Effect of H 2 H 2 O on the Reduction of Olivine Pellets in CO CO 2 Gas

Transcription

1 , pp Effect of H 2 H 2 O on the Reduction of Olivine Pellets in CO CO 2 Gas Antti KEMPPAINEN, 1) * Olli MATTILA, 2) Eetu-Pekka HEIKKINEN, 1) Timo PAANANEN 2) and Timo FABRITIUS 1) 1) Laboratory of Process Metallurgy, University of Oulu, P.O Box 4300, FI Finland. 2) Ruukki Metals Oy, Rautaruukintie 155, Raahe, FI Finland. (Received on March 30, 2012; accepted on June 7, 2012) Large amounts of injectants are used in the blast furnace (BF) process to reduce coke consumption, but this changes the gas composition in the BF shaft where iron ore reduction takes place. H 2 and H 2O gases change markedly in the gas atmosphere at high injection levels, which makes it important to investigate their effects on the reduction of iron oxides in a CO CO 2 atmosphere. The gaseous H 2 and H 2O content in the BF shaft atmosphere is approximately 8%. In the present work olivine pellets were reduced in H 2 H 2O CO CO 2 and CO CO 2 atmospheres with equal reducing potentials of H 2 and CO by fixing the H 2/H 2O and CO/CO 2 ratios. No significant differences in the reduction rates of iron oxides were found between the H 2 H 2O CO CO 2 and CO CO 2 atmospheres at high temperatures but at lower temperatures H 2 H 2O CO CO 2 had higher reduction rate. Activation energies determined for hematite to magnetite reduction for both gas mixtures indicated better initial stage and later stage reduction in the H 2 H 2O CO CO 2 atmosphere. Field Emission Scanning Electron Microscope (FESEM) analysis was carried out on samples, and wüstite relics were found in the inner parts of pellets reduced to iron in the CO CO 2 gas but not in the samples reduced in the H 2 H 2O CO CO 2 gas. KEY WORDS: thermogravimetric experiments; iron oxide; reduction; hydrogen; water vapor; carbon monoxide; carbon dioxide. 1. Introduction Large amounts of injectants, e.g. coal or oil together with oxygen enrichment, are commonly used in the blast furnace (BF) process to reduce coke consumption. This procedure alters the gas composition in the BF shaft, where the reduction of iron ore takes place. At high injection levels gas composition changes markedly in terms of H 2 and H 2O gases in the reducing gas atmosphere, which makes it important to investigate the effects of these gases on the reduction of iron oxides in a CO CO 2 atmosphere. Several authors have carried out reduction experiments on iron oxides in CO, 1 3) H 2 4,5) or CO H 2 1,2,6) mixtures and have shown that an addition of H 2 to CO speeds up the reduction of iron oxides when pure reductive gases are used. 2,6,7) Various activation energies have been reported in the literature for iron oxide reduction with CO, H 2 and CO H 2 gases, mainly on account of the different conditions under which the reduction experiments have taken place. The reduction experiments reported here were carried out on olivine pellets in equilibrated CO CO 2 and H 2 H 2O CO CO 2 gases, conditions that have not been reported in the literature earlier, and the activation energies were determined for hematite to magnetite reductions. 2. Experimental The laboratory experiments in the reduction of olivine * Corresponding author: antti.kemppainen@oulu.fi DOI: pellets were carried out in a thermogravimetric analysis furnace (TGA). Low magnetite grade pellets weighing between 4.0 and 4.3 grams were selected in order to enhance comparison, and these were reduced at temperature range and at and C with CO/CO 2 gas composition ratios of 15/85, 65/35 and 90/10, reducing the hematite pellets to magnetite, wüstite and iron, respectively. The pellets were pre-heated in an N 2 atmosphere to the desired temperature and then kept under isothermal conditions in a reducing atmosphere. Chemical analysis was used to determine the total iron-bound removable oxygen. Chemical composition of low magnetite grade olivine pellets is shown in Table 1. The degree of reduction of pellets was calculated by weight loss method using the Eq. (1): Reduction (%) Weight of oxygen removed from iron oxides =... (1) Total weight of removable oxygen in iron oxides Table 1. Chemical analysis of the low magnetite grade olivine pellets. Component Content [mass-%] Fe tot 67.1 FeO 0.1 CaO 0.38 SiO MgO 1.25 Al 2O ISIJ

2 Table 2. Compositions of the gas mixtures, gas flow rates and temperatures used in the reduction experiments. Experiment H 2 (%) H 2O (%) CO (%) CO 2 (%) Time (min) Flow (l/min) Temperature ( C) Fig. 2. Temperatures and H 2O/H 2 ratios in the reduction gas mixtures of the performed experiments shown on an Fe Fe 1 yo Fe 3O 4 phase diagram. 8) Fig. 1. Temperatures and CO 2/CO ratios in the reduction gas mixtures, shown on an Fe Fe 1 yo Fe 3O 4 phase diagram. 8) Since the H 2 H 2O gas mixture content of the total gases in the BF shaft was estimated to be 8%, the effect of a H 2 H 2O mixture on the reduction of the iron ore pellets in the CO CO 2 mixtures was investigated by replacing 8% of the CO CO 2 flow with H 2 H 2O, added to the mixture in a ratio at which the equilibrium partial pressure of oxygen for the carbon dioxide and water vapor formation reactions were equal in the total gas flow. Thus the reduction potentials of hydrogen and carbon monoxide were equal during the reduction of the iron ore pellets. The computed gas composition ratios for 8% H 2/H 2O flows at temperatures of , and C were 1.1/6.9, 4.2/3.8 and 6.4/1.6, respectively. The experimental conditions are illustrated in the Fe Fe 1 xo Fe 3O 4 phase diagrams presented in Figs. 1 and 2, and the compositions of the gas mixtures, gas flow rates and reduction temperatures are shown in Table Results 3.1. Reduction Rates The effects of the H 2 H 2O additions on the rate of reduction of the iron ore pellets in the CO CO 2 mixtures are shown in Figs Previous experiments had shown that the gas flow rates used here were high enough to prevent the reduction reactions from being restricted by mass transfer of the gaseous phase. A 8% H 2 H 2O addition to the CO CO 2 mixture at a fixed ratio increases the reduction rate at temperatures 750 and 800 C, as shown in Figs. 3 and 4. At 850 C reduction rate is increased slightly by the H 2 H 2O addition as shown in Fig. 5. At temperatures 900 and 1000 C the effect on the reduction rate is not significant, as shown in Figs. 6 and 7, and at C the reduction rates achieved in the experiments performed with the CO CO 2 and H 2 H 2O CO CO 2 mixtures are almost identical, as seen in Fig. 8. The higher reduction rates at temperatures 750 and 800 C can be explained by the occurrence of water-gas shift reaction (WGSR), which has its ΔG=0 at 827 C. It has been shown that at high temperatures WGSR has high reaction rate and introducing a small quantity of hydrogen into gas mixture can produce remarkable effect on reduction. The reducing time is shorted greatly since H 2 accelerates the reduction process and the product H 2O reacts with CO to create H 2, which takes part to in the reduction again. It has been proposed that part of H 2 does not participate in the reduction but it acts as something like catalyst which speeds up the reduction greatly. 9 11) The high amount of water vapor in the feed gas mixture (6.9%) at C temperatures also prefers WGSR to occur. Due to WGSR the gas consist probably more hydrogen, which has higher reduction potential than carbon monoxide and thereby increases the rate of reduction. Water-gas shift reaction is given in Eq. (2) below: CO+H 2 O H 2 +CO 2... (2) At temperatures above 850 C reduction rates with H 2 H 2 O CO CO 2 gas are not significantly higher than with CO CO 2 mixtures which indicates that with equilibrated mixtures 8% H 2 H 2 O addition has no significant effect on the reduction rate. The reverse WGSR above 850 C can be 2012 ISIJ 1974

3 Fig. 3. Reduction rate curves for pellets at 750 C in CO CO 2 H 2 Fig. 6. Reduction rate curves for pellets at 900 C in CO CO 2 H 2 Fig. 4. Reduction rate curves for pellets at 800 C in CO CO 2 H 2 Fig. 7. Reduction rate curves for pellets at C in CO CO 2 H 2 H 2O gas mixtures with a CO/CO 2 ratio of 65/35. Fig. 5. Reduction rate curves for pellets at 850 C in CO CO 2 H 2 also considered to explain the ineffectiveness of hydrogen as if hydrogen reacts to form CO and H 2 O at higher temperatures, it will not have increasing effect on the rate of reduction. According to the literature the first 0 5% H 2 addition to CO should raise the reduction rate significantly in an atmosphere consisting purely of reductive gases, 12) and the Fig. 8. Reduction rate curves for pellets at C in CO CO 2 H 2 H 2O gas mixtures with a CO/CO 2 ratio of 90/10. present experiment shows that such effect can be detected with CO CO 2 H 2 H 2 O gas mixtures under the conditions used here at temperatures 750 and 800 C but not at temperatures above 850 C ISIJ

4 3.2. Activation Energies The activation energy is defined as the energy barrier that must be surmounted to enable the occurrence of the bond redistribution steps required to convert the reactants into products. 13) Activation energies were determined here for CO CO 2 H 2 H 2O and CO CO 2 gas mixtures for hematite to magnetite reduction steps. Reactions which were expected to occur during the reduction for hematite to magnetite as shown in Eqs. (3) and (4). 3Fe 2 O 3 +CO=2Fe 3 O 4 +CO 2... (3) 3Fe 2O 3+H 2=2Fe 3O 4+H 2O... (4) The reduction rate curves in Figs. 3 5 appears to be linear the first 4% of reduction. After 4% of reduction the reduction curves appears to change to non-linear, which indicates Fig. 9. Apparent rate constant (k) for the olivine pellets calculated from the tangent of the fraction of oxygen removed as a fraction of time at 2% reduction. of change in the reaction mechanism. Therefore slopes for the determination of apparent activation energies were obtained from 2% and 8% of reduction in the reduction rate curves to determine activation energies for both stages. Since unreacted core-model is known to be problematic in iron ore reduction an Arrhenius equation was used to determine the reaction rate constant. The reaction rate constant k can be expressed as an Arrhenius equation in the form of either Eqs. (5) or (6) below.: or k = k e 0 EA RT... (5) EA 1 ln k = + ln k... (6) 0 R T where k 0 is the frequency factor for the reaction, E A the activation energy, R the gas constant (8.314 J K 1 mol 1 ), T the temperature (K) and e Napier s constant (2.718). Arrhenius plots obtained from calculated ln(k) and 1/T values at 2% and 8% reduction at temperature range C are shown in Figs. 9 and 10 and the apparent activation energies determined for the CO CO 2 and H 2 H 2O CO CO 2 gases are shown in Table 3. The activation energies at 2% and 8% reduction are within the range of the values reported in the literature, although the considerable differences in the experimental conditions (in terms of particle size, the partial pressures of the reducing and carrier gases, etc.) cause some variation between estimated activation energies quoted by different authors. In the reductions carried out by Piotrowski et al., 14) for instance, the Fe 2O 3 powder of mean size of 91 μm was reduced under a 30 ml/min gas flow rate, which differs considerably from the conditions used in the current experiments. El-Geassy 2) reduced Fe 2O 3 briquettes at a 1 l/min total gas flow rate and estimated the activation energies to be 56.5 kj/mol for CO and 28.8 kj/mol for H 2 CO, which are close to the values obtained in the current study. At 2% reduction the activation energies of kj/mol determined for CO and kj/mol for H 2 CO indicate higher reduction rate at initial reduction stage with H 2 H 2O Table 3. Apparent activation energies (E a) determined for the CO CO 2 and H 2 H 2O CO CO 2 gas mixtures at 2% and 8% reduction (kj/mol). Gas mixture E a at 2% red. E a at 8% red. CO CO H 2 H 2O CO CO Table 4. Estimated activation energies reported in the literature for the initial stages in the reduction of iron oxides. Fig. 10. Apparent rate constant (k) for the olivine pellets calculated from the tangent of the fraction of oxygen removed as a fraction of time at 8% reduction. Author Composition of reducing mixture E a(kj/mol) Piotrowski et al. 14) 10% CO + 90% N El-Geassy et al. 2) 100% CO 56.5 Piotrowski et al. 14) 10% H % N El-Geassy et al. 2) 100% H Piotrowski et al. 14) 4.3% H % CO + 90% N El-Geassy et al. 2) 50% H % CO ISIJ 1976

5 Fig. 11. addition. At 8% reduction determined activation energies of kj/mol for CO and kj/mol for H 2 CO indicate also higher reduction with H 2 H 2O addition. The effect of an H 2 addition to the CO in lowering the activation energy shown in the literature was detected here at both 2% and 8% reduction stages. The values for the activation energies of the initial stages in the reduction of iron oxides reported in the literature are shown in Table Sample Characterization Characterization of the iron oxide phases by Field Emission Scanning Electron Microscope showed that the pellets in experiments from 1 to 4 and from 7 to10 were reduced to magnetite (Fe 3O 4), those in experiments 5 and 11 were reduced to wüstite (FeO) and those in experiments 6 and 12 to iron (Fe). Larger wüstite relics were found in the inner parts of the pellet sample in experiment 6, but not in the pellet sample in experiment 12, as shown in light microscope images in Fig. 11. In Fig. 11 is also shown that smaller wüstite relics and other phases were found in the pellet samples of the experiments 6 and Discussion Optical light microscope images of the pellets reduced in experiments 6 (a) and 12 (b). The fact that the reduction rate achieved with 100% CO is much lower than with 100% H 2 may be due to the much lower diffusivity of CO and the probability of blockage by carbon deposited in the pores of the initially formed solid iron (2CO=CO 2+C). 1) According to the literature, pure hydrogen is a superior reducing agent at first, but later, at a high degree of reduction, the rate slows down. In the case of CO, the last portion of the oxygen is removed more quickly although initially the rate is sluggish. The longer time taken by hydrogen for complete reduction is probably due to the formation of a dense non-porous iron shell. Much quicker reduction than that obtained either with H 2 or with CO can be effected if both of them are present in the reducing gas. This is due to the carburizing effect of the CO gas on the iron. The carbon diffuses to the wüstite/iron interface, reacts to form CO+CO 2 and a high gas over-pressure is built up inside which breaks the surrounding iron film, thus permitting further gas exchange at the oxide/metal interface. In the case of hydrogen only, the steam over-pressure is not sufficient to burst the shell open. With a CO H 2 gas mixture, however, the CO performs the bursting, opening up the passage for hydrogen, with its greater reducing power. It has been proposed that a 0 5% H 2 addition to CO will increase the degree of reduction significantly (a 2 5% increase for each 1% H 2), but that a further 5% will give much less improvement (1% for each 1% of additional H 2). 9) El-Geassy 2) detected a significant increase in the rate of reduction in the initial stages when reducing Fe 2O 3 compacts at 600 to C in a gas mixture comprising 25% H 2 and 75% CO. He also found that the addition of CO to an H 2 atmosphere lowered the reduction rate in the initial stages due to the poisoning effect of CO but increased it in later stages due to the side reactions of the deposited carbon. Moon et al., 7) also studying the reduction behaviour of hematite compacts in H 2 CO gas mixtures at C, observed a decrease in the reduction rate with CO content in the gas over the whole temperature range studied. In the present experiments the reduction potentials of H 2 and CO were set as equal by fixing the CO/CO 2 and H 2/H 2O ratios with respect to the CO 2 and H 2O formation reactions. It was then seen that the effect of 8% H 2 H 2O addition to the CO CO 2 gas had an increasing effect on the rate of reduction of olivine pellets at temperatures 750 and 800 C, slightly increasing effect at 850 C and a negligible effect at 900, and 1150 C. As for the reason of higher reduction rate at lower temperatures can be considered water-gas shift reaction (WGSR) which increases the amount of hydrogen and thereby increases the reduction rate. At higher temperatures, where reverse WGSR is thermodynamically is favorable, no significant differences in the reduction rates achieved by CO CO 2 and H 2 H 2O CO CO 2 gas mixtures can be detected, which indicates that 8% H 2 H 2O addition does not affect reduction rate significantly at temperatures above 850 C when reduction potentials of H 2 and CO are set as equal by fixing the CO/CO 2 and H 2/H 2O ratios. The estimated activation energies for hematite to magnetite reduction indicated higher degree of reduction with H 2 H 2O CO CO 2 than with CO CO 2 at 2% and 8% reduction. For the reason for higher reduction rate can be considered better diffusivity of hydrogen to inner parts of the pellet. Activation energies of reduction at 2% and 8% for H 2 H 2O CO CO 2 mixture indicates of pore diffusion mechanism and mixed control mechanism for CO CO 2 mixture but ascertaining the rate controlling mechanism at the different reduction stages would need further investigation such as processing the reduction data with developed mathematical equations for proposed reduction mechanisms and morphological observations. 15) Determination of rate controlling mechanism is not made here but is planned for future research work. The only visible differences in the reduced pellets were wüstite relics found in the inner parts of the pellet reduced to iron in CO CO 2, the reason for the appearance of which is probably that H 2 is thermodynamically more efficient than CO for performing wüstite reduction at temperatures above 821 C. 12) In addition, the smaller molecule size of H 2 relative to CO provides more efficient diffusion to the inner parts of the pellet. 5. Conclusions The conclusions reached regarding the effect of an 8% H 2 H 2O addition to the CO CO 2 atmosphere used for the reductions of olivine pellets at temperatures C with fixed CO/CO 2 and H 2/H 2O ratios may be summarized as follows: ISIJ

6 8% H 2 H 2O addition to the CO CO 2 gas had an increasing effect on the rate of reduction of olivine pellets at temperatures 750 and 800 C and slightly increasing effect at 850 C. At temperatures 900, and C no significant influence on the reduction rate was detected by replacing 8% of the CO CO 2 mixture with an H 2 H 2O mixture at a ratio where the equilibrium partial pressures of oxygen for the carbon dioxide and water vapor formation reactions were equal. The accelerating influence of hydrogen shown in the literature with pure reductive gases was detected at temperatures C but not at temperatures above 900 C with H 2 H 2O CO CO 2 mixtures. The activation energies determined for hematite to magnetite reductions were in the range of reported values in the literature and indicated higher degree of reduction with the H 2 H 2O CO CO 2 gas than with CO CO 2 at 2% and 8% reduction. Scanning electron microscope analysis pointed to wüstite relics in the inner parts of the pellet reduced to iron in CO CO 2 atmosphere which indicates better wüstite reduction with the H 2 H 2O CO CO 2 gas than with CO CO 2. Acknowledgements This research is a part of the Energy Efficiency & Lifecycle Efficient Metal Processes (ELEMET) research programme coordinated by the Finnish Metals and Engineering Competence Cluster (FIMECC). Rautaruukki Oyj and the Finnish Funding Agency for Technology and Innovation (TEKES) are acknowledged for funding this work. Mr Tommi Kokkonen of the University of Oulu is acknowledged for his technical support. REFERENCES 1) A. A. El-Geassy, K. A. Shehata and S. Y. Ezz: Trans. Iron Steel Inst. Jpn., 17 (1977), ) A. A. El-Geassy: J. Mater. Sci., 21 (1986), ) K. Mondal, H. Lorethova, E. Hippo, T. Wiltowski and S. B. Lalvani: Fuel Process. Technol., 86 (2004), 33. 4) E. T. Turkdogan and J. V. Vinters: Met. Trans. AIME, 3 (1972), ) M. V. C. Sastri, R. P. Viswanath and B. Viswananthan: Int. J. Hydrogen Energ., 7 (1982), ) N. Towhidi and J. Szekely: Ironmaking Steelmaking, 8 (1981), ) I. J. Moon, C. H. Rhee and D. J. Min: Steel Res., 69 (1998), ) L. Von Bogdandy and H. J. Engell: The Reduction of Iron Ores - Scientific Basis and Technology, Springer-Verlag, Berlin, (1971), 38. 9) M. Keqin, L. Jiaxin and Z. Jingzhi: J. EIM, 10 (1993), 1. 10) H. Ono-Nakazato, T. Yonezawa and T. Usui: ISIJ Int., 43 (2003), ) J. Li, P. Wang, L. Zhou and M. Cheng: ISIJ Int., 47 (2007), ) A. K. Biswas: Principles of Blast Furnace Ironmaking, Cootha Publishing House, Brisbane, (1981), ) A. K. Galwey and M. E. Brown: Thermochim. Acta, 386 (2002), ) K. Piotrowski, K. Mondal, H. Lorenthova, L. Stonawski, T. Szymanski and T. Wiltowski: Int. J. Hydrogen Energ., 30 (2005), ) I. Sohn and M. Jung: Steel Res. Int., 82 (2011), ISIJ 1978