Question

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

• A. Hydrated $Al _{2} O _{3}$ precipitates, when $CO _{2}$ is bubbled through a solution of sodium aluminate.
• B. Addition of $Na _{3} AlF _{6}$ lowers the melting point of alumina.
• C. $CO _{2}$ is evolved at the anode during electrolysis.
• D. The cathode is a steel vessel with a lining of carbon.

Answer 1

Answer: (A), (B), (C), (D)

Sodium meta aluminate is precipitated in the form of $Al ( OH )_{3}$ or $Al _{2} O _{3} \cdot xH _{2} O$.
i.e hydrated aluminium hydroxide by passing $CO _{2}$ gas.
$2 NaAlO _{2}+ CO _{2}( g )+3 H _{2} O (\ell) \longrightarrow \underset{ Or Al _{2} O _{3} \cdot kH _{2} O }{2 Al ( OH )_{3} } \downarrow+ Na _{2} CO _{3}$
In electrolytic reduction of $Al _{2} O _{3}$ by Hall-Haroult process,
cryolite $\left( Na _{3} AlF _{6}\right)$ and fluorspar $\left( CaF _{2}\right)$ are added in order to reduce the melting temperature of $Al _{2} O _{3}$.
Anode is made up of graphite,
So, $C ( s )+2 O ^{2-} \longrightarrow CO _{2}( g )+4 e ^{-}$
Cathode is a steel vessel lined with graphite.