Question

Name the principal ore of aluminium. Explain the significance of leaching in the extraction of aluminium ?

Answer 1

Hint: The ore that can be regarded as the main ore of aluminium contains an hydrated oxide of Aluminium. It contains various impurities such as silicon dioxide, titanium oxide etc that are removed during the process of leaching in various steps and at the end we get pure alumina.

Complete step by step answer:
We know that the principal ore of aluminium is the bauxite ore. It consists of hydrous aluminium oxides, aluminium hydroxides, clay minerals and some insoluble materials like quartz, hematite etc. It has bauxite ores in Madhya Pradesh and Jharkhand. Now, let us understand the process of leaching.
The leaching is the technique in which the solute is extracted from its carrier substance by using the solvent. The solute in this case will be aluminium.

The process of leaching of alumina from bauxite is known as Hall’s process. It involves three steps as-
Step 1.) In the first step, the aluminium oxide is converted into sodium meta-aluminate by fusing the bauxite ore and sodium hydroxide together. This fused mass is treated with water. Some insoluble residues are present that contain the impurities of silica and iron oxide. These impurities are later filtered.
$A{l_2}{O_3} + 2NaOH + 3{H_2}O\xrightarrow{{473 - 523K}}2Na\left[ {Al{{(OH)}_4}} \right]$

Step 2.) In the second step, the filtrate is warmed first and then it is neutralized by passing carbon dioxide through solution. As a result, the precipitate of aluminum hydroxide is formed.
$2Na\left[ {Al{{(OH)}_4}} \right] + C{O_2} \to A{l_2}{O_3}.x{H_2}O + 2NaHC{O_3}$

Step 3.) In the third step, the precipitates obtained of aluminium hydroxide are washed, dried and then heated to get $A{l_2}{O_3}$.

$A{l_2}{O_3}.x{H_2}O\xrightarrow{{1470K}}A{l_2}{O_3} + x{H_2}O$
 Now, comes the significance of leaching.
The process of leaching concentrates the pure alumina ($A{l_2}{O_3}$) from the ore. During the process of leaching, the alumina gets dissolved as sodium meta-aluminate and the impurities such as $Si{O_2},F{e_2}{O_3}{\text{ and Ti}}{{\text{O}}_2}$ are left which are filtered out. This process gives us the pure alumina.

Note: The bauxite resources are very less for production of aluminium for years. Some other materials that could be used for alumina production instead of bauxite are clay minerals, alunite, power plant ash and oil shale etc. In 2017, India was one among top 10 bauxite producing countries.