Leaching is a hydrometallurgical process in which a solvent (the leaching agent) is used to selectively dissolve and extract valuable minerals, metals, or compounds from ore, concentrate, or processed material. It is one of the foundational unit operations in mineral processing and metallurgical extraction, applicable across a wide range of mining commodities including gold, bauxite (aluminum), copper, uranium, and others.
The leaching process relies on chemical reactions between the solvent and the target mineral species, resulting in the formation of soluble metal complexes that can be separated from the insoluble gangue fraction. In gold mining, cyanide leaching is the most widely practiced technique globally. Sodium cyanide solution reacts with metallic gold in the presence of oxygen to form the soluble gold-cyanide complex [Au(CN)2]⁻, which is then recovered using activated carbon or zinc precipitation. Heap leaching is a variant where dilute cyanide solution is percolated through stacked crushed ore on a lined pad.
In bauxite processing through the Bayer Process, leaching involves digesting crushed bauxite in concentrated sodium hydroxide solution (caustic soda) at temperatures ranging from 140°C to 280°C and elevated pressures. This dissolves aluminum hydroxides from the bauxite, producing a supersaturated sodium aluminate solution. The efficiency and selectivity of the leaching reaction depends on the mineralogy of the bauxite (gibbsitic vs. boehmitic vs. diasporic), caustic concentration, temperature, and particle size.
Environmental, safety, and cost considerations are central to the design and operation of leaching systems, as the reagents involved (cyanide, caustic, acids) carry significant handling risks and regulatory requirements. Innovations in leaching technology continue to improve metal recoveries and reduce environmental impacts.