Microwave-assisted self-reduction of composite briquettes of zinc ferrite and carbonaceous materials

Abstract

Microwave carbothermic reduction of zinc ferrite was studied based on self-reduction of briquettes composed of zinc ferrite and different reducing agents, including coke, coal and biochar, in a vertical microwave furnace from both thermodynamic and experimental perspectives. The thermodynamic analysis showed that zinc ferrite could be reduced using the carbonaceous materials in a temperature range of 910–1250 °C. The experimental results demonstrated that the reducing agents had distinct microwave absorption capabilities, confirmed by the comparisons of corresponding microwave penetration depths (Dp, in this study Dp,coke> > Dp,coal > Dp,biochar) and reflection losses (RL, in this study RLcoke> > RLcoal > RLbiochar) based on the microwave permittivity and permeability measurements. This finding was in agreement with their different roles in the reduction, which produced reduced briquettes and volatilized matters via a series of stepwise reactions. Compared with conventional reduction, it was also found that the presence of microwave led to a much higher reduction efficiency. The coke exhibited a much lower reduction efficiency than coal and biochar. This observation was more obvious in microwave-assisted reduction where the coal and biochar contributed to high iron metallization degree (97%) and reduction degree (above 99%) after reduction at 1050 °C for 15 min even with low fixed carbon contents. Further analysis of the phase composition and microstructure of the reduced briquettes obtained in microwave-assisted reduction showed that metallic iron particles were formed but aggregated in different morphologies, probably associated with the growth of the metallic particles adjacent to the residual carbonaceous reducing agents. Meanwhile, the analysis of the corresponding volatized matter revealed that it was mainly constituted by highly pure metallic zinc particles which presented a hexagonal structure. The results indicated that in comparison with coke, the other two carbonaceous materials, especially biochar, could serve as more efficient and environmentally friendly reducing agents for treatment of zinc ferrite and zinc-bearing materials by its combined usage with microwave external field.