What is leaching? Explain with an example.

(N/A) Leaching is a process of concentration of ore,which is used when the ore is soluble in a suitable solvent.
Principle: The powdered ore is treated with a suitable solvent in which the ore is soluble,but the impurities are not. The ore is then recovered from the solution by suitable chemical methods.
Example-$1$: Leaching of Alumina from Bauxite.
Bauxite,the principal ore of aluminium,usually contains $SiO_2$,iron oxides,and titanium oxide $(TiO_2)$ as impurities.
Concentrated $NaOH$ solution is used to digest the powdered ore at $473-523 \ K$ and $35-36 \ bar$ pressure. This process extracts $Al_2O_3$ as sodium aluminate.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \rightarrow 2Na[Al(OH)_4]_{(aq)}$
The impurities like $SiO_2$ also dissolve in $NaOH$ to form sodium silicate,while other impurities remain insoluble.
The aluminate in solution is neutralised by passing $CO_2$ gas,which results in the precipitation of hydrated $Al_2O_3$. The solution is seeded with freshly prepared $Al_2O_3$ samples to induce precipitation.
$2Na[Al(OH)_4]_{(aq)} + 2CO_{2(g)} \rightarrow Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$
The sodium silicate remains in the solution,and the hydrated alumina is filtered,dried,and heated to get pure $Al_2O_3$.
$Al_2O_3 \cdot xH_2O_{(s)} \xrightarrow{1470 \ K} Al_2O_{3(s)} + xH_2O_{(g)}$
Example-$2$: Leaching of Silver and Gold.
In the metallurgy of silver and gold,the respective metal is leached with a dilute solution of $NaCN$ or $KCN$ in the presence of air $(O_2)$,which supplies the necessary oxygen.
$4M_{(s)} + 8CN^-_{(aq)} + 2H_2O_{(aq)} + O_2(g) \rightarrow 4[M(CN)_2]^-_{(aq)} + 4OH^-_{(aq)}$ (where $M = Ag$ or $Au$)
The metal is then recovered by displacement using zinc.
$2[M(CN)_2]^-_{(aq)} + Zn_{(s)} \rightarrow [Zn(CN)_4]^{2-}_{(aq)} + 2M_{(s)}$