Understanding sulfate transport phenomena during electrochemical acid recovery from waste MgSO4: A modelling approach

Electrochemical membrane separation is an effective technique for minimizing process waste, particularly when waste-specific mitigation practices do not offer a pathway for resource recovery. Primary nickel production generates considerable amounts of magnesium sulfate waste solutions, which are typically stored in evaporation ponds or disposed into water bodies. By electrochemically treating these magnesium sulfate solutions through water splitting, sulfuric acid and magnesium hydroxide can be produced and reused within the nickel production process, reducing the need for additional process reagents. This water splitting technology involves the transport of multi-ionic species within the electrochemical cell. Investigating sulfate transport at waste solution concentrations poses challenges due to its varying speciation throughout the electrolyser, which can significantly influence faradaic efficiency. In this work, a 1-D numerical model based on a Finite Element Method is developed to understand the flux behaviour of all of ions involved during magnesium sulfate water electrolysis. Results showed that proton back-diffusion, rather than migration, is responsible for 85 % of the total proton leakage. It also revealed that HSO₄⁻ is the major species balancing the membrane charge under the evaluated conditions. High current densities increased the rate of sulfate membrane speciation, while increasing the MgSO₄ catholyte concentration beyond 1 M showed minimal impact on sulfate flux. The electrolyte potential drop calculated by the model ranged from 0.83 to 1.5 V. The model results are validated against our previously published experimental work and demonstrated a good agreement with experimental acid recovery.