Process intensification of low-concentration magnesium sulfate leaching of ion-adsorption rare earth ores by pulsed microwave-assisted gradient injection

Abstract: Rare earth elements (REEs), particularly middle and heavy REEs, are strategic metals critical to advanced technologies. Ion-adsorption type rare earth ores (IAREOs) constitute the primary global source of these REEs, making the development of efficient and environmentally friendly extraction methods highly significant. Conventional in-situ leaching using magnesium sulfate suffers from high reagent consumption and prolonged percolation cycles. Here, we introduce a novel approach combining low-concentration magnesium sulfate leaching with pulsed microwave irradiation, and establish a microwave-coupled column leaching system simulating in-situ conditions. Comparative experiments with conventional heating leaching reveal that, under low microwave energy density (1.19 W/g), a pulse regime of 20 s on-5 min off optimally enhances leaching performance. Relative to conventional heating leaching, pulse microwave assisted leaching accelerated seepage by 32%–39% and increased REEs extraction efficiencies by 6.42%–10.73%. Notably, the synergistic enhancement became substantially more pronounced when the initial pH of the leaching solution was adjusted to 3.0. The study further explores pulsed microwave-assisted gradient injection leaching. A two-stage injection scheme (stage I: 1.25 wt% MgSO₄, L/S of 2:5; stage II: 0.5 wt% MgSO₄, L/S of 3:5), corresponding to a total MgSO₄ concentration of 0.8 wt%, achieves a REEs recovery of 90.12%, representing a 20.54% increase compared with conventional continuous leaching at the same concentration. Mechanistic investigations, including Zeta potential, mineral microstructure, and particle size distribution analyses before and after leaching, indicate that pulsed microwave enhancement arises from selective enhancement of seepage pathways, interfacial modification of mineral particles, and accelerated ion migration, involving both thermal and non-thermal effects from microwave heating. The integration of pulsed microwave with low-concentration, gradient injection leaching synergistically increases REEs recovery while shortening leaching cycles and reducing leachant usage by 22.2%. This work provides a promising strategy for sustainable, high-efficiency exploitation of IAREOs.