When minerals dissolve in water, they dissociate into positively charged cations and negatively charged anions. The demineralization process exchanges these ions with hydrogen (H+) and OH– in the ion exhange resins producing "pure" water in the effluent.
Although both the cation and the anion resin are responsible for the quality of a deionization (DI) system effluent, it is the cation resin that is the big contributor to leakage. Improving the leakage characteristics of DI cation will reduce effluent conductivity, drop the pH and will lead to better silica removal efficiency.
The effluent of a dual bed DI system is generally alkaline or high pH (8.5 to 10). This is because the low pH of the cation reaction (pH 2.5 to 3.0) causes some of the residual sodium left on the resin from the previous regeneration to "leak" during the run. Sodium is the least tightly held of the cations. In the anion bed, this sodium is converted to sodium hydroxide (NaOH), which causes the high pH. One ppm of Na+ in the effluent will give a pH of about 9.5 and 4 ppm will result in a pH of around 10.0. This pH will also give rise to an increase of silica in the final effluent (leakage from the anion). It is, therefore, apparent that improving the leakage performance of the cation resin is key to overall good product quality from the system.
One way to reduce overall cation leakage is to increase the acid dosage during regeneration of the cation resin. In fact, the regenerant level is about the only controllable variable. However, there is a question in the economics for increasing the acid dose compared to the increase in capacity and reduction of leakage. Typical acid levels are approximately 8 pounds per cubic feet of resin.
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