A specific depressant, , is introduced into a flotation tank containing a mixture of Galena () and S — Metallurgy and Isolation of Elements Chemistry Question

💡 Solution & Explanation

$NaCN$ acts as a selective depressant for $ZnS$. It reacts with $ZnS$ to form the stable, water-soluble coordination complex sodium tetracyanozincate(II), $Na_2[Zn(CN)_4]$. Because it dissolves, it cannot float, allowing $PbS$ to be skimmed off in the froth.