Synergistic adsorption of polar and nonpolar reagents on oxygen-containing graphite surfaces: Implications for low-rank coal flotation

Conventional oily collectors cannot improve the floatability of low-rank coal because of the presence of containing-oxygen groups on the mineral surface, and the effect of the application of a single polar reagent is also limited. In this study, an innovative approach using molecular dynamics simulations to study the adsorption of mixed collectors on an oxygen-containing graphite surface was carried out. The adsorption structures of the collectors were clearly revealed, which has important implications for flotation recovery. In the simulations, when a nonpolar collector, dodecane, was used alone, it was only adsorbed on the carbon atom sites, and the hydrophilic oxygen-containing groups were not covered and remained exposed. In contrast, when a polar collector, dodecanoic acid, was used alone, the molecules self-aggregated, which reduced the surface coverage and increased the collector consumption. However, the use of a mixture of nonpolar and polar molecules resulted in a good dispersion over the mineral surface and significantly improved the coverage of oxygen-containing groups. The nonpolar molecules could be adsorbed on the oxygen-containing sites, being directed by the polar molecules, resulting in a synergistic effect. In addition, water contact angle tests show that the mixed collector can significantly improve the hydrophobicity of the low-rank coal surface, which is beneficial for improving the floatability of coal particles. Flotation tests verified the theoretical and experimental results: when the mass ratio of dodecanoic acid was 40%, the combustible matter recovery was significantly increased compared to that using a single collector. Consequently, the addition of a polar component reduces the required dosage of the traditional oily collector. This investigation provides a better understanding of the interaction mechanism of mixed collectors in low-rank coal flotation. (C) 2019 Elsevier Inc. All rights reserved.