Extraction of Aluminium — Educational Guide
1. What is Aluminium?
Aluminium (Al) is a lightweight, silvery‑white metal with atomic number 13. It is widely used because of its low density, corrosion resistance, electrical conductivity, and recyclability.
Common properties: low density (~2.7 g/cm³), good thermal and electrical conductor, forms a protective oxide layer (Al₂O₃).
2. Major uses of Aluminium
- Transport: aircraft, cars, ships (lightweight structures).
- Construction: window frames, cladding, roofing.
- Packaging: cans, foil.
- Electrical: conductors, wiring, components.
- Consumer: cookware, appliances, smartphones.
- Recycling is important — recycling aluminium saves ~95% energy vs primary production.
3. Where aluminium comes from (ore)
The primary ore is bauxite. Bauxite is a mixture of hydrated aluminium oxides (gibbsite Al(OH)₃, boehmite, diaspore) plus impurities (iron oxides, silica).
4. Overview of the extraction — two main stages
- Purification (Bayer process) — convert bauxite to pure alumina (Al₂O₃).
- Electrolytic reduction (Hall–Héroult process) — electrolyse alumina dissolved in molten cryolite to produce aluminium metal.
5. Bayer process (detailed steps)
- Crush and grind bauxite to increase surface area.
- Digest with concentrated NaOH at 150–200 °C under pressure: aluminium compounds dissolve as sodium aluminate.
Al₂O₃ + 2 NaOH → 2 NaAlO₂ + H₂O
- Filter to remove undissolved impurities (red mud: Fe₂O₃, TiO₂, silica).
- Precipitate Al(OH)₃ by cooling and seeding the solution:
NaAlO₂ + 2 H₂O → Al(OH)₃ ↓ + NaOH
- Calcine the Al(OH)₃ to obtain pure alumina:
2 Al(OH)₃ → Al₂O₃ + 3 H₂O (on heating)
Notes: The byproduct 'red mud' is highly alkaline and requires careful disposal or treatment. Alumina (Al₂O₃) from Bayer process is white and powdery.
6. Hall–Héroult process (electrolysis)
Alumina is dissolved in molten cryolite (Na₃AlF₆) to lower melting point and increase conductivity. The electrolysis cell (pot) has a carbon-lined steel container (cathode) and carbon/graphite anodes.
Cathode reaction (reduction):
Anode reaction (oxidation):
The oxygen produced reacts with the carbon anodes to form CO or CO₂:
Overall simplified cell reaction:
Practical notes:
- Operating temperature ~940–980 °C (molten cryolite + alumina bath).
- High current (hundreds of kA per cell) — very energy intensive.
- Carbon anodes are consumed and must be replaced regularly — source of CO₂ emissions in primary aluminium production.
7. Energy and environmental considerations
- Primary aluminium production consumes a lot of electricity — the carbon footprint depends heavily on the energy source.
- CO₂ emissions come from both carbon anode consumption and electricity (if fossil fuels are used).
- Red mud (Bayer process waste) is alkaline and needs safe disposal or resource recovery (research ongoing on red mud valorisation).
- Recycling aluminium uses ≈95% less energy and produces far less emissions — recycling is highly encouraged.
8. Recycling aluminium
Used aluminium (cans, scrap, components) is melted and re‑cast. Energy for recycling is much lower than primary production. Aluminium can be recycled indefinitely without losing properties.
9. Quick summary (cheat‑sheet)
- Ore: Bauxite.
- Refining: Bayer process → Al₂O₃ (alumina).
- Extraction: Hall–Héroult electrolysis of alumina in cryolite → Al metal.
- Equation: 2 Al₂O₃ → 4 Al + 3 O₂.
- Recycling: Saves ~95% energy vs. primary production.
10. Chemical equations (all in one place)
Bayer: Al2O3 + 2 NaOH → 2 NaAlO2 + H2O Precipitation: NaAlO2 + 2 H2O → Al(OH)3 ↓ + NaOH Calcination: 2 Al(OH)3 → Al2O3 + 3 H2O Electrolysis (overall): 2 Al2O3 → 4 Al + 3 O2
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