Leaching

Leaching is a CHEMICAL (rather than purely physical) method of concentration, based on the solubility of the metal-bearing compound of the ore in a suitable solvent, combined with the chemistry of what happens once it dissolves. The crushed ore is treated with a solvent chosen so that the metal of interest is converted into a soluble salt or complex ion, while the gangue remains insoluble and can simply be filtered off. Because leaching directly converts the metal into solution, it often does double duty as a first step toward extraction as well as a concentration method. Four worked examples of leaching, distinguished by the solvent/reagent used, are given below.

Cyanide leaching. The standard method for concentrating gold ore: the crushed ore is leached with an aerated (oxygen-containing), dilute solution of sodium cyanide, which converts the gold into a soluble cyanide complex while the gangue (aluminosilicate) stays insoluble:

4Au(s) + 8CN⁻(aq) + O2(g) + 2H2O(l) → 4[Au(CN)2]⁻(aq) + 4OH⁻(aq)

Once gold is in solution as [Au(CN)2]⁻, it is recovered by reduction: reacting the deoxygenated leached solution with metallic zinc reduces the gold back to its elemental (zero oxidation state) form, a step called CEMENTATION:

Zn(s) + 2[Au(CN)2]⁻(aq) → [Zn(CN)4]²⁻(aq) + 2Au(s)

Ammonia leaching. Used for ores containing nickel, copper and cobalt together: treating the crushed ore with aqueous ammonia under suitable pressure selectively leaches these three metals as their soluble ammine complexes -- [Ni(NH3)6]2+, [Cu(NH3)4]2+ and [Co(NH3)5H2O]3+ respectively -- leaving behind the gangue, iron(III) oxides/hydroxides and aluminosilicate, which do not form soluble ammine complexes under these conditions.

Alkali leaching. The industrial method used to obtain pure alumina from bauxite, an important ore of aluminium: bauxite is heated with a solution of sodium hydroxide (or sodium carbonate) in the temperature range 470-520 K at about 35 atm, converting the aluminium oxide into soluble sodium meta-aluminate while the impurities (iron oxide and titanium oxide) remain behind, insoluble:

Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4

The hot sodium meta-aluminate solution is decanted away from the insoluble impurities, cooled and diluted; passing CO2 gas through it neutralises the solution and precipitates hydrated alumina:

2NaAl(OH)4 + 2CO2(g) → Al2O3.3H2O(s) + 2NaHCO3(aq)

The precipitate is filtered off and then strongly heated to around 1670 K to drive off all the water of hydration and obtain pure alumina, Al2O3.

Acid leaching. Sulphide ores such as ZnS and PbS can be leached by treating them with hot aqueous sulphuric acid, which oxidises the insoluble sulphide into a soluble sulphate plus elemental sulphur: …