Cyanide Leaching
Also: Cyanidation, Cyanide leach, Carbon-in-Leach, Heap leaching
A hydrometallurgical process for extracting gold in which crushed ore is treated with a dilute sodium cyanide solution to bring gold selectively into solution.
Cyanide leaching has, since the late 19th century, been the world's dominant process for extracting gold from oxidic ores. Crushed rock is brought into contact with a weakly alkaline sodium cyanide solution (NaCN), which selectively dissolves gold - and, to a lesser extent, silver. The recovered metal is then brought up to refining grade by affinage and finally delivered to refineries as a doré bar. More than three quarters of annual mine production is obtained in this way.
Chemical background
The basic reaction (MacArthur-Forrest process, 1887) is:
4 Au + 8 NaCN + O2 + 2 H2O -> 4 Na[Au(CN)2] + 4 NaOH
Gold forms the stable dicyanoaurate(I) complex ([Au(CN)2]-). Oxygen acts as the oxidising agent; a pH value of 10-11 keeps the solution stable and prevents the formation of the highly toxic hydrogen cyanide gas (HCN).
Process variants
| Variant | Application | Feature |
|---|---|---|
| Heap leaching | Poor ores (< 1 g/t Au) | Solution trickles over a heaped pile |
| Carbon-in-Leach (CIL) | Richer ores | Activated carbon adsorbs gold directly in the reactor |
| Carbon-in-Pulp (CIP) | Oxidic ores | Separation of leaching and adsorption |
| BIOX / bio-oxidation | Sulphidic ores | Bacteria oxidise sulphides before leaching |
After adsorption, gold is recovered from the activated carbon by elution with hot leach and deposited electrolytically or precipitated with zinc dust (Merrill-Crowe process). The resulting raw gold - the so-called doré - typically contains 60-90% gold and passes on to affinage.
Environment and regulation
Cyanide is highly toxic and persists in the environment if it escapes uncontrolled - as, for example, in the tailings-dam failure at Baia Mare (Romania, 2000). The international International Cyanide Management Code (ICMC) has, since 2002, set voluntary but market-relevant standards for the transport, handling and detoxification of the effluent. EU member states have banned industrial cyanide heap leaching since 2011 (Mining Waste Directive), so that the method plays practically no role in Europe today, while in the Southern Hemisphere it remains standard.
Impact on the gold price
The ore grade in g/t of gold and the leaching recovery decisively determine production costs (AISC). Falling ore grades worldwide are forcing mines to use more energy and reagents, which drives up production costs and, in the long term, influences the level of the gold price. The Melt Value Calculator shows the material value the finished, refined metal has on the current spot market.
In brief
Cyanide leaching is what makes modern large-scale industrial gold mining economically possible - it is efficient but ecologically demanding. Strict oversight and certified cyanide management are today the minimum standard for responsible primary production.