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  4. In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperature and pressures: Al2 O3 (s)+2OH- (aq) →2Al2 O-2(aq)+H2O (l) Solid impurities such as Fe2O3 and SiO2 are removed and then Al(OH)-4 is reprecipitated: 2Al(OH)4- → Al2 O3. 3H2O (s)+2OH- (aq). In the industrial world :

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Q. In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperature and pressures :
$Al_{2} O_{3} \left(s\right)+2OH^{-} _{(aq) }\to2Al_{2} O^{-}_{2(aq)}+H_{2}O _{(l)}$
Solid impurities such as $Fe_{2}O_{3}$ and $SiO_{2}$ are removed and then $Al\left(OH\right)^{-}_{4}$ is reprecipitated :
$2Al\left(OH\right)_{4}^{-} \to Al_{2} O_{3}. 3H_{2}O_ {(s)}+2OH^{-}_ {(aq)}.$ In the industrial world :

WBJEEWBJEE 2009General Principles and Processes of Isolation of Elements

Solution:

Industrially, in the extraction of alumina, temperature and pressure are dropped and the super saturated solution seeded. Sometimes $CO _{2}$ is also used to precipitate alumina.