Waste recycling of coal fly ash for design of highly porous whisker-structured mullite ceramic membranes

Coal fly ash, a solid state waste massively produced from coal combustion, is considered to be highly hazardous to the environment due to its persistently toxic trace elements. High-value added waste recycling is a promising technique to address this issue. In this work, a waste-to-resource strategy is proposed for design of highly porous whisker-structured mullite ceramic membranes derived from waste coal fly ash and Al(OH)₃ as raw materials and MoO₃ as a single sintering additive. These were characterized in terms of their dynamic sintering behavior, shrinkage, bulk density, porosity, phase evolution, microstructure, pore size distribution, N₂ permeation flux, and mechanical strength. Addition of molybdenum trioxide effectively inhibited the sintering densification of membranes while at the same time forming a metastable low viscosity liquid at lower temperatures. This enables formation of a novel and more highly porous whisker-interlocked structure and accelerates the growth of mullite whiskers with controllable morphologies. Without degradation of mechanical properties, the open porosity increased significantly from 41.65 ± 0.13% to 58.14 ± 0.15% with increasing MoO₃ content from 0 to 20 wt.% without any pore-forming agent, while shrinkage and pore size decreased. The method proposed in this study is expected not only to give a new and facile insight for high-value added recycling of waste coal fly ash but also to fabricate low-cost high performance ceramic membranes with novel structures for further environmental applications.