The important information in this chapter is summarized below.

Ion Exchange Summary

Demineralize is defined as the process whereby impurities present in the incoming fluid (water) are removed by exchanging impure ions with H+ and OR ions resulting in the formation of pure water.

Ion exchange is a process used extensively in nuclear facilities to control the purity and pH of water by removing undesirable ions and replacing them with acceptable ones.

Mixed-bed demineralizer is a vessel containing resin that is a uniform mixture of cation and anion resins in a specific volume ratio depending on their specific gravities. Normally the ratio is 2 parts cation resin to 3 parts anion resin.

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.

Anion is an ion with a negative charge. Common anions include , and OH-. An anion resin is one that exchanges negative ions.

Decontamination factor (DF) is a ratio of the concentration (or activity) of the fluid at the inlet compared to the concentration (or activity) at the effluent, which defines the effectiveness of the ion exchange process. Mathematically it is:

Polymers 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.

Affinity is often used to describe the attraction between a resin and a given ion. This affinity can be described quantitatively by experimental determination of a parameter called the relative affinity coefficient.

Resin beads are long-chain cross-linked polymers that contain sites occupied by exchangeable ions. The general order of affinity serves as a useful guide in understanding many ion exchange processes. Cation and anion resins are named according to the identity of the ion occupying the exchange sites, such as hydrogen, hydroxyl, and chloride. Mixed-bed resins are used to remove both cations and anions.

The effectiveness of any ion exchanger is directly related to the relative affinities between a resin and different ions. In order of decreasing strength, the relative affinities between a cation resin and various cations are:

Similarly, the relative affinities of the anion resin for various anions are:

The higher the relative affinity the more effective the ion exchanger. This effectiveness is expressed by the Decontamination Factor.

The reaction for removal of NaCl and CaS0₄ by a mixed-bed ion exchanger such as one containing HOH resin is as follows: