HOT SODA LIME PROCESS
Soda lime is a process used in water treatment to remove Hardness from water. This process is now obsolete but was very useful for the treatment of large volumes of hard water. Addition of lime (CaO) and soda (Na2CO3) to the hard water precipitates calcium as the carbonate, and magnesium as its hydroxide. The amounts of the two chemicals required are easily calculated from the analysis of the water and stoichiometry of the reactions. The lime‐soda uses lime, Ca (OH)2 and soda ash, Na2CO3, to precipitate hardness from solution.
Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime. In this process Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)2) and soda ash (Na2CO3).
REACTIONS:
As slacked lime is added to a water, it will react with any carbon dioxide present as follows:
Ca(OH)2 + CO2 →CaCO3 ↓ +H2O....(1)
The lime will react with carbonate hardness as follows:
Ca(OH)2 + Ca(HCO3 )2 →2CaCO3 ↓ +2H2O.....(2)
Ca(OH)2 + Mg(HCO3 )2 →MgCO3 + CaCO3 ↓ +2H2O.....(3)
The product magnesium carbonate in equation 3 is soluble. To remove it, more lime is added:
Ca(OH)2 + MgCO3 →CaCO3 ↓ +Mg(OH)2 ↓.....(4)
Also, magnesium non-carbonate hardness, such as magnesium sulfate, is removed:
Ca(OH)2 + MgSO4 →CaSO4 + Mg(OH)2 ↓.....(5)
Lime addition removes only magnesium hardness and calcium carbonate hardness. In equation 5 magnesium is precipitated, however, an equivalent amount of calcium is added. The water now contains the original calcium non-carbonate hardness and the calcium non-carbonate hardness produced in equation 5. Soda ash is added to remove calcium non-carbonate hardness:
Na2CO3 + CaSO4 → Na2SO4 + CaCO3 ↓.....(6)
