Question

Observe the following statements regarding purification of bauxite: 1\. During Hall's process, silica is removed as Si (vapour). 2\. Bauxite ore contaminated with \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) is purified in Baeyer's process. 3\. During Serpeck's process, AIN is formed. The correct answer is (a) 1,2 and 3 are correct (b) only 1 and 3 are correct (c) only 2 and 3 are correct (d) only 1 and 2 are correct

Short answer

Option (c) only 2 and 3 are correct.

Step-by-step solution

Analyzing Statement 1

Statement 1 claims that during Hall's process, silica is removed as Si (vapour). The Hall's process is primarily used for the extraction of aluminum from alumina, and it does not involve the removal of silica as Si (vapour). Therefore, Statement 1 is incorrect.

Analyzing Statement 2

Statement 2 claims that bauxite ore contaminated with \(\text{Fe}_2\text{O}_3\) is purified in Baeyer's process. The Baeyer's process is indeed used to remove iron oxides from bauxite during aluminum extraction. Therefore, Statement 2 is correct.

Analyzing Statement 3

Statement 3 claims that during Serpeck's process, AlN is formed. The Serpeck process involves the reaction of bauxite with carbon and nitrogen to form aluminum nitride (AlN), making this statement correct.

Evaluating the Options

Based on our analysis, Statement 2 and Statement 3 are correct, while Statement 1 is incorrect. Therefore, the correct option that includes both correct statements is (c) only 2 and 3 are correct.

Key concepts

Hall's Process

Hall's Process, named after Charles Martin Hall, is a crucial method used in the extraction of aluminum from bauxite ore. The process primarily involves two main steps: refining bauxite to obtain alumina and then reducing the alumina to produce pure aluminum.

• **Refining Bauxite**: The bauxite is first purified to extract alumina ( ext{Al}_2 ext{O}_3 ext{)}. This purification does not involve the removal of silica as Si vapor. This is a common misconception. Instead, impurities like iron oxide ( ext{Fe}_2 ext{O}_3 ext{)} are removed.
• **Electrolytic Reduction**: Once pure alumina is obtained, it is dissolved in molten cryolite ( ext{Na}_3 ext{AlF}_6 ext{)} to lower the melting point, making it more efficient for electrolysis. Through electrolysis, aluminum is deposited on the cathode, and oxygen is released at the anode.

Hall's Process revolutionized the aluminum industry as this method makes aluminum extraction more cost-effective and efficient, paving the way for its use in various industries.

Baeyer's Process

Baeyer's Process is essential in purifying bauxite ore, particularly those contaminated with iron oxide ( ext{Fe}_2 ext{O}_3 ext{)}. Unlike other methods that deal with silica or other impurities, this process excels at separating alumina from iron oxides.

• **Extraction of Alumina**: The bauxite ore is treated with a strong solution of sodium hydroxide ( ext{NaOH} ext{)}. This causes the alumina to dissolve, forming sodium aluminate, leaving behind impurities like iron oxides as a residue.
• **Precipitation and Separation**: By cooling and neutralizing the sodium aluminate solution, alumina is precipitated as hydrated alumina ( ext{Al(OH)}_3 ext{)}. This precipitate is then filtered and washed.
• **Calcination**: Finally, the hydrated alumina is heated to remove water, yielding pure alumina ( ext{Al}_2 ext{O}_3 ext{)} ready for the electrolytic reduction in Hall's Process.

The effectiveness of Baeyer's Process lies in its ability to yield high-purity alumina suitable for subsequent aluminum production processes.

Serpeck's Process

Serpeck's Process, introduced by Karl Josef Bayer, is less commonly used but noteworthy for its ability to process certain types of bauxite that other methods struggle with. This process is particularly useful for bauxites with silica impurities.

• **Reaction Process**: In this method, bauxite is treated with coke (carbon) and nitrogen at high temperatures. This reaction results in the formation of aluminum nitride (AlN).
• **Advantages**: By converting alumina to aluminum nitride, Serpeck's Process effectively deals with the presence of silica impurities by preventing them from interfering with aluminum recovery. This process is initiated when traditional methods like Baeyer's Process are ineffective due to the composition of the ore.
• **Challenges**: Despite its unique approach, the Serpeck's Process is less frequently employed due to its complexity and the difficulty in handling nitrogen efficiently at the required high temperatures.

While not as widely used, Serpeck's Process remains a pivotal development in scenarios involving challenging bauxite compositions, illustrating the versatility required in ore processing techniques.