Problem 42 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite? [Adv. 2020] (a) Hydrated \(\mathrm{Al}_{2} \mathrm{O}_{3}\) precipitates, when \(\mathrm{CO}_{2}\) is bubbled through a solution of sodium aluminate. (b) Addition of \(\mathrm{Na}_{3} \mathrm{AlF}_{6}\) lowers the melting point of alumina. (c) \(\mathrm{CO}_{2}\) is evolved at the anode during electrolysis. (d) The cathode is a steel vessel with a lining of carbon.

Step-by-Step Solution

Answer

All four statements (a), (b), (c), and (d) are true.

Step 1: Analyze Statement (a)

Statement (a) claims that hydrated \( \mathrm{Al}_{2} \mathrm{O}_{3} \) precipitates when \( \mathrm{CO}_{2} \) is bubbled through a solution of sodium aluminate. This is true. When \( \mathrm{CO}_{2} \) is passed through sodium aluminate, \( \text{NaAlO}_2 \), it reacts to form \( \text{Al(OH)}_3 \), a hydrated form of \( \text{Al}_2\text{O}_3 \), which then precipitates.

Step 2: Analyze Statement (b)

Statement (b) claims that the addition of \( \mathrm{Na}_{3} \mathrm{AlF}_{6} \) lowers the melting point of alumina. This is true. \( \mathrm{Na}_{3} \mathrm{AlF}_{6} \), also known as cryolite, acts as a flux to lower the melting point of alumina from about 2000°C to 900–1000°C, making the reduction process feasible.

Step 3: Analyze Statement (c)

Statement (c) claims that \( \mathrm{CO}_{2} \) is evolved at the anode during electrolysis. This is true for the Hall-Héroult process where carbon anodes are used, and oxygen produced at the anode combines with carbon to form \( \mathrm{CO}_{2} \) gas.

Step 4: Analyze Statement (d)

Statement (d) claims that the cathode is a steel vessel with a lining of carbon. This statement is true because, in the Hall-Héroult process, the steel vessel lined with carbon indeed serves as the cathode.

Key Concepts

Hall-Héroult Process

The Hall-Héroult process is the primary method used to extract aluminium from its ore, bauxite. It is an essential electrochemical procedure that occurs in a specialized cell designed for aluminium production. The process revolves around the electrolytic reduction of alumina (aluminium oxide, \( \mathrm{Al}_2\mathrm{O}_3 \)).
In this process:

• Alumina is dissolved in molten cryolite, which serves as a solvent to effectively lower its melting point.
• The electrolysis cell contains a steel vessel lined with carbon, which acts as the cathode, while carbon anodes are submerged in the cryolite-alumina mix.
• During electrolysis, the electric current passes through the solution, breaking down alumina into aluminium and oxygen gas.

The aluminium forms at the cathode and settles at the bottom of the cell, where it can be periodically siphoned off. Oxygen, on the other hand, is released at the anode, where it reacts with the carbon anode to release \( \mathrm{CO}_{2} \), explaining the gas evolution during electrolysis.
This method is widely used because it allows for efficient aluminium production on a large scale.

Sodium Aluminate

Sodium aluminate is a compound with the formula \( \text{NaAlO}_2 \), and it plays a vital role in the Bayer process, which is a precursor to aluminium extraction. The compound is formed when bauxite ore is treated with sodium hydroxide (NaOH) at high pressures and temperatures, ultimately leading to the extraction of alumina.
In the context of aluminium extraction:

• When carbon dioxide \( \mathrm{CO}_2 \) is bubbled through a solution of sodium aluminate, a reaction takes place, leading to the precipitation of aluminium hydroxide \( \text{Al(OH)}_3 \).
• The process is essential since aluminium hydroxide can be further processed through calcination to produce pure alumina \( \text{Al}_2\text{O}_3 \).

This precipitation reaction is crucial as it ensures the purity and quality of the alumina extracted, which will be used in the subsequent Hall-Héroult process. Sodium aluminate solutions are dynamically involved in the primary stages of making aluminium, underscoring their importance in refining processes.

Cryolite

Cryolite, chemically known as \( \text{Na}_3 \text{AlF}_6 \), is a key ingredient in the aluminium extraction industry. It is primarily used as a flux in the Hall-Héroult process to dissolve alumina and enable electrolytic reduction.

• One significant benefit of adding cryolite to alumina is its ability to lower the melting point from a very high 2000°C to approximately 900–1000°C.
• The reduced melting point makes it energy-efficient and economically viable to carry out the electrolytic reduction, allowing large-scale aluminium production.

Cryolite's unique properties also facilitate the setting up of ideal ionic conditions within the cell for electrolysis. By providing a medium that efficiently conducts electricity, it enhances the current efficiency and production yield. Thus, cryolite is indispensable in ensuring the practical and efficient extraction of aluminium at an industrial scale.