Question

NaCN is sometimes added in the froth flotation process as a depressant when $ZnS$ and $PbS$ minerals are expected because:
(A) $ZnS$ forms soluble complex $N{a_2}[Zn{(CN)_4}]$ while $PbS$ from froth
(B) $Pb{(CN)_2}$ s is precipitated while no effect on $ZnS$ from froth
(C) $PbS$ forms soluble complex $N{a_2}[Zn{(Pb)_4}]$ while $ZnS$ form froth
(D) $NaCN$ is never added in froth flotation process

Answer 1

In the Froth flotation process, depressants help to separate two sulphide ones by selective prevention of froth formation by one ore and allowing the other to come info froth. Sodium cyanide is added so that it can form complex with the unwanted sulphide so that wanted sulphide ore can be easily extracted.

Complete step by step solution:
Froth floatation mainly used for separation of sulphide ores.
Froth flotation is the process of separation of beneficial minerals from a mixture by creating froth on which minerals separate out.
A common depressant used in the froth flotation of lead sulphide is sodium cyanide when the ore containing $ZnS$ and $PbS$ is subjected to froth flotation process in the presence of $NaCN$ , then only $PbS$ forms the froth. This happens because $NaCN$ reacts with $ZnS$ to form $N{a_2}[Zn{(CN)_4}]$ .
The complete reaction is as follows-
$4NaCN + ZnS \to N{a_2}[Zn{(CN)_4}] + N{a_2}S$
The complex is formed which prevents ZnS from coming to froth. Therefore NaCN acts as a depressant which prevents ZnS in froth floatation reaction. Therefore, we conclude that ZnS and PbS can be separated by using depressant by adjusting the proportion of oil to water. Depressant used for this purpose is NaCN. It selectively prevents ZnS from coming to the froth.
Hence, the correct option is option (A).

Note:
An important criterion of separation of minerals by froth flotation method is that size of particles of the ores must be very small equivalent to powder form. There are many other ways for separation of minerals with their ore such as magnetic separation, gravity separation, electrostatic , chemical leaching etc.