Effect of the heating rate and premelting process on the melting point and volatilization of a fluorine-containing slag

Abstract

The melting point and volatilization characteristics of a fluorine-containing slag were investigated under different heating rates and premelting processes. The “hemisphere method” was used to detect the melting point, and the results showed that the measurements for the fluorine-free slag increased with increasing heating rate, and the deviation reached 60 °C and was affected by the hysteresis and fractional melting. The measured values and volatilization of the fluorine-containing slag decreased with increasing heating rate. The weight loss reached 16.8%, and the melting point deviation reached 90 °C, which was primarily affected by the volatility. The melting point of the synthetic fluorine-containing slag was 70 °C higher than that of the premelted slag due to flux volatilization of 8.3%. Scanning electron microscopy-energy dispersive spectroscopy revealed a large amount of CaF2 on the surface of the melted slag. The internal crystals in the synthetic slag were mainly diamond-shaped calcium fluoroaluminate (3CaO·3Al2O3·CaF2) and those in the premelted slag were needle-shaped cuspidine (3CaO·2SiO2·CaF2) that formed during secondary crystallization. Two factors that impacted the volatilization were proposed: one is the content of free CaF2, and the other is the slag structure. Based on these factors, volatilization models for synthetic and premelted fluorine-containing slags were established. This work is of significance for relevant measurement specifications to ensure the repeatability and comparability of the physicochemical properties, especially for volatile-containing slags.