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ABSTRACT
An oxidized coal surface is more negatively charged and hydrophilic than that of fresh coal because of the introduction of polar oxygen functional groups. During flotation, the decreased attractive hydrophobic force between bubbles and coal particles and the increased repulsive electrostatic force result in a low recovery for oxidized coal. In the present study, preconditioning with positive-charged microbubbles (PCMs), generated using a microbubbles generator and a cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB), was proposed to enhance oxidized coal flotation. Conventional column flotation tests were also carried out for comparison. The results show that many oxygen functional groups (-OH and C-O) existed on the oxidized coal surface as indicated by X-ray photoelectron spectroscopy (XPS). A flotation recovery of 84.33% with an ash recovery of 82.97% was obtained when PCM was used, much higher than that of conventional flotation (a recovery of 17.82% with an ash recovery of 9.64%). PCM preferably adsorbed at the negative-charged hydrophilic sites on oxidized coal in the preconditioning stage. An additional attractive electrostatic double-layer force between oxidized coal and negative flotation bubbles was introduced, enhancing PCM preconditioning. However, the worst flotation selectivity was observed using PCM compared with that of conventional flotation based on Fuerstenau upgrading curves analysis.
Funding
This work was supported by the Fundamental Research Funds for Central Universities [Grant 2017XKZD04].