cyanidation

Cyanidation is a hydrometallurgical process for extracting metals from ores by dissolving them in cyanide solutions. It is the dominant method for commercially recovering gold and, to a lesser extent, silver from a wide range of ore types, especially low-grade material.

In gold cyanidation, ore is ground to a fine particle and placed in leaching circuits where a dilute cyanide solution, typically sodium cyanide or potassium cyanide, is added under strongly alkaline conditions (pH about 10–11) with controlled aeration. Gold dissolves as the soluble dicyanoaurate complex [Au(CN)2]−. Lime is used to maintain alkalinity and stabilize the slurry, and leaching times vary with ore characteristics.

Recovery of dissolved gold is usually achieved by adsorption onto activated carbon in carbon-in-pulp (CIP) or carbon-in-leach (CIL) circuits, or by zinc precipitation in the Merrill-Crowe process. The loaded carbon is processed to recover gold, which is then eluted and typically recovered by electrowinning or smelting to produce dore bars.

Cyanide-bearing effluents require treatment to destroy cyanide prior to discharge. Destruction methods include chemical oxidation with chlorine or hypochlorite, sodium metabisulfite, or other oxidants, performed under alkaline conditions, along with proper containment and tailings management to meet regulatory standards.

Cyanidation offers high gold recovery and suitability for large-scale operations, but it poses environmental and safety risks due to cyanide toxicity and potential spills. The method has a long history dating to the late 19th century, notably with the development of the MacArthur-Forrest process for gold dissolution and zinc-based precipitation for recovery.