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Virginia Tech | require 7-8 minutes of residence time. Some industrial columns have been designed to
operate with residence times of 12-15 minutes or more. It is also important to note that
the active volume for columns is defined as the volume of pulp contained between the
bottom of the froth and the top of the air spargers. The calculated volume is also typically
reduced by 10-20% to account for the volume occupied by air bubbles. This volume is
substantially less than the total volume of the column tank structure.
2.2.3 Froth Carrying Capacity
Figure 7 shows a typical relationship between combustible recovery and residence
time for a coal flotation column. The volumetric feed flow rate to the column was
steadily increased in this series of tests in order to reduce the mean residence time. As
shown, the test data obtained at 2% solids correlated well with the theoretical
performance curve predicted using Levenspiel’s (1972) equation. However, as the solids
content of the feed stream was increased to 5% solids, the recovery dropped sharply and
deviated substantially from the theoretical curve. This deviation was more pronounced
when the solids content was increased to 10% solids. The drop in recovery with
increasing solids content can be attributed to limitations associated froth carrying
capacity. When this occurs, there is insufficient bubble surface area to carry all of the
floatable particles through the froth. While the carrying capacity restriction can often be
ignored for conventional flotation circuits, it is of great importance in the design of
column cells. This is largely due to the fact that the specific surface area of the cell (ratio
of the cross-sectional area to the volume) is much higher for conventional cells.
19 |
Virginia Tech | range of 0.06 to 0.24 tph/ft2, with an average of about 0.12 tph/ft2 for 100 mesh x 0 feeds.
The values at the lower end of this range typically correspond to coal feeds of much finer
particle size (325 mesh x 0). Once this value is known, the required column cross-
sectional area can be determined by dividing the expected clean coal tonnage (tph) by the
carrying capacity (tph/ft2). However, for scale-up purposes, it is generally more
convenient to calculate the clean coal tonnage for a full-scale column from a smaller test
column using:
2
Large Column tph ⎛Large Diameter⎞
=⎜ ⎟ [7]
⎜ ⎟
SmallColumn tph ⎝ SmallDiameter ⎠
According to this relationship, a 14 ft diameter column should have a clean coal capacity
that is nearly twice that of a 10 ft diameter column [i.e., (14/10)2 = 1.96]. Field data
suggest that this relationship holds valid for coal columns as large as 15 ft diameter.
However, this suggestion appears to contradict data reported by Amelunxen (1990)
stating that froth mobility problems make overflow lip length (and not cross-sectional)
the appropriate scale-up parameter, i.e:
Large Column tph ⎛Large Diameter⎞
= ⎜ ⎟ [8]
⎜ ⎟
SmallColumn tph ⎝ SmallDiameter ⎠
This expression may be more appropriate for columns that operate with unstable froths
created by inadequate frother additions, improper water distributors, or coarser coal
feeds.
2.2.4 Froth Wash Water
The most important feature of a column is the wash water system. A froth depth
of about 2-3 ft is typically required to ensure good distribution of the wash water and to
21 |
Virginia Tech | prevent short-circuiting. In addition, the flow of wash water must exceed the volumetric
flow of water reporting to the clean coal product to prevent entrainment of the high-ash
slimes. In most cases, less than about 1% of the feed water will report to the froth product
if the wash water is properly controlled. The amount of water carried by the froth can be
calculated from:
⎛100 ⎞
Q = 4C⎜ −1⎟ [9]
⎝ P ⎠
Where Q is the water demand (gpm/ft2), C the froth carrying capacity (tph/ft2) and P is
the froth percent solids. For example, a column cell producing 0.12 tph/ft2 of dry clean
coal at 18% solids will carry about 2.2 gpm/ft2 of water from the pulp into the froth [ i.e.,
4 x 0.12 (100/18-1) = 2.2].
Entrainment should theoretically be eliminated when the number of dilution
washes (defined as the froth water demand divided by the wash water addition rate)
reaches a value of one. However, as shown in Figure 8, froth mixing usually requires that
1.5 dilution washes be used to fully suppress hydraulic entrainment. This constraint
dictates that a wash water flow rate of at least 3.3 gpm/ft2 [i.e., 1.5 x 2.2 = 3.3] be used in
the current example to prevent the entrainment of high-ash slimes. Field data collected
obtained from columns operating in the coal industry suggest that a wash water flow rate
of 3.0-3.5 gpm/ft2 is normally adequate for most commercial installations. However,
higher gas and frother addition rates will typically increase the froth water demand and,
as a result, the amount of wash water required. Excessive wash water flows should be
avoided since the extra wash water passing downward through a column will create an
undesirable reduction in the slurry retention time and, hence, a potential reduction in
22 |
Virginia Tech | the depth of the drained froth and the extent of froth drainage to be varied by raising or
lowering the distributor. Changes to the vertical position of the water distributor can be
used to somewhat control the split of water between the clean coal and refuse streams. In
some cases, multi-level concentric distribution rings may also be used to overcome
problems associated with poor froth mobility. The inner rings are typically located above
the outer rings to reduce drainage and improve the fluidity of the froth in the center of the
column. In other cases, the water distributor may be located just above the top of the
froth. This arrangement does not allow the froth mobility to be controlled by adjusting
the distributor location, but it does make if easier to identify and correct plugging
problems that may severely impact the performance of the distribution network.
2.2.5 Gas Sparging
The air sparging system is the most important, and perhaps most controversial,
component in the design of a coal flotation column. Ideally, the spargers should produce
small, uniformly sized bubbles at a desired aeration rate. The spargers should also be
non-plugging, wear-resistant and allow for easy, on-line servicing. Three primary types
of column sparging systems are used:
• porous bubblers fabricated from filter cloth, punctured rubber tubes or sintered
metals/plastics (e.g., Ken-Flote and many “homemade” columns),
• high-velocity air or air-water injectors (e.g. EIMCO, CPT/Eriez and Minnovex
columns), and
• circulating systems incorporating inline or external agitators such as static mixers
(e.g., Microcel column).
24 |
Virginia Tech | reported by Honaker et al. (1995). This statement does not, however, imply that sparger
selection is unimportant. The test data indicate that the porous bubbler and static mixer
generally produced higher overall recoveries than the high-velocity air injector. In
addition, the static mixer was generally less sensitive to changes in operating conditions
and tended to produce data points higher on the separation curve. On the other hand,
systems such as the porous bubbler have been shown to plug easily and require frequent
maintenance in industrial operations. Capital costs for the static mixer system are
generally higher due to the purchase of a slurry pump and wear can become an issue
when very coarse feeds (>100 mesh) are treated. All these factors need to be carefully
considered prior to selecting a sparging system.
2.2.6 Aeration Rate
Differences in spargers can normally be attributed to variations in the amount of
bubble surface area generated by each device. This capability is commonly reported in
terms of the superficial bubble surface area rate (S ), which is defined as the total bubble
b
surface area per unit of time passing through a given column cross-sectional area. This
value can be calculated by:
S = 30.5V /D [4]
b g b
in which V is the superficial gas rate (cfm/ft2) and D is the bubble diameter (mm). The
g b
superficial air rate may be calculated by dividing the volumetric gas flow rate (cfm) by
the column cross sectional area (ft2). The impact of S on flotation recovery is illustrated
b
by the test results given in Figure 10. These data show that recovery increases sharply as
S increases above 50 sec-1 and eventually reaches a plateau at 100-150 sec-1. These
b
26 |
Virginia Tech | coal applications. These values correspond to total aeration rates of 540-770 scfm of air
for a full-scale 14 ft diameter column. The gas rates at the lower end of the range would
generally be used for spargers that generate smaller bubbles, while the higher gas rates
are typically needed for less efficient spargers. A proper combination of gas rate and
bubble size will generally provide a gas holdup in the flotation pulp in the range of 15-
18%. The holdup can be determined by installing pressure transducers at two different
levels along the height of the flotation column. A low air holdup indicates that the
production capabilities of the column are being underutilized.
Caution should also be used during the metering of gas flow rates. A properly
designed system should be equipped with a flow meter that is calibrated to read correctly
at a specified operating pressure. The operating pressure should be held constant by
placing a pressure regulator ahead of the flow meter. By placing the air flow control
valve after the flow meter, the flow meter will always operate at its design pressure. If the
flow meter is placed after the control valve, then the operating pressure and true gas flow
rate are both unknown. Improper metering of the gas flow rate can be a particularly
serious problem when laboratory and pilot-scale tests are conducted for the purpose of
collecting scale-up information.
A great deal of confusion also exists regarding the specification of compressors
for column applications. In fact, several installations of columns in the coal industry have
required the purchase of additional compressors to reach the original design flow rate.
Much of this confusion is related to improper use of gas flow terminology (Sullair
Bulletin, 1992). For example, column manufacturers normally report gas flow rates as a
“standard” volumetric flow per time. This value is only valid at 1 atm (14.7 psia) of
28 |
Virginia Tech | pressure and 20oC (68oF) of dry air. The “actual” flow rate specified by compressor
manufacturers is typically reported in terms of “inlet” conditions or “free air.” Although
this amount of air enters the compressor, it is not necessarily the amount of air delivered
to the column due to compressor seal leakage. As a result, the “actual” flow may be only
95% of the “inlet” flow. Furthermore, corrections to the gas flow rate must be made to
account for differences in elevation (atmospheric pressure) and humidity. Air temperature
generally has little impact on the capacity of an oil-flooded screw compressor, but may
affect the performance of an air-cooled compressor. These complications generally
require that professionals be consulted to ensure that the compressor is properly sized for
the specified air requirements. This item is not an issue with the Jameson cell that
requires no compressed air.
2.2.7 Froth Handling
Froth handling is a major problem associated with coal flotation columns.
Concentrates containing large amounts of ultrafine (325 mesh x 0) coal generally become
excessively stable, creating serious problems related to backup in launders and
downstream handling. Attempts to overcome this problem by selecting weaker frothers or
reducing frother dosage have not been successful and generally lead to lower column
recoveries. Therefore, several circuit modifications have been adopted by the coal
industry to deal with the froth stability problem. For example, column launders need to be
considerably oversized with steep slopes to reduce backup. Horizontal froth travel
distances must be kept as short as possible, while adequate vertical head must be
provided between downstream operations and the column launder. Most of the newer
column installations include a deaeration tank to permit time for the froth to collapse (see
29 |
Virginia Tech | 2.3 Residence Time Modeling
2.3.1 Mean Residence Time
A considerable period of contacting time is necessary in flotation systems to
ensure that all coal particles have the opportunity to collide and adhere to air bubbles.
This contacting time is commonly referred to as the flotation time or residence time. As a
rule-of-thumb, a mean residence time of at least 3½-4 minutes is typically required in
conventional flotation machines to achieve good recoveries of well-floating bituminous
coals. The required residence time may be even longer for difficult-to-float coals that are
very fine or superficially oxidized. The mean residence time (τ) can be estimated by
dividing the active cell volume (V) by the volumetric flow rate (Q) of slurry passing
through the cells, i.e.:
τ=V /Q [10]
Note that the active volume of the cell discounts the volume of the impeller mechanism
and any hold-up of gas due to rising air bubbles. For example, a bank of four 14.2 m3
machines (85% active volume) fed 817.6 m3/hr of slurry would have τ≈3.5 minutes (4
cells x 14.2 m3/cell x 0.85 / 817.6 m3/hr x 1 hr/60 min = 3.5 minutes). While this rule-of-
thumb is useful for a first-pass evaluation, the specification of a minimum residence time
can be very misleading in some cases.
The mean residence time represents the average length of time that it takes an
element of fluid to travel through the flotation cell. In a plug-flow or batch system, all
fluid elements are exposed to the same residence time (Figure 12). Likewise, in a
perfectly-mixed system, some elements begin to exit immediately after being introduced
31 |
Virginia Tech | are difficult to duplicate in industrial practice. As a result, most commercial flotation
machines are designed with multiple agitated tanks arranged in a cell-to-cell bank in an
attempt to approach plug-flow behavior. A simple calculation shows that the total
recovery (R ) for a bank of N tanks in series is:
N
R =R +R (1−R )+R (1−R )2 +R (1−R )3 +...+R (1−R )N =1−(1−R )N [6]
N i i i i i i i i i i
where R is the fraction recovery defined at τ=τ/N. For a bank of perfectly-mixed cells,
i i
Equations (2)-(4) can be combined and mathematically simplified to yield:
N
⎛ N ⎞
R =1−⎜ ⎟ [13]
N ⎝N +kτ⎠
As expected, the performance curve provided by this equation approaches that of a plug-
flow system as N approaches infinity. However, in commercial practice, banks are
typically limited to four or five tanks in series since additional cells provide little
incremental improvement in recovery compared to the increased cost of purchasing
additional cells within the bank.
2.3.2 Residence Time Distributions
Unfortunately, field studies indicate that the performance curve described by
Equation [11] is often inadequate to describe the kinetic response of individual flotation
cells. Instead, most industrial machines operate with 0<Pe<∞. Therefore, Pe must be
determined to establish the appropriate recovery versus kτ curve for industrial flotation
systems. Pe can be estimated from an experimental residence time distribution (RTD)
curve. The RTD is obtained by:
34 |
Virginia Tech | (i) adding a tracer substance to tag an incoming element of fluid to a flotation
machine and
(ii) monitoring the concentration of the tracer in the tailings as a function of
time.
An example of RTD data for a four cell bank of conventional cells is shown in Table 2.
The mean residence time (τ) can be calculated from these data using (Levenspiel, 1972):
∑tCΔt
τ= [14]
∑CΔt
where C is the tracer concentration at any time t and Δt is the time increment between
samples [Δt= (t +t )/2]. This procedure gives τ=3.47 minutes for the data provided in
i+1 i-1
Table 3. Pe is related to the spread of the RTD, which can be quantified by the
Table 3. Example of RTD data for a four-cell bank of conventional machines.
Time Δt C tCΔt CΔt t2CΔt
(min) (min) (ppm) (ppm-min2) (ppm-min) (ppm-min3)
0 -- 0 0 0 0
1 1 480 480 480 480
2 1 780 1560 780 3120
3 1 740 2220 740 6660
4 1 560 2240 560 8960
5 1.5 370 2775 555 13875
7 2.5 130 2275 325 15925
10 4 15 600 60 6000
15 --- 0 0 0 0
∑=12,150 ∑=3,500 ∑=55,020
∑tCΔt 12,150 ∑t2CΔt 55,020
τ= = =3.47min σ2 = −1= −1=0.304
∑CΔt 3,500 θ τ2∑CΔt 3.472(3,500)
35 |
Virginia Tech | 3.0 EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Dye Mixing Studies
Conventional flotation machines are typically arranged in series with the tailings
from each cell providing feed to the next. This multi-stage arrangement makes it possible
for conventional machines to maintain good coal recoveries even at relatively short
residence times. Column cells, on the other hand, are usually fed in parallel and operate
independently from each other. This layout dictates substantially more residence time be
provided for columns to maintain good recoveries of floatable particles (Nicol and
Bensley, 1988). However, even at long residence times, the lack of downstream
scavenging creates a potential for feed to bypass to tailings in a relatively short time.
In order to qualitatively assess the severity of mixing and short-circuiting in a
column flotation cell, a series of tests were conducted using colored dye (dark red) to
visually observe the mixing behavior of fluid passing through a clear Plexiglas flotation
cell. The 30-inch diameter cell was filled with fluid to a height of 60 inches to provide a
length-to-diameter of 2:1, which is geometrically similar to many industrial column cells.
The flow rate of gas to the cell, which was equipped with a MicrocelTM sparging, was
held constant at 2.0 cm/sec for all tests. A feed flow rate equivalent to a superficial
velocity of 0.25 cm/sec was maintained by injecting fresh water through a 4-way feed
distributor placed near the top of the liquid level. This feed flow rate provided a total
volumetric residence time of approximately 10 minutes. However, when corrected for air
hold-up, the mean residence was probably in the range of 7-8 minutes. The transport of
37 |
Virginia Tech | timed sequence of images shows that the dye is first visible through the column wall at
about 4 seconds. Surprisingly, the dye appeared to be thoroughly mixed throughout the
entire cell volume after only about 13-14 seconds. This finding suggests that column cells
operated under similar conditions are likely to be well-mixed as opposed to plug-flow.
Contributors to the high degree of internal mixing are likely to include the small length-
to-diameter ratio and the agitation and circulation provided by the sparger pump.
Another series of tests were conducted using a column cell equipped with baffles
designed to minimize mixing. The baffles consisted of four horizontal partitions that split
the cross-sectional of the column into four “pie” slices of equal area. The baffles passed
from just below the top of the liquid level, down pass the the sparger inlets, and
terminated just above the central tailings discharge. Unfortunately, the test data showed
that the dye appeared to be mixed throughout the cell volume after only about 15-16
seconds (Figure 16). In addition, the data showed that the partitions tended to provide
unequal distribution of the feed in each quadrant, which would be an undesirable result
for industrial operations. Therefore, the qualitative data obtained in this study suggests
that this type of baffling cannot effectively minimize large internal mixing.
3.2 RTD Measurements
In order to quantitatively establish the extent of mixing in an industrial column, an
RTD measurement was conducted at an industrial column flotation plant. The resultant
RTD curve is plotted in Figure 17. The RTD calculations showed that the single column
operated with a relatively long residence time of τ=12.0 minutes. However, the
calculations showed that the column was relatively well mixed, as indicated by the low
39 |
Virginia Tech | column B. The columns were sampled before and after the modification. RTD
measurements were also taken for the modified two-stage circuit. The performance
calculations and experimental results for these tests are summarized in Table 4. As
expected, the modification reduced axial dispersion by increasing Pe from 1.58 to 4.22
(see Figure 17). The modification did not substantially change the mean residence time
since twice the feed flow passed through twice the cell volume after the modification.
However, the change in feeding configuration shifted the performance for the overall
circuit from a recovery of 73.7% (two-run average) for the single column to 79.8% (two-
run average) for the two-column bank. This improvement agrees very well with the
79.3% recovery projected from the series flow calculation given in Equation [16].
The results obtained from the trial run were promising enough to move ahead
with the reconfiguration of the entire five column circuit to a cell-to-cell system. In order
Table 4. Example of RTD data for single- and two-stage column circuits.
Feed Clean Tails Yield Recovery
Circuit Type
Ash (%) Ash (%) Ash (%) (%) (%)
Single-Stage Circuit (τ=11.9 min, Pe= 1.58)
Run #1 54.83 15.02 79.84 38.58 72.59
Run #2 52.45 13.86 79.59 41.29 74.80
Average 53.64 14.44 79.72 39.94 73.69
Two-Stage Circuit (τ=12.9 min, Pe= 4.22)
Run 1 - Column 1 56.55 15.94 83.21 39.63 76.67
Run 1 - Column 2 83.21 17.87 84.25 1.57 7.66
Run 1 - Column 1&2 56.55 15.98 84.25 40.58 78.46
Run 2 - Column 1 51.39 18.28 77.27 43.87 73.75
Run 2 - Column 2 77.27 18.78 82.23 7.82 27.93
Run 2 - Column 1&2 51.39 18.33 82.23 48.26 81.09
Average (Column
53.97 17.16 83.24 44.42 79.77
1&2)
43 |
Virginia Tech | 4.0 SUMMARY AND CONCLUSIONS
1. Column flotation cells can produce a higher overall separation performance than
conventional flotation machines when processing high-ash coal feeds. If properly
operated, the separation performance achieved by columns approaches the theoretical
maximum cleanability predicted by release analysis.
2. The improved separation performance of flotation columns can be largely attributed
to froth washing which minimizes the nonselective hydraulic entrainment of ultrafine
slimes into the froth product. Wash water rates in the range of 3.0-3.5 gpm/ft2 are
typically required to maintain the proper number of dilution washes needed to prevent
entrainment.
3. The capacity of industrial coal flotation columns is normally controlled by limitations
associated with froth carrying capacity. This constraint, which is very sensitive to
variations in particle size, establishes the total column cross-sectional area (column
number and diameter) required for a given application. Carrying capacities commonly
fall in the range of range of 0.06 to 0.24 tph/ft2, with an average of about 0.12 tph/ft2
for 100 mesh x 0 feed coals. The lower carrying capacities typically correspond to
finer feeds (325 mesh x 0).
4. Data obtained to date suggest that the selectivity of coal columns is largely
independent of column size and sparger design. The spargers must be capable of
dispersing air into small bubbles at gas rates typically exceeding 3.5 scfm/ft2 in order
47 |
Virginia Tech | to maintain acceptable coal recoveries and production rates. Sparger performance can
be measured by the parameter S , which represents the amount of bubble surface area
b
generated per unit time for a given column cross-sectional area. For optimum
performance, S values greater than approximately 80-100 sec-1 should be maintained.
b
5. Column cells can have both positive and negative impacts on downstream operations
in coal plants. For example, the ability of columns to removal of clay slimes and
reduce total ash content can lead to a significant increase in the filtration rate and a
lowering of the equilibrium moisture content of the clean coal filter cake. On the
other hand, difficulties associated with the increased stability of column froths may
create serious material handling problems and require the installation of deaeration
tanks or possibly the use of expensive defoaming agents. These factors must be fully
addressed prior to completing the design of any industrial circuit.
6. Residence time distribution (RTD) measurements conducted at a coal flotation plant
indicates that the poor recovery performance observed in some column flotation
installations may be partly due to the inadvertent short-circuiting of feed to tailings
cause by strong axial mixing. Mathematical calculations using a first-order kinetic
model indicate that this inherent shortcoming can be minimized by reconfiguring
banks of column cells to operate as a cell-to-cell circuit as opposed to the parallel
feeding arrangement commonly used by column manufacturers.
7. The feed piping at a coal column flotation plant was modified from a parallel to a
series layout. The modification improved the recovery by nearly 5 percentage points,
48 |
Virginia Tech | Fundamental Studies of Bitumen Aeration
Juan Ma
ABSTRACT
In the oil sand industry, bitumen is separated from sands by aerating the heavy oil so that
it can float out of a flotation vessel, leaving the unaerated sands behind. A bubble-against-plate
apparatus equipped with a high-speed camera has been developed to record the optical interference
patterns of the wetting films formed on a flat surface and subsequently obtain the temporal and
spatial profiles of the films offline using the Reynolds lubrication theory. The technique has been
used to study the interaction mechanisms between air bubbles and bitumen. It has been found that
the film thinning kinetics increases in the order of asphaltene, bitumen, and maltene, and that the
kinetics increases sharply with increasing temperature.
In addition to obtaining kinetic information, the temporal and spatial profiles of the wetting
films have been used to derive appropriate hydrodynamic information that can be used to
determine the disjoining pressures () in the wetting films. The results obtained in the present
work show that < 0 for maltene and bitumen, while > 0 for asphaltene at temperatures in the
range of 22 to 80 °C. The disjoining pressure data have been analyzed by considering the
contributions from the hydrophobic and steric forces in addition to the classical DLVO forces. It
has been found that the hydrophobic force increases with increasing temperature, which
corroborates well with contact angle data. Dynamic contact angle measurements show that air
bubbles attach on bitumen with relatively small contact angles initially but increase sharply to
>90o. The extent and the kinetics of contact angle change increase sharply with increasing
temperature. These findings suggest that the primary role of temperature may be to increase
ii |
Virginia Tech | Chapter 1
Introduction
1.1 Background
Canadian oil sands reserves are estimated to contain as much as 1.7 to 2.5 trillion barrels
of crude oil, of which about 170 billion barrels are economically recoverable using existing
technologies. These numbers make Canada the third-largest country in the world in proven oil
reserves, after Saudi Arabia and Venezuela. This huge oil sands resource would be sufficient to
meet Canada’s need for crude oil for 186 years at an oil consumption rate of 2.5 million barrels
per day. Canada’s oil sands deposits, covering 141,000 square kilometers (54,132 square miles),
are located in three main regions within the province of Alberta: Peace River, Athabasca (Fort
McMurray area) and Cold Lake. Among them, the Athabasca area, which is located in the northeast
of Alberta, has the world’s largest oil sands deposit.1
Oil sand, also known as tar sand, bituminous sand, or bitumen-impregnated sandstone/rock,
is a type of unconventional petroleum deposit that contains a carbonaceous material with a high
viscosity at reservoir temperature. This carbonaceous material is normally referred to as bitumen.
At room temperature, bitumen, like cold molasses, is so viscous and difficult to flow that it must
be treated to reduce its viscosity before it can be transported. In the upgraders and refineries,
bitumen is further treated to produce usable fuels such as gasoline, jet fuel, heating oil and diesel
fuel.
Formed 50 to100 million years ago and first discovered in 1715, Alberta’s oil sands occur
naturally as a mixture of sands, fines (- 44µm particles including silt, clay and other minerals),
water and bitumen. It typically contains 7 to 14 wt.% bitumen, 3 to 5 wt.% water, and 83 to 88
1 |
Virginia Tech | wt.% solids. In an oil sand matrix, a film of water surrounds the sand grains, which in turn is
surrounded by bitumen. A portion of the clays and fines are associated with the bitumen at the
bitumen-water interface.2
Two predominant methods currently exist to extract bitumen from oil sands: surface
mining followed by bitumen extraction and in-situ extraction. Surface mining, or open-pit mining,
is used when oil sands are close to the surface, for deposits at a depth of less than 75 meters.
Surface mining represents the most significant source for bitumen production and is currently the
main extraction method. The problem, however, is that only about 20% of the oil sands are shallow
enough to be recovered by surface mining. The other method, in-situ extraction, is used to recover
oil sands that are hundreds of meters underground. Approximately, 80% of oil sands deposits are
situated deeply underground and are only recoverable through in-situ technologies. Most in-situ
methods involves injecting steam into the oil sands deposits to heat and soften the bitumen so that
it can be pumped to the surface through wells. Steam-Assisted Gravity Drainage (SAGD) and
Cyclic Steam Stimulation (CSS) are the two main in-situ methods. The present work is related to
surface mining, which is detailed in the following sections.
1.2 Process Description
Numerous scientists and engineers have investigated economic and efficient ways to
recover bitumen from oil sands. Among them, Dr. Karl Clark was the first to develop a successful
extraction process in the 1920s.3 His process, which is referred to as the Clark Hot Water Process,
laid the foundation for today’s oil sands extraction technologies. Despite some modifications over
the ensuing decades, Clark’s hot water process is still used in commercial operations by many oil
sand operators.
2 |
Virginia Tech | Figure 1.1 shows a generic flow diagram of the bitumen recovery process.4 The basic
operations of bitumen production include mining, utilities, extraction, froth treatment, water
management (tailings treatment) and upgrading, all of which are interrelated. For example, mining
affects extraction and the extraction, in turn, affects upgrading. As shown in Figure 1.1, in
commercial bitumen recovery, oil sands are mined in open-pit mines using large shovels and then
carried by trucks to the crushers, where large clumps of oil sands are broken down into
transportable sizes. The oil sands are then treated with a warm recycle process water to transform
dry oil sands into a slurry in mixing boxes, stirred tanks, cyclo-feeders or rotary breakers. Initially,
this slurry was then fed to tumblers; more recently, the tumblers have been replaced by pipelines,
known as hydrotransport process, which helps reduce the cost of transporting ore from mine sites
to processing plants. Whether in tumblers or hydrotransport pipelines, the lumps of oil sands are
Figure 1.1 Generalized scheme for oil sands processing using water-based extraction
processes.4
3 |
Virginia Tech | then broken and dispersed in water. While sand grains coated with bitumen are suspended in water,
the bitumen is detached (or liberated), so that it can interact with air bubbles and become aerated.
Chemical additives such as caustic soda can be added during the slurry conditioning stage.
The oil sand slurry is transported to large gravity separation vessels, known as primary
separation vessels (PSV), in which the aerated bitumen float to the surface while unaerated sands
sink to the bottom. The aerated bitumen floating to the top of the PSV is subsequently skimmed
off from the slurry. Small amounts of bitumen (or middlings) not recovered via the PSV are
recovered in a bank of mechanical flotation cells or cyclo-separators.5
The bitumen froth product, normally containing 60% bitumen, 30% water and 10% solids,
is de-aerated, diluted with hydrocarbon solvents (usually naphtha) to reduce bitumen density and
viscosity, and subsequently separated from the water and solids using centrifuge, cyclones, and/or
lamella thickener in the froth treatment process. Naphtha is recovered so that it can be used again.
Later, the diluted bitumen containing 1.5 to 2.5% water and 0.4 to 0.6% solids is upgraded via a
combination of thermal cracking and catalytic hydrocracking processes to obtain synthetic crude
oils (SCO) and hydrocarbon gases. An alternative is to use a paraffinic solvent (mainly hexane) at
a sufficiently high diluent-to-bitumen ratio to precipitate asphaltenes, forming composite
aggregates that trap the water and solids in the diluted bitumen froth. In this way diluted bitumen
with less than 0.5% solids plus water can be obtained.6
Tailings from the extraction plant, which is a process byproduct composed of water, clay,
sand, and small amount of residual bitumen, are then pumped to the tailings pond where water is
recycled to be used in the extraction plant. At Sunsor and Syncrude (the two major oil sand
companies in Canada), gypsum is added to mature fine solids to consolidate the fines together with
coarse sands into a non-segregating mixture that is subsequently disposed of in a geotechnical
4 |
Virginia Tech | manner. At Albian (a relatively new oil sand company), cyclones are used, with the underflow
(coarse tailings) being directly pumped into a tailings pond, while the overflow (fine tailings)
pumped to thickeners where they are treated with flocculants, then thickened and finally
discharged into a tailings pond.
The industrial bitumen-extraction process described above entails two fundamental steps:
the liberation of bitumen from sand grains, and the aeration of liberated bitumen to air bubbles.
Both steps are very important for the success of bitumen flotation. The bitumen liberation and
subsequent stabilization against heterocoagulation between the released bitumen and sand or clay
particles are prerequisites for bitumen flotation. NaOH is added to the slurry to create a weakly
alkaline environment that favors the liberation of bitumen from oil sands. There is an optimum
amount of added NaOH that leads to maximum bitumen recovery.2 The role of added NaOH was
reported to ionize organic acids in bitumen to generate natural surfactants, which are surface active
species that facilitate bitumen liberation.7
Once the bitumen is liberated, bitumen aeration, which involves the attachment and
engulfment of liberated bitumen to air bubbles, is critical to bitumen flotation since bitumen has
almost the same density as water over the temperature ranges used in the extraction process. The
attachment/engulfment of bitumen to air bubbles decreases the apparent density of bitumen so that
it can float to the top of the separation cell and be collected.1 Bitumen aeration occurs in the oil
sand slurry conditioning stage, at which point liberated bitumen attaches to air bubbles entrained
in oil sands or extra air bubbles added to the slurry, and in flotation cells used to extract remaining
bitumen in the middlings from the primary separation cell. Research shows that the aeration of
bitumen can also enhance the effective release of bitumen from sand grains.8
5 |
Virginia Tech | Despite the importance of bitumen aeration, several fundamental mechanisms involved in
this process are still not fully understood. For example, what is the driving force for the attachment
and subsequent engulfment of bitumen onto air bubbles? Does attachment have the same driving
force as engulfment? Will the conditions favorable for bitumen liberation enhance bitumen
aeration? What role will each bitumen component play in bitumen aeration? To answer these
questions, the present work focuses on studies of bitumen aeration—mainly the thinning of the
wetting film of water formed between bitumen and air bubble. Additionally, the engulfment of
bitumen over air bubbles is described.
1.3 Bitumen Composition
The fact that so many chemicals are present in bitumen makes its chemistry complex. The
elemental composition of bitumen differs from the crude source. Typically, bitumen contains
carbon (80 to 88 wt.%) and hydrogen atoms (8 to 12 wt.%), affording a hydrogen-to-carbon (H/C)
molar ratio of around 1.5, which is intermediate between that of aromatic structures and saturated
alkanes.9 In addition, bitumen contains heteroatoms such as sulphur (0 to 9 wt.%), nitrogen (0 to
2 wt.%), oxygen (0 to 2 wt.%), as well as trace metals such as vanadium (up to 2000 ppm) and
nickel (up to 200 ppm). Sulfur, generally the most ubiquitous polar atom, appears in the form of
sulfides, thiols, and to a lesser extent, sulfoxides; oxygen exists in the form of ketones, phenols
and, to a lesser extent, carboxylic acids; and nitrogen in the form of pyrrolic and pyridinic
structures. Most metals appear in the form of complexes such as metalloporphyrins.
Constituents of bitumen include maltenes and asphaltenes. Asphaltenes are defined as the
insoluble part of bitumen in low molecular weight paraffins (i.e., heptane, pentane, and hexane),
but soluble in light aromatic hydrocarbons (i.e., toluene and benzene). Maltenes are defined as the
soluble part of bitumen in both types of solvents, which can be further separated into saturates,
6 |
Virginia Tech | aromatics, and resins. At present, the composition of bitumen is usually given in so-called SARA
fractions, which refers to Saturates, Aromatics, Resins and Asphaltenes, as shown in Figure 1.2.
1.3.1 Maltenes
1.3.1.1 Saturates
Saturates have a H/C ratio close to 2, usually accounting for 5 to 15 wt.% of the total
bitumen.10 They contain a few crystalline n-alkanes, with a number-average molecular weight of
about 600 g/mol. Fourier-Transform Infra-Red spectroscopy (FTIR) shows that different
branching structures and some long aliphatic chains are present in saturates. At room temperature,
they appear as a colorless or lightly colored liquid. Their density at 20 °C is around 0.9 g/cm3.
1.3.1.2 Aromatics
Aromatics, also referred to as naphthene aromatics, are the most abundant components of
bitumen; in fact, they account for 30 to 45 wt.% of the total bitumen. The carbon skeleton of an
aromatic is slightly aliphatic with lightly condensed aromatic rings. Aromatics have a number-
average molecular weight of about 800 g/mol, and at room temperature they form a yellow to red
liquid. They are more viscous than saturates at the same temperature. They have a density close
(inferior) to 1 g/cm3 at 20 °C.10
Bitumen
Maltene Asphaltene
Saturates Aromatics Resins
Figure 1.2 Separation of bitumen into its various components
7 |
Virginia Tech | 1.3.1.3 Resins
Resins also account for 30 to 45 wt.% of the total bitumen; moreover, depending on choice
of solvent they could even outnumber the percentage of aromatics. Resins have a H/C ratio
between 1.38 and 1.69, with a number-average molecular weight of about 1100 g/mol.10 It has
been shown that resins have a similar composition to that of asphaltene, but exhibit a less
condensed aromatic structure and a lower molar mass. They typically contain fused aromatic rings,
with a most probable structure corresponding to 2 to 4 fused rings, and a few polar groups. When
saturates and aromatics are oily liquids at room temperature, the resins form a black solid. Their
density at 20 °C is close to 1.07 g/cm3.
1.3.2 Asphaltenes
Asphaltenes represent a solubility class of macromolecules present in crude oil and
bitumen. They have a H/C ratio between 0.98 and 1.56 depending on the asphaltene source, and
usually gather traces of transition metals of the whole bitumen (e.g., Ni, Va, Fe), thereby forming
complexes such as metallo-porphyrins.9 They account for 5 to 20 wt.% of the total bitumen. The
molecular weight of asphaltenes has been controversial for decades, with estimates ranging from
700 to 3500 g/mol.11 Some estimates of the average molecular weight of asphaltene are as high as
9500 g/mol.12 Asphaltenes are by far the most studied bitumen fraction due to their viscosity-
building role and their importance in the processing of crude oil. They exist as a black powder at
room temperature and are largely responsible for the black color of bitumen. They have a density
of about 1.15 g/cm3 at 20 °C. Moreover, asphaltenes contain fused aromatic rings, most probably
4 to 10 fused rings, together with some pending aliphatic chains and polar groups such as carboxyl,
amide, thiol, and hydroxyl groups.9 In comparison to other bitumen molecules, asphaltenes contain
8 |
Virginia Tech | more condensed aromatic rings and more polar groups. Because of their many condensed aromatic
rings, asphaltenes form almost planar molecules.
1.4 Thin Liquid Film
A thin liquid film (TLF) is formed when two interfaces of a liquid are brought into close
proximity, normally a few hundred nanometers or less. TLFs are ubiquitous in industrial processes
and among scholarly researchers, and also play a vital role in our daily lives. There are six types
of TLF, which are categorized according to the phases of the interface:
1) Solid/liquid/solid film: A film confined between two solid surfaces, which is very common
in colloidal chemistry. For instance, a water film between two quartz or mica surfaces is
often studied as a model system by atomic force microscopy (AFM) to investigate surface
forces. Another example is the oil film between rocks in an oil reservoir, which is of interest
to the oil and gas industry.
2) Liquid/liquid/liquid film: A film between two liquid droplets that is immiscible with the
liquid in between. This type of TLF is common in emulsions, such as milk (an emulsion of
fat in water and proteins).
3) Gas/liquid/gas film: A foam lamella separating gas phases, which is frequently found in
soap, shampoo, and the froth flotation industry.
4) Solid/liquid/liquid film: A film between a solid surface and a liquid surface. For instance,
the water film between rock and oil in an oil reservoir.
5) Solid/liquid/gas film: A film between a solid and a gas bubble—i.e., a wetting film. For
example, the water film between a mineral particle and an air bubble in flotation.
6) Liquid/liquid/gas film: A film between a liquid surface and a gas bubble, such as the water
film between an oil and air bubble.
9 |
Virginia Tech | There are many different types of TLFs associated with the bitumen extraction process:
those between bitumen and air bubble, between bitumen and sand grain, between bitumen and clay
particle such as kaolinite, illite or montmorillonite, between air bubble and sand grain, between air
bubble and clay particle, between sand grain and clay particle, between two bitumen droplets,
between two air bubbles, between two sand grains, and between two similar or different clay
particles. In the present work, we focus on the wetting film between bitumen and air bubble. Other
types of TLFs will only be discussed briefly (or not included in this study).
1.4.1 Disjoining Pressure
TLFs are known to be unstable and seek to thicken or thin themselves depending on the
pressure in the film ( ) relative to that in the adjacent bulk ( ) of the same fluid under the
same thermodynamic conditions. Such a difference is defined, originally by Derjaguin in 1955, as
the “disjoining pressure” Π(h),13 h being the thickness of the TLF,
[1.1]
Thermodynamically, disjoining pressure can be defined as the change in excess Gibbs free
energy per unit area of a flat TLF (G) with the film thickness (h),14, 15
[1.2]
at constant pressure (P), temperature (T), and chemical potential of solutes (μ ).
s
Disjoining pressure quantifies the driving force for spontaneous thinning or thickening of
a film. With a positive disjoining pressure, i.e., Π > 0, the film thickens; with a negative disjoining
pressure, i.e., Π < 0, it thins. When Π = 0, it is in an equilibrium state. The process of thickening
is equivalent to separating (or “disjoining”) its bounding surfaces, hence its name. When h
becomes sufficiently large that the liquid in the film behaves the same as in the bulk, Π tends to
be zero.
10
P
film
P
b u lk
(h) P P
film bulk
h G h
P T
S
( ) ( / )
, , |
Virginia Tech | Disjoining pressure determines the stability of a TLF. A TLF should be stable
thermodynamically when the disjoining pressure derivative is negative, i.e., ∂Π/∂h < 0. In other
words, the disjoining pressure vs. h curve should have a negative slope. When the disjoining
pressure derivative is positive, i.e., ∂Π/∂h > 0, a TLF should be unstable, thin by itself, and rupture
catastrophically, such as the wetting film between a hydrophobic particle and an air bubble in
flotation.
The most common method for measuring a film with a thinning thickness is to use optical
interference techniques in conjunction with microscopy, which was originally developed by
Derjaguin and Kusakov16 to measure > 0. In 1967, Laskowski and Kitchener17 recognized that
the disjoining pressure in wetting films must be negative for bubble-particle attachment to occur
in flotation. However, the authors recognized the difficulty of measuring negative disjoining
pressure due to several reasons—principally fast kinetics of film thinning, deformation of the
air/water interface, and complex interactions between hydrodynamic and surface forces. It was
only recently that CAST developed a technique for measuring negative disjoining pressure using
a modified thin film pressure balance (TFPB) technique with high-speed video microscopy.18, 19
This new method is based on monitoring the profiles of the wetting films in nanoscale, and then
analyzing the profiles using the Reynolds lubrication theory to determine various pressures. Such
a technique is possible because bubbles deform in response to changes in pressure.
1.4.2 Disjoining Pressure Isotherm
As indicated in Eqs. [1.1] and [1.2], disjoining pressure Π is a function of the film thickness
h. Π(h), called the disjoining pressure isotherm, depends on the makeup of the film, the adjoining
bulk phases and the interfaces between them.
11 |
Virginia Tech | There are many contributors to disjoining pressure. The classical Derjaguin-Landau-
Verwey-Overbeek (DLVO) theory is based on the contribution from the van der Waals force
present in a TLF (
12
d
) and the electrostatic force between overlapping electrical double layers
( ), as shown below,
e
d
e
[1.3]
DLVO theory is used to explain the stability of a colloidal system, but only to a limited
degree. Specifically, discrepancies between DLVO theory and experimental results have been
reported in a number of cases,20, 21 indicating that other contributions must also be considered, as
shown in the following equation,
[1.4]
where , , , , represent the contribution from hydrophobic force, steric force,
hydration force, and hydrogen bonding, respectively. There might be other forces as well, such as
supramolecular structuring forces.
1.4.2.1 Van der Waals Force
Van der Waals dispersion forces are ever-present between two macroscopic objects and
can be important both for both small and large separations. There are three origins for Van der
Waals forces: 1) electrostatic interactions between permanent charge distributions in the molecules,
such as dipoles, quadrupoles, etc. (Keesom’s dipole – dipole interaction); 2) electrostatic
interactions between permanent charge distributions and induced charge distributions (Debye’s
dipole – induced dipole interaction); and 3) the dispersion forces associated with the oscillations
of the orbital electrons and interactions with the synchronous induced dipoles in neighboring
molecules (London’s fluctuating dipole-induced dipole interaction ).22
h
d
s
h
e
y d
ra
tio n
h
s
h y d ra tio n
H
H |
Virginia Tech | Hamaker,23 who was responsible for elucidating the mechanisms of the van der Waals
interaction, suggested that the van der Waals force between two macroscopic objects could be
obtained by adding up the forces acting between all the molecules in one body and all those in the
other body. The Hamaker constant, A, which is often used in interaction laws, is conventionally
defined as
[1.5]
where and are the number of atoms per unit volume (number density) in the two bodies,
and C is the coefficient in the atom-atom pair potential. Typical values for the Hamaker constants
of condensed phases, either solid or liquid, are about 10-19 or 10-20 J for interactions in a vacuum
or air. For gas phases, the Hamaker constant is generally taken to be zero due to the low molecular
density in such media.
The pairwise additivity assumptions in Eq. [1.5] ignore the influence of neighboring atoms
on the interaction between any pair of atoms. The Lifshitz theory24 avoided the additivity problem
and provided an alternative way to calculate the nonretarded Hamaker constant macroscopically
for the interaction of material (1) and (2) in the medium (3), as shown below
[1.6]
where k is Boltzmann constant, T is the absolute temperature (in Kelvin), is the Planck’s
constant (6.63×10-34J·s), is main electronic adsorption frequency (3×1015 Hz), , and
represent the refractive indices of the three media, and and refer to the static dielectric
constants of the three media. Eq. [1.6] is an approximate expression based on the assumption that
the absorption frequencies of all three media are the same.
13
2
1 2
A
1
C
2
A
1 3 2
3
4
k T ( 1
1
3
3
) ( 2
2
3
3
)
3 h
8
p
2
e
( n 21 n 23 ) 1 / 2 ( n 22
(
n
n
21
23
) 1
n
/ 2
2 ) (3(
n
n
21
22
n
n
23
23
)
)
1 / 2 ( n 22 n 23 ) 1 / 2
e
h
p
1
n
1
n
2
n
3
2
3 |
Virginia Tech | attraction, while negative values suggest van der Waals repulsion. If A is intermediate in value
33
between A and A , the Hamaker constant will be negative, which is the case for flotation.
11 22
If the materials separating the liquid film are the same,
[1.12]
A is thus always positive in spite of the relative values of A and A . Therefore, van der
131 11 33
Waals interactions are always attractive for like materials, such as foam films and films between
two like particles or liquid droplets. The greater the difference between Hamaker constants of the
material and the medium, the greater the van der Waals attraction.
In bitumen flotation, where the case is bubble (1)-bitumen (2) interaction in the medium of
water (3), the Hamaker constant of air bubble tends to zero (A = 0), as mentioned earlier, and the
11
Hamaker constant of bitumen is greater than that of water (A >A ). Therefore, the Hamaker
22 33
constant of the film is negative (A <0) according to Eq. [1.11], indicating repulsive van der
132
Waals interactions between air bubbles and bitumen in flotation.
1.4.2.2 Electrostatic Force
Interfaces rarely carry a net charge. They are normally charged because of following
mechanisms: 1) preferential adsorption/desorption of lattice ions, 2) specific adsorption of ions, 3)
ionization of surface functional groups, 4) isomorphic substitution, and 5) depletion of electrons.
When a surface acquires a charge, counterions gather in the vicinity of the charged surface and
neutralize the charge, forming an “electric double layer.” If two surfaces bearing an electric double
layer approach one another, their overlap can contribute to the total disjoining pressure.
The electrical double-layer interaction between a particle and an air bubble can be
determined from the Poisson-Boltzmann equation, as shown below:
15
A
1 3 1
( A
1 1
A
3 3
) 2 |
Virginia Tech | ez
2 ez n ()exp i [1.13]
0 i i kT
where is the dielectric constant of the solution, is the permittivity of vacuum, is the
potential, e is the charge on electron, is the number per unit volume of the electrolyte ions
of type i with valence . Two boundary conditions are needed to solve the Poisson-Boltzmann
equation.
The electrical double-layer interaction also depends on the charging mechanisms at the
surfaces. Three cases are considered:
1) Both surfaces at constant surface potentials.
In this case, the surface potentials are known and remain the same for all separations. Zeta
potential is often used rather than surface potential. The disjoining pressure between a particle and
an air bubble can be calculated using the Hogg-Healey-Fuerstenau (HHF) approximation:26, 27
[1.14]
where is the reciprocal Debye length, and are the zeta potentials at the solid/water and
air/water interfaces, respectively. Eq. [1.14] holds exactly for surface potentials less than 25/z mV
i
(the condition of the Debye-Hückel linearization) and for small-scaled separations (the condition
of the Derjaguin approximation). A comparison between the HHF expression and the exact
solution of the Poisson-Boltzmann equation shows that even at 75 to 100 mV the divergence is not
excessive except for very short distances between the surfaces.26
For two surfaces with potentials of the unlike sign, the interaction is attractive to each other
at all separation distances, as expected. For two surfaces with potentials of the same sign but of
unequal magnitude, the interaction is repulsive at large separations, but attractive at small
16
0
e 2 s i n
0
h
z
(
i
2
h )
(
21 22
n
)
(i
c o
s
)
e c h ( h ) 2
1 2
c o t h ( h )
1
2
|
Virginia Tech | separations. When the surfaces have potentials of the same sign and of equal magnitude, the
interaction is repulsive at all separations.27
2) Both surfaces at constant surface charges.
If a system cannot regulate its charge during the interaction, a constant charge interaction
is appropriate. In this case, charge per unit area on the surfaces (charge densities) remains constant.
The disjoining pressure is given by28
[1.15]
Where and are the zeta potentials at the isolated solid/water and air/water interfaces
before the interaction, respectively.
When the two surfaces have unlike sign charges of unequal magnitude, the interaction is
attractive at large separations, but becomes repulsive at small separations. When the two surfaces
are of unlike sign charges with equal magnitude, the interaction is attractive at all separations.
When the two surfaces have the same sign charges, the interaction is repulsive at all separation
distances.29
3) One surface at constant potential and the other at constant charge.
In this case, one surface has constant potential at all separations while the other has constant
surface charge. The disjoining pressure can be obtained by29
[1.16]
The electrical double-layer interaction in this case falls between that at constant potential and at
constant charge. As shown above, for two surfaces having the same initial potential, the
electrostatic interactions are different for systems due to the differences in the charging
mechanisms of their surfaces.
17
e
0
2
2 2
1 2
c o
s
s h (
i n h 2
h
(
)
h )
21 22
1
2
e
0
2
2 2
1 2
s i n
c
h
o
(
s h
h
2
)
( h
(
)
21 22 )
|
Virginia Tech | Given the fact that the third case is complicated, the first and second cases have been
favored in scholarly research. The constant potential case (#1) represents an ideal regulation of the
surface charge. When the surface charge cannot be regulated during the interaction, a constant
charge model should be used. In flotation, if a particle slides over an air bubble, the location of the
particle-bubble interaction sites changes continuously, making it difficult to achieve a perfect
regulation of the surface charge. In such cases the interaction at constant surface charge is more
appropriate than at constant surface potential. If the location of the particle-bubble interaction does
not change over the bubble surface, constant surface potential interaction may occur.
According to Eqs. [1.14] to [1.16], one must know surface potential to calculate the
electrostatic double-layer interaction. Surface potential is often substituted by zeta potential, which
can be measured by electrokinetic methods such as electrophoresis, electroosmosis, or streaming
potential. In bitumen flotation, the zeta potential of bitumen emulsion is mainly measured using
electrokinetic methods. Research shows that the bitumen zeta potential becomes increasingly more
negative when the solution pH increases from 4 to 6, but then levels off at pH>8.30, 31 The iso-
electric point of bitumen in water has been reported to be about pH = 3.
Measuring the zeta potential of gas bubbles is more difficult than that of solid particles due
to the inconveniencies associated with the generation of gas bubbles in the measuring cells and the
rise of bubbles in liquid solutions. Different values have been reported depending on the solution
pH, electrolyte concentration, surfactant addition, etc.32-37 In general, gas bubbles are negatively
charged in pure water and inorganic solutions such as NaCl and KCl at pH > 2-3. The magnitude
of the zeta potential decreases with increasing salt concentration, suggesting that Na+, K+, Cl- ions
act as indifferent ions that control the effective diffuse layer thickness, but do not specifically
adsorb on the gas-water interface. When surfactants are present in the solution, the zeta potential
18 |
Virginia Tech | of gas bubbles is determined by the type of surfactant—specifically, negative for an anionic
surfactant and positive for a cationic surfactant.
1.4.2.3 Steric Force
Although DLVO theory can be applied to many systems, the situation becomes more
complicated when considering flotation. In this case, non-DLVO forces such as steric force and
hydrophobic force must also be considered. When bitumen is in contact with water, asphaltene is
extracted from bitumen and deposited on the bitumen/water interface. The polar groups of
asphaltene are exposed toward the aqueous phase.38 In this manner, the bitumen surface would
become relaxed or swelled, forming brush-like structures. It has been reported that asphaltene is
responsible for the steric repulsion in bitumen flotation systems.38-41
The steric interaction has been studied both theoretically and experimentally.42-44
Normally, stabilizing polymers contain anchor groups and buoy groups. Anchor groups are
components that are attached either chemically or physically to a surface to prevent the escape of
polymers as polymer-coated surfaces approach each other. Buoy groups, or stabilizing moieties,
are the adsorbed layers that are as solvent-compatible as possible and have sufficient molar mass
to provide needed adlayer thickness. Anchor groups usually account 10 to 25% of the total
adsorbing macromolecule, with the remaining volume occupied by the buoy groups.
When two polymer-coated surfaces are brought into contact and start to interpenetrate, the
potential energy of the steric interaction may be comprised of: 1) mixing effect — the excess
chemical potential of the adsorbate and solvent due to overlapping of layers; 2) volume restriction
effect — elastic energy arising from compression of the polymer layers, which makes it difficult
to press two polymer layers together; or 3) changes in surface free energy, which could occur from
moving polymers laterally away from the surface or because of the desorption of polymers.45 The
19 |
Virginia Tech | third factor, however, can be discarded since typically the adsorbed molecules are assumed to be
anchored irreversibly and locally.
The most important properties of the system in terms of steric repulsion are 1) the adlayer
thickness or the polymer tail length L, and 2) the solvency of the buoy blocks of the polymer in
the medium. These two properties are related to each other. While the adlayer thickness depends
primarily on the molar mass of the buoy groups of the polymer, solvency can also play a role. In a
good solvent, interactions between buoy groups of polymers and solvent molecules are
energetically favorable; thus, the polymer coils will expand or extend. In a poor solvent, polymers
prefer to interact with themselves and the polymer coils will shrink. The length of polymer tails is
roughly equivalent to the end-to-end distance of these free polymer segments under the same
conditions as the system. At low surface coverage and in a good solvent, the thickness of the
adsorbed polymer layer should be roughly equal to the Flory radius (R ):
F
[1.17]
where l and n are the length and the number of the monomers, respectively.
At higher coverage, the grafted chains are so close to each other that they are forced to
extend away from the surface much farther than R , thereby forming a “brush layer.” For a brush
F
in a good solvent, L becomes:46
[1.18]
where s is the mean distance between attachment points.
Once two brush-bearing surfaces are within a distance of 2L from each other, there will be
a repulsive pressure between them. According to the Alexander-de Gennes theory,44 the disjoining
pressure is given by:
20
R
F
n 3 / 5 l
5/3
R
L s F
s |
Virginia Tech | 21
s
k
s
T
3
(
2 L
h
) 4 / 9 (
2
h
L
) 3 / 4
[1.19]
1.4.2.4 Hydrophobic Force
In addition to steric force, another non-DLVO force—namely, the hydrophobic force—is
also critical for bitumen flotation because both bitumen and air bubble surfaces are hydrophobic.
Israelachvili and Pashley47 were the first to directly measure the short-range attractive hydrophobic
force between two curved mica surfaces using surface force apparatus (SFA). The researchers
rendered the mica surfaces moderately hydrophobic using a cationic surfactant
hexadecyltrimethylammonium bromide (CTAB), having an advancing water contact angle of
about 60°.48 Since then, many other investigators have measured both short- and long-range
hydrophobic forces between two solid surfaces using SFA and atomic force microscopy (AFM).49-
52 All results indicate the presence of an attractive hydrophobic force between hydrophobic solid
surfaces.
Measuring the surface forces between a solid surface and an air bubble were made possible
by AFM.53 Assessing particle-bubble interactions using AFM was first reported by Ducker et al.54
and Butt55 in 1994; other related studies soon followed.56-62 Attractive forces were observed
between a hydrophobized silica particle and an air bubble in water.
The hydrophobic forces (F ) measured in experiments are commonly represented using
h
the following form,47
F h
h Cexp( ) [1.20]
R D
where R is the radius of curvature of the hydrophobic surfaces interacting with each other, C and
D (decay length) are fitting parameters.
According to the Derjaguin approximation,63 which is shown in the following equation, |
Virginia Tech | [1.21]
one can obtain Eq. [1.22] to calculate the disjoining pressure contributed by hydrophobic force:
[1.22]
The origin of the hydrophobic force is still controversial. Some believe it is of entropic
origin, arising from the rearrangement of water molecules at hydrophobic surfaces.64-66 When
water molecules come into contact with a small nonpolar molecule or bubble, they re-orientate or
re-structure to form “clathrate cages” or “gas hydrates.”67 When water molecules are in contact
with a hydrophobic-water interface, including the air-water interface, they also form clathrate-like
structures.68, 69 When two hydrophobic surfaces approach each other, the water structure changes
in the overlapping boundary layers of water.70 Based on this concept, Eriksson et al.66 derived a
theoretical model for hydrophobic force, which assumes that the intervening water between two
surfaces is progressively more ordered with decreasing h. Most recently, Yoon and coworkers71-73
determined the thermodynamic functions of hydrophobic interactions by conducting surface force
measurements at several different temperatures. The researchers showed that macroscopic
hydrophobic interactions entail decreases in both the excess entropy (Sf) and the excess enthalpy
(Hf) of thin liquid films (TLF) of water confined between hydrophobic surfaces.
Other investigators believe that hydrophobic force is due to the bridging of nanoscopic
bubbles preexisting on the hydrophobic force. This theory is supported by three important
observations: 1) discontinuities (or “steps”) on force vs. distance curves,52, 74 2) the disappearance
of attractive forces in deaerated water,75 and 3) the presence of nanobubbles on hydrophobic
surfaces detected by AFM imaging.76 In contrast, other researchers argue that 1) steps on the force
curve can be avoided by modifying the conditions and procedures involved in force
22
F
R
h
h
h
d h
h
2
2
C
D
e x p (
h
D
)
|
Virginia Tech | measurement,77 2) long-range attractions still exist in degassed solutions,78, 79 and 3) AFM studies
do not show nanobubbles on hydrophobic surfaces;80 rather, optical measurements confirm that
water between two hydrophobic surfaces has the same refractive index as the bulk water.81
There are other possibilities for the origin of hydrophobic force, including electrostatic
origin,82 separation-induced cavitation,83 hydrodynamic fluctuating correlation,84 etc.
In bitumen flotation, all , and are positive (or repulsive); therefore, the total
disjoining pressure does not afford a negative value that is required for bitumen-bubble attachment.
Adding a hydrophobic component to the disjoining pressure allows the total disjoining pressure to
be negative when is negative.
1.5 Dissertation Outline
Despite the importance of bitumen aeration in water-based bitumen extraction processes,
some of the fundamental mechanisms involved in bitumen aeration are still not fully understood.
Thus, the principal objective of this thesis work is to investigate the thinning of wetting films
formed between bitumen and air bubbles. To facilitate this objective, we developed a bubble-
against-plate apparatus equipped with a high-speed camera to record the optical interference
patterns of the wetting films formed on a flat bitumen surface. It is necessary to use a high-speed
camera as the wetting films formed on hydrophobic surfaces are unstable and hence thin fast. The
recorded interference patterns were then used to obtain the temporal and spatial profiles of the
films offline. The film profiles were then analyzed to determine the disjoining pressure () using
the method and algorithms developed by Pan, et al.18 These authors derived the following
expression for disjoining pressure,
23
h
d
e
s |
Virginia Tech | 24
2
R
r
r
r
h
r
1 2
r
r r
1
h 3
r
r 0
r
h
t
d r
d r
[1.23]
where is the air/water interfacial tension, R the bubble radius, r the radial position, h the wetting
film thickness, and µ the fluid viscosity.
Chapter 1 gives an overview of the state-of-the-art oil sands extraction process and a brief
review of the recent scientific research conducted on the role of colloidal forces in the extraction
processes.
Chapter 2 describes the temperature effect on the kinetics of the thinning of wetting films
of water on bitumen. A modified thin film pressure balance (TFPB) technique has been used to
monitor the changes in the profiles of the TLFs between an air bubble and a flat substrate coated
by bitumen, maltene, or asphaltene at temperatures ranging from 22 to 80 °C. The kinetic
information in the vertical and radial directions, i.e., 𝜕ℎ⁄𝜕𝑡 and 𝜕ℎ⁄𝜕𝑟, respectively, have been
derived from the film profiles to determine the disjoining pressure () in the TLFs. In addition,
water contact angles on bitumen surfaces at temperatures in the range of 22 to 80 °C have been
measured using captive bubble method. Based on these results and the thermodynamic data
available in the literature, a model for bitumen aeration is proposed.
Chapter 3 studies the role of colloidal forces in bitumen aeration. The thin film pressure
balance (TFPB) technique has been used i) to study the kinetics of thinning of the wetting films
formed on bitumen and its hydrophilic and hydrophobic components, i.e., asphaltene, and maltene,
and ii) to determine the disjoining pressures () in the films from the curvature changes recorded
during the process of film thinning. The results will be analyzed in view of the extended DLVO
theory, which includes the contributions from the electrical double-layer force, van der Waals force,
hydrophobic force, and steric force that may be present in the TLFs. |
Virginia Tech | Chapter 4 deals with the effects of solution chemistry on the stability of wetting films on
bitumen. A new bubble-against-plate apparatus has been developed and used to study effects of
immersion time, pH, KCl, Ca2+, Mg2+ ion concentrations, and the approach speed on bubble-
surface interactions.
Chapter 5 includes the summary of the present work and suggestions for future work.
References
1. Masliyah, J.H., Z. Xu, and J.A. Czarnecki, Handbook on Theory and Practice of Bitumen
Recovery from Athabasca Oil Sands, Kingsley Publishing Services, 2011.
2. Sanford, E.C., Processibility of athabasca oil sand: Interrelationship between oil sand
fine solids, process aids, mechanical energy and oil sand age after mining, The Canadian
Journal of Chemical Engineering. 61(4), p. 554-567, 1983.
3. Clark, K. and D. Pasternack, Hot Water Seperation of Bitumen from Alberta Bituminous
Sand, Industrial & Engineering Chemistry. 24(12), p. 1410-1416, 1932.
4. Masliyah, J., et al., Understanding Water‐Based Bitumen Extraction from Athabasca
Oil Sands, The Canadian Journal of Chemical Engineering. 82(4), p. 628-654, 2004.
5. Cymerman, G.J. and T. Kwong, Improvements in the Oil Recovery Flotation Process at
Syncrude Canada Ltd., in Processing of Hydrophobic Minerals and Fine Coals:
Proceedings of the 1st UBSMcGill Bi-Annual International Symlposium on
Fundamentals of Mineral Processing, CIM, Vancouver, BC, 1995.
6. Tipman, R.N. and Y.-C. Long, patent US 5876492, 1999.
7. Sanford, E.C. and S. F.A., Processability of Athabasca Tar Sand Using a Batch
Extraction Unit: The Role of NaOH, CIM Bull. 72(803), p. 164-169, 1979.
25 |
Virginia Tech | Chapter 2
Temperature Effect on the Stability of Wetting Films of
Water on Bitumen
Abstract
For air bubbles to attach on mineral (or bitumen) particles, the wetting films of water
formed on the surfaces must rupture. Thermodynamically, the films can rupture when the
disjoining pressures () of the films are negative. In the present work, the disjoining pressures of
the wetting films formed on bitumen have been measured at temperatures in the range of 22 to
80 °C. The results show that becomes less negative with decreasing temperature. However, it
remains negative until the temperature is decreased to 35 °C, but becomes positive at 22 °C. These
results are consistent with the industrial experiences in bitumen flotation.
2.1 Introduction
In 2012, Canada produced 1.8 million bbl/day of oil from the oil sands resources in Alberta,
which is expected to grow to 5.2 million bbl/day by 2030, according to the projections from the
Canadian Association of Petroleum Producers (CAPP). The amount of heavy oil (bitumen)
deposited in the form of oil sands is estimated to be 2.5 trillion bbls, which is five-times larger
than the conventional oil reserves in Saudi Arabia. At present, approximately 55% of the bitumen
35 |
Virginia Tech | is produced from mining followed by flotation, with the rest produced by in-situ extraction
methods, e.g., cycle steam stimulation (CSS) and steam-assisted gravity drainage (SAGD).
The oil sands ores are composed of sand grains with a median size of 65 mesh (208 µm)
and a top size of 14 mesh (1,168 µm), a layer of hydrated water (or innate water), and bitumen that
forms a continuous phase between sand grains.3 The innate water contains ultrafine clay particles
such as kaolinite and illite. The ores containing typically 7 to 14% by weight of bitumen are
processed by flotation, with recoveries usually in the range of 93%, while those of the in situ
processes are much lower (40 to 70%). In general, the ores containing large amounts of ultrafine
particles are of low grades and suffer from low recoveries. Also, the ores containing divalent
cations such as Ca2+ and Mg2+ ions are difficult to process.5
The basic bitumen extraction processes used today are the same as the original hot-water
processing method developed by Clark in the 1920s.7 In this process, bitumen is detached (or
liberated) from sand grains by agitating an aqueous ore slurry in hot water (~80 °C) in the presence
of caustic soda to raise the pH to 8.0 – 8.5. Initially, the agitation was provided by a tumbler along
with steam injection. In early 1990’s, the tumblers have been replaced by pipelines, in which
bitumen is liberated and aerated in transit. The aerated bitumen droplets float in a primary
separation vessel (PSV) to be separated from the sand grains and clays. The middlings that are
insufficiently aerated are directed to a bank of conventional and/or column flotation cells to recover
additional bitumen. The froth products, containing typically 60% bitumen, 30% water and 10%
solids, are de-aerated, diluted in naphtha (or n-alkane), and centrifuged to remove solids. The clean
bitumen is then upgraded by a combination of thermal and catalytic hydrocracking processes to
obtain synthetic crude oils (SCO).
36 |
Virginia Tech | Much work has been done to lower operating temperatures to reduce the energy
consumption. In 1990, Sury9 developed a cold water process for bitumen extraction, in which an
oil sand matrix was attrition-scrubbed in water in the presence of appropriate conditioning agents
to achieve liberation and disperse the liberated bitumen. The aqueous slurry was then subjected to
flotation. The conditioning agents included typical flotation reagents such as kerosene, diesel oil,
and frother, e.g., methyl-isobutyl-carbinol (MIBC). Bitumen recoveries were ~90 % at 5 to 10 °C
and ~95% at 15 to 35 °C. In 2000, a similar process was tested at the Aurora plant, Syncrude
Canada, Ltd., at 25 °C.10 In 2002, the operating temperature was raised to 35 to 40 °C to obtain
higher and more consistent recoveries.
Despite the simplicity in the extraction process and the significant improvement achieved
in recent years, some of the mechanisms involved in the different steps of the extraction process
are not fully understood. Contrary to the general belief that bitumen-in-water emulsions are
stabilized by the surfactants that are naturally present in bitumen (e.g., naphthenate), it has been
shown that the emulsions are stabilized by the asphaltene polymers present in bitumen.11 This
mechanism was proposed on the basis of the surface forces measured between two bitumen-coated
mica surfaces using a surface force apparatus (SFA). Recognizing the role of asphaltene as an
emulsifier may help explain the long conditioning times and high temperatures required for
bitumen extraction.
In general, the bubble-bitumen attachment (or aeration) requires long induction times
usually in the range of seconds, while the bubble-particle attachment in mineral flotation occurs in
milliseconds.11-13 Further, air bubbles attach on mineral surfaces, forming finite contact angles,
while in bitumen flotation small bubbles are engulfed into bitumen droplets. Despite the
differences noted above, the fundamental mechanisms are the same in that the wetting films of
37 |
Virginia Tech | water formed on the substrates (mineral and bitumen) must be broken for the flotation to occur. It
is more difficult for air bubbles to break the wetting films formed on bitumen than those formed
on mineral most probably due to the presence of asphaltene polymers exposed on the former.
When an air bubble is pressed against the surface of a bitumen droplet (or a mineral particle)
under turbulent conditions, it deforms; and the curvature change associated with the deformation
creates an excess pressure (p), which is equal to 2/R, where is the surface tension and R is the
radius of curvature. The excess (or Laplace) pressure causes the thin liquid film (TLF) of water
(or wetting film) formed between the two macroscopic surfaces to thin. As the film thins further,
the surface forces in the film come into play and create a disjoining pressure () at h < 200 to 300
nm, in which case the excess pressure should become,
2
p [2.1]
R
Eq. [2.1] shows that the excess pressure becomes large when < 0, which will accelerate the film
thinning and cause the film to rupture at a critical thickness (h ). This will result in bubble-bitumen
cr
attachment and subsequent engulfment. If > 0, however, the wetting film is stable and the
bubble-particle attachment becomes difficult.
The highest value of in a disjoining pressure isotherm ( vs. h curve) may represent the
energy barrier (E ) for bubble-particle attachment. In bitumen flotation, E is high most probably
1 1
due to the repulsive steric force created by the asphaltene exposed on bitumen surface11 and due
to the high negative -potentials created by the alkali (KOH) added to the system to help liberate
the bitumen from sand grains. According the bubble-particle attachment model proposed by
Luttrell and Yoon (1992),9
E
P exp 1 [2.2]
a E
k
38 |
Virginia Tech | it is necessary to apply a high kinetic energy (E ) to overcome the energy barrier and achieve a
k
high probability of attachment (P ). An alternative to applying a high E would be a long
a k
conditioning time, which may be provided by the pipeline transport (or hydrotransport) system
used in industry today.
Still another alternative is to reduce the energy barrier (E ) by appropriate means. It has
1
been shown that the induction time for bubble-bitumen interaction decreases at lower pH,11
suggesting a corresponding decrease in E . However, flotation at low pH is impractical in a sense
1
that bitumen liberation requires a high pH. In the present work, the effects of temperature on E
1
have been studied by measuring at temperatures in the range of 22 to 80 °C. The measurements
were conducted using the modified thin film pressure balance (TFPB) technique described
previously.10,11 Since this technique is capable of measuring pressures in unstable films, the
measurements have been extended to the TLFs of water formed on hydrophobic substrates such as
maltene and bitumen at elevated temperatures of up to 80 °C. The results are in good agreement
with industry practice in terms of temperature effect on bitumen extraction. It is hoped that the
results of the present work will be useful for better understanding the role of temperature in
bitumen extraction by flotation.
2.2 Experimental
2.2.1 Materials
A sample of bitumen (99 wt.%) was provided by Suncor Energy, Inc. Reagent grade
toluene (>99.5% purity) was purchased from Spectrum Chemical Mfg. Corp., and HPLC grade n-
heptane (99.3% purity) was obtained from Fisher Scientific. Millipore ultrapure water with a
resistivity of 18.2 MΩ·cm was obtained using a Direct Q3 water purification system. Silicon
wafers (100 crystal planes, University Wafer) were used as the substrates for coating bitumen,
39 |
Virginia Tech | asphaltene and maltene. Sulfuric acid (H SO , 98% purity, VMR international) and hydrogen
2 4
peroxide (H O , 29.0–32.0% purity, Alfa Aesar) were used to clean the silicon substrates.
2 2
2.2.2 Separation of Asphaltene and Maltene
The bitumen sample was dissolved in toluene at a toluene/bitumen ratio of 5:1 by volume.
The solution was then stirred using a magnetic stirrer for 2 hr, centrifuged for 30 min to remove
the solids, and subsequently put under a fume hood to allow the toluene to naturally evaporate.
After the toluene evaporation was complete, n-heptane was added to the toluene- and solids-free
bitumen at a heptane/bitumen ratio of 40:1 by volume. The n-heptane and bitumen mixture were
stirred by means of a magnetic stirrer for 2 hr, during which time maltene dissolved into n-heptane
while asphaltene precipitated out. The mixture was left overnight for the asphaltene to settle at the
bottom. The supernatant n-heptane was carefully removed by means of a pipette and placed under
a fume hood to let the n-heptane naturally evaporate, leaving maltene behind. The amount of the
evaporated n-heptane was small.
The asphaltene precipitate was repeatedly washed for 17 times with an excess amount (~1L)
of n-heptane to remove any maltene that may have co-precipitated. In each washing step, the
mixture was agitated for 2 hr and left to stand overnight to allow the asphaltene to precipitate and
the supernatant was carefully removed. After the last washing step, the supernatant n-heptane
became colorless. The asphaltene precipitate was left under a fume hood to allow n-heptane to
evaporate and thereby obtain a sample of purified asphaltene.
2.2.3 Preparation of Bitumen-, Maltene-, and Asphaltene-coated Surfaces
Silicon wafer was first oxidized in an oxidation furnace at 1070 °C for ~40 min using wet
oxidation procedure. The oxidized silicon wafers were then cut into 12x12 mm pieces and used as
substrates for bitumen, asphaltene, and maltene. Before the coating, the silicon substrates were put
40 |
Virginia Tech | in an ultrasonic bath of water for ~1 hr and then cleaned by immersing in a Piranha solution (3:7
by volume of H O /H SO ) at 120 °C for 1 hr, followed by rinsing with an adequate amount of
2 2 2 4
ultrapure water and pure nitrogen (N ) blow-drying. The silicon substrate was placed in a spin-
2
coater (Laurell, WS-400-6NPP), rinsed with toluene, and then spun at 6,000 rpm for 30 s to dry
off the toluene. When the substrate was free of toluene, a small amount of (2 or 3 drops) of a
bitumen-in-toluene solution (2.5 mg/mL) was placed on the substrate and let it stand for 10 s. The
substrate was then spun at 3,500 rpm for 20 s before placing another drop of the bitumen solution
and continuing spinning for additional 10 s. The spinning continued for an additional 1 minute at
6,000 rpm to evaporate the solvent and obtain a smooth and uniform bitumen coating. The spin-
coated bitumen was then placed in a particulate matter-free laminar hood for ~30 min to ensure
maximum removal of toluene from the coating. The procedure described above was also employed
for the preparation of asphaltene- and maltene-coated silicon surfaces.
2.2.4 Measurement of Film Thinning Kinetics
Figure 2.1 shows the modified thin film pressure balance (TFPB) apparatus used in the
present work to study the kinetics of film thinning. The original TFPB apparatus was designed for
the study of foam films (or the water films between two air bubbles) by Scheludko and Exorowa.14
The equipment was modified to study the kinetics of film thinning for wetting films.15 As shown
in the inset, a flat surface of interest is placed on the top of a film holder (2.0 mm diameter glass
tubing) with its surface facing downward, so that the surface is in contact with the water in the
film holder. In the present work, the flat silicon wafers coated with bitumen, asphaltene, or maltene
were used as samples of interest.
In a given experiment, the silicon wafer-glass tubing assembly was inserted in a glass cell
surrounded by a water jacket, in which water of known temperature was circulated. The film
41 |
Virginia Tech | Figure 2.1 Schematics of the modified TFBC apparatus used for the study of wetting
films.
holder-water jacket assembly was placed on an inverted microscopic stage (Olympus IX51) to
monitor the changes in film thicknesses as a function of time. A mercury arc short lamp (100 W,
Olympus) was used as a light source with a bandpass interference filter (#65-643, Edmund Optics)
to produce a monochromatic green light source (λ=546 nm). In a given experiment, the liquid in
the film holder is slowly withdrawn by turning the knob of the piston. Once the interference
patterns (Newton rings) began to appear in the microscopic field of view, the film was allowed to
thin spontaneously while recording the images by means of a high-speed camera (Fastec Imaging,
HS400115). The interference patterns recorded were used to reconstruct timed thickness profiles
of a wetting film across the entire film holder with a nanometer resolution. The spatial and
temporal film profiles of the wetting film were then used to monitor the changes in film thicknesses
(h) as a function of time and radial distance of the film at any point of the film. The experimental
42 |
Virginia Tech | data were used to obtain information on the film thinning kinetics and calculate the disjoining
pressure () in the thin liquid films (TLFs) according to Eq. [2.3], as derived by Pan. et.al.,16
43
2
R
r
r
r
h
r
1 2
r
r r
1
h 3
r
r 0
r
h
t
d r
d r
[2.3]
where is the air/water interfacial tension, R the bubble radius, r the radial position, h the wetting
film thickness, and µ the fluid viscosity. The experiments were conducted at temperatures in the
range of 22 to 80 °C.
2.2.5 Contact Angle Measurements on Bitumen Film
Flat plates of silicon wafers were coated with bitumen using the spin-coating technique
described in the previous section (2.2.3). The thicknesses of the coatings were approximately 11
nm. The hydrophobicity of the bitumen-coated surfaces were determined by measuring water
contact angles using the captive bubble technique. For each measurement, an air bubble was
brought to a bitumen-coated surface from underneath. The measurements were conducted using a
Ramé-Hart contact angle goniometer, Model 250, F4 series. The measurements were conducted in
a double-jacketed rectangular glass cell with optical windows filled with ultrapure water. The
measurements were conducted at different temperatures in the range of 22 to 80 oC under
controlled temperature conditions.
Effect of temperature was also studied by measuring contact angles on relatively thick
(0.5 mm) films of bitumen formed on clean Teflon plates. Several bitumen-coated Teflon plates
were placed in an oven for 3 to 4 min at 120 °C to soften the bitumen and obtain flat surfaces,
which were subsequently cooled at room temperature in a dust-free cabinet. To measure contact
angles at an elevated temperature, a bitumen-coated Teflon plate was placed in an oven at a desired
temperature for a few minutes and then placed in ultrapure water at approximately the same |
Virginia Tech | temperature. The water temperature was controlled by circulating the water from a constant
temperature bath through a double-walled glass cell that was designed for captive bubble contact
angle measurement. The Ramé-Hart contact angle goniometer was used to record the images of
air bubbles interacting with bitumen-coated surfaces at a speed of 30 fps and to determine dynamic
contact angles offline using a protractor.
2.3 Results and Discussion
2.3.1 Temperature Effect on the Film Thinning Kinetics
Figure 2.2 shows the changes in Newton rings (or interference patterns) of the wetting films
formed on bitumen-, asphaltene-, and maltene-coated silicon surfaces obtained at 80 °C. As shown,
the Newton rings changed with time due to film thinning until the film ruptured. From the light
intensities of the interference patterns, one can calculate the film thickness (h) using the micro-
interferometric technique and obtain temporal and spatial profiles of the wetting films, as shown
Figure 2.2 Changes of Newton rings with time due to the decrease in the thickness of the
wetting film of water formed on maltene (a), bitumen (b) and asphaltene (c) surface, while
the air bubble was approaching the substrate at 80 °C.
44 |
Virginia Tech | As shown in Figure 2.3, the films formed on maltene and bitumen surfaces thinned fast and
ruptured at 0.12 and 0.16 s, respectively. On asphaltene, the film thinned substantially slower, with
the rupture occurring at 0.615 s. Note here that the wetting film before rupture formed on
asphaltene were flatter than those formed on the other two substrates most probably due to the
presence of a greater resistance force to film thinning.
Figure 2.4 shows the effect of temperature on the kinetics of film thinning on bitumen. The
film thicknesses (h) measured at the film centers, i.e., r = 0, are plotted vs. time (t) at different
temperatures. As shown, the film thinning kinetics represented by the slope of the h vs. t curves
increased with increasing temperature, which explains the advantage of floating bitumen at higher
temperatures. According to the kinetics curves presented in Figure 2.4, bitumen flotation would
be possible at temperatures as low as 35 °C. At 22 °C, it took >80 s for the wetting film to rupture,
400
300
Bitumen
Natural pH
200
)
m
(n 22 0C
h 100 35
80
0685 5
0 40 80
) t (s)
m
200
n
(
h
22 0C
35
80 55
68
0
0 4 8 12
t (s)
Figure 2.4 Effect of temperature on the kinetics of film thinning on bitumen in water at natural
pH of about 5.7.
46 |
Virginia Tech | which would make it difficult to float bitumen at such a low temperature. The results presented in
Figure 2.4 are consistent with the industrial experiences reported in the literature. The original hot
water extraction process was designed to work at 80 to 85 °C. During the 1990s, the operating
temperatures were reduced to 70 to 80 °C to save energy. In 2000, the Aurora plant of Syncrude
Canada, Ltd., tested the cold water processing at approximately 25 °C.10 However, the operating
temperature has been increased to 35 to 40 °C, which is consistent with the results obtained in the
present work. At present, most plants operate at temperatures in the range of 40 to 55 °C to ensure
operation reliability and high bitumen recovery. The results that the film thinning became faster at
higher temperatures are also in agreement with the induction time measurements reported by Gu
et al.,12, 17 who found that the induction time of bubble-bitumen attachment decreased greatly with
increasing temperature.
Figures 2.5 and 2.6 show the h vs. t curves for the wetting films formed on maltene and
asphaltene, respectively, at temperatures in the range of 35 to 80 °C. As shown, the kinetics of film
thinning increased with increasing temperature. Also, the thin liquid film of water thins much
faster on maltene than on asphaltene at a given temperature, indicating a greater resistance on
asphaltene surface.
Comparison of the results presented in Figures 2.4, 2.5, and 2.6 shows that the kinetics of
film thinning increased in the order of asphaltene, bitumen, and maltene, which in turn suggests
that the hydrophobicity of the three different materials increases in the same order. This finding is
in good agreement with the work by Yoon and Rabinovich,18 who conducted direct force
measurements using an atomic force microscope (AFM) between bitumen, asphaltene and maltene
surfaces in water and found that there was a long-range attractive force between maltene surfaces
47 |
Virginia Tech | while a strong repulsive steric force between asphaltene surfaces. The steric force may be
responsible for the slow kinetics of film thinning on asphaltene-coated surfaces.
2.3.2 Temperature Effet on Disjoning Pressure
Figure 2.7 shows the disjoining pressure () in the wetting films of water formed on
maltene, asphaltene, and bitumen surface at temperatures in the range of 20 to 80 °C. The
disjoining pressure was calculated using a spatial and temporal profile obtained in the present work.
Figure 2.3 shows the data obtained at 80 °C, while the profiles obtained at other temperatures are
not shown in this communication. The film profiles were used to derive the kinetic information on
film thinning, i.e., 𝜕ℎ⁄𝜕𝑡 and 𝜕ℎ⁄𝜕𝑟, which were then substituted into Eq. [2.3] and obtain . As
shown in Figure 2.7, both maltene and bitumen exhibited negative disjoining pressures ( < 0) at
temperatures between 35 to 80 °C. On asphaltene, > 0 at all temperatures except at 80 °C. These
results suggest that maltene and bitumen can be more readily floated than asphaltene. In general,
decreases with increasing temperature, which is consistent with the industrial experience that
bitumen flotation becomes easier at higher temperatures.
2.3.3 Contact Angle Measurements at Different Temperatures
The results obtained in the present work suggest that the hydrophobicity of bitumen
increased with increasing temperature. In order to better understand this observation, the contact
angles of water on bitumen-coated surfaces, on both thin and thick bitumen films, were measured
at different temperatures using the captive bubble method. Figure 2.8 shows the dynamic contact
angles measured on thin bitumen films (11 nm thick) formed on silicon wafer at different
temperatures. At a given temperature, the contact angle decreased with time, as was also observed
by others.19 Note here that contact angles decreased with increasing temperature, which is contrary
49 |
Virginia Tech | 90
22 oC
60
)
o 55 35
(
68
30
0
0 20 40 60
t (min)
Figure 2.8 Dynamic water contact angles measured on the thin bitumen films at
different temperatures. The thickness of the bitumen films were 11 nm.
to what is expected in view of increasing flotation recovery with temperature.20 It is also contrary
to our findings that film thinning kinetics on bitumen increased with temperature and the disjoining
pressure became more negative at a higher temperature.
The decrease in contact angle with time may be attributed to changes in the orientation of
the surfactants present in bitumen, as suggested by Ren, et al.19 It is more likely, however, that the
bitumen films used for the contact angle measurement were so thin that parts of the films dissolved
away into water and, thereby, exposed the hydrophilic silicon dioxide substrates. The higher the
temperature, the faster and more bitumen would dissolve into the solution. Therefore, a series of
contact angle measurements were conducted on a thick bitumen film of 0.5 mm to avoid the
exposure of silicon dioxide substrates toward water. Figure 2.9 shows the images of the air bubbles
adhering on bitumen surfaces at different temperatures, while Figure 2.10 shows the dynamic
contact angles measured from the images by extrapolating the curvatures of the bubbles. A close
51 |
Virginia Tech | examination of the images shows that a type of liquid, most probably maltene, collects near the
three-phase contact points, and the amount of the liquid increases with temperature.
When bitumen is exposed to water, asphaltene (or the surfactant naturally present in
bitumen) may be exposed to the surface, rendering the surface less hydrophobic. As the
temperature increases, maltene will diffuse through the asphaltene layer and restore the
hydrophobicity of the bitumen surface, causing the contact angle to increase, as shown in Figure
2.10. Furthermore, part of the air bubble may be covered by maltene. As shown, the contact angles
increased with time and temperature, indicating that the diffusion of maltene is slow at room
temperature, but the rate of diffusion increases with temperature due to a corresponding decrease
in viscosity.
Figure 2.11 depicts a model showing how bitumen contact angles increase with time at a
high temperature. Initially, the contact angle is relatively low due to the presence of asphaltene on
the surface, which is hydrophilic at an alkaline pH in the range of 8 to 10. At a pH close to its
pK , its functional groups are ionized and strongly interact with water (or become hydrophilic). At
a
an elevated temperature, maltenes diffuse into the asphaltene layer on the surface and increase its
Figure 2.11 A model for the attachment of air bubble to the surface of bitumen at a high
temperature: a) initially, an air bubble attaches to a bitumen surface coated by asphaltene
with a low contact angle; b) maltene diffuses into the asphaltene layer on the surface and
increases the hydrophobicity and hence the contact angle. This is possible due to the low
viscosity of maltene at higher temperatures. Eventually, the bitumen layer saturated with
maltene begins to cover the bubble surfaces.
53 |
Virginia Tech | Table 2.1 Dynamic Viscosity Data for Bitumen, Maltene, n-Dodecane, Hexacane and
Naphthalene.
Viscosity (mPa·s)
20 °C 30 °C 40 °C 50 °C 60 °C 75 °C
Athabasca bitumen1 7x105 ~105 ~4x104 ~104 ~5x103 ~103
Lloydmin bitumen2 38601 11591 3999 1604 746
Maltene extracted from
2585 1017 431 219 119
Lloydmin bitumen2
n-Dodecane (C H )4 ~1.344 0.911 0.659
12 26
Hexadecane (C H )6 ~3.061 1.829 1.231
16 34
Naphthalene (C H )8 2.483 1.422
10 8
hydrophobicity, causing the contact angle to increase, as shown. (Recall that in the present work
the contact angle measurements began as soon as a sample was placed in water.) When the
asphaltene layer is saturated with maltene, an oily substance begins to coat the bubble surface, as
shown in the images of the dynamic contact angle measurements (see Figure 2.9). At the same
time, the contact angles increase, and the profiles of the bubbles change accordingly. As the bubble
profiles become flat, the bubble-bitumen contact area increases, which may facilitate the diffusion
of air into bitumen underneath through the asphaltene layer. The time it takes for the contact angle
to increase and reach an equilibrium decreases with increasing temperature most probably due to
the decrease in viscosity. It can be seen from Table 2.1 that the dynamic viscosity of bitumen and
maltene decreased dramatically with temperature. For example, the viscosity of Athabasca
bitumen decreases by nearly three orders of magnitude as temperature increases from 20 to 75 °C.
The viscosity data presented in Table 2.1 were experimentally measured using different types of
54 |
Virginia Tech | viscometers by previous researchers. Details about the viscometers used could be found in the
cited papers.1, 2, 4, 6, 8 The increase in contact angle with time and temperature, as shown in Figure
2.10, may be attributed qualitatively to the increased saturation of the asphaltene layer with
maltene at longer contact times and higher temperatures. Furthermore, the hydrophobicity of
maltene itself increases with temperature, as shown in the disjoining pressure measurements (see
Figure 2.7a).
2.3.4 Thermodynamic Analysis Based on Acid-base Theory
As noted in the foregoing paragraph, it appears that the oily substance coats the bubble
when the asphaltene layer is saturated with maltene. It was thought first that maltene could readily
coat bubble surface due to the hydrophobic interaction between maltene and air bubble as both are
hydrophobic. A simple thermodynamic analysis suggests, however, that maltene is less likely to
coat the bubble than the asphaltene saturated with maltene (or bitumen). The free energy change
associated with the spreading of an oily substance on an air bubble (
55
G
s
) can be obtained using
the following relation,
G
s o o w w
/
[2.4]
where is the surface tension of oil,
o o / w
is the interfacial tension between oil and water, and
w
is the surface tension of water. Eq. [2.4] shows that the spreading of an oily substance on an
air bubble entails the expansion of the oil/air and oil/water interfaces at the expense of the air/water
interface of the bubble. In the present work, maltene may be considered the oily substance.
According to the acid-base theory,21
2 dd 2 2 [2.5]
o/w o w o w o w o w |
Virginia Tech | where
56
do and dw are the dispersion components of the oil and water interfaces, respectively; o
and w are the acidic components of oil and water, respectively; o and w are the basic
components of oil and water, respectively.
Substituting Eq. [2.5] into Eq. [2.4], one obtains,
G
s
o
do dw o w o w 2 2 2 2 [2.6]
If maltene is as hydrophobic as other hydrocarbon oils ,
o
and do should be in the range of 17 to
25 mJ/m2, while dw = 21.8 mJ/m2, w = w = 25.5 mJ/m2, and o and o are approximately zero,
assuming maltene is completely apolar.22 Assuming also that
o
= 17 mJ/m2 for maltene, Eq. [2.6]
gives G
s
= -4.5 mJ/m2, suggesting that maltene can coat the bubble surface. If = 25 mJ/m2
o
rather than 17 mJ/m2, the free energy change becomes positive, and thus maltene cannot coat air
bubbles. Since the surface tensions of oils decrease with increasing temperature in general, it is
thermodynamically possible that maltene can coat air bubbles at relatively high temperatures. It is
unfortunate that detailed thermodynamic information for maltene is not available in the literature.
The authors contacted the University of Alberta looking for a correct value of for maltene to
o
no avail. The data do not exist as yet. It would be fruitful to determine the missing thermodynamic
data in the future.
The surface tension data for bitumen are better known than those for maltene. According
to the data reported by Moran et al.,23 = 30 mJ/m2 for bitumen, and are in the range of 11
o o/w
to 20 mJ/m2. Substituting these values into Eq. [2.4] along with = 72 mJ/m2, one obtains that
w
G
s
is in the range of -22 to -31 mJ/m2. It appears, therefore, that bitumen (or asphaltene saturated
with maltene) is more likely to coat air bubbles during bitumen flotation. The reason for this is that |
Virginia Tech | the polar groups of asphaltene can interact with the surrounding water molecules and give rise to
a more negative free energy of spreading (
57
G
s
), as is obvious from Eq. [2.6]. Conceptually, a
hydrophobic oil will not spread at the air/water interface or bubble surface because it cannot
interact with the water surrounding bubbles. If a surfactant is added to the oil, however, it can
spread because the polar heads of the surfactant molecules interact with the surrounding water
molecules and reduce free energy. Asphaltene acts effectively as a surfactant in the bitumen
extraction system and facilitates the spreading of bitumen (or maltene) on bubble surfaces. As
mentioned earlier, asphaltene contains polar groups such as carboxyl, sulfide, amide, and hydroxyl
groups. In the spreading process, asphaltene molecules in bitumen tend to adsorb at the air/water
interface because of these polar groups, lowering the bitumen/water interfacial tension and hence
the Gibbs free energy change for spreading. Asphaltene acts as a spreading agent for bitumen.
It should be noted here that maltene is not completely apolar because one of its components,
resin, contains polar groups such as carboxylates and sulfates.24 Compared to asphaltene, however,
resin is less polar.25 Asphaltene still plays a major role in facilitating the spreading of bitumen onto
air bubbles.
2.3.5 A Bitumen Aeration Model
Based on the contact angle measurements shown in Figures 2.9 and 2.10 and the
thermodynamic considerations discussed above, a conceptual model for bitumen aeration may be
suggested as depicted in Figure 2.12. When a small air bubble collides with a bitumen droplet in a
hydrotransport pipeline or in a secondary flotation cell, the bubble will deform and create an excess
pressure (p) in the wetting film of water formed in between the bitumen droplet and the air bubble.
At this point in time, the film will thin due to the curvature pressure (p ). When the thickness
cur
reaches the range of 250 to 300 nm, the film thinning process may encounter a resistance from the |
Virginia Tech | Figure 2.12 A dynamic bitumen aeration model: a) an air bubble attaches to an asphaltene-
coated bitumen surface, forming a low contact angle; b) maltene diffuses into the asphaltene
layer on the surface and increases the contact angle of the layer, and subsequently covers bubble
surface; c) when the air bubble is covered by the maltene-saturated asphaltene, the bubble enters
bitumen droplet, completing aeration. Thermodynamically, maltene-saturated asphaltene (or
bitumen) spreads on air bubble better than maltene. Thus, asphaltene may act as a spreading
agent for maltene.
colloidal forces in the form of a positive disjoining pressure ( > 0). The film thinning will stop
when p = , because the excess pressure in the film becomes zero. If < 0, on the other hand,
cur
the excess pressure will increase, as p = p - , and hence accelerate the film thinning process
cur
and eventually rupture the wetting film. Once the film has ruptured, a contact angle is formed.
Initially, the contact angle is small as depicted in Figure 2.12a. The experimental data
presented in Figures 2.9 and 2.10 show actually that initial contact angles are less than 90°. In
time, however, contact angles increase above 90o, as depicted in Figure 2.12b. According to the
Young’s equation,
cos b b/w [2.7]
w
the contact angle increase should be due to the changes in interfacial tensions involved, Eq. [2.7]
suggests that contact angle () should be less than 90o when . The contact angle should
b b/w
increase above 90° when . Therefore, the dynamic contact angle behavior of bitumen as
b b/w
manifested in Figures 2.9 and 2.10 must be due to the changes in interfacial tensions. It should be
58 |
Virginia Tech | noted also that the secret of achieving high contact angles, which is essential for successful
aeration, is to increase the bitumen/water interfacial tension (
59
b / w
).
As has already been noted, it is necessary to increase the disjoining pressure for bitumen
liberation, which can be achieved by agitating a bitumen slurry at a reasonably high temperature
in the presence of an alkali (KOH). Under this condition, the basic functional groups of asphaltene,
which includes carboxylic acids, carbonyls, phenols, pyroles, and pyridinic functional groups,
dissociate and become polar.26-29 As a result, highly-charged asphaltene molecules are exposed on
bitumen surface and create steric and electrical double-layer forces to create a highly-positive
disjoining pressure in the wetting films of water between bitumen and silica sands. At the same
time, the exposure of the polar asphaltene should render the bitumen surface polar by increasing
the acidic and basic components of surface tensions, i.e., b and . This should in turn decrease
b
the bitumen/water interfacial tension ( ) according to the acid-base theory,
b/w
b w
b
w
db dw b w b w
/
2 2 2 [2.8]
After bitumen is liberated from sand grains, bitumen aeration is desired. The aeration of
bitumen consists of two substeps: 1) the attachment of bitumen droplets to air bubbles and 2) the
subsequent engulfment of air bubbles by bitumen. The bubble-bitumen attachment, like in the case
of mineral flotation, involves the thinning of the wetting film of water formed between bitumen
droplets and air bubbles and the subsequent film rupture at a critical thickness forming a three
phase contact line. The bubble engulfment involves the spreading of the bitumen onto air bubbles.
Attachment (or film rupture) is the prerequisite of the engulfment. Without the film rupture,
engulfment will not happen and the bitumen flotation is not possible. If a bitumen droplet and an |
Virginia Tech | air bubble collide, their mutual attachment is thermodynamically favorable if the Gibbs free energy
change associated in the attachment (G ) is negative. G can be given as the following equation,
a a
G ( ) [2.9]
a b b/w w
which represents a process of a new air/bitumen interface being created at the expense of the
air/water and bitumen/water interfaces. If G is negative, the attachment is favored and
a
spontaneous. If G is positive, the bitumen should not attach. From Eq. [2.9], one can see that
a
high bitumen/water interfacial tension (
60
b / w
) will favor the bitumen-air attachment
For bitumen-bubble attachment, it is necessary to render the disjoining pressure in the
wetting films formed between air bubble and bitumen negative, for which the asphaltene molecules
exposed on bitumen surface need to become less polar so that the bitumen/water interfacial tension
is increased. The most likely way to achieve this goal will be to allow the maltene at the interior
of a bitumen-in-water emulsion droplet to diffuse out to the exterior through the asphaltene layer
exposed on the surface and render the droplet surface hydrophobic. According to Eq. [2.8], one
must reduce the acidic and basic components of bitumen surface tension, which can be readily
accomplished when the maltene exposed on the surface masks the polar groups of the asphaltene
on the surface. Once this has been accomplished, air bubble begins to penetrate the asphaltene
droplet, as shown in Figure 2.12b, with the asphaltene acting effectively as an emulsifier for
maltene, as has already been discussed.
In effect, bitumen droplets rendered hydrophilic during the liberation step are rendered
hydrophobic in a hydrotransport pipeline prior to aeration. The hydrophilic-to-hydrophobic
transition is induced by the diffusion of maltene through the asphaltene layer. Most of the
diffusion-controlled processes are slow. One way to expedite the diffusion is to reduce the
viscosity of maltene, which can be accomplished by increasing temperature. It appears, therefore, |
Virginia Tech | that the role of temperature in bitumen aeration may be to decrease the viscosity of maltene. That
contact angle data obtained at different temperatures and presented in Figures 2.9 and 2.10 may
support this view.
After the bitumen-bubble attachment has occurred, bitumen would spread spontaneously
over an air bubble (Figure 2.12b and 2,12c) if the Gibbs free energy change associated in spreading
is negative (G < 0), as discussed previously. (G ) can be calculated as
s s
G [2.10]
s b b/w w
Eq. [2.10] is essentially the same as Eq. [2.4]. Contrary to bitumen-air attachment, low
bitumen/water interfacial tension (
61
b / w
) will favor the bitumen spreading over air bubbles.
Figure 2.12c shows that an air bubble enters a bitumen droplet and remains spherical. The
air bubble may form a coaxial interlayer between the maltene core and the layer of bitumen
exposed on the surface. According to Eq. [2.4], however, G
s
will become more negative if
o
is small, which can be achieved if the bubble remains spherical so that the air/oil (or air/maltene)
interfacial area remains small.
Eqs. [2.9] and [2.10] give the following relation,
G
a
G
s
2
b / w
[2.11]
which shows that G is always more negative than or equal to G . Based on Eq. [2.11], there
a s
are three possibilities, representing three possible cases in bitumen aeration. First, if G is positive,
a
G has to be positive too. In this case, bitumen will neither attach nor spread on air bubble surfaces
s
and therefore flotation won’t occur. Second, if G is negative, but G is positive, in which case
a s
bitumen will attach but won’t spread on air bubbles. This case will occur when the processing
temperature of the bitumen extraction is low, e.g., < 35 °C, and bitumen droplets will attach to air
bubbles as discrete particles.30, 31 In this case, the success of flotation depends on whether the slurry |
Virginia Tech | is sufficiently quiescent that the attached bitumen won’t be sheared away from the bubble. Finally,
if both G and G are negative, then bitumen will attach and engulf air bubbles, which happens
a s
at high processing temperatures, e.g., > 45 °C. Once the bitumen engulfs an air bubble, only very
high mechanical shear would cause it to be stripped away. Therefore, this is the best condition for
successful bitumen aeration.31
Our thermodynamic analysis shows that bitumen/water interfacial tension (
62
b / w
) plays an
important role in bitumen extraction form oil sands. Low bitumen/water interfacial tension will be
favorable for bitumen liberation from oil sands as well as the engulfment of bitumen over air
bubbles. On the other hand, high bitumen/water interfacial tension is beneficial for the attachment
between bitumen and air bubble. Therefore, our analysis suggests that an optimum bitumen/water
interfacial tension is needed for bitumen extraction in industrial operations. Bitumen/water
interfacial tension could be controlled by the slurry pH. A high pH will facilitate the ionization of
polar molecules at the bitumen/water interface and therefore reduce the bitumen/water interfacial
tension. It was reported that an optimum amount of NaOH was needed to achieve a maximum
bitumen recovery.32 An over-dose of NaOH would cause a too low bitumen/water interfacial
tension, making bitumen-bubble attachment difficult. It could also cause bitumen to emulsify,
leading to small bitumen droplets that are undesirable for bitumen flotation.33
2.4 Summary and Conclusion
The wetting films of water formed on bitumen and its components (maltene and asphaltene)
thin faster with increasing temperature, which provides an explanation for the improved bitumen
extraction at higher temperatures. At a given temperature, the kinetics increase in the order of
asphaltene, bitumen and maltene. |
Virginia Tech | From the experimental data obtained in the film thinning kinetics studies, the disjoining
pressures in the wetting films of water formed on bitumen, maltene, and asphaltene have been
determined. The results show that < 0 for maltene and bitumen, while > 0 for asphaltene. In
general, disjoining pressure decreases with temperature with all three substrates. That the
disjoining pressure for bitumen decreases with temperature provides an explanation for the
increased kinetics of bubble-bitumen interaction and hence improved extraction at higher
temperatures.
The dynamic contact angle measurements conducted on bitumen show that, initially,
contact angles are small but subsequently increase above 90o, obviously due to changes in the
interfacial tensions involved. It appears that the changes in interfacial tensions are brought about
by the diffusion of maltene from underneath the asphaltene layer formed on the surface of bitumen,
causing an increase in the surface hydrophobicity of bitumen.
References
1. Helper, L.G. and C. Hsi, AOSTRA Technical Handbook on Oil Sands, Bitumen and Heavy
Oils, AOSTRA technical publication series 6, Alberta Oil Sands Technology and Research
Authority, Edmonton, AB, 1989.
2. Luo, P. and Y. Gu, Effects of asphaltene content on the heavy oil viscosity at different
temperatures, Fuel. 86(7), p. 1069-1078, 2007.
3. Plitt, L.R., Athabasca Tar Sands, in Milling Practice in Canada, p. 371-378, Harpell's
Press Cooperative, Ste. Anne de Bellevue, Quebec, 1978.
63 |
Virginia Tech | Chapter 3
Colloidal Forces in Bitumen Aeration
Abstract
The thin film pressure balance (TFPB) technique has been used to study the thinning
kinetics of the wetting films formed on bitumen, asphaltene, and maltene and to determine the
disjoining pressures () in the films from the curvature changes recorded during the process of
film thinning. It was found that film thinning kinetics decreased in the order of maltene, bitumen
and asphaltene. The results also showed that < 0 on maltene and bitumen, while > 0 on
asphaltene, indicating that asphaltene may serve as a kinetic barrier for bitumen aeration. The
disjoining pressure data obtained in the present work have been analyzed in view of the extended
DLVO theory. The results suggest that hydrophobic force is the driving force for bitumen aeration.
3.1 Introduction
In the water-based bitumen extraction from Athabasca oil sands, bitumen aeration is
critically important. Without aeration bitumen would not float because it has almost the same
density as water.1 Bitumen aeration is one of the two fundamental steps in the extraction process,
the other being bitumen liberation from sand grains. These two steps entails three-phase
interactions involving bitumen-bitumen, bitumen-solids (silica and clays), and bitumen-bubble. It
has been generally recognized that colloidal forces play a crucial role in controlling these
interactions and thus the stability of the thin liquid films (TLFs) formed between the three
macroscopic phases.
67 |
Virginia Tech | In the past two decades, several researchers have reported their investigations of colloidal
forces between two bitumen surfaces.2-8 They studied various factors affecting colloidal forces and
some also investigated adhesion forces. In this work, we focus on the nature of colloidal forces.
Yoon et. al.2, 3 reported the first direct measurement of colloidal forces between two bitumen
surfaces. They prepared bitumen-coated mica or silica surfaces by means of a Langmuir-Blodgett
trough (L–B deposition) and conducted surface force measurements using SFA and AFM. They
observed long-range repulsive electrostatic force at a separation distance above 70 nm and strong
repulsive force at a shorter range that cannot be explained by the classical DLVO theory. The
authors suggested that the non-DLVO repulsive forces were steric forces due to the asphaltenes
exposed on the bitumen surface in an aqueous environment. Wu et al.4-6 on the other hand,
investigated the colloidal forces between two micron-sized bitumen droplets in water based on the
collision trajectories of two colliding bitumen droplets and the hydrodynamic force applied to
break up a bitumen doublet into two droplets. They suggested that the enhanced repulsion between
bitumen was due to the heterogeneous protrusion structure on the bitumen surface. Laroche et al.7
used the same techniques and compared colloidal forces between deasphalted bitumen (i.e.
maltene) droplets and whole bitumen droplets and found that the repulsion increases in the
presence of asphaltene. More recently, Liu et al.8 measured colloidal forces between two bitumen
surfaces also using AFM as Yoon did. The difference was that Liu et al. prepared the bitumen
substrate using spin-coating method and the bitumen sphere using dip-coating technique. In
addition to the repulsive long-range electrostatic force and short-range steric force, they also
detected attractive hydrophobic force at a separation distance around 10 nm in the presence of
1mM KCl in the solution.
68 |
Virginia Tech | For bitumen-sand interaction, Liu et al.9 were the first to directly measure the surface force
between bitumen and silica with AFM and found that at long separation distances bitumen-silica
repulsion could be described by DLVO theory while at shorter distance (< 2–3 nm) an additional
repulsive force of steric origin was observed. Long et. al.10 found that the repulsive force between
bitumen and silica increases with increasing temperature. Later, Zhao et al.11 measured the surface
force between bitumen and silica also using AFM in synthetic plant solutions, actual plant water,
and the foam and residue solutions. In the synthetic electrolyte solutions, they found similar results
as Liu et al.9. While in plant process water and the foam and residue solutions, the measured forces
were weaker than the DLVO forces. More recently, Hogshead et al.12 used AFM to measure
bitumen-silica interaction forces in both an aqueous solution and an ionic liquid. When Utah
bitumen was used, colloidal forces collected in water and ionic liquid appeared similar. For the
Alberta bitumen, the attractive force was longer-ranged and stronger in water than in the ionic
liquid. For bitumen-clay interactions, Liu et al.13 measured the surface forces between bitumen-
kaolinite and bitumen-montmorillonite using AFM. Repulsive forces were detected for both
systems.
Many researchers studied the bitumen-bubble interactions.2, 14-24 As far as colloidal forces
are concerned, only a couple papers have been published in the open literature. In these studies,22,
23 the forces between an air bubble and a bitumen surface were measured with AFM, in which the
air bubble was placed on a hydrophobic substrate, while bitumen was coated on silica spheres by
dip-coating. Long range repulsive forces were initially observed due to electrostatic double layer
forces. It was also reported that an external force applied to the AFM cantilever was needed to
exceed a threshold value before the probe jumped into contact with the air bubble, forming stable
three-phase contact line.
69 |
Virginia Tech | Studies of the wetting film formed between bitumen and air bubble can provide useful
information for better understanding the colloidal forces acting between the two macroscopic
surfaces. In 1967, Laskowski and Kitchener25 recognized that the disjoining pressure () in
wetting films must be negative for bubble-particle attachment to occur in flotation. However, the
authors recognized the difficulty in measuring the negative disjoining pressures because the
wetting films with < 0 are unstable and rupture too quickly. It was only recently that CAST
developed a technique to measure negative disjoining pressures using a modified thin film pressure
balance (TFPB).26, 27 This new method is based on monitoring the profiles of the wetting films in
nanoscale and analyzing the profiles using the Reynolds lubrication theory to determine the excess
pressure in the film. This was made possible by monitoring the deformation of bubbles during the
course of bubble-particle interactions.
Bitumen is a colloidal dispersion of asphaltene micelles in maltenes.28, 29 Asphaltenes are
the insoluble part of a bitumen in low molecular weight paraffins (e.g., heptane, pentane, and
hexane) but soluble in light aromatic hydrocarbons (e.g., toluene and benzene). Maltenes are the
soluble part in both types of the solvents and consists of saturates, aromatics and resins. The polar
components of a bitumen are asphaltenes and resins, but asphaltenes are more polar than resins.
These polar molecules have strong affinity toward water and adsorb at oil/water interfaces,
contributing to the stabilization of the emulsions formed in bitumen extraction process.30 For the
bitumen-in-water emulsions, some researchers31-33 suggested that they are stabilized by natural
surfactants, mainly carboxylates and sulfates, present in bitumen, while others2, 3 proposed that
asphaltenes are the stabilizers. For the water-in-bitumen emulsions, both asphaltenes and resins
are responsible for the stabilization of the emulsions, but asphaltenes are the key stabilizer.34-37 In
these regards, one may expect that the asphaltenes and/or resins may play a significant role. It is,
70 |
Virginia Tech | therefore, the objective of this chapter to study the role and extent that asphaltenes play in bitumen
aeration.
In the present study, asphaltenes and maltenes were separated from bitumen to determine
the colloidal forces present in the wetting films of water formed on these substrates and to discuss
their roles in bitumen aeration. The kinetics of thinning of the wetting films formed on bitumen,
asphaltene, and maltene were monitored using the TFPB technique. The disjoining pressure was
determined based on the temporal and spatial profiles of the wetting films by the method developed
by Lei et al.27 The disjoining pressure data obtained in the present work have been analyzed in the
view of the extended DLVO theory.3, 38
3.2 Experimental
3.2.1 Materials
A sample of bitumen (99 wt%) was provided by Suncor Energy, Inc. Reagent grade toluene
(>99.5% purity) was purchased from Spectrum Chemical Mfg. Corp., and HPLC grade n-heptane
(99.3% purity) was obtained from Fisher Scientific. Millipore ultrapure water with a resistivity of
18.2 MΩ·cm was obtained using a Direct Q3 water purification system. Silicon wafers (100 crystal
planes, University Wafer) were used as the substrates for coating bitumen, asphaltene and maltene.
Sulfuric acid (H SO , 98% purity, VMR international) and hydrogen peroxide (H O , 29.0–32.0%
2 4 2 2
purity, Alfa Aesar) were used to clean the silicon substrates.
3.2.2 Separation of Asphaltene and Maltene
The bitumen sample was dissolved in toluene at a toluene/bitumen ratio of 5:1 by volume.
The solution was then stirred using a magnetic stirrer for 2 hr, centrifuged for 30 min to remove
the solids, and subsequently put under a fume hood to allow the toluene to naturally evaporate.
After the toluene evaporation was complete, n-heptane was added to the toluene- and solids-free
71 |
Virginia Tech | bitumen at a heptane/bitumen ratio of 40:1 by volume. The n-heptane and bitumen mixture were
stirred by means of a magnetic stirrer for 2 hr, during which time maltene dissolved into n-heptane
while asphaltene precipitated out. The mixture was left overnight for the asphaltene to settle at the
bottom. The supernatant n-heptane was carefully removed by means of a pipette and placed under
a fume hood to let the n-heptane naturally evaporate, leaving maltene behind. The amount of the
evaporated n-heptane was small.
The asphaltene precipitate was repeatedly washed for 17 times with an excess amount (~1L)
of n-heptane to remove any maltene that may have co-precipitated. In each washing step, the
mixture was agitated for 2 hr and left to stand overnight to allow the asphaltene to precipitate and
the supernatant was carefully removed. After the last washing step, the supernatant n-heptane
became colorless. The asphaltene precipitate was left under a fume hood to let n-heptane to
evaporate and thereby obtain a sample of purified asphaltene. The amount of the evaporated n-
heptane was small.
3.2.3 Preparation of Bitumen-, Maltene-, and Asphaltene-coated Surfaces
Silicon wafers were first oxidized in an oxidation furnace at 1070 °C for ~40 min using
wet oxidation procedure. The oxidized silicon wafers were then cut into 12x12 mm pieces and
used as substrates for bitumen, asphaltene, and maltene. Before the coating, the silicon substrates
were subjected to ultrasonic vibration in water for ~1 hr and then cleaned by immersing in a
Piranha solution (3:7 by volume of H O /H SO ) at 120 °C for 1 hr, followed by rinsing with an
2 2 2 4
adequate amount of ultrapure water and pure nitrogen (N ) blow-drying. The silicon substrate was
2
placed in a spin-coater (Laurell, WS-400-6NPP), rinsed with toluene, and then spun at 6,000 rpm
for 30 s to dry off the toluene. When the substrate was free of toluene, a small amount of (2 or 3
drops) of a bitumen-in-toluene solution (2.5 mg/mL) was placed on the substrate and let it stand
72 |
Virginia Tech | for 10 s. The substrate was then spun at 3,500 rpm for 20 s before placing another drop of the
bitumen solution and continuing spinning for additional 10 s. The spinning continued for an
additional 1 minute at 6,000 rpm to evaporate the solvent and obtain a smooth and uniform bitumen
coating. The spin-coated bitumen was then placed in a particulate matter-free laminar hood for
~30 min to ensure maximum removal of toluene from the coating. The procedure described above
was also employed for the preparation of asphaltene- and maltene-coated silicon surfaces.
3.2.4 Measurement of Film Thinning Kinetics
Figure 3.1 shows the modified thin film pressure balance (TFPB) apparatus used in the
present work to study the kinetics of film thinning, which is the same as Figure 2.1. The original
TFPB apparatus was designed for the study of foam films (or the water films between two air
bubbles) by Scheludko and Exorowa.39 The equipment was modified to study the kinetics of film
Figure 3.1 Schematics of the modified TFBC apparatus used for the study of wetting
films.
73 |
Virginia Tech | thinning for wetting films.40 As shown in the inset, a flat surface of interest is placed on the top of
a film holder (2.0 mm diameter glass tubing) with its surface facing downward, so that the surface
is in contact with the water in the film holder. In the present work, the flat silicon wafers coated
with bitumen, asphaltene, or maltene were used as samples of interest.
In a given experiment, the silicon wafer-glass tubing assembly was inserted in a glass cell
surrounded by a water jacket, in which water of known temperature was circulated. The film
holder-water jacket assembly was placed on an inverted microscopic stage (Olympus IX51) to
monitor the changes in film thicknesses as a function of time. A mercury arc short lamp (100 W,
Olympus) was used as a light source with a bandpass interference filter (#65-643, Edmund Optics)
to produce a monochromatic green light source (λ=546 nm). In a given experiment, the liquid in
the film holder is slowly withdrawn by turning the knob of the piston. Once the interference
patterns (Newton rings) began to appear in the microscopic field of view, the film was allowed to
thin spontaneously while recording the images by means of a high-speed camera (Fastec Imaging,
HS400115). The interference patterns recorded were used to reconstruct timed thickness profiles
of a wetting film across the entire film holder with a nanometer resolution. The spatial and
temporal film profiles of the wetting film were then used to monitor the changes in film thicknesses
(h) as a function of time and radial distance of the film at any point of the film. The experimental
data were used to obtain information on the film thinning kinetics and calculate the disjoining
pressure () in the thin liquid films (TLFs) according to Eq. [3.1], as derived by Pan. et.al.,27
74
2
R
r
r
r
h
r
1 2
r
r r
1
h 3
r
r 0
r
h
t
d r
d r
[3.1]
where is the air/water interfacial tension, R the bubble radius, r the radial position, h the wetting
film thickness, and µ the fluid viscosity. |
Virginia Tech | air bubble and maltene, bitumen, and asphaltene surfaces in water at 55 °C. The profiles are similar
to those obtained at 80 °C (see Figure 2.3 in Chapter 2). From a thickness of 300 nm, the films
formed on maltene thinned fastest and ruptured at 0.6 s. On asphaltene, the film thinned 20 times
slower than on maltene, with the rupture occurring at 12.05 s. On bitumen, which contains both
maltene and asphaltene, the film thinned slower than on maltene and faster than on asphaltene.
The film thickness (h) vs. time (t) plots in Figure 3.3 presents the kinetics of film thinning at the
center of the wetting film, which increased in the order of asphaltene, bitumen, and maltene. It is
obvious from Figure 3.3 that the film on asphaltene seemed “metastable” for about 3 s before its
rupture, corresponding to the flattening of the film shown in Figure 3.2.
The film profiles presented in Figure 3.2 have been used to obtain kinetic information such
as 𝜕ℎ⁄𝜕𝑡 and 𝜕ℎ⁄𝜕𝑟 (r is the radius position) that can be used to calculate the disjoining pressure
() according Eq. [3.1].27
300 55 0C
Natural pH
) 200
m
n
(
h Asphaltene
100
Bitumen
Maltene
0
0 4 8 12
t (s)
Figure 3.3 Kinetics of film thinning on maltene, bitumen, and asphaltene at 55 °C. It decreases
in the order of maltene, bitumen and asphaltene.
76 |
Virginia Tech | Figure 3.4 shows the results of obtained for maltene, bitumen, and asphaltene at different
time and radial positions. In these calculations, 67.1 mN/m, µ = 0.5x10-3 Pas at 55 °C, and R
= 2 mm were used. As shown, the disjoining pressures were negative in the wetting films formed
on both maltene and bitumen, i.e., < 0, while it is positive, i.e., > 0, on asphaltene. A vertical
pressure balance shows that
77
p p
c u r
[3.2]
where p is the pressure of the liquid in the wetting film and p is the pressure due to curvature
cur
change and surface tension (Laplace pressure). Eq. [3.2] shows that a negative disjoining pressure
helps increase the film thinning process, while a positive disjoining pressure causes retardation.
Thus, the results presented in Figure 3.4 explain why the kinetics of film thinning increased in the
order of asphaltene, bitumen, and maltene. The change in disjoining pressure with film thickness
at the center of the films on maltene, bitumen, and asphaltene were shown in Figure 3.5. As shown,
on maltene and bitumen, the disjoining pressure becomes increasingly negative as the film
thickness decreases, while on asphaltene it becomes increasingly positive. = -450 Pa on maltene
and = -160 Pa on bitumen were obtained just before the wetting films rupture. Therefore, the
fast kinetics observed with maltene can be attributed to the highly negative disjoining pressure.
It has been shown that the hydrophobic force, and hence the magnitude of negative
disjoining pressure, increase with increasing surface hydrophobicity.41 The results obtained in the
present work show, therefore, that maltene is the most hydrophobic, followed by bitumen.
Asphaltene, on the other hand, is hydrophilic as shown by the positive disjoining pressures
measured (see Figures 3.4b and 3.5). Asphaltene being least hydrophobic is understandable
because its functional groups, such as carboxylic, amide, thiol, hydroxyl, and carbonyl, are |
Virginia Tech | hydrophilic. Maltene being the most hydrophobic is also understandable, because it contains
hydrocarbons. The hydrophobicity of bitumen lies in between asphaltene and maltene, suggesting
that asphaltene is exposed to the bitumen/water interface in aqueous.
The results obtained in the present work suggest that the hydrophobicity of maltene is
compromised by the presence of asphaltene on the surface. It is likely that the hydrophilic groups
of asphaltene are exposed to the aqueous phase, the extent of which will determine the
hydrophobicity of bitumen and its floatability. That bitumen is still hydrophobic despite the likely
exposure of asphaltene on the surface suggests that the surface may not be completely covered by
the hydrophilic groups of asphaltene, which is very likely since more than 80% of bitumen is
maltene. Further study is necessary to more fully understand the hydrophobicity of bitumen.
Asphaltene
0
Bitumen
)
a
P
( -200
Maltene
-400 55 0C
Natural pH
0 100 200 300
h (nm)
Figure 3.5 Changes in the disjoining pressure () at the center of the wetting films of water
formed on maltene, bitumen, and asphaltene at 55 °C. Thermodynamically, the film can rupture
when < 0.
79 |
Virginia Tech | 3.3.2 Extended DLVO Theory
It is well known that colloidal forces play an important role in bitumen extraction. The
classical DLVO theory42, 43 recognizes two colloidal (or surface) forces, which include the
electrical double-layer force (F ) and the van der Waals dispersion force (F ). One can readily
e d
convert the forces to disjoining pressures () or vice versa from the Derjaguin approximation,44
80
F
R
2 G ( h ) [3.3]
and the definition of disjoining pressure,
h G h
P T
s
( ) ( / )
, ,
[3.4]
in which F is the surface force, R is the radius of curvature of the macroscopic surface(s) involved,
and G(h) is the free energy of a wetting film which varies with film thickness h or the separation
between two surfaces. According to the DLVO theory, the disjoining pressure of a thin liquid film
(TLF) can be given by,
e
d
[3.5]
where the subscripts e and d represent the electrical double-layer and van der Waals dispersion
forces, respectively.
The DLVO theory has long been used as the governing law in a wide range of industries,
including the bitumen industry. The theory can predict the stability of the thin liquid films (TLFs)
formed between any two surfaces involving macroscopic particles, air bubbles, oils, etc. For
example, the disjoining pressure in the TLF between bitumen and an air bubble affects the bitumen
aeration during the hydrotransport process.
When a particle approaches an air bubble, the TLF (or wetting film) formed in between
them must be thinned quickly and rupture (or break) to form a finite contact angle (). |
Virginia Tech | Thermodynamically, wetting films rupture when > 0, as is well known. The role of colloidal
forces in bubble-particle interactions can be better appreciated using the Frumkin-Derjaguin
isotherm,45
81
c o s 1 ( 1 /
2 3
)
h
0
d h [3.6]
in which
2 3
is the interfacial tension between air bubble and water (or surface tension of water),
and is the disjoining pressure in a wetting film which varies with film thickness h. The integral
of from h = h to represents the Gibbs free energy change associated with bubble-particle
o
interaction (G). Here, the parameter h represents the thickness of the water film in equilibrium
o
with the vapor phase inside an air bubble attached to a hydrophobic surface. The values of h are
o
usually less than 1 nm.
The disjoining pressure in the wetting film due to the van der Waals force is given by,
d 6
A
1 3
h
23
[3.7]
where A is the Hamaker constant, while the disjoining pressure due to the electrostatic double-
132
layer force is given by
e 2 s i n
0
h (
2
h )
(
21 22
c o s e c h ( h ) 2
1 2
c o t h ( h )
[3.8]
in which
0
is the permittivity in vacuum, the dielectric constant of water,
1
and
2
the
double-layer potentials at the solid/water and air/water interfaces, respectively, and is the
reciprocal Debye length. In this report, the subscripts 1, 2, and 3 represent solid, gas, and water
phases, respectively.
Substituting Eqs. [3.7] and [3.8] into the DLVO theory (Eq. [3.5]) and subsequently into
the Frumkin-Derjaguin isotherm (Eq. [3.6]), one obtains the following equation, |
Virginia Tech | 82
c o s
0
1
1
2 3
1
A
2
1 3 2
h 20
0
2
1 2
e
e
x
x
p
p
(
( 2
h
0
h
)
0
) 1
21 22
[3.9]
In wetting films, both of the terms within the bracket are positive values, because A is negative
132
and ψ and ψ are negative under most flotation conditions including bitumen aeration. Under these
1 2
conditions, contact angles cannot be greater than zero and consequently G > 0 for the bitumen-
bubble interaction. The only way to have > 0 and G < 0, would be to create a hydrophobic force
in wetting films, so that one can use the following relation,
c o s 1
1
2 3
-
1
A
2
1 3 2h
20
o
2
1 2
e
e
x
x
p
p
(
(
2
h
0h
)
0
-
) - 1
21 - 22
C
2
e x p (
h
D
o )
[3.10]
in which the third term in the bracket represents the contribution from the hydrophobic force ( )
h
to the total disjoining pressure () in a wetting film.
According to Eq. [3.10], the hydrophobic force (and hence the negative disjoining pressure)
must be large enough to overcome the sum of the repulsive van der Waals and double-layer forces
in order to break the wetting film and form a finite contact angle that is greater than zero. The three
terms within the bracket constitute the extended DLVO theory,46
e
d
h
[3.11]
where represents the disjoining pressure due to the hydrophobic force in a TLF, as proposed
h
earlier by Yoon’s group at Virginia Tech.26, 27
3.3.3 Disjoining Pressure Isotherms
According to the extended DLVO theory (Eq. [3.11]), the disjoining pressure () in a
wetting film has three components, i.e., double-layer ( ), van der Waals dispersion ( ), and
e d
hydrophobic force ( ) components. It will be of interest to construct disjoining pressure
h
isotherms, in which changes in the disjoining pressure components vs. film thickness are plotted. |
Virginia Tech | a) Maltene
In Figure 3.6, the changes in , , and vs. h are plotted for the wetting film of water
e d h
formed on maltene at 55 °C. The triangles represent the experimental data obtained in the present
work, and the solid line represents a fit to the extended DLVO theory (Eq. [3.11]). There are three
dashed lines, representing calculated using Eq. [3.7] with a A value of -1 x 10-20 J, and
d 132 e
calculated using Eq.[3.8] with ψ = - 65 mV for the surface potential of maltene, and ψ = - 26 mV
1 2
for that of air bubble, and κ-1 = 50 nm for the Debye length, and
83
h
2
C
D
e x p
h
D
[3.12]
Figure 3.6 The disjoining pressure isotherm ((h)) obtained at the center of the wetting film
of water formed on maltene at 55 °C. The triangles represent the experimental data presented
in Figure 3.5. The solid line represents a fit to the extended DLVO theory, which includes
contributions from: van der Waals force ( ) with a Hamaker constant of A = -1 x 10-20 J;
d 132
electrostatic force ( ) with surface potential of ψ = - 65 mV for maltene, surface potential
e 1
of ψ = - 26 mV for air bubble, and Debye length of κ-1 = 50 nm; and hydrophobic force ( )
2 h
with fitting parameters of C = -0.43 mN/m and D = 50 nm. |
Virginia Tech | where C (= -0.4 mN/m) and D (= 50 nm), known as decay lengths, are the best fit parameters. The
figure legend shows all other parameters used to construct the disjoining pressure isotherms.
The vs. h isotherms show that far exceeds in magnitude the sum of and , so that
h e d
is net negative at all separations studied. The decay length of 50 nm is very large as compared
to the hydrophobic forces measured between hydrophobic surfaces, partly because air bubbles in
water are the most hydrophobic entity known to date.47, 48 Air bubbles in water have an interfacial
tension of 72 mN/m, while hydrocarbon oils have interfacial tensions of 50 mN/m. Interfacial
tensions are excellent measures of hydrophobicity.
b) Asphaltene
Figure 3.7 shows the disjoining pressure isotherms obtained at the center of the wetting
film of water formed on asphaltene at 55 °C. As shown, > 0 indicating that asphaltene is
hydrophilic. As suggested previously,2 the sharp increase in may represent the repulsive steric
disjoining pressure ( ). Thus, the extended DLVO theory may be written as follows,
s
84
d
e
s
[3.13]
where
s
k
s
T
3
(
2 L / h ) 4 / 9 ( h / 2 L ) 3 / 4
[3.14]
In Eq. [3.14], s represents the mean distance between grafted points of a polymer, and L is the
length of the polymer tail. Eq. [3.14] was derived by de Gennes for the interaction between two |
Virginia Tech | Figure 3.7 The disjoining pressure isotherm ((h)) obtained at the center of the wetting film of
water formed on asphaltene at 55 °C. The triangles represent the experimental data presented in
Figure 3.5. The solid line represents a fit to the extended DLVO theory, which includes
contributions from: van der Waals force ( ) with a Hamaker constant of A = -1 x 10-20 J;
d 132
electrostatic force ( ) with surface potential of ψ = - 68 mV for asphaltene, surface potential
e 1
of ψ = - 26 mV for air bubble, and Debye length of κ-1 = 65 nm; and repulsive steric force ( )
2 s
between asphaltene polymers with a brush length of L = 63 nm and a grafting distance of s = 23
nm.
surfaces grafted by “brush-like” polymers.49 The best-fit parameters used to fit the experimental
data are presented in the figure legend.
c) Bitumen
Figure 3.8 shows the disjoining pressure isotherms for the TLF of water formed on bitumen
at 55 °C. The extended DLVO theory that was used to represent the system included four different
components:
[3.15]
d e s h
85 |
Virginia Tech | Figure 3.8 The disjoining pressure isotherm ((h)) obtained at the center of the wetting film of
water formed on bitumen at 55 °C. The triangles represent the experimental data. The solid line
represents a fit to the extended DLVO theory, which includes contributions from: van der Waals
force ( ) with a Hamaker constant of A = -1 x 10-20 J; electrostatic force ( ) with surface
d 132 e
potential of ψ = - 57 mV for bitumen, surface potential of ψ = - 30 mV for air bubble, and Debye
1 2
length of κ-1 = 45 nm; repulsive steric force ) between bitumen polymers with a brush length
s
of L = 32 nm and a grafting distance of s = 26 nm, and hydrophobic force ( ) with fitting
h
parameters of C = -0.2 mN/m and D = 54 nm.
where the subscripts d, e, s, and h represent the dispersion, double-layer, steric, and hydrophobic
disjoining pressures, respectively. The model parameters that were used to fit the experimental
data are given as figure legends.
As shown in Chapter 2, disjoining pressure measurements were also conducted with
bitumen substrates at other three different temperatures, i.e., 22, 35 and 80 °C, and their disjoining
pressure isotherms were constructed using the extended DLVO theories (Eq. [3.15]) with the
86 |
Virginia Tech | Table 3.1 Parameters used to fit the disjoining pressure of the wetting film of water
formed on bitumen at different temperatures
T A ψ ψ κ-1 s L C D
132 1 2
( °C) ( J ) ( mV ) ( mV ) ( nm ) ( nm ) ( nm ) ( mN/m ) ( nm )
22 -1 x 10-20 -50 -24 80 30 68.3 0 0
35 -1 x 10-20 -55 -20 33 26 28.3 -0.12 47
55 -1 x 10-20 -57 -30 45 26 32 -0.2 54
80 -1 x 10-20 -110 -45 50 26 32 -1.32 55
fitting parameters shown in Table 3.1. The disjoining pressure component due to hydrophobic
force ( ) have been obtained and plotted together in Figure 3.9. As shown, becomes
h h
increasingly negative (or strong) and longer-ranged at higher temperatures, which is consistent
with the flotation data reported in the literature.1 Further, this finding is in agreement with the
contact angle data obtained on the thick films of bitumen immersed in water (Chapter 2, Figures
2.9 and 2.10).
That hydrophobic forces measured in the wetting films of bitumen becomes stronger with
increasing temperature, as shown in Figure 3.9, is contrary to what has been reported previously
by Yoon’s group at Virginia Tech.50, 51 The group conducted a series of AFM force measurements
with silica and gold surfaces hydrophobized by different surfactant coatings. The results obtained
with the gold and silica surfaces hydrophobized with n-hexadenane (C-16) thiol and
octadecylchlorosilane (OTS), respectively, showed that hydrophobic forces increase with
decreasing temperature, which is contrary to what has been observed in the present work with
bitumen and presented in Figure 3.9.
87 |
Virginia Tech | A rigorous thermodynamic analysis of the AFM force data obtained previously by the
Yoon group showed that the excess entropy (Sf) in the thin liquid films (TLFs) confined between
hydrophobic surfaces decreases with decreasing film thickness, which has been attributed to the
formation of H-bonded structures in the TLFs. The free energy of the water molecules in the TLFs
confined between hydrophobic surfaces are higher than those in thick films, because much of the
water molecules in the vicinity of hydrophobic surfaces cannot form H-bonds with the confining
surfaces. The excess free energy is expended to form H-bonded structures, known as low density
liquid (LDL).
It has been found also that the entropy decrease (Sf< 0) becomes more significant at lower
temperatures, which indicates that water molecules can more readily form structures at lower
88
-
-
-
1
1
5
0
5
0
0
0
0
0
0
0
0
5 5 o C
1
8
0
0
0
o C
2 0 0 3 0 0
)
a
P
(
h
3 5 o C
h ( n m )
N a tu a l p H
B itu m e n
Figure 3.9 Hydrophobic disjoining pressure isotherms ( ) on bitumen at different
h
temperatures in water of a natual pH of about 5.7. |
Virginia Tech | temperatures, which is consistent with our daily experiences that water forms H-bonded structures
at lower temperatures (i.e., ice).
It is well known that water molecules form low density liquid (LDL) in supercooled
water.52 More recent spectroscopic studies conducted using x-ray absorption spectroscopy (XAS)
and x-ray Raman scattering (XRS) at Stanford University showed that LDL is also formed in bulk
water at ambient temperatures.53, 54 By monitoring the OH-vibrations of water molecules using the
Raman and FTIR experiments, the Mallanace group at the University of Messina, Italy, showed
that the population of LDL species at ambient temperatures are much higher than anticipated.55, 56
According to Stillinger, water molecules behave similarly under supercooled conditions and in
hydrophobic interactions,52 suggesting that LDL can be more readily formed in the TLFs confined
between hydrophobic surfaces at ambient temperature. In supercooled water, water molecules are
prevented from forming ice by mechanical agitation or other external forces. Under this condition,
they can form LDL to minimize free energy. In confined spaces, water cannot interact with the
hydrophobic surfaces. One way to expand the excess free energy is to form LDL. Thus, the
hydrophobic forces operating in colloid chemistry and mineral flotation originate from the
structural changes of water in TLFs relative to the water structure in the bulk.
As has already been noted, the hydrophobic forces measured with gold and silica surfaces
coated with C-16 thiol and OTS-coated surfaces decreased with increasing temperature, which is
contrary to what has been observed with bitumen-coated surface in the present work. The AFM
force measurements were carried out in narrow temperature ranges, i.e., 10 to 40 °C, so that the
chemistry of the hydrophobic surfaces would not change significantly with temperature. The only
change that can occur over the narrow temperature range would be the water structure, or, rather,
the tendency for low-density-liquid (LDL) to form in confined spaces.50, 51 With bitumen, the
89 |
Virginia Tech | structure and properties of the substrate, i.e., bitumen, can change significantly over the larger
temperature range investigated in the present work, i.e., 22 to 80 °C. After all, the bonding between
hydrocarbons constituting bitumen was in liquid form; therefore, its structure can be more readily
changed than the structure of a solid, such as gold or silica. It has been shown that the surface
tension of bitumen decreases significantly with increasing temperature,57 which is a manifestation
of changing intermolecular bond energy. Surface tensions of solids do not change significantly
over the narrow range of temperatures employed in the present work. In general, the lower the
surface energy (or surface tension), the higher the hydrophobicity. Therefore, the increase in
bitumen contact angle with temperature observed in the present work may be attributed to the
decrease in surface free energy of bitumen, which corroborate well with the surface tension data
noted above. Yoon’s group showed also that the higher the contact angle, the stronger and longer-
ranged the hydrophobic force.41 Thus, there are two opposing factors affecting the hydrophobic
forces in a bitumen flotation system. One is the contact angle, and the other is water structure. It
appears that the former plays a more significant role than the latter.
Figure 3.10 compares the disjoining pressure components due to the steric forces present
in the TLFs formed on bitumen at different temperatures. As shown, is maximum at 55 °C,
s
followed by those obtained at 35 °C and 22 °C. The data seem to suggest that asphaltene polymers
are more readily exposed at higher temperatures.
In Table 3.1, the Hamaker constant (A ) for the asymmetric interaction between air bubble
132
1 and bitumen 2 in the medium of water 3 is predicted using the geometric mean combining rule
A A A [3.16]
132 131 232
where A and A are the Hamaker constants for symmetric interactions. From the values of A
131 232 131
= 3.7 × 10-20 J and A = 2.8 × 10-21 J reported in the literature,8, 58 one can obtain the value of A
232 132
90 |
Virginia Tech | = -1.0 × 10-20 J for the bitumen and bubble interaction, which is the same as used by Englert et
al.22 The -potentials of the bitumen/water interface given in the table were obtained from the data
reported by Dai et al.59 and Liu et al.60 The bitumen zeta potential used at 80 °C is more negative
than those at lower temperatures, which is in agreement with Dai’s findings59 and other materials
such as quartz,61 sodium kaolinite,61, 62 and others.63 Zeta potentials of air bubble in water were
close to the value reported by McShea et al.64 The rest of the parameters in the table were best-fit
parameters obtained on the basis of the experimental disjoining pressure isotherms. Better fit could
have been obtained if the zeta potentials of bitumen and air bubbles were directly measured on the
bitumen samples used in the present work at the relevant temperatures of interest. The zeta
91
3
2
1
0
0
0
0
0
0
0
0
3 5 o
5
C
5 o
5
C
0 1 0 0 1 5 0
)
a
P
(
s
h ( n m )
N a t
2
u
2
a l p
o C
H
B itu m e n
Figure 3.10 Steric disjoining pressure isotherms ( ) on bitumen at different
s
temperatures in water of a natual pH of about 5.7. |
Virginia Tech | potentials can be readily measured by electrokinetic methods, and the structural information on
polymers can be measured using the dynamic light scattering, viscometric methods, and the small-
angle neutron scattering (SANS) techniques.
3.3.4 Disjoining Pressure in Liberation
Unlike the oil sands in Utah, the sand grains in Canadian oil sands are thought to be
surrounded by a thin film of water, known as connate water, as shown in Figure 3.11a. The first
step toward extracting bitumen is to create a positive disjoining pressure in the innate water, i.e.,
> 0. According to the extended DLVO theory discussed above, there are two ways to create a
positive disjoining pressure. One is to increase the steric repulsion ( ), and the other is to increase
s
the double-layer repulsion ( ). Both of these parameters are optimized by controlling
e
temperature, pH, and electrolyte concentrations. At an elevated temperature, asphaltene can be
more readily exposed to the surface of bitumen due to decreased viscosity. As shown in Table 2.1,
the viscosity of Athabasca bitumen decreases by nearly three orders of magnitude as temperature
increases from 20 to 75 °C. The presence of asphaltene polymers creates steric repulsion ( ) and
s
Figure 3.11 a) For liberation, it is necessary to create a positive disjoining pressure in the TLFs
of water between bitumen and sand grains, i.e., > 0; for air bubble-bitumen attachment, it is
necessary to create negative disjoining pressures in the TLFs, i.e., < 0.
92 |
Virginia Tech | hence a positive disjoining pressure. Further, an increase in pH should ionize the polar groups of
the asphaltene polymers and, thereby, increase the negative surface charge and increase the
repulsive electrostatic disjoining pressure ( ). In principle, any electrolyte in process water
e
should be detrimental to bitumen liberation as it can cause the surface charge to decrease (e.g., by
Ca2+ ion adsorption) and the electrical double-layer to be compressed. However, the double-layer
compression becomes discernable only at very high electrolyte concentrations.
Figure 3.11b shows that a bubble deforms when it encounters a bitumen surface in a
hydrotransport pipeline. The deformation creates an excess pressure (p) in the film, causing the
film to thin. At this point in the film thinning process, the excess pressure is solely dependent upon
the radius R of the bubble and surface tension, i.e., (p = 2/R). As the film thinning continues
cur
and reaches a thickness range of 250 to 300 nm, however, disjoining pressure () comes into
play. The film thins further if < 0. If > 0, however, film thinning stops when becomes equal
to the curvature pressure (p ), because p becomes zero according to Eq. [3.2]. The film is stable
cur
at this equilibrium thickness (h ); therefore, the contact angle () becomes zero, and no flotation
e
will occur. As shown in Eq. [3.10], > 0 only when < 0 by virtue of the hydrophobic force
present in the wetting film. Since the disjoining pressure in the film must be high to liberate
bitumen, the hydrophobic force should be strong and long-ranged enough to create a negative
disjoining pressure, which is a necessary condition for aeration. One way to increase the
hydrophobic force is to increase the temperature for bitumen aeration. In mineral flotation, a
collector is added to render the surface hydrophobic. In bitumen aeration, temperature is raised to
increase contact angle and hence hydrophobic force as has been discussed in foregoing sections.
93 |
Virginia Tech | 3.4 Summary
In the present work, the bitumen was separated into maltene and asphaltene in order to
study the role of these two components in the interaction between bitumen and air bubble. The
kinetics of thinning of the wetting films formed on bitumen, asphaltene, and maltene were
monitored using the TFPB technique. The results showed that the kinetics decreased in the order
of maltene, bitumen and asphaltene. The disjoining pressure in the wetting film was determined
based upon the temporal and spatial profiles of the wetting films obtained using the modified TFPB
method. It was found that < 0 on maltene and bitumen, while > 0 on asphaltene, indicating
that asphaltene may serve as a kinetic barrier for bitumen aeration. Finally, the disjoining pressure
data obtained in the present work were analyzed in view of the extended DLVO theories.
Comparison of the disjoining pressure isotherms for maltene, bitumen, and asphaltene showed that
the electrostatic and steric forces are responsible for the difficulties that may be encountered in
bitumen-bubble interactions, while the hydrophobic force serves as the driving force for the fruitful
interactions.
References
1. Masliyah, J.H., Z. Xu, and J.A. Czarnecki, Handbook on Theory and Practice of Bitumen
Recovery from Athabasca Oil Sands, Kingsley Publishing Services, 2011.
2. Yoon, R.H., D. Guzonas, and B.S. Aksoy, Role of Surface Forces in Tar Sand
Processing, in Proceeding of the 1st UBC-McGill Bi-Annual International Symposium
Vancouver, BC, Canada, 1995.
94 |
Virginia Tech | Chapter 4
The Effect of Solution Chemistry on the Stability of Wetting
Films on Bitumen
Abstract
The solution chemistry plays an important role in bitumen aeration. In this study, the effects
of solution chemistry on the thinning of wetting films between bitumen and air bubble were
studied. It was found that the thinning kinetics of wetting films decreased with increasing
immersion time. The addition of electrolyte KCl compressed the double layer and increased the
thinning kinetics. The presence of calcium and magnesium ions did not show significant effects
on the thinning kinetics when 1 mM KCl was present. In addition, the effect of approach speed of
air bubble to bitumen surface was investigated. It was found that dimples were formed at high
approaching speeds. There existed a critical film thickness, above which the wetting film thinned
faster at higher approaching speeds and below which the film thinned faster at lower approaching
speeds.
4.1 Introduction
Properties of the thin liquid films (TLFs) formed between particles, bubbles, and drops
control their interactions with each other. In Athabasca bitumen flotation in Canada, air bubbles
collide with bitumen droplets and create wetting films between them. If the TLFs are unstable,
bitumen droplets coalesce, attach to air bubbles, and float. In contrast, no flotation is possible if
102 |
Virginia Tech | the TLFs are stable. Thus, the stability of wetting films is of critical importance in bitumen flotation,
and is strongly dependent on the chemistry of the flotation system, which includes factors such as
temperature, pH, electrolyte concentration, the presence of fines, etc.
The effect of processing temperature on bitumen extraction has been studied extensively.1-
7 It has been reported that bitumen recovery does not significantly change in the range of 50 to
80 °C, but sharply decreases with decreasing temperatures below 35 °C (see Chapter 2).
The solution pH is another critical operating parameter, which controls the surface charges
of the particles in bitumen slurry. As early as 1920s, in the Clark’s Hot Water Extraction Process,
chemicals such as NaOH and Na CO were added to the oil sands slurry to increase the solution
2 3
pH to achieve satisfactory bitumen recovery.8 It was found that there was an optimal concentration
of reagent to obtain maximum bitumen recovery.9 Sanford and Seyer10 found that the role of NaOH
addition was to derive natural surfactants by ionizing organic acids in the bitumen. However, a
subsequent study indicated that only a small fraction of NaOH was needed to produce sufficient
natural surfactants needed for bitumen flotation, and that the bulk of added NaOH reacted with
polyvalent metal carbonates, clays and sulfates contained in the oil sands.11 One study also
produced a report that increasing slurry pH favors bitumen detachment (or liberation) from sand
grains.4 Specifically, the zeta potential of the bitumen surface in an aqueous solution becomes
more negative with increasing pH.4, 12, 13 After the pH had reached 8, the zeta potential leveled off.
Moreover, the zeta potential of an asphaltene surface becomes increasingly negative as the pH is
increased.14-17 Force measurements conducted by AFM showed that the repulsive force between
bitumen-silica, asphaltene-silica, bitumen-bitumen and asphaltene-asphaltene in an aqueous
solution increased with increasing pH.14, 18-21
103 |
Virginia Tech | Divalent cations such as Ca2+ and Mg2+ ions are always present in a bitumen extraction
system. Calcium ions can be introduced into a bitumen extraction system through gypsum, which
is added as a process aid to form consolidated tailings in the recycling of process water from
tailings.22 The presence of Ca2+ ions could decrease the absolute values of the negative zeta
potentials of bitumen and clay surfaces due to their adsorption on these surfaces.13, 18, 23 A sizable
number of studies also confirmed that the presence of calcium and magnesium ions has a
detrimental effect on bitumen recovery.11, 18, 24, 25 However, Kasongo et al.23 found that calcium
alone appears to have a marginal effect on bitumen recovery. When both calcium and
montmorillonite were present, however, bitumen recovery decreased sharply. In support of
Kasongo’s findings, Gu et al.26 reported that calcium and fine solids had a synergic effect on the
induction time of air-bubble attachment. The researchers proposed that when calcium ions are
combined with fine solids, the adsorbed positive calcium ions on fines would interact with negative
carboxylic groups on bitumen surfaces, acting as a ‘‘binder’’ between bitumen and fine solids. A
coating of fine solids on a bitumen surface changes its wettability from hydrophobic to more
hydrophilic in nature, thereby reducing bitumen-bitumen coalescence and air bubble-bitumen
attachment.
Although earlier studies have advanced our knowledge of bitumen extraction from oil
sands, we still do not fully understand the stability of the wetting films formed between bitumen
and air bubbles. The present study was designed to fill that knowledge gap by investigating the
various factors that impact the thinning of the wetting films of water formed on bitumen.
Specifically, a bubble-against-plate (BAP) apparatus was used to measure the kinetics of the
thinning of wetting films of water formed between bitumen and bubble under different conditions.
Optical interference patters (Newton rings) were recorded using a high-speed camera and then
104 |
Virginia Tech | analyzed offline to calculate the film thickness, which changed with time at different radial
positions of the film. The results will be useful in elucidating the mechanisms involved in bitumen
aeration.
4.2 Experimental
4.2.1 Materials
A sample of bitumen (99 wt%) was provided by Suncor Energy, Inc. Reagent grade toluene
(>99.5% purity) was purchased from Spectrum Chemical Mfg. Corp. Anhydrous CaCl (99%, Alfa
2
Aesar) and MgCl (99%, Alfa Aesar), and KCl (99%, Alfa Aesar) were used to study the effect of
2
electrolyte on the film thinning. Diluted hydrochloric acid and NaOH were used to adjust solution
pH. Millipore ultrapure water with a resistivity of 18.2 MΩ·cm was obtained using a Direct Q3
water purification system. Silicon wafers (100 crystal planes, University Wafer) were used as the
substrates for coating bitumen. Sulfuric acid (H SO , 98% purity, VMR international) and
2 4
hydrogen peroxide (H O , 29.0–32.0% purity, Alfa Aesar) were used to clean the silicon substrates.
2 2
4.2.2 Preparation of Bitumen-coated Surfaces
Silicon wafers were first oxidized in an oxidation furnace at 1070 °C for ~40 min using
wet oxidation procedure. The oxidized silicon wafers were then cut into 12x12 mm pieces and
used as substrates for bitumen, asphaltene, and maltene. Before the coating, the silicon substrates
were put in an ultrasonic bath of water for ~1 hr and then cleaned by immersing in a Piranha
solution (3:7 by volume of H O /H SO ) at 120 °C for 1 hr, followed by rinsing with an adequate
2 2 2 4
amount of ultrapure water and pure nitrogen (N ) blow-drying. The silicon substrate was placed in
2
a spin-coater (Laurell, WS-400-6NPP), rinsed with toluene, and then spun at 6,000 rpm for 30 s to
dry off the toluene. When the substrate was free of toluene, a small amount of (2 or 3 drops) of a
bitumen-in-toluene solution (2.5 mg/mL) was placed on the substrate and let it stand for 10 s. The
105 |
Virginia Tech | substrate was then spun at 3,500 rpm for 20 s before placing another drop of the bitumen solution
and continuing spinning for additional 10 s. The spinning continued for an additional 1 minute at
6,000 rpm to evaporate the solvent and obtain a smooth and uniform bitumen coating. The spin-
coated bitumen was then placed in a particulate matter-free laminar hood for ~30 min to ensure
maximum removal of toluene from the coating.
4.2.3 Measurement of Film Thinning Kinetics
In the TFPB technique described in previous chapters, a small volume of liquid is used to
measure the thickness of wetting films. Also, the experimental procedure is complex and delicate.
The objective of the present work is to study the effect of solution chemistry on the stability of
thin liquid films, for which it would be desirable to use larger volumes of liquids. Figure 4.1 shows
the bubble against plate (BAP) apparatus designed and constructed in the present work for this
purpose. In a given experiment, a solution of known ionic composition is placed in a rectangular
Figure 4.1 Schematic representation of the bubble-against-plate (BAP) apparatus used to
monitor the film thinning kinetics of wetting films.
106 |
Virginia Tech | Plexiglas cell with dimensions of 38x38x14 mm. The bottom plate of the cell is thick enough to
fixate a short glass tubing (~1.8 mm OD) at the center. Once a bubble of 3 mm diameter is formed
on the tip of the glass tubing by means of an air-tight syringe, a flat silicon plate (9x9x0.5 mm)
coated with bitumen is brought close to the bubble surface within a distance of 15 µm. The bubble
is then moved upward by means of the piezoelectric crystal at an approach velocity of 0.2 µm/sec.
As the film thickness is reduced, optical interference patters (Newton rings) begin to appear. The
Newton rings are recorded as a function of time by means of a high-speed camera using the same
optical system employed for the TFPB system. The recorded Newton rings are analyzed off-line
to determine the changes in film thickness (h) as a function of time (t) and radial distance (r). In
the present work, all experiments have been carried out at room temperature, i.e., 22 °C.
4.3 Results and Discussion
4.3.1 Effect of Immersion Time
Figure 4.2 shows the results of the film thickness measurements conducted using the
bubble against plate (BAP) apparatus shown in Figure 4.1. The measurements were conducted
after immersing bitumen-coated silicon plates for different periods of time, i.e., 5, 15, 30, and 60
min. As shown, the kinetics of film thinning decreased with increasing immersion time, indicating
that the bitumen surface becomes less hydrophobic at longer immersion times. The results show
also that the equilibrium film thickness (h ) increased with increasing immersion time; h increased
e e
from 118 nm to 156 nm when the immersion time was increased from 5 min to 1 h. Note, however,
that neither the thinning kinetics nor the equilibrium thickness increased further after 30 min of
immersion time, indicating that 30 min is sufficient for the bitumen film to reach an equilibrium
with water at room temperature. This finding is in agreement with the work conducted by Liu, et
al.19
107 |
Virginia Tech | 300
5 min
15 min
30 min
1 h
)
m
n 200 Increasing time
(
h
100
0 10 20 30 40 50
t (s)
Figure 4.2 Thinning kinetics of the wetting films formed on bitumen at different
immersion times in water at pH 8.4 to 7.5.
The time-dependent behavior of bitumen may be attributed to the slow kinetics of the polar
groups of the asphaltene and resin, e.g., carboxyl, amide, thiol, and hydroxyl groups,27 migrating
through the polymeric matrix at room temperature. When a bitumen surface is exposed to air,
molecules in bitumen lie collapsed on the surface.12 Upon exposure to water, the molecules swell,
as indicated by the increase in h , and the polar groups migrate toward the bitumen/water interface,
e
as evidenced by the slow film thinning kinetics. The slow kinetics of the ‘swelling’ and the
migration of polar species may be due to the high viscosity of bitumen at room temperature. At
higher temperatures, the kinetics of migration are faster, and the equilibrium thickness can be
reached faster. The results presented in Figure 4.2 are consistent with the general observation that
the surface chemistry of bitumen in contact with water is time-dependent.18-20
108 |
Virginia Tech | 4.3.2 Effect of Electrolyte KCl
Figure 4.3 shows the effect of KCl on the thinning kinetics of the wetting films formed on
bitumen. The experiments were conducted at a natural pH of about 5.7 and room temperature (~22
°C). In the absence of KCl, the wetting film thinned slowly and reached a thickness of 153 nm in
50 s. The film did not rupture. In the presence of 1 mM KCl, the film thinned faster and ruptured
within 9 s at a critical film thickness (h ) of 34 nm.
cr
As shown in the foregoing section, the bitumen surface is negatively charged in water due
to the exposure of the negatively-charged polar groups on the surface, with its zeta potential being
around -60 mV.13 The air bubble surface is also negatively charged with different values of
negative zeta potential reported in the literature.28-31 Therefore there exists a repulsive electrostatic
force between bitumen and air bubble, creating a positive disjoining pressure and thus causing the
thinning rate to decrease and stop after reaching the equilibrium thickness.
109
3
2
1
0
0
0
0
0
0
0
0
1 m M
1
K
0
C l
2 0 3 0 4 0 5 0
)
m
n
(
h
ru p tu re
0 m M
t
K
( s
C
)
l
Figure 4.3 Effect of KCl on the thinning kinetics of the wetting films formed on
bitumen in aqueous solution of pH 8.4 to 7.5 and at 22 °C. |
Virginia Tech | In the presence of KCl, the electrical double-layers at the bitumen/water and air/water
interfaces are compressed, causing the positive disjoining pressure to decrease. There are two
consequences of the double-layer compression: one is to increase the kinetics of film thinning, and
the other is to decrease the film thickness, both of which are clearly manifested in Figure 4.3. As
the film thickness becomes thinner, the film is subjected to a stronger hydrophobic force, which is
the major destabilizing force for wetting films, and therefore ruptures.
4.3.3 Effect of Solution pH
Figure 4.4 shows the effect of solution pH on the kinetics of film thinning in 1 mM KCl
solution. As shown, the kinetics of film thinning decreased with increasing pH. The pH
adjustments were made by adding aliquots of HCl and NaOH solutions. At pH < 5, the film
ruptured within less than 3 s, while at pH ~8, the film was stable for as long as 9 s before it finally
ruptured. These values are close to what has been reported earlier.12 In general, the film rupture or
induction time for bitumen flotation is orders of magnitude longer than observed in mineral
110
3
2
1
0
0
0
0
0
0
0
0 3 6
3
5
8
.1.3.4
-4-7 .8.5
9
)
m
n
(
h
In c r e a s in g p
t
H
( s )
p H
Figure 4.4 Effect of solution pH on the thinning kinetics of the wetting films formed
on bitumen in a 1 mM KCl solution. The short arrows indicate the critical rupture
thicknesses (h ) of the wetting films.
cr |
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