Fast extraction and enrichment of rare earth elements from waste water via microfluidic-based hollow droplet

•Microfluidic-based hollow droplet was used to intensify the extraction of REEs.•Extraction performance was highly enhanced compared with conventional methods.•The extraction process could be finished within a few seconds.•A model was proposed to predict the intensification of extraction efficiency. The conventional solvent extraction set-up for low-concentration rare earth elements (REEs) at high phase ratios has shortcomings including large factories, long mixing times, high energy consumption, extractant loss and easy emulsification. In this work, a solvent extraction system for Nd(III) using 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) was investigated to enhance extraction in a microfluidic device to solve the above problems. The effects of residence time and phase ratio on the extraction efficiency of a liquid-liquid system were studied. The results showed that extraction efficiency increased as the residence time increased significantly, and a longer channel was required to reach equilibrium for higher phase ratios than for others owing to the decreased oil flow rate and increased mass transfer distance. We also investigated the effects of outlet length, phase ratio, and gas flow rate on the extraction efficiency of a gas-liquid-liquid system. The results indicated that only 0.3m of the outlet channel was required to reach equilibrium for the entire process, and the overall volume mass transfer coefficient kLa was 5–50times larger than in systems without a gas phase. The enrichment factor could reach 200–450 with an initial concentration of 30–90ppm, and the extraction efficiency was higher than 90% even at a high phase ratio of 200. A theoretical model was built to predict the intensification of extraction by the gas-filled hollow droplet. Finally, europium and erbium were chosen as the typical medium and heavy rare earth systems to prove the applicability of this method.