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Water treatment by ion-exchange methods

General information on ion-exchange resins and principles of ion-exchange processes

The preliminary treatment is not enough for make-up water used in boilers and feed water used in the heat networks. The final stage of water treatment that relates to change of its ion composition till the complete removal of dissolved impurities, is performed by means of the ion-exchange method.

The principle of ion exchange is based on ability of the special materials (ion-exchange resins) to change ion composition of the processed water in order to achieve desired parameters. Ion-exchange resins are insoluble macromolecular substances that are capable of ion exchange reactions due to special functional groups presented in them. In other words, ion-exchange resins are able to capture positive or negative ions from the solution in exchange for the number of other ions contained in the ion exchange resin with the same charge. Depending on the negative and positive charge of the exchanging ions the ion-exchange resins are divided into cation-exchange resins and anion-exchange resins.

Ion exchange reaction proceeds in compliance with the rules common for regular electrolytes, namely the rules of ion exchange equivalence and process reversibility. Moreover, in the ion exchange technology the following factors are of significant importance: ability of predominant absorption of some ions compared with other ones, such ability being called selectivity, and the rate of establishment of the ion exchange equilibrium (ion exchange kinetics).

After Na+ cations in cation-exchange resin are replaced with Ca2+ ions (forward reaction), the backward process (regeneration) usually takes place during which the cation-exchange resin's ability to collect Ca2+ ions from the treated water is regenerated.

Flowing through the ion-exchange resin primary saturated with A ions, the treated solution containing B ions is in constant contact with new layers of the fresh (not exhausted) ion-exchange resin. In the course of time upper layers of ion-exchange resin release all A ions and lose their ion exchange capacity, the layers become somewhat exhausted. As consequence the boundary area where the exchange takes place will move lower and lower downstream the layer. Under the certain conditions this area reaches the fixed width. Distribution of exchanging ions concentration in this area is called "filtration front". In the lower layers (area C) ion-exchange resin is still entirely saturated by A ions. When the lower front line of the boundary area reaches the end of the ion-exchange resin layer, the breakthrough of B ion in filters occurs that is a signal to stop filter for the regeneration.

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* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)