Electrical properties and the surface characteristics of dispersions of colloidal particulates in coal-derived liquid

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Abstract
The conductivity, permittivity, dielectric loss, and electrolytic polarization of solvent‐refined coal dispersions are measured with respect to frequency (10–105 Hz), to concentration in the range from solids‐free liquid to nearly dry solid, and to some extent temperature in the range 290–313 K. Some measurements are also made on the oil, asphaltene, carbene, and insoluble carboid/mineral fractions isolated from centrifuge sediment and supernatant liquid. Washed insoluble matter samples having bulk mineral ash contents from 3 to 63% are also examined, using optical and scanning electron microscopy and surface analysis via electron spectroscopy. The asphaltenes, and to a lesser extent the carbenes, are distributed between the liquid and the particle phases; the latter contains all of the carboids, covering a core of mineral matter. Electrolytic polarization at the electrodes gives the ion concentration, which is about 1.7×10−9 molar for the liquid. Conductivity probably involves both ionic and electronic mechanisms, with the former arising mainly from alphaltenes. The presence of particulate matter decreases the conductivity and electrolytic polarization compared to the solids‐free liquid; this is particularly marked up to about 8% solids concentration and is likely to reflect association of acidic asphaltene and basic carbene ionizable species at the outer surfaces of the particles. Induced surface polarization of this associated ionic species, for which the maximum surface charge density is about 1.8×1010 charges/cm2, appears to be responsible for the increased permittivity in the presence of solids. The frequency dependence of the conductivity and permittivity correspond to Cole‐Cole dielectric relaxation, and suggest a narrow size distribution, with a mean of about 5 μm in diameter, for the relaxing species. The temperature dependence of conductivity and permittivity indicate that the concentration and mobility components of the activation energies are nearly equal. A small electrophoretic separation and a larger low‐frequency dielectrophoretic separation of particles and fluid are predicted.