What Is Lime Plaster?
Lime-gypsum softening method refers to the method of simultaneously adding lime and gypsum solution to hard water to remove carbonate hardness and sodium alkalinity. It is a kind of pharmaceutical softening method. Lime mainly removes carbonate hardness, and gypsum cooperates with lime to remove sodium salt alkalinity. It is used when the alkalinity in the raw water is greater than the hardness and the user requires both hardness and alkalinity.
- Lime-gypsum softening method refers to the method of simultaneously adding lime and gypsum solution to hard water to remove carbonate hardness and sodium alkalinity. It is a kind of drug softening method. Lime mainly removes carbonate hardness, and gypsum cooperates with lime to remove sodium salt alkalinity. When the alkalinity in the raw water is greater than the hardness, and the user requires both hardness and alkalinity, [1]
- CaO + H 2 O = Ca (OH) 2
- CO 2 + Ca (OH) 2 --- CaCO 3 + H 2 O
- Ca (HCO 3 ) 2 + Ca (OH) --- 2CaCO 3 + 2H 2 O
- Mg (HCO 3 ) 2 + 2Ca (OH) 2 --- 2CaCO + Mg (OH) 2 + 2H 2 O
- If alkalinity> hardness, excess HCO 3 should also be removed
- If Fe ions are present in the water, Ca (OH) 2 is also consumed
- Therefore, the amount of lime added:
- [CaO] = [CO 2 ] + [Ca (HCO 3 ) 2 ] +2 [Mg (CO 3 ) 2 ] + [Fe] + a
- To minimize the hardness of the carbophosphate, lime + coagulation precipitation can be performed simultaneously.
- Note: The lime method can only reduce carbonate hardness and alkalinity in water [2]
- Removes carbonate and non-carbonate hardness
- CaSO 4 + Na 2 CO 3 --- CaCO 3 + Na 2 SO 4
- MgSO 4 + Na 2 CO 3 --- MgCO 3 + Na 2 SO 4
- MgCO 3 + Ca (OH) 2 --- CaCO 3 + Mg (OH) 2
- But soda ash is too expensive, this method is generally not used [2]
- When the alkalinity of raw water is greater than hardness, that is, negative hardness (when alkalinity is present) appears:
- 2NaHCO 3 + CaSO 4 + Ca (OH) --- 2CaCO 3 + Na 2 SO 4 + 2H 2 O [1]