State briefly the principles which serve as the basis for the following operations in metallurgy.
(i) Froth floatation process
(ii) Zone refining
(iii) Refining by liquation.

Explanation

(i) Froth floatation process : The process is based on the principle that the sulphide ores of certain metals are wetted by oil like pine oil in the presence of air while the gangue impurities by water. This helps in the separation of impurities from these ores.
(ii) Zone refining : It is based on the principle that when impure metal like germanium or gallium in the molten state is allowed to cool, only the metal crystallises while the impurities remain in the molten state. This helps in their separation.
(iii) Refining by liquation : This helps in the purification of low melting metals like. `Bi, Sn, Pb, Hg` etc. and the process is based on the principle that when impure metal is heated in an inert atmosphere of carbon monoxide on the sloping hearth of a reverberatory furnace, the metal melts and flows down the sloping surface leaving behind the impurities which do not melt.