INFLUENCE OF PRE-SINTERING TEMPERATURE OF COARSE ANDALUSITE PARTICLES ON THERMAL SHOCK RESISTANCE OF MULLITE-CORUNDUM MATERIAL

The crystal structure of andalusite (Al₂O₃·SiO₂) belongs to the orthorhombic system, and the thermal expansion coefficient of his particles is characterized by anisotropy. At high temperature, it irreversibly transforms into mullite and SiO₂-rich glass phase, and its thermal expansion coefficient changes accordingly. During the mulliteization process, the crystal axis will change and become long columnar mullite crystals. The microcracks caused by the mismatch of thermal expansion coefficients in the sample will affect the thermal shock resistance of the sample, and the pre-sintering of andalusite particles can alleviate the above effect
Changing the pre-burning temperature can control the degree of mulliteization, and the thermal expansion coefficient of some mullite coarse particles will also change, which will affect the thermal expansion coefficient difference between the coarse andalusite particles and the matrix, thereby affecting the thermal shock resistance of the sample. In this work, 20% (w) coarse andalusite particles (granularity of 5-3 mm) pre-fired at 1300-1600 ℃ were added to the mullite-corundum refractory to explore the effect of andalusite pre-sintering temperature on the The effect of crack size was studied, and the effect of pre-sintering temperature on thermal shock resistance of mullite-corundum refractory was studied.
test
1.1 Raw materials
The raw materials are: South African andalusite coarse particles without pre-sintering and pre-sintering at 1300, 1400, 1500, 1600 ℃ for 3 hours, the particle size is 5~3mm, w(Al₂O₃)>57%, w(SiO₂)≈40 %; sintered mullite particles, particle size 3~1 and ≤1mm, w(Al₂O₃)≈69%; tabular corundum powder, w(Al₂O₃)>98%, particle size ≤0.044mm (325 mesh); active oxidation Aluminum powder, w(Al₂O₃)>99%, particle size ≤0.044mm (325 mesh); SiO₂ micropowder, w(SiO₂)>95%, particle size d50=100nm≤. The binder is pulp waste liquid.
1.2 Sample preparation
The sample formula (w) is: 5~3mm andalusite aggregate (not pre-fired or pre-fired at different temperatures) 20%, 3-1 and ≤1mm mullite aggregate 20% each, ≤0.044mm The tabular corundum powder is 31%, the activated alumina powder ≤0.044mm is 6%, and the SiO₂ micropowder is 3%. Weigh andalusite and mullite aggregates respectively according to the proportions, and mix all the fine powders (tabular corundum, activated alumina and SiO₂ micropowder) weighed together and put them in a ball mill for pre-mixing for 2 hours. First add the aggregate into the mixer and mix it with the pulp waste liquid for 3 minutes, then add the premixed powder and mix it for 15 minutes. The uniformly mixed mud is pressed into a long sample of 25mm×25mm×125mm with a steel mold on a pressure testing machine at a pressure of 200MPa. After drying at 110°C for 24h, it is placed in a laboratory electric furnace and kept at 1450°C for 3h. fired.
In addition, the fine powder part of the formula is taken for batching, and the matrix sample is made by mixing, molding and firing in the same way as above, which is used for thermal expansion test.
1.3 Performance Testing
The phase composition of the andalusite particles after pre-burning was analyzed by BRUKERD8Focus×diffraction analyzer, the scanning range was 10°~70°, the voltage was 40kV, the current was 30mA, and the step size was 0.02°; according to GB/T7320-2008, the calcination was measured by the ejector rod method. Thermal expansion of post-matrix samples at 25-950°C. According to GB/T2997-2000, the bulk density and apparent porosity of the samples after burning are tested, the linear change rate after burning is tested according to GB/T5988-2007, the flexural strength at room temperature is tested according to GB/T3001-2007, and the flexural strength at room temperature is tested according to YB/T376.2 In 1995, the thermal shock resistance of the fired samples was tested (characterized by the retention rate of flexural strength after 5 times of air-cooled thermal shocks at 950°C), and the elastic modulus was measured using a normal temperature elastic modulus tester (DEMA-01); ZEISSLICMA scanning electron microscope analyzes the microstructure of the fired sample. The sample needs to be cured with resin before the test, and then corroded by hydrofluoric acid for 15s and then sprayed with gold.
Results and discussion
2.1 Phase analysis of andalusite coarse particles after calcination
After calcination at 1300°C, the main phases are andalusite and a small amount of quartz, indicating that mullite has not yet started; Part of it is mullite; it is all mullite after pre-sintering at 1600 °C, indicating that it has all been mullite. It can be seen that the residual andalusite content in the aggregate after pre-sintering decreases with the increase of pre-sintering temperature, and the mullite conversion rate of andalusite increases with the increase of pre-sintering temperature.
2.2 Physical properties of the sample
With the increase of the pre-sintering temperature of the coarse andalusite particles, the expansion of the sample decreases gradually until it shrinks. Andalusite is converted into mullite and SiO2-rich glass phase during the pre-sintering process, and with the increase of pre-sintering temperature, the degree of mulliteization of andalusite increases, and the SiO2-rich glass phase also increases; in mullite-corundum During the sintering process of the sample, the residual andalusite will continue to mullite. On the one hand, with the increase of the andalusite pre-sintering temperature, the amount of residual andalusite decreased, so the volume expansion of the coarse andalusite particles continued to mullite during the sintering process of the sample gradually decreased; With the increase of the sintering temperature, the SiO2-rich glass phase increases, so that the effect of the liquid phase to promote sintering is gradually strengthened. Based on these two reasons, the fired sample changes from expansion to contraction with the increase of the pre-fired temperature of coarse andalusite particles.
With the increase of andalusite calcination temperature, the elastic modulus of calcined samples increased continuously, from 20.23GPa with uncalcined andalusite to 36.98GPa with 1600℃ calcined andalusite. With the increase of pre-sintering temperature, the degree of mulliteization of coarse andalusite particles increases, the difference in thermal expansion coefficient between aggregate and matrix decreases, and the size of microcracks caused by thermal expansion coefficient mismatch decreases gradually. The elastic modulus of andalusite increased with the addition of andalusite pre-sintering temperature.
With the increase of the pre-sintering temperature of andalusite, the flexural strength at room temperature of the fired samples gradually increased, but the strength retention rate decreased gradually after being subjected to air-cooled thermal shocks at 950℃ for 5 times. This may be because with the increase of pre-sintering temperature, the degree of mulliteization of andalusite increases, and the difference in thermal expansion coefficient between aggregate and matrix decreases. During sintering and cooling, the thermal expansion coefficient of aggregate and matrix is ​​mismatched. The size of the micro-cracks is also gradually reduced, while the smaller-sized micro-cracks cannot play a role in relieving thermal stress, preventing new cracks and crack propagation during the thermal shock process, resulting in a gradual decrease in the thermal shock resistance of the sample. . Therefore, compared with adding pre-fired andalusite large particles, the mullite-corundum refractory with unpre-fired andalusite coarse particles (5~3mm) has better thermal shock resistance.