In this study, the effects of air bubbles and nanobubbles on flotation performance and kinetics of oxidized coal were investigated. The surface properties of the coal sample before and after oxidation were characterized by a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The nanobubbles on highly oriented pyrolytic graphite (HOPG) were observed by an atomic force microscope (AFM). The interaction between coal and conventional bubbles in the absence and presence of nanobubbles was explained by induction time. Flotation results showed that oxidized coal flotation in the presence of nanobubbles resulted in 10% higher combustible matter recovery than conventional air bubble flotation. Moreover, it was found that the flotation of oxidized coal in the absence and presence of nanobubbles can be best described using the first-order model with the rectangular model. AFM images analysis showed that a large number of nanobubbles were produced and attached to the oxidized coal surface. The induction times of the oxidized coal in the absence and presence of nanobubbles were 1000 and 39 ms, respectively, indicating that the existence of nanobubbles effectively promotes the interaction between oxidized coal and macroair bubbles. In addition, the agglomeration between oxidized coal particles also occurred spontaneously in the presence of nanobubbles, which was helpful in improving the combustible matter recovery and flotation rate of oxidized coal.
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