Availability of anode raw materials in the growing aluminum industry results in a wider range of petroleum cokes being used to produce carbon anodes. The boundary between anode grade cokes and what previously was considered non-anode grades are no longer as distinct as before, leading to introduction of cokes with higher sulfur and higher trace metal impurity content in anode manufacturing. In this work, the chemical nature of sulfur in five industrial cokes, ranging from 1.42 to 5.54 wt pct S, was investigated with K-edge XANES, while the reactivity of the cokes towards CO2 was measured by a standard mass loss test. XANES identified most of the sulfur as organic sulfur compounds. In addition, a significant amount is identified (16 to 53 pct) as S-S bound sulfur. A strong inverse correlation is observed between CO2-reactivity and S-S bound sulfur in the cokes, indicating that the reduction in reactivity is more dependent on the amount of this type of sulfur compound rather than the total amount of sulfur or the amount of organic sulfur.
Lower quality petroleum coke is required to be used in anode manufacture containing S and the metal impurities V, Ni and Fe. These affect the anode performance in the aluminium production process. The chemical identity of the metal species in the coke is not known.Industrial petroleum cokes with high S and containing V, Ni and Fe were analysed by XANES and EXAFS. XANES spectra were compared with standards. EXAFS was used to compare the impurity metal structures with known crystal structures. V is present largely as hexagonal V3S4. Ni is present largely as hexagonal NiS. Fe is present as hexagonal FeS.
XAS measurement and analysis
The quality of coke materials available for anodes for the aluminium industry is changing and industrial cokes with higher impurity levels are now introduced. The cokes in the anodes must meet specifications with respect to impurity levels to ensure proper operation in the electrolysis cells, and a desired quality of the aluminium metal. The presence of sulfur has been observed to reduce the CO2 reactivity and a certain level of sulfur is therefore targeted in the anodes. In this work, the significance of varying sulfur and metal impurity content in industrial cokes were evaluated with respect to CO2 reactivity, accessible surface area, pore size distribution, surface oxide groups and crystallite reactive edge planes. While relatively similar cokes are observed to give a lower reactivity with increasing sulfur content, cokes that have distinct differences in surface properties can have dissimilar reactivity despite identical sulfur content. Correlations between pore size distribution and presence of S-S bound sulfur, possibly condensed Sx, was also observed.
Lower quality petroleum coke is required to be used in anode manufacture containing S and the metal impurities V, Ni and Fe. These affect the anode performance in the aluminium production process. The chemical identity of the metal species in the coke is not known. Industrial petroleum cokes with high S and containing V, Ni and Fe were analysed by XANES and EXAFS. XANES spectra were compared with standards. EXAFS was used to compare the impurity metal structures with known crystal structures. V is present largely as hexagonal V3S4. Ni is present largely as hexagonal NiS. Fe is present as hexagonal FeS.
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