Oxidation of byproduct calcium sulfite hemihydrate from coal-fired power plants

Abstract

The flue gas desulfurization by-product from the TU Electric Martin Lake power plant near Tatum, Texas was characterized using thermal analysis, x-ray diffraction, microprobe and infrared spectroscopy. The byproduct, called gypsite, consisted of a mixture of calcium sulfate dehydrate, calcium carbonate, a solid solution of calcium sulfate and calcium sulfite hemihydrate, and trace quantities of silicon and iron. Pure CaSO3.(1/2)H20 was used to prepare solid solutions with different sulfate concentrations. The solid solutions so prepared were used to determine the critical solubility limit of calcium sulfate in calcium sulfite hemihydrate in the solid solution (CaSO4)a (CaSO3)1-a.(I/2)H20 at room temperature, Gypsite can be oxidized to calcium sulfate in slurry or solid form. Oxidation of calcium sulfite in the gypsite slurry form was carried out at temperatures between 30 and 80 'C and for slurry concentrations of 4 to 8 % solids. Sulfuric acid was used to convert calcium carbonate to calcium sulfate dehydrate. The effect of amount of sulfuric acid added was also studied under different conditions. Air and hydrogen peroxide were used as the oxidizing agents. Oxidation using hydrogen peroxide is much faster compared to oxidation using air. Hydrogen peroxide can be used for oxidizing slurries with high solids concentration. The exothermic heat generated during the reaction of hydrogen peroxide with gypsite was used to directly obtain calcium sulfate hemihydrate. The effect of various transition metal salts on the oxidation of gypsite in slurry using air as an oxidant was also studied. Manganese salts were found to significantly increase the rate of oxidation, while molybdenum, vanadium, aluminum, cobalt, zinc, iron and nickel salts fail to show any significant increase in gypsite oxidation rate. When air is used as the oxidizing agent, the rate of oxidation decreases with increasing slurry concentration. Sulfur dioxide is released when sulfuric acid is added to the slurry during the oxidation process. Scanning electron microscopy and particle size analysis were used to characterize the initial byproduct and oxidized product obtained under different conditions. The addition of gypsum seeds during the oxidation of the gypsite led to increased particle size. The particle size increased as the amount of added gypsum seeds increased. These results suggest that oxidation of FGD byproduct slurries may be an economically feasible process. Future studies must be done on the pilot plant scale. For this purpose, a process flow diagram for a pilot plant for FGD byproduct oxidation was developed.