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The interaction between long-flame coal particles and oily bubbles controls the crucial processes of attachment and mineralization in oily-bubble flotation thus dominates the recovery of coal. In this study, the interaction energy between long-flame coal particles and diesel oily bubbles was estimated by both DLVO and EDLVO theories, while the attachment of coal particles to oily bubbles was evaluated by induction time measurements and oily-bubble flotation tests. The results indicated that the isoelectric points of oily bubbles and coal particles were observed at about pH 3.5 and 2.3, respectively. Over the entire range of pH 2–11, the attachment between coal particles and oily bubbles was confirmed by the results of the induction time and oily-bubbles flotation tests. Additionally, it was found that the flotation recovery of coals was inversely related to the induction time. The attachment could be accurately predicted by the EDLVO theory over the tested pH range. On the contrary, it showed a great deviation for the DLVO theory to predict the attachment over the tested pH range except at pH 3. The reason for the deviation in prediction may be that the hydrophobic force on the interface of coal particles and oily bubbles is too great to be ignored. [ABSTRACT FROM AUTHOR]

The surface properties of coal play a crucial role in its flotation, therefore it is of great significance to investigate the changes in both chemical structure and surface morphology in oxidation process of low rank coal. In this paper, the mechanism of difficult floatation of oxidized coal was highlighted. The surface hydrophobicity and floatability of Meizhiyou long-flame coal before and after oxidation processing with H 2 O 2 solution at different concentration were evaluated by induction time and contact angle analyses as well as flotation tests, while the changes in its surface properties in the oxidation process were analyzed by XPS, FTIR, BET and SEM measurements. The maximum increase extent of induction time between bubbles and oxidized coal particles reached to 13217% while the maximum reduction extent of combustible matter recovery reached to 95.28%, which indicated the floatability of oxidized coal particles almost decreased exponentially along with the increase of concentration of H 2 O 2 solution. Additionally, the results suggested the active chemical functional groups of >CH 2 peak at 1437 cm −1 , CH 3 peak at 1373 cm −1 and C O C peak at 1215 cm −1 were susceptible to oxidation. After oxidation processing, on the one hand, the content of hydrophobic groups on coal surface showed an obvious decrease while that of hydrophilic groups showed a significant increase. What’s more, the content of carbonyl group had a close correlation with the extent of oxidation of the coal sample. On the other hand, it was found that the surface morphology of coals was significantly changed after oxidation process, and there were more pores and cracks on the coal surface. As a result, the contact angle of coals showed a significant decline after oxidation processing and it declined more quickly with the increase of contact time. Consequently, the poor flotation performance of oxidized coals may be attributed to the dramatic changes in both surface chemical structure and surface morphology in oxidation process. [ABSTRACT FROM AUTHOR]