Cyanide Leaching
Also: Cyanide leaching, Cyanidation, Carbon-in-Leach, Heap Leaching
A hydrometallurgical process for gold recovery in which crushed ore is treated with a dilute sodium cyanide solution to selectively dissolve the gold.
Cyanide leaching has been the world's dominant process for extracting gold from oxide ores since the late 19th century. Crushed rock is brought into contact with a weakly alkaline sodium cyanide (NaCN) solution, which selectively dissolves gold — and to a lesser extent silver. The recovered metal is subsequently brought to refining quality through affinage and finally delivered to refineries as a doré bar. More than three quarters of annual mine production is recovered 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 of 10–11 keeps the solution stable and prevents the formation of highly toxic hydrogen cyanide gas (HCN).
Process variants
| Variant | Application | Feature |
|---|---|---|
| Heap Leaching | Low-grade ores (< 1 g/t Au) | Solution trickles over a stacked heap |
| Carbon-in-Leach (CIL) | Richer ores | Activated carbon adsorbs gold directly in the reactor |
| Carbon-in-Pulp (CIP) | Oxide 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 (Merrill-Crowe process). The resulting crude gold — the so-called doré — typically contains 60–90 % gold and passes to 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 Cyanide Code (ICMC) has set voluntary but market-relevant standards for transport, handling and detoxification of effluents. EU Member States have banned industrial cyanide heap leaching since 2011 (Mining Waste Directive), so the method plays practically no role in Europe today, while it remains standard in the Southern Hemisphere.
Impact on the gold price
The ore grade in g/t gold and the leaching recovery decisively determine production costs (AISC). Falling ore grades worldwide force mines to use more energy and reagents, which drives production 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 industrial gold mining economically viable — it is efficient but environmentally demanding. Strict oversight and certified cyanide management are today the minimum standard for responsible primary mining.