Electrochemical Extraction of Aluminium -- Hall-Heroult Process

The Hall-Heroult process is the standard industrial ELECTROCHEMICAL extraction of aluminium metal from alumina (Al2O3), and is the worked example of electrochemical metallurgy given in this unit.

Cell setup. Electrolysis is carried out in an iron tank that is lined internally with carbon; this carbon lining itself serves as the CATHODE of the cell. Carbon blocks are immersed in the electrolyte and serve as the ANODE. The electrolyte is prepared as a 20% solution of alumina (obtained from the bauxite ore via the alkali-leaching route of Section 1.2.3) dissolved in molten CRYOLITE, with about 10% calcium chloride also added; the calcium chloride's role is to help LOWER THE MELTING POINT of the overall fused mixture, so that the electrolysis can be run at a more manageable temperature. The fused electrolyte mixture is maintained above 1270 K throughout electrolysis.

Electrode/ionisation reactions. Alumina first ionises in the fused mixture: Al2O3 → 2Al3+ + 3O2⁻. At the CATHODE, the Al3+ ions are reduced and deposit as molten aluminium metal: 2Al3+(melt) + 6e⁻ → 2Al(l). At the ANODE, the O2⁻ ions are oxidised to liberate oxygen gas: 6O2⁻(melt) → 3O2 + 12e⁻. Because the anode itself is made of carbon, this liberated oxygen reacts further with the anode material, so two additional anode reactions also occur: C(s) + O2⁻(melt) → CO + 2e⁻ and C(s) + 2O2⁻(melt) → CO2 + 4e⁻. These two reactions mean the carbon anodes are SLOWLY CONSUMED (burned away) over the course of electrolysis and must be periodically replaced -- this consumption of the anode is an intrinsic, unavoidable feature of the Hall-Heroult design, not a fault. …