Hall-Héroult process

Aluminium metal cannot be produced by conventional carbothermic reduction: its oxide Al₂O₃ is too stable (ΔG_f = −1582 kJ/mol). The solution, found simultaneously by Charles Hall (USA) and Paul Héroult (France) in 1886, is electrolysis in a molten cryolite bath. A modern Hall-Héroult cell is ~10 × 4 m, holds ~25 cm of liquid bath at 950-980 °C, and runs at 4-4.5 V for currents of 200,000 to 600,000 A (recent AP60-type plants exceed 600 kA). The cathode is a carbon lining at the cell bottom; anodes are carbon blocks (~1.5 × 0.7 × 0.5 m) suspended in the bath and gradually lowered as they're consumed. Alumina, fed to the bath by automatic injection, dissolves as oxofluoroaluminates (AlOF₂⁻, Al₂OF₆²⁻). At the cathode, liquid aluminium deposits (density 2.3 > bath at 2.1 — Al accumulates at the bottom and is siphoned every 24 h). At the anode, the oxygen released immediately attacks the carbon: that's why anodes are consumable and the process emits CO₂. The overall balance is: 2 Al₂O₃ + 3 C → 4 Al + 3 CO₂. Theoretical electrical consumption is 5.99 kWh/kg Al; best modern plants run at 12.5-13 kWh/kg Al, an energy efficiency of ~46 %. World production is ~70 Mt/year (2023), dominated by China (~60 %) — where electricity is largely coal-fired, making Chinese aluminium particularly carbon-intensive (~16 t CO₂/t Al vs ~4 t CO₂/t Al for Icelandic or Quebec hydroelectric aluminium).