Refining of the Extracted MetalThe impure metal extracted from its ore may be refined (or purified) by one or more of the following methods:
- Electrolytic, and
- Zone Refining.
In the liquation method (also used for concentrating the ore), the impure metal is heated on the sloping floor of a furnace. The pure metal flows down (lower melting) while the impurities (higher melting) are left behind. Low melting metals like tin, lead and bismuth are purified by this method.
Low boiling metals like zinc and mercury are purified by distillation. In this method, impure metals are heated to vaporisation. The vapours are condensed to obtain pure metals.
Oxidation methods for refining metals are used when the impurities are easily oxidisable. For example, lead (Pb) as an impurity in silver (Ag) is removed by heating in a boat-shaped crucible. The impurity is easily oxidized to litharge (PbO) and is removed by air. Pure silver is left behind.
Electrolytic Refining of Copper
In the electrolytic method, a block of impure metal is made the anode and a thin sheet of pure metal, the cathode. The electrolytic cell contains a soluble salt of the metal as the electrolyte. On passing electric current, the pure metal gets deposited on the cathode, through the electrolyte. The less electropositive impurities settle down below as anode mud, while the more electropositive impurities pass into the solution. Metals like copper, silver, gold, zinc, nickel, etc., are purified by this method. Electrolytic refining of copper (impure copper - anode; pure copper - cathode; copper sulfate solution - electrolyte) is shown in the figure.
When an impure metal is solidified, crystals of the pure metal are deposited in preference to the impurities present which remain in their molten form. Based on this principle, the zone refining process is used to obtain metals of very high purity.
Zone RefiningA circular heater is fitted around a rod of an impure metal and is slowly moved along the rod. At the heated zone, the rod melts. As the heater moves along, pure metal crystallises out of the melt while the impurities are carried away in the molten zone to the end of the rod.
The process is repeated and after several passes, the impurities are completely swept to the end which is finally discarded. Silicon, germanium and gallium, which are used for making semiconductors are purified by this technique.