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

Fine Gold Recovery Purpose of study

– Alternatives to The study sets out to identify methods capable now, or in the near future, of Mercury and Cyanide recovering gold traditionally lost by Robin Grayson placer gold mines and artisanal miners, Eco-Minex International Ltd., but without resorting to dangerous Apt.14, Bldg. 40, 1/40000 Microdistrict, Sukhbaatar mercury or controversial cyanide. The District, Ulaanbaatar 210644, P.O.B. 242, Mongolia. study tracks the rise and fall of gold E-mail: [email protected] recovery systems across the world. 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, Geology before lecturing at Wigan Mining College for ten years. know-how has failed to spread between Robin is a specialist in placer gold and ecology and is currently artisanal miners, recreational miners compiling Best Available Techniques (BAT) for Placer Gold Miners. and mining companies, and between He is Steppegold on the famous Alaska Gold Forum placer and hard-rock gold specialists. (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)

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

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 Figure 2. GOLD SIZE CLASSIFICATION version of the chart, the inches and millimetre equivalent The new international nomenclature for gold size, as presented by members of the Alaska Gold Forum. Gold smaller than one of the North American mesh systems are added. micron is termed ‘sub-micron gold’.

67 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

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)

68 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 <0.1mm, then traditional sluices often recover only 20% of the gold, and for over a century miners and scientists have been dissatisfied. Figure 4. USPTO PATENT SEARCH A choice of method exists: Quick Search by words, an Advanced Yet most placer miners persist with the humble Search using patent classification and a Patent Number Search. sluice; not surprising as a sluice is unsurpassed for its remarkable concentration ratio, often as high as 20,000:1. A traditional sluice cannot capture fine gold; not a problem if the gold is coarse. But when milled hardrock ore and placers are rich in fine gold, or difficult-to-settle flat gold, then a traditional sluice invariably loses of a fortune in gold and may cause a company to crash due to lack of cash to cover its costs. How to catch fine gold? The main emphasis has been to supplement gravity by adding mercury to catch gold by amalgamation. For recovering >60μ gold the addition of mercury is highly effective; for recovering gold smaller than 60μ, mercury is ineffective [12]. To recover <60μ Figure 5. USPTO PATENT SEARCH gold, conventional wisdom invokes either extreme To search for patents by number, type the numbers in the box. gravitational devices such as advanced centrifuges or chemical leaching methods such as cyanide. So complete is the dominance of advanced centrifuges and cyanide leaching that other methods have become neglected and overlooked. Raising awareness of these neglected alternatives was one of the main objectives of the present study.

Search for Best Available Techniques (BAT)

The author undertook a global search for equipment Figure 6. USPTO PATENT SEARCH By clicking on the Images button, the patent can be viewed. and methods that showed promise of being BAT (Best Available Techniques) for recovering fine gold. This involved reading about 500 patents (US, Canada, UK, New Zealand, Australia, Russia), reviewing about 500 technical reports (US, Canada, UK, New Zealand, Australia, China, Indonesia, Laos, Philippines, Guinea, Brazil, Peru etc.), contacting 50+ gold recovery specialists worldwide, 50+ equipment manufacturers and 50+ recreational miners. The author focussed on searching the patents held in the archives of the United States Patent and Trademark Office (USPTO), not only as this is the biggest archive of patents, but also because it is now searchable on-line via Internet, both via the USPTO website (www.uspto.gov) and via the Google’s patent meta-search engine. The present study is thought to be the most wide ranging review undertaken of equipment and methods Figure 7. VIEWING A US PATENT relevant to the recovery of fine gold. That said, a US patents are viewed one page at a time. The free AlternaTIFF file has to be downloaded first to make the patent page visible. significant number have doubtless been overlooked and Later the pages can be assembled into a single PDF document await discovery – in the field and in patent offices. using Adobe Professional software.

69 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

Profiles of different methods of recovering fine gold

Introduction The study gathered methods of recovering gold to 1980 - 1990 compile a ‘World list’. To merit inclusion, a method has to 30: bioleaching – 1980s research in Wales and California have a clear description, plausible within the realm of 31: biooxidation – 1980s research in B.C. and California science, and some data exist on the ability to recover gold 32: agglomeration – 1980s research in Australia and China of different sizes. The ‘World list’ is composed of 75 33: oleophilic adhesion – 1980s research in Alberta methods that each meets all three criteria. The reader is cautioned that over 400 methods that failed to meet these 34: magnetic coated gold – 1980s research in Colorado criteria are omitted and maybe 10% merit fresh attention. 35: flat bar riffles – 1980s research in Yukon, 1990s in Mongolia The ‘World list’ can be much increased if and when 36: angle-iron riffles – 1980s research in Canada the patent systems of Australia, New Zealand, Russian 37: expanded metal grating riffles – 1980s research in Canada Federation, Brazil, China etc become freely accessible. For 38: expanded metal mesh riffles – 1980s research in Canada instance the author has declined to pay to inspect over 50 39: McCann’s small sluice – 1980s research in California relevant Australian patents at 50$ each. 40: hydraulic riffles – 1980s research in NZ and Canada Hand-held manually-driven gold recovery devices – 41: Graefe’s E-tank – 1980s research in California pans, bowls, bateas, dulongs, lotoks etc. are not included 42: Cleaveland/IHC jig – 1980s research in USA and Holland as they are reviewed adequately elsewhere [13-19]. 43: Lashley’s ASAT E-tower – 1980s research in New Mexico The list is arranged chronologically to indicate the 44: Osterberg’s E-tower – 1980s research in California waxing and waning of different methods through time. 45: Younge’s horizontal centrifuge – 1980s research in B.C. Doing so was difficult, bearing in mind the time lag between the invention, the patent and commercialisation. 46: Mozley MGS centrifuge – 1980s research in Cornwall 47: Kelsey centrifugal jig – 1980s research in Australia Prior to 1970 48: Yunxi bowl – 1960s-90s research in Yunnan TM 1: mercury – amalgamation of gold 49: Knelson bowl – 1980s research in British Columbia TM 2: cyanide – chemical leaching of gold 50: Falcon C bowl – 1980s research in British Columbia 3: chlorine – chemical leaching of gold 51: Lemmon’s vanner – 1980s research in the Yukon 4: iodine – chemical leaching of gold 52: Brosseuk’s helix cylinder – 1980s research in B.C. TM 5: bromine – chemical leaching of gold 53: Gemeni table – 1980s research in Colorado 6: thiocyanate – chemical leaching of gold 54: Mark-7 Reichert spirals – 1980s research in USA 7: thiourea – chemical leaching of gold 1990 - 2000 8: nitric acid – chemical cleaning of gold 55: bromine leaching – 1990s research in Indiana 9: aqua regia – chemical leaching of gold 56: gold-paraffin floatation – 1990s research in Brazil 10: borax – smelting of gold 57: Damn Fine SluiceTM – 1990s research in New Mexico 11: froth floatation – 1930s research in Idaho and USSR 58: Cleangold® sluice – 1990s research in Oregon 12: riffled sluices – 1960s-1970s research in China and USSR 59: PyramidTM E-tank – 1990s research in California 13: simple jigs – 1970s research in China 60: GekkoTM in-line pressure jig – 1990s research in Australia 14: pan-am duplex jigs – Alaska tests 61: FalconTM SB bowl – 1980s research in British Columbia 15: Knudsen bowl – Alaska tests 62: ItomakTM bowl – 1990s research in Novosibirsk 16: Gilkey bowl – Alaska tests 63: BGS shaking table – 1990s research in UK 17: helix wheel (gold wheel) – 1900s research in Colorado 64: GoltronTM machine – 1990s research in Utah 18: Wilfley shaking table – 1890s research in Colorado 65: U-TechTM reverse polarity table – 1990s research in Arizona 19: shaking tables – 1960s-1970s research in China 20: shaking tables – 1960s research in USSR 2000 - 2007 21: Pinched sluice – historical usage 66: HGP leaching – 2000s research in New Jersey 22: Reichert cone – 1960s research in Australia 67: iGoli chlorine leaching – 2000s research in South Africa 23: Humphrey spirals – 1940s research in Colorado 68: tincture of iodine leaching – 2000s research in Japan 1970 - 1980 69: gold-binding proteins – 2000s in research in Washington 70: phytomining – 2000s research in New Zealand 24: thiosulphate leaching – 1970s research in Canada 71: Loewen electrostatic sluice – 2000s research in Alberta 25: Duke’s E-sludge tank – 1970s research in Georgia 72: Popandson sluice – 2000s research in USA 26: Visman’s CWC – 1970s research in Yukon 73: reflux classifier – 2000s research in Australia 27: Neffco bowl – 1970s research in Utah 74: Ecologic E-tower – 2000s research in New Zealand 28: Bartles’ crossbelt – 1970s research in Cornwall 75: helix belt – 2000s research in Canada and USA 29: Bartles-Mozley orbital tables – 1970s research in Cornwall

70 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

Out-competing mercury – artisanal and small-scale gold miners

Mercury reduction is not good enough Gravitational challengers to mercury Worldwide, millions of artisanal gold miners use mercury as routine with appalling risk to human health Eleven ‘gravitational’ competitors to mercury are capable and the environment [20]. The mercury issue, coupled of immediate use as Best Available Techniques (BAT): with issues of poverty, wealth creation, child labour, Cleangold® Sluice (#58). A low-cost innovation human rights, land degradation and conflict with formal that recovers 90% of very fine gold down to about 30μ. mining, has spawned artisanal mining projects worldwide. The device has performed well in demonstrations in North One of the largest is the is the Global Mercury Project America, Guyana and the Philippines, and is capable of (GMP) on ‘Removal of Barriers to Introduction of Cleaner comprehensively out-competing mercury. Artisanal Gold Mining and Extraction Technologies’ under DFS sluice (#57). A simple in-stream sluice the aegis of the United Nations Industrial Development allowing recreational miners to recover fine gold. Its Organization (UNIDO) [21]. Remarkably the GMP has not potential for artisanal miners has recently been noted via systematically investigated gravitational or chemical the PopandSon sluice (#71), a development of the DFS alternatives to mercury, and instead focussed mostly on sluice, recovering 90% of fine gold down to 50μ with ‘tiny’ mercury REDUCTION. Such projects are simple to do by raised expanded metal mesh. It can be used as a high consultants, socio-economists and ecologists with no prior banker and has ‘dampers’ to reduce surging. knowledge of mineral separation; quick success assured BGS shaking table (#63). A home-built over-the- by simple measures such as promoting retorts. ‘Reduction’ shoulder hand-cranked gold recovery device that can is only short-term local ‘sticking plaster’ liable to compete with mercury in recovering >90% of very fine marginalise the technical scientific approach essential for gold down to about 40μ. mercury ELIMINATION, such as by Cleangold® [22,23], Duke’s E-tank (#25). A wash-plant recovering the British Geological Survey [4,24], Projekt-Consult [25] 90% of fine gold down to 50-60μ and requiring very little and the Alaska Gold Forum. As a result the GMP Training water. The device seems ideal for larger artisanal Manual [26] is a fascinating anecdotal compilation devoid operations, and can be made locally at low cost. of a coherent roadmap for mercury elimination. Graefe’s E-tank (#41). A wash-plant recovering 90% of fine gold down to 30μ and requiring very little Mercury elimination by out-competing water. The device is small, ideal for small operations and For eliminating mercury by out-competing it, 15 can be made locally at moderate cost. candidates for Best Available Techniques (BAT) were ASAT E-tower (#43). A tranquil elutriation tower, identified. Most are cheap, pose little risk to the capable of recovering >95% of gold down to 20μ and still environment and human health, and none are overly catch useful amounts of 5μ gold. Scaling up for industrial complicated. This refutes the mantra that alternatives to mining proved difficult and R&D ceased. However the mercury are too costly or too technical for ASM to adopt. original device seems ideal for artisanal miners and can out-compete mercury and reduce the case for cyanide. Chemical challengers to mercury Osterberg’s E-tower (#44). A tranquil elutriation Four ‘chemical’ competitors to mercury are capable of tower intended to assist recreational miners to recover immediate use as Best Available Techniques (BAT): fine gold from black sand concentrates. It seems ideal for HGP leaching (#66). A non-toxic chemical leaching artisanal miners and can compete with mercury. method able to compete directly with mercury on cost, Ecologic E-tower (#72). A turbulent E-tower with efficiency and speed. Small units recently performed well an innovative pedal-powered water pump. The device is in extensive tests in Ghana with artisanal gold miners designed for recreational and artisanal miners to recover compared with mercury – www.habercorp.com. coarse and fine gold and is being marketed worldwide. Chlorine leaching (#3). A long-neglected chemical Neffco bowl (#27). This device is superior to other leaching method that performed adequately in basic tests single-wall bowl centrifuges in recovering fine gold but no by WWF in the Guianas, and forms the basis of MINTEK’s tests have been published. It is used by a few recreational iGoli process (#67) that is gaining interest among miners on small offshore dredge platforms in Alaska and artisanal and small-scale gold miners in Southern Africa as can be adapted to meet the needs of artisanal miners. an alternative to mercury. Younge’s horizontal centrifuge (#45). A simple Borax smelting (#10). A traditional method of motorised cylinder able to recover >90% of very fine gold preparing dore gold from clean concentrate. However, down to at least 75μ, yet requiring very little water. The artisanal miners in the Philippines seem to use borax device seems especially suitable for arid regions, and can smelting at an earlier stage as an alternative to mercury. be built in a simple workshop. Nitric acid cleaning (#8). A method used by Helix wheel (gold wheel) (#17). A device widely artisanal miners in Kyrgyzstan to free gold from sulphide used for decades by recreational miners and industrial ores. It is an alternative to mercury in some situations. miners, mostly for upgrading. Know-how transfer to In the medium-term, tincture of iodine (#68) and artisanal miners is warranted – especially of the many phytomining (#70) may become viable. types of small gold wheels made for recreational miners.

71 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

Out-competing mercury – industrial gold mining

Popular sentiment is that mercury amalgamation has can be put in parallel on a truck-mounted mobile wash- been eliminated in industrial gold mining due to plants to achieve industrial-scale production. The device is regulations in the Soviet Union, European Union, USA, ideal for arid regions, and can be built in a workshop. Canada etc, and by the voluntary action of stock-market Neffco bowl (#27). This device is superior to other listed mining companies. Even so, a significant number of single-wall bowl centrifuges in recovering fine gold but no placer and hardrock gold mines still persist in using tests have been published. The device is used by industrial mercury amalgamation as the method of choice for gold placer gold mines and is ideal for incorporating into mobile recovery. Often such mines are in remote regions far from wash-plants – www.neffcomining.com. regulators, or are in regions such as China where mercury Helix wheel (gold wheel) (#17). Large high- amalgamation is tolerated by government. capacity models were made in the 1970s and 1980s and A global search was made for Best Available proved effective in recovering fine gold in industrial gold Techniques (BAT) that might tempt ‘rogue’ companies to mining. The patents have expired and a greater range of cease mercury amalgamation voluntarily on grounds of robust synthetic moulding materials exist to enable ease of use and enhanced profitability with low capital and production to be resumed at lower cost. operating costs as preconditions. ‘Rogue’ companies are High performance jigs. The pan-am duplex jigs motivated solely by profit, not health or environment. (#14) and IHC Cleaveland jig (#42) recover 90% of very fine gold down to 60μ in tests, and similar claims are Chemical challengers to mercury… made for Terrawash jigs, Ross jigs, Goldfield jigs and Cyanide leaching (#2). Cyanide out-competes many other wash-plants that incorporate high- mercury amalgamation for recovering gold <70μ and performance jigs. Such jigs are used in industrial placer poses little or no risk to human health or environment if gold mining for recovering fine placer gold, and some are the company has strong heath and safety procedures in used for recovering gold from milled hardrock. A few such place. But ‘rogue’ companies often have lax management as the Terrawash jigs are compact enough to incorporate and encouraging them to switch from mercury to cyanide into fully-mobile land-based wash-plants. may solve one problem by creating another. GekkoTM In-line pressure jig (#60). An advanced Alternatives to mercury include many leaching jig able to recover 90% of very fine gold down to 60μ and systems other than cyanide (e.g. #4, #5, #6, #7, #9, probably even finer. IPJs are popular in hardrock gold #24, #30, #31 and #55) but most are inappropriate for recovery circuits and are easy to install in-line. IPJs are promoting to ‘rogue’ companies due to the high risk of also favoured in industrial placer mining being compact inadequate safeguards to health of operatives or to the enough for fully mobile land-based wash-plants. local environment. Helix cylinders (#52). A cylindrical Archimedes Two exceptions exist, both safe eco-friendly chemical screw recovering 90% of very fine gold down to 60μ and leaching alternatives to mercury that might be effective perhaps finer. Units are made for industrial placer mining for ‘rogue’ companies as Best Available Techniques (BAT): and are compact for fully mobile land-based wash-plants. HGP leaching (#66). A non-toxic chemical leaching HPC Helix belt (#73). A belted Archimedes screw method able to compete directly with mercury and that offers similar advantages to a helix cylinder. cyanide on cost, efficiency and speed. Small units have Potential gravitational challengers… been tested in the USA and the process is capable of up- scaling for industrial use – www.habercorp.com. Large efficient sluices. The Yukon and BC tests iGoli chlorine leaching (#67) has so-far been showed the most efficient to be angle-iron riffles (#36) demonstrated effective only for artisanal and small-scale to catch coarse gold plus raised expanded metal mining. However now that health and environmental grating riffles (#37) for smaller gold, plus raised concerns have been fully addressed the iGoli method can expanded metal mesh (#38) as riffles for even smaller be scaled-up for industrial gold mining operations. gold. Such permutations, properly tuned and set on unbacked NomadTM matting recover >90% of moderately Gravitational challengers to mercury… fine gold (>150μ). The challenge is how to incorporate Ten ‘gravitational’ competitors to mercury are capable sections of smaller and smaller sizes of expanded metal of immediate use as Best Available Techniques (BAT): mesh as tiny riffles for extremely fine gold, as with the Duke’s E-tank (#25). A truck-mounted mobile DFS sluice (#57) and PopandSon sluice (#71), in wash-plant able to process 130m3/hour of loose pay order to recover 90% of fine gold down to 50μ and so gravel, recovering 90% of very fine gold down to 50-60μ outcompete mercury in industrial wash-plants. and requiring very little water. It seems ideal for large ASAT E-tower (#43). An extraordinary device, operations, and can be made locally at low cost. capable of recovering >95% of gold down to 20μ and still Younge’s horizontal centrifuge (#45). A simple catch useful amounts of 5μ gold. Scaling up for industrial motorised cylinder able to process 4-10m3/hour of loose mining proved difficult and R&D ceased. pay gravel, recovering >90% of very fine gold down to at Lemmon’s vanner (#51). A large dry wash-plant least 75μ and requiring very little water. Several cylinders claimed to recover >90% of 20-250μ placer gold.

72 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

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 Figure 8. EASE OF GRAVITATIONAL METHODS Techniques (BAT) capable of challenging cyanide leaching. Gravitational methods are unrivalled at recovering COARSE GOLD, Five are also BAT for out-competing mercury: but percentage recovery falls dramatically with FINE GOLD. (drawing: Robin Grayson). 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. 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 Figure 9. EASE OF LEACHING METHODS Chemical leaching is unrivalled at recovering EXTREMELY FINE GOLD, The smaller the gold particles, the more difficult but percentage recovery falls dramatically with COARSE GOLD gravitational recovery becomes. This is not only due to the because large gold particles take too long to dissolve completely. sharp decline ‘settling velocity’ and the increased role of (drawing: Robin Grayson). 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 Figure 10. EASE OF AMALGAMATION useless at recovering <20μ gold. Nevertheless two Amalgamation is effective at recovering MODERATELY FINE GOLD, gravitational techniques recover >90% of <20μ gold and so but amalgamation cannot recover VERY FINE GOLD <70μ and are Best Available Techniques (BAT) capable of reducing percentage recovery falls dramatically with COARSE GOLD because the interior of large gold particles resist amalgamation. cyanide leaching or – perhaps – out-competing it: (drawing: Robin Grayson). Cleangold® sluice (#58). A low-cost device that recovers 90% of very fine gold down to about 30μ. It out- competes mercury and reduces the case for cyanide. ASAT E-tower (#43). An extraordinary device, capable of recovering >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 Figure 11. SQUEEZE ON AMALGAMATION The dominance of mercury amalgamation is being challenged from both capable of competing with cyanide leaching. But the both sides – by better leaching methods being able to leach coarser high cost, tight screening, training and supervision are gold more efficiently than before, and by better gravitational major deterrents for artisanal and small-scale miners. methods being able to settle finer and finer gold.. (drawing: Robin Grayson).

73 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

The ‘Perfect Mousetrap’ by 1970

Mercury and cyanide reigned supreme… By 1970 a curious situation had arisen. While perhaps 95% of miners continued with inefficient sluices, jigs Taking 1970 as an arbitrary starting point for became fairly popular to clean up the concentrates. This discussion, mercury and cyanide reigned supreme. meant that jigging with its high gold recovery was being Mercury amalgamation (#1) had dominated recovery used AFTER heavy gold losses by inefficient sluices. It of fine gold (>70μ), especially fine free gold in certain meant that the full potential of jigs was not being used. placers and non-refractory hardrock ores after milling. Simple centrifuges were common but primitive, with Cyanide leaching (#2) was (and remains) the most the Knudsen bowl (#15) being the market leader. Often popular leaching method and had become prevalent in mercury was added in an effort to recover fine gold; and parts of the west but had yet to spread far worldwide. specially built mercury centrifuges (‘forced amalgamators’) Cyanide proved to be effective at leaching <70μ gold that were popular led by the Gilkey bowl (#16). is too tiny for recovery by mercury amalgamation and had Other gold recovery devices were available – gold begun to replace mercury outright in some districts. wheels (#17), traditional shaking tables (#18, #19, By 1970, many other chemical methods of recovering #20), pinched sluices (#21), Reichert cone (#22) and gold were known, but as yet none significantly challenged Humphrey spirals (#23), but these remained suitable only the duopoly of mercury and cyanide. for clean-ups as they could not cope with coarse feed. Chlorine, iodine and bromine leaching (#3, #4, #5) had all been known since the 1800s but almost vanished due to competition from mercury and then cyanide. Thiocyanate leaching (#6) was still little known. Thiourea leaching (#7) had been neglected since the 1930s; it would be the 1980s before it received fresh attention. Nitric acid (#8) was (and remains) little used although this method has been known since the 1800s. Aqua regia leaching (#9) had become popular in recovering gold from goldsmith scrap, but did not gain favour among miners. Borax smelting (#10) had been routine for over a century. But now artisanal gold miners found it useful in upgrading concentrates, rather than in making dore gold. Froth floatation (#11) had become widespread in mineral processing, yet the high cost of chemicals and the demand for technical expertise meant that this method had been neglected for placer gold for three decades [2].

Gravity methods still primitive… By 1970, in spite of 130 years of research and innovation, gravitational methods of recovering gold remained crude and offered no real challenge to mercury. Sluices (#12) remained primitive; it was only in the 1980s that clear-sided flumes were used to witness how a sluice recovered black sand and gold. By 1970 it was beginning to be realised that screening off large material could vastly improve the recovery of gold by a sluice [27]. The primitive sluice designs are still liked in Russia, Mongolia, China and elsewhere [28]. Jigs (#13) were improving following installation on gold and tin dredges – led by the Pan-American duplex jig (see figure 8) and by the Yuba jig. For the first time more than 90% of gold of 100μ could be consistently recovered. These advanced jigs could recover some of the fine gold but were rarely able to recover >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 Figure 12. METHODS OF GOLD RECOVERY BY 1970 sluices plus mercury. See text for details of each method. (compiler: Robin Grayson)

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1: 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 adding Hg to a milling device (e.g. Muller mill = edge mill) to capture gold and leave amalgam in the milled residue; ² adding Hg to a pan, sluice, centrifuge, Hg-centrifuge (forced amalgamator) or other device to assist the capture of gold; ² adding Hg to concentrate to recover gold without the time and trouble of upgrading the concentrate further; and ² adding Hg to devices to recover gold lost to the tailings. 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 Figure 13. MERCURY-GOLD AMALGAM Soft bead of HgAu amalgam ready for firing off the mercury to gold particles larger than 60-70μ [12]. For mercury leave gold. (photo: Peter Appel of GEUS) amalgamation to be effective, preconditions apply: ² the gold particle must have a clean surface available; Until 50 years ago, mercury (Hg) was the method-of- ² mercury must be put in direct contact with the gold particle; choice for industrial-scale recovery of hardrock gold, and ² mercury must be clean enough; to a lesser degree for recovery of placer gold also. ² the gold must be already liberated from the matrix, OR ² the gold has its surface exposed to adhere to the mercury. Since then, with the increased recognition of the harmful impact of mercury on human health and After amalgamation, the resulting lumps of amalgam ecosystems, mercury use by companies and recreational are retrieved by squeezing out excess mercury through a miners has become strictly controlled and in some regions fine fabric or chamois leather. The amalgam paste is banned and eliminated. Mercury has been virtually retrieved by hand and the mercury driven off by heating to eliminated in industrial placer gold mines in the USA, leave a residue of impure gold containing traces of mercury. Canada, New Zealand, Australia, Russian Federation, Adoption by placer gold miners Kazakhstan, Kyrgyzstan and Mongolia. Yet mercury is prevalent in large placer gold mines in South/Central Mercury amalgamation is entrenched as the ‘global America, Africa and China. norm’ for gold recovery from concentrates by artisanal Companies shun mercury for six interlocking reasons: placer gold miners. Mercury has been eliminated amongst ² human health of employees and local people; placer mining companies in the former Soviet Union and is ² environmental protection; highly restricted, strictly controlled and virtually eliminated ² insurance risks and liabilities; ² legal prohibition; in industrial and recreational placer mining in the west. ² viable alternatives to mercury now exist; and ² mercury is ineffective at recovering gold <70μ [12].

Figure 14. 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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2: 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 0.02- 0.05%, and must be kept strongly alkaline (pH 10-11). Cyanide leaching can be by different methods: ² 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]; ² agitated leaching – very weak cyanide solution is added to Figure 15. CYANIDE LEACHING vats that are agitated by paddles or by blowing in A Chinese operation near Zuunkharaa in north Mongolia, leaching compressed air to keep the material in suspension; and fine gold from mercury-laden tailings of edge mills. (photo: ² heap leaching – very weak cyanide solution percolates down Jőrgen Hartwig of Projekt-Consult) through crushed/milled ore heaped in a ‘heap basin’ lined Cyanide leaching has been used to recover gold since with leach-proof materials e.g. clay, asphalt or tarpaulin. the 1890s but only since the 1960s with the advent of 2nd stage – recovering gold from solution heap leaching has cyanide become the method-of-choice Having leached the gold and dissolved it into solution for leaching gold from milled hardrock, and has potential as gold cyanide complexaqueous, the solid gold is recovered for leaching gold from placer concentrates. by a choice of methods, such as traditional methods Cyanidation uses a very weak cyanide solution to tested on placer gold concentrates in Alaska [33]: dissolve (‘leach’) fine gold into solution, and then ² absorption by activated carbon – 99.85% Au recovery; ² absorption by ion-exchange resin – 96.31% Au recovery; or precipitates it as easy-to-recover gold. It leaches gold that ² precipitation by zinc dust – 99% recovery. – due to flatness, small size or attached quartz – is lost by The zinc dust method, as typified by the Merill-Crowe simple gravitational devices or mercury. process, first removes oxygen from the cyanide solution Cyanide has risks if handled carelessly or gains and then mixes in a fine zinc powder and recovers the fine access to streams or wells. The cyanide solution must be gold precipitate (<50μ) on a precoat filter. kept strongly alkaline to prevent the generation of highly toxic hydrogen cyanide gas. A problem is cyanide vapour Adoption by placer gold miners rising from ponds. Such concerns have triggered the gold The author is unaware of the cyanide leaching being industry to seek cost-effective alternatives [32] used at large-scale placer gold mines. Yet for small-scale Coarse gold typical of most placers takes too long to and artisanal mining, cyanide leaching is now widespread leach. Marcello Veiga noted that for a 0.21mm gold for hardrock ores and ‘difficult’ lateritic ores in Peru, particle to dissolve in cyanide took over 60 hours! Ecuador, Columbia, Brazil, Philippines and China.

Figure 16. 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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3: Chlorine – chemical leaching of gold Chlorine had been used to recover gold for a couple Frothing and foaming may push off the lid causing of hundred years, notably to leach gold from residues of spills, especially if the concentrate has calcite or dolomite Hg amalgamation. Later chlorination found favour in large as vein material, rocks (e.g. limestone) or coatings. The gold mining operations in Australia and the USA but has foaming is carbon dioxide CO2 gas liberated when HCl now largely disappeared [34]. Chlorination is safer and dissolves carbonate. simpler than cyanide, and can challenge mercury. When dissolution seems over, the lidded bucket is By the 1970s, chorination had been largely forgotten. stirred again and left overnight to allow the process to Yet, as noted by WWF-Guianas (www.wwfguianas.org), a terminate. Next day the acidity is checked by pH paper basic version of chlorination could replace amalgamation (litmus test). If not slightly acid then the acid was and cyanide, and the following account is based largely on completely used, so more 15% HCl is added until the their report [35]. Two common chemicals are required: process is complete. Then the bucket is stood for several ² bleach – 14-16% sodium hypochlorite NaOCl (household hours to settle, and surplus water gently tipped away. bleach) but products often have additives that may interfere st with the process. Training and precautions are ESSENTIAL. 1 stage – leaching gold into solution ² hydrochloric acid – 30% HCl strength, as for cleaning A plastic pole is used to make a hollow in the swimming pools. Training and precautions are ESSENTIAL. concentrate, and the bleach gently stirred in. Shortly after, However bleach (sodium hypochorite solution) forms the 30% HCl is poured in, taking MAXIMUM precautions, comparatively stable trihalomethanes (THMs) and stirring the concentrate with the pole. haloacetic acids (HAAs) claimed to be carcinogenic and Gold is dissolved (‘leached’) by attack by nascent posing other health risks [36]. Only in 2001 was this chorine to form gold chloride (AuCl2) in solution: addressed by Mintex’s ‘iGoli method’, as discussed in a goldsolid + sodium hypochloriteaqueous + hydrochloric acidaqueous later section [37-40]. = gold chlorideaqueous + sodium chlorideaqueous + waterliquid 2nd stage – recovering gold from solution Operation The liquid is decanted from the bucket and filtered.

The floor is sealed, and a tap plus plastic garden The filtrate includes leached gold as gold chlorideaqueous. hose kept ready to flush spillages of bleach and acid. The To precipitate the gold from the pregnant solution, operators must be trained and clad in rubber gloves, several methods are available: sodium metabisulphate, rubber apron, laboratory eye-protectors and rubber boots. zinc metal (chunks, bars or powder), oxalic acid, Preparation – removal of carbonates ferrosulphate or sodium nitrate. Sodium metabisulphate The first task is to dissolve carbonates as they can was used by WWF-Guianas [35]. Zinc often has cadmium interfere with leaching gold. A stock of STRONG 30% HCl (Cd) as impurity that is toxic if released. is added to water to produce 15% dilute HCl. The Adoption by placer gold miners concentrate is put in a lidded plastic bucket and 15% HCl added to submerge the concentrate, using a plastic rod to Chlorination is a neglected method for recovering stir. A long handle cut from a plastic sweeping brush is gold from placer concentrates and the author is unaware ideal. of any companies using this method, although some artisanal placer miners may be doing so.

Figure 17. 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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4: 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 Operation rise in popularity of cyanide leaching and mercury The Harrison method in outline is as follows. First a amalgamation, and the high cost of iodine. Unlike test batch of 0.43 kilos of ore is reduced to a <50-75μ chlorination, recycling of leachate is of paramount powder and leached without any preliminary roasting step. importance for commercial viability of the method. High 1st stage – leaching gold into solution rates of recycling of iodine can be achieved although To prepare the leachate, 3.8 kilos of solid potassium requiring additional plant layout. Fortunately iodine is a iodide are added to four litres of water and then 1.9 kilos good lixiviant for gold so only very low concentrations of of iodine crystals added. The water is agitated until all the iodine are required. Three methods are noted below. crystals dissolve. Then 0.45 litres of concentrated nitric acid The Prichard method of iodine leaching was invented is added in small steps with agitation. The leachate consists by Loius M. Pritchard of Idaho and patented in 1907 (US of water, potassium iodide, hydroiodic acid, free iodine and #861,535) and uses an excess of iodine dissolved in potassium nitrate able to “completely dissolve all tellurides potassium iodide in aqueous solution. The gold so and selenides, and sulphides of gold present”. dissolved is recovered by adding mercury which reduces The ore is added to the leachate and agitated for an the gold in solution to a metallic state whereupon it forms hour to form a pregnant solution. This is filtered and the an amalgam that is then washed free. But the Prichard residue washed in a concentrated solution of potassium method is unsatisfactory in failing to precipitate colloidal iodide to dissolve insoluble silver iodide, until no yellow gold, and gold recovery is sometimes only 25%. precipitate remains. The wash water is thoroughly mixed The Harrison method of iodine leaching was invented with the filtered pregnant solution. by George D. Harrison of Detroit and patented in 1942 2nd stage – recovering gold from solution (US #2,304,823). It was said to be effective with placer Mercury is added to the pregnant solution to form concentrates and difficult ores such as platinum ores and amalgams of gold, silver and base metals. This residue is gold telluride ores. The lixiviant is an aqueous solution of filtered off and to it is added a hot solution of four parts iodine and potassium iodide, plus nitric acid to prevent the concentrated nitric acid and one part water to prevent the formation of insoluble gold salts. gold becoming colloidal. After cooling for 30 minutes, the An in-situ method of iodine leaching of gold ore was gold is recovered in a furnace. invented by Kent McGrew and Jack Murphy of Wyoming Some gold and other precious metals remain in the and patented in 1985 (US #4,557,759) as a safer filtrate. This is treated with nitric acid (see patent for alternative to in-situ cyanide leaching. The gold leached by details) and the gold precipitated is recovered by filtration. the iodine is recovered by activated charcoal, and the iodine regenerated for re-use by electrochemical oxidation. Adoption by placer gold miners E-goldprospecting (www.e-goldprospecting.com) has a good account of the pros and cons of iodine leaching. Iodine leaching is a neglected method for recovering An updated tincture method of iodine leaching was placer gold and the author is unaware of any companies, invented in 2006 [40] and is dealt with in a later section. recreational miners or artisanal miners doing so.

Figure 18. 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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5: 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 Operation competition from cyanide and mercury amalgamation. The Fink and Putnam method in outline is as follows. Some bromine leaching methods are outlined below. First the ore is finely powdered and preferably roasted. The Schaeffer method of bromine leaching was Carbonates need not be removed as bromine solvents can invented by Charles A. Schaeffer of New York State and dissolve gold in the presence of carbonates. patented in 1882 (US #267,723). Bromine in aqueous 1st stage – leaching gold into solution solution is used to leach the gold as gold bromide in Bromine and sodium bromide are sourced from solution over 24 hours. Silver bromide forms as sludge. specialised suppliers. Elemental bromine is particularly The gold is recovered by precipitation by adding oxalic hazardous and extremely difficult to store or transport acid or iron sulphate to the pregnant solution. By putting safely. Sodium bromide is more stable and cheaper. sludge into solution with sodium or calcium hyposulphite, The powered ore is added to water in a leach tank. silver can be precipitated by adding calcium sulphide. The leachate is prepared by adding sodium chloride @ Bromine is a highly corrosive fuming liquid generating a 100 grams/litre, sodium bromide @ 1gram/litre and finally suffocating vapour, making this method hazardous. chlorine @ 1.4 grams/litre. Throughout the first 15 The Engelhardt method of bromine leaching was minutes, sulphuric acid is added @ 1.25 grams/litre to invented by Ernest C. Engelhardt of South Dakota and maintain acidity. The reactive mixture is allowed to stand patented in 1893 (US #509,368) and uses bromine in for a further ten minutes for leaching to finish. In tests 23- dilute hydrochloric acid to leach the gold as gold bromide carat gold leaf dissolved in barely four minutes, and only in solution. Adding acid increases the solubility of bromine 11.1% was lost to the tailings. from 2-3%vol to 13-15%vol and the process shortened. 2nd stage – recovering gold from solution The MacArthur method of bromine leaching was To recover the gold from the pregnant solution, invented by John S. MacArthur of Scotland and patented several methods are available: sodium metabisulphate, in 1889 (US #411,047) and uses “perbromide of iron” in zinc metal (chunks, bars or powder), oxalic acid, aqueous solution to leach the gold as gold bromide in ferrosulphate or sodium nitrate. solution in a vat heated close to boiling point. Silver, The solvent power of the barren liquid is restored by copper, lead and zinc must be removed first by ferric salts. either adding chlorine or a hypochlorite and a mineral The leach is reused until exhausted; then the “perbromide acid. According to the patent, “Except for losses due to of iron” is regenerated using bromine. Gold is recovered by vaporisation and solution left in the tailings, practically all filtering the pregnant solution through coke or charcoal. of the free and combined bromine is recovered.” The Fink and Putnam method of bromine leaching was invented by Colin G. Fink and Garth Louis Putnam of Adoption by placer gold miners New York and patented in 1942 (US #2,283,198). They discovered leaching gold in aqueous bromine solutions is Bromine leaching is a neglected method for recovering accelerated by chloride and bromide ions. placer gold [41], and the author is unaware of any companies, recreational miners or artisanal miners doing so.

Figure 19. 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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6: Thiocyanate – chemical leaching of gold Thiocyanate leaching was invented in the 1890s but was largely ignored due to the popularity of iodine and Operation bromine leaching, and the rise of cyanide leaching. The Wan-LeVier method is summarised as follows. Thiocyanate leaching is particularly effective on The thiocyanate is sourced from specialised suppliers. difficult sulphide-rich refractory hardrock gold ore. This is Preparation – oxidation of sulphides because thiocyanate leaching requires an extremely acidic The ore is finely powdered by milling and added to a environment and breaking down the sulphides in the ore heap leach facility. Bio-oxidation of sulphides is generates highly acidic solutions. accomplished by Thiobacillus ferrooxidans, Leptospirillum The Kendall method of thiocyanate leaching was ferrooxidans, Sulfobocillus thermosulfidooxidans, Sedula, invented by Edward E. Kendall of New York State and Metallospheara or Acidianus brierley as in US patents patented in 1899 and 1901 (US #625,564 and #671,704). #5,246,486, #5,332,559, #5,834,294, #5,127,942 and Ammonium or potassium thiocyanate is used to leach #5,244,493. Bio-oxidation takes about 90 days – much silver and gold, and the pregnant solution directed to an faster than with cyanide leaching as acidic products are agitated trough where potassium sulphide is added to not a problem. precipitate silver sulphide that is recovered by settling and 1st stage – leaching gold into solution filtering. The supernatant liquid is directed to a reaction The leach solution is first conditioned to adjust the tank for gold recovery using comminuted zinc metal. concentration and molar ratio of dissolved thiocyanate and The Barrick method of thiocyanate leaching was dissolved ferric iron, acidity and temperature. The invented by Kenneth T. Thomas, Christopher Fleming, leachate is extremely acidic, ranging from pH 1 to 3. Andrew R. Marchbank and David Dreisinger of Canada and The leach solution is introduced to the milled ore and patented in 1988 (US #5,785,736), the patent assigned to leaching takes from days to months in a heaped leach Barrick Gold Corporation of Toronto. facility, or hours in a heated pressurised autoclave. The Wan-LeVier method of thiocyanate leaching was nd invented by Rong Yu Wan and K. Marc LeVier of Colorado 2 stage – recovering gold from solution and a patent was applied for in 2004 (US #0197249ki). Acid The pregnant thiocyanate leach solution contains thiocyanate solution is used to leach precious metals as a dissolved gold in the form of gold-thiocyanate complex, precious metal-thiocyanate complex. The leach solution and the pregnant solution is removed for further may contain a large molar ratio of ferric iron to processing to recover the dissolved gold. thiocyanate. The precious metal is removed from the Residual solids depleted in gold may be subjected to pregnant thiocyanate solution by transferring the precious additional metal recovery operations or to further metals to precious metal-cyanide complex and then treatment for disposal in an appropriate manner. loading this onto absorbent material. The residual cyanide Adoption by placer gold miners in the thiocyanate leach solution is converted to thiocyanate for further leaching. Thiocyanate leaching is a neglected method for E-goldprospecting (www.e-goldprospecting.com) has recovering placer gold and the author is unaware of any a good account of thiocyanate leaching. companies, recreational miners or artisanal miners doing so.

Figure 20. 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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7: Thiourea – chemical leaching of gold Thiourea has been heralded for decades as an alternative to cyanide, but as yet few if any industrial Operation operations have proved to be a commercial success. In Thiourea is usually supplied in powder form sourced theory, thiourea can be used to recover gold from milled from specialised manufacturers. A weak solution of hardrock, and has potential for leaching gold from placer thiourea is prepared, and the first stage is the oxidation of concentrates. Thiourea leaching can proceed four or five thiourea to form formamidine disulphide: times faster than cyanide leaching, making thiourea more 3+ 2+ + 2CS(NH2)2 + 2Fe = C2S2(NH)2(NH2)2+ 2Fe + 2H effective at dissolving large gold particles, such as those The role of the formamidine is to oxidise the gold to typical of placer gold. [42] form a gold-thiourea complex: Thiourea CS(NH2)2 is an organic compound that is + 2+ classed by INCHEM/WHO as, “toxic. Known animal 2Au + C2S2(NH)2(NH2)2 + 2CS(NH2)2 + 2H = 2Au(CS(NH2)2) carcinogen and probable human carcinogen. May cause Importantly, “formamidine acts as an oxidiser as well irreversible effects. May affect fertility. May be fatal if as a complexing agent, supplying about 50% of the swallowed. May cause allergic skin reaction. May cause ligands to the complexation” and due to this thiourea skin ulcers, liver damage. Handle as a carcinogen. Gloves, leaching of gold is faster than cyanide leaching [43]. The safety glasses, good ventilation. Protect against spills and overall equation for thiourea leaching is: the spread of dust.” An end product is cyanamide that 3+ 2+ 2+ 2Au + 4CS(NH2)2 + 2Fe = 2Au(CS(NH2) + 2Fe contains the cyanide radical and reacts with acids to form To drive the equation to the right, thiourea must be a highly toxic gas. Cyanamide is toxic if swallowed, present in excess, and “the ratio of complexing and harmful to the skin and is an eye irritant. oxidising agents must be carefully adjusted’ to avoid The thiourea method uses a weak solution of excessive oxidation of the thiourea solution and thiourea in the presence of an oxidising agent to dissolve consequent excessive use of reagents [43]. (‘leach’) fine gold into solution, and then precipitate it as In a final step, the formamidine breaks down easy-to-recover gold. irreversibly to cyanamide and elemental sulphur. In thiourea leaching of gold, ferric iron (Fe3+) is used The sulphur is a potential problem to the success of as an oxidising agent, it being the most effective the thiourea method, for it forms a fine grained sticky compared to alternatives such as hydrogen peroxide, coating which can inhibit the leaching of gold. sodium peroxide, ozone, potassium permanganate and formamidine disulphide. In contrast, cyanide leaching uses Adoption by placer gold miners oxygen as an oxidising agent direct from the air. Sufficient ferric iron (Fe3+) should already be The author is unaware of thiourea leaching being liberated and available to make the addition of more used at large-scale placer gold mines, large or small. The oxidising agent either limited or unnecessary for a highly main deterrent is the uncertain technology, variable oxidised hardrock ore, or in a typical placer ore. results and difficulty of controlling the process efficiently.

Figure 21. 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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8: Nitric acid – chemical cleaning of gold Operation 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 Figure 22. NITRIC ACID CLEANING local regulations and international norms. The ‘acid site’ Extremely toxic fumes being generated by hot concentrated nitric must be out-of-doors in a well-lit fenced off area away acid poured onto dry concentrate. After a few seconds the brown from other people. All non-essential personnel must be fumes are completely broken down to harmless nitrogen. Artisanal miners in Kyrgyzstan (photo: Peter Appel of GEUS) excluded to minimise exposure to risk. Only one operator is needed, but a second operative should be within 10 Hot concentrated nitric acid (HNO ) helps to recover 3 metres to respond to any emergency. It should not be fine gold from concentrates. Peter Appel of the Denmark- attempted if raining, snowing or in high wind. Greenland Geological Survey noted the method being The concentrate is put in a heat-resistant, acid- used by artisanal gold miners in Kyrgyzstan to liberate resistant, pan on a small stove inside the ‘acid site’ and gold from sulphide ores [18]. It appears over 90% of gold warmed up. Then the operator uses a long-handled pot to of 100μ to 300μ is recoverable, but tests are needed to pour hot, concentrated nitric acid into the pan of dry confirm what percentage of <100μ can be recovered. concentrate. The operator refrains from leaning forward However if mercury is present, from previous processing, and must wear protective clothing and eye-protectors. from contamination or from native mercury or cinnabar, Immediately reaction starts, the operator steps back and then potentially explosive chemicals may form. vacates the area BEFORE heavy brown fumes appear. Nitric acid has been used in gold recovery for at least The brown fumes are of nitrogen oxides and are 150 years as a minor process step. Paul B. Queneau and EXTREMELY TOXIC and even trace amounts cause severe John D. Prater of Utah invented a nitric acid method of lung problems. The process is exceedingly dangerous. recovering base metals and gold, patented in 1974 (US However, if the process is carried out outdoors in an open #3,793,429) assigned to Kennecott Copper. This method place then the brown fumes are blown away after a few adds nitric acid continuously to decompose pyrite and minutes. After a short time in the atmosphere the brown arsenopyrite to liberate copper, gold etc. but achieves only fumes disintegrate into harmless nitrogen and oxygen. 80% gold recovery, and requires the feed to be <53μ. These limitations were overcome by Rein Raudsepp, Adoption by placer gold miners Ernest Peters and Morris J.V. Beattie of Vancouver whose The nitric acid method does not appear to be being nitric acid method was patented in 1987 (US #4,647,307), used by placer mining companies but is being used by achieving 99.3% gold recovery in laboratory conditions. artisanal placer gold miners in parts of South America and However the patented process is complex and it does not elsewhere [45]. seem to have become commercialised.

Figure 23. 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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9: Aqua regia - chemical leaching of gold Aqua regia was invented by Iranian alchemist Abu Operation Musa Jabir ebn Hayyan about 800 A.D. This followed from This text is based on recovering gold from scrap by his discovery of hydrochloric acid upon mixing common Shor International www.shorinternational.com. salt with sulphuric acid. The two acids mix quietly – avoid splashes, protect Aqua regia is a mixture of three to four volumes of eyes and work in the open or under a fume hood. Both concentrated hydrochloric acid to one volume of acids emit acrid fumes. No heat is evolved when mixing concentrated nitric acid. It is a corrosive, fuming, but the aqua regia at once starts to emit chlorine gas aggressive liquid and must only be used by a trained slowly for several days. Never stopper an aqua regia chemist following strict precautions in a properly equipped bottle for chlorine may build up and explode it. The aqua laboratory or outside space, and only after a hazard regia is used immediately, or days or weeks later. analysis has been prepared. Typically 1-2 kilos of scrap are put in an empty 6-litre Neither of the acids in aqua regia can dissolve gold, Ehrlenmeyer flask under a fully ventilated fume hood, or but in combination are very aggressive in dissolving gold. outside. The aqua-regia is added slowly, such as drop-by- The fuming and yellow colour of aqua regia are due drop from a bottle set on a shelf above the reaction flask. to the reaction of nitric acid HNO3 with hydrogen chloride Dangerous fumes of nitrogen oxides are generated; form water H20 plus two chemicals that are yellowish and being heavier than air they require either a very good volatile - nitrosyl chloride NOCl and chlorine Cl2. The fume hood, or for the process to be done outside. newly-formed nitrosyl chloride decomposes to nitric oxide When bubbling ceases and no more brown fumes are NO and chlorine. produced then a little hydrochloric acid is added. A further Nitric acid HNO3 is a powerful oxidizer, which will spurt of activity may occur if the original hydrochloric acid actually dissolve a virtually undetectable amount of gold, has been exhausted. When reaction has ceased, the forming gold ions (Au3+). pregnant solution is poured off into a glass or plastic - + 3+ Ausolid + 3NO3 aq + 6H aq = Au aq + 3NO2 gas + 3H2Oliquid container, leaving the solid residue in the reaction vessel. Hydrochloric acid HCl supplies chloride ions (Cl-) in The process is repeated with more aqua regia until large amounts which attack the gold to produce no bubbling or brown fumes are seen, even if the reaction - chloraurate anions AuCl4 in solution: vessel is gently warmed and gently agitated. All the gold 3+ - - Au aq + 4Cl aq = AuCl4 aq has now been dissolved. The contents of the reaction vessel are vacuum This is an equilibrium reaction favouring formation of filtered, and the gold precipitated from the pregnant chloraurate anions. It results in a removal of gold ions solution as described in www.shorinternational.com. from solution and allows further oxidation of gold by the nitric acid, and so more gold is dissolved. In addition, gold Adoption by placer gold miners may be oxidized by free chlorine in the aqua regia. 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].

Figure 24. 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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10: Borax – early smelting of gold

Operation 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 Figure 25. BORAX SMELTING Smelting concentrate to recover gold in the Philippines. (photo: cheap and already used by the artisanal miners for Peter Appel of GEUS, Geological Survey of Denmark and Greenland) burning amalgam. The heating continues until first the ‘Early smelting’ is possible on unclean concentrate to borax melts and later the gold melts. recover fine gold that might be lost if upgrading were to Borax depresses the melting point of gold. The be attempted by amalgamation or gravitational means. molten gold is heavier than the other heavy minerals The ‘early smelting’ method of recreational miners is which float off as slag to leave a nice gold pellet. described at www.nuggethunters.org [46,47]. The ore is The entire process only takes a few minutes. screened or milled at 2mm and the black sand soaked in Adoption by placer gold miners acidic acid for couple of days to help break down sulphides, then carefully dried. The flux is of anhydrous ‘Early smelting’ assisted by borax has been used for borax Na2B4O7 (5 parts), #70 silica sand SiO2 (40 parts), many years by small-scale gold miners in the Benguet soda ash Na2CO3 (10 parts) and sodium nitrate NaNO3 area north of Manila in the Philippines as an alternative to (20 parts) as oxidizer. The ingredients are mixed and mercury or cyanide. Elsewhere borax-assisted smelting is stored in a container and kept dry. sometimes used by artisanal miners, but only after Flux (1-2 parts) is added to the concentrate (1 part) mercury amalgamation or cyanide leaching. and mixed well. The mixture is spooned into the crucible Smelting assisted by borax is common among and dampened with rubbing alcohol. The mixture is fired recreational and industrial gold miners in North America, by the acetylene torch up to 1,100°C and the heat kept Russia and Mongolia upon concentrates that are clean. until the mixture is molten and a bright yellow white. But ‘early smelting’ is advocated for recreational miners by The molten material is poured into a cast-iron mould, www.nuggethunters.org [46,47] for recovering fine gold requiring special clothes and care [46,47,48]. from concentrates that are not particularly clean.

Figure 26. 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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11: Froth floatation – 1930s research in Idaho and USSR

Froth floatation of gold is possible due to gold’s surface hydrophobicity – “the antithesis of gravity Adoption by placer gold miners concentration" [27] and therefore froth floatation can In 1916 Lang suggested that the platy shape of recover up to 100% of very fine to moderately fine gold placer gold in black sand of the Pacific Coast of North (<150μ), much the same as cyanide leaching. America should make it amenable to recovery by The first paper on froth floatation of gold appeared floatation [50]. The first research report on placer gold as late as 1914 [49]. Today froth floatation is a widely floatation was by Arthur Fahrenwald in 1933 [51] and used method for recovering many types of minerals 1937 saw publication of floatation recovery of placer gold including gold, platinum, chromite, fluorspar and coal. on Idaho gold dredges [3,52]. Reagents were amyl Regarding placer gold, deterrents are the cost of xanthate and Aerofloat-15 plus either pine oil or cresylic reagents, the need to recycle effluent and the challenge of acid frother. Gold recoveries were 47-76% – the less floating placer gold due its depressed hydrophobicity, slimes then the better the floatation [3]. Contemporary buoyancy and floatability due to impurities and coatings. tests in the Soviet Union with similar reagents on clean-up But placer gold is often irregular in shape (due to natural tailings and <150μ fraction of placer ores yielded 75-90% leaching) – even porous – and is often remarkably flat, recovery at concentration ratios of 25:1 to 42:1 [3,53,54]. and paradoxically it is these factors that make Research resumed in the 1970s in China [27] where gravitational settling difficult yet froth floatation easier. 78-99% recovery was attained with rougher concentration Operation ratios of several hundred [3]. Floatation tests of Soviet Union coastal marine sands For placer ore, the process is most appropriate to achieved 70-100% recovery of 75-125μ gold [2,27]. A gold recovery from fine tailings or from concentrate. graph by Wang and Poling shows >95% recovery was The feed consists of finely milled hardrock ore, or possible for “coastal type gold” <150μ with a retention else fine tailings or placer concentrate. time of 15 minutes, and 100% recovery possible for Slurry is made by adding water, and fed continuously <120μ with a retention time of only 5 minutes. into a floatation tank. The Soviet Union appears to have operated the In the floatation tank, the slurry is agitated and air world’s only full-scale floatation circuit for placer gold, a bubbles injected. The gold particles attach themselves to six-cell floatation circuit in the 1930s that scavenged both the meniscus of the rising bubbles. This is due to gold ‘fine’ and ‘minute’ gold from gravity tailings aboard a being hydrophobic – and by selectively enhancing this bucket-line dredge [2,27]. It processed 300 tons of solids tendency by adding chemicals known as ‘collectors’; by per day and although the gold recovery was “satisfactory” controlling the collectors using ‘conditioners’; by the floatation circuit was deemed uneconomic at the then stimulating wetting by ‘wetting agents’; by stimulating prevailing low gold price. frothing by ‘frothing agents’ and by controlling pH. Today, froth floatation is a neglected method for The resulting froth is then removed and the gold recovering placer gold and the author is unaware of any recovered by either gravity settling or chemical means. commercial placer operations using this method.

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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12: 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 Figure 28. RUSSIAN EXPANDED METAL RIFFLES Sluice-boxes with expanded metal mesh riffles freshly installed in that are still favoured in the Russian Federation and a Soviet bucket-line dredge in Mongolia. (photo: Robin Grayson) Mongolia. But the results were poor compared to the Yukon tests on expanded metal riffles a decade later.

Figure 29. GOLD RECOVERY BY SIMPLE RIFFLED SLUICES – China tests Poor performance of simple riffled sluices [27]. (compiler: Robin Grayson)

Figure 30. GOLD RECOVERY BY SIMPLE RIFFLED SLUICES – Soviet Union tests Poor performance of simple riffled sluices [2]. (compiler: Robin Grayson)

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13: Simple jigs – 1960s-1970s research in China Operation 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). Figure 31. SIMPLE JIGS The drive is significant, either hydraulic or mechanical. Overhead view of a conventional 2x4 cell rectangular jig. On the upstroke, the erupting water intermingles (drawing: Robin Grayson, adapted from Nio 1978 [55]) with the jig bed, and causes all of the jig bed to be ‘jigged’ A simple jig consists of a square jig cell comprised of – the steel balls may slightly rise and fall, but the layer of a lower water-filled chamber (hutch) covered by a jig smaller particles on or near the jig bed are pushed screen above which slurry is introduced. Resting on the jig upwards – allowing Stokes Law to operate. The smaller screen are large heavy particles (e.g. steel balls) that particles become sufficiently agitated to become a constitute the jig bed. fluidised bed like quicksand – the thixotropic state. The Small square jigs are often arranged in series (to loosened heavies fall rapidly to burrow into the protective increase recovery) or in parallel (to increase capacity). jig bed, while lights are swept away as tailings. A simple square jig is typical of most jigs in exhibiting On the downstroke, water is pulled downwards by a mix of continuous discharge of fine gold and batch suction, and the upper part of the jig bed becomes a hard discharge of coarse gold. layer – the dilatant state. The suction plus gravity pulls Although easy to build and simple to operate, simple dense particles down to the bottom of the jig bed where square jigs are unsuitable for efficient placer gold mining: coarse gold and gold nuggets accumulate as ‘jig bed ² the jig’s footprint is large, demanding too much space on concentrate’ awaiting cleanout during batch discharge. dredges where space is a premium, and too bulky to easily make into a mobile land-based processing unit; Finer gold is flushed through the jig screen into the ² water consumption is high to very high – a serious problem bottom of the hutch to be tapped off the bottom as ‘hutch for land-based units if water is scarce and demanding large concentrate’ continuous discharged via a spigot. tailings ponds for water storage and recirculation; and ² high % recovery of fine gold recovery is difficult to achieve. Adoption by placer gold miners In China in the 1960s-70s experiments by placer Simple square jigs used to be fairly popular in placer scientists of the Minerals Processing Laboratory of the gold mining, particularly in wash-plants on-board dredges, Kunming Institute of Metallurgy in Yunnan Province but have virtually disappeared with the advent of more determined the percentage gold recovery of simple jigs modern jigs, although a few are seen in remote mines and [27]. Gold recovery falters at 0.8mm, is only 90% by are occasionally used for upgrading concentrates. 0.3mm, and collapses to 50% at 0.1mm.

Figure 32. GOLD RECOVERY BY SIMPLE JIGS – China tests Poor performance of simple jigs [3,27]. (compiler: Robin Grayson)

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14: Pan-American duplex jigs – Alaska tests Operation 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 (‘hard- packed’) state. Small gold is sucked down into the upper Figure 33. PAN-AM DUPLEX JIGS hutch to fall and settle on the bottom of the lower hutch A duplex of Pan-American jigs. (photo: courtesy of the for continuous discharge with black sand via a spigot. manufacturer, Delta Mining & Manufacturing Co of Nashville, Gold particles too big to pass though the jig screen Tennessee - www.graymfg.com/mineral.html) remain stranded on the screen awaiting recovery when In the early 1930s the engineers of the Bulolo Gold jigging stops for cleaning the screen – a batch discharge. Dredging Company designed the Pan-American placer jig The upstroke of the lower hutch pushes more water (Pan-Am jig) that is compact and tolerates wave motion in through the screen than replaced in the downstroke – small dredge ponds or even open sea. By the late 1930s compensated by inlets injecting water in the upper hutch. the Pan-Am jig was widely used on dredges for recovery of alluvial cassiterite (SnO2 tin ore), gold and diamonds. Adoption by placer gold miners The Pan-Am jig is a balanced pair of jig cells known Pan-Am duplex jigs are fairly popular in many as a duplex jig, saving 50% of energy in jigging. Each cell regions, and can be seen in action in Alaska, Yukon, South is underlain by a conical hutch of two parts joined by an

America, and Africa but are rare in Russia and Mongolia. annular diaphragm of flexible rubber to allow up-and-

Pan-Am duplex jigs are made in many regions, e.g.: down oscillation of the lower hutch. Standard 42inch x ² USA – IRD of Carson City, Nevada 42inch cells (about 1m x 1m) have seven variables [56]: www.ird-jigs.com ² amount of ragging – typically 425lbs (193 kilos) per cell; ² USA – Delta Mining & Mnfr Co of Nashville, Tennessee ² type of ragging – typically 3/16 inch (4.75mm) steel shot; www.graymfg.com/mineral.html ² ² feed pulp density – 30% to 60% (w/w); USA – Goldfield Engineering Co, of Lindon, Utah 3 3 3 www.goldfieldeng.com ² feed rate – 20 yd to 30yd per hour (15 to 23m per hour); ² Thailand – Dove Engineering ² hutch water added – 2.3 to 4.5m3 per hour per jig cell; www.dovemining.com ² stroke length – ¾ inch to 1½ inch (19 to 38mm); and ² China – China National Gold Corporation (CNGC) ² stroke frequency – 120 to 200 cycles per minute. www.chinagold.org.placer.html

Figure 34. 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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15: 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 1- inch 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 limiting the feed to <1/8th inch. The rotation speed has to Figure 35. KNUDSEN BOWL Knudsen Bowl showing the central feed pipe, rubber riffles and 3 be at least 80 rpm or “…the bowl started dumping large struts that enable the position of each blade to be adjusted. quantities of slurry directly onto the ground and drive (photo: Steve Gaber of the Alaska Gold Forum) mechanism.” Optimum recovery of gold >65 Tyler mesh The Knudsen bowl was invented by George Knudsen (about >0.2mm) is at 105 rpm with 68 gallons/minute of of California and patented in 1942 (US #2,272,675). It water and a pulp density of 5-20% solids. eclipsed the first rubber-riffled centrifuge – the Ainlay bowl Tests by ‘Dredger’ of Alaska Gold Forum in New invented by Thomas Ainlay of Nebraska and patented in South Wales, Victoria, Tasmania and New Zealand suggest 1928 and 1932 (US #1,658,874 and #1,853,249). Knudsen bowls lose about 20% gold, mainly invisible fine The Knudsen bowl has particular advantages: gold whose recovery can be much improved by: ² bowl of non-magnetizable material e.g. aluminium: ² using a binocular microscope to detect and monitor fine gold; ² resistant to wear, and easily, completely and quickly cleaned; ² screening the feed at minus 250 mesh (e.g. a second pass); ² tough rubber riffles as liner, easily and quickly removed; and ² adding 2 variable speed v-belt pulleys, with lockable lever ² separation helped by “…agitation resulting from the wobbling speed control, enabling precise adjustment of the bowl’s rpm; action of the bowl and by the kneading action of the soft ² ensuring a person watches the feed rate; flexible rubber ribs of the riffle member”. ² attaching a good small test sluice to the outlet of the bowl; The Knudsen bowl is typically 12 to 36-inch in ² checking tailings with a x40 to x80 binocular microscope; and ² cleaning the bowl regularly. diameter and mounted on a vertical drive shaft. The wall slopes outward to create a gradient of g forces increasing Adoption by placer gold miners upwards, inducing the slurry to climb the wall. The Knudsen bowl was used worldwide, notably in The need for fine pre-screening makes the Knudsen North America and New Zealand, and copycats in Africa bowl unsuitable for wash-plants, but it has merit for and South America. The Knudsen survives being rugged, upgrading if two or even three bowls are put in series. easy to use, cheap, durable and ease of adding mercury.

Figure 36. 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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16: Gilkey bowl – Alaska tests Operation 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. Figure 37. GILKEY BOWL A 6-inch Gilkey bowl, showing the flow of the slurry feed and The gold particles in the film of slurry crossing the redirection by the ‘striking surface’ to minimise flouring of mercury are forced to amalgamate. After a period the feed mercury. (redrawn from James Anderson [59] by Robin Grayson) is shut off and then the power shut off to slowly bring the The Gilkey bowl was invented by Walter W. Gilkey of spinning bowl to rest. The gold is then recovered from the Washington State during the 1960s, and only advanced mercury by squeezing through a fine cloth to retain the versions patented. The basic version is a symmetrical steel Au-Hg amalgam as a paste, and the amalgam is then bowl with open mouth upwards, dynamically balanced subjected to firing and retorting to separate and recover around a drive spindle below the bowl. Part of the inner the gold and recycle the mercury. wall is a broad cylindrical channel that holds the mercury. The Gilkey bowl was one of the last of the ‘forced Adoption by placer gold miners amalgamators’ that had been popular for over a century The Gilkey bowl and other ‘centrifugal forced to recover ‘fine’, ‘flat’ and ‘flour’ gold by adding mercury to amalgamators’ were popular on large gold dredges in North a spinning bowl to centrifugally press gold into mercury. America. The Gilkey bowl was a focus of research of the Many forced amalgamators were invented and patented, Mineral Industry Research Laboratory in Alaska [59,60] in such as the McKlellar amalgamator (US #1,003,118 of the 1970s but manufacture then ceased. A few Gilkey bowls 1911); the Taber amalgamator (US #1,457,560 of 1923); may still be in use in parts of Africa and South America. and the Lorentsen machine (US #1,866,111 of 1932).

Figure 38. 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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17: Helix wheel (gold wheel) – 1900s research in Colorado Operation 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 Figure 39. HELIX WHEEL Avista geologist using a gold wheel in near-freezing conditions. battery. The speed is critical and can be controlled (photo: Avista Ltd of Bishkek) between 15 to 22 rpm. The best gold wheels are of The helix wheel (gold wheel), commonly called a moulded polypropylene plastic for lightness and TM spiral panning machine, is a flattened Archimedes screw smoothness. The 7-spiral Little Camel has a wheel 16.5 with the helix no longer turning inside a cylinder but inches across. To enhance gold recovery: spiralling smaller and smaller to a central discharge hole. ² pre-screen the concentrate to <0.6 mm; ² for fine gold, pre-screen to 0.15 to 0.30 mm; The gold wheel was invented by Henry Earle of ² if it clogs with black sand, add a teaspoon of normal sand; Denver and patented in 1911 (US #987,866). ² material must be limited to keep it fluidised; and Some gold wheels are designed to recover fine gold, ² to cut surface tension, add a little Cascade or Jet-Dry anti- whereas others focus on maximising throughput. spotting agent (not detergent). By arranging two to six gold wheels, Johnny Hilmer Kleven of California invented a method of increasing the Adoption by placer gold miners washing capacity to 1.5-2.0 tons/hour, patented in 1977 and Gold wheels are popular worldwide with recreational 1978 (US #4,008,152 and US #4,110,206), and he later miners, artisanal miners, prospectors and companies for invented a single multi-step wheel that accomplishes the same upgrading concentrate. Many makers exist in the USA and (US #4,267,036). in South America, Africa, China and Russian Federation. Industrial-scale gold wheels appeared in the 1980s Yet the gold wheel’s popularity is uneven and the reason led by PMX Industries [61], and Keith B. Cleland was unclear. awarded patents in 1983 and 1984 for solving how to The modern small portable gold wheel was invented build large wheels with 60-100 helical riffles converging on by Angus Nicholls in the USA and his Little CamelTM gold a single central port (US #4,389,308 and US #4,406,783). wheel is still made by Camel Mining Inc (www.desfox.com). Production of large gold wheels ceased after a decade.

Figure 40. GOLD RECOVERY BY GOLD WHEELS – generalised Recovery of placer gold by helix wheels (gold wheels) [61] (compiler: Robin Grayson)

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18: Wilfley shaking table – 1890s research in Colorado Operation 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 Figure 41. WILFLEY SHAKING TABLE A Wilfley shaking table made in Australia. (photo: courtesy of the streams each dominated by a particular mineral. manufacturer, Motive Traction Pty Ltd - www.motive-traction.com.au) The shaking motion has a slow westward stroke and The Wilfley shaking table was invented by Arthur R. rapid return eastward stroke – often with a bump. This Wilfley of Denver, Colorado and patented in 1897 (US induces settled particles to crawl in a juddering manner #590,675). The device proved enormously popular being westward along the table with the thin film of slurry. able to consistently recover fine particles of dense The shaking is usually very rapid with a frequency of minerals and with a high concentration ratio. 4 to 5.5 strokes per second. The shaking displacement is Many thousands of Wilfley tables were made and are usually half to one inch to-and-fro. still manufactured. Dozens of variations emerged, such as A set of low riffles aligned east-west guide the the Deister table [61] patented by William F. Deister and heavies ever westward to fall off the south-west corner of Emil Deister of Indiana (US #1,642,843). Today shaking the table into a hopper as a continuous discharge. tables are made in the USA, UK, Australia, Russia, China, Meanwhile, a spray bar introduces clean ‘wash water’ Thailand and elsewhere. along the north edge of the table, sending a thin film of Shaking tables are thin-film devices whereby heavy clean water southward to encounter the riffles and the particles are induced to settle from a flowing film of slurry westward flowing slurry. The wash water mixes with the while light particles are washed away as tailings. The thin- slurry and overrides the riffles taking the lighter particles film needs a large surface area and therefore some sort of with it to spill over the southern edge as a continuous table shape is essential, ranging from small laboratory discharge of tailings. tables to production tables 7x15ft in size. Adoption by placer gold miners A shaking table can recover >90% of gold from 3mm down to about 70μ, and still able to recover >70% of 50μ Wilfley shaking tables remain popular amongst placer gold, and useful amounts of 30μ gold. gold miners in many regions of the world. A key factor is Disadvantages include: low capacity, bulky size, high that miners like to see the gold separating. water usage, and need for having a stable.

Figure 42. 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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19: Shaking tables – 1960s-1970s research in China Operation 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, Figure 43. CHINESE SHAKING TABLES surface texture and purity; presence of other minerals Chinese-built shaking tables of traditional design recovering gold (light or heavy) attached to the gold; nature of other from slurry fed from a ball mill at a Chinese-owned hardrock gold mine in Bayanhogor Aimag in Mongolia. (photo: Robin Grayson) particles present; dilution of the feed; the fineness of the feed; and the problem of any traces of oil or grease. For over a century shaking tables have remained popular in China as elsewhere for clean-up of concentrate Adoption by placer gold miners from placer gold wash-plants and milled hardrock ore. Shaking tables are popular amongst placer gold In the 1970s, placer scientists of the Minerals miners in many regions of the world. A key factor for Processing Laboratory of the Kunming Institute of many miners is that they see the gold separating. They Metallurgy in Yunnan Province determined the percentage can be seen in action in Alaska, Yukon, Alberta, British gold recovery of conventional shaking tables [27]. It is Columbia, California, Central America, South America, unclear what type of shaking tables were tested or the Africa, Australia, New Zealand, S.E. Asia, China, Mongolia size of the feed, but the results showed gold recovery and the Russian Federation. Manufacturers of shaking starts to falter at 0.5mm and is only 90% by 0.2mm, and

tables are numerous and widely spread, for instance: collapsed to 75% at 0.1mm. ² United Kingdom – Holman-Wilfley Ltd of England These results are disappointing, and a modern www.holmanwilfley.co.uk shaking table if operated carefully performs significantly ² Australia – Motive Traction Pty Ltd Inc of New South Wales better. However in the 1970s the observed performance www.motive-traction.com.au ² USA – Outokumptechnology Inc of Florida of shaking tables in the China tests would have been www.outokumptechnology.com considered acceptable, bearing in mind the chronic ² Thailand – Dove Engineering performance of simple jigs and sluices at that time. www.dovemining.com ² China – China National Gold Corporation (CNGC) www.chinagold.org.placer.html

Figure 44. 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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20: Shaking tables – 1960s research in the Soviet Union Operation 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 Figure 45. SOVIET SHAKING TABLE conventional Soviet-made shaking table, with five sizes of Shaking table of traditional Soviet design at the Sharin Gol Mine placer gold described as “porous, acicular” [62]. Again it is of Polymet Potala Ltd in Mongolia. (photo: Robin Grayson) not entirely clear what the correct English translation In spite of the popularity of the Wilfley shaking table should be. The performance was poor, with >90% and its derivatives, little has been published their ability to recovery only possible for 300μ gold and larger. Recovery recover gold of different size or flatness. Many reports fell to only 80% for 150μ gold, 55% for 100μ gold and affirm shaking tables are “effective” or “successful” in under 35% for 50μ gold. recovering gold of this or that size or shape, yet rarely It is unclear what the parameters of the Soviet tests mention the amount of gold lost. were, rendering it impossible to interpret the results. For Studies on the ability of shaking tables to recover tin instance the feed is assumed to have been screened at (cassiterite SnO ) led people to assume a shaking table 2 3mm as is standard practice, but if screened at say 5mm would perform better with gold; gold being so dense. This then performance would have been compromised. assumption is dubious, for gold, especially placer gold, is The results cast doubt on the ability of shaking tables often leached and porous so its density is reduced; gold is to perform well at recovering fine placer gold in industrial- markedly hydrophobic making it prone to float on a scale operations, and demonstrate the adverse effect on shaking table; and gold is often so flat its settling velocity recovery if the gold particles are porous or flat. is less than expected. Conversely, cassiterite is typically not leached, not porous, not hydrophobic and not flat. Adoption by placer gold miners Only in the Soviet Union and China does it seem Shaking tables are popular amongst placer gold proper scientific tests were conducted on the ability of miners in many regions of the world. A key factor for traditional shaking tables to recover gold, and later by the many miners is that they see the gold separating. British Geological Survey [24].

Figure 46. 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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21: 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 ‘pinch- point’, 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 a means to adjust the inclination of the sluice; and ² a means to adjust the angle and position of ‘splitters’. Pinched sluices cannot exceed about 70% recovery Figure 47. PINCHED SLUICE of heavy minerals, as this is the limit of the pinching General outline of a pinched sluice. (drawing: Robin Grayson, after Michael Priester, Projekt-consult – www.projekt-consult.de) mechanism [25]. The author suggests it is possible that gold particles may continue to be crowded and upgraded Pinched sluices (fanned sluices) have been used for – experiments are required. centuries [25]. A pinched sluice is a small sluice that A pinched sluice appears, in the opinion of the tapers towards the discharge end. Slurry rushing down the author, to have two potential applications that are quite sluice gets crowded, the slurry is forced to deepen, and different and should not be confused: the crowded denser particles gravitate towards the bottom ² traditional application – producing an underflow, forcing the lighter particles to rise above them. As a result middlings flow and a tailings flow; and the slurry discharge is stratified – a very dense ‘underflow’ ² additional application – controlling surges in the feed rate. of valuable concentrate, a medium density middle flow Some pinched sluices have permanent magnets (‘middlings’) and a low density upper flow (‘tails’). The installed beneath them to encourage magnetite and other discharge is divided by splitters into separate streams – magnetic particles to join the underflow. This could assist concentrate flow, middlings flow and tailings flow. in creating a traction carpet of dense particles at the base A pinched sluice is a low cost way to produce of the slurry, and in theory this would assist in the concentrate, yet the middlings need to be recirculated. ejection of low density particles and the burrowing down Most pinched sluices are used to recover mineral sands of tiny gold particles. More research is warranted. [63-66] and are a neglected device in gold recovery. A sister device to a pinched sluice is the Wright Adoption by placer gold miners impact plate invented by Douglas Charles Wright of New Pinched sluices are rarely seen in placer gold mines South Wales and patented in 1978 (US #4,078,997). in spite of their obvious potential.

Figure 48. 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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22: 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 quartz – terminal Settling Velocity cut from 0.9 to 0.4cm/sec; ² magnetite – terminal Settling Velocity cut from 2.3 to 2.7cm/sec; ² gold – terminal Settling Velocity cut from 7.6-9.8 to 7.1-9.3cm/sec. 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 Figure 49. REICHERT CONES particles such as quartz become increasingly vulnerable to Example of Reichert cones. (photo: courtesy of Dale Henderson of the manufacturer Roche Mining – www.rochemt.com.au) ejection from the ever-denser slurry. Gold concentrate is removed by annular slots in the The Reichert cone was invented by Ernst Reichert of cone. Consistent recovery of gold particles >45μ have Queensland who applied for an Australian patent in 1966 been reported [67]. At the Snake River in Idaho, USA, and was awarded a US patent in 1968 (US #3,379,310). gold was recovered using a Reichert cone in conjunction The device packs pinched sluices in a circle, then with a conventional shaking table. Gold recovery >85% dispenses with their side walls to create a single cone with was noted by Thomas Ferree [68], with 44% of recovered a central discharge hole. The flow is free of edge-effects gold being <75μ. without side walls. The cone is of lightweight structural glass reinforced Adoption by placer gold miners plastic (GRP), laminated with polyurethane. The cones are stacked in series to repeat the benefit of crowding. Reichert cones have not gained much foothold in placer gold miners. They have been used in Queensland Multiple stages of upgrading are achieved. Stacks of cones 3 are mounted in circular frames over 6 metres tall. The by a 20m /hour plant to recover fine gold from tailings. In 3.5-metre cone processes up to 350 tons/hour. Water Snake River Idaho, a sand and gravel company recovered consumption is less than for sluices or jigs [61]. The unit has gold with Reichert cones while selling gravel. no moving parts and very low operating costs, but needs screens, cyclones and slurry pumps.

Figure 50. 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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23: Humphrey spirals – 1940s research in Colorado Operation 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 Figure 51. HUMPHREY SPIRALS faster particles that are less retarded by friction. As a Humphrey spiral illustrated on the front page of the classic ‘Tools consequence the heavy particles spiral along the inside for Mining Book’ by Michael Priester and colleagues [25] walls of the channel, while the faster lighter particles spiral Modern spirals began with Frank Pardee of further out towards the outer rim of the channel. Pennsylvania who was awarded patents in 1899, 1924 and The heavier particles are recovered as concentrate 1939 (US #629,590, US # 1,516,926 and US #2,145,315). from discharge outlets on the inside of the channel. The Humphrey spiral was invented by Ira B. Separation precision can be improved by adding additional Humphrey of Denver USA who applied for US patents in water during the sorting process. 1943 and was awarded patents in 1947 (US #2,431,559 Key variables are the cross section of the channel, the and US #2,431,560) and 1955 (US #2,700,469). diameter of the spiral, the number of windings Advantages include low cost, long life, small (revolutions), slope of the channel and the positioning and footprint, good recovery of fine gold and ease of visually number of discharge outlets and supplementary-water checking to see if material is separating properly. intakes. The basic Humphrey spiral is 3 metres tall with 5-6 windings of the channel and is capable of processing 0.8- Adoption by placer gold miners 12 tons/day of concentrate depending on the design of Humphrey spirals enjoyed great popularity for coal the channel and the particle size. Early production models cleaning and mineral sand recovery, but were very rarely were of cast iron sections and required numerous pipes used in placer gold recovery. Early models were difficult for supplementary-water intake and discharge outlets for placer miners due to the weight and the need not only making it a rather complicated, very heavy and difficult to close screening and a steady feed of slurry. The later adjust; and rapid wear of the rubber lining and irregular model 7 Reichert spirals gained a stronger niche amongst wash water distribution caused production problems [61]. placer mining companies for fine gold recovery. Later Humphrey spirals were of lightweight material and more compact being a double helix.

Figure 52. GOLD RECOVERY BY HUMPHREY SPIRALS Recovery of placer gold by Humphrey spirals, according to diverse fragmentary sources. (compiler: Robin Grayson)

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The ‘Perfect Mousetrap’ 1970-80 Helix cylinders, after languishing for over half a century, became a focus of research during the 1970s, Cyanide begins to challenge mercury… being able to recover both fine and coarse gold as a mobile wash-plant in placer operations. Development By 1980 mercury amalgamation (#1) was beginning peaked in the 1980s and early 1990s, and many types are to be challenged, but still dominated fine gold recovery in still in production. placer mines and most hardrock gold mines. The Other gold recovery devices were available – gold alternatives had increased but were still limited and rarely wheels, traditional shaking tables, pinched sluices, in general use. Reichert cone and Humphrey spiral, but these were Cyanide leaching (#2) made technical advances and suitable only for clean-ups as they could not cope with heap leaching was proving effective in recovering fine gold coarse feed. These limitations attracted some innovators, in hardrock ores, including the vast tailings of hardrock and gold wheels were arranged stepwise to boost gold mines in North America. While becoming common in throughput and permit coarser feed, and a multi-step the west, heap leaching had yet to spread globally. wheel was invented. Cyanide’s success overshadowed all other leaching, and As well as witnessing an increase in the throughput of research focussed on how to leach ‘difficult’ refractory clean-up devices, the 1970-80 interval also saw the start of gold ores that were not amenable to cyanide leaching. miniaturisation to suit recreational miners and prospectors, a Thiosulphate leaching (#24) after being neglected trend that continues unabated to the present day. for over half a century received fresh attention but progress was slow.

Gravity methods make slight progress… Between 1970 and 1980, gravitational methods of recovering gold made some limited breakthroughs and the role of independent inventors began to become important again for the first time since the 1920s. Duke’s E-tank (#25) was the first elutriated sluice tank of modern times but, in spite of enabling small placer gold miners to catch fine gold down to 50μ, Duke’s device failed to raise interest and manufacture was short-lived. Compound water cyclones (#26) emerged as a distinctive hydrocyclone. Rather than dewatering to create sludge as in a typical hydrocyclone, a CWC removes heavy particles as underflow and light particles as overflow (tailings). Research teams in Canada, USA, South Africa and the Soviet Union made good progress at recovering fine gold, but interest in the west faded by the end of the 1980s due to lack of funding and operational difficulties. It seems that CWC are still made in the Russian Federation. Neffco bowls (#27) were perfected by Larry Neff of Utah, and performed significantly better than the Knudsen bowl and all other traditional centrifuges, and better than the ‘forced amalgamators’ such as the Gilkey bowl. Neffco bowls are able to recover fine gold down to 30μ yet they gained only a limited share of the North American market although they are still manufactured. Bartles’ crossbelt (#28) revitalised vanners and became popular for industrial-scale clean-ups, particularly in recovering hardrock gold. Decked orbital tables (#29) of Richard Mozley revived interest in shaking tables, being able to recover 90% of 5μ gold in tightly screened industrial-scale clean-ups. This suited hardrock gold operations; mercury amalgamation continued to dominated placer gold recovery. Jigs remained popular for recovery of hardrock and placer gold, but made little progress was made in improving recovery of finer gold. On the other hand, significant reductions in energy and water requirements Figure 53. METHODS OF GOLD RECOVERY BY 1980 were achieved. See text for details of each method. (compiler: Robin Grayson)

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24: Thiosulphate leaching – 1970s research in Canada Thiosulphate leaching has the potential to replace cyanide leaching, being relatively cheap, environmentally Operation less hazardous and capable of leaching gold from difficult 1st stage – leaching gold into solution refractory hardrock ores such as carbonaceous or Carlin Sodium thiosulphate pentahydrate Na2S2O3.5H2O types. In contrast, cyanide cannot leach gold from (‘hypo’) is usually supplied in powder form sourced from refractory ores without a preliminary step [69]. specialised manufacturers. A solution of hypo is prepared, Thiosulphate leaching is by sodium thiosulphate and then ammonia added to make the leachate strongly Na2S2O3, an essentially non-toxic colourless crystalline alkaline (10 to 10.5pH). Copper ions must also be present compound that is more familiar as the pentahydrate, as essential oxidising agent. The ammonia and copper are Na2S2O3.5H2O known as sodium hyposulphite or ‘hypo’ as catalyst-like for they are neither produced nor consumed used with silver in traditional photography. but recycled. In practice, copper may precipitate cupric Although hypo had been known since the early 1900s sulphide that inhibits leaching, while ammonia may escape to be useful in leaching gold, it was only in the 1970s that Hypo tends to be unstable, avoidable by adding it was subjected to detailed study, leading to a patent sulphite ions to regenerate the thiosulphate and prevent being awarded to Roman N. Genik-Sas-Berezowsky, silver precipitating too early as insoluble silver sulphide. Verner Sefton and Lynton Gormely of Canada in 1978 (US The milled ore is added to the thiosulphate leach with #4,070,182) assigned to Sherritt Gordon Mines Ltd. a pulp density of 40 to 45% solids. Hypo consumption For three decades thiosulphate leaching has been may be as much as 30 kilos per ton or ore, but can be cut heralded as being close to challenging cyanide leaching to 13 kilos/ton by adding reducing agents as chelates. [69]. It has been the subject of four to five US patents 2nd stage – recovering gold from solution every year since the late 1970s and the focus of hundreds The pregnant thiosulphate leach solution contains of research papers. In spite of this, thiosulphate leaching dissolved gold in the form of gold-thiosulphate complex, has not yet been properly commercialised. A breakthrough and the pregnant solution is removed for further seems tantalisingly close as shown by the patent awarded processing to recover the dissolved gold. to Jinxing Ji, Christopher Fleming, Paul West-Sells and Activated carbon or resins are ineffective for Ralph Hackl of Canada and patented in 2006 (US recovering the gold from the pregnant leach solution. #7,066,983) assigned to Placer Dome Inc. Instead gold is recovered by cementation method The thiosulphate method uses a solution of hypo using zinc, iron or copper. Under controlled conditions, Na2S2O3.5H2O in the presence of an oxidising agent to gold recovery from suitable ores can exceed 90%. dissolve (‘leach’) fine gold as a strong complex 3- [Au(S2O3)2] , and then to precipitate easy-to-recover Adoption by placer gold miners gold. The preferred oxidising agent is copper ions. In The author is unaware of thiosulphate leaching being contrast, thiourea leaching uses ferric iron (Fe3+) whereas used at large-scale placer gold mines, large or small. The cyanide leaching uses oxygen direct from the air. main deterrent is the uncertain technology, variable Adding more oxidising agent is unnecessary for a Cu- results and difficulty of controlling the process efficiently. bearing ore.

Figure 54. 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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25: Duke’s E-sludge tank – 1970s research in Georgia Operation 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 Figure 55. DUKE’s E-SLUDGE TANK has a rectangular hopper-shaped tank in which pay gravel Duke’s E-Tank on a flat-bed truck. A manifold injects water to is dumped by a conveyor, front-end loader, etc. The slurrify the base of the mound of dryish pay-gravel, creating stiffish slurry that undergoes density stratification. The stratified material should be dry or moist and should not be wetted slurry flows down the floor of the tank to the right, discharging in by spray-bars. Water is injected into the base of the tank a ‘recovery box’ where dense basal slurry is trapped in slots. by means of a manifold of perforated water pipes, at a (drawing: Robin Grayson, adapted from Duke’s patent) pressure of 5-35psi but typically 5-10psi. Unlike a jig, E- Duke’s E-tank was invented by Arthur Duke of tower or sluice, the water requirement is very low – each Georgia USA and patented in 1976 (US #3,951,787). tank is about a metre wide but a tank requires a mere Gold-separating devices classed as E-tanks include Duke’s 7.6m3/hour of water to process about 22m3/hour of loose E-tank, Graefe’s E-tank and Pyramid’s E-tank. pay gravel per hour. This is an order of magnitude less Elutriated sludge tanks (E-tanks) is a term coined by water than a Yukon-style sluice needs to process the same the author [28] for devices that inject water from below volume of pay gravel. into a tank containing a bed of pay gravel that is quite The pile of gravel in the tank cavitates from below thick (say >10 cm) to create a fairly stiff slurry just watery due to contact with injected water. The slurrified basal enough to stimulate gravitational settling of dense gravel is free to stratify (dense material at the bottom) particles and rising of low-density particles. The settled and creeps along the base of the tank tilted at about 12°. heavy concentrate can be a continuous discharge from The slowly discharging sluggish porridge-like slurry near the base of the E-tank (e.g. Duke’s E-tank), or travels down a chute tilted at 15-35° and the dense lower remains in the tank as a lag deposit to await batch material is trapped by slots. According to the inventor, discharge (e.g. Graefe’s E-tank). 95% recovery is normal, and that recovered gold can be Unlike an E-tower, the E-tank’s contents remain as as small as 8μ! However no tests are published. porridge-like slurry from top to bottom, and the slurry is not watery enough for classic ‘hindered settling regime’. Adoption by placer gold miners Instead the contents resemble a thixotropic quicksand. Since the award patent in 1976, the author has been Unlike a jig, an E-tank contains slurry throughout. unable to trace any record of Duke’s E-tank being used by There is no jig screen, no ragging, no hutch chamber and placer gold miners, in spite of its potential. slurry is not pulsed upwards (i.e. no jigging).

Figure 56. 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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26: Visman’s compound water cyclone – 1970s research in the Yukon Operation Feed is passed through a fine mesh screen (e.g. 1- 3mm), 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

Figure 57. VISMAN’s COMPOUND WATER CYCLONE stage, rather than replacing say a sluice, jig or centrifuge. Cross-section of Visman’s compound water cyclone (CWC). Dense Results of tests are variable, and it is concluded by particles report to the UNDERFLOW, light particles report to the the author that as yet – in spite of considerable research – OVERFLOW. (drawing: Robin Grayson from US patent #3,353,673) the theoretical basis for CWC is not firmly established and Visman’s compound water cyclone (CWC) was this is an impediment to developing a device that invented by Jan Visman of Alberta and patented in 1965 consistently produces good results in terms of fine gold (US #3,353,673) and reissued in 1967 (US Re#26,720). recovery and concentration ratio. Research on the ability of compound water cyclones to recover placer gold began in the Yukon in the 1970s Adoption by placer gold miners [70] and continued in the 1980s in British Columbia [71] Experiments on placer gold recovery by CWCs have and Alaska [11,56,72,73]. been conducted from the early 1970’s to recent times in CWCs were developed to maximise concentration by North America (British Columbia, Yukon and Alaska) and particle density. They are of squat shape due to their Soviet Union [74] wide-angled cones, and internally have long ‘vortex In spite of early success in recovering placer gold, finders.’ CWCs are potentially excellent gold recovery R&D on compound water cyclones did not produce fully devices by virtue of low cost, ease of operation, and no reliable CWCs for placer gold mines. Manufacture has moving parts. Instead it uses the energy of pumped ceased in the west, although they may be still being made inflowing slurry ‘guided’ by the internal shape of the in the Russian Federation. device to achieve centrifugal concentration.

Figure 58. 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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27: 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 looks like a reverse auger as the groove appears to auger

down to the bottom of the bowl” (source: Figure 59. NEFFCO BOWL Examining the inside of a Neffco bowl, showing the spiral riffles. PopandSonminers). The drag of the water may aid (photo: courtesy of DanfromNY on the Alaska Gold Forum) heavies to move to the sidewall and lights to spiral up. The Neffco bowl was invented by Larry Neff of Utah. The slurry is continually swept down the spiral He began experimenting with centrifugal recovery of flour groove. As the slurry spirals down, the heavies end up at gold in 1976 and by 1980 had invented the novel Neffco the bottom of the groove while the lighter particles escape bowl – it has a single long riffle groove that spirals upwards and out the top. The end result is that a high downwards. Other bowls have grooves in rings. The percentage of heavies are driven to the bottom of the Neffco bowl is made by Neffco Mining, of Salt Lake City, bowl. Periodically the bowl is stopped for batch discharge. Utah – www.neffcomining.com. The rpm is fixed, and control is exerted by altering The maker claims the Neffco bowl, when part of a the water input (source: Zooka). The water level is complete wash-plant, recovers “better than 95% of the of maintained 1.5 inches below the rim and at the rim, with the flour gold in most materials we have tested”. less water for finer cuts. Anecdotal evidence indicates the Neffco bowl is capable of The 24-inch Neffco bowl will run 1-8 tons per hour of recovering fine gold. But there are reports of very rounded solids, uses a ½hp motor, rotates twice per second, uses gold particles sometimes flowing (rolling?) straight out of 125 gallons per minute of water and is claimed to be able the unit although these can be caught before or to capture 500-mesh gold (25μ). afterwards with a standard sluice (source: Zooka of AGF). Adoption by placer gold miners To achieve capacity, often 4 or more bowls are run in parallel, and – presumably because of the escape of large The Neffco bowl is fairly popular among placer gold high sphericity gold – a long length of expanded metal miners in North America, mainly for cleaning concentrate. sluice is used to scavenge the tailings. The Neffco bowl A few mines use Neffco bowls in parallel as the core of achieves a concentration ratio of 2,000 to 1. their primary wash-plants, plus a sluice to catch the round To clean concentrates, material is run through two gold that escapes. Some recreational miners in Alaska use bowls in series, and the second captures about 5% extra. Neffco bowls aboard small offshore dredges.

Figure 60. 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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28: 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” [61]. The orbital shear is closely controlled and adjusted to optimise recovery. Figure 61. BARTLES’ CROSSBELT Lights fail to settle on the belt due to the action of Layout simplified from the patent. (drawing: Robin Grayson). the orbital shear and so flow off the sides of the belt, The Bartles crossbelt is a vanner invented by Richard made possible by the sides of the belt gently sloping Owen Burt of Cornwall and patented in the UK and then in sideward at 1.5° to 3°. 1977 in the United States (US #4,060,482), and assigned The belt is much wider than conventional tables and to Bartles (Carn Brea) Ltd of Cornwall. this allows a greater spreading area for valuable products, The Bartles crossbelt consists of a 2.44m wide therefore allowing distinct cuts to be made between the endless PVC belt that passes over a pair of rollers, one gold concentrate and the middlings. being the drive roller. A unique feature of the belt is its Adoption by placer gold miners central longitudinal ridge from which the belt slopes slightly to its sides. The Bartles crossbelt was intended primarily for An orbital shaking motion of 250-400 rpm is imparted hardrock mills and tin recovery, and sold worldwide. The to the moving belt by a rotating weight, made possible by author is unaware of it having ever being applied to the belt assembly being freely suspended by four wires recovery of very fine gold, although it has potential. The from a supporting frame. The orbital shaking motion is invention is an alternative to a shaking table but induced by an out-of-balance rotating drive shaft driven manufacture ceased a decade ago. by an infinitely variable D.C. electric motor.

Figure 62. GOLD RECOVERY BY BARTLES’ CROSSBELT Gold recovery by Bartles’ crossbelt, according to Silva 1986 [61]. (compiler: Robin Grayson)

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29: Bartles-Mozley multi-deck tables – 1970s research in Cornwall Operation 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]. Figure 63. BARTLES-MOZLEY MULTI-DECK TABLES The deck and its riffles undergo an orbital horizontal Layout simplified from the patent. (drawing: Robin Grayson). motion imparted by an out-of-balance electric motor. The The Bartles-Mozley multi-deck concentrator consists of orbital horizontal motion has a large amplitude of 5 to 18 40 fibreglass decks (tables) each 3.6ft x 5ft arranged in cm at a frequency of 0.8 to 3 Hz. The orbital horizontal two sections of 20 decks each, suspended by cables. Each motion energises the light particles to inhibit them settling deck is riffled and connected by ½-inch plastic formers and they remain suspended in the flowing film of water that define the pulp channel. Good recovery is from 100μ that overrides the riffles to discharge as tailings. to 5μ, “and in some cases as small as 1 micron” [61]. Meanwhile high density particles settle on the table The device soon became “probably the most widely and remain restrained by the riffles. used slimes table today” [61] due to its high throughput, After running for about 35 minutes, feed is stopped small footprint, low labour requirement, low power and the decks tilted slightly to drain, then tilted steeply to consumption, and low water consumption. [75,76]. After allow the concentrates to be flushed into a collection more than a decade its popularity collapsed due to sump. The tables are then returned to the original competition from new centrifuges (e.g. KnelsonTM bowl orientation and processing recommences. and FalconTM C bowl) and better chemical leaching. Several types of multi-deck concentrators existed. Adoption by placer gold miners Richard H. Mozley of Cornwall patented a variant in the UK The Bartles-Mozley separator was intended primarily and then in 1981 the United States (US #4,251,358), and for hardrock mills and tin recovery, and sold worldwide. assigned to the National Research Development The author is unaware of it having ever being applied to Corporation. A modern variant was patented by Paul recovery of very fine gold, although it has potential. Marriot of Cornwall in 1989 (US #5,148,922) but allowed to lapse in 1996.

Figure 64. 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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The ‘Perfect Mousetrap’ 1980-90 Cyanide strongly challenges mercury… The Yunxi bowl (#48) achieved high % recovery of 3-37μ cassiterite in China, but little known in the west. By 1990 mercury amalgamation (#1) was under KnelsonTM (#49) and FalconTM C (#50) bowls proved strong challenge from cyanide and improved gravity capable of out-competing mercury and recovered >10μ methods, yet global usage of mercury soared with the rise gold that was too slow to easily recover by leaching. in artisanal mining. Conversely mercury was now tightly Lemmons’ vanner (#51) recovered 90% of 20μ gold restricted in placer mines in the Soviet Union, Mongolia, in field tests in the Yukon, but failed to gain much interest. Europe, Canada, USA, Australia and New Zealand. Broessek’s helix cylinder (#52) recovered 90% of Cyanide leaching (#2) dominated industrial hardrock 30μ gold and became popular in industrial placer mining. mining outside the Soviet Union and China, and became The GemeniTM table (#53) surpassed most entrenched amongst artisanal gold miners worldwide due traditional shaking tables and remains popular today. to its ability to leach fine gold left after amalgamation. The mark VII Reichert spiral (#54) recovered 80- The search for alternatives to cyanide leaching 90% of 40-50μ gold but hampered by tight screening. continued. New avenues included bioleaching (#30), and biooxidation (#31) to render ores amenable to leaching. Agglomeration (#32) notably coal-gold agglomeration (CGA) rekindled interest in froth floatation of gold but CGA failed to compete with cyanide. Oleophilic adhesion (#33), familiar as the grease table for recovering diamonds but neglected in gold recovery since the 1930s, was revisited by Jan Kruyer but his sticky mesh belt failed to gain commercial attention. Gravity methods make major progress… 1980-90 witnessed an upsurge of interest in gravity by miners, inventors and governments, driven by: ² draconian legislation in Russia, Mongolia, Canada and USA limiting or banning mercury in placer gold mining; ² rising world gold prices making it profitable to mine low-grade and fine gold deposits IF % gold recovery was high; ² further expansion of recreational mining in affluent countries; ² Hg/Pb remediation spurred demand for gravity equipment; ² projects to alleviate poverty by ASM worldwide; ² projects to combat mercury usage by ASM worldwide; and ² aversion to cyanide leaching by some communities. Magnetic coated gold (#34) was a surprising innovation by did not become commercialised. Industrial sluices (#35-38) were transformed by the Yukon tests, and now recovered 90% of 150μ gold. McCann’s small sluice (#39) for recreational miners went further and could catch 90% of 70μ gold. Hydraulic sluices (#40) were perfected in New Zealand. No better than a Yukon sluice for fine gold, they became leaders as mobile wash-plants with skid-mounted trommels. Graefe’s E-Tank (#41) marketed as the Keene Hydromatic Jig recovered 90% of 50μ gold. After a decade of interest from recreational miners, manufacture ceased. Cleaveland/IHC jig (#42) “came ashore” when the tin price collapsed, and competed with new advanced jigs – Ross jigs, Goldfield jigs, Arctic Miner jigs etc. Lashley’s ASAT E-tower (#43) recovered 90% of 20- 30μ gold, but R&D ceased as commercial viability neared. Osterberg’s E-tower (#44) was also effective but secured only a limited market among recreational miners. Younge’s horizontal centrifuge (#45) recovered 90% of 70μ gold yet required very little water but failed to gain market share and manufacture ceased.

The MozleyTM ‘Multi-Gravity Separator’ (#46) and the Figure 65. METHODS OF GOLD RECOVERY BY 1990 TM Kelsey centrifugal jig (#47) advanced vibrating centrifuges See text for details of each method. (compiler: Robin Grayson) and won markets in fine ore recovery – except for gold.

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30: Bioleaching – 1980s research in Wales and California Bioleaching is the extraction of metal from its ore by Operation means of microbes. An introductory account can be read This text is based on the Geobiotics method of at: http://en.wikipedia.org/wiki/Bioleaching. bioleaching as presented in US patent #5,378,437. The Pooley method of bioleaching was invented by Mass cultivation of the chosen micro-organisms is Frederick D. Pooley of Wales and patented in 1987 (US undertaken in outdoor ponds in a sunlit site close to the #4,497,778) with limited success. ore body. The culture pond is lined with plastic or The Geobiotics method of bioleaching was invented concrete and the water body is 10-30m wide and 20-50m by Dennis Kleid, William Kohr and Francis Thiobodeau of deep. The culture pond is fitted with a pumping system to California who applied for a patent in 1992, awarded in permit harvesting and recirculation of the growth media. 1995 (US #5,378,437). It met with greater success. The Algae are the most convenient. Each species has method is suitable for ores of grades as low as 0.02 Troy particular tolerance of physical and chemical conditions, ounces per ton. Advantages are: ² economic: simple and cheap, and few technicians required; and nutritional requirements including phosphorous, ² environmental: the microbes are natural and easy to cultivate. nitrogen, sulphur, iron, manganese, trace elements and Disadvantages are: ions. The pond is kept strongly alkaline, pH7-10, by ² economic: bacterial leaching process is very slow; adding lime or phosphate buffer. Following guidance of + ² environmental: Sulphuric acid and H ions can leak and turn the patent, annual yields of 40 dry tons per hectare are surface water and groundwater acidic, and heavy metals such as iron, zinc and arsenic be leached by acid mine drainage. possible, and the microbes are sprayed onto the ore from the pond at the time of maximal cyanide production. Suitable microbes include: ² algae – Chlorella vulgaris, Cyanopora paradoxa and Bioleaching may be done in three settings: Cyanidium caldarium; ² tank bioleaching – milled ore in a tank; ² blue-green cyanobacteria – Anacystis nidulans; ² heap bioleaching – milled ore in a heap; or ² bacteria – Chromobacterium violatum, Chromobacterium ² in-situ bioleaching – blasted ore in situ. flavum, certain Bacillus species (pyocyaneus, flourescens, Oxidation of gold to gold-cyanide complexes violaceous, mesentericus, nitrificans), certain Pseudomonas species (aeruginosa, fluorescens, aureofaciens, cyanogena, commences once the microbes contact the ore, often with + - liquifaciens, cepacia); and the gold (I) ion – [Au ][CN ]2. Biosorption is “automatic ² fungi –notably Marasmius oreades (‘Fairy Rings’), the Snow and immediate” even if the microbes are dead. Mould basidiomycete and some Fusarium species. The fluid with microbes and biosorbed gold is pumped Cyanogenesis is the same in all these microbes, by into a settling pond or tank at least 3m deep and allowed to the oxidative decarboxylation of the glycine in a process settle, aided by flocculants. The sludge of living and dead stimulated by methionine or other methyl-group donors: microbes is harvested and the biosorbed gold recovered. NH2CH2COOH J HCN + CO2 + 4[H] An interesting approach is to co-culture microbes: Adoption by placer gold miners ² bacteria producing glycine in bulk; Bioleaching has not been adopted by placer gold ² same bacteria able of absorbing gold-cyanide ion complexes; ² micro-organisms liberating methionine in bulk; and miners as it poses too many challenges. Eventually a ² algae able to produce cyanide from a glycine substrate. simplified version may be acceptable.

Figure 66. 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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31: Biooxidation – 1980s research in British Columbia and California Biooxidation oxidises both iron and sulphur under acidic conditions, causing the solubilisation of iron as ferric Operation (III) ion and sulphide as sulphate ion. This liberates the The ore is first batch tested to determine if encapsulated gold making it accessible to leaching. biooxidation is effective. Batch testing may require six During the 1980s, biooxidation became the focus of months due to the time required for the bacteria to adapt intense research effort, offering a low-cost means of to the substrate and the time gap between inoculation of preparing refractory hardrock ores to make them the ore and its oxidation. The testing and evaluation can responsive to leaching such as cyanide leaching. be accelerated using a device such as the Oxidor column Biooxidation research continues unabated and a few reactor. Suitable cultures include the following species of milestone patents are outlined below. bacteria, either alone or in combination: Thiobacillus The Hackl biooxidation method was invented by thiooxidans, Thiobacillus ferrooxidans, Sulfobacillus Ralph P. Hackl, Frank W. Wright and Albert Bruynsteyn of thermosulfidooxidans, Metallosphera sedula and British Columbia, patented in 1991 (US #4,987,081) and Leptospirillium ferrooxidans. assigned to GB Biotech Inc of British Columbia. The A bacterial culture is developed that can grow in high method cultures of at least three species of bacteria - acidity and high metal content. The bacteria suspension is Thiobacillus thiooxidans, Thiobacillus ferrooxidans and used to inoculate ore stacked in the open air resting on a Leptospirillium ferrooxidans. The cultures are subjected to pad system. Biooxidation has a choice of settings: increasing concentrations of dissolved arsenic and low pH ² tank biooxidation - for refractory ores of relatively high grade to raise their tolerance. ² heap biooxidation - for refractory ores of relatively low grade. The Kohr biooxidation method was invented by Typically 180 to 600 days is required to oxidise the William J. Kohr of California, patented in 1995 (US iron and sulphur in the ore. This puts pressure on cash- #5,573,575) and assigned to Biotech Inc of California. flow, increased the mine footprint and adds to production Refractory sulphide ore is crushed and separated into a costs. Care is needed in the heap design to ensure fine fine and coarse fraction. The coarse fraction is stacked in materials do not plug the voids essential for aeration and a heap, and a concentrate produced from the fine liquid flow. Plugging results in starvation of nutrients, fraction. Alternatively biooxidation can be assisted by carbon dioxide and oxygen and uneven distribution of the forming particulates that are then heaped (US bacteria. Adequate air flow is essential to cool the heap #5,246,486) and polymer agglomeration may be from the exothermic effects of biooxidation. beneficial (US #5,332,559). After biooxidation the resulting oxidised ore is highly Biooxidation of carbonaceous and carbonaceous- acidic and, for leaching by cyanide must first be treated sulphidic ores is difficult, and requires a specific carbon- with lime to raise the pH substantially. deactivating microbial assemblage (US #5,244,493). The Oxidor column reactor for testing and evaluating Adoption by placer gold miners refractory ores was invented by Andrew Carter of Texas Biooxidation is inappropriate to placer gold ores as and patented in 2002 (US #6,498,031), assigned to sulphides are rare and gold is in the form of free particles. Oxidor Corporation.

Figure 67. GOLD RECOVERY BY BIOOXIDATION Biooxidation can oxidise sulphide ores sufficient for leaching. (compiler: Robin Grayson)

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32: Agglomeration – 1980s research in Australia and China Operation 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 Figure 68. COAL-GOLD AGGLOMERATION with the gold or to act as a carrier of the gold particles. Flowsheet for placer gold recovery by CGA. (drawing: Robin Grayson) The first step is to create coal-oil agglomerates about Agglomeration ‘snowballs’ extremely fine gold with 5mm in diameter using coal dust bound by kerosene, light coaly or oily material to produce large particles that can gas oil or fuel oil. Agglomerates smaller than 500μ seem then be recovered easily by floatation. to be more effective in recovering gold. Coal-gold agglomeration (CGA) makes use of gold The second step is to add the coal agglomerate to being hydrophobic (resists water-wetting) and oleophilic the gold-bearing slurry in the special reactor. The gold (easily wetted by oils). CGA only works with fine gold, particles, due to their oleophilic nature, continue to enter making CGA appropriate for placer tailings with fine gold the agglomerate particles until the operator considers the that would otherwise be lost, and for milled hardrock ore. target gold concentration has been reached. CGA began with BP plc in Australia who built a pilot Then the gold enriched oil-saturated activated carbon plant processing 1 ton/hour of placer gold concentrate. A particles are agitated and the agglomerated gold is patent was awarded to Mark Cadzow, Graham Elkes, mechanically separated. Gavin Ewin and David Mainwaring in 1986 (US Finally gold is recovered from the agglomerates by #4,585,548) and assigned to BP Australia. The team then burning them, the coal and oil incinerating to leave gold tried CGA for low-grade hardrock ores with <1 gram/ton and ash. The gold is then separated from the ash. of gold, patented in 1990 (US #4,976,781). In China, Zhao Bing and colleagues found CGA had Adoption by placer gold miners “many advantages” over cyanide and claimed 88% Early success in recovering placer gold with CGA did recovery of gold in amalgamation tailings. In Africa, lab not lead to its adoption by placer gold miners. Yet it might experiments by Kotze and Petersen achieved 85% gold allow high % gold recovery from ‘difficult’ placers, such as recovery from artificial gold-slurry mixtures [77-82]. fine gold in laterites (as in much of South America, Africa, A CGA pilot by Envi-Tech Inc under the Canada- Australia and parts of Kazakhstan, Mongolia and China), Alberta MDA project in 1993/94 indicated gold recovery without the use of mercury or cyanide. Recent work in from agglomeration-adsorption technology may be 95- Turkey shows high % recovery is possible for gold 99% but no results seem to have been published. between 53μ and 300μ [83].

Figure 69. 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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33: 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 Operation ideal being oleophilic, and proven to be naturally The text is adapted from the account of the greasy hydrophobic [84,85]. Conversely magnetite and quartz are belt described by Royer Luckenbach in his patents. oleophobic and hydrophilic. Yet gold recovery by oleophilic Hardrock ore is milled to 2.5mm – a major advantage adhesion failed to challenge froth floatation, in spite of over froth floatation that requires much finer milling. requiring less water and less grinding, ease of Placer ore is screened at 2.5mm. Gold in the oversize regenerating oils/greases/waxes and general simplicity. In is separated in a simple device such as sluice. The contrast, oleophilic adhesion became the standard means <2.5mm fraction is subjected to oleophilic adhesion. of recovering diamonds on grease tables and grease belts. Water is added to the <2.5mm feed to create a slurry Oleophilic adhesion was formerly known as the of about 25% solids by volume. Hydrophilic particles are ‘contact method of ore concentration’, for which Royer wetted by adding a trace of wetting agent such as sodium Luckenbach of New York was awarded two patents in silicate (see patents) or liquid non-frothing detergent. This 1923 and a third in 1931 (US #1,448,928, US #1,478,237 weakens surface tension and sinks ‘float gold’. and US 1,792,544). The patents propose a sticky coating The ‘greasy belt’ is an endless rubber belt moving of oil, grease or wax being smeared on an endless belt between rollers, one of which is a drive roller. The belt is (e.g. a Frue vanner) to which gold particles would be coated in a thin sticky (‘tacky’) coating of oil, grease or attracted and remain attached even when the belt inverts wax but not so liquid that it might drip free when the over an end roller where black sand and quartz are shed moving belt inverts on passing over the end roller. as tailings. The gold is removed from the moving inverted Luckenbach suggests a flexible resin binder such as belt by a scraper, and a roller reapplies a sticky smear of rubber is added to the coating to make it waterproof. oil, grease or wax. Luckenbach added sodium silicate to The slurry issues as a thin stream onto the moving the smear as a wetting agent to deter settling of endless belt and the gold adheres to the sticky coating by magnetite and other gangue minerals, and his patents oleophilic adhesion. At the end, the water and gangue mention an extraordinary range of suitable oils, greases minerals are shed as tailings whereas the gold and other and waxes – including candle wax, candle tar, coal tar, oleophilic particles remain stuck to the inverted belt from horse grease, bitumen and lard. which they are removed by a scraper. The scraper also The Lurgi method was invented by Ernst Bierbrauer removes some or all of the sticky coating. of Germany and patented in 1940 and 1942 (US #2,189,698 The inverted belt passes across a roller that applies a and US #2,291,447) but not for gold recovery. fresh sticky coating and then turns ‘right-way-up’ over its The Kruyer method was invented by Jan Kruyer of end roller to again capture oleophilic particles from slurry. Alberta and patented in 1983 (US #4,511,461) and rather than using a sticky solid belt uses a sticky mesh belt. Adoption by placer gold miners Rather than scraping the belt to collect the adhering Oleophilic adhesion does not appear to be being used oleophilic particles, the belt is squeezed between rollers or by placer gold mining companies, artisanal miners or alternatively be blown or shaken off. recreational miners in spite of its apparent simplicity.

Figure 70. 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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34: Magnetic coated gold – 1980s research in Colorado A rather unexpected way to recover placer gold is, having first removed magnetite and other magnetic Operation minerals to then selectively make the gold particles This text is based on the Kindig and Turner method magnetic and remove them by magnetic means. presented in US patent #4,229,209. James K. Kindig and Ronald L. Turner of Golden, Iron carbonyl liquid is sourced from specialist Colorado gained patents in clean coal technology having suppliers and stringent precautions are applied during discovered that by warming coal with iron carbonyl vapour transport, storage and use regarding risk of fire and risk of the pyrite became magnetic and could then be removed acute poisoning. more easily (US #3,938,966 and #4,175,924). The placer ore is either dry-screened or else wet- Kindig and Turner then adapted the method for screened and then dried. Magnetic minerals are removed placer gold with good results and they were awarded a as fully as possible. patent in 1980 (US #4,229,209) and assigned it to Hazen The dry non-magnetic fraction is fed to a rotating kiln Research Inc. First they removed the magnetite and serving as a reaction vessel to bring the material into suchlike using magnetic separators. Next they put the direct contact with iron carbonyl vapours in the presence non-magnetic placer into a rotating kiln with iron carbonyl of a gas such as nitrogen that is inert to the reaction. vapour in an inert nitrogen atmosphere. Important The objective is to cause the decomposition of the variables include the temperature, pressure, type of iron carbonyl to form a magnetic skin on the gold particles carbonyl used, gas composition etc. but not on the other particles present. Typically about 0.5 The iron carbonyl selectively attaches itself to the to 4 kilos of carbonyl are added per ton of feed. gold particles and decomposes to make a magnetic coat of Generally a reaction time of from half an hour to an iron for the gold particles. hour is adequate, at 110-130°C. The higher the

Fe(CO)5  Fe + 5CO temperature the more complete is the gold recovery, but at higher temperatures the iron carbonyl is liable to coat Iron carbonyl Fe(CO)5 is a straw-yellow liquid that is other minerals beside gold particles. cheap, but unfortunately it requires special precautions as After the treatment, the magnetic coated gold is it is not only flammable but is also toxic if inhaled. removed by dry magnetic separators such as a low to The inventors recovered 93.3% of placer gold from medium separator with a magnetic drum having field Clear Creek, Colorado; then 76.5-93.3% of placer gold strengths of up to about 2,500 gauss, electromagnetic from the Vulture placer in Arizona, and later on their tests drum separators up to about 7,000 gauss or induced roll were getting 99.98% gold recovery. separators up to about 11,000 gauss. Of interest is that the Kindig and Turner method requires no water at all, making it of potential value in Adoption by placer gold miners enabling large-scale placer gold recovery in arid regions. However, the method has not been tested on gold <100μ This 'making gold magnetic' process unsuitable for and therefore further research is warranted. artisanal miners, but seems to have merit for large-scale placer mining operations – particularly in dry deserts as the process requires no water.

Figure 71. 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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35: Flat bar riffles – 1980s research in Yukon, 1990s research in Mongolia Operation 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. Figure 72. FLAT BAR RIFFLES Generally the riffle sets are orientated with the riffles Slanted flat bar riffles on ribbed rubber matting at a placer mine slanted down-sluice. This helps to stimulate vortices and in the Zaamar Goldfield of Mongolia. (photo: Jeanie Barnett of GSA) shields metalwork from damage and abrasion from stones. Flat bar riffles consist of flat metal bars inclined For clean sands, a 30° slant is preferred. For clay-rich across a sluice box to trap black sand and gold. They have sands the slant may reach 45°. Sometimes the riffles are been popular for at least 70 years. slanted upstream to act as a nugget catcher. 1980s tests in the Yukon, Canada Flat bar riffles are commonly used in conjunction with Flat bar riffles on unbacked NomadTM matting were a Siberian-style PgSh wash-plant typified by violent slanted at various angles to the sluice-box floor in tests by surging and wide fluctuation in flow and density of slurry. Randy Clarkson and Owen Peer [8]. When slanting 15° Lacking a lip, a flat bar riffle is less able to guide sand upstream, flat bar riffles choked with sand faster than if into a vortex, and sand exits its vortex instead of being led slanting 15° downstream. Slanting downstream produces into the next vortex. This “severely reduces the opportunity a slower vortex with its eye closer to the centre, and the for gravels and gold to enter the riffles” and “the turbulence… vortex launches material at a lower angle. Performance …destroys effective vertical segregation.” [8] was inferior to Hungarian riffles. Vortices cease after a few hours choked with 1990s tests in Mongolia sediment, yet washing continues for an 8-hour shift. Slanted flat bar riffles on ribbed rubber matting were Flat bar riffles are less strong than angle iron tested by a Soviet team led by Ms. Tsevel Delgertsoo in (Hungarian riffles) and more prone to bending. 1991-95 at four placer gold mines – about 25 tests in all. Adoption by placer gold miners Each test consisted of panning to estimate the head- grade, measuring the volume washed in an 8-hour shift Slanted bar riffles on square-ribbed black rubber (1-2,000m3) and sampling tails every 15 minutes across mats are the norm for placer gold mines in the former the sluice. Careful panning was done in a gold room. Soviet Union and Mongolia. Recovery was 60-70% for medium to large gold. Fine gold was not fully tested – it was often present but lost.

Figure 73. 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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36: Angle-iron (Hungarian) riffles – 1980s research in Canada Operation 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 TM Figure 74. ANGLE-IRON RIFFLES sluice-box and pressed down on the Nomad matting. A set of 1-inch riffled sluices being made ‘on the spot’ at the The riffle sets are secured by metal or wooden chocks. Sharin Gol mine of Polymet Potala Ltd in Mongolia The welder has The angle iron is positioned with one of its flat sides ensured each riffle has a 15° tilt. (photo: Robin Grayson) uppermost to act as a short slick plate and splitter to The origin of the term ‘Hungarian riffles’ is unclear guide the bottom flow into the vortex. Its other flat side and predates World War II. By the time of the Yukon tests obstructs the flow to retain the vortex and trap heavies. [8] the term had become synonymous with ‘angle iron Slurry is fed at 48.8m3/hour per metre width. A very riffles’ set across the width of a sluice-box. large concentration ratio is possible (i.e. vast amounts of 1980s tests in British Columbia and Yukon, Canada black sand are shed to produce a gold concentrate). Lab tests using gold tracer in the University of British The Yukon tests showed angle iron riffles maintain Columbia by James Hamilton and George Poling [7] the captured black sand in a loose state for a long time, showed angle-iron riffles if on unbacked NomadTM matting so continuing to be able to recover gold. This enables can recover >90% of >0.3mm gold, and 85% of 150μ clean-ups to be needed only once every 24 hours. gold, subject to control of the feed and the riffle size, Adoption by placer gold miners angle and spacing. Tests in the Yukon by Randy Clarkson and Owen Angle-iron riffles are the norm in North America and Peer [8] confirmed the findings, and included gold tracers, common in most of the world but not everywhere. In gold radiotracers, flume observation tanks and testing Siberia and Mongolia, inclined flat bar riffles is the norm. performance of sluice-boxes of placer mines [86-90]. Many artisanal miners use basic ‘Hungarian’ riffles of wood.

Figure 75. 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 Recovery of placer gold by 2x2-inch angle-iron riffles @ 4-inch spacing, on backed NomadTM matting [86]. (compiler: Robin Grayson)

Figure 77. ANGLE-IRON RIFFLES ON BACKED NOMAD MATTING – Yukon field tests Recovery of placer gold by 3 x3-inch angle-iron riffles @ 6-inch spacing, on backed NomadTM matting [86]. (compiler: Robin Grayson)

Figure 78. ANGLE-IRON RIFFLES ON BACKED NOMAD MATTING – Yukon field tests 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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37: Expanded metal grating riffles – 1980s research in Canada

Operation 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

Figure 79. EXPANDED METAL GRATING RIFFLES slightly better (source – Zooka of Alaska Gold Forum). Raised expanded metal grating suitable for using as large A sheet of expanded metal grating is cut to fit snugly expanded metal riffles. (photo: Robin Grayson) in the sluice box, and secured by metal or wooden chocks. Expanded metal grating was invented in the early Several sections may be fitted into a sluice box, butted 1880s. The first innovative use of expanded metal as together with no overlap. Each equates to a ‘riffle set’. riffles in a sluice was by Robert Hodgson Postlethwaite, a The grating is inserted with the raised lips facing upstream British subject at the Risdon Iron and Locomotive Works in to serve as riffles. San Francisco. He applied for a patent in 1897, awarded The riffles are “coarse” 4lbs/ft2 raised expanded 1900 (US #652,900). It was only in the 1980s that the metal grating identical to 4.0# grating of the Expanded effectiveness as riffles was proved by scientific tests. Metal Manufacturers Association (EMMA) 'standards', 1980s tests in Yukon, Canada downloadable: www.naamm.org/emma/literature.php. Randy Clarkson and Owen Peer [8] tested relatively Expanded metal riffles achieve a very large “coarse” 4lbs/ft2 expanded metal grating and finer 1-10H concentration ratio (i.e. shed vast amounts of black sand expanded metal mesh. In flume tests, both displayed, to achieve a gold-rich concentrate), as do flat bar riffles “similar deposition and vortex patterns” and the mesh and angle-iron (Hungarian) riffles. developed “smaller and more numerous vortices”. Expanded metal grating riffles can maintain the They observed that the grating remained firmly in captured black sand in a loose state for a long time, so place whereas the mesh warped off the NomadTM matting continuing to be able to recover gold. This enables clean- causing “excessive scour”. ups to be needed only once every 24 hours. The grating has to be ‘Raised (R) = Standard (S)’ and Adoption by placer gold miners not ‘Flattened (F)’. According to Vincent Ruth of Continental Wire Cloth, “the applications that this product Raised expanded metal riffles of grating type are dominates would be used when designing something that used worldwide by placer miners. 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)

114 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

38: Expanded metal mesh riffles – 1980s research in Canada

Operation 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). Figure 81. EXPANDED METAL MESH RIFFLES A roll of raised expanded metal mesh type 1-10H is Raised expanded metal mesh suitable for using as small expanded metal riffles. (photo: Robin Grayson) unrolled and cut to fit the sluice box, and secured by metal or wooden chocks. Several sections may be butted Expanded metal mesh seems to have been used for together with no overlap. Each equates to a ‘riffle set’. riffles a little later than grating. It was only in the 1980s The mesh is inserted with the raised lips facing upstream the effectiveness of mesh was proved by scientific tests. to serve as riffles. 1980s tests in British Columbia, Canada When unrolling the mesh, flatten it. Keep the sluice James Hamilton and George Poling [7] tested 1-10H narrow to reduce warping. Clamping too tight may expanded metal mesh. The mesh is ‘Raised (R) = compress the NomadTM matting and warp the mesh. Tying Standard (S)’, not ‘Flattened (F)’. the mesh to the floor of the sluice-box inhibits warping The riffles are 1-10H raised expanded metal mesh but prolongs cleanups; quick release bolts are better. identical to the 1-10H expanded metal mesh manufactured Expanded metal riffles achieve a very large by Continental Wire Cloth Inc of Calgary. Their product 1- concentration ratio (i.e. shed vast amounts of black sand 10H has not changed since the early 1980s according to to achieve a gold-rich concentrate), as do flat bar riffles Vincent Ruth, see: www.cwcloth.com/expanded.htm. and angle iron (Hungarian) riffles. Of concern is the susceptibility of 1-10H mesh to lose Expanded metal 1-10H mesh can maintain the gold by the mesh warping to permit scouring of the captured black sand in a loose state for a long time, so matting beneath. Even a small surge is likely to cause gold continuing to be able to recover gold. This enables clean- losses for, as pointed out by Randy Clarkson and Owen ups to be needed only once every 24 hours. Peer [8] the live sorting crescent is so shallow it is vulnerable to being ejected. The cause of scouring is due Adoption by placer gold miners to the ease of warping of the mesh, plus two variables: Expanded metal riffles of mesh are popular ² a surge of water, due to too much or too little water; and/or worldwide amongst placer gold miners. ² a surge of changed slurry, due to too much or too little solids.

Figure 82. 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 The superiority of NomadTM matting in recovering placer gold compared to close weave matting. (compiler: Robin Grayson)

Figure 84. GOLD RECOVERY BY RAISED EXPANDED METAL MESH ON BACKED NOMAD MATTING – Yukon tests Recovery by expanded metal mesh (type 1-10H ?) on backed NomadTM matting. (compiler: Robin Grayson from Clarkson 1989 [86])

Figure 85. GOLD RECOVERY BY RAISED EXPANDED METAL MESH ON BACKED NOMAD MATTING – Yukon tests 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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39: McCann’s small sluice – 1980s research in California Operation 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 Figure 86. McCANN’s SMALL SLUICE General arrangement, details omitted. (drawing: Robin Grayson) encourage a steady outflow. Upon escaping from the hopper, the slurry McCann’s sluice was invented by John C. McCann of encounters wash-water flowing over a deflector plate. The California and patented in 1985 (US #4,525,270). This slurry and wash-water flow over a perforated plate that was among the most outstanding innovations to the sluice has numerous small holes through which dense fine made by recreational miners in North America and particles fall into a quiet zone of slowly moving water that elsewhere who, since sometime before the 1970s, have is where most of the fine gold tends to be caught. Larger been making incremental improvements to their small particles pass down the main sluice lined with rubber v- sluices in an effort to recover more fine gold. By the 1980s groove riffles capture 90-98% of the visible gold, assisted the innovative surge had become quite remarkable. by a section of fine expanded metal mesh and a single McCann’s sluice is a complete wash-plant satisfying a “shaped riffle”. The manufacturer recommends adding a litany of wishes of recreational miners: ² small, lightweight, portable device; heavy duty HFBE vibrator to assist recovery of fine gold. ² minimises water use by recycling; Tailings water flows into a filter bag at the end of the ² minimises energy consumption; sluice that retains the tailings and the cleaned water fills a ² has a good concentration ratio of 1,000:1; heavy duty 5-gallon water bucket. From here the water is ² has an adjustable slope; recirculated many times an hour by a small submersible ² maintains a steady flow; 3 ² recovers 90-98% of visible gold; and pump (capacity 1.89m /hour) requiring 1.5 amps at 12 ² recovers 60-80% of gold particles as fine as 30-40μ. volts allowing 20-30 hours use on a car battery. McCann’s sluice removes technical justification for Adoption by placer gold miners mercury. Yet in over two decades since the device was patented and thousands sold to recreational gold miners McCann’s wash-plant is made by Micro-Sluice Gold worldwide, the device has been overlooked by researchers Products of Wisconsin, USA (www.micro-sluice.com) and using public funds in efforts to improve large gold sluices marketed as the Micro-Sluice, with over 3,700 units sold in for companies [90,91] and gold sluices for artisanal miners 16 countries over the last 20 years. The device is popular [21,26,92]. with recreational gold miners and has potential for artisanal miners especially in arid regions.

Figure 87. 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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40: Hydraulic riffles – 1980s research in New Zealand and Canada Operation 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 Figure 88. HYDRAULIC METAL RIFFLES slick plate to calm the slurry and engender laminar flow to A set of hydraulic metal riffles showing the manifold supplying the elutriation water under pressure. This set was manufactured by allow the slurry to stratify with heavies concentrating near PAuSE Ltd in New Zealand. (photo: Robin Grayson) the bottom. After the slick plate the stratified slurry Hydraulic riffles appeared over a century ago. In their crosses the first set of hydraulic riffles where 32-48 3 modern form they began in New Zealand in the 1970-80s. m /hour of clean water is injected 15-35kpa pressure from Lindsay Guy Herron of Queenstown invented compact a manifold into the black sand to maintain a fluidised bed hydraulic riffles patented in New Zealand (NZ #216,327, that traps the gold. The hydraulic riffles rest on unbacked TM filed 1986, lapsed 1997) and the United States (US Nomad matting to assist capture of gold. #4,863,588, awarded 1989, lapsed 1993). The slurry continues over a second slick plate that A typical sluice box for hydraulic riffles is short and encourages more density stratification and passes over wide, contrasting with the long and narrow sluice box for the second set of hydraulic riffles to recover more gold. conventional riffles. Hydraulic riffles inject pressurised Finally the slurry passes over an end section of raised clean water into the black sand from below. This keeps expanded metal mesh that catches gold from the tails and the bed loose and fluidised indefinitely, so gold particles helps to verify that the system is functioning properly. continue to fall into the black sand and accumulate. Adoption by placer gold miners Randy Clarkson noted “…unlike conventional riffles that rely on the formation of vortices, hydraulic riffles rely Hydraulic riffles are popular in New Zealand, and primarily on the settling velocity of gold” [86-90]. In the have spread to the Yukon (Canada), Alaska (USA), Yukon tests, hydraulic riffles performed well at “extremely Mongolia, South America, Australia and doubtless low feed rates” and “low water flows” but at high feed elsewhere. In Mongolia, the adoption of hydraulic riffles rates are “only suitable for coarse gold recovery (nugget has been slow, but of the 200 or so wash-plants more traps)”. The Yukon tests on hydraulic riffles were limited than a dozen now have hydraulic riffles including: Cold and the author is unaware of any other tests. They are Gold Mongolia Ltd (New Zealand); Ochir LG Ltd (British- unlikely to be as efficient as tuned conventional sluices in Dutch-Mongolian www.ochirlg.com); Gatshuurt Ltd; Jump recovering fine gold, but have the overriding advantage of Ltd; Gazar Holdings Ltd; G&U Gold Ltd, and Barmash JSC. 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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41: Graefe’s E-tank – 1980s research in California Operation 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 35- 75μ. 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 Figure 90. GRAEFE’s E-TANK Cut-away side view of Graefe’s E-tank showing the injection towards the spillway lip and so a skimmer plate is welded water rising from below. (drawing: Robin Grayson) on to prevent it escaping. Graefe’s E-tank was invented by Ralph Graefe of Batch discharge is rather cumbersome. The tank is California and patented in 1985 (US #4,523,989). Rights progressively tilted more and more steeply to discharge were transferred to Keene Engineering Inc who sold it as its upper contents as tailings. Then the residual the ‘Keene Hydromatic Jig.’ This is a misnomer as it lacks concentrate flushed out with water by opening the bung the key features of a jig. in the discharge pipe in the base of the tank. Graefe’s E-tank is an elutriated sludge tank that not Adoption by placer gold miners only has its contents slurrifed by injecting water from below, but is vibrated at 180 cycles/second to prevent Graefe’s E-tank was marketed as the ‘Keene ‘hard-cake’ and to ensure that particles are distributed Hydromatic Jig’ to recreational miners but manufacture throughout the depth of the slurry by density, not by size. ceased after a decade or so. Members of the Alaska Gold According to test results in Ralph Graefe’s patent, Forum (AGF) are re-evaluating the device and it may yet the device can recover 100% of gold as fine as 70μ, enjoy a revival at least amongst recreational miners. Graefe’s falling slightly to a very impressive 93% of 30μ gold. The E-tank is a complete wash-plant, but small. Although ideal test results show a 60° slope to the walls of the tank for recreational and artisanal miners it is difficult to recover far more fine gold than a 45° slope. envisage how it might be scaled up for industrial mining.

Figure 91. 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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42: Cleaveland jig and IHC jig – 1980s research in USA and Holland Operation 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 Figure 92. IHC JIG PLANT <6mm that report as slurry to a “second stage”, a small An IHC jig plant recovering fine gold in Sumatra, Indonesia. secondary jig who jig screen in turn produces tailings of (photo: IHC Holland – www.ihcholland.com) <4mm that are recycled back to the primary jig. A “third The Cleaveland jig was invented by Norman stage” is sometimes added to the series. Cleaveland of New Mexico and patented in 1982 (US Coarse gold and nuggets remain on the jig screens #4,310,413). The jig is an innovative circular jig intended for recovery as a batch discharge at clean-up, while small for large tin dredges, a compact star-pattern of jigs with gold is produced as a continuous discharge with black the slurry introduced at the centre [93]. sand via spigots at the bottom of the hutches of the jigs. IHC Holland of the Netherlands and Alluvial Dredges Adoption by placer gold miners Ltd (ADL) of Scotland and Australia made Cleaveland-type jigs. IHC made vast research into jigs led by the Mineral IHC trapezoidal jigs were installed on the Bema Technological Institute (MTI) in the Netherlands [94-97]. Dredge, a famous bucket-line gold dredge off the coast of IHC developed the Cleaveland jig further as a circular Alaska. Several winterised gold bucket-line dredges with array of trapezoidal jigs with sawtooth jigging motion, a IHC jigs operated in NE China. IHC jig-plants of 1, 2 and motion now emulated in most jigs. The short-lived fierce 3-modules were installed in placer gold mines in Alaska, upward stroke prevents the loss of fine gold by Peru, Columbia, Ghana, Indonesia, Mongolia and suppressing the phase of hindered settling is suppressed. elsewhere, gaining a reputation for recovering >90% of IHC marketed its jigs for large tin dredges. After the tin moderately fine gold. Failure to gain wider popularity is price collapsed, IHC marketed the ‘IHC trapezoidal jig’ for attributed to the high price, bulkiness, limited mobility, recovery of placer gold, hardrock gold and diamonds on and low concentration ratio. dredges, pontoons and dry land [98-105].

Figure 93. 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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43: 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). Operation Today elutriation towers use a rising column of water Clean water is introduced into a 1-inch, 2-inch or 4- and typically use a teeter (hindered settling) for recovery. inch diameter column either near its base or part-way up, However there are many types of E-tower and a at a controlled rate as determined by experiments. systematic classification is long overdue. Rising up inside the column, the water enters a Around 1968 the United States Bureau of Mines teeter zone characterised by hindered settling. began a Heavy Metals Program and awarded a research Finely screened concentrate (e.g. <0.2mm) is contract to the Minerals Industry Research Laboratory spooned into the tower from above and falls through the (MIRL) of the University of Alaska, to test and improve the rising water down as far as the teeter zone. recovery rate of -100 screen mesh gold in the Alaskan Extremely dense particles such as gold fall through placer deposits. MIRL built a lab scale E-tower and ran the teeter zone to accumulate in the bottom of the device hundreds of successful tests on placer material, and much as a lag deposit to await batch discharge of the findings were published [109,110]. Fairly dense particles accumulate as a hovering In the late 1980s ASAT continued the research and fluidised mass (teeter bed) in the teeter zone. The built ASAT towers of several sizes (1-inch, 2-inch and 4- continuous arrival of new particles from above into the inch) for lab tests, de-sliming clean-ups and full-scale teeter zone causes particles to become increasingly production [111]. crowded and then only hindered settling can occur Walter Lashley of ASAT invented a superior form of The hindered settling means that dense only dense elutriation tower (E-tower) in the 1980s that attracted particles can get to the base of the teeter bed, and in considerable attention for its ability to recover fine gold. doing so they eject upwards less dense particles. During the early 1990s an environmental mining As a result the teeter bed becomes vertically equipment R&D group took over the testing on the E- stratified with the densest particles at its base overlain by towers from Walter Lashley and did considerable field lighter particles tests and is reported to have made vast improvements on The lighter particles are vulnerable to ejection with ASAT’s original design. Over 120 units were installed on the rising water as tailings whereas the heavier particles several mining projects in the USA and Mexico and are protected from scouring away by the carpet of lighter reported to have worked very well. particles resting upon them. An advanced form of ASAT E-tower is capable of After processing many spoonfuls of material, the recovering <5μ gold in a controlled environment in a lab, enriched gold concentrate is siphoned off from the device. and can recover 50% of <10μ in field experiments which is significantly better than most other recovery devices. Adoption by placer gold miners Variants of E-towers relevant to simple recovery of E-towers became popular for a while among placer fine placer gold include the ‘Gold Funnel’ of Rodney gold companies and recreational miners in North America Charles Christensen of California patented in 1997 (US but currently there seems to be neither manufacturers nor #5,692,620). R&D interest, and awareness among miners is very low.

Figure 94. 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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44: 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 through a perforated plate secured to a bottom ring; then ² through a disc of felted, fibrous synthetic polymer; then ² through a screen with openings so fine that upward flow is substantially unaffected in its lateral uniformity; and finally ² through a coarse screen. Screened concentrate is spooned into the tower from above. The water rises with enough velocity to non- turbulently 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 Figure 95. OSTERBERG’s E-TOWER a teetered bed settler as hindered settling seems minor. Siphoning of tailings. (adapted by Robin Grayson from the patent) The patent suggests the fluidised bed be stirred Osterberg’s E-tower was invented by Daniel Osterberg gently at the start to encourage gold to fall to the bottom. of California, patented in 1984 (US #4,451,359), and The upper portion of the fluidised bed is deemed to marketed as the Quick Gold Separator. be depleted of gold and is siphoned off as tailings. The device causes black sand to fluidise and particles The dense gold particles remain behind and more to hover. For ‘hovering’, the water velocity has to slacken concentrate is spooned into the top of the column. upwards – impossible in a cylinder is of uniform diameter. After processing many spoonfuls of material, the Slackening is by the water escaping from the constricted enriched gold concentrate is siphoned off from the device. space in the flow distributor. If a teetered bed E-tower Adoption by placer gold miners then hovering is accompanied by hindered settling in the teetering fluidised bed, displacing lighter particles upward. Osterberg’s E-tower gained some popularity in North The author suggests that Osterberg’s device is a America among recreational gold miners but production multiple E-tower with many rising columns jetting upward, eventually ceased in spite of the clear potential of the each decelerating once free of the flow distributor. device. The device is undergoing technical evaluation by Osterberg’s E-tower seems capable of achieving high Megan Rose (Gold_Tutor) and details are posted on the percentage fine gold recovery and merits testing. Colorado Prospectors’ Forum [112].

Figure 96. 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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45: Younge’s horizontal centrifuge – 1980s research in British Columbia Operation 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 Figure 97. YOUNGE’s HORIZONTAL CENTRIFUGE with solids and the ejected water flows into the next Younge’s horizontal centrifuge in an innovative wash-plant by compartment. This flow "…shears over the longitudinal Roger Wagner. (photo: Leonard Leeper - www.golddredger.com) riffles and in doing so forms a vortex area between Younge’s horizontal centrifuge was invented by Earl adjacent longitudinal riffles…" and, "…this vortex keeps G. Younge of Langley in British Columbia for which he was particles of low specific gravity in suspension and they are granted patents in Canada in 1981 and 1983 (CA ultimately carried along through a succession of annular #1,110,206 and #1,153,336) and similar patents in the compartments...” and so are ejected as tailings. USA in 1981 and 1982 (US #4,265,743 and #4,347,130) The cylinder has a high rotation speed and so the entitled ‘Method and Apparatus for Extraction of Gold diameter of the cylinder can be rather small: From Placer Gravel’ and ‘Placer Mineral Concentrator and ² 5ft long 16-inch diameter cylinder, with five 1.25-inch high Process’. longitudinal riffles and three 1.375-inch high annular riffles, In this account the device is termed a horizontal rotates at 140-170 rpm and processes 8 tons/hour (4m3); centrifuge to clarify its distinctive character – a spinning ² 7.5ft long 20-inch diameter cylinder, with six 1.75-inch high cylinder with its interior wall equipped with two sets of longitudinal riffles and four 2-inch high annular riffles, rotates 3 riffles – one set of longitudinal riffles running its entire at 128 rpm and processes 20 tons/hour (10m ). length, and at right angles to them a second set of riffles Adoption by placer gold miners forming annular rings around the wall. The simplest horizontal centrifuge is a scrubber- Younge’s horizontal centrifuge is occasionally seen at centrifuge – a scrubber rotating so fast the material is not placer gold mines in North America but does not seem to only scrubbed but also spun so fast it attains a significant have gained wide acceptance, presumably due to lack of degree of density classification. Some scrubbers may awareness of its considerable advantages regarding fine unwittingly be centrifuges. gold recovery and low water usage.

Figure 98. 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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46: Mozley MGS centrifuge – 1980s research in Cornwall Operation 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

Figure 99. MOZLEY MGS CENTRIFUGE discharge rate of the concentrate. A differential scraper Schematic cutaway diagram of an Axsia-Mozley Multi-Gravity mechanism enables a longer residence time for the slurry. Separator MGS. (diagram: courtesy of Ian Daniels of the Additional cleaning water is added to assist the removal of manufacturer Axsia Mozley Ltd - www.natcogroup.com) slimes and enrichment of the concentrate. The Mozley ‘Multi-Gravity Separator’ was invented by The Axsia-Mozley Multi-Gravity Separator is Richard Mozley of Cornwall and patented in 1990 (US manufactured in 3 models: #4,964,845) after applying for a UK patent in 1986. ² MGS C900 – compact lab unit, 1 ton/hour of solids; Today re-branded as the Axsia-Mozley MGS, the ² MGS C902 – medium-capacity, 3 tons/hour of solids; device is manufactured by Axsia Mozley of Cornwall, part ² MGS MeGaSep™ – large-capacity, 30 tons/hour of solids. of the NATCO Group of Houston USA. The larger MGS models have special features: The Axsia-Mozley MGS combines the high apparent ² a proprietary gearbox drive that gives: improved reliability; gravity of a centrifuge with the principle of a shaking table zero maintenance and sealed-for-life lubrication. [113-116]. The device is a gently inclined cylinder that ² the MeGaSep has hydraulic fluid drive, remote power pack and simple control technology. rotates to create an apparent gravitational field of 8g to 22g. The cylinder also pulsates along its shaft in the Adoption by placer gold miners manner of a shaking table. The cylinder is lined with a few special riffles. The Axsia-Mozley Multi-Gravity Separator is being Key advantages of the Axsia-Mozley MGS are: used for the separation of ores of gold, tantalum, tin, zinc, ² fine to ultra-fine separation (down to 2μ); copper, silver, lead and coal. However, the author has not ² better capacity than some other ultra-fine separators; seen any reports of the MGS being used in placer gold ² efficient use of energy; recovery. ² minimum maintenance or supervision; and ² requires no chemicals or physical reagents.

Figure 100. 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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47: 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 apparent gravitational field, up to 50 times gravity, results Figure 101. KELSEY CENTRIFUGAL JIG in a major improvement in separation efficiency, A model J1800 Kelsey centrifugal jig. (photo: courtesy of Dale particularly of very fine minerals, by significantly reducing Henderson of makers Roche Mining – www.rochemt.com.au) the effect of forces that hinder fine particle separation.” The Kelsey centrifugal jig was invented by Hindered settling is accentuated by the centrifugal Christopher G. Kelsey of Australia and patented in force together with the pulsing of the ragging bed. The Australia in 1985 (PH9037, PG0122) and 1990 in the USA pulsing is via pulse arms connected to pads to work (US #4,898,666). The first commercial sale was of a lab- against the jig’s flexible diaphragm. “Water contained sized machine in 1989. Geo Logics Inc developed the within the concentrate hutches presses against the device, and automatic screen cleaning was introduced in diaphragm, at a frequency and amplitude set by the 1999. In 2001 Geo Logics was bought by Roche Mining operator, thus dilating the ragging bed. The level of who continues to make the jig (www.rochemt.com.au). dilation impacts on the amount of material able to pass to The Kelsey centrifugal jig maximises its effectiveness concentrate.” The pulsed shockwaves have two effects: and efficiency by combining the pulsation principle of a jig ² dilating the ragging bed to allow minerals to enter it; and with the high apparent gravitational field of a centrifuge. ² accentuating differences in acceleration between particles of The main models and operating variables are: different density. Separation of particles of similar size and ² J200 KCJ – lab test unit, 15-100 kg/hour of solids; shape but different density slows at their terminal velocity. ² J1300 MkII KCJ – smallest commercial unit, 2-30 tons/hour; The shockwaves repeatedly stop the particles, limiting their ² J1800 KCJ – largest commercial unit, 5-60 tons/hour. time at terminal velocity to maintain a high rate of separation. ² feed-related variables – feed type; feed density; feed rate; feed size; and density difference between minerals. Adoption by placer gold miners ² jig set-up variables – screen aperture size; ragging type, The Kelsey centrifugal jig is used to recover hardrock ragging density, ragging size distribution and depth. ² jig control variables – rotational speed (induced gravity field); gold, platinum, cassiterite, monazite sand, coal etc [117- pulsing frequency; pulse amplitude (distance travelled during 122]. It performed well with fine flat placer gold from each pulse stroke); and rate of adding hutch water. offshore Alaska [122] but is not seen in placer gold mines.

Figure 102. 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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48: 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 The author has not seen a description of how the Yunxi bowl operates. Figure 103. YUNXI BOWL Generalised arrangement of an 80cm diameter Yunxi bowl, after Tin ore is first screened to <74μ and a suitable slurry Y. Sun [123,124]. (drawing: Robin Grayson) prepared. The slurry is gravity fed into the rotating Yunxi The Yunxi bowl was developed by the Yunnan Tin bowl and the heavies are slammed on the inside wall of Corporation in the early 1960s in China for recovering the centrifuge. The amount of enhanced gravity is 102 g, 51g and 30g for bowls of diameters of 40cm, 80cm and extremely fine cassiterite (SnO2) [124]. For decades unknown in the west, over 20 years the 1.6m respectively. Yunxi bowl was the world’s best centrifuge for recovering The heavies remain on the bowl wall as concentrate fine minerals, until the advent of the KnelsonTM bowl, to await batch discharge, while the lighter particles are FalconTM bowls, MozleyTM MGS bowl and KelseyTM ejected as a continuous discharge of tailings. centrifugal jig. For three decades the Yunxi bowl remained After a period, the device is stopped for batch radically different from other bowl centrifuges until the discharge of concentrate. The stopping, discharging and birth of the Russian ItomakTM bowl. restarting are automatically controlled. The Yunxi bowl is a short centrifuge that rotates on a Adoption by placer gold miners horizontal axis. The bowl is near-parallel sided, sloping 3- 5° to its discharge end. A limitation is the low capacity, About 1,000 Yunxi bowls were installed in tin, e.g. about 30 tons/day for an 80cm bowl [124]. tungsten and iron ore plants in China [128]. However the In the late 1980s the Yunxi bowl spawned the SL- author has found no evidence of either the Yunxi bowl or type separator (SLS) with “injection flowing film its successor the SLS bowl being used in gold recovery.

Figure 104. 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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49: 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 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 98- 99.9% of fine placer gold [148]. Figure 105. KNELSONTM BOWL ² lab testing – Ikh Alt Mine in Zaamar Goldfield, Mongolia. Pay A 3-inch KnelsonTM concentrator. with ceramic bowl ready to insert. (photo: KnelsonTM Inc – www.knelsongravitysolutions.com) gravels sampled in the harsh winter and trucked to Ulaanbaatar to determine grade using a 3-inch KnelsonTM The KnelsonTM bowl is a centrifugal bowl inside a bowl fitted with a vibrating sluice and spray bar [149]. water jacket, elutriated water being injected under ² pilot mining – Toson Terrace Mine in Zaamar Goldfield, pressure via holes to prevent packing of the black sand Mongolia. KnelsonTM concentrators proved very effective in trapped in the riffles of the bowl wall. test mining [150], being superior to Russian-style sluices. TM The Knelson bowl was invented by Bryan Knelson Setbacks occurred in 1997-99 with the failure of Java Gold of British Columbia in the 1970s [129,130]. He patented Inc at this mine using KnelsonTM concentrators – due to the device in 1986 (US #4,608,040) as an advance on the management not technology [151]. elutriated centrifugal bowl patented 50 years before by ² full-scale mining – Howley Mine of Metana Minerals NL in Arnold Nesbitt McNicol in Australia (AU #17487/34 and AU Western Australia. Initial test recovery was disappointing with TM #22055/35). Bryan Knelson continued innovating, gaining a trommel-jig washing plant. Knelson bowls increased gold recovery by 35%, permitting reworking of a million m3 of jig more patents in 1988 (US #4,776,833) and 1991 tailings despite high clay content. [129]. (US#4,983,156). Later patents cover refinements, notably ² full-scale mining – Akrokeri-Ashanti Ltd (AAGM) in Ghana. continuous discharge (US#5,338,254) and mercury Partial success, but mine closed due to various factors, TM recovery (US #5,368,541). The Knelson bowl became a including lack of mobility of the wash-plants. popular subject for theses [131-137] and tests [138-148].

Figure 106. 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)

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TM 50: Falcon C bowl – 1980s research in British Columbia Operation 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 Figure 107. FALCONTM C BOWL upper part of the centrifuge termed the ‘retention zone’. Generalised view of a FalconTM C bowl, modified from the patent. Gold arriving in the retention zone displace lighter (drawing: Robin Grayson) particles that are liberated into the jacket-like discharge The FalconTM C bowl is the original Falcon invented chamber to be ejected as tailings. by Steve McAlister after noting fine gold in the sand/slime Since 1990, the device is fitted with an AutoPAC that stream of gravel mines in British Columbia. A prototype controls the rpm of the rotor and feed valve and was tested in 1981. By 1986 the first commercial FalconTM automates a 90-minute cycle time interrupted by a mere was operational and patented in 1989 (US #4,824,431) 30-40 seconds for automatic water flushing of concentrate [152-155]. Although largely eclipsed by later versions, down the hollow drive shaft. experiments continue to deepen understanding of the The percentage recovery of very fine gold is high, but device [156] – www.concentrators.net. the concentration ratio is low. Therefore the concentrate is The FalconTM C bowl has a very smooth inner wall. It not rich enough for smelting and the device is best used differs fundamentally radically from the KnelsonTM C bowl as a pre-concentrator and for upgrading tailings. by NOT having riffles or elutriation water and so lacks pores and has no water jacket. Rather than trapping gold Adoption by placer gold miners in a fluidised bed maintained by water injection, the The Falcon C bowl was originally designed to recover TM Falcon C bowl uses high apparent gravity to induce placer gold, but has been superseded by the Falcon SB density stratification in a thin flowing film. bowl in placer gold operations.

Figure 108. 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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51: Lemmon’s vanner – 1980s research in the Yukon Operation 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 Figure 109. VANNER as a vanner then belt should be vibrating but it is unclear A vanner is a vibrating endless belt moving upslope taking if the motor vibrates the belt significantly or not. heavies with it to discharge at the top end as valuable As the belt travels upward, the material “...is sifted 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 Lemmon’s vanner was invented by Norvel Lemmons you have a downward flow of water as the material moves of Arkansas and patented in 1989 (US #4,826,018). Tests upward. The gangue washes down while the gold particles on Liard River placers in the Yukon achieved recoveries stick to the belt with great tenacity - especially when the “consistently above 95% and some approached 99%” for gold appears in the form of flat particles (platelets)”. This placer gold of 20-250μ. However it is unclear if Lemmons’ comment is intriguing as it suggests that Ammen’s belt vanner was commercialised, but it merits further study. may be more disposed to recover flat gold than is possible Experiments to recover placer gold with other types with most other devices. of vanner were made in Alaska [157] and elsewhere but Ammen’s belt proved was effective in recovering none were successfully commercialised. Meriting “99.99%” of “fine gold” that was put as tracer in sand fed investigation is that gold particles grip tenaciously to to the device. certain synthetic surfaces due to electrostatic charges. A gently inclined endless belt can recover gold. If the Adoption by placer gold miners belt is driven upslope and slurry poured on it, then the slurry will flow down the belt to topple off as tailings over The author is unaware of vanners or belt the end roller. Meanwhile dense particles such as gold will concentrators being used by company-scale placer gold cling to the belt by friction and be hauled with the belt to miners at present. Such devices are used by artisanal gold the top roller where they topple into a concentrate bin. miners in South America and perhaps elsewhere, notably Vanners are a simple but marked improvement, by gold recovery from milled hardrock ore. introducing some gentle vibration to the slowly moving belt and this generally enhances gold recovery.

Figure 110. 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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52: Brosseuk’s helix cylinder – 1980s research in British Columbia Operation 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. Figure 111. THE GOLD MACHINE One of the smaller portable helix cylinders invented by Raymond In contrast, the lighter particles and wash water Brosseuk, ideal for prospecting and evaluating deposits. (photo: override the riffles to continue along the floor of the courtesy of www.extrac-tec.com) cylinder to emerge as a continuous discharge of tailings. Brosseuk’s helix cylinder was invented by Raymond Brosseuk of British Columbia, patented in 1992 (US Adoption by placer gold miners #5,108,584) and marketed as ‘The Gold Machine’ and has Tests of helix cylinders in the 1980s in North been the most successful helix cylinder for use in large- American placer mines [61] later led to some uptake scale placer gold mining. worldwide. Today several types of large helix cylinders are Externally a helix cylinder resembles a scrubber made in North America for placer gold recovery, notably: (blind trommel). Both are long cylindrical drums tilted to ² The Gold Machine (Brosseuk’s helix cylinder), cause slurry fed in at the raised end to discharge at the precursor of the HPC helix belt – www.extrac-tec.com TM lower end. But the interior of a helix cylinder is lined by ² Gold Claimer Rotary Riffle – production unclear ² Golden Boy Rotary Separator – www.goldenboyinc.com transverse riffles that are helical, each spiralling round and round along the length of the cylinder to produce an Miniature helix cylinders gained and retain a niche in Archimedes screw [159]. the recreational mining market as small-scale placer gold Brosseuk’s helix cylinder followed earlier innovations recovery units: ² Dixie Doodlebug – famous machine, production ceased during the 1970s and 1980s, such as the PMX helix and ² Mountain Goat – www.desfox.com [160] the TRI-R helix, but little has been published apart from ² Gold Screw – www.goldscrew.com 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. 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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53: GemeniTM table – 1980s research in Colorado Operation 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 Figure 113. GEMENITM TABLE the table deck to achieve good performance. In modern A GemeniTM table showing streaks of black sand and yellow models, a direct drive system has a geared motor driving streaks of gold. (photos: courtesy of MD Technologies Ltd – a crank connected to the table deck. To absorb overrun, www.gravityrecovery.com) the crank has a sprung connection system, and a bump The GemeniTM table was invented by Henry W. stop system provides a fine tuning mechanism. Table Rodgers of Colorado and patented in 1986 (US tuning is by adjusting a single screw. #4,758,334). The device is an innovative shaking table designed to recover fine gold to directly produce a clean Adoption by placer gold miners smeltable concentrate to produce dore bars. Often the The GemeniTM table is now seen in most placer gold cleaned concentrate is good enough to do so. mining regions of the world. It appears to be significant The GemeniTM table is claimed by the maker [161], better than other shaking tables, but is also significantly and confirmed by users and researchers [3,61,162], to be more expensive. Locally-made copy-cats are made in capable of producing a very clean gold concentrate from some parts of South America and perhaps elsewhere. gold-bearing black-sand concentrates when fed <1mm The switch to the GemeniTM table has been slow and material. Recovery is excellent down to about 40-50μ incomplete, perhaps due to the higher capital cost, the [161]. conservatism of most placer companies and slightly more

demanding housing and skill.

Figure 114. 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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54: 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 concentrate stream – taken for further upgrading; ² middlings stream – recycled (acts as buffer, inhibiting gold Figure 115. MARK-7 REICHERT SPIRALS loss if slurry surges or fluctuates); Mark 7 Reichert spirals installed on a mobile placer gold wash- ² plant in the USA. (photo: courtesy of John Strain) tailings stream – discarded; ² water stream – low in solids, either discarded or recycled. Reichert spirals were developed in Australia in the Compared to the traditional Humphrey spirals, all the 1960s by using lightweight materials and reassessing the concentrate off-take ports are eliminated from the spiral hydrodynamics of the Humphrey spiral. except on its final turn. This simplification dispenses with Reichert spirals are made of lightweight fibreglass, a a large amount of expensive tubing, and eliminates the material that permitted many improvements and need for the addition of any top-up wash water. variations. Over 20 models exist, the spirals contoured to separate particles of a particular range of densities. Adoption by placer gold miners Mark-7 Reichert spirals have been produced since 1982 and its spirals are designed to recover cassiterite Mark-7 Reichert spirals proved to be successful in 3 recovering moderately fine placer gold in California, particles with a density of 6 to 7g/cm and it then proved Arizona and Colorado USA, and proved effective at effective at recovering fine gold in tests in North America scavenging for fine gold from washed sand concentrates [61,163-166]. The mark-7 model has no moving parts, and can operate 24 hours a day, 365 days a year, for at sand pits [164-166]. But the take-up of spirals by placer many years. Compared with earlier spirals, the mark-7 is gold miners has been low, being seen only as a valuable easier to operate and requires less water. processing stage rather than a key component of a wash- For tests, a spiral is mounted vertically on a support plant. An exception is gold placers that are fine well- column. For production, identical spirals are mounted on washed sand of high energy coasts and large swift rivers. the same column as a double or triple helix to increase the capacity from ‘one-start’ to ‘two-start’ or ‘three-start’.

Figure 116. 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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The ‘Perfect Mousetrap’ 1990-2000

Mercury resists elimination… The BGS shaking table (#62) recovered 90% of 40μ By 2000 mercury amalgamation (#1) had been gold, but a decade later awaits fully marketing to the virtually eliminated in formal placer and hardrock gold- artisanal miners for which is was designed. mining in the west by other methods. But intense efforts The GoltronTM machine (#63) advanced in both by governments and international donors failed to directions, recovering not only fine gold but also nuggets. eliminate mercury usage by artisanal miners worldwide in The U-TechTM reverse polarity table (#64) recovered spite of localised success with mercury reduction. very fine gold, and gained and retained market share. Cyanide leaching (#2) became further entrenched worldwide as method-of-choice of the hardrock gold mining industry, as well as cyanide usage becoming further entrenched worldwide by ASM. Bio-leaching gained market share for difficult refractory hardrock ores, but thiosulphate leaching remained dogged by problems in spite of immense research. Bromine leaching (#55) resurfaced but did not become popular. Gold-paraffin floatation (#56) surfaced in Brazil but much more research is required to perfect the method.

Gravity methods make limited progress… Industrial sluices continued to improve with the Yukon know-how rippling worldwide, but surpassed by the ‘Damn Fine Sluice’ (#57) recovered 90% of 50μ gold; stunning for a cheap device. Overnight it became popular with recreational miners, but know-how failed to spread to artisanal or industrial miners. The Cleangold® sluice (#58) smashed the 40-50μ barrier of conventional sluices, its magnetically-held riffles routinely recovering 90% of 10-20μ gold, out-competing mercury and the device is being promoted to artisanal miners worldwide, catching gold better than mercury. Placer miners were preoccupied in introducing improvements invented a decade earlier such as better sluices, helix cylinders, Falcons, Knelsons, and better jigs. Many innovations of the previous decade waned – some such as Bartles’ crossbelt and Mozley’s tilting table eliminated due to the superiority of cyanide heap leaching and advanced centrifuges. Yet the author does not attribute the demise of Duke’s E-tank, Graefe’s E-tank, ASAT E-towers, Younge’s horizontal centrifuge, compound water cyclones or Lemmon’s vanner to competition. It seems these innovations failed in the market-place due to poor marketing, insufficient R&D and conservatism of mining companies, especially placer companies. The PyramidTM E-tank (#59) seemed to recover extremely fine gold but manufacture ceased before it could be independently tested. The GekkoTM in-line pressure jig (#60) of Australia revitalised the stale jig market, being ideal for hardrock gold circuits and for placer gold recovery. The ItomakTM bowl (#61) of the Russian Federation proved popular as a low-cost centrifuge for placer and hardrock, but cannot recover >90% of gold <100μ. The leading brands of centrifuges repositioned; the Knelson and Falcon bowls now in the global market for hardrock gold recovery, with less attention on placer gold. Figure 117. METHODS OF GOLD RECOVERY BY 2000 See text for details of each method. (compiler: Robin Grayson)

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55: Bromine leaching – 1990s research in Indiana The Dadgar method of bromine leaching was invented An innovative alternative invented by Ahmad Dadgar by Ahmad Dadgar and co-workers of Great Lakes and Charles C. Shin of Great Lakes Chemical Corporation Chemical Corporation in Indiana and patented in 1997 (US in Indiana and patented in 1990 (US #4,936,910). This #5,620,585). It may yet help to revive interest in bromide recovers gold by passing the pregnant gold solution leaching, using perbromides with desirable characteristics through acid-washed Sphagnum peat moss in a suitable such as high bromine levels, low bromine vapour pressure contacting apparatus. and stability even in freezing conditions. The sphagnum peat moss (live and dead) is chopped and screened to retain the 75μ to 1.5mm fraction. This Operation enables water to pass through. The screened moss is The Dadgar method in outline is as follows. washed with a protic acid such as 0.5-1.0M HCl (10 grams The ore is reduced to a fine powder and leached of dried sieved moss per 100mL HCl) and then rinsed with without any preliminary roasting step. deionised water until the filtrate reaches pH4. 1st stage – leaching gold into solution The acid-treated sphagnum moss is packed in a Bromine is sourced from specialised suppliers as column and the pregnant gold solution passed through in sodium bromide, as elemental bromine is particularly the manner of a typical ion-exchange column. hazardous and extremely difficult to store or transport Experiments prove acid-treated sphagnum peat moss safely. Sodium bromide is more stable and cheaper. is able to recover about 32 milligrams of gold per gram of The powdered ore is delivered as a continuous feed moss dry-weight when it is then at maximum capacity. to the first of two cascade agitated leach tanks where it is The process is very rapid (10-20 minutes) and is mixed with an aqueous bromide solution. The resulting indifferent to temperature variation in the range of 20 to slurry overflows from the first leach tank to the second 50°C. At least 99.9% of the gold contained in the leachate leach tank and overflows again to a thickener. Solids can be recovered by the sphagnum moss. produce a sludge that passes through a countercurrent 3rd stage – final recovery of gold for sale washing system of several thickeners, the final thickener The sphagnum moss is incinerated in a muffle being fed with an aqueous washing medium. Solids collect furnace at 750°C to recover the metallic gold. in the bottom of the final thickener as tailings, while the Adoption by placer gold miners liquid fraction is a pregnant gold solution. 2nd stage – recovering gold from solution The Dadgar method has potential for recovering fine The pregnant gold solution can be passed through placer gold from concentrates but awaits field tests and ion exchange columns, as is usual, to recover the gold. promotion to placer companies, recreational miners and artisanal miners.

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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56: 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 Operation as an alternative to mercury amalgamation [167], and is This account is based on the bench experiments of currently being assessed by Eco-Minex in Mongolia. Hamelmann and Lins in Brazil [167]. The gold-paraffin wax process as developed by The concentrate is finely divided and made into Christina Hamelmann and Fernando Lins of CETEM, Brazil, aqueous slurry of 25-30% solids. exploits the preference for gold particles to adhere to The slurry is heated in a container to 70°C, just 2 hydrophobic materials and thereby can be separated from degrees centigrade above the melting point of the slurry. The selected hydrophobic material is paraffin-wax paraffin-wax used (about 68°C). which is non-toxic, low-cost and widely available. The The temperature of the slurry is maintained at 70°C, concentrate used in the tests was from Minas Gerais with and pieces of paraffin wax are added and allowed to melt. a gold grade of 11 grams/ton. A mechanical stirrer disperses the melted paraffin to The CETEM researchers found that the greater the encourage it to contact the particles in the slurry. By this volume of paraffin wax compared to the volume of the simple means, the gold particles being hydrophobic sample then the greater the gold recovery by the gold- adhere to the globules (droplets) of paraffin wax whereas paraffin wax method. Attempts were made to obtain an black sand and quartz do not. electrostatic attraction between the gold particles and the When the stirring ceases, the paraffin globules rise to paraffin globules by controlling the acidity at pH 3, as the the surface to form a low-density paraffin phase above the isoelectric point for gold is pH 2 and pH 3-5 for paraffin water phase. The gold is locked in the paraffin when it [168]. In theory at pH 3 the gold particles should be solidifies as a floating solid. The solidified gold-containing negative and paraffin globules positive, and some paraffin is removed from the surface of the aqueous improvement in gold recovery was achieved at pH 3 phase and further processed to remove the gold. suggesting that this mechanism was occurring. The researchers also found xanthate as a gold collector Adoption by placer gold miners enhanced gold recovery. The gold-paraffin wax method requires further The CETEM researchers succeeded in achieving gold experimentation to improve % gold recovery sufficient to recoveries of 40-50% by the gold-paraffin wax method justify interest by placer gold miners. [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.e- goldprospecting.com/html/gold-paraffin_process.html.

Figure 119. 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)

135 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

57: Damn Fine SluiceTM (DFS) – 1990s research in New Mexico The Damn Fine SluiceTM (DFS) was invented by Phil Operation Hontz of New Mexico in the 1990s. The device was never The DFS is an in-stream sluice, positioned in shallow patented as it had been discussed widely on internet fast-flowing water. If necessary rocks are arranged to forums and is a direct descendant of earlier innovations form a temporary dam or weir to ensure flow is adequate based on the research on larger sluices in the Yukon tests and fast. The legs are adjusted to ensure the DFS is by Owen Poor and Randy Clarkson [8] and more sloping downstream and yet is level across its width. particularly the sluices tested by James Hamilton and Pay gravel is screened at about 2mm and the George Poling [7] that had raised expanded metal mesh oversize discarded after checking for nuggets. The riffles on unbacked NomadTM matting (miner’s moss). undersize is put on the leading edge of the slick-plate, a These sluices achieved >90% gold recovery down to small quantity at a time. about 150μ nominal diameter. The pay gravel is swept through the sluice by the The Damn Fine SluiceTM is a considerable advance on water current. After the small mound of pay gravel has these earlier sluices in being able to recover >90% gold been cleared by this means a fresh mound is added. The recovery down to about 50μ nominal diameter, although current causes the pay gravel to spread out across the rigorous testing does not appear to have been done. width of the slick plate to assume laminar (non-turbulent) The DFS is manufactured by the Damn Fine flow consisting of a bottom-hugging traction carpet of Equipment Co (www.damnfinesluice.com). This is part of black sand overlain by a traction carpet of lights. relentless effort by recreational miners in North America to The traction carpet of black sand is pulled into the recover fine gold using small sluices for small dredges, vortices (rollers) of the riffles and the gold burrows into high bankers and clean-ups. the underlying layer of NomadTM matting. The lighter The Damn Fine SluiceTM consists of a smooth slick minerals and surplus black sand are swept out as tailings. plate followed by a section of tiny raised expanded metal Eventually the NomadTM matting becomes hard mesh fitted on matting, all set in a short sluice-box. The packed. Clean-up is rapid, as the riffles are easily removed unit is about 1.19m long and 25cm wide, and weighs 3.63 by turning the wing-nuts on the ends of bolts that secure kilos. It includes a pair of adjustable legs to aid setting up. them, and the matting is lifted clear. The riffles, matting The DFS is an in-stream sluice, with a flared intake and sluice-box are then flushed clean into a bowl or (‘fixed wings’) to help funnel water into the mouth of the suchlike if needs be with a few drops of detergent added sluice and to aid stability. The first section is a long slick to founder any float gold. Later the contents of the bowl plate to encourage laminar flow to guide heavy particles are cleaned by panning, tabling or other means. into a section of tiny raised expanded metal riffles clamped on “heavy duty” unbacked NomadTM matting. Adoption by placer gold miners The DFS, being an in-stream sluice, lacks a hopper and screen, and has no means of recirculating water. Nor The outstanding success of the DFS in recovering fine is it designed to catch gold nuggets. placer gold led to its immediate and continuing popularity among recreational miners and has inspired many other devices such as the PopandSon sluice.

Figure 120. 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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58: Cleangold® sluice with magnetic riffles – 1990s research in Oregon Operation 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. Figure 121. CLEANGOLD® SLUICE Gold particles are actively trapped by the fluidised A Cleangold® sluice, after attracting magnetic particles to create bed – not by the magnetic fields but by the fluidised bed corduroy-like ridges that trap fine gold. (photo: Robin Grayson) being a thixotropic carpet in which heavy particles such as The Cleangold® sluice was invented by David Plath gold are trapped and burrow down by gravity alone. of Oregon and patented in 1999 (US #5,927,508). The Cleaning the sluice takes only a few seconds using a Cleangold® sluice uses magnetic strips embedded in a plastic scraper to scrape the concentrate into a plastic bin. non-magnetic rubberised sheet inserted in a plain Care is needed to decide when the sluice needs to be aluminium sluice to attract, hold and accumulate cleaned. Cleaning too frequently renders excessive the ferromagnetic minerals in corduroy-like ridges that serve further upgrading required, and cleaning too infrequently as riffles capable of trapping very fine gold. risks the sluice being over-full of gold and other extremely Overall the Cleangold® sluice is a low-cost, highly heavy minerals whereupon its effectiveness may suffer.’ efficient appropriate technology solution to upgrading gold It is clear the Cleangold® sluice can recover at least and is capable of recovering much fine gold lost by 95% of gold present, including most of the very fine gold. panning, sluicing and amalgamation [22,23,169-173]. It appears to be superior to mercury in recovering very Several different versions are produced by Cleangold LLC fine gold, and appears capable of recovering a significant equating to a gold pan, a trough and a sluice insert proportion of extremely fine gold. Positive comments have (www.cleangold.com). been made in several independent reports. Tests briefly mentioned in the paper by Lars Hylander and David Plath [169] claim 60-70% recovery of gold Adoption by placer gold miners “down to 0.005mm” at first pass, and in a second pass a The innovative Cleangold® sluice has great potential recovery of “a further 60-70% of the remaining fines”. for placer gold recovery by artisanal miners, recreational This suggests four permutations for the overall result – miners and by mining companies. The equipment is new 84%, 88%, 91% and 98% recovery. and is currently penetrating artisanal markets in Surinam [22], Guianas [23] and Philippines [36, 170,171].

Figure 122. 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)

137 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

59: Pyramid E-tank (Pyramid rotary jig®) – 1990s research in California Operation 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 Figure 123. PYRAMID E-TANK slurrifying by rotary stirring. The gold and other heavy (drawing: Robin Grayson based on advert by the manufacturer) particles spiral down, while the lights spiral upward. This The Pyramid E-tank was invented in the early 1990s seems to be in thixotropic sludge-like slurry. by Pyramid Industries of California and marketed as the After several minutes of operation, the barren tailings Pyramid Rotary Jig®. Manufacture ceased and an advert is are bled through the tank wall via drain taps at two levels. the sole source of data, but it seems to be an elaboration Then the tank is refilled and the process repeated “over of the ‘Heavy Mineral Separator’ invented by Laurence H. and over again” until an “ultra-rich concentrate” is Konvalin of California, patented in 1983 (US #4,389,309). achieved. A concentration ratio as large as 2000:1 is The Pyramid E-tank lacks jig screen, jig bed, ragging, possible and the concentrate is removed from the bottom hutch chamber or vertical jig motion. The advert stresses of the tank via a concentrate tap as a batch discharge. it differs from hutch jigs and diaphragm jigs. Rather the Continuous operation is possible with automatic feeders. invention belongs to a family of gravitation devices termed Adoption by placer gold miners ‘elutriated sludge tanks’ (E-tanks) that seem particularly suited to recovering very fine gold [28]. The Pyramid E-tank seems to have enjoyed a brief The Pyramid Rotary Jig® is here termed the ‘Pyramid period of moderate interest from recreational miners in E-tank’. Model #T50 consists of a circular tank tapering North America but then swiftly vanished into obscurity. It from 22-inch diameter at the top to 18-inch diameter at is unclear if any such devices are still in use in spite of the bottom. The taper is said to be significant. their clear potential for fine gold recovery.

Figure 124. 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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60: GekkoTM in-line pressure jig – 1990s research in Australia Operation 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 Figure 125. GEKKOTM IN-LINE PRESSURE JIG separation interface on the top of the jig bed. The jig bed A skid-mounted IPJ at a placer gold mine in Australia. (photo: is pulsed by a central drive shaft driven by the central courtesy of Nigel Grigg of Gekko – www.gekkos.com) drive ram at the unit’s base. The pulsing achieves episodic The GekkoTM in-line pressure jig (IPJ) was invented ‘quicksand’ (thixotropic) conditions. by Alexander Gray of Victoria who was awarded patents in During the dilation phase, gold separation occurs by 1995 in Australia (AU #W095/26,232) and 2000 in the density, plus segregation by differences in size and shape. USA (US #6,079,567). The innovation was intended for In the suction phase, heavies are drawn through the recovery of low-grade high volume placer gold; later for ragging and screen to fall into the hutch – drawn off by a hard rock grinding circuits [174-178]. spigot as a continuous discharge. Nuggets stay on the jig A slurry pump drives the entire system. The screen awaiting batch discharge. The ragging is a 25mm negligible pressure drop ensures a 15-metre head to thick carpet of lead shot covering the 2-3mm holes of the pump tailings up to a settling area, and allows jigs to be jig screen. The IPJ 1500 uses about 200 kilos of lead shot arranged in series if so desired. The IPJ has a moveable, and Gekko has begun making ‘synthetic’ ragging. rather than fixed, jig screen. The operator can dictate the Adoption by placer gold miners height the jig bed rises and falls; and all particles are raised to the same height regardless of particle density. Many IPJs are in hardrock gold mills. Placer applications Jigging is a sawtooth pulse rather than simple pulse, and include tin (New South Wales) and diamonds onshore and the operator can select the upstroke and downstroke offshore in Namibia and South Africa. About 20 are speeds independently, as well as adjust the stroke installed at placer gold mines, in Australia, New Zealand, duration via electronic control of the hydraulic drive. Papua New Guinea, North Korea, Guinea and Peru. 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. 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)

139 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

TM 51: Falcon SB bowl – 1990s research in British Columbia Operation 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 Figure 127. FALCONTM SB BOWL the concentrate bed in preference to lighter minerals that Looking down a Falcon SB bowl, showing the smooth lower are ejected as continuously discharging tailings. separation area overlain by a riffled upper separation area. The Falcon SB achieves a concentration ratio of (photo: Falcon Concentrators Inc – www.concentrators.net) 1,000 or more and so the volume of concentrate is low. The FalconTM Superbowl (SB) was invented by Steve Periodically the feed is stopped to permit the McAlister to complement the original FalconTM B bowl. The concentrate to be rinsed out of the bowl as a batch new SB bowl was tested on a placer gold deposit in British discharge. A typical operating cycle is 2 to 4 hours. Columbia and patented in 1995 (US #5,462,513) Recently Falcons have been awarded a patent for [125,153]. The floor of the Superbowl has an impeller, continuous discharge (US #6,796,934). and the lower two-thirds of the wall are a smooth ‘migration zone’. However the upper third of the wall Adoption by placer gold miners bears horizontal riffles with pores for water injection in the Falcon Superbowls are occasionally used in wash- intervening grooves, acting as a ‘retention zone’. This is plants in placer gold operations: based on the elutriated centrifugal bowl patented 50 years ² Crescent Mining, Guyana – Falcon SB250; before by Arnold Nesbitt McNicol in Australia (AU ² Sunshine Fields Corp, Philippines – Falcon SB250; #17487/34 and AU #22055/35). ² Promotora Minera Salemex SA de CV, Mexico – Falcon SB750; Recovery of fine gold by the Falcon and Knelson ² Umico Ltd, Lukarasi Project, Tanzania – Falcon SB750; bowls are somewhat similar [125,134,140, 142,145,180- ² Nolan Gold Mine, Alaska – Falcon SB2500 in1° recovery; ² Garraway Resources, Guyana – Falcon SB2500; and 183]. ² Apollo Gold, Venezuela – Falcon SB2500.

Falcon Superbowls are occasionally used to recover

gold as a by-product of sand and gravel mining:

² Teichert Aggregates, California – Falcon SB2500 cleaning jig tails, plus a Falcon SB750 cleaning table tails; and ² Montezuma Aggregates, USA – Falcon SB2500 in automated 1° recovery, plus an SB750.

Figure 128. 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)

140 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

TM 62: Itomak bowl – 1990s research in Novosibirsk Operation 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 Figure 129. ITOMAKTM BOWL The smallest ItomakTM concentrator, model KG 0.1, at the placer stage of a wash-plant. Depending on the model they can gold mine of Polymet Polata Ltd at Sharin Gol in Mongolia. The processing 2-15m3 of solids. Slurry screened at <3mm is top cover is removed to display the tilted riffled bowl. (photo: fed via a slurry pipe to discharge at the ‘far wall’ of the Robin Grayson) spinning bowl. The bowl rotates in the horizontal plane, TM The Itomak bowl is a novel centrifugal concentrator again differing from most western bowls. The rotor rotates developed by SMA ITOMAK [184,185,186], a company at 500-650 rpm to slam the slurry against the wall of the spun out of a Novosibirsk scientific centre: bowl. Injection of fluidisation water helps prevent packing. www.itomak.com. ItomakTM bowls, large and small, are stopped for The bowl is riffled and spins tilted in smaller models batch discharge of concentrate. and horizontally with the larger models, unlike western bowl centrifuges – KnelsonTM, FalconTM, Knudsen, NeffcoTM, Adoption by placer gold miners GoldkatchaTM etc – whose bowls spin round a vertical axis. Itomaks have been used in placer mining since 1996, The larger ItomakTM bowls resembles the Chinese Yunxi and are installed in Western Siberia, the Urals, in Yakutya- and SLS bowls that spin round a horizontal axis. Sakha, Buriatia, Hakasya, Mining Altai, Krasnoyarsk, The horizontal attitude has some advantages, such Khabarovsk edge of the Magadan and Amur areas, and also as the rotary mechanism being above the water when at Kazakhstan, Vietnam, Tanzania, Mongolia and South Africa. rest.

Figure 130. GOLD RECOVERY BY ITOMAKTM BOWL – Novosibirsk tests Recovery of placer gold by Itomak centrifuge bowl, based on information from the manufacturer. (compiler: Robin Grayson)

141 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

63: BGS shaking table – 1990s research in United Kingdom Operation 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 Figure 131. BGS SHAKING TABLE cam. Hand cranking at a comfortable one turn per second Top view of the Mongolian home-made version of the hand- cranked BGS shaking table. (photo: Robin Grayson) translates into five bumps per second – enough to operate the shaking table. Rebound from the bump is by means of The BGS shaking table was developed by the British a rubber band. If desired the crank can be modified to be Geological Survey (BGS) as part of the DFID/BGS powered by a bicycle, motor cycle or a motor, either Technology Development Research (TDR) project R6226 electric or diesel. ‘Mitigation of mining-related mercury pollution hazards’. BGS trials at Kias Creek in the Philippines were [24]. The task was to design, construct and test a cheap, successful; the table was easily set up and adjusted to a simple shaking table that could be produced for use by stable configuration and heavy mineral concentrates were small-scale miners in developing countries. The BGS readily recovered. A hand lens showed much fine-grained shaking table is being evaluated by recreational gold gold had been recovered. Laboratory examination miners in North America and by the Support for Artisanal revealed most of the gold was only around 40µ in size and Mining (SAM) project in Mongolia (www.sam.mn). grains as small as 10µ had been recovered. BGS laboratory trials showed the BGS shaking table is Trials at Acupan in the Philippines were problematical as good as – and probably slightly more effective than – and it was far more difficult to set up the table in a stable the commercial Wilfley shaking table for in recovering configuration. Laboratory examination of the concentrates fine-grained gold. However the BGS laboratory trials were showed that significant amounts of gold had been carried out in almost perfect conditions: ² the samples were washed and deslimed prior to tests; recovered from ores and tailings and substantial amounts ² the table was set up on large flat benches; and were very fine-grained, around 30µ in size. ² a well controlled, even pressure water supply was available. Adoption by placer gold miners Field trials show the BGS shaking table to be an effective device. Its use is more difficult in adverse For recreational gold miners the BGS shaking table conditions where material to be processed is muddy, offers a simple rapid means of upgrading concentrates in stability is a problem and water pressure is variable. the field rather than “bringing them home” as well as Under optimum conditions, it seems that the BGS removing any temptation to resort to mercury. shaking table can be expected to routinely recover almost For artisanal gold miners the BGS shaking table is all moderately fine gold (100µ to 1mm) and probably small, lightweight, transportable, affordable and offers a >90% of very fine gold down to 50µ. possible alternative to mercury for upgrading concentrates.

Figure 132. 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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64: GoltronTM machine – 1990s research in Utah Operation Gold-bearing black-sand concentrates as coarse as 3- 5mm 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 feed- rate 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 >1.19mm gold in the nugget trap; TM Figure 133. GOLTRONTM MACHINE ² 0.42-1.19mm gold on the ‘coarse side’ of the Golton table; TM View of a Goltron ready for action. (photo: courtesy of Goldfield ² <1.19mm gold on the ‘fine side’ of the Golton table. International – www.goldfieldeng.com) The GoltonTM unit has a power requirement of TM The Goltron unit is a recent innovation by Goldfield 110/220 volts, single phase at 50/60 hertz, to vibrate the International of Utah (www.goldfieldeng.com) designed to table and/or screens. recover not only fine gold but a full range of gold sizes The manufacturer claims that recovery efficiencies including nuggets to directly produce a clean smeltable are excellent, and that the unit is capable of achieving concentrate. It seems to be the only shaking table set-up “gold recovery down to minus 400-mesh” (37μ). However capable of catching everything from nuggets to fine gold. test results have yet to be published. The manufacturers claim it is “fastest, most efficient system that does not rely on chemicals or amalgamation” Adoption by placer gold miners to separate fine gold. In seems possible that upgraded The GoltronTM Unit is new and is gaining some TM concentrate from a Goltron table may be pure enough acceptance by placer gold miners at least in North for direct smelting to produce dore bars, but little America. information has been published.

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)

143 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

65: U-TechTM reverse polarity table – 1990s research in Arizona Operation 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 pre- treatment 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 Figure 135. REVERSE POLARITY TABLE reciprocating motion than possible with a conventional Side view of the drive mechanism, U-Tech RP-4 shaking table, Central Mineral Laboratory in Ulaanbaatar. (photo: Davasambuu shaking table, by using spring steel plates instead of linear of the Swiss-funded Support for Artisanal Mining project) sliding bearings. The U-TechTM reverse polarity (RP) table was A special feature is spinning rare earth magnets invented by Darvin Wade of North Virginia and patented in positioned underneath the deck surface. By reversing the 2001 (US #6,308,835). The RP table has a reverse magnetic polarity of magnetite at about 800 cycles/minute polarity spinning magnet system under the table top that (elliptical polarization of the magnetite) magnetite rises to aids separation of black sands. The RP table is built by U- the surface and is then more easily washed into the tails Tech Co. in Arizona, USA - www.goldequipment.com/gold- by the transverse flow of water. This is aided by the mining-equipment-concentrating-table-RP-4.html magnetite particles delivering additional magnetism by The U-TechTM RP-4 table has a 12x48inch cleaning aligning their magnetic poles to produce long chains. deck, weighs 60lb and fits in a large car boot. U-Tech The tailings leaving the table pass a ‘tails nugget claims the RP-4 table “saves down to 1 micron (Lab Test)” trap’ before exiting as a continuous discharge. TM www.goldequipment.com/gold-mining-equipment-warranty.html An optional deck screen added to the U-Tech RP-4 Larger units such as the RP-16 table can process >8 table operates as a combined shaker screen, magnetite tons/hour (4 tons of black sands) of sand-sized feed. The separator and gold gravity concentrator. RP-16-D gravity concentrating table weighs about 2,200 Adoption by placer gold miners lbs and requires 60-80 gallons of water/minute. According to U-Tech, “we have users claiming the U- Reverse polarity RP tables gained some popularity in TECH heavy mineral concentrators are saving down to one North America, but in spite of the claimed advantages of micron” and, “will recover 99% of microscopic gold from your the U-TechTM RP table they have not yet begun to magnetite concentrates and is designed to run continuously", seriously challenge the entrenched position of the also “we have users claiming the U-TECH heavy mineral traditional shaking tables or the advanced types of concentrators are saving down to -500 particle size", and shaking table such as the GemeniTM table. “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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The ‘Perfect Mousetrap’ by 2007

Mercury still resists elimination… Mercury amalgamation (#1) still persists as method- of-choice of artisanal miners throughout the world, often in tandem with cyanide leaching of tailings to recover <70μ gold that mercury cannot amalgamate. In artisanal and small-scale mining, mercury amalgamation and cyanide leaching (#2) face competition from HGP leaching (#66) that uses eco-friendly non-toxic proprietary chemicals and is cost-effective with artisanal miners in Ghana. Competition also comes from Mintek’s iGoli process (#67) in Africa and may arise from tincture of iodine leaching (#68) based on recent Japanese tests. In industrial mining, leaching is becoming more elaborate with many viable options emerging (see chart) capable of recovering gold from difficult refractory ores. Microbial methods are gaining in importance, and gold- binding proteins (#69) are being investigated. The absorption of gold by higher plants holds promise that phytomining (#70) may become commercially viable.

Gravity methods poised for victory? By 2007, improved gravity methods seem poised to eliminate mercury the first time in human history, due to the efforts of recreational miners. Sluices seemed to have reached their evolutionary peak with the Yukon tests of the 1980s when 150μ was the limit for 90% recovery. With placer gold miners digesting the Yukon research findings and improving their gold recovery, scant attention was paid to the Yukon tests NOT having been designed to recover really fine gold, due to the scarcity of fine gold in most of the Yukon. Recreational sluices become year-by-year more innovative and more efficient at recovering ever finer gold. Recreational sluices recover 90% of fine gold down to 50μ by using much smaller riffles than industrial sluices in the Yukon tests did. Rather than using raised expanded metal grating favoured in the Yukon tests, recreational sluices use raised expanded metal mesh as in the University of British Columbia tests [7] but much tinier. The Popandson sluice (#71) of Steve and Jason Gaber is a development of the Damn Fine SluiceTM and recovers >90% of 50μ gold. In 2006 it caught 30-50μ gold in a qualitative test. Research into small sluices is ongoing by recreational miners. Loewen’s electrostatic sluice (#72) may further enhance fine gold recovery of small sluices used by recreational miners, as electrostatic adhesion is barely used in wet sluices, though the norm for drywashers. Reflux classifiers (#73) are undergoing intensive research in Australia and commercial recovery of fine gold seems realistic within the next few years. EcologicsTM E-tower (#74) of New Zealand is a low- cost pulsatory type of agitated E-tower that recovers gold with ease down to 100μ and probably finer. Helix belts (#75) evolved from helix cylinders and are now attracting wide attention from placer gold miners. Figure 137. METHODS OF GOLD RECOVERY BY 2007 See text for details of each method. (compiler: Robin Grayson)

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66: HGP leaching – 2000s research in New Jersey Operation 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 chemically- based 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 Merrill- Figure 138. HABER GOLD PROCESS Crowe and carbon methods. A mobile test unit for the Haber Gold Process (HGP). ² hardrock ore – the rock is first crushed and milled to 80-120 (photo: courtesy of Haber Inc – www.habercorp.com) mesh (130-200μ) – an advantage over using cyanide that The Haber gold process (HGP) was developed by typically requires milling to at least 200 mesh (74μ). Norman Haber of New Jersey for hardrock gold ores, but it Therefore it seems highly likely that HGP will prove effective has potential for placer gold recovery. The HGP is a at recovering gold less than 200μ in size. ² placer ore – with placers there is no requirement to mill the chemical leaching process using a non-toxic lixiviant ore, as the commercial gold content is ‘free’ gold. The Haber (extracting solution) of proprietary composition. Haber Inc Gold Process should be effective for placer concentrates (www.habercorp.com) says the chemicals used are screened at 1mm or even 5mm, for a spherical gold particle “readily available”. of 200μ diameter has hydraulic equivalence to a spherical About 100 tons of different types of gold ore have magnetite particle twice this diameter and a quartz particle several times this diameter. The Haber Gold Process should been processed in small-scale HGP pilot plants which are be effective at recovering fine gold in the tailings of claimed to extract gold in bulk in “significantly less time” conventional wash-plants. than with cyanide. The tested ores responded much faster with the HGP. Reagent costs are similar to cyanide but the Adoption by placer gold miners costs of the HGP are claimed to be “an order of magnitude The Haber gold process (HGP) is undergoing lower” than for a cyanide system. commercialisation trials in Ghana for applications to Haber Inc. claims the following advantages of HGP combat and replace mercury usage by thousands of leaching over cyanide leaching: artisanal and small-scale gold miners (ASM). Initial ² routinely recovers more gold; ² processing rates are significantly faster; progress has been very encouraging and publication of ² effective with more types of gold ore; and detailed results is awaited with interest. ² overall cost of processing is same or less.

Figure 139. 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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67: 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 Operation Mining and Beneficiation Division (SSMB) of Mintek in 1st stage – leaching gold into solution South Africa (www.mintex.co.za). The process equipment is made of PVC and other The iGoli process is designed to leach gold from plastics, as chlorine in the leach would attack mild steel >0.1% gold concentrate to produce 99.9% gold product. and other metals. Use of plastics allows transparent The iGoli process is a modern version of chlorination, vessels allowing the operator to witness the progress. and uses a mixture of pool acid (dilute hydrochloric acid), The feed is of finely screened concentrate with a bleach (sodium hypochlorite) and sodium metabisulphate grade of >0.1% gold. A batch is added to the reaction to leach and recover metallic gold [38,39,187]. vessel where the gold is dissolved (leached) by a mixture Assessment of concentrate of pool acid (dilute hydrochloric acid) and bleach (sodium Tests by Mintex show that concentrates from hypochlorite). Gold is leached by attack by nascent different areas require slightly different recipes for the chorine to form gold chloride (AuCl2) in solution: iGoli process. Mintek SSMB requires a 2-kilo sample of goldsolid + sodium hypochloriteaqueous + hydrochloric acidaqueous concentrate containing at least 1gram/ton to determine = gold chlorideaqueous + sodium chlorideaqueous + waterliquid the optimum recipe [187]. In some placers and a few Leaching the gold usually takes a number of hours. hardrock ores, carbonates are present in such abundance Any fumes that may form are scrubbed in the that they interfere with leaching gold and have to be first attached vessel using sodium hydroxide solution. removed [35]. 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 small-

scale 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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68: 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 Operation rise in popularity of cyanide leaching and mercury Tincture of iodine is obtained from a lab supplier. amalgamation and the high cost of iodine. Advances prior If placer ore, the pay gravel is finely screened, the to 2000 are dealt with in Section 4. coarse fraction being subjected to conventional In the last 25 years much has been claimed about gravitational separation, the fine fraction being subjected ‘secret’ lixiviant formulations, and methods of precipitating to leaching. If hardrock, the ore is milled very finely gold from streams and seawater – most is quackery. before being subjected to leaching In contrast, the tincture method of iodine leaching is 1st stage – leaching gold into solution fully explained, repeatable and verifiable, as invented in The fine ore is added to a little water in a tank and 2006 Hiroyasu Murakami and Y. Nakao of the National kept agitated by stirring. Tincture of iodine is stirred in, Institute of Advanced Industrial Science and Technology - and is dark brown due to the presence of I3 ions. These (AIST) of Japan: ‘A trial of extracting gold from stream are an effective oxidant and in the presence of I- ions sediment and high Au/Ag ore using halogen-containing reacts to form the stable gold-iodine complex: organic system’ [40]. - - - 2Au + 3I3  2[AuI4] + I To dissolve the gold, a halogen-containing organic Lab experiments show an hour is sufficient for the system (HOS) is used, composed of iodine I2, sodium tincture of iodine to leach most of the gold – faster than iodide NaI and ethanol C2H5OH, a mixture closely akin to cyanide can. Then dissolution declines and full leaching of ‘tincture of iodine’. Tincture of iodine is usually 10% gold from a saturated gold solution might take six hours. elemental iodine in ethanol, and a component of 2nd stage – recovering gold from solution emergency survival kits to disinfect wounds and to sanitize To recover the dissolved gold from the pregnant surface water for drinking. solution, ascorbic acid is added to reduce the iodine: To precipitate the gold from solution, ascorbic acid - I2 J 2I C6H8O6 is added – a chemical familiar as vitamin C. - This reaction results in a deficiency in I causing the Iodine being expensive, it is important to regenerate 3 tincture to lose its dark brown colour and become a poor the tincture of iodine. Hydrogen peroxide H2O2 is added to solvent. It is now possible to precipitate gold. Deposition oxidise the iodide I- back to iodine I2. The regenerated starts when the tank is diluted by about 70% water by tincture of iodine can once again dissolve gold. volume. The gold appears as colloidal gold and next as The Japanese researchers recovered 79.9% gold fine-grained particles. The gold is removed by filtration. from a stream sediment sample, and 84% gold from a hardrock sample in the Kitakami mountains of northern Adoption by placer gold miners Japan. It seems possible that tincture of iodine may become popular among recreational and artisanal gold miners for fine gold recovery.

Figure 141. 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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69: Gold-binding proteins – 2000s in research in Washington Gold-binding proteins (GBPs) have recently been recognised as having potential applications in recovering Operation fine gold and in gold exploration. GBPs are proteins that GBP-enabled magnetic separation of extremely fine have a high specificity and affinity for gold [188]. gold particles from magnetite-rich slurry. A “magnetic A research team at the University of Washington led mineral binding agent including a gold-specific protein” is by Clement Furlong with funding from Placer Dome Inc added to a sample to form a complex of magnetic mineral investigated gold-binding proteins and registered a patent and gold. A magnetic field is applied and the complex in 2005 (US #6,875,254). The patent is highly innovative removed from the liquid. In a lab test, 3μ gold beads were but broad-brush in character and the methods it proposes coated with GBP antibodies and bound to magnetic beads are not yet commercial. However the speed of advance in to form a complex. Being magnetic, the complex was parallel subjects such as immunology and microbiology pulled to the wall of a microcentrifuge in a magnetic field may lead to sudden breakthroughs. while the other material settled to the bottom of the tube. The patent stretches the meaning of GBPs to In a very different test, gold was bound to natural embrace all “gold-specific non-naturally occurring binding magnetite by means of a reagent with both gold-and- ligand to gold” in a protein, polypeptide, peptide, protein magnetite-binding-domains to form a complex that could fragment, oligonucleotide, carbohydrate, antibody, be separated by magnetic methods. chelating agent, magnetic agent, hydrophobic agent or GBP-assisted floatation – a GBP is modified to form a any combination of these. hydrophobic reagent by reacting with valeric anhydride to

The patent envisages many types of gold recovery form a GBP with C5 hydrophobic tails (C5-GBP). A mechanisms involving GBP, or rather “binding ligands to laboratory experiment showed C5-GBP bound to extremely gold”, such as: fine gold (“micron gold”) could reside at the oil-water ² GBP-enabled magnetic separation to recovering extremely interface. However in the author’s opinion this result fine gold particles from magnetite-rich slurry. should be treated with caution for gold itself is ² GBP-assisted floatation using GBPs modified to form a hydrophobic and the merit of adding C5 hydrophobic tails hydrophobic reagent. is not demonstrated. ² GBP-assisted microbial recovery of sub-micron gold (<1μ). 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.

Adoption by placer gold miners

Gold-binding proteins (GBPs) have yet to be

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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70: Phytomining – 2000s research in New Zealand Operation 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 Figure 143. PHYTOMINING bioaccumulate the metals in their roots, shoots and Small-scale field trials of gold phytomining trial in Brazil. leaves. (photo: courtesy of Dr. Chris Anderson of Massey University - The lixiviant chemical may be toxic to animals and http://ite.massey.ac.nz/staff/rhaverka/Phytomining.htm) man but harmless to the crop, such as cyanide that may Phytomining is a still largely experimental. Research be broken down in the soil. is investigating plants able to grow on toxic soils polluted After a few days or weeks, the crop is harvested and by mine waste or from natural high toxic metal anomalies. processed by incineration to recover gold, mercury and Not only is a ‘ground cover’ of plants produced, but also other metals in the ash. some plants absorbed such large amounts of toxic metals Researchers at Massey University in New Zealand are that ‘bioremediation’ is sometimes possible by cropping testing the use of common crops such as rapeseed to soak the plants to remove the metals. up toxic contaminants from soil at abandoned gold mining The next step has been very recent – to investigate if sites, and to return the land to safe agricultural use. The valuable metals can be mined by cropping such plants, the idea is that the gold harvested during the operation covers plants absorbing the valuable metals front the soil – and the cost of clean-up and provides revenue for the so a new scientific line of investigation emerged for which education and training of the communities to create term ‘phytomining’ has been coined. The first sustainable incomes by farming the land. experimental success was as commercial production of nickel metal from plants grown on soils with abnormally Adoption by placer gold miners high concentrations of nickel. Phytomining is making rapid progress. The author Recently there has been some success by New suggests several routes to commercialisation may emerge: Zealand researchers with gold recovery from crops of ² phyto-reclamation as an incentive to cleaning up tailings; and plants grown on soils with high gold content [189-192] ² gold recovery from difficult placers, e.g. fine gold in laterites. see: http://ite.massey.ac.nz/staff/rhaverka/Phytomining.htm.

Figure 144. 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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71: 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 Operation means of drywashers and related waterless equipment. This text is based on the Loewen electrostatic sluice Little has been published on the gold recovery achieved, as presented in US patent #7,012,209. other than generalised comments that dry methods Placer pay gravel is first screened to say 15mm and usually recover less than wet. gold recovered by sluices, jigs or similar gravitational Electrostatics has received little attention in water- devices. Tailings are screened <1mm and fed into the based gravitational recovery of gold. Yet there is a feed hopper of the Loewen electrostatic sluice. significant amount of anecdotal evidence that Alternatively milled hardrock ore is screened <1mm electrostatics are helpful, particularly in wet recovery of and fed directly into the feed hopper of the Loewen fine placer gold. Two examples are noted below. electrostatic sluice. ‘Differential Charging Recovery Systems’ (DCRS) The device consists of a simple inclined gravitational were invented by Robert Barefoot of Calgary and patented sluice, typically “ten feet in length and is four inches wide in 1990 (US #4,975,182). DCRS turning screened pay with one-and-one-half-inch high sides.” gravel into watery slurry in which a positive electrostatic The inclined sluice is lined with “a material which charge was induced in the water droplets and gold incurs a positive charge when immersed in water, particles by subjecting the slurry to high-velocity spinning especially water having a pH value between 4 and 8.” in a cyclone-like surge tank. Then the positively charged The patent suggests vinyl (PVC) to be a suitable gold particles encounter negatively charged surfaces and material. The material has transverse ribs that serve as are forced to settle by the strong force of electrical riffles and the gold is trapped in the intervening grooves. attraction and the relatively weaker force of gravity. The Water is added to the feed to make it very thin patent claimed “highly efficient recovery of the invisible slurry, “a good ratio would be nine parts water to one part gold (less than 320 mesh)” using a 180 tons/day mobile [screened feed].” test unit. Feeding the sluice too quickly with slurry would cause DCRS is somewhat dauntingly complicated and one the grooves to plug. The patent does not specify the version depends on mercury amalgamation. The system preferred flow rate at which “the gold can be observed failed to be commercialised for many reasons, and some settling out during the process, as most of the gold will are noted at www.barefootscureamerica.com. settle out in the first three feet of the sluice.” The Loewen electrostatic sluice was invented by After the batch feed has been exhausted, the sluice Wayne W. Loewen of Alberta and patented in 2006 (US is allowed to run clean. Then the contents are flushed into #7,012,209) and is refreshingly simple. Gold is recovered a ‘clean-up pail’ and its contents are allowed to settle and in a wet sluice lined by ribbed plastic (e.g. polyvinyl the water decanted to leave a rich gold concentrate. chloride PVC) than is positively electrostatic when immersed in water, and therefore catches negatively Adoption by placer gold miners electrostatic fine gold particles by a combination of The Lowen electrostatic sluice is very new and has electrostatic attraction and gravitational settling. yet to be marketed to placer gold miners.

Figure 145. 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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72: PopandSon sluice – 2000s research in USA Operation 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 Figure 146. POPANDSON SLUICE 88.9cm of style #3/16 raised expanded mesh clamped on Carrying a standard over-the shoulder PopandSon sluice in the TM Gobi Desert of Mongolia. (photo: Robin Grayson) unbacked Nomad 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 (74- Figure 147. RANGE OF RIFFLE SIZES The large, medium and tiny raised expanded metal mesh in a 149μ) the best performance of the top section was 92- ‘standard’ PopandSon sluice. (photo: Robin Grayson) 94% using style #3/16 expanded metal mesh, but fell to The PopandSon sluice was invented in 2005 by Steve 86% using style #1/2 expanded metal mesh. and Jason Gaber (PopandSonminers of Alaska Gold Forum For 200-325 mesh gold (44-74μ) at a gentle slope of AGF) in Washington State. The PopandSon sluice is a 16mm per metre length, the recovery was only 65% in development of the innovative Damn Fine SluiceTM (DFS) the top section but 85% for both sections. invented by Phil Hontz of New Mexico and made by the Steve and Jason Gaber suggest a “fairly large drop-off in Damn Fine Equipment Co (www.damnfinesluice.com) in recovery efficiency somewhere below 200 mesh (44 microns)”. the 1990s. The DFS is a simple cheap device consisting of Experimental work is limited to narrow sluices (6.5 a smooth slick plate followed by a section of tiny raised inches = 165mm) for recreational miners and clean-ups. expanded metal mesh fitted on matting, all set in a short Yet the PopandSon sluice might be scaled-up for industrial sluice-box. This is part of relentless effort by recreational wash-plants if four issues are solved: a) screening feed to miners in North America to recover fine gold using small about 2mm; b) preventing warping; c) preventing surging; 3 sluices for small dredges, high bankers and clean-ups. and d) reducing water – for each m of loose placer water 3 Steve and Jason Gaber conducted bench tests with usage is very high – 19.6m for steep angle. tungsten (W) powder and gold (Au) tracers [193] in a Adoption by placer gold miners version of the PopandSon sluice consisting of an aluminium sluice-box lined with simple thin ribbed rubber Expanded metal riffles of tiny mesh are increasingly followed by unbacked NomadTM matting with style #3/16 popular amongst recreational placer gold miners, and aluminium raised expanded metal mesh fitted on top. have potential for artisanal miners and mining companies.

Figure 148. 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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73: Reflux classifier – 2000s research in Australia Operation 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 Figure 149. REFLUX CLASSIFIER fine particles fail to clear the top of the plates and so Generalised layout of a modern reflux classifier (drawing: Robin Grayson, redrawn from article by Zhou et al 2006 [195].) these particles collide with the plates. The ensuing friction slows the dense fine particles and they slide down the The ‘modern’ reflux classifier was invented by Kevin plates back into the E-tower. The plates are inclined at Galvin of New South Wales who applied for an Australian about 70° degrees – shallow enough to ensure dense patent in 2000 (application 09/890,487) and was awarded particles hit the plates, yet steep enough to ensure a USA patent in 2004 (US 6,814,241). Commercialisation particles slide down it [199]. is by Ludowici Mineral Processing Equipment Pty Ltd – Other particles fail to settle and continue upwards to www.ludomin.com/products/reflux_classifier.htm. escape as tailings. The ‘modern’ reflux classifier follows innovations such To ensure fine gold will settle yet quartz remain in as the 'Method and apparatus for cleaning sand or grading suspension the ratio of plate length to plate-plate gap is sand…' patented in 1947 by A.B. Morris (US #2,426,839). as great as 200:1 (‘aspect ratio) [195,197], far more than A reflux classifier is a combination of E-tower and the 40:1 used to separate coal from mineral matter. lamella settler. More than one set of lamella is possible, but a set of lamella caps the E-tower as an ‘inclined Adoption by placer gold miners section’ for best results [194-199]. This inhibit heavy particles escaping with the overflow water, enabling fine The modern reflux classifier may prove to have major particles to be “removed or classified in a more concentrated applications for recovering fine placer gold for artisanal form" and is more tolerance of feed fluctuations. gold miners, recreational miners and placer companies.

Figure 150. 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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74: Ecologic E-tower – 2000s research in New Zealand Operation 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. Figure 151. ECOLOGIC CONCENTRATOR Oblique views of Ecologic’s ‘Gold Concentrator’ showing the The device is stopped to gather the concentrate simple screen (photo: Ecologics Ltd – www.ecologics.co.nz) settled in the bottom the cone, by flushing out with a little The Ecologic concentrator is an innovative type of water. It is then upgraded by panning or other means. elutriation tower (E-tower) recently developed by The inventor considers coarse gold recovery is 95% Ecologics Ltd of New Zealand (www.ecologics.co.nz). and fine gold recovery 80-90%. In some field tests only Marketed as the Ecologic ‘gold concentrator’, the 0.2% of the gold reported to the tailings. Generalised test device is a remarkable ‘catch-all’ able to catch all shapes results have been published in South Africa [200]. and sizes of gold particles, large and small. This is Adoption by placer gold miners technically challenging, for the high-energy Newtonian setting regime needed to catch nuggets and eject large The Ecologic ‘gold concentrator’ is marketed quartz is also capable of flushing out fine gold and flat worldwide to artisanal placer gold miners and interest is gold with the tailings. being shown by some recreational miners.

Figure 152. 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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75: Helix belt – 2000s research in Canada and USA Operation 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. Figure 153. HPC HELIX BELT The belt’s motion and water flow cause heavies to Material ascending the helix belt of the HPC-10 wash-plant. settle on the helix belt. Any heavies escaping over a rib (photo: courtesy of Extrac-TEC www.extrac-tec.com) are trapped and re-processed by the next rib. Once settled The Extrac-TEC HPC helix belt is called by the makers on the helix belt, heavies are inexorably hauled up the a “transverse spiral concentration belt”. The device is a slope by tangential motion of the ribs (riffles). highly innovative form of Archimedes screw, unique to the Arriving at the top end of the helix belt, the heavies Extrac-TEC HPC systems. The helix belt, helix cylinder and continuously discharge into the concentrate sluice. helix wheel (gold wheel) are three different classes of The bed-profile, inclination, characteristics of the Archimedes screw used for heavy mineral separation. reverse helix, water flow and belt speed are configured to The helix belt has ribs that serve as riffles arranged produce the solids density desired (0.1 to 40% by weight). in a helix. The profile of the helix belt is sagged into a The helix belt ensures concentrate fed to the sluice is bow-shaped gutter-like trough that is gently tilting to drain properly pre-concentrated, and prevents surging and so the tailings down the trough, whereas the settled heavies flow is steady down the sluice and optimized for gold are hauled up the trough by the moving riffles. recovery. This reduces water consumption, and the Development began in 1986, the first prototype was manufacturer claims that gold recovery is increased “down completed in 1988, and the first patent was granted in to 40 microns”. A high concentration ratio is achieved and 1993 according to the company website. “…independent so the final concentrate is reasonably clean. analyses showed recovery efficiency close to 95%”. From 1999-2002 the company focused on using the Adoption by placer gold miners HPC technology for its own placer operations in Canada The Extrac-TEC HPC wash-plant with its helix belt has such as in Anderson Creek in the Yukon. become widely available over the last few years, and According to the company (www.extrac-tec.com) some are operational in most continents, and early fresh patents were filed in 2003 for the more advanced versions were sold in Canada, China, Mexico and USA. ‘Generation-2’ version using the helix belt.

Figure 154. 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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Acknowledgements IZTM of Irkutsk for information on sluices; staff of Zavod Trud of Novosibirsk for data on tables, Trud jigs and special placer equipment; Dr. David Grant MBA of Connectpro for The author is pleased to acknowledge the valuable contributions and huge encouragement tireless encouragement, interest and support; Professor Dr. given by members of the Alaska Gold Forum (AGF), Ş. Levent Ergьn of Hacettepe University of Turkey for and specifically: guidance on pinched sluices; Dr.Gary Beaudoin, formerly of Jim Alaska and Steve Herschbach for keeping AGF such a Java Gold regarding tests with Knelson Concentrators at great forum; Steve and Jason Gaber (PopandSonminers) Zaamar; Jim Bouma formerly of Knelson Inc for guidance TM for detailed technical advice on PopandSon sluices to on Knelson bowls; Steve McAlister of Falcon Inc for TM catch very fine gold; David Bryce (Zooka) for valuable guidance on Falcon bowls; Greg Hakonson, President of advice on small sluices, matting, Neffco bowls and a wide Eldorado Placers Ltd for advice on Z-sluices in the Yukon; range of placer gold equipment; Phil Hontz (PhilNM) for Matt Norris for sharing his experience of placer gold mining; advice on the Damn Fine Sluice; John Strain (Ausco_9999) Vincent Ruth of Continental Wire Cloth for guidance on expanded metal for riffles; Jeanie Barnett of the Geological for advice on Reichert cones, Reichert spirals and ASAT E- Society of America for photos of flat-bar riffled sluices; Dr. towers; Jim Halloran (Geo_Jim) for helping gathering Michael Styles and Dr. Clive. Mitchell of the British placer references worldwide and drawing attention to Geological Survey (BGS) for advice on the BGS shaking many recovery methods; Roger Brown (Poprivet) for ideas table; Megan Rose (Gold_Tutor) of Colorado Prospectors on how to recover gold; Allan Coty of Alaska for clarifying Forum for guidance on Osterberg’s E-tower; Greig the design of the Keene Hydromatic Jig; Timothy McNulty Oppenheimer of Extrac-Tec HPC for insight into Archimedes (Dragline) for encouragement and many ideas about gold screws and helix belts; David Plath of Cleangold for recovery. Dan Murphy of New York (Dan_inNY) for know- guidance on the Cleangold® sluice; Randy Smith of how of Neffco bowls in recovering offshore gold in Alaska; Goldfield Engineering for information on Goldfield jigs and Karl Readel (MidwestDredger) for stimulating ideas on Goltron tables; J Larry Wood of Terra Resources Ltd of tracers; Joe Scheckler (INDY) for help and wisdom on Alaska for technical information on Artic MinerTM mobile many aspects of mining in North America; Randall Seden jigs; staff of Delta Mining and Manufacturing Co of (Seden) for advice on placer gold recovery equipment, Nashville, Tennessee for information on Pan-Am jigs; notably vanners and E-towers; Michael (Stickwhipper) for Rickford Vieira of WWF Guianas for research results on gold advice on a wide range of equipment including the elusive TM recovery by chemicals without mercury; Nigel Grigg of Pyramid Jig; Leonard Leeper (Leonard) for special Gekko Systems Inc for guidance on In-Line Pressure Jigs advice on Younge’s horizontal centrifuges for recovering (IPJs); Dale Henderson of Roche Mining for data on fine gold – and many other AGF members. Reichert spirals, Reichert cones and Kelsey centrifugal jigs; Special thanks are due to miners and geologists in Mike Wort, Mineral Processing Consultant for guidance on Alaska, Yukon, British Columbia, Washington recovery techniques; staff of Motive Traction Inc for State, California, Nevada, Utah, New York, United information on tables; staff of MD Technologies for test Kingdom, the Netherlands, Denmark, Germany, reports of the GemeniTM tables; Dr. Baatar Tumenbayar of Russian Federation, Kazakhstan, Kyrgyzstan, Sans Frontiere and Eco-Minex regarding mercury Mongolia, Thailand, Australia and New Zealand: amalgamation, sluices and KnelsonTM tests; Minjin Batbayar Dr. Peter Appel of the Greenland and Denmark Geological MSc for assistance in the field; Battulga of MRPAM for Survey GEUS for advice on nitric acid and borax; Ian fieldwork and discussions; N. Davaasambuu, Ms. D. Daniels of Axia Mozley for advice on Mozley Multi-Gravity Enkhtuya, B. Dashbal and N. Nyamtsend of the SAM project Separators; Bill Rigby of Richards Engineering for for valuable discussions about washing equipment; information on Richards-Yuba jigs; Jonathan Simon of Od Lkhagvadorj ‘George’ Tumur for discussions on wash- Consult for discussions and fieldwork; Dr. Michael Priester plants; Ms. Tsevel Delgertsoo for unique advice on Russian of Projekt Consult for sharing his vast knowledge of gold and Mongolian wash-plants; Mrs. Chimed-Erdene Baatar recovery methods; Peter Seidel and Veith Seidel of Polymet MBA for assessment of informal miners in north-central TM Potala for access to their Itomak centrifuges, shaking Mongolia; Mrs. Garamjav of Monpolymet for access to her tables and sluices; Gerrit Roelof Bazuin of Ochir LG for bucket-line dredges and terrace mines; Dr. Jadambaa for discussions on IHC jigs, Knelson bowls and PAuSE hydraulic advice and photos on impacts of placer gold mining; riffles; staff of IHC Holland for reports on IHC jigs; Willem managers of Zaamar Uls for discussions about large sluices; Kramer of ADMS for advice on IHC jigs and dredges; Neil Cullen of Ecologics of New Zealand for information on Vladimir Kozevnikov formerly of SMEDA Aktobe for the Ecologics E-tower. Dr. Kevin Galvin of New South Wales discussions on SME placer mining; members of Aktobe for advice on hydraulic classifiers; Dr. Richard Haverkamp Geological Society for sharing knowledge of Kazakhstan and Dr. Chris Anderson of New Zealand for advice on placers; Gennady Potapenko, Leading Geologist of the State phytomining gold; Jim Loveridge of PAuSE of New Zealand Geological Agency of Kyrgyzstan, for guidance on Soviet for advice on hydraulic riffles; staff of Dove Engineering of wash-plants; Valery Rogalsky of AVISTA for advice on gold Thailand for help on jigs and tables; managers of Small recovery in Kyrgyzstan; Akylbek Rakaev of the Small Mining Supplies of Harare for advice on the GoldKatcha bowl. Diggers Artel regarding wash-plants and local centrifuges in This study was made possible by support from Kyrgyzstan; Nurlan Djoldoshev and the PROMA team in Bishkek for fact-finding; Anatoly Lazardi of Itomak of Eco-Minex International Ltd. Novosibirsk for information on ItomakTM centrifuges, staff of

156 World Placer Journal – 2007, Volume 7, pages 66-161. www.mine.mn

UNIDO/UBC/CETEM, Rio de Janeiro, 94 pages. References Download: www.facome.uqam.ca/pdf/veiga_01.pdf 22. Kristina Shafer, Lars D. Hylander and David Plath (2006). Novel 1. Donald M. Hausen (2000). Characterizing the Textural Features of solution to reduce or eliminate mercury pollution from artisanal and small Gold Ores for Optimising Gold Extraction. Journal of Mining (JOM) volume scale gold mining. [ABSTRACT] Abstracts of 8th International Conference 52, page 14. Download: on Mercury as a Global Pollutant. Lancaster, PA, USA. http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/JOM- Download: http://opus.uu.se/publication.xml?id=88550 0004-14/$FILE/JOM-0004-14F.pdf?OpenElement 23. David Plath (2002). CleangoldTM in the Guianas. PowerPoint 2. O.V. Zamyatin, A.G. Lopatin, N.P. Sammikova and A.D. Chugunov Presentation. 17 slides. (1975). The Concentration of Auriferous Sands and Conglomerates. 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Tools for Overseas Geology Series, British Geological Survey BGS, Keyworth, Mining, Techniques and Processes for Small Scale Mining. GATE, Vieweg- Nottingham, England, UK, 34 pages. Download: www.bgs.ac.uk Verlag, 537 pages. 5. D.F. Stewart and P.W. Ramsay (1993). Improving the simple sluice View: http://sleekfreak.ath.cx:81/3wdev/CD3WD/APPRTECH/ box. International Journal of Mineral Processing, volume 39, pages 119-136. G10TOE/INDEX.HTM 6. D.F. Stewart and P.W. Ramsay (1993) – as above. 26. Marcello M. Veiga, Randy F. Baker, Stephen M. Metcalf, Bern Klein, 7. George W. Poling and James Frederick Hamilton (1986). Fine Gold Gillian Davis, Andrew Bamber and Patience Singo (2006). Manual for Recovery of Selected Sluicebox Configurations. University of British Training Artisanal and Small-Scale Gold Miners: Removal of Barriers to Columbia. Prepared for Canada/Yukon Economic Development Agreement. Introduction of Cleaner Artisanal Gold Mining and Extraction Technologies. Indian and Northern Affairs Canada; Northern Affairs: Yukon Region, iv Global Mercury Project, United Nations Industrial Development +77 pages. Download: Organization (UNIDO), 76 pages. www.geology.gov.yk.ca/publications/tech/fine_gold_recovery_sluiceboxes.pdf Download: http://web.uvic.ca/~gmp/documents/documents.htm 8. Randy Clarkson and Owen Peer (1990). An Analysis of Sluicebox 27. Weqian Wang (1979). A Study on Methods for Fine Placer Gold Riffle Performance. New Era Engineering Corporation, Yukon. Report for Processing, Non-ferrous Metals (China), volume 4, pages 6-12. the Klondyke Placer Miners Association. 31 pages. 28. Robin Grayson (2007).The Gold Miners Book – BAT Best Available Download: www.geology.gov.yk.ca/publications/tech/analysis_sluice.pdf Techniques for Placer Gold Miners. Published on CD by Eco-Minex 9. John M. West (1971). How to Mine and Prospect for Placer Gold. International Ltd. 1,200 pages. Purchase: www.mine.mn Bureau of Mines publication, Information Circular #8517. 29. Baatar Tumenbayar, Minjin Batbayar and Robin Grayson (2000). Read: http://imcg.wr.usgs.gov/usbmak/ic8517.html Environmental hazard in Lake Baikal watershed posed by mercury placer in 10. Daniel E. Walsh and P. Dharma Rao (1988). A Study of Factors Mongolia. World Placer Journal volume 1, pages 134-159. Download: Suspected of Influencing the Settling Velocity of Fine Gold Particles. www.mine.mn Mineral Industry Research Laboratory (MIRL) Publication #76: University of 30. Baatar Tumenbayar and Robin Grayson (2001). Mercury pollution in Alaska at Fairbanks, 52 pages. $5.00 from www.uaf.edu/sme/Mirlpub.html North Mongolia. Abstract of the 6th International Conference on 'Mercury as th 11. P. Dharma Rao, David R. Maneval and Daniel E. Walsh (1984). Field Global Pollutant' held in Minamata, Japan, 15-19 October 2001. Investigation of Hydrocyclones for the Recovery of Fine Gold, Phase III. 31. Baatar Tumenbayar (2003). Action Research on Mercury in the Boroo June 1984, MMRRI Final Report. Mineral Industry Research Laboratory Area, Mongolia. Japan International Cooperation Agency (JICA). 79 pages. (MIRL), University of Alaska at Fairbanks. Publication OF 84-14, 96 pages. 32. L. Kozin and V. Melekhin (2004). Extraction of Gold from ores and $10.10 from: www.uaf.edu/sme/Mirlpub.html concentrates by leaching with the use of cyanides and alternative reagents. 12. Eoin H. MacDonald, (1983). Alluvial Mining: The Geology, Publisher: MAIK Nauka Interperiodica. Russian Journal of Applied Technology, and Economics of Placers. Publisher: Chapman & Hall, Kluwer Chemistry, volume 77, pages 1573-1592. Academic Publishers, hardcover, 508 pages. ASIN: 0412246309 33. Cleland N. Conwell (1980). Gold recovery from placer concentrates 13. Robin Grayson (2006). Gold recovery in gold pans by cyanidation: Alaska Division of Geological & Geophysical Surveys, – the term ‘panning’. World Placer Journal, volume 6, pages 1-21. Miscellaneous Publication #29, pages1-6. Download: www.mine.mn Download: www.dggs.dnr.state.ak.us/pubs/ 14. Robin Grayson (2006). Gold recovery in cones in Laos 34. A.K. Williams (no date). Super Clorox. Prospector's Paradise website, – the term ‘dulanging’. World Placer Journal, volume 6, pages 36-41. San Pedro, Costa Rica. Download: www.mine.mn View: www.prospectorsparadise.com/html/super_clorox.html 15. Robin Grayson (2006). Gold recovery in wooden trays in Russia – 35. Rickford Vieira (2004). Mercury-Free Gold mining Technologies: the term 'lotoking'. World Placer Journal, volume 6, pages 22-35. Possibilities for Adoption in the Guianas, Technical Paper #1 by WWF- Download: www.mine.mn Guianas Regional Program Office, 8 pages. 16. Chimed-Erdene Baatar (2006). Gold recovery on mats in Mongolia Download: www.wwfguianas.org/technicalpapers/mercfreetech.pdf – the term ‘matadoring’. World Placer Journal, volume 6. 36. Lars D. Hylander, David Plath, Conrado R. Miranda, Sofie Lucke, Download: www.mine.mn Jenny Őhlander and Ana T.F. Rivera (2007). Comparison of different gold 17. Chimed-Erdene Baatar (2006). Gold recovery in bowls in Mongolia recovery methods with regard to pollution control and efficiency. Clean, – the term ‘bowling’. World Placer Journal, volume 6. volume 35, pages 52-61. Download: www.mine.mn 37. Sidney Mahkatsi (2006). The iGoli mercury-free gold extraction 18. Robin Grayson (2006). Gold recovery in buckets in Kyrgyzstan method. Paper presented to the Communities and Small-scale Mining th – the term ‘bucketing’. World Placer Journal, volume 6, pages 42-51. (CASM Annual General Meeting, 11-15 November 2006 held in Antsirabe, Download: www.mine.mn Madagascar, 14 pages. 19. Peter W. U. Appel (2005). Small-scale mining – hazards and 38. Sidney Mahkatsi and Rob Guest (2003). The iGoli mercury-free gold opportunities in Kyrgyzstan and Mongolia. Geological Survey of Denmark extraction process. Urban Health and Development Bulletin, South Africa, and Greenland Bulletin #7, pages 77-80. volume 6, page 62. Download: www.geus.dk/publications/bull/nr7/nr7_p77-80.pdf 39. Anon (2007). The iGoli mercury-free gold extraction process. 20. Lars Hylander Lars and Markus Meili (2005). The Rise and Fall of Manuscript from Rob Guest of Mintek’s Small Scale Mining and Mercury: Converting a Resource to Refuse After 500 Years of Mining and Beneficiation Division (SSMB), 5 pages. www.mintek.co.za Pollution. Critical Reviews in Environmental Science and Technology, 40. Hiroyasu Murakami and Y. Nakao (2006). A trial of extracting gold from volume 35, pages 1-36. stream sediment and high Au/Ag ore using halogen-containing organic 21. Marcello M. Veiga (1997). Introducing new technologies for system. CCOP-GSJ/AIST-GAI CASM-Asia Workshop on the State-of-the-Art th abatement of global mercury pollution in Latin America. of Science and Technology to Protect the Environment and People, 27-29

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November 2006, Bandung, Indonesia. PowerPoint presentation, 16 slides. 64. Ю.L. Ergьn and S. Ersayэn (1998). Characterization of flow on a Download: www.dim.esdm.go.id/makalah/2006_murakami_CCOP.pdf pinched sluice and its effect on separation. Chapter 8 in: Innovation in 41. A.K. Williams (no date). Halide Leaching and Recovery. Prospector's Physical Separation Technologies, Institute of Mining and Metallurgy Paradise website, San Pedro, Costa Rica. London (IMM), pages 85-98. View: www.prospectorsparadise.com/html/leaching.html 65. Ю.L. Ergьn and S. Ersayэn (2002). Studies on pinched sluice 42. Jinshan Li and Jan Miller (2006). A review of gold leaching in acid concentration: Part 1 – The effects of operating variables and sluice thiourea solutions. Mineral Processing and Extractive Metallurgy Review, geometry on the performance of pinched sluices. Minerals Engineering, volume 27, pages 177-214. volume 15, pages 423-435. 43. Anon (2000). Gold Leaching using Thiourea. View: 66. Ю.L. Ergьn and S. Ersayэn (2002). Studies on pinched sluice www.e-goldprospecting.com/html/gold_leaching_using_thiourea.html concentration: Part2 – Characterization of flow over a pinched sluice. Minerals Engineering, volume 15, pages 437-446. 44. A.K. Williams (no date). Acids and Bases. Prospector's Paradise website, San Pedro, Costa Rica. 67. D. Erik Spiller (1983). Gravity separation of gold – then and now. th View: www.prospectorsparadise.com/html/acids.html Paper presented at the National Western Mining Conference, held on 10 February 1983 in Denver, Colorado, 7 pages. 45. Marcello M. Veiga, Shefa Siegel, Patrick Schein, Cheick Santigui Camara, Joachim Dejean, Djibril Kamara and Amadou Diouf (2006). 68. Thomas J. Ferree (1981). The Reichert cone concentrator, an rd Technical Mission to the Artisanal Gold Mines in Upper Guinea. Blacksmith innovation for a fine gold recovery system. Proceedings of the 3 Annual Institute and UNIDO Report, 13 pages. Conference on Alaskan Placer Mining., held at Fairbank, Alaska USA. Download: http://web.uvic.ca/~gmp/countries/guinea/ 69. M.G. Aylmore and D.M. Muir (2001). Thiosulfate Leaching of Gold - a Guinea%20Veiga%20Blacksmith%202006%20report%20v%204%20final.pdf Review. Minerals Engineering, volume 14, pages 135-174. 46. Anon (undated). The Poormans Method of Smelting Gold. 70. Jan Visman and N.E. Andersen (1975). Application of Compound Nuggethunter. View: www.nuggethunters.org/Poormans-Smelting.html Water Cyclones for the Processing of Placer Sand from Burwash Creek, 47. Anon (undated). Smelting Gold. Nuggethunter. Yukon Territory, Canada. Canada Centre for Mineral and Energy View: www.nuggethunters.org/smelting.html Technology, Energy Research Laboratories Report ERP/ERL #75 (CF) – 41 (IR), Department of Energy, Mines and Resources, Edmonton, Alberta, 48. A.K. Williams (no date). Melting and Smelting. Prospector's Paradise Canada, 120 pages. website, San Pedro, Costa Rica. View: www.prospectorsparadise.com/html/smelting.html 71. Cecil Urlich (1985). Compound Water Cyclones: Recovery of Micron Size Precious Metals from an Ancient Beach Deposit at Sombrio Point, 49. W. Motherwell (1914). Floatation Test at Mount Morgan. Min. and Vancouver Island. 5th Annual RMS-ROSS Seminar on Placer Gold Mining, Sci. Press, 27th June issue, #53140. Vancouver, British Columbia. 50. H. Lang (1916). Black sand of the Pacific Coast. Mining Sci. Press, 72. Daniel E. Walsh (1988). Evaluation of the 4-inch Compound Water volume 3, pages 811-813. Cyclone as a Fine Placer Gold Concentrator Using a Radiotracer Technique, 51. Arthur W. Fahrenwald (1933). Floatation of Gold from River Sand A Thesis. Mineral Industry Research Laboratory (MIRL) Report #70, and Black Sand. Min. J. (Phoenix, Arizona), volume 16, #23, 3-4. University of Alaska at Fairbanks, 260 pages. 52. Arthur W. Fahrenwald, J. Newton, W.W. Staley and R.E. Shaffer $10.00 from: www.uaf.edu/sme/Mirlpub.html (1939). A metallurgical study of Idaho placer sand: Idaho Bureau of Mines 73. Daniel E. Walsh and P. Dharma Rao (1988). Study of the compound and Geology, Pamphlet #51, 10 pages. water cyclone's concentrating efficiency of free gold from placer material, $2.50 from: www.idahogeology.org/products/ Canadian Institute of Mining Engineers Bulletin #? pages 53-61. 53. G.P. Slavnin (1936). Flotation of placer gold. Soviet Zolotoprom, 74. A.G. Lopantin et al (1971). In 1976, V.M. Man’kov and N.P. volume 2, pages 49-54. Sannikova published encouraging results of Soviet tests on placer gold 54. G.P. Slavnin (1938). Flotation of gold from concentrate tailings. recovery using “wide cone cyclones” (MIRL Report #70 by Daniel Walsh). Tsvet. Metal., volume 5, pages 46-49. 75. Chris Mills and R.O. Burt (1979). Thin Film Concentrating Devices and the 55. Tjoe Hauw Nio (1978). Mineral dressing by IHC jigs. Gravity Bartles-Mozley Concentrator. Mining Magazine, volume 141, pages 32-39. Separation Technology. Paper in: Gravity Separation Technology; Papers 76. Chris Mills (1978). Thin Film Concentrating Devices and the Bartles- presented at a Short Course on Gravity Separation Technology, held 13- Mozley Concentrator in Particular. Short Course on Gravity Concentration th 17 October 1978 at the University of Nevada, Reno, USA. 64 pages. Technology, University of Nevada, Reno, October, 13 pages. 56. Daniel E. Walsh, P. Dhoma Rao and Donald J. Cook (1987). Study of 77. W. Kotze and F.W. Petersen (1998). The coal gold agglomeration a Static Screen, Jig, Spiral and a Compound Water Cyclone in a Placer Gold (CGA) process to recover free gold. 17th Annual Mineral Processing Recovery Plant. Mineral Industry Research Laboratory (MIRL) Publication Symposium, Cape Town, South Africa, 5-7th August 1998. #73: University of Alaska at Fairbanks. 78 pages. 78. F.W. Petersen and W. Kotze (1998). Effect of operating parameters $10.00 from: www.uaf.edu/sme/Mirlpub.html on the performance of the Coal-Gold-Agglomerate process. 4th 57. Michael Richard Mark Anthony (1986). Investigation and Evaluation International Conference on Clean Technologies for the Mining Industry of the Knudsen Bowl Concentrator. Phase II Report. State of Alaska held 13-15th May 1998 at Santiago, Chile. Department of Natural Resources. 79. F.W. Petersen and W. Kotze (1998). The use of coal-oil mixtures to 58. Michael Richard Mark Antony (1995). Laboratory and Field Testing of recover free gold through agglomeration. Coal Indaba ‘98, Johannesburg, the Knudsen Bowl Concentrator. University of Alaska, Fairbanks, E.M. South Africa, 17-18th November 1998. thesis, 64 pages. 80. W. Kotze and F.W. Petersen (1999). Coal-gold agglomeration - a 59. William James Anderson (1975). Parameter Impact on the viable process for clean small scale mining, paper submitted to Extraction Performance Ability of the Centrifugal Forced Amalgamator. Mineral Metallurgy Africa ’99, Johannesburg, South Africa, 1-2nd November 1999. Industry Research Laboratory (MIRL) Contract Report MT-1: University of 81. L.M. Moses and F.W. Petersen (2000). Flotation as separation Alaska at Fairbanks, 97 pages. $10.20. Download: technique in the coal gold agglomeration process. Minerals Engineering, www.blm.gov/ak/jrmic/docs/usbm/cr/AuBeachSands/CR_AuBeachSands.pdf volume 13, pages 255-264. 60. Joseph Kuo-Hua Wang (1977). Centrifugal Amalgamation in Dredge 82. UDSM (2002). Progress Report for Project on Mercury-free Recovery Recovery Systems. Master of Science thesis, University of Fairbanks at of Gold (Coal Agglomeration Process - CGA). Chemical Engineering Alaska. Mineral Industry Research Laboratory (MIRL) Contract Report MT- Department, University of Dar-Es-Salaam, Tanzania. 2: University of Alaska at Fairbanks, 73 pages. Order: www.uaf.edu/sme/Mirlpub.html 83. S. Sen, A. Seyrankaya and Y. Cilingir (2005). Coal-oil assisted flotation for the gold recovery. Journal of Minerals Engineering, volume 18, 61. Michael Silva (1986). Placer Gold Recovery Methods. California pages 1086-1092. Department of Conservation, Division of Geology and Mines. Special Publication #87, 32 pages. 84. E.M. Coelho (1972). Floatation of Oxidized Copper Minerals: an Download: www.consrv.ca.gov/cgs/geologic_resources/gold/SP87.pdf Infrared Spectroscopic Study. PhD thesis, University of British Columbia. 62. O.V. Zamyatin and A.T. Konyukova (1970). Extracting various sizes 85. E.M. Coelho and George W. Poling (1973). Spectroscopic studies of of gold from sands on a concentrating table. Soviet Journal of Non-Ferrous organic acid absorption on copper oxides and on gold, paper presented at Metals, volume 43. Colloid Symposium, Ottawa. 63. B. Jeyadevan and G.K.N.S. Subasinghe (1989). The effect of solid 86. Randy Clarkson (1989). Gold Losses at Klondyke Placer Mines. Gold parameters in a pinched sluice concentrator. Journal of Mineral Processing, Recovery Project (Phase 1). Report for Klondyke Miners Association. New volume 29, pages 31-50. Era Engineering, Yukon. 42 pages + Appendix of 2 pages and 19 figures.

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Northern Affairs Program, Whitehorse, Yukon. 107. Del Ackels and James Madonna (1985). Placer Mining in Alaska with Download: www.geology.gov.yk.ca/publications/tech/gold_loss.pdf IHC jigs. Alaskan Prospectors & Miners News. Fall Edition. 11 pages. 87. Randy Clarkson (1990). The Use of Radiotracers to Determine Gold Reprint on request: www.ihcholland.com Losses at Klondike Placer Mines. Report for Klondike Placer Miners 108. Del Ackels (1985). Some aspects of gold recovery with IHC jigs. 7th Association. Northern Affairs Program, Whitehorse, Yukon. Annual Conference on Alaskan Placer Mining, March 1985. Mineral Industry Download: www.geology.gov.yk.ca/publications/tech/the_use_radio.pdf Research Laboratory (MIRL), University of Alaska at Fairbanks. 88. Randy Clarkson (1990). Evaluation of Yukon sluicing operations 109. Donald J. Cook and P. Dharma Rao (1973). Distribution, Analysis, using radiotracer gold: KPMA Gold Recovery Project update. Pages 68-77 and Recovery of Fine Gold from Alluvial Deposits. Mineral Industry in: Cathy Farmer and Daniel E. Walsh (editors and compilers) Proceedings Research Laboratory (MIRL) Publication #32. University of Alaska at of the 12th Annual Alaskan Conference on Placer Mining. Held 22-24th Fairbanks. 101 pages. $5.00 from: www.uaf.edu/sme/Mirlpub.html March 1990 in Fairbanks, Alaska. Sponsored by: Alaska Women in Mining, 110. Daniel E. Walsh (1991). Elutriator Design Manual for Coarse Heavy Alaska Miners Association, Placer Miners of Alaska, and School of Mineral Mineral Recovery from Sluice Box Concentrate. Mineral Industry Research Engineering UAF, iv + 93 pages. Laboratory (MIRL) Publication #91. University of Alaska at Fairbanks. 47 89. Randy Clarkson (1990). The use of nuclear tracers to evaluate the pages. $8.75 from: www.uaf.edu/sme/Mirlpub.html gold recovery efficiency of sluices. Canadian Institute of Mining and 111. Arthur Lashley (1995?). Building and Using an ASAT Elutriation Metallurgy (CIMM) Bulletin, April 1994, pages 29-37. tower - 95 minute video. American Society for Applied Technology (ASAT), 90. Randy Clarkson (1990). Placer Gold Recovery Research, Final Silver City, New Mexico. Summary. Report for Klondike Placer Miners Association. Northern Affairs Obtainable from: www.asat.volant.org/Publications.html Program, Whitehorse, Yukon, 49 pages. 112. Megan Rose (2004). Does Gold Separator Work? [Osterberg’s E- Download: www.geology.gov.yk.ca/publications/tech/placerrecovery.pdf tower]. Colorado Prospectors’ Forum. 91. Charles H. McDonald (editor) (1990). Fine Gold Recovery of Selected View: www.coloradoprospector.com/forums/index.php?showtopic=470 Sluice Box Configurations. Report of Contract YEDA-MR-001, pages 1-11. 113. B.S. Chan, Richard H. Mozley and G.J.C. Childs (1989). The Multi- Funded by the Canada/Yukon Economic Development Agreement (YEDA). Gravity Separator (MGS) - A mine scale machine. Proceedings of Download: www.geology.gov.yk.ca/publications Symposium on Mineral Processing in the United Kingdom, held at Leeds. 92. Michael T. Styles, J. Simpson and E.J. Steadman (2002). Good 114. B.S. Chan, Richard H. Mozley and G.J.C. Childs (1991). Extended Practice in the Design and Operation of Large Sluice Boxes. BGS trials with the high tonnage Multi-Gravity Separator. International Commissioned Report CR/02/029N prepared for DFID KAT Project R7120 Symposium on Gravity Separation Technology, Minerals Engineering, ‘Recovering the Gold of the Developing World’, published by the British volume 4, pages 489-496, Pergamon Press Geological Survey (BGS), 39 pages. 115. A. Traore, P. Conil, R. Houot and M. Save (1995). An evaluation of 93. Mort J. Richardson (1984). The evolution and current application of the Mozley MGS for fine particle gravity separation. Minerals Engineering, the MKII Cleaveland Circular Jig to alluvial gold recovery. World Dredging & volume 8, pages 767-778. Marine Construction, July 1984. 116. Anon (2002). Axsia Mozley Multi-Gravity Separation Systems. 4-page 94. Tjoe Hauw Nio (1985). Some aspects of gold recovery with IHC jigs. brochure. Download: www.axsia.com/PDFContent/Minerals/ IHC Holland NV. 16 pages + 11 pages of figures. Multi-Gravity-Separation-Systems.pdf 95. Tjoe Hauw Nio and Jack M. Donkers (1984). Mining and Recovery of 117. J.T. Waldram and P.G. Capps (1986). Concentration of fine grained Placer Minerals. Pages 36-40 in: Daniel E. Walsh and M. Susan Wray heavy minerals using the Kelsey centrifugal Jig. A world source of Ilmenite, th (editors and compilers) Proceedings of the 6 Annual Conference on Rutile, Monazite and Zircon. Australian Institute of Mining and Metallurgy, th Alaskan Placer Mining. Held 28-29 March 1984 in Fairbanks, Alaska. Perth, October 1986. On request: www.rochemt.com.au University of Alaska at Fairbanks, Mineral Industry Research Laboratory 118. David Geralty (no date). The Kelsey Centrifugal Jig - a new (MIRL) Report #69, 77 pages. $6.00 from: www.uaf.edu/sme/Mirlpub.html dimension to gold processing. 5 pages. On request: www.rochemt.com.au 96. Tjoe Hauw Nio and Henk van Muijen (1985). Notes on IHC jigs in 119. H. Wyslouzil (1990). Evaluation of the Kelsey Centrifugal Jig at Rio gravity concentration. IHC Holland NV. 12 pages. Kemptville Tin. Proceedings of 22nd Annual Meeting of Canadian Mineral 97. Tjoe Hauw Nio (1988). IHC Jigs in Gravity Concentration. Paper Processors, Ottawa, January. 1990, pages 461-472. presented at International Congress on ‘Dredge Mining Systems’ held 10- On request: www.rochemt.com.au 13th October 1988 at Baltimore, Ohio, USA. 41 pages. 120. T. Malvik, K.L. Sandvik and A. Rein (1997). Scandinavian 98. Willem Kramer (1984). Alluvial Washing Plants. IHC Offers Floating experiences with the Kelsey centrifugal jig. The Richard Mozley Memorial and Shore-based Units for Dredge Mining Projects. World Dredging and Symposium, June 1997 Falmouth, Cornwall, England. Marine Construction, July issue, pages 20-22. 121. Anon (2002). Kelsey Centrifugal Jig. Roche Mining. Mineral Available on request: www.ihcholland.com Technologies (MT). 3-page leaflet. On request: www.rochemt.com.au 99. Willem Kramer (1988). IHC Alluvial Mining Equipment for Efficient 122. Anon (2002). Kelsey Centrifugal Jig - Gold. Roche Mining. Mineral Dredging and Separation. Dredging and Port Construction, volume 25, 2 Technologies (MT). Application Sheet, 7 pages. pages. Available on request: www.ihcholland.com On request: www.rochemt.com.au 100. IHC Holland (1983). IHC floating and shore-based treatment plants. 123. Y. Sun (1982). Gravity separation. Metallurgy Press, Beijing, China, Symposium on Dredging and Mining Technology. November 1983, Chapter 310 pages [Chinese text]. M5. IHC Holland N.V. 8 pages + 19 pages of photos/figs. Available on request: www.ihcholland.com 124. Jinghong Ling (1998). A Study of a Variable Speed 3-inch Knelson Concentrator. PhD thesis, Mc.Gill University, Montreal, 244 pages. 101. IHC Holland (1983). Mineral dressing by IHC jigs. Symposium on Download: www.collectionscanada.ca/obj/s4/f2/dsk1/tape11 Dredging and Mining Technology. November 1983, Chapter M6. IHC /PQDD_0023/NQ50209.pdf Holland N.V. 32 pages + 5 pages diagrams and 11 pages photos. Available on request: www.ihcholland.com 125. Jie Xiao (1998). Testing a New Gold Centrifugal Concentrator. Master of Engineering Thesis, McGill University, Montreal, 131 pages. 102. IHC Holland (1991). IHC Jig Versus Other Gravity Separators. [Falcon Superbowl] Download: Brochure published by IHC Holland N.V. 6 pages, reprinted from: Ports and www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0029/MQ50677.pdf Dredging #128. Available on request: www.ihcholland.com 126. Y. Lu (1994). Injection-Flowing Film Centrifugation separation of fine 103. IHC Holland (1991). Gravity Separation for Gold Mining. Brochure and ultrafine refractory slimes. Metallurgical Industry Press, 1994. (Chinese). published by IHC Holland N.V. 6 pages. Available on request: www.ihcholland.com 127. Xiang Ren, Quan Li, Yumin Zhang and De Liu (1994). A new centrifugal separator for recovering minerals from fine and ultrafine sizes. 104. IHC Holland (1997). Alluvial Mining Equipment. Brochure published Pages 349-355 in: Turgut Yalcin (editor) Innovations in Mineral Processing, by IHC Holland N.V. 6 pages. Laurentian University in Sudbury, Ontario, 6-8th June 1994. 514 pages. Available on request: www.ihcholland.com 128. X. Fan (1991). Retrospect and Prospect of Gravity Separation. 105. Henk van Muijen (1993). Some Innovating Development in Gold Methods of Ore Dressing Abroad, 7/8 pages 21-26. Mining with IHC Equipment. 1993 ISGMT, pages 1-9 plus 21 figures. Available on request: www.ihcholland.com 129. Byron Knelson and Ron Edwards (1990). Development and Economic Application of Knelson Concentrators in Low Grade Alluvial Gold Deposits. 106. Gerrit Roelof Bazuin, Tsevel Delgertsoo and Robin Grayson (2001). AusIMM Annual Conference: ‘The Mineral Industry of New Zealand’, held at Placer Gold Recovery in Mongolia with IHC trapezoidal sawtooth jigs. World Roturua 18-21st March 1990, pages 123-128. Placer Journal, volume 2, pages 14-35. Download: www.knelsongravitysolutions.com Download: www.mine.mn

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130. Byron Knelson (1992). The Knelson Concentrator: metamorphosis 152. Steve McAlister and K.C. Armstrong (1988). Development of the from crude beginning to sophisticated worldwide acceptance. 'Minerals Falcon concentrators. Society for Mining, Metallurgy, and Exploration, Inc. Engineering '92' Conference in Vancouver, Canada, Camborne School of Annual Meeting, Orlando, 9-11th March 1998. Mines & Minerals Engineering Journal, 6 pages. Download: www.concentrators.net 131. A. Putz (1994). An investigation of the gravity recovery of gold. 153. Steve McAlister (1992). Case study in the use of the Falcon gravity M.Eng. thesis, McGill University, 230 pages. concentrator. Proceedings of the 24th Annual Meeting of the Canadian 132. Frederique Vincent (1997). A comparison of Knelson Concentrator Mineral Processors, Ottawa, 1992, page 20. and Jig peformance for gold recovery, Master of Engineering Thesis, McGill Download: www.concentrators.net University. Montreal. Download: 154. Andre R. Laplante, M. Buonvino, A. Veltmeyer, J. Robitaille and G. www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0032/MQ50673.pdf Naud (1994). A study of the Falcon concentrator. Canadian Metallurgical 133. Bo Zhang (1997). Recovering gold from high density gangues with Quarterly, volume 33, pages 279-288. Knelson Concentrators. Master of Engineering thesis, McGill University, 155. Colin H.A. Sprake (1998). Falcon fine recovery. Falcon Concentrators Montreal, 77 pages. Download: Inc. 2 pages. www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0032/MQ50680.pdf Download: www.concentrators.net/pdf/scriteria.pdf 134. Rickford Vieira (1997). The performance of the Knelson and Falcon 156. Brent Gee, Peter Holtham, Robert Dunne and Simon Gregory (2005). centrifuges for fine gold concentration. Master of Engineering Report, Recovery of fine gold particles using a Falcon ‘B’ separator. Julius University of British Columbia, July 1997. Kruttschnitt Mineral Research Centre, Queensland; and Newmont Australia. 135. Jinghong Ling (1998). A Study of a Variable Speed 3-inch Knelson Download: http://espace.library.uq.edu.au/eserv.php Concentrator. PhD thesis, Mc.Gill University, 244 pages. ?pid=UQ:9451&dsID=Holtham_Gee.pdf 136. Liming Huang (1996). Upgrading of Gold Gravity Concentrates, A 157. Fred Wilkinson (1987). Countercurrent Sluicing on Ketchem Creek. th Study of the Knelson Concentrator, PhD thesis McGill University, 229 pages. [Alaska]. Proceedings of the 9 Annual Alaska Conference on Placer Download: Mining, pages 229-233 in: Mary Albanese and Bruce Campbell (compilers) th http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ29960.pdf (1987). Placer Mining – Jobs for Alaska. Proceedings of the 9 Annual Alaskan Conference on Placer Mining, held 18-25th March 1987 in 137. Frederick C. Woodcock (1994). Use of a Knelson Unit to Quantify Fairbanks, Alaska. Miscellaneous Publication #9 of the Alaska Department Gravity Recoverable Gold in an Ore. Master of Engineering Thesis, McGill of Natural Resources, 315 pages. Download: www.dggs.dnr.state.ak.us/pubs/ University, Montreal, 128 pages. 158. C.W. Ammen (1984). Recovery and Refining of Precious Metals. Van 138. Cecil Urlich (1984). Recovery of Fine Gold by Knelson Hydrostatic Norstrand Reinhold, USA, 328 pages. [vanner belt] Cone and Compound Water Cyclone Technologies. 4th Annual RMS-ROSS Seminar on Placer Gold Mining, Vancouver, British Columbia. 159. Chris Rorres (2000). The turn of the screw: optimal design of an Archimedes Screw. Journal of Hydraulic Engineering, January 2000, pages 139. G. Caceres, Ph. Joly, D. Goffaux and J. Frenay (1996). Application of 72-80. Download: www.mcs.drexel.edu/~crorres/screw/screw.pdf the Knelson concentrator to small-scale mining in the Atacama region, Chile. In: M. Sanchez, F. Vergara and S. Castro (editors) Clean Technology 160. Jim Klein (2000?). 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Resume of High Capacity Gravity Separation Section C, volume 105, pages C126-C132. Equipment for Placer Gold Recovery. Report prepared for Teichert 142. Evandro Costa e Silva, Nilce Alvestdas dos Santos and Vanessa de Aggregates of Sacramento, California. Mineral Industry Research Macedo Torres (1999). Centrifugal Concentrators – a New Era in Gravity Laboratory (MIRL), University of Alaska at Fairbanks. Publication #AR-3, 20 Concentration – the Experience of the CVRD Research Center. 9 pages. pages. $3.00 from www.uaf.edu/sme/Mirlpub.html 143. Andre R. Laplante (2000). Testing Requirements and Insight for 164. C.N. Robinson and Thomas J. Ferree (1983). Fine gold recovery th Gravity Gold Circuit Design. Paper presented at the Randol Gold and Silver using Reichert technology. 5 Annual Conference on Alaskan Placer Mining, Forum, Vancouver 2000, 12 pages. Download: March 1983. Mineral Industry Research Laboratory (MIRL), University of http://knelsongravity.xplorex.com/sites/knelsongravity/files/reports/report20s.pdf Alaska at Fairbanks. Publication #68, 84 pages. $6.00 from: www.uaf.edu/sme/Mirlpub.html 144. Rickford Vieira (2000). Evaluation of a 7.5-inch Knelson Concentrator in the Geologically Defined Proto-Mahdia. Guyana Geology and Mines 165. D. Erik Spiller (1983). Gravity separation of gold – then and now. th Commission Report, September 2000. Paper presented at the National Western Mining Conference, held on 10 February 1983 in Denver, Colorado, 7 pages. 145. V.A. Bocharov, V.G. Urikov and V.V. Gurikov (2002). Analysis of gold-containing products separation processes in Knelson and Falcon SB 166. Kelly Dolphin (1984). Considerations for applying Reichert Mark VII th concentrators. Obogashchenie rud (Mineral Processing), volume 2002, #2. Spirals in Alaska. 6 Annual Conference on Alaskan Placer Mining. Details: http://rudmet.ru/en/ University of Alaska, Mineral Industry Research Laboratory (MIRL) Report #46, pages 29-33. Order from: www.uaf.edu/sme/Mirlpub.html 146. A.V. Bogdanovich and A.M. Vasilyev (2005). Study of operation of gravity separators designed to concentrate fine-grained materials. 167. Cristina R.A. Hamelmann and Fernando Antonio Freitas Lins (1997). Obogashchenie Rud (Mineral Processing), volume 2005 #1. A Non-polluting Technology to Recover Gold: The Gold-Paraffin-Process. th Details: http://rudmet.ru/en/ 20 International Mineral Processing Congress, Aachen 1997, volume 3, pages 659-668. 147. T. Coulter and G.K.N. Subasinghe (2005). A mechanistic approach to modelling Knelson concentrators. Minerals Engineering, volume 18, pages 168. D.W. Thompson and I.R. Collins (1992). Electrical properties of the 9-17. Gold Aqueous Solution Interface. Journal of Colloids and Interface Science, volume 152, pages 197-204. 148. Luis A. Meza, Willy Hartmann and Carlos A. 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Meech (2003). 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/dc/dcboard.php?az=show_topic&forum =181&topic_id=62871&mesg_id=62871&page=4

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