Question

In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperatures and pressures.
$A{l_2}{O_3}\left( s \right) + 2O{H^ - }\left( {aq} \right) \to 2Al{O_2}^ - \left( {aq} \right) + {H_2}O\left( l \right)$
Solid impurities such as $F{e_2}{O_3}$ ​ and $Si{O_2}$ are removed and then $Al{\left( {OH} \right)_4}^ -$ is reprecipitated $2Al{\left( {OH} \right)_4}^ - \to A{l_2}{O_3}.3{H_2}O\left( s \right) + 2O{H^ - }\left( {aq} \right)$.
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

In the Bayer process, the alkaline solution is cooled and treated with bubbling $C{O_2}$ to precipitate alumina $(A{l_2}{O_3})$, but the seeding of saturated solution by dropping the temperature and pressure makes the process feasible.

Complete step by step answer:
Alumina is the common name given to aluminium oxide $(A{l_2}{O_3})$. Alumina is produced from bauxite which is an ore from which it is mined from. The Bayer process is carried out in four different steps. The first step is, after the bauxite is crushed, washed and dried, it is dissolved with caustic soda at very high temperatures. In the next step, the mixture is filtered to remove the impurities which is called “red mud,” and is properly discarded. The remaining alumina solution is then transferred to tall tanks called precipitators.
$A{l_2}{O_3} + 2NaOH \to 2NaAl{O_2} + {H_2}O$
The original process was that the alkaline solution was cooled and then it was treated by the bubbling Carbon dioxide $\left( {C{O_2}} \right)$ through it. It is a method by which aluminium hydroxide precipitates.
$2NaAl{O_2} + 3{H_2}O + C{O_2} \to 2Al{\left( {OH} \right)_3} + N{a_2}C{O_3}$
This process gives way to seeding the supersaturated solution with high purity $Al{\left( {OH} \right)_3}$ crystal.
In the precipitator tank, the hot solution starts to cool and very small particles are added, the aluminium hydroxide seeds. The aluminium hydroxide seeds stimulate the precipitation of solid aluminium hydroxide crystals. The aluminium hydroxide settles at the bottom of the tank and is removed. In the final step, the aluminium hydroxide is washed of any remaining caustic soda and heated to remove excess water. After this process, alumina $(A{l_2}{O_3})$ emerges as a fine white powder which looks much like sugar used in baking, but is hard enough to scratch a pane of glass.

Therefore, the correct answer is option (C).

Note: The Bayer process was invented in 1887. It is the primary process by which alumina $(A{l_2}{O_3})$ is extracted from the bauxite ore. The Bayer process is still used to produce almost all of the world’s alumina supply. Alumina is used for key industrial purposes in the production of spark plug insulators and metallic paint, and its use as a fuel component for solid rocket boosters other than producing aluminium.