Principle

• Zeolites are hydrated sodium alumina silicates having general formula as $Na_2O \ \ Al_2O_3 \ \ xSiO_2 \ \ yH_2O$ and are capable of exchanging their Na-ions for hardness causing ions like $Ca^{2+} \ \ and \ \ Mg^{2+}$.

• Zeolites have a exchange capacity per unit weight. These calcium and magnesium zeolite on treatment with a solution of NaCl can replace $Ca^{2+}$ and $Mg^{2+}$ with $Na^+$ ion and thus an exhausted zeolite gets regenerated chemical representation of zeolite: $Na_2Z$.

Working:

1. Zeolite softening is carried out on large cylindrical tanks which hold zeolite material on perforate platform or between layers of sand.

2. The tank has 2 inlets through one inlet hard water is passed and other for passing saturated NaCl solution.

3. There are 2 outlets, through one softened water exits and through the other wash water containing $CaCl_2$ and $MgCl_2$.

Reactions:

$MgSO_4 + Na_2Z → MgZ + Na_2SO_4$

$CaCl_2 + Na_2Z → CaZ + 2NaCl$

$Mg(HCO_3)_2 + Na_2Z → MgZ + 2NaHCO_3$

$Ca(HCO_3)_2 + Na_2Z → CaZ + 2NaHCO_3$

Regenerated reactions:

Exhausted zeolite (CaZ or MgZ)

$CaZ + 2NaCl → Na_2Z + CaCl_2 \\ MgZ + 2NaCl → Na_2Z + MgCl_2$

Advantages of zeolite process

1. Residual hardness of water about 10ppm

2. No generation of sludge as there is no precipitation.

3. Equipment is compact, operating cost is low as NaCl is used to regenerate exhausted zeolite.

4. Easily handled and supervised by trained people.

Disadvantages

1. Presence of Na-salts like NaHCO3 and Na2CO3 in softened water can cause caustic embrittlement and boiler corrosion. Therefore not suitable for boilers.

2. Iron and Manganese zeolites cannot be easily regenerated.

   ∴ Fe and Mn impurities in water are problematic

3. Highly acidic water can affect the mineral, therefore not suitable.

4. Raw water should be free of turbidity as it can clog the pores of zeolite.