Hot Process Softening

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

Silica reduction is accomplished by adsorbing silica on the magnesium hydroxide precipitate. If not enough magnesium is present, magnesium oxide (MgO) can be added to provide the necessary adsorption sites. 

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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... 

Hoi-Lime Zeolite Softening. In this process hydrated lime is employed to react with the bicarbonate alkalinity of the raw water. The precipitate is calcium carbonate and is filtered from the solution. To reduce silica, the natural magnesium of the raw supply can be precipitated as magnesium hydroxide, which acts as a natural absorbent fur silica. These reactions are carried out in a vat or tank that is located just head of the zeolite softener tank. The effluent from this tank is filtered and then introduced into the zeolite softener. There is always some residual hardness leakage from the hot-process softener to be removed in the final zeolite process. The hot lime process operates at about 220T (I04°C). At this temperature the potential for the exchange of sodium for hardness ions is greater than at ambient temperature, and the result is a lower hardness effluent than is achieved at ambient temperatures. This system is shown schematically in Fig. 2. 

Figure 8.19 Sludge-blanket (upflow) hot process softener. Courtesy of Res-Con, Inc.

Steam is used to heat the process. Figure 8.19 shows the cross section of a sludge-blanket (upflow) hot process softener. The operations of the sludge blanket hot process unit is similar to that for sludge-blanket clarifiers (see Chapter 8.1.1.1). Figure 8.20 shows the cross section of a downflow hot process softener. The downflow units rely on recirulation pumps to provide sludge contact. 

Hot-processing softening — An alternative method of oil removal, although not necessarily the most desirable, is to return the eoiuU u.saU m a hot-process softener. In the softener, the oil is removed by sorption on the normal precipitates formed during the softening process. This method effectively removes influent oil at concentrations of approximately 30 to 40 ppm. Effluent oil concentrations of less than 5 ppm can be obtained. 

The hot-process softener, sometimes used for treating boiler feedwater, uses steam to heat the water to more than 200°F (93°C). Hot-process hme treatment has been used to remove silica from boiler feedwater where extremely low concentrations requiring ion exchange are not necessary. The sUica is removed by adsorption on freshly precipitated magnesium hydroxide. The removal is therefore dependent on the amount of Mg(OH)2 precipitated. The effective-... 

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