The chemical analysis and structural evaluation for the multi-component calcium ferrites were carried out, focusing on the microstructural classifications 1H-ACF, 2H-CF and M-FCF used in the 'Development of environmentally friendly steelmaking process technology (COURSE 50)'. The synthesis was carried out with reference to the multi-component calcium ferrite compositions observed in 1H-ACF, 2H-CF and M-FCF. The crystal structure of 1H-ACF and 2H-CF is strongly influenced by the final temperature reached during sintering. And the crystalline phases observed in 1H-ACF and 2H-CF are SFCA and SFCA-I, with a small amount of SFCA-III. On the other hand, the structure of the SiO₂-rich M-FCF was SFCA. The crystal structure of 2H-CF in “sinter 5” was evaluated by single-crystal X-ray diffraction analysis. The multicomponent calcium ferrites in the 2H-CF region of the actual sintered ores reflect the fact that SFCA, SFCA-I and SFCA-III are of the M_14+6nO_20+8n (M = Si, Fe, Al, Ca, Mg) polysomatic series, and are therefore either SFCA with structural defects or SFCA-I. In the 2H-CF region, SFCA-III, rich in Fe₂O₃ and poor in MgO, was also found and the model structural composition was Ca_6.66Fe_39.43Al_3.28Mg_2.63O₇₂, with the MgO component playing an important role in the formation of spinel sheets. SFCA-III, described in detail for the first time in this report, is richer in Fe₂O₃ than those of SFCA and SFCA-I, suggesting an interesting active role as a new component operating in a more reducing environment with Fe²⁺ present.

Multi component calcium-ferrites (CFs) including in iron ore sinter are key component of sinter properties, such as strength and reducibility. Those in a sinter are crystallized from oxide melts and form various morphologies such as columnar, needle, and fine textures depending on the formation process. Ca₂(Fe,Ca)₆(Fe,Al,Si)₆O₂₀ (SFCA), Ca₃(Ca,Fe)(Fe,Al)₁₆O₂₈ (SFCA-I) and SFCA-III phases are representative multicomponent CFs, which are included in a sinter. Those texture and types of the crystal structure are not matched one by one. Since their compositional ranges and chemical properties have not yet been fully clarified, and they have micron ordered grain size, it is difficult to determine CF phases only by EPMA chemical composition analysis. In this study, phase determination of CFs in sinter was conducted by Electron backscatter diffraction (EBSD) analysis, which has better spatial resolution than Electron Probe Micro Analyzer (EPMA). Needle like CFs in the sinter with size of several microns were analyzed and confirmed presence of SFCA, SFCA-I and SFCA-III with the needle like texture. Crystal grains determined as SFCA showed higher Si content compared to those of SFCA-I, and this result is consistent with phase diagram. Mg concentration of SFCA-III found in sinter was in the range of 0.6–2.6mass%, which was lower than that of previously reported single crystal structure analysis sample.

To reduce the reducing agent ratio and CO₂ emissions in blast furnace operation, it is important to control the material structure of sintered ore, which affects its metallurgical and mechanical properties. Multicomponent calcium ferrites (also called CF or SFCA (silico-ferrite of calcium and aluminum)), which is a type of melting and solidification structure, has attracted considerable interest recently, and the chemical composition and crystal structure of each CF have been researched. Although the crystal structure of CF has conventionally been analyzed mainly by XRD, the atomic arrangement could not be observed directly. Therefore, in this study, CF was investigated at the atomic level by scanning transmission electron microscopy (STEM). This research revealed that acicular CF, which was previously understood to be SFCA-I, has a SFCA (≠ SFCA-I) structure. It was also found that columnar CF had a non-periodic SFCA structure induced with a magnetite-like structure. Furthermore, a CF in which SFCA and SFCA-I were alternately stacked repeatedly was also discovered. This research clarified the fact that CF has a non-periodic structure at the atomic level.

This study investigates the phase stability and decomposition behavior of SFCA (Silico-Ferrite of Calcium and Aluminum) at 1573 K under varying oxygen partial pressures. SFCA, a key mineral phase in sintered ore, plays a critical role in enhancing the mechanical strength and reducibility of sinter. Understanding its stability under different conditions is essential for optimizing sinter production processes.

Equilibrium and decomposition experiments were conducted under oxygen partial pressures ranging from atmospheric levels to 10⁻⁶ atm. The results revealed that SFCA remains stable up to 5×10⁻³ atm, while it decomposes at lower oxygen partial pressures. The decomposition products include magnetite, melilite, and other SFCA groups, indicating that SFCA undergoes a stepwise transformation rather than direct breakdown. Additionally, the liquid phase in equilibrium with SFCA shifts toward the CA₆ (CaAl₆O₁₀) region of the CCC plane as oxygen partial pressure decreases, resulting in an expansion of the liquidus-only region.

The findings highlight the narrow stability range of SFCA, with an estimated oxygen partial pressure limit around 10⁻³ atm. These results provide valuable insights into the formation and transformation of SFCA under sintering conditions, where local oxygen partial pressures fluctuate due to coke combustion.

The effect of Al₂O₃ on the compositional region of silico-ferrite of calcium and aluminum (SFCA) and the liquid phase and the phase equilibria, including SFCA, was investigated in a CaO–SiO₂–Fe₂O₃–5 mass%Al₂O₃ system at 1240°C in air. To obtain the desired composition, reagent-grade CaCO₃, SiO₂, Fe₂O₃, and Al₂O₃ powders were weighed, mixed, and equilibrated at 1240°C in air. Each obtained sample was divided into two parts: one was pulverized into a powder and analyzed by XRD, and the other was subjected to microstructural observation and compositional analysis using EPMA. The results revealed that the compositional region of SFCA lies on the CF₃–CA₃–C₄S₃ plane and is C/S = 2.77–7.60 for 5 mass% Al₂O₃. Compared with the SFC composition region for 0 mass% Al₂O₃, the compositional range of SFCA extended in the CF₃–C₄S₃ direction, suggesting that the addition of Al₂O₃ contributes to the stability of SFCA. Furthermore, the liquid-phase region was divided into a ferrite melt with a high Fe₂O₃ concentration and a silicate melt with a high SiO₂ concentration, both of which shifted to the lower Fe₂O₃ side compared to the liquidus isotherm in the CaO–SiO₂–Fe₂O₃ system. Unlike CaO–SiO₂–Fe₂O₃, SFCA-I (SFC-I) was observed in the CaO–SiO₂–Fe₂O₃–5 mass%Al₂O₃ system, thus indicating that the addition of Al₂O₃ contributes to the stability of SFCA-I.

The reducibility and mechanical properties of iron ore sinter in blast furnace is critical to effective plant operation. The reduction reaction of sinters progresses heterogeneously owing to microstructures with various mineral phases and pore networks. The reduction process was investigated by semi-microbeam synchrotron X-ray multimodal analysis. Heterogeneous chemical state evolution of Fe and trigger sites of crack formation were visualized using two-dimensional Fe K-edge X-ray absorption near-edge structure analysis and were discussed based on reduction gas transfer. The elemental composition map and X-ray diffraction microanalysis were also combined to reveal the microprocesses during the reduction, such as calcium ferrite decomposition and crystal grain growth.

The declining quality of iron ore fines and increasing re-use requirement of steelmaking slag have led to higher Al₂O₃ content in sinter products. High grade concentrated hematite ore (concentrate) has been emerged as a potential solution for reducing Al₂O₃ levels of sinter. However, research regarding on the melting and assimilation behavior of concentrates in sintering process is limited. On the other hand, our previous work demonstrated that Parallel Granulation with Inclined Mixing of Limestone, based on the creation and mixing of pseudo-particles with high and low CaO/Fe₂O₃ ratios, can improve the sinter strength and productivity. To investigate the melting and assimilation characteristics of concentrates, we conducted melt dripping tests various kind of iron ore, revealing that adhesive layers with high concentrate content have low melt retention capacity. Furthermore, sinter pot tests under single granulation condition showed that when a large amount of concentrate is blended, fragile sinter cake is formed due to insufficient melting and assimilation. Sinter pot tests using Inclined Mixing of Limestone demonstrated that melting and assimilation are promoted by positioning concentrate near abundant melt sources.

Reduced Multicomponent Calcium Ferrite was analyzed using STEM-EDS (scanning transmission electron microscope - energy dispersive spectrometer) and 3DAP (three-dimensional atom probe) in order to clarify the reducibility of SFCA (silico-ferrites of calcium and aluminum) based on the microstructure analysis. The morphological observation and crystal structure analysis using STEM-EDS and electron diffraction revealed that the amorphous Ca–Si–O oxides, spinel phase (Fe,Mg)(Fe,Al)₂O₄ and brownmillerite phase (Ca,Fe)₂(Fe,Al)₂O₅ were formed in the metallic iron obtained by the reduction of SFCA. Since the relatively reducible spinel phase was observed after reduction, it is suggested that the formation of spinel phase (Fe,Mg)(Fe,Al)₂O₄ by the dissolution of Mg into spinel phase affect the reducibility of SFCA. It was also found that each oxide was dispersed in metallic iron in granular form, either singly or in complex form. The three-dimensional analysis by performing 3DAP clarified that the presence of the Fe–O enriched region with a width of about 2 nm existed at the interface between amorphous Ca–Si–O oxide and metallic iron. This result suggests that the morphology of amorphous Ca–Si–O oxides affect the reducibility of SFCA.

In this study, influence of crystal structure and chemical composition for Silico-Ferrite of Calcium and Aluminum (SFCA) on its reducibility are examined. Eight types of the powder samples containing SFCA and SFCA-I phases were prepared using chemical reagents by the heat treatments in air. The samples were heated up to 800°C in the different atmospheres of CO–CO₂–H₂–H₂O systems. The reducibility of the samples was evaluated using the peak intensity ratio identified by XRD before and during the reduction experiment. The intensity of SFCA peak is not decreased up to 700°C in CO–CO₂ atmosphere, whereas the intensity become weak in CO–CO₂–H₂–H₂O atmosphere. The intensity of SFCA-I peak is decreased above 500°C in all atmospheric condition and the reduction reactions are enhanced by the addition of H₂–H₂O gas. Decrease in intensity of SFCA peak is independent of Fe composition, whereas that of SFCA-I is decreased with decreasing Fe concentration. The difference in reducibility is attributed to the difference in the crystal structure of multi-component CF. SFCA and SFCA-I are composed of pyroxene and spinel units. Since the pyroxene unit contains more gangue minerals than spinel unit, it implies that the pyroxene unit shows low reducibility than the spinel units. Since SFCA-I contains more the spinel units than SFCA, SFCA-I is easily reduced than SFCA.

In this study, the method to extract multi-component calcium ferrite (CF) into actual iron ore sinter is proposed and its reducibility is investigated under N₂–CO–CO₂ atmosphere. Actual iron ore sinter are pre-reduced at 500°C for 3 h in N₂–CO–CO₂–H₂–H₂O atmosphere and then sample particles containing mainly multi-component CF are successfully obtained through crashing and wet magnetic separation.

In the isothermal reduction at 700–800°C, the reduction rate shows positive correlation with compositions of SFCA-I and SFCA-III up to early stage of reduction, and the negative correlation is obtained with SFCA. However the relationship is reversed, for example at 750°C for approximately 90 s. After approximately 380 s, there is no longer any correlation between SFCA and the reduction rate. It indicates that SFCA-I and SFCA-III are already reduced to intermediate products up to 90 s and the reduction of SFCA affects the reduction rate up to 360 s at 750°C.

In the heating reduction using in-situ XRD, the formation of wustite is identified along with the decrease in the intensity of multi-component CF from around 600°C. The intensity of SFCA-III decreases from around 550°C and disappears at 760°C, whereas the SFCA-I decreases more gradually. It is known as the multi-component CF consists of series of spinel and pyroxene structure. Since the proportion of S units is SFCA-III > SFCA-I > SFCA, it implies that the reduction of Fe ions advances in S units than in P units.

In this study, the influence of Fe²⁺/Fe³⁺ ratios in SFCA-I on its reducibility are examined. Five types of powder samples containing SFCA-I with different Fe²⁺/Fe³⁺ ratios were prepared using chemical reagents by solid phase reaction at high temperature. The samples were heated up to 800°C in the different atmospheres of CO–CO₂–H₂–H₂O systems. The reducibility of the samples was evaluated using the integrated intensity ratio identified by XRD before and during the reduction experiment.

The integrated intensity ratio of SFCA-I is not decreased up to 700°C in CO–CO₂ atmosphere, whereas the intensity become weak in CO–CO₂–H₂–H₂O atmosphere. The integrated intensity ratio of SFCA-I remains at 800°C under CO–CO₂ atmosphere. On the other hands, the integrated intensity ratio disappears under the CO–CO₂–H₂–H₂O atmosphere. The increase in the Fe²⁺/Fe³⁺ ratio in SFCA-I leads to the increase in the concentration of Fe in SFCA-I and significant decrease the integrated intensity under different reducing gas conditions at 750°C. The increase in the amount of Fe²⁺ may lead to enter Fe²⁺ into the spinel unit, and this may contribute to the reduction of SFCA-I.

The reduction disintegration index (RDI) of sintered ore, which is the main raw material of blast furnaces, greatly affects blast furnace operation. In order to improve RDI without deteriorating the reducibility index (RI), sintered ore having a CaO concentration of 20 mass%, which is higher than the conventional 10 mass% when using porous Australian iron ore, was produced, and its effects on the mineral structure, porosity, RI and RDI were evaluated.

In the sintered ore having a CaO concentration of 20 mass%, hematite decreased and calcium-ferrite (sum of other component system calcium-ferrite and binary calcium-ferrite), the SFCA-I/SFCA ratio and porosity increased in comparison with that having a CaO concentration of 10 mass%.

When Australian iron ore was used as a raw material for sintered ore, RI increased in sintered ore having a CaO concentration of 20 mass% compared with the one having a CaO concentration of 10 mass%. In addition, RDI was improved in the sintered ore having a CaO concentration of 20 mass% compared with the that having a CaO concentration of 10 mass%. This is due to the formation of binary calcium-ferrite instead of secondary hematite, which deteriorates RDI.

Conventionally, it has been known that the product yield of the upper part of the sintering layer is extremely low, because of the heat loss caused by transferring heat toward the space above sintering layer, and of the large amount of unburned carbon in upper sintering layer.

As a countermeasure, REMO-tec (Re-ignition Method for Optimization of Total Energy Consumption) has been developed. Here, REMO-tec is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method has an effect on improving sinter yield with maintaining high sinter reducibility.

This paper focuses on gas flow rate control between first ignition and re-ignition for the purpose of further improving product yield.

It has been clear that certain decrease of gas flow rate between these two ignitions, has an effect on increasing heat storage amount in coke combustion zone at re-ignition caused by controlling vertical position of coke combustion zone.

This method has been applied in Kimitsu No.3 sinter plant. Product yield improves by 0.3% with increasing high temperature holding time.

REMO-tec, developed by Nippon Steel, is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method improves sinter yield while maintaining high reducibility, thereby making REMO-tec a technology that contributes to reduce CO₂ emissions in both sintering and blast furnace operations. This study aims to investigate the effects of re-ignition and coke breeze reduction on the mineral phase and pore structure of sintered ore through pot tests, providing a comprehensive evaluation linked to sintering operational parameters. Sinter pot tests were conducted with a 130 mm layer of raw materials, focusing on the upper part of the sintering layer. Experimental conditions included a base condition with single ignition and REMO-tec with re-ignition under three levels of coke breeze blending ratios (4.1, 3.3, and 2.9%). Microstructure observation and pore structure analysis were conducted using an optical microscope and image processing, respectively. REMO-tec extended the high-temperature holding time while maintaining the maximum bed temperature, leading to the formation of an acicular calcium ferrite under low coke conditions (3.3 and 2.9%) with a peak temperature of approximately 1250°C. Additionally, pore structure analysis revealed that the reducibility of sintered ore correlates with the volume of pores smaller than 200 µm. Consequently, producing high-FeO sintered ore with an acicular calcium ferrite matrix and a large volume of pores smaller than 200 µm through REMO-tec under low bonding agent conditions is the most desirable approach for balancing sintering operational parameters and reducing CO₂ emissions in the ironmaking process.

Conventionally, it has been known that the product yield of the upper part of the sintering layer is extremely low, because of the heat loss caused by transferring heat toward the space above sintering layer, and of the large amount of unburned carbon in upper sintering layer.

As a countermeasure, REMO-tec (Re-ignition Method for Optimization of Total Energy Consumption) has been developed. Here, REMO-tec is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method has an effect on improving sinter yield with maintaining high sinter reducibility. This effect leads to improving sinter reducibility without decreasing sinter yield by decreasing control of coke breeze content in sinter mixture.

This paper focuses on coke combustion efficiency as combustion ratio of carbon in coke breeze for considering improvement of sinter yield through sinter pot test. Here, carbon combustion ratio is defined as proportion of actual heat generation at combustion to ideal heat generation as complete combustion (C+O₂→CO₂) of all carbon in coke. And it can be calculated based on component analyses of exhaust gas.

As the result, it was confirmed as shown bellows.

1) By re- ignition, the unburned coke remaining in the upper layer of the sinter packed bed was burned, which has a role of extending keeping time over 1200°C especially in the upper layer of sinter packed bed.

2) Due to the effect of 1), the increasing amount of heat supply at “REMO-tec” case was equivalent to the same as the experimental case of increasing coke breeze content, at which increasing heat amount at blending coke breeze content was four times larger of the heat amount at re-ignition. (For a 430 mm layer pot test)

3) In addition, since the re-ignition heat is donated to the upper layer (surface layer), the amount of heat consumption in the upper layer of the sinter packed bed increases and the amount of heat consumption in the lower layer decreases compared to the case of increasing coke breeze content, which results in decrease of the difference between heat consumption in upper layer and that in lower layer.

In addition, these effects have been also confirmed at the commercial sinter plant.