Question

Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite?

• A. Hydrated $Al _{2} O _{3}$ precipitates, when $CO _{2}$ is bubbled through a solution of sodium aluminate.
• B. Addition of $Na _{3} AlF _{6}$ lowers the melting point of alumina.
• C. $CO _{2}$ is evolved at the anode during electrolysis.
• D. The cathode is a steel vessel with a lining of carbon.

Answer 1

The Correct Option is A, B, C, D

(A) The extraction of aluminum is achieved through the Hall-Héroult process, where Al₂O₃ is obtained as a precipitate by bubbling CO₂ through a solution of sodium aluminate, resulting in the reaction:

\(2\text{Na}[ \text{Al}(\text{OH})_4](\text{aq.}) + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + 2\text{Al}(\text{OH})_3(\downarrow) \text{ or } \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} (\text{ppt})\)

(B) The electrolytic reduction of pure alumina occurs in a steel box lined with carbon (cathode), along with the presence of cryolite (Na₃AlF₆) and fluorspar (CaF₂), which decrease the melting point and enhance the electrolyte's conductivity.

(C) In the electrolysis process of the Hall-Héroult method, graphite rods serve as anodes. At the cathode, aluminum ions are reduced:

\(\text{Al}^{3+} + 3e^- \rightarrow \text{Al}\)

At the anode, the liberated oxygen reacts with the carbon of the anode to form CO and CO₂:

\(\text{C} + \text{O}_2^- \rightarrow \text{CO} + 2e^-\)
\(\text{C} + 2\text{O}_2^- \rightarrow \text{CO}_2 + 4e^-\)

(D) In this context, the cathode consists of a steel vessel with a carbon lining.