Lime Soda Softening
The lime (Ca(OH)2) reacts with the bicarbonate hardness
(Ca(HCO3)2 ) to precipitate calcium carbonate (CaCO2) and magnesium hydroxide
(Mg(OH)2 ). The soda ash reacts with the noncarbonate hardness such as magnesium
sulfates and chlorides to form the same fairly insoluble products. These
precipitates are allowed to settle out, and the settled water is usually
clarified by filtration.
Ca(HCO3)2 + Ca(OH)2 ===> 2CaCO3 + 2 H2O
Mg(HCO3)2 + 2Ca(OH)2 ===> 2CaCO3 + Mg(OH)2 + 2H2O
The reactions between soda ash and noncarbonate sulfate hardness are as
CaSO4 + Na2CO3 ===> CaCO3 + Na2SO4
MgSO4 + Na2CO3 + Ca(OH)2 ===> CaCO3
+ Mg(OH)2 + Na2SO4
Finally any free carbon dioxide must be also removed by adding enough lime
to raise the pH value to an optimum value required for the process.
CO2 + Ca(OH)2 ===> CaCO3 + H2O
The previous equations appear to indicate complete precipitation of hardness;
actually the reactions do not go to completion, and some residual
hardness is always left in solution. The residuals can
be decreased by doses of lime and soda ash in excess of those suggested
by the stoichiometry of the reactions. The excess increases the amounts
of dissolved carbonate and hydroxide anions in the softened water, providing
a common ion effect that depresses the solubilities of calcium carbonate
and magnesium hydroxide.