CIL is the workhorse of the leaching family described on tailings gold recovery methods, and while it has been referenced across this site as the standard final stage of a retreatment flowsheet, this page is where the mechanism, residence time, and carbon management actually get worked through. It became the industry-standard leach configuration precisely because combining leaching and adsorption in the same tanks removes a whole separate stage of equipment, tankage, and pumping that a sequential process needs, without sacrificing recovery on well-behaved feed.
How does the simultaneous leach-and-adsorb mechanism work?
CIL dissolves gold into cyanide solution and adsorbs it onto activated carbon in the same tank at the same time, rather than treating leaching and adsorption as two separate stages. As gold dissolves off particle surfaces into solution, the activated carbon suspended in the same slurry immediately adsorbs it, which keeps dissolved gold concentration in solution low and helps drive the leach reaction further toward completion. That simultaneous design is the key difference from carbon-in-pulp, which separates the two steps instead.
How is a CIL tank train physically arranged?
A CIL tank train is a series of agitated tanks, commonly six to eight in a full-scale circuit, connected so slurry flows continuously from the first tank to the last while carbon advances in the opposite direction. Each tank is mechanically agitated to keep both the ore particles and the carbon granules suspended, since either settling out defeats the process: settled ore stops leaching efficiently, and settled carbon stops adsorbing. Air or oxygen is typically sparged into the earlier tanks specifically, where dissolved gold concentration in solution is highest and the leach reaction most needs the oxygen to proceed.
The cascading tank arrangement matters because it approximates a single long plug-flow reactor using a series of well-mixed stages, giving the circuit predictable, tunable residence time without needing a single impractically long tank. Adding or removing tanks from the train, or adjusting the slurry flow rate through it, is the primary lever a plant has for changing total residence time without a full redesign.
Oxygen supply itself is worth calling out as a real design choice, not an incidental detail. Some circuits rely on simple air sparging, while others inject pure oxygen, generated on site or delivered as liquid oxygen, where leach kinetics or high cyanide-consuming sulphide content make plain air an insufficient oxygen source. The choice affects both capital cost, oxygen generation equipment is a meaningful line item, and achievable leach rate, and it is decided from the same bottle-roll and pilot testwork that sets residence time overall.
What residence time does CIL need on tailings specifically?
CIL tanks on tailings retreatment are typically sized for 24 to 48 hours of residence time, and where a project's material sits in that range depends on how oxidized the tailings are and how much preg-robbing carbonaceous material is present. Tailings that have weathered and oxidized in storage can sometimes leach faster than an equivalent fresh sulphide ore, but preg-robbing material needs the longer end of that range, or a pre-treatment step, to give the carbon a fair chance of capturing gold before it re-adsorbs onto the preg-robbing material instead.
Residence time requirements are established through bottle-roll and pilot-scale leach testwork on representative samples, not assumed from a generic industry figure, because two dumps with similar head grades can leach at meaningfully different rates depending on mineralogy and weathering history. A project that under-specifies residence time based on an assumed figure risks leaving recoverable gold in the tailings stream leaving the circuit, while over-specifying it ties up capital in tankage the leach kinetics don't actually require. A typical bottle-roll test tracks gold extraction against elapsed leach time on a laboratory sample, producing an extraction curve that shows where recovery genuinely plateaus, the point beyond which additional residence time buys negligible extra gold and simply adds cost.
How is carbon managed through the circuit?
Carbon in a CIL circuit advances countercurrent to the slurry flow, meaning fresh carbon is added at the tank the slurry is about to leave, while the most heavily loaded carbon is screened out at the tank the slurry first entered. That countercurrent arrangement maximizes the concentration gradient driving adsorption at every stage. Once screened out, loaded carbon moves to elution, typically a Zadra or AARL process as described on tailings reprocessing equipment, where the gold is stripped back into solution ahead of electrowinning.
Screens separating carbon from slurry at each transfer point are a frequent source of operational trouble if not properly sized and maintained, since a torn or blinded screen either loses valuable loaded carbon downstream with the slurry, a direct gold loss, or restricts flow enough to bottleneck the whole train. Regular screen inspection is accordingly treated as a core operating discipline on any CIL circuit rather than a minor maintenance item.
What reagent and safety considerations are specific to CIL?
CIL needs careful cyanide dosing control, adequate dissolved oxygen, and lime addition to hold the alkaline pH the leach reaction requires, all of which are covered from the chemistry side on cyanidation basics: chemistry of gold leaching. Cyanide is dosed to match consumption rather than added in excess, both for cost control and because base-metal sulphides in the feed can consume far more cyanide than the gold-dissolution reaction alone requires.
Safe cyanide management extends well beyond the leach tanks themselves into storage, handling, and final tailings detoxification before the barren slurry is discharged to a tailings storage facility. The International Cyanide Management Code sets out a widely referenced framework for exactly this full chain of custody, from delivery through use to eventual detoxification, and a project's alignment with that framework is a legitimate, verifiable point of due diligence for investors and communities alike, distinct from and in addition to the physical integrity standards a tailings storage facility itself must meet under frameworks like the Global Industry Standard on Tailings Management.
How is CIL performance actually measured?
CIL performance is measured through the same metallurgical balance used across this site: comparing head grade against tails grade to establish how much gold the circuit actually recovered, alongside carbon loading efficiency to confirm the adsorption side of the process is keeping pace with leaching. A CIL circuit can be leaching gold successfully while still under-recovering overall if the carbon side is under-performing, which is why both halves of the balance are tracked separately.
Carbon loading is itself reported as mass of gold adsorbed per mass of carbon, and a well-run circuit tracks this figure tank by tank along the train rather than only at the point carbon is finally screened out for elution. A loading profile that falls off faster than expected in the early tanks can flag a leach kinetics problem upstream, while a profile that stays too flat across the train can point to carbon that is aged, fouled, or otherwise under-performing and due for replacement with fresh activated carbon.
How does CIL performance change with tailings feed, versus fresh ore?
Tailings feed differs from fresh ore in oxidation state, moisture carried into the circuit, and the proportion of preg-robbing carbonaceous material present, and all three change how CIL should be tuned. Oxidized tailings can sometimes leach faster than an equivalent fresh sulphide feed, which can shorten the residence time needed, while a higher proportion of preg-robbing material pushes in the opposite direction and argues for longer contact time or a pre-treatment step ahead of the leach tanks.
How CIL compares to the alternative sequencing
The decision between CIL's simultaneous approach and CIP's sequential one is a genuine engineering tradeoff, not a default, covered directly on carbon-in-pulp explained and the head-to-head comparison that follows it.
Why gravity recovery ahead of CIL still matters even with a well-run leach circuit
Every gram of gold captured by an upstream gravity stage is a gram CIL never has to leach, which directly reduces cyanide consumption, residence time demand, and carbon inventory pressure on the leach circuit. A well-specified gravity stage ahead of CIL, described across the gravity equipment pages in this set, is one of the most direct ways to lower a CIL circuit's operating cost per tonne rather than trying to optimize the leach chemistry alone.
Why this stage is central to the investment case for a retreatment project
CIL is typically where the majority of a retreatment project's total recovered gold ounces are produced, since it captures the fine and surface-bound gold that gravity equipment cannot reach on its own. Its residence time, reagent consumption, and recovery performance, established through proper testwork rather than assumed from general industry figures, are accordingly among the most closely scrutinized technical inputs in any feasibility study or investment memorandum for a Ghanaian tailings retreatment opportunity.