Cyanide Leaching
Also: Cyanidation, Carbon-in-leach, Heap leaching
A hydrometallurgical process for extracting gold in which crushed ore is treated with a dilute sodium-cyanide solution to selectively bring gold into solution.
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Cyanide leaching has been the world's dominant process for recovering gold from oxidised ores since the late nineteenth century. Crushed rock is brought into contact with a weakly alkaline sodium-cyanide (NaCN) solution that selectively dissolves gold — and, to a lesser extent, silver. The recovered metal is then brought to refining quality by refining/affinage and finally delivered to refineries as a doré bar. More than three-quarters of annual mine production is won this way.
Chemical background
The base reaction (the MacArthur–Forrest process, 1887) reads:
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 oxidiser; a pH of 10–11 keeps the solution stable and prevents the formation of highly toxic hydrogen-cyanide gas (HCN).
Process variants
| Variant | Application | Distinctive feature |
|---|---|---|
| Heap leaching | Low-grade 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) | Oxidised ores | Leaching and adsorption separated |
| BIOX / bio-oxidation | Sulphide ores | Bacteria oxidise sulphides before leaching |
After adsorption, gold is recovered from the activated carbon by elution with hot solution and deposited electrolytically or precipitated with zinc dust (the Merrill–Crowe process). The resulting raw gold — the doré — typically contains 60–90 % gold and goes on to refining/affinage.
Environment and regulation
Cyanide is highly toxic and persists in the environment if it escapes uncontrolled — as in the tailings-dam failure at Baia Mare (Romania, 2000). Since 2002 the international International Cyanide Management Code (ICMC) has 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 (the Mining Waste Directive), so the method today plays practically no role in Europe, while it remains standard in the southern hemisphere.
Influence on the gold price
The ore grade in g/t of gold and the leaching recovery rate largely determine production costs (AISC). Falling ore grades worldwide force mines to use more energy and reagents, which drives up mining costs and over the long term influences the gold price level. The melt value calculator shows the material value of the finished refined metal at the current spot market.
In brief
Cyanide leaching is what makes modern large-scale gold mining economically viable in the first place — it is efficient but environmentally demanding. Strict oversight and certified cyanide management are today the minimum standard for responsible primary mining.