Sintered magnesia brick is one of the magnesia refractory materials with periclase as the main crystal phase. Because of its high refractoriness, good resistance to alkaline slag and iron slag such as CaO, FeO, etc., it is widely used in converters and electric furnaces, cement rotary kiln and glass kiln regenerator, etc. However, the thermal shock resistance of sintered magnesia bricks is poor, and it is usually damaged after 3 to 6 times of thermal shock in water at 1100 ℃. Studies have shown that CaZrO3 has good refractory properties,adding it to refractory materials can improve the thermal shock resistance,high temperature resistance. CaTiO3, as a refractory material with excellent high-temperature thermal stability and high-temperature mechanical properties, which is less studied at present, has high-temperature resistance similar to CaZrO3. In this study, with reference to the literature , the effect of CaTiO3 addition on the properties of sintered magnesia bricks was studied.
1. 1 Synthesis and detection of CaTiO3 powder
Take w(CaC03)=99% analytically pure calcium carbonate and w(TiO2)=95% titanium dioxide as raw materials, mix uniformly according to n(Ca):n(Ti):1, and add 3%(w) of total mass Polyvinyl alcohol is used as a binding agent. After being calcined at 1550 ℃ for 2.5 hours, CaTiO3 is synthesized. Then the sintered sample is crushed as CaTi03 fine powder ≤0.076 mm (200 mesh), and XRD is used for qualitative and quantitative analysis. Analyze the microscopic morphology with SEM scanning electron microscope.
1.2 Production and performance testing of sintered magnesia brick
1.2. 1 Sample production
The raw materials for producing sintered magnesia brick samples are sintered magnesia particles (with a particle size of 3-1, ≤1 mm) as aggregates, sintered magnesia fine powder with a particle size of ≤0.088 mm and CaTi03 fine powder with a particle size of ≤0.076mm as the matrix. Use paper pulp waste liquid as binding agent. The chemical composition of the test raw materials is shown in Table 1.
Table 1. Chemical composition of raw materials
Add different contents of CaTi03 fine powder to the sintered magnesia brick according to Table 2, and observe the performance changes before and after adding calcium titanate to the magnesia brick. When compounding, premix the aggregates with a particle size of 3-1, ≤1 mm and 3% (w) pulp waste for 10 minutes, and then add sintered magnesia fine powder and CaTiO3 fine powder in sequence, and then mix for 10 minutes. The mixed materials are pressed into a strip sample of 130 mm × 40 mm × 35 mm at 500 MPa under a four-column hydraulic press DSBS-200A (maximum working pressure 1 800 N), and then the sample is placed in a KYH high temperature oxidation resistance test furnace It is sintered at 1580 ℃ for 4 hours, and then cooled to room temperature with the furnace.
Table 2.Experimental formula
1.2.2 Performance testing
After burning at 1580℃, test the porosity and bulk density according to CB/T 2997-2000, measure the compressive strength at room temperature according to GB/T 5072.1-2008, and according to YB/T 2206.2-1998 (water cooling method) to measure thermal shock resistance, measure the starting temperature of load softening on the HRY-02 high temperature softening tester according to YB/T 370-95.
The static crucible method was used to carry out the slag penetration resistance test (characterized by the percentage of penetration area) at 1580 ℃ for 3 hours. The chemical composition (w) of the test slag was: CaO 36.81%, A12O3 19.05%, SiO2 13.6%, Fe2O3 12.12%, MgO 8.99 %, F 5.26%, MnO 2.76%, P2O5 0.69%, TiO2 0.31%, Cr2O3 0.14%, WO3 0.13%. The German Zeiss scanning electron microscope was used to analyze the microstructure of the anti-slag sample.
2 Results and discussion
2.1 Phase composition and microstructure of synthetic CaTi03 powder
The XRD pattern of the synthesized CaTi03 powder is shown in Figure 1. It can be seen from Figure 1 that the main crystal phase of the synthesized calcium titanate is CaTi03 phase, which is relatively pure. The content of CaTi03 phase is roughly calculated by XRD refinement and semi-quantitative analysis to be 93% (w).
Figure1.XRD pattern of synthesized CaTiO3 powder
The SEM photo of the synthesized CaTiO3 powder is shown in Figure 2. It can be seen that the crystal grain morphology of the synthesized CaTiO3 is smooth curved or cubic, and the crystal grain size is about 4 μm.
Figure 2. SEM picture of synthesized CaTiO3 powder
2.2 The influence of the amount of synthetic CaTi03 powder added on the performance at room temperature
Figure 3 shows the influence of the amount of CaTiO3 powder added on the volume density, apparent porosity and compressive strength of the sample after firing.
It can be seen from Figure 3 that when the amount of CaTi03 powder added is 1% to 5% (w), the bulk density and compressive strength at room temperature of the sample increase, and the apparent porosity decreases, but the increase is slower than that afterwards. . The possible reason is that with the addition of CaTiO3 powder, CaO-Ti02 produces a liquid phase at high temperatures (as can be seen from the phase diagram of the Ca-Ti binary system, perovskite produces a liquid phase at 1475 ℃) to promote the sintering of the material. In addition, due to the large thermal expansion coefficient of MgO, volume expansion occurs during the sintering process, filling pores, resulting in a decrease in apparent porosity, an increase in bulk density and compressive strength at room temperature; when the amount of CaTiO3 powder added is 5% to 7% (w), the bulk density and compressive strength increase significantly, and the apparent porosity decreases. When the amount of 7% (w) is added, the apparent porosity is the smallest. The possible reason is: when adding 7% (w) CaTiO3 powder, the content of calcium titanate increases, and the amount of liquid phase increases accordingly. The volume expansion of MgO is accompanied by micro-cracks inside the magnesia brick. Because the density of calcium titanate is higher, the titanium dioxide produced at high temperature migrates down the crack pores and fills the pores, so the performance of the sample changes significantly; when the addition amount exceeds 7% (w), the bulk density and compressive strength are small, and the apparent porosity increases. The possible reason is that too much calcium titanate generates too much liquid phase, which causes the tendency of microcracks due to the volume expansion of MgO to be larger than the filling pores, and microcracks dominate.
2.3 The influence of the amount of synthetic CaTiO3 powder added on thermal shock resistance
Observing the appearance of the samples after thermal shock, it was found that after 4 times of 1100℃ water-cooled thermal shock, each sample had microcracks, and after 6 times, the front and side of each sample were almost completely damaged. Among them: the test without CaTiO3 powder the damage of the sample is more serious. There are large cracks in the horizontal and vertical directions of the brick, and the overall damage is serious; with the increase of the content of CaTiO3, the damage of the pattern is slightly alleviated, and it can be seen that the damage of the brick body gradually deviates from the whole to one end. Although the sample is damaged when the powder is added at 7% (w), the overall damage of the brick body is lighter, and the thermal shock resistance is relatively good. The reason for the analysis may be: the thermal expansion coefficient of periclase is larger than that of calcium titanate. The larger the thermal expansion coefficient, the greater the thermal stress generated by thermal shock and the worse the thermal shock resistance. It is also due to the thermal expansion coefficient between materials the difference causes micro-cracks in the sample, which can release the internal thermal stress of the material; with the increase of the number of thermal shocks, the stress concentration effect is relieved, and the cracks no longer expand. Secondly, calcium titanate contains some TiO2. The thermal conductivity of TiO2 is relatively high, which can appropriately improve the thermal shock resistance of the sample.
2.4 The influence of the amount of synthetic CaTiO3 powder added on the anti-slag penetration performance
The cross-sectional photo of the slag-resistant sample after burning at 1550℃(heat preservation 3h,1580℃) is shown in Figure 4. It can be seen that the percentage of slag penetration area of each sample: 0# sample is about 30%, 1# sample is about 36% , 3# sample is about 45%, 5# sample is about 50%, 7# sample is about 70%, 9# sample is about 85%. Therefore, the order of the degree of penetration is: 9#sample>7#sample>5#sample>3#sample>1#sample>0#sample.
Figure 5 shows the scanning electron micrograph of the sample after partial anti-slag. It can be seen that the 0# and 1# samples have a small amount of calcium titanate added, and the liquid phase required for the reaction is mainly provided by some oxides in the brick. An appropriate amount of liquid phase promotes the sintering of the magnesia brick. The added CaO and TiO2 are due to the density is relatively large, distributed along the liquid phase in the boundary and matrix of the sintered magnesia particles, filling the internal pores of the sample, so the bulk density and compressive strength increase; with the further addition of calcium titanate, excessive CaTiO3 the inside of the brick body hinders the solid phase sintering between the mass points. When the amount of liquid inside the sample is too much, it also reduces the adhesion inside the sample, which is not conducive to the sintering of the sample. In addition, with the expansion of the volume of magnesia, the microcracks inside the sample increase, which leads to the penetration of steel slag along the pores. With the addition of CaTiO3, the penetration effect gradually increases.
2.5 The influence of the amount of synthetic CaTiO3 powder added on the starting temperature of load softening.
Fig. 6 shows the change of the starting temperature of the sample load softening after firing with the addition of calcium titanate. It can be seen that with the increase of the replacement amount of CaTiO3 powder, the load softening start temperature of the sample first increases and then decreases. The load softening start temperature is the highest when the addition amount is 1% (w). The possible reason is that the amount of liquid phase in the magnesia brick is mainly provided by Al2O3, CaO, SiO2 and other harmful oxides when the added amount is relatively small, which promotes the sintering of the magnesia brick and enhances the compactness of the magnesia brick. For a long time, due to the high temperature decomposition of CaTiO3 powder into CaO and TiO2, the amount of liquid phase inside the sample is too much and the viscosity of the liquid phase is reduced, so the load softening of the sample begins to decrease, so that the sample has a wide range of load deformation temperature.
(1) Adding an appropriate amount of synthetic CaTiO3 powder can increase the volume density and compressive strength of the sample at room temperature, and reduce the apparent porosity.
(2) With the increase in the amount of CaTiO3 powder added, due to the difference in thermal expansion coefficients of calcium titanate and sintered magnesia, the thermal shock resistance of the material after adding CaTiO3 powder is improved, and the addition amount is 7% (w). .
(3) As the amount of CaTiO3 powder added increases, the slag penetration resistance of the sample becomes worse.
(4) After adding CaTiO3 powder, the load softening starting temperature of the sample increases first and then decreases. The effect is best when 1% (w) is added.