Development of a thermodynamic model for carbon crystallites and their impurities

碳晶体是由一层层的石墨烯层构成的。它们的尺寸以平均晶体长度（Lc）为特征，通常在纳米级范围内。它们构成了用于铝生产的Hall-Heroult工艺的大部分碳阳极（在煅烧的焦炭中）和石墨阴极。它们也存在于锂电池的石墨阳极中。在Hall-Heroult工艺中，每年要使用数百万吨的碳阳极。硫是这些阳极中的主要杂质（通常为2-4 wt.%），来自石油焦。在焦炭煅烧过程中，在处理碳阳极之前进行，形成碳晶，并与硫和其他杂质（H, Na， Ca等）结合在一起。最终，硫在这个过程中被消耗，被氧化并排出到大气中。但与此同时，焦炭中的硫和几种杂质正或负地参与了空气和CO2对碳氧化的催化作用。这个项目的短期目标是获得一个热力学模型，以吉布斯能量函数的形式，在铝工业使用的碳材料中发现碳晶体及其主要杂质（S， H, Na，…）。该模型将取决于杂质的质量分数、温度、总压和平均晶体长度（Lc）。这包括硫在石墨烯层（S， H）的末端C原子中的附着，以及由Na， Li和k形成的所谓石墨插层化合物。该模型将与晶体的结构有关，并打算用于碳材料中杂质的行为，作为焦炭煅烧，阳极烘烤和电解（阳极+阴极）过程中操作条件的函数。该模型也可用于锂电池的应用。该项目的长期目标是将上述模型与申请人已经获得的碳材料中无机杂质（盐，基于C-S-O-Na-K-Ca-Fe-Ni-Al-F的氧化物）的其他热力学模型相结合，以获得Hall-Heroult工艺中阳极过量碳消耗（25-40%）的过程建模能力，从而减少能源和环境影响。

Carbon crystallites are formed of stacks of graphene layers. Their size is characterized by the average crystallite length (Lc), which is typically in the nanometric range. They compose the majority of carbon anodes (in the calcined coke) and graphite cathodes used in the Hall-Heroult process for aluminum production. They are also present in graphite anodes in lithium batteries. In the Hall-Heroult process, millions of tons of carbon anodes are used every year. Sulfur is a major impurity in these anodes (typically 2-4 wt.%), coming from petroleum cokes. During coke calcining, done before processing the carbon anodes, the carbon crystallites are being formed, and sulfur and other impurities (H, Na, Ca, etc.) are bonding to them. Ultimately, sulfur is being consumed in the process and is oxidized and rejected to the atmosphere. But in the meantime, sulfur and several impurities in the coke are taking part positively or negatively in the catalysis of carbon oxidation by air and by CO2. The short-term objective of this project is to obtain a thermodynamic model, in the form of a Gibbs energy function, for carbon crystallites and their major impurities (S, H, Na, ...) found in carbon materials used by the aluminum industry. The model will be dependent on the mass fractions of the impurities, the temperature, the total pressure, and the average crystallite length (Lc). This includes the attachment of sulfur in the terminal C atoms of the graphene layers (S, H) and the so-called graphite intercalation compounds formed with Na, Li and K. The model will be related to the structure of the crystallites, and is intended to be used for the behavior of impurities in carbon materials as a function of operating conditions during coke calcining, anode baking and electrolysis (anode + cathode). The model can also be used in Li batteries applications. The long-term objective of this project is to couple the above model with other thermodynamic models already obtained by the applicant for inorganic impurities in carbon materials (salts, oxides based on C-S-O-Na-K-Ca-Fe-Ni-Al-F) to gain process modeling capabilities of the excess carbon consumption (25-40%) of anodes in the Hall-Heroult process, thus reducing the energy and the environmental impacts.