Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO2 (-MgO)-Al2O3 slags

Abstract

The viscosity of CaO-SiO 2 (-MgO)-Al 2 O 3 slags was measured to clarify the effects of Al 2 O 3 and MgO on the structure and viscous flow of molten slags at high temperatures. Furthermore, the infrared spectra of the quenched slags were analyzed to understand the structural role of Al 2 O 3 in the polymerization or depolymerization of silicate network. The Al 2 O 3 behaves as an amphoteric oxide with the composition of slags; that is, the alumina behaves as a network former up to about 10 mass pct Al 2 O 3 , while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al 2 O 3 . This amphoteric role of Al 2 O 3 in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al 2 O 3 on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO 4 ]-tetrahedra into the [SiO 4 ]-tetrahedral units, and this was confirmed by the infrared (IR) spectra of the quenched slags. The influence of alumina on the viscosity decrease can be explained on the basis of a decrease in the DOP by the increase in the relative fraction of the [AlO 6 ]-octahedral units. The relative intensity of the IR bands for the [SiO 4 ]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO 4 ]-tetrahedra with high NBO/Si increased with increasing Al 2 O 3 content greater than the critical point, i.e., about 10 mass pct in the present systems. The variations of the activity coefficient of slag components with composition indirectly supported those of viscosity and structure of the aluminosilicate melts.

The Influence of Al₂O₃/SiO₂ Ratio on the Viscosity of Mold Fluxes

“…Finally, the variation range of alumina content is much higher than that reported in earlier papers, and the total amount of alumina and silica is maintained constant in the present experiment, i.e., the amount of alumina increases corresponding to the decrease of the amount of silica, which also differs with reported by earlier papers. [35][36][37] For these reasons, it is difficult to compare the current results to the earlier studies but it appears that for the mold-slag compositions investigated here, Al 2 O 3 does not exhibit amphoteric behavior and instead continuously increases the viscosity.Based on the experimental data, the activation energies for viscous flow of mold fluxes bearing high alumina content were evaluated according to Weymann-Frenkel's equation, The results are presented in Fig. 4 and it can be seen, that the activation energy increases with increasing of X Al 2 O 3 /X SiO 2 , and the degree of activation energy increase was found to decrease at the ratios above 0.5.…”

“…[35][36][37] The variation of viscosity in quaternary slag systems CaO-SiO 2 -10wt%MgO-Al 2 O 3 (CaO/SiO 2 ϭ1.0 or 1.3) showed the similar tendency with that in the ternary slag systems. 35,36) The CaO/SiO 2 ratios (basicity) in the slags in- Table 1. vestigated in this study are significantly lower than those in the aforementioned studies. Thus, the degree and type of polymerization of Si is expected to be different and furthermore the slag structure is affected by other constituents, such as CaF 2 .…”

“…It should be pointed out that the results obtained in this study are not in agreement with the common idea that an increase of alumina content results in the increase of viscosity, following by a decrease with further increasing of alumina content. [35][36][37] The viscosity in ternary slag systems CaO-SiO 2 -Al 2 O 3 (CaO/SiO 2 ϭ1.0 or 1.3) were found to increase with increasing of alumina and to reach a maximum value at the amount of 10 wt%, followed by a decrease with further increase of alumina in the range of 10 to 15 wt%. [35][36][37] The variation of viscosity in quaternary slag systems CaO-SiO 2 -10wt%MgO-Al 2 O 3 (CaO/SiO 2 ϭ1.0 or 1.3) showed the similar tendency with that in the ternary slag systems.…”

“…[35][36][37] The viscosity in ternary slag systems CaO-SiO 2 -Al 2 O 3 (CaO/SiO 2 ϭ1.0 or 1.3) were found to increase with increasing of alumina and to reach a maximum value at the amount of 10 wt%, followed by a decrease with further increase of alumina in the range of 10 to 15 wt%. [35][36][37] The variation of viscosity in quaternary slag systems CaO-SiO 2 -10wt%MgO-Al 2 O 3 (CaO/SiO 2 ϭ1.0 or 1.3) showed the similar tendency with that in the ternary slag systems.35,36) The CaO/SiO 2 ratios (basicity) in the slags in- Table 1. vestigated in this study are significantly lower than those in the aforementioned studies. Thus, the degree and type of polymerization of Si is expected to be different and furthermore the slag structure is affected by other constituents, such as CaF 2 .…”

Thermal Conductivity of Molten Slags: A Review of Measurement Techniques and Discussion Based on Microstructural Analysis

“…For example, recently reported analyses of the microstructure of various slag systems using Fourier Transform-Infrared Spectroscopy (FT-IR), Raman Spectroscopy, Nuclear Magnetic Resonance (NMR) might provide further insight into the relationship between structure and thermal conductivity. [8][9][10] In this study, typical measurement methods for the thermal conductivity of molten slags, such as the hot-wire and laser flash methods, are briefly presented and evaluated. In addition, the influence of molten slag microstructure on the © 2014 ISIJ composition and temperature dependence of thermal conductivity is discussed with regards to selected microstructural analysis results.…”