Question

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

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

Answer 1

The correct option is C $A{l}_2{O}_3.2H_{2}O\stackrel{\Delta }{\to }A{l}_2{O}_3+2H_{2}O$

• Bauxite is the principal ore of aluminium. The impurities present in bauxite are $Si{O}_2$, iron oxides and titanium oxide ($Ti{O}_2$).
  
• Concentration is carried out by heating the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This process is called digestion.
• $A{l}_2{O}_3$ is extracted out as sodium aluminate. The impurity, $Si{O}_2$ too dissolves forming sodium silicate. Other impurities are left behind.
  

$A{l}_2{O}_3+2NaOH+3H_{2}O\to 2Na\left[Al\right(OH{)}_4]$

• The sodium aluminate 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.
  

$2Na\left[Al\right(OH{)}_4]+C{O}_2\to A{l}_2{O}_3.x{H}_{2}O+2NaHC{O}_3$

• Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure$A{l}_2{O}_3$.
  

$A{l}_2{O}_3.x{H}_{2}O\to A{l}_2{O}_3+x{H}_{2}O$

• Hence, option C is correct answer.