A comprehensive mathematical model is proposed to study the transport phenomena in an electrodialysis (ED) process employed to recover lithium hydroxide and sulfuric acid from the lithium sulphate solution derived from a recycling process of spent lithium-ion battery material. The model is developed based on the conservation equations of mass and ions, and considers electrolyte solutions consisting of mono- and multivalence ions. The concentration polarization at ion exchange membranes (IEMs) and their adjacent diffusion boundary layers as a function of the applied current, inlet concentrations, and flow rate are computed. Experimental data from a three-compartment ED cell are used for validation. A parametric study is performed to evaluate the impact of parameters on transmembrane fluxes of ion and water. It is revealed that increasing current leads to enhancement of the transmembrane water and concentration polarization across IEMs. Feeding solutions consisting of smaller ions result in lower water transfer through IEMs. Raising the lithium concentration at the dilute channel increases the LiOH concentration due to reduced transmembrane water transfer. Using the uncertainty propagation method, it is found that current and counter-ion radius are the most influential parameters affecting the outlet concentration of concentrate channel and transmembrane water transfer.
In the hydrometallurgical recycling of spent lithium-ion batteries, a lithium sulphate solution (Li2SO4) can be obtained as a by-product. Electrodialysis (ED) was employed in this study to split Li2SO4 into lithium hydroxide (LiOH) and sulfuric acid (H2SO4) solutions, which can be reused in the recycling process to create a closed-loop process. A three-compartment ED cell with a dilute channel filled with a Li2SO4 solution and two concentrate channels separately filled with LiOH and H2SO4 solutions was developed. The dilute and concentrate channels were separated by cation-exchange and anion-exchange membranes, respectively. High ion recovery ratios of Li+ and SO42- of 94.3 and 87.5%, respectively, were achieved at a current density of 833 A·m-2. The effects of the current density, inlet concentrations, and initial fluid volume on the overall efficiency of the cell were studied. Electro-osmosis played an important role during ED, particularly on the functioning of the cation-exchange membrane. Increasing the initial solution volume in the concentrated compartments can enhance current efficiency and ion recovery. In conclusion, the present study provides insights into the transport of coupled species through an ED cell, and the findings may guide future designs and operations of ED cells for optimal efficiency.
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