Efficient Leaching and Separation of Iron, Aluminum, and Calcium from Carbon-Rich Components in Coal Gasification Fine Slag Using Organic Acids
Coal gasification fine slag (CGFS) is a solid waste generated in large quantities during coal gasification, containing residual carbon and inorganic ash rich in SiO2, Al2O3, CaO, Fe2O3, and MgO. The carbon-rich components (CGFS-H) of CGFS, typically comprising 20–50% residual carbon, present both environmental challenges and opportunities for resource recovery. This study systematically investigates the selective leaching behavior of Fe3+, Al3+, and Ca2+ from CGFS-H using three organic acid extractants: citric acid, tartaric acid, and tetrasodium iminodisuccinate (IDS-4Na). The results demonstrate distinct selectivity: IDS-4Na exhibits the highest leaching yield and selectivity for Fe3+, achieving a single leaching yield of 41.2% while suppressing Ca2+ and Al3+ leaching to below 4%, with a selectivity ratio of Fe3+ to Al3+ and Ca2+ of 10.73. Tartaric acid effectively leaches both Fe3+ and Al3+, with single yields of 38.7% and 33.5%, respectively, while Ca2+ leaching remains below 5%, yielding a selectivity of Fe3+ and Al3+ relative to Ca2+ of 14.73. Citric acid preferentially leaches Ca2+, achieving a single yield of 71.5%, but also leaches Fe3+ and Al3+ at 35.2% and 39.1%, respectively, resulting in a low selectivity ratio of Ca2+ to Fe3+ and Al3+ of 0.96. Based on these selective affinities, a green stepwise separation method was developed using sequential leaching with IDS-4Na, tartaric acid, and citric acid. Under optimal conditions, cumulative leaching yields of 79.8% for Fe3+, 65.08% for Al3+, and 78.6% for Ca2+ were achieved. XRD, XRF, and SEM analyses elucidate the complexation mechanisms, indicating that the synergistic effects of selective coordination between structurally diverse organic acids and metal ions drive the process. This advancement provides a critical foundation for developing Ca/Fe/Al hydrotalcite materials using CGFS-H as a sustainable feedstock, promoting resource-efficient utilization of coal gasification fine slag.