Question

Explain the difference between Bayer’s process and Hall’s process by explaining the Bayer’s process.

Answer 1

• Difference between Bayer’s process and Hall’s process:
  In Bayer’s process, aluminium ore is heated with concentrated caustic soda while in Hall’s process, it is heated with aqueous sodium carbonate to obtain water-soluble sodium aluminate. In Bayer’s process, the solution of sodium aluminate is diluted with water to obtain aluminium hydroxide while in Hall’s process, sodium aluminate is converted to aluminium hydroxide by reaction with CO₂.
• Bayer’s process:

1. 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.
2. Aluminium oxide, being amphoteric in nature, dissolves in an aqueous NaOH solution, forming water-soluble sodium aluminate.
   \[\ce{\underset{\text{oxide}}{\underset{\text{Aluminium}}{Al2O3.2H2O_{(s)}}} + \underset{\text{hydroxide}}{\underset{\text{Sodium}}{2NaOH_{(aq)}}} -> \underset{\text{aluminate}}{\underset{\text{Sodium}}{2NaAlO_{2(aq)}}} + \underset{\text{Water}}{3H2O_{(l)}}}\]
3. The iron oxide present in the gangue does not dissolve in an aqueous NaOH solution and is separated by filtration. However, silica from the gangue dissolves in an aqueous NaOH solution forming water-soluble sodium silicate.
4. The solution of sodium aluminate is diluted with water and then cooled to 50 °C. This gives aluminium hydroxide as a precipitate.
   \[\ce{\underset{\text{aluminate}}{\underset{\text{Sodium}}{NaAlO_{2(aq)}}} + \underset{\text{Water}}{2H2O_{(l)}} -> \underset{\text{hydroxide}}{\underset{\text{Sodium}}{NaOH_{(aq)}}} + \underset{\text{hydroxide}}{\underset{\text{Aluminium}}{Al(OH)_3}\downarrow}}\]
5. The aluminium hydroxide is then filtered, washed, dried and calcined by heating at 1000 °C to get pure aluminium oxide, called alumina.
   \[\ce{\underset{\text{hydroxide}}{\underset{\text{Aluminium}}{2Al(OH)_{3(s)}}} ->[heat][1000 °C] \underset{\text{oxide}}{\underset{\text{Aluminium}}{Al2O_{3(s)}}} + \underset{\text{vapour}}{\underset{\text{Water}}{3H2O_{(g)}}}}\]