Improving the floatability of coal with varying surface roughness through hypobaric treatment

Surface roughness has a great impact on coal floatability, and usually the effect is found to be negative. In this paper, hypobaric treatment was introduced to improve the floatability of coal with varying surface roughness. The floatability was characterized by induction time, and the micro-bubble nucleation theory was proposed to explain the enhancement mechanism of hypobaric treatment. The results show that the induction time was increased with increasing surface roughness. However, hypobaric treatment was proven to be an effective method in mitigating the adverse effects of surface roughness on floatability. Under natural conditions, neither Wenzel nor Cassie model could be used to describe the wetting regime of coal surface. Instead, a mixed wetting state, where water partially wets the asperities on coal surface and partially sits on entrapped air pockets, was more appropriate. It was difficult to drain the entrapped water at the upper part of the grooves; hence, bubble-coal attachment was prevented, leading to the increased induction time at high surface roughness. After hypobaric treatment, the diffused gas molecules in water preferred to nucleate at the sites of air pockets, leading to the formation of micro-bubbles. These micro-bubbles would act as the bridge between macroscopic bubbles and the coal surface, significantly decreasing induction time and improving the coal contact angle and floatability.