Ind. Eng. Chem. Res. 2005, 44, 4753-4761
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Role of Acidified Sodium Silicate in Low Temperature Bitumen Extraction from Poor-Processing Oil Sand Ores H. Li, Z. A. Zhou,† Z. Xu, and J. H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
Process aids are generally required to improve bitumen recovery from poor-processing oil sand ores containing a relatively high amount of divalent metal ions and fine solids/clays. In this paper, the role of acidified sodium silicates as a dispersant/depressant of clay fines in bitumen extraction was evaluated using a laboratory hydrotransport extraction system (LHES) at low temperature (35 °C). Bitumen recovery experiments showed that adding acidified silicates during the bitumen extraction process resulted in a higher degree of bitumen liberation from sand grains, a faster bitumen flotation rate, and a better bitumen froth quality than adding caustic. Solution chemistry analysis demonstrated that acidified sodium silicate is a better process aid than caustic because it has three functions: to precipitate calcium and magnesium in the process water, which minimizes the synergistic effect of divalent cations in inducing a clay coating on the bitumen surface and clay gelation; to maintain an adequate pulp slurry pH for better bitumenair bubble attachment; and to disperse/depress clay fines from flotation by its specific species. Introduction Rapid expansion of Athabasca oil sands exploration has brought a fast process evolution and development. A low energy extraction process, i.e., using warm water (50 °C) or cold water (25 °C) as the processing medium, is proposed to save thermal energy and reduce greenhouse gas emissions while maintaining bitumen recovery efficiency similar to the conventional Clark hot water extraction (CHWE) process operating at 80 °C. In the low energy extraction process, bitumen flotation is more akin to minerals flotation in terms of bitumenair bubble attachment. The notable progress in this area includes the development of the OSLO (Other Six Lease Owners) cold water process and Syncrude low energy extraction (LEE) process.1,2 One of the common features of these processes is the use of process aids, such as kerosene and methyl isobutyl carbinol (MIBC), to improve the bitumen extraction performance. The use of these process aids has been based on the experience of coal flotation and has not been fully understood on a scientific basis in bitumen extraction. Oil sands slurry digestion using pipelines, instead of tumblers as in the initial hot water process, is adopted to facilitate the operation and provides the necessary hydrodynamic conditions for oil sands digestion and bitumen-air bubble attachment. However, the impact of these new processes on oil sands processibility requires further study. One of the challenges facing the Canadian oil sands industry is to improve the processability of poor- or difficult-processing oil sand ores, which normally contain a relatively large amount of fine minerals and clays (say, >20% passing 44 µm). The poor processability of the poor-processing oil sand ores is often attributed to * To whom correspondence should be addressed. E-mail:
[email protected]. † Now with Mineable Oil Sands, Alberta Research Council. E-mail:
[email protected].
the interactions between the fine clays and bitumen triggered by the chemical species released from the bitumen and fines. Such interactions could result in a possible coating of clay fines on the bitumen surface, thereby reducing the bitumen surface hydrophobicity. These fines on bitumen present a barrier for bitumen attachment to air bubbles, leading to a low bitumen flotation recovery. In addition, the fine solids in the slurry may also interfere with and retard the coalescence of bitumen droplets. The resultant fine bitumen droplets exhibit low flotation kinetics. It is, therefore, evident that minimizing the heterocoagulation of fine clays with bitumen and bubbles and dispersing fine clays in the slurry are key factors in improving the processability of oil sand ores with a high fines content. It has been realized from oil sands extraction practice that for poor-processing ores, simply adjusting the physical parameters, such as temperature, aeration, mechanical agitation, etc., is not necessarily sufficient to obtain a high bitumen recovery with an acceptable bitumen froth quality.3 Certain chemical additives of dispersing functions (such as caustic, silicates, and phosphates,4 etc.) are required to improve bitumen recovery from poor-processing oil sand ores.5 In fact, silicates were used in the early oil sands extraction research in the context of the hot water extraction process in the 1920-30s (as chemical additives to enhance bitumen recovery).6 The use of caustic in the commercial operation could stem from the consideration of its low cost and easy availability. Although the effectiveness of the inorganic dispersants in enhancing bitumen recovery has been tested from time to time,5,7 no clear conclusions could be drawn as to what kind of chemicals would perform the best. The recent development of experimental and analytical techniques in oil sands research has made it possible to examine the role of the operating parameters in each individual subprocess of bitumen extraction. The preliminary work conducted by Schramm et al.,8 using BEU tests, suggested that, for kerosene/MIBC to be effective
10.1021/ie048998k CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005
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Figure 1. Laboratory hydrotransport extraction system (LHES).
in improving bitumen recovery from poor-processing oil sand ores at a low operating temperature (say, 25 °C), more than 20 000 ppm kerosene/MIBC was needed. This level of chemical addition is impractically high. They attributed the effectiveness of the added kerosene/MIBC to the reduced bitumen viscosity. They also found that some organic solvents could improve bitumen froth quality without sacrificing bitumen recovery.9,10 In this case, the role of the bitumen/water interfacial tension was claimed to be responsible for the improved performance. To keep up with the fast pace in oil sands exploration expansion and development, and to identify possible solutions to counteract the problems related to the poor processibility of poor-processing ores, a systematic study was initiated in our research group to investigate the effect of different chemical aids, such as inorganic dispersant, polymer flocculant, and organic complex agents, on oil sands processibility using lowtemperature extraction processes. This communication focuses on the role of acidified sodium silicates as the dispersing agent in bitumen extraction based on the considerations of its effective removal of divalent metal ions and high dispersing ability of clay fines with less effect on slurry pH. To our best knowledge, there are no reports in the open literature on the use of acidified sodium silicate in bitumen extraction. Both bitumen extraction experiments and fundamental analyses were performed to reveal the role and identify the mechanism of added chemicals. Experimental Section 1. Methodology and Materials. To simulate the commercialized oil sands operations using hydrotransport pipelines, a laboratory hydrotransport extraction system (LHES) was built for oil sands conditioning and bitumen extraction experiments (Figure 1).11 It consists of a recirculating pipeline made of glass with an attached water jacket, slurry pump, and separation vessel for feeding and froth collection. A slurry pump from the Weatherford artificial lift system (Sejo Leopoldo-RS, Brazil) with a special rotator design was chosen in such a way, by oversizing, that its shearing effect on oil sands lumps and bitumen-air aggregates within the pump is minimized. As a result, any changes in the hydrodynamic conditions of the system and their effect on bitumen extraction could be attributed to the shear conditions inside the pipeline, not those created by the pump. The slurry temperature inside the pipeline was controlled by a thermal circulating water bath. One charge-coupled device (CCD) camera was used for online monitoring of bitumen liberation, with the signals automatically recorded and stored by a computer.
Transition ores and process water from the Aurora mine area of Syncrude Canada Ltd. were used in this study. The transition ore is located between estuarine and marine ore bodies. The difference of these kind of ores from good-processing ores is that transition ores, in general, contain a much higher content of fines. The mined oil sand ores were homogenized, packed at 600 g each in plastic bags, and stored in a deep freezer at -29 °C to minimize oxidation. The assay of the transition ore showed 9.2% bitumen, 7.3% water, and 83.5% solids containing 33% fines passing 44 µm. The process water, analyzed with atomic absorption, contained 47.0 ppm Ca2+, 15.0 ppm Mg2+, 15.5 ppm K+, and 550 ppm Na+ with a pH of 8.2. The chemicals used included reagent-grade sodium hydroxide, sodium silicate, and sulfuric acid, all purchased from Fisher Scientific. An acidified sodium silicate solution of pH ∼10 was prepared by mixing the sodium silicate solution (5 wt %, pH 12.8) with sulfuric acid at a 5:1 mass ratio prior to each test. 5:1 is the lowest sodium silicate/sulfuric acid ratio to prepare acidified sodium silicate solution without gelation, as determined experimentally. Unless otherwise stated, all experiments were conducted using the Aurora process water at a slurry temperature of 35 °C. 2. Experimental Procedure. 2.1. Bitumen Flotation. For each bitumen extraction test, 1 kg of a prepared oil sands sample was defrosted, chopped, homogenized, and fed into the hydrotransport pipeline in which 3 L of process water containing the desired chemicals had been preheated to a given temperature (usually around 35 °C). The oil sands slurry was conditioned with chemicals for 5 min prior to the introduction of air through a stainless steel needle of size 18 × 4 in. (Fisher Scientific). Right after the air rate was adjusted to 195 mL/min, the flotation timing was started. The liberated bitumen droplets were aerated and escaped from the pipe into the separation vessel to form a froth layer on the slurry interface inside the separation vessel (Figure 1). Over a 60-min flotation period, six froth samples were collected separately over incremental flotation times of 3, 10, 20, 30, 40, and 60 min and placed in Whatman filtration thimbles (Fisher Scientific) to obtain the information on the bitumen flotation kinetics. During the entire experimental run, the CCD camera was turned on to collect information on bitumen liberation, with captured images stored on a computer. The bitumen froth and feed samples inside the thimbles were assayed using the Syncrude standard procedure, which uses the Dean Stark Method,2 to determine the content of bitumen, solids, and water. In this method, the bitumen is extracted by flux of toluene, whereas the water evaporated is collected in a water trap with the solids left inside the thimble. The bitumen flotation recovery was calculated based on the ratio of bitumen weight in the froth to that in the feed. 2.2. Measurement of Bitumen Liberation. To estimate the effect of chemical addition on the degree of bitumen liberation from the sand grains, a high-speed black-and-white CCD camera was mounted just in front of a specifically designed square section of the glass pipeline to monitor the “darkness” of the oil sands slurry. Images of the flowing oil sands slurry were captured on-line by an interfaced computer using an interactive image analysis technique. The images were digitized and recorded for each picture element (pixel)
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Figure 3. Bitumen liberation with (3 mM NaOH or 10 mM acidified sodium silicate) and without chemical addition.
Figure 2. Images showing the effect of chemical additions (NaOH at 3 mM or acidified sodium silicate at 10 mM) on bitumen liberation after 60 min flotation.
as gray level, which distinguishes whether the picture element is bitumen. By analyzing the variations in gray scale intensities, the degree of darkness of the images was taken as an inverse measure of the degree of bitumen liberation from the sand grains.11 The threshold gray levels of zero for the lowest intensity (black) pixels and a maximum numerical value of 256 for the highest intensity (white) pixels were used for this purpose. 2.3. Measurement of Zeta Potential Distribution. To interpret the flotation results, measurements of the zeta potential distribution were conducted,12,13 which allowed the effectiveness of the chemicals as dispersant to be assessed. Prior to measurement, the suspensions were prepared with Aurora process water in two groups: emulsified bitumen droplets (treated by ultrasonication) or clay-fines obtained from bitumen flotation tailings (referred as tailing fines) and a mixture of the emulsified bitumen and tailing fines. The suspensions were then conditioned by a magnetic stirrer for 5 min, with or without chemical addition in the process water. About 35 mL of suspension was transferred to an electrophoresis cell with the help of a 60-mL syringe. Zeta potential distribution was measured by a Zetaphoremeter III (SEPHY/CAD) equipped with a laserilluminator and a digital video image capture system (CCD camera). A detailed description of the operation principles of the technique can be found elsewhere.13 Results and Discussion 1. Bitumen Extraction Experiments. The results of bitumen extraction experiments presented and discussed here are with optimal chemical dosages, i.e., 3 mM sodium hydroxide (recommended dosage by Bichard5) and 10 mM acidified sodium silicate (based on an optimum level determined from chemical dosage experiments) to process water volume. These dosages are equivalent to 360 and 3660 g/t oil sand ores, respectively. 1.1. Bitumen Liberation. As the first step in bitumen recovery from oil sand ores, bitumen liberation plays an important role in enhancing bitumen extraction. A high degree of liberation means more bitumen droplets being separated from sand grains. Figure 2
Figure 4. Effect of chemical addition (3 mM NaOH or 10 mM acidified sodium silicate) on bitumen recovery.
shows the images of a slurry taken by the CCD camera at the beginning (at time 0, Figure 2a) and the end (at 60 min) of flotation experiments under different chemical conditions. The degree of darkness on these images reflects the different extents of unliberated bitumen. The use of the darkness of recorded images as a measure of the degree of liberation is well documented.11,14 Without chemical addition, the image obtained after 60 min of flotation is a dark gray color (Figure 2b), suggesting that the bitumen liberation was limited. With the sodium hydroxide addition (Figure 2c), the image became light gray, indicating an improvement in bitumen liberation. In contrast, the image taken with the addition of acidified sodium silicate appeared much lighter, showing a better bitumen liberation. A quantitative representation of bitumen liberation with/without chemical addition is shown in Figure 3. Adding chemicals (sodium hydroxide or acidified sodium silicate) produced a quite high level of bitumen liberation. Toward the end of flotation, the degree of bitumen liberation reached >90%, representing a 20% increase as compared with the case without chemical addition. One of the possible reasons for the low bitumen liberation without chemical addition could be a poor dispersion of solids or clay fines at a lower slurry pH (pH 8.2) due to a strong adhesion between bitumen and sand grains at this pH,12 making the separation of bitumen from sand grains more difficult. 1.2. Bitumen Flotation. Figure 4 compares cumulative bitumen recoveries as a function of flotation time in the presence and absence of chemical addition obtained with the LHES. Without chemical addition,
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bitumen recovery was the lowest, at 65% after a 60min flotation. An increase of bitumen recovery by 20% and 30% was observed with sodium hydroxide and acidified sodium silicate, respectively. The slurry pH changed from 8.2 without chemical addition to 8.8 with sodium hydroxide or acidified sodium silicate. An observation from this figure is that the bitumen recoveries with sodium hydroxide and acidified sodium silicate addition are quite different, although their addition resulted in a similar slurry pH. This observation suggests that slurry pH within the range listed above is not a determining factor for the improved bitumen recovery. In other words, the OH- ions released from sodium hydroxide are not as powerful a dispersant as the silicate ions. In fact, sodium hydroxide has been found to have very limited ability to improve the processibility of poor-processing oil sand ores.15 Sodium silicate is known to be a good dispersant for silicate and clay minerals.16 The improved dispersion of clay particles by acidified silicates contributes to a significant improvement in bitumen recovery. Moreover, the use of acidified sodium silicate has less effect on slurry pH, even at a high dosage, and can precipitate divalent ions at this pH, which will be discussed later. As demonstrated recently,3 the coating of the solids on the bitumen surface at a lower pH could change the bitumen surface hydrophobicity. The removal of divalent cations as well as the slime coating by the silicates will ensure that bitumen surface hydrophobicity will be maintained. Consequently, a much faster flotation kinetics was obtained with the addition of acidified silicates than with that of sodium hydroxide, as observed in Figure 4. 1.3. Bitumen Froth Quality. The presence of clay fines in oil sands and high concentrations of divalent ions, such as calcium and magnesium, in the process water may also impact bitumen froth quality due to the undesired heterocoagulation of bitumen-clay fines activated by these ions.3,12 Dispersion of clays by chemical additives is anticipated to minimize this heterocoagulation and, hence, to improve both bitumen recovery and froth quality. With sodium hydroxide, the slurry pH can be increased to as high as 11 by high chemical dosage in order to obtain the desired dispersing effect. As a result, the pH-dependent surface charge of both bitumen and clay fines became more negative due to an increased ionization of surface species and/or the adsorption of hydroxyl ions. Therefore, the electrostatic (columbic) repulsive force between bitumen and clay was increased, thereby minimizing fines/bitumen heterocoagulation. Since a high dosage of sodium hydroxide was required in order to efficiently disperse the clay fines, an excessively high pH would result in emulsification of bitumen to produce bitumen droplets too fine to be efficiently recovered. More importantly, contact angle measurement has clearly shown that the hydrophobicity of bitumen decreases rapidly when the pH is >10, with sodium hydroxide being used as the pH modifier. At a slurry pH >11, detachment of air bubbles from the bitumen surface was observed.3 As a compromise, the slurry pH of bitumen flotation is often set ∼8.5. With this pH limitation, the dosage of sodium hydroxide was controlled to ∼3 mM because of the acidic nature of the ores. Under the given water chemistry conditions, a high froth quality could not be obtained because of the lack of a strong repulsive force between the bitumen and clay fines.
Figure 5. Effect of chemical addition (3 mM NaOH or 10 mM acidified sodium silicate) on bitumen content of froth.
Figure 6. Water content of froth obtained in flotation with (3 mM NaOH and 10 mM acidified sodium silicate) and without chemical addition.
The dilemma inhered in using sodium hydroxide as the pH modifier or dispersant could be resolved by using acidified sodium silicate with three characteristics that will be discussed in the following section of analysis of oil sands slurry: (a) The slurry pH increased slowly as the chemical dosage increased and can be maintained at an optimal range, as measured ∼pH 8.8. This pH level is beneficial to the bitumen liberation and suitable for the dispersion of clay fines. (b) The main species of acidified sodium silicate caused more effective dispersion/depression of clay fines but had less effect on bitumen surface properties such as hydrophobicity. (c) Forming precipitates with silicates reduced the amounts of calcium and magnesium ions in the process water. Therefore, a higher bitumen recovery and better froth quality can be obtained with acidified sodium silicate than with sodium hydroxide. The experimental results of froth quality, such as the content of bitumen, water, and solids, shown in Figures 5, 6, and 7, respectively, confirmed the effectiveness of acidified sodium silicate. Figure 5 shows that the cumulative bitumen content in froths without chemical addition was only 23 wt %. The addition of sodium hydroxide increased bitumen content to 32 wt %. With acidified sodium silicate, bitumen content as high as 40 wt % was obtained. The improvement in froth quality by sodium hydroxide or acidified silicate is attributed to the effective dispersion/ depression of clay fines, and the effectiveness of these two chemicals are quite different (8% difference in bitumen content).
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These solution equations listed above show that bicarbonate in water undergoes a series of ionizations to release either H+ or OH-, depending on the solution pH. Among the equilibrium constants pK1a, pK2a, and pK2h, pK2h has the smallest value, indicating that the solution of bicarbonate is always alkaline. Indeed, the Syncrude Aurora process water has a pH ∼8.5, due to the presence of a relatively high bicarbonate content. However, when the oil sands were mixed with the process water, the slurry pH dropped slightly to ∼8.2. Considering the pKa values of carboxylic acids, one of the reasons for this pH drop could be the ionization of weak organic acids on the bitumen droplets at this alkaline pH to form, for example, carboxylate: Figure 7. Solids content of froth obtained in flotation with (3 mM NaOH and 10 mM acidified sodium silicate) and without chemical addition.
Figure 6 shows that the water content in froth was reduced from 58% to 49% and 45% (by weight) by the addition of sodium hydroxide and acidified sodium silicate, respectively. Compared to the case without chemical addition, the acidified sodium silicate reduced the most water content, by 13%, in the cases with chemicals. Figure 7 shows the corresponding solids content reduction from 19% to 18% and 16% with sodium hydroxide and acidified sodium silicate, respectively. Compared with the case without chemical addition, sodium hydroxide addition showed a negligible effect on reducing solids content, by only 1% if experimental error is considered. However, with the acidified sodium silicate, 3% less solids were recovered to the froth. These results clearly indicate that the clay fines in the froth are difficult to be dispersed with only a pH adjustment, and acidified sodium silicate performed better than sodium hydroxide because the acidified sodium silicate has a stronger dispersing ability than that of sodium hydroxide, which will be discussed later. 2. Analysis of Oil Sands Slurry. To further discuss the advantages of acidified sodium silicate over sodium hydroxide, the effect of their addition on the properties of the oil sands slurry, including alkalinity, clay fines dispersion, and divalent ions removal, was investigated by solution chemistry analysis. 2.1. Alkalinity. It has been well-established from laboratory research and industrial practice that a weak alkalinity of the oil sands slurry is needed for a good recovery of bitumen from oil sand ores. Alkalinity serves not only to improve bitumen liberation from sand grains but also to disperse the clays and minimize the bitumen-fines coagulation,7 albeit, it is not desirable for tailings management and water recycling. The alkalinity of the oil sands slurry can come from different sources. The presence of a relatively high amount of bicarbonate in the process water (say, about 600-1000 ppm), as is the case in this study, could contribute to the alkalinity of the slurry as per the following equations:17
HCO3- + H2O h H2CO3 + OH-
pK2h ) 5.64 (1)
H2CO3 h HCO3- + H+
pK1a ) 6.35
(2)
HCO3- h CO32- + H+
pK2a ) 10.33
(3)
R-COOH + OH- h R-COO- + H2O
pKa ) 4.25 (4)
It was found that bitumen behaves as an amphoteric material, i.e., both acidic and basic aqueous solutions tend to be neutralized when mixed with bitumen.5,18 As shown in eq 4, the higher the pH, or the higher the alkalinity, the further the reaction proceeds from left to right to form more caboxylates, which requires more base to be added to the solution to make the bitumen and solids surface more negatively charged. The increase in ionization favors bitumen liberation from the sand grains and minimizes bitumen-fines coagulation. One way to achieve the desired alkalinity is to add caustic. Sodium hydroxide is the simplest compound among the chemicals tested. In water, sodium hydroxide dissociates, releasing OH- ions as shown in eq 5:
NaOH h Na+ + OH-
(5)
Here, the OH- ions from sodium hydroxide act as a dispersant as they adsorb onto solid particles. It is wellestablished that hydrogen and hydroxide ions are potential determining ions for oxides such as silica and other metal oxides.16 The hydroxide ions, for example, adsorb on oxide minerals to increase the negative surface charge density and, hence, to disperse the particles by increasing the electrostatic repulsion. Another way is to add silicates. For a solution of sodium silicate, more ionic species are present than in the the case with sodium hydroxide, as shown below:16
Na2SiO3 + H2O f H2SiO3 + 2Na+ + 2OH- (6) H2SiO3 h HSiO3- + H+
pK1h ) 9.43
(7)
HSiO3- h SiO32- + H+
pK2h ) 12.56
(8)
On the basis of these equalities, three anionic species are formed in sodium silicate solutions: OH-, HSiO3-, and SiO32-. The OH- ion accounts for the increase in the solution pH, whereas the other two are the main dispersing/depressing species of silicates usually used in mineral processing. Therefore, using sodium silicate can be expected to achieve a better dispersion than that obtained when using sodium hydroxide that does not have these dispersing species. Moreover, the dispersing ability among these silicate species is different, and their concentration distributions in solution are highly dependent on the solution pH. The effect of pH on the percent species distribution is shown in Figure 8, in which the total concentration of sodium silicate is 1 M.
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Figure 8. Calculated distribution of silicate species on the basis of dissociation constants for silicic acid at different pHs (total 1 M).
At an acidic pH below 7, the main component is H2SiO3. As pH is increased above 8-9, the main species becomes HSiO3-. The SiO32- concentration is always low over the whole pH range. It is evident that at an operating pH of 8.5, HSiO3- is the main species of sodium silicate to act as dispersant/depressant. Unfortunately, the shortcoming of directly using sodium silicate for bitumen extraction is the increase of the slurry pH at a high dosage, which is similar to the use of sodium hydroxide. While increasing slurry pH may be beneficial for dispersing clays in the slurry and minimizing bitumen-fines coagulation, a too-high pH would cause bitumen emulsification19 and affect bitumen-air bubble attachment by reducing the bitumen surface hydrophobicity.3 The desired situation, therefore, would be to have more chemical species in the solution which depress or disperse fine particles and, at the same time, keep the slurry pH in an optimum range. To accomplish this, one practical option is to prepare acidified sodium silicate. The benefits of using acidified sodium silicate are not only to lessen the effect on the slurry pH but also to produce more species that have astronger dispersing ability. According to the research results of Gong et al.,20 a difference in dispersing ability was found among the species of sodium silicate. These species can be classified into three categories: polymeric silicate species, monomeric silicates, and colloidal amorphous silica particles. Among them, polymeric silicate and small colloidal silica particles have shown stronger dispersing/depressing abilities. Accordingly, efforts need to be devoted to produce more polymeric species from sodium silicate solution. This can be achieved by acidifying sodium silicate by the following two reactions.
Sodium silicate is acidified to silicic acid in acid solution: Na2SiO3 + 2H+ + H2O h Si(OH)4 + 2Na+
(9)
The silicic acid then polymerizes: 2Si(OH)4 h (OH)3SiOSi(OH)3 + H2O
(10)
Long chain polymeric silicate species are formed in the last reaction by acidification. These chains then form a gel matrix that fills with water. No OH- ions are produced from the above two reactions, and only one compound of (OH)3SiOSi(OH)3 is generated for the purpose of dispersion/depression. The mechanism of
Figure 9. Effect of chemical dosage on process water pH.
Figure 10. Zeta potential distributions of individual bitumen and clay fines in process water without chemical addition (pH 8.5).
silicic acid polymerization involves the formation of siloxane bonds with the condensation of silanol (SiOH) groups to form siloxane (Si-O-Si).21 The most significant factor controlling the rate of silicic acid polymerization is the pH. Usually the reaction rate of polymerization is the most rapid within a pH range from 5 to 7. A suitable mass ratio of silicate and acid becomes critical in preparing acidified sodium silicate. The use of acidified sodium silicate is justified in our study. It was found that the addition of acidified sodium silicate into Aurora process water (containing 47 and 15 ppm calcium and magnesium, respectively) showed a weaker effect on the pH change than the addition of sodium hydroxide, as shown in Figure 9. It was noted that the final pH values reached by sodium hydroxide addition was higher than that reached by sodium silicate or acidified sodium silicate addition at the same chemical concentration (mM). Acidified sodium silicate resulted in the lowest variation in pH among these three chemicals. 2.2. Dispersing Ability of Acidified Sodium Silicate. Acidified sodium silicate has been proven to be an effective depressant to remove unwanted activation of gangue minerals by calcium ions.22,23 The role of acidified silicates in dispersing clay suspension is validated by measurements of zeta potential distributions as discussed below. Figure 10 shows the zeta potential distribution of bitumen and tailing fines measured individually in the absence of chemical addition. It can be seen that both tailing fines and bitumen carry a negative surface charge, with bitumen being slightly more negative. The
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Figure 11. Zeta potential distributions of a bitumen/clay mixture with an acidified sodium silicate addition: (A) 0 mM, pH 8.5; (B) 4 mM, pH 9.5; (C) 10 mM, pH 9.8; (D) 20 mM, pH 10.8).
characteristic peak of zeta potential for the fines is about -30 to -35 mV, while for bitumen it is about -50 to -55 mV. For bitumen and tailing fines mixtures without chemical addition, Figure 11A showed only one broad distribution peak, with the peak value close to that obtained for tailing fines. This observation would suggest that bitumen is partially covered by the fines, i.e., a slime coating or heterocoagulation. However, the addition of 4 mM of acidified sodium silicate (Figure 11B) caused some separation of the zeta potential distribution into two peaks centered around -35 and -55 mV, but with a noticeable overlap. The peak values were very close to that of the individual tailing fines and bitumen, respectively, thereby indicating that the added acidified silicate did not prevent clay fines from coagulation with bitumen. The split of zeta potential distributions became more evident as shown in Figure 11C at a higher dosage of acidified sodium silicate of 10 mM, the optimal dosage determined by bitumen extraction experiments. It is clear that, at this level of acidified sodium silicate addition, little attachment of clay fines on bitumen droplets takes place. Further increasing the dosage of acidified sodium silicate to 20 mM (Figure 11D) did not cause significant change from the case with a 10 mM silicate addition. The above results clearly show that acidified sodium silicate effectively minimized the heterocoagulation between bitumen and clay fines, and a dosage of at least 10 mM of acidified sodium silicate is required. These observations are consistent with the bitumen flotation experiments using LHES, where a much higher bitumen recovery and higher froth quality were observed with acidified sodium silicate addition than in the case without chemical addition or in the case with sodium hydroxide addition. 2.3. Removal of Divalent Metal Ions (Ca and Mg) by Acidified Sodium Silicate. In a commercial oil sands extraction system, different ionic species are present in the process water or slurry. The presence of divalent cations such as calcium and magnesium not only induces bitumen-clay heterocoagulation but also leads to gelation of the clay suspension. All these conditions could deteriorate bitumen-air bubble attachment and bitumen flotation recovery. Therefore, adopting a suitable strategy to remove divalent cations
could be beneficial to bitumen recovery and froth quality. This can be achieved by sequestration or chelating (holding ions in solution), by precipitation (forming an insoluble substance), or by ion exchange (trading with electrically charged particles). In the case of bitumen extraction, chemicals that possess both the functions of dispersing clay fines and of removing metal ions could be better candidates for enhancing bitumen/ fines separation. An economical way is to use acidified silicates, because soluble silica can react with all multivalent cationic metal ions to form corresponding insoluble metal silicates, as will be demonstrated below. 2.3.a. Solution Chemistry Consideration. Solution chemistry analysis provides a good approach to evaluate the role of pH in controlling the chemical concentration required to remove divalent ions. Solubility-pH diagrams of Ca2+-SiO32- and Mg2+-SiO32- are constructed to show the relation of Ca2+ and Mg2+ precipitation with solution pH. Although acidified sodium silicate was used in bitumen flotation, a sodium silicate system is examined for illustrative purposes. For a given solution system, if the ion concentration of calcium or magnesium is above the solubility product limit, the formation of calcium or magnesium silicate is anticipated and is governed by:24,25
Ca2+ + SiO32- h CaSiO3 (s) KspCa-Si ) 8.3 × 10-12 (11) Mg2+ + SiO32- h MgSiO3 (s) KspMg-Si ) 4 × 10-12 (12) where KspCa-Si or KspMg-Si is the corresponding solubility product constant, which defines the solubility limit. According to coordination chemistry, the true ion concentration is determined by the conditional solubility product Lsp:
LspCa-Si ) [Ca2+]e[SiO32-]e ) KspCa-SiRCaRSi (13) LspMg-Si ) [Mg2+]e[SiO32-]e ) KspMg-SiRMgRSi (14) [Ca2+]e, [Mg2+]e, and [SiO32-]e are the total soluble calcium, magnesium, and silicate concentration, respectively, represented by subscript “e”. The R’s in eqs 13 and 14 are the fraction species at a given state. All pertinent calcium and magnesium hydrolysis as well as silicate protonation reactions are listed below:24
Ca2+ + OH- h CaOH+ Ca2+ + 2OH- h Ca(OH)2
KCa1 ) 101.3 (15) KCa2 ) 10-5.19 (16)
RCa ) 1 + KCa1[OH-] + KCa2[OH-]2 Mg2+ + OH- h MgOH+ Mg2+ + 2OH- h Mg(OH)2
(17)
KMg1 ) 102.58 (18) KMg2 ) 1011.15 (19)
RMg ) 1 + KMg1[OH-] + KMg2[OH-]2
(20)
H+ + SiO32- h HSiO3-
K1H ) 1012.56 (21)
H+ + HSiO3- h H2SiO3
K2H ) 109.43 (22)
RSi ) 1 + K1H[H+] + K1HK2H[H+]2
(23)
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Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005
Figure 12. Conditional solubility product of calcium and magnesium silicate (log[M]e[SiO3]e, where M represents Ca or Mg) as a function of pH for an initial ion concentration of 1mM.
Solving eqs 11-23 with initial concentrations of Ca2+ and Mg2+ allows the calculation of the conditional solubility products and minimum silicate concentrations required to precipitate the cations at given concentrations. Figure 12 shows the conditional solubility product of calcium silicate and magnesium silicate as a function of pH (with initial ion concentrations of 1mM). The region above the curve represents a system of higher concentration product where bulk calcium silicate or magnesium silicate precipitation is anticipated. The higher the solution pH, the lower the conditional solubility product, and the higher is the possibility for calcium or magnesium to be precipitated by acidified silicate. In other words, the minimum silicate concentration required to precipitate calcium or magnesium ion is low at a high pH. According to the calculation, as high as 80 M of silicate concentration is required to precipitate calcium ions and as high as 40 M is required to precipitate magnesium at pH 6, whereas, at pH 8.5, the concentrations are reduced to only 8 × 10-3 and 4 × 10-3 M silicate, respectively. 2.3.b. Ion Removal by Acidified Sodium Silicate. In Figure 13a, the curves for the calculated ion removal by eqs 11-23 based on an effective SiO32- concentration from sodium silicate predict that the silicate can dramatically reduce the ion concentrations of both calcium and magnesium from 1 mM to ∼0 when 20 mM silicate is added. To demonstrate the effectiveness of acidified sodium silicate on ion removal, a bulk solution with 1 mM calcium chloride or 1 mM magnesium chloride added to deionized water was prepared. Four 100 mL samples were taken from the bulk solution into four beakers. To each beaker, a given amount of acidified sodium silicate (0, 5, 10, and 20 mM) was added and the solution was agitated for 1 min. When necessary, HCl solution was added to readjust the solution pH to 8.5 in order to minimize the effect of acidified silicate addition on the solution pH. Then, 30 mL of sample was taken from each beaker and was centrifuged at 15 000 rpm for 30 min to remove any precipitates. The supernatant of the centrifuged solutions was analyzed by atomic absorption spectrometer (Spectr AA 220 FS) for Ca2+ and Mg2+ ions. The results are shown in Figure 13b. It can be seen that the ion concentrations of both Ca2+ and Mg2+ decreased rapidly with the addition of acidified sodium silicate. Above 10 mM acidified sodium silicate, that is, the optimal concentration used in bitumen extraction, both ion concentrations were reduced to
