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Timo Paananen, M.Sc.Tech Laboratory of Process Metallurgy Department of Process and Environmental Engineering University

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Timo Paananen, M.Sc.Tech Laboratory of Process Metallurgy Department of Process and Environmental Engineering University

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    1. Blast Furnace Seminar 30. – 31.3.2004 Timo Paananen, M.Sc.Tech Laboratory of Process Metallurgy Department of Process and Environmental Engineering University of Oulu EFFECT OF ALUMINA ON REDUCTION OF IRON OXIDES My field of research is burden materials of blast furnace. Actually I have studied and am still studying reduction of iron oxides and effects of impurities on reduction of iron oxides. My field of research is burden materials of blast furnace. Actually I have studied and am still studying reduction of iron oxides and effects of impurities on reduction of iron oxides.

    2. Blast Furnace Seminar 30. – 31.3.2004 Effect of alumina on reduction of iron oxides Introduction Solubility of alumina in iron oxides Experiments and results Analysis Conclusion This presentation consist of five topics. First I will present some previous studies from the same field. Before going forward to experiments and results I will show a couple of phase diagrams about solubility of alumina in iron oxides. After the results I would like to present scanning electron microscopy images and energy dispersive analysis. In the end of the presentation I have some conclusions about the subject.This presentation consist of five topics. First I will present some previous studies from the same field. Before going forward to experiments and results I will show a couple of phase diagrams about solubility of alumina in iron oxides. After the results I would like to present scanning electron microscopy images and energy dispersive analysis. In the end of the presentation I have some conclusions about the subject.

    3. Blast Furnace Seminar 30. – 31.3.2004 Previous studies of authors Dissolved alumina in iron oxide retards the reduction by forming a hercynite cover between the iron oxide and iron* Reduction is promoted by the formation of hercynite, because the reduction proceeds along the grain boundaries intersected by the hercynite.** The fundamendal and thoroughgoing study about effect of alumina on reduction of iron oxides was not presented very much. The most of researches are concerned with correlation between to some tests and content of alumina of burden material and something like that. Few of studies are going to make clear or explain the phenomena that is the reason for each process like disintegration, retardation or promoting of reduction. In the two doctoral thesis where are studied the field more thoroughly are presented contradictory results. An other one is made almost 30 years and the other one 15 years ago. The fundamendal and thoroughgoing study about effect of alumina on reduction of iron oxides was not presented very much. The most of researches are concerned with correlation between to some tests and content of alumina of burden material and something like that. Few of studies are going to make clear or explain the phenomena that is the reason for each process like disintegration, retardation or promoting of reduction. In the two doctoral thesis where are studied the field more thoroughly are presented contradictory results. An other one is made almost 30 years and the other one 15 years ago.

    4. Blast Furnace Seminar 30. – 31.3.2004 Disintegration during reduction Measured for example using RDI-test Deleterious effect of alumina on RDI Correlation between the concentration of dissolved alumina and degree of disintegration of iron oxides has been shown in many papers and this is associated with* hematite-magnetite phase transformation effect of alumina on amount of calciumferrite The effect of alumina on decreasing of burden material strenght has been reported in many studies. Correlation between reduction disintegration index and alumina is reported to be clear.The effect of alumina on decreasing of burden material strenght has been reported in many studies. Correlation between reduction disintegration index and alumina is reported to be clear.

    5. Blast Furnace Seminar 30. – 31.3.2004 Solubility of alumina into iron oxides Solubility of alumina in magnetite is above 950 oC high because magnetite and hercynite has complete solid solution above 870 oC. That means as content of alumina cations in spinel structure about 30 weight percent.Solubility of alumina in magnetite is above 950 oC high because magnetite and hercynite has complete solid solution above 870 oC. That means as content of alumina cations in spinel structure about 30 weight percent.

    6. Blast Furnace Seminar 30. – 31.3.2004 Ternary phase diagram for system Fe-Al-O Solubility of alumina in wustite at 1000 oC is only about 0.5 weight percent. When alumina content becomes higher and over the solubility limit a new phase must be formed. This new phase is hercynite.Solubility of alumina in wustite at 1000 oC is only about 0.5 weight percent. When alumina content becomes higher and over the solubility limit a new phase must be formed. This new phase is hercynite.

    7. Blast Furnace Seminar 30. – 31.3.2004 Preparation of samples before reduction Magnetite concentrate (96.1 wt. % Fe3O4) Alumina additive was corumdum with a purity of 99.99% Dissolution temperature (at 900 oC for 2h) heated three times ”in magnetite conditions” quenched to water not heated after the last grinding/pressing process Dissolution/sintering temperature (at 1395 oC for 2h) quenched to water Primary test material was magnetite concentrate, whose iron oxide content was about 98 percent including magnetite and ferrioxide. The concentrate was ground and sieved under 200 mesh grain size. The mixed material, including magnetite and right amount of alumina, was pressed briquettes with purified ethanol. After that the samples were prepated two different ways. In the first case samples were heated 3 times at 900 oC for 2 hours. In order to achieve porous structure for more extensive surface area of reaction between solid and gas phases the samples were not heated after third grinding and pressing process. In the other case the heating temperature was almost 14 hundred degrees in order to achieve denser and sintered structure. Primary test material was magnetite concentrate, whose iron oxide content was about 98 percent including magnetite and ferrioxide. The concentrate was ground and sieved under 200 mesh grain size. The mixed material, including magnetite and right amount of alumina, was pressed briquettes with purified ethanol. After that the samples were prepated two different ways. In the first case samples were heated 3 times at 900 oC for 2 hours. In order to achieve porous structure for more extensive surface area of reaction between solid and gas phases the samples were not heated after third grinding and pressing process. In the other case the heating temperature was almost 14 hundred degrees in order to achieve denser and sintered structure.

    8. Blast Furnace Seminar 30. – 31.3.2004 Experiments TGA Sample was held in platinum basket Briquette warmed up to 950 oC in argon Reduced by CO/CO2 -gas (90/10) The experiments were executed thermogravimetric analysator and briquette was held in platinum basket hanging on a gravimeter. Briquette was warmed up to 950 oC in argon atmosphere and reduction was made using CO/CO2-atmosphere where 90 % was CO and the end CO2 The experiments were executed thermogravimetric analysator and briquette was held in platinum basket hanging on a gravimeter. Briquette was warmed up to 950 oC in argon atmosphere and reduction was made using CO/CO2-atmosphere where 90 % was CO and the end CO2

    9. Blast Furnace Seminar 30. – 31.3.2004 Here can be seen the reduction curves of porous samples as a function of time for different alumina contents. It is not easy to observe differences from these curves but it will be easier to see differences in the next image.Here can be seen the reduction curves of porous samples as a function of time for different alumina contents. It is not easy to observe differences from these curves but it will be easier to see differences in the next image.

    10. Blast Furnace Seminar 30. – 31.3.2004 In this image the x-axis is reduction curve for pure magnetite sample and the others are difference from that a funciton of time. In addition those pictures from a to d are briquettes after reduction. There can be seen in pictures that the higher is alumina content the more is disintegration of samples.In this image the x-axis is reduction curve for pure magnetite sample and the others are difference from that a funciton of time. In addition those pictures from a to d are briquettes after reduction. There can be seen in pictures that the higher is alumina content the more is disintegration of samples.

    11. Blast Furnace Seminar 30. – 31.3.2004 SEM image of reduced samples (3000x magnification) The mechanism of disintegration was studied using Scanning electron microscopy but analysing was difficult. The diameter of analyse spot using energy dispersive spectrometer is near 1 micrometer or over and these small phases middle of the white iron phase are a lot of smaller. That is the reason that was prepared sintered briquettes. The aim was to get same phenomena bigger and easier to study.The mechanism of disintegration was studied using Scanning electron microscopy but analysing was difficult. The diameter of analyse spot using energy dispersive spectrometer is near 1 micrometer or over and these small phases middle of the white iron phase are a lot of smaller. That is the reason that was prepared sintered briquettes. The aim was to get same phenomena bigger and easier to study.

    12. Blast Furnace Seminar 30. – 31.3.2004 Reduction curves of sintered (1395 oC) samples These are reduction curves of doped and un-doped samples, where aluminium is assumed to bound with FeO as hercynite (FeAl2O3) These are reduction curves of doped and un-doped samples, where aluminium is assumed to bound with FeO as hercynite (FeAl2O3)

    13. Blast Furnace Seminar 30. – 31.3.2004 Reduction difference of sintered samples Al2O3-addition = [0], [3.78] wt.%, T = 950 oC This image is made same way than earlier. The x-axis is reduction curve of un-doped sample and gray and black ones are difference from that. Black curve was calculated as assumption that alumina was formed hercynite and did not reduce on prevailing conditions.This image is made same way than earlier. The x-axis is reduction curve of un-doped sample and gray and black ones are difference from that. Black curve was calculated as assumption that alumina was formed hercynite and did not reduce on prevailing conditions.

    14. Blast Furnace Seminar 30. – 31.3.2004 SEM image of reduced samples using 500x magnification SEM-images using 500 times magnification did not reveal anything differences between doped and un-doped reduced samples. SEM-images using 500 times magnification did not reveal anything differences between doped and un-doped reduced samples.

    15. Blast Furnace Seminar 30. – 31.3.2004 SEM image of reduced samples using 3000x magnification On magnification 3000, in doped structure can be seen a lot of small inclusions in iron.On magnification 3000, in doped structure can be seen a lot of small inclusions in iron.

    16. Blast Furnace Seminar 30. – 31.3.2004 SEM image of reduced samples using 6000x magnification Those small phases are able to analyse atleast almost on magnification 6000. The inclusions seemed to be a quite near from composition of hercynite. This not yet reveal the disintegration mechanism.Those small phases are able to analyse atleast almost on magnification 6000. The inclusions seemed to be a quite near from composition of hercynite. This not yet reveal the disintegration mechanism.

    17. Blast Furnace Seminar 30. – 31.3.2004 Experiments interrupted after 30 minute reduction In order to be able to see the front of reduction, interrupted experiments were executed. The samples were reduced for 30 minutes and then quenced to water.In order to be able to see the front of reduction, interrupted experiments were executed. The samples were reduced for 30 minutes and then quenced to water.

    18. Blast Furnace Seminar 30. – 31.3.2004 SEM images from reduced briquettes interrupted after 30 minutes reduction Now the difference between doped and undoped could be seen even on 250 times magnification. In near the surfaces doped sample was disintegrated and undoped not. In addition undoped sample does not appear iron phase but magnetite and wustite phases can be seen. In the doped sample can be observed a few iron phases. The gray area is texture of magnetite and wustite that can be seen on higher magnification. Undoped sample can still be seen wustite and magnetite phases similar to lower magnification with the exception of a fayalite stripe come from impurities of magnetite concentrate. In the doped sample can be observed darker relic phases both iron and wustite phases. When we take even a little bit higher magnification there can be seen relic phases and cracks in the same orientation.Now the difference between doped and undoped could be seen even on 250 times magnification. In near the surfaces doped sample was disintegrated and undoped not. In addition undoped sample does not appear iron phase but magnetite and wustite phases can be seen. In the doped sample can be observed a few iron phases. The gray area is texture of magnetite and wustite that can be seen on higher magnification. Undoped sample can still be seen wustite and magnetite phases similar to lower magnification with the exception of a fayalite stripe come from impurities of magnetite concentrate. In the doped sample can be observed darker relic phases both iron and wustite phases. When we take even a little bit higher magnification there can be seen relic phases and cracks in the same orientation.

    19. Blast Furnace Seminar 30. – 31.3.2004 SEM-EDS analysis from sample interrupted after 30 minutes redution Analysing was still difficult, but relic phases clear included high content of alumina. These analysis are semi-quantitative, because the particles are smaller than the analysis spot.Analysing was still difficult, but relic phases clear included high content of alumina. These analysis are semi-quantitative, because the particles are smaller than the analysis spot.

    20. Blast Furnace Seminar 30. – 31.3.2004 Reduction ”front” inside grains The aim was to achieve a topochemical and clear reduction front for making analysing and observe of phenomena easier. However, as can be seen in both images, even inside of grains the reduction seems to favour certain internal lattice planes of mangetite host. In the right image neighbour grains that are randomly oriented which can be seen from the different orientation of the wustite lamellae.The aim was to achieve a topochemical and clear reduction front for making analysing and observe of phenomena easier. However, as can be seen in both images, even inside of grains the reduction seems to favour certain internal lattice planes of mangetite host. In the right image neighbour grains that are randomly oriented which can be seen from the different orientation of the wustite lamellae.

    21. Blast Furnace Seminar 30. – 31.3.2004 Wustite and minerals of spinel group as magnetite and hercynite have cubic crystal structure. In the reduction process, crystallographic orientation of the host magnetite and wustite are detected to be epitaxial. Therefore it is possible to count theoretical volumetric change during reduction using lattice constants of any minerals. This image shows that the volumetric change from magnetite to wustite is 6 to 8 percent durin reduction. If alumina content is high and the reducing mineral is near composition of hercynite, the volumetric change is 16 to 18 percent. This causes very strong tensions on the interface of phases and which presumably cause the fracturing of the structure. Wustite and minerals of spinel group as magnetite and hercynite have cubic crystal structure. In the reduction process, crystallographic orientation of the host magnetite and wustite are detected to be epitaxial. Therefore it is possible to count theoretical volumetric change during reduction using lattice constants of any minerals. This image shows that the volumetric change from magnetite to wustite is 6 to 8 percent durin reduction. If alumina content is high and the reducing mineral is near composition of hercynite, the volumetric change is 16 to 18 percent. This causes very strong tensions on the interface of phases and which presumably cause the fracturing of the structure.

    22. Blast Furnace Seminar 30. – 31.3.2004 Stepwise reduction of alumina-bearing magnetite described with scanning electron microscope In these images stepwise reduction of alumina doped magnetite is represented. In three images on the left can be seen mangetite base material where amount of wustite phase is increasing. In the images on the right of slide can be seen iron phase on the surface of grain. The main phase in those images is wustite where is hercynite phases as inclusions. Those inclusions continue many places as cracks.In these images stepwise reduction of alumina doped magnetite is represented. In three images on the left can be seen mangetite base material where amount of wustite phase is increasing. In the images on the right of slide can be seen iron phase on the surface of grain. The main phase in those images is wustite where is hercynite phases as inclusions. Those inclusions continue many places as cracks.

    23. Blast Furnace Seminar 30. – 31.3.2004 Stepwise reduction of alumina-bearing magnetite to wüstite schematically described The same thing is described scematically on this slide. Briefly summary: in the first step, aluminum cation have to accumulate in magnetite and only part of dissolved aluminium can remain in wustite (0.5 wt.%). In the second step, amount of wustite increases and in the third step the content of aluminium in magnetite phase increases and the composition of the phase approaches that of hercynite.The same thing is described scematically on this slide. Briefly summary: in the first step, aluminum cation have to accumulate in magnetite and only part of dissolved aluminium can remain in wustite (0.5 wt.%). In the second step, amount of wustite increases and in the third step the content of aluminium in magnetite phase increases and the composition of the phase approaches that of hercynite.

    24. Blast Furnace Seminar 30. – 31.3.2004 Conclusion Reduction was promoted by alumina Disintegration of iron oxide structure associated with magnetite-wüstite phase transformation Microscopic reduction front is not able to achieve but reduction goes along lattice planes Here are the conclusion of the study. Reduction seems to be promoted by alumina at least in the beginning of the reduction. This is probably result of increased surface area between solid and gas phases that is correspondingly result of disintegration of structure. Disintegration of iron oxide structure seems to be associated with phase transformation of magnetite to wustite. Microscopic reduction front is not able to achieve but reduction goes along lattice planesHere are the conclusion of the study. Reduction seems to be promoted by alumina at least in the beginning of the reduction. This is probably result of increased surface area between solid and gas phases that is correspondingly result of disintegration of structure. Disintegration of iron oxide structure seems to be associated with phase transformation of magnetite to wustite. Microscopic reduction front is not able to achieve but reduction goes along lattice planes

    25. Blast Furnace Seminar 30. – 31.3.2004 THANK YOU FOR ATTENTION

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