Process for electro-dialysis

In a process for electro-dialyzing an electrolyte solution in an electro-dialysis apparatus, comprising the steps of:

• providing a plurality of alternately counterposed cation exchange membranes and anion exchange membranes;
• providing a plurality of frames interposed between the ion exchange membranes, thereby forming dilution chambers and concentration chambers alternately between the counterposed ion exchange membranes;
• providing supply and discharge passageways for the electrolyte solution, through the frames, through which the electrolyte solution is introduced to and discharged from the respective dilution chambers and concentration chambers,
• providing supply and discharge passageways for air bubbles, formed through the frames, through which air bubbles are introduced to lower parts and discharged from upper parts of the dilution chambers;
• applying an electric potential to a pair of electrodes respectively at both ends of the plurality of the dilution chambers and the concentration chambers, thereby diluting the electrolyte solution in the dilution chambers and concentrating the electrolyte solution in the concentration chambers by migration of ions due to electrophoresis of ions in the electrolyte solution and selective ion permeability of the ion exchange membranes;
• passing electrolyte solution diluted in the dilution chambers and electrolyte solution concentrated in the concentration chambers through their respective chambers and passageways;
• passing air through the air passageways so as to introduce air bubbles to the lower portions of the dilution chambers and discharge the air from the upper portions of the dilution chambers;
• measuring the limiting current density (i.sub.o) without air bubbles in the chambers;
• monitoring the limiting current density (i) during operation when the air bubbles are introduced in the chambers;
• determining the ratio of i/i.sub.o ;
• measuring the electrical resistance between the electrodes (R.sub.o) without air bubbles in the chambers;
• monitoring the electrical resistance (R) between the electrodes during operation when the air bubbles are introduced in the chambers;
• determining the ratio R/R.sub.o ;
• and
• controlling the superficial velocity of air bubbles (Va) through the dilution chambers during operation so that Va is maintained within the range of superficial air velocity that maintains R/R.sub.o less than one and i/i.sub.o constant and Va optimum with respect to both conductivity of electrolyte and resistance between electrodes, whereby the thickness of the diffusion layers formed by concentration polarization near the ion exchange membranes in the dilution chambers are maintained smaller than when no air bubbles are introduced thereto.