Evolution of drying kinetics and properties of pyrolusite during microwave heating

Microwave heating is a developing noncontact heating method in dewatering technology. It has been proved to be effective to dewater high moisture content in mineral substance via the application of the dielectric loss of materials. In this study, the effects of microwave power levels (250–550 W) and different sample masses (20–40 g) on microwave drying behavior of pyrolusite have been investigated. For the purpose of the mechanism and morphological changes to the pyrolusite in the process of drying, thermogravimetric analysis, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were used, respectively. The experiment results demonstrated that an increase in the microwave power as well as sample mass was found to increase the drying rate during pyrolusite drying processes, with there are four main mass loss stages from 20 to 900 °C. The SEM-EDS analysis results show that morphological changes with the presence of more pores and appearance in spiral cross-linking way reduced the particle size and the shrinkage of the faces and edges, in which it can been concluded that the microwave drying is an effective method to produce the high-grade pyrolusite. By analysis, the increased pore structure and shrinkage phenomena can be observed, which increase the specific surface area of manganese ore effectively and may benefit the subsequent reduction process of high valence manganese, after the process of microwave heating.