The unburned carbon (UC) content of fly ash is a vital parameter that governs its use in mortar and concrete. Flotation is an effective methodology for removing the UC contained in fly ash. However, the specific physicochemical properties of the surface of UC make it hydrophilic. In this study, we use a coal tar-based (CTB) collector to remove the UC contained in fly ash by flotation. The adsorption mechanism of CTB collector and diesel on the surface of UC was characterized with Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), induction time tests, and molecular dynamics (MD) simulations. The levels of oxygen-containing functional groups and aromatic ring compounds present in the CTB collector were considerably higher than those in diesel. The intensity of the absorption peaks of the hydrophobic functional groups (-CH₃, -CH₂) on UC surface increased significantly after CTB collector treatment. The XPS results demonstrated that the surface oxygen-containing functional group contents on the untreated UC, diesel-treated UC, and CTB collector-treated UC decreased in that order. The CTB collector significantly enhanced the floatability of the UC particles. The MD indicated that the aromatic compounds (dibenzofuran and naphthalene, etc.) in the CTB collector interacted strongly and adsorbed at deep locations with the UC surface compared to the alkanes (dodecane, etc.). The alkanes are indispensable as the collector in the fly ash flotation process and synergize with aromatic compounds to improve the mobility of water molecules on the UC surface.
 

Fly ash flotation; CTB collector; FTIR; XPS; SEM-EDS; Induction time；Molecular dynamics simulation