How can we separate a mixture of two sulphide ores using froth floatation method?
In the froth floatation process, the role of the depressants is to separate two sulphide ores. They do so by  preventing one ore from forming froth. 

In this process, we want some substances to settle down and some to float. Now ideally, we want the sulphide of the substance that we wish to extract tonfloat and the gangue to sink. However, sometimes the ore we are using also has other sulphides present in it as impurities. For example, lead sulphide generally has zinc sulphide also present as impurity which will also float along with the lead sulphide. To make sure that lead sulphide floats but zinc sulphide do not float, we add depressants.
Commonly used depressant is $\ce{NaCN}$.
To separate two sulphide ores ($\ce{ZnS and Pbs}$), $\ce{NaCN}$ selectively allows $\ce{PbS}$ to come with froth, but prevents ZnS. This happens because of the reaction of $\ce{NaCN}$ with $\ce{ZnS}$ to form $\ce{Na2[Zn(CN)4]}$.
The complete reaction is,
$$\ce{4NaCN + ZnS  →  Na2[Zn(CN)4] + Na2S}$$