Effect of Caustic Type on Bitumen Extraction from Canadian Oil Sands

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Effect of Caustic Type on Bitumen Extraction from Canadian Oil Sands Christopher Flury,† Artin Afacan,† Marjan Tamiz Bakhtiari,† Johan Sjoblom,‡ and Zhenghe Xu*,† †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Ugelstad Laboratory, Norwegian University of Science and Technology, N-7491 Trondheim, Norway



ABSTRACT: The role of different types of caustics in bitumen extraction from Canadian oil sands is investigated by analyzing their effect on subprocesses involved in bitumen extraction. Both sodium hydroxide and ammonium hydroxide were shown to improve bitumen liberation, which is favorable for bitumen recovery, but increased induction time of bitumen−bubble attachment, which is harmful for bitumen recovery. Although a similar performance was observed at pH of ∼8.5, at pH 11.3, the use of ammonium hydroxide led to a shorter induction time of bitumen−bubble attachment and, hence, better bitumen recovery than the use of sodium hydroxide to achieve the same slurry pH. The better performance of ammonium hydroxide is attributed to a more hydrophobic surface and a less negative zeta potential of bitumen in the process water because of the release of less amount of natural surfactants in ammonium hydroxide solutions than in sodium hydroxide solutions of the same solution pH. Overall, ammonium hydroxide was found to be a suitable replacement for sodium hydroxide. Although this study was mainly concerned with mining−extraction operations, the major findings derived from this study could be applied readily to in situ thermal bitumen production operations, because ammonia can be delivered with steam to oil sands formations.



its recovery.4 The addition of NaOH alters the following physical and chemical characteristics of the extraction systems: (i) increasing pH of solution and, hence, ionization of the natural surfactant,5 (ii) extracting the natural surfactant from bitumen,6 (iii) hydrolyzing sand grains and, hence, increasing wettability of the sands, (iv) dispersing the clays by imposing negative surface charges on the edge surface of clays, and (v) scavenging divalent cations, such as calcium and magnesium ions, by forming respective carbonates, in particular when the pH of the slurry is increased above 9.5. Increasing ionization of the natural surfactant at the bitumen−water interface with an increasing addition of NaOH reduces bitumen−water interfacial tension and increases wettability of bitumen. Similarly, increasing natural surfactant extraction from bitumen with increasing NaOH addition also decreases bitumen−water interfacial tension because of the increased adsorption of the natural surfactant at the bitumen− water interface. Although both of these effects enhance bitumen liberation from sand grains, they could be detrimental to bitumen aeration needed for bitumen flotation, because of the increase in wettability of bitumen from the increased adsorption of the natural surfactant and its ionization at the bitumen−water interface. Increasing natural surfactant generation and ionization with increasing pH by NaOH addition could also make bubbles more hydrophilic, imposing a stable liquid film between bitumen and air bubbles and reducing bitumen−bubble attachment. In addition, lowering interfacial tension is anticipated to emulsify bitumen to stable droplets of increasingly smaller sizes, leading to poor aeration. These negative effects lead to low bitumen recovery. As a result, there

INTRODUCTION Oil sands are also known as tar sands or bituminous sands. Canada’s bitumen resources are located almost entirely 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 reserves. Of the total of 170.4 billion barrels remaining in established reserves, about 20% is considered recoverable by surface mining methods.1 In 2012, the production of bitumen from Alberta oil sands exceeded 1.6 million barrels/day, with about 50% of the production by surface mining. Developing cutting edge technologies based on the fundamental understanding of bitumen recovery processes provides opportunities to secure fossil fuel supplies while minimizing environmental consequences. At present, commercial recovery of bitumen from the mineable Athabasca oil sands deposits uses almost exclusively the warm-water-based extraction technology. For energy-saving purposes, the process temperature is kept under 50 °C.2,3 The bitumen liberation is an essential subprocess of a typical warmwater-based extraction process and generally involves bitumen displacement along a sand grain to a bitumen globule and its detachment from the sand grain. In current commercial mineable oil sands operations, bitumen liberation from sand grains occurs during the hydrotransport of oil sands slurry by pipelines for a few kilometers prior to separation of aerated bitumen in a gravity separation vessel. The forces of bitumen adhesion to the sand grains and those pulling the bitumen away from the sand grains determine the rate of bitumen liberation. As an essential subprocess of successful bitumen extraction, bitumen liberation is highly impacted by physical, chemical, and hydrodynamic characteristics of extraction systems. Appropriate addition of caustics, mostly in the form of sodium hydroxide (NaOH), is known to improve bitumen liberation and, hence, © 2013 American Chemical Society

Received: September 4, 2013 Revised: November 15, 2013 Published: November 18, 2013 431

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decreased the induction time of bitumen−air bubble attachment, and at the same time, facilitated bitumen liberation.12 The present study focuses on understanding the role of different types of caustics in bitumen liberation and aeration, which are two essential subprocesses for bitumen recovery from oil sands. An in situ bitumen liberation flow visualization cell (BLFVC) and a custom-built induction timer are used to determine bitumen liberation kinetics and bitumen−air bubble attachment, respectively. At the same time, the amount of carboxylic surfactant released and zeta (ζ) potential of bitumen droplets with ammonia and NaOH addition are determined. The results are correlated with the results of batch bitumen extraction using a Denver flotation cell to illustrate the benefit of using ammonia as an alternative of caustics.

is an optimal addition of NaOH to maximize its benefit on bitumen liberation without sacrificing bitumen aeration.6 The amount of NaOH added to oil sands extraction is normally between 0.01 and 0.2% (weight percent of oil sands ore) to achieve an operating slurry pH of ∼8.5 and optimum recovery. Raising pH by NaOH addition is known to enhance hydrolysis of sands, mostly silica or quartz. The increase in hydrolysis of solids increases wettability of solids, which, in turn, facilitates bitumen liberation without an adverse effect on bitumen aeration.7 Making the sand more hydrophilic also reduces flotation of sands, which improves the quality of bitumen froth. Increasing pH also increases charges of solids, such as clays, resulting in an increased repulsion between bitumen and clays, which, in turn, minimizes slime coating of clays on bitumen surfaces. Scavenging of divalent cations in the form of calcium or magnesium carbonates at elevated pH will contribute to reducing slime coating and, hence, a cleaner bitumen surface for enhanced bitumen aeration.8 Both of these effects would unfortunately lead to a more stable suspension of fine clays, causing difficulties in tailings management. Despite the widespread use of NaOH in bitumen extraction, the recognized adverse impact of its use leaves the opportunity to find the alternative caustics. One such alternative is ammonia, NH3(g). Upon contact with water, ammonia becomes protonated in the following manner: NH3(g) + H 2O(l) ↔ NH4 + + OH−



EXPERIMENTAL SECTION

Materials. To have the generality of the study, two Athabasca oil sands ores from the Aurora mine, supplied by Syncrude Canada, were used. The characteristics of these two ores are given in Table 1. The

Table 1. Composition of Oil Sands Ores Used in This Study composition (wt %) ore name

solids

bitumen

water

fines (wt % of solids)

A1 ore C ore

84.5 86.2

12.3 12.6

3.2 1.2

0.7 12.1

(1) Dean−Stark apparatus was used to determine the bitumen, solid, and water contents of each ore. In the Alberta oil sands industry, fines were defined as solid particles less than 44 μm. Solids were wet-sieved using a 44 μm sieve to separate the fine from the solids. The biggest difference between these two ores was their fines content, representing two different processabilities. It should be noted that bitumen chemistry between these two ores could be different and, hence, could have different effects on bitumen recovery from these two ores. However, the objective of this study was to investigate the effect of different types of caustics on the processability of oil sands ores, and hence, investigating the effect of bitumen chemistry and fines content on bitumen recovery is beyond the scope of the current study. Unless otherwise stated, the plant process water from Syncrude Canada was used in this study. On average, the plant process water contained 56 ppm Ca2+, 20 ppm Mg2+, 750 ppm Na+, and 21 ppm K+. The pH of the process water was ∼8.2 and adjusted using concentrated (1 M) sodium hydroxide (NaOH) or ammonium hydroxide (NH4OH) solutions to desired values. Bitumen Liberation. The liberation of bitumen from oil sands ores was examined using a novel BLFVC developed recently in our research group. The oil sands sample stored in a deep freezer was left out at room temperature overnight in a sealed glass sample bottle to thaw the sample. At the beginning of each run, a thin layer of bitumen (vacuum distillation unit feed) was spread on a piece of filter paper, followed by a layer of oil sands ore. The filter paper with the ore sample was placed in the glass sample holder. The oil sands ore, which, otherwise, would have been eroded by the liquid flowing on top of it, could now be efficiently kept intact on the sample holder with the help of a low-level vacuum applied below the glass frit in the BLFVC.13 A 1L glass jar was used as the feed solution container and placed in a water bath (Contraves, Rheotherm 115), which was used to control the temperature of the feed solutions. The feed solution prepared from Syncrude plant process water with desired pH was circulated through the flow cell using a peristaltic pump (Masterflex, C/L), while the video was taken in real time at 10 frames per seconds (fps) rate using a stereo-optical microscope from Olympus (SZX 10), equipped with a high-resolution charge-coupled device (CCD) camera. The flow rate of the solution was kept constant at 32 mL/min for all runs, unless otherwise stated. The temperature of the process water flowing over the oil sands sample was determined using three K-type

releasing hydroxide ions (OH−) and, hence, increasing pH of the system. This caustic would provide all of the same benefits as NaOH, yet it would also add a new cationic ion in ammonium (NH4+) into the system. With an increase in pH, the ammonium deprotonates into neutral ammonia (NH3), which would decrease the risk of an increasing amount of cations in the system. In addition, ammonia in its natural vapor form can be injected with high-temperature steam into the ground for enhancing bitumen production by in situ thermal production methods, such as the steam-assisted gravity drainage (SAGD) process. Furthermore, ammonia as a byproduct of upgrading facilities is readily available at the vicinity of oil sands operations. All of these factors make ammonia a viable candidate to replace NaOH as the caustic used in the bitumen extraction process. Ammonium hydroxide (NH4OH) has been suggested as a caustic in several patents. Choules suggested addition of ammonium hydroxide or another lyotropic salt to enhance bitumen extraction.9 In that study, ammonium was referred to as the floating agent. He reported the use of an ammonium phosphate salt at lower concentrations as an effective recovery agent and concluded that a combination of various salts containing the lyoptropic ions could lead to the same results. In a separate patent, Myers recommended mixing of ammonium hydroxide with tannic acid as process aids to improve bitumen recovery.10 There were several problems noted in the patent though, with the largest problem being the separation of the bitumen from the solution after being lifted above the ground as the produced fluids. To offset the adverse effect of caustic addition on bitumen aeration, Wang et al. examined use of short-chain organic amines to compensate for negative charges and increased wettability of bitumen surfaces caused by caustic addition, making the bitumen surface more hydrophobic.11 Their study showed that adding short-chain amines into the solution indeed made the bitumen surface more hydrophobic, 432

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thermocouples mounted at the inlet, middle, and outlet of the cell, respectively. The videos transferred to the computer and displayed on the monitor were used later on to determine bitumen liberation kinetics. The bitumen liberated during this period was carried away from the cell with the flowing process water. Images were extracted at specific times during each run and analyzed using GNU Image Manipulation Program (GIMP), which allowed for a custom-defined threshold of a certain color. The extracted images were opened with a Photoshop program and analyzed frame by frame. For each image, individual sand grains were selected manually. The brightness of each pixel on the sand grain selected was subjected to a comparison to the preset threshold value to assign the pixel to the bitumen (black) or sand (white). The fraction of the sand grain that was clear (fraction of white on the grain), i.e., the number of white pixels divided by the total number of pixels of the sand grain, was determined for every sand grain in the image. Once all of the sand grains were analyzed, the average percentage of the clear sands grains was determined using the equation given below and defined as the degree of bitumen liberation (DBL).

DBL (%) =

∑ percent cleared on the i th grain number of grains

the free end of the capillary tube in the test solutions using a micro syringe. The bubble driven by the speaker diaphragm at a speed of 40 mm/s was made in contact with the bitumen surface for a given period of time and then retracted at the same speed. A light source was placed on either side of the sample cell to help illuminate the cell, and a CCD camera attached to a macro lens was installed and connected to the computer to record the entire attachment process. This visualization capability allowed for not only the determination of a successful bitumen−bubble attachment but also the accurate control of the bubble size at 0.25 mm and initial gap between the bubble and bitumen at 0.4 mm. For a given contact time, this process was repeated 20 times and the percentage of contacts that made attachment, i.e., the number of attachments over the total number of trials, was plotted as a function of the contact time. In this study, the contact time with a 50% attachment was considered as the induction time. Unless otherwise stated, all of the measurements were carried out at 35 °C. Bitumen Flotation. Bitumen flotation tests were conducted in a Denver flotation cell. A 2-L stainless-steel flotation cell was modified to have a water jacket. A Neslab water bath (EX-111) was connected to the water jacket to keep the cell at a desired temperature. The agitation of the slurry was provided by an impeller connected to a 1/2 HP Baldor Industrial Motor. The speed of the impeller was measured using a tachometer. Air was allowed to pass through the impeller shaft at a constant flow rate measured using a precalibrated rotameter (Matheson). Prior to each test, the ore was allowed to thaw at room temperature for 2 h. The flotation cell was filled with process water at the desired temperature of 35 °C. This temperature is close to the lower limit of temperatures at which current commercial oil sands plants are operating. At this temperature, the operation is more sensitive to process aids, such as caustic addition. Tests at this temperature ensure a better response of the process to the caustic addition. The pH of the process water was adjusted to desired values using sodium hydroxide or ammonium hydroxide. A precisely weighed 300 g ore was placed in the flotation cell under the agitation at 1500 rpm. Agitation without air addition lasted for 10 min to condition the slurry. The air at 150 mL/ min was then allowed to flow into the flotation cell, while bitumen froth was collected at 5 min intervals into separate thimbles for bitumen, solids and water content analysis using the standard Dean− Stark apparatus. From the results of Dean−Stark analysis, cumulative bitumen recovery over a flotation period of t, R(t) is calculated by

(2)

As an example, bitumen liberation from the sand surface to form small bitumen droplets is shown in Figure 1. The image taken at 20 s into

Figure 1. Video images of bitumen liberation on A1 ore. the run remains very dark, with the solids being covered with a thick layer of bitumen film, while the image taken 300 s into the run shows the bitumen forming spherical droplets on the sand surface. After 300 s into the run, the majority of the sand grains are clear, representing a high degree of bitumen liberation. Induction Time Measurement. To determine the effect of sodium and ammonium hydroxide addition on bitumen aeration, the induction time of bitumen−air bubble attachment, defined as the minimum time of an air bubble in contact with bitumen for the bubble to attach to the bitumen surface, was measured using a custom-built induction timer.14 The vacuum distillation unit feed bitumen was used in the induction time measurement. The bitumen was placed in a half sphere shape pocket on a circular Teflon disk. The bitumen surface was made even by a razor blade and allowed to relax for approximately 30 min in a covered dish. The sample, now flat and mirror-shining (smooth) to the naked eye, was transferred into a small rectangular glass cell that had been filled with the testing solution. The sample was allowed to equilibrate in the solution for 30 min prior to each measurement. The rectangular glass cell hosting a bitumen sample in processing water was placed on a three-axial micro translation stage. A portable heater and J-type thermocouple were built in the stage to allow for the temperature of the stage to be kept at a desired and constant value. A glass capillary tube was attached to the diaphragm of a speaker, which was placed above the translation stage. The speaker was connected to a charge amplifier driven by a computer to control the movement of the capillary tube, such as the displacement, approach, and reaction speed and duration of the tube at the set displacement. An air bubble of 1.5 mm in diameter was generated at

R(t ) =

bitumen in froth (g) × 100 bitumen in feed (g)

(3)

Surfactant Analysis. The Fourier transform infrared spectroscopy (FTIR) method, which was initially developed by Syncrude Canada for quantification of naphthenic acids in the process water, was used to quantify carboxylic surfactants released in the tailings water after each flotation test.15−17 Surfactant released because of caustic addition was analyzed by first acidifying 50 g of the process or tailings water from the flotation tests to pH 2.3 using hydrochloric acid (HCl), followed by the extraction of the acidified neutral surfactant using a 2:1 volume ratio of dichloromethane (DCM). After homogenization for 2 min, the mixture was allowed to stand still for a few minutes, during which DCM settled to the bottom of the mixture. The extraction was repeated for a second time. After this two-stage extraction, DCM collected was placed in a vented fume hood to evaporate DCM for approximately 3 h or until all DCM was evaporated. Air filtered using a 45 μm filter was used for DCM evaporation. The dried sample was redissolved in an accurately weighed 20 g of DCM. The FTIR spectrum of the solution was taken. The height of the peaks at 1743 and 1706 cm−1 corresponding to carboxylic functional groups in the form of monomers and dimers, respectively, was recorded to calculate the concentration of the carboxylic surfactant in the solution. The overall absorbance at these two wavenumbers was used to quantify the concentration of carboxylic surfactants based on the standard curve. The standard curve was obtained using a commercial mixture of naphthenic acids purchased from Sigma-Aldrich. The FTIR spectra were obtained using a Bio-Rad FTS 6000 (Cambridge, MA), equipped 433

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with a deuterated triglycine sulfate (DTGS)-type detector. A KBr super-sealed liquid cell with a 3 mm path length was used to hold the sample in the instrument, and the sample compartment of the instrument was continuously purged with dry air. The resolution of the instrument was set to 4 cm−1, and the spectra were collected in the wavenumber range of 4000−400 cm−1. The background and sample spectra were the results of 128 co-added scans.

In the context of bitumen liberation kinetics at a given pH, there is little difference between sodium and ammonium hydroxide as pH modifier, although higher pH favors bitumen liberation kinetics and an overall degree of bitumen liberation for both ores investigated in this study. The small difference between the two caustics is within the experimental error, as shown by error bars of five runs under the identical conditions. The negligible difference at pH 8.5 between the two caustics as pH modifier is anticipated because only a very small volume (0.1% of the total volume) of sodium or ammonium hydroxide stock solution is needed to bring the pH to this level. To reach pH 11.3, a much larger volume (3−3.5% of the total volume) of ammonium hydroxide than that of sodium hydroxide (1.2− 1.6% of the total volume) stock solution is needed because of the dissociation reaction of the ammonium ion to ammonia (reverse of reaction 1), which would add an extra hydrogen ion, compounded by the escape of gaseous ammonia molecules from solutions. The release of extra hydrogen would lower the pH, unless additional hydroxide ions are added, leading to a larger consumption of ammonium hydroxide than sodium hydroxide to reach the same alkaline pH. Despite a large amount of ammonium hydroxide addition than sodium hydroxide addition to reach pH 11.3, the bitumen liberation kinetics is essentially the same for these two caustics, indicating that it is the pH or concentration of hydroxide ions and not specific ions that determine bitumen liberation kinetics. Increasing pH is known to increase bitumen displacement for both model oil sands and real ores.13,18,19 Takamura and Chow suggested that, at higher pH values, the bitumen and sand surfaces are more negatively charged, leading to a stronger repulsive force and, hence, enhanced bitumen liberation.20 Increasing pH also increases ionization of natural surfactant at the bitumen−water interface, reducing its interfacial tension and, hence, enhancing bitumen liberation. In addition, increasing pH would release more natural surfactant from bitumen, leading to a higher concentration of natural surfactant in solution and, hence, at the bitumen−water interface, favoring bitumen liberation. Furthermore, increasing pH leads to increased hydrolysis and ionization of hydroxyl groups on sand grains and clay surfaces, as shown in Scheme 1, which leads to more hydrophilic surfaces and, hence, enhances bitumen liberation.



RESULTS AND DISCUSSION Bitumen Liberation. The clear still images obtained with our novel BLFVC, as shown in Figure 1, allow us to study how the various caustic additions affect bitumen liberation. When bitumen receding on sand grains in sequential images are analyzed, the degree of bitumen liberation defined in eq 2 can be obtained as a function of time. The liberation tests were conducted at a set temperature of 35 °C, and pH 8.5 and 11.3. Two ores were studied using the in situ BLFVC. As shown in Table 1, these two ores were very similar, with the biggest difference being in the fines content. The effect of different caustic addition and pH on the bitumen liberation kinetics for these two ores is shown in Figures 2 and 3. These two figures

Figure 2. Bitumen liberation from A1 ore at 35 °C using different caustics as pH modifiers in process water.

Scheme 1. Effect of Increasing pH on Surface Hydrolysis and, Hence, Surface Hydrophilicity and Charge of Silica (Sand) Surfaces

Figure 3. Bitumen liberation from C ore in process water at 35 °C using different caustics as pH modifiers.

It is also interesting to note a slower bitumen liberation kinetics and lower overall degree of bitumen liberation for A1 ore than for C ore, as a result of increasing fines content in the ores. This finding suggests that the presence of excessive fines is detrimental to bitumen liberation, even if the liberation is conducted at the same pH. The exact reason as to why fines content has a negative impact on bitumen liberation remains to be explored.

show a rapid liberation of bitumen from sand grains, mostly occurring within the first 50 s. The subsequent increase in the liberation time to 500 s increased the degree of bitumen liberation by only approximately 10%. For both ores at a given pH of either 8.5 or 11.3, the trend of quick initial bitumen displacement is similar for both sodium hydroxide and ammonium hydroxide. 434

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Bitumen Flotation. To understand the role of bitumen liberation in bitumen flotation kinetics, bitumen flotation tests were performed using a Denver flotation cell. In the Denver cell, the impeller provides the mechanical agitation, which is absent in an in situ bitumen liberation flow visualization cell (FIBLVC) used to study bitumen liberation. The flotation tests were conducted at slurry pH of 8.5 and 11.3, adjusted using sodium or ammonium hydroxide. The process temperature was kept constant at 35 °C, the same temperature as used in bitumen liberation tests. The results of bitumen recovery experiments are shown in Figures 4 and 5 for A1 ore and C ore, respectively. The error bars are the typical relative errors.

In contrast, when the pH was increased by ammonium hydroxide to 11.3, a much significant improvement in bitumen recovery was observed for both ores, from 30 to 85% for A1 ore and from 30 to 55% for C ore. Considering that the only difference between the sodium and ammonium hydroxide addition is the counterions of sodium versus ammonia and both exhibited similar improvement on bitumen liberation, this finding clearly suggests the importance of counterions in caustics to bitumen aeration and, hence, bitumen recovery by flotation. It appears that ammonium counterions are more effective than sodium counterions in neutralizing the surface charge of bitumen and air bubbles. This leads to a less reduction in bitumen surface hdyrophobicity by ammonium hydroxide addition than by sodium hydroxide addition, thereby exhibiting higher bitumen recovery. This hypothesis will be investigated below by measuring ζ potential, induction time, and natural surfactant concentration released during bitumen extraction at these two different pH values modified with sodium and ammonium hydroxide. Induction Time Measurement. To better distinguish the role of bitumen liberation and aeration in bitumen recovery, the induction time of bitumen−air bubble attachment was measured. To investigate how different caustics affect the bitumen and air bubble attachment (aeration) in process water the pH of solutions used in induction time measurement was adjusted using either sodium or ammonium hydroxide, as done in bitumen liberation and flotation tests. In this study, vacuum distillation unit feed bitumen was used in the induction time measurement. The results of the induction time measurement are shown in Figure 6. As in the case of bitumen liberation and

Figure 4. Effect of pH modifiers, sodium and ammonium hydroxide, on bitumen recovery of A1 ore in process water.

Figure 5. Effect of pH modifiers, sodium and ammonium hydroxide, on bitumen recovery of C ore in process water.

Figure 6. Effect of the caustic type, sodium or ammonium hydroxide, on the induction time of an air bubble attaching to a bitumen surface in process water.

For both ores at pH 8.5, adjusting pH using either sodium or ammonium hydroxide showed a negligible effect on bitumen recovery. This observation is not unexpected, because only a very small amount of caustics was needed to reach this pH, as discussed in the Bitumen Liberation section above. Increasing pH from 8.5 to 11.3 by sodium hydroxide addition showed a marginal increase in bitumen recovery for A1 ore, but depressed bitumen recovery from 30 to 10% for C ore. This is in great contrast to the improved bitumen liberation for both ores by sodium hydroxide addition to increase pH from 8.5 to 11.3. Although puzzling, this opposite effect of sodium hydroxide addition on bitumen recovery for different ores illustrates the critical importance of controlling caustic addition. The presence of optimal sodium hydroxide addition for bitumen recovery is well-documented in the literature.4,6

flotation, there exists essentially no difference in the induction time of an air bubble attaching to a bitumen surface at pH 8.5 for two different pH modifiers. In contrast, the induction time increased significantly from 800 to 2000 ms when pH of solution was increased from pH 8.5 to 11.3 by sodium hydroxide, corresponding to a significant reduction in bitumen recovery from 30 to 10% for C ore, despite an increase in overall bitumen liberation and bitumen liberation kinetics with the same pH increase. However, when ammonium hydroxide was used to increase pH from 8.5 to 11.3, a significant reduction in the induction time from 800 to 450 ms was observed. Such a reduction in the induction time corresponds well with an observed increase in bitumen recovery from 30 to 85% for A1 ore and from 30 to 435

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55% for C ore. Clearly the observed opposite effect of pH adjustment using sodium and ammonium hydroxide on bitumen recovery is mainly by their effect on bitumen aeration, because both improve bitumen liberation kinetics to a similar level. This finding leads us to study the effect of different caustic addition on the wettability of bitumen by measuring the contact angle of an air bubble on bitumen in water of different pH values adjusted with either sodium or ammonium hydroxide. Contact Angle Measurement. The angle of the bubble (θb) attaching to a bitumen surface in process water was determined using a drop shape analyzer (DSA, Kruss). For contact angle measurement, bitumen was spread onto a circular Teflon disc and placed in a rectangular glass container. Solutions of different pH were prepared using process water and sodium or ammonium hydroxide as pH modifiers. The bitumen in the rectangular cell was allowed to equilibrate with the process water of the desired pH for 20 min. An air bubble generated by a micro syringe was then brought in contact with the bitumen surface. Once attached, the syringe was removed from the bubble, while the images of the bubble were recorded for the subsequent contact angle measurement between bitumen and the air−water interface using the DSA software. This angle was then subtracted from 180° to obtain the contact angle measured through the solution phase. The contact angle measured as such could be considered as a static receding contact angle. The increase in pH from 8.5 to 11.3 by sodium hydroxide addition caused a significant decrease in contact angle from 58° to 38°. This reduction in contact angle is responsible for the observed increase in induction time of bubble−bitumen attachment and, hence, the reduction in bitumen recovery. In contrast, the increase in pH of process water from 8.5 to 11.3 by ammonium hydroxide addition did not cause any noticeable change in contact angle (only from 58° to 53°). Despite a negligible change in contact angle with increasing pH from 8.5 to 11.3 by ammonium hydroxide addition, the induction time of bitumen−air bubble attachment decreased. This finding suggests an important role of surface charge in liquid film drainage and, hence, induction time for a given surface wettability, although the surface charge also has a significant impact on surface wettability. At pH 11.3, the presence of a high concentration of ammonium counterions neutralizes a negative surface charge at the bitumen−water interface, compensating for the effect of increased ionization of the natural surfactant at higher pH. Zeta (ζ) Potential Measurement. The ζ potential of both bitumen droplets and air bubbles was measured using a microelectropheresis-based zetaphoremeter (CAD, Paris, France) to investigate how different caustics affect the surface charge and its role in bitumen liberation and aeration. Bitumen emulsions and air bubble dispersions were prepared using the similar method by Liu et al. and Wu et al., respectively.21,22 The results of the ζ potential measurements at pH 8.5 adjusted using both sodium and ammonium hydroxide solutions were similar at −36 mV, as anticipated because of the limited addition of caustics. The ζ potential values obtained in this study agree well with the values reported in the literature for bitumen emulsion and air bubble dispersion, with pH being adjusted using sodium hydroxide.21,23 The results of ζ potential measurement for the bitumen emulsions and air bubble dispersions at pH 11.3 are summarized in Table 2. The measured ζ potential values for bitumen emulsions agree well with literature values when sodium hydroxide was used as pH

Table 2. Zeta (ζ) Potential of Bitumen Emulsions and Air Bubbles at pH 11.3 in Tailings Water ζ potential (mV) bitumen emulsions

bubble dispersions

pH modifiers

C ore

A1 ore

air

NaOH NH4OH

−80.4 −73.3

−79.4 −71.9

−38.8 −22.4

modifier.21 For all of the cases, the ζ potential was more negative at pH 11.3 than at pH 8.5 for the bitumen droplets, suggesting increased adsorption and/or ionization of the natural surfactant at the bitumen−water interface. ζ potential measured at pH 11.3 with sodium hydroxide as the pH modifier was more negative than that with ammonium hydroxide as the pH modifier, suggesting either suppressed release of the natural surfactant in process tailings water or suppressed ionization and charge neutralization of the natural surfactant at the bitumen− water interface by an increased concentration of ammonium cations. The ζ potential of air bubbles in solution also became more negative with increasing pH to 11.3 when using sodium hydroxide as the pH modifier. This corresponds well with literature data and is a result of increased ionization with increasing pH of the natural surfactant present in the process water and, hence, at the air−water interface. It can also be seen that, at pH 11.3, the ζ potential of air bubbles is less negative with ammonium hydroxide as the pH modifier than with sodium hydroxide as the pH modifier. Again, an increased concentration of ammonium counterions played a significant role in reducing the charge of the natural surfactant at the air− water interface. At this elevated ammonium ion concentration (∼35 mM), the well-known Hofmeister effect of ammonium ions may be at play to exhibit specific adsorption at air−water and/or bitumen−water interfaces. Regardless of the working mechanism of ammonium ions, reducing this ζ potential by almost half, compounded with the reduction in the ζ potential of the bitumen−water interface, will greatly reduce the electrical repulsion between bitumen and air bubbles, enhancing drainage of the liquid film between the air bubble and the bitumen surface, which, in turn, significantly reduces the induction time of bitumen−air bubble attachment, leading to a great chance of bitumen−air bubble attachment. Overall, the less negative ζ potential of bitumen and air bubbles at pH 11.3 with ammonium hydroxide as the pH modifier compared to the case of sodium hydroxide as the pH modifier leads to a greater bitumen recovery, as seen in the Denver cell flotation experiments. Surface Tension Measurement. To evaluate the effect of different caustics on natural surfactant release, the surface tension of tailings water was determined. The tailings water was obtained by extracting bitumen from A1 ore and C ore using a Denver flotation cell. Ammonium hydroxide and sodium hydroxide were used as pH modifiers to adjust the pH of the process water to the desired values of 8.5 and 11.3. The surface tension of process water or tailings water was measured using a Kruss K12 processor tensiometer. The results are summarized in Table 3. At pH 11.3, the surface tension of tailings water with sodium hydroxide as the pH modifier is lower than that with ammonium hydroxide as the pH modifier. The lower surface tensions can be attributed to the release of a greater amount of natural surfactants from the bitumen during bitumen 436

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similar surface wettability and surface charge of bitumen and surface tension of tailings water as if no pH modifiers were added. The increase in bitumen recovery when using ammonium hydroxide at pH 11.3 for both A1 and C ores can be attributed to both increased bitumen liberation and reduced induction time of bitumen−air bubble attachment as a result of specific interaction of cationic ammonium counterions at increased concentration, neutralizing the surface charge of bitumen− water and air−water interfaces. Because a greater amount of ammonium hydroxide is needed to raise the pH to 11.3, there is a possibility for the slurry to become supersaturated with ammonia, leading to in situ ammonia gas nucleation on hydrophobic bitumen surfaces. The presence of gas nuclei is known to enhance bitumen aeration25,26 and, hence, to contribute to the observed increase in bitumen recovery. However, the agitation of the slurry may introduce CO2 from entrained air into the slurry and, hence, reduce the pH of the slurry. As shown in eq 1, the reduction in pH, even though small, may break the equilibrium between ammonium ions and ammonia in the right direction to make the slurry unlikely to be supersaturated by ammonia. Regardless of the mechanisms of action, technically, ammonium hydroxide is more favorable than sodium hydroxide as a process aid to improve bitumen recovery from oil sands ores.

Table 3. Surface Tension of Tailings Water with (at pH 11.3) and without Caustic Addition surface tension (mN/m) pH modifier

C ore

A1 ore

NaOH NH4OH no caustic

49.3 56.8 65.9

39.7 45.2

extraction.24 The higher concentration of the natural surfactant in tailings water of bitumen extraction using sodium hydroxide as the pH modifier than using ammonium hydroxide as the pH modifier would lead to a higher coverage of the natural surfactant at bitumen−water and air−water interfaces, making both more negatively charged and bitumen less hydrophobic, which was observed in ζ potential and contact angle measurements, accounting for the longer induction time and lower bitumen recovery with sodium hydroxide as the pH modifier as compared to the case of ammonium hydroxide as the pH modifier. Release of Natural (Carboxylic) Surfactants. To confirm the increased release of natural surfactants by caustic addition, in particular when sodium hydroxide was used as the pH modifier, the concentration of natural surfactants, mainly carboxylic surfactants, in tailings water and original process water, was determined by solvent extraction and the FTIR analysis method. The results obtained from the extraction tests at pH 11.3 are shown in Table 4. From this table, it can be seen



CONCLUSION In this study, the role of two different caustics, sodium and ammonium hydroxide, in bitumen extraction was investigated by determining their effect on bitumen liberation and aeration. From this study, the following conclusions were drawn: (1) Both sodium and ammonium hydroxide addition to pH 8.5 and 11.3 improved bitumen liberation similarly for both ores (A1 and C ores). (2) Increasing pH by sodium hydroxide to pH 11.3 either decreased bitumen recovery (A1 ore) or showed no effect on bitumen recovery (C ore). (3) A large increase in bitumen recovery was seen on both A1 and C ores by increasing pH to 11.3 when ammonium hydroxide was used. (4) In comparison to sodium hydroxide addition, increasing pH to 11.3 by ammonium hydroxide addition led to a shorter induction time, a more hydrophobic bitumen surface, a less negative surface charge of bitumen and air bubbles in tailings water, a higher surface tension of tailings water, and a decreased release in the natural surfactant, accounting for the observed increase in bitumen recovery. (5) This study clearly shows that technically ammonium hydroxide is a viable alternative for sodium hydroxide as the bitumen extraction process aids. (6) Ammonium hydroxide can be readily integrated into in situ thermal bitumen production methods, such as steam-assisted gravity drainage (SAGD), to improve bitumen recovery by enhanced bitumen liberation because ammonia can be readily delivered with steam to the oil sands formation.

Table 4. Carboxylic Surfactant Concentration in Tailings Water with (at pH 11.3) and without Caustic Addition, Analyzed Using FTIR carboxylic surfactant concentration (ppm) pH modifier

C ore

A1 ore

NaOH NH4OH no caustic

64.0 52.3 50.4

74.5 54.9 NA (not available)

that, for both cases, adjusting pH using sodium hydroxide released a higher concentration of surfactants into the solution than that using ammonium hydroxide. At such a high pH of 11.3, these natural surfactants carry a negative charge. An increased adsorption of these negatively charged natural surfactants at bitumen−water and air−water interfaces with increasing their concentration by increasing pH using sodium hydroxide leads to a more negative ζ potential of the bitumen− water and air−water interfaces. A more negative ζ potential increases the repulsive electrical double layer forces between bitumen and air bubbles, thus increasing film drainage resistance and, hence, induction time. Such an increase in the induction time with increasing pH by sodium hydroxide addition leads to a significantly lower bitumen flotation recovery, as seen in Figures 4 and 5, despite its increase in bitumen liberation kinetics, as shown in Figures 2 and 3. In summary, we can conclude that the similar bitumen recovery at pH 8.5 using either sodium or ammonium hydroxide as the pH modifier can be attributed to a similar degree of bitumen liberation and bitumen−air bubble attachment. These findings can be attributed to the small amount of the caustic at approximately 1 mL of either sodium or ammonium hydroxide stock solution added to 1 L of process water needed to adjust the process water to pH 8.5, leading to a



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The financial support for this work from the Natural Sciences and Engineering Research Council of Canada under Industrial 437

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(20) Takamura, K.; Chow, R. S. A mechanism for initiation of bitumen displacement from oil sand. J. Can. Pet. Technol. 1983, 22, 22−30. (21) Liu, J.; Zhou, Z.; Xu, Z.; Masliyah, J. Bitumen−clay interactions in aqueous media studied by zeta potential distribution measurement. J. Colloid Interface Sci. 2002, 252, 409−418. (22) Wu, C.; Nesset, K.; Masliyah, J.; Xu, Z. Generation and characterization of submicron size bubbles. Adv. Colloid Interface Sci. 2012, 179, 123−132. (23) Elmallidy, A. M.; Mirnezami, M.; Finch, J. A. Zeta potential of air bubbles in presence of frothers. Int. J. Miner. Process. 2008, 89, 40− 43. (24) Schramm, L. L.; Smith, R. G. Influence of natural surfactants on interfacial charges in the hot-water process for recovering bitumen from the Athabasca oil sands. Colloids Surf. 1985, 14, 67−85. (25) Zhou, Z. A.; Xu, Z.; Finch, J. A.; Masliyah, J. H.; Chow, R. S. On the role of cavitation in particle collection in flotationA critical review. II. Miner. Eng. 2009, 22, 419−433. (26) Zhou, Z. A.; Chow, R. S.; Cleyle, P.; Xu, Z. H.; Masliyah, J. H. Effect of dynamic bubble nucleation on bitumen flotation. Can. Met. Q. 2010, 49, 363−372.

Research Chair program in Oil Sands Engineering is gratefully acknowledged. The authors thank Syncrude Canada for supplying the oil sands ore samples, bitumen samples, and process water. The authors would also like to thank Dr. Einar Eng Johnsen of Statoil (Trondheim, Norway) for his invaluable discussions and technical advice throughout the course of this study.



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