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Effect of illite clay and divalent cations on bitumen recovery.

The adverse effect of illite clay on bitumen recovery was found to be related to its acidity. The addition of calcium or magnesium ions to the flotation deionized water had a marginal effect on bitumen recovery when measured using a Denver flotation cell. However, the co-addition of illite clay and divalent cations caused a significant reduction in bitumen recovery. The effect was found to be compounded at a lower process temperature and low pH values. Zeta potential distributions of illite suspensions and bitumen emulsions were measured individually and as a mixture to investigate the effect of divalent cations on the interaction between bitumen and illite clay. The presence of 1 mM calcium or magnesium ions in deionized water had a significant effect on the interactions between bitumen and illite clay. Slime coating of illite onto bitumen was not observed in zeta potential distribution measurements performed in alkaline tailings water.

On a trouve que l'effet adverse de l'argile d'illite sur la recuperation de bitume etait relie a son acidite. L'ajout d'ions de calcium ou de magnesium a l'eau deionisee de flottation a un effet marginal sur la recuperation de bitume lorsqu'on la mesure avec une cellule de flottation de Denver. Toutefois, l'ajout combine d'argile d'illite et de cations divalents entraine une reduction significative de la recuperation de bitume. On a trouve que les effets etaient combines a une faible temperature de procede et de faibles valeurs de pH. Les distributions de potentiel zeta des suspensions d'illite et des emulsions de bitume ont ete mesurees individuellement et dans le melange afin d'etudier les effets des cations divalents sur l'interaction entre le bitume et l'argile d'illite. La presence de 1 mM d'ions de calcium ou de magnesium dans l'eau deionisee a un effet significatif sur les interactions entre le bitume et l'argile d'illite. On n'a pas observe de couche de boues d'illite sur le bitume dans les mesures de distributions de potentiel zeta obtenues dans de l'eau de rejets alcaline.

Keywords: bitumen recovery, zeta potential distribution, divalent cations, illite clays, pH

INTRODUCTION

At present, approximately 35% of Canada's petroleum needs can be met from the Athabasca oil sands. As conventional sources of oil and gas are depleted, it is inevitable that the oil sands will play a greater role in meeting North America's petroleum needs. The main technology used today to extract bitumen from the Athabasca oil sands is a lower temperature version of the Clark Hot Water Process (CHWP). A typical bitumen extraction process involves the following essential steps: First, oil sand lumps are crushed and transported to slurry preparation where hot water is mixed with the ore. The resulting slurry is pumped into a hydrotransport slurry pipeline where slurry conditioning takes place. During the slurry transport, bitumen is liberated from the sand grains and the liberated bitumen becomes aerated by the entrained air in the slurry. A gravity separator is then used to recover the aerated bitumen as bitumen froth. After removing the water and solids from the froth, the bitumen is ready for upgrading. It is evident that the extraction process requires (Hepler and Hsi, 1989; Hepler and Smith, 1994): liberation of bitumen from the sand grains; attachment and/or engulfment of the liberated bitumen with air bubbles; and flotation of bitumen-air aggregates to form a bitumen-rich froth. Clearly, interactions between bitumen and clay minerals or air bubbles play a key role in bitumen recovery as they affect bitumen aeration.

It is well known that extraction techniques to recover bitumen from mined oil sand ore use large volumes of water. Currently, to produce one barrel of bitumen, 2.5 to 4.0 barrels of imported water are required (National Energy Board, 2004). With the increasing need to produce bitumen and with an increasingly limited water supply, producers have to reclaim water from mature fine tailings. To achieve this purpose, gypsum is usually added into the tailings system to accelerate the settling of fine solids and the release of water. However, the use of gypsum is anticipated to increase the concentration of calcium ions in the recycle water system. Magnesium is another divalent ion that is usually present in bitumen extraction slurries. A number of researchers (Sanford, 1983; Takamura and Wallace, 1988; Smith and Schramm, 1992; Zhou et al., 1999; Kasongo et al., 2000) have reported that in industrial and laboratory tests, the levels of ions and clay minerals present in the slurry have a significant impact on bitumen recovery. In order to elucidate the role of ore characteristics and water chemistry on bitumen flotation, a systematic research program was initiated in our research group. Kasongo et al. (2000) developed a novel "doping" method that involved the addition of a prescribed amount of calcium and/or clays into a good processing estuarine oil sands ore during batch flotation tests performed using deionized water. They found that the addition of calcium ions (up to 40 ppm) or clay (kaolinite, illite or montmorillonite) at 1 wt.% of the processed oil sands had a marginal effect on bitumen recovery. However, a sharp reduction in bitumen recovery was observed when both calcium ions and montmorillonite clays were added together. Further analysis revealed that the adverse impact of calcium ions and montmorillonite clays on bitumen recovery was associated with a stronger affinity of calcium for montmorillonite clays than for kaolinite or illite clays. A novel technique was developed by Liu et al. (2002) to investigate the interactions between bitumen and clays in an aqueous solution using zeta potential distribution measurements. For a single component suspension, a single zeta potential distribution peak was obtained under a given solution condition. In the case of a two-component mixture, the measured zeta potential distributions showed either one or two distribution peaks, depending on the chemical make up of the aqueous solution and the type/amount of clays present. Using this method, Liu et al. (2002) found that a stronger attraction existed between bitumen and montmorillonite clay than that between bitumen and kaolinite clay when 40 ppm of calcium ions were present in deionized water. This finding is in excellent agreement with the flotation results reported by Kasongo et al. (2000). Xu et al. (2003) conducted an electrokinetic study of clay interactions in coal flotation. Their study further demonstrated that zeta potential distribution measurements are a powerful tool for studying slime coating phenomena and are useful in diagnosing flotation systems.

The primary clay minerals observed in various oil sands extraction process streams are kaolinite and illite (Bichard, 1987; Budziak et al., 1988; Omotoso and Mikula, 2004). To our best knowledge, only a few studies (Kasongo, 2000; Wallace et al., 2004) were conducted to study the effect of illite on bitumen recovery. In this study, "doping" flotation tests using deionized water and electrokinetic studies were carried out to further investigate the effect of illite clays on bitumen recovery. The effect of magnesium ions was investigated and compared with calcium ions. In addition, the effects of temperature and tailings water chemistry are discussed.

EXPERIMENTAL

Materials

Bitumen from vacuum distillate feed and high grade north mine estuarine oil sands ore (F11B) were provided by Syncrude Canada Ltd. and used in zeta potential distribution measurements and flotation tests, respectively. The oil sands ore was homogenized and stored in a refrigerator maintained at -29[degrees]C to minimize oxidation. Samples were allowed to thaw at room temperature prior to being used in flotation tests. The average composition of the oil sands ore (based on the analyses of 10 small, random sample bags) is shown in Table 1. The analysis error was less than 0.5%.

Illite clay was purchased from the Clay Minerals Society at Purdue University. It was ground in a porcelain ball mill for 5 h. A 44 [micro]m sieve was used to remove the coarse solids. The fine solids were used for all the tests in this study. The average particle size was determined to be 10.9 [micro]m. Reagent grade Ca[Cl.sub.2] x 2[H.sub.2]O (Fisher) and Mg[Cl.sub.2] x 6[H.sub.2]O (BDH) were used as the source of calcium and magnesium ions, respectively. Reagent grade NaOH (Fisher) was used as a pH modifier. Deionized water with a resistivity of 18.2 MO cm, prepared with an Elix 5 followed by a Millipore ultra water system, was used in all the experiments, unless otherwise specified.

Procedures

Bitumen flotation tests

To simulate the commercial Clark Hot Water Extraction (CHWE) process, Sanford and Seyer (1979) developed a Batch Extraction Unit (BEU), which has been extensively used to conduct research on oil sands processability. More recently, Kasongo et al. (2000) and Zhou et al. (2004) used a modified Denver flotation cell to conduct their flotation tests. The Denver flotation cell is more sensitive at detecting the effects of operating parameters on bitumen recovery than BEU tests at temperatures below 50[degrees]C. In this study, a modified Denver flotation cell was used. In order to control the temperature, a water jacket connected to a thermal water bath was attached to a one-litre stainless steel flotation cell. For baseline tests, 300 g of F11B oil sands ore and 950 ml of heated deionized water with the pH adjusted to 8.5 were placed in the cell. After the slurry was conditioned at 1500 rpm for 5 min, air was introduced at a rate of 150 mL/min. Bitumen froth was collected as a function of time for a period of 18 min. The final pH of the tailings slurry was recorded. All other flotation tests were performed using a similar procedure with the exception that clay and/or magnesium or calcium salt were added into the slurry prior to conditioning. Unless otherwise specified, deionized water was used in all the flotation tests.

Bitumen assays

A Dean-Stark method (Bulmer and Star, 1979) was used to obtain the composition of oil sands ore and flotation froth (i.e., the content of bitumen, solids, and water).

Tailings analyses

Four 50 mL centrifuge tubes were filled with the tailings slurry that was obtained upon completion of a bitumen flotation test. The filled tubes were centrifuged in an Allegra[TM] 64 Centrifuge at 15 000 g for 30 min. The supernatant was set aside for water analysis and zeta potential distribution measurements. A SpetrAA (220F Varian) atomic absorption spectrometer was used to determine the concentrations of divalent cations in the supernatant and hence in the tailings water. The detection limit was 0.1 ppm for divalent cations. The analysis of the baseline tests showed that the concentrations of calcium and magnesium ions were 0.2 and 0.4 ppm, respectively, confirming that the F11B oil sand ore contained a negligible amount of soluble divalent cations.

Sample preparation for electrokinetic studies

For the zeta potential distribution measurements, a bitumen-in-water emulsion was prepared by sonication of about 1 g of bitumen in 100 ml of 1 mM KCl solution using a Model 550 Sonic Dismembrator (Fisher) for 18 min. The clay suspension was prepared using a similar procedure but with an ultrasonic bath (Fisher) instead of the Sonic Dismembrator. Each sample was diluted prior to zeta potential distribution measurements as the instrument can measure the zeta potential of only relatively dilute suspension/emulsions. To prepare a bitumen-clay suspension, small amounts of prepared bitumen emulsion and clay suspension were diluted and mixed at a specified ratio and then conditioned in an ultrasonic bath (Fisher) for 18 min before zeta potential distribution measurements.

Zeta potential distribution measurements

Zeta potential distribution measurements were carried out with a Zetaphoremeter IV[TM] (CAD). The instrument was equipped with an electrophoresis chamber consisting of two electrode compartments and a connecting rectangular cell, a laser illuminator, and a digital video image capture and viewing system. The computerized operating system captured the image of moving particles in a stationary plane under a known electric field. A built-in image processing software analyzed the captured images and then provided a histogram of the electrophoretic mobilities. The data collected were then converted to a zeta potential distribution as desired. All the measurements were performed with the addition of 1 mM KCl as a background electrolyte.

RESULTS AND DISCUSSION

Bitumen Flotation

Effect of illite clay and divalent cations

Figure 1 shows bitumen fl otation recovery as a function of flotation time. The addition of 1 mM magnesium ions and/or illite clay at 0.5% by weight of total oil sands ore had little effect on bitumen flotation kinetics or bitumen recovery. Kasongo et al. (2000) reported a similar observation. To explore whether the amount of illite clay could be a factor affecting bitumen recovery, flotation tests were performed with an increasing amount of clays (up to 5%). The results summarized in Figure 2 show that at 35[degrees]C, bitumen recovery decreased slightly with an increasing amount of illite clay addition. At 25[degrees]C, the decrease was more pronounced. The pH of the produced flotation tailings as a function of the amount of illite clay addition is plotted in Figure 3. The data show that the tailings water pH decreased with an increasing amount of illite clay addition. This pH decrease is due to the acidity of illite clay (Bichard, 1987; Du et al., 1997). As the pH of the produced tailings water at a given level of clay addition is not affected by temperature, one can conclude that the depression in bitumen recovery observed at 25[degrees]C for a given clay addition is not due to the effect of pH but rather due to the effect of processing temperature.

[FIGURES 1-3 OMITTED]

For the flotation system containing 1 mM of magnesium ions, the bitumen recoveries experienced a sharper reduction (when compared to the control case) when the clay level exceeded a certain value for both operating temperatures, as shown in Figures 4 and 5. The control tests were conducted in the absence of divalent cations at the respective temperature. The pH of the produced tailings water is plotted as a function of the amount of illite clay addition (Figure 6). As was the case where no cation addition (control case) was made, the variation of the tailings water pH with clay content is not sensitive to temperature. However, the pH drop observed in the presence of illite clay was larger when divalent cations were present than when they were not present. Here, the control curve represents the case of no divalent cation addition for 25 and 35[degrees]C as was previously shown in Figure 3. As mentioned earlier, at a given processing temperature and clay addition level, the bitumen recovery is affected by the addition of divalent cations. Since the tailings water pH at a given clay content is lower due to the addition of the divalent cations, the depression in bitumen recovery at a given temperature as shown in Figures 4 and 5 could be a consequence of pH effects.

[FIGURES 4-6 OMITTED]

It is well recognized that water based bitumen extraction processes perform better at a mildly alkaline pH (pH 8.5) (Clark and Pasternack, 1932; Sanford, 1983; Bichard, 1987; Dai and Chung, 1995; Liu et al., 2004, 2005). At acidic pH values, bitumen liberation from the sand grains becomes more difficult and low bitumen recovery would be expected. It appears that the effect of illite clay addition on bitumen recovery is also related to its acidic nature. The effect of illite clay and magnesium ion on bitumen recovery can therefore be attributed to incomplete bitumen liberation and slime coating of illite on the liberated bitumen. To reconfirm whether pH is a key factor in affecting bitumen recovery for a given processing temperature, flotation tests were conducted at a controlled pH of 8.5. The pH was controlled by adding NaOH solution into the flotation slurry. For the flotation slurry containing illite and/or magnesium ions, the bitumen recovery at pH 8.5 increased greatly at both temperatures as shown in Figures 7 and 8. This observation on the effect of NaOH addition is in agreement with previous work (Clark, 1929; Sanford, 1983; Bichard, 1987; Dai and Chung, 1996). To detect the change of divalent cations in the flotation system, atomic adsorption (AA) analysis was conducted on the flotation tailings water samples. The measured divalent cation concentrations were plotted as a function of the amount of illite addition under the different conditions (Figure 9). It shows that the divalent cation concentrations first experienced a sharp decrease and then decreased slightly with an increasing amount of illite addition for the flotation tests without pH control. For the pH controlled flotation test, the divalent ion concentrations were close to zero. At pH 8.5, calcium and magnesium ions should not precipitate (Dai et al., 1992). Therefore, the decrease in divalent cation concentration may be related to the reaction or adsorption of the cations with surfactants or clays at pH 8.5 (Smith and Schramm, 1992).

[FIGURE 9 OMITTED]

Zeta Potential Distribution Measurements

Effect of pH and divalent cations

As shown in Figures 3 and 6, the pH of the tailings water dropped significantly (as low as 4.9) during the flotation tests performed with the addition of magnesium ions and illite clay. To determine whether slime coating occurs between bitumen and illite clays at such a low pH, zeta potential distribution measurements were conducted at pH 4.9. The results in Figure 10 suggest some slight interaction between bitumen and illite for a system without the addition of magnesium ions. However, the results shown in Figure 11 suggest a strong slime coating of illite on bitumen occurred when magnesium ions were present. When clay coats a bitumen surface, there is a lower likelihood of successful air-bitumen attachment. As a result, bitumen recovery decreases. These observations correlate well with the flotation test results, i.e. the bitumen recovery was much lower for the flotation system with the co-addition of illite and magnesium ions than that with the addition of illite alone.

[FIGURES 10-11 OMITTED]

During the flotation tests, when the pH of the slurry was controlled at 8.5 by adding NaOH, depression of bitumen recovery due to the addition of divalent cations and illite clays was not observed. To determine whether or not slime-coating phenomena disappear at pH 8.5, zeta potential distribution measurements were conducted at this pH. As shown in Figure 12, the zeta potential distribution measurement results indicate slime coating of illite on bitumen occurs. This finding contradicts the observations from the flotation tests (where a large recovery drop was not observed). To explain this discrepancy, supernatant from the produced tailings water of the test with pH control was used to conduct the zeta potential distribution measurements (instead of deionized water). The results in Figure 13 suggest a little slime coating of bitumen by illite clay, which correlates well with the flotation test results. The difference in the zeta potential distribution measurements using deionized water and the measurements performed in the supernatant from the produced tailings water may be due to the presence of chemical species in the tailings water generated by the added NaOH. Zeta potential distribution measurements were repeated using the supernatant instead of deionized water to check whether the supernatant from the flotation tests without pH control has the same effect on the interactions between bitumen and illite as the supernatant from the flotation tests with pH control. The results in Figure 14 show that the acidic supernatant did not have the same effect as the alkaline supernatant in preventing the slime coating.

[FIGURES 12-14 OMITTED]

To summarize the effect of various additives on bitumen recovery, Figure 15 shows a critical role of flotation slurry pH in bitumen recovery from oil sands ore. The results here are obtained using a good processing ores doped with calcium, magnesium and/or fine clays. It is evident that for a given temperature, bitumen recovery correlates well with flotation slurry pH. Flotation at pH below 8 is not recommended as there is a sharp drop in bitumen recovery, in particular at low operating temperatures. The general trend shown in Figure 15 is significant as it advices us a caution in operation whenever dealing with ores containing a significant amount of fines which are acidic. In this case, control of flotation slurry pH by caustic addition is advised.

[FIGURE 15 OMITTED]

Use of Plant Recycle Water

Initially, bitumen recovery tests and the subsequent zeta potential distribution measurements were conducted using deionized water as discussed in the previous sections. To that end, no complications were introduced as otherwise would through the use of plant recycle water. However, subsequent bitumen recovery tests were performed using Aurora process water with the addition of 5 wt.% of illite clays and 1 mM magnesium ions. Table 2 gives the divalent cation concentration and pH of Aurora process water. The results in Table 3 show that the clay and cation additions had no detrimental effect on bitumen recovery. As well, zeta potential distribution measurements showed no clay slime-coating on bitumen.

CONCLUSIONS

1. The detrimental effect of illite clay on bitumen recovery was due to its acidity, and its negative effect could be reconciled by the addition of NaOH.

2. The presence of 1 mM calcium or magnesium ions alone in deionized water had little effect on bitumen recovery. But when co-added with illite clay, both had the same adverse effect on molar equivalent basis when tests were performed in solutions prepared using deionized water.

3. To some extent, temperature mitigated the adverse effect on bitumen recovery caused by the addition of illite clays and divalent cations.

4. Slime coating of illite on bitumen was not observed in the measurements performed using alkaline tailings water.

5. Zeta potential distribution measurements are a useful tool for studying slime coating phenomena and diagnosing flotation systems.

6. The co-presence of illite clay and divalent cations had no detrimental effect on bitumen recovery when tests were performed using plant recycle water.

ACKNOWLEDGEMENTS

Financial support for this work by NSERC Industrial Research Chair in Oil Sands Engineering is greatly appreciated.

REFERENCES

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Budziak, C. J., E. I. Vargha-Butler, R. G. V. Hancock and A. W. Neumann, "Study of Fines in Bitumen Extracted from Oil Sands by Heat-Centrifugation," Fuel 67(12), 1633-1638 (1988).

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Omotoso, O. E. and R. J. Mikula, "High Surface Areas Caused by Smectitic Interstratification of Kaolinite and Illite in Athabasca Oil Sands," Applied Clay Sci. 25, 37-47 (2004).

Sanford, E. C. and F. A. Seyer, "Processibility of Athabasca Tar Sand Using a Batch Extraction Unit: The Role of NaOH," CIM Bullitin 164-169 (March 1979).

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Xinlin Ding (1), Chris Repka (2), Zhenghe Xu (3) and Jacob Masliyah (3) *

* Author to whom correspondence may be addressed. E-mail address: jacob.masliyah@ualberta.ca

(1.) SNC-Lavalin Inc., Calgary, AB, Canada T2P 3H5

(2.) Baker Petrolite Corporation, Fort McMurray, AB, Canada T9K 1P1

(3.) Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6

Manuscript received April 15, 2006; revised manuscript received August 10, 2006; accepted for publication August 14, 2006.
Table 1. Composition of F11B oil sand ore (mass %)

Bitumen Water Solids Fines ([dagger])

14.5 3.2 82.3 9.5

([dagger]) The fines are defined as the solids smaller than 44[micro]m
and the fines content is expressed as a percentage of solids in this
size fraction with respect to the total solids.

Table 2. Divalent cation concentration and pH
of Aurora process water

Calcium (ppm) Magnesium (ppm) pH

27.8 16.2 8.2

Table 3. Bitumen recovery test results using Aurora process water

 25[degrees]C

Conditions pH Bitumen recovery

Control: no additives 8.5 88.9%

With addition of
5 wt.% illite clays and 24 ppm 8.3 88.0%
(1mM) magnesium

 35[degrees]C

Conditions pH Bitumen recovery

Control: no additives 8.4 93.8%

With addition of
5 wt.% illite clays and 24 ppm 8.3 92.5%
(1mM) magnesium

Figure 7. Effect of pH on bitumen recovery using deionized
water at T = 25[degrees]C

 Flotation without Flotation with
 pH control pH control
without [Mg.sup.2+] 5.8 8.5

with 24ppm [Mg.sup.2+] 4.9 8.5

Figure 8. Effect of pH on bitumen recovery using deionized water
at T = 35[degrees]C

 Flotation without Flotation with
 pH control pH control

without [Mg.sup.2+] 5.6 8.5

with 24ppm [Mg.sup.2+] 4.9 8.5
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Date:Dec 1, 2006
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