Continuous ion-exchange membrane electrodialysis of mother liquid discharged from a salt-manufacturing plant and transport of Cl- ions and SO42- ions

Mother liquid discharged from a salt-manufacturing plant was electrodialyzed at 25 and 40 degrees C in a continuous process integrated with SO42- ion low-permeable anion-exchange membranes to remove Na2SO4 and recover NaCl in the mother liquid. Performance of electrodialysis was evaluated by measuring ion concentration in a concentrated solution, permselectivity coefficient of SO42- ions against Cl- ions, current efficiency, cell voltage, energy consumption to obtain one ton of NaCl and membrane pair characteristics. The permselectivity coefficient of SO42- ions against Cl- ions was low enough particularly at 40 degrees C and SO42- transport across anion-exchange membranes was prevented successfully. Applying the overall mass transport equation, Cl- ion and SO42- ion transport across anion-exchange membranes is evaluated. SO42- ion transport number is decreased due to the decrease of electro-migration of SO42- ions across the anion-exchange membranes. SO42- ion concentration in desalting cells becomes higher than that in concentration cells and SO42- ion diffusion is accelerated across the anion-exchange membranes from desalting cells toward concentrating cells.