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09 March 2012

Water softening process - Lime soda process

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 2nd 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

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