An on-line multi-frequency electrical resistance tomography (mfERT) device with a melt-resistive sensor and noise reduction hardware has been proposed for crystalline phase imaging in high-temperature molten oxide. The melt-resistive sensor consists of eight electrodes made of platinum-rhodium (Pt-20mass%Rh) alloy covered by non-conductive aluminum oxide (Al₂O₃) to prevent an electrical short. The noise reduction hardware has been designed by two approaches: (1) total harmonic distortion (THD) for the robust multiplexer, and (2) a current injection frequency pair: low $f^L$ and high $f^H$, for thermal noise compensation. THD is determined by a percentage evaluation of k-th harmonic distortions of ZnO at $f=0$.1~10,000 Hz. The $f^L$ and $f^H$ are determined by the thermal noise behavior estimation at different temperatures. At $f <1$00 Hz, the THD percentage is relatively high and fluctuates; otherwise, THD dramatically declines, nearly reaching zero. At the determined $f^L\ge$ 10,000 Hz and $f^H\approx$ 1,000,000 Hz, thermal noise is significantly compensated. The on-line mfERT was tested in the experiments of a non-conductive Al₂O₃ rod dipped into conductive molten zinc-borate (60ZnO-40B₂O₃) at 1000~1200 $°$C. As a result, the on-line mfERT is able to reconstruct the Al₂O₃ rod inclusion images in the high-temperature fields with low error, ${\mathsf{\varsigma }}_{f^L, T}$ = 5.99%, at 1000 $°$C, and an average error$⟨{\mathsf{\varsigma }}_{f^L}⟩$ = 9.2%.

Under the operating temperatures employed in steelmaking, most slags and fluxes often contain solids, such as undissolved CaO and its reaction products; thus, they are more viscous than their fully liquid states. However, few studies have considered the dielectric interactions of solid particles with the liquid matrix in such systems. In the present study, the viscosity of suspensions of dispersed particles consisting of polyethylene beads in a matrix of silicone oil or aqueous glycerol at room temperature was measured. Then, empirical models for estimating the viscosity based on the Einstein–Roscoe equation were proposed. Furthermore, the viscosity of suspensions of CaO and MgO particles dispersed in a matrix of CaO–Al2O3–SiO2–MgO slag at 1773 K was measured, and the feasibility of the proposed viscosity equations was investigated. As expected, the viscosities of the suspensions of polyethylene beads dispersed in silicone oil and glycerol increased with an increasing bead volume fraction. Under comparable measurement conditions, the viscosities of the glycerol suspensions were higher than those of the silicone oil suspensions. The proposed viscosity models based on the Einstein–Roscoe equation and the capillary number reproduced the viscosity of the silicone oil suspensions but underestimated that of the glycerol suspensions. The trend of increasing viscosity of the molten slag suspensions with dispersed CaO and MgO particles was similar to that of the room-temperature suspensions, exhibiting Bingham non-Newtonian behavior. The viscosity model composed with the results from the glycerol aqueous suspensions underestimated the slag viscosity, which can be attributed to the repulsive forces in the high-polarity liquid matrix.

CaO-based slag used in hot metal pretreatment and converters in steelmaking processes typically contains dispersed gas phases. This is called foaming slag, which is known to degrade the quality of slag. The rheological behavior of this slag is dependent on the dispersed part of the gas phase. This gas is generated by the chemical reaction between the hot metal and the slag. In this study, simulated foaming slag was prepared by reacting sodium hydrogen carbonate and oxalic acid in glycerol, which disperses carbon dioxide. Next, we systematically investigated the effects of the volume fraction of the dispersed gas phase and the proportion of glycerol on the viscosity and bubble diameter. According to the model used in this study, the bubbles were smaller than those in the model in which the gas was directly dispersed. The bubble size increased as the gas phase ratio and liquid viscosity increased, likely because the bubble growth is promoted by increase in the gas phase ratio and liquid phase viscosity, and the frequency with which the bubbles contact one other. The increase of the gas phase ratio at low liquid-phase viscosity and low shear rate caused an increase in both apparent viscosity and relative viscosity, which was obtained by dividing the apparent viscosity by liquid-phase viscosity. However, these increases in viscosity were not observed at a high shear rate. This is likely because the mechanism of bubble diffusion and flow is affected by the liquid-phase viscosity and shear rate. We found that the model in this study exemplified a Herschel-Bulkley fluid. In addition, we proposed an equation for measuring viscosity from the gas phase ratio.

Iron droplet that entrapped in the refining slag reduces the yield of iron, so it is necessary to accurately estimate the settling rate of iron particle in the slag. However, since slag is a solid-liquid coexisting fluid, the sedimentation rate of iron particle cannot be derived even using the Stokes’ equation based on the viscosity of the liquid phase. In this study, using silicone oil in which polyethylene particles were suspended as a slag substitute sample, the sedimentation rate of stainless steel balls was measured, and the apparent viscosity was derived with the Stokes method. The effects of the viscosity of the silicone oil, the solid phase ratio, and the diameter of the metal balls on this apparent viscosity were investigated. It was confirmed that the apparent viscosity changes depending on the diameter of the descending metal balls.

In the process of steelmaking refining, slag used in the preliminary treatment of molten iron or the converter is a multiphase melt. It contains solid particles that cannot dissolve entirely from the raw materials and gas bubbles generated through reactions, thus making it a multiphase molten material with dispersed components. The flow characteristics of the suspensions, foams, and emulsions significantly affect the separation of iron particles in the slag and the behavior of slag discharge. Multiphase melts typically behave as non-Newtonian fluids, so the evaluation, focusing on viscosity, is crucial to understanding their flow characteristics. This report reviews recent advances in slag visualization techniques for understanding the flow of multiphase molten materials. The results are as follows:Through studies of viscosity measurements of multiphase melts using rotational viscometers, falling-ball methods, and dam break methods in cold experiments with simulated slag and high-temperature experiments with slag compositions close to practical operation, it has been shown that the apparent viscosity increases with an increase in the volume fraction of solid particles and bubbles. The complex behaviors of the viscosity measurement values could have been reproduced with high precision using predictive models based on recent developments in machine learning. In terms of process evaluation, the application of mesh and mesh-free methods is advancing as methods of computational fluid dynamics (CFD) that take non-Newtonian behavior into account, providing valuable insights into evaluating slag discharge properties and more.