Nobuhiro Maruoka

Dissolution Rate of Various Limes into Steelmaking Slag

Abstract In the steelmaking process, various solid oxides such as lime and dolomite are used as flux. It is well known that the dissolution rate of these oxides into the molten slag has an important role to increase the reaction rate between the steel and slag. Although CaO effectively removes phosphorus from iron, undissolved CaO remains in the slag. The dissolution rate of solid oxides has been previously measured using a sintered rod that was immersed and rotated in the molten slag. However, because this sintered rod was dense, it was difficult to estimate the dissolution rate of the solid oxides used in the actual operations. In this study, the dissolution rate of various limes into the slag was evaluated by measuring the compositional changes in the slag after the addition of these oxides into the molten slag. In an experiment, slag was melted in an iron crucible and Ar gas was injected from the bottom of the crucible. An iron rod was used for stirring. Various limes were added to the molten slag and the slag was sampled at each interval. The slag samples were analyzed using XRF after 1:10-dilution with glass beads followed by fine grinding. A sintered rod of lime made from reagent powder of CaO was used as a dense oxide and the metallurgical lime, used in industrial operation, was used as a porous oxide after preheating at 1273 K. The dissolution rate of metallurgical lime was also measured. Three types of slag were used to evaluate the effect of reaction layer between the lime and molten slag on the dissolution rate of CaO–FeO–SiO2, B2O3-CaO-FeO, and Al2O3-CaO-FeO slag. The CaO–FeO–SiO2 slag caused the formation of a dense reaction layer of 2CaO–SiO2 around the lime. It was found that the influence of porosity of lime on the dissolution rate was not so strong when the reaction layer formed. On the other hand, when the reaction layer did not form, the dissolution rate of the porous lime was higher than that of the dense lime because of the penetration effect of slag into the lime. The dissolution rate of metallurgical lime was fast compared with dense and porous limes due to the gas formation internally.

Effects of Top Layer, Nozzle Arrangement, and Gas Flow Rate on Mixing Time in Agitated Ladles by Bottom Gas Injection

This research investigates mixing phenomena in bottom gas-stirred ladles using water modeling, which incorporates hexane as the top layer. The effects of slag thickness, nozzle position, number of nozzles, and gas flow rate on mixing time have been investigated. Conditions to improve mixing time have been identified. A single nozzle located at two-thirds of the ladle radius was found to produce the shortest mixing time. Under extremely low gas flow rates, an unusual behavior was observed, where the top layer promoted a decrease in mixing time.

Dissolution Behavior of Dicalcium Silicate and Tricalcium Phosphate Solid Solution and other Phases of Steelmaking Slag in an Aqueous Solution

Abstract Most of the phosphorus in slag forms a solid solution of dicalcium silicate (C2S) and tricalcium phosphate (C3P), and the process used to separate this solid solution from the matrix phase is the same technology used to separate P from other valuable elements such as Mn and Cr containing in the matrix phase. Although it is known that the solubility of C2S in an aqueous solution is much greater than that of C3P, the solubility of the solid solution and that of the matrix phase have yet to be investigated. To clarify the possibility of selectively extracting P from slag through a leaching process, the dissolution behaviors of the solid solution at various compositions and that of the matrix phase were investigated. The following results were obtained: The dissolution ratio of Ca to the aqueous solution at pH = 7 was close to 1.0 in the case of pure C2S and decreased greatly with increasing C3P content. The dissolution ratio of P was about 0.1 and did not change relative to the C3P content. When the ratio of C3P in the solid solution was higher than 0.3, hydroxyapatite (HAP) formation was observed in the residue. The dissolution ratio of P increased for 30 min, and after reaching the maximum value, started to decrease owing to the precipitation of HAP. The dissolution ratio of each element from a glassy slag sample (matrix phase) was lower than that from the solid solution at every pH level. In this study, the possibility to extract a solid solution containing P without dissolving the matrix phase was found through the use of an aqueous solution at pH = 7, although the dissolution ratio of P was not sufficiently high.

Modeling of Ascending/Descending Velocity of Metal Droplet Emulsified in Pb-Salt System

Metal–slag emulsion is an important process to enhance the reaction rate between the two phases; thus, it improves the heat and mass transfer of the process significantly. Various experimental studies have been carried out, and some system specific relations have been proposed by various investigators. A unified, theoretical study is lacking to model this complex phenomenon. Therefore, two simple models based on fundamental laws for metal droplet velocity (both ascending and descending) and bubble velocity, as well as its position at any instant of time, have been proposed. Analytical solutions have been obtained for the developed equations. Analytical solutions have been verified for the droplet velocity, traveling time, and size distribution in slag phase by performing high-temperature experiments in a Pb-salt system and comparing the obtained data with theory. The proposed model has also been verified with published experimental data for various liquid systems with a wide range of physical properties. A good agreement has been found between the analytical solution and the experimental and published data in all cases.

Dissolution Behavior of Nutrient Elements from Fertilizer Made of Steelmaking Slag, in an Irrigated Paddy Field Environment

To produce fertilizer made of steelmaking slag, the combination of mineralogical phases is necessary to be appropriate not only for refining at high temperature, but also for supplementing nutrient elements in paddy fields. In this research, first, the variations in the chemical composition and mineralogical structure of various brands of fertilizer made of steelmaking slag were investigated. The basicity (CaO/SiO2), free-CaO content, total Fe content, and Fe2+/Fe3+ ratio of fertilizer were found to vary widely depending on the brand. In addition, regarding the mineralogical phases, solid solution consisting of 2CaO·SiO2 and 3CaO·P2O5, MgO–FeO, CaO–Fe2O3, free CaO, metallic Fe, and the other phase which included liquid phases at refining temperature were observed. The mass fraction of each phase also varied depending on the brand. Second, the dissolution behavior of each mineralogical phase in the fertilizers into water that simulated the early stage of flooding condition was studied. Regarding the dissolution of Ca, free CaO dissolved in a short time, and the solid solution of 2CaO·SiO2 and 3CaO·P2O5 dissolved slowly. Fe barely dissolved from any mineralogical phase except the other phase which consisted of CaO–SiO2–FeO. A clear relationship was observed between the dissolution of Fe and the mass fraction of other phase as well as the ratio of Fe2+ to Fe3+ in Fe oxides in fertilizer made of steelmaking slag.

Production and utilisation of iron and steelmaking slag in Japan and the application of steelmaking slag for the recovery of paddy fields damaged by Tsunami

Abstract In Japan, about 40 million tonnes of slag is generated annually during the iron and steelmaking process. This large amount of byproduct warrants investigation on its disposal. In this study, the production, chemical characteristics, and use of iron and steel slag in Japan, especially converter slag, have been reviewed. Then, the necessity for sodium removal and nutrient supply for the recovery of the paddy fields damaged by the Tsunami, which resulted from the great east Japan earthquake, has been explained. Since converter slag can serve the above two requirements simultaneously, owing to its chemical properties, investigations on efficient utilisation of converter slag have been carried out. With the perspective of both agriculture and engineering science, the positive effects of converter slag on desalting, pH improvement, and supply of nutrients such as silicon have been found.

Novel Recycling Process of Mn by Sulfurization of Molten Slag from a By-Product of Steelmaking Process

We propose a novel process to recycle Mn from steelmaking slag. The first step is to sulfurize the slag, producing a liquid sulfide phase (matte) without P. High-purity Fe-Mn alloys can then be made by desulfurizing the matte. However, to our knowledge, there have been no reports to date concerning the sulfurization of P and Mn contained in molten slag. Therefore, knowledge of the distribution of Fe, Mn and P between the matte and the molten slag is required to determine the feasibility of this process. In this study, the equilibrium distributions of Mn, Fe, Ca and P between a FeS-MnS matte and FeO-MnO-SiO2-MgO-P2O5 slag with/without CaO are investigated under controlled partial pressures of oxygen and sulfur. It was found, as P is not present in the matte, that a separation of P from Mn was accomplished. The increament of (CaOCMgO)=SiO2 in slag can improve the concentrations of Mn and Fe in the matte. The content of Ca in the matte was less than 2 mass%, even when the concentration ratio of CaO=SiO2 in the slag was unity.

Rapid Dissolution of Quicklime into Molten Slag by Internally Formed Gas

In steelmaking process, quicklime is used to produce CaO-based slag. Although rapid dissolution of quicklime is required for high-efficiency refining, it is known that the rate decreases when dicalcium silicate (C2S) layer forms around the quicklime by reacting with slag. The equation that driving force is the difference of CaO content between in slag and a liquid phase of slag saturated by C2S has been often used for estimating the dissolution rate of lime, in which this saturated value is thermodynamically determined. The authors, however, revealed that the quicklime used in actual operation showed much faster dissolving rate than that of completely calcined lime that is covered by C2S layer during dissolution into slag. This was caused by a gas formation due to a thermal decomposition of residual limestone existed in quicklime. In this study, the dissolution rate of quicklime with the gas formation is quantitatively investigated.

In Situ Observation of Dross Formation During Melting of Al–Mg Alloy

Al–Mg alloy is one of the most important industrial aluminum products used in can lids, panel, wheel, welding materials etc. It is known, however, that magnesium in the alloy enhances dross formation on the molten aluminum surface during melting process. Suppression of the dross formation is one of the most important tasks in aluminum industry. In the present work, in situ observation of dross formation during melting of Al–Mg alloy was conducted. By the air oxidation of molten Al–Mg alloy at 800 °C, horizontally projected area of dross phase formed on the surface of the melt and weight gain of the alloy were measured. It was observed that the projected area of dross phase drastically increased after an incubation period and then it changed a little. On the other hand, weight gain of the alloy continuously increased and it became faster with increasing magnesium content of the alloy. These findings help to clarify the dross formation mechanism.

Thermodynamic properties of lead oxide in a mixture of stainless steelmaking and nonferrous smelting slags

Abstract In our previous paper, a slag modification process involving the mixing of stainless steelmaking and nonferrous smelting slags was proposed for preventing the disintegration of the stainless steelmaking slag. In order to use this method, the behavior of heavy metals especially PbO contained in the nonferrous slag has to be assessed. In the present study, the activity coefficient of PbO in CaO-SiO2-FetO-Al2O3-MgO and CaO-SiO2-FetO slags saturated with iron was measured at 1673 K. The results showed that the activity coefficient of PbO increased with basicity and had a maximum value when the basicity was approximately 1.0. The equilibrium PbO content in the modified slag had a minimum value that corresponded to a mixing ratio of 0.6. The trend was similar to the change in the removal ratio of PbO observed in the previous study. Therefore, the change in the oxygen potential and the change in the activity coefficient of PbO can be considered the cause of this trend.