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.

Slag foaming is a phenomenon caused by the generation of CO bubbles due to the reaction between iron oxide in slag and carbon in pig iron. The purpose of this study is to explore the controlling factors of slag foaming by observing the bubble formation behavior caused by the chemical reaction between iron oxide and Fe-C alloy in slag. 0.06 g of Fe-C alloy was charged to the bottom of the BN crucible, and 6.0 g of slag (SiO₂:CaO:Fe₂O₃ = 40:40:30) was charged on top of it. The crucible was placed in an infrared image heating furnace, and the temperature was rapidly raised to 1370°C at a rate of 1000°C/min in a N₂ stream, then held for a predetermined time and rapidly cooled. After rapidly cooling, the internal structure of the sample was observed using a high-resolution X-ray CT device. The spherical equivalent volume is calculated based on the number of bubbles observed and their equivalent circle diameter, and the relationship between the volume ratio of small bubbles in the slag volume and the distance from the bottom of the crucible is calculated, and the bubble density and volume ratio are calculated. It was suggested that the value tends to increase as the distance from the bottom of the crucible increases.

Foaming slag generated in the steelmaking process, especially in hot-metal pretreatment and electric arc furnaces, is a gas-liquid coexistent fluid with CO gas generated by the interfacial reaction between slag containing iron oxide and hot metal or carbonaceous materials. In addition, it is essential to understand the flow behavior of foaming slag during slag-tapping and the sedimentation behavior of iron particles, which affects iron yield, and to expand our knowledge of the viscosity of gas-liquid coexisting fluids for CFD modeling of these phenomena. In the present study, the apparent viscosity of a foaming slag was systematically investigated, which was generated by reacting CaO-SiO₂-Fe_xO slag with Fe-C alloy and varying the composition, gas phase ratio, and shear rate of the slag. By adding Fe-C alloy powder to the slag, bubbles were continuously generated in the molten slag, and foaming slag suitable for viscosity measurement could be prepared. It was found that the higher the amount of Fe-C alloy powder, the larger the gas phase ratio of the foaming slag due to an increase in the number of bubbles generated. The relative viscosity of the foaming slag was found to increase with the gas phase ratio. The higher the rotation speed, the smaller the relative viscosity of the foaming slag indicating shear-thinning characteristics. The relationship between shear rate and shear stress calculated from the viscosity of the foaming slag did not show general non-Newtonian fluid behavior.

To meet the increasing demand for low-impurity steel products, powder top blowing has been applied to the steelmaking process. Powder reagents penetrating deeper into the molten metal lead to longer resident time and higher efficiency of refining. Many studies have been performed on the basis of cold model experiments with a single liquid phase for clarifying the penetration behavior of the particle. However, the effects of the second liquid phase have been reported little whereas molten slag often exists on the surface of molten metal in the steelmaking process.

In the present work, the sphere was penetrated into the fluids consisting of a silicone oil layer and water bath. The time variation of the penetration depth of the sphere was measured with a high-speed camera. Effects of the type and size of the sphere, entry velocity, and oil depth were estimated. As a result, the stagnation of penetration occurred under the condition of no air column behind the sphere. On the other hand, a thin oil layer led to no stagnation and deeper penetration due to promoting the formation of air or oil columns. However, an oil layer thicker than 2 mm suppressed the penetration by decreasing the kinetic energy under the condition of high viscosity. The same penetration behavior was observed with a smaller sphere. However, the behavior was more sensitive to the effect of buoyant force because the size of the residual bubble on the surface of the sphere became relatively bigger than the sphere.

In the steel making process, most slags and fluxes often contain solids phase such as CaO. It is well known that the suspension in which solid phase are suspended has higher viscosity than that of pure matrix liquid. Therefore, it is expected that the viscosity of slag containing solid phase will increase. In this study, terminal settling velocity of particle in suspension has been measured. The suspensions consist of silicone oil matrix and polyethylene beads, and the settling particles are bearing balls made of stainless steel. As a result of the higher viscosity of suspension, the terminal settling velocity of bearing ball becomes slower than that in pure silicone oil. It was clarified that the retardation of the terminal velocity and the increasing of drag coefficient depend only on the volume fraction of solid phase (the polyethylene beads) of the suspension, and it is independent of the size of the suspended beads and the viscosity of the matrix liquid. A correlation equation for predicting the drag coefficient of particles in suspension was proposed.

The recovery rate of iron is reduced if iron particles suspended in the refining slag do not sediment. The sedimentation rate of particle iron in the foaming slag is slower than in the slag in the single-phase liquid. Iron particles are especially likely to remain in the foaming slag. To predict the sedimentation rate of iron particles in the slag, it is necessary to derive an accurate viscosity of the foaming slag. However, it is difficult to estimate an appropriate value because the state of gas-liquid multiphase fluid changes the condition with time. Its apparent viscosity varies depending on the measurement method because it is a non-Newtonian fluid. In this study, to understand the sedimentation behavior of iron particles in foaming slag, a gas-liquid multiphase fluid was generated by glycerin solution. Its apparent viscosity was estimated by the Stokes equation using the falling-ball method. The sedimentation rate of stainless steel, titanium, and glass balls with a diameter of 2 mm were measured in a glycerin aqueous solution gas-liquid fluid. The sedimentation rate was non-uniform because the gas-liquid fluid's state differed depending on the position. The apparent viscosity of the fluid increased with an increase in the gas phase ratio. The variation of apparent viscosity with the conditions of the falling-ball method was also discussed. Furthermore, a comparison was made between the present results and the apparent viscosity measured by the rotational technique.

In the steelmaking process, molten slag is foamed through gas injection and gas generation reactions, and molten iron droplets get mixed and trapped in the slag. A settling velocity of an iron droplet in the foaming slag are very important, because a residence time of an iron droplet in the slag is directly calculated the settling velocity. According to the previous research, the settling velocity is expected to be slower than in regular non-foaming slag. However, it has yet to be quantitatively clarified. This study measured the settling velocities of particles through a foaming liquid of glycerin-water solution. A dimensionless correlation equation for particle settling velocity in the formed liquid was proposed by conducting a dimensional analysis of the experimental data. Using the obtained equation, we have predicted the settling velocity of iron particles in the foaming slag. It was clarified that the settling velocity of iron particles is highly affected by a volume fraction of gas phase in the foaming slag. There is a certain threshold for the velocity, and the velocity abruptly became zero when it falls below that threshold.

We experimentally investigated the rupture conditions of a thin film of an aqueous surfactant solution when a spherical particle with a finite falling velocity penetrates the film. When the sphere passes through the film, the film wraps around the sphere, and a gas layer is maintained between the film and the spherical surface. When the velocity of the sphere is small, perforation occurs in the wrapping film below the equator of the sphere and the contact line moves along on the sphere surface. The energy instability occurs at a certain position of the contact line on the sphere surface, leading to rupture of the entire thin film. As the sphere velocity is increased, the perforation of the wrapping film occurs above the equator. In this condition, the probability of thin film rupture increases, since the perforation of the wrapping film immediately leads to rupture of the entire film. The motion of the gas between the thin film and the spherical surface was considered analytically from the balance between surface tension and viscous force. According to the result, the velocity condition above which the wrapping thin film could exist beyond the equator of the sphere was evaluated.