Cyanidation is the dissolution of gold (and silver) by an alkaline cyanide solution. The reaction — known as the Elsner equation — combines metallic gold with cyanide and oxygen to form a soluble gold-cyanide complex: 4 Au + 8 NaCN + O₂ + 2 H₂O → 4 Na[Au(CN)₂] + 4 NaOH. This dissolved gold is then recovered downstream via carbon adsorption (ADR) or zinc precipitation (Merrill-Crowe).
The Chemistry
Sodium cyanide (NaCN) dissociates in water to release free cyanide (CN⁻). In the presence of oxygen and at alkaline pH (typically 10.5–11.0), CN⁻ forms a soluble complex with metallic gold and silver. Lime (CaO) is added to maintain pH and prevent cyanide gas formation. Typical operating concentration: 200–500 ppm NaCN.
Why Cyanide
Cyanide is the industry standard because it is selective (preferentially dissolves Au and Ag over most base metals), cheap compared to alternatives, and well-understood after 130 years of commercial use. Most of the world's gold has been recovered this way.
Safety & Environment
Cyanide is toxic but degrades naturally in sunlight and oxygen. Modern operations use the INCO SO₂/air or peroxide destruction process to lower cyanide concentrations below regulatory limits before tailings disposal. The International Cyanide Management Code provides industry-standard safe-handling practices.
Alternatives
Thiosulfate, glycine, and chloride-based lixiviants exist but remain niche due to selectivity, cost, or scale-up challenges. Cyanide remains the dominant method for over 95% of global primary gold production.
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