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Effect of V2O5 on the properties of mullite ceramics synthesized from high-aluminum fly ash and bauxite
Journal of Hazardous Materials 166 (2009) 1535–1539
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Effect of V2 O5 on the properties of mullite ceramics synthesized from high-aluminum fly ash and bauxite Jin-Hong Li ∗ , Hong-Wen Ma, Wen-Hui Huang State Key Laboratory of Geological Processes and Mineral Resources, National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 10 May 2008 Received in revised form 28 September 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Mullite ceramics Fly ash V2 O5
a b s t r a c t In this communication, high-strength mullite ceramics was prepared from bauxite and high-aluminum fly ash that is a by-product of coal combustion in thermal power plants. The effects of the doping V2 O5 on the bulk density, apparent porosity, bending strength and microstructure of mullite ceramics were studied in detail. It was indicated that 5–10 mol% V2 O5 reduced the sintering temperature by 50 ◦ C. The apparent porosity and water absorption of the mullite ceramics decreased with increasing V2 O5 content. Mullite ceramics with bending strength as high as 108 MPa were obtained at 1500 ◦ C with the addition of 10 mol% V2 O5 . X-ray diffraction analysis suggested that the prepared ceramics was mainly in phase of mullite, and scanning electron microscope images confirmed that it mostly existed in the shape of a long parallelepiped. This research may provide a new method in utilizing the vast resources of fly-ash waste from power plants in the production of low-cost mullite-based engineering materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mullite is a unique family of oxides belonging to the aluminium silicate family. It has an orthorhombic structure, consisting of a three-dimensional framework, with alternate corner-sharing among the [AlO6 ] octahedra and the [SiO4 ] (or [AlO4 ]) tetrahedra [1]. Because of their excellent mechanical properties, high melting points (1830 ◦ C), low coefficients of thermal expansion (4.5 × 10−6 K−1 ), excellent creep resistance, good chemical stability, and high strength at high temperatures, mullite and mullite ceramics have been widely used in many fields [2,3]. To reduce their cost, many studies have been carried out in their preparation, characterization, and sintering, including their synthesis from various cheaper natural mineral materials, such as kaolinite [4–6], the sillimanite group (sillimanite, andalusite, kyamite) [7–9], staurolite, and topaz [10]. Fly ash is a by-product of coal combustion in thermal power plants and the huge amount of the recovery is a most important economic concern as well as environmental problem, requiring urgent attention [11,12]. Because high-aluminum fly ash contains appreciable amounts of silica and alumina [13], it may be useful in manufacturing mullite ceramics. Until now, there have only been a few reports of the preparation of mullite from fly ash. Huang et al. reported the preparation of mullite using a fly ash and alumina mixture [14]. Jung et al. synthesized yttria-stabilized zirconia (3YSZ)-doped mullite from waste fly ash, Al2 O3 powder, and
∗ Corresponding author. Tel.: +86 10 82322039; fax: +86 10 82322974. E-mail address: [email protected] (J.-H. Li). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.11.059
3YSZ [15]. Dong et al. reported the preparation of low-cost mullite ceramics from natural bauxite and industrial waste fly ash [16]. Park et al. manufactured mullite whiskers by firing compacts of coal fly ash and NH4 Al(SO4 )2 ·12H2 O powder with a small quantity of NaH2 PO4 ·2H2 O [17]. However, new methods are still being explored with an aim of reducing the sintering temperature and densifying the final mullite products. It has been reported that the addition of V2 O5 reduces the temperature of the mullite phase formation in a mixture of Al2 O3 and SiO2 , whereas Nb2 O5 and Ta2 O3 inhibit mullitization [18]. A further study indicated that the rare earth element oxides (Y2 O3 , La2 O3 , and CeO2 ) had a positive effect on mullitization behavior and lowered the mullite formation temperature by about 100 ◦ C [19]. In this study, we synthesized mullite ceramics from a high-aluminum fly ash and bauxite, and linked the physical properties of the final mullite to the doping content of V2 O5 . 2. Experimental Bauxite (no. CB-1) (Al2 O3 88.60, SiO2 4.91, TiO2 3.55, other 2.94 [wt%]) and fly ash (no. BF-1) (SiO2 48.13, TiO2 1.66, Al2 O3 39.03, Fe2 O3 2.94, FeO 0.77, MgO 1.05, CaO 3.30, Na2 O 0.21, K2 O 0.69, MnO 0.017, P2 O5 0.63, and loss on ignition 0.90 [wt%]) from Beijing Thermal Power Plant were used as received. A series of 29.9 wt% bauxite was added to 70.1 wt% fly ash to dope it with V2 O5 (Beijing Chemical Reagent Ltd., Beijing, China) in a molar ratio of 5 mol% (2.18 wt%), 10 mol% (4.50 wt%), or 20 mol% (9.59 wt%). The mixed powders were ball milled for 2 h. The particle size distribution of the mixture is shown in Fig. 1. The specific surface area of the milled mixture was
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Fig. 1. Particle size distribution of the ground mixture of fly ash and bauxite with V2 O5 .
3.73 m2 g−1 , which is approximately twofold higher than that of the original fly ash. The treated mixture was dried at 105 ◦ C for 12 h, and then pressed into bar compacts (40 mm × 6 mm × 6 mm) with an isostatic pressure of 136 MPa. The compacts were heated to the designated temperatures (1100 ◦ C, 1200 ◦ C, 1300 ◦ C, 1400 ◦ C, and 1500 ◦ C) at a rate of 10 ◦ C/min, and kept at these temperatures for 4 h. All the samples were cooled down naturally. A procedure defined by the American Society for Testing and Materials (ASTM) C373, based on Archimedes’ principle, was used to calculate the bulk density and apparent porosity of the final mullite specimens [20]. The particle size distribution was determined using a laser particle-size analyzer (Mastersizer 2000, Malwern Corporation, Britain). Water absorption was measured from the percentage increase in weight of “surface dry” samples after they had been submerged in water for 24 h. The bending strength of the rectangular specimens was determined by the three-point bending technique at room temperature on a Reger Universal Testing Machine (Shenzhen, China). The lower span for the bending test was 20 mm and the loading rate was 0.5 mm/min. 3. Results and discussions Table 1 summarizes the physico-mechanical parameters of the prepared specimens with different content of the V2 O5 and the sintering temperatures. As expected, an increase in temperature accelerates the bulk densification process. The apparent porosity, and water absorption decreases with the increase of sintering temperatures from 1100 ◦ C to 1500 ◦ C (runs 1–5). At 1500 ◦ C, the apparent porosity, and water absorption decreases with the doping content of V2 O5 (runs 5–7). It seems that bulk density is not influenced by V2 O5 at the temperature of 1500 ◦ C and the bulk density remains at 2.7 g cm−3 in the content of V2 O5 5∼20 mol%. According to the formula D = D0 exp(−Q/RT) [21], the sintering process is dominated by surface diffusion at low temperatures, and by body diffusion at high temperatures. Only body diffusion influences the densification of the final products. Therefore, temperature is the major element in densifying the final product in this process, however the porosity decreases greatly with V2 O5 doping at 1500 ◦ C. This indicates V2 O5 favors the formation of the liquid phase and promotes the sintering process and densification at high firing temperatures.
Fig. 2. Plot of the mullite content (a) and bending strength (b) versus sintering temperature and V2 O5 doping content.
The plot of mullite content versus sintering temperature and V2 O5 doping content is illustrated in Fig. 2a. The mullite content increases with an increase in the sintering temperature from 1100 ◦ C to 1500 ◦ C. At a same temperature in 1100–1300 ◦ C, the content of mullite increases with the doped V2 O5 , i.e. the highest content of mullite appears in the case of 20 mol% V2 O5 . However, at the sintering temperature of 1400–1500 ◦ C the highest percentage of mullite was found in the product that was prepared with
Table 1 Influence of temperature and V2 O5 content on the bulk density, apparent porosity and water absorption properties of the prepared samples. Run
Temperature (◦ C)
V2 O5 (mol%)
Bulk density (g cm−3 )
Apparent porosity (%)
Water absorption (%)
1 2 3 4 5 6 7
1100 1200 1300 1400 1500 1500 1500
5 5 5 5 5 10 20
1.74 1.89 2.04 2.15 2.70 2.74 2.75
42.9 37.9 32.7 28.6 4.15 1.42 0.93
24.6 20.0 16.0 13.3 1.53 0.52 0.34
J.-H. Li et al. / Journal of Hazardous Materials 166 (2009) 1535–1539
5 mol% V2 O5 . Fig. 2b illustrates the bending strength at various temperatures and V2 O5 contents. With a content of 5 mol% V2 O5 , the bending strength increased significantly from 1100 ◦ C to 1500 ◦ C, and a reverse of trend in porosity was found in this region. This can be explained by the disappearance of the gas pores, and the stress surrounding the pores was removed at a certain V2 O5 doping content [22]. The highest bending strength (108 MPa) was achieved at 10 mol% V2 O5 content. This effect is attributable to the low melting point of V2 O5 (690 ◦ C), which enhances the formation of the liquid phase, thus facilitating sintering, although it does not enhance the mechanical strength of the products [23]. However, the anisotropic grain growth of the mullite phase increases its strength as the temperature increases. This effect is similar to that reported by Mechnich et al., whereby microstructural homogeneity was improved and the mechanical properties could be tuned by changing the amounts of doped yttria and ceria [24]. Therefore, the mechanical properties depend on a compromise between these two factors. Here, V2 O5 accelerated mullite formation through tran-
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sient and metastable melting, yielding homogeneous and relatively dense ceramics at 1400–1500 ◦ C. The microstructures of the samples were recorded by scanning electron microscopy (SEM; JEOL JSM-4300F), and are shown in Fig. 3. In the product sintered at 1100 ◦ C with 5 mol% V2 O5 , some puncheon-shaped mullites appear among glassy spheres that is attributed to the unreacted fly ash. When the temperature was increased to 1200 ◦ C, more puncheons were embedded in the vitreous phase and fewer pseudomorph appeared (Fig. 3b) because there is less fly ash in the final product. At 1300 ◦ C (Fig. 3c), there are relatively better-developed and numerous cubic puncheon-shaped crystals with aspect ratios of 4–6. Fig. 3d shows a larger tetragonal section of the puncheon, with an aspect ratio of about 6 at 1500 ◦ C, which indicates a higher growth rate at this temperature. According to energy dispersive X-ray spectroscopy analysis (Fig. 3d inset), the calculated formula unit of the mullite product was Al3.96 Fe0.12 Ti0.08 V0.02 Si1.82 O9.91 . When the percentage of V2 O5 increases to 10 mol% and 20 mol%, mullite crystal grows into longer
Fig. 3. SEM images of samples sintered at 1100 ◦ C with V2 O5 5 mol% (a); 1200 ◦ C, 5 mol% (b); 1300 ◦ C, 5 mol% (c); 1500 ◦ C, 5 mol% (d); 1500 ◦ C, 10 mol% (e); and 1500 ◦ C, 20 mol% (f). The samples were etched with 30 wt% hydrofluoric acid solution at 25 ◦ C for 30 s before they were characterized.
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hedral transition metal is associated with the removal of Al3+ from the structure, the entry of cations with varying oxidation states, i.e. V3+ or V4+ , results in a reduction in V2 O5 by the components of the fly ash and also compensates for the excess positive charge introduced by the impurities in the fly ash and bauxite [25]. The subsequent lattice parameters are larger than those of the original mullite because the radii of V3+ (0.74 Å) and V4+ (0.63 Å) are larger than that of Al3+ (0.53 Å). 4. Conclusions In this research, we produced low-cost mullite ceramics at a relatively low sintering temperature using high-aluminium fly ash and bauxite as raw materials. The effects of doping with V2 O5 on the properties of the mullite ceramics are described. The apparent porosity and water absorption decreased with an increase in the V2 O5 content at 1500 ◦ C. The prepared mullite mainly existed in the shape of long parallelepipeds when doped with V2 O5 . It underwent a transition from small puncheons to large cuboids with an increase in temperature, until an aspect ratio of about 6 was reached at 1500 ◦ C. Doping with V2 O5 enlarges the cell lattice parameters of the mullite, because of the entrance of V3+ or V4+ and the substitution Al3+ in the cells of the mullite. This research should facilitate the utilization of fly-ash waste in the production of highly pure mullite ceramics. Acknowledgments This research was supported financially by National Natural Sciences Foundation of China (No. 40602008), the State Key Laboratory of Geological Processes and Mineral Resources (GPMR0739), National Laboratory of Mineral Materials ( No. 08A002), and National Basic Research Program of China (2006CB202202).
Fig. 4. XRD profiles of the samples sintered at different temperatures with 5 mol% of V2 O5 (a) and with different V2 O5 contents sintered at 1500 ◦ C (b). m: mullite; a: corundum; c: cristobalite.
cubic puncheon (Fig. 3e and f), Therefore, doping with V2 O5 favors the yield of whisker-shaped mullite. To confirm the phase transition in this process, the samples were analyzed with X-ray diffraction (XRD; X’PERT SW). Peaks of mullite appeared in the sample sintered at 1100 ◦ C with 5 mol% V2 O5 (Fig. 4), but the ␣-Al2 O3 and cristobalite peaks were also found. The amorphous background of the profile was derived from the unreacted glass phase of the original fly ash. As the sintering temperature was increased to 1300 ◦ C, all the samples showed complete mullitization. All the diffraction peaks were perfectly indexed to the orthorhombic mullite structure. The mullite lattice constants calculated from the XRD data are given in Table 2, and are larger than those for Joint Committee on Powder Diffraction Standards (JCPDS) no. 15-0776. Taking into account that the incorporation of an octaTable 2 Calculated lattice parameters of mullite prepared under different conditions. Temperature (◦ C)
1100 1200 1300 1400 1500 1500 1500
V2 O5 (mol%)
5 5 5 5 5 10 20 JCPDS(15-0776)
Lattice parameter ao (Å)
bo (Å)
co (Å)
Vcell (Å3 )
7.563 7.554 7.561 7.556 7.562 7.568 7.569 7.545
7.708 7.700 7.706 7.699 7.707 7.714 7.707 7.689
2.887 2.888 2.891 2.889 2.891 2.894 2.893 2.884
168.48 168.04 168.44 168.04 168.49 168.94 168.99 167.35
References [1] W.E. Cameron, Nonstoichiometry in sillimanite: mullite compositions with sillimanite-type superstructures, Phys. Chem. Miner. 1 (1977) 265–272. [2] S. Ananthakumar, M. Jayasankar, K.G.K. Warrier, Microstructural, mechanical and thermal characterisation of sol–gel-derived aluminium titanate–mullite ceramic composites, Acta Mater. 54 (2006) 2965–2973. [3] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite—a review, J. Eur. Ceram. Soc. 28 (2008) 329–344. [4] C.Y. Chen, G.S. Lan, W.H. Tuan, Preparation of mullite by the reaction sintering of kaolinite and alumina, J. Eur. Ceram. Soc. 20 (2000) 2519–2525. [5] Y.F. Chen, M.C. Wang, M.H. Hon, Transformation kinetics for mullite in kaolinAl2 O3 ceramics, J. Mater. Res. 18 (2003) 1355–1362. [6] Y.W. Kim, H.D. Kim, C.B. Park, Processing of microcellular mullite, J. Am. Ceram. Soc. 88 (2005) 3311–3315. [7] S. Rahman, U. Feustel, S. Freimann, Structure description of thermic phase transformation sillimantie–mullite, J. Eur. Ceram. Soc. 21 (2001) 2471–2478. [8] M.L. Bouchetou, J.P. Ildefonse, J. Poirier, et al., Mullite grown from fired andalusite grains: the role of impurities and of the high temperature liquid phase on the kinetics of mullitization and consequences on the thermal shocks resistance, Ceram. Int. 31 (2005) 999–1005. [9] F. Mazel, M. Gonon, G. Fantozzi, Manufacture of mullite subrstrates from andalusite for the development of thin film solar cells, J. Eur. Ceram. Soc. 22 (2002) 453–461. [10] X. Miao, Porous mullite ceramics from natural topaz, Mater. Lett. 38 (1999) 167–172. [11] J.H. Li, H.W. Ma, H.W. Zhao, Preparation of sulphoaluminate–alite composite mineralogical phase cement clinker from high alumina fly ash, Key Eng. Mater. 334–335 (2007) 421–424. [12] J.H. Li, H.W. Ma, Y. Chao, Preparation and characterization of -Sialon ceramics from high aluminum fly ash via carbothermal reduction-nitridation, Mater. Sci. Forum. 561–565 (2007) 587–590. [13] K. Nikolaos, R. Zeng, V. Perdikatsis, Mineralogy and geochemistry of Greek and Chinese coal fly ash, Fuel 86 (2006) 2301–2309. [14] X. Huang, J.Y. Hwang, B.C. Mutsuddy, Properties of mullite synthesized from fly ash and alumina mixture, Ceram. Int. 44 (1995) 65–71. [15] J.S. Jung, H.C. Park, R. Stevens, Mullite ceramics derived from coal fly ash, J. Mater. Sci. Lett. 20 (2001) 1089–1091. [16] Y.C. Dong, X.Y. Feng, X.F. Feng, et al., Preparation of low-cost mullite ceramics from natural bauxite and industrial waste fly ash, J. Alloys Compd. 460 (2008) 599–606.
J.-H. Li et al. / Journal of Hazardous Materials 166 (2009) 1535–1539 [17] Y.M. Park, T.Y. Yang, S.Y. Yoon, Mullite wiskers derived from coal fly ash, Mater. Sci. Eng. A 454–455 (2007) 518–522. [18] L.B. Kong, Y.B. Gan, J. Ma, et al., Mullite phase formation and reaction sequences with the presence of pentoxides, J. Alloys Compd. 351 (2003) 264–272. [19] L.B. Kong, T.S. Zhang, J. Ma, et al., Mullite phase formation in oxide mixtures in the presence of Y2 O3 ,La2 O3 and CeO2 , J. Alloys Compd. 372 (2004) 290–299. [20] Annual Book of ASTM Standards, 2000, C-20, Standard Test Methods for Apparent Porosity, Water Absorption, Apparent Specific Gravity and Bulk Density of Burned Refractory Brick Shapes by Boiling Water.
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[21] M. Schmücker, H. Schneider, Kinetic of mullite grain growth in alumino silicate fibers, J. Am. Ceram. 88 (2005) 488–490. [22] S. Gupta, M. Dubikova, D. French, et al., Effect of CO2 gasification on the transformations of coke minerals at high temperatures, Energy Fuels 21 (2007) 1052–1061. [23] X.Y. Kong, Z.L. Wang, S.J. Wu, Rectangular single-crystal mullite microtubes, Adv. Mater. 5 (2003) 1445–1449. [24] P. Mechnich, H. Schneider, M. Schmücker, et al., Accelerated reaction bonding of mullite, J. Am. Ceram. Soc. 81 (1998) 1931–1937. [25] H. Schneider, Transition metal distribution in mullite, Ceram. Trans. 6 (1990) 135–158.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Effect of V2 O5 on the properties of mullite ceramics synthesized from high-aluminum fly ash and bauxite Jin-Hong Li ∗ , Hong-Wen Ma, Wen-Hui Huang State Key Laboratory of Geological Processes and Mineral Resources, National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 10 May 2008 Received in revised form 28 September 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Mullite ceramics Fly ash V2 O5
a b s t r a c t In this communication, high-strength mullite ceramics was prepared from bauxite and high-aluminum fly ash that is a by-product of coal combustion in thermal power plants. The effects of the doping V2 O5 on the bulk density, apparent porosity, bending strength and microstructure of mullite ceramics were studied in detail. It was indicated that 5–10 mol% V2 O5 reduced the sintering temperature by 50 ◦ C. The apparent porosity and water absorption of the mullite ceramics decreased with increasing V2 O5 content. Mullite ceramics with bending strength as high as 108 MPa were obtained at 1500 ◦ C with the addition of 10 mol% V2 O5 . X-ray diffraction analysis suggested that the prepared ceramics was mainly in phase of mullite, and scanning electron microscope images confirmed that it mostly existed in the shape of a long parallelepiped. This research may provide a new method in utilizing the vast resources of fly-ash waste from power plants in the production of low-cost mullite-based engineering materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mullite is a unique family of oxides belonging to the aluminium silicate family. It has an orthorhombic structure, consisting of a three-dimensional framework, with alternate corner-sharing among the [AlO6 ] octahedra and the [SiO4 ] (or [AlO4 ]) tetrahedra [1]. Because of their excellent mechanical properties, high melting points (1830 ◦ C), low coefficients of thermal expansion (4.5 × 10−6 K−1 ), excellent creep resistance, good chemical stability, and high strength at high temperatures, mullite and mullite ceramics have been widely used in many fields [2,3]. To reduce their cost, many studies have been carried out in their preparation, characterization, and sintering, including their synthesis from various cheaper natural mineral materials, such as kaolinite [4–6], the sillimanite group (sillimanite, andalusite, kyamite) [7–9], staurolite, and topaz [10]. Fly ash is a by-product of coal combustion in thermal power plants and the huge amount of the recovery is a most important economic concern as well as environmental problem, requiring urgent attention [11,12]. Because high-aluminum fly ash contains appreciable amounts of silica and alumina [13], it may be useful in manufacturing mullite ceramics. Until now, there have only been a few reports of the preparation of mullite from fly ash. Huang et al. reported the preparation of mullite using a fly ash and alumina mixture [14]. Jung et al. synthesized yttria-stabilized zirconia (3YSZ)-doped mullite from waste fly ash, Al2 O3 powder, and
∗ Corresponding author. Tel.: +86 10 82322039; fax: +86 10 82322974. E-mail address: [email protected] (J.-H. Li). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.11.059
3YSZ [15]. Dong et al. reported the preparation of low-cost mullite ceramics from natural bauxite and industrial waste fly ash [16]. Park et al. manufactured mullite whiskers by firing compacts of coal fly ash and NH4 Al(SO4 )2 ·12H2 O powder with a small quantity of NaH2 PO4 ·2H2 O [17]. However, new methods are still being explored with an aim of reducing the sintering temperature and densifying the final mullite products. It has been reported that the addition of V2 O5 reduces the temperature of the mullite phase formation in a mixture of Al2 O3 and SiO2 , whereas Nb2 O5 and Ta2 O3 inhibit mullitization [18]. A further study indicated that the rare earth element oxides (Y2 O3 , La2 O3 , and CeO2 ) had a positive effect on mullitization behavior and lowered the mullite formation temperature by about 100 ◦ C [19]. In this study, we synthesized mullite ceramics from a high-aluminum fly ash and bauxite, and linked the physical properties of the final mullite to the doping content of V2 O5 . 2. Experimental Bauxite (no. CB-1) (Al2 O3 88.60, SiO2 4.91, TiO2 3.55, other 2.94 [wt%]) and fly ash (no. BF-1) (SiO2 48.13, TiO2 1.66, Al2 O3 39.03, Fe2 O3 2.94, FeO 0.77, MgO 1.05, CaO 3.30, Na2 O 0.21, K2 O 0.69, MnO 0.017, P2 O5 0.63, and loss on ignition 0.90 [wt%]) from Beijing Thermal Power Plant were used as received. A series of 29.9 wt% bauxite was added to 70.1 wt% fly ash to dope it with V2 O5 (Beijing Chemical Reagent Ltd., Beijing, China) in a molar ratio of 5 mol% (2.18 wt%), 10 mol% (4.50 wt%), or 20 mol% (9.59 wt%). The mixed powders were ball milled for 2 h. The particle size distribution of the mixture is shown in Fig. 1. The specific surface area of the milled mixture was
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J.-H. Li et al. / Journal of Hazardous Materials 166 (2009) 1535–1539
Fig. 1. Particle size distribution of the ground mixture of fly ash and bauxite with V2 O5 .
3.73 m2 g−1 , which is approximately twofold higher than that of the original fly ash. The treated mixture was dried at 105 ◦ C for 12 h, and then pressed into bar compacts (40 mm × 6 mm × 6 mm) with an isostatic pressure of 136 MPa. The compacts were heated to the designated temperatures (1100 ◦ C, 1200 ◦ C, 1300 ◦ C, 1400 ◦ C, and 1500 ◦ C) at a rate of 10 ◦ C/min, and kept at these temperatures for 4 h. All the samples were cooled down naturally. A procedure defined by the American Society for Testing and Materials (ASTM) C373, based on Archimedes’ principle, was used to calculate the bulk density and apparent porosity of the final mullite specimens [20]. The particle size distribution was determined using a laser particle-size analyzer (Mastersizer 2000, Malwern Corporation, Britain). Water absorption was measured from the percentage increase in weight of “surface dry” samples after they had been submerged in water for 24 h. The bending strength of the rectangular specimens was determined by the three-point bending technique at room temperature on a Reger Universal Testing Machine (Shenzhen, China). The lower span for the bending test was 20 mm and the loading rate was 0.5 mm/min. 3. Results and discussions Table 1 summarizes the physico-mechanical parameters of the prepared specimens with different content of the V2 O5 and the sintering temperatures. As expected, an increase in temperature accelerates the bulk densification process. The apparent porosity, and water absorption decreases with the increase of sintering temperatures from 1100 ◦ C to 1500 ◦ C (runs 1–5). At 1500 ◦ C, the apparent porosity, and water absorption decreases with the doping content of V2 O5 (runs 5–7). It seems that bulk density is not influenced by V2 O5 at the temperature of 1500 ◦ C and the bulk density remains at 2.7 g cm−3 in the content of V2 O5 5∼20 mol%. According to the formula D = D0 exp(−Q/RT) [21], the sintering process is dominated by surface diffusion at low temperatures, and by body diffusion at high temperatures. Only body diffusion influences the densification of the final products. Therefore, temperature is the major element in densifying the final product in this process, however the porosity decreases greatly with V2 O5 doping at 1500 ◦ C. This indicates V2 O5 favors the formation of the liquid phase and promotes the sintering process and densification at high firing temperatures.
Fig. 2. Plot of the mullite content (a) and bending strength (b) versus sintering temperature and V2 O5 doping content.
The plot of mullite content versus sintering temperature and V2 O5 doping content is illustrated in Fig. 2a. The mullite content increases with an increase in the sintering temperature from 1100 ◦ C to 1500 ◦ C. At a same temperature in 1100–1300 ◦ C, the content of mullite increases with the doped V2 O5 , i.e. the highest content of mullite appears in the case of 20 mol% V2 O5 . However, at the sintering temperature of 1400–1500 ◦ C the highest percentage of mullite was found in the product that was prepared with
Table 1 Influence of temperature and V2 O5 content on the bulk density, apparent porosity and water absorption properties of the prepared samples. Run
Temperature (◦ C)
V2 O5 (mol%)
Bulk density (g cm−3 )
Apparent porosity (%)
Water absorption (%)
1 2 3 4 5 6 7
1100 1200 1300 1400 1500 1500 1500
5 5 5 5 5 10 20
1.74 1.89 2.04 2.15 2.70 2.74 2.75
42.9 37.9 32.7 28.6 4.15 1.42 0.93
24.6 20.0 16.0 13.3 1.53 0.52 0.34
J.-H. Li et al. / Journal of Hazardous Materials 166 (2009) 1535–1539
5 mol% V2 O5 . Fig. 2b illustrates the bending strength at various temperatures and V2 O5 contents. With a content of 5 mol% V2 O5 , the bending strength increased significantly from 1100 ◦ C to 1500 ◦ C, and a reverse of trend in porosity was found in this region. This can be explained by the disappearance of the gas pores, and the stress surrounding the pores was removed at a certain V2 O5 doping content [22]. The highest bending strength (108 MPa) was achieved at 10 mol% V2 O5 content. This effect is attributable to the low melting point of V2 O5 (690 ◦ C), which enhances the formation of the liquid phase, thus facilitating sintering, although it does not enhance the mechanical strength of the products [23]. However, the anisotropic grain growth of the mullite phase increases its strength as the temperature increases. This effect is similar to that reported by Mechnich et al., whereby microstructural homogeneity was improved and the mechanical properties could be tuned by changing the amounts of doped yttria and ceria [24]. Therefore, the mechanical properties depend on a compromise between these two factors. Here, V2 O5 accelerated mullite formation through tran-
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sient and metastable melting, yielding homogeneous and relatively dense ceramics at 1400–1500 ◦ C. The microstructures of the samples were recorded by scanning electron microscopy (SEM; JEOL JSM-4300F), and are shown in Fig. 3. In the product sintered at 1100 ◦ C with 5 mol% V2 O5 , some puncheon-shaped mullites appear among glassy spheres that is attributed to the unreacted fly ash. When the temperature was increased to 1200 ◦ C, more puncheons were embedded in the vitreous phase and fewer pseudomorph appeared (Fig. 3b) because there is less fly ash in the final product. At 1300 ◦ C (Fig. 3c), there are relatively better-developed and numerous cubic puncheon-shaped crystals with aspect ratios of 4–6. Fig. 3d shows a larger tetragonal section of the puncheon, with an aspect ratio of about 6 at 1500 ◦ C, which indicates a higher growth rate at this temperature. According to energy dispersive X-ray spectroscopy analysis (Fig. 3d inset), the calculated formula unit of the mullite product was Al3.96 Fe0.12 Ti0.08 V0.02 Si1.82 O9.91 . When the percentage of V2 O5 increases to 10 mol% and 20 mol%, mullite crystal grows into longer
Fig. 3. SEM images of samples sintered at 1100 ◦ C with V2 O5 5 mol% (a); 1200 ◦ C, 5 mol% (b); 1300 ◦ C, 5 mol% (c); 1500 ◦ C, 5 mol% (d); 1500 ◦ C, 10 mol% (e); and 1500 ◦ C, 20 mol% (f). The samples were etched with 30 wt% hydrofluoric acid solution at 25 ◦ C for 30 s before they were characterized.
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hedral transition metal is associated with the removal of Al3+ from the structure, the entry of cations with varying oxidation states, i.e. V3+ or V4+ , results in a reduction in V2 O5 by the components of the fly ash and also compensates for the excess positive charge introduced by the impurities in the fly ash and bauxite [25]. The subsequent lattice parameters are larger than those of the original mullite because the radii of V3+ (0.74 Å) and V4+ (0.63 Å) are larger than that of Al3+ (0.53 Å). 4. Conclusions In this research, we produced low-cost mullite ceramics at a relatively low sintering temperature using high-aluminium fly ash and bauxite as raw materials. The effects of doping with V2 O5 on the properties of the mullite ceramics are described. The apparent porosity and water absorption decreased with an increase in the V2 O5 content at 1500 ◦ C. The prepared mullite mainly existed in the shape of long parallelepipeds when doped with V2 O5 . It underwent a transition from small puncheons to large cuboids with an increase in temperature, until an aspect ratio of about 6 was reached at 1500 ◦ C. Doping with V2 O5 enlarges the cell lattice parameters of the mullite, because of the entrance of V3+ or V4+ and the substitution Al3+ in the cells of the mullite. This research should facilitate the utilization of fly-ash waste in the production of highly pure mullite ceramics. Acknowledgments This research was supported financially by National Natural Sciences Foundation of China (No. 40602008), the State Key Laboratory of Geological Processes and Mineral Resources (GPMR0739), National Laboratory of Mineral Materials ( No. 08A002), and National Basic Research Program of China (2006CB202202).
Fig. 4. XRD profiles of the samples sintered at different temperatures with 5 mol% of V2 O5 (a) and with different V2 O5 contents sintered at 1500 ◦ C (b). m: mullite; a: corundum; c: cristobalite.
cubic puncheon (Fig. 3e and f), Therefore, doping with V2 O5 favors the yield of whisker-shaped mullite. To confirm the phase transition in this process, the samples were analyzed with X-ray diffraction (XRD; X’PERT SW). Peaks of mullite appeared in the sample sintered at 1100 ◦ C with 5 mol% V2 O5 (Fig. 4), but the ␣-Al2 O3 and cristobalite peaks were also found. The amorphous background of the profile was derived from the unreacted glass phase of the original fly ash. As the sintering temperature was increased to 1300 ◦ C, all the samples showed complete mullitization. All the diffraction peaks were perfectly indexed to the orthorhombic mullite structure. The mullite lattice constants calculated from the XRD data are given in Table 2, and are larger than those for Joint Committee on Powder Diffraction Standards (JCPDS) no. 15-0776. Taking into account that the incorporation of an octaTable 2 Calculated lattice parameters of mullite prepared under different conditions. Temperature (◦ C)
1100 1200 1300 1400 1500 1500 1500
V2 O5 (mol%)
5 5 5 5 5 10 20 JCPDS(15-0776)
Lattice parameter ao (Å)
bo (Å)
co (Å)
Vcell (Å3 )
7.563 7.554 7.561 7.556 7.562 7.568 7.569 7.545
7.708 7.700 7.706 7.699 7.707 7.714 7.707 7.689
2.887 2.888 2.891 2.889 2.891 2.894 2.893 2.884
168.48 168.04 168.44 168.04 168.49 168.94 168.99 167.35
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