Explain Hall's process used for concentration of bauxite ore.

Explanation

i.In this method, the powdered bauxite ore is soaked in aqueous `Na_2CO_3` which dissolves the main ingredient of ore, alumina to form soluble sodium aluminate. The undissolved gangue that is left behind is filtered off.
`underset"Aluminium oxide"(Al_2O_3. 2H_2O_"(s)") + underset"Sodium carbonate"(Na_2CO_(3"(aq)")) to underset"Sodium aluminate"(2NaAlO_(2(aq))) + underset"Carbon dioxide"(CO_2 uarr) + underset"Water"(2H_2O_"(l)")`
ii.The filtrate is warmed and neutralized by passing carbon dioxide through it resulting in the precipitation of aluminium hydroxide.
`underset"Sodium aluminate"(2NaAlO_(2(aq))) + underset"Water"(3H_2O_"(l)") + underset"Carbon dioxide"(CO_(2(g))) to underset"Aluminium hydroxide"(2Al(OH)_3 darr) + underset"Sodium carbonate"(Na_2CO_3_((aq)))`
iii. The precipitate of aluminium hydroxide obtained in the process is filtered, washed, dried and calcined at `1000^@C` to get pure alumina.
`underset"Aluminium hydroxide"(2Al(OH)_(3(s))) underset(1000^@C) overset"heat"to underset"Aluminium oxide"(Al_2O_(3(s))) + underset"Water vapour"(3H_2O_((g)))`