( iii ) Zone-refining This method is based upon the principle of fractional crystallisation , i . e ., difference in solubilities of impurities in molten and solid state of metal . Semiconductors like silicon , germanium , gallium arsenide and indium antimonide are purified by this method . Elements of very high purity are obtained by this method .
( iv ) Vapour phase refining In this method , crude metal is made free from impurities by first convertmg it Into its volatile compound by heating with a chemical reagent at low temperature . After this , the volatile compound is decomposed by heating to some higher temperature to give pure metal .
( a ) van Arkel method This method is used for preparing ultra-pure metal used in space technology ( e . g ., Ti , Zr , etc .)
( b ) Mond ’ s process It is used for refining of nickel .
( v ) Chromatographic method Adsorption chromatography is generally used . The impure metal is dissolved in a suitable solvent and the solution is allowed to run slowly into an adsorbent column packed with alumina ( Al2O3 ). The metal and the impurities present are adsorbed at different rates . These are then eluted with suitable eluent ( solvent ). In this method .
weakly adsorbed component is eluted first and the strongly adsorbed component is eluted afterwards .