Sintering Behavior of Silica Filler Sands for Sliding Nozzle in a Ladle

Transcription

1 , pp Sintering Behavior of Silica Filler Sands for Sliding Nozzle in a Ladle Yusuke KOBAYASHI, 1) Hidekazu TODOROKI, 1) * Fumiaki KIRIHARA, 1) Waki NISHIJIMA 2) and Hiroshi KOMATSUBARA 3) 1) Technical Research Center, Nippon Yakin Kogyo Co., Ltd., 4-2 Kojima-cho, Kawasaki-ku, Kawasaki, Kanagawa, Japan. 2) Technology Development Dept, Kawasaki Plant, Nippon Yakin Kogyo Co., Ltd., 4-2 Kojima-cho, Kawasaki-ku, Kawasaki, Kanagawa, Japan. 3) Production Dept, Kawasaki Plant, Nippon Yakin Kogyo Co., Ltd., 4-2 Kojimacho, Kawasaki-ku, Kawasaki, Kanagawa, Japan. (Received on March 17, 2014; accepted on May 8, 2014) A study has been carried out to clarify the sintering behavior of silica filler sand. Sintering experiments were firstly conducted and the experimental data were compared with the thermodynamic calculations by FactSage and MELTS software. It was found that sintering of the silica sand proceeded through the evolution of liquid phases, which were originated from alkali feldspars. The liquid proportion was found to increase with an increase in temperature. It wan considered that the viscous liquid phases connecting silica particles contributed to the strength of the sintered sand. The thermodynamic calculations revealed that MELTS software could sufficiently simulate the phases in equilibrium with each other. The comparison of the experimental data with the thermodynamic simulations demonstrated that the whole system of the silica sand did not attain equilibrium. It was hence inferred that filler sands should be designed to prevent the state fully sintered by controlling the particle size. KEY WORDS: filler sand; sintering; sliding gate; ladle. 1. Introduction Filler sands are usually used for the sliding nozzle of a ladle to keep the sliding gate refractory apart from molten steel. Filler sands, put into the space upon the sliding gate, are sintered with taking heat from molten steel. The sands should be sintered to an appropriate extent to keep stable operations without any problems. The probable problems are typically classified into the following two phenomena. A surplus extent of sintering results in a failure to open the gate to pour molten steel, while an insufficient extent of sintering results in leakage of molten steel. Therefore, it is necessary to understand the sintering behavior of filler sands. However, very few studies have been so far carried out regarding filler sands. Matsui et al. 1) has studied the effect of concentrations of alkali oxides on the sintering behavior of silica filler sands to prevent surplus sinter that leads to blockage on pouring. They presumed that the sinter proceeded with the generation of liquid phases in between silica particles. Volume fractions of liquid phases were calculated using thermodynamic software with varying alkali oxide contents as a function of temperature. As a result, it was revealed that the liquid fraction increased with increasing alkali oxide contents as well as with increasing temperature. Finally, an optimum condition * Corresponding author: todoroki@nyk.co.jp DOI: was found to prevent surplus sinter and the direction to successful operations was suggested. Another literature 2) has pointed out that filler sands may affect cleanliness of molten steel. It is considered that improper ejection of filler sands may lead to the formation of large inclusions. Consequently, it is important to understand the sintering behavior of filler sands to stabilize practices with ladles and to keep steel cleanliness. With this background, a study has been undertaken to clarify the sintering behavior. Sintering experiments were firstly conducted using silica filler sands. After that, microscopic observation was carried out to understand how liquid phases generate. Furthermore, the experimental data were compared with thermodynamic calculations to confirm if the prediction is probable by calculations. 2. Experimental Procedure Sintering experiments have been firstly conducted to understand the effect of temperature on the sintering state. In addition, the effect of particle size has been determined to better understand the sintering behavior from the view points of equilibrium and kinetics. Thermodynamic calculations were further carried out to see if the prediction is probable Effect of Temperature A box type furnace schematically illustrated in Fig. 1 was ISIJ

2 used for the sintering experiments. After the furnace was heated up to C, a platinum crucible containing 15 g of sand was placed in the furnace. An R-type thermocouple protected by the alumina tubing was contacted with the crucible to allow the temperature measurements as accurately as possible. Subsequently, the specimen was heated to the aimed temperatures with a heating rate of 40 K/minute, after which the specimen was held for 2 hours to be sintered under an air atmosphere. The aimed temperatures were chosen at 1 000, 1 250, 1 400, and C to understand the evolution of sintering with an increase in temperature. After the sintering experiments, the specimens were taken out of the crucible with crushing the sintered sand by a hammer because the specimens were stuck on the crucible wall especially at the higher temperatures of and C. Therefore, the specimens after taken out were broken to pieces of granular lumps. The chemical composition of the silica filler sand employed in this study is shown in Table 1. The silica sand consists mainly of silica with a small amount of Al 2 O 3, Na 2 O and K 2 O. Density of the sand was measured as 2.57 g/cm 3 by Archimedian method. The appearance of the sand is shown in Fig. 2 and the sand material has granular morphology with relatively uniform size distribution. The distribution of the particle size was measured using sieves as shown in Table 2. It can be seen that the majority of the particles have the size of 0.5 to 1 mm. Table 1. Fig. 1. Experimental apparatus. Chemical composition of the silica filler sand (mass%). SiO 2 Al 2O 3 K 2O Na 2O Table 2. Distribution of particle size. Size (mm) mass% Minerals: Silica (SiO 2), Alkali feldspar (Na,K)(Si,Al) 4O 8 Fig. 2. Appearance of the sand specimen Effect of Particle Size As mentioned above, the size of the sand particles are relatively large so that the sand has been crushed to have a smaller size distribution to determine the effect of particle size on sintering behavior. A vibrating mill was used to crush the sand to have the sand particles smaller than mm. The crushed sand specimen was applied for the sintering experiments with the procedure which was basically the same as stated above. The specimens were heated at 1250 C for 12 hours and at C for 4 hours Observation After every experiment, macroscopic observation by eyesight was made to evaluate how sintering proceeded depending on the condition. It was so difficult to express the sintering state quantitatively. For reference, therefore, the sintering index indicated in Table 3 with the schematic illustrations was used to express the sintering state in this study. Index 1 corresponds to the sand without being sintered. Index 3 corresponds to the state that the sand can be easily broken with a hammer implying the medium sintered state. Index 5 corresponds to the state being stiffly sintered. After the eyesight observation, the specimens were mounted with resin material penetrated into the cavities. They were thereafter polished with SiC abrasive papers and finished with diamond paste of 1 μm particles. Au was coated on the specimen surface to allow microscopic observations by an SEM (Scanning Electron Microscope; HITACHI S-3500N). Image analysis of SEM image pictures was performed to quantitatively indicate the sintering state associated with the sintering index. Porosities in an image were specified to obtain area percent with the help of image analysis software of WinROOF version 7.0. Five images were taken for analysis. Thereby, porosity proportions were obtained and the average values were used for further considerations. Qualitative analysis was also carried out by an EDS (Energy Dispersive X-ray Spectrometry; HORIBA EMAX-7000) to observe the element distribution in order to understand the evolution of liquid phases. Quantitative analysis was also carried out as for every phase after checking the composed phases. Before the observations of the sintered specimens, the original sand particles of Fig. 2 were quantitatively analyzed to identify the mineral phases as well. Five points were randomly measured for each phase and the average values were taken for analysis Thermodynamic Calculation Recently, it is of general recognition that thermodynamic software is a powerful tool to simulate the phase evolutions. In this study, thermodynamic software of FactSage version 6.3 3) was applied for the calculation. Further, thermodynamic software of MELTS version ) typically used by geologists to simulate magma constituents was also applied for the calculation. In the both calculations, the conditions were taken as SiO 2 Al 2O 3 K 2O Na 2O quaternary system under an air atmosphere ISIJ 1824

3 Table 3. Description of sintering index. State of the sand after the sintering experimentation Sintering index Illustration The sand is remained as is. 1 The sand is sherbet-like and can be easily taken out with a spoon. 2 The sand can be broken to separated pieces by hitting with a hammer. 3 The sand can be broken to larger pieces by hitting with a hammer. 4 The sintered sand is hard to be broken even using a hammer. 5 Fig. 3. Appearance of the filler sand specimens and element distributions with a schematic illustration at C. Table 4. Chemical compositions of typical particles contained in the silica filler sand (mass%). SiO 2 Al 2O 3 K 2O Na 2O Remarks Silica K-rich Alkali Feldspar Na-rich Alkali Feldspar Average values of five points are listed. 3. Results and Discussion 3.1. Mineral Phases in the Silica Sand Table 4 shows the analytical result of the particles contained in the original silica sand of Fig. 2. The sand consists of pure silica particles and two types of the particles composed of SiO 2, Al 2 O 3, Na 2 O and K 2 O. It was obvious from the observation that majority of the particles were silica with a minor amount of the latter minerals. It should be noted that the distributions of the SiO 2, Al 2 O 3, Na 2 O and K 2 O were confirmed to be completely homogeneous by the mapping analysis. Referring to the book of rock-forming minerals, 5) the particles composed of SiO 2, Al 2 O 3, Na 2 O and K 2 O phases are identified as K-rich and Na-rich alkali feldspars as denoted in Table 4. Comparing the two types of feldspars, K-rich alkali feldspars was even more dominant consistently with the chemical composition of the sand as shown in ISIJ

4 Table Sintering Behavior Figure 3 shows the appearance of the silica filler sand specimens along with the element distributions typically observed at each temperature. As indicated below the macroscopic pictures, the sintering index increases with increasing temperature. This result implies that sintered sand becomes stronger with an increase in temperature. Subsequently, the porosity proportions were measured for the specimens except at C. Figures 4(a) and 4(b) show a typical example of the SEM image and the measured porosity proportions plotted against temperature, respectively. As can be seen, the porosity proportion decreases with increasing temperature, which may imply that melting proceeds as temperature rises. Further, the experiments with the crushed sand brought lower values compared at the same temperatures. As a result, the porosity proportion could quantitatively express the sintering state. This melting behavior and the lower proportions with the crushed sand will be described later in detail. In the mappings of Fig. 3 that show the element distributions, the red, blue and green colors indicate the distributions of Al, Si and K, respectively. One can see that the particles are as they are before the experiment at C and that the sand specimen contains silica particles together with alkali feldspars that contain Al 2O 3, Na 2O and K 2O. This result is well consistent with the observation of the original silica sand, as described above. At C, it can be seen that the red regions are more spread in the silica matrix than at 1000 C. The red regions appear to penetrate into between the silica particles at and C. Finally at C, the red regions are more dominated than at the lower temperatures. Furthermore, the gradation at C in the colors is dimmer than at C. Therefore, it is so difficult to differentiate the regions that an illustration was given for the phase distribution at C. The red regions of the pictures higher than C are determined as liquid phase as described below. The red regions observed in the specimens after the sintering experiments were analyzed as listed in Table 5 and shown in Fig. 5. To make sure if the red regions are molten, we have referred to the phase diagrams of KAlSi 2O 6 SiO 2 and KAlSi 3O 8 NaAlSi 3O 8 pseudo-binary systems. 5) The diagrams tell us that the melting point of alkali feldspars is 1150 C. Thus the red regions seen above C can be determined as liquid phases, which are originated from the contained alkali feldspars. This result shows that the silica sand sinters through the evolution of liquid phases. Besides, dilution of Al 2O 3, Na 2O and K 2O in the liquid phase is obvious with increasing temperature implying that silica dissolves into the liquid phases at the elevated temperatures. At C, silica occupies the liquid phase as much as 88.1 mass%. Meanwhile, every silica phase observed in Fig. 3 was Table 5. Chemical compositions of the liquid phases (mass%) and liquid proportion (mass%). Sand temperature SiO 2 Al 2O 3 K 2O Na 2O Liquid proportion Liquid phase was not generated Original (0.5 mm ~) Crushed (~ mm) Average values of five points are listed. Fig. 4. (a) SEM image of the specimen after sintering experiment of C and (b) measured porosity proportion. Fig. 5. Effect of temperature on liquid proportion and composition ISIJ 1826

5 analyzed as pure silica which was the same as in the original sand. This implies that the silica phases do not contain Al 2O 3, Na 2O and K 2O. Condensation of these minor oxide species in the liquid phase can indicate the liquid proportion assuming that the density of the solid is the same as that of the liquid. In this study, employing Al 2O 3 as the oxide, one can obtain the proportion of liquid phases by the following equation: (mass% Al2O 3) total /(mass% Al2O 3) in liquid phase (1) = liquid proportion (mass%) Here, (mass% Al 2O 3) total is 4.6% as listed in Table 1. The obtained values of the liquid proportion are listed in Table 5 along with the plots in Fig. 5. As can be seen, liquid proportion increases with increasing temperature. This increase is relatively gradual in the range from to C, above which the extent in the increase is facilitated. At C, liquid proportion attains as high as 63 mass%. The area% of the liquid region in the element distributions provided in Fig. 3 was measured using WinROOF software to confirm the validness of Eq. (1). The measured data can be plotted on Fig. 5 because area% is equivalent to mass% under the assumption that the density of the solid is the same as that of the liquid. It is evident that the data obtained from the images well agree to the values obtained from Eq. (1), proving that this consideration is valid. To better understand the evolution behavior of the liquid phase, the compositions were plotted on the phase diagram of SiO 2 Al 2O 3 K 2O system 6) in Fig. 6, counting Na 2O equivalently as K 2O. As mentioned above, no phase change takes place until C. Thereafter alkali feldspars start to melt at C. Silica particles are considered to be dissolved into the liquid phases with increasing temperature. Then, SiO 2 is getting more dominant in the liquid phase at C and closer to the bulk composition. The viscosity of the liquid phase at C is estimated to be as high as 10 8 (poise). 7) Basically, filler sand plays a role of plugging to prevent leakage of molten steel. It is considered from this experiment that this role can be attributed to the viscous property of the liquid phases, with which the sand particles are able to be connected tightly with each other. In other words, sintered sand can behave as if it is a refractory lid over a sliding gate at the interface between molten steel and sand. With getting closer to the gate, the temperature becomes lower with keeping disconnected states without being sintered. It seems that this combination can allow molten steel to be safely poured after opening a sliding gate Thermodynamic Calculations According to the diagram of Fig. 6, the sand should be all molten because the liquidus temperature is below C at the bulk composition. This reason may be attributed to the fact that the whole system does not reach equilibrium in the sintering experiments. Therefore, thermodynamic considerations were made to better understand the sintering behavior. Figures 7 and 8 show the results by FactSage and MELTS, respectively. Basically, the both results show that alkali feldspars start to melt at around 800 C by reacting with silica. With an increase in temperature, silica phases decrease while liquid phases increase, and finally the whole system becomes molten at around C. However, the behaviors in the alkali feldspars are different from each other at the temperatures lower than C. In particular, Fig. 7. Variation of phase proportion as time calculated by FactSage. Fig. 6. Liquid compositions plotted on SiO 2 Al 2O 3 K 2O ternary phase diagram ISIJ

6 decomposition of feldspars to form corundum can be realized in Fig. 7, which has not been observed in the sintering experiments. This fact is attributed to the definition of the solid solution of alkali feldspars. It is known that K-rich and Na-rich alkali feldspars form a solid solution. 5) FactSage Fig. 8. Variation of phase proportion as time calculated by MELTS. Fig. 9. Liquid proportion plotted against temperature. version 6.3 does not consider the definition of this solid solution. This result tells us that MELTS software should be more accurate for the calculation of the present sand system. In the sintering process, the evolution of the liquid phases is the most important to be accounted for. The liquid proportions obtained by the both calculations are therefore plotted in Fig. 9 along with the experimentally obtained values. As realized, the experimental values are closer to the liquid proportion calculated by FactSage. Besides, the tendency is quite similar to each other in the gradual increase from to C and the rapid increase above C. However, the fact closer to the line by FactSage is inconsistent with the above explanations about the corundum formation. We therefore assumed that the deviation between the experimental results and the equilibrium state was unexpectedly very large at every temperature. Therewith, further experiments were performed to understand the factors of kinetics Effect of Particle Size The effect of particle size on the evolution of liquid phase has been examined to ascertain which software can correctly predict the sintering behavior with respect to equilibrium. That is why the holding times have been decided to be prolonged. Figure 10 shows the experimental results using the crushed sand, indicating that the crushed sands have more sintered than the original sands. The appearances of the sintered specimens by the crushed sands look like rocks. Particularly, the sintering index increased dramatically from 2 to 5 with the crushed sand at C. This behavior of more sintered with the crushed sand is consistent with the abovementioned results of Fig. 4. The microstructures of the specimens are shown in Fig. 11. Clearly, the liquid regions are more spread in the sintered specimens by the crushed sands. Quantitative analysis shows that silica contents in the liquid phases are higher than by the original sands as provided in Table 5. The liquid proportions obtained by Eq. (1) are also higher than by the original sands. The liquid proportions were plotted in Fig. 12 along with the lines calculated by the software. It is evident that the liquid proportions experimentally obtained are in good agreement with the line by MELTS. Especially, the value at C is right on the simulation line. The exper- Fig. 10. Effects of particle size and holding time on sintering index ISIJ 1828

7 Fig. 11. Effects of particle size and holding time on microstructures of the specimens. Fig. 12. imental data of C position 10 mass% lower than the line. This could be caused by the viscous liquid phases which obstruct the dissolution of the silica particles. This result of agreement indeed proves that MELTS software is able to accurately simulate the present sand system. The sintering behavior can be predicted by thermodynamic simulation, as described above. It should be notified, however, that the deviation exists between the simulations based on equilibrium and the actual sintered situations. The particle size of filler sands could be empirically determined to prevent the state fully sintered to avoid complete blockage. It is expected with this study that designing the size distribution can be more optimized by accounting for the sintering behavior. 4. Conclusions Calculated liquid proportion curves plotted against temperature along with experimental data. A study has been undertaken to clarify the sintering behavior of silica filler sand. Sintering experiments were firstly conducted and the experimental data were compared with the thermodynamic calculations by FactSage and MELTS software. The following conclusions were drawn: (1) The sintering of the silica sand proceeded through the evolution of liquid phases, which were originated from the contained alkali feldspars. (2) The liquid proportion increased with an increase in temperature as a result of the dissolution of silica phases into the liquid phases. (3) It was considered that the viscous liquid phases connecting silica particles contributed for the strength after sintering. (4) According to the thermodynamic calculations, MELTS software could sufficiently simulate the phases in equilibrium with each other. (5) The comparison of the experimental data with the thermodynamic simulations demonstrated that the whole system of the silica sand did not attain equilibrium. (6) It was inferred that the filler sand should be designed to prevent the state fully sintered by controlling the particle size with accounting for the sintering behavior. REFERENCES 1) T. Matsui, K. Miura, S. Hanakiri and S. Iwamoto: Taikabutsu, 50 (1998), ) A. W. Cramb: Scand. J. Metall., 26 (1997), 2. 3) C. W. Bale, E. Bélisle, P. Chartrand, S. A. Decterov, G. Eriksson, K. Hack, I.-H. Jung, Y.-B. Kang, J. Melançon, A. D. Pelton, C. Robelin and S. Petersen: Calphad, 33 (2009), ) M. S. Ghiorso and R. O. Sack: Contrib. Mineral. Petrol., 119 (1995), ) W. A. Deer, R. A. Howe and J. Zussman: An Introduction to the Rock- Forming Minerals, Longman Science & Technical, Harlow, (1992), ) Slag Atlas, 2nd ed., ed. by Verein Deutscher Eisenhüttenleute (VDEh), Verlag Stahleisen GmbH, Düsseldorf, (1995), ) Handbook of High Temperature Properties of Molten Steel and Slag, ISIJ, Tokyo, (1971), ISIJ