Fluidization behavior and reducibility of iron ore fines during hydrogen-induced fluidized bed reduction

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Fluidization behavior and reducibility of iron ore fines during hydrogen-induced fluidized bed reduction Daniel Spreitzer ∗ , Johannes Schenk Chair of Ferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria

a r t i c l e

i n f o

Article history: Received 9 September 2019 Received in revised form 30 October 2019 Accepted 26 November 2019 Available online xxx Keywords: Fluidized bed Iron ore reduction Hydrogen Fluidization behavior Kinetic analysis

a b s t r a c t A laboratory fluidized bed reactor was used to investigate the fluidization behavior and reducibility of various iron ore fines. Hydrogen was chosen as a reducing agent across a temperature range of 873–1073 K. The magnetite ore used exhibited strong sticking behavior after the initiation of metallic iron formation. All other tested ores fluidized sufficiently well when subjected to the same high reduction temperatures. Parallel kinetic analysis was conducted using a previously developed model to include three rate-limiting step types. The trend of apparent activation energy was correlated with the degree of reduction. Additionally, the influence of varying the specific gas rate was investigated. The results show the variation in reducibility as a result of different interactions, which influence the rate-limiting mechanisms of nucleation and the undertaken chemical reactions, which vary as a function of temperature and degree of conversion. The apparent activation energies, determined from the reduction of wüstite to metallic iron, were in the range of 15–60 kJ/mol, depending on the iron ore used and the degree of conversion. The change in apparent activation energy deriving from the increased specific gas rate can be explained by an increasing nucleation effect, especially at lower reduction temperatures. © 2020 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Introduction Conventional ironmaking technologies, such as blast furnaces combined with basic oxygen converters, have improved over the last decades in terms of energy consumption and productivity. Owing to the improved efficiencies, the energy demand of the global steel industry has decreased to a level of 40% when compared with the 1960s (World Steel Association, 2019). At present, these technologies, especially blast furnaces, are working close to their thermodynamic limitation, which is why there is no possibility for a further increase in the efficiency of the blast furnace process itself. Pulverized coal injection has been developed in recent years to substitute expensive coke with cheaper coal (Lyalyuk, Tarakanov, Kassim, Otorvin, & Pinchuk, 2017; Nomura & Callcott, 2011). As a result of the blast furnace process concept, lumpy input materials such as sinter or pellets and coke are required. Separate energy-demanding process steps for the preparation of the input materials are required, including sinter or pelletizing plants and coking plants. Hence, numerous direct and smelting reduc-

∗ Corresponding author. E-mail addresses: [email protected] (D. Spreitzer), [email protected] (J. Schenk).

tion processes have been developed over the last few decades to circumvent such processes and to improve the efficiency of ironmaking to be consistent with environmental issues (Chatterjee, 2012, 2014; Hasanbeigi, Arens, & Price, 2014; Xu & Cang, 2010). Some of these processes are based on fluidized bed technologies that have the additional advantage of directly using iron ore fines without prior agglomeration (Schenk, 2011). An example of a smelting reduction process using the fluidized bed technology for pre-reduction on an industrial scale is the Finex® process (Thaler et al., 2012). Proven industrial direct reduction processes based on fluidized bed technologies are the Finmet® and Circored® processes (Elmquist, Weber, & Eichberger, 2002; Hillisch & Zirngast, 2001; Lucena, Whipp, & Albarran, 2007; Nepper, Sneyd, Stefan, & Weckes, 2011). Aside from the direct use of iron ore fines, fluidized bed direct reduction processes have the possibility of using only hydrogen as a reducing agent, as performed in the Circored® process. An almost CO2 -free ironmaking process is possible, which is in-line with future climate targets regarding CO2 emissions. Direct reduction processes operate in a temperature range below the melting temperature of all phases involved in the process. Consequently, there is no possibility to remove gangue from the iron ore used. Iron ores with high iron content and a low gangue ratio are required, which is a challenge for the beneficiation processes of the ores. In addition to the lower gangue content, higher reducibility perfor-

https://doi.org/10.1016/j.partic.2019.11.006 1674-2001/© 2020 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Spreitzer, D., & Schenk, J. Fluidization behavior and reducibility of iron ore fines during hydrogeninduced fluidized bed reduction. Particuology (2020), https://doi.org/10.1016/j.partic.2019.11.006

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Nomenclature A AReactor a Ea Fetot f(x) g k k(T) m n O p R T t u0 umf ut V˙ w x xi

Pre-exponential factor, s−1 Free reactor area, m2 Nucleation rate constant, s−1 Apparent activation energy, kJ/mol Iron amount in the sample portion, mol Mathematical function Gravitational constant, m/s2 Rate constant, s−1 Arrhenius rate constant, s−1 Mass of sample portion, g or kg or t Kinetic exponent Oxygen bonded on iron, mol Pressure drop, mbar Gas constant, J/(mol K) Reduction temperature, K Reduction time, s Superficial gas velocity, m/s Minimum fluidization velocity, m/s Terminal velocity, m/s Flow rate, NL/min or Nm3 /h Weight factor Conversion Molar ratio of species i

Abbreviations BET Brunauer–Emmett–Teller GOD Gas oxidation degree JMA Johnson, Mehl and Avrami Degree of reduction RD RMSD Root mean square deviation Specific gas rate SGR SRC Standard reduction conditions

mances are required for direct reduction processes because of the lower operating temperatures and the absence of any liquid phases compared with conventional blast furnace processes. The scope of the work herein is to investigate the fluidization behavior and the reducibility of different iron ores during hydrogen-induced fluidized bed reduction at the laboratory scale to gain further knowledge of the fluidization and reduction procedures. A method, based on apparent activation energy against the degree of reduction (RD), and kinetic studies using a model based on the Johnson, Mehl and Avrami (JMA) equation, were applied to evaluate and compare the reducibility (Spreitzer & Schenk, 2019). Experimental Fig. 1 shows the layout of the laboratory fluidized bed reactor used, which has already been previously detailed (Spreitzer & Schenk, 2019). The principal components of the facility comprise the reactor itself, a three-stage electrical resistance heating furnace and a gas supply system. The fluidized bed reactor, with an inner diameter of 68 mm, was equipped with an internal cyclone and a dust filter unit to clean the off-gas and return discharged material to the fluidized bed. The gas flow into the fluidized bed reactor was controlled by mass flow controllers (EL-Flow F201CV/AV, Bronkhorst, the Netherlands). The addition of water vapor to the reducing gas mixture, if required, involved the use of a gas humidifier (Evaporator W-303A-333-K, Bronkhorst). Preheating of the gas to the desired temperature occurs while passing through the gas supply pipe, which is located inside the furnace. To evaluate RD, the weight of the entire reactor was measured during the

Fig. 1. Layout of the laboratory fluidized bed reactor used: 1: gas supply pipe from gas mixing unit; 2: gas preheating section; 3: fluidized bed reactor; 4: dust filter unit; 5: pressure regulator; 6: differential pressure transducer; 7: three-stage electrical heating furnace; 8: scale.

experiment using scale (Toledo XP64000L, Mettler, Switzerland). The pressure inside the system was controlled by a pressure regulator (28–23131, Masoneilan, France) which allows a maximum pressure of 1.4 bar. Measurements of pressure differential (Smart pressure transmitter, Kobold, Germany) were conducted across the gas distributor and the fluidized material to determine the fluidization behavior of the material. Thermocouples (type N) were used below the distributor and inside the material to measure the gas temperature and the temperature of the sample portion, respectively. The three-stage heating system controlled the temperature up to 1300 K. A gas distributor with 33 orifices was chosen for all experiments. For the experiments, the iron ore sample was charged into the reactor prior to heating the system to the required reduction temperature. The sample mass was restricted to 400 g as a result of the reactor dimensions, with preheating subjected to a constant nitrogen (Linde, Germany, purity 99.999 vol%) flow. After a temperature equilibrium period, the gas atmosphere was changed to reduction conditions. The weight change and all other important parameters such as differential pressure, gas flow, and temperature were recorded during the entire experiment. The reduction was assumed to be complete after a constant weight was maintained. During the fluidized bed reduction, attrition and elutriation of the particles will occur resulting in the formation and discharging of fine particles. However, for the experiments presented herein, this phenomenon was discarded because the discharged fine particles were collected in the dust filter unit. The dust filter mass was measured together with the reactor system, therefore, the attrition and elutriation had no effect on the overall weight signal. After a constant weight signal

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Table 1 Overview of the process conditions.

Sample mass (g) Grain size (mm) SiO2 addition (g) Temperature (K) Gas composition (vol%) Total flow rate (NL/min)a Gas velocity (m/s) a

Standard reduction conditions (SRC)

Specific gas rate (SGR) Ore A

400 0.25–0.5 – 873–1073 65 H2 , 35 N2 25.9 0.35–0.43

230 130

Standard ambient pressure and temperature: p = 1 atm, T = 298.15 K.

Fig. 3. Baur–Glaessner diagram of the reduction conditions.

Fig. 2. Experimental determination of the minimum fluidization velocity at ambient temperature using N2 as the fluidization gas.

was achieved, the reducing gas flow was isolated and the sample cooled down to ambient temperature under a nitrogen atmosphere. Four different iron ores were investigated: two natural hematite-based ores, one limonite-based ore, and one magnetitebased ore. To investigate the fluidization behavior and reducibility of the ores in a fluidized state, the standard reduction conditions (SRC) were defined. The experimental conditions are summarized in Table 1, which show the reducing gas mixture comprising hydrogen (Linde, Germany, purity 99.999 vol%) and nitrogen at a ratio of 65–35 vol%. Nitrogen was added to the gas mixture to support fluidization. Experiments were conducted across a temperature range of 873–1073 K, at 50 K increment increases. The experimental molar gas flow rate was kept constant, which ended in different superficial gas velocities at varying temperatures. All gas velocities were such to maintain a bubbling fluidized bed. The experimental measured minimum fluidization velocity umf of the ores used, in a completely reduced state at ambient temperature in the presence of nitrogen, was 0.082 m/s, as shown in Fig. 2. umf is defined as the gas velocity where the pressure drop across the material becomes constant with increasing gas velocity. The constant pressure drop value across the material should be equal to the theoretical value, which can be calculated using Eq. (1) (Hauzenberger, Reidetschlager, Schenk, & Mali, 2004): pBed =

mg AReactor

,

(1)

where m is the mass of the sample portion inside the reactor, g is the gravitational constant, and A the free reactor area. Using Eq. (1), the pressure drop created by the material in a completely fluidized state should be 7.89 mbar, which is in good agreement with the measured values. umf calculated under the same conditions, using the Ergun equation for the pressure drop at the conditions of minimum fluidization, is 0.092 m/s with a density of 2450 kg/m3 for the reduced solids. The terminal velocity ut for discharging the particles was calculated to be 0.821 m/s, ending at a range of u0 /umf = 4.35 and

u0 /ut = 0.49 for a superficial gas velocity u0 of 0.4 m/s inside the reactor, which should be only a representative value for the actual conditions. For the calculations, the umf and ut equations, shown in Spreitzer and Schenk (2019), were applied. Gas density and viscosity were assumed to be 1.145 kg/m3 and 18.6 ␮Pa s, respectively. The sphericity of the solids was fixed to 0.86, which is defined as the ratio of the surface of a sphere to the surface of the particle with the same volume. Thus, the sphericity for an ideal sphere is 1. Previous iron ore umf measurements have been determined to define the sphericity, to obtain similar comparisons between experimental and theoretical umf . The investigations have shown that a value in the range of 0.82–0.90 is suitable. Therefore, a value of 0.86 should only provide an assumption close to the actual value of the used sponge iron particles. Table 1 summarizes the process conditions for the SRC tests and the captures the changes made to achieve a higher specific gas rate (SGR). Therefore, the iron ore input amount was partially substituted with SiO2 . All other parameters remained constant. Fig. 3 shows the different process conditions within the Baur–Glaessner diagram, which demonstrates the stability areas of different iron oxides as a function of temperature and gas oxidation degree (GOD). The GOD, in the absence of CO and CO2 in the gas mixture, is defined as the ratio of water to the sum of water and hydrogen in the reducing gas mixture, as shown in Eq. (2). GOD =

xH2 O xH2 O + xH2

.

(2)

A low GOD value represents a high reduction potential of the gas mixture. To form metallic iron during reduction, both a desired temperature and GOD in the stability area of iron are required. The equilibrium between Fe and FeO is observed to increase to higher GOD values with increasing temperature. Thus, the reduction force increases as a function of increasing temperature. During the hydrogen-induced reduction of a hematite-based iron ore, the following reactions occur: 3 Fe2 O3 + H2 → 2Fe3 O4 + H2 O

(3)

(1 − y) Fe3 O4 + (1 − 4y) H 2 → 3Fe(1−y) O + (1 − 4y) H2 O,

(4)

Fe(1−y) O + H2 → (1 − y)Fe + H2 O

(5)

At reduction temperatures above 843 K, the reduction proceeds from hematite Fe2 O3 to magnetite Fe3 O4 (Eq. (3)), continues to wüstite FeO (Eq. (4)) and ends in the formation of metallic iron Fe (Eq. (5)). Below 843 K, the reduction proceeds from magnetite directly to metallic iron because of the fact that wüstite is not stable. These reduction sequences can be also observed in the Baur–Glaessner diagram shown in Fig. 3. Generally, the overall reduction of iron oxides by hydrogen is endothermic. For that reason, energy is required to ensure a constant reduction temperature.

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Table 2 Iron ore grade properties.

Fluidization behavior and kinetics of reduction Kinetic studies To further investigate the reduction behavior, the curve of weight change obtained during reduction was converted into a curve showing RD derived from the chemical analysis of the iron ore used. Detailed information regarding the procedure of the test evaluation is given in (Spreitzer & Schenk, 2019). RD is defined as follows: RD =



1−

O 1.5Fetot



(6)

where Fetot represents the total iron amount in the sample and O the total amount of oxygen bonded on iron (mol). Different iron oxides exhibit varying RDs: 0, 11.1%, 33.3%, and 100% for Fe2 O3 , Fe3 O4 , FeO, and Fe, respectively. The calculated curve showing RD as a function of time can also be used to determine the reduction rate against time using the first deviation. The reduction rate of the isothermal gas–solid reaction is defined as follows (Pijolat, Favergeon, & Soustelle, 2011; Simon, 2004): dx = k (T ) f (x) , dt

(7)

k(T) shows the temperature-dependent Arrhenius rate constant and f(x) gives a mathematical function. f(x) depends on the kinetic model used and remains constant at a certain temperature and gaseous concentration. In the current case, x represents RD. Setting f(x) to 1 ends in a so-called model-free analysis. The relationship among k(T), temperature T, and apparent activation energy Ea is given by the Arrhenius equation, Eq. (8) (Ozawa, 1992): Ea

k (T ) = Ae− R∗T ,

(8)

where A denotes the pre-exponential factor and R represents the gas constant. To determine Ea , a combination of Eqs. (7) and (8) is required, which results in Eq. (9). The logarithmic form of Eq. (9) is given in Eq. (10), which evaluates Ea via linear regression as a function of conversion. This method allows the determination of the trend of Ea against RD. Ea dx = Ae− RT f (x) , dt

ln

 dx  dt

=−

Ea + ln (A) + ln [f (x)] . RT

(9) (10)

To gain information regarding the mechanism that limits the kinetics of reduction, the JMA model was used, which is shown in Eq. (11) (Avrami, 1939, 1940; Avrami, 1941; Johnson & Mehl, 1939): n

x = 1 − e−at ,

1

Ore A

Ore B

Ore C

Ore D

63.6 0.45 3.48 2.07 0.25–0.50 11.83

62.2 1.09 6.00 1.1 0.25–0.50 2.59

61.9 0.42 2.99 2.14 0.25–0.50 20.75

57.3 19.66 7.05 n.a. 0.25–0.50 n.a.

(13), where xt is the total conversion at time t, x0 represents the conversion at the beginning of the analysis and w1,2,3 the corresponding weight factor of three different rate-limiting steps. xt = x0 + w1 x1 + w2 x2 + w3 x3 .

(13)

Inserting Eq. (11) to Eq. (13) results in Eq. (14). x0 was set to be zero at the beginning of the reduction. The fitting procedure was conducted using the solver function of Microsoft Excel by variation of w1,2,3 , a1,2,3 , and n1,2,3 to minimize the root mean square deviation (RMSD). The definition of RMSD is shown in Eq. (15) (Piotrowski et al., 2005):



xt = w1 1 − e−a1 t



RMSD =



n1





+ w2 1 − e−a2 t

xcalc − xexp n−1

n2





+ w3 1 − e−a3 t

n3



,

(14)

2 ,

(15)

where xcalc are the values obtained by the fitting procedure and xexp the values derived from the experiments; n represents the number of data sets. A similar procedure was used (Chen, Zheng, Chen, & Bi, 2017; Chen, Zheng, Chen, Yu, & Yue, 2017; Monazam, Breault, Siriwardane, Richards, & Carpenter, 2013) for the reduction of Fe3 O4 to Fe by carbon monoxide and for the reduction of Fe2 O3 by methane, respectively, were the w or n were fixed to a particular RD or a fixed rate-limiting mechanism, respectively. In the present work, w and n were not fixed to a particular RD to be able to evaluate if more than one rate-limiting step acts together in parallel. This type of analysis was performed because of the low accuracy of conventional kinetic analyses using basic models of gas–solid reactions (Khawam & Flanagan, 2006). Conventional kinetic analyses often show a high number of models, which fit well to the experimental results (Chen, Zheng, Chen, Bi et al., 2017; Chen, Zheng, Chen, Yu et al., 2017; Jeong, Lee, & Bae, 2015; Pineau, Kanari, & Gaballah, 2006; Pineau, Kanari, & Gaballah, 2007; Piotrowski et al., 2005; Spreitzer & Schenk, 2019). As a result, precisely determining one rate-limiting step is not possible. Conventional kinetic analyses only provide an indication as to which rate-limiting steps are acting during the reduction.

(11) Results and discussion

where x represents the conversion (RD), a is the nucleation rate constant, t the reduction time, and n the kinetic exponent. The value of n can be linked to the rate-limiting step. If n is <1, the mechanism is assumed to be diffusion controlled; if n is close to 1, the progress is limited by reaction kinetics. A value of n > 1.5 shows that the reaction progress can be described by limitation because of nucleation phenomena. A value of n = 1.5 represents a zero-nucleation rate. If n is between 1.5 and 2.5, the nucleation rate decreases. n = 2.5 shows a constant nucleation rate and n > 2.5 confirms an increasing nucleation rate (Málek, 1995). The rate constant depends on a and n and is defined as follows: k = an

Fetot (wt%) Fe2+ (wt%) SiO2 (wt%) Al2 O3 (wt%) Grain size (mm) Specific surface area BET (m2 /g)

(12)

To evaluate more than one rate-limiting step occurring in parallel, a combination of three mechanisms was chosen to reproduce the experimental results as accurately as possible, shown in Eq.

To check the variation in reducibility of the iron ore fines, four iron ore types were investigated. Two iron ores were based on hematite (Ore A and Ore B), one based on limonite (Ore C), and the fourth based on natural magnetite (Ore D). Table 2 shows the iron ore properties related to chemical analysis, grain size, and specific surface area. The grain size was defined to be 0.25–0.50 mm. However, all iron ores had not exactly the same mean particle diameter. The fractions between 0.25 and 0.50 mm were extracted by sieving from a given grain size distribution ranging from 0.063 to 2.8 mm, which was similar for all iron ores used. Therefore, the difference in mean diameter should be negligible. The specific surface areas were determined from the Brunauer–Emmett–Teller model (BET). The results show that Ore C has the highest initial specific surface area, followed by Ore A and Ore B. Generally, a high initial porosity of the iron ore is conducive for a high reduction rate because of enhanced

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Fig. 5. RD comparison of different ores at: (a) 873 K; (b) 1023 K.

Fig. 4. Progress of reduction (RD) as a function of temperature: (a) Ore A; (b) Ore B; (c) Ore C.

permeability of the reducing gas to the reaction interface (Edstrom, 1953). Accordingly, Ore C should show the best reducibility. Fig. 4 shows RD as a function of reduction time for Ores A, B, and C at different temperatures. For Ore B, a successful experiment at 1073 K was not possible as a result of fluidization difficulties at high temperatures (sticking phenomenon). For all experiments, an initial reduction up to ∼10% rapidly occurred, representing a reduction from Fe2 O3 to Fe3 O4 . The fast, initial reduction preceded a near constant slope of reduction rate until a particular reduction extent for Ore A and Ore B, depending on the temperature. Thereafter, the reduction rate slowed until a complete reduction was achieved. The deceleration of the reduction rate appears across almost the same reduction range for Ore B, which is in the range of 90%. In the case of Ore A and Ore C, the deceleration depends significantly on the reduction temperature. For Ore A the deceleration took place when the extent of reduction was between 75% and 90%. For Ore C, this effect is even more pronounced and took place when the extent of reduction was between 65% and 90%. Furthermore, Ore C also exhibits different behaviors during the second stage of reduction. A constant reduction rate is not observed, especially at lower reduc-

tion temperatures, whereby, the reduction rate increased at a RD of 25%–35%. Fig. 5 shows the direct comparison of the different ores at reduction temperatures of 873 and 1023 K. The reducibility is observed to be markedly different at low reduction temperatures. Ore C shows the least reducibility potential compared with the other ores, which is somewhat unexpected because Ore C possesses the highest initial porosity and should, therefore, show the best reducibility. Further kinetic analysis should define the reasons for this different behavior. At high reduction temperatures, all three ores show almost the same reducibility. A complete reduction could be achieved for all ores after ∼3500 s. Furthermore, for all experiments with Ores A, B, and C, except the experiment with Ore B at a reduction temperature of 1073 K, a stable fluidization was achieved across the entire reduction time. For Ore D, which is a magnetite-based iron ore, guaranteeing stable fluidization during the complete reduction procedure was not possible. Immediately in the range after the first metallic iron phase was formed, the particle sticking phenomenon started (Du et al., 2016; Zhang, Gong, Wang, & Guo, 2011; Zhong, Wang, Gong, & Guo, 2012). At a reduction temperature of 1073 K, the pressure drop decreased drastically in the area where the first iron precipitation occurred, as demonstrated in Fig. 6. At 923 K, the pressure drop loss rate reduced, however, this observation started in the range of the first metallic iron formation. After de-fluidization, the samples were further reduced at 923 and 1073 K to a RD of 80% for 71 and 150 min total reduction time, respectively. Fig. 7 shows the polished micro sections of these reduced samples, as a function of temperature. The remaining dense wüstite core can be observed in both samples. In both cases, an iron layer was formed around the wüstite core. In the case of a lower reduction temperature, the metallic iron formed shows a significantly higher porosity compared with higher reduction temperatures. Additional time was required to reach a RD of 80% at a higher reduction temperature, because the dense iron layer avoids

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Fig. 6. RD and pressure drop across the distributor and material for the reduction of Ore D at a reduction temperature of 1073 K.

direct contact of the reducing gas mixture at the reaction interface, indicating that diffusion or nucleation is the reduction rate-limiting step (Fruehan, Li, Brabie, & Kim, 2005). As a result of the fluidization difficulties of Ore D, further kinetic analysis was performed using only the data derived from Ores A, B, and C. A general trend regarding the re-oxidation behavior was also observed during the experiments in the presence of Ore A. After reduction, the sample portions were cooled down to ambient temperature under a nitrogen atmosphere. The first contact with oxygen of the directly reduced iron took place during the discharge. Samples reduced at temperatures lower than 973 K, and in the presence of oxygen, started to oxidize immediately. At reduction temperatures >973 K, the samples remained stable under ambient conditions. The reason for this behavior could also be observed in the polished micro sections of the partly reduced samples of Ore A, with a RD of ∼75% at reduction temperatures of 873 and 1073 K, shown in Fig. 8. In the case of a lower reduction temperature, the iron formation is uniformly distributed across the entire particle with the final iron product being highly porous. A homogeneous distribution of small iron nuclei can be observed across the particle. The iron formed at higher reduction temperatures exhibited a lesser degree of porosity, and concomitantly, a lower specific surface area according to the polished micro sections, which hinders instant re-oxidation.

To gain further knowledge regarding reducibility, the trends of Ea against RD for Ores A, B, and C were determined according to the aforementioned procedure. Fig. 9 shows the curves for the different ores. For all iron ores used, Ea initially increase at the initial stage of reduction until a first peak can be observed. This first peak occurs at a RD between 10% and 14%, which is in the range corresponding to a complete reduction from Fe2 O3 to Fe3 O4 . Thereafter, Ea shows a decrease until a RD of ∼30%. Next, in the case of Ore B, Ea remains at a nearly constant level until a high RD is observed. The behavior of Ore A and Ore C is different from that of Ore B, hence, Ea increases again in the area of reduction from FeO to Fe. Thus, a second peak can be observed. Overall, Ea for all ores varies between 10 and 65 kJ/mol. The ore with the highest reducibility potential, Ore B, shows the lowest Ea with no secondary peak observed in the area of reduction from FeO to Fe. The ore that demonstrates the least reducibility potential, Ore C, shows the highest Ea and an intensive peak during reduction from FeO to Fe. Further kinetic analysis, using the JMA model, should clarify the different trends of Ea . Fig. 10 shows examples of the results from the fitting procedure, employing the JMA model, for the three different ores at a reduction temperature of 923 K. As shown, all results fit quite well to the experimental data, and exhibit low RMSD values in all cases. The n-values in the figures represent the appropriate n term. At this reduction temperature, the initial stage of reduction shows n1 as ∼1 for Ores A and B, indicating a kinetically-controlled reaction. For Ore C, the value of n1 is <1, which signifies a limitation by diffusion. The later stage of reduction is controlled by a mixture of reaction kinetics and nucleation for Ores A and B. The n2 value for Ore C is ∼1.5, which can also be associated with a limitation by nucleation. Therefore, the overall reduction from Fe3 O4 to FeO and Fe is limited by nucleation at a given reduction temperature, which explains the increasing reduction rate for Ore C across the RD area of 25%–35%. Table 3 shows the results of the fitting procedure for the different ores as a function of reduction temperature. The behavior of Ore A has previously been explained in (Spreitzer & Schenk, 2019). n1 is observed to change from a kinetically-controlled reaction to diffusion controlled with increasing reduction temperature. In the same way, the corresponding w increases. The rate constant shows an unexpected trend, since a decrease is observed with increasing temperature, which may result from the fact that the weight factor reaches a value bigger

Fig. 7. Polished micro sections of partly reduced samples at different temperatures—Ore D: (a) 923 K; (b) 1023 K. Gray areas: wüstite, white areas: metallic iron.

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Table 3 Multistep kinetic analyses for different iron ores. Ore A

Weight factors Nucleation rate constants (s−1 ) Kinetic exponents

Rate constants (s−1 ) Root mean square deviation

w1 w2 w3 a1 a2 a3 n1 n2 n3 k1 k2 k3 RMSD

873 K

923 K

973 K

1023 K

1,073 K

0.0915 0.8535 0.0549 0.0117 5.31 × 10−5 2.06 × 10−9 1.17 1.29 2.61 0.0231 4.94 × 10−4 4.71 × 10−4 0.0045

0.1032 0.7607 0.1362 0.0140 7.29 × 10−5 2.06 × 10−9 1.07 1.28 2.65 0.01858 5.88 × 10−4 5.28 × 10−4 0.0048

0.1379 0.6277 0.2344 0.0109 1.39 × 10−4 2.07 × 10−9 1.05 1.22 2.76 0.0138 6.99 × 10−4 7.31 × 10−4 0.0049

0.2691 0.3865 0.3443 0.0286 1.20 × 10−4 2.07 × 10−9 0.69 1.24 2.86 0.006 7.16 × 10−4 9.07 × 10−4 0.0061

0.2371 0.0722 0.6907 0.0176 5.38 × 10−7 8.61 × 10−7 0.84 1.86 2.04 0.008 4.33 × 10−4 1.06 × 10−3 0.0070

Ore B

Weight factors Nucleation rate constants (s−1 ) Kinetic exponents

Rate constants (s−1 ) Root mean square deviation

w1 w2 w3 a1 a2 a3 n1 n2 n3 k1 k2 k3 RMSD

873 K

923 K

973 K

1023 K

1073 K

0.1394 0.5602 0.3000 0.0094 3.83 × 10−5 2.07 × 10−9 0.96 1.38 2.63 0.008 6.41 × 10−4 4.92 × 10−4 0.0046

0.1581 0.5463 0.2956 0.0044 7.84 × 10−5 2.07 × 10−9 1.13 1.30 2.68 0.008 7.07 × 10−4 5.80 × 10−4 0.0047

0.2588 0.3402 0.4010 0.0104 1.25 × 10−4 2.07 × 10−9 0.88 1.22 2.77 0.006 6.37 × 10−4 7.26 × 10−4 0.0051

0.2787 0.2579 0.4634 0.0044 1.29 × 10−4 2.07 × 10−9 1.03 1.22 2.84 0.005 6.42 × 10−4 8.66 × 10−4 0.0040

No results because of fluidization problems (sticking).

Ore C

Weight factors Nucleation rate constants (s−1 ) Kinetic exponents

Rate constants (s−1 ) Root mean square deviation

w1 w2 w3 a1 a2 a3 n1 n2 n3 k1 k2 k3 RMSD

873 K

923 K

973 K

1023 K

1073 K

0.1506 0.4153 0.4340 0.0588 1.00 × 10−6 2.06 × 10−9 0.68 1.89 2.41 0.015 6.77 × 10−4 2.48 × 10−4 0.0055

0.1387 0.7420 0.1193 0.0601 1.26 × 10−5 2.07 × 10−9 0.74 1.47 2.65 0.023 4.62 × 10−4 5.29 × 10−4 0.0049

0.1759 0.7229 0.1011 0.0577 1.60 × 10−5 2.07 × 10−9 0.63 1.48 2.85 0.011 5.82 × 10−4 9.03 × 10−4 0.0080

0.1394 0.6625 0.1981 0.0617 5.67 × 10−5 2.07 × 10−9 0.82 1.38 2.86 0.034 8.38 × 10−4 9.17 × 10−4 0.0055

0.2205 0.4919 0.2876 0.0327 1.03 × 10−4 2.07 × 10−9 0.77 1.34 2.92 0.012 1.07 × 10−3 1.06 × 10−3 0.0061

than 0.11 when the reaction from Fe3 O4 to FeO is still being undertaken. Hence, temperature does not appear to play an important role during the initial stages of reduction because the reduction from Fe2 O3 to Fe3 O4 is rapid. Further reduction is hindered by a combination of reaction kinetics and nucleation, represented by n2 ∼ 1 and n3 > 1.5. At the highest reduction temperature, only nucleation limits the reduction rate, whereby at the lowest temperature, w3 , representing nucleation, is low. Thus, the reduction rate is controlled mainly by reaction kinetics. The effect of limitation by reaction kinetics declines as a function of increased temperature, which is in good agreement with the polished micro sections, shown in Fig. 8, where iron nuclei are observed across the entire particle at a reduction temperature of 873 K. Such observations indicate that RD is only limited by reaction kinetics. At high temperatures, iron nuclei are not observed in the particle, hence, nucleation limits the reduction rate. The different re-oxidation behaviors can therefore be described by such analyses. For Ore B, the behavior is slightly different. The initial stage of the process is also kinetically-controlled with n1 ∼ 1. Further reductions at later stages of the process are controlled by a combination

of reaction kinetics and nucleation. However, the reaction kinetics rate constant k2 is observed to be almost constant with increasing temperature. With respect to nucleation, k3 is increased as a function of temperature, which is the reason why only the nucleation process influences the observed difference in reduction rate, where w3 also increases as a function of temperature, but to a lesser degree when compared with Ore A. In the case of Ore C, which shows the lowest reduction rate, the initial stage of reduction is limited by diffusion, shown by n1 < 1. For w1 , no general trend can be observed. n2 is significantly higher compared with the other ores, which corresponds to limitation by nucleation until a temperature of 973 K is reached. n2 decreases with increasing temperature. n3 again shows a limitation by nucleation. k values increase for both the n2 and n3 cases; a general trend regarding w2 and w3 cannot be observed. During the initial reduction, the rate of diffusion limitation suggests for formation of dense magnetite. Thereafter, further reduction of the dense magnetite becomes increasingly difficult and is principally limited by nucleation, which slows down the reduction rate and increases Ea , especially at temperatures up to 923 K. Thurnhofer,

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Fig. 8. Polished micro sections of partly reduced samples at reduction temperatures—Ore A: (a) 873 K; (b) 1073 K. Gray areas: wüstite, white areas: metallic iron.

Fig. 9. Comparison of apparent activation energy Ea against RD for different iron ores.

Fig. 10. Fitting results of the experimental data using the Johnson, Mehl and Avrami (JMA) model for different iron ores at a reduction temperature of 923 K: (a) Ore A; (b) Ore B; (c) Ore C.

Schachinger, Winter, Mali, and Schenk, (2005) investigated the influence of varying pre-reduction conditions on the final reduction behavior and observed that the final RD reached is influenced by the pre-reduction conditions. Such influences may also be problematic during the reduction of Ore C under actual conditions. At higher temperatures (1023 and 1073 K), reduction becomes limited by a mixture of reaction kinetics and nucleation, with a growing importance of nucleation with rising temperatures. The poor reducibility of Ore C, especially at low reduction temperatures, can be explained by this observed difference in behavior compared with Ore A and Ore B. The fitting of data by applying the JMA model shows that the trends of Ea can be further elucidated. The presence of the first peak for all the ores occurs because of the different changes in reduction rates at different RDs, depending on temperature, which is observed by the general trend of increasing w1 as a function of increasing temperature. Consequently, there is a need to consider the different equilibrium gas compositions at different reduction temperatures. At a low reduction temperature (873 K), the equilibrium between Fe3 O4 and FeO is close to that of FeO and Fe. At higher temperatures, the difference is significantly larger, as seen

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Table 4 Determined activation energy Ea values derived from the model analysis. Ea (kJ/mol)

Ore A

Ore B

Ore C

Ea2 Ea3

19.30 33.88

∼0 28.45

47.42 35.07

Fig. 12. Comparison of the trends of Ea as a function of RD for different specific gas rates. Table 5 Multistep kinetic analyses for the increased specific gas rate during the reduction of Ore A.

Fig. 11. RD with increased specific gas rates at different temperatures.

in the Baur–Glaessner diagram in Fig. 3. Variation in the reduction potential also influences the first peak associated with Ea . Another point of consideration is the presence of the second peak for Ore A and Ore C associated with the reduction of FeO to Fe compared with Ore B, where this peak is not observed. Table 4 shows the Ea values determined by the rate constants derived from the fitting procedure, were k-values from the four best-fitting temperatures were used, checked by the coefficient of determination. As shown, the Ea of Ore B, when kinetically-controlled, is almost zero, hence, only the limitation by nucleation is responsible for Ea , which remains constant in that case. For the other ores, both terms show Ea . As a result of changing w, Ea also changes as a function of RD. Overall, if more than one rate-limiting step is proceeding simultaneously, Ea changes as a function of conversion.

w1 w2 w3 a1 a2 a3 n1 n2 n3 k1 k2 k3 RMSD

873 K

923 K

973 K

1023 K

1073 K

0.0967 0.5242 0.3791 0.0059 9.22 × 10−4 5.26 × 10−7 1.21 0.99 1.99 0.0145 8.63 × 10−4 6.92 × 10−4 0.0088

0.1016 0.5140 0.3834 0.0053 5.54 × 10−4 5.26 × 10−7 1.16 1.07 2.05 0.0109 9.03 × 10−4 8.70 × 10−4 0.0105

0.1706 0.4179 0.4115 0.0063 4.69 × 10−4 5.26 × 10−7 1.17 1.08 2.16 0.0128 8.53 × 10−4 1.23 × 10−3 0.0084

0.2218 0.2416 0.5366 0.0016 3.95 × 10−5 4.26 × 10−7 1.43 1.38 2.27 0.0108 6.47 × 10−4 1.55 × 10−3 0.0083

0.2245 0.3393 0.4362 0.0072 8.25 × 10−4 4.26 × 10−7 1.13 1.08 2.29 0.0126 1.36 × 10−3 1.67 × 10−3 0.0086

Effect of different specific gas rates on the kinetics of reduction To determine the influence of a higher specific gas rate on the reducibility of Ore A, the iron ore content was partly substituted by silica. Instead of 400 g iron ore, 230 g were charged into the reactor together with 130 g silica. Silica was added to achieve a near constant sample volume, and therefore, the same fluidization conditions. The process parameters, such as gas composition and temperature, are similar to the SRC. As a result, the specific gas rate increased from 2520 to 4383 Nm3 H2 (t h)−1 , defined as follows:





VH˙ 2 (Nm3 /h)

(16)

Fig. 13. Fitting results of the experimental data for the higher specific gas rate of Ore A using the JMA model at a reduction temperature of 923 K.

The RDs against time, as a function of temperature, for the increased specific gas rates are shown in Fig. 11. RD can again be divided into three stages; a temperature dependent, fast, initial reduction within a range between 10% and 20%, followed by a near constant reduction rate up to a reduction of 80%–90%, depending on temperature, and a slow reduction rate at the final stage until complete reduction was reached. Fig. 12 shows the Ore A trend comparison of Ea against RD for different specific gas rates. As shown, a bi-modal shaped curve is again observed. However, the observed differences are in the position and height of the first and second peaks. Table 5 shows the results of the fitting procedure for an increased specific gas rate, which was performed in the same way as previously described. Fig. 13 shows the fitting results at a reduction temperature of 923 K. The main difference compared with the reduction at a lower specific

gas rate is the ratio between the limitation by reaction kinetics and nucleation. At a lower specific gas rate, the reduction at low temperatures is mainly kinetically-controlled, whereas at high reduction temperatures, the reduction is principally controlled by nucleation. At higher specific gas rates, the observed ratio changes are consistent, however, at low reduction temperatures, the limitation by nucleation is also present. In terms of a lower specific gas rate, the sample mass was only 230 g compared with 400 g for a higher specific gas rate. Thus, the available particle surface was lower. As a conclusion, a lower available surface area for reduction increases the impact of nucleation at low a reduction temperature, which can also be observed in the case of Ore B, which is the ore with the lowest specific surface area. At higher temperatures, the system acts in the other direction because the chemical reaction proceeds sufficiently rapid enough to form an abundance of nuclei. Hence, the

SGR

Nm3 H2 (t · h)

=

mOre (t)

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system is again controlled by the mechanisms of reaction kinetics and nucleation. For the first Ea peak, the influence of equilibrium gas composition is not as high for lower specific gas rates because of the smaller sample amount. The peak height decreases and shifts to higher RDs. The final stage of reduction, exhibiting a slow reduction rate, is only kinetically-controlled, which is also observed in Fig. 13. Ea determined by the fitting procedure for n2 and n3 , representing limitation by chemical reaction and nucleation, are 8.18 and 36.76 kJ/mol, respectively, promoting a reduction in the value of the chemical reaction when compared with lower specific gas rates (19.30 kJ/mol), which corresponds to the observed reduced level of the second peak. Conclusions A laboratory fluidized bed reactor was used to investigate the fluidization behavior and reducibility of various iron ore grades using hydrogen as a reducing agent at temperatures between 873 and 1073 K. Kinetic analyses using the JMA model were performed to elucidate the limiting factors during the reduction. The influence of changing the specific gas rate was also investigated. A stable fluidization of hematite- and limonite-based iron ores is possible during the entire reduction procedure across the temperature range studied, except for Ore B, which exhibited issues with fluidization at a reduction temperature of 1073 K. The experiments demonstrated that fluidization of the magnetite-based iron ore (Ore D) is not possible under the actual process conditions. De-fluidization, across all temperatures studied, was immediately observed in the area of first metallic iron formation. The dependence of instant re-oxidation of the reduced material, after a first contact with oxygen, on the reduction temperature can also be explained by the kinetic analyses. If highly porous iron is formed at low reduction temperatures (limitation by chemical reaction, not nucleation) the product is vulnerable to re-oxidation. A high specific surface area of the initial iron ore does not necessarily indicate good reducibility. The limonitic ore, which exhibits the highest initial specific surface area showed the least reducibility potential compared with the other ores. The trends of Ea against RD show the highest values for the ore with the least reducibility potential. Thus, lower Ea values represent enhanced reducibility. The reduction of FeO to metallic Fe is always controlled by a combination of nucleation and reaction kinetics. The different interactions between these two phenomena are responsible for the different reduction behaviors, which can also be linked to the different trends of Ea . The Ea value for the reduction of FeO to metallic Fe varies between 15 and 60 kJ/mol, depending on the iron ore and RD. An increased specific gas rate does not influence the general trend regarding Ea or the kinetic analyses, however, in comparison with a lower specific gas rate, the limitation by nucleation is also an important factor at low reduction temperatures. Conflict of interests The authors declare that they have no conflict of interest. Acknowledgements The authors acknowledge the financial support from the project E3 -SteP (Enhanced Energy Efficient Steel Production), which is funded by the Austrian Research Promotion Agency (FFG). References Avrami, M. (1939). Kinetics of phase change. I. General theory. The Journal of Chemical Physics, 7(12), 1103–1112.

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