Key Takeaways & Executive Findings
- •• Equimolar substitution of MgO for CaO in high-alumina slag leads to a viscosity minimum at 15 mol% MgO, offering a strategy to optimize slag fluidity. • Spectroscopic analyses (FTIR, Raman, XPS) reveal progressive polymerization of the silicate network with increasing MgO substitution, evidenced by increased bridging oxygen and decreased non-bridging oxygen. • The viscosity minimum arises from a complex interplay between structural polymerization and bond strength evolution of non-bridging oxygens, challenging the simple depolymerization model. • The findings provide theoretical and data support for utilizing high-alumina ores in blast furnace operations, potentially expanding the allowable Al2O3 content range.
Abstract
Currently, the Al2O3 content in the high-alumina slag systems within blast furnaces is generally limited to 16wt%–18.5wt%, making it challenging to overcome this limitation. Unlike most studies that concentrated on managing the MgO/Al2O3 ratio or basicity, this paper explored the effect of equimolar substitution of MgO for CaO on the viscosity and structure of a high-alumina CaO–MgO–Al2O3–SiO2 slag system, providing theoretical guidance and data to facilitate the application of high-alumina ores. The results revealed that the viscosity first decreased and then increased with higher MgO substitution, reaching a minimum at 15mol% MgO concentration. Fourier transform infrared spectroscopy (FTIR) results found that the depths of the troughs representing [SiO4] tetrahedra, [AlO4] tetrahedra, and Si–O–Al bending became progressively deeper with increased MgO substitution. Deconvolution of the Raman spectra showed that the average number of bridging oxygens per Si atom and the (Q_i) ratio increased from 2.30 and 1.02 to 2.52 and 2.14, respectively, indicating a progressive polymerization of the silicate structure. X-ray photoelectron spectroscopy (XPS) results highlighted that non-bridging oxygen content decreased from 77.97mol% to 63.41mol% with increasing MgO concentration, whereas bridging oxygen and free oxygen contents increased. Structural analysis demonstrated a gradual increase in the polymerization degree of the tetrahedral structure with the increase in MgO substitution. However, bond strength is another important factor affecting the slag viscosity. The occurrence of a viscosity minimum can be attributed to the complex evolution of bond strengths of non-bridging oxygens generated during depolymerization of the [SiO4] and [AlO4] tetrahedral structures by CaO and MgO.
1. Introduction
Viscosity is one of the most important physicochemical properties of aluminosilicate melts, controlling the transport phenomena in high-temperature processes [1]. For instance, viscosity has a direct impact on several indicators during the process of blast furnace ironmaking, including desulfurization, slag–iron separation, heat transfer, and FeO reduction [2–4]. In the preparation of glass and glass ceramics, viscosity is related to the molding ability of the glass material as well as the crystallization ability and densification of the glass ceramics [5–6]. Therefore, an accurate knowledge of the values of viscosity and variations in viscous behavior is essential for the optimization and improvement of metallurgical processes as well as for the preparation of glass and glass ceramics materials. Among them, understanding the effect of mutual substitution of alkaline earth cations on the viscosity of slag is currently the focus of attention in the field of aluminosilicate melts.
The CaO–MgO–Al2O3–SiO2 system is an important basic system for metallurgical slags [7], industrial glass materials [8], and glass ceramics [9]. Previous studies on this slag system have focused on the effects of basicity and MgO/Al2O3 ratio on the viscosity and structure [10–11]. Qiu et al. [11] found that increasing either the MgO/Al2O3 ratio or the CaO/SiO2 ratio decreased the viscosity of CaO–SiO2–17.5wt%Al2O3–MgO–2wt%TiO2 slag. In addition, the changes in the viscosity and structure of slag triggered by the substitution of CaO by MgO have received much attention from scholars [12–19]. Kim et al. [1] investigated the effect of MgO/CaO molar ratio on the viscosity of CaO–MgO–15mol%Al2O3–55mol%SiO2 slag with the (CaO+MgO)/Al2O3 molar ratio of 2, and found that the viscosity decreased monotonically as CaO was gradually replaced by MgO. Li and Ning [12] measured the evolution behavior of the viscosity of CaO–MgO–SiO2–18wt%Al2O3 slag with a fixed CaO/SiO2 mass ratio of 1 or 1.2 as a function of MgO content, and the results revealed that the viscosity decreased with increasing MgO content. The same pattern of viscosity variation was demonstrated in the studies of Saito et al. [13], Machin et al. [14], and Kim et al. [15]. However, Johannsen and Brunion [16], Veit and Rüssel [17] obtained the opposite results after measuring the viscosity of CaO–MgO–Al2O3–SiO2 slag with fixed Al2O3 and SiO2 contents, noting that the viscosity increased monotonically with the substitution of CaO by MgO. In addition to the monotonic variation, L
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Yong Hou, Shuo Zhang, Jie Dang, Jia Guo, Hanghang Zhou, Xuewei Lü (2025). Viscosity and structure relationship with equimolar substitution of CaO with MgO in the CaO–MgO–Al2O3–SiO2 slag melts. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-2913-9
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Frequently Asked Questions
What is the effect of equimolar substitution of MgO for CaO on slag viscosity?
The viscosity first decreases and then increases with higher MgO substitution, reaching a minimum at 15 mol% MgO concentration.
How does MgO substitution affect the slag structure?
Spectroscopic analyses (FTIR, Raman, XPS) show progressive polymerization of the silicate network, with increased bridging oxygen and decreased non-bridging oxygen content.
Why does the viscosity minimum occur?
The viscosity minimum is attributed to the complex evolution of bond strengths of non-bridging oxygens generated during depolymerization of [SiO4] and [AlO4] tetrahedra by CaO and MgO.
What is the significance of this study for blast furnace operations?
The findings provide theoretical guidance and data to facilitate the application of high-alumina ores, potentially overcoming the limitation of Al2O3 content in blast furnace slag systems.
What techniques were used to analyze the slag structure?
Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used.
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