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 SiO 2 /Al 2 O 3 ratios, while Brunauer–Emmett–Teller measurements revealed a predominantly microporous structure (∼0.6 nm) and a high specific surface area (728.8 m 2 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.