In the carbon-in-leach process, gold dissolution by cyanide and adsorption of the gold complex onto activated carbon take place simultaneously in a train of agitated tanks. Because it removes a separate adsorption stage and copes better with naturally carbonaceous (preg-robbing) ores, CIL is the most widely used gold process worldwide.
Cyanidation chemistry
Gold dissolves in cyanide solution in the presence of oxygen according to Elsner's equation:
4Au + 8NaCN + O₂ + 2H₂O → 4NaAu(CN)₂ + 4NaOH
- Pulp pH is held at about 10–11 with lime to prevent the formation of toxic HCN gas.
- Free cyanide is typically a few hundred ppm and is set from bottle-roll tests.
- Dissolved oxygen often limits the leach rate and may be supplemented with air or oxygen injection.
The CIL circuit
After grinding (typically to a P80 of about 75 µm) and pre-leach, the pulp flows through a train of 6 to 8 tanks. Activated carbon (usually coconut-shell, 6×12 mesh) is held at about 10–25 g/L and advanced counter-current to the pulp using interstage screens. Loaded carbon leaves the first tank and barren pulp leaves the last.
- Total leach residence time: typically 24–36 hours.
- Loaded carbon: typically several thousand grams of gold per tonne of carbon.
Elution, electrowinning and smelting
After acid washing, loaded carbon is stripped in an elution column with caustic-cyanide solution at elevated temperature (about 110–140 °C, AARL or Zadra methods). Gold is electrowon onto steel-wool cathodes, dried and smelted with fluxes in an induction furnace to produce doré bullion. Carbon is thermally regenerated in a rotary kiln at about 650–750 °C and returned to the circuit.
Environmental considerations
Tailings are treated in a cyanide destruction unit (for example SO₂/air) before discharge to the tailings facility. Compliance with the International Cyanide Management Code and safe design of tanks and monitoring systems are core design requirements.
Company experience
AriaHangard designed a 450 kg/y tank-leaching gold plant — the first fully domestic design of its kind, with recovery above 90% — and designed and supervised the recovery of gold-bearing carbon from tailings at the Muteh Gold Complex.