A novel process to enrich alumina and prepare silica nanoparticles from high-alumina fly ash

Extraction of alumina from high-alumina fly ash (HAFA) enhances the value-added utilization of HAFA. However, this industrial application has been restricted by the high silica content of HAFA. Thus, alumina enrichment via the separation of silica is a key technique for HAFA utilization. In this study, a novel method of alumina enrichment by ammonium fluoride activation was proposed. The influence of the ammonium fluoride to silica in HAFA molar ratio (F/Si), the calcination temperature, and calcination time on the alumina to silica mass ratio (Al/Si) of the calcined product were investigated. The results showed that the Al/Si ratio reached 8.83 when HAFA with a F/Si ratio of 5.0 was calcined at 850 °C for 60 min. Silica in HAFA was transformed into gaseous ammonium fluosilicate, which was separated from the calcined product by condensation, thereby increasing the Al/Si ratio. However, impurities in HAFA inhibited further improvement of the Al/Si ratio. Subsequently, dissolution of ammonium fluosilicate with a surfactant produced silica nanoparticles with a surface area of 454.2 m2/g and a recyclable ammonium fluoride solution through an ammonification process. Thus, this work provides a novel strategy for the high-value utilization of HAFA.