A XANES Study of Sulfur Speciation and Reactivity in Cokes for Anodes Used in Aluminum Production
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ODUCTION
HIGH-QUALITY carbon anodes are critical to the economy of aluminum production. Anode carbon, which is consumed during electrolysis, makes up around 11 to 13 pct of the cost of aluminum production.[1] Anodes are produced by mixing calcined petroleum coke, recycled anode butts, and coal tar pitch before being subject to a baking cycle up to 1200 C. Smelters require anodes with high-density, low-impurity levels (e.g., V, Ni, Fe, Si, Na, Ca, Mg, and Al), and low thermal expansion to achieve predictable performance in the cells, usually achieved by blending different cokes. Sulfur is usually specified around 1.5 to 2 wt pct based on operational experience, and in some cases restricted due to limitations on SO2 to the atmosphere for smelters without SO2 scrubbing. The availability of high-quality anode grade calcined petroleum cokes for use in anodes is declining, resulting in challenges for the aluminum industry. One reason for
GØRIL JAHRSENGENE and ANN MARI SVENSSON are with the NTNU Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] HANNAH C. WELLS and RICHARD G. HAVERKAMP are with the Massey University, Palmerston North 4222, New Zealand. STEIN RØRVIK and ARNE PETTER RATVIK are with the SINTEF Industry, 7465 Trondheim, Norway. Manuscript submitted September 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
the reduction in coke quality is that sour crude oils, with high sulfur and other impurity content and higher specific gravity, are now favored by many refineries because they are more available and of lower cost than the lightweight, low sulfur, sweet oils.[2] Petroleum coke is produced from the heavy residual fractions of crude oil, the fraction that tends to be highly concentrated in impurities (including sulfur), and with improved techniques for extracting the volatile fractions the quality of the coke decreases. This results in cokes with higher sulfur content, usually accompanied by an increase in metal trace impurities. An almost linear relationship between sulfur and vanadium content is observed in most cokes,[3] although the relationships tend to be more scattered for cokes very high in vanadium or sulfur. Vanadium promote carbon reactivity with air, resulting in an increase in anode consumption if the anode cover material is not completely sealing the top and sides of the anode.[4] It is important to understand that there is not a shortage of coke, it is only the availability of good anode-grade cokes that are not covering the demand of the aluminum industry. Hence, the smelters have to learn to cope with this gradual decline in availability of high-quality raw materials. Because of the limited availability of high-quality cokes, the use of shot coke, previously described as fuel grade, has been investigated as one alternative.[2,5] Sulfur is present in cokes in a variety of forms. In crude oil, more than 1500 sulfur compounds have been identified.[6] During calcination of green coke and
baking of the anode, thermal processes may ch
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