Microwave carbo-thermal reduction for recycling of Cr from Cr-containing steel making wastes

In this study, Cr-containing steel making slag and pickling sludge were reduced with graphite by microwave (MW) heating. The reduced metal (alloy) particles were obtained and their compositions were determined. Pure Cr ₂O₃ was reduced at lower temperature than expected from thermodynamics. Fundamental studies on MW heating of Cr₂O₃ were performed using a multi-mode and a single mode MW applicators in order to investigate the enhanced reaction kinetics. It was demonstrated that heating of Cr₂O₃ requires incubation period before onset of the sudden temperature rise. Cr₂O₃ was preferentially heated in the electric (E-) field of MW and metal pieces were formed. It was pointed out that the MW heated specimen had inhomogeneous temperature distribution, the temperature could be measured at the localized region and/or it is an average of the temperature distribution in a miro-scale. The reduction might have occurred in the high temperature region, preferentially. In order to account for generation of the inhomogeneous distributions, two possibilities are considered. First, as the enhanced heating behavior in E-field, dielectric heating mechanism of Cr₂O₃ is likely, and its permittivity (loss factor) has large temperature dependence. Therefore, small initial temperature fluctuation was amplified to become the local temperature difference (hot spot). Second, occurrence of local arcing enhanced the kinet-ics and formation of metal pieces.