Demineralisation is the Process of removing the mineral salts from water by ion-exchange. Impurities that remains dissolved in water dissociate to form positive and negative charged particles known as ions. These impurities or compounds are called electrolytes. Generally, all natural water has electrolytes in varying concentrations. An ion-exchange vessel holds ion-exchange resin of the required type through which water is allowed to pass. The selective ions in the water are exchanged with ions or radicals loosely held by the resin. In this way, the water is passed through several vessels or a mixed bed vessel so that both positive and negative ions are removed and water is dematerialized.
The two-bed deionizer consists of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form. Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions.
To keep the water electrically balanced, for every monovalent cation, e.g. Na+, one hydrogen ion is exchanged and for every divalent cation, e.g. Ca2+, or Mg2+, two hydrogen ions are exchanged. The same principle applies when considering anion-exchange.
The desalinized water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).