Outline the principles of refining of metals by the following methods.
a) Zone refining
b) Electrolytic refining
c) poling
d) vapour phase refining

A) Zone refining:
i) This method is based on the principle that the impurities are more soluble in the molten state than in the solid state of the metal.
ii) A circular mobile heater is fixed at one end of impure metal.
iii) The molten zone moves along with the heater moves forward the pure metal crystallises out of the melt and the impurities pass into the adjacent molten zone.
iv) This process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated, this end is cut off.
b) Electrolytic refining:
i) This process is used for less reactive metals like Cu, Ag etc.
ii) In this process anode is made by impure metal and a thin of pure metal acts as cathode. on electrolysis metal dissolves from anode and pure metal gets deposited at cathode.
At Anode: $\mathrm{M}\to {\mathrm{M}}^{\mathrm{n}+}+{\mathrm{ne}}^{-}$
At cathode: ${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{ne}}^{-}\to \mathrm{M}$
c) Poling: This method is used when metals having same metal oxides as impurities. the impure metal is melted and is then covered by carbon powder. then it is stirred with green wood poles the reducing gases formed from the green wood and the cabon reduce the oxides to metal.
Ex:- Cu, Sn
d) vapour phase refining: In this method, the metal is converted into its volatile compound and collected. It is then decomposed to give pure metal. conditions in the process are
i) metal to be refined should form a volatile compound, while impurities do not.
ii) volatile compounds should be easily decomposable so that pure metal can be recovered easily.