Sulfur speciation in aluminum smelting anodes

Carbon anodes used in aluminum smelting typically contain 1-4 wt% sulfur, originating from the petroleum cokes used in anode fabrication. When the anode is consumed, sulfur gases are emitted, in particular COS and SO2, which are detrimental to the environment. In addition, the presence of sulfur affects the reactivity of the anode. To understand the mechanisms of sulfur gas formation and the influence of sulfur on anode reactivity, it is important to know the chemical form sulfur takes in the anode. In this study, XANES (X-ray absorption near-edge structure) spectroscopy was used to determine the sulfur speciation of petroleum cokes from major suppliers and of anodes with differing thermal histories. It was found that organic sulfur containing five- and six-membered ring structures were the dominant sulfur species in the cokes studied, reflecting the origins of the coke, and that these species were stable with anode baking and usage.