WATER TRETMENT: WARM LIME SOFTENING
The warm lime softening process operates in the temperature range of 120-140 o F (49-60 o C). The solubilities of calcium, magnesium, and silica are reduced by increased temperature. Therefore, they are more effectively removed by warm lime softening than by cold lime softening. This process is used for the following purposes:
· To recover waste heat as an energy conservation measure. The water to be treated is heated by a waste stream, such as boiler blowdown or low-pressure exhaust steam, to recover the heat content.
· To prepare feed to a demineralization system. The lower levels of calcium, magnesium, and especially silica reduce the ionic loading on the demineralizer when warm lime-softened water is used rather than cold lime-softened water. This may reduce both the capital and operating costs of the demineralizer. However, most strong base anion resins have a temperature limitation of 140 o F (60 o C); therefore, additional increases in temperature are not acceptable for increasing the effectiveness of contaminant reduction.
· To lower the blowdown discharge from cooling systems. Cooling tower blowdown may be treated with lime and soda ash or caustic to reduce calcium and magnesium levels so that much of the blowdown may be returned to the cooling system. Silica levels in the recirculating cooling water are also controlled in this manner.
In any warm lime or warm lime-soda ash process, temperature control is critical because temperature variations of as little as 4 o F/hr (2 o C/hr) can cause gross carryover of the softener pricipitates.