Outline the principles of refining of metals by the following methods: (i) Zone refining (ii) Electrolytic refining (iii) Vapour phase refining
(i) Zone refining It is used for refining Si,B,Ga,In etc. Impurities are more soluble in the molten state of metal than in solid state. There is a circular mobile heater at one end of a rod of impure metal. Heater and the molten zone moves from one end to another end. Pure metal crystallizes out of the melt and the impurities pass onto the adjacent molten zone. Repetition of this process several times segregates impurities at one end of a rod. The end with impurities can then be cut off. (ii) Electrolytic refining It is carried out for refining of impure zinc. The anode is impure zinc and cathode is pure zinc. The electrolyte is a mixture of zinc sulphate and dilute sulphuric acid (small amount). When current is passed, zinc is deposited on the cathode and is collected. At anode : Zn→Zn2++2e− At cathode: Zn2++2e−→Zn (iii) Vapour phase refining Metal is converted into volatile compound and then decomposed to obtain pure metal. This method is used for refining Ni,Zr and Ti. Ni is heated with CO to form volatile nickel tetracarbonyl. Ni+4CO330−350KNi(CO)4 At high temperature, nickel tetracarbonyl decomposes to pure Ni. Ni(CO)4450−470KNi+4CO This is Mond's process for refining Ni.
