Water containing calcium and magnesium ions is called 'hard water'. These ions have tendency to associate with other ions and compounds to form hard scales on the surface they get in touch with.
Water softening is used for removal of calcium and magnesium ions, which are the hardness (scale) forming constituents of water. Ion exchange resin having exchangeable sites in ‘Na' form is used for this process.
When water is passed over an ion exchange bed in a softening process, Ca2+ and Mg2+ ions are exchanged for the equivalent Na+ ions. The water obtained contains only Sodium salts, which do not produce boiler scales as they are readily soluble in water.
At the end of softening cycle or service cycle, resin bed gets 'exhausted' and there is a need of regeneration before it can remove hardness again.
After every service cycle, the resin is backwashed with water followed by regeneration with 5-15% of NaCl solution. This surplus of sodium ions make the exhausted resin to release hardness ions picked up during softening /service cycle and resin is back to its original sodium form to take on the next cycle. Ion exchange cannot remove suspended matter.
Softening process is used for scale prevention in low and medium pressure boilers.