Question
Describe the extraction of aluminium from bauxite using the Hall-Héroult process. Include the purification of ore and the electrolytic process with electrode reactions.
Solution — Step by Step
The principal ore of aluminium is bauxite (Al₂O₃·2H₂O — hydrated aluminium oxide). It also contains impurities: SiO₂ (silica), Fe₂O₃ (iron oxide), and TiO₂ (titania). Before electrolysis, the ore must be purified to obtain pure alumina (Al₂O₃). This is done by the Bayer’s process.
Bauxite is dissolved in hot concentrated NaOH (sodium hydroxide) solution:
Alumina dissolves as sodium meta-aluminate. The impurities (SiO₂ also dissolves but can be managed; Fe₂O₃ and TiO₂ do not dissolve) are filtered off as red mud.
The filtrate (NaAlO₂ solution) is then diluted and seeded with Al(OH)₃ crystals to precipitate aluminium hydroxide:
The precipitate is filtered, washed, and heated (calcined) to give pure alumina:
Aluminium is a very reactive metal (high position in the electrochemical series, very negative reduction potential: V). It cannot be reduced from its oxide by carbon (coke) or carbon monoxide at economically feasible temperatures — Al₂O₃ is too stable. Electrolysis is the only practical method.
Pure Al₂O₃ has a very high melting point (~2050°C) — too high to electrolyse in the molten state directly. So it is dissolved in molten cryolite (Na₃AlF₆) at about 950–1000°C, which acts as both a solvent and a flux. A small amount of fluorspar (CaF₂) is added to lower the melting point further and increase conductivity.
Cathode: Carbon-lined steel vat (acts as the cathode). Molten aluminium collects here and sinks to the bottom (denser than the electrolyte).
Anode: Carbon (graphite) rods suspended in the electrolyte.
At cathode (reduction):
At anode (oxidation):
The oxygen liberated at the anode reacts with the carbon anode:
This is why the carbon anodes gradually burn away and must be replaced periodically — a significant operational cost.
Molten aluminium (purity ~99.5%) sinks to the bottom and is tapped off.
Why This Works
Aluminium’s very high reactivity means it cannot be displaced from its ore by a cheaper reducing agent. Electrolysis provides the energy to force the otherwise non-spontaneous reduction. The clever engineering is using cryolite to dissolve Al₂O₃ — bringing the operating temperature down from 2050°C to ~950°C, which is economically and technically feasible.
The Hall-Héroult process was independently invented by Charles Hall (USA) and Paul Héroult (France) in the same year (1886) — one of chemistry’s remarkable coincidences.
Alternative Method — Quick Summary Table
| Step | Process | Key Chemical |
|---|---|---|
| Ore purification | Bayer’s process (NaOH treatment) | NaAlO₂ → Al(OH)₃ → Al₂O₃ |
| Electrolyte | Al₂O₃ dissolved in cryolite | Na₃AlF₆ at 950°C |
| Cathode product | Aluminium metal | Al³⁺ + 3e⁻ → Al |
| Anode product | CO₂ (carbon anodes burn) | C + O₂ → CO₂ |
Common Mistake
Students often state that “Al₂O₃ is directly melted and electrolysed.” This is wrong — Al₂O₃ melts at over 2000°C, which is impractical. It is dissolved in molten cryolite to lower the operating temperature. The cryolite is the solvent, not the material being reduced.
Also, students forget that the carbon anodes burn away (react with liberated oxygen). Writing only the half-reactions at anode without mentioning the burning of carbon anode leaves the answer incomplete.
For board exams, the standard question is: “Why is cryolite used in Hall-Héroult process?” Answer: Al₂O₃ has a very high melting point (2050°C). Dissolving it in cryolite brings the operating temperature down to about 950°C, making electrolysis economically and practically feasible.