Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO 2 (-MgO)-Al 2 O 3 slags
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10/3/04
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Amphoteric Behavior of Alumina in Viscous Flow and Structure of CaO-SiO2 (-MgO)-Al2O3 Slags JOO HYUN PARK, DONG JOON MIN, and HYO SEOK SONG The viscosity of CaO-SiO2 (-MgO)-Al2O3 slags was measured to clarify the effects of Al2O3 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 Al2O3 in the polymerization or depolymerization of silicate network. The Al2O3 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 Al2O3, while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al2O3. This amphoteric role of Al2O3 in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al2O3 on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO4]-tetrahedra into the [SiO4]-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 [AlO6]-octahedral units. The relative intensity of the IR bands for the [SiO4]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO4]-tetrahedra with high NBO/Si increased with increasing Al2O3 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.
I. INTRODUCTION
THE amphoteric behavior of alumina in aluminosilicate melts has been an issue for several decades in the fields of metallurgy as well as glass and geo-sciences. Thus, many researchers have focused on the structural role of alumina in aluminosilicate systems by measuring the physical properties such as viscosity and thermal or electrical conductivities,[1–5] measuring the spectroscopic properties such as infrared (IR) and Raman spectra,[6–9] and by employing thermodynamic calculations.[10–13] In alkaline (or alkaline earth) silicate melts containing alumina, Al-O coordination would be tetrahedral, i.e., [AlO4] just like [SiO4]-tetrahedral units, when the molar ratio Al2O3/MO (MO is basic oxide) is less than unity.[1,14–17] When this ratio exceeds 1.0, Al3 ions would have octahedral coordination with oxygen and enter interstices in the structure. These structural changes in aluminosilicate melts could affect their physical properties. The viscosity of the MO-SiO2-Al2O3 slags increases as MO is replaced by Al2O3, for a given SiO2 content, until the molar ratio Al2O3/MO is about unity as a result of a decrease in the concentration of nonbridging oxygen in the melt, and vice versa above this ratio. Turkdogan and Bills indicated that th
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