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III. Purification of the crude metal , The metal obtained by the reduction of the ore is usually contaminated with impurities like unchanged ore, other metals present in the ore and non metals from the anions in the ore., For example, the copper obtained from its sulphide ore, a compound of copper iron pyrites (CuFeS₂), contains some copper sulphide, iron and sulphur., It is purified by suitable methods including electrolysis. The process of obtaining the pure metal from the impure metal is called refining of the metal., Refining of the metal involves several types of processes. Some refining methods are: a) Distillation (b) Poling (c) Liquation (d) Electrolysis , a) Distillation: , This method is very useful for purification of low boiling metals like zinc and mercury containing high boiling metals as impurities. , The extracted metal in the molten state is distilled to obtain the pure metal as distillate.

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b) Poling:, The molten metal is stirred with logs (poles) of green wood. The impurities are removed either as gases or they get oxidized and form scum (slag) over the surface of the molten metal. Blister copper is purified by this method., The reducing gases, evolved from the wood, prevent the oxidation of copper. , c) Liquation: , In this method a low melting metal like tin can be made to flow on a sloppy surface to separate it from high melting impurities., d) Electrolytic refining: , In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal., The required metal gets deposited on the cathode in the pure form. The metal, constituting the impurity, goes as the anode mud. The reactions are: