Ind. Eng. Chem. Res. 2005, 44, 621-626
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Computational Calorimetric Study of the Iron Ore Reduction Reactions in Mixtures with Coal Vladimir Strezov,*,† Gui-su Liu,‡ John A. Lucas,§ and Louis J. Wibberley| Graduate School of the Environment, Macquarie University, NSW 2109, Australia, MobotecUSA Inc., 107 Edinburgh Drive South, Suite 203, Cary, North Carolina 27511, Discipline of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia, and Commonwealth Scientific and Industrial Research Organisation Energy Centre, 10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia
Thermal analysis on two coals (semi-anthracite and high-volatile coking coal), iron ore, and their corresponding mixtures was performed using a computer-aided thermal analysis technique. Samples were heated to 1000 °C at a typical rate of 10 °C/min under an argon atmosphere. It was found that the iron ore undergoes several reactions prior to its reduction, which resulted in an endothermic heat effect. The iron ore reduction commenced at temperatures as low as 580 °C and progressively increased at higher temperatures. Coal devolatilization was found to play an important role in iron ore reduction for the coal-ore mixtures at temperatures below 920 °C, while the effect of char gasification resulting in CO as a reducing gas was dominant at higher temperatures. No apparent difference in the effect of coal devolatilization on reduction reactions was observed when low- and high-volatile matter coal was mixed with the iron ore. The main difference was detected only in the temperature range where char gasification became prominent and was predominantly responsible for the reduction of the iron ore. Similarities in the endothermic and exothermic peaks were found at different heating rates, indicating a unified reaction mechanism between them. However, the peaks shifted toward the higher temperature range under increased heating rates. Introduction The industrialized world is still dependent on iron and steel manufacturing for its development. There is, however, an increased pressure on steel producers for the development of environmentally sustainable ironmaking and steelmaking operations. Conventional ironmaking technologies involve separate cokemaking and sintering plants to feed blast furnaces1 in the process of reduction of iron oxides to metallic iron. Both front ends of the blast furnace operations are causing environmental concerns because of CO2 and particulate emissions. Additionally, as they are physically separated from the main blast furnace, the intermediate products, coke breeze and iron ore fines, are discarded, and some amounts of dust and organic pollutants may be discharged into the environment. In this respect, a new currently emerging generation of smelting operations has been developed, such as FASTMET, COREX, ROMELT, DIOS, HIsmelt, etc., that consist of direct reduction of iron ores. One of the methods for direct iron ore reduction is based on the use of mixtures of iron ore with coal, thereby avoiding the necessity for making coke and sinter. In this procedure, the reduction is driven by the coal volatile products liberated during devolatilization, as well as carbon monoxide, regenerated according to the Boudouard reaction. Direct reduced ironmaking (DRI) pro* To whom correspondence should be addressed. Tel.: +61 2 9850 6959. Fax: +61 2 9850 7972. E-mail: vstrezov@ gse.mq.edu.au. † Macquarie University. ‡ MobotecUSA Inc. § The University of Newcastle. | Commonwealth Scientific and Industrial Research Organisation Energy Centre.
cesses have advantages over the conventional blast furnace operations with low pollution effects and low capital intensive operation, and they can provide successful smelting with low-grade thermal coal. These processes enhance the reducing properties of coal, hence avoiding the necessity for use of premium coking coals. An advanced new utilization opportunity is, in this respect, generated for the high-volatile thermal coals, which have a low profile within the steel industry. The fundamental reactions that occur during heating of coal-ore pellets are very complex. Coal devolatilization, iron ore reduction, char gasification, and heat and mass transfer are coupled in this system and occur simultaneously. Optimization of the direct iron ore reduction process depends on the thermal behavior of the reduction reactions, and a careful study of the reduction steps is required. Defining the thermal properties, including specific heat and enthalpy, thermal regions, and the ability to predict the thermal behavior of the iron ore reduction steps is in this respect of great importance. A range of experimental and theoretical works have previously been published on iron ore reduction of coalore mixtures and its kinetics.2-20 Coal devolatilization is an important process in iron ore reduction of the coalore pellets.8,11,18,19 It has been observed that hematite is reduced to magnetite by CO from the volatiles between 400 and 550 °C.5,6 Evolved hydrogen plays an important role in the reduction of iron oxides below 800 °C. Reduction of iron ore by carbon is known to take place by means of reducing gaseous intermediates CO and H2 21 produced through char gasification with CO2 and H2O. Gasification of carbon has been identified as the controlling step for nonisothermal iron ore reduction in ore-coal pellets at high temperatures.15
10.1021/ie030835t CCC: $30.25 © 2005 American Chemical Society Published on Web 01/06/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 3, 2005 Table 1. Analysis of Coals Used in the Study Gregory coking coal (GR)
Figure 1. Schematic diagram of the experimental setup.
Optimization of the process conditions in the direct iron ore reduction operations is becoming increasingly important and will essentially determine the future developments in this industry. In this respect, sensitive and comprehensive instrumental and analytical measurement methods are required for investigation of the process variables and their influence on the kinetics and reactions of the reduction. The purpose of this work is to investigate the fundamentals of the reactions during heating of coal-ore mixtures using a computational calorimetric method. Computational calorimetry is carried out by heating coal-ore samples under defined heat-transfer conditions and calculating the sample thermal properties by employing an inverse numerical technique. Experimental Section Computer-Aided Thermal Analysis. In this work, a computer-aided thermal analysis was performed on coal, iron ore, and mixtures of coal and iron ore. The thermal analysis technique has been detailed elsewhere,22 and in the following, a summary of the experimental procedure is presented. Each sample was packed inside the 10.6-mm-i.d. quartz sample tube and was insulated on the sides with alumina ceramics. The quartz glass tube was then covered with carbon soot on its outer surface to ensure that uniform emissivity is achieved over the entire glass surface. The glass tube with the packed sample was located concentrically to a graphite heating element, as shown in Figure 1. The assembly was placed inside an infrared furnace, and both the graphite and sample tube were kept under an inert atmosphere with separate flows of argon gas. Three 1-mm-diameter chromel-alumel thermocouples were used during the measurements: one embedded inside the graphite tube and used to control the heating rate of the furnace and two thermocouples placed on the surface and in the center of the sample. The temperature collection was conducted at a 1-Hz logging rate, and the data were stored in a computer. Thermal analysis was performed with the measured temperature data by applying an inverse numerical technique to solve the heat conduction equation (1), where Cp )
FCp
∂T ∂T ∂ r )k ∂t ∂r ∂r
( )
(1)
specific heat (J/kg‚K), k ) thermal conductivity (W/m‚K), F ) density (kg/m3), T ) temperature (K), t ) time (s), and r ) radius (m). The sample was divided in a grid pattern with a defined number of nodes n across the radius. The heat balance for each node was calculated based on the principle that heat accumulated by the node equals the difference of the heat input and heat release from the
South Walker thermal coal (SW)
VM ash FC S P
Proximate Analysis (% ad) 33.5 6.5 58.0 0.65 0.025
13.3 15.0 69.7 0.70 0.05
C H N S O
Ultimate Analysis (% daf) 85.1 5.5 2.1 0.7 6.6
89.20 4.20 1.70 0.80 4.10
node. The boundary conditions of the system were the temperatures measured at the center and surface of the sample, zero heat flux in the center of the sample, and surface heat flux calculated assuming radiative heat transfer from the graphite tube to the sample described by eq 2, where Q ) heat flux (W/m2), σ ) Stefan-
Q ) F1-2σ(Tg4 - Ts4)
(2)
Boltzmann constant, Tg ) graphite temperature (K), Ts ) sample surface temperature (K), and F1-2 ) radiation shape factor estimated through calibration as a function of the graphite temperature.22 A computational matrix, using eq 3, was developed to estimate the apparent specific heat of heated material based on the initial mass of a dried sample. The detailed solution procedure is provided elsewhere.22,23 The volu-
[
∆x2π t (T0 - T0t-1) + 4∆t n-1 ∆x2π 2π∆x2i t 1 n - (Tnt - Tnt-1) + (Ti - Tit-1) ∆t 4 ∆t i)1 (3)
FCp ) 2πn∆xQ(t)/
( )
∑
]
metric specific heat estimated from the acquired temperature data using eq 3 is apparent, including the heats ∆H developed from decomposition, transformation, or reaction (Cp ) Cpi + ∆H/∆T). During an endothermic reaction, the apparent specific heat is increased; hence, the heat is consumed by the sample. In contrast to this, if an exothermic reaction is developed during heating, the apparent specific heat will show a decrease. The performance of the above-mentioned measurement method was tested previously22 on a range of different materials, and an accuracy of approximately (2% was found in the measurement data. The maximum temperature was limited to 1000 °C, and the results are plotted against the average of the two sample temperatures. Samples. An Australian semi-anthracite (South Walker, SW), high-volatile coking coal (Gregory, GR), and a Western Australian iron ore (Mount Newman, MN) dominated by hematite (Fe2O3) were used in this work. The main purpose for selection of semi-anthracite and high-volatile coking coal was to detect the influence of the volatile matter on the reduction reactions. The second concern here was whether the semi-anthracites and/or anthracites, which have currently very limited use, could be applied in the emerging ironmaking operations. The proximate and ultimate analyses for the
Ind. Eng. Chem. Res., Vol. 44, No. 3, 2005 623 Table 2. Properties of MN Iron Ore species
wt %
species
wt %
species
wt %
Fe SiO2 Al2O3 CaO
62.1 4.33 1.98 1.24
Mn MgO P TiO2
0.14 0.15 0.05 0.07
Zn LOI C H2O
0.001 4.38 0.106 4.0
coals used in this work are shown in Table 1, and the properties of MN iron ore are listed in Table 2. The samples were dried under vacuum at 80 °C for 2 h before proceeding with each experiment. The single coals, iron ore, and their mixtures were then correspondingly packed in the silica sample tube and heated at the prescribed heating rate from room temperature up to 1000 °C. The average particle diameters for coal and iron ore were -80 and -50 µm, respectively. The porosities for the packed single coal, iron ore, and coal-ore mixtures at a mass ratio of 20:80 were approximately 0.44, 0.49, and 0.45 with packing densities of 900, 2850, and 2100 kg/m3, respectively. Results Thermal Analysis of Single Coals and Iron Ore. Thermal analysis for single SW and GR coals was performed at a heating rate of 10 °C/min under an argon atmosphere, as shown in Figure 2a. Upon heating, coals start to decompose, forming tars, hydrocarbons, and oxides of carbon and hydrogen. During this process, coal undergoes serious structural changes, and if the coal has coking capabilities, it forms a plastic phase during the heating process and resolidifies as a coke. Thermal coals, such as the semi-anthracite used in this study, do not go through the plastic stage; volatiles are formed by bond breaking of the coal macromolecular structure and escape through an intraparticle diffusion. The inability to form a plastic layer upon resolidification, which yields in a physically strong and permeable carbon-rich structure, made the thermal coals unsuitable for the conventional blast furnace ironmaking. Regardless of their thermoplastic properties, the decomposition of coal is a complex phenomenon where devolatilization reactions are coupled in multiple regions of activity,24 which are summarized here. For the single SW coal, the first endothermic reaction with a peak appearing at about 560 °C was due to the secondary devolatilization, whereas the exothermal peak at 790 °C was related to the release of hydrogen.25 The coking coal GR had a distinctive behavior at low temperatures. As shown in Figure 2a, a rapid and significant exotherm occurred for GR coal between 420 and 460 °C, which is thought to be caused by both physical- and chemical-related coal plastic changes.25 The thermogravimetric-Fourier transform infrared analysis25 has shown the maximum rates of mass loss over this temperature range. In addition, high swelling has been found in this region, and its intensity has been well correlated with the exotherm of the coal. Following the tar formation, the secondary devolatilization and hydrogen release occurred over a temperature range of 500-1000 °C, which was similar to that of the reactions occurring in the thermal coal. The gas analysis of this coal has been performed extensively and was reported previously.26 It was emphasized that the evolution of CO starts from 450 °C and completes at about 950 °C, with a maximum evolution rate at 720 °C, whereas H2 starts to evolve from 495 °C and continued at 1000 °C, with a maximum rate at 785 °C.
Figure 2. Specific heat of the samples: (a) single SW and GR coals; (b) single MN iron ore and kaolinite.
The thermal analysis of the iron ore MN is shown in Figure 2b. Once the heating of MN was completed, it was cooled to room temperature and reheated again in order to detect the phase transformations of the sample. The apparent specific heat of a single iron ore exhibited several sharp peaks, as indicated in Figure 2b. The first peak at 120 °C was related to the endothermic water vaporization, and the second at 340 °C was caused by dehydroxylation of goethite [FeO(OH), hydrated iron oxide] with formation of hematite. The third endothermic peak at around 600 °C was due to breakdown of kaolinite. The presence of kaolinite in the iron ore has been confirmed previously by the X-ray diffraction analysis.26 The energy content for breakdown of kaolinite in the iron ore sample appeared to have significant values. However, the thermal analysis of kaolinite alone, as shown in Figure 2b, revealed a strong endothermic energy intensity during decomposition, which is reflected in the increased heating energy requirements for the iron ore samples, even when kaolinite is present in the ores at low quantities. The fourth peak in the iron ore specific heat, which appeared at 685 °C, was caused by magnetic transformation of Fe2O3,27 whereas the fifth trough was attributed to the partial reduction of Fe2O3, most likely caused by the carbon present in the iron ore. The rerun curve showed that all peaks, except for the magnetic transformation, have disappeared. Thermal Analysis of the Coal-Ore Mixture. A comparison of the thermal behavior of the coal-ore mixture with one coal and iron ore is shown in Figure 3a. The apparent specific heat of the mixture, at a mass ratio of 20:80 coal-iron ore, exhibits similarities with the pure iron ore, prior to 600 °C. Dehydroxylation and decomposition of kaolinite are the dominant reactions for the mixture, with peaks similar to those of iron ore without coal. Primary coal devolatilization began below 600 °C as shown for SW coal alone; however, the intensity of the reaction is reduced by the presence of
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what is commonly known as the Boudouard reaction:
C + CO2 f 2CO
Figure 3. Thermal analysis of mixtures of iron ore and coal. (a) Comparison of the specific heats of coal, iron ore, and their mixture. (b) Effect of coal and coke on the specific heat of iron ore during reduction. (c) Effect of the coal type on the specific heat of iron ore during reduction.
iron oxides.5,6 Above 600 °C, the curve was substantially different from that of either the single iron ore or coal. An exothermic trough was detected at around 690 °C, followed by a strong endothermic reaction. Previous studies have indicated that in this temperature range the predominant coal volatile species are CO and H2,25 which both become sources for reduction of the iron oxides, forming metallic iron in the following order:
2Fe2O3 + CO/H2 f 2Fe3O4 + CO2/H2O ∆H° ) -52869/-11715 J/g‚mol (4) Fe3O4 + CO/H2 f 3FeO + CO2/H2O ∆H° ) 36250/77405 J/g‚mol (5) FeO + CO/H2 f Fe + CO2/H2O ∆H° ) -17305/23849 J/g‚mol (6) While coal devolatilization provides a rich source of CO and H2, carbon char is also an essential reductant source for CO generation. In the latter case, CO is a product of highly endothermic carbon gasification via
∆H° ) 172464 J/g‚mol (7)
The effect of this reaction alone on the reduction of iron ore could be investigated through thermal treatment of a mixture of iron ore and coke. A comparison between the thermographs during reduction of SW-MN and coke-MN mixtures is provided in Figure 3b. Coke is a carbon-rich and volatile-free material; therefore, the effect of coal devolatilization to the reduction is in this case excluded. The reduction is first initiated with the carbon from coke by solid-state reduction (3Fe2O3 + C f 2Fe3O4 + CO), triggering further the carbon gasification and iron ore reduction reactions. A comparison between the results in Figure 3b shows that the reduction of iron ore with coke occurs at temperatures above 920 °C, suggesting that at temperatures above this point the effect of carbon gasification on the reduction is becoming predominantly effective. Similarly, for the SW-MN mixture at temperatures below approximately 920 °C, the reduction reactions of iron ore can be related to the effect of coal devolatilization products (CO and H2) to the overall reduction. The effect of carbon gasification to the reduction of iron ore at temperatures beyond this point is also observed for the SW-MN mixture, although this reaction is significantly smaller than that in the case when the iron ore is mixed with coke. Figure 3c shows the thermal reactions during reduction of an iron ore sample in mixtures with SW and GR. The results provide a comparison of the effect of the coal type on the reduction process. Surprisingly, the curves for GR-MN and SW-MN are quite similar in the temperature range where reduction is expected, having the same characteristic peak temperatures T1 and T2, although the volatile content between the coals is considerably different. This was attributed to the higher amounts of tars in the case of GR, evolving at temperatures too low to initiate tar gasification in the current mixtures (400-450 °C); therefore, the tars made no contribution to the reduction process, whereas the amounts of reducing gases (CO and H2) appear to be similar for both coals. The endothermic reaction for GR-MN is found to be slightly higher than that of the SW-MN mixture at temperatures above 900 °C. This is most likely due to the differences in the char gasification properties of the examined coals. Five mixtures of SW coal and MN iron ore (mass ratios of 20:80, 30:70, 40:60, 50:50, and 70:30) were further subjected to thermal analysis. The specific heat determined as a function of the temperature for these samples is shown in Figure 4a. As observed, the first two peaks at 330 and 580 °C, respectively, are similar for each mixture, representing dehydroxylation and decomposition reactions with the formation of hematite. The sharpness of the peaks seemed to be related to the mass percentage of iron ore in the mixture. It is suggested that no significant reaction between iron ore and coal occurred below this temperature. Specific heats of the coal-ore mixtures exhibited series of exothermic and endothermic reactions at temperatures above 600 °C; however, each mixture behaved differently, as shown in Figure 4a. The characteristic temperatures T1, T2, and T3 were dependent on the amount of coal present in the coal-ore mixtures, and they all decreased with an increase in the mass ratio of coal. The intensity of these peaks also decreased. Figure
Ind. Eng. Chem. Res., Vol. 44, No. 3, 2005 625
Figure 4. Reduction of MN iron ore with SW coal. (a) Effect of the mass ratio on the specific heat of SW-MN mixtures. (b) Characteristic temperatures of T1, T2, and T3 vs mass percentages of coal in SW-MN mixtures.
4b shows a good linear correlation of the characteristic temperatures T1, T2, and T3 correlated with the percentage of coal in the coal-ore mixture. The characteristic peak temperatures were correlated with the dominant reaction occurring in the mixture; for instance, the peak at T1 was caused by the exothermic reduction of Fe2O3 by CO released from coal devolatilization. Reduction by H2 at this stage was considered to be insignificant because the amount of released H2 was small in this temperature range.25,26 With an increase of the coal mass ratio, larger amounts of volatiles were released, which promoted the reduction process. From the analysis of the phase diagram of the Fe-C-O system,28 it was understandable that the reduction of Fe2O3 was initiated at temperatures as low as 600-800 °C under reducing atmosphere (high CO/CO2 ratio due to the volatile release). Increased CO concentration in the system would drive the reduction reaction toward low temperatures, which is subject to an equilibration limitation. Similarly, the peak at T2 was associated with the endothermic reduction of Fe3O4 by CO and/or H2. Hydrogen started to evolve at this temperature range and began making its contribution to the reduction. The peak T3 corresponded to the competitive reactions of carbon gasification and iron ore reduction, forming metallic iron and resulting in an overall endothermic reaction. Figure 5a shows the effect of the mass ratio on the specific heat of the GR-MN mixtures. Similar to the SW-MN mixtures, consistent peaks were found at approximately 120, 320, and 580 °C, which represented the water vaporization, dehydroxylation, and decomposition of kaolinite, respectively. As shown in Figure 5b,
Figure 5. Reduction of MN iron ore with GR coal. (a) Effect of the mass ratio on the specific heat of GR-MN mixtures. (b) Characteristic temperatures of T1, T2, and T3 vs mass percentages of coal in GR-MN mixtures.
Figure 6. Effect of the heating rate on the specific heats of GR-MN mixtures at a mass ratio of 20:80.
above 600 °C the characteristic temperatures T1, T2, and T3 were well correlated with the mass percentage of coal in the mixtures. Figure 6 shows the specific heats of GR-MN mixture (mass ratio of 20:80) measured at heating rates of 10, 50, and 100 °C/min. First, the heating rate increases the volatile yield of coal as well as the devolatilization rate,29 because of the enhanced secondary reactions.30 Second, an increase in the heating rate delays most of the reactions, in this case coal devolatilization and iron ore reduction. These reactions are kinetically controlled, and the completion of the reaction depends on the temperature at which the reaction occurs, as well as the holding time at that temperature. As shown in Figure 6, the intensities of the peaks at different heating rates are similar; however, in the case of 100 °C/min, the peaks were delayed by 50-90 °C compared to the
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heating rate of 10 °C/min. These results suggest that the reaction mechanisms for coal-ore mixtures heated under different heating rates are unified. Conclusion The work presented here provides an attempt to apply the computational calorimetric method as an instrumental study of the reduction reactions in order to investigate the effect of coal volatile matter on the iron ore reduction. Two types of coals, a low-volatile semianthracite and a high-volatile coking coal, as well as one iron ore were thermally characterized, during which a series of decomposition reactions were detected. Coals went through a complex reaction mechanism involving primary and secondary devolatilization and a hightemperature dehydrogenation, while the decomposition of iron ore was mainly related to the breakdown of the metallic hydroxyl groups. In the mixtures of coal and iron ore, the CO and H2 produced during coal devolatilization become sources for stepwise reduction of the iron oxides. Coal carbon gasification produces additional CO at elevated temperatures, accelerating the reduction operation. The bulk of this study was performed on mixtures of coal and iron ore at a mass ratio of 20:80, which has been confirmed to be sufficient for the reduction to be completed and to achieve a metallization degree of approximately 99%.26 The reduction reactions observed in the current study commenced as an exothermic reaction at 580 °C and shifted to an overall endothermic reaction starting from 750 °C. Coal devolatilization was found to play an important role in the prereduction of iron ore at temperatures below approximately 920 °C, while the reduction by char gasification dominated above this point, which was confirmed by the thermal study of the mixture of iron ore and coke fines. At the temperature range of approximately 580-920 °C, where coal devolatilization products CO and H2 were the predominant reducing sources, the rank of coal did not show any major influence on the reduction reactions. The main reason was that with the high-volatile matter coals the largest amount of mass is lost during heating through the evolution of tars, which have no contribution to the reduction of iron ores because the amounts of the devolatilization products CO and H2 for both high- and low-volatile coals used in this study appear to be of similar order. The main difference that was observed during thermal analysis of the mixtures of iron ore with both coals and the coke fines was in the intensity of the high-temperature endothermic reaction at temperatures above 900-920 °C, which was related to the iron ore reduction through CO produced by coal char gasification. It was apparent that the gasification properties of coal char under the current conditions are influenced by the coal type. Literature Cited (1) Hinnela¨, J.; Saxe´n, H.; Petterson, F. Modeling of the Blast Furnace Burden Distribution by Evolving Neural Networks. Ind. Eng. Chem. Res. 2003, 42, 2314. (2) Abraham, M. C.; Ghosh, A. Kinetics of reduction of iron oxide by carbon. Ironmaking Steelmaking 1979, 1, 14. (3) Carvalho, R. J. D.; Netto, P. G. Q.; D’Abreu, J. C. Kinetics of reduction of composite pellets containing iron ore and carbon. Can. Metall. Q. 1994, 33, 217. (4) Coetsee, T.; Pistorius, P. C.; De Villiers, E. E. Ratedetermination steps for reduction in magnetite-coal pellets. Miner. Eng. 2002, 15, 919.
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Resubmitted for review April 2, 2004 Revised manuscript received November 1, 2004 Accepted November 2, 2004 IE030835T
