Refining: the process of purifying impure metals is called refining.
(A) Liquation process: the process is based upon the difference in fusability of the metal and the impurities.when the impurities are less fusible the the metal itself, this process is used.the impure metal such as Bi, Sn, Hg, Pb,etc. is placed on the sloping hearth of the furnace and gentely heated. The metal melts and flows down.
(B) Distillation: the process is used for those metals which are easily volatile. The impure metals are heated and the vapours are seperately condensed in a receiver. This is used for purification of Zn, Cd, Hg, etc.
(C) Coupellation: the impure metal is heated in the couple or voal shaped crucible made of base ash or cement and removed with the blast of air. Impurityof lead in silveris removed by this process.
(D) Poling : it is used for the purification of those metals which contains their own oxide as impurity. The molten impure metal is stirred with green poles of wood.
(E) Electrorefining : metals like Cu, Hg, Au, Ni, Cr, Al, etc.are purified by this method.the impure metal is made as anode and a strip of pure metals is made is cathode while electrolyte solution consists of a solution of a suitable salt of metal.
(F) Zone refining: when highly pure metals are required, this method is applied for purification. This method is based on the difference in the solubility of impuritiesin the molten or solid state of metal. This method is used in the purification of germanium gallium silicon etc. which are used as semiconductor.
(G) Van arkel process : this method is used for the purfiaction of ultra pure metals. The impure metal is converted into a volatile compound which is then decomposed electrically to getpure metals. Ti, Zr, Hf, Si, etc are refined by this method.
(H) Mond process: nickel is purified by this method. The impur nickel is treated with CO at 330-350K. when volatile nickel carbonyl is formed.the carbonyl is subjected to 450-470K when it is decomposes giving pure nickel.
(I) Chromatographic method: it is based on the selective distribution of various contituents of a mixturebetween two phases, a stationary phase and a moving phase. The stationary phase either be solid or lightly bound liquid on the solid support. The moving phse may be a liquid or a gas. Different types of chromatographic mathods have been developed.
(J) Leaching: leaching involves the treatment of the ore with is suitable reagent to make it soluble while impurities remain insoluble. The ore or the metal is recovered from solution by a suitable chemical method.
e.g natives ore of Ag and Au are treated with aqueous solution of nacn in the presence of oxygen. The Au and Ag particles get dissolved forming complex cyanides. Ag or Au is recovered from solution by addtion of electropositive metal Zn.
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