2016 Volume 56 Issue 8 Pages 1325-1332
Wetting between liquid iron and coke influences liquid flow in the lower part of a blast furnace, which strongly affects the operation of the furnace. With increasing fluidity, the blast furnace performs more favourably and efficiently. To further improve blast furnace operation, the wetting behaviour of liquid iron on coke must be correctly understood. The effects of ash in the coke on reactive wetting in concave formations, such as holes formed at the contact area, must be considered. This study aims to elucidate the effects of the ash component in coke on the reactive wetting behaviour of carbon-unsaturated liquid iron on a simulant coke substrate and concave formations thereof. In this study, reactive wetting between the iron samples and the substrates was measured by a sessile drop method with a molten injection system at 1673 K. The results revealed that the apparent contact angle significantly decreased with time in the first 300 s after contact. After 300 s, the contact angle stabilized at a constant equilibrium value. The initial contact angles depended not only on the carbon concentration of the liquid iron, but also on the Al2O3 content in the substrates. Concave geometries formed when the carbon-unsaturated iron samples were wetted on substrates containing 0, 5, and 10 vol% Al2O3. The effect of Al2O3 on the carbon dissolution reaction was the main factor affecting the reactive wetting behaviour of substrates against liquid iron.
In the lower part of a blast furnace, increases in the fluidity of liquid flow decrease liquid hold-up1) on the voids in the coke layer, contributing to an increased gas permeability.1,2,3,4,5) In turn, this causes the blast furnace to perform more favourably and efficiently, and consequently, reduces fuel consumption of the blast furnace and suppresses CO2 emissions. Therefore, the behaviour of the liquid phase dripping through this area is significant to the blast furnace operation. Kawabata et al.6,7) and Ohgusu et al.8) reported that the wetting behaviour of the liquid phase on solid materials was one of the most important properties during the dripping period. In order to clarify the liquid flow behaviour in the blast furnace, the wetting behaviour of liquid iron samples on simulant coke is investigated in this study.
Concurrent flows of liquid iron and molten slag occur in the lower part of the blast furnace. The role of the molten slag in this region is also important because the flow of liquid iron through a coke-packed bed is influenced not only by the physical properties of the iron, but also by interaction with the molten slag.9) In this study, the wettability of liquid iron on solid coke is considered a first step in understanding the liquid flow behaviour in blast furnace operations. In the next step and in future studies, the effect of liquid slag interactions on the wetting behaviour of the iron sample on coke will be investigated.
An initial liquid iron phase is formed when the temperature exceeds 1573 K in a cohesive zone of the blast furnace; once formed, the liquid iron begins to drip through the coke-packed bed.5,10) The liquid iron can absorb carbon from the coke bed and initially has a carbon concentration of 2.5–3 mass%. The flowing of the liquid iron over the coke-packed bed may cause the iron to reach a carbon concentration of approximately 4.5 mass%.5) Therefore, determining the carbon contents of the liquid iron is necessary for the investigation of the iron’s wettability on coke at 1673 K in this study.
Several factors, such as the structure of graphite, the ash component in carbonaceous materials, and the carbon concentration in the iron sample, affect the wetting behaviour of liquid iron on a carbonaceous material substrate.11,12,13,14,15,16,17,18) The effects of the ash component on the wettability of liquid phases on carbonaceous materials have been extensively explored.11,12,13,14,15,16,17,18) These studies implied that an interfacial layer between the liquid iron and the carbonaceous substrate could form because carbon dissolves into molten iron at high temperatures, leaving behind a layer containing the complex components of ash.11,12,13,14) Additionally, a gradual increase of ash area at the interface occurred with time, which prevented the transfer of carbon from the substrate to the iron sample.13)
These results indicate that ash is very important in the wetting of substrates by molten iron because the ash components could affect carbon transfer from the substrate to the iron sample, thus changing the driving force for the wetting behaviour. However, the wetting behaviour between solid and liquid phases depends on both mass transfer from the solid substrate to the liquid phase and the interfacial morphology.19) When carbon-unsaturated liquid iron is wetted on a substrate of carbonaceous materials, the interfacial morphology may change by carbon dissolution, which can affect the wettability of the substrate by the liquid phase. The change in interfacial morphology would occur by the formation of a concave hole, located at the contact area between the iron sample and the substrate. However, information on the reactive wetting behaviours between carbon-unsaturated liquid iron and carbonaceous materials is limited, because only a few studies on concave morphological formations and the effects of ash components on the formation of such concave structures have been performed.
The aim of this study is to elucidate the effects of ash components in coke on the reactive wetting behaviour of carbon-unsaturated liquid iron on simulant coke substrate and the formation of concave geometries at the interfacial contact area between the iron and substrate.
To simplify the ash component in the simulant coke, Al2O3 was used as simulant ash. In this study, the apparent contact angles of the iron sample on the substrate were measured using the sessile drop method with a molten sample injection system. This method can prevent reactions between the iron sample and substrate during the heating process to measurement temperature. In the conventional sessile drop method, reactions between sample and substrate during the heating process can affect the measured contact angle values.
Iron samples were fabricated in a high-frequency induction heating furnace under an inert gas atmosphere. High-purity electrolytic iron and specified amounts of graphite powder were prepared with purities of 99.90%. A mixture containing 100 g of iron and graphite was placed in an Al2O3 crucible measuring 36 mm in diameter and 45 mm in height. The crucible was set into the centre of the furnace and heated to 1673 K under a flowing 99.99% pure argon gas atmosphere. After a holding time of 300 s to allow total dissolution of the graphite powder, the molten iron in the Al2O3 crucible was retrieved using a SiO2 tube with a diameter of 4 mm and then quenched in water. These solid iron samples were cracked into sample particles with grain sizes of 1–2 mm, suitable for placement in the Al2O3 funnel of the injection system of the wettability measurement equipment. Some of the sample particles were analysed for the initial carbon content in the iron samples by a carbon sulfur analyzer (EMIA-320V2, Horiba Scientific Co., Ltd.). The carbon contents of the iron samples are given in Table 1.
SiO2 is the main component of ash in coal and coke; however, it is unstable at temperatures exceeding 1873 K because it can react with graphite during the fabrication of simulant coke.2) In the present study, Al2O3 powder was used as ash in the simulant coke because Al2O3, another main component of coke ash, is stable with graphite at 1873 K.2) Simulant coke substrates were formed from 99.90% pure graphite and 99.99% pure Al2O3 powder using a graphite-element hot press furnace (FVPHP-R-3, Fuji Dempa Kogyo Co., Ltd.). Graphite powder (20 g) with grain sizes of ~45 μm was mixed well with Al2O3 powder with average grain sizes of ~3 μm at concentrations of 0, 5, and 10 vol% by a planetary mixer (ARE-250, Thinky Co.). The proportions of 5 and 10 vol% of Al2O3 in the mixed powder weigh 1.99 and 4.21 g, respectively. The weight of Al2O3 was calculated by Eq. (1).
An apparatus was employed to measure the change of apparent contact angle with time, shown schematically in Fig. 1. The main parts of the furnace include a sample stage and an injection system. The injection system consists of an Al2O3 pushing bar and an Al2O3 funnel. The prepared substrate was placed on the sample stage at the centre of the furnace. To minimize the effect of droplet vibration, caused by the dropping impact, on the initial carbon angle measurement, the substrate was set 10 mm below the injection system. For the in-situ observation of the high-temperature wettability of droplets on the sample stage, the furnace was equipped with a sapphire window to allow a camera to record movies and take pictures of the droplet shape on the substrate.
Schematic of sessile drop method with molten sample injection system.
To measure the contact angle of molten iron on the substrate, 1.5 g of iron-carbon sample particles was placed in the Al2O3 funnel, which was set in the injection system. The substrate and the injection system were heated to 1673 K from room temperature at the heating rate of 15 K/min (0.25 K/s) and then held isothermally at this temperature for 1800 s. The inert atmosphere in the furnace was maintained by flowing 99.99% purity argon gas at a rate of 0.5 L/min (0.83 × 10−6 m3/s). After holding at this temperature, the molten iron-carbon sample was pushed from the Al2O3 funnel by the Al2O3 pushing bar and dropped onto the substrate. From this moment of injection, a digital camera was used to capture images of the droplet wetting on the substrate at the experimental temperature every 60 s for a period of 1800 s. The contact angle value at the first moment of contact between the iron sample and the substrate should be the initial contact angle. However, the droplet vibrated during the first several seconds of contact. This vibration impeded the precise measurement of the initial contact angle. Thus, the initial contact angle was defined as the contact angle value measured 5 s after the first moment of contact between the two phases. The effects of the droplet vibration on the triple line movement and reactions between the iron and substrate in this 5-second period were neglected in this study. The captured images were used to measure the contact angle between the substrates and the iron samples. Finally, the furnace was cooled by turning off the furnace power.
The cooled samples were mounted in resin and cut in half to observe the interfacial phenomena between the substrates and iron samples by optical microscopy. The cross-sections of the samples were polished with 1-μm diamond paste. An analysis of Al2O3 distribution was performed by analysing images of the substrates using optical micrographs.
Figure 2 plots the apparent contact angle variation over time of the iron-3.27 mass% carbon sample, MS1, wetting on substrates of SN0, SN5, and SN10, containing 0, 5, and 10 vol% Al2O3, respectively. The contact angles significantly decrease with time in the initial 300 s of contact. After this period, they stabilize at constant values, apparently equilibrium contact angles. For the iron-3.70 mass% carbon sample, MS2, and iron-4.26 mass% carbon sample, MS3, in contact with all experimental substrates, the variations of contact angles with time are plotted as shown in Figs. 3 and 4, respectively. A gradual decrease in the contact angles occurs in the initial 300 s of contact, and then the contact angles maintain constant values. These results show similarities in behaviour to that of MS1 wetting on SN0, SN5, and SN10 substrates.
Dynamic wetting of MS1 sample on simulant coke substrate.
Dynamic wetting of MS2 sample on simulant coke substrate.
Dynamic wetting of MS3 sample on simulant coke substrate.
From these results, the reactive wetting behaviour of the iron samples on the substrates can be divided into two periods. The first period, or early stage, is defined as the contact time of 0 to 300 s. In this period, the contact angles gradually decrease from an initial value. The second period is defined from 300 s to the end of the observation period, referred to as the constant stage, when the contact angle values remain apparently unchanged.
During the early stage, the Al2O3 amount in the substrate obviously influences the reactive wetting behaviour between the carbon-unsaturated iron and the substrate, which causes differences in contact angle values between the substrates. The contact angle value in the early stage decreases with increasing Al2O3 amounts in the substrate. Our previous study2) reported the negative effect of Al2O3 on the rate of carbon dissolution. The study showed that the difference of carbon dissolution rates was related to the amount of Al2O3 in the substrate, which affected the effective reaction area ratio at the reaction interface. Therefore, during the early stage in this study, it is thought that the reactive wetting behaviour depends on the amount of Al2O3 in the substrate, which influences the carbon dissolution reaction rate until carbon reaches saturation in the iron sample.
During the constant stage, the results show apparent equilibrium contact angles of 82, 87, and 90° when the MS1, MS2, and MS3 samples are wet on the substrates, as shown in Figs. 2, 3, and 4, respectively. No obvious difference in apparent equilibrium contact angle occurs by varying the amount of Al2O3 in the substrates.
From above discussions, it was regarded in this study that the early stage and the constant stage are the carbon unsaturated stage and the carbon saturated stage, respectively.
Figure 5 shows optical micrographs of the interfacial region between the SN10 substrate and the iron samples of MS1, MS2, and MS3. Figure 6 shows images of this area for SN0, SN5, and SN10 substrates in contact with the MS1 iron sample. Concave geometries are clearly formed at the interfacial regions in all cases.
Optical microscope images of concave formation at the interface area between MS1, MS2, and MS3 samples and SN10 substrate.
Optical microscope images of concave formation at the interface area between MS1 sample and SN0, SN5, and SN10 substrates.
The interfacial region images show that carbon has obviously dissolved from the substrates into the iron samples without residual ash in the reacted layer, and concave formations have developed as shown in Figs. 5 and 6. The concave formations have been thought to depend on the amount of carbon dissolved into the iron sample from the substrate. Therefore, it is necessary to estimate the amount of carbon dissolution to evaluate the relationship between this amount and the formation of concave regions. However, it is difficult to directly determine the amount of dissolved carbon, because the interface between iron sample and substrate has a particularly complicated shape. This impedes the clear separation of the iron sample from the substrate. Thus, the method of image analysis was applied to calculate the amount of carbon dissolution from the images, similar to our previous method.2) Equation (2) was used to calculate the amount of carbon dissolution.
Estimation of carbon dissolution amount from optical micrograph at interface.
The effects of the Al2O3 amount in the substrates and the initial carbon content of the iron samples on the carbon dissolution amount are shown in Fig. 8. The amount of carbon dissolution decreases with increases in the Al2O3 amount and in the initial carbon content of the iron sample. This trend of varied carbon dissolution with the initial carbon concentration of the iron samples agrees well with previous studies.2,3,4,8,11) From these results, Al2O3 is concluded to impede carbon transport phenomena from the substrate to the molten iron sample; larger initial carbon contents in the iron sample also cause decreases in the carbon absorption capacity of the iron sample.
Relationship between Al2O3 amount in the substrate and carbon dissolution amount.
Interfacial energy values between the liquid and solid phases are calculated at the moment of time defined as 0 of contact between the iron sample and the substrate, which is the initial interfacial energy between the liquid and solid phases. The effect of carbon dissolution on the macroscopic morphology is assumed negligible at this moment. The triple line formed by the intersection of the iron liquid sample, the solid substrate, and the gas phases has moved on the flat surface of the substrate. In this case, Young’s equation (Eq. (3)) can be applied to calculate the interfacial energy between the liquid and solid phases.21,22)
To calculate the value of γLS, the values of γLV and γSV in Eq. (3) must be determined. The value of γLV is estimated for the iron sample. It is assumed to be constant with varied carbon content in the iron-carbon liquid phase.23,24,25) The value of γLV is 1.73 J/m2, which is the literature value at 1673 K.26) Secondly, the values of γSV in Eq. (3) are estimated for the SN0, SN5, and SN10 substrates. We assume that the interfacial energy between the solid and vapour phases is a combination of the interfacial energies between the solid and vapour phases of graphite and Al2O3 components in the substrate and that the interfacial energies between pores and the vapour phase is negligible. Therefore, the values of γSV of the substrates are obtained from Eq. (4).
Evaluation of the occupied Al2O3 area on the surface of the simulant coke substrate from image analysis of cross-sectional observations.
Because of concave formations at the points of contact between carbon-unsaturated liquid iron and the simulant coke substrate, Young’s equation could not be applied. Additionally, the formation of concave geometry is observed clearly at the contact area between the substrate and carbon-unsaturated iron, as shown in Figs. 5 and 6. Hence, Neumann’s equation, as shown in Eq. (7), is applied in this reactive wetting case.21,26,28)
Schematic of the geometry at the triple position.
From the above calculations of the initial and equilibrium interfacial energies between liquid and solid phases, the subtraction from the initial interfacial energy to the equilibrium interfacial energy were determined, defined in this study as the variation in interfacial energy. Figure 11 shows the relationship between the variation in interfacial energy and the amount of Al2O3 in the substrate. Larger amounts of Al2O3 in the substrate cause lager variations in the interfacial energy.
Relationship between interfacial energy variation and Al2O3 amount in the substrate.
Both the carbon dissolution reaction and the interfacial energy variation are affected by the amount of Al2O3 in the substrate. Al2O3 affects the formation of concave regions at the interfacial area between the carbon-unsaturated liquid iron and the simulant coke substrate.
When carbon dissolution occurred from the substrate to the iron droplet, the Al2O3 particles moved from the substrate into the iron droplet. Once the Al2O3 particles had moved from the substrate, the particles did not accumulate on the surface, and on the interface between the substrate and the iron droplet or on the triple line, as shown in Figs. 5 and 6. Additionally, the moved Al2O3 particle’s volume from the substrate is less than 1 vol.% in the iron droplet. Therefore, the Al2O3 particles had no effect on the wetting behaviour after having moved from the substrate.
The initial contact angles could be measured without interactions between the iron sample and the substrate during the heating process before the contact angle measurement started, because the sessile drop method with the molten sample injection system prevented these interactions.
Figure 12 shows the variation of initial contact angles with the Al2O3 amount in the substrates for iron samples MS1, MS2, and MS3 wetting at 1673 K. The initial contact angles depend not only on the carbon concentrations of the iron samples, but also on the amount of Al2O3 in the substrate. The initial contact angles are increased with increases in the carbon concentration of the iron samples. As a result, the initial contact angle also correlates with the amount of Al2O3 in the substrates. Figure 12 shows almost linear increases in the initial contact angles with increasing the amount of Al2O3 in the substrates in wetting the substrates by iron samples with equal carbon contents.
Variation of initial contact angle with Al2O3 amount in the substrate for the iron samples.
The reactive wetting behaviour of the iron samples and the substrates show that the apparent equilibrium contact angle is unchanged for iron samples with the same carbon contents wetting substrates with different Al2O3 amounts, as shown in Figs. 2, 3, and 4. The apparent equilibrium contact angles are 82, 87, and 90° in the wetting of iron samples MS1, MS2, and MS3 on the substrates, respectively.
However, the amount of carbon dissolution and the interfacial energy are changed with the amount of Al2O3 in the substrate, as shown in Fig. 8. These changes suggest that the relationship between the apparent equilibrium contact angle and the carbon dissolution behaviour from the substrate to iron sample would be complicated rather than direct. This relationship may be understood by investigating the dihedral contact angles.
The relationship of the dihedral contact angle θL with the amount of Al2O3 in the substrate is plotted in Fig. 13. Larger amounts of Al2O3 in the substrate cause decreases in the dihedral contact angles. This trend could be related to the decrease in concave geometry volumes, caused by the decreased amount of carbon dissolution, as shown in Fig. 8. The carbon dissolution amount decreases, decreasing θL for substrates SN0, SN5, and SN10 by the iron samples. This indicates that larger Al2O3 amounts in the substrates cause the θL between the iron samples and the substrates to decrease. Although the relationship between concave formations and θL remains to be clarified, this will be addressed in a future work.
Relationship between Al2O3 amount in the substrate and dihedral contact angle.
To understand the role of Al2O3 in the reactive wetting behaviour and the formation of concave geometry at the iron-coke interface in blast furnaces, the wetting behaviour of carbon-unsaturated liquid iron samples and simulant coke substrates was investigated using the sessile drop method with a molten sample injection system. This study revealed the following results.
The apparent contact angle experienced a significant decrease over the first 300 s of contact between liquid iron and simulant coke substrate. The contact angle stabilized at a constant value, apparently equilibrium contact angles, of 82, 87, and 90° in the wetting of iron samples containing 3.27, 3.70, and 4.26 mass% carbon, respectively, with substrates containing less than 10 vol% Al2O3.
The initial contact angles increased with increasing carbon concentrations in the iron samples. The initial contact angle also correlated with the amount of Al2O3 in the substrates. The initial contact angles increased almost linearly with increasing amounts of Al2O3 in the substrates.
Concave geometries formed when the carbon-unsaturated liquid iron was wetted on the substrates containing 0, 5, and 10 vol% Al2O3. The effect of Al2O3 on the carbon dissolution reaction was the main factor affecting the wettability of the simulant coke substrates against the iron samples; the wettability decreased with increasing amount of Al2O3 in the substrates.
The authors express their gratitude for the scientific advice from the Research Group for “Optimization of Transport Phenomena for Low Carbon Blast Furnace” in ISIJ, and their thanks to experimental help from Prof. Kunihiko Nakashima and Prof. Noritaka Saito of Kyushu University.