Question

Explain Bayer’s process of concentration of bauxite with a chemical equation.

Answer 1

1. Bauxite is the main ore of aluminium with silica (SiO₂), ferric oxide (Fe₂O₃) and titanium oxide (TiO₂) as impurities in it.
2. In this process, the bauxite ore is first crushed and then it is leached by heating with hot concentrated caustic soda (NaOH) solution under high pressure for 2 to 8 hrs at 140 to 150 °C in a tank called a digester.
3. Aluminium oxide, being amphoteric in nature, dissolves in an aqueous NaOH solution, forming water-soluble sodium aluminate.
   \[\ce{\underset{\text{oxide}}{\underset{\text{Aluminium}}{Al2O3.2H2O_{(s)}}} + \underset{\text{hydroxide}}{\underset{\text{Sodium}}{2NaOH_{(aq)}}} -> \underset{\text{aluminate}}{\underset{\text{Sodium}}{2NaAlO_{2(aq)}}} + \underset{\text{Water}}{3H2O_{(l)}}}\]
4. The iron oxide present in the gangue does not dissolve in an aqueous NaOH solution and is separated by filtration.
5. However, silica from the gangue dissolves in an aqueous NaOH solution forming water-soluble sodium silicate.
6. The solution of sodium aluminate is diluted with water and then cooled to 50 °C. This gives aluminium hydroxide as a precipitate.
   \[\ce{\underset{\text{aluminate}}{\underset{\text{Sodium}}{NaAlO_{2(aq)}}} + \underset{\text{Water}}{2H2O_{(l)}} -> \underset{\text{hydroxide}}{\underset{\text{Sodium}}{NaOH_{(aq)}}} + \underset{\text{hydroxide}}{\underset{\text{Aluminium}}{Al(OH)3\downarrow}}}\]
7. The aluminium hydroxide is then filtered, washed, dried and calcined by heating at 1000 °C to get pure aluminium oxide, called alumina.
   \[\ce{\underset{\text{hydroxide}}{\underset{\text{Aluminium}}{2Al(OH)_{3(s)}}} ->[heat][1000°C] \underset{\text{oxide}}{\underset{\text{Aluminium}}{Al2O_{3(s)}}} + \underset{\text{Water vapour}}{3H2O_{(g)}}}\]