Miyuki Hayashi

Continuous pig iron making by microwave heating with 12.5 kW at 2.45 GHz.

Abstract A continuous process of pig iron making using microwave of 2.45 GHz was constructed in a microwave furnace with maximum power of 12.5 kW. Pig iron was produced from the mixed powder of magnetite ore and carbon such as coal, coke and graphite. Molten pig iron initially poured from reaction chamber after about 40 min when temperature attained over 1200°C. After temperature attained at 1400°C, 50 or 200 g of mixed powder was added several times in regular interval and pig iron continuously dropped out from reaction chamber. When mixed powder was supplied, temperature in reaction chamber decreased by about 200°C and recovered during 5 min. Finally, the recovery time of temperature lengthened. In order to make the scale-up of the microwave furnace for iron making, it has been discussed how to improve energy efficiency and to make a preferable construction.

MAGNETIC STRUCTURE OF AS-QUENCHED SILICATE GLASSES CONTAINING IRON OXIDES

The effect of valence states of iron on the magnetic structure of as-quenched silicate glasses containing iron oxides has been investigated. The glasses, all having the same compositions, are melted in various oxygen pressures to control the ratios of Fe3+ ions to the total number of iron ions and quenched at the same speed, and the magnetic properties are measured. There are only a few reports which focus particularly on the relation between valence states of iron and the magnetic properties of the glasses. Here, the dc magnetizations and ac susceptibilities of the glasses are explained based upon a proposed magnetic structure model in which both microcrystalline clusters and free iron ions in the glass matrix bear magnetism. (In this article free iron ions mean iron ions which are distributed in the glass matrix.) The number density of microcrystalline clusters is highest in a sample of x=0.73 (x: the ratio of Fe3+ ions to the total number of iron ions) and it becomes smaller as the value of x becomes s...

Surface-plasmon-like modes of graphite powder compact in microwave heating

We determine the mechanism of rapid and selective heating of nonmagnetic conductive particles by the electric and magnetic fields of microwaves. We investigated the dependencies of the heating behaviors of carbonpowders on the radius and electrical conductivity for various relative densities. In these experiments, strong microwave absorption was observed in magnetic field at certain radii and ratios of the crystallite size to the radius. Mie theory for a single particle could account for the high heating rates generated by the microwavemagnetic field in sintering experiments. In the dependence of the heating behavior on the relative density, that H field exhibited the maximum absorption at certain relative densities of the graphitepowders. These surface plasmon-like modes were observed in graphite, but were not observed when an E field was applied. Multiparticle systems such as graphitepowder were found to have significantly different heating behaviors from a single particle. Microwave heating of metal particles is expected to be affected by the structure and shape of the particles.

Magnetic interaction between magnetite particles dispersed in calciumsilicate glasses

Abstract The effect of the measuring process on the magnitude of the inter-particle interaction has been investigated by measuring the superparamagnetic relaxation for magnetite particles dispersed in calciumsilicate glasses using thermoremanent magnetization (TRM), AC susceptibility and Mossbauer spectroscopy measurements: The temperature dependence of the remanent magnetization in TRM measurements can be explained by Neels theory which describes the superparamagnetic relaxation of a noninteracting particle. On the other hand, the relation between log 10 τ m ( τ m : the measurement time) and 1 T B ( T B : the average blocking temperature) in AC susceptibility measurements can be interpreted using Dormanns model in which the inter-particle interaction is taken into account. Therefore, it can be concluded that the measuring process for the superparamagnetic relaxation can be more or less sensitive to the inter-particle interaction.

Thermodynamic approach to physical properties of silicate melts

Abstract The thermophysical properties of silicate melts are strongly structure dependent. It is well known that the viscosity of slags increases with increasing degree of polymerisation of the silicate anion. Even the thermodynamic properties of slags are dependent on the species type and population in the melt. Thus, a link between the thermophysical and thermochemical properties of silicate melts is logically expected. The present paper elucidates the salient features of Darkens excess stability approach to the Gibbs energy of solution as applied to the viscosities of silicate melts. It is demonstrated that the second derivatives of the viscosities of binary silicate melts with respect to composition indicate maxima corresponding to the existence of stable compounds in these systems. The concept has been successfully applied to the following systems: Al2O3–SiO2, CaO–SiO2, FeO–SiO2, MgO–SiO2 and MnO–SiO2. In all cases, the second derivative plots of viscosities with respect to composition show peaks corresponding to the metasilicates. The second derivatives of the activation energies of viscous flow with respect to temperature have earlier been shown to reflect the formation of associates/embryos in homogeneous silicate melts, indicating the readiness of the melt to separate a solid phase. Thermodynamic coupling of thermal diffusivities in the case of the CaO–Al2O3–SiO2 system from laser flash measurements of these slags, as a function of temperature, has been examined as part of the present study. Densities have been estimated from integral molar enthalpies in the case of silicate systems, and the results are presented.

Effect of particle size and relative density on powdery Fe3O4 microwave heating.

Abstract In recent years, microwave energy is expected to be a heat source of high temperature process aiming for CO2 reduction and energy conservation owing to the possibility of volumetric heating. In order to examine the applicability of microwave heating to ironmaking, it is important to investigate the microwave heating of raw materials of ironmaking such as Fe3O4. In this study, the effect of particle size and relative density on microwave absorptivity of powdery Fe3O4 was elucidated by the heating curves. Powdery Fe3O4 samples having different particle sizes and relative densities and bulk Fe3O4 samples were heated at the positions of the H (magnetic) and E (electric) field maxima in a 2.45 GHz single-mode microwave cavity. Sample temperatures abruptly increase and become constant after a while. At a constant temperature, the energy balance is attained, i.e., the rate of microwave energy absorption is equal to the rate of thermal energy dissipation. Assuming that the thermal energy dissipation rate due to convection and radiation heat fluxes is only a function of the sample temperature, the microwave absorptivity could be evaluated by the temperature at the steady state. It has been found that the microwave absorptivity of Fe3O4 powder decreases with an increase in relative density. On the other hand, the microwave absorptivity hardly depends on the particle size, which may be due to its quite a large penetration depth of Fe3O4 compared to metal.

Thermochemical and thermophysical property measurements in slag systems

This paper reviews developments on experimental methods and results of thermochemical and thermophysical property measurements of molten silicate slag systems and its theoretical achievements. Several selected topics are focused on, including experimental procedure and measurements of viscosity, density, surface-interfacial tension, thermal conductivity, thermal diffusivity and velocity and absorption coefficient of ultrasonic waves. Thermal conductivity and diffusivity of slags have been mainly measured by the transient techniques such as the laser flash and hot wire methods. Most of the measurements for velocity and absorption coefficient of ultrasonic waves are carried out using a pulse technique. The reliable data for thermochemical and thermophysical properties are required for the optimisation of metallurgical processes and the data is needed in order to improve the numerical models of processing. For academic interest, the results were discussed from the viewpoint of slag structure, as these properties are closely related to the slag structure.

Thermal diffusivities of uniaxially cold-pressed Fe2Mo powders

Fe2Mo powders have been produced from Fe2MoO4 powders by gas-solid reduction using pure H-2 gas at 1023 and 1173 K. The thermal diffusivity of the cold-pressed Fe2Mo powders having a relative densi ...

Effects of relative density on microwave heating of various carbon powder compacts microwave-metallic multi-particle coupling using spatially separated magnetic fields

We have investigated the microwave heating characteristics of non-magnetic conductive multi-particle systems using spatially separated electric and magnetic fields (Emax and Hmax, respectively) to determine the effects of the multi-particle structure on microwave heating. Pure carbon, carbon black, and artificial graphite multi-particle systems exhibited peak microwave absorption at specific relative densities only under Hmax. These absorptions can be categorized into two types: one originates from coupling between metal spheres, while the other originates from a heterogeneous distribution of particles.

Thermal, optical and surface/interfacial properties of molten slag systems

Abstract In modelling mass and heat transfer steps in metallurgical processes, it is important to have knowledge of the physical properties of slags, the most important among these being the surface and interfacial tensions, thermal diffusivities, optical properties and viscosities. A critical review is presented of work reported in the past two decades relating to the following properties of slag systems: (i) surface/interfacial tensions and related interfacial phenomena; (ii) thermal diffusivities and thermal conductivities; (iii) velocities and coefficients of absorption of ultrasonic waves; (iv) optical properties. A perspective for further work is also provided.