LIME-SODA ASH SOFTENING
Chemical precipitation is one of the more common methods used to soften water. The chemicals
normally used are lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3).
Lime is used to remove the chemicals that cause the carbonate hardness. Soda ash is used to remove
the chemicals that cause the non-carbonate hardness.
When lime and soda ash are added, the hardness-causing minerals form nearly insoluble precipitates.
When calcium hardness is removed in a chemical softener, it is precipitated as calcium carbonate
(CaCO3). When magnesium hardness is removed in a chemical softener, it is precipitated as
magnesium hydroxide (Mg(OH)2). These precipitates are removed by the conventional processes of
coagulation/flocculation, sedimentation, and filtration. Because the precipitates are very slightly
soluble, some hardness remains in the water--usually about 50 to 85 mg/l (as CaCO3). This hardness
level is desirable to prevent corrosion problems associated with water being too soft and having little
or no hardness ions.
1. What are the chemical reactions that happen with lime addition?
CO2. + Ca(OH)2 -> CaCO3 + H2O
Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 + 2H20
Mg(HCO3)2 + Ca(OH)2 -> CaCO3 + MgCO3 + 2H20
MgCO3 + Ca(OH)2 -> CaCO3 + Mg(OH)2
2 What are the chemical reactions with soda ash?
MgSO4 + Ca(OH)2 -> Mg(OH)2 + CaSO4
CaSO4 + Na2CO3 -> CaCO3 + Na2SO4
Na2CO3 = sodium carbonate or soda ash
For each molecule of calcium bicarbonate hardness removed, one molecule of lime is used.
For each molecule of magnesium bicarbonate hardness removed, two molecules of lime are used.
For each molecule of non-carbonate calcium hardness removed, one molecule of soda ash is used.
Each molecule of non-carbonate magnesium hardness requires one molecule of lime plus one
molecule of soda ash.
3. What are the types of lime-soda ash processes?
Conventional lime-soda ash treatment
When the content of the water has little magnesium hardness only the calcium ions need to be
removed. This is termed conventional lime-soda treatment. Only enough lime and soda ash are added
to the water to raise its pH to between 10.3 and 10.6. The calcium hardness will then be removed
from the water, but little if any magnesium hardness will be removed.
Excess lime treatment
When the magnesium hardness of water is more than about 40 mg/l (expressed as CaCO 3),
magnesium hydroxide scale will deposit in household hot-water heaters operated at normal
temperatures of 140 to 150° F. To reduce this magnesium hardness, more lime than is used in the
conventional process must be added to the water. The extra lime will raise the pH above 10.6 so that
the magnesium hydroxide will precipitate out of the water. This process is known as Excess Lime
Treatment.
Blow Down Process:
When a boiler is generating steam, the feedwater continuously carries dissolved mineral
impurities into the boiler. These dissolved impurities stay behind in the boiler water when steam
is made; only pure water leaves the boiler as a gas. These dissolved impurities concentrate in
the boiler water until the water can no longer hold all the impurities in solution. Once these
impurities have reached a saturation level they will begin to drop out forming a sludge or scale.
If this sludge is not properly removed from the boiler deposits will begin to form on the boiler
water-side surfaces. To prevent an excessive accumulation of dissolved and undissolved solids
which will interfere with proper boiler operation, a blowdown program must be established.
The boiler blowdown process involves the periodic or continuous removal of water from a
boiler to remove accumulated dissolved solids and/or sludges. During the process, water is
discharged from the boiler to avoid the negative impacts of dissolved solids or impurities
on boiler efficiency and maintenance.
Reasons for boiler blow down are:
1.
To remove the precipitates formed as a result of chemical addition to the boiler water.
2.
To remove solid particles, dirt, foam or oil molecules from the boiler water. This is mainly done by scum valve
and the procedure is known as “scumming.”
3.
To reduce the density of water by reducing the water level.
4.
To remove excess water in case of emergency.