Melting Rate of Iron Oxide Pellets into Iron Melt*

Transcription

1 UDC : : Melting Rate of Iron Oxide Pellets into Iron Melt* By Akira SATO,** Ryuichi NAKAGAWA,** Shiro Akira FUKUZAWA** and Tsuyoshi OZAKI** YOSHIMATS U,** Synopsis The melting rate of iron oxide pellets into iron melt and the reduction rate of pellets by carbon in iron melt were obtained by measuring the evolution rate of CO gas. The pellets were made from iron oxide powder by pressing and sintering and were not crushed into pieces when added onto the iron melt. The effects of the iron oxide phase, the oxide additives in pellets, and the temperature and the carbon content of iron melts on the melting and the reduction rates were investigated. The following results were obtained: (1) It has been found that the heat transfer was not the rate-determining step for the two rates. It is considered that the carbon diffusion in a boundary layer on the iron melt surface could be the rate-determining step. (2) The total reduction rate was constant for reduction degrees from about 20% to about 70%, where solid iron oxides were reduced by carbon dissolved in iron melt. (3) The apparent activation energy of the melting of pellets into the carbon saturated iron melt was 35 for Fe203; 18 and 41 for Fe304 above and below 1470 C, respectively; 44 and 79 kcal/mol for FeO above and below 1470 C, respectively. (4) The melting rate of Fe203, Fe304 and FeO pellets into the iron melt at 1570 C was proportional to C057, C045 and G042 respectively, where C denotes the weight percentage of carbon in an iron melt. The minimum amount of CO gas evolved was obtained at about 2%C. I. Introduction The melting behavior of reduced iron pellets into iron melt in the continuous iron- and steel-making process has been studied at National Research Institute for Metals since It was shown that the melting rate of reduced iron pellets into iron melt depends greatly on the residual oxygen content, the content and composition of gangue in pellets, the carbon content of iron melt, the temperature and the molten slag.1 3~ For a constant residual oxygen content the melting rate is same for wustite, magnetite, and hematite at higher temperatures, but it is different at lower temperatures.4~ In this report, the melting rate of iron oxide pellets into iron melt and the reduction rate of iron oxide pellets by carbon in iron melt were obtained by measuring the evolution rate of CO gas. The effects of the phase of iron oxide, the oxide additives in pellets, and the temperature and the carbon content of an iron melt on the melting and the reducing rates of iron oxide pellets have been investigated. II. Experimental Iron oxide powder used were pure hematite [special grade chemical reagents (C.A.) and equivalent to first grade C.A.], pure magnetite (equivalent to first grade C.A.), and wustite of 98.3% purity made by reducing pure hematite of first grade C.A. l0-5o g of each kind of iron oxide powder before and after sintering was formed into a pellet by pressing. The pressing under 4 to 6 t/cm2 was performed using a die and a punch having 7 cm2 of sectional area. Sintering of iron oxide powder and pellets was performed in air or Ar gas at ~ C for 1-4h. A hematite pellet of 10 g containing oxide powder of CaO (95.2% CaO ), A1203 (first grade C.A. ), MgO (first grade C.A.), Si02 (99.8%Si02) or CaO-Si02 (CaO/ Si02=1) was sintered in air at C for 1 h after pressing at 4 t/cm2. The density of a pellet was calculated from the weight measured by a direct-vision balance and the height and the diameter measured by a micrometer. A Tammann furnace for controlled atmosphere melting was used.2~ The iron melt was kept at a desired temperature within ±5 C, and the amount of CO gas evolved was measured with an integrating gas meter. Because the measurable range of this meter was /h, nitrogen gas was flowed at the rate of 601/h in order to measure a small amount of CO gas evolved. Graphite crucibles and recrystallized alumina crucibles of 55 mm ID and 190 mm L were used for iron melts saturated and unsaturated with carbon, respectively. Iron-carbon alloys were prepared in graphite crucibles in an induction furnace. The experimental procedure was as follows. An iron melt weighing about 1.5 kg was melted in a nitrogen atmosphere and held at an experimental temperature for 30 min and its surface was cleaned by scraping before and after adding a small amount of Fe203. Samples for chemical analysis were taken after stirring the iron melt. A pellet charged previously in the apparatus was dropped onto the iron melt after the flow rate of nitrogen gas was recorded every 30 s for 2 min. From that moment, the flow rate was recorded every 5 s, and the recording was continued for 2 min or longer after the end of reaction was visually recognized. The reproducibility was confirmed by comparing the results of repeated runs in each experimental condition. The time required for complete melting of pellets was determined as the time from the moment of dropping the pellet to the last evolution of CO gas. The melting rate of pellets (g/s) was obtained by dividing the weight of pellet by the above time. The * ** Originally published in Tetsu-to-Hagane, 67 (1981), 303, in Japanese. English version received November 12, National Research Institute for Metals, Nakameguro, Meguro-ku, Tokyo Research Article (879)

2 0880) Transactions ISIJ, Vol. 21, 1981 reduction rate of solid iron oxides by carbon in iron melt, which was considered to play a main role in the process of melting, was discussed by using various reduction rate equations reported in Refs. 5) to 11). III. Results Table 1 shows the apparent density and the porosity of pellets, the latter of which was calculated with the true density in Ref, 12). The X-ray diffraction patterns of iron oxides were not changed by sintering. Pellets used mainly in this report were those marked by in Table 1. The diameter and the height of a hematite pellet (15 g), a magnetite pellet (30 g), and a wustite pellet (20 g) are approximately 30~ x 4, 30~ x 11 and 30~ x 7 mm, respectively. Figure 1 shows the effects of the temperature of iron melt saturated with carbon and the phase of iron oxide on the relation between the total reduction and the time. The relations from about 20% to about 70% of the total reduction are linear, showing that the reduction of solid iron oxide by the carbon in iron melt proceeds at a constant rate. The reducing rate of solid iron oxide increases with increasing temperature. Figure 2 shows the dependence of the time for melting of pellets on the temperature of iron melt saturated with carbon and the phase of iron oxide. The time required for melting of pellets increases with decreasing temperature and with decreasing oxygen content in iron oxide. Particularly, the weight of Fe0 pellet melted into the carbon saturated iron melt at C for 10 min was only 2.2 g. This pellet was quenched into water and dried at 105 C for 6 h. The structure of this pellet was examined by an optical microscope, an EPMA and an X-ray diffractometer. The transformation of iron oxide from solid to liquid was observed at the last stage of melting above C. Figure 3 shows the dependence of the time required for melting of pellets on the carbon content in iron Fig. 2. Dependence of the time for melting the temperature of carbon saturated of pellets on iron melts. Fig. 1. Changes of the temperature for total reduction rate with time carbon saturated iron melts. and Fig. 3. Dependence of the the carbon content time for melting in iron melts at of pellets on 1570 C. Table 1. Density and porosity of pure iron oxide pellets.

3 Transactions ISIJ, Vol. 21, 1981 (881) melt at 1570 C and the phase of iron oxide. Experiments with iron melts of carbon content below 2% at C could not be performed, because the inside diameter of crucible decreased due to the sticking of a mixture of splashed metallic iron and iron oxide. The sticking was not formed at C. The time required for melting markedly increases at the carbon content below 1 %. The minimum time required for melting is not recognized for iron oxide pellets. For partially reduced pellets, it was recognized 2.5'-.'3.5%C.2'4 ) Figure 4 shows the dependence of the time required for melting of pellets into the carbon saturated iron melt at 1520 C on the content of oxide additives in hematite pellets. The time required for melting increases with increasing contents of CaO, Si02 and CaO-Si02. The time for pellets containing 16% is about 1.5N3 times longer than that of a pellet without additive. The time required for melting of pellets containing up to 8%A1203 increases similarly to the above pellets, but that of a pellet containing 16%A1203 is extremely long. The time required for melting of pellet containing 2%MgO is shorter than that of pellet without additive, but only 1.7 g of a pellet containing l6%mgo melted into iron for 10 min. This pellet was quenched into water and dried at 105 C for 6 h. The structure of this pellet was examined like the wustite pellet at C. I V. Discussion 1. Time Required for Melting The time required for melting of pellets includes the time required of heating pellets, the time required for reducing solid iron oxide by carbon in iron melt, and the time required for reducing partly or fully liquefied iron oxide by carbon in iron melt. In the development of a new iron- and steel-making process, the time required for disappearing of pellets, which is equal to the time required for melting, is considered to be most important and will be discussed at first. Figure 5 shows variations with temperature of the melting rate (g/ s) into the carbon saturated iron melt. The temperature dependence of the melting rate of hematite pellets is nearly the same as that of pure iron pellets. The temperature dependence of the melting rate of magnetite and wustite pellets above 1470 C differs from that below C. The apparent activation energy of melting of pellets obtained from these relations is 35 for hematite; 18 and 41 for magnetite above and below 1470 C, respectively; 44 and 79 kcal/mol for wustite above and below 1470 C, respectively. The change of the apparent activation energy at 1470 C is considered to be due to the liquefaction of iron oxide.13) Because the melting point of FeO is reported to be N C,'2'14'15) iron oxide liquefies above 1470 C. This is confirmed from the observation that the solid iron oxide liquefied at the last stage of melting above 1470 C. Figure 6 shows relations between the melting rate and the carbon content (wt%) in iron melt at C. Below 4%C, the melting rate of hematite, magnetite and wustite pellets is proportional to (wt%c)o.57, (wt%c) 45, and (wt%c) 42, respectively. At 1520 C, experiments with iron melts below 2%C could not be performed, and the dependence of the melting rate on the carbon content was slightly stronger than that at C. Figure 7 shows the dependence of the ratio of the actual amount of CO gas evolved to the theoretical amount on the carbon content in iron melt in alumina crucibles at C. The ratios of hematite, magnetite and wustite pellet in a graphite crucible are 1. 1, 0.96 and 1.3, respectively. The Fig. 5. Arrhenius-type plot of the saturated iron melts. melting rate into carbon Fig. 4. Dependence of the time for melting of pellets carbon saturated iron melts at C on content of oxide additives in hematite pellets. into the Fig. 6. Relations between the melting rate in iron melt at 1570 C. and the wt%c Research Article

4 (882) Transactions ISIJ, Vol. 21, 1981 Fig. 7. Dependence of the ratio of the measured volume of CO gas evolved to the theoretical volume on the carbon content in iron melt in alumina crucibles at 1570 C. reason why these values exceed unity would be due to the errors of the CO measurements ( ±5 % ) and the pellet weight measurements. Moreover, the marked high value of Fe0 pellets is because the purity was 93.8 %Fe0 and the rest was Fe203. The ratio for a high cation iron melt in an alumina crucible was nearly the same as the ratio for a graphite crucible. The amount of CO gas evolved shows a minimum near. 2%C. For the experimental results at 1520 C, the amount of CO gas evolved decreased with decrease of the carbon content in iron melt above 2%C. The minimum amount of CO gas evolved from 2%C melt would be due to the evolution of C02 gas.2,7,9,10) Most of gas evolved from a high carbon iron melt would be CO gas, while the amount of C02 gas would increase with decreasing carbon content down to about 2 %. But the amount of CO gas produced as a result of reaction of C02 gas with the graphite cylinder mounted on crucible would increase, because the gas evolving rate decreases with decreasing the carbon content below 2%. Therefore, the ratio would again increase with decreasing the carbon content below 2 %. 2. Reduction of Solid Iron Oxides by the Carbon in Iron Melt Solid pellets were observed to react with carbon in iron melt except for the last stage of melting. Consequently, the reduction of solid iron oxides by carbon in iron melt is considered to play the most significant role in the process of melting of iron oxide pellets into iron melt. Figure 1 shows the effects of the temperature of a carbon saturated iron melt and the phase of iron oxide on the total reduction. The reaction rate at a range of reduction from 20% to 70% is nearly constant5~9) and is controlled by the reduction of solid iron oxide by carbon in iron melt. The increasing reaction rate at a range of reduction below 20% would be due to the heating of pellets. The decreasing reaction rate at a range of reduction above 70% would be due to the liquefaction of iron oxide and the decrease of the reaction area during melting Fig. 8. Relations between K and 1/T. tion per cent/s) were obtained Fig. 1. of a pellet. A constant reducing rate of solid iron oxide is an indication that the reaction of zeroth order is controlled by the chemical reaction.8) However, as described later, the estimated thickness of the diffusion layer at reaction interface was not thin enough to deny the carbon transport in iron melt as the controlling step. In order to compare the present results with those obtained by Dancy,5) the data in Fig. 1 were analyzed with the equation of the first and the second order of reaction. But, both order of reaction did not fit the results. Moreover, the equation used by Kondahov et a1.10) and Kato et a1.,11) did not fit the present data either. Figure 8 shows the relations between K and 1 / T. Values of K (reduction per cents) were obtained from the data in Fig, 1. The data points for Fe203 and Fe304 represented by a straight line, but the point of Fe0 at C is much below a best fit line of Fe0 above C. The apparent activation energy of the reduction of solid Fe203, Fe304 and Fe0 by carbon in iron melt was 23, 23 and 42 kcal/mol, respectively. Table 2 shows the results of the present work and other studies. The apparent activation energy obtained in this work is nearly the same as the values reported in other work. The thickness of the carbon diffusion layer at reaction interface in iron melt was estimated in the following equation assuming that the diffusion of oxygen in solid iron oxide could be neglected. N/A = K1 C~ b, K1= D/o where, N: the CO gas evolution rate (mol/s) A : the area of reaction interface (cm2) K1: the constant of carbon transfer in iron melt (cm/s) Cab: the carbon content in bulk iron melt (mol/cm3) D : the diffusion coefficient of carbon in iron melt (cm2/s) o : Values of K (reducfrom the data in the thickness of the diffusion layer at reaction interface (cm). The area of reaction interface was calculated as the contact area between a solid iron oxide pellet and

5 Transactions ISIJ, Vol. 21, 1981 (883) Table 2. Experimental results in the present work and other work. the iron melt on the condition of its floating. Using the data in Refs. 16) and 17) the estimated thickness was 24x N10.3 cm for the carbon saturated iron melt and 8 X 10_4 cm for the iron melt of 1 %C at 1570 C. These values are smaller than those for the melting of steel scrap into an iron-carbon melt by Mori and Nomura.l8~ In the experiment by Mori and Nomura, the melt was stirred by induction of high frequency. The small values obtained in this work would be due to the stirring of the reaction interface by the bubbling of CO gas evolved. However, the estimated values are not small enough to deny the carbon transfer in iron melt as the rate controlling step. Figure 9 shows the dependence of K (reduction per cents from about 20% to about 70% of total reduction) on the content of oxide additives in pellets reduced by the carbon saturated iron melt at C. Ca0-Si02 (simultaneous addition of 1; 1), A1203 and Si02 decreased the reduction rate. The reduction rate slightly increased with increasing addition of Ca0 and Mg0 up to about 2%, and decreased greatly above 8 % of their addition. Fig. 9. Dependence of K (reduction per cents from about 20% to about 70% of total reduction) on the content of oxide additives in pellets reduced with carbon saturated iron melts at C. 3. Heating of Iron Oxide Pellets Figure 10 shows the effect of the heating of pellets on the total reduction. The heating of pellets was performed by suspending them with a Pt wire at about 2 cm above the iron melt for 5 min at C and C, and for 2.5 min at C. Because a liquid droplet from all iron oxide pellets dropped in 3.5 min after heating at 1420 C, the above heating conditions were considered to be adequate. The effect of heating on the reduction rate differed in different phases of iron oxide. The heating increased the reduction rate of wustite pellets, but it decreased the rate of hematite and magnetite pellets. In particular, the difference was remarkable at 1320 C. Since the time required for melting of wustite pellets heated at 1370 C and 1420 C was about 90% of the time for a cold one, the heat transfer could not be the controlling Fig. 10. Effect of the heating of pellets on the time dependence of the total reduction. The heating of pellets was performed by suspending them with Pt wire at about 2 cm above the iron melt for 5 min (1320 C) and 2.5 min (1420 C). step. The heating of Fe203 and Fe304 in nitrogen gas containing 0.5 ppm of oxygen is known to transform these phases to FeO.19~ This was also confirmed from an observation in this study that the amount of CO

6 ( 884 ) Transactions ISIJ, Vol. 21, 1981 gas evolved from a heated Fe203, Fe304 or FeO pellet was less than that from a cold one by 29%, 19% or 3%, respectively. Therefore, the decrease of the reduction rate by heating is considered to be due to the transformation of Fe203 and Fe304 to FeO. However, the reason why the FeO formation decreases the reduction rate is unknown yet. Photograph 1 shows the microstructures of the melting interface of iron oxide pellets floated for 10 min on the carbon saturated iron melt (not shown). At the melting interface of an FeO pellet (a) at C, a dense layer consisted of metallic iron and iron oxide was observed, while MgO enriched and condensed in the progress of melting was observed in an Fe203 16%MgO pellet (b). The structure in the middle of these pellets was very porous. Photograph 2 shows the electron and the X-ray images of the melting interface of a wustite pellet in Photo. 1. The white phase is confirmed to be metallic iron because the 0!L a intensity is nearly zero. The existense of metallic iron and wustite were confirmed by the X-ray diffraction of this pellet. Photograph 3 shows the electron and the X-ray images of the melting interface of an Fe203 16%MgO pellet in Photo. 1. MgO is known to be enriched and condensed near the melting interface and also dissolved in solid FeO.20~ The transformation of Fe203 to FeO was confirmed by X-ray diffraction of this pellet. Photo. 1. Microstructures of the melting interface of iron oxide pellets floated for 10 min on the carbon saturated iron melt (lower not shown). Photo. 2. Electron and X-ray images at the melting interface of wustite pellet floated for 10 min on the carbon saturated iron melt (lower, not shown) at C.

7 Transactions ISIJ, Vol. 21, 1981 (885) Photo. 3. Electron and X-ray images at the melting interface of an Fe203 16%Mg0 pellet floated for 10 min on the carbon saturated iron melt (lower, not shown) at 1520 C. V. Conclusion Uncrushed iron oxide pellets were added onto iron melt. The melting rate of pellets into iron melt and the reduction rate of pellets by carbon in iron melt were obtained by measuring the evolution rate of CO gas. The effects of the phase of iron oxide, the oxide additives in pellets, the temperature, and the carbon content of iron melt on the melting and the reduction rates were investigated. The following results were obtained : (1) The melting and the reduction rates of heated hematite and magnetite pellets were smaller than those of cold pellets, while those of heated wustite pellets were slightly greater than those of cold pellets. This indicates that the heat transfer was not the ratedetermining step. (2) A constant reduction rate was observed at the range of reduction from about 20% to 70%, and hence the chemical reaction is considered to be the rate-determining step. (3) The estimated thickness of the diffusion layer of carbon in iron melt was 2N4 x 10.3 cm for the carbon saturated iron melt, and 8 x 10.4 cm for the iron melt of 1 %C at C. These values are not small enough to deny the carbon transfer in iron melt as the rate-determining step. (4) The apparent activation energy of melting of pellets into the carbon saturated iron melt was 35 for Fe2O3; 18 and 41 for Fe3O4 above and below 1470 C; 44 and 79 kcal/mol for FeO above and below 1470 C. The change of the apparent activation energy at 1470 C is considered to be due to the liquefaction of iron oxide. (5) The melting rate of hematite, magnetite and wustite pellets into the iron melt at 1570 C was proportional to (wt%c)o.57, (wt%c)o.45, and (wt%c) 42, respectively. The minimum amount of CO gas evolved was observed at about 2%C, and this is considered to be due to the maximum CO2 gas evolution. (6) Addition of CaO-SiO2 (simultaneous addition of 1: 1), A12O3 and SiO2 decreased the melting and the reduction rates of hematite pellets. The melting and the reduction rates of pellets containing CaO or MgO were slightly larger than those of pellets without CaO or MgO below 4% addition, but much smaller above 8% of addition. (7) The melting and the reduction rates of wustite pellets in the carbon saturated iron melt at C were extremely small as a result of the formation of a dense layer consisted of metallic iron and iron oxide at the reaction interface. On the other hand, the rates of hematite pellets containing 8 % and 16%MgO were extremely small as a result of the enrichment and the condensation of MgO at the reaction interface. Research Article

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