Calcium may be present in water solution as bicarbonate, sulfate, chloride, or nitrate. It may also be produced in water solution when lime is used to neutralize waste acid.

With the exception of calcium carbonate and calcium sulfate, most calcium compounds are very soluble. The solubility of calcium sulfate compounds varies with temperature. Gypsum (CaSO4 • 2H2O) has a solublity, in mg/l, of about 1800 at 32°F, 2100 at 100°F, and about 1700 at 212°F. The solublity of calcium carbonate in pure wa-

ter is small—about 15 mg/l. However, when precipitated, it produces supersaturated solutions that are relatively stable at water tempertures below 200°F.

Precipitation in the presence of a common ion—either calcium or carbonate—reduces solubility. Precipitation in the presence of about 5% by weight of calcium carbonate virtually eliminates supersaturation. This same phenomenon is noted when calcium fluoride is precipitated. Precipitated calcium compounds are crystalline and relatively easy to dewater by vacuum filtration at rates from 20 to 50 lb/sq ft/hr.

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