Coal gasification fine slag (CGFS) is a significant source of solid waste requiring improved treatment methods. This study primarily investigates the mechanism of ultrasonic treatment in optimising flotation-based decarbonization of CGFS and its impact on CGFS modified with surfactants. The objective is to maximise the carbon ash separation effect to support the clean and efficient utilisation of CGFS. Flotation experiments revealed optimal conditions at an ultrasonication power of 180 W for 2 min and a slurry concentration of 60 g/L, resulting in a residual ash content of 82.59%. Particle size analysis, scanning electron microscopy (SEM), and Brunner−Emmet−Teller (BET) measurements demonstrate the efficacy of ultrasound in extracting inorganic minerals from the surface and pores of residual carbon, consequently reducing both pore and particle sizes. Fourier transform infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analyses indicate alterations in the surface chemistry of CGFS induced by ultrasound treatment. The content of hydrophilic groups decreased from 31.64% to 29.88%, whereas the COO- group content decreased from 13.13% to 8.43%, consequently enhancing hydrophobicity. Adsorption experiments demonstrate an increase in surfactant adsorption capacity following ultrasonic treatment. Furthermore, ultrasonic treatment facilitates the desorption of surfactants previously adsorbed onto the surfaces of CGFS residue. Therefore, optimal flotation is obtained by applying ultrasonic pretreatment to CGFS before adding flotation chemicals. Upon the addition of Polysorbate (Tween-80), the residual ash content increased 90.17%.
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