The extraction of aluminium

The extraction of aluminium occurs in three stages;

• Purification of the ore bauxite.
• Dissolving pure alumina in molten cryolite (Na₃AIF₆)
• Electrolysis in the cell.

The process

Aluminium is extracted from bauxite (hydrated aluminium oxide). Bauxite, Al₂O₃ . 2H₂O is impure aluminium oxide and contains impurities such as iron (III) oxide and silicon (IV) oxide. The purification process begins when bauxite is mixed with hot concentrated sodium hydroxide solution.

The amphoteric aluminium oxide dissolves forming a solution of sodium aluminate.

The impurities do not react or dissolve in sodium hydroxide hence they are filtered often as sludge. The filtrate, sodium aluminate is then seeded with a bit of aluminium hydroxide solution. This causes the precipitation of more aluminium hydroxide.

The aluminium hydroxide (Al(OH)₃) precipitate is filtered, washed and dried. It is then heated strongly to remove water and to obtain pure aluminium oxide (Al₂O₃). The sodium hydroxide is used again for further purification.

At the anode, oxygen is produced.

           2O^2-_(aq)                    O_2(aq) + 4e^-

The oxygen readily burns away the anode (self-baking), hence the anode has to be replaced from time to time.

             C_(s) + O_2(g)                         CO_2(g)

At the cathode aluminium is obtained and is tapped off

The pure alumina (Al³⁺) has a very high melting temperature of 1200°C which is not readily available. Because of this it is dissolved in molten cryolite which lowers the melting point and increases ionization.

Electrolysis of the purified alumina is carried out in a shallow steel tank lined with carbon (graphite) which acts as the cathode of the cell. The anode which is also made of carbon is suspended in the electrolyte at the top of the cell.

The electrolyte forms a solid crust at the top, preventing the re-oxidation of aluminium.

Because of the high electrical energy needed in this extraction, aluminium companies must be located where there is a cheap source of electricity. In Cameroon it is located in Edea.

Uses or aluminium and properties of use

Aluminium is widely used in

• Making cooking utensils. This is because it is cheap, light, non-poisonous, and resistant to corrosion and conducts heat.
• Making overhead cables because of its low density (lightness)
• Making electric wires because it is a good conductor of electricity and it is ductile.
• Making roofing sheets, because it is malleable, resistant to corrosion, reflective and light.
• Making paints because of its high reflectivity
•  Making alloys because of its low tensile strength and resistant to corrosion.