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 follows:

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.

2007 Arizona Board of Regents for The University of Arizona