Effect of Separate Refining and Co-refining of BCTMP/KP on Paper

Effect of Separate Refining and Co-refining of BCTMP/KP on Paper
Effect of Separate Refining and
Co-refining of BCTMP/KP
on Paper Properties
By Y. Gao, F. Huang, V. Rajbhandari, K. Li, Y. Zhou
Abstract: The effect of PFI separate refining and co-refining on paper properties was investigated for
aspen bleached CTMP (BCTMP) and eucalyptus kraft pulp (EKP). The results showed that for a given
freeness, separate BCTMP/EKP PFI refining required more energy than BCTMP/EKP co-refining;
compared with BCTMP/EKP separate refining, BCTMP/EKP co-refining produced handsheets with
improved surface smoothness and physical strength, while there was no significant difference in opacity and
light scattering between these two refining processes. The results indicate that, in paper mill practice, it is
possible to choose an appropriate addition level of BCTMP in KP and an appropriate refining process to
maintain acceptable paper bulk and paper smoothness.
uring the last decades, bleached
(BCTMP), also called high-yield pulp
(HYP), has attracted an increasing
amount of interest among papermakers in order
to reduce costs and improve the quality of woodfree printing and writing paper [1, 2]. In the HYP
process, pulps are produced mainly through a mild
chemical treatment and the action of mechanical
forces, with a yield of 80-90% and retention of
most of the lignin in the wood [3]. Compared
with chemical pulps, e.g. kraft pulp (KP), HYP
contains a higher content of fines. HYP fibres
are rigid, coarse, and cannot easily collapse. These
differences between HYP and KP can result
in different paper properties. Paper from HYP
normally has higher bulk, bending stiffness, and
dimensionally stability. The light scattering power,
opacity, and printability obtained from HYP are
also superior to those from chemical pulps mainly
because of the higher amount of fines in HYP.
Due to these specific attributes, HYP has been
used in a variety of paper grades to reduce production cost and improve paper quality [4].
The high bulk of HYP can be used to reduce
the basis weight at a given caliper in wood-free
printing and writing papers. Higher bulk also
increases paper stiffness at a given basis weight,
high fines of content and the HYP may also
improve sheet formation. In addition, the higher
opacity of HYP provides optical benefits, particularly in lightweight specialty grades, such as
lightweight coated (LWC) paper.
In recent years, the focus on HYP production and product development has been moving
towards optimizing the process to tailor-make
HYP with some specific pulp properties for a
specific end-use in paper and board, particularly in
28 PULP & PAPER CANADA July/August 2009
the production of wood-free papers. In wood-free
papers, such as LWC and printing and writing
paper grades, HYP is used to replace hardwood
kraft pulp and gives the final sheets higher bulk
and opacity. The typical substitution rate of HYP
in KP is 5 to 15% for coated and 10 to 25% for
uncoated paper grades.
Although the bulk and opacity improvement
can be achieved by the addition of HYP, the paper
surface smoothness may be affected. Previous
studies [5, 6, 7] indicated that thick-walled and
Department of Chemical
Engineering, University
of New Brunswick,
Fredericton, N.B.
Department of Chemical
Engineering, University
of New Brunswick,
Fredericton, N.B.
Department of Chemical
Engineering, University
of New Brunswick,
Fredericton, N.B.
K. LI,
Department of Chemical
Engineering, University
of New Brunswick,
Fredericton, N.B.
Tembec Inc.,
Temiscaming, Que.
Bulk, cm3/g
PPS Roughness, µm
FIG. 1. Bulk versus freeness.
FIG. 2. Roughness versus freeness.
TABLE I. Sample identification.
Sample ID
Se-Re 10%BCTMP/90%EKP
Se-Re 20%BCTMP/80%EKP
Co-Re 10%BCTMP/90%EKP
Co-Re 20%BCTMP/80%EKP
coarse HYP fibres may not only deteriorate paper surface smoothness but may also
cause surface roughening upon rewetting
in the printing process. To compensate
the surface smoothness loss caused by the
addition of HYP and to improve paper
printability, HYP should be fine-tuned
to tailor-make final product qualities. In
mill practice, further refining of HYP is
quite often employed to reduce the fibre
coarseness in the mixed furnish. In some
mills, hardwood KP is refined to about 400
mL CSF, and HYP is mixed in hardwood
KP without refining, which is referred to
separate refining as in this paper. In other
paper mills, HYP is co-refined with hardwood KP.
The issue of separate or co-refining a
mixed furnish of kraft pulps has drawn
considerable attention amongst researchers
and papermakers [8, 9, 10, 11], since the
fibre characteristics of each type of pulp are
quite often significantly different. It is difficult to conclude whether separate refining or co-refining provides better quality of
the finished product. The method of refining a mixed furnish to achieve an optimal
product quality is largely dependent upon
the individual components of the furnish,
e.g. HYP and kraft pulp, the furnish composition, e.g. percentage of HYP and kraft
pulp in the furnish, and the pulp species,
e.g. aspen KP or eucalyptus KP. In addition, some investigations revealed that for a
given tensile strength, the energy efficiency
TABLE II. Pulp properties of eucalyptus KP (EKP) and aspen BCTMP.
separate refining
separate refining
and equipment investment between separate refining and co-refining were significantly different [12, 13].
Most published works on the comparison of separate refining and co-refining
were based on hardwood and softwood
kraft pulps. There is little published data
in the literature on separate refining and
co-refining of HYP and KP. In this study,
the effect of separate refining and corefining of aspen BCTMP and eucalyptus KP is investigated. The influence of
the BCTMP substitution ratio on paper
physical strength and surface smoothness
are discussed.
Aspen BCTMP 325/85 from a Canadian
pulp mill and a bleached eucalyptus kraft
pulp (EKP) from a French pulp mill were
used in this investigation.
For separate refining, EKP was refined
with a PFI mill to 480, 450, 400 and 350
ml CSF, and then mixed with 10% and
20% aspen BCTMP, respectively. For
co-refining, EKP was mixed with 10%
and 20% aspen BCTMP, and then refined
with a PFI mill to CSF of 480, 450, 400
and 350 ml, respectively. For convenience,
a short form ID was given to each pulp
sample and different refining process, as
listed in Table I.
CSF freeness, ml
Fibre length, mm
Fines, %
Coarseness, g/m
Tensile index, N.m/g
Opacity, % ISO
Bulk, cm3/g
PPS roughness, µm
Brightness, ISO %
Eucalyptus Aspen
kraft pulp BCTMP
Handsheet making and paper property
Handsheets of a basis weight of 60±2 g/m2
(o.d.) were prepared according to TAPPI
standard (T205 sp-02). The thickness
of the handsheets was measured with an
L&W micrometer, and paper bulk was
calculated based on the weight of handsheet. The tensile strength of the handsheets were tested with a tensile strength
tester by L&W. Opacity, brightness, and
light scattering were measured with a
TechniBrite Micro-1c tester. Roughness
was measured with a Parker surf tester, and
internal bond was tested by a TMI station
(Model 80-01-03). All the measurements
were conducted according to the relevant
TAPPI standard methods.
SEM analysis of paper surface characteristics
A Jeol JSM-6400 scanning electronic
microscope (SEM) was used to study
the fibre morphology characteristics of
the paper surfaces and cross-sections; the
handsheets were coated with carbon and
gold before the surface observation. The
cross-section was obtained as per the fol-
July/August 2009 PULP & PAPER CANADA 29
Internal bond, J/m2
Tensile index, N.m/g
Fig. 3. Tensile index versus freeness.
lowing: the sample sheet was first cut
by a sharp blade, and then it was vertically installed between two metal plates to
expose the cross-section, it was then coated
with carbon and gold as done in surface
Basic properties of EKP and BCTMP
The basic properties of eucalyptus KP
(EKP) and aspen BCTMP are summarized in Table II. Aspen BCTMP pulp has
higher opacity (80.5%) than EKP (75.8%),
which is mainly due to the higher fines
content in the pulp (23.1% vs. 10.0%).
The coarseness of aspen BCTMP (0.174
g/m) is almost twice as high as that of
EKP (0.095 g/m). Coarser fibres render
aspen BCTMP pulp higher in bulk (2.48
cm3/g) than the EKP (1.83 cm3/g), which
is the reason for BCTMP to be a partial
substitute for KP to improve paper bulk.
However, the higher coarseness of BCTMP pulp results in higher sheet roughness
than the EKP pulp (7.02 µm vs. 5.67 µm).
The physical strength is also different
between BCTMP pulp and EKP pulp.
Compared with EKP pulp, BCTMP has
lower tensile index (17.88 N.m/g vs. 28.06
N.m/g) which is mainly attributed to its
coarser, shorter and stiffer fibres [14].
The above comparisons indicate that
EKP and BCTMP pulps have significant
differences in physical and optical properties. Therefore, it is reasonable to achieve
desired paper properties such as higher
bulk and better opacity by way of mixing
a certain quantity BCTMP with KP [15].
Paper physical properties
Bulk and roughness
Paper bulk is dependent, to some extent,
on the fibre components in the network,
30 Fig. 4. Internal bond versus freeness.
and coarser fibres render higher bulk [16,
17]. For a given bulk, the fibre consumption is lower when using coarser fibres, as a
result, the production cost is reduced.
Figure 1 shows how the handsheet bulk
properties developed for different pulp furnishes. In comparison with the EKP pulp,
the bulk in a blend furnish of BCTMP/
EKP was improved in either separate
refining or co-refining, which was mainly
due to the coarser and stiffer BCTMP
fibres in the furnish. A higher substitution ratio (20%) of BCTMP results in
higher bulk. For the same substitution
ratio, separate refining and co-refining
bring about different results. With separate
refining, the average bulk gain (20%) of
20%BCTMP/80%EKP was almost two
times higher than that of co-refining (7%).
In the separate refining process, BCTMP
was mixed with EKP without refining,
therefore, the intact BCTMP fibres contribute more to bulk than when refined
with EKP.
Figure 2 shows that the influence of
BCTMP on paper surface roughness
depends on its addition levels, especially in
the case of separate refining. In comparison
with EKP, the blend furnish produced
sheets with higher roughness in either separate refining or co-refining. As mentioned
above, the BCTMP fibres are coarser and
stiffer, which not only contribute to the
bulk gain but also negatively affect paper
smoothness. In addition, in both refining processes, higher addition (20%) of
BCTMP gives the sheet higher roughness.
Interestingly, compared with the
EKP, the roughness increase of corefining BCTMP/EKP was not significant, especially in the case of Co-Re
10%BCTMP/90%EKP. A possible explanation is that in the case of co-refining,
PULP & PAPER CANADA July/August 2009
BCTMP fibres were modified by the
refining process, which gives the sheet
lower roughness. This indicates that with
a substitution ratio of 10 to 20%, co-refining of BCTMP/KP can achieve similar
roughness as 100% KP furnish.
Physical strength
At a given freeness, although separate
refining of BCTMP/EKP had a positive
effect on paper bulk and produced acceptable roughness, the physical strength of
the sheets was lower, as shown in Figs.
3 and 4. In Fig. 3, for the blends of 10%
BCTMP/90%EKP, the average loss of
tensile strength was 6% in the separate
refining, while it was 4% for co-refining.
Additional blends (moving from 10% to
20%) had significant effects on the tensile
loss: 14% for both separate refining and
It should be noted that these findings
are contradictory with the previous studies: adding a certain HYP (10 to 15%) in
KP did not necessarily weaken the paper’s
physical strength in disk refining [1]. The
possible explanations might include the
differences in the refining effects on pulp
fibres between the lab PFI mill and disk
refiner. In a comparative study between
PFI mill and commercial refiners in refining energy, refining intensity, and other
factors governing action on pulp, Kerekes
and others [18, 19] indicated that the PFI
mill is a high energy, low intensity refining
device which produces a refining effect
that differs significantly from a disk refiner
in paper mills. As a result, the pulp fibres
refined in a PFI mill revealed higher internal fibrillation, lower external fibrillation,
and fibre shortening, which are mainly
due to the fact that the PFI mill imposes a
greater proportion of compressive to shear
Opacity, % ISO
Light scattering coefficient, m2/kg
FIG. 5. Opacity and freeness.
FIG. 6. Light scattering coefficient and freeness.
FIG. 7.SEM image: top view of paper surface (Se-Re
FIG. 8.SEM image: top view of paper surface (Co-Re
forces than an industrial refiner. Therefore,
for a given freeness, the sheet physical
strength produced in PFI mill may be
inferior to those in disk refiner.
Similar trends can be observed in the
relation of freeness to internal bond, as
shown in Fig. 4. The decrease of the internal bond in co-refined BCTMP/EKP was
less important than in the case of separate
refining. In co-refining, BCTMP fibres
were modified by splitting, delaminating, and unraveling actions of mechanical
refining, which were favourable for the
bonding potential of fibres. Compared
with the un-treated BCTMP fibres in
separate refining, the refined BCTMP in
co-refining contributes more to the paper’s
physical strength, i.e internal bond.
In addition, to get the same internal bond from different pulp furnishes
indicated a different required freeness.
For example, to achieve 300 J/m2, EKP,
Co-Re 10%BCTMP/90%EKP, and CoRe 20%BCTMP/80%EKP were intended
to refine to 400, 350, 370 mL CSF,
respectively. This may suggest that mixing BCTMP in EKP furnish needs more
refining energy to get the same physical
strength as that of the individual EKP
refining. With a higher percentage of KP
being mixed in a BCTMP furnish, more
energy is consumed.
The above analysis indicates that replacing KP with BCTMP would deteriorate the
physical strength of final product, especially
in the case of separate refining of BCTMP/
KP. In mill practice, this disadvantage could
be compensated for by process modification and development. For instance, with
advancement and modernization in the
design of paper machines, the web tolerance on paper’s physical strength would be
largely improved. In addition, some new
products which are less exigent on physical
strength could be developed to fulfill different customer requirements.
Optical properties
As indicated in Table II, BCTMP contains a higher fraction of fines (23.1%)
than EKP (10%). Compared with chemical pulp fibres, mechanical pulp fines have
greater specific surface area owing to the
abundant content of fibril-like and flake-
like particles in the fines [20]. Fines have
strong effects on the structures and properties of fibre networks, particularly rendering a high opacity and light scattering
coefficient [21], which are favourable for
printing and writing papers [22].
As expected, adding BCTMP to KP
improved optical properties, such as opacity
(Fig. 5) and light scattering coefficient (Fig.
6) compared with the EKP. Interestingly,
for both opacity and light scattering, there
was no significant difference between separate refining and co-refining processes. This
finding suggests that the gain in the optical
properties is dependent more on the nature
(or grade) of the BCTMP, and less on their
addition ratio, as reported in [1].
SEM analysis
Figures 7 and 8 show the surface characteristics of separate and co-refining of the same
blend furnish: 20%BCTMP/80%EKP
with the same freeness level (350 ml).
These SEM images indicate that there
is no significant difference between the
top views of the surfaces. The relatively
smooth EKP fibres and fibrillated BCTJuly/August 2009 PULP & PAPER CANADA 31
MP fibres were evenly distributed on the sheet surfaces.
Although there is no evident fibre difference from the topview surfaces between the separate refining and co-refining
(Figs. 7 and 8), the SEM cross-sectioned view images (Figs.
9 and 10) revealed an apparent difference in surface roughness
between these two refining processes. In separate refining, the
sheet surface was uneven and some tube-like uncollapsed fibres
were loosely combined together, which renders higher sheet
bulk. In co-refining, the sheet surface showed relatively smooth
and collapsed fibres which were tightly compacted, and resulted
in lower paper bulk. This observation can explain the reason that
the separate refining produced handsheets with higher bulk and
roughness, but lower physical strength when compared with those
of co-refining.
Refining energy
In this study, the PFI revolutions were used to evaluate pulp
energy consumption in refining. Figure 11 shows the freeness
development at different PFI mill revolutions. The initial freeness before refining was 680 ml for EKP and 570 ml for aspen
BCTMP (Table II). After refining, the freeness-revolution curves
(trends) were different among the different pulp furnishes. For a
given freeness, higher energy consumption was found for BCTMP/EKP in a separate refining process than that of BCTMP/
EKP in a co-refining process and EKP individual refining.
In BCTMP/EKP separate refining, the freeness reduction
was completely attributed to the EKP refining alone since the
BCTMP was directly added into the EKP, after EKP refining,
and suffered no subsequent refining action in the blend furnish.
Therefore, for a given refining freeness, BCTMP/EKP separate
refining consumes more energy than the EKP individual refining
and BCTMP/EKP co-refining. Furthermore, in separate refining, a higher percentage of EKP replacement (20%) required
more PFI revolutions to obtain the same freeness level than a
lower percentage of replacement (10%).
It should be pointed out that for the BCTMP/EKP co-refining, although a different proportion (10% and 20%) of BCTMP
was mixed in EKP, they had similar freeness/revolution trends
as the EKP. This suggests that adding up to 20% of the aspen
BCTMP to the EKP had no significant effect on the freeness
development, compared with the EKP alone. This means that for
32 PULP & PAPER CANADA July/August 2009
FIG. 10. Paper cross-section SEM image (Co-Re
Freeness, mL
FIG. 9.Paper cross-section SEM image (Se-Re
FIG. 11. Freeness and PFI revolutions.
a paper mill, replacing up to 20% of eucalyptus bleached KP with
aspen BCTMP may not require a significant increase in energy
consumption in the refining operation.
Partial substitution (10 to 20%) EKP with BCTMP would
improve paper bulk, sheet opacity, and light scattering coefficient,
however, physical strength would be reduced.
For a given freeness, compared with BCTMP/EKP separate
refining, BCTMP/EKP co-refined produced handsheets with
improved surface smoothness and physical strength, while there
were no significant differences in opacity and light scattering
between these two refining processes.
A higher percentage of BCTMP in EKP resulted in higher
bulk and higher roughness, but weakened physical strength.
It is possible to choose an appropriate addition level of BCTMP in EKP and a refining process to maintain acceptable paper
bulk and paper smoothness.
For a given freeness, BCTMP/EKP PFI separate refining
required more energy than BCTMP/EKP co-refining.
The authors would like to acknowledge the financial support of
Natural Science and Engineering Research Council of Canada
(NSERC) and the Atlantic Innovation Fund of Canada (AIF).
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Basestock Properties, PAPTAC 90th Annual Meeting,
Montreal, Canada: C95-C101 (2004).
3. ZHOU, Y., CANNELL, E. Bleached High Yield
Pulp: Process, Installations, and End Uses, PAPTAC
91st Annual Meeting, Montreal, Canada: B231-B239
4. ZHOU, Y., ZOU, X. Achieving Desired End-use
Performance by Using HYP in Wood-free Coated
Papers, International Mechanical Pulping Conferences, Quebec, Canada:15-19 (2003).
5. REME, P.A., HELLE, T., JOHNSON, P.O. Fibre
Characteristics of Some Mechanical Pulp Grades, Nordic Pulp & Paper Res. J. 13(4): 263-268 (1998).
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Length, Fabric Structures, and Machine Drainage
Characteristics, Tappi J. 1(9): 3-9 (2002).
7. NESBAKK, T., HELLE, T., The Influence of the
Pulp Fibre Properties on Supercalendered Mechanical Pulp Handsheets, J. Pulp Paper Sci. 28 (12): 406-409
8. PECKHAM, J.R., MAY, M.N. Refining of Softwood
and Hardwood Kraft Pulps Separately and As Mixtures, Tappi J. 42(7): 556 (1959).
9. BAKER, C. Optimization of Paper-mill Refining
Systems, Proceeding of 3rd International Refining
Conference, Atlanta, USA: 20(1995).
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into the Pilot Scale Refining of Blending Papermaking Furnishes, Appita, J., 54(6): 547-551 (2001).
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Pilot Refiner, Appita J., Vol, (4): 301-307 (2003).
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PAPTAC 90th Annual Meeting, Montreal, Canada:
B143-B148 (2004).
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Bristow, J.A., and Kolseth, P (ed.) Paper Structure and
Properties, Marcel Dekker, New York (1986).
the Distributions of Mass, Thickness and Density in
Paper, Appita J. 54 (4): 385-389 (2001).
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PFI Mills, 6th Pira International Refining Conference,
Toronto, Canada: 12 (2001).
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Résumé: Nous avons analysé les effets sur les propriétés du papier de la séparation du co-raffinage et du raffinage sur PFI d’une pâte chimico-thermomécanique blanchie (PCTMB) de tremble
et d’une pâte kraft d’eucalyptus (PKE). Les résultats indiquent que, pour un degré d’égouttage
donné, le raffinage séparé de la PCTMB et de la PKE exige davantage d’énergie que le co-raffinage
de ces deux pâtes. Comparativement au raffinage séparé, le co-raffinage a produit des formettes
dont le lissé de la surface et la résistance physique étaient améliorés, tandis qu’il n’y avait pas
de différence notable entre les deux procédés de raffinage en ce qui a trait à l’opacité et à la diffusion de la lumière. Les résultats indiquent que, en usine, il est possible d’ajouter une quantité
appropriée de PCTMB dans la pâte kraft et de sélectionner un procédé de raffinage permettant de
maintenir un papier doté d’un bouffant et d’un lissé acceptables.
Reference: GAO, Y., HUANG, F., RAJBHANDARI, V., LI, K., ZHOU, Y. Effect of Separate Refin-
ing and Co-refining of BCTMP/KP on Paper Properties. Pulp & Paper Canada 110 (6): T99-T104
(July/August 2009). Paper presented at the PAPTAC 94th Annual Meeting in Montreal, February
5-7, 2008. Not to be reproduced without permission of PAPTAC. Manuscript received December
13, 2007. Revised manuscript approved for publication by the Review Panel Dec. 2008.
July/August 2009 PULP & PAPER CANADA 33
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