The reduction of calcium sulfate to calcium sulfide with carbon monoxide was studied using various catalysts. Ferric oxide, stannous sulfate, and vanadium pentoxide were found to have a pronounced catalytic effect on the reduction reaction. The ferric oxide was the most active catalyst and resulted in about 97% reduction of the calcium sulfate in 45 min at 680°C when at the optimum concentration of about 9 wt %. The system at 660°C showed reproducible oscillations of SO4 content with time. In addition, calcium sulfide was found to autocatalytically favor its own rate of formation. A mechanism involving the formation of active carbon
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