Fine Gold Recovery

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

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Fine Gold Recovery

Purpose of study

Robin Grayson

The study sets out to identify methods capable now, or in the near future, of recovering gold traditionally lost by placer gold mines and artisanal miners, but without resorting to dangerous mercury or controversial cyanide. The study tracks the rise and fall of gold recovery systems across the world.

– Alternatives to Mercury and Cyanide

Eco-Minex International Ltd., Apt.14, Bldg. 40, 1/40000 Microdistrict, Sukhbaatar District, Ulaanbaatar 210644, P.O.B. 242, Mongolia. E-mail: [email protected]

About the author

The study clarifies the meaning of ‘fine gold’, for the term has been applied in a strikingly divergent manner. A new nomenclature for gold size is presented that is simple to use. A ‘World list’ of 75 different methods of recovering gold is compiled, described and discussed.

Robin graduated in Geology and Zoology from Manchester University in 1970 where he completed a Masters Degree in

While some methods are well-known, know-how has failed to spread between artisanal miners, recreational miners and mining companies, and between placer and hard-rock gold specialists.

Geology before lecturing at Wigan Mining College for ten years. Robin is a specialist in placer gold and ecology and is currently compiling Best Available Techniques (BAT) for Placer Gold Miners. He

is

Steppegold

on

the

famous

Alaska

Gold

Forum

(http://bb.bbboy.net/alaskagoldforum).

The article notes the neglect of major international projects on artisanal mining and mercury abatement to assess many alternative methods of recovering gold, and the lack of a clear technical focus in such projects. The overall conclusion is that mercury and cyanide can be out-competed quickly if cheap affordable alternatives are tested and promoted. For this to happen, a shift in donor-funding is required, away from socioeconomic-led projects to appropriate technology-led projects.

Figure 1.

Testing a PopandSon sluice in northern Mongolia. The sluice uses tiny mesh riffles to recover much more fine gold than any industrial sluice in the country can do. (photo: Robin Grayson)

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

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What is fine gold? The Tower of Babel… For more than a century a sterile debate has lingered about how tiny do gold particles need to be to term them as ‘fine gold’. Even when large regions such as the former Soviet Union and United States managed to impose some standardisation in their territories, other regions evolved quite different definitions based on their own tradition of screen sizes. Some examples of the widely differing nomenclature are presented in figure 3. The task of defining fine gold has been compounded by the United States clinging to the archaic Anglo-Saxon system of measurements (e.g. fractions of an inch) while the metric system of measurement prevails in most regions of the world and is the norm in science. Further complexity and confusion have been added by some authors seeking to define fine gold in terms of disparate criteria such as the limit of the human eye in resolving gold particles, the limit of traditional pans and sluices in catching tiny gold, and by the limit of mercury in amalgamating with tiny gold. Vagueness and uncertainty is compounded by placer miners needing to highlight relatively fine gold encountered in this or that location whether it be stream, terrace, valley, mine or borehole. The expression “relatively fine” retains its value but only in a comparative sense, and being subjective cannot serve as the basis for regional or international nomenclature. In the absence of a simple standardized scheme, comparing the performance of different gold recovery devices remains tedious and prone to confusion – to the dubious advantage only of vendors of wash-plants. The comparison of placers from region to region, or even from borehole to borehole, remains prone to misunderstanding – risking uncertainty in prospecting, exploration, defining the resource, calculating the reserve, mine planning and in mine management.

Demolishing the Tower of Babel… In an effort at international harmonisation, the author has devised – with the help of members of the Alaska Gold Forum (ASF) – a simple classification intended for international use in describing placer gold and hardrock gold. It is presented in figure 2. After many efforts and permutations, a classification was arrived at that is easy to remember, easy to use and is based on the logarithmic scale of the metric system of measurement. To assist visual appreciation of size charts, the scale is colour-coded, the colours being standard colours of most word processing packages and MS Paint software. A standard chart was designed embodying the new nomenclature and colour-coding, with the North American Tyler mesh and U.S. mesh classification added for convenience of North American users. In the expanded version of the chart, the inches and millimetre equivalent of the North American mesh systems are added.

Figure 2.

GOLD SIZE CLASSIFICATION

The new international nomenclature for gold size, as presented by members of the Alaska Gold Forum. Gold smaller than one micron is termed ‘sub-micron gold’.

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

Proposed international scheme for gold sizes

Figure 3.

DIVERGENT NOMENCLATURE FOR DESCRIBING GOLD SIZE

Eleven different nomenclatural schemes as used in the USA, Canada, Britain and the Russian Federation. The numbers refer to sources given in the list of references [1,2,3,4,5,6,7,8,9,10,11] (drawing: Robin Grayson)

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

www.mine.mn

Choosing the recovery method Lingering dissatisfaction… The effectiveness of gold recovery systems is a never-ending topic of debate and dispute – particularly regarding their ability to recover fine gold. If gold is 1mm or more in nominal diameter, it is easy to recover by a wide choice of devices capable of recovering >90% of the gold. But if the gold is 60μ gold the addition of mercury is highly effective; for recovering gold smaller than 60μ, mercury is ineffective [12]. To recover 90% of very fine gold down to about 40μ. Duke’s E-tank (#25). A wash-plant recovering 90% of fine gold down to 50-60μ and requiring very little water. The device seems ideal for larger artisanal operations, and can be made locally at low cost. Graefe’s E-tank (#41). A wash-plant recovering 90% of fine gold down to 30μ and requiring very little water. The device is small, ideal for small operations and can be made locally at moderate cost. ASAT E-tower (#43). A tranquil elutriation tower, capable of recovering >95% of gold down to 20μ and still catch useful amounts of 5μ gold. Scaling up for industrial mining proved difficult and R&D ceased. However the original device seems ideal for artisanal miners and can out-compete mercury and reduce the case for cyanide. Osterberg’s E-tower (#44). A tranquil elutriation tower intended to assist recreational miners to recover fine gold from black sand concentrates. It seems ideal for artisanal miners and can compete with mercury. Ecologic E-tower (#72). A turbulent E-tower with an innovative pedal-powered water pump. The device is designed for recreational and artisanal miners to recover coarse and fine gold and is being marketed worldwide. Neffco bowl (#27). This device is superior to other single-wall bowl centrifuges in recovering fine gold but no tests have been published. It is used by a few recreational miners on small offshore dredge platforms in Alaska and can be adapted to meet the needs of artisanal miners. Younge’s horizontal centrifuge (#45). A simple motorised cylinder able to recover >90% of very fine gold down to at least 75μ, yet requiring very little water. The device seems especially suitable for arid regions, and can be built in a simple workshop. Helix wheel (gold wheel) (#17). A device widely used for decades by recreational miners and industrial miners, mostly for upgrading. Know-how transfer to artisanal miners is warranted – especially of the many types of small gold wheels made for recreational miners.

Mercury elimination by out-competing For eliminating mercury by out-competing it, 15 candidates for Best Available Techniques (BAT) were identified. Most are cheap, pose little risk to the environment and human health, and none are overly complicated. This refutes the mantra that alternatives to mercury are too costly or too technical for ASM to adopt.

Chemical challengers to mercury Four ‘chemical’ competitors to mercury are capable of immediate use as Best Available Techniques (BAT): HGP leaching (#66). A non-toxic chemical leaching method able to compete directly with mercury on cost, efficiency and speed. Small units recently performed well in extensive tests in Ghana with artisanal gold miners compared with mercury – www.habercorp.com. Chlorine leaching (#3). A long-neglected chemical leaching method that performed adequately in basic tests by WWF in the Guianas, and forms the basis of MINTEK’s iGoli process (#67) that is gaining interest among artisanal and small-scale gold miners in Southern Africa as an alternative to mercury. Borax smelting (#10). A traditional method of preparing dore gold from clean concentrate. However, artisanal miners in the Philippines seem to use borax smelting at an earlier stage as an alternative to mercury. Nitric acid cleaning (#8). A method used by artisanal miners in Kyrgyzstan to free gold from sulphide ores. It is an alternative to mercury in some situations. In the medium-term, tincture of iodine (#68) and phytomining (#70) may become viable.

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Out-competing mercury – industrial gold mining can be put in parallel on a truck-mounted mobile washplants to achieve industrial-scale production. The device is ideal for arid regions, and can be built in a workshop. Neffco bowl (#27). This device is superior to other single-wall bowl centrifuges in recovering fine gold but no tests have been published. The device is used by industrial placer gold mines and is ideal for incorporating into mobile wash-plants – www.neffcomining.com. Helix wheel (gold wheel) (#17). Large highcapacity models were made in the 1970s and 1980s and proved effective in recovering fine gold in industrial gold mining. The patents have expired and a greater range of robust synthetic moulding materials exist to enable production to be resumed at lower cost. High performance jigs. The pan-am duplex jigs (#14) and IHC Cleaveland jig (#42) recover 90% of very fine gold down to 60μ in tests, and similar claims are made for Terrawash jigs, Ross jigs, Goldfield jigs and many other wash-plants that incorporate highperformance jigs. Such jigs are used in industrial placer gold mining for recovering fine placer gold, and some are used for recovering gold from milled hardrock. A few such as the Terrawash jigs are compact enough to incorporate into fully-mobile land-based wash-plants. GekkoTM In-line pressure jig (#60). An advanced jig able to recover 90% of very fine gold down to 60μ and probably even finer. IPJs are popular in hardrock gold recovery circuits and are easy to install in-line. IPJs are also favoured in industrial placer mining being compact enough for fully mobile land-based wash-plants. Helix cylinders (#52). A cylindrical Archimedes screw recovering 90% of very fine gold down to 60μ and perhaps finer. Units are made for industrial placer mining and are compact for fully mobile land-based wash-plants. HPC Helix belt (#73). A belted Archimedes screw that offers similar advantages to a helix cylinder.

Popular sentiment is that mercury amalgamation has been eliminated in industrial gold mining due to regulations in the Soviet Union, European Union, USA, Canada etc, and by the voluntary action of stock-market listed mining companies. Even so, a significant number of placer and hardrock gold mines still persist in using mercury amalgamation as the method of choice for gold recovery. Often such mines are in remote regions far from regulators, or are in regions such as China where mercury amalgamation is tolerated by government. A global search was made for Best Available Techniques (BAT) that might tempt ‘rogue’ companies to cease mercury amalgamation voluntarily on grounds of ease of use and enhanced profitability with low capital and operating costs as preconditions. ‘Rogue’ companies are motivated solely by profit, not health or environment.

Chemical challengers to mercury… Cyanide leaching (#2). Cyanide out-competes mercury amalgamation for recovering gold 90% of moderately fine gold (>150μ). The challenge is how to incorporate sections of smaller and smaller sizes of expanded metal mesh as tiny riffles for extremely fine gold, as with the DFS sluice (#57) and PopandSon sluice (#71), in order to recover 90% of fine gold down to 50μ and so outcompete mercury in industrial wash-plants. ASAT E-tower (#43). An extraordinary device, capable of recovering >95% of gold down to 20μ and still catch useful amounts of 5μ gold. Scaling up for industrial mining proved difficult and R&D ceased. Lemmon’s vanner (#51). A large dry wash-plant claimed to recover >90% of 20-250μ placer gold.

Gravitational challengers to mercury… Ten ‘gravitational’ competitors to mercury are capable of immediate use as Best Available Techniques (BAT): Duke’s E-tank (#25). A truck-mounted mobile wash-plant able to process 130m3/hour of loose pay gravel, recovering 90% of very fine gold down to 50-60μ and requiring very little water. It seems ideal for large operations, and can be made locally at low cost. Younge’s horizontal centrifuge (#45). A simple motorised cylinder able to process 4-10m3/hour of loose pay gravel, recovering >90% of very fine gold down to at least 75μ and requiring very little water. Several cylinders

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Out-competing cyanide – artisanal and small-scale gold miners Popular sentiment is that no techniques exist that are cheap and simple enough to challenge cyanide leaching of fine gold as practiced by artisanal and small-scale miners. Contrary-wise, the study found several candidates for Best Available Techniques (BAT) as partial to complete alternatives to cyanide leaching.

Chemical challengers to cyanide Six chemical methods were found suitable for use by artisanal and small-scale miners as Best Available Techniques (BAT) capable of challenging cyanide leaching. Five are also BAT for out-competing mercury: HGP leaching (#65). – USA and Ghana. Chlorine leaching (#3) – Guyana. iGoli chlorine leaching (#67) – South Africa. Borax smelting (#10) - Philippines. Nitric acid cleaning (#8) – Africa.

Figure 8.

EASE OF GRAVITATIONAL METHODS

Figure 9.

EASE OF LEACHING METHODS

Figure 10.

EASE OF AMALGAMATION

Figure 11.

SQUEEZE ON AMALGAMATION

Gravitational methods are unrivalled at recovering COARSE GOLD, but percentage recovery falls dramatically with FINE GOLD. (drawing: Robin Grayson).

Aqua regia leaching (#9). A sixth challenger to cyanide leaching, but involving drastic chemical attack of concentrates. As with most other chemical methods it demands skilled personnel, training, protective clothing, strict supervision and proper premises.

Gravitational challengers to cyanide

Chemical leaching is unrivalled at recovering EXTREMELY FINE GOLD, but percentage recovery falls dramatically with COARSE GOLD because large gold particles take too long to dissolve completely. (drawing: Robin Grayson).

The smaller the gold particles, the more difficult gravitational recovery becomes. This is not only due to the sharp decline ‘settling velocity’ and the increased role of particle shape in governing particle movement. It is also due to the enhanced role of effects such as hydrophobicity, oleophobicity, electrostatics, magnetism, surface tension, fluid viscosity, vortices, buoyancy, Brownian motion etc. Conversely the faster the chemical leaching becomes. This is due to smaller particles having a large surface area to volume ratio and therefore leach much faster than larger particles. It is for this reason that granulated sugar dissolves faster than a sugar lump. It follows that most gravitational techniques that prove acceptable at recovering gold down to say 50μ prove useless at recovering 90% of 95% of gold down to 20μ and still catch useful amounts of 5μ gold. It out-competes mercury and reduces the case for cyanide. In addition, a few artisanal and small-scale miners may be able to use advanced centrifuges, albeit with considerable difficulty: KnelsonTM bowl (#49) and FalconTM bowl (#50). Both are capable of recovering extremely fine gold and both capable of competing with cyanide leaching. But the high cost, tight screening, training and supervision are major deterrents for artisanal and small-scale miners.

Amalgamation is effective at recovering MODERATELY FINE GOLD, but amalgamation cannot recover VERY FINE GOLD 70μ), especially fine free gold in certain placers and non-refractory hardrock ores after milling. Cyanide leaching (#2) was (and remains) the most popular leaching method and had become prevalent in parts of the west but had yet to spread far worldwide. Cyanide proved to be effective at leaching 90% of gold finer than 90μ. Therefore mercury remained supreme, even on dredges. Jigs were perceived as being too big and too expensive for normal land-based placer gold mines and the typical placer miners persisted with their inefficient sluices plus mercury.

Figure 12.

METHODS OF GOLD RECOVERY BY 1970

See text for details of each method. (compiler: Robin Grayson)

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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)

²

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 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