Water softening process - Lime soda process

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Hydrated lime Ca(OH)₂ and sodium Carbonate are used in this method to precipitate Calcium and Magnesium ions. Particularly, Ca(OH)₂ is used for Carbonate ions and NaCO₃ is used for non Carbonate  ions.

For temporary hardness,

Ca(HCO₃) + Ca(OH)₂ --> 2CaCO₃↓+ 2H₂O

Mg(HCO₃) + Ca(OH)₂ --> 2MgCO₃↓+ 2H₂O

Here, the solubility of Ca(HCO₃) is lower than that of Mg(HCO₃). CaCO₃ precipitate first while water is treated with Ca(OH)₂ and at 100⁰C temperature the solubility of CO₃ is 12-75 ppm.

For permanent hardness.

CaSO₄ + NaCO₃ --> CaCO₃ + Na₂SO₄

MgCl₂ + Ca(OH)₂ --> CaCl₂ + Mg(OH)₂

In 2^nd reaction from above CaCl₂ is produced which is replaced by CaCO₃ by using Na₂CO₃.

CaCl₂ + Na₂CO₃ = CaCO₃ + NaCl

The main parts of the process are:

1. Reagent tanks (Soda lime + Coagulants)
2. Reaction tank
3. Filter
4.  Soft water storage tank.

Process:

The lime and soda are entered into the reagent tank. At the same time, predetermined amount of raw hard water is pumped into the Reaction tank. A large propeller acts as an agitator into the Reaction tank. The agitation is increased to get more amount of ppm, steam is passed through the sideway pipe to increase the temperature of the mixer. The water is filtered to remove CaCO₃ after precipitation is completed and finally passed to the soft water storage tank.

 Precipitation rate can be increased by:

1. By increasing Temperature.
2. By using excess reagent and stirring.
3. By using sand grain.

The residual hardness that presents in water as CaCO₃ can be decreased by either hot or cold method. For cold method it can be reduced to 5-20 ppm and for hot process it is 5-15 ppm.

After processing with lime soda method if the water contains some hardness, then it can be removed by adding phosphate. In this process, some NaOH are also used.

10CaCO₃ + 6Na₃(PO₄) + 2NaOH --> {Ca(PO₄)₂}₃Ca(OH)₂ + NaCO₃