J. Cent. South Univ. (2021) 28: 690-698

DOI: https://doi.org/10.1007/s11771-021-4638-5

Reducing process of sinter in COREX shaft furnace and influence of sinter proportion on reduction properties of composite burden

SHI Ben-jing(师本敬)^{1, 2, 3}, ZHU De-qing(朱德庆)⁴, PAN Jian(潘建)⁴,HU Bing(胡兵)^{1, 2, 3}, WANG Zhao-cai(王兆才)^{1, 2, 3}

1. Zhongye Changtian International Engineering Co., Ltd., Changsha 410205, China;

2. Process Research Institute, National Engineering Research Center of Sintering and Pelletizing Equipment System, Changsha 410205, China;

3. Key Laboratory of Hunan Province for the Synergetic Control and Resource Reuse of the Multi-Pollutants of Flue Gas, Changsha 410205, China;

4. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: In order to reduce the materials cost of COREX ironmaking process, sinter has been introduced into the composite burden in China. This work explored the reducing process of sinter in COREX shaft furnace to clarify its reduction properties change and then the effect of sinter proportion on metallurgical performance of composite burden was investigated. The results show that the reducing process of sinter in COREX shaft furnace was basically same with that in blast furnace but sinter seems like breaking faster. Under reducing condition simulated COREX shaft furnace, sinter possessed the worst reduction degradation index (RDI) and undifferentiated reduction index (RI) compared with pellet and iron ore lumps. Macroscopic and microscopic mineralogy changes indicated that sinter presents integral cracking while pellet and lump ore present surface cracking, and no simple congruent relationship exists between cracks of the burden and its ultimate reduction degradation performance. The existence of partial metallurgical performance superposition between composite and single ferrous burden was confirmed. RDI_+6.3≥70% and RDI_+3.15≥80% were speculated as essential requirements for the composite burden containing sinter in COREX shaft furnace.

Key words: sinter; COREX shaft furnace; reducing process; composite burden; reduction degradation index (RDI); metallurgical performance

Cite this article as: SHI Ben-jing, ZHU De-qing, PAN Jian, HU Bing, WANG Zhao-cai. Reducing process of sinter in COREX shaft furnace and influence of sinter proportion on reduction properties of composite burden [J]. Journal of Central South University, 2021, 28(3): 690-698. DOI: https://doi.org/10.1007/s11771-021-4638-5.

1 Introduction

As a mature smelting reduction technology for iron making, the COREX process is the first one that realized industrial iron making production among various smelting reduction processes. Since non-coking coal could account for more than 90% of the fuel requirement for iron making, COREX process has grown correspondingly less dependent on the metallurgical coking coal resource which is indispensable for ironmaking in blast furnace (BF) process [1-4]. China has been the country that produced the most iron and steel on the world for years, and also has the advantage of vast non-coking coal resources reserve for developing COREX process as a promising alternative route to BF process.

In the past decades, Baosteel which is a large-scale iron and steel enterprise in China successively introduced two sets of COREX devices and started producing, but both had poor economic performance. Then they were shut down in a few years and moved to Xinjiang Province where the coal resources were abundant and cheaper [5, 6]. Apart from technical flaws, the supply shortage and higher price of pellets in China has become important factors that increase the iron making cost in COREX process, which causes its operational economy to decrease and affects further development [7]. In order to reduce materials cost, the sinter that was produced with surplus capacity and lower cost compared to pellet was innovatively introduced into the burden for COREX process in Bayi steel. However, the gas distribution in the shaft furnace became abnormal when the sinter proportion in composite burden exceeded a certain level and even the operation control system was alerted for hanging. Consequently, the sinter proportion in composite burden was limited to only 20%-25% [8]. Considering different reduction characteristics of sinter, pellet and iron ore lump, these abnormal conditions accompanied with high proportion of sinter in shaft furnace burden were speculatively attributed to the worse reduction degradation performance of sinter. There is a great deal of research indicating that the low-temperature reduction degradation of sinter could greatly affect the permeability of the burden and BF operation [9-11]. Whereas, to what extent the sinter proportion could affect the reduction degradation performance and reduction index of composite burden in COREX shaft furnace was not entirely clear. This is a critical issue for increasing the charging proportion of sinter in composite burden and reducing material cost of COREX process further. Besides, reducing gas in COREX shaft furnace contains 15%-20% H₂ and CO+H₂ occupy exceeds 90%. In consideration of the significant difference of reduction condition between COREX and BF processes, metallurgical properties of sinter in shaft furnace also should be better understood.

The objectives of this work are to clarify the reduction properties of sinter, including reduction degree, reduction degradation and mineralogy changes in specific stage during reducing process in COREX shaft furnace, and to illuminate the effect of sinter proportion on the metallurgical performance of composite burden, which will provide reference for the improvement of accurate predictions and charging operation of the COREX shaft furnace. At reducing condition simulating COREX shaft furnace, the reduction characteristic, reduction degradation characteristic and mineralogy of sinter in different stages during reducing process were tested and analyzed. Simultaneously, the reduction index (RI) and reduction degradation index (RDI) of sinter were compared with pellet and iron ore lump at specific reducing condition. Then the variation in metallurgical performance of the composite burden with different proportions of sinter was investigated experimentally and theoretically.

2 Experimental

2.1 Materials

The materials used in the present work included sinter, pellet and iron ore lump. The sinter samples were prepared in lab scale with a 180 mm diameter and 1000 mm length sinter pot. The pellet and lump ore were taken from the production site of COREX shaft furnace. The chemical compositions of materials used in the present work are shown in Table 1. It indicates that the iron grade of sinter is obviously lower than pellets or lump ore. But given that some additional fluxes such as limestone, dolomite and silica were needed for the shaft furnace, taking sinter into the composite burden is a viable option. All the materials were prepared with a size of 10-12.5 mm. Besides, reducing agents used in this work such as CO, H₂, CO₂ and N₂, were all standard cylinder gas with a purity of 99.9%.

Table 1 Chemical compositions of materials (wt%)

2.2 Methods

All the reducing tests in the present work were carried out in a static gas-solid reaction apparatus as shown in Figure 1. Each test employed 500 g of samples, including sinter, pellet and iron ore lump. The sample was reduced in a reaction tube of 75 mm diameter and 800 mm length, which could resist the temperature above 900 °C. In order to simulate reducing condition in COREX shaft furnace, the reducing temperature and gas composition were adapted accordingly.

Figure 1 Schematics of experimental apparatus for reduction experiments:

2.2.1 Reduction properties of sinter in shaft furnace

The experiments for reduction characteristic and reduction degradation characteristic of sinter in different reducing stages in COREX shaft furnace were conducted firstly. The sinter samples were put into the reaction tube and heated with a heating rate of 2.5 °C/min from air temperature. Meantime, 13.3 L/min pure N₂ was introduced into the tube, and then it was replaced by reducing gas with a same gas flow as the temperature reached 250 °C. Simulating the reducing condition in COREX shaft furnace, the original reducing gas consisted of 35% CO, 50% CO₂ and 15% H₂. Its composition changed gradually to 72% CO, 8% CO₂ and 20% H₂ in the next 240 min, while the temperature increased simultaneously to 850 °C, as shown in Figure 2.

When the temperature reached 375, 425, 450, 475, 500, 525, 575, 625, 700, 750, 800 and 850 °C, respectively, the heating and reducing gas were both interrupted and 13.3 L/min pure N₂ was introduced into the tube again to cool the reduced sample. The cooled sinter samples were weighed to calculate their reduction degree (RD):

Figure 2 Changes of temperature and reducing gas composition during reduction

             (1)

where m₁ is the mass of sinter before reduction; m₂ is the mass of sinter after reduction; w₁ and w₂ are the FeO and total Fe content of sinter, respectively.

After being weighed, the sinter samples were charged into a with tumbling drum 130 mm diameter and 200 mm length. The drum rotated for 10 min with a 30 r/min speed. After tumbling, the particle size distribution of samples was determined by sieving with 6.3, 3.15 and 0.5 mm mesh. Then the reduction degradation degree (RDD) was calculated as follows:

                       (2)

                      (3)

                      (4)

where m is the mass of reduced sinter sample; m₃ and m₄ are the mass of particles above 6.3 and 3.15 mm, respectively; m₅ is the mass of particles below 0.5 mm.

For the mineralogy of samples, the optical microscopy (Leica DMI5000M, Germany) was employed to analyze the changes of mineralogy of sinter during different reduction stages. The scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) (PhenomPro, Waltham, MA, USA) was used to distinguish mineral phases in sinter.

2.2.2 Metallurgical performance of burden in shaft furnace

The metallurgical performance of burden was evaluated by RI and low-temperature RDI. In terms of RI, samples including sinter, pellet and iron ore lump were reduced at 820 °C for 180 min with a 13.3 L/min reducing gas, which contained 68% CO, 9% CO₂ and 23% H₂. Then the RI was determined by the changes in mass of samples before and after reduction. Its calculation was in the same way as that of the RD, which is shown in Eq. (1). For the tests of RDI, samples were reduced at 550°C for 30 min with a 13.3 L/min reducing gas, which consisted of 35% CO, 50% CO₂ and 15% H₂. Then the RDI of samples was tested and calculated by the same methods for RDD analysis, as shown in Eqs. (2)-(4).

3 Results and discussion

3.1 Reducing process of sinter in COREX shaft furnace

In actual production, the temperature and reducing gas concentration increase gradually in the downward direction of COREX shaft furnace. During reduction process that simulated COREX shaft furnace, the variations of RD and RDD are shown in Figure 3. It indicates that the reduction degree kept rising in the whole reduction process, while the reduction degradation performance deteriorated rapidly before 550 °C and then got a slight upturn. With 550 °C as a division line, the reduction process of sinter in shaft furnace underwent two stages. In the first stage that temperature increased from 375 to 550 °C, the RDD_+6.3 and RDD_+3.15 of sinter decreased from nearly 100% to 29.85% and 62.65%, respectively,while the RDD_-0.5 rose from zero to 6.62%. Meantime, the RD increased to 8.86% gradually. In the second stage that temperature exceeded 550 °C, the RDD_+6.3 and RDD_+3.15 of sinter increased gradually from each nadir to 42.82% and 75.50%, respectively, while the RD picked up quickly to 92.81%.

Figure 3 Variations of RD and RDD of sinter during reduction process in COREX shaft furnace

Earlier researches that were conducted in condition simulated blast furnace, which is a non-hydrogen reduction process, have shown that the phenomenon of sinter degradation took place within a low temperature zone (400-600 °C) in the upper shaft of blast furnaces [10-13]. Experimental results in this work indicate that the reduction properties of sinter under reducing condition simulated COREX shaft furnace were similar to those in BF. However, the sinter had obvious higher reduction degree and worse reduction degradation performance. Meanwhile, the reduction degradation of sinter mainly occurred in the temperature-rising reduction stage of 375 to 550 °C when the corresponding reduction degree was quite low. Consequently, sinter broke faster in COREX shaft furnace than in blast furnace.

During reduction, the volumetric expansion of the iron oxide phases generated by transformation of hematite (Fe₂O₃) to magnetite (Fe₃O₄) was generally deemed as the main factor that causes degradation [12-14]. By means of EDS measurement of original sinter for mineral identification as shown in Figure 4 and Table 2, optical microscopic analysis for mineralogy of sinter during reduction process conducted in this work confirmed this point of view.

Figure 5 shows the microstructure of sinter samples in different specific reduction stages. It indicates that the mineralogy of sinter remained typical microstructure and mineral composition of usual high-basicity sinter, which consists of Fe₂O₃, Fe₃O₄, silicoferrites of calcium and aluminum (SFCA), glass phase and some small pores, after undergoing reduction process lasted to 375 °C, as shown in Figure 5(a). Figure 5(b) shows that the mineralogy of sinter reduced in reduction process lasted to 450 °C changed significantly. It can be clearly seen that Fe₂O₃ has been partly reduced to Fe₃O₄ and many cracks occurred in surrounding area. Figure 5(c) shows the internal reducing reaction preceded to a greater level after sinter was reduced in stage lasted to 550 °C. It indicates that more Fe₂O₃ has transformed into Fe₃O₄ compared with former stage. Meanwhile, many longer cracks occurred and cross linking with each other. The degradation of sinter peaked at this reduction stage. When the reduction process lasted to 700 °C,metallic iron could be observed diffusely inside the sinter as shown in Figure 5(d). The change in mineralogy of sinter visually shows the degradation mechanism of sinter in COREX shaft furnace and corresponds well to former relevant research results.

Figure 4 Chemical compositions of different mineral phases in sinter:

Table 2 Chemical composition of different mineral phases (%)

3.2 Reduction properties of different shaft furnace burden

Under reducing condition simulated COREX shaft furnace, the RDI and RI of sinter, pellet and iron ore lumps were investigated respectively. The results are shown in Figure 6. It shows that the possessed pellet has both the best reducibility and reduction degradation performance among three kinds of burden，followed by lump ore. The sinter had a remarkable RI that reached 87.97%. Though it was lower than pellet (92.10%) or lump ore (89.91%), but as to RDI, sinter was much lower than other two kinds of burden. The RDI_+6.3 and RDI_+3.15 of sinter were only 24.52% and 61.83%, respectively, which were less than that of the pellet by 67.04% and 30.64%, also less than that of the lump ore by 50.61% and 21.83%. Meanwhile, the sinter has the highest RDI_-0.5 (7.58%) among three kinds of burden. Compared with the pellet and lump ore, the worst low-temperature reduction degradation performance of sinter means that more powder would be generated in particular position or zone in the COREX shaft furnace after sinter was proportioned into burden, which has a significant adverse effect on the reducing gas distribution and then affects the operation for production.

Figure 5 Mineralogy of sinter in specific stage during reduction process under different temperature:

Figure 7 shows macroscopic appearance of the pellet, iron ore lump and sinter after reduction that for RDI-testing. It illustrates that more cracks appeared on the surface of iron ore lump than other two kinds of burden, and little difference on surface crack between the pellet and sinter was observed. This visual evaluation result is entirely different from the results of RDI test as shown in Figure 5, which means that there is no simple congruent relationship between macroscopic cracks on the burden generated during reduction and its ultimate reduction degradation performance.

Figure 6 RDI and RI of different kinds of burden in COREX shaft furnace

In industrial furnaces for iron making, the iron ores are especially susceptible to external pressure generated by the burden besides of the innerstress accompany with reduction. In the RDI tests, the rotation of tumbling drum could be regarded as the simulation of external pressure. So the reduction degradation is a result of double effects of external loads and internal stress as stated in previous relative studies [15-17]. These researches also reported that the highest porosity of sinter compared with pellet and iron ore lump made it easier for the reducing gas to enter inside sinter in reduction process. With the going of reduction, the innerstress generated by reduction was widespread in the sinter, which has been confirmed by the cracks inside sinter as shown in Figure 5. This resulted in the fact that sinter presents integral cracking while pellet and lump ore present surface cracking. Consequently, the degradation of sinter was the most serious under the external-load function.

3.3 Influence of sinter on RDI and RI of composite burden in shaft furnace

In order to clarify the effect of sinter on the metallurgical of composite burden in COREX shaft furnace, the RDI and RI of composite burdens with different sinter proportion were investigated in corresponding reducing condition. The pellet and lump ore occupied 80% and 20%, respectively, in the initial composite burden. When the sinter proportion increased, the ratios of pellet and iron ore lumps decreased in proportion. The variations of RDI and RI of composite burden with different sinter proportions are shown in Figure 8. It suggests that with the proportion of sinter in composite burden increased from 0 to 100%, the RDI_+6.3 and RDI_+3.15 of burden decreased from 87.60% and 88.64% to 24.52% and 61.43%, respectively, and the RDI_-0.5 rose from 3.04% to 7.58%. Meanwhile, the RI of burden also slightly fell from 92.81% to 87.97%. Overall, the effects of increasing sinter proportion on RDI_+6.3 and RDI_+3.15 of the composite burden were more distinct than that on RDI_-0.5 or RI.

Figure 7 Macroscopic appearance of different burden after reduction for RDI-testing:

Figure 8 Effect of sinter ratios on RDI and RI of COREX furnace burden

According to the experience in industrial production, the COREX shaft furnace behaved normally when the sinter proportion was 25% in composite burden but performed abnormally when the sinter proportion reached 30%. While in the results of laboratory tests, under the above two conditions of sinter proportion, the RDI_+6.3 of composite burden was 71.23% and 68.13% respectively, and the RDI_+3.15 of composite burden was 81.96% and 79.46%, respectively, as shown by points A-D in Figure 8. Thus the critical points of the RDI_+6.3 and RDI_+3.15 for good operation of shaft furnace could be speculated to around 70% and 80%, respectively. The points could be understood as the least requirements of low-temperature reduction degradation performance when prepared composite burden for the COREX shaft furnace.

Besides, a previous research concerning metallurgical properties of ferrous burdens in blast furnace showed that it is superimposed for mix burden in reducibility (RI) and reduction degradation (RDI_+3.15) [18]. In this study, the relationships of theoretical and experimental metallurgical performance between single burden and composite burden were compared. The theoretical results were obtained by weighted calculation, given the RDI and RI of single burden and its proportion in composite burden. As shown in Figure 9, the calculated values for RDI and RI of composite burden were basically the same as the experimental values, and this marks the existence of metallurgical performance superposition among different kinds of ferrous burden in COREX shaft furnace. In order to further evaluate the reliability of the superposition quantitatively, the fitted curves of theoretical and experimental values of RDI and RI of composite burden were compared and analyzed. Table 3 shows characteristic parameter of above fitted curves. As to RDI_+6.3, RDI_+3.15 and RI, the characteristic parameters of fitted curves presented few differences between theoretical and experimental values. Meanwhile, the fitted curves of RDI_-0.5 for theoretical and experimental values possessed significant difference.

Figure 9 Comparison of experimental and theoretical values of RDI and RI of composite burden

These results demonstrated that for the composite burden in COREX shaft furnace, its metallurgical performance such as RDI_+6.3, RDI_+3.15 and RI can be preliminary predicted by weighted calculation, taken respective metallurgical performance and proportion in composite burden of each single burden. Moreover, the metallurgical performance of composite burden can also be optimized by improving respective metallurgical performance of each kind of burden. Consequently, to further increase current sinter proportion in composite burden and make full use of the advantages that sinter brought COREX ironmaking process, the worse reduction degradation performance of sinter compared with pellet and lump ore has been a major problem which requires particular attention and additional work.

Table 3 Characteristic parameters of fitted curves for RDI and RI of composite burden

4 Conclusions

The reduction properties of sinter and its effect on the metallurgical performance of composite burden in COREX shaft furnace were investigated in simulated reduction conditions, and conclusions were drawn as follows:

1) During reduction process simulating COREX shaft furnace, the reduction degradation of sinter peaked at 550 °C and then relieved slightly, while its reduction degree increased rapidly during the temperature zone of 550 to 850 °C. The mineralogical changes of sinter during reduction process in COREX shaft furnace were basically the same with that in BF, but the sinter seems like breaking faster in COREX shaft furnace than in BF.

2) Under reducing condition in COREX shaft furnace, sinter possessed almost undifferentiated and remarkable RI compared with pellet and iron ore lumps, but its RDI_+6.3 and RDI_+3.15 were considerably lower than pellet or iron ore lumps. There is no simple congruent relationship between macroscopic cracks of the burden generated by reduction and its ultimate reduction degradation performance. Sinter presents integral cracking while the pellet and lump ore present surface cracking, so the sinter degraded most seriously under the external-load function during reduction.

3) Sinter proportion had distinct adverse effect on RDI_+6.3 and RDI_+3.15 of the composite burden for COREX shaft furnace. There was superposition for RDI_+6.3, RDI_+3.15 and RI between composite and single ferrous burden. RDI_+6.3≥70% and RDI_+3.15≥ 80% were speculated as the low-temperature reduction degradation performance requirements when prepared composite burden for COREX shaft furnace.

Contributors

The overarching research goals were developed by ZHU De-qing, SHI Ben-jing and PAN Jian. SHI Ben-jing conducted the research experiments and analyzed the results. The initial draft of the manuscript was written by SHI Ben-jing. HU Bing and WANG Zhao-cai edited the draft. All authors replied to reviewers’comments and revised the final version.

Conflict of interest

SHI Ben-jing, ZHU De-qing, PAN Jian, HU Bing and WANG Zhao-cai declare that they have no conflict of interest.

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(Edited by ZHENG Yu-tong)

中文导读

COREX竖炉中烧结矿的还原过程及其对综合炉料还原性能的影响

摘要：为了降低COREX炼铁工艺的成本，中国在综合炉料中加入了烧结矿。本文研究了烧结矿在COREX竖炉中的还原过程，阐明了其还原性质的变化，并研究了烧结矿配比对综合炉料冶金性能的影响。结果表明，在COREX竖炉中烧结矿的还原过程与其在高炉中类似，但还原粉化发生得更快；在模拟COREX竖炉还原条件下，与球团矿和块矿相比，烧结矿的还原粉化性能(RDI)最差而还原性能(RI)差别不大。宏观和微观矿相分析表明，还原时烧结矿呈现整体开裂而球团矿和块矿均呈现表面开裂，炉料自身产生的裂纹与其最终还原粉化性能之间不存在简单的对应关系。COREX竖炉综合炉料与单一种类炉料之间存在部分冶金性能叠加性，配入烧结矿时综合炉料的低温还原粉化性能RDI_+6.3和RDI_+3.15应分别不低于70%和80%。

关键词：烧结矿；COREX竖炉；还原过程；综合炉料；还原粉化性能(RDI)；冶金性能

Foundation item: Project(2019JJ51007) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2020-07-24; Accepted date: 2020-11-18

Corresponding author: SHI Ben-jing, PhD, Intermediate Engineer; Tel: +86-731-82920527; E-mail:shi_benjing@126.com; ORCID: https://orcid.org/0000-0003-1315-6613; ZHU De-qing, PhD, Professor; Tel: +86-731-88836942; E-mail: dqzhu@csu.edu.cn