Study On The EDI For Production Of Ultra-pure Water

Electrodeionization (EDI) is a novel separation process combining with ion-exchange. The advantage of producing ultra-pure water with EDI is non-pollution and continuous. EDI with reverse osmosis (RO) is an inevitable development tendency to producing ultra-pure water.The mechanism of water dissociation and membrane stack scaling are key problems in the EDI process and its application. In this paper, performances of the EDI process for the production of ultra-pure water and the removal of hardness ions were investigated. The purpose of the study is not only to discuss the rules and influential factors of water dissociation and stack scaling in the EDI process, but also find methods to prevent stack scaling.Through investigation of the Current-Voltage curses and the pH-Current curses during the EDI process under different condition, this experiment summarizes the mechanism of water dissociation in the EDI process. Under low voltage, the pH value of the concentrated water and dilute water is descending along with the increase of the voltage, because water dissociation on the surface of cation-exchange membrane takes place more early and severely than on the surface of anion exchange membrane; Through an certain point of voltage, the current and pH value of the concentrated water and dilute water are continuously rising with the increase of the voltage due to water dissociation on the surface of anion-exchange membrane and resins. The flow flux of the dilute water affects little on water dissociation, but the resistance of the dilute water will descend with the flow flux of the dilute water increased. However, when the flow flux of the concentrated water increases, water dissociation will became weak. Water dissociation will be difficult to take place as conductivity in the feed water is high. At the same time, this study suggested that the optimum operating voltage was 40-45V and the optimum water recover rate was 80%-90%.With the mass balance for hardness ions in the EDI process, operating voltage has an important influence on two different working regimes of EDI process. Under low voltage, EDI works in resin-enhanced regime and the resins in the dilute water remain in the salt forms; under high voltage, the resins are electrochemically converted to the hydrogen and hydroxide forms, and deionization is consistent with a model of continuous regenerated mixed resin bed. The characteristics of the two different regimes are discussed. In this experiment, the demarcation voltage of two different working regimes of EDI process was 25-30V. Base on above conclusions, when the voltage was above 35V, the resistivity of dilute water can be above 13MΩ·cm and removal rate of hardness ions can be up to 96%; scaling is easy to occur in the EDI stack working in resin-enhanced regime. Feed water with high concentration of hardness ions and carbon dioxide can result in scaling.