gold extraction

Gold occurs principally as a native metal, i.e., gold itself. Sometimes it is alloyed to a greater or lesser extent with silver, which is called electrum. Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles (known as colour) embedded in rock minerals. Other forms of gold are the minerals calaverite (AuTe), aurostibnite (AuSb₂), and maldonite (Au₂Bi). These latter three, although rarer that native gold, can be slow to react with cyanide and thus difficult to process.{{cite journal|doi=10.1016/0892-6875(94)90114-7|title=Review of gold extraction from ores |year=1994 |last1=La Brooy |first1=S.R. |last2=Linge |first2=H.G. |last3=Walker |first3=G.S. |journal=Minerals Engineering |volume=7 |issue=10 |pages=1213–1241 |bibcode=1994MiEng...7.1213L }} Still other gold-containing ores include various tellurides (sylvanite, nagyagite, petzite, and krennerite).

Certain contaminants in ores can interfere with the extractability of gold by cyanide. These interfering agents are called "preg-robbing ores". For example, gold can bind tightly to carbon, resisting normal cyanide extraction. Gold cyanides bind also to some clays.

File:Potassium dicyanoaurate.svg.]]

While the romantic picture of gold mining focuses on nuggets, the reality is that gold is typically recovered from ores containing >10 ppm of the metal. Thus, the main challenge is concentrating this trace amount.{{cite book |doi=10.1002/14356007.a12_499|chapter=Gold, Gold Alloys, and Gold Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2000 |last1=Renner |first1=Hermann |last2=Schlamp |first2=Günther |last3=Hollmann |first3=Dieter |last4=Lüschow |first4=Hans Martin |last5=Tews |first5=Peter |last6=Rothaut |first6=Josef |last7=Dermann |first7=Klaus |last8=Knödler |first8=Alfons |last9=Hecht |first9=Christian |last10=Schlott |first10=Martin |last11=Drieselmann |first11=Ralf |last12=Peter |first12=Catrin |last13=Schiele |first13=Rainer |isbn=3527306730 }}

The principal technology is the cyanide process, in which gold is leached from the ore by treatment with a solution of cyanide. The first step is comminution (grinding) to increase surface area and expose the gold to the extracting solution. The extraction is conducted by dump leaching or heap leaching processes. Sodium cyanide is produced on a billion-ton/year scale mainly for this purpose. "Black cyanide", a carbon-contaminated form of calcium cyanide (Ca(CN)₂) is often used because it is cheap. The crude ore is washed with a c. 0.3% solution of cyanide in air, often repeatedly, and the aqueous extract is collected and refined further. Recovery from solution typically involves adsorption on activated carbon, the carbon in pulp process.

Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience preg-robbing.

Leaching through bulk leach extractable gold, or BLEG, is also a process that is used to test an area for gold concentrations where gold may not be immediately visible.

Amalgamation with mercury can be used to recover very small gold particles, and mercury is still widely used in small-scale artisanal mining across the world.{{cite web|url=http://geology.com/publications/getting-gold/gold-extraction.shtml|title=Gold Extraction - Gold Mining - Washing, Amalgamation, Leaching, Smelting|publisher=geology.com|access-date=2008-03-20|archive-url=https://web.archive.org/web/20130816134805/http://geology.com/publications/getting-gold/gold-extraction.shtml|archive-date=2013-08-16|url-status=dead}} Mercury forms a mercury-gold amalgam with smaller gold particles, and then the gold is concentrated by boiling away the mercury from the amalgam. This is effective in extracting very small gold particles, but the process is hazardous due to the toxicity of mercury vapour. Large-scale use of mercury stopped in the 1960s. However, mercury is still used in artisanal and small-scale gold mining (ASGM).Feijoo, M. D. A., Walker, T. R. (2018). Correspondence to the Editor Re: Artisanal and small-scale gold mining impacts in Madre de Dios, Peru: Management and mitigation strategies. Environment International, 111, 133-134. doi:10.1016/j.envint.2017.11.029 One mechanism by which mercury is employed in hydraulic mining is as an "undercurrent", in which the flow of smaller grains is diverted over mercury-coated copper plates. High flow velocities associated with hydraulic mining cause flouring of mercury, the wearing down of mercury particles that contributes to mercury loss into the environment.{{cite web|url=https://pubs.usgs.gov/fs/2005/3014/|title=Mercury Contamination from Historical Gold Mining in California|last1=Alpers|first1=Charles A.|last2=Hunerlach|first2=Michael P.|last3=May|first3=Jason T.|last4=Hothem|first4=Roger L.|date=November 2005|access-date=1 February 2024|id=Fact Sheet 2005-3014 Version 1.1|publisher=U.S. Department of the Interior, U.S. Geological Survey}}

Over {{convert|10000000|lbs|kg}} of mercury contaminated the environment in California as a result of placer mining in the late nineteenth and early twentieth centuries. Stamp mill mining contributed an additional {{convert|3000000|lbs|kg}} of mercury contamination. Mercury contamination in California waterways is a major contemporary environmental issue, as is groundwater pollution, mostly by inorganic mercury.{{cite web|url=https://www.waterboards.ca.gov/gama/docs/coc_mercury.pdf|title=Groundwater Fact Sheet Mercury (Hg)|website=The California Water Boards|publisher=California Environmental Protection Agency, State of California|date=October 2017|access-date=1 February 2024}}

File:Gold-Quartz-273364.jpg. The appearance is typical of very good gold-quartz ore.]]

A "refractory" gold ore is an ore that has ultra-fine gold particles disseminated throughout its gold occluded minerals. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment before cyanidation can be used. A refractory ore generally contains sulphide minerals, organic carbon, or both. Sulphide minerals are impermeable minerals that occlude gold particles, making it difficult for the leach solution to form a complex with the gold. Organic carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon. This so-called "preg-robbing" carbon is washed away because it is significantly finer than the carbon recovery screens typically used to recover activated carbon.

Pre-treatment options for refractory ores include:

1. Roasting
2. Bio-oxidation, such as bacterial oxidation
3. Pressure oxidation
4. Albion process

The refractory ore treatment processes may be preceded by concentration (usually sulphide flotation). Roasting is used to oxidize both the sulphur and organic carbon at high temperatures using air and/or oxygen. Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation is an aqueous process for sulphur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. The Albion process utilises a combination of ultrafine grinding and atmospheric, auto-thermal, oxidative leaching.

{{Main | Gold parting}}

Parting is a process by which gold is purified to a commercially-tradeable standard, typically ≥99.5%. Removal of silver is of particular interest since the two metals often co-purify. The standard procedure is based on the Miller process. The separation is achieved by passing chlorine gas into a molten alloy. The technique is practiced on a large scale (e.g. 500 kg). The principle of the method exploits the nobility of gold, such that at high temperatures, gold does not react with chlorine, but virtually all contaminating metals do. Thus, at c. 500 °C, as the chlorine gas is passed through molten mixture (again, mainly gold), a low-density slag forms on top, which can be decanted from the liquid gold. Silver chloride and other precious metals can be recovered from this slag. The slag layer is often diluted with a flux like borax to facilitate the separation.

Alternative methods exist for parting gold. Silver can be dissolved selectively by boiling the mixture with 30% nitric acid, a process sometimes called inquartation. Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid.{{cite book |doi=10.1002/0471238961.1909122205201809.a01.pub3|chapter=Silver and Silver Alloys |title=Kirk-Othmer Encyclopedia of Chemical Technology |year=2010 |last1=Etris |first1=S. F. |pages=1–43 |isbn=978-0471238966 }} Electrolysis using the Wohlwill process is yet another approach.

File:Jsmacarthur-X-007r.jpg developed the cyanide process for gold extraction in 1887.]]

Image:Hydraulic mining in Dutch Flat, California, between 1857 and 1870.jpg mine in Dutch Flat, California sometime between 1857 and 1870.]]

The smelting of gold began sometime around 6000 – 3000 BC.Gold Field Mineral Services (GFMS)-(goldavenue.com) & I Podleska and T Green – [http://info.goldavenue.com/Info_site/in_arts/in_civ/in_civ_overview.html goldavenue] {{webarchive|url=https://web.archive.org/web/20020401114537/http://info.goldavenue.com/Info_site/in_arts/in_civ/in_civ_overview.html |date=2002-04-01 }} Retrieved 2012-07-01Carnegie Mellon University Dept. of Material Science and Engineering – {{cite web |url=http://neon.mems.cmu.edu/cramb/Processing/history.html |title=History of Metals |access-date=2008-09-12 |url-status=dead |archive-url=https://web.archive.org/web/20081211052028/http://neon.mems.cmu.edu/cramb/Processing/history.html |archive-date=2008-12-11 }} Retrieved 2012-07-01G Leick – [https://books.google.com/books?id=jhbSbuplQ28C&dq=history+of+gold+jewelry+Mesopotamia&pg=PA79 Historical Dictionary of Mesopotamia] {{Webarchive|url=https://web.archive.org/web/20230514024016/https://books.google.com/books?id=jhbSbuplQ28C&dq=history+of+gold+jewelry+Mesopotamia&pg=PA79 |date=2023-05-14 }} Scarecrow Press, 16 Dec 2009 Retrieved 2012-07-01 ISBN

0810863243 According to one source the technique began to be in use in Mesopotamia or Syria.R.J. Forbes – [https://books.google.com/books?id=9q83AAAAIAAJ&dq=gold+smelting&pg=PA169 Studies in Ancient Technology, Volume 1] {{Webarchive|url=https://web.archive.org/web/20230513170932/https://books.google.com/books?id=9q83AAAAIAAJ&dq=gold+smelting&pg=PA169 |date=2023-05-13 }} Brill Archive, 1993 Retrieved 2012-07-01 In ancient Greece, Heraclitus wrote on the subject.M Y Treister – [https://books.google.com/books?id=dcTexDa4I0kC&dq=early+history+of+gold+smelting&pg=PA172 The Role of Metals in Ancient Greek History] {{Webarchive|url=https://web.archive.org/web/20230513170928/https://books.google.com/books?id=dcTexDa4I0kC&dq=early+history+of+gold+smelting&pg=PA172 |date=2023-05-13 }} BRILL, 1996 Retrieved 2012-07-01 {{ISBN|9004104739}}

According to de Lecerda and Salomons (1997) mercury was first in use for extraction at about 1000 BC,B Lottermoser – [https://books.google.com/books?id=BzyGinS4CbAC&dq=mercury+used+Phoenicians+gold&pg=PA27 Mine Wastes: Characterization, Treatment and Environmental Impacts] {{Webarchive|url=https://web.archive.org/web/20230513170930/https://books.google.com/books?id=BzyGinS4CbAC&dq=mercury+used+Phoenicians+gold&pg=PA27 |date=2023-05-13 }} Springer, 2 Aug 2010 Retrieved 2012-07-22 {{ISBN|3642124186}} according to Meech and others (1998), mercury was used in obtaining gold until the latter period of the first millennia.R Eisler – [https://books.google.com/books?id=gqc8wUvsI8cC&dq=gold+extraction+amalgamation+history&pg=PA239 Mercury Hazards to Living Organisms] {{Webarchive|url=https://web.archive.org/web/20230513170931/https://books.google.com/books?id=gqc8wUvsI8cC&dq=gold+extraction+amalgamation+history&pg=PA239 |date=2023-05-13 }} CRC Press, 14 Mar 2006 Retrieved 2012-07-21 {{ISBN|0849392128}}(secondary)L Drude De Lacerda, W Salomons – [https://books.google.com/books?id=cb0AHTBBE7UC&dq=history+of+gold+extraction+amalgamation&pg=PA1 Mercury from Gold and Silver Mining: A Chemical Time Bomb?] {{Webarchive|url=https://web.archive.org/web/20230513170927/https://books.google.com/books?id=cb0AHTBBE7UC&dq=history+of+gold+extraction+amalgamation&pg=PA1 |date=2023-05-13 }} Springer, 1998 Retrieved 2012-07-21 {{ISBN|3540617248}}(secondary) Guido Küstel – [https://books.google.com/books?id=B39NAAAAYAAJ&dq=amalgamation+extraction+of+gold&pg=PA59 Nevada and California processes of silver and gold extraction] {{Webarchive|url=https://web.archive.org/web/20230513170930/https://books.google.com/books?id=B39NAAAAYAAJ&dq=amalgamation+extraction+of+gold&pg=PA59 |date=2023-05-13 }} F. D. Carlton, 1863 Retrieved 2012-07-21(secondary) A Tilloch – [https://books.google.com/books?id=Gx1RAAAAYAAJ&dq=extraction+of+gold&pg=PA26 Philosophical Magazine, Volume 52] {{Webarchive|url=https://web.archive.org/web/20230514024014/https://books.google.com/books?id=Gx1RAAAAYAAJ&dq=extraction+of+gold&pg=PA26 |date=2023-05-14 }} Retrieved 2012-07-21

A technique known to Pliny the Elder was extraction by way of crushing, washing, and then applying heat, with the resultant material powdered.JM Stillman – [https://books.google.com/books?id=hdaAGF5Y1N0C&dq=Pliny+gold+extraction+techniques&pg=PA56 Story of Alchemy and Early Chemistry]{{Dead link|date=June 2024 |bot=InternetArchiveBot |fix-attempted=yes }} Kessinger Publishing, 1 Jan 2003 Retrieved 2012-07-22(primary source "Pliny") S Venable – [https://books.google.com/books?id=TBGzEK4laJ0C&dq=ancient+gold+extraction&pg=PA202 Gold: A Cultural Encyclopedia] {{Webarchive|url=https://web.archive.org/web/20230514024011/https://books.google.com/books?id=TBGzEK4laJ0C&dq=ancient+gold+extraction&pg=PA202 |date=2023-05-14 }} ABC-CLIO, 2011 Retrieved 2012-07-22 {{ISBN|0313384312}}(secondary) M Page – [https://books.google.com/books?id=-IIEhFx0OfwC&dq=Pliny+gold+extraction+techniques&pg=PA43 The First Global Village: How Portugal Changed the World] {{Webarchive|url=https://web.archive.org/web/20230514024017/https://books.google.com/books?id=-IIEhFx0OfwC&dq=Pliny+gold+extraction+techniques&pg=PA43 |date=2023-05-14 }} Leya, 2006 Retrieved 2012-07-22 {{ISBN|9724613135}}

Like all metals, gold is insoluble in a water. Gold does however exhibit the distinctive properties that in the presence of cyanide ions, it dissolves in the presence of oxygen (or air). This transformation was reported in 1783 by Carl Wilhelm Scheele, but it was not until the late 19th century, that the reactions were exploited commercially. The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits being found tended to be pyritic ore. The gold was difficult to extract from such ores.

A process known as chlorination was once used to treat pyritic gold ore. Typically, the ore was roasted and then treated with chlorine gas. The residue was extracted to give an aqueous solution of gold chloride. It was used, notably at the Mount Morgan mine, where it remained in use until 1911. The chloride process became obsolete with the development of the cyanide process.{{Cite news |date=2 December 1891 |title=The Macarthur-Forrest Process |url=https://trove.nla.gov.au/newspaper/article/44074505 |work=Barrier Miner |pages=2}}{{Cite journal |last=McQueen |first=Ken |date=October 2012 |title=Early Developments in Treating Pyritic and Refractory Gold Ores in Australia |url=https://www.mininghistory.asn.au/wp-content/uploads/7.-McQueen.COMPLETED7.Vol_.10.compressed.pdf |journal=Journal of Australasian Mining History |volume=10 |pages=88–97 |via=mininghistory.asn.au}}

In 1887, John Stewart MacArthur, working in collaboration with brothers Dr Robert and Dr William Forrest for the Tennant Company in Glasgow, Scotland, developed the MacArthur-Forrest Process for the extraction of gold ores. By suspending the crushed ore in a cyanide solution, up to 96 percent gold was extracted.{{cite news|url=http://suertegold.wordpress.com/in-the-beginning-there-was-gold-1/day-52/|title=Methods to recover Gold II|newspaper=Rio Suerte Gold|date=14 May 2013|access-date=24 July 2014|archive-date=16 November 2020|archive-url=https://web.archive.org/web/20201116012643/https://suertegold.wordpress.com/in-the-beginning-there-was-gold-1/day-52/|url-status=live}}L Gmelin -[https://books.google.com/books?id=j_A4AAAAMAAJ&dq=Bagration+gold&pg=PA38 Hand book of chemistry, Volume 8] {{Webarchive|url=https://web.archive.org/web/20230513170932/https://books.google.com/books?id=j_A4AAAAMAAJ&dq=Bagration+gold&pg=PA38 |date=2023-05-13 }} Printed for the Cavendish Society, 1853 Retrieved 2012-07-21{{cite book|url=https://books.google.com/books?id=kgGAVmMBQoEC&q=Scheele+gold+cyanide&pg=PA18|author=RG Bautista (TB Drew)|title=Advances in Chemical Engineering, Volume 9|date=10 April 1974|publisher=Academic Press, 1974|access-date=2012-07-21|isbn=0120085097|archive-date=2023-05-13|archive-url=https://web.archive.org/web/20230513170933/https://books.google.com/books?id=kgGAVmMBQoEC&q=Scheele+gold+cyanide&pg=PA18|url-status=live}}WH Brock - [https://books.google.com/books?id=Dd75mKOE4BMC&dq=MacArthur-Forrest+process&pg=PA414 William Crookes (1832-1919) and the Commercialization of Science] {{Webarchive|url=https://web.archive.org/web/20230513170931/https://books.google.com/books?id=Dd75mKOE4BMC&dq=MacArthur-Forrest+process&pg=PA414 |date=2023-05-13 }} Ashgate Publishing, Ltd., 2008 Retrieved 2012-07-17 {{ISBN|0754663221}}R Eisler – [https://books.google.com/books?id=B3qXQswP8zIC&dq=heap+leaching+gold+extraction+history&pg=PA353 Eisler's Encyclopedia of Environmentally Hazardous Priority Chemicals] {{Webarchive|url=https://web.archive.org/web/20230514024009/https://books.google.com/books?id=B3qXQswP8zIC&dq=heap+leaching+gold+extraction+history&pg=PA353|date=2023-05-14}} Elsevier, 8 Aug 2007 Retrieved 2012-07-17 {{ISBN|044453105X}}J Park – [https://books.google.com/books?id=vyYFAAAAMAAJ&dq=gold+extraction+history&pg=PA1 The Cyanide Process of Gold Extraction] {{Webarchive|url=https://web.archive.org/web/20230514024013/https://books.google.com/books?id=vyYFAAAAMAAJ&dq=gold+extraction+history&pg=PA1 |date=2023-05-14 }} C. Griffin, limited, 1820 Retrieved 2012-07-17{{cite book |url=https://books.google.com/books?id=OuoV-o_Xf-EC&q=gold+history&pg=PA1 |author=J Marsden, I House |title=The Chemistry of Gold Extraction |year=2006 |publisher=SME, 5 Jun 2006 |access-date=2012-07-17 |isbn=0873352408 |archive-date=2023-05-13 |archive-url=https://web.archive.org/web/20230513170929/https://books.google.com/books?id=OuoV-o_Xf-EC&q=gold+history&pg=PA1 |url-status=live }}

The process was first used on a large scale at the Witwatersrand in 1890, leading to a boom of investment as larger gold mines were opened up. In 1896, Bodländer confirmed that oxygen was necessary for the process, something that had been doubted by MacArthur, and discovered that hydrogen peroxide was formed as an intermediate.Habashi, Fathi [http://www.ucv.ve/cifi/16%5CArticuloh.htm Recent Advances in Gold Metallurgy] {{webarchive|url=https://web.archive.org/web/20080330234816/http://www.ucv.ve/cifi/16%5CArticuloh.htm |date=2008-03-30 }}

The method known as heap leaching was first proposed in 1969 by the United States Bureau of Mines,R Eisler (sourced originally in Marsden and House) and was in use by the 1970s.GW Ware – [https://books.google.com/books?id=VYjCNFvMW3gC&dq=heap+leaching+gold+extraction+history&pg=PA21 Reviews of Environmental Contamination and Toxicology] {{Webarchive|url=https://web.archive.org/web/20230513170929/https://books.google.com/books?id=VYjCNFvMW3gC&dq=heap+leaching+gold+extraction+history&pg=PA21 |date=2023-05-13 }} Springer, 15 Jul 2004 Retrieved 2012-07-17 {{ISBN|0387208445}}

• Digger gold
• Ore genesis

{{reflist}}

Category:Gold

Category:Metallurgical processes

de:Gold#Gewinnung