Removal of AAEMs from Zhundong coal through ionic liquids leaching technique: leaching process and mechanism investigation

Zhundong coal, with low costs, large reserves, and high reactivity, contains abundant alkali and alkaline earth metals (AAEMs), leading to fouling and corrosion during combustion. This study investigates the removal of AAEMs from Zhundong coal using four ionic liquids ([PMIm]Cl, [BMIm]Cl, [PrSO3HMIm]Cl, and [BSO3HMIm]Cl). Single-factor experiments evaluated the effects of ionic liquid concentration, temperature, reaction time, and solid-to-liquid ratio on AAEM removal, determining optimal conditions. The results indicate that acidic ionic liquids demonstrate significantly enhanced removal efficiency for water-soluble, ammonium acetate-soluble, and acid-soluble AAEMs compared to their nonacidic ionic liquids, achieving approximately twice the total AAEM removal rate. Under optimal conditions, [BSO3HMIm]Cl achieved removal ratios of 83.51% for Na, 52.73% for K, 44.58% for Mg, and 67.77% for Ca. Kinetic analysis revealed that acidic systems involve two mechanisms: intraparticle diffusion and chemical reactions. Post-treatment analysis showed that ionic liquids cause coal swelling, structural loosening, increased micropore surface area, and functional group changes. Nonacidic ionic liquids remove slag-forming metals through swelling, while acidic ionic liquids combine swelling, ion exchange, and metathesis reactions. This study offers insights into AAEM removal for cleaner, more efficient coal utilization.