Question

Which of the following is related to extraction of aluminium from bauxite:

• A. Baeyer process
• B. Hall’s method
• C. Froth floatation process
• D. Roasting

Answer 1

Answer: (A), (B)

Hall’s method

Baeyer process:
Finely powdered bauxite is digested with an aqueous solution of sodium hydroxide in an autoclave at ${150}^{\circ }\text{C}\text{and}80$ atmospheric pressure. Alumina present in bauxite dissolves as sodium meta aluminate. The other impurities remain insoluble. These are filtered out.
$A{l}_2{O}_3.2H_{2}O+2NaOH⟶2NaAl{O}_2+3H_{2}O$
The filtrate containing sodium meta aluminate is agitated with freshly precipitated $Al(OH{)}_3\text{for}36$ hours. Sodium meta aluminate hydrolyses to $Al(OH{)}_3$, $NaAl{O}_2+2H_{2}O⟶NaOH+Al(OH{)}_3$.
Then $Al(OH{)}_3$ is washed, filtered, dried and heated at about ${1500}^{\circ }\text{C}$. Alumina is obtained.

Hall’s method:
In this method, bauxite ore is fused with $N{a}_{2}C{O}_3$ and the sodium meta illuminate formed is extracted with water. Carbon dioxide gas is passed through the sodium meta illuminate solution at temperature $50-{60}^{\circ }\text{C}$, $Al(OH{)}_3$ gets precipitated. $Al(OH{)}_3$ thus obtained is filtered, dried and heated in a furnace at about ${1500}^{\circ }\text{C}$ to obtain $A{l}_2{O}_3$.