Oxidation of Flue Gas Desulfurization Waste and the Effect on Treatment Modes

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Abstract
Wastes from a limestone FGD system are mainly CaSO3 with lesser amounts of CaSO4. This thixotropic mixture has a high water content, impeding water conservation, and it exhibits physical properties making land fill applications difficult. Complete oxidation to CaSO4 can be achieved in a two stage SO2 absorption system at differently maintained pH levels. Forced oxidation converts the flaky CaSO3 particles to monoclinic gypsum crystals, thereby eliminating the thixotropic properties of the sludge. Oxidation was completed at a pH of 4.5, at an air stoichiometry of 3(11.8 sec contact time) at temperature of 76°F, and at a slurry depth of 10 ft. The oxidized slurry, when filtered, yielded greater than 80% solids, with liquid cyclones shown to be acceptable separation devices. Settling tests resulted in sedimentations lasting for minutes, instead of hours as obtained for the unoxidized sludge, and these corresponded well with the decrease in viscosity, measured as 1200 cps for the unoxidized, compared with 1 to 2 cps for the oxidized slurry.

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