The cleaning of ultrafine coal using microbubbles
Mechanized mining techniques that are commonly used in the coal industry produce a large amount of fine particles. These fines are extremely difficult to recover by conventional flotation methods, mainly because of the large size of the bubbles produced relative to the size of the particles. Hydrodynamic analyses have shown, however, that the use of smaller air bubbles can improve the flotation rate of these fines and, hence, the coal recovery. In the present work, a microbubble generator has been developed that produces bubbles smaller than 100 microns in diameter.
Batch flotation tests conducted on samples from five different coal seams have demonstrated that the microbubble flotation process produces improved recoveries and often cleaner products than the conventional flotation process. The higher recoveries are a result of the increased bubble-particle collision efficiencies obtained with smaller bubbles, and also the larger number of bubbles produced in the microbubble process. The improved selectivity has been explained tentatively by the longer froth residence time in the microbubble process, along with the increased bubble loading and the reduced turbulence around the microbubbles. Various techniques have proven successful in further improving the selectivity by reducing the entrainment and/or entrapment of ash in the froth.
To better understand the mechanisms of microbubble flotation, basic information regarding surface tension, contact angles, viscosity, streaming currents of microbubbles, electrophoretic mobilities of coal and mineral particles, and the stability of microbubble suspensions has been obtained using two non-ionic frothers. Microbubble flotation results obtained using each of these frothers have also been compared.