(I) 4Au / Ag (s) + 8CN – (aq) + 2H 2 O(aq) + O 2 (g) ⎯→ 4[Au / Ag (CN) 2 ] – (aq) + 4OH–(aq)

2[Au / Ag (CN) 2 ] – (aq) + Zn(s) ⎯→ 2Au / Ag (s) + [Zn(CN) 4 ] 2– (aq)

(II) This method is based on the fact that gangue and ore particles have different degree of wettability with water and pine oil; the gangue particles are preferentially wetted by water while the ore particles are wetted by oil.

(III) Electrolytic reduction (Hall-Heroult process) :

The purified Al 2 O 3 is mixed with Na 3 AlF 6 (cryolite) or CaF 2 (fluorspar) ) which lowers the melting point of the mixture and increases conductivity. The fused matrix is electrolysed. The electrolytic reactions are :

Cathode : Al 3+ (melt) + 3e – ⎯→ Al(l)

Anode : C(s) + O 2– (melt) ⎯→ CO(g) + 2e –

C(s) + 2O 2– (melt) ⎯→ CO 2 (g) + 4e –

(IV) This process is used when metals are required in very high purity, for specific application. For example pure Si and Ge are used in semiconductors and hence are purified by this method. Zone refining method is based on the principle that an impure molten metal on gradual cooling will deposit crystals of the pure metal, while the impurities will be left in the remaining part of the molten metal.