In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperature and pressures :
\(\mathrm{Al}_2 \mathrm{O}_3(\mathrm{~s})+2 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{Al}_2 \mathrm{O}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(1)\)

Solid impurities such as \(\mathrm{Fe}_2 \mathrm{O}_3\) and \(\mathrm{SiO}_2\) are removed and then \(\mathrm{Al}(\mathrm{OH})_4^{-}\)is reprecipitated : \(2 \mathrm{Al}(\mathrm{OH})_4^{-} \rightarrow \mathrm{Al}_2 \mathrm{O}_3 \cdot 3 \mathrm{H}_2 \mathrm{O}(\mathrm{s})+2 \mathrm{OH}^{-}\)(aq) . In the industrial world :

1. A Carbon dioxide is added to precipitate the alumina
2. B Temperature and pressure are dropped and the supersaturated solution seeded
3. C Both (1) and (2) are practised
4. D The water is evaporated