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64thAppitaAnnualConference&Exhibition
Incorporatingthe2010PanPacificConference
Melbourne,Australia
18‐21April2010
__________________
CORRECTION
Manuscripttitled“Effectofpitchpreparationonitscolloidalpropertiesanddeposition
behaviour”
byKStack,RLee,RRao,DRichardson,GGarnierandTLewis
publishedonPage273oftheproceedingswasinadvertentlyincludedinplaceofthecorrect
manuscript
“Measurementofpitchdepositionbyimpingingjetmicroscopy:Effectofdivalentsalts”
by
RolandLee,KarenStack,TrevorLewis,GilGarnier,DesRichardsonandTheoVandeVan
Peer reviewed
Appita 2010
Measurement of pitch deposition by
impinging jet microscopy: Effect of
divalent salts.
ROLAND LEE1, KAREN STACK1, TREVOR LEWIS1, GIL
GARNIER2, DES RICHARDSON3, THEO VAN DE VAN4
1 University of Tasmania,
2 Monash University,
3 Norske-Skog, Boyer, Tas Australia
4McGill University,Montreal, Canada.
Abstract
Papermakers for many years have had to contend
with the wood resins that are released during pulping
and papermaking. As paper mills reduce water
consumption by further process loop closure, there is
potential for the problems to increase. Accumulated
organic and inorganic material can lead to deposits
on machinery, poor process control, loss in
efficiency, and lower product quality. There are a
number of factors, which have an impact on the
deposition tendencies of colloidal particles as a result
of different inorganic materials or salts. Along with
the valency of the salt, the concentration of salt, the
properties of the colloidal particles, and the chemical
nature of the surface are important. Variations in
colloidal stability and deposition are observed even
for salts of the same valency.
With the use of impinging jet microscopy (IJM) the
deposition of pitch on to hydrophobic and
hydrophilic model surfaces was measured and the
effect on the rate of deposition was quantified with
changing cation. On both model surfaces the pitch
deposition was seen to be slightly faster when
calcium at 800 mg/L was present than magnesium at
the same concentration. This concentration is around
twice the critical coagulation concentration. The
concentration of calcium in process water in some
paper mills can be 200 – 300 mg/L. With further
process loop closure this could rise to levels being
investigated in this study. The concentration of
magnesium would normally be much lower than this
unless magnesium based alkalis were to be used.
The rate of deposition onto the model hydrophobic
surface was far greater (up to a 2.5 times) than on the
hydrophilic surface for both salts. Contact angle
measurements inferred that in the air-surface
environment the hydrophobicity of the surface
doesn’t affect its affinity for neat pitch suggesting
that the pitch may be reforming on the surface. IJM
results show variation in the pitch shape on the model
surfaces. It is possible that molecular reorganization
of the components occurs at the surface and water
interfaces. On the hydrophilic surface the pitch
particle size for both salts is about 0.33-0.35 µm
while for the hydrophobic surface the particle size is
about 5 times more for the calcium salt than the
magnesium salt. Film thinning or spreading of the
pitch particles was observed to occur on the
hydrophobic surface with calcium and to a lesser
extent with magnesium salt. With time film thinning
will affect the chemistry expressed by the surface and
change the interaction of other process components in
the paper making process.
Introduction
Significant volumes of water are used in the pulp and
paper industry in the pulping of wood chips and
formation of the paper. World’s best practice for
water consumption in manufacture of mechanical
paper grades is around 14m3 tonne-1 of product (1).
The most water efficient of Norske Skog’s mills
operates at around 9 m3 tonne-1, but climatic
conditions can be such that even further reductions
would be welcome. Further closure of the process
water loop however would lead to an increase in the
amount of organic and inorganic material recycled in
the process. The accumulated organic and inorganic
material can lead to deposits on machinery, poor
process control, loss in efficiency, and lower product
quality.
The organic material includes compounds like resin
acids, fatty acids, and fatty acid esters, known
collectively as “wood resins” or “pitch” These
constitute a very complex mixture, making the
prediction of how they react with other system
components difficult (2-9). Inorganic ions such as
calcium or aluminium react with the soluble resin
acids to form sticky deposits of metal soaps that are
not seen on the addition of magnesium and sodium
(6, 7, 9-14).
Previous work has focused on investigation of
changes to the concentrations of wood resins in the
sample, along with changes in the salt concentrations
and salt type (9, 15-17). Recent work using a
photometric dispersion analyzer (PDA) has shown
variation in the rate of aggregation of the wood resins
is dependent on the salt present, for example growth
of pitch particles is faster in the presence of
magnesium salts compared to calcium salts (18).
For particle-surface interactions there are a number of
key factors involved in deposition including
hydrodynamic conditions in the region of the surface,
and particle transport, adsorption and adhesion.
Direct quantitative measurement of the deposition
rate is made possible with the use of the impinging
jet microscopy (IJM) and video imaging. IJM gives
quantitative information of particles absorbed onto
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Appita 2010
the surface with well-defined hydrodynamic and
physical-chemical properties (19).
Figure 1 shows the schematic view of the impinging
jet consisting of two plates (collector plate and
bottom confiner plate) where r is the radius of the jet
h is the distance between the plates and particle
counts were collected at the Stagnation point. Within
the impinging jet the flow of particles forms a
Newtonian flow (20-23). This enters through a
circular hole impinging onto the collector plate. The
rate of deposition and flow distribution is defined by
the dimensionless ratio of h/r. The experimental rate
of deposition (j) is defined as the number of particles
on the surface (ns) with time (t):
j = ns /t
Figure 1: Schematic view of a radial impinging jet.
A solid substrate when placed in contact with solvent
rich environment will be covered in a thin film of
fluid. The technique allows the observation of only
those particles that are absorbed to the surface.
The technique has been used to probe numerous
deposition problems including the competitive
deposition of PEI coated particles and absorption of
PEI to a surface (22). The kinetics of adhesion for
phosphatidylcholine liposomes to quartz surfaces
(23) and the deposition of particles onto cellulose
films (24) have also been studied using impinging jet.
The nature of both the surface and chemical
composition of the material interacting with the
surface are important. Qin et al showed that when a
droplet of water was introduced to a surface coated
with pitch components the pitch contact angle and
wettablity is dependant on its chemical composition
and the surface (15-17).
In this paper the direct measurement of the deposition
of colloidal pitch particles on different substrates has
been conducted with the use of impinging jet
microscopy. This enables the effect of different salts
on the pitch “stickiness” and the effect of the surface
physiochemical characteristics on the rate of
deposition to be studied.
Methods
Pitch Preparation
A thermomechanical pulp (TMP) made from Pinus
Radiata was collected from the primary refiners at
Norske Skog, Boyer, Tasmania. The pulp was air-
dried and soxhlet extracted for 8 hours with hexane.
Aqueous wood resin dispersions of 100 mg/L
concentration were prepared by the addition of
dissolved extracted wood resin in acetone (99.5%
purity, Sigma-Aldrich) to a 1 mM KNO3 solution in
distilled water with a pH of 5.5. Dialysis of the
dispersion was performed using cellulose membrane
tubing with a molecular mass cut off of 12,000 amu
(Sigma-Aldrich D9402-100FT), to remove acetone.
The wash water used was 1 mM KNO3 pH adjusted
to 5.5. This was changed every hour for the first 5 h
and then at 24 h.
All electrolytes used were dissolved in distilled
water. Constant volumes were added to the
impinging jet sample solution, such that the final
volume had the required concentration of salt. CaCl2
and KNO3 were purchased from BDH (99.8 purity
%). MgCl2 (99.8% purity) was obtained from Merck.
Hydrophobic conversion of glass slides
Microscope glass slides (CANEMCO & MARIVAC
Frosted End Microscope Slides 75 x 25 mm) where
immersed in a 50/50 solution of trimethylchlorosilane
and pyridine (≥99% Sigma- Aldrich ) at 60°C for 12
h, removed and cleaned with hexane (99.8% Sigma-
Aldrich ) and air dried.
Contact angle measurements
The contact angle measurements were conducted
with the use of a Data Physics OCA 20. A 10 µL
drop of water was placed on the surface and the
contact angle assessed with SCA20 software. This
procedure was the same for the contact angle of pitch
with the model surfaces. A 100 µL drop of pure
extracted pitch, prior to formation of the pitch
colloids, was introduced to the model surfaces and
the contact angle of the pitch on the surface was
determined.
Impinging jet
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Appita 2010
The dialysed pitch colloidal dispersion was made up
to salt concentration of 20 mM with the desired salt
and stirred for 10 min to allow particles to aggregate.
The h/r ratio within the jet was kept at 1.7 for all
experiments, with a constant flow rate of 70 mL/min
through the impinging jet.
Magnification of 10 times objective was achieved
with an Autoplan microscope. Images were captured
with the use of an IMI tech Han series digital camera.
Results
Contact angle measurements for the model
hydrophilic and hydrophobic surfaces with water and
pitch were made. Figure 2 depicts the contact of
water with untreated glass slide with a contact angle
calculated to be 18o. This shows a high affinity
between the surface and the water confirming that the
surface is hydrophilic.
Figure 2: Water contact angle with model
hydrophilic surface (18 °).
In contrast to this in figure 3 the contact angle post
silylation of the surface with trymethylchlorosilane
and water is 118°. This indicates that the model
surface is hydrophobic and so can be used to model
similar surfaces found in a real system.
Figure 3: Water contact angle with model
hydrophobic surface (118 °).
Droplets of neat wood resin were placed onto the
model surfaces in an air environment as shown in
figure 4 with a contact angle of 18° for both surfaces.
It is apparent from the contact angle measurements
that the surface has little or no effect on the contact
angle of pitch in the absence of water.
Figure 4: Wood resin contact angle with model
hydrophobic surface (18°).
The rate of deposition onto the two surfaces was
assessed by counting the total number of particles on
the surface at various times. Figures 5 and 6 show the
typical droplet formation of particles of pitch from 20
mM of MgCl2 and 20 mM CaCl2 respectively onto a
hydrophilic collector plate.
Figure 5: Pitch deposition onto a model
hydrophilic surface with MgCl2.
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Appita 2010
Figure 6: Pitch deposition onto a model
hydrophilic surface with CaCl2.
For both salts the pitch was seen to form on the glass
as a droplet. This droplet may be distinguished from
the background via the ring encircling the particle
from the reflection of light and the difference
between the refractive indices of water and pitch.
In figure 7 the deposition of pitch onto a hydrophobic
surface is shown. It can be seen from the absence of
the “halo” around the particles that the droplet
formation changes with the change of surface
chemistry and the particles are no longer distinct
droplets on the surface.
Figure 7: Pitch deposition onto model
hydrophobic surface with CaCl2.
Over a period of time the deposited pitch forms an oil
film on the hydrophobic surface as seen in figure 8.
This phenomenon is not seen with the model
hydrophilic surface even over a prolonged period of
time. The effect is known as film thinning. The oil
film was found to obscure the rate of deposition. In
order to overcome this effect, the surface was cleaned
after each run and the zero time was taken as the time
the pitch first contacted the cleaned surface.
Figure 8: Pitch deposition onto a model
hydrophobic surface with CaCl2. after a period of
time.
Figure 9 presents the average particle size of the pitch
on the model surfaces after 60 sec of deposition. It
can be seen that pitch particles on the hydrophilic
surfaces are smaller than those on hydrophobic
surfaces. Furthermore the large difference between
the hydrophobic surfaces for the calcium and
magnesium salts is related to the amount of film
thinning or wetting of the surface that is experienced.
More film thinning was observed with the calcium
salt and as a result a larger particle size was measured
with a larger amount of variation in the particle size
as indicated by the error bars.
0
0.5
1
1.5
2
2.5
3
3.5
4
Mg hydrophilic Mg hydrophobic Ca hydrophilic Ca hydrophobic
P articles S iz e
(
um
)
Mg hydrophilic
Mg hydrophobic
Ca hydrophi lic
Ca hydrophobic
Figure 9: The average particle size for the
deposited pitch after 60 sec of deposition onto the
surface.
The particles were found to deposit faster on to the
hydrophobic surfaces as shown in Figure 10. The rate
of pitch deposition is equal to particles /second or the
slope of the lines of best fit in figure 10. On the
hydrophilic surface the pitch deposition rate in the
presence of magnesium salt is 41 counts/sec and this
increased to 103 counts/sec with the change in
surface to a hydrophobic one. This indicates a 2.5
fold increase in the particle-surface interaction. In the
presence of calcium salt, this increase in the affinity
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Appita 2010
of the pitch particles is also seen. However the
change is not as marked, with a 1.8 fold increase.
0
1000
2000
3000
4000
5000
6000
7000
8000
0 1020304050607080
Time (sec)
particle count (j)
hydrophilic
Ca
hydrophilic
Mg
hydrophobic
Ca
hydrophobic
mg
Figure 10: Flux of particles to the surface with
different salt types (CaCl2 and MgCl2) and surface
conditions (hydrophobic and hydrophilic).
Discussion
The wood resin dispersions being deposited onto the
surfaces were at pH 5.5 which is below the resin acid
and fatty acid pKa’s. As such fatty acid and resin
acid components in the wood resins will be mostly
un-ionised. These acids contain a hydrophilic head
group (the carboxylic acid) and a hydrophobic tail.
The molecules can reorient themselves so that the
polar group will interact with a hydrophilic surface or
the non-polar groups can orientate and interact with
the hydrophobic surface. The low contact angle
observed for neat pitch on the two different surfaces
(figure 4) can be explained by this ability of the
molecules to reorient. However orientation of the
component molecules of pitch in a water environment
is restricted due to water-pitch interactions. This may
then change the manner in which the pitch can
deposit onto a surface in water and the form it will
take.
The large size difference in the particles between the
magnesium salts and the calcium salts on a
hydrophobic surface and the differences in the
amount of film thinning that occurred indicate that
the pitch particles are interacting with the surface in
different manners. It is possible that the small number
of ionized resin components is reacting with the
calcium metal salts in solution forming non-polar
metal soaps. Due to the hydrophobicity of the core of
the colloid and the mobility of components in the
colloidal particles the metal resonates may move to
the core of the particles. This change results in an
increase in the concentration of non-polar component
in the particle and the consumption of the resins.
Further work is being undertaken to study this and
determine if this does occur.
As all experiments are conducted above the critical
coagulation concentration, the deposition of pitch
will readily occur through compression of electrical
double layer promoting aggregation and
destabilization of the particles. Deposition is noted to
occur on both surfaces (figures 6 and 7) however
there is variation in the rate of deposition due to the
change in surface polarity. This indicates that pitch
will deposit onto both hydrophilic and hydrophobic
surfaces but the wood resins have a higher affinity for
the hydrophobic surface. Hydrophobic modification
of the surface results in the reduction of the surface
charge. As a result the electrical double layer
associated with the surface is reduced, and hence the
repulsive forces associated with it. The, interactions
that have to be overcome for fast particle-surface
interaction, will be proportionally reduced. The
results indicates that the critical deposition
concentration (CDC) for pitch onto the model
hydrophilic surface is greater than the 20mM salt
added to the wood resin solution., Further work is
required to find the CDC’s for pitch onto the model
surfaces used.
The film thinning that occurred on the hydrophobic
surface enables the pitch to conform easily to the
surface as an oil coating. This will with time obscure
further deposition of particles onto the model surface.
It is possible that the more hydrophobic components
of pitch migrate to the solid hydrophobic surface and
the hydrophilic components to the pitch-water
interface and so change the chemistry and nature of
the surface solution interactions. Further work will
be conducted to study this effect in more detail.
The salt concentration levels investigated in this
study correspond to levels of 800 mg/L which are
meant to represent mill conditions of increased water
closure. Calcium concentrations of levels of 200-
300mg/L are not uncommon in many mills and so the
levels investigated are within levels that would occur
with increased water closure. The results indicate
that surfaces that are hydrophobic in nature within
the paper machine will form oil films with higher
deposition rates than on hydrophilic surfaces. The
formation of the pitch oil film layer will then absorb
the fibres and process material that comes into
contact with it. This will result in the tacky deposits
observed in the mill that contains a mixture of fibres
and wood resins. Unless magnesium based process
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Appita 2010
additives were to be used, concentrations of
magnesium will remain much lower than this.
Conclusion
In the air-surface environment the hydrophobicity of
the surface doesn’t affect its affinity for neat pitch.
However, pitch in a water environment is more
structured due to water-pitch interactions, as seen by
the differences in appearance of the deposited pitch
on the hydrophobic and hydrophilic surfaces in the
IJM experiments.
On both model surfaces the pitch deposition was seen
to be faster in the presence of calcium salt. The rate
of deposition was 62 counts/sec for the hydrophilic
surface and 115 counts/sec for the hydrophobic
surface, a 1.8 fold increase in the rate of deposition.
A 2.5 fold increase in the interaction of pitch with the
surface is noted from the rate of pitch deposition onto
the hydrophobic surface compared to the hydrophilic
surfaces in the presence of magnesium salt.
Deposition onto the hydrophilic surface for
magnesium is 41 counts/sec, increasing to 103
counts/sec with the change in surface to the
hydrophobic model surface.
Over time, the formation of an oil film will change
the nature of the surface solution interactions. It is
inferred that surfaces that are hydrophobic in nature
within the paper machine will form these oil films
more readily then hydrophilic surfaces. The
formation of these films will result in the tacky
deposits observed in the mill that contains a mixture
of fibres and wood resins.
Acknowledgments
Financial support for this project was provided by
Norske-Skog Paper and an ARC Linkage grant
McGill University department chemistry’s technical
support, Jean-Phillipe Guay and Alfred Kluck, are
acknowledged for construction of the impinging jet
and support stand.
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