Investigation of Bituminous Coal Hydrophobicity and its Influence on Flotation

The hydrophobicity of bituminous coal surface has a significant effect on the separation of coal and mineral matters by flotation process. In this work, water contact angles on bituminous coals displaying different brightness in terms of specular reflectance were measured to characterize their surface hydrophobicity. It is found that water contact angle on coal increases with specular reflectance for both bright and dull coals, and the bright coals have higher contact angle and specular reflectance than the dull coals. The oxidation of coal surface will render it less hydrophobic, but has little effect on the specular reflectance. Results from petrographic analysis show remarkable differences in volume percentage of maceral groups for bright and dull coals. Bright coals contain more vitrinite, less inertinite and mineral mater, than the dull coals. X-ray photoelectron spectroscopy (XPS) analysis indicates that the dull surface has more hydrophilic, oxygen-bearing functional groups. These results imply that vitrinite is more hydrophobic than inertinite, and should float better than the latter. The combustible recoveries of bench scale flotation tests increase with the corresponding contact angles and their relationship can be accurately described by a sigmoidal equation. The maximum combustible recovery for vitrinite is 96.76% under the current operation conditions. The results also indicates that in order to reach the targeted combustible recovery in flotation, the contact angle of coal particles needs to reach certain value.