Refining of Metals

Except in the electrolytic reduction method, metals produced by any other method are generally impure. The impurities may be in the form of

  • (i) other metals
  • (ii) unreduced oxide of the metal
  • (iii) non-metals like carbon, silicon, phosphorus, sulphur, etc.
  • (iv) flux or slag

Liquation

Easily fusible metals like tin, lead, etc. are refined by this process. In this method, the impure metal is poured on the sloping hearth of a reverberatory furnace and heated slowly to a temperature little above the melting point of the metal. The pure metal drains out leaving behind infusible impurities.

Poling

Poling involves stirring the impure molten metal with green logs or bamboo. The hydrocarbons contained in the pole reduce any metal oxide present as impurity. Copper and tin are refined by this method.

Distillation

Volatile metals like zinc and mercury are purified by distillation. The pure metal distils over, leaving behind non-volatile impurities. 

Electrolytic Refining

A large number of metals like copper, silver, zinc, tin, etc. are refined by electrolysis. A block of impure metal is made the anode and a thin sheet of pure metal forms the cathode of the electrolytic cell containing suitable metal salt solution which acts as an electrolyte.

On passing current, pure metal deposits at the cathode sheet while more electropositive impurities are left in solution. Less electropositive metals do not dissolve and fall away from the anode to settle below it as anode mud.