Study on removal of ultrafine graphite by nanobubbles-assisted flotation technique from graphite slime slurry

Abstract

Nanobubbles on solid surface could play a key role in developing novel nanobubbles-assisted flotation techniques for the removal of fine even ultrafine coal/minerals from relevant coal/mineral slurry, however surface nanobubbles nucleation process and its effect on the interaction between particles have not been studied yet thoroughly and systematically. In this work, HOPG, silicon wafer and alumina were employed as substrates for atomic force microscope imaging. Nanobubbles were produced with temperature difference. The results show that nanobubbles were nucleated at HOPG, hydrophobized alumina and silicon wafer surface while not founded at hydrophilic alumina and silicon wafer surface, indicating that surface hydrophobicity plays a more important role in nanobubbles nucleation compared with surface roughness. The force mapping results show that the graphite particle was subjected to a repulsive force during trace in the presence of nanobubbles and a high adhesion force between the graphite and the substrate was shown as the graphite was retraced away from the substrate. The high adhesion force is attributed to bridging effect of nanobubbles between particles. The larger graphite agglomerations were induced to form in the presence of nanobubbles. Besides, The collision and adhesion performance between graphite particles and a captive bubble was improved with nanobubbles nucleation. One hand, the formation of graphite agglomerations resulted in an increased collision probability between graphite and a bubble. On the other hand, the nucleation of nanobubbles on graphite surface could be helpful for graphite-bubble adhesion. Both of above two aspects should be responsible for the improved removal efficiency of graphite from graphite slurry with flotation technique.