suitable for using as small expanded metal riffles. (photo: Robin Grayson)

Expanded metal mesh seems to have been used for riffles a little later than grating. It was only in the 1980s the effectiveness of mesh was proved by scientific tests. 1980s tests in British Columbia, Canada James Hamilton and George Poling [7] tested 1-10H expanded metal mesh. The mesh is ‘Raised (R) = Standard (S)’, not ‘Flattened (F)’. The riffles are 1-10H raised expanded metal mesh identical to the 1-10H expanded metal mesh manufactured by Continental Wire Cloth Inc of Calgary. Their product 110H has not changed since the early 1980s according to Vincent Ruth, see: www.cwcloth.com/expanded.htm. Of concern is the susceptibility of 1-10H mesh to lose gold by the mesh warping to permit scouring of the matting beneath. Even a small surge is likely to cause gold losses for, as pointed out by Randy Clarkson and Owen Peer [8] the live sorting crescent is so shallow it is vulnerable to being ejected. The cause of scouring is due to the ease of warping of the mesh, plus two variables: ² ²

Adoption by placer gold miners Expanded metal riffles of mesh worldwide amongst placer gold miners.

a surge of water, due to too much or too little water; and/or a surge of changed slurry, due to too much or too little solids.

Figure 82.

are

popular

GOLD RECOVERY BY RAISED EXPANDED METAL MESH ON UNBACKED NOMAD MATTING – B.C. tests

Recovery of placer gold by expanded metal mesh, type 1-10H on unbacked NomadTM matting. (compiler: Robin Grayson from Poling and Hamilton [7])

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38 continued: Expanded metal mesh riffles –research in Canada and USA

Figure 83.

GOLD RECOVERY BY SMALL SLUICE WITH DIFFERENT MATTING – Zooka Tests

Figure 84.

GOLD RECOVERY BY RAISED EXPANDED METAL MESH ON BACKED NOMAD MATTING – Yukon tests

Figure 85.

GOLD RECOVERY BY RAISED EXPANDED METAL MESH ON BACKED NOMAD MATTING – Yukon tests

The superiority of NomadTM matting in recovering placer gold compared to close weave matting. (compiler: Robin Grayson)

Recovery by expanded metal mesh (type 1-10H ?) on backed NomadTM matting. (compiler: Robin Grayson from Clarkson 1989 [86])

Inability of expanded metal mesh (type 1-10H?) to retain medium-coarse placer gold. (compiler: Robin Grayson from Clarkson 1989 [86])

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McCann’s small sluice – 1980s research in California

Operation

Figure 86.

Feed is best screened at 2.5 to 5mm, although the manufacturer says no pre-screening is required. Pay gravel is fed by trowel either dry or wet into the feed hopper and the device will process about 136 kilos/hour of solids. Rather than the wash water being added from above, the wash water is added from three holes in the rear of the hopper. The resultant slurry passes through a static screen in the base of the hopper with shaped holes encourage a steady outflow. Upon escaping from the hopper, the slurry encounters wash-water flowing over a deflector plate. The slurry and wash-water flow over a perforated plate that has numerous small holes through which dense fine particles fall into a quiet zone of slowly moving water that is where most of the fine gold tends to be caught. Larger particles pass down the main sluice lined with rubber vgroove riffles capture 90-98% of the visible gold, assisted by a section of fine expanded metal mesh and a single “shaped riffle”. The manufacturer recommends adding a heavy duty HFBE vibrator to assist recovery of fine gold. Tailings water flows into a filter bag at the end of the sluice that retains the tailings and the cleaned water fills a heavy duty 5-gallon water bucket. From here the water is recirculated many times an hour by a small submersible pump (capacity 1.89m3/hour) requiring 1.5 amps at 12 volts allowing 20-30 hours use on a car battery.

McCANN’s SMALL SLUICE

General arrangement, details omitted. (drawing: Robin Grayson)

McCann’s sluice was invented by John C. McCann of California and patented in 1985 (US #4,525,270). This was among the most outstanding innovations to the sluice made by recreational miners in North America and elsewhere who, since sometime before the 1970s, have been making incremental improvements to their small sluices in an effort to recover more fine gold. By the 1980s the innovative surge had become quite remarkable. McCann’s sluice is a complete wash-plant satisfying a litany of wishes of recreational miners: ² ² ² ² ² ² ² ²

small, lightweight, portable device; minimises water use by recycling; minimises energy consumption; has a good concentration ratio of 1,000:1; has an adjustable slope; maintains a steady flow; recovers 90-98% of visible gold; and recovers 60-80% of gold particles as fine as 30-40μ.

McCann’s sluice removes technical justification for mercury. Yet in over two decades since the device was patented and thousands sold to recreational gold miners worldwide, the device has been overlooked by researchers using public funds in efforts to improve large gold sluices for companies [90,91] and gold sluices for artisanal miners [21,26,92].

Figure 87.

Adoption by placer gold miners McCann’s wash-plant is made by Micro-Sluice Gold Products of Wisconsin, USA (www.micro-sluice.com) and marketed as the Micro-Sluice, with over 3,700 units sold in 16 countries over the last 20 years. The device is popular with recreational gold miners and has potential for artisanal miners especially in arid regions.

GOLD RECOVERY BY McCANN’s SMALL SLUICE

Recovery of placer gold by McCann’s small sluice, according to the original patent. (compiler: Robin Grayson)

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Damn Fine SluiceTM (DFS) – 1990s research in New Mexico

Operation

The Damn Fine SluiceTM (DFS) was invented by Phil Hontz of New Mexico in the 1990s. The device was never patented as it had been discussed widely on internet forums and is a direct descendant of earlier innovations based on the research on larger sluices in the Yukon tests by Owen Poor and Randy Clarkson [8] and more particularly the sluices tested by James Hamilton and George Poling [7] that had raised expanded metal mesh riffles on unbacked NomadTM matting (miner’s moss). These sluices achieved >90% gold recovery down to about 150μ nominal diameter. The Damn Fine SluiceTM is a considerable advance on these earlier sluices in being able to recover >90% gold recovery down to about 50μ nominal diameter, although rigorous testing does not appear to have been done. The DFS is manufactured by the Damn Fine Equipment Co (www.damnfinesluice.com). This is part of relentless effort by recreational miners in North America to recover fine gold using small sluices for small dredges, high bankers and clean-ups. The Damn Fine SluiceTM consists of a smooth slick plate followed by a section of tiny raised expanded metal mesh fitted on matting, all set in a short sluice-box. The unit is about 1.19m long and 25cm wide, and weighs 3.63 kilos. It includes a pair of adjustable legs to aid setting up. The DFS is an in-stream sluice, with a flared intake (‘fixed wings’) to help funnel water into the mouth of the sluice and to aid stability. The first section is a long slick plate to encourage laminar flow to guide heavy particles into a section of tiny raised expanded metal riffles clamped on “heavy duty” unbacked NomadTM matting. The DFS, being an in-stream sluice, lacks a hopper and screen, and has no means of recirculating water. Nor is it designed to catch gold nuggets.

Figure 120.

The DFS is an in-stream sluice, positioned in shallow fast-flowing water. If necessary rocks are arranged to form a temporary dam or weir to ensure flow is adequate and fast. The legs are adjusted to ensure the DFS is sloping downstream and yet is level across its width. Pay gravel is screened at about 2mm and the oversize discarded after checking for nuggets. The undersize is put on the leading edge of the slick-plate, a small quantity at a time. The pay gravel is swept through the sluice by the water current. After the small mound of pay gravel has been cleared by this means a fresh mound is added. The current causes the pay gravel to spread out across the width of the slick plate to assume laminar (non-turbulent) flow consisting of a bottom-hugging traction carpet of black sand overlain by a traction carpet of lights. The traction carpet of black sand is pulled into the vortices (rollers) of the riffles and the gold burrows into the underlying layer of NomadTM matting. The lighter minerals and surplus black sand are swept out as tailings. Eventually the NomadTM matting becomes hard packed. Clean-up is rapid, as the riffles are easily removed by turning the wing-nuts on the ends of bolts that secure them, and the matting is lifted clear. The riffles, matting and sluice-box are then flushed clean into a bowl or suchlike if needs be with a few drops of detergent added to founder any float gold. Later the contents of the bowl are cleaned by panning, tabling or other means.

Adoption by placer gold miners The outstanding success of the DFS in recovering fine placer gold led to its immediate and continuing popularity among recreational miners and has inspired many other devices such as the PopandSon sluice.

GOLD RECOVERY BY DAMN FINE SLUICETM - generalised

Recovery of placer gold by Damn Fine SluiceTM according to reports by many users. (compiler: Robin Grayson)

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PopandSon sluice – 2000s research in USA

Operation

Figure 146.

POPANDSON SLUICE

Figure 147.

RANGE OF RIFFLE SIZES

Initial testing with chunky tungsten powder as tracer suggested by Steve Bryce (Zooka of AGF) indicated high percentage gold recovery at fast flows and feeds. Using 100-200 mesh placer gold tracer run over 88.9cm of style #3/16 raised expanded mesh clamped on unbacked NomadTM matting resting on ribbed rubber mat and sloped at 16mm per metre length, achieved nearly 100% recovery. At a steeper slope of 36.5mm per metre, gold recovery was 95-97%. The steeper slope allowed a much higher feed rate. The test sluice has two sections of sluice liner, each about 43cm long. The top section is the primary test bed, and the end section is to scavenge gold in the tailings of the top section. For recovery of 100-200 mesh gold (74149μ) the best performance of the top section was 9294% using style #3/16 expanded metal mesh, but fell to 86% using style #1/2 expanded metal mesh. For 200-325 mesh gold (44-74μ) at a gentle slope of 16mm per metre length, the recovery was only 65% in the top section but 85% for both sections. Steve and Jason Gaber suggest a “fairly large drop-off in

Carrying a standard over-the shoulder PopandSon sluice in the Gobi Desert of Mongolia. (photo: Robin Grayson)

The large, medium and tiny raised expanded metal mesh in a ‘standard’ PopandSon sluice. (photo: Robin Grayson)

The PopandSon sluice was invented in 2005 by Steve and Jason Gaber (PopandSonminers of Alaska Gold Forum AGF) in Washington State. The PopandSon sluice is a development of the innovative Damn Fine SluiceTM (DFS) invented by Phil Hontz of New Mexico and made by the Damn Fine Equipment Co (www.damnfinesluice.com) in the 1990s. The DFS is a simple cheap device consisting of a smooth slick plate followed by a section of tiny raised expanded metal mesh fitted on matting, all set in a short sluice-box. This is part of relentless effort by recreational miners in North America to recover fine gold using small sluices for small dredges, high bankers and clean-ups. Steve and Jason Gaber conducted bench tests with tungsten (W) powder and gold (Au) tracers [193] in a version of the PopandSon sluice consisting of an aluminium sluice-box lined with simple thin ribbed rubber followed by unbacked NomadTM matting with style #3/16 aluminium raised expanded metal mesh fitted on top.

Figure 148.

recovery efficiency somewhere below 200 mesh (44 microns)”. Experimental work is limited to narrow sluices (6.5 inches = 165mm) for recreational miners and clean-ups. Yet the PopandSon sluice might be scaled-up for industrial wash-plants if four issues are solved: a) screening feed to about 2mm; b) preventing warping; c) preventing surging; and d) reducing water – for each m3 of loose placer water usage is very high – 19.6m3 for steep angle.

Adoption by placer gold miners Expanded metal riffles of tiny mesh are increasingly popular amongst recreational placer gold miners, and have potential for artisanal miners and mining companies.

GOLD RECOVERY BY POPANDSON SLUICE

Recovery of placer gold by the PopandSon sluice based on bench tests with gold and tungsten tracers [193]. (compiler: Robin Grayson)

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Loewen electrostatic sluice – 2000s research in Alberta

Electrostatics has been found to be useful in assisting gravitational recovery of placer gold for over a century by means of drywashers and related waterless equipment. Little has been published on the gold recovery achieved, other than generalised comments that dry methods usually recover less than wet. Electrostatics has received little attention in waterbased gravitational recovery of gold. Yet there is a significant amount of anecdotal evidence that electrostatics are helpful, particularly in wet recovery of fine placer gold. Two examples are noted below. ‘Differential Charging Recovery Systems’ (DCRS) were invented by Robert Barefoot of Calgary and patented in 1990 (US #4,975,182). DCRS turning screened pay gravel into watery slurry in which a positive electrostatic charge was induced in the water droplets and gold particles by subjecting the slurry to high-velocity spinning in a cyclone-like surge tank. Then the positively charged gold particles encounter negatively charged surfaces and are forced to settle by the strong force of electrical attraction and the relatively weaker force of gravity. The patent claimed “highly efficient recovery of the invisible gold (less than 320 mesh)” using a 180 tons/day mobile test unit. DCRS is somewhat dauntingly complicated and one version depends on mercury amalgamation. The system failed to be commercialised for many reasons, and some are noted at www.barefootscureamerica.com. The Loewen electrostatic sluice was invented by Wayne W. Loewen of Alberta and patented in 2006 (US #7,012,209) and is refreshingly simple. Gold is recovered in a wet sluice lined by ribbed plastic (e.g. polyvinyl chloride PVC) than is positively electrostatic when immersed in water, and therefore catches negatively electrostatic fine gold particles by a combination of electrostatic attraction and gravitational settling.

Figure 145.

Operation This text is based on the Loewen electrostatic sluice as presented in US patent #7,012,209. Placer pay gravel is first screened to say 15mm and gold recovered by sluices, jigs or similar gravitational devices. Tailings are screened <1mm and fed into the feed hopper of the Loewen electrostatic sluice. Alternatively milled hardrock ore is screened <1mm and fed directly into the feed hopper of the Loewen electrostatic sluice. The device consists of a simple inclined gravitational sluice, typically “ten feet in length and is four inches wide

with one-and-one-half-inch high sides.” The inclined sluice is lined with “a material which incurs a positive charge when immersed in water, especially water having a pH value between 4 and 8.” The patent suggests vinyl (PVC) to be a suitable material. The material has transverse ribs that serve as riffles and the gold is trapped in the intervening grooves. Water is added to the feed to make it very thin slurry, “a good ratio would be nine parts water to one part [screened feed].” Feeding the sluice too quickly with slurry would cause the grooves to plug. The patent does not specify the preferred flow rate at which “the gold can be observed

settling out during the process, as most of the gold will settle out in the first three feet of the sluice.” After the batch feed has been exhausted, the sluice is allowed to run clean. Then the contents are flushed into a ‘clean-up pail’ and its contents are allowed to settle and the water decanted to leave a rich gold concentrate.

Adoption by placer gold miners The Lowen electrostatic sluice is very new and has yet to be marketed to placer gold miners.

GOLD RECOVERY BY LOEWEN’s ELECTROSTATIC SLUICE

Recovery of placer gold by Loewen electrostatic sluice according to the patent and diverse assumptions. (compiler: Robin Grayson)

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Cleangold® sluice with magnetic riffles – 1990s research in Oregon

Operation

Figure 121.

The Cleangold® sluice creates a fluidised bed of black sand held in position by complex magnetic fields trapping a carpet of magnetite. Normal riffles are redundant, substituted by a set of corduroy-like ribs of magnetite held by the magnetic fields. After a few minutes, the magnetic fields attract and hold on the otherwise smooth floor of the Cleangold® sluice a carpet of magnetite particles from the black sand. If magnetite is rare, Cleangold LLC recommends a little black sand is brought from elsewhere to fire up the sluice. Gold particles are actively trapped by the fluidised bed – not by the magnetic fields but by the fluidised bed being a thixotropic carpet in which heavy particles such as gold are trapped and burrow down by gravity alone. Cleaning the sluice takes only a few seconds using a plastic scraper to scrape the concentrate into a plastic bin. Care is needed to decide when the sluice needs to be cleaned. Cleaning too frequently renders excessive the further upgrading required, and cleaning too infrequently risks the sluice being over-full of gold and other extremely heavy minerals whereupon its effectiveness may suffer.’ It is clear the Cleangold® sluice can recover at least 95% of gold present, including most of the very fine gold. It appears to be superior to mercury in recovering very fine gold, and appears capable of recovering a significant proportion of extremely fine gold. Positive comments have been made in several independent reports.

CLEANGOLD® SLUICE

A Cleangold® sluice, after attracting magnetic particles to create corduroy-like ridges that trap fine gold. (photo: Robin Grayson)

The Cleangold® sluice was invented by David Plath of Oregon and patented in 1999 (US #5,927,508). The Cleangold® sluice uses magnetic strips embedded in a non-magnetic rubberised sheet inserted in a plain aluminium sluice to attract, hold and accumulate ferromagnetic minerals in corduroy-like ridges that serve as riffles capable of trapping very fine gold. Overall the Cleangold® sluice is a low-cost, highly efficient appropriate technology solution to upgrading gold and is capable of recovering much fine gold lost by panning, sluicing and amalgamation [22,23,169-173]. Several different versions are produced by Cleangold LLC equating to a gold pan, a trough and a sluice insert (www.cleangold.com). Tests briefly mentioned in the paper by Lars Hylander and David Plath [169] claim 60-70% recovery of gold “down to 0.005mm” at first pass, and in a second pass a recovery of “a further 60-70% of the remaining fines”. This suggests four permutations for the overall result – 84%, 88%, 91% and 98% recovery.

Figure 122.

Adoption by placer gold miners The innovative Cleangold® sluice has great potential for placer gold recovery by artisanal miners, recreational miners and by mining companies. The equipment is new and is currently penetrating artisanal markets in Surinam [22], Guianas [23] and Philippines [36, 170,171].

GOLD RECOVERY BY CLEANGOLD® SLUICE - generalised

Recovery of placer gold by Cleangold® sluice according to tests reported by Hylander and Plath [169]. (compiler: Robin Grayson)

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Hydraulic riffles – 1980s research in New Zealand and Canada

Operation

Figure 88.

Washed pay gravel screened at 25mm is fed into the sluice box at a rate of about 25m3/hour of solids. Higher throughput is by two or more sluices in parallel. The slurry first enters a boil box that serves as a nugget trap, where the trap shape ensures turbulence to prevent clogging with fines, assisted by 9-13.5m3/hour of clean water injected at 15-35kpa pressure from a manifold. Exiting the boil box, the slurry passes over a slick plate to calm the slurry and engender laminar flow to allow the slurry to stratify with heavies concentrating near the bottom. After the slick plate the stratified slurry crosses the first set of hydraulic riffles where 32-48 m3/hour of clean water is injected 15-35kpa pressure from a manifold into the black sand to maintain a fluidised bed that traps the gold. The hydraulic riffles rest on unbacked NomadTM matting to assist capture of gold. The slurry continues over a second slick plate that encourages more density stratification and passes over the second set of hydraulic riffles to recover more gold. Finally the slurry passes over an end section of raised expanded metal mesh that catches gold from the tails and helps to verify that the system is functioning properly.

HYDRAULIC METAL RIFFLES

A set of hydraulic metal riffles showing the manifold supplying the elutriation water under pressure. This set was manufactured by PAuSE Ltd in New Zealand. (photo: Robin Grayson)

Hydraulic riffles appeared over a century ago. In their modern form they began in New Zealand in the 1970-80s. Lindsay Guy Herron of Queenstown invented compact hydraulic riffles patented in New Zealand (NZ #216,327, filed 1986, lapsed 1997) and the United States (US #4,863,588, awarded 1989, lapsed 1993). A typical sluice box for hydraulic riffles is short and wide, contrasting with the long and narrow sluice box for conventional riffles. Hydraulic riffles inject pressurised clean water into the black sand from below. This keeps the bed loose and fluidised indefinitely, so gold particles continue to fall into the black sand and accumulate. Randy Clarkson noted “…unlike conventional riffles

Adoption by placer gold miners Hydraulic riffles are popular in New Zealand, and have spread to the Yukon (Canada), Alaska (USA), Mongolia, South America, Australia and doubtless elsewhere. In Mongolia, the adoption of hydraulic riffles has been slow, but of the 200 or so wash-plants more than a dozen now have hydraulic riffles including: Cold Gold Mongolia Ltd (New Zealand); Ochir LG Ltd (BritishDutch-Mongolian www.ochirlg.com); Gatshuurt Ltd; Jump Ltd; Gazar Holdings Ltd; G&U Gold Ltd, and Barmash JSC.

that rely on the formation of vortices, hydraulic riffles rely primarily on the settling velocity of gold” [86-90]. In the Yukon tests, hydraulic riffles performed well at “extremely low feed rates” and “low water flows” but at high feed rates are “only suitable for coarse gold recovery (nugget traps)”. The Yukon tests on hydraulic riffles were limited and the author is unaware of any other tests. They are unlikely to be as efficient as tuned conventional sluices in recovering fine gold, but have the overriding advantage of compactness for ease of low-cost mobile mining.

Figure 89.

GOLD RECOVERY BY HYDRAULIC RIFFLES WITHOUT MATTING – Yukon tests

Recovery of placer gold by hydraulic riffles one of NZ-style, the other unknown. (compiler: Robin Grayson from Clarkson 1989 [86])

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Simple jigs – 1960s-1970s research in China

Operation

Figure 31.

The pay gravel is first disaggregated, size-sorted and oversize rejected in a screening plant. The slurry feed passes across the jig bed that rests on the jig screen. At the same time, water erupts through the holes in the jig screen from the hutch below. The water is pushed up by some means, usually a rubber diaphragm inserted in the hutch as a pulsator – pushing up (upstroke) and sucking down (downstroke). The drive is significant, either hydraulic or mechanical. On the upstroke, the erupting water intermingles with the jig bed, and causes all of the jig bed to be ‘jigged’ – the steel balls may slightly rise and fall, but the layer of smaller particles on or near the jig bed are pushed upwards – allowing Stokes Law to operate. The smaller particles become sufficiently agitated to become a fluidised bed like quicksand – the thixotropic state. The loosened heavies fall rapidly to burrow into the protective jig bed, while lights are swept away as tailings. On the downstroke, water is pulled downwards by suction, and the upper part of the jig bed becomes a hard layer – the dilatant state. The suction plus gravity pulls dense particles down to the bottom of the jig bed where coarse gold and gold nuggets accumulate as ‘jig bed concentrate’ awaiting cleanout during batch discharge. Finer gold is flushed through the jig screen into the bottom of the hutch to be tapped off the bottom as ‘hutch concentrate’ continuous discharged via a spigot.

SIMPLE JIGS

Overhead view of a conventional 2x4 cell rectangular jig. (drawing: Robin Grayson, adapted from Nio 1978 [55])

A simple jig consists of a square jig cell comprised of a lower water-filled chamber (hutch) covered by a jig screen above which slurry is introduced. Resting on the jig screen are large heavy particles (e.g. steel balls) that constitute the jig bed. Small square jigs are often arranged in series (to increase recovery) or in parallel (to increase capacity). A simple square jig is typical of most jigs in exhibiting a mix of continuous discharge of fine gold and batch discharge of coarse gold. Although easy to build and simple to operate, simple square jigs are unsuitable for efficient placer gold mining: ²

²

²

the jig’s footprint is large, demanding too much space on dredges where space is a premium, and too bulky to easily make into a mobile land-based processing unit; water consumption is high to very high – a serious problem for land-based units if water is scarce and demanding large tailings ponds for water storage and recirculation; and high % recovery of fine gold recovery is difficult to achieve.


In China in the 1960s-70s experiments by placer scientists of the Minerals Processing Laboratory of the Kunming Institute of Metallurgy in Yunnan Province determined the percentage gold recovery of simple jigs [27]. Gold recovery falters at 0.8mm, is only 90% by 0.3mm, and collapses to 50% at 0.1mm.

Figure 32.

Simple square jigs used to be fairly popular in placer gold mining, particularly in wash-plants on-board dredges, but have virtually disappeared with the advent of more modern jigs, although a few are seen in remote mines and are occasionally used for upgrading concentrates.

GOLD RECOVERY BY SIMPLE JIGS – China tests

Poor performance of simple jigs [3,27]. (compiler: Robin Grayson)

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Helix wheel (gold wheel) – 1900s research in Colorado

Operation

Figure 39.

Using the Little CamelTM as an example, a gold wheel is a rotating tilted pan with spiral ribs (‘riffles’) on its upper surface. The Little CamelTM has seven spirals but other makes vary from one to seven. A cup of concentrate is added every 10-15 seconds to a point on the gold wheel. Water is supplied by a header tank or 12-volt submersible pump, and added via a perforated pipe to gently flush concentrate across the face of the pan. Gold particles are trapped by the riffles and migrate to an exit hole in the centre for recovery. Tailings are discharged over the lip of the rotating pan. Rotation is by a 12-volt motor, adaptable to a car battery. The speed is critical and can be controlled between 15 to 22 rpm. The best gold wheels are of moulded polypropylene plastic for lightness and smoothness. The 7-spiral Little CamelTM has a wheel 16.5 inches across. To enhance gold recovery:

HELIX WHEEL

Avista geologist using a gold wheel in near-freezing conditions. (photo: Avista Ltd of Bishkek)

The helix wheel (gold wheel), commonly called a spiral panning machine, is a flattened Archimedes screw with the helix no longer turning inside a cylinder but spiralling smaller and smaller to a central discharge hole. The gold wheel was invented by Henry Earle of Denver and patented in 1911 (US #987,866). Some gold wheels are designed to recover fine gold, whereas others focus on maximising throughput. By arranging two to six gold wheels, Johnny Hilmer Kleven of California invented a method of increasing the washing capacity to 1.5-2.0 tons/hour, patented in 1977 and 1978 (US #4,008,152 and US #4,110,206), and he later invented a single multi-step wheel that accomplishes the same (US #4,267,036). Industrial-scale gold wheels appeared in the 1980s led by PMX Industries [61], and Keith B. Cleland was awarded patents in 1983 and 1984 for solving how to build large wheels with 60-100 helical riffles converging on a single central port (US #4,389,308 and US #4,406,783). Production of large gold wheels ceased after a decade.

Figure 40.

² ² ² ² ²

pre-screen the concentrate to <0.6 mm; for fine gold, pre-screen to 0.15 to 0.30 mm; if it clogs with black sand, add a teaspoon of normal sand; material must be limited to keep it fluidised; and to cut surface tension, add a little Cascade or Jet-Dry antispotting agent (not detergent).

Adoption by placer gold miners Gold wheels are popular worldwide with recreational miners, artisanal miners, prospectors and companies for upgrading concentrate. Many makers exist in the USA and in South America, Africa, China and Russian Federation. Yet the gold wheel’s popularity is uneven and the reason unclear. The modern small portable gold wheel was invented by Angus Nicholls in the USA and his Little CamelTM gold wheel is still made by Camel Mining Inc (www.desfox.com).

GOLD RECOVERY BY GOLD WHEELS – generalised

Recovery of placer gold by helix wheels (gold wheels) [61] (compiler: Robin Grayson)

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Brosseuk’s helix cylinder – 1980s research in British Columbia

Operation

Figure 111.

Feed is introduced as slurry via a slurry pipe inserted at the front end of the helix cylinder. The pipe discharges about midway along the cylinder, the slurry landing on the helical riffles. Heavies are caught in the grooves between the riffles and being inside an Archimedes screw are carried up the length of the helix by the rotation of the riffles to the lip of the cylinder where they disgorge as a continuous discharge of concentrate. In contrast, the lighter particles and wash water override the riffles to continue along the floor of the cylinder to emerge as a continuous discharge of tailings.

THE GOLD MACHINE

One of the smaller portable helix cylinders invented by Raymond Brosseuk, ideal for prospecting and evaluating deposits. (photo: courtesy of www.extrac-tec.com)

Brosseuk’s helix cylinder was invented by Raymond Brosseuk of British Columbia, patented in 1992 (US #5,108,584) and marketed as ‘The Gold Machine’ and has been the most successful helix cylinder for use in largescale placer gold mining. Externally a helix cylinder resembles a scrubber (blind trommel). Both are long cylindrical drums tilted to cause slurry fed in at the raised end to discharge at the lower end. But the interior of a helix cylinder is lined by transverse riffles that are helical, each spiralling round and round along the length of the cylinder to produce an Archimedes screw [159]. Brosseuk’s helix cylinder followed earlier innovations during the 1970s and 1980s, such as the PMX helix and the TRI-R helix, but little has been published apart from the review by Michael Silva [61]. Good examples include the truck-mounted helix cylinder patented by Loyd Harris of Oregon in 1979 (US #4,178,238) and a helix cylinder patented by Richard and Isabelle Tice of Washington State in 1982 (US #4,339,043). A helix cylinder can vary from a small cylinder (1ft diameter and 5ft long) for upgrading of concentrates to a large cylinder (8ft diameter and 40ft long) as a rougher.

Figure 112.

Adoption by placer gold miners Tests of helix cylinders in the 1980s in North American placer mines [61] later led to some uptake worldwide. Today several types of large helix cylinders are made in North America for placer gold recovery, notably: ²

² ²

The Gold Machine (Brosseuk’s helix cylinder), precursor of the HPC helix belt – www.extrac-tec.com Gold ClaimerTM Rotary Riffle – production unclear Golden Boy Rotary Separator – www.goldenboyinc.com

Miniature helix cylinders gained and retain a niche in the recreational mining market as small-scale placer gold recovery units: ² ² ²

Dixie Doodlebug – famous machine, production ceased Mountain Goat – www.desfox.com [160] Gold Screw – www.goldscrew.com

GOLD RECOVERY BY BROESSEUK’s HELIX CYLINDER

Recovery of placer gold by Brosseuk’s helix cylinder, according to the original patent. (compiler: Robin Grayson)

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Helix belt – 2000s research in Canada and USA

Operation

Figure 153.

Pay gravel is dumped in a hopper that feeds a scrubbing/screening trommel that is an integral part of the Extrac-TEC HPC wash-plant. Screening is at 6mm, 13mm or 25mm. The washed undersize is fed as slurry to the helix belt. The helix belt rotates as a reverse helix that functions as an Archimedes screw. The helix belt is tilted at a gentle angle sufficient to ensure water and lights fed onto it are washed down-slope. Water and lights travel down the central valley of the helix belt by spilling over each rib (riffle) sequentially. Arriving at the bottom end of the helix belt, the lights discharge as tailings. The belt’s motion and water flow cause heavies to settle on the helix belt. Any heavies escaping over a rib are trapped and re-processed by the next rib. Once settled on the helix belt, heavies are inexorably hauled up the slope by tangential motion of the ribs (riffles). Arriving at the top end of the helix belt, the heavies continuously discharge into the concentrate sluice. The bed-profile, inclination, characteristics of the reverse helix, water flow and belt speed are configured to produce the solids density desired (0.1 to 40% by weight). The helix belt ensures concentrate fed to the sluice is properly pre-concentrated, and prevents surging and so flow is steady down the sluice and optimized for gold recovery. This reduces water consumption, and the manufacturer claims that gold recovery is increased “down to 40 microns”. A high concentration ratio is achieved and so the final concentrate is reasonably clean.

HPC HELIX BELT

Material ascending the helix belt of the HPC-10 wash-plant. (photo: courtesy of Extrac-TEC www.extrac-tec.com)

The Extrac-TEC HPC helix belt is called by the makers a “transverse spiral concentration belt”. The device is a highly innovative form of Archimedes screw, unique to the Extrac-TEC HPC systems. The helix belt, helix cylinder and helix wheel (gold wheel) are three different classes of Archimedes screw used for heavy mineral separation. The helix belt has ribs that serve as riffles arranged in a helix. The profile of the helix belt is sagged into a bow-shaped gutter-like trough that is gently tilting to drain the tailings down the trough, whereas the settled heavies are hauled up the trough by the moving riffles. Development began in 1986, the first prototype was completed in 1988, and the first patent was granted in 1993 according to the company website. “…independent analyses showed recovery efficiency close to 95%”. From 1999-2002 the company focused on using the HPC technology for its own placer operations in Canada such as in Anderson Creek in the Yukon. According to the company (www.extrac-tec.com) fresh patents were filed in 2003 for the more advanced ‘Generation-2’ version using the helix belt.

Figure 154.

Adoption by placer gold miners The Extrac-TEC HPC wash-plant with its helix belt has become widely available over the last few years, and some are operational in most continents, and early versions were sold in Canada, China, Mexico and USA.

GOLD RECOVERY BY THE EXTRAC-TEC HPC HELIX BELT – generalised

Recovery of placer gold by the Extrac-TEC HPC helix belt, according to information from the manufacturer. (compiler: Robin Grayson)

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Wilfley shaking table – 1890s research in Colorado

Operation

Figure 41.

The points of the compass are used for clarity in describing the operation of the Wilfley shaking table. The feed is screened to <3mm and fed into a small hopper above the north-east corner of the shaking table, where it is mixed with clean water. The resulting slurry is introduced to the north-east corner of the shaking table and begins to spread southwards as a thin film. The feed fan outs towards the edge of the table, allowing the operator to see exactly what is happening, and to decide where to subdivide the fan into distinct streams each dominated by a particular mineral. The shaking motion has a slow westward stroke and rapid return eastward stroke – often with a bump. This induces settled particles to crawl in a juddering manner westward along the table with the thin film of slurry. The shaking is usually very rapid with a frequency of 4 to 5.5 strokes per second. The shaking displacement is usually half to one inch to-and-fro. A set of low riffles aligned east-west guide the heavies ever westward to fall off the south-west corner of the table into a hopper as a continuous discharge. Meanwhile, a spray bar introduces clean ‘wash water’ along the north edge of the table, sending a thin film of clean water southward to encounter the riffles and the westward flowing slurry. The wash water mixes with the slurry and overrides the riffles taking the lighter particles with it to spill over the southern edge as a continuous discharge of tailings.

WILFLEY SHAKING TABLE

A Wilfley shaking table made in Australia. (photo: courtesy of the manufacturer, Motive Traction Pty Ltd - www.motive-traction.com.au)

The Wilfley shaking table was invented by Arthur R. Wilfley of Denver, Colorado and patented in 1897 (US #590,675). The device proved enormously popular being able to consistently recover fine particles of dense minerals and with a high concentration ratio. Many thousands of Wilfley tables were made and are still manufactured. Dozens of variations emerged, such as the Deister table [61] patented by William F. Deister and Emil Deister of Indiana (US #1,642,843). Today shaking tables are made in the USA, UK, Australia, Russia, China, Thailand and elsewhere. Shaking tables are thin-film devices whereby heavy particles are induced to settle from a flowing film of slurry while light particles are washed away as tailings. The thinfilm needs a large surface area and therefore some sort of table shape is essential, ranging from small laboratory tables to production tables 7x15ft in size. A shaking table can recover >90% of gold from 3mm down to about 70μ, and still able to recover >70% of 50μ gold, and useful amounts of 30μ gold. Disadvantages include: low capacity, bulky size, high water usage, and need for having a stable.

Figure 42.

Adoption by placer gold miners Wilfley shaking tables remain popular amongst placer gold miners in many regions of the world. A key factor is that miners like to see the gold separating.

GOLD RECOVERY BY WILFLEY SHAKING TABLE – generalised

Recovery of placer gold by Wilfley shaking table, based on comments of the British Geological Survey [24]. (compiler: Robin Grayson)

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Shaking tables – 1960s-1970s research in China

Operation

Figure 43.

Typically the concentrate is screened at 3mm, and fed to the shaking table as either slurry or spooned into a small hopper on the corner of the shaking table where water is added to produce the desired slurry. Shaking tables operate as a thin-film separator, and a vast range of shaking tables existed by the 1970s. Decks are of wood or fibreglass; deck coverings include linoleum, plastic, rubber or fibreglass; riffles vary in height, width, spacing and orientation; shaking varies in amplitude, frequency, length, forward velocity and return velocity, and may by to-and-fro or orbital. Even for a particular shaking table there are many critical variables, such as the gold particles’ size, flatness, surface texture and purity; presence of other minerals (light or heavy) attached to the gold; nature of other particles present; dilution of the feed; the fineness of the feed; and the problem of any traces of oil or grease.

CHINESE SHAKING TABLES

Chinese-built shaking tables of traditional design recovering gold from slurry fed from a ball mill at a Chinese-owned hardrock gold mine in Bayanhogor Aimag in Mongolia. (photo: Robin Grayson)

For over a century shaking tables have remained popular in China as elsewhere for clean-up of concentrate from placer gold wash-plants and milled hardrock ore. In the 1970s, placer scientists of the Minerals Processing Laboratory of the Kunming Institute of Metallurgy in Yunnan Province determined the percentage gold recovery of conventional shaking tables [27]. It is unclear what type of shaking tables were tested or the size of the feed, but the results showed gold recovery starts to falter at 0.5mm and is only 90% by 0.2mm, and collapsed to 75% at 0.1mm. These results are disappointing, and a modern shaking table if operated carefully performs significantly better. However in the 1970s the observed performance of shaking tables in the China tests would have been considered acceptable, bearing in mind the chronic performance of simple jigs and sluices at that time.

Figure 44.

Adoption by placer gold miners Shaking tables are popular amongst placer gold miners in many regions of the world. A key factor for many miners is that they see the gold separating. They can be seen in action in Alaska, Yukon, Alberta, British Columbia, California, Central America, South America, Africa, Australia, New Zealand, S.E. Asia, China, Mongolia and the Russian Federation. Manufacturers of shaking tables are numerous and widely spread, for instance: ²

²

²

²

²

United Kingdom – Holman-Wilfley Ltd of England www.holmanwilfley.co.uk Australia – Motive Traction Pty Ltd Inc of New South Wales www.motive-traction.com.au USA – Outokumptechnology Inc of Florida www.outokumptechnology.com Thailand – Dove Engineering www.dovemining.com China – China National Gold Corporation (CNGC) www.chinagold.org.placer.html

GOLD RECOVERY BY TRADITIONAL SHAKING TABLES – China tests

Recovery of placer gold by traditional shaking table of uncertain type, results of tests in China [27]. (compiler: Robin Grayson)

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Shaking tables – 1960s research in the Soviet Union

Operation

Figure 45.

Government-funded placer scientists in the Soviet Union undertook intensive tests during the 1960s and 1970s on the ability of shaking tables to recover gold [62]. In one test, the Soviet scientists appraised a conventional Soviet-made shaking table, probably modelled closely on Wilfley or Deister tables, with four sizes of placer gold described as “laminar, lumpy” [62]. It is not quite clear what the English translation should be. The performance was less good than expected for placer gold by western users of shaking tables, with >90% recovery only possible for 150μ gold and larger. Recovery fell to only 80% for 90μ gold and under 60% for 50μ gold. In a second test, the Soviet scientists tested a conventional Soviet-made shaking table, with five sizes of placer gold described as “porous, acicular” [62]. Again it is not entirely clear what the correct English translation should be. The performance was poor, with >90% recovery only possible for 300μ gold and larger. Recovery fell to only 80% for 150μ gold, 55% for 100μ gold and under 35% for 50μ gold. It is unclear what the parameters of the Soviet tests were, rendering it impossible to interpret the results. For instance the feed is assumed to have been screened at 3mm as is standard practice, but if screened at say 5mm then performance would have been compromised. The results cast doubt on the ability of shaking tables to perform well at recovering fine placer gold in industrialscale operations, and demonstrate the adverse effect on recovery if the gold particles are porous or flat.

SOVIET SHAKING TABLE

Shaking table of traditional Soviet design at the Sharin Gol Mine of Polymet Potala Ltd in Mongolia. (photo: Robin Grayson)

In spite of the popularity of the Wilfley shaking table and its derivatives, little has been published their ability to recover gold of different size or flatness. Many reports affirm shaking tables are “effective” or “successful” in recovering gold of this or that size or shape, yet rarely mention the amount of gold lost. Studies on the ability of shaking tables to recover tin (cassiterite SnO2) led people to assume a shaking table would perform better with gold; gold being so dense. This assumption is dubious, for gold, especially placer gold, is often leached and porous so its density is reduced; gold is markedly hydrophobic making it prone to float on a shaking table; and gold is often so flat its settling velocity is less than expected. Conversely, cassiterite is typically not leached, not porous, not hydrophobic and not flat. Only in the Soviet Union and China does it seem proper scientific tests were conducted on the ability of traditional shaking tables to recover gold, and later by the British Geological Survey [24].

Figure 46.

Adoption by placer gold miners Shaking tables are popular amongst placer gold miners in many regions of the world. A key factor for many miners is that they see the gold separating.

GOLD RECOVERY BY TRADITIONAL SHAKING TABLES – Soviet Union tests

Recovery of placer gold by Soviet shaking tables in the Soviet Union. (compiler: Robin Grayson, after Zamyatin and Konyukova [62])

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BGS shaking table – 1990s research in United Kingdom

Operation

Figure 131.

The BGS shaking table is compact and light enough to be carried over the shoulder by a strong individual. The table is manually driven, using bicycle gears and chains plus rubber bands made from car tyre inner tubes. The drive is hand cranked – one turn of the handle translates into five bumps to the table via an eccentric cam. Hand cranking at a comfortable one turn per second translates into five bumps per second – enough to operate the shaking table. Rebound from the bump is by means of a rubber band. If desired the crank can be modified to be powered by a bicycle, motor cycle or a motor, either electric or diesel. BGS trials at Kias Creek in the Philippines were successful; the table was easily set up and adjusted to a stable configuration and heavy mineral concentrates were readily recovered. A hand lens showed much fine-grained gold had been recovered. Laboratory examination revealed most of the gold was only around 40µ in size and grains as small as 10µ had been recovered. Trials at Acupan in the Philippines were problematical and it was far more difficult to set up the table in a stable configuration. Laboratory examination of the concentrates showed that significant amounts of gold had been recovered from ores and tailings and substantial amounts were very fine-grained, around 30µ in size.

BGS SHAKING TABLE

Top view of the Mongolian home-made version of the handcranked BGS shaking table. (photo: Robin Grayson)

The BGS shaking table was developed by the British Geological Survey (BGS) as part of the DFID/BGS Technology Development Research (TDR) project R6226

‘Mitigation of mining-related mercury pollution hazards’. [24]. The task was to design, construct and test a cheap, simple shaking table that could be produced for use by small-scale miners in developing countries. The BGS shaking table is being evaluated by recreational gold miners in North America and by the Support for Artisanal Mining (SAM) project in Mongolia (www.sam.mn). BGS laboratory trials showed the BGS shaking table is as good as – and probably slightly more effective than – the commercial Wilfley shaking table for in recovering fine-grained gold. However the BGS laboratory trials were carried out in almost perfect conditions: ² ² ²

the samples were washed and deslimed prior to tests; the table was set up on large flat benches; and a well controlled, even pressure water supply was available.


Field trials show the BGS shaking table to be an effective device. Its use is more difficult in adverse conditions where material to be processed is muddy, stability is a problem and water pressure is variable. Under optimum conditions, it seems that the BGS shaking table can be expected to routinely recover almost all moderately fine gold (100µ to 1mm) and probably >90% of very fine gold down to 50µ.

Figure 132.

For recreational gold miners the BGS shaking table offers a simple rapid means of upgrading concentrates in the field rather than “bringing them home” as well as removing any temptation to resort to mercury. For artisanal gold miners the BGS shaking table is small, lightweight, transportable, affordable and offers a possible alternative to mercury for upgrading concentrates.

GOLD RECOVERY BY BGS SHAKING TABLE – based on UK tests

Recovery of placer gold by the hand-cranked BGS shaking table, tested under laboratory conditions [24]. (compiler: Robin Grayson)

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GemeniTM table – 1980s research in Colorado

Operation

Figure 113. TM

Feed properties are critical to the success of a GemeniTM table. The feed size must be <1mm, and ideally the feed should have been passed through a magnetic separator remove any tramp metal. The feed density requires constant checking with a pulp meter, and a flowmeter is required to maintain the correct rate of flow. The water should be filtered to remove suspended solids and organic material. Water usage is substantial and has to be at constant pressure, best achieved by positioning a small header tank three metres above the table deck. The room height has to be correspondingly high. The GemeniTM table is made of fibreglass supported on a steel frame. It is designed to be used inside a building with a solid floor to which the unit is then securely bolted. It is important to be securely fastened to the floor, as all the drive motion must be transferred to the table deck to achieve good performance. In modern models, a direct drive system has a geared motor driving a crank connected to the table deck. To absorb overrun, the crank has a sprung connection system, and a bump stop system provides a fine tuning mechanism. Table tuning is by adjusting a single screw.

GEMENITM TABLE

A Gemeni table showing streaks of black sand and yellow streaks of gold. (photos: courtesy of MD Technologies Ltd – www.gravityrecovery.com)

The GemeniTM table was invented by Henry W. Rodgers of Colorado and patented in 1986 (US #4,758,334). The device is an innovative shaking table designed to recover fine gold to directly produce a clean smeltable concentrate to produce dore bars. Often the cleaned concentrate is good enough to do so. The GemeniTM table is claimed by the maker [161], and confirmed by users and researchers [3,61,162], to be capable of producing a very clean gold concentrate from gold-bearing black-sand concentrates when fed <1mm material. Recovery is excellent down to about 40-50μ [161].

Figure 114.

Adoption by placer gold miners The GemeniTM table is now seen in most placer gold mining regions of the world. It appears to be significant better than other shaking tables, but is also significantly more expensive. Locally-made copy-cats are made in some parts of South America and perhaps elsewhere. The switch to the GemeniTM table has been slow and incomplete, perhaps due to the higher capital cost, the conservatism of most placer companies and slightly more demanding housing and skill.

GOLD RECOVERY BY GEMENITM TABLE

Recovery of placer gold by GemeniTM table, according to the original patent and testing [161,162]. (compiler: Robin Grayson)

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U-TechTM reverse polarity table – 1990s research in Arizona

Operation

Figure 135.

The feed is concentrate screened at <¾-inch. Water is added to the RP-4 table at a rate of 8-14 gallons/minute via a water distribution bar. The unit includes a pretreatment feed sluice tray described as a “sluice box moulded into deck”. The slurry spreads across the table – a very smooth 1-piece riffled deck and launder tray moulded from ABS plastic (truck bed liner material). The U-TechTM RP-4 is driven by a HP motor, 115/220v, AC 60 Hz, 8.6 Amp to produce a smoother reciprocating motion than possible with a conventional shaking table, by using spring steel plates instead of linear sliding bearings. A special feature is spinning rare earth magnets positioned underneath the deck surface. By reversing the magnetic polarity of magnetite at about 800 cycles/minute (elliptical polarization of the magnetite) magnetite rises to the surface and is then more easily washed into the tails by the transverse flow of water. This is aided by the magnetite particles delivering additional magnetism by aligning their magnetic poles to produce long chains. The tailings leaving the table pass a ‘tails nugget trap’ before exiting as a continuous discharge. An optional deck screen added to the U-TechTM RP-4 table operates as a combined shaker screen, magnetite separator and gold gravity concentrator.

REVERSE POLARITY TABLE

Side view of the drive mechanism, U-Tech RP-4 shaking table, Central Mineral Laboratory in Ulaanbaatar. (photo: Davasambuu of the Swiss-funded Support for Artisanal Mining project)

The U-TechTM reverse polarity (RP) table was invented by Darvin Wade of North Virginia and patented in 2001 (US #6,308,835). The RP table has a reverse polarity spinning magnet system under the table top that aids separation of black sands. The RP table is built by UTech Co. in Arizona, USA - www.goldequipment.com/goldmining-equipment-concentrating-table-RP-4.html The U-TechTM RP-4 table has a 12x48inch cleaning deck, weighs 60lb and fits in a large car boot. U-Tech claims the RP-4 table “saves down to 1 micron (Lab Test)” www.goldequipment.com/gold-mining-equipment-warranty.html Larger units such as the RP-16 table can process >8 tons/hour (4 tons of black sands) of sand-sized feed. The RP-16-D gravity concentrating table weighs about 2,200 lbs and requires 60-80 gallons of water/minute. According to U-Tech, “we have users claiming the U-

Adoption by placer gold miners Reverse polarity RP tables gained some popularity in North America, but in spite of the claimed advantages of the U-TechTM RP table they have not yet begun to seriously challenge the entrenched position of the traditional shaking tables or the advanced types of shaking table such as the GemeniTM table.

TECH heavy mineral concentrators are saving down to one micron” and, “will recover 99% of microscopic gold from your magnetite concentrates and is designed to run continuously", also “we have users claiming the U-TECH heavy mineral concentrators are saving down to -500 particle size", and “demonstrated to move one micron size dry particles”.

Figure 136.

GOLD RECOVERY BY U-TECHTM REVERSE POLARITY TABLE

Recovery of placer gold by U-Tech reverse polarity table, according to the original patent and maker’s website. (compiler: Robin Grayson)

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GoltronTM machine – 1990s research in Utah

Operation Gold-bearing black-sand concentrates as coarse as 35mm is fed into the variable feed-rate hopper of the GoltonTM unit. The feed rate is 150lbs/hour of solids and 300-600 gallons/hour of water. Feed properties are less critical than for conventional tables, or advanced tables such as the GemeniTM table. The GoltonTM variable feedrate hopper contains an auger that dispenses steady amounts of feed onto a coarse wet vibrating screen with 1.19mm apertures. Oversize (>1.19mm) is routed to a carpeted nugget trap. Undersize is re-screened on a fine wet vibrating screen of 35-mesh (0.42mm). The GoltonTM table is unique in having a coarse side and a fines side. Oversize re-screened material (>0.42mm) is directed to the ‘coarse side’ of the GoltonTM table, while the undersize re-screened material (<0.42mm) is directed to the fines side of the GoltonTM table. Gold is recovered in three locations: ²

Figure 133.

GOLTRONTM MACHINE

²

View of a Goltron ready for action. (photo: courtesy of Goldfield International – www.goldfieldeng.com)

²

>1.19mm gold in the nugget trap; 0.42-1.19mm gold on the ‘coarse side’ of the GoltonTM table; <1.19mm gold on the ‘fine side’ of the GoltonTM table.

The GoltonTM unit has a power requirement of 110/220 volts, single phase at 50/60 hertz, to vibrate the table and/or screens. The manufacturer claims that recovery efficiencies are excellent, and that the unit is capable of achieving “gold recovery down to minus 400-mesh” (37μ). However test results have yet to be published.

The GoltronTM unit is a recent innovation by Goldfield International of Utah (www.goldfieldeng.com) designed to recover not only fine gold but a full range of gold sizes including nuggets to directly produce a clean smeltable concentrate. It seems to be the only shaking table set-up capable of catching everything from nuggets to fine gold. The manufacturers claim it is “fastest, most efficient

system that does not rely on chemicals or amalgamation”


to separate fine gold. In seems possible that upgraded concentrate from a GoltronTM table may be pure enough for direct smelting to produce dore bars, but little information has been published.

The GoltronTM Unit is new and is gaining some acceptance by placer gold miners at least in North America.

Figure 134.

GOLD RECOVERY BY GOLTRONTM MACHINE - generalised

Recovery of placer gold by the Goltron machine, based on information issued by the manufacturer. (compiler: Robin Grayson)

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Bartles-Mozley multi-deck tables – 1970s research in Cornwall

Operation

Figure 63.

This account is based on Silva’s description of the Bartles-Mozley multi-deck concentrator [61]. The feed consist of slurry of 15-35% solids that have been finely screened, ideally 100μ. The slurry is fine enough to be termed slime. The 40-deck unit is able to process about 20 tons/hour of solids. The feed pipe tops up a feed box from where a flexible pipe conveys feed to each deck, the feed being spread across the width of the deck by means of a manifold with twelve discharge holes. The slurry flows down the deck sloping 1.3° to 2.5°, and encounters transverse riffles “10 to 100 thousandths of an inch high, spaced apart by 1 to 3 inches” [patent]. The deck and its riffles undergo an orbital horizontal motion imparted by an out-of-balance electric motor. The orbital horizontal motion has a large amplitude of 5 to 18 cm at a frequency of 0.8 to 3 Hz. The orbital horizontal motion energises the light particles to inhibit them settling and they remain suspended in the flowing film of water that overrides the riffles to discharge as tailings. Meanwhile high density particles settle on the table and remain restrained by the riffles. After running for about 35 minutes, feed is stopped and the decks tilted slightly to drain, then tilted steeply to allow the concentrates to be flushed into a collection sump. The tables are then returned to the original orientation and processing recommences.

BARTLES-MOZLEY MULTI-DECK TABLES

Layout simplified from the patent. (drawing: Robin Grayson).

The Bartles-Mozley multi-deck concentrator consists of 40 fibreglass decks (tables) each 3.6ft x 5ft arranged in two sections of 20 decks each, suspended by cables. Each deck is riffled and connected by ½-inch plastic formers that define the pulp channel. Good recovery is from 100μ to 5μ, “and in some cases as small as 1 micron” [61]. The device soon became “probably the most widely used slimes table today” [61] due to its high throughput, small footprint, low labour requirement, low power consumption, and low water consumption. [75,76]. After more than a decade its popularity collapsed due to competition from new centrifuges (e.g. KnelsonTM bowl and FalconTM C bowl) and better chemical leaching. Several types of multi-deck concentrators existed. Richard H. Mozley of Cornwall patented a variant in the UK and then in 1981 the United States (US #4,251,358), and assigned to the National Research Development Corporation. A modern variant was patented by Paul Marriot of Cornwall in 1989 (US #5,148,922) but allowed to lapse in 1996.

Figure 64.

Adoption by placer gold miners The Bartles-Mozley separator was intended primarily for hardrock mills and tin recovery, and sold worldwide. The author is unaware of it having ever being applied to recovery of very fine gold, although it has potential.

GOLD RECOVERY BY BARTLES-MOZLEY ORBITAL TABLES – generalised

Gold recovery by Bartles-Mozley orbital tables, according to Michael Silva [61]. (compiler: Robin Grayson)

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Pinched sluice – historical usage

Operation Feed is screened at 100μ to 1mm and should have no more than 5% clays [25]. Slurry is fed by gravity from a trommel or screen via a chute into the head of the pinched sluice or arrives by pipe fed by a slurry pump. The floor and walls of the pinched sluice must be free of obstructions to achieve laminar flow. Laminar flow permits gravitational settling and stratification of slurry. In compelling the slurry to converge to a ‘pinchpoint’, wear on the floor and walls may be severe, and a replaceable liner such as smooth rubber or wear-resistant material is advisable. A pinched sluice should incorporate: ² ²

Figure 47.

Pinched sluices cannot exceed about 70% recovery of heavy minerals, as this is the limit of the pinching mechanism [25]. The author suggests it is possible that gold particles may continue to be crowded and upgraded – experiments are required. A pinched sluice appears, in the opinion of the author, to have two potential applications that are quite different and should not be confused:

PINCHED SLUICE

General outline of a pinched sluice. (drawing: Robin Grayson, after Michael Priester, Projekt-consult – www.projekt-consult.de)

Pinched sluices (fanned sluices) have been used for centuries [25]. A pinched sluice is a small sluice that tapers towards the discharge end. Slurry rushing down the sluice gets crowded, the slurry is forced to deepen, and the crowded denser particles gravitate towards the bottom forcing the lighter particles to rise above them. As a result the slurry discharge is stratified – a very dense ‘underflow’ of valuable concentrate, a medium density middle flow (‘middlings’) and a low density upper flow (‘tails’). The discharge is divided by splitters into separate streams – concentrate flow, middlings flow and tailings flow. A pinched sluice is a low cost way to produce concentrate, yet the middlings need to be recirculated. Most pinched sluices are used to recover mineral sands [63-66] and are a neglected device in gold recovery. A sister device to a pinched sluice is the Wright impact plate invented by Douglas Charles Wright of New South Wales and patented in 1978 (US #4,078,997).

Figure 48.

a means to adjust the inclination of the sluice; and a means to adjust the angle and position of ‘splitters’.

²

²

traditional application – producing an underflow, middlings flow and a tailings flow; and additional application – controlling surges in the feed rate.

Some pinched sluices have permanent magnets installed beneath them to encourage magnetite and other magnetic particles to join the underflow. This could assist in creating a traction carpet of dense particles at the base of the slurry, and in theory this would assist in the ejection of low density particles and the burrowing down of tiny gold particles. More research is warranted.

Adoption by placer gold miners Pinched sluices are rarely seen in placer gold mines in spite of their obvious potential.

GOLD RECOVERY BY PINCHED SLUICES – generalised

Recovery of placer gold by pinched sluices; generalised information compiled from fragmentary sources. (compiler: Robin Grayson)

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Reichert cone – 1960s research in Australia

Operation Feed is pre-screened as cones are unable to concentrate particles >0.5mm, and recovery falls if clay is >5% of the feed. The cone is sensitive to changes in slurry feed density (55-70% solids) Slurry feed is poured into the cone evenly around its circumference, and flows to the central hole. The slurry becomes progressively crowded, due to all the particles converging upon the central hole. The increased packing of suspended particles increases the overall density of the lower slurry. Suppose the slurry fluid attains 2g/cm3, then: ² ² ²

Figure 49.

By repeated crowding, the free settling regime becomes a hindered settling regime, cutting the terminal Settling Velocity of all particles even more, and the lighter particles such as quartz become increasingly vulnerable to ejection from the ever-denser slurry. Gold concentrate is removed by annular slots in the cone. Consistent recovery of gold particles >45μ have been reported [67]. At the Snake River in Idaho, USA, gold was recovered using a Reichert cone in conjunction with a conventional shaking table. Gold recovery >85% was noted by Thomas Ferree [68], with 44% of recovered gold being <75μ.

REICHERT CONES

Example of Reichert cones. (photo: courtesy of Dale Henderson of the manufacturer Roche Mining – www.rochemt.com.au)

The Reichert cone was invented by Ernst Reichert of Queensland who applied for an Australian patent in 1966 and was awarded a US patent in 1968 (US #3,379,310). The device packs pinched sluices in a circle, then dispenses with their side walls to create a single cone with a central discharge hole. The flow is free of edge-effects without side walls. The cone is of lightweight structural glass reinforced plastic (GRP), laminated with polyurethane. The cones are stacked in series to repeat the benefit of crowding. Multiple stages of upgrading are achieved. Stacks of cones are mounted in circular frames over 6 metres tall. The 3.5-metre cone processes up to 350 tons/hour. Water consumption is less than for sluices or jigs [61]. The unit has no moving parts and very low operating costs, but needs screens, cyclones and slurry pumps.

Figure 50.

quartz – terminal Settling Velocity cut from 0.9 to 0.4cm/sec; magnetite – terminal Settling Velocity cut from 2.3 to 2.7cm/sec; gold – terminal Settling Velocity cut from 7.6-9.8 to 7.1-9.3cm/sec.

Adoption by placer gold miners Reichert cones have not gained much foothold in placer gold miners. They have been used in Queensland by a 20m3/hour plant to recover fine gold from tailings. In Snake River Idaho, a sand and gravel company recovered gold with Reichert cones while selling gravel.

GOLD RECOVERY BY REICHERT CONES

Recovery of placer gold by Reichert cones, according to Erik Spiller and Thomas Feree [67,68]. (compiler: Robin Grayson)

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Humphrey spirals – 1940s research in Colorado

Operation

Figure 51.

Feed is slurry of 25-35% solids, with the solids screened to 3mm or preferably to 1-2mm. As well as the feed being finely screened, care is required to ensure that the feed rate and feed consistency is maintained as constant as possible to ensure satisfactory results The slurry descends the chute of the Humphrey spiral, the chute twisted into typically six windings (turns). The heaviest particles fall to the bottom of the channel where their velocity is retarded by friction. Upon slowing, the heavy particles are less affected by centrifugal forces generated by the spiral flow of slurry than are the lighter faster particles that are less retarded by friction. As a consequence the heavy particles spiral along the inside walls of the channel, while the faster lighter particles spiral further out towards the outer rim of the channel. The heavier particles are recovered as concentrate from discharge outlets on the inside of the channel. Separation precision can be improved by adding additional water during the sorting process. Key variables are the cross section of the channel, the diameter of the spiral, the number of windings (revolutions), slope of the channel and the positioning and number of discharge outlets and supplementary-water intakes.

HUMPHREY SPIRALS

Humphrey spiral illustrated on the front page of the classic ‘Tools for Mining Book’ by Michael Priester and colleagues [25]

Modern spirals began with Frank Pardee of Pennsylvania who was awarded patents in 1899, 1924 and 1939 (US #629,590, US # 1,516,926 and US #2,145,315). The Humphrey spiral was invented by Ira B. Humphrey of Denver USA who applied for US patents in 1943 and was awarded patents in 1947 (US #2,431,559 and US #2,431,560) and 1955 (US #2,700,469). Advantages include low cost, long life, small footprint, good recovery of fine gold and ease of visually checking to see if material is separating properly. The basic Humphrey spiral is 3 metres tall with 5-6 windings of the channel and is capable of processing 0.812 tons/day of concentrate depending on the design of the channel and the particle size. Early production models were of cast iron sections and required numerous pipes for supplementary-water intake and discharge outlets making it a rather complicated, very heavy and difficult to adjust; and rapid wear of the rubber lining and irregular wash water distribution caused production problems [61]. Later Humphrey spirals were of lightweight material and more compact being a double helix.

Figure 52.

Adoption by placer gold miners Humphrey spirals enjoyed great popularity for coal cleaning and mineral sand recovery, but were very rarely used in placer gold recovery. Early models were difficult for placer miners due to the weight and the need not only close screening and a steady feed of slurry. The later model 7 Reichert spirals gained a stronger niche amongst placer mining companies for fine gold recovery.

GOLD RECOVERY BY HUMPHREY SPIRALS

Recovery of placer gold by Humphrey spirals, according to diverse fragmentary sources. (compiler: Robin Grayson)

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Mark-7 Reichert spirals – 1980s research in California, Arizona and Colorado

Operation The feed is screened at <2mm and poured as slurry in the spiral channel and helter-skelter down the spiral with the denser particles settling as dense basal slurry or traction carpet. The channel’s cross-section has a continuously variable profile that takes its deep axis outward during the descent, and the stream of dense particles is constrained to follow the axis of the channel. At the bottom of the ‘helter-skelter run’, the stream of heavy concentrate is collected via a take-off port on the final turn of the spiral. As well as the concentrate stream, three other streams are collected at the bottom of the spiral: ²

Figure 115.

²

MARK-7 REICHERT SPIRALS

Mark 7 Reichert spirals installed on a mobile placer gold washplant in the USA. (photo: courtesy of John Strain)

² ²

Reichert spirals were developed in Australia in the 1960s by using lightweight materials and reassessing the hydrodynamics of the Humphrey spiral. Reichert spirals are made of lightweight fibreglass, a material that permitted many improvements and variations. Over 20 models exist, the spirals contoured to separate particles of a particular range of densities. Mark-7 Reichert spirals have been produced since 1982 and its spirals are designed to recover cassiterite particles with a density of 6 to 7g/cm3 and it then proved effective at recovering fine gold in tests in North America [61,163-166]. The mark-7 model has no moving parts, and can operate 24 hours a day, 365 days a year, for many years. Compared with earlier spirals, the mark-7 is easier to operate and requires less water. For tests, a spiral is mounted vertically on a support column. For production, identical spirals are mounted on the same column as a double or triple helix to increase the capacity from ‘one-start’ to ‘two-start’ or ‘three-start’.

Figure 116.

concentrate stream – taken for further upgrading; middlings stream – recycled (acts as buffer, inhibiting gold loss if slurry surges or fluctuates); tailings stream – discarded; water stream – low in solids, either discarded or recycled.

Compared to the traditional Humphrey spirals, all the concentrate off-take ports are eliminated from the spiral except on its final turn. This simplification dispenses with a large amount of expensive tubing, and eliminates the need for the addition of any top-up wash water.

Adoption by placer gold miners Mark-7 Reichert spirals proved to be successful in recovering moderately fine placer gold in California, Arizona and Colorado USA, and proved effective at scavenging for fine gold from washed sand concentrates at sand pits [164-166]. But the take-up of spirals by placer gold miners has been low, being seen only as a valuable processing stage rather than a key component of a washplant. An exception is gold placers that are fine wellwashed sand of high energy coasts and large swift rivers.

GOLD RECOVERY OF REICHERT SPIRALS – based largely on recovery of cassiterite

Recovery of placer gold by mark7 Reichert spirals, according to the cassiterite and placer gold tests [164-166]. (compiler: Robin Grayson)

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Duke’s E-sludge tank – 1970s research in Georgia

Operation

Figure 55.

The inventor claims the pay gravel need not be screened as blockage by large stones are easily removed. Yet dry screening to reject >10-15cm oversize would surely be advantageous; and the oversize could be fed into a conventional small wash-plant parked alongside. The inventor recommends a set of six units be mounted in parallel on a flat-bed truck, collectively able to process about 130m3/hour of loose pay gravel. Each unit has a rectangular hopper-shaped tank in which pay gravel is dumped by a conveyor, front-end loader, etc. The material should be dry or moist and should not be wetted by spray-bars. Water is injected into the base of the tank by means of a manifold of perforated water pipes, at a pressure of 5-35psi but typically 5-10psi. Unlike a jig, Etower or sluice, the water requirement is very low – each tank is about a metre wide but a tank requires a mere 7.6m3/hour of water to process about 22m3/hour of loose pay gravel per hour. This is an order of magnitude less water than a Yukon-style sluice needs to process the same volume of pay gravel. The pile of gravel in the tank cavitates from below due to contact with injected water. The slurrified basal gravel is free to stratify (dense material at the bottom) and creeps along the base of the tank tilted at about 12°. The slowly discharging sluggish porridge-like slurry travels down a chute tilted at 15-35° and the dense lower material is trapped by slots. According to the inventor, 95% recovery is normal, and that recovered gold can be as small as 8μ! However no tests are published.

DUKE’s E-SLUDGE TANK

Duke’s E-Tank on a flat-bed truck. A manifold injects water to slurrify the base of the mound of dryish pay-gravel, creating stiffish slurry that undergoes density stratification. The stratified slurry flows down the floor of the tank to the right, discharging in a ‘recovery box’ where dense basal slurry is trapped in slots. (drawing: Robin Grayson, adapted from Duke’s patent)

Duke’s E-tank was invented by Arthur Duke of Georgia USA and patented in 1976 (US #3,951,787). Gold-separating devices classed as E-tanks include Duke’s E-tank, Graefe’s E-tank and Pyramid’s E-tank. Elutriated sludge tanks (E-tanks) is a term coined by the author [28] for devices that inject water from below into a tank containing a bed of pay gravel that is quite thick (say >10 cm) to create a fairly stiff slurry just watery enough to stimulate gravitational settling of dense particles and rising of low-density particles. The settled heavy concentrate can be a continuous discharge from near the base of the E-tank (e.g. Duke’s E-tank), or remains in the tank as a lag deposit to await batch discharge (e.g. Graefe’s E-tank). Unlike an E-tower, the E-tank’s contents remain as porridge-like slurry from top to bottom, and the slurry is not watery enough for classic ‘hindered settling regime’. Instead the contents resemble a thixotropic quicksand. Unlike a jig, an E-tank contains slurry throughout. There is no jig screen, no ragging, no hutch chamber and slurry is not pulsed upwards (i.e. no jigging).

Figure 56.

Adoption by placer gold miners Since the award patent in 1976, the author has been unable to trace any record of Duke’s E-tank being used by placer gold miners, in spite of its potential.

GOLD RECOVERY BY DUKE’s ELUTRIATED SLUDGE TANK

Recovery of placer gold by Duke’s E-tank, according to the original patent. (compiler: Robin Grayson)

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Graefe’s E-tank – 1980s research in California

Operation

Figure 90.

GRAEFE’s E-TANK

Cut-away side view of Graefe’s E-tank showing the injection water rising from below. (drawing: Robin Grayson)

Graefe’s E-tank was invented by Ralph Graefe of California and patented in 1985 (US #4,523,989). Rights were transferred to Keene Engineering Inc who sold it as the ‘Keene Hydromatic Jig.’ This is a misnomer as it lacks the key features of a jig. Graefe’s E-tank is an elutriated sludge tank that not only has its contents slurrifed by injecting water from below, but is vibrated at 180 cycles/second to prevent ‘hard-cake’ and to ensure that particles are distributed throughout the depth of the slurry by density, not by size. According to test results in Ralph Graefe’s patent, the device can recover 100% of gold as fine as 70μ, falling slightly to a very impressive 93% of 30μ gold. The test results show a 60° slope to the walls of the tank recover far more fine gold than a 45° slope.

Figure 91.

The unit is wheeled by the operator. Pay gravel is spaded into a hopper-screen above the tank. Screening can be ½-¾ inch but finer screening aids gold recovery. Water is injected via a manifold in the base of the tank at 1 to 8 psi – the larger the gravel the higher the pressure. The finer the material then the less the throughput 8 tons/hour for 0.3-5mm falling to 2 tons/hour for 3575μ. The Keene website says “…up to 4 cubic yards” of material can be processed per hour, and “…can operate on as little as 10 gallons of water per minute.” The patent states 7-8 gallons/minute (1.6-1.8m3/hour) is required to process coarse material, and only 1-2 gallons/minute (0.2-0.5m3/hour) to process fine material. A little additional water is required from a hand-held spray bar to inhibit hard-cake from forming. Gold larger than 75μ sinks to the bottom of the tank to await batch discharge. But <75μ gold is carried towards the spillway lip and so a skimmer plate is welded on to prevent it escaping. Batch discharge is rather cumbersome. The tank is progressively tilted more and more steeply to discharge its upper contents as tailings. Then the residual concentrate flushed out with water by opening the bung in the discharge pipe in the base of the tank.

Adoption by placer gold miners Graefe’s E-tank was marketed as the ‘Keene Hydromatic Jig’ to recreational miners but manufacture ceased after a decade or so. Members of the Alaska Gold Forum (AGF) are re-evaluating the device and it may yet enjoy a revival at least amongst recreational miners. Graefe’s E-tank is a complete wash-plant, but small. Although ideal for recreational and artisanal miners it is difficult to envisage how it might be scaled up for industrial mining.

GOLD RECOVERY BY GRAEFE’s ELUTRIATED SLUDGE TANK (Keene Hydromatic Jig)

Placer gold recovery by Graefe’s E-tank, according to the original patent. (compiler: Robin Grayson)

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Pyramid E-tank (Pyramid rotary jig®) – 1990s research in California

Operation

Figure 123.

The tank is first filled with screened material – how much screening is unclear. When the tank is full of sediment, pressurised water injected from below – how much pressure is unclear. As with other E-tanks, little water is required. Water consumption varies with the gold size to be recovered – 2mm gold requires 2,725 litres/hour for a throughput of 4 tons of solids, down to very fine gold (<100μ) requiring 454 litres/hour for a throughput of 0.9 tons, and for the finest gold 114 litres/hour for 0.5 tons of solids. Once the pressurized water is added, the stirrer blades are able to begin to rotate – how fast a rotation is unclear. The blades are simple metal bars welded at intervals along a central solid metal bar that acts as the drive axle turned by a 110-volt A.C. electric motor mounted above the tank. The device is innovative in slurrifying by rotary stirring. The gold and other heavy particles spiral down, while the lights spiral upward. This seems to be in thixotropic sludge-like slurry. After several minutes of operation, the barren tailings are bled through the tank wall via drain taps at two levels. Then the tank is refilled and the process repeated “over and over again” until an “ultra-rich concentrate” is achieved. A concentration ratio as large as 2000:1 is possible and the concentrate is removed from the bottom of the tank via a concentrate tap as a batch discharge. Continuous operation is possible with automatic feeders.

PYRAMID E-TANK

(drawing: Robin Grayson based on advert by the manufacturer)

The Pyramid E-tank was invented in the early 1990s by Pyramid Industries of California and marketed as the Pyramid Rotary Jig®. Manufacture ceased and an advert is the sole source of data, but it seems to be an elaboration of the ‘Heavy Mineral Separator’ invented by Laurence H. Konvalin of California, patented in 1983 (US #4,389,309). The Pyramid E-tank lacks jig screen, jig bed, ragging, hutch chamber or vertical jig motion. The advert stresses it differs from hutch jigs and diaphragm jigs. Rather the invention belongs to a family of gravitation devices termed ‘elutriated sludge tanks’ (E-tanks) that seem particularly suited to recovering very fine gold [28]. The Pyramid Rotary Jig® is here termed the ‘Pyramid E-tank’. Model #T50 consists of a circular tank tapering from 22-inch diameter at the top to 18-inch diameter at the bottom. The taper is said to be significant.

Figure 124.

Adoption by placer gold miners The Pyramid E-tank seems to have enjoyed a brief period of moderate interest from recreational miners in North America but then swiftly vanished into obscurity. It is unclear if any such devices are still in use in spite of their clear potential for fine gold recovery.

GOLD RECOVERY BY PYRAMID E-TANK (Pyramid rotary jig) – generalised

Recovery of placer gold by Pyramid E-tank based on claims of the manufacturer’s advertisement. (compiler: Robin Grayson)

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Lashley’s ASAT E-tower – 1980s research in New Mexico

An early elutriation tower was invented by Lewis Jennings of New York and patented in 1849 (US #8,410). Today elutriation towers use a rising column of water and typically use a teeter (hindered settling) for recovery. However there are many types of E-tower and a systematic classification is long overdue. Around 1968 the United States Bureau of Mines began a Heavy Metals Program and awarded a research contract to the Minerals Industry Research Laboratory (MIRL) of the University of Alaska, to test and improve the recovery rate of -100 screen mesh gold in the Alaskan placer deposits. MIRL built a lab scale E-tower and ran hundreds of successful tests on placer material, and much of the findings were published [109,110]. In the late 1980s ASAT continued the research and built ASAT towers of several sizes (1-inch, 2-inch and 4inch) for lab tests, de-sliming clean-ups and full-scale production [111]. Walter Lashley of ASAT invented a superior form of elutriation tower (E-tower) in the 1980s that attracted considerable attention for its ability to recover fine gold. During the early 1990s an environmental mining equipment R&D group took over the testing on the Etowers from Walter Lashley and did considerable field tests and is reported to have made vast improvements on ASAT’s original design. Over 120 units were installed on several mining projects in the USA and Mexico and reported to have worked very well. An advanced form of ASAT E-tower is capable of recovering <5μ gold in a controlled environment in a lab, and can recover 50% of <10μ in field experiments which is significantly better than most other recovery devices. Variants of E-towers relevant to simple recovery of fine placer gold include the ‘Gold Funnel’ of Rodney Charles Christensen of California patented in 1997 (US #5,692,620).

Figure 94.

Operation Clean water is introduced into a 1-inch, 2-inch or 4inch diameter column either near its base or part-way up, at a controlled rate as determined by experiments. Rising up inside the column, the water enters a teeter zone characterised by hindered settling. Finely screened concentrate (e.g. <0.2mm) is spooned into the tower from above and falls through the rising water down as far as the teeter zone. Extremely dense particles such as gold fall through the teeter zone to accumulate in the bottom of the device as a lag deposit to await batch discharge Fairly dense particles accumulate as a hovering fluidised mass (teeter bed) in the teeter zone. The continuous arrival of new particles from above into the teeter zone causes particles to become increasingly crowded and then only hindered settling can occur The hindered settling means that dense only dense particles can get to the base of the teeter bed, and in doing so they eject upwards less dense particles. As a result the teeter bed becomes vertically stratified with the densest particles at its base overlain by lighter particles The lighter particles are vulnerable to ejection with the rising water as tailings whereas the heavier particles are protected from scouring away by the carpet of lighter particles resting upon them. After processing many spoonfuls of material, the enriched gold concentrate is siphoned off from the device.

Adoption by placer gold miners E-towers became popular for a while among placer gold companies and recreational miners in North America but currently there seems to be neither manufacturers nor R&D interest, and awareness among miners is very low.

GOLD RECOVERY BY ASAT’s E-TOWER

Recovery of placer gold from black sand concentrate by Lashley’s ASAT E-tower, according to diverse reports. (compiler: Robin Grayson)

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Osterberg’s E-tower – 1980s research in California

Operation Clean water is introduced into a 4-inch diameter column near its base, at 2 gallons/minute. Once inside, the water passes through a distributor in a sequence designed to create a non-turbulent up-flow: ² ² ²

²

Figure 95.

Screened concentrate is spooned into the tower from above. The water rises with enough velocity to nonturbulently fluidise the concentrate yet without ejecting particles with the overflow of wash water. The upwelling water must decelerate for black sand to hover as a fluidised bed, the deceleration being due to the rising non-turbulent plumes of wash water have more width once free of the flow distributor. Photos posted by Megan Rose (Gold_Tutor) [112] suggest the device is not a teetered bed settler as hindered settling seems minor. The patent suggests the fluidised bed be stirred gently at the start to encourage gold to fall to the bottom. The upper portion of the fluidised bed is deemed to be depleted of gold and is siphoned off as tailings. The dense gold particles remain behind and more concentrate is spooned into the top of the column. After processing many spoonfuls of material, the enriched gold concentrate is siphoned off from the device.

OSTERBERG’s E-TOWER

Siphoning of tailings. (adapted by Robin Grayson from the patent)

Osterberg’s E-tower was invented by Daniel Osterberg of California, patented in 1984 (US #4,451,359), and marketed as the Quick Gold Separator. The device causes black sand to fluidise and particles to hover. For ‘hovering’, the water velocity has to slacken upwards – impossible in a cylinder is of uniform diameter. Slackening is by the water escaping from the constricted space in the flow distributor. If a teetered bed E-tower then hovering is accompanied by hindered settling in the teetering fluidised bed, displacing lighter particles upward. The author suggests that Osterberg’s device is a multiple E-tower with many rising columns jetting upward, each decelerating once free of the flow distributor. Osterberg’s E-tower seems capable of achieving high percentage fine gold recovery and merits testing.

Figure 96.

through a perforated plate secured to a bottom ring; then through a disc of felted, fibrous synthetic polymer; then through a screen with openings so fine that upward flow is substantially unaffected in its lateral uniformity; and finally through a coarse screen.

Adoption by placer gold miners Osterberg’s E-tower gained some popularity in North America among recreational gold miners but production eventually ceased in spite of the clear potential of the device. The device is undergoing technical evaluation by Megan Rose (Gold_Tutor) and details are posted on the Colorado Prospectors’ Forum [112].

GOLD RECOVERY BY OSTERBERG’s E-TOWER

Recovery of placer gold from black sand concentrate by Osterberg’s E-tower, tentatively based on diverse reports. (compiler: Robin Grayson)

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Reflux classifier – 2000s research in Australia

Operation

Figure 149.

This account is conjectural, as the reflux classifier has yet to be tested with fine gold. The device will process feed up to 5mm size. Intuitively feed would be screened into fractions (e.g. >1mm, 1-2.5mm and 2.5-5mm) each directed to a different reflux classifier. In practice a reflux classifier is most advantageous for the <1mm fraction. Feed is introduced towards the top of the E-tower and begins to fall through the water column. This is opposed by the up welling of injected fluidisation water and a wobbling ‘teetered’ mass of fluidised particles results, only the densest and heaviest particles reaching the bottom as final concentrate. The rising column of water lifts the finest particles (including very fine gold) and up well between the inclined plates of the lamella. In the inclined section, the trajectories of the densest fine particles fail to clear the top of the plates and so these particles collide with the plates. The ensuing friction slows the dense fine particles and they slide down the plates back into the E-tower. The plates are inclined at about 70° degrees – shallow enough to ensure dense particles hit the plates, yet steep enough to ensure particles slide down it [199]. Other particles fail to settle and continue upwards to escape as tailings. To ensure fine gold will settle yet quartz remain in suspension the ratio of plate length to plate-plate gap is as great as 200:1 (‘aspect ratio) [195,197], far more than the 40:1 used to separate coal from mineral matter.

REFLUX CLASSIFIER

Generalised layout of a modern reflux classifier (drawing: Robin Grayson, redrawn from article by Zhou et al 2006 [195].)

The ‘modern’ reflux classifier was invented by Kevin Galvin of New South Wales who applied for an Australian patent in 2000 (application 09/890,487) and was awarded a USA patent in 2004 (US 6,814,241). Commercialisation is by Ludowici Mineral Processing Equipment Pty Ltd – www.ludomin.com/products/reflux_classifier.htm. The ‘modern’ reflux classifier follows innovations such as the 'Method and apparatus for cleaning sand or grading sand…' patented in 1947 by A.B. Morris (US #2,426,839). A reflux classifier is a combination of E-tower and lamella settler. More than one set of lamella is possible, but a set of lamella caps the E-tower as an ‘inclined section’ for best results [194-199]. This inhibit heavy particles escaping with the overflow water, enabling fine particles to be “removed or classified in a more concentrated form" and is more tolerance of feed fluctuations.

Figure 150.

Adoption by placer gold miners The modern reflux classifier may prove to have major applications for recovering fine placer gold for artisanal gold miners, recreational miners and placer companies.

HYPOTHETICAL GOLD RECOVERY BY REFLUX CLASSIFIER

Hypothetical recovery of gold by the reflux classifier’ based on work by Zhou and colleagues [195]. (compiler: Robin Grayson)

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Ecologic E-tower – 2000s research in New Zealand

Operation

Figure 151.

A trowel-full of pay gravel is put on the static simple screen that sits on top of the conical device. The device can process up to 2m3/hour of easy-to-dig pay gravel, less so if clayey. Water is supplied by the innovative pedal-powered Ecologics water pump patented worldwide in 2006 (WO #2006071127). About 100 litres of water are re-circulated until it becomes too contaminated. Both pump and concentrator can use water that is clayey or silty. The material on the screen is swashed from below by agitated water and the fines fall though the screen. Washed oversize remains on the screen and is checked for nuggets then discarded. Black sand and gold fall to the bottom of the cone, in spite of the turbulent up-flow, while fine light particles are swept out. Water is introduced violently from the bottom of the device. Rather than a delicately balanced teeter, this is an up-welling mass of energetic water pumped in pulses via a flexible plastic pipe from Ecologic’s pedal-powered water pump. The pulsing action keeps the water in the cone agitated and causes it to continuously spill over the rim of the top of the cone. The device is stopped to gather the concentrate settled in the bottom the cone, by flushing out with a little water. It is then upgraded by panning or other means. The inventor considers coarse gold recovery is 95% and fine gold recovery 80-90%. In some field tests only 0.2% of the gold reported to the tailings. Generalised test results have been published in South Africa [200].

ECOLOGIC CONCENTRATOR

Oblique views of Ecologic’s ‘Gold Concentrator’ showing the simple screen (photo: Ecologics Ltd – www.ecologics.co.nz)

The Ecologic concentrator is an innovative type of elutriation tower (E-tower) recently developed by Ecologics Ltd of New Zealand (www.ecologics.co.nz). Marketed as the Ecologic ‘gold concentrator’, the device is a remarkable ‘catch-all’ able to catch all shapes and sizes of gold particles, large and small. This is technically challenging, for the high-energy Newtonian setting regime needed to catch nuggets and eject large quartz is also capable of flushing out fine gold and flat gold with the tailings.

Figure 152.

Adoption by placer gold miners The Ecologic ‘gold concentrator’ is marketed worldwide to artisanal placer gold miners and interest is being shown by some recreational miners.

GOLD RECOVERY BY ECOLOGIC E-TOWER

Recovery of placer gold by the Ecologic ‘gold concentrator’ based on manufacturer’s information. (compiler: Robin Grayson)

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Visman’s compound water cyclone – 1970s research in the Yukon

Operation

Figure 57.

Feed is passed through a fine mesh screen (e.g. 13mm), then pumped as slurry into a CWC at a controlled rate. The slurry pressure induces the slurry to slam against the internal wall of the cyclone, and spin vigorously while subjected to high g forces (e.g. 40-50 g). A gold particle’s residence time in a CWC is a mere second [72] before being ejected in the underflow. It is unclear how separation is achieved so rapidly. The densest particles, fine and coarse, are continuously discharged as concentrate from the base of the CWC with the underflow. The lightest particles are continuously discharged from the top of the CWC with the overflow. Unfortunately the Alaska tests did not produce a high concentration ratio, and therefore the compound water cyclone can be inserted into a wash-plant as a useful but non-essential stage, rather than replacing say a sluice, jig or centrifuge. Results of tests are variable, and it is concluded by the author that as yet – in spite of considerable research – the theoretical basis for CWC is not firmly established and this is an impediment to developing a device that consistently produces good results in terms of fine gold recovery and concentration ratio.

VISMAN’s COMPOUND WATER CYCLONE

Cross-section of Visman’s compound water cyclone (CWC). Dense particles report to the UNDERFLOW, light particles report to the OVERFLOW. (drawing: Robin Grayson from US patent #3,353,673)

Visman’s compound water cyclone (CWC) was invented by Jan Visman of Alberta and patented in 1965 (US #3,353,673) and reissued in 1967 (US Re#26,720). Research on the ability of compound water cyclones to recover placer gold began in the Yukon in the 1970s [70] and continued in the 1980s in British Columbia [71] and Alaska [11,56,72,73]. CWCs were developed to maximise concentration by particle density. They are of squat shape due to their wide-angled cones, and internally have long ‘vortex finders.’ CWCs are potentially excellent gold recovery devices by virtue of low cost, ease of operation, and no moving parts. Instead it uses the energy of pumped inflowing slurry ‘guided’ by the internal shape of the device to achieve centrifugal concentration.

Figure 58.

Adoption by placer gold miners Experiments on placer gold recovery by CWCs have been conducted from the early 1970’s to recent times in North America (British Columbia, Yukon and Alaska) and Soviet Union [74] In spite of early success in recovering placer gold, R&D on compound water cyclones did not produce fully reliable CWCs for placer gold mines. Manufacture has ceased in the west, although they may be still being made in the Russian Federation.

GOLD RECOVERY BY COMPOUND WATER CYCLONES – Alaska tests

Recovery of placer gold by CWCs in tests in Alaska by the Minerals Industry Research Laboratory [56,72,73]. (compiler: Robin Grayson)

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Bartles’ crossbelt – 1970s research in Cornwall

Operation The feed consist of 15-35% solids that have been finely screened, ideally 150μ. The slurry is fine enough to be termed slime. Slurry is fed to the device at a rate of about 500 kilos per hour of solids. The slurry is introduced via a feed box to about half the length of the central ridge of the belt. Heavies settle on the belt and remain on it, moving along with the belt, the belt travelling forward at a rate of 3-8mm per second. The moving belt passes through a ‘cleaning zone’ where middlings are washed off the belt. The heavies remain on the belt to be discharged over the roller when the belt starts to turn upside down. The Bartles crossbelt is “particularly effective” for recovering material from 20 to 150μ and “consistently

outperforms conventional fine sands and slimes tables”

Figure 61.

[61]. The orbital shear is closely controlled and adjusted to optimise recovery. Lights fail to settle on the belt due to the action of the orbital shear and so flow off the sides of the belt, made possible by the sides of the belt gently sloping sideward at 1.5° to 3°. The belt is much wider than conventional tables and this allows a greater spreading area for valuable products, therefore allowing distinct cuts to be made between the gold concentrate and the middlings.

BARTLES’ CROSSBELT

Layout simplified from the patent. (drawing: Robin Grayson).

The Bartles crossbelt is a vanner invented by Richard Owen Burt of Cornwall and patented in the UK and then in 1977 in the United States (US #4,060,482), and assigned to Bartles (Carn Brea) Ltd of Cornwall. The Bartles crossbelt consists of a 2.44m wide endless PVC belt that passes over a pair of rollers, one being the drive roller. A unique feature of the belt is its central longitudinal ridge from which the belt slopes slightly to its sides. An orbital shaking motion of 250-400 rpm is imparted to the moving belt by a rotating weight, made possible by the belt assembly being freely suspended by four wires from a supporting frame. The orbital shaking motion is induced by an out-of-balance rotating drive shaft driven by an infinitely variable D.C. electric motor.

Figure 62.

Adoption by placer gold miners The Bartles crossbelt was intended primarily for hardrock mills and tin recovery, and sold worldwide. The author is unaware of it having ever being applied to recovery of very fine gold, although it has potential. The invention is an alternative to a shaking table but manufacture ceased a decade ago.

GOLD RECOVERY BY BARTLES’ CROSSBELT

Gold recovery by Bartles’ crossbelt, according to Silva 1986 [61]. (compiler: Robin Grayson)

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Lemmon’s vanner – 1980s research in the Yukon

Operation

Figure 109.

A good example of the operation of a vanner in recovering placer gold is of the home-made belt concentrator devised by C.W. Ammen [158]. The device is “a smooth rubber belt moving uphill at a rather slow pace.” The belt is of extremely smooth white rubber belting from a food machinery manufacturer. The unit has a hand-held water hose allowing the operator to vary the water pressure, volume and direction. The belt is driven by a gear-reducer motor with an autotransformer, giving a range of speeds. Strictly speaking, to be classed as a vanner then belt should be vibrating but it is unclear if the motor vibrates the belt significantly or not. As the belt travels upward, the material “...is sifted

VANNER

A vanner is a vibrating endless belt moving upslope taking heavies with it to discharge at the top end as valuable concentrate. Light particles pour off the bottom roller as tailings. (drawing: Robin Grayson).

onto the belt about midway between top and bottom. At this point, a fine water spray is played on the belt, so that you have a downward flow of water as the material moves upward. The gangue washes down while the gold particles stick to the belt with great tenacity - especially when the gold appears in the form of flat particles (platelets)”. This

Lemmon’s vanner was invented by Norvel Lemmons of Arkansas and patented in 1989 (US #4,826,018). Tests on Liard River placers in the Yukon achieved recoveries “consistently above 95% and some approached 99%” for placer gold of 20-250μ. However it is unclear if Lemmons’ vanner was commercialised, but it merits further study. Experiments to recover placer gold with other types of vanner were made in Alaska [157] and elsewhere but none were successfully commercialised. Meriting investigation is that gold particles grip tenaciously to certain synthetic surfaces due to electrostatic charges. A gently inclined endless belt can recover gold. If the belt is driven upslope and slurry poured on it, then the slurry will flow down the belt to topple off as tailings over the end roller. Meanwhile dense particles such as gold will cling to the belt by friction and be hauled with the belt to the top roller where they topple into a concentrate bin. Vanners are a simple but marked improvement, by introducing some gentle vibration to the slowly moving belt and this generally enhances gold recovery.

Figure 110.

comment is intriguing as it suggests that Ammen’s belt may be more disposed to recover flat gold than is possible with most other devices. Ammen’s belt proved was effective in recovering “99.99%” of “fine gold” that was put as tracer in sand fed to the device.

Adoption by placer gold miners The author is unaware of vanners or belt concentrators being used by company-scale placer gold miners at present. Such devices are used by artisanal gold miners in South America and perhaps elsewhere, notably gold recovery from milled hardrock ore.

GOLD RECOVERY BY LEMMON’s VANNER – Lizard River tests, Yukon

Recovery of placer gold by Lemmon’s vanner, according to the original patent. (compiler: Robin Grayson)

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Younge’s horizontal centrifuge – 1980s research in British Columbia

Operation

Figure 97.

Pay gravel is introduced into the slightly raised feed end. The pay gravel is pre-washed by intense focussed spray bars in the feed hopper, but only a little water is used and no water is added in the device itself. The horizontal centrifuge needs a 50:50 mix of solids and water, a major advantage over other types of wash-plant. The slurry falls on the floor of the spinning cylinder, and is lifted up the wall by the rising longitudinal riffles. The increasing tilt of the riffles in their curved ascent causes the water and lighter particles to cascade back down to the floor. The heavies remain pinned against the wall of the spinning cylinder by enhanced g forces. The compartments between the longitudinal riffles fill with solids and the ejected water flows into the next compartment. This flow "…shears over the longitudinal

YOUNGE’s HORIZONTAL CENTRIFUGE

Younge’s horizontal centrifuge in an innovative wash-plant by Roger Wagner. (photo: Leonard Leeper - www.golddredger.com)

riffles and in doing so forms a vortex area between adjacent longitudinal riffles…" and, "…this vortex keeps particles of low specific gravity in suspension and they are ultimately carried along through a succession of annular compartments...” and so are ejected as tailings.

Younge’s horizontal centrifuge was invented by Earl G. Younge of Langley in British Columbia for which he was granted patents in Canada in 1981 and 1983 (CA #1,110,206 and #1,153,336) and similar patents in the USA in 1981 and 1982 (US #4,265,743 and #4,347,130) entitled ‘Method and Apparatus for Extraction of Gold From Placer Gravel’ and ‘Placer Mineral Concentrator and Process’. In this account the device is termed a horizontal centrifuge to clarify its distinctive character – a spinning cylinder with its interior wall equipped with two sets of riffles – one set of longitudinal riffles running its entire length, and at right angles to them a second set of riffles forming annular rings around the wall. The simplest horizontal centrifuge is a scrubbercentrifuge – a scrubber rotating so fast the material is not only scrubbed but also spun so fast it attains a significant degree of density classification. Some scrubbers may unwittingly be centrifuges.

Figure 98.

The cylinder has a high rotation speed and so the diameter of the cylinder can be rather small: ²

²

5ft long 16-inch diameter cylinder, with five 1.25-inch high longitudinal riffles and three 1.375-inch high annular riffles, rotates at 140-170 rpm and processes 8 tons/hour (4m3); 7.5ft long 20-inch diameter cylinder, with six 1.75-inch high longitudinal riffles and four 2-inch high annular riffles, rotates at 128 rpm and processes 20 tons/hour (10m3).

Adoption by placer gold miners Younge’s horizontal centrifuge is occasionally seen at placer gold mines in North America but does not seem to have gained wide acceptance, presumably due to lack of awareness of its considerable advantages regarding fine gold recovery and low water usage.

GOLD RECOVERY BY YOUNGE’S HORIZONTAL CENTRIFUGE

Recovery of placer gold by Younge’s horizontal centrifuge, according to the original patent. (compiler: Robin Grayson)

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ItomakTM bowl – 1990s research in Novosibirsk

Operation

Figure 129.

The smaller ItomakTM models (KG-0.1 and KG-1.0) are for upgrading. Concentrate screened at <2mm is fed manually into the hopper above the centrifuge. Water is added via a hand-held spray to create slurry that flows down a slurry pipe to discharge close to the bottom of the inside of the bowl. Here the slurry collides with a raised discharge plate that has six oblique tangential extensions to help fling the solids to reach the base of the riffled wall of the centrifuge. The rotor rotates at 1,250 rpm in model KG-01 and 700 rpm in the somewhat larger model KG-1.0. Injection of fluidisation water helps prevent packing of the riffles. The smaller ItomakTM models (KG-0.1 and KG-1.0) are distinctive and probably unique amongst centrifugal bowl concentrators in that the bowl is tilted out of the vertical and the centrifuge therefore spins around an inclined axis of rotation. The larger ItomakTM models are designed for the final stage of a wash-plant. Depending on the model they can processing 2-15m3 of solids. Slurry screened at <3mm is fed via a slurry pipe to discharge at the ‘far wall’ of the spinning bowl. The bowl rotates in the horizontal plane, again differing from most western bowls. The rotor rotates at 500-650 rpm to slam the slurry against the wall of the bowl. Injection of fluidisation water helps prevent packing. ItomakTM bowls, large and small, are stopped for batch discharge of concentrate.

ITOMAKTM BOWL

The smallest ItomakTM concentrator, model KG 0.1, at the placer gold mine of Polymet Polata Ltd at Sharin Gol in Mongolia. The top cover is removed to display the tilted riffled bowl. (photo: Robin Grayson)

The ItomakTM bowl is a novel centrifugal concentrator developed by SMA ITOMAK [184,185,186], a company spun out of a Novosibirsk scientific centre: www.itomak.com. The bowl is riffled and spins tilted in smaller models and horizontally with the larger models, unlike western bowl centrifuges – KnelsonTM, FalconTM, Knudsen, NeffcoTM, GoldkatchaTM etc – whose bowls spin round a vertical axis. The larger ItomakTM bowls resembles the Chinese Yunxi and SLS bowls that spin round a horizontal axis. The horizontal attitude has some advantages, such as the rotary mechanism being above the water when at rest.

Figure 130.

Adoption by placer gold miners Itomaks have been used in placer mining since 1996, and are installed in Western Siberia, the Urals, in YakutyaSakha, Buriatia, Hakasya, Mining Altai, Krasnoyarsk, Khabarovsk edge of the Magadan and Amur areas, and also Kazakhstan, Vietnam, Tanzania, Mongolia and South Africa.

GOLD RECOVERY BY ITOMAKTM BOWL – Novosibirsk tests

Recovery of placer gold by Itomak centrifuge bowl, based on information from the manufacturer. (compiler: Robin Grayson)

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FalconTM SB bowl – 1990s research in British Columbia

Operation

Figure 127.

The feed is screened at 2-6mm and fed as slurry from above via a slurry pipe that discharges near the bottom of the bowl where it is flung by an impeller towards the wall. Rotation is very fast to achieve 50 to 300g and this slams the concentrate against the wall and induces density stratification of the slurry. Upon rising up the outwardly sloping wall, the stratified slurry passed over a concentrate bed fluidised from behind by back-pressure water. The fluidised bed enables the Falcon SB to process and retain particles in the concentrate bed in preference to lighter minerals that are ejected as continuously discharging tailings. The Falcon SB achieves a concentration ratio of 1,000 or more and so the volume of concentrate is low. Periodically the feed is stopped to permit the concentrate to be rinsed out of the bowl as a batch discharge. A typical operating cycle is 2 to 4 hours. Recently Falcons have been awarded a patent for continuous discharge (US #6,796,934).

FALCONTM SB BOWL

Looking down a Falcon SB bowl, showing the smooth lower separation area overlain by a riffled upper separation area. (photo: Falcon Concentrators Inc – www.concentrators.net)

The FalconTM Superbowl (SB) was invented by Steve McAlister to complement the original FalconTM B bowl. The new SB bowl was tested on a placer gold deposit in British Columbia and patented in 1995 (US #5,462,513) [125,153]. The floor of the Superbowl has an impeller, and the lower two-thirds of the wall are a smooth ‘migration zone’. However the upper third of the wall bears horizontal riffles with pores for water injection in the intervening grooves, acting as a ‘retention zone’. This is based on the elutriated centrifugal bowl patented 50 years before by Arnold Nesbitt McNicol in Australia (AU #17487/34 and AU #22055/35). Recovery of fine gold by the Falcon and Knelson bowls are somewhat similar [125,134,140, 142,145,180183].

Adoption by placer gold miners Falcon Superbowls are occasionally used in washplants in placer gold operations: ² ² ² ² ² ² ²

Crescent Mining, Guyana – Falcon SB250; Sunshine Fields Corp, Philippines – Falcon SB250; Promotora Minera Salemex SA de CV, Mexico – Falcon SB750; Umico Ltd, Lukarasi Project, Tanzania – Falcon SB750; Nolan Gold Mine, Alaska – Falcon SB2500 in1° recovery; Garraway Resources, Guyana – Falcon SB2500; and Apollo Gold, Venezuela – Falcon SB2500.

Falcon Superbowls are occasionally used to recover gold as a by-product of sand and gravel mining: ²

²

Figure 128.

Teichert Aggregates, California – Falcon SB2500 cleaning jig tails, plus a Falcon SB750 cleaning table tails; and Montezuma Aggregates, USA – Falcon SB2500 in automated 1° recovery, plus an SB750.

GOLD RECOVERY BY FALCONTM SB BOWL – based on recovery of tungsten tracer

Recovery by FalconTM Super Bowl, according to lab experiments with tungsten tracer [140]. (compiler: Robin Grayson)

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FalconTM C bowl – 1980s research in British Columbia

Operation

Figure 107.

The feed is screened at say 2mm and fed either dry or as slurry from above via a feed pipe that delivers the feed in an ‘impeller zone’ near the bottom of the bowl. Here the feed is flung tangentially to the foot of the wall by the high apparent gravity of the spinning bowl plus the action of the impeller. The impeller reduces the energy needed to rotate the bowl and helps to drive the slurry. Rotation is very fast to attain 20 to 300g [152]. The feed is driven by the high apparent gravity up the lower part of the centrifuge is gently conical, tapering outwards at 8-14°, and termed the ‘migration zone’. Within this zone, the flowing film becomes highly stratified with the gold against the wall and the lighter particles and liquid forming a superficial layer. Rising up the outwardly tapering wall of the migration zone, the flowing film enters the cylindrical upper part of the centrifuge termed the ‘retention zone’. Gold arriving in the retention zone displace lighter particles that are liberated into the jacket-like discharge chamber to be ejected as tailings. Since 1990, the device is fitted with an AutoPAC that controls the rpm of the rotor and feed valve and automates a 90-minute cycle time interrupted by a mere 30-40 seconds for automatic water flushing of concentrate down the hollow drive shaft. The percentage recovery of very fine gold is high, but the concentration ratio is low. Therefore the concentrate is not rich enough for smelting and the device is best used as a pre-concentrator and for upgrading tailings.

FALCONTM C BOWL

Generalised view of a FalconTM C bowl, modified from the patent. (drawing: Robin Grayson)

The FalconTM C bowl is the original Falcon invented by Steve McAlister after noting fine gold in the sand/slime stream of gravel mines in British Columbia. A prototype was tested in 1981. By 1986 the first commercial FalconTM was operational and patented in 1989 (US #4,824,431) [152-155]. Although largely eclipsed by later versions, experiments continue to deepen understanding of the device [156] – www.concentrators.net. The FalconTM C bowl has a very smooth inner wall. It differs fundamentally radically from the KnelsonTM C bowl by NOT having riffles or elutriation water and so lacks pores and has no water jacket. Rather than trapping gold in a fluidised bed maintained by water injection, the FalconTM C bowl uses high apparent gravity to induce density stratification in a thin flowing film.

Figure 108.

Adoption by placer gold miners The Falcon C bowl was originally designed to recover placer gold, but has been superseded by the Falcon SB bowl in placer gold operations.

GOLD RECOVERY BY FALCONTM C BOWL – based on recovery of gold tracer

Recovery by FalconTM C bowl, according to lab experiments with gold tracer, and users claims. (compiler: Robin Grayson)

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KnelsonTM bowl – 1980s research in British Columbia

Operation Gold is recovered in an inner bowl in a centrifugal field with an apparent gravitational field of 60g. By injecting pressurised water into the inner bowl maintains a fluidised bed of black sand into which gold particles can burrow. Black sand is dislodged to make room. Basic models have batch discharge (manual or automatic), the operation being stopped to permit the flushing out of concentrate. In larger models, discharge may be continuous, enabling uninterrupted operation.

Adoption by placer gold miners The KnelsonTM bowl did not become widely popular in placer gold recovery due to cost and need to pre-screen: ²

Figure 105.

KNELSONTM BOWL

²

A 3-inch KnelsonTM concentrator. with ceramic bowl ready to insert. (photo: KnelsonTM Inc – www.knelsongravitysolutions.com)

The KnelsonTM bowl is a centrifugal bowl inside a water jacket, elutriated water being injected under pressure via holes to prevent packing of the black sand trapped in the riffles of the bowl wall. The KnelsonTM bowl was invented by Bryan Knelson of British Columbia in the 1970s [129,130]. He patented the device in 1986 (US #4,608,040) as an advance on the elutriated centrifugal bowl patented 50 years before by Arnold Nesbitt McNicol in Australia (AU #17487/34 and AU #22055/35). Bryan Knelson continued innovating, gaining more patents in 1988 (US #4,776,833) and 1991 (US#4,983,156). Later patents cover refinements, notably continuous discharge (US#5,338,254) and mercury recovery (US #5,368,541). The KnelsonTM bowl became a popular subject for theses [131-137] and tests [138-148].

²

²

²

lab testing – Bajo Cauca and El Bagre placers in Antioquia, Columbia. Coarse gold recovery had been 40-70% with sluice boxes and jigs. Fine gold recovery had been low. Tests with 3-inch and 7.5-inch KnelsonTM bowls yielded recoveries of 9899.9% of fine placer gold [148]. lab testing – Ikh Alt Mine in Zaamar Goldfield, Mongolia. Pay gravels sampled in the harsh winter and trucked to Ulaanbaatar to determine grade using a 3-inch KnelsonTM bowl fitted with a vibrating sluice and spray bar [149]. pilot mining – Toson Terrace Mine in Zaamar Goldfield, Mongolia. KnelsonTM concentrators proved very effective in test mining [150], being superior to Russian-style sluices. Setbacks occurred in 1997-99 with the failure of Java Gold Inc at this mine using KnelsonTM concentrators – due to management not technology [151]. full-scale mining – Howley Mine of Metana Minerals NL in Western Australia. Initial test recovery was disappointing with a trommel-jig washing plant. KnelsonTM bowls increased gold recovery by 35%, permitting reworking of a million m3 of jig tailings despite high clay content. [129]. full-scale mining – Akrokeri-Ashanti Ltd (AAGM) in Ghana. Partial success, but mine closed due to various factors, including lack of mobility of the wash-plants.

GOLD RECOVERY BY KNELSONTM BOWL – based on recovery of tungsten tracer Recovery of placer gold by KnelsonTM bowl, according to lab experiments with tungsten tracer [140]. (compiler: Robin Grayson) Figure 106.

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Yunxi bowl – 1960-1990s research in Yunnan

centrifugation IFFC” [125-127]. The bowl’s rotation is adjustable from 83 g to a phenomenal 1,500 g. The fast rotation creates a stratified bed of moving particles that climbs the wall to the rim where they are dislodged by a high pressure water jet. The SLS bowl has six models with capacities from 110 kilos to 12 tons/hour, all designed to recover fine and ultrafine cassiterite particles as a continuous discharge. Adjusting the operating parameters enables high recovery of 1μ cassiterite [125], and commercial tests showed 55-60% recovery of cassiterite from <10μ tailings with an upgrading ratio of 10 [127]. Using the SL-600 Separator on cassiterite slimes, the most suitable size range for cassiterite recovery was 3-37 µm [125, 127].

Operation Figure 103.

The author has not seen a description of how the Yunxi bowl operates. Tin ore is first screened to <74μ and a suitable slurry prepared. The slurry is gravity fed into the rotating Yunxi bowl and the heavies are slammed on the inside wall of the centrifuge. The amount of enhanced gravity is 102 g, 51g and 30g for bowls of diameters of 40cm, 80cm and 1.6m respectively. The heavies remain on the bowl wall as concentrate to await batch discharge, while the lighter particles are ejected as a continuous discharge of tailings. After a period, the device is stopped for batch discharge of concentrate. The stopping, discharging and restarting are automatically controlled.

YUNXI BOWL

Generalised arrangement of an 80cm diameter Yunxi bowl, after Y. Sun [123,124]. (drawing: Robin Grayson)

The Yunxi bowl was developed by the Yunnan Tin Corporation in the early 1960s in China for recovering extremely fine cassiterite (SnO2) [124]. For decades unknown in the west, over 20 years the Yunxi bowl was the world’s best centrifuge for recovering fine minerals, until the advent of the KnelsonTM bowl, FalconTM bowls, MozleyTM MGS bowl and KelseyTM centrifugal jig. For three decades the Yunxi bowl remained radically different from other bowl centrifuges until the birth of the Russian ItomakTM bowl. The Yunxi bowl is a short centrifuge that rotates on a horizontal axis. The bowl is near-parallel sided, sloping 35° to its discharge end. A limitation is the low capacity, e.g. about 30 tons/day for an 80cm bowl [124]. In the late 1980s the Yunxi bowl spawned the SLtype separator (SLS) with “injection flowing film

Figure 104.

Adoption by placer gold miners About 1,000 Yunxi bowls were installed in tin, tungsten and iron ore plants in China [128]. However the author has found no evidence of either the Yunxi bowl or its successor the SLS bowl being used in gold recovery.

CASSITERITE RECOVERY BY YUNXI BOWL – China tests

Recovery of cassiterite by the Yunxi bowl and its successor the SLS bowl according to Jie Xiao [125]. (compiler: Robin Grayson)

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Neffco bowl – 1970s research in Utah

Operation Feed material is screened at 0.3 to 1.5mm, then preferably to <100 Tyler mesh as larger solids may block the groove. If feed is dry it must first be thoroughly wetted, for instance in a longish water trough. The Neffco bowl runs full of water as it spins. Slurry is fed down the central tube to the bottom of the bowl and works its way to the sidewall. The bowl and its spiral groove drag the water and induce the water to spin. “It Figure 59.

looks like a reverse auger as the groove appears to auger down to the bottom of the bowl” (source:

NEFFCO BOWL

PopandSonminers). The drag of the water may aid heavies to move to the sidewall and lights to spiral up. The slurry is continually swept down the spiral groove. As the slurry spirals down, the heavies end up at the bottom of the groove while the lighter particles escape upwards and out the top. The end result is that a high percentage of heavies are driven to the bottom of the bowl. Periodically the bowl is stopped for batch discharge. The rpm is fixed, and control is exerted by altering the water input (source: Zooka). The water level is maintained 1.5 inches below the rim and at the rim, with less water for finer cuts. The 24-inch Neffco bowl will run 1-8 tons per hour of solids, uses a ½hp motor, rotates twice per second, uses 125 gallons per minute of water and is claimed to be able to capture 500-mesh gold (25μ).

Examining the inside of a Neffco bowl, showing the spiral riffles. (photo: courtesy of DanfromNY on the Alaska Gold Forum)

The Neffco bowl was invented by Larry Neff of Utah. He began experimenting with centrifugal recovery of flour gold in 1976 and by 1980 had invented the novel Neffco bowl – it has a single long riffle groove that spirals downwards. Other bowls have grooves in rings. The Neffco bowl is made by Neffco Mining, of Salt Lake City, Utah – www.neffcomining.com. The maker claims the Neffco bowl, when part of a complete wash-plant, recovers “better than 95% of the of the flour gold in most materials we have tested”. Anecdotal evidence indicates the Neffco bowl is capable of recovering fine gold. But there are reports of very rounded gold particles sometimes flowing (rolling?) straight out of the unit although these can be caught before or afterwards with a standard sluice (source: Zooka of AGF). To achieve capacity, often 4 or more bowls are run in parallel, and – presumably because of the escape of large high sphericity gold – a long length of expanded metal sluice is used to scavenge the tailings. The Neffco bowl achieves a concentration ratio of 2,000 to 1. To clean concentrates, material is run through two bowls in series, and the second captures about 5% extra.

Figure 60.

Adoption by placer gold miners The Neffco bowl is fairly popular among placer gold miners in North America, mainly for cleaning concentrate. A few mines use Neffco bowls in parallel as the core of their primary wash-plants, plus a sluice to catch the round gold that escapes. Some recreational miners in Alaska use Neffco bowls aboard small offshore dredges.

GOLD RECOVERY BY NEFFCO BOWL – generalised

Recovery of placer gold by Neffco bowl, according to manufacturer’s information and comments by users. (compiler: Robin Grayson)

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Gilkey bowl – Alaska tests

Operation

Figure 37.

First, mercury is poured into the stationary bowl, a little less than the capacity of the mercury channel in the side-wall. The bowl is then rotated to drive the mercury into the channel. Next the screened feed slurry is introduced, and the feed density has to be strictly controlled at 20% solids [59,60]. The feed slurry is introduced under a small head of gravity via a central feed delivery pipe, pouring into the middle of the bottom of the bowl. From here the slurry is forced to slam into the sidewall. To minimise flouring, and loss of mercury, the slurry first hits a ‘striking surface at the bottom of the sidewall. Then the slurry flows up the inner wall of the spinning bowl to cross the rotating mercury held in the recess of the channel. The tailings are ejected from the spinning rim of the bowl as a continuous discharge and the risk of mercury being accidentally lost is high so the tailings discharge is immediately intercepted by a mercury trap. The gold particles in the film of slurry crossing the mercury are forced to amalgamate. After a period the feed is shut off and then the power shut off to slowly bring the spinning bowl to rest. The gold is then recovered from the mercury by squeezing through a fine cloth to retain the Au-Hg amalgam as a paste, and the amalgam is then subjected to firing and retorting to separate and recover the gold and recycle the mercury.

GILKEY BOWL

A 6-inch Gilkey bowl, showing the flow of the slurry feed and redirection by the ‘striking surface’ to minimise flouring of mercury. (redrawn from James Anderson [59] by Robin Grayson)

The Gilkey bowl was invented by Walter W. Gilkey of Washington State during the 1960s, and only advanced versions patented. The basic version is a symmetrical steel bowl with open mouth upwards, dynamically balanced around a drive spindle below the bowl. Part of the inner wall is a broad cylindrical channel that holds the mercury. The Gilkey bowl was one of the last of the ‘forced amalgamators’ that had been popular for over a century to recover ‘fine’, ‘flat’ and ‘flour’ gold by adding mercury to a spinning bowl to centrifugally press gold into mercury. Many forced amalgamators were invented and patented, such as the McKlellar amalgamator (US #1,003,118 of 1911); the Taber amalgamator (US #1,457,560 of 1923); and the Lorentsen machine (US #1,866,111 of 1932).

Figure 38.

Adoption by placer gold miners The Gilkey bowl and other ‘centrifugal forced amalgamators’ were popular on large gold dredges in North America. The Gilkey bowl was a focus of research of the Mineral Industry Research Laboratory in Alaska [59,60] in the 1970s but manufacture then ceased. A few Gilkey bowls may still be in use in parts of Africa and South America.

GOLD RECOVERY BY GILKEY BOWL – Alaska tests

The Gilkey bowl achieves 60% gold recovery at the 1st pass, 90% at the 2nd stage and 99% at the 3rd stage. [60]. (compiler: Robin Grayson)

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Knudsen bowl – Alaska tests

Operation Pre-screened feed enters from above via a slurry pipe that discharges in the centre of the spinning bowl about 1inch from the base. The slurry is flung sideward to slam against the spinning wall. The climbing film of slurry is impeded by concentric ribs of tough rubber riffles. Dense particles slam in the grooves between the riffles; Light particles climb the riffles to escape as tailings. Lab tests by Mark Anthony [57,58] show the inventor’s instructions to be in error. With <3/8 inch gravel “the rocks

could not be washed out of the riffles with a high-pressure hose and had to be removed by pulling them out of the rubber riffles with a pair of pliers.” This was solved by Figure 35.

limiting the feed to <1/8th inch. The rotation speed has to be at least 80 rpm or “…the bowl started dumping large

KNUDSEN BOWL

Knudsen Bowl showing the central feed pipe, rubber riffles and 3 struts that enable the position of each blade to be adjusted. (photo: Steve Gaber of the Alaska Gold Forum)

quantities of slurry directly onto the ground and drive mechanism.” Optimum recovery of gold >65 Tyler mesh

The Knudsen bowl was invented by George Knudsen of California and patented in 1942 (US #2,272,675). It eclipsed the first rubber-riffled centrifuge – the Ainlay bowl invented by Thomas Ainlay of Nebraska and patented in 1928 and 1932 (US #1,658,874 and #1,853,249). The Knudsen bowl has particular advantages:

(about >0.2mm) is at 105 rpm with 68 gallons/minute of water and a pulp density of 5-20% solids. Tests by ‘Dredger’ of Alaska Gold Forum in New South Wales, Victoria, Tasmania and New Zealand suggest Knudsen bowls lose about 20% gold, mainly invisible fine gold whose recovery can be much improved by:

² ² ² ²

bowl of non-magnetizable material e.g. aluminium: resistant to wear, and easily, completely and quickly cleaned; tough rubber riffles as liner, easily and quickly removed; and separation helped by “…agitation resulting from the wobbling

² ² ²

action of the bowl and by the kneading action of the soft flexible rubber ribs of the riffle member”.

² ² ²

The Knudsen bowl is typically 12 to 36-inch in diameter and mounted on a vertical drive shaft. The wall slopes outward to create a gradient of g forces increasing upwards, inducing the slurry to climb the wall. The need for fine pre-screening makes the Knudsen bowl unsuitable for wash-plants, but it has merit for upgrading if two or even three bowls are put in series.

Figure 36.

²

using a binocular microscope to detect and monitor fine gold; screening the feed at minus 250 mesh (e.g. a second pass); adding 2 variable speed v-belt pulleys, with lockable lever speed control, enabling precise adjustment of the bowl’s rpm; ensuring a person watches the feed rate; attaching a good small test sluice to the outlet of the bowl; checking tailings with a x40 to x80 binocular microscope; and cleaning the bowl regularly.

Adoption by placer gold miners The Knudsen bowl was used worldwide, notably in North America and New Zealand, and copycats in Africa and South America. The Knudsen survives being rugged, easy to use, cheap, durable and ease of adding mercury.

GOLD RECOVERY BY KNUDSEN BOWL – Alaska tests

The Knudsen bowl recovers 100% of gold >0.2mm, plummeting to 70% for 100μ gold, according to Alaska tests [57,58]. (compiler: Robin Grayson)

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Kelsey centrifugal jig – 1980s research in Australia

Operation The slurry feed is 25-40% solids and enters a chamber consisting of a circular screen mounted vertically, plus a particle bed. The chamber rotates at 30-45 rpm to create a force of up to 50g. Water injected into the inner chamber subjects the slurry to a jig-like pulsation force to alternately expand and compress the particle bed. This stimulates denser minerals to move towards the bed and so be collected after passing through the screen into an inner chamber and discharged via small holes. Lighter particles overflow the chamber as tailings. According to the maker, “the ability to change the Figure 101.

apparent gravitational field, up to 50 times gravity, results in a major improvement in separation efficiency, particularly of very fine minerals, by significantly reducing the effect of forces that hinder fine particle separation.”

KELSEY CENTRIFUGAL JIG

A model J1800 Kelsey centrifugal jig. (photo: courtesy of Dale Henderson of makers Roche Mining – www.rochemt.com.au)

The Kelsey centrifugal jig was invented by Christopher G. Kelsey of Australia and patented in Australia in 1985 (PH9037, PG0122) and 1990 in the USA (US #4,898,666). The first commercial sale was of a labsized machine in 1989. Geo Logics Inc developed the device, and automatic screen cleaning was introduced in 1999. In 2001 Geo Logics was bought by Roche Mining who continues to make the jig (www.rochemt.com.au). The Kelsey centrifugal jig maximises its effectiveness and efficiency by combining the pulsation principle of a jig with the high apparent gravitational field of a centrifuge. The main models and operating variables are: ² ² ² ²

²

²

Hindered settling is accentuated by the centrifugal force together with the pulsing of the ragging bed. The pulsing is via pulse arms connected to pads to work against the jig’s flexible diaphragm. “Water contained

within the concentrate hutches presses against the diaphragm, at a frequency and amplitude set by the operator, thus dilating the ragging bed. The level of dilation impacts on the amount of material able to pass to concentrate.” The pulsed shockwaves have two effects: ² ²

J200 KCJ – lab test unit, 15-100 kg/hour of solids; J1300 MkII KCJ – smallest commercial unit, 2-30 tons/hour; J1800 KCJ – largest commercial unit, 5-60 tons/hour. feed-related variables – feed type; feed density; feed rate; feed size; and density difference between minerals. jig set-up variables – screen aperture size; ragging type, ragging density, ragging size distribution and depth. jig control variables – rotational speed (induced gravity field); pulsing frequency; pulse amplitude (distance travelled during each pulse stroke); and rate of adding hutch water.

Figure 102.

dilating the ragging bed to allow minerals to enter it; and accentuating differences in acceleration between particles of different density. Separation of particles of similar size and shape but different density slows at their terminal velocity. The shockwaves repeatedly stop the particles, limiting their time at terminal velocity to maintain a high rate of separation.

Adoption by placer gold miners The Kelsey centrifugal jig is used to recover hardrock gold, platinum, cassiterite, monazite sand, coal etc [117122]. It performed well with fine flat placer gold from offshore Alaska [122] but is not seen in placer gold mines.

GOLD RECOVERY BY KELSEY CENTRIFUGAL JIG

Recovery of placer gold by Kelsey centrifugal jig, according to the patent and test on Alaska Gulf placers [122]. (compiler: Robin Grayson)

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GekkoTM in-line pressure jig – 1990s research in Australia

Operation

Figure 125.

The maximum feed for a standard IPJ is 25mm – remarkably large. The slurry is pumped along a feed pipe into the top of the jig. Entering the distributor, the slurry is turned around upwards to spread out in the deceleration chamber. The jig is pressurised, sealed, and completely filled with water and slurry. The lack of a water surface is a deterrent to flour gold floating away. The spreading motion, and unimpeded large circular area available for it, causes the slurry to slow to a velocity incapable of holding so much suspended solids. So the ‘heavies’ drop towards the jig bed and are trapped in the separation interface on the top of the jig bed. The jig bed is pulsed by a central drive shaft driven by the central drive ram at the unit’s base. The pulsing achieves episodic ‘quicksand’ (thixotropic) conditions. During the dilation phase, gold separation occurs by density, plus segregation by differences in size and shape. In the suction phase, heavies are drawn through the ragging and screen to fall into the hutch – drawn off by a spigot as a continuous discharge. Nuggets stay on the jig screen awaiting batch discharge. The ragging is a 25mm thick carpet of lead shot covering the 2-3mm holes of the jig screen. The IPJ 1500 uses about 200 kilos of lead shot and Gekko has begun making ‘synthetic’ ragging.

GEKKOTM IN-LINE PRESSURE JIG

A skid-mounted IPJ at a placer gold mine in Australia. (photo: courtesy of Nigel Grigg of Gekko – www.gekkos.com)

The GekkoTM in-line pressure jig (IPJ) was invented by Alexander Gray of Victoria who was awarded patents in 1995 in Australia (AU #W095/26,232) and 2000 in the USA (US #6,079,567). The innovation was intended for recovery of low-grade high volume placer gold; later for hard rock grinding circuits [174-178]. A slurry pump drives the entire system. The negligible pressure drop ensures a 15-metre head to pump tailings up to a settling area, and allows jigs to be arranged in series if so desired. The IPJ has a moveable, rather than fixed, jig screen. The operator can dictate the height the jig bed rises and falls; and all particles are raised to the same height regardless of particle density. Jigging is a sawtooth pulse rather than simple pulse, and the operator can select the upstroke and downstroke speeds independently, as well as adjust the stroke duration via electronic control of the hydraulic drive. The jig is very compact, treating up to five times more for same area of jig screen. The trommel or shaking screen can be low as the slurry is lifted to the jig by slurry pump. Water demand is lower than for most other jigs.

Figure 126.

Adoption by placer gold miners Many IPJs are in hardrock gold mills. Placer applications include tin (New South Wales) and diamonds onshore and offshore in Namibia and South Africa. About 20 are installed at placer gold mines, in Australia, New Zealand, Papua New Guinea, North Korea, Guinea and Peru.

GOLD RECOVERY BY GEKKOTM IN-LINE PRESSURE JIG - generalised

Recovery of placer gold by Gekko in-line pressure jig, based on information from the manufacturer. (compiler: Robin Grayson)

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Cleaveland jig and IHC jig – 1980s research in USA and Holland

Operation

Figure 92.

Feed is screened to <12.5mm and pumped as slurry to the narrow end of the primary jig bed of the IHC jig plant. The trapezoidal shape of the jig bed widens towards the tailings end to counteract and minimise the unwanted acceleration in cross-flow typical of a normal type of jig. The trapezoidal primary jigs can be clustered in a compact 12-cell star with a single central feed – a major advantage aboard large dredges. In land-based placer gold mining, the optimum configurations are 1-cell, 2-cell and 3-cell trapezoidal jigs otherwise the wash-plant is too bulky and trucking placer becomes excessive [106]. When integrated into a skid-mounted wash-plant the units are semi-mobile [107,108]. The jig screen of the primary jig produces tailings of <6mm that report as slurry to a “second stage”, a small secondary jig who jig screen in turn produces tailings of <4mm that are recycled back to the primary jig. A “third stage” is sometimes added to the series. Coarse gold and nuggets remain on the jig screens for recovery as a batch discharge at clean-up, while small gold is produced as a continuous discharge with black sand via spigots at the bottom of the hutches of the jigs.

IHC JIG PLANT

An IHC jig plant recovering fine gold in Sumatra, Indonesia. (photo: IHC Holland – www.ihcholland.com)

The Cleaveland jig was invented by Norman Cleaveland of New Mexico and patented in 1982 (US #4,310,413). The jig is an innovative circular jig intended for large tin dredges, a compact star-pattern of jigs with the slurry introduced at the centre [93]. IHC Holland of the Netherlands and Alluvial Dredges Ltd (ADL) of Scotland and Australia made Cleaveland-type jigs. IHC made vast research into jigs led by the Mineral Technological Institute (MTI) in the Netherlands [94-97]. IHC developed the Cleaveland jig further as a circular array of trapezoidal jigs with sawtooth jigging motion, a motion now emulated in most jigs. The short-lived fierce upward stroke prevents the loss of fine gold by suppressing the phase of hindered settling is suppressed. IHC marketed its jigs for large tin dredges. After the tin price collapsed, IHC marketed the ‘IHC trapezoidal jig’ for recovery of placer gold, hardrock gold and diamonds on dredges, pontoons and dry land [98-105].

Figure 93.

Adoption by placer gold miners IHC trapezoidal jigs were installed on the Bema Dredge, a famous bucket-line gold dredge off the coast of Alaska. Several winterised gold bucket-line dredges with IHC jigs operated in NE China. IHC jig-plants of 1, 2 and 3-modules were installed in placer gold mines in Alaska, Peru, Columbia, Ghana, Indonesia, Mongolia and elsewhere, gaining a reputation for recovering >90% of moderately fine gold. Failure to gain wider popularity is attributed to the high price, bulkiness, limited mobility, and low concentration ratio.

GOLD RECOVERY BY IHC TRAPEZOIDAL JIGS DERIVED FROM CLEAVELAND CIRCULAR JIGS

Placer gold recovery extrapolated from recovery of placer tin, as claimed in IHC technical literature. [96] (compiler: Robin Grayson)

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Pan-American duplex jigs – Alaska tests

Operation

Figure 33.

The upstroke of the rocker arm lifts the lower hutch and compresses the diaphragm, forcing water up into the upper hutch and up through the static screen to lift in the ragging and its blanket of black sand, so fluidising the sand into a thixotropic (‘loose’) state. Less dense particles are flung up and swept away with the tailings. Dense particles remain. Gold particles burrow in the black sand down into the heavy ragging. The downstroke extends the diaphragm, sucking water down from the upper hutch and down through the static screen to suck the ragging and its blanket of black sand, compressing the black sand into a dilatant (‘hardpacked’) state. Small gold is sucked down into the upper hutch to fall and settle on the bottom of the lower hutch for continuous discharge with black sand via a spigot. Gold particles too big to pass though the jig screen remain stranded on the screen awaiting recovery when jigging stops for cleaning the screen – a batch discharge. The upstroke of the lower hutch pushes more water through the screen than replaced in the downstroke – compensated by inlets injecting water in the upper hutch.

PAN-AM DUPLEX JIGS

A duplex of Pan-American jigs. (photo: courtesy of the manufacturer, Delta Mining & Manufacturing Co of Nashville, Tennessee - www.graymfg.com/mineral.html)

In the early 1930s the engineers of the Bulolo Gold Dredging Company designed the Pan-American placer jig (Pan-Am jig) that is compact and tolerates wave motion in small dredge ponds or even open sea. By the late 1930s the Pan-Am jig was widely used on dredges for recovery of alluvial cassiterite (SnO2 tin ore), gold and diamonds. The Pan-Am jig is a balanced pair of jig cells known as a duplex jig, saving 50% of energy in jigging. Each cell is underlain by a conical hutch of two parts joined by an annular diaphragm of flexible rubber to allow up-anddown oscillation of the lower hutch. Standard 42inch x 42inch cells (about 1m x 1m) have seven variables [56]: ² ² ² ² ² ² ²

Adoption by placer gold miners Pan-Am duplex jigs are fairly popular in many regions, and can be seen in action in Alaska, Yukon, South America, and Africa but are rare in Russia and Mongolia Pan-Am duplex jigs are made in many regions, e.g.: .

²

amount of ragging – typically 425lbs (193 kilos) per cell; type of ragging – typically 3/16 inch (4.75mm) steel shot; feed pulp density – 30% to 60% (w/w); feed rate – 20 yd3 to 30yd3 per hour (15 to 23m3 per hour); hutch water added – 2.3 to 4.5m3 per hour per jig cell; stroke length – ¾ inch to 1½ inch (19 to 38mm); and stroke frequency – 120 to 200 cycles per minute.

Figure 34.

²

²

²

²

USA – IRD of Carson City, Nevada www.ird-jigs.com USA – Delta Mining & Mnfr Co of Nashville, Tennessee www.graymfg.com/mineral.html USA – Goldfield Engineering Co, of Lindon, Utah www.goldfieldeng.com Thailand – Dove Engineering www.dovemining.com China – China National Gold Corporation (CNGC) www.chinagold.org.placer.html

GOLD RECOVERY BY PAN-AMERICAN DUPLEX JIGS

Good recovery of gold by Pan-Am duplex jigs during tests by MIRL in Alaska [56]. (compiler: Robin Grayson)

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tincture of iodine leaching – 2000s research in Japan

Iodine leaching had been widely used to recover gold in the late 1800s and early 1900s, then dwindled with the rise in popularity of cyanide leaching and mercury amalgamation and the high cost of iodine. Advances prior to 2000 are dealt with in Section 4. In the last 25 years much has been claimed about ‘secret’ lixiviant formulations, and methods of precipitating gold from streams and seawater – most is quackery. In contrast, the tincture method of iodine leaching is fully explained, repeatable and verifiable, as invented in 2006 Hiroyasu Murakami and Y. Nakao of the National Institute of Advanced Industrial Science and Technology (AIST) of Japan: ‘A trial of extracting gold from stream sediment and high Au/Ag ore using halogen-containing organic system’ [40]. To dissolve the gold, a halogen-containing organic system (HOS) is used, composed of iodine I2, sodium iodide NaI and ethanol C2H5OH, a mixture closely akin to ‘tincture of iodine’. Tincture of iodine is usually 10% elemental iodine in ethanol, and a component of emergency survival kits to disinfect wounds and to sanitize surface water for drinking. To precipitate the gold from solution, ascorbic acid C6H8O6 is added – a chemical familiar as vitamin C. Iodine being expensive, it is important to regenerate the tincture of iodine. Hydrogen peroxide H2O2 is added to oxidise the iodide I- back to iodine I2. The regenerated tincture of iodine can once again dissolve gold. The Japanese researchers recovered 79.9% gold from a stream sediment sample, and 84% gold from a hardrock sample in the Kitakami mountains of northern Japan.

Figure 141.

Operation Tincture of iodine is obtained from a lab supplier. If placer ore, the pay gravel is finely screened, the coarse fraction being subjected to conventional gravitational separation, the fine fraction being subjected to leaching. If hardrock, the ore is milled very finely before being subjected to leaching 1st stage – leaching gold into solution The fine ore is added to a little water in a tank and kept agitated by stirring. Tincture of iodine is stirred in, and is dark brown due to the presence of I3- ions. These are an effective oxidant and in the presence of I- ions reacts to form the stable gold-iodine complex: 2Au + 3I3- 2[AuI4]- + ILab experiments show an hour is sufficient for the tincture of iodine to leach most of the gold – faster than cyanide can. Then dissolution declines and full leaching of gold from a saturated gold solution might take six hours. 2nd stage – recovering gold from solution To recover the dissolved gold from the pregnant solution, ascorbic acid is added to reduce the iodine: I2 J 2IThis reaction results in a deficiency in I3 causing the tincture to lose its dark brown colour and become a poor solvent. It is now possible to precipitate gold. Deposition starts when the tank is diluted by about 70% water by volume. The gold appears as colloidal gold and next as fine-grained particles. The gold is removed by filtration.

Adoption by placer gold miners It seems possible that tincture of iodine may become popular among recreational and artisanal gold miners for fine gold recovery.

GOLD RECOVERY BY TINCTURE OF IODINE LEACHING

Recovery of placer gold by tincture of iodine and vitamin C in tests by Hiroyasu Murakami and Y. Nakao [40]. (compiler: Robin Grayson)

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Mozley MGS centrifuge – 1980s research in Cornwall

Operation

Figure 99.

The feed requires fine screening. The exact degree of screening is unclear, but seems to be about 0.5mm. The screened feed is prepared as slurry with 25 to 35% solids. The slurry is piped into the MGS centrifuge via a tube. The slurry is decelerated by a circular device that spreads the slurry across the internal wall of the cylinder. Within the cylinder, a stratifying mechanism separates the denser particles from the lighter particles. By lowering or steepening the angle of inclination (‘tilt’) of the centrifuge, the retention time of the slurry can be increased or decreased accordingly, along with the discharge rate of the concentrate. A differential scraper mechanism enables a longer residence time for the slurry. Additional cleaning water is added to assist the removal of slimes and enrichment of the concentrate. The Axsia-Mozley Multi-Gravity Separator is manufactured in 3 models:

MOZLEY MGS CENTRIFUGE

Schematic cutaway diagram of an Axsia-Mozley Multi-Gravity Separator MGS. (diagram: courtesy of Ian Daniels of the manufacturer Axsia Mozley Ltd - www.natcogroup.com)

The Mozley ‘Multi-Gravity Separator’ was invented by Richard Mozley of Cornwall and patented in 1990 (US #4,964,845) after applying for a UK patent in 1986. Today re-branded as the Axsia-Mozley MGS, the device is manufactured by Axsia Mozley of Cornwall, part of the NATCO Group of Houston USA. The Axsia-Mozley MGS combines the high apparent gravity of a centrifuge with the principle of a shaking table [113-116]. The device is a gently inclined cylinder that rotates to create an apparent gravitational field of 8g to 22g. The cylinder also pulsates along its shaft in the manner of a shaking table. The cylinder is lined with a few special riffles. Key advantages of the Axsia-Mozley MGS are: ² ² ² ² ²

² ² ²

The larger MGS models have special features: ²

²

a proprietary gearbox drive that gives: improved reliability; zero maintenance and sealed-for-life lubrication. the MeGaSep has hydraulic fluid drive, remote power pack and simple control technology.

Adoption by placer gold miners The Axsia-Mozley Multi-Gravity Separator is being used for the separation of ores of gold, tantalum, tin, zinc, copper, silver, lead and coal. However, the author has not seen any reports of the MGS being used in placer gold recovery.

fine to ultra-fine separation (down to 2μ); better capacity than some other ultra-fine separators; efficient use of energy; minimum maintenance or supervision; and requires no chemicals or physical reagents.

Figure 100.

MGS C900 – compact lab unit, 1 ton/hour of solids; MGS C902 – medium-capacity, 3 tons/hour of solids; MGS MeGaSep™ – large-capacity, 30 tons/hour of solids.

GOLD RECOVERY BY MOZLEY MGS CENTRIFUGE – generalised

Recovery of gold by the Axsia-Mozley Multi-Gravity Separator, based on incomplete information [113-116]. (compiler: Robin Grayson)

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List of References 1. Donald M. Hausen (2000). Characterizing the Textural Features of Gold Ores for Optimising Gold Extraction. Journal of Mining (JOM) volume 52, page 14. Download: http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/JOM-0004-14/$FILE/JOM-0004-14F.pdf?OpenElement 2. O.V. Zamyatin, A.G. Lopatin, N.P. Sammikova and A.D. Chugunov (1975). The Concentration of Auriferous Sands and Conglomerates. Nedra Press: Moscow. 260 pages. (English translation is available from Geological Survey of Canada). 3. Wenqian Wang and George W. Poling (1983). Methods for recovering fine placer gold. Canadian Institute of Mining and Metallurgy (CIMM) Bulletin, volume 76, pages 47-56. 4. Clive. J. Mitchell, E.J. Evans and Michael T. Styles (1997). A Review of Gold Particle-size and Recovery Methods. Technical Report #WC/97/14, Overseas Geology Series, British Geological Survey BGS, Keyworth, Nottingham, England, UK, 34 pages. Download: www.bgs.ac.uk 5/6. D.F. Stewart and P.W. Ramsay (1993). Improving the simple sluice box. International Journal of Mineral Processing, volume 39, pages 119-136. 7. George W. Poling and James Frederick Hamilton (1986). Fine Gold Recovery of Selected Sluicebox Configurations. University of British Columbia. Prepared for Canada/Yukon Economic Development Agreement. Indian and Northern Affairs Canada; Northern Affairs: Yukon Region, iv +77 pages. Download: www.geology.gov.yk.ca/publications/tech/fine_gold_recovery_sluiceboxes.pdf 8. Randy Clarkson and Owen Peer (1990). An Analysis of Sluicebox Riffle Performance. New Era Engineering Corporation, Yukon. Report for the Klondyke Placer Miners Association. 31 pages. Download: www.geology.gov.yk.ca/publications/tech/analysis_sluice.pdf 9. John M. West (1971). How to Mine and Prospect for Placer Gold. Bureau of Mines publication, Information Circular #8517. Read: http://imcg.wr.usgs.gov/usbmak/ic8517.html 10. Daniel E. Walsh and P. Dharma Rao (1988). A Study of Factors Suspected of Influencing the Settling Velocity of Fine Gold Particles. Mineral Industry Research Laboratory (MIRL) Publication #76: University of Alaska at Fairbanks, 52 pages. $5.00 from www.uaf.edu/sme/Mirlpub.html 11. P. Dharma Rao, David R. Maneval and Daniel E. Walsh (1984). Field Investigation of Hydrocyclones for the Recovery of Fine Gold, Phase III. June 1984, MMRRI Final Report. Mineral Industry Research Laboratory (MIRL), University of Alaska at Fairbanks. Publication OF 84-14, 96 pages. $10.10 from: www.uaf.edu/sme/Mirlpub.html 12. Eoin H. MacDonald, (1983). Alluvial Mining: The Geology, Technology, and Economics of Placers. Publisher: Chapman & Hall, Kluwer Academic Publishers, hardcover, 508 pages. 13. Robin Grayson (2006). Gold recovery in gold pans – the term ‘panning’. World Placer Journal, volume 6, pages 1-21. Download: www.mine.mn 14. Robin Grayson (2006). Gold recovery in cones in Laos – the term ‘dulanging’. World Placer Journal, volume 6, pages 36-41. Download: www.mine.mn 15. Robin Grayson (2006). Gold recovery in wooden trays in Russia – the term 'lotoking'. World Placer Journal, volume 6, pages 22-35. Download: www.mine.mn 16. Chimed-Erdene Baatar (2006). Gold recovery on mats in Mongolia – the term ‘matadoring’. World Placer Journal, volume 6. Download: www.mine.mn 17. Chimed-Erdene Baatar (2006). Gold recovery in bowls in Mongolia – the term ‘bowling’. World Placer Journal, volume 6. Download: www.mine.mn 18. Robin Grayson (2006). Gold recovery in buckets in Kyrgyzstan – the term ‘bucketing’. World Placer Journal, volume 6, pages 42-51. Download: www.mine.mn 19. Peter W. U. Appel (2005). Small-scale mining – hazards and opportunities in Kyrgyzstan and Mongolia. Geological Survey of Denmark and Greenland Bulletin #7, pages 77-80. Download: www.geus.dk/publications/bull/nr7/nr7_p77-80.pdf 20. Lars Hylander and Markus Meili (2005). The Rise and Fall of Mercury: Converting a Resource to Refuse After 500 Years of Mining and Pollution. Critical Reviews in Environmental Science and Technology, volume 35, pages 1-36. 21. Marcello M. Veiga (1997). Introducing new technologies for abatement of global mercury pollution in Latin America. UNIDO/UBC/CETEM, Rio de Janeiro, 94 pages. Download: www.facome.uqam.ca/pdf/veiga_01.pdf 22. Kristina Shafer, Lars D. Hylander and David Plath (2006). Novel solution to reduce or eliminate mercury pollution from artisanal and small scale gold mining. [ABSTRACT] Abstracts of 8th International Conference on Mercury as a Global Pollutant. Lancaster, PA, USA. Download: http://opus.uu.se/publication.xml?id=88550 23. David Plath (2002). CleangoldTM in the Guianas. PowerPoint Presentation. 17 slides.

Download: http://casmsite.org/Documents/Salvador/33-Cleangold-Plath.pdf 24. Clive J. Mitchell, E.J. Evans and Michael T. Styles (1997). The design, construction and testing of a simple shaking table for gold recovery: laboratory testing and field trials. British Geological Survey (BGS) Report #WC/97/061, DFD contract #R6226, 27 pages plus Appendices. Download: www.bgs.ac.uk/dfid-kar-geoscience/database/reports/colour/WC97061_COL.pdf 25. Michael Priester, Thomas Hentschel and B. Benthin (1993). Tools for Mining, Techniques and Processes for Small Scale Mining. GATE, Vieweg-Verlag, 537 pages. View: http://sleekfreak.ath.cx:81/3wdev/CD3WD/APPRTECH/G10TOE/INDEX.HTM 26. Marcello M. Veiga, Randy F. Baker, Stephen M. Metcalf, Bern Klein, Gillian Davis, Andrew Bamber and Patience Singo (2006). Manual for Training Artisanal and Small-Scale Gold Miners: Removal of Barriers to Introduction of Cleaner Artisanal Gold Mining and Extraction Technologies. Global Mercury Project, United Nations Industrial Development Organization (UNIDO), 76 pages. Download: http://web.uvic.ca/~gmp/documents/documents.htm 27. Weqian Wang (1979). A Study on Methods for Fine Placer Gold Processing. Non-ferrous Metals (China), volume 4, pages 6-12. 28. Robin Grayson (2010).The Gold Miners Book – BAT Best Available Techniques for Placer Gold Miners. Eco-Minex International Ltd. 1,700 pages. Purchase: www.mine.mn 29. Baatar Tumenbayar, Minjin Batbayar and Robin Grayson (2000). Environmental hazard in Lake Baikal watershed posed by mercury placer in Mongolia. World Placer Journal volume 1, pages 134-159. Download: www.mine.mn 30. Baatar Tumenbayar and Robin Grayson (2001). Mercury pollution in North Mongolia. Abstract of 6th International Conference on 'Mercury as Global Pollutant' held in Minamata, Japan, 15-19th October 2001. 31. Baatar Tumenbayar (2003). Action Research on Mercury in the Boroo Area, Mongolia. Japan International Cooperation Agency (JICA). 79 pages. 32. L. Kozin and V. Melekhin (2004). Extraction of Gold from ores and concentrates by leaching with the use of cyanides and alternative reagents. Publisher: MAIK Nauka Interperiodica. Russian Journal of Applied Chemistry, volume 77, pages 15731592. 33. Cleland N. Conwell (1980). Gold recovery from placer concentrates by cyanidation. Alaska Division of Geological & Geophysical Surveys, Miscellaneous Publication #29, pages1-6. Download: www.dggs.dnr.state.ak.us/pubs/ 34. A.K. Williams (no date). Super Clorox. Prospector's Paradise website, San Pedro, Costa Rica. View: www.prospectorsparadise.com/html/super_clorox.html 35. Rickford Vieira (2004). Mercury-Free Gold mining Technologies: Possibilities for Adoption in the Guianas. Technical Paper #1 by WWF-Guianas Regional Program Office, 8 pages. Download: www.wwfguianas.org/technicalpapers/mercfreetech.pdf 36. Lars D. Hylander, David Plath, Conrado R. Miranda, Sofie Lucke, Jenny •hlander and Ana T.F. Rivera (2007). Comparison of different gold recovery methods with regard to pollution control and efficiency. Clean, volume 35, pages 52-61. 37. Sidney Mahkatsi (2006). The iGoli mercury-free gold extraction method. Paper presented to the Communities and Small-scale Mining (CASM Annual General Meeting, 11-15th November 2006 held in Antsirabe, Madagascar, 14 pages. 38. Sidney Mahkatsi and Rob Guest (2003). The iGoli mercury-free gold extraction process. Urban Health and Development Bulletin, South Africa, volume 6, page 62. 39. Anon (2007). The iGoli mercury-free gold extraction process. Manuscript from Rob Guest of Mintek’s Small Scale Mining and Beneficiation Division (SSMB), 5 pages. www.mintek.co.za 40. Hiroyasu Murakami and Y. Nakao (2006). A trial of extracting gold from stream sediment and high Au/Ag ore using halogen-containing organic system. CCOP-GSJ/AIST-GAI CASM-Asia Workshop on the State-of-the-Art of Science and Technology to Protect the Environment and People, 27-29th November 2006, Bandung, Indonesia. PowerPoint presentation, 16 slides. Download: www.dim.esdm.go.id/makalah/2006_murakami_CCOP.pdf 41. A.K. Williams (no date). Halide Leaching and Recovery. Prospector's Paradise website, San Pedro, Costa Rica. View: www.prospectorsparadise.com/html/leaching.html 42. Jinshan Li and Jan Miller (2006). A review of gold leaching in acid thiourea solutions. Mineral Processing and Extractive Metallurgy Review, volume 27, pages 177-214. 43. Anon (2000). Gold Leaching using Thiourea. View: www.e-goldprospecting.com/html/gold_leaching_using_thiourea.html 44. A.K. Williams (no date). Acids and Bases. Prospector's Paradise website, San Pedro, Costa Rica. View: www.prospectorsparadise.com/html/acids.html 45. Marcello M. Veiga, Shefa Siegel, Patrick Schein, Cheick Santigui Camara, Joachim Dejean, Djibril Kamara and Amadou Diouf (2006). Technical Mission to the Artisanal Gold Mines in Upper Guinea. Blacksmith Institute and UNIDO Report, 13 pages. Download: http://web.uvic.ca/~gmp/countries/guinea/Guinea%20Veiga%20Blacksmith%202006%20report%20v%204%20final.pdf

46. Anon (undated). The Poormans Method of Smelting Gold. Nuggethunter. View: www.nuggethunters.org/Poormans-Smelting.html 47. Anon (undated). Smelting Gold. Nuggethunter. View: www.nuggethunters.org/smelting.html 48. A.K. Williams (no date). Melting and Smelting. Prospector's Paradise website, Costa Rica. View: www.prospectorsparadise.com/html/smelting.html 49. W. Motherwell (1914). Floatation Test at Mount Morgan. Min. and Sci. Press, 27th June issue, #53140. 50. H. Lang (1916). Black sand of the Pacific Coast. Mining Sci. Press, volume 3, pages 811-813. 51. Arthur W. Fahrenwald (1933). Floatation of Gold from River Sand and Black Sand. Min. J. (Phoenix, Arizona), volume 16, #23, 3-4. 52. Arthur W. Fahrenwald, J. Newton, W.W. Staley and R.E. Shaffer (1939). A metallurgical study of Idaho placer sand. Idaho Bureau of Mines and Geology, Pamphlet #51, 10 pages. $2.50 from: www.idahogeology.org/products/ 53. G.P. Slavnin (1936). Flotation of placer gold. 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University of Alaska, Fairbanks, E.M. thesis, 64 pages. 59. William James Anderson (1975). Parameter Impact on the Performance Ability of the Centrifugal Forced Amalgamator. Mineral Industry Research Laboratory (MIRL) Contract Report MT-1: University of Alaska at Fairbanks, 97 pages. $10.20. Download: www.blm.gov/ak/jrmic/docs/usbm/cr/AuBeachSands/CR_AuBeachSands.pdf 60. Joseph Kuo-Hua Wang (1977). Centrifugal Amalgamation in Dredge Recovery Systems. Master of Science thesis, University of Fairbanks at Alaska. Mineral Industry Research Laboratory (MIRL) Contract Report MT-2: University of Alaska at Fairbanks, 73 pages. Order: www.uaf.edu/sme/Mirlpub.html 61. Michael Silva (1986). Placer Gold Recovery Methods. California Department of Conservation, Division of Geology and Mines. Special Publication #87, 32 pages. Download: www.consrv.ca.gov/cgs/geologic_resources/gold/SP87.pdf 62. O.V. Zamyatin and A.T. Konyukova (1970). 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Gold phytomining. Novel Developments in a Plant-based Mining System. Download: www.gold.org//discover/sci_indu/gold2003/pdf/s36a1355p976.pdf?PHPSESSID=008570ced09611e1c09ef1d58d2a54d3 190. Christopher Anderson, Robert B. Stewart; C. Wreesmann; G. Smith; and John A. Meech (2003). Bio-nanotechnology and phytomining: the living synthesis of gold nanoparticles by plants. In: editors: John A. Meech; Y. Kawazoe; J.F. Maguire; V. Kumar; and H. Wang, Proceedings of the 4th International Conference on the Intelligent Processing and Manufacturing of Materials (IPMM) Sendai, Japan, 18th-23rd May 2003. CD-ROM. 191. A.E. Lamb (2002). Methods for the recovery of gold from plant ash. M.Tech. thesis, Massey University, Palmerston North, New Zealand. 192. A.E. Lamb, Christopher Anderson and Richard Haverkamp (2001). The Induced Accumulation of Gold in the Plants Brassica Juncea, Berkheya Coddii and Chicory. Chemistry in New Zealand, volume 65, pages 34-36. 193. Steve Gaber and Jason Gaber (2004). Sluice Bench Testing, Phase 1. [PopandSon Sluice] View: www.49ermike.com 194. A. Acrivos and E. Herbolzheimer (1979). Enhanced sedimentation in settling tanks with inclined walls. Journal of Fluid Mechanics, volume 92, 435-457. 195. J. Zhou, K. Walton, D. Laskovski, P. Duncan and Kevin P. Galvin (2006). Enhanced separation of mineral sands using the Reflux Classifier. Minerals Engineering, volume 19, pages 1573-1579. 196. D. Laskovski, P. Duncan, P. Stevenson, J. Zhou and Kevin P. Galvin (2006). Segregation of hydraulically suspended particles in inclined channels. Chemical Engineering Science, volume 61, pages 7269-7278. 197. Elham Doroodchi, J. Zhou, D. Fletcher and Kevin P. Galvin (2006). Particle size classification in a fluidised bed containing parallel inclined plates. Minerals Engineering, volume 19, pages 162-171. 198. Kevin Galvin, Elham Doroodchi, A.M. Callen, N. Lambert and S.J. Pratten (2002). Pilot plant trial of the reflux classifier. Minerals Engineering, volume 15, pages 19-25. 199. Kevin P. Galvin, A. Callen, J. Zhou and Elham Doroodchi (2005). Performance of the reflux classifier for gravity separation at full scale. Minerals Engineering, volume 18, pages 19-24. 200. Jeannette McGill (2005). Sustainable Development framework for technology provision to small-scale mining sectors. Natural Resources and the Environment, Division of CSIR of South Africa. Presentation to CASM meeting in South Africa. Download: www.casmsite.org 201. Steve Gaber and Jason Gaber (2007). More fine gold sluice testing. [PopandSon Sluice] View: http://bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=422&postnum=0&highlight=] 202. Phil Hontz [PhilNM] (2007). Re: Zooka and Popandson start the ASM sluice program. [Discussion on the Damn Fine Sluice DFS] View: http://bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=527&postnum=150

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75 Gold Recovery Methods

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