Question

# Extraction of gold and silver involves leaching was $C{N^ - }$ ion. Silver is later recovered by:A.Displacement with $Zn$B.LiquationC.DistillationD.Zone refining

Hint: During the extraction of metal, it was made to react with a species with which it can form a water-soluble complex. The impurities formed can be easily isolated by filtration. Further, the use of a more electropositive species acts as a good reducing agent to free the metal from the complex.

The extraction of silver and gold is done by the cyanide process also known as Mac Arthur - Forrest process. During the extraction of silver and gold, the respective ore is leached with a dilute solution of $NaCN$ or $KCN$. For example considers $A{g_2}S$ is the ore of silver, and then the $NaCN$ reacts with $A{g_2}S$ to give a complex in presence of ${O_2}$ which is expressed as:
$A{g_2}S + 4NaCN\xrightarrow{{}}2Na[Ag{(CN)_2}] + N{a_2}S$.
$Na[Ag{(CN)_2}]$ is soluble in water and is treated with $Zn$. The zinc metal is more electropositive in nature and can easily reduce the silver from the complex generating metallic silver during the reaction.
$2Na[Ag{(CN)_2}] + Zn \to N{a_2}[Zn{(CN)_4}] + 2Ag \downarrow$
The precipitated silver can be removed by filtration. In a similar way, the gold is extracted by leaching with $C{N^ - }$ ion and then displacement with $Zn$.
So silver is later recovered by displacement with $Zn$ of the intermediate complex formed by leaching with $C{N^ - }$ ion.