Direct numerical simulation of continuous lithium extraction from high Mg²⁺/Li⁺ ratio brines using microfluidic...

A novel ion concentration polarization-based microfluidic device is proposed for continuous extraction of Li⁺ from high Mg²⁺/Li⁺ ratio brines. With simultaneous application of the cross-channel voltage that drives electroosmotic flow and the cross-membrane voltage that induces ion depletion, Li⁺ is concentrated much more than other cations in front of the membrane in the microchannel. The application of external pressure produces a fluid flow that drags a portion of Li⁺ (and Na⁺) to flow through the microchannel, while keeping most of Mg²⁺ (and K⁺) blocked, thus implementing continuous Li⁺ extraction. Two-dimensional numerical simulation using a microchannel of 120 µm length and 4 µm height and a model, highly concentrated brine, shows that the system may produce a continuous flow rate of 1.72 mm/s, extracting 25.6% of Li⁺, with a Li⁺/Mg²⁺ flux ratio of 2.81×10³, at a pressure of 100 Pa and cross-membrane voltage of 100 times of thermal voltages (25.8 mV). Fundamental mechanisms of the system are elaborated and effects of the cross-membrane voltage and the external pressure are analyzed. These results and findings provide clear guidance for the understanding and designing of microfluidic devices not only for Li⁺ extraction, but also for other ionic or molecular separations.