Synthesis of ultrafine boehmite from waste coal fly ash by integrating sol-gel process with sodium aluminate decomposition and hydrothermal treatment

Boehmite, as an important functional material, has wide applications in catalysts, flame retardants, and lithium battery separator coatings. In this study, a novel method is proposed for the synthesis of nano/micron-sized boehmite by integrating the sol-gel method, sodium aluminate decomposition, and hydrothermal process. This synthesis employs AlCl 3 — derived from hydrochloric acid extraction of Al 2 O 3 in waste coal fly ash — to synthesize boehmite with mean particle sizes ranging from 0.13 to 4.31 μm. For representative nano-sized boehmite synthesis, optimal parameters are pH of 4, hydrothermal temperature of 200 ℃, and duration of 12   h. The Al(OH) 3 precursor preparation requires a caustic ratio of 1.5, Na 2 O concentration of 160   g/L, seed coefficient of 0.15, decomposition temperature of 65 ℃, duration of 24   h, and agitation rate of 90   r/min. The typical micron-sized boehmite synthesis is optimized at a pH of 12 with hydrothermal treatment at 180 ℃ for 12   h. Under these conditions, nano-sized boehmite exhibited a mean particle size of 180.55   nm, while micron-sized boehmite displayed a mean particle size of 4.31 μm. Nano-sized boehmite exhibits fibrous, hexagonal, and rhombic morphologies, while micron-sized boehmite is predominantly hexagonal. The transformation mechanism proceeds via dissolution-precipitation of the Al(OH) 3 precursor.