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Effect of shear on the yield stress and aggregate structure of flocculant-dosed, concentrated kaolinite suspensions
Minerals Engineering 123 (2018) 95–103
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Effect of shear on the yield stress and aggregate structure of flocculantdosed, concentrated kaolinite suspensions Ravi Neelakantan1, Farid Vaezi G.2, R. Sean Sanders
T
⁎
Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Mineral tailings Polymer flocculation Thickening Shear energy input FBRM Aggregate size Vane yield stress
The rheology of paste or thickened tailings, which are mineral tailings treated with polymers and subsequently dewatered, is complex and time-dependent. The complex rheological behavior is exacerbated by the permanent change in rheology that occurs when the mixture is exposed to shearing (e.g. in pumps and pipes), primarily through breakdown and restructuring of the aggregates of particles that comprise the dispersed phase. While the effects of shearing on structural changes of aggregates in a dilute suspension are reasonably well characterized, the relationship in more concentrated suspensions treated with polymer flocculants is not well understood. In the present study, a custom-built, concentric cylinder shearing apparatus is used to shear a large volume of flocculant-dosed (thickened) kaolinite suspension. The evolution of the particle size distribution (PSD) and vane yield stress were monitored as a function of shear energy input. Size distributions were obtained using the Focused Beam Reflectance Measurement (FBRM) technique. Concentrated suspensions were prepared using two different anionic polymers and at two suspension pH values to evaluate the effect of polymer structure and water chemistry on changes in the vane yield stress and aggregate size. Changes in both measured parameters correlated directly with shear energy input. During shearing, a large population of smaller aggregates, in the 10–100 μm size range, is generated. Aggregate size reduction occurs in concert with structural changes; together, they dictate the value of the equilibrium (fully-sheared) yield stress. At pH 7, the aggregates were able to restructure due to edge-edge associations between kaolinite particles. This phenomenon was not observed at pH 8.5, resulting in an equilibrium yield stress ∼1/3 the value of that measured at pH 7. The chemical composition of the polymer flocculants did not affect the yield stress or aggregate size over the shear energy input range studied here.
1. Introduction
remaining tailings, as thickener underflow slurry (suspension), is highly concentrated and is transported via pipeline to a disposal area for further dewatering, consolidation and subsequent land reclamation (Alberta Energy Regulator, 2009; U.S Department of the Interor, 1977). One of the sensitive and very important characteristics of a thickened tailings mixture is its rheological behavior. For example, it is critical to prevent the settling of coarse particles in laminar pipe flow (Hanks, 1986; Thomas, 1977), while sustaining sufficient strength for subsequent land reclamation, which ultimately requires yield strengths in excess of 10 kPa (Alberta Energy Regulator, 2009). Kaolinite represents one of the most predominant clay types encountered in mineral tailings and is often used in model studies due to its well characterized chemistry and morphology (van Olphen, 1963; Addai-Mensah, 2007; Mpofu et al., 2003; McFarlane et al., 2005; Klein and Hurlbut, 1993). Derakhshandeh (2016) demonstrated the
The mining industry produces millions of tonnes of fluid fine tailings annually, and this is expected to increase with rising demand for precious minerals (Wang et al., 2014). The long-term storage of fluid fine tailings incurs both an increased operating cost and concerns of environmental sustainability. Environmental protection legislation is enforcing increasingly strict regulations on water use in mineral processing operations. Some open-pit mining operations are located in arid climates where water usage is further restricted by the lack of available water. Tailings produced in open-pit mining operations typically contain clays, silts, sand, and residual ore suspended in a large volume of water. In tailings treatment processes, rapid destabilization of the fineparticle (clay) suspension is achieved using polymer flocculants added in a thickener to facilitate the immediate recovery of water. The
⁎
Corresponding author. E-mail address: [email protected] (R.S. Sanders). Present address: PARC, A Xerox Company, Palo Alto, CA, USA. 2 Present address: Teck Resources Ltd, Trail, British Columbia, Canada. 1
https://doi.org/10.1016/j.mineng.2018.03.016 Received 12 November 2017; Received in revised form 27 February 2018; Accepted 13 March 2018 Available online 09 May 2018 0892-6875/ © 2018 Elsevier Ltd. All rights reserved.
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are referred to here as ‘fragments’. Shearing also decreased porosity and increased aggregate fractal dimension, DF, indicating that the structural change was due to both fragmentation and compaction, particularly at higher shear rates and longer shearing times. It was found that the porosity and aggregate size were related through a power-law relationship, i.e.
applicability of model kaolinite tailings for fluid mechanics studies of fluid fine tailings. Fine tailings and kaolinite suspensions had similar rheological behavior for a given particle size, zeta potential and solids volume concentration; and, as expected, at higher concentrations, both exhibited time- and shear-dependent behavior (Adeyinka et al., 2009). Kaolinite has two different basal faces – one composed of octahedral alumina and the other composed of tetrahedral silica. At the so-called “edge” surface, which accounts for roughly 10% of the particle surface area, aluminol and silanol bonds are exposed (Addai-Mensah, 2007). The edge iso-electric point occurs at pH 5.25 (Braggs et al., 1994) while that of the silica basal face is pH ∼2 (Masliyah et al., 2011), and that of the alumina basal face is between pH 6 and pH 8 (Gupta and Miller, 2010). Kaolinite particles also undergo isomorphous substitution of lower valence cations (van Olphen, 1963) along the silica and alumina basal planes resulting in a permanent negative charge. The kaolinite particle anisotropy produces a variety of aggregate structures depending on the pH, including edge-edge, edge-face and face-face associations. Under appropriate conditions (e.g. high pH and low salt concentration), kaolinite particles are dissociated due to strong repulsive electrostatic interactions. The effects of pH on kaolinite surface chemistry influence polymer flocculant selection, where the appropriate charge, charge density, pendant group and molecular mass (Mpofu et al., 2003; Hogg, 2000) is required to efficiently destabilize the suspension. The most commonly used polymer flocculants for destabilizing mineral tailings are high molecular mass anionic polyacrylamides (PAM A), which are favored due to the high initial settling rates (ISR) achieved at relatively low dose and cost (Hogg, 2000; Sworska et al., 2000; Nasser and James, 2007, 2006; McFarlane et al., 2005). PAM A with high molecular weight flocculates particles via a bridging mechanism (Mpofu et al., 2003; Fan et al., 2000; Ovenden and Xiao, 2002), which requires that the polymer is of sufficient length to surpass the electric double layer of the particle and come in contact with other particles. Adsorption occurs at the edge sites of kaolinite through hydrogen bonding interactions (Lee et al., 1991; Nabzar and Pefferkorn, 1985); however, repulsions between negatively charged groups on the polymer and particle surface limit adsorption (Mpofu et al., 2003). Electrostatic repulsions at the surface can also be beneficial, as they allow the polymer to adopt a more extended conformation which is advantageous to bridging and produces stronger aggregates (Nasser and James, 2006). The resulting flocculated aggregates are highly porous, thus entrapping water, and require further compaction to increase the overall solids loadings of the tailings. Vaezi et al. (2011) flocculated dilute kaolinite clay suspensions with PAM A (Magnafloc® 1011) at a constant shear rate of 145 s−1 and found that aggregates initially produced were very large (200–1400 μm) and quite porous. Aggregates greater than 400 μm in diameter were the most porous with internal void fractions of 0.90 ⩽ ε ⩽ 0.95. The authors observed a trend between the aggregate porosity (or aggregate effective density) and diameter such that aggregate porosity increased with diameter. Aggregates smaller than 200 μm in size had porosities of 0.60 ⩽ ε ⩽ 0.80 , meaning there was less water contained in the aggregate structure. As flocculation time progressed, the aggregates decreased in size due to fragmentation and reached a dynamic steady state. Similar results were observed by Klein (2014), who studied flocculation of dilute (8% solids by mass) tailings and observed a decrease in aggregate size when mixing time increased, with the size eventually reaching an equilibrium after approximately one hour. Moreover, the rate of aggregate breakage has been shown to increase with mixing intensity and floc fragility (Yukselen and Gregory, 2004). Vaezi (2013) studied dilute, polymer-flocculated kaolinite suspensions to investigate the effect of shearing on aggregate structural properties, including size, shape, porosity, and fractal dimension. Upon shearing, the mean aggregate diameter decreased significantly, indicating that the large aggregates were broken into smaller ones, which
(1−ε ) αL(DF − 3)
(1)
where DF is the fractal dimension and L is the aggregate representative diameter. The fractal dimension was found to increase with shearing time, indicating aggregate compaction. For example, at a shear rate of 1350 s−1, DF increased from 2.27 to 2.62 after 60 s of shearing. Further complicating the issue is the resulting rheology of the flocculated suspension. The rheological behavior of flocculant-dosed clay suspensions is strongly affected by the degree of flocculation, water chemistry, and clay surface physico-chemical properties (Nguyen and Boger, 1998). The high solids loadings and presence of high molecular mass polymers cause the resulting suspension to exhibit non-Newtonian behavior, the exact nature of which is essentially impossible to predict a priori. Most often, such conditions will produce a suspension yield stress: for example, studies have shown that flocculant-dosed (PAM A) kaolinite and smectite suspensions can have yield stresses surpassing 150 Pa at 40% solids by mass, which can increase to 300 Pa (or greater) at higher flocculant doses (Mpofu et al., 2003; McFarlane et al., 2005). Typically, suspension yield stress has a cubic or exponential correlation with solids volume fraction (Shook et al., 2002; Coussot and Piau, 1995) and also depends on suspension chemistry (Thomas, 1961). Only very few studies of the effect of shearing on concentrated suspensions of flocculant-dosed tailings are available in the literature. Gillies et al. (2012) measured the rheology of flocculated oil sand tailings in pipeline transport. Initial yield stress, rate of yield stress reduction, and equilibrium (fully-sheared) yield stress depended on the composition of the ore from which the tailings were prepared. Salinas et al. (2009) demonstrated that the geometry of the shearing apparatus (rotating cylinder, vane or impeller) used to study the yield stress reduction did not affect the behavior of flocculated mineral tailings (e.g. the yield stress at E = 200 kJ/m3 for a sample of tailings was the same regardless of how the shear energy was imparted). However, initial and fully-sheared yield stresses, as well as rates of yield stress reduction, varied from one ore sample to another. Treinen et al. (2010) successfully demonstrated the use of a bench-top concentric cylinder rheometer for quantifying Bingham yield stress reduction of flocculated fine particle (90% of particles < 75 µm) suspensions as a function of shear energy input. The aforementioned studies provide practical methods of characterizing shear energy-induced rheology changes; however, no fundamental links are drawn between the size or structure of the aggregates in the suspension and the observed rheology. To the authors’ knowledge, there is no study in the literature that examines the underlying mechanisms of shear-induced rheology degradation – specifically the relationship between aggregate structure and yield stress as a function of shear energy to which the suspension is exposed. The objective of the present study is therefore to examine how the shearing of highly concentrated, polymer flocculant-dosed kaolinite suspensions decreases aggregate size due to fragmentation, thus affecting both aggregate structure and suspension yield stress. There are some studies that describe the effects of shear on aggregate structure, and others that report the effect of shear on flocculated suspension rheology. However, there appears to be no study that links the two phenomena for flocculant-dosed suspensions, especially at high solidsloadings. In the present study the two main parameters, aggregate size and suspension yield stress, are measured as a function of shear energy input. A model tailings suspension composed of kaolinite is used because of such suspensions are well characterized and exhibit highly reproducible rheological behavior that has been shown to be similar to that of industrial fine tailings (Hogg, 2000). The suspensions are dosed 96
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Fig. 1. Flow chart of experimental procedure.
with two different PAM A flocculants, Magnafloc® LT27AG and Magnafloc® 1011, to study the effect of molecular weight and pendant group on shear energy-induced rheology changes. The effect of altered kaolinite surface potential is investigated by testing suspensions at different pH. The yield stress of each suspension is measured using a vane rheometer to avoid the effects of wall-slip, which are often encountered in high yield stress (> 20 Pa) measurements (Dzuy and Boger, 1983). Particle size distributions (measured as chord length distributions) are tracked with the Focused Beam Reflectance Measurement (FBRM) technique, which measures particles in the size range of 1–1000 μm and is often used to track trends in particle size (Blanco et al., 2002, 1996; Yoon and Deng, 2004; Heath et al., 2002) (e.g. flocculation).
2.3. Polymer solutions The two polymers used in the present study were obtained from CIBA Chemicals. Magnafloc® 1011 is a ‘very high’ molecular mass PAM A of 30% charge and unknown chemical composition and Magnafloc® LT27AG, is an ‘ultra-high’ molecular mass PAM A of known chemical composition, poly(acrylamide-co-acrylate), and medium charge density. Polymer stock solutions (10 g/L) were prepared by stirring 1.0 g of polymer solid with 100 mL of deionized water at 60 RPM for 12 h (overnight). A pitched-blade impeller, 5 cm in diameter, was placed roughly 5 mm from the base of beaker that was 7.5 cm in diameter. The beaker was covered with Parafilm M® during stirring to minimize evaporation. Fresh 1 g/L feed solutions were prepared daily by mixing 10 mL of 10 g/L stock solution with 90 mL of deionized water, followed by 30 min of stirring at 200 RPM. Polymer stock solutions were stored in a cool, dark location and discarded after two days.
2. Experimental method 2.1. Introduction Rather than describing the experimental procedures in detail, a flow chart showing the major steps is shown in Fig. 1. A more detailed description is provided elsewhere (Neelakantan, 2016). Each test required the preparation of a dilute, 8% solids (by mass) kaolinite suspension. The suspension was concentrated to 41% solids by mass using a combination of polymer flocculant and a gravity filtration apparatus. Roughly 620 cm3 of concentrated (thickened) suspension was required for each test, as this volume was sufficient to fill the concentric cylinder shearing apparatus two times. Half of the concentrated suspension provided an initial, unsheared measurement, by placing that sample in the shearing apparatus and immediately removing it without adding any shearing energy. Three vane yield stress measurements, as well as a FBRM measurement, were taken using the unsheared sample. The other half of the sample was then sheared for a specified duration using the shear cell. The same measurements were then repeated for the sheared sample. The concentric cylinder shearing apparatus and procedures followed to obtain FBRM measurements for the concentrated suspensions are described in detail in the following sections, as both are unique to this study.
2.4. Flocculation tests Kaolinite clay suspensions were prepared as per Section 2.2 and 500 mL samples were flocculated in 500 mL beakers (9 cm in diameter, with the impeller placed 6 cm from the bottom) at 350 RPM. The 1 g/L polymer solution was dispensed over a period of 40 s and stirring continued for an additional 20 s at which point the aggregates reached their maximum diameter, which was monitored with FBRM. The mixer was stopped and the mixture was gently poured into a 500 mL stoppered graduated cylinder and inverted four times. The initial settling rate (ISR) was measured as the time required for the mudline to pass from the 450 mL mark to the 350 mL mark (5.5 cm). Based on ISR tests, the optimal polymer dose of 100 g/tonne was selected, as flocculant concentrations greater than that did not improve settling rate or supernatant clarity. 2.5. Concentrated suspension preparation A concentrated, paste-like suspension was prepared by dewatering the flocculant-dosed kaolinite suspension. Due to the dramatic increase in suspension rheology with increasing solids volume fraction, the target suspension concentration was 41 ± 1% solids (by mass), which is 21 ± 1% by volume. A Buchner funnel was fitted with a 1 mm stainless steel mesh bowl and a 5 μm industrial filter paper to dewater the large aggregates. The Buchner funnel was 20 cm in diameter and the stainless steel mesh bowl was fitted such that the base of the bowl did not come in contact with the bottom of the Buchner funnel. In a 4 L container, 3.5 L of dilute clay suspension was prepared as outlined in Section 2.2, and flocculation was performed as per Section 2.4. The flocculated suspension was left to settle for 30 min before being transferred to the Buchner funnel for filtration. The supernatant
2.2. Clay suspension A mass of 305 g of kaolinite (Kentucky Tennessee Clay Co.) with a mean primary particle size of 1.2 μm and an inherent density of 2560 kg/m3 (reported by the supplier) was mixed with 3.5 L of deionized water to produce an 8% solids by mass (3.5% by volume) suspension. Sufficient KCl was added to produce a background electrolyte concentration of 1 mM. The mixture was stirred in a 4 L beaker at 1000 RPM for 30 min, while the pH was adjusted with concentrated KOH or HCl solution as required. The size distribution was measured using the FBRM and was found to be unchanged after 30 min of conditioning. 97
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Torque (N·m)
0.4 0.3 0.2 0.1 0.0 0
300 Time (s)
600
Fig. 3. A typical torque versus time trace of flocculated suspension (here with Magnafloc® LT27AG, pH 8.5) in the concentric cylinder shearing apparatus. The black line is the fitted power-law function.
(25 mm and 29.5 mm, respectively), L is the length of the spindle (400 mm), P is the power, t is shearing time, and k is a constant which accounts for mechanical energy loss. The power, P, is integrated using a Riemann sum:
∫0
was decanted and the remaining suspension was gently poured onto the filter paper for dewatering. The suspension was left to dewater for 90 min, after which a consistent solids concentration was attained.
t
(P−k ) dt
m=1
(P (tm) + P (tm + dt )) dt 2
(3)
Vane yield stress measurements were conducted using a six-bladed FL100 vane (D = 22 mm, H = 16 mm) attachment on a Haake Viscotester 550. The vane was rotated at very low angular velocity (0.0075 rad/s) and the maximum measured torque value was used to calculate the vane yield stress (Dzuy and Boger, 1983):
A custom-designed concentric cylinder shearing apparatus was fabricated for this program. It is shown in Fig. 2. The concentric cylinder shear cell has a 4.5 mm gap and accommodates approximately 310 cm3 of fluid, which was needed for subsequent vane yield stress measurements (Nguyen and Boger, 1998). Dzuy and Boger (1983) define the required volume for vane yield stress measurements to avoid effects from the wall of the container. The container holding the suspension must be at least twice the diameter and twice the height of the vane. Given the dimensions of the vane (D = 22 mm, H = 16 mm) the required volume is approximately 100 cm3 which is easily met by the capacity of the apparatus. An IKA Eurostar 60 with a programmable interface was used. Shearing data was measured using a Burster GmbH model 8661-5005-V0110 torque sensor with USB interface, providing 20 data points per second of power P , torque T , and spindle speed output, ω . The motor was mounted directly to a wall to reduce vibrations and hence signal noise. Shear rate and shearing time were combined in a single parameter, i.e. shear energy input. Shearing times were varied from 10 s to 10 min at 120 RPM to test a broad range of energy input values. Shear energy input was calculated using (Treinen et al., 2010):
∫0
∑
2.7. Vane yield stress measurements
2.6. Concentric cylinder shearing apparatus and shearing protocols
1 2πL (R22−R12)
n
Pdt =
where P is power at time t, n is the number of data points and dt is the time interval between data points (0.05 s). The minimum spindle speed ω to obtain complete shearing across the gap was determined experimentally. Treinen et al. (2010) and Salinas et al. (2009) noted that complete shearing across the gap for a flocculated suspension in a concentric cylinder apparatus is obtained when a steady power-law decay is observed after reaching peak torque. A minimum spindle speed of 105 rpm was required to obtain complete shearing across the gap and all the shearing experiments were conducted at spindle speeds of 120 rpm. A typical torque versus time trace is shown in Fig. 3, with the fitted power-law curve shown in black.
Fig. 2. Dimensions of concentric cylinder shear cell (A) shown in mm in-line with torque sensor (B) and belt driven by a digital motor (C).
E=
t
τV = T / K K=
πD3 H 1 ⎛ + ⎞ 2 ⎝D 3⎠
(4)
(5)
where τv is the vane yield stress, H is vane height, D is vane diameter, and T is torque. The sheared sample was first gently poured from the concentric cylinder shearing apparatus into a 300 mL beaker. Three vane yield stress measurements were taken from each concentrated suspension and averaged, for both the initial and sheared samples. The vane placement was staggered within the sample to provide three separate measurements and a more representative averaged yield stress value. 2.8. Aggregate size distributions of concentrated suspensions As part of this study, numerous methods were used to directly measure the size distributions of the concentrated suspensions using FBRM. Unfortunately, most methods did not provide representative, reproducible results, often because of probe tip fouling and/or multiple scattering of the reflected laser light from the FBRM probe. Fouling could be delayed with intense mixing; however this affected aggregate
(2)
where R1 and R2 are the inner and outer radii of the concentric cylinder 98
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size, thus producing a highly time-dependent measurement. A rapid dilution method was developed to prevent fouling and minimize the amount of shearing energy (and hence aggregate size reduction) imparted to the suspension. The rapid dilution technique allowed reasonable estimates of the aggregate size distribution in the concentrated suspension, as will be discussed in Section 3. The following procedure was developed in order to produce a dilute, homogenous system with minimal shear energy input prior to measurement:
Table 1 Initial (unsheared) suspension properties.
τV (0) (Pa) x (0) (μm)
Magnafloc® LT27AG 100 g/tonne pH 8.5
Magnafloc® 1011 100 g/tonne pH 8.5
Magnafloc® LT27AG 100 g/tonne pH 7
130 ± 10 100 ± 10
130 ± 20 90 ± 10
160 ± 10 110 ± 10
steady state where dx / dt is nearly constant at ∼−1 μm/s during the period 16 ≤ t ≤ 60 s, as shown in Fig. 4. The small, constant value of dx/dt indicates that the aggregate size is relatively constant; in other words, aggregate size reduction is minimal due to dilution and mixing.
● 420 mL of deionized water, adjusted to have the same pH as the suspension, was stirred at 350 RPM with the FBRM probe placed at a 45° angle and 3 cm from the base of a 500 mL beaker. ● Roughly 80 cm3 of concentrated suspension was quickly added to the stirred beaker.
3.2. Effects of shear energy input The rapid dilution procedure took less than 20 s. The first chord length distribution obtained with the FBRM after in situ dilution was taken. As is discussed in the following section, this method provides a reasonable estimate of the aggregate size distribution in the concentrated suspension.
Initial, unsheared properties of the concentrated suspensions tested during the present study are shown in Table 1, where the initial cubicweighted mean aggregate size is referred to x (0) and the vane yield stress as τV (0) . The unsheared suspension properties appeared to be relatively similar, indicating that differences in charge density, molecular mass, and pendant group did not have any pronounced effect, although this should not be taken to suggest they generally have no effect. The suspension pH mildly affected x (0) and τV (0) , with the increase in τV (0) and x (0) at pH 7 resulting primarily from the increased adsorption strength to the kaolinite clay particle edges. The frequency distributions of the unsheared suspensions were monomodal with chord lengths ranging from 10 to 1000 µm and a mode of ∼400 μm, as shown in Fig. 5. Although only the frequency distributions for one of the suspensions is shown here, the general trends were very similar for the Magnafloc® 1011-dosed suspension, while the size distributions of the pH 7 suspension are discussed subsequently. As shown in Fig. 5, the largest aggregates were quickly broken down at E = 200 kJ/m3. With additional energy input, the distributions became bimodal, with the large aggregates fragmenting to produce a significant population of smaller aggregates in the 1–100 μm size range. Continued shear exposure caused the aggregates to fracture even further and, as a result of the bridging mechanism through which flocculation had occurred, they could not re-flocculate. The fragments reach an equilibrium size which is still much larger than the kaolinite suspension particles, indicating the polymer-dosed aggregates do not totally break down even after significant shear exposure. The steadystate size distribution represents an equilibrium between hydrodynamic forces, which break apart aggregates, and attractive forces (van der Waals, bridging, hydrogen bonding). The highest rate of fragmentation occurs between E = 0 and
3. Results and discussion 3.1. Aggregate size measurement using FBRM The rapid dilution technique, described above, was used to obtain chord length distributions with the FBRM. The dilution/homogenization process was tracked using the volume-number (or cube-weighted) chord length, x (Trotter and Dhodapkar, 2014): 3 1/3
∑ ni di ⎞ x = ⎜⎛ i ⎟ ⎝ ∑i ni ⎠
(6)
This volume-number mean chord length was selected as it is more appropriate for larger particles (Heath et al., 2002; Yoon and Deng, 2004) and also has been found to most accurately track the shift from a monomodal to bimodal distribution, the importance of which is explained later in this discussion. The homogenization process during dilution is demonstrated in Fig. 4, which shows the time derivative of the cube-weighted mean chord length, x, while the concentrated suspension is stirred into the deionized water. Note that the concentrated suspension is added at t = 0. The global maximum at t = 6 s indicates the point at which the first aggregates reach the FBRM. The global minimum immediately follows at t = 8 s, which results partly from the smallest aggregates reaching the probe tip and partly from the homogenization of the diluted mixture. Following homogenization, the suspension reaches a dynamic
1
60
0 kJ/m3 200 kJ/m3 400 kJ/m3 1000 kJ/m3 Kaolinite
0.8
40
Frequency
dx/dt ( m/s)
50
30 20 10
0.6 0.4 0.2
0
0
-10
0.1
0
10
20
30
40
50
60
1
10
100
1000
CL ( m)
Time (s)
Fig. 5. Frequency distributions showing the effect of shear energy input on aggregate size distribution for concentrated suspensions prepared with 100 g/ tonne Magnafloc® LT27AG at pH 8.5. ‘Kaolinite’ indicates the size distribution of the 8% solids (by mass) kaolinite suspension prior to flocculant addition.
Fig. 4. Variation in mean aggregate size with time (dx/dt) during paste dilution and aggregate size measurement using FBRM (41% w/w solids suspension produced using Magnafloc® LT27AG, pH 8.5). 99
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a
1
b
1
LT27AG 0.8
0.8
1011
0.6
x'
v'
0.6
0.4
0.4
0.2
0.2
0
0
0
500
1000
1500
0
500
1000
E (kJ/m3)
1500
E (kJ/m3)
Fig. 6. (a) Normalized mean aggregate size, x′ and (b) normalized vane yield stress, τv′ for polymer-dosed suspensions mixed at pH 8.5.
E = 200 kJ/m3, as is clearly shown in Fig. 6a, where the normalized mean aggregate size, x ′ = x (E )/ x (0) , is plotted as a function of shear energy input. It should be noted that although the mean value does not reflect the bimodal nature of the aggregate size distributions, it very clearly captures the production of a significant population of smaller aggregates (cf. Figs. 5 and 6a). The value of x′ rapidly approaches a steady state for E > 400 kJ/m3. As shown in Fig. 6b, the dimensionless yield stress τv′ (where τv′ = τv (E )/ τv (0) ) decreases most rapidly from E = 0 to E = 200 kJ/m3, and in fact shows a nearly identical trend to that of x′. For E > 400 kJ/ m3 the vane yield stress approaches an equilibrium that is 10% of its initial value at pH 8.5 (e.g. τv′ (E = 1000) = 0.1). This is a far more dramatic decrease than what has been reported in the literature (Thomas, 1961; Gillies et al., 2012; Salinas et al., 2009), where decreases of 25–50% of the initial yield stress were observed. This difference in yield stress reduction can be explained by key differences in the suspension characteristics. For example, the aforementioned studies were for suspensions containing coarse particles, which contribute to the mixture yield stress (they have been shown to increase yield stress by up to 80%) (Rahman, 2011) but are not susceptible to shear degradation. In addition, these studies involved suspensions with different clay mineralogy and mixture chemistry those used in the present study. Therefore, one must be cautious in extrapolating results from one polymer-amended suspension to another.
1 0 kJ/m3 400 kJ/m3 1000 kJ/m3
0.8
F
0.6 0.4 0.2 0 1
10
100
1000
Chord Length ( m) Fig. 7. Cube-weighted, cumulative chord length distribution, F , for suspensions flocculated with 100 g/tonne Magnafloc® LT27AG at pH 8.5.
those used in the present study, and thus provides a reasonable estimate. The average shear rate in the present study is 106 s−1 and so the lowest shear rate measurements (145 s−1) from Vaezi (2013) are considered. At that shear rate, the fractal dimension (of Eq. (1)) was found to be DF = 2.27, such that
Cf (E ) = Cf (0)(x 0 / x )(2.27 − 3) or C′f = (1/ x ′)−0.73
(7)
Eq. (7) accounts for the change (reduction) in apparent solids volume fraction with aggregate size reduction. The relationship between solids volume fraction and yield stress has been expressed by numerous authors (Shook et al., 2002; Coussot and Piau, 1995; Thomas, 1963) using an exponential fit. Here, we use the correlation of Coussot and Piau (1995):
3.3. Relationship among aggregate size, structure and suspension yield stress As mentioned previously, and shown in Fig. 6a and b, the changes in x′ and τ′v with shear energy input appear to be closely correlated. In this study, the primary cause of the yield stress reduction is related to changes in aggregate structure. When the large porous flocs are fragmented into smaller aggregates, they release entrapped water, thus reducing their effective volume in the suspension. Particularly, aggregates greater than 400 µm in diameter under the present conditions can be expected to have void fractions of 0.90 ⩽ ε ⩽ 0.95, meaning they are 90–95% water by volume (Yukselen and Gregory, 2004). As a suspension is sheared to E = 1000 kJ/m3, the fraction of these large aggregates (CL > 400 μm) is reduced from Cf = 0.20 to Cf = 0.02 , as shown in Fig. 7. The result is a dramatic reduction in the apparent volume of the suspension, as entrapped water from large aggregates is released. To demonstrate how aggregate restructuring leads to an apparent solids volume fraction reduction, which in turn causes the suspension yield stress to decrease, we apply aggregate fractal dimension data from Vaezi (2013) and Eq. (1) to calculate the reduced apparent volume, C′f , as a function of energy input. The aggregate structure data from Vaezi (2013) was obtained using the same flocculant, kaolinite and pH as
τv = Ae BCf
(8)
where ‘A’ and ‘B’ are fitted constants which depend on the chemistry and mineralogy of the suspension. By substituting C′f (E ) for Cf into Eq. (8) and applying least squares regression to determine the values of A and B, we are able to obtain a reasonably good fit of the normalized vane yield stress data. Both the measured data, τv′ (E ) , and the fitted values, τv″ (E ) , obtained using Eqs. (7) and (8) and the best-fit values of A and B (A = 0.00939; B = 4.98), are shown in Fig. 8. Also shown in Fig. 8 is the apparent solids volume, C′f (E ) , which decreases to about 55% of its initial value at E > 400 kJ/m3, and remains roughly constant thereafter. It should be noted that the point of this exercise was show that the two measured parameters (aggregate size and suspension yield stress) can be directly related by considering the effect of aggregate structural changes that occur during shearing. In other words, the reduction in 100
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on aggregate size and yield stress reduction are studied here. In Fig. 9a, the reduction in vane yield stress with shear energy input is shown for two Magnafloc® LT27AG flocculant-dosed kaolinite suspensions: one at pH 8.5 and one at pH 7. Fig. 9b shows the corresponding cube-weighted mean aggregate size reduction for the two suspensions. As shown in Fig. 9a, the reduction of τv′ (E ) with energy input is essentially identical for the two suspensions at E ≤ 200 kJ/m3, but the trends become dissimilar at greater values of E. Interestingly, x′(E) changes with energy input in a similar way for the two suspensions. Clearly, though, there is a difference in the reaction of the two suspensions to shearing. To investigate the matter in greater detail, the aggregate size frequency distributions are considered. Recall that for the sheared pH 8.5 suspension, a bimodal size distribution began to appear at E = 200 kJ/m3 and was clearly apparent at E = 400 kJ/m3, as shown in Fig. 5. In Fig. 10a, volume (cube-weighted) frequency distributions for the two suspensions, measured at E = 300 kJ/m3, are shown. Note how the frequency chord length distribution for the pH 7 suspension is not bimodal, while as mentioned previously, the pH 8.5 suspension already exhibits a bimodal distribution. A bimodal distribution is not established until E > 500 kJ/m3 for the pH 7 suspension. As shown in Fig. 10b, the aggregates undergo a densification event before fragmentation; in other words, they reduce in size (see Fig. 10b), which, as discussed earlier, results in structural conformation that reduces their porosity. The densification occurs due to the aggregates being more deformable at pH 7 as a result of lower surface potential and polymer charge density, which has been reported to produce more flexible aggregates (Nasser and James, 2008). The lowered surface potential of kaolinite results in stronger adsorption, while the reduced ionization of acrylic groups allows the polymer to bend resulting in stronger and more deformable aggregates. Once E > 500 kJ/m3 at pH 7, a large population of fragments is produced and τv′ (E ) reaches an equilibrium value that is three times greater than observed at pH 8.5, as shown in Fig. 9b. Leong et al. (1995) and Kapur et al. (1997) both observed that the rheology of a fine particle suspension is primarily governed by the smallest fraction of particles present – in this case, the population of fragmented aggregates. At pH 8.5, which is well above the IEP of kaolinite clay, both the faces and edges of each particle possess a strong negative charge (van Olphen, 1963; Braggs et al., 1994; Van Olphen and Hsu, 1978; Grimm, 1968) and, as a result, have few structural associations, thus entrapping less water. The increase in τv′ at equilibrium at pH 7 is an indication of enhanced inter-particle bonding between kaolinite particles within the fragments. The increase in inter-particle bond strength resulting from the decrease in pH from 8.5 to 7 is inherent to the properties of
τv' (measured) Cf' (calculated) τv'' (fit)
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200
400
600
800
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E (kJ/m3) Fig. 8. Dimensionless apparent volume fraction, C′f , and a comparison of dimensionless experimentally observed normalized yield stress, τv′, and dimensionless predicted yield stress, τv″.
C′f (E ) due to aggregate fragmentation is the dominant factor in the reduction of τv′ (E ) at pH 8.5. This simple approach does not consider, for example, the way that aggregate morphology is likely to change with shearing and fragmentation. For example, Vaezi (2013) reported that as a result of shearing, aggregate shape was modified (in addition to aggregate size and structure) and that aggregates became more spherical in shape as shear rate increased. This morphological change may affect interactions between fragments, and therefore may also play a considerable role in the continued decrease in yield stress with energy input.
3.4. Effect of suspension pH Profiles of τv′ (E ) and x ′ (E ) were independent of flocculant type (Magnafloc® LT27AG and Magnafloc® 1011) tested at pH 8.5, as shown in Fig. 6. It can be concluded that flocculant structure, charge density, and pendant group type, for the two polymers used in this study, did not affect aggregate size or yield stress evolution at a suspension pH of 8.5. At a lower pH, e.g. when the suspension pH is 7, the surface charge of kaolinite reduces in magnitude, thereby altering the polymer-particle dynamics. The polymer bridging mechanism relies on electrostatic repulsions between the negatively charged particle surface and anionic pendant groups to project the polymer beyond the electric double-layer. On the other hand, the reduced electrostatic repulsion results in higher adsorption strength to the particle surface. A reduction in pH affects particle-particle interactions as well, as the kaolinite edge approaches its IEP, i.e. the zeta potential becomes less negative, thus increasing the importance of attractive van der Waals forces. These combined effects
1
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'
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E
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Fig. 9. (a) Dimensionless yield stress, τv′ and (b) Dimensionless chord length, x′, as a function of shear energy input for suspensions flocculated with Magnafloc® LT27AG at pH 8.5 and pH 7. 101
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Fig. 10. (a) Effect of pH on chord length distribution at E = 300 kJ/m3 (b) Evolution of chord length distribution at pH 7 for suspensions flocculated with Magnafloc® LT27AG.
reduction (shear degradation) of flocculant-dosed tailings suspensions with two common polymer flocculants used internationally in the mining and mineral processing industries. The decrease in yield stress is related to the reduced apparent solids volume fraction of the suspension, which is caused by aggregate fragmentation and compaction upon shearing. After significant shear energy input, a substantial population of fragments that are 1–100 μm in size is established, which dominates the rheological behavior of the suspension. The surface potential of kaolinite particles, which is altered with pH, influences fragment structure causing changes to the equilibrium yield stress. At pH values near the IEP of the edge surface, relatively shear-resistant and porous aggregates (compared to a higher pH suspension) dramatically augment the equilibrium (fully-sheared) yield stress of the suspension. Finally, the use of the thoroughly specified mixing and shearing conditions presented in this study will facilitate the future investigation of the effects of polymer type and suspension chemistry on (a) the structure of the aggregates (Mpofu et al., 2003; Nasser and James, 2007, 2006, 2008) and fragments (Leong et al., 1995, 1993) and (b) shear-induced yield stress reduction.
1
pH 8.5 pH 7 Kaolinite
Frequency
0.8 0.6 0.4 0.2 0 1
10
100
Chord Length ( m) Fig. 11. Chord length distributions of fragments produced at high shear energy input values (E > 1000 kJ/m3) flocculated with Magnafloc® LT27AG.
kaolinite. At pH 7, which is nearer the isoelectric point of the kaolinite edge (pH 5.25), the edge-edge electrostatic repulsions are reduced, allowing attractive van der Waals forces to become more important. This facilitates both edge-edge and edge-face associations between kaolinite particles within the fragments, which entraps more water and results in larger fragments. The population of fragments produced is greater at pH 7 than at pH 8.5, even at very high values of shear energy input (E > 1000 kJ/m3), as shown in Fig. 11. The number of fragments in the 1–20 µm range is considerably less at pH 7 compared to pH 8.5, which indicates the greater degree of association of kaolinite particles at pH 7. Qualitatively, this fact is reinforced by the chord length frequency distribution of fragments at pH 8.5, which more closely approaches the unflocculated kaolinite distribution than the fragment chord length distribution at pH 7. The three distributions are compared in Fig. 11. The increase in the entrapment of water due to the fragment population being larger and more porous results in the increase of the apparent solids volume fraction of the suspension, thus producing a greater equilibrium yield stress. These results are also consistent with studies of fine particle suspension rheology, in which a maximum yield stress is observed at the isoelectric point of the suspended particles (Addai-Mensah, 2014; Scales et al., 1998; Johnson et al., 2000).
Acknowledgment The authors would like to thank the Institute for Oil Sands Innovation (IOSI) for providing the primary source of funding for this project and for access to the FBRM. We are grateful also for the guidance of Q. Liu (IOSI Director) and A. Dunmola (Industrial project sponsor). Additional research funding and infrastructure was provided through the support of the NSERC Industrial Research Chair in Pipeline Transport Processes (held by RS Sanders). The contributions of Canada’s Natural Sciences and Engineering Research Council (NSERC) and the Industrial Sponsors (Canadian Natural Resources Limited, CNOOC-Nexen Inc., Saskatchewan Research Council’s Pipe Flow Technology Centre™, Shell Canada Energy, Suncor Energy, Syncrude Canada Ltd., Total SA, Teck Resources Ltd., and Paterson & Cooke Consulting Engineering Ltd.) are recognized with gratitude. References Addai-Mensah, J., 2007. Enhanced flocculation and dewatering of clay mineral dispersions. Powder Technol. 179 (1), 73–78. http://dx.doi.org/10.1016/j.powtec.2006. 11.008. Addai-Mensah, J., 2014. Colloidal forces, rheology and implications. Pre-Conference Workshop at the 19th International Conference on Hydrotransport, pp. 1–3. Adeyinka, O.B., Samiei, S., Xu, Z., Masliyah, J.H., 2009. Effect of particle size on the rheology of athabasca clay suspensions. Can. J. Chem. Eng. 87 (3), 422–434. http:// dx.doi.org/10.1002/cjce.20168. Alberta Energy Regulator. Directive 074: Tailings Performance Criteria and Requirements for Oil Sands Mining Schemes. < https://www-aer-ca.login.ezproxy.library.ualberta.
4. Conclusions A cost-effective, time-efficient and reproducible method of studying rheology degradation has been developed and tested. This method avoids the use of a pipe loop while also avoiding the pitfalls typically associated with concentric cylinder rheometry when studying high yield stress fluids. As well, we present a fundamental mechanism for the yield stress 102
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Nasser, M.S., James, A.E., 2006. The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite suspensions. Sep. Purif. Technol. 52 (2), 241–252. http://dx.doi.org/10.1016/j.seppur. 2006.04.005. Nasser, M., James, A., 2006. Settling and sediment bed behaviour of kaolinite in aqueous media. Sep. Purif. Technol. 51 (1), 10–17. http://dx.doi.org/10.1016/j.seppur.2005. 12.017. Nasser, M., James, A., 2007. Effect of polyacrylamide polymers on floc size and rheological behaviour of kaolinite suspensions. Colloids Surf. Physicochem. Eng. Aspects 301 (1), 311–322. http://dx.doi.org/10.1016/j.colsurfa.2006.12.080. Nasser, M., James, A., 2008. Compressive and shear properties of flocculated kaolinite–polyacrylamide suspensions. Colloids Surf. Physicochem. Eng. Aspects 317 (1), 211–221. http://dx.doi.org/10.1016/j.colsurfa.2007.10.021. Neelakantan, R., 2016. Effect of Shear Energy Input on the Rheology of Flocculant-Dosed Kaolinite Suspensions. [Master of Science]. University of Alberta, Edmonton, AB. Nguyen, Q., Boger, D., 1998. Application of rheology to solving tailings disposal problems. Int. J. Miner. Process. 54 (3), 217–233. http://dx.doi.org/10.1016/S03017516(98)00011-8. Ovenden, C., Xiao, H., 2002. Flocculation behaviour and mechanisms of cationic inorganic microparticle/polymer systems. Colloids Surf. Physicochem. Eng. Aspects 197 (1), 225–234. http://dx.doi.org/10.1016/S0927-7757(01)00903-7. Rahman, M.H., 2011. Yield Stresses of Mixtures with Bimodal Size Distributions. University of Alberta, Edmonton, AB [Master of Science]. Salinas, C., Martinson, R., Cooke, R., Ferrada, O., 2009. Shear and rheology reduction of thickened tailings. Proceedings of 12th International Seminar on Paste and Thickened Tailings. Vina Del Mar, Chile. Scales, P.J., Johnson, S.B., Healy, T.W., Kapur, P.C., 1998. Shear yield stress of partially flocculated colloidal suspensions. AIChE J. 44 (3), 538–544. http://dx.doi.org/10. 1002/aic.690440305. Shook, C.A., Gillies, R.G., Sanders, R.S.S., 2002. Pipeline Hydrotransport: With Applications in the Oil Sand Industry. SRC Pipe Flow Technology Centre. Sworska, A., Laskowski, J., Cymerman, G., 2000. Flocculation of the syncrude fine tailings: Part II. Effect of hydrodynamic conditions. Int. J. Miner. Process. 60 (2), 153–161. http://dx.doi.org/10.1016/S0301-7516(00)00013-2. Thomas, D., 1961. Transport characteristics of suspensions: III. Laminar-flow properties of flocculated suspensions. Am. Inst. Chem. Eng. 7 (3), 431–437. http://dx.doi.org/ 10.1002/aic.690070317. Thomas, D.G., 1963. Transport characteristics of suspensions VII. Relation of hinderedsettling floc characteristics to rheological parameters. AIChE J. 9 (3), 310–316. http://dx.doi.org/10.1002/aic.690090308. Thomas, A., 1977. A rational design philosophy for long distance slurry pipelines. Chem. Eng. Aust. 2 (1), 22. Treinen, J.M., Cooke, R., Salinas, C., 2010. Energy induced rheology reduction of flocculated slurries. Proceedings of 18th International Conference on Hydrotransport. Rio de Janeiro, Brazil, p. 487. Trotter, R., Dhodapkar, S., 2014. A guide to characterizing particle size and shape. CEP Mag. 36. U.S Department of the Interor. Surface Mining Control and Reclamation Act of 1977. Section 102, 1977. Vaezi, G., 2013. Effect of Laminar Shear on the Aggregate Structure of Flocculant-Dosed Kaolinite Slurries. University of Alberta, Edmonton, AB [PhD Dissertation]. Vaezi, G., Sanders, R., Masliyah, J., 2011. Flocculation kinetics and aggregate structure of kaolinite mixtures in laminar tube flow. J. Colloid Interf. Sci. 355 (1), 96–105. http:// dx.doi.org/10.1016/j.jcis.2010.11.068. Van Olphen, H., Hsu, P.H., 1978. An introduction to clay colloid chemistry. Soil Sci. 126 (1), 59. van Olphen, H., 1963. Clay mineralogy. In: An Introduction to Clay Colloid Chemistry for Clay Technologists, Geologists and Soil Scientists. John Wiley & Sons, Inc, New York, USA, p. 59. Wang, C., Harbottle, D., Liu, Q., Xu, Z., 2014. Current state of fine mineral tailings treatment: a critical review on theory and practice. Miner. Eng. 58, 113–131. http:// dx.doi.org/10.1016/j.mineng.2014.01.018. Yoon, S., Deng, Y., 2004. Flocculation and reflocculation of clay suspension by different polymer systems under turbulent conditions. J. Colloid Interf. Sci. 278 (1), 139–145. http://dx.doi.org/10.1016/j.jcis.2004.05.011. Yukselen, M.A., Gregory, J., 2004. The reversibility of floc breakage. Int. J. Miner. Process. 73 (2), 251–259. http://dx.doi.org/10.1016/S0301-7516(03)00077-2.
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Effect of shear on the yield stress and aggregate structure of flocculantdosed, concentrated kaolinite suspensions Ravi Neelakantan1, Farid Vaezi G.2, R. Sean Sanders
T
⁎
Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Mineral tailings Polymer flocculation Thickening Shear energy input FBRM Aggregate size Vane yield stress
The rheology of paste or thickened tailings, which are mineral tailings treated with polymers and subsequently dewatered, is complex and time-dependent. The complex rheological behavior is exacerbated by the permanent change in rheology that occurs when the mixture is exposed to shearing (e.g. in pumps and pipes), primarily through breakdown and restructuring of the aggregates of particles that comprise the dispersed phase. While the effects of shearing on structural changes of aggregates in a dilute suspension are reasonably well characterized, the relationship in more concentrated suspensions treated with polymer flocculants is not well understood. In the present study, a custom-built, concentric cylinder shearing apparatus is used to shear a large volume of flocculant-dosed (thickened) kaolinite suspension. The evolution of the particle size distribution (PSD) and vane yield stress were monitored as a function of shear energy input. Size distributions were obtained using the Focused Beam Reflectance Measurement (FBRM) technique. Concentrated suspensions were prepared using two different anionic polymers and at two suspension pH values to evaluate the effect of polymer structure and water chemistry on changes in the vane yield stress and aggregate size. Changes in both measured parameters correlated directly with shear energy input. During shearing, a large population of smaller aggregates, in the 10–100 μm size range, is generated. Aggregate size reduction occurs in concert with structural changes; together, they dictate the value of the equilibrium (fully-sheared) yield stress. At pH 7, the aggregates were able to restructure due to edge-edge associations between kaolinite particles. This phenomenon was not observed at pH 8.5, resulting in an equilibrium yield stress ∼1/3 the value of that measured at pH 7. The chemical composition of the polymer flocculants did not affect the yield stress or aggregate size over the shear energy input range studied here.
1. Introduction
remaining tailings, as thickener underflow slurry (suspension), is highly concentrated and is transported via pipeline to a disposal area for further dewatering, consolidation and subsequent land reclamation (Alberta Energy Regulator, 2009; U.S Department of the Interor, 1977). One of the sensitive and very important characteristics of a thickened tailings mixture is its rheological behavior. For example, it is critical to prevent the settling of coarse particles in laminar pipe flow (Hanks, 1986; Thomas, 1977), while sustaining sufficient strength for subsequent land reclamation, which ultimately requires yield strengths in excess of 10 kPa (Alberta Energy Regulator, 2009). Kaolinite represents one of the most predominant clay types encountered in mineral tailings and is often used in model studies due to its well characterized chemistry and morphology (van Olphen, 1963; Addai-Mensah, 2007; Mpofu et al., 2003; McFarlane et al., 2005; Klein and Hurlbut, 1993). Derakhshandeh (2016) demonstrated the
The mining industry produces millions of tonnes of fluid fine tailings annually, and this is expected to increase with rising demand for precious minerals (Wang et al., 2014). The long-term storage of fluid fine tailings incurs both an increased operating cost and concerns of environmental sustainability. Environmental protection legislation is enforcing increasingly strict regulations on water use in mineral processing operations. Some open-pit mining operations are located in arid climates where water usage is further restricted by the lack of available water. Tailings produced in open-pit mining operations typically contain clays, silts, sand, and residual ore suspended in a large volume of water. In tailings treatment processes, rapid destabilization of the fineparticle (clay) suspension is achieved using polymer flocculants added in a thickener to facilitate the immediate recovery of water. The
⁎
Corresponding author. E-mail address: [email protected] (R.S. Sanders). Present address: PARC, A Xerox Company, Palo Alto, CA, USA. 2 Present address: Teck Resources Ltd, Trail, British Columbia, Canada. 1
https://doi.org/10.1016/j.mineng.2018.03.016 Received 12 November 2017; Received in revised form 27 February 2018; Accepted 13 March 2018 Available online 09 May 2018 0892-6875/ © 2018 Elsevier Ltd. All rights reserved.
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are referred to here as ‘fragments’. Shearing also decreased porosity and increased aggregate fractal dimension, DF, indicating that the structural change was due to both fragmentation and compaction, particularly at higher shear rates and longer shearing times. It was found that the porosity and aggregate size were related through a power-law relationship, i.e.
applicability of model kaolinite tailings for fluid mechanics studies of fluid fine tailings. Fine tailings and kaolinite suspensions had similar rheological behavior for a given particle size, zeta potential and solids volume concentration; and, as expected, at higher concentrations, both exhibited time- and shear-dependent behavior (Adeyinka et al., 2009). Kaolinite has two different basal faces – one composed of octahedral alumina and the other composed of tetrahedral silica. At the so-called “edge” surface, which accounts for roughly 10% of the particle surface area, aluminol and silanol bonds are exposed (Addai-Mensah, 2007). The edge iso-electric point occurs at pH 5.25 (Braggs et al., 1994) while that of the silica basal face is pH ∼2 (Masliyah et al., 2011), and that of the alumina basal face is between pH 6 and pH 8 (Gupta and Miller, 2010). Kaolinite particles also undergo isomorphous substitution of lower valence cations (van Olphen, 1963) along the silica and alumina basal planes resulting in a permanent negative charge. The kaolinite particle anisotropy produces a variety of aggregate structures depending on the pH, including edge-edge, edge-face and face-face associations. Under appropriate conditions (e.g. high pH and low salt concentration), kaolinite particles are dissociated due to strong repulsive electrostatic interactions. The effects of pH on kaolinite surface chemistry influence polymer flocculant selection, where the appropriate charge, charge density, pendant group and molecular mass (Mpofu et al., 2003; Hogg, 2000) is required to efficiently destabilize the suspension. The most commonly used polymer flocculants for destabilizing mineral tailings are high molecular mass anionic polyacrylamides (PAM A), which are favored due to the high initial settling rates (ISR) achieved at relatively low dose and cost (Hogg, 2000; Sworska et al., 2000; Nasser and James, 2007, 2006; McFarlane et al., 2005). PAM A with high molecular weight flocculates particles via a bridging mechanism (Mpofu et al., 2003; Fan et al., 2000; Ovenden and Xiao, 2002), which requires that the polymer is of sufficient length to surpass the electric double layer of the particle and come in contact with other particles. Adsorption occurs at the edge sites of kaolinite through hydrogen bonding interactions (Lee et al., 1991; Nabzar and Pefferkorn, 1985); however, repulsions between negatively charged groups on the polymer and particle surface limit adsorption (Mpofu et al., 2003). Electrostatic repulsions at the surface can also be beneficial, as they allow the polymer to adopt a more extended conformation which is advantageous to bridging and produces stronger aggregates (Nasser and James, 2006). The resulting flocculated aggregates are highly porous, thus entrapping water, and require further compaction to increase the overall solids loadings of the tailings. Vaezi et al. (2011) flocculated dilute kaolinite clay suspensions with PAM A (Magnafloc® 1011) at a constant shear rate of 145 s−1 and found that aggregates initially produced were very large (200–1400 μm) and quite porous. Aggregates greater than 400 μm in diameter were the most porous with internal void fractions of 0.90 ⩽ ε ⩽ 0.95. The authors observed a trend between the aggregate porosity (or aggregate effective density) and diameter such that aggregate porosity increased with diameter. Aggregates smaller than 200 μm in size had porosities of 0.60 ⩽ ε ⩽ 0.80 , meaning there was less water contained in the aggregate structure. As flocculation time progressed, the aggregates decreased in size due to fragmentation and reached a dynamic steady state. Similar results were observed by Klein (2014), who studied flocculation of dilute (8% solids by mass) tailings and observed a decrease in aggregate size when mixing time increased, with the size eventually reaching an equilibrium after approximately one hour. Moreover, the rate of aggregate breakage has been shown to increase with mixing intensity and floc fragility (Yukselen and Gregory, 2004). Vaezi (2013) studied dilute, polymer-flocculated kaolinite suspensions to investigate the effect of shearing on aggregate structural properties, including size, shape, porosity, and fractal dimension. Upon shearing, the mean aggregate diameter decreased significantly, indicating that the large aggregates were broken into smaller ones, which
(1−ε ) αL(DF − 3)
(1)
where DF is the fractal dimension and L is the aggregate representative diameter. The fractal dimension was found to increase with shearing time, indicating aggregate compaction. For example, at a shear rate of 1350 s−1, DF increased from 2.27 to 2.62 after 60 s of shearing. Further complicating the issue is the resulting rheology of the flocculated suspension. The rheological behavior of flocculant-dosed clay suspensions is strongly affected by the degree of flocculation, water chemistry, and clay surface physico-chemical properties (Nguyen and Boger, 1998). The high solids loadings and presence of high molecular mass polymers cause the resulting suspension to exhibit non-Newtonian behavior, the exact nature of which is essentially impossible to predict a priori. Most often, such conditions will produce a suspension yield stress: for example, studies have shown that flocculant-dosed (PAM A) kaolinite and smectite suspensions can have yield stresses surpassing 150 Pa at 40% solids by mass, which can increase to 300 Pa (or greater) at higher flocculant doses (Mpofu et al., 2003; McFarlane et al., 2005). Typically, suspension yield stress has a cubic or exponential correlation with solids volume fraction (Shook et al., 2002; Coussot and Piau, 1995) and also depends on suspension chemistry (Thomas, 1961). Only very few studies of the effect of shearing on concentrated suspensions of flocculant-dosed tailings are available in the literature. Gillies et al. (2012) measured the rheology of flocculated oil sand tailings in pipeline transport. Initial yield stress, rate of yield stress reduction, and equilibrium (fully-sheared) yield stress depended on the composition of the ore from which the tailings were prepared. Salinas et al. (2009) demonstrated that the geometry of the shearing apparatus (rotating cylinder, vane or impeller) used to study the yield stress reduction did not affect the behavior of flocculated mineral tailings (e.g. the yield stress at E = 200 kJ/m3 for a sample of tailings was the same regardless of how the shear energy was imparted). However, initial and fully-sheared yield stresses, as well as rates of yield stress reduction, varied from one ore sample to another. Treinen et al. (2010) successfully demonstrated the use of a bench-top concentric cylinder rheometer for quantifying Bingham yield stress reduction of flocculated fine particle (90% of particles < 75 µm) suspensions as a function of shear energy input. The aforementioned studies provide practical methods of characterizing shear energy-induced rheology changes; however, no fundamental links are drawn between the size or structure of the aggregates in the suspension and the observed rheology. To the authors’ knowledge, there is no study in the literature that examines the underlying mechanisms of shear-induced rheology degradation – specifically the relationship between aggregate structure and yield stress as a function of shear energy to which the suspension is exposed. The objective of the present study is therefore to examine how the shearing of highly concentrated, polymer flocculant-dosed kaolinite suspensions decreases aggregate size due to fragmentation, thus affecting both aggregate structure and suspension yield stress. There are some studies that describe the effects of shear on aggregate structure, and others that report the effect of shear on flocculated suspension rheology. However, there appears to be no study that links the two phenomena for flocculant-dosed suspensions, especially at high solidsloadings. In the present study the two main parameters, aggregate size and suspension yield stress, are measured as a function of shear energy input. A model tailings suspension composed of kaolinite is used because of such suspensions are well characterized and exhibit highly reproducible rheological behavior that has been shown to be similar to that of industrial fine tailings (Hogg, 2000). The suspensions are dosed 96
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Fig. 1. Flow chart of experimental procedure.
with two different PAM A flocculants, Magnafloc® LT27AG and Magnafloc® 1011, to study the effect of molecular weight and pendant group on shear energy-induced rheology changes. The effect of altered kaolinite surface potential is investigated by testing suspensions at different pH. The yield stress of each suspension is measured using a vane rheometer to avoid the effects of wall-slip, which are often encountered in high yield stress (> 20 Pa) measurements (Dzuy and Boger, 1983). Particle size distributions (measured as chord length distributions) are tracked with the Focused Beam Reflectance Measurement (FBRM) technique, which measures particles in the size range of 1–1000 μm and is often used to track trends in particle size (Blanco et al., 2002, 1996; Yoon and Deng, 2004; Heath et al., 2002) (e.g. flocculation).
2.3. Polymer solutions The two polymers used in the present study were obtained from CIBA Chemicals. Magnafloc® 1011 is a ‘very high’ molecular mass PAM A of 30% charge and unknown chemical composition and Magnafloc® LT27AG, is an ‘ultra-high’ molecular mass PAM A of known chemical composition, poly(acrylamide-co-acrylate), and medium charge density. Polymer stock solutions (10 g/L) were prepared by stirring 1.0 g of polymer solid with 100 mL of deionized water at 60 RPM for 12 h (overnight). A pitched-blade impeller, 5 cm in diameter, was placed roughly 5 mm from the base of beaker that was 7.5 cm in diameter. The beaker was covered with Parafilm M® during stirring to minimize evaporation. Fresh 1 g/L feed solutions were prepared daily by mixing 10 mL of 10 g/L stock solution with 90 mL of deionized water, followed by 30 min of stirring at 200 RPM. Polymer stock solutions were stored in a cool, dark location and discarded after two days.
2. Experimental method 2.1. Introduction Rather than describing the experimental procedures in detail, a flow chart showing the major steps is shown in Fig. 1. A more detailed description is provided elsewhere (Neelakantan, 2016). Each test required the preparation of a dilute, 8% solids (by mass) kaolinite suspension. The suspension was concentrated to 41% solids by mass using a combination of polymer flocculant and a gravity filtration apparatus. Roughly 620 cm3 of concentrated (thickened) suspension was required for each test, as this volume was sufficient to fill the concentric cylinder shearing apparatus two times. Half of the concentrated suspension provided an initial, unsheared measurement, by placing that sample in the shearing apparatus and immediately removing it without adding any shearing energy. Three vane yield stress measurements, as well as a FBRM measurement, were taken using the unsheared sample. The other half of the sample was then sheared for a specified duration using the shear cell. The same measurements were then repeated for the sheared sample. The concentric cylinder shearing apparatus and procedures followed to obtain FBRM measurements for the concentrated suspensions are described in detail in the following sections, as both are unique to this study.
2.4. Flocculation tests Kaolinite clay suspensions were prepared as per Section 2.2 and 500 mL samples were flocculated in 500 mL beakers (9 cm in diameter, with the impeller placed 6 cm from the bottom) at 350 RPM. The 1 g/L polymer solution was dispensed over a period of 40 s and stirring continued for an additional 20 s at which point the aggregates reached their maximum diameter, which was monitored with FBRM. The mixer was stopped and the mixture was gently poured into a 500 mL stoppered graduated cylinder and inverted four times. The initial settling rate (ISR) was measured as the time required for the mudline to pass from the 450 mL mark to the 350 mL mark (5.5 cm). Based on ISR tests, the optimal polymer dose of 100 g/tonne was selected, as flocculant concentrations greater than that did not improve settling rate or supernatant clarity. 2.5. Concentrated suspension preparation A concentrated, paste-like suspension was prepared by dewatering the flocculant-dosed kaolinite suspension. Due to the dramatic increase in suspension rheology with increasing solids volume fraction, the target suspension concentration was 41 ± 1% solids (by mass), which is 21 ± 1% by volume. A Buchner funnel was fitted with a 1 mm stainless steel mesh bowl and a 5 μm industrial filter paper to dewater the large aggregates. The Buchner funnel was 20 cm in diameter and the stainless steel mesh bowl was fitted such that the base of the bowl did not come in contact with the bottom of the Buchner funnel. In a 4 L container, 3.5 L of dilute clay suspension was prepared as outlined in Section 2.2, and flocculation was performed as per Section 2.4. The flocculated suspension was left to settle for 30 min before being transferred to the Buchner funnel for filtration. The supernatant
2.2. Clay suspension A mass of 305 g of kaolinite (Kentucky Tennessee Clay Co.) with a mean primary particle size of 1.2 μm and an inherent density of 2560 kg/m3 (reported by the supplier) was mixed with 3.5 L of deionized water to produce an 8% solids by mass (3.5% by volume) suspension. Sufficient KCl was added to produce a background electrolyte concentration of 1 mM. The mixture was stirred in a 4 L beaker at 1000 RPM for 30 min, while the pH was adjusted with concentrated KOH or HCl solution as required. The size distribution was measured using the FBRM and was found to be unchanged after 30 min of conditioning. 97
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Torque (N·m)
0.4 0.3 0.2 0.1 0.0 0
300 Time (s)
600
Fig. 3. A typical torque versus time trace of flocculated suspension (here with Magnafloc® LT27AG, pH 8.5) in the concentric cylinder shearing apparatus. The black line is the fitted power-law function.
(25 mm and 29.5 mm, respectively), L is the length of the spindle (400 mm), P is the power, t is shearing time, and k is a constant which accounts for mechanical energy loss. The power, P, is integrated using a Riemann sum:
∫0
was decanted and the remaining suspension was gently poured onto the filter paper for dewatering. The suspension was left to dewater for 90 min, after which a consistent solids concentration was attained.
t
(P−k ) dt
m=1
(P (tm) + P (tm + dt )) dt 2
(3)
Vane yield stress measurements were conducted using a six-bladed FL100 vane (D = 22 mm, H = 16 mm) attachment on a Haake Viscotester 550. The vane was rotated at very low angular velocity (0.0075 rad/s) and the maximum measured torque value was used to calculate the vane yield stress (Dzuy and Boger, 1983):
A custom-designed concentric cylinder shearing apparatus was fabricated for this program. It is shown in Fig. 2. The concentric cylinder shear cell has a 4.5 mm gap and accommodates approximately 310 cm3 of fluid, which was needed for subsequent vane yield stress measurements (Nguyen and Boger, 1998). Dzuy and Boger (1983) define the required volume for vane yield stress measurements to avoid effects from the wall of the container. The container holding the suspension must be at least twice the diameter and twice the height of the vane. Given the dimensions of the vane (D = 22 mm, H = 16 mm) the required volume is approximately 100 cm3 which is easily met by the capacity of the apparatus. An IKA Eurostar 60 with a programmable interface was used. Shearing data was measured using a Burster GmbH model 8661-5005-V0110 torque sensor with USB interface, providing 20 data points per second of power P , torque T , and spindle speed output, ω . The motor was mounted directly to a wall to reduce vibrations and hence signal noise. Shear rate and shearing time were combined in a single parameter, i.e. shear energy input. Shearing times were varied from 10 s to 10 min at 120 RPM to test a broad range of energy input values. Shear energy input was calculated using (Treinen et al., 2010):
∫0
∑
2.7. Vane yield stress measurements
2.6. Concentric cylinder shearing apparatus and shearing protocols
1 2πL (R22−R12)
n
Pdt =
where P is power at time t, n is the number of data points and dt is the time interval between data points (0.05 s). The minimum spindle speed ω to obtain complete shearing across the gap was determined experimentally. Treinen et al. (2010) and Salinas et al. (2009) noted that complete shearing across the gap for a flocculated suspension in a concentric cylinder apparatus is obtained when a steady power-law decay is observed after reaching peak torque. A minimum spindle speed of 105 rpm was required to obtain complete shearing across the gap and all the shearing experiments were conducted at spindle speeds of 120 rpm. A typical torque versus time trace is shown in Fig. 3, with the fitted power-law curve shown in black.
Fig. 2. Dimensions of concentric cylinder shear cell (A) shown in mm in-line with torque sensor (B) and belt driven by a digital motor (C).
E=
t
τV = T / K K=
πD3 H 1 ⎛ + ⎞ 2 ⎝D 3⎠
(4)
(5)
where τv is the vane yield stress, H is vane height, D is vane diameter, and T is torque. The sheared sample was first gently poured from the concentric cylinder shearing apparatus into a 300 mL beaker. Three vane yield stress measurements were taken from each concentrated suspension and averaged, for both the initial and sheared samples. The vane placement was staggered within the sample to provide three separate measurements and a more representative averaged yield stress value. 2.8. Aggregate size distributions of concentrated suspensions As part of this study, numerous methods were used to directly measure the size distributions of the concentrated suspensions using FBRM. Unfortunately, most methods did not provide representative, reproducible results, often because of probe tip fouling and/or multiple scattering of the reflected laser light from the FBRM probe. Fouling could be delayed with intense mixing; however this affected aggregate
(2)
where R1 and R2 are the inner and outer radii of the concentric cylinder 98
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size, thus producing a highly time-dependent measurement. A rapid dilution method was developed to prevent fouling and minimize the amount of shearing energy (and hence aggregate size reduction) imparted to the suspension. The rapid dilution technique allowed reasonable estimates of the aggregate size distribution in the concentrated suspension, as will be discussed in Section 3. The following procedure was developed in order to produce a dilute, homogenous system with minimal shear energy input prior to measurement:
Table 1 Initial (unsheared) suspension properties.
τV (0) (Pa) x (0) (μm)
Magnafloc® LT27AG 100 g/tonne pH 8.5
Magnafloc® 1011 100 g/tonne pH 8.5
Magnafloc® LT27AG 100 g/tonne pH 7
130 ± 10 100 ± 10
130 ± 20 90 ± 10
160 ± 10 110 ± 10
steady state where dx / dt is nearly constant at ∼−1 μm/s during the period 16 ≤ t ≤ 60 s, as shown in Fig. 4. The small, constant value of dx/dt indicates that the aggregate size is relatively constant; in other words, aggregate size reduction is minimal due to dilution and mixing.
● 420 mL of deionized water, adjusted to have the same pH as the suspension, was stirred at 350 RPM with the FBRM probe placed at a 45° angle and 3 cm from the base of a 500 mL beaker. ● Roughly 80 cm3 of concentrated suspension was quickly added to the stirred beaker.
3.2. Effects of shear energy input The rapid dilution procedure took less than 20 s. The first chord length distribution obtained with the FBRM after in situ dilution was taken. As is discussed in the following section, this method provides a reasonable estimate of the aggregate size distribution in the concentrated suspension.
Initial, unsheared properties of the concentrated suspensions tested during the present study are shown in Table 1, where the initial cubicweighted mean aggregate size is referred to x (0) and the vane yield stress as τV (0) . The unsheared suspension properties appeared to be relatively similar, indicating that differences in charge density, molecular mass, and pendant group did not have any pronounced effect, although this should not be taken to suggest they generally have no effect. The suspension pH mildly affected x (0) and τV (0) , with the increase in τV (0) and x (0) at pH 7 resulting primarily from the increased adsorption strength to the kaolinite clay particle edges. The frequency distributions of the unsheared suspensions were monomodal with chord lengths ranging from 10 to 1000 µm and a mode of ∼400 μm, as shown in Fig. 5. Although only the frequency distributions for one of the suspensions is shown here, the general trends were very similar for the Magnafloc® 1011-dosed suspension, while the size distributions of the pH 7 suspension are discussed subsequently. As shown in Fig. 5, the largest aggregates were quickly broken down at E = 200 kJ/m3. With additional energy input, the distributions became bimodal, with the large aggregates fragmenting to produce a significant population of smaller aggregates in the 1–100 μm size range. Continued shear exposure caused the aggregates to fracture even further and, as a result of the bridging mechanism through which flocculation had occurred, they could not re-flocculate. The fragments reach an equilibrium size which is still much larger than the kaolinite suspension particles, indicating the polymer-dosed aggregates do not totally break down even after significant shear exposure. The steadystate size distribution represents an equilibrium between hydrodynamic forces, which break apart aggregates, and attractive forces (van der Waals, bridging, hydrogen bonding). The highest rate of fragmentation occurs between E = 0 and
3. Results and discussion 3.1. Aggregate size measurement using FBRM The rapid dilution technique, described above, was used to obtain chord length distributions with the FBRM. The dilution/homogenization process was tracked using the volume-number (or cube-weighted) chord length, x (Trotter and Dhodapkar, 2014): 3 1/3
∑ ni di ⎞ x = ⎜⎛ i ⎟ ⎝ ∑i ni ⎠
(6)
This volume-number mean chord length was selected as it is more appropriate for larger particles (Heath et al., 2002; Yoon and Deng, 2004) and also has been found to most accurately track the shift from a monomodal to bimodal distribution, the importance of which is explained later in this discussion. The homogenization process during dilution is demonstrated in Fig. 4, which shows the time derivative of the cube-weighted mean chord length, x, while the concentrated suspension is stirred into the deionized water. Note that the concentrated suspension is added at t = 0. The global maximum at t = 6 s indicates the point at which the first aggregates reach the FBRM. The global minimum immediately follows at t = 8 s, which results partly from the smallest aggregates reaching the probe tip and partly from the homogenization of the diluted mixture. Following homogenization, the suspension reaches a dynamic
1
60
0 kJ/m3 200 kJ/m3 400 kJ/m3 1000 kJ/m3 Kaolinite
0.8
40
Frequency
dx/dt ( m/s)
50
30 20 10
0.6 0.4 0.2
0
0
-10
0.1
0
10
20
30
40
50
60
1
10
100
1000
CL ( m)
Time (s)
Fig. 5. Frequency distributions showing the effect of shear energy input on aggregate size distribution for concentrated suspensions prepared with 100 g/ tonne Magnafloc® LT27AG at pH 8.5. ‘Kaolinite’ indicates the size distribution of the 8% solids (by mass) kaolinite suspension prior to flocculant addition.
Fig. 4. Variation in mean aggregate size with time (dx/dt) during paste dilution and aggregate size measurement using FBRM (41% w/w solids suspension produced using Magnafloc® LT27AG, pH 8.5). 99
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a
1
b
1
LT27AG 0.8
0.8
1011
0.6
x'
v'
0.6
0.4
0.4
0.2
0.2
0
0
0
500
1000
1500
0
500
1000
E (kJ/m3)
1500
E (kJ/m3)
Fig. 6. (a) Normalized mean aggregate size, x′ and (b) normalized vane yield stress, τv′ for polymer-dosed suspensions mixed at pH 8.5.
E = 200 kJ/m3, as is clearly shown in Fig. 6a, where the normalized mean aggregate size, x ′ = x (E )/ x (0) , is plotted as a function of shear energy input. It should be noted that although the mean value does not reflect the bimodal nature of the aggregate size distributions, it very clearly captures the production of a significant population of smaller aggregates (cf. Figs. 5 and 6a). The value of x′ rapidly approaches a steady state for E > 400 kJ/m3. As shown in Fig. 6b, the dimensionless yield stress τv′ (where τv′ = τv (E )/ τv (0) ) decreases most rapidly from E = 0 to E = 200 kJ/m3, and in fact shows a nearly identical trend to that of x′. For E > 400 kJ/ m3 the vane yield stress approaches an equilibrium that is 10% of its initial value at pH 8.5 (e.g. τv′ (E = 1000) = 0.1). This is a far more dramatic decrease than what has been reported in the literature (Thomas, 1961; Gillies et al., 2012; Salinas et al., 2009), where decreases of 25–50% of the initial yield stress were observed. This difference in yield stress reduction can be explained by key differences in the suspension characteristics. For example, the aforementioned studies were for suspensions containing coarse particles, which contribute to the mixture yield stress (they have been shown to increase yield stress by up to 80%) (Rahman, 2011) but are not susceptible to shear degradation. In addition, these studies involved suspensions with different clay mineralogy and mixture chemistry those used in the present study. Therefore, one must be cautious in extrapolating results from one polymer-amended suspension to another.
1 0 kJ/m3 400 kJ/m3 1000 kJ/m3
0.8
F
0.6 0.4 0.2 0 1
10
100
1000
Chord Length ( m) Fig. 7. Cube-weighted, cumulative chord length distribution, F , for suspensions flocculated with 100 g/tonne Magnafloc® LT27AG at pH 8.5.
those used in the present study, and thus provides a reasonable estimate. The average shear rate in the present study is 106 s−1 and so the lowest shear rate measurements (145 s−1) from Vaezi (2013) are considered. At that shear rate, the fractal dimension (of Eq. (1)) was found to be DF = 2.27, such that
Cf (E ) = Cf (0)(x 0 / x )(2.27 − 3) or C′f = (1/ x ′)−0.73
(7)
Eq. (7) accounts for the change (reduction) in apparent solids volume fraction with aggregate size reduction. The relationship between solids volume fraction and yield stress has been expressed by numerous authors (Shook et al., 2002; Coussot and Piau, 1995; Thomas, 1963) using an exponential fit. Here, we use the correlation of Coussot and Piau (1995):
3.3. Relationship among aggregate size, structure and suspension yield stress As mentioned previously, and shown in Fig. 6a and b, the changes in x′ and τ′v with shear energy input appear to be closely correlated. In this study, the primary cause of the yield stress reduction is related to changes in aggregate structure. When the large porous flocs are fragmented into smaller aggregates, they release entrapped water, thus reducing their effective volume in the suspension. Particularly, aggregates greater than 400 µm in diameter under the present conditions can be expected to have void fractions of 0.90 ⩽ ε ⩽ 0.95, meaning they are 90–95% water by volume (Yukselen and Gregory, 2004). As a suspension is sheared to E = 1000 kJ/m3, the fraction of these large aggregates (CL > 400 μm) is reduced from Cf = 0.20 to Cf = 0.02 , as shown in Fig. 7. The result is a dramatic reduction in the apparent volume of the suspension, as entrapped water from large aggregates is released. To demonstrate how aggregate restructuring leads to an apparent solids volume fraction reduction, which in turn causes the suspension yield stress to decrease, we apply aggregate fractal dimension data from Vaezi (2013) and Eq. (1) to calculate the reduced apparent volume, C′f , as a function of energy input. The aggregate structure data from Vaezi (2013) was obtained using the same flocculant, kaolinite and pH as
τv = Ae BCf
(8)
where ‘A’ and ‘B’ are fitted constants which depend on the chemistry and mineralogy of the suspension. By substituting C′f (E ) for Cf into Eq. (8) and applying least squares regression to determine the values of A and B, we are able to obtain a reasonably good fit of the normalized vane yield stress data. Both the measured data, τv′ (E ) , and the fitted values, τv″ (E ) , obtained using Eqs. (7) and (8) and the best-fit values of A and B (A = 0.00939; B = 4.98), are shown in Fig. 8. Also shown in Fig. 8 is the apparent solids volume, C′f (E ) , which decreases to about 55% of its initial value at E > 400 kJ/m3, and remains roughly constant thereafter. It should be noted that the point of this exercise was show that the two measured parameters (aggregate size and suspension yield stress) can be directly related by considering the effect of aggregate structural changes that occur during shearing. In other words, the reduction in 100
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1
on aggregate size and yield stress reduction are studied here. In Fig. 9a, the reduction in vane yield stress with shear energy input is shown for two Magnafloc® LT27AG flocculant-dosed kaolinite suspensions: one at pH 8.5 and one at pH 7. Fig. 9b shows the corresponding cube-weighted mean aggregate size reduction for the two suspensions. As shown in Fig. 9a, the reduction of τv′ (E ) with energy input is essentially identical for the two suspensions at E ≤ 200 kJ/m3, but the trends become dissimilar at greater values of E. Interestingly, x′(E) changes with energy input in a similar way for the two suspensions. Clearly, though, there is a difference in the reaction of the two suspensions to shearing. To investigate the matter in greater detail, the aggregate size frequency distributions are considered. Recall that for the sheared pH 8.5 suspension, a bimodal size distribution began to appear at E = 200 kJ/m3 and was clearly apparent at E = 400 kJ/m3, as shown in Fig. 5. In Fig. 10a, volume (cube-weighted) frequency distributions for the two suspensions, measured at E = 300 kJ/m3, are shown. Note how the frequency chord length distribution for the pH 7 suspension is not bimodal, while as mentioned previously, the pH 8.5 suspension already exhibits a bimodal distribution. A bimodal distribution is not established until E > 500 kJ/m3 for the pH 7 suspension. As shown in Fig. 10b, the aggregates undergo a densification event before fragmentation; in other words, they reduce in size (see Fig. 10b), which, as discussed earlier, results in structural conformation that reduces their porosity. The densification occurs due to the aggregates being more deformable at pH 7 as a result of lower surface potential and polymer charge density, which has been reported to produce more flexible aggregates (Nasser and James, 2008). The lowered surface potential of kaolinite results in stronger adsorption, while the reduced ionization of acrylic groups allows the polymer to bend resulting in stronger and more deformable aggregates. Once E > 500 kJ/m3 at pH 7, a large population of fragments is produced and τv′ (E ) reaches an equilibrium value that is three times greater than observed at pH 8.5, as shown in Fig. 9b. Leong et al. (1995) and Kapur et al. (1997) both observed that the rheology of a fine particle suspension is primarily governed by the smallest fraction of particles present – in this case, the population of fragmented aggregates. At pH 8.5, which is well above the IEP of kaolinite clay, both the faces and edges of each particle possess a strong negative charge (van Olphen, 1963; Braggs et al., 1994; Van Olphen and Hsu, 1978; Grimm, 1968) and, as a result, have few structural associations, thus entrapping less water. The increase in τv′ at equilibrium at pH 7 is an indication of enhanced inter-particle bonding between kaolinite particles within the fragments. The increase in inter-particle bond strength resulting from the decrease in pH from 8.5 to 7 is inherent to the properties of
τv' (measured) Cf' (calculated) τv'' (fit)
0.8 0.6 0.4 0.2 0 0
200
400
600
800
1000
1200
1400
E (kJ/m3) Fig. 8. Dimensionless apparent volume fraction, C′f , and a comparison of dimensionless experimentally observed normalized yield stress, τv′, and dimensionless predicted yield stress, τv″.
C′f (E ) due to aggregate fragmentation is the dominant factor in the reduction of τv′ (E ) at pH 8.5. This simple approach does not consider, for example, the way that aggregate morphology is likely to change with shearing and fragmentation. For example, Vaezi (2013) reported that as a result of shearing, aggregate shape was modified (in addition to aggregate size and structure) and that aggregates became more spherical in shape as shear rate increased. This morphological change may affect interactions between fragments, and therefore may also play a considerable role in the continued decrease in yield stress with energy input.
3.4. Effect of suspension pH Profiles of τv′ (E ) and x ′ (E ) were independent of flocculant type (Magnafloc® LT27AG and Magnafloc® 1011) tested at pH 8.5, as shown in Fig. 6. It can be concluded that flocculant structure, charge density, and pendant group type, for the two polymers used in this study, did not affect aggregate size or yield stress evolution at a suspension pH of 8.5. At a lower pH, e.g. when the suspension pH is 7, the surface charge of kaolinite reduces in magnitude, thereby altering the polymer-particle dynamics. The polymer bridging mechanism relies on electrostatic repulsions between the negatively charged particle surface and anionic pendant groups to project the polymer beyond the electric double-layer. On the other hand, the reduced electrostatic repulsion results in higher adsorption strength to the particle surface. A reduction in pH affects particle-particle interactions as well, as the kaolinite edge approaches its IEP, i.e. the zeta potential becomes less negative, thus increasing the importance of attractive van der Waals forces. These combined effects
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Fig. 9. (a) Dimensionless yield stress, τv′ and (b) Dimensionless chord length, x′, as a function of shear energy input for suspensions flocculated with Magnafloc® LT27AG at pH 8.5 and pH 7. 101
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reduction (shear degradation) of flocculant-dosed tailings suspensions with two common polymer flocculants used internationally in the mining and mineral processing industries. The decrease in yield stress is related to the reduced apparent solids volume fraction of the suspension, which is caused by aggregate fragmentation and compaction upon shearing. After significant shear energy input, a substantial population of fragments that are 1–100 μm in size is established, which dominates the rheological behavior of the suspension. The surface potential of kaolinite particles, which is altered with pH, influences fragment structure causing changes to the equilibrium yield stress. At pH values near the IEP of the edge surface, relatively shear-resistant and porous aggregates (compared to a higher pH suspension) dramatically augment the equilibrium (fully-sheared) yield stress of the suspension. Finally, the use of the thoroughly specified mixing and shearing conditions presented in this study will facilitate the future investigation of the effects of polymer type and suspension chemistry on (a) the structure of the aggregates (Mpofu et al., 2003; Nasser and James, 2007, 2006, 2008) and fragments (Leong et al., 1995, 1993) and (b) shear-induced yield stress reduction.
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Chord Length ( m) Fig. 11. Chord length distributions of fragments produced at high shear energy input values (E > 1000 kJ/m3) flocculated with Magnafloc® LT27AG.
kaolinite. At pH 7, which is nearer the isoelectric point of the kaolinite edge (pH 5.25), the edge-edge electrostatic repulsions are reduced, allowing attractive van der Waals forces to become more important. This facilitates both edge-edge and edge-face associations between kaolinite particles within the fragments, which entraps more water and results in larger fragments. The population of fragments produced is greater at pH 7 than at pH 8.5, even at very high values of shear energy input (E > 1000 kJ/m3), as shown in Fig. 11. The number of fragments in the 1–20 µm range is considerably less at pH 7 compared to pH 8.5, which indicates the greater degree of association of kaolinite particles at pH 7. Qualitatively, this fact is reinforced by the chord length frequency distribution of fragments at pH 8.5, which more closely approaches the unflocculated kaolinite distribution than the fragment chord length distribution at pH 7. The three distributions are compared in Fig. 11. The increase in the entrapment of water due to the fragment population being larger and more porous results in the increase of the apparent solids volume fraction of the suspension, thus producing a greater equilibrium yield stress. These results are also consistent with studies of fine particle suspension rheology, in which a maximum yield stress is observed at the isoelectric point of the suspended particles (Addai-Mensah, 2014; Scales et al., 1998; Johnson et al., 2000).
Acknowledgment The authors would like to thank the Institute for Oil Sands Innovation (IOSI) for providing the primary source of funding for this project and for access to the FBRM. We are grateful also for the guidance of Q. Liu (IOSI Director) and A. Dunmola (Industrial project sponsor). Additional research funding and infrastructure was provided through the support of the NSERC Industrial Research Chair in Pipeline Transport Processes (held by RS Sanders). The contributions of Canada’s Natural Sciences and Engineering Research Council (NSERC) and the Industrial Sponsors (Canadian Natural Resources Limited, CNOOC-Nexen Inc., Saskatchewan Research Council’s Pipe Flow Technology Centre™, Shell Canada Energy, Suncor Energy, Syncrude Canada Ltd., Total SA, Teck Resources Ltd., and Paterson & Cooke Consulting Engineering Ltd.) are recognized with gratitude. References Addai-Mensah, J., 2007. Enhanced flocculation and dewatering of clay mineral dispersions. Powder Technol. 179 (1), 73–78. http://dx.doi.org/10.1016/j.powtec.2006. 11.008. Addai-Mensah, J., 2014. Colloidal forces, rheology and implications. Pre-Conference Workshop at the 19th International Conference on Hydrotransport, pp. 1–3. Adeyinka, O.B., Samiei, S., Xu, Z., Masliyah, J.H., 2009. Effect of particle size on the rheology of athabasca clay suspensions. Can. J. Chem. Eng. 87 (3), 422–434. http:// dx.doi.org/10.1002/cjce.20168. Alberta Energy Regulator. Directive 074: Tailings Performance Criteria and Requirements for Oil Sands Mining Schemes. < https://www-aer-ca.login.ezproxy.library.ualberta.
4. Conclusions A cost-effective, time-efficient and reproducible method of studying rheology degradation has been developed and tested. This method avoids the use of a pipe loop while also avoiding the pitfalls typically associated with concentric cylinder rheometry when studying high yield stress fluids. As well, we present a fundamental mechanism for the yield stress 102
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