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

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 1 823 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.