Question

is the principle ore of aluminium and is concentrated by process.

• A. Bauxite
• B. Bayer's
• C. Cassiterite
• D. Froth flotation

Answer 1

Answer: (A), (B)

The correct options are
A Bauxite
B Bayer's

Bauxite, $A{l}_2{O}_3.2H_{2}O$, is the principle ore of aluminium.
Bayer's process is used to concentrate this ore. The reactions involved are given below-
${Al}_2{O}_3{.2H}_{2}O\left(s\right)+NaOH\left(aq\right)\to {2NaAlO}_2\left(aq\right)+{3H}_{2}O\left(l\right)$
$2NaAl{O}_2\left(aq\right)+2H_{2}O\left(l\right)\to NaOH\left(aq\right)+Al\left(OH{)}_3\right(s)$
${Al\left(OH\right)}_3\left(s\right)\to {Al}_2{O}_3\left(s\right)+{3H}_{2}O\left(l\right)$

- $F{e}_2{O}_3$ and $Si{O}_2$ are present as impurities.
-Bauxite being amphoteric reacts with $conc.NaOH$ and forms sodium aluminate $\left(NaAl{O}_2\right)$.
- $Si{O}_2$ reacts with $NaOH$ to form soluble sodium silicate $\left(N{a}_{2}Si{O}_3\right)$ while $F{e}_2{O}_3$ remains unaffected.
-The impurities are then filtered out. The filtrate contains $NaAl{O}_2$ and $N{a}_{2}Si{O}_3$.
-Small amount of freshly prepared $Al(OH{)}_3$ is added which hydrolyses $NaAl{O}_2$ and precipitates white $Al(OH{)}_3$
- $Al(OH{)}_3$ ppt is calcined to get $A{l}_2{O}_3$.