Dissolution rate of various limes into 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, B₂O₃-CaO-FeO, and Al₂O ₃-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.