Phase evolution and sintering characteristics of porous mullite ceramics produced from the flyash-Al(OH)3 coating powders

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Abstract

Mullite precursors, waste flyash coated with aluminum hydroxide, were used to prepare low cost porous mullite-based ceramics by reaction sintering. The samples with different alumina contents (0–41.20 wt.%) were sintered at several temperatures from 1000 to 1500 °C. Phase evolution, sintering characteristics and microstructures were investigated in terms of alumina coating content and heat treating temperature. The X-ray diffraction (XRD) results showed, for the 33.30 wt.% alumina coated samples, the mullitization began to occur at 1250 °C via the reaction of α-alumina coating and silica (cristobalite and silica-rich glassy phase) in flyash, and completed at around 1400 °C. With increasing alumina content, both the crystallinity of mullite phase and aspect ratio of mullite crystals were decreased. Our results also demonstrated that the introduction of aluminum hydroxide coating had a positive effect on improving open porosity by inhibiting sintering shrinkage. Compared with flyash, the aluminum hydroxide coated samples showed a more wide sintering temperature range and well-controlled open porosity.

Introduction

Porous ceramics have great potential in many applications because of their high separation efficiency, corrosion and thermal resistance as well as mechanical strength and structural stability. Mullite is extensively studied because it exhibits some additional properties such as low specific gravity, high creep resistance, low thermal conductivity, good infrared transparency and low dielectric constant [1], [2]. In particular, the unique combination of low linear thermal expansion coefficient (LTEC) (4.5–5.6 × 10−6 K−1 between 20 and 1000 °C) and good thermal shock resistance makes porous mullite-based ceramics promising engineering materials for the high temperature applications where the environment temperature is changed rapidly [3].

A lot of efforts have been made to prepare porous mullite ceramics using various starting materials, such as industrial grade and chemical method derived mullite. These processes are at the cost of expensive starting materials, which are synthesized beforehand. In recent years, to reduce the cost of fabrication, the reaction sintering technique has been frequently used to prepare porous mullite or zirconia–mullite ceramics directly utilizing natural materials such as kaolin [4], [5], [6], [7], [8], topaz [9], zircon [10], [11] and SiC [12]. The advantages of this process include simple processing procedures and relatively low sintering temperature, which are suitable for large-scale industrial production. In many cases, however, alumina precursors were added by the mechanical mixing method for mullitization in the form of aluminum, α-alumina or aluminum hydroxide. It is quite difficult to mix the starting materials homogeneously.

Flyash, an abundantly available industrial waste, contains large amount of reactive SiO2 (mainly in the form of quartz crystals and amorphous impurity-containing silica-rich vitreous micro-spheres). It is interesting to utilize this waste to decrease pollution of the environment and to produce high value ceramic products. Recently, some researchers have attempted to prepare dense mullite from this industrial waste by reaction-sintering with industrial α-alumina addition [13], [14], [15], [16]. However, little research has been done to prepare porous mullite ceramics using coal flyash. In particular, most of flyash particles have nearly spherical shape, which were formed from molten glass bodies during rapid cooling from high temperature [17]. This characteristic makes flyash suitable for the preparation of porous materials after classification.

In the present work, therefore, low cost porous mullite-based ceramics were prepared with flyash waste and industrial grade aluminum chloride as raw materials. First, mullite precursors were produced using a heterogeneous precipitation route to assure a homogeneous mixing of flyash and aluminum hydroxide. For these precursors, aluminum hydroxide as a source of alumina was deposited on the surfaces of flyash particles. Then, the porous sintered bodies with mullite as the crystalline phase were prepared by the dry pressing-sintering process from the above mullite precursors. The effect of sintering temperature and alumina coating contents on crystalline phases, sintering characteristics and microstructures were evaluated.

Section snippets

Main starting materials

Commercially available aluminum chloride (AlCl3·6H2O) was used as a source of alumina in this study. Flyash waste was obtained from Hefei thermal power plant (Anhui province, China). Raw powders were classified by sedimentation using tap water as a medium to remove fine particles. Then, the beneficiated flyash (D50 = 8.60 μm) was collected and then analyzed by an X-ray fluorescence spectrometer (XRF-1800, Shimadzu Corporation, Japan). The chemical composition (in wt.%) of as-used flyash is: 47.66%

Characterization of mullite precursors

The typical SEM microphotographs (Fig. 2) reveal that aluminum hydroxide was deposited on the surfaces of flyash particles based on different alumina contents (0, 23.10 and 33.30 wt.%). The mullite precursors basically maintain the spherical morphology of flyash. As the concentration of AlCl3 aqueous solution increases, the surface of the coated particles become slightly rough, indicating that more aluminum hydroxide was deposited. During the precipitation process of Al3+ cation, the

Conclusions

In this work, porous mullite ceramics were prepared with flyash and aluminum chloride as the starting materials. As a source of alumina, aluminum hydroxide was deposited on the surfaces of flyash particles by the heterogeneous-precipitation. Then the samples with and without aluminum hydroxide were fired at different temperatures to create a porous structure.

For the flyash samples, the transformation of quartz to cristobalite occurred at 1100–1250 °C, and therefore mullite-cristobalite porous

Acknowledgements

This work was financially supported by Ministry of Science and Technology of China under contract no. 2003CB615700. The editor and reviewers are gratefully acknowledged for their good advice.

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