Question

Which of the following methods is best suited for the concentration of bauxite ore?

• A. Hydraulic washing
• B. Hydrometallurgy
• C. Magnetic separation
• D. Leaching

Answer 1

Answer: (B), (D)

The correct options are
B

Hydrometallurgy

D

Leaching

The principal ore of aluminium - bauxite - usually contains $Si{O}_2$, iron oxides, and $Ti{O}_2$ as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of $NaOH$ at 473-523 K and 35-36 bar pressure. Thus, $A{l}_2{O}_3$ is leached out as sodium aluminate leaving the impurities behind.

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

Of course, the $Si{O}_2$ impurities get dissolved in NaOH to form sodium silicate. This exists in the solution along with sodium aluminate, from which $Al(OH{)}_3$ is seeded out by using a seed crystal. This is the famous Bayer’s process. Before recovering the ore from this solution, what do you think should be done with other insoluble impurities like $Ti{O}_2$ and $F{e}_2{O}_3$?

Hall’s process and Serpeck’s process can also be used for the concentration of bauxite ore. Even after concentration, we only get pure alumina $A{l}_2{O}_3$.