Basic Principles
5
Cat. No. 01024000
Basic Principles
The Deionization Process
Deionization or demineralization is the process of removing up to 95 percent of the dissolved minerals such as calcium,
magnesium, sodium, manganese, sulfates, chlorides, nitrates and silica from the water supply by way of ion exchange.
Dissolved minerals in the water consist of two parts—a metallic part which is a positively charged ion and a non-metal-
lic part which is a negatively charged ion. Two separate resins are utilized for the Deionization process; the cation resin
exchanges the positive ions for hydrogen ions (H+) while the anion resin exchanges the negative ions for hydroxide ions
(OH-). The cation resin is regenerated with hydrochloric acid (HCl), and the anion resin is regenerated with sodium hy-
droxide (NaOH), also known as caustic soda.
Cations:
Ammonium (NH
4
+
)
Sodium (Na
+
)
Potassium (K
+
)
Magnesium (Mg
++
)
Calcium (Ca
++
)
Ferrous iron (Fe
++
)
Copper (Cu
++
)
Zinc (Zn
++
)
Aluminum (Al
+++
)
Chrome (Cr
+++
)
Ferric iron (Fe
+++
)
Hydrogen (H
+
)
Anions:
Chloride (Cl
–
)
Bicarbonate (HCO
3
–
)
Nitrate (NO
3
–
)
Bisulfate (HSO
4
–
)
Carbonate (CO
3
– –
)
Silicate (SiO
3
– –
)
Sulfate (SO
4
– –
)
Carbon dioxide (CO
2
)
Silica (SiO
2
)
Hydroxide (OH
–
)
H
2
O
CATION
ANION
Figure 2. Overview of deionization process.
The raw water enters the top of the first column and flows down through the resin. The cationic part of the dissolved
minerals in the raw water is exchanged for hydrogen (H+) as the water passes through the cation resin. The water then
flows out the bottom of the first column and into the top of the second column and flows down through the anion resin.
The anionic part of the dissolved minerals is exchanged for hydroxide (OH-) as the water passes through the anion resin.
Because the cations were exchanged for hydrogen and the anions were exchanged for hydroxide, the water exiting the
bottom of the anion column is H
2
O with most of the minerals removed.
Cation and strong base anion resins are attracted to certain ions more than others; this phenomenon is referred to as
affinity or selectivity. Table 1 shows the strong acid cation resin affinities and the strong base anion affinities.
Table 1. Strong acid cation and strong base anion affinities.
Cation Affinities
Strong Base Anion Affinities
Less
More
Aluminum
Sulfate
Iron
Carbonate
Calcium
Nitrate
Magnesium
Chloride
Potassium
Bicarbonate
Sodium
Silica
