The following questions are relevant to the extraction of Aluminium :

(i) State the reason for addition of caustic alkali to bauxite ore during purification of bauxite.

(ii) Give a balanced chemical equation for the above reaction.

(iii) Along with cryolite and alumina, another substance is added to the electrolyte mixture. Name the substance and give one reason for the addition.

(i) Bauxite is heated under pressure with conc. caustic soda (NaOH) for 2 to 8 hours. Bauxite dissolves and forms sodium meta aluminate because of the amphoteric nature of aluminium, leaving behind insoluble impurities called red mud. Red mud consists of ferric oxide, sand, etc. which are removed by filtration.

(ii) Balanced chemical equation for the conversion of impure bauxite to sodium aluminate is given below:

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}$2\text{O}3.2\text{H}2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

(iii) Fluorspar is added to the electrolyte mixture as it lowers the fusion point of the mixture i.e., the mixture fuses around 950°C instead of 2050°C.