Morphological and Mineralogical Characterization of Oil Sands Fly

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Energy & Fuels 2005, 19, 2121-2128

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Morphological and Mineralogical Characterization of Oil Sands Fly Ash Heemun Jang* and Thomas H. Etsell Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received April 26, 2005. Revised Manuscript Received June 29, 2005

The feasibility of extracting metal values from the oil sands fly ash has been studied in some detail; however, little work has been completed on the mineral phases in the fly ash and their phase changes during ash formation from oil sands coke. Characterization of the ash is essential to obtain sufficient information about the mineral assemblage and its processability. In this study, coke obtained from Syncrude and Suncor, which are the two major oil producers in northern Alberta, Canada, was investigated to elucidate the morphology, ash formation behavior, and the mineralogy, by ashing it at various temperatures. Both high temperature ashing and low temperature ashing (LTA) were employed, and the ash structure was investigated using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The results clearly showed that a slight difference in chemistry and mineralogy between two samples greatly affected the properties of ash at high temperatures. Formation of oil sands fly ash was explained by the movement of iron-titanium-related minerals and amorphous aluminosilicate melts.

Introduction Alberta’s oil sands, located in the north of Alberta, Canada, contain the largest crude bitumen resource in the world, and crude bitumen production is drastically increasing with the development of bitumen upgrading techniques. The EUB (Alberta Energy and Utilities Board) estimates that approximately 27.7 billion m3 (174 billion barrels) are considered potentially recoverable in this area.1 It reported that, in 2003, Alberta produced 55.9 million m3 (352 million barrels) of crude bitumen, and the two major tar sands oil producers, Suncor Energy Inc. and Syncrude Canada Ltd., produced more than 325 000 barrels of crude oil per day. While the major oil producers plan to increase their crude oil production, a significant amount of waste products are also generated and stockpiled. During upgrading processes, approximately 22% of the bitumen is converted to petroleum coke at Suncor, and around 15% of bitumen is converted to coke at Syncrude with two-thirds of the coke being stockpiled.2 The balance is consumed internally in the plants yielding another type of waste product, fly ash. In Suncor, fly ash is produced when the coke is burned in boilers that generate steam and electricity. The temperature of the boilers is similar to that of conventional power plants. On the other hand, in Syncrude, fly ash is produced when the coke obtained from its fluid cokers is burned in the cokers themselves * Corresponding author. Telephone: (780) 492-6620. Fax: (780) 4922881. E-mail: [email protected]. (1) Statistical Series (ST) 2004-98. Alberta’s Reserves 2003 and Supply/Demand Outlook 2004-2013; Alberta Energy & Utilities Board (EUB): Edmonton, 2004. (2) Griffin, P. J.; Etsell, T. H. In Future of Heavy Crude Tar Sands, Int. Conf., 2nd. (Calgary); Meyer, R. F., Wynn, J. C., Olson, J. C., Eds.; McGraw-Hill: New York, 1982; pp 1286-1293.

to maintain the coker temperature, which is normally held at around 630 °C.2,3 In 2002, approximately 250 and 50 tonnes of fly ash were produced by Suncor and Syncrude per day, respectively.4 Chemical compositions of Suncor and Syncrude fly ash after removing all carbon from the coke have been reported in detail.4 Briefly, as compared to coal fly ash, significant amounts of V, Ni, and Ti are contained in oil sands fly ash. On the other hand, the amount of base elements such as Ca, Na, and Mg is much lower in oil sands fly ash than in coal ash. The size and shape of Suncor fly ash is quite similar to that of coal fly ash because both their coke and their coal undergo phase transformations to form fly ash at similar temperatures. Suncor fly ash generally consists of both ash spheres and irregular carbon particles, and the size is quite fine (80% < 45 µm). Holloway and Etsell5 found that the size of the spheres was around 2-30 µm. Cenospheres and pleurospheres were also observed in Suncor fly ash. Microcrystals growing on the surface of spherical particles are found in some fly ash samples. Griffin and Etsell2 confirmed the presence of these microcrystals on fine spherical particles. They reported that the microcrystals were between 0.1 and 0.8 µm in width and 0.5-5 µm in length. In addition, these crystals were characterized with much higher levels of Fe, Ti, and Ni than the surrounding aluminosilicate spheres. (3) Hammond, D. G.; Lampert, L. F.; Mart, C. J.; Massenzio, S. F.; Phillips, G. E.; Sellards, D. L.; Woerner, A. C. Review of Fluid Coking and Flexicoking Technologies; AlChE Paper 44c; ExxonMobile Research and Engineering, 2003. (4) Holloway, P. Vanadium Recovery from Oil Sands Fly Ash. M.Sc. Thesis, University of Alberta, Edmonton, 2002. (5) Holloway, P.; Etsell, T. H. In Vanadium Geology, Processing and Applications; Proceedings of the International Symposium on Vanadium (Montreal); Taner, M. F., Riveros, P. A., Dutriziac, J. E., Gattrell, M. A., Perron, L. M., Eds.; CIM: Montreal, 2002; pp 227-242.

10.1021/ef050123a CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005

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Syncrude fly ash is somewhat different in particle morphology because their ash is produced at relatively lower temperatures. Almost all particles are flaky aluminosilicates with a few spheres and microcrystals. Free quartz particles are also reported in many Syncrude ash samples, and their size ranges from 10 to 30 µm.5 Along with environmental concerns, assessment of the commercial utilization of these waste products is under consideration. A large number of studies dealing with oil sands fly ash have focused on leaching behavior because fly ash contains appreciable amounts of metal values, such as vanadium, titanium, and nickel. Although some physical, chemical, and mineralogical properties of oil sands fly ash have been uncovered from these studies, they are insufficient to explain and predict the behavior of ash. This paper describes a more detailed chemical, mineralogical, and morphological characterization of oil sands coke ash, with an analysis of ash formation behavior. In this study, oil sands coke, not as-received fly ash, was used to generate ash from the low-temperature ashing (LTA) temperature to 1200 °C at 100 °C intervals. Samples and Experimental Procedure Samples. Coke used in this study was obtained from Suncor and Syncrude, which are the major commercial mining companies in northern Alberta. Suncor uses a delayed coking technique to reject carbon from the hydrocarbon molecules, creating a waste mineral, petroleum coke, whereas Syncrude uses a fluid coking technique for the same purpose. In fluid coking, the residence time of feed in the cokers is significantly reduced and the excellent heat transfer inside the cokers breaks up more volatiles, giving rise to a lower yield of coke. These differences mainly determine the difference in physical and chemical properties. The properties of both Suncor and Syncrude coke are summarized in Table 1, which is modified from Furimsky.6 The appearance was also slightly different: Suncor coke was in the form of lumps and Syncrude coke was in a powder form. Experimental Procedure. Both Suncor and Syncrude cokes were ashed at 100 °C intervals from 400 to 1200 °C before analysis. All of the samples were combusted in a muffle furnace until constant weight was achieved, and then the ash content of each sample was calculated. To preserve the original mineral phases of the coke, a LTA technique was also used. Each coke sample was placed in two sample chambers of model LTA-302 purchased from LFE Corp., MA, which were held in an activated oxygen-plasma atmosphere generated by a radio frequency field.7 The main advantage of using the low temperature asher, or radio frequency asher, is that the organically bonded minerals can be broken apart without mineral alterations as activated oxygen generated from the radio frequency oscillator reacts with organic substances in the coke. The operating temperature of the sample chambers was 65 °C, and ashing duration was 14 days. The particle size used in this experiment was from 90 to 150 µm in the case of Syncrude coke. The samples were randomly taken from this size distribution. Suncor coke was partly crushed, and same size particles were used. A combination of scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) was used to investigate morphology and mineralogy in the samples. For chemical composition of coke ash samples in all temperature ranges, (6) Furimsky, E. Fuel Process. Technol. 1998, 56, 263-290. (7) LFE Corporation. Operation and Maintenance, No. 255539, Waltham, MA, 1978.

Jang and Etsell Table 1. Proximate and Ultimate Analysis of Oil Sands Coke (wt %)6 cokes

Suncor (1) Suncor (2) Syncrude (1) Syncrude (2)

moisture ash volatiles fixed carbon

N/A 3.99 12.48 83.53

Proximate N/A 3.8 12.5 83.7

0.69 7.52 6.10 85.69

0.25 4.83 4.99 89.95

carbon hydrogen H/C nitrogen sulfur oxygen

84.02 3.67 0.50 1.38 5.73 1.21

Ultimate 83.7 3.7 0.53 1.8 5.7 1.3

80.94 1.56 0.23 1.73 6.15 1.41

83.74 1.77 0.25 2.03 6.52 0.88

SiO2 AlO3 Fe2O3 TiO2 P2O5 CaO MgO SO3 Na2O K2O BaO SrO V2O5 NiO MnO Cr2O3 total

37.00 21.31 11.47 3.34 N/A 4.91 2.91 5.39 0.68 0.91 N/A N/A N/A 1.28 N/A N/A 94.55

Ash Composition 42.18 41.26 22.72 25.94 11.85 12.14 3.28 4.84 0.29 0.35 3.45 1.63 1.59 1.40 2.53 1.87 0.75 1.16 1.93 1.93 0.03 0.14 0.02 0.06 4.40 3.21 1.21 N/A N/A 0.29 N/A 0.08 96.23 98.63

37.64 24.33 11.42 4.63 0.40 2.94 1.46 2.88 1.67 1.72 0.09 0.11 4.94 N/A 0.27 0.09 98.35

Table 2. Quantities of Residues on Burning (wt %) residues on burning temperature (°C)

Suncor coke

Syncrude coke

LTA 400 500 600 700 800 900 1000 1100 1200

12.45 3.04 3.06 2.75 2.82 2.43 2.74 2.25 2.43 2.13

12.83 8.48 8.86 8.95 8.73 8.55 7.91 7.48 7.87 4.82

SEM/EDS analyses were performed on a Hitachi S-2700 scanning microscope link eXL EDS system. The formation of different phases was identified by a Rigaku X-ray diffractometer operating at 40 kV and 110 mA. X-ray data were collected using Cu KR radiation in the range between 10° and 90° 2θ and steps of 0.05° 2θ with a count time of one second per step.

Results and Discussion Chemical Composition. When the oil sands coke samples are burned at different temperatures until constant weight is achieved, the residue would include ash only or ash and volatiles depending on the temperature. The residues of Suncor and Syncrude coke on burning from 400 to 1100 °C range from 2.25 to 3.06 wt % and from 7.48 to 8.95 wt %, respectively. Table 2 shows the quantities of residues on burning for both Suncor and Syncrude coke. They are very close to the actual ash contents of oil sands fly ash.6 The difference between LTA and 400 °C explains the difference in delayed coking and fluid coking techniques provided that only carbon is removed by LTA. Tables 3 and 4

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Table 3. Elemental Composition of Suncor Coke Ash Treated from LTA Temperature to 1200 °C (wt %) temperature (°C) elements

LTA

400

500

600

700

800

900

1000

1100

1200

Na Mg Al Si P S K Ca Ti V Mn Fe Ni

0.45 0.92 13.53 20.97 0.36 10.78 2.25 1.75 5.05 5.40 0.23 12.33 2.24

0.69 0.96 14.32 21.51 0.37 3.80 1.89 1.90 4.12 4.10 0.36 11.14 1.76

0.69 1.22 15.96 22.70 0.42 1.65 1.95 2.33 4.71 5.00 0.43 15.46 2.43

0.85 1.19 16.21 22.68 0.43 1.30 2.13 2.35 5.29 5.32 0.55 16.62 2.48

0.67 1.14 15.75 21.78 0.47 1.65 2.08 2.10 5.56 5.59 0.37 15.83 2.42

0.71 0.95 16.22 22.00 0.30 0.00 2.22 2.66 6.29 6.57 0.36 15.07 2.82

0.72 1.07 15.81 21.03 0.32 0.00 1.79 2.65 5.89 5.95 0.51 17.23 2.69

0.68 1.05 15.49 21.84 0.35 0.00 1.83 2.24 5.38 5.67 0.38 16.85 2.79

1.15 1.11 15.11 19.36 0.28 0.00 1.77 1.81 4.31 3.92 0.30 15.35 2.97

0.65 1.74 17.56 21.76 0.37 0.00 2.61 2.19 3.61 4.59 0.93 15.82 2.84

Table 4. Elemental Composition of Syncrude Coke Ash Treated from LTA Temperature to 1200 °C (wt %) temperature (°C) elements

LTA

400

500

600

700

800

900

1000

1100

1200

Na Mg Al Si P S K Ca Ti V Mn Fe Ni

0.97 0.70 11.91 29.02 0.40 11.41 2.19 5.58 2.93 3.68 0.27 11.13 1.63

1.50 0.87 15.27 30.09 0.50 3.66 2.45 6.57 3.08 4.33 0.40 12.48 1.92

1.28 0.99 16.41 26.16 0.40 4.66 2.87 6.10 3.43 4.26 0.33 12.29 1.82

1.60 1.09 17.07 27.29 0.30 3.54 2.64 5.57 3.29 3.62 0.35 12.03 1.76

1.55 1.09 17.01 27.53 0.47 2.36 2.98 6.35 3.74 4.24 0.41 12.70 1.78

1.77 0.98 16.46 26.75 0.40 0.00 2.94 7.18 3.87 4.26 0.45 14.17 1.98

1.70 0.87 16.54 26.36 0.33 0.00 2.97 7.65 3.82 4.23 0.52 14.52 1.62

1.68 1.01 16.12 26.69 0.34 0.00 2.95 6.82 3.79 4.39 0.45 14.98 1.87

1.75 1.24 14.82 22.17 0.27 0.00 2.04 4.62 2.91 3.47 0.34 12.44 1.85

1.21 0.92 14.28 29.35 0.00 0.00 2.58 5.66 3.61 3.84 0.59 18.60 2.24

show the chemical composition of Suncor and Syncrude coke ash with temperature varying from the LTA temperature to 1200 °C. As found in coal fly ash, aluminum, silicon, and iron are the major elements. However, contrary to coal fly ash, significant amounts of V, Ni, and Ti also appear as major elements in these samples. Na, Mg, K, and Mn are found as minor elements. Although the quantity of each element is quite similar, there are slight differences in composition between Suncor and Syncrude coke ash. As compared to Suncor coke ash, Syncrude coke ash contains higher Si (25-27 wt % vs 20-22 wt % in Suncor ash). The amount of Al and Fe remains at the same level as that in Suncor ash, but vanadium, nickel, and titanium are slightly lower in their quantities, whereas quantities of Ca, K, and Na are somewhat higher. There are no significant changes in composition as temperature varies. However, sulfur found in the original coke sample drastically decreases in quantity as temperature increases to 500 °C and disappears at around 800 °C. When the temperature increases from the LTA temperature to 500 °C, the quantity of sulfur decreases from 10.78% to 1.65% in Suncor ash. In the case of Syncrude ash, it decreases from 11.41% to 4.66%. It appears that the loss of sulfur at lower temperature (e500 °C) results from organic sulfur, which is broken apart from the organic matrix and is released as SO2 and SO3 during combustion, whereas the loss at higher temperature (600-800 °C) is attributed to the decomposition of calcium sulfates, such as gypsum and anhydrite, which are found in XRD studies. It is obvious that, in Suncor coke ash, only a small amount (around 15%) of the sulfur accounts for the formation of calcium sulfates in early stages of combustion and, in Syncrude

coke ash, up to 41% of the sulfur exists as calcium sulfates. Due to the higher quantity of Ca in Syncrude ash (6-7.5%, as compared to 1.7-2.6% in Suncor ash), more Ca compounds are expected to be present. This could explain the higher amount of sulfur in Syncrude ash at temperatures from 600 to 800 °C. Morphology. Figures 1 and 2 show the SEM micrographs of Suncor ash and Syncrude ash, respectively. SEM micrographs of oil sands coke ash samples obtained in the low-temperature range (from the LTA temperature to 600 °C) show that the predominant phase in the ash samples is the amorphous aluminosilicate minerals. EDS point analyses indicate that these particles are somewhat homogeneous in terms of their chemical composition. It appears that, as all organic combinations are broken apart at an early stage of combustion, Al, Si, Ca, Na, K, Fe, V, Ti, and Ni are combined all together in the amorphous aluminosilicate particles. These particles are flakelike, with a size less than 3 µm. No different shapes of particles are found in this temperature range. Starting at around 800 °C, these flaky particles seem to combine with each other to make bigger particles by means of migration of amorphous aluminosilicate melts. These glassy melts cover from 2 to around 10 neighboring particles at 900 °C, and, at a later stage, they form much bigger particles. Although the quantity of amorphous aluminosilicates decreases as temperature increases, the movement of melts continues up to 1200 °C resulting in much bigger particles. At 1200 °C, numerous small particles are entrapped in the aluminosilicate melt. At 800 °C, the average particle size is found to be around 6 µm, and at 1100 °C, it seems to average around 15-20 µm. Around 60 µm size particles

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Figure 1. SEM micrographs of Suncor coke ash (×5000). Samples were ashed using Suncor coke at (A) LTA (×5000); (B) 400 °C (×5000); (C) 500 °C (×5000); (D) 600 °C (×5000); (E) 700 °C (×5000); (F) 800 °C (×5000); (G) 900 °C (×5000); (H) 1000 °C (×5000); (I) 1100 °C (×1500); (J) 1200 °C (×1500).

are also easily found at this temperature. At 1200 °C, the particle size is even bigger, and one particle shown in Figure 1J is around 300 µm when examined at a lower magnification.

The shape of particles also experiences a change as temperature changes. Very small thin flaky particles found to be predominant at low temperature become spherical when the combustion temperature is 1000 °C

Characterization of Oil Sands Fly Ash

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Figure 2. SEM micrographs of Syncrude coke ash. Samples were ashed using Syncrude coke at (A) LTA (×5000); (B) 400 °C (×5000); (C) 500 °C (×5000); (D) 600 °C (×5000); (E) 700 °C (×5000); (F) 800 °C (×5000); (G) 900 °C (×5000); (H) 1000 °C (×1500); (I) 1100 °C (×5000); (J) 1200 °C (×5000).

or higher. Both cenosphere-like and pleurosphere-like particles are identified in this high-temperature range. Figure 3 shows a pleurosphere-like particle and a cenosphere-like particle found in other Suncor ash samples formed at 1100 °C. However, perfect spheres

that are normally found in the as-received fly ash were not identified in these samples. As-received Suncor fly ash samples contained both spheres with smooth surfaces and spheres with rough coatings.5 However, spheres with smooth surfaces were

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Figure 3. Pleurosphere-like particle (left) and cenosphere-like particle found in Suncor ash 1100 °C samples (×500).

Figure 4. Micrograph of needles (×5000) and EDS point analyses on a needle and the matrix of 800 °C Suncor coke ash. Points 1 and 2 indicate the matrix, and point 3 indicates the root of a needle.

rarely found in this study. One would postulate that spheres with a smooth surface could form when the aluminosilicate melt solidifies rapidly right after coalescence, not allowing many microcrystals to be contacted. If the ash formation temperature is high enough (or cooling rate is sufficiently low) for microcrystals to contact the aluminosilicate melt, all of the spheres might be coated with rough microcrystals. No visible crystalline structures are found in lowtemperature samples. Although XRD analyses indicate that hematite crystallizes along with calcium sulfates at 500 °C, no particles enriched in iron are found. However, thin acicular microcrystal structures are found in 700 °C and higher Suncor samples. At 700 °C, these needles are around 1 µm in diameter, 2-6 µm long, and the size increases as the temperature increases. When the combustion temperature is around 1000 °C, these needles appear to react with the matrix (flaky-shaped aluminosilicates), which is fused and

coalesced with other individual particles, and are partly covered by this melt. At the later stages, these needles are completely entrapped in the melt, which becomes spherical as it solidifies around entrapped CO2 or SO2 gas bubbles. These needles have also been reported in many works,2,4,8,9 and the composition, shapes, and sizes of needles are consistent with ones found in this study. EDS analyses on needles indicate that the needles contain higher fractions of iron, nickel, and titanium, whereas the melts are generally enriched in Si, Al, Ca, and K. Figure 4 illustrates EDS point analyses on a 800 °C Suncor sample. In the case of Syncrude ash, the presence of microcrystals is first found at 800 °C, which is retarded by about 100 °C as compared to Suncor ash. The size of a (8) Schneider, L. G. Extraction of Vanadium from Oil Sands Fly Ash, M.Sc. Thesis, University of Alberta, Edmonton, 1983. (9) Gomez-Bueno, C. O.; Spink, D. R.; Rempel, G. L. Metall. Trans. 1981, 12B, 341-352.

Characterization of Oil Sands Fly Ash

needle is very small, less than 0.2 µm in diameter and 1 µm long. Fe, Ti, Ni, and other elements that might be combined in needles are obviously less mobile on heating as compared to Suncor ash. This is probably because of the higher silica content, which normally forms amorphous materials at an elevated temperature, and higher Ca, K, and other base elements that interfere with the movement of needles. EDS point analyses indicate that the composition of needles is similar to that of Suncor ash. In addition to needles, another type of microcrystal is found in high-temperature samples (1000 °C or higher). These euhedral- to subhedral-shaped microcrystals have high amounts of iron with an average quantity of titanium and low aluminum and silicon (Figure 1I,J and Figure 2J). Along with needles, these microcrystals are also a major component of a spherical particle at high temperatures. It is worthwhile for the possibility of metal recovery to conclude that Al, Si, Ca, K, and Na are enriched in the aluminosilicate melts, whereas Fe, Ti, and Ni are enriched in the needles. Vanadium is found both in needles and in aluminosilicate melts in samples obtained from 700 to 1200 °C. Based on numerous spot analyses, it is concluded that higher fractions of vanadium are associated with the aluminosilicate matrix and lower fractions are associated with Fe, Ti-rich microcrystals. Ash Formation Behavior. Among major constituents of oil sands coke ash, Fe, Ti, and Ni are relatively active so that they crystallize as thin acicular-shaped particles at 700 °C in Suncor ash (800 °C in Syncrude ash samples). On the other hand, amorphous materials enriched in Al, Si, Ca, Na, and K are fused and coalesced at 800 °C or higher, and cover small particles which are either flaky aluminosilicates or aluminosilicate particles connected to acicular particles. When these melts solidify around entrapped gas bubbles, spherical particles may form. In this case, small particles initially entrapped in the melt are lined up at the surface of the hollow spheres (cenospheres). These particles could also be covered by other hollow spheres, so that pleurospheres would form. The degree of movement, that is, the activation energy required for the melt to migrate, seems to control particle size of the ash. With this point of view, it could be concluded that the higher the temperature, the bigger the size. It is also concluded that surface enrichment of minor or trace elements occurs due to migration of these elements from the interior toward the surface of the particles. SEM micrographs of Suncor ash clearly show that the heavy metal ions are mobile enough even after being covered by the aluminosilicate melts. Similar to the ash formation mechanisms of coal fly ash,10-14 the governing mechanism parameters for the oil sands ash formation are found to be coagulation and coalescence. More detailed studies are ongoing to define the ash formation mechanism, including other possible (10) Couch, G. Understanding Slagging and Fouling in pf Combustion; CR/72, IEA Coal Research: London, 1994. (11) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2001, 80, 1333-1340. (12) McLennan, A. R.; Bryant, G. W.; Stanmore, B. R.; Wall, T. F. Energy Fuels 2000, 14, 150-159. (13) Wu, H.; Bryant, G.; Wall, T. Energy Fuels 2000, 14, 745-750. (14) Spears, D. A. Appl. Clay Sci. 2000, 16, 87-95.

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Figure 5. Mineral phases of Suncor ash identified by XRD analyses at different temperatures. Table 5. Minerals Identified in Oil Sands Coke Ash silicates

quartz cristobalite mullite microcline sillimanite kaolinite illite

SiO2 SiO2 3Al2O3‚2SiO2 KAlSi3O8 Al2SiO5 Al2Si2O5(OH)4 K0.7Al2(Si,Al)4O10(OH)2

feldspars

anorthite albite

CaAl2Si2O8 NaAlSi3O8

oxides

hematite pseudobrookite mayenite hercynite

Fe2O3 Fe2TiO5 Ca12Al14O33 FeAl2O4

sulfates

anhydrite gypsum

CaSO4 CaSO4‚2H2O

mechanisms such as the vaporization behavior of volatiles and physicochemical reaction between particles, which have not been confirmed in this study. Mineralogy. The mineral phases identified in Suncor and Syncrude ash from the LTA temperature to 1200 °C are summarized in Table 5 and Figures 5 and 6. In LTA samples, which may represent the mineral phases of the original coke, kaolinite, illite, gypsum, quartz, and microcline are the main mineral phases in Suncor ash. In Syncrude LTA samples, kaolinite is not identified. As well, anhydrite is dominant instead of gypsum. At intermediate temperatures (400-700 °C), hematite, sillimanite, and anorthite are newly crystallized. Whereas the high intensity peaks of quartz and microcline are still found, kaolinite, illite, gypsum, and anhydrite are completely decomposed in this temperature range. At high temperatures (700-1200 °C), mullite, cristobalite, hercynite, albite, and pseudobrookite become the major constituents in Suncor ash. However, mullite and cristobalite are not identified in Syncrude ash because no kaolinite has been found as a precursor in LTA and intermediate temperature ranges. Instead, the highintensity peaks of anorthite and albite become stronger. Based on the mineralogy of LTA samples, it is found that the flaky particles found in oil sands fly ash are a complex mixture of clays (mainly kaolinite and illite), calcium sulfate (gypsum and anhydrite), quartz, amorphous aluminosilicates enriched in Ca, K, and Na, and amorphous aluminosilicates enriched in Fe, Ti, and Ni. As clays decompose as well as hematite and pseudobrookite (needles) newly crystallize at temperatures higher than 700 °C, Al and Si become abundant in the amorphous aluminosilicates. The presence of crystallites such as hercynite, mullite, and sillimanite at high

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Figure 6. Mineral phases of Syncrude ash identified by XRD analyses at different temperatures.

temperatures indicates that a much higher fraction of Al is found in the crystallites and a higher fraction of Si remains in amorphous materials. It is, thus, concluded that higher Si in Syncrude ash results in more amorphous materials at elevated temperatures. Excess Ca, K, and Na also appear in the melts. Except pseudobrookite, which can be visible through SEM analyses, the size of other crystallites is so small that they are normally embedded in the aluminosilicate melts. Conclusions Oil sands ash directly produced from Suncor and Syncrude coke was characterized mainly as to chemical composition, mineralogy, and morphology. In the temperature range between the LTA temperature and 1200 °C, numerous characteristics of coke ash were collected and compared. As well, along with hightemperature ashing, a low-temperature ashing technique revealed the nature of oil sands coke. The ash formation behavior during combustion was also explained in detail. SEM micrographs of both Suncor and Syncrude ash are not overly different. However, a slight difference in chemistry between two samples greatly affects the

nature of ash at high temperatures. Formation of oil sands coke ash is explained by the movement of irontitanium-related minerals and by the movement of aluminosilicate melts. It is concluded that the growth of needles, which results from migration toward the surface of melts, is direct evidence of surface enrichment of heavy metals indicated in many research works. It is also concluded that the speed of melt migration controls the size of a particle. When the melts move farther, bigger particles are formed. The factors affecting the speed of melt migration may include temperature, chemical distribution of each element that affects the melt viscosity, and the amorphocity of the ash. Although some variables exist, the results from this study could be used to determine the temperature at which the ash forms. As well, a good understanding of the mineralogy and the mechanism of ash formation behavior could lead to future improvements and optimization of recovery of metal values from oil sands ash. Acknowledgment. We thank Suncor Energy Inc. and Syncrude Canada Ltd. for providing the coke samples used in this study. EF050123A