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Roasting process design for arsenopyrite gold ore pretreatment

Arsenopyrite is one of the most stubborn carriers of refractory gold, locking fine particles in sulphide matrices that resist direct cyanidation. Operations in the Eastern Goldfields, including well-known leases near Kalgoorlie-Boulder, frequently encounter this mineralogy where conventional leaching returns disappointing yields. A well-designed roasting stage breaks that lock, oxidising the sulphide lattice so that downstream leaching can reach what was previously trapped.

Pretreatment by oxidative roasting remains a mature route, even as biological and pressure oxidation plants have grown in number across Australia. For project owners balancing ore variability, throughput targets, and capital constraints, roasting still offers predictable metallurgical outcomes and a robust engineering track record. The challenge sits in tailoring the design to the specific arsenic-to-iron ratio, the carbonate content, and the downstream flowsheet the operator already runs.

This guide walks through the design considerations that shape a reliable roasting circuit, with particular attention to Australian operating conditions, regulatory frameworks, and integration with established infrastructure.

Understanding arsenopyrite behaviour during pretreatment

Arsenopyrite (FeAsS) breaks down under heat and oxygen in a controlled sequence, releasing sulphur dioxide and arsenic trioxide while leaving porous iron oxide calcine behind. The gold that was once locked becomes accessible, but only if the residence time, temperature profile, and oxygen partial pressure are tuned carefully. Too little oxygen leaves unreacted cores; too much fuses the calcine into a dense, less permeable product that reduces leaching kinetics and increases reagent consumption.

Mineralogy drives the design. Highly pyritic ores generate more heat during oxidation, which can push a furnace into runaway temperatures if not moderated. Arsenic-rich material, common around historic Victorian fields and parts of the Yilgarn Craton, demands tighter temperature control to prevent condensation of arsenic-bearing dust in downstream ductwork. Engineers usually characterise the ore through thermogravimetric analysis and bench-scale roasts before committing to furnace geometry.

Moisture, carbonate, and clay content also matter. Clay-rich feed from open pits in the Pilbara and the Tanami can agglomerate in the feed system, while carbonates consume acid in any subsequent leach. Decisions on mobile crushing plant deployment shape the particle size entering the roaster and the consistency of feed presentation, and they belong in the front-end design package rather than as an afterthought.

Thermodynamics of oxidative roasting

The reactions follow a known sequence, with FeAsS converting first to arsenate intermediates, then to iron oxide, sulphur dioxide, and arsenic trioxide vapour. The peak exotherm typically sits between 550 and 650 °C, with operating windows generally held in the 600 to 750 °C range. Above roughly 750 °C, the calcine begins to sinter, reducing porosity and trapping gold within dense hematite shells.

Partial roasting can be appropriate for feeds with low sulphide content, while full oxidation is preferred for highly refractory material where gold is finely disseminated. The decision affects downstream gas volumes, dust loads, and arsenic capture duty, which in turn drive the gas-cleaning train.

Engineers usually run mass and energy balances around the entire roasting island, treating the furnace, waste-heat boiler, and gas cleaning as one system. The latent heat in roaster offgas is rarely left unused in Australian projects, where power is expensive at remote sites and operators typically recover energy through waste-heat steam generation that supports plant heating and leach solution warming.

Furnace selection and reactor engineering

Fluidised bed roasters have become the dominant choice for arsenopyrite feeds, particularly in campaigns above roughly 100 tonnes per day. They deliver excellent heat transfer, uniform temperature, and high specific throughput. Multiple hearth designs remain in operation at some legacy sites and can suit lower-throughput or variable feeds where operators prefer gentler temperature ramps.

The choice of refractory lining, distributor plate design, and bed material all influence longevity. For Australian projects where shutdown windows must align with FIFO crew change cycles and wet-season access restrictions, mean time between rebuilds carries a direct operational weight. Many owners now specify modular refractory packages that can be replaced in sections during planned maintenance, reducing the length of each outage.

Instrumentation and control strategies have matured considerably. Bed temperature measurement at multiple points, oxygen profiling in the freeboard, and continuous offgas analysis let operators detect early signs of agglomeration or underblow. Specialist input from experienced engineering partners is often sought for the front-end design of furnace geometry, distributor design, and the integration of advanced process control packages.

Gas handling and arsenic capture

Roaster offgas carries arsenic trioxide and sulphur dioxide that must be captured before release. Quench cooling, bag filtration, and wet scrubbing are standard, with the scrubbing liquor sent to an arsenic fixation circuit that produces a stable iron arsenate or scorodite residue. Scorodite is widely accepted as a stable disposal form, though Australian regulators require demonstration of long-term stability beyond short-term leaching characteristics.

Sulphur dioxide is usually captured as sulphuric acid through a contact plant, particularly at larger operations where the acid can be used on-site or sold into the broader market. Smaller or junior operators running campaigns of a few hundred tonnes per day sometimes neutralise SO₂ with slaked lime, producing gypsum that goes to tailings. Both routes are workable, but the economics differ sharply depending on acid market access and freight costs to remote operations.

Dust collected in the baghouse returns to the roaster or joins the calcine stream, depending on arsenic content. Operators typically monitor arsenic partitioning carefully, because volatile arsenic that condenses in cooler sections creates maintenance burdens and occupational exposure concerns. Enclosed handling, sealed transfer points, and bag-loading systems protect both worker safety and recovery yield.

Connecting roasted calcine to downstream recovery

After roasting, the calcine cools, neutralises if needed, and reports to a leach train that may be CIL, CIP, or heap-based. The carbon-in-pulp route remains common in Australia, particularly for higher-grade campaigns. Calcined material is generally more amenable to standard cyanidation than raw sulphide, with recoveries often climbing from the 30 to 50 percent range into the 85 to 95 percent range once the sulphide matrix is gone.

Carbon management, attrition losses, and adsorption contact all shape gold-in-carbon grade and elution circuit loading. Operators commonly revisit the carbon adsorption stage design once roasted calcine is available, because slurry characteristics shift significantly. Viscosity, fines content, and residual reagent demand change once the sulphide is removed, and the adsorption kinetics improve noticeably with the more porous calcine surface.

Water balance deserves fresh attention at this stage. Roasted calcine generates a different bleed profile, and the recycling loops that worked for raw sulphide slurry may need adjustment before steady-state operation is reached.

Australian regulatory landscape and ESG obligations

Australian mining operates under a layered framework. Each state sets its own mineral tenement and environmental protection rules, with Western Australia governed through the Department of Mines, Industry Regulation and Safety and the Department of Water and Environmental Regulation. Nationally, the Environment Protection and Biodiversity Conservation Act triggers federal review where projects could affect matters of national environmental significance.

Roasting projects attract additional scrutiny because of arsenic handling and air emissions. Operators must demonstrate that arsenic-bearing residue will remain stable over the long term and that ambient air quality around the plant will meet the relevant guidelines. ESG performance has also moved from voluntary reporting to lender requirements, with many banks and bondholders requiring standards aligned with the Towards Sustainable Mining framework before financing is approved.

Cultural heritage considerations add another layer, particularly on the mainland where many leases overlap with areas of significance to Aboriginal communities. Engagement, agreement-making, and heritage surveys are typically sequenced well ahead of construction. Remote Pilbara and Northern Territory sites also raise logistics planning obligations, including seasonal road access restrictions that shape equipment delivery windows.

Practical recommendations for project teams

Project owners who treat the roasting circuit as an integrated system, rather than a stand-alone island, consistently achieve better outcomes than those who specify it in isolation. The strongest designs connect feed preparation, the roaster, the gas-cleaning train, and the downstream leach, with each stage shaping the others. Operating discipline in the second and third years usually decides whether the plant delivers the financial outcomes the feasibility study promised.

To discuss a roasting flowsheet tailored to a particular ore body, or to scope an integrated plant package that aligns with Australian operating and regulatory realities, contact the team at Lozova.org and request a consultation with a process engineer who has worked on similar Australian feeds.