Question

Discuss the Bayer’s method for extraction of alumina from bauxite ore with chemical equation.

Answer 1

Hint: Initially, a semi-finished product, alumina is extracted from the ores. Then the metallic aluminium is produced electrolytically from the alumina. The process includes purification of bauxite and then the alumina is produced from bauxite.

Complete step by step solution:
Bauxites are the most abundant used raw materials used for aluminium extraction. Other sources of aluminium are alunites, leucistic lavas, labradorites etc.
Bauxite is the most important aluminium ore. Bayer’s process is considered as the main method of producing aluminium. There are four different extraction methods- Deville process, Serpeck’s process, Hall-Heroult process and Bayer’s process.
Bayer’s process includes two steps-Purification of bauxite and dissolving of bauxite in cryolites. The chemical formula of bauxite is \[{\text{A}}{{\text{l}}_2}{{\text{O}}_3}\].
Step 1:
Dissolve bauxite in aqueous sodium hydroxide, \[{\text{NaOH}}\] by digestion. There forms the aluminium hydroxide,${\text{Al}}{\left( {{\text{OH}}} \right)_3}$.
${\text{A}}{{\text{l}}_2}{{\text{O}}_3} + 2{\text{NaOH}} \to 2{\text{NaAl}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}$
                                     Sodium aluminate
$2{{\text{H}}_2}{\text{O}} + {\text{NaAl}}{{\text{O}}_2} \to {\text{Al}}{\left( {{\text{OH}}} \right)_3} + {\text{NaOH}}$
${\text{Al}}{\left( {{\text{OH}}} \right)_3}$is then treated with $3{\text{M}}$ \[{\text{NaOH}}\]at $120 - {140^ \circ }{\text{C}}$
Bauxite, \[{\text{A}}{{\text{l}}_2}{{\text{O}}_3}\] with higher oxide content is treated at higher temperature say, $200 - {250^ \circ }{\text{C}}$, $35{\text{atm}}$with $5 - 7{\text{M}}$ \[{\text{NaOH}}\].
The insoluble impurities formed are separated by filtration.
${\text{Al}}{\left( {{\text{OH}}} \right)_3}$is precipitated by \[{\text{C}}{{\text{O}}_2}\]and heated at $ \sim {1200^ \circ }{\text{C}}$to produce bauxite ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$
$2{\text{Al}}{\left( {{\text{OH}}} \right)_3} \to {\text{A}}{{\text{l}}_2}{{\text{O}}_3} + 3{{\text{H}}_2}{\text{O}}$
Step 2:
Cryolite contains $5 - 7\% .{\text{Ca}}{{\text{F}}_2},5 - 7\% .{\text{Al}}{{\text{F}}_3},2 - 8\% .{\text{A}}{{\text{l}}_2}{{\text{O}}_3}$. The purified bauxite is dissolved in cryolites. It is then electrolyzed at ${950^ \circ }{\text{C}}$in a carbon lined steel as a cathode with hard carbon rods as an anode. Lithium carbonate,${\text{L}}{{\text{i}}_2}{\text{C}}{{\text{O}}_3}$ is also used. The reactions involves are given below:
${\text{A}}{{\text{l}}_2}{{\text{O}}_3} \to 2{\text{A}}{{\text{l}}^{3 + }} + 3{{\text{O}}^{2 - }}$
At cathode, $2{\text{A}}{{\text{l}}^{3 + }} + 6{{\text{e}}^ - } \to 2{\text{Al}}$
Aluminium gets reduced from \[ + 3\]oxidation state to \[0\]oxidation state.
At anode, $3{{\text{O}}^{2 - }} \to 3{\text{O}} + 6{{\text{e}}^ - }$
Oxygen gets oxidized from $ - 2$ to \[0\]oxidation state.

Additional information:
The product formed after digestion is referred to as red mud. It has constituents like calcium and sodium hydroxide. They are very harmful, which may cause pollution. But this red mud is used in the production of ceramic.

Note: Aluminium has a low density. It is non-toxic, has high thermal conductivity, and corrosion resistance. It is the second most malleable metal and sixth most ductile metal. It is cheaper than copper. This process can occur only in industries.