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Effect of Weathering on Surface Characteristics of Solids and Bitumen from Oil Sands Sili Ren, Trong Dang-Vu, Hongying Zhao, Jun Long, Zhenghe Xu,* and Jacob Masliyah Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, AB, Canada T6G 2G6 ReceiVed August 2, 2008. ReVised Manuscript ReceiVed October 26, 2008
The effect of weathering on the surface chemical composition and wettability of the oil sand solids and bitumen was studied by contact angle measurement, film flotation technique, XPS analysis, and ellipsometry thickness measurement. It was found that mild bitumen oxidation occurred during ore weathering. However, it has negligible effect on the bitumen surface wettability. In contrast, weathering forced more organic matters to adsorb onto the mineral solid surfaces, which made the solids more hydrophobic. A mechanism on how the organic matters adsorb on the solid surface due to weathering was put forward. The loss of the formation water due to weathering resulted in an intimate contact of bitumen with the mineral solids and thus offered the opportunity for the organic matters to adsorb, which was responsible for the enrichment of organic matters on the mineral solid surface. It was further observed that hot water washing partially removed the organic matters from the solids surface and hence decreased the solids hydrophobicity.
Introduction In oil sand processing, low bitumen recovery and poor bitumen froth quality are often obtained while processing weathered/oxidized ores.1-6 We refer to weathered/oxidized oil sand ores as ores that have been exposed to the external environment for an extended period of time and/or are not deeply buried under the overburden. Early studies indicated that the oxidation of various minerals in the ores and the change of bitumen chemistry were responsible for the observed poor processability.1-4 However, there is no direct evidence as to how the weathering affects the surface physicochemical properties of oil sand components and thus the processability. It has been reported that the solids in oil sand ores are always associated with toluene-insoluble organic matters (TIOM, humic-like matter).7-12 These humic matters made the solids moderately hydrophobic. The degree of hydrophobicity depends on the content of toluene-insoluble organic carbons on the solids. * To whom correspondence should be addressed. Phone: (1-780) 4927667. Fax: (1-780) 492-2881. E-mail:
[email protected]. (1) Bowman, C. W. Proc 7th World Pet. Congr. 1967, 3, 583. (2) Mikula, R. J.; Munoz, V. A.; Wang, N.; Bjornson, B.; Cox, D.; Moisan, B. Wiwchar, k. J. Can. Pet. Technol. 2003, 42, 50. (3) Sanford, E. C. Can. J. Chem. Eng. 1983, 61, 554. (4) Wallace, D.; Henry, D. In Aging of Oil Sand During Storage; Engineering Foundation Conference on the Processing of Energy Minerals: Shale, Tar Sands and Coal; Henniker, NH, 1984. (5) Wallace, D.; Henry, D.; Takamura, K. A. Fuel Sci. Technol. Int. 1989, 7, 699. (6) Liu, J. J.; Xu, Z. H.; Masliyah, J. Energy Fuels 2005, 19, 2056. (7) Kotlyar, L. S.; Sparks, B. D.; Kodama, H.; Grattanbellew, P. E. Energy Fuels 1988, 2, 589. (8) Sparks, B. D.; Kotlyar, L. S.; O’Carroll, J. B.; Chung, K. H. J. Pet. Sci. Eng. 2003, 39, 417. (9) Axelson, D. E.; Mikula, M. R. J.; Potoczny, Z. M. Fuel Sci. Technol. Int. 1989, 7, 659. (10) Kotlyar, L. S.; Sparks, B. D.; Woods, J.; Capes, C. E.; Schutte, R. Fuel 1995, 74, 1146. (11) Kotlyar, L. S.; Deslandes, Y.; Sparks, B. D.; Kodama, H.; Schutte, R. Clays and Clay Minerals 1993, 41, 341. (12) Kotlyar, L. S.; Sparks, B. D.; Deslandes, Y.; Schutte, R. Fuel Sci. Technol. Int. 1994, 12, 923.
The form of toluene-insoluble organic carbon could range from methyl (CH3), polymethylene [-(CH2)n-], methine, alkylbenzenes, unsubstituted and bridgehead aromatic carbons to a very small amount of carboxyl carbon.13 The organic-rich solids (ORS) are believed to play a role in forming and stabilizing the hydrophobic globules, which have a higher affinity to bitumen and therefore adversely affect bitumen recovery and froth quality.8,9,14 Therefore, the content of the ORS appeared to be a better indicator for oil sands processability than others, such as bitumen or fines content.8 In addition to the adverse influence of the ORS on oil sand processability, the ultrafine (less than 0.3 µm) ORS remaining in bitumen could cause serious problems in bitumen upgrading.8,15 The presence of the TIOM on the solids surface, on the other hand, could accelerate the flocculation of fine solids in the tailings to form gels, hereby significantly affecting the settling of tailings fine solids.10-12 Clearly, the ORS not only deteriorate the oil sand processability but also cause severe problems in bitumen upgrading and tailings settling. To our best knowledge, how the organic matters adsorb onto the solids surface and what factors affect the adsorption are not clearly understood. Therefore, investigating the effect of ore weathering on surface physicochemical properties of oil sand components would improve our understanding of the processability of weathered ores. In this paper, we present the results of surface characterization of bitumen and solids samples from both good processing and weathered ores, aiming at understanding the effect of weathering on the adsorption of organic matters on solid surfaces. Experimental Section Ore Weathering. Three ores, a good processing ore (GPO), a laboratory weathered ore (LWO), and a naturally weathered ore (13) Darcovich, K.; Kotlyar, L. S.; Tse, W. C.; Ripmeester, J. A.; Capes, C. E.; Sparks, B. D. Energy Fuels 1989, 3, 386. (14) Kotlyar, L. S.; Sparks, B. D. AOSTRA J. Res. 1990, 6, 41. (15) Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Raymond, S.; Le Page, Y.; Shelfantook, W. Petr. Sci. Technol. 1998, 16, 1.
10.1021/ef800635t CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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Table 1. Composition of the Oil Sand Ores Investigated in This Study finesa in solids bitumen water solids (wt %) composition (wt %)
type of ore good GPO processing ore laboratory LWO weathered ore naturally NWO weathered ore a
provider
14.3
3.4
82.3
12.7
Syncrude
14.2
0.2
85.6
13.2
12.2
2.6
85.2
12.4
prepared by GPO weathering Suncor
Fines are defined as microsolids smaller than 44 µm.
(NWO) were used in this study. The composition of these ores is given in Table 1. The LWO was prepared by weathering the GPO in an oven under controlled conditions as follows. A one-centimeter thick layer of the GPO in a pan was placed in an oven at 60 °C with air ventilation for 7 days. This weathering procedure caused a weight loss of 3.9%. Through Dean-Stark analysis, the indigenous formation water remaining in the treated ore was reduced from 3.4 to 0.2 wt %. The difference between the total weight loss and water loss indicates some (0.7 wt %) loss of volatile light hydrocarbons (organics). Preparation of Bitumen Samples and Solid Particles. Various bitumen samples and solid particles were extracted from the ores by toluene washing. After adding the toluene to the ore and shaking the mixture for several minutes, the obtained mixture was centrifuged at 20 000g force for 30 min. The upper bitumen solution was collected and adjusted to certain concentration for preparing bitumen films on silicon wafers at a later stage. Repetitive washing with toluene of solids was then conducted until the supernatant appeared colorless. Solid particles were then collected and dried in a vacuum oven at room temperature. To prepare bitumen films, single-crystal silicon wafers (Silicon Valley Microelectronics Inc., Santa Clara, CA) were cut into 12 × 12 mm2 pieces and used as the substrate. The silicon substrates were cleaned by immersing them in a piranha solution [a mixture of 7:3 (v/v) 98% H2SO4 and 30% H2O2] at 90 °C for 30 min, followed by rinsing with an adequate amount of ultrapure water, and N2 blow-drying. As a result of Si-OH groups generated on the silicon surface by piranha solution treatment, the cleaned silicon substrates were extremely hydrophilic with a water contact angle of almost 0°. The bitumen surface was prepared by the spin-coating technique with a P6700 spin-coater (Specialty Coating Systems Inc.), following the procedure given elsewhere.16 Briefly, the bitumen-in-toluene solution was first centrifuged at 20000g force for 30 min to remove any remaining fine solids and then further diluted to 2.5 mg/mL bitumen concentration. Within 25 s, ten drops of the prepared bitumen solution were added onto a substrate surface spinning at 2000 rpm. The substrate was then spun at 5000 rpm for 1 min to ensure a smooth, uniform bitumen surface without toluene. In preparing the bitumen surface on the silicon wafer, bitumen obtained from a commercial oil sands operation was also used for comparison. To better understand the effect of bitumen weathering on adsorption of organic matters on the sand surface, silicon wafers with a thin surface layer of silica were used as a model for sand grains. They were modified by contacting with diluted bitumen derived from the different ores. The adsorption of organics on the silicon wafer surface was performed by two different methods. In the first method (I), silicon wafers were immersed in a bitumen-toluene solution (2.5 mg/mL) for 15 min. The wafers were then washed exhaustively with toluene, followed by N2 blow-drying. The silicon wafers modified with the above procedure using bitumen from GPO, LWO, and NWO are referred to as I-G, I-L, and I-N, respectively. In the second method (II), a spin-coating technique was used. A layer of bitumen from the GPO was first spin-coated on the silicon wafer. The bitumen-coated silicon wafers obtained as such were (16) Liu, J. J.; Xu, Z. H.; Masliyah, J. Langmuir 2003, 19, 3911.
Figure 1. A schematic view (a) and photograph (b) of the film flotation apparatus.
divided into two groups. In the first group, samples were immediately washed exhaustively with toluene followed by N2 blowdrying. These modified silicon wafers were coded as II-G. In the second group, bitumen-coated silicon wafers were weathered in the oven under normal air environment at 60 °C for 7 days. The bitumen-coated silicon wafers after weathering were thoroughly washed using toluene. The silicon wafers modified as such were coded as II-W. Wettability Characterization. Water contact angle measurements were performed with a drop shape analysis system (DSA10, Kru¨ss) using the sessile drop method. The contact angle was measured through the water phase. Variation of contact angle with time was determined for the modified silicon wafer surface. For the bitumen samples, only the initial water contact angle was recorded. For each sample surface, contact angles at five different locations were measured, and the average values are reported. Surface wettability of the solid particles was evaluated using two methods. In the first method, partitioning of the solids in the mineral oil phase and water phase was determined. Briefly, 1 g of dried solid sample was placed in a glass bottle followed by the addition of mineral oil and deionized water at a volume ratio of 1:1. After shaking the bottle, the sample was allowed to settle/cream for 1 h, and then the bottle was photographed. In the second method for a quantitative determination, the film flotation technique was used. The film flotation technique has been widely used for determining the surface wettability of mineral particles.17-19 In the film flotation tests, a series of methanol/water solutions was prepared with water concentration ranging from 0 to 100 vol % at 10 vol % concentration intervals. This set of solutions covers a surface tension range of probing liquid from 22.4 mN/m (pure methanol) to 72.5 mN/m (deionized water). The apparatus as shown in Figure 1 for film flotation tests was designed in our laboratory. The apparatus was filled with a methanol solution to the lower rim level of the circular ring, which was made to restrict the floating particles within the ring. A precisely weighed aluminum pan was placed below the ring through the opening. Approximately 0.05 g of solids with particle size in a range of 106-250 µm obtained using dry sieving was gently sprinkled over the solution surface to form a thin layer of solids. Particles would either remain floating on the solution surface or sink, depending on the wettability of the solids and the surface tension of probing liquids. After 1 (17) Fuerstenau, D. W.; Diao, J.; Hanson, J. S. Energy Fuels 1990, 4, 34. (18) Fuerstenau, D. W.; Williams, M. C. Particle Characterization 1987, 4, 7. (19) Pawlik, M.; Laskowski, J. S.; Melo, F. Coal Preparation 2004, 24, 233.
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Figure 2. Contact angle of water on various surfaces of bitumen from (1) GPO, (2) LWO, (3) NOW, and (4) plant bitumen.
min, the aluminum pan with sunk particles was slid out of the ring and taken out of the probing liquid. The particles floating on the surface of the probing liquid were collected carefully. After drying of both the collected floated and sunk particles in the oven at 120 °C for 12 h, the weight of particles in each fraction was determined. The percentage of the particles floating was plotted as a function of the probing liquid surface tension. The particles that just sunk can be considered to have the critical surface tension equal to the surface tension of the probing liquid. The solids of higher critical surface tension are considered to be less hydrophopbic. The partition curve obtained with this modified film flotation procedure was found to be quite reproducible. Surface Characterization by X-ray Photoelectron Spectroscopy (XPS). The surface chemical composition of various bitumen and solid samples was determined by XPS. Solid particles of sizes less than 44 µm were used. The bitumen samples were prepared by spin-coating a thin layer (>30 nm) of bitumen on silicon wafers. The XPS characterization was performed with an AXIS 165 spectrometer (Kratos Analytical). The base pressure in the analytical chamber was lower than 5 × 10-8 Pa and the working pressure was better than 3 × 10 -7 Pa. A monochromatic Al KR (hν 1486.6 eV) source of 210 W was used. The analyzer was operated in fixed analyzer transmission (FAT) mode. The highresolution XPS spectra were obtained at a pass-energy of 20 eV with a step of 0.1 eV. The sampling spot size was 700 × 400 µm. The binding energy of C1s in hydrocarbon at 284.8 eV was used as reference. Thermogravimetric Analysis (TGA). Thermogravimetric analysis of the solids with sizes less than 44 µm was performed on a NETZSCH thermal analyzer (STA 409PC) under atmospheric environment at a heating rate of 10 °C/min up to 600 °C. Thickness Measurements. The layer thickness of the bitumen coatings and the toluene-insoluble organics on silicon wafer surface was measured with a Gaertner L116S ellipsometer, which was equipped with a He-Ne laser (632.8 nm) set at an incident angle of 70°. A real refractive index of 1.58 was assumed for all the films.20 For each sample, six replicable measurements were carried out, and the average value is reported. The precision and repeatability were both at subangstrom resolution.
Results Wettability. The contact angles measured for different bitumen surfaces prepared on silicon wafers are shown in Figure 2. It was found that the bitumen surfaces were hydrophobic with a water contact angle around 96°. Among the surfaces investigated, there was no detectable difference in the contact angle, indicating that weathering has little effect on bitumen surface wettability measured by contact angle. Visual observations of particle partition are shown in Figure 3. For solids isolated from the GPO, the water phase was “muddy”, while the mineral oil phase was relatively clear. In contrast, for the solids isolated from the two weathered ores, (20) Taylor, S. D.; Czarnecki, J.; Masliyah, J. Fuel 2001, 80, 2013.
Figure 3. Photographs of total solids distribution in the mineral oil and water phases: (a) solids from GPO, (b) solids from LWO, and (c) solids from NWO.
Figure 4. Film flotation of solids from GPO, LWO, and NWO: (a) weight percent of the floating solid particles as a function of the surface tension of probing solution and (b) frequency distribution of the critical surface tension of solids.
the mineral oil phase was muddy while the water phase was clear. Under gravity, solids encapsulated in large oil drops settled to the bottom of the bottle for the case when the partition was performed using solids isolated from the two weathered ores. The results clearly show that solids from the GPO were mostly hydrophilic while solids isolated from the LWO and NWO were clearly hydrophobic. For a quantitative evaluation of the surface wettability of these solids, the film flotation technique was used to determine the critical surface tension distribution of solids with particle sizes between 106 and 250 µm. Figure 4a shows the weight percentage of the particles floating on the probing liquid as a function of the surface tension of the probing liquids. Clearly solids from the GPO are of the least hydrophobicity, while solids from the NWO are the most hydrophobic among the three tested solids. To have a more intuitive view of the results, the data in Figure 4a are fitted to a mathematical function that allowed us to
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Figure 5. TGA analysis of the solids from GPO, LWO, and kaolin clay. Table 2. Mass Content (%) of Elements on Various Fine Solids Surface (
