Question

When bauxite is boiled with aqueous $NaOH$ solution, sodium metal aluminate solution is formed. This part in the metallurgy of aluminium is called:

• A. liquation
• B. amalgamation
• C. leaching
• D. calcination

Answer 1

The correct option is C leaching

In leaching, a suitable reagent (not water) dissolves the ore and not the impurities. Thus, leaching is a chemical method of separation/concentration of the ore from impurities.
Leaching is used in extraction of Al from bauxite.
$A{l}_2{O}_3\left(s\right)+2NaOH\left(aq\right)+3H_{2}O\left(l\right)\to 2Na\left[Al\right(OH{)}_4\left]\right(aq)$
Sodium Aluminate is filtered. The filtrate is neutralised by passing $C{O}_2$ gas.
$2Na\left[Al\right(OH{)}_4\left]\right(aq)+C{O}_2(g)\to A{l}_2{O}_3.x{H}_{2}O(s)+2NaHC{O}_3(aq)$
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to about ${1200}^{\circ }C$ to get pure Alumina $\left(A{l}_2{O}_3\right)$.
$A{l}_2{O}_3.x{H}_{2}O\left(s\right)\stackrel{1470K}{\to }A{l}_2{O}_3\left(s\right)+xH2O\left(g\right)$
Hence option (c) is correct.