Localized Heating and Reduction of Magnetite Ore with Coal in Composite Pellets Using Microwave Irradiation

, pp. 817 822 Localized Heating and Reduction of Magnetite Ore with Coal in Composite Pellets Using Microwave Irradiation Kotaro ISHIZAKI, 1) Kazuhiro NAGATA 1) and Tetsuro HAYASHI 2) 1) Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552 Japan. 2) Seki Branch Laboratory, Gifu Research Institute of Industrial Product Technology, Oze 1288, Seki, Gifu 501-3265 Japan. (Received on December 12, 2006; accepted on March 9, 2007) Magnetite iron ore-coal composite pellets were microwave irradiated in N 2 atmosphere with a power supply gradually increased in 2 min intervals from 0.2 up to 2 kw at 2.45 GHz. Electron probe microanalysis (EPMA), X-ray image of SEM micrographs, back scattered electron image (BSE) and Fe, O, S and C mappings of composite pellets irradiated up to different temperatures were obtained. Under the present experimental conditions, pellets heated up to about 800 C without reduction. Above this temperature the reduction occurred stepwise; Fe 3 O 4 reduced to FeO between about 800 C and 1 000 C and then FeO reduced to Fe from about 1 000 to 1 250 C average temperatures. The measured temperature appears to reasonable represent the average temperature of the pellet. However inside of the reacting mass localized slightly different temperatures may exist. Point carbon contents of reduced iron inside the composite pellet irradiated up to 1 150 C were from almost 0% to 2% with no correlation between with their spatial location inside the pellet. It is concluded that carbon in the microwave irradiated pellet acting as a reducing and heating agent at the same time generates localized reduction microenvironments. KEY WORDS: magnetite ore; coal; composite pellets; microwave irradiation; localized reduction microenvironment. 1. Introduction The effectiveness of microwave energy in carbothermic reduction processes has been extensively demonstrated. Carbon and the vast majority of heavy metals oxides respond to microwave heating making the microwave assisted carbothermic reduction of metal oxides possible. If the metal oxide is poor receptor then added carbon plays the role of microwave heating accelerator. 1) A chronological summary of published findings on the application of microwave energy to reduce iron oxides by carbon puts the work of Standish and coworkers 2,3) as the first reports on the subject. Their results showed that applying a constant microwave power of 1.4 kw both, magnetite concentrates and hematite fines were satisfactorily and rapidly reduced by either charcoal or coke, but the reduction was faster with the former. Few years later, Aguilar and Gomez 4,5) compared the reduction kinetics of iron ore pellets by carbon with conventional and microwave heating. Almost simultaneously Zhong et al. 6) reported on the carbothermic reduction of magnetite ore concentrates by microwave heating as an alternative to solve the slow heat transfer problem of conventional reduction processes. Coal as the carbon source in the carbothermic reduction by microwave heating was found to be more effective than coke. The higher effectiveness of coal was interpreted to be due to the evolution of H 2, CH 4 and CO from volatile matter in the coal, which acted as initiators of the reduction reaction. According to these authors, the slow reduction rate achieved with coke resulted from the fact that in this case the initial reaction requires direct solid-solid contact between the magnetite and the coke. In this study the reduction temperature was kept constant by controlling the microwave power supplied. The reduction rate increased dramatically from 900 to 1 000 C, and above this temperature, a 90% reduction was easily obtained in 10 min. The reduction rate was also dramatically increased by using a 20% excess carbon. Without excess carbon, a higher microwave power output was necessary to maintain the test temperatures. Mourao et al. 7) also studied the carbothermic reduction of iron ore with charcoal or coke in the form of composite pellets. The iron oxide composition of the ore was not shown. Composites of 4 5 g weight with and without cement were irradiated with 1.1 kw constant microwave power. In this study charcoal again proved to be more efficient than coke, and by the addition of cement the attained temperature was higher and the reduction rate was faster. Their results also showed that an increase in the amount of carbonaceous material as well as thermal insulation of the pellets promoted higher temperatures and higher reaction rates. The carbothermic reduction of 1, 2 and 3 kg iron ore concentrates, coal, and lime mixtures with a microwave power supply of 15 kw was evaluated by Chen et al. 8) In this study the heating rate of the mixtures was also ob- 817 2007 ISIJ

Table 1. Chemical composition of raw materials in mass%. served to increase by increasing the ratio of coal in the mixture. The authors of the present communication investigated the feasibility of producing pig iron from magnetite ore and coal mixtures by microwave irradiation. 9,10) It was clearly shown that the mixtures were heated very rapidly and pig iron produced at lower temperature of about 1 350 C within 5 min shorter time than by conventional heating. It was also shown that the rate of temperature increase of the composite pellets depended on the microwave power. 10) Other important applications of microwave assisted carbothermic processes related to the treatment of steelmaking slag to recycle and recover iron and phosphorus as well as in the production of magnesium vapor for desulphurization or deoxidation of molten iron have been investigated by Morita and coworkers. 11 14) Graphite powder was used in the carbothermic reduction of synthesized and industrial steelmaking slag. The results showed that the larger the carbon equivalent in the Fe C alloy product the higher the fractional reduction of iron and phosphorus in the slag. 12) Yoshikawa and Morita reported that the initial graphite particle size was an important factor for the heating and reduction behavior of magnesium oxide by graphite in powder mixtures microwave irradiated. 13) The results of all these investigations of microwave induced carbothermic reductions show that; the reductions are occurring at temperatures lower than by conventional heating, the rates are faster and the form and quantity of the carbon employed plays an important role in the course of the reduction. In the present work, iron phase changes with temperature in the reduction of iron ore by coal under gradually increased microwave irradiation was investigated and compared with reported results of direct reduction using radiation heating. Fig. 1. 2. Experimental 2.1. Sample Preparation Spherical composite pellets with about 20 mm diameter were prepared using a pelletizer from a mixture of magnetite ore (named Romeral) (ca. 50 mm diameter), coal (named Robe River) (ca. 40 mm diameter) and bentonite powder as a binder. Table 1 summarizes the chemical compositions of the raw materials. The amount of coal was determined by taking into account the amount of carbon required for reducing iron ore completely to form 87%CO 13%CO 2 gas and for producing pig iron with 2 mass% of carbon. The content of bentonite was 2 mass%. This carbon proportion was used considering that using microwave the reduction would be mainly producing CO and a small proportion of CO 2. The weight and density of the pellets ranged from 7.6 9.6 g and 1.95 2.04 g/cm 3 respectively. Schematic illustration of the chamber indicating the position of the pellet. 2.2. Experimental Procedure A microwave generator with 5 kw maximum power operating at 2.45 GHz was employed. Pellets were placed in an alumina crucible in the middle of the microwave oven chamber and covered with an alumina insulator. Silicon carbide as an exothermic auxiliary substance was pasted inside of the alumina insulator to heat it and compensate the heat lost from the pellet. The chamber was first evacuated to 0.03 torr and then filled with 99.9% N 2 gas. The temperature of pellets above 600 C was monitored using a pyrometer from the upper part of the chamber through a window. The pyrometer was a two color radiation thermometer. A schematic illustration of the experimental apparatus has been presented elsewhere. 10) One pellet was placed inside of a crucible and located in the center of the chamber. In every experiment the location of the pellet was exactly at the same altitude and horizontal location inside of the chamber as shown in Fig. 1. In order to investigate the progress of reaction, pellets were heated up to 800, 1 050, 1 150 and 1 250 C by a step power supply and cooled to room temperature inside the chamber. A schematic of the microwave step power supply pattern is shown in Fig. 2(a). A 0.2 kw initial power was increased in 2 min intervals to 0.7 kw, 1 kw, 1.5 kw, 1.75 kw and finally 2 kw. The 2 kw power was applied until the end of the experiment. 2.3. Analysis The pellets cooled from 800, 1 050, 1 150 and 1 250 C were cut in quarters. One quarter of the reacted pellet was 2007 ISIJ 818

Fig. 2. (a) The step power supply pattern. (b) Temperature increase of 20 mm diameter pellets irradiated up to 800, 1 050, 1 150 and 1 250 C by the step power supply shown in (a). put in resin and the cross section was mechanically polished for observation by electron probe microanalysis (EPMA; Shimadzu EPMA-1610). X-ray image of SEM micrographs, back scattered electron image (BSE) and Fe, O, S and C mappings were obtained. In line and point qualitative and quantitative analyses were performed by EPMA. The compositions of the samples were calibrated using ZAF2 and ZAF3 programs installed in the EPMA-1610. The carbon concentration was analyzed all over the surface in about 100 different points. The diameter of the beam was adjusted to 1 mm. Calibration curve method was used to obtain the concentrations. Four standards placed in resin were analyzed simultaneously with the sample to obtain the calibration curve. X-ray diffractometer (XRD); Rigaku RINT-TTR-3C/PC was used to identify the crystal phases of reactants and products. Fig. 3. BSE image of the initial and irradiated composites at the center, middle and out positions. The gray color shows the area of iron ore, the white color shows the area of reduced iron and the black color shows the area of bentonite, coal, slag and empty space included all together. The square shows the area of Fig. 4(a). 3. Results 3.1. Temperature Increase of Pellets The temperature increase of pellets heated up to different temperatures is shown in Fig. 2(b). All samples were heated from room temperature but the pyrometer used to measure the temperature could not detect below 600 C. This is the reason why the time-temperature profiles start from about 600 C and heating patterns below this temperature can not be discussed. The temperature profiles show that the samples were constantly heated and their temperature increased linearly without showing the plateau caused by the endothermic reaction of iron ore observed in the profiles of similar composite pellets when radiation heat was used. 15) The time-temperature profile of the sample heated up to 1 050 C has about 100 s delay compared with other samples. This delay was interpreted in the previous publication as the different reflection of microwave due to the condensation of evolved volatiles on the chamber walls of the oven. 10) However, in the range of temperature data, a clear change in the heating rate is observed around 1 000 C. Average heating rates were calculated from all temperaturetime measurements shown in Fig. 2(b). Below 1 000 C the average heating rate is 1.8 C/s and from 1 000 to 1 250 C it drops to almost half; 1.1 C/s. Possible explanations for the register change in heating rate in relation with the reduction process will be discuss later. 3.2. Phase Transformations In the previous report 10) ; the progress of the reduction was followed by XRD analyses of the reaction products present in pellets heated up to different temperatures. The results can be summarized as follows; in the pellet heated up to 800 C, the majority of iron phase was magnetite (Fe 3 O 4 ) with a little amount of wustite (FeO). The patterns of the pellet heated up to 1 050 C indicated the presence of wustite (FeO) and iron (Fe) phases in similar proportions. At 1 150 C, the proportion of reduced iron (Fe) increased and at 1 250 C only reduced iron (Fe) was detected. In the present communication, the advancement of the reduction of the same pellets was followed by microanalysis of the pellets cross sections. BSE images were taken in the center, middle and outer positions of the cross sections. The results are shown in Fig. 3. The contrast of these images was adjusted to show areas of Fe and Fe O in white and gray colors respectively and black areas correspond to bentonite, coal, slag or empty spaces. Images of a pellet before heating are included as reference. In the initial pellet 819 2007 ISIJ

and the one heated up to 800 C only gray areas are observed. Considering the results of XRD analyses, in these images, the gray areas correspond to magnetite. The images of the pellet heated up to 1 050 C show the presence of Fe, white areas, as well as gray areas that from the XRD results correspond to FeO. In the previous communication, 10) XRD patterns of this pellet taken in the center and near the surface show only the presence of FeO and Fe phases but the proportion of Fe phase appears to be a little larger near the surface. This is in agreement with the images shown in Fig. 3. Increase of white areas, reduced Fe, is observed in pellets heated up to 1 150 and 1 250 C in accordance with XRD results. A close examination of the micrographs revels that at all temperatures the proportion of reduced iron, white color, is larger near the surface; out in Fig. 3 than in the center. The results of a parallel study by the authors 16) about the selectivity of microwave energy consumption in the reduction of Fe 3 O 4 by carbon black in powder mixtures revealed that microwaves are absorbed and used first by high microwave absorbing materials. The study also showed that above around 650 C, when pure magnetite losses its microwave absorption capability, the sample receives some external radiant heating from SiC. Either or both factors may explain the fact that, at these temperatures, there is more reduced iron near the surface of the pellet. Fig. 4. (a) EPMA image of the square portion indicated in the BSE image of the magnetite ore and coal composite pellet sample irradiated up to 1 050 C in the middle, Fig. 2. The white color (W) shows the areas of iron, the dark gray (DG) corresponds to the areas of iron plus oxygen and the light gray (LG) shows the areas of carbon plus sulfur. The white line indicates the place where Fe and O analyses were performed. The Fe and O profiles are shown in (b) as gray and black lines respectively. These line profiles show that there are 3 areas A, B and C with different Fe and O concentrations. 3.3. Iron and Oxygen Distribution Patterns An EPMA image of the square portion indicated in the BSE image of the magnetite ore and coal composite pellet sample heated up to 1 050 C in the middle, Fig. 3, is shown in Fig. 4(a). In this image, areas of Fe, Fe plus O and C plus S (coal) are shown in white (W), dark gray (DG) and light gray (LG) colors respectively. The black color areas correspond to other elements or empty space. This image shows that almost all the reduced iron is in the area between coal (LG) and Fe plus O (DG) and that near by Fe (W) there is coal. The white line in this figure indicates where analysis of Fe and O was carried out. The Fe and O profiles are shown Fig. 4(b) as black line and gray dot lines respectively. These line profiles show 3 areas; A, B and C with different Fe and O intensities. Area A has a high intensity of iron and low oxygen. In B the proportions of Fe and O are almost equal, and in C the intensity of O is higher than Fe. Point analyses of these areas were taken. The content in area A was Fe: 87.2 mol%, the content in area B was Fe: 42.5 mol% and O: 47.7 mol% and the content in area C was Fe: 40.7 mol% and O: 48.8 mol%. This result is in agreement with electron microscope observations reported by Chen et al. 17) They indicated that in the self-reduction process of iron ore concentrates containing coal heated by microwave; magnetite grains were initially reduced around coal grains to form metal iron granular structure. Fig. 5. (a) Cross section of the composite irradiated up to 1 150 C showing the points of carbon measurements. (b) Distribution of carbon concentration at these measurement points. 3.4. Dispersion in Carbon Content Figure 5(a) shows a cross section view of the step heated composite up to 1 150 C. In this figure, the points where the carbon content was analyzed are shown by white dots. The analysis points correspond to reduced iron only. Figure 5(b) shows the carbon concentration of reduced iron as a function of distance from the center of the sample. The results show that the carbon concentration of reduced iron is totally heterogeneous with no clear distribution pattern between the center and the surface. The range was from almost 0% to 2% concentration. There is clear difference between these results and the carbon content of conventionally heated composite pellets investigated by Nagata et al. 15) In the case of conventional heating the carbon distribution shows a clear pattern of carbon content from the surface to the center of the pellet both, after reduction as well as after carburization. The carbon content in reduced iron was 0.25 to 0.35 mass% near the surface of pellet and 0.15 mass% at the center. 2007 ISIJ 820

4. Discussion 4.1. Reduction Behavior In the previous report, 10) a good correlation was found between the weight loss % of 10, 15 and 20 mm diameter pellets microwave irradiated with the step power supply shown in Fig. 2(a) and the temperature of the pellet. The correlation indicated that the reduction started at about 820 C in accordance with XRD results. XRD patterns of the composite pellet microwave irradiated up to 1 050 C showed that at this temperature all the magnetite was reduced to wustite and some reduced iron. The out, middle and center micrographs of the pellet heated up to this temperature presented in Fig. 3 also show small amounts of reduced iron. The change of heating rate of pellets recorded at about 1 000 C as seen in Fig. 2(b) may be taken as an indication of the beginning of the reduction of wustite to iron and or a faster depletion of carbon by gasification compared with temperatures below 1 000 C. There is also agreement between the micrographs shown in Fig. 3 and the XRD results presented in the previous communication 10) showing that at 1 250 C; all the wustite has been reduced to iron. Combining the results of temperature increase patterns, iron phases present at different temperatures in the course during the reduction and microanalysis results, the reduction behavior of magnetite ore-coal composite pellets irradiated under a power supply gradually increased in 2 min intervals from 0.2 up to 2 kw can be summarized in three stages. I) Pellets heated up to about 800 C without reduction. II) Fe 3 O 4 reduced to FeO between about 800 C and 1 000 C. In this stage the rate of temperature increase was about 1.8 C/s. III) FeO reduced to Fe from about 1 000 to 1 250 C. In this stage the rate of temperature increase was about 1.1 C/s. Liu et al., 18) using very advanced experimental techniques, performed thermal investigations of direct iron ore reduction with coal in mixtures heated by radiation from a surrounding graphite cylinder under a constant heating rate of 10 C/min. The reduction behavior outlined above based on the results of the present work is in agreement with their findings indicating that the iron oxides underwent stepwise reduction. There is also very good agreement between the range of temperatures for each reduction presented by these authors and the reduction temperatures of the present work. The reduction of Fe 3 O 4 to FeO was between 740 and 870 C, but the reduction rate was lower between 740 and 800 C than between 800 and 870 C. FeO was subsequently reduced to Fe between 870 and 1 200 C but the rate of reduction was rapid between 950 and 1 100 C. The 10 C/min constant heating condition of Liu et al. experiment made the temperature of the reaction mass to be uniform and an accurate follow up of the reduction as a function of temperature was possible. The agreement of phase transformations temperatures in the present work with the data presented by Liu et al., 18) may be taken as an indication that the temperature data in Fig. 2(b) closely represents the average temperature of the pellet. However inside of the reacting mass localized slightly different temperatures may exist. Aguilar and Gomez 5) noted that in a microwave irradiated iron ore pellet partially submerged in carbon; the temperature between the pellet and the carbon was higher. These differences may result in localized microenvironments as discuss below. 4.2. Localized Heating and Reduction Microwave energy transforms into heat inside materials with high loss factor and the heat is deposited directly inside the sample. In conventional heating, heat enters the sample through its surface and propagates inside due to heat transfer processes mainly thermal conduction. 19) Iron ore-coal composite pellets irradiated by a gradually increased microwave power supplied reduced similarly as a mixture under constant radiant heating rate in the work of Liu et al. 18) However a big difference is found in the increase of temperature of pellets heated by microwave; Fig. 2(b), compared with composite pellets, almost identical to the ones used in this study, heated in an electric furnace by Nagata et al. 15) The difference may be explained by the fact that in the case of the Liu et al. 18) study; the mixtures were under homogeneous thermal equilibrium at each temperature and therefore the reaction occurred homogenously throughout the mass but in the case of Nagata et al. 15) this was not the case. Under microwave irradiation the role of carbon is as a reduction agent and heating agent at the same time because carbon couples quickly with electromagnetic radiation, and generates heat owing to the Joule effect. 20) Therefore, inside the microwave irradiated pellet heating and reduction are taking place locally meanwhile in the case of Nagata et al., 15) the reduction reaction of iron oxides progresses from the surface to the center of the pellet. Therefore for mixtures under homogeneous thermal equilibrium like in the study of Liu et al. 18) ; the temperature gradient from the surface to the center of the mass was avoided same as when the sample is process under microwave irradiation. Even though the quantity of reduced iron is slightly higher near the outer of the pellet where the microwaves first reach the sample, the heterogeneous distribution of carbon concentration in the iron products inside the pellet heated up to 1 150 C shown in Fig. 5(b) indicate that the reduction is occurring locally inside the reacting mass without any pattern. Nagata et al. 15) explained the observed gradient of carbon content as follows. Since the reduction started from the surface, CO 2 gas generated by the reduction diffuses through the surface iron layer and gasifies residual carbon in the layer to produce CO gas and this causes the carbon content to be lower near the surface of the pellet. Differences in local environments inside of the reacting mass where the reduction is taking place may explained the scatter distribution of carbon concentration measured in one hundred different reduced iron products of the whole pellet. Even though the proportion of reduce iron is larger near the surface, as seen in Fig. 3, there is no gradient in carbon content between surface and center as in the case of Nagata et al. 15) study. Therefore there are no fundamental differences in the nature of the reaction occurring near the surface or at the center. This observation confirms that the microwave radiation is absorbed and used in order of the absorbing power of the materials on their path within the irradiated mass as previously demonstrated. 16) 821 2007 ISIJ

Another evidence in support of localized reduction comes from the examination a portion of the image of the iron ore-coal composite pellet reacted up to 1 050 C at the middle, Fig. 3, shown in Fig. 4(a) and the oxygen and iron profiles shown in Fig. 4(b). Iron (W) is present in between reduced iron-ore (DG) and coal (LG) and the profiles indicate that there has been reduction reaction where the iron ore was in contact with coal. At this temperature the oxide phase is wustite that has just started to be reduced and is being reduced to iron near by coal. In the same imagine there is another coal in contact with iron ore but in this case the reduction to iron has not yet started. It is therefore concluded that inside the microwave irradiated composite pellet localized reduction microenvironments are generated. 5. Conclusion From the current work the following conclusions have been driven out: (1) Magnetite iron ore-coal composite pellets microwave irradiated in N 2 atmosphere with a power supply gradually increased in 2 min intervals from 0.2 up to 2 kw at 2.45 GHz heated up to about 800 C without reduction. (2) Above this temperature the reduction occurred stepwise; Fe 3 O 4 reduced to FeO between average temperatures of about 800 C and 1 000 C and then FeO reduced to Fe from about 1 000 to 1 250 C. (3) The measured temperature appears to reasonable represent the average temperature of the pellet. However inside of the reacting mass localized slightly different temperatures may exist. (4) The carbon content measured in one hundred places where reduced iron was present all over the cross section of a composite irradiated up to 1 150 C was from almost 0% to 2%. (5) There is no correlation between point carbon contents of reduced iron inside the composite pellet irradiated up to 1 150 C with spatial location. (6) Inside the microwave irradiated composite pellet, localized reduction microenvironments are generated. (7) Generation of microenvironments is interpreted as due to the fact that carbon in the pellet is acting as a heating and reducing agent at the same time. REFERENCES 1) K. E. Haque: Int. J. Miner. Process., 57 (1999), 1. 2) N. Standish and W. Huang: ISIJ Int., 31 (1991), 241. 3) N. Standish and H. Wornen: Iron Steelmaker, 18 (1991), 59. 4) I. Gomez and J. A. Aguilar: Materials Research Society Symp. Proc., Vol. 366, Dynamics in Small Confining Systems II, (1995), 347. 5) J. A. Aguilar and I. Gomez: J. Microwave Power Electromagn. Energy, 32 (1997), 67. 6) S. Zhong, H. E. Geotzman and R. L. Bleifuss: Miner. Metall. Process., 300 (1996), 174. 7) M. B. Mourao, I. Parreiras de Carvalho, Jr. and C. Takano: ISIJ Int., 41 (2001), S27. 8) J. Chen, L. Liu, J. Zeng, R. Ren and J. Liu: Iron Steel, 39 (2004), 1. 9) K. Nagata, K. Ishizaki and T. Hayashi: Proc. of the 5th Japan Brazil Symp. on Dust Processing-Energy-Environment in Metallurgical Industries, Vol. 1, (2004), 617. 10) K. Ishizaki, K. Nagata and T. Hayashi: ISIJ Int., 46 (2006), 1403. 11) K. Morita, M. Guo, Y. Miyazaki and N. Sano: ISIJ Int., 41 (2001), 716. 12) K. Morita, M. Guo, N. Oka and N. Sano: J. Mater. Cycles Waste Manag., 4 (2002), 93. 13) T. Yoshikawa and K. Morita: Mater. Trans., 44 (2003), 722. 14) K. Morita: Met. Technol. (Jpn.), 76 (2006), 882. 15) K. Nagata, R. Kojima, T. Murakami, M. Susa and H. Fukuyama: ISIJ Int., 41 (2001), 1316. 16) K. Ishizaki and K. Nagata: ISIJ Int., 47 (2007), 811. 17) J. Chen, L. Liu, J. Zeng, R. Ren and J. Liu: J. Chin. Electron Microsc. Soc., 24 (2005), 114. 18) G. Liu, V. Strzov, J. A. Lucas and L. J. Wibberley: Thermochim. Acta, 410 (2004), 133. 19) Y. V. Bykov, K. I. Rybakov and V. E. Semenov: J. Phys. D, Appl. Phys., 34 (2001), R55. 20) E. Fagury-Neto and R. H. G. A. Kiminami: Ceram. Int., 27 (2001), 815. 2007 ISIJ 822