Excess Lime-soda Softening:
The process is used for the removal of all forms of magnesium hardness. Excess lime is added in order to precipitate the insoluble magnesium hydroxide.
To the calcium chloride and calcium sulphate produced, soda ash (Na2CO3) is added to convert these to calcium carbonates as in the removal of permanent calcium hardness as mentioned under Lime-soda softening.
MgCl2 + MgSO4 + 2Ca (OH)2 → 2Mg(OH)2 + CaCl2 + CaSO4
CaCl2 + CaSO4 + 2Na2CO3 → 2CaCO3 + Na 2SO 4 + 2NaCl
Lime-soda treatment generally results in the reduction of total dissolved solids of treated water.
All forms of chemical precipitation softening produces considerable volumes of sludge. Sometimes lime is recovered by calcining the calcium carbonate sludge and slaking with water.
CaCO3 → CaO + CO2
CaO + H2O → Ca(OH)2
In most of the cases, hardness due to calcium is predominant in water. The addition of lime alone removes only the carbonate hardness portion of the calcium hardness. Soda ash is required to remove the non-carbonate hardness. Lime alone achieves sufficient reduction of hardness because in many cases greater portion of the hardness in water is temporary.