Question

In Hall's process the main reagent is mixed with:

• A. $NaF$
• B. $Na_{3}AlF_{6}$
• C. $AlF_{3}$
• D. none of these

Answer 1

Answer: (B)

$Na_{3}AlF_{6}$

Answer 2

Chief ore of aluminium is bauxite $Al _{2} O _{3} \cdot 2 H _{2} O$. Firstly bauxite is concentrated by chemical method. (Baeyer's method) using $NaOH$ in which pure alumina $\left( Al _{2} O _{3}\right)$ is obtained. Bauxite has impurity of sand $\left( SiO _{2}\right)$ only $Al _{2} O _{3}$ reacts with $NaOH$ $\underset{\text{ Bauxite}}{Al _{2} O _{3}}+ NaOH \longrightarrow \underset{\text{(soluble in water)}}{NaAlO _{2}}+ H _{2} O$ Impurities remain in the solution. It is then filtered and the filterate is strongly heated $\underset{\text{filterate}}{NaAlO _{2}}+ H _{2} O \longrightarrow \underset{\text{white ppt}}{Al ( OH )_{3}}+ NaOH$ This solution is again filtered to obtain white precipitate of $Al ( OH )_{3}$ which is strongly heated to obtain pure $Al _{2} O _{3}$. $Al ( OH )_{3} \xrightarrow{\Delta}Al _{2} O _{3}+ H _{2} O$ Aluminium being highly electropositive is reduced by electrolytic reduction (Hall's process). During Hall's process alongwith molten $Al _{2} O _{3}$ small amount of cryolite $\left( Na _{3} AlF _{6}\right)$ and fluorspar $\left( CaF _{2}\right)$ are also added as they decrease the melting point of $Al _{2} O _{3}$ and increase its electrical conductivity.