Coal Flotation And Fine Coal Utilization PDF Download

Fine coal flotation has been a longstanding problem in industry. Coal particles below approximately 38 microns in diameter are difficult to float, and the process consumes large amounts of reagents. Hydrodynamic analyses have shown, however, that the use of air bubbles smaller than those that are generated in conventional flotation machines (0.2 to 3 mm diameter) can improve the flotation rate and, hence, the coal recovery. Theoretically, a tenfold reduction in average bubble size should result in a thousandfold increase in the flotation rate constant at a given gas flow rate. Therefore, work has been done to use microbubbles less than 100 microns in diameter for the flotation of fine coal particles. Seven different U.S. coal samples have been tested in the present work. The feed size varies from -100 mesh to -500 mesh. Flotation kinetics tests have been conducted on some of these coal samples as a function of bubble size at a constant gas flow rate. The results show a drastic improvement in flotation rate with the use of microbubbles, which may account for the improved recoveries obtained with the microbubble flotation technique. In addition, test results obtained with ultrafine coal samples ( -20 microns) indicate that the microbubble flotation process is more selective than conventional flotation. This improved selectivity has been explained tentatively by the increased bubble loading and the reduced turbulence around the microbubbles. Various techniques have been employed to further enhance the selectivity of the process by minimizing the ash entrapment problem. To better understand the mechanisms of microbubble flotation, basic information regarding surface tension, contact angle, viscosity, streaming currents of microbubbles, electrophoretic mobilities of coal and mineral matter, and stability of microbubble suspensions has been obtained. 50 references, 42 figures, 9 tables.