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∘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∘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.