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

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$Pb (CN)_{2}$ is precipitated while no effect on ZnS

ZnS forms soluble complex ${Na}_{2} [Zn (CN)_{4}]$

PbS forms soluble complex ${Na}_{2} [Pb (CN)_{4}]$

they cannot be separated by adding NaCN

answer is B.

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Detailed Solution

When ZnS and PbS minerals are present together then HaCN is added to separate them in the froth floatation process as a depressant, because ZnS forms a soluble complex ${Na}_{2} [Zn (CN)_{4}]$ .

By interacting with ZnS to create complex ${Na}_{2} [Zn (CN)_{4}]$ , NaCN inhibits ZnS from forming the froth and functions as a depressant. While both ores may be readily separated, it does not stop PbS from creating froth.

Hence, option B is correct.

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