Stability of coal-deprived particles in organic media

Modern coal-derived liquids, such as those used herein, are new to the scientific community, and a physical and thermodynamic property data base did not exist. Characterization and property determinations were thus necessary prerequisites to the search for a valid mechanism. The results of this effort constitute a significant contribution to the information available on these materials for general use by designers and experimenters.

A method of solubility class fractionation originally developed for petroleum asphalts was adapted to coal liquids. The behavior of the resulting component classes was found to be the key factor in delineating the mechanisms of the stability of particles in coal liquids. The component classes - asphaltols, resins, and oils - were separated according to their solubility in pyridine, benzene, pentane, and propane, respectively. Important physical and thermodynamic properties (viscosity, density, dielectric constant, and conductivity) of these fractions were determined as a function of temperature to aid in subsequent determinations of properties related to particle stability. In many cases, these quantities have not previously been available to other investigators and are much in demand.

Paraffins had previously been shown by a number of experimenters to produce agglomeration of particles in coal liquids. Microscopic examination of various solutions of the above component classes with and without n-decane provided definitive evidence that agglomeration of solids did not occur in oils and resin solutions containing solids, unless asphaltenes were present. This observation provided early insight into the mechanism of agglomeration by adsorption of asphaltenes on particle surfaces.

Empirical determinations of the stability of coal particle dispersions in pure hydrocarbon solvents were made by measuring their settling characteristics in solutions containing Aerosol OT (sodium di-2-ethylhexyl sulfosuccinate, commonly called AOT). Rapid settling was observed in all solvents in the absence of AOT. The stability of the coal-derived particles was less than that of a known reference material, carbon black, but was still classified as moderate (30% increase in light transmittance in 4 hours). Asphaltenes appeared to be highly stable, with practically no change in transmittance after seven days. Particle stability in coal liquids could not be measured by this technique (spectrophotometer) due to the inherent opacity of the liquids.

A special cell with a depth of ~100 µm had to be constructed to obtain qualitative zeta potentials (a measure of the charge associated with particles, and thus their stability) in semiopaque coal liquids. While the significant wall effects in this cell precluded the assignment of quantitative numbers to the zeta potential, the qualitative determinations made on various particles provided an order-of-magnitude relationship between the coal-derived materials. Assuming the zeta potential of solids in oils and resins to be 1, the zeta potentials of asphaltenes and asphaltols were higher by factors of 2.7 and 1.9, respectively.

A 1-mm-deep cell was constructed to obtain quantitative estimates of zeta potential in the "oils" fraction of the coal liquids. The depth of this cell was sufficient to allow determination of the proper level required for measurement but not to attenuate the transmitted light necessary for the measurements in the coal-derived oil. The results obtained with this cell for carbon black in pure organic liquids were in good agreement with the few literature values available and with values from a third instrument, a commercially available "Zeta Meter." Zeta potentials for coal-derived particles calculated by Von Smoluchowski's equation were found to be as high as -60 mV in cyclohexane with AOT, -45 mV in benzene with AOT, and +24 mV in coal-derived oil. (These values would be -90, -67.5, and +36, respectively, using the Hückel equation.)

Stability criteria for particles in aqueous electrolyte systems have been well established. A few authors have attempted an extrapolation to organic systems, generally concluding that, if significant charge exists, the solubility criteria for aqueous systems can be used by taking into account the extended double layer of charge that exists in organic systems. With this consideration, the double layer repulsion van der Waals attraction model indicates that the +24 mV charge on the coal-derived particles is sufficient for stability without the necessity to consider other factors such as steric hindrance. The zeta potentials measured herein were also shown to be related to the empirical stability as determined by settling characteristics.

A mechanism for establishing the surface charge was developed based on conclusive observations from this work and is supported by the experiments of others. The proposed "Coaloid" model explains the high stability of particles in coal liquids.

The stability of coal-derived materials during long-term storage was also an area of particular interest. Elemental analyses (C, H, N, O, and S), ultimate analyses (ash, heat content, and solids content), and selected physical property (density and viscosity) determinations were performed each month during the aging period. Examination of aging effects over nearly a year showed that the materials did not change significantly under typical storage conditions.