Question

Which of the following techniques is used in the manufacture of aluminium from bauxite?
(A) Reduction with magnesium
(B) Reduction with coke
(C) Electrolytic reduction
(D) Reduction with iron

Answer 1

The method which is commonly used to convert metal hydroxides or hydrated metal oxides to their respective oxides is calcination.
Aluminium is obtained from alumina by the Hall and Heroult process.

Complete step by step answer:
Many metals are easily obtained from their respective oxides by reduction, hence, metals are extracted in a way that involves the following two chemical processes:
Conversion of the metal ore into its corresponding metal oxide or de-electronation of the ore.
Reduction or electronation of the metal oxide into the free metal.
Usually, metals are present in ores as carbonates, sulphides and hydrated oxides or hydroxides. For hydrated oxides, the ores are converted into the corresponding metal oxides by calcination.
Bauxite is an aluminium ore having the formula ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}$ . Thus, it is a hydrated oxide ore. So, it will be converted into the respective oxide by calcination.
Now, calcination involves heating of the ore strongly below its melting point either in absence or limited supply of air. During calcination, at first moisture is driven out and volatile impurities of sulphur, arsenic, phosphorus are removed as their volatile oxides. Finally, water is removed from the hydrated oxide.
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\xrightarrow{{{\text{Heat}}}}{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$
Thus, calcinations converts bauxite into alumina, ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ .
The fused alumina is a poor electricity conductor and so cryolite and feltspar are added to it to make alumina a good electricity conductor and also to reduce the melting point to around 1140 Kelvin.
The Hall and Heroult process involves the conversion of mixture of alumina and cryolite to aluminium by electrolysis. It is performed in an electrolytic cell made of iron using carbon electrodes.
The reactions which occur during electrolysis:
Cathode: ${\text{A}}{{\text{l}}^{{\text{3 + }}}}\left( {{\text{melt}}} \right){\text{ + 3}}{{\text{e}}^{\text{ - }}} \to {\text{Al}}\left( {\text{l}} \right)$
Anode:
${\text{C}}\left( {\text{s}} \right){\text{ + }}{{\text{O}}^{{\text{2 - }}}}\left( {{\text{melt}}} \right) \to {\text{CO}}\left( {\text{g}} \right){\text{ + 2}}{{\text{e}}^{\text{ - }}} \\\ {\text{C}}\left( {\text{s}} \right){\text{ + 2}}{{\text{O}}^{{\text{2 - }}}}\left( {{\text{melt}}} \right) \to {\text{C}}{{\text{O}}_2}\left( {\text{g}} \right){\text{ + 4}}{{\text{e}}^{\text{ - }}} \\\$
Aluminium liberated at cathode melts at the high temperature of the cell. The molten metal is heavier than the molten electrolyte and so it sinks to the bottom of the cell. The metal obtained in this way is about $99.95$ percent pure.

So, the technique is used in the manufacture of aluminium from bauxite is electrolytic reduction and hence the correct option is C.

Note:
- Bauxite is also the main source for another rare metal gallium.
-During the extraction of aluminium from bauxite ore by Baeyer process, gallium accumulates in the sodium hydroxide liquor.
- The gallium thus obtained can be extracted by methods like ion-exchange resin method.