Reduction Behaviour of Olivine Iron Ore Pellets in the Experimental Blast Furnace

Transcription

1 Reduction Behaviour of Olivine Iron Ore Pellets in the Experimental Blast Furnace Si Hyung Lee 1), Rita Khanna 1), Bo Lindblom 2), Mats Hallin 2) and Veena Sahajwalla 1) 1) Centre for Sustainable Materials Research and Technology, School of Materials Science and Engineering, University of New South Wales, Sydney, NSW2052, Australia, 1) LKAB, SE Luleå, Sweden An experimental study was conducted to determine the reduction behaviour of olivine iron ore pellets and associated reduction mechanisms in the experimental blast furnace (EBF) located at Luleå. Two sets of EBF samples, namely slowly annealed excavated samples and rapidly quenched probe samples of olivine bearing iron ore pellets were examined in detail. Pellet samples were analysed using SEM, XRD and SIROQUANT analysis to quantitatively determine iron ore phase transformations during descent in the EBF. In the tested EBF campaign, up to 75% of reduction occurred at less than 1100 C, i.e. before the pellet reached the cohesive zone while rest of 25% reduction was completed when pellets reached a temperature of 1300 C and hence within the cohesive zone. The reduction degree of pellets was found to have a linear correlation with distance from the stock line of the EBF. This study showed that the presence of olivine did not have a significant effect on reduction degree for temperatures less than 1100 C in the upper zone of the EBF. However, olivine increased the reduction rate in the final stage of reduction for temperatures in excess of 1100 C in the cohesive zone, which was attributed to the formation of an increased amount of molten FeO containing slag within the pellet. This study is expected to make important contributions towards further improvements in the pellet design as well as the optimization of blast furnace operation and efficiency. Keywords: Iron oxide reduction, Blast furnace, cohesive zone, reduction degree DOI: /SRI09SP031; submitted on 5 January 2009, accepted on 20 April 2009 Introduction With their ability to adapt to changing conditions regarding raw material availability, energy resources and hot metal demand, ironmaking technologies have reached a high degree of excellence. New challenges, especially with respect to the environment and CO 2 emissions, are the strong drivers for technological progress in the 21 st century. Energy has become one of the most important factors in the steel industry, impacting the cost and environment greatly. In addition to the cost factor, nearly 70% of energy is produced from coal-based thermal power stations, which also produce huge amounts of CO 2 gas [1]. With Japan setting a target for a 50% reduction in energy consumption and Europe for a 60% reduction in CO 2 emissions, consistent efforts are being made worldwide towards sustainable developments in the steel industry to safeguard and improve environment [2, 3]. These targets have not yet been achieved. In a significant effort towards developing an eco-conscious and responsible steel industry, a consortium of 48 European companies and organisations have launched a long term project called ULCOS, an acronym for Ultra Low CO 2 Steelmaking, to search for new concepts for making steel that have a potential of reducing CO 2 emissions by more than 30% [4]. Among the processing steps in an integrated steel plant, ironmaking which is currently dominated by the blast furnace, consumes about 75-80% of energy and generates the most CO 2 emissions. Although alternative ironmaking technologies such as COREX and FINMET are being developed [5, 6], changeover to new technologies is possible only in a timeframe of several decades due to huge capital requirements and very long average plant life in the steel industry. The blast furnace is therefore expected to retain its dominant position in the production chain from iron ore to crude steel. The reduction of iron ore and the carburization of reduced iron at high temperatures are the two key processes in blast furnace ironmaking that consume the highest amounts of energy [7]. It will be a very valuable technology innovation if it were possible to accelerate the reduction of iron ore and succeeding in carburization at lower temperatures. An indepth knowledge of iron oxide reduction in the furnace for various types of iron bearing materials as a function of temperature, heating rate, gas composition and flow rate is therefore of great importance. However, there is little recent information reported with regard to the reduction degree of iron oxides within commercial furnaces as this is difficult to measure during operation and opportunities to examine the internal state of a commercial furnace have been very limited. In 1997, LKAB, a Swedish iron ore company, built an experimental pilot plant blast furnace (EBF) in Luleå, Sweden. The EBF, with a flexible design for tests of different process concepts, is the only facility of its kind in the world with a size large enough to simulate the operation of a commercial size blast furnace, and at the same time small enough for research purposes [8, 9]. The EBF provides a very good opportunity to understand the blast furnace operation and the behaviour of burden materials such as iron ores and cokes during their passage through the furnace. A number of studies have been carried out by researchers using EBF and laboratory scale tests. Most of the cohesive zone studies were carried out using the softening under load (reduction under load) test equipment to evaluate the softening and melting properties of iron ore burdens through changes in contraction and pressure drop during experiments. These were however performed under simulated conditions, and it was 702

2 #3 #4 #2 #5 #1 Figure1. Description of the EBF and sample positions. Table 1. Chemical analysis of the olivine iron ore pellets (wt. %). Sample Fe 2 O 3 SiO 2 CaO MgO Al 2 O 3 Pellet A Pellet B Pellet C Pellet D Figure 2. A schematic representation of the TGA experimental arrangement. Table 2. Excavated samples- pellet type and their position within EBF. Sample No Depth (mm) Distance from centre (mm) #1. Pellet C Pellet D Pellet A #2. Pellet C Pellet D Pellet A #3. Pellet B Pellet D Pellet A #4. Pellet A Pellet C Pellet D #5. Pellet B Pellet C difficult to obtain realistic blast furnace conditions [10, 11]. The reduction behaviour in the cohesive zone needs to be investigated in great detail. In this study, a detailed analysis has been carried out on probed/excavated samples from the EBF. These have been supplemented with laboratory based fundamental investigations. The objective of the present study is to investigate the reduction behaviour of olivine iron ore pellets in the blast furnace from the shaft zone to the cohesive zone. A critical analysis and comparison of controlled laboratory and plant scale investigations is expected to lead to a clear understanding of ore reduction behaviour in the experimental blast furnace. Experimental Probed and excavated specimens from EBF. Four different types of olivine based iron ore pellets were investigated in this study. Table 1 shows the detailed chemical composition of pellets on which laboratory experiments and EBF campaign were carried out. Labels A, B, C and D represent ore pellets blended with different amounts of CaO, MgO and SiO 2. In order to evaluate the pellet reduction behaviour in the shaft and the cohesive zone, probed and excavated samples from EBF were prepared and treated for various analyses. Figure 1 shows a cross section of the EBF and positions of both probed and excavated samples. The upper shaft probes were located at about 1m below the stock level and the temperature range of this position was ~500 C -700 C. The position of the lower probe was 3.4m below the stock level and the temperature was ~850 C C. Up to four sets of pellets were sampled by upper and lower probes during operation. Upper probe samples were collected from three radial locations (1-wall, 2-middle, 3-opposite wall), and lower probe samples were collected from four radial locations (1-wall, 2-middle, 3-middle, 4-opposite wall). For sake of convenience, the following labelling style has been used throughout this manuscript: sample marked as UA- 3 represents an upper probe specimen, A type pellet and located at radial position 3. These pellet samples were used to determine the reduction behaviour in the EBF shaft zone. During excavation, reduced ore pellets were collected from 5 different locations below the lower shaft probe of the furnace. These specimens were however located at different positions along the radial direction; this aspect could be important while analysing their reduction behaviour. Each sample set consisted of 4 baskets located at different positions. The excavated pellet types and their location have been provided in Table

3 Magnetite Wustite Iron U61 Magnetite Wustite ( 0 ) Laboratory based reduction investigations. To investigate the reduction behaviour of olivine pellets under controlled conditions, a reduction test study was also carried out in the temperature range 700 C to 1200 C using a thermogravimetric analyzer (TGA) [12]. The TGA allowed the required experimental conditions such as gas composition and temperature to be controlled during in-situ recording of mass changes. A schematic diagram of the experimental set up of the TGA is shown in Figure 2. The TGA consists of a vertical tube furnace with four Super Kanthal (trademark of Kanthal Corporation) resistance heating elements. The furnace temperature was controlled using a thermocouple external to the reaction tube. A 60mm inner diameter re-crystallized alumina tube was used as the reaction tube, with gas seals at both ends. The temperature accuracy was 5 C for the isothermal zone of 5 cm width located at the centre of the furnace. A hematite sample Iron Figure 3. Comparison of XRD patterns of probed pellets A. Mineral Phase(Wt.%) 100% % 80 60% 60 40% 40 20% 20 0% 0 Hem Mag Wus M.Fe U U L71 L72 L L74 U61 U62 U63 L71 L72 L73 L74 Probed sample No. Figure 4. Comparison of percentages of iron phases in probed pellets based on SIROQUANT analysis. tablet was made by chemical agent with 10mm diameter and 10mm height size. Also, the olivine pellet was grinded and prepared as a tablet with the same size. Temperature profile and gas composition were simulated under blast furnace condition. X-ray diffraction analysis. X-ray diffraction data for mineral phase characterization of reduced pellets was obtained using a Siemens D5000 diffractometer fitted with an incident-beam monochromator using Cu K radiation (30KV, 30mA). Scanning was performed from 5 to 105 at a rate of 0.6 /min with a step size of Since the conditions of sample preparation, such as particle size and flatness of the surface of a packed sample, could affect the accuracy of X-ray diffraction patterns, great care was taken in material preparation. The sample was grounded finely and screened between 0.125mm-0.174mm. The packing method of the sample to the X-ray sample holder was kept identical in all cases in an attempt to obtain similar background levels in all cases. SIROQUANT software was used as a tool for the quantitative estimation of various mineral phases present [13]. This program calculates a theoretical XRD profile and fits it to the measured pattern by full-matrix leastsquares refinement of the following Rietveld parameters: phase scales, line asymmetry, phase preferred orientation, phase line widths, instrument base-line, and line-shape parameter for each phase, and the phase unit cell dimensions. After fitting the peak intensity and width, relative proportions of mineral phases can be calculated from the area under the peak. Error outputs for the SIROQUANT weight percentages were calculated from the least-squares variance-covariance matrix and estimated standard deviations. During SIROQUANT analysis, it was assumed that the only mineral phases present were ferrous oxides and metallic Fe. These results were later compared with the reduction degrees calculated from chemical analysis data. Results and Discussion Determination of the reduction degree. Detailed XRD patterns measured for 7 probed pellets, 3 from the upper shaft probe and 4 from the lower shaft probe (Pellet A series) are shown in Figure 3. As can be seen clearly, various peak intensities in the XRD patterns showed significant differences. These patterns were then used for a quantitative analysis of phase transformations during reduction of pellets from the areas under the peaks corresponding to various mineral phases. Detailed SIROQUANT results are shown in Figure 4. the upper probe sample UA-1 showed large amounts of hematite and magnetite with very small amounts of wustite and metallic iron. The upper probe specimens in the middle and closer to the opposite wall showed better reduction with most of hematite being reduced. While significant amounts of wustite were present, there was very little metallic iron present. The lower probe results showed much higher levels of reduction, with phases predominantly consisting of wustite and metallic iron and very small amounts of magnetite. No well defined patterns were however observed along the radial direction. 704

4 Assuming that initial Fe in the pellet was composed of Fe 2 O 3 alone, the oxygen mole fraction before reduction could be obtained. The reduction degree of the iron ore pellets can be calculated from the mole fractions of oxygen before and after reduction. The values computed from the XRD analysis were found to be slightly higher than the corresponding results from chemical analysis. During SIROQUANT analysis, the mineral phases in pellets were assumed to consist only of various iron oxides and metallic iron. Gangue minerals such as MgO and SiO 2 were not considered in the calculation which could have led to a slightly higher magnitude of the reduction degree. From the chemical analysis results on pellets, the total iron oxide content was found to range from 94.9 to 95.1 wt%, indicating the presence of wt% gangue minerals in the pellets samples. The reduction degree from XRD analysis was recalculated by accounting for the contribution from gangue minerals; this resulted in a much better linear correspondence between chemical analysis and XRD analysis results with a slope of (Figure 5). Reduction degree of probed pellets in the shaft zone. Detailed reduction results on 20 ore samples collected by upper and lower probes are presented in Table 3. For pellet A, the upper probe sample closest to the wall (UA- 1) showed least reduction, with significant amounts of unreduced hematite still present. Hematite levels were much lower for the sample probed from the middle of the shaft zone (UA-2) and one from the opposite wall (UA-3). A similar trend was observed for pellets B with the sample from the middle showing high levels of reduction. However, pellet UB-3 closer to the opposite wall showed poor reduction. Pellets C also showed poor reduction close to the wall. No well defined trends were observed between the three types of pellets; it is quite likely that effects of marginal differences in their composition could have been masked by the local variations in gas and temperature profiles or position changes caused by localised sinking within the furnace. All lower probe samples showed much higher levels of reduction. The reduction of both hematite and magnetite had reached near completion for all 12 samples under investigation. Wustite and metallic iron were the main components in lower probe pellets. Once again, no well defined trends were observed between the various types of pellets or with their radial location. Reduction degree of excavated pellets in the cohesive zone. Pellets were excavated from five locations within the EBF after the campaign. The 18 basket samples which consisted of 5 sets were used for this study. Figure 6 shows the average mineral compositions of each sample set as calculated from XRD patterns. The metallic iron content increased linearly with increasing depth except for sample sets 2 and 3. The sample set 2 was located at 200mm distance from the centre; sample set 3 was located at 600mm from the centre (Table 2). Reduction was generally found to be higher in the central regions; set 2 which was much closer to the centre than set 3, showed a higher degree of reduction. The average reduction degree of these samples as a function of EBF depth has been plotted in Figure 7. The average reduction degree of the Reduction degree from XRD analysis(%) y = x R 2 = Reduction degree from chemical analysis(%) Figure 5. Comparison of reduction degree calculated from SIROQUANT analysis and from chemical analysis. Mineral Phase(Wt.%) 100% 80% 60% 40% 20% 0% 0 Hem Mag Wus M.Fe #14 #11 #22 #20 #24 Excavated sample No. Figure 6. Comparison of reduction degree calculated from SIROQUANT analysis and from chemical analysis. Table 3. Reduction behaviour of probed samples as measured using SIROQUANT analysis (wt. %). No. Hematite Magnetite Wustite Metallic iron Reduction degree % Set I Type A Pellets UA UA UA LA LA LA LA Set II- Type B Pellets UB UB UB LB LB LB LB Set III- Type C Pellets UC UC LC LC LC LC

5 Reduction degree(%) EBF Depth from stock level(mm) Figure 7. Relationship of average reduction degrees of excavated samples as a function of EBF depth. Distance from stock level (mm) Reduction degree(%) Results in this study Figure 8. Comparison of the reduction degree of excavated pellets in this study with other pellet types charged in EBF baskets. Reduction degree(%) EBF depth from stock level(mm) Figure 9. Reduction degree of pellet as function of depth from the stock line. excavated olivine pellets showed a good trend along the depth of the EBF considering the radial position. These results were compared with the values obtained from chemical analysis; a good correlation was observed between the two sets of results. Figure 8 shows a plot of reduction degrees for different types of ore pellets charged in baskets to the EBF over several campaigns [9]. The reduction degrees of the excavated pellets analysed in this study have been marked with a black circle symbol; the present results showed a good agreement with the reduction degrees trends reported in previous studies. Hallin et al. [9] had found that the cohesive zone was located at between m below the stock level for most types of pellet brands after excavation. Therefore, the olivine pellets excavated for this study should have been located in the upper part of the cohesive zone of EBF as well. In Figure 9, we have plotted the reduction degrees of both probed and excavated olivine pellets in the EBF. The reduction rate showed a change in slope at approximately 3 m below the stock level. Due to the lack of samples from the upper part of the cohesive zone, the exact change-over point could not be accurately identified. These results indicate that the ore reduction rate along the EBF depth had two distinct stages, one before and other after around 3 meters below from the stock level. J. Sterneland has reported on the reduction degree profile in the EBF as a function of depth from the stock-line and had observed three steps in reduction kinetics [10, 11]. The three stages included initial fast reduction (I) followed by a slow reduction rate (II), and finally again a high reduction rate (III). The high speed of the reduction at stage III was caused by high temperatures and high reducing ability of the gas, resulting in the prevailing Boudouard reaction and leading to direct reduction of FeO. Taking the temperature profile and gas composition in the blast furnace into account, these results appear to be reasonable; however, they are not sufficient to explain the sharp increase of reduction degree at stage III. Although an increase in CO levels is expected to accelerate the reduction process, the influence of direct reduction needs to be examined further as the rate of direct reduction of iron oxide in solid state may have little effect on the increasing reduction rate. Laboratory investigations using TGA. In this study, the reduction degree of pure hematite and olivine based pellets was also determined under simulated conditions of temperature distribution and gas composition of blast furnace using thermal gravimetric analysis. Figure 10 shows the reduction behaviour of both olivine pellet and pure hematite agent under the simulated blast furnace condition. To fully understand the effect of gas composition, CO gas levels were increased with increaseing temperatures. The reduction rates for both hematite and olivine based pellets showed little difference until the temperature of 400 C with marginal differences later on. The increase at 800 C was caused probably by an increase in CO/(CO+CO 2 ) ratio. At 1000 C, the gas composition was changed again but there was no corresponding increase in the reduction rate. The absolute magnitude of differences in the reduction degrees between 800 C-1100 C could be due to the 706

6 Reduction degree (%) Temperature( 0 C) Figure 10. Reduction degree as a function of temperature for an olivine pellet and hematite agent. Gas composition in different temperature zones has been provided in the figure. 800 C 1000 C 1200 C Figure 11. Microscopic view of the pellet samples after reduction in the laboratory furnace at different temperatures. formation of magnesium bearing ferrite, formed by the diffusion of MgO into FeO or Fe 3 O 4. Mg bearing ferrite is known as a reduction retarding compound during the iron ore reduction process [14-17]. Therefore, the reduction degree of pure hematite had a higher value than that of olivine based pellet. However, for temperatures higher than 1100 C, the reduction rate of two samples showed significant differences with olivine pellets having a higher reduction rate. An olivine pellet consisted predominantly of hematite, MgO and SiO 2, the only difference compared to hematite was the existence of MgO, SiO 2 and gangue minerals in the sample. Therefore, the difference in reduction rate over 1100 C between the two samples could only be caused by small differences in chemical composition. Also, it should be noted that the influence of these differences became significant only in certain temperature ranges. From laboratory scale experiments on the reduction behaviour of pellets containing MgO and CaO, Sang Ho Yi has found that MgO fluxed pellets have a lower reduction degree below the cohesive zone leading to a decrease in the softening temperature [18]. The presence of MgO was seen to increase the melting temperature of the FeO containing slag. Our results shown in Figure 10 indicate that an increase of metallic iron content caused by the higher reduction degree of pellets could also have an influence on the melting temperature. 707

7 Figure 11 shows microscopic images of pellet samples reduced at various temperatures in the laboratory scale test. At 800 C, the metallic Fe which is shown as bright white regions was observed on the surface of wustite particles. At 1000 C, the thickness of the metallic Fe layer had increased but without a significant change in shape. However at 1200 C, the shape of metallic Fe was seen to be quite different compared to the corresponding shape at 1000 C, wherein metallic iron phase appeared precipitated from the melts. This result on metallic Fe morphology can be directly related to observed differences in reduction rate of olivine pellets at temperatures between 1000 C and 1200 C (Figure 10). From the laboratory scale reduction test using both pure hematite and olivine pellet, it was found that the change of reduction rate over 1100 C between the two samples was caused not only by a change in gas composition but also by the changes in morphology of reduced iron at different temperatures. This new aspect needs to be taken into account to fully understand changes in reduction rate as a function of temperature. Studies on the softening and melting behaviour in the blast furnace have found that the initial formation of liquid phase containing FeO affects the softening temperature [14-18]. But these have not discussed the influence of melt formation on the reduction rate. Our results indicate that the changes of reduction rate in the EBF could be explained by the changes of gas composition and melt formation containing FeO between the temperature of 1100 C and 1200 C. Conclusions Olivine pellets from an experimental blast furnace were examined by X-ray diffraction and chemical analysis to determine the degree of reduction of ore pellets during their descent from top towards the lower parts of the EBF. The reduction behaviour of olivine pellets and pure hematite was also investigated under simulated conditions of temperature and gas composition in a laboratory TGA furnace to clarify the reduction mechanisms of olivine pellets in an operating blast furnace. Key conclusions of this study are given below. This study demonstrated that X-ray diffraction analysis can be used to successfully determine the reduction degree of olivine pellets in different parts of the experimental blast furnace. These results were found to be consistent with assessments of the reduction degree based on a detailed chemical analysis. Average reduction degree of iron oxide was seen to increase as the pellets descended towards the lower zone of the EBF; up to 75% reduction was completed before the pellet reached the cohesive zone; the remaining reduction was completed within the cohesive zone. The reduction degree of the ore pellet showed a linear correlation with distance from the stock line of the EBF. This study showed that the presence of olivine did not much influence the reduction degree of iron ore pellets for temperatures below 1100 C in the upper shaft zone of the EBF. However, olivine was found to increase the rate of reduction in the advanced stages of reduction in the cohesive zone for temperatures in excess of 1100 C. This effect was attributed to the formation of an increased amount of molten iron within the pellet. References [1] W. K. Lu and G. R. Bryer: Science and Technology of Innovative Ironmaking for aiming at Energy Half Consumption, Tokyo, Japan (2003), [2] K. Wang, C. Wang: Energy Policy, 35 (2007), [3] D. Cang, Y. Zong: Science and Technology of Innovative Ironmaking for aiming at Energy Half Consumption, Tokyo, Japan (2003), [4] [5] A. Hassan, R. Whipp: Iron & Steel Scrap, Scrap Subsitutes and Direct Steelmaking, Atlanta USA (1995). [6] H. M. W. Delport and P. J. Holaschke: COREX Symposium, South Africa (1990), [7] J. G. Peacy: The Iron Blast Furnace-Theory and Practice, Pergamon Press (1979). [8] A. Dahlstedt, M. Hallin and J.-O. Wikström: 4 th European Coke and Ironmaking Congress Proc., (2000), [9] M. Hallin: International BF Lower Zone Symposium, Wollongong, Australia (2002), [10] J. Sterneland: Some aspects on the reduction of olivine pellets in laboratory scale and in an experimental blast furnace, Ph.D. Thesis (2002). [11] J. Sterneland and M. Andersson: Ironmaking and Steelmaking, 30 (2003), No. 4, [12] M. Grigore, R. Sakurovs, D. French and V. Sahajwalla: ISIJ International, 46 (2006), No. 4, [13] T. Hilding, S. Gupta, V. Sahajwalla, B. Bjorkman and J. O. Wikstrom: ISIJ International, 45 (2005), No. 7, [14] T. Borider: Scandinavian J. of Metallurgy, 16 (1987), [15] J. H. Kaspersma and R. H. Shay: Metallurgical and Materials Transactions B, 12B (1981), [16] A. A. El-Geassy: ISIJ International, 37 (1997), No. 9, [17] A. A. El-Geassy: ISIJ International, 36 (1996), No. 11, [18] S-H. Yi: Scandinavian J. Metallurgy, 28 (1999),