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That timing can matter greatly for preg-robbing ore and for ores with unusual leaching or adsorption kinetics. However, the available technical literature does not establish a universal recovery, cost or capital winner. A defensible choice requires ore-specific mineralogy, leach and adsorption testwork, equipment design and economic modelling.
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The fundamental difference between CIP and CIL
The U.S. Environmental Protection Agency stated the distinction plainly in its December 2022 metal-mining fact sheet: “The major difference between CIP and CIL is that, in CIP, leaching and carbon adsorption occur sequentially and in CIL, leaching and carbon adsorption occur simultaneously.”
| Aspect | Carbon-in-pulp (CIP) | Carbon-in-leach (CIL) |
|---|---|---|
| Process sequence | Leaching and adsorption are separate operations. Leached pulp leaves the leach circuit and then enters carbon-contact tanks. | Leaching and adsorption take place in the same series of agitated tanks. |
| When carbon enters | After the dedicated leach stage. | While leaching is still occurring. |
| What happens in the tank train | The leach circuit contains no adsorption carbon; adsorption is handled downstream. | Leaching generally dominates in the first tanks, while adsorption becomes more important in later tanks as carbon contacts the pulp. |
| Gold-bearing carbon | Loaded carbon is removed from the adsorption section for elution. | Loaded carbon is likewise removed for elution. |
| Final gold recovery | Pregnant eluate from elution proceeds to a recovery step such as electrowinning or zinc precipitation. | The same downstream route is used: elution followed by a recovery step such as electrowinning or zinc precipitation. |
| Universal performance advantage | Not established. | Not established. |
Both circuits therefore belong to the same family of activated-carbon gold-recovery flowsheets. The choice is not between carbon adsorption and a completely different recovery mechanism; it is primarily a decision about when adsorption occurs and how that timing interacts with the ore.
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How a CIP circuit works
1. Prepare and leach the ore pulp
Ground ore is mixed with water to form pulp and treated in a series of agitated leach tanks. Cyanidation dissolves recoverable gold, but no adsorption carbon is present in these dedicated leach tanks in a conventional CIP arrangement.
2. Transfer leached pulp to adsorption tanks
After the planned leach residence time, the pulp moves to a separate train containing granular activated carbon. The separation gives the operator distinct leach and adsorption sections, with each section designed around its own residence-time and mixing requirements.
3. Load gold onto activated carbon
Dissolved gold contacts the carbon and is adsorbed onto its internal surface. Carbon and pulp must be mixed effectively, while screens retain carbon as slurry moves through the circuit.
4. Remove loaded carbon
Carbon that has accumulated gold is screened from the slurry and directed to the elution section. Screen performance is essential: carbon loss to the tailings stream is a direct loss of the captured product and can also disturb the carbon inventory in the adsorption train.
5. Elute and recover the gold
Elution strips the adsorbed gold from the loaded carbon into a concentrated solution, commonly called pregnant eluate. The eluate then goes to a recovery operation such as electrowinning or zinc precipitation. CIP does not eliminate these downstream steps; it only places adsorption after leaching.
How a CIL circuit works
1. Add carbon to the leach train
In CIL, activated carbon is mixed directly with the ore pulp in a series of agitated leach tanks. The tanks perform both functions instead of having a completely carbon-free leach section followed by a completely separate adsorption section.
2. Let leaching and adsorption overlap
The first tanks normally provide the strongest leaching duty. As pulp progresses, more dissolved gold is available for adsorption and the adsorption duty becomes increasingly important. The EPA technical resource describes fresh carbon being added countercurrent to the pulp so that active carbon meets pulp later in the train, where dissolved-gold concentrations are lower and adsorption can polish the solution.
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3. Screen and transfer carbon
Interstage screens keep carbon in the intended tank or move it in the designed direction while allowing slurry to continue through the circuit. Agitation, screen integrity and carbon movement must be considered together; a problem in one can appear as a recovery problem elsewhere.
4. Elute and recover gold from the carbon
Once carbon is loaded, it follows the same broad downstream route as CIP carbon: elution produces pregnant eluate, and the eluate is treated by a recovery method such as electrowinning or zinc precipitation.
Why the timing of adsorption matters
Preg-robbing ores
Some ore constituents can adsorb dissolved gold before the plant captures it on its intended activated carbon. This behavior is known as preg-robbing. Introducing carbon during leaching gives the carbon an opportunity to compete for dissolved gold as it is generated, so CIL is a potential response to preg-robbing.
That is a possibility, not a guaranteed result. The extent of preg-robbing, the speed of gold dissolution, the speed of carbon loading and the amount and quality of carbon in the circuit determine whether simultaneous leaching and adsorption improves the outcome. The response must be demonstrated with representative ore testing.
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Gold recovery depends on two linked rates: how quickly gold dissolves from the ore and how quickly dissolved gold loads onto carbon. M. D. Adams’s 2011 review recommends modelling these ore-specific rates and testing alternative circuit designs rather than applying one generic residence-time rule to every deposit.
A circuit with slow dissolution may need more leach residence time even if adsorption is rapid. Conversely, rapid dissolution with slow or inhibited adsorption may require a different carbon-contact arrangement. Combining the functions in CIL does not remove either kinetic constraint; it makes their interaction a central design issue.
Contact with competing species and dissolved gold
Carbon performance can be affected by other dissolved or suspended constituents. The practical question is not simply whether carbon is present, but whether it remains active, adequately mixed and exposed to dissolved gold for long enough to achieve the target loading and solution profile.
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Practical comparison for a project team
| Decision dimension | CIP implication | CIL implication | What must be verified |
|---|---|---|---|
| Preg-robbing control | Adsorption starts after the dedicated leach stage, so gold may be exposed to preg-robbing material before carbon is introduced. | Carbon is present while gold is dissolving and may compete with preg-robbing surfaces. | Preg-robbing tests using representative ore and the proposed cyanidation conditions. |
| Residence-time design | Leach and adsorption residence times can be designed as separate sections. | Tank duties change along the train; early tanks emphasize leaching and later tanks emphasize adsorption. | Ore-specific leach and adsorption-rate data and a mass-balance model. |
| Carbon handling | Carbon is concentrated in the downstream adsorption section. | Carbon is present throughout the combined leach-and-adsorption train and must be moved and screened reliably. | Carbon inventory, screen capacity, transfer rates and expected carbon losses. |
| Agitation | Separate mixing requirements can be specified for leach and adsorption tanks. | Each tank must support the changing balance between leaching and adsorption. | Solids concentration, slurry rheology, power draw and mixing design. |
| Carbon fouling | Fouling can reduce adsorption performance in the dedicated carbon circuit. | Fouling can affect carbon while it is exposed to leaching slurry, potentially reducing performance across the combined train. | Fouling mechanisms, carbon-activity tests and a treatment or replacement strategy. |
| Downstream gold recovery | Loaded carbon goes to elution and then to electrowinning or zinc precipitation. | Loaded carbon follows the same broad downstream sequence. | Elution capacity, eluate chemistry and recovery-equipment design. |
| Capital and operating cost | No generally applicable cost advantage is established. | No generally applicable cost advantage is established. | Project-specific equipment quantities, power, reagents, labour, maintenance and carbon losses. |
| Gold recovery | No universal recovery advantage is established. | No universal recovery advantage is established. | Metallurgical testwork on the actual ore types and variability domains. |
A defensible method for choosing between CIP and CIL
Step 1: Define the ore domains before choosing the circuit
Do not base the decision on a single composite if the orebody contains materially different zones. Record mineralogy, gold association, expected grade variability and the presence of constituents that may adsorb dissolved gold. An engineering overview from Allnorth notes that different portions of an orebody may warrant different treatment; that broad observation must be validated by testwork rather than adopted as a rule.
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Measure how rapidly gold dissolves under the proposed cyanidation conditions and how much gold remains in solution or solids at each stage. The result should identify whether the proposed leach residence time is adequate and whether slow dissolution is likely to control the circuit.
Step 3: Measure adsorption and preg-robbing behaviour
Run adsorption tests on leached pulp and tests in which carbon is present during leaching. Compare:
- Gold remaining in solution at equivalent reaction times.
- Gold remaining on solids after leaching and adsorption.
- Carbon loading and loading rate.
- Evidence that ore constituents are competing with activated carbon for dissolved gold.
- Changes in carbon activity or fouling during contact with the pulp.
A CIL response is justified only when the combined circuit produces a demonstrable metallurgical benefit or a better overall design balance for the tested ore.
Step 4: Model alternative tank duties
Use the measured leach and adsorption rates to model both a sequential CIP arrangement and a combined CIL arrangement. The model should show gold dissolution, dissolved-gold concentration, carbon loading, slurry flow, carbon movement and the expected tailings loss for each tank or stage.
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Step 5: Check equipment and operating consequences
Compare the equipment required for each alternative, including:
- Agitated tank volume and residence time.
- Mixing duty at the planned solids concentration.
- Interstage and carbon-recovery screens.
- Carbon transfer, storage and handling systems.
- Elution capacity for the expected loaded-carbon flow.
- Carbon-fouling management and carbon-performance monitoring.
- Instrumentation needed to detect solution, solids and carbon losses.
The Minerals Research Institute of Western Australia’s project summary identifies agitation, screening, carbon performance, elution and fouling as significant process and operating topics. They should be treated as design work packages, not as secondary details after the circuit name has been selected.
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Step 6: Compare economics under explicit assumptions
Build capital and operating estimates from the actual design alternatives. Include tankage, screens, pumps, agitation power, reagent consumption, carbon make-up or losses, elution equipment, maintenance and labour. Test the result against ore-grade, recovery, throughput and carbon-performance sensitivities.
The reviewed sources do not provide a current, universally transferable percentage recovery improvement or cost saving for either process. Any estimate should therefore carry its ore type, test conditions, design basis and date with it.
Operating issues that can determine the result
Agitation and solids suspension
Both circuits rely on well-mixed pulp. Poor suspension can create dead zones, reduce contact between dissolved gold and carbon and produce misleadingly low adsorption performance. In CIL, the required mixing duty may change along the tank train because the leaching and adsorption roles are not uniform from the first tank to the last.
Screening and carbon losses
Carbon must be retained and moved deliberately while slurry passes through the circuit. Damaged, overloaded or poorly selected screens can allow carbon to escape with tailings or send carbon to the wrong tank. A reconciliation of carbon additions, transfers, loaded-carbon removals and measured losses is an essential operating control.
Carbon activity and fouling
Activated carbon is the recovery medium in both CIP and CIL, but its performance can deteriorate when fouled by material in the process stream. Fouling is identified as a major process problem in the MRIWA summary. Monitoring carbon loading and activity, identifying the source of fouling and defining a cleaning, replacement or treatment response are part of the flowsheet design.
Elution capacity
A plant can achieve good adsorption and still become constrained downstream if the elution section cannot process loaded carbon at the required rate. Compare the expected carbon-loading profile and removal frequency with elution capacity rather than sizing the adsorption section in isolation.
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Changing ore characteristics
Ore blending can change leach rates, preg-robbing intensity, slurry behaviour and carbon fouling. Operating rules should specify how to recognize a change in ore response and how to adjust carbon concentration, tank residence time, blending or other controllable variables within the approved design envelope.
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Common selection mistakes and how to avoid them
“CIL always recovers more gold”
Simultaneous leaching and adsorption can help a preg-robbing ore, but the available sources do not support a universal recovery ranking. Use comparative testwork and a calibrated model.
“CIP is obsolete because CIL combines two steps”
Combining functions does not automatically produce a better circuit. CIP can provide separate control of leaching and adsorption, which may suit an ore whose kinetics or operating constraints favour distinct stages.
“A generic residence time is transferable”
Leach and adsorption rates vary with ore characteristics and process conditions. Adams’s review specifically supports modelling ore-specific rates instead of applying one generic rule.
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“The downstream plant is the same, so it does not affect selection”
Both circuits require elution and a gold-recovery step, but the amount and timing of loaded carbon reaching elution can differ. Carbon handling and elution capacity must be checked against the selected tank arrangement.
“A single composite sample proves the design”
Variability can change the preferred circuit. Test the major ore domains and identify whether a blend, separate campaign or different treatment route is needed.
Illustrative cases
Case 1: Ore with confirmed preg-robbing
Testwork shows that ore constituents remove a measurable portion of dissolved gold when carbon is absent, while carbon present during leaching captures more of the dissolved gold. CIL becomes a credible candidate. The project should still verify carbon fouling, screen performance, elution loading and economics before selecting it.
Case 2: Fast adsorption but slow leaching
If carbon loads gold rapidly once gold dissolves, but dissolution is slow, adding carbon does not remove the need for adequate leach residence time. The design team should model whether a CIL train can provide the required leach duty or whether a separately controlled leach section is more practical.
Case 3: Distinct ore zones with different responses
One zone may be relatively non-preg-robbing while another adsorbs dissolved gold strongly or fouls carbon. A single plant-wide assumption can conceal this difference. Domain-specific tests may support blending rules, operating changes or different treatment arrangements for different mining periods.
Case 4: Expansion of an existing adsorption plant
An existing CIP facility may have established screens, carbon handling and elution capacity. Converting or adding CIL tanks should be evaluated against the actual constraints of that plant, including available tank volume, mixing power, screen layout, carbon inventory and shutdown requirements. The fact that a CIL circuit is compact in concept does not prove that a retrofit will be cheaper or easier.
Questions to require in a design review
- Which ore domains were tested, and how representative were the samples?
- What are the measured leach and adsorption rates for each important domain?
- Was preg-robbing measured under both sequential and simultaneous conditions?
- What carbon concentration, activity and loading assumptions were used?
- How are carbon losses measured and reconciled?
- What screen duty, aperture and capacity are required at each transfer point?
- What fouling mechanism is expected, and what is the response plan?
- Can the elution circuit process the predicted loaded-carbon flow?
- Which capital and operating assumptions drive the economic comparison?
- How sensitive is the result to ore variability, throughput, grade and recovery?
Bottom line
CIP leaches first and adsorbs dissolved gold afterward; CIL places activated carbon in the leach train so leaching and adsorption overlap. CIL is worth testing when preg-robbing or other ore behaviour makes early carbon contact valuable, while CIP may be preferable when separate control of leaching and adsorption offers a better fit. Neither process is a universal winner. Select the circuit only after ore-specific kinetics, preg-robbing tests, carbon and screen design, elution capacity and project economics have been evaluated together.
Quick Recap
Sources used for the technical explanation
- U.S. Environmental Protection Agency, TSCA Chemical Data Reporting Fact Sheet: Reporting Manufactured Chemical Substances from Metal Mining and Related Activities, December 2022.
- U.S. Environmental Protection Agency, Technical Resource Document: Gold Extraction and Beneficiation of Ores and Minerals, Volume 2.
- Australian Industrial Chemicals Introduction Scheme, PEC31 Sodium Cyanide, 2020.
- Minerals Research Institute of Western Australia, Carbon in-pulp gold processing technology, January 1983.
- Goldcorp, Red Lake Gold Operation NI 43-101 Technical Report, filed with the U.S. Securities and Exchange Commission, 2008.
- M. D. Adams, Factors influencing the rate of gold cyanide leaching and adsorption on activated carbon, and their impact on the design of CIL and CIP circuits, 2011.
- ALS Global, Carbon-In-Pulp (CIP) and Carbon-In-Leach (CIL), circa 2013.
- Allnorth, The Fundamentals of Responsible Gold Flowsheet Design, 2023.
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