Effect of varying Al2O3 contents of CaO–Al2O3–SiO2 slags on lumped MgO dissolution

Abstract This study utilized the single hot thermocouple technique to examine the dissolution behavior of lumped magnesium oxide (MgO) in CaO–Al2O3–SiO2 ternary slags. The aluminum oxide (Al2O3) content in the slag (C/S = 1) varied from 10% to 30%; the MgO sphere with a diameter of 1 mm was placed in molten slags at 1,550 °C. Results showed that the dissolution rate decreased as the Al2O3 content increased up to 20%. Over 20% Al2O3, MgAl2O4 was formed at the interface of MgO and it did not fully melt at 30% Al2O3. The dissolution behavior and the formation of MgAl2O4 were analyzed by a phase diagram provided by Factsage 7.0 software. In the case of less than 20% Al2O3 content, apparent sphere radii were measured; the shrinking core model was then applied to understand the dissolution mechanism. The dissolution rate of both slags was controlled by boundary layer diffusion. The dissolution rate at 20% Al2O3 slag appeared to fit the behavior to the boundary layer diffusion, although it deviated during the middle stage of the dissolution because of MgAl2O4 formation. The 10% Al2O3 slag fitted well to the boundary layer diffusion curve; the obtained diffusion coefficient was 0.94 × 10−9 m2/s.