Bioleaching transforms coal waste into safe material while producing high purity ferrous sulphate proving mine liabilities can become valuable resources

A new study has shown that bioleaching can strip more than 90% of sulphur and iron from coal waste, neutralising its acid-generating potential and creating a saleable by-product.

At the Life of Mine | Mine Waste and Tailings Conference 2025 in Brisbane, Professor Susan Harrison, Executive Dean of the Faculty of Engineering, Architecture and Information Technology at the University of Queensland, presented the paper “Bioleaching of Sulfur Enriched Discards Applied to Circular Economy”, co-authored with M. Gcayiya and J. Amaral Filho.

The research focused on pyrite-enriched coal discards – a challenging waste stream that, when left untreated, generates acid rock drainage (ARD) over decades, polluting waterways and creating enduring liabilities for mine operators.

“Treatment is one thing,” Harrison told delegates. “But our goal is to remove the ARD risk entirely rather than just delaying it.”

Targeting a high-liability waste stream

The team began by separating coal discards into three density fractions using dense medium separation:

• Pyrite-rich fraction – high in sulphur and the main ARD risk driver
• Intermediate mineral fraction – potentially suitable for construction aggregates, soil fabrication, or rare earth element recovery
• Coal-rich fraction – saleable for energy production

The research concentrated on the pyrite-rich fraction. By oxidising this material under controlled conditions, the team could mimic and accelerate the natural ARD process – but in a managed, contained environment.

Harnessing microorganisms to do the work

The bioleaching process used iron- and sulphur-oxidising microorganisms, well-known in metallurgical applications, to convert pyrite (FeS₂) to ferric sulphate. This acidifies the system and releases soluble iron. In pilot column tests, the researchers achieved more than 90% removal of both sulphur and iron, reducing the net acid producing potential from 555 to –155 kilograms of sulphuric acid per tonne – effectively transforming the material into a non–acid-forming residue.

“We demonstrated that with optimal conditions, the process can be completed in under a year,” Harrison said. “That’s a huge improvement over the uncontrolled leaching that would happen over decades in a waste dump.”

From waste to product

The ferric sulphate-rich leachate was then reduced to ferrous sulphate and crystallised as ferrous sulphate heptahydrate (melanterite). This high-purity product – with yields above 90% – has established uses as a coagulant in water treatment and potential applications in agriculture and industrial chemistry.

The crystallisation step, which currently uses ethanol as a precipitating agent, is still being optimised to improve efficiency and reduce reagent volumes. Importantly, the ethanol can be recovered and reused, adding to the process’s sustainability profile.

Circular economy in action

Harrison emphasised that integrating bioleaching into operational mine waste management could deliver a double win – eliminating the long-term ARD liability while generating a revenue stream from the recovered product.

“This isn’t about moving the problem down the line,” she said. “By recovering the pyrite as a usable form of iron sulphate, we’re closing the loop and ensuring the residual material is safe for disposal.”

The approach fits squarely within circular economy principles:

• Risk removal – Eliminates ARD potential at source rather than relying on covers or water treatment
• Value recovery – Extracts a saleable chemical product from waste
• Sustainability – Reduces the footprint and future cost of mine closure by addressing the problem during operations

Industry implications

The work also has broader implications for policy and practice. With increasing pressure on mining companies to demonstrate progressive rehabilitation and safe closure, technologies that address environmental risks while offering a commercial return are likely to gain traction.

“Doing this while the mine is still running is critical,” Harrison stressed. “You have the infrastructure, you have the material handling systems, and you can integrate the process into ongoing operations rather than retrofitting after closure.”

If adopted at scale, the method could reshape how the coal sector – and potentially other mining sectors with sulphide-rich waste – view tailings and coarse rejects. Instead of long-term liabilities sitting in storage facilities, these materials could become inputs to new value chains.

Professor Susan Harrison presenting her research on bioleaching pyrite-enriched coal discards to remove ARD risk and recover valuable by-products at the Life of Mine | Mine Waste and Tailings Conference 2025. Photo: Jamie Wade.