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CJCE-14-0567.R1 - OBSERVATIONS ON THE YIELDING BEHAVIOR OF OIL SAND SLURRIES UNDER
VANE AND SLUMP TESTS
Dear Dr. Pawlik,
The article has now been added to the queue of articles to copyedit prior to exporting to the
typesetting phase of production as the figures are now per the publishing requirements. Ms.
Pearce will be in contact with you directly if there are any edits.
Best regards,
Ms. Kyra Van Den Bos
The Canadian Journal of Chemical Engineering
Observations on the yielding behavior of oil sand slurries under vane and slump tests
Leopoldo Gutierrez and *Marek Pawlik
University of British Columbia
Norman B. Keevil Institute of Mining Engineering
517-6350 Stores Road
Vancouver, BC,
Canada
*
Corresponding author: Tel. 1 604 827 5034; fax: 604 822 5599; [email protected].
ABSTRACT
Yield stress measurements were carried out on slurries prepared with five different ore samples of
varying contents of bitumen and fines in the sand fraction. These rheological measurements were
performed using the vane, slump, and relaxation methods, as well as by extrapolation of equilibrium flow
curves to zero shear rate. In the case of the vane tests it was found that the yield stress values agreed better
with the results obtained from the other techniques when the yield stresses were calculated using the torque
value at the point of departure from linearity on the torque-time curves. It was found that the yield stress
values of oil sands slurries increased with an increase in bitumen content in the ores. High-bitumen ores
tended to yield within a volume of the slurry extending well beyond the geometry of the vane. In contrast,
low-bitumen ores yielded much closer to the vane edges. As a result, the torque value at the point of
departure from linearity on the torque-time curve was recommended for calculating the yield stress of
high-bitumen ore slurries. On the other hand, the maximum torque value on the torque-time curve can be
used for determining the yield stress of low bitumen ores.
Keywords: oil sands, yield stress, bitumen, sand
Introduction
Oil sand ores can be described as mixtures of three components, i.e., sand (80-85 %), a viscous
form of petroleum called bitumen (4-14 %) which is the valuable component, and finally intrinsic water (215%) dispersed within the ore matrix. It is believed that bitumen is not in direct contact with the
hydrophilic grains of sand, and a film of water surrounds the sand grains (Mossop 1980; Takamura 1982).
Bitumen in these ores is extracted using a process which consists of several inter-related unit operations,
i.e., mining, ore conditioning, bitumen-froth recovery, froth treatment and upgrading, and finally water
management (Kasongo et al. 2000). Ore conditioning is very important because it is during this stage when
bitumen liberation from the surfaces of sand particles takes place. In the contemporary practise ore
conditioning is generally done in hydrotransport pipelines where oil sands are mixed with warm/hot water
and some pH modifiers to produce mixtures of solid contents varying between 60 and 70 wt%.
The factor that makes oil sand slurries different compared to mineral slurries that are typically
encountered in mineral processing operations is the presence of hydrophobic bitumen. Attractive forces
acting between two bitumen-coated surfaces are significantly increased by a contribution of hydrophobic
forces. It was determined (Liu et al. 2005) that the force constant of attractive interactions between bitumen
surfaces was on the order of 10-19 J, which is 20 times higher than the Hamaker constant of interactions
between quartz particles of around 5x10-21 J (Franks 2002). This result could not be explained by
considering only the effect of van de Walls forces and an attractive hydrophobic force component had to be
added to the overall force balance. From a practical point of view and considering that the zeta potential of
bitumen and quartz are very similar over a wide range of pH, it is reasonable to expect that a strong
internal network characterized by a high yield stress, can develop in these types of slurries, especially when
bitumen liberation is low, that is, bitumen coats the sand particles and at a sufficiently high bitumen
content it starts forming a continuous oily phase. Model slurries prepared from pure quartz particles exhibit
much lower viscosities and yield stresses than slurries prepared using bitumen-coated quartz particles with
a bitumen content of only 0.7% (wt.) (Gutierrez and Pawlik 2012).
Although the conditioning stage of oil sand slurries in hydrotransport pipelines seems to be the
most affected by the rheology of these types of slurries, a proper bench-scale measurement of the rheology
of oil sand slurries is not a simple task. Most of the available rheological studies in this area have been
conducted in order to understand the behavior of pure bitumen at different conditions (Mossop 1980; Clark
and Pasternack 1932; Basu et al. 1996; long et al. 2007) and of oil-in-water emulsions with additions of
solids (Pal and Masliyah 1990; Yan et al. 1991). Our work on this important subject (Gutierrez and Pawlik
2012) showed that some properties of oil sand slurries, such as bitumen liberation/detachment from the
sand grains, could be followed using rheological data.
Several definitions of the yield stress were proposed with some significant physical differences.
Some authors (Bingham 1930; Lang and Rha 1981; Nguyen and Boger 1983) defined the yield stress as the
minimum shear stress at which continuous deformation is observed, marking the transition from elastic to
viscous behavior (Keentok 1982; Bingham, 1922). Other authors (Scott Blair, 1933), who showed that
plastic deformation could be detected and measured at shear stresses below the yield stress, preferred to
define the yield stress as the value below which no flow can be detected under the experimental conditions,
particularly over the time scale of the test. This last definition also led to suggestions that the yield stress
may even not exist (Barnes and Walters, 1985). Several methods and procedures were proposed to estimate
the yield stress of concentrated suspensions with good results for a variety of applications (Lang and Rha
1981; Nguyen and Boger 1983; Nguyen and Boger 1985; Keentok 1982; Scott Blair 1935; De Kee et al.
1980; Cokelet et al. 1963; Magnin and Piau 1987; Pashias et al. 1996). However, the applicability of those
various techniques to measure the yield stress of oil sand slurries has to be carefully analyzed. A first
characteristic to consider is the potential adhesion of bitumen to measuring surfaces and geometries such as
bobs, cups, metal parts, etc. Bitumen adheres to steel surfaces with the degree of this adhesion fluctuating
depending on the experimental conditions, i.e., temperature, ore grade, and clays content (Xu et al. 2004).
Another characteristic is the phenomenon of water migration in a sample that is confined in a container. In
concentrated slurries, water will tend to migrate from the center of the container to the top surface of the
slurry. This phenomenon produces an increase in the solids concentration of the sample in the center of the
container leading to non-homogeneous conditions, producing rheological results that can be difficult to
interpret. A third feature to consider is the existence of significant thixotropy and the time-dependent
rheological behavior of oil sand slurries.
The main objective of the current work is to measure/estimate the yield stress of concentrated oil
sand slurries and to identify conditions (and limitations) under which various techniques can be applied.
Although the main focus is on the vane and slump tests, other rheological techniques will also be used, i.e.,
extrapolation of equilibrium flow curves, and the stress relaxation method.
Material and methods
Samples and reagents
Five oil sand ores of varying quality were supplied by Canadian Natural Resources Ltd. Table 1
shows a characterization of these ores in terms of bitumen, water, solids, fines contents and BET surface
area (Dean-Stark analysis - Bulmer and Starr, 1979). As can be seen from the wide range of bitumen
concentrations and fines contents covered by these samples, the quality of these ores was diverse including
good (high bitumen and low fines), and poor processing ores. Table 1 also shows information regarding the
particle size distributions of the tested samples, i.e., mean size ( ), standard deviation (S), and coefficient
of variation (CV=
S/ ). The coefficient of variation can be used to quantify the degree of polydispersity
of a distribution. It can be seen that the particles size distributions of all the samples are similar which
means they have the same degree of polydispersity.
It has to be noted that all the oil sands ores were stored in freezers at approximately -4 °C so that
possible changes in properties due to weathering or aging were kept at minimum.
Table 1. Characterization of the tested oil sand ores.
Sand
Bitumen
Water
Solids
<44 m
< 3 m
BET
wt.%
wt.%
wt.%
vol.%
vol.%
m2/g
m
m
%
Ore 1
10.7
3.6
84.8
26.4
5.6
1.5
112
8
7
Ore 2
10.6
3.0
85.7
41.5
7.9
1.7
67
5
8
Ore 3
9.4
3.9
86.1
35.1
6.3
2.2
85
7
8
Ore 4
6.4
4.3
89.3
32.6
6.9
2.6
93
8
9
Ore 5
5.9
4.7
88.9
55.2
8.7
4.0
48
4
8
S
S/
The data reported in Table 1 show that ores 4 and 5 have bitumen contents lower than 8 wt.% and
high concentrations of fines (< 44 μm) in the sand fraction. Although ores 1, 2, and 3 display bitumen
contents above 9 wt.%, the concentration of fines in the sand fraction is for all of them higher than 20
vol.% (cut off value for defining an ore as poor or good). The majority of the detailed experimental work
was performed on ores 1 and 5, as the best and poorest ore types, respectively. However, the main
comparative study of the slump and vane tests was performed for all the ores.
Procedures
Oil sand slurries were prepared by manually mixing the ore samples with a predetermined volume
of a dilute sodium chloride (0.01 mol/L) solution as a background electrolyte for 3 minutes, which was
followed by a resting time of 0.5 minutes before performing the tests. Some precautions were taken in
order to remove as much air as possible which was done by gently stirring the slurries with a glass rod. All
the yield stress measurements were carried out at room temperature (21 ºC).
The vane method was implemented using 4 different four-bladed vanes connected to a Haake
Rotovisco VT550 rheometer. The vanes were of a constant diameter (Dv) of 1.9 cm, and of different
heights (Hv) of 2.9, 4.1, 4.7 and 6.0 cm. The vanes were machined out of brass. The procedure started by
gently introducing the vane into the tested sample contained in a beaker. The dimension of the vanes and
beaker were such that the ratios of beaker-to-vane heights, and beaker-to-vane diameters were larger than 3
so that rigid boundary effects were kept at minimum (Nguyen and Boger 1983; Nguyen and Boger 1985).
After this, the vane was rotated at a constant speed, and the torque required to keep a constant rotational
speed was recorded as a function of time in order to determine the values of the maximum torque (Tm), and
the torque of departure from linearity (Tdl) of the torque-time curve. These torque values were used to
calculate the yield stresses according to the procedure described by Nguyen and Boger (1983). All these
measurements were performed in triplicates.
The point of departure from linearity, Tdl, of a torque-time/vane rotation curve was determined by
fitting an increasing number of the initial data points on the curve with a straight line, and finding such a
torque value (data point) on the curve at which the coefficient of determination (R2) of the linear fit
decreased below 0.995. For example, linear equations of the type Y = a*X were first fitted to the initial
three data points as can be seen in Figure 1 (A). As the R2 in this case is 1 (>0.995), the linear equation was
successively fitted to the fourth (Figure 1 (B)), fifth (Figure 1 (C)) and sixth (Figure 1 (D)) data points,
until the value of R2 decreased below 0.995. In the example presented in Figure 1, R 2 was below 0.995
when the linear equation was fitted to the first six data points. Then, the torque of departure from linearity
was calculated as an average between the 5th and 6th torque data points, in this example T dl equaled
(0.28+0.33)/2 = 0.31 Ncm.
Fig. 1. Example of determination of the torque of departure from linearity (Tdl) of the torque-vane
rotation curve from vane tests.
The second rheological technique used to estimate the yield stress of oil sands slurries was the
“slump test”. This test was originally developed to evaluate the flow properties of concrete (Christensen
1991), and the technique was gradually modified for measuring the flow behavior of very concentrated
slurries (Pashias et al. 1996). Slump tests were done using a single PVC cylinder of 77 mm diameter and
100 mm height. The procedure consisted of first placing the PVC cylinder filled with a slurry over a flat
surface. Then, the cylinder was evenly lifted so that the column of slurry settled under its own weight. The
height of the slump was measured, and the yield stresses were calculated using the procedure proposed by
Pashias et al. (1996).
Direct extrapolation to zero shear rate of the corresponding equilibrium flow curves was also used
to estimate yield stresses. The data for the flow curves were obtained from stress decay experiments
performed using a Haake Rotovisco VT550 rotational viscometer connected to an elongated fixture
originally designed to measure properties of suspensions likely to settle (Klein, 1992). This fixture
consisted of a concentric cylinder, bob-in-cup, double-gap arrangement with gap sizes of 2.5 mm and 3.03
mm for the inner and outer gaps, respectively. The shearing surfaces of this fixture were grooved to
minimize wall slip effects. The procedure to obtain the rheological data for the equilibrium flow curves
started by inserting the hollow bob into the study sample confined in the cup of the elongated fixture. After
a resting time of 1 minute the rheometer was started. The shear rate was increased from 0 to 20 s -1 over a
period of 120 seconds. The shear rate was kept constant at that value for 90 seconds, after which it was
suddenly changed from 20 s-1 to a different target value. The samples were sheared at these final values for
a period of 200 seconds so that equilibrium shear stresses could be reached. Since the bob of the fixture is
essentially a hollow cylinder, inserting such a bob into the sample minimized the distortion of the slurry
network. This was an important measuring aspect since the tested oil sands slurries were expected to
display a strongly thixotropic response.
The yield stresses of concentrated oil sands slurries were also estimated using the relaxation
method (Whorlow, 1980). The same elongated fixture from the stress decay experiments was used again.
The procedure followed in these measurements consisted of pre-shearing oil sand slurries at a given shear
rate for a period of 180 seconds. After this, the shear rate was suddenly lowered to zero, and the shear
stresses were recorded as a function of time during an additional 180 seconds. Pre-shearing was carried out
at shear rates of 10, 20, 30, and 40 s-1.
All the different techniques could most reliably be used only over a solids content range.
Suspensions with lower solids contents, typically below 68% (wt), were tested using the rotational
rheometer to obtain equilibrium flow curves and stress relaxation data within the measuring limits of the
instrument. Vane and slump tests were performed on more concentrated suspensions with solids contents
of up 72-76% (wt). At the same time, the vane and slump methods did not give reliable and reproducible
results in the solids content range below about 66-64% (wt.) since neither technique was sensitive enough
to measure the corresponding low yield stresses. Although these transition solids contents were orespecific, several sets of data were collected in the solids concentration range between 64-68% (wt)
common to all the measuring methods. Therefore, whenever possible, the yield stress results are presented
over the full solids range although in several figures the results are divided between the higher and lower
solids content ranges.
Results and discussion
The effect of bitumen on the yield stress of concentrated slurries was evaluated through
experiments performed on slurries prepared in the solids content range from 64 to 73% (wt.). These tests
were done on slurries prepared using ores, and on slurries prepared only from sand fractions of the ores. In
this way, it was possible to assess the contribution of bitumen to the rheological behavior of the ore
slurries. The sand fractions were separated from bitumen by repeatedly washing the ores with toluene.
Slurries of ores 1 and 5, as well as of their corresponding sand fractions, were prepared and tested in order
to study two ores of extreme qualities. The results obtained from the vane, slump, relaxation, and flow
curve extrapolation methods are presented in the following sections.
Vane tests
In order to obtain reliable data from vane tests it is essential to first analyze the effect of the vane
rotational speed on the measurements of the T m and Tdl values. Nguyen and Boger (1983) found for
suspensions of bauxite residue that the maximum torque on the torque-time curve was not affected by the
rotational speed of the vane below a speed of 8 rpm, above which the torque increased. They proposed that
the increase in the maximum torque observed beyond 8 rpm was due to viscous resistance effects. Figure 2
shows the effect of the vane rotational speed on T m and Tdl for slurries prepared with ore 1 at 68 wt.%
solids (a), and with ore 5 at 72 wt.% solids (b). These results show that both values of torque are relatively
constant in the range of rotational speeds below 1 rpm. Nguyen and Boger (1983) recommended using the
lowest rotational speed possible. However, in order to maintain the time scale of the experiments at
minimum and to reduce the effect of water migration and ore segregation it was decided to use the highest
rotational speed at which the torque value was still unaffected by vane rotation, which according to Figure
2 (a and b) was 1 rpm.
Another aspect that deserves some additional discussion is the general shape of the torque-time
curve. Figures 2 (c) and (d) show examples of torque-time curves for ores 5 and 1 respectively. Figure 2 (c)
shows torque-time curves for slurries of ore 5 at 72 wt.% solids, as well as two curves obtained from
testing of samples of pure bitumen. This figure shows that it can be very difficult to clearly identify T m due
to the rather flat shape of the torque-time curves. However, if the maximum values of torque are directly
taken, it can be seen that these values are reached at large vane rotations between 0.75 and 1.2 rad (4369°). Tdl values were also determined for these data following the procedure outlined in Figure 1. These
results show that Tdl values are obtained at vane rotations below 0.25 rad (<14°), which is in agreement
with the results obtained by Nguyen and Boger (1983) who reported rotation angles of around 0.35 radians
(20 °) at the yield point. Besides, angles of rotations less than 0.25 rad agree with what could reasonably be
expected from a material deforming under conditions of elastic deformation (before yielding). These
observations suggest that yielding in oil sand slurries occurs at T dl, and that a disagreement between the
yield stresses calculated using Tm or Tdl should be expected. In order to verify which torque value should
be used in the calculation of the yield stress, the vane results will be compared with the results obtained
using other rheological techniques.
The torque-time curves obtained from vane tests performed on samples of pure bitumen are also
illustrated in Figure 2 (c). It can be seen from these results that the torque remains constant over the entire
timescale of the measurement. The fact that torque remains constant during these experiments shows that
bitumen does not exhibit a yield stress, a conclusion that agrees with those obtained by other authors who
showed that bitumen was a Newtonian fluid (Basu et al., 1996; Gutierrez, 2009). Another interesting
observation is that the torque values for bitumen are in the range of 0.22-0.42 Ncm, which are significantly
lower than those obtained for the slurries of ores 1 and 5. The fact that bitumen alone does not display a
yield stress, and that the values of torque are significantly lower compared to the values obtained from
testing slurries of ores 1, and 5, suggest that it is the combination of bitumen and sand what generates the
conditions for the existence of high yield stress and viscosity.
The reasons why ore 5 does not display a clear maximum on the torque-time curves are not very
clear. However, ore 5 stands out from the other tested ores as having a fines content twice as high as that of
ore 1, and a higher specific surface area. It has to be noted that after yielding takes place in a suspension,
particles start sliding over other particles, and the resistance to flow will depend on the number of
collisions and contacts between the flowing particles. It could perhaps be argued that after yielding takes
place, the flow resistance remains high due to a large number of contacts between the fine particles of ore 5
so the slurry structure can quickly recover. In contrast, in the case of slurries of ore 1, after Tm collisions
are less frequent due to a smaller number of fine particles.
Fig.2. Effect of vane rotational speed on the maximum torque (T m), and on the torque of departure
from linearity (Tdl) obtained for slurries of ore 1 at 68 wt.% solids (a), and of ore 5 at 72 wt% solids
(b), using a single vane of 1.9 cm diameter and 2.9 cm height. Torque-time curves for slurries of ore 5
(poor ore) at 72 wt.% solids (c), ore 1 (good-average ore) at 68 wt.% solids (d).
Figure 3 shows graphs of Tm and Tdl values plotted against the vane height (H v) for slurries of ores
1 and 5 at different solids contents. These results verify that these relationships are indeed straight-lines,
and consequently the yield stress values can be determined using the procedure described by Nguyen and
Boger (1983). It is important to note that the reproducibility of these measurements was very good as can
be observed from the standard deviations () of experiments performed in triplicates (see legends in Figure
3).
Fig.3. Maximum torque (Tm) versus vane height (Hv) (a and c), and torque of departure from linearity
(Tdl) versus vane height (Hv) (b and d) obtained from experiments on slurries of ore 1 and 5 at different
solids contents.
Figure 4 shows the vane yield stresses plotted as a function of solids content for slurries of ores 1
and 5, as well as for slurries prepared with only the sand fractions of ores 1 and 5. The resulting pH values
for slurries of ore 1 were on the order of 7.0-7.3, and 6.5-6.8 for slurries of ore 5. The first observation that
can be made is that the yield stresses (obtained either using Tm or Tdl values) for slurries of ore 1 are higher
than those obtained from slurries of ore 5. At the same solids content, slurries of ore 1 contain more
bitumen than the ones of ore 5, which leads to a stronger inter-particle aggregation and higher yield
stresses in the slurries prepared with ore 1. Removal of bitumen from the ores decreases the yield stresses
to almost zero at lower solids contents. This effect of increasing bitumen content on the rheology of quartzbitumen mixtures was also described by Gutierrez and Pawlik (2012). Another factor contributing to the
differences in the yield stresses of slurries of ores 1 and 5 is the degree of bitumen weathering in these
samples. As was shown in our previous publication (Gutierrez and Pawlik, 2014), the amount of humic
acids per mass of bitumen in the oil sand ores was significantly higher in ore 5 than in ore 1. Accordingly,
bitumen in ore 5 is expected to be less hydrophobic which leads to weaker hydrophobic forces and lower
yield stresses.
It is also important to note that the slurries prepared with the solids from ore 5 give higher yield
stresses than those prepared with the solids from ore 1. This result should be expected since the solids from
ore 5 are much finer and have a higher surface area than the solids of ore 1 although both types of solids
have the same degree of polydispersity. At the same time, the high fines content in ore 5 does not generate
high yield stresses in suspensions of the entire ore. Another interesting aspect observed from Figure 4 is
related to the difference between the yield stresses calculated using T m and Tdl. It can be seen that this
difference is significantly higher in the case of slurries of ore 1 than in the case of slurries of ore 5, which
suggests that bitumen plays a very significant role in generating this difference already at low solids
contents. The difference between T m and Tdl can also be substantial for solids only, but at very high solids
contents. This in turn suggests that interparticle contacts (aggregation) also contribute to the difference.
Fig.4. Vane yield stresses of slurries of ores 1, and 5, as well as for the sand fractions of ores 1, and 5.
If yielding occurs at T dl, the structure would be broken down at this torque, and the suspension
would deform permanently after T dl, with the torque values decreasing with time during the rest of the test.
However, as can be seen in Figure 2 (d) torque keeps increasing after T dl until Tm is reached for slurries of
ore 1. The explanation for this type of behavior can be associated with two phenomena. First, the method
proposed by Nguyen and Boger (1983) to estimate the yield stress assumes that yielding takes place on a
cylindrical surface of radius Dv/2 defined by the vane geometry. However, the exact dimensions of this
cylindrical yielding surface do not seem to correspond with the height and diameter of the vane as the
difference between Tm and Tdl can be large. As was discussed by Keentok et al. (1985), a fracture zone or
yielding volume of thickness  is generated, and in this case the slurry yields not just on one plane but on a
number of yielding planes in the volume section located between D v/2 and Dv/2+. According to this
analysis, it can be argued that slurry volumes located right on the cylindrical shearing surface defined by
the vane geometry yield at Tdl, after which additional layers of slurry farther away from the vane start to
yield, and the total torque still increases after Tdl. This advance of the yielding plane away from the vane
edges continues until the Tm is reached. Therefore, the results shown in Figure 4 can be analyzed following
this concept of a yielding volume rather than of a single yielding plane. For slurries of ore 1, which have
high bitumen contents, the difference between T m and Tdl is large because interparticle aggregation and
cohesion within the slurry are enhanced by the bitumen phase, and propagation of shearing affects a larger
volume of the slurries. In the absence of bitumen, suspensions of solids alone do not exhibit a large
difference between Tm and Tdl as they yield along the vane surfaces. Even for solids only, this difference
can be increased at higher solids contents at which interparticle aggregation promotes the formation of
extensive structuring within the slurry. It is this internal slurry structure, whether produced by bitumen or
by interparticle aggregation that leads to yielding within a volume rather than on a well-defined plane.
To verify how the deformation of the slurry propagates to planes away from the vane, a test was
carried out in which the vane was inserted only half way into slurries of ores 1 and 5. The idea was to draw
a white straight line on the surface of the slurry, and to follow the deformation and the position of the white
line as a function of time and vane rotation angle. The extension of the deformation of this line from its
zero time position gives a measure of how advanced is the propagation of shearing generated by the vane.
Figure 5 shows changes in the position of the line at different times and rotation angles for high and low
bitumen ores. These results show that for the slurries of ore 1 the deformation of the white line extends
almost across the whole gap between the vane and the cup. In the case of the slurry of ore 5, the
deformation of the white line was localized to a section closer to the vane, and did not extend to the outer
cup. These visual observations qualitatively confirm the concept that yielding of oil sand slurries does not
take place at the cylindrical surface of the vane, but rather over a distance farther away from the vane tips.
Moreover, this distance appears to increase with the bitumen content in the ore. Since high bitumen ores
produce the largest discrepancy between T m and Tdl values, the difference between these two torque values
seems to result from the presence of bitumen. As noted earlier, slurries of solids from oil sand ores
basically yield at Tm = Tdl. It can be postulated that a sufficiently high amount of bitumen creates a
continuous highly viscous medium, compared to a low-viscosity aqueous phase, which produces a solids-
in-bitumen suspension of very strong cohesion and elasticity. As the vane rotates in such a medium, the
deformation extends/propagates much farther away from the vane ends.
A key question as to which torque value represents the yield point will experimentally be tested
using other techniques for yield stress measurements.
Fig.5. Schematic of extension of the deformation of the time zero line for high and low bitumen ores.
Slump tests
Slump tests were performed on slurries prepared from ores 1 and 5, as well as using only solids
from those ores. Figure 6 shows the yield stress values obtained using this method. As can be seen from
this figure, the yield stresses of slurries of ore 1 are significantly higher than the values obtained for
slurries of ore 5, which is in agreement with the results obtained from vane tests. This result again reveals
the high degree of cohesion existing in the slurries prepared with ore 1. Figure 6 also displays the slump
yield stresses for slurries prepared with the sand fractions of ores 1, and 5. It can be seen that the yield
stresses of slurries prepared from solids from ore 5 are higher than those from solids of ore 1 which is also
in agreement with the vane tests. Similarly to the vane results, the high content of bitumen in ore 1 leads to
a large difference between the yield stress values for ore suspensions and the yield stresses of the sand
suspensions. At the same time, the effect of the much smaller bitumen content in ore 5 on the yield stress
of the sand and of ore suspensions is rather small, and the two sets of the yield data are very close together.
Fig.6. Comparison of yield stresses determined from slump tests on slurries of ores 1, and 5, as well as
slump results from tests on slurries prepared from solids (sand) from ores 1 and 5.
Relaxation method
Attempts were made to use the relaxation method to measure the yield stress of concentrated
slurries for ores 1 and 5. However, the high bitumen content of ore 1 led to significant levels of bitumen
build-up on the rotating surfaces of the concentric cylinder rheometer, which made it impossible to perform
reliable and reproducible measurements. It is interesting to note that the build-up of bitumen on the
shearing surfaces of the rheometer was not observed during experiments on slurries of artificial mixtures of
fresh bitumen with fine quartz tested at 45 wt.% solids (Gutierrez and Pawlik, 2012) probably due to the
lower bitumen liberation of those slurries. Consequently, the relaxation method was only used to measure
the yield stress of concentrated slurries of ore 5.
Figure 7 (a-c) shows the results of relaxation tests on slurries of ore 5 tested at three different
solids concentrations, i.e., 64, 66, and 68 wt.%. The first observation that can be made about these results is
that after the shear rate is switched to zero, the shear stress increases as a function of time until reaching a
steady-state value. This phenomenon is observed at all the solids concentrations tested in these
experiments. This result is associated with the thixotropic behavior of these slurries. After shearing is
stopped there is a recovery of the internal structure of the slurry, which leads to an increase of the shear
stress as a function of time. In addition to this, it can be observed that the shear stresses obtained at the
steady-state conditions, corresponding to the yield stresses, slightly depend on the initial shear rate used in
these experiments. These results show that the yield stresses increase when the initial shear rates decrease
which is another demonstration of the time-dependent nature of these slurries (Cheng, 1986). In order to
compare the results of the relation method with those obtained from the vane and slump techniques,
average values of the different yield values obtained at different shear rates were taken.
Flow curve extrapolation (equilibrium flow curves from stress decay tests)
Yield stresses were also estimated by extrapolation of flow curves to zero shear rates. The
rheological data were obtained from equilibrium flow curves generated from stress decay experiments. The
idea behind these experiments was again to obtain additional rheological data and to gain more confidence
in the yield stresses estimated using the vane and slump tests for ore 5. As in the case of measurements
using the relaxation method, the determination of rheological flow curves of slurries of ore 1 was not
possible, thus, only slurries of ore 5 were investigated. Figure 7 (d-f) illustrates the corresponding
equilibrium flow curves for slurries of ore 5 tested at solids contents of 64, 66, and 68 wt.%. Fitting of the
experimental data was done using the Herschel-Bulkley (HB) model presented in Equation 1.
̇
( )
Where B is the Herschel-Bulkley yield stress, KHB is called the consistency parameter and n the
power-law index. The HB model was only used to fit the data at shear rates above 8 s -1.
The first observation that can be obtained from these experimental results is the shear thinning
behavior of these slurries, as can be detected from the values of the parameter n in the HB model which is
less than 1 in all the cases. Another very important observation is that oil sand slurries exhibit static yield
stresses as defined by Cheng (1986).The static values are higher than those determined by extrapolation of
the HB model. This type of behavior was previously reported in suspensions of bentonite and waxy crude
and fuel oils, and was explained by the presence of more than one type of internal structure in the
suspension (Cheng, 1986). One very sensitive structure is readily broken at very low shear rates, and a
second stronger structure exists at moderate to high shear rates. The occurrence of a static yield stress is
related to the sequential breaking of these two structures. In the case of ore 5, it seems that shear rates on
the order of 5-10 s-1 promote the formation of a stronger network through collisions between bitumencoated particles. It is noteworthy that such a rheological response is not observed in the case of fine quartz
suspensions (Scott, 1982).
Fig.7. Stress relaxation curves of slurries of ore 5 obtained using the elongated fixture designed by
Klein (1992) (a-c), and equilibrium flow curves generated from stress decay data (d-f).
Comparison of the yield stress values obtained using the vane, slump, relaxation, and flow curve
extrapolation methods
Figure 8 (a) shows the yield stress values measured using the vane, relaxation, and extrapolation
methods for slurries of ore 5 in the solids concentration range from 64 to 68 wt.%. These results show that,
apart from the yield stress values calculated using T m at 68 wt.% solids, all the other results fall in a band
of standard deviation of around ±20-25 Pa. Figure 8 (b) shows the yield stresses obtained from vane and
slump measurements for slurries of ore 5 in the solids content range from 66 to 73 wt.%. The results are
split between two figures in order to highlight any differences in the yield stresses particularly at lower
solids contents. These data show that the yield stresses calculated from T dl values agree with the values
from slump tests over a wide range of solids contents. However, the yield stresses calculated from T m
values are higher in the whole range of solids contents. As was already discussed, the process of
interparticle aggregation and networking in suspensions is promoted by higher solids contents and by
higher amounts of bitumen, and the plane of yielding generated by the vane propagates to positions away
from the cylindrical plane surrounding the vane, which creates the difference between the yield stresses
calculated with Tm and Tdl.
Fig.8. (a) Yield stresses estimated using the slump, vane, flow curve extrapolation, and relaxation
method for slurries of ore 5 prepared at solids concentrations between 64 and 68 wt.%. (b) Yield
stresses estimated using the slump, and vane methods for slurries of ore 5 prepared at solids
concentrations between 66 and 73 wt.%.
Figure 9 shows the yield stresses obtained from vane and slump tests, for slurries of ore 1 in the
solids content range from 64 to 70 wt.%. It can be seen that for slurries of this relatively good processing
ore the difference between the yield stress values calculated using T m and Tdl is rather high over the entire
solids content range although the difference is more significant at higher solids contents. Although the
yield stresses calculated using Tdl are between 30 to 100 Pa higher than those from slump tests, the
experimental data suggest that the value of T dl should be taken to calculate the yield stress of oil sands
slurries of high bitumen ores (good ores) since in such a case the agreement between the yield stresses
obtained from these two techniques is much better.
Fig.9. Yield stresses estimated using the slump and vane methods for slurries of ore 1 prepared at
solids concentrations between 64 and 70 wt.%.
Figure 10 presents the data obtained from vane and slump tests on slurries of the sand fractions of
ores 1 and 5, respectively. These results show that the yield stresses obtained using T dl agree well with the
values obtained from slump tests in the whole range of solids content. In contrast, the yield stresses
calculated using Tm, only agree with the rest of the data in the low range of concentrations, and significant
departures can be observed at higher solids contents.
Fig.10. Yield stresses estimated using the slump and vane tests for slurries of solids of ore 1 (a) and 5
(b).
Effect of ore quality on the yield stress
The effect of ore quality on the yield stress was also studied. In this case, slurries of ores 1, 2, 3, 4,
and 5 were prepared at 70 wt.% solids and tested through the vane and slump techniques. The results are
shown in Figure 11. The first observation that can be made is that slurries of high-bitumen ores (ores 1, 2,
and 3) display higher yield stress values than slurries prepared from low-bitumen ores, such as ores 4 and
5. It should also be remembered that the relative amount of humic matter per mass of bitumen is much
higher in the case of ores 4, and 5, so the hydrophobicity of bitumen also varies from ore 1 to ore 5
(Gutierrez and Pawlik, 2014). Bitumen in ore 1 can be expected to be more hydrophobic than bitumen in
ore 5, and the wettability of bitumen also contributes to the trend in Figure 11. As shown by Gutierrez
(2013) even mild oxidation of an otherwise good quality ore drastically lowered the yield stress of a
concentrated slurry prepared from the oxidized ore to a point where no slump could be measured and the
slurry flowed freely. As a result, a low amount of bitumen with a higher amount of oxygen functional
groups in poor processing ores does not generate high yield stresses and has a small overall influence on
slurry rheology.
Another important observation that can be obtained from Figure 11 is related to the disagreement
between the yield stresses calculated using Tm and Tdl values. These results show that yield stresses
calculated using Tdl are in close agreement with the yield stress values obtained from the slump tests.
However, a significant disagreement is observed in the case of yield stresses calculated using T m, with this
difference being more pronounced in the case of slurries of high-bitumen ores 1, 2, and 3. These results
illustrate well the previously discussed effect of bitumen content on the discrepancy between yield stresses
calculated using either Tm or Tdl.
Fig.11. Yield stresses of slurries of ores 1, 2, 3, 4, and 5 at 70 wt.% solids.
Conclusions
The rheology of oil sands slurries depends on the bitumen content of the oil sands slurries as well
as on the quality or processability of the ores from which the corresponding slurries are prepared. At the
same solids content, slurries prepared from good processing ores display high yield stresses, while slurries
of poor processing ores display low yield stresses. Bitumen acts as a high-viscosity binder increasing the
internal cohesion of the slurry although the strength of the internal slurry structure, as measured by the
yield stress of the slurry, also depends on the hydrophobicity of bitumen.
Observations made during the vane test indicated that concentrated slurries of high-bitumen ores
yielded within a volume of the slurry that extended far beyond the cylindrical surface defined by the
geometrical dimensions of the vane. This propagation of the yielding plane beyond vane dimensions was
much less pronounced in concentrated slurries of low bitumen ores. Therefore, it was proposed that this
unusual behavior of concentrated oil sands slurries was caused by the presence of bitumen, which at
sufficiently high concentrations created a continuous high-viscosity medium of high elasticity. As a result,
the calculation of the yield stress from torque-vs-time curves generated using the vane method required a
careful analysis of the shape of the curves. It was found that the torque value at the point of departure from
linearity (Tdl) along the initial part of the curve, rather than the maximum torque value on the curve (Tm),
gave yield stress values that agreed very well with those obtained with other measuring techniques. The
difference between the maximum torque on the torque-time curve and the torque at the point of departure
from linearity was large for high bitumen ores which suggested that this difference was produced by
bitumen. Additional vane tests on slurries prepared only from the solids extracted from the ores showed
that the difference between T m and Tdl was very small and either Tm or Tdl could be used to assess the yield
stress. However, even for those bitumen-free slurries the difference between T m and Tdl increased with
increasing solids content, which suggested that the difference between those two torque values generally
originated from extensive aggregation between particles within a concentrated slurry. Under such
conditions the Tdl value rather than Tm should be used for calculating yield stresses from the vane method.
Acknowledgements
This study was made possible through the financial assistance provided by a collaborative
research and development grant from the Natural Sciences and Engineering Research Council (NSERC)
and Canada Natural Resources Limited (CNRL). Leopoldo Gutierrez also acknowledges the support by
“Centro CRHIAM Proyecto Conicyt Fondap 15130015”.
References
Barnes HA, Walters K (1985) The yield stress myth?. Rheologica Acta 24:323-326
Basu S, Nandakumar K, Masliyah JH (1996) A study of oil displacement on model surfaces. Journal of
Colloid and Interface Science 182: 82-94
Bingham EC (1922) Fluidity and Plasticity. McGraw-Hill, New York
Bingham EC (1930) Rheology definitions make progress. Journal of Rheology 1(5): 507-516
Bulmer, JT, Starr, J (1979) Syncrude analytical methods for oil sand and bitumen processing. The Alberta
Oil Sands Technology and Research Authority, 46-52
Cheng DC-H (1986) Yield stress: A time-dependent property and how to measure it. Rheologica Acta 25:
542-554
Clark KA, Pasternack DS (1932) Hot water separation of bitumen from Alberta bituminous sand. Ind. Eng.
Chem. 24:1410-1416
Cokelet GR, Merrill EW, Gilliland ER (1963) The Rheology of Human Blood-Measurement Near and at
Zero Shear Rate. Transactions of the Society of Rheology 7:303-317
Christensen G (1991) Modelling the flow of fresh concrete: the slump test. PhD Dissertation, Princeton
University
De Kee D, Turcotte G, Fildey K, Harrison B (1980) New method for the determination of yield stress.
Journal of Texture Studies 10:281-288
Franks GV (2002) Zeta potential and yield stresses of silica suspensions in concentrated monovalente
electrolytes: isoelectric point shift and additional attraction. Journal of Colloid and Interface Science
249:44-51
Gutierrez LE (2009) Probing mineral-bitumen liberation using rheological measurements. MASc Thesis,
The University of British Columbia, Vancouver, Canada
Gutierrez LE, Pawlik M (2012a) Influence of pH and temperature on the rheology of aqueous quartzbitumen suspensions. Journal of Rheology 56:687-706
Gutierrez LE, Pawlik M (2014) Influence of humic acids on oil sand processing. Part I: Detection and
quantification of humic acids in oil sand ores and their effect on bitumen wettability, International Journal
of Mineral Processing 126:117-125
Gutierrez LE (2013) A role of humic matter and ore oxidation in rheology of oil sand slurries and in
bitumen extraction. PhD Thesis, The University of British Columbia, Vancouver, Canada
Kasongo T, Zhou Z, Xu Z, Masliyah J (2000) Effect of clays and calcium ions on bitumen extraction from
athabasca oil sands using flotation. Canadian Journal of Chemical Engineering 78:674-681
Keentok M (1982) The measurement of the yield stress of liquids. Rheologica Acta 21:325-332
Keentok M, Milthorpe JF, O’Donovan E (1985) On the shearing zone around rotating vanes in plastic
liquids: theory and experiment. Journal of Non-Newtonian Fluid Mechanics 17:23-35
Klein B (1992) Rheology and stability of magnetite dense media. PhD Dissertation, The University of
British Columbia, Vancouver, Canada
Lang ER, Rha C (1981) Determination of the yield stress of hydrocolloid dispersions. Journal of Texture
Studies 12:47-62
Liu J, Xu Z, Masliyah J (2005) Colloidal forces between bitumen surfaces in aqueous solutions measured
with atomic force microscope. Colloids and Surfaces A: Physicochem. Eng. Aspects 260:217-228
Long J, Drelich J, Xu Z, Masliyah J (2007) Effect of Operating Temperature on Water‑Based Oil Sands
Processing. The Canadian Journal of Chemical Engineering 85:726-738
Magnin A, Piau JM (1987) Shear rheometry of fluids with a yield stress. Journal of Non-Newtonian Fluid
Mechanics 23:91-106
Mossop GD (1980) Geology of the Athabasca oil sands. Science 207:145-152
Nguyen QD, Boger DV (1983) Yield stress measurement for concentrated suspensions. Journal of
Rheology 27(4):321-349
Nguyen QD, Boger DV (1985) Direct yield stress measurement with the vane method. Journal of Rheology
29(3):335-347
Pal, R, Masliyah, J (1990) Rheology of oil in water emulsions with added solids. The Canadian Journal of
Chemical Engineering, Vol. 68, 24-28, 1990
Pashias N, Boger DV, Summers J, Glenister DJ (1996) A fifty cent rheometer for yield stress
measurement. Journal of Rheology 40(6):1179-1189
Scott KJ (1982) The effect of surface charge on the Rheology of concentrated aqueous quartz suspensions.
Chemical Engineering research Group-Council for Scientific and Industrial Research, Report CENG 423,
Pretoria, South Africa
Scott Blair GW (1933) On the nature of Yield-Value. Journal of Applied Physics 4:113-118
Scott Blair GW (1935) The thixotropy of heather honey. Journal of Phys. Chem. 39(2):213-220
Takamura K (1982) Microscopic structure of Athabasca oil sand. The Canadian Journal of Chemical
Engineering 60:538-545
Whorlow RW (1980) Rheological Techniques. Ellis Horwood Limited, Chichester Eng. and New York
Xu Y, Dabros T, Friesen WI, Maciejewski WB, Czarnecki J (2004) Adhesion of bitumen to a metal surface
in a flowing oil sands slurry. The Canadian Journal of Chemical Engineering 82:807-812
Yan Y, Pal R, Masliyah J (1991) Rheology of oil-in-water emulsions with added solids. Chemical
Engineering Science 46:985-994