Enhanced solvent extraction of rare earth elements in ultra-high phase ratio with pore-throat microchannels

To enrich and recover low-concentration (101–102 ppm) rare earth elements from ores and industrial wastes, high phase-ratio solvent extraction is favored. However, high phase-ratio solvent extraction is limited by low mass transfer efficiency in the continuous phase due to the insufficient contact between large volume continuous phase and sparse droplets. To realize efficient solvent extraction of rare earth ions from low concentrations (100 ppm) aqueous phase to oil phase with high phase ratio, we introduce a type of microchannel with sequential pore-throat geometry. This sequential pore-throat geometry creates capillary barriers and effectively retains dispersed droplets, leading to a reduction of apparent water–oil volume ratio and thereby major mass transfer enhancement in the continuous phase. We experimentally highlight its advantage over classic uniform microchannel for solvent extraction of rare earth ions under high phase ratios: in a double pore-throat microchannel, extraction equilibrium can be reached within 30 s under high phase ratios of 50–250; for an extreme phase ratio of 500:1, extraction efficiency can achieve 77 % within a quadruple pore-throat channel, while that within a uniform microchannel is only below 40 %. The superiority of sequential pore-throat microchannel for extraction at a high phase ratio is reproduced using different types of rare earth ions. We thus conclude that sequential pore-throat geometry can drastically promote the performance of microfluidic-based solvent extraction at extreme phase ratios, for rare earth enrichment and potentially for other relevant applications.