Electrochemical separation of sulfuric acid from magnesium sulfate solutions: Application for nickel laterite processing

Abstract

Nickel has wide range of applications, and its production is expanding to meet the growing demand especially for energy storage applications. Hydrometallurgical nickel extraction from laterite ores requires a significant amount of sulfuric acid, primarily due to acid-consuming minerals such as magnesium silicates. The reaction of acid and magnesium silicates results in magnesium sulfate waste solution which is an environmental liability. Electrochemical separation can be used to produce acid from these waste solutions which can be reused on-site to increase nickel recovery from the resource. In addition, the generated magnesium hydroxide can be used for carbon capture or as a precipitant in the nickel process. In this work, a single membrane electrolyser was used to convert magnesium sulfate solutions into sulfuric acid and a magnesium hydroxide precipitate. The performance of the electrolyser was evaluated in terms of faradaic efficiency, acid production energy intensity, current density and sulfate recovery extent. This evaluation involved varying the electrolyser potential as well as the initial concentrations of catholyte and anolyte. The results indicated that when starting with a MgSO4 concentration of 1 M or lower and applying a 4 V electrolyser potential, 90 % faradaic efficiency can be maintained up to an anolyte concentration of 0.16 M H2SO4. Higher voltages, concentrations and conductivities across the anolyte and catholyte has a detrimental impact on the faradaic efficiency due to water splitting. High electrolyser potential enhances sulfate transport but also results in excessive Mg(OH)2 precipitation, which scales the cathode; an operational challenge that will need to be managed. Sulfate recovery ranged between 43 and 49 % after 4 h when testing initial anolyte concentrations of 0.1 and 0 M, with an average energy intensity of 2.94 and 4.44 kWh/kg-H2SO4, respectively.