Explanation

(a)
Baeyer’s process or concentration of bauxite ore-Impure bauxite is treated with NaOH with which it forms water soluble sodium meta aluminate complex.
$Al_{2}O_{3}\ .\ 2H_{2}O + 2NaOH\ \ \overset{\ \ \ \ \ 420 - 425K\ \ \ \ \ }{\rightarrow}\ 2NaAlO_{2} + 3H_{2}O$
sod. metaaluminate
Impurities such as ${Fe}_{2}O_{3},\ TiO_{2}$ and silica are left behind. Pure alumina is recovered from solution.
$NaAlO_{2} + 2H_{2}O\ \rightarrow Al(OH)_{3} + NaOH$
$\ \ \ \ \ \ \ \ \ \ \ 2Al(OH)_{3}\overset{\ \ \ \ \ \mathrm{\Delta}\ \ \ \ \ }{\rightarrow}\ Al_{2}O_{3} + 2H_{2}O$