Name: Ellingham Diagram: Introduction
Uploaded: 2019-07-19
Duration: 7 min 42 s
Description: The Ellingham diagram proved to be a cornerstone for the process of reducing metal oxides to their native form. Watch this video to learn about the Ellingham...

Question

Describing the leaching of aluminium ore.

Accepted Answer

In the extraction of aluminium, the significance of leaching is to concentrate pure alumina ${Al}_{2} O_{3}$ from bauxite ore. Bauxite usually contains silica, iron oxide, and titanium oxide as impurities. In the process of leaching, alumina is concentrated by digesting the powdered ore with a concentrated solution of $NaOH at 473 - 523 K and 35 - 36 bar .$

Under these conditions, alumina dissolves as sodium silicate leaving the impurities behind. The impurities are then filtered and the solution is neutralized by passing carbon dioxide gas.

${Al}_{2} O_{3} (s) + 2 NaOH (aq) + 3 H_{2} O (l) \overset{}{\to} 2 Na [Al (OH)_{4}] (aq) Impure Alumina Soluble$

${SiO}_{2} + 2 NaOH (aq) \to {Na}_{2} {SiO}_{3} (aq) + H_{2} O (l) Soluble$

In this process, ${Al}_{2} O_{3}$ hydrated gets precipitated and sodium silicate remains in the solution.

$2 Na [Al {(OH)}_{4}] (aq) + {CO}_{2} (g) \to {Al}_{2} O_{3} x 2 H_{2} O (s) + 2 {NaHCO}_{3} (aq)$

Precipitation is induced by seeding the solution with freshly prepared samples of hydrated ${Al}_{2} O_{3}$ . Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina.

${Al}_{2} O_{3} . 2 H_{2} O (s) \overset{heat}{\to} {Al}_{2} O_{3} (s) + {xH}_{2} O (g) Hydrated Alumina pure Alumina$

Describing the leaching of aluminium ore.

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