Cold lime softening

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

The removal of other species requires the addition of sodium aluminate (NajAl O ). Reduction in the concentration magnesium is only about 10%. The addition of sodium aluminate also helps with the reduction in silica, as the magnesium precipitate, magnesium hydroxide, adsorbs silica. More complete removal requires the addition of soda ash (NajCOg) and warmer temperatures, as shown in Table 8.10. 

Cold lime and lime-soda softening is conducted in a solids-contact clarifier (see Chapter 8.1.1.). Cold lime softeners are typically followed by filtration and sodium softening to remove the balance of hardness from the water. 

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. 

Ca(HC03)2 = calcium bicarbonate CaC03 = calcium carbonate precipitate 

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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. 

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. 

Figure 11.2 Actual product flow rate data from a facility operating on cold-lime softened Delaware River water.

In many cases, additional pretreatment is required prior to the HERO process. Additional pretreatment can include cold lime softening, sodium softening, and ultrafiltration or microfiltration. These pretreatment unit operations are required when the total hardness is very high and/or the concentration of suspended solids is high. 

Ca(A102)2 may have a lower solubility at pH 10. Nevertheless, the lower aluminum at pH 10 is a significant advantage since as with copperas, cold lime softening and clarification can be done in one step. Figure 6 shows the reduction achieved in the Brazos River water hardness as a function of pH. Approximately 18 ppm less hardness was obtained with the AlCla than with copperas. 

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