Role of Preconditioning Cationic Zetag Flocculant in Enhancing

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Role of Preconditioning Cationic Zetag Flocculant in Enhancing Mature Fine Tailings Flocculation Chen Wang,* Chao Han, Zehui Lin, Jacob Masliyah, Qingxia Liu, and Zhenghe Xu Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada S Supporting Information *

ABSTRACT: The ongoing generation of mature fine tailings (MFT) or fluid fine tailings (FFT) from surface mining activities of the oil sands industry in Canada has been a contentious issue for many years. In the absence of large-scale processing facilities, many far-reaching consequences from extensive stockpiling of FFT will plague the industry for many years to come. Application of polymeric flocculants to treating FFT for efficient solid−water separation has been well-established. However, most commercially used flocculants carry a negative charge and yield incomplete capture of suspended fine solids and hence relatively turbid recycle water. This inefficient flocculation of fine solids limits the effort of process water recycling and severely strains most downstream dewatering processes, such as filtration. Cationic flocculants offer a promising alternative in terms of overall solids capture and recycle water quality, although the associated high cost hindered much of its commercial applications. In this work, we introduce a method to deploy a commercial cationic flocculant (Zetag 8110). Heating and increasing pH of the flocculant solution in oil sands process water led to more effective fines flocculation and a supernatant of 1100 NTU), indicating that insufficient dosage was applied (left side of Figure 2). In fact, further tests showed an extremely high dosage of Zetag at 6−8 kg/t to achieve a clear supernatant. When Zetag solutions hydrolyzed at room temperature were applied to the same MFT samples, improvements in floc growth became evident, as shown in Figure 1, although the floc growth remained a slow process. Flocs generated by the Zetag solutions aged for 7 days showed a noticeably better performance with a single-stage growth rate of 2.29 ± 0.2 μm/s. However, there was no improvement in the supernatant turbidity with Zetag solutions aged for 7 days at ambient temperatures. In contrast, the supernatant turbidity, as shown on right side of Figure 2, was reduced to 70 ± 4 NTU, accompanied by the formation of large, discrete, and shearresistant flocs when the same MFT samples were flocculated by the same dosage of Zetag aged for 7 months under ambient conditions. A two-stage floc growth process was observed, with the growth rate being 2.83 ± 0.3 and 13.9 ± 0.8 μm/s for the first and second growth stages, respectively. The much higher growth rate over the second stage of floc growth is likely a result of flocculation among the small flocs formed during the first stage of flocculation. The flocs settled immediately with much higher solids content sediments shown by a smaller sediment height on right side of Figure 2 when slurry agitation was stopped. These observations presented an interesting opportunity of the use of Zetag family flocculants in oil sands tailings treatment. When the Zetag solution was hydrolyzed to a desired extent, it is possible to achieve a rapid and nearcomplete solid−liquid separation while reducing the overall flocculant dosages and hence the cost. It is important to note that the experiments with the results shown in Figure 1

Figure 2. Clarity of the supernatant and photographs of 5 wt % MFT flocculated by freshly prepared (left) and extensively aged (right) Zetag solutions.

involved a 5 min flocculant addition as a result of relatively high dosages applied. This long flocculant addition period was used to avoid local overdosing of the flocculant that may arise from adding flocculant too quickly. Despite the good efficacy of the aged Zetag solution, a 7 month aging is not practical or acceptable to use for further research efforts. Duplicate experiments for this case were deemed sufficient for demonstration purposes and providing a direction in applying Zetag flocculant to MFT treatment. To take advantage of this aging phenomenon in a more feasible manner, it is necessary to promote the kinetics of hydrolyzing Zetag solutions as described in method B, where high pH or high temperature was proposed to speed up the hydrolysis of Zetag over the same time frame as in method A (e.g., 18 h). Flocculation by Zetag Solutions Prepared Using Method B. Figure 3 (panels a and b), Figure 4, and Figure 5 show the FBRM results obtained using Zetag solutions hydrolyzed at elevated temperatures and/or solution pH prior to their addition to diluted MFT. All of the heated solutions were cooled to ambient temperature before their use in flocculation tests. Figure 3 (panels a and b), Figure 4, and Figure 5, each batch of flocculant was added at the 1 min mark. Although the FBRM results were highly reproducible in most cases, it was not always possible to obtain high-quality data for some systems shortly after the 3 min mark due to the formation of large flocs that amass near and block the FBRM probe, leading to large signal fluctuations. Nevertheless, all trends in these figures are D

DOI: 10.1021/acs.energyfuels.6b00108 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 4. Comparison of flocculating 5 wt % MFT by 1.5 kg/t Zetag hydrolyzed for 18 h at 22 °C and three different pH values (8.5, 8.67, and 8.8). Flocculation was conducted at pH 8.5 for all of the cases to emphasize the effect of Zetag hydrolysis at different pH values on flocculation by the resulting Zetag flocculant.

Figure 3. (a) Flocculation of 5 wt % MFT by 1.5 kg/t Zetag hydrolyzed for 18 h at pH ∼ 7.7 and two different temperatures (60 and 70 °C). Additional experiments were repeated with Zetag hydrolyzed at 70 °C to show large variations in the flocculant performance. (b) Comparison of flocculating 5 wt % MFT by 1.5 kg/t Zetag hydrolyzed for 18 h at pH ∼ 7.7 and two different temperatures (75 and 80 °C).

Figure 5. Comparison of flocculating 5 wt % MFT by 1.5 kg/t Zetag hydrolyzed for 18 h at 70 °C and three different pH values (8.5, 8.67, and 8.8). Flocculation was conducted at pH 8.5 for all of the cases to emphasize the effect of Zetag hydrolysis at different pH values on flocculation by the resulting Zetag flocculant.

sufficient to qualitatively determine the extent of floc growth and likelihood of floc breakage, with larger and shear-resistant flocs along with a clear supernatant being the more desirable characteristics. The results in Figure 3a show similar two-stage floc growth patterns (first, 2.53 ± 0.7 μm/s; second, 13.57 ± 2.2 μm/s) and maximum sustained floc sizes by Zetag hydrolyzed at 70 °C for 18 h (method B) to the case obtained using Zetag solutions aged for 7 months by method A (results shown in Figure 1), accompanied by an average supernatant turbidity of 153 ± 10 NTU. These results demonstrate excellent flocculation performances at a significantly reduced dosage (1.5 kg/t) of Zetag hydrolyzed at an elevated temperature of 70 °C. The effective flocculation led to relatively large variations (∼100 μm) in the maximum attained floc sizes and less clear two-stage floc growth mechanisms with the first- and second-stage growth rate at 0.46 and 9.78 μm/s, respectively (see the Supporting

Information). The varying amount of adipic acid present in Zetag appeared to be responsible for this discrepancy. Without solution pH adjustments, the pH of Zetag solution by method B could deviate slightly from 7.7, potentially causing these aforementioned variations. The Zetag solutions hydrolyzed at temperatures below 70 °C (e.g., at 60 °C) by method B showed a less efficient flocculation with a single-stage growth rate of 4.24 ± 0.3 μm/s and smaller measured average floc sizes of ≤400 μm that were more susceptible to shear, leading to an average supernatant turbidity greater than 1100 NTU. Although the flocculation performance using Zetag hydrolyzed by method B at 60 °C could be improved with higher Zetag dosages, our objective is to maximize the benefit of Zetag hydrolysis by keeping the Zetag dosage constant at 1.5 kg/t. E

DOI: 10.1021/acs.energyfuels.6b00108 Energy Fuels XXXX, XXX, XXX−XXX

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supernatant of ≤200 NTU turbidity from 5 wt % solids MFT. The pertinent fundamental mechanisms of improved flocculation performance by controlled hydrolysis of Zetag at 70 °C and pH 8.5 were investigated as follows. Verification of Zetag Hydrolysis by Method B. Figure 6 shows the concentration of choline extracted in DI water from

Figure 3b shows the effect of increasing the temperature of hydrolysis above 70 °C using method B on flocculation efficiency of the hydrolyzed Zetag. Flocculation by Zetag hydrolyzed at 75 and 80 °C exhibits a two-stage floc growth mechanism with similar growth rates: first at 2.88 ± 0.02 and 2.58 ± 0.04 μm/s and second at 13.81 ± 2.1 and 12.65 ± 2.2 μm/s, respectively. As shown in Figure 3b, increasing the temperature of Zetag hydrolysis decreases the maximum sustained floc sizes (∼50 μm) and progressively deteriorates supernatant quality. Hydrolysis at temperatures greater than 80 °C was not investigated as a result of the risk of polyacrylamide decomposition by breaking carbon−carbon bonds and resultant chain scission. The results collectively show the best benefits of increasing the temperature of Zetag hydrolysis using method B to 70 °C without pH adjustment. To illustrate the effect of Zetag hydrolysis occurring at the pH near the value of 5 wt % MFT on MFT flocculation without the additional effect of disturbing the pH of diluted MFT (in cases of hydrolysis pH > 8.5), it was necessary to equalize the pH of Zetag solutions prepared using method B with the pH of MFT slurry immediately before its addition. Figure 4 shows that progressively larger but less shear-resistant flocs were formed by increasing hydrolysis pH from 8.5 to 8.8 at 22 °C. Flocculation using Zetag solutions hydrolyzed under these conditions yielded a slower and single-stage floc growth with growth rates ranging from 0.91 ± 0.1 to 1.39 ± 0.05 and 2.67 ± 0.3 μm/s at pH 8.5, 8.67, and 8.8, respectively, accompanied by a more turbid supernatant. Evidently, changing solely the pH of Zetag hydrolysis using method B is not sufficient to achieve a desired flocculation performance. It was therefore necessary to explore the combined benefits of increasing both the hydrolysis temperature and pH using method B for the optimum flocculation performance. The results in Figure 5 show that, at the optimized temperature of 70 °C, increasing pH of Zetag hydrolysis from 7.7 to 8.5 while flocculating diluted MFT at the same pH of 8.5 resulted in maximum two-stage growth rates of 4.20 ± 0.4 and 16.10 ± 2.3 μm/s. Although supernatant turbidity increased to 198 ± 2 NTU, the solids content of ∼0.07% TSS in the supernatant is still far below the specification of 0.5% TSS (industrial standard) for recycle of the released water to the extraction process. The results demonstrated an excellent synergy of the beneficial roles of increased Zetag hydrolysis at a higher temperature and pH. Increasing hydrolysis pH further above 8.5 significantly deteriorated both the floc growth rate and supernatant quality. For example, the floc growth rates by Zetag hydrolyzed at pH 8.67 were reduced from optimal values of 4.20 ± 0.4 and 16.10 ± 2.3 μm/s to 1.07 ± 0.2 and 5.53 ± 0.6 μm/s, respectively, for the first and second growth stages. For the hydrolysis of Zetag at pH 8.8, only a single-stage flocculation at a growth of 2.18 ± 0.2 μm/s was observed. These results indicate a significant reduction in binding of the hydrolyzed Zetag polymers with the suspended particulates. Because the flocculation experimental conditions used in this study were too mild to break the carbon−carbon bonds of the polyacrylamide chains, as indicated by a constant viscosity of Zetag solutions after shearing under the identical conditions, the progressive affinity losses are believed to be related to the increments in ionic strength of the Zetag solutions. The results above suggest an optimal condition of Zetag hydrolysis at 70 °C and pH 8.5 under agitation for 18 h. With such hydrolyzed Zetag as the flocculant, we were able to consistently obtain large and shear-resistant flocs with a

Figure 6. Amount of choline extracted into DI water from Zetag solutions prepared by methods A and B at 70 °C and pH 8.5, with the latter showing an optimal flocculation performance. The dashed horizontal line denotes the calculated maximum choline concentration assuming that all quaternized amine groups are extracted in DI water.

freshly prepared Zetag solutions by methods A and B at 70 °C and pH 8.5, with the latter showing an optimal flocculation performance. As anticipated, only a small fraction of choline (0.17 ppm) was extracted in DI water from hydrolyzed Zetag solutions by method A, whereas almost all choline (1.69 ppm) was extracted from Zetag solutions hydrolyzed by method B. It is evident that Zetag hydrolysis occurred even by method A as a result of the slightly alkaline pH (∼7.7) of Zetag solutions. The low choline yield over 18 h of hydrolysis by method A suggests that most cationic amine groups remain attached to Zetag polymers under this condition. It is interesting to note that the choline concentration determined after hydrolysis using method B is slightly higher than the calculated concentration, most likely as a result of the measurement error in the cationic content reported by the vendor for the original Zetag. Nevertheless, all of the cationic amine groups appeared to be dissociated from the polyacrylamide backbone and, hence, became extractable ions when Zetag was hydrolyzed by method B at 70 °C and pH 8.5, producing an A-PAM with ∼10% anionic charge along with dissociated cationic choline ions in aqueous solutions. Viscosity of Hydrolyzed Zetag Solutions. To examine whether hydrolysis of Zetag broke its backbone and hence reduced the MW of the hydrolyzed Zetag, the viscosity of 0.1 wt % Zetag solutions prepared in DI water with or without hydrolysis and dialysis was measured using an Ubbelohde viscometer. It should be noted that the process water could not be used in this set of viscosity measurements of Zetag solutions, because the presence of various organic and inorganic ions in the process water would introduce an additional effect on Zetag solution viscosity as a result of their interactions with charges of Zetag molecules. F

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viscosity increase is within the experimental error. The dialysis of hydrolyzed Zetag polymer solutions is believed to reduce significantly concentrations of ionic species, e.g., choline, Na+, and OH−, present in the hydrolyzed solutions. Negative charges on the polymer backbone will attract some soluble cations and repel all anions during ion migration, which causes slightly disproportional ejections of cations and anions from the hydrolyzed Zetag solutions. As a result, it is possible that dialysis following Zetag hydrolysis preserves residual cations that lead to self-association of negatively charged polyacrylamide backbones, contributing to a minor increase in viscosity. Hypothesized Role of Zetag Polymer Hydrolysis. On the basis of the results above, hydrolysis of Zetag solutions using method B appears to lead to almost complete dissociation of cationic quaternary amine groups from the polyacrylamide backbone structures. The cationic choline ions released as a result of Zetag hydrolysis can coagulate the suspended clays, ultrafines, and bitumen in the MFT slurry via charge neutralization. The disentanglement of polyacrylamide backbones is anticipated to either enhance directly the flocculation of suspended particles by bridging via hydrogen bonding with the particle surfaces or become attracted to the particles coagulated with choline. In the classical work on flocculation by Kitchener,9 it was stated that flocculation preceded by coagulation is more efficient than flocculation alone, which is supportive of the hypothesized beneficial role of charge neutralization by cationic choline released during hydrolysis of Zetag solutions. These two mechanisms collectively lead to an overall increase in affinity between the hydrolyzed Zetag flocculant and suspended solids, which may be responsible for the excellent flocculation performance achieved at much reduced Zetag dosages. Figure 8 compares the flocculation performance of Zetag hydrolyzed under optimal conditions by method B to two commercial A-PAM flocculants used in the oil sands industry (Hychem AF246 and Magnafloc 1011). Clearly, the hydrolyzed Zetag exhibits superior performance when applied to 5 wt % diluted MFT, as shown by a much larger SWM chord length of flocs and a clear supernatant of much lower turbidity.

On the basis of the structure of Zetag molecules shown in Scheme 1, one would expect a decrease in the Zetag MW from 12.7 million Da (baseline) to ∼11.8 million Da once all quaternary amine groups were dissociated by hydrolysis and removed by dialysis. However, the results in Figure 7 show a

Figure 7. Changes in viscosity-based MW of Zetag (prepared in DI water) upon hydrolysis and/or dialysis.

more significant decrease in solution viscosity, corresponding to a viscosity average MW of 3.51 million Da, as compared to the anticipated viscosity average MW of 11.8 million Da. Clearly, such a significant reduction in the viscosity average MW upon hydrolysis of Zetag could not be explained solely by choline removal from the Zetag. Following the previous assertion that the hydrolysis conditions in this study were too mild to cause polyacrylamide chain scission, it is possible that Zetag self-associates (entangles) in DI water as a result of incomplete hydrolysis. For Zetag solutions hydrolyzed using method A, for example, about 11% of the cationic amine groups on Zetag are converted to negatively charged carboxylic groups, which could electrostatically attract any remaining cationic amine groups. Selfentanglement of Zetag molecules could occur as a result of intra- and intermolecular attractions to become coiled, which causes a significant increase in viscosity of resulting polymer solutions. This self-entanglement also reduces the amount of cationic sites on the hydrolyzed Zetag molecules that are available for the attachment to solid particles in 5 wt % MFT, leading to higher polymer dosages required for effective flocculation. Both of these hypothesized consequences have been encountered in this study and thus are plausible. Upon hydrolysis, the backbone of remaining polyacrylamide chains carries exclusively a net negative charge that causes electrostatic repulsion among the remaining polymer segments, leading to reduced entanglement of polymer backbones and, hence, a decrease in viscosity, which was indeed observed. The increased extension of polymer chains allows for better bridging of fine particles, making more efficient use of the polymer for fine particle flocculation, which is considered as a possible dosage reduction mechanism offered by Zetag solutions hydrolyzed by method B. Interestingly, there appears to be a slight increase in viscosity after dialysis of Zetag solutions hydrolyzed by either method A or B. The increase is more pronounced for Zetag solutions hydrolyzed by method B than by method A, in which case the

Figure 8. Flocculation performance of Hychem AF246 (MW of 14.5 million Da, with ∼10% anionic charge), Magnafloc 1011 (MW of 17.5 million Da, with ∼27% anionic charge), and Zetag hydrolyzed by method B, all at 1.5 kg/t dosage. G

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Energy & Fuels Future Research Opportunities. It would be interesting to examine the links between the extent of Zetag hydrolysis and flocculation performance in terms of floc size, morphology and strength, and turbidity of supernatants. A further systematic study on the positive role of choline in flocculation would also be beneficial. We have carried out some exploratory work in this regard. On the basis of the results in Figure 6, tests were conducted by adding 0.051 kg of choline per tonne of fines to 5 wt % MFT, followed by the addition of anionic flocculants AF246 or Magnafloc 1011. However, no significant impact on the floc size was determined by FBRM until the choline dosage reached ∼0.1−1 wt % fines (see the Supporting Information). The large specific surface area of the dispersed fines and the low MW of choline appear to necessitate such high choline dosages to show any effect. In comparison with Zetag hydrolyzed by method B, it seems that in situ-generated choline ions function collectively with the polyacrylamide backbone during flocculation, instead of sequentially. Another approach to illustrate the role of choline is to deliberately remove choline from the hydrolyzed Zetag solutions. However, the removal of choline via dialysis is difficult if not impossible to perform as a result of complex chemistry of Zetag solutions prepared using process water. When such a complex saline solution is subjected to the dialysis using the aforementioned dialysis bags, a huge number of species will be extracted to the exterior water phase alongside choline ions. Such extraction causes enormous changes in polymer solution chemistry and properties, making the subsequent analyses a non-trivial task. Attempts to circumvent this issue were carried out by hydrolyzing or dialyzing a concentrated (0.5 wt %) Zetag solution in DI water, followed by process water dilution to 0.1 wt % with thorough mixing. As shown in the Supporting Information, such a dilution procedure produced far inferior results, emphasizing the importance of in situ generation of choline and conformation change of the hydrolyzed Zetag in flocculation of diluted MFT. Further quantitative analysis on polymer solution chemistry, viscosity, and titration in combination with measuring dynamic light scattering can yield new insights into the dosage reduction mechanisms responsible for Zetag 8110 and other cationic polyacrylamide polymers. Ultimately, we hope to spur new developments in novel polymer flocculants or new methods to deploy existing polymer flocculants, which can effectively and efficiently process oil sands tailings to positively aid any downstream dewatering processes on large scales.

while retaining its excellent flocculation performance of 5 wt % solids MFT by controlled hydrolysis. It was hypothesized that the dissociation of cationic functional groups from the polyacrylamide backbone structure can lead to an increased number of solids−polymer bonding sites, leading to simultaneous use of both parent and daughter reagents in fine particle or MFT flocculation through an overall increase in the solids− polymer affinity and hence a drastic reduction in polymer dosages. Applying Zetag solutions hydrolyzed using enhanced hydrolysis to MFT slurries of 5 wt % solids yields the formation of large and compact flocs with a significant reduction in turbidity of supernatants to