Reduction of sulfur dioxide to elemental sulfur by a cyclic process involving calcium sulfide and sulfate

A new process for converting sulfur dioxide to elemental sulfur has been investigated. It involves reacting sulfur dioxide with calcium sulfide to produce elemental sulfur, and reducing the produced calcium sulfate by hydrogen to regenerate calcium sulfide.; This research consisted of a thermodynamic analysis of both reaction systems, the investigation of the rates and yields of the reactions under various conditions, and the determination of the reactivity of calcium sulfide reduced from nickel-catalyzed calcium sulfate with sulfur dioxide gas and regenerated calcium sulfate with hydrogen gas, over several cycles.; The thermodynamic analyses showed that calcium sulfide is suitable for recovering elemental sulfur from sulfur dioxide gas, and hydrogen gas is effective to reduce calcium sulfate to calcium sulfide. The advantage of this suggested process is that little gaseous product, other than sulfur, is produced with calcium sulfate, which is reduced to reusable calcium sulfide without generating harmful off-gas.; At temperatures of 973 to 1153 K under sulfur dioxide partial pressures 5.0 to 60.1 kPa, elemental sulfur was recovered by reacting sulfur dioxide with calcium sulfide. The rate of this reaction was measured at the same conditions using a thermogravimetric analysis technique. At 1073 K under a sulfur dioxide partial pressure of 25.8 kPa, 48% of the fresh calcium sulfide powder was converted to calcium sulfate in one hour. Under the same conditions, 55% of the calcium sulfide powder reduced from nickel-catalyzed calcium sulfate powder was converted to calcium sulfate in one hour even after a third cycle. After the fifth cycle, the rate remained nearly the same as the fresh calcium sulfide powder. A pore-blocking model was found to fit the reaction rate reasonably well. The reaction is first order with respect to sulfur dioxide partial pressure at all cycles.; Calcium sulfate was reduced to calcium sulfide at temperatures of 973 to 1153 K under hydrogen partial pressures 1.7 to 86.1 kPa. The rate of this reaction was measured. At 1073 K and a hydrogen partial pressure of 86.1 kPa, 60% of the fresh calcium sulfate powder in the absence, and 95% in the presence, of nickel catalyst was converted to calcium sulfide in one hour. The reaction rate of nickel-catalyzed calcium sulfate regenerated by hydrogen was nearly the same as that reduced in the first cycle even after the tenth cycle. The reaction order changed somewhat with each cycle. A nucleation and growth model was found to fit the reaction rate.