(i) Zone refining:
Thismethod is based on the principle that impurities are more soluble in the moltenstate of metal (the melt) than in the solid state. In the process of zonerefining, a circular mobile heater is fixed at one end of a rod of impuremetal. As the heater moves, the molten zone of the rod also moves along withit. As a result, pure metal crystallizes out of the melt and the impuritiespass to the adjacent molten zone. This process is repeated several times, whichleads to the segregation of impurities at one end of the rod. Then, the endwith the impurities is cut off. Silicon, boron, gallium, indium etc. can bepurified by this process.
(ii) Electrolyticrefining;
Electrolyticrefining is the process of refining impure metals by using electricity. In thisprocess, impure metal is made the anode and a strip of pure metal is made thecathode. A solution of a soluble salt of the same metal is taken as theelectrolyte. When an electric current is passed, metal ions from theelectrolyte are deposited at the cathode as pure metal and the impure metalfrom the anode dissolves into the electrolyte in the form of ions. Theimpurities present in the impure metal gets collected below the anode. This isknown as anode mud.
(iii) Vapour phaserefining
Vapourphase refining is the process of refining metal by converting it into itsvolatile compound and then, decomposing it to obtain a pure metal. To carry outthis process,
(i) the metal should form a volatile compoundwith an available reagent, and
(ii) the volatile compound should be easilydecomposable so that the metal can be easily recovered.
Nickel,zirconium, and titanium are refined using this method.