Electrodialytic concentration of NaCl for the chlor-alkali industry

Abstract

Production of chlorine and sodium hydroxide in the membrane electrolysis plants requires saturated brines, free of calcium, magnesium, and sulfates. On the other hand, desalination plants produce waste brines, which are considered as an environmental hazard. Electrodialysis with univalent permselective membranes can simultaneously preconcentrate the sodium chloride solution up to the level required by the chlor-alkali industry and decrease the salinity of the desalination plant brines. In order to assess the bivalent ions transport across univalent permselective membranes during the NaCl concentration process, a series of batch-scale experiments were performed in an electrodialyzer of 4.5 cm2 effective membrane area and four membrane pairs. The tested membranes were Neosepta CMS/ACS and Aciplex K-192/A-192, the tested concentration range was 0.3–3.0 g/L in case of Ca2+, 0.6–6.0 g/L in case of Mg2+, 1–20 g/L in case of SO42-, and 35–300 g/L in case of NaCl. The results allowed the estimation of empirical mass transport coefficients in the membrane. Based on the estimated parameters and a hydrodynamic model of a single-pass electrodialyzer, an electrodialyzer was designed and the energy consumption of saturated brine production was estimated.