Lime-Soda Water Softening 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).

Following are the reactions that takes place in this process:

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)

To precipitate CaCO3 requires a pH of about 9.5; and to precipitate Mg(OH)2 requires a pH of about 10.8, therefore, an excess lime of about 1.25 meq/l is required to raise the pH.

The amount of lime required: lime (meq/l) = carbon dioxide (meq/l) + carbonate hardness (meq/l) + magnesium ion (meq/l) + 1.25 (meq/l)

The amount of soda ash required: soda ash (meq/l) = non-carbonate hardness (meq/l)

After softening, the water will have high pH and contain the excess lime and the magnesium hydroxide and the calcium carbonate that did not precipitate. Recarbonation (adding carbon dioxide) is used to stabilize the water. The excess lime and magnesium hydroxide are stabilized by adding carbon dioxide, which also reduces pH from 10.8 to 9.5 as the following:

CO2 + Ca(OH)2 → CaCO3↓ + H2O

CO2 + Mg(OH)2 → MgCO3 + H2O

Further recarbonation, will bring the pH to about 8.5 and stablize the calcium carbonate as the following:

CO2 + CaCO3 + H2O → Ca(HCO3)2

It is not possible to remove all of the hardness from water. In actual practice, about 50 to 80 mg/l will remain as a residual hardness.

Limitation of Soda Lime Process:
Lime soda softening cannot produce a water at completely free of hardness because of the solubility (little) of CaCO3 and Mg(OH)2. Thus the minimum calcium hardness can be achieved is about 30 mg/L as CaCO3, and the magnesium hardness is about 10 mg/L as CaCO3. We normally tolerate a final total hardness on the order of 75 to 120 mg/L as CaCO3, but the magnesium content should not exceed 40 mg/L as CaCO3 (because a greater hardness of magnesium forms scales on heat exchange elements).