Water softening process - Demineralization process

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Demineralization process is the modern water softening process. By applying this process the water obtained may be as pure as distilled water. Because not only the ions are removed those are responsible for water hardness but also some dissolved salt i.e. FeCO₃, CaCl₂ etc. can be removed. In this process strong acid cation resin and strong base anion resin are used. Cation in water like Sodium, Calcium, Magnesium removed by cation resin. And for anion like Chloride, Sulphate anion resin are used.

 Process: The demineralization plant consist of both cation and anion unit. Control over cation and anion unit is essential. Predetermined amount of resin bed are taken according to the composition of hard water. Then, the process is done by two steps:

1. Hydrogen Cation exchanger: Cation resin can be in Hydrogen (R-SO₃H) or Sodium form (R-SO₃Na). Sulphonated resin (Cation acid resin) in Hydrogen form is used as cation exchanger for water softening.

CaCl₂ + H₂R --> CaR↓ + 2HCl

Ca(HCO₃) + H₂R --> CaR↓ + 2H₂O + 2CO₂

2. Anion exchanger: Amino resin (anion base resin) is used and produced HCl is removed.

HCl + HOR₁ --> ClR₁↓ + H₂O (soft water)

Regeneration: We know, regeneration is done to get back the resins which are used to softening water are exhausted and we have to apply some acid & alkali to regenerate them again.

For step1, regeneration is performed by using acid. Less than 2% H₂SO₄ is used.

CaR + H₂SO₄ --> CaSO₄ + H₂R (regenerated resin)

For step2, regeneration is done by using 1% solution of NaOH.

ClR₁ + NaOH --> NaCl + HOR₁ (regenerated resin)