Lime soda process is one of the water softener systems. In this system calcium hydroxide and soda ash are used as reagents. By this process soluble magnesium and calcium salts are removed as calcium carbonate and magnesium hydroxide precipitated. After removal of this precipitated, we obtain soft water. Both temporary water hardness and permanent water hardness are removed by this lime softening water softener systems. Before water treatment calculate the amount of temporary and permanent water hardness, and then estimate the amount of lime and soda ash and added control with careful. In case of temporary water hardness the following reactions are take places:
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Mg(HCO3)2 + Ca(OH)2 → 2CaCO3 + MgCO3 + 2H2O
MgCO3 + Ca(OH)2 → Mg(OH)2 + CaCO3
In case of permanent water hardness the following reactions are take places:
CaSO4 + Na2CO3→ CaCO3 + Na2SO4
MgSO4 + Na2CO3→ MgCO3 + Na2SO4
MgCO3 + Ca(OH)2 → Mg(OH)2 + CaCO3
There are two type lime-soda process; cold lime soda process and hot lime soda process.
Cold lime soda process
In the cold lime soda process the reactions take place at normal temperature. The raw water is passed into a tank with a static continuous flow, at the same time a calculated amount of chemical mixture of lime and soda ash are also added a static continuous flow and then thoroughly mixed with a stirrer in this mixture tank. Afterward this water is allowed to settle. When the settlement becomes complete the water is passed by continuous up-flow process into another tank which is called filter bed. Then we get soften water.
The cold lime soda process is not suitable for all purposes. It is limited for those purposes where partially softened water needed because this process fails to precipitate completely all the hardness forming salts. The cold lime soda process is used for municipal water and cooling water softener systems. The reason in case of municipal purpose, it is not necessary to remove all water hardness forming ions. Same way cooling water should be free from temporary water hardness. In this process it is possible to remove magnesium water hardness nearly completely but calcium hardness may be remains more than 40ppm.
Hot lime soda process
In the hot lime soda process the reactions take place at higher temperature near about boiling point of water. The chemical mixing process is same as the cold lime soda process, but steam is applied in mixture tank. As a result precipitation becomes almost complete very quickly. This process is more effective than cold lime soda process. The hot lime soda process water softener systems are exclusively used for boiler purpose. In this process it is possible to remove magnesium water hardness completely and calcium hardness may be remains very low. Moreover dissolved corrosive gases also removed by this hot lime soda process.