What is the role of depressant in froth floatation process?

Explanation

The depressants are used to prevent certain types of particles from forming the froth with bubbles in froth floatation process. This helps to separate two sulphide ores. For example , in case of an ore containing zinc sulphide (ZnS) and lead sulphide (PbS), sodium cyanide (NaCN) is used as a depressant . It forms a layer of zinc complex `Na_2 [Zn (CN)_4]` with ZnS on the surface of ZnS and therefore, prevents it from forming the froth. Therefore, it acts as a depressant.
`ZnS + 4NaCN to underset("Sodium tetracyanidozincate (II)")(Na_2[Zn(CN)_4])+ Na_2 S`
However, NaCN does not prevent PbS from forming the froth adn allows it to come with the froth.