Rapid microwave-assisted reduction of ferromanganese spinel with biochar: Correlation between phase transformation and heating mechanism

Abstract

The reduction of ferromanganese spinel (MnFe2O4) in microwave field using biochar as reducing agent was investigated in this study. It was found that the MnFe2O4-biochar composite briquette possessed good microwave absorption capability with microwave penetration depth less than 20 mm at 2.45 GHz, enabling its efficient self-reduction under microwave irradiation. The MnFe2O4 was initially reduced to monoxide and then transformed to metallic iron and MnO. The iron metallization degree of reduced briquette reached 90.19% under the conditions of the C/MnFe2O4 molar ratio (n(C)/n(MnFe2O4)) of 3, reduction temperature of 1050 °C, and dwell time of 15 min. The selective heating effect of microwave induced the transformation of MnFe2O4 and promoted the breaking of chemical bond between Mn and Fe based on the density functional theory analysis. The microwave heating mechanism was associated with the phase transformation in the reduction process. At low temperatures, the dielectric loss due to interfacial polarization and dipolar polarization and the magnetic loss originated primarily from natural resonance controlled the heating process. As the temperature exceeded 572.9 °C, the composite briquette had a sharp decrease of magnetic response. The formation of (FeO)x(MnO)1-x led to slight fluctuations of permittivity with increasing temperature. When the temperature was higher than 903.5 °C, the conductive loss increased dramatically because of the formation of metallic iron, which could speed up the reduction process due to the “lens effect” of microwave. Based on the combined use of microwave energy and sustainable reducing agent with a focus on the correlation between phase transformation and microwave heating mechanism, this study has validated the feasibility of the route for comprehensive utilization of spinel-structured metallurgical solid wastes, showing the bright prospect of its industrial applications.