Conversion of manganese-chromium spinel to sulphides in H2-H2O-H2S atmospheres

Abstract

The conversion of MnCr₂O₄ to sulphides was investigated at 1073 K, 1173 K and 1273 K in H₂-H₂O-H₂S mixtures. From thermogravimetric studies on the sulphidation of the bulk spinel, the stability limit of MnCr₂O₄ was determined. When a certain sulphur pressure is exceeded corresponding to the reaction MnCr₂O₄ + 3/2S₂ = MnS + 2CrS + 2O₂, the spinel is converted to a porous layer of mixed sulphides (Mn,Cr)S and (Cr,Mn)S. The partial pressure dependences indicate that the reaction rate is controlled by a surface reaction, i.e. the sulphur transfer from H₂S at the interface sulphide-spinel which is easily accessible through the large pores in the sulphide layer. Similar kinetics and morphology of the reaction products were observed when MnCr₂O₄ layers obtained by preoxidation of Fe-Cr-Mn alloys were sulphidized. The results provide information on the stability of oxide layers formed on Mn-containing high temperature alloys in oxidizing-sulphidizing atmospheres.