Analysis of the oxidation reactions of CaS

Abstract

The chemical reactions of dense pellets of solid CaS in oxidizing atmospheres were studied by using continuous thermogravimetric analysis and iodimetric titration of SO₂ in the off-gas. The experiments covered a temperature range of 1223 to 1853 K, and Ar-O₂ mixtures varying from 1 to 100 pct O₂. The oxidation of CaS was found to be a complex process involving the formation of CaO and CaSO₄. Within the experimental conditions stated, three different processes were observed: 1) Weight loss with the formation of CaO and SO₂ which occurred at high temperatures and low partial pressures of O₂. 2) Weight gain which occurred at low temperatures and high partial pressures of O₂. 3) Combination of a) weight gain with the overall formation of CaSO₄ and b) weight loss with the overall oxidation of CaS to CaO and the decomposition of CaSO₄ leading to an oscillatory behavior which occurred at intermediate temperatures and intermediate to high partial pressures of O₂. The boundaries between these three processes are distinct. The rate of oxidation of the sulfide is limited first by the transport of O₂ across the gas boundary layer and later by the diffusion of O₂ through the porous reaction product. From these results and the Ca-S-0 stability diagram, it is possible to predict the experimental conditions which will produce processes of weight gain, weight loss and oscillatory behavior.