Q.Explain the electrometallurgy of aluminium.

Step 1. Aluminium's oxide (Al2O3) is far too thermodynamically stable to be reduced by carbon (Section 1.3.2), so aluminium must be extracted electrochemically instead -- this is the Hall-Heroult process (Section 1.5.1).

Step 2. Cell setup: electrolysis is carried out in an iron tank lined internally with carbon, which itself serves as the CATHODE; carbon blocks immersed in the electrolyte serve as the ANODE.

Step 3. Electrolyte: a 20% solution of alumina (obtained from bauxite via alkali leaching) dissolved in molten cryolite, with about 10% calcium chloride added to lower the melting point of the fused mixture; the melt is maintained above 1270 K.

Step 4. Ionisation and electrode reactions: alumina ionises, Al2O3 → 2Al3+ + 3O2-. At the cathode, Al3+ is reduced to molten aluminium: 2Al3+(melt) + 6e- → 2Al(l). At the anode, O2- is oxidised to liberate oxygen: 6O2-(melt) → 3O2 + 12e-; because the anode is carbon, this oxygen further reacts with the anode itself, C + O2- → CO + 2e- and C + 2O2- → CO2 + 4e-, so the carbon anodes are slowly consumed and need periodic replacement. …

Walk through the Hall-Heroult cell setup, electrolyte composition, and the full set of electr …

• Forgetting that the carbon ANODE is consumed during electrolysis (via the CO/CO2-forming side reactions) -- unlike the carbon cathode lining, which is not consumed the sa …