Question

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

• A. $Pb(CN{)}_2$ precipitated while no effect on $ZnS$
• B. $ZnS$ forms soluble complex $N{a}_{2}Zn(CN{)}_4$
• C. $PbS$ forms soluble complex $N{a}_{2}Pb(CN{)}_4$
• D. both $\left(a\right)$ and $\left(b\right)$

Answer 1

Answer: (B)

$ZnS$ forms soluble complex $N{a}_{2}Zn(CN{)}_4$

$NaCN$ prevents $ZnS$ from forming the froth by reacting with it to from complex $N{a}_2\left[Zn\right(CN{)}_4]$ and acts as a depressant. While it does not prevent $PbS$ from forming froth and both the ores can easily be separated.