Heap Leaching: Pad Design & Operating Guide

Last updated: 2026-10-25

Heap leaching recovers 60–90% of the copper in oxide ores and 55–80% of the gold in crush-agglomerated precious-metal ores, at a fraction of the cost of milling. Because crushed ore stacked on an engineered pad behaves as a self-renewing packed-bed reactor, success depends on pad design, agglomeration, irrigation and solution management — the core topics of this guide and of the wider leaching technology family.

Key Takeaways

• Crush size (P80) governs both metal recovery and reagent demand; coarser crushes trade recovery for throughput.
• Agglomeration with cement or lime prevents fines migration and channeling on clay-bearing ores.
• Drip irrigation at 5–15 L/h/m² keeps the heap unsaturated and protects solution inventory.
• A geomembrane-lined pad with leak detection is standard; drainage design controls solution head over the liner.
• Match recovery to tenor: SX-EW for copper oxide PLS, carbon adsorption or Merrill-Crowe for gold.

What Is Heap Leaching?

Heap leaching is a hydrometallurgical extraction method that recovers dissolved metals from crushed or run-of-mine ore by percolating a chemical lixiviant — dilute sulfuric acid (H₂SO₄) for copper oxide minerals, a dilute sodium cyanide (NaCN) solution for gold and silver — through a large engineered stack, then capturing the metal-bearing drainage at its base. The technique has been applied to copper oxides since the early twentieth century and to precious-metal ores at commercial scale since the late 1960s, and it now accounts for a substantial share of the world’s primary copper and a significant fraction of newly mined gold.

Leaching in practice

Leaching Reactor Volume Calculator from Residence Time in practice

A heap leach pad is a multi-layer civil structure that supports the stacked ore while isolating the pregnant leach solution (PLS) from the subgrade through a geomembrane liner, a drainage layer and a leak-detection system. The pad is not a passive floor: its grading, liner sequence and collection piping determine how completely the solution can be recovered, how much static inventory is held beneath the heap, and how the facility performs through its full operating life and eventual closure.

Agglomeration is a conditioning step that binds fine particles onto coarse fragments using cement, lime or acid as a binder, producing a granule that resists compaction and preserves permeability. Without it, ores containing more than roughly 15–20% fines tend to stratify during stacking, blinds the pore network and lets solution race through preferential channels instead of contacting mineral surfaces.

The flowsheet logic is simple. Metal-laden PLS drains from the heap to a pond and reports to a downstream recovery circuit — solvent extraction and electrowinning (SX-EW) for copper, or carbon adsorption followed by elution and electrowinning for gold. The spent solution, known as raffinate, is re-strengthened with acid or cyanide and pumped back to the top of the heap. This closed loop means the only consumables are lixiviant make-up, acid consumed by gangue minerals, and pumping energy. Where a project sits between percolation leaching and agitation leaching, see our overview of leaching methods or the dedicated heap leaching system equipment page.

Key Design Parameters

Every percolation-leach project is a negotiation between recovery, throughput and reagent cost, and the parameters in the table below set the terms of that negotiation.

Crush size (P80). Finer grinding exposes more mineral surface and shortens the diffusion path, but it also generates fines that destroy permeability. Gold ores are commonly crushed to a P80 of 6–19 mm and agglomerated; copper oxide ores often leach acceptably at a P80 of 12–50 mm; secondary sulfide (chalcocite) bioheaps may be crushed even coarser, or not at all. The optimum is found in column testing, not in a textbook — the leaching recovery calculation resource shows how column results translate to heap-scale projections.

Lift height. Ore is placed in lifts of typically 4–10 m. Lower lifts oxygenate and drain better and suit slow-leaching or clayey ores; higher lifts reduce pad footprint and handling cost. Multi-lift operation builds final heights of 30–120 m, with each new lift compacting the ones beneath it — a key reason permeability degrades with depth.

Irrigation rate. Drip irrigation at 5–15 L/h/m² (roughly 0.002–0.006 US gpm/ft²) keeps the heap unsaturated. Higher rates push more solution through per day but shorten residence time, dilute the PLS and can flood the pore network, driving out the oxygen that sulfide bioleaching requires.

Cure time and lixiviant strength. Acid-cured copper heaps typically receive 1–7 days of concentrated acid application before full irrigation, accelerating the attack on secondary copper minerals. Cyanide heaps operate at 0.2–0.5 g/L NaCN and pH 10.5–11.0, held above pH 10.3 (the point where HCN gas evolution becomes dangerous) by lime addition.

Permeability and fines. Stacked, agglomerated ore should maintain a saturated hydraulic conductivity well above 10⁻⁴ m/s so that irrigation never saturates the bed. Fines below 75 µm should be limited to roughly 20–30% even after agglomeration.

The single most consequential parameter is gangue acid consumption (GAC): every kilogram of acid consumed by calcite, dolomite or iron oxides is a kilogram that never leaches copper. Ore with a GAC above roughly 50–70 kg/t H₂SO₄ rarely supports an economic oxide heap, and this number should be established on representative drill-core before any other design work.

When Does Heap Leaching Beat Tank Leaching?

Heap leaching wins on cost; tank leaching wins on speed and recovery. The decision therefore hinges on ore grade, throughput and mineralogy. Heap leaching is normally preferred for low-grade ores (roughly below 1 g/t Au or below 0.5–1% Cu in oxide form), for large tonnages that would make a mill uneconomic, and for oxide or secondary sulfide minerals that dissolve readily in acid or cyanide. Agitated tank leaching (CIL or conventional cyanidation) is justified when grade is high enough to pay for grinding and when refractory mineralogy demands fine liberation. For a full side-by-side treatment, see our heap versus tank leaching comparison.

how Agitated Leaching Tank Design is decided

The gray zone — mid-grade ores, moderate clay, borderline tonnage — is exactly where a well-run test program earns its keep, because a 3–5% recovery difference can swing the flowsheet choice either way.

How Do You Design a Heap Leach Pad?

Pad design integrates civil, geotechnical and hydrometallurgical engineering into one facility, and the sequence below reflects the order in which decisions lock in cost.

1. Site selection and subgrade preparation. Choose a gentle slope (typically 2–5%) with stable foundations and minimal cut-and-fill. Strip topsoil, key into competent material, and place a compacted low-permeability soil layer (often 300–600 mm) as the liner subbase.
2. Liner system. Install a 1.5–2.0 mm HDPE geomembrane over the prepared subbase, with a geosynthetic clay liner (GCL) or compacted clay beneath it on most modern facilities, plus leak detection between the layers. Most regulators now require this double-liner arrangement for new percolation leach facilities, and liner QA/QC (seam testing, subgrade smoothness) deserves full-time supervision.
3. Solution collection and ponds. Lay corrugated collection piping on drainage bedding above the liner, sloped to the PLS outlet. Size the PLS, raffinate and storm ponds to contain the operating inventory plus a regulatory design storm, and provide emergency containment so a pond overtopping cannot reach the environment.
4. Ore delivery and stacking. Select between truck dumping (flexible, but risks compaction from wheel traffic) and conveyor stacking with a radial stacker (uniform, low compaction, higher CAPEX). Specify lift-building procedure and traffic control so wheeled equipment never travels on freshly irrigated surfaces.
5. Agglomeration and curing. Feed ore through a rotary drum or agglomerating conveyor with the binder (cement, lime or acid) dosed into the wetting stream. Design moisture to 8–14% and allow the cure/ripening period from the parameter table before irrigation starts.
6. Irrigation and aeration. Install drip laterals spaced 0.6–1.0 m with emitters at 0.6–1.0 m, protected by pressure filters and flushed on schedule. Where sulfide bioleaching is intended, add forced aeration through a perforated piping network at the base of each lift — oxygen supply, not solution supply, is usually rate-limiting in sulfide heaps.
7. Rinse, drain-down and closure planning. Design the rinse protocol (typically irrigating with water until effluent pH and metal concentrations meet closure criteria), the drain-down period (weeks to months), and the final cover — an evapotranspiration or low-permeability cap that sheds rainfall indefinitely.

Two cross-cutting rules apply throughout. First, design the pad for the whole life: liner exposure to UV during staged construction, static and dynamic loads of 100 m-plus ore columns, and decades of acid or cyanide contact. Second, instrument early — piezometers, solution head sensors on the liner and flow meters per irrigation sector turn the pad from a black box into something a metallurgist can actually control.

Why Do Heaps Fail to Leach Uniformly?

Uneven leaching is the most common reason heap operations miss their recovery forecast, and the failure modes repeat across commodities.

1. Ponding on the lift surface. Symptom: standing solution on top of the heap, declining irrigation flow per sector. Cause: compaction from stacking equipment, fines washed down into the surface, or over-wet agglomerates that slaked under irrigation. Remedy: rip or scarify the surface, restack affected lifts at lower moisture, enforce traffic exclusion zones, and tighten agglomeration quality control on moisture and binder.
2. Channeling and preferential flow. Symptom: solution appears at the drains within hours of application, with PLS tenor far below column-test predictions. Cause: particle segregation during stacking creates coarse and fine bands, so water follows the coarse lanes and bypasses the rest. Remedy: agglomerate the feed, reduce drop heights at the stacker, use mixers rather than conveyors alone, and verify granule stability with tumble tests.
3. Precipitate blinding in the heap. Symptom: recovery plateaus well below plan after an initially strong start. Cause: secondary minerals — gypsum in high-calcium ores, jarosite or ferric hydroxide where iron and pH are uncontrolled — precipitate in pores as solution chemistry evolves down the heap. Remedy: manage acid strength and pH along the flow path, increase wash ratios periodically, and consider interlift solution injection to restart stalled zones.
4. Acid front stall. Symptom: effluent pH rises, copper in PLS declines, yet shallow piezometers show solution moving normally. Cause: gangue acid consumption along the flow path exhausts the lixiviant before it reaches the base of deep lifts. Remedy: raise acid feed strength within metallurgical limits, extend cure time on new lifts, reduce lift height, or slow irrigation to extend residence.
5. Heap slumping and differential settlement. Symptom: surface depressions, solution pooling, and in severe cases damage to collection piping. Cause: binder-deficient or over-wet agglomerates that cannot carry load, or stacking lifts onto a lift that had not drained. Remedy: re-dose binder and moisture in agglomeration, enforce drain-down time between lifts, and rebuild settlement zones rather than irrigating through them.

The common thread is that most uniformity failures originate upstream in feed preparation, not in the pad itself. A pilot-scale column and agglomeration program — such as the pilot testing services offered with our equipment packages — is far cheaper than re-handling a million tonnes of impermeable ore.

Cost and Energy Considerations

Costs below are typical ranges for greenfield projects and should be treated as scoping-level figures; ore character, topography and labor market move them materially.

Capital. For a 10,000 t/d oxide copper leach operation (pad, ponds, irrigation, SX-EW plant, excluding mining fleet and owner’s costs), capital intensity of roughly US$40–90 per annual tonne of ore capacity is typical, versus several times that figure for a milling and flotation or agitated-leach route. The pad and liner system alone commonly represents 10–20% of process plant capital, and pond sizing after a design storm is a frequent source of overrun.

Operating cost. Copper oxide heap leaching typically runs at US$1.5–3.5/t ore processed (excluding mining), of which sulfuric acid dominates: consumption of 15–40 kg/t at delivered acid prices of US$80–150/t gives US$1.2–6.0/t depending on gangue acidity — see the acid consumption analysis for copper oxide ores. Gold heap leaching typically costs US$2–5/t ore, with NaCN at 0.2–0.5 kg/t (US$0.4–1.5/t at US$2,000–3,000/t delivered) and cement or lime binder at US$0.5–2.5/t.

Energy. Crushing is the largest electrical load at 1–3 kWh/t depending on circuit and hardness. Stacking consumes 0.3–0.8 L of diesel per tonne (or its electrical equivalent on conveyor systems). Solution pumping depends on head: as a rule of thumb, lifting solution 100 m costs about 0.3 kWh/m³, which translates to roughly 0.05–0.3 kWh/t ore at normal irrigation rates (approximately 0.02–0.12 kWh/t at 30–120 m heads in SI terms). The absence of grinding — which alone consumes 10–20 kWh/t in a milling circuit — is the structural reason heap leach costs sit below tank leaching, and the reason the economics improve further at remote sites where diesel-generated power is expensive.

Two assumptions underpin these figures and deserve explicit testing during feasibility. Reagent prices are volatile — delivered acid cost has moved by a factor of two within single years — so a sensitivity of plus or minus 30% on reagent pricing should accompany any scoping study. And operating cost excludes closure and water management, which for a lined pad with long-term drain-down and treatment obligations can add a meaningful per-tonne provision that early-stage models routinely omit.

What Do Operating Data Show?

Two anonymized cases illustrate how design parameters convert into operating results. Both datasets are illustrative composites drawn from published industry experience, not from any single named client.

Copper oxide heap, South America (illustrative). An 18,000 t/d operation crushing to P80 20 mm on ore with 22 kg/t gangue acid consumption irrigated at 10 L/h/m². PLS tenor stabilized near 2.4 g/L Cu, raffinate returned at 6–8 g/L H₂SO₄, and overall copper recovery reached 78% at approximately 240 days of leach time. About 60% of ultimate recovery was achieved within the first 60 days — the fast-acid stage — with the remainder drawn out over months as secondary minerals dissolved.

Gold heap, West Africa (illustrative). An 8,000 t/d crush-agglomerated operation at P80 12 mm, bound with 1.2 kg/t lime, leached with 0.3 g/L NaCN at pH 10.8 and drip rates of 8 L/h/m². Recovery climbed to 74% by day 90, with carbon loaded to 2,500–3,500 g/t Au in the adsorption columns feeding the elution circuit. Cyanide consumption ran 0.35 kg/t, most of it to reactive gangue rather than gold dissolution.

Three patterns repeat across such datasets. Recovery is strongly front-loaded: the first month of irrigation typically delivers half or more of the ultimate value, which is why cure practices and early irrigation control matter so much. Solution tenor is inversely related to irrigation rate — running faster buys tonnage but dilutes PLS, which raises downstream SX-EW or adsorption cost per kilogram recovered. And measured recovery almost always lands a few percentage points below column-test projections, a calibration gap best absorbed by conservative scale-up factors rather than optimistic assumptions. For ore-type-specific results, see our copper oxide ore application notes.

Frequently Asked Questions

What ore types are best suited to heap leaching?

Oxide copper ores (chrysocolla, malachite, atacamite), secondary sulfides such as chalcocite, and free-milling gold ores are the classic candidates. The ore should liberate sufficiently at crush sizes rather than requiring fine grinding, contain limited clays or be amendable to agglomeration, and show manageable gangue acid consumption. Highly refractory sulfide ores generally need oxidation pretreatment and are poor heap candidates.

How long does a complete heap leach cycle take?

Oxide copper heaps typically leach for 90–240 days per lift, gold heaps for 45–120 days, and sulfide bioleach heaps for one to four years because bacterial oxidation is slow. Multi-lift operations overlap these cycles, so the pad continuously discharges PLS even though any individual lift takes months to wash, drain and retire from service.

What liner system does a heap leach pad require?

Modern practice is a composite liner: a compacted low-permeability soil or geosynthetic clay layer topped by a 1.5–2.0 mm HDPE geomembrane, with leak detection between liners on most regulated facilities. The geomembrane resists the lixiviant in use — HDPE handles sulfuric acid and cyanide solutions well — and seams are vacuum- or spark-tested throughout construction.

Why is agglomeration used before stacking?

Agglomeration binds fines onto coarse particles with cement, lime or acid, converting a segregating, impermeable feed into stable granules. Without it, fines migrate during stacking and irrigation, clog the pore network and force solution into channels. Agglomerated ore irrigates uniformly, supports higher lift heights, and sustains permeability through the entire leach cycle.

How is copper recovered from heap leach solution?

Pregnant leach solution at roughly 1–6 g/L copper feeds a solvent extraction plant, which upgrades and purifies it to 40–50 g/L, and the loaded electrolyte goes to electrowinning to produce LME-grade copper cathode directly on site. The barren raffinate returns to the heap after acid re-dosing, closing the loop without any intermediate copper product.

How is gold recovered from a heap leach operation?

Gold-bearing solution flows through columns of activated carbon, which adsorbs the gold-silver cyanide complex. Loaded carbon is stripped in an elution (ADR) circuit, gold is electrowon or precipitated with zinc, and the barren solution is re-dosed with NaCN and lime before returning to the heap. Where solution tenors are high and silver ratios unfavorable, Merrill-Crowe zinc precipitation can replace carbon.

Can heap leaching operate in cold or wet climates?

Yes, with adjustments. Solution temperatures below about 10 °C slow both chemical leaching and bacterial oxidation, so cold-climate heaps may use solution heating, event ponds for extreme rainfall, drip (rather than sprinkler) irrigation to cut evaporative and freeze losses, and reduced winter application rates. High rainfall mainly affects pond sizing and water balance, not feasibility.

Further Reading and Next Steps

More background, calculators and technical notes are available in our resource library. If you would like these parameters applied to your orebody, our team can review your assay and mineralogy data and propose a test program.

Conclusion

Heap leaching remains the lowest-cost route to metal for low-grade oxide and free-milling ores, but its economics are earned through disciplined engineering rather than assumed. Crush size, agglomeration quality, lift height, irrigation rate and liner design interact — a mistake in any one of them surfaces months later as ponding, channeling or a recovery shortfall that no downstream adjustment can fully repair. The operations that consistently hit 60–90% recovery share one habit: they characterize the ore honestly, pilot the flowsheet early, and instrument the heap so problems are found in weeks, not quarters. If you are evaluating heap leaching for a project, the next step is a structured flowsheet review. Contact Lexin-tech with your ore data to discuss a leach flowsheet or request a technical proposal for a complete pad, irrigation and recovery system.

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. Fuerstenau, M. C. & Han, K. N. (eds.) (2003). Principles of Mineral Processing. SME, Littleton, CO.
3. Kappes, D. W. (2002). “Copper heap leaching: the chemistry and the physics.” Presented at the ALTA 2002 Copper Hydrometallurgy Forum, Perth, Australia.
4. Ghorbani, Y., Franzidis, J.-P. & Petersen, J. (2016). “Heap leaching technology — current state, trends and applications.” Minerals Engineering, Vol. 84, pp. 2–9.
5. Watling, H. R. (2006). “The bioleaching of sulphide minerals with emphasis on copper sulphides — a review.” Hydrometallurgy, Vol. 84, pp. 81–108.
6. Dixon, D. G. & Petersen, J. (2003). “Comprehensive modelling study of chalcocite column and heap bioleaching.” Canadian Metallurgical Quarterly, Vol. 42, No. 4, pp. 497–508.
7. U.S. Geological Survey (2025). Mineral Commodity Summaries 2025. Reston, Virginia.

Changelog

2026-10-25 — first published.