Effective Removal of Fe and Ca From Coal Fly Ash for High‐Purity and High‐Surface‐Area Zeolite X Synthesis: A Comparative Study of Acid Refluxing and Acid Roasting Methods

Coal fly ash (CFA) is an abundant industrial waste rich in silica and alumina and represents a viable feedstock for zeolite synthesis; however, high levels of calcium and iron limit zeolite purity and crystallisation. This study evaluates acid refluxing and acid roasting as pre-treatment strategies for the simultaneous removal of Ca and Fe from high calcium, sub-bituminous CFA. Acid refluxing was performed using hydrochloric acid under controlled reflux conditions, while acid roasting involved thermal treatment with hydrochloric acid in a muffle furnace. The effects of temperature (80°C-115°C), treatment time (0.5-2 h) and acid concentration (5%-37%) were systematically investigated. Both methods reduced impurity contents to below 2%, with acid refluxing achieving higher removal efficiencies (99% Ca and 97% Fe) than acid roasting (95% for both). Zeolite X synthesised from reflux-treated CFA exhibited improved crystallinity and purity. X-ray fluorescence spectroscopy and X-ray diffraction analyses confirmed enhanced SiO₂/Al₂O₃ ratios, while Brunauer-Emmett-Teller measurements revealed a predominantly microporous structure (∼0.6 nm) and a high specific surface area (728.8 m² g⁻¹), compared with mesoporous, low-surface-area materials obtained via acid roasting. These results demonstrate the importance of pre-treatment in controlling zeolite properties derived from CFA.