Thiosulfate leaching for non-cyanide gold extraction in Australia

Thiosulfate leaching is emerging as a credible non-cyanide option for gold extraction, but it is not a simple substitution. The process combines alkaline thiosulfate with oxidising conditions to form a soluble gold complex, after which activated carbon, resin adsorption or precipitation can recover the metal. Its value depends on ore-specific kinetics, reagent consumption and responsible residue management.

For Australian mine owners, the case is strongest where cyanide handling is difficult, conventional carbon adsorption is impaired or a brownfield site needs a compact retrofit. Representative tests must still compare thiosulfate with established cyanide flowsheets, including recovery, water use, operating labour and closure impacts.

Kalgoorlie-Boulder, Leonora, the Pilbara and Mt Isa illustrate how different Australian mining regions can be. This article outlines the chemistry, test program, equipment, regulatory considerations and staged route to operating a thiosulfate plant.

Why operators are seeking a non-cyanide route

The potential advantage is strongest for deposits where gold can be dissolved selectively without activating undesirable minerals. Carbonaceous ores that exhibit preg-robbing may benefit because adsorption behaviour differs from cyanide, although some carbon still binds gold–thiosulfate complexes and requires resin adsorption or a preg-robbing stage.

Regulatory and community expectations also shape the business case. Removing cyanide can simplify hazardous-chemical controls, but it does not eliminate duties of care, operator competence or discharge monitoring. An Australian plan should state the risks of every reagent and avoid describing thiosulfate as harmless.

Attention to human factors is equally important. Workplace reward programs need clear safety rules because the mechanics of a hold and win promotion show how entertainment-led rewards can blur the line between motivating safe work and pressuring shortcuts. Production incentives should never sit ahead of statutory controls.

Core chemistry and critical process controls

In the common alkaline thiosulfate process, gold forms a soluble bis(thiosulfato)aurate complex. Oxygen supplies oxidising capacity, while copper ions can accelerate dissolution. This permits leaching in a mildly alkaline circuit, typically around pH 8–10, without the strongly alkaline conditions used in many cyanide plants.

Sodium thiosulfate is straightforward to dose as a bulk reagent, but ammonium thiosulfate can improve leach kinetics in some ores. Ammonia may also be added as a promoter. Those benefits must be balanced against reagent cost, ammonia volatilisation, nitrogen in effluent and the load placed on downstream water treatment.

Control of pH, dissolved oxygen, oxidation-reduction potential, temperature and free thiosulfate is essential. Excessive acidity promotes thiosulfate breakdown, while insufficient reagent or oxygen can leave recoverable gold in solution. Laboratory optimisation should therefore test chemistry and recovery together rather than maximising dissolution alone.

How thiosulfate compares with cyanide

Cyanide still offers fast leaching, mature carbon-circuit operation and extensive operating experience. It can remain the lower-risk choice when an existing plant performs well and reagent consumption is modest. The comparison changes when the ore is carbonaceous, the site has strict chemical controls or the project needs a non-cyanide flowsheet from the outset.

Thiosulfate avoids cyanide toxicity and cyanide-bearing waste, but it is not environmentally inert. Oxidation can convert thiosulfate to sulphite, sulphate and other sulfur species that increase oxygen demand. Ammonia, if used, adds monitoring and treatment requirements. Tailings pore water, plant drainage and occupational exposure therefore need whole-of-site assessment.

Recovery equipment also behaves differently. Activated carbon may adsorb the gold–thiosulfate complex more slowly and can be affected by competing anions, while ion-exchange resins can provide effective adsorption and a practical regeneration route. A pilot circuit should compare carbon, resin and precipitation routes, then follow the selected option through elution or stripping, electrowinning and smelting.

Test work that supports an investment decision

A sound test program starts with representative core, run-of-mine and process samples, not a single bulk sample. Bottle rolls and small stirred tanks should compare sodium and ammonium thiosulfate, oxygen availability, copper addition, pulp density and adsorption. Lozova’s mineral processing equipment range can support early scoping, but the test matrix must be set by the ore rather than by available hardware.

Measure dissolved gold, residual thiosulfate, sulphite, sulphate, ammonia, pH and dissolved oxygen throughout each cycle. Include carbon assays and preg-robbing tests where organic carbon is present. Changes in solution chemistry should be tracked with mass balances so that apparent dissolution is not confused with gold that is later lost to tails or plant surfaces.

Scale testing should progress from bottle rolls to agitated bench cells, then to a continuous pilot capable of handling representative slurry. Compare thiosulfate with cyanide on split samples and include sensitivity to grind size, leach time, reagent dosage and circuit configuration. Metallurgical accounting, rather than a single head-grade calculation, should establish recovery and forecast operating cost.

Equipment and plant configuration for a pilot

A modular pilot can include crushing and grinding to a controlled particle size, slurry storage, agitated leach tanks, oxygen sparging, hydrocyclone classification, adsorption columns, elution and electrowinning. If the flowsheet includes concentrate treatment, flotation and gravity separation remain valuable upstream and downstream stages. Tank geometry should provide adequate mixing and oxygen transfer without excessive reagent residence time.

Slurry handling deserves equal attention. Incorrect cyclone pressure or worn apices can upset classification, consume reagents and obscure leach performance. Lozova’s polyurethane wear parts explains how liners can extend apex wear life in abrasive circuits.

Equipment selection should account for Australian maintenance conditions: dust, heat, long travel times and variable operator experience. Local fabrication capability, spare-part availability and clear guarding are more valuable than a low purchase price. A supplier offering engineering, procurement, commissioning, plant management and operational support can reduce the interfaces that often delay a pilot becoming reliable production infrastructure.

Delivering the project in Australia

Imported sodium or ammonium thiosulfate requires correct commercial documents, safety data sheets, storage design and customs import declarations through the Australian Border Force. Dangerous-goods and workplace-storage assessments depend on the formulation and any ammonia used. Shipments through Fremantle or Port Hedland should be planned around mine schedules, while buffer stock protects a remote operation from delayed deliveries.

Environmental approval also varies by jurisdiction. In Western Australia, tailings and groundwater impacts engage state resources regulators and the Environmental Protection Authority of Western Australia, while site-specific waste acceptance criteria can affect process design. Elsewhere, state environmental authorities and local landholders may impose different discharge, noise and dust conditions. These requirements should be translated into measurable plant limits at the design stage.

Remote and fly-in-fly-out operations need modular skids, simple controls and maintainable equipment that technicians can service with limited downtime. Perth provides a base for engineering, procurement and project management, but commissioning support must reach the mine. Specialist suppliers such as Genoptics SPR can be evaluated for optical measurement or controlled spray applications, supporting clearer tank conditions and more consistent reagent addition.

Recommended route to bankable operation

A practical project should advance through linked technical and commercial gates rather than jump directly from a promising bottle roll to full-scale EPC expenditure. The following actions protect both recovery and capital value:

• Sample every geometallurgical domain and preserve split samples for independent checks.
• Benchmark thiosulfate against cyanide using the same ore, grind and reporting basis.
• Optimise sodium and ammonium thiosulfate with oxygen, copper and promoter additions.
• Test carbon, resin and precipitation recovery at continuous pilot scale.
• Build a complete water, reagent, tailings and energy balance for the selected circuit.
• Include Australian customs, licensing, maintenance and workforce requirements in the feasibility model.

After pilot results are stable, engineering should convert the best flowsheet into an operating design with mass-balance guarantees, reagent control strategies, contingency limits and a commissioning plan. Procurement should package long-lead items early, while management systems define operator competency, maintenance intervals and responsible reagent handling.

Australian mine owners and project stakeholders ready to evaluate this route can request an ore-testing and flowsheet review from Lozova’s engineering team. Bring representative samples, current cyanide benchmark data and the intended scale; the resulting package can define a thiosulfate pilot, commercial comparison and turnkey path to production.