Aluminium Industry - Bauxite

Aluminium is produced by a smelter process. Smelter means to produce a metal from its ore. Smelting uses heat and a chemical reducing agent to decompose the ore, leaving the metal behind. The reducing agent is usually carbon.
The smelting process uses a huge amount of electrical energy.
  1. Aluminium is widely used as a metal.
        • malleable
        • ductile
        • electrical conductor
        • very strong
        • not heavy, light
        • etc.

  2. Aluminium mineral ore is called bauxite.

  3. Bauxite consists mainly of aluminium oxide Al2O3.
    But it contains impurities: silicon dioxide SiO2 and iron(III)oxide Fe2O3.

  4. Bauxite is treated with sodium hydroxide (NaOH) to produce pure Al2O3 called alumina.

  5. The waste produced is an alkaline suspension of iron (III) oxide which is called red-mud. Red mud waste is unstable, and alkali from red mud dams can seep through and contaminate water supplies.

  6. The alumina (pure Al2O3) is dissolved in cryolite (Na3AIF).
    The cryolite is used to lower the working temperature.
    The melting point of Al2O3 is 20500C. But when dissolved in cryolite, the melting point becomes 900 to 10000C.
    Thus the cryolite helps to decrease energy costs.
    The molten pure Al2O3 can now undergo electrolysis.

  7. The electrolytic cell has the anode and cathode made of carbon.

  8. Aluminium is formed at the cathode (negative electrode) by a reduction half reaction.

  9. Al+3 + 3e- → Al

  10. Oxygen is produced at the anode (positive electrode) by an oxidation half reaction.
    2O-2 → O2 + 4e-1

  11. Net Reaction
    2Al2O3(s) → 4Al(l) + 3O2(g)

  12. Problems with the process.
        • red mud waste is unstable
        • alkali from red mud dams can seeps through and contaminate water supplies.
        • fluoride emissions are produced during the electrolysis.
        • electrical energy costs are very high

  13. Recycling is desirable especially when using a smelter process. It is much cheaper to recycle aluminium than to manufacture it. Recycling uses lesser energy.