Gold extraction is the extraction of gold from dilute ores using a combination of chemical processes. Gold mining produces about 3,600 tons annually, and another 300 tons is produced from recycling. Since the 20th century, gold has been principally extracted in a cyanide process by leaching the ore with cyanide solution. The gold may then be further refined by gold parting, which removes other metals (principally silver) by blowing chlorine gas through the molten metal. Historically, small particles of gold were amalgamated with mercury, and then concentrated by boiling away the mercury. The mercury method is still used in some small operations.
Types of ore
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 than native gold, can be slow to react with cyanide and thus difficult to process. 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.
Concentration
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.
Cyanidation (and thiosulfate) 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.
Mercury amalgamation Amalgamation with mercury can be used to recover fine gold particles, and mercury continues to be used in artisanal and small-scale gold mining worldwide. Mercury forms a mercury-gold amalgam with smaller gold particles, and then the gold is concentrated by heating the amalgam to evaporate the mercury. 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). 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. Over 10,000,000 pounds (4,500,000 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 3,000,000 pounds (1,400,000 kg) of mercury contamination. Mercury contamination in California waterways is a major contemporary environmental issue, as is groundwater pollution, mostly by inorganic mercury.
Refractory gold processes
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:
Roasting Bio-oxidation, such as bacterial oxidation Pressure oxidation 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.
Gold refining and parting
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