The oxidation mechanism of nickel concentrate from ambient up to 1000 degrees C was investigated by thermogravimetric (TG) and differential thermal analysis (DTA) to improve the understanding of the oxidation behavior of the nickel concentrate during industrial roasting. The reaction products at intermediate temperatures were analyzed by X-ray powder diffraction (XRD). SEM/EDS, electron probe micro-analysis (EPMA), and chemical analysis. A reaction scheme was deduced, in which the preferential oxidation of iron sulfide species took place in the temperature range of 350-700 degrees C, forming Ni1-xS and Fe2O3. Subsequently, the resulting Ni1-xS was transformed into Ni3 +/- xS2, Ni3 +/- xS2 core melted at 813 degrees C and accelerated its oxidation forming NiO and NiSO4. At 942 degrees C, NiSO4 decomposed and the complete oxidation of the remaining nickel sulfide took place due to the absence of the protective sulfate shell. The kinetic results indicate that the diffusion of O-2 through the sample bed controls the reaction rate if the bed depth is larger than approximately 125 mu m. (C) 2012 Elsevier B.V. All rights reserved.