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Coal is being examined as a replacement for both liquid fuels and carbon production. However, determining how to most effectively and economically utilize each coal will depend on the structure of the coal and the chemistry to convert the coal into the products of interest. A general indicator of coal structure is coal rank. Low rank coals have higher oxygen content and contain some ether linkages compared to medium to high rank coals. Anthracite is the highest rank of coal, and is an abundant and inexpensive natural resource typically used as the fuel for power generation. Most anthracites contain 92–98 wt.% carbon, virtually all of which is present as aromatic carbon in large polycyclic sheets that may contain 30 or more fused aromatic rings. Because of the high carbon content of anthracite coals, perhaps using anthracite to produce high-value carbon materials rather than as a fuel makes sense economically and when considering one of the main products of anthracite combustion is carbon dioxide; researchers have been exploring modifying the structure of anthracite to produce value-added marketable products, including synthetic graphite production, filler for cathodes in aluminum smelting, anthracite-based pitch, and activated carbons. 

Anthracite has also been examined for use as a filler in the production of cathodes for aluminum smelting. Before using anthracite as a cathode filler, it must be treated at temperatures far above 1000 °C. This can be achieved in two ways: 1) gas calcined anthracite (GCA) where fuel gas is heated to ~ 1300 °C or 2) electrically calcined anthracite (ECA), which uses electrical current to temperatures of 1600–2200 °C, the median temperature 1800 °C. Expansion (a.k.a. “puffing” or “exfoliation”) during treatment prohibits anthracite use as a cathode filler, and this can be minimized by selecting an anthracite with low sulfur content and/or volatile matter, or by pretreatment by ECA at or above 2000 °C. Low calcination temperature (1200–1700 °C) by GCA may also minimize expansion.

Several other pretreatments have been examined in order to improve the graphitization behavior of anthracites. By adding a hydrogenation process with hydrogen donor solvents, Atria et al. were able to improve the quality of graphitic carbon produced from anthracite . They suggested this approach increased the available carbon by hydrogenating and liquefying fragments of the anthracite. In addition to increased hydrogenation, they also found that reduced particle size and lower oxygen content could lead to greater degrees of graphitization. Gonzalez et al. explored the effect of anthracite particle size and H/C ratio on graphitization behavior, and found that an increased H/C ratio in the coal appeared to improve the graphitization of the coal, but found that reduced particle size slightly inhibited graphite formation.

In recent work in our laboratory, we have been ball milling anthracite in the presence of a hydrocarbon solvent (cyclohexene). We have demonstrated this process leads to carbon transformations particle size reduction, and low-temperature hydrogen evolution, with the hydrogen a product of dehydrogenation of the cyclohexene during millin. As discussed above, both hydrogenation and particle size will affect the transformation of the coal at calcination and graphitization temperatures. The objective of this work is to explore the effect of hydrogenative ball milling and calcination of anthracite as a pretreatment to graphitization. We utilized primarily X-ray diffraction (XRD) and temperature programmed oxidation (TPO) to assess the degree of graphitization (DOG) of the materials with various pretreatments, and discuss seeming discrepancies between the two assessment techniques. Electron microscopy (scanning [SEM] and transmission [TEM]) are presented for select samples to track particle size and morphology.

Source:  https://www.sciencedirect.com/science/article/abs/pii/S037838200900215X