: 210
Aluminium extraction requires electrolysis because its oxide (Al2O3) is too stable for chemical reduction. The process has two stages.
Stage 1 — Bayer's process (purification): Bauxite + 2NaOH -> 2NaAlO2 + 3H2O (Al2O3 dissolves; Fe2O3 filtered as red mud). NaAlO2 + CO2 + 2H2O -> Al(OH)3 + NaHCO3. Al(OH)3 calcined to pure Al2O3.
Stage 2 — Hall-Heroult electrolysis: Pure Al2O3 dissolved in molten cryolite (Na3AlF6) which lowers the melting point from 2345 K to ~1240 K. CaF2 added for fluidity. Cathode (carbon lining): Al3+ + 3e- -> Al(l) collected at bottom. Anode (graphite rods): O2- -> O2 which reacts with graphite: C + O2 -> CO/CO2 (anode consumed — requires periodic replacement).
Energy consumption is ~15 kWh/kg Al, making it extremely energy-intensive. Aluminium smelters are strategically located near cheap hydroelectric power. The role of cryolite (lowers m.p., increases conductivity, dissolves Al2O3) is frequently tested in JEE.