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How does leaching selectively dissolve metal compounds from ore?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

Leaching is a hydrometallurgical process that selectively dissolves specific metal compounds from ore using an aqueous solvent. The selectivity of this process relies on choosing a solvent that effectively reacts with the target metal while leaving the surrounding waste rock (gangue) largely unaffected.123

The choice of solvent is determined by the chemical composition of the ore:
* Oxides: Often leached with sulfuric acid or sodium hydroxide.1
* Precious Metals: Frequently dissolved using cyanide solutions.1
* Native Ores, Carbonates, and Sulfides: Can be treated with ammonium hydroxide.1
* Metal Complexes: Some metals, such as nickel, cobalt, and copper, are extracted as ammine complexes using aqueous ammonia, where differences in complex stability and solubility allow for selective separation.4

By converting metals into soluble forms—often through the formation of coordination complexes—the metal can be separated from the insoluble residue. Once dissolved, the resulting "pregnant" solution is separated from the waste and processed to recover the metal through precipitation or other refining techniques.1432