Effects of the composition on the structure and viscosity of the CaO–SiO2-based mold flux

• The formation of the molten oxide slags involves the equilibrium and redistribution processes of O2 − ions.
• The network breakers have different O2 − donor abilities and have different effect on the properties of molten slag.
• The O2 − offered by MeO/Me2O is preferentially used to break the Si–O–Si bands.
• The coordination number of Al is strongly affected by the SiO2 content.

In this study, to investigate the effects of composition on the structure and viscosity of CaO–SiO2-based mold flux, the structure of molten slag was investigated by Raman spectroscopy, and the effect of the components on the viscosity was analyzed. The results show that the formation of the molten slag with multicomponent oxides involves the equilibrium and redistribution processes of O2 − ions. The network breakers offered O2 − ions and the network formers accepted the O2 − ions. Al3 + ions can act as both the network formers (O2 − acceptor) and the network breakers (O2 − donor). The Al3 + ions were absorbed into the silicate structure, mainly acting as the network formers in the mold flux with a high CaO/SiO2 ratio, but mainly as the network breakers in the mold flux with a low CaO/SiO2 ratio. Because the network breakers (Li2O, Na2O, CaO, and MgO) have different O2 − donor powers, the effect of equimolar substitutions between the network breakers was different in two aspects: the degree of polymerization of the molten slags and the viscosity properties. CaF2 also can dissolve into Ca2 + and F−, and F− can depolymerize the molten slag and decrease the viscosity and therefore is also used as a network breaker. The equimolar substitutions between the network breakers can be used to adjust the structural and the viscosity properties of the molten slags. Hence, the proportional control of the network breakers is very important to adjust the structure and viscosity properties during the design of mold flux.