Cyanide Leaching Equipment: 9 Downtime Causes

Last updated: 2026-10-26

Nine recurring failure points account for most unplanned stops in a gold carbon-in-leach (CIL) circuit, and every one of them sits inside the cyanide leaching equipment train — agitators and gearboxes, interstage screens, airlift pumps, carbon transfer systems, and safety instrumentation. Plants that systematically address these nine items commonly lift CIL plant availability from below 90% to above 96% within one maintenance year.

This troubleshooting guide is part of our broader leaching coverage; if your project is still comparing extraction routes, our heap leaching guide covers the percolation alternative, and the leaching recovery calculation resource shows how downtime and recovery interact in the economics.

Key Takeaways

• Nine equipment items cause most CIL stoppages; gearboxes and interstage screens lead the list.
• Interstage screen blinding is the top silent availability killer: it bypasses carbon before anyone notices.
• Airlift pump blockage usually traces to tramp oversize and carbon sludge, not to pump sizing.
• Carbon fines losses quietly bleed 0.5–2% of gold production when screen machines wear.
• Scheduled maintenance on the cyanide leaching equipment train pays back within weeks at typical throughputs.

What Counts as Cyanide Leaching Equipment?

Cyanidation dissolves gold and silver into a dilute sodium cyanide (NaCN) solution, and the machinery that keeps that reaction running is what we mean by the equipment train. Cyanide leaching equipment is the complete set of mechanical assets between the grinding circuit discharge and the tailings pumpbox: leach tanks and their agitators, interstage screens, air and oxygen delivery systems, carbon transfer hardware, safety screen machines, and the instrumentation that keeps the operation safe. In a carbon-in-leach flowsheet, where leaching and adsorption happen in the same series of tanks, every item works wet, abrasive and loaded with activated carbon — one of the harshest duty cycles in the plant.

Tank Leaching vs Heap Leaching, step by step

Agitated Leaching Tank Design: selection rules

Leaching explained

A mechanical agitator is a rotating impeller assembly that keeps ore particles suspended, disperses air or oxygen through the pulp, and prevents sanding at the tank bottom. In most CIL plants it is the single most critical asset in the train, because an agitator stop in a 40% solids slurry gives you minutes, not hours, before the settled solids bury the impeller and turn a repair into a tank excavation.

An interstage screen is an internal sieve panel fitted at the top of each tank that retains the activated carbon in the tank while letting pulp pass forward. Two configurations dominate: static screens with no moving parts, and pumped screens where an external pump forces pulp through the panel. The choice between them changes hydraulics, blinding behavior and carbon attrition — see our carbon-in-pulp versus carbon-in-leach comparison for the flowsheet context.

An airlift pump is a gas-lift device that moves carbon slurry between tanks by injecting compressed air into a vertical riser, with no rotating parts in contact with the abrasive, carbon-laden pulp. A dewatering screen machine is a vibrating sieve unit that drains loaded carbon ahead of elution and catches carbon fines before they reach the tailings line. Alongside these go carbon transfer pumps (vertical cantilever or peristaltic types) and the safety instrumentation — hydrogen cyanide (HCN) gas detectors, level probes and interlocks — that turns a hazardous chemistry into a controlled process. Tank internals complete the picture: rubber-lined or polyurethane-coated shells, baffles to stop vortexing, and wear-protected impellers sized for the gold leaching tank duty you actually have, not the duty the brochure describes. Specifying cyanide leaching equipment well at purchase is the cheapest availability program you will ever run; most of the nine failures below are designed in before the first tank is filled.

The Equipment Train, Item by Item

The table below lists each item, its function, and the failure mode that most often stops it. Later sections quantify the downtime and the fixes. Tank-side selection logic for the vessels and agitators themselves — the upstream of everything in this table — is covered in our gold leaching tank equipment overview.

Three observations from commissioning and audit work are worth more than the table. First, the items that stop a plant hardest — agitators — are also the ones operators baby the most; the items that bleed money quietly — screens and carbon losses — get almost no attention until a recovery audit forces the issue. Second, dual-drive agitators are not redundant drives: losing one pinion usually forces a full stop because the surviving drive is not rated for the torque alone. Third, instrument trips (HCN, level, vibration) cause more recorded “stops” than mechanical breakages, and most of them are calibration and housekeeping problems, not hardware problems.

Why Does My CIL Circuit Keep Stopping? 9 Downtime Causes

Every stoppage below is drawn from the same failure library: what the operator sees, what is actually going wrong, and what fixes it permanently rather than for one shift. The sequence runs roughly from the most expensive to the most frequent.

1. Agitator gearbox failure — Symptom: rising gearbox vibration and oil temperature over weeks, metal particles on the oil analysis report, then a sudden trip with the impeller buried in settled solids. Root cause: oil contamination by water or carbon dust through a failed shaft seal, missed oil changes, orundersized bearings on tanks run above design solids density. Fix: an oil analysis program with quarterly sampling and hard alarms on water-in-oil; seal replacement at every tank entry; for chronic cases, upgrade to a gearbox one frame larger than the absorbed power suggests, because CIL agitators see torque spikes during carbon transfers that catalog duty cycles never capture.
2. Dual-drive agitator load imbalance — Symptom: one motor of a dual-drive unit runs hot or trips on overload while its twin runs light; vibration grows at the pinion mesh frequency. Root cause: worn couplings or a settled tank foundation putting the two pinions out of phase, so one drive carries most of the torque. Fix: laser-align both pinions at every overhaul, verify foundation level annually, and interlock the drives so a single-drive condition stops the unit before the loaded motor burns — running on one drive is not an operating mode.
3. Interstage screen blinding — Symptom: tank carbon inventory drifts, screen pressure differential climbs, and pulp level in the affected tank rises until it spills. Root cause: slimes, drill steel fragments, rubber lining debris and wood chips wedged in the screen apertures; on pumped screens, mis-set pump speed resuspending the same fines endlessly. Fix: specify screen apertures against your carbon size (typically 0.7–1.0 mm panels for 6×12 mesh carbon), install upstream trash screens and keep them serviced, and pulse or back-flush panels on a schedule rather than waiting for level alarms. Blinding that recurs within days usually means the aperture is wrong for the carbon, not that the operators are careless.
4. Interstage screen structural failure — Symptom: carbon appears in downstream tanks and on the tailings safety screen; recovery drops before anyone understands why. Root cause: cracked support frames from fatigue, panels flexing under pulldown of an oversized carbon transfer pump, or corrosion of fasteners. Fix: inspect frames at every carbon inventory count (weekly at most plants), replace deformed panels immediately rather than welding them a third time, and match transfer pump flow to the screen’s rated pass-forward capacity — pulling harder than the screen can pass is how panels die young.
5. Airlift pump blockage — Symptom: carbon transfer between two tanks slows or stops; air blows back or the lift runs full of slurry without moving it. Root cause: airlift pump blockage by compacted carbon sludge in the riser foot, oversize tramp material, or lost air pressure because a compressor or distribution line serves another consumer first. Fix: fit foot valves and flush connections at each riser base, purge risers with high-pressure air or water on every shift change, screen the tank contents against tramp oversize, and give the airlift system a dedicated air header so a downstream consumer cannot starve it mid-transfer.
6. Carbon transfer pump wear — Symptom: transfers that took 40 minutes now take two hours; the pump loses prime on carbon-rich slurry. Root cause: abrasive slurry eroding the impeller and casing of a vertical cantilever pump, or peristaltic hose perforation that drips cyanide-laden slurry in the pump bay. Fix: measure and record transfer flow monthly, replace impellers on measured wear rather than calendar guesswork, and keep one rebuilt spare per duty pump. On peristaltic units, track hose hours and rotate hoses front-to-back to double service life.
7. Screen machine panel damage — Symptom: carbon granules visible in the dewatering screen undersize, and gold values in the fines baghouse or tailings. Root cause: torn or worn screen panels on the dewatering or safety screen machine, often started by carbon dropped from height during loading. Fix: inspect panels daily — a two-minute walk-by with a flashlight — and stock a full panel set; add rubber-lined chutes to cut drop height; reconcile carbon inventory weekly so a torn panel shows up as missing grams, not as a recovery miss three months later.
8. Tank liner and impeller wear — Symptom: slurry weeping at a tank shell weld, or steel fragments appearing on downstream screens. Root cause: rubber or polyurethane liner wear-through along the impeller tip line and at baffle roots, accelerated by running impeller tip speeds above roughly 5 m/s or by sanding episodes after agitator trips. Fix: ultrasonic thickness surveys of the shell annually, liner wear profiling at each shutdown, and — the part everyone skips — a written procedure for restarting agitators after a stop, because re-suspending a settled bed at full speed is what chews liners fastest.
9. HCN gas alarms and NaCN safety interlocks — Symptom: repeated nuisance trips of the HCN detection system shutting down tank-bay work, or conversely, alarms finding sensors reading 20–30% below true concentration. Root cause: sensor drift past its calibration interval, detectors mounted where airflow never reaches them, or dust-clogged sensor housings — a NaCN safety instrument that is dirty is worse than one that is absent, because it teaches everyone to ignore alarms. Fix: monthly bump tests, six-monthly full calibration, detector siting reviewed against actual ventilation, and PPE (personal protective equipment) discipline written into the same permit system as the interlocks. Real HCN exposure incidents and false trips both trace back to the same root: instruments treated as furniture instead of as safety-critical equipment.

Notice the pattern: causes 1, 2, 6 and 8 are managed by condition monitoring; causes 3, 4, 5 and 7 are managed by routine inspection and inventory reconciliation; cause 9 is managed by discipline. No exotic technology is required — only a schedule someone actually owns.

How Often Should Cyanide Leaching Equipment Be Serviced?

Interval recommendations are cheap; the discipline to keep them is not. The table below reflects intervals we consider workable for a mid-size CIL plant (4–10 tanks, 5,000–20,000 t/d). Shorten them for coarse, sharp feed or above-design tonnage; lengthen nothing without oil analysis or thickness data to justify it.

Two cautions from the field. Condition-based beats calendar-based wherever a measurable signal exists — an oil sample every quarter catches more gearbox failures than a calendar ever will. And intervals that require a shutdown belong in the production plan as fixed annual slots; plants that wait for “a convenient weekend” run their agitators 30% past sensible wear limits, then pay for it in cause 1.

Why Is Your Circuit Losing Carbon as Fines?

Carbon fines losses are the most under-reported cost in gold cyanidation. Every gram of carbon that breaks away as fines carries adsorbed gold with it, and fines below roughly 0.4 mm pass straight through normal interstage apertures unless a dedicated fines screen or safety screen machine catches them. Plants routinely lose 15–40 g of carbon per tonne of ore treated; well-run plants hold it near 10 g/t, and the difference at 2.5 g/t head grade is worth a full percentage point of recovery — often US$200,000–600,000 a year at mid-size throughputs.

Where do the fines come from? Three mechanisms dominate. Mechanical attrition in over-driven transfer pumps and high-tip-speed agitators grinds good granules down; osmotic and thermal shock — cold water on warm carbon during elution, or untreated make-up carbon added straight to a hot circuit — cracks particles along their pore structure; and worn screen machine panels let intact granules escape, which inventory reconciles as “fines” even though the physical mechanism is theft, not breakage.

The fixes are unglamorous. Buy attrition-hard carbon and pre-soak and screen new make-up carbon before it enters the circuit. Match transfer pump speed to duty instead of running full throttle. Install or repair the safety screen over the tailings line and sample its undersize weekly for carbon. And run the carbon inventory reconciliation in the maintenance table above — plants that count carbon weekly find torn panels in days, while plants that count quarterly fund an anonymous donation of gold-bearing carbon to the tailings dam every month.

What Does One Day of Downtime Cost?

The honest answer is “far more than your maintenance budget,” so let us build the number openly. Take a mid-size plant at 5,000 t/d treating ore at 2.5 g/t Au with 90% dissolution in the leach. Gold dissolved per day is 5,000 × 2.5 × 0.90 = 11,250 g, or about 362 troy ounces. At a spot price of US$2,600/oz (illustrative; treat as a range), gross gold value lost is roughly US$940,000 per day. A full-day outage costs nearly all of that, because a CIL circuit has no ore buffer to keep paying from: once the tanks stop, recovery stops with them.

Assumptions and exclusions, stated plainly: the calculation is gross metal value at spot, not net smelter return; reagent and power savings during the stop are not credited (they rarely exceed 5–10% of the loss); prices move daily; and deferred — not permanently lost — production is the correct accounting only if the plant truly catches up later, which milling capacity constraints often prevent. Even on the most conservative reading, one avoided day of downtime pays for an entire year of the preventive maintenance program in the table above, several times over.

Field Data: Availability Before and After a Screen Upgrade

The following figures are illustrative composites assembled from plant audit experience and published industry norms, not attributable to any single named client. They describe a 6-tank CIL plant with pumped interstage screens, 5,200 t/d throughput.

Before the intervention, recorded mechanical availability of the leaching train averaged 89.2% over twelve months, with 14 unplanned stops per month logged across the circuit — of which 5 involved interstage screen blinding or panel faults and 3 involved carbon transfer delays. Carbon losses measured 38 g/t of ore, roughly 2.5 percentage points of which walked out as gold-loaded fines. Residual gold on tailings ran 0.42 g/t against a 0.31 g/t design target.

After replacing the pumped screens with larger-aperture static panels on the first three tanks, adding shift-purge provisions to every airlift riser, and enforcing the weekly carbon count, the same circuit ran 96.1% availability over the following twelve months, with unplanned stops down to 4 per month. Carbon losses fell to 13 g/t and tailings gold to 0.35 g/t — a recovery gain of about 1.1 percentage points, worth on the order of US$500,000 per year at the throughput and grade above. Total intervention cost, including the screen package and three shutdowns, was under one-fifth of the first year’s gain. The equipment was not exotic; the discipline was.

Frequently Asked Questions

How long should a CIL tank agitator run between major overhauls?

With quarterly oil analysis, healthy seals and a stable torque load, a quality agitator gearbox in cyanide leaching equipment service typically runs 5–8 years between major overhauls, and 10+ years is achievable on smaller tanks. Overhauls get forced early by water contamination through failed shaft seals and by sanding episodes after trip events. Track oil trend data and impeller wear profiles rather than trusting the calendar alone.

What is the difference between static and pumped interstage screens?

Static screens pass pulp through the panel using only the natural hydraulic gradient between tanks, so they have no moving parts and cannot pump carbon forward, but they are more prone to blinding at high slimes loads. Pumped screens use an external pump to force pulp through the panel, handling higher throughput per square meter but adding a pump maintenance item and more carbon attrition. High-tonnage or high-slime circuits usually justify pumped screens; smaller plants often run static panels more cheaply.

How much carbon loss is acceptable in a CIL plant?

Benchmark plants hold total carbon losses near 10–15 g per tonne of ore treated, combining fines and intact granule escape. Losses above 25–30 g/t usually mean a torn screen machine panel, an over-driven transfer pump, or uncontrolled fines generation, and at typical grades each 10 g/t of avoidable loss is worth roughly 0.3–0.5 percentage points of gold recovery. The weekly carbon inventory reconciliation is the single most cost-effective control.

Why do airlift pumps block more often than centrifugal pumps?

An airlift has no impeller to shear compacted material, so sludge, tramp oversize and settled carbon simply accumulate in the riser foot until flow stops. Blocks concentrate where the riser meets the tank floor and where air pressure dips because a shared compressor header is being drawn down by another consumer. Dedicated air supply, foot valves, and a purge routine at shift change reduce airlift pump blockage to a rare event rather than a daily fight.

What HCN gas alarm levels should a cyanide leaching plant use?

Follow the International Cyanide Management Code practice and your regulator’s exposure limits: typical arrangements alarm at 4–5 ppm HCN (warn, don PPE, evacuate non-essential staff) and at 10 ppm (evacuate, investigate with SCBA). Detectors need monthly bump tests and six-monthly calibration because drift is constant in humid, dusty tank bays. Never treat a repeat alarm as a nuisance trip until an instrument technician has verified the sensor — the code assumes the alarm is real every time.

Does preventive maintenance on this equipment really pay back?

Yes, and quickly. On the cost basis above, one avoided downtime day at a 5,000 t/d plant is worth roughly US$0.9 million, while a full annual program of oil analysis, screen inspections, airlift purging, HCN calibration and carbon counting typically costs tens of thousands of dollars per year in parts and labor. The audit experience behind this article is consistent: plants moving from reactive to scheduled maintenance gain 3–7 percentage points of CIL plant availability within the first year.

Conclusion

Cyanide leaching equipment rarely fails spectacularly; it fails predictably. Agitator gearboxes announce themselves in the oil for months before they seize, interstage screens blind gradually as apertures fight the wrong carbon size, airlifts silt up one shift at a time, and carbon fines leave the plant in gram-sized increments that only a weekly inventory count will ever reveal. The nine causes in this guide account for the overwhelming majority of unplanned stoppages we see across gold cyanidation circuits, and every one of them has a fix that is procedural as much as mechanical: condition monitoring, scheduled inspection, and inventory discipline, applied to a train that is otherwise left alone until it stops. At the economics in the tables above, availability is not a maintenance department statistic — it is the project’s return on investment expressed as a percentage. If you are specifying a new CIL circuit or fighting an existing one, our team can review your current layout, maintenance records and screen design, and our pilot testing services can confirm agitator, screen and carbon-handling parameters against your actual ore before you commit capital. You will find more hydrometallurgy technical guides in our resource library.

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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. Darling, P. (ed.) (2011). SME Mining Engineering Handbook, 3rd edition, Vol. 2 — chapter on gold processing. SME, Littleton, CO.
3. U.S. Geological Survey (2026). Mineral Commodity Summaries 2026 — gold. Reston, Virginia. usgs.gov
4. International Cyanide Management Institute. The International Cyanide Management Code for the Manufacture, Transport and Use of Cyanide in the Production of Gold. cyanidecode.org
5. Stanley, G. G. (ed.) (1987). The Extractive Metallurgy of Gold in South Africa, Vol. 2 — chapter on the application of activated carbon to gold recovery. South African Institute of Mining and Metallurgy, Johannesburg.
6. Logsdon, M. J., Hagelstein, K., Mudder, T. I. (1999). The Management of Cyanide in Gold Extraction. International Council on Metals and the Environment, Ottawa.

Changelog

2026-10-26 — first published.