Cyanide Leaching
Also: Cyanide leaching, Cyanidation, Carbon-in-leach, Heap leaching
Hydrometallurgical process for gold extraction in which crushed ore is treated with a dilute sodium cyanide solution to selectively dissolve gold.
Like what you see and read?
We pour our heart into keeping preciousmetalprices.com fast, clean and free — no paywalls, no clutter, just reliable facts and live prices. If it helps you, the nicest way to say thank you is to pass it on. Every share helps a fellow investor discover us and keeps the project alive. 💛
Like what you see and read?
We pour our heart into keeping preciousmetalprices.com fast, clean and free — no paywalls, no clutter, just reliable facts and live prices. If it helps you, the nicest way to say thank you is to pass it on. Every share helps a fellow investor discover us and keeps the project alive. 💛
Cyanide leaching has been the world's dominant process for extracting gold from oxidic ores since the late 19th century. Crushed rock is brought into contact with a weakly alkaline sodium cyanide solution (NaCN) that selectively dissolves gold — and to a lesser extent silver. The recovered metal is subsequently brought to refinery grade through refining and finally delivered as doré bars to refineries. 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 of 10–11 keeps the solution stable and prevents the formation of the highly toxic hydrogen cyanide gas (HCN).
Process variants
| Variant | Application | Key feature |
|---|---|---|
| Heap leaching | Low-grade ores (< 1 g/t Au) | Solution trickles over a heaped stockpile |
| Carbon-in-leach (CIL) | Higher-grade ores | Activated carbon adsorbs gold directly in the reactor |
| Carbon-in-pulp (CIP) | Oxidic ores | Separation of leaching and adsorption |
| BIOX / Biooxidation | Sulphidic ores | Bacteria oxidise sulphides prior to 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 is sent for refining.
Environment and regulation
Cyanide is highly toxic and persists in the environment if released uncontrolled — as occurred, for example, in the tailings dam breach at Baia Mare (Romania, 2000). The international International Cyanide Management Code (ICMC) has set voluntary but market-relevant standards for the transport, handling and detoxification of wastewater since 2002. EU member states have banned industrial cyanide heap leaching since 2011 (Mining Waste Directive), so the method plays virtually no role in Europe today, but remains standard practice in the southern hemisphere.
Influence on the gold price
The ore grade (g/t gold) and leaching recovery together largely determine production costs (AISC). Declining ore grades worldwide are forcing mines to use more energy and reagents, driving up production costs and influencing the gold price level over the long term. The melt value calculator shows what material value the fully refined metal has at the current spot market.
In brief
Cyanide leaching makes modern large-scale gold mining economically viable — it is efficient, but environmentally demanding. Rigorous oversight and certified cyanide management are today the minimum standard for responsible primary mining.