Oxidation Kinetics of Dense Pentlandite

Dense plates of pentlandite, Fe4.5Ni4.5S8, were oxidized at 783, 823 and 863 K in a mixed O2–N2, gas stream. Partial pressure of O2 gas in the mixed gas was 20 kPa. Iron was preferentially oxidized, and a duplex oxide was formed on the sample surface: The outer and inner oxides were Fe2O3 and Fe3O4, respectively. A small amount of nickel was contained in Fe3O4. A very small amount of SO2 gas was evolved in the initial 100 s of oxidation, and no SO2 gas was detected thereafter. The analysis of elements using EPMA in the inner sulfide revealed, on the other hand, that sulfur was oxidized slowly during the oxidation for 72 ks. The thickness of Fe3O4 layer increased rapidly at the initial stage of oxidation, and then both layers of Fe3O4 and Fe2O3 grew in accordance with the parabolic rate law. The activation energies for the growth of Fe3O4 and Fe2O3 layers were 170 and 160 kJ·mol−1, respectively.