An EXAFS and XANES study of V, Ni, and Fe speciation in cokes for anodes used in aluminum production

The main ingredient in pre-baked carbon anodes used in the aluminum industry, is petroleum coke. Today, the aluminum industry faces challenges regarding the availability of what is considered anode grade coke. The increasing amount of impurities (e.g. sulfur and metals) in the crude oil end up in the low-quality product, coke. Petroleum coke that can be used in the aluminum production is calcined, producing calcined petroleum coke (CPC), and coke that have previously only been used as fuel, needs to be considered for CPCs, despite the high impurity content [1]. Impurities in coke are known to affect certain reactions in the cell; sulfur is believed to reduce CO2 reactivity, while metals are suggested as catalyzers increasing the CO2 and air reactivity [2]. The exact mechanisms of these reactions are not known but can be assumed to depend on the chemical composition of the impurities. The chemical sulfur compounds in CPC are known to mainly be organically polycyclic thiophenes and thiazines, as part of the carbon sheets [3, 4]. Metals have been assumed to be present as organic complexes, similarly to the compounds found in crude oil [5], but the nature of the metals have not previously been determined. Previous investigations by the authors [6], using synchrotron radiation, identified significant quantities of an additional sulfur compound to the mentioned organic sulfur. This was identified as a compound with S-S bonds, possibly including metal sulfides, but the high amount quantified in some cokes did not correlate to the metal content in the respective cokes. Further investigations on metal speciation in cokes were therefore suggested. X-ray absorption spectroscopy (XAS) techniques were discovered to be applicable to CPCs to decide the chemical compounds of the metals V, Ni, and Fe. By comparing the recorded fluorescence spectra of cokes to those of known compounds, using the X-ray absorption near edge structure (XANES) region of the spectra, Ni and Fe were suggested to be metal sulfides, and possibly some metal porphyrin. Using the extended X-ray absorption fine structure (EXAFS) region, and comparing to information on local structure, i.e. nearest atoms and bond lengths associated with variable space groups, found in large databases, all three metals were discovered to be bound as hexagonal metal sulfides (V3S4, NiS and FeS respectively). The metal sulfides were also discovered to be highly dispersed in the cokes, and not present as inclusions [7].

Keywords

Coke, Aluminium, Anodes, Industry, Impurities, Metals, Sulfur, Carbon dioxide, Synchrotron radiation, X-ray Spectroscopy

Citation

Jahrsengene, G., Wells, H. C., Sommerseth. C., Ratvik, A. P., Lossisu, L. P., Sizeland, K. H., Kappen. P., Svensson, A. M., & Haverkamp, R. G. (2020). An EXAFS and XANES study of V, Ni, and Fe speciation in cokes for anodes used in aluminum production. Paper presented to TMS 2020 and 149th TMS Annual Meeting and Exhibition, San Diego, California, 23- 27 February 2020, (pp. 275). Retrieved from https://www.tms.org/tms2020/downloads/TMS2020-Technical-Program-WEB.pdf