Mechanisms of Separation and Crystal Growth of Mullite Grains during Preparation of Mullite-Based Ceramics from High Alumina Coal Fly Ash

High-alumina coal fly ash (HAFA: 45% Al2O3 and 40% SiO2) is regarded as a special solid waste that is generated from power plants in northwestern China. It is regarded as an important substitute for bauxite and is applied to prepare mullite-based ceramics. In this work, a hydrometallurgy-pyrometallurgy synergistic process is proposed to resolve the lower Al2O3/SiO2 mass ratio (Al/Si) and lower degree of crystallinity that can promote the formation of compact mullite-based ceramics. During the activation-desilication process, the inert Al-O-Si is activated to form more active Si-O-H in acid activation, which can be decomposed easily in the desilication process, and the Al/Si mass ratio increases from 1.17 to 2.80, so the mullite grains and metastable phase can be exposed. During the sintering process, mullite grains and the metastable phase tended to axial growth, part of the metastable amorphous Q(4)(3,2,1Al) structure was transformed to Q(4)(4Al) structure (mullite), and then the staggered spatial structure was formed such that the bulk density and apparent porosity of the mullite-based ceramic reached 2.85 g/cm(3) and 0.5%, respectively. This process not only consumes more HAFA but also helps alleviate the shortage of bauxite, which will promote the development of clean coal-fired power generation.