Recycling of mullite from high-alumina coal fly ash by a mechanochemical activation method: Effect of particle size and mechanism research

Highlights: • HAFA shows a structure with mullite crystal core and silica-rich amorphous shell. • Al₂O₃/SiO₂ mass ratio was elevated to 3.02 via mechanochemical activation method. • Effect of particle size after milling on desilication process was investigated. • Hydroxysodalite coating layer was suppressed by decreasing particle size. • Mullite refractory obtained from DHAFA exhibits excellent physical properties. High-alumina coal fly ash (HAFA) is a special solid waste since its alumina content can reach 40–50 wt%, which is seen as a potential resource for mullite material production. However, obtaining an ideal mullite material from HAFA is difficult because of its low Al₂O₃/SiO₂ mass ratio. In this work, the microstructure characteristics of HAFA were systematically analyzed by combining multiple characterization techniques. It was found that HAFA had a core-shell structure with a mullite/corundum crystal core and a silica-rich amorphous phase shell. The novel mechanochemical activation–desilication process was used to remove amorphous phase from HAFA and elevate the Al₂O₃/SiO₂ mass ratio. In particular, the effect of particle size after mechanical treatment and mechanism of the desilication process were extensively investigated. On decreasing the particle size, a high leaching rate of alumina was achieved during mechanochemical activation, thus generating a hydroxysodalite coating layer as desilication was suppressed, and the amorphous phase was effectively removed. The mineralogical phase of the desilicated HAFA is mainly mullite and corundum, and the Al₂O₃/SiO₂ mass ratio was elevated from 1.29 to 3.02. Mullite refractory obtained from the desilicated HAFA exhibited excellent physical properties. This study provides insights into further high-valued utilization of HAFA.