Effect of MgO addition on the structural behaviour of ions in CaF ₂ –Al ₂ O ₃ -based electroslag systems

Molecular dynamics simulations were conducted to elucidate the effects of MgO/Al 2 O 3 and MgO/CaF 2 ratio variations on the structure and viscosity of CaF 2 −Al 2 O 3 −MgO melts. Results indicate that in basic environments, Al 3+ acts as a network former, strongly bonding with O 2− ions and predominantly existing in aluminum−oxygen tetrahedral configurations. Increasing the MgO/Al 2 O 3 ratio promotes the substitution of F − by O 2− in [AlO 3 F] 4− units, fostering the formation of non-bridging oxygen structures. This leads to a decrease in highly polymerised Q 4 and Q 5 units, causing network depolymerisation and a concurrent rise in free fluorine, which effectively reduces melt viscosity. Conversely, increasing the MgO/CaF 2 ratio significantly depletes F − ions. This forces [AlO 3 F] 4− structures to either incorporate O 2− to form [AlO 4 ] 5− tetrahedra or to create three-coordinated oxygen by interacting with Al−O−Al linkages. The resulting enhancement of network polymerisation and the demand for cation charge compensation synergistically elevate the melt's viscosity.