Fluoride vaporization and crystallization of CaF2–CaO–Al2O3–(La2O3) slag for vacuum electroslag remelting

Abstract

The vaporization and crystallization of CaF2–CaO–Al2O3 slags with different La2O3 contents for electroslag remelting were investigated by various analytical methods. The results show that the main volatiles from investigated slag within 7.4 wt% La2O3 content at 1723 K are CaF2 and AlF3. The weight loss of CaF2–CaO–Al2O3-(La2O3) slags decrease with increase of La2O3 content in both thermo gravimetric experiment and vacuum condition. The addition of La2O3 within 7.4 wt% reduces the viscosity and breaking temperature of investigated slag, and the change of viscosity is not the main reason to inhibit the fluoride vaporization. The vapor pressure of CaF2 and AlF3 at 1723 K decreases with increase of La2O3 content because of the changes in component activity. Thus, fluoride vaporization becomes weakened. The crystalline phases of CaF2–CaO–Al2O3 slag are 11CaO·7Al2O3·CaF2 and CaF2, while the crystalline phases of slag within 7.4 wt% La2O3 content are (CaLa)Al3O7, La(AlO3) and CaF2. The La2O3 addition can decrease the crystallization temperature of CaF2 and prevent the precipitation of 11CaO·7Al2O3·CaF2.