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Preparation and properties of lightweight mullite refractories with low thermal conductivity and high strength based on fly ash WAN ZHUOFU, SANG SHAOBAI* —The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081,China. —National-provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology, Wuhan 430081, China. —email: [email protected] Introduction Mullite refractories are widely used in industrial furnaces because of their high strength, excellent thermal shock resistance and high refractoriness under load. Due to a series of negative impact on the environment of high temperature processes, as the basic material of heating furnace, the lightweight mullite refractory with high porosity and low thermal conductivity has been paid more and more attention. Obviously, thermal conductivity is a significant index for energy saving. Increasing porosity has become a main method to reduce the thermal conductivity of refractories. In recent few decades, several processing methods have been utilized to fabricate lightweight mullite refractory, such as in-situ decomposition synthesis, pore forming agent method, foaming method, gel-casting method, etc., which not only reduces the density of refractories, but also can obtain a lower thermal conductivity. However, the low strength limits the application of lightweight mullite refractories and, most importantly, its inapplicability to work linings. Therefore, new methods have been adopted to improve the strength of mullite refractories. Mullite whiskers are generally considered as reinforcing materials to enhance the mechanical properties of ceramic composites. As the main solid waste discharged from coal-fired power plants, fly ash is the fine ash captured by various means after burning coal. Similar to the traditional raw materials for mullite synthesis, fly ash containing main mineral composition of mullite, quartz and a certain amount of glass phase also belongs to the Al 2 O 3 -SiO 2 system raw materials, which is a very suitable raw material for mullite synthesis. TThe phase composition of low grade bauxite is mostly Al2O3-SiO2 system minerals. Among them, diaspore and pyrophyllite decompose in situ to form micropores at high temperature, which increases the porosity of the specimens. In this work, lightweight mullite refractories are prepared with fly ash, fine low grade bauxite powder and fine α-Al2O3 powder as raw materials. In order to develop lightweight mullite refractories with low thermal conductivity and high strength, the influence of the proportion between fly ash and low grade bauxite fine powder and sintering temperature on the structure and properties of lightweight mullite refractories was studied. Experiment The specific formulas were shown in Table 1, which the Al 2 O 3 content of the formulas was controlled at 60wt% as well as F60 means the mass ratio of fly ash and low grade bauxite is 60:40, and so on. After that, the raw materials of different formulas was mixed via the Eirich mixer (EL5, Eirich Group China Ltd.) at the rotational speed of 800 r/min for 5 minutes three times. 12 hours later, the specimens of different formulas were pressed by a hydraulic press (TYE-2000B, Wuxi jianyi instrument machinery Co., China) under the pressure of 110MPa into cylindrical specimens 36mm × H36mm), rectangle parallelepiped specimens (140mm × 25mm × 25mm), and disc specimens 180mm × H20mm), then dried at 110C for 12h. Finally, the green specimens were fired at 1400 , 1500 , 1600 for 3 hours in an electric furnace (PBDR16-15-16YZ, Precondar Heat Resistant Testing Equipment Co., Ltd, China). The flow chart of the experiment is shown in Fig. 1. Table 1 Experimental formula Bauxite (wt%) Fly ash (wt%) α-Al 2 O 3 (wt%) F60 30.88 46.33 22.79 F80 14.12 56.48 29.40 F100 - 65.04 34.96 Results and discussion Fig. 2 The morphology of F100 fired specimens at 1400 (a), 1500 (b) and 1600 (c)and fired specimens of F60 (d), F80 (e) and F100 (f) sintered at 1500 ℃ after hydrofluoric acid corrosion Microstructure As shown in Fig. 2 (a ~ c), the morphology of columnar mullite in the specimens sintered at 1600 can be clearly seen after hydrofluoric acid corrosion. It can be seen from the figure that with the increase of sintering temperature, the columnar mullite in the fired specimens becomes stronger and better developed. It is worth noting that the columnar mullites in the sintered samples merge at 1600 , which makes the bonding between the columnar mullites closer, which is one of the main reasons for the high strength of the sintered samples at 1600 . However, as shown in Fig. 2 (d ~ e), the change of formula has little effect on the morphology of columnar mullite. Table 2 Specimens of density and porosity Temperature (℃) NO. Bulk density (g/cm 3 ) True density (g/cm 3 ) Apparent porosity (%) Closed porosity (%) Total porosity (%) 1400 F60 1.87 3.07 38.44 0.64 39.08 F80 1.77 3.06 39.03 2.46 42.39 F100 1.71 3.01 41.50 1.85 43.35 1500 F60 1.96 3.03 30.49 4.67 35.17 F80 1.89 3.06 32.99 5.29 38.28 F100 1.82 3.04 35.50 4.71 40.21 1600 F60 2.45 3.07 3.70 16.57 20.27 F80 2.42 3.07 3.02 17.94 20.96 F100 2.28 3.01 19.63 4.56 24.19 Fig. 4 Thermal conductivity of fired specimens at 1600℃ (a) and F100 formula (b) at different temperatures Pore structure The porosity of lightweight refractories is related to the thermal conductivity of lightweight refractories. Table 2 shows the apparent porosity, closed porosity and total porosity of fired specimens with different compositions at different sintering temperatures. The total porosity of the fired specimens at 1600 is more than 20%, while at 1400 , the total porosity can reach 43.35%. Fig. 3. Pore size distribution of fired specimens with different formula (a) and sintering temperature (b) Fig.3 shows the pore size distribution of F100 samples at different sintering temperatures. It can be seen from Fig.3 (a) that the average pore size of the sintered sample decreases with the increase of the amount of low grade bauxite. It can be found from Fig.3 (b) that the average pore size of the sample increases gradually with the increase of sintering temperature. However, the average pore diameter of the sintered samples at three sintering temperatures is less than 9 μ m, and the average pore diameter of the sintered samples sintered at 1400 is only 1.963 μ m. It shows that in this study, the internal pores of the sintered samples are micron pores. Thermal conductivity The total porosity of refractories is closely related to its thermal conductivity. Fig.4 shows the thermal conductivity of sintered samples at different temperatures. At the high temperature of 1000 , the thermal conductivity of the sintered sample is between 0.450~1.098W/(m·k), which meets the thermal insulation requirements of lightweight refractories. Strength Fig.5 shows the strength of fired specimens with different formulations at different sintering temperatures. The compressive strength and flexural strength of the sintered samples obtained at 1600 are about 140MPa and 45MPa. It can meet the needs of most working conditions. (a) (b) Fig.5 Cold compressive strength (a) and cold fracture strength (b) of fired specimens Fracture behavior The fracture behavior of the sintered sample was characterized by analyzing the load-displacement curve shown by Fig.9. It can be seen from the diagram that all the fired specimens are brittle fracture. For the same formula, the maximum load of the sintered sample increases with the increase of temperature at different sintering temperatures. In order to characterize the fracture toughness of sintered samples, the brittleness of sintered samples can be characterized by calculating the ratio of fracture energy (G f ) to flexural strength (R). Usually, the higher the ratio, the lower the brittleness of refractories. It can be seen from Table3 that the G f /R of F100 is the highest at 1400 , which indicates that F100 has the best fracture toughness when the sintering temperature is 1400 . Fig. 6 Force-displacement curves of the fired specimens of F100 formula (left) and different formulas at 1600℃ (right). Table 4 Relative parameters of the fracture toughness of the fired specimens. F100 at 1400℃ F100 at 1500℃ F100 at 1600℃ F80 at 1600℃ F60 at 1600℃ Fracture energy G f (J/m2) 298.21 362.93 593.20 353.04 522.17 G f / R 13.89 12.95 12.10 8.30 10.92 Conclusion In this study, lightweight mullite refractories with mullite staggered structure were synthesized by fly ash, low grade bauxite and α-Al 2 O 3 fine powder. With the increase of the amount of fly ash in the formula, the porosity and pore diameter of the specimens will increase, and the thermal conductivity will decrease. With the increase of sintering temperature, the mechanical properties, pore size and thermal conductivity of the materials will increase obviously, and the porosity will decrease obviously. (a) (b)

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Page 1: Preparation and properties of lightweight mullite

Preparation and properties of lightweight mullite refractories with low thermal conductivity and high strength based on fly ash

WAN ZHUOFU, SANG SHAOBAI*—The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081,China.—National-provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology, Wuhan 430081, China.—email: [email protected]

IntroductionMullite refractories are widely used in industrial furnaces because of their high strength, excellent thermal shock resistance and high refractoriness under load. Due to a series of negative impact on the environment of hightemperature processes, as the basic material of heating furnace, the lightweight mullite refractory with high porosity and low thermal conductivity has been paid more and more attention. Obviously, thermal conductivity is asignificant index for energy saving. Increasing porosity has become a main method to reduce the thermal conductivity of refractories. In recent few decades, several processing methods have been utilized to fabricate lightweightmullite refractory, such as in-situ decomposition synthesis, pore forming agent method, foaming method, gel-casting method, etc., which not only reduces the density of refractories, but also can obtain a lower thermalconductivity. However, the low strength limits the application of lightweight mullite refractories and, most importantly, its inapplicability to work linings. Therefore, new methods have been adopted to improve the strength ofmullite refractories. Mullite whiskers are generally considered as reinforcing materials to enhance the mechanical properties of ceramic composites.As the main solid waste discharged from coal-fired power plants, fly ash is the fine ash captured by various means after burning coal. Similar to the traditional raw materials for mullite synthesis, fly ash containing main mineralcomposition of mullite, quartz and a certain amount of glass phase also belongs to the Al2O3-SiO2 system raw materials, which is a very suitable raw material for mullite synthesis. TThe phase composition of low grade bauxiteis mostly Al2O3-SiO2 system minerals. Among them, diaspore and pyrophyllite decompose in situ to form micropores at high temperature, which increases the porosity of the specimens. In this work, lightweight mulliterefractories are prepared with fly ash, fine low grade bauxite powder and fine α-Al2O3 powder as raw materials. In order to develop lightweight mullite refractories with low thermal conductivity and high strength, the influenceof the proportion between fly ash and low grade bauxite fine powder and sintering temperature on the structure and properties of lightweight mullite refractories was studied.

ExperimentThe specific formulas were shown in Table 1, which the Al2O3 content of the formulas was controlled at 60wt% as well as F60 means the mass ratio of fly ash and low grade bauxite is 60:40, and so on. After that, the raw

materials of different formulas was mixed via the Eirich mixer (EL5, Eirich Group China Ltd.) at the rotational speed of 800 r/min for 5 minutes three times. 12 hours later, the specimens of different formulas were pressed by ahydraulic press (TYE-2000B, Wuxi jianyi instrument machinery Co., China) under the pressure of 110MPa into cylindrical specimens (Φ36mm × H36mm), rectangle parallelepiped specimens (140mm × 25mm × 25mm),and disc specimens (Φ180mm × H20mm), then dried at 110C for 12h. Finally, the green specimens were fired at 1400 ℃, 1500 ℃, 1600 ℃ for 3 hours in an electric furnace (PBDR16-15-16YZ, Precondar Heat ResistantTesting Equipment Co., Ltd, China). The flow chart of the experiment is shown in Fig. 1.

Table 1 Experimental formula

Bauxite (wt%) Fly ash (wt%) α-Al2O3(wt%)

F60 30.88 46.33 22.79 F80 14.12 56.48 29.40 F100 - 65.04 34.96

Results and discussion

Fig. 2 The morphology of F100 fired specimens at 1400 ℃ (a), 1500 ℃ (b) and 1600 ℃(c)and fired specimens of F60 (d), F80 (e) and F100 (f) sintered at 1500 ℃ after

hydrofluoric acid corrosion

MicrostructureAs shown in Fig. 2 (a ~ c), the morphology of columnar mullite in thespecimens sintered at 1600 ℃ can be clearly seen after hydrofluoricacid corrosion. It can be seen from the figure that with the increase ofsintering temperature, the columnar mullite in the fired specimensbecomes stronger and better developed. It is worth noting that thecolumnar mullites in the sintered samples merge at 1600 ℃, whichmakes the bonding between the columnar mullites closer, which is oneof the main reasons for the high strength of the sintered samples at1600 ℃. However, as shown in Fig. 2 (d ~ e), the change of formulahas little effect on the morphology of columnar mullite.

Table 2 Specimens of density and porosity

Temperature (℃)

NO. Bulk

density (g/cm3 )

True density (g/cm3 )

Apparent porosity (%)

Closed porosity (%)

Total porosity (%)

1400

F60 1.87 3.07 38.44 0.64 39.08

F80 1.77 3.06 39.03 2.46 42.39

F100 1.71 3.01 41.50 1.85 43.35

1500

F60 1.96 3.03 30.49 4.67 35.17

F80 1.89 3.06 32.99 5.29 38.28

F100 1.82 3.04 35.50 4.71 40.21

1600 F60 2.45 3.07 3.70 16.57 20.27 F80 2.42 3.07 3.02 17.94 20.96 F100 2.28 3.01 19.63 4.56 24.19

Fig. 4 Thermal conductivity of fired specimens at 1600℃ (a) and F100 formula (b) at different temperatures

Pore structureThe porosity of lightweight refractories is related to the thermalconductivity of lightweight refractories. Table 2 shows the apparentporosity, closed porosity and total porosity of fired specimens withdifferent compositions at different sintering temperatures. The totalporosity of the fired specimens at 1600 ℃ is more than 20%, whileat 1400 ℃, the total porosity can reach 43.35%.

Fig. 3. Pore size distribution of fired specimens with different formula (a) and sintering temperature (b)

Fig.3 shows the pore size distribution of F100 samples at differentsintering temperatures. It can be seen from Fig.3 (a) that theaverage pore size of the sintered sample decreases with theincrease of the amount of low grade bauxite. It can be found fromFig.3 (b) that the average pore size of the sample increasesgradually with the increase of sintering temperature. However, theaverage pore diameter of the sintered samples at three sinteringtemperatures is less than 9 μ m, and the average pore diameter ofthe sintered samples sintered at 1400 ℃ is only 1.963 μ m. Itshows that in this study, the internal pores of the sintered samplesare micron pores.

Thermal conductivityThe total porosity of refractories is closely related to its thermalconductivity. Fig.4 shows the thermal conductivity of sinteredsamples at different temperatures. At the high temperature of 1000℃, the thermal conductivity of the sintered sample is between0.450~1.098W/(m·k), which meets the thermal insulationrequirements of lightweight refractories.

StrengthFig.5 shows the strength of fired specimens with different formulationsat different sintering temperatures. The compressive strength andflexural strength of the sintered samples obtained at 1600 ℃ are about140MPa and 45MPa. It can meet the needs of most working conditions.

(a) (b)

Fig.5 Cold compressive strength (a) and cold fracture strength (b) of fired specimens

Fracture behavior

The fracture behavior of the sintered sample was characterized byanalyzing the load-displacement curve shown by Fig.9. It can be seenfrom the diagram that all the fired specimens are brittle fracture. For thesame formula, the maximum load of the sintered sample increases withthe increase of temperature at different sintering temperatures.

In order to characterize the fracture toughness of sintered samples, thebrittleness of sintered samples can be characterized by calculating theratio of fracture energy (Gf) to flexural strength (R). Usually, the higherthe ratio, the lower the brittleness of refractories. It can be seen fromTable3 that the Gf/R of F100 is the highest at 1400 ℃, which indicatesthat F100 has the best fracture toughness when the sintering temperatureis 1400 ℃.

Fig. 6 Force-displacement curves of the fired specimens of F100 formula (left) and different formulas at 1600℃ (right).

Table 4 Relative parameters of the fracture toughness of the fired specimens.

F100 at 1400℃

F100 at 1500℃

F100 at 1600℃

F80 at 1600℃

F60 at 1600℃

Fracture energy Gf (J/m2)

298.21 362.93 593.20 353.04 522.17

Gf / R 13.89 12.95 12.10 8.30 10.92

ConclusionIn this study, lightweight mullite refractories with mullite staggered structure were synthesized by fly ash, low grade bauxite and α-Al2O3 fine powder. With the increase of the amount of fly ash in the formula, the porosity andpore diameter of the specimens will increase, and the thermal conductivity will decrease. With the increase of sintering temperature, the mechanical properties, pore size and thermal conductivity of the materials will increaseobviously, and the porosity will decrease obviously.

(a) (b)