cyanidation

Cyanidation is a hydrometallurgical process for extracting gold and silver from crushed ore using aqueous cyanide solution. The ore is ground to a fine powder, mixed with water, and treated with sodium cyanide or calcium cyanide at a pH of 10 to 11. Under these conditions, cyanide ions form soluble complexes with gold and silver, leaching them from the rock matrix while leaving most base metals behind. The pregnant solution is then separated from the ore solids by filtration or settling, and the precious metals are recovered by precipitation or adsorption onto activated carbon.

The chemistry relies on the extremely high stability of the gold-cyanide complex, Au(CN)2-, which forms under mildly alkaline conditions. Oxygen must be present in solution for the reaction to proceed; without it, leaching stops. Temperature, pH, cyanide concentration, and oxygen levels all affect extraction rate and recovery. Typical operations run at 20 to 30 degrees Celsius with cyanide concentrations between 0.5 and 2 grams per liter. Under these conditions, 70 to 90 percent of gold can be extracted in 24 to 48 hours depending on ore grade and fineness.

Cyanidation displaced older methods like roasting and gravity separation because it is faster, recovers finer particles that gravity cannot retrieve, and requires less capital equipment. It is the dominant process for large-scale gold mining worldwide. Heap leaching, where cyanide solution is percolated through stacked ore on a pad, requires days or weeks but cuts costs for lower-grade material. Vat leaching, used for richer ores, achieves faster kinetics in agitated tanks.

Complications and constraints

Cyanide is a poison, and managing it safely is a major operational and regulatory concern. Some ores contain minerals that consume cyanide without dissolving gold, raising leach times and solution costs. Copper, zinc, and iron minerals in the ore can also form stable complexes with cyanide, competing with gold leaching. Carbonaceous material in ore can absorb dissolved gold, preventing recovery. Pre-treatment by roasting or oxidative leaching is sometimes needed to handle these obstacles.

Tailings disposal and cyanide destruction are critical to permitting and closure. Cyanide is typically destroyed by oxidation with chlorine, hydrogen peroxide, or permanganate before tailing storage. Some jurisdictions now require cyanide recovery and recycling. The term itself comes from the cyan color of certain cyanide salts, though the solution used in leaching is colorless. The process name has become generic despite the use of different cyanide reagents and recovery paths in practice.