Surface Coating Improves Coal–Water Slurry Formation of Shangwan Coal

Abstract

Shangwan coal is a low-rank coal from inner Mongolia. Shangwan coal particles show electronegativity in the slurry, because there are many oxygenated functional groups, such as the carboxyl group, hydroxyl group, and carbonyl group on the surface and in the core of the particles. Besides, because these functional groups are all hydrophilic, Shangwan coal shows strong water-retaining capacity. Hence, coal–water slurries (CWSs) made from Shangwan coal contain little free water, and their coal concentration is low. In this paper, we used four different chemicals to coat the surface of Shangwan coal and investigate the corresponding effects on slurry preparation. Tests of contact angle indicate that the physical characteristics of coal particles are adjusted, because surfactants cover the coal surface and the hydrophobicity of the coal surface is increased. Measuring the ζ potential confirms that the absolute value of the ζ potential of modified coal is higher than that of unmodified coal, for instance, about 18.6 mV when using the surfactant Span-40. The oxygen functional groups are covered by chemicals. Therefore, the hydrophilic groups on the surface of Shangwan coal could not combine with water, because the surfactant separates water from the hydrophilic groups on the surface of coal. Results of ζ potential also demonstrate that the surfactants improve the adsorption of dispersing agents on the surface of Shangwan coal, i.e., from −77.5 (unmodified coal) to −91.4 mV. Using Span-40 for surface coating, the maximum achieved coal concentration is increased from 58 to 61.5 wt %.