Thermodynamic modelling of decomposition processes in the Mn-O and Mn-O-H systems

An understanding of the decomposition processes in the Mn-O and the Mn-O-H systems is important for several applications such as the reduction of manganese ores or the production of pure manganese oxides. Precise thermodynamic data is required for modelling these thermal processes and in particular data is lacking for Mn5O8. It would be advantageous to more accurately model these processes, but this depends on the availability of reliable thermodynamic data. In the present research, the currently available data for the Mn-O system was evaluated in order to obtain consistent decomposition temperatures of the various manganese oxides, as calculated using the Equilibrium Composition program of HSC Chemistry® 7.1. Subsequently, these temperatures were compared to the experimentally determined values in the literature. Also, the effects of variables such as atmospheric composition and pressure on the temperatures were evaluated. Additionally, the thermal conditions required for the formation of Mn5O8, were delineated. The thermodynamic data utilized for Mn5O8 were as follows: ΔH° = −2444.5 kJ/mol, ΔS° = 280.2 J/(mol·K) and Cp(T)=227.8+182.2×10−3T−16.7×105T−2−60.4×10−6T2J/(mol·K). The selected data utilized in this research, can be used for accurate thermodynamic calculations and as examples, phase stability diagrams for the Mn-O, Mn-O-H and Mn-O-N systems were determined.