Carbon in Pulp vs Carbon in Leach: Which Wins?

Last updated: 2026-10-26

In the carbon in pulp vs carbon in leach decision, carbon in leach (CIL) wins for most modern gold plants: running leaching and adsorption in the same tanks removes 20–30% of vessel count and neutralizes preg-robbing ores that can strip 5–15% of recovery from a CIP circuit. Carbon in pulp (CIP) stays ahead only on clean, fast-leaching, low-slime feeds.

Key Takeaways

• CIL leaches and adsorbs simultaneously; CIP completes leaching first, then adsorbs in a separate train.
• CIL tolerates preg-robbing and high-slime ores that can cost a CIP circuit 5–15% of recovery.
• CIP delivers higher loaded-carbon grades but needs roughly two more tanks per train.
• Carbon inventory and attrition losses run higher in CIL; screen duty differs in timing, not count.
• Adsorption and tank-design fundamentals are covered in our leaching pillar guide.

What Are Carbon in Pulp and Carbon in Leach?

Both circuits recover dissolved gold from cyanide leach slurry using granular activated carbon, and both end at the same place: loaded carbon reporting to an elution (stripping) circuit, electrowinning or zinc precipitation, and a tailings disposal system. The difference is not the chemistry — it is where adsorption sits in the flowsheet.

ASME VIII Div 1 vs Div 2 explained

the Leaching guide

Carbon in pulp is a gold adsorption circuit that contacts fully leached slurry with granular activated carbon in a series of agitated tanks, typically six to eight, after cyanide leaching has run to completion in a separate leach train. Dissolved gold exists as the aurocyanide complex Au(CN)₂⁻; carbon adsorbs this complex onto its internal pore structure, and slurry flows down the train while carbon is advanced countercurrently, so the freshest carbon meets the most barren slurry.

Carbon in leach is a combined leach-adsorption flowsheet that dissolves gold with dilute sodium cyanide (NaCN) and adsorbs the Au(CN)₂⁻ complex onto carbon in the same vessels at the same time, so a single tank train performs two duties that CIP assigns to two separate trains. Leach kinetics and adsorption kinetics overlap: gold dissolving in tank 3 is already being captured in tanks 1 and 2.

Preg-robbing ore is a carbonaceous feed that contains natural organic carbon or graphitic matter which re-adsorbs the aurocyanide complex from solution, stealing back gold that the plant has already dissolved. This behavior is the single strongest argument in the CIP vs CIL comparison, because the two circuits handle it very differently.

Carbon inventory is a working stock of activated carbon that the plant holds inside its adsorption tanks at all times, advanced forward against the slurry flow and withdrawn periodically for elution and thermal regeneration. Inventory size, carbon grade and attrition rate drive a large share of operating cost, and they differ measurably between the two circuits.

Both circuits grind feed to a fine size — typically P80 (80% passing size) of 53–75 µm — because gold liberation, not adsorption, is usually the rate-limiting step. Where the overall choice between agitated tanks and percolation leaching is still open, see our tank versus heap leaching comparison.

Process Flowsheet Differences: Carbon in Pulp vs Carbon in Leach

The two flowsheets share crushing, grinding and classification, and they share the carbon elution and regeneration island at the back end. They diverge entirely in the middle, where CIP separates leaching from adsorption and CIL fuses them into one train.

Three consequences follow from this sequence difference, and each one ends in a practical conclusion.

Vessel count. Because CIL folds adsorption into the leach train, a plant that would need 8 leach tanks plus 7 adsorption tanks under CIP — 15 agitated vessels — can often meet the same metallurgical duty with 9–12 CIL tanks. Fewer tanks mean less steel, less concrete, fewer agitators and a smaller building footprint. For a greenfield project, CIL therefore wins the vessel-count comparison in nearly every case where the ore does not demand a separate high-tenor leach.

Solution tenor profile. In CIP, the adsorption feed is fully leached pregnant solution at peak tenor, commonly 2–10 mg/L Au, so the first carbon loads to high grades quickly and the adsorption efficiency per tank is high. In CIL, solution tenor is suppressed from the start because carbon is removing gold as fast as cyanide dissolves it. That is good for locking in recovery, but it means carbon loads more slowly and to lower final grades.

Screening timing. Both circuits need an interstage screen at each tank to retain carbon while passing slurry, so screen count per tank is identical; the difference is what the screens see. CIP screens receive fully leached, already-conditioned slurry; CIL screens receive fresh, debris-laden slurry in the first tanks, where residual coarse gold and grinding grit are more likely to blind apertures. Screen selection for a CIL first tank deserves extra attention — our gold CIL tank equipment page covers the mechanical details, and our gold leaching tank resource guide covers sizing.

Why Does CIL Tolerate Preg-Robbing and High-Slime Ores?

Preg-robbing is a race: once gold dissolves into solution, either activated carbon or the ore’s own carbonaceous matter captures it. In a CIP plant the leach train runs for 24–48 h with no activated carbon present, so natural organic carbon — humic acids, graphitic carbon, bituminous matter — has that entire window to re-adsorb Au(CN)₂⁻ onto surfaces from which it will never report to elution. The loss is silent: feed and tailings solids assays look normal, yet the dissolved gold reports nowhere.

In a CIL plant, activated carbon enters the first tank with the slurry. Activated carbon has a far higher affinity and faster uptake kinetics for Au(CN)₂⁻ than natural carbonaceous matter does, so it captures the complex minutes after dissolution and holds it until elution. In effect, CIL converts a chemical competition the plant loses into one it wins. Plants treating carbonaceous ores commonly report recovery improvements of 5–15 percentage points when they move from CIP to CIL duty; the exact gain is ore-specific and should be confirmed by diagnostic test work.

High-slime ores reward CIL for a different reason: residence time. Slimes — material finer than roughly 20 µm — raise slurry viscosity, slow the diffusion of cyanide and oxygen to gold surfaces, and lengthen the leach. A CIP flowsheet facing a 40-hour leach must build a 40-hour leach train and then a separate adsorption train behind it. A CIL flowsheet absorbs the long leach inside its single train, and adsorption proceeds for free during those same hours. The conclusion is direct: when long leach residence and heavy adsorption duty coincide, CIL delivers both in one set of tanks and CIP pays for two.

Head-to-Head Comparison: Carbon in Pulp vs Carbon in Leach

The table below compresses the decision into eight parameters. Ranges are typical for refractory-gold and free-milling plants of 0.5–5 Mt/a and should be confirmed by test work for any specific ore.

Residence time is the first-order difference: CIL converts adsorption hours into leach hours, so total train time drops and, with it, tank count. Tank count drives the CAPEX line, and for a greenfield gold plant the CIL route typically saves 10–20% of the tanks-and-structure capital — real money at the 10–30 million dollar scale of a 2 Mt/a adsorption island. Carbon inventory moves the other way: because CIL adsorbs at suppressed tenor, it needs more carbon in circuit to achieve the same capture rate, and that carbon arrives at elution at lower grade, which raises elution frequency per kilogram of gold produced. Screen duty is a timing issue rather than a count issue — both circuits screen every adsorption tank — but a CIL first tank must tolerate the grittiest slurry in the plant. On commissioning risk, the honest summary is that CIP’s separation of duties makes each train easy to start up, while CIL’s interaction of leaching and adsorption makes early tuning harder but the tuned endpoint metallurgically stronger. For most ore bodies, the head-to-head record therefore reads: CIL on capital and robustness, CIP on carbon economy and startup simplicity.

Commissioning sequence differences are worth spelling out, because they change the startup program:

1. Water and air checks. Both circuits start identically — agitators, air lifts and interstage screens are proven with water before any slurry.
2. Leach chemistry first (CIP). A CIP plant tunes cyanide strength (0.3–0.5 g/L NaCN typical), pH 10.5–11.0 and dissolved oxygen with no carbon in circuit, then introduces carbon to an already-stable leach.
3. Combined chemistry (CIL). A CIL plant must establish leach and carbon transfer simultaneously; the practical method is to run the first days at reduced throughput with carbon transfer cadence slowed, watching tailings solution tenor hourly.
4. Screen integrity sweeps. In CIL, inspect first-tank screens twice per shift during startup; coarse debris loading here is the classic cause of carbon carryover into tailings.
5. Inventory build-up. Grow carbon inventory in stages — roughly 50%, 80%, then full design inventory — while confirming loaded-carbon grades against the design curve.
6. Performance sign-off. Accept the circuit on tailings solution losses (target typically below 0.02 mg/L Au) sustained over 72 h, not on a single good shift.

Which Should You Choose? Decision Rules by Ore Type

Ore mineralogy, not vendor preference, decides this question. The matrix below gives the rules we apply when reviewing a client’s flowsheet options.

Four of those rows deserve the full rule form. For preg-robbing carbonaceous ores, CIL is preferred because activated carbon in the first tank captures the aurocyanide complex within minutes of dissolution, before natural organic carbon can reabsorb it. For high-slime, slow-leaching ores, CIL is preferred because the extended residence the ore demands is absorbed by a single train instead of a leach train plus an adsorption train. For clean, fast-leaching free-milling ore, CIP is preferred because a completed high-tenor leach feed loads carbon to 3,000–5,000 g/t Au, shrinking carbon inventory, elution frequency and gold locked up in circuit. For retrofit projects where serviceable leach tanks already exist, CIP is preferred because the adsorption train is the only new capital and the existing vessels keep earning. Where the ore itself might be better served by percolation leaching at lower grade, our heap leaching guide is the next reference point.

How Does Carbon Management Differ Between CIP and CIL?

Carbon management is where CIP vs CIL trade-offs become line items on the operating budget, and three quantities move.

Inventory size. CIL plants carry the larger stock. Adsorbing at low solution tenor means each tonne of carbon collects gold more slowly, so the plant must hold roughly 1.5–2 times the CIP inventory to achieve the same daily gold capture. A useful first-pass rule: design carbon inventory near 20–40 times the daily gold production rate in grams, refined by kinetic test work.

Loaded-carbon grade and elution frequency. The mirror image of the larger inventory is lower grade. CIP loaded carbon at 3,000–5,000 g/t Au delivers more gold per elution cycle; CIL carbon at 1,000–2,500 g/t Au means more elution cycles per kilogram of gold, more reagent per cycle, and more thermal regeneration passes. Each elution-regeneration cycle also strips fines from the carbon, so CIL plants typically consume more make-up carbon — a figure to confirm in test work, since attrition is strongly affected by carbon hardness, transfer pumping design and screen wear.

Screen and transfer duty. Both circuits advance carbon countercurrently through interstage screens, but a CIL plant’s screens work around the clock on slurry that still contains coarse particles, so stainless wedge-wire or polymer screens in the 0.6–0.8 mm range, sized to retain carbon granules of 1.4–2.0 mm while passing slurry, need more frequent inspection. The management conclusion: budget CIL carbon management as a defined operating discipline — scheduled inventory audits, weekly screen inspections and monthly attrition testing — rather than as an afterthought to the leach.

One further difference deserves a note: gold locked up in circuit. Carbon inventory is also a gold warehouse. A CIL plant holding 250 t of carbon at an average 800 g/t has roughly 200 kg of gold sitting in the tanks — working capital, yes, but also a delayed cash conversion that finance teams should model alongside the metallurgy.

Cost & Energy

Figures below are scoping-level ranges for gold plants of 0.5–5 Mt/a, expressed in US dollars, excluding mining and owner’s costs; they are typical ranges, to be confirmed by test work and vendor quotations.

Capital. For the agitated-leach island itself (tanks, agitators, screens, air lifts, piping, exclusive of grinding), CIP typically falls at US$25,000–40,000 per tonne per day of capacity and CIL at roughly 10–20% less, because the single train eliminates 3–5 agitated vessels and their drives. The elution and carbon regeneration island is essentially identical in both cases; a 500 kg-per-cycle elution plant with regeneration kiln commonly adds US$2–5 million to either flowsheet.

Operating cost. Agitation is the largest electrical load in the adsorption area: expect 0.8–1.5 kWh/t ore processed across the tank train, with CIL at the lower end thanks to fewer agitated vessels and CIP at the higher end. Activated carbon consumption runs 10–25 g/t ore in a well-run CIP plant and 15–35 g/t in a comparable CIL plant, reflecting the higher elution frequency; at delivered carbon prices of US$3,000–5,000/t, that difference is worth roughly US$0.02–0.05 per tonne of ore — small next to reagent costs, but not zero. Cyanide consumption (0.3–1.0 kg/t NaCN, ore-dependent) and lime (0.5–2 kg/t) are effectively identical between the circuits. Elution reagent and power costs scale directly with elution frequency, so the CIP carbon-economy advantage shows up here as a modest saving, typically US$0.05–0.15/t ore.

The net. Across a 10-year model, CIL’s capital saving usually dominates for greenfield builds while CIP’s carbon and energy economies claw back a few dollars per tonne; neither gap is large enough to overrule mineralogy.

Field Data: Retrofit Decision at a 1.2 Mt/a Gold Plant

The following case is an illustrative composite assembled from industry experience, with numbers representative of a real class of problem rather than any single named operation.

A 1.2 Mt/a (≈3,300 t/d) gold plant had been built as a conventional CIP circuit: eight leach tanks at 4 h each, seven adsorption tanks at 1 h each, grinding to P80 63 µm. After three years, mining reached a zone with 0.4% total organic carbon, and diagnostic test work confirmed genuine preg-robbing behavior: bottle-roll tests with carbon present recovered 91% of gold, while the same tests without carbon plateaued at 82%.

Plant data told the same story. Tailings solution losses had climbed from 0.015 mg/L Au to 0.048 mg/L Au, worth approximately 96 kg of gold per year at design solution flows, and loaded carbon grades had slipped from 3,800 to 2,600 g/t Au as the plant compensated with faster carbon transfer.

The retrofit converted the first four leach tanks to CIL duty by installing interstage screens and air-lift carbon transfer, leaving the last four leach tanks and the full adsorption train as a CIP polishing stage — a hybrid arrangement that placed carbon into the leach path exactly where preg-robbing was occurring. Capital cost was approximately US$2.8 million, dominated by screens, transfer systems and interconnects. Measured results over the following six months: tailings solution losses fell to 0.014 mg/L Au, overall recovery rose by 2.7 percentage points, and carbon inventory grew from 190 t to 240 t to support the added duty. At the prevailing gold price, the payback was under 11 months.

The lesson generalizes: the CIP vs CIL choice is not always binary, and a staged conversion that injects carbon where the ore demands it can capture most of the CIL benefit at a fraction of greenfield cost. For how such recovery deltas are projected from bottle-roll and column data, see our leaching recovery calculation resource.

Frequently Asked Questions

How many tanks does each circuit actually need?

A CIP plant typically runs 6–8 leach tanks plus 6–8 adsorption tanks, totaling 12–16 agitated vessels. A CIL plant combines both duties in a single train of 8–12 tanks, each providing 2–4 h of residence. The exact counts depend on leach kinetics from bottle-roll testing, target recovery and throughput; the reliable pattern is that CIL needs roughly 20–30% fewer tanks for the same ore duty.

Why does CIL carbon load to a lower gold grade than CIP carbon?

Carbon grade tracks the solution tenor it sees. In CIL, carbon removes gold as soon as it dissolves, so the average solution tenor stays low and each carbon particle accumulates gold slowly, finishing at 1,000–2,500 g/t Au. In CIP, carbon meets fully leached pregnant solution at 2–10 mg/L Au and loads rapidly to 3,000–5,000 g/t Au. Higher grade means fewer elution cycles and less carbon make-up per kilogram recovered.

Can an existing CIP plant be converted to CIL?

Often, yes, and the conversion can be partial. Adding interstage screens and air-lift transfer to the first leach tanks creates a hybrid: carbon enters the leach path where preg-robbing occurs while the existing adsorption train continues as a polishing stage. The capital is modest — screens, transfer piping, and carbon inventory — but the change must be verified with kinetic test work confirming that simultaneous adsorption does not slow gold dissolution.

Does the choice affect gold locked up in inventory?

Yes, materially. Carbon in circuit is a gold warehouse: a CIL plant carrying 250 t of carbon at an average 800 g/t holds about 200 kg of gold, whereas a CIP plant typically holds less carbon at higher average grade, and its in-circuit gold can be broadly similar or lower. Finance teams should model this working capital and the delayed cash conversion alongside the metallurgical and capital comparisons.

Is CIL suitable for silver-rich ores?

It works, but with a caveat. Silver competes with gold for carbon adsorption sites and elutes differently, so silver-rich feeds load carbon faster but to mixed metal values, and Merrill-Crowe zinc precipitation sometimes replaces carbon entirely at high silver-gold ratios. For moderately silver-bearing ores, both CIP and CIL function; the CIP vs CIL decision criteria — preg-robbing, slimes, kinetics — remain the same as for gold-only ores.

How is carbon kept out of the tailings?

Interstage screens at every tank retention point hold carbon back while slurry passes through, and a final safety screen on the tailings line catches any escapee. Carbon granules are 1.4–2.0 mm while screen apertures are typically 0.6–0.8 mm. In CIL plants, first-tank screens see the grittiest slurry, so screen inspection frequency and spare screen inventory matter more than in CIP, where screens see clean, fully leached slurry.

Conclusion

The carbon in pulp vs carbon in leach question resolves on ore character, and the record is consistent. CIL wins where mineralogy attacks the CIP flowsheet: carbonaceous preg-robbing ores, high-slime slow-leaching feeds, fine-grind refractory material and any greenfield project that wants 20–30% fewer tanks and 10–20% less adsorption-area capital. CIP wins where the ore is clean and fast-leaching and the operator wants maximum carbon-loading efficiency, lower inventory, richer loaded carbon and the simplest possible startup. Between those poles sit hybrid retrofits, which prove daily that the two circuits are ends of a spectrum rather than mutually exclusive designs. What never works is choosing on capital cost alone and discovering the preg-robbing behavior in the first year of operation. Put your ore through bottle-roll kinetics, a diagnostic preg test and a small-scale carbon adsorption program before the flowsheet is frozen — our pilot testing services team can design and run that program with you. More hydrometallurgy technical guides, calculators and application notes are available in our resource library.

References

1. Marsden, J. O. & House, C. I. (2006). The Chemistry of Gold Extraction, 2nd edition. Society for Mining, Metallurgy & Exploration (SME), Littleton, CO.
2. Stange, W. (1999). “The process design of gold leaching and carbon-in-pulp circuits.” Journal of the South African Institute of Mining and Metallurgy, Vol. 99, No. 1, pp. 13–25.
3. Fleming, C. A. (1992). “Hydrometallurgy of precious metals recovery.” Hydrometallurgy, Vol. 30, pp. 127–162.
4. La Brooy, S. R., Linge, H. G. & Walker, G. S. (1994). “Review of gold extraction from ores.” Minerals Engineering, Vol. 7, No. 10, pp. 1213–1241.
5. SME Mineral Processing and Extractive Metallurgy Handbook (2019). Society for Mining, Metallurgy & Exploration, Englewood, CO.
6. U.S. Geological Survey (2026). Mineral Commodity Summaries 2026 — gold. Reston, Virginia. https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-gold.pdf

Changelog

2026-10-26 — first published.

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