Investigation of the effect of P2O5 and CaF2 on the crystallization behavior of La2O3-bearing calcium–silicate–aluminum slags using the single hot thermocouple technique

Abstract

The effects of P2O5 and CaF2 components of rare earth (RE)-bearing slags on their crystallization behavior were investigated using single hot-thermocouple technique (SHTT). The time temperature transformation (TTT) diagram exhibited single-nose curves within the examined slag composition range, in agreement with the individual CaLa4(SiO4)3O-type phase detected by an isothermal crystallization experiment in a furnace. Additionally, an increase in the P2O5 content shortened the incubation time and increased the nucleation rate of the columnar CaLa4(SiO4)3O-type phase in the 1350–1150 °C temperature range, while an increase in the CaF2 content had the opposite effects on the incubation time and nucleation rate. The sizes of the CaLa4(SiO4)3O-type phases in the slags with CaF2 were larger than those in the slags with P2O5, as verified by both in situ SHTT images and the crystallization experiment. The continuous cooling transformation (CCT) diagram suggested that an increase in the P2O5 content increased the initial crystallization temperature, whereas an increase in the CaF2 content decreased it. An analysis of the isothermal crystallization kinetics revealed that the columnar CaLa4(SiO4)3O-type phase exhibited phase diffusion-controlled growth. An increase in the P2O5 content tended to decrease the crystallization activation energy, while an increase in the CaF2 content increased the activation energy.