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 Fe_7.9S₈ to Fe_2.37Ni _5.53S₈ 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, Fe ₂(SO₄)₃ and residual mss were identified in the final phases after 24 h oxidation of the mss composition Fe_7.9S ₈; hematite and Ni₁₇S₁₈ for compositions Fe_6.15Ni_1.54S₈ and Fe_2.37Ni _5.53S₈; hematite, Ni₁₇S₁₈ and pentlandite for Fe_6.4Ni_1.6S₈. Given a constant iron to nickel atomic ratio of 4:1, the sample with lower metal concentration, Fe_6.15Ni_1.54S₈, showed a faster oxidation rate than its metal richer counterpart, Fe_6.4Ni_1.6S ₈. 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. Vyazovkin's theory of changing activation energy (E_a) with reaction extent (y) was employed in the current kinetic study. The activation energy was determined using a model-free method. The oxidation of Fe_6.4Ni_1.6S₈ exhibited a higher E_a than Fe_6.15Ni_1.54S₈ over the course of reaction. The activation energy increases with y from 67.1 to 103.3 kJ mol^-1 for mss composition Fe_6.15Ni_1.54S ₈; 76.1 to 195.0 kJ mol^-1 for Fe_6.4Ni _1.6S₈. Bulk compositions Fe_7.9S₈, Fe_2.37Ni_5.53S₈ were selected to give a constant metal to sulfur atomic ratio of 7.9:8. Oxidation of Fe_2.37Ni _5.53S₈ achieved equilibrium within 1 h, compared to 5 h for Fe_7.9S₈.