Gold Leaching Tank Sizing & Selection Guide

Last updated: 2026-10-26

A gold leaching tank is normally sized for a total residence time of 24–48 hours — the single number that fixes tank farm volume, agitator power and capital cost for a carbon in leach (CIL) circuit. Derive the residence time from test work, split it across 6–8 tanks, then select agitators, materials and interstage screens; this guide walks that sequence within the wider leaching technology family.

Key Takeaways

• Residence time from test work — 24–48 h is typical — fixes tank farm volume; never copy another plant’s number.
• Six to eight tanks in series approach plug-flow behavior and out-recover a single vessel of equal volume.
• Design agitation for 40–50% w/w solids; specific power input falls as tank diameter grows.
• Slow-running hydrofoil impellers and dual-drive agitators limit carbon attrition at 10–20 g/L carbon.
• Rubber-lined carbon steel is the default shell material; upgrade to 316L or duplex only when chemistry demands it.

What Is a Gold Leaching Tank?

A gold leaching tank is an agitated, baffled vessel that holds finely ground ore as a 40–50% solids slurry in contact with a dilute sodium cyanide (NaCN) solution, so that gold dissolves into the liquid phase while dissolved oxygen and reagent are distributed uniformly through the charge. In a carbon in leach (CIL) circuit the same vessel also holds granular activated carbon, which adsorbs the dissolved gold-cyanide complex as the slurry flows from tank to tank. Typical installations are open-top, flat-bottomed steel tanks of 1:1 to 1:1.1 diameter-to-height ratio, arranged in series of six to eight in any serious gold CIL plant.

Tank Leaching vs Heap Leaching in practice

Agitated Leaching Tank Design

Leaching, step by step

An agitated leaching tank is a mechanically stirred reactor that keeps solids in suspension, transfers oxygen from a sparged air stream into the slurry, and prevents sanding during normal and emergency shutdown. Stirring is provided by a vertically mounted shaft carrying one or two large, slow-running impellers, usually hydrofoil or pitched-blade types, driven through a right-angle gearbox. Where mechanical agitation is not used, a Pachuca tank is an air-agitated column that suspends the slurry with compressed air injected near the cone or flat bottom — simpler mechanically, but taller, noisier and harder to tune, which is why most modern plants choose mechanical units. Our agitated leaching tank page shows representative configurations.

CIL itself is a flowsheet arrangement that combines leaching and adsorption in the same tank series; its sibling, carbon in pulp (CIP), leaches first in agitated tanks and adsorbs afterward in separate carbon stages. The choice affects residence time, carbon inventory and screen count — the trade-offs are set out in our carbon in pulp vs carbon in leach comparison. An interstage screen is a static sieve-bend separator that retains carbon in each tank while allowing slurry to pass forward between stages.

One more comparison frames the whole decision: tank leaching vs heap leaching is a trade of capital for recovery and speed. Agitation in tanks recovers 85–95% of gold in one to two days from finely ground ore, while percolation heap leaching takes weeks to months on coarse crush but requires no mill. Tanks earn their place wherever grade, clay content or refractory mineralogy justify grinding — for the equipment side of that decision, see our gold CIL tank system page.

Key Design Parameters

Every CIL tank design starts from the same set of variables, each interacting with the rest. The table collects the typical ranges; the paragraphs below explain how each parameter is set.

Residence time. Carbon in leach residence time is the total slurry hold-up divided by slurry flow, and it is the one parameter that should never be borrowed from another project. Bottle-roll and stirred-tank kinetic tests establish the dissolution curve; CIL VariAbility testing or a continuous pilot run adds the adsorption side. Most free-milling ores reach the bulk of ultimate dissolution within the first half of the leach, with the tail of the curve bought at disproportionate capital cost. Total design residence time of 24–48 hours covers the majority of plants; ores with slow kinetics or preg-robbing behavior push toward the upper end (typical range, to be confirmed by test work).

Solids density. Operating at 40–50% solids by weight shrinks tank volume, cuts per-tonne reagent consumption and reduces pumping energy, but raises slurry viscosity and therefore agitation power. Above roughly 50% solids, suspension quality deteriorates sharply in most gold slurries and carbon movement through the circuit becomes erratic. The density test work establishes a settling and viscosity profile; the design point usually sits just below the viscosity knee.

Tank geometry and stage count. For structural and mixing reasons, large CIL tanks are built close to a 1:1 diameter-to-height ratio; diameter is limited by shell steel thickness, roof loads and agitator torque rather than by metallurgy. Stage count is a kinetics decision, covered in the sizing section below.

Agitation intensity. Industry reviews of CIL agitation report specific power inputs from about 1 kW/m³ in small reactors down to below 0.05 kW/m³ in the largest 10–15 m diameter tanks, because suspension duty scales favorably with impeller diameter. Impeller tip speed is capped — commonly in the 3–5 m/s band — to limit carbon degradation (exact cap to be confirmed with the agitator vendor for each duty).

Carbon concentration and screens. CIL tanks carry 10–20 g/L of carbon in suspension, a compromise between adsorption rate and attrition losses. Interstage screens with apertures of roughly 600–800 µm retain carbon particles (typically 6×12 mesh) while passing ground slurry; aperture is confirmed against the carbon specification and the grind size (typical range, to be confirmed by test work).

Solution chemistry. Free NaCN of 0.2–0.5 g/L, pH held at 10.5–11.0 with lime, and dissolved oxygen maintained above roughly 5–10 mg/L (by air sparging or oxygen injection where kinetics demand) define the chemical envelope. These values interact with agitation: better oxygen dispersion shortens residence time — the cheapest volume you will ever buy.

How Do You Size a Gold Leaching Tank?

Sizing is a cascade of simple calculations in which each step constrains the next, and it should always be run from measured data forward — never from a competitor’s drawing backward. The worked logic below uses a 2,500 tonnes per day (t/d) example with illustrative numbers; the structure applies at any scale.

1. Fix the residence time from test work. Plot gold dissolution and adsorption versus time from bottle-roll, stirred-tank and — where justified by grade — continuous pilot CIL runs. Choose the time where the recovery curve flattens against the marginal value of extra volume. For the example, assume 30 h total carbon in leach residence time from the laboratory program (typical range, to be confirmed by test work).
2. Convert tonnage to slurry flow. At 2,500 t/d, dry solids run 104 t/h. At 45% w/w solids with an ore specific gravity of 2.7 and slurry specific gravity of about 1.45, the volumetric slurry flow is roughly 166 m³/h. This figure — not the dry tonnage — drives every downstream volume calculation, so density control matters as much as mill throughput.
3. Multiply flow by residence time to get total live volume. 166 m³/h × 30 h ≈ 4,980 m³ of live slurry. Add an allowance for carbon displaced volume, air hold-up and emergency shutdown margin — commonly 10–15% on top of live volume, sized with the mechanical engineer rather than assumed.
4. Choose the number of stages. Each gold leaching tank in series is a stage whose exchange with its neighbors drags the circuit toward plug-flow behavior; six to eight stages is the practical optimum for CIL, recovering meaningfully more than four while avoiding the screen and agitator count penalty of ten or more. Splitting 4,980 m³ across seven stages gives about 715 m³ per tank.
5. Set tank geometry. At 1:1 to 1:1.1 diameter-to-height ratio, 715 m³ corresponds to roughly a 9.5 m diameter shell with 10.5 m of liquid depth. Check shell thickness, wind and seismic loads, and the agitator gearbox torque before freezing the diameter — the cheapest volume is the volume you do not have to build twice.
6. Size the agitators. Specify suspension of the coarsest slurry fraction under the densest expected duty, then check tip speed against the carbon attrition limit. For mid-size tanks of this class, installed specific power around 0.08–0.15 kW/m³ is representative (vendor design to confirm); each agitator in the example carries on the order of 55 kW.
7. Close the loop on carbon, screens and interstage transfer. Verify carbon hold-up (10–20 g/L), interstage screen area against the slurry flux, and whether interstage pumping or pumpless screen arrangements suit the elevation profile of the tank farm.

Two checks finish the sizing. First, translate the predicted recovery curve into a leaching recovery calculation so the tank farm value — not just its cost — is on the table at the design review. Second, stress the design against the mine plan: if a harder, slower-leaching ore arrives in year five, the residence time margin decided now determines whether the plant flexes or underperforms.

Which Agitator and Materials of Construction Fit the Duty?

Agitator and materials choices are made once and lived with for decades, so they deserve more scrutiny than the paint schedule usually receives.

Mechanical versus air agitation. Mechanical agitators win on suspension control, oxygen transfer efficiency and floor space; Pachuca columns survive where air is cheap, heads are generous and duty is undemanding. For a modern gold CIL plant the mechanical unit is the default, and the interesting choice is within the mechanical family: two large slow impellers (typically an axial-flow lower impeller pumping downward, with a radial or up-pumping upper impeller) at 8–15 rpm class speeds.

The dual-drive agitator. A dual-drive agitator is a drive arrangement that splits torque between two independent motor-gearbox sets acting on the same agitator shaft, so that the failure of one drive leaves partial agitation instead of a stationary shaft settling onto a sand bed. The premium buys insurance: an unplanned stoppage can otherwise mean sanded tanks, manual excavation and weeks of lost production. Specify dual drive where power supply is weak, spares logistics are slow, or the slurry settles fast; a single drive with robust standby power is defensible where those risks are small.

Materials of construction. At pH 10.5–11.0, cyanide solution is not aggressively corrosive to steel, so abrasion — not corrosion — usually governs. That is why carbon steel with a 4–6 mm rubber lining is the industry default shell for a gold leaching tank: rubber shrugs off sanding particles, tolerates cyanide, and can be spot-repaired without hot work (a genuine safety advantage in cyanide service). Stainless 316L suits low-abrasion duties — solution, reagent and small agitated vessels — where lining quality control is harder to guarantee. Duplex grades such as 2205 earn their premium in hot, high-chloride or otherwise aggressive slurries where neither rubber nor 316L lasts. Confirm every selection against the actual solution analysis rather than the general table below.

What Goes Wrong: Operating Problems & Fixes

Most tank-farm failures announce themselves early through agitator current, screen condition and assay trends. The recurring modes follow:

1. Sanding at the tank bottom. Symptom: agitator current spikes or drops, coarse assay in tank discharge, grinding sound on restart. Cause: power input below suspension requirement, slurry density drifting above design, or coarse tail from the milling circuit. Fix: restore density to design, verify impeller immersion and speed against vendor curves, and consider a bottom-sweep impeller retrofit before sand accumulates into an excavation job.
2. Interstage screen blinding or rupture. Symptom: carbon escaping forward into later stages and into tailings, or slurry backing up and overflowing a stage. Cause: screen apertures fouled by tramp oversize, plastic or wood chips, or a torn panel from fatigue. Fix: install upstream trash screening, rotate or back-flush screens on schedule, and stock spare panels matched to the carbon specification.
3. Excessive carbon attrition. Symptom: fine carbon in screens and tailings, rising gold losses on fines, carbon make-up climbing. Cause: tip speed too high, sharp-edged screen surfaces, or carbon cycling through too many adsorption-regeneration rounds. Fix: cap tip speed, round and inspect screen contact surfaces, and track fines generation per cycle as a standing KPI.
4. Poor stage efficiency and short-circuiting. Symptom: dissolved gold profile flat across stages, adsorption far below laboratory prediction. Cause: slurry short-circuiting between stages, insufficient carbon inventory, or carbon backflow through poorly sealed screens. Fix: verify stage-by-stage solution assays, restore carbon concentration to 10–20 g/L, and seal screen bypass paths.
5. Oxygen starvation. Symptom: leach kinetics slower than test work predicted, DO readings below target, MnO₂ or peroxide demand climbing. Cause: sparge air maldistribution, slurry density above design, or biological/chemical oxygen demand from ore constituents. Fix: rebalance sparge headers, trim density, and confirm oxygen demand on fresh composite samples.
6. Liner wear and debonding. Symptom: exposed steel patches, rubber blisters, iron contamination in solution trending up. Cause: impeller tip speed or solids size beyond lining duty, or adhesive failure at shell imperfections. Fix: schedule ultrasonic lining surveys, repair with cold-bonded patches promptly, and correct the agitation condition that created the wear pattern.
7. Shaft seal and gland leakage. Symptom: slurry weeping at the tank roof nozzle, gearbox oil contamination. Cause: seal face wear, shaft deflection from impeller imbalance, or misalignment after maintenance. Fix: monitor runout at every overhaul, rebalance impellers after liner repairs, and convert to upgraded seal arrangements where leakage repeats.

Cost & Energy

Figures below are scoping-level typical ranges, stated with their assumptions; they are not quotations and move with ore, site and market conditions.

Capital. In a gold plant, the agitated tank farm is commonly among the largest line items after grinding. Cost scales with shell steel weight, agitator torque and screen count rather than with volume alone: doubling tank diameter roughly quadruples volume but far more than doubles steel and drive cost, which is why the stage-count-versus-diameter trade is a genuine economic decision. A mid-size rubber-lined steel tank with agitator, drive and interstage screen typically lands in the low hundreds of thousands of US dollars per unit (indicative only; confirm by vendor quotation). Foundations and tank-farm piping add materially in seismic or remote locations.

Energy. Agitation energy per tonne of ore follows from specific power, residence time and slurry volume per tonne. For plants of the 700 m³-per-tank class running 0.08–0.15 kW/m³, expect roughly 2–5 kWh per tonne of ore treated (assumes 30 h residence, 45% w/w solids). Smaller high-intensity vessels can exceed this range; the largest tanks fall well below it. Air sparging adds a modest compressor load unless oxygen enrichment is selected for slow ores.

Reagents and carbon. NaCN consumption of 0.25–0.6 kg/t and lime of 0.5–2.5 kg/t are common envelopes for free-milling ores (to be confirmed by test work on your gangue and sulfide content). Activated carbon make-up — driven by attrition and regeneration losses — typically runs in the tens of grams per tonne of ore and responds directly to the tip-speed and screen discipline described in the problems section.

The OPEX lever that matters most. Because residence time converts directly into installed volume, every hour trimmed from the test-work-justified figure saves shell steel, agitator power and screens for the life of the plant. Accurate kinetics are therefore the largest single cost lever in CIL tank design.

Field Data: A 2,500 t/d CIL Tank Farm

The following case is an illustrative composite from industry design practice; figures are rounded and anonymized, not from any named client.

The plant treats 2,500 t/d of free-milling ore through seven tanks of about 715 m³ each — 9.5 m diameter with 10.5 m liquid depth, rubber-lined carbon steel with dual-drive agitators of 55 kW per tank. At 45% w/w solids the slurry flow is approximately 166 m³/h, giving a measured mean residence time of 29.6 h against the 30 h design target. Installed agitation totals 385 kW, or about 0.078 kW/m³ of live volume, and the whole farm draws near 3.7 kWh per tonne of ore treated.

Operational data from the composite: carbon held at 14–16 g/L per tank, total circuit inventory near 74 t of activated carbon; interstage screens at 700 µm passing full slurry flow without overflow at design rate. Dissolved gold fell from above 2 mg/L in the first stage to below 0.2 mg/L by stage six, and overall gravity-inclusive recovery averaged about 91% (illustrative). Carbon attrition, tracked as fines generated per adsorption cycle, held near 3–4% after tip speed was trimmed early in the ramp-up — the single most profitable adjustment made in the first year.

Three lessons generalize: conservative slurry-density control, not extra volume, delivered the design margin; the oxygen profile, not agitation speed, set the leach rate in the first three stages; and the screen and attrition metrics predicted carbon losses months before the monthly make-up figures did.

Frequently Asked Questions

What residence time should I design a gold leaching tank circuit for?

Let test work decide, not industry averages. Bottle-roll and stirred kinetic tests give the dissolution curve; CIL VariAbility or pilot runs add adsorption. Most plants land at 24–48 h total, but preg-robbing or slow-leaching ores can need materially more. A residence time chosen from a neighboring plant’s flowsheet usually costs either recovery (too short) or capital (too long) for the life of the mine.

How many tanks should a CIL circuit have?

Six to eight tanks in series is the practical optimum. Each additional stage pushes the circuit closer to plug-flow behavior, recovering more gold per unit of total volume, but adds agitators, interstage screens and height for interstage flow. Below about five stages the kinetics penalty becomes significant; above about ten, the incremental recovery rarely pays for the extra mechanical equipment and maintenance.

What is a dual-drive agitator and when is it worth the premium?

It is an agitator whose shaft is driven by two independent motor-gearbox sets sharing the torque. If one drive fails, the other keeps the slurry moving partially, preventing sanding and a weeks-long excavation. The premium is worth paying where grid power is unreliable, spares are weeks away, or the slurry settles rapidly — conditions common in remote gold operations.

Why is rubber-lined carbon steel the default material for leach tanks?

At pH 10.5–11.0 the cyanide slurry is mildly corrosive, so abrasion from the sand fraction governs material life. Rubber lining absorbs particle impact, tolerates cyanide chemistry, and can be repaired with cold-bonded patches — no hot work inside a cyanide facility. Stainless steel is reserved for low-abrasion duties, and duplex grades for hot or high-chloride slurries where both rubber and 316L underperform.

How much activated carbon does a CIL tank hold?

Carbon in leach circuits operate at 10–20 g/L of carbon in suspension. A 700 m³ tank therefore holds roughly 7–14 t of carbon, and a seven-tank farm accumulates 50–100 t of circulating inventory. Higher concentrations speed adsorption but raise attrition losses and screen loading, so the value is optimized against measured fines generation rather than set at the top of the range.

Can an existing CIP plant be converted to CIL?

Usually yes, with care. Conversion adds carbon and interstage screens to the first leach tanks, which changes agitation duty, screen count and carbon handling logistics, and it shortens the dedicated adsorption section. It suits ores whose leaching is largely complete before adsorption begins; where leaching and adsorption genuinely overlap, the conversion typically improves recovery per unit volume. Confirm with stage-by-stage solution profiling first.

How do interstage screens work between leach tanks?

An interstage screen is a static sieve-bend separator mounted at the tank launder: slurry and carbon flow across it, slurry passes through apertures of roughly 600–800 µm to the next tank, and carbon is returned to the same tank, creating counter-current movement of carbon against slurry. Designs are either pumped or pumpless, the latter using tank elevation differential to drive flow and reducing carbon degradation from pumping.

Conclusion

Sizing and selecting a gold leaching tank is a disciplined chain, not a catalogue choice: residence time from honest test work, slurry flow from controlled density, total volume split across six to eight stages, geometry checked against mechanical limits, agitators specified for the densest duty the ore will deliver, and materials chosen for abrasion before corrosion. Get the sequence right and the tank farm quietly delivers 85–95% recovery for decades; skip a step and the failure appears as sanded bottoms, carbon losses or a recovery shortfall that no downstream equipment can recover. The numbers in this guide are typical ranges, and your ore will argue with some of them — that argument is exactly what test work is for. Before locking tank count or agitator specifications, put your ore through structured kinetics and adsorption testing: our team runs and interprets these programs daily, and the pilot testing services page explains the packages available. Additional calculators and background are available in our more hydrometallurgy technical guides.

Leaching Reactor Volume Calculator from Residence Time, step by step

References

1. Marsden, J. O. & House, C. I. (2006). The Chemistry of Gold Extraction, 2nd edition. Society for Mining, Metallurgy & Exploration (SME), Littleton, CO. — Chapters on agitated leaching, CIP and CIL design criteria.
2. SME Mining Engineering Handbook, 3rd edition (2011). Peter G. Darling, editor-in-chief. SME, Littleton, CO. — Gold processing plant design sections.
3. Wills, B. A. & Finch, J. A. (2016). Wills’ Mineral Processing Technology, 8th edition. Butterworth-Heinemann. — Cyanidation and agitation fundamentals.
4. U.S. Geological Survey (2025). Mineral Commodity Summaries 2025. Reston, Virginia. https://www.usgs.gov/centers/national-minerals-information-center/mineral-commodity-summaries
5. International Cyanide Management Code for the Manufacture, Transport and Use of Cyanide in the Production of Gold. https://www.cyanidecode.org
6. ALTA 2023 Gold-PM Proceedings. “Agitators in gold cyanide leaching (CIL/CIP reactors)” session papers, ALTA Metallurgical Services, Melbourne.

Changelog

2026-10-26 — first published.