Structural Changes Occurring during Reduction of Hematite and Magnetite Pellets Containing Coal Char*

Transcription

1 Structural Changes Occurring during Reduction of Hematite and Magnetite Pellets Containing Coal Char* By Carlos E. SEA TON,** James S. FOSTER*** and Julio VELASCO**** Synopsis The structural changes that occurred during the reduction of hematite and magnetite pellets containing coal char, in the temperature range between 900 C and 1200 C, were studied. The behavior of the pellets was complex. Significant temperature gradients within the pellet were attained during the process, and the reduction was accompanied by catastrophic swelling of the pellets between 900 C and C, while shrinkage was observed at and 1200 C. Observed microstructural features such as intragranular porosity and cracking of oxide grains resulted from severe lattice disturbances occurring upon transformation of hematite to magnetite. The catastrophic swelling observed for hematite and magnetite pellets 900 and C was due to the filamentary or whisker growth of iron outward from the wustite surfaces. The whisker formation appears to be due to a changing reducing potential of the gas phase within the pellet coupled with the presence of calcium oxide on the wustite surface. The shrinkage reported at and 1200 C is accounted for in terms of sintering of iron filaments. The pellets strength was found to decrease between 900 C and C, and increased at higher temperatures. The weakness at low temperatures was due to the absence of bonding between iron filaments or whiskers. The high strength obtained at higher temperatures (1100 and 1200 C) was produced by sintering of iron filaments. I. Introduction The structural changes occurring during iron oxide reduction have been extensively investigated in recent years. In connection with direct reduction, the main reason for such interest is that the structure and properties developing during reduction will determine whether the material will withstand the pressures and other conditions within the reactor and how the reduced material will behave during transportation and handling. Failures by degradation or disintegration of iron oxide pellets during blast furnace operations have been reported,1-3> and they were associated with abnormal catastrophic swelling of the agglomerate during the reduction stages.4,5) The reduction of hematite to magnetite and wustite is usually accompanied by volume changes and pore formation. Brill-Edward, Daniell and Samuel,s~ studied the reduction of hematite pellets to magnetite and wustite between 400 C and C under H2/ H2O atmospheres. They found that the volume increases for the stages Fe203 -~ Fe304 and Fe203 -~ FeO were between 20 % and 27 %, and that the volume changes were due to the extensive intergranular and transgranular cracking which developed during the reduction of hematite to magnetite. Other investigations have confirmed their observations7'8) ; however, volume changes between 20 % and 27 % are considered normal and acceptable. The abnormal swelling of iron oxide pellets occurs during the conversion of wustite to iron. Watanabe and Yoshinaga4} investigated the gaseous (CO) reduction of hematite pellets. They found abnormal swelling of iron ore pellets and suggested that it was caused by fibrous growth of metallic iron from the wustite surface and was fundamentally related with the intergrowth or microtwin texture observed microscopically in hematite grains. Later Fuwa and Ban-ya9~ investigated the swelling of iron ore during reduction with CO/CO2 mixtures at temperatures between 600 C and 1200 C. They demonstrated that the abnormal or catastrophic swelling takes place during the reduction of wustite to iron and is due to the growth of iron in the form of fibers or whiskers from the wustite surface. These authors proposed a mechanism by which iron will nucleate at the wustite surface once the critical supersaturation of ferrous ions and electrons was reached. Once the nuclei are formed, the ferrous ions and electrons in the vicinity of the nuclei will diffuse toward them, attaching to the base at the wustite-iron interface, pushing the tip outward and causing the fibrous iron growth. They stated that the amount of fibrous iron would depend on the rate of chemical reaction at the surface of the grain, the diffusion coefficient of iron, and the diffusion path. Since the whiskers or fiber formation requires the nucleation and growth of iron outward from the wustite surface, most theories have associated this mode of reduction with a low density of nucleation sites on the wustite surface. Thus, nucleation was thought to be inhibited by: 1) impurites adsorbed on the wustite; 2) an unfavorable crystallographic structure produced during the reduction of higher oxides, or 3) an irregular or non-uniform reducing potential of the gas phase. The view that impurities caused whisker formation was suggested by Bleifuss.10~ He observed whisker formation during the carbon monoxide reduction of calciferrous magnetite pellets made by the reduction of hematite containing lime with CO-CO2 mixtures. Bleifuss attributed the whisker growth to the formation of a calcium-rich phase on the wustite surface * * * Received June 21, ISIJ Department of Metallurgy, Michigan Technological University, Houghton, Michigan 49931, U.S.A. On leave from Dept. de Ciencia de los Materiales, Universidad Simon Bolivar, Caracas 1080, Venezuela. Formerly Department of Metallurgy and Materials Science, Michigan Technological University. Now at Department of Metallurgy, University of Missouri at Rolla, Missouri 65401, U.S.A. Metallurgy Department at Venalum, Puerto Ordaz, Venezuela.

2 0 (498) Transactions ISIJ, Vol. 23, 1983 during the reduction of magnetite. Thus the calciumrich phase acted as a barrier to iron nucleation which was limited to a few calcium-free sites on the wustite. As reduction continued, iron had to migrate to these points either by volume or surface diffusion. Similar results were obtained by von Ende et al.11 In their opinion, the presence of calcium ions on the wustite surface leads to preferential nucleation and whiskerlike growth of iron that did not occur in the absence of calcium. The whiskers were not observed at high CO JCO2 ratios. Lu12~ studied the reduction of iron ore mixtures of hematite, magnetite and artificial wustite with carbon monoxide. Whisker formation was observed. He associated this phenomena with a reducing atmosphere that changes its composition from that in equilibrium with wustite to a higher reduction potential. This effect of the gaseous atmosphere in promoting preferential nucleation has also been observed by Wright13) during hydrogen reduction of hematite and by Rao14~ who observed preferential nucleation of iron at edges and grain boundaries during the reduction of wustite by carbon monoxide. In later work, Lu15~ stated that the conditions for fiber or whisker growth were that of the above plus the presence of impurities such as calcium oxide on the wustite surface. The CaO was believed to reduce the activity of wustite which in turn raised the COf CO2 ratio required for the reduction to take place at certain sites on the wustite surface. Most of the previous work deals with the structural changes which occur during gaseous reduction of iron ores. The changes that take place during iron oxide reduction with carbon have not been examined in detail. Such studies are of great importance since they should allow a better appraisal of the feasibility of processes such as the Dwight-Lloyd and McWaneis~ or the MTU cold bond17~ when coke or char is incorporated in the pellets during the agglomeration process. This work studies the structural changes occurring during the reduction of iron oxide-coal char pellets and relates them to the volume and strength changes. II. Experimental Procedure The chemical composition analysis of the hematite and magnetite concentrate is given in Table 1 along with the chemical composition of the bituminous coal char. The screen analysis of the coal char showed 100 % passing 325 mesh. The screen size ranges of the high purity lime and silica used were 100 % passing 200 mesh and 325 mesh, respectively. Both the hematite and magnetite concentrates were 80 % passing 325 mesh. The relative percentages of pellets constituents is shown in Table 2. Agglomeration of the hematite and magnetite-coal char pellets used in this research was done by using a low temperature process developed and patented by Goksel and Volin17~ at the Institute of Mineral Research at Michigan Technological University. The raw materials, mixed in the proportions indicated in Table 2, were pelletized in a laboratory scale disc pelletizer. The green pellets were then exposed to a high water vapor pressure, 200 psig, in an autoclave at 198 C for 2 hr. Hardening of the pellets is attributed to the formation of a bonding gel under the temperature-steam pressure conditions existing in the autoclave. A second batch of pellets was made with the lime addition being reduced from 7 to 2 % while keeping the hematite-coal char ratio constant. No silica was added. The original batch was made up to have a basicity ratio close to two for the slag which would result from melting the pellets in either iron or steelmaking. The second batch was made in order to determine the effect of the lime content on the reduction behavior of the pellets, and on the microstructural changes that accompanied the reduction. The reduction experiments were conducted in a constant temperature environment in a mullite reaction tube 40 inches long and 2 inches I.D., which was mounted horizontally in a tube furnace. The pellets were reacted under a flowing nitrogen atmosphere and were examined before and after exposure to determine changes of size and weight. The weight change under different temperature and time conditions were determined, and they were used to obtain the fraction of the reaction as a function of time. The change in size was used to determine the percent volume change by using the following relationship : f V x 100 Percent volume change = V V Table 1. Chemical compositions of iron ores and coal char used. Table 2. Pellet compositions.

3 Transactions ISIJ, Vol. 23, 1983 (499) where V f and Vo are the apparent pellet volume of the reduced and agglomerated pellets, respectively. The oxides present at different reduction stages were determined by X-ray diffraction analysis of powder samples using a Norelco diffractometer with chromium radiation. Fracture surfaces and polished sections of reduced pellets were observed using a JEOL scanning electron microscope (SEM), in order to follow the microscopic structural changes occurring during the course of reaction. The fracture surfaces were prepared by breaking the pellets in either a spring loaded Chatillon press or a Dillon hydraulic press for low or high compressive strengths, respectively. III. Result and Discussion 1. Volume Changes The volume changes that have been observed during the reduction of hematite and magnetite pellets are shown in Figs. 1 and 2 where the percentage of swelling or shrinkage as a function of time for all the tested temperatures is presented. Both figures indicate that hematite and magnetite pellets experienced extensive swelling when reduced in the temperature range between 900 C and C. The swelling percentage was greater for the hematite pellets and increased as the percentage of calcium oxide increased. For instance, Fig. 1 clearly shows that the swelling percent increased from 43 to 80 % at C and from 43 to 120 % at 900 C, when the calcium oxide content increased from 2 to 7 %. At temperatures between C and 1200 C, all the pellets swelled at first, but shrunk as reduction progressed. Table 3 shows the time required for the conversion of hematite or magnetite pellets to wustite. The values are approximations, since the reduction was not completely stepwise. However, they do demonstrate that both the abnormal swelling reported at lower temperatures and the early swelling observed at higher temperatures are related to the conversion of wustite to iron. Later, it will be shown that all the volume changes can be accounted for in terms of the fiber or whisker growth of iron. 2. Microstructural Changes The microstructural changes that accompanied the reduction were functions of furnace temperature, time of exposure and relative portion of the pellet studied. The pellet behavior is generally complex, with a substantial temperature gradient observed under all experimental conditions. Photographs 1 to 3 show examples of hematite and magnetite pellets microstructure when reduced between C and C. As can be seen, the iron nucleates on the wustite surface and grows outward from it in the form of fibers or whiskers. The wustite grain resulting from the reduction of hematite, Photo. 1(A) is highly porous and contains several voids and cracks while that of wustite produced from magnetite, Photo. 2(A) is non-porous and shows no evidence of cracks and voids. These surface differences had a direct effect I IIYI G, Fig. 2. Table Fig. 1. Percent volume changes as, time for hematite pellets. Reduced between 900 C and 1200 C. Percent volume changes as, time for magnetite pellets. Reduced between C and 1200 C. 3. Time required for conversion of hematite or magnetite pellets to wustite. on the resulting microstructure. The whiskers resulting from reduction of hematite tend to be long and thin; they bend as they get taller and weld at the tip to give the appearance shown in Photo. 1(B). Similar results were reported by Lu12~ during the study of gaseous reduction of iron oxide. Whiskers or fibers resulting from the reduction of magnetite,

4 (500) Transactions ISIT, Vol. 23, 1983 Photo. 1. Fractured section of hematite pellets. Reduced at C showing filamentary growth of iron, and cracks and voids in the unreduced wustite grain (A) 10 min of exposure (B) 20 min of exposure. Photo. 2(B), are shorter, have a wider base than those formed from hematite, and tend to spread over the wustite surface. These have a slight resemblance with the whiskers of conical shape recently reported by Nicole and Rist.18~ Thus, it appears that the large crack formation and higher porosity of the wustite grain resulting from hematite provided large surface areas which enhanced the reduction reaction and increased the number of sites where iron nucleation could be attained. Once the nuclei formed, the cracks and voids acted as barriers which favored the growth of tall and thin whiskers over the short, wide and conical shape like reported for the magnetite pellets. Photograph 3 also shows that within a single wustite grain there are areas of early and advanced stage of iron growth. For example, the white and small protuberances surrounded by shallow depressions observed in Photo. 3(A) are iron. The proximity between these protuberances suggest that the large iron fibers observed at the lateral sides of the same wustite grain may represent the growth of several nuclei that existed over a small region on the wustite surface. Photograph 3(B) indicates that such a difference in the stage of iron growth is related to proximity of the Photo. 2. Fractured section of magnetite pellets. Reduced at 1000 C (A) 60 min of exposure. Lower magnification indicating iron filaments, throughout the pellet, growing from wustite particles (B) 40 min of exposure. Higher magnification shows in more detail the iron filaments. wustite grain to the char particles. The whisker growth of iron during the reduction of hematite and magnetite pellets was also observed at temperatures between C and C. Photograph 4(A) shows how the iron in the surface region of hematite pellet continues to grow outward from the porous wustite surface. However, notice that the whiskers are not as long, and that the high temperature of reduction ultimately causes the sintering of the iron observed in Photo. 4(B). Similar results were obtained for the surface region of magnetite pellets. The behavior of the core or center region of the magnetite pellets was quite different. Photograph 5 illustrates this point rather well. As can be seen in Photo. 5(A), iron grows, spreading itself over the wustite surface in a manner that resembles a " dendritic like " structure. This phenomena is related to the reported temperature gradient between the surface and core of the pellet.19) Thus during early stages the conditions at the core were similar to those attained during reduction at 900 and C. This originated a few nuclei over the wustite surface, and they grew fast as temperature and hence the rate

5 Transactions 3. Photo. Fractured section of magnetite pellet. Reduced at C. (A) single wustite grain showing large filaments at lateral side and small white iron protuberances in central side (B) wustite grains with preferential growth of iron filaments near the char particles indicated by black arrows. of oxygen removal increased. Photograph 5(B) shows the core of the pellet when the temperature gradient is no longer present. Obviously, the iron growth rate is higher than that obtained at lower temperatures. Notice also that, as predicted by Fuwa and Ban-ya,9) the wustite surface recedes as the iron grows out. As time increases, the iron sinters and the final structure obtained is similar to that reported in Photo. 4(B). 3. Conditions Leading to Whisker Growth The iron nucleation over the wustite surface re- quires the attainment of a critical supersaturation in order to overcome the energy barrier associated with the new phase formation. The nucleation and growth of new solids phases during reduction of sulfides and oxides have been discussed by C. Wagner20) whose fundamental ideas have been applied by Fuwa and Ban-ya,9~ and Nicolle and Rist18~ to propose mechanisms that would explain the growth of iron in the form of whisker of fibers. Today, it is generally accepted that the growth of iron in the form of whiskers is related to changes in the reducing potential of the Photo. 4. ISIJ, Vol. 23, 1983 (501) Fractured section of hematite pellets. Reduced at and 1200 C (A) reduced at C for 10 min. Notice the beginning of sintering of early formed filaments. (B) reduced at 1200 C for 25 min. Notice advanced sintering of iron; there is no resemblance to the original microstructure. gas phase, and to the inhibition of nucleation at certain sites on the wustite surface. 1. Effect of the Gaseous Phase The solid reduction of iron oxides can be described by two concurrent heterogeneous reactions. They are: FeO+CO C02+C = Fe+CO2 = 2C0...(1)...(2) Reaction (1) represents the reduction of wustite to produce iron, while reaction (2) illustrates the generation, by the gasification of carbon, of the carbon monoxide needed for the reduction to continue. The kinetics results of this study19)have demonstrated that the rate of the reduction process is controlled by reaction (2). That is, the removal of oxygen from the wustite lattice takes place as fast as the carbon monoxide is supplied by the gasification of the char. The results have demonstrated that the fibrous growths of iron tend to be favored in the areas of wustite grains close to char particles. This fact, and the observed temperature gradient during the reduction

6 (502) Transactions ISIJ, Vol. 23, 1983 Fig. 3. Pellets crushin temperature. g strength as a function of reduction Photo. 5. Fracture section from the core of magnetite pellets. Reduced at and 1200 C. (A) 5 min of reduction at 1200 C (B) 15 min of reduction at C. Notice the " Dendritic like " spreading of iron on the wustite surface. process must have produced variations in the gas composition within the pellets. That is, the gas composition changed radially from surface to core of the pellets due to the thermal gradient, and locally depending upon the distribution of char particles. Since reaction (2) is rate controlling, the (CO/C02) ratio must have changed continuously from that of equilibrium with iron and wustite, to one of higher reducing potential. Lu12~ proved that this is one of the conditions necessary for fibrous growth of iron. 2. Effect of Calcium Oxide Calcium oxide has been associated with the fibrous growth of iron during the reduction of wustite. The results of the present work support that view. The swelling of the pellets increased from 80 to 140 % when the calcium oxide content increased from 2 to 7%. In the results of previous investigators, the calcium oxide was reported as dissolved or forming compounds with iron oxide or silica. The reaction between these oxides was considered to occur during firing of hematite or magnetite pellets. Goksel and Volin17~ have shown that the agglomeration of the pellets used in this work occurs via a hydrothermal process by which calcium, iron and silicon oxides react at 198 C and 200 psig to form gels of considerable strength. These conditions are not likely to produce the dissolution of the calcium oxide in the iron oxide lattice. Thus, if there is diffusion of CaO into the wustite or reaction between them to form compounds, it must occur during the reduction process. None of these possibilities were studied. However, the lower reduction rate and low percentage of reduction attained at 900 and C could be accounted for by a low rate of the gasification of carbon reaction combined with a decrease in the wustite activity produced by its interaction with calcium oxide. This fact, and the observed catastrophic swelling produced between 900 C and C, renders support to the idea of an effective inhibition of iron nucleation on the wustite surface produced by the calcium oxide. The role of this oxide is not clearly understood. The evidence obtained in this work agrees with previous findings but can not be used in support of any of the mechanisms that have been proposed thus far. A more fundamental approach must be taken in order to understand not only the extense filamentary growth produced at lower temperatures, but also the " dendritic like " structure obtained at higher temperature which suggest partition or impurity segregation during the growth of iron over the wustite surface. 4. Pellets Strength The influence of the structural changes that occurred during reduction on the pellets strength is shown in Fig. 3, but the results show that, at all temperatures, there is a lowering of the pellets crushing strength during early reduction stages. This suggests the repture of the interparticle bond produced by the hydrothermal agglomeration process. It thus follow that a new hardening mechanism must be established if the crushing strength is to increase as reduction proceeds. Obviously, this new hardening mechanism did not occur for the hematite and magnetite pellets reduced at C. They had rather low strength

7 Transactions ISIJ, Vol. 23, 1983 (503) values. This is directly related to the extensive swelling observed at that temperature which produced a pellet of spongy appearance that disintegrated readily. The results also show that the crushing strength increases with temperature and time of exposure in the range to C. This agrees with previous findings.21) The increase in strength is due to sintering of the iron particles. Macroscopic examination of the fractured surfaces of pellets reduced at and C revealed the presence of a surface layer surrounding a central core. The increasing thickness of the surface shell with the time of exposure and the observed differences in the sintering from surface to core suggest that the above result is related to the temperature gradient caused by the rapid heating conditions of this experiment. This produced rapid reduction and subsequent formation of an outer sintered iron layer which advanced toward the center as reduction progressed. This is in agreement with previous work22~ where similar defects were obtained as a result of large temperature gradients induced by severe heating conditions during the induration of magnetite pellets. The conclusion that the observed structure is generally due to rapid heating rates is also supported by the fact that this structure was not observed during the examination of the fractured surfaces of the pellets reduced under relatively mild heating conditions and longer sintering periods. I V. Conclusions (1) The reduction of hematite and magnetite pellets containing bituminous coal char is accompanied by catastrophic swelling of the pellets at the temperature range between 900 C and 1000 C. The abnormal swelling was due to the growth of iron in the form of fibers or whiskers and occurs during the reduction of wustite to iron. (2) The fiber or whisker like growth of iron appear to be caused by a continuous change in the reducing potential of the gas phase, and by the inhibition of nucleation at certain sites over the wustite surface due to the influence of calcium oxide. (3) The pellets strength as a function of reduction, decreased at C and C and 1200 C. The lowering of the strength at 1000 C was related to the whisker formation which produced a weak pellet characterized by the lack of bonding between iron particles. The increase in strength observed at and 1200 C was due to sintering of the iron particles. This process occurred concurrently with the reduction reaction. Acknowledgements This study was supported jointly by the Venezuelan Government, and the Institute for Minerals Research of Michigan Technological University. The authors wish to express their appreciation to Dr. M. A. Goksel and Dr. W. L. Freyberger for their invaluable comment throughout this work. REFERENCES 1) N. Ponghis, R. Vidal, A. Bragard and A. Poos : Iron Making Proceedings, Raw Materials, II, RIME, Chicago, (1967) ) A. Ishimitsu, K. Sugahara and M. Hirato : T etsu-to-hagane, 50 (1964), ) R. L. Bleifuss: Proceedings ICSTIS, I, Suppl. to Trans. ISIJ, 11 (1971), 52. 4) S. Watanabe and M. Yoshinaga: Trans. RIME, 245 (1968), 1. 5) J. K. Wright: Trans. ISIJ, 17 (1977), ) Brill-Edwards, B. L. Daniell and R. L. Samuel: JISI, 203 (1965), ) H. Brill-Edwards, H.E.N. Stone and B. L. Daniel!: JISI, 207 (1969), ) R. D. Walker, N. S. Ford and D. L. Carpenter: Proceedings ICSTIS, I, Suppl. to Trans. ISIJ, 11 (1971), ) T. Fuwa and S. Ban-ya: Trans. ISIJ, 9 (1969), ) R. L. Bleifuss: Trans. AIME, 247 (1970), ) H, von Ende, K. Grebe and S. Thomalla: Stahl. Eisen, 91 (1971), ) W. K. Lu: Scand. J. Met., 2 (1973), ) J. K. Wright: Proc. Aus. Inst. Min. Met., 265 (March, 1978), 1. 14) Y. K. Rao: Met. Trans., lob (1979), ) W. K. Lu : Scand. J. Met., 3 (1974), ) T. E. Ban, D. C. Violetta and C. D. Thompson : Chemical Engineering Progress Symposium Series N 43, AIChE, 59 (1963), ) M. A. Goksel and M. E. Volin: Trans. AIME, 244 (1969), ) R. Nicolle and A. Rist: Met. Trans., lob (1979), ) C. E. Seaton, J. S. Foster and J. Velasco: Trans. ISIJ, 23 (1983), ) C. Wagner: Proceedings of The Chipman Conference: Steelmaking, MIT Press, Mass., (1965), ) S. Taniguchi, M. Ohmi and H. Fukuhara: Trans. ISIJ, 18 (1978), ) J. R. Wynnycky and W. A. McDordy: Met. Trans., 5 (1974), 2207.