Acid Curing and Agglomeration for Copper Heap Leaching

Last updated: 2026-11-08

Acid curing of crushed ore before stacking typically lifts copper oxide heap leach recovery by 5–15 percentage points, at a cost of only a few dollars per tonne of ore. Within the wider leaching flowsheet, the agglomeration drum is where a heap gets a permeable structure — or inherits fines that channel and strand metal. This guide covers the mechanism, drum design numbers, cure chemistry, and test work.

Key Takeaways

• Acid curing pre-conditions crushed ore with concentrated sulfuric acid so dissolution starts before leach solution arrives.
• Fines migrate downward during solution flow, clogging channels and causing preferential flow that strands un-leached ore.
• A rotary agglomeration drum at 30–60 s retention and L/D of 2.5–4:1 produces stable, permeable agglomerates.
• Typical cure strength is 10–50 kg H₂SO₄ per tonne of ore; the exact dose must come from test work.
• Stacking practice — lift height, tipping impact, traffic — can undo a well-designed curing circuit.

What Is Acid Curing and Why Does Crushed Ore Need It?

Four definitions frame the practice for engineers new to heap operations.

Acid curing is a pre-leach conditioning step that applies concentrated sulfuric acid, usually with added water, to crushed ore and holds the moistened bed for a ripening period so that acid-sensitive minerals begin dissolving before dilute leach solution is applied. It is most valuable on oxide copper feeds where acid-soluble minerals such as malachite, azurite, and chrysocolla dominate the grade.

Ore agglomeration is a mechanical conditioning step that tumbles crushed ore with liquid binder — acid solution, raffinate, or water — so that fines coat coarse particles and adhere into rounded, permeable granules. It is the physical complement to the chemical cure, and on most modern copper oxide projects the two are performed in the same drum, which is why the combined operation is often called acid curing and agglomeration in a single breath.

Ripening is the controlled hold period that follows acid addition, typically several hours to several days, during which acid diffuses into particle microcracks and reacts with gangue and ore minerals. It is a zero-capital part of the flowsheet — the ore simply waits in the cure stockpile — yet it materially changes how the heap wets up.

Channeling is the failure mode that agglomeration exists to prevent: solution finding preferential flow paths through the heap, bypassing most of the ore, and exiting as poorly loaded barren solution. It is the direct consequence of uncontrolled fines and uneven wetting.

The economic logic is straightforward. Acid-soluble copper minerals react quickly once they contact strong acid, so curing front-loads dissolution into a period when the ore is stacked thin and accessible, rather than buried under 6–10 m of lift. Cured, agglomerated ore also wets up uniformly, which shortens the time to first production and reduces the acid Inventory that sits unreacted in dead zones. Recovery arithmetic for the wider circuit is covered in our leaching recovery calculation page.

Why Do Fines Kill a Heap Leach?

Crushing circuits are unavoidable generators of fines. A typical three-stage crush to −12 mm (−0.47 in) will carry 15–30% material finer than 1 mm, and softer oxide ores — the very ores that suit acid curing best — produce the highest fines fractions (typical range, confirm with test work on your ore).

Three mechanisms then conspire against permeability. First, segregation during stacking: fine particles concentrate in the core of each tipped load and along the lift surface, while coarse fragments roll to the toe. Second, fines migration during operation: as leach solution percolates, it carries slimes downward until they lodge in interparticle voids, progressively sealing the lower lifts like sediment in a filter. Third, slumping: a moistened fines-rich bed loses shear strength, compacts under the weight of lifts above, and collapses into low-permeability lenses that solution cannot enter.

The operational symptoms are distinctive and worth memorizing. Ponds form on the lift surface. Solution issues from the heap toe within hours of irrigation, still near-neutral in pH and near-barren in copper — the signature of bypass flow. When the heap is finally excavated, operators find hard, dry, barely-attacked ore blocks sitting beside washed-out channels. Recovery plateaus 10–20 points below the bottle-roll prediction, and no amount of extra irrigation or acid in solution recovers the deficit, because the solution simply never reaches the metal.

Agglomeration attacks all three mechanisms at once. Tumbling in a drum with liquid and acid physically coats fines onto coarse particles, so the fines can no longer travel independently. The acid in the binder starts dissolving clay edges and precipitating iron, which “sets” the granule structure much like cement hydration. And the granules themselves are rounded and equi-dimensional, so a bed of them resists compaction far better than a bed of crushed fragments and slimes. This is why acid curing and agglomeration in a single drum has become near-standard for clay-rich copper oxide projects in Chile, Peru, and the southwestern United States, and why it is equally relevant to gold-silver oxide ores — see our gold oxide ore application note — and to agglomerated nickel laterite feeds in atmospheric acid circuits.

Acid Curing and Agglomeration: Key Design Parameters

Table 1 collects the design envelope for a rotary drum performing combined acid curing and agglomeration. Every range is a typical industry envelope to be confirmed by test work on your specific ore.

Three interactions inside that table deserve a second read. Retention time, slope, and rotational speed are not independent: for a given drum, retention time is approximately drum volume divided by volumetric flow, and raising the slope or speed shortens it. Designers therefore fix the retention time first — the metallurgical requirement — and back-calculate slope and speed. Moisture and acid addition are also coupled, because most plants add acid pre-diluted with raffinate rather than neat, so every kilogram of acid brings water with it; the water balance must close against the 8–12% moisture target, not just the acid target. Finally, cure strength interacts with gangue mineralogy: high-carbonate gangue consumes acid and generates CO₂, so the dose is set by acid consumption testing rather than by copper head grade alone — the method is on our acid consumption for copper oxide ores page.

How Do You Select Drum Size, Speed, and Acid Strength?

Selection follows a fixed sequence: metallurgy first, chemistry second, machine last.

Step 1 — Fix the retention time from ore behavior. Run agglomeration tests at several tumble durations and measure green strength (drop test), permeability of the cured bed, and fines release after handling. Weak ores — high clay, soft oxidation rims — need the upper end of the 30–60 s band; hard, low-clay ores agglomerate in 30 s. If the ore cannot form stable granules at 60 s, the answer is more binder moisture or higher acid, never a longer drum.

Step 2 — Fix the acid dose and cure duration from cure testing. Cure small columns or sealed drums at 10, 20, 30, 40, and 50 kg/t and track acid-soluble copper extraction after ripening. Oxide ores dominated by malachite and azurite often reach target extraction at 15–25 kg/t with 24–48 h ripening. Chalcocite and other secondary sulfides respond poorly to sulfate-only cures; adding chloride to the cure liquor — 5–20 kg/t — forms cuprous chloride complexes that accelerate the sulfide reaction, a practice supported by the broader chloride hydrometallurgy literature and discussed for reactor contexts on our chloride leaching reactor selection page. Chloride-cured ores demand corrosion-resistant downstream handling, and environmental review, before adoption.

Step 3 — Size the drum from throughput and retention. Required holdup equals throughput multiplied by retention time; required drum volume is holdup divided by a design fill fraction, conventionally 8–15% of the internal volume. Then choose L/D within 2.5–4:1, set the slope in the 3–8° band to trim retention, and check peripheral speed against the 0.8–1.5 m/s window. The worked example below shows the arithmetic in full.

Step 4 — Design the acid and water delivery. Acid is added through a distributor or spray bar at the feed end, ideally onto the falling curtain of ore where mixing is instantaneous. Neat acid gives maximum cure strength per liter of liquid but risks local over-acidification and heat spikes; pre-dilution with raffinate at 50–70% strength is the common compromise. Instrument flow to ore ratio, not just flow, because ore feed rate swings are the norm in crushing plants.

Step 5 — Protect the cure through stacking. A cured granule is a fragile asset. Truck tipping from more than about 2–3 m (6–10 ft) shatters agglomerates and re-liberates fines; scraper stacking or telescopic chutes reduce drop height at the point of deposition. Lift height matters too: 6–8 m lifts impose more compaction than 4–5 m lifts, and each additional lift of dead loading squeezes the bed below. Radar or laser surveys that track heap subsidence give early warning of slumping before ponds appear.

How Does Stacking Practice Undo Good Acid Curing?

More acid curing and agglomeration projects fail in the stacking phase than in the drum, and the failure is invisible until the first ponds form. The physics is simple: an agglomerate cured to 10% moisture is a moist, weakly cemented ball. Dropping it three meters onto a hard surface cracks it; driving a 40 t haul truck over the lift surface crushes it; dumping the next lift on top compresses it. Every one of those events returns fines to the free state that agglomeration paid to immobilize.

The countermeasures are operational, cheap, and consistently underweighted in project schedules. Constrain tipping height with spotters or mechanical height limiters. Keep haul trucks off freshly stacked lifts, using dozer routes outside the active drip area. Prefer grasshopper conveyors and radial stackers over truck stacking where topography allows — a radial stacker with a low drop point never re-damages the bed. Where truck stacking is unavoidable, plan lift geometry so each tipping point is used once. Plants that treat the cured granule as a product, with handling procedures to match, routinely hold permeability through 6+ lifts; plants that treat it as crushed ore lose the benefit within the first one.

What Test Work Should Precede Agglomerator and Cure Design?

Test work proceeds through three tiers, and skipping tiers is the most common cause of redesigned drums and missed recoveries.

Tier 1 — Characterization and bottle rolls. Standard bottle roll leach tests on ground or coarse samples establish the acid consumption curve, the acid-soluble copper fraction, and the ultimate extractable recovery under agitated conditions. Bottle rolls deliberately destroy agglomerates, so they bound the chemistry but say nothing about bed hydrology.

Tier 2 — Agglomeration tests. Small drum or sealed-jar tests tumble crushed feed with varying moisture, acid dose, and duration, then evaluate the product: granule size distribution, green and cured drop strength, fines release after sieving, and permeability measured as drainage rate under a static head. This tier fixes the retention time, moisture, and acid dose for the drum design and typically needs 50–200 kg of representative sample per variant.

Tier 3 — Column tests. Column percolation leaching on cured, agglomerated feed — typically 1–4 m columns at 100–200 mm top size — measures recovery, solution flow behavior, and acid consumption under conditions that respect bed structure. Comparing a column of agglomerated feed against one of untreated feed on the same ore quantifies the agglomeration uplift directly, and that measured delta is the number that justifies the drum in the financial model. Residence-time reasoning for the columns, and for the heap itself, follows the method on our leach residence time calculation page.

A practical rule: the column is the referee. If bottle-roll recovery is 85% but the agglomerated column stalls at 65%, the gap is hydrology, and hydrology is exactly what acid curing and agglomeration were hired to fix.

Troubleshooting Acid Curing and Agglomeration: Six Failures and Their Fixes

Field problems in acid curing cluster into six recognizable patterns, presented below as phenomenon, cause, and remedy.

1. Weak cure leading to slumping and channeling. Phenomenon: heap surface ponds within weeks of start-up, toe flow is fast and barren, and excavated areas show intact, unleached ore blocks. Cause: acid dose or moisture below the level needed to set the granule structure, so agglomerates break down under stacking load and release migrating fines. Remedy: re-run cure testing at higher acid and moisture, raise the in-drum dose in 5 kg/t increments while monitoring acid consumption in the raffinate circuit, and audit stacking drop heights in parallel — the two causes produce identical symptoms.
2. Over-wetting and mud balls. Phenomenon: drum discharge contains large, wet, plastic lumps rather than discrete granules; material builds up on drum walls and carry-back scorches the shell. Cause: total liquid addition — acid solution plus wash water plus raffinate moisture — exceeds roughly 12% w/w, collapsing interparticle friction so the charge slumps instead of cascading. Remedy: install or recalibrate a moisture analyzer on drum feed, close the water/acid ratio control loop against the setpoint, and interlock the acid pump with the belt scale so the liquid-to-ore ratio survives feed rate swings.
3. Uneven acid distribution. Phenomenon: cure piles with sharply bimodal pH — smoking acid-rich zones beside nearly neutral material; downstream pH excursions; patchy heap wet-up. Cause: single-point acid addition onto a non-cascading bed, plugged distributor ports, or acid added before the drum rather than into the falling curtain. Remedy: relocate addition to a multi-port spray bar across the curtain, add a recirculating flush to keep ports open, and verify distribution by tray sampling across the discharge belt.
4. Drum short-circuiting. Phenomenon: retention measured by tracer or visual testing comes out under 20 s; fines remain uncoated in the discharge. Cause: slope or speed above design, damaged lifters or dam rings, or L/D below 2.5:1 from a reused or undersized drum. Remedy: measure actual retention with a tracer, restore dam height, and re-derive the slope-speed setpoint from the required holdup.
5. Agglomerates break down in storage or on the belt. Phenomenon: granules intact at drum discharge but degraded to fines by the time they reach the stacker. Cause: ripening interrupted — the cure needs its 4 h to several days of hold before handling — or transfer points with excessive drop heights and belt scraper damage. Remedy: schedule a minimum cure stockpile inventory equal to the ripening requirement, and reduce every transfer drop to under 1.5 m (5 ft) where practical.
6. Escalating acid consumption without recovery gain. Phenomenon: acid usage per tonne of copper climbs month over month while recovery stays flat. Cause: carbonate gangue consuming acid neutrally, or chloride-bearing cure liquor raising acid demand in sulfide zones. Remedy: re-run the acid consumption curve on current ore blends, and shift the dose strategy from a fixed kg/t to a grade- and mineralogy-based recipe fed from the mine plan.

What Does Acid Curing Cost?

The cost stack has three components: acid, capital, and power — in that order of magnitude for most oxide projects.

All figures are indicative planning ranges, confirm with test work and vendor quotations.

Acid. The acid curing dose of 10–50 kg H₂SO₄ per tonne overlaps the solution-phase acid consumption, and on well-run oxide heaps total acid consumption typically lands at 20–60 kg/t of ore (typical range, confirm with test work; wet-basis ore, concentrated acid metered at the drum). At delivered acid prices of roughly $150–250/t H₂SO₄ (indicative, region-dependent), the cure contribution alone runs about $2–10 per tonne of ore. Because cured heaps recover copper faster and more completely, the acid is usually the cheapest reagent in the project; the sensitivity analysis in the financial model should still test acid price at ±30%.

Drum capital. A rotary drum agglomerator serving 500–1,500 t/h of crushed ore — materials of construction from mild steel with rubber lining to 316L for chloride service — carries indicative installed equipment costs of roughly $0.5–2.5 million (indicative budgetary range only, excludes civils, acid storage, and stacking systems; confirm with vendor quotations). Adding the acid distribution skid, storage, and controls typically brings the package to the upper half of that band.

Power. Drum drive power scales with holdup and speed; a practical planning figure is 0.3–0.7 kWh per tonne of ore agglomerated (typical range, confirm with vendor data). At $0.08/kWh this is under $0.06/t — negligible against acid, which is exactly why design effort should be spent on dose optimization rather than drive efficiency.

The avoided costs complete the picture. A 5-point recovery uplift on an ore containing 5 kg of copper per tonne is 0.25 kg of additional copper per tonne of ore treated — worth several dollars per tonne at any plausible copper price, dwarfing the $2–10/t cure cost.

A Synthetic Worked Example: Sizing an Agglomeration Drum for a 20,000 t/d Oxide Heap

The following example is synthetic and declared as such: the numbers are chosen to be representative for a copper oxide project, not drawn from any specific operation. All figures are wet-basis ore unless stated.

Feed and dose. Throughput is 20,000 t/d = 833 t/h at 92% availability of the crushing-agglomeration line (8,066 h/y assumed). Cure testing on this hypothetical ore — 1.1% total Cu, 0.75% acid-soluble Cu, moderate carbonate — returns an optimum in-drum dose of 25 kg H₂SO₄/t and 10% final moisture, with 48 h ripening. Daily acid demand is 20,000 × 25 = 500 t/d of acid; at $180/t delivered (indicative), cure acid costs $90,000/d, or $4.50/t of ore.

Drum holdup. At the design retention time of 45 s, required holdup is 833 t/h × (45/3,600) h = 10.4 t. At a loose cured-bulk density of 1.6 t/m³, that is 6.5 m³ of live bed. With a design fill fraction of 12% of internal volume, required internal volume is 6.5 ÷ 0.12 ≈ 54 m³.

Drum geometry. Adopting L/D = 3:1, the internal volume of a drum of diameter D and length 3D is π(D/2)² × 3D ≈ 2.36 D³. Setting 2.36 D³ = 54 gives D³ ≈ 22.9, so D ≈ 2.85 m and L ≈ 8.5 m (9.4 ft × 27.9 ft) — a compact machine, which is why agglomeration duty is often filled by a single drum even on large heaps.

Speed check. Critical speed for a 2.85 m drum is 42.3/√D ≈ 25 rpm. Operating at 35% of critical gives 8.8 rpm, and peripheral speed is π × 2.85 × 8.8/60 ≈ 1.31 m/s (4.3 ft/s) — inside the 0.8–1.5 m/s design window, confirming the geometry and speed are self-consistent.

Drive power. At 0.5 kWh/t (mid-range planning figure), drive demand is 833 × 0.5 ≈ 420 kW installed, with the operating draw typically lower.

Ripening inventory. A 48 h cure requires two days of stacked, cured inventory ahead of irrigation — here 40,000 t, or roughly a 4–5 m deep, 2-hectare cure pad section. The land and loader hours for that stockpile are real costs that first-time heap developers underestimate; they belong in the capex and opex models next to the drum itself.

Benefit cross-check. If agglomeration and curing lift column recovery on this ore from 62% to 72% of contained copper — the kind of uplift Tier 3 test work measures on clay-rich feeds — the additional copper is 20,000 t/d × 1.1% × 0.10 = 22 t/d of copper. Against $90,000/d of acid and a low-seven-figure drum package, the economics of acid curing are not close; the risk is not whether to agglomerate but whether the dose and handling are right.

Frequently Asked Questions

What does acid curing actually do to the ore?

Acid curing applies concentrated sulfuric acid, often with water, to crushed ore and holds it for hours to days so acid-soluble minerals begin dissolving before dilute leach solution reaches them. It also initiates gangue reactions — carbonate neutralization, clay destabilization — that consume acid early in a controlled, accessible setting. The result is faster wet-up, higher and steadier recovery, and fewer fines-related flow problems once the heap is irrigated.

How much acid is used in a typical cure?

Typical cure strengths range from 10 to 50 kg of H₂SO₄ per tonne of ore (typical range, confirm with test work). High acid-soluble copper with carbonate gangue pushes toward the upper half of the band; low-carbonate, low-grade oxide ores often cure successfully at 15–25 kg/t. The correct number comes from cure testing across a dose series, cross-checked against the total acid consumption curve for the ore, not from copying a neighboring project.

How long should the cure or ripening period last?

Ripening runs from roughly 4 hours to 7 days depending on mineralogy (typical range, confirm with test work). Simple acid-soluble oxide ores usually need 24–48 hours; chalcocite and other secondary sulfides benefit from multi-day cures, often with chloride additions to accelerate the sulfide reaction. The practical constraint is inventory: the ripening time equals the stockpile residence you must build and fund ahead of irrigation.

Why are chlorides added to some cures?

Chloride forms stable cuprous chloride complexes that pull the dissolution of chalcocite, covellite, and other secondary copper sulfides forward, so a chloride-bearing cure — typically 5–20 kg Cl⁻ per tonne of ore (confirm with test work) — can convert a sluggish secondary sulfide feed into one that responds like an oxide. The trade-offs are corrosion of downstream equipment, tighter liquor chemistry control, and careful management of chloride in barren solution and environmental discharge streams.

Can agglomeration work without acid, or acid without agglomeration?

Yes, and each alone delivers about half the benefit. Water-only agglomeration fixes fines migration but leaves the chemistry cold; acid curing without tumbling improves dissolution but leaves the fines free to migrate and slump. Most copper oxide projects combine both in one drum because the incremental cost of acid in the binder liquor is small. Gold-silver oxide heaps frequently agglomerate with water or weak cyanide solution instead, since their chemistry differs.

What agglomeration test work is the minimum before design?

Minimum credible scope: a dose-series cure study, small-scale agglomeration tests fixing retention time, moisture, and binder chemistry with strength and permeability measurements, and paired columns — agglomerated versus untreated feed — to quantify the recovery uplift. Bottle rolls establish chemistry but cannot see bed hydrology. Skipping the paired columns leaves the project justifying the drum on literature numbers instead of its own ore, which lenders and boards rightly challenge.

How does stacking damage cured ore, and what is acceptable?

Tipping from excessive height, haul-truck traffic on fresh lifts, and the dead load of subsequent lifts all crush agglomerates and re-release fines. Practical limits: keep tipping drop to about 2–3 m (6–10 ft) or less, keep trucks off freshly cured material, and favor low-drop stacking methods where topography allows. Sites that treat the cured granule as a fragile product hold heap permeability through many lifts; sites that do not lose the benefit within the first one.

Conclusion

Acid curing is the highest-leverage, lowest-cost step in a copper oxide heap leach flowsheet: a 10–50 kg/t acid dose and one well-sized rotary drum routinely buy 5–15 points of recovery, faster wet-up, and immunity to the fines migration that quietly strands metal in every uncured heap. The design logic is disciplined but not complex — fix retention time and dose from test work, size the drum from holdup arithmetic, protect the cured granule through stacking, and let paired columns prove the uplift before capital is committed. If you are evaluating agglomeration for a new heap project or troubleshooting an existing one, request a quote with your ore characteristics, crush size, and tonnage, and our process team will review your drum, cure, and stacking design against your specific ore before capital is committed. The full set of process guides is in our resource library, and heap-specific practice continues on our heap leaching page.

References

1. U.S. Geological Survey, Mineral Commodity Summaries — Copper. Reston, Virginia: U.S. Department of the Interior.
2. Society for Mining, Metallurgy & Exploration (SME), Mineral Processing and Extractive Metallurgy Handbook — sections on heap leaching, ore pretreatment, and agglomeration.
3. F. Habashi, A Textbook of Hydrometallurgy. Métallurgie Extractive Québec — chapters on leaching of oxide and secondary sulfide copper ores.
4. Perry’s Chemical Engineers’ Handbook, 9th edition — rotary drum equipment sizing, retention time, and scale-up principles.
5. Hydrometallurgy (Elsevier journal) — peer-reviewed literature on sulfuric acid curing, agglomerated heap leaching, and chloride-assisted curing of secondary copper sulfides.
6. D. G. Dixon, heap leach modeling and agglomeration literature in extraction and process metallurgy conference volumes (MetSoc of CIM / TMS).
7. Mining Engineering (SME) journal, articles on heap leach stacking, agglomeration, and cure practice.

Changelog

• 2026-11-08: First published.

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