Evaluating the Cyclic CO ₂ Capture Performance of NaX and NaY Zeolites Synthesized from Blast Furnace Slag under Sulfur-Containing Flue Gas

Blast furnace slag (BFS) and blast furnace gas (BFG) are byproducts of the ironmaking process. The silicon and aluminum in BFS can be utilized to prepare zeolite adsorbents, and the carbon dioxide (CO2) and carbon monoxide (CO) in BFG have a great potential for recovery and purification. Integrating the two enables a “waste-treats-waste” strategy. In this work, FAU-type zeolites were synthesized through a hundred-liter scale from BFS via acid leaching and hydrothermal conversion, yielding BFS-X and BFS-Y with Si/Al ratios of 1.4 and 2.0, respectively. Static adsorption measurements indicate that BFS-X exhibits higher CO2 adsorption capacity (82.04 cm3/g) and stronger binding sites, whereas BFS-Y demonstrates lower and more uniform isothermal adsorption heat (Qst), making it easier to desorb in VPSA systems. Ideal adsorption solution theory (IAST) calculations indicate that BFS-Y exhibits a CO2/CO selectivity of 32.61, outperforming the BFS-X selectivity of 18.29 and demonstrating superior selectivity. In a twin-column vacuum pressure swing adsorption (VPSA) test simulating blast furnace gas at 313 K and an operating pressure of 0.2 MPa, the BFS-Y-type sphere achieved higher CO2 product purity (64.35%) and better sulfur tolerance, while the BFS-X type achieved a slightly higher CO2 recovery rate (77.70%); the productivity of the two was similar. Sulfur adsorption cycling experiments and DFT calculations further indicated that FAU-X has stronger H2S affinity and sulfur deposition capacity compared to FAU-Y. In summary, FAU-type zeolites derived from BFS─particularly the BFS-Y spherical pellets─provide a scalable pathway for the valorization of BFS, yielding adsorbents that balance selectivity, cycling performance, and sulfur resistance in CO2 capture applications, making them suitable for VPSA processes.