However, an additional 25 ppm of calcium would be expected in the treated water due to the following reactions: MgSO 4 If noncarbonate magnesium hardness is present in an amount greater than 70 ppm and an excess hydroxyl alkalinity of about 5 ppm is maintained, the magnesium will be reduced to about 70 ppm, but the calcium will increase in proportion to the magnesium reduction.įor example, in cold lime treatment of a water containing 110 ppm of calcium, 95 ppm of magnesium, and at least 110 ppm of alkalinity (all expressed as calcium carbonate), calcium could theoretically be reduced to 35 ppm and the magnesium to about 70 ppm. Noncarbonate or permanent calcium hardness, if present, is not affected by treatment with lime alone. Figures 7-1 and 7-2 show these relationships. Magnesium reduction is a function of the amount of hydroxyl (OH -) alkalinity excess maintained. If the proper chemical control is maintained on lime feed, the calcium hardness may be reduced to 35-50 ppm. When hydrated lime, Ca(OH) 2, is added to the water being treated, the following reactions occur: CO 2 Precipitation softening accomplished at ambient temperatures is referred to as cold lime softening. A combination of lime and soda ash, along with coagulant and flocculant chemicals, is added to raw water to promote a precipitation reaction. Reduction of noncarbonate hardness, by contrast, requires chemical addition. In effect, heating reverses the solution reaction: Ca(HCO 3) 2 The significance of "carbonate" or "temporary" hardness as contrasted to "noncarbonate" or "permanent" hardness is that the former may be reduced in concentration simply by heating. These salts are caused by mineral acids present in rain water or the solution of naturally occurring acidic minerals. Hardness may also be present as a sulfate or chloride salt, referred to as noncarbonate or permanent hardness. These compounds result from the action of acidic, carbon dioxide laden rain water on naturally occurring minerals in the earth, such as limestone. In almost every raw water supply, hardness is present as calcium and magnesium bicarbonate, often referred to as carbonate hardness or temporary hardness. Waters with moderate to high hardness and alkalinity concentrations (150-500 ppm as CaCO 3) are often treated in this fashion. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The water is treated with lime or a combination of lime and soda ash (carbonate ion). This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. Precipitation softening processes are used to reduce raw water hardness, alkalinity, silica, and other constituents. Precipitation process (chemical) control.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |