Production of dental stone from flue-gas gypsum

Abstract

Flue-gas gypsum, a waste product obtained from desulfurization process of the Mae Moh power plant, Lampang, was precleaned by ultrasonicating and acid leaching method. Precleaned FGD gypsum had a purity of approximately 98% and a light color because impurities such as fly ash and iron oxide, were removed. It was transformed to alpha-hemihydrate by classical vapor process. In this technique, the FGD gypsum briquettes pressed at 180 bars were impregnated in solution of either sodium succinate or succinic acid, and calcined to alpha-hemihydrate in an autoclave at a steam pressure of 6 bars for 1 hour. The alpha-hemihydrate obtained had alpha-HH content of 94-99% and was consisted of low-aspect-ratio crystals, resulting in a good flowabiity. At a water to plaster ratio of 0.32, the dental stone having equivalent properties to natural gypsum was successfully produced from ultrasonicated FGD gypsum without additive and had a maximum compressive strength of 28.75 MPa.