Ion Exchange
A process in which Contaminant ions are exchanged from water is called ion exchange. This process occurs in ion exchange resin Initially, resin is loaded with hydrogen (H ) ion for cations and hydroxide (OH-) ion for anions. As water passes through the resin, the contaminant ions in the water displace the loaded ions from the sites on the resin. This is because the resin has a greater affinity for the contaminant ions. Affinity for most resins is based loosely on ionic size and charge
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Regeneration
Over a period of time most of the cation resin gets converted into Ca, Mg, Na form and it has no Hydrogen Ions (H ) to replace Ca, Mg, Na ions and the anion resin gets converted into Cl, SO4, SiO2 form and it has no Hydroxyl Ions (OH-) to replace Cl, SO4, SiO2 ions.. This is called the resin exhaustion or end of service cycle. The exhausted resin has to be brought back into Hydrogen (H ) Hydroxyl (OH-) form to continue the process and use it again and again.
The procedure of converting exhausted resin into Hydrogen (H ) Hydroxyl (OH-) form is called regeneration
Separate Bed DM Systems
Ion exchange occurs across at least two different columns. The first is a cation column, which is filled with resin loaded with positive hydrogen ions (H ). As water passes through this column, the positively charged impurities in the water are exchanged for the hydrogen ions
A similar exchange takes place in the second column, the anion column. This column is filled with resin loaded with negative hydroxide ions (OH-). As the now cation-free water from the cation column passes through, any negatively charged contaminants are removed from the water and replaced by hydroxide ions from the resin. The water now has a neutral pH. hydroxide [OH-] and hydrogen [H ] ions exchanged in the columns combine, forming water [H2O)
Mixed Bed Systems
In a Mixed Bed System water is purified by removing all ions in one pass through an ion exchange bed. The bed consists of cation and anion resins in mixed condition, so that ions are actually removed in the same two step ion exchange process. All the traces of Cations and Anions of the dissolved solids and silica in the water are replaced by Hydrogen ion (H) and Hydroxyl Ion (OH-) respectively. We get very high purity demineralized water at the outlet of the MB.
