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JEE Main levelSingle correct optionDifficulty 2 of 3mt135

A mixture of $PbS$ and $ZnS$ is subjected to froth flotation. Sodium cyanide is added so that

A $ZnS$ is prevented from rising in the froth and $PbS$ alone floats
Bboth sulphides rise in the froth together
C $PbS$ is oxidised to $PbSO X_{4}$ and floats
D $NaCN$ acts as a collector for $ZnS$

Answer: A

NaCN

acts as a depressant: it forms a complex layer such as

Na X_{2} [Zn (CN) X_{4}]

on the zinc sulphide surface, which stops it from being carried up by the froth. This allows selective separation of

PbS

.

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