Question

Consider the following steps involved in the extraction of Aluminium. What is \(Z\)? \[ \text{Bauxite} \xrightarrow[\;523K\;]{\text{Hot conc. NaOH}} X(aq) \xrightarrow[\;CO_2\;]{} Y \xrightarrow[\;1473K\;]{} Z \xrightarrow[\;\text{electrolysis}\;]{} Al \]

• A. \(Al(OH)_3\)
• B. \(Al_2(CO_3)_3\)
• C. \(Al_2O_3\)
• D. \(Al(HCO_3)_3\)

Hint:

In aluminium metallurgy always remember: \[ \text{Bauxite} \rightarrow Al(OH)_3 \rightarrow Al_2O_3 \rightarrow Al \] Electrolysis is carried out on alumina, not directly on bauxite.

Answer 1

The Correct Option is C

Concept: The extraction of aluminium from bauxite involves purification of bauxite by Bayer's process followed by electrolytic reduction of alumina through Hall-Heroult process. The important sequence is: \[ \text{Bauxite} \rightarrow \text{Sodium aluminate} \rightarrow Al(OH)_3 \rightarrow Al_2O_3 \rightarrow Al \] Only pure alumina (\(Al_2O_3\)) is used in the electrolytic extraction of aluminium.

Step 1: Formation of sodium aluminate Bauxite is treated with hot concentrated NaOH. \[ Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4] \] Thus \(X\) is sodium aluminate solution.

Step 2: Precipitation of aluminium hydroxide Passing carbon dioxide through sodium aluminate solution gives aluminium hydroxide. \[ 2NaAlO_2 + CO_2 + 3H_2O \rightarrow 2Al(OH)_3 + Na_2CO_3 \] Thus \(Y = Al(OH)_3\).

Step 3: Conversion into alumina On heating aluminium hydroxide to high temperature: \[ 2Al(OH)_3 \rightarrow Al_2O_3 + 3H_2O \] Hence \[ Z = Al_2O_3 \]

Step 4: Electrolysis The Hall-Heroult process electrolyses molten alumina dissolved in cryolite to produce aluminium metal. Therefore the substance immediately before electrolysis must be: \[ \boxed{Al_2O_3} \] Hence the correct option is \[ \boxed{\text{(C)}} \]