Question

In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperatures and pressures.
${Al}_2{O}_3\left(s\right)+2O{H}^{-}\left(aq\right)\to 2Al{O}_2^{-}\left(aq\right)+H_{2}O\left(l\right)$
Solid impurities such as ${Fe}_2{O}_3$ and $Si{O}_2$ are removed and then $Al{\left(OH\right)}_4^{-}$ is reprecipitated
$2Al{\left(OH\right)}_4^{-}⟶{Al}_2{O}_3\cdot 3H_{2}O\left(s\right)+2O{H}_{\left(aq\right)}^{-}$.
In the industrial world __________.

• A. carbon dioxide is added to precipitate the alumina
• B. temperature and pressure are dropped and the supersaturated solution seeded
• C. both (A) and (B) are practised
• D. the water is evaporated

Answer 1

The correct option is C both (A) and (B) are practised

In original process the alkaline solution was cooled and treated with bubbling $C{O}_2$ to precipitate alumina.

but the seeding of saturated solution by dropping the temperature and pressure makes the process feasible.