PREPARATION OF A COAL SURFACE FOR CONTACT-ANGLE MEASUREMENTS

Coal specimens of different ranks were polished using silicon carbide abrasive papers (with a grit from #60 to #1200) and alumina powder of varying size (from 5 to 0.05 mu m). The coal surface roughness and con tamination (by alumina powder) were examined with both scanning electr on microscopy and atomic force microscopy. The water advancing and rec eding contact angles were measured on such surfaces by varying the bub ble size, using the captive bubble technique. It was found that silico n carbide paper abraded all components of the coal surface, i.e. both organic and inorganic matter, to a similar depth. The roughness of the coal surface due to polishing with silicon carbide abrasive papers af fected the contact angle hysteresis and the contact angle vs. bubble s ize relationship. Polishing of coal specimens with alumina powder redu ced the microroughness of the coal surface but produced rough features at the macro level and caused mineral inclusions rising above the smo oth organic matter. This phenomenon results from the heterogeneity of coal specimens consisting of minerals and macerals with different hard ness values. The roughness at the macro level was easily distinguishab le and had a significant impact on the measured contact angles when th e coal surface was polished with coarse alumina powders, 5 and 1 mu m in diameter. The effect of surface roughness on the advancing and rece ding water contact angles was significantly reduced (if not completely eliminated) when the coal surface was polished with a fibrous cloth ( CHEMOMET) in the final step, after having been polished with 0.05 (0.0 6) mu m alumina powder. Microscopic observation of the coal surfaces r evealed that an appropriate ultrasonic treatment (8-10 min in an ultra sonic bath filled with water) and mechanical cleaning (polishing with a CHEMOMET cloth) of coal samples were required to remove the alumina particles left on the surface due to the previous polishing procedure. An improved methodology for coal surface preparation, prior to contac t angle measurements, as proposed in this paper, includes polishing wi th a series of abrasive papers and 0.05 (0.06) mu m alumina powder, po lishing and cleaning with a fibrous cloth (e.g. CHEMOMET), and, finall y an extended cleaning in an ultrasonic bath filled with water.