Chemistry – Metals - Extraction of metals | e-Consult

The Hall-Héroult process extracts aluminium from alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆). Alumina has a very high melting point (around 2072 °C), making direct electrolysis impractical. Cryolite acts as a solvent, significantly lowering the melting point of the mixture to around 950-980 °C. This reduction in melting point drastically reduces the energy required for electrolysis.

Carbon anodes are used because they are relatively inert to the electrolyte under the high temperatures and voltages used in the process. The anodes provide a surface for the oxidation of the fluoride ions and the subsequent formation of oxygen gas. The carbon is gradually consumed during this process.

The electrochemical reactions are as follows:

• At the Cathode (Reduction): Al³⁺(l) + 3e^- → Al(l) Aluminium ions from the electrolyte are reduced to molten aluminium, which deposits at the bottom of the electrolytic cell.
• At the Anode (Oxidation): 2O^2-(l) → O₂(g) + 4e^- Fluoride ions (F^-) in the electrolyte are oxidized to oxygen gas (O₂) on the anode. This is a crucial reaction as it maintains the electrical conductivity of the electrolyte.

The continuous consumption of the carbon anodes is a significant operational cost of the Hall-Héroult process. The anodes must be replaced regularly to maintain the efficiency of the electrolysis.