Study On Ion-exchange Packing Materials In EDI Stacks And The Packing Forms

Electrodeionization(EDI) technique is based upon a perfect combination of electrodialysis(ED)and conventional ion-exchange(IE), which makes use of the advantages of both ED and IE. It can product ultrapure water continuously and directly. The difference between EDI and ED is that it is devised to pack the diluting compartment of ED system with ion-exchange materials. So the choice of ion-exchange materials and the packing form of ion-exchange materials are the keys of EDI module.Nowadays, the ion-exchange resin is studied to be the main ion-exchange material for EDI modules. Also there are a few research reports on ion exchange fiber and others new materials, but which have not been used in commercial modules widely. Currently, there are three different kinds of the packing forms of ion-exchange materials, such as mixed bed form, layered bed form and separate bed form. The difference of packing materials or packing forms will influence the EDI process remarkably. So it is significative to start the study on packing materials and packing forms for understanding and realizing EDI process.The different packing forms of mixed bed and layered bed of ion-exchange resin are studied. For the first time, by the method of material balanceable calculation, the steady state of the stack operation is confirmed. Under the steady state, all of the experimental data are obtained. The characteristic curves of voltage-current density (U-i), the pH-current density (pH-i) and resistivity-current density are gained. Through the analysis of characteristic curves of the steady state, the inherence rule of ion-exchange and water decomposition in the module can be achieved.Based on analysis of the characteristic curves and the water decomposition, two points of view are concluded. One is that the ion removal is restricted by the water decomposition, ion transfer and ion exchange orderly as the current density of EDI module is rising gradually. The other is about water dissociation. Under the lower operating current density, water decomposition occurs mainly on the area between cation ion-exchange membrane and anion ion-exchange resin. Along with the rise of current density, water decomposition also exists between cation and anion ion-exchange resin.Moreover, the proportion of anion and cation ion-exchange resin and packing forms of resin are varied. The results indicate that the effect of proportion on EDI process is very obvious in mixed bed EDI module. Even the EDI process can not generate when the proportion is 1/2. There are also some results that involves the layered-bed EDI module, the more layers in diluted compartment, the better for performance of EDI module, the more powerful for capability of removal ions.About removal of the weak-ionized species (CO2,NH3,SiO2), the study shows that both capability of removal ions and product water resistivity decrease as the accretion of the weak-ionized specie concentrate in feed water. At the same time, the removal rate of weak-ionized species is decline. And the effect of CO2 on EDI process is the most obvious.Finally the performance of EDI module with ion-exchange fiber is studied, the fiber is processed before packing in diluted compartment. The purpose of the study is to aid in the development of an ion exchange object for resemble EDI module more conveniently. It can produce water above 15M(?)cm.