Introduction

CIL (Carbon-In-Leach) is one of the most mature cyanidation technologies for treating hard rock gold ore. It is widely adopted by medium and large scale gold mines globally. Unlike gravity separation used for alluvial sand gold, CIL targets fine-grained gold locked inside rock matrix. All theories, chemical reactions and process parameters in this article follow standard extractive metallurgy principles and industrial plant operation data. No exaggerated recovery figures or unproven techniques. This article explains the chemical mechanism, complete process flow, equipment functions, critical control parameters and the difference between CIL and CIP.

Basic Chemical Principle of Gold Cyanidation

Gold metal is chemically stable under natural conditions. In alkaline cyanide solution and with sufficient dissolved oxygen, solid gold will dissolve into aqueous solution to form soluble gold cyanide complex ion. The simplified Elsner Equation is the core reaction:

4Au + 8NaCN + O₂ + 2H₂O = 4Na[Au(CN)₂] + 4NaOH

• Sodium cyanide (NaCN): provides cyanide ion CN⁻ to form gold complex.
• Dissolved oxygen: essential oxidizing agent; insufficient oxygen severely reduces leaching speed.
• Alkaline environment (lime CaO): prevent cyanide hydrolysis and toxic HCN gas release; maintain pH 10.5–11.5.

Activated carbon has strong adsorption capacity toward Au(CN)₂⁻ complex. CIL core feature: leaching reaction and carbon adsorption happen simultaneously inside the same leaching tank.

CIL vs CIP: Core Process Difference

Many miners confuse CIL and CIP. The distinction is clear in metallurgical engineering:

• CIL (Carbon-In-Leach): Leaching and carbon adsorption occur at the same time in identical tanks. Carbon moves counter-current with slurry. As soon as gold dissolves, carbon immediately adsorbs gold complex, maintaining low solution gold concentration and accelerating further gold dissolution.
• CIP (Carbon-In-Pulp): Ore leaching finishes completely first in dedicated leach tanks. Then fully leached pulp flows into separate adsorption tanks for carbon loading.

Advantage of CIL: higher overall leaching efficiency, shorter total retention time, suitable for ore with fast-leaching fine gold. It reduces the risk of gold re-precipitation in pulp.

Full Step-by-Step CIL Processing Flow

The whole circuit can be divided into 6 major stages: Crushing & Grinding → Pre-leach thickening → CIL Leaching-Adsorption → Carbon Screening → Desorption & Electrowinning → Smelting.

Stage 1: Crushing and Fine Grinding
Run-of-mine hard rock passes through jaw crusher, cone crusher and ball mill. The target fineness for most gold ores: 70%–90% passing 200 mesh (0.074 mm). Grinding liberates fine gold particles from gangue rock. Unliberated locked gold cannot be contacted by cyanide solution and will remain unrecovered.

Stage 2: Thickening & Pre-conditioning
Ball mill discharge slurry enters thickener to adjust pulp density to 40%–45% solid mass ratio. Lime is added to adjust pH value. For ores containing harmful sulfide minerals, pre-aeration may be applied to reduce cyanide consumption.

Stage 3: CIL Counter-Current Leaching & Adsorption (Core Circuit)
Slurry flows sequentially forward through 5–8 connected leach tanks. Sodium cyanide is dosed into the front tanks. Activated carbon flows backward (counter-current): loaded carbon is transferred from the last tank to the first tank. Agitators continuously stir pulp to keep solids suspended, and air is injected to supply dissolved oxygen. While gold dissolves, carbon adsorbs dissolved gold complex continuously. Total retention time generally ranges from 24–48 hours, determined by ore mineral characteristics.

Stage 4: Carbon Screening & Separation
Inter-tank screens separate larger activated carbon particles from fine ore slurry. Loaded carbon enriched with gold is pumped out from the first tank and sent to desorption. Barren pulp after final tank undergoes cyanide detoxification before tailings discharge.

Stage 5: Desorption and Electrowinning
Gold is stripped from loaded carbon using hot caustic-cyanide solution (high temperature high pressure desorption commonly used). The obtained high-grade gold-bearing solution flows into electrowinning cells. Under direct current, metallic gold deposits onto steel wool cathodes.

Stage 6: Smelting & Carbon Regeneration
Gold sludge from cathodes is washed, filtered and smelted with flux inside furnace to produce gold bullion. Stripped carbon goes to kiln thermal regeneration to recover adsorption activity, then returns to CIL tanks for recycling.

Key Standard Operating Parameters for CIL Plant

• Pulp density: 40%–45% solids by weight
• pH value: 10.5 ~ 11.5 (maintained by lime)
• Cyanide concentration in solution: 0.2–0.5 g/L NaCN
• Dissolved oxygen: ≥6–8 mg/L
• Activated carbon particle size: 6×12 mesh or 8×16 mesh coconut shell carbon
• Carbon concentration in pulp: 10–25 g/L
• Normal overall gold recovery for free-milling ore: 85%–92%

Important note: For refractory gold ore (high arsenopyrite, pyrite), direct CIL yields low recovery. Pre-oxidation (roasting, pressure oxidation or bio-oxidation) is required before cyanidation.

Harmful Gangue Minerals & Impurities

Certain minerals consume cyanide and oxygen, increasing reagent cost and lowering gold recovery:

• Pyrite, arsenopyrite: react with cyanide and oxygen, high reagent consumption.
• Copper minerals (chalcopyrite): form stable copper cyanide complexes, compete for cyanide.
• Antimony, tellurium-bearing gold ores: difficult to leach under standard CIL conditions.
• Clay, carbonaceous matter: “preg-robbing” phenomenon – naturally occurring carbon absorbs dissolved gold and causes permanent loss.

Safety & Environmental Compliance Reminders

Cyanide is highly toxic. All CIL projects must satisfy local mining environmental regulations:

• Strictly maintain pH above 10.5 to avoid generation of toxic hydrogen cyanide gas.
• Tailings slurry must complete cyanide detoxification (SO₂-air process or hydrogen peroxide method) before discharge.
• Build anti-seepage liners for leach tanks and tailings storage facility to prevent underground water contamination.
• Operators require professional safety training and protective equipment.

Frequently Asked Questions

Q1: Can CIL process alluvial sand gold directly?
A: Not economically feasible. Alluvial gold can be recovered efficiently via gravity separation. CIL requires fine grinding and expensive cyanide reagents; it is designed for hard rock ore where gold is locked inside rock.

Q2: Why cannot recovery reach 100% in CIL plant?
A: Main reasons: fine gold locked in unground gangue, preg-robbing carbonaceous minerals, incomplete leaching, gold loss in fine tailings. Free-milling ore normally cannot exceed 92%–94% recovery with standard CIL.

Q3: How long is activated carbon service life?
A: Carbon gradually wears into fine powder during circulation. After multiple regeneration cycles, carbon activity declines. Normally carbon replenishment rate is 5–10 kg per tonne of ore processed.

Conclusion

CIL gold processing relies on cyanide leaching chemistry and simultaneous activated carbon adsorption. Its core advantage is combining leaching and adsorption in one circuit to improve reaction driving force. Performance heavily depends on ore mineralogy, grinding fineness, dissolved oxygen supply and stable reagent control. Before constructing a CIL plant, laboratory leach test and mineralogical analysis are mandatory. Unsuitable ore types will lead to high reagent consumption and low gold recovery. Contact us to arrange ore testing and provide matched CIL plant design proposal.