Fundamentals of High Temperature Processes
Improvement of Filtration Performance of Foam Ceramics Using External Electric Field
2021 年 61 巻 8 号 p. 2317-2319
詳細
2021 年 61 巻 8 号 p. 2317-2319
To enhance filtration performance of foam ceramics, effects of electric field on filtration performance of foam ceramics were studied. Results show that external electric field can change the wettability between melt and foam ceramics and can improve filtration performance of foam ceramics. Meanwhile, chemical composition of melt is not affected by external electric field.
With development and maturity of foam ceramics production technology, foam ceramics have been widely used in many fields, such as flue gas filtration, sewage purification, melt filtration, fire resistance, and heat insulation.1,2,3,4,5,6) Using melt filtration as an example, it has a large specific surface area and contributes to formation of the cake effect because of the three-dimensional pore structure inside the foamed ceramics. Foam ceramics made from Al2O3 and kaolinite were first invented as early as 1978, and these foamed ceramics were first used in aluminum alloy casting and filtration.7) In recent years, foam ceramic filter materials have also been used in the field of melt purification nonferrous metals and small castings.8,9,10,11,12) However, there are still some problems in applications of foam ceramics that must be improved. For example, when filtration efficiency of foam ceramics is lower, it is easy to cause melt condensation and to create a blockage at the filter outlet. Low strength of a ceramic structure leads to the structure to break and fall into the melt to form new inclusions. Also, a long time filtration leads to decreased filtration performance. With gradual optimization of production technology and improved preparation performance of foam ceramics, problems such as condensation blockage and ceramic shedding must be effectively solved.13,14) However, research progress on how to maximize filtration performance of foam ceramics is relatively slow. This problem not only restricts use efficiency of foam ceramics but also limits wider application and development of foam ceramics in the field and process of high temperature melt purification.
External electric fields are an auxiliary application technology that has been widely used in many material processes and manufacturing fields.15,16,17,18,19) For example, in the field of regulating and controlling inclusions, the movement state of inclusions can be well-controlled and changed using an external electric field.20,21,22,23) If an external electric field is introduced into foam ceramics filtration, can it enhance the filtration ability of foam ceramics filtration?
On the basis of the above background, the influence of an external electric field on the inclusion filtration properties of foam ceramics filtered steel was investigated in this paper. It is hoped that the relevant results can provide new methods and a research basis for improving filtration performance of foam ceramics and will make some contributions to further improving overall filtration performance of foam ceramics in the future.
The experimental process and corresponding filter model are shown in Fig. 1. In experiment, first, a pre-installed filter device was placed in a muffle furnace that was used for preheating and heat preservation to avoid the direct solidification of molten steel because of large temperature differences. The corresponding preheating temperature was 1000°C, and argon gas was introduced to protect the device from oxidation. Then, the raw material was quickly heated in the induction heating furnace for 60 s to make the raw material melt completely. The composition of the steel that was used in the experiment is shown in Table 1. To improve the experimental effects, analytically pure powder of MnS (3% in total) was added to the steel as inclusions. When the molten steel was completely melted and the composition was homogeneous, the molten steel was poured directly into the graphite crucible of the foam ceramics filter plate (length is 100 mm, width is 100 mm, height is 20 mm, and the unit density of pores is 20 PPI (pores per linear inch)). The molten steel was then cooled to room temperature after it was filtered using foam ceramics. Finally, the molten steel that was obtained after filtration was sampled and analyzed to investigate the effects that the electric field had on the filtration performance of foam ceramics.
Experimental process and device diagram. (Online version in color.)
| C | Si | Mn | P | S | Mg |
|---|---|---|---|---|---|
| 3.56 | 2.87 | 0.29 | 0.04 | 0.014 | 0.02 |
Figure 2 show the internal mass and distribution of inclusions in the steel that was obtained using different filtration conditions. Compared to the steel that was obtained with an electric field, the number of inclusions in the steel is still relatively large under normal casting and filtration conditions; also, the size of inclusions is relatively large. However, after using an electric field to assist foam ceramics filtration, the quality of the steel was significantly improved. After solidification, the number of inclusions in the steel decreased obviously, and in particular, large-sized inclusions are almost gone.
Distribution of inclusions after casting and filtration. (Online version in color.)
As seen in Fig. 3, with the application of an external electric field, the number density of inclusions or areal proportion in the steel after filtration is obviously improved compared with the corresponding values for the steel obtained with normal treatment. The average number density of inclusions decreased from 64.56 to 46.11 (in 4 × 10−3 cm2), and the relative number density of inclusions decreased by 28.58%. Also, the areal proportion decreased from 0.0376% to 0.0187, which is a decrease of 50.27%. This further confirms that the filtration ability of ceramics was significantly improved with the application of an electric field and that a large number of inclusions (especially large-sized inclusions) were effectively filtered out.
Average number density and average proportion of inclusions after filtration. (Online version in color.)
As seen in Fig. 4, the composition of inclusions in the steel did not change significantly, whereas the size and number of inclusions decreased. The main component of inclusions was still MnS, and there was a certain amount of carburization. This is mainly because the graphite in the crucible that was used in the melting and filtering process of the molten steel in this experiment dissolve into the molten steel. However, in the actual continuous casting process, the cooling rate of the molten steel was faster, and the molten steel did not contact the refractory, which contained carbon, for a long time; this will not affect the composition of molten steel.
Composition and morphology of residual inclusions in steel. (Online version in color.)
From the above results, it is seen that the external electric field effectively improves foam ceramics filtration performance and does not have any effect on the composition of molten steel.
The composition of molten steel is not affected under an electric field, and this indicates that an electric field only improves the filtration performance of SiC foam ceramics. The electro-wetting behavior of SiC at high temperature was analyzed to explore the reason that an electric field improves the filtration performance of foam ceramics. The results are shown in Fig. 5.
Changes in contact angle for a SiC substrate and steel under an electric field.
As seen in the Fig. 5, in the absence of an external electric field (voltage of 0 V), the contact angle was 132° after the molten steel spread and stabilized on the SiC substrate for a long time. With an increase in electric field strength, wettability between molten steel and SiC gradually improved. In particular, when the applied voltage was less than 5 V, the effect that an electric field has on the wetting effect was most obvious, and at this point, the contact angle decreased to 116°. Thus, compared with conventional wetting behavior, there will be a certain electric field force at the interface during the electric wetting process, and the corresponding force can be roughly expressed as shown in Fig. 6.24) The improved wettability increases the contact area between molten steel and the ceramic matrix in the process of flowing foam ceramics to enhance the adsorption capacity of foam ceramics.
Schematic diagram of changes in interfacial wetting behavior with an applied electric field. (Online version in color.)
However, with increased voltage, the free energy of the interface that forms via charge accumulation approaches the limit and approaches equilibrium with the surface energy of the solid-liquid phase; this causes the electro-wetting to gradually become saturated. Meanwhile, because of influences of material properties and external environments, the solid-liquid contact angle under an electric field also has a threshold value. Finally, the influence that electric field strength has on wettability gradually decreases or does not change. Under the experimental conditions, when the applied voltage reached 15 V, the contact angle decreased to 103°.
4.2. Changes of Inclusions Flowing through Foam Ceramics after Applying an Electric FieldAs seen in the Fig. 7, under normal conditions, when the foam ceramics filter that has inclusions is placed in molten steel, the molten steel flows through the pores of the foam ceramics. In this stage, the inclusions that flow through and make contact with the foam ceramics are intercepted and adhere to the macroporous surface of the foam ceramics. However, these pores also contain a large number of micropores. The molten steel cannot make contact with these tiny pores because the wettability between molten steel and foam ceramics is poor. Therefore, the filtration effect is low. Also, with accumulation of inclusions on foam ceramics during the filtration process, these micropores are blocked by adherent inclusions. With an increase in the number of inclusions that are absorbed by ceramics, the pore size of foam ceramics gradually decreases, and the surface of the ceramic becomes rough. These changes also reduce the contact between the ceramics and molten steel. Finally, the filtration effect of the ceramics gradually decreased.
Changes of inclusions flowing through foam ceramics after applying an electric field. (Online version in color.)
When an electric field is applied to foam ceramics, filtration capacity of foam ceramics is improved. Because the interface energy and interfacial tension between molten steel and foam ceramics are changed by the application of an external electric field, contact and wettability between molten steel and ceramics are improved.24,25) Although the wettability between molten steel and ceramic is still poor (as shown in Fig. 5, contact angle > 90°), but these results are measured statically. However, in the actual process, there are not only occur the action of electric field force but also has the flow pressure of molten steel. Under these conditions, the interface between molten steel and ceramic may be contacted and wetted. Therefore, more molten steel maybe can flow through and make contact with pores in the ceramic. With the increase of contact area, contact distance, and contact time between molten steel and foam ceramics, more inclusions enter and contact these pores. This beneficial effect increases the probability of more and more inclusions adhering to the ceramic surface and improve the filtration efficiency of foam ceramics. Moreover, the newly formed clogging structure does not affect the contact area between molten steel and the ceramics. Finally, after an electric field is applied, the filtration efficiency of ceramics can be guaranteed for the entire casting process. Therefore, as mentioned in previous results, foam ceramics filtration effect can be effectively improved after electric field is applied.
(1) An external electric field effectively improves foam ceramics filtration performance and does not have any effect on the composition of molten steel and inclusions.
(2) An external electric field changes the wettability between molten steel and foam ceramics, improving contact between molten steel and ceramics. More molten steel can flow through and make contact with pores in the ceramics.
(3) Although an electric field can change the wettability of materials, wettability is affected by many factors. Thus, when an electric field is increased too much, the influence that electric field strength has on wettability is gradually decreases or does not change.
The authors would like to express their gratitude for the financial support from the National Natural Science Foundation of China (Grant No. 51974074, No. 51874083, No. 51932008, No. 52074070), and the Fundamental Research Funds for the Central Universities (N2025036).
