reduction kinetics of hematite concentrate particles by

7th International Symposium on High-Temperature Metallurgical Processing Edited by: Jiann-Yang Hwang, Tao Jiang, P. Chris Pistorius, Gerardo R.F. Alvear F., Onuralp Yücel, Liyuan Cai, Baojun Zhao, Dean Gregurek, and Varadarajan Seshadri TMS (The Minerals, Metals & Materials Society), 2016

REDUCTION KINETICS OF HEMATITE CONCENTRATE PARTICLES BY CO+H2 MIXTURE RELEVANT TO A NOVEL FLASH IRONMAKING PROCESS Yousef Mohassab1, Feng Chen1,2, Mohamed Elzohiery1, Amr Abdelghany1, Shengqin Zhang1,3, and Hong Yong Sohn1 1

Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, USA 2 School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, China 3 School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Keywords: Hematite, CO+H2 mixtures, Reduction kinetics, Flash ironmaking Abstract The kinetics of hematite concentrate reduction by mixtures of hydrogen and CO of various compositions has been investigated as part of the development of a flash ironmaking process at the University of Utah. This process produces iron directly from iron oxides concentrates by the gas-solid flash reaction based on the partial oxidation of natural gas, resulting in a significant reduction in energy consumption and greenhouse gas emission. The reduction kinetics of hematite concentrate of an average particle size 21.3 µm by the above mentioned gases in the temperature range 1423 to 1623 K (1150 to 1350 ºC) was investigated. Hematite concentrate particles can be reduced to > 90% by any of these reductants in several seconds of residence time typically available in a flash reactor. The activation energy ranged from 214 kJ/mol for hydrogen to 231 kJ/mol for CO. Introduction The high energy consumption and CO2 emissions in the conventional blast furnace process have given rise to the need for alternative ironmaking technologies. Although the blast furnace has high production rates among other advantages, it faces problems arising from high energy consumption and CO2 emissions due to the use of coke and the need for sintering, pelletization and cokemaking. At the University of Utah, a novel flash ironmaking process is under development [1-10], in which iron ore concentrate is flash-reduced by gaseous reductants at temperatures above 1473 K (1200 ºC). This flash ironmaking is more energy efficient and drastically reduces the emissions of carbon dioxide based on the elimination of sintering, pelletization and cokemaking steps required for the blast furnace process. Although hydrogen is the cleanest and most efficient reductant, other gaseous reductants can be used as well in the flash ironmaking process. Reformed natural gas and coal gas, which are mainly composed of CO and H2, are also possible reductants in this novel process. Most of the previous work [11-17] on the reduction of iron oxides was conducted at temperatures lower than the minimum temperature, 1473 K (1200 ºC), expected for the new flash process. Furthermore, the majority focused on the reduction of hematite samples with particle size larger 1

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than 100 µm, which is significantly larger than the concentrate particles for the novel flash process. Additionally, in the rapid reduction of fine hematite particles at high temperatures, the reduction does not proceed by forming the next lower oxides in order; rather, a combination of hematite, magnetite, wustite and iron is present at any given moment. There had been little work on the gaseous reduction of iron oxide concentrates before the conception of the new flash ironmaking technology. As an integral part of the novel flash ironmaking process, the goal of this research was to perform a systematic measurement of the hematite concentrates particles reduction kinetics by mixtures of H2 and CO of various compositions and derive the rate expressions in the temperature range 1423 to 1623 K (1150 ºC to 135 ºC). Rate equations that contain the effects of process variables in a flash ironmaking reactor, including temperature and partial pressures of reductant gases, were developed. Experimental Work The experimental apparatus, the materials, and the detailed procedures have been described elsewhere [9, 10], the summary of which is presented in this paper. Hematite concentrate used in this study was from the Yuanjiacun Range, Shanxi Province, China, with average particle size of 21.3 μm. About 30 mass pct magnetite was originally contained in the ore and then removed by magnetic separation to separate hematite particles. The chemical composition of the magnetically concentrated hematite sample (about 88 wt pct hematite and 3.4 wt pct magnetite) is presented in Table I. Table I. Chemical Composition of Hematite Concentrate Used in This Work Component

Total Fe

FeO in Fe3O4

SiO2

Al2O3

K2O

Na2O

P

S

wt pct

63.98

1.05

5.53

0.82

0.06

0.02

0.03

0.01

A drop-tube reactor system consisted of a vertical tubular furnace housing an alumina tube, a pneumatic powder feeder, gas delivery lines, a powder cooling and collecting system, and an offgas burning system, was used in this work to reduce hematite concentrate particles. The isothermal zone ( Liso ) where the temperature was maintained within ±20 K started at the bottom of cylindrical honeycomb and extended 91 cm downward. When the target temperature was reached, reductant gas (hydrogen/carbon monoxide mixtures) and nitrogen were charged into the reactor at predetermined flow rates after the system was appropriately purged by nitrogen. After a thorough reductant gas leak test, the hematite particles were fed into the reactor tube. After a certain amount of the concentrate particles were supplied into the reactor, the syringe pump was stopped and the vibrator was kept running for 3 minutes longer to discharge the powder in the delivery lines. Then, the flow of the reductant gases was stopped and the flow rate of nitrogen increased to purge the whole system for 15 to 20 minutes. The valve of the powder collection bin was closed and the collector was disconnected from the reactor and quenched by water to room temperature to cool the reduced powder to avoid the re-oxidation when the fine particles contact with air. The reduced sample was transferred into a vial sealed with a cap for further analysis by ICP, SEM, and XRD. 2

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Previous work [9, 10] has described the definition of parameters, including the degree of reduction (X), the residence time () and pct excess reductant gas. It is noted that the barometric pressure at Salt Lake City is 0.85 atm (1 atm = 101.32 kPa), which was used in all the experimental design and analysis in this work. In addition, pct excess H2 or CO is > 500 in the reduction experiments by single gaseous reductant, and > 300 pct in the reduction experiments by CO+H2 mixture. Results Effect of Experimental Variables Reduction Temperature: Previous work [18, 19] indicated that the magnetite reduction rate by hydrogen was fast enough in this novel flash system when the temperature was higher than 1373 K (1100 ºC). The experiments were carried out under 1423 K (1150 ºC) to 1623 K (1350 ºC) in this work to examine the effect of reduction temperature on the reduction degree of hematite concentrate particles. Reductant Gas Partial Pressure: It was concluded from previous work [19, 20] that high reduction degree of magnetite concentrate particles could be obtained within a short residence time in this novel flash reduction system. N2 was added to change the partial pressure of reductant gas. Hematite Reduction Kinetics by Single Reductant Gas Hematite concentrate reduction kinetics by CO in the temperature range 1473 to 1623 K (1200 to 1350 ˚C) and that by H2 in the temperature range 1423 to 1623 K (1150 to 1350 ˚C) were investigated by Sohn and coworkers [10-11]. It was found that within a few seconds of residence time, a reduction degree of over 90 pct was achieved using either CO or H2 at temperatures above 1573 K (1300 ˚C), as shown in Figures 1 and 2. The nucleation and growth rate equation with the Avrami parameter n = 1.0 adequately described both the carbon monoxide and hydrogen reduction kinetics of hematite concentrate particles, as detailed elsewhere [9-10]. In addition, the dependence with respect to gas partial pressure was 1st-order for both CO and H2. The activation energy ranged from 214 kJ/mol for hydrogen to 231 kJ/mol for CO. The following complete rate equations (Eq. [1] and [2]) are, respectively, the hematite kinetics rate expressions separately by CO and H2: 231000

pCO2 dX ) CO  1.91 107  e RT  ( pCO  )  (1  X ) dt K 214000 pH2O dX ( ) H2  4.41  107  e RT  ( pH2  )  (1  X ) dt K (

[1] [2]

where X is the fraction of oxygen removed from iron oxide (degree of reduction), R is 8.314 J/mol·K, T is in K, p is in atm, and t is in seconds. The agreement between the calculated reduction degrees vs time using the developed rate equation and the experimental results is satisfactory, as shown in Figures 3 and 4.

3

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120

Reduction Degree (pct)

100 80 60

40 1473 K pH = 0.2 atm 1523 K pH = 0.2 atm 1573 K pH = 0.2 atm

20 0

0

2

4

6

8

Residence Time (s)

Figure 1. Reduction degree of hematite by H2 vs residence time. The solid lines represent the calculated reduction degree using the developed rate expression, Eq. [1]. (pct excess H2 > 500 in all experiments.)


100

75

50

25

0

1473 K pCO = 0.6 atm 1573 K pCO = 0.85 atm 1623 K pCO = 0.3 atm 0.0

2.0

4.0

6.0

8.0

Residence Time (s)

Figure 2. Reduction degree of hematite by CO vs residence time. The solid lines represent the calculated reduction degree using the developed rate expression, Eq. [2]. (pct excess CO > 500 in all experiments.) 4

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Calculated Reduction (pct)

100

75

50 R² = 0.96 25

0

0

25

50

75

100

Exprimental Reduction Degree (pct)

Figure 3. Comparison between the calculated reduction degree (using Eq. [1]) and the experimental results for all the runs made in the reduction by H2.

Calculated Reduction Degree (pct)

100

75

50

R² = 0.91 25

0

0

25

50

75

100

Experimental Reduction Degree (pct)

Figure 4. Comparison between the calculated reduction degree (using Eq. [2]) and the experimental results for all the runs in the reduction by CO.

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Hematite Reduction Kinetics by CO+H2 Mixtures Hematite reduction kinetics by CO+H2 mixtures, was conducted to represent the case of using natural gas or coal gas as the reductant/fuel in the flash ironmaking process. The hematite reduction rate by CO+H2 mixtures is complicated due to the simultaneous reduction by the two reductants. It was found that within a few seconds of residence time, a reduction degree of over 90 pct was achieved using either CO+H2 at temperatures above 1573 K (1300 ˚C), as shown in Figure 5. It is also clear that adding CO to the H2 boosts the reduction kinetics as compared with the reduction by a single gas H2 or CO. It was found, however, that increasing CO partial pressure from 0.05 to 0.2 atm while holding hydrogen partial pressure at 0.2 atm did not affect the reduction kinetics of hematite concentrate, as Figure 5 shows. More reduction experiments by CO+H2 mixtures are being conducted. 100

1573 K


80

60

40 pH pH pH pH

20

0

0.0

1.0

=0.1 atm - pCO=0.2 atm =0.1 atm - pCO=0.1 atm =0.1 atm - pCO=0.05 atm =0.1 atm

2.0

3.0

4.0

Residence Time (s)

Figure 5. Reduction degree comparisons between single reductant gas (H2/CO) and CO+H2 mixtures at 1573 K (1300 ºC). Conclusions The reduction kinetics of hematite concentrate particles of an average particles size 21.3 µm by gaseous reductants, including H2, CO and their mixtures, were investigated. The dependences of rate on partial pressure of gaseous reductant, residence time and temperature were determined. The reduction temperature ranges of hematite concentrate particles by H2, CO and their mixtures were 1423 to 1623 K (1150 to 1350 ºC).

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The results clearly indicated that the hematite concentrate can be reduced to greater than 90 pct by H2, CO and their mixtures in the several seconds of residence time typically available in a flash reactor. The nucleation and growth rate equations with an Avrami parameter n = 1 well describe the kinetics of hematite reduction by H2 and CO. The reduction rate has a 1st-order dependence on the partial pressure of reductant gas, including H2 and CO. The activation energy of carbon monoxide reduction of hematite concentrate was 231 kJ/mol in contrast to 214 kJ/mol for the hydrogen reduction. Complete rate equations were developed respectively to satisfactorily represent the reduction kinetics of hematite concentrate particles by H2 and CO and are suitable for the design of a flash reactor. Acknowledgments The authors thank Omar Kergaye, Andrew Laroche, Caio Melo, and Tuvshinbat Ganbat for the help with the experimental runs and the analytical work using ICP. The authors acknowledge the financial support from the U.S. Department of Energy under Award Number DEEE0005751 with cost share by the American Iron and Steel Institute (AISI) and the University of Utah. Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References 1. 2. 3. 4. 5.

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