In leaching of alumina from bauxite by Bayer’s process, the ore is treated with NaOH (aq). Concentration is carried out by heating the powdered ore with a concentrated solution of NaOH at $473-523K$ and 35 – 36 bar pressure. $A{l}_2{O}_3$ is extracted out as sodium aluminate.
$A{l}_2{O}_3\left(s\right)+2NaOH\left(aq\right)+3H_{2}o\left(l\right)\to 2Na\left[Al{\left(OH\right)}_4\right]\left(aq\right)$
The sodium aluminate formed is neutralised by passing $C{O}_2$ gas and hydrated $A{l}_2{O}_3$ is precipitated.
$2Na\left[Al{\left(OH\right)}_4\right]\left(aq\right)+C{O}_2\left(g\right)\to A{l}_2{O}_3.x{H}_{2}O\left(s\right)+2NaHC{O}_3\left(aq\right)$