Question

$NaCN$ is sometimes added in the froth flotation process as a depressant when $ZnS$ and $PbS$ are to be separated in the given sample because:

• A. $Pb(CN{)}_2$ is precipitated, no effect on $ZnS$
• B. $ZnS$ forms a soluble complex, $PbS$ goes up with the froth
• C. $NaCN$ reacts with $ZnS$ to form $N{a}_2\left[Zn\right(CN{)}_4]$
• D. $Zn(CN{)}_2$ is precipitated, no effect on $PbS$

Answer 1

Answer: (B), (C)

$NaCN$ reacts with $ZnS$ to form $N{a}_2\left[Zn\right(CN{)}_4]$

A depressant helps in separating two sulphide ores by selective prevention of froth formation by one of the ores allowing the other one to form froth.
ZnS forms a soluble complex $N{a}_2\left[Zn\right(CN{)}_4]$ with NaCN while PbS does not and goes up with the froth.
$4NaCN+ZnS\to N{a}_2\left[Zn\right(CN{)}_4]+N{a}_{2}S$