Study on the thermal decomposition mechanism of Mg(NO3)2·6H2O from the perspective of resource utilization of magnesium slag

Abstract

ABSTRACT The magnesium slag (magnesium nitrate hydrate Mg(NO3)2·6H2O) produced in the nitric acid leaching process of laterite nickel ore can be effectively recycled by thermal decomposition. To this end, this study placed great emphasis on disclosing the thermal decomposition mechanism of Mg(NO3)2·6H2O. Firstly, thermal decomposition paths of Mg(NO3)2·6H2O were revealed through Thermogravimetry-Mass Spectrometry, Differential Scanning Calorimetry and powder X-ray diffraction. It was found that the thermal decomposition of Mg(NO3)2·6H2O was a multistep endothermic reaction involving two dehydration stages and one denitration stage. The two dehydration stages were characterized by the evolution of H2O, with the formation of magnesium nitrate dihydrate and anhydrous magnesium nitrate. The denitration stage was characterized by the simultaneous evolution of O2 and NO2, with the formation of MgO. The conventional kinetic analysis was not suitable for describing such complex multistep reaction behaviour. Thus, the kinetic rate data (dα/dt-T) for the overall reaction were separated into those for three contributing stages by mathematical peak deconvolution. Then, the complete kinetic interpretations of the separated reaction stages for Mg(NO3)2·6H2O pyrolysis were achieved by the Friedman method and the master plots method. Finally, the original experimental α-T curves were successfully simulated using the resulting kinetic triplets.