Question

Complete the following statement using every given options.
During the extraction of aluminium..............
a. Ingredients and gangue in bauxite.
b. Use of leaching during the concentration of ore.
c. Chemical reaction of transformation of bauxite into alumina by Hall’s process.
d. Heating the aluminium ore with concentrated caustic soda.

Answer 1

a. Bauxite is the main ore of aluminium. Silica (SiO₂), ferric oxide (Fe₂O₃) and titanium oxide (TiO₂) are the impurities present in bauxite.
Ingredient in bauxite are molten cryolite (Na₃AIF₆), fluorspar (CaF₂).

b. The separation of impurities(silica (SiO₂), ferric oxide (Fe₂O₃) and titanium oxide (TiO₂)) in bauxite ore is done by leaching process using either Bayer’s method or Hall’s method.

c. In the Hall’s process the bauxite is in powdered form and then leached by heating with aqueous sodium carbonate in the digester to form water soluble sodium aluminate. Then the insoluble impurities are filtered out. The filtrate is warmed and neutralised by passing carbon dioxide gas through it. This results in the precipitation of aluminium hydroxide.
$$\begin{gathered} A{l}_2{O}_{3}2H_{2}O\left(s\right)+N{a}_{2}C{O}_3\left(aq\right)\to 2NaAl{O}_2\left(aq\right)+C{O}_2+2H_{2}O\left(l\right) \\ 2NaAl{O}_2\left(aq\right)+3H_{2}O+C{O}_2\left(g\right)\to 2Al(OH{)}_3+N{a}_{2}C{O}_3 \end{gathered}$$
The precipitate of Al(OH)₃ obtained in both, Bayer’s and Hall’s processes is filtered, washed, dried and then calcined by heating at 1000⁰C to obtain alumina.
$2Al(OH{)}_3\to A{l}_2{O}_3+3H_{2}O$

d. When aluminium ore is heated with caustic soda (NaOH) solution under high pressure for 2-8 hours at 1400-1500C, water soluble sodium aluminate is formed.
$A{l}_2{O}_3+2NaOH\to 2NaAl{O}_2+H_{2}O$