A leaching agent, also known as a lixiviant, is the chemical solvent or reagent used in a leaching process to selectively dissolve and extract target minerals or metals from ore or concentrate. The choice of leaching agent is determined by the mineralogy of the ore, the target metal, the required selectivity, environmental considerations, regulatory requirements, and economic feasibility.
In gold mining, sodium cyanide (NaCN) is by far the most widely used leaching agent for gold and silver extraction. It forms stable, water-soluble gold-cyanide complexes at neutral to slightly alkaline pH in the presence of dissolved oxygen. Alternative leaching agents for gold include thiosulfate, thiourea, chlorine/hypochlorite, and halide systems, which are being explored in response to the environmental concerns associated with cyanide use. Thiosulfate leaching, for instance, is gaining attention for the treatment of preg-robbing and carbonaceous ores where cyanide performs poorly.
In bauxite refining (the Bayer Process), the leaching agent is sodium hydroxide (caustic soda, NaOH) dissolved in water at high concentration. The caustic soda selectively dissolves aluminum hydroxide minerals while leaving iron oxides, titanium minerals, and residual silica (as desilication products) behind as red mud. The concentration and temperature of the caustic solution are carefully controlled to maximize alumina extraction efficiency.
In iron ore processing, sulfuric acid or hydrochloric acid may serve as leaching agents for the removal of impurities such as phosphorus or silica. In diamond processing, since diamonds are not typically leached, chemical reagents are used mainly for disaggregating host rock or treating associated mineral contaminants. A leaching agent must be effective, selective, recoverable, affordable, and manageable from a safety and environmental standpoint.