Electrolytic refining: Impure metal (anode) dissolves; pure metal deposits on cathode. Electrolyte = soluble salt of the metal. More reactive impurities remain in solution; less reactive (Ag, Au) fall as anode mud. Used for Cu, Zn, Ni, Ag, Au, Al. Zone refining: Impurities are more soluble in melt than solid. A molten zone is swept along a rod → impurities carried to one end. For ultra-pure semiconductors: Ge, Si, Ga, In. Mond process (Ni): Ni + 4CO (330 K) → Ni(CO)4 (volatile) → Ni + 4CO (450 K). Only Ni forms volatile carbonyl at this temperature. Van Arkel process (Zr, Ti): Metal + I2 (870 K) → MI4 (volatile) → M + 2I2 (1800 K on hot W filament). Iodine is recycled.