Explain briefly the extraction of aluminium from bauxite.

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Solution

The extraction of the metal from bauxite involves the three main steps.
Concentration of Bauxite $({A l}_{2} O_{3} .2 H_{2} O)$
Concentration of ore is done by leaching which helps in removing impurities like ${F e}_{2} O_{3}, S i O_{2}$ , etc. Bauxite is treated with $N a O H$ to form sodium aluminate which on further reaction with $C O_{2}$ gives pure Alumina.
${A l}_{2} O_{3} + 2 N a O H + 3 H_{2} O ⟶ 2 N a [A l {(O H)}_{4}] 2 C O_{2} - ------ \to - 2 N a H C O_{3} {A l}_{2} O_{3} . x H_{2} O Δ - --- \to 1470 K {A l}_{2} O_{3} Pure alumina + x H_{2} O$
Electrolytic reduction of Alumina $({A l}_{2} O_{3})$
Electrolytic reduction of pure alumina takes place in iron box (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. Graphide rods act as anode.
At cathode:-
${A l}^{3 +} + 3 e^{-} ⟶ A l$
At anode:- The oxygen liberated at anode reacts with the carbon of anode to produce $C O$ and $C O_{2}$ .
$C + O^{2 -} ⟶ C O + 2 e^{-}$
$C + 2 O^{2 -} ⟶ C O_{2} + 4 e^{-}$
By this process $98.8 %$ pure aluminium is obtained.

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Explain the significance of the following in the extraction of aluminium.
(i) Bauxite
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