Novel separation of the differing forms of unburned carbon present in fly ash using density gradient centrifugation

Although the increasing amount of unburned carbon in fly ash is known to impede the efficient use of ash in the cement industry, very little is known about the characteristics of the unburned carbon. Three different types of carbon particles have been identified microscopically: inertinite, isotropic coke, and anisotropic coke, with little information on their relative properties. This work presents the first isolation of these three types of carbon. Two fly ashes, identified as Dale and WEPCO, obtained from different power plants, were examined in this study. The methodology used includes a preliminary triboelectrostatic enrichment, followed by density gradient centrifugation (DGC) with a high-density lithium polytungstate media (2.85 g cm^-3 max). One of the parent fly ash samples, Dale, was also demineralized, then separated in a similar manner without the preliminary triboelectrostatic carbon-enrichment step. For the three separations, maximum purities ranged from 77 to 85 vol % for inertinite, 63-79 vol % for isotropic coke, and 69-76 vol % for anisotropic coke. The most efficient enrichment was obtained for inertinites, considering this material comprises <9% of the DGC feed. In all cases, maximum purities were obtained at approximately 1.50-1.65 g cm^-3 for inertinite, at 1.72-1.78 g cm^-3 for isotropic coke, and at approximately 1.88-1.95 g cm^-3 for anisotropic coke. The density of like carbon forms isolated from the two fly ashes appears to be quite similar, regardless of the source of the fly ash.