Separation of divalent ions from seawater concentrate to enhance the purity of coarse salt by electrodialysis with monovalent-selective membranes

In this study, an electrodialysis (ED) system which was divided into three-stage operation was designed to treat seawater concentrate. The experiment was carried using a laboratory ED-cell with an effective area of 189 cm2. Two types of monovalent selective ion-exchange membranes were investigated: CIMS/ACS and CSO/ASV. The effect of applied current density during ED process was also studied. The experimental results indicate that the separation performance for divalent ions (i.e., Ca2 +, Mg2 +) with CIMS/ACS membranes stack was superior to CSO/ASV membranes stack; furthermore, a lower current density can increase the selectivity in monovalent ions to divalent ions with either the CIMS membrane or the CSO membrane. The current efficiency and energy consumption were optimal at a current density of 4 mA/cm2 by using CIMS/ACS membranes stack as the first stage of system in this experiment. Furthermore, the desalination rate (70%) was chosen as the experimental operation endpoint of the first-stage ED operation based on the experimental results. Moreover, the latter two-stage operation was used to concentrate brine to produce coarse salt after evaporation process. Finally, the repeated batch experiments confirmed the system feasibility for treating seawater concentrate to produce coarse salt with the purity of ~ 85% under continuous operation.