Question

Name the important ore of silver. Write all the steps and reactions involved in the Cyanide process for the extraction of silver from its ore.

Answer 1

The important ore of silver are Argentite (${Ag}_{2}S$) and Horn Silver $\left(AgCl\right)$.
Extraction of Silver from ore by cyanide process: In this process the finely powered ore is concentrated by froth flotation process and then treated with dilute $NaCN$ and a current of air is passed into the solution so that $Ag$ present in the ore is converted into soluble sodium argento cyanide complex.
${Ag}_{2}S+4NaCN\Rightarrow \underset{Sodiumargentocyanidecomplex}{2Na\left[Ag{\left(CN\right)}_2\right]}+{Na}_{2}S$
The current of air passed into the solution oxidises ${Na}_{2}S$ formed in the above reaction into ${Na}_2{S}_2{O}_3,{Na}_2{SO}_4$ and $S$ thus enables the above reversible reaction to proceed in forward direction.
$2{Na}_{2}S+2O_2+H_{2}O⟶{Na}_2{S}_2{O}_3+2NaOH$
$4{Na}_{2}S+5O_2+2H_{2}O⟶2{Na}_2{SO}_4+4NaOH+2S$
Then the solution containing $Na\left[Ag{\left(CN\right)}_2\right]$ is treated with $Zn$ dust so that $Ag$ gets precipitated.
$2Na\left[Ag{\left(CN\right)}_2\right]+Zndust⟶{Na}_2\left[Zn{\left(CN\right)}_4\right]+2\underset{ppt}{Ag\downarrow }$
Precipitated $Ag$ is purified by electrolytic method. The cathode is pure silver, anode is block of impure silver and electrolyte is $6$% $Ag{NO}_3$ with $1$% $H{NO}_3$. On passing current, impurities get deposited at the anode and pure silver at the cathode. $Zn$ and $Cu$ remain in solution while $Au$ settles down as anode mud.