What is the role of depressant in froth-floatation process?

Explanation

In forth - floatation process, depressant plays an important role in preventing the frothing of one type of sulphide ore particles. For example, to separate lead sulphide (PbS) ore from zinc sulphide (ZnS) ore, NaCN is used as a depressant. Here, NaCN forms a zinc complex, `Na_(2)[Zn(CN)_(4)]` on the surface of ZnS and prevents it from forming the froth, while PbS forms the froth and is separated from ZnS ore.
`4NaCN+ZnS rarr underset("Sodium tetracyanozincate (II)")(Na_(2)[Zn(CN)_(4)]+Na_(2)S)`