An integrate study of the effects of CaF2 on the viscous behavior and structure of CaO-SiO2-MgO-Al2O3-CaF2 blast-furnace slag

Abstract

Understanding the physical and chemical characteristics of fluorine-containing blast furnace slag is crucial for using fluorine-bearing ores in blast furnace melting. This paper comprehensively analyzes the effects of CaF₂ on the viscous flow characteristics and structural changes of CaO-SiO₂-MgO-Al₂O₃-CaF₂ blast furnace slag at 1773K. Viscosity measurement results suggest that a higher concentration of CaF₂ contributes to a reduction in slag viscosity. Increasing the CaF₂ content consistently decreases the viscosity of the slag, though the magnitude of this effect diminishes with higher CaF₂ levels. FT-IR shows changes in structure: the diminishing intensity of the [SiO₄]-tetrahedral vibration band and the T-O-T bond strength, shallowing of the groove depth. Besides, a slight narrowing of the shoulder width of the [AlO₄]-tetrahedral structure, but the change is insignificant. Raman results indicate a depolymerization of Si-based structural units. As the CaF₂ content increases from 0 % to 4 %, there is a rise in simpler Q⁰ and Q¹ structural units and a reduction in more complex Q² units. The Q³ structural units show a decreasing trend, without significant change upon further increase. The ratio of non-bridging oxygen to silicon (NBO/Si) increases from 1.95 to 2.28 with the addition of CaF₂, suggesting a lower level of slag polymerization. Fluorine ion serves as a network modifier within the slag structure. XPS results show a decline in the relative concentration of bridging oxygen, whereas the relative concentrations of NBO and free oxygen increase. Finally, ²⁷Al MAS NMR reveals that adding CaF₂ reduces the proportion of [AlO₄] and raises the proportion of [AlO₅] and [AlO₃F] units.