Kinetic study on reduction in carbon composite magnetite pellet employing thermogravimetry and quadruple mass spectrometry

Abstract

The carbothermic reduction in magnetite was investigated from a kinetic viewpoint in the temperature range of 900–1200°C employing thermogravimetric analysis and quadruple mass spectrometry. The critical temperature for the activation of carbon gasification was found to be 1055°C by non-isothermal reduction experiments. The carbon gasification reaction was activated after all the Fe₃O₄ was reduced to wüstite, i.e. at 25% reduction, and that the Fe catalysed nature of wüstite reduction to Fe was not identified before 25% reduction. The activation energy for the reduction in Fe₃O₄ to wüstite was evaluated to be 85·5 kJ mol⁻¹, and the reduction stage is believed to be limited by mixed control of Fe₃O₄ reduction and carbon gasification. From 40·7 kJ mol⁻¹ of the activation energy for the reduction in wüstite to Fe, the second half reduction stage might be limited by mixed control of wüstite reduction by CO and the gaseous diffusion. It is believed that the wüstite reduced from Fe₃O₄ has lower reactivity for the reduction to Fe compared with that originated from Fe₂O₃. In the present study, it was considered that the changeover in reaction mechanism might be carried out from mixed control of the reduction in Fe₃O₄ to wüstite and carbon gasification to the reduction in wüstite to Fe with increasing reduction.

Introduction

Owing to the shortage of high quality hematite ores, it is required to utilise magnetite ores in conventional blast furnace ironmaking or direct reduction iron (DRI) production.¹ The proportion of DRI usage in electric arc furnace (EAF) operation has been increasing because high quality and low residual scrap, which had traditionally been used in EAFs to produce high quality steel, is in shortage.² About 90% DRI is produced by the gas based direct reduction (DR) processes, such as MIDREX or HYL III, in those limited regions where natural gases can be supplied at relatively low cost.¹ In comparison with gas based DR process, the coal based DR processes employing rotary kiln or rotary hearth furnace are receiving worldwide attention owing to the effective usage of magnetite ores and non-coking coals in the form of coal composite iron ore agglomerates.¹ However, extensive understanding of DRI production technology using magnetite ore is limited.

The fundamental investigations on the mechanism and kinetics of carbothermic reduction in iron oxide were extensively carried out for the carbon composite iron ore agglomerates. Rao³ found that the availability of CO governs the reduction process in hematite and carbon powder mixtures and that the carbon gasification reaction constitutes the rate limiting step for the overall process. Srinivasan and Lahiri⁴ performed experiments in a nitrogen atmosphere to study the reduction in hematite pellets by graphite in the temperature range of 925–1060°C. They showed a continuous decrease in CO₂/CO ratio during reduction, the values being far away from FeO/Fe equilibrium for wüstite reduction by CO. In addition, the activation energy of the reduction process has been found to decrease with progress in reduction, indicating a possible changeover in reaction mechanism. Fruehan⁵ confirmed that the reduction in Fe₂O₃ to FeO takes place by means of the gaseous intermediates CO and CO₂, and that the overall rate is controlled by the carbon oxidation by CO₂. Fruehan⁵ also pointed out that the reduction in Fe₂O₃ to FeO is faster than that in FeO to Fe because its CO₂/CO equilibrium ratio is higher and hence the oxidation rate of carbon is faster. Szendrei and Berge⁶ studied the reduction in hematite and graphite composite using thermogravimetric analysis (TGA) equipped with differential thermal analysis (DTA), simultaneously conducting direct in situ analysis of reaction product gases during reduction. Coetsee et al.⁷ reported that heat transfer was found not to be limiting for the reduction reaction. In addition, for average rate constants, the chemical reactions are under mixed control by the reduction in iron oxide and carbon gasification. Chellan et al.⁸ studied the DR in mixed magnetite and coal pellets using induction heating. Yang et al.⁹ reported that the DR at the initial stage dominates carbothermic reduction in hematite–graphite composite and that it is determined by the indirect reduction together with the carbon gasification reaction. Recently, Zhu et al.¹⁰ studied the DR behaviours of composite binder magnetite pellets in coal based grate rotary kiln processes. They found that preheated pellets possess a self-curing function, which results in depressing degradation and preventing accretion in rotary kiln. However, most of the previous researches about carbothermic reduction have been carried out using TGA with some samples characterised by X-ray diffraction (XRD) without direct in situ monitoring of reaction product gases in the progress of reduction.

In the current study, the carbothermic reduction in magnetite in an Ar atmosphere has been investigated employing thermogravimetry and quadruple mass spectrometry (QMS) gas analysis with the intention of understanding in detail the reduction progress of carbon composite iron ore agglomerates in the rotary kilns. The kinetics and mechanisms of carbothermic reduction in Fe₃O₄ would greatly be improved through sophisticated analyses of the compositional change of the reaction gases using the precisely calibrated QMS, which can facilitate the clear interpretation of the chemical reactions taking place in the carbon composite Fe₃O₄ pellets.

Experimental

Materials

Reagent grade of Fe₃O₄ (1 μm and 99·8% purity) and graphite powder (5 μm and 98·0% purity) with a molar ratio of 1∶4 were homogeneously mixed using a ball mill. The pellets were formed to cylindrical shape (10 mm diameter and 7 mm height) weighing 1·5 g (±0·01 g) by a hydraulic press under the pressure of 50 MPa for 1 min with distilled water added to ensure complete compactness. The pellets were air dried in an oven at 150°C before use.

Experimental procedure

The pellet was placed into a cylindrical Al₂O₃ crucible (diameter 14 mm, height 20 mm). Then, the crucible was suspended and connected using a Pt wire to the balance part of a TGA apparatus (RUBOTHERM, Germany). The isothermal reduction experiments were performed rapidly by heating up the TGA furnace at 100°C min⁻¹ to the target temperature in the range of 900–1200°C, and the reaction temperature was controlled within ±3°C. The flowrate of Ar carrier gas was preliminarily determined to be 250 mL min⁻¹. The reduction degree of magnetite–graphite composite was calculated based on the continuous weight change of the pellet. The quantitative compositions of the product gases were continuously monitored by connecting TGA to the quadruple mass spectrometer (IPI, GAM400) preliminarily calibrated. In addition, the quantitative phase compositions of the reduced pellets or the pellets withdrawn and quenched in the progress of reduction were evaluated by applying Rietveld refinement of X-ray powder diffraction (XRD) patterns.

Calculation of reduction degree

The entire reduction can be expressed by the following reaction 1 In the current experiments, the magnetite and graphite were mixed with a molar ratio of 1∶4. The reduction degree (%) was defined as the ratio of pellet weight decrease to the theoretical weight change, which corresponds to 0·4006 for magnetite in case reaction (1) goes to completion 2 where W _o is the initial pellet weight before reduction (g), ΔW is the pellet weight decrease (g) and MW is the molecular weight (g mol⁻¹).

Results and discussion

Non-isothermal reduction in magnetite–graphite pellets

Non-isothermal reduction in magnetite and graphite pellets was investigated to identify some chemical reactions taking place in the reduction progress by means of TGA-DSC (differential scanning calorimetry) thermal analysis. In the experiments, QMS was coupled with the TGA-DSC combination for direct in situ analyses of the reaction product gases for more thorough confirmation of some reactions. As shown in Fig. 1, ∼187·025 mg Fe₃O₄–graphite was placed into the sample position of the TGA-DSC apparatus (Mettler TGA/DSC1, Switzerland), and the temperature was raised to 1200°C at 10°C min⁻¹ in an Ar atmosphere of 50 mL min⁻¹.

Thermal analysis results by TGA-DSC coupled with QMS for reduction in magnetite with graphite [sample: 187·025 mg (Fe₃O₄+4C pellet)]

The weight of Fe₃O₄+graphite pellet started to decrease at ∼700°C with increasing temperature. As was shown in the gas composition data by QMS in Fig. 1, reduction in Fe₃O₄–graphite pellet continued to proceed until all the Fe₃O₄ was reduced to wüstite, which corresponds to exactly 25% reduction. In particular, the QMS data indicate that the first CO₂ gas was generated in the temperature range of 700–900°C. In addition, with increasing temperature, the second generation of CO₂ gas was detected at 900–1050°C. 3 4 It is believed that the CO₂ formation was ascribed to the combined reactions of equations (3) and (4). That is, CO gas generated by equation (3) was immediately used to reduce other Fe₃O₄ around it, which results in the production of CO₂. Because the gas–solid reaction of equation (4) is much faster than a solid–solid reaction of equation (3), only CO₂ gas can be identified by QMS in the reduction stage during heating. This can be supported by DSC data in Fig. 1 since the combined reaction of equations (3) and (4) is definitely endothermic. This was inferred from the change of DSC curve showing the weak endothermic characteristics in Fig. 1.

As shown in Fig. 1, the reduction in Fe₃O₄–graphite pellet rapidly proceeded right after 25% of reduction degree by the fast increase in reduction degree. This can be explained by the QMS data, indicating that the partial pressure of CO abruptly increased, which is due to the carbon gasification. The carbon gasification reaction confirmed by the abrupt increase in partial pressure of CO was supported by DSC curve in Fig. 1. That is, as the temperature reached ∼1055°C, in the non-isothermal reduction, some dramatic endothermic reaction was detected by DSC curve, which must be the carbon gasification reaction. The onset temperature of carbon gasification reaction was reported to be ∼1070°C by Szendrei and Berge⁶ for the non-isothermal reduction in hematite–graphite pellet. It is quite interesting that the carbon gasification was activated right after the completion of Fe₃O₄ reduction to wüstite. This indicates that carbon gasification was activated by solid Fe reduced from wüstite right after all the Fe₃O₄ was reduced to FeO. The catalytic effect of metallic Fe was also reported by previous investigations.^3,6 That is, the nucleation and growth of iron phase formed by wüstite reduction worked as a catalyst for the acceleration of the carbon gasification reaction. Another noticeable observation was that small amount of CO gas was generated at ∼1150°C according to the QMS data. It is believed that this is ascribed to the reduction in remaining wüstite to iron by the dissolved carbon in iron, taking the eutectic point (4·3 wt-%C, 1147°C) of Fe–C phase diagram into account:¹¹ 5

Effect of temperature on isothermal reduction

The reduction experiments were performed at six different temperatures in the range of 900–1200°C as shown in Fig. 2. The change of reduction degree at temperatures higher than 1000°C shows similar patterns to those previously reported,^9,12 that is, the reduction process can be classified into three zones. The reduction reaction at the initial stage proceeded quite slowly up to ∼25% reduction. However, the reduction rate in the second stage became extremely fast and the final reduction reached higher than ∼90%. The reduction at the third stage became slow again.

Effect of temperature on reduction degree of magnetite–graphite pellet

As indicated in Fig. 3, the reduction in Fe₃O₄–graphite pellet at 1000°C took ∼45 min out of the entire reduction time of 55 min to reach 25% reduction. Twenty-five per cent reduction corresponds to the state where all the magnetite reduces to wüstite with respect to the reduction completion of Fe₃O₄+graphite pellet by equation (1). That is, 25% reduction represents the state by equation (6) and can be evaluated by equation (7) based on equation (2) 6 7 Furthermore, 25% reduction can be confirmed by the theoretical calculation of the reduction in magnetite (molar ratio of O/Fe = 1·33) to wüstite (molar ratio of O to Fe = 1·00) by taking the relative molar ratio of oxygen to iron of magnetite and wüstite into account. As is similar to the QMS data in the non-isothermal reduction, some amount of CO₂ was detected in the initial period of the reduction in comparison with CO. This also indicates that CO gas generated by DR in Fe₃O₄ was used to reduce Fe₃O₄ by gaseous reduction by equations (3) and (4).

Effect of reaction time on reduction degree of magnetite–graphite pellet at 1000°C

Another interesting result in Fig. 3 is that the partial pressure ratio of CO₂ and CO, in the duration between 25 and 45 min was almost constant during this period from the QMS data. The data indicates that the CO/CO₂ gases in reaction (8) are closely in equilibrium with Fe₃O₄/wüstite. The constant variation of was reported by Srinivasan and Lahiri.⁴ This can also indicate that carbon gasification reaction takes place relatively slowly at 1000°C. Turkdogan¹³ previously reported that CO consumption rate by the reduction in iron oxide is higher than the CO generation by non-catalytic oxidation of coke in the temperature range of 900–1100°C. The report by Turkdogan supports the current result in the reduction progress from Fe₃O₄ to wüstite.¹³ According to the QMS data, it is believed that the carbon gasification reaction is not in highly activated state at 1000°C. On the other hand, it took just 8 min for the reduction to proceed from 25% of reduction degree to completion. This indicates that the wüstite was reduced to metallic Fe with slight CO gas generated by mild reaction of carbon gasification right after all the magnetite was reduced to wüstite and that the ratio of reaches less than the value for FeO/Fe equilibrium at 1000°C. The decisive driving force is the carbon gasification activated by increased CO₂ and catalysed by metallic Fe, which makes great contribution to the reduction in wüstite to metallic Fe by providing the strongly reducing CO gas. This results in quickly reaching the final reduction in higher than 90% 8 9 However, as indicated in Fig. 4, isothermal reduction at 1100°C showed that the reduction in magnetite–graphite pellet proceeded very fast and its reduction reached ∼96% in <27 min. In addition, it took just 5 min for the reduction to be completed starting from 25% reduction, which is slightly faster than the reduction pattern at 1000°C. It is believed that the obvious difference between the reduction in Fe₃O₄–graphite pellet at 1000 and 1100°C is ascribed to the result in Fig. 1. That is, it was observed that the onset temperature of the carbon gasification reaction was ∼1055°C in terms of DSC and QMS data. Generally, it has been widely known that 1100°C is the critical temperature above which the carbon gasification reaction goes to completion.¹¹ In other words, the reduction rate of Fe₃O₄–graphite pellet strongly depends on the temperature of reduction where the carbon gasification reaction can be positively activated as is clearly demonstrated by the QMS data. It was noted that the gasification reaction rate rapidly increases, which results in sharply improving the reduction right after the reduction reaches 25% (wüstite stage). This can be explained by the previous interpretation concerning the catalytic effect due to the rapid nucleation and spread of metallic iron.⁶ The autocatalytic nature of wüstite reduction was previously inferred by Rao.³ However, in the current investigation, Fe catalysed nature of wüstite reduction by Rao was not found before 25% reduction.

Effect of reaction time on reduction degree of magnetite–graphite pellet at 1100°C

Determination of rate controlling step by evaluation of activation energies for reduction in Fe₃O₄–graphite pellet

To estimate the activation energies for the reduction in magnetite to wüstite and for that of wüstite to metallic Fe, the uniform internal reduction model^5,14 was applied to the current iron ore and carbon composite system 10 11 where f represents the fractional reduction degree, k is an apparent rate constant and E _A is the activation energy. Equation (11) indicates that the activation energy for the corresponding reduction reaction can be evaluated by plotting the logarithms of the apparent rate constant against the reciprocal of temperature. From the data of ln (1−f) with time in the temperature range of 1000–1150°C, the apparent rate constants for the reduction in Fe₃O₄ to wüstite and for that in wüstite to Fe were estimated by differentiating the data curve in the period of time corresponding to each reduction stage.

As is clear from the results plotted in Fig. 5, the rate constants for the reduction in Fe₃O₄ to wüstite is one to two orders of magnitude smaller than those for the reduction in wüstite to Fe and the difference between the rate constant of each reduction stage becomes smaller with increasing temperature. The activation energy values of 85·5 kJ mol⁻¹ evaluated in the current system can be compared with the previously reported activation energies (64–74 kJ mol⁻¹) for the reduction in magnetite to wüstite under carbon monoxide.^15,16 In general, CO consumption rate in the reduction in iron ore is larger than CO generation rate by non-catalytic oxidation of coke.¹³ That is, the reduction rate of iron oxide is faster than the rate of carbon gasification reaction in the temperature range of 900–1100°C. Considering that the current activation energy was evaluated in the temperature range of 1000–1150°C, it is believed that the reduction in Fe₃O₄ to wüstite is limited by mixed control of Fe₃O₄ reduction and carbon gasification.⁷

Temperature dependence of apparent rate constants for reduction in Fe₃O₄ by graphite

On the other hand, 40·7 kJ mol⁻¹ of the activation energy was obtained for the reduction in wüstite to metallic Fe. This value is much smaller than those by CO gas, which ranges from 116 to 151 kJ mol⁻¹.^15,17,18 The order of magnitude of the greatly decreased activation energy values indicates that the reduction in wüstite to metallic Fe might be limited by mixed control of wüstite reduction by CO and the gaseous diffusion through the reduced Fe layer for which 8–21 kJ mol⁻¹ of activation energies have been reported.¹¹ The activation energy for the reduction in wüstite to Fe is smaller than that for the reduction in magnetite to wüstite, which is in good agreement with the previous results.^4,19 That is, lower values of activation energy at high degrees of reduction are apparent since the activation energy decreases with increasing the reduction degree of iron oxide.

Reducibility of wüstite strongly depends on its activity, which is affected by the degree of oxidation from which it is originated. That is, the reduction rate of wüstite to Fe can be determined by its originality, i.e. hematite and magnetite. To compare the reduction rates of wüstites that originated from magnetite and hematite respectively, it was undertaken to compare the activation energies of wüstite to Fe. The activation energy for the reduction in wüstite that originated from magnetite, 40·7 kJ mol⁻¹, was larger than that for the reduction in wüstite that evolved from hematite, 25·9 kJ mol⁻¹.²⁰ It is believed that the wüstite reduced from Fe₃O₄ has lower reactivity for the reduction to Fe than that originated from Fe₂O₃, which results in the larger temperature dependence of reduction rate constant of wüstite reduced from magnetite to Fe. Therefore, the current result is in good agreement with the general knowledge about the reducibility of wüstite depending on its origin.

Compositional change of product gases in progress of reduction

The DR in iron ore with solid carbon can be carried out usually under vacuum by equation (12) 12 The actual reduction reaction usually takes place through gaseous intermediates at atmospheric pressures and can be expressed by the following reactions²¹ 13 14 That is, carbon must pass through the intermediate stage of CO formation before reaction with iron oxides. As is clear in the current system, it can be considered that the initial formation of CO is ascribed to direct solid–solid reaction between magnetite (Fe₃O₄) and carbon. The gas–carbon and gas–oxide reactions proceed in parallel and influence each other through the composition of the resulting CO–CO₂ atmosphere. It is clear that preliminary knowledge of the gas composition will facilitate the theoretical interpretation of the kinetics and mechanisms of reduction. For this reason, the change of gas composition in terms of CO% in the CO+CO₂ gas mixture during the reduction at 1100°C was plotted in Fe–O–C equilibrium diagram as shown in Fig. 6.

Progressive change of reaction gas generated in reduction progress of Fe₃O₄–graphite pellet quickly heated from room temperature to 1100°C in Fe–O–C diagram

According to the compositional change of product gas by QMS in Fig. 6 and the heating profile in Fig. 4, the reduction progress can be classified into several stages in terms of CO% in CO+CO₂ gas mixture. First of all, the first stage indicates that CO% in the atmosphere gradually increases while the magnetite–graphite pellet was being heated in the temperature range of 400–750°C. This clearly indicates that CO gas was generated by the solid DR between magnetite and graphite as previously shown in Fig. 4. Compared with the result in the reduction in hematite–graphite pellet,²⁰ the proportion of CO in the gas mixture is higher, which indicates that the reduction rate of Fe₃O₄ by CO is relatively lower in the first stage. The second stage is the rapid decrease in CO due to CO₂ gas generated by the reduction in Fe₃O₄ to wüstite, which results in reaching the local minimum amount of CO gas at ∼1050°C. The third stage represents the rapid generation of CO gas by carbon gasification reaction catalysed by metallic Fe. From this stage, the reduction rate of magnetite–graphite pellet starts to be accelerated towards the equilibrium concentration of CO determined by the gasification curve in Fig. 6, which plays the decisive role in reducing the wüstite into metallic Fe. The %CO in the gas slightly decreases due to the further reduction in wüstite into Fe and again increases by carbon gasification reaction; the reduction is then finished. In particular, in case no more carbon is available, the gas composition is determined by the wüstite/Fe equilibrium curve of Fe–O–C diagram. The indication in Fig. 6 is that the critical temperature and some accumulation of CO₂ gas are required for the activation of carbon gasification, which results in improving the rapid reduction in wüstite to Fe.

Evaluation of phase changes in reduction progress by XRD patterns of pellets (Rietveld refinement)

To quantitatively confirm the phase changes in the reduction progress of magnetite – graphite pellet, the pellets obtained at several reduction degrees were analysed with XRD and the results are shown in Fig. 7. The specific reduction degrees were preliminarily chosen considering the weight change data of magnetite–graphite reduction at 1100°C as shown in Fig. 4. Considering the heating rate of 100°C min⁻¹ from room temperature to 1100°C, the pellets were withdrawn from the TGA at the specific time and quenched to room temperature in an Ar atmosphere. The pellets were then weighed to evaluate the reduction.

X-ray diffraction (Cu K _α radiation) patterns of pellets at different reduction degrees

For the identification of the phases that evolved in the reduction progress, the positions of the major and minor peaks for graphite, magnetite, wüstite and iron are displayed in Fig. 7. The XRD patterns obtained using Cu K_α radiation in Fig. 7 indicate that the initial pellet consists of magnetite and graphite. The strongest line of magnetite appeared at the diffraction angle of 2theta = 35·47° for the crystal plane (311) and that of graphite was detected at 2θ = 26·60° for the crystal plane (006). As the reduction proceeded, it was clearly observed that the peak intensity of magnetite reduces while those of wüstite (Fe_0·942O) and iron were successively confirmed.

The XRD patterns in Fig. 7 clearly show that almost all the magnetite in the pellet transformed to wüstite when the reduction reached ∼23%. As previously mentioned, 25% reduction corresponds to the fraction of oxygen to be removed from magnetite when all the magnetite is reduced to wüstite. According to the QMS data, the CO₂ gas started to decrease and CO gas gradually increases after some duration of constant values. That is, the carbon gasification reaction gradually starts to be activated from this point. This means that the reaction rate constant of reaction (16), k ₁₆ becomes larger than that of reaction (15), ∼25% reduction, which makes great contribution to the rapid reduction in wüstite to Fe. This process can reasonably be explained by the abrupt increase in %CO in the product gas as shown in Fig. 4 15 16 17 X-ray diffraction patterns in Fig. 7 have been usually utilised for identifying the phases of concern in a qualitative manner. It has been undertaken to improve the method of analysing the quantitative compositions of the constituting phases of a powder sample by the Rietveld refinement.^22–24 In this study, the quantitative phase compositions for Fe₃O₄, wüstite and metallic Fe were evaluated using the TOPAS (Total Pattern Analysis Solution) software based on Rietveld refinement, and the results are shown in Table 1 and Fig. 8. In case the reduction degree reached 23%, it was noted that a significant amount of Fe₃O₄ was reduced to wüstite. As the reduction proceeded toward 25%, the wüstite phase content gradually increased, while the magnetite concentration decreased. As shown in Table 1, according to Rietveld refinement, it was confirmed that metallic Fe was not formed at reductions <25%, which theoretically corresponds to the completion of Fe₃O₄ reduction to wüstite. This indicates that the carbon gasification reaction was not catalysed by metallic Fe until all the magnetite was reduced to wüstite at 25% reduction. According to the QMS data in Fig. 4, CO₂ generation was gradually increased by the continuous reduction in Fe₃O₄ to wüstite and the carbon reactivity for activating the carbon gasification reaction was improved due to the catalytic effect of metallic Fe formed by reaction (17), as was previously proposed by Vastola and Walker.²⁵ Then, the CO produced by reaction (16) made great contribution to the accelerated reduction in wüstite to metallic Fe.

Change of phase composition in progress of reduction

Table 1.

Phase compositions of pellets in reduction progress, which was evaluated by Rietveld refinement

Reduction degree/%	Phase composition/wt-%
Reduction degree/%	Fe₃O₄	Wüstite (Fe_0·942O)	Fe
23	22·3	74·8	Trace
40	14·6	67·9	17·0
60	6·6	51·1	42·2
99	0·0	Trace	99·0

Possible changeover of reaction mechanism during reduction progress

In case some specific reaction mechanism is identified in the progress of non-isothermal reduction progresses, the slopes of the lines of temperature dependence of reduction rate in the chemical control region decrease with increasing reduction degree.¹¹ That is, the reduction becomes increasingly diffusion controlled since chemical reaction has higher activation energy than gaseous diffusion. This indicates that the activation energies decrease in the chemical control region with increasing reduction.⁴

According to the measurements made about the non-catalytic oxidation of coke granules in CO–CO₂ mixtures by Turkdogan,¹³ it was shown that the reduction rate of iron oxide giving rise to CO₂ is higher than that of carbon gasification and that carbon gasification becomes prominent only >1100°C. The result can be supported by the previous study carried out by Otsuka and Kunii¹⁹ who evaluated the activation energies of carbon gasification and iron oxide reduction by CO to be 250–420 and ∼54 kJ mol⁻¹ respectively. As previously mentioned, it was found that the reaction rate constants were varied depending on the reduction progress and that the temperature dependence of rate constants was evaluated in terms of the activation energy in the different reduction stage. Furthermore, the constant variation of in the reduction stage of Fe₃O₄ to wüstite strongly suggests that the reduction in Fe₃O₄ to wüstite is in equilibrium.

Based on the major results, the changeover in reaction mechanism of the reduction in magnetite–graphite pellet can schematically be expressed by Fig. 9. At low reduction where carbon gasification is not activated, the carbon gasification reaction can be rate controlling. As the reduction increases, the rate of carbon gasification reaction is close to that of the reduction in Fe₃O₄ to wüstite and then the rate of carbon gasification becomes higher than that of wüstite reduction to metallic Fe. That is, the changeover in reaction mechanism is carried out from carbon gasification to the reduction in wüstite by CO due to the activation of carbon gasification reaction with increasing reduction. However, in case the isothermal reduction in magnetite–graphite pellet is performed in the temperature range where the carbon gasification reaction is activated to some degree, carbon gasification reaction may not be rate controlling throughout the course of reduction, the final stages being probably controlled by the reduction in wüstite by CO. It is believed that the reduction rate of magnetite is under mixed control of the reduction in Fe₃O₄ to wüstite and carbon gasification in the reduction progress up to 25% of reduction degree, considering that the current activation energy was evaluated in the temperature range of 1000–1150°C.⁷

Changeover of reaction mechanism during reduction progress

Future work

The present results were obtained for the reduction in reagent grade Fe₃O₄ and graphite pellet by coupling TGA, QMS and/or DSC. As previously mentioned, due to the shortage of high grade iron ores and coals, new alternative ironmaking processes should be developed for the extensive utilisation of low grade ores and coals, which results in the decrease in the production cost of liquid iron. The experimental technique developed in the current study can be useful means of investigating the efficient combination of ores and coal pellet used in the DR process. Therefore, additional research should be followed about the reduction in commercial iron ore and coal pellet by employing TGA–QMS–DSC in complementary manner in order to find the optimal reduction conditions.

Conclusions

The reduction in magnetite–graphite pellet has been investigated from a kinetic viewpoint employing thermogravimetric measurement technique and QMS gas analysis in the temperature range of 900–1200°C. From the findings, the following conclusions were obtained.

According to the non-isothermal reduction experiment of Fe₃O₄–graphite by coupling TGA–DSC–QMS, it was found that the critical temperature was 1055°C, where the carbon gasification reaction started to be activated.

In the reduction progress of magnetite–graphite pellet, it was observed that the carbon gasification reaction was activated right after all the Fe₃O₄ was reduced to wüstite, i.e. at 25% of reduction degree, and that the Fe catalysed nature of wüstite reduction to Fe was not identified before 25% reduction.

Considering that the activation energy of 85·5 kJ mol⁻¹ for the reduction in Fe₃O₄ to wüstite was evaluated in the temperature range of 1000–1150°C, it is believed that the reduction in Fe₃O₄ to wüstite can be limited by mixed control of Fe₃O₄ reduction and carbon gasification.

The activation energy for the reduction in wüstite to Fe was estimated to be 40·7 kJ mol⁻¹ and the reduction stage might be limited by mixed control of wüstite reduction by CO and the gaseous diffusion through the reduced Fe layer.

It is believed that the wüstite reduced from Fe₃O₄ has a lower activity for the reduction to Fe compared with that originated from Fe₂O₃, which results in larger temperature dependence of reduction rate constant of wüstite to Fe.

According to Rietveld refinement, it was confirmed that metallic Fe was not formed in the reduction degree <25% whose point theoretically corresponds to the completion of Fe₃O₄ reduction to wüstite.

The experimental approach developed in the current study can be applied to the control of the reduction progress of iron ore and coal composite in DR processes by in situ analysing the reaction gas compositions.

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Kinetic study on reduction in carbon composite magnetite pellet employing thermogravimetry and quadruple mass spectrometry

Abstract

Introduction

Experimental

Materials

Experimental procedure

Calculation of reduction degree

Results and discussion

Non-isothermal reduction in magnetite–graphite pellets

Effect of temperature on isothermal reduction

Determination of rate controlling step by evaluation of activation energies for reduction in Fe3O4–graphite pellet

Compositional change of product gases in progress of reduction

Evaluation of phase changes in reduction progress by XRD patterns of pellets (Rietveld refinement)

Phase compositions of pellets in reduction progress, which was evaluated by Rietveld refinement

Possible changeover of reaction mechanism during reduction progress

Future work

Conclusions

References

Determination of rate controlling step by evaluation of activation energies for reduction in Fe₃O₄–graphite pellet