Study of Bitumen Liberation from Oil Sands Ores by Online

(7, 8) studied the displacement of the bitumen/water interface on a glass surface ..... periods of time in an oven at 60 °C and ambient pressure with...
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Study of Bitumen Liberation from Oil Sands Ores by Online Visualization Sundeep Srinivasa, Chris Flury, Artin Afacan, Jacob Masliyah, and Zhenghe Xu* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada S Supporting Information *

ABSTRACT: A novel visualization cell was designed to study the kinetics of bitumen liberation from oil sands. This novel visualization cell allows for direct observation of bitumen recession from sand grains in real time under various experimental conditions, thereby providing a better understanding of bitumen liberation and the critical role of process conditions in bitumen extraction from oil sands ores. Although direct recession of bitumen from sand grains is found to be the primary mechanism of bitumen liberation, the presence of entrained air in oil sands ores greatly enhances bitumen liberation via bitumen spreading over air bubbles. Imaging analysis of the recorded real-time bitumen liberation process allowed for quantitative analysis of bitumen liberation kinetics. A rapid bitumen recession and, consequently, high bitumen recovery were observed for a good processing ore, in contrast to a slower bitumen liberation and lower bitumen recovery for a high-fines ore, which was considered to be a poor processing ore. The weathering (aging) of good processing ore was found to significantly reduce bitumen liberation kinetics, leading to a lower bitumen recovery, even though the bitumen content and solids composition of the ore remained the same. These findings confirmed the critical role of bitumen liberation in bitumen extraction. Increasing the process water temperature was found to increase significantly bitumen liberation kinetics and led to a higher degree of bitumen liberation. While high pH facilitated bitumen liberation, the presence of excessive salts (16 000 ppm sodium) was found to be detrimental to bitumen liberation, in particular at high pH. The bitumen liberation study using this novel visualization cell was extremely valuable for identifying and understanding critical operating parameters that control bitumen liberation and, hence, ore processability, providing a scientific basis for designing breakthrough technology to improve processability of oil sands ores and reducing the environmental impact of oil sands development.

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INTRODUCTION Oil sands are also known as tar sands or bituminous sands. Canada’s bitumen resources in oil sands are located mostly within the province of Alberta. Canada boasts the second largest proven crude oil reserve globally, only after Saudi Arabia, and accounts for 15% of the total world oil reserves. Of the total 170 billion barrels of bitumen remaining in the established reserves, about 20% is considered recoverable by surface mining methods. In 2010, Alberta’s production of bitumen was 1.6 million barrels/day, with surface mining accounting for 53% of the production.1 Developing cutting edge technologies based on the fundamental understanding of bitumen recovery processes provides opportunities to secure fossil fuel supplies while minimizing environmental consequences of oil sands development. Oil sands nominally contain about 85% mineral solids, 10% bitumen, and 5% connate water by weight. The mineral solids range in size from 1 mm silica sand grains to sub-micrometersize mineral clays. Within the oil sands ore, the mineral solids are impregnated with bitumen, which, in most cases, forms a continuous phase. To recover the bitumen from the oil sands, bitumen must be first separated from the sand grains, i.e., must be liberated from the sand grains. At present, commercial recovery of bitumen from the mineable Athabasca oil sands deposits uses almost exclusively the warm-water-based extraction technology. The bitumen liberation from the sand grains is an essential subprocess of a typical warm-water extraction process and generally involves © 2012 American Chemical Society

bitumen displacement along a sand grain to a bitumen globule and its detachment from the sand grain. The hydrophilic nature of the sand grains in Athabasca oil sands is the key for bitumen recovery using water-based extraction methods.2,3 At current commercial oil sands processing operations, bitumen liberation from the sand grains occurs during the hydrotransport of oil sands slurry by pipelines for a few kilometers prior to its separation in a gravity separation vessel. The success of bitumen recovery from oil sands largely depends upon the degree of bitumen liberation in the slurry hydrotransport pipelines. One of the goals in oil sands research and development is to find a robust process to recover bitumen from poor oil sands ores while reducing processing energy consumption, which requires a fundamental understanding of subprocesses involved in bitumen extraction. Our understanding of bitumen extraction at each step has been extended from the macroscopic scale down to the molecular level with the development and adoption of advanced analytical instruments.4 However, bitumen liberation from oil sands ores has never been studied in real time at the microscopic level under environments similar to the hydrotransport pipelines used in the industry, although experiments have been attempted using pseudo- or model oil sands.5 Received: January 28, 2012 Revised: April 13, 2012 Published: April 13, 2012 2883

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Figure 1. Experimental setup for in situ bitumen liberation study: (a) schematic view of experimental system and (b) dimensions (top) and photograph (bottom) of the BLFVC. All units are in millimeters.

in oil sands ore would be quite different from the case studied in the pseudo-oil sands system. Other researchers10−15 focused on understanding bitumen liberation, by extrapolating the results from experiments using a Denver flotation cell, which included both subprocesses of bitumen liberation and aeration. The present study deals with understanding bitumen liberation kinetics by direct visualization of bitumen liberation from real oil sands under dynamic/laminar fluid flow conditions. The objective of this study is to design a unique novel setup that allows for bitumen liberation kinetics from an oil sands ore to be visualized in real time in situ under dynamic flow conditions. In this study, an imaging analysis procedure is developed to extract quantitative information from the unique and novel set of results obtained from the novel flow visualization cell. The effect of the temperature and pH, salt addition, weathering of ores, and kerosene addition on bitumen liberation is investigated. Additional experiments are performed on ores with varying percentage of fines (defined as the percentage of mineral solids smaller than 44 μm) to help us better understand the impact of fines on bitumen liberation from good processing oil sand ores.

Bitumen recovery from the oil sands is strongly influenced by physical, chemical, and hydrodynamic conditions, with interfacial phenomena playing a critical role in successful recovery of bitumen from oil sands. The influence of physical parameters on bitumen recovery was studied by Lam et al.6 for various ores at temperatures of 15 and 50 °C. They concluded that bitumen viscosity is an important parameter affecting bitumen recovery. Dai et al.5 investigated the chemical parameters affecting bitumen recovery and proposed a mechanism for the warm-water extraction process based on their results using model oil sands. They found a significant improvement in bitumen recovery by adding sodium hydroxide to either “connate” water or processing water. However, incomplete bitumen/sand separation or oil/water emulsifications were present with a deficiency or overdose of sodium hydroxide. Basu et al.7,8 studied the displacement of the bitumen/water interface on a glass surface by determining the static and dynamic contact angles of bitumen on a glass surface in aqueous solutions of different temperatures and pH. At a weakly alkaline pH, the equilibrium “contact angles” of bitumen on the glass slide measured through the bitumen in water were lower, leading to a better recovery. Kasongo et al.9 studied the effect of calcium ions and clays on bitumen extraction by doping ions and/or clays into good processing oil sands ores. They identified the wettability of bitumen as an important parameter governing bitumen recovery and noted stronger adsorption tendencies of calcium ions on montmorillonite than on kaolinite or illite, causing slime coating of montmorillonite on bitumen and, hence, lower bitumen recovery. The majority of research initiated to study bitumen liberation from oil sands has been carried out using pseudo- or synthetic oil sands. In the study by Basu et al.,7 for example, a glass slide resembling the sand grain, with processed bitumen (coker feed) being coated as a film on the glass slide was used as a model of oil sands. In their study, the glass did not have similar surface properties to a sand grain nor did the processed bitumen represent the bitumen in its natural state in the oil sands ore. Furthermore, the contact angle of bitumen with the glass slide did not represent real contact of bitumen with sands in real oil sands ore. As a result, the adhesion of bitumen and sand grain

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EXPERIMENTAL SETUP

Bitumen liberation from oil sands is studied using a novel bitumen liberation flow visualization cell (BLFVC) system designed exclusively for this study. The BLFVC as shown schematically in Figure 1b is designed on the basis of the concept initially attempted by Walker.16 The BLFVC consists of a low-vacuum line connected to an oil sands sample holder. The oil sands sample holder consists of a glass frit of pore sizes between 40 and 100 μm, fused into a solid glass sample holder. A thin layer of oil sands ore is placed on the top of the glass frit and kept in the glass sample holder. The oil sands ore, which otherwise would have been crumbled and eroded away by the liquid flowing on top of it, can now be efficiently kept intact on the sample holder with the help of a low-level vacuum applied from below the glass frit in the BLFVC. Figure 1a shows the schematic diagram of the experimental system, and Figure 1b shows the detailed dimensions (top) and photograph (bottom) of BLFVC. A 1 L glass jar is used as the feed solution container and is placed in a water bath (Contraves, Rheotherm 115), which is used to control the temperature of feed solutions. The feed solution is circulated through the flow cell using a peristaltic pump (Masterflex, C/L). Bitumen liberation is recorded in real time using a stereo-optical microscope 2884

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equipped with a high-resolution charge-coupled device (CCD) camera from Olympus (SZX 10). The CCD camera captures and transfers high-quality images to the computer and monitor at 10 frames per seconds (fps) rate.

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EXPERIMENTAL PROCEDURE

Prior to each experiment, the oil sands sample stored in a deep freezer was left out overnight in a sealed glass sample bottle. The 1 L glass jar was filled with 500 mL of the desired feed solution and placed in a water bath at least 2 h prior to experiment. Unless otherwise stated, all solutions were prepared using deionized water with a resistivity of 18.2 MΩ cm. The pH of feed solutions was adjusted using 1 N NaOH and/ or 1 N HCl solutions. The feed solution from the exit valve of the BLFVC was recirculated back to the 1 L glass jar in all of the cases. In this study, the flow rate of the feed solution was maintained constant at 0.633 mL/s, which corresponds to a liquid linear flow velocity of 0.0025 m/s. At the start of each run, a layer of bitumen (vacuum distillation unit feed) was spread on a filter paper of the same diameter as the glass frit. Approximately 3 g of oil sands were pressed onto the bitumen-smeared filter paper and placed into the glass sample holder. Once placed into the glass sample holder, the excess oil sands were removed from the top and sides using a knife to obtain a flat oil sands top surface, as shown in the bottom panel of Figure 1b. The glass sample holder with oil sands was then carefully placed on an O-ring in the packet of the cell. A metal plate with a central hole was placed on top of the glass sample holder and pressed tight with a set of screws to seal the glass sample holder. The entire cell was then sealed with a glass slide on an O-ring and a steel cover pressed on the glass slide by four screws from the top. For each run, the bitumen liberation process was recorded as a function of time and the results were quantified by imaging analysis of the snapshot images to determine bitumen liberation kinetics. Unless otherwise stated, a high-grade oil sands ore containing 14.8 wt % bitumen, 0.8 wt % water, and 82.9 wt % mineral solids, of which 2.5 wt % were fines with a particle size less than 44 μm as defined by the oil sands industry, was used in all of the experiments. This ore represents a fairly high ore grade with very low fines content. The degree of bitumen liberation was calculated by determining the percentage of clear sand grains out of total sand grains visible on each frame. Clear grains in our analysis were the sand grains that did not have visible bitumen other than bitumen globules that were readily detachable from the surface under the proper hydrodynamic conditions encountered in commercial hydrotransport slurry pipelines. Figure 2 shows a typical snapshot image of a high-grade oil sands ore sample after 375 s of contact with the flowing feed solution at pH 8.5 and 35 °C. The white bar in this photograph and subsequent photographs is 1 mm in real scale. In Figure 2, the solid circles represent bitumen globules on sand grains, which according to our assumption were capable of disengaging from the sand grains, while the short-dashed circles indicate bitumen films, which have not yet formed globules that can disengage from the sand grain. The grain marked with the long-dashed circle is considered as a clean sand, while the dotted circles mark an air bubble rubbing the bitumen from surrounding sand grains with the dynamic process being shown in the video (played 4x slower in the Supporting Information). The images extracted at different time intervals from the video were analyzed grain-by-grain. The number of clear sand grains as circulated by the solid and long-dash circles in Figure 2 was counted as a function of time. The degree of liberation kinetics curves were constructed by calculating the percentage of clear sand grains (solid and long-dash circled) among the total number of sand grains in the view as a function of time. It should be noted that this method of evaluating degree of liberation was compared with the percentage of cleared areas of sand grains in the view. Both methods produced similar liberation kinetics profiles, although the absolute values of the degree of liberation bitumen (DBL) varied slightly.

Figure 2. Typical snapshot image of oil sands ore after 375 s of contact with flowing liquid at pH 8.5 and 35 °C and quantification criterion for bitumen liberation: grains with solid circles were considered as liberated with globules of bitumen; grains with long-dashed circles were considered as clean; and grains with short-dotted circles were considered as unliberated. The dotted circles represent air bubbles rubbing the bitumen from surrounding sand grains.

by bitumen recession from sand grains to form bitumen globules prior to detachment/liberation. Bitumen engulfing on air bubbles formed from entrained air was found to contribute to bitumen liberation, as shown in the video (see the Supporting Information). These two bitumen liberation modes are highlighted in Figure 3. In this figure, the bitumen recedes from the original ore, as seen in Figure 3A, to form droplets highlighted with solid circles in Figure 3C, while entrained air bubbles rubbing bitumen off the sand grains are shown with dotted circles in panels D−F of Figure 3. This phenomenon of an air bubble rubbing the bitumen from surrounding sand grains is best described in the video (see the Supporting Information). It describes air bubbles as circled in panels D−F of Figure 3 rotating among the sand grains, during which bitumen on the sand grains is transferred to the surface of air bubbles by engulfing the air bubble. A similar bitumen liberation mechanism was proposed by Lelinski et al.17 The focus of this paper is on bitumen recession from sand grains. Effect of the Solution Temperature and pH on Bitumen Liberation. The effect of the temperature and solution pH on bitumen liberation was studied by carrying out the liberation experiments at three different temperatures of 23, 30, and 46 °C for each solution pH of 7.8, 9.7, and 11.3. The results are shown in Figure 4. For each data point in this type of bitumen liberation curve, about 100 grains were counted to ensure that the values obtained are statistically representative. The lines in this and subsequent figures are trend lines intended to guide the eyes of the readers. Figure 4 shows a varying degree of increase in bitumen liberation kinetics, depending upon pH. At pH 7.8, for example, a significant increase in bitumen liberation kinetics was observed as the temperature increased from 30 to 46 °C, while at pH 11.3, a significant increase in bitumen liberation kinetics was observed between 23 and 30 °C, above which only a minimal further increase in bitumen liberation kinetics with temperature was observed. It is not surprising to observe intermediate bitumen liberation kinetics at pH 9.7 compared to the cases at pH 7.8 and 11.5.

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RESULTS AND DISCUSSION Bitumen Liberation. In this study with direct visualization, bitumen liberation from the sand grains was identified mainly 2885

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Figure 3. Two bitumen liberation modes: (A−C) illustrating bitumen receding on sand grains and (D−F) showing bitumen liberation by bitumen engulfment on entrained air bubbles from surrounding sand grains over time from t = 0 to 300 s.

Figure 4. Effect of the temperature on bitumen liberation kinetics at pH 7.8, 9.7, and 11.5 using a high-grade ore. The snapshot images after 300 s of liberation period as the insets at pH 11.5 show clearly a much cleaner sand surface at (A) 46 °C than (B) 23 °C.

increase in bitumen liberation kinetics with increasing process temperature was reported by Luthra et al.,19 when they determined the color change of oil sands slurry confined in a Couette glass cylinder as a function of the agitation time. Increasing the temperature is known not only to reduce the viscosity of bitumen but also to increase the repulsive force

The insets of Figure 4 for pH 11.3 provide a visual observation of enhanced bitumen liberation at 46 °C than 23 °C. Using a laboratory hydrotransport extraction system (LHES), Wallwork et al.18 observed a similar increase in bitumen liberation kinetics with increasing process temperature by determining the color change of the flowing slurry. A similar 2886

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between bitumen and silica, leading to a faster recession.5 Through colloidal force measurements using an atomic force microscope, Long et al.20 showed that increasing the water temperature decreases the adhesion forces between bitumen and sand grains, which disappear at 32−35 °C, leading again to a faster bitumen liberation. The findings from our in situ bitumen liberation study agree well with theoretical predictions by Long et al.20 and Ding et al.15 The observed enhancement of bitumen liberation kinetics at lower temperature with increasing solution pH appears to be linked with the enhanced release of natural surfactant from bitumen with increasing solution pH; i.e., at a given temperature, more natural surfactant is released at higher pH. The results in Figure 4 also show a significant increase in bitumen liberation kinetics with increasing solution pH at a given temperature. The solution with the highest pH took the least time to reach its highest degree of bitumen liberation, highlighting the importance of pH on bitumen liberation kinetics. Takamura et al.21 suggested that, at high pH, both silica and bitumen surfaces are negatively charged and a strong repulsive force separates them. Dubey et al.22 used a base/acid ratio to describe the wetting properties of oil. They suggested that, at neutral and alkaline pH, the presence of acid-type natural surfactants in bitumen gives bitumen a high negative charge. High pH also facilitates hydrolysis of sand grains, leading to more hydrophilic and highly negatively charged sands surfaces. Both of these factors are beneficial for bitumen liberation. Through numerous experiments, Basu et al.7 showed a higher “contact angle” of bitumen on glass substrates as measured through bitumen in higher pH solutions, which is equivalent to a lower contact angle as measured through the aqueous phase, i.e., 180° minus the measured “contact angle”, and conventionally used in textbooks and our study. A higher “contact angle” of bitumen as measured through the bitumen phase on solids in higher pH aqueous solutions indicates an easier liberation of bitumen because the bitumen tends to ball up under the stronger cohesion force of bitumen than the adhesion force of bitumen with solids in water. Effect of Weathering on Bitumen Liberation. Weathering is known to drastically reduce the bitumen recovery from oil sands.23,24 Slow bitumen recession was proposed to be the reason for low recovery from weathered ores. To test whether bitumen liberation is a limiting step in processing weathered oil sands ores and hence bitumen recovery, the high-grade oil sands ore was weathered (aged) for different periods of time in an oven at 60 °C and ambient pressure with air ventilation. The results of the in situ bitumen liberation tests performed at pH 10 and solution temperature of 46 °C are shown in Figure 5. It is evident that the bitumen liberation kinetics decreased drastically with increasing weathering time. The high-grade ore without weathering achieved 90% bitumen liberation within 300 s, while the ore weathered for 3 days achieved only 45% bitumen liberation after 800 s. For the ore weathered for 1 week, negligible bitumen liberation was observed even after 800 s. Image A of Figure 5 taken after 500 s liberation clearly shows that the majority of bitumen films have been washed away from the sand grains of unweathered high-grade ore by the flowing fluid. In contrast, a substantial amount of bitumen remained on the sand grains of the ore weathered for 3 days, even after experiencing a prolonged liberation period of 800 s, as shown in image B of Figure 5. The bitumen that remained on sand grains is only partially receded to non-spherical shape. Image C of Figure 5 shows a negligible recession of bitumen from sand

Figure 5. Effect of ore weathering on bitumen liberation kinetics: (A) high-grade ore without weathering, (B) high-grade ore weathered in the laboratory at 60 °C for 3 days, and (C) high-grade ore weathered in the laboratory at 60 °C for 7 days. All images were taken at 500 s of bitumen liberation tests at pH 10 and 46 °C. Ore weathering leads to severe degradation in bitumen liberation.

grains even after experiencing a liberation period of 900 s when the ore was weathered for 1 week. Careful inspection of these images reveals a decreased contact angle of bitumen on sand in flowing processing water with increasing ore weathering time. The poor bitumen liberation with ore weathering could be attributed in part to the loss of connate (formation) water. Such a loss increases the hydrophobicity of the solids, forcing a stronger attachment of bitumen films to the sand grain. Mikula et al.25 characterized bitumen properties of weathered ores using confocal microscopy. They observed a loss of aliphatic carbons and a relative increase in hydroxyl groups of the bitumen when ores are weathered or oxidized. They concluded that these changes in bitumen chemistry are the dominant parameter responsible for lower bitumen liberation of weathered ores. Ren et al.26 and Dang-Vu et al.27 reported a significant increase in hydrophobicity of solids from weathered oil sands ore. They showed that solids extracted from weathered ores are the most hydrophobic among all of the ores tested, suggesting difficult bitumen liberation from sand grains of weathered ores. They found that laboratory weathering caused a significant decrease in bitumen recovery, which confirms the findings from this study. Effect of the Salt (NaCl) Concentration on Bitumen Liberation. In commercial oil sands processing, no processaffected water is discharged to the environment. With the mined ore containing salts, the salt content in the tailings pond water that is used in ore processing rises with time. The increase in the salt content in the processing water that is continually recycled from the tailings ponds is of concern to the industry because it can be detrimental to ore processability. It is therefore important to understand the effect of process water salinity (NaCl concentration) on bitumen liberation. The results in Figure 6 show a progressive decrease in bitumen liberation kinetics with increasing NaCl concentration when the bitumen liberation tests were conducted at pH 10 and 46 °C. For example, without NaCl addition, more than 90% bitumen was liberated from sand grains within 300 s. With the addition of 4000 ppm NaCl to the feed solution, a maximum bitumen liberation at 50% was achieved after 800 s. A further increase in the NaCl concentration to 16 000 ppm led to negligible bitumen liberation even after 1000 s. 2887

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Figure 7. Effect of fines content on bitumen liberation at pH 11.3 and 46 °C. Snapshot images taken after 800 s show a much poorer degree of bitumen liberation from high-fines ore A2 than from lower fines ore A1.

Figure 6. Effect of the salinity of feed solution on bitumen liberation kinetics at pH 10 and 46 °C: (A) without NaCl addition, (B) with 4000 ppm NaCl addition, (C) with 8000 ppm NaCl addition, and (D) with 16 000 ppm NaCl addition. Insets are snapshot images after 600 s of liberation.

fines ore. Under the same processing condition of pH 11.3 and 46 °C, bitumen liberation from lower fines ore A1 reached 95% within 450 s, in contrast to 60% bitumen liberation from higher fines ore A2 after 600 s. Images A1 and A2 of Figure 7 taken after 800 s show a distinct difference: bitumen forms droplets on the sand grains of A1 ore, which contains nearly no fines, while no visible bitumen droplets are seen for A2 ore containing 12.6% fines. The presence of fines severely reduced bitumen recession and, hence, bitumen liberation. The slower bitumen recession in the presence of fines is attributed to the poor separation of bitumen from fines, possibly because of its strong binding with fines. The presence of fines in bitumen is anticipated to hinder the recession of bitumen to become globules, as seen in A1 ore.30 High fines are also known to contribute to a higher divalent cation concentration of connate water because of cation-exchange characteristics of clays. A divalent cation, such as calcium, reacts with the carboxylic groups in the bitumen, which triggers adsorption of surfactant on clay and sand surfaces, making them hydrophobic (oil-wet), thereby contributing to depression of bitumen recession from sand grains. Effect of Diluent (Kerosene) Addition on Bitumen Liberation. Schramm et al.12 studied the importance of solvent addition in increasing bitumen recovery and froth quality using a flotation cell. However, there was no direct evidence whether solvent addition helped bitumen liberation and/or aeration because their study was based on a batch extraction unit. With the novel in situ bitumen liberation visualization cell, it is possible to study independently the effect of kerosene addition on bitumen liberation. Because we showed earlier that weathering caused a significant decrease in bitumen recovery of oil sands ore, a 7-day weathered ore was chosen to study the effect of kerosene addition on bitumen liberation. In this study, the weathered ore was soaked with a given volume of kerosene (2 drops or 0.2 mL of kerosene for 3 g of weathered high-grade ore) for 15 min prior to the liberation test. The results in Figure 8 show a drastic increase in bitumen liberation kinetics with kerosene addition, reaching 100% bitumen liberation in 320 s, in contrast to negligible bitumen liberation without kerosene addition. The snapshot image taken at 300 s shows clear sand grains, indicating effective bitumen liberation. Recently, Wang et al.31 reported a similar enhance-

Insets A−D in Figure 6 are snapshot images after 600 s of bitumen liberation from the high-grade ore in flowing feed solutions of different NaCl concentrations. As shown in image A of Figure 6, the sand grains without NaCl addition are essentially either free of bitumen or bitumen sitting on the sand grains in the form of globules, suggesting a highly hydrophilic nature of solids. Increasing the NaCl addition makes bitumen recession much more difficult. At 16 000 ppm NaCl addition, for example, image D of Figure 6 shows a negligible bitumen recession. A similar depression of bitumen liberation by NaCl addition was also observed (not shown here) when the tests were conducted at pH 7.8 and 46 °C, although to a less of an extent.28 Increasing the salt concentration in industrial process water is known to reduce electrostatic repulsion between bitumen and sand grains, leading to a stronger adhesion and, hence, a low degree of bitumen liberation.29 It is also possible that increasing the salt concentration hinders the production of natural surfactants, contributing to poor bitumen liberation. Effect of Fines on Bitumen Liberation. Clays are known to affect bitumen flotation. The effect of fines, defined as solids less than 44 μm in size, on bitumen recovery has been studied extensively. Using Denver flotation cell, Sparks et al.14 and Ding et al.15 found that the presence of fines reduces bitumen recovery significantly. In a bitumen recovery study using a good processing ore doped with different types of clays, Kasongo et al.9 identified a detrimental role of smectite clays with divalent cation addition. However, whether the bitumen liberation is a limiting factor for depressed bitumen recovery with fines addition remains unresolved. In this study, an attempt is made to study this aspect by conducting bitumen liberation tests on two ores (A1 and A2) having similar bitumen, water, and solids contents but differing in their fines content. Table 1 shows the composition of these two ores. The results in Figure 7 show a much slower bitumen liberation kinetics from the higher fines ore than from lower Table 1. Composition (wt %) of A1 and A2 Ores ore type

bitumen

water

solid

fines

A1 A2

11.8 11.1

2.2 3.3

84.4 83.6

0.1 12.6 2888

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The financial support for this work from the Natural Sciences and Engineering Research Council of Canada (NSERC) under the Industrial Research Chair Program in Oil Sands Engineering is gratefully acknowledged. The authors thank SyncrudeCanada, Ltd. for providing ore samples and bitumen samples.

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Figure 8. Effect of kerosene addition on the degree of liberation at pH 8.5 and 25 °C, showing a significant improvement in bitumen liberation kinetics and degree of bitumen liberation (A) with kerosene soaking than (B) without kerosene addition. Images are taken after 300 s of flow with kerosene addition to the ore sample.

ment of bitumen liberation with kerosene or fatty acid methyl ester (FAME) addition, although a bitumen film on glass slides was used in their study. Bitumen recovery tests conducted by Harjai13 using the Denver flotation cell showed that soaking oil sands ore in kerosene before extraction improved bitumen recovery significantly. Our study supports his findings, and improved bitumen liberation is considered as a major reason for the improved bitumen recovery from weathered ores by solvent addition. The lower the viscosity, the greater the recession rate. Schramm et al.32 demonstrated that a viscosity reduction of bitumen from 500 to about 3 Pa s can increase bitumen recovery to an acceptable level.

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CONCLUSION In this study, a novel in situ bitumen liberation visualization flow cell was designed and tested for viewing bitumen liberation directly from an oil sand ore under various processing conditions. A high solution temperature and high pH led to a faster bitumen liberation rate and higher degree of bitumen liberation, while weathering of ores severely depressed the degree of bitumen liberation and bitumen liberation kinetics, leading to poor bitumen recovery. Kerosene addition to oil sands ore prior to bitumen liberation facilitated bitumen liberation from the sand grains, leading to improved bitumen recovery. The presence of fines in the oil sand ore and high salt (NaCl) concentrations showed a detrimental impact on bitumen liberation, thereby providing a fundamental explanation on depressed bitumen recovery from high-fines and/or high-electrolyte ores.

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ASSOCIATED CONTENT

S Supporting Information *

Air bubbles rubbing the bitumen from surrounding sand grains (AVI video). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 2889

dx.doi.org/10.1021/ef300170m | Energy Fuels 2012, 26, 2883−2890

Energy & Fuels

Article

(32) Schramm, L. L.; Stasiuk, E. N.; Yarranton, H.; Maini, B. B.; Shelfantook, B. J. Can. Pet. Technol. 2003, 42, 55−61.

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dx.doi.org/10.1021/ef300170m | Energy Fuels 2012, 26, 2883−2890