Surface free energy and floatability of low-rank coal

Measurements were made of zeta potential of low-rank coals, the surface of which was previously covered with apolar liquid to different degrees. The zeta potentials were then used for determination of the film pressure of this liquid, making it possible to calculate the apolar Lifshitz-van der Waals (gamma(s)(LW)) component of the coal surface free energy. Electron donor (gamma(s)(-)) and electron acceptor (gamma(s)(+)) polar components were determined by measurement of water and glycerol contact angles on coal plates. The values of gamma(s)(-) and gamma(s)(+) were then compared with the non-dispersion component, gamma(s)(n), of the surface free energy. The values of the surface free energy components confirmed that the hydrophobic properties of a coal surface increase with increasing degree of coalification. This was also confirmed by tests on the natural floatability of these coals in a Hallimond single-bubble tube. Copyright (C) 1996 Elsevier Science Ltd.