Taphole clay has been subjected to higher demands due to the development of blast-furnace smelting technologies. Taphole clay is no longer a consumable refractory but a functional one. It is not only responsible for sealing the taphole but also protecting the bottom side walls of furnaces and maintaining the stability of tapholes [1,2,3,4]. During operation, molten iron-slag constantly scour the taphole channel as well as the side walls. Taphole clay that is easily eroded or lost will damage the side walls and bottom of the furnace. It may also cause rapid expansion of taphole channel. Taphole clay must have good mechanical properties and corrosion-resistant. TiO2, a common raw material for refractory materials, is used widely in many of them [5,6]. The titanium ore furnace protective technology is becoming more mature, and it has become a standard method for blast furnace operation. The introduction of TiO2 to the taphole-mud system has become a hot topic of research in taphole-mud, and titanium containing taphole-mud is being gradually used in different blast furnaces in major steel mills. There are very few reports about the effects of TiO2 in taphole-mud, despite its widespread use. This study examined the effects of TiO2 in Al2O3-SiC and C taphole mud on its density, mechanical properties as well as oxidation resistance and slag resistant.
1 Test:
The following raw materials were used for the test: brown corundum, rutile TiO2, clay (particle sizes =0.074mm), sericite (particle sizes =0.045mm), blast furnace slag (particles size =0.088mm), environmentally friendly tar and ball pitch. The Table 1 shows the chemical composition of main raw materials.
The aggregates with a particle diameter greater than 1mm were milled and mixed in a mixer during 1 minute. After that, 1/3 of environmentally friendly tar, which was previously mixed, was added, mixed, and milled again for 5 minutes. Next, the remainder of pre-mixed materials were added, mixed, and milled again for 3 minutes, and then the remaining environmentally friendly material was added, mixed, and milled again for 10 minutes, before the material was discharged. The Marschall value was determined according to literature [9] to determine the plasticity. The mixture was pressed at 30 MPa into strip samples of 140 mmx25mmx25mm, cylindrical samples of ph50mmx45mm, and crucibles with an outer diameter ph50mmx45mm and an interior diameter ph20mmx20mm. The samples were removed from the oven after 24 hours of drying at 200 degrees Celsius.
Io=So/S
Where: Io represents the oxidation index (%); So represents the sum of the area of the oxidation and decarburization layers, in mm2; and S is total cross-sectional surface area, in mm2. In an atmosphere of air, 12 g blast furnace slag was placed into a dried crucible. Its chemical composition can be found in Table 1. After cooling in the furnace, samples were cut along the central axis to the crucible's hole symmetrically. This was done to observe erosion and penetration by the slag. SEM (PHILIPS 30 TMP) was used to observe the microstructures of the etched sample. The X-ray EDS spectrometer was then used to analyze the chemical composition and element distribution of the micro-regions.
2 Results and discussion
The Marshall value of taphole clay
The effect of TiO2 addition on the Marshall value of taphole clay is shown in Figure 1. It can be seen that with the increase of TiO2, the Marshall value of cannon clay shows a slightly wave-like downward trend, but the change range is small.
Effect of TiO2 addition on the Marshall value of taphole clay
Linear change after firing
The effect of TiO2 addition on the linear change rate of samples after heat treatment at different temperatures is shown in Figure 2. It can be seen that: after heat treatment at 1100℃, shrinkage occurred, and after heat treatment at 1450℃, expansion occurred, and the linear shrinkage rate and linear expansion rate increased with the increase of TiO2.
Effect of TiO2 addition on the linear change rate of samples after heat treatment at different temperatures
Heat-treated samples have a higher density and greater strength
Figure 3 shows the effect of TiO2 on volume density and apparent porousness of samples after heat treatments at different temperatures. Figure 3 shows that for the samples treated at 1100, the volume density first increases, then decreases with an inflection at TiO23%(w); the apparent porosity decreases gradually. After heat treatment at 1400, the volume density decreases, and the apparent porosity increases. When TiO2 remains the same, apparent porosity is lower in the 1450 heat treated samples than it is for the 1100 heat treated samples.
The cross-section photos of each sample after the anti-erosion test are shown in the figure. It can be seen that no obvious slag erosion and slag penetration occurred in each sample, and the pore shape of the crucible was basically consistent with its initial shape (the part marked by the red line), indicating that they all have good resistance to slag erosion and slag penetration.
During the heat treatment at 1 100 and 1 450 ℃, the sample sintered under the action of high temperature and liquid phase, resulting in densification and shrinkage of the sample[12]. At the same time, the expansion effect of the mullite reaction of clay, sericite, kyanite, etc. in the sample caused the sample to expand. An appropriate amount of expansion can squeeze the pores around the in-situ mullite and improve the density of the sample; however, the excessive formation of in-situ mullite will lead to excessive expansion, causing the in-situ mullite to be cracked and cracks to appear[13], which in turn reduces the density of the sample. During the heat treatment at 1 100 ℃, the sample eventually showed shrinkage, probably because sintering shrinkage played a dominant role; with the increase of TiO2, the amount of liquid phase in the sample increased, the post-sintering shrinkage increased, the apparent porosity decreased, and the room temperature compressive strength and room temperature flexural strength increased. During the heat treatment at 1 450 ℃, the sample eventually showed expansion, probably because the expansion caused by mullite played a dominant role. As the amount of TiO2 added increases, the amount of mullite generated in the sample increases (see Figure 8), the expansion effect increases, and therefore the expansion after firing increases. The decrease in density caused by the increase in microcracks exceeds the increase in density caused by liquid phase-promoted sintering, which ultimately leads to an increase in the apparent porosity of the sample with the increase in TiO2. This may be because the increase in the number of needle-shaped mullite improves the mechanical properties of the sample more than the decrease in the mechanical properties of the sample caused by the decrease in density, which ultimately leads to an increase in the room temperature compressive strength and room temperature flexural strength of the sample with the increase in TiO2.
Therefore, the addition of TiO2 will be beneficial to the improvement of the erosion resistance of Al2O3-SiC-C taphole clay, and at the same time, it will help maintain the stability of the iron mouth by generating Ti(C,N) in situ.
Conclusion
(1) After heat treatment at 1 100℃ for 3h, the samples all shrank, and their linear shrinkage increased with the increase of TiO2; at the same time, the density and room temperature strength of the samples also increased with the increase of TiO2. (2) After heat treatment at 1 450℃ for 3h, the samples all expanded, and their linear expansion increased with the increase of TiO2; the density of the samples decreased with the increase of TiO2; the room temperature strength of the samples increased first and then decreased with the increase of TiO2, reaching the maximum when TiO2 was 3% (w). (3) After adding TiO2, the oxidation resistance and slag erosion resistance of the taphole clay were slightly improved.
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