Describe Bayer's process for concentration of bauxite ore.

Explanation

i.Bauxite is the main ore of aluminium with silica `(SiO_2)`, ferric oxide `(Fe_2O_3)` and titanium oxide `(TiO_2)` as impurities in it.
ii. In this process, the bauxite ore is first crushed and then it is leached by heating with hot concentrated caustic soda (NaOH) solution under high pressure for 2 to 8 hrs at 140 to `150^@C` in a tank called digester.
iii. Aluminium oxide, being amphoteric in nature, dissolves in aqueous NaOH solution, forming water soluble sodium aluminate.
`underset"Aluminium oxide"(Al_2O_3.2H_2O_((s))) + underset"Sodium hydroxide"(2NaOH_((aq))) + underset"Water"(3H_2OO_((l)))`
iv. The iron oxide present in the gangue does not dissolve in aqueous NaOH solution and is separated by filtration.
v.However, silica from the gangue dissolves in aqueous NaOH solution forming water soluble sodium silicate.
v.The sodium of sodium aluminate is diluted with water and then colled to `50^@C` .This gives aluminium hydroxide as a precipitate .
`underset"Sodium aluminate"(NaAl_(2(aq))) + underset"Water"(2H_2O_(l)) to underset"Sodium hydroxide"(NaOH_((aq))) + underset"Aluminium hydroxide"(Al(OH)_3 darr)`
vii. The aluminium hydroxide is then filtered, washed, dried and calcined by heating at `1000^@C` to get pure aluminium oxide, called 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)))`