Quantcast Principles of Ion Exchange

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WATER TREATMENT PROCESSES DOE-HDBK-1015/2-93 Principles of Water Treatment CH-04 Rev. 0 Page 4 There are two general types of ion exchange resins: those that exchange positive ions, called cation resins, and those that exchange negative ions, called anion resins.  A cation is an ion with a 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 include Cl , SO   , and OH .  An anion resin is one that exchanges negative ions. Chemically, both types - -- - 4 are  similar and belong to a group of compounds called polymers, which are extremely large molecules that are formed by the combination of many molecules of one or two compounds in a repeating structure that produces long chains. A mixed-bed demineralizer is a vessel, usually with a volume of several cubic feet, that contains the resin.  Physically, ion exchange resins are formed in the shape of very small beads, called resin beads, with an average diameter of about 0.005 millimeters.  Wet resin has the appearance of damp, transparent, amber sand and is insoluble in water, acids, and bases.  Retention elements or other suitable devices in the top and bottom have openings smaller than the diameter of the resin beads.  The resin itself is a uniform mixture of cation and anion resins in a specific volume ratio depending on their specific gravities.  The ratio is normally 2 parts cation resin to 3 parts anion resin. In  some  cases,  there  may  be  chemical  bonds  formed  between  individual  chain  molecules  at various points along the chain.  Such polymers are said to be cross-linked.  This type of polymer constitutes the basic structure of ion exchange resins.  In particular, cross-linked polystyrene is the polymer commonly used in ion exchange resins.  However, chemical treatment of polystyrene is required to give it ion exchange capability, and this treatment varies depending on whether the final 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 1 and Figure 2, beginning with the formation of cross-linked polystyrene.  The polymer itself is a covalent compound.  By the chemical reactions indicated in Figure 2, hydrogen atoms covalently bonded 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 is 3 2 3 3 covalently bonded to the polymer, but each also contains an atom that is bonded to the radical group  by  a  predominantly  ionic  bond.    In  the  two  examples  above,  H  in  SO  H  and  Cl  in 3 CH  N(CH  )  Cl are the ionically-bonded atoms.  Sometimes these are written as SO    H   and 2 3 3 3 - + CH N(CH )   Cl  to emphasize their ionic characters.  These ions (H  and Cl ) are replaceable by 2 3 3 + - + - 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 by an  exchangeable ion.  All of the resin, except the exchangeable ion, is inert in the exchange process.  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 not participate in exchange reactions.  The term R is inexact because it is used to represent the inert portion  of  both  cation  and  anion  resins,  which  are  slightly  different.    Also,  the  structure represented 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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