2025 Volume 65 Issue 11 Pages 1754-1759
The increase of hydrogen (H2) proportion in the blast furnace affects the reduction degradation of the ferrous burden materials. As one of the main ferrous burden materials, iron ore sinter consistently shows higher degradation than other burden materials. The typical analysis of reduction degradation of sinter utilises standard tests and microstructure observation from the sample after reduction. However, this method has limitations in terms of the view being limited in 2D and the analysis using the same samples was difficult.
To overcome the limitation, in this study, an alternative method to study the reduction degradation of iron ore sinter using X-ray micro-computed tomography (MCT) was introduced. The sinter analogue was prepared using an infra-red furnace and the change in their structure before and after reduction was compared from the MCT scan results. As the MCT scan was considered a non-destructive method, the same sample can be compared in 3D which allows the identification of cracks. Based on the MCT scan results, the cracks formed due to the reduction were identified and supported the previously suggested mechanism of reduction degradation. Cracks were observed across the whole samples and indicated a preference to form in the area where magnetite and SFCA initially existed. With the improvement of the segmentation technique along with microstructure analysis, the use of sinter analogue and MCT could be considered a promising method for studying the reduction degradation of sinter under more complex gas mixtures.
The injection of hydrogen through the tuyeres of the blast furnace is considered a promising solution to reduce CO2 emissions while the steel industry transitions to other alternative low-carbon ironmaking technologies. The injection of H2 into an experimental blast furnace successfully reduced CO2 emissions by about 16% while maintaining constant pig iron temperature and productivity.1) However, the increase of H2 proportion in the reducing gas will affect the reduction degradation of ferrous burden materials,2,3) affecting the productivity in the blast furnace.4) Sinter, as one of the main ferrous burden materials in the blast furnace,5) consistently showed a higher degradation than lump and pellets.6,7)
The reduction degradation is the condition when ferrous materials generate fines particles after reduction at low temperature in the range of 400–700°C. The fundamental cause of degradation is the volume expansion during the reduction of hematite to magnetite which leads to the generation of stress and the formation of cracks.6) The generation of fines particles lowers the gas permeability inside the blast furnace8) and decrease the productivity.4) The lower value of degradation is desirable to prevent excess fines generation and secure smooth gas flow inside the blast furnace.
Sinter structure generally consisted of hematite nuclei surrounded by a bonding matrix. It has been proposed that the reduction degradation in sinter starts with the reduction of easily accessible hematite which leads to the crack initiation and crack propagation to the surrounding matrix.6) Further growth and branching of cracks are driven by the reduction of newly exposed hematite along the walls of the cracks. Cracks become a channel for the gas to penetrate the previously inaccessible areas of the sinter.
It was reported that the secondary hematite content in the sinter highly affects the degradation.9,10) However, another study found that both types of hematite (primary and secondary) undergo severe cracking.11) Moreover, there is no strong relationship between total hematite content and degradation value,12) indicating that the overall phase proportion is important for the degradation of sinter. Some studies utilised the fracture toughness to compare the resistance of the phase to crack propagation and found that different phases in the sinter have different fracture toughness values.12,13,14) Hematite has the highest value of fracture toughness followed by magnetite or silico-ferrite of calcium and aluminium (SFCA), while silicate or glass shows the lowest value.12,14) These authors propose that sinter with a higher composite fracture toughness will likely have a lower degradation.
Real-time microradiology has been used to study the degradation of sinter under CO–N2 gas mixtures.15) In this study, it was reported that cracks initiated from pores and propagated along the neighbouring pores. Cracks started to form at 450°C, and the initiation of cracks occurred preferentially on big macro-pores (>800 μm). However, there is limited discussion about the role of matrix phase in this study. Mizutani et al.16) utilised Acoustic Emission (AE) to detect the formation of the cracks during the reduction degradation test of sinter, lump, and pellets. They observed that the AE energy attributed to the reduction degradation differed from the AE energy due to thermal stress during the cooling period of the test. The total AE energy due to reduction degradation and thermal stress shows a good relationship with the reduction degradation index (RDI) value, suggesting that the application of AE can be used to evaluate the degradation behaviour quantitatively.
Observation of microstructure shows that cracks were found on the magnetite formed from the reduction,2) and many cracks were found in the surrounding area where hematite has been partly reduced to magnetite.17) Other studies observed that cracks were formed indiscriminately across all the phases in the sinter.12) Calcium ferrite was not considered as the origin of cracks but still propagated the cracks.11) Another study found that calcium ferrite influenced the crack formation and degradation of sinter, which depended on the reduction conditions and calcium ferrite type.18) The crack density which is calculated based on the cross-section images of the reduced samples also shows a good correlation with the degradation index.2)
While the analysis of reduction degradation through microstructure observation gave a lot of details into phase morphology, texture, chemical composition, and the cracks location, it has limitations in terms of the view being limited in 2D and carried out after the completion of the experiment. Moreover, the effect of reduction on the samples cannot be analysed using the same samples as the sample preparation will damage the samples. Concerning this condition, computed tomography has the potential to overcome these limitations.
Computed Tomography is a non-destructive technique that has been used to analyse the structure of materials in 3D. The use of computed tomography in the iron ore sintering process has been performed to study the structure and properties of sinter.19,20,21,22,23,24,25) The analysis of sinter bed pore structure using CT shows that pore branch structure was important for bed permeability.24) Nushiro et al.23) and Kasai et al.21) applied an in-situ approach to study the sinter bed structure by combining the sinter pot with CT apparatus. Under this experimental setup, the change in the bed structure as a function of time or the sintering process could be obtained. With the aid of CT, the interrupted test of sinter pot experiments was used to study the shrinkage of the sinter bed during the sintering process and the pore structure at different sintering zones.20,26)
Despite the extensive use of CT in the sintering process, there is limited study about reduction degradation phenomena in sinter using CT. A previous study utilised CT to study the reduction degradation; however, it was limited only to the pellet.27,28) Therefore, in this study, the reduction degradation of iron ore sinter was studied using iron ore sinter analogue and CT to gain an insight into the formation of the cracks in the sinter during reduction. Previous study on the CT scan of industrial and pot grate sinter showed a large variability in the structure of similar samples.29) The resolution of the CT scan for the industrial and pot sinter particles was between 10–20 μm19,29) while using the analogue the resolution could reach 4.5 μm.22) The sinter analogues were selected to reduce the variability in the sinter being studied and to increase the resolution of the scans.
The reduction of analogues was performed at 500°C under simplified gas mixtures of H2–N2. The crack’s location after reduction was analysed to assess whether cracks show any preferential location for their formation. The findings from this preliminary study will help in the study of reduction degradation under more complex gas mixtures, including the conventional blast furnace and H2-injection blast furnace gas.
Sinter analogues were prepared from Australian iron ore using an infra-red rapid heating furnace. The equipment and procedures are described in a previous work.30) A green analogue was prepared from the −1 mm fraction of the ore, and it was fluxed to reach a basicity of 2.0, SiO2 of 5.4%, MgO of 1.8%, and containing Al2O3 of 1.9%. The green analogues were sintered using similar temperature profiles and gas atmosphere with previous work30) to produce sinter analogues. The gas atmosphere during heating was controlled at pO2 of 5×10−3 atm using a bottled gas mixture (0.5% O2 in N2) and switched to air during cooling (pO2 of 0.21 atm). The maximum sintering temperature (Tmax) was selected as 1320°C with the holding time at this temperature of 1 minute, followed by the cooling process at the rate of 1°C/s. After sintering, the analogues have a mass of 0.5 g, about 8.9 mm in height and 5.2 mm in diameter.
The sinter analogue was then mounted, ground, and polished to analyse the microstructure using a reflective light microscope. The pore was segmented based on the greyscale value using ImageJ software, and the pore area was obtained to represent the porosity of the analogue. Another analogue was prepared for the reduction test and CT scan analysis.
The reduction of sinter analogues was performed by isothermal reduction at 500°C for 60 minutes using a thermogravimetric analyser (NETZCH STA 449 F5 Jupiter). Temperature profile and gas atmosphere were controlled using NETZCH Proteus-80 software. The gas mixture is 30%H2-70%N2 with a total gas flow rate of 200 mL/min. Careful treatment has been performed to minimize the damage or fracture of the reduced analogues before the scanning process using X-ray Micro Computed Tomography (MCT).
The CT scan for the sinter sample before and after the reduction was acquired at the National Laboratory for X-ray Micro Computed Tomography (CTLab) at the Australian National University (ANU) using a space-filling trajectory.31) The acquisition time and the resultant voxel size for the CT scan was approximately 25 hours and 3.2 um, respectively, for both scans acquired for the sample before and after reduction. The acquired 2D projections were reconstructed into a 3D volume using a multi-grid iterative image reconstruction method that is preconditioned with a volume-space filter.32) The scan acquired for the sample after reduction was digitally registered to the scan acquired before the reduction sample for alignment of pore-scale features.33)
Subsequent image processing and analysis were carried out using the image analysis tool WebMango developed by the Australian National University (ANU) (webmango.anu.edu.au).34) To remove regions containing the core holder and air outside of the samples, the reconstructed 3D data were cropped into a cylindrical volume with diameter and height equal to 4.2 mm and 7.8 mm, respectively.
For data segmentation, which is the process of identifying the two physical phases, i.e. the air and the solid phases in the 3D reconstructed data, each voxel in the scan was assigned an integer based following a “Converging Active Contours” (CAC) routine.34) In this method, lower and higher intensity thresholds were chosen based on the intensity histogram, where voxels with grayscale levels smaller than the lower threshold were classified as air and voxels with grayscale levels higher than the upper threshold were classified as solid. With the segmented data, the porosity of the samples was calculated by counting the number of voxels occupied by the air and the solid.
The microstructure of the sinter analogue before the reduction is shown in Figs. 1(a) and 1(b). The sinter structure mainly consists of hematite bonded by the bonding phase of SFCA and magnetite. Under the reflective light microscope, pores were identified as a black or dark grey colour, primary and secondary hematite as a white colour, magnetite as a pinkish colour, and SFCA as a grey colour. Figure 2 shows the example of the microstructure in more detail. The porosity of sinter analogues was calculated based on the segmentation results of Fig. 1(b) using ImageJ software. The size of the area was selected to be the same as the cropped data of the CT scan (width 4.2 mm and height 7.8 mm). The resulting porosity calculated using this method was 41.2%.
The data obtained from MCT contains stacked images that can be constructed into 3D structures. Figure 3(a) shows an example of an image acquired from MCT before reduction. The grey levels in the image correspond to X-ray attenuation, a denser phase will appear brighter while a lighter phase darker. The dark area was identified as a pore while the grey and white areas were identified as solid. Figure 3(b) shows the segmentation of pores and solids to obtain the porosity of the analogue. The calculated porosity (in 3D) was 35.4%, which was about 5.8% lower compared with the porosity calculated from the cross-section images from the light microscope (2D). The difference between the calculated values was expected as they are calculated under different dimensions (2D vs 3D) and due to the difference in the image resolution. The pixel size for the image from the light microscope is 0.64 μm, while the CT image has a voxel size of 3.2 μm. With a higher resolution, more porosity can be segmented in the 2D image, resulting in a higher reported porosity.
Within the analogue structure, pores can be classified into open and closed pores. Open pores are connected to the atmosphere, meaning that the gas can diffuse or penetrate through these pores to reach the reaction interface. Meanwhile, closed pores are isolated, and were likely filled with the gas entrapped during the sintering process. Under a reflective light microscope, the pores that have not been filled with resin and hardener will appear darker and sometimes identified as closed pores. However, this identification method highly relies on the viscosity of the resin or hardener to fill the pore during mounting. In this study, an additional MCT scan using Xenon gas was utilised to identify the closed pores.
Figures 4(a) and 4(b) show the images acquired without and with Xenon gas during MCT scanning. During the scanning with Xenon gas, the Xenon gas penetrated to the pores in the analogue. The pores that cannot be penetrated by the Xenon gas will be filled with the existing gas, possibly air, which is trapped during the sintering process. Due to the difference in density between Xenon gas and air, the open pores appeared as a grey area while the closed pores as a dark area. Figure 4(c) shows a closed pores distribution, and the total closed pores percentage is 8.8% (by volume). This result means that most pores in the analogue were in the form of open pores, which provide access for the reducing gas to reach the reaction interface during the reduction process.
Figure 5 shows the image before and after reduction obtained from MCT with the Xenon gas. The images were aligned using the image registration method to compare the structure of the analogues at the same location. Comparing the image before the reduction (Fig. 5(a)), it was clear that after the reduction (Fig. 5(b)) some cracks appeared in various locations. The formation of cracks due to the reduction process was confirmed using the technique used in this study (CT scan) and showed a good agreement with previous studies using an in-situ approach.15,16)
Detailed observations from Fig. 5(b) show that most cracks were connected between pores. It suggests that the cracks were initiated near the pores and propagated to the surrounding areas until they reached another pore. Since the cracks initially formed because of the reduction of hematite, the cracks likely began in areas close to the open pores, where the reduction process was more likely to initiate. The propagation of the cracks then depends on the continuation of stress generated from the reduction and the fracture toughness of the materials. Some closed pores were converted to open pores after the reduction (see red circle in Fig. 5) because of crack formation. This finding supports the previously proposed mechanism which suggests that the crack acts as a channel that allows reducing gas to penetrate previously inaccessible regions of the sinter.6) Figure 6 shows the distribution of the closed pores before and after the reduction, showing a significant reduction of closed pores from 8.8% to 0.5%.
Observation of the cracks also reveals that they were formed across the whole sample. It means that the cracks did not propagate only through a specific weaker phase. This finding supports the previous microstructure observation on the sinter after reduction where cracks were identified across all the phases in the sinter.7)
Based on the observation from Fig. 1, the MCT data after reduction was segmented into three phases by using additional CAC segmentation for the solid phase by picking grayscale intensity thresholds. The three phases are phase A (brighter phase) which is assumed to be hematite (primary/secondary), phase B (grey phase) which is assumed to be a mixture of magnetite and SFCA, and phase C which belongs to pore and crack.
An attempt has been made to reveal the cracks by subtracting the MCT data before the reduction and after the reduction. However, the result was not satisfactory with lots of noise appearing. The cracks were then obtained through segmentation based on the grey levels, and additional image processing was applied to separate pores from cracks. For the identification of each crack, an “erosion dilation” filter was applied to the segmented data of the MCT scans after reduction, resulting in the separation between pore and crack for phase C. For the erosion dilation filter, a radius of three pixels was applied. Using this technique, a total of 533 individual cracks were obtained and labelled.
To determine if the crack has a preference to form in phase A or B, the interfacial area (IFA) between the crack and these two phases was calculated and compared. The IFA between the three phases was calculated using a three-phase extension of the algorithm of Ohser and Mücklich35) that computes unbiased surface area from voxelated datasets. Here, the solid phase corresponding to a higher ratio of air-solid IFA indicates preferred crack formation. The average ratio for phase B was 81.7% which indicates that the cracks preferred to form at the area where initially assumed to be magnetite and SFCA. There is no correlation found between the size of the cracks and the air-solid IFA ratio in this study.
However, the segmentation results in Fig. 7(b) show that some portions of cracks cannot be segmented as cracks due to a similar grey scale level with the bonding phase B. Based on this condition, the cracks segmented in this study were limited to cracks that have significantly different grey levels from other solid phases. Moreover, improvement of segmentation is necessary to provide better information to answer the crack preferences questions. For future study, one might start with a simpler system such as hematite and calcium ferrite rather than a complex sinter system. It might help in obtaining a better segmentation with the aid of images from a light microscope or scanning electron microscope (SEM).
Overall, the combination of sinter analogue and MCT to study the reduction degradation shows a promising result to complement another technique such as microstructure analysis. The comparison of cracks formed after reduction with more complex gas mixtures including a simulated blast furnace gas compositions is planned for future works.
The reduction degradation phenomena in the iron ore sinter were studied using iron ore sinter analogue and X-ray MCT to obtain an insight into the formation of the cracks during the reduction. The main findings of this preliminary study can be summarized as follows:
• The use of X-ray MCT to compare the sinter structure before and after reduction allows the observation of the crack’s formation. It was observed that cracks were formed due to the reduction process, leading to the degradation of the sinter.
• The formation of cracks caused some closed pores to be converted to open pores, which allow the reducing gas to reach the previously inaccessible regions for reduction and the extension of crack formation.
• Cracks were formed across the whole sample in different phases, suggesting that the propagation of cracks does not occur only in a specific weak phase.
• Segmentation of phases from MCT data indicates that cracks have a preference to form on the region initially assumed to be magnetite and SFCA. For future work, improvement of segmentation with the aid of images from a light microscope or SEM was suggested to obtain more accurate information.
The authors declare that they have no conflict of interest.
The authors acknowledge the financial support and permission to publish this paper from BHP. The authors also thank Dr Salman Khoshk Rish for the assistance in the reduction test and Dr Michael Turner for conducting the CT imaging.