Reduction kinetics of iron oxide pellets with H 2 and CO mixtures

International Journal of Minerals, Metallurgy and Materials Volume, Number 7, July 015, Page 1 DOI: 10.1007/s1613-015-0000-0 Reduction kinetics of iron oxide pellets with H and CO mixtures Hai-bin Zuo 1), Cong Wang 1), Jie-ji Dong ), Ke-xin Jiao ), and Run-sheng Xu ) 1) State Key Laboratory of Advanced Metallurgy, University of Science and Technology of Beijing, Beijing 100083, China ) School of Metallurgical and Ecological Engineering, University of Science and Technology of Beijing, Beijing 100083, China (Received: 9 May 014; revised: 18 September 014; accepted: September 015) Abstract: Reduction of hematite pellets using H CO mixtures with a wide range of H /CO by molar (1:0, 3:1, 1:1, 1:3, and 0:1) at different reducing temperatures (1073, 1173, and 173 K) was conducted in a program reducing furnace. Based on an unreacted core model, the effective diffusion coefficient and reaction rate constant in several cases were determined, and then the rate-control step and transition were analyzed. In the results, the effective diffusion coefficient and reaction rate constant increase with the rise in temperature or hydrogen content. Reduction of iron oxide pellets using an H CO mixture is a compound control system; the reaction rate is dominated by chemical reaction at the very beginning, competition during the reduction process subsequently, and internal gas diffusion at the end. At low hydrogen content, increasing temperature takes the transition point of the rate-control step to a high reduction degree, but at high hydrogen content, the effect of temperature on the transition point weakens. Keywords: iron oxide pellets; reduction kinetics; kinetics models; hydrogen; carbon monoxide 1. Introduction Traditional ironmaking production is based on a carbon-consuming metallurgical process, leading to major environmental problems caused by the gigantic amount of CO emitted [1 3]. Hydrogen has been proven to be a good reductive agent as well as an environment-friendly material because it uses innocuous H O as its reducing product [4 7]. Substituting carbon for hydrogen in the ironmaking process would radically eliminate the environmental crisis caused by greenhouse gas emissions; however, the earth cannot supply a hydrogen resource used directly. In the iron and steel industries, a few gas-based direct-reduction iron (DRI) processes, such as HYL and Midrex, have employed the mixing gas enriched with hydrogen as the reducing agent, and, in order to mitigate CO emission, coke oven gas and natural gas have been introduced into blast furnaces from tuyeres in some plants during the ironmaking process, resulting in a promotion of hydrogen content in blast furnace gas. Full oxygen blast furnaces with top gas recycling and COREX processes are also characterized by high CO and H content in the gas [8 10]. In these processes, H content in the reducing gas ranges widely, often fluctuating from a few percent to dozens. In this paper, a pellet-reduction process with a wide range of H content syngas was investigated to reveal the requirements for pellet reduction and its dynamic parameters. While the reduction of iron oxide with H and CO has been extensively studied, the kinetics of reduction with H CO mixtures has not been adequately investigated for a large range of H content. Piotrowski et al. [11] investigated the effect of gas composition on the kinetics of iron oxide reduction from Fe O 3 to FeO at a temperature range from 973 K to 1183 K through thermogravimetric experiments. Liu et al. [1] also assessed the apparent activation energy needed to reduce iron oxides by a CO and CO H mixture. Pineau et al. [13 14] analyzed the apparent activation energy needed for the reduction of Fe O 3 and Fe 3 O 4 by H in the temperature range of 493 K to 953 K and 483 K to 13 K, respectively. Ono-Nakazato et al. [15] studied the reduction of FeO 1.05 powder packed bed with an H CO mixture. Bonalde et al. [16] reported on the kinetic analysis of iron oxide reduction through a batch of experiments on the reduction of fired hematite pellets at 113 K using hydrogen, Corresponding author: Hai-bin Zuo E-mail: zuohaibin@metall.ustb.edu.cn University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 015

Int. J. Miner. Metall. Mater., Vol., No. 7, July 015 carbon monoxide, and Midrex gas. Mousa et al. [17] studied the reduction behavior of iron ore pellets with the simulated coke oven gas and natural gas between 973 K and 153 K. Although the reduction of iron ore using CO, H, or a mixture of the two has been widely investigated, the reduction development under a wide range of H /CO molar ratios at different temperatures still remains to be elucidated. Therefore, the purpose of this investigation was to study the kinetics of reduction in those cases to determine the rate-controlling step and its transit rules upon the gas composition and temperature, with the goal of providing scientific direction for reactor design and operational parameter determination.. Experimental.1. Sample preparation Iron oxide pellets used in the experiments were sampled from one batch of production in a commercial iron and steel plant. The chemical composition of the pellets is shown in Table 1. The pellets selected had a good spherical shape with the diameter carefully controlled at 1 mm. Table 1. Chemical composition of the iron oxide pellets wt% TFe FeO SiO Al O 3 MgO CaO S P 68..86 3.67 0.30 0.39 0.5 0.018 1.18.. Reduction experiments The schematic drawing of experimental apparatus for pellet reduction using an H CO mixture with different molar ratios of H to CO is shown in Fig. 1. The reducing gases (H and CO) were provided by individual commercial compressed gas cylinders, and a mass flow meter coupled to the outlet of cylinder was used to control the gas (N ) flow. Sample weight was acquired every 5 s by an electrical balance (Mettler Toledo AL04-IC) placed on a platform atop the furnace. The precision of balance was 0.1 mg and the data were automatically recorded by a computer. A sample basket was hung from the balance via a steel wire, as shown in Fig. 1. The wire length was designed to ensure that the basket was kept in the constant temperature zone. The basket was woven with Fe Cr Al alloy wire with 0.5 mm diameter to hold six pellet particles. An R-type thermocouple (Pt/Pt-13%Rh) was equipped beneath the basket to control the furnace temperature. After drying, the pellets were loaded into the basket and set into a vertical quartz tube embedded into a silicon molybdenum furnace. The power of the silicon molybdenum furnace was 8 kw, and the maximum heating rate and highest temperature were 100 K min 1 and 1873 K, respectively. The heating behavior and target temperature were well controlled by an intellective program control unit with ± K precision. The pure nitrogen gas was introduced from the bottom of tube at a rate of 5 L min 1 to evacuate the air in the tube. The sample was heated from room temperature to the target temperature (1073, 1173, and 173 K) with a 50 K min 1 heating rate, and the N gas was switched to the reducing gas with different H /CO molar ratios (1:0, 3:1, 1:1, 1:3, and 0:1) at a rate of 5 L min 1, then the data acquisition system was started simultaneously. The reducing gas flowed up through the tube, reacting with samples, then evacuated from the top of tube. The exhaust gas was removed by a ventilator to protect the operators from the poison. When the sample mass stopped changing, the reducing gas was replaced with nitrogen and the sample was cooled to room temperature..3. Mathematical model and resolution 1 Compressed gas cylinder; Program control unit; 3 Thermocouple; 4 Samples; 5 Silicon molybdenum furnace; 6 Outlet of flow; 7 Electrical balance Fig. 1. Schematic drawing of experimental apparatus for reaction. An unreacted core model has been widely used to describe the kinetics and mechanism of iron oxide reduction by H, CO, and mixtures of the two, and its applicability has been confirmed, especially for pellet reduction [18]. Considering the reduction from FeO to iron to be the most difficult step with the maximum extent of deoxygenization, it could be thought that only one single reaction interface, the iron/wustite interface, existed inside the whole pellet, and the reduction only occurred in that interface. In this case, the overall rate of reduction, including intrinsic reduction, pore diffusion, and gas-film mass transfer, could be described by

H.B. Zuo et al., Reduction kinetics of iron oxide pellets with H and CO mixtures 3 V ( ) 4πr c c 0 b e t = 1 r0( r0 ri) K r 0 + + kg Deffri k r rec + i ( 1 K ) where V t is the reaction rate, mol s 1 ; r 0 the initial radius of pellet, m; c b the concentration of reducing agent, mol m 3 ; c e the equilibrium concentration of reducing agent, mol m 3 ; r i the radius of the un-reacted core of the pellet, m; k g the mass transfer diffusion coefficient in gas-film, m s 1 ; D eff the effective diffusion coefficient of gaseous species, m s 1 ; k rec the reaction rate constant, m s 1 ; and K the equilibrium constant of chemical reaction. X was defined as the reduction fraction (deoxygenization amount dividing initial oxygen amount). When the oxygen content in pellet was uniform, X was presented as X 3 r i = 1 r0 According to the conservation of mass, the reaction rate can be expressed as the following equation: Vtdt = 4πri d0dri (3) where d 0 is the oxygen density of the pellet, mol m 3 ; and t the time, s. Applying Eqs. () and (3) to Eq. (1), a relationship between the reduction fraction and time is given by X r 3k 6 0 + 1 3( 1 X) 3 ( 1 X) g D + + eff K ( ) 1 c 3 b ce 1 1 X = t (4) krec ( 1+ K ) rd 0 0 where K is calculated according to the Gibbs free-energy change of reaction, as provided in Table. Table. Gibbs free-energy change ( ΔG reactions (1) () ) of the chemical Reaction ΔG FeO + H = Fe + H O 3430 16.16T FeO + CO = Fe + CO 13160 + 17.1T CO + H O = CO + H 30460 + 8.14T Using the mixing gas, all three reactions listed in Table occurred simultaneously at the experimental temperature, leading to a complex reduction process relying on the reaction rate and chemical balance of the three reactions. However, no matter what degrees of the three reactions were, the gas-gas reaction (CO + H O = CO + H ) did not change the molar ratio of the reducing agent (H + CO) and products (H O + CO ). Therefore, the mixture was regarded as a single gas phase. The reaction becomes A + FeO = Fe + B, with A and B representing H CO and CO H O, respectively, and the equilibrium constantis given by K = CB C (5) A where C B and C A are the concentrations of product and reducing agent, respectively. In this case, the gas gas reaction was neglected because it did not affect C B /C A, and the equilibrium constant of reaction was decided by the initial composition of mixture and the equilibrium constants of the two gas solid reduction reactions individually. In previous publications, researchers have paid less attention to the calculation of K of the mixing gas; only a simple formula was given by Eq. (6) [19]. K = xcokco + xh K (6) H where xh and x CO are the molar fractions of H and CO in the mixture, and KH and KCO are the equilibrium constants of the two reduction reactions where FeO is reduced by H and CO, respectively. Because Eq. (6) did not provide a satisfactory result, especially when KCO was quite different from K H, a new method for calculating K needed to be established. Considering the chemical reaction balance, more than one mole of CO needs to be reduced to get one mole of Fe, the real consumption of CO is called the redundant coefficient of CO, donated by n, and the reaction is given as FeO + n CO = Fe + CO + (n 1) CO (7) In this circumstance, to get one mole of Fe, n moles of CO must be provided; and in the residual gas, KCO = PCO / P CO = 1/( n 1). Therefore, one mole of CO could be reduced to get K CO /( K CO + 1) moles of iron. Similarly, one mole of H could be reduced to get KH /( K H + 1) moles of iron, and one mole of mixture gas for K / ( K + 1) moles of iron as well. The total iron production using a certain quantity of CO or H individually as the reducing agent should be the same as that using the mixing gas consisting of equal quantities of CO and H when the reduction reactions reach balance. The relationship of iron production is given as K K CO H K xco + xh = (8) KCO + 1 KH + 1 K + 1 Then the equilibrium constant, K, is expressed by Eq. (9). K = xcokco ( KH + 1) + x ( ) H K H K CO + 1 ( K + 1)( K + 1) x K ( K + 1) x K ( K + 1) CO H CO CO H H H CO (9) The gas transfer coefficient, k g, in Eq. (4) could be calculated using an empirical formula according to the similarity principle as listed in Eq. (10).

4 Int. J. Miner. Metall. Mater., Vol., No. 7, July 015 1 1 kd g 3.0 0.6Re Sc D = + (10) where d is the pellet diameter, m; D the diffusion coefficient, m s 1 ; Re the Reynolds number, and Sc the Schmidt number. Mass transfer coefficients at different temperatures are listed in Table 3. Table 3. Mass transfer coefficient of gas in gas film at different temperatures Temperature / K 1073 1173 173 Mass transfer coefficient / (m s 1 ) 0.196 0.09 0.1 In the reduction experiments, the extent of reduction, X, was improved gradually along with reducing time and was calculated accurately according to the weight loss of sample. The parameters C 1, C, F, and t 1 are defined as follows. rd 0 0 C1 = 6D c c C ( ) eff b e K rd 0 0 = k K c c ( 1+ )( ) ( X ) 1 3 rec b e (11) (1) F = 1 1 (13) rd 0 0X t1 = 3k c c ( ) g b e (14) where C 1, C, and t 1 are the corresponding total reaction times when the reaction rate is only controlled by interior diffusion, chemical reaction, and exterior diffusion, respectively, and F the ratio of reaction time and total reaction time. Eqs. (11), (1), (13), and (14) may be applied to Eq. (4) to get Eq. (15). t t 1 = C1 F F + C (15) ( 3 ) F Plotting (t t 1 )/F according to 3F F and making a linear fitting, C 1 and C correspond to the slope and intercept, respectively. Then, the effective diffusion coefficient and the rate constant of reaction were easily obtained from Eqs. (11) and (1). 3. Results and discussion 3.1. Effect of temperature The effect of temperature on the extent of reduction was investigated using mixing gases with different H /CO molar ratios. Fig. shows the change of reduction degrees with time when the CO/H ratios are 1:0 and 0:1. It is clear that the increase in temperature causes an increase in the extent of reduction after the same reducing time, whether the reducing agent is H or CO, as observed previously [16,18]. When using H as the reducing agent, the times for a 98% reduction degree at 1073 and 173 K are 37.5 and 19.5 min, respectively, reducing by 18 min. The reasons are as follows, first, the equilibrium content of H decreases with increasing temperature because it is an endothermal reaction, leading to a higher reducing potential at high temperature, and as a result, the driving force of reaction is enhanced; second, the high temperature contributes to a high mass transfer coefficient, as shown in Table 3, which attributes to an intensification of gas molecule motion and a mitigation of diffusion resistance. Moreover, the reaction rate constant is also increased, in accordance with the expression of Arrhenius equation, leading to a decrease in reaction resistance as well. The reduction reaction, thus, is accelerated as a result of above three factors. Fig.. Change of reduction degree with time: (a) CO:H =0:1; (b) CO:H =1:0. Similarly, using CO as the reducing agent, the reduction degree is improved from 50.01% at 107 K to 83.38% at 173 K after the same reducing time of 60 min. With increasing temperature, although the driving force of reaction decreases, caused by the increase of equilibrium content of CO, a greater degree of decrease in the resistances of internal diffusion and chemical reaction leads to a promotion in the reaction rate. It can thereby be deduced that the effect of

H.B. Zuo et al., Reduction kinetics of iron oxide pellets with H and CO mixtures 5 temperature on gas diffusion and chemical reaction resistance may exceed the reduction potential. According to these results, improving the reducibility of burden and optimizing the microstructure must be the preconditions to enhance indirect reduction by lowering the reaction temperature. 3.. Effect of mixture gas composition Different gases have different transfer properties and reaction capacities, presenting a specific characteristic reaction process. Fig. 3 shows the effect of gas composition on the reduction process. As reported in previous investigations [17,19], the fastest reaction occurred by using H, the lowest rate was obtained by using CO, and the reduction rate using a mixture gas of H and CO was intermediate. According to the thermodynamic calculations, the reductive ability of CO is higher than that of H when the temperature is lower than 1163 K. However, from Fig. 3(a), it can be seen that the reaction rate increases gradually with the increase of H content in the mixture. This can be attributed to the higher penetration capacity of H than CO. The dwindling of diffusion resistance becomes a dominating factor compared with the weakening of the reaction driving force caused by substituting CO with H. Contrasting Fig. 3 (a) and Fig. 3 (b), a more rapid increase in reaction rate is obtained with increase in the H content in the mixture gas when the temperature is between 1163 and 1173 K. This is because hydrogen has higher reducing and diffusing capacities than CO at temperatures above 1163 K. As the temperature increases, the differences caused by substituting H for CO decrease gradually, as shown in Fig. 3(c). Further quantitative analysis about the effects of H on reduction will be discussed. Fig. 3. Change of reduction degree with reducing time: (a) 1073 K; (b) 1173 K; (c) 173 K. 3.3. Effective diffusion coefficient and rate constant of reaction F and t 1 were calculated individually according to Eqs. (13) and (14), then (t t 1 )/F and 3F F were inserted into the calculation using the experimental data automatically recorded by computer. The relationship between (t t 1 )/F and 3F F was calculated and a linear fitting was generated, as shown in Fig. 4. The fitting equation parameters according to Eq. (15) are listed in Table 4. Fig. 4 shows that (t t 1 )/F and 3F F present a good linear relationship with the reduction degree (R ) over 93.0%, except for the cases at 173 K. This is mainly due to that, when the reaction temperature reaches 173 K, the pellets possibly soften and deform, leading to a more complex and complicated reaction process. Nevertheless, the values of R at 173 K are over 90%, which demonstrates the reliability of the linear fittings. The effective diffusion coefficient and reaction rate constant were obtained subsequently according to slope and intercept, and are listed in Table 5.

6 Int. J. Miner. Metall. Mater., Vol., No. 7, July 015 Fig. 4. Relationship of (t t 1 )/F and 3F F : (a) 1073 K; (b) 1173 K; (c) 173 K. Table 4. Fitting equations at different conditions Temperature / K CO: H by molar (t t 1 )/F = C 1 (3F F ) + C C C 1 R 1:0 113.11075 308.37784 0.9783 3:1 8.85644 106.69014 0.98341 1073 1:1 71.57943 36.87597 0.94945 1:3 5.48409 19.7794 0.93889 0:1 46.9840 4.08568 0.97418 1:0 9.643 15.76375 0.96934 3:1 81.5436 5.465 0.96843 1173 1:1 56.1930 15.37461 0.9555 1:3 4.53178 1.08444 0.9631 0:1 8.96776.74955 0.9970 1:0 61.14809 78.5633 0.919 3:1 59.6889 15.58486 0.93637 173 1:1 43.76141 6.4896 0.90150 1:3 39.47810 3.5947 0.95769 0:1 3.75906.0993 0.97835 Note: R is the relative coefficient, corresponding to the reduction degree. Table 5. Effective diffusion coefficient and reaction rate constant at different conditions CO: H by molar D eff / (10 4 m s 1 ) k rec / (10 m s 1 ) 1073 K 1173 K 173 K 1073 K 1173 K 173 K 1:0 0.05 0.14 0.8 0.50 0.673 1.103 3:1 0.15 0.713 1.86 0.686 0.76 1.130 1:1 0.446 1.131.845 0.794 1.107 1.541 1:3 0.866 1.381 4.681 1.083 1.461 1.708 0:1 4.145 5.835 7.443 1.10.145.838

H.B. Zuo et al., Reduction kinetics of iron oxide pellets with H and CO mixtures 7 The rate constant of the chemical reaction escalates with the rise in reaction temperature and hydrogen content in the mixing gas. Furthermore, the increase in speed of the reaction rate constant with temperature is amplified when the hydrogen content is promoted in the mixture. The effects of temperature have been demonstrated effectively by the Arrhenius equation. On the contrary, regarding the role of hydrogen in improving the increasing rate of the rate constant, it may be attributed to the endothermal reaction of hydrogen reduction and exothermic reaction of carbon monoxide reducing the wustite. The results illustrate that the effect of temperature on rate constant for the endothermal reaction exceeds that for the exothermic reaction when temperature is increased, contributing to a higher rate of increase for the rate constant in the condition of high hydrogen content. The effective diffusion coefficients for different hydrogen contents were also obtained in our research. The diffusion coefficient is determined by the temperature and physical properties of gas, such as molecular size, viscosity, and pressure. The effective diffusion coefficient increases with increasing temperature or hydrogen content in the mixture. The effect of temperature on gas diffusion is easily explained by the diffusion mechanism; when the temperature is increased, the molecular motion increases, leading to the energy growth and diffusion enhancement. In terms of hydrogen, it has a higher penetration capacity than carbon monoxide due to a smaller molecule size and lower viscosity, which results in lower diffusion activation energy. Moreover, the effective diffusion coefficient drops drastically when mixing CO into H compared with pure H ; however, the reaction rate constant does not decrease so rapidly. For example, compared with using pure H at 1073 K, the effective diffusion coefficient declines to about one fifth after mixing CO into H in a molar ratio of 1:3, while the rate constant only decreases from 0.0110 to 0.01083 m s 1, only decreased by about 9.1%. Similar laws appear at 1173 and 173 K. The possible reasons for the drastic reduction in effective diffusion coefficient when mixing a little CO into H are considered as the intrinsic properties of gas; CO has the higher viscosity and bigger diameter than H, which lower the fluidity of the mixture; beyond that, large CO molecules block the diffusion path, holding back the hydrogen passing through, even if only a little CO is introduced. With continuous increase in CO content, the obstacle from large CO slows down, leading to the reduction rate of the effective diffusion coefficient slowing down consequently. 3.4. Resistance analysis in different reduction stages The diffusion and reaction resistances were calculated respectively based on the effective diffusion coefficients and reaction rate constants obtained by the above experiments, without external diffusion considered because of the high flow rate of the reducing gas. The relative resistance expression is presented in Fig. 5. Fig. 5. Change of relative resistance with reduction degree: (a) 1073 K; (b) 1173 K; (c) 173 K.

8 Int. J. Miner. Metall. Mater., Vol., No. 7, July 015 As many papers reported, the rate-controlling step transits from the chemical reaction at the beginning to the complex control of reaction and diffusion, and finally to the diffusion factor. Fig. 5 reveals that when using pure CO as the reducing agent, the chemical reaction controls the reaction rate to a higher reduction degree with the increase of temperature. This is because the promotion of effective diffusion coefficient has an advantage over the increase of the reaction rate constant with the increase of temperature, as seen in Table 5. When mixing more H into the reducing mixture, the high penetration capacity of H contributes to a big increase in the effective diffusion coefficient and further prolongs the rate-control period of the chemical reaction. When pure H is used as the reducing agent, the resistance of diffusion and chemical reaction both decrease with the rise of temperature, while the transit points where the diffusion resistance equals the chemical reaction resistance are almost the same. This result indicates that the temperature has almost the same effect on chemical reaction and diffusion. Furthermore, increasing H content in the mixture could weaken the effects of temperature on the transit point of dominating resistance. Moreover, when the temperature is 1173 K, there are the similar transit points for the mixing gas (H : CO=1:3, 1:1, and 3:1 by molar). This illuminates that when the reducing agent is mixing gas, H has almost equal impact on the diffusion and reaction rate constants. When temperature is 173 K, the effects of H on diffusion precede those on the reaction rate constant, resulting in a backward step for the reaction resistance transit point. 4. Conclusions (1) Increasing hydrogen content in the reducing gas mixture or increasing reaction temperature can clearly accelerate reduction reactions, and more hydrogen can lead to a more rapid increase in reaction rate with the increase of temperature due to the endothermal reaction of hydrogen reduction partly replacing the exothermic reaction of carbon monoxide reduction. () The effective diffusion coefficient and the rate constant of chemical reaction are simultaneously enhanced with increasing temperature or increasing hydrogen content in the mixture. The effect of temperature on the reaction rate constant is influenced by the hydrogen content. A higher hydrogen content leads to a higher intensity of impact; the similar rules exist for the effective diffusion coefficient. Moreover, adding just a little CO into the H would decrease the gas effective diffusion coefficient drastically compared with pure hydrogen. 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