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The Cyanide Process is a mining technique for extracting gold from low-grade ore via the use of cyanide compounds. It is at present the most important and most-often used process for this purpose. Due to the highly toxic nature of cyanide, the process is controversial.
History
The process traces to 1887, though the original discovery was made in 1783 by the chemist Carl Wilhelm Scheele, the discoverer of cyanide.
The modern cyanide process was developed in Glasgow, Scotland in 1887 by J. S. MacArthur and others.
The reaction
The chemical reaction is called the Elsner Reaction; its stoichiometry is:
- 4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH
The process and its uses
The ore is ground to a fine powder in a revolving cylinder with steel balls, and may be further concentrated by flotation, given the presence of certain impurities. It is combined with a dilute solution of sodium cyanide or potassium cyanide while bubbling air through it, forming a solution called "slime".
The negatively charged cyanide anions release the gold cations from the ore as a metal complex. The gold oxidizes to form the soluble aurocyanide metallic complex, NaAu(CN)2.
The solution is separated from the ore by methods such as filtration, and then the gold is displaced by adding zinc dust, which precipitates the gold: zinc has a higher affinity for the cyanide ion than gold.
Silver may also precipitate, and unreacted zinc. The precipitate is further refined, e.g., by smelting, to remove the zinc and by treating with nitric acid to dissolve the silver.
Mines formerly thought played out may be revived via the cyanide process.
Controversy
The process is controversial, due to the highly toxic nature of cyanide; see also Summitville mine, the worst environmental mining disaster in United States history, which rendered 17 miles of a Colorado river devoid of life. The American state of Montana and several countries have banned cyanide mining.
External links and references
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