Sustainable application of coal fly ash: Alumina recovery from coal fly ash by low-temperature roasting with potassium persulfate

Abstract

This study introduces a novel process for recovering Al₂O₃ from coal fly ash (CFA), utilizing potassium persulfate (K₂S₂O₈) as an activator, and explores the decomposition behavior of both crystalline and amorphous alumina phases within CFA. The experiment involved mixing K₂S₂O₈ with CFA in K₂S₂O₈/Al₂O₃ molar ratios ranging from 0.5:1 to 4:1, followed by calcination at varying temperatures and durations to optimize alumina extraction. The results indicated that the alumina extraction efficiency of 91 % was achieved with a molar ratio of 3:1 (K₂S₂O₈/Al₂O₃), calcination temperature of 405 °C, and reaction time of 2 h. X-ray photoelectron spectroscopy, scanning electron microscopy, and reaction kinetics were employed to analyze the reaction mechanism. The findings revealed that K₂S₂O₈ preferentially reacted with the amorphous alumina phase, leading to the breakage of Al–O–Si bonds. The release of SO₂ and O₂ during the reaction further facilitated the breakdown of the crystalline phase by disrupting aluminum–oxygen tetrahedra. Simultaneously, the silicon-oxygen tetrahedra decomposes and forms SiO₂ gel layer, wrapping the remaining aluminum oxygen octahedron, thus preventing further alumina extraction. This study contributes to a deeper understanding of the decomposition behavior of crystalline and amorphous alumina phases in CFA during sulfate roasting. The findings provide a theoretical foundation for refining alumina recovery processes and offer significant improvements to existing sulfate roasting methods.