Thiosulphate leaching is the most commercially mature alternative to the cyanidation chemistry described on cyanidation basics: chemistry of gold leaching, and it is presented here with the same honest framing this site applies throughout: a genuine technical option for specific circumstances, not a general-purpose upgrade over the industry-standard reaction. It has a real, if limited, history of commercial-scale deployment, most notably on preg-robbing carbonaceous gold ores where cyanide-based circuits have historically struggled regardless of how the plant is configured.
How does thiosulphate chemistry differ from cyanidation?
Thiosulphate leaching dissolves gold using an ammoniacal thiosulphate solution, typically catalyzed by a small amount of copper in solution, forming a soluble gold-thiosulphate complex rather than the gold-cyanide complex the Elsner equation produces. The copper acts as an oxidant and catalyst in this system, cycling between oxidation states to keep the leach reaction proceeding, a meaningfully more complex reagent chemistry than cyanidation's comparatively simple, single-reagent mechanism. This added chemical complexity is one of the central practical challenges of running thiosulphate at industrial scale: keeping the copper catalyst, ammonia, and thiosulphate concentrations all in the right balance simultaneously is a more demanding process control task than managing cyanide concentration and pH alone.
How is gold actually recovered from a thiosulphate leach solution?
Standard activated carbon, the workhorse adsorbent behind CIL and CIP throughout this recovery methods set, does not adsorb the gold-thiosulphate complex efficiently the way it adsorbs gold-cyanide complex, which means a thiosulphate circuit generally needs a different recovery technology downstream of leaching. Ion exchange resins are the most established route, selectively binding the gold-thiosulphate complex out of solution in a manner functionally analogous to carbon adsorption in a cyanide circuit, though using resin beads and different elution chemistry rather than activated carbon and the Zadra or AARL processes described elsewhere on this site.
This resin-based recovery step is itself an added piece of process infrastructure and expertise a project must build or acquire, distinct from the carbon handling equipment and operator experience a conventional cyanide circuit already relies on. For a project without existing resin-based recovery experience, this is a genuine additional learning curve worth weighing alongside the leach chemistry itself when considering thiosulphate, and it is a cost and complexity factor that belongs in the same economic comparison as the reagent chemistry itself, not a minor footnote to it.
Why hasn't thiosulphate displaced cyanide as the default?
Thiosulphate has not displaced cyanide as the default gold lixiviant primarily because of reagent consumption and cost volatility, since thiosulphate itself decomposes under some pulp conditions faster than cyanide does, requiring higher reagent dosing and more careful chemical management to sustain an effective leach. The multi-reagent system, thiosulphate, ammonia, and copper together, is also more complex to manage in continuous plant operation than cyanide's simpler chemistry, and gold recovery on typical feed types has generally proven lower and less consistent with thiosulphate than with cyanide under otherwise comparable conditions.
None of this makes thiosulphate a failed technology; it makes it a specialized one whose economics have not yet generally beaten cyanide's on typical ore and tailings types. This honest framing matters because thiosulphate is sometimes presented in less careful sources as a straightforward, environmentally superior swap for cyanide, when the real picture is a genuine set of tradeoffs rather than a clear across-the-board improvement.
Operating a thiosulphate circuit reliably at industrial scale also demands a level of process chemistry expertise and continuous monitoring that many established mining operations have simply built cyanide operating experience around instead over decades of practice. This institutional-knowledge gap is a real, if less frequently discussed, factor behind thiosulphate's limited adoption alongside its chemical and cost challenges: switching lixiviant systems means retraining operators and rebuilding operating procedures from a much smaller base of industry experience to draw on.
When does thiosulphate genuinely make sense?
Thiosulphate's clearest and most established niche is preg-robbing carbonaceous ore, material where carbonaceous matter re-adsorbs dissolved gold-cyanide complex out of solution before carbon can capture it, the same preg-robbing problem described on cyanidation basics: chemistry of gold leaching. The gold-thiosulphate complex is less prone to this re-adsorption than the gold-cyanide complex is, which gives thiosulphate a genuine performance advantage specifically on preg-robbing feed that cyanide-based leaching struggles with regardless of circuit design.
This advantage is specific and mechanistic, not general. It applies because of a particular chemical interaction between the carbonaceous material and each type of dissolved gold complex, not because thiosulphate is broadly superior at dissolving gold. Applying that same reasoning to a dump without a genuine preg-robbing problem does not carry the advantage over, since the underlying mechanism that makes thiosulphate useful on preg-robbing ore simply is not present.
Thiosulphate can also become relevant in specific regulatory contexts where cyanide use faces genuine restriction or community opposition strong enough to threaten a project's social license, though this is a project-specific and jurisdiction-specific consideration rather than a general reason to prefer thiosulphate everywhere.
Is thiosulphate genuinely more environmentally favorable than cyanide?
Thiosulphate is often assumed to be automatically more environmentally benign than cyanide simply because it is not cyanide, but the honest answer is more nuanced. Thiosulphate itself and its breakdown products are generally less acutely toxic than cyanide, which is a real advantage, but the ammonia used in the leach system carries its own environmental management requirements around discharge and water quality, and the copper catalyst needs its own containment and recovery consideration rather than being an environmentally free component of the system.
A fair comparison weighs the full reagent suite each system requires, not just the headline lixiviant, and a project genuinely motivated by environmental considerations should commission a proper comparative assessment of both systems' full reagent and discharge profiles rather than assuming thiosulphate wins simply by avoiding the word cyanide. A rigorous cyanide management program built around the International Cyanide Management Code, covered on the cyanidation chemistry page, can in many cases deliver comparable real-world environmental outcomes to a thiosulphate system without taking on thiosulphate's added chemical complexity and generally lower recovery.
Has thiosulphate been used on tailings retreatment specifically?
Whether thiosulphate is worth considering for a specific Ghanaian tailings retreatment project is a testwork question, not an assumption in either direction. A dump confirmed through diagnostic leach testwork to carry genuine preg-robbing carbonaceous material, the specific condition where thiosulphate has a real, demonstrated advantage, is a legitimate candidate for evaluating thiosulphate leaching alongside standard cyanidation. A dump without that specific problem gains little from thiosulphate's added chemical complexity and generally lower typical recovery, and standard cyanidation remains the more proven, lower-risk choice.
Most of the legacy Ghanaian tailings addressed elsewhere on this site are described as free-milling and directly leachable by standard cyanidation, without a documented preg-robbing problem driving the discussion. This page exists to give a technical reader the vocabulary for the specific case where that assumption does not hold, not to suggest thiosulphate is a routine consideration for Ghanaian tailings retreatment generally.
What other lixiviants exist beyond thiosulphate?
Thiosulphate is the most commercially proven of several non-cyanide leaching approaches, a broader field covered on glycine and other emerging lixiviants, which surveys chemistry with genuinely less commercial maturity than thiosulphate has achieved.
Why this fits into the responsible cyanide management conversation
Understanding thiosulphate as a real, if conditional, alternative is part of a complete picture of responsible cyanide management, alongside the handling, detoxification, and international code compliance already covered on the cyanidation chemistry page, since a project's willingness to evaluate genuine alternatives where they make technical sense is itself a credibility signal for ESG-conscious investors and communities.
Testwork needed before any lixiviant substitution decision
Substituting thiosulphate for cyanide on a specific project is never a decision made from general industry literature alone; it requires dedicated bench-scale and pilot testwork on the actual tailings material in question, comparing recovery, reagent consumption, and cost directly against a parallel cyanidation test under matched conditions, before any substitution could be responsibly recommended.
How this fits into an investor's technical due diligence
An investor reviewing a project that proposes thiosulphate leaching should expect to see the specific diagnostic result that justified the choice, namely a confirmed preg-robbing problem that standard cyanidation testwork could not resolve, alongside a direct cost and recovery comparison against cyanide under matched testwork conditions. A proposal that recommends thiosulphate without that supporting evidence warrants closer scrutiny rather than automatic acceptance.