:Image:Dicyanoaurate(I)-3D-balls.png|thumb|right|300px|Dicyanoaurate(I) ([Au(CN)2]−) is the product of heap-leaching of low-grade gold ore.
Leaching involves the use of aqueous solutions to extract metal from metal-bearing materials. The extracting solution is called a
lixiviant. The lixiviant is optimized in terms of
pH,
oxidation-reduction potential, presence of
chelating agents, and temperature. In a simple implementation, a slurry of the pulverized ore in the lixiviant solution is filtered to yield a solution containing the metal ion(s) of interest. For example copper in its carbonate minerals such as
malachite dissolve in aqueous
sulfuric acid. On the other hand, copper sulfide minerals, which are more prevalent, are not amenable to hydrometallurgy, at least until they have been roasted. Hydrometallurgy is used to extract
rare earths. By using chelating agents, one can selectively extract certain metals.. Some leaching reactor configurations are in-situ, heap,
vat leaching,
tank, and
autoclave.
In-situ leaching is also called "solution mining" involves pumped extracting solution into the deposit. The
Beverley uranium deposit is an example of in-situ leaching. In heap leaching, crushed (and sometimes agglomerated) ore is piled in a heap on top of an impervious sheet. Leach solution is sprayed over the top of the heap and allowed to percolate downward through the heap. The heap design usually incorporates collection sumps, which allow the "pregnant" leach solution (i.e. solution with dissolved valuable metals) to be pumped for further processing. An example is
gold cyanidation, where pulverized ores are extracted with a solution of
sodium cyanide, which, in the presence of air, dissolves the gold, leaving behind mostly nonprecious residue. ==Solution concentration and purification==