Leaching Recovery Calculation: Worked Examples (2026)

A leaching recovery calculation tells you, in one number, how much of the metal in the feed actually dissolved and reported to solution — for a typical copper oxide heap, a well-run operation extracts 75–90% of the copper and a gold CIL circuit 85–92% of the gold. This guide sets out the three standard accounting routes, the formulas behind each one, and fully worked numeric examples so you can reproduce every step on your own plant data.

By the Lexin Hydromet Process Engineering Team. All figures in the worked examples below are illustrative numbers chosen to demonstrate the arithmetic, not production data from any specific operation.

Key Takeaways

• There are three standard routes to a leaching recovery calculation: head-assay/tail-assay solid balance, solution (PLS) metal balance, and whole-flowsheet metal accounting — each answers a slightly different question.
• The assay-based formula E = (F − T)/F × 100% is simple, but only valid when head and tail assays are on the same dry-tonne basis and the residue has been fully washed.
• Assay-based and solution-based recoveries routinely disagree by 1–5 percentage points; the gap comes from moisture, sampling error, unwashed tail losses, and solution losses to ponds or spillage.
• Dry tonnes vs wet tonnes is the single most common arithmetic error; every assay must be corrected to a dry basis before it enters the balance.
• Recovery is not just a KPI — it is a design input. The recovery-vs-time curve sets residence time, which in turn sets tank volume and count in agitated circuits.

What “Leaching Recovery” Actually Means

Leaching recovery is a mass-balance ratio that expresses the fraction of a metal contained in the feed material that has been dissolved into the leach liquor by the end of the process. It is usually quoted as a percentage, and it is the number that plant managers report to owners, that EPC teams use to size downstream circuits, and that metallurgists interrogate when extraction drifts.

Tank Leaching vs Heap Leaching in practice

Leaching, step by step

Before doing any arithmetic, be precise about which recovery you mean, because the term is used loosely and the variants differ materially:

Dissolution (leach) recovery measures metal dissolved from the solid into solution within the leach itself. It ignores what happens afterwards — losses in washing, clarification, or entrainment. This is the number the assay-based formula returns.

Overall (flowsheet) recovery measures metal in the final saleable product — copper cathode, gold doré — divided by metal in the feed over an accounting period. It absorbs every intermediate loss, including solution left in tailings, spills, and electrowinning or carbon-processing inefficiencies. It is always equal to or lower than dissolution recovery.

Washed recovery is an intermediate concept used in heap and dump leaching: dissolution recovery minus the metal that remains entrained in the residue moisture after washing. A heap can dissolve 88% of the copper yet report only 84% to PLS if the residue leaves the pad carrying unwashed liquor at high tenor.

A pregnant leach solution (PLS) is a metal-bearing liquor that drains from a leach pad or overflows an agitated tank, and it is the reference stream for solution-based accounting. Because the choice of reference stream changes the answer, state explicitly in every report which recovery is being quoted. The broader context of how these definitions fit into leaching flowsheets is covered in our pillar resource.

Leaching Recovery Calculation: Formula Summary

The table below summarises the three standard routes. Every formula assumes SI-consistent units and dry-basis assays unless stated otherwise.

Two consistency rules apply to all three routes. First, mass and metal must both be on a dry basis: convert wet tonnages using W_dry = W_wet × (1 − m), where m is the moisture fraction. Second, the time window must match: a PLS balance over one shift is meaningless if the feed rate is a monthly average.

Route A: Head-Assay / Tail-Assay Solid Balance

The assay-based route is the fastest leaching recovery calculation and the only one available for laboratory bottle rolls and most column tests. You need two numbers: the metal assay of the feed solid and the metal assay of the washed residue solid, both on a dry basis and in the same units.

E = (F − T) / F × 100%

where F is the dry head assay and T is the dry tail assay. A tail assay is the metal content of the solid residue that remains after leaching and washing, determined by fire assay, acid digestion, or an equivalent analytical method.

Worked Example 1 — Copper Oxide Heap Leach (Assay Basis)

A composite of run-of-mine ore from an acid-curable copper oxide deposit is column-leached for 60 days. The numbers below are illustrative.

Given:

• Wet head sample: 1.080% Cu at 6.0% moisture (assay reported on a wet basis)
• Washed, dried residue: 0.170% Cu (dry basis)

1. Correct the head assay to a dry basis. The wet-basis assay must be divided by (1 − m): F = 1.080 / (1 − 0.060) = 1.080 / 0.940 = 1.149% Cu. Note that this correction changes the assay in the fourth significant figure at low moisture, but at 15–20% moisture the error from skipping it exceeds one full recovery point.
2. Apply the assay-based formula. E = (F − T) / F × 100% = (1.149 − 0.170) / 1.149 × 100% = 0.979 / 1.149 × 100% = 85.2%.
3. Sanity-check against acid-soluble copper. Sequential assay of the head shows acid-soluble Cu of 1.02% and total Cu of 1.149%. The maximum possible extraction on an acid leach is therefore 1.02/1.149 = 88.8%. The achieved 85.2% means 96% of the acid-soluble copper dissolved — a useful normalisation when comparing ores with different gangue mineralogy. If acid dosing looks high relative to this extraction, review our guide to acid consumption in copper oxide ores.

One caution on step 3: many oxide ores contain a copper fraction (chrysocolla, wad, cupriferous clays) that is only partially acid-soluble, so the acid-soluble ceiling is an estimate, not a physical law. Column testwork at representative crush size and drip density remains the definitive measurement — see our notes on leaching kinetics for how extraction curves develop over time.

Worked Example 2 — Gold CIL (Assay Basis with Dissolved-Loss Correction)

A carbon-in-leach (CIL) circuit treats 2.40 g/t Au head ore. CIL needs one extra step because unrecovered gold exists in two places: locked in the solid residue and dissolved in the solution entrained in the residue moisture. Illustrative figures:

Given:

• Dry head assay: F = 2.40 g/t Au
• Washed solid tail assay: T = 0.35 g/t Au
• Tail slurry moisture: 25% by mass (0.25 t solution per t dry solid)
• Dissolved Au in tail solution: 0.05 mg/L

1. Compute the solid-based recovery. E_solid = (2.40 − 0.35) / 2.40 × 100% = 2.05 / 2.40 × 100% = 85.4%.
2. Quantify the dissolved loss held in residue moisture. Each tonne of dry solid carries 0.25 t of solution ≈ 250 L. Dissolved Au lost = 250 L × 0.05 mg/L = 12.5 mg per tonne of solid = 0.0125 g/t equivalent.
3. Recompute recovery on a total-unrecovered basis. Total unrecovered = 0.35 + 0.0125 = 0.3625 g/t. E_total = (2.40 − 0.3625) / 2.40 × 100% = 2.0375 / 2.40 × 100% = 84.9%.
4. Judge the magnitude. A 0.5 percentage-point gap sounds small, but on a 2.0 Mt/a plant it is roughly 2.0 Mt × (0.854 − 0.849) × 2.40 g/t ≈ 24 kg of gold per year. Whether the tail thickeners or wash wheels justify upgrading is exactly this kind of calculation.

The lesson generalises: for precious-metal circuits, always ask whether the reported tail assay is solid-only or includes the moisture fraction, and always record tail slurry density and dissolved tenor alongside the solid assay. Without those three numbers, the leaching recovery calculation cannot be audited later.

Route B: Solution / PLS-Based Metal Balance

Where the assay route compares solids, the solution route compares metal in the liquor against metal in the feed. It is the standard method for operating heap leach and agitated circuits where PLS flow and tenor are measured continuously but head samples arrive only per shift or per day.

E = (Q_p × C_p) / (Q_f × G_f) × 100%

where Q_p is PLS flow rate, C_p is PLS tenor, Q_f is dry ore feed rate, and G_f is dry head grade. To be complete, the metal recovered from residue washing (wash liquor returning to the pond) should be added to the numerator, and metal lost in residual moisture leaving the circuit subtracted.

Worked Example 3 — Copper Heap Leach (Solution Basis)

Illustrative figures for a mid-size copper oxide operation:

Given:

• Dry ore feed: 180 t/h at 1.62% Cu
• PLS flow: 900 m³/h at 2.95 g/L Cu
• Residue (stacked tailings) moisture after washing: 18% by mass
• Copper tenor of solution entrained in residue: 0.28 g/L Cu

1. Compute metal in the feed. M_feed = 180 t/h × 1.62% = 180 × 16.2 kg/t = 2,916 kg/h Cu.
2. Compute metal reporting to PLS. M_PLS = 900 m³/h × 2.95 g/L. Since 1 m³ = 1,000 L, this is 900,000 L/h × 2.95 g/L = 2,655,000 g/h = 2,655 kg/h Cu.
3. Compute the entrained-solution loss. Residue solution flow = 180 t/h × 0.18 = 32.4 t/h ≈ 32.4 m³/h. Metal lost = 32.4 m³/h × 0.28 kg/m³ = 9.1 kg/h Cu.
4. Assemble the solution-based recovery. M_accounted = 2,655 + 9.1 = 2,664.1 kg/h. E_solution = 2,664.1 / 2,916 × 100% = 91.4%.
5. Compare against the uncorrected version. Ignoring the wash credit, 2,655 / 2,916 = 91.0%. The wash correction adds 0.4 points here; in a poorly washed operation the same correction can move the number by two points or more, because unwashed residue leaving at PLS tenor (2.95 g/L instead of 0.28 g/L) would carry roughly 96 kg/h of dissolved copper — a 3.3-point swing.
6. Close the balance and reconcile the residual. Metal unaccounted = 2,916 − 2,664.1 = 251.9 kg/h, about 8.6% of feed. In a real reconciliation this residual is decomposed into sampling error, assay bias, ore on pad not yet fully leached (inventory), solution losses to the pond, and evaporation. A persistent residual in one direction is a red flag for a metering or sampling problem, not a “lost” recovery.

The solution route is sensitive to instrument quality in a way the assay route is not. Flowmeter error propagates linearly: a 3% low bias on PLS flow understates recovery by roughly 3 points of the dissolved fraction. Tenor from on-stream analysers needs periodic cross-checking against laboratory titration or AAS.

The table below gathers the three worked examples so the arithmetic can be checked at a glance. Every figure is illustrative.

Why Do Assay-Based and Solution-Based Recoveries Disagree?

When both routes are applied to the same circuit over the same window, the leaching recovery calculation rarely returns the same number twice. Differences of one to five percentage points are normal, and the direction of the gap is diagnostic. The main mechanisms:

• Moisture and dry-basis inconsistency. If the head assay is dry-basis but the tonnage feeding the solution balance is wet, or if two laboratories dry samples at different temperatures, the two routes diverge systematically.
• Sampling bias. Head samples from a conveyor cut underestimate coarse fragments and overestimate fines; tail samples from a slurry line are biased by stream velocity gradients. Sampling standards from ISO and ASTM address cut frequency, cutter geometry, and sample mass — follow their generic principles (correct cutter opening, proportional sampling, adequate composite mass) even where a specific standard is not contractually invoked.
• Unabridged tail wash losses. The solid tail assay only measures metal locked in particles. Metal dissolved in the residue moisture is invisible to Route A but real to Route B. This is the classic reason solution-based recovery reads lower than assay-based recovery.
• Solution losses outside the measured envelope. Pond seepage, evaporation, spillage, and water balance exports carry dissolved metal that the PLS flowmeter never sees. Route B then overstates recovery; whole-flowsheet accounting exposes the difference.
• Inventory effects. In heaps, metal dissolved today may not reach PLS for days or weeks. Over short windows the solution route misattributes inventory changes to recovery; only long-window or flowsheet-level accounting settles it.
• Locked-cycle versus single-pass thinking. A single-pass laboratory test measures one contact. Locked-cycle testing is a procedure that recycles intermediate streams — wash liquors, barrens, middlings — through successive cycles until the build-up of dissolved species stabilises, so the measured recovery reflects what a recirculating plant can actually achieve. Single-pass results typically look better than locked-cycle results because impurity build-up and competing losses are absent.

The professional habit is to run both routes monthly, publish the gap, and investigate any systematic drift. A shrinking or growing gap tells you more about plant health than either number alone.

Which Accounting Pitfalls Most Often Distort Leaching Recovery Calculation?

Of these, the dry/wet tonne error deserves emphasis because it is silent: the arithmetic looks perfect, and the wrong answer is self-consistent. Make dry-basis conversion a documented, checklisted step rather than a habit.

How Does Leaching Recovery Feed Tank and Residence-Time Sizing?

Recovery is not only an outcome — during design it is an input. A leaching recovery calculation performed on kinetic testwork is what converts a target extraction percentage into vessel volume, which makes it the first step of tank sizing rather than the last line of a monthly report. Sizing an agitated leach circuit is an exercise in choosing the residence time at which the target recovery is achieved at acceptable reagent and power cost, then converting that time into vessel geometry.

Leaching Reactor Volume Calculator from Residence Time, step by step

The logic chain runs as follows. Kinetic testwork produces an extraction-versus-time curve, typically fitted to a model such as E = E_max × (1 − e^(−kt)) or a shrinking-core variant. From the target recovery — say 88% when E_max is 92% — you solve for the required time t. Residence time then fixes total slurry volume:

V_total = Q_slurry × t

where Q_slurry is the design slurry volumetric flow. Dividing V_total by a chosen tank working volume (with 10–15% freeboard and a settling-safe agitator aspect ratio) gives tank count. A worked illustration: at 250 t/h dry solids, 50% solids by mass, and slurry density 1.52 t/m³, slurry flow is roughly (250/0.5) / 1.52 ≈ 329 m³/h. If the kinetics demand 24 h of residence, V_total ≈ 7,900 m³ — eight tanks of 1,000 m³ working volume. Halve the target recovery-informed residence time and you halve capital; push recovery two points higher and the last tanks may earn nothing but operating cost. This trade-off, and the agitator power and gas-dispersion constraints that accompany it, is developed in our agitated leaching tank design guide, with the corresponding equipment options described at agitated leaching tanks.

Because the recovery-time curve is the hinge of the whole sizing exercise, its accuracy matters more than any single number on the P&ID. Curve extrapolation beyond the tested range is where designs go wrong; a pilot campaign de-risks it, and our pilot testing team can take an ore sample through column or continuous pilot work to a defensible design curve. Budget contingency should also recognise that ore hardness, clay content, and agglomeration quality all shift the curve between the test product and the commercial feed.

Metal Accounting Good Practice

A leaching recovery calculation is only as credible as the measurement chain beneath it, and the following practices keep that chain auditable: The following practices separate defensible metallurgical accounting from convenient arithmetic:

• Check assays independently. Submit split samples to a second laboratory at a fixed frequency (commonly 1 in 20) and track the bias. Inter-laboratory drift of 0.05 g/t on a gold tail assay moves recovery by two points.
• Follow sampling standards generically. ISO and ASTM sampling standards codify cutter geometry, sample mass, and cut frequency for a reason: most recovery disputes trace back to a sampling violation, not to chemistry. Adopt the principles — even where certification is not required.
• Reconcile across three levels. Monthly, compare (a) assay-based, (b) solution-based, and (c) product-based recovery. Persistent disagreement above about two points is an audit item.
• Track inventory explicitly. Metal on the pad, in tanks, in carbon, and in the pond is not recovery. A reserve estimate that treats inventory drawdown as extraction will flatter a shrinking operation and punish a growing one.
• Document the basis. Every published recovery figure should carry a footnote: dry basis, accounting window, wash-loss treatment, and assay method. Ambiguity here is how two truthful people report different numbers from one plant.
• Certify the balance chain. Weighbridge calibration, flowmeter proving, and analyser cross-checks belong on the same maintenance schedule as any rotating equipment, because they are, economically, equipment.

FAQ (Frequently Asked Questions)

What is the difference between leaching recovery and extraction?
In most hydrometallurgical usage the two are synonyms: both describe the fraction of metal dissolved from the feed solid. Some organisations reserve “extraction” for the solvent-extraction (SX) step specifically, so on a copper SX-EW flowsheet “leach extraction” and “SX extraction” are different numbers. Always define the term on the first page of any report.

Can I calculate leaching recovery from head and tail assays alone?
Yes — E = (F − T)/F × 100% — provided both assays are on the same dry basis and the tail has been washed so that dissolved metal in residual moisture is not part of the picture. For precious metals, also record tail slurry moisture and dissolved tenor so the dissolved loss can be added. Without the wash condition, the assay route flatters recovery.

Why is my solution-based recovery lower than my assay-based recovery?
Usually because dissolved metal is leaving in residue moisture, pond seepage, or spillage — streams the PLS meter does not capture but the solid tail assay partially ignores. Systematic instrument bias on flow or tenor is the second suspect. Run both routes over the same accounting window and decompose the gap stream by stream.

How do I correct assays for moisture?
Convert wet-basis assays to dry with A_dry = A_wet / (1 − m), where m is the moisture fraction (6% moisture means m = 0.06). Convert tonnages the opposite way: dry tonnes = wet tonnes × (1 − m). The correction is small at 5% moisture but exceeds two recovery points at 20% moisture on a 1.5% Cu ore — never skip it.

What recovery should I promise in a feasibility study?
Use the recovery consistently achieved across the full testwork programme — column, locked-cycle, and pilot — at design head grade and mineralogy, then apply the discount your organisation’s standard requires. Promising the best bottle-roll result is the fastest route to a plant that underperforms its model and a board that asks why.

How often should leaching recovery be recalculated?
Operationally, daily from the solution balance and monthly from the full three-route reconciliation. Strategically, recalculate whenever feed source, crush size, or reagent regime changes materially, since each shifts the recovery-time curve that your residence-time design depends on.

Does higher leaching recovery always mean more profit?
Not necessarily. The marginal recovery comes at the price of longer residence time, more acid or cyanide, more wash water, and larger capital per tonne of metal. The optimum is where the marginal value of recovered metal equals the marginal cost of recovering it — a curve, not a maximum.

Conclusion

A leaching recovery calculation is straightforward arithmetic sitting on top of demanding measurement discipline. The three routes — solid assay balance, solution/PLS balance, and whole-flowsheet metal accounting — will agree within a point or two only when dry-basis conversion, representative sampling, adequate residue washing, and matched time windows are all in place. The worked examples above show that a skipped moisture correction or an unmeasured tail-moisture loss can move the answer by several points, which on a large copper or gold operation translates into tonnes of unaccounted metal and real design errors in tank sizing.

Treat recovery as a designed quantity, not just a reported one: anchor it to a tested recovery-time curve, size the tanks from that curve, and keep the three accounting routes in monthly reconciliation. For ore-specific testwork, equipment selection, or an independent review of your flowsheet mass balance, contact the Lexin Hydromet process engineering team — we supply custom leaching, solid-liquid separation, SX, and evaporation-crystallisation equipment and support the testwork that de-risks it. More calculation guides and technical resources are available in our /resources/ hub.

References

1. Society for Mining, Metallurgy & Exploration (SME). SME Mineral Processing Handbook. SME, Littleton, CO.
2. Wills, B.A. & Finch, J.A. Wills’ Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery, 8th edition. Butterworth-Heinemann.
3. Havlík, T. Hydrometallurgy: Principles and Applications. CRC Press / Woodhead Publishing.
4. Marsden, J.O. & House, C.I. The Chemistry of Gold Extraction, 2nd edition. Society for Mining, Metallurgy & Exploration (SME).
5. Davenport, W.G., King, M., Schlesinger, M. & Biswas, A.K. Extractive Metallurgy of Copper, 4th edition. Pergamon/Elsevier.
6. U.S. Geological Survey (USGS). Mineral Commodity Summaries (copper and gold chapters), annual editions. U.S. Department of the Interior.
7. ISO and ASTM International sampling standards — generic principles of particulate material sampling (cutter design, sample mass, cut frequency) as referenced in plant sampling practice.

Changelog

• 2026-10-19: First published.