Gold Processing8 min read
CIL is easier to operate when the two flows are read separately. The slurry moves through the tank train and carries the unliberated and dissolved gold forward. Activated carbon…
Author: Wayne Ncube
CIL is easier to operate when the two flows are read separately. The slurry moves through the tank train and carries the unliberated and dissolved gold forward. Activated carbon moves in the opposite direction, becoming richer as it meets a more heavily loaded solution. Keeping those two stories straight turns a complex plant into a sequence of inspectable steps.
Independence cyanide mill foundations, Victor, Colorado. Photo: James St. John, Wikimedia Commons, CC BY 2.0. The image is historical context, not a CIL equipment reference.
The tank scene makes the hidden control variables visible: the impeller moves solution, the vessel retains solids and carbon, and the baffles change the flow pattern. In a real CIL tank, the air lance adds the oxygen that the chemistry needs.
In the example CIL circuit, the thickener underflow is pumped to a stock tank through a feed splitter. Cyanide can be dosed in the stock tank and at later points if required. The slurry passes through agitated tanks, with air supplied down hollow agitator shafts. Carbon is held back by interstage screens and moved counter-current by transfer pumps.
From the first adsorption tank, loaded carbon is pumped to a loaded-carbon screen. The screen separates clean carbon from slurry; the slurry returns to the CIL circuit while the carbon moves to the loaded vessel. Acid washing removes inorganic fouling before the carbon enters the elution column. After elution and electrowinning, the carbon is regenerated, cooled, screened, and returned.
| Pattern | Carbon path | Typical trade |
|---|---|---|
| Classic CIL (this article) | Carbon inside leach tanks, counter-current to slurry | Immediate adsorption as gold dissolves; higher inventory and abrasion |
| CIP-style split | Carbon introduced after substantial leaching, outside full leach residence | Lower carbon in aggressive leach; dissolved gold waits longer for capture |
The illustrative CIL circuit described here is CIL-like: carbon is held by interstage screens and moved counter-current by transfer pumps while leaching and adsorption overlap. Ore type (preg-robbing, slow leach) decides which pattern is chosen — the control discipline of following both streams still applies.
Oxygen support also matters. Some circuits use hydrogen peroxide dosing in specific tanks when dissolved oxygen is the constraint; that is a chemistry decision with its own addition and monitoring rules, not a default “additive for recovery.”
This sequence is useful on shift because every handoff has an owner:
A simple first calculation is:
residence time = active slurry volume / slurry volumetric flow
If six tanks each have 2,400 m³ of active volume and the measured flow is 300 m³/h, the nominal total residence time is 48 hours. If only five tanks are available, the same calculation gives 40 hours. A pump fault or bypass therefore changes the process, not just the equipment status.
The calculation still needs a reality check. Confirm that the tank is not empty, that flow meters are calibrated, and that the active volume matches the operating level. When the pulp density changes, the dry-tonnes flow can remain constant while the volumetric flow changes. That distinction matters when the plant is targeting a residence-time basis.
Carbon can leave the normal inventory through several paths:
A useful check is not only the carbon-in-tank estimate. Sample the overflow and inspect the filter or screen for escaped carbon. Then inspect the tailings stream and account for the carbon that is recovered by the fine-carbon system. A high carbon-in-tank reading can coexist with poor recovery if it is concentrated in the wrong tank.
A short daily sheet can make the loop visible:
| Variable | What it changes | First response to drift |
|---|---|---|
| Dissolved oxygen | Gold dissolution rate | Check air supply, sparger condition, and measured demand |
| Free cyanide | Available leaching reagent | Check dose, consumption, and pH before increasing it |
| pH | Cyanide safety and reagent balance | Check lime rate and carbonate or acid load |
| Pulp density | Mixing, carbon suspension, and residence time | Reconcile flow, tonnes, and level |
| Agitation | Mass transfer and carbon attrition | Check impeller condition and operating speed |
| Carbon concentration | Adsorption capacity | Check transfer pumps, screens, and tank distribution |
| Screen condition | Carbon retention | Inspect wiper, mesh, seal, and overflow evidence |
The best control room decision is usually the next test, not the biggest immediate setpoint change. Record the observation, the time, the operating state, and the action so that the next shift can distinguish a transient from a developing failure.
Carbon transfer is not continuous flow in the process sense. It is a scheduled movement of a solid inventory. A typical sequence is to withdraw loaded carbon from the adsorption end, charge the next vessel, circulate the acid wash, rinse, transfer the carbon to the elution column, and return regenerated carbon near the barren end. The timing matters because a tank can contain carbon at the right average concentration while still having the wrong distribution.
A carbon batch record should answer five questions:
Without those answers, a poor gold result cannot be assigned to adsorption, elution, regeneration, or simply an unlucky assay.
A failed interstage screen has several signatures, and they can appear before the tailings grade moves:
The safe response is to isolate the symptom, protect the carbon inventory, and collect evidence before making a broad chemistry change. A gold plant is vulnerable to the temptation to treat every downstream symptom as a cyanide problem.
The residence-time calculation can be made more useful by writing the uncertainty explicitly:
τ = (N_active × V_usable) / Q_slurry
If the tank level is uncertain by ±10%, the active volume carries that same uncertainty. If the flow meter is biased, the calculated time is biased in the opposite direction. If a tank is bypassed, the denominator stays similar while the numerator drops. That is why a residence-time estimate should be paired with a measured level, a calibrated flow signal, and a record of bypass status.
The six-tank teaching example provides a useful sanity check: 6 × 2,400 m³ / 300 m³/h = 48 hours. That is not automatically the plant's operating time. At a different slurry density or flow rate, the volumetric relationship changes even when dry tonnes per day remain constant.
Counter-current
carbon movement relative to slurry
2 bed volumes/h
illustrative elution flow target
0.8 mm
illustrative tail screen aperture
Elution reverses adsorption with hot caustic-cyanide solution, low ionic-strength water, controlled flow, and suitable pressure. The pregnant eluate goes to electrowinning. Barren eluate is managed separately and the carbon moves to reactivation.
Acid washing removes inorganic fouling, including salts and mineral matter. Thermal reactivation removes organic material in a steam environment at high temperature. If the kiln is too cool, foulants remain; if it is too hot or residence time is disturbed, carbon can degrade or clinker. A carbon-inventory problem may therefore originate outside the adsorption tanks.
Cyanide, hot caustic, pressure, hydrogen cyanide risk, and kiln combustion hazards require site-specific controls. The operating values in this article are a study aid, not a procedure for carrying out reagent or pressure work.