Role of prewetting/immersion time in the attachment time between air bubble and Taixi oxidized coal

Taixi oxidized coal surface has many hydrophilic functional groups as well as many cracks and holes. The effect of prewetting time on the flotation behavior of Taixi oxidized coal had been observed in our previous studies, and it was surmised that the oxidized coal surface was easier wrapped by water shell because the hydrophilic functional groups could be bonded with water through hydrogen bond and the cracks and holes could also be filled up with water when the coal was prewetted in the flotation cell. However, the mechanism of how the prewetting/immersion process can affect the flotation performance of Taixi oxidized coal is still poorly understood and should be investigated clearly. This paper was to conduct the fundamental studies on the effect of prewetting/immersion time in the attachment time between air bubble and Taixi oxidized coal. It was found that the attachment time increased quickly with the increase of prewetting/immersion time. The micro-bubbles are produced on the oxidized coal surface when the coal particles are immersed in the water. The micro-bubbles may be primarily from the air entrapped in the cracks and holes on the oxidized coal surface. As the prewetting/immersion time increased, the micro-bubbles would dissolve and get away from coal surface, and then the cracks and holes would be fully filled up with the water. Finally, the oxidized coal surface would be wrapped by a thick hydration shell which could prevent the oxidized coal surface from being attached by the bubbles, and hence the bubble-coal attachment time increases with the increase of prewetting/immersion time.