Question

Bauxite is the principal ore used in the commercial extraction of aluminium. The Bayer’s process is used to refine bauxite into pure alumina, with caustic soda playing a crucial role in the initial stage of the process. Based on this, answer the following questions: 

(a) Explain the reason behind the addition of caustic soda during the Bayer’s process. 
(b) Write a balanced chemical equation representing the reaction between bauxite and caustic soda during the Bayer’s process. 
 

Hint:

The Bayer’s process is specifically a chemical method of refining, whereas the subsequent Hall-Héroult process is an electrolytic method of extraction.

Answer 1

In the Bayer’s process, bauxite (Al₂O₃·2H₂O) is purified to obtain pure alumina using caustic soda (NaOH).

(a) Reason for adding caustic soda:
Caustic soda is added because aluminium oxide (Al₂O₃) is amphoteric in nature.
It reacts with sodium hydroxide to form a soluble compound called sodium aluminate.

This helps in separating alumina from impurities like iron oxide (Fe₂O₃) and silica (SiO₂), which are insoluble in NaOH.
Thus, caustic soda is used to dissolve alumina selectively and leave behind impurities.

(b) Balanced chemical equation:
The reaction between bauxite (hydrated aluminium oxide) and caustic soda is:

Al₂O₃·2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O

Here:
- Aluminium oxide reacts with NaOH to form sodium aluminate (NaAlO₂)
- Water is also produced

Final Answers:
(a) Caustic soda dissolves amphoteric alumina to form soluble sodium aluminate, helping in removal of impurities.
(b) Al₂O₃·2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O