Key Takeaways & Executive Findings
- •• • IDS-4Na achieves a single leaching yield of 41.2% for Fe3+ while suppressing Ca2+ and Al3+ to <4%, with a selectivity ratio of 10.73, enabling high-purity iron recovery from CGFS-H for downstream metal extraction. • • Tartaric acid co-leaches Fe3+ (38.7%) and Al3+ (33.5%) with Ca2+ yield below 5%, achieving a selectivity of 14.73 for Fe/Al over Ca, which is critical for separating iron and aluminum from calcium in complex slag matrices. • • Citric acid preferentially leaches Ca2+ with a single yield of 71.5%, but also extracts Fe3+ (35.2%) and Al3+ (39.1%), yielding a low Ca selectivity ratio of 0.96, indicating the need for stepwise processing to achieve efficient calcium separation. • • Sequential leaching with IDS-4Na, tartaric acid, and citric acid yields cumulative extractions of 79.8% Fe3+, 65.08% Al3+, and 78.6% Ca2+, demonstrating a green, scalable route for recovering valuable metals from coal gasification slag, which is essential for producing hydrotalcite materials and reducing waste.
Abstract
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.
1. Introduction
Coal gasification fine slag (CGFS) is an unavoidable byproduct of coal gasification, a cornerstone technology for clean coal utilization in China. The slag contains 20–50% residual carbon and inorganic ash rich in SiO2, Al2O3, CaO, Fe2O3, and MgO. Current disposal methods, such as landfilling and co-combustion, fail to recover the valuable metal content, leading to resource waste and environmental contamination. While prior research has focused on separating residual carbon for porous materials, the comprehensive utilization of the inorganic fraction, particularly the selective recovery of iron, aluminum, and calcium, remains underexplored. The challenge lies in the complex matrix of CGFS, where these metals are often locked in aluminosilicate phases, requiring aggressive reagents that compromise selectivity and environmental sustainability.
This study addresses the bottleneck by employing organic acids with distinct chelating properties to selectively leach Fe3+, Al3+, and Ca2+ from the carbon-rich components (CGFS-H). The novelty lies in the stepwise use of tetrasodium iminodisuccinate (IDS-4Na), tartaric acid, and citric acid, each chosen for its preferential affinity toward specific metal ions. This approach not only achieves high extraction yields—79.8% for Fe, 65.08% for Al, and 78.6% for Ca—but also enables separation through sequential leaching, eliminating the need for harsh inorganic acids. The findings provide a sustainable pathway to transform CGFS from a waste stream into a feedstock for high-value hydrotalcite materials, aligning with circular economy principles and resource efficiency goals.
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NAN Tianhao, ZHOU Anning, HAN Rui, HAN Chunmeng, CHEN Heng, ZHANG Ningning, LI Bingying (2026). Efficient Leaching and Separation of Iron, Aluminum, and Calcium from Carbon-Rich Components in Coal Gasification Fine Slag Using Organic Acids. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(25)60612-8
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Frequently Asked Questions
What is the selectivity mechanism of IDS-4Na for Fe3+ over Al3+ and Ca2+ in CGFS-H, and how does it affect the purity of the leachate?
IDS-4Na, a tetrasodium salt of iminodisuccinic acid, exhibits a high affinity for Fe3+ due to its chelating structure that forms stable complexes with trivalent iron. Under optimal conditions, it achieves a single leaching yield of 41.2% for Fe3+ while suppressing Al3+ and Ca2+ leaching to below 4%, resulting in a selectivity ratio of 10.73. This high selectivity ensures that the iron-rich leachate has minimal contamination from aluminum and calcium, which is crucial for downstream purification and the production of high-purity iron compounds.
How does the stepwise leaching process achieve cumulative yields of 79.8% Fe, 65.08% Al, and 78.6% Ca, and what are the optimal conditions for each step?
The stepwise process leverages the selective affinities of the three organic acids. First, IDS-4Na is used to leach Fe3+ preferentially, achieving a yield of 41.2%. Then, tartaric acid is applied to co-leach Fe3+ and Al3+, with yields of 38.7% and 33.5%, respectively, while Ca2+ remains below 5%. Finally, citric acid is used to leach Ca2+, achieving a yield of 71.5%. The cumulative yields are the sum of the individual steps, reaching 79.8% for Fe, 65.08% for Al, and 78.6% for Ca. Optimal conditions include specific acid concentrations, temperatures, and leaching durations, which are detailed in the full paper but are not fully disclosed in the abstract.
What are the scalability challenges of using organic acids for metal recovery from CGFS, particularly regarding cost and environmental impact compared to inorganic acids?
Organic acids such as citric acid, tartaric acid, and IDS-4Na are generally more expensive than inorganic acids like sulfuric acid. However, they offer advantages in selectivity, reducing the need for extensive purification steps. Additionally, they are biodegradable and less corrosive, lowering environmental and equipment costs. The stepwise approach minimizes reagent consumption by using each acid for its specific purpose, potentially offsetting higher unit costs. A detailed techno-economic analysis is not provided in the abstract, but the high yields and selectivity suggest that the process could be economically viable, especially when considering the value of recovered metals and the avoidance of waste disposal costs.
How does the presence of residual carbon in CGFS-H affect the leaching efficiency and selectivity of the organic acids?
The carbon-rich components (CGFS-H) contain 20–50% residual carbon, which can influence leaching by adsorbing organic acids or metal ions, potentially reducing efficiency. However, the study demonstrates that effective leaching is achievable, indicating that the carbon matrix does not significantly hinder the complexation process. The organic acids likely interact with metal ions on the surface of mineral phases, and the carbon may act as a porous support, enhancing contact. The exact impact is not quantified in the abstract, but the high yields suggest that the carbon content does not pose a major obstacle.
What is the potential for using the leached metal solutions to synthesize Ca/Fe/Al hydrotalcite materials, and what are the expected properties?
The leached solutions containing Fe3+, Al3+, and Ca2+ can be used as precursors for hydrotalcite-like compounds, which are layered double hydroxides with applications in catalysis, adsorption, and flame retardancy. The stepwise separation ensures that the metal ratios can be precisely controlled, which is critical for tailoring the hydrotalcite composition and properties. The abstract states that this work provides a foundation for developing such materials, but specific synthesis conditions and resulting properties are not detailed. Future work would involve co-precipitation or other methods to form hydrotalcite, with expected properties depending on the Mg/Al/Fe ratios and synthesis parameters.
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