Discuss the extraction of aluminium from bauxite.

Aluminium is the most abundant metal on Earth. It is extracted from bauxite (Al₂O₃.2H₂O) in 2 steps (i) to obtain alumina from bauxite (ii) to obtain alumina metal from alumina.

i) To obtain alumina from bauxite

Bauxite contains impurities such as iron oxide and sand (silica) is also present. Alumina is obtained from bauxite by Bayer’s process, in this process bauxite powder is taken and concentrated (45%) sodium hydroxide solution is added and then heated in closed vessel at 433 K temperature and 5 to 6 bar pressure, by hours by doing this aluminium oxide present in bauxite is converted into sodium aluminate which is soluble in water.

Al₂O₃(s) + 2NaOH (aq) → 2NaAlO₂ (aq) + H₂O (l)

Iron oxide is insoluble in water so it is removed by filtration but silica form sodium silicate which is soluble in water. Now, sodium aluminate and sodium silicate are there as filtrate. Excess of water is added and continuously stirred so that hydrolysis of sodium aluminate takes place and forms aluminium hydroxide and sodium silicate remains as impurity.

2NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH

The precipitate is washed repeatedly with water, dried and on heating at 1473 K temperature pure aluminium oxide (alumina) is obtained.

ii) To obtain aluminium from alumina

To obtain aluminium from alumina is done by electrochemical reduction and also known as Hall-Heroult method. In this process cryolite (Na₃AlF₆) is added so that electrolysis could be done easily and this mixture also works as good electric conductor, further the melting point is also brought down by addition of feldspar(CaF₂).

The mixture of alumina, feldspar and cryolite is electrolysed in a vessel of iron having inner surface layered with carbon. In this the rods of carbon are joined by copper clamp which acts as anode and carbon layered graphite acts as cathode. When electric current is passed molten aluminium is deposited on cathode and dioxygen gas is produced at cathode. The molten aluminium is collected at the bottom of the cell is taken out.

Cathode: 2Al⁺³ + 6e^-→ 2Al

Anode: 3O^-2→ 3/2 O₂ 6e^-

The dioxygen gas produced at anode reacts with the rod of carbon and forms carbon dioxide, as a result anode gets corroded and should be frequently replaced.