Question

In aluminium extraction by the Baeyer’s process alumina is extracted from bauxite by sodium hydroxide at high temperature and pressure.
$A{l_2}{O_3} + 2O{H^ - } \to 2Al{O_2}^ - + {H_2}O$
Solid impurities such as $F{e_2}{O_3}{\text{ and }}Si{O_2}$ are removed and then $Al{(OH)_3}$ is reprecipitated.
$2Al(OH)_4^{ - 1} \to A{l_2}{O_3} + {H_2}O + 2O{H^ - }$

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 has evaporated.

Answer 1

Recall the leaching process of alumina from its ore called bauxite. The aluminate is formed during the middle of the process which is neutralised by passing carbon dioxide gas and at this stage solution is also seeded with freshly samples of hydrated alumina.

Complete step by step answer:
For the purpose of extraction of aluminium, bauxite ore is used. This is the principle ore of aluminium. It usually contains silica oxides, iron oxides, titanium oxides as impurities and the separation of alumina from the impurities in its ore is done by baeyer's process.
Concentration is carried out by digesting the powered ore with a concentrated solution of NaOH. This way $A{l_2}{O_3}$ is leached out and sodium aluminate leaving the impurities behind :
$A{l_2}{O_3} + 2NaOH + 3{H_2}O \to 2Na[Al{(OH)_4}]$

The aluminate in solution is neutralised by passing carbon dioxide gas and hydrated $A{l_2}{O_3}$is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated alumina which induces the precipitation.
$2Na[Al{(OH)_4}] + C{O_2} \to A{l_2}{O_3}.x{H_2}O + 2NaHC{O_3}$

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure $A{l_2}{O_3}$.
$A{l_2}{O_3}.x{H_2}O\xrightarrow {{1470K}}A{l_2}{O_3} + x{H_2}O$

Thus, in the extraction process of aluminium carbon dioxide in the industrial world, $C{O_2}$ is added to precipitate the alumina as well as temperature and pressure is dropped and the supersaturated solution seeded.
So, the correct answer is “Option C”.

Note: Leaching is the process in which an ore is treated with a suitable reagent as to make it soluble and the impurities remain insoluble. Baeyer’s process is the principle industrial method of refining bauxite ore to produce alumina. Remember that $C{O_2}$ gas is passed to neutralise the aluminate and temperature and pressure also dropped in the leaching process of alumina.