Question

In the Parke’s process of Ag-extraction, Zn is removed from (Zn-Ag) alloy by using:
A) Cupellation
B) Electrolytic refining
C) Distillation
D) Fractional Crystallization

Answer 1

The desilverization of argentiferous lead (PbS + 0.01 to 1 % Ag) is Parke's process. This is based on the distribution principle. The two-phase system is formed from Molten zinc and molten lead. Silver is soluble in the molten zinc than in the molten lead.

Complete Solution :
-Discussing option, A, Cupellation is said to be the refining process in metallurgy where ores or alloyed metals are treated under very high temperatures and have controlled operations to separate noble metals, like gold and silver, from base metals, like lead, copper, zinc, arsenic, antimony, or bismuth, present in the ore.
-The process is based on the principle that precious metals do not oxidize or react chemically, unlike the base metals, so when they are heated at high temperatures, the precious metals remain apart, and the others react, forming slags or other compounds.
$Ag(s) + 2Pb(s) + {O_2} \to 2PbO(absorbed) + Ag(I)$

-Discussing option B, In the electrolytic refining of Zn, the following reaction takes place, i.e.
${\text{Anode}} \to {\text{impure zinc}}$
${\text{Cathode}} \to {\text{strip of pure zinc}}$
${\text{Electrolyte}} \to {\text{acidified zinc sulphate solution}}$

At anode:
${\text{Zn}} \to {\text{Z}}{{\text{n}}^{2 + }} + 2{e^ - }$ (impure zinc)
At cathode:
${\text{Z}}{{\text{n}}^{2 + }} + 2{e^ - } \to {\text{Zn}}$ (pure zinc)

- Considering option C, The two-phase system of Molten zinc and molten lead. We can say that Silver is more soluble in molten zinc than the molten lead. The Zinc - silver alloy is found to be lighter than lead and freezes fast. The solidified Zinc - Silver alloy can be removed from molten lead. Zinc can be separated by distillation from Zinc - Silver alloy. As zinc is volatile at ${\text{920}}^\circ {\text{C}}$. Hence on heating zinc goes into vapor phase and Silver remains at the bottom of the vessel.

-Discussing option D, the method of refining substances based on differences in their solubility id called Fractional crystallization. An example is separating ${\text{KN}}{{\text{O}}_3}$ and ${\text{KCl}}{{\text{O}}_3}$.
So, the correct answer is “Option C”.

Note: In the Parkes process, the silver and gold are absorbed in an alloy of zinc. As Silver is more miscible in molten zinc than in molten lead. The Zinc metal is added because it is immiscible in lead.