Clean microwave carbothermic reduction of hematite using biomass-derived products: Dual role of biochar and pyrolysis gas

Abstract

Hematite is one of the major wastes in rare earth production and its utilization requires reduction. However, the traditional carbothermic reduction requires a large amount of fossil energy, and the process of coking and reduction results in a significant waste of thermal energy. Therefore, there is an urgent need to develop new green carbothermic reduction technologies. In this study, the strategy of whole-component utilization of biomass pyrolysis products was employed to carry out microwave thermal reduction of hematite using biochar and pyrolysis gas derived from integrated fruit peel waste. Biochar shows excellent microwave-thermal conversion ability: it can participate in microwave carbothermic reduction reaction as both heating medium and reduction medium in the reduction process. Under the synergistic effect of biochar and pyrolysis gas, the reduction temperature of hematite was lowered, the metallization rate of the reduction products was enhanced, and the whole-component utilization of biomass pyrolysis products was realized. The biochar-gas coupling process has been demonstrated to facilitate the metallization of hematite to a significant extent. A 93.32 % hematite metallization rate was achieved within 30 min of microwave reduction at 1000°C, with only 15.62 % biochar consumption. The results show the dual contributions of biochar and pyrolysis gas under microwaves, providing a new approach to biomass waste and tailings utilization.