Note: Descriptions are shown in the official language in which they were submitted.
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EFFICIENT PRODUCTION OF A CONTAINERBOARD
TO BE USED AS FLUTING
TECHNICAL FIELD
The invention relates to a method of producing a containerboard to be used
as fluting.
BACKGROUND
Neutral Sulfite Semi-Chemical (NSSC) pulping is an old process that it is well
known in the field of paper pulping and in use in many pulp mills around the
world. One of the reasons for using NSSC pulping is the high yield.
In NSSC pulping, the cooking liquor comprises sulfite, such as Na2S03 or
(NH4)2503 and a base, such as NaOH or Na2CO3. "Neutral" means that the
pH of the NSSC cooking liquor is generally between 6 and 10. Normally, the
cooking time is between 0.5 and 3 hours and the cooking temperature is 160-
185 C. The NSSC pulp comprises comparatively high amounts of residual
lignin, such as 15-20 %, which make the NSSC pulp stiff. The NSSC pulping is
"semi-chemical" in the sense that it comprises mechanical
treatment/grinding (after the chemical (cooking step)).
The NSSC pulp is for example used to produce containerboard that is
subsequently corrugated to form the fluting of corrugated board.
Examples of mills using the NSSC pulping method are: BillerudKorsnas' mills
in Gruvon (PM 6) and Skarblacka (PM4), Sweden; Mondi Swiecie S.A.'s mill
in Swiecie (PM 4), Poland; Mondi's (Powerflute's) mill in Koupio, Finland;
Stora Enso Oyj's mill in Heinola, Finland (Heinola Fluting Mill); S.C. Celrom
S.A.'s mill in Drobeta, Romania; Packaging Corp. of America's mills in Filer
City (PM1, PM2 & PM3), Tomahawk (PM2 & PM4) and Wallula (PM2),
United States; Ilim Group's mills (PM1 and PM3) in Korjazma, Russia;
Permsky Karton's mill (PM2) in Perm, Russia; WestRock's mills in Longview
(PMio) and Stevenson (PM1 & PM2), United States; International Paper's
mills in Mansfield (PM2) and Pine Hill (PM2), United States; Georgia-Pacific
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LLC' s mills in Big Island (PM1 & PM3) and Cedar Springs, United States;
Cascades Containerboard Packaging's mill in Trenton, Canada; Sappi's
Tugela mill (PM2) in South Africa; Lake Utopia Paper's mill in St. George,
Canada; Graphic Packaging International's mill in West Monroe, United
States; Greif Bros Corp's mill in Riverville, United States; Hood Container
Corp's mill in New Johnsonville, United States; and Sonoco's mill in
Hartsville (PM in), United States.
EP3o26173 discloses a method of producing a containerboard (to be used as
fluting) of increased SCT strength from pulp comprising NSSC pulp.
According to the method, a web formed from the pulp comprising NSSC pulp
is pressed in a shoe press, in which the line load is at least 1200 kN/m.
Further, it is generally acknowledged in the prior art that the SCT strength
of
NSSC-based containerboard to be used as fluting is increased when the NSSC
pulp is subjected to more refining. As an example, the rebuild of PM4 in
Swiecie in 2015 involved a rebuild of the refining system to allow for more
refining of the NSSC pulp in order to increase SCT strength. For the same
purpose, Billerud AB (now BillerudKorsnas AB) increased the refining
capacity in the NSSC mill in Gruvon, Sweden back in 2005. Powerflute's
NSSC mill and Stora Enso's mill in Heinola improved their refining capacity
in 2010 and 2011, respectively.
SUMMARY
The object of the present disclosure is to increase the efficiency of the
method
disclosed in EP3o26173 while maintaining the SCT strength of the product at
a high level.
In the context of the present disclosure, the increased efficiency may be an
increased energy efficiency (i.e. a reduced amount of energy consumed for
producing one tonne of paper) and/or a higher productivity (i.e. that a higher
amount of paper can be produced by the paper machine used for the
method).
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To meet the above-mentioned objects, there is provided a method of
producing of a containerboard having a geometric SCT index of 37.0-42.0
Nm/g when measured according to ISO 9895:2008, comprising the steps of:
- providing a pulp having a Schopper-Riegler (SR) value of 15-19 when
measured according to ISO 5267-1:1999, wherein at least 70% by dry weight
of the pulp is NSSC pulp;
- forming a web from the pulp;
- pressing the web in a press section comprising a shoe press, wherein the
shoe press comprises a shoe press belt having discontinuous grooves and the
line load in the shoe press is in the range of 1400-2000 kN/m;
- drying the web from the press section in a drying section to obtain said
containerboard.
The method may also be described as follows:
A method of producing of a containerboard having a geometric SCT index of
37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprising
the steps of:
- forming a web from a pulp in a forming section comprising a head box,
wherein at least 70% by dry weight of the pulp is NSSC pulp and wherein the
pulp in the head box has a Schopper-Riegler (SR) value of 15-19 when
measured according to ISO 5267-1:1999;
- pressing the web in a press section comprising a shoe press, wherein the
shoe press comprises a shoe press belt having discontinuous grooves and the
line load in the shoe press is in the range of 1400-2000 kN/m;
- drying the web from the press section in a drying section to obtain said
containerboard.
The above range for the Schopper-Riegler value of the pulp (15-19) reflects a
very low degree of refining, which means low energy consumption in that
stage of the papermaking process. The inventor has also shown that a
reduction of the refining energy results in that less energy (i.e. less steam)
is
.. needed for drying the paper web formed from the pulp in the drying section.
Surprisingly, the inventors have found that the low degree of refining is not
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associated with an impaired SCT strength when the production is carried out
according to the present disclosure.
Regarding the pressing of the web in the shoe press, the inventors have found
that increasing the line load to 1400 kN/m or more generally does not
increase dewatering of the web unless the right type of shoe press belt is
selected. When the right belt is selected, however, such an increase of the
line
load effectively reduces the amount of steam needed to dry the paper web in
the downstream drying section.
The present disclosure can thus make the production of NSSC-based
containerboard more energy efficient, which is of particular interest since
the
yield of the NSSC pulping process is so high that the remaining biofuel (that
is combusted in the recovery boiler) is insufficient for the papermaking
process. Consequently, NSSC mills are dependent on external energy. If the
NSSC mill is not integrated with an adjacent sulphate mill that produces a
.. surplus of biofuel, it may be necessary to use fossil fuel as the external
energy.
Examples of such non-integrated NSSC mills are the Heinola Fluting Mill and
the Powerflute Mill. Further, the speed of many papermaking processes is
limited by the drying capacity of the drying section. This limitation is a
particular problem when containerboard of higher grammages is produced.
By providing a web that requires less drying energy in the drying section, the
present disclosure allows for higher speed and thus higher productivity in the
making of NSSC-based containerboard.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing the shoe press line load and the resulting
specific
steam consumption (medium pressure steam at about 8 bar) in the drying
section during the inventive trial described below. The shoe press line load
was 1300 kN/m during a first period ("ist") and a third period ("3rd"). During
a second period ("2ndTh
) the shoe press line load was 1700 kN/m.
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DETAILED DESCRIPTION
There is thus provided a method of producing a containerboard having a
geometric SCT index of 37.0-42.0 Nm/g. The containerboard is intended for
use as fluting (i.e. corrugated medium) in corrugated board. As understood
5 by the skilled person, the method is intended to be used on a full-scale
paper
machine, i.e. a machine adapted to produce at least 50,000 tons of
containerboard per year, normally at least 100,000 tons of containerboard
per year. The grammage of the containerboard may be in the range of 100-
240 g/m2, such as 110-240 g/m2, such as 120-240 g/m2, such as 120-200
g/m2 or 140-230 g/m2, such as 150-230 g/m2. Grammage is measured
according to ISO 536:2012.
Preferably, the geometric SCT index of the containerboard is 38.0-42.0
Nm/g, such as 38.0-41.0 Nm/g.
To obtain the geometric SCT index, the compressive strength in the machine
direction (MD) and the cross direction (CD) of the containerboard is first
measured using a short-span compressive tester (SCT) measured according
to ISO 9895:2008. To calculate the compressive strength index, the
compressive strength (N/m) is divided by the grammage. The unit of the SCT
index is thus Nm/g. The geometric SCT index is calculated as the square root
of the product of the SCT index in MD and CD:
geometric SCT index = -i(SCT index (MD) * SCT index (CD)).
The compressive strength is considered to be more important in CD than in
MD. The SCT index in the CD of the containerboard may for example be
above 28 Nm/g, such as at least 29 Nm/g. An upper limit for the SCT index
index may for example be 32 Nm/g.
The method comprises the step of:
- providing a pulp having a Schopper-Riegler (SR) value of 15-19 when
measured according to ISO 5267-1:1999, wherein at least 70% by dry weight
of the pulp is NSSC pulp.
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In one embodiment, the SR value is 16-19. In another embodiment, the SR
value is 15-18.
The SR value referred to above (and in the claims) is the SR value that the
pulp has in the head box (i.e. the chamber from which the pulp is caused to
flow onto the wire of the wire section). To obtain this SR value, the pulp of
the present disclosure may be subjected to refining, such as LC refining,
between the NSSC pulping process and the head box.
Preferably, at least 80% by dry weight of the pulp is NSSC pulp. In one
example, at least 85% or 88% by dry weight of the pulp is NSSC pulp.
The NSSC pulp may comprise hardwood NSSC pulp. Preferably, at least 70%
by dry weight of the NSSC pulp is hardwood NSSC pulp, such as birch NSSC
pulp. In one example, at least 80% by dry weight of the NSSC pulp is
hardwood NSSC pulp, such as birch NSSC pulp.
It has been reported in the prior art that a relatively low yield of the NSSC
pulp is needed to obtain a large increase in strength. The results presented
in
the Examples section are however obtained using a NSSC pulp of a relatively
high yield, more precisely about 82 %. The yield of the NSSC pulp of the
present disclosure may thus be 75%-85%, preferably 79%-85 %, such as 8o-
84%.
In addition to the NSSC pulp, the pulp may comprise recycled fibers, reject
pulp and/or clippings (typically corrugated box plant clippings). The above-
mentioned LC refining may be carried out before or after mixing with such
other fibres.
"NSSC pulp" is obtained from "NSSC pulping", which in turn is defined in the
background section. The NSSC pulp of the present disclosure may for
example be sodium-based NSSC pulp, which means that the cooking liquor of
the NSSC cook comprised Na2S03.
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The method further comprises the step of:
- forming a web from the pulp (that has a SR value of 15-19), typically in a
wire section (as conventional in paper making).
In the method, the head box consistency may for example be 0.50%-1.20%,
such as o.80%-1.20%, such as 0.90%4.15%. The higher consistencies are
particularly relevant when a large proportion (e.g. at least 80%) of the NSSC
pulp is derived from hardwood.
In one embodiment, the head box consistency is 1.05%4.20% and the
grammage of the containerboard is 140-240 g/m2, such as 140-240 g/m2.
The method further comprises the step of:
- pressing the web in a press section comprising a shoe press.
The nip length in the shoe press may for example be 200-330 mm, such as
250-300 mm. The shoe press is typically a double felted shoe press.
The shoe press comprises a shoe press belt having discontinuous grooves.
Such a shoe press belt is commercially available. One example is the
BlackBelt G DG marketed by Valmet. Another example is Valmet Black Belt H
DG marketed by Valmet. The hardness of the elastomer in the shoe press belt
may for example be 93-95 Shore A.
The line load in the shoe press is in the range of 1400-2000 kN/m, preferably
1500-2000 kN/m, such as 1600-2000 kN/m, such as 1600-1800 kN/m.
In the shoe press, the web may be subjected to a press impulse of 105-280
kPa*s, such as 105-190 kPa*s, such as 105-135 kPa*s. The highest press
impulses are obtained when the line load is relatively high and the web speed
is relatively low (because of a high grammage of the produced
containerboard).
The press section may comprise another press arranged upstream the shoe
press. The other press is typically double felted and may for example be a
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jumbo press or a shoe press. The line load of the other press may be 100-300
kN/m, such as 150-250 kN/m, in particular in case of a jumbo press.
Finally, the method further comprises the step of:
- drying the web from the press section in a drying section to obtain said
containerboard. As explained above, the method of the present disclosure
facilitates a reduced steam consumption in the drying section and/or
production at a relatively high speed, such as 700-850 m/min or even 750-
850 m/min. When the method of the present disclosure is carried out, the
steam consumption in the drying section may be as low as 1.20-1.35 tonne
steam per tonne paper. This steam is normally "medium pressure steam",
which refers to steam having a pressure of 6-11 bar, such as 6-io bar,
typically
about 8 bar.
EXAMPLES
First reference trial
.. Over a first period of about three months, NSSC containerboard (intended
for
fluting) of various grammages was produced in the Gruvon mill, Sweden.
Data from the production is presented in table 1. The pulp was a mixture of
about 90% NSSC pulp from birch and about io% kraft pulp. The pulp was
refined. The resulting SR numbers are presented in table 1. The press section
.. had two double felted nips; first a jumbo press and then a shoe press with
a
shoe press belt having blind drilled holes. The line load of the jumbo press
was i8o kN/m.
Over a second period of about three months, NSSC containerboard (intended
for fluting) of the same grammages was produced in the Gruvon mill,
Sweden. Data from the production is presented in table 2. Again, the pulp
was a mixture of about 90% NSSC pulp from birch and about io% kraft pulp
and it was refined. The resulting SR numbers are presented in table 2. The
press section was the same as during the first period except that another shoe
press belt was used. This new shoe press belt had discontinuous grooves. The
characteristics of the grooves were:
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width: 1.30 mm;
land: 1.80 mm;
depth: 1.30 mm;
open area: 38 %;
void volume: 460 ml/m2.
From the data of tables 1 and 2, the change of shoe press belt does not appear
to have had any particular effect. It is notable that the specific steam
consumption was not reduced during the second period despite that the
average SR number was slightly lower than during the first period. It is also
notable that the average geometric SCT index was almost the same during the
second period as during the first period.
Table 1. First reference trial, first period (shoe press belt having blind
drilled
holes). "SSC" means specific steam consumption. The SR numbers were
measured online.
Gram- SR Head box Head box Line load SSC
SCT SCT
mage number slice lip consis- shoe press (ton/ton)
Geo Geo
(kg/m2) (mm) tency (%) (kN/m)
index
0.110 17.9 15.7 0.92 1 300 1.44 4.3
39.1
0.120 18.6 16 1.01 1 300 1.46 4.7
39.2
0.130 19.2 16.2 1.03 1 300 1.5 5.1
39.2
0.140 19.9 16.7 1.07 1 300 1.53 5.4
38.6
0.150 19.4 17.3 1.08 1 296 1.53 5.8
38.7
0.160 19.4 17.9 1.11 1 300 1.53 6.1
38.1
0.175 19.2 20.4 1.06 1 300 1.54 6.9
39.4
Average 19.1 1 299 1.504
38.9
Table 2. First reference trial, second period (shoe press belt having
discontinuous grooves). "SSC" means specific steam consumption. The SR
numbers were measured online.
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Gram- SR Head box Head box Line load SSC
SCT SCT
mage number slice lip consis- shoe press (ton/ton)
Geo Geo
(kg/m2) (mm) tency (%) (kN/m)
index
0.110 19 15.8 0.92 1 300 1.5 4.3
39.1
0.120 18.8 16.1 1.00 1 275 1.49 4.7
39.2
0.130 19 16.4 1.02 1 289 1.49 5
38.5
0.140 18.3 16.6 1.08 1 341 1.53 5.4
38.6
0.150 18 17.1 1.09 1 309 1.56 5.7
38.0
0.160 18 17.9 1.11 1 325 1.54 6.2
38.8
0.175 16.8 19.5 1.11 1 300 1.48 6.9
39.4
Average 18.3 1 306 1.513
38.8
Second reference trial
NSSC containerboard (intended for fluting) having a grammage of 140 g/m2
was produced in the Gruvon mill, Sweden. The pulp was a mixture of about
5 90% NSSC pulp from birch and about io% kraft pulp. The pulp was refined.
The resulting SR numbers are presented in table 3 along with other data from
the production. The press section had two double felted nips; first a jumbo
press (line load = 180 kN/m) and then a shoe press with a shoe press belt
having blind drilled holes. During a first period of about 28 hours, the shoe
10 press line load was 1500 kN/m. During a second period of about 34 hours,
the shoe press line load was again 1300 kN/m. Data from the production in
presented in table 3.
The data in table 3 show that the increased line load of 1500 kN/m did not
reduce the specific steam consumption.
Table 3. Second reference trial (grammage = 140 g/m2, shoe press belt having
blind drilled holes). "SSC" means specific steam consumption. The SR
numbers were measured online.
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Period Average Head box Head box Line load Average SSC
SR slice lip consis- shoe press (ton/ton)
number (mm) tency (%) (kN/m)
First 15.9 17.0 1.05 1500 1.54
Second 16.1 17.1 1.05 1300 1.52
Inventive trial
NSSC containerboard (intended for fluting) having a grammage of 160 g/m2
was produced in the Gruvon mill, Sweden. The pulp was a mixture of about
90% NSSC pulp from birch and about io% kraft pulp. The pulp was refined.
The resulting SR numbers are presented in table 4 along with other data from
the production. The press section had two double felted nips; first a jumbo
press (line load = 180 kN/m) and then a shoe press having a shoe press belt
having discontinuous grooves with the characteristics described above under
the first reference trial. During a first period of about 2.5 hours, the shoe
press line load was 1300 kN/m. The shoe press line load was then gradually
increased and during a second period of about two hours, the shoe press line
load was 1700 kN/m. The shoe press line load was then gradually decreased
and during a third period of about one hour, the shoe press line load was
.. again 1300 kN/m. Data from the production is presented in table 4 (see also
figure 1).
The data in table 4 and figure 1 show that in case of a shoe press belt having
discontinuous grooves, increasing the line load above 1300 kN/m (e.g. to
1700 kN/m) reduced the specific steam consumption considerably. On
average, the specific steam consumption was 5 % lower at 1700 kN/m than at
1300 kN/m.
Further, the average geometric SCT index of the containerboard produced
during the second period was 38.3 Nm/g. During the same period, the SCT
index in the cross direction was 29.4 Nm/g.
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Table 4. Inventive trial (grammage = 160 g/m2, shoe press belt having
discontinuous grooves). "SSC" means specific steam consumption. The SR
numbers were measured online.
Period Average Head box Head box Line load shoe Average
SR slice lip consis- press (kN/m) SSC
number (mm) tency (%) (ton/ton)
First 17.2 18.1 1.10 1300 1.38
Second 17.3 18.o 1.11 1700 1.33
Third 17.3 18.1 1.10 1300 1.41
