Effect of Basicity on the Structure, Viscosity and Crystallization of CaO-SiO2-B2O3 Based Mold Fluxes

Abstract

In this study, the structure, viscosity characteristics, and crystallization behavior of CaO-SiO2-B2O3 based melts were studied combining molecular dynamics (MD) simulation, Fourier transform infrared (FTIR) spectroscopy, rotating viscometer test, and FactSage thermodynamic calculation. The results showed that, in the ternary CaO-SiO2-B2O3 glass system, stable structural units of [SiO4]4− tetrahedral, [BO3]3− trihedral and [BO4]5− tetrahedral were formed, and the Si-O and B-O structure depolymerize with the basicity increase from 1.15 to 1.25, then the B-O structure become complex with the basicity further increase to 1.35. In fluorine-free mold fluxes, with the basicity increases, the viscosity at 1300 °C increases, the liquidus temperature decreases and then increases, the network structure polymerizes, it indicates that the structural complexity rather than the melting property change plays a predominant role in increasing the viscosity at 1300 °C. Moreover, due to the changes in crystallization phase and solid solution ratio, the viscosity-temperature curve of fluorine-free slag shows the characteristics of alkaline slag and the break temperature increase with the basicity increase. The MD simulation, FTIR experiment, viscosity test, and FactSage calculation results are verified and complemented each other.

Effect of B2O3 on the Melt Structure and Viscosity of CaO–SiO2 System

“…The [SiO 4 ] 4− tetrahedron structural units can be represented by Q n (Si), where the superscript n represents the number of BO, and Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 correspond to island, diners, ring or chain, slice or layer, 3D frame or mesh tetrahedron, respectively. [ 38 ] In the CaO–SiO 2 –B 2 O 3 ternary system, the CN of B–O hardly change with basicity, indicating that the proportion of [BO 3 ] 3− and [BO 4 ] 5− in the system is relatively stable, the magnitude of atomic ratio of [BO 4 ] 5− and [SiO 4 ] 4− is ≈1:100, Si 4+ ions are more predominant than B 3+ ions in forming network structures, and the main network structures are the Q n (Si) units, correspondingly. Therefore, we focused on the effect of basicity and the addition of B 2 O 3 on the change of Q n (Si) in CaO–SiO 2 melts.…”