Heap Leach vs CIL: Two Roads to an Ounce

Once gold-bearing ore comes out of the pit or stope, the real chemistry begins. The two dominant methods heap leaching and carbon-in-leach (CIL) represent fundamentally different trade-offs.

Heap leach: cheap, slow, low-recovery

In a heap leach, crushed ore is stacked on a lined pad and irrigated with a dilute cyanide solution. The pregnant solution drains through the pile, is collected, and the gold is recovered via carbon columns or zinc precipitation. Capex is low. Operating costs are low. But recovery rates rarely exceed 70%.

• Heap leach capex: $5-15 per tonne capacity
• CIL plant capex: $30-60 per tonne capacity
• Heap leach recovery: 50-75%
• CIL recovery: 88-96%
• Heap leach cycle time: 60-180 days per lift

CIL: expensive, fast, high-recovery

CIL grinds ore to a fine slurry, mixes it with cyanide and activated carbon in agitated tanks, and recovers the loaded carbon for elution. Recoveries above 90% are routine. The capex bill is two to four times that of heap leach, and the energy footprint is heavier.

"Heap leach is for the patient operator. CIL is for the operator with a balance sheet." mining engineering textbook, paraphrased

When to choose which

Heap leach works best on oxide ores with simple mineralogy, low grade, and big tonnage. Think Nevada, northern Mexico, the Atacama. CIL is the answer for higher-grade or refractory sulphide ores where every percentage point of recovery matters because the rock itself is more valuable.

Environmental considerations

Both methods rely on cyanide, the dominant lixiviant in modern gold processing. Heap leaches concentrate risk in the pad liner: a leak under a 50-million-tonne stack is hard to fix. CIL concentrates risk in tailings dams, which have produced some of the most catastrophic mining failures of the past decade. Bio-leach and thiosulphate alternatives exist but are not yet at scale.

Takeaway: when you read a mine's technical report, scroll to the processing flowsheet. It tells you more about long-term economics than almost any other section.