NaCN is sometimes added in the froth flotation process as a depressant when mineral contains ZnS and PbS because,

Pb(CN)₂ is precipitated while there is no effect of ZnS.

ZnS forms soluble complex ${\text{Na}}_{\text{2}}{\text{[Zn(CN)}}_{\text{4}}\text{]}$ while PbS forms froth.

PbS forms soluble complex ${\text{Na}}_{\text{2}}{\text{[Pb(CN)}}_{\text{4}}\text{] }$ while ZnS forms froth.

silicious impurities settle down on the bottom