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 in the ores into metallic states.
(B) Leaching agent to bring the gold and silver into the 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

Hint: The cyanide solution acts as a leaching agent in this extraction process. A coordinate compound is formed that helps in the overall reaction.

Complete step-by-step reaction:
Extraction of metals is done by carrying out a series of steps. At each stage the metal is gradually more of itself than the impurities it once surrounded itself with. The first process in this series is known as “concentration of ores”. The impurities are known as “gangue” and are removed by a variety of methods which greatly depend on the type of ore, the place where it is found and the cost of its overall production.
In case of metals like gold and silver, a process called leaching is used to concentrate the metals. This process is only applicable to ores which are soluble in some kind of solution. In case of gold and silver a cyanide solution is used. It is a dilute sodium cyanide solution ($NaCN$) for silver and dilute potassium cyanide solution ($KCN$) for gold. The metals are leached away in the form of coordinate compounds in the presence of oxygen. Later the metals are obtained by a simple displacement reaction using zinc metal. The equations of the above processes are as below:
- Leaching of the metal
$4M(s)+8C{{N}^{-}}(aq)+2{{H}_{2}}O(aq)+{{O}_{2}}(g)\to 4{{[M{{(CN)}_{2}}]}^{-}}(aq)+4O{{H}^{-}}$
Here “M” is gold or silver.
- Getting pure metal through displacement reaction with zinc
$2{{[M{{(CN)}_{2}}]}^{-}}(aq)+Zn(s)\to {{[Zn{{(CN)}_{4}}]}^{2-}}(aq)+2M(s)$
Therefore the answer to the above question is option (B) Leaching agent to bring the gold and silver into the solution as cyanide complexes and thus separate these metals from the ores.

Note: The leaching process is only carried out when the ore is soluble in a particular solution and not the substances that make it impure. Many times some other substance is also able to dissolve in the leaching solution along with the ore but this is later separated.
The displacement reaction carried out in the above processes is only possible because zinc has a higher tendency to oxidise than gold or silver and therefore zinc acts as a reducing agent.