The depressants help in the separation of two sulphide ores. For example, in the case of an ore containing PbS (galena) and ZnS (Zinc blende), the depressant is sodium cyanide (NaCN). It prevents ZnS from coming to the froth but allows PbS to come with froth in the froth floatation process ZnS forms a soluble complex with NaCN.