Contact angles on heterogeneous coal surface: The effect of mineral matter

Coal is an organic sedimentary rock of complex structure, composed of both organic matter as well as inorganic matter. It is demonstrated in this study that the discrete mineralogical nature of coal largely influences the wetting of the coal surface by water. Both advancing and receding contact angles were measured using captive-bubble and sessile-drop techniques with an automatic image analysis system. The distribution of mineral inclusions on the coal surface was examined by scanning electron microscopy. The coal surface region, on which the contact angles were measured, was separated from the bulk part by micro-slicing the coal samples, and the amount and size distribution of mineral grains in the coal surface region were determined through low-temperature ashing, followed by mass balance and particle size analyses. A significant scatter in contact angle values (up to +/-15 degrees) is reported for samples of the same coal, indicating poor reproducibility of heterogenous structures of coal at a microscopic level. Further, it was found that the increasing amount of mineral matter in coal samples reduced the contact angle value, although the scattered data clearly indicate that there are some other factors responsible for the contact angle variation. The results reveal that an important factor in analysis of contact angle variation on coal surfaces is the size of the hydrophilic mineral inclusions. Both the advancing and receding contact angles decrease with increasing size of the mineral grains and this effect is more pronounced for receding contact angles.