Membrane-based Ion-ion Separation Processes: Nanofiltration and Electrodialysis

Abstract

Selective ion-ion separation is a critical research area due to its potential applications in resource extraction and recovery. One promising application is extracting lithium (Li) from brines rich in magnesium (Mg) to meet the growing demand of Li batteries. Nanofiltration (NF), a pressure-driven filtration process, and electrodialysis (ED), an electrical separation process, are two promising membrane processes for selective ion-ion separation. However, understanding ion-ion separation in NF and ED from microscopic ion transport to process-scale performance remains a challenge. This dissertation aims to advance the understanding of ion-ion separation with experiments and modeling, for tackling the specific challenge of Li extraction from brine lakes with a high mass ratio of Mg over Li. The dissertation first demonstrates the cross-method incomparability of ion-ion selectivity and the deficiency of using selectivity as the only performance metric. A standard protocol for membrane selectivity evaluation is recommended. A new framework for evaluating ion-ion separation based on the success criteria of product purity and recovery is established for NF at both material level and process level. Next, a multi-pass NF process with brine recirculation is proposed to achieve high Li/Mg selectivity without sacrificing Li recovery. The dissertation then extends the counter-ion condensation model to mixed salts scenarios for describing ion partition and mobility inside ion exchange membranes when used in ED. Finally, the performance of NF and ED based Li/Mg separation is compared under a unified mass transport model from coupon-scale to module-scale. The impacts of membrane properties and operating conditions on the separation performance are investigated and an important operational tradeoff between Li/Mg selectivity and Li recovery at the module-scale is elucidated for both NF and ED. This dissertation can contribute to the development of novel membranes with superior ion-ion selectivity and to process innovation for precise ion-ion separations.