IntroductionGold typically occurs at very low concentrations in ores - less than 10 g/t or 0.001% (mass basis). At these concentrations the use of aqueous chemical (hydrometallurgical) extraction processes is the only economically viable method of extracting the gold from the ore. Typical hydrometallurgical gold recovery involves a leaching step during which the gold is dissolved in an aqueous medium, followed by the separation of the gold bearing solution from the residues, or adsorption of the gold onto activated carbon. After elution from the activated carbon the gold is further concentrated by precipitation or electrodeposition.
Gold is one of the noble metals and as such it is not soluble in water. A complexant, such as cyanide, which stabilizes the gold species in solution, and an oxidant such as oxygen are required to dissolve gold. The amount of cyanide in solution required for dissolution may be as low as 350 mg/l or 0.035% (as 100% NaCN).
Alternative complexing agents for gold, such as chloride, bromide, thiourea, and thiosulfate form less stable complexes and thus require more aggressive conditions and oxidants to dissolve the gold. These reagents present risks to health and the environment, and are more expensive. This explains the dominance of cyanide as the primary reagent for the leaching of gold from ores since its introduction in the later part of the 19th century.