Outline the principles of refining of metals by the following methods :
(a) Electrolytic refining
(b) Zone refining
(c) Vapour phase refining

Explanation

(a) The crude metal is refined by electrolysis. It is carried out in an electrolytic cell containing aqueous solution of the salts of the metal of interest. The rods of impure metal are used as anode and thin strips of pure metal are used as cathode. The metal of interest dissolves from the anode, pass into the solution while the same amount of metal ions from the solution will be deposited at the cathode. During electrolysis, the less electropositive impurities in the anode, settle down at the bottom and are removed as anode mud.
(b) Zone refining: This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of the rod of the impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystalises out of melt and the impurities pass on into the adjacent molten zone. This process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated.

This method is very useful for producing semiconductor and other metals of very high purity like germanium, silicon, boron etc.
(c) Vapour phase refining: In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal. There are two requirements:
(i) the metal should form a volatile compound with an available reagent.
(ii) the volatile compound should be easily decomposable, so that recovery is easy.
Example : The Mond process for refining nickel is based on vapour phase refining. In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl `Ni+4 COoverset(330-350K)rarrunderset("Nickel tetracarbonyl")(Ni(CO)_(4))`
The carbonyl is decomposed to pure metal at high temperature.
`NI(CO)_(4)overset(450-470K)rarrNi+4CO`