Development of low-fluoride slag for electroslag remelting: Role of Li2O on the crystallization and evaporation of the slag

Abstract

To reduce environmental pollution caused by fluoride from conventional electroslag remelting (ESR)-type slag and meet the requirements of vacuum ESR, it is strongly needed to develop low-fluoride and fluoride-free slag. The crystallization behaviors and evaporation of CaF2-CaO-Al2O3-MgO-Li2O slag as a candidate for low-fluoride ESR-type slag were studied. The sequence of crystal precipitation in CaF2-CaO-Al2O3- MgO-Li2O slag during cooling process was 11CaO·7Al2O3·CaF2 to CaO, followed by CaF2. The dominant crystalline phase in the slag was 11CaO·7Al2O3·CaF2. The liquidus temperature and crystallization temperature of slag decreased significantly with increasing Li2O content from 1.59 mass% to 4.46 mass%. Increasing Li2O contents suppressed the crystallization behaviors of ESR-type CaF2-CaO-Al2O3-MgO-Li2O slag. The weight loss of the slag melts increased with increasing Li2O content in the slag from 1.59 mass% to 4.46 mass%. The gaseous species evaporated from the slag melts were mainly LiF and contained a few amounts of CaF2. A proper amount of Li2O could be considered as an effective component for the design of low-fluoride ESR-type slag. Serious fluoride evaporation of LiF from CaF2-CaO-Al2O3- MgO-Li2O slag melts takes place when Li2O content exceeds a critical value.