Gold extraction
The first step is comminution (grinding) to increase surface area and expose the gold to the extracting solution.
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.
This is effective in extracting very small gold particles, but the process is hazardous due to the toxicity of mercury vapour.
[5] 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.
[6] 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.
These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes.
These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold.
The Albion process utilises a combination of ultrafine grinding and atmospheric, auto-thermal, oxidative leaching.
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.
[15][16][17][18] A technique known to Pliny the Elder was extraction by way of crushing, washing, and then applying heat, with the resultant material powdered.
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.
[24][25][26][27][28][29][30] 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.
