In froth flotation, the job of a depressant is to separate two sulphide ores by selectively preventing one ore from foaming. For example, to separate two sulphide ores, PbS and ZnS, NaCN is utilised. NaCN forms a combination with zinc on the surface of zinc sulphide, Na₂ [Zn( CN )₄], preventing ZnS from foaming selectively.

4NaCN + ZnS ⇒ Na₂ [Zn( CN )₄] + Na₂S