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Six major refining methods exist, each suited to specific metals based on their physical and chemical properties.
Electrolytic refining (most widely used): Impure metal = anode, pure metal = cathode, metal salt solution = electrolyte. Anode dissolves, pure metal deposits on cathode. More reactive impurities stay in solution; less reactive (Ag, Au) settle as anode mud. Used for Cu, Zn, Ni, Ag, Au, Al.
Distillation: For volatile metals (Zn, Hg). Crude metal heated, vapour condensed to get pure metal.
Liquation: For low-melting metals (Sn, Pb, Bi). Gentle heating melts pure metal which flows away from higher-melting impurities.
Zone refining: For ultra-pure semiconductors (Ge, Si, Ga, In). Principle: impurities are more soluble in melt than solid. A molten zone swept along rod carries impurities to one end.
Mond process (Ni only): Ni + 4CO (330 K) -> Ni(CO)4 (volatile) -> Ni + 4CO (450 K). Only nickel forms volatile carbonyl at this mild temperature.
Van Arkel process (Zr, Ti, Hf): Metal + I2 (870 K) -> MI4 (volatile) -> M + 2I2 (1800 K on hot W filament). Iodine is recycled.
JEE tip: Match the refining method to the metal. The most common trap is confusing Mond (Ni) with Van Arkel (Zr, Ti).