Dissolution kinetics and mechanisms of calcium aluminate inclusions in CaO-Al₂O₃-SiO₂-(MgO) steelmaking slags

The dissolution kinetics and mechanisms of solid calcium aluminate inclusions (CaO∙2Al₂O₃ (CA2) and CaO∙6Al₂O₃ (CA6)) in CaO-Al₂O₃-SiO₂-(MgO) metallurgical slags at 1550 °C were investigated using high temperature confocal laser scanning microscopy (HT-CLSM). The effects of slag viscosity, CaO/Al₂O₃ (C/A) ratio, and MgO content on the dissolution time of CA6 and slag MgO content on that of CA2 particles were examined by tracking the time dependent changes of particle projection areas. The obtained results showed that the dissolution kinetics of CA2 and CA6 particles was enhanced by an increase in slag MgO content. Moreover, increasing C/A ratio of slag or decreasing slag viscosity improved the dissolution rate of CA6 particles. Post dissolution analysis using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) combined with thermodynamic calculations revealed the dissolution paths of CA6 particles in slag S3 with C/A ratio 3.8 and S6 with 8.0 wt% MgO, where the dissolution time is out of expectation. It was found that an intermediate solid layer melilite formed around the undissolved CA6 particle in slag S3 with C/A ratio 3.8, reducing its dissolution rate. Conversely, the formation of randomly dispersed intermediate solid products around the undissolved CA6 particle in slag S6 with 8 wt% MgO did not impend their dissolution rate. Finally, based on the obtained findings, two distinct dissolution mechanisms were proposed advancing the understanding of solid inclusion dissolution in metallurgical slags. The findings obtained from this study aim to provide new insights to further improve steel cleanliness for a longevity of the product service life.