Recently, microwave energy is expected to be a heat source for high temperature process aiming at CO2 reduction and energy conservation owing to the possibility of volumetric and selective heating. In order to examine the applicability of microwave heating to ironmaking, a basic research may be required with respect to the heating mechanism of raw and product materials of ironmaking as well as the carbothermic reduction mechanism of iron ore. To carry out this research, the authors have developed a single mode microwave furnace combined with X‐ray diffraction analysis. By using this, the authors have elucidated the effects of electric and magnetic fields of microwave on carbothermic reduction of magnetite powders, and have observed the selective heating of materials. It has been found that the Fe3O4–C mixed powders are more rapidly heated by the magnetic field than by the electric field just after the onset of heating. The rapid heating owing to the magnetic field is associated with the magnetic loss of Fe3O4 powders. With respect to the magnetic field heating, once the temperature increases up to 800–1000°C, the temperature does not increase and the reduction hardly progresses, which may stem from the decrement in the microwave absorption of the sample. The main heat source is C at this stage. On the other hand, temperature monotonically increases up to around 1000°C and reaction gradually progresses for the electric field heating. It is considered that the main energy absorber is C at the beginning, and then is possibly changed to FeO at elevated temperatures.

Microwave heated materials often reach a quasi-stable temperature resulting in thermal runaway. To control the quasi-stable temperature in microwave processing, it is important to predict the quasi-stable temperature of the steady state. We demonstrated that the microwave heating behavior of hematite varies significantly with its initial temperature. In microwave heating experiments, the temperature of hematite that had not been preheated did not increase, whereas hematite that had been preheated to 410 • C or higher was heated to a temperature of 1020 • C. The microwave heating behavior can be accurately predicted by considering the steady-state energy balance.

Articles you may be interested inEffects of relative density on microwave heating of various carbon powder compacts microwave-metallic multiparticle coupling using spatially separated magnetic fieldsIn materials processing research using microwave heating, there have been several observations of various phenomena occurring known as microwave effects. One significant example of such a phenomenon is increased reaction kinetics. It is believed that there is a possibility that this might be caused by localized heating, were some reactants would attain a higher than apparent temperature. To examine whether such thermal gradients are indeed possible, mixed powders of two microwave non-absorbers, alumina and magnesia, were mixed with graphite, a known absorber, and heated in a microwave furnace. During microwave irradiation, the local temperatures of the respective sample constituents were measured using an in situ x-ray diffraction technique. In the case of the alumina and graphite sample, a temperature difference of around 100 C could be observed. V C 2014 AIP Publishing LLC. [http://dx.