An ore sample of galena (PbS) is contaminated with zinc blende (ZnS). Name one chemical which can be used to concentrate galena selectively by froth floatation process.

Explanation

Chemical used is sodium cyanide which acts a depressant and prevents ZnS from the formation of froth but does not prevent PbS from forming the froth. This is because NaCN forms a zinc complex, `Na_2 [Zn(CN)_4]` on the surface of ZnS and thereby prevents it from the formation of froth.