Mechanism of shale oil as an effective collector for oxidized coal flotation: From bubble–particle attachment and detachment point of view

[Display omitted] •More unsaturated fatty acids and long-chain hydrocarbons promoted the application of shale oil in oxidized flotation.•Surface hydrophobicity of oxidized coal was enhanced and resulted in a high flotation recovery.•Shale oil shortened the induction time and promoted bubble–oxidized coal attachment.•Shale oil inhabited the bubble–oxidized coal detachment more efficiently than that of diesel. Due to the high oxygen content on oxidized coal surfaces, it is difficult to realize high recovery efficiency during oxidized coal flotation with traditional oily reagents, whereas a polar mineral oil may be an effective alternative flotation collector. In this study, shale oil, which is found in abundant reserves and has many promising applications, was adopted to explore its effect on oxidized coal flotation, while traditional diesel was used for comparison. The surface physicochemical properties of coal before and after oxidization were analyzed by X-ray photoelectron spectroscopy and the chemical compositions of the collectors were characterized via Fourier transform infrared spectroscopy and gas chromatography/mass spectrometry. The strengthening mechanism of oxidized coal flotation using shale oil was systematically explained based on studies of bubble–particle attachment and detachment by induction time, wrap angle, and bubble–particle attachment and detachment visualization tests. Flotation results prove that shale oil can evidently improve the clean coal yield. Compared with diesel, the induction time in the presence of shale oil can be shortened from 500 to 103 ms. Shale oil can promote bubble–oxidized coal attachment and form a larger three-phase contact line, and thus prolong or even hinder the detachment process of bubble–particle more efficiently. More particles were attached to the surface of the bubbles in the presence of shale oil. The above phenomenon was attributed to the unsaturated fatty acids and greater number of long-chain hydrocarbons present in shale oil, which may result in a more strongly hydrophobic oxidized coal surface and promote bubble–particle attachment.