WATER TREATMENT PROCESSESDOE-HDBK-1015/2-93Principles of Water TreatmentCH-04Rev. 0Page 4There are two general types of ion exchange resins: those that exchange positive ions, calledcation resins, and those that exchange negative ions, called anion resins. A cation is an ion witha positive charge. Common cations include Ca , Mg , Fe , and H . A cation resin is one that+++++++exchanges positive ions. An anion is an ion with a negative charge. Common anions includeCl , SO , and OH . An anion resin is one that exchanges negative ions. Chemically, both types----4are similar and belong to a group of compounds called polymers, which are extremely largemolecules that are formed by the combination of many molecules of one or two compounds ina repeating structure that produces long chains.A mixed-bed demineralizer is a vessel, usually with a volume of several cubic feet, that containsthe resin. Physically, ion exchange resins are formed in the shape of very small beads, calledresin beads, with an average diameter of about 0.005 millimeters. Wet resin has the appearanceof damp, transparent, amber sand and is insoluble in water, acids, and bases. Retention elementsor other suitable devices in the top and bottom have openings smaller than the diameter of theresin beads. The resin itself is a uniform mixture of cation and anion resins in a specific volumeratio depending on their specific gravities. The ratio is normally 2 parts cation resin to 3 partsanion resin.In some cases, there may be chemical bonds formed between individual chain molecules atvarious points along the chain. Such polymers are said to be cross-linked. This type of polymerconstitutes the basic structure of ion exchange resins. In particular, cross-linked polystyrene isthe polymer commonly used in ion exchange resins. However, chemical treatment of polystyreneis required to give it ion exchange capability, and this treatment varies depending on whether thefinal product is to be an anion resin or a cation resin.The chemical processes involved in producing anion and cation resins are outlined in Figure 1and Figure 2, beginning with the formation of cross-linked polystyrene. The polymer itself is acovalent compound. By the chemical reactions indicated in Figure 2, hydrogen atoms covalentlybonded to the original polymer at certain sites are replaced by functional groups (called radicals)such as SO H (sulfonic acid) and CH N(CH ) Cl (quaternary ammonium). Each such group is3233covalently bonded to the polymer, but each also contains an atom that is bonded to the radicalgroup by a predominantly ionic bond. In the two examples above, H in SO H and Cl in3CH N(CH ) Cl are the ionically-bonded atoms. Sometimes these are written as SO H and2333-+CH N(CH ) Cl to emphasize their ionic characters. These ions (H and Cl ) are replaceable by233+-+-other ions. That is, H will exchange with other cations in a solution, and Cl will exchange with+-other anions.In its final form, an ion exchange resin contains a huge, but finite, number of sites occupied byan exchangeable ion. All of the resin, except the exchangeable ion, is inert in the exchangeprocess. Thus, it is customary to use a notation such as R-Cl or H-R for ion exchange resins.R indicates the inert polymeric base structure and the part of the substituted radical that does notparticipate in exchange reactions. The term R is inexact because it is used to represent the inertportion of both cation and anion resins, which are slightly different. Also, the structurerepresented by R contains many sites of exchange, although only one is shown by the notation,such as R-Cl. Despite these drawbacks, the term R is used for simplicity.
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