Study On The Mechanism Of Continuous Deionization In The EDI Process

With the development of scientific technology, purity water, especially ultrapure water has applied widely in many engineering fields such as power, semiconductor, biology, pharmacy, and chemical engineering, etc. Electrodeionization (EDI) is a membrane separation process associating electrodialysis with ion-exchange. It combines the advantages of ion exchange and electrodialysis. In addition, it can lessen the problems produced in each of these separate technologies. EDI with reverse osmosis (RO) is an inevitable development tendency to produce super-purity water.Nowadays, a majority of EDI devices adopted in industries are imported and they are expensive. This project makes use of homemade ion-exchange membrane and ion-exchange resin so as to develop the homemade EDI technology. Through investigatation of the Voltage-Current curves and the pH-Current curves during the EDI process under different conditions, this experiment summarizes the mechanism of ion transfer analyzing diverse factors that affects the operation. EDI devices demand the feedwater with low conductivity. Furthermore, the pH value of the concentrating chambers is descending along with the increase of the current due to water decomposing, in the end it comes to the balance. However, the flow flux of feedwater affects little on the resistance of the diluting chambers, so the change range of voltage-current curve is narrow at the different flux. When the flow flux is great, the ions have no time to transfer from the diluting chambers and the quality of product will become bad. At the same time, the different thickness in the diluting chambers lead to the different transferred time. Compared to the thin diluting chambers, the thick diluting chambers need bigger voltage to obtain the same current because the thick diluting chambers filled with more resins.Base on the theory above, through analyzing the layers of resin, the mechanism of EDI is described with "Cavity" Transfer. When the ions in the surface transfers under the effect of the DC electric potential, the resin particles are left a lot of cavities, moreover, it forms the structure of double electric layer. The ions in solution are attracted by these cavities and enter into the layers of resin. Finally these ions transfers to the concentrating chambers under the effect of the DC electric potential and are removed from the device.