Elsevier

Materials Chemistry and Physics

Volume 213, 1 July 2018, Pages 518-524
Materials Chemistry and Physics

Preparation and characterization of mullite powders from coal fly ash by the mullitization and hydrothermal processes

https://doi.org/10.1016/j.matchemphys.2018.04.056Get rights and content

Highlights

  • The mullitization increased with increasing the boehmite addition.

  • The mullite phase firstly increased and then decreased above 1200 °C.

  • The optimal mullitization was achieved at 1200 °C with 12% boehmite.

  • The mullite powder consisted of prismatic and isotropic crystallites.

  • The evolution of different crystal phases was analyzed in detail.

Abstract

Coal fly ash generated from thermal power plants is the largest single solid waste, and its reutilization is indispensable for the environmental protection and resource sustainability. In this work, mullite powders have been prepared from coal fly ash by the mullitization and hydrothermal processes. In the mullitization process, the milled fly ash was mixed with boehmite sol, and sintered to increase the mullite content. The mullite content firstly increased and then decreased with the increase of sintering temperature above 1200 °C, while it increased progressively with the increasing boehmite addition, especially above 8%. The mullite content of 63.8% was achieved with 12% boehmite at 1200 °C. In the subsequent hydrothermal process, the sintered powders were treated in the alkaline solution, and immersed in the acidic solution. The optimal mullite powder consisted of 96.4% mullite and 3.6% quartz. The secondary prismatic crystallites (0.3–1 μm) coexisted with the primary spherical crystallites (∼0.1 μm). The combined processing route sheds light on the potential production of mullite powders from coal fly ash.

Introduction

Coal fly ash that has been generated from the thermal power plants is the largest single solid wastes in China. The annual production amounts to 565 million tonnes in 2016, while the reutilization rate is less than 70% [1]. The fly ash reutilization not only prevents the environmental pollutions [2], but also reduces the production cost of industrial products. The related reutilizations involve construction materials, ceramics, agriculture, catalysis, deep separation, adsorbents, zeolites, etc [[3], [4], [5], [6], [7]].

Coal fly ash has been usually employed as the main component of cements and concretes, resulting in the significantly reduced production costs and improved performance [8]. The fly ash-based geopolymers were regarded as green construction materials, showing the excellent mechanical properties and durability comparable to that of hydrated Portland cements [9,10]. Specifically, alumina powders could be extracted from fly ash by the sintering and leaching methods, which have been the alternative routes to the conventional alumina production from bauxite mineral [11,12]. The recovery rate of alumina was increased to 96.6% by the combination of CaCl2 calcination and H2SO4 leaching [13].

Coal fly ash was also used as the starting material to prepare mullite-based ceramics and porous membranes [14]. When the powder mixture of fly ash and alumina was sintered at 1500 °C, the mullite ceramic had an optimal fracture strength of 395 MPa [15]. With the assistance of composite catalysts, light-weight and high strength mullite network was prepared by direct sintering from the powder mixture of fly ash, bauxite and kaolin [16]. The alkali-activated fly ash was sintered into the mullite ceramic tiles, with the relative density of 91% and flexural strength of 110 MPa [17]. By the way, mullite foams and membranes have been prepared from activated fly ash, enabling the reutilizations of fly ash in the thermal insulators, soundproof materials, gas purification and wastewater treatment [[18], [19], [20]].

In addition to the alumina powder, mullite powder was recycled from fly ash by deep desilication and molten salt methods [21,22]. Generally, fly ash with high mullite content is favorable for the recycling of mullite, whereas most fly ashes from the thermal power plants contain the mullite contents below 20% [23,24]. Regarding the cost-effective production, the mullite content should be remarkedly increased by the addition of exotic aluminum source. In this work, fly ash was mixed and sintered with the boehmite sol to greatly increase the mullite content. The variation of mullite phase with the sintering temperature and boehmite addition was studied in terms of the phase composition analysis. After the hydrothermal treatment and acidic immersion, the purified mullite powder was characterized by the phase composition and particle morphology.

Section snippets

Materials

The fly ash powder was obtained from a thermal power plant in Hebei province. The chemical reagents of sodium hydroxide and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. The distilled water was used in the milling and washing procedures. As the milling media, zirconia beads with the diameters of 2–5 mm were purchased from Guangzhou Pleased Grinding Media Ltd.

Preparation of mullite powders

Fig. 1 shows the flow chart of processing route. To remove the residual carbon, coal fly ash was calcined at

Characterization of fly ash powders

Fig. 2 shows the SEM image and XRD patterns of decarbonized and milled fly ash powders. The milled powder showed the agglomerated and spherical nanoparticles (∼100 nm), which adhered to submicron-sized grains, confirming the enhanced surface energy of particles after the milling process. The size distribution presented an average particle size of 19.6 μm, consistent with the serious agglomeration of primary particles. In Fig. 2(b), pristine fly ash consisted of mullite (Al2.272Si0.728O4.864,

Conclusions

Mullite powders have been successfully prepared from coal fly ash by the mullitization and hydrothermal processes. The formation secondary mullite phase was facilitated by increasing the sintering temperature, but retarded above 1200 °C. The boehmite addition below 8% was nearly ineffective on the mullitization due to its dissolution in the glass melt. Above 8% boehmite, the mullite phase increased progressively with increasing the boehmite content, implying the competitive relationship between

Acknowledgements

This work was financially supported by the China Scholarship Council (Grant No. [2016] 5113) and the Postgraduate Creative Fund of Nanchang Hangkong University (YC2016024).

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