In the present study, a hybrid Al(OH)3-polyacrylamide (Al-PAM) was synthesized and used in combination with a partially hydrolyzed polyacrylamide (HPAM) to process a low-grade oil sand ore. It was found that Al-PAM was capable to improve bitumen froth quality and tailings settling. But it led to deterioration in bitumen recovery due to the formation of large bitumen lumps during the bitumen extraction process. To resolve this problem, HPAM was added in combination with Al-PAM as a dual system. The use of the dual system at a low dosage achieved a holistic improvement in bitumen recovery, froth quality, and tailings settling. To understand the role of Al-PAM and the dual system in the bitumen extraction process, bitumen-clay, bitumen-bitumen, and clay-sand interaction and adhesion forces were directly measured using an atomic force microscope (AFM). The measured forces indicate that bitumen recovery and tailings settling are controlled by the colloidal interaction and adhesion forces between the oil sand components.
Dans la présente étude, un polyacrylamide-Al(OH)3 hybride (Al-PAM) a été synthétisé et utilisé conjointement à un polyacrylamide partiellement hydrolysé (HPAM) afin de traiter un minerai de sables bitumineux de faible teneur. On a trouvé que l'Al-PAM était capable d'améliorer la qualité de l'écume de bitume et la sédimentation des résidus. Mais celui-ci donne une moins bonne récupération du bitume en raison de la formation de larges agrégats de bitume lors du procédé d'extraction. Afin de résoudre ce problème, du HPAM a été ajouté à de l'Al-PAM comme système dual. L'utilisation de ce système à faible dose a permis l'amélioration générale de la récupération de bitume, de la qualité de l'écume et de la sédimentation des résidus. Pour comprendre le rôle de l'Al-PAM et du système dual dans le procédé d'extraction de bitume, l'interaction bitume-argile, bitume-bitume et argile-sable ainsi que les forces d'adhésion ont été mesurées directement à l'aide d'un microscope de force atomique (AFM). Les forces mesurées indiquent que la récupération de bitume et la sédimentation des résidus sont contrôlées par les forces d'interaction et d'adhésion colloidales entre les composantes des sables bitumineux.
Oil sand ores mined in the Athabasca area of Alberta, Canada are a mixture of bitumen (∼10 wt%), mineral solids (∼85%), and water (∼5%). The mineral solids consist mainly of coarse sand grains and clay fines. To recover bitumen from the oil sands, water-based processes derived from the pioneering Clark Hot Water Extraction (CHWE) technology (Clark and Pasternack, 1932) are widely used in the industry. In these processes, hot or warm water is added to oil sands to form a slurry from which bitumen is liberated from the sand grains and recovered as bitumen froth by flotation (Masliyah et al., 2004). The remainder of the slurry is discharged as tailings into thickeners or tailings ponds. The bitumen froth is subjected to further cleaning to remove entrapped fine solids and emulsified water before bitumen upgrading.
To improve bitumen recovery, chemical aids are often needed in bitumen extraction (Sanford, 1983; Taylor, 1988; Hepler and Smith, 1994). Particularly, for the processing of low-grade oil sand ores with a higher content of fines (mineral solid particles less than 44 µm), process aids have to be used to obtain an acceptable bitumen recovery. Although the use of conventional process aids, such as caustic, does improve bitumen recovery and bitumen froth quality, they lead to difficulties in tailings treatment by dispersing the fine particles in the tailings. An attempt was made recently in our group (Li et al., 2005) to overcome such problems. A hydrolyzed polyacrylamide (HPAM) with a high molecular weight and a medium charge density (a commercial polymer flocculant with a trade name of Percol 727) was used to process a low-grade ore. It was found that the addition of this polymer in the bitumen extraction process not only improved bitumen recovery but also enhanced tailing settling. However, it led to a deteriorated bitumen froth quality.
From both economic and technical perspectives, it is always desirable to achieve a holistic improvement in bitumen recovery, froth quality, and tailings settling. With the successful use of HAPM in improving both bitumen recovery and tailings settling, such an ambitious goal becomes attainable as long as bitumen froth quality can be improved. In this regard, we made a preliminary attempt in the present study.
In order to improve bitumen froth quality, it is essential to understand the role of HPAM in the bitumen extraction process. Long et al. (2006) employed the technique of single molecule force spectroscopy to measure the adhesion forces of single Percol 727 molecules on the surfaces of various oil sand components, such as bitumen, sand, and clay, using an atomic force microscope (AFM). They found that the polymer would preferentially adsorb onto the clay surface than onto the bitumen surface. It is the selective adsorption of HPAM on clay that benefited both bitumen recovery and tailings settling. When the polymer was used as a process aid in the extraction process, it induced the formation of large flocs of fine particles, thus reducing the number of individual fine particles in the oil sand slurry. As a result, the chance for slime coating, that is, the coating of bitumen surface by a layer of fine solids, to take place was reduced. This in turn enhanced the attachment of air bubbles to bitumen droplets and thus improved the flotation efficiency and consequently bitumen recovery. The formation of large floccules also increased the settling rate of the fine solids in the tailings. Because the produced large floccules were normally loose and irregular in shape, they could be brought up to the bitumen froth by aerated bitumen droplets and air bubbles during the flotation process, thereby leading to a poor froth quality. Therefore, it is desirable to find/design polymer aids capable of enhancing flocculation of fine particles so as to improve bitumen froth quality. The formed floccules by these polymers must be denser than those formed by HPAM.
Among various attempts to enhance fine clay flocculation, the use of polymer flocculants, such as HPAM, in combination with microparticles was found to be effective. The microparticles can be either polymers or inorganic substances. For example, Xiao et al. (1999) used cationic polymeric microparticles with an anionic polyacrylamide (PAM) flocculant to treat clay fines and found that such a system was substantially more efficient than either component alone. The adsorption of the cationic polymeric microparticles on the clay surface and the complexation between the microparticles and anionic polymers were the main driving forces to significantly promote clay flocculation. Asselman and Garnier (2000) found that anionic bentonite microparticles could significantly enhance the PAM-induced hetero-flocculation between wood fibres and fines. Ovenden and Xiao (2002) used cationic colloidal alumina microparticles (Al2O3) with various linear PAM polymers in clay flocculation. They found that a strong synergy between the cationic microparticles and anionic polymers resulted in effective flocculation.
Instead of a simple combination of microparticles and polymer flocculants, further efforts have been made to synthesize microparticle–polymer hybrid products for more efficient flocculation. Yang et al. (2004) synthesized a novel flocculant of hybrid Al(OH)3–polyacrylamide (Al-PAM) and found that an ionic bond existed between Al(OH)3 colloids and PAM chains in the Al-PAM. The flocculation efficiency of Al-PAM in treating kaolinite suspensions was much better than that of a commercial PAM and a PAM/AlCl3 blend because the Al-PAM-induced floccules were denser and larger and of a spherical shape.
Based on the above discussion, it appears that Al-PAM is a polymer that can flocculate clay fines in a desirable manner as described earlier for improving bitumen froth quality. Therefore, a hybrid Al-PAM was synthesized in the current study and used as a process aid for low-grade oil sand ore processing. It was found that Al-PAM was indeed able to improve bitumen froth quality, but at the cost of bitumen recovery. In this study, an attempt is made to use Al-PAM with HPAM for low-grade oil sand ore processing. The goal is to achieve the aforementioned holistic improvement in bitumen recovery, bitumen froth quality, and tailings settling.
Al-PAM Synthesis and Characterization
Hydrogen bonding is the main driving force for PAM polymers to adsorb on clay surface (Sabah and Erkan, 2006). For HPAM, it is therefore not sufficient to enhance their affinity on clay surface by adjusting such molecular properties as charge density and molecular weight (Michaels, 1954; Gu and Doner, 1993). New functional groups must be integrated into the PAM molecular structure. These groups should provide a stronger binding with clay surface than hydrogen bonding, thus being able to enhance their adsorption on clay surface. Clay minerals, in general, consist of two basic units: alumina octahedral and silica tetrahedral. Aluminum, silicon, and oxygen are the main mineral elements. Because there is a strong affinity between aluminum and oxygen in the form of -O-Al-O-, integrating Al(OH)3 as a functional group into the PAM molecular structure may help the adsorption of the polymer on a clay surface because the resulting hybrid Al-PAM can strongly attach to the clay surface in the form of -O-Al-O[-Al-PAM.
All chemicals used for the synthesis of Al-PAM, including acrylamide, ammonium carbonate, aluminum chloride, ferric chloride hexahydrate, sodium bisulphite, ammonium persulphate, and acrylamide, were purchased from Fisher Scientific (Ottawa, Canada). The synthesis procedure has been described in detail elsewhere (Yang et al., 2004). Briefly, it had three steps. Step one was to prepare an Al(OH)3 colloid solution by a slow and dropwise addition of an ammonium carbonate solution into an aluminum chloride solution at room temperature during which the following reaction occurred,
Strong agitation was required to obtained Al(OH)3 colloidal particles of uniform size. The second step was to synthesize the hybrid polymer, that is, the polymerization of acrylamide monomers in an Al(OH)3 colloidal solution with (NH4)2S2O8-NaHSO3 as an initiator. Typically, a 0.3 mL solution of 0.075 wt% NaHSO3 and 0.15 wt% (NH4)2S2O8 was added into a 30 mL Al(OH)3 colloidal solution in a 2000 mL flask. Nitrogen gas was introduced to the flask for 20 min before the addition of the initiator. After 4.5 g of acrylamide monomers were added, the flask was sealed and the polymerization was initiated and proceeded for 8 h at 40°C. The final step was to extract and purify the reaction product by dissolving the product in deionized water, precipitating impurities, and extracting pure hybrid polymer with an acetone solution. After the extracted material was dried at room temperature in a vacuum oven, the final product of hybrid polymer was obtained.
Bitumen Extraction Test
The oil sand ore used for the bitumen extraction tests was a transition ore provided by Syncrude Canada Ltd (Fort McMurray, Alberta, Canada). The ore contained 8.8 wt% bitumen, 8.7% connate water, and 82.5% solids. In the mineral solids, 25.9% were fines (mineral particles smaller than 44 µm). Because the ore had a high fine content, it is considered to be low grade with poor processability. The water used in the extraction tests was a recycle process water from Aurora plant of Syncrude Canada Ltd. Atomic absorption spectrometry (AAS) analysis showed that this process water contained 47.0 ppm calcium and 15.0 ppm magnesium. Its pH was about 8.2.
Two polymers, including the aforementioned HPAM (Percol 727) purchased from Ciba Specialty Chemicals (Basel, Switzerland) and the synthesized Al-PAM, were used as process aids in the bitumen extraction. Percol 727 is a partially HPAM with a high molecular weight of ∼17.5 million Daltons and a medium charge density of ∼27%. These two polymers were used individually or together in the bitumen extraction tests. Polymer solutions were prepared at a concentration of 0.05 wt% using deionized water one day prior to their use in the extraction tests.
Bitumen extraction experiments were conducted in a laboratory hydrotransport extraction system (LHES), which has been described in detail in an earlier paper (Li et al., 2005). For each bitumen extraction experiment, 5 L of Aurora process water preheated to 35°C were added in the LHES followed by the addition of a polymer solution at a desired dosage. It should be noted that in the current study, polymer dosage in ppm refers to mg of polymer per litre of oil sand slurry. (To convert this slurry volume-based ppm to mg of polymer per kilogram of oil sand, a multiplying factor of 3.8 should be used. For example, 20 ppm on the basis of oil sand slurry volume, mg/L, is equivalent to 76 ppm on the basis of oil sand mass, mg/kg.) A 1.5 kg of the oil sand sample was then fed into the LHES system. The formed slurry was conditioned by circulating in the LHES at a constant temperature of 35°C for 5 min before air was introduced at a flow rate of 200 mL/min. A timer was then started to indicate the flotation time. Bitumen froth was collected six times in a flotation period of 60 min at different time intervals.
The collected froth samples were assayed using the industrial standard procedure (Dean Stark) to determine the content of bitumen, solids, and water. The weight ratio of bitumen in the froth to that in the feed represents the bitumen recovery. Froth quality was determined using weight ratios of bitumen to solids (B/S) and bitumen to water (B/W) in the froth.
After each bitumen extraction test, the tailings slurry left in LHES was collected and used for tailings settling tests according to the following procedure. Fifty millilitres of tailings sample directly taken from the tailings slurry were poured into a 50 mL graduated cylinder. To mix well the sample, the cylinder was sealed by a glass stopper and gently turned upside down several times. It was then placed on a bench and the settling test was started. The descent of the solids/liquid interface (mud line) was observed and recorded as a function of settling time.
Colloidal Force Measurement
To understand the role of Al-PAM and the Al-PAM + HPAM dual system in bitumen extraction, direct surface force measurements were carried out in the current study. The interactions between a clay particle and a bitumen surface and between two bitumen surfaces as well as between a clay particle and a silica surface in aqueous media were measured.
A Nanoscope E AFM with a vendor-supplied fluid cell (Digital Instruments, Santa Barbara, CA) was used for the surface force measurements. Clay particles used as force probes were chosen under an optical microscope from a great number of particles obtained from the tailings slurry of the bitumen extraction experiment without chemical addition. The chosen clay particles were attached to the apex of gold-coated silicon nitride cantilevers (lever type 100 µm wide) with a spring constant of 0.58 N/m. Bitumen probes were prepared by first dipping silica spheres (8 µm in diameter) in a bitumen-in-toluene solution to produce a thin layer of bitumen coating. The bitumen-coated spheres were then attached to the cantilever (Liu et al., 2005). The bitumen substrate surface was prepared by coating a thin layer (∼100 nm) of bitumen onto 10 × 10 mm2 silica wafers (NANOFAB, University of Alberta, Canada) using a spin-coater. The characteristics of prepared bitumen surface can be found elsewhere (Liu et al., 2003). The silica wafers had an oxidized surface layer of ∼0.6 µm and were also used as substrates in the force measurements to represent sand grains. The liquid medium used in the force measurements was the supernatant of the tailings slurries produced from the bitumen extraction tests. The detailed procedure of AFM force measurements has been described elsewhere (Ducker and Senden, 1992; Long et al., 2005). Briefly, a sample surface moves towards and away from a cantilever tip (colloid probe) by the extension and retraction of an AFM piezotube. The force acting between the probe and surface is determined from the deflection of the cantilever using Hooke's law. Each force plot represents a complete extension–retraction cycle of the piezo. When a sample surface approaches a probe, the long-range interaction force 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).
All force measurements were conducted after an incubation time of 60 min. Preliminary experiments showed that 60 min were sufficient for the two surfaces immersed in the aqueous medium to equilibrate. For each given condition, the force measurement was performed at different locations of the substrate surface and several probe–substrate pairs were used to obtain representative results. All force measurements were conducted at room temperature of 22 ± 1°C.
RESULTS AND DISCUSSION
Effect of Al-PAM on Bitumen Extraction
The results of bitumen recovery obtained with the addition of hybrid Al-PAM at dosages of 0, 2.5, 5, 10, and 20 ppm in the extraction process are given in Figure 1a. Surprisingly, the bitumen recovery was decreased with the addition of Al-PAM from 0, 2.5 to 5 ppm. When 10 ppm of Al-PAM was added, the recovery was still lower than that of no polymer addition (0 ppm, ∼50% recovery). At a further higher dosage of 20 ppm, the recovery is only marginally higher than 50%. Clearly, the lower bitumen recoveries obtained with the addition of Al-PAM were not anticipated. Such lower bitumen recoveries may be related to the following phenomenon observed during the extraction experiments. With the addition of Al-PAM, bitumen formed large black lumps with a size of about 0.5–2 cm. Figure 1b shows a sample of a bitumen lump. Since Al-PAM strongly attached to clay surface, its addition in the extraction process resulted in the flocculation of fine clay particles. However, it also resulted in a strong coalescence between the bitumen droplets. The large bitumen lumps formed were too heavy to be brought up to the bitumen froth by air bubbles with a size of about 0.1–1 mm. Thus the bitumen recovery decreased.
Figure 2 shows the effect of Al-PAM addition on the bitumen froth quality. As shown in Figure 2a, the B/S ratio in the froth increased from about 1.5 to 1.7 with the addition of Al-PAM from 0 to 10 ppm. Further increasing the dosage to 20 ppm shows little effect on the B/S ratio. These results indicate that the amount of solids in the froth decreased by the addition of Al-PAM and that the quality of the bitumen froth was improved. Although both Al-PAM and HPAM can induce flocculation of clay fines, the size, shape, and structure of the floccules formed by these two polymers are different. Yang et al. (2004) found that clay floccules formed by Al-PAM were larger and denser than those formed by HPAM. These floccules also had a spherical shape compared to an irregular shape induced by HPAM. In addition to the common adsorption mechanism of the PAM chains on clay particles by hydrogen binding, the cationic Al(OH)3 cores in Al-PAM also attracted the negatively charged clay particles by electrostatic interactions. Such a synergetic effect resulted in the formation of dense spherical floccules. Therefore, when Al-PAM was used in the bitumen extraction process, the formed floccules of fine solids could not be easily brought to the bitumen froth by the air bubbles and/or bitumen droplets, leading to an increased B/S ratio.
Figure 2b shows the effect of Al-PAM dosage on the B/W ratio. With the addition of Al-PAM from 0 to 10 ppm, the B/W ratio significantly increased from approximately 0.25 to 0.48. These numbers indicate that the amount of water in the froth was substantially decreased through the addition of Al-PAM. This decrease could be due to changes in the interfacial properties between water and other components, such as bitumen and air bubbles in the presence of Al-PAM. To show such an effect, we measured the contact angles of produced water on a bitumen surface. The produced water was taken from the supernatant of tailings slurry produced by the bitumen extraction process with Al-PAM added at different dosages. Figure 3 shows the contact angle as a function of the Al-PAM dosage used in the bitumen extraction process. Without polymer addition, the contact angle is about 75°. However, the contact angle significantly increased to about 98° when 5 ppm or more Al-PAM was used in the extraction process. As the contact angle increases, the amount of water brought by bitumen droplets to the froth decreases.
Figure 4 shows the effect of Al-PAM addition on the tailings settling. Even at the lowest Al-PAM dosage used in the extraction process, the settling was substantially improved by the addition of Al-PAM as compared with the case of no polymer addition. The final sediment volumes for all the dosages used are nearly the same. As mentioned earlier, Al-PAM can induce larger and denser spherical floccules. Therefore, faster settling and smaller final sediment volumes would be anticipated when Al-PAM was used.
Role of Al-PAM in Tuning Colloidal Interactions
To understand the effect of Al-PAM addition on bitumen extraction, the long-range interaction and adhesion forces between bitumen and fine clay particles and between bitumen and bitumen were measured by AFM. Figure 5 shows the measured forces between bitumen and clay particles. Because the clay particles were rough and irregular, as would be anticipated, the force data obtained are highly scattered. As shown in Figure 5, the long-range interactions between clay and bitumen are repulsive albeit weak. Comparing with the force profile of no polymer addition (circles), the addition of Al-PAM shows little effect on the clay–bitumen interactions. However, the clay–bitumen adhesion forces as shown in the inset of this figure were substantially decreased by the addition of Al-PAM. In the case of no polymer addition (0 ppm), the adhesion force is about 2.3 mN/m. Such a large adhesion force indicates that the binding between bitumen and clay particles is strong, leading to hetero-coagulation between bitumen and clays and thus to difficulties in the separation of bitumen from the solids. As a result, the bitumen recovery would be low. This anticipation is consistent with the result of bitumen recovery obtained (50%). When Al-PAM was added at a dosage of 5 or 10 ppm, the bitumen–clay adhesion forces became small (about 0.3–0.4 mN/m). Hence, the separation of bitumen from the solids should be easier comparing with the case of no polymer addition. An improved bitumen recovery would be anticipated. However, the experimental results show that the bitumen recovery was decreased by the addition of Al-PAM (Figure 1a). This apparent discrepancy, as explained earlier, is due to the formation of large bitumen lumps (Figure 1b), which are too heavy to float. Analysis on samples obtained from the bitumen lumps shows that they contain ∼54 wt% bitumen, 17% solids, and 29% water. The B/S ratio is about 3, which is higher than the B/S ratio of the bitumen froth obtained (∼1.7, Figure 2a). The higher B/S ratio in the bitumen lumps suggests that the separation of bitumen from the solids was in fact improved by the addition of Al-PAM. This is indeed what would be anticipated from the measured forces.
To understand the coalescence of bitumen droplets and thus the formation of the large bitumen lumps, we also measured the long-range interaction and adhesion forces between two bitumen surfaces. The results are presented in Figure 6. In the case of no polymer addition, the long-range interactions are purely repulsive (squares). Because bitumen is normally negatively charged in an alkaline solution (Liu et al., 2003), this repulsion originates from the electrostatic interactions. When Al-PAM was added (triangles for 5 ppm and diamonds for 10 ppm), the repulsive forces remain unchanged until a separation distance of approximately 10 nm. Within 10 nm, attractive forces are observed. In such a short distance, the extruded polymer chains adsorbed on one surface could attach to the other surface by mainly electrostatic attractions between the positively charged Al(OH)3 cores in the Al-PAM molecules and the negatively charged sites of bitumen surface, and/or by such interactions as hydrogen bonding and van der Waals forces during the approach process in the force measurements. The inset of Figure 6 shows the measured adhesion forces between two bitumen surfaces. In the case of no polymer addition, the adhesion force is about 6–7 mN/m. When Al-PAM was added, the adhesion forces become extremely high, reaching about 50 mN/m. In the force measurements, we observed that both bitumen surfaces strongly attached to each other. The presence of an attractive long-range interaction and a strong adhesion between bitumen surfaces facilitates coalescence of bitumen droplets, leading to the formation of large bitumen lumps. This finding is consistent with the experimental observation (Figure 1b).
As the addition of Al-PAM in the bitumen extraction process also improves tailings settling (Figure 4), to understand such phenomena, the interaction and adhesion forces between clay particles and silica wafers (representing sand grains in oil sands) were measured and the results are presented in Figure 7. Although the measured long-range interaction forces are purely repulsive in all cases, the repulsions are depressed by the increasing addition of Al-PAM. The observed attractions in a very short separation distance of about 2 nm may result from the electrostatic attraction between the aluminum cores of Al-PAM and the negatively charged sites of the silica or clay surface, or from the van der Waals or other interactions. The inset of Figure 7 shows the adhesion forces between clay and silica. In the case of no polymer addition, the adhesion force is zero. The combination of a strong long-range repulsion and a zero adhesion indicates that the solid particles in the tailings slurry would remain in a well-dispersed state, resulting in very slow settling. When Al-PAM was used, the adhesion force substantially increased from 0 to about 2 mN/m. Such strong adhesion forces were induced by the formation of polymer bridges between the solid surfaces. Such a bridging effect leads to flocculation of fine solid particles. Consequently, a fast settling of fine solids in the tailings slurry was achieved (Figure 4).
Effect of Al-PAM and HPAM Dual Systems on Bitumen Recovery
From the results presented above, one can conclude that Al-PAM is able to improve the separation of bitumen from fine solids, bitumen froth quality, and tailing settling. However, it led to deteriorated bitumen recovery due to the formation of large bitumen lumps during the extraction process. As shown in Figure 6, Al-PAM caused attractive long-range interactions and very strong adhesion forces between bitumen surfaces. As a result, coalescence between bitumen droplets occurred readily. This finding indicates that the affinity of Al-PAM to the bitumen surface is very high. To resolve this negative impact, a process aid with less affinity to the bitumen should be used. In an earlier study (Li et al., 2005), HPAM was used as a process aid to process a similar low-grade ore. No bitumen lumps were observed in the extraction process and the bitumen recovery was significantly improved. However, the addition of HPAM, as mentioned earlier, resulted in poor bitumen froth quality. Considering the different performance of Al-PAM and HPAM, we were prompted to use both Al-PAM and HPAM together in the bitumen extraction process. In these tests, 5 ppm of Al-PAM with 5, 15, or 20 ppm of HPAM were used in the bitumen extraction process. The results of using such dual-polymer systems are presented in this section.
Figure 8 shows the results of bitumen recovery with the addition of the Al-PAM and HPAM dual systems. For comparison, the results for the case of no polymer addition and for the case with the addition of Al-PAM or HPAM alone are also plotted in this figure. With the addition of HPAM alone, the highest bitumen recovery, about 67%, was achieved at a HPAM dosage of 20 ppm. With the co-addition of the two polymers at 5 + 5 ppm (Al-PAM + HPAM), the bitumen recovery increased to about 78%. The highest bitumen recovery of ∼86% was obtained with the co-addition of the two polymers at 5 + 15 ppm. These results indicate that the use of the dual system can substantially improve bitumen recovery as compared with the recoveries obtained at no polymer addition (50%) and at the addition of one polymer alone (67% for HPAM at 20 ppm and 45% for Al-PAM at 5 ppm).
Figure 9 shows the effect of the co-addition on bitumen froth quality. As shown in Figure 9a, when the dual system was used at a dosage of 5 + 5 ppm, the B/S ratio is nearly the same as that with the addition of Al-PAM alone at 5 ppm (or 5 + 0 ppm). Increasing the HPAM dosage in the dual system (e.g. at 5 + 15 and 5 + 20 ppm) decreased the B/S ratio. Figure 9b shows the results of B/W ratio. Comparing with the B/W ratio obtained in the case of Al-PAM addition alone at 5 ppm, the B/W ratio was decreased by the use of the dual system. However, the B/W ratios at 5 + 5, 5 + 15, and 5 + 20 ppm are still higher than that obtained without chemical addition or that with the addition of HPAM alone.
Figure 10 shows two photographs of bitumen froths obtained without polymer addition and with the addition of the dual-polymer system at 5 + 5 ppm. In the case of no polymer addition, the froth as shown by the photograph (Figure 10a) appears greyish brown, indicating a high solid content and a poor froth quality. In contrast, the photograph of Figure 10b shows that the froth obtained with the co-addition of the two polymers at 5 + 5 ppm is shining and black, suggesting a good froth quality.
Figure 11 shows the effect of the co-addition on tailing settling. While compared to the case of no polymer addition (triangles), the addition of a polymer, whether it is individual HPAM or Al-PAM, or a dual system, can substantially improve the tailings settling. There is no significant difference among the final sediment volumes obtained with the addition of HPAM or Al-PAM alone, or both together.
In summary, the results presented in Figures 8 to 11 show that the use of an Al-PAM + HPAM dual system led to a holistic improvement in bitumen recovery, bitumen froth quality, and tailings settling. Considering all the aspects of bitumen recovery, bitumen froth quality, and tailings settling, the optimal dosage for the co-addition is about 5 + 5 ppm.
Colloidal Forces in the Presence of the Al-PAM + HPAM Dual System
To understand the effect of the Al-PAM and HPAM dual system on bitumen recovery, the colloidal interactions between bitumen and clay and between bitumen and bitumen were measured in the supernatant of tailings slurry obtained with the Al-PAM + HPAM dual system added in the extraction process at 5 + 5 ppm. Figure 12 shows the measured long-range interaction forces between bitumen and clay particles (open symbols). Different symbols present different experimental runs. For comparison, a force profile obtained in the case of no polymer addition (filled circles) is also shown in this figure. Clearly, the long-range interactions are all repulsive. Compared with the force profile of no polymer addition (filled circles), some force curves (e.g. the diamonds) show that the long-range interaction forces are affected only marginally by the co-addition of the two polymers. Although the repulsion force becomes slightly stronger in some cases (e.g. the triangles, inverted triangles, and hexagons), the effect of the polymer co-addition is not prominent. The inset shows a histogram of the corresponding adhesion forces obtained. Although a wide range of adhesion forces from 0 to 1.5 mN/m were measured, the average adhesion force is as small as about 0.28 mN/m. For the case of no polymer addition, the adhesion force is about 2.3 mN/m (Figure 5). These results indicate that the clay–bitumen adhesion was substantially reduced by the polymer co-addition. As a result, the bitumen recovery should be improved by the use of the dual polymer system. This anticipation is verified by the results of bitumen recovery. As presented in Figure 8, the bitumen recovery increased from ∼50% with no polymer addition to ∼78% with the dual polymer co-addition at 5 + 5 ppm.
As the use of Al-PAM alone also reduces the clay–bitumen adhesion force (inset of Figure 5), the lower bitumen recovery (∼45% at 5 ppm of Al-PAM addition) is, as discussed earlier, mainly due to the formation of the large bitumen lumps. When the dual polymer system was used in the extraction process, no bitumen lumps were observed. This indicates that the presence of HPAM in the dual system significantly altered the interactions between bitumen and bitumen. Figure 13 shows the results of measured long-range interaction forces between bitumen and bitumen for the case of the co-addition of Al-PAM and HPAM at 5 + 5 ppm in the extraction process. This figure shows that the long-range interaction forces between bitumen and bitumen in the presence of the dual polymer system are still repulsive (triangles) although the repulsion is depressed when compared with the forces obtained with no polymer addition (squares). In direct contrast, attractive forces were measured between bitumen and bitumen for the case of Al-PAM alone (Figure 6). The inset of Figure 13 shows a histogram of the adhesion forces between bitumen and bitumen in the presence of the dual polymer system. From this histogram, the average adhesion force obtained is about 0.36 mN/m. As shown in the inset of Figure 6, the bitumen–bitumen adhesion forces are about 6–7 mN/m in the case of no polymer addition and as high as 50 mN/m in the case of Al-PAM addition alone at 5 or 10 ppm. These results indicate a significant impact of the polymer co-addition on the interactions between bitumen droplets. Because the long-range interaction forces are repulsive and the adhesion forces are weak, the use of the dual system prevents the coalescence of bitumen droplets. This resolved the major problem in the case of Al-PAM alone: the formation of large bitumen lumps. Thus, higher bitumen recovery is anticipated by the polymer co-addition. This agrees well with the experimental results of bitumen recovery.
To understand the role of the dual polymer system in tailings settling, the interaction and adhesion forces between clay particles and silica wafers (representing sand grains in oil sands) were also measured in the supernatant of tailings slurry obtained with the dual system added in the extraction process at 5 + 5 ppm. Figure 14 shows the results. Compared with the force profile of no polymer addition (filled circles), the presence of the dual system significantly depresses the repulsion between clay and silica. In some cases, it results in attractive forces (e.g. triangles and squares). The inset of Figure 14 shows a distribution of adhesion forces between clay and silica. A wide range of adhesion force from about 0.5 to 4.5 mN/m were obtained. The average adhesion force is about 2.83 mN/m. The combination of possible attractive long-range interactions and strong adhesion forces suggests that the solid particles in the tailings slurry would be flocculated and that a fast tailings settling would be anticipated. This is confirmed by the results of tailings settling shown in Figure 11.
To achieve a holistic improvement in bitumen recovery, froth quality, and tailings settling in the processing of low-grade oil sand ores, a hybrid Al-PAM was synthesized and used in combination with a partially HPAM as process aids in bitumen extraction. An AFM was used to directly measure the bitumen–solid, bitumen–bitumen, and solid–solid interactions so as to understand the role of Al-PAM and the Al-PAM + HAPM dual system in bitumen extraction and tailings settling. From the results obtained, the following conclusions can be drawn.
Al-HPAM was capable to improve both bitumen froth quality and tailings settling. But, it led to deterioration in bitumen recovery due to the formation of large bitumen lumps in the extraction process. This problem was resolved by the co-addition of HPAM. The expected holistic improvement in bitumen recovery, tailings settling, and froth quality was achieved by the co-addition of Al-PAM and HPAM at a low dosage.
The measured surface forces indicate that the presence of Al-PAM induced attractive long-range interactions and strong adhesion forces between bitumen surfaces, leading to the coalescence of bitumen droplets and consequently to the formation of large bitumen lumps. The dual system lowered the bitumen–clay and bitumen–bitumen adhesion forces but increased solid–solid adhesion forces, resulting in higher bitumen recovery and improved bitumen froth quality as well as enhanced tailings settling.
The financial support for this work from NSERC Industrial Research Chair Program in Oil Sands Engineering (held by JHM) is gratefully acknowledged.