Irreversible long fibre collapse at high temperature TMP reject refining

TMP REFINING
Irreversible long fibre collapse
at high temperature TMP
reject refining - initial studies.
BY S. NORGREN, H. HÖGLUND AND R. BÄCK
Abstract: Effects of preheating temperature on energy consumption, fibre collapse and pulp
quality in refining of spruce TMP reject material were studied. At preheating to temperatures well
above the softening temperature of lignin thick-walled fibres were collapsed to a higher degree.
Shives and fibres in the BMN +10 fraction were efficiently reduced. An increase in sheet density was
obtained. Energy consumption was reduced at a certain freeness level. This study indicates that an
improved surface smoothness can be achieved as result of high temperature reject refining.
INTRODUCTION
Thermomechanical pulp (TMP) is the most
commonly used mechanical pulp in newspaper
today. TMP is also more and more frequently
used as an alternative to the traditional groundwood pulp, as a base in high quality printing
papers, such as super-calendered magazine
papers (SC papers) and lightweight coated magazine papers (LWC papers).
The demand for surface smoothness is much
higher in SC and LWC papers than in newspaper.
In thin magazine papers, stiff and uncollapsed
long fibres in mechanical pulp can be a problem
[1,3,4,5]. This is most evident in TMP-based products, as TMP contains a much higher degree of
long fibre fraction than groundwood pulp. The
reason might be that the long fibres in TMP have
not been treated enough, nor at optimum conditions in the process. The long fibre fraction in
groundwood pulp for magazine papers is very
small and the fibres are usually considered to be
of a high quality.
Primary refining in the TMP process determines the basic characteristics of the pulp, including the characteristic of the long fibre fraction.
The second stage of refining only changes the
character of the pulp marginally [8,9,11]. In the
reject system the long fibre content is high. The
characteristic of the long fibre fraction in the final
pulp is modified to a high extent during reject
refining. However, there are only a few research
reports that describe the effects of different processing conditions in reject refining in general.
On the other hand, a lot of studies have reported
on the effects of different chemical treatments.
How sulphonation of screen rejects affects
pulp properties in reject refining is one area that
has been investigated thoroughly [12-14]. In
these studies it is shown that the strength properties of the long fibres can be improved considerably. In spite of this, chemical treatment of
mechanical pulp rejects is very seldom used in
applications in paper mills.
A suitable alternative method of treating the
long fibres would be to carry out the reject refining after preheating the pulp to a temperature
above the softening temperature of lignin.
Researchers have demonstrated that the soften-
Pulp & Paper Canada T 170
ing temperature is of great importance in the primary refining stage, in order to ensure the quality of mechanical pulp [17,18]. In the Thermopulp concept it has been shown that second stage
refining at high temperatures decreases energy
consumption [7]. It has also been shown that
refining using the Thermopulp process gives a
higher sheet density, which can indicate that the
fibres are more collapsed [6].
In this investigation preheating temperatures
well above the lignin softening point are used in
the reject refining process, i.e. a “thermo reject
refining” is carried out. The purpose of this temperature treatment of the long fibres is to achieve
a higher degree of long fibre collapse. More collapsed long fibres should result in a smoother
and denser sheet, which means that LWC and SC
papers can be produced using a higher proportion of mechanical pulp.
EXPERIMENTAL
The effect of the preheating temperature on
energy consumption and pulp quality in the
refining of spruce TMP reject was studied in trials
at a pilot plant and during full-scale operations.
The trials were carried out on high freeness
screen reject with a high content of long fibres
from the Ortviken TMP plant in Sundsvall, Sweden. The effects of preheating to temperatures
below and above the lignin softening temperature were compared.
Pilot Plant Trials
Pulps from the screw press in the reject refining
line at the SCA Ortviken mill, Fig. 2, were used in
a reject refining pilot plant study at Metso Paper
R&D Centre in Sundsvall. The layout of the pilot
plant is shown in Fig. 1.
Reject refining was carried out in an OVP-20”
single disc refiner equipped with standard segments, Table I. Refining took place after a short
preheating to temperatures below and well above
the softening temperature of lignin i.e. 125°C
and 185°C respectively.
Full-scale Thermo Reject Trial
As the results from the pilot trials were encouraging, it was decided to make further tests in the
S. NORGREN
Mid-Sweden University, FSCN,
Sundsvall, Sweden
e-mail: [email protected]
H. HÖGLUND
Mid-Sweden University, FSCN,
Sundsvall, Sweden
R. BÄCK
SCA Graphic Research AB
Sundsvall, Sweden
105:7 (2004) ❘❘❘
47
TMP REFINING
First stage preheating at 0,19 MPa
Steam
Cyclone
Screw
press
Plug screw
Plug screw
Fibers
Out: 0,82 MPa
Plug screw
Thermomixer
In: 0,80 MPa
FIG. 1. The pilot plant at Metso R&D Centre.
FIG. 2. Schematic picture of the reject system at Ortviken.
Steam pressures refer to conditions in high temperature
refining.
TABLE I. Equipment and conditions in the pilot plant trial.
TABLE II. Equipment and conditions mill trial.
Equipment, Conditions
Refiner type
Refiner size
Refiner plates
Disc speed
Production rate
Preheating temp.
Preheating time
Equipment, Conditions
Single disc
20”
Standard, 5811
1500 rpm
1-2 kg/min
125°C, 185°C
~1 min
Pulp characteristics before reject refining
Material
Freeness
Long fibre content
Dry Content
Spruce TMP reject
~600 ml
~70%
~37%
reject refining system at the Ortviken mill. In these trials, as in
the pilot plant trials, the main objective was to compare the
effect on the quality of the pulp of preheating to temperatures
below and above the softening temperature of lignin. Initial
studies were carried out to find the limitations of the refiner system, e.g. how much could we increase the inlet pressure without
getting problems with the stability of the process? Another crucial question was to find out if it was possible to achieve the same
results in the mill system as those achieved in the pilot plant
study. In the mill, the retention time in the preheating system
was very short. It is known from previous reported trials with the
Thermopulp® process that the disc-gap in the refiner is small in
high temperature refining [6,7]. This can result in runnability
problems. However, the initial trials gave us valuable information about how to operate the system to avoid these. Parameters
such as differential pressure in the refiner casing, plug water
flow, safety values of control loop and so on was optimized. This
was of vital importance in order to find a strategy for running
the system continuously without problems.
The reject refining system at the Ortviken mill is shown in
Fig. 2 [10]. The refiner used was a single disc refiner, RGP262,
equipped with a standard set of refiner plates. After several pretrials it was decided to use a pressure of 0.19 MPa in the first preheating stage, Fig. 2. A 24-hour trial, which is reported in this
paper, was carried out without giving rise to any runnability
problems. Important parameters for the reject refining system
are shown in Table II. The table also shows the inlet and house
pressures for the refiner in reject refining in the standard system
and in the high temperature process.
Pulp Testing
Pulps from the trials were analyzed with standard pulp and
paper testing methods (ISO) on 65 g/m2 laboratory sheets,
48
❘❘❘ 105:7 (2004)
Refiner type
Refiner size
Refiner plates
Disc speed
Production
Inlet pressure
House pressure
Preheating time
Single disc
62”
Standard, SP2622GSH602+
P262203SH
1500 rpm
100 t/d
0.30 MPa, 0.80 MPa
0.40 MPa, 0,82 MPa
A few seconds
Pulp characteristics before reject refining
Material
Freeness
Long fibre content
Dry Content
Spruce TMP reject
~600 ml
~70%
~37%
which were made according to the Rapid-Köthen method [2]. In
this method the laboratory sheets are dried at 95°C with an
applied pressure of 96 KPa.
A SEM analysis (Scanning Electron Microscopy) of cross-section fibre dimensions was carried out on long fibres from the
BMN + 30 fractions in the pilot plant trials. A minimum of 300
fibres were measured from each sample to achieve statistically
relevant results.
RESULTS AND DISCUSSION
Pilot Plant Trials
Long, stiff and uncollapsed TMP fibres cause disturbances in the
sheet structure that result in an inferior surface smoothness in
thin wood-containing magazine papers. It is evident that more
efficient reject refining could reduce the problem. This pilot
plant study was carried out in order to discover if it was possible
to increase the degree of fibre collapse by using a process in
which the fibre material was preheated to a temperature well
above the softening temperature of lignin before the reject
refiner stage.
Fibre collapse was measured on BMN +30 fractions in a SEM
analysis. Fibre lumen width was used as a measure of fibre collapse. Fibres were divided into three groups according to their
wall thickness in the analysis.
Figure 3 show that preheating to a temperature well above
the softening temperature of lignin, “thermo reject refining”,
affected the thick-walled fibres in reject refining very significantly. Under such conditions they collapsed to a much higher
degree than in the reference system. The number of fibres that
have a lumen width at or close to zero was increased from 30%
to over 50% as a result of high temperature refining.
T 171 Pulp & Paper Canada
TMP REFINING
Thermo reject
Unrefined screened reject
Reference
FIG. 3. This figure shows how the lumen width was affected in reject refining in the fibre fraction with a wall thickness > 2,6µm. Reference process: Preheating temperature
125°C Thermo reject: Preheating temperature 185°C.
FIG. 4. Fibres wall and lumen thickness after refining under
different conditions. In the figure are fibres with medium
wall thickness in three intervals are shown, (A =>2,5 µm,
B=1,15 - 2,5µm C=<1,15µm).
FIG. 5. High temperature pre-treatments in reject refining
have a great impact on energy consumption.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
FIG. 6. Density at a certain freeness increased at high temperature refining.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
Figure 4 shows that the wall thickness was only reduced to a
minor extent but the lumen width was reduced considerably in
the reject refining process. High temperature refining seems to
be an efficient way to collapse the fibres with the thickest walls,
but the collapsibility of early wood fibres was not improved
through this technique. The large effect on the thickest-walled
material is in agreement with earlier findings from refining studies of the Thermopulp process [6,7]. In these studies it was
shown that the sheet density increased more in high temperature refining of thick-walled pine chips than in the refining of
the thinner-walled spruce chips.
Full-scale Thermo Reject Trial
As shown in Fig. 5, the high temperature refining process
reduced energy consumption at a certain freeness level by about
15%. This result could be expected, as the conditions during
“thermo reject” refining are the same as those in the previously
reported Thermopulp process. However this is still a very interesting and important result.
Figure 6 shows that the density of the sheet increased as a result
of the high temperature refining technique. The increase in density indicates that we have achieved a more collapsed fibre structure.
The tensile index did not decrease as a result of high temperature refining, in spite of the significant reduction in energy
consumption, Fig. 7.
Figure 8 shows that the bonding strength in the sheet structure, measured by the Scott-Bond method, was improved at a
certain freeness level as a result of high temperature refining.
Pulp & Paper Canada T 172
FIG. 7. High temperature reject refining does not appear to
affect the tensil index.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
This indicates that the adhesion forces between the fibres were
improved. The reason for this could be that fibres were
collapsed to a higher extent, which resulted in an increased
bonding area.
Figure 9 shows that the tear index dropped significantly at a
CSF below 150 ml during high temperature refining in a single
stage operation. This shows that it is not desirable to refine to
105:7 (2004) ❘❘❘
49
TMP REFINING
FIG. 8. The Scott-bond vs. freeness diagram shows an
increased bonding strength between the ”thermo reject
refined“ fibres.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
FIG. 9. The tear index indicates fibre cutting at the lower
freeness values. Under these conditions strength was
reduced (compare figure 7).
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
FIG. 10. High levels of shive reduction were achieved at
”thermo reject refining“.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
FIG. 11. Long fibre fractions from BaurMcNet shows that
the proportion of stiffest fibres (BMN +10) decreased as a
result of high temperature refining.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
TABLE III. Comparative results from reject refining at SCA,
Ortviken.
FIG. 12. Bendtsen roughness on the topside of the laboratory sheet was reduced as a result of “thermo reject refining”.
▲ = High temperature refining (thermo reject)
■ = Reference, refining at standard conditions.
such a low CSF value under these specific conditions. It is a wellknown problem that fibre length is significantly reduced at certain processing conditions in single disc refiners. However, preserving a high tear index and at the same time reducing the
surface roughness in the sheets is a general problem. The tear
index is improved by a high content of long, stiff fibres [15, 16].
50
❘❘❘ 105:7 (2004)
Freeness
Energy
BMN 10-30
BMN -200
Shive content
Sommerville
Density
Tensile index
Tear index
Light scattering
Brightness ISO
Roughness
Scott-bond
Thermo
reject
Ref.
Thermo
reject
Ref.
CSF ml
KWh/t
%
%
%
150
820
47
9,5
0,25
150
980
47,5
12
0,60
185
800
48
8,5
0,35
185
930
49
11,5
0,80
kg/m3
kNm/kg
Nm2/kg
m2/kg
%
ml
J/m2
530
52,5
7,0
43
59,5
140
275
510
53
8,0
44
61
150
225
520
51,5
7,5
42
59
145
245
500
52
8,0
43
60
160
210
These fibres are also the main contributor to surface roughness.
It is obvious that a compromise must be made between the objectives of achieving a high tear index and a high paper smoothness.
To be able to minimize surface roughness it is essential to
reduce the shive content and the proportion of the stiffest fibres
in the pulp. Figure 10 illustrates how the shive content is
T 173 Pulp & Paper Canada
TMP REFINING
reduced more efficiently in “thermo
reject refining” than in reject refining
under standard conditions. At freeness
higher than 150 ml the shive content was
more than halved. The amount of fibres
with the greatest degree of stiffness,
defined as fibres in the BMN +10 fraction,
was also reduced more efficiently as a
result of high temperature refining, Fig.
11. This was done without any significant
reduction of long fibres in the 10-30 mesh
fraction at CSF > 150 ml.
Refining with the high temperature
technique can possibly diminish the
roughness problem caused by long stiff
fibers. This is indicated in Fig. 12. It shows
how the roughness on the topside of laboratory sheets was reduced.
In Table III, some mechanical and optical properties from the “thermo reject refining” and the reference trials are compared.
CONCLUSIONS
The results in this study indicate that “Thermo reject refining” ie reject refining after
preheating to a temperature value well
above the softening temperature of lignin
can be a suitable method of reducing the
surface roughness problem. The initial
study of this refining technique showed that:
• Energy consumption at a certain freeness level was reduced.
• Shives and the BMN +10 fractions were
significantly reduced.
• A higher degree of fibre collapse was
achieved.
• Sheet density was increased.
• Sheet roughness was reduced
FUTURE ACTIVITIES
This study will be followed by more fullscale trials. Pulps from these trials will be
examined using the ESEM technique. In
these studies we will examine to what
extent the fibre collapse that was achieved
as a result of high temperature refining
is irreversible.
Institutt for kjemisk prosessteknologi. NTNU: Trondheim (2000)
2. EN ISO 5269-2; Preparation of laboratory sheets for
physical testing - Part 2: Rapid-Köthen Method. 1998,
CEN. P 13.
3. FORSETH, T., HELLE, T. Moisture-Induced Roughening During “Water Coating” of Precalandered
Wood-Containing Paper. J. Pulp Pap. Sci. 24(10): 301307 (1998).
4. FORSETH, T., WIIK, K., HELLE, T. Surface Roughening Mechanism for Printing Paper Containing
Mechanical Pulp. Nordic Pulp and Paper Research Journal.
1: 67-71 (1997).
5. FORSETH, T., HELLE, T. Effect on Moistening on
Cross Sectional Details of Calandered Mechanical
Paper. In CPPA, 82nd annual technical meeting. Montreal. (1996)
6. JACKSON, M., DANIELSSON, O., FALK, B. Thermopulp™- a New Energy Efficient Mechanical Pulping
Process. In 5th International Conference on New Available Techniques, Stockholm, Sweden. p 229-244 (1996).
7. HÖGLUND, H., BÄCK, R., FALK, B., JACKSON, M.
Thermopulp™ - a New Energy Efficient Mechanical
Pulping Process. In International Mechanical Pulping
Conference. Ottawa, Ontario, Canada. P213-225
(1995).
8. HEIKKURINEN, A., VAARASALAO, J., KARNIS, A.
Effect of Initial Defiberization on the Properties of
Refiner Mechanical Pulp. In International Mechanical
Pulping Conference, Minneapolis. p. 303 (1991).
9. STATIONWALA, M., MILES, K. B., KARNIS, A. The
Effect of First Stage Refining Condition on Pulp Properties and Energy Consumption. In International
Mechanical Pulping Conference Minneapolis. p. 321
(1991).
10. ENGSTRAND, B., SUNDBLAD, P. Ortviken - The
Uncommon Paper Mill. In International Mechanical
Pulping Conference, Vancouver, p 91-97 (1987).
11. FALK, B., JACKSSON, M., DANIELSSON, O. The
Use of Single and Double Disc Refiner Configuration
for the Production of TMP for Filled Super Calandered and Light Weight Coated Grade. In International Mechanical Pulping Conference, Vancouver. P.
137(1987).
12. GOEL, K. Upgrading Mechanical Pulps by Chemical Treatment of Rejects Prior to Refining. Pulp &
Paper Canada 88(11) : 69-73 (1987).
13. GUMMERUS, M., RATH, B. Sulphite Treatment of
TMP Rejects. Part 2. Effect of Different Treatment
Conditions and Refining on the Properies of Rejects
Pulps. Paperi ja Puu 68(4) : 269-282 (1986).
14. GUMMERUS, M. Sulphite Treatment of TMP
Rejects. Part 3. Properties of Remixed Pulps. Paperi ja
Puu 68(8) : 534-544 (1986)
15. JACKSON, M., WILLIAMS, G. J. Factors Limiting
the Strength Characteristics of Thermomechanical
Pulp. In International Mechanical Pulping Conference. Toronto, Ontario, Canada. p 37-48 (1979).
16. MOHLIN, U-B. Properties Of TMP Fractions and
Their Importance for the Quality of Printing Papers.
In International Mechanical Pulping Conference.
Toronto, Ontario, Canada. p 57-83 (1979).
17. ATACK, D. On the Characterization of Pressurized
Refiner Mechanical Pulps. Svensk Papperstidning 75(3):
89-94 (1972).
18. KORAN, Z. Surface Structure of Thermomechanical Pulp Fibres Studied by Electron Microscopy. Wood
and Fibre 2(3): 247-258 (1970).
Résumé: Les effets de la température de préchauffage sur la consommation d’énergie, l’affaissement des fibres et la qualité de la pâte lors du raffinage des refus de PTM d’épinette ont été
étudiés. Lors du préchauffage à des températures bien au-delà de la température de ramollissement de la lignine, les fibres à paroi épaisse se sont davantage affaissées. Les bûchettes et les
fibres de la fraction BMN +10 ont été efficacement réduites. La densité de la feuille s’est accrue.
La consommation d’énergie a été réduite à un certain indice d’égouttage. Cette étude indique
qu’on peut améliorer le lissé grâce à l’utilisation d’une température élevée lors du raffinage.
Reference: NORGREN, S., HÖGLUND, H, BÄCK, R. Irreversible long fibre collapse at high
temperature TMP reject refining - initial studies. Pulp & Paper Canada 105(7): 170-174 (July, 2004).
Paper presented at the 2003 Intl. Mechanical Pulping Conference in Quebec City, QC, June 2-4,
2003. Not to be reproduced without permission of PAPTAC. Manuscript received on September 13,
2003. Revised manuscript approved for publication by the Review Panel on November 24, 2003.
Keywords: THERMOMECHANICAL, PULPING, COLLAPSE, REJECTS REFINERS, HIGH
TEMPERATURE, SMOOTHNESS, PRINTING PAPERS, SOFTENING POINT.
ACKNOWLEDGEMENT
Many people have contributed to the
results of this study. We would like to take
this opportunity to thank:
SCA Ortviken and Metso Paper, for
their invaluable support and co-operation
on full-scale trials and trials at the pilot
plant.
The staff of the SCA Graphic Research
paper testing laboratory for their support
regarding methods and the provision
of equipment.
We would also like to express our
thanks for the financial support from the
Swedish Energy Agency, the Foundation
for Knowledge and Competence and the
European Union, Objective 1, Region of
South Forest Counties.
LITERATURE
1. REME, P.A. Some Effects of Wood Characteristics
and the Pulping Process on Mechanical Pulp Fibres.
Pulp & Paper Canada T 174
105:7 (2004) ❘❘❘
51

Download Report