High Temperature Softening and Melting Interactions Between Newman Blend Lump and Sinter

Abstract

In this work, the softening and melting (S&M) behaviour and whole blast furnace (BF) performance of Newman Blend Lump (NBLL), plant sinter, and sinter-NBLL mixture were studied using S&M under load test and numerical BF modelling. Both physical and chemical interactions between sinter and lump were confirmed in the S&M process. Significant improvements were found in the S&M behaviour of the sinter-NBLL mixture because of the physical and chemical interaction. The physical interaction was examined using X-ray/Neutron Computed Tomography (CT) scanning on the samples from interrupted S&M under load tests. The void fraction in the ferrous layer of the sinter-NBLL mixture was found to be similar to the sinter and was higher than that for NBLL. The chemical interaction was investigated by analysing the Ca transfer from sinter to NBLL, which indicated that Ca transfer started around 1200°C in the S&M process. FactSage was used to assist in the interpretation of the S&M test results. It was found that the NBLL sample starts to melt at a lower temperature compared to other burdens used in the present study, which also agreed well with the CT scan results. The whole BF performance of different ferrous burdens was studied using the experimental results as inputs. The sinter-NBLL mixture behaved more like the sinter than the NBLL; compared with the sinter only burden with the same total basicity, the sinter-NBLL combination formed a more permeable CZ, had a lower total BF pressure drop, and a higher gas utilization rate.

1. Introduction

Over the past few years, the rapid expansion of the world’s ironmaking and steelmaking industry has faced growing challenges, including environmental and economic issues. For instance, according to Zhang,¹⁾ 50% of the sintering capacity of some steel mills in China was shut down in 2017 to save the environment from potential pollution during the winter heating season. However, one of the possible solutions to this problem is to utilize a large fraction of iron ore lump in the ferrous burden as it can be charged directly into the ironmaking blast furnace (BF). Furthermore, iron ore lump is more environmentally friendly and economically effective compared to sinter and pellet as no agglomeration process is needed.^2,3) Iron ore agglomeration through sintering and pelletizing is estimated to produce 270 kg of CO₂ per tonne of hot-rolled coil.⁴⁾ So, increasing the lump ratio in the BF burden can reduce the cost of hot metal and overall energy consumption, especially the greenhouse emissions.^5,6,7,8,9)

In China, the utilization of averaged lump ore ratio in large BFs (> 4000 m³) was less than 9.6% which is lower than the other steelmaking countries like Japan, wherein the proportion of used lump ore was more than 20%.^10,11) One of the main reasons for the low lump utilization is the perception that the properties of individual lump are not as good as those of sinter or pellet, particularly the softening and melting (S&M) process when the ferrous burden materials turn from solid to liquid. However, the Chinese steel mills increased the utilization of lump ore in recent years; for example, the fraction of Australian lump ore utilization increased from 17% to 23% in a 2000 m³ BF in Hebei province.^{12,13,14,15,16,17)}

In laboratory experiments, the S&M properties of ferrous materials are usually investigated using the S&M under load test (or Reduction Under Load). In general, the individual test of lump ore shows that it has a higher pressure drop and lower softening temperature compared to sinter. However, several studies have proven that the reaction between sinter and lump ore in the S&M process, which is termed high-temperature interaction, can significantly improve the S&M properties of the ferrous burden materials.^{12,13,14,16,17)} Very few studies are available on the high-temperature interaction mechanism and this is still not fully understood.^2,18) Therefore, the present study aims to carry out a series of comprehensive S&M under load tests for lump ore, sinter, and their mixture, including tests carried out to completion as well as interrupted tests designed to investigate the interaction mechanism. Specifically, the aims were to:

a) Examine the S&M behaviour of lump (Newman Blend Lump (NBLL), a high-grade hematite goethite Australian lump ore), sinter, and mixed burden of lump and sinter.

b) Examine the structural and metallurgical aspects of the interaction. In particular, the variation of ferrous burden voidage is tested using synchrotron X-ray CT scanning, and the metallurgical interaction is studied using an optical and scanning electron microscope (SEM).

c) Quantify the melt fraction of ferrous burden using the FactSage.

d) Evaluate the BF performance using experimental results-based numerical modelling.

2. Experimental

2.1. Raw Materials

In the present study, three different raw materials were used to carry out the S&M test, namely higher basicity sinter (SH1), NBLL, and mixed burden (MB1, composed of 79% SH1 and 21% NBLL). The chemical compositions and basicity of related raw materials are summarised in Table 1.

Table 1. Chemical compositions (wt%) and basicity of raw materials.

Materials	FeO	TFe	CaO	SiO2	Al2O3	MgO	P	R2	R4
SH1	7.60	56.70	10.04	5.43	1.87	1.76	0.067	1.85	1.62
MB1	6.00	57.98	7.94	5.07	1.77	1.41	0.070	1.57	1.37
NBLL	–	62.80	0.05	3.70	1.40	0.10	0.080	0.01	0.03

R₂: binary basicity, mass ratio of (CaO/SiO₂), R₄: quaternary basicity, mass ratio of (CaO+MgO)/(SiO₂+Al₂O₃)

2.2. Softening and Melting under Load Test

Figures 1(a)–1(b) presents the schematic of the experimental rig used in the present study. The details of the experimental rig and testing conditions for the S&M under load test have been described in earlier studies.^2,3,18,19) Briefly, iron ore samples (diameter: ~10.0–12.5 mm, height: 70 mm) were sandwiched between two layers of coke (diameter: ~10.0–12.5 mm) in a graphite crucible (60 mm inner diameter) and subjected to a programmed, time-dependent temperature, reducing gas (flow rate: ~14.0 L/min, 30%CO + 70%N₂) and load (~1.0 kPa). It is assumed that the bed contraction measured experimentally using 10–12.5 mm particles is applicable to the blast furnace particle size distribution used in the global model.

Fig. 1.

(a) Schematic of softening and melting rig and (b) enlarged view of the crucible.¹⁹⁾ (Online version in color.)

During the test, the sample temperature, the contraction of the burden, and the pressure drop across the burden were measured, and the degree of reduction was monitored by analysing the off-gas composition using an infrared gas analyser. Melting in the bed at a higher temperature was confirmed by visualising the dripping of molten droplets in the collecting tray underneath the test rig. In addition, a series of interrupted tests were carried out at different temperatures varied from 1000°C to 1450°C wherein the samples were heated first and then cooled quickly by turning off the furnace and passing N₂ gas through the bed. After cooling, the graphite crucibles containing the ferrous and coke particles were scanned using Computed tomography (CT) scanning technology.

2.3. Scanning Techniques

The quenched samples were scanned using the Synchrotron X-ray CT facility at the Imaging and Medical Beam Line (IMBL) (3 GeV energy), Australian Synchrotron, Melbourne, to understand the 3D void network and structure.²⁾ Noted that the quenched samples at 1450°C were scanned using neutron CT at the Australian Centre for Neutron Scattering (ACNS) to avoid the higher attenuation of beam light from the ferrous layer. For details of CT scanning properties such as beam intensity, camera resolution, filters, etc., the readers are referred to Liu et al.²⁾

The CT and neutron scattering images were reconstructed using parallel computing systems and used for further image processing. The voxel size of the images from X-ray CT and neutron scattering were ~31.5 and 50 μm/voxel, respectively. Different layers such as coke, ferrous and void were then segmented from the scanned images based on the artificial intelligence method using the material simulator GeoDict (v2021). After segmentation, a quantitative analysis was carried out over these images to estimate the volume fraction of coke, ferrous and void layers as a function of bed height and reconstruct the three-dimensional (3D) structure.

In addition, the scanned samples were set in resin, sectioned, and polished to study the two – dimensional (2D) structures and the chemical composition of the ferrous materials using optical microscopy and TIMA (Tescan Integrated Mineral Analyser). The TIMA system comprises the TIMA hardware and Version 1.5.50 TIMA software. Featuring four Energy Dispersive X-ray (EDX) detectors attached to the chamber of the VEGA (thermionic emission – tungsten) platform, the system is capable of three types of analysis (Modal, Liberation and Bright Phase Search) with different acquisition modes (High resolution mapping, Point spectrometry, Line mapping and Dot mapping).²⁰⁾

2.4. BF Modelling Using S&M under Load Test Results

As discussed in a former work,¹⁸⁾ a combination of experimental study and numerical modelling was proposed to further the knowledge in the complex BF ironmaking process. The measured contraction – temperature (Sr-T) curves from S&M under load tests were inputted to a global BF model (a 2D BF model with the dual aim of visualizing the internal state of the furnace and predicting the effect of changes in operation) to predict the BF performance of different ferrous burdens.^21,22,23) The method is as follows:²⁴⁾

a) the experimental contraction – temperature curves were converted to dimensionless Sr-T relationships using the regression analysis.   

$$S_r=a+b{T}^{\prime }+C{T^{\prime }}^2+ d{T^{\prime }}^3+e{T^{\prime }}^4+ f{T^{\prime }}^5$$(1)

  

$$T^{\prime }=\left(T_{sample}-T_{ms}\right)/T_0$$(2)

where T’, T_sample and T_ms are the dimensionless temperature, the sample temperature and the initial softening temperature, respectively; a, b, c, d, e, and f are constants. Noted that the value of T₀ is 1000°C.

b) to incorporate the Sr-T relationships in the model, the initial definition of the CZ (the temperature range between 1200°C and 1400°C) in the global model was replaced by the temperature range between 0.05 and 0.75 contractions of ferrous burdens and the code was modified to SR = MIN(MAX(0, a+bT′+CT′²+dT′³+eT′⁴+fT′⁵), 0.75).

c) the modified Sr-T relationships were inputted to the global BF model based on a calibration run for a practical 3200 m³ BF, with a predominantly sinter burden mix.

3. Results and Discussion

3.1. S&M Behaviour

Figures 2(a)–2(d) shows the averaged S&M under load test results for SH1, NBLL, and MB1. Similar to former studies of C. E. Loo, L. T. Matthews and D. P. O’dea³⁾ and X. Liu, T. Honeyands, S. Mitra, G. Evans, B. Godel, R. G. Acres, F. Salvemini, D. O’Dea and B. Ellis,²⁾ the 100% NBLL burden contracted much faster than the sinters below 1400°C and the lowest softening temperature (T_s, the temperature at which the burden contraction reaches 50%) was obtained for the NBLL lump, followed by higher basicity sinter (SH1). In other words, the softening temperature of individual ferrous materials increased with the basicity; but the melting temperature (T_m, the temperature at which the pressure drops return to that at T_s) of SH1 was higher than that for NBLL. Furthermore, the pressure drop across the 100% lump burden (NBLL) was obviously higher than the sinter at the same contraction level, and a much higher S_value (the area between the pressure drop-temperature curve and 50% contraction baseline) was obtained. In general, it is inferred from these laboratory tests that, compared with sinter, 100% NBLL lump burden could form a thicker and less permeable cohesive zone (CZ) with a higher position in the BF.

Fig. 2.

S&M under load test results for lump, sinters, and mixture, (a) averaged contraction vs. temperature, (b) averaged contraction vs. pressure drop, (c) averaged indices of S&M test, and (d) degree of reduction. (Online version in color.)

However, NBLL lump demonstrated a very different behaviour when mixed with sinter, as it would be in a practical BF operation. As shown in Figs. 2(a) and 2(b), the contraction – temperature and pressure drop – contraction curves of the mixed burden (MB1) were both similar to the results for 100% higher basicity sinter (SH1) rather than the lump. In addition, the softening temperature of MB1 lay between the results for NBLL and SH1 only burdens, while the melting temperature, maximum pressure drops, and S_value of the MB1 were all lower than both individual components (see Fig. 2(c)). From Fig. 2(d), it can be observed that the averaged sinter and mixed burden degree of reduction follow a similar trend, while the degree of reduction of lump demonstrates different behaviour. At the beginning of the S&M process, NBLL had a higher reduction degree than sinter up to a temperature of 950°C. However, above this temperature, the reduction degree of MB1 and SH1 exceeded the reduction degree of NBLL. Finally, the degree of reduction of NBLL increased significantly at the end of the test due to the direct contact between liquid iron-bearing slag and coke. These results proved that the interaction between sinter and lump largely improved the S&M behaviour of the mixture and made it better than both its individual components.

3.2. Physical Interaction in S&M Process

The samples from interrupted S&M under load test at 1000°C, 1100°C, 1200°C, and 1300°C were scanned using the X-ray CT scanning, and the 1450°C sample was scanned using the neutron CT. Figure 3 shows the percentage of voidage in ferrous layers as a function of temperature for different burdens. It can be observed that the high basicity sinter SH1 presented a linear relationship with the temperature while that of NBLL decreased as a function of temperature. Below 1200°C, the voidage of ferrous layers for SH1 was always higher than for the mixed burden MB1 and NBLL. However, the voidage of the ferrous layer for 1300°C MB1 sample was even higher than that for SH1 and NBLL, indicating that MB1 formed a more permeable burden at that temperature. For instance, the reconstructed 3D structure of the three ferrous burdens at 1450°C is shown in Fig. 4. It appears that the bed height of the NBLL burden was clearly smaller compared to the burden SH1 and MB1, and an apparent penetration of the ferrous material into the bottom coke layer was also noticed. Noted that it was not always possible to completely segregate the coke and void layer in the middle ferrous layer due to very close grey scale value of these two phases. This anomaly has been reported in our previous work¹⁹⁾ in detail.

Fig. 3.

Voidage of ferrous layers for different ferrous burdens. The dotted lines represent the fitted curves. (Online version in color.)

Fig. 4.

Reconstructed 3D structure of different ferrous burdens at 1450°C (a) SH1 (b) MB1 and (c) NBLL. (Online version in color.)

Figure 5 shows the 2D NBLL bed structure at different temperatures using optical microscopy. At 1000°C, the samples showed a heterogeneous reduction across the burden, i.e., higher reduction of the top section as reducing gas was introduced from the top. Some particles showed a typical distinctive dark core (mainly wustite), and light shell (mainly metallic iron) structure and evident internal cracks have been formed due to the dehydration of goethite and reduction of hematite.²⁵⁾ Some particles have been compressed to connect with each other due to the load but many voids still existed in the burden. It was noted that no melt was found in the cross section of the burden. After the temperature was raised to 1100°C, the burden was further compressed decreasing the inter-particle voids in the burden, the metallic iron shells were thicker and more intra-particle cracks and voids were generated without melt existing in the inter-particle voids; in addition, the coke and ferrous layers became slightly intermixed at this temperature. In the temperature range from 1000°C to 1100°C, the contraction of the burden was relatively fast and consistent. Therefore, the NBLL contraction was mainly caused by the particle movement in the bed because of changes at the inter-particle contact, which resulted from particle size and shape changes while still predominately in the solid-state.

Fig. 5.

Optical microscopic images of two-dimensional (2D) bed structure of NBLL. Below is higher magnification of above images.

At 1200°C, the lump samples were further reduced but still showed the metallic iron shell – wustite core structure, the particles were in more intimate contact and deformed into each other, resulting in a much lower bed voidage and an obvious bed shrinkage compared with that for 1100°C. Slag or melts were found in the inter-particle pores and intra-particle pores of the metallic iron shell. In the case of some particles, clear gaps can be seen between the metallic iron shell and the wustite core, which might be caused by the melting and flowing out of the core.²⁷⁾ Due to the low inter-particle space for shrinkage and insufficient melt, the bed contraction was limited, and the contraction rate began to decrease. There were no significant changes in the bed when the temperature reached 1300°C except that the metallic iron shells were thicker with further reduction; the contraction rate was consistently low at this region due to the lack of void space and formation of melts.

At 1450°C, the NBLL burden showed a very distinct empty metallic iron shell structure, which was caused by the melting and flowing out of the core,²⁷⁾ resulting in an increase in the bed voidage measured by CT scanning. Due to the lack of support by the cores, the metallic iron shells were easy to deform and compress, so the sample presented a very sharp increase in the contraction rate, and undoubtedly, followed by a fast decrease in the contraction rate after the compression of the empty metallic iron shells.

Figure 6 shows the 2D bed structure of SH1 at different temperatures after CT scanning. Firstly, it is very clear that the reduction was relatively uniformly across the bed; the voidage in SH1 was visibly higher than that for NBLL at the same temperature; unlike the lump particles, SH1 particles didn’t have the distinct shell and core structure as the reduction was more uniform. Although the bed voidage was slightly decreasing, there was no significant changes in the bed structure from 1000°C to 1200°C, and the bed was still very permeable. It also can be seen from the microstructures that the intra-particle porosity of SH1 particles were also decreasing with temperature, indicating the rapid contraction of SH1 in this temperature range was caused by the coalescence of both inter and intra-particle porosities. At 1300°C, the SH1 burden was further squeezed, and an obvious reduction in the void fraction was identified, but the voidage was obviously higher than that of NBLL at the same temperature; it can be inferred that the contraction in this step was mainly caused by the changes in the inter-particle structures. At 1450°C, both the inter and intra-particle porosity were reduced to a low level and slag phases flowed out of the samples.

Fig. 6.

Optical microscopic images of two-dimensional (2D) bed structure of SH1. Below is higher magnification of above images.

Figure 7 showed the 2D bed structure of MB1, which mainly represents the interface between SH1 and NBLL at different temperatures. Until 1200°C, the MB1 bed behaved very similar to that of SH1; the bed was quite permeable with the NBLL particles showing metallic iron shell – wustite core structure, or in other words, before the chemical interaction, sinter was dominant in the mixed burden, and NBLL behaved independently. At 1300°C, although the SH1-NBLL interfaces were tightly compressed, the inter-particle pores between SH1 particles were still obvious. Therefore, the bed permeability can be maintained. When the sample temperature reached 1450°C, no empty metallic iron shell structures were found. There was no clear difference between the macro structures of sinter and lump, and the lump structure appeared coarser than that of SH1.

Fig. 7.

Optical microscopic images of two-dimensional (2D) bed structure of MB1. Below is higher magnification of above images.

3.3. Chemical Interaction in S&M Process

It has been proven that chemical interaction exists between basic sinter and lump in the S&M process,^2,28) which is mainly caused by the Ca transfer from sinter to lump. Therefore, the CT scanned samples were analysed using TIMA to study the Ca transfer behaviour. Figure 8 shows the BSE (backscattered electron) images and Ca maps of the SH1-NBLL interface at different temperatures. As shown, no Ca was found in the lump until 1200°C although well – connected interfaces have been formed between the SH1 and NBLL particles. At 1200°C, Ca was found in the NBLL particle and the diffusion depth was around 300 μm, the Ca concentration in NBLL particle decreased with the distance from the SH1 – NBLL interface and was clearly lower than that for SH1, i.e., the Ca transfer from SH1 to NBLL started around 1200°C under the experimental condition. At 1300°C, although there still was an obvious gradient between the Ca concentration of SH1 and NBLL, the Ca was detected in almost the entire NBLL particle, and no gap existed between SH1 and NBLL particles. At 1450°C, little difference was found in the Ca concentration of NBLL and SH1 particles, and the particles can only be differentiated by the metallic iron structure.

Fig. 8.

Ca map of SH1-NBLL interface at different temperatures. (Online version in color.)

In summary, both physical and chemical interactions exist between SH1 and NBLL. The changes in the bed structure caused by the physical interaction generated more voids in the burden, while the chemical interaction improved the mineral composition of the melts. Therefore, the S&M behaviour of the SH1-NBLL burden was largely improved.

3.4. FactSage Calculation

The liquid slag formation during the S&M process was modelled by thermodynamic equilibrium calculations. A system of slag containing CaO, MgO, FeO, Al₂O₃, and SiO₂ was considered for these calculations (see Table 1). The composition of slag samples was calculated at various stages of S&M test based on the degree of reduction and its corresponding temperature (Fig. 2(d)). FactSage software (v. 8.0) was used to equilibrate slag components, and it was assumed that oxygen was provided by oxides present in the slag system. Thermodynamic data was extracted from FactPS and FTOxid databases and based on the results of minimization of Gibbs free energy, stable phases were determined.

Figure 9 plots the percentage of liquid slag as a function of temperature during the S&M test. Based on this figure, increasing temperature caused an increment in the amount of liquid slag for all samples. However, the melting behaviour of each sample is different. For instance, the NBLL sample starts to melt at lower temperature and reaches 60% liquid at about 1170°C, while the SH1 and MB1 samples reach this value at about 1250°C. On the other hand, NBLL contains a considerable amount of liquid slag (more than 60%) in a wider temperature range compared to the two other samples. MB1 and SH1 samples require about 80 degrees higher temperatures to reach the same amount of liquid slag. This enables us to load mixed burdens instead of lumps in blast furnaces and obtain similar softening and melting behaviour to sintered burdens. One of the advantages of the mixed sinter-lump burden over 100% lump is the higher softening temperature which, in turn, promotes the kinetics of iron oxide reduction.

Fig. 9.

Percentage of liquid slag as a function of temperature for different burden. (Online version in color.)

In addition, results of S&M test also verified that softening temperature (Ts) of NBLL is much lower than that of two other samples, which is consistent with thermodynamic predictions. Furthermore, according to the 2D bed structure for the NBLL sample (Fig. 5), the slag phase appeared in these images at 1200°C, which also agreed well with the FactSage prediction of 1160°C. Also, the formation of liquid slag at a lower temperature for NBLL samples and filling the intra-particle spaces explains the lower permeability in NBLL samples and consequently less voidage during the S&M test (see Fig. 4).

3.5. Whole BF Performance

The combination of the numerical modelling and experimental study provides a possibility to extend the work from S&M behaviour to the whole BF performance. The calculated Sr-T relationships that were used as inputs to the global BF model are shown in Eqs. (3), (4), (5). The comparison between the experimental contraction – temperature curve and the calculated Sr-T relationships are shown in Fig. 10. As shown, all the calculated Sr-T relationships showed great consistency (R²>0.999) with the measured contraction – temperature curves.   

$$\mathrm{S}\mathrm{H}1:\hspace{1em}{S}_r=0-0.154T^{\prime }+3.774{T^{\prime }}^2-2.026{T^{\prime }}^3$$(3)

  

$$\mathrm{M}\mathrm{B}1:\hspace{1em}{S}_r=0-0.217T^{\prime }+3.609{T^{\prime }}^2-2.160{T^{\prime }}^3$$(4)

  

$$\begin{array}{l}\mathrm{N}\mathrm{B}\mathrm{L}\mathrm{L}:\hspace{1em}{S}_r=0.9427T^{\prime }-10.567{T^{\prime }}^2+70.148{T^{\prime }}^3\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}-139.01{T^{\prime }}^4+87.115{T^{\prime }}^5\end{array}$$(5)

Fig. 10.

Experimental and modelling Sr-T relationships for different burden samples. (Online version in color.)

The global BF modelling results for different ferrous burdens are shown in Table 2 and Fig. 11. For the 100% NBLL burden, it showed a much thicker W-shaped CZ with a very broad roof near the wall and a significantly higher position in the BF (top side temperature at 869°C), a much higher pressure drop, and an obviously lower gas utilization rate. For the mixed burden MB1, it behaved more like the basic sinter SH1 rather than the NBLL, and the top-side of its CZ was also between that for SH1 and NBLL; the bottom side of the CZ was lower than both individual components, showing an opposite trend with the experimental results. Compared with the sinter-only burden SH1, the MB1 had a relatively thicker CZ but a lower total pressure drop and higher top gas utilization (see Table 2). These results indicated that mixing ~20% lump with the basic sinter has a similar whole BF performance with the sinter only burden, and it performed much better than the lump only burden.

Table 2. BF modelling results for different burdens.

Burdens	CZ properties/°C			Total pressure drop/atm	Top gas CO2/(CO+CO2)/%
	Top	Bottom	Thickness
NBLL	869	1436	567	0.64	55.7
SH1	1060	1463	403	0.49	59.0
MB1	996	1473	477	0.50	58.6

Fig. 11.

Solid temperature isotherms (°C) in BF from the global modelling for the different burdens (NBLL, SH1, MB1). (Online version in color.)

In general, the numerical modelling results showed consistent results with the S&M under load test but with much more useful information on the whole BF performance. It needs to be noted that this work is an initial study applying the experimental results to the numerical BF modelling; some simplified relationships and conditions were used, including that the variation of coke and flux rates for different burdens was not considered. More realistic conditions and more complex relationships, e.g., the bed voidage – pressure drop, or pressure drop – temperature curves from the experiments, are currently being considered as inputs to the global BF modelling to improve the accuracy of the results. Further studies involving various combinations of lump and sinter burdens are also currently underway to quantify the extent of the lump-sinter interactions, and to establish the effect of the interaction on overall blast furnace performance.

4. Conclusions

In this work, both the physical and chemical interaction between a basic sinter and Newman Blend Lump were examined, and the whole BF performance of different ferrous burdens were evaluated using a global BF model based on the experimental results. The following conclusions were obtained:

(a) A mixed burden comprising ~21% NBLL lump and basic sinter (R = 1.85) performed better in the S&M under load test than the individual raw materials, exhibiting a much narrower CZ and higher burden permeability, very similar to that for 100% sinter. More importantly, compared with 100% sinter, the MB1 also showed a comparatively narrower CZ temperature range, lower maximum pressure drop, and higher permeability, suggesting the mixed burden composed of sinter with higher basicity and the porous lump is more appropriate for BF practice. In addition to decreasing the proportion of sinter required, a mixed burden incorporating lump and sinter with higher basicity has the added benefits of decreasing sinter plant fuel rate, increasing productivity, improving sinter strength, size, reduction degradation, and reducibility.

(b) X-ray and neutron CT was able to scan samples from interrupted S&M under load test to directly measure the bed voidage in ferrous and coke layers, quantifying the physical interaction. The sinter-lump mixture had a similar ferrous layer voidage with that for the sinter and higher than that for the lump.

(c) Under the experimental conditions, the Ca transfer from SH1 to NBLL started around 1200°C, then diffused through most of the lump particle at 1300°C but with an obvious concentration gradient, before the Ca was uniformly distributed in SH1 and NBLL at 1450°C.

(d) FactSage thermodynamic modelling predicts that the NBLL sample starts to melt at lower temperatures and reaches 60% liquid at about 1170°C, while the SH1 and MB1 samples reached this value at about 1250 and 1260°C, respectively.

(e) The BF modelling results also indicated that the whole BF performance of SH1-NBLL mixture was very similar to that of SH1; the sinter-lump mixture showed a narrower CZ, lower total pressure drop, and higher gas utilization rate. It needs to be noted that this work is an initial study applying the experimental results to the numerical BF modelling; some simplified relationships and conditions were used, including that the variation of coke and flux rates for different burdens was not considered. More realistic conditions and more complex relationships, e.g., the bed voidage-pressure drop, or pressure drop – temperature curves from the experiments, are currently being considered as inputs to the global BF modelling to improve the accuracy of the results.

Acknowledgements

The authors gratefully acknowledge the funding of the Australian Research Council in supporting the ARC Research Hub for Advanced Technologies for Australian Iron Ore at the Newcastle Institute for Energy and Resources and industrial partner BHP for their financial support and permission to publish this paper. Authors would also like to acknowledge the assistance provided by Dr. Anton Maksimenko at IMBL, Australian Synchrotron, Melbourne for tomography; Dr. Floriana Salvemini for assisting with the neutron scanning at DINGO, ANSTO, Sydney.

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