What is the role of depressant in froth floatation process?

In the froth floatation method, the role of depressants is to prevent certain types of particles from forming the froth with the air bubbles. For example, NaCN is used as a depressant to separate lead sulfide ore (PbS)  from zinc Sulphide ore (ZnS).NaCN forms a zinc complex Na2[Zn(CN)4] on the surface of ZnS thereby preventing it from the formation of froth.

 4 NaCN + ZnS -----Na2[Zn(CN)4]