Molecular dynamics analysis of the microstructural behavior of fluoride ions in CaO–SiO2-based and CaO–Al2O3-based slag systems

To further clarify the structural behaviour of F − in CaO–SiO 2 –CaF 2 –MgO and CaO–Al 2 O 3 –CaF 2 –MgO slags, the microstructure of F ions in slags with different CaF 2 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 CaF 2 content, the average distance between Si–Si and Al–Al atom pairs increases, and CaF 2 plays the role of network diluent. F − ions are mainly coordinated with Ca 2+ , and there is a dynamic equilibrium between Ca 2+ and coordination anions (O 2− 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 O 2− in the Ca–O structure, Ca 2+ 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 O 2− 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 Q n structural units. In aluminates, the electrically neutral [AlO 3 F] 4− tetrahedral structure is formed as F replaces O atoms in [AlO 4 ] 5− tetrahedra, where the released O atoms are absorbed by other [AlO 4 ] 5− tetrahedra to form unstable highly coordinated Al. However, since the formed [AlO 3 F] 4− tetrahedral structure is neutral, it can, together with Ca 2+ , 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 CaF 2 in reducing viscosity is diminished.