Effects of B2O3 on melting characteristic temperature and evaporation of CaF2-CaO-Al2O3 based ESR slag

The evaporation behaviors of ESR slag (CaF2-CaO-Al2O3-MgO-(B2O3)) for B-containing rack steel was laboratorially investigated to improve the stability of ESR process and the quality of ESR ingots. The thermogravimetric measurement, FactSage 8.3, kinetic analysis and molecular dynamics simulation methods were applied in this study. Additionally, the melting characteristic temperatures of the slag was tested by the hemisphere method. The results showed that B2O3 reduced the melting characteristic temperatures and mass loss of slag. Increasing the B2O3 content increased the vapor pressure of AlF3, OBF, and BF3, consequently promoting their evaporation. The main evaporating product in the slag was found to be CaF2, followed by minor amounts of MgF2, AlF3, OBF, AlOF, and BF3. Notably, the apparent activation energy for fluoride evaporation was calculated to fall within the range of 189.43-388.24 kJ mol(-1), with the evaporation process following a nucleation and growth mechanism. The order of the self-diffusion coefficients of ions in the slag was determined to be F- > Mg2+>Ca2+>O2- approximate to B3+>Al3+. With the increase of B2O3 content in the slag, the self-diffusion coefficient of each ion underwent an initial decrease followed by a significant increase, illustrating that diffusion did not serve as the controlling step of the evaporation reaction.