THE DEIONIZATION PROCESS

The purpose of this article is to give basic information about the two-column (segregated) deionization, utilizing an automatic system controlling the regeneration process.
The terms “Pure Water” and “High Quality”, often used when discussing deionized water, denote only the dissolved mineral materials or substances of the water.
As water as such is an excellent solvent, when it reaches the point of its use, it contains other dissolved minerals or suspended materials.
Carbon dioxide is dissolved in it when water in the form of rain falls. The air borne particulate matter contaminates the reservoir water. Silica slowly and gradually gets dissolved in the reservoir water from the reservoir walls.
Water, as it passes through ground to underground water supplies dissolves and suspends additional minerals such as iron, magnesium and calcium, along with additional silica. As it is transferred from the point of natural storage to end consumer, water again is contaminated by iron primarily from iron piping.
Water contains dissolved minerals in the form of ions. Several commercially available process may be employed to reduce the dissolve mineral contents of water, which include reverse osmosis, distillation and deionization. Our discussion will concentrate mainly on the deionization process in this article.
The process of removal or reduction of negative ions (anions) and positive ions (cations) is commonly referred to as deionization, which is employed by passing water first through a column of cation exchange resin, then through a column of anion exchange resin. The result of this process is water that has most of the dissolved minerals removed from it.
The deionized water quality that is produced will be determined by several factors.
As the untreated water passes through the cation resin column, the positively charged ions of calcium, magnesium, sodium, potassium, etc are exchanged for hydrogen ions that were placed on the cation resin during regeneration of the resin with hydrochloric acid. The hydrogen ions combine with the carbonates, chlorides, nitrates, sulfates, silicates, etc that are in the water, forming weak solutions of carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, silicic acid, etc. The decationized water will have a low pH (about 3.0) as a result of the weak acids present in the water.
The decationized water then passes through the anion resin column where the carbonates, chlorides, nitrates, sulfates, etc, are split away from their respective acids and are exchanged for hydroxide ions that were placed on the anion resin during regeneration process of the resin with sodium hydroxide. The available hydrogen ions as a result of the splitting of the acids are free to combine with the hydroxide ions.
The result is as follows:
Hydrogen (H+) + Hydroxide (OH-) = HOH =H2O
H2O is water.
The weakly basic anion resin cannot split carbonic acid and silicic acids; therefore, the final water quality is lower than if strongly basic resin were used in the process.

USES OF DEIONIZED WATER:
The uses of deionized water vary significantly. Some of the applications in common are:
Rinsing of electronic components
Manufacture and rinsing of glassware
Final rinse of surgical and laboratory instruments prior to sterilization
Preparation of laboratory chemicals
Preparation of process chemicals and cosmetics
Final rinse at car washes

With constant refinements, the use of deionized water is growing gradually and steadily in the chemical industries, pharmaceutical industries and electronics.
The desired quality of the treated water is dependant on the application and is expressed in terms of the conductance of the water in micromho/cm.
The use of most common anion resin (called weakly basic), a water quality of 100 micromhos/cm to 10 micromhos/cm can be obtained. For silica and carbon dioxide removal, strongly basic anion resin must be used, and the resulting water quality can be 20 micromhos/cm to 1 micromhos/cm.

To get a water quality better than 1micromho/cm, mixed bed deionizers must be used.