Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite?

C O_{2}

is evolved at the anode during electrolysis

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Addition of

N a_{3} A l F_{6}

lowers the melting point of alumina.

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Hydrated

A l_{2} O_{3}

precipitates, when

C O_{2}

is bubbled through a solution of sodium aluminate.

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The cathode is a steel vessel with a lining of carbon.

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Solution

The correct option is D The cathode is a steel vessel with a lining of carbon.
Answer: A, B, C, D
a)Extraction of aluminium (Hall's process and Hall Heroult's electrolytic cell):
The process involved in extraction of aluminium is Hall Heroult’s process. During process, $A l_{2} O_{3}$ is obtained as precipitate when $C O_{2}$ is bubbled through a solution of sodium aluminate. The following reaction takes place:
$2 N a A l O_{2} + C O_{2} (g) + 3 H_{2} O (l) \to 2 A l (O H)_{3} (o r A l_{2} O_{3} . x H_{2} O) + N a_{2} C O_{3}$

b) Electrolytic reduction of pure alumina takes place in steel box with lining of carbon (cathode) with cryolite( $N a_{3} A l F_{6}$ ) and fluorspar( $C a F_{2}$ ) which lowers the melting point and increases the conductivity of electrolyte.

c) Electrolysis process in Hall’s process:
Graphite rods acts as anode:
At cathode: $A l^{3 +} + 3 e^{-} \to A l$

At anode: The oxygen liberated at anode reacts with the carbon of anode to produce CO and CO₂.
$C + O^{2 -} \to C O + 2 e^{-}$

$C + 2 O^{2 -} \to C O_{2} + 4 e^{-}$

d) Here the cathode is a steel vessel with a lining of carbon.

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