Carbon-in-Leach vs Carbon-in-Pulp: Choosing the Right Activated Carbon Grade for Gold Recovery

Gold recovery from cyanide-leached ore pulp using activated carbon has been the dominant hydrometallurgical process in large-scale gold mining since the late 1970s, replacing the zinc cementation (Merrill-Crowe) process at most operations processing low-grade disseminated ore. Two variants of the technology have evolved: Carbon-in-Pulp (CIP) and Carbon-in-Leach (CIL). Both use activated carbon as the adsorbent for the gold-cyanide complex, but they differ fundamentally in how the leaching and adsorption stages are configured.

The choice between CIL and CIP — and the specification of the correct activated carbon grade for each — is one of the most consequential technical decisions in gold plant design. This guide provides a thorough explanation of both processes, compares their technical requirements for activated carbon, and gives precise grade specification guidance for procurement and metallurgical engineers.

Western Carbon supplies activated carbon for gold recovery applications to mining operations across Africa, Asia, and the Americas. Our Westgold grade is specifically engineered for CIL and CIP circuit performance.

2. What Is Carbon-in-Pulp (CIP)?

In a Carbon-in-Pulp process, cyanide leaching of the milled ore is completed as a separate, prior stage before the leach pulp contacts activated carbon. The typical flow sequence is: milling, classification, thickening, leaching in a series of agitated tanks (6 to 12 tanks at 24 to 72 hours retention time), followed by transfer of the leached pulp to the CIP adsorption circuit. The CIP circuit consists of a series of agitated tanks (typically 4 to 8) in which activated carbon is contacted counter-currently with the leach pulp.

Counter-current operation means the leach pulp flows in one direction (tank 1 to tank N), while the carbon is advanced in the opposite direction (from the final tank back toward the first) by pumping. This arrangement ensures that the most gold-rich leach solution contacts the freshest carbon (lowest existing gold loading) in the first CIP tank, maximising the thermodynamic driving force for adsorption throughout the circuit.

3. What Is Carbon-in-Leach (CIL)?

In a Carbon-in-Leach process, activated carbon is added to the leach tanks simultaneously with cyanide. Both the leaching reaction (dissolution of gold from the ore) and the adsorption reaction (transfer of gold-cyanide complex from solution to carbon surface) occur in the same tank at the same time. A CIL circuit typically consists of 6 to 12 agitated tanks, each containing both a cyanide addition point and a carbon inventory, with carbon advancing counter-currently through the circuit in the same manner as CIP.

The simultaneous nature of CIL means that gold is captured by the carbon almost as soon as it dissolves, before it can be lost to preg-robbing materials in the ore or to solution losses. This is the fundamental advantage of CIL over CIP for certain ore types.

4. CIL vs CIP: A Detailed Process Comparison

Why CIL Has Become Dominant

Over the past two decades, CIL has displaced CIP as the default configuration at most new gold plants for three reasons: lower capital cost (fewer tanks), better performance on the increasing proportion of preg-robbing ore bodies being mined, and improved gold recovery from lower-grade ore. The World Gold Council estimates that the majority of large-scale gold operations commissioned after 2000 use CIL as their primary recovery technology, though CIP remains operational at many established plants.

5. Preg-Robbing Ores and Why CIL Dominates

Preg-robbing is the phenomenon in which natural organic carbon compounds within the ore matrix adsorb dissolved gold-cyanide complex from solution before it can be captured by the process carbon. This natural carbonaceous material may include graphite, naturally occurring coal or charcoal inclusions, and bio-organic residues. The gold adsorbed by preg-robbing material is lost to tailings because it cannot be easily recovered from the mineralised material.

Preg-robbing is measured by comparing the gold dissolution rate in the presence of the ore versus the gold dissolution rate in the absence of the ore. An ore that reduces gold recovery by more than 5% relative to the ore-free baseline is classified as preg-robbing. Many deposits in West Africa, the Carlin Trend in Nevada, parts of Australia, and the Witwatersrand Basin in South Africa contain preg-robbing ore that cannot be economically processed by CIP.

CIL solves this problem because the manufactured activated carbon, with its high surface area, activity, and gold affinity, outcompetes the natural carbonaceous material for the gold-cyanide complex. The key requirement is that the manufactured carbon must adsorb gold faster than the natural material — which means adsorption kinetics are as important as equilibrium capacity in CIL carbon specification.

6. Activated Carbon Grade Requirements for Gold Recovery

The activated carbon used in CIL and CIP circuits must meet a specific set of physical and chemical requirements that are more demanding than those for water treatment or gas-phase applications. The following table provides the standard specification framework used by gold mining operations worldwide.

The particle size specification of 6×12 mesh is driven by the inter-stage screen design of the CIL or CIP circuit. Inter-stage screens typically have apertures of 0.8 to 1.0 mm. The carbon must be retained on these screens while the ore particles (typically 75 to 150 microns after milling) and solution pass through. Fines generated by carbon attrition (below 0.8 mm) pass through the screens and report to the tailings, carrying adsorbed gold as an unrecoverable loss.

7. Gold Loading Kinetics and Adsorption Rate

For CIL circuits in particular, gold adsorption rate is a critical performance parameter that cannot be captured by the iodine number alone. The rate at which gold-cyanide complex transfers from solution to the carbon surface determines how much gold is recovered per unit residence time in each tank.

The Adsorption Rate Test

The standard method is a bottle roll test: activated carbon at a known mass is contacted with a solution of known gold concentration under controlled temperature, pH, and agitation conditions. The gold concentration in solution is measured at intervals (typically 30, 60, 90, and 120 minutes) to construct an adsorption kinetics curve. The rate constant (K) from this test is compared between candidate carbon grades to select the highest-activity option.

A carbon grade that passes the minimum iodine number specification but has slow adsorption kinetics may allow gold to remain in solution long enough to be adsorbed by preg-robbing materials in a CIL circuit, negating the benefit of the simultaneous leach-adsorption design. This is why sophisticated gold operations use kinetic rate testing, not just iodine number, as their primary carbon acceptance criterion.

Factors Affecting Adsorption Rate

Several factors influence adsorption rate in an operating circuit: solution gold concentration (higher concentration drives faster adsorption), carbon inventory per tank (more carbon per unit volume means more available surface), temperature (adsorption rate increases with temperature), cyanide concentration, and pH. The carbon itself contributes through its micropore accessibility and surface chemistry, including the presence of surface oxygen groups that can inhibit or enhance gold loading depending on their type.

8. Elution, Stripping, and Thermal Regeneration

Loaded carbon from the first (leading) tank of the CIL or CIP circuit is advanced for elution — the stripping of adsorbed gold from the carbon surface back into a small volume of solution (pregnant eluate) from which gold is recovered by electrowinning.

The AARL and Zadra Elution Methods

Two dominant elution processes are used in the gold industry. The Anglo American Research Laboratories (AARL) method uses a pre-soaking step with a sodium hydroxide and ethanol solution followed by hot water elution at 110 to 120 degrees Celsius at elevated pressure. The Zadra method uses continuous sodium hydroxide and sodium cyanide elution at 80 to 95 degrees Celsius at atmospheric pressure. The AARL method is faster (6 to 12 hours vs 24 to 72 hours for Zadra) and is the current industry standard at most modern plants.

Thermal Regeneration

After elution, the stripped carbon (which still contains organic and inorganic fouling compounds accumulated from the leach pulp) is thermally regenerated in a rotary kiln at 700 to 750 degrees Celsius in a steam or inert atmosphere. Regeneration burns off organic fouling compounds and restores the carbon’s micropore surface. Carbon that is properly regenerated can recover 90 to 95% of its original iodine number activity per cycle. Coconut shell carbon’s superior thermal stability means it withstands more regeneration cycles before its hardness and activity fall below the minimum specification for circuit use.

Carbon Inventory Management

Gold plant metallurgists track the health of the carbon inventory by monitoring the iodine number of the regenerated carbon after each elution-regeneration cycle. When the iodine number of returned carbon falls below the site-specific minimum (typically 950 to 1000 mg/g), that batch is removed from circuit and replaced with fresh carbon. The removed carbon is either sold to a carbon reprocessor or disposed of according to site environmental management plans.

9. Carbon Losses, Fines, and Gold Inventory Management

Carbon losses in CIL and CIP circuits occur through four mechanisms: mechanical attrition (generating fines that pass through inter-stage screens), losses during screening and pump transfers, catastrophic screen failure (allowing coarse carbon to pass), and dissolution/chemical degradation of the carbon structure. Each loss mechanism contributes to what the mining industry calls the “carbon inventory loss rate,” typically expressed as kilograms of carbon lost per tonne of ore processed.

The Financial Cost of Carbon Fines

Carbon fines in the tailings stream carry a disproportionate amount of gold because fine particles have a higher surface-area-to-mass ratio — they adsorb gold efficiently but cannot be screened out. A gold operation processing 5 million tonnes of ore per year with a carbon loss rate of 100 g/tonne and a loaded carbon gold grade of 5000 g/tonne is losing 2500 kg of activated carbon per year — and if each tonne of fine carbon carries the full circuit average gold loading, the gold loss associated with carbon fines can be financially significant.

The primary lever for reducing this loss is carbon hardness. A carbon grade with an abrasion number of 85 vs 75 can reduce attrition-driven fine generation by 30 to 50% in a high-shear pumping environment. This is the fundamental economic justification for specifying premium coconut shell carbon over lower-cost alternatives in gold recovery.

10. Who Uses CIL and CIP: Global Mining Applications

CIL and CIP technology is used at gold operations on every continent where primary gold ore is processed. The following industry groups are the primary users of activated carbon for gold recovery.

11. Related Reading

12. Frequently Asked Questions