Surface properties of aged coal and their effects on bubble–particle attachment during flotation

Aged coal after mining often shows poor flotation performance due to the formation of hydrophilic oxygenated functional groups. In this study, different meta-bituminous aged coal was characterized by XPS and SEM analysis, and contact angle, zeta potential and bubble–particle attachment measurements. Following aging process, the changes in flotation behavior of the coal samples were quantified at different diesel dosages. It was found that the percentage of hydrophobic functional groups of the fresh coal was about 1.31 times higher than that in the case of the aged coal. The contact angle of the fresh coal was 78.9° while that for the aged coal was 36.7°. It means that the hydrophilicity of the aged coal samples was significantly higher than that of the fresh coal. The results of zeta potential, FTIR, XPS and SEM measurements agreed well with the contact angle results. The combustible flotation recovery of the fresh coal in the presence of collector was 98% while that of the aged coal was about 20%. Despite the significant differences in the surface properties of the fresh and aged coal, the induction time slightly changed i.e. the induction time for the most hydrophobic coal (i.e the fresh coal in the presence of collector) was 10 ms while that for the least hydrophobic coal (i.e. the aged coal in the absence of collector) was 11.6 ms. The attachment efficiency and flotation rate shows an exponential decay relationship with induction time.