CIL Plant Design: Residence Time, Carbon Screens and Elution Temperature

By the YELI Technical Team - Updated October 2026 - 8 min read

Short answer: CIL plant design decides what your carbon has to do before you buy a single bag. Leaching and adsorption residence time sets the number and volume of tanks; the interstage screen aperture sets the minimum particle size you can run without losing carbon to tails; carbon concentration in the pulp sets the standing inventory; and the elution design — atmospheric Zadra at roughly 90-100 °C, or a pressurised circuit typically 115-160 °C depending on the reference — sets the thermal and pressure duty your carbon must survive. Specify 6x12 mesh coconut shell carbon at iodine 1000-1300 mg/g and hardness at least 98% and the design works; specify soft carbon and every one of those design decisions leaks value.

Most carbon problems in a CIL plant are blamed on the carbon. In practice, a large share of them are set in place at the design stage: the tank train is too short for the ore's leaching kinetics, the interstage screens are the wrong aperture for the carbon size, or the elution circuit runs hotter than the carbon grade can survive cycle after cycle.

1. Residence time: the first design decision that affects your carbon

CIL merges leaching and adsorption in the same tank train. That means residence time is shared: the ore needs enough time to leach, and the carbon needs enough contact time to load. When a plant is designed with leaching kinetics in mind only, the adsorption side is what suffers — carbon never reaches its equilibrium loading, and the last tank does very little work.

Design references commonly work with a total CIL residence time of the order of 20-30 hours across the tank train, split between leaching and adsorption duty. <em>(Industry-typical range; your own testwork decides.)</em> The practical symptoms of too little residence time are easy to recognise:

• Loaded carbon assays lower than the design value.
• Soluble gold in the last tank stays high instead of collapsing.
• Operators compensate by raising carbon concentration, which raises attrition and carbon consumption instead of fixing the cause.

2. Carbon concentration and transfer strategy

The carbon concentration in the adsorption tanks is the single number that couples the design to your monthly carbon bill. Industry practice commonly runs in the 10-25 g/L range (10-25 kg/m&sup3; of pulp) across the train, held higher in the first tanks and lower in the last. <em>(Industry-typical.)</em>

Two consequences follow from that range:

• Standing inventory. 400 m&sup3; of pulp at 15 g/L is about 6 t of carbon sitting in the circuit, and that inventory has to be paid for, eluted and regenerated whether or not the plant is running at design throughput.
• Transfer frequency. Carbon usually moves counter-current, one stage at a time. Transferring too often increases the number of pump and eductor cycles, and every cycle abrades the particles. Transferring too rarely leaves the first tank loaded and the last tank starved.

Rising fines in the tailings screen oversize, with a stable iodine value on incoming carbon, means the transfer and pumping regime is the problem — not the carbon grade.

3. Interstage screens: aperture, blinding and maintenance

The interstage screen is what keeps carbon in the circuit. Aperture selection follows from the carbon cut: 6x12 mesh carbon (1.70-3.35 mm) runs against screens typically in the 0.8-1.0 mm range, so that whole particles are retained with margin while pulp passes freely. <em>(Industry-typical.)</em>

Screen blinding is the most under-maintained failure mode in CIL and CIP circuits:

• What blinding looks like. Pulp flow through the screen drops, tank levels become uneven, and carbon starts reporting to the next stage unevenly or to tails.
• What causes it. Wood fibre and vegetation from the ore, flotation reagents and oils, calcium carbonate scale from lime dosing, and carbon fines packing the apertures.
• What it costs. A blinded screen raises the effective carbon loss to tails, and the fines that pass carry gold with them. It also distorts the adsorption profile across the train, so the plant's own data stops making sense.
• How to manage it. Inspect on a fixed schedule rather than on failure; keep a spare screen set; wash with dilute acid where scale is the cause; and treat high fines generation upstream (harder carbon, gentler transfer) rather than only cleaning downstream.

4. Elution design: Zadra, AARL and the temperature question

Elution design is usually quoted as a temperature, and that single number drives the carbon specification more than any other part of the flowsheet.

<em>All values are industry-typical ranges quoted in design references, not a YELI process commitment; final setpoints come from your plant design and metallurgical testwork.</em>

What matters for the carbon buyer is the consequence: a circuit that runs hot and pressurised cycles the carbon harder, so hardness and attrition resistance matter more, not less, in a hot strip circuit. The same carbon that performs acceptably in a mild atmospheric strip can shed unacceptable fines in a 150 °C cycle.

5. What the design implies for the carbon specification

Read backwards, a CIL flowsheet produces a short specification list:

Any of these numbers missing from a quotation means the quotation cannot be checked against the design.

6. Design-stage checklist for the carbon buyer

• Tank train volume and total residence time, split between leach and adsorb duty.
• Carbon concentration target per tank (g/L) and the resulting standing inventory in tonnes.
• Interstage screen aperture and the carbon cut it implies.
• Elution route and the temperature the carbon will actually see each cycle.
• Expected carbon consumption (g/t ore) and monthly make-up.
• COA parameters required for acceptance, and the right to reject on the shipped lot.

Frequently asked questions