The report gives results of an experimental study of techniques suitable for forcing the oxidation of calcium sulfite (a throwaway product of flue gas desulfurization scrubbers now operating in the U.S.) to gypsum, over a range of scrubber operating conditions applicable to the use of high-sulfur coals. Potential advantages of converting this product to gypsum include: (a) reduction of the total waste production, (b) improved settling and dewatering properties of the sludge, and (c) avoidance of chemical fixation of the sludge. It is shown that appropriate oxidizer designs can achieve complete conversion with reasonable air stoichiometries at atmospheric pressure without catalysts. Gypsum filterable to 80% solids can be obtained in either two- or single-stage scrubbers. In either case, efficiency is determined primarily by the physical absorption of oxygen from the air injected into the oxidizer, when the oxidation step is conducted within the scrubbing loop. The study shows that the required oxygen transfer efficiencies can be obtained either with an air-sparged tower or an air ejector, when used to aerate the slurry in the scrubber effluent hold tank. Merits of the various approaches are compared and discussed in terms of the quality of gypsum produced, operability of the system, air-feed pressure requirements, and predictability of performance when scaled-up to larger systems.

Program Element no. EHE624. Issued June 1977. Includes bibliographical references.