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

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%. The study further explores pulsed microwave-assisted gradient injection leaching. A two-stage injection scheme (stage I: 1.25 wt% MgSO 4 , L/S of 2:5; stage II: 0.5 wt% MgSO 4 , L/S of 3:5), corresponding to a total MgSO 4 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, demonstrating the significant enhancement effect of pulsed microwave irradiation on the low-concentration gradient injection system. Under the above gradient injection conditions, adjusting the initial pH of the leaching solution to 3 increased the REEs leaching efficiency to 92.44%, indicating that pH adjustment can further enhance the synergistic effect between the leaching process and microwave irradiation. Furthermore, by comparing the zeta potential, microstructural morphology, and particle-size distributions of ore samples before and after leaching across different ore layers, together with a comparison with conventional thermally assisted leaching, the principal mechanisms underlying microwave-enhanced leaching were elucidated from the perspectives of thermal and non-thermal effects. These mechanisms mainly involve selective enhancement of seepage, modification of mineral particle interfaces, and accelerated ion migration. 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.