What is the role of depressant in froth floatation process?

Explanation

The role of depessant is to prevent one type of sulphide ore particles from forming the froth with air bubbles ,NaCN is used as a depressant to seqartate lead sulphide (PbS) one from zinc sulphide (Zn s)Ore . NaCN forms a zinc complex `Na_(2)[Zn(CN)_(4)]` on the surface of ZnS thereby preventing it from the formation of froth .
`4NaCN+ZnSto underset("zincate"(II))underset("sodium tetracyano-")Na_(2)[zm(CN)_(4)]+Na_(2)S`
in this consition , only lead sulphide forms froth and thus can be separated from zinc sulphide ore.