Study of electrodialysis concentration process of inorganic acids and salts for the two-stage conversion of salts into acids utilizing bipolar electrodialysis

Abstract

The electrodialysis concentration process of sodium chloride, sodium sulfate, lithium chloride, ammonium nitrate, sulfuric and nitric acids were studied. Using the model of electrodialysis concentration, the transport coefficients of the Ralex CM-PP/Ralex AMH-PP membrane pair were calculated. It has been established that for salt solutions, the key factors determining the maximum attainable electrolyte concentration in the concentration chamber are the electromigration current efficiency and the hydration number of the electrolyte. For acid solutions, the most important factors determining their concentration are the osmotic and electroosmotic transport of water through the membrane. Besides, it was found that in the process of electrodialysis concentration of acids, proton leakage occurs through the strongly basic anion-exchange membrane from the concentration chamber to the desalination chamber, which significantly reduces the current efficiency. To obtain concentrated solutions of sulfuric acid with a low content of salt ions, a two-stage scheme for the recovery of the sodium sulfate into sulfuric acid using bipolar electrodialysis (BMED) and an electrodialyzer-concentrator (EDC) is proposed. A two-stage process, including BMED and EDC, for the conversion of sodium sulfate into sulfuric acid allows one to obtain acid with a concentration of up to 1.52 M by sulfate ion with a content of sodium ions of not more than 0.005 M. At a reasonable current density of 2 A/dm2 the obtained concentration of acid was 1.16 M by sulfate ion with current efficiency of 0.89 for BMED and 0.26 for EDC, total specific energy consumption of 0.83 kWh/mol SO42-, and acid flux of 0.024 mol SO42-·dm−2·h−1 (0.23 kg H2SO4·m−2·h−1). The obtained specific characteristics of the process are comparable with the characteristics of bipolar electrodialysis using industrial bipolar membranes.