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Calcium ferrite containing aluminum (CFA) is a precursor of the low-temperature bonding phase in the sintering process of iron ore fines for blast furnace ironmaking. Thus, improving the formation of CFA at lower temperature is very important for saving energy, improving efficiency and production. In this paper, the formation process of CFA was investigated at 1200 °C by reactions of alumina (Al2O3), respectively with a mixture of calcium oxide (CaO) and hematite (Fe2O3) and monocalcium ferrite (CF) as a recognized initial product, as well as reaction of Al-containing hematite (Hss) with CF. The result confirmed that CF is an intermediate product formed easily in the sintering process, and it may react with excessive Fe2O3 to generate an alpha-calcium iron oxide (Ca2Fe15.50O25) as a new phase. It was found that CFA can be formed directly by reactions of CF with Hss and Ca2Fe15.50O25 with Al2O3, while the reaction of CF with Al2O3 is more helpful in generating Ca2Fe15.5O25 rather than CFA, simultaneously forming a calcium aluminum oxide (CaAl2O4, CA; CaAl4O7, CA2). It was revealed that the appearance of CA and CA2 is a main reason to hinder CFA formation in the sintering process of iron ore fines.

Al2O3 is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al2O3 effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al2O3 content increased. The crystallization sequence of samples containing Al2O3 was observed as Ca4Fe14O25 (C4F14) → Fe2O3 → Ca3.18Fe15.48Al1.34O36 (SFCA-I) → CaFe2O4 (CF) → Ca5Si2(Fe, Al)18O36 (SFCA) → γ-Ca2SiO4 (C2S). The generation pathway of SFCA-I was found to be C4F14 + Si4+ + Al3+ → SFCA-I. Increasing the cooling rate can promote the formation of C4F14, SFCA-I, Fe2O3 and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C2S to γ-C2S. When the Al2O3 content reached or exceeded 2.5 mass pct, the viscosity of Fe2O3-CaO-SiO2-Al2O3 melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.

The glass phase is one of the binding phases in high-basicity sinter, which is mainly formed during a high-temperature cooling process while cannot crystallize in time. The phase still involves the “structure” information of the binding phase’s liquid phase in the sinter. In addition, the generation of glassy phases can seriously deteriorate the metallurgical properties of sintered ore. However, the formation mechanism and crystallization process of glass phases are still unclear. In this work, the glass phase and the crystallized samples of the CaO-SiO2-Al2O3 system were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy and Raman spectroscopy. The effect of alkalinity (R) and Al2O3 on crystallization and the relationship between crystallization and structure are discussed. The results showed that the chemical composition significantly influences the crystallization of the CaO-SiO2-Al2O3 glass. Decreasing basicity (R = 0.8–1.2, the mass ratio of CaO and SiO2) favors the crystallization of the glass phase, while increasing the content of Al2O3 (9–12%) can inhibit the crystallization of the glass phase. In addition, the crystallization order of the 45mass%CaO-45mass%SiO2-10mass%Al2O3 sample is CaSiO3 → CaAl2O4. Raman spectroscopic analysis showed that increase of slag basicity promoted the aggregation degree (Q3/Q2), resulting in deterioration of the glass phase crystallization. and that the glass phase crystallization deteriorated as the aggregation degree increased. However, increasing the Al2O3 content has little effect on the agglomeration degree but does promote the formation of SiO4 tetrahedra (Q0), which results in the deterioration of glass-phase crystallization.

Secondary hematite (SH) is a serious factor resulting in reduction degradation of iron ore sinter in a blast furnace; however, until now, a quantitative study for SH formation had not been reported. In this work, the effects of gangue composition, including MgO, Al2O3 and SiO2, on the solid-state formation in the sintering process of iron ore fines were investigated quantitatively. It shows that the SH formation decreased from 67.84% to 46.11%, 35.44% and 22.37% after adding 1.0%, 3.0% and 5.0% MgO, respectively, while for Al2O3, the amount increased to 69.38%, 69.98% and 70.56%, respectively. For SiO2, the amount changed to 68.14%, 61.59% and 47.96%, respectively. Simultaneously, the magnetite (magnesioferrite) formation increased from 8.24% to 34.79%, 50.26% and 70.45% after adding 1.0%, 3.0% and 5.0% MgO, respectively. For Al2O3 and SiO2, the amount changed to 8.95%, 8.37%, 7.62% and 7.62%, 11.10%, 18.77%, respectively, compared with no gangue. This indicates that the SH formation increased with decrease in magnesioferrite. It was found that the decrease in SH formation relates to the diffusion of Mg2+ in magnesioferrite, which inhibits the solid-state formation of SH kinetically. A supposition was suggested that a maghemite existed at a high temperature, and decreased with an increase in MgO addition. This would be another reason to improve the degradation performance of iron ore sinter.