Kinetic Study of NaF Gas Formation During Mold Flux Melting Based on Different Fluorides by Experimental and Computational Methods

Abstract

Low-basicity fluorine-containing mold flux, which forms a porous slag film with NaF pores, is expected to solve the contradiction between heat transfer and lubrication in the continuous casting of peritectic steel. Herein, the kinetic mechanism and the influence of NaF gas formation based on the different fluorine raw materials (NaF, CaF₂, and Na₃AlF₆) at 0.9 basicity and 6 wt% fluorine are investigated by the combination of experiments and calculations. The results show that CaF₂ is the first to generate NaF gas at 1189 K, followed by Na₃AlF₆ (1238 K) and NaF (1328 K). NaF raw material produces the most NaF gas with the lowest activation energy in the three raw materials, due to high Na ion kinetic energy, the simple degree of polymerization, and Na-F₁ bonds. Correspondingly, NaF bubbles in the molten slag are the largest based on their aggregation. Na₃AlF₆ and CaF₂ raw materials rank second and third, with F atoms primarily bonded as Al-F_x and Ca-F_x and the higher proportion of $Q_{\text{Si}-O-\text{Al}}^2$ and $Q_{\text{Si}-O-\text{Si}}^4$ structure. Notably, at 0.9 basicity and 6 wt% fluorine, Na₃AlF₆ as raw material generates a suitable amount of gas, making diffuse and fine NaF bubbles in the molten slag.