Extraction of gold and silver involves leaching the metal with CN^- ion. The metal is recovered by

Extraction of gold and silver involves leaching the metal with CN^- ion. The metal is recovered by displacement of metal by some other metal from the complex ion. This is an oxidation reaction.
$\begin{array}{r}4\mathrm{Au}\left(\mathrm{s}\right)+8\mathrm{CN}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)⟶\\ 4{\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]}^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\end{array}$
$4{\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]}^{-}\left(\mathrm{aq}\right)+\mathrm{Zn}\left(\mathrm{s}\right)⟶$$2\mathrm{Au}\left(\mathrm{s}\right)+{\left[\mathrm{Zn}(\mathrm{CN}{)}_4\right]}^{2-}\left(\mathrm{aq}\right)$
Here, Zn acts as a reducing agent.