In the cyanide gold extraction process, the core function of the gold leaching agent (usually sodium cyanide or potassium cyanide) is to dissolve gold through a chemical reaction. The mechanism is as follows: cyanide reacts with gold and oxygen in an alkaline solution (pH 10-11) to form a stable water-soluble gold-cyanide complex [Au(CN)₂]⁻, with the reaction formula: 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻. This process is highly selective, effectively dissolving fine gold particles (even as low as 1 ppm) while reducing interference from impurities such as iron and copper.
The efficiency of the leaching agent is affected by multiple factors: 1. Oxygen concentration: Oxygen participates in the reaction as an oxidant; oxygenation or the addition of peroxides can accelerate dissolution. 2. pH control: An alkaline environment (commonly adjusted with lime) prevents the volatilization of cyanic acid (HCN), ensuring safety. 3. Temperature and Time: Leaching is typically carried out at room temperature, but heating (50-80°C) can shorten the leaching cycle. The advantages of cyanidation are low cost and high recovery rate (over 90%), but it faces two major challenges: - Environmental risks: Cyanide is highly toxic and requires tailings treatment (such as ozone oxidation or SO₂-air process) to degrade residues. - Ore adaptability: Ores containing carbon or sulfides require pretreatment (such as roasting). Modern processes often combine activated carbon adsorption or zinc replacement to recover dissolved gold, and tightening environmental regulations have driven the development of non-cyanide leaching agents (such as thiourea and thiosulfate), but cyanidation still dominates due to its maturity.
