Question

How is aluminium extracted from its ores?

Answer 1

Aluminium is mainly extracted from bauxite. The extraction of aluminium from bauxite is carried out in the following steps.

Purification of bauxite: About 56% of bauxite is Al₂O₃, with impurities including iron, silicon, and titanium oxides. Before bauxite is processed, these impurities must be eliminated in order to get pure aluminium. The following procedures are used to purify bauxite.

(a) Hall’s process: Bauxite is crushed and fused with sodium carbonate when sodium meta aluminate is formed.

\[\ce{Na2CO3 + Al2O3 * 2H2O ->[Heat] \underset{aluminate}{\underset{Sodium meta}{2NaAlO2}}}\]

When sodium meta aluminate enters the solution and the iron oxide impurities precipitate, the fused mass is lixivated with water. After filtering, the clear solution is heated to 323–333K and carbon dioxide is injected through it to cause aluminium hydroxide to precipitate.

\[\ce{2NaAlO2 + CO2 + 3H2O -> Na2CO3 + 2Al(OH)3 \downarrow}\]

The precipitate of Al(OH)₃ is now ignited to 1473 K when pure alumina (Al₂O₃) is obtained.

\[\ce{2Al(OH)3 ->[1473 K] Al2O3 + 3H2O}\]

Baeyer’s process: When there is a greater proportion of i',:on oxide in bauxite, this procedure is employed. To turn any ferrous oxide into ferric oxide, the finely ground bauxite ore is first roasted at a low temperature. When aluminium oxide enters the solution while NaAlO₂ and ferric oxide do not dissolve, the roasted ore is now digested using a hot, concentrated solution of caustic soda at 423 K under pressure. Sodium silicate (Na₂SiO₃) is created when some of the silica that is present as an impurity dissolves in sodium hydroxide.

\[\ce{Al2O3 * 2H2O + 2NaOH -> 2NaAlO2 + 3H2O}\]

\[\ce{SiO2 + 2NaOH -> NaSiO3 + H2O}\]

Sodium silicate and sodium meta aluminate are present in the filtered solution. A tiny amount of recently precipitated aluminium hydroxide from the prior process is added to this after it has been diluted with water. After stirring the combination for a few hours, sodium silicate is left in the solution as aluminium hydroxide precipitates. To obtain pure alumina, this is filtered and burnt.

\[\ce{NaAlO2 + 2H2O -> NaOH + Al(OH)3}\]

\[\ce{2AlOH3 ->[1473 K] Al2O3 + 3H2O}\]