PURPOSE: To separate and recover high-purity Nail from a liq. mixture of Na, Ca, Al and SO4 by arranging a univalent ion permselective cation-exchange membrane, univalent ion permselective anion-exchange membrane, non- permselective cation-exchange membrane and non-permselective anion-exchange membrane in this order as the membranes.

CONSTITUTION: A liq. mixture 31 is introduced into a desalted liq. circulating line by a pump 21 and sent to an electrodialysis cell 1, and a DC voltage is impressed thereon. Consequently, the Na in the liq. mixture sent to the desalting chambers 16 and 18 is permeated through a univalent ion permselective cation- exchange membrane 12 and tune Cl through a univalent ion permselective anion- exchange membrane 13 and both are introduced into a concentration chambers 17, and NaCl concentrated to high purity is recovered. Meanwhile, the Ca in the liq. mixture 31 sent to desalting chambers 18 and 20 is permeated through a non-permselective cation-exchange membrane 1 4 and the SO4 through a non-permselective membrane 15 and both are introduced into a concentration chamber 19, and CaSO4 is concentrated. The NaCl concd. liq. 34 is introduced into an NaCl concd. liq. circulating line by a concentration pump 22, and NaCl is discharged.