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Article

Characterizing the Vertical Heterogeneity in Ultra-High Bed Sintering: From Mixture Properties to Stratified Phase Composition and Sinter Strength

by
Yuchao Zhao
1,2,
Hongzhuang Dong
1,2,
Peng Li
1,
Wenzheng Jiang
1,*,
Qiang Zhong
1,* and
Mingjun Rao
1
1
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
2
Shandong Iron & Steel Group Co., Ltd., Jinan 250101, China
*
Authors to whom correspondence should be addressed.
Metals 2025, 15(12), 1282; https://doi.org/10.3390/met15121282
Submission received: 30 October 2025 / Revised: 20 November 2025 / Accepted: 21 November 2025 / Published: 24 November 2025
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Abstract

With the growing demand for efficiency, low consumption, and environmental sustainability in the iron and steel industry, ultra-high bed sintering technology emerges as a research hotspot due to its advantages in significantly reducing fuel consumption and pollutant emissions. However, studies on the influence of fuel on mineralization behavior under ultra-high bed sintering conditions remained limited. This study systematically analyzes the effects of particle size, chemical composition, alkalinity, and MgO/Al2O3 ratio on mineralization behavior using a 500 m2 sintering machine, while evaluating the tumbler strength and phase composition of the sinter. The results reveal that particle size segregation in the mixture was primarily caused by the upper layer, with the lower layer having a lesser impact on overall segregation. Chemical composition also exhibited significant segregation, particularly in TFe and fuel distribution along the bed height. Fuel segregation was pronounced vertically but negligible horizontally. Under the current fuel distribution, uneven heat distribution was observed, with excessive heat in the lower layer leading to increased liquid phase formation, reduced porosity, and improved sinter strength downward along the bed. Additionally, the phase composition varied markedly across layers: hematite content gradually increases from top to bottom, calcium ferrite (SFCA) content peaks in the middle layers, and magnetite decreases with bed depth.

1. Introduction

The iron and steel industry, a cornerstone of national economic development, directly influences economic strength and comprehensive competitiveness [1,2]. As the primary raw material for blast furnace ironmaking, sinter quality critically impacts the technical and economic performance of ironmaking processes [3]. With rapid industry growth, demand for high-quality sinter has surged, necessitating advancements in sintering technology [4,5]. Traditional sintering processes face challenges such as high energy consumption and pollution, which conflict with green and low-carbon development goals [6,7,8].
Ultra-high bed sintering technology offers a promising solution by improving sinter quality, reducing fuel consumption, and lowering emissions [9,10,11]. However, the increased bed height complicates heat and mass transfer, mineral formation, and phase evolution. The nonlinear rise in airflow resistance can lead to combustion zone offset and uneven temperature fields, affecting key binder phases like calcium ferrite (SFCA) [12,13,14]. Moreover, oxidation–reduction kinetics, liquid phase fluidity, and condensation behavior under ultra-high bed conditions differ fundamentally from conventional sintering, rendering existing mineralization models inadequate [15,16].
Fuel characteristics—type, ratio, and combustion properties—play a pivotal role in mineralization behavior and sinter quality [17,18]. Surface fuel addition has been shown to reduce energy consumption and emissions while improving heat utilization [19]. However, uneven heat distribution due to fuel segregation results in variable liquid phase quantities and properties [20]. For instance, mathematical models highlight the impact of sintering machine speed on bed temperature distribution [21], and double-layer sintering has been proposed to address permeability and productivity limitations [17].
Despite these advancements, systematic studies on mineralization behavior under ultra-high bed conditions are scarce. This paper investigates the effects of particle size, chemical composition, alkalinity, and MgO/Al2O3 ratio on mineralization, providing theoretical and technical support for optimizing ultra-high bed sintering processes.

2. Materials and Methods

Experiments were conducted on a 500 m2 sintering machine (Taiyue Metallurgical, Laiwu, China) equipped with a shuttle distributor, 11 rollers, and a mud roller. The mixture sampling was conducted as follows: After the shuttle completed feeding but before ignition, the sinter trolley was stopped at a predetermined position. The mixture was sampled from unignited trolleys and divided into eight layers (120 mm each, including base material) at longitudinal, and eight columns (700 mm each) at transverse. Approximately 4 kg of samples were collected per point for shrinkage analysis. Sintered products were similarly sampled (4 layers × 4 columns, 100 kg per point) to measure composition (TFe, FeO), strength, phase composition, and microstructure. The sampling diagram is shown in Figure 1. Particle size distribution was determined via dry vibrating screening, and drum strength was tested per GBT24531-2009 [22].

3. Results and Discussion

3.1. Mixture Segregation Analysis

Figure 2 illustrates the particle size distribution of each layer in the 500 m2 sintering machine. An analysis of Figure 2 revealed that the average particle size of the sample section mixture was 3.43 mm. Along the bed height direction, the mixture on the trolley surface exhibited an average particle size of 2.65 mm, representing a deviation of 22.74% from the overall average. At the trolley bottom, particle size reached 4.08 mm on average, with a deviation of 18.95% from the overall mean. Notably, this variation span reached 41.69% between these two locations. These findings indicated that the 500 m2 sintering machine exhibited substantial segregation during material distribution. Meanwhile, segregation was also observed in the transverse direction of the trolley. Particle size distribution in the 0~2.8 m region exceeded the overall average, characterized by higher concentrations of coarse particles in both surface and bottom layers, whereas the 2.8~5.5 m region showed finer particles relative to the overall average, manifested as an increase in surface fine particulate matter from 2.28 mm to 2.5 mm and a decrease in bottom coarse particulate matter from 7.13 mm to 6.69 mm. This phenomenon could be attributed to the large footprint of the sintering machine, the 5.5 m trolley length, and segregation mechanisms inherent in the shuttle distributor operation [23].
The segregation evaluation of the particle size of the mixture in the vertical direction is shown in Figure 3. In Figure 3, segregation evaluation parameters are defined as follows: Surface layer extreme difference = (maximum value in surface layer) − (minimum value in surface layer). Bottom layer extreme difference = (maximum value in bottom layer) − (minimum value in bottom layer). Relative extreme difference = (absolute extreme difference/average content of that size fraction) × 100%. Figure 3 revealed that segregation intensity varied considerably among different size fractions with increasing particle size. For the 1~3 mm fraction, the absolute range reached 19.56%; however, given its 33.53% proportion in the mixture, the relative range was only 58%. Conversely, the +8 mm fraction exhibited a relatively large absolute range due to its low proportion in the mixture, resulting in a relative range as high as 140%. Particles sized 3~5 mm demonstrated the smallest absolute and relative ranges, with the latter measuring 24.7%. Furthermore, analysis indicated that the 1~3 mm, 5~8 mm, and +8 mm fractions in the surface layer exhibited significantly higher segregation ranges compared to the bottom layer, suggesting that the segregation of these fractions was predominantly attributed to the upper bed mixture, whereas the lower layer exerted a minimal influence on overall bed segregation.
As illustrated in Figure 4, the mixture exhibited an average TFe content of 50.40%. Significant segregation was observed along the bed height. Specifically, TFe content in the surface and bottom layers measured 47.91% and 50.20%, respectively, yielding a difference of 2.29%. This disparity was attributed to particle size segregation. Although minor segregation occurred in the transverse direction, the magnitude remained negligible.
Figure 5 illustrates the correlation between TFe content and ternary particle size distribution (S < 3 mm, M = 3~5 mm, L > 5 mm) in the mixture. Figure 5 reveals that TFe content increased proportionally with the coarse particle fraction, while intermediate particles exerted minimal influence. Conversely, an increased fine particle proportion led to a reduced TFe content. This trend was primarily attributed to the preferential aggregation of dolomite and limestone within the fine particle fraction.
MgO/Al2O3 in the mixture is an important index to ensure the smooth operation of the blast furnace. The distribution of MgO/Al2O3 in each layer of the 500 m2 sintering machine is shown in Figure 6. It can be seen from Figure 6 that the average MgO/Al2O3 of the whole material layer section was 1.05, and the MgO/Al2O3 in a large part of the area was about 0.94~1.04. Among them, the range corresponding to the first layer to the third layer was as high as 1.24 because of the low content of Al2O3. The segregation of MgO/Al2O3 was mainly reflected in the transverse segregation, and the longitudinal segregation was small.
Figure 7 presents the basicity distribution across different layers of the 500 m2 sintering machine. As shown in Figure 7, the mixture exhibited an average basicity of 1.96 in this section. Along the bed height, basicity in the surface layer reached 2.12, whereas the bottom layer exhibited only 1.77, resulting in a variation of 0.35. Such basicity variation along the bed height readily induced substantial differences in sinter properties. In the transverse direction, basicity in the 0~2.8 m region was significantly higher than the average, while the opposite side registered lower values. Correlation with particle size segregation results revealed that high-basicity mixtures demonstrated superior granulation efficiency and larger particle formation. Consequently, transverse segregation in the 0~2.8 m region resulted in corresponding basicity segregation.
The above phenomenon can be explained by the phase diagram of the CaO-SiO2-Fe2O3-1.54%Al2O3-1.61%MgO system in air atmosphere, drawn by the phase diagram module of FactSage 8.1 thermodynamic software. As demonstrated in Figure 8, decreasing basicity (where Rmin, Raverage, and Rmax represent the basicity of the bottom layer, average basicity, and surface layer basicity, respectively) resulted in an overall expansion of the liquid phase region, accompanied by reduced liquidus temperature. Notably, the magnified region in the figure corresponds to the Fe2O3 content range across different layers of the 500 m2 sintering machine. Within this range, basicity variation exerted a more pronounced influence on the liquid phase region compared to Al2O3 content fluctuation. Liquid phase evolution during sintering exerts a decisive influence on the microstructure, mechanical properties, and metallurgical performance of sinter. Optimal liquid phase formation can significantly enhance sinter mechanical strength and facilitate the development of high-bonding mineral phases (such as SFCA) through promoting particle rearrangement, dissolution-precipitation, and pore-filling mechanisms.
Figure 9 presents the fuel distribution of the 500 m2 sintering machine along the trolley bed. As illustrated in Figure 9, significant fuel segregation occurred along the vertical direction of the material layer, whereas no obvious segregation was observed in the horizontal direction. Vertically, the surface layer exhibited an average fuel content of 5.17%, while the bottom layer showed only 4.12%, yielding a substantial difference of 1.05%. Notably, fuel concentration was considerably higher in the surface region.
Within the 0~0.12 m surface zone, corresponding to the ignition layer during sintering, elevated fuel ratios were detected. This distribution pattern promotes incomplete combustion or localized reducing atmospheres due to oxygen deficiency during ignition, consequently increasing the ferrous content in the sinter product [23]. Throughout the 0.12~0.84 m material layer depth, average fuel content decreased from 3.50% to 1.73%, representing a 1.77% gradient. This reduced segregation magnitude suggests that fine fuel particles were predominantly utilized in the 500 m2 sintering machine. During granulation, poor adhesion characteristics prevented effective pellet formation, and subsequent bedding segregation concentrated these fine particles primarily in the surface layer.
Based on the fuel distribution, the heat distribution of each layer of the 500 m2 sintering machine is calculated according to the heat storage theory [24,25,26,27], and the results are shown in Figure 10. It can be seen from Figure 10 that the heat income of the material layer gradually increases along the height of the material layer, and the heat income of the sixth layer reached the peak. The total heat storage rate of the material layer increased from the first layer to 90% of the eighth layer, and the available heat storage rate of the material layer remained at about 55% since the third layer. It can be seen that, under the fuel distribution of the 500 m2 sintering machine, there was a large problem of uneven heat distribution. The heat in the lower layer was too high, and the amount of liquid phase generated was large, resulting in fewer voids in the lower layer. At the same time, the relatively low fuel ratio, high temperature, and long-term high temperature duration in the lower layer would lead to the gradual decrease in FeO in the sinter from top to bottom.

3.2. Segregation Analysis of Sinter

Figure 11 presents the TFe content distribution across each layer of sinter in the 500 m2 sintering machine. As illustrated in Figure 11, the average TFe content in the sinter was 53.62%, with a maximum value of 53.94%, a minimum of 53.29%, and a range of 0.65%. The surface layer exhibited an average content of 53.51%, while the bottom layer showed 53.46%. Vertically, TFe content increased initially and then decreased from top to bottom, which was attributed to flux segregation in the surface layer. Horizontal distribution was nonuniform, with TFe content in the left-side middle layer being notably higher.
The distribution of FeO content in each layer of sinter in the 500 m2 sintering machine is shown in Figure 12. As illustrated in Figure 12, the average FeO content in the sinter was 10.47%, while the surface layer exhibited an elevated content of 11.81%, substantially exceeding normal sinter production requirements. The bottom layer showed an average ferrous iron content of 9.77%, representing a 2.04% difference from the surface layer. Ferrous iron content also exhibited segregation in the transverse direction of the trolley. Generally, edge regions displayed lower values, while central areas showed higher concentrations. Ferrous iron content across the entire trolley varied by as much as 3.75%. Based on the component distribution in the mixture, FeO formation is primarily influenced by fuel distribution and oxygen potential [28]. Elevated fuel content in the surface layer resulted in low oxygen potential during combustion, consequently producing high FeO content in the sinter. Due to edge effects, oxygen potential on both sides of the trolley was higher, thereby yielding slightly lower FeO content.
Figure 13 presents the tumbler strength of sinter in this section. As shown in Figure 13, the strength of the sinter increased significantly along the bed depth, and the tumbler index rose from 70.6% in the first layer to 80.5% in the fourth layer. From the first layer to the second layer, the tumbler index exhibited the most substantial increase of 5.96%. As the bed depth decreased, the increase gradually diminished. This change in the tumbler index is mainly due to the growth of SFCA and the decrease in porosity [29,30].
Ferrous content in sinter commonly serves as an indirect indicator of fuel distribution and low-strength olivine phase content [31,32]. As evident from Figure 13, an increased fuel ratio in the mixture corresponded to a decreased tumbler strength of the sinter. This phenomenon primarily occurs because fuel combustion leaves voids in the sinter; while moderate fuel increases promote liquid phase formation, excessive fuel induces liquid phase over-melting, generating large-pore thin-walled structures. Fuel distribution analysis revealed that fuel was predominantly concentrated in the surface layer. However, owing to the short high-temperature duration in the surface layer, limited liquid phase was generated, and residual voids exerted a detrimental effect, consequently causing surface layer strength to be substantially lower than that of the bottom layer.
Figure 14 presents the microstructure of sinter ore in different layers. As illustrated in Figure 14, in the first layer, a small amount of interwoven SFCA and magnetite phase was clearly observed between incompletely melted iron ore, with cracks formed due to condensation shrinkage. Pores between iron ore particles were interconnected by cracks, and porous sinter was intermixed with unburned mixture, indicating insufficient surface heat. Additionally, fuel combustion and carbonate decomposition generate gas and form voids in the liquid phase. At fast cooling rates, these voids solidify in the sintered product, forming loose and porous structures that exhibit poor strength. Along the bed depth, porosity and pore size decreased significantly, which is likely attributed to reduced cooling rates and heat accumulation. Compared with the first layer, the SFCA proportion in the second layer increased substantially. Interwoven structures of SFCA, magnetite, and silicate constituted the dominant features of the second and third layers. However, the second layer was dominated by needle-like SFCA, while the third layer was characterized by columnar SFCA. This morphological variation may be influenced by heat accumulation and the crystallization rate. At slower cooling rates, SFCA grains undergo growth. The fourth sinter layer was predominantly composed of massive SFCA and granular magnetite embedded with some secondary hematite, exhibiting very low porosity and rough fracture surfaces. This structure primarily results from abundant liquid phase formation due to high heat, while slower cooling rates promote dense structure crystallization. Concurrently, mineral phase transformation associated with chemical composition segregation represents another key factor governing the above structure, with hematite content increasing gradually due to decreased fuel and enhanced oxidizing atmosphere.
In order to quantitatively analyze the change in the sintered mineral phase, the phase composition of different layers of sinter was counted, with results presented in Figure 15. As illustrated in Figure 15, phase composition varied considerably among different sinter layers. The content of hematite gradually increased from top to bottom, the content of calcium ferrite increased first and then decreased, and the magnetite content decreased progressively along the material layer. Statistical analysis revealed that the content of silicate in the sinter was relatively low, so FeO mainly existed in the form of magnetite.

4. Conclusions

(1)
Under the condition of ultra-high bed sintering, the segregation of particle size in the material layer was mainly caused by the upper mixture, while the segregation of the lower layer exerted less influence on the segregation of the whole material layer than the upper layer. Due to the segregation of particle size, the chemical composition in the mixture also showed serious segregation. The segregation of TFe and fuel in the height direction of the material layer was significant, while the segregation of MgO/Al2O3 and alkalinity appeared in the width direction of the trolley.
(2)
TFe content in sinter initially increased, and then decreased. The average FeO content in the bottom layer was 9.77%, with a 2.04% difference from the surface layer. FeO content also exhibited segregation in the transverse direction of the trolley, generally displaying lower values at the edges and higher concentrations in the central regions.
(3)
Sinter strength improved significantly along the bed depth. With increasing FeO content, the drum strength of the sinter decreased. This phenomenon resulted from mixture fuel segregation, which was predominantly concentrated in the surface layer. However, owing to the short high-temperature duration in the surface layer, a limited liquid phase was generated, and residual voids exerted a detrimental effect, consequently causing the surface layer strength to be substantially lower than that of the bottom layer.
(4)
Phase composition of different sinter layers varied considerably. Hematite content increased gradually from top to bottom, calcium ferrite content initially increased then decreased, and magnetite content decreased progressively along the material layer.

Author Contributions

Conceptualization, Y.Z., Q.Z. and W.J.; methodology, Q.Z. and Y.Z.; software, H.D. and P.L.; validation, M.R.; investigation, M.R. and P.L.; resources, H.D. and P.L.; writing—original draft preparation, Y.Z.; writing—review and editing, Q.Z. and W.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52274290) and the National Key Research and Development Program of China (No. 2022YFC3900904).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Authors Yuchao Zhao and Hongzhuang Dong were employed by the company Shandong Iron&Steel Co., LTD. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Metals 15 01282 g001
Figure 1. Sampling partition diagram: (a) blending materials; (b) sintering ore.
Figure 1. Sampling partition diagram: (a) blending materials; (b) sintering ore.
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Figure 2. Particle size distribution of each layer of 500 m2 sintering machine.
Figure 2. Particle size distribution of each layer of 500 m2 sintering machine.
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Figure 3. Evaluation of segregation of mixture particle size in the direction of material layer.
Figure 3. Evaluation of segregation of mixture particle size in the direction of material layer.
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Figure 4. TFe distribution diagram of each layer of 500 m2 sintering machine.
Figure 4. TFe distribution diagram of each layer of 500 m2 sintering machine.
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Figure 5. The relationship between TFe distribution and ternary particle size of mixture.
Figure 5. The relationship between TFe distribution and ternary particle size of mixture.
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Figure 6. MgO/Al2O3 distribution diagram of each layer of 500 m2 sintering machine.
Figure 6. MgO/Al2O3 distribution diagram of each layer of 500 m2 sintering machine.
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Figure 7. Alkalinity distribution map of each layer of mixture.
Figure 7. Alkalinity distribution map of each layer of mixture.
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Figure 8. Phase diagram of CaO-SiO2-Fe2O3-1.54%Al2O3-1.61%MgO system in air atmosphere.
Figure 8. Phase diagram of CaO-SiO2-Fe2O3-1.54%Al2O3-1.61%MgO system in air atmosphere.
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Figure 9. Fuel distribution map of each layer of 500 m2 sintering.
Figure 9. Fuel distribution map of each layer of 500 m2 sintering.
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Figure 10. The heat distribution of each layer of 500 m2 sintering.
Figure 10. The heat distribution of each layer of 500 m2 sintering.
Metals 15 01282 g010
Metals 15 01282 g011
Figure 11. The distribution of TFe content in each layer of sinter.
Figure 11. The distribution of TFe content in each layer of sinter.
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Metals 15 01282 g012
Figure 12. The distribution of FeO content in each layer of sinter.
Figure 12. The distribution of FeO content in each layer of sinter.
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Metals 15 01282 g013
Figure 13. The change in drum strength along the height direction of material layer.
Figure 13. The change in drum strength along the height direction of material layer.
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Metals 15 01282 g014
Figure 14. Microstructure of sinter in different material layers: SFCA, calcium ferrate complex; M, magnetite; S, silicate; P, porous. (a) 50×; (b) 50×; (c) 50×; (d) 50×; (e) 100×; (f) 100×; (g) 100×; (h) 100×.
Figure 14. Microstructure of sinter in different material layers: SFCA, calcium ferrate complex; M, magnetite; S, silicate; P, porous. (a) 50×; (b) 50×; (c) 50×; (d) 50×; (e) 100×; (f) 100×; (g) 100×; (h) 100×.
Metals 15 01282 g014
Metals 15 01282 g015
Figure 15. The phase composition proportion along the height direction of the material layer.
Figure 15. The phase composition proportion along the height direction of the material layer.
Metals 15 01282 g015
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MDPI and ACS Style

Zhao, Y.; Dong, H.; Li, P.; Jiang, W.; Zhong, Q.; Rao, M. Characterizing the Vertical Heterogeneity in Ultra-High Bed Sintering: From Mixture Properties to Stratified Phase Composition and Sinter Strength. Metals 2025, 15, 1282. https://doi.org/10.3390/met15121282

AMA Style

Zhao Y, Dong H, Li P, Jiang W, Zhong Q, Rao M. Characterizing the Vertical Heterogeneity in Ultra-High Bed Sintering: From Mixture Properties to Stratified Phase Composition and Sinter Strength. Metals. 2025; 15(12):1282. https://doi.org/10.3390/met15121282

Chicago/Turabian Style

Zhao, Yuchao, Hongzhuang Dong, Peng Li, Wenzheng Jiang, Qiang Zhong, and Mingjun Rao. 2025. "Characterizing the Vertical Heterogeneity in Ultra-High Bed Sintering: From Mixture Properties to Stratified Phase Composition and Sinter Strength" Metals 15, no. 12: 1282. https://doi.org/10.3390/met15121282

APA Style

Zhao, Y., Dong, H., Li, P., Jiang, W., Zhong, Q., & Rao, M. (2025). Characterizing the Vertical Heterogeneity in Ultra-High Bed Sintering: From Mixture Properties to Stratified Phase Composition and Sinter Strength. Metals, 15(12), 1282. https://doi.org/10.3390/met15121282

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