Outline the principles of refining metals by the following methods :
(i) Zone refining
(ii) Electrolytic refining
(iii) Vapour phase refining

Explanation

(i) Zone refining : This process is based upon the principle that impurities are more soluble in the molten state of the metal than in the solid form.
(ii) Electrolytic refining : It is used for less reactive metals like Cu, Ag, Al, etc. In it, anode is made by impure metal and a thin strip of pure metal acts as cathode.
On passing electricity, metal dissolves form anode and pure metal gets deposited at cathode.
At anod `MrarrM^(n+)+n e^(-)`
`{:("Impure"),("Metal"):}`
At cathode `M^(n+)+n e^(-)rarrM` Pure Metal
Impurities settle down below anode in the form of anode mud.
(iii) Vapour phase refining : This method is based upon the following facts :
(a) Metal to be refined should form a volatile compound, while impurities do not.
(b) Volatile compound should be easily decomposable so that pure metal can be recovered easily.