Magnetic preconcentration of clay‑hosted rare earth ores: optimisation of hydrodynamic and magnetic operating conditions

• WHIMS upgraded clay-hosted REE ore into REE-rich concentrates. • WHIMS preconcentration was evaluated under varying magnetic and flow conditions. • REE grade and recovery depended on magnetic-hydrodynamic interactions. • Critical detachment threshold defines optimal WHIMS operation for REE recovery. • Mineral locking and detachment limited maximum REE recovery. Recently, there have been numerous discoveries of clay deposits, which contain rare earth elements (REE) largely hosted in secondary minerals. Preliminary hydrometallurgical studies have shown poor recovery rates (<60%) at high lixiviant concentration. This study forms part of ongoing efforts to identify tactical beneficiation strategies to be considered in flowsheets for REE extraction in these clay deposits. To this end, the feasibility of deploying magnetic separation as a preconcentration step was investigated with the view of producing rich REE stream for downstream hydrometallurgical extraction. Box-Behnken design combined with response surface methodology was employed to model and optimize wet high intensity magnetic separator (WHIMS) operating conditions. Specifically, the relative impact of applied magnetic field intensity, matrix packing density, and pulp flow rate was analysed for REE minerals recovery. The ANOVA results show that the response surface model captures statistically meaningful trends for REE concentrate grade (F = 2.98, p = 0.033), while the recovery response exhibits a weaker overall model fit (F = 1.5, p = 0.23), consistent with the greater inherent variability of recovery behaviour in clay‑rich WHIMS systems. Tests data revealed that throughput is a grade-positive variable and when magnetic conditions are sufficient. It was observed that hydraulic–magnetic coupling plays a central role in controlling REE minerals recovery efficiency when processing typical Australian clay hosted deposits. TREE grade was maximised under operating conditions that promoted selective capture rather than maximum particle capture, namely high throughput, moderate matrix packing density, and low to moderate pulp flow rates. Recovery losses at elevated pulp flow rates were attributed to particle detachment, while mineral locking limited the recovery of a proportion of REE-bearing minerals. Overall, the test data suggest that magnetic separation offered huge potential with high REE upgrade–recovery performance, clearly justifying its possible inclusion in future beneficiation flowsheet design.