Aluminium is one of the most widely used metals in the world. It is strong, lightweight, and corrosion-resistant, which makes it useful for aircraft, drink cans, window frames and electrical cables. But extracting aluminium from its ore is not simple — because aluminium is too reactive to be extracted by reduction with carbon. Instead, we use electrolysis.
The process used to extract aluminium is called the Hall–Héroult process, and it involves the electrolysis of molten alumina (aluminium oxide), usually obtained from the ore bauxite.
Why the Alumina Must Be Molten
Aluminium oxide has a very high melting point (over 2000 °C). To reduce the temperature needed for electrolysis, the alumina is dissolved in molten cryolite. Na₃AlF₆). This lowers the melting point to around 950 °C and reduces energy cost. Only when molten can the ions move freely, which is essential for electrolysis to work.
What Happens During Electrolysis?
When a voltage is applied:
Electrode | Attracts | Ion | Reaction |
Cathode (negative) | Cations |  | (reduction) |
Anode (positive) | Anions |  | Lose electrons (oxidation) |
Cathode (Reduction reaction)
Liquid aluminium metal is formed and collects at the bottom of the cell.
Anode (Oxidation reaction)
Oxygen gas forms at the anode. However — there is a complication! The anodes are made of carbon, and the oxygen reacts with the hot carbon to form carbon dioxide:
This means the anodes burn away and must be replaced regularly, increasing the cost of the process.
Summary of Key Points
Alumina is dissolved in molten cryolite to lower the melting point
Electrolysis splits the compound into elements
Aluminium is reduced at the cathode
Oxygen is formed at the anode
Carbon anodes react with oxygen, producing CO₂
Environmental Considerations
Although aluminium is recyclable and reduces waste long-term, producing it by electrolysis:
Uses large amounts of electricity
Releases carbon dioxide
Has a significant carbon footprint unless powered by renewable energy
🎯 Exam Question (6 marks)
Explain how aluminium is extracted from aluminium oxide by electrolysis. Include the role of the cryolite and the reactions at each electrode.
✅ Model Answer (6/6)
Electrolysis is a separation technique used to separate ions in molten or aqueous liquids.
Aluminium oxide (Alumina) contains aluminium ions and oxygen (oxide) ions which are free to move because the substance is liquid. When an electric current is passed through the liquid, the charged ions are attracted to electrodes of opposite charge.
During electrolysis, Al3+ ions move to the cathode, where they gain electrons and are reduced:
Oxygen ions O2−move to the anode, where they lose electrons and are oxidised:
The oxygen reacts with the carbon anode to form carbon dioxide. Liquid aluminium collects at the bottom of the cell.
The melting point of pure alumina (aluminium oxide, Al₂O₃) is approximately 2,072°C . This extremely high temperature is due to the strong ionic bonds within its crystal lattice. Aluminium oxide is dissolved in molten cryolite to lower the melting point, so less energy is needed, making the process cheaper.
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