Question

$NaCN$ is sometimes added in the froth flotation process as a depressant when $ZnS$ and $PbS$ minerals are expected because ______.

• A. $Pb(CN{)}_2$ is precipitated while no effect on $ZnS$.
• B. $ZnS$ forms soluble complex $N{a}_2\left[Zn\right(CN{)}_4]$ while $PbS$ forms froth
• C. $PbS$ forms soluble complex $N{a}_2\left[Pb\right(CN{)}_4]$ while $ZnS$ forms froth
• D. $NaCN$ is never added in froth floatation process

Answer 1

The correct option is B $ZnS$ forms soluble complex $N{a}_2\left[Zn\right(CN{)}_4]$ while $PbS$ forms froth

Both $ZnS$ and $PbS$ forms froth with collector in the froth flotation process.
Addition of $NaCN$ (depressant), selectively prevents $ZnS$ from coming to the froth by forming a soluble complex $N{a}_2\left[Zn\right(CN{)}_4]$ but allows $PbS$ to come with the froth. So, option (b) is the correct option.