The mechanism of gold leaching agents is based on chemical complexation and redox reactions. Taking the cyanide method as an example, sodium cyanide (NaCN) reacts with gold in an alkaline solution (pH 10-11). Oxygen acts as an oxidant, oxidizing elemental gold (Au⁰) to Au⁺, which then combines with CN⁻ to form a stable dicyanoaluminate complex ion (Au(CN)₂⁻), thus dissolving the gold. The reaction formula is: 4Au + 8CN⁻ + O₂ + 2H₂O → 4Au(CN)₂⁻ + 4OH⁻. Non-cyanide leaching agents such as thiourea (CS(NH₂)₂) work synergistically with oxidants (such as Fe³⁺ or H₂O₂) under acidic conditions (pH 1-2) to form the Au(CS(NH₂)₂)₂⁺ complex. Thiourea has the advantages of low toxicity and fast reaction speed, but it is more expensive.
Leaching efficiency is regulated by multiple factors:
1. Oxidant concentration: Sufficient oxygen or Fe³⁺ accelerates gold oxidation;
2. pH value: Cyanide requires alkalinity to prevent HCN volatilization, while thiourea requires acidity to stabilize the complex;
3. Temperature: Moderate heating (usually 20-30°C) promotes reaction kinetics;
4. Ore characteristics: Gold encapsulation requires pre-oxidation treatment (such as roasting or bio-oxidation) to expose gold particles.
In addition, environmentally friendly leaching agents (such as thiosulfates and halides) can replace cyanide through similar complexation mechanisms, but conditions need to be optimized to balance efficiency and economy.
