Development of a thermodynamic model for carbon crystallites and their impurities

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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.