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Reduction of 1-3 mm iron ore by H2 in a fluidized bed
International Journal of Minerals, Metallurgy and Materials Volume 16, Number 6, December 2009, Page 620
Metallurgy
Reduction of 1-3 mm iron ore by H2 in a fluidized bed Jian-ming Pang, Pei-min Guo, Pei Zhao, Chao-zhen Cao, Ding-guo Zhao, and Duo-gang Wang The State Key Laboratory for Advanced Steel Processes and Materials, Central Iron and Steel Research Institute, Beijing 100081, China (Received 2008-12-11)
Abstract: The reduction of 1-3 mm fine powder of iron ore by H2 was conducted in a lab-fabricated kg class high temperature fluidized bed. The results show that the differential pressure in the fluidized bed, which has small fluctuation with time, increases with the increase of gas flowing velocity. The utilization ratio of gas decreases when the reaction lasts longer time indicating that the reaction is faster at the beginning of reduction and becomes slower in the latter process. The higher the reaction temperature is, the higher the utilization ratio of gas is, but the difference of utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas and the metallization ratio can reach 9% and 84% respectively at 750°C for 20 min, which shows the reduction reaction by H2 is very fast. The increase of metallization ratio with gas velocity performs quite good linearity, which shows that a higher velocity of reducing gas can be used to improve the productivity of the reactor when H2 is used as reducing gas. With the increase of charge height, the metallization ratio decreases, but the utilization ratio of gas increases. The reaction temperature can be reduced to 700-750°C from 800-850°C when H2 is used as reducing gas. Key words: hydrogen; fluidized bed; fine powder; iron ore; ironmaking; reduction reaction
[This work was financially supported by the National Nature Science Foundation of China (No.50474006) and the National Science and Technology Support Program for the 11th Five-Year Plan of China (No.2006BAE03A12 and No.2006BAE03A05).]
1. Introduction The blast furnace ironmaking process has been continuously developed and improved in some aspects, such as raw material pre-treatment, coke quality control, and process automation and diagnosis. Thus high productivity and superior energy efficiency are accomplished. Nevertheless, it has little operational flexibility and is limited in use of coke and sintering ore, which can cause serious environment pollutions. Thus high-cost capitals have to be invested for anti-pollution equipments to satisfy the strict environmental regulations. These force the steel industries all of the word to develop new ironmaking processes as alternatives to the blast furnace, such as Corex, Dios, Hismelt and Finex [1-2]. The reduction in fluidized bed reactors offers some advantages, such as good dispersion of feed, uniform temperature in the reactor, and excellent heat and mass transfers. Some of the fluidized bed processes can be used to produce hot briquetted iron (e.g. Finmet and Circored process). Other of them can be used as Corresponding author: Pei-min Guo, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
pre-reduction reactors in smelting reduction process (e.g. Finex) [3-4]. At present, the Finex process is still stayed in industrially experimental stage. One of the key problems of the fluidized bed reactor is the low efficiency if the CO-rich gas is used. However, when H2 is used in the reduction process, the efficiency can be improved, and the CO2 emission is decreased. Under the sponsoring of the National Science and Technology Support Program for the 11th Five-Year Plan of China, the technology of H2 reduction for fine powder of iron ore in the fluidized bed is in development [5-13]. This paper was focused on the experimental study of the H2 reduction for fine powder of iron ore (1-3 mm in size) in a lab-fabricated kg class high temperature fluidized bed, which might accumulate the experience on the fluidized bed processes.
2. Experimental Hematite (Australian iron ore) was used as the experimental material, and its composition is shown in Also available online at www.sciencedirect.com
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
Table 1. The particle size of the iron ore, obtained by screening, is around 1-3 mm, and its bulk density is Table 1. TFe 62.81
SiO2 3.01
Al2O3 2.18
621
2020 kgm3. Additionally, H2 and N2 with high purity were also used.
Composition of hematite CaO 0.01
MgO 0.07
The reduction was conducted in a lab-fabricated kg class high temperature fluidized bed, which included a distribution chamber, gas pre-heater, high temperature fluidized bed (100 mm in diameter), cyclone scrubber, gas cooling system, bag-type dust collector, and feeder system. The schematic diagram of the fluidized bed system is showed in Fig. 1. The temperature, pressure and flow velocity of gas were controlled during the reduction, in which the gas with 50vol% H2 and 50vol% N2 was used as the reducing gas.
P2O5 0.202
wt% S 0.026
FeO 0.39
Burning loss 4.85
3. Results and discussion 3.1. Control of inlet gas temperature For the gas-solid reaction in the fluidized bed, the temperature of inlet gas, an important factor, can be controlled well by the gas pre-heater. Fig. 2 shows the variations of the temperature of inlet gas with time at 700qC when the gas velocity (Ug) is set at 0.8 ms-1. All curves appear smooth, which means that the temperature drift is very small.
Fig. 1. Schematic diagram of the lab-fabricated kg class fluidized bed system. 1—N2 cylinder; 2—H2 cylinder; 3—mixing vessel; 4—gas pre-heater; 5—alumina balls; 6—fluidized bed; 7—feeder system; 8—cyclone scrubber; 9—bag-type dust collector.
The reduction procedure could be described as follows: (1) put certain amount of fine powder of iron ore into the fluidized bed by using feeder facility; (2) heat the fluidized bed and gas pre-heater to an expected temperature; (3) after maintaining the system at the expected temperature for a few hours, keep the pure N2 flowing through the fluidized bed for 20-40 min to wash the system free from O2; (4) switch the pure N2 flow to the flow of the gas mixture of 50vol% H2 and 50vol% N2, and keep this reductive atmosphere for 10-30 min; (5) switch back to pure N2 atmosphere again and keep this atmosphere until the system was cooled down to room temperature. The phase of the samples was analyzed by X-ray diffraction (XRD, X’Pert Pro, PANalytical, Co target O=1.78897, 2ș: 20-80°, scanning rate: 5qmin1). Prior to XRD analysis, the sample was mixed uniformly by grindings. The metallization ratio and the reduction degree of samples were calculated by using the method suggested in Ref. [14], and the utilization ratio of H2 was calculated according to the quantity of the ventilated H2 and the reduction quantity.
Fig. 2. Variations of the temperature of inlet gas with different time at 700°C: (a) 10 min; (b) 20 min; (c) 30 min.
3.2. Change of pressure difference in the fluidized bed The pressure difference in the fluidized bed is an important operational factor. When the fine powder of iron ore is in the critical fluidization condition, the theoretical value of the pressure difference in the fluidized bed can be calculated by the following equation [15]: ǻp
L mf ( ȡ s ȡ g )(1 İ mf ) g
(1)
where ǻp is the pressure difference, Pa; L mf the
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
charge height in the critical fluidization condition, m; ȡ s the particle density, kgm3; ȡ g the gas density, kgm3; İ mf the void fraction of the bed in the critical fluidization condition; and g the gravitational ac-
those of metallization ratio.
celeration, 9.81 ms2. The theoretical value of the pressure difference is 1280 Pa in the critical fluidization condition when the fine powder of iron ore is used in the experiments. Thus when the value of the bed pressure difference is over 1280 Pa, the fine powder of iron ore are in the fluidization condition. Fig. 3 shows variations of the pressure difference with reduction time at different gas velocities at 700°C. The value of pressure difference increases with gas velocity, and for a certain gas velocity, the fluctuation of ǻp is not over 548.8 Pa. All the values of the bed pressure difference are over 1280 Pa in all experiments, which means that the fine powder of iron ore is in the fluidization condition when the gas velocity is at 0.8, 1.2 or 1.6 ms.
Fig. 3. Pressure difference vs. reduction time at different gas velocities.
3.3. Influence of temperature on metallization ratio and reduction degree Fig. 4 illustrates the XRD patterns of samples that were reduced at various temperatures. The metallization ratio and reduction degree were calculated by the peak heights of related phases. Fig. 5 illustrates the influence of reaction temperature on the metallization ratio and reduction degree at 650, 700 and 750°C at the gas velocity of 0.8 ms1 and the still charge height of 6 cm. The metallization ratio reaches 84% for 20 min reduction process at 750°C, more than 75% at 700°C for 20 min, and 75% at 650°C for 30 min, which indicates the reduction reaction by H2 is quite fast. The changing profiles of reduction degree with reduction time are similar to
Fig. 4. X-ray diffraction patterns of samples reduced at various temperatures: (a) 750°C; (b) 700°C; (c) 650°C.
Due to the fast reducing characteristic of H2 at relatively low temperature, the sticking problem will be avoided. 3.4. Influence of temperature on the utilization ratio of gas Here the total gas utilization ratio and stage gas utilization ratio were defined firstly. The total utilization ratio was defined as the average utilization ratio in the entire reducing period; the stage gas utilization ratio was a gas utilization ratio in a certain period. As shown in Fig. 6, the utilization ratio of gas de-
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
creases with reduction time when the gas velocity is 0.8 ms1 and the still charge height is 6 cm, which shows that the reaction is fast at the beginning of the reduction process, then it becomes slower in the latter period. The higher the reaction temperature, the higher the utilization ratio of gas is, but the difference of
Fig. 5.
Fig. 6.
623
utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas can reach 9% and the metallization ratio 84% for 20 min reduction at 750°C, which shows the reduction reaction by H2 is very fast.
Influence of temperature on metallization ratio (a) and reduction degree (b).
Influence of temperature on gas utilization ratio: (a) total utilization ratio; (b) stag utilization ratio.
The utilization ratio of H2 is only 34% at 750°C when the reaction approaches its thermodynamic equilibrium [16]. In our experiments, the still charge height is 6 cm and the apparent gas velocity is 3 ms1. In this case, only 20 ms is need for the gas to flow through the sample bed. Our measurements showed that the utilization ratio of gas of 9% can be achieved in 20 ms reduction, which means this gas-solid reaction is quite fast. 3.5. Influence of gas velocity on ore reduction As shown in Fig. 7, the increase of metallization ratio with gas velocity performs quite good linearity when the reduction is conducted at 700°C for 10 min with the still charge height of 6 cm and H2 as reducing gas, which indicates that the H2 reduction is quite fast. With the increase of gas velocity, the total gas
utilization ratio maintains at a higher level than the data reported in other studies [16]. Thus, under these circumstances, increasing the gas velocity can reduce the reduction time. For example, at 700°C, when the gas velocity is at 1.6 ms1, the metallization ratio can reach 76% for only 10 min reduction, but when the gas velocity is at lower velocity of 0.8 ms1, it has to spend 20 min to approach a similar metallization ratio, which shows a higher velocity of reducing gas can be used to improve the productivity of the reactor. 3.6. Influence of charge height on reduction Since the diameter of the bed is fixed, the charge height corresponds to the mass of the sample. The influence of charge height on the ore reduction at 700°C with a gas velocity of 0.8 ms1 and a reduction time of 10 min is represented in Fig. 8. The metallization
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
ratio and reduction degree decrease with the increase of charge height. However, in the same case, the gas utilization ratio increases. The above results can be understood by the following idea: when more ore powder was put into the bed, the charge height was increased, if the gas flow rate was kept the same, the fine powder of iron ore per kilogram would react with less H2, which led to the decline of both metallization ratio and reduction degree; on the other hand, if the charge height was increased, the amount of H2 would react with more powder, the contact time between gas and powder was prolonged, therefore, the utilization ratio of gas increases.
height should be decreased.
Fig. 8. Influence of charge height on the ore reduction (a) and gas utilization ratio (b).
4. Conclusions
Fig. 7. Influence of gas velocity on reduction of iron ore (a) and gas utilization ratio (b).
Fig. 8 shows that the relationship between charge height and gas utilization ratio deviates from the linearity, which is probably due to the moisture in reducing gas. The moisture in gas may reduce the reduction ability. It is obvious that the influence of charge height on metallization ratio is contrary to that on gas utilization ratio. Thus if we like to increase the gas utilization ratio, the charge height should be increased; while we like to increase the metallization ratio, the charge
(1) The utilization ratio of gas decreases when the reaction lasts longer time, which indicates that the reaction is faster at the beginning of reduction, while it becomes slower in the latter process. The higher the reaction temperature, the higher the utilization ratio of gas is, but the difference of utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas can reach 9% and the metallization ratio 84% for 20 min reduction at 750°C, which shows the reduction reaction by H2 is very fast. (2) The reaction temperature can be reduced to 700-750°C from 800-850°C when H2 is used as reducing gas. (3) The increase of metallization ratio with gas velocity performs quite good linearity, which shows a higher velocity of reducing gas can be used to improve the productivity of the reactor when H2 is used as reducing gas. (4) With the increase of charge height, the metallization ratio decreases, but the utilization ratio of gas
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
increases.
References [1] X.D. Jin, Choice of non-coking ironmaking process, Iron Steel (in Chinese), 33(1998), No.4, p.9. [2] S. Joo, H. Kim, I.O. Lee, et al., FINEX®: a new process for production of hot metal from fine ore and coal, Scand. J. Metall., 28(1999), No.4, p.178. [3] A. Hassan and R. Whipp, The Finmet process—an operational and technical update, Stahl Eisen, 124(2004), No.4, p.53. [4] D.W. Zhang, P.M. Guo, and P. Zhao, Progress in modern ironmaking process, Iron Steel Vanadium Titanium (in Chinese), 27(2006), No.4, p.26. [5] P. Zhao and P.M. Guo, Preparation of Iron Production in Use of Iron Ore Reduction at Low Temperature (in Chinese), Chinese Patent, ZL10000815.6, 2006-08-23. [6] P. Zhao and P.M. Guo, A Preparation Method of Iron Production (in Chinese), Chinese Patent, ZL10039264.4, 2006-08-23. [7] P. Zhao and P.M. Guo, Passivation Method of Ultrafine Iron Powder (in Chinese), Chinese Patent, ZL10113861.6, 2009-05-20. [8] P. Zhao and P.M. Guo, A Preparation Method of Micron and Sub-micron Iron Powder (in Chinese), Chinese Patent, ZL10063632.2, 2009-05-20.
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[9] P. Zhao and P.M. Guo, Study and outlook of nano-metallurgy process, [in] CSM 2005 Annual Meeting Proceedings (in Chinese), Beijing, 2005, p.677. [10] P. Zhao and P.M. Guo, Study on promotion of metallurgical reactions by nanocrystallization of reactants, Iron Steel (in Chinese), 40(2005), No.6, p.6. [11] P. Zhao, P.M. Guo, and D.W. Zhang, Study on reduction sequence of hematite at low-temperature non-equilibrium state, Iron Steel (in Chinese), 41(2006), No.8, p.12. [12] P.M. Guo, D.W. Zhang, and P. Zhao, Study on catalytic mechanism of reducing hematite by carbon at low temperature, Iron Steel Vanadium Titanium (in Chinese), 27(2006), No.4, p.1. [13] D.W. Zhang, Fundamental Research on the New Ironmaking Process Based on Fast Reduction at Low Temperature [Dissertation] (in Chinese), Central Iron and Steel Research Institute, Beijing, 2007, p.61. [14] P.M. Guo, D.W. Zhang, and P. Zhao, A new method for the measurement of degrees of reduction and metall ization of iron oxide, Spectrosc. Spectral Anal. (in Chinese), 27(2007), No.4, p. 816. [15] Z.S. Wu, L.T. Ma, and Z.W. Wang, Fluidization Supporting Technology and Application (in Chinese), Chemical Industry Press, Beijing, 2006, p.71. [16] A.M. Squires and C.A. Johnson, The H-iron process, J. Met., 9(1957), No.4, p.586.
Metallurgy
Reduction of 1-3 mm iron ore by H2 in a fluidized bed Jian-ming Pang, Pei-min Guo, Pei Zhao, Chao-zhen Cao, Ding-guo Zhao, and Duo-gang Wang The State Key Laboratory for Advanced Steel Processes and Materials, Central Iron and Steel Research Institute, Beijing 100081, China (Received 2008-12-11)
Abstract: The reduction of 1-3 mm fine powder of iron ore by H2 was conducted in a lab-fabricated kg class high temperature fluidized bed. The results show that the differential pressure in the fluidized bed, which has small fluctuation with time, increases with the increase of gas flowing velocity. The utilization ratio of gas decreases when the reaction lasts longer time indicating that the reaction is faster at the beginning of reduction and becomes slower in the latter process. The higher the reaction temperature is, the higher the utilization ratio of gas is, but the difference of utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas and the metallization ratio can reach 9% and 84% respectively at 750°C for 20 min, which shows the reduction reaction by H2 is very fast. The increase of metallization ratio with gas velocity performs quite good linearity, which shows that a higher velocity of reducing gas can be used to improve the productivity of the reactor when H2 is used as reducing gas. With the increase of charge height, the metallization ratio decreases, but the utilization ratio of gas increases. The reaction temperature can be reduced to 700-750°C from 800-850°C when H2 is used as reducing gas. Key words: hydrogen; fluidized bed; fine powder; iron ore; ironmaking; reduction reaction
[This work was financially supported by the National Nature Science Foundation of China (No.50474006) and the National Science and Technology Support Program for the 11th Five-Year Plan of China (No.2006BAE03A12 and No.2006BAE03A05).]
1. Introduction The blast furnace ironmaking process has been continuously developed and improved in some aspects, such as raw material pre-treatment, coke quality control, and process automation and diagnosis. Thus high productivity and superior energy efficiency are accomplished. Nevertheless, it has little operational flexibility and is limited in use of coke and sintering ore, which can cause serious environment pollutions. Thus high-cost capitals have to be invested for anti-pollution equipments to satisfy the strict environmental regulations. These force the steel industries all of the word to develop new ironmaking processes as alternatives to the blast furnace, such as Corex, Dios, Hismelt and Finex [1-2]. The reduction in fluidized bed reactors offers some advantages, such as good dispersion of feed, uniform temperature in the reactor, and excellent heat and mass transfers. Some of the fluidized bed processes can be used to produce hot briquetted iron (e.g. Finmet and Circored process). Other of them can be used as Corresponding author: Pei-min Guo, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
pre-reduction reactors in smelting reduction process (e.g. Finex) [3-4]. At present, the Finex process is still stayed in industrially experimental stage. One of the key problems of the fluidized bed reactor is the low efficiency if the CO-rich gas is used. However, when H2 is used in the reduction process, the efficiency can be improved, and the CO2 emission is decreased. Under the sponsoring of the National Science and Technology Support Program for the 11th Five-Year Plan of China, the technology of H2 reduction for fine powder of iron ore in the fluidized bed is in development [5-13]. This paper was focused on the experimental study of the H2 reduction for fine powder of iron ore (1-3 mm in size) in a lab-fabricated kg class high temperature fluidized bed, which might accumulate the experience on the fluidized bed processes.
2. Experimental Hematite (Australian iron ore) was used as the experimental material, and its composition is shown in Also available online at www.sciencedirect.com
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
Table 1. The particle size of the iron ore, obtained by screening, is around 1-3 mm, and its bulk density is Table 1. TFe 62.81
SiO2 3.01
Al2O3 2.18
621
2020 kgm3. Additionally, H2 and N2 with high purity were also used.
Composition of hematite CaO 0.01
MgO 0.07
The reduction was conducted in a lab-fabricated kg class high temperature fluidized bed, which included a distribution chamber, gas pre-heater, high temperature fluidized bed (100 mm in diameter), cyclone scrubber, gas cooling system, bag-type dust collector, and feeder system. The schematic diagram of the fluidized bed system is showed in Fig. 1. The temperature, pressure and flow velocity of gas were controlled during the reduction, in which the gas with 50vol% H2 and 50vol% N2 was used as the reducing gas.
P2O5 0.202
wt% S 0.026
FeO 0.39
Burning loss 4.85
3. Results and discussion 3.1. Control of inlet gas temperature For the gas-solid reaction in the fluidized bed, the temperature of inlet gas, an important factor, can be controlled well by the gas pre-heater. Fig. 2 shows the variations of the temperature of inlet gas with time at 700qC when the gas velocity (Ug) is set at 0.8 ms-1. All curves appear smooth, which means that the temperature drift is very small.
Fig. 1. Schematic diagram of the lab-fabricated kg class fluidized bed system. 1—N2 cylinder; 2—H2 cylinder; 3—mixing vessel; 4—gas pre-heater; 5—alumina balls; 6—fluidized bed; 7—feeder system; 8—cyclone scrubber; 9—bag-type dust collector.
The reduction procedure could be described as follows: (1) put certain amount of fine powder of iron ore into the fluidized bed by using feeder facility; (2) heat the fluidized bed and gas pre-heater to an expected temperature; (3) after maintaining the system at the expected temperature for a few hours, keep the pure N2 flowing through the fluidized bed for 20-40 min to wash the system free from O2; (4) switch the pure N2 flow to the flow of the gas mixture of 50vol% H2 and 50vol% N2, and keep this reductive atmosphere for 10-30 min; (5) switch back to pure N2 atmosphere again and keep this atmosphere until the system was cooled down to room temperature. The phase of the samples was analyzed by X-ray diffraction (XRD, X’Pert Pro, PANalytical, Co target O=1.78897, 2ș: 20-80°, scanning rate: 5qmin1). Prior to XRD analysis, the sample was mixed uniformly by grindings. The metallization ratio and the reduction degree of samples were calculated by using the method suggested in Ref. [14], and the utilization ratio of H2 was calculated according to the quantity of the ventilated H2 and the reduction quantity.
Fig. 2. Variations of the temperature of inlet gas with different time at 700°C: (a) 10 min; (b) 20 min; (c) 30 min.
3.2. Change of pressure difference in the fluidized bed The pressure difference in the fluidized bed is an important operational factor. When the fine powder of iron ore is in the critical fluidization condition, the theoretical value of the pressure difference in the fluidized bed can be calculated by the following equation [15]: ǻp
L mf ( ȡ s ȡ g )(1 İ mf ) g
(1)
where ǻp is the pressure difference, Pa; L mf the
622
International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
charge height in the critical fluidization condition, m; ȡ s the particle density, kgm3; ȡ g the gas density, kgm3; İ mf the void fraction of the bed in the critical fluidization condition; and g the gravitational ac-
those of metallization ratio.
celeration, 9.81 ms2. The theoretical value of the pressure difference is 1280 Pa in the critical fluidization condition when the fine powder of iron ore is used in the experiments. Thus when the value of the bed pressure difference is over 1280 Pa, the fine powder of iron ore are in the fluidization condition. Fig. 3 shows variations of the pressure difference with reduction time at different gas velocities at 700°C. The value of pressure difference increases with gas velocity, and for a certain gas velocity, the fluctuation of ǻp is not over 548.8 Pa. All the values of the bed pressure difference are over 1280 Pa in all experiments, which means that the fine powder of iron ore is in the fluidization condition when the gas velocity is at 0.8, 1.2 or 1.6 ms.
Fig. 3. Pressure difference vs. reduction time at different gas velocities.
3.3. Influence of temperature on metallization ratio and reduction degree Fig. 4 illustrates the XRD patterns of samples that were reduced at various temperatures. The metallization ratio and reduction degree were calculated by the peak heights of related phases. Fig. 5 illustrates the influence of reaction temperature on the metallization ratio and reduction degree at 650, 700 and 750°C at the gas velocity of 0.8 ms1 and the still charge height of 6 cm. The metallization ratio reaches 84% for 20 min reduction process at 750°C, more than 75% at 700°C for 20 min, and 75% at 650°C for 30 min, which indicates the reduction reaction by H2 is quite fast. The changing profiles of reduction degree with reduction time are similar to
Fig. 4. X-ray diffraction patterns of samples reduced at various temperatures: (a) 750°C; (b) 700°C; (c) 650°C.
Due to the fast reducing characteristic of H2 at relatively low temperature, the sticking problem will be avoided. 3.4. Influence of temperature on the utilization ratio of gas Here the total gas utilization ratio and stage gas utilization ratio were defined firstly. The total utilization ratio was defined as the average utilization ratio in the entire reducing period; the stage gas utilization ratio was a gas utilization ratio in a certain period. As shown in Fig. 6, the utilization ratio of gas de-
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
creases with reduction time when the gas velocity is 0.8 ms1 and the still charge height is 6 cm, which shows that the reaction is fast at the beginning of the reduction process, then it becomes slower in the latter period. The higher the reaction temperature, the higher the utilization ratio of gas is, but the difference of
Fig. 5.
Fig. 6.
623
utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas can reach 9% and the metallization ratio 84% for 20 min reduction at 750°C, which shows the reduction reaction by H2 is very fast.
Influence of temperature on metallization ratio (a) and reduction degree (b).
Influence of temperature on gas utilization ratio: (a) total utilization ratio; (b) stag utilization ratio.
The utilization ratio of H2 is only 34% at 750°C when the reaction approaches its thermodynamic equilibrium [16]. In our experiments, the still charge height is 6 cm and the apparent gas velocity is 3 ms1. In this case, only 20 ms is need for the gas to flow through the sample bed. Our measurements showed that the utilization ratio of gas of 9% can be achieved in 20 ms reduction, which means this gas-solid reaction is quite fast. 3.5. Influence of gas velocity on ore reduction As shown in Fig. 7, the increase of metallization ratio with gas velocity performs quite good linearity when the reduction is conducted at 700°C for 10 min with the still charge height of 6 cm and H2 as reducing gas, which indicates that the H2 reduction is quite fast. With the increase of gas velocity, the total gas
utilization ratio maintains at a higher level than the data reported in other studies [16]. Thus, under these circumstances, increasing the gas velocity can reduce the reduction time. For example, at 700°C, when the gas velocity is at 1.6 ms1, the metallization ratio can reach 76% for only 10 min reduction, but when the gas velocity is at lower velocity of 0.8 ms1, it has to spend 20 min to approach a similar metallization ratio, which shows a higher velocity of reducing gas can be used to improve the productivity of the reactor. 3.6. Influence of charge height on reduction Since the diameter of the bed is fixed, the charge height corresponds to the mass of the sample. The influence of charge height on the ore reduction at 700°C with a gas velocity of 0.8 ms1 and a reduction time of 10 min is represented in Fig. 8. The metallization
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
ratio and reduction degree decrease with the increase of charge height. However, in the same case, the gas utilization ratio increases. The above results can be understood by the following idea: when more ore powder was put into the bed, the charge height was increased, if the gas flow rate was kept the same, the fine powder of iron ore per kilogram would react with less H2, which led to the decline of both metallization ratio and reduction degree; on the other hand, if the charge height was increased, the amount of H2 would react with more powder, the contact time between gas and powder was prolonged, therefore, the utilization ratio of gas increases.
height should be decreased.
Fig. 8. Influence of charge height on the ore reduction (a) and gas utilization ratio (b).
4. Conclusions
Fig. 7. Influence of gas velocity on reduction of iron ore (a) and gas utilization ratio (b).
Fig. 8 shows that the relationship between charge height and gas utilization ratio deviates from the linearity, which is probably due to the moisture in reducing gas. The moisture in gas may reduce the reduction ability. It is obvious that the influence of charge height on metallization ratio is contrary to that on gas utilization ratio. Thus if we like to increase the gas utilization ratio, the charge height should be increased; while we like to increase the metallization ratio, the charge
(1) The utilization ratio of gas decreases when the reaction lasts longer time, which indicates that the reaction is faster at the beginning of reduction, while it becomes slower in the latter process. The higher the reaction temperature, the higher the utilization ratio of gas is, but the difference of utilization ratio among the different temperatures becomes smaller with time. The utilization ratio of gas can reach 9% and the metallization ratio 84% for 20 min reduction at 750°C, which shows the reduction reaction by H2 is very fast. (2) The reaction temperature can be reduced to 700-750°C from 800-850°C when H2 is used as reducing gas. (3) The increase of metallization ratio with gas velocity performs quite good linearity, which shows a higher velocity of reducing gas can be used to improve the productivity of the reactor when H2 is used as reducing gas. (4) With the increase of charge height, the metallization ratio decreases, but the utilization ratio of gas
J.M. Pang et al., Reduction of 1-3 mm iron ore by H2 in a fluidized bed
increases.
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