When ZnS and PbS minearls are present together, then NaCN is added to separate them in the froth floatation process as a depressant, because :

Options

A. `Pb(CN)_(2)` is precipitated while no effect on ZnS
B. ZnS forms soluble complex `Na_(2)[Zn(CN)_(4)]` while PbS forms froth
C. PbS forms soluble complex `Na_(2)[Pb(CN)_(4)]` while ZnS forms froth
D. NaCN is never added in froth floatation process

Answer

B

Explanation

Depressant is a substance that is added to suppress the floating characteristic of metal sulphide present as an impurity.
`PbS+underset(("Impurity"))(ZnS) overset(4NaCH)to underset("Soluble complex")(Na_(2)[Zn(CN)_(4)])+PbS`