Vanadium Speciation in Petroleum Cokes for Anodes used in Aluminium Electrolysis

The expanding aluminium industry is facing challenges with availability in raw materials. A wider range of petroleum cokes are being taken into use, blurring the border between anode grade and fuel grade coke in anode production. Vanadium, a common impurity in coke, is known to be a catalyst for air-reactivity burn-off, which may increase the consumption rate of the anodes. Additionally, vanadium is an unwanted impurity in the finished aluminium. Levels of vanadium and sulfur are typically correlated in petroleum cokes, but as vanadium is present mostly as porphyrins in crude oil, it is assumed this is the case in the resulting coke as well, rather than as vanadium-sulfur compounds. In this work the actual speciation of vanadium was investigated using X-ray absorption spectroscopy techniques. Six different industrial cokes were characterized with XANES, and they all seem to have the approximate same vanadium speciation. Based on the comparison with different vanadium reference standards, it was shown that vanadium is not present as the previously anticipated porphyrin. Based on the EXAFS spectrum, bonds between vanadium and sulfur were identified. Close to identical speciation of the vanadium was found in all the cokes, and this was reflected in a linear increase in air reactivity with vanadium content.