Lime softening temperatures

Gold Lime Softening. Precipitation softening accompHshed at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca(OH)2, is added to the water being treated, the following reactions occur ... 

Warm Lime Softening. The warm lime softening process operates in the temperature range of 49—60°C. The solubiHties of calcium, magnesium, and siHca are reduced by increased temperature. Therefore, they are more effectively removed by warm lime softening than by cold lime softening. 

Lime softening can be conducted cold (ambient), at warmer temperatures, or hot, where steam is used to heat the process. The differences among the three options are in the removal of hardness, alkalinity, and silica. Table 8.10 lists approximate effluent from cold, warm, and hot lime softeners.16... 

Cold lime softening is conducted at ambient temperatures and involves feeding calcium as lime (Ca(OH)2) to precipitate out calcium... 

The cold lime process is slow, and gets slower as the temperature gets colder. For very cold waters, there is the danger of postprecipitation elsewhere in the facility, as some of the reactions will not be completed in the lime softener, and will continue into the transfer lines. 

Warm lime softening is conducted at 120 - 140°F, where the solubilities of calcium and magnesium are reduced. Temperature control is critical. A change of as little as 4°F can cause carryover of softener precipitates. Conventional cold lime softening equipment is used for warm lime softening. 

Hot lime softening (also known as hot process softening) is conducted at 227 - 240°F. At these temperatures, the lime softening... 

The solubihty of sihca is a function of pH and temperature. Lime softening and lime plus soda ash softening are most effective in removing sihca. Other options are process related (a) run the RO system at reduced recovery, (b) increase the feed water temperature - sihca solubihty increases with temperature, and (c) use silica inhibitors. Colloidal silica is difficult to remove by IX because it is not ionised, and can foul the resins when the levels are high. Colloidal silica flocculates easily in boundary layers resulting in severe fouhng. It can be removed by UF membranes with a MW CO of up to 100,000 Da. Since the solubihty of the sihca increases below a pH of about 6.0 and above a pH of about 9.0, the actual solubihty of sihca in the concentrate stream is further affected by the pH of reject water. 

Hot lime softening The softening process operates at 90°C. The increased temperature increases the potential of the softener to exchange sodium for hardness ions and results in a lower hardness than that which can be achieved. 

Hot Process Softening. Hot process softening is usually carried out under pressure at temperatures of 108—116°C. At the operating temperature, hot process softening reactions go essentially to completion. This treatment method involves the same reactions described above, except that raw water COg is vented and does not participate in the lime reaction. The use of lime and soda ash permits hardness reduction down to 0.5 g/gal, or... 

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