question_answer 30)

Outline the principles of refining of metals by following methods : (i) Zone refining               (ii) Electrolytic refining (iii) Vapour phase refining.

Answer:

                (i) Zone refining. This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. (ii) Electrolytic refining. A large number of metals such as copper, silver, gold, nickel, chromium etc. are refined by this method. In this method, the impure metal (e.g. Copper) is made anode while cathode is a rod of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal (e.g., CuSO4 solution). When electric current is passed, metal ions from the electrolyte are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as metal ions. (iii) Vapour phase refining. In this method, impure metal is first converted into its volatile compound by heating with a chemical reagent at lower temperature. After this, the volatile compound is decomposed by heating to give the pure metal. e.g., Mond's process for extraction of nickel and Van-Arkel Method for preparing ultra pure titanium are based on the principle of vapour phase refining.