Understanding the mechanism and kinetics of pentlandite oxidation in extractive pyrometallurgy of nickel

Pentlandite oxidation behavior in the temperature range 530-600 °C was investigated by microscopic and kinetic methods. A clear four-step reaction sequence was identified through monitoring the reaction progress by Rietveld quantitative phase analysis of quenched samples. The first step involves the transformation of pentlandite to monosulfide solid solution (mss), (Fe, Ni)S. This is followed by the transformation from mss to α-NiS in the second step, the oxidation from Fe ₃O ₄ to Fe ₂O ₃ in the third step, and the oxidation of α-NiS to NiO in the final step. Avrami/Arrhenius analysis indicated rapid transformation of pentlandite to mss with rate constant increasing from 3.8 × 10 ^-4 s ^-1 at 530 °C to 2.1 × 10 ^-3 s ^-1 at 600 °C and an activation energy of 140 kJ mol ^-1; the rate constant for the transformation from mss to α-NiS increases from 6.7 × 10 ^-5 s ^-1 at 530 °C to 4.4 × 10 ^-4 s ^-1 at 600 °C with an activation energy of 151 kJ mol ^-1, and for the oxidation from Fe ₃O ₄ to Fe ₂O ₃ increases from 3.2 × 10 ^-5 s ^-1 at 530 °C to 1.7 × 10 ^-4 s ^-1 at 600 °C with an activation energy of 127 kJ mol ^-1. Such clear reaction mechanism and the associated kinetic data can be applied to optimize the operation conditions in industrial rosters and flash smelters to improve process efficiency and Ni recovery.