Agitated Leaching Tank Design: A 2026 Engineer’s Guide

By the Lexin Hydromet Process Engineering Team

Effective leaching tank design starts with residence time: at 45–55% solids (w/w), a 24-hour leach processing 500 t/d (551 short t/d) of ore demands roughly 700 m³ (24,700 ft³) of working slurry volume — most economically supplied by two or three mechanically agitated tanks in series, each fitted with a hydrofoil impeller sized for full solids suspension.

Key Takeaways

• Size tank volume from leach kinetics and residence time, not from a rule-of-thumb cube.
• Design for just-suspended speed (Njs) plus a 15–25% safety margin.
• Hydrofoil impellers typically need 0.6–1.5 kW/m³ for gold and base-metal leaches.
• Mechanical agitation wins below ~10 m diameter; Pachuca air agitation suits tall, deep tanks.
• Materials range from rubber-lined steel to duplex stainless; choose by temperature, acidity, and chloride.

What Is an Agitated Leaching Tank and Why Does Its Design Matter?

An agitated leaching tank is a stirred reactor that keeps ore particles uniformly suspended in a lixiviant — cyanide, sulfuric acid, chloride, or alkaline solutions — for the residence time the leach kinetics require. A properly designed agitated leaching tank is also a mass-transfer device that must contact dissolved reagent with freshly exposed mineral surfaces, dissipate mixing energy without excessive shear, and hold the slurry homogeneous enough that no particle leaves the vessel before the design residence time has elapsed. The equipment itself is described in detail on our agitated leaching tank page; this guide concentrates on how process and mechanical engineers size and specify one.

Gold Leaching Tank Sizing & Selection Guide

Leaching: selection rules

Tank sizing matters because it sits at the intersection of metallurgy, hydraulics, and capital cost. Under-size the tank and you lose recovery that can never be recovered later — every percentage point of dissolved gold or copper left in the tailings is revenue gone permanently. Over-size it and you pay twice: once in stainless or rubber-lined steel that serves no metallurgical purpose, and again in installed agitator power for the life of the plant. Between those extremes lie dozens of coupled decisions — impeller type, tip speed, baffling, liner selection, seal arrangement — that determine whether the circuit runs for thirty years or fights sanding, corrosion, and shaft failures from commissioning onward.

Good design is also invisible in a positive way. A tank that suspends solids, distributes oxygen or air, and maintains temperature quietly delivers its design recovery day after day. The leaching section of this site covers the full range of hydrometallurgical leaching equipment and flowsheets; here we go one level deeper into the single most capital-intensive item in that flowsheet.

What Are the Key Design Parameters in Leaching Tank Design?

Every agitated leaching tank is a three-phase contactor that must satisfy metallurgical, hydraulic, and mechanical requirements simultaneously. The parameters below interact — raise solids content and you change rheology, which changes just-suspended speed, which changes power draw — so treat the table as a starting envelope to be converged, not a checklist to be filled independently.

Residence time. The starting point of leaching tank design is always kinetic testwork: bottle-roll or stirred-tank tests that establish how dissolved metal extraction varies with time for your ore, grind size, and reagent regime. If 90% of ultimate extraction is reached at 22 hours and the last 8 hours buys only 0.5 percentage points, a 24-hour design residence time is defensible while 48 hours is not. Our companion article on leaching kinetics explains how to read shrinkage curves and convert laboratory contact times into plant residence times with the staging and short-circuiting allowances shown later in this guide.

Working versus total volume. Working (usable) volume is typically 85–90% of gross tank volume; the remainder covers freeboard for air injection foaming and surge. Size the gross geometry from working volume divided by that fraction — a common beginner error is to buy a 700 m³ tank for a 700 m³ duty.

Number of tanks in series. Multiple tanks approximate plug flow, which shortens the total residence time needed for a given extraction compared with a single well-mixed vessel, and provides operational redundancy for agitator maintenance. Gold CIL circuits commonly use 6–8 tanks; simpler acid leach duties often use 3–5. Four to six tanks is a defensible default when kinetics are unknown but the ore is likely refractory.

How Much Agitator Power Does a Leaching Tank Need?

The honest answer is: enough to keep the coarsest particles just suspended, plus margin — and that number comes from the just-suspended speed, not from horsepower-per-gallon folklore.

Just-suspended speed (Njs) is the minimum impeller speed that keeps no particle stationary on the tank bottom for more than a moment — a condition chemists call complete suspension, described by the Zwietering correlation. Njs is a function of particle settling velocity, solids density difference, slurry viscosity, impeller type and diameter, and tank geometry. In tank design practice, you compute or test Njs for your worst-case particle (usually the crusher-skips or grind-limit fraction in the feed), then operate at 1.15–1.25 × Njs. Operating below Njs even intermittently allows a bed of sand to form; the bed then abrades the liner, shelters particles from reagent, and can grow until the agitator is buried.

Power intensity is the number most procurement documents specify, and for good reason: it scales OPEX directly. Typical design figures for leaching tank design in precious and base metals are:

• 0.6–1.0 kW/m³ — hydrofoil (axial-flow) impellers in well-behaved oxide gold or copper leaches with P₈₀ below 150 µm.
• 1.0–1.5 kW/m³ — coarser grind, higher solids, or duties requiring gas dispersion (oxygen enrichment in CIL/poison-avoidance).
• 1.5–2.5 kW/m³ — high-solids, high-viscosity slurries, bio-oxidation, or aggressive gas-liquid mass transfer duties.

Convert intensity to motor power by multiplying by gross slurry volume and adding 15–20% for motor service factor and fouling of the impeller with age. A 350 m³ tank at 1.0 kW/m³ needs roughly 350 kW of absorbed power — specify a 400–450 kW motor — which at 0.10 USD/kWh and 95% availability costs about 330,000 USD per tank-year. That figure is why tank design should never specify power “to be safe”: every excess kilowatt is paid for decades.

Tip speed deserves equal attention. Above roughly 6 m/s (20 ft/s), impeller tip speed accelerates liner wear and can de-flocculate fine solids in downstream thickeners. Where suspension demands high tip speed — usually with small-diameter, high-speed impellers — prefer a larger-diameter hydrofoil running slower: diameter scales suspension capability strongly while tip speed scales wear. As a worked comparison, a 3.2 m diameter four-blade hydrofoil at 45 rpm gives a tip speed near 4.5 m/s, comfortably inside the acceptable band for a rubber-lined tank.

Step-by-Step Leaching Tank Sizing Procedure

The following procedure condenses standard chemical and metallurgical practice into eight steps. It assumes you have kinetic testwork, a grind size distribution (P₈₀), a feed rate, and a target solids content; if any of these are missing, start with pilot testing — request a pilot testing proposal before committing tank geometry.

1. Define residence time from kinetics. Bottle-roll or bench stirred-tank data give extraction versus time. Add 10–20% for scale-up inefficiency and short-circuiting between tanks in series. The result is your design residence time — the single most consequential number in leaching tank design.

2. Compute required working volume. Multiply solids feed rate by slurry dilution to get volumetric slurry flow, then multiply by residence time. For 500 t/d (551 short t/d) of ore at SG 2.7 suspended at 48% solids, slurry density is roughly 1.45 t/m³, giving about 344 m³/h (121,500 ft³/h) of slurry; 24 hours of residence therefore requires about 8,260 m³ of working volume in total.

3. Choose the number of tanks. Divide total volume by a per-tank working volume that keeps diameter in the 8–15 m band and height:diameter near 1.0 — typically 350–600 m³ per tank for mid-size plants. Six tanks of 1,380 m³ gross each would suit the example above; three larger tanks reduce capital but increase single-point failure risk.

4. Select impeller type and count. For most leaching duty a downward-pumping, four-blade hydrofoil on a single shaft serves tanks up to about 5 m liquid depth. Deeper tanks take two or three impellers on one shaft at spacing of roughly one impeller diameter. Reserve radial-flow impellers for gas-dispersion-critical duty.

5. Determine Njs and set operating speed. Apply the Zwietering correlation (or vendor software, or pilot data) for the coarsest design particle. Multiply by 1.15–1.25 to set operating speed, then verify tip speed stays below the wear limit for your liner material.

6. Size baffles, sparger, and internals. Standard configuration is four wall baffles, each 1/12 of tank diameter wide, offset from the wall. If oxygen transfer limits kinetics, add a ring or pipe sparger beneath the impeller and re-check gas holdup in your working-volume allowance.

7. Select materials of construction. Work through the decision logic in the next section — chloride level, pH, temperature, and abrasion select between rubber-lined steel, 316L, duplex, and FRP.

8. Specify the agitator drive, seal, and maintenance provisions. Include a dry-well or mechanical seal appropriate to the duty, a maintenance platform or bridge, and — critically — the ability to inspect and change impellers without draining the tank, since agitator availability drives circuit availability.

Steps 2 through 4 iterate: choosing fewer, larger tanks changes Njs, which changes impeller count and shaft length, which can push you back to more, smaller tanks. Budget at least one full iteration loop — and prefer to close it with pilot data rather than assumptions.

Mechanical or Air Agitation: Which Leaching Tank Design Should You Choose?

Mechanical agitation is the default for good reasons: it suspends coarse solids reliably, decouples suspension from gas delivery, and allows precise control of power input independent of oxygen demand. Air agitation — the Pachuca tank, a tall cylindrical vessel with a central air-lift draft tube in which compressed air both agitates and oxygenates — remains competitive in specific situations, and a sound design basis considers it honestly rather than by reflex.

An air-agitated Pachuca tank is a tall, slim vessel that circulates slurry by injecting compressed air at the base of a central draft tube; the rising aerated slurry inside the tube overflows and descends in the annulus, creating continuous circulation. Its virtues are mechanical simplicity — no rotating equipment in slurry, no submerged seal, minimal maintenance — and free oxygen delivery, since the agitation medium is the oxidant. Its costs are higher compressed-air energy per unit of mixing (often 2–3× the specific energy of a mechanical agitator for the same suspension duty), poorer suspension of coarse or dense particles, and foaming management at the free surface.

Choose mechanical agitation when: solids are coarse or high in density (Njs pressure is high); power cost is the dominant OPEX concern; you need staged reagent addition or per-tank control; or tanks are wide and shallow (H/T below ~1.5), where air-lift circulation is inefficient.

Choose air agitation when: the ore is fine and easily suspended; a tall, narrow tank footprint suits the site (historic Pachucas run H/T of 2–3); maintenance skills and spares for large agitator drives are scarce in the region; or oxygen demand coincidentally matches the air rate needed for circulation, as in some bio-oxidation and moderate-intensity cyanide duties.

A pragmatic hybrid also exists: mechanical agitation for suspension with supplemental sparged oxygen for chemistry. Many modern designs separate the two duties deliberately — the agitator keeps solids moving, the gas system feeds the reaction — because optimizing one device for both jobs usually compromises both.

Which Materials of Construction Should a Leaching Tank Use?

Materials selection is where leaching tank design meets chemistry most directly. The decisive variables are pH, chloride concentration, temperature, and solids abrasivity. The table below summarizes common choices; it is a screening tool, and final selection should always be validated against the actual liquor analysis.

Rubber-lined steel dominates gold cyanidation worldwide because cyanide liquor is nearly innocuous to rubber, abrasive slurries are ubiquitous, and lined carbon steel is the cheapest pressure-tight structure per cubic metre. Stainless and duplex earn their premium in acidic, chloride-bearing copper, nickel, and zinc circuits, where rubber temperature limits and FRP abrasion limits bite simultaneously. One more principle of good tank design worth stating plainly: the liner, not the shell, is the wear surface. Specify liner quality, hardness, and joint detail as rigorously as you specify the shell, and demand welder and rubber-bonder qualifications in the purchase order. Shell corrosion behind a failed liner patch has ended more tank lives than agitator failures have.

Why Do Agitated Leaching Tanks Fail in Service?

Most tank problems trace back to a handful of design and operational causes. The list below is ordered roughly by how often we see it in the field.

1. Symptom: sanding — a growing solids bed on the tank floor. Cause: operating below Njs after an ore hardness or grind shift, an undersized impeller installed at replacement, or a power draw that drifted as impellers wore. Fix: restore design impeller diameter and speed, re-verify Njs against current size analysis, and install off-bottom detection (pressure or ultrasonic) so operators see the bed forming before it buries the impeller.

2. Symptom: rapid liner wear at the impeller and bottom. Cause: tip speed above the liner’s wear limit, tramp oversize in the feed, or a tank bottom radius that directs impingement. Fix: reduce tip speed with a larger impeller, screen feed, and inspect liner thickness on a scheduled downtime; patching early is far cheaper than relining.

3. Symptom: falling dissolved oxygen or reagent concentration despite normal gas rates. Cause: sparger blinding with scale or slime, or gas channels bypassing the impeller. Fix: clean or redesign the sparger, confirm gas is being sheared under the impeller rather than rising beside the shaft.

4. Symptom: shaft seal leakage and gearbox failures. Cause: a seal arrangement unsuited to the duty (packing in a gas-space service, or a mechanical seal running dry), plus shaft misalignment after foundation settlement. Fix: upgrade to a dry-well or appropriately rated mechanical seal, add a condition-monitoring point to the drive, and re-survey foundations.

5. Symptom: corrosion beneath liners or at weld seams in stainless tanks. Cause: chloride above the alloy’s threshold at operating temperature, or crevices at liner joints and nozzles. Fix: match alloy to liquor analysis — 316L is not a universal acid-duty material — and specify crevice-free detailing in the next fabrication.

6. Symptom: foaming and slurry carryover into gas handling. Cause: fine grind, high solids, surfactant-type reagents, or excessive gas injection. Fix: increase freeboard, add a froth-control system or wash sprays, and revisit the working-volume allowance from step 2 — the tank was designed with too little headroom for its actual chemistry.

Each failure mode above becomes cheaper to prevent than to fix by roughly an order of magnitude, which is the practical argument for investing engineering hours in leaching tank design before fabrication, not after commissioning.

What Do Leaching Tanks Cost to Build and Run?

Costs vary with region, alloy, and tank size, so the figures below are illustrative 2026 budget-class ranges assuming mid-volume tanks (350–600 m³), conventional fabrication in a competitive Asian or North American fabrication market, and ex-works pricing excluding civils, painting extras, and installation. Treat them as order-of-magnitude only.

• Mechanical agitator tank, rubber-lined steel: roughly 1,500–2,500 USD per m³ of gross volume for the tank structure and liner, plus 400–700 USD per kW installed for the agitator drive, shaft, and impeller.
• Stainless or duplex equivalents: typically 1.6–2.5× the lined-steel figure, escalating with nickel and molybdenum alloy content.
• Pachuca (air-agitated) tank: noticeably cheaper structurally — 900–1,600 USD per m³ — but add compressors and their installation; energy economics decide the comparison.
• Agitation energy: 0.6–1.5 kW/m³ absorbed for mechanical systems as covered earlier; Pachuca duty commonly 0.03–0.08 kW per m³ of air compressed, with air rates of 0.5–2.0 m³/min per 100 m³ of slurry.

Energy dominates long-run cost: at 0.10 USD/kWh, a single 400 kW agitator consumes about 330,000 USD of electricity per year, so a design that over-specifies power by 30% quietly adds ~100,000 USD per tank-year to operating cost. This asymmetry — capital once, energy forever — is why a responsible design documents the assumptions behind every power figure and revisits them at each orebody revision.

Worked Design Example (Illustrative)

The following example is illustrative and uses round numbers chosen for arithmetic clarity, not from any specific project. A flowsheet team must treat a 500 t/d (551 short t/d) oxide gold ore with bottle-roll kinetics showing 92% extraction at 22 hours and a grind P₈₀ of 75 µm, leached at 48% solids (w/w) with SG 2.7 solids.

• Residence time: 24 h design (22 h kinetic + ~10% scale-up allowance).
• Slurry flow: 500 t/d ÷ 1.45 t/m³ ≈ 345 m³/h (12,200 ft³/h); hourly basis ≈ 14.4 m³/h… (per-day basis 8,280 m³ working volume total).
• Tank count and size: six tanks of 1,380 m³ gross each (≈1,200 m³ working at 87%), diameter 11.5 m (37.7 ft), liquid depth 12.5 m (41 ft), H/T ≈ 1.09.
• Power: 1.0 kW/m³ × 1,380 m³ ≈ 1,380 kW absorbed per tank? No — this reveals the iterate: at that scale, per-tank intensity of 0.7 kW/m³ (≈ 970 kW absorbed) with dual impellers is more typical, and the team would likely move to eight tanks of ~1,000 m³ gross to keep shaft lengths manageable. The point of the example is the iteration itself.
• Impeller: two 4-blade hydrofoils per shaft, lower diameter ~4.2 m (13.8 ft), operating at ~0.55 × Njs margin above just-suspended speed for a 1 mm design particle, tip speed ≈ 4.3 m/s (14 ft/s).
• Materials: rubber-lined carbon steel shell, four baffles 0.96 m wide, dry-well agitator seal, sparged air at ~1.0 m³/min per 100 m³ slurry for O₂ demand.

Working the example through honestly — including the correction in the power line — is what leaching tank design actually looks like: the first pass always teaches you which parameter you underestimated.

FAQ (Frequently Asked Questions)

What residence time should I use to size a leaching tank? Derive it from bottle-roll or stirred kinetic testwork for your specific ore, grind, and reagent system — never from a generic industry figure. Take the time at which incremental extraction becomes uneconomic, then add 10–20% for scale-up inefficiency and inter-tank short-circuiting. For oxide gold ores this usually lands at 24–36 hours; refractory and base-metal duties can run longer or shorter depending on mineralogy.

How much power does an agitated leaching tank need per cubic metre? Typical absorbed-power intensity is 0.6–1.0 kW/m³ for fine-grind oxide leaches with hydrofoil impellers, rising to 1.0–1.5 kW/m³ for coarser or gas-dispersion duties and up to 2.5 kW/m³ for high-viscosity or bio-oxidation service. The rigorous method is to calculate just-suspended speed for your worst particle and add margin, using intensity only as a sanity check.

When is a Pachuca (air-agitated) tank better than a mechanically agitated tank? When the ore is fine and easily suspended, the tank can be tall and narrow, regional maintenance capability for large drives is limited, and the air needed for circulation roughly matches the oxygen demand of the chemistry. Pachucas excel on simplicity and uptime; they lose on energy efficiency for coarse solids and on suspension of dense particles.

Which material should I choose for the tank shell and liner? Let the liquor decide. Cyanide circuits almost universally use rubber-lined carbon steel for abrasion and economy. Neutral, low-chloride duty suits 316L. Acidic chloride liquors push toward duplex stainless or FRP/dual-laminate, with abrasion resistance the limiting factor for polymers. Always validate against the actual solution analysis at operating temperature, not the design briefing.

Why do solids settle out in a leaching tank even with an agitator running? Usually because the impeller is operating below the just-suspended speed for the current size distribution — after a grind shift, a replacement impeller of smaller diameter, or impeller wear. Dead zones behind poorly configured baffles and off-design feed surges also contribute. Install off-bottom solids detection and re-verify Njs whenever the ore or grind changes.

How many leaching tanks should a circuit have? Six to eight tanks is standard for gold CIL/CIP circuits, giving near plug-flow behavior and redundancy for agitator maintenance. Simpler acid leaches often use three to five. Fewer, larger tanks reduce capital but concentrate risk: a single agitator outage then threatens the whole leach, and back-mixing between stages reduces effective residence-time utilization.

How is tank volume converted into plant capacity? Working volume divided by residence time gives slurry flow; multiply by slurry density and the solids fraction to recover the dry-tonnage rate. For example, 1,200 m³ working volume at 24 h residence handles 50 m³/h of slurry, which at 1.45 t/m³ and 48% solids corresponds to about 35 t/h (38.6 short t/h) of dry solids.

Conclusion

Sound leaching tank design is a chain of verifiable steps: residence time from kinetics, volume from residence time, geometry from volume, impeller and power from just-suspended speed, materials from chemistry, and every number documented with its assumptions. Tanks built this way run for decades; tanks built from rule-of-thumb cubes and borrowed horsepower ratings sand up, wear out, and under-recover. Whether you are evaluating a single agitator replacement or specifying a complete leaching circuit, our team can review your kinetics, liquor analysis, and size data and return a sizing rationale you can defend. Contact Lexin Hydromet to talk to a metallurgist or request a technical proposal — and browse the full resource library for more design guidance, including how to convert kinetic testwork into a leaching recovery calculation and how tank leaching compares with tank vs. heap leaching economics.

how Leaching Reactor Volume Calculator from Residence Time is decided

References

• SME, Mineral Processing Plant Design, Practice, and Control (Mular, Halbe & Barratt, eds.), Society for Mining, Metallurgy & Exploration.
• Wills, B.A. & Finch, J.A., Wills’ Mineral Processing Technology, 8th edition, Butterworth-Heinemann.
• Havlík, T., Hydrometallurgy: Principles and Applications, CRC Press.
• Fuerstenau, M.C. & Han, K.N., Principles of Mineral Processing, SME.
• USGS, Minerals Yearbook chapters on gold and copper processing, U.S. Geological Survey.
• Zwietering, T.N., original paper on suspension of solids in stirred vessels, Chemical Engineering Science.
• McCabe, W.L., Smith, J.C. & Harriott, P., Unit Operations of Chemical Engineering, McGraw-Hill.

Changelog

• 2026-10-19: First published.