Question

How is alumina separated from silica in the bauxite ore associated with silica? Give equations.

Answer 1

Bauxite will be dissolved to form sodium aluminate. Impurity will be left out behind.$C{O_2}$ is passed to neutralize the aluminate leading to precipitation of the hydrated alumina, also called aluminum hydroxide. This aluminum hydroxide is filtered, washed, and dried to obtain alumina.

Complete answer: To separate alumina from silica in the bauxite ore associated with silica the process used is called leaching. In this process first, the powdered ore is heated with a concentrated NaOH solution at a temperature of around $473 - 523K$ and pressure around $35 - 36$ bars. This leads to the leaching out of alumina ($A{l_2}{O_3}$) in the form of sodium aluminate and silica ($Si{O_2}$) is left out behind as impurities in the form of sodium silicate.

$$\mathop {A{l_2}{O_3}_{(S)}}\limits_{\left( {{\text{Alumina}}} \right)} \; + {\text{ }}2NaO{H_{(aq)}}\; + {\text{ }}{H_2}{O_{(l)}} \to 2\mathop {Na\left[ {Al{{\left( {OH} \right)}_4}} \right]{\;_{(aq)}}}\limits_{{\text{(sodiu}}{{\text{m}}_{}}{\text{alumina)}}} \\\ \\\ \mathop {Si{O_2}}\limits_{{\text{(silica)}}} \; + {\text{ }}2NaO{H_{(aq)}}{\text{ }} \to {\text{ }}\mathop {N{a_2}Si{O_{3(aq)}}}\limits_{{\text{(sodium silicate)}}} \; + {\text{ }}{H_2}{O_{(l)}} \\\$$

After this, $C{O_2}$ gas is passed through the resulting solution this $C{O_2}$ neutralizes the aluminate in the solution and this results in the precipitation of hydrated alumina. The precipitation is induced when the solution is seeded with the freshly prepared samples of hydrated alumina.
$\mathop {2Na{{[Al{{\left( {OH} \right)}_4}]}_{\left( {aq} \right)}}}\limits_{{\text{(sodium aluminate)}}} + {\text{ }}C{O_{2\left( g \right)}}\; \to {\text{ }}\mathop {A{l_2}{O_3}.x{H_2}{O_{\left( s \right)}}}\limits_{{\text{(hydrated alumina)}}} + \mathop {2NaHC{O_{3\left( {aq} \right)}}}\limits_{\left( {{\text{sodium hydrogen carbonate}}} \right)}$
When this process is taking place, sodium silicate remains in the solution. The obtained hydrated alumina is first filtered, then dried, and finally heated to get pure alumina back.
$\mathop {A{l_2}{O_3}.x{H_2}{O_{\left( s \right)}}}\limits_{{\text{(hydrate}}{{\text{d}}_{}}{\text{alumina)}}} \;\xrightarrow{{1470K}}{\text{ }}\mathop {A{l_2}{O_{3\left( s \right)}}}\limits_{{\text{(alumina)}}} \; + {\text{ }}x{H_2}{O_{\left( g \right)}}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}$

Note:
In the extraction of aluminum leaching plays an important role. Leaching concentrates the pure alumina ($A{l_2}{O_3}$) from the bauxite ore. Bauxite contains many impurities like silica, iron oxide, titanium oxide. Bauxite is the main source of the world’s aluminum. Aluminum is the most abundant metal in the earth’s crust and the third most abundant element.