Effect of middle-temperature pyrolysis on the surface hydrophobicity of sub-bituminous coal

Middle-temperature pyrolysis is widely used to convert sub-bituminous coal into gas/liquid products and the coal char, which benefits the utilization of low rank coal resources. However, the coal char usually contains high-ash content because the volatile components in coal release from coal particle forming gas/liquid products while most of high-ash mineral components remain in the coal char. Therefore, the upgrading of the coal char is usually required to meet the requirement of calorific value for burning. It is necessary to find out the effect of middle-temperature pyrolysis on the surface hydrophobicity of coal. In this study, the effects of pyrolysis temperature (700, 800, and 900°C) and pyrolysis time (30 and 90 min) on the surface hydrophobicity of sub-bituminous coal were comprehensively investigated. X-ray photoelectron spectroscopy (XPS), attachment time, and flotation tests were used to reveal the changes of surface hydrophobicity and floatability of sub-bituminous coal before and after middle-temperature pyrolysis. The XPS results indicated the content of hydrophilic oxygen-containing functional groups was reduced while the content of hydrophobic functional groups on coal surface was increased after the pyrolysis. The attachment time of coal particle-bubble was reduced while the flotation recovery of coal was increased after the pyrolysis. The surface hydrophobicity and floatability of sub-bituminous coal were enhanced by middle-temperature pyrolysis, which makes the upgrading of the coal char feasible.