In froth floatation-process, the role of the depressant is to prevent one type of sulphide ore particles from forming the froth with air bubbles. For eg. NaCN is used as a depressant to separate lead sulphide (PbS) ore from zinc sulphide (ZnS) ore. The reason being that NaCN forms a zinc complex, Na2[Zn (CN)4] on the surface of ZnS thereby preventing from the formation of the froth. Under these conditions, only PbS forms froth and hence can be separated from ZnS ore.
4NaCN + ZnS → Na2 [Zn (CN)4] + Na2S
Sod. tetracy emozincate (II)
