FUNDAMENTALS OF DEIONIZATION BY ION EXCHANGE
All naturally-occurring water contains dissolved mineral salts. In solution, salts separate into positively-charged cations and negatively-charged anions. Deionization can reduce the amounts of these ions to very low levels through the process of ion exchange.
Cations are removed by cation exchange resin. It replaces sodium, calcium, magnesium and other cations with hydrogen ions (H ). This exchange produces acids which must be removed or neutralized by anion exchange resin.
Two types of anion resin are used for deionization: weak base resin and strong base resin. Weak base resin absorbs strong acids, while strong base resin exchanges chlorid, sulate, and alkaline anions for hydroxide ions (OH). The hydrogen ions from the cation exchange process combine with the hydroxide ions from the anion exchange process to form water (HOH or HO).
Because the deionization process is so effective, the water quality is usually measured by the water's resistance to electric current (in Ohm-cm). Deionized water quality depends on a variety of factors, including raw water composition, ion exchange resin types and quantities and the number of resin tanks in the system.
Two-bed deionizers use separate tanks, one containing cation resin, the other containing anion resin. A two-bed weak base deionizer typically produces water with electrical resistance of about 50,000 Ohm-cm. A two-bed strong base deionizer typically produces water with electrical resistance of about 200,000 Ohm-cm.
In a mixed-bed deionizer, cation and anion resins are thoroughly mixed in a single tank. The mixed resins act like a series of alternating cation and anion exchange tanks to produce very high quality water. A mixed-bed deionizer typically produces water with greater than 10,000,000 Ohm-cm resistance, which is equivalent to less than 0.05 mg/L of sodium chloride.
The resins need regeneration when they no longer produce the desired quality water. In the case of a two-bed deionizer, the cation tank is backwashed for 5 to 10 minutes, then washed with a 6 percent solution of hydrochloric acid. Then the anion tank is backwashed and washed with a 5 percent solution of sodium hydroxide. After rinsing residual chemicals from each tank, water flows through both tanks to drain until the water reaches the desired quality.
In a mixed-bed deionizer, the resins have to be separated before regeneration. After regeneration and rinsing they have to be remixed, using air, before returning to service.
Although the process is fairly simple in concept, its application is complicated by variables in raw water composition, treated water quality needs, resin selection, chemical dosages and control system requirements. Culligan International Company has trained professionals to help in the selection of the proper deionizer system.
