In the froth floatation process, the role of the depressants is to separate two sulphide ores by selectively preventing one ore from forming froth.

For example, to separate two sulphide ores, $\mathrm{PbS}$ and $\mathrm{ZnS},\mathrm{NaCN}$ is utilised. $\mathrm{NaCN}$ forms a combination with zinc on the surface of zinc sulphide, ${\mathrm{Na}}_2\left[\mathrm{Zn}{\left(\mathrm{CN}\right)}_4\right]$, preventing $\mathrm{ZnS}$ from foaming selectively.

$4\mathrm{NaCN} + \mathrm{ZnS}\Rightarrow {\mathrm{Na}}_2\left[\mathrm{Zn}{\left(\mathrm{CN}\right)}_4\right]+{\mathrm{Na}}_2\mathrm{S}$