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On the Role of Temperature in Oil Sands Processing J. Long, Z. Xu, and J. H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received December 3, 2004. Revised Manuscript Received April 5, 2005
Bitumen recovery from oil sands was found to severely deteriorate at operating temperatures lower than a “critical” value, suggesting a substantial change in one or more key process variables. A sharp increase in bitumen viscosity at lower temperatures has been considered as the major contributor to such deterioration. On the basis of the fact that the addition of selected chemicals does improve bitumen recovery without affecting bitumen viscosity, there must be other physicochemical factors that affect bitumen recovery and undergo a sharp change with temperature. In the present study, the interaction and adhesion forces between bitumen and sand grains or clays in water were measured as a function of temperature using an atomic force microscope. The results show that the measured adhesion force between clay and bitumen decreases with increasing temperature until a critical value of about 32-35 °C, above which the adhesion force disappears. As the adhesion force between clay and bitumen controls clay slime coating on bitumen surface and subsequently bitumen aeration, increase in the adhesion force with decreasing processing temperature would lead to slime coating and lower bitumen recovery. The effect of a chemical additive, methylisobutyl carbinol (MIBC), on the colloidal forces was also studied. The results show that MIBC addition can reduce the adhesion force between clay and bitumen, thus facilitating bitumen aeration.
1 Introduction Up to the early 1990s, the bitumen in oil sands had been commercially recovered using the Clark hot water process (CHWP)1-6 operating at 70-80 °C with caustic addition. However, such a bitumen extraction process requires an input of considerable thermal energy. Much of this thermal energy is not recoverable and is lost in the discharge of slurry tailings. To minimize the thermal energy input, conventional hot water process plants must also be located close to a supply of thermal energy, thus necessitating costly transportation of the oil sands from mining sites to central processing units. The separation of bitumen from oil sands at low temperatures would significantly reduce thermal energy consumption and allow extraction plants to be located in proximity to the mine face, thus minimizing the cost of transporting the oil sand ores. Tremendous efforts6-13 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Helper, L. G.; His, C. AOSTRA Technical Publication Series #6; Alberta Oil Sands Technology and Research Authorities: Edmonton, Alberta, Canada, 1989. (2) Clark, K. A. Trans. Can. Inst. Min. Metall. 1944, 47, 257-274. (3) Clark, K. A. Can. Inst. Min. Metall. Bull. 1929, 22, 1385-1395. (4) Clark, K. A.; Pasternack, D. S. Ind. Eng. Chem. 1932, 24, 14101416. (5) Clark, K. A. Can. Oil Gas Ind. 1950, 3, 46-50. (6) Hepler, L. G.; Smith, R. G. AOSTRA Technical Publication Series #14; Alberta Oil Sands Technology and Research Authorities: Edmonton, Canada, 1994. (7) Sury, K. N. U.S. patent 4 946 597, August 7, 1990. (8) Sury, K. N. CA patent 1 302 327, June 2, 1992. (9) FTFC (Fine Tailings Fundamentals Consortium). Advances in Oil Sands Tailings Research; Alberta Oil Sands Technology and Research Authorities: Edmonton, Canada, 1995. (10) Kessick, M. A. AOSTRA J. Res. 1991, 7, 279-281. (11) Kessick, M. A. CA patent 2 101 240, Jan. 14, 1994.
have been made to lower the operating temperature of the bitumen recovery process. In recent years, several industrial extraction processes have been successfully operated at about 40-50 °C. These processes are referred to as warm water process. In 2000, a low-energy extraction (LEE) process (also known as cold-water process) was designed and initially operated at a temperature of about 25 °C, at the Aurora plant of Syncrude Canada Ltd. In 2002 this process, however, was shifted to operate at a higher temperature of 35-40 °C to ensure operation reliability and higher bitumen recovery. The effect of temperature on bitumen recovery has been widely investigated with various types of oil sand ores.14-20 It has been found that bitumen recovery was not significantly affected within the temperature range of about 50-95 °C.14,15,18 However, bitumen recovery (12) Calta, J. S. U.S. patent 5 746 909, May 5, 1998. (13) Allcock, G.; Siy, R.; Spence, J.; Sury, K. U.S. patent 6 007 708, Dec. 28, 1999. (14) Bichard, J. A. AOSTRA Technical Publication Series #4; Alberta Oil Sands Technology and Research Authority: Edmonton, Canada, 1987. (15) Stasiuk, E. N.; Schramm, L. L.; Yarranton, H.; Shelfantook, B. Shear and interfacial phenomena involved in reducing process temperature for the recovery of bitumen from Athabasca oil sand, 227th ACS National Meeting, Anaheim, CA, March 28-April 1, 2004. (16) Dai, Q.; Chung, K. Fuel 1995, 74, 1858-1864. (17) Dai, Q.; Chung, K. Fuel 1996, 75, 220-226. (18) Schramm, L. L.; Stasiuk, E. N.; Yarranton, H.; Maini, B. B.; Shelfantook, B. J. Can. Pet. Technol. 2002, 42, 55-61. (19) Ding, X.; Xu, Z.; Masliyah, J. Effects of Divalent Ions, Illite Clays and Temperature on Bitumen Recovery, 54th Canadian Chemical Engineering Conference, Calgary, Canada, October 3-6, 2004. (20) Seyer, F. A.; Gyte, G. W. AOSTRA Technical Publication Series #6; Alberta Oil Sands Technology and Research Authorities: Edmonton, Canada, 1989; Chapter 4.
10.1021/ef0496855 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/19/2005
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Table 1. Properties of the Aurora Process Water and the Transition Ore Used Ca2+ ppm Aurora process water Aurora transition ore
Mg2+ ppm
pH
comments
47.0 15.0 8.2 Recycle pond water from the Aurora plant of Syncrude Canada, Ltd. One of the problem ores from the Aurora area of Syncrude Canada, Ltd., with 9.2 wt % bitumen, 7.3% water, and 83.5% solids. The solids contain 33% of fines (less than 44 µm in size)
decreased at temperatures lower than 50 °C. Sharp decrease in bitumen recovery was observed particularly at temperatures lower than 35 °C. The early study by Bichard14 showed that bitumen recovery of a poor processing oil sand ore (called Area D, Hole 11, tar sand in the reference) was significantly reduced from about 90% at 37.8 °C (100 °F) to 40% at 26.7 °C (80 °F). Schramm et al.18 observed an order of magnitude reduction in primary bitumen recovery for an averagegrade ore from 88% at 50 °C to 8% at 25 °C. Ding et al.19 found a good bitumen recovery of 85-90% at 35 °C for a good processing ore even with addition of illite clay, calcium, or magnesium ions. However, the recovery reduced significantly to 70% at 25 °C, or even as low as 30% when illite, calcium, or magnesium ions were added to the oil sand slurry. All the abovementioned results indicate that one or more key process variables underwent a substantial change when the processing temperature was reduced. Stasiuk et al.15 and Schramm et al.18 suggested that bitumen viscosity was the main contributor to the dramatic reduction in bitumen recovery when the temperature was decreased to ambient temperature of 25 °C. They suggested that a bitumen viscosity threshold existed at about 3 Pa‚s18 and that the role of increasing temperature was to reduce bitumen viscosity below the threshold.15 Bitumen viscosity is known to increase sharply with decreasing temperature.6,20 The effect of bitumen viscosity by changing processing temperature on bitumen recovery is, therefore, evident and critical. However, previous studies15,18,21 also indicate that selected chemicals, such as MIBC (methylisobutyl carbinol), can significantly increase bitumen recovery without affecting bitumen viscosity. Schramm et al.18 found that addition of kerosene and MIBC significantly improved bitumen recovery. With kerosene addition at an optimal dosage of 20 mg/g (oil sand basis) in ambient extraction tests at 25 °C, the recovery increased from 8% (without kerosene addition) to 79% for an average-grade ore. Together with 20 mg/g kerosene addition, MIBC addition at an optimal dosage of 1 mg/g increased the recovery to 98%, which is much higher than the recovery of 88% obtained at 50 °C (without chemical addition). The effect of kerosene addition is clear as the added kerosene would decrease bitumen viscosity18 and enhance bitumen separation from sand grains. However, the highest recovery obtained with kerosene addition (79%) is still lower than the recovery obtained at 50 °C (88%) although the viscosity was reduced by kerosene addition to the same level of the bitumen at 50 °C. It is clear that other factors than bitumen viscosity must also affect bitumen recovery. In addition, as MIBC addition did not affect bitumen viscosity, the mechanism responsible for the increase of 19% in recovery (from 79 to 98%) by MIBC addition remains to be determined.18 (21) Li, H.; Long, J.; Xu, Z.; Masliyah, J. Synergetic role of polymer flocculant in low-temperature bitumen extraction and tailings treatment. Energy Fuels 2005, in press.
It is well-known that the separation of bitumen from oil sands is controlled by the interactions between bitumen and the solids in the oil sands. A possible way to establish the effect of temperature and chemical additives is to directly measure the interaction forces between bitumen and the solids as a function of temperature or with the addition of these chemicals. The recently developed surface force apparatus (SFA)22,23 and atomic force microscope (AFM)24 made quantitative investigations of such interactions possible. Using an AFM, Liu et al.25 successfully measured the interaction and adhesion forces between bitumen and silica. In their study, silica spheres were used as the model for sand grains and the force measurements were carried out in simple KCl electrolyte solutions. On the basis of the work by Liu et al.,25 the current study focuses on an in-depth investigation that addresses the effect of temperature and MIBC addition on the interactions between bitumen and sand grains or clay fines. To better represent the interactions between bitumen and solids in oil sand processing, both clay particles, directly chosen from an oil sand tailings slurry, and model silica spheres were used in the force measurements. Process recycle water obtained from a commercial operation site was used as the aqueous media. The goal is to identify the reason or the mechanism for the aforementioned sharp reduction in bitumen recovery with decreasing temperature and thus to better understand the role of temperature and chemical additives in oil sand processing. Such an understanding not only benefits the current commercial operations of bitumen recovery but also lays a foundation for the development of new lowtemperature processes to recover bitumen from oil sands. 2 Experimental Section 2.1 Materials. Vacuum-distillation-feed bitumen provided by Syncrude Canada Ltd. was used in this study. Silica microspheres (∼8 µm in diameter), used as a model sand grain for colloidal force measurements, were purchased from Duke Scientific Co. (United States). Silicon wafers with an oxidized surface layer of ∼0.6 µm was obtained from NANOFAB (University of Alberta, Canada) and used in the force measurements. Clay particles obtained from tailings slurry, with a pseudospherical shape, were also used as the probe for the force measurements. These particles were chosen under an optical microscope from a great number of particles, which were directly obtained from the tailings slurry of a bitumen extraction process using a transition ore and Aurora process water. The details of the extraction process can be found elsewhere.21 The ore was one of the problem ores from the Aurora region of Syncrude Canada Ltd. and its composition is given in Table 1. The Aurora process water was recycle process pond water from Aurora commercial plant and some (22) Tabor, D.; Winterton, R. H. S. Nature (London) 1968, 219, 1120-1121. (23) Israelachvili, J. N.; Adams, G. E. Nature (London) 1976, 262, 774-777. (24) Binnig, G.; Rohrer, H. IBM J. Res. Dev. 1986, 30, 355-369. (25) Liu, J.; Xu, Z.; Masliyah, J. H. Langmuir 2003, 19, 3911-3920.
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relevant properties of the process water are also given in Table 1. The Aurora process water was used as the aqueous medium for the force measurements. 2.2 Surface force measurement. A Nanoscope E atomic force microscope (AFM) with a vendor-supplied fluid cell (Digital Instruments, Santa Barbara, CA) was used for the surface force measurement. Gold-coated silicon nitride cantilevers also from Digital Instruments were chosen. A colloid probe, a silica sphere or a clay particle, was attached onto the apex of the cantilever (lever type 100-µm wide) with a spring constant of 0.58 N/m. The details of preparing such colloid probes and using AFM for force measurements are provided elsewhere.25,26 Briefly, in the force plot mode, the X and Y voltages applied to the AFM piezo tube are held at zero while a triangular waveform is applied to the Z piezo tube. As a result of the applied voltage, the sample surface attached to the piezotube moves toward and away from the cantilever tip (the colloid probe) by the extension and retraction of the piezotube. The force acting between the probe and the surface is determined from the deflection of the cantilever by using Hooke’s law. Each force plot represents a complete extensionretraction cycle of the piezo. When a sample surface approaches a probe, the long-range interaction between the two surfaces is measured while the adhesion (or pull-off) force can be obtained during the retraction process. For quantitative comparison, the measured long-range interaction force (F) and adhesion force (pull-off force) were normalized by probe radius (R). Force measurements were performed in a fluid cell where clay probes interacted with bitumen surface in aqueous solutions. The bitumen surface was prepared by coating a thin layer (∼100 nm) of bitumen onto 10 × 10 mm2 silica plates using a spin-coater. A detailed description on the preparation of the bitumen surface and the characteristics of the prepared bitumen surface can be found elsewhere.25 To adjust and control the temperature of the aqueous medium in the fluid cell, a sample heater with a temperature controller (Digital Instruments, Santa Barbara, CA) was added onto the AFM. The heater provided controlled heat to the bottom of the sample from ambient temperature to 60 °C with a resolution of 0.1 °C. The actual temperature of the aqueous medium in the cell was measured using a thermocouple thermometer (Dual J-T-E-K, Cole-Parmer Instrument CO., Illinois, United States). All force measurements were conducted after an incubation time of 30 min for each set of temperature. Preliminary experiments showed that 30 min was sufficient for the two surfaces immersed in the aqueous medium to equilibrate. For silica probes, the measurement under each test condition was performed at different locations on bitumen surface. For clay probes, as the surface of the probes was quite irregular, each force measurement was performed several times with different probes to obtain more representative results.
3 Results 3.1 Silica-Bitumen Interaction. To study the interactions between bitumen and sand grains in oil sands processing environment, the interaction and adhesion forces between a silica sphere and a bitumen surface were measured in the Aurora process water. Figure 1 shows the effect of temperature on the interaction forces. Over the temperature range, from the ambient up to about 40 °C, the measured long-range interactions at a separation distance less than 15 nm are monotonically repulsive. The repulsive force increases with increasing temperature. At the lowest temperature of 21.2 °C, a weak long-range repulsive (26) Ducker, W. A.; Senden, T. J. Langmuir 1992, 8, 1831-1836.
Long et al.
Figure 1. Effect of temperature (T, °C) on the long-range interaction forces (F/R) between silica and bitumen in the Aurora process water. Solid curves represent the DLVO fitting with Hamaker constant of 3.3 × 10-21 J, best fitted decay length (κ-1, nm), and surface potentials of silica and bitumen (VSi and VB, mV).
Figure 2. Effect of temperature on the adhesion force between silica and bitumen in the Aurora process water. The measured adhesion force deceases with increasing temperature until about 32 °C. At temperatures higher than 32 °C, no adhesion is noted.
force is still observed. As shown by the solid lines, at separations greater than 2-3 nm, all the measured force profiles can be well fitted with the classical DLVO theory.27 Details on the DLVO fitting are discussed in section 4. Figure 2 shows the results of adhesion force between silica and bitumen in the Aurora process water. The adhesion force was obtained from the retracting branches of the AFM force profiles. The results in Figure 2 show that adhesion between bitumen and sand grains exists only at temperatures lower than about 32 °C and that the adhesion force increases with decreasing temperature. At temperatures higher than 32 °C, no adhesion force is observed. The adhesion force between two particles generally originates from the molecular/atomic interactions within the contact area, such as electrostatic interaction and chemical and hydrogen bonding. These contact forces are highly sensitive to surface composition. In the present system, the silica surface at pH ∼8.2 is dominated by ≡SiO- groups. The bitumen surface bears various types of natural surfactants25 that could become dissociated. The dissociated anionic surfactants (RCOOand ROSO3-) dominate the bitumen surface. In addition (27) Isrealachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: San Diego, 1992.
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Figure 4. Effect of temperature on the adhesion force between tailing particles and bitumen surface in the Aurora process water.
Figure 3. Effect of temperature (T, °C) on long-range interaction force (F/R) between tailing particles and bitumen surface in the Aurora process water. Curves: DLVO fitting. Vt: fitted surface potential (mV) of the tailing particles. Other symbols are the same as used in Figure 1.
to possible hydrogen bonding, the presence of divalent cations (calcium and magnesium) in the process water is most likely responsible for the observed adhesion through chemical binding of anionic groups on both the silica and bitumen surfaces. With increasing temperature, both the silica and bitumen surfaces became more negatively charged. The divalent ions acting as a bridge to join the anionic groups on both surfaces might be neutralized. Also, the possible hydrogen bonding might be lost with increasing temperature. As a result, the adhesion force decreases or even disappears at higher temperature. 3.2 Bitumen-Clay Interaction. To simulate the interactions between bitumen and clays in oil sands, clay particles obtained from the tailing slurry were used as the probe for the force measurements. Figure 3 shows the measured long-range interaction force between a clay particle and a bitumen surface in the Aurora process water as a function of temperature. For each temperature, the force was obtained with different clay probes. Although nearly spherical particles were chosen as the probe, the shape of each particle is not as spherical as the model silica spheres, thus resulting in a scatter in the measured force. Figure 3 shows that the long-range interaction force changes progressively from attractive (Figure 3a) to repulsive (Figure 3c) with increasing medium temperature. At a lower temperature, for example, 21.2 °C, the long-range interaction force is attractive (Figure 3a). Attraction is observed until the temperature is increased to about 30.8 °C (Figure 3b). At this temperature, the detected force is very small. At a higher temperature, for example, 40.6 °C as shown in Figure 3c, the long-range interaction force becomes purely repulsive. To fit these force profiles by the DLVO theory, the decay length and the bitumen
surface potential for each temperature used are the same as those used in Figure 1 since the same liquid was used in all the force measurements. The fitting indicates that the surface potential of clay particles changes little with temperature. The attractive force profiles at low temperatures possibly result from the adsorption of calcium and magnesium ions on both bitumen and tailings particle surfaces. The adhesion force between clay and bitumen as a function of temperature is shown in Figure 4. From this figure, it can be observed that the adhesion force decreases from about 1.5 mN/m at 21 °C to zero at about 33 °C. At a higher temperature, the adhesion force remains negligible. Comparing Figures 4 and 2, one finds that the adhesion force between silica and bitumen is much weaker than that between clay and bitumen at low temperatures. For clay particles, the edge and face of a clay lamella carry opposite charges.28 Normally, the edge is positively charged while the face has a negative charge. This dual characteristic of clay surface charges results in heterogeneity in terms of charge densities on the surface of clay particles although the net charge of clay particles is negative. Such heterogeneity may contribute to the strong adhesion detected between clay and bitumen in addition to the bridging effect of divalent ions. 3.3 Silica-Bitumen Interaction in the Presence of MIBC. The effect of MIBC on the long-range interaction forces between bitumen and silica is shown in Figure 5 while the inset of this figure shows the change of the adhesion force with the MIBC concentration. The force measurements were carried out in the Aurora process water at room temperature with MIBC added in the process water at 0, 50, 500, and 5000 ppm. Figure 5 shows that the long-range interaction force is purely repulsive and changes little with MIBC addition. These force profiles are well fitted with the DLVO theory (the solid curve), indicating the long-range interaction force is predominated by electrostatic double layer interactions. MIBC itself has no charge and tests15,18 have shown that it does not affect the surface charge of both bitumen and fine solids. Therefore, a marginal effect of MIBC addition on the electrostatic double layer interac(28) Swartzen S.; Matijevi, E. Chem. Rev. 1974, 74, 385-400.
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4 Discussion
Figure 5. Effect of MIBC on the interaction and adhesion forces between silica and bitumen in the Aurora process water at room temperature. The solid curve represents the DLVO fitting for the force profile without MIBC addition using Hamaker constant of 3.3 × 10-21 J, decay length of 2.7 nm, and surface potential of -24 mV for bitumen and -20 mV for silica, respectively.
Figure 6. Effect of MIBC on interaction and adhesion forces between clay particles and bitumen surface in the Aurora process water at room temperature. The solid curves represent the DLVO fitting for the force profile without MIBC addition using Hamaker constant of 3.3 × 10-21 J, decay length of 2.6 nm, and surface potential of -17 mV for bitumen and -5 mV for clay, respectively.
tions between the two surfaces as shown in Figure 5 is not unexpected. As well, the effect of MIBC addition on the adhesion force between silica and bitumen, as shown in the inset of Figure 5, is marginal at all MIBC concentrations. 3.4 Bitumen-Clay Interactions in the Presence of MIBC. The effect of MIBC on the interaction and adhesion forces between bitumen and clay is shown in Figure 6. The force measurements were also carried out in the Aurora process water at room temperature with 0, 50, 100, 500, 1000, and 5000 ppm MIBC addition. At a low MIBC concentration of 0-100 ppm, the long-range interaction force appears slightly attractive. At a high MIBC concentration above 500 ppm, the force becomes purely repulsive. As shown in the inset of Figure 6, the effect of MIBC addition on the adhesion force between bitumen and clay is evident. Without MIBC addition, the adhesion force is at the level of 1.5 mN/m or higher. The adhesion force decreases with increasing MIBC addition from 0 to 500 ppm. In the range of MIBC concentration from 500 to 1000 ppm, the adhesion force remains small. A further increase in MIBC concentration to 5000 ppm caused notable increase in adhesion force.
4.1 Fitted Decay Length and Surface Potentials. In this study, the force-distance fitting was carried out with the classical DLVO theory, which considers only the van der Waals forces and electrostatic double layer forces.27 The van der Waals forces were calculated by Hamaker’s microscopic approach.27 Using 6 × 10-20,29 3.7 × 10-20, and 6.5 × 10-20 J27 as the Hamaker constant for bitumen, water, and silica, respectively, the combined Hamaker constant for the bitumen/water/ silica system was calculated to be ca. 3.3 × 10-21 J.25 The electrostatic double layer force, on the other hand, was calculated numerically by solving the nonlinear Poisson-Boltzmann equation for both surfaces with constant surface potential as boundary conditions. During the fitting exercise, surface potentials for both surfaces as well as the decay length, κ-1, were set as adjustable parameters.25 As shown by the solid lines in Figure 1, at separations greater than 2-3 nm, all the measured force profiles can be well fitted with the DLVO theory. The excellent fit shown in this figure suggests that the long-range repulsive forces are predominantly from the electrostatic double layer interactions. The observed repulsive force at separation distances less than 2-3 nm is inconsistent with the attractive force regime as predicted by the DLVO theory, suggesting the presence of an additional repulsive force. Although the exact reason for this deviation is not clear, considering a 2-3 nm range, this additional repulsive force would appear to originate from brushlike surfaces or small protrusions at the bitumen/water interface, resulting in a steric type of repulsion.25 As shown in Figure 1, the best fitted decay length (κ-1, nm) increases slightly with increasing temperature. The magnitude of the decay length depends solely on the properties of the liquid and not on any properties of the surface. For a 2:2 electrolyte, for example, MgSO4, the decay length at 25 °C is κ-1 ) 0.152/x[MgSO4].27 With this expression, we obtain κ-1 ≈ 3 nm for [MgSO4] ) 2.5 mM (equivalent to about 60 ppm Mg2+ in the solution). As the composition of the Aurora process water used in the force measurements was quite diverse, it is impossible to theoretically calculate the decay length. However, considering the fact that there were 47 ppm of calcium and 15 ppm of magnesium in the water, the fitted decay length of 2.7 nm at 25.6 °C is considered to be very reasonable. The effect of temperature on the best fitted decay length is shown in Figure 7. Theoretically, κ-1 is proportional to the square root of temperature: κ-1 ∝ xT 27 where temperature T is in degrees Kelvin. From Figure 7, a good linear fitting between κ-1 and xT can be observed. This indicates a good agreement of the fitted decay length with the theoretical prediction in terms of the effect of temperature. The fitted surface potentials of both silica and bitumen as a function of temperature are shown in Figure 8. Over the temperature range tested, the silica and bitumen in the Aurora process water at pH ∼8.2 are both negatively charged. This is consistent with the measured results of Dai and Chung 16 and Liu et al.25 (29) Vincent, B. J. J. Colloid Interface Sci. 1973, 42, 270-285.
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Their measurements also indicate that bitumen surface is more negatively charged than silica surface. The more negative value obtained in the present fitting practices for each temperature was, therefore, assigned as the surface potential of bitumen. The negative charge of the bitumen surface can be explained by the dissociation of the carboxyl/sulfonate groups of the surfactants that are naturally present in bitumen30 while the dissociation of surface silanol groups are responsible for the negative charge of the silica surface.31 From Figure 8, it can be observed that the fitted surface potentials of both silica and bitumen become more negatively charged with increasing temperature. Such a trend agrees well with the measured surface potentials for bitumen16 and silica16,31,32 as a function of temperature. For the silica surface, a thermodynamic analysis31 suggests that increasing temperature favors the formation of H3SiO4groups, thus resulting in a more negative charge. As the surface potentials of both silica and bitumen increase (more negatively charged) with increasing temperature, the repulsion between them becomes stronger (Figure 1). Therefore, from the perspective of surface potentials, a high temperature is favorable for the separation of bitumen from silica sands. 4.2 Critical Temperature and Bitumen Recovery. In a water-based bitumen extraction process, oil sand lumps are mixed with recycle process water. The prepared oil sands slurry is then introduced into con-
ditioning hydrotransport pipelines or tumblers, where the oil sand lumps are sheared and lump sizes are reduced. Within the tumblers or the hydrotransport pipelines, bitumen is released or liberated from the sand grains. As bitumen has nearly the same density as water, air is often introduced to form air bubbles that attach to bitumen. The aerated bitumen is then separated and recovered from the slurry by floating to the top of the slurry. Such a process includes two essential micro-subprocesses: bitumen liberation and aeration.33 These two micro-subprocesses determine bitumen recovery. “Liberation” is the recession of bitumen from sand grain surface with subsequent detachment. It is a prerequisite step in bitumen extraction process. “Aeration” is a process in which the liberated bitumen droplets attach to air bubbles to achieve effective flotation. Both bitumen liberation and aeration are affected or even controlled by the interactions between bitumen and the solids in oil sands. The adhesion force determines solid attachment onto bitumen, while the long-range forces are the key for dispersion or coagulation of solids-bitumen colloidal systems. For a dynamic colloidal system such as bitumen extraction process, both the long-range forces and adhesion forces have to be considered. It is evident that for the bitumen liberation process, a repulsive long-range colloidal force and a zero adhesion force between bitumen and sand grains are desirable for easy bitumen liberation. As shown in Figure 1, the long-range interaction force between bitumen and silica sands is always repulsive and the repulsive force becomes stronger with increasing temperature. At temperatures higher than 32 °C, there is no adhesion between bitumen and silica sands (Figure 2). Therefore, the bitumen and sand grains are at a stable colloidal state and little bitumen-sand attachment or coagulation occurs. The results suggest that bitumen liberation from sand grains can be achieved by increasing the temperature up to 32 °C or higher. In contrast, at temperature lower than 32°, although the long-range interaction force is still repulsive (Figure 1), the adhesion force is nonzero and the adhesion force increases with decreasing temperature (Figure 2). For such a system, a careful balance between the hydrodynamic force and the colloidal force is required to create a favorable condition for bitumen liberation. As the adhesion force is weak between silica and bitumen (Figure 2), under the hydrodynamic conditions encountered in a bitumen extraction process, silica sand grains and bitumen droplets usually can obtain sufficient hydrodynamic energy to overcome the small adhesion force and become separated from each other. This indicates that the effect of the adhesion force between silica and bitumen on bitumen liberation is insignificant. Overall, it appears that bitumen liberation is mainly controlled by bitumen viscosity. For the bitumen aeration process, a phenomenon called “slime coating” has been found to have a profound impact.33 Slime coating is defined as a layer of fine gangue particles coating on valuable minerals.34,35 In oil sands processing, slime coating of bitumen is referred
(30) Takamura, K.; Chow, R. S. Colloids Surf. 1985, 15, 35-48. (31) Ramachandran, R.; Somasundaran, P. Colloids Surf. 1986, 21, 355-369. (32) Dunstan, D. E. J. Colloid Interface Sci. 1994, 166, 472-475.
(33) Liu, J.; Xu, Z.; Masliyah, J. AIChE J. 2004, 50, 1917-1927. (34) Fuerstenau, D. W. Fine Particle Processing; The Society of AIME: New York, 1980; pp 669-705. (35) Sivamohan, R. Int. J. Miner. Process. 1990, 28, 247-288.
Figure 7. Effect of temperature on the fitted decay length. T: temperature in degrees Kelvin.
Figure 8. Effect of temperature on the fitted surface potentials of silica and bitumen by the DLVO theory.27
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to as the coating of fines onto the surface of bitumen droplets. The presence of slime coating not only reduces the bitumen flotation rate and recovery by setting up a steric barrier retarding bitumen drops to contact air bubbles but also deteriorates the froth quality by carrying fine solids that are attached to the bitumen to the froth product, which bears significant implications for subsequent froth treatment. Slime coating is controlled by the interaction and adhesion forces between bitumen and clay fines in oil sands slurry. Figures 3a and 4 show that at temperatures lower than 32 °C, an attractive long-range interaction force and an adhesion force exist between bitumen and clay particles in the Aurora process water. Such forces could induce a strong heterocoagulation between bitumen and the fines, possibly resulting in slime coating of the bitumen surface and preventing an intimate contact of air bubbles with bitumen. Thereby, the aeration efficiency and subsequent bitumen recovery can deteriorate. In contrast, at temperatures higher than 32 °C, the long-range interaction force becomes repulsive (Figure 3c) and the adhesion force is extremely weak (Figure 4). Thus, the fine particles cannot strongly attach to the bitumen surface and any possibly attached fine particles on the bitumen can be removed by the hydrodynamic forces during a bitumen extraction process. As a result, contact of air bubbles with a solid-free bitumen leads to an effective air-bitumen attachment and hence flotation. The above discussion indicates that at low temperatures, the adhesion force between clay and bitumen plays a critical and controlling role in bitumen aeration and subsequently bitumen recovery. A temperature of 32-35 °C can be considered to be a critical temperature for oil sands processing. Bitumen extraction processes should operate at a temperature higher than 35 °C. Bitumen recovery operating at a temperature lower than this critical value will sharply deteriorate. This is consistent with the results of bitumen recovery tests15-20 and commercial operations. 4.3 Effect of MIBC on Bitumen Recovery. Figure 5 shows that the long-range interaction force between bitumen and silica is repulsive and the repulsive force changes little with MIBC addition. Also, the adhesion force between silica and bitumen is very small (inset of Figure 5) and the effect of MIBC addition on the adhesion force is negligible. These results indicate that MIBC addition has little impact on bitumen liberation from sand grains. However, MIBC addition does impose a significant impact on bitumen aeration. As shown in Figure 6,
Long et al.
without MIBC addition or with MIBC addition at a low concentration, the presence of attractive long-range interaction force and adhesion force between bitumen and clay could induce heterocoagulation of bitumen and clay. Such a condition would lead to slime coating, thus possibly resulting in poor bitumen-air attachment and low bitumen recovery. At desired MIBC addition, the long-range interactions change progressively from attractive to repulsive and the adhesion force decreases substantially and eventually disappears. As a result, little slime coating occurs and thereby a higher bitumen recovery can be achieved. At a very high concentration of 5000 ppm, an adhesion force was measured again, leading to a deteriorated bitumen recovery. This is consistent with the bitumen recovery results of Schramm et al.18 as they found that high MIBC addition decreased the bitumen recovery. 5 Conclusions In the present study, bitumen-solid interaction and adhesion forces were directly measured as a function of temperature using atomic force microscopy. The following conclusions can be drawn. (1) The adhesion force between bitumen and clay in commercial process water decreases with increasing temperature until a critical value of 32-35 °C. At temperatures higher than the critical temperature, the adhesion force becomes very small or even disappears. Bitumen recovery should operate at a temperature higher than this critical temperature. (2) The adhesion force between silica and bitumen is quite small and thus imposes little impact on bitumen liberation, which is mainly controlled by bitumen viscosity. In contrast, bitumen aeration is significantly affected or even controlled by the adhesion force between clay and bitumen as mentioned in 1. (3) The role of MIBC addition in bitumen recovery is to reduce the adhesion force between clay and bitumen, thus facilitating bitumen aeration. (4) The current study suggests that increased adhesion force and bitumen viscosity coupled with decreased surface charge of sand grains and bitumen are likely responsible for the deteriorated bitumen recovery at low temperatures. Acknowledgment. Financial support from NSERC Industrial Research Chair in Oil Sands Engineering (held by J.H.M.) is greatly acknowledged. EF0496855
