Phase evolution and kinetics of the oxidation of monosulfide solid solution under isothermal conditions

In this work, the effects of stoichiometry on phase evolution during the oxidation of mss (monosulfide solid solution) were investigated. A series of mss samples, ranging from Fe7.9S8 to Fe2.37Ni5.53S8 were synthesized from pure components. Samples with grain size 53–90 μm were oxidized at 830 and 850 K in air in a muffle furnace. The Rietveld quantitative phase analysis method was used to identify and quantify the phase information from powder X-ray diffraction (XRD) profiles. Hematite was observed and accounted for most of the oxidized iron. Nickel in mss was not oxidized to NiO under current isothermal conditions; instead, it was finally transformed to Ni17S18. Hematite, Fe2(SO4)3 and residual mss were identified in the final phases after 24 h oxidation of the mss composition Fe7.9S8; hematite and Ni17S18 for compositions Fe6.15Ni1.54S8 and Fe2.37Ni5.53S8; hematite, Ni17S18 and pentlandite for Fe6.4Ni1.6S8. Given a constant iron to nickel atomic ratio of 4:1, the sample with lower metal concentration, Fe6.15Ni1.54S8, showed a faster oxidation rate than its metal richer counterpart, Fe6.4Ni1.6S8. The mean oxidation rates for these two samples are 1.85 × 10−4 and 1.22 × 10−4 s−1 respectively for 1.5 h heating at 830 K. Vyazovkinʹs theory of changing activation energy (Ea) with reaction extent (y) was employed in the current kinetic study. The activation energy was determined using a model-free method. The oxidation of Fe6.4Ni1.6S8 exhibited a higher Ea than Fe6.15Ni1.54S8 over the course of reaction. The activation energy increases with y from 67.1 to 103.3 kJ mol−1 for mss composition Fe6.15Ni1.54S8; 76.1 to 195.0 kJ mol−1 for Fe6.4Ni1.6S8. Bulk compositions Fe7.9S8, Fe2.37Ni5.53S8 were selected to give a constant metal to sulfur atomic ratio of 7.9:8. Oxidation of Fe2.37Ni5.53S8 achieved equilibrium within 1 h, compared to 5 h for Fe7.9S8.