As we know froth floatation method makes use of the wetting properties of mineral particles.

Mineral particles (sulphide) become wet with oils, while gangue particles get wet by water

According to my textbook (NCERT Class 12), we can use the froth-flotation method to separate a mixture of sulphide ores also using depressants.

How do these depressants actually work in this process?


closed as too broad by Gaurang Tandon, M.A.R., Tyberius, Mithoron, aventurin Mar 28 '18 at 15:29

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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, $$\ce{4NaCN + ZnS → Na2[Zn(CN)4] + Na2S}$$


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