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.