Copper heap leaching with sulfuric acid: concentration and consumption basics require separating the free acid strength of the circulating solution from the total acid consumed by the ore. Crushed copper ore is stacked in lifts and irrigated with raffinate from solvent extraction, re-acidified to maintain free H₂SO₄ at 5–20 g/L. Pregnant leach solution from oxide and mixed sulfide heaps typically carries 1–5 g/L Cu at pH 1.5–2.0. Heap lift heights are commonly 6–10 m, and solution application rates are frequently 0.1–0.3 L min⁻¹ m⁻². Sulfuric acid consumption is reported as kilograms H₂SO₄ per dry metric ton of ore and is dominated by gangue reactions, not copper grade. Free acid concentration and acid consumption are linked because the heap is a flowing acid reactor with a moving neutralization front. Insufficient free acid slows chrysocolla and malachite dissolution; excessive free acid accelerates gangue attack and transfers iron, aluminum, and silica into solution.

Solution free acid is measured by titration. pH is determined electrometrically in accordance with ASTM D1293-18, and acidity is determined by titration in accordance with ASTM D1067-16. pH should not be converted directly to free acid concentration because sulfate speciation and ionic strength alter the second dissociation of sulfuric acid. In a commercial heap, the vertical acid profile is non-uniform. Bulk raffinate may be controlled at 10 g/L free acid while the lower portion of a lift can show temporarily higher residual acid after carbonate depletion. Early-cycle acid consumption is commonly elevated as calcite and dolomite release CO₂; later-cycle consumption declines but does not disappear because silicate gangue continues to react.

What Determines the Effective Acid Concentration in a Heap?

The effective acid concentration at a mineral surface is lower than the bulk irrigation concentration because protons are consumed by carbonate minerals, chlorite, biotite, and feldspar along the solution path. For oxide copper minerals, design guidance generally sets bulk raffinate acid at 5–15 g/L H₂SO₄, with pH values from 1.5 to 2.0. The lower threshold of 5 g/L is not a copper solubility limit but an operational safeguard against slow chrysocolla dissolution, particularly in silicate-hosted copper mineralization. Above 20 g/L, silica dissolution, ferric iron extraction, and gypsum precipitation can become more significant. These are consistency boundaries rather than thermodynamic discontinuities; the actual setpoint is selected from column test data and solvent extraction constraints.

Acid strength is also influenced by solution contact time and preferential flow. Drip emitters create localized wetted zones, and acid may bypass low-permeability ore domains. In such cases, increasing bulk acid concentration does not necessarily raise the effective acid concentration inside dense fine-grained regions. Operators therefore use total acid flux, the product of free acid concentration and irrigation rate, as the more informative control variable. A heap receiving 0.2 L min⁻¹ m⁻² at 10 g/L free acid delivers approximately 2.88 kg H₂SO₄ m⁻² day⁻¹, equivalent to 28.8 t day⁻¹ over 10,000 m² of active pad area.

Acid consumption forecasting is based on column percolation testing conducted under ASTM E2242-13, not on static rock digestions alone. Column tests charge crushed ore into columns of 100–300 mm diameter, irrigate with dilute sulfuric acid, and measure net acid consumption over sequential leach cycles. The results are reported as kilograms H₂SO₄ per dry metric ton of ore. Split-core column tests on the same orebody frequently show relative differences of 10–20% in acid consumption due to spatial distribution of carbonate stringers. For low-carbonate oxide ore, copper dissolution acid requirement alone is often less than 5 kg H₂SO₄/t.

When Gangue Mineralogy Dictates Sulfuric Acid Consumption

Acid consumption in copper heap leaching is controlled less by copper grade than by gangue mineral reactivity. Calcite neutralizes acid according to CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O. Stoichiometrically, 1.00 kg of calcite consumes approximately 0.98 kg H₂SO₄. Dolomite consumes approximately 1.06 kg H₂SO₄ per kilogram of mineral because two moles of acid react with one mole of dolomite. An ore containing 2.0 wt% calcite contains 20 kg calcite per metric ton, resulting in approximately 19.6 kg H₂SO₄/t acid demand from calcite alone if complete reaction occurs. This demand is additive to copper mineral dissolution and slow silicate reactions.

The reaction product gypsum has limited solubility. Gypsum solubility in water at 25 °C is approximately 2.4 g/L as CaSO₄·2H₂O, and high sulfate process water can reduce this further. Where carbonate-rich ore is leached, gypsum precipitation can blind drip emitters, coat particle surfaces, and reduce heap permeability. Field-scale acid consumption above 80 kg H₂SO₄/t is typically uneconomic for direct acid heap leaching unless ore blending or pre-neutralization of carbonate gangue is implemented. Ferric-bearing silicates such as biotite and chlorite consume acid through incongruent dissolution, releasing magnesium, iron, and aluminum. Feldspar dissolution is slower but contributes to long-cycle acid demand. These silicate reactions can continue for months after carbonate neutralization has ceased, producing a time-dependent acid demand curve with an early carbonate spike and a later silicate tail.

Drip irrigation networks on commercial copper heaps are typically constructed with pressure-compensating emitters spaced at 0.5–1.5 m, delivering 1.0–4.0 L/h per emitter. At an application rate of 0.2 L min⁻¹ m⁻² and a free acid concentration of 10 g/L H₂SO₄, sulfuric acid delivery is approximately 2.88 kg m⁻² day⁻¹. This mass-balance relationship explains why acid concentration and irrigation rate must be considered jointly. Reducing acid concentration from 10 g/L to 8 g/L at constant irrigation lowers acid flux by 20%, which may be sufficient to reduce gangue attack but can also extend leach cycle duration. Published data for this specific configuration is limited, but the mass-balance relationship is direct and is used in heap leach control rooms.

Irrigation Rate, Pregnant Leach Solution Chemistry and Solvent Extraction Limits

The free acid concentration in the raffinate returning to the heap must remain compatible with copper solvent extraction. Pregnant leach solution pH below 1.5 can reduce copper extraction by shifting the extraction isotherm toward proton exchange and can transfer more ferric ion to the organic phase. Most solvent extraction circuits are designed for PLS pH of 1.5–2.0 and copper concentrations of 1–5 g/L Cu. Sulfuric acid addition is therefore not a simple copper recovery lever; it is balanced against solvent extraction selectivity, electrolyte acid carry, and impurity transfer. Monitoring pH per ASTM D1293-18 and acidity per ASTM D1067-16 provides the input for that balance. Sulfate in process water can be determined by ASTM D4327-17 to track gypsum scaling potential.

Concentrated sulfuric acid is usually injected into the raffinate return line, followed by a static mixer or in-line dilution skid to prevent localized high-acid zones. Direct acid addition to the heap surface without adequate mixing can create preferential acid channels and uneven leach kinetics. Total acid addition is the sum of acid added during agglomeration and acid injected into the raffinate. Plant mass balance requires daily reconciliation of acid addition, PLS acid, raffinate acid, and heap discharge acid. The difference is the apparent acid consumption for that operating period.

Agglomeration and Curing Practices Adjust Acid Demand Before Stacking

Before stacking, crushed ore may be agglomerated in rotating drums with sulfuric acid and water. Acid addition during agglomeration is typically 2–8 kg H₂SO₄/t, water addition is commonly 8–12 wt% for moist agglomerate stability, and cure duration is generally 24–72 h. A cure period allows acid to react with carbonate minerals and begin copper dissolution before irrigation starts. Concentrated sulfuric acid is added through metering pumps with dilution rings or injection lances because direct acid addition to dry ore can create hot spots and non-uniform agglomerate strength. After curing, the heap is built in lifts of 6–10 m and irrigated. Agglomeration acid is part of total consumption, so plant accounting must include both agglomeration and irrigation additions. Published data for exact agglomeration acid splits in commercial oxide copper heaps is limited; the range is site-specific and depends on crush size, clay content, and available cure time.

Monitoring Free Acid, Sulfate and Acid Mass Balance

Process control of acid concentration and consumption is anchored to daily mass balance: acid added to raffinate, acid in PLS, acid in raffinate, and acid in heap discharge. The difference is consumption. Sampling frequency depends on ore variability; online pH analyzers and routine titration are used at most operating plants. If free acid concentration drifts below 5 g/L, operator response may be to reduce irrigation rate rather than increase acid concentration to maintain total acid flux. Conversely, if free acid exceeds 20 g/L and copper recovery does not increase, acid addition is reduced to limit impurity dissolution. These control bands are applied with geological block data and column test results. No single acid concentration is universally optimal for all copper heaps; the operational window depends on mineralogy, crush size, cure efficiency, and solvent extraction circuit constraints.

Heap leach acid control is also constrained by local water chemistry. High calcium and sulfate concentrations in recycled process water promote gypsum scaling in drip lines and on heap surfaces. Silica dissolved from chrysocolla and silicate gangue can form colloidal silica and later contribute to crud formation in solvent extraction. These operational boundaries are managed by maintaining free acid within the tested range and by blending ore types rather than by relying solely on after-the-fact sulfate measurements. Acid consumption data from daily mass balance, column tests, and geochemical block models are therefore treated as interdependent inputs to heap leach management.