Dissolution of ZrO₂, Al₂O₃, MgO and MgAl₂O₄ particles in a B₂O₃ containing commercial fluoride-free mould slag

Non-metallic inclusions composed of ZrO₂, Al₂O ₃, MgO and MgAl₂O₄ are associated with problems during the continuous casting of steels and so it is desirable that such particles dissolve completely if they appear in the slag. The dissolution behaviour of particles of these oxides in a fluorine-free slag containing 1.5wt% B₂O₃, was studied in situ using Confocal Scanning Laser Microscopy (CSLM). The effects of particle type, initial size, and slag temperature were investigated. Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) was performed to chemically analyze quenched samples, in order to identify any surface reaction or diffusion layers formed. Analytical prediction models were compared to the experimental data to relate the kinetics to possible rate-limiting steps. Thermodynamic solubility limits for use in the model were determined using commercial CALPHAD based software. The dissolution rates of Al₂O₃, MgO and MgAl₂O₄ were found to be comparable to one another whereas the dissolution rate of ZrO₂, is four times slower. The surface reaction appears to be controlling the rate of dissolution, with the activation energy for ZrO₂ being 128.8 kJ/mol and for MgAl₂O ₄ being 77.8 kJ/mol. This implies that the removal of ZrO₂ particles by dissolution in this type of slag is not feasible for the typical residence times expected, and strategies that prevent the incorporation of these particles should be used.