In the extraction of Aluminium, the silica impurity is removed by

Analyze the question: The question asks how silica (SiO₂) impurity is removed when extracting aluminium from its ore, bauxite (Al₂O₃·2H₂O).

Concept: Purification of bauxite (Bayer process)

1. Bauxite is crushed and digested with hot concentrated NaOH solution.
2. Amphoteric Al₂O₃ dissolves as sodium aluminate: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
3. Iron(III) oxide (Fe₂O₃) does not dissolve in NaOH. It is removed by filtration as "red mud".
4. Silica (SiO₂) is acidic, so it dissolves in NaOH to form sodium silicate: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
5. The solution is then diluted and seeded with Al(OH)₃ crystals, which precipitates pure Al(OH)₃. The sodium silicate remains in solution.
6. Al(OH)₃ is heated to give pure Al₂O₃, which is then electrolysed in molten cryolite.

Evaluating the options:

• A. Electrolysis: this is the final step that extracts aluminium metal, not impurity removal.
• B. Filtration: this removes the insoluble red mud (Fe₂O₃), not the dissolved silica.
• C. Reaction with H₂SO₄: this is not part of the standard Bayer process.
• D. Precipitation: aluminium hydroxide is precipitated out, leaving the silicate behind in solution. This is how silica is separated.

Note on the key: The wording of the question is loose, but of the options given, precipitation of Al(OH)₃ is the step that separates the aluminium from the dissolved silica.

Answer: D. precipitation

Exam tip: Remember the Bayer process as "Dissolve, Filter, Precipitate, Heat, Electrolyse". Iron oxide is removed by filtration, while silica stays dissolved as silicate when pure Al(OH)₃ is precipitated.