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Cyanide Leaching in the Mining Industries

Cyanide leaching has been used by the mining industries for more than 100 years to extract noble metals. Minerals from which most of the noble metals are obtained are increasingly poorer, so the leaching solution from these processes contain substantial quantities of silver and copper along with gold. In most cases the copper ion concentration is over 100 times higher than the other two metals.The dissolution of gold in cyanide solution is essentially an electrochemical process. The overall reaction is accompanied by the cathodic reduction of oxygen at the surface of the gold particle.

The remarkable success of cyanide as lixiviant for gold is due to the enormous stability of the dicyanoaurate ion. This allows gold to be leached efficiently at a very low concentration of cyanide (<0.01 mole dm-3), and the dicyanoaurate complex remains in alkaline solution even when the free cyanide concentration falls to zero. This contributes to the selectivity of the process.

Another advantage of cyanide is that the reaction takes place in an alkaline environment. The dissolution of gold requires cyanide to be present as the free cyanide ion (CN–). Therefore, the composition of cyanide solution is determined by the hydrolysis reaction.

Hence, it is very obvious that the high alkalinity suppresses the forward reaction, leading to higher cyanide ion concentration in the system. A high pH is also necessary for safety and economic reasons, as HCN is a volatile and poisonous gas that is purged from the leach slurry during vigorous air agitation. An electrochemical study by Dorin and Woods4 has shown a maximum dissolution of gold, silver,palladium, and platinum in cyanide solution at pH 10–10.5.Cyanide loss is very low at pH 11.5, as it mostly eliminates the loss of cyanide through HCN formation. It is 0.1 kg/t NaCN in the 10–10.5 pH range as compared to 2–3 kg at 9.5 pH.4,5 An important advantage of leaching under alkaline conditions is that the dissolution of base metals is substantially reduced, resulting in clearer effluents than those generally produced in an acidic leaching system.The noncorrosive nature of alkaline cyanide solution also means that cheaper materials can be used for the construction of a gold recovery plant.