Question

How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.

Answer 1

Baeyer’s process: When there is a greater proportion of iron 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.

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

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

Serpek’s process: When there is a greater proportion of silica present in the bauxite ore as an impurity, this procedure is employed. This method involves heating powdered bauxite ore to 2073 K in a nitrogen current while combining it with carbon (coke). The ore’s aluminium oxide transforms into aluminium nitride, while SiO₂ impurities are converted to silicon, which is volatile and evaporates as vapour.

\[\ce{Al2O3 + 3C + N2 -> 2AlN + 3CO}\]

\[\ce{SiO2 + 2C -> Si \uparrow + 2CO \uparrow}\]

Water breaks down the resulting aluminium nitride to produce aluminium hydroxide, which, when burnt, produces pure alumina.

\[\ce{AlN + 3H2O -> Al(OH)3 + NH3}\]

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