For $\mathrm{ZnS},\mathrm{KCN}$ is used as depressant.

In the metallurgy of silver and gold, the respective metal is leached with a dilute solution of $\mathrm{NaCN}$ or $\mathrm{KCN}$ in the presence of air, which supplies ${\mathrm{O}}_2$. The metal is obtained later by replacement reaction.

$4\mathrm{M}\left(\mathrm{s}\right)+8{\mathrm{CN}}^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4{\left[\mathrm{M}{\left(\mathrm{CN}\right)}_2\right]}^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \left[\mathrm{M}=\mathrm{Ag} \mathrm{or} \mathrm{Au}\right]$

$2{\left[\mathrm{M}{\left(\mathrm{CN}\right)}_2\right]}^{-}\left(\mathrm{aq}\right)+\mathrm{Zn}\left(\mathrm{s}\right)\to {\left[\mathrm{Zn}{\left(\mathrm{CN}\right)}_4\right]}^{2-}\left(\mathrm{aq}\right)+2\mathrm{M}\left(\mathrm{s}\right)$

Copper metal is not involved in this process.