(ii) Purification of lead
|Thin sheets of pure lead—Cathode|
|Electrolyte—A solution of lead silico fluoride PbSiF6 containing 8 – 10% of H2<>/subSiF6|
(e) Special methods
(i) Mond’s process : Nickel is purified by this method. Impure nickel is treated with carbon monoxide at 60–80°C when volatile compound, nickel carbonyl, is formed. Nickel carbonyl decomposes at 180°C to form pure nickel and carbon monoxide which can again be used.
(ii) Van-Arkel process : This methods is generally applied for obtaining ultrapure metals. The impure metal is converted into a volatile compound while the impurities are not affected. The volatile compound is then decomposed electrically to get the pure metal.
Ti, Zr, Hf, Si, etc., have been refined by this method. The method is quite expensive.
(iii) Zone refining of Fractional crystallisation : Elements such as Si, Ge, Ga, etc., Which are used as semi-conductors are refined by this method. Highly pure metals are obtained. The method is based on the difference in solubility of impurities in molten and solid state of the metal. A movable heater is fitted around a rod of the impure metal. The heater is slowly moved across the rod. The metal melts at the point of heating and as the heater moves on from one end of the rod to the other end, the pure metal crystallises while the impurities pass on the adjacent melted zone.
Summary of the Extraction of Metals
Different metallurgical processes can be broadly divided into three main types.