Effect of Al2O3 and MgO on crystallization and structure of CaO–SiO2–B2O3-based fluorine-free mold flux

Abstract

The crystallization behavior and melt structure of the CaO–SiO2–B2O3-based fluorine-free mold flux were investigated. The results show that the crystallization of the mold flux was first inhibited and then promoted with the increase in Al2O3 content from 4 to 12 wt.%. However, it was enhanced by MgO in the range of 2–10 wt.%. The results of Fourier transform infrared spectroscopy and Raman spectroscopy showed that Al2O3 worked as a network former in the mold flux melt when its content was in the range of 4–8 wt.%, whereas it worked as the network breaker to provide O2− when its content was in the range of 8–12 wt.%. In addition, the combined effects from the charge compensation by Mg2+ and the network broken by O2− led to the increase in some typical T–O–T (Al–O–Al, B–O–B, etc.) and simpler structural units (Q0(Si), B–O− in the [BO2O−], etc.) when the MgO content was in the range of 2–6 wt.%. The continuous increase in O2− provided by the addition of MgO from 6 to 10 wt.% further depolymerized the network of the melt and finally caused fast crystallization.