In froth-floatation process, depressants help to separate two sulphide ores by selective prevention of froth formation by one ore and allowing the other to come into froth.
For example, in order to separate two sulphide ores $(ZnS$ and $PbS), NaCN$ is used as a depressant. It selectively allows $PbS$ to form the froth, but prevents $Zn$ S from coming to froth. This happens due to the fact that $NaCN$ reacts with $ZnS$ to form $Na _{2}\left[ Zn ( CN )_{4}\right]$
$4 NaCN + ZnS \longrightarrow Na _{2}\left[ Zn ( CN )_{4}\right]+ Na _{2} S $