Extraction of Aluminium from Bauxite Process

To determine the correct statement(s) regarding the extraction of aluminium metal from bauxite ore, let us analyze each statement individually based on the metallurgical processes involved (Bayer's process and the Hall–Héroult process):

1. Analysis of Statement (A): In Bayer's process for the leaching of bauxite ore, the crude bauxite is digested with a concentrated solution of $\text{NaOH}$ at high temperature and pressure to form soluble sodium aluminate: $\text{Al}_2\text{O}_3(s) + 2\text{NaOH}(aq) + 3\text{H}_2\text{O}(l) \rightarrow 2\text{Na}[\text{Al}(\text{OH})_4](aq)$ The resulting sodium aluminate solution is then neutralized by passing carbon dioxide ($\text{CO}_2$) gas through it, which lowers the $\text{pH}$ and causes the precipitation of hydrated aluminium oxide ($\text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O}$): $2\text{Na}[\text{Al}(\text{OH})_4](aq) + \text{CO}_2(g) \rightarrow \text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O}(s) + \text{Na}_2\text{CO}_3(aq) + (3-x)\text{H}_2\text{O}(l)$ Therefore, Statement (A) is correct.

2. Analysis of Statement (B): Pure alumina ($\text{Al}_2\text{O}_3$) has a very high melting point ($\approx 2323\text{ K}$ or $2050^\circ\text{C}$) and is a poor conductor of electricity in the solid state. During the Hall–Héroult process, cryolite ($\text{Na}_3\text{AlF}_6$) and/or fluorspar ($\text{CaF}_2$) are added to the alumina. This mixture serves two functions:

   • It significantly lowers the melting point of alumina to around $1173\text{ K}$ ($950^\circ\text{C}$).
   • It increases the electrical conductivity of the molten bath. Therefore, Statement (B) is correct.

3. Analysis of Statement (C): In the Hall–Héroult electrolytic cell, carbon (graphite) rods serve as the anode. During electrolysis, oxygen ions ($\text{O}^{2-}$) are discharged at the carbon anode, where they react with the carbon rods at high temperature to form carbon monoxide ($\text{CO}$) and carbon dioxide ($\text{CO}_2$) gases: $\text{C}(s) + \text{O}^{2-}(\text{melt}) \rightarrow \text{CO}(g) + 2e^-$ $\text{C}(s) + 2\text{O}^{2-}(\text{melt}) \rightarrow \text{CO}_2(g) + 4e^-$ Thus, $\text{CO}_2$ gas is evolved at the anode. Therefore, Statement (C) is correct.

4. Analysis of Statement (D): The cell used for the electrolysis of alumina consists of a heavy steel vessel lined on the inside with carbon (graphite), which acts as the cathode. At the cathode, aluminium ions are reduced to liquid aluminium metal: $\text{Al}^{3+}(\text{melt}) + 3e^- \rightarrow \text{Al}(l)$ Therefore, Statement (D) is correct.

Conclusion: All four statements (A), (B), (C), and (D) are correct.