RECOVERY OF CALCIUM AND MAGNESIUM VALUES

IN well PLANTS water by softening the lime-soda hard, process, clear well water by the limeda process, the sludge comprises calcium carbonate, magnesium hydroxide, and the insolubles present in the lime. For plants softening hard, turbid waters, clay is an added component. Although recalculation has proved both successful and profitable for the very few plants employing it, its wide adoption has been delayed because no successful method for the physical separation of the calcium carbonate from the other components has been developed. At Dayton, Ohio, where clear well water high in magnesium is softened, stack gas from the 150-ton/day lime kiln is used to dissolve selectively the gelatinous Mg(OH)2 from the crystalline CaC03, and high-quality chemical lime containing 92-93 per cent CaO has been continuously produced since 1958. (See Fig. 1.) The supernatant from the carbonatedsludge thickener containing the dissolved magnesium bicarbonate is discharged into the Mad River.1 The necessity of meeting new and rigorous standards for such waste discharges has made it necessary to find another method for its disposition. This need has led to the discovery of a relatively simple and inexpensive method for recovering the magnesium values in the form of very pure magnesium carbonate. An additional step makes possible its use for processing the sludges containing clay that are produced when softening hard, turbid waters. For all such plants, present chemical treatment costs will be substantially reduced because both calcium and magnesium values are recovered as high-quality quicklime and magnesium carbonate; all sludge water is recovered and recycled ; and clay, the only solid waste remaining, presents no pollution problem since it may be disposed of as land fill.