Basics of Ion Exchange

Ion exchange is a best technology to remove dissolved contaminant from water, like calcium and magnesium hardness, dissolved ions etc. Dissolved contaminants of water contains cations and anions.

Dissolved substances are present in water as ions, which are electrically charged. Positively charged ions are known as cations and negatively charged ions are known as anions. In water, cation are always equal to anions, or wise a versa.

Water contain following types of ions

• A monovalent monoatomic cation: the sodium ion Na⁺
• A divalent monoatomic cation: the calcium ion Ca⁺⁺
• A monovalent polyatomic cation: the ammonium ion NH₄⁺
• A monovalent monoatomic anion: the chloride ion Cl^–
• A monovalent polyatomic anion: the nitrate ion NO₃^–
• A divalent polyatomic anion: the carbonate ion CO₃^2-
• Another divalent polyatomic anion: the chromate ion (metallic complex) CrO₄^2-

Dissolved salts are in ionized form and ions are able to move freely in water. They are not attached to ions of opposite charge. Water is always neutral, as sum of charge is same for all cations and all anions.

Sum of Cations = Sum of Anions

Salts are crystallised substances containing a fixed proportion of cations and anions. When you dissolve a salt into water, its cations and anions are dissociated.

Sodium carbonate (Na₂CO₃) you have sodium cations Na⁺ and carbonate anions CO₃^2-, so that you need 2 sodium ions for each carbonate ion. Calcium chloride is made of calcium ions Ca²⁺ and chloride ions Cl^–.

When water is boiled upto complete dry, we see residue at the bottom of plate that is because of dissolved contaminants in water. This dry residual is called Total dissolved solides or TDS.

Impurities in Water:

Water is universal solvent, because water has capability to dissolve a variety of substances. Also water content some insoluble substances which are not dissolved. These substances to be separated through simple filtration method.

Soluble content is removed by Ion Exchange process.

Ion Exchange Resin:

Ion exchange resin is a polymeric compound, which has active function group to act as active site to remove dissolved cation or anion. It is a polymer insoluble matrix normally in the form of small beads of diameter from 0.5 to 1 mm. The beads are typically porous, providing a high surface area. The trapping of ions occurs with the accompanying releasing of other ions; thus the process is called ion-exchange. There are multiple types of ion-exchange resin. Most commercial resins are made of polystyrene sulfonate.

Application of Ion Exchange:

There is wide application of ion exchange in process industry, out of that some are listed below.

1. Waste softening
2. Demineralization
3. Metal separation
4. Juice purification
5. Sugar Deashing/ sugar syrup purification
6. Pharmaceutical application

Steps involved in Ion Exchange process:

1. Regeneration of IX bed
2. Service cycle
3. Exhaustion of IX bed
4. Regeneration of IX bed

Regeneration Step involve following cycles

1. Backwash (For co- flow system)
2. Injection of regenerant
3. Displacement
4. Rinse