Question

When ZnS and PbS minerals are present together, then NaCN is added to separate them in the froth floatation 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 (a) and (b)

Answer 1

NaCN prevents ZnS from forming the froth by reacting with it to form complex $N{a_2}[Zn{(CN)_4}]$ and behaves as a depressant. But it does not prevent PbS from forming Froth and both the ores can easily be separated.

Complete step by step process:
Froth floatation is a process based on selectivity of material where material is separated as they are either hydrophobic or hydrophilic.
NaCN when added to ZnS or PbS, start acting as depressant because NaCN reacts with ZnS and forms a soluble complex with ZnS, this is $N{a_2}[Zn{(CN)_4}]$ and thus prevent ZnS from forming froth whereas NaCN doesn't react with PbS and doesn't form a complex. So, it doesn't prevent PbS from forming froth. This way PbS ore is left behind and can be separated easily and can be extracted and afterwards ZnS can also be separated from the complex, $N{a_2}[Zn{(CN)_4}]$ . Thus, NaCN only reacts with ZnS and not PbS so PbS is left out. NaCN used to stop ZnCN in forming froth whereas it allows PbS to form froth.
Therefore, when ZnS and PbS minerals are present together, then NaCN is added to separate them in froth flotation process as a depressant, because ZnS forms soluble complex $N{a_2}[Zn{(CN)_4}]$ .

When PbS reacts with NaCN there is no reaction whereas when ZnS reacts with NaCN, a soluble complex is formed, $N{a_2}[Zn{(CN)_4}]$
.
$4NaCN + ZnS \to N{a_2}[Zn{(CN)_4}] + N{a_2}S$

Therefore, the correct answer is option (B).

Note: Froth floatation is a technique which is commonly used in the mining industry. Ores containing multiple metals such as copper, lead, zinc can be extracted by this process.