De-ionizer

Deionizer helps to remove almost all minerals in the water by using the resins.The resins are organic substances, industrially prepared.Their special characteristic is that they have a “mobile” functional group, on a base called matrix. The functional group is a part chemically balanced between the matrix of the resins itself and the ions dissolved in the water. The cation resins will have the functional group balanced with cations, while anion resins will be with anion. This way allows the exchange of ions between the resins and the water.

A cation resin can exchange cations when its functional group is “equipped” with a sodium ion or hydrogen ion. The first case, the resin can exchange only ions of calcium and magnesium, and releasing sodium (Na+) to the water, so these resins can be used for softening only (see bulletin “softening”).

The second case, instead, the resin can exchange all the cations in water, releasing hydrogen ion (H+). This way allows to get water completely free of cations, and containing only the acids (the anions) related to the dissolved salts. An anion resin, instead, can exchange anions when its functional group is “equipped” with a hydroxyl (OH–). Therefore, whether a water completely free of cations flows through an anion resins, the final result will be a water completely free of cations and anions, hence a water completely free of salts (deionized water).

The chemical reactions of the above descriptions are:

Cation Exchange:

Anion Exchange:

Deionization (sum of both):

Where C+ and A– stands for the ions (salinity as dissociated form), while H+ and OH– will join to form water (H2O).

The exhausted resins can be regenerated, meaning to re-build the original form, according to the aforementioned reactions. For example, if we increase very much the concentration of H+ ions (see cation exchanging reaction) the reaction itself will be balanced towards left part of the same, so the resins in H+ form is reproduced and cations are released.

The same will happen for anion exchanging whether we increase concentration of OH– ions.

Since H+ ions are typical of acid and OH– of alkaline, the regeneration of the resins is featured by flowing hydrochloric acid (HCl) or sulphoric acid (H2SO4) through cation resins and caustic soda (NaOH) through anion resins.

The practical application of the above theorical principle is a little bit more complicated that how it appears. There are many other factors, not considered until now, that can influence the ion exchanging and the deionization process. One of them is the fact that it is quite impossible to exchange all cations and anions (then to remove it off water); the aforementioned reactions is a balance, then it is obvious that there will be always a leakage of some ions and, therefore, a residual of salinity in treated water.

When deionized water is required, it is important to specify the quality, hence the max allowable value of salinity, according to the application of a deionizer. It will allow to select and/or design the most suitable deionization system for the application, and according to the quality of the available raw water. Indeed, several type of resins (cation and anion) are available, with very different chrematistics of structure, performances, etc. The utilization of these resins is usually selected according to the special case, in order to always identify the most suitable deionization system.