Hall-Héroult process
Electrolysis at 950-980 °C of alumina dissolved in molten cryolite (Na₃AlF₆) to produce aluminium metal. Universal since 1886 — it alone consumes ~3 % of world electricity.
Decomposition driven by electric current
Key reaction
Operating conditions
- Temperature
- 950-980°C
- Pressure
- 1bar
- Catalyst
- Cryolithe Na₃AlF₆ fondue + AlF₃ + CaF₂
- Phase
- liquid
How it works
How it works
Key components
The role of each main part, and the elements / compounds it involves.
Reduction cell (pot)
Electrolysis reactor — holds the molten cryolite bath and the liquid aluminium produced.
Rectangular cell ~10 × 4 × 1.2 m, steel shell internally lined with carbon blocks (cathode) then an insulating frozen-alumina crust. The cell tilts slightly and a siphon draws off the molten Al collected at the bottom. A modern plant lines up 200-300 cells in series, forming a 'potline' fed by a single giant rectifier.
10 × 4 × 1,2 m · 200-600 kA · 4-4,5 V · 200-300 cuves en série
Carbon anodes
Supply electrons and are consumed by reaction with the released oxygen.
Pre-baked carbon blocks (Söderberg technology nearly extinct) made from calcined petroleum coke + coal-tar pitch, baked at 1200 °C. ~1.5 × 0.7 × 0.5 m, ~1 t each. A cell holds 20-30 anodes, replaced every 25-30 days. Consumption: ~400 kg carbon per tonne Al — this is what makes the process an irreducible CO₂ emitter (at least ~1.5 t CO₂/t Al just from the anodes).
Coke Pb + brai · cuit 1200 °C · ~400 kg C/t Al · changement /25-30 j
Electrolytic bath (cryolite + AlF₃)
Molten ionic solvent that dissolves alumina and carries current between anode and cathode.
Na₃AlF₆ (cryolite) + AlF₃ (8-12 % excess) + CaF₂ (~5 %) + 2-4 % dissolved Al₂O₃. This composition lowers the melting point of Al₂O₃ from 2050 °C to ~960 °C — the central trick of the process. Natural cryolite (Greenland) being exhausted, it's synthesized from fluorspar + hydrofluoric acid. Fluoride losses: 15-25 kg F/t Al — partially captured by the off-gas treatment system.
Na₃AlF₆ + 8-12 % AlF₃ + 5 % CaF₂ · Tf ~960 °C · 2-4 % Al₂O₃
See also :al2o3Alumina feeders
Automatically deliver alumina to the bath to maintain 2-4 % dissolution.
Pneumatic dosers injecting 1-2 kg of Al₂O₃ every 1-3 minutes per cell, controlled by bath resistance (which rises as alumina depletes). Below 1 %, the 'anode effect' kicks in: high voltage, formation of PFCs (CF₄, C₂F₆) — greenhouse gases 6,500-9,200× more potent than CO₂. Fine control is therefore climate-critical.
1-2 kg/min · contrôle par résistance · seuil 1 % critique (PFC)
See also :al2o3Tapping siphon
Withdraws the molten aluminium pooled at the cell bottom without stopping electrolysis.
Refractory steel tube lowered into the cell from the top, set under suction by a vacuum bell. Tapping of 1-2 t Al at 950 °C in 5-15 min, ~24 times/day. Tapped Al still contains 0.1-0.3 % impurities (Fe, Si) and is then refined in a holding furnace before casting into ingots, billets or plates.
1-2 t/cuve/coulée · ~24×/jour · 0,1-0,3 % impuretés résiduelles
Physical and chemical principles
The fundamental laws that make this process possible — and the constraints they impose.
Electrochemical decomposition
Theoretical decomposition potential of Al₂O₃ is 1.18 V at 977 °C. Real voltage (4-4.5 V) includes anode and cathode overpotentials, ohmic drop in the bath, and connection resistance. The gap is where energy inefficiency sits — hence the constant effort on geometry and bath quality.
E°(977 °C) = 1,18 V ; U_industrielle = 4-4,5 VThermal stability of the bath
Joule heating (R·I²) of the current in the bath produces the heat that keeps it molten. The cell runs at a precarious thermal equilibrium: too hot, the carbon lining erodes; too cold, alumina stops dissolving and the anode effect kicks in. A frozen-alumina 'crust' on the walls acts as a buffering thermal insulator.
Compounds involved
Input
By-product
World production
Main applications
- Transportation (automotive, aerospace, rail)28 %
- Construction (frames, façades)25 %
- Packaging (cans, foils)17 %
- Electrical (overhead lines, conductors)12 %
- Machinery, durables, miscellaneous18 %
Decarbonizing aluminium
- Anodes inertes (Elysis, RUSAL) — démonstrateurs 450 kA
- Hall-Héroult bas-carbone (Islande, Norvège, Québec)
- Cellules de très grande taille (600+ kA, AP60)
- Boucle de recyclage post-consumer (97 % de récupération sur les canettes)
Similar or competing processes
Related industrial processes — alternative chemistry, alternative technology.
- bayer
Upstream process — supplies the pure alumina that Hall-Héroult electrolyses.