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Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
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*%# ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2013, 20(2). 08-13
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions LIN Yin-he,
GUO Zhan-cheng,
TANG Hui-qing
(State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China) Abstract: To process optimization and improve the degree of reduction, a two-step experiment was designed. The ex periment was carried out in the micro-fluidized bed. The reactor in the micro-fluidized bed is operated as a differential reactor to ensure an equal temperature and residence time with the reactor volume. The experiment used Brazilian iron ore and reducing gas of CO. T h e operating temperature was 400 to 800 'C and the residence time was between 10 and 60 min. In correspondence with experiment, microscopic technique was applied too. T h e test shows that temperature and residence time of the pre-reduction stage have an important effect on the degree of reduction. By using two-step experiment, the maximum value of reduction degree increases by 44. 1% compared with the maximum value of tradi tional reduction experiment. Microscopic analysis shows that the specific surface area, surface morphology and tex ture of reduced iron ore after pre-reduction stage have an important effect on the degree of final reduction too. Key words : iron ore reduction; two-step experiment; specific surface area; morphology; pre-reduction degree; final reduction degree
In order to compensate the decrease of high quality steel scrap, direct reduced iron becomes an important industrial material of production in steel plants. DRI (direct reduced iron) is produced in indus trial fluidized bed (FINMET, FIOR and FINEX) C l ] . The industrial fluidized bed is one-step technology of iron-making. It requires relatively high initial condi tions of iron ore (surface morphology, porosity). The reduction degree is affected by initial conditions of iron ore too much. The reduction degree is gener ally low. The experiment was considered to change initial conditions of iron ore and get higher reduction degree. Therefore, the two-step experiment was de signed. The first step was pre-reduction stage. The iron ore in this stage got all kinds of initial condi tions. The second step was final reduction stage. The products resulting from pre-reduction stage were used as reactant of final reduction stage. It was used to study the influence of the changes of pre-re duction conditions on final reduction degree.
1
Experimental
1.1
Experimental setup To investigate the influence of the temperature
and residence time of pre-reduction on their final re duction degree under atmospheric pressure, a microfluidized bed reactor was used. The operating condi tions of the micro-fluidized bed reactor are as follows: 1) Working pressure is in atmosphere pressure. 2) Operating temperature is 400 to 800 "C. 3) Reducing gas is pure CO. 4) Operating time is from 10 to 60 min. The micro-fluidized bed is a differentially oper ating reactor. It can promise that each particle of the sample in reactor is in the same process conditions at any time during reduction'· 2-4 -'. The basic principle of the experimental setup is shown in Fig. 1. Fluidizing agent and iron ore were respectively put in reactor and sample injector. The inert gas flowing through the reactor prevented iron ore from reacting with other materials in air during the pre heating phase. After reaching the desired process temperature, the inert gas was switched to reducing gas. When concentration of CO was kept steady, iron ore was shot into reactor by sample injector and reac ted with CO gas. After the demanded reduction time, the reducing gas was switched "back to inert gas for preventing from reoxidation. When temperature of re-
Foundation Item:Item Sponsored by National Natural Science Foundation of China (50834007) Biography:LIN Yin-he(1980—), Male, Doctor! E-mail: [email protected]; Received Date: August 12, 2011 Corresponding Author : GUO Zhan-cheng(1963 — ) , Male, Doctor, Professor; E-mail: [email protected]
Issue 2
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
Gas filter/ condenser r "
i
| I'
Atmosphere
L
Weedle vavle\ I |
Thermocouple Fluidizing agerrt quartz sand | Furnace electric I resistance ' Quartz-*^' _ 3 J tubular reactor
fl Process
A
But t h e experiment used a differential reactor. It can ensure t h e t e m p e r a t u r e and concentration being a constant. A n d another aspect is the experimental conditions. T h e former experiment was under high p r e s s u r e condition and did not study the normal p r e s s u r e condition. T h e r e f o r e , t h e experiment does some exploratory research. 1. 3
Determination of degree of reduction T h e pre-reduction degree (R) is calculated by Eqn. ( 1 ) :
Fig. 1 Basic principle of experimental setup
R= Γ %-
actor had been cooled d o w n to room t e m p e r a t u r e , the reduced iron ore samples were removed from t h e reactor and a part of t h e m was analyzed by a titriometric m e t h o d ( i r o n chloride m e t h o d ) for overall Fe, ot > Fe° and F e 2 + [ 5 _ 6 ] . T h i s step above was pre-reduction stage. T h e rest of t h e reduced iron ore was used to re peat t h e above steps on conditions of final reduction. T h a t was t h e final reduction stage. T o get knowledge about s t r u c t u r a l c h a n g e s , microscopic morphology of raw iron ore particles as well as t h a t of reduced iron ore particles w a s i n v e s t i g a t e d in reflected light on S E M of E V O I 8 Special Edition in Carl Zeiss N T S . 1. 2
Reduction tests T o investigate t h e influence of pre-reduction on final reduction d e g r e e , two-step experiment w a s car ried out. T h e reduction t e s t s w e r e carried out w i t h Brazilian iron ores ( T a b l e 1) at different t e m p e r a t u r e s and residence times. S t a n d a r d operating condi tions were defined in which t e m p e r a t u r e s and resi dence times of t h e pre-reduction stage were varied. A f t e r w a r d s t h e samples w e r e reduced under final r e duction conditions in t h e final reduction stage. T h e t e m p e r a t u r e , m a s s flow, reaction times and varia tion of oxygen in iron ores w e r e recorded. T h e t w o - s t e p experiment is different from t h e formers. T h e most commonly used equipments of the formers w e r e large-scale fluidized bed. T h e s e in s t r u m e n t s w e r e integral reactor. Various parts of the reactor had different t e m p e r a t u r e s and concentrations. Table 1 Chemical analysis of Brazilian iron ores (mass percent, %) Fe,ot Fe 2 0 3 FeO 65
92.85
0
• 9 ·
Si0 2
CaO
MgO
AI2O3
3.7
0.07
0.11
2.35
S
Ignorance 0.92
(1)
Jo U |
w h e r e , t is reaction t i m e ; O, is total m a s s of oxygen in iron o r e ; and O m is variation of oxygen in iron ore per second. T h e n t h e p r o d u c t s resulting from pre-reduction reaction w e r e used as reactant in final reduction stage. So the final reduction degree (jRf) is calculat ed by Eqn. ( 2 ) :
M: a
(2)
0 Of
w h e r e , Of is total m a s s of oxygen in product resul ting from pre-reduction s t a g e ; and O n is variation of oxygen in reactant per second.
2
Results and Discussion
T h e pre-reduction is carried out at 400 to 700 - C , and time is 10 min. Final reduction is performed at 800 °C , and time is 20 min. S t a r t i n g from standard conditions, t e m p e r a t u r e and residence time of t h e pre-reduction stage were varied. Final reduction conditions were the same for all experiments. 2.1
Variation of temperature in pre-reduction stage T h e curve of the final reduction degree versus the t e m p e r a t u r e of pre-reduction is s h o w n in Fig. 2. 0.65
/
0.60
/~"\
0.55
\ \
0.50
\
0.45 0.40
\
\
■
400 Fig. 2
500
T/V
600
Degree of final reduction versus pre-reduction temperature
700
Journal of Iron and Steel Research, International
• 10 ·
The final reduction degree increases with increasing pre-reduction temperature up to 500 "C. Hereafter, final reduction degree at higher t e m p e r a t u r e decreases up to a pre-reduction t e m p e r a t u r e of 600 "C. W h e n prereduction t e m p e r a t u r e is 600 to 625 -C , final reduc tion degree increases again w i t h a further increase of the pre-reduction t e m p e r a t u r e . But w h e n further in creasing the t e m p e r a t u r e of pre-reduction reaction, degree of final reduction does not have great changes. T o find out the reasons of this p h e n o m e n o n and to improve the understanding for different reduction behaviors depending on pre-reduction t e m p e r a t u r e , microscopic morphologies of original ore as well as the partly-reduced material were investigated. W h e n t h e t e m p e r a t u r e of pre-reduction reaction is less t h a n 500 °C , interfacial reaction rate is low and little quantity of m a g n e t i t e is generated in prereduction stage. T h e magnetite resulting from prereduction reaction appears on t h e surface of ore grains with small particles [ F i g . 3 ( a ) ] . It increases specific surface area of reactant in final reduction re action. T h e quantity of m a g n e t i t e increases as prereduction t e m p e r a t u r e increases. It m e a n s t h a t spe cific surface area of reactant in final reduction reac tion increases with increasing pre-reduction tempera ture. T h e r e f o r e , degree of final reduction increases as pre-reduction t e m p e r a t u r e increases. W h e n the t e m p e r a t u r e of pre-reduction reaction is 500 to 600 °C , rate of interfacial reaction rises. Particles of magnetite produced by pre-reduction r e action bond with each o t h e r and form a dense hard shell of F e 3 0 4 on surface of fine ore [ F i g . 3 ( b ) ] . It prevents reduction gas from continuing to react with fine ore. T h e proportion of dense hard shell on sur
Fig. 3
Vol. 20
face of fine ore further increases as pre-reduction t e m p e r a t u r e increases. T h e r e f o r e , degree of final reduction decreases w h e n pre-reduction t e m p e r a t u r e increases. T h e dense magnetite is reduced to porous w u s tite w h e n t e m p e r a t u r e of pre-reduction stage is 600 to 625 °C [ F i g . 3 ( c ) ] . T h e proportion of porous w u s t i t e on surface of fine ore increases as pre-reduc tion t e m p e r a t u r e increases and specific surface area of products resulting from pre-reduction reaction al so increases. P r o d u c t s resulting from pre-reduction reaction are used as reactant in final reduction stage. Pre-reduction t e m p e r a t u r e increases specific surface area of reactant in final reduction. T h e r e f o r e , de gree of final reduction increases as pre-reduction t e m p e r a t u r e increases. When pre-reduction temperature is above 625 °C, almost all magnetite on surface of fine ore is transformed to w u s t i t e [ F i g . 3 ( d ) and ( e ) ] . P r o d u c t s resulting from pre-reduction stage are used as reactant in final reduction stage. W h e n reactant produced by pre-re duction stage is in final reduction conditions, w u s tite is reduced to metallic iron. T h e iron appears on surface of w u s t i t e w i t h form of w h i s k e r , bonds with each o t h e r and forms a dense metal shell of iron [Fig. 3 ( f ) ] , which prevents reduction reaction continuing to happen. Therefore, degree of final reduction does not have great change as pre-reduction t e m p e r a t u r e in creases. According to the analyses above, the materials with different m i c r o s t r u c t u r e s w e r e formed in different pre-reduction conditions. These materials were used for reactant in final reduction stage. It has a significant influence on their final reduction degree. The materials
Reduced iron ores after pre-reduction stage (a) , ( b ) , ( c ) , (d) and after final-reduction stage (e) , (f)
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
Issue 2
with different m i c r o s t r u c t u r e s can memorize their different corresponding pre-treating p r o c e s s M . 2. 2
Variation of residence time Starting from the standard operating condi t i o n s , the residence time in t h e pre-reduction stage was varied from 10 to 60 min. F r o m Fig. 2 of final reduction degree versus prereduction temperature, the pre-reduction experiments were respectively carried out in the t e m p e r a t u r e range of 400 to 500 °C , 500 to 600 "C and 600 to 700 *C to investigate influence of the pre-reduction time on t h e degree of final reduction. T h e points of 4 5 0 , 550 and 625 °C were selected as t e m p e r a t u r e points of residence time e x p e r i m e n t s in t h e pre-reduction stage. T h e results are s h o w n in Fig. 4. Increasing residence time in the pre-reduction stage causes an increase of the final reduction degree at 450 -C ( F i g . 4 ) . T h e effect can be explained by the magnetite formed in the pre-reduction stage. In this pre-reduction s t a g e , the magnetite resulting from pre-reduction reaction appears on the surface of ore grains with small particles [ F i g . 5 ( a ) and ( b ) ] . T h e magnetite particles become more and more as pre-re duction time increased. It increases t h e specific sur face area of iron ore after pre-reduction stage too. The 0.85
f
0.75 0.65 *0.55 0.45
■
^ ^
■625 t ; • 550 "C A 450 Έ
/
X
_
0.35 0.25
10
20
30
40
50
60
i/min Fig. 4
Final reduction degree versus pre-reduction time
Fig. 5
• 11 ·
iron ore produced by pre-reduction reaction is used as reactant of final reduction reaction and it contrib utes to an increase in specific surface area of reactant in final reduction stage. Therefore, final reduction de grees increase as pre-reduction time increases. Grains of m a g n e t i t e produced by pre-reduction stage further increase as residence time increase at the pre-reduction t e m p e r a t u r e of 550 °C. Magnetite particles resulting from pre-reduction stage bond w i t h each o t h e r and form a dense hard shell on the surface of iron ore. T h e a m o u n t s of dense magnetite layer further increase w h e n residence time increases [ F i g . 5 (c) and ( d ) ] . T h e layers of dense magnetite prevent reduction gas in t h e final reduction stage from diffusing into particle interior of reactant. T h e r e f o r e , final reduction degree decreases w h e n residence time of pre-reduction stage increases. T h e dense m a g n e t i t e is reduced to w u s t i t e at a pre-reduction t e m p e r a t u r e of 625 "C. W u s t i t e shows a fine-pored s t r u c t u r e which increases specific sur face area of reaction product of pre-reduction stage. T h e porous w u s t i t e increases w h e n residence time increases. T h e r e f o r e , degrees of pre-reduction in crease as residence times increase [ F i g . 5 ( e ) and ( f ) ] . T h e n , reaction product after pre-reduction stage is used as reactant of final reduction stage. T h e porous w u s t i t e is reduced to metallic iron· ( F e ) on s t a n d a r d operating conditions of final reduction. Metal iron appears in whisker f o r m , bonds with each other and forms a dense hard shell of iron on the surface of w u s t i t e [ F i g . 5 ( g ) ] . T h e dense iron shell prevents reduction gas from entering particle interior. So final reduction gas only reacts with the external surface of particle after pre-reduction. T h e r e f o r e , degree of final reduction does not have great changes with residence time increase. According to Ref. [ 4 ] , the thickness of dense shell
Reduced iron ore after pre-reduction stage (a) , (b) , (c), (d) , (e), (f) at different residence time and after final reduction stage (g)
increases with increasing residence time and block the reaction when pre-reduction temperature is at 450 °C. It is different from the results of this experi ment. The difference between reference and this ex periment is the quantity of Fe 3 0 4 produced by reac tion under different experimental conditions. In this condition that experimental conditions are high pres sure, Newman hematite ore and 10 min of residence time, Fe 3 0 4 produced by reaction appears on iron ores and the quantity of Fe 3 0 4 is too much. Fe 3 0 4 on surface of iron ores bonds with each other and forms a dense shell. It blocks the reaction. But in this part of experiment (normal pressure, Brazilian iron ores and residence time of 10 min) , Fe 3 0 4 produced by re action is little and does not bond with each other. It increases the specific surface area of iron ores. There fore, reaction rate of final reduction stage increases with increasing residence time of pre-reduction stage. It helps final reduction. But if residence time of pre-reduc tion stage is long enough, Fe 3 0 4 on surface of iron ores will bond with each other and form a hard shell. Ac cording to the analyses above, the condition of forming a hard shell is Fe3 0 4 quantity produced by reaction on surface of iron ores. When temperature of pre-reduction stage is at 550 °C , reaction rate increases with increasing reacTable 2 Reduction temperature/'C Reduction time/min Reduction degree/%
tion temperature. Fe 3 0 4 quantity produced by reac tion can bond with each other and form a hard shell. Therefore, the experiment is not like Ref. [ 6 ] that residence time of pre-reduction stage on final reduc tion degree is only one effect. But the influence changes when the temperature of pre-reduction stage changes and also changes when residence time changes. 2. 3
Comparison of reduction degree The traditional experiment uses similarly exper imental condition shown in Table 2 , but only the residence time of experiment is 30 min (The pre-re duction time plus final reduction time of two-step experiment equals the residence time of traditional experiment). The temperature of traditional experi ment is in the range of 400 to 800 °C and the reduc tion degree increases as the temperature increasing. When the temperature is equal to 800 °C , the reduc tion degree reaches its maximum value and equals 40%. Hence, it compares the maximum value with reduction degree of the two-step experiment, and the result is shown in Table 2. The maximum value of reduction degree of the two-step experiment in creased 23. 5% comparing with traditional experiment. Therefore, the two-step experiment is of great benefit
Degree of two-step reduction versus degree of traditional reduction Two-step experiment
Traditional experiment
450 500 550 600 650 700 10 (Pre-reduction time)+20 (Final reduction time) 40.9 56.1 63.5 52.7 40.2 45.6 45.8
800 30 40
400
to reduction degree. Then the pre-reduction time of the two-step ex periment was increased to 30 min ( The reduction time of the two-step experiment is equal to 50 min) , and reduction degree increased to 84. 1 % when the temperature of pre-reduction is 450 'C. But the resi dence time of traditional reduction similarly in-' creased to 50 min, the reduction degree did not have great change. The reduction degree of the two-step experiment was increased by 44. 1 % comparing with traditional reduction and the result is shown in Fig. 3. Therefore, the two-step experiment is of great benefit to increasing the reduction degree.
3
Vol. 20
Journal of Iron and Steel Research, International
• 12 ·
Conclusions
1) Pre-reduction has a significant influence on their final reduction degree. The iron ore particles seem to memorize their corresponding pre-treating process.
2) Pre-reduction time and pre-reduction tempera ture have the same influence on the degree of final re duction when pre-reduction stage is respectively in temperature area (400 to 500 °C , 500 to 600 "C , 600 to 625 "C and greater than 625 ' O . 3) Specific surface area is the pnly factor influ encing final reduction degree. Final reduction degree increases as specific surface area increases when prereduction temperature is below 625 °C. But while prereduction temperature is above 625 °C, final reduc tion degree does not have great changes as specific surface area increases. In this time, in order to in crease final reduction degree, whisker appearing on surface of wustite in final reduction stage is the most considering things. 4) Dense magnetite does not always prevent the improvement of final reduction degree and it is also possible to help final reduction degree increasing.
Issue 2
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
When dense magnetite appears on surface of fine ores in the form of small particles, it is good for in creasing specific surface area of products resulting from pre-reduction reaction and increasing final re duction degree too. When dense magnetite produced by pre-reduction reaction bonds with each other, forms a dense hard shell of magnetite and prevents reduction gas from continuing to react with reactant, it decreases final reduction degree. 5 ) Pre-reduction conditions have a significant effect on reduced iron ore texture. Dense textures with a low porosity formed in the pre-reduction process have a good or bad influence on the final re duction degree. Such textures are formed after low prereduction temperature. Porous textures were formed at pre-reduction temperature greater than 600 °C. The specific surface area of iron ore increased at higher pre-reduction temperature. 6) The two-step reduction greatly increases re
• 13 ·
duction degree, and the reduction degree can reach 84. 1% when pre-reduction time and temperature is respectively 30 min and 450 °C. References: [1] [2]
[3]
[4]
[5] [6]
Kumaran V. Kinetic Theory for a Vibro-Fluidized Bed [J]. Rev Metall Cah Inf Tech, 1998, 364 : 163. Christoph Feilmayr, Albin Thumhofer, Franz Winter, et al. Reduction Behavior of Hematite to Magnetite Under Fluidized Bed Conditions [ J ] . ISIJ Int, 2004, 44(7): 1125. Habermann A, Winter F, Hofbauer H, et al. An Experimental Study on the Kinetics of Fluidized Bed Iron Ore Reduction [ J ] . ISIJ Int, 2000, 40(10): 935. Thumhofer A, Schachinger M, Winter F, et al. Iron Ore Re duction in a Laboratory-Scale Fluidized Bed Reactor-Effect of Pre-Reduction on Final Reduction Degree [ J ] . ISIJ Int, 2005, 45(2): 151. Gmbh V S. Analytical Instruction for Internal Use [M]. Linz: [s. n. ] , 2003. Eder N, Pawlik C, Schuster S, et al. Memory-Effekte Von Fei neisenerzen Bei Hohen Drucken und Temperaturen [M]. Wien: [s. n. ] , 2004 (in German).
*%# ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2013, 20(2). 08-13
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions LIN Yin-he,
GUO Zhan-cheng,
TANG Hui-qing
(State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China) Abstract: To process optimization and improve the degree of reduction, a two-step experiment was designed. The ex periment was carried out in the micro-fluidized bed. The reactor in the micro-fluidized bed is operated as a differential reactor to ensure an equal temperature and residence time with the reactor volume. The experiment used Brazilian iron ore and reducing gas of CO. T h e operating temperature was 400 to 800 'C and the residence time was between 10 and 60 min. In correspondence with experiment, microscopic technique was applied too. T h e test shows that temperature and residence time of the pre-reduction stage have an important effect on the degree of reduction. By using two-step experiment, the maximum value of reduction degree increases by 44. 1% compared with the maximum value of tradi tional reduction experiment. Microscopic analysis shows that the specific surface area, surface morphology and tex ture of reduced iron ore after pre-reduction stage have an important effect on the degree of final reduction too. Key words : iron ore reduction; two-step experiment; specific surface area; morphology; pre-reduction degree; final reduction degree
In order to compensate the decrease of high quality steel scrap, direct reduced iron becomes an important industrial material of production in steel plants. DRI (direct reduced iron) is produced in indus trial fluidized bed (FINMET, FIOR and FINEX) C l ] . The industrial fluidized bed is one-step technology of iron-making. It requires relatively high initial condi tions of iron ore (surface morphology, porosity). The reduction degree is affected by initial conditions of iron ore too much. The reduction degree is gener ally low. The experiment was considered to change initial conditions of iron ore and get higher reduction degree. Therefore, the two-step experiment was de signed. The first step was pre-reduction stage. The iron ore in this stage got all kinds of initial condi tions. The second step was final reduction stage. The products resulting from pre-reduction stage were used as reactant of final reduction stage. It was used to study the influence of the changes of pre-re duction conditions on final reduction degree.
1
Experimental
1.1
Experimental setup To investigate the influence of the temperature
and residence time of pre-reduction on their final re duction degree under atmospheric pressure, a microfluidized bed reactor was used. The operating condi tions of the micro-fluidized bed reactor are as follows: 1) Working pressure is in atmosphere pressure. 2) Operating temperature is 400 to 800 "C. 3) Reducing gas is pure CO. 4) Operating time is from 10 to 60 min. The micro-fluidized bed is a differentially oper ating reactor. It can promise that each particle of the sample in reactor is in the same process conditions at any time during reduction'· 2-4 -'. The basic principle of the experimental setup is shown in Fig. 1. Fluidizing agent and iron ore were respectively put in reactor and sample injector. The inert gas flowing through the reactor prevented iron ore from reacting with other materials in air during the pre heating phase. After reaching the desired process temperature, the inert gas was switched to reducing gas. When concentration of CO was kept steady, iron ore was shot into reactor by sample injector and reac ted with CO gas. After the demanded reduction time, the reducing gas was switched "back to inert gas for preventing from reoxidation. When temperature of re-
Foundation Item:Item Sponsored by National Natural Science Foundation of China (50834007) Biography:LIN Yin-he(1980—), Male, Doctor! E-mail: [email protected]; Received Date: August 12, 2011 Corresponding Author : GUO Zhan-cheng(1963 — ) , Male, Doctor, Professor; E-mail: [email protected]
Issue 2
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
Gas filter/ condenser r "
i
| I'
Atmosphere
L
Weedle vavle\ I |
Thermocouple Fluidizing agerrt quartz sand | Furnace electric I resistance ' Quartz-*^' _ 3 J tubular reactor
fl Process
A
But t h e experiment used a differential reactor. It can ensure t h e t e m p e r a t u r e and concentration being a constant. A n d another aspect is the experimental conditions. T h e former experiment was under high p r e s s u r e condition and did not study the normal p r e s s u r e condition. T h e r e f o r e , t h e experiment does some exploratory research. 1. 3
Determination of degree of reduction T h e pre-reduction degree (R) is calculated by Eqn. ( 1 ) :
Fig. 1 Basic principle of experimental setup
R= Γ %-
actor had been cooled d o w n to room t e m p e r a t u r e , the reduced iron ore samples were removed from t h e reactor and a part of t h e m was analyzed by a titriometric m e t h o d ( i r o n chloride m e t h o d ) for overall Fe, ot > Fe° and F e 2 + [ 5 _ 6 ] . T h i s step above was pre-reduction stage. T h e rest of t h e reduced iron ore was used to re peat t h e above steps on conditions of final reduction. T h a t was t h e final reduction stage. T o get knowledge about s t r u c t u r a l c h a n g e s , microscopic morphology of raw iron ore particles as well as t h a t of reduced iron ore particles w a s i n v e s t i g a t e d in reflected light on S E M of E V O I 8 Special Edition in Carl Zeiss N T S . 1. 2
Reduction tests T o investigate t h e influence of pre-reduction on final reduction d e g r e e , two-step experiment w a s car ried out. T h e reduction t e s t s w e r e carried out w i t h Brazilian iron ores ( T a b l e 1) at different t e m p e r a t u r e s and residence times. S t a n d a r d operating condi tions were defined in which t e m p e r a t u r e s and resi dence times of t h e pre-reduction stage were varied. A f t e r w a r d s t h e samples w e r e reduced under final r e duction conditions in t h e final reduction stage. T h e t e m p e r a t u r e , m a s s flow, reaction times and varia tion of oxygen in iron ores w e r e recorded. T h e t w o - s t e p experiment is different from t h e formers. T h e most commonly used equipments of the formers w e r e large-scale fluidized bed. T h e s e in s t r u m e n t s w e r e integral reactor. Various parts of the reactor had different t e m p e r a t u r e s and concentrations. Table 1 Chemical analysis of Brazilian iron ores (mass percent, %) Fe,ot Fe 2 0 3 FeO 65
92.85
0
• 9 ·
Si0 2
CaO
MgO
AI2O3
3.7
0.07
0.11
2.35
S
Ignorance 0.92
(1)
Jo U |
w h e r e , t is reaction t i m e ; O, is total m a s s of oxygen in iron o r e ; and O m is variation of oxygen in iron ore per second. T h e n t h e p r o d u c t s resulting from pre-reduction reaction w e r e used as reactant in final reduction stage. So the final reduction degree (jRf) is calculat ed by Eqn. ( 2 ) :
M: a
(2)
0 Of
w h e r e , Of is total m a s s of oxygen in product resul ting from pre-reduction s t a g e ; and O n is variation of oxygen in reactant per second.
2
Results and Discussion
T h e pre-reduction is carried out at 400 to 700 - C , and time is 10 min. Final reduction is performed at 800 °C , and time is 20 min. S t a r t i n g from standard conditions, t e m p e r a t u r e and residence time of t h e pre-reduction stage were varied. Final reduction conditions were the same for all experiments. 2.1
Variation of temperature in pre-reduction stage T h e curve of the final reduction degree versus the t e m p e r a t u r e of pre-reduction is s h o w n in Fig. 2. 0.65
/
0.60
/~"\
0.55
\ \
0.50
\
0.45 0.40
\
\
■
400 Fig. 2
500
T/V
600
Degree of final reduction versus pre-reduction temperature
700
Journal of Iron and Steel Research, International
• 10 ·
The final reduction degree increases with increasing pre-reduction temperature up to 500 "C. Hereafter, final reduction degree at higher t e m p e r a t u r e decreases up to a pre-reduction t e m p e r a t u r e of 600 "C. W h e n prereduction t e m p e r a t u r e is 600 to 625 -C , final reduc tion degree increases again w i t h a further increase of the pre-reduction t e m p e r a t u r e . But w h e n further in creasing the t e m p e r a t u r e of pre-reduction reaction, degree of final reduction does not have great changes. T o find out the reasons of this p h e n o m e n o n and to improve the understanding for different reduction behaviors depending on pre-reduction t e m p e r a t u r e , microscopic morphologies of original ore as well as the partly-reduced material were investigated. W h e n t h e t e m p e r a t u r e of pre-reduction reaction is less t h a n 500 °C , interfacial reaction rate is low and little quantity of m a g n e t i t e is generated in prereduction stage. T h e magnetite resulting from prereduction reaction appears on t h e surface of ore grains with small particles [ F i g . 3 ( a ) ] . It increases specific surface area of reactant in final reduction re action. T h e quantity of m a g n e t i t e increases as prereduction t e m p e r a t u r e increases. It m e a n s t h a t spe cific surface area of reactant in final reduction reac tion increases with increasing pre-reduction tempera ture. T h e r e f o r e , degree of final reduction increases as pre-reduction t e m p e r a t u r e increases. W h e n the t e m p e r a t u r e of pre-reduction reaction is 500 to 600 °C , rate of interfacial reaction rises. Particles of magnetite produced by pre-reduction r e action bond with each o t h e r and form a dense hard shell of F e 3 0 4 on surface of fine ore [ F i g . 3 ( b ) ] . It prevents reduction gas from continuing to react with fine ore. T h e proportion of dense hard shell on sur
Fig. 3
Vol. 20
face of fine ore further increases as pre-reduction t e m p e r a t u r e increases. T h e r e f o r e , degree of final reduction decreases w h e n pre-reduction t e m p e r a t u r e increases. T h e dense magnetite is reduced to porous w u s tite w h e n t e m p e r a t u r e of pre-reduction stage is 600 to 625 °C [ F i g . 3 ( c ) ] . T h e proportion of porous w u s t i t e on surface of fine ore increases as pre-reduc tion t e m p e r a t u r e increases and specific surface area of products resulting from pre-reduction reaction al so increases. P r o d u c t s resulting from pre-reduction reaction are used as reactant in final reduction stage. Pre-reduction t e m p e r a t u r e increases specific surface area of reactant in final reduction. T h e r e f o r e , de gree of final reduction increases as pre-reduction t e m p e r a t u r e increases. When pre-reduction temperature is above 625 °C, almost all magnetite on surface of fine ore is transformed to w u s t i t e [ F i g . 3 ( d ) and ( e ) ] . P r o d u c t s resulting from pre-reduction stage are used as reactant in final reduction stage. W h e n reactant produced by pre-re duction stage is in final reduction conditions, w u s tite is reduced to metallic iron. T h e iron appears on surface of w u s t i t e w i t h form of w h i s k e r , bonds with each o t h e r and forms a dense metal shell of iron [Fig. 3 ( f ) ] , which prevents reduction reaction continuing to happen. Therefore, degree of final reduction does not have great change as pre-reduction t e m p e r a t u r e in creases. According to the analyses above, the materials with different m i c r o s t r u c t u r e s w e r e formed in different pre-reduction conditions. These materials were used for reactant in final reduction stage. It has a significant influence on their final reduction degree. The materials
Reduced iron ores after pre-reduction stage (a) , ( b ) , ( c ) , (d) and after final-reduction stage (e) , (f)
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
Issue 2
with different m i c r o s t r u c t u r e s can memorize their different corresponding pre-treating p r o c e s s M . 2. 2
Variation of residence time Starting from the standard operating condi t i o n s , the residence time in t h e pre-reduction stage was varied from 10 to 60 min. F r o m Fig. 2 of final reduction degree versus prereduction temperature, the pre-reduction experiments were respectively carried out in the t e m p e r a t u r e range of 400 to 500 °C , 500 to 600 "C and 600 to 700 *C to investigate influence of the pre-reduction time on t h e degree of final reduction. T h e points of 4 5 0 , 550 and 625 °C were selected as t e m p e r a t u r e points of residence time e x p e r i m e n t s in t h e pre-reduction stage. T h e results are s h o w n in Fig. 4. Increasing residence time in the pre-reduction stage causes an increase of the final reduction degree at 450 -C ( F i g . 4 ) . T h e effect can be explained by the magnetite formed in the pre-reduction stage. In this pre-reduction s t a g e , the magnetite resulting from pre-reduction reaction appears on the surface of ore grains with small particles [ F i g . 5 ( a ) and ( b ) ] . T h e magnetite particles become more and more as pre-re duction time increased. It increases t h e specific sur face area of iron ore after pre-reduction stage too. The 0.85
f
0.75 0.65 *0.55 0.45
■
^ ^
■625 t ; • 550 "C A 450 Έ
/
X
_
0.35 0.25
10
20
30
40
50
60
i/min Fig. 4
Final reduction degree versus pre-reduction time
Fig. 5
• 11 ·
iron ore produced by pre-reduction reaction is used as reactant of final reduction reaction and it contrib utes to an increase in specific surface area of reactant in final reduction stage. Therefore, final reduction de grees increase as pre-reduction time increases. Grains of m a g n e t i t e produced by pre-reduction stage further increase as residence time increase at the pre-reduction t e m p e r a t u r e of 550 °C. Magnetite particles resulting from pre-reduction stage bond w i t h each o t h e r and form a dense hard shell on the surface of iron ore. T h e a m o u n t s of dense magnetite layer further increase w h e n residence time increases [ F i g . 5 (c) and ( d ) ] . T h e layers of dense magnetite prevent reduction gas in t h e final reduction stage from diffusing into particle interior of reactant. T h e r e f o r e , final reduction degree decreases w h e n residence time of pre-reduction stage increases. T h e dense m a g n e t i t e is reduced to w u s t i t e at a pre-reduction t e m p e r a t u r e of 625 "C. W u s t i t e shows a fine-pored s t r u c t u r e which increases specific sur face area of reaction product of pre-reduction stage. T h e porous w u s t i t e increases w h e n residence time increases. T h e r e f o r e , degrees of pre-reduction in crease as residence times increase [ F i g . 5 ( e ) and ( f ) ] . T h e n , reaction product after pre-reduction stage is used as reactant of final reduction stage. T h e porous w u s t i t e is reduced to metallic iron· ( F e ) on s t a n d a r d operating conditions of final reduction. Metal iron appears in whisker f o r m , bonds with each other and forms a dense hard shell of iron on the surface of w u s t i t e [ F i g . 5 ( g ) ] . T h e dense iron shell prevents reduction gas from entering particle interior. So final reduction gas only reacts with the external surface of particle after pre-reduction. T h e r e f o r e , degree of final reduction does not have great changes with residence time increase. According to Ref. [ 4 ] , the thickness of dense shell
Reduced iron ore after pre-reduction stage (a) , (b) , (c), (d) , (e), (f) at different residence time and after final reduction stage (g)
increases with increasing residence time and block the reaction when pre-reduction temperature is at 450 °C. It is different from the results of this experi ment. The difference between reference and this ex periment is the quantity of Fe 3 0 4 produced by reac tion under different experimental conditions. In this condition that experimental conditions are high pres sure, Newman hematite ore and 10 min of residence time, Fe 3 0 4 produced by reaction appears on iron ores and the quantity of Fe 3 0 4 is too much. Fe 3 0 4 on surface of iron ores bonds with each other and forms a dense shell. It blocks the reaction. But in this part of experiment (normal pressure, Brazilian iron ores and residence time of 10 min) , Fe 3 0 4 produced by re action is little and does not bond with each other. It increases the specific surface area of iron ores. There fore, reaction rate of final reduction stage increases with increasing residence time of pre-reduction stage. It helps final reduction. But if residence time of pre-reduc tion stage is long enough, Fe 3 0 4 on surface of iron ores will bond with each other and form a hard shell. Ac cording to the analyses above, the condition of forming a hard shell is Fe3 0 4 quantity produced by reaction on surface of iron ores. When temperature of pre-reduction stage is at 550 °C , reaction rate increases with increasing reacTable 2 Reduction temperature/'C Reduction time/min Reduction degree/%
tion temperature. Fe 3 0 4 quantity produced by reac tion can bond with each other and form a hard shell. Therefore, the experiment is not like Ref. [ 6 ] that residence time of pre-reduction stage on final reduc tion degree is only one effect. But the influence changes when the temperature of pre-reduction stage changes and also changes when residence time changes. 2. 3
Comparison of reduction degree The traditional experiment uses similarly exper imental condition shown in Table 2 , but only the residence time of experiment is 30 min (The pre-re duction time plus final reduction time of two-step experiment equals the residence time of traditional experiment). The temperature of traditional experi ment is in the range of 400 to 800 °C and the reduc tion degree increases as the temperature increasing. When the temperature is equal to 800 °C , the reduc tion degree reaches its maximum value and equals 40%. Hence, it compares the maximum value with reduction degree of the two-step experiment, and the result is shown in Table 2. The maximum value of reduction degree of the two-step experiment in creased 23. 5% comparing with traditional experiment. Therefore, the two-step experiment is of great benefit
Degree of two-step reduction versus degree of traditional reduction Two-step experiment
Traditional experiment
450 500 550 600 650 700 10 (Pre-reduction time)+20 (Final reduction time) 40.9 56.1 63.5 52.7 40.2 45.6 45.8
800 30 40
400
to reduction degree. Then the pre-reduction time of the two-step ex periment was increased to 30 min ( The reduction time of the two-step experiment is equal to 50 min) , and reduction degree increased to 84. 1 % when the temperature of pre-reduction is 450 'C. But the resi dence time of traditional reduction similarly in-' creased to 50 min, the reduction degree did not have great change. The reduction degree of the two-step experiment was increased by 44. 1 % comparing with traditional reduction and the result is shown in Fig. 3. Therefore, the two-step experiment is of great benefit to increasing the reduction degree.
3
Vol. 20
Journal of Iron and Steel Research, International
• 12 ·
Conclusions
1) Pre-reduction has a significant influence on their final reduction degree. The iron ore particles seem to memorize their corresponding pre-treating process.
2) Pre-reduction time and pre-reduction tempera ture have the same influence on the degree of final re duction when pre-reduction stage is respectively in temperature area (400 to 500 °C , 500 to 600 "C , 600 to 625 "C and greater than 625 ' O . 3) Specific surface area is the pnly factor influ encing final reduction degree. Final reduction degree increases as specific surface area increases when prereduction temperature is below 625 °C. But while prereduction temperature is above 625 °C, final reduc tion degree does not have great changes as specific surface area increases. In this time, in order to in crease final reduction degree, whisker appearing on surface of wustite in final reduction stage is the most considering things. 4) Dense magnetite does not always prevent the improvement of final reduction degree and it is also possible to help final reduction degree increasing.
Issue 2
Reduction Behavior With CO Under Micro-Fluidized Bed Conditions
When dense magnetite appears on surface of fine ores in the form of small particles, it is good for in creasing specific surface area of products resulting from pre-reduction reaction and increasing final re duction degree too. When dense magnetite produced by pre-reduction reaction bonds with each other, forms a dense hard shell of magnetite and prevents reduction gas from continuing to react with reactant, it decreases final reduction degree. 5 ) Pre-reduction conditions have a significant effect on reduced iron ore texture. Dense textures with a low porosity formed in the pre-reduction process have a good or bad influence on the final re duction degree. Such textures are formed after low prereduction temperature. Porous textures were formed at pre-reduction temperature greater than 600 °C. The specific surface area of iron ore increased at higher pre-reduction temperature. 6) The two-step reduction greatly increases re
• 13 ·
duction degree, and the reduction degree can reach 84. 1% when pre-reduction time and temperature is respectively 30 min and 450 °C. References: [1] [2]
[3]
[4]
[5] [6]
Kumaran V. Kinetic Theory for a Vibro-Fluidized Bed [J]. Rev Metall Cah Inf Tech, 1998, 364 : 163. Christoph Feilmayr, Albin Thumhofer, Franz Winter, et al. Reduction Behavior of Hematite to Magnetite Under Fluidized Bed Conditions [ J ] . ISIJ Int, 2004, 44(7): 1125. Habermann A, Winter F, Hofbauer H, et al. An Experimental Study on the Kinetics of Fluidized Bed Iron Ore Reduction [ J ] . ISIJ Int, 2000, 40(10): 935. Thumhofer A, Schachinger M, Winter F, et al. Iron Ore Re duction in a Laboratory-Scale Fluidized Bed Reactor-Effect of Pre-Reduction on Final Reduction Degree [ J ] . ISIJ Int, 2005, 45(2): 151. Gmbh V S. Analytical Instruction for Internal Use [M]. Linz: [s. n. ] , 2003. Eder N, Pawlik C, Schuster S, et al. Memory-Effekte Von Fei neisenerzen Bei Hohen Drucken und Temperaturen [M]. Wien: [s. n. ] , 2004 (in German).