The Chemistry of Lime-Soda Ash Precipitation Water Softening

There are atleast 7 different chemical reactions that take place during the Lime-Soda Ashwater softening process.

Quick lime(CaO) is added to the water as a pre-treatment. It is typically the first stepin softening. This quick lime combineswith water to form calcium hydroxide (lime), which then reacts with carbondioxide to form calcium carbonate. This is done at an optimum pH of 10.3

Reaction: CaO + H2O Ca(OH)2then see next reaction:

Carbon Dioxidein the water reacts with Lime [Ca(OH)2] to form calcium carbonate,which will precipitate out. If there is a lot of dissolved CO2, thewater can be expensive to treat. Because the goal of adding Lime to the wateris to soften it, it may be cost effective to aerate the water prior to dosingit with Lime.

Reaction: CO2 + Ca(OH)2 CaCO3 + H2O

CalciumBicarbonate is commonly removed from water by adding lime. The chemicalscombine to form water and calcium carbonate which will precipitate out becauseit is insoluble.

Reaction: Ca(HCO3)2+ Ca(OH)2 2CaCO3 + 2H2O

Magnesium bicarbonate reacts with lime to formmagnesium carbonate. Keep in mind that magnesium carbonate is soluble. You needto add more lime to convert the magnesium to magnesium hydroxide, which isinsoluble and will settle out. Magnesium precipitation requires a pH of about11.0 to 11.3 (caused by adding more lime)

Reaction:Mg(HCO3)2+ Ca(OH)2 MgCO3 + 2H2O&

MgCO3+ Ca(OH)2 Mg(OH)2+CaCO3

If there ismagnesium sulfate in the water, you will need two reactions to remove it.Magnesium sulfate reacts with lime to for calcium Sulfate and magnesiumhydroxide. The calcium sulfate does not precipitate out, so see the nextreaction.

Reaction: MgSO4 + Ca(OH)2 CaSO4 + Mg(OH)2

If the hardnesscausing chemical is Calcium sulfate, you can add soda ash to form calciumcarbonate, which is insoluble, and sodium sulfate, which doesn't contribute tohardness.

Reaction: CaSO4 + Na2CO3 Na2SO4 + CaCO3

Practical Limit of Lime-Soda Ash precipitationare 30 mg/l of calcium as CaCO3 and 10 mg/l of Mg(OH)2expressed as CaCO3. Thefollowing flow chart outlines the various reactions that occur in the water.