Question
One of the processes used for concentration of
ores is $$Froth{\text{ }}floatation{\text{ }}process.$$ This process is
generally used for concentration of $$sulphide{\text{ }}ores.$$ Sometimes in this process we add $$NaCN$$ as a depressant. $$NaCN$$ is generally added in case of $$ZnS$$ and $$PbS$$ minerals. What is the purpose of addition of $$NaCN$$ during the process of Froth floatation ?
A.
$$NaCN$$ causes reduction by precipitation
B.
A soluble complex is formed by reaction between $$NaCN$$ and $$ZnS$$ while $$PbS$$ forms froth
C.
A soluble complex is formed by reaction between $$NaCN$$ and $$PbS$$ while $$ZnS$$ forms froth
D.
A precipitate of $$Pb{\left( {CN} \right)_2}$$ is produced while $$ZnS$$ remain unaffected.
Answer :
A soluble complex is formed by reaction between $$NaCN$$ and $$ZnS$$ while $$PbS$$ forms froth
Solution :
$$NaCN + ZnS \to \mathop {N{a_2}\left[ {Zn{{\left( {CN} \right)}_4}} \right]}\limits_{{\text{Soluble complex}}} $$
A layer of this zinc complex is formed on the surface of $$ZnS$$ and due to this $$ZnS$$ is prevented from the froth formation while $$PbS$$ form froth. ( i.e., $$NaCN$$ is added as depressant for $$ZnS$$ )