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.