Energy & Fuels 1988,2, 163-170 the concentrated coal suspensions and that of very dilute suspensions, rather than unreliability of the mass transport method. The fact that bulk slurry properties correlate better with mass transport data than with the microelectrophoresis data leads to the same conclusion. The differences between electrophoretic mobilities measured in very dilute and in concentrated suspensions can be attributed in part to the higher ionic strengths in the concentrated slurries, which could be duplicated in microelectrophoresis experiments, and in part to particle-particle interactions and interactions between coal and soluble minerals in the concentrated suspensions, which could not be duplicated in dilute suspensions.
Conclusions For the 12 bituminous coal samples included in this study mass transport electrophoretic mobility of 50 wt % slurries correlated with slurry rheology and stability toward sedimentation better than electrophoretic mobility measured by microelectrophoresis in dilute suspensions, as previously found for four other bituminous c0a1s.l~ For the three coal samples on which a comparison was made, the pH dependence of ESA measurements in concentrated slurries was similar to that of mass transport measurements. Near mass transport isoelectric points, flocculation led to good sedimentation stability and maximum apparent viscosity. For coals containing only small quantities of soluble inorganic minerals, the lowest ash coals had the highest isoelectric points, near pH 7, while the highest ash coal had an isoelectric point below pH 3. The isoelectric point appears to depend upon the ratio of carbonaceous to clay areas on the heterogeneous surface. Oxidized coals
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had highly negative mass transport electrophoretic mobilities at pH 7, with very poor stability toward sedimentation, low viscosity, and hard-packed sediments. Particle-particle interactions may cause some of the discrepancies between microelectrophoretic and mass transport mobilities for these coals. Coals containing large quantities of soluble inorganic minerals formed high ionic strength slurries and had very low mass transport electrophoreticmobilities, accompanied by flocculation, across the pH range. For these slurries, the differences between mass transport and microelectrophoresis mobilities can be accounted for by the ionic strength variation between dilute and concentrated suspen sions. In the low ionic strength slurries, apparent viscosity was highest and shear thinning was most marked at the smallest median particle size where the average interparticle distance is smallest and the surface area per gram is largest. As the median size gets larger, surface chemical effects become less important until suspension properties depend only upon hydrodynamic effects.
Acknowledgment. This research was supported by the Pittsburgh Energy Technology Center, Office of Fossil Energy, U.S.Department of Energy, under Grant No. DE-FG22-84PC70765. We also thank Haya Atlas and Christopher A. Povirk for laboratory assistance during this study. Several former undergraduate students at Carnegie Mellon University, Karl Robinson, Michael Francis, Michael Banyas, and Robert Klemm, also carried out portions of the laboratory work. We especially wish to acknowledge the contribution of the late Geoffrey D. Parfitt, who initiated this study.
Effect of Weathering on Solvent Extraction of Coalst' David H. Buchanan,* Kenneth R. Osborne, Linda C. Warfel, Wanping Mai, and Douglas Lucas Chemistry Department, Eastern Illinois University, Charleston, Illinois 61920 Received July 31, 1987. Revised Manuscript Received January 19, 1988 Solvent extraction of coal, including use of anaerobic conditions, membrane filtration of extracts, and monitoring of solvent removal by FT-IR spectroscopy, produced colloid-free extracts as judged by ultracentrifugation. Material balances were typically 91-102%. Sequential extraction of several bituminous coals with toluene followed in order by THF, DMF, and pyridine showed that oxidation lowered the total extract yield from Illinois coals, shifted the distribution from polar-soluble toward nonpolar-soluble material, and increased the quantity of colloids isolated by membrane filtration of extracts. FT-IRspectra of the colloids revealed carboxyl groups and mineral matter. Each solvent in the listed order dissolved all of the material soluble in the previous solvents plus new material. Size exclusion chromatography of the extracts in pyridine revealed an increase in average molecular size from toluene-soluble through pyridine-solublematerial. Solvents such as DMF or pyridine, which bind tightly to coal surfaces, are conveniently removed by good wetting agents such as 80% (v/v) methanol/water. Extracts of unoxidized coals are very sensitive to air oxidation, producing insoluble precipitates with increased phenol contents as judged by FT-IRspectra. Oxidized extracts or extracts of oxidized coals lack the air-sensitive material and do not produce precipitates upon additional air exposure.
Introduction Extraction of solid fuels with solvents at low (
