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In the cyanide process for extraction of gold and silver from ores, the cyanide solution acts as a
reducing agent to reduce the gold and silver compounds present in the ores into the metallic states
leaching agent to bring gold and silver into solution as cyanide complexes and thus separate these metal from impurities
leaching agent to dissolve all other constituents of ores leaving gold and silver as metals
leaching agent to bring gangue completely into solution
Solution
Silver and gold are extracted by the cyanide process (Mac Arthur – Forrest process). After the preliminary crushing and concentration by froth floatation process, the ore (crushed auriferous rocks in the case of gold) is leached with dilute ($0.4 – 7\, \%)$ solution of sodium cyanide made alkaline by adding lime kept agitated by a current of air. Silver (or gold) pass into solution as argentocyanide (or aurocyanide).
It is based on the fact that gold/silver dissolves in dilute solution of sodium cyanide in presence of atmospheric oxygen with the formation of complex cyanide.
Reaction:
$Ag _2 S +4 NaCN \rightarrow 2 Na \left[ Ag ( CN )_2\right]+ Na _2 S$
Silver is obtained by reaction with $Zn$ :
$2 Na \left[ Ag ( CN )_2\right]+ Zn \rightarrow Na _2\left[ Zn ( CN )_4\right]+2 Ag$
