Question

In the cyanide process for extraction of gold and silver from ores, the cyanide solution acts as a :

• A. Reducing agent to reduce the gold and silver compounds present in the ores into the metallic states
• B. Leaching agent to bring the gold and silver into solution as cyanide complexes and thus separate these metals from the ores
• C. Leaching agent to dissolve all the other constituents of the ores leaving the gold and silver as metals
• D. Leaching agent to bring the ores into solution

Answer 1

The correct option is B Leaching agent to bring the gold and silver into solution as cyanide complexes and thus separate these metals from the ores

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:

$A{g}_{2}S+4NaCN\to 2Na\left[Ag\right(CN{)}_2]+N{a}_{2}S$

Silver is obtained by reaction with Zn:

$2Na\left[Ag\right(CN{)}_2]+Zn\to N{a}_2[Zn\left(CN{)}_4\right]+2Ag$