Measurement of Physical Properties of Slag Formed around the Raceway in the Working Blast furnace

Transcription

1 , pp Measurement of Physical Properties of Slag Formed around the Raceway in the Working Blast furnace Shinichi INABA, Yoshio KIMURA, Hiroyuki SHIBATA 1) and Hiromichi OHTA 2) Kobelco Research Inst., Inc. Kakogawa Labs., , Onoecho, Ikada, Kakogawa, Hyogo Japan. 1) Inst. for Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai Japan. 2) Department of Materials Science, Ibaraki University, Nakanarusawa, Hitachi Japan. (Received on June 15, 2004; accepted in final form on August 27, 2004 ) Slags around raceway were synthesized using reagents. The chemical compositions of the synthesized slags were chosen to be those of slags obtained from the working blast furnace. Viscosity, density, surface tension, wettability to coke and thermal conductivity of the slags were measured in order to investigate physical properties of tapping slags. KEY WORDS: liquid slag; raceway; blast furnace; viscosity; density; surface tension; wettability; contact angle; thermal conductivity. 1. Introduction Recently, many kinds of materials such as a pulverized coal, plastics and a fine iron ore have been injected into tuyeres of a blast furnace. The slag is formed rapidly reacting with a coke ash and the injected matters in the very high temperature region. Characteristics of slags are the key issue of the operation conditions of the blast furnace, especially gas permeability and fluidity of the liquid, and thermal preservation in the lower part of the blast furnace. However, the characteristics of slags during operation have not been well known. The chemical compositions of slag sampled during the operation around the raceway have been reported on the literatures. Then slags were synthesized using reagents to reproduce the slags around the raceway of the blast furnace and their physical properties were investigated. The tapping slags sampled from the working large blast furnace at the high heat-level and the low-heat level operations were also investigated. 2. Experiment 2.1. Sample Preparation The chemical compositions of slag samples are determined based on the chemical analyses of slags taken from the working blast furnaces. 1 4) The chemical reagents are mixed as to be the target composition. The mixture of reagents are melted in the iron crucible stirring two hours at K under the inert gas atmosphere. After quenching the molten mixture into the water, solidified slag is dug out of crucible and crushed. The powdered slag is melted again in the same conditions before. This process is repeated three times in order to equilibrate FeO content of slag with the iron crucible. The dimension of iron crucible is 76 mm inner diameter, 130 mm depth, 6.6 mm wall thickness and 9 mm bottom thickness. Table 1 shows the chemical compositions of the target and the synthesized slags. Slag, and are imaged from the slag around the raceway, the incomplete slag formation of SiO 2 in coke and the dropping slag under the raceway, respectively. Slag is imaged slag between raceways. As found in Table 1, the compositions of every synthesized slag are similar to the own target. Slag and are the tapping slag taken from the working blast furnace at the high heat-level and the low-heat level operations, respectively. The both chemical compositions show a small fluctuation by the sampling point because of fine iron particles suspended in slag. The operation conditions at the high heat level have indicated the tapping temperature of K, silicon content of pig iron [Si] 0.53% and manganese content [Mn] 0.38%. The pig iron has been produced under the operation of the low heat level. Under this operation, the pig iron was tapped at K with the chemical composition [Si] 0.24% and [Mn] 0.37%. For the measurement of thermal conductivity, Slags around the raceway are newly synthesized in the same conditions for Slags. Slags are corresponding to Slags on the situation in the blast furnace, respectively. Slags is the standard slag with three components Al 2 O 3 CaO SiO 2 for certifying the pertinence of measured data by the comparison with published thermal conductivity. These chemical compositions are shown in Table Measurements Viscosity Viscosity of slag has been measured by the oscillatingplate viscometer developed by Iida et al. 5) When the oscillating thin plate with constant driving force is immersed into the liquid, the amplitude of vibration of the plate diminishes depending upon the viscous resis ISIJ 2120

2 Table 1. Chemical compositions of slags at target, synthesis and after viscosity test. tance. The change of vibration amplitude gives the viscosity. It is well known that the product (r m) of the density and the viscosity of liquid is given as Eq. (1) when the oscillating-plate is vibrating with the sympathetic frequency. ρ µ K E 2 a 1...(1) E where, R K M2 2 π fa a r: density of liquid (kg/m 3 ), m: viscosity of liquid (Pa s), E a : amplitude of vibration in the air (m), E: amplitude of vibration in liquid (m), R M : part of peculiar mechanical impedance of viscometer (kg m/s), f a : sympathetic frequency in the air (Hz), A: surface area both sides of oscillatingplate (m 2 ). As Eq. (1) is made up of the rough approximate based on the many assumptions, the order 2 in the equation changes a little with the measurement conditions actually. 6) Then, looking for the instrumental constant K using liquid with given density and viscosity in advance, the measurement of E a and E should introduce the product r m in Eq. (1). Viscosity should be calculated from the measurement of density. In this work, the value K has been found from the measurement using the JIS standard liquid for viscosity. The characteristics of the oscillating-plate viscometer are as follows; i) For a quick method, the continuous measurement can be done in a little swing of temperature. ii) A thin and small oscillating-plate brings about a wide space in the sample container to insert a thermocouple which gives a precise temperature directly. iii) Viscosity can be measured in a wide range of temperature. iv) Difficult to control the atmosphere, completely. v) The measurement of precise viscosity needs to furnish the precise density. vi) Unsuitable to measure the absolute viscosity data precisely. On the measurement of slag viscosity, slag sample in the iron crucible is heated up to K under Ar gas blowing. After confirming the constant given temperature, viscosity is continuously measured descending the temperature with 2Kmin 1 by the automatic control using personal computer. The dimension of crucible for the viscosity measurement is 48 mm outer diameter, 38 mm inner diameter, 100 mm height and 8 mm bottom thickness made of iron. The oscillating plate size of 20 mm 20 mm 1 mm made of iron is decided to be suitable for this viscosity measurement on the pre-test. The accuracy of measuring data has been assured by the pre-test using the standard solution at room temperature. Since the liquid slag containing FeO lead to a small change of the composition during the measurement, the every slag sample has been measured once Surface Tension and Density Surface tension and density have been measured by the maximum bubble pressure method at the maximum temperature of K. In this method, a narrow pipe is immersed into the liquid slag and made a bubble on the tip by the gas pressure. The gas pressure bursting the bubble P is measured with different depth H in the liquid. The force balance between the bubble bursting pressure and the surface tension g is given as the following Eq. (2), considering the liquid weight equivalent to the tip depth H. 7) pr 2 P 2prg rgh...(2) Changing Eq. (2), 2γ ρgh P...(3) r 2 π r where r: inner radius of narrow pipe making a bubble, r: density, g: acceleration of gravity. Measuring the bubble bursting pressure with different tip depth, the relation between P and H shows the straight line. Surface tension is given by the gas pressure at H 0 equivalent to the liquid surface and density corresponds to the inclination of straight line (DP/DH) ISIJ

3 Density is also needed to calculate the thermal conductivity Wettability (Contact Angle) to Coke Coke for the blast furnace is cut to the plate with size of mm and polished the surface coming into contact with liquid slag by #1 200 polishing paper. The coke plate set up horizontally and the powdered slag packed into the graphite container above the coke plate are heated in the furnace controlled in the Ar gas atmosphere at the temperature of K. After confirming the given temperature, molten slag is put calmly onto the coke plate through the small bottom hole and the contact angle to the coke plate is measured using the telescope. It has been considered that slag dripped on the coke will flow down immediately after the only short stay in the blast furnace. The change of contact angle of slags to coke has been measured within 10 min after the contact Specific Heat Specific heat of liquid slag has been measured using Differential Scanning Calorimeter (DSC). The atmosphere is the Ar gas with the flowing rate of 50 ml min 1 and the temperature is controlled to the descending rate of 5Kmin 1 from to K. The measuring accuracy of this instrument has been assured 5% about sapphire at K Thermal Conductivity Thermal conductivity of liquid slag has been measured by the new laser-flash method developed by Ohta et al. 8) Schematic diagram of the apparatus is shown in Fig. 1. The cell is made of platinum with 0.2 mm thick and its size is 5 mm in depth and 20 mm in diameter. A laser pulse irradiates the bottom surface of the platinum cell. The resultant temperature response is monitored with respect to the same bottom surface by an InSb infrared detector. The upper part of the cell is completely open, so that bubbles can be released from this side. Theoretical temperature response has been determined under the following conditions. (1) The system is kept in thermal equilibrium before laser irradiation. (2) The heat flow is one dimensional direction. (3) The melt layer is semi-infinite. (4) The contact thermal resistance of the cell/melt interface is negligible. (5) The temperature distribution in the platinum layer is isothermal because the platinum cell is thin and its thermal diffusivity is considerably larger than that of the liquid slag layer. The measurement time is set from 4 to 12 ms after laser irradiation to satisfy these conditions. The temperature decay T d (t) can be given by Eqs.(4) and (5). T d (t) T 0 exp (h 2 t) erfc (h t )...(4) bs h...(5) ρ d Cl d d where T 0, C d, r d, b s and l d are the maximum temperature Fig. 1. Schematic diagram of the apparatus for thermal conductivity. 7) 3. Results and Discussion 3.1. Viscosity Figure 2 shows the relation between the viscosity and the temperature on Slags around the raceway. Every curve has noises in greater or lesser degree. These noises are because that FeO has been oxidized during the measurement and that the some compounds may come to suspend in the liquid slag. Considering the main four components Al 2 O 3, CaO, MgO and SiO 2 of Slag for example, the composition is put into the region near eutectic line be- rise of the temperature response, the specific heat, density, the thermal effusivity and the thickness of platinum plate, respectively. The subscripts d and s indicate the platinum and the sample, respectively. Here, time t is defined as the elapsed time after irradiating a laser pulse. The values of T 0 and h in Eq. (4) can be estimated by fitting the measured temperature decay using the conventional least squares method. The thermal diffusivity value of the sample liquid, a s, is easily obtained from the following simple equation with respect to thermal effusivity, b s : bs αs ρscs...(6) Thermal conductivity (l: W m 1 K 1 ) of liquid slag is calculated from Eq. (7) as the product of the thermal diffusivity (a s : m 2 s 1 ) and the heat content (r s C s : J m 3 K 1 ). l a s r s C s...(7) It is known that the obtained values are not affected by the radiative and convective heat transfer because the measurement is carried out within 12 ms. 9) 2004 ISIJ 2122

4 Fig. 2. Effect of temperature on the viscosity of slag around the raceway. tween Melilite and Merwinite in the system 15 mass% Al 2 O 3 CaO MgO SiO 2. 10) It is supposed that when the slag is cooled down, some solid compounds are educed from the liquid changing the composition. But any compound has not been confirmed at present work. The chemical compositions on some samples have been checked after the viscosity measurement. The results are shown in Table 1 and do not exhibit the large difference in the chemical composition between the synthesized slag and the slag after the viscosity measurement. Any explication and reliable evidence are not found about the appearance of these noises. Sugiyama et al. 11) have measured the viscosity of slag with five component 15mass% Al 2 O 3 CaO FeO 5mass% MgO SiO 2 (CaO/SiO 2 1.2) changing FeO content exclusively. They have used a rotating cylinder method with the rotor made of pure iron. Comparing Fig. 2 to Sugiyama et al., both results tend to lower the viscosity with the increase of FeO content. The present work shows a little lower viscosity and lower beginning temperature of the viscosity increase than that of Sugiyama. Viscosities of Slags and show almost the same behavior over about K of the tapping temperature, though the operations of blast furnace are different in the heat level, as shown in Fig. 3. Slag at the high heat level operation begins to solidify at about 40 K higher temperature than Slag. Mroz 12) has measured a blast furnace final slag with the basicity CaO/SiO and the chemical composition of Fe: 1.1%, CaO: 44.3%, SiO 2 : 38.8%, Al 2 O 3 : 8.4, MnO: 0.5% and MgO: 5.2%. The viscosity temperature curve reported by Mroz shows slightly higher viscosity and slower increase with descending temperature than that of Fig Surface Tension and Density Results of surface tension and density are summarized in Table 2. Density of standard slag is put on kg m 3 Fig. 3. Comparison of the viscosity of Sslags and at the operation with different heat level. from the literature. 10) Densities of Slags, and with low content of FeO are lower than that of other slags. Judging from Slags and, the increase of temperature leads to the small decrease of density but brings about no change of surface tension. Hino et al. 13) have studied the surface tension of Fe t O Gehlenite (2CaO SiO 2 Al 2 O 3 ) CaO SiO 2 system as one of the primary slags in a blast furnace. Their surface tension decreases with the increase of Fe t O content by 10 mass% and are almost constant over 10 mass% Fe t O with a relatively large fluctuation. Surface tensions in Table 2 are in the almost same range of Hino, et al. and do not show any effect of FeO content Wettability (Contact Angle) to Coke The results of wettability are summarized in Table 3 and Fig. 4. The contact angles of slag to coke are little changed within 10 min after the contact except Slag. These angles are degree higher than the data measured by Sato et al. 14) whose data shows comparatively large fluctuations over one h. Slag with high FeO content begins to decrease after 5 min and lowers about 7 degree at 10 min. The ISIJ

5 Table 2. Surface tension and density of slags. Table 3. Change of contact angle of slag droplet on the coke surface. (unit; degree) Fig. 4. Change of contact angle between coke and liquid slag with time. Fig. 6. An example of the theoretical and measured temperature response of slag just after irradiating a laser pulse. indicate the phase transformation; the appearance of some solid phases. Fig. 5. Effect of temperature on the specific heat of slag. measured contact angle of Slag at 10 min is close to the value of Sato, et al Specific Heat Figure 5 shows the effect of temperature on the specific heat. With respect to Slags and with high FeO content, the specific heat could not be measured because they chemically reacted with platinum cell. As shown in Fig.5, the specific heats of Slags, and slightly increase with descending temperature and are in the range of J g 1 K 1. The measured specific heat of Slags and shows a peak around K and K, respectively. The peaks of measured specific heat 3.5. Thermal Conductivity Figure 6 is an example of the temperature response just after irradiating a laser pulse on the liquid slag at K. The measured temperature response, indicated by black circles, has been coincident with the theoretical curve and proved the reliance of the measured data.. The thermal diffusivities were calculated from Eqs. (5) and (6), using the physical properties of platinum, the measured values of h, specific heat and density of the slags. The thermal conductivities were also investigated from Eq. (7). These values are summarized in Table 4. The specific heats in this table show only the value at the temperature corresponding to the thirmal conductivity measurement. The specific heats of Slags and are estimated using the value from literature 15) as follows. It is assumed that the increase of 10% Fe in the slag brings about the decrease of the specific heat 0.07 J g 1 K 1 and that the specific heat of slag containing less 1% FeO is equal to that of blast furnace slag 1.81 J g 1 K 1. On these assumptions, the specific heats of Slags and are presumed to be 1.70 J g 1 K 1 and 1.59 J g 1 K 1 considering the decrease of specific heat corresponding to increase of FeO content, respectively. And the specific heat for Slag is quoted from the literature. 16) The temperature dependencies of the measured thermal conductivity are observed in Fig. 7. The thermal conductiv ISIJ 2124

6 Table 4. Thermal conductivity and its relating measured and calculated values of liquid slags. Fig. 7. Relationship between thermal conductivity and temperature of liquid slags. ity of every sample has less dependency on the temperature and comes into the range of Wm 1 K 1. The thermal conductivities of Slag and Slag show small peaks at about K and K, respectively. Comparing Fig. 7 to Fig. 5 with respect to Slag, the temperature of the peak on the thermal conductivity is corresponding to the ones of the measured specific heat, though the difference between both temperatures is recognized a little. This suggests that consumption of heat by transformation of the sample such as melting of the solid in the solid-liquid coexistent molten slag makes the apparent thermal conductivities high. It is supposed that the values at the peaks are not directly corresponds to the physical properties of slags. El Gammal et al. 17) have reported about 2 Wm 1 K 1, but not exceed this value in the freezing range at K on the same slag composition of 40% CaO 40%SiO 2 20%Al 2 O 3. Their method is a modified stationary hollow cylinder process. This involves the determination of the absorption coefficient and the reflecting power of the slag by measuring the emission intensity of the slag relative to a reference emitter, a black body emitter by preference. Tapping Slags and from the blast furnace are in the higher thermal conductivity. They have almost the same value at near tapping temperature of K. And Slag at the higher heat level indicates a small increase with descending temperature and Slag at the lower heat level conversely. It is supposed that the thermal conductivity of liquid slag is related to the structure of slag such as the crystallization, the density change and the precipitation in the liquid. The physical meaning on the thermal conductivity of liquid slag will be made clear by the advanced research based on the industrial needs in the near future. 4. Conclusion Slags are synthesized with chemical reagents imaging the condition around raceway and its physical properties are measured. And the tapping slags sampled from the working large blast furnace at the high heat-level and the low-heat level operations have been studied. Proposed data are available to develop a new process and to analyze the process operation on the ironmaking technology. Acknowledgement Present work has been supported by Special Co ordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) ISIJ

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