Thiosulfate Leaching vs Cyanide: Cost and Recovery

Last updated: 2026-11-02

Thiosulfate leaching typically recovers 85-95% of gold from carbonaceous and copper-bearing ores where cyanidation falls below 60%, at an indicative reagent cost of USD 110-400 per ounce recovered versus USD 55-175 per ounce for cyanide on free-milling ore. This page gives process engineers the chemistry, cost structure, recovery routes, equipment deltas, and ore-by-ore verdicts needed to commit.

Key Takeaways

• Thiosulfate leaching wins decisively on carbonaceous, copper-gold, and arsenic-bearing refractory ores where cyanide underperforms.
• Gold-thiosulfate complexes load poorly on activated carbon, so recovery requires resin-in-pulp, ion exchange, or cementation.
• Consumption of 10-40 kg/t thiosulfate dominates operating cost; degradation to tetrathionate and sulfate is the main driver.
• Cyanide stays cheaper on free-milling oxide ore: lower reagent cost, proven carbon recovery, and mature detox practice.
• Confirm kinetics and consumption with bottle roll, column, and continuous mini-plant work — start from the leaching hub.

What Is Thiosulfate Leaching and How Does the Chemistry Work?

Thiosulfate leaching is a hydrometallurgical gold extraction process that dissolves metallic gold as the stable anionic complex Au(S2O3)2³⁻ in an alkaline, oxygen-containing thiosulfate solution. Preg-robbing is a behavior in which natural carbon in the ore re-adsorbs dissolved gold that is already in solution, silently stealing recovery from the pregnant liquor. Tetrathionate is a polythionate oxidation byproduct (S4O6²⁻) that forms when thiosulfate degrades during leaching, consuming reagent and fouling downstream recovery media.

In the standard ammonia-thiosulfate (ATS) system, the gold dissolution reaction is:

4 Au + 8 S2O3²⁻ + O2 + 2 H2O → 4 Au(S2O3)2³⁻ + 4 OH⁻

Copper is the workhorse catalyst. Cu(II), added as copper sulfate and held in solution as the ammine complex Cu(NH3)4²⁺, oxidizes Au(0) to Au(I); Cu(I) is then re-oxidized by dissolved oxygen, closing the redox cycle. Ammonia does double duty: it stabilizes Cu(II) in solution and buffers the pulp in the pH 9.5-10.5 window where the gold complex is stable and ammonia loss is manageable. Oxygen demand is modest compared with pressure oxidation, but it is real — starve the system of O2 and Cu(II) collapses, and leaching stops within hours. Kinetics at a healthy catalyst concentration are broadly comparable to cyanidation, though the exact rate depends on ore mineralogy and must be confirmed by test work.

Two practical notes. First, thiosulfate is consumed both by the (desired) gold complexation pathway and by oxidative side reactions, so free thiosulfate must be measured and topped up on assay, not by timer. A solution assay suite covering thiosulfate, tetrathionate, copper, and free ammonia belongs in the operating manual from day one, because every control decision in this system reads off those four numbers. Second, unlike cyanidation, the system tolerates saline and brackish make-up water well, which matters for operations in water-stressed regions — an advantage worth checking early in your water balance.

Why Does Thiosulfate Leaching Suit Carbonaceous, Copper-Bearing, and Refractory Ores?

The decisive property of the Au(S2O3)2³⁻ complex is that activated carbon adsorbs it only weakly. In a conventional cyanide circuit, preg-robbing carbonaceous matter strips aurocyanide out of solution faster than carbon can recover it, and recovery can collapse below 60% — sometimes far below. In a thiosulfate system the natural carbon has almost nothing to grab, so the gold stays in solution until engineered resin or cementation recovers it. For ores where organic carbon content exceeds roughly 0.5-1.0% (indicative), switching lixiviant is usually more economic than adding kerosene blanketing or pre-flotation de-carbonization. The practical screening test is simple and cheap: run paired bottle rolls, one cyanide and one thiosulfate, on the same carbonaceous sample and compare the solution profiles. When the cyanide curve rolls over mid-leach while the thiosulfate curve keeps climbing, the ore has told you which circuit it belongs in. Quantify the gap before designing anything: a recovery delta of five points on a 2 g/t ore is a different business case than a delta of thirty points, and the delta is what pays for the higher reagent consumption and the resin circuit.

Copper-gold ores are the second clear-cut case. Cyanidation of copper-bearing ore burns NaCN at runaway rates — each percent of reactive oxide copper can consume multiples of the cyanide needed for gold alone — and the resulting weak-acid-dissociable copper complexes load the detox circuit and the carbon. The consumed cyanide is pure operating cost with no recovery benefit, and on some porphyry copper-gold ores the cyanide bill alone has made projects uneconomic at conventional flowsheets. Thiosulfate inverts the problem: dissolved copper is the catalyst the circuit needs, and copper minerals dissolve slowly enough that they mostly serve the metallurgy rather than poison it. The copper inventory becomes a controlled reagent, dosed and assayed like any other, instead of an uncontrolled consumer. Arsenian and antimonial refractory ores (arsenopyrite, realgar, stibnite-hosted gold) round out the list; these sulfide matrices resist cyanidation through passivation and speciation effects, while thiosulfate systems attack fine gold in sulfide-hosted textures more effectively, especially after fine grinding. Where the sulfide host is highly refractory, expect a pre-oxidation step upstream of either lixiviant; thiosulfate then competes on recovery and detox cost rather than on raw chemistry. Compare the hardware implications against conventional cyanide leaching equipment before locking a flowsheet.

How Is Gold Recovered From Thiosulfate Leaching Solutions?

This is the single biggest engineering difference between the two routes. Aurocyanide loads onto activated carbon at commercial loadings with ease; the aurothiosulfate complex does not. Carbon loadings from thiosulfate liquors are typically one to two orders of magnitude lower (indicative), which is why a straight CIP/CIL copy of a cyanide plant will not work. Three recovery routes are used instead:

Strong-base resin in a resin-in-pulp (RIP) flowsheet is the industry reference case for thiosulfate gold plants. Resin loadings in the range of 5-20 kg Au per tonne of resin (indicative, ore-specific) are reported in the research literature, with elution achievable without high-pressure cyanide-stripping hardware. The trade is selectivity management: copper ammine complexes and tetrathionate compete for resin sites, so a copper removal or resin-wash step belongs in the design. Elution is the second benefit: strong-base resin strips at ambient pressure with an acidic thiocyanate or nitrate eluant in typical flowsheets studied to date, so the high-pressure, high-temperature Zadra-style stripping hardware of a cyanide plant disappears. Budget for resin inventory, safety screen design, and a resin-wash circuit in place of carbon regeneration; the flowsheet is different machinery, not more machinery. Where your project keeps a cyanide option open, review the benchmark carbon flowsheet in our CIL vs CIP comparison and translate your metallurgical assumptions with the leaching recovery calculation method — the recovery-percentage arithmetic is identical, only the media change.

What Does Thiosulfate Leaching Cost per Tonne and per Ounce?

Reagent consumption is where thiosulfate systems live or die economically. NaCN dosing on a well-run free-milling circuit is 0.3-1.0 kg/t. Thiosulfate dosing is 10-40 kg/t — one to two orders of magnitude higher — because a large fraction of the reagent oxidizes to tetrathionate and ultimately sulfate through side reactions with Cu(II), sulfide minerals, and dissolved oxygen before it ever complexes gold. Ammonia make-up of 2-8 kg/t is typical where condensation and scrubber recycle are absent; well-designed circuits cut this substantially. Copper catalyst losses of roughly 0.1-0.5 kg/t CuSO4 equivalent (indicative) follow the copper tenor of the ore. None of these numbers is a rule of thumb you can build a bankable model on — they are ranges to be fixed by test work.

Assumptions stated: per-ounce figures convert the per-tonne totals at 2.0 g/t head grade with 85% (thiosulfate) and 90% (cyanide) recoveries, i.e. 1.70 and 1.80 g Au per tonne of ore respectively; reagent unit costs are indicative and vary by region and contract. Read the table honestly: on refractory and preg-robbing ores the cyanide column also carries flotation, blanketing, or oxidation pre-treatment plus failed-recovery write-offs, which is exactly where the thiosulfate column wins even at higher reagent spend. On genuinely free-milling oxide ore, it does not — see the decision table below.

Energy and consumables beyond reagents sit in a similar band for both routes at agitated-tank scale: mill and classifier power dominate either flowsheet, and agitator plus aeration power runs at broadly comparable intensity. Thiosulfate leaching circuits do add compressor or oxygen-supply capacity for the Cu(II) regeneration duty, and a small acid or caustic dosing package for pH trim, but they delete the detox circuit’s SO2 supply, lime, and air-blowing load entirely — on many sites that deletion offsets the added oxygen cost within 10-20% (indicative). Where a heap option is evaluated, thiosulfate dripper-line and pond materials must be checked against ammonia, which standard cyanide-compatible polyethylene typically tolerates but fittings and elastomers may not. Confirm every line of this paragraph against vendor quotes and your site power tariff; none of it substitutes for a site-specific estimate.

How Do Equipment, Materials, and Effluent Requirements Differ?

Ammonia is hard on metallurgy. Brass, bronze, and other copper-bearing alloys are excluded from pumps, pipework, and instrument fittings because ammoniacal solutions attack them; wetted agitator and pump components move to 316L stainless steel or comparable alloys. Carbon steel may survive in selected low-corrosion duties after test work confirms it, but budget stainless for the contactors. You also delete an entire plant section: there is no cyanide destruction circuit (no SO2/air INCO loop, no peroxide plant), which removes meaningful capex and a skilled-operator burden from the flowsheet.

What replaces it: a resin-in-pulp or resin-in-column contactor train with associated elution and regeneration, in place of carbon columns and a 650-700 °C regeneration kiln. Agitated tanks see one further difference — with no suspended carbon to keep in suspension, pulp density in agitated thiosulfate duty is often run higher, in the 45-55% solids band (indicative), which shrinks tankage per unit throughput; verify your gold leaching tank mixing and air-lift design against the denser pulp.

Effluent is a genuine trade, not a free win. Tailings liquor carries ammonia — managed with pH control and, where required, stripping — and carries thiosulfate and its degradation products, which exert oxygen demand in the tailings facility as they naturally oxidize. Neither is a cyanide-species liability, and both avoid the acute-toxicity permitting profile of WAD cyanide, but your water balance and tailings dam DO monitoring must be engineered for them from day one. Three consequences follow. First, the detox plant footprint and its reagent supply contracts vanish from the capital estimate, which is a real saving on any size of operation. Second, solution bleed and make-up water demand rise, because sodium, sulfate, and copper tenor all accumulate in a closed circuit and must be purged. Third, permitting conversations change shape rather than disappear: regulators familiar with cyanide codes will ask about ammonia emission limits, nitrogen loading in tailings return water, and degradation behavior of polythionates, so have the chemistry answers prepared before the meeting, not after it.

Which Operating Envelope Should You Target?

Hold the circuit inside these bands and the chemistry stays predictable. Drift outside them and either leaching stalls or reagent burn accelerates.

The ORP line deserves emphasis. Pushing ORP up with excess oxidant does not buy faster gold dissolution; it buys faster thiosulfate destruction and higher consumption. The control objective is a stable Cu(II) concentration in the 5-20 mM band, read from solution analysis, with oxygen delivered to match — not an aggressive redox target.

When Should You Stay With Cyanide?

For each ore class there is a decisive answer, not a shrug. Free-milling oxide ore stays with cyanide: NaCN at 0.3-1.0 kg/t, carbon recovery that actually loads, a mature regulatory playbook, and no thiosulfate consumption penalty — the gold oxide ore flowsheet already in service is the cheaper circuit and remains so. Low-grade, long-leach heap duty behaves the same way. Everything else on the short list below moves to thiosulfate, conditional only on confirming the consumption numbers in test work.

If your deposit sits in the “use thiosulfate” rows, the incremental cost column in the table above is the price of recovery you cannot buy with cyanide at any spend.

How Should You Sequence Pilot Test Work and Troubleshoot the Circuit?

Commit to thiosulfate leaching in sequence, not in one leap. Stage one: bottle rolls on 1-3 kg charges across the grade domains — 24-96 h tests that fix kinetics, reagent consumption, and ORP response. Stage two: locked-column tests on a lab-scale column percolation leaching unit at 10-100 kg per column, 2-8 weeks, to confirm crush size, agglomeration, and percolation behavior if heap duty is contemplated. Stage three: a continuous mini-plant — a small resin RIP loop or, for a hybrid study, a lab-scale carbon-in-leach reactor as the cyanide control — run over weeks on a few hundred kilograms of blended ore, closed-loop, so salinity build-up, resin fouling, and make-up rates surface before they surprise you at scale. The mini-plant stage is non-negotiable for thiosulfate: closed-loop operation is where cumulative effects appear that no single batch test can show. Run it long enough to cycle the resin through several load-elute rounds, hold the solution inventory constant, and log conductivity, copper, and polythionate alongside gold tenor every shift. The output of this stage is the design consumption, the resin fouling rate, and the bleed requirement — the three numbers your capital and operating estimates actually depend on.

Common failure modes and the fix:

1. Falling gold tenor. Dissolution stalls mid-leach — check ORP first; low ORP means Cu(II) depletion, so add oxygen and CuSO4 until the redox recovers.
2. High thiosulfate consumption. Consumption above 40 kg/t signals degradation runaway — cut aeration intensity, verify sulfide reactivity, and re-baseline the Cu(II) setpoint downward.
3. Ammonia loss. Falling pH with rising NH3 odor points to over-aeration or hot liquor — trim air rate, trim temperature, and check the scrubber before dosing more ammonia.
4. Resin fouling. Loading capacity decays over cycles — add a wash stage for polythionates and copper, and screen for silica or organic fouling before condemning the resin.
5. Slow late-stage kinetics. Gold tenor plateaus above target tail — test finer grind or longer residence before blaming the lixiviant; refractory textures often set the floor.
6. Salinity build-up. Conductivity and sulfate climb with bleeds — increase solution bleed and verify make-up water quality against the assay schedule.
7. Scale and precipitate. Gypsum or metal hydroxide scale in lines and on resin — control pH, manage calcium from lime, and add antiscalant where make-up water is hard.

Each symptom above traces to a control variable in the operating envelope table — fix the variable, not the symptom.

Frequently Asked Questions

Is thiosulfate leaching commercially proven?
Yes, with caveats. The chemistry is well established in the research literature and several operations have run thiosulfate-based circuits or campaigns, but the installed base is far smaller than cyanidation. The practical consequence is that flowsheet risk sits in recovery media selection and reagent management rather than in dissolution chemistry, so bankable numbers must come from your own continuous mini-plant campaign, not from vendor claims.

Why doesn’t activated carbon work with thiosulfate solutions?
The aurothiosulfate complex Au(S2O3)2³⁻ is strongly anionic and hydrophilic, and it adsorbs onto activated carbon far more weakly than aurocyanide does — loadings are typically one to two orders of magnitude lower, which is commercially uneconomic. Strong-base anion exchange resins, by contrast, hold the complex firmly, which is why resin-in-pulp rather than carbon-in-pulp is the standard recovery route for thiosulfate liquors.

How much thiosulfate is consumed per tonne of ore?
Typical consumption spans 10-40 kg/t, with the low end approached on clean, low-sulfide ore and the high end on reactive sulfide-bearing material. The span reflects not just gold complexation but oxidation of thiosulfate to tetrathionate and sulfate driven by Cu(II) and ore sulfides. Because the degradation side reactions dominate, consumption is ore-specific and must be fixed by bottle roll and column test work before any design number is used.

Is ammonia in thiosulfate leaching a safety or permitting problem?
It is manageable, not free. Ammonia handling requires compatible materials — no copper alloys in wetted service — plus pH control to keep the speciation toward NH4+ in circuit, and monitoring of vapor near open tanks. Compared with cyanide, the acute-toxicity permitting burden is lower and there is no destruction circuit, but stripping losses, odor, and tailings-dam nitrogen loading all need engineering attention from the start.

Can thiosulfate leaching be run as a heap leach?
It can be studied as one, and column work is the gate. Thiosulfate is compatible with saline make-up water and with percolation of agglomerated crush, but long residence times and open-to-air columns drive reagent oxidation and ammonia loss, so percolation behavior, consumption, and evaporative losses must be demonstrated on large-diameter columns over weeks. Heap thiosulfate should be treated as a test work outcome, not an assumed flowsheet.

What ORP should a thiosulfate leach be controlled at?
Hold the solution in roughly the +80 to +250 mV band versus Ag/AgCl, which corresponds to maintaining Cu(II) at 5-20 mM in the ammonia system. Higher ORP does not accelerate gold dissolution; it accelerates thiosulfate destruction to tetrathionate and sulfate and drives consumption. Control the Cu(II) concentration by assay, deliver oxygen to match, and resist chasing aggressive redox targets during commissioning.

How long does a thiosulfate leaching test program take?
Plan on a staged campaign of several months: bottle rolls in days per test across grade domains, locked columns over two to eight weeks each, then a continuous mini-plant campaign of several weeks with closed-loop solution. The continuous stage is where consumption, fouling, and make-up salinity become real numbers. Budget laboratory capacity accordingly — the program is longer than a cyanide bottle-roll package, and shortcuts here surface later as plant-time write-offs.

Conclusion

Thiosulfate leaching is the decisive choice for carbonaceous, copper-gold, and arsenian refractory ores, and cyanide remains the decisive choice for free-milling oxide ore. Pull your ore through the three-stage test work sequence above, then request a quote for a pilot-scale leaching circuit sized to your confirmed consumption and recovery numbers.

References

• Marsden, J. O. and House, C. I., The Chemistry of Gold Extraction, 2nd edition, SME.
• Wills, B. A. and Finch, J. A., Wills’ Mineral Processing Technology, 8th edition, Butterworth-Heinemann.
• Society for Mining, Metallurgy and Exploration (SME), Mineral Processing Handbook (latest edition).
• U.S. Geological Survey, Mineral Commodity Summaries, Gold chapter (annual editions).
• Commonwealth Scientific and Industrial Research Organisation (CSIRO), published thiosulfate gold leaching research papers.
• AusIMM, conference and journal papers on thiosulfate and non-cyanide gold leaching (Proceedings series).
• Breuer, P. L. and Jeffrey, M. I., journal papers on copper-catalysed thiosulfate gold dissolution (published in hydrometallurgy journals).

Changelog

• 2026-11-02: First publication.