Identify the processes involved in extraction of aluminium from bauxite

Extraction of aluminium from bauxite mainly involves:

• Purification of bauxite by Bayer process
• Electrolytic reduction of alumina by Hall–Héroult process

Check each option one by one

Option A

"the reaction of $\mathrm{Al_2O_3}$ with coke $(\mathrm C)$ at a temperature $>2500^\circ\mathrm C$"

This is not used for extraction of aluminium.

Reason: $\mathrm{Al_2O_3}$ is a very stable oxide and cannot be reduced by carbon in the usual metallurgical way. In fact, at such high temperatures, carbide formation may occur instead of obtaining pure aluminium.

So, A is incorrect.

Option B

"the neutralization of aluminate solution by passing $\mathrm{CO_2}$ gas to precipitate hydrated alumina $\left(\mathrm{Al_2O_3\cdot 3H_2O}\right)$"

In Bayer process, bauxite is digested with hot aqueous NaOH to form sodium aluminate solution. From this aluminate solution, hydrated alumina is precipitated.

One way to represent precipitation is by passing $\mathrm{CO_2}$: $2\mathrm{NaAlO_2}+\mathrm{CO_2}+3\mathrm{H_2O}\rightarrow \mathrm{Al_2O_3\cdot 3H_2O}+\mathrm{Na_2CO_3}$

Thus, this step is involved in extraction/purification.

So, B is correct.

Option C

"the dissolution of $\mathrm{Al_2O_3}$ in hot aqueous $\mathrm{NaOH}$"

In Bayer process, alumina present in bauxite dissolves in hot concentrated NaOH due to amphoteric nature: $\mathrm{Al_2O_3}+2\mathrm{NaOH}+3\mathrm{H_2O}\rightarrow 2\mathrm{Na[Al(OH)_4]}$ or equivalently as sodium aluminate formation.

Hence this is indeed part of aluminium extraction.

So, C is correct.

Option D

"the electrolysis of $\mathrm{Al_2O_3}$ mixed with $\mathrm{Na_3AlF_6}$ to give $\mathrm{Al}$ and $\mathrm{CO_2}$"

In Hall–Héroult process, alumina is dissolved in molten cryolite $\left(\mathrm{Na_3AlF_6}\right)$ and electrolysed.

Cathode reaction: $\mathrm{Al^{3+}}+3e^-\rightarrow \mathrm{Al}$

Oxygen formed at the anode reacts with carbon anodes: $\mathrm{C}+\mathrm{O_2}\rightarrow \mathrm{CO_2}$

Therefore overall aluminium is obtained and $\mathrm{CO_2}$ is produced.

So, D is correct.

Final derived answer

Correct options are: $\boxed{B,\ C,\ D}$

Comparison with stored correct answer

Stored correct answer: B, D, C

This matches exactly with the derived set of correct options.