THE STABILIZATION OF COAL-OIL MIXTURES

In this study a more fundamental understanding was developed of the stabilization of coal-oil mixtures (COM) by three methods: (1) the addition of water, (2) the addition of chemical additives; and (3) the control of the particle size distribution (PSD) of the coal (so-called ultrafine COM). Aggregation of the coal particles was determined to control both the sedimentation and rheological properties of the COM. The degree of coal particle aggregation depended upon the nature and PSD of the coal powder, on the nature of the oil used, and the use of chemical additives, such as water and/or surfactants.; Sedimentation stability of COM prepared with coarse particles (80% < mesh, 65% < 325 mesh) is achieved by the aggregation of the coal particles. This leads to the formation of a network of particles and/or aggregates. The oil is entrapped in the interstices of the network and thus sedimentation is prevented or severely retarded.; The particle size distribution of the coal: (1) directly affects the sedimentation rate of the coal and (2) affects the degree of particle aggregation. The sedimentation stability of ultrafine COM is not directly due to the fineness of the powder but due to the formation of a network of flocculated particles.; The effect of the water in a COM depends strongly on the relative hydrophilic/oleophilic nature of the coal powder. The wettability of coal powders was evaluated by the Pickering emulsion test and a spherical agglomeration test. The results indicated that some samples particularly high ranking coals were oleophilic, while lower ranking coals were hydrophilic. The coal-oil-water (COW) mixtures prepared from hydrophilic coals were more sedimentatively stable than the COW mixtures prepared from oleophilic coals. The stabilization of hydrophilic coals involves the development of a particle network which arises as a result of the formation of water bridges between the coal particles. The strength of the structure arises from capillary forces which promote the adhesion of the particles in the network. The stabilization of oleophilic coals by water involves the formation of an emulsion of water-in-oil, which may be stabilized by the coal fines in the oil.; Anionic SAA were least sensitive to the coal type and in general enhanced the aggregation stability of the suspension. The stabilization may be either the result of the electric double layer mechanism or the solvation mechanism.; The effect of cationic SAA, nonionic SAA and polymer additives depended upon the specific chemical structure of the SAA, the water content of the COM and the type of coal. For most of these additives, no strong correlations between the relative hydrophilic/oleophilic nature of the coals and the effectiveness of the SAA was observed. This suggests that the specific chemical nature of the coal's surface, rather than the mean oleophilicity or hydrophilicity may control the nature of the SAA/coal interactions.