Thermal decomposition mechanism and kinetics of bastnaesite in suspension roasting process: A comparative study in N2 and air atmospheres

Abstract

To investigate the thermal decomposition behavior and reaction kinetics of bastnaesite in suspension roasting, the gas and solid products of bastnaesite roasted in N2 and air atmospheres were examined using a gas analyzer, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Subsequently, the kinetic parameters of bastnaesite in the suspension roasting process were derived and calculated using the isothermal method. The results show that the decomposition product of bastnaesite in N2 is CeOF. However, once the roasting temperature exceeds 600 °C, CO is generated in addition to CO2, and all the XRD diffraction peaks of CeOF are shifted to the right, indicating that CO2 can oxidize CeOF and lead to the transformation of Ce(III) into Ce(IV). When roasted in air, the decomposition product CeOF can be completely converted to CeF3 and Ce7O12 as it easily oxidizes. Additionally, the reaction rate of bastnaesite in air is higher than that of N2, and the starting reaction temperature is lower than that of N2. A large number of irregular cracks and holes appear on the surface of solid-phase products following suspension roasting, which are due to the thermal decomposition of bastnaesite that produces CO2 as well as the reconstruction of the lattice of the solid-phase products. The reaction kinetic model of bastnaesite roasted in N2 (temperature range 600–750 °C) and air (temperature range 500–575 °C) conforms to the A3/2 model with the mechanism function G(α)=–ln(1–α)2/3, and the reaction activation energy is 59.78 kJ/mol and lnA is 1.65 s−1 in N2 atmosphere. In air, the reaction activation energy is 100.30 kJ/mol and lnA is 9.63 s−1.