Effect of Divalent Cations and Surfactants on SilicaBitumen Interactions

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Effect of Divalent Cations and Surfactants on Silica-Bitumen Interactions Hongying Zhao, Jun Long, Jacob H. Masliyah, and Zhenghe Xu* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6

In oil sands processing, the separation of bitumen from sand grains is controlled by interfacial interactions between the bitumen and the sand. In this study, the effects of calcium and magnesium cations, surfactants, and their combination on the sand-bitumen interactions and consequently on bitumen liberation were investigated. Surface forces between silica and bitumen in industrial-plant process water and in water containing calcium and magnesium in amounts equivalent to those found in the plant process water were directly measured using an atomic force microscope. Zeta potential measurements were carried out for silica, bitumen, and their mixture to study the coagulation behavior between silica and bitumen. It was found that divalent cations are detrimental to bitumen liberation from sand grains because they decrease the long-range repulsive force and increase the adhesion force between silica and bitumen. On the other hand, surfactants were found to facilitate bitumen liberation by reducing silica-bitumen adhesion. 1. Introduction Interaction forces between colloidal particles/droplets are central to the fundamental understanding of various processes involved in many disciplines such as biotechnology, pulp and paper technology, and mineral processing. For example, in oil sands processing, the separation of bitumen from oil sands is controlled by the interactions between the bitumen and sand grains. Oil sands are unconsolidated sand deposits impregnated with high-molar-mass viscous petroleum, which is normally referred to as bitumen.1 Over 1.7 trillion barrels of bitumen are locked up in Alberta’s oil sands deposits. With the depletion of conventional crude oil and the increase of worldwide oil demand, recovering bitumen from Alberta’s oil sands becomes increasingly important. To recover bitumen from oil sands, water-based extraction processes have been widely used in commercial plants.1 In such processes, lumps of oil sands are mixed with hot water and process aids to form a slurry. Bitumen is separated from the sand grains within the slurry. The liberated bitumen is aerated by entrained air bubbles and recovered as a bitumen froth by gravity separation and flotation. Typically, the average diameter of the bitumen-air aggregate flowing to the froth layer in a commercial separation vessel is 1.0 ( 0.6 mm, and the equivalent diameters of the bubbles and bitumen drops contained in the aggregate are of the same order of magnitude.2,3 In the water-based extraction of bitumen from oil sands, bitumen liberation from the sand grains and its subsequent stabilization against heterocoagulation with the sand grains or mineral fines are prerequisite for bitumen extraction. Because silica sands constitute the main minerals in oil sand ores, the successful separation of bitumen from silica would greatly enhance the bitumen recovery and lead to a high-quality froth. Because the liberation step is essentially related to the sandbitumen interactions and is affected by water chemistry, the effects of solution characteristics on bitumen liberation such as the pH and the type and concentration of cations and surfactants have been studied extensively.4-21 Using a bitumen pick-up method, Dai and Chung11 found that NaOH aids in the release of bitumen droplets from sand particles. By observing the rates of bitumen displacement and detachment from glass plates in * To whom correspondence should be addressed. Tel.: (1-780) 4927667. Fax: (1-780) 492-2881. E-mail: [email protected].

model systems, Basu et al.4,7 found that the rate of bitumen displacement on the glass surface is higher at low pH, whereas the equilibrium contact angle of bitumen is greater at high pH. On the basis of these findings, they suggested that a pH cycle might be considered to facilitate bitumen liberation. Bitumen liberation was also found to be dependent on operating temperature.7,9,11,14 A higher temperature not only decreases the viscosity of bitumen and increases surface charge of sand grains and bitumen, but also reduces the adhesion force between the sand grains and the bitumen.7,11,14 The presence of both calcium and clays, as observed by Basu et al.,6 has a negative impact on bitumen liberation from sand grains. Using an atomic force microscope (AFM), Liu et al.13 found that calcium ions could depress the long-range repulsion and increase the adhesion between silica and bitumen in prepared solutions. Takamura and Chow20 found that surfactants facilitate the initiation of bitumen displacement from oil sands. Schramm et al.18,19 found that bitumen recovery is related to the concentration of surfactants in solution and that there is an optimal surfactant concentration at which a maximum bitumen recovery can be obtained. Surfactants generated from bitumen or present in bitumen are believed to facilitate bitumen recovery by lowering the interfacial tensions and increasing the surface charges of various components in oil sands slurries.15-18,22 Drelich and Miller12 found that the lower the bitumen-water interfacial tension, the higher the bitumen recovery. A synergetic effect of various surfactant constituents on the coagulation of silica and bitumen was found by Zhou et al.21 Rowe et al.23 and Basu et al.8 found that surfactants, including octylphenol ethoxylate (Triton X-100), sodium dodecyl sulfate (SDS), and a mixture of methylisobutyl carbinol (MIBC) and kerosene, could enhance bitumen displacement by water on a glass surface. It should be noted that most of the existing studies on silicabitumen interactions and bitumen liberation, as discussed above, were conducted primarily in laboratory-simulated aqueous solutions rather than in plant process water. However, a few investigations have indicated that bitumen liberation in plant process water might differ significantly from that in simple electrolyte solutions. For example, Walker et al.24 found the receding rate of bitumen from a glass surface is low in prepared calcium solutions but high in industrial-plant process water containing equivalent amounts of calcium ions. The industrial-

10.1021/ie060348o CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7483 Table 1. Properties of the Aurora Plant Process Water and the Foam and Residual Solutions property

process water

foam solution

residual solution

inorganic ion (mM) K+ Na+ Mg2+ Ca2+ ClNO3SO42HCO3pH conductivity (mS/cm) surface tension, γ (mN/m)

0.37 22 0.79 1.2 12 0.03 0.66 11 7.8-8.5 2.1 ( 0.2 63.4-68.9

0.36 21 0.75 1.2 11 0.03 0.63 10 8.7-9.1 2.1 ( 0.2 44.0-56.5

0.35 21 0.79 1.2 12 0.03 0.64 11 9.2-9.5 2.1 ( 0.2 64.2-70.7

plant process water, which contains not only electrolytes but also hydrocarbons and, in particular, natural surfactants, is used in industrial bitumen extraction processes. Therefore, it is important to study the interactions between silica and bitumen not only in simple electrolytes solutions but also in industrialplant process water. From the above discussion, one notes that studies on the interactions between bitumen and sands by direct force measurement are scarce, and most existing studies on bitumen liberation were not carried out in industrial process water. The effects of various species present in industrial process water, such as divalent cations and natural surfactants, individually or synergetically, on solids-bitumen interactions and thus on bitumen liberation have not been investigated systemically. Consequently, the mechanism of bitumen liberation in the context of industrial operations is not well understood. To that end, in the present study, we measured the interaction forces between silica and bitumen in industrial-plant process water and various electrolyte solutions. To study the effects of surfactants on the silica-bitumen interaction, we used foam fractionation to separate industrial-plant process water into surfactant-enriched foam and residual solutions in which the interaction forces between silica and bitumen were measured. To complement the results of the force measurements, zeta potential distributions of bitumen, silica, and their mixture were measured and used to infer the coagulation behavior of silica and bitumen. The objective was to understand the effects of divalent cations, surfactants, and their combination on silica-bitumen interactions and thus on bitumen liberation. The general goal was to provide new insights into the mechanisms of bitumen liberation. The effects of other electrolytes such as sodium, chloride, sulfate, and bicarbonate will be considered in a separate study. 2. Experimental Section 2.1. Materials. The industrial-plant process water used in the current study was recycle pond water from the Aurora plant of Syncrude Canada Ltd. (Fort McMurray, Canada). The main characteristics of the plant process water are listed in Table 1. Vacuum-distillation-feed bitumen (Syncrude Canada Ltd., Fort McMurray, Canada) was used to prepare bitumen surfaces and bitumen suspensions. Silica sands of 5-µm diameter provided by U.S. Silica Co. (Berkeley Springs, WV) were used to prepare silica suspensions for zeta potential distribution measurements. Silica microspheres of 8-µm diameter were purchased from Duke Scientific Co. (Fremont, CA) and used as model sand grains for colloidal force measurements. Silica wafers (Silicon Valley Microelectronics Inc., Santa Clara, CA) were used as substrates to prepare bitumen surfaces for the force measurements. Ultra-high-purity KCl (>99.999%, Aldrich) was used

Figure 1. Schematic of the foam fractionation setup.

as the supporting electrolyte. All other salts and solvents were of reagent grade and were purchased from Fisher Scientific Co. (Ottawa, Canada). Reagent-grade HCl and NaOH were used as pH modifiers. CaCl2‚2H2O (99.9965%) and MgCl2‚6H2O (ca. 100%) were used as sources of divalent cations. Toluene was used as the solvent in the preparation of bitumen solutions. Chloroform and ethanol were used as cleaning solvents. Deionized water with a resistivity of 18.2 MΩ cm was prepared with an Elix 5 apparatus followed by a Millipore-UV Plus Ultra water purification system (Millipore Ltd., Willowdale, Canada) and was used throughout this study where applicable. 2.2. Foam Fractionation. Foam fractionation was performed to obtain surfactant-enriched foam solutions from the plant process water to study the effect of natural surfactants present in the industrial process water on the interactions between silica and bitumen. The technique of foam fractionation has been reported in the literature.17,25-27 A schematic of the foam fractionation setup is shown in Figure 1. The fractionation column was a 150-mL cylindrical vessel (26 mm in diameter) equipped with a gas distributor, which was a glass filter (ACE Glass Inc., Toronto, Canada) with a pore size of 10 µm. The gas distributor was placed slightly above the bottom of the vessel and connected to a water flask. For each foam fractionation test, 100 mL of the plant process water was placed into the column. Pure nitrogen (99.99% v/v) saturated with deionized water in the flask was introduced into the column through the gas distributor at the bottom of the column. Gas bubbles with diameters of several micrometers were generated by gas dispersion. As surface-active compounds tend to adsorb at the gas-liquid interface, the surfactants present in the plant process water accumulated at the gas bubble-liquid interface. After a bubbling period of about 15 min, a stable foam phase was formed at the top of the column. Then, the aeration rate was increased slightly (starting from 120 cm3/min) to let the foam slowly overflow from the column at a foam depth of 4 cm to minimize water carry-over. The foam was first collected to a total volume of 20 mL of liquid, which was used as the foam solution. The bubbling was continued until the volume of the liquid remaining in the column was about 20 mL; this liquid was collected and used as the residual solution. This foam fractionation process was repeated several times to collect sufficient foam and residual solutions for subsequent analysis and relevant measurements. 2.3. Solution Characterization and Contact Angle Measurement. In this study, the plant process water and the foam and residual solutions were characterized by a variety of analytical techniques. The concentrations of inorganic cations were analyzed by atomic absorption spectrometry. The con-

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centration of HCO3- was measured by acid-base titration. Other inorganic anions were analyzed by ion chromatography. The surface tensions of various liquids were determined by the ring method28 using an automatic tensionmeter K12 (KRUSS USA, Charlotte, NC). Static contact angles were measured using a captive-bubble method or sessile-drop method28 with a drop shape analysis instrument DSA10 (KRUSS USA, Charlotte, NC). 2.4. Zeta Potential Distribution Measurements. 2.4.1. Sample Preparation. A stock silica suspension was prepared by mixing 1 g of silica particles (5 µm in diameter) with 100 mL of 1.0 mM KCl solution. A stock bitumen emulsion was prepared by emulsifying 1 g of bitumen in 100 mL of 1.0 mM KCl solution for about 15 min using a 550 sonic dismembrator (Fisher Scientific Co.). The emulsion was allowed to cream29 for about 2 h. The average diameter of the bitumen droplets prepared as such was determined with a Mastersizer instrument (Malvern Instruments, Inc., San Bernadino, CA) to be around 8 µm, which is in the range of values reported by Liu et al.30 For each zeta potential distribution measurement, a dilute suspension of silica or bitumen was prepared by adding several drops of the stock silica suspension or bitumen emulsion to 50 mL of the solution of interest. The dilute suspension contained about 0.01-0.1 wt % of bitumen or silica and was stirred with a magnetic mixer for 5 min. A silica-bitumen mixture was prepared by mixing equal volumes of their respective dilute suspension and emulsion. The mixture was conditioned in an ultrasonic bath (Fisher) for 10 min. 2.4.2. Zeta Potential Distribution Measurements. The distribution histogram of the zeta potential and the corresponding average value were measured using a Zetaphoremeter III apparatus (SEPHY-CAD Instrumentations, France), which determines the rate at which these particles move in a known electric field. The measured mobility of these particles is converted to zeta potential by the instrument software using the Smoluchowski equation.31 In this article, for each sample, the measurement was repeated three times at room temperature (22 ( 0.1 °C). The average value and standard deviation are reported on the basis of the three measurements. 2.4.3. Interpretation of Zeta Potential Distributions. In the case of a binary mixture, the measured zeta potential distributions of the individual components and their mixture can be used directly to study particle interactions and the slime coating phenomenon.30,32 This concept was developed and illustrated in detail by Liu et al.30 For the system of silica and bitumen, the zeta potential distributions of the bitumen droplets and silica particles are measured individually and assumed to be centered at different zeta potential values. When the bitumen droplets and silica particles are mixed together under the same physicochemical conditions, the measured zeta potential distribution of the mixture can be interpreted in terms of the interactions between the two components. A bimodal zeta potential distribution would be anticipated if silica and bitumen do not coagulate with each other. Only one peak would appear for the zeta potential distribution if silica and bitumen strongly interact with each other. 2.5. Atomic Force Microscopy (AFM). 2.5.1. Substrate Surface and Probe Preparation. Bitumen surfaces were prepared by coating a thin layer of bitumen on a silica surface with a WS-400A-6NPP/LITE spin coater (Laurell Technologies Co., North Wales, PA). Silica wafers (14 × 14 mm2) were used as the substrate for bitumen coating after being washed with chloroform to remove adsorbed organic contaminants, rinsed with deionized water and then ethanol, and finally blow-dried

with ultra-pure-grade nitrogen. Bitumen was dissolved in toluene at a concentration of 0.3 wt %. The bitumen solution was centrifuged at 45 000g force for 30 min to remove fine solids entrapped in the bitumen. About 0.05 mL of the solids-free bitumen-in-toluene solution was placed slowly on a cleaned silica wafer spinning on the spin-coater at 2000 rpm for 20 s and then 5000 rpm for 1 min. The spin-coated bitumen was dried for more than 12 h in a particle-free horizontal laminar hood. From the volume of bitumen solution used in spin-coating and the area of the wafer coated, the thickness of the prepared bitumen films was estimated to be less than 100 nm. The surface of the prepared bitumen film was found to be acceptably smooth for colloidal force measurements.13 To measure the colloidal forces, gold-coated silicon nitride cantilevers from Digital Instruments (Santa Barbara, CA) were used. Cantilevers with a width of 100 µm and a spring constant of 0.58 N/m were chosen for the force measurements. A colloidal silica sphere of 8-µm diameter was attached to the cantilever at a location close to the apex using an extremely small amount of epoxy resin.13 Prior to each set of force measurements, the prepared probes were thoroughly rinsed with deionized water and ethanol and then blow-dried with ultrapure nitrogen. The probes were then exposed to an ultraviolet light for more than 5 h to remove any possible organic contaminants. The exact size of the silica probes was determined by scanning electronic microscopy (SEM) after the force measurements. 2.5.2. Colloidal Force Measurements. The interaction forces between a bitumen surface and a silica probe in various liquids were measured using a Nanoscope E AFM instrument (Digital Instruments, Santa Barbara, CA) with a vendor-provided liquid cell. Prior to the force measurements, both the probe and bitumen surfaces were immersed in a test liquid in the fluid cell and allowed to equilibrate for at least 30 min. For each liquid tested, the force measurement was repeated with at least three new silica probe and bitumen surface pairs. For each pair, the measurements were performed at more than five different points on the bitumen surface. For quantitative comparisons, the measured long-range interaction force (F) and the adhesion force (Fadh) were normalized by the probe radius (R). The maximum loading force used in the force measurements was in the range of 5.5-7.7 nN. All experiments were conducted at room temperature (22 ( 1 °C). 2.5.3. AFM Force Curve Interpretation. In AFM force measurements, each force profile obtained represents a complete extension-retraction cycle of the AFM piezo. When a sample surface approaches a probe, the long-range interaction force between the two surfaces is measured, and the adhesion (or pulloff) force can be obtained during the retraction process after contact has been made. To determine the dominant long-range forces between silica and bitumen in an aqueous medium, the classical DLVO theory, which considers only the electrostatic double-layer and van der Waals forces,33,34 was employed to analyze the measured longrange interaction forces. The van der Waals forces are calculated by Hamaker’s microscopic method in the form of eq 1, given below for a sphere of radius R interacting with a flat surface at a distance D.33 Using Hamaker constants of 6.5 × 10-20, 6.0 × 10-20, and 3.7 × 10-20 J for silica, bitumen, and water, respectively, the combined Hamaker constant for the bitumen/ water/silica system was calculated to be 3.3 × 10-21 J.33 Using the Derjaguin approximation, the electrical double-layer force, given by eq 2, was calculated numerically by solving the nonlinear Poisson-Boltzmann equation (eq 3) with boundary conditions of constant surface potentials for both surfaces. A

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Visual Basic program running on an Excel spreadsheet, developed by Liu et al.,13 was used for the theoretical calculation of the DLVO forces. In the calculations, the Debye-Huckel theory33 was used to evaluate the decay length, and the measured zeta potential values were used as an approximation of the surface potentials.

Fv A )- 2 R 6D FE R

{

) -2π kBTni∞

0

d2ψ dx2

∑i

(1)

[ ( )] ∑ ( ) 1 - exp

- zieψ

-

kBT

( )}

0 dψ 2

2

dx

(2)

-zieψ

) -e

zini∞ exp

i

kBT

(3)

where Fv and FE are the van der Waals and electrical doublelayer forces, respectively; R is the radius of a probe sphere; A is the Hamaker constant; D is the distance between surfaces;  and 0 are the permittivities of the medium and vacuum, respectively; e is the elementary charge; z is the valence of the electrolyte; ni∞ is the bulk electrolyte concentration; kB is the Boltzmann constant; ψ is the electric potential; and T is the temperature. 3. Results and Discussion 3.1. Characteristics of the Plant Process Water and the Foam and Residual Solutions. To understand the role of water chemistry, especially the role of divalent cations and surfactants, in bitumen liberation, it is essential to know the properties of the plant process water. As shown by the results in Table 1, calcium and magnesium ions were the major divalent cations present in the process water, at approximately 1.2 and 0.8 mM, respectively. The pH value of the plant process water was in the range of 7.8-8.5. The conductivity of the process water was approximately 2.1 mS/cm. Sodium ions were present at the highest concentration among inorganic ions (about 22 mM). Except for chloride ions, bicarbonate was present at the highest concentration (about 11 mM) among anions. The surface tension of the process water was in the range of 65-69 mN/m, which is slightly lower than that of pure water (72.6 mN/m). The chemistry of the foam and residual solutions obtained from the plant process water is also included in Table 1. The concentrations of inorganic ions present in the foam and residual solutions are nearly the same as those in the plant process water, as anticipated. However, the surface tension of the foam solution, 44.0-56.5 mN/m, is much lower than that of the plant process water. This finding indicates that surface-active compounds do exist in the plant process water and can be concentrated by foam fractionation. This is in agreement with the findings of Schramm et al.15-18 The surface tension of the residual solution was about 64.2-70.7 mN/m. It was also found that the average pH value of the foam solution was about 8.9, which is slightly higher than that of the process water but lower than that of the residual solution (∼9.3). Because of the complex nature of the process water, the exact reasons resulting in such a difference in pH values are not clear. 3.2. Results of Zeta Potential Distribution Measurements. To study the coagulation behavior of silica and bitumen, the zeta potential distributions of bitumen, silica, and their mixture were measured in the prepared electrolyte solutions, the plant

Table 2. Average Zeta Potentials (ζ) of Silica and Bitumen in Various Liquids solution

bitumen (ζB)

silica (ζS)

pH

1 mM KCl 1 mM KCl + 1 mM CaCl2 process water foam solution residual solution

-65.0 ( 0.5 -34.5 ( 0.5

-54.0 ( 0.7 -24.5 ( 0.4

7.9 ( 0.2 8.0 ( 0.2

-44.3 ( 0.1 -40.6 ( 0.3 -43.7 ( 0.3

-30.9 ( 1.5 -26.8 ( 0.3 -28.0 ( 0.5

8.2 ( 0.3 8.8 ( 0.3 9.3 ( 0.3

process water, and the foam and residual solutions. The average zeta potentials of bitumen droplets and silica particles in these liquids are reported in Table 2. In a 1 mM KCl solution at pH 7.9, the zeta potentials of silica and bitumen were -54 and -65 mV, respectively. When 1 mM calcium was added to the KCl solution and the pH was adjusted to 8.0, the zeta potentials of silica and bitumen increased to -25 and -35 mV, respectively, as anticipated by double-layer compression and possible specific adsorption of calcium ions. To ascertain whether this change in zeta potential is due to ionic strength or calcium adsorption, the zeta potential of silica was measured in a 9 mM KCl solution. The measured zeta potential was found to be about -45 mV. This value is more negative than that for a solution of 1 mM KCl and 1 mM CaCl2. This finding makes it clear that the less negative value of zeta potential of silica in calcium solution results from the specific adsorption of calcium ions onto the negatively charged silica surface. Compared to the prepared 1 mM calcium solution, the plant process water had a higher divalent cation concentration (about 0.8 mM magnesium and 1.2 mM calcium). However, the zeta potentials of bitumen and silica in the plant process water, -44 and -31 mV, respectively, were slightly more negative than those in the 1 mM calcium solution. The results indicate that either other negative ions adsorbed onto the surfaces of silica and bitumen or some divalent cations combined with anions present in the water, losing their affinity to the surfaces. The measured zeta potential distributions of bitumen, silica, and their mixture in the prepared solution, the plant process water, and the foam and residual solutions are shown in Figure 2. In the prepared 1 mM calcium solution, a distinct peak is observed for the mixture of silica and bitumen, as shown in the bottom graph of Figure 2a. This peak is close to that of silica alone (middle graph of Figure 2a), suggesting that the bitumen surface was coated with the silica particles. In contrast, a bimodal distribution (bottom graphs of Figure 2b-d) for the mixture of silica and bitumen was observed in the plant process water and the foam and residual solutions. This indicates that silica and bitumen did not coagulate in these liquids, even though they contained about 0.8 mM magnesium ions and 1.0 mM calcium ions. 3.3. Results of Colloidal Force Measurements. 3.3.1. Effect of Divalent Cations on Silica-Bitumen Interactions in the Prepared Solutions. The above zeta potential results indicate that divalent cations have a significant impact on the coagulation behavior between silica and bitumen in prepared electrolyte solutions. To better understand the role of divalent cations, AFM was used to quantitatively measure the interaction forces between silica and bitumen. The force measurements were carried out in 1.0 mM KCl solutions with the addition of calcium and magnesium at a pH of around 8.2. The measured longrange forces, as shown by the force profiles in Figure 3a, were monotonically repulsive. However, the repulsive forces decreased upon calcium addition (circles). The long-range repulsive forces were further depressed by the addition of both calcium and magnesium (triangles). The adhesion force between silica and bitumen, shown in Figure 3b, was zero in 1.0 mM

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Figure 2. Zeta potential distributions of emulsified bitumen, silica suspension, and their mixture in various liquids: (a) 1.0 mM KCl +1 mM CaCl2 solution at pH 8.5, (b) plant process water at pH 8.3, (c) foam solution at pH 8.9, (d) residual solution at pH 9.2.

KCl solutions, but it increased to approximately 1.9 mN/m in the presence of 1.0 mM calcium and increased further to about 2.5 mN/m in the presence of both calcium and magnesium. The depression of the long-range repulsive forces and the increase of the adhesion force in the presence of divalent cations indicate that both calcium and magnesium cations have a negative impact on the separation of bitumen from silica. As shown in Figure 3a, the measured force profiles agree well with the DLVO force profiles (solid curves) at separations greater than about 10 nm. This indicates that the dominant forces between silica and bitumen in these prepared electrolyte solutions are the electrical double-layer forces. At separation distances down to several nanometers, the measured repulsive forces are much stronger than those predicted by the DLVO theory. Such a deviation could originate from a number of factors, such as steric and hydration interactions.13,35,36 3.3.2. Silica-Bitumen Interactions in the Plant Process Water and the Foam and Residual Solutions. Figure 4 compares the long-range interaction and adhesion force between silica and bitumen obtained in the plant process water with that obtained in the prepared divalent cation solution. The long-range interaction forces shown in Figure 4a are similar for the two cases. However, the average adhesion force in the plant process water is only about 0.4 mN/m, which is much smaller than that obtained in the divalent cation solution (2.5 mN/m). From the perspective of the adhesion force, these results indicate that the release of bitumen from a silica surface would be easier in the plant process water than in an electrolyte solution containing an equivalent amount of divalent cations. At a separation distance greater than 6 nm, the long-rang interaction force profile obtained in the plant process water matches the theoretical DLVO profile given by the solid curve. In the

calculation of the DLVO forces, a calculated decay length of 1.5 nm and measured zeta potentials of -44 and -31 mV for bitumen and silica, respectively, were used. When the separation distance was less than 6 nm but greater than 1 nm, the measured forces were much less than the calculated DLVO forces. This indicates that a deviation between the calculated DLVO forces and the measured forces exists for silica and bitumen in the plant process water. Because surfactants and other hydrocarbon compounds are present in the plant process water, the adsorption of these compounds could result in a sandwich structure between the probe and substrate. The deviation between the measured and theoretical forces could thus result from an underestimation of the separation between the silica and bitumen surfaces. To verify whether the surfactants contribute to the decrease in the adhesion force between silica and bitumen, the interactions between silica and bitumen were also studied in the foam and residual solutions. Figure 5 shows the forces measured in the foam and residual solutions. For comparison, the forces obtained in the plant process water are also included in this figure. The long-range forces between silica and bitumen in the plant process water and the foam and residual solutions (scattered curves), as shown in Figure 5a, are all repulsive albeit not strong, and there is little difference among these force profiles. Figure 5b shows the adhesion force distributions in these liquids. The smallest average adhesion force, about 0.3 mN/m, was obtained in the foam solution, whereas the largest, about 1.2 mN/m, was obtained in the residual solution. As the foam solution was surfactant-enriched, the adhesion force results indicate that surfactants indeed play a critical role in decreasing the adhesion force between silica and bitumen. 3.3.3. Effect of SDS on Silica-Bitumen Interactions in the Prepared Solutions. It is well-known that surfactants play an

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Figure 3. Interactions between silica and bitumen in various prepared solutions at pH 8.2: (a) Normalized long-range forces (F/R). Solid curves represent the DLVO force profiles calculated using Abws ) 3.3 × 10-21 J with the following calculated decay lengths (κ-1) and measured zeta potentials of bitumen (ζB) and silica (ζS): solution A (square), κ-1 ) 9.07 nm, ζB ) -65.0 mV, and ζS ) -54.0 mV; solution B (circle), κ-1 ) 6.10 nm, ζB ) -34.5 mV, and ζS ) -24.5 mV; solution C (triangle), κ-1 ) 4.54 nm, ζB ) -32.0 mV, and ζS ) -21.0 mV. (b) Adhesion force as a function of total divalent cation concentration.

Figure 4. Interactions between silica and bitumen in calcium and magnesium solution (solution C) and plant process water (solution D): (a) Normalized long-range forces (F/R). Solid curves represent the DLVO force profiles calculated using Abws ) 3.3 × 10-21 J with the following calculated decay lengths (κ-1) and measured zeta potentials of bitumen (ζB) and silica (ζS): solution C (triangle), κ-1 ) 4.54 nm, ζB ) -32.0 mV, and ζS ) -21.0 mV; solution D (circle), κ-1 )1.52 nm, ζB ) -44.3 mV, and ζS ) -30.9 mV. (b) Distributions of the adhesion force in these two liquids.

important role in bitumen recovery from oil sands.15,18 Both carboxylate and sulfate/sulfonate anionic surfactants have been found in oil sands process water.16,17 Liu et al.37 suggested that dodecylamine hydrochloride (DAH), sodium palmtitic acid (NaPa), and sodium dodecyl sulfate (SDS) are present in industrial-plant process water. To understand the role of surfactants in bitumen liberation, the effect of a negatively charged surfactant, SDS, on the interactions between silica and bitumen was studied. A series of SDS solutions was prepared by adding SDS to 1.0 mM calcium solutions. The long-rang interaction force profiles obtained are shown in Figure 6a. When the concentration of SDS was increased from 0.001 to 10 mM, the measured longrange force profiles were all repulsive and close to each other. When the SDS concentration was 0.001 mM, the force profile could be fitted reasonably well using the DLVO theory (lower solid line). The fitting parameters are reported in Table 3. When the SDS concentration was in the range of 0.001-1.0 mM, the zeta potentials of silica and bitumen in these solutions did not show any noticeable change. However, as the concentration of SDS was increased to 10.0 mM, which is slightly higher than the critical micelle concentration of SDS in electrolyte solutions (4-8 mM),38 the negative zeta potentials of bitumen and silica increased from -38 to -70 mV and from -29 to -50 mM, respectively. The more negative zeta potentials of silica and bitumen suggest a substantial adsorption of negatively charged SDS molecules on the negatively charged bitumen/silica surface, possibly through a calcium bridge between the SDS molecules and the silica surface. As shown in Figure 6a, when the SDS concentration was 10 mM, a deviation similar to that observed in the plant process water existed between the measured force

profile and the DLVO fitting curve (upper solid line) at separations of less than 8 nm. Because of the adsorption of SDS molecules and its micelles near the surfaces, this deviation between the measured and theoretical forces could result from an underestimation of the separation distance between the silica and bitumen surfaces. Figure 6b shows the normalized adhesion force as a function of the SDS concentration. The adhesion force decreased with increasing SDS addition, and it became virtually zero at an SDS concentration of 10.0 mM. This result indicates that the addition of negatively charged surfactants can decrease the adhesion force between silica and bitumen, thus facilitating the bitumen liberation process. 3.3.4. Silica-Bitumen Interactions in the Plant Process Water and Foam Solution with Calcium Addition. The above results indicate that divalent cations have a significant impact on bitumen liberation from silica in the prepared electrolyte solutions by increasing the adhesion force and decreasing longrange forces between the silica and bitumen. However, in the industrial-plant process water, the presence of natural surfactants plays an important role in alleviating the negative effect of divalent cations on silica-bitumen interactions. To determine the synergetic effect of divalent cations and surfactants on silica-bitumen interactions and to estimate the maximum amount of divalent ions that can be present in plant process water without causing a significant negative impact on bitumen liberation, a series of solutions was prepared by adding calcium chloride to the plant process water at concentrations of 0.0, 1.0, 2.4, and 9.3 mM. No immediate precipitation was observed during the solution preparation process. The total calcium concentration (the added calcium plus the original calcium

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Figure 5. Interactions between silica and bitumen in plant process water and the foam and residual solutions: (a) Normalized long-range forces (F/ R) and (b) distributions of the adhesion force. γ denotes surface tension.

present in the plant process water at a concentration of about 1.2 mM) in these solutions was analyzed by the atomic absorption method, and the values obtained were 1.2, 2.2, 3.6, and 10.5 mM, respectively. Figure 7 shows the long-range force profiles measured in these solutions. The results show that 1 mM additional calcium (upper triangles) depressed the repulsion between silica and bitumen as compared to the case of no calcium addition (circles). The long-range forces changed dramatically to be attractive when 2.4 mM calcium was added. The attractive long-range forces were further increased as more calcium, i.e., 9.3 mM, was added to the plant process water. Similar measurements were performed in the foam solution with a series of calcium additions. When calcium was added into the foam solution, an immediate precipitation occurred. The measured final total calcium concentration in these liquids was 1.1, 3.1, 5.8, and 9.0 mM, respectively. The obtained long-range force profiles are shown in Figure 8. A similar trend of decreased long-range repulsive forces followed by an induced attractive force with increasing calcium addition was observed. Figure 9 shows the relationship between the measured adhesion force and the total concentration of divalent cations, which was calculated by adding the concentration of magnesium (about 0.8 mM) to the total calcium concentration. The adhesion force increased with calcium addition, but at the same concentration of total divalent cations, the adhesion force in the plant process water was slightly higher than that in the foam solution. It is clear that the foam solution has a higher tolerance for divalent ions than the plant process water in terms of causing the same adhesion force, further illustrating the importance of surfactants in process water in alleviating the negative impact of divalent cations on bitumen liberation.

Figure 6. Interactions between silica and bitumen in 1.0 mM calcium solutions with various amounts of SDS at pH 7.8: (a) Normalized longrange forces (F/R). Solid and dashed curves represent DLVO fitting results obtained using Abws ) 3.3 × 10-21 J with the calculated decay length and measured zeta potentials given in Table 3. (b) Normalized adhesion force as a function of SDS concentration.

Figure 7. Normalized long-range forces (F/R) between silica and bitumen in the plant process water with calcium addition. Table 3. Composition, pH, Debye Length, and Zeta Potentials of Silica and Bitumen in Various SDS Solutions solution

[SDS] (mM)

[Ca2+] (mM)

[KCl] (mM)

pH

κ-1 (nm)

ζB (mV)

ζS (mV)

1 2 3 4 5

1.0 × 10-3 1.0 × 10-2 1.0 × 10-1 1.0 10

1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 1.0 1.0

7.8 7.8 7.7 7.7 7.8

4.53 4.53 4.48 4.06 2.42

-37.0 -35.3 -34.0 -37.5 -70.3

-27.8 -30.0 -26.7 -29.0 -50.4

3.4. On the Role of Divalent Cations and Surfactants in Tuning the Silica-Bitumen Interactions. The results presented in this study indicate that the presence of divalent cations, such as calcium and magnesium, causes a decrease in repulsive longrange force but an increase in the adhesion force between silica and bitumen, thus leading to coagulation of silica and bitumen, which is detrimental to bitumen liberation. In contrast, the results obtained using the plant process water indicate that the presence

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Figure 8. Normalized long-range forces (F/R) between silica and bitumen in the foam solution with calcium addition.

Figure 9. Normalized adhesion force versus total divalent cation concentration in the plant process water (PW) and foam solution with extra calcium addition.

of surfactants results in a decrease in the adhesion force and consequently alleviates the negative impact of divalent ions. To control bitumen liberation, it is essential to understand how these divalent cations and surfactants tune the silica-bitumen interaction forces. It is well established that silica and bitumen surfaces are both negatively charged at alkaline pH as shown by schematics a and b of Figure 10. For bitumen, this is due to the presence of dissociated anionic surfactants (such as RCOO- and ROSO3-) on the surface.39 For the silica surface, it is due to the ionization of surface silanol groups (-SiOH).40 When divalent cations are present in the aqueous solutions, they act as a binder to connect the negatively charged silica and bitumen surfaces together (Figure 10c), leading to a strong adhesion force and coagulation. When both divalent cations and anionic surfactants are present in the aqueous phase, e.g., in the process water, they can combine together in the way shown in Figure 10d. Thus, the number of free divalent cations in the liquid, which act as a binder between silica and bitumen, decreases. As a result, the adhesion force between silica and bitumen also decreases, leading to a reduced probability of silica-bitumen heterocoagulation. 4. Conclusions In this study, the effects of divalent cations, surfactants, and their combination on interactions between silica and bitumen were investigated by surface force and zeta potential distribution measurements in both industrial-plant process water and water containing calcium and magnesium in amounts equivalent to those found in plant process water. It was found that calcium and magnesium cations have a negative impact on bitumen liberation because they increase the adhesion force and decrease the long-range repulsive forces between silica and bitumen. This

Figure 10. Schematics showing the impact of calcium and surfactants on silica-bitumen interactions. Negatively charged (a) bitumen and (b) silica surfaces in alkaline solution. (c) Calcium acts as a binder between the silica and bitumen surfaces. (d) Possible combination of calcium cations with anionic surfactants present in the solution.

is consistent with the observation in the zeta potential distribution measurements, which indicate heterocoagulation between silica and bitumen in the presence of calcium. The presence of surfactants can alleviate the negative impact of divalent cations and thus facilitate bitumen liberation. Acknowledgment Financial support from the NSERC Industrial Research Chair in Oil Sands Engineering (held by J.H.M.) is gratefully acknowledged. Literature Cited (1) Masliyah, J.; Zhou, Z.; Xu, Z.; Czarnecki, J.; Hamza, H. Understanding Water-Based Bitumen Extraction from Athabasca Oil Sands. Can. J. Chem. Eng. 2004, 82, 628-654. (2) Warszynski, S. N.; Zembala, M.; Malysa, K. Bitumen-Air Aggregates Flow to Froth Layer: I. Method of Analysis. Miner. Eng. 2000, 13 (14-15), 1505-1517. (3) Warszynski, S. N.; Zembala, M.; Malysa, K. Bitumen-Air Aggregates Flow to Froth Layer: II. Effect of Ore Grade and Operating Conditions on Aggregate Composition and Bitumen Recovery. Miner. Eng. 2000, 13 (14-15), 1519-1532. (4) Basu, S.; Nandakumar, K.; Masliyah, J. On Bitumen Recovery from Oil Sands. Can. J. Chem. Eng. 1997, 75 (2), 476-479. (5) Basu, S.; Kanda, W. C.; Nandakumar, K.; Masliyah, J. H. The Effect of Hydrophobic and Hydrophilic Clays on Bitumen Displacement by Water on a Glass Surface. Ind. Eng. Chem. Res. 1998, 37, 959-965. (6) Basu, S.; Nandakumar, K.; Lawrence, S.; Masliyah, J. Effect of Calcium Ion and Montmorillonite Clay on Bitumen Displacement by Water on a Glass Surface. Fuel 2004, 83 (1), 17-22. (7) Basu, S.; Nandakumar, K.; Masliyah, J. A Study of Oil Displacement on Model Surfaces. J. Colloid Interface Sci. 1996, 182 (1), 82-94. (8) Basu, S.; Nandakumar, K.; Masliyah, J. H. Effect of NaCl and MIBC/ Kerosene on Bitumen Displacement by Water on a Glass Surface. Colloids Surf. A 1998, 136 (1-2), 71-80.

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ReceiVed for reView March 22, 2006 ReVised manuscript receiVed August 11, 2006 Accepted August 21, 2006 IE060348O