1 Introduction
As we all know, white fused corundum and sintered corundum are the main raw materials for the production of alumina bricks, especially high alumina bricks containing 90% A12O3. These two types of corundum have been widely used as raw materials in the past 10 years. However, it is difficult to find basic information on the performance of fired high alumina bricks containing fused silica and mullite in the existing scientific literature.
The purpose of this research work is to explore the influence of additives on the properties of fired high alumina bricks using fused corundum as raw material.
2 Experiments
In the research work, industrial fused corundum with a particle size of 0~5mm is used as the main raw material. The additives used are fused silica and mullite. The main chemical components of the raw materials are listed in Table 1. The content of Al2O3 in fused corundum and mullite is >99% and ~70%, respectively. The composition of refractory sample molding materials is shown in Table 2. The bonding agent used in the manufacture of bricks is M. In order to explore the influence of additives, the main raw materials used in the brick samples were mixed with 0-3β% fused silica and 0-3β% mullite, and then formed on a 500t friction brick press Experimental bricks. The bricks were dried at 150°C for 24 hours, and then fired in a tunnel kiln at 1450°C for 6 hours.
Table 1. Chemical composition of raw materials/%
After firing. The physical properties and thermal mechanical properties of the samples were measured. Such as volume density, open porosity, normal temperature pressure resistance and high temperature flexural strength. After repeated heating of the sample for 3 to 5 times, a supplementary high-temperature flexural strength test is performed. The sample fired at 1400°C was quickly cooled in water to determine the resistance to thermal cracking. In addition, the X-ray phase composition analysis method was used to study the microstructure and the X-ray spectroscopy electron probe microscopic analyzer was used for phase composition analysis.
Table 2. Composition of molding ingredients/%
3 Analysis results
3.1 The influence of fused silica additive
Figure 1 shows the volume density changes when the brick samples contain different amounts of fused silica. It can be seen from Figure 1 that the density of the test bricks (A1～A3) containing fused silica is lower than that of the test bricks (A0) using fused corundum as the raw material, and the density value decreases with the increase of the silica content. , When the silica content is greater than 2a%, there is almost no change in density.
Figure 1. The relationship between volume density and silicon dioxide content
Figure 2 shows the relationship between the porosity and the amount of fused silica added. The sample with silica (A1, A2) has an increased porosity. When the amount of silicon dioxide added is 3α%, the porosity of the sample drops significantly, even lower than that of the sample A0 without silicon dioxide.
Figure 2 . The relationship between open porosity and silicon dioxide content
Figure 3 shows the effect of adding fused silica on the pressure resistance. As the amount of silicon dioxide added increases, the pressure resistance tends to remain unchanged. This may be the result of changes in the microstructure and unevenness of the microstructure. As shown in Figure 4, some large pores or holes were found around the fused silica in the matrix. This indicates that a large volume expansion has occurred around the silica particles during the firing and cooling period, which will inevitably affect the changes in the microstructure. In addition, the concentration of stress in the area where the uneven expansion occurs in the base material results in an insignificant tendency for pressure resistance to change. However, as can be seen from Figure 5, the adhesion strength and compactness of the sample containing silicon dioxide are higher than that of the matrix A0, which is composed of only fine particles of fused corundum aggregate. In addition, the volume expansion of the silicon dioxide and the high adhesion strength between the silicon dioxide and the corundum raw material also affect the pressure resistance and the irregular changes.
Figure 3. The relationship between withstand voltage and silicon dioxide content
Figure 6 shows the relationship between the pressure resistance and the silicon dioxide content. The compressive strength of all samples containing silicon dioxide is higher than that of samples not containing silicon dioxide. According to speculation, this may be the result of a relatively large amount of silicon dioxide and other impurities in the matrix. With the increase in the number of additives, the pressure strength is slightly higher.
Figure 4. Microstructure around fused silica
Figure 5. Comparison of the microstructure of the sample without silicon dioxide (A0) and the sample with silicon dioxide added
Figure 6. The relationship between pressure resistance and silicon dioxide content
Figure 7 shows the diffraction diagram of the influence of fused silica additives. as illustrated . With the increase in the number of additives, the spectral line intensity of the α-crystalline phase gradually increases. Obviously, the fused silica, which is in the initial phase of amorphous phase, undergoes phase transformation during the sintering process at 1450°C. It is transformed into α-sided quartz, its volume swells, and the bond strength of the matrix increases.
Figure 7. Diffraction pattern of the sample
Figure 8 shows the change in high temperature flexural strength with the increase in the amount of fused silicon dioxide and after repeated heating and cooling cycles. It can be seen from Figure 8 that the samples (A1～A3) after heating and cooling cycles of different times have a tendency to increase in high temperature flexural strength with the increase of fused silica content, but the addition amount is 2 α% Exception (A2). This shows that the high-temperature flexural strength of refractory bricks using fused corundum as the raw material can be improved when fused silica is added. When the added amount is 3α%, regardless of the number of heating-cooling cycles, the high-temperature flexural strength can reach the highest index.
Figure 8. After different times heating and cooling cycles. The relationship between the high temperature flexural strength of the sample and the amount of silicon dioxide added
Figure 9. The relationship between thermal cracking resistance and the amount of silicon dioxide added
Figure 9 shows the effect of adding fused silica on the thermal cracking resistance. The method for measuring the thermal crack resistance is as follows: The sample is heated to 1400°C and then cooled in air in a cycle. After 5 cycles of this test, the material is studied. And make a comparative analysis of its surface. As shown in Figure 9, vertical cracks were found on the sides of the samples (A1 and A3). Obvious cross-shaped cracks were observed in the lower part. From the perspective of the width and length of the crack expansion, the order of the excellent thermal cracking resistance is as follows: A1> A2> A3. This shows that the amount of fused silica added has an effect. This can improve the thermal cracking resistance of refractory bricks made of fused corundum.
3.2 The influence of mullite
The experimental results show that the addition of mullite (Al2O3 content is 70%) will affect the volume density of the samples (A0, B1～B3), and it is related to the amount of mullite added (see Figure 10)
Figure 10. The relationship between volume density and mullite content
It can be seen from Figure 10 that when the mullite content increases. The bulk density of all samples gradually decreased. The sample with an additive amount of 3α% has the lowest volume density.
Figure 11 shows the change in porosity when the mullite content increases. With the addition of the number of agents increased from 15.3% to 16.9%. The porosity of all samples (A0, B1～B3) increased, and sample B3 had the highest porosity. This shows that compared with the sample (A0) using fused corundum as the raw material without additives, the addition of mullite (70% Al2O3) will increase the porosity.
Figure 11. The relationship between open air porosity and diarylite content
Figure 12 shows the change in pressure resistance of samples with different mullite content. It can be seen from Figure 12 that the index value of sample B1 (β%) is significantly increased, while the index value of samples B2 (2β%) and B3 (3β%) slightly increases. Therefore, it can be considered as. After adding mullite, the compressive strength of the samples (Bl～B3) has been improved.
Figure 12. The relationship between pressure resistance and mullite content
Figure 13 shows the influence of the amount of mullite added on the flexural strength. All samples containing mullite (regardless of the amount of mullite added) have higher flexural strength than refractory bricks made of fused corundum (no additives). With the increase in the number of agents added, the flexural strength of the sample does not increase much. This is due to the reaction between the low melting point impurities in the mullite and the fine particles of the corundum aggregate, which increases the adhesion force in the matrix.
Figure 13. The relationship between flexural strength and laystone content
Figure 14 shows the change in the high temperature flexural strength of the sample when the mullite content increases and after the number of heating-cooling cycles.
Figure 14. After different heating and cooling cycles. The relationship between the high temperature flexural strength of the sample and the mullite content
It can be seen from Figure 14. After different heating-cooling cycles. The high temperature flexural strength of the samples (B1～B3) is higher than that of the samples without mullite. Among them, sample B1 has the highest high temperature flexural strength. When the added amount of mullite exceeds β% (sample B1), its high temperature flexural strength tends to decrease.
All the samples (A0, B1～B3) were analyzed by X-ray. As shown in Figure 15, the main phase is α-corundum, and no major changes have been observed on the diffraction diagram. This shows that when mullite was added, no new phases were formed or any other phase changes occurred.
Figure 15. Diffraction diagram of the sample
Figure 16 shows the microstructure analysis and energy dispersive X-ray analysis of the sample matrix. The figure shows the obvious difference in the SiO2 content of the material matrix. This can be explained as: when the content of SiO2 in the matrix increases, its compressive strength, flexural strength and high temperature flexural strength also increase.
Figure 16. Scanning electron microscope analysis of sample matrix and energy dispersion X-ray analysis
Figure 17. The relationship between thermal cracking resistance and pressure resistance at room temperature
The thermal cracking resistance of samples containing silicon dioxide has also been studied. However, no cracks were found on the surface of all the samples.
In order to evaluate the thermal cracking resistance, the sample was air-cooled and its pressure resistance was measured. At the same time, a comparative analysis was made on the pressure resistance of the samples that were not cooled by air. Figure 17 shows that. When mullite is added, the strength of the base material is increased, so that its resistance to thermal cracking is also improved. This is the same as when adding mullite to the pressure strength, flexural strength and high temperature flexural strength.
4 Conclusion
The effect of adding fused silica and mullite on the performance of refractory bricks based on fused corundum was studied. And get the following conclusion:
(1) When adding the best amount of fused silica. The mechanical properties of materials such as pressure resistance, flexural strength, and high-temperature flexural strength have all been improved.
(2) When the optimal amount of mullite (AlO, 70% content) is added, higher mechanical properties and higher thermal cracking resistance can be achieved.
