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Fine Gold Recovery – Alternatives to Mercury and Cyanide

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Fine Gold Recovery – Alternatives to Mercury and Cyanide 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.
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