Development and scale-up of particle agglomeration processes for coal beneficiation

The development of two modified agglomeration processes for coal beneficiation is presented separately in Parts I and II of this dissertation. Part I is based on research which was conducted to study the mechanism and characteristics of a gas-promoted oil agglomeration process. Part II is based on research which was carried out to develop a newer and more innovative method for agglomerating coal particles with microscopic gas bubbles in aqueous suspensions.; In Part I, the development of a gas-promoted oil agglomeration process for cleaning coal was carried out with scale model mixing systems in which aqueous suspensions of ultrafine coal particles were treated with a liquid hydrocarbon and a small amount of air. The resulting agglomerates were recovered by screening. During batch agglomeration tests the progress of agglomeration was monitored by observing changes in agitator torque in the case of concentrated suspension. A key parameter turned out to be the minimum time te required to produce compact spherical agglomerates. Other important parameters included the projected area mean particle diameter of the agglomerates recovered at the end of a test as well as the ash content and yield of agglomerates. Batch agglomeration tests were conducted with geometrically similar mixing tanks which ranged in volume from 0.346 to 11.07 liters.; It was shown that gas bubbles trigger the process of agglomeration and participate in a very complex mechanism involving the interaction of particles, oil droplets, and gas bubbles. The process takes place in stages involving dispersion of oil and gas, flocculation, coagulation, and agglomerate building.; Numerous agglomeration tests were conducted with two kinds of coal in concentrated suspensions to determine the important characteristics of the process and to study the effects of the following operating parameters: i-octane concentration, air concentration, particle concentration, tank diameter, impeller diameter, and impeller speed. Several excellent correlations between the minimum time required to produce spherical agglomerates or a final agglomerate diameter and the operating parameters were obtained by using the general linear regression method. In addition, the results provided a basis for size scale up of an agglomeration system.; In Part II, the technical feasibility of a gas agglomeration method for cleaning coal was demonstrated by means of bench-scale tests conducted with a mixing system which enabled the treatment of ultrafine coal particles with a colloidal suspension of microscopic gas bubbles in water. A suitable suspension of microbubbles was produced by agitation and a small amount of i-octane. When the suspension of microbubbles and coal particles was mixed, agglomeration was rapid and small spherical agglomerates were produced. Since the agglomerates floated, they were separated from the nonfloating tailings in a settling chamber.; By employing this method in numerous agglomeration tests of moderately hydrophobic coals with 26 wt. % ash, it was shown that the ash content could be reduced to 6–7 wt. % while achieving a coal recovery of 75 to 85% on a dry, ash-free basis by using a solids concentration of 3 to 5 w/w %, air saturation of 5 to 15 psig, and i-octane concentration of 1.0 v/w % based on the coal weight. It was also shown that the process of agglomeration can be reversed by subjecting an aqueous suspension of agglomerates to a pressure sufficient to redissolve the microbubbles.