Question

Common impurities present in bauxite are
A. \[{\text{CuO}}\]
B. ${\text{ZnO}}$
C. ${\text{CaO}}$
D. ${\text{Si}}{{\text{O}}_2}$

Answer 1

Hint: Bauxite is a mixture of different compounds. They include aluminium compounds, iron compounds, silicon compounds and titanium compounds. They have a comparatively high amount of aluminium compounds.

Complete step by step solution:
Bauxite is a clay-like material which is reddish brown in color. It is the main ore of aluminium. It is also a source of gallium. It is not a single composition, more a range of hydrated aluminium oxides and
hydroxides. It is found as a weathering product in soils in hot subtropical and tropical climates. Only 30-60% of bauxite’s composition is included by aluminium. Bauxite contains aluminium in the form of hydrated aluminium hydroxides. They are gibbsite, boehmite, and diaspora. Rest of them are a mixture of iron oxides, titanium oxides and silica. The iron oxides are goethite and hematite. It also includes clay kaolinite. Therefore it must be refined to get the pure aluminium metal. This is done by Bayer’s process.
This process involves three stages:
1) Preparation of pure alumina from bauxite
2) Electrolysis of alumina
3) Purification of aluminium.

We know, bauxite contains several impurities. From the given options the fourth option ${\text{Si}}{{\text{O}}_2}$ which is silicon dioxide, is relevant to the answer.

Therefore option D is correct.

Additional information:
Pure alumina is extracted from bauxite by dissolving it in hot caustic soda, i.e., sodium hydroxide. It is then followed by purification and precipitation of aluminium oxide. Aluminium oxide is the raw material for extracting into pure aluminium by electrolysis.

Note: Gibbsite is alumina trihydrate while boehmite is alumina monohydrate. Bauxites are mainly used for aluminium production. It is also used for production of refractory material, chemicals and cements.