混相流 Vol. 30(2016) No. 2

A lot of steel engineers researched various means to decrease reducing agents at a blast furnace in order to reduce CO₂ emissions. For example, injection of H₂, waste plastics and carbon neutral materials such as biomass into a blast furnace is better alternative. When iron oxide is reduced by H₂ gas, CO₂ don' t form. We have studied optimal reduction conditions of iron oxide. In this work, we focused on H₂-CO gas mixtures reduction of iron oxide. In order to prepare samples, reagent Fe₂O₃ was mixed with SiO₂ and CaO in the mass ratio of Fe₂O₃：SiO₂：CaO = 80：7.14：12.86. Then the mixture was pelletized by handrolling and sintered at 1220℃ (Sinter-A, a porous sample) or 1250℃ (Sinter-B, a relatively porous sample). The formed pellets were reduced from Fe₂O₃ to FeO at 800℃ and from FeO to Fe 1100℃. From weight loss curves of samples, chemical reaction rate content (k_c) and effective diffusion coefficient (D_e) in a product layer were calculated by using an unreacted core model. The values of k_c and D_e in H₂ reduction were much higher than those values in CO reduction. The particle size of the reduced Fe in H₂ reduction was smaller than that one in CO reduction.

In the blast furnace, there is a packed bed consists of coke and iron ore, and there is a counter flow of the condensed and gas phases. Controlling the liquid and solid flow and preventing accumulation of the liquid and the powder are important issues for the productivity and the stability of the blast furnace process. The flow of the packed bed of burden, and liquid and gas flows in the cohesive zone had been investigated. Here, studies on the permeability of the cohesive and dripping zone of the blast furnace are introduced, and how to control the physical properties of the liquid and powder are discussed for increasing the gas permeability in the packed bed. In addition, progress of numerical simulation with installing influence of the physical properties of liquid, gas and powder is introduced.

Studies on the carburization rate of metallic iron through CaO-SiO₂-Al₂O₃ slag phase and its mechanism have been introduced as a fundamental of slag-design for the low-temperature and high-speed of iron carburization. The carbon solubility in slag increases with an increase of slag basicity, and the rate of iron carburization through slag is faster when the slag basicity is higher. From these facts, it is concluded that the iron carburization proceeds faster when the slag basicity is higher, the carbon solubility is higher, and the partial pressure of oxygen is lower. On the other hand, the carburization rate is much slower than the rate of direct carburization or carburization through slag including iron oxide (FeO). That is to say, when iron ore agglomerates including carbon source are used as a raw material, in order to enhance the carburization reaction, it is important to proceed ‘melting and carburization’ according to the following mechanism: slag component is melted not after the reduction of iron oxide (formation of metallic iron) by the carbon source, but before iron oxide is perfectly reduced to metallic iron so that iron oxide can dissolve into slag.

In most previous works, liquid hold-ups were studied by using a fixed bed soaked prior to experiments. In the present study, the characteristics of liquid hold-ups and liquid flow were investigated with initially unsoaked bed. Contact angles (θ) for these particle/liquid (tap water) systems were about 70° and 10° for fluorine-coated particles and non-coated particles. Under bad wettability condition (θ≒70°), total and static hold-ups for initially unsoaked bed packed with small balls are remarkably smaller than those for initially soaked bed. In initially unsoaked bed, total and static hold-ups under bad wettability condition (θ≒70°) indicate maximum values at about D_p =10 mm and decrease abruptly in proportion to a decrease in particle size, despite an increase in the specific surface area. Under good wettability condition (θ≒10°), total and static hold-ups are smaller than those of bad wettability condition (θ≒70°), because only restricted liquid droplets and/or liquid rivulets are formed within the packed bed for initially unsoaked bed, nevertheless liquid is easy to spread out on the solid surface under good wettability condition.

Ironmaking process is currently focused on the decreased reducing agent operation of a blast furnace in order to decrease CO₂ emissions. Because the coke ratio is reduced in this operation, causing a huge pressure drop, it has become increasingly important to understand the in-furnace phenomena using the non-empirical method. The discrete element method (DEM), Euler-Lagrange coupling method (DEM-CFD), and fully-Lagrangian method (e.g. SPH, MPS) simulation were carried out to analyze the momentum, heat and mass transfer between the fluid and the particles in a packed bed. These developed numerical methods are using discrete element type model, which can track the movement of the solid or the fluid phase directly. The optimum bed structure for decreased reducing agent operation can be clarified by application of these methods. Moreover, three-dimensional information for understanding in-furnace local flow structure can be also obtained with high spatial resolution. This paper explains the outline of the latest numerical model of blast furnace.

New technologies for increasing the utilization of iron-scraps are strongly demanded. One of them is liquid-liquid immiscibility phenomena for metals, which can realize the separation, concentration and refining of metals. It have been reported that several alloy systems cause two immiscible liquids. In this report, the progress of researches on the following three systems for liquid-liquid immiscibility phenomena was investigated. (1) In Fe-Cu-C system, it has been reported that a uniform liquid in an Fe-Cu system separates into two immiscible phases, i.e. a Cu-rich phase and an Fe-rich phase, when C is added to the system, and in addition the minor metals contained in an Fe-Cu-C system are distributed between the Cu-rich and Fe-rich phases. In recent years, the distribution behavior of rare metals and the stability of immiscibility under cooling process etc. have been studied. (2) In Fe-Ag system, the copper removal has been tried based on the distribution of copper and the low mutual solubility for liquid Fe phase and liquid Ag phase. The oxidation removal of tramp-elements (Cu and Sn) from carbon saturated iron via silver has been proposed recently. (3) In Fe-Ca system, the refining technology of iron by using Ca and its potential has been discussed.

Microbubble flows are employed in a variety of engineering applications, such as the cleaning of surfaces, drag reduction in ship hulls and various chemical plant processes. Especially for surface cleaning, the measurement of the bubble and liquid flow speed is required for the better understanding of cleaning efficiency. PIV is a useful tool for the measurement of the fluid velocity, because it is possible to measure planar flow structures without disturbing the flow field. To apply PIV to microbubble flows, the separation of microbubbles and liquid in the acquired images is required. In the present study, we present a velocity measurement system for gas-liquid two phase flows that employs phase discrimination based on the different optical properties of the bubbles and solid tracer particles. By using two cameras, the 3-dimensional position and velocity of both phases in microbubble impinging jet was successfully measured.

Cavitation due to a suction vortex (vortex cavitation) is an important problem in some fluid machinery fields, such as fast reactors. In this study, a water experiment on vortex cavitation is carried out using a simple cylindrical vessel with a suction nozzle. In order to understand the fundamental behavior of vortex cavitation, its instantaneous occurrence behavior is grasped by visualization using high speed camera. Velocity distribution around vortex, which causes cavitation, is also quantitatively grasped by means of Particle Image Velocimetry. From visualization measurements, vortex cavitation is considered to be triggered by a wall nuclei and the cavity develops immediately toward the suction nozzle once triggering occurs on the bottom wall. In addition, distribution of pressure decrease along vortex center estimated based on Burgers model and measured velocity distribution shows monotone increase from the bottom wall toward the suction nozzle. As the results, the cavity is thought to develop toward the suction nozzle intake immediately, if some triggering of cavitation occur on the bottom wall.

This paper presents an investigation of liquid thread breakup from tip of a spike with applying high-voltage AC. Microscale interface behavior is visualized by using high-speed camera. An interface of a magnetic-fluid spike is oscillating with a twice frequency of the one of applying AC voltage. Applying more than a certain voltage makes an elongation of liquid thread from a tip of a spike. Then, micro droplets are generated by breaking up of liquid thread. Applying AC voltage allows continuous generation of droplets. It is found that the diameter of generated droplet is increase with applying voltage. The interface behavior becomes random above a certain voltage.