Effect of Na₂O on Dissolution Rate of Alumina in CaO–Al₂O₃–MgO–SiO₂ Slag

Abstract

As the last step of inclusion removal in tundish, the dissolution of Al₂O₃ in tundish slag is very important and has received many interests by previous researchers. In the present work, the effect of Na₂O addition on dissolution of Al₂O₃ in CaO–Al₂O₃–MgO–SiO₂ slag at 1823 K was investigated by using rotating finger method. The interface between rod and molten slag was investigated using SEM-EDS technique. The dissolution of Al₂O₃ in CaO–Al₂O₃–MgO–SiO₂–Na₂O system increases with increase of rotating speed and immersing time. A linear relationship between logarithm of dissolution rate and logarithm of periphery velocity was found with a slope of 0.53, which indicates that rate determining step for dissolution of Al₂O₃ is the mass transfer of solute in boundary layer of molten slag. The dissolution rate of alumina in CaO–Al₂O₃–MgO–SiO₂–Na₂O slag increases with increase of Na₂O content, which could be attributed to the kinetic and thermodynamic factors. The decrease of viscosity of molten slag with increasing Na₂O content could lead to increased mass transfer in boundary layer of molten slag. On the other hand, the increase of Na₂O would lead to increase of thermodynamic driving force of Al₂O₃ dissolution. Some intermediate compounds such as MgAl₂O₄ and CaAl₄O₇ were found at interface between alumina rod and slag, indicating indirect dissolution of Al₂O₃ in slags.