Ultrasonic flotation cleaning of high-ash lignite and its mechanism

Lignite, as a low rank coal reserve plays an important role in energy supply. However, fine lignite is difficult to upgrade using the conventional flotation because of its high surface hydrophilicity. This investigation was to enhance the flotation recovery of high-ash lignite combined with wet-screening, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and induction time measurer to reveal the mechanism of simultaneous ultrasonic treatment enhancing the lignite flotation. Flotation results indicate the clean coal yield is greatly increased whereas the clean coal ash content is highly reduced by simultaneous ultrasonic flotation compared with the conventional flotation. SEM and XPS results show coal surface is physically cleaned and hence the clay coating is reduced by simultaneous ultrasonic flotation. The removal of high-ash mineral particles from coal surface benefits the coal-bubble attachment. Furthermore, ultrasonic treatment creates many micro-bubbles that enhance the floatability of lignite through micro-bubble-based coating on coal surface. The micro-bubbles play a bridging role in the coal-bubble attachment. The water film on lignite surface may be also unstable and easy to rupture during bubble-coal attachment under the condition of ultrasonic treatment.