What happens when $NaCN$ is added in ore containing $PbS$ and $ZnS$ during concentration by froth floatation method?

Hint: the answer to this question includes the fact that $NaCN$ is used as a depressant which allows $PbS$ coming to froth and therefore the answer lies in the product of this reaction.

Complete Solution :
We have studied about the concepts of chemistry which includes separation techniques.
One among them is the froth floatation process which is the method used to separate hydrophobic materials from the hydrophilic materials.
- Froth floatation is used in many applications. It is mainly used for the processing of minerals, recycling of paper and also for the waste – water treatment in industries.
- It was the first method to be used in the mining industry in the history of the 20th century.
- Froth floatation method involves the following steps. The ore is first reduced to fine particles by crushing and also grinding which makes the minerals to exist as separate grains physically. This method is also called liberation. The crushing and grinding method is called communition.
- For froth floatation, the ground ore is mixed with water that forms slurry and the material required is rendered hydrophobic by adding surfactants.
Therefore on adding $NaCN$ which acts as surfactant or depressant to the ore containing $PbS$ and $ZnS$ where only $PbS$ forms froth because sodium cyanide reacts with $ZnS$ to give $N{{a}_{2}}[Zn{{(CN)}_{4}}]$

Thus the reaction is as given below,
\[4NaCN+ZnS\to N{{a}_{2}}[Zn{{(CN)}_{4}}]+N{{a}_{2}}S\]
Therefore, this is the correct answer.

Note: Note that only sulphide ore can be concentrated by froth floatation method because pine oil selectively wets the sulphide ore and brings it to the froth. Sometimes lime is used to maintain slurry alkalinity.