Molecular dynamics analysis of the microstructural behavior of fluoride ions in CaO–SiO 2 -based and CaO–Al 2 O 3 -based slag systems

Abstract

To further clarify the structural behaviour of F⁻ in CaO–SiO₂–CaF₂–MgO and CaO–Al₂O₃–CaF₂–MgO slags, the microstructure of F ions in slags with different CaF₂ contents was resolved by molecular dynamics simulation. The results showed that no stable coordination structure was formed between Si and F, and no chemical bond could be formed; while Al and F and Ca and F could form relatively stable structures, both of which could be bonded. With the increase of CaF₂ content, the average distance between Si–Si and Al–Al atom pairs increases, and CaF₂ plays the role of network diluent. F⁻ ions are mainly coordinated with Ca²⁺, and there is a dynamic equilibrium between Ca²⁺ and coordination anions (O²⁻ and F⁻) in both systems, and the total coordination number is maintained between six and seven, which is distributed in the network structure in the form of Ca–F clusters. As F⁻ replaces the position of O²⁻ in the Ca–O structure, Ca²⁺ ions in the silicate slag system are not sufficient to balance the negative charge, and the charge balance can only be maintained by sharing the O²⁻ at the top angle, leading to more interconnected silicate network structures, resulting in a shift in the oxygen type in the melt and the appearance of more highly polymerised Qⁿ structural units. In aluminates, the electrically neutral [AlO₃F]⁴⁻ tetrahedral structure is formed as F replaces O atoms in [AlO₄]⁵⁻ tetrahedra, where the released O atoms are absorbed by other [AlO₄]⁵⁻ tetrahedra to form unstable highly coordinated Al. However, since the formed [AlO₃F]⁴⁻ tetrahedral structure is neutral, it can, together with Ca²⁺, balance the Al–O network structure with negative charges, making the system contain a large amount of Al–O–Al bridging oxygen structure and the effect of CaF₂ in reducing viscosity is diminished.