Question

Draw the labelled diagram of electrolytic cell of alumina and write the chemical reactions taking place in it?

Answer 1

If Bauxite contains $F{e}_2{O}_3$ as impurity, it is purified by Baeyer's process.
Electrolysis of alumina (Hall-Heroult process): Electrolysis of alumina is done in an iron tank fitted with a lining of gas carbon which acts as cathode. Graphite rods act as anode and a bulb is fitted in parallel as indicator for alumina content. The electrolyte is a mixture of alumina and cryolite $\left(N{a}_{3}Al{F}_6\right)$. The melting point of alumina is very high $\left({2050}^{o}C\right)$. But, in presence of cryolite $\left(N{a}_{3}Al{F}_6\right)$ and fluorspar $\left(Ca{F}_2\right)$, it melt at ${870}^{o}C$. Then pure alumina is added to the tank which dissolve in cryolite mixture.
Reaction: Reaction take place as below
$N{a}_{3}Al{F}_6\to 3NaF+Al{F}_3$
$Al{F}_3\to A{l}^{3+}3F^{-}$
$3F^{-}\to 3e^{-}+3F$ (oxidation) at anode
$A{l}^{3+}3e^{-}\to Al$ (reduction) at cathode
$2A{l}_2{O}_3+12F\to 4Al{F}_3+3O_2\uparrow$