Give the depressing agents used in the froth flotation process and why we use depressing agents in that process?

Explanation

Generally sodium cyanide and sodium carbonate are used as an depressing agents for forth floatation process.These reagents are used to selectively prevent other metal sulphides from coming to the forth.For example,When impurities such as ZnS is present in galena (PbS),sodium cyanide(NaCN) is added to depresses the flotation property of ZnS by forming a layer of zinc complex `Na_(2)[Zn(CN)_(4)]` on the surface of zinc sulphide