Leaching

Leaching is used whenever the desired compound in an ore is soluble in some suitable solvent, while the gangue is not. The ore is dissolved out selectively, leaving the unwanted material behind as an insoluble residue.

(a) Leaching alumina out of bauxite

Bauxite — the chief ore of aluminium — is never pure; it typically carries $SiO_2$, iron oxides, and $TiO_2$ as impurities. Concentration proceeds as follows:

Digestion. The powdered ore is heated with a concentrated NaOH solution at 473–523 K and 35–36 bar pressure. This step, called digestion, dissolves the aluminium oxide as soluble sodium aluminate:

The $SiO_2$ impurity is also soluble under these conditions — it dissolves as sodium silicate — while the other impurities (mainly iron oxides and titania) are insoluble and are simply filtered off.

Neutralisation and seeding. $CO_2$ gas is then passed through the sodium aluminate solution. This neutralises it and precipitates hydrated alumina:

A small amount of freshly prepared hydrated $Al_2O_3$ is added to the solution just before this step to induce the precipitation — a trick called seeding, essentially giving the precipitate nucleation sites to grow from. The sodium silicate impurity, unlike the aluminate, stays dissolved and doesn't precipitate alongside it.

Calcination. The filtered, dried hydrated alumina is finally heated strongly (around 1470 K) to drive off the water of hydration and recover pure $Al_2O_3$:

(b) Leaching in the metallurgy of silver and gold

Silver and gold ores are leached with a dilute solution of NaCN or KCN in the presence of air (which supplies the $O_2$ needed). The metal dissolves as a soluble cyanide complex:

$4M(s) + 8CN^-(aq) + 2H_2O(aq) + O_2(g) \rightarrow 4[M(CN)_2]^-(aq) + 4OH^-(aq) \qquad (M = Ag\ \text{or}\ Au) \qquad(6.4)$ …