Key Takeaways & Executive Findings
- •• Increased cooling rates promote CaFe2O4 (CF) precipitation but inhibit Ca2Fe2O5 (C2F) formation in both Al2O3- and MgO-doped calcium ferrite systems. • Al2O3 addition lowers the initial crystallization temperature of calcium ferrite, whereas MgO addition raises it due to enhanced MF and C2F precipitation. • Kinetic analysis using Ozawa and Malek methods reveals distinct crystallization mechanisms: logarithmic law initially, then reaction-order or exponential law for Al2O3-doped systems, and (1−α)2 for MgO-doped systems. • These findings provide critical thermodynamic and kinetic insights for optimizing sinter ore composition and improving blast furnace slag performance.
Abstract
Al2O3 and MgO serve as the primary gangue components in sintered ores, and they are critical for the formation of CaO–Fe2O3–xAl2O3 (wt%, C–F–xA) and CaO–Fe2O3–xMgO (wt%, C–F–xM) systems, respectively. In this study, a nonisothermal crystallization thermodynamics behavior of C–F–xA and C–F–xM systems was examined using differential scanning calorimetry, and a phase identification and microstructure analysis for C–F–xA and C–F–xM systems were carried out by X-ray diffraction and scanning electron microscopy. Results showed that in C–F–2A and C–F–2M systems, the increased cooling rates promoted the precipitation of CaFe2O4 (CF) but inhibited the formation of Ca2Fe2O5 (C2F). In addition, C–F–2A system exhibited a lower theoretical initial crystallization temperature (1566 K) compared to the C–F system (1578 K). This temperature further decreases to 1554 K and 1528 K in the C–F–4A and C–F–8A systems, respectively. However, in C–F–xM system, the increased MgO content raised the crystallization temperature. This is because that the enhanced precipitation of MF (a spinel phase mainly comprised Fe3O4 and MgFe2O4) and C2F phases suppressed the CF precipitation reaction. In kinetic calculations, the Ozawa method revealed the apparent activation energies of the C–F–2A and C–F–2M systems. Malek’s method revealed that the crystallization process in C–F–2A system initially followed a logarithmic law ( or ), later transitioning to a reaction order law ((1−α)−1 or (1−α)−1/2, n = 2/3) or the function of the exponential law. In C–F–2M system, it consistently followed the sequence ƒ(α) = (1−α)2 (α is the crystallization conversion rate; n is the Avrami constant; ƒ(α) is the differential equations for the model function of C2F and CF crystallization processes).
1. Introduction
In modern metallurgical processes, the composition of blast furnace slag plays a decisive role in determining pig iron quality and production efficiency [1]. The formation pathways and performance of blast furnace slag are closely linked to the physicochemical properties of sinter ore, which include the mineral composition, microstructural features, and phase transformation mechanisms. Specifically, calcium ferrites (CFs), serving as critical binding phases in sinter ore, enhance both the mechanical strength and metallurgical performance of sinter. This enhancement in turn affects the fluidity, desulfurization thermodynamic equilibrium, and stability of the blast furnace slag.
Al2O3 and MgO, as key gangue components in sinter ores, participate in composite reactions to form CF solid solutions or compounds, contributing to the complex SiO2–Fe2O3–CaO–Al2O3–MgO multiphase system. During the sintering process, the formation of CFs is governed by high-temperature physicochemical reactions involving liquid-phase generation and crystallization dynamics [2]. The addition of Al2O3 and MgO modulates liquid-phase behavior, directly influencing the thermodynamic stability of CFs and their microstructural evolution [3].
Early research on the CaO–Fe2O3 system primarily focused on the crystallization behaviors of the CaFe2O4 (CF) and Ca2Fe2O5 (C2F) phases. Sosman and Merwin [4] was the first to systematically report the formation mechanism of binary calcium ferrites, but did not observe the presence of the CaO·2Fe2O3 (CF2) phase. Subsequent experiments [5] verified the formation of the CF2 phase, filling the gap in this field. Hughes’ research [6] indicated that the formation of the 4CaO·7Fe2O3 (C4F7) phase required the presence of FeO, highlighting the crucial influence of the component ratio on phase evolution. To gain a deeper understanding of the formation of the CF phase, the research gradually shifted to more complex multi-component systems. Scholars such as Edström [7], Schürmann and Wurm [8], and Evrard et al. [9] successively identified intermediate phases such as CaO·FeO·Fe2O3 (CWF) and CaO·3FeO·Fe2O3 (CW3F). After that, the multi-component calcium ferrite
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Rende Chang, Chengyi Ding, Hongming Long, Xuewei Lü, Tiejun Chun, Xiaoqing Xu, Zhiming Yan, Xuchao Wang, Sheng Xue, Wei Lü (2025). Thermodynamics and kinetics of alumina and magnesium oxide in calcium ferrite sintering process. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3070-x
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Frequently Asked Questions
What is the role of Al2O3 and MgO in calcium ferrite sintering?
Al2O3 and MgO are key gangue components that influence the formation of calcium ferrite phases, affecting the thermodynamic stability and crystallization behavior during sintering.
How does cooling rate affect the crystallization of calcium ferrite systems?
Increased cooling rates promote the precipitation of CaFe2O4 (CF) but inhibit the formation of Ca2Fe2O5 (C2F) in both Al2O3- and MgO-doped systems.
What is the effect of Al2O3 content on crystallization temperature?
Increasing Al2O3 content lowers the initial crystallization temperature of calcium ferrite, from 1578 K in the undoped system to 1528 K in the C–F–8A system.
What kinetic models describe the crystallization process?
For Al2O3-doped systems, crystallization initially follows a logarithmic law, then transitions to a reaction-order law or exponential law. For MgO-doped systems, it consistently follows the model ƒ(α) = (1−α)2.
Why does MgO addition raise the crystallization temperature?
MgO addition enhances the precipitation of MF (spinel phase) and C2F phases, which suppresses the CF precipitation reaction, thereby raising the crystallization temperature.
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