Relationship between structure and thermodynamic properties in the CaO-SiO2-BO1.5 slag system

Abstract

To understand the relationship between thermodynamic properties and structures in the CaO-SiO2- BO1.5 slag system, the thermodynamic properties of BO1.5 and SiO2 were measured by a chemical equilibrium technique and the local structures of boron(B) and silicon(Si) were investigated by 11B and 29Si magic angle spinning-nuclear magnetic resonance (MAS-NMR) measurements. Activity coefficients of BO 1.5 increased with an increase in the BO1.5 content of the slag system. Activity coefficients of SiO2 increased with increasing BO1.5 content and decreased with increasing CaO/SiO2 ratio. By using 11B MAS-NMR, the existence of two types of B sites was confirmed-three- and fourcoordinated B sites. The relative fraction of four-coordinated B increased with an increase in the BO1.5 content and decreased with an increase in the CaO/SiO2 ratio. 29Si MAS-NMR results revealed that bridging oxygen bonding to Si atoms increased with an increase in the BO1.5 content and decreased with decreasing CaO/SiO2 ratio. The number of non-bridging oxygen atoms bonded to the tetrahedrally coordinated Si atom (NBO/T) was calculated using 29Si and 11B MAS-NMR under specific assumptions. By comparing NBO/Ts calculated from these two methods, the number of non-bridging oxygen atoms bonded to three-coordinated B in the studied composition region was estimated to be one. Finally, by comparing the thermodynamic properties with the results of MAS-NMR measurements, it was found that the composition dependences on BO1.5 and SiO 2 activity coefficients were dependent on changes in the local structure of B and Si with composition. © 2013 ISIJ.