Physical properties and structure of boron-containing slags during reduction period of AOD process

Abstract

The effect of basicity and content of boron oxide on viscosity, crystallization temperature, phase composition, and structure of the СаО – SiO2 – B2O3 – 12 % Cr2O3 – 3 % Аl2O3 – 8 % МgO fluorine-free slag system in the range of boron oxide content 3 – 6 % and basicity 1.0 – 2.5 is studied by vibrational viscometry, thermodynamic phase composition modeling (HSC Chemistry 6.12 (Outokumpu)), and Raman spectroscopy. It was found that physical properties of the studied slags mainly depend on the balance between the degree of structure polymerization, nature of the bond with it, and phase composition. With a low basicity of 1.0, slags are “long” and an increase in the content of boron oxide from 3 to 6 % makes them more fusible, reducing the crystallization temperature of the slag from 1340 to 1224 °C, and its viscosity from 1.0 – 0.8 to ~0.25 Pa·s at 1600 – 1660 °C, despite the significant complication of the structure, reflected in the growth of the bridging oxygen index BO from 1.10 to 1.49. With an increase in basicity, slags transfer from “long” to “short” and the content of calcium oxide increases, which, being a donor of free oxygen ions (O2–), acts as a modifier of the slag structure. Thus, with a basicity of B = (CaO/SiO2) = 2.5, slags have a simpler structure (BO = 0.50 – 0.53) relative to slags with a basicity of 1.0, while the addition of boron oxide complicates it only slightly (an increase in BO from 0.5 up to 0.53). Increasing the concentration of B2O3 lowers the crystallization temperature from 1674 to 1605 °C and the viscosity from 1.0 to 0.3 Pa·s at 1660 °C as a result of the formation of low-melting compounds (mostly 2CaO·B2O3).