A mechanism of Pennsylvania anthracite graphitization involving carbide formation and decomposition.

摘要

Four Pennsylvania anthracites were selected for this research based on presumed geological differences, in terms of mineral matter type and content, as well as amount of fixed carbon content. The LCNN, UAE, Joddo, and Summit anthracites were all heat-treated to the same temperatures for the same period of time, based on heating rate and soak time at maximum temperature. The heat-treatment temperatures were 2000, 2200, 2500, and 2640°C with a soak time of one hour. After heat-treatment it was found that all the anthracites graphitized, as evidenced by the presence of a weak (112) X-ray diffraction peak, but that the Summit anthracite achieved the highest degree of graphitization, in terms of d-spacing closest to 3.354 Å and largest crystallite dimensions. It was also determined that a non graphite phase was present in the 2200°C heat-treated anthracites, and that this non-graphite phase must be a reaction product, or carbide, formed from the carbon of the anthracite and metallic elements of the minerals. The carbide phase was no longer present in the 2500°C heat-treated anthracites, meaning that the carbide must have decomposed.; The possibility that this carbide formation and decomposition was involved in promoting graphitization was examined by demineralizing the Summit anthracite and adding the minerals rutile, quartz, iron oxide, and calcite back into this demineralized sample, as well as to the least graphitizing Joddo anthracite. These four minerals were selected because of computer controlled scanning electron microscopy (CCSEM) data, plasma emission data, and literature on industrial production of carbides and carbon electrode additives. The result of this demineralization and re-mineralization was that the Jeddo sample with added minerals achieved a higher degree of graphitization than the raw Jeddo anthracite, and that the demineralized Summit anthracite could no longer be considered a gaphitizing carbon because the (112) peak was absent from the 2600°C heat treated demincralized sample. However, when external minerals were added to the demineralized Summit anthracite, and the mixture was heat treated, the (112) peak re-appeared, and the re-mineralized Summit aithmite had a lower d-spacing and larger crystallite dimensions than the raw Summit sample. Kaolinite was also added to the raw Jeddo anthracite, but this addition had little impact on the graphitizability of the anthracite, indicating that not all minerals arc involved in promoting graphitization.; The mechanism by which these minerals enhance graphitization is by the formation of a carbide from the mineral and disordered carbon of the anthracite. As with any graphitizing carbon, the removal of disordered carbon is necessary for the development of planar graphene layers that are able to stack in an ABAB configuration. So, in this system, the formation of the carbide essentially means the removal of disordered carbon and its conversion into ordered structures. As temperature is increased, the carbide can decompose into the gaseous metal and carbon atom; the metal can react with more disordered carbon and the carbon atom can add to the edge of a graphene layer or fill a hole in the layer. The reaction mechanism for each mineral—rutile, quartz, calcite, and iron oxide—and the properties of the carbide formed from them are slightly different The specific reactions are discussed in Chapter 4.