Mechanical testing of anthracite to assess its surface energy and temperature dependence

In order to assess the applicability of mechanical method for testing the surface energy of coal, the surface energy of ten coal specimens of two macroscopic lithotypes (semidull and semibright) with metamorphic degree in anthracite stage and a carbonaceous mudstone specimen from a certain mine were tested and analyzed at different temperatures. The results showed that the measurements of surface energy are affected by the test methods, test temperature, coal macerals and minerals, and coal surface roughness. For coal and rock solid materials with strong heterogeneity, the mechanical method is simpler than the optical method and the error is smaller. The surface energy of coal specimens varies from 22.08 to 28.08 mJ.m(-2) at room temperature of 293 K by mechanical method. With the increase of temperature, the surface energy of the specimens decreases linearly, and the coefficient of variation of the measured surface energy of various specimens exponentially increased. The surface energy of carbonaceous mudstone specimen is the largest, followed by semidull coal specimens and semibright coal specimens. In contrast, the order of decrease rate of surface energy with temperature increases is semibright coal, semidull coal and carbonaceous mudstone. The surface energy of the specimens exhibited a negative exponentially decreasing trend as the surface roughness increased. The study results provide a basis for further improving the test methods of the surface energy of heterogeneous solid materials, and the application of engineering methods to improve the extraction efficiency of coalbed methane by changing temperature.