Highlights

• Researchers demonstrated effective leaching of monazite concentrate using oxalic acid at moderate temperatures, achieving over 65% phosphate dissolution.
• The study revealed two distinct monazite phases.
• Free oxalate concentration, not temperature, governs dissolution efficiency.
• Potential for more sustainable and cost-effective rare earth element extraction with lower temperature processing and reduced environmental impact.

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A new peer-reviewed study published in Hydrometallurgy (Elsevier, October 3, 2025) by lead author M.S. Henderson with co-authors L.G. Dyer and B. Tadesse from Curtin University’s Western Australian School of Mines (WASM) has demonstrated that monazite concentrate—a key rare earth phosphate mineral—can be effectively leached with oxalic acid at moderate temperatures (≈45 °C) while achieving over 65% phosphate dissolution, more than double prior benchmarks.

Study Summary: Dissolving a Long-Standing Challenge

The Curtin team conducted systematic leaching experiments across temperatures from 30 °C to 95 °C, assessing mineralogical and thermodynamic factors limiting rare earth element (REE) extraction. The researchers achieved high phosphate solubilization using a multi-stage, cross-flow leach system, simultaneously reducing undesirable iron dissolution, a persistent issue in acidic REE processing. The study further identified two distinct monazite phases—one reactive, one refractory—offering insight into why traditional high-temperature approaches (> 65 °C) often stall.

Electron microscopy confirmed that while ash-layer formation is rare, oxalate availability sharply influences leach performance. The authors conclude that free oxalate concentration—not temperature alone—governs dissolution efficiency, implying a delicate balance between chemical consumption, precipitation, and redox stability.

Implications: Lower Heat, Lower Cost, Higher Promise

For REE producers, particularly those developing monazite-based hydrometallurgical flowsheets, these results could mark a turning point. Operating below 50 °C could significantly reduce capital and energy intensity while minimizing corrosion and waste handling burdens typical of sulfuric or hydrochloric acid systems. The findings bolster the potential of organic-acid leaching as a greener, scalable alternative for REE recovery—especially where environmental or regulatory constraints restrict high-temperature acid treatment.

If validated at pilot scale, Curtin’s process could enable simpler reagent recycling, improved thorium-free REE oxalate generation, and more sustainable feedstock utilization—particularly relevant to emerging midstream projects in Australia and North America seeking non-Chinese refining routes.

Limitations: Complexity in Chemistry

The study also underscores limitations that could temper immediate industrial adoption. Reaction kinetics remain heterogeneous and mineral-specific, oxalate loss through redox degradation (notably by cerium(IV) or manganese) reduces reagent efficiency, and partial phosphate reprecipitation can halt dissolution beyond 65%. Furthermore, scaling from laboratory to continuous operation may introduce new challenges in oxalate balance, fluid dynamics, and impurity management.

Conclusion

Henderson et al.’s research deepens the scientific foundation for low-temperature REE extraction, illuminating both promise and pitfalls in organic-acid hydrometallurgy. As global magnet-metal demand surges, such advances could underpin more resilient, environmentally responsible refining strategies—but only if the complex interplay between mineralogy and solution chemistry can be mastered in the field.

Citation: Henderson M.S., Dyer L.G., Tadesse B. (2025). Understanding limitations to monazite concentrate dissolution in oxalic acid. Hydrometallurgy (Elsevier). https://doi.org/10.1016/j.hydromet.2025.106582 (opens in a new tab)

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