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Abstract

A process is described for making flue gas gypsum from desulfurization of power plant exhaust gas. The process comprises treating the flue gas in a wash tower with a wash water suspension containing calcium oxide, calcium hydroxide or calcium carbonate and converting the scrubbed sulfur dioxide by oxidation with input atmospheric oxygen and by crystallization to calcium sulfate dihydrate crystals. A cyclic flow is drawn from the wash tower sump and delivered by spray nozzles to the wash tower. Further the process comprises continuously drawing an additional crystal suspension from the wash tower sump which after separation of a crystal slurry is partly fed back to the wash tower sump. The lower run-off of a cyclone separator receiving the crystal suspension is dewatered, the flue gas gypsum is separated and a clear effluent is returned to the wash tower. A partial flow is separated from the upper run-off of the cyclone separator and a solid free fluid replaces it. By control of the amount of the partial flow separated from the upper run-off the grain size of the removed gypsum or calcium sulfate dihydrate crystals is precisely adjusted and the proportion of finely divided solids in the wash tower sump is adjusted so that at least 10 weight %, advantageously 20 to 70 weight %, of the solid material contained in the crystal slurry remains in the upper run-off of the cyclone separator.

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