Dewatering of Oil Sands Tailings with Novel Chitosan-Based Flocculants

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Environmental and Carbon Dioxide Issues

Dewatering of Oil Sands Tailings with Novel Chitosan-based Flocculants Leonardo Pennetta de Oliveira, Sarang Prakash Gumfekar, Fernanda Lopes Motta, and Joao B. P. Soares

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Dewatering of Oil Sands Tailings with Novel Chitosan-based Flocculants Leornado Pennetta de Oliveira1, Sarang Gumfekar1, Fernanda Lopes Motta1, João BP Soares1,* 1

Department of Chemical and Materials Engineering, University of Alberta, 9211 116 St, Edmonton, AB T6G 1H9, Canada *Corresponding author: [email protected]

Abstract Mature fine tailings need to be dewatered to reduce the environmental impact caused by oil sands extraction. Polymer flocculants are commonly used to accelerate this process. In this work, we modified

chitosan,

a

naturally

occurring

biopolymer,

with

3-chloro-2-hydroxypropyl

trimethylammonium chloride (Chito-CTA), and also grafted polyacrylamide to chitosan (Chito-gPAM). We compared the dewatering performance of these two flocculants with that of a commercial cationic polyacrylamide (C-PAM). Chito-CTA and Chito-g-PAM dewatered tailings at rates of 18.27 m/h and 20.72 m/h, respectively. The dewatering ability of Chito-CTA and Chito-g-PAM, measured in terms of capillary suction time (CST), was below 10 seconds, whereas the value for CPAM was 82.3 seconds at optimum dosage. The turbidity of the supernatant obtained after flocculation with Chito-CTA or Chito-g-PAM was below 10 NTU, while C-PAM produced turbid supernatants. We studied the effect of flocculant microstructure on the specific resistance to filtration of the sediments. Chito-g-PAM produced sediments with the lowest resistance, 2.99×1012 m/kg, while C-PAM’s sediments had a much higher resistance of 40.26×1012 m/kg. We also used focussed beam reflectance measurement technique to determine floc size evolution, floc stability, and time required to induce floc formation. Our results indicate that chitosan-based polymers may be successfully used to treat oil sands mature fine tailings. Key words: Flocculation; Chitosan; Mature Fine Tailings; Dewatering; Oil Sands -1ACS Paragon Plus Environment

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1. Introduction The largest oil sands reserves in the world are in Canada. Bitumen is extracted from oil sands deposits using hot water and caustic in the Clark hot water extraction process.1,2 This process produces wastewater containing clays, called fluid fine tailings (FFT).3 Approximately 1 billion m3 of FFT has been deposited in tailings ponds, covering an area of approximately 176 km2.4 Over approximately two years, larger FFT particles settle to the bottom of the ponds, forming a fluid with yogurt-like consistency called mature fine tailings (MFT). MFT contains water, dissolved salts, clays, residual bitumen, and naphtha products that are toxic to the environment.2 If left unattended, tailing ponds are hard to dewater.3 The oil sands industry must develop efficient and sustainable processes to recycle water to the bitumen extraction process, thus reducing the withdrawal of fresh water from the Athabasca River, and to reclaim the land currently occupied by tailing ponds.5 Current polymer flocculation/dewatering technologies use acrylamide-based flocculants such as polyacrylamide (PAM). PAM, however, is super hydrophilic and makes loosely packed flocs that retain a large content of water. Several studies reported the combinations of chemical and mechanical dewatering techniques, such as flocculation using polymers followed by filtration or centrifugation, as summarized in a recent article.6 Some researchers enhanced the dewatering of clay suspension by synthesizing hydrophobically modified polymers, because hydrophobic groups present in the flocculant cause the sediments to retain less water.7 Although a range of innovative flocculating agents are currently under investigation, no widely-accepted technology can successfully treat MFT.8,9 Recently, natural polymer flocculants have been used to bridge particles in suspension.10-12 Biopolymers are attractive because they are non-toxic, renewable, biodegradable, and easily available.10,13-16 Biopolymers that may be used as flocculants are usually made up of starch, chitosan, cellulose, konjac glucomanna, tamarind kernel, alginate, and guar gum.14,17,18 -2ACS Paragon Plus Environment

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Chitosan-based flocculants have been used to treat wastewater contaminated with dissolved and undissolved inorganic, organic, and biological contaminants, including suspended solids, heavy metals, humic acid, dyes, algae, and bacteria.19 Despite the advantages of chitosan, its low water solubility makes it hard to use as a flocculant for MFT. Additionally, the primary amine groups that are abundant in chitosan need to be protonated to form a cationic polyelectrolyte. Cationic flocculants are preferred to flocculate MFT because cations neutralize the negative charges on MFT clay surface.20 Thankfully, the performance of chitosan can be improved by modifying it with suitable functional groups.21 We increased the water solubility of chitosan, and made it cationic, by modifying chitosan with 3-chloro-2-hydroxypropyl trimethylammonium chloride (CTA). ChitoCTA is a linear polymer with short cationic pendant groups. Recently, Das et al. demonstrated that a biobased PAM-grafted flocculant efficiently dewatered and aggregated kaolin, which is abundant in MFT.22 To combine the advantages of both synthetic and natural polymers, we also grafted PAM onto chitosan to create a better flocculant for MFT. These new polymers were used to flocculate and dewater MFT. Their performance was measured through several metrics: 1) initial settling rate (ISR), 2) capillary suction time (CST), 3) supernatant turbidity, 4) solids content of sediments, and 5) specific resistance to filtration (SRF). We also monitored floc size evolution during flocculation with these polymers using focussed beam reflectance measurement (FBRM). Our results indicate that chitosan-based polymers may constitute a viable class of flocculants for MFT.

2. Materials and Methods 2.1. Materials -3ACS Paragon Plus Environment

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Chitosan (75 wt.% deacetylation, molecular weight 310,000-375,000 Da), 3-chloro-2hydroxypropyl trimethylammonium chloride (CTA) (60 wt. % solution in H2O), acrylamide, ceric ammonium nitrate (CAN), acetone (99.9 %), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. The reference cationic polyacrylamide (C-PAM) had a molecular weight of approximately 10 million Da and 20 % cationic charge density. We selected a cationic PAM as reference material since Chito-CTA is also cationic. Isopropanol (99.9 %) and hydrochloric acid (HCl) were purchased from Fisher Scientific. Imperial Oil (Fort McMurray, AB, Canada) provided the MFT samples used in our investigation.

2.2. Synthesis of Chitosan Modified with CTA (Chito-CTA) The chitosan used in study is made by the deacetylation of chitin (Figure 1A). We dispersed chitosan in distilled water (2.8 g/L), heated it to 33 °C for 30 minutes, and added 15 mmol NaOH (1 mol/L stock solution). After mixing the suspension for 15 minutes, we added an aqueous solution of CTA (60 wt. % solution in H2O) and maintained the temperature at 33 °C for 18 hours. We stopped the reaction by adding HCl until the pH of the mixture dropped below 7. CTA caused the quaternization of chitosan, improving its solubility in water. We varied the degree of quaternization by changing the molar ratio CTA/ chitosan from 163 to 1304. We precipitated and washed the reaction mixture in excess isopropanol three times to remove unreacted CTA. Finally, we dried the washed Chito-CTA under vacuum at 40 °C for 24 h. The scheme we follow to modify chitosan with CTA is shown in Figure 1B.

2.3. Synthesis of Chitosan Grafted with PAM (Chito-g-PAM) We dissolved chitosan in 1 % acetic acid aqueous solution (0.0046 g/mL) under continuous nitrogen flow for 30 min. Next, we added CAN initiator (0.0015 g/mL) to the reaction mixture under continued stirring for 15 min. Further, we added 0.15 g acrylamide dissolved in 50 mL water -4ACS Paragon Plus Environment

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dropwise to the reaction mixture, keeping the reaction for 3 h at room temperature. The produced polymer was precipitated using excess acetone and dried under vacuum at 40 °C for 48 h. This procedure is depicted in Figure 1C.

Figure 1. Chemical reactions: (A) deacetylation of chitin to chitosan, (B) chitosan to chitosan modified with CTA (chito-CTA), and (C) chitosan grafting with PAM (chito-g-PAM).

2.4. Characterization of Flocculants and MFT

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We analyzed chitosan, Chito-CTA, and Chito-g-PAM by Fourier transform infrared (FTIR) spectroscopy using an Agilent Technologies Cary 600 Series FTIR spectrometer in the range of 4004000 cm−1.

We used the Dean−Stark extraction method to determine the amount of solids, water, and bitumen in the MFT sample (Table 1). The details of this method are described elsewhere.5 Atomic absorption spectroscopy (AAS) (Varian 220FS) was used to quantify the concentration of major ions in the MFT sample (Table 2). The MFT used to perform the flocculants tests in this work was the same characterized by Thompson et al.23 We measured the zeta potential of polymer solutions synthesized in this study using a Zetasizer Nano unit (Malvern). The solution concentration was maintained at 1000 mg/L in deionized water at pH 7. Table 1: MFT composition using the Dean-Stark method. Compound Weight % Water

62.7

Solids

32.3

Bitumen

4.2

Table 2: MFT ion composition measured using atomic absorption spectroscopy. Ion

Quantity (ppm)

Na+

251.6

K+

17.3

Ca2+

25.6

Mg2+

12.4

2.5. MFT Flocculation


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Flocculation experiments were carried out with 150 mL of a 5 wt. % solids mixture prepared by diluting MFT with deionized water in a 250 mL baffled beaker. We dosed the diluted MFT with 3000, 5000, 7000, and 10000 ppm of flocculant (on a weight basis relative to the weight of solids in the MFT). The MFT suspension was mixed in two stages: 1) at 600 rpm using a 45 degree pitch blade turbine (PBT) - 4 blades impeller (4.8 cm diameter) for 1 min to ensure a uniform dispersion of solids, 2) at 300 rpm for 1 min, while the desired amount of flocculant was added into the MFT suspension. After the second mixing step, we immediately transferred the mixture to a 100 mL graduated cylinder, and recorded the mudline position (solid-liquid interface) for 1 h. We used the same impeller in the first and second stage. The initial settling rate (ISR) was calculated from the slope of linear part of the settling profile. After settling for 24 hours, the supernatant was collected, and the turbidity was measured using a Hach 2100AN turbidimeter. The CST of the suspension after flocculation was measured using a Triton Electronics 319 multi-purpose CST apparatus using a standard Triton filter paper (7 cm × 9 cm). CST determines how fast sediments can be dewatered, by measuring the time taken by water to travel a certain radial distance over filter paper.24 The specific resistance to filtration (SRF) (m/kg) of MFT after flocculation was calculated using Equation (1),

SRF =

2 #1

where, P is the pressure (Pa), A is the filter area (m2), b is the slope calculated from the plot of t/V against V, V is the volume of filtrate (m3), t is the filtration time (s), µ is the viscosity of filtrate


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(Pa·s), and ω is the mass of solids cake formed per unit filtrate volume (kg/m3). A detailed procedure to measure SRF is described elsewhere.25-27

2.6. Floc Size Measurement We used focused beam reflectance measurement instrument (FBRM G400, Mettler-Toledo, USA) to measure the floc size evolution in real time before and after flocculant addition.20,28,29 The FBRM probe consists of a fast rotating laser beam, which is immersed in a beaker containing the MFT suspension. The incident laser beam gets reflected from the clay particles in the MFT, and is captured by the FBRM probe. The multiplication of the beam reflectance speed and the duration of the reflected signal gives the chord length, which is a measurement of floc/particle size. We also obtained floc size distribution in MFT before and after addition of the flocculants. These experiments were carried out with only 7000 ppm of Chito-CTA, Chito-g-PAM, and C-PAM.

3. Results and Discussion 3.1. Characterization of Chito-CTA and Chito-g-PAM The detailed synthesis of Chito-CTA and Chito-g-PAM is described in the Experimental Section, and their chemical structures are shown in Figure 1. The FTIR spectra of unmodified chitosan, CTA, PAM, Chito-CTA, and Chito-g-PAM are compared in Figure 2. The spectrum for chitosan shows the characteristic O-H stretch at 3000 cm-1, N-H bend at 1400 cm-1, and bridge-O stretch at 1100 cm-1.14 To confirm the modification of chitosan with CTA, we first obtained the FTIR of CTA, showing the signature methyl group peak of the quaternary ammonium at 1488 cm1 15

.

In Chito-CTA, this peak shifts to 1371 cm-1. Two functional groups on chitosan, amine and

hydroxyl, may react with CTA. The spectrum of Chito-CTA shows that hydroxyl groups are present, implying that CTA reacted with amine groups. PAM can be identified by its characteristic amide -8ACS Paragon Plus Environment

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C═O peak at 1670 cm-1, which is also present in the spectrum of Chito-g-PAM. The intensity of the hydroxyl peaks in the spectrum of Chito-g-PAM is low, implying that PAM was grafted to chitosan at the hydroxyl groups. We also confirmed the modification of chitosan by measuring the zeta potential at pH 7. As expected, increasing the molar ratio of CTA to chitosan increased the zeta potential of the polymer solution, as shown in Figure 3. Chito-g-PAM and commercial C-PAM had zeta potentials of 39.5 ± 4.07 and 37.3 ± 3.12 mV, respectively.

*

Chitosan

* * CTA

Transmitance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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*

PAM

* Chito-CTA

*

Chito-g-PAM

4000

3600

* 3200

2800

2400

2000

1600

1200

800

-1

Wavenumber (cm )

Figure 2. FTIR spectra of unmodified chitosan, CTA, PAM, Chito-CTA, and Chito-g-PAM.


400

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Figure 3. Zeta potentials of the polymers synthesized in this study.

3.2. Initial settling rate (ISR) We flocculated 5 wt% MFT with Chito-CTA with varying CTA/chitosan molar ratio, Chito-gPAM, and commercial C-PAM. Figures 4A and 4B compares these results. Flocculation experiments with Chito-CTA were carried out at varying dosages from 3000 to 10000 ppm. At 3000 ppm, settling in MFT was significant only at CTA/chitosan molar ratio of 1304. As we increased the dosage up to 10000 ppm, settling was induced at lower CTA/chitosan ratios, indicating that the sufficient cation density required to induce the solid-liquid separation in MFT was achieved. When the Chito-CTA sample had lower cation charge density, higher dosages were required to initiate the flocculation. At 7000 and 10000 ppm dosages, ISR increased up to 18 m/h, due to the increase in electrostatic interactions between negatively charged clays in MFT and positively charged cations on the polymer backbone.30 Figure 4B compares the effect of different dosages of Chito-CTA -10ACS Paragon Plus Environment

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(CTA/chitosan molar ratio of 1304) and Chito-g-PAM on ISR. Chito-g-PAM always outperformed Chito-CTA, while C-PAM did not effectively flocculate the MFT sample. We could not clearly distinguish the solid-liquid interface, which is needed to calculate ISR, for C-PAM; therefore, ISR using C-PAM is not shown in Figure 4.

Figure 4. Initial settling rate of MFT flocculated using (A) Chito-CTA of varying CTA to chitosan molar ratios and dosages, and (B) different dosages of Chito-CTA (CTA/chitosan = 1304) and Chito-g-PAM.

3.3. Capillary Suction Time (CST)

CST is a measure of the dewaterability of sludges. It is often used to quantify the performance of MFT flocculants.31 A lower CST correlates with higher dewaterability. Figure 5A shows the effect of CTA/chitosan molar ratio and polymer dosage on CST. Pure MFT has a CST higher than 300 s, but CST decreased with increasing dosage of Chito-CTA at any given CTA/chitosan molar ratio. In addition, CST decreased when the CTA/chitosan molar ratio increased from 163 to 1304. Typically, CST values of less than 20 s are acceptable in oil sands industry. We speculate that Chito-CTA -11ACS Paragon Plus Environment

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formed dense flocs that were nearly closed, which limited the diffusion of water into the flocs and caused the CST to decrease with increasing polymer dosage. Figure 5B compares the CST for MFT sediments produced by adding Chito-CTA (CTA/chitosan = 1304), Chito-g-PAM, and C-PAM at different dosages. The commercial C-PAM did not significantly reduce the CST even at the highest tested dosage, while Chito-CTA and Chito-g-PAM reduced the CST to industrially acceptable limits. At lower dosages of 3000 and 5000 ppm, Chito-g-PAM dewatered MFT faster than ChitoCTA, likely because of its longer PAM branches.

Figure 5. CST of MFT flocculated using (A) Chito-CTA of varying CTA to chitosan molar ratios and dosages, and (B) different dosages of Chito-CTA (CTA/chitosan = 1304), Chito-g-PAM, and CPAM. We also measured the composition of MFT after the dewatering using Chito-CTA, Chito-gPAM, and C-PAM. Table 3 shows the composition of MFT after dewatering. Table 3: The MFT composition after dewatering using the flocculants developed in this study.

Solids (wt. %)

Chito-CTA

Chito-g-PAM

C-PAM

11.96±4.13

20.48±5.61

34.53±10.10


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Water (wt. %)

88.04±4.13

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79.52±5.61

65.47±10.10

3.4. Supernatant Turbidity

The quality of the water recovered after the flocculation was measured by its turbidity. The presence of electrostatically stable particles in the supernatant increases its turbidity. Figure 6A shows the turbidity of the supernatant collected after 24 h of MFT flocculation using Chito-CTA of varying CTA/chitosan molar ratios and dosages. In general, the turbidity of supernatant decreased with increasing polymer dosage, because either more flocs were formed, or flocs grew larger when more polymer was added to the suspension. Other researchers reported similar observations for MFT flocculation.20 The supernatant turbidity also decreased significantly for higher CTA/chitosan molar ratios. The effect of CTA/chitosan molar ratio may be explained in terms of increased charge neutralization of fine particles in the supernatant due to the increased cation density at higher CTA/chitosan molar ratios. Figure 6B compares supernatant turbidities obtained using Chito-CTA (CTA/Chitosan = 1304), Chito-g-PAM, and C-PAM. The commercial C-PAM produced very turbid supernatant, while chitog-PAM produced the clearest supernatants at all dosages investigated in this study. The graft microstructure of the Chito-g-PAM seemed to be best suited to capture the majority of the fine particles that are otherwise difficult to neutralize using linear polymers. Chito-CTA produced acceptable supernatants only at 7000 and 10000 ppm dosages.


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Figure 6. Turbidity of the supernatant collected after 24 h of MFT flocculation using (A) ChitoCTA of varying CTA to chitosan molar ratios and dosages, and (B) different dosages of Chito-CTA (CTA/Chitosan = 1304) and Chito-g-PAM.

3.5. Specific Resistance to Filtration (SRF)

The SRF, calculated with Equation (1), quantifies the filterability of MFT after its treatment with a flocculant. Figure 7A shows the SRF of MFT after treatment with Chito-CTA of varying CTA/ chitosan molar ratios and dosages. As we increased the dosage of Chito-CTA, SRF dropped to a minimum of 3.80 x 1012 m/kg using 7000 or 10000 ppm of Chito-CTA (molar ratio 1304). At higher dosages, Chito-CTA forms more compact flocs that expel water more easily. Figure 7B compares the SRF for Chito-CTA (CTA/chitosan = 1304), Chito-g-PAM, and CPAM. Both chitosan-based flocculants produced sediments with significantly lower SRF values than the commercial C-PAM flocculant. Although Chito-CTA may become polar due to the quaternary modification using CTA, the incomplete deacetylation makes it partially hydrophobic, causing the MFT flocs to expel water more easily. -14ACS Paragon Plus Environment

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Chito-g-PAM and Chito-CTA produced sediments with similar SRF values, except at the 3000 ppm dosage. The overall higher SRF of C-PAM likely arose from C-PAM’s ability to hold significant amounts of water in the flocs. Chito-CTA and Chito-g-PAM, therefore, offer good alternatives for post flocculation treatment processes that involve filtration at moderate pressures to recover the water and densify the sediments.

Figure 7. Specific resistance to filtration of MFT after the treatment using (A) Chito-CTA of varying CTA to chitosan molar ratios and dosages, and (B) different dosages of Chito-CTA (CTA/chitosan = 1304), Chito-g-PAM, and C-PAM.

3.6. Floc Formation in MFT

Figure 8A shows the real-time evolution of floc size in MFT using 10000 ppm of Chito-CTA, Chito-g-PAM, and C-PAM (CTA/chitosan = 1304). Figure 8A provides information about highest floc size, decrease in floc size over time, floc stability, and the time required to induce the floc formation.


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The commercial C-PAM formed flocs with largest sizes (32 µm), but floc size significantly decreased over the analysis period. On the other hand, flocs formed using Chito-CTA and Chito-gPAM grew up to maximum of 25 µm, but the floc size did not change significantly over the flocculation period. Flocs formed using C-PAM were not very stable, perhaps because the flocs break due to inefficient/weak adsorption of C-PAM onto the MFT clay surfaces. Contrarily, flocs formed with Chito-CTA and Chito-g-PAM reached stable sizes after approximately 300 s. The time required for the adsorption of polymer chains onto particles, and subsequent floc formation, is called characteristic adsorption time. It is defined as

=

− ln1 − 2

where, f is the fraction of total polymer needed to adsorb onto the clay surface to induce flocculation, k is the frequency of collisions between polymer molecules and clay particles per unit time, and N is the initial number of particles per unit volume in the suspension. It is evident from the Figure 8A that the characteristic adsorption time was highest for C-PAM and lowest for Chito-gPAM. The graft microstructure of Chito-g-PAM possibly helped it adsorb onto the clay surfaces and form flocs faster than Chito-CTA and C-PAM. Figure 8B compares the floc size distribution when a dosage of 10000 ppm of Chito-CTA, Chito-g-PAM, and C-PAM was used. We used the square weighted counts because it resolves even small changes in floc size. Various researchers have also recommended that the square weighted counts represent an unbiased change in the system.32-34 The average of 10 particle size distributions (PSD) for pure MFT and for flocs produced with each flocculant are compared in Figure 8B. Chito-CTA and Chito-g-PAM made flocs with smoother PSD than C-PAM, and the PSD for Chito-CTA is shifted to higher particles sizes, which agrees with -16ACS Paragon Plus Environment

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the results in Figure 8A, where the cord length of flocs made with Chito-CTA is higher than Chitog-PAM. The PSD results for C-PAM, however, are less definitive. Figure 8A shows that the flocs with the highest cord lengths are formed with C-PAM. This result, however, contradicts the PSD for CPAM in Figure 8B, which is very similar in range to that of flocs made with Chito-g-PAM. This apparent contradiction may be explained to the noisy results obtained when C-PAM was used to flocculate MFT. It seems that C-PAM produced flocs that were unstable and fluctuated in size throughout the flocculation. We visually observed that C-PAM produced sludge/sediments with significantly large water entrainment that were likely shear-sensitive. Large flocs settle faster and may not be detected by the FPRM probe, while smaller flocs, resulting from the breakage of the weak larger flocs may be re-suspended and detected by the FBRM probe, causing the observed distortion in the PSD reported in Figure 8B.

40 (A)

Chito-CTA Chito-g-PAM C-PAM

30

Counts (squared weighted)

32

Chord length (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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28 26 24 22

35 30

MFT Chito-CTA Chito-g-PAM C-PAM

(B)

25 20 15 10 5

20

0 0

100

200

300

400

500

600

700

Time (s)

10

100 Chord length (µm)

Figure 8. (A) Real-time evolution of floc size in MFT using 10000 ppm of Chito-CTA (CTA/chitosan = 1304), Chito-g-PAM, and C-PAM, (B) floc size distribution in MFT and MFT flocculated with 10000 ppm of Chito-CTA, Chito-g-PAM, and C-PAM. -17ACS Paragon Plus Environment

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Dewatering of MFT mainly involves costs for flocculant raw materials, energy required for the reactions/mixing, and mechanical operations such as filtration/ centrifugation/ freeze-thaw etc. Rodrigues et al. considered similar factors for analyzing techno-economic feasibility of wastewater treatment using coagulation/flocculation.35 Since the chitosan, the main raw material, is obtained from abundantly available (second largest biopolymer in the world) chitin in crustaceans, it is cheaper than commercial synthetic monomers used for the synthesis of flocculants. In addition, chitosan comes as a polymer of varying molecular weight; therefore, there is no cost/energy for polymerization of monomers. Further, there is no known adverse footprint of flocculation using the chitosan-based flocculants since the chitosan is a linear polysaccharide and does not degrade into toxic byproducts. The overall operational cost may increase with flocculant dosage but it will be much lesser than flocculants made from synthetic monomers. 4. Conclusions Although several papers have been published on the use of chitosan as flocculants, these polymers have been rarely used to flocculate mature fine tailings. Our work intended to fill the knowledge gap. We

synthesized

two

flocculants:

chitosan

modified

with

3-chloro-2-hydroxypropyl

trimethylammonium chloride (Chito-CTA) and chitosan grafted with polyacrylamide (Chito-gPAM). We compared the flocculation performance of these two flocculants with a commercial cationic polyacrylamide (C-PAM) flocculant. CTA caused the quaternization of chitosan that improved its solubility in water. We varied the degree of quaternization by changing the mole ratio of CTA to chitosan from 163 to 1304. Free radical polymerization of acrylamide produced grafts of polyacrylamide onto chitosan backbone. Chito-g-PAM settled MFT faster than Chito-CTA, while CPAM could not even form a distinct solid-liquid separation mudline.


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Page 20 of 26

Conventionally, calcium ions are added to the mature fine tailings suspension to neutralize the negative charges on clay and subsequent bridging is facilitated by the polymeric flocculants. The recycled water contains the calcium ions, which create operational issues such as reduced efficiency of bitumen extraction and equipment fouling. It is important to point out that no calcium was added to help flocculation, which is desirable to the re-use of the recovered water from mature fine tailings flocculation with chitosan based polymers presented in this work. Chito-g-PAM dewatered mature fine tailings quicker than Chito-CTA and C-PAM, as evidenced by capillary suction time measurements. In general, the hydrophobic core of chitosan reduced the amount water entrapped in the flocs in comparison with the commercial high molecular weight C-PAM. Among the three flocculants Chito-g-PAM exhibited higher fraction of hydrophobicity that caused faster dewatering of mature fine tailings. Supernatant turbidity was significantly lower at all dosages tested for Chito-g-PAM, and was within the acceptable industrial limits. Chito-CTA and Chito-g-PAM offered similar specific resistance to filtration at all dosages tested in this work. Focused beam reflectance measurement enabled us to determine the highest floc size, decrease in floc size over the period of time, stability of flocs, and time required to induce the floc formation in mature fine tailings. Chito-CTA and Chito-g-PAM produced flocs with maximum cord length of 25 µm. which were stable throughout the flocculation. C-PAM created flocs/sludge with uneven floc size that seemed to randomly fluctuate during the flocculation. Chito-CTA produced flocs with broad floc size distribution, showing its ability to capture particles with a wide range of sizes. Our results show how to make novel chitosan-based polymers to treat oil sands tailings. Our chitosan-based flocculants are promising candidates to effectively dewater mature fine tailings, although further modifications are certainly needed to be done to make these polymers commercially competitive.


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Energy & Fuels

Acknowledgements Authors gratefully acknowledge the financial support from the Campus Alberta Innovation Program Chair in Interfacial Polymer Engineering for Oil Sands Processing. The authors also thank Dr. Marco da Silva for his help with the molecular weight analysis of the polymers studied in this work.

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Dewatering of Oil Sands Tailings with Novel Chitosan-Based Flocculants

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