The terms ion exchange, deionization and demineralization are often used synonymously to refer to the same process. However, deionization and demineralization may be accomplished by technologies other than ion exchange (see reverse osmosis). Ion exchange is a process used extensively in nuclear facilities, industrial processes and medical and pharmaceutical operations to control the purity and pH of water by removing undesirable ions and replacing them with acceptable ones.
Specifically, it is the exchange of ions between a solid substance (called a resin) and an aqueous solution, typically municipal, potable water but it frequently involves process or even waste water (e.g., the plating industry). Depending on the identity of the ions that a resin releases to the water, the process may result in water purification or in control of the concentration of a particular ion in a solution. An ion exchange is the reversible exchange of ions between a liquid and a solid.
This process is generally used to remove undesirable ions from a liquid and substitute acceptable ions from the solid (resin). The devices in which ion exchange occurs are commonly called demineralizers. This name is derived from the term demineralize, which means the process whereby impurities present in the incoming fluid (water) are removed by exchanging impure ions with H and OH ions, in the formation of pure water. H+ and OH- are present on the sites of resin beads contained in the demineralizer tanks or columns.
Cation Exchange Resins vs. Anion Exchange Resins
There are two general types of ion exchange resins: those that exchange positive ions, called cation exchange resins, and those that exchange negative ions, called anion exchange resins. A cation is an ion with a positive charge. Common cations include Ca+2, Mg+2, Fe+2, and H+1. A cation resin is one that exchanges positive ions. An anion is an ion with a negative charge. Common anions include Cl-1, SO4-2, and OH-1 . An anion resin is one that exchanges negative ions. Chemically, both types 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 demineralizer is a vessel, usually with a volume of several cubic feet, that contains the resin.
A demineralizer may contain an intimate mixture of both cation exchange resins and anion exchange resins and is called a mixed bed. Two-bed demineralizers have two vessels, the first containing cation resin followed by a separate vessel containing anion resin. Physically, ion exchange resins are formed in the shape of very small beads, called resin beads, with an average diameter of about 0.5 millimeters. Wet resin has the appearance of damp, transparent, amber sand and is insoluble in water, acids, and bases. The ratio is normally 2 parts cation resin to 3 parts anion resin.
The ion exchange resin is contained in a vessel with a volume of several cubic feet. Retention elements at the top and bottom consist of screens, slotted cylinders, or other suitable devices with openings smaller than the resin beads to prevent the resin from escaping from the vessel. When the resin bed is a uniform mixture of cation and anion resins in a volume (typically in a ratio of 2 parts cation resin to 3 parts anion resin). This arrangement is called a mixed-bed resin, as opposed to an arrangement of cation and anion resins in discrete layers or separate vessels.
The use of different volumes of the two types of resins is due to the difference in exchange capacity between cation and anion resins. Exchange capacity is the amount of impurity that a given amount of resin is capable of removing, and it has units of moles/ml, equivalents/ml, or moles/gm. The anion resin is less dense than the cation resin; thus, it has a smaller exchange capacity, and a larger volume is needed for anion resins than for the cation resins to obtain equal total exchange capabilities.
The "reactivation" process is called regeneration and is carried out using a strong acid for the cation (as a source of hydronium ions) and liquid caustic (sodium hydroxide) as a source of hydroxyl ions for the anion. Resin is regenerated on site using hydrochloric or sulfuric acid and sodium hydroxide (caustic) to regenerate the cation and anion respectively. Controls cause the resin to backwash and then draw a set amount of regenerant chemical for a specified period of time and at a specified flowrate, followed by a slow and fast rinse. In the case of mixed bed resin, a controlled backwash causes the resin to separate and two manifolds function to direct acid to the cation and caustic to the anion. Alternatively, the resin may be furnished in vessels by the WaterProfessionals® and regenerated off site, with the many advantages shown under deionization.