2056
Energy & Fuels 2005, 19, 2056-2063
Processability of Oil Sand Ores in Alberta Jianjun Liu,§ Zhenghe Xu,*,‡ and Jacob Masliyah*,‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received April 1, 2005
Oil sand ores mined at different locations in Alberta have different physical and chemical properties that dictate their processability by water-based bitumen extraction technology. In this study, the processability of one good processing ore and three poor processing ores has been investigated using a Denver flotation cell. The floatability of oil sand ores was found to vary significantly among the examined oil sand ores. Surface forces between bitumen-silica and bitumen-fines were measured using an atomic force microscope (AFM) to examine the mechanism of varying processability of oil sand ores. Factors examined include bitumen grade, fines content, divalent cation concentration, and weathering/aging. The results indicate that the processability of the ores cannot be simply evaluated from bitumen and fines content alone. Fines, divalent cations, and weathering/aging can significantly affect the processability of oil sand ores, either individually or collectively.
Introduction Water-based extraction processes have been developed to recover bitumen from Athabasca oil sand ores in Alberta.1,2 In this process, two crucial steps, liberation and aeration, dictate the processability of oil sand ores.3 Liberation of bitumen from sand grains is the prerequisite for bitumen extraction, which is controlled by interactions between bitumen and silica sand grains. Aeration of liberated bitumen droplets with air bubbles is essential for the liberated bitumen droplets to float. Aeration is dependent on the hydrophobicity of the bitumen surface and the size of the bitumen droplets. The hydrophobicity of the bitumen surfaces is affected by water chemistry and interactions between bitumen and fine mineral solids (slime coating).3,4 Any factor that causes poor liberation or poor aeration would result in a poor bitumen recovery from oil sand ores. Oil sand ores mined at different locations may possess different physical and chemical properties, which dictate the kinetics of bitumen liberation and aeration. Bitumen extraction from good processing ores has been successful in a number of commercial operations, and a total * Corresponding authors. E-mail: (J.M.)
[email protected]; (Z.X.)
[email protected]. § Present address: Global Mining and Metals, Nalco Company, Naperville, IL 60563. ‡ University of Alberta. (1) Hepler, L. G.; Hsi, C. AOSTRA Technical handbook on oil sands, bitumen, and heavy oils, AOSTRA technical publication series #6; Alberta Oil Sands Technology and Research Authority: Edmonton, AB, 1989. (2) Hepler, L. G.; Smith, R. G. The Alberta oil sands: industrial procedures for extraction and some recent fundamental research, AOSTRA technical publication series #14; Alberta Oil Sands Technology and Research Authority: Edmonton, AB, 1994. (3) Masliyah, J.; Zhou, Z.; Xu, Z.; Czarnecki, J.; Hamza, H. Can. J. Chem. Eng. 2004, 82, 628-654. (4) Liu, J. Ph.D. Thesis. Role of Colloidal Interactions between Oil Sand Components in Bitumen Recovery from Oil Sands, University of Alberta, 2004.
bitumen recovery above 93% is not uncommon. For poor processing ores, however, many technical challenges exist to achieve a satisfactory bitumen recovery and froth quality. Evidently, understanding the governing mechanism of processability for different types of oil sand ores is essential to resolve the challenges encountered in the existing extraction processes and to develop a more versatile and effective bitumen extraction process. It has been commonly accepted that the processability of oil sand ores is fairly well-correlated to the bitumen and fines content of the ores.1-8 High-grade (high bitumen and low fines content) ores possess good processability, whereas low-grade (low bitumen and high fines content) ores often exhibit poor processability. The poor processability for high fines ores is attributed to strong slime coating (i.e., the attachment of fines to bitumen surfaces).4,9-11 The presence of slime coating not only reduces the bitumen flotation rate and recovery by setting up a steric barrier retarding bitumen droplets to contact with air bubbles but also deteriorates the bitumen froth quality by carrying fine materials to the bitumen froth product. However, high-grade ores do not always possess good processability. The poor processability for some high-grade ores has, from time to time, been encountered in both industrial operations and laboratory tests. For example, a high-grade ore that was stockpiled could suffer a low flotation recovery and a (5) Sanford, E. C. Can. J. Chem. Eng. 1983, 61, 554-567. (6) Smith R. G.; Schramm, L. L. Fuel Proc. Technol. 1992, 30, 1-14. (7) Zhou, Z. A.; Xu, Z.; Masliyah, J. H.; Czarnecki, J. Colloid Surf. A 1999, 148, 199-211. (8) Kasongo, T.; Zhou, Z.; Xu, Z.; Masliyah, J. Can. J. Chem. Eng. 2000, 78, 674-681. (9) Liu, J.; Zhou, Z.; Xu, Z.; Masliyah, J. J. Colloid Interface Sci. 2002, 252, 409-418. (10) Liu, J.; Xu, Z.; Masliyah, J. AIChE J. 2004, 50, 1917-1927. (11) Liu, J.; Xu, Z.; Masliyah, J. Can. J. Chem. Eng. 2004, 82, 655666.
10.1021/ef050091r CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
Processability of Oil Sand Ores in Alberta
Energy & Fuels, Vol. 19, No. 5, 2005 2057
Table 1. Composition (wt %) of Oil Sand Samples fines bitumen solids (-44 µm) water
oil sands good processing ore high electrolytes ore high fines ore weathered ore
12.6 12.8 6.3 15.2
80.4 86.4 86.6 83.2
9.6 10.2 40.4 4.8
6.1 0.8 7.1 1.2
source Syncrude Syncrude Syncrude Suncor
poor froth quality.12-15 Therefore, the processability of oil sand ores cannot be simply evaluated from bitumen and the fines content alone. Some other factors beyond bitumen and the fines content could affect the processability of oil sand ores. In the current study, the processability of four different types of oil sand ores has been tested in a Denver flotation cell. Effect of fines content, divalent cations content, and weathering/aging on the processability of oil sand ores was investigated by measuring surface forces using AFM between bitumen and silica, which is used as models of coarse sands, and between bitumen and fines extracted directly from oil sand ores in the corresponding process water. Within the context of the oil sands industry, fines are defined as mineral solids having a diameter less than 44 µm. It is well-established in the industry that fines are wellcorrelated with mineral solids less than 2-5 µm. Experimental Procedures Materials. Four types of oil sand ores, one good processing ore and three poor processing ores, provided by Syncrude Canada, Ltd. and Suncor Energy, Inc. are used for the flotation tests. Three poor processing ores are specified as high electrolyte ore, high fines ore, and weathered ore. The main components of these samples are shown in Table 1. Bitumen extracted from the good processing ore with toluene is used for surface force measurement. It should be noted that solventextracted bitumen from good and poor processing ores showed indistinguishable surface properties in terms of zeta potential, induction time for air bubble-bitumen attachment, and colloidal force between bitumen and silica sand grains.4 Silica microspheres (∼8 µm), from Duke Scientific Co., were treated with dimethyldichloro silane (8 wt % in toluene) to render them hydrophobic and were used as models of sand grains for the weathered ore in the colloidal force measurements, whereas silica microspheres without further treatment were used to represent the sand grains for the other three ore types. Silicon wafers of 1-0-0 crystal planes were purchased from MEMC electronic Materials (Italy) and used as the substrate for the preparation of bitumen surfaces by the spin-coating method. CaCl2 (99.9965% Fisher) was used as the source of calcium ions. Hydrogen peroxide (3%, Fisher) was used to age oil sands in aqueous solutions. Reagent grade toluene (Fisher) was used as solvent to extract bitumen from oil solids. Mineral oil (reagent grade, Fisher) was used as the oil phase in evaluating surface wettability of solids from oil sands samples. The water used in this study was prepared with an Elix 5 followed by a Millipore ultrapurification water system with a resistivity of 18.2 MΩ cm. Flotation Test. Bitumen batch flotation tests were carried out in the deionized water in a 1 L Denver flotation cell with a water jacket for temperature control. For each test, 300 g of the oil sand sample was dispersed in 400 mL of deionized water at 50 °C. After being conditioned under mechanical agitation at 1500 rpm for 5 min, 600 mL more of deionized (12) Schramm, L.; Smith, R. AOSTRA J. Res. 1987, 3, 195-213. (13) Schramm, L.; Smith, R. AOSTRA J. Res. 1987, 3, 215-224. (14) Wang, N.; Mikula, R. J. Can. Pet. Technol. 2002, 42, 8-10. (15) Mikula, R.; Munoz, V.; Wang, N. J. Can. Pet. Technol. 2003, 42, 50-54.
water was added, and 150 mL/min airflow was introduced. Immediately, the bitumen froth was collected as a function of time for a total of 16 min. The collected froth samples were taken to Dean-Stark for bitumen assay. The tailings remaining in the cell were allowed to settle for 30 min to remove coarse sand grains. After 30 min of settling, the supernatant was collected and transferred to a centrifuge tube and centrifuged at 15 000 rpm for 30 min. The sediments in the centrifuge tube, referred to as fines, were used as mineral fines probes for surface force measurements. The upper clear water in the centrifuge tube was further filtered with ultra-fine filter paper. The filtrate, referred to as the tailing water, was taken to an atomic absorption spectrometer for chemical analysis and used as the probing medium in surface force measurements. Aging Procedure. Aging of the good processing ore was carried out in a well-controlled oven under different conditions. (i) Aging in air: a certain amount of oil sand was placed in an alumina tray as a layer 0.5 cm thick and heated in the oven with air vented at a given temperature for a given period of time. (ii) Aging in a vacuum: a certain amount of oil sand was placed in an alumina tray as a layer 0.5 cm thick and was inserted into the oven. The oven was first evacuated at -26 in. Hg vacuum for 4 h and flashed with nitrogen at a flow rate of 20 mL/min for 4 h at room temperature. The alternative evacuation and flashing lasted for 1 day. The sample was then heated to a desired temperature and remained at that temperature for 7 days before it was cooled to room temperature under a vacuum of -26 in. Hg. (iii) Aging in aqueous solution: a certain amount of oil sand was soaked in an aqueous solution, which was sealed in a bottle. The sealed bottle was heated in the oven at 50 °C for 7 days. Surface Force Measurement (AFM Technique). The surface force measurement was carried out with a Nanoscope E atomic force microscope (AFM, Digital Instrument, Santa Barbara, CA). The AFM consists of a piezoelectric translation stage, a cantilever tip, a laser beam system, a split photodiode, and a fluid cell. The working principle of AFM for colloidal force measurement can be found in open literature.16,17 In the present experiments, probe particles of the spherical model silica and pseudo-spherical fines about 5-10 µm in diameter were glued with a two-component epoxy (EP2LV, Master Bound, Hackensack, NJ) onto the tip of a short, wide beam AFM cantilever under an optical microscope. Bitumen substrates were prepared by spin-coating bitumen on silica wafers withaP6700spin-coater(SpecialtyCoatingSystemsInc.).4,10,11,18,19 The spin-coated bitumen substrate was glued onto a magnetic plate that was mounted on the piezoelectric translation stage. The cantilever substrate with a probe particle was mounted in the fluid cell. The tailing water was injected into the fluid cell slowly, and the system was allowed to stabilize for 1 h before force measurements. When the piezo stage brought the bitumen substrate to approach or retreat from the probe particle in the vertical direction, the force between the two surfaces caused the cantilever spring to deflect upward or downward, depending on the nature of the force between them. The deflection of the cantilever spring was detected by the position-sensitive laser beam that was focused on the upper surface of the spring cantilever and reflected to the split photodiode through a mirror. From the displacement of the piezo stage and the deflection of the spring cantilever, the longrange force and adhesion force (pull-off force) between the substrate and the probe particle can be obtained. Adhesion force data are collected under loading forces of 8-10 mN/m (the loading force is defined as the maximum force in the (16) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831-1836. (17) Kappl, M.; Butt, H. J. Part Part Syst. Charact. 2002, 19, 129144. (18) Liu, J.; Xu, Z.; Masliyah, J. Langmuir 2003, 19, 3911-3920. (19) Liu, J.; Xu, Z.; Masliyah, J. Colloid Surf. A 2005, 260, 217228.
2058
Energy & Fuels, Vol. 19, No. 5, 2005
Liu et al. Table 2. Flotation Rate Constant (k) and Ultimate Recovery (RM) Obtained by Fitting the Flotation Experimental Results to Eq 1 oil sands ore
k (min-1)
RM (%)
good processing ore high electrolytes ore high fines ore weathered ore
1.76 0.45 0.28 0.25
95.7 90 73 72
Table 3. pH and Electrolyte Concentration of the Corresponding Tailing Water
Figure 1. Raw data of a typical probing cycle in AFM surface force measurements.
oil sands
pH
Ca (ppm)
Mg (ppm)
good processing ore high electrolytes ore high fines ore weathered ore
8 7.8 7.5 7.5
0.1 28 6.1 2
0.5 6 5.8 1.8
in the flotation cell follows a first-order process, flotation recovery (R, %) as a function of flotation time (t, min) can be described by the following equation:
R ) RM(1 - e-k(t-τ))
Figure 2. Floatability of different oil sand ores. (a) Recovery as a function of flotation time and (b) bitumen/solid ratio as a function of flotation time. 0: good processing ore; O: high electrolytes ore; 4: weathered ore; and 3: high fines ore. constant compliance region). It is found that the adhesion forces are independent of the loading forces in the range examined. A typical set of raw data of probing surface forces is shown in Figure 1. In this figure, the repulsive force barrier and adhesion force are of great concern since they control coagulation behaviors between two particles. To ensure that the measured force profiles are representative, the force measurement was repeated with at least six bitumen-silica or bitumen-fines pairs. The average values of the repulsive force barriers or the adhesion forces are reported. All the force measurements were carried out at room temperature (22 °C). Hydrophobicity Characterization of Solids. To characterize the hydrophobicity of solids, partitioning of the solids in water and mineral oil phases was measured. Solids were first extracted from oil sand by repeatedly removing bitumen with toluene. Partitioning measurements were carried out in 20 mL glass bottles. For each test, the glass bottle was filled with 9 mL of deionized water and 9 mL of mineral oil, followed by the addition of 2 g of collected solids. After being shaken for about 5 min, the mixture was allowed to phase separate for 2 h. The solids remaining in the water phase and oil phase were considered to be hydrophilic and hydrophobic, respectively. The percentage of solids distributed in the mineral oil phase over the total added solids represents a measure of the solids’ hydrophobicity.
Results Floatability of Oil Sands. The flotation tests for one good processing ore and three poor processing ores were carried out. The flotation recovery and froth quality (bitumen/solid ratio) as a function of flotation time are shown in Figure 2. These oil sands exhibit significantly different flotation behaviors. To better evaluate flotation kinetics, the comparison of the flotation rate constant is preferred. Assuming that the batch flotation process
(1)
where RM (%) is the ultimate flotation recovery; k (min-1) is the flotation rate constant; and τ (min) is the time delay related to the flotation test itself. The flotation recovery data in Figure 2a can be used to estimate RM and k values, which allow for a better overall assessment of the flotation performance. The fitted values for various oil sand ores examined are summarized in Table 2. The good processing ore is of a higher flotation rate and a higher ultimate bitumen recovery, whereas poor processing ores give both lower flotation rates and lower ultimate bitumen recovery. Froth quality (bitumen/solid ratio) shown in Figure 2b for the good processing ore is higher than that for the poor processing ores. Various levels of multivalent cations in the corresponding tailing water are also observed for different types of oil sand ores, as shown in Table 3. These values reflect the level of multivalent cations in the oil sands since the cations are released from oil sands. More importantly, higher level divalent cations in the processing water imply that surface potentials of bitumen and solids, and therefore repulsive forces between bitumen and solids, could be significantly depressed through the compression of electric double layers. The depressed repulsive force barriers can cause poor liberation, or serious slime coating, or both, which are detrimental to bitumen extraction. Hydrophobicity of solids in oil sand plays a critical role in controlling colloidal interactions between bitumen and solids. In this regard, partitioning of the solids in water and mineral oil phases is a good marker for the hydrophobicity of the solids. The solids remaining in the water and oil phase are referred to as being hydrophilic and hydrophobic, respectively. Visual observations of coarse and fine solid distributions in water and oil phases are shown in Figure 3a, and the percentage of solids distributed in the mineral oil phase over the total solid added is presented in Figure 3b. The results of Figure 3a,b show the following increasing order in hydrophobicity of the solids: good processing ore, high electrolytes ore, high fines ore, and weathered ore. For the good processing ore, both coarse sand and fines are hydrophilic. For the high electrolytes ore, the
Processability of Oil Sand Ores in Alberta
Energy & Fuels, Vol. 19, No. 5, 2005 2059
Figure 4. Effect of calcium ions on floatability of the good processing ore. 0: without calcium added; O: 40 ppm calcium added; and 4: 400 ppm calcium added.
Figure 5. Flotation results of the good processing ore artificially aged in air for 7 days at different aging temperatures. (a) Recovery as a function of flotation time and (b) bitumen/solid ratio as a function of flotation time. 0: fresh ore; O: 22 °C; 4: 35 °C; and 3: 50 °C. Figure 3. Partitioning of solids from oil sand ores in water and mineral oil phases. (a) Visualization and (b) percentage of solids distributed in the mineral oil phase over total solids added.
coarse sand and most of the fines are hydrophilic. For the high fines ore, however, the coarse sand is hydrophilic, but the fines are hydrophobic. For the weathered ore, both coarse sand and fines are hydrophobic. From the colloidal interaction point of view, the hydrophobic feature of the solids can induce strong attractive and adhesion forces between bitumen and solids, thereby leading to poor liberation, serious slime coating, or both. Calcium Effect on Floatability. Divalent cations such as calcium and magnesium are inevitable ions in bitumen flotation systems. Their effect on the floatability of bitumen is of great concern. To clarify this issue, flotation tests of the good processing ore with the addition of calcium ions at different concentrations were carried out. As shown in Figure 4, the addition of calcium ions at 40 ppm has only a marginal effect on the floatability. This observation is in agreement with results reported in literature.8 When the calcium ion concentration increased to 400 ppm, however, both flotation recovery and froth quality for the good processing ore deteriorated. This finding indicates that divalent ions above a certain level can affect the processability of oil sand ores. Weathering/Aging Effect on Floatability. Weathering or aging is a natural process occurring during the formation of oil sand deposits, especially for those near the surface or with shallow overburden. Weathering/ aging may also occur during the storage or storage piling of oil sand ores prior to treatment. To understand the effect of weathering/aging on the processability of oil
Figure 6. Weight loss of oil sand sample aged artificially for 7 days under different ambient conditions as a function of aging temperature. 0: in air and O: in a vacuum.
sand ores, the good processing ore was artificially aged under controlled conditions before flotation. The flotation results for the good processing ore that was artificially aged in air for 7 days at different temperatures are shown in Figure 5. Compared with the fresh ore, bitumen floatability of the artificially aged oil sand samples deteriorated significantly. With increased aging temperature, both bitumen recovery and froth quality decreased. The results confirm that weathering/aging does have a significant impact on bitumen floatability. Detailed analysis indicates that the weight of the treated oil sand and the hydrophobicity of the solids were changed during the aging process. The results in Figures 6 and 7 show a continuous increase in weight loss of the oil sand sample and in hydrophobicity of the solids with increased aging temperature during the aging process. Weight loss may result from
2060
Energy & Fuels, Vol. 19, No. 5, 2005
Liu et al.
Figure 7. Percentage of solids distributed in mineral oil phase over total solids added as a function of aging temperature for the oil sand samples aged artificially for 7 days under different conditions. 0: in air; O: in a vacuum; 4: in 0.3% H2O2 solution; and 3: in deionized water.
Figure 9. Flotation results of the good processing ore aged artificially at 50 °C for 7 days in aqueous solutions. (a) Recovery as a function of flotation time and (b) bitumen/solid ratio as a function of flotation time. 0: fresh ore; O: in deionized water; and 4: in 0.3% H2O2.
Table 4. Evaporation of Components (on Basis of 100 g of Fresh Ore) during the Process of Artificially Aging the Good Processing Ore at 50 °C for 7 Days oil sands
fresh ore
ore aged in air
ore aged in a vacuum
water content (g)a water loss (g)b total weight loss (g)c organics loss (g)d
6.1 0 0 0
1.2 4.9 5.3 0.4
1.2 4.9 5.4 0.5
a Water content was determined by the Dean-Stark method. Water loss (g) ) water content in fresh ore (g) - water content in aged ore (g). c Total weight loss (g) ) weight of fresh ore (g) weight of aged ore (g). d Organics loss (g) ) total weight loss (g) water loss (g).
b
Figure 8. Flotation results of the good processing ore artificially aged in a vacuum for 7 days at different temperature. (a) Recovery as a function of flotation time and (b) bitumen/solid ratio as a function of flotation time. 0: fresh ore; O: 22 °C; 4: 35 °C; and 3: 50 °C.
evaporation of both connate water and light hydrocarbon (organics). To distinguish water evaporation from light hydrocarbon evaporation during the aging process, the connate water in the oil sands before and after aging was analyzed. The results in Table 4 show that the weight loss is mainly due to water evaporation (4.9 g/100 g of fresh ore) and only a small fraction from light hydrocarbon loss (0.4 g/100 g of fresh ore). Figure 8 shows flotation results for the good processing ore artificially aged in a vacuum at different temperatures. Compared with the untreated ore, the aged ore suffers a remarkable decrease in both flotation recovery and froth quality (bitumen/solids) with increasing aging temperature. At a higher temperature, a higher weight loss during aging is observed, and the solids become more hydrophobic (Figure 7). Compared
Figure 10. Normalized repulsive force barrier and adhesion force between bitumen and solids in their tailing water for different oil sand ores. (a) Bitumen-silica and (b) bitumenfines.
with the case of aging in air, the aging of oil sand ore in a vacuum shows lower floatability deterioration, and a smaller increase in the hydrophobicity of solids, although the total weight loss (water and organic losses) is quite similar (Table 4). The floatability of the good processing ore aged in aqueous solutions at 50 °C for 7 days is shown in Figure 9. When the good processing ore was aged in the 0.3% H2O2 solution, both flotation recovery and bitumen/solid ratio decreased, whereas when it was aged in deionized water, only a marginal change was observed in bitumen floatability. As shown in Figure 7, solids from the ore aged in H2O2 solution exhibited some degree of hydrophobicity, whereas solids from the ore aged in deionized water remained hydrophilic. Colloidal Forces between Bitumen and Solids. For a colloidal system under shear such as in the bitumen extraction process, consideration of both longrange and adhesion forces is critical to gain a holistic view of the process itself. Comparison of long-range repulsive force barriers and adhesion forces allows for a better prediction of coagulation behavior in a dynamic colloidal system. In the case that there exists a strong repulsive force barrier and a weak adhesion force, for example, two particles are unlikely to attach to each other. Even if they become attached to each other, they can be easily detached. If there is a weaker repulsive force barrier and a stronger adhesion force, two particles are likely to attach to each other under favorable mixing conditions to overcome weak repulsion. Repulsive force barriers and adhesion forces measured between bitumen and solids in the tailing water for the tested oil sand ores are shown in Figure 10. Bitumen liberation from sand grains is mainly determined by the adhesion forces between bitumen and
Processability of Oil Sand Ores in Alberta
silica sands. From the results of the hydrophobicity tests shown in Figure 3, it was established that sand grains from the good processing, high electrolyte, and high fines ores are hydrophilic, while those from the weathered ore are hydrophobic. Since it is difficult to pick up the right sized sand grains from each ore to perform surface force measurements, model silica spheres were used. To further represent the hydrophobic nature of sand grains from the weathered ore, the results labeled with weathered ore were obtained with model silica spheres silanized to become hydrophobic. This silanation would represent an extreme case of hydrophobization, which serves the purpose of comparison. For the good processing ore, high electrolyte ore, and high fines ore, the weaker adhesion forces between bitumen and model silica shown in Figure 10a indicate a good liberation of bitumen from silica sands, whereas for the weathered ore, a stronger adhesion force between bitumen and model silica is obtained. In this case, poor liberation is anticipated. Slime coating in the bitumen extraction system is controlled by both repulsive force barriers and adhesion forces between bitumen and fines. Figure 10b shows the results obtained using fines from their corresponding tailings. As shown in Figure 10b, a strong repulsive force barrier and a weak adhesion force are observed between bitumen and fines from the good processing ore. For the high electrolyte ore, particularly for the high fines ore and the weathered ore, weak repulsive force barriers and strong adhesion forces are determined. These results imply a negligible slime coating in bitumen extraction systems of the good processing ore and a possible strong slime coating in bitumen extraction for the other three types of oil sands, especially in the case that there is a sufficient amount of fines present. Discussion Good Processing Ore. The good processing ore has the features of high bitumen content, low solid fines, and low divalent cation content with negligible weathering. As shown in Figure 2 and Table 2, processing of the good processing ore leads to good recovery and froth quality, with the flotation rate constant, ultimate recovery, and bitumen/solid ratio being 1.8 min-1, 95.7%, and 4.5, respectively. Since both coarse sand and fines are hydrophilic (Figure 3) and the divalent cation concentration is low (Table 3), stronger repulsive forces and weaker adhesion forces between bitumen-silica and bitumen-fines are not unexpected (Figure 10). A system with such characteristics would have good bitumen liberation from the silica sand surface and negligible slime coating, thereby accounting for the observed good processability. High Fines Ore. The high fines ore has the characteristics of low bitumen and high fines solid content with some divalent cations being present. The impact of the fines on the bitumen flotation kinetics has been wellrecognized. As shown in Figure 2, this type of ore responded to flotation with a low flotation rate and recovery along with a poor froth quality. A certain level of calcium and magnesium ions in the tailing water depresses the repulsive component of the long-range forces between bitumen and solids mainly by electric double layer compression. The hydrophobic
Energy & Fuels, Vol. 19, No. 5, 2005 2061
nature of the fines triggers an attractive hydrophobic force and enhances the adhesion force between bitumen and fines. As a result, depressed repulsive force barriers and enhanced adhesion forces are observed between bitumen and solids, more so for bitumen and fines pairs. According to the measured force profiles (Figure 10), a repulsive force barrier with an equivalent adhesion force between bitumen and silica would lead to a reasonable bitumen liberation, and a strong adhesion force with a negligible repulsive force barrier between bitumen and fines would lead to a strong slime coating. The slime coating is the main reason for the observed poor processability of the high fines ore. High Electrolytes Ore. The high electrolyte ore used in the tests has the characteristics of high bitumen content, low fines content, and hydrophilic solids with high divalent ions. As shown in Figure 2, this type of oil sand ore also suffers a low flotation rate and poor froth quality. This is not expected from the characteristics of a high bitumen grade, low fines content, and hydrophilic solids, which are similar to that of the good processing ore. However, the high electrolyte ore contains much higher concentrations of divalent ions in the tailing water in comparison with other tested oil sand ores (Table 3). When compared with the effect of calcium addition on the processability of the good processing oil sand (Figure 4), the divalent ions at levels of 28 ppm calcium and 6 ppm magnesium in the tailing water are not very high. Therefore, the cation levels are anticipated to have a marginal impact on the processability of the high electrolyte ore. However, careful examination indicates that the role of the divalent cations released from the oil sand being tested into the process water is different from that of those intentionally added into the slurry even though their concentration shows a similar level in the tailing water. Supposing that the divalent cations in the tailing water mainly came from the connate water of the oil sands, the divalent cation concentration in the connate water could reach about 2500 ppm calcium and 500 ppm magnesium calculated using the data from Tables 1 and 3 for the high electrolytes ore. It is conceivable that such high levels of divalent ions in the connate water would impact the surface properties of bitumen and solids, such as zeta potentials and hydrophobicity, and hence the interactions between bitumen and solids. The measured long-range repulsive force barriers and adhesion forces (Figure 10) tend to suggest a weak coagulation between bitumen and silica and a strong heterocoagulation between bitumen and fines in the tailing water. This means that the liberation of bitumen from silica sands is not favorable, especially at the initial stage where the dominating interaction forces between bitumen and sands is in the medium of connate water containing an excessively high concentration of divalent cations. Slime coating, on the hand, is strong but limited to some extent since the fines content is only about 10% for the high electrolytes ore. The presence of divalent ions can also reduce the hydrophobicity of the bitumen surface by forming precipitates of divalent ions with the carboxylate surfactants on the bitumen surface.10 A certain degree of slime coating, combined with the reduced hydrophobicity of bitumen surfaces, hinders the aeration of bitumen with air bubbles. The deterio-
2062
Energy & Fuels, Vol. 19, No. 5, 2005
rated aeration with unfavorable bitumen liberation is mainly responsible for the observed poor processability of the high electrolyte ore. Weathered Ore. The weathered ore used in this study contains high bitumen and low solid fines content with a moderate level of divalent ions. From a composition point of view, good processability is anticipated for this type of ore. The results of Figure 2 and Table 2, however, show an extremely poor processability for the weathered ore. The flotation rate constant (0.25 min-1) and ultimate recovery (73%) are similar to those for the high fines ore. The ratio of bitumen to solids in the froth is only about 0.25, corresponding to a bitumen content of 20% on a water-free basis, which is only marginally higher than the bitumen content in the oil sand ore itself. This observation would suggest that bitumen is not liberated from the sand grains during the flotation process. A distinct feature of the weathered ore from other ores is that both coarse sand and fine solids from the weathered ore are hydrophobic (Figure 3a). To better mimic the silica sand with a hydrophobic nature from the weathered ore, the model silica spheres in colloidal force measurements were silanated to become hydrophobic. Although this represents an extreme case of strongly hydrophobic sand grains, the results can show a trend for the purpose of comparison. In the tailing water, strong adhesion forces without repulsive force barriers between bitumen and hydrophobized model silica spheres are determined (Figure 10a). A poor liberation of bitumen from solids is not unexpected for such systems. Since the fines content is low, the degree of slime coating would be limited for the weathered ore. Therefore, the poor liberation of bitumen from silica sand appears to be the main cause for the poor processability of the weathered ore. Weathering/aging is a complex physicochemical process. Aging of a good processing oil sand in air and vacuum leads to weight loss and to increased hydrophobicity of the mineral solids. It can be hypothesized that the weathering/aging effect would have two consequences: evaporation and oxidization. Each of these processes makes the solids hydrophobic. Evaporation includes removal of connate water and volatile hydrocarbons, whereas oxidation can affect both bitumen and mineral solids. When aged in air, the good processing ore sample showed the worst flotation performance (Figure 5). In addition, the solids become most hydrophobic (Figure 7), accompanied with a certain degree of weight loss (Figure 6). In this case, both evaporation and oxidization can play a role in affecting bitumen recovery. When the oil sand ore sample is aged in a vacuum, evaporation plays an effective role, while oxidization is minimized. As shown in Figures 6 and 7, the weight loss (evaporation) of the oil sand sample and hydrophobicity of the solids are dependent on aging temperature. From the analysis of connate water before and after aging, it is not difficult to distinguish the water loss from the loss of light hydrocarbon (organics) during the aging process. The results in Table 4 show that the weight loss is mainly due to the water evaporation (4.9 g/100 g of fresh ore) and that the evaporation of light hydrocarbon (0.4 g/100 g of fresh ore) only accounts for less
Liu et al.
than 10% of the total weight loss. The consequence of the connate water loss is thinning or even removal of the water film (if it exists) between the bitumen and the sand grains. This in turn leads to an increase in divalent ion concentration in the remaining connate water. The direct contact between bitumen and sand grains coupled with a high concentration of divalent ions in the connate water would undoubtedly make the sand grain surface more hydrophobic via the transfer of surfactants from bulk bitumen to the solid surface. It can be concluded that evaporation of the connate water during weathering/aging of oil sands, or loss of water from a water film at the solids surface,20 is one of the main reasons for the deteriorated processability of the weathered/aged ores, although the effect of the organic evaporation needs to be established. At higher aging temperatures, an increased loss of water and organics by evaporation is responsible for the severe depression of floatability (Figure 8). Compared with the results of Figure 5, the degree of deterioration in floatability by aging in a vacuum is less severe than that by aging in air, even though the weight loss by evaporation is quite similar (Figure 6 and Table 4). This comparison would lead us to conclude that oxidization can also contribute to the deterioration of oil sand floatability. When the oil sand ore sample is aged in 0.3% H2O2 solution, only oxidization is in effect, and evaporation can be considered negligible. Hydrogen peroxide (H2O2) is a strong oxidizing agent. Oxidization would occur for both bitumen and minerals. From near infrared spectroscopy (NIR) and light microscopy (LM) analysis, there is convincing evidence of bitumen oxidization in the weathered ore.15 Efforts have been made to quantify the degree of weathering by analyzing NIR spectra or froth morphology. NIR spectrum or froth morphology is a good indicator of bitumen oxidization for weathered/aged ore. However, surface properties important for bitumen extraction, such as zeta potential, induction time of bitumen attaching to air bubbles, hydrophobicity (contact angle) and surface forces between bitumen and silica, showed very little difference between the bitumen samples from fresh oil sands or from weathered oil sands.4 It is clear that oxidation of bitumen in oil sand ore is a good indicator in identifying the type of oil sands, but oxidized bitumen itself does not contribute to the poor processability. To trace down the cause of poor processability of weathered/aged ores, the solids need to be examined. The solids from the aged ore in H2O2 show a noticeable increase in hydrophobicity (Figure 7). This suggests that some minerals are oxidized to cause a change in the surface properties of solids by weathering/aging. In this case, mineral pyrite in oil sands is likely to be oxidized to generate ferric ions, which can adsorb on the silica surface. The adsorption of iron species on the silica surfaces can activate adsorption of natural surfactants, rendering the silica sands hydrophobic. For the purpose of comparison, the good processing ore is aged in deionized water at the same elevated temperature (50 °C). In this case, both evaporation and oxidization are minimized. The floatability of the aged ore is affected (Figure 9) only marginally, and the solids (20) Czarnecki, J.; Radoev, B.; Schramm, L.; Slavchev, R. Adv. Colloid Interface Sci. 2005, in press.
Processability of Oil Sand Ores in Alberta
remain hydrophilic. This finding would indicate (Figure 7) that any aging/weathering process with negligible evaporation and oxidization would not impact the processability of the oil sands. General Remarks. This study clearly demonstrates that processability of oil sand ores cannot be simply evaluated from bitumen grade and fines content alone. Any factor that causes poor liberation or slime coating would result in a poor processability. In this regard, fines content, divalent cation concentration in the tailing water, and the hydrophobic nature of solids are indicators for the processability of oil sands. They can individually or collectively impact liberation, aeration, or both, and hence the processability of oil sands. These findings bear important practical implications for improving bitumen extraction efficiency. For the high fines ore, for instance, slime coating should be mitigated through addition of chemicals such as silicates, phosphates, or polymers. To process a high electrolyte ore, elimination of divalent cations is critical. In this case, chelation or precipitation of divalent cations by polymers or inorganic chemicals such as caustic and bicarbonates can be practiced. Extension of conditioning time and increasing temperature at the liberation stage would be beneficial. To process the weathered/aged ore, the hydrophobicity of the solids needs to be addressed. For this purpose, increasing the temperature and solution pH would be good venues. In addition, some strong chelating chemicals would help shield active sites on the solids surface to mitigate solid hydrophobicity. Nevertheless, there remains much work to validate these hypotheses.
Energy & Fuels, Vol. 19, No. 5, 2005 2063
Conclusions Different types of oil sand ores are found to have diverse processabilities. Their processability cannot be simply evaluated from bitumen and fines content alone. Fines content, divalent cation content, and weathering/ aging can, either individually or collectively, significantly affect the processability of oil sands. Fines content, divalent cation concentration in the tailing water, and the hydrophobic nature of solids are indicators for the processability of oil sands. A good processability was observed for the good processing ore, which is attributed to favorable bitumen liberation and aeration. A poor processability was observed for the high fines ore, high electrolyte ore, and weathered ore. Their poor processability is mainly attributed to serious slime coating and poor bitumen liberation. These findings provide some guidelines for improving the processability of poor processing ores. Acknowledgment. The authors acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the NSERC Industrial Research Chair in Oil Sands Engineering (J.M.). Useful discussion with Dr. Randy Mikula and Mr. Brad Komishke and provision of oil sand samples by Syncrude Canada, Ltd. and Suncor Energy, Inc. are also acknowledged. EF050091R
