( 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.