Microwave Induced Solid–Solid Reactions between Fe₃O₄ and Carbon Black Powders

Abstract

The reduction of 1 μm diameter magnetite by 22 μm diameter carbon black in powder mixture under a constant microwave power of 2.8 kW at 2.45 GHz up to 1200°C was investigated. The initial mixture mass was 6 g in every experiment. Temperature and mass loss of the reacting mixture as a function of time were continuously recorded. The temperature measurement was from 220°C and the mass loss from the start of irradiation. Five different mixtures were irradiated; all of them up to 800°C, three up to 1000°C and two up to 1200°C. All mixtures registered measurable mass losses below 220°C. Total reduction of magnetite to wustite was accomplished below an average temperature of 651°C. Reduction rates as a function of the irradiation time were calculated. The reduction presented three distinctive stages; a initial very fast stage with an average maximum reduction rate of 0.017 g/s, a second stage of constant very low reduction rate 0.0017 g/s, and a third one with a maximum average reduction rate of 0.0045 g/s. During the first stage all the magnetite was reduced to wustite and some wustite to iron. The reduction of magnetite to wustite is shown to take place through a series of consecutive reactions involving solid–solid reactions of magnetite with carbon black as well as solid–gas reactions. Under microwave irradiation true direct reduction occurs at much lower temperatures than under conventional heating. The results can not be explained to occur by thermal effects but by the so called microwave effects.