Question

Which of the following reactions does not take place during leaching for concentration of bauxite?

• A. $A{l}_2{O}_3\left(s\right)+2NaH\left(aq\right)+3H_{2}O\left(l\right)\to 2Na\left[Al\right(OH{)}_4\left]\right(aq)$
• B. $2Na\left[Al\right(OH{)}_4\left]\right(aq)+C{O}_2(g)\to A{l}_2{O}_3.x{H}_{2}O(s)+2NaHC{O}_3(aq)$
• C. $A{l}_2{O}_3\left(s\right)+2NaOH\left(aq\right)+3H_{2}O\left(l\right)\to 2Na\left[Al\right(OH{)}_4\left]\right(aq)$
• D. $A{l}_2{O}_3.x{H}_{2}O\left(s\right)\stackrel{1470K}{\to }A{l}_2{O}_3\left(s\right)+xH2O\left(g\right)$

Answer 1

The correct option is A $A{l}_2{O}_3\left(s\right)+2NaH\left(aq\right)+3H_{2}O\left(l\right)\to 2Na\left[Al\right(OH{)}_4\left]\right(aq)$

Bauxite is the principal ore of aluminium. The impurities present in bauxite are $Si{O}_2$, iron oxides and titanium oxide $\left(Ti{O}_2\right).$

Concentration is carried out by heating the powdered ore with a concentrated solution of $NaOHat473-523K$ and $35-36$ bar pressure. This process is called digestion.

$A{l}_2{O}_3\left(s\right)+2NaOH\left(aq\right)+3H_{2}O\left(l\right)\to 2Na\left[Al\right(OH{)}_4\left]\right(aq)$
The sodium aluminate presente present in solution is neutralised by passing $C{O}_2$ gas and hydrated $A{l}_2{O}_3$ is precipitated. At this stage, small amount of freshly prepared sample of hydrated $A{l}_2{O}_3$ is added to the solution. This is called seeding. It induces the precipitation.
$2Na\left[Al\right(OH{)}_4\left]\right(aq)+C{O}_2(g)\to A{l}_2{O}_3.x{H}_{2}O(s)+2NaHC{O}_3(aq)$

Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure $A{l}_2{O}_3.x{H}_{2}O\left(s\right)\stackrel{1470K}{\to }A{l}_2{O}_3\left(s\right)+xH2O\left(g\right)$
Hence, option (a) is correct answer.