Kinetic studies of fluoride evaporation from slags

In view of the environmental problems associated with the evaporation of fluoride from slags and fluxes, a systematic study of the fluoride emission from slags at temperatures, relevant to steelmaking and casting, has been initiated in the present work. The loss of fluoride from slag melts was monitored by Thermogravimetric Analysis (TGA). The present paper reports the fluoride evaporation in the three binary systems Al₂O₃-SiO ₂, CaO-SiO₂ and MgO-SiO₂ with additions of CaF₂ (ca. 3.5, 5 and 9wt%). The thermogravimetric experiments were carried out in argon atmosphere in the isothermal mode in the temperature range 1 673-1 873 K. The gas flow was kept above the starvation rate in order to avoid the gas phase mass transfer step. From the results, the Arrhenius activation energies for the evaporation reaction were evaluated. The activation energies were found to be dependent on temperature and slag chemistry for the various slags studied. In binary systems, the activation energy for fluoride evaporation was found to depend on the square of the activity of SiO₂ in the slag melt. This correlation obtained in the case of the binary systems could be validated in the case of the results obtained for the ternary system Al ₂O₃-CaO-SiO₂ with 5 and 10wt% of CaF ₂. The present results are considered to be useful in estimating the fluoride emissions from industrial slags and mould fluxes.