State that the main ore of aluminium is bauxite and that aluminium is extracted by electrolysis

Metals – Extraction of Aluminium (IGCSE Chemistry 0620)

Learning Objective

State that the main ore of aluminium is bauxite and that aluminium is extracted from this ore by electrolysis (the Hall–Héroult process). Explain why aluminium cannot be reduced by carbon using the reactivity‑series argument.

1. The Ore – Bauxite

• Composition: mainly aluminium hydroxide minerals
  • Gibbsite – Al(OH)₃
  • Boehmite – γ‑AlO(OH)
  • Diaspore – α‑AlO(OH)
• Impurities such as Fe₂O₃, TiO₂, silica are present and must be removed before aluminium can be produced.

1.1 From Bauxite to Alumina – Bayer Process

1. Crush bauxite and digest with hot concentrated NaOH → soluble sodium aluminate. Al₂O₃·xH₂O + 2 NaOH → 2 NaAlO₂ + (x+1) H₂O
2. Filter to remove insoluble impurities.
3. Cool the solution; aluminium hydroxide precipitates. NaAlO₂ + 2 H₂O → Al(OH)₃↓ + NaOH
4. Calcine the precipitate at ≈ 1100 °C to give pure alumina (Al₂O₃). 2 Al(OH)₃ → Al₂O₃ + 3 H₂O

2. Why Aluminium Needs Electrolysis

• In the reactivity series aluminium lies **above carbon**: Al  >  C (more reactive).
• Therefore carbon cannot reduce Al₂O₃ to metal (unlike iron).
• The strong Al–O ionic bonds give a high lattice energy; only a large external electrical energy input can break them.

3. Hall–Héroult Electrolytic Cell

3.1 Cell Components

• Electrolyte: molten mixture of Al₂O₃ (≈ 30 %) and cryolite (Na₃AlF₆). Cryolite lowers the melting point from ~2072 °C to ~950 °C.
• Cathode (steel lining, negative): reduction of aluminium ions. Al³⁺ + 3 e⁻ → Al(l)
• Anode (carbon blocks, positive): oxidation of oxide ions; oxygen reacts with carbon. 2 O²⁻ → O₂(g) + 4 e⁻ O₂ + C → CO₂
• Molten aluminium (denser) collects at the bottom and is tapped off periodically.

3.2 Overall Reaction

2 Al₂O₃(l) → 4 Al(l) + 3 O₂(g)

3.3 Key Operating Conditions

• Temperature: ≈ 950 °C (maintained by external heating).
• Current: very high (hundreds of kilo‑amperes) – the rate of aluminium production is directly proportional to the current (Faraday’s laws).
• Energy consumption: ≈ 13–15 kWh kg⁻¹ Al.

4. Stoichiometry & Faraday’s Laws (AO2)

Worked example: How much aluminium can be produced from 100 g of alumina (Al₂O₃)?

1. Molar mass Al₂O₃ = (2 × 27.0) + (3 × 16.0) = 102 g mol⁻¹.
2. Moles of Al₂O₃ = 100 g ÷ 102 g mol⁻¹ ≈ 0.98 mol.
3. From the overall equation, 2 mol Al₂O₃ give 4 mol Al → 1 mol Al₂O₃ gives 2 mol Al.
4. Moles of Al produced = 0.98 mol × 2 = 1.96 mol.
5. Mass of Al = 1.96 mol × 27.0 g mol⁻¹ ≈ 53 g.

Thus 100 g of alumina can yield about 53 g of aluminium (theoretical yield).

5. Redox, Rate & Energy (AO1/AO2)

• Redox: Al³⁺ is reduced (gain of electrons); O²⁻ is oxidised (loss of electrons).
• Rate of production: proportional to the electric current (Faraday’s first law). m = (Q · M) / (n · F) where Q = I t.
• Energy: The high lattice energy of Al₂O₃ and the endothermic melting of the cryolite bath account for the large electricity requirement.

6. Amphoteric Behaviour of Alumina (AO1)

• With acids: Al₂O₃ + 6 HCl → 2 AlCl₃ + 3 H₂O
• With bases: Al₂O₃ + 2 NaOH + 3 H₂O → 2 Na[Al(OH)₄]

This links the extraction topic to the wider syllabus on oxides and their reactions.

7. Periodic‑Table Context (AO1)

• Aluminium is a Group 13 metal. Its ionisation energy is higher than that of the transition metals but lower than non‑metals, placing it above carbon in the reactivity series.
• Consequences:
  • Carbon reduction (as used for iron) is impossible.
  • Electrolysis is the only practical industrial route.

8. Comparison of Extraction Methods for Common Metals

9. Summary – Key Points to Remember

• The principal ore of aluminium is bauxite.
• Aluminium is extracted by the Hall–Héroult electrolytic process, not by carbon reduction.
• Alumina (Al₂O₃) is first obtained from bauxite by the Bayer process.
• In the electrolytic cell:
  • Cathode: Al³⁺ + 3 e⁻ → Al(l)
  • Anode: 2 O²⁻ → O₂(g) + 4 e⁻ (oxygen reacts with carbon to form CO₂).
• Aluminium lies above carbon in the reactivity series; therefore carbon cannot reduce Al₂O₃, making electrolysis essential.
• The process is energy‑intensive (≈ 13–15 kWh kg⁻¹ Al) and requires large currents.
• Al₂O₃ is amphoteric – it reacts with both acids and bases.
• Understanding the Hall–Héroult cell provides a basis for the broader topics of redox, electrochemistry, and metal extraction in the IGCSE syllabus.