Carbon-in-Leach (CIL) Process
Carbon-in-Leach (CIL) process, which combines cyanide leaching and adsorption onto activated carbon, has become one of the most important technologies used in gold cyanidation. Since the 1970s, extensive development has been carried out on the elution of gold-loaded activated carbon and thermal regeneration of stripped carbon, enabling activated carbon to be recovered and reused. Today, CIL is widely applied due to its relatively low capital and operating costs and its ability to economically process low-grade gold ores.
CIL is particularly suitable for oxidized gold ores with relatively low silver content. Compared with several alternative processing routes, CIL can offer lower capital requirements and significant economic advantages. CIL process generally includes the following major stages:
- Removal of impurities
- Pre-thickening
- Leaching and adsorption
- Elution and electrowinning
- Gold recovery and refining
- Activated carbon regeneration
- Tailings pressure filtration
- Water treatment
Impurity Removal from Pulp: before the adsorption stage, impurities such as wood chips and other coarse debris must be removed from the cyanide pulp. This prevents gold from being adsorbed onto wood particles and subsequently being lost with the activated carbon. It also prevents screen openings from becoming blocked by coarse debris.
Pre-Thickening: to achieve the appropriate pulp density for leaching, the pulp is generally thickened using thickeners before entering the leaching circuit. Pulp density affects not only the required leaching residence time, but also the suspension and mixing behavior of activated carbon within the pulp.
Leaching and Adsorption: after impurity removal and thickening, the pulp is introduced into a series of high-efficiency leaching and adsorption tanks, typically arranged in series. A typical CIL circuit may consist of approximately 5–8 tanks, depending on plant capacity and process requirements. Cyanide is generally added in the initial leaching tanks, while activated carbon is introduced into the downstream tanks. Activated carbon moves through the circuit in counter-current flow relative to the slurry. As gold is adsorbed onto the activated carbon, the carbon becomes gold-loaded carbon. Following completion of the adsorption process, carbon is recovered from the pulp using carbon recovery screens, where the activated carbon is separated from the pulp and transferred to the downstream elution circuit.
Elution and Electrowinning: gold-loaded activated carbon is transferred to an elution circuit, where the adsorbed gold is stripped from the carbon. The resulting gold-bearing solution is subsequently treated by electrowinning to recover metallic gold. The main equipment used in this section includes elution columns, acid-washing tanks, heaters, filters, pregnant solution tanks, and electrowinning cells.
Smelting: gold-bearing material recovered from electrowinning can be subjected to smelting after appropriate treatment and impurity removal. The resulting molten gold is cast into gold ingot. Depending on the refining route and process conditions, the purity of the resulting ingot can reach approximately 99.99% after further refining.
Activated Carbon Regeneration and Reuse: following gold elution, the activated carbon is initially acid-washed to remove carbonates and other accumulated inorganic deposits. After repeated cycles of adsorption and elution, the activated carbon gradually loses part of its adsorption capacity. To restore its adsorption performance, the carbon is subjected to thermal regeneration and can subsequently be returned to the adsorption circuit for reuse. The main equipment in this section includes an activated carbon regeneration kiln, quench tank, carbon sizing or screening equipment, and associated auxiliary systems.