Hydrodynamic and surface chemistry effects in coal-oil agglomerate flotation.

Abstract

Oil agglomeration combined with bubble flotation has proven to be an effective means of upgrading and recovering waste fine coal when low-oil floc-type agglomerates are formed. Low-oil recovery is desirable, since less ash is incorporated into the agglomerates and the expense of using oil is reduced. In this state, the flocs are too small and weak for screen separation but can be successfully recovered by flotation as even a small amount of oil imparts a much improved surface character, and agglomerate strength is less important. Reduction of the amount of wetting phase used in the oil agglomeration process for advance coal cleaning and utilization technologies is important to the economics of the process. An analysis was made of the agglomerate surface properties and on the size enlargement characteristics and possible mechanisms of the process taking place when floc-type coal-oil agglomerates are formed at oil levels less than about 2% oil by weight. Since this product exists not as dense agglomerates but only as small aggregates of a few particles, the adhesion technique was used to determine the surface properties of agglomerates made from a high purity coal. The surface properties closely matched those of the oil at agglomerate oil-levels above 2% by weight. Particle-size measurements by laser diffraction were then correlated to the surface properties as a function of the oil-level and of agglomerate particle packing. Subsequently, the surface free energies of agglomerates made from coals of several different ranks were measured by the adhesion technique. Oil levels from 0 to 10% by weight were used. As was expected, it was found that higher-rank coals are more easily oil-wetted and that the surface properties of the agglomerates correlate with agglomeration recovery and ash removal data. It has been illustrated in pilot studies that oil-agglomeration together with flotation gives an enhanced combustibles recovery and ash separation. To corroborate successful flotation results for such agglomerates, a computational fluid dynamics simulation was formulated to model their recovery. This simulation employed the finite volume method, and featured three-phase flow as well as the collection of particles by bubbles. Additionally, a novel formulation of a boundary element method for unsteady full Navier-Stokes flow was undertaken to model the collisional aspects of micro-flotation. This method was successfully implemented, but problems with numerical instability in the required flow regimes prevented a more detailed micro-flotation study in the present work. The surface properties of coal-oil agglomerates were incorporated into a model based on the resolved hydrodynamics of the flotation process. The overall result was an integrated model where individual local mechanisms could be summed to determine recovery values which compared very well to a wide range of data. (Abstract shortened by UMI.)