Sulfur Speciation in Aluminum Smelting Anodes - ACS Publications

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Ind. Eng. Chem. Res. 2004, 43, 1690-1700

Sulfur Speciation in Aluminum Smelting Anodes Suzanne J. Hay,† James B. Metson,*,‡ and Margaret M. Hyland† Department of Chemical and Materials Engineering and Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand

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 fiveand 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. 1. Introduction Petroleum coke, the major component of the consumable anodes used in aluminum smelting, is also the dominant source of sulfur to the process. Depending on the coke source used, the sulfur content of the resulting anode can vary from 1 to 4 wt %. Petroleum coke is the residuum fraction of petroleum refining and is typically the lowest-value product, so it is the fraction in which sulfur species tend to be concentrated, to optimize the quality of the other fractions. Sulfur is incorporated into crude oil when H2S and polysulfides produced by anaerobic bacteria react with either unstable organic structures1 or, particularly in marine environments, ferrous ions to form pyrite in the early stages of petroleum deposit formation.2 More than 1500 sulfur compounds have been identified in crude oil, with a dominance of organic sulfur containing fiveand six-membered ring structures.1 The sulfur species present in any one coke will therefore depend on the location and mechanism of formation of the parent crude oil. As progressively more sour crude oils are refined, there is an industry trend toward increasing sulfur content of the coke and, thus, of the anodes subsequently produced for smelting.3 The sulfur impurity and the chemical form of the sulfur in anodes need further consideration for several reasons, in particular the generation and environmental significance of COS and SO2 gases and the recognized influence of sulfur on anode reactivity.4 With respect to anode reactivity, sulfur has a beneficial effect, binding Na impurities in the anode and rendering them less active in catalyzing the nonelectrolytic consumption of the anode, particularly through reaction with CO2.4 However, this advantage of higher anode sulfur must be balanced against the negative impacts of higher sulfur gas emissions, changes in electrochemical consumption, and a potential increase in porosity associated with desulfurization on baking.5 * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (64 9) 3737 5999. Fax: (64 9) 3737 422. † Department of Chemical and Materials Engineering. ‡ Department of Chemistry.

It has been established that COS is the dominant sulfur gas to form at the anode as the anode is consumed during aluminum electrolysis;6,7 however, little of this species survives in the air draft drawn over the cell for fume capture. In most smelters, this cell gas is then transported through ducting to a dry scrubber that functions primarily to remove gaseous and particulate fluorides. Measurements of COS gas throughout the emissions capture system show that some COS survives in the cell ducting leading to the dry scrubber and that there is little, if any, reduction in COS emissions due to scrubbing.8 All of the remaining COS is released directly into the atmosphere. Recent publications suggest that the aluminum industry is a significant anthropogenic emitter of COS into the atmosphere.9,10 Carbonyl sulfide is implicated in ozone depletion reactions and also has one of the longest lifetimes of all sulfur gases in the atmosphere.11 SO2 gas forms largely via oxidation of COS when the gases produced at the anode are released into the cell hooding. It is the dominant sulfur gas entering the dry scrubber and is poorly scrubbed in the dry scrubber, but subsequent wet scrubbing is used in some jurisdictions to effectively remove this gas.7,8 Currently, there is legislation in parts of U.S., Europe, and especially Scandinavia to limit emissions of this gas,12 and it is anticipated that similar legislation elsewhere in the world will follow in the future. Knowledge of the chemical form of sulfur in the anode is the logical starting point for understanding the mechanism of sulfur gas emission and sulfur influence on reactivity. Carbon anodes are made from coke filler (85%) and 15-17% coal tar pitch binder. The filler is usually a mixture of calcined petroleum coke (65%) and a recycled fraction of anodes that have been partially consumed in the cell. This fraction is known as the “butts”. The formed anode is baked to about 1100 °C prior to use in the reduction cell. The coke in the anode is thus subjected to three thermal processes that raise its temperature to ∼1000 °C: calcination prior to anode forming, anode baking, and thermal history during its life in the aluminum cell. At each stage, the chemical form of sulfur can also change.

10.1021/ie0301031 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004

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XANES (X-ray absorption near-edge structure) spectroscopy has previously been successfully used to determine sulfur speciation in carbonaceous materials such as coals and oil shales13-15 and potentially offers a method for doing the same in anodes and cokes. It can probe the nature of both organic and inorganic sulfur bonding, with sufficient sensitivity to address this application. More common methods for obtaining chemical structure information such as XPS (X-ray photoelectron spectroscopy) either do not have sufficient sensitivity or do not provide sufficient chemical information to determine the nature of sulfur in complex structures such as coke and anodes.4 In the present study, XANES spectroscopy was applied to determine the nature of the sulfur species present in petroleum cokes from major suppliers to the aluminum industry and to determine the influence of heat treatment on the form of sulfur in anodes. 2. XANES Spectroscopy For a given element, absorption of an X-ray photon at an energy around the ionization threshold of a particular orbital leads to excitation of electrons into low-lying unoccupied states above the Fermi level and subsequent emission of electrons or photons. The spectrum is generally reported as total (or partial) electron, Auger, or fluorescence yield as a function of the incident X-ray photon energy, as this energy is scanned over the edges corresponding to various electron shells. The K and L edges of sulfur were measured by choosing the appropriate photon energy ranges to excite electrons from the K (∼2400 eV) and L (∼160 eV) shells in the sulfur species. Photoelectrons resulting from such excitations can be scattered from neighboring atoms. The backscattered wave interferes constructively or destructively with the outgoing wave, depending on its wavelength and the distance to neighboring atoms.13 The conventional extended X-ray absorption fine structure (EXAFS) region, which starts ∼50 eV above the edge, is dominated by single scattering events.13 The fine structure in the 0-50 eV near-edge region reflects both the density of states above the Fermi level, or above the band gap for semiconductors, and the influence of multiple scattering.13 The absorption spectrum for sulfur is reported as either the fluorescence yield (FLY) or total electron yield (TEY), with the analyses representing different sampling depths in the material. The spectrum shows an envelope of peaks representing the partial density of states above the Fermi level. The position of the edge and the structure of the envelope provide a detailed chemical fingerprint of the sulfur speciation in the anode or coke sample. The XANES spectra for sulfur in anodes and cokes were measured at both the K and L edges in an energy range extending to 50 eV beyond the edge, which is at 2472.5 eV for elemental sulfur at the K edge and 162.7 eV at the L edge. The K edge is useful for determining major differences in sulfur speciation, such as oxidation state changes, from the peak shift of the edge, whereas the lower-lying L edge gives richer structural information and better peak resolution, allowing, for example, for the distinction between similar organic sulfur species.14 3. Methodology The experiments in this study were performed at the Synchrotron Radiation Center (SRC) in Stoughton, WI,

Table 1. Petroleum Coke and Anode Samples Studied Using XANES name

S (%)

C6575 C6628 C6440 C6438 C4130 C6448 C6435 C6445 anode coke

Cokes 1.75 3.67 2.60 1.03 2.73 2.29 0.81 1.32 1.37

401118 425102 425703 101405 401118 401118 butt

Anodes 1.19 1.14 1.11 1.14 1.09 0.65 1.05

equivalent tempa (baking) (°C)

944 1117 1124 1280 1400 1500

a Note: Equivalent temperature is calculated from the L c determining the degree of graphitization of the sample.17

on the 1 GeV Aladdin storage ring. Synchrotron radiation is a unique form of radiation, providing a continuous spectrum of photon energies over a wide range between the soft and hard X-ray regions at a flux that is several orders of magnitude more intense than that of conventional X-ray sources.15 Eight petroleum cokes, representing the major suppliers to the aluminum industry, were analyzed using XANES spectroscopy. Additionally, six anode samples, with varying baking temperatures, and one anode butt sample (the remainder of a used anode, exposed to temperatures ranging from 450 to 900 °C top to bottom16) were analyzed, along with their parent petroleum coke. The sulfur content of each petroleum coke and anode are detailed in Table 1. Coke sulfur contents were supplied with coke samples. Anode sulfur contents were measured using X-ray fluorescence (XRF) spectrometry (2 g of ground sample, mixed with 3 g of boric acid powder and pressed into a pellet for analysis). Samples were generally analyzed as ground powders. XANES spectroscopy was used as a fingerprinting technique, and so, reference sulfur species were also analyzed to represent all major forms of inorganic and organic sulfur that might be present in cokes and anodes. These spectra were then compared to the coke and anode spectra to determine the sulfur speciation of the samples of interest. The choice of reference compounds was guided by the range of formal oxidation states and chemical environments that have been suggested to exist in other coals, asphaltenes, etc. The compounds, their sources, formal oxidation states, and representative chemical environments are summarized in Table 2. Separate beamlines were required to access the K and L edges as each monochromator is suited only for a narrow range of photon energy; a DCM (double-crystal monochromator) was used to measure the S K edge (photon resolution ) 0.9 eV) and a Grasshopper monochromator to measure the S L edge (photon resolution e 0.4 eV). Both beamlines are part of the Canadian Synchrotron Radiation Facility (CSRF) at the SRC. Both the fluorescence yield (FLY) and the total electron yield (TEY) were measured for each sample at both the K and L edges. Spectra were normalized to the beam current, I0, to account for fluctuation and decay in the beam

1692 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 2. Reference Compounds Used for the XANES Study Representing Different Sulfur Species

current. Spectra were also calibrated in energy by setting the elemental S K-edge white-line positions to 2472.5 eV (at the K edge) and 162.7 eV (at the L edge) and were calibrated to the pyrite and sulfate edges at regular intervals over the measurement period. The edge position was determined using derivative curves. A linear background subtraction was then performed by extrapolating a background line from the preedge region. Spectra were finally normalized to the height of the maximum for the L edge and to the height of the jump edge for the K edge.14 Semiquantitative analysis was also carried out using K-edge data (semiquantitative analysis at the L edge is not possible because of the complexity of the spectra) by summing various reference spectra together and performing a least-squares analysis to obtain the best fit to a chosen sample spectra.22 4. Reference Sulfur Species Results Selected reference XANES spectra of sulfur species from this study are shown in Figures 1 (organic) and 3 (inorganic) at the S K edge and in Figures 2 (organic) and 4 (inorganic) at the S L edge. It was found that, although the peak shift in the K edge, as a function of oxidation state, was usually sufficient to distinguish inorganic species, such as S22-, S8, and SO42- (Figure 1), the fine structure at the L edge was more useful for organic species, such as the straight-chain and cyclic structures in Figure 4. Having confirmed that a large range of sulfur species could be fingerprinted in this

way, these XANES reference spectra were then compared to spectra of petroleum cokes and anodes. The XANES spectra for the inorganic reference compounds, Figures 1 and 2, were collected in the total electron yield (TEY) mode. The results are plotted in order of oxidation state of the sulfur atom, as this is an important factor in determining the position of the edge on the energy scale. In general, the recorded inorganic spectra and measured peak energies agreed well with previous work.18-21 It was possible to distinguish, especially at the K edge, peaks associated with S-S, S-O, and SdO bonding. The general trend of increasing peak position with increasing oxidation state was also found in this work. A notable exception was ZnS, S2-, where the lack of S-S bonding in this species results in its main edge peak being higher than those for pyrite, FeS2 (oxidation state -1) and elemental sulfur (oxidation state 0), which both have S-S bonding. Overall, several distinctive features were observed from the XANES spectra of inorganic sulfur, the first of which was the low-energy single peak (about 2472 eV at the K edge) and doublet (162.5-164 eV at the L edge). The second was the presence of oxidized sulfur, which resulted in a shift to higherenergy peaks: 2482 eV at the K edge and 172-173 and 180-181 eV at the L edge. Thus, the K edge resolves complex chemical shifts of the edge in response to oxidation state changes and the nature of sulfur bonding, whereas the L edge provides a more detailed fingerprint of the specific species.


Figure 1. XANES S K-edge inorganic reference spectra.

TEY spectra at the K and L edges for the organic reference compounds studied are given in Figures 3 and 4, respectively. Important S functional groups that might exist in carbonaceous materials are represented here. These include alkyl and aryl sulfides, six-membered heterocyclic ring sulfur, and thiophenic sulfur.14 Two oxidized forms of organic sulfur were also considered (S bonded to one or three O atoms). Generally, the organic reference spectra obtained were in agreement with previous measurements14,21,22 and exhibited a

reasonable correlation between the main edge peak and the oxidation state, especially at the K edge, but also to a certain extent at the L edge. Four organic compounds where S is in a formal oxidation state of 0 were studied. These are L-methionine, an alkyl sulfide (C-S-C), and three compounds in which sulfur is present in ring structures, namely, phenothiazine, an organic six-membered heterocyclic ring, and two five-membered-ring sulfur (thiophenic) compounds, dibenzothiophene and thianaphene-2-car-

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Figure 2. XANES S L-edge inorganic reference spectra.

boxylic acid. At the K edge, all four give very similar spectra: a main peak at 2473.6 or 2473.8 eV, followed by one or two smaller low-energy peaks. These compounds are better distinguished from each other at the L edge, where there is greater shift in the largest edge peak. This shift is 0.6 eV between L-methionine (straightchain S) and phenothiazine (S in a six-membered heterocyclic ring), where the edge is centered around a peak at 165.3 eV, and a further 1 eV shift from phenothiazine to thianaphene-2-carboxylic acid (thiophen-

ic sulfur), which has its main edge peak at 166.2 eV. There are additional differences in peak shape that further aid identification: the L edge of L-methionine is characterized by a triplet, whereas four peaks can be distinguished at the edge for phenothiazine and thianaphene-2-carboxylic acid. Unfortunately, the low melting point of dibenzothiophene made it too unstable for its L edge to be measured accurately. As the formal sulfur oxidation state increases, the Kand L-edge spectra also shift to higher energies. At the


Figure 3. XANES S K-edge organic reference spectra.

K edge, the shift is 2 eV for the +2 oxidation state of D,L-methionine sulfoxide and approximately 7.5 eV for the +6 oxidation state. The doublet structure in the K-edge spectrum of 5-sulfosalicylic acid is a result of two different S bonding environments, one singly bonded to oxygen and the other doubly bonded. At the L edge, a distinctive triplet peak shape (closer in shape to L-methionine) was observed for +2 D,L-methionine sulfoxide, approximately 1.5 eV higher in energy than the highest-energy, 0 oxidation state reference compound.

A broad multicomponent peak was found at still higher energy for +6 5-sulfosalicylic, centered at 171.4 eV. Generally, the organic sulfur structures were easier to distinguish at the L edge than at the K edge, because of better peak resolution and the more complex and distinctive peak envelope. This reflects the reduced lifetime broadening of the transitions originating in the L shell, relative to the more deeply buried K shell. All of the organic compounds had K- and L-edge spectra


Figure 4. XANES S L-edge organic reference spectra.

that were significantly different from those of any of the inorganic compounds. 5. Petroleum Coke Results Eight petroleum coke samples from the major suppliers to the aluminum industry were analyzed using XANES spectroscopy. The results, presented in Figure 5 for the S K edge and in Figure 6 for the S L edge, compare three typical cokes with some organic reference

compounds, namely, L-methionine (C-S-C), phenothiazine (S in a six-membered ring), and thianaphene-2carboxylic acid (S in a five-membered ring), to identify the nature of bonding at the main edge peak. At the K edge (Figure 5), it is clear from the edge position that the major species is organic and is consistent with a formal oxidation state of 0. Semiquantitative analysis of the K edge, summarized in Table 3, generally indicated five-membered-ring sulfur as the


Figure 5. XANES S K-edge spectra comparing petroleum cokes with reference species.

Figure 6. XANES S L-edge spectra comparing petroleum cokes with reference species.

dominant sulfur species. The L-edge results (Figure 6) provide considerable extra detail, but a slightly conflicting interpretation. The major species in the 1.32% S coke most resembles sulfur in a five-membered ring. On the other hand, the peak shape and position of the 2.73% S coke is more like those of sulfur in a six-membered ring. The 3.67% S coke is perhaps a mixture of the two types of ring structures. Higher-energy peaks around 2476 and 2480 eV either could be the result of the presence of cyclic sulfur (cyclic sulfur reference compounds also have peaks here) or could be oxidized sulfur, such as sulfoxide and sulfate, respectively. In addition to the dominant presence of thiophenic and other cyclic sulfur, semiquantitative analysis also detected small quantities of sulfoxides and elemental sulfur in some samples. Lesser amounts of inorganic sulfides (S22-) and sulfates were commonly detected in the petroleum cokes. Ring structures, such as thiophenic sulfur, as well as inorganic sulfides and sulfates were also the dominant species found in a previous study of coal sulfur speciation.14 Generally, cokes from different sources have organic sulfur ring structures as the dominant sulfur species, with slight variation from one supplier to the next. This suggests that speciation in the raw materials is similar and that the calcination process preserves similar sulfur structures, regardless of the coke source. Thus, the S-containing five- and sixmembered rings in crude oil persist into the coke.

Table 3. Sulfur Compositiona in Cokes Calculated Using TEY Reference Spectra to Model Coke Spectra at the K Edge

6. Anode Results Four anode core samples and the parent coke were compared using XANES spectroscopy to determine

S type

reference compound used to model

five-membered ring dibenzothiophene five-membered ring thianaphene-2-carboxylic acid six-membered ring phenothiazine C-S-C L-methionine R-(SdO)-R D,L-methionine sulfoxide R-(SO3)-H 5-sulfosalicylic acid S22FeS2 S8 50% S8 in graphite SO42Na2SO4 a

coke %S 1.32 2.73 3.67 0.09 0.23 0.00 0.62 0.47 0.59 0.22 0.00 0.00 0.00 0.03 0.00 0.05

0.05 0.00 0.07 0.00 0.13 0.00 0.05

0.00 0.00 0.09 0.00 0.19 0.13 0.00

Values given in the table are normalized to a total of 1.

whether the baking process changed the sulfur speciation. Figure 7 shows the S K-edge XANES results, while Figure 8 shows the L-edge results for these anode samples. Both the K-edge and L-edge results confirm the predominance of S in ring structures, although analysis of the K-edge data (Table 4) suggests a fivemembered ring, whereas the peak shape of the L-edge results suggests six-membered. Two of the four anode core samples analyzed, baked at 944 and 1280 °C and containing 1.19% S and 1.14% S, respectively, are also shown. Semiquantitative analysis at the K edge (see Table 4) consistently favors thiophenic sulfur as the major sulfur species. Sulfoxides, inorganic sulfides, elemental sulfur, and sulfate were also detected in small quantities using this analysis method. Although the peak intensity does vary from one spectrum to another,


Figure 7. XANES S K-edge spectra comparing anode samples with reference sulfur ring species.

Figure 8. XANES S L-edge spectra comparing anode samples with reference sulfur ring species.

there are no trends to indicate that this might be due to, for example, oxidation to sulfate on heating. Although the higher baking temperature resulted in additional desulfurization, the sulfur speciation did not change significantly from coke to anode within typical baking temperatures. The next factor considered was the effect of anode usage. An anode butt sample containing 1.05% S, made from the same parent coke was analyzed using XANES spectroscopy. This anode would have been exposed to electrolysis conditions (975 °C in cryolitic electrolyte) for a period of approximately 21 days. The decrease of about 0.3% S indicates that further desulfurization occurs as the anode is being consumed in the cell; however, once again, sulfur speciation was stable with usage, as shown in Figures 7 and 8 and Table 4. Finally, two anode samples from the 944 °C core sample were rebaked in the laboratory at temperatures representing the extremes of anode baking, namely, 1400 and 1500 °C, for 150 min. The anode sample rebaked at 1500 °C, shown in Figures 7 and 8 and Table 4, was substantially desulfurized, with only 0.65% S remaining after heat treatment. However, once again, the organic sulfur ring structures were stable. Predictions of Anode Sulfur Gas Formation. COS formation at the anode at equilibrium, as well as oxidation to SO2 in the cell hooding predicted using thermodynamic data, agrees with experimental measurements in smelters. HSC Chemistry23 was used to perform a Gibbs energy minimization to calculate the sulfur species composition at equilibrium at the anode and in the cell hooding.

Figure 9 shows the thermodynamic equilibrium at 970 °C when sulfur in the anode is present as thiophene (C4H4S, sulfur in a five-membered ring), along with elemental carbon to balance the 2 wt % S in the anode (0.082 kmol total), in an increasingly oxidizing atmosphere ranging between 0-0.2 and 0.002 kmol of F2(g) [to account for the formation of HF(g) in competition with H2S(g)]. The formation of COS(g) was favored at equilibrium between 0.01 and 0.08 kmol of O2(g). At higher O2 concentrations, the formation of SO2(g) was favored, and at very low O2 concentrations, elemental sulfur was the favored species at equilibrium. Small amounts of CS2(g) and H2S(g) also formed at low O2 concentrations. H2S(g) formation was generally not favored at equilibrium over HF(g) formation. Given the known dominance of COS in the anode gas,7,8 the gas composition from Figure 9 at O2(g) ) 0.076 kmol was chosen to represent anode gas most closely and used in further thermodynamic calculations to determine the effect of a changing CO2/CO ratio. The starting sulfur composition for these calculations was therefore 90.74% COS, 8.24% SO2, 0.84% H2S, and 0.18% CS2. The calculated equilbrium compositions (Figure 10) show that COS(g) is the favored sulfur species at all CO2/CO ratios below 90:1. At typical CO2/ CO ratios, therefore, COS(g) could be expected to be the dominant sulfur species in anode gas. As the anode gases leave the cell and are mixed with the air draft in the cell hood, there are two important factors that influence stable sulfur species: a significant drop in temperature (from 970 °C to about 100 °C in the ducting) and dilution with air [i.e., increased O2(g) and humidity levels]. These two factors were investi-

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1699 Table 4. Sulfur Compositiona in the Source Coke, Core Anode Samples, and an Anode Butt, Calculated Using TEY Reference Spectra to Model Sample Spectra at the K Edge

a

S type

reference compound used to model

944

five-membered ring five-membered ring six-membered ring C-S-C R-(SdO)-R R-(SO3)-H S2 2S8 SO42-

dibenzothiophene thianaphene-2-carboxylic acid phenothiazine L-methionine DL-methionine sulfoxide 5-sulfosalicylic acid FeS2 50% S8 in graphite Na2SO4

0.04 0.71 0.00 0.00 0.06 0.00 0.13 0.00 0.06

anode baking temp (°C) 1280 1500 0.04 0.65 0.00 0.00 0.06 0.00 0.13 0.04 0.08

0.04 0.60 0.00 0.00 0.08 0.00 0.17 0.04 0.07

coke

butt

0.08 0.65 0.00 0.00 0.10 0.00 0.12 0.00 0.05

0.00 0.78 0.00 0.00 0.07 0.00 0.05 0.05 0.06

Values given in the table are normalized to a total of 1.

Figure 9. Thermodynamic equilibrium compositions for sulfur species at 970 °C for the starting composition 0.001 kmol of C4H4S (thiophene), 0.082 kmol of C, 0.001 kmol of F2(g), 0.002-0.2 kmol of O2(g).

Figure 11. Equilibrium composition of anode gas (from Figure 9 calculations) as it is cooled from 1000 to 0 °C in excess (99 mol %) air (2 mol % H2O, remainder 79% N2/21% O2).

despite the thermodynamic prediction, 100% conversion of COS is not achieved in smelters.7,8 7. Conclusions

Figure 10. Sulfur species thermodynamic equilibrium at 970 °C for the starting composition at O2(g) ) 0.076 kmol from Figure 9 (anode gas composition from thiophenic S), where the CO2/CO ratio is changing [CO2(g) + CO(g) ) 20 kmol].

gated separately, and then in combination, with respect to the anode gas from Figure 9. COS was the stable sulfur gas between 300 and 1000 °C, but was completely oxidized to SO2 at 970 °C if air was present. Of the two factors (temperature drop and air dilution), addition of air seems to be the more important factor in determining the equilibrium composition, as shown in Figure 11. In this case, the anode gas was mixed with 99 mol % air and cooled from 1000 °C. SO2 is initially stable to below around 450 °C, when H2SO4 becomes the major sulfur species. Although the formation of H2SO4 has not been reported in actual smelter measurements, thermodynamics correctly predicts that addition of air favors oxidation of COS into SO2, which is observed in smelter studies.7,8 Kinetics must also be important because,

This study has demonstrated that, even in materials of relatively low sulfur content such as the petroleum cokes and anodes used in the aluminum industry, XANES spectroscopy can be used to distinguish among different sulfur species. Knowledge of the speciation of sulfur in these materials is important in understanding the evolution of sulfur-containing gases from the reduction cell and especially of COS as the anode is consumed. Cokes from calciners sourcing cokes manufactured from different crude oils have organic sulfur ring structures as the dominant sulfur species, with slight variation from one supplier to the next. Sulfur speciation does not change from coke to anode for the typical baking temperatures and soak times to which prebake anodes are subjected. Organic sulfur rings are also very stable with anode usage and even after extended heat treatment and significant desulfurization. Under the appropriate conditions, calculations of equilibrium compositions indicate COS gas formation can be predicted at the anode, when sulfur is in the form of thiophene (S in a five-membered ring). Acknowledgment We gratefully acknowledge the assistance of Raymond Perruchoud at R&D Carbon and Alan Tomsett at Comalco for providing coke and anode samples. Access to the DCM and Grasshopper beamlines at the Aladdin ring of the SRC, University of Wisconsin-Madison, was provided through the Canadian Synchrotron Radiation Facility. SRC is supported by the NSF (Grant DMR-


0084402). We thank the beamline scientists Dr. Yongfeng Hu, Dr. Astrid Ju¨rgensen, and Dr. Kim Tan at the CSRF for assistance in the collection of the synchrotron data and the reviewers of this paper for their constructive comments. Literature Cited (1) Hunt, J. M. Petroleum Geochemistry and Geology; W. H. Freeman and Co.: New York, 1996. (2) Diessel, C. F. K. Coal-Bearing Depositional Systems; SpringerVerlag: Berlin, 1992. (3) Vogt, M. F.; Waller, J. H.; Zabreznik, R. D. The Problem of Sulfur Content in Calcined Petroleum Coke. JOM 1990, July, 33. (4) Hume, S. M.; Fischer, W. K.; Perruchoud, R. C.; Metson, J. B.; Baker R. T. Influence of Petroleum Coke Sulphur Content on the Sodium Sensitivity of Carbon Anodes. In Light Metals 1993; Das, S. K., Ed.; TMS: Warrendale, PA, 1993; p 535. (5) Vogt, M. F.; Reis, K.; Smith, M. Anode Desulfurization on Baking. In Light Metals 1995; Evans, J. W., Ed.; TMS: Warrendale, PA, 1995; p 691. (6) Kimmerle, F. M.; Noe¨l, L.; Pisano, J. T.; Mackay, G. I. COS, CS2 and SO2 Emissions from Prebaked Hall-He´roult Cells. In Light Metals 1997; Huglen, R., Ed.; TMS: Warrendale, PA, 1997; p 153. (7) Utne, I.; Paulsen, K. A.; Thonstad, J. The Emission of Carbonyl Sulphide from Prebake and So¨derberg Aluminium Cells. In Light Metals 1998; Welch, B., Ed.; TMS: Warrendale, PA, 1998; p 293. (8) Tveito, K.; Tonheim, J.; Paulsen, K. A.; Thonstad, J. Carbonyl Sulphide (COS) Emissions from Prebake Aluminium Cells. In Greenhouse Gases in the Metallurgical Industries: Policies, Abatement and Treatment; Pickles, C. A., Ed.; Canadian Institute of Mining, Metallurgy and Petroleum: Toronto, 2001; p 291. (9) Harnisch, J.; Borchers, R.; Fabian, P. COS, CS2 and SO2 in Aluminium Smelter Exhaust. Environ. Sci. Pollut. Res. 1995, 2 (4), 229. (10) Harnisch, J.; Borchers, R.; Fabian, P.; Kourtidis, K. Aluminium Production as a Source of Atmospheric Carbonyl Sulphide (COS). Environ. Sci. Pollut. Res. 1995, 2 (3), 161. (11) Schlesinger, W. H. Biogeochemistry: An Analysis of Global Change; Academic Press: New York, 1997. (12) Strømmen, S. O.; Bjørnstad, E.; Wedde, G. SO2 Emission Control in the Aluminium Industry. In Light Metals 2000; Peterson, R. D., Ed.; TMS: Warrendale, PA, 2000; p 351.

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Received for review February 3, 2003 Revised manuscript received December 18, 2003 Accepted January 21, 2004 IE0301031

Sulfur Speciation in Aluminum Smelting Anodes - ACS Publications

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