Electrochemical principle of metallurgy
Similar to thermodynamic principles, electrochemical principles also find applications in metallurgical process. The reduction of oxides of active metals such as sodium, potassium etc., by carbon is thermodynamically not feasible. Such metals are extracted from their ores by using electrochemical methods. In this technique, the metal salts are taken in a fused form or in solution form. The metal ion present can be reduced by treating it with some suitable reducing agent or by electrolysis.
Gibbs free energy change for the electrolysis process is given by the following expression
ΔG° = -nFE°
Where n is number of electrons involved in the reduction process, F is the Faraday and E0 is the electrode potential of the redox couple.
If E0 is positive then the ΔG is negative and the reduction is spontaneous and hence a redox reaction is planned in such a way that the e.m.f of the net redox reaction is positive. When a more reactive metal is added to the solution containing the relatively less reactive metal ions, the more reactive metal will go into the solution. For example,
Cu (s) + 2Ag+ (s) → Cu 2+ (aq) + 2Ag (s)
Cu2+ (aq) + Zn (s) → Cu (s) + Zn 2+ (aq)
In this method, electrolysis is carried out in an iron tank lined with carbon which acts as a cathode. The carbon blocks immersed in the electrolyte act as a anode. A 20% solution of alumina, obtained from the bauxite ore is mixed with molten cyrolite and is taken in the electrolysis chamber. About 10% calcium chloride is also added to the solution. Here calcium chloride helps to lower the melting point of the mixture. The fused mixture is maintained at a temperature of above 1270 K. The chemical reactions involved in this process are as follows.
Ionisaiton of alumina Al2O3 → 2Al3+ + 3O2-
Reaction at cathode 2Al3+ (melt) + 6e- → 2Al (l)
Reaction at anode 6O2- (melt) → 3O2 + 12e-
Since carbon acts as anode the following reaction also takes place on it.
C (s) + O2- (melt) → CO + 2e-
C (s) + 2O2- (melt) → CO2 + 4e-
Due to the above two reactions, anodes are slowly consumed during the electrolysis. The pure aluminium is formed at the cathode and settles at the bottom. The net electrolysis reaction can be written as follows.
4Al3+ (melt) + 6O2- (melt) + 3C (s) → 4Al (l) + 3CO2 (g)