Nobuhiro Maruoka

Paddy fields located near the coast of northeastern Japan have suffered significant damage from the tsunami caused by the earthquake. The restoration of a damaged paddy field can be achieved with the washing out of Na by desalting treatment and fertilization with silica. The concentration of Na can be effectively reduced with Ca supplements. As steelmaking slag contains CaO and SiO₂, it has a strong potential to become an economic solution for the recovery of fields. To evaluate the effect of fertilizer made of steelmaking slag, a new soil-filled column testing method was developed to simulate the paddy field conditions. The column was composed of a plow layer containing pore water and soil with fertilizer, along with a layer of surface water. Every day, water sample was collected from the bottom of the column and the same amount of fresh water was supplied for approximately 2 months. The original soil and the soil after desalting were used with and without fertilizer made of steelmaking slag. The influence of soil particle size was also investigated. During the experiment, the pH increased, while the oxidation and reduction potential decreased day by day; thus, the typical changes under paddy field conditions were simulated. Supplementation with fertilizer made of steelmaking slag led to the increase in Ca content and pH. In contrast, most of the Na was washed out by water exchange during the experiment; however, an initial decrease in its content was observed by the addition of fertilizer made of steelmaking slag.

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–SiO ₂ , B ₂ O ₃ -CaO-FeO, and Al ₂ O ₃ -CaO-FeO slag. The CaO–FeO–SiO ₂ slag caused the formation of a dense reaction layer of 2CaO–SiO ₂ 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.

A kinetic model to simulate the reactions in a ladle furnace was developed in the previous paper. The following parameters were considered in this model; (1) ratio of the entrapment of slag in the molten steel, (2) ratio of the floatation of the deoxidation products and inclusions originating from the slag, (3) ratio of the agglomeration of deoxidation products with inclusions originating from the slag and (4) ratio of the volume of the bulk zone to the total volume of molten steel and that of slag phase. These parameters were optimized using sensitivity calculation by comparison with operational results as the parameters affected the amount and composition of inclusions. Then, the method to suppress the formation of MgO·Al₂O₃ spinel-type inclusion was discussed using the optimized parameters. The calculated results showed that the formation of MgO·Al₂O₃ spinel-type inclusion could be suppressed by optimizing the additional amount of Al, initial content of MgO in the slag, and slag basicity in addition to the Ca treatment. The changes in the inclusions calculated using the kinetic model were in good agreement with those predicted by the phase stability diagram. The developed model was useful for optimizing the operation of a ladle furnace.

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