Influence mechanism of Ca-Fe flux on fusibility and crystallization behavior of coal fly ash

The preparation of high-strength inorganic fibers by coal fly ash is an important method to achieve its high-value utilization. Due to high content of SiO₂ and Al₂O₃ in coal fly ash, it is necessary that Ca-Fe flux should be added to decrease melting temperature of coal fly ash during homogenization of slag and avoid crystallization behavior during preparation of fibers. In this paper, the influence mechanism of Ca-Fe flux on the fusibility and crystallization behavior of coal fly ash under air atmosphere was investigated. The results show that calcium oxide (CaO) and iron oxide (Fe₂O₃) does not show synergistic fluxing effect on fusibility of coal fly ash under air atmosphere, moreover, the fluxing effect of CaO on fusibility is more obvious than that of Fe₂O₃. At high temperature, Fe₂O₃ cannot react with silicon-aluminum components to form low melting point minerals, while CaO is beneficial to transform refractory mullite into anorthite, which greatly reduces the melting point of fly ash. However, excess CaO (>30%) leads to formation of calcium feldspar, which increases melting point of fly ash obviously. During the cooling process, iron precipitates in the form of hematite while calcium precipitates in the form of anorthite, and the crystallization temperature of hematite is lower than that of anorthite. When content of CaO is 32.46%, the melting point of coal fly ash is the lowest, and there is no crystallization during the cooling process. Therefore, CaO is suitable to adjust fusibility and crystallization behavior of coal fly ash during preparation of inorganic fiber.