Study on fluoride evaporation from casting powders

Abstract

Casting powders are commonly used in continuous casting of steels. The chemical composition of these powders is based on various oxides, carbonaceous materials and fluoride compounds. The purpose of fluorite (CaF2) addition is to control viscosity, fluidity temperature and cuspidine crystallization during casting. Unfortunately, fluoride compounds are lost in the vapour phase during the casting operations, due to the high vapour pressure of these compounds. In view of the environmental problems associated with the evaporation of fluoride from the casting powders, a kinetic study on the fluoride evaporation was carried out. Three commercial casting powders with different fluoride contents (between 2.6 and 10.6 mass%) were selected in this study. Powders characterization includes the melting behaviour determined by hot stage microscopy, the physical properties such as viscosity estimated by Fact Sage 7.1 and surface tension calculated on the base of contact angle measurements. The crystallization tendency of the samples was determined by microscopy observations on samples quenched from 1300 °C. The emission study includes thermal analysis (DTA–TG) tests. The different phases present in the system were predicted through thermodynamic simulation. The gaseous species predicted were: NaF, KF, (NaF)2, NaAlF4, KAlF4 and SiF4. It was possible to establish that the fluoride gases emissions occur when the samples present liquid phases. The type of fluoride gas is determined by the chemical composition of the casting powders, while the percentage of emissions depends on both the viscosity and surface tension of liquids.