Kinetics of the Thermal Decomposition of Rhodochrosite

Manganese is a widely used element in the steel industry; its main source is a mineral named rhodochrosite (MnCO3). For industrial usage, rhodochrosite is reduced to different manganese oxides by means of nodulation furnaces. In this study, rhodochrosite was thermally analyzed at temperatures ranging from 100 °C to 1200 °C. XRD (Powder X-ray diffraction), XRF (X-ray fluorescence), AAS (Atomic Absorption Spectrometry), and FESEM-EDX (Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectrometry) were used to characterize the mineral and the residues were analyzed by XRD and FTIR (Fourier-transform infrared spectroscopy) to determine the stoichiometry of the thermal decomposition reactions. Three mass losses were observed, the first attributed to the transformation from carbonate to manganese (III) oxide, the second to the reduction to manganese tetroxide, and the third to the decomposition of calcium carbonate (CaCO3) present as a contaminant in the studied mineral. Thermal decomposition kinetics shows that the first mass loss required 17.91 kJ mol−1, indicating a control by mass transport-controlled process. For the second and third mass loss, the apparent activation energy of 112.41 kJ mol−1 and 64.69 kJ mol−1 was obtained respectively, indicating that both mass loss events were rate-controlled.