Question

Sodium cyanide is added as a depressant in the froth floatation process when the ore contains a mixture of ${ZnS}$ and ${PbS}$. This is because

• A. $\ce{Pb(CN)_2}$ gets precipitated without any effect on $\ce{ZnS}$
• B. $\ce{ZnS}$ forms soluble complex while $\ce{PbS}$ forms froth
• C. $\ce{PbS}$ forms soluble complex while $\ce{ZnS}$ forms froth
• D. $\ce{Zn(CN)_2}$ gets precipitated without any effect on $\ce{PbS}$.

Answer 1

Answer: (B)

$\ce{ZnS}$ forms soluble complex while $\ce{PbS}$ forms froth

Answer 2

${NaCN}$ forms a soluble complex with ${ZnS}$ thus, it selectively prevents ${ZnS}$ from coming to the froth.but allows ${PbS}$ to come with the froth.
{4NaCN + ZnS ->\underset{\text{ (Soluble complex). }}{ {Na_2 [Zn(CN)_4 ] }}+ Na_2 S }