Note: Descriptions are shown in the official language in which they were submitted.
1~64s~9
I~EE 024 P2 -1-
HIGH YIELD CHEMIC~L PULPING
Background Of The Invention
The present invention relates to a chemical
pulping process for the production of cellulose pulps.
More particularly, the present invention relates to a
modification of conventional chemical pulping processes
which improves yield, reduces steam requirements, and
produces a pulp which is more easily bleached and which
possesses improved brightness stability.
Chemical pulping processes are characterized in
that ~he wood fibers are released by dissolving the lignin
which binds them together. Because lignin and other
non-cellulosic portions of the wood chips are removed in
the process, chemical pulping processes typically provide
yields of 40-50% based on the dry chips.
The rate limiting step in any pulping process is
the penetration of the pulping chemicals into the chips.
While there are several theories for chip impregnation in
chemical pulping, one theory is that the liquor diffuses
through the network of chip lumen and pits to remove the
lignin. A typical wood chip contains several million
fibers depending on species and chip si~e. Each fiber is
a miniature cylinder. The rniddle of that cylinder is the
lumen. The lumen of each fiber is connected to the lumen
of adjoining fibers by openings or windows called piks.
Liquor penetration i9 limited by the microscopic size of
the lurnen and pits through the chip. Licluor in the lumen
must still penetrate the fiber wall to remove the lic3nin
in the middle lamella. Permeability of the fiber wall is
MEE 024 P2 -2-
poor and a significant portion of the wall must be
dissolved to make it porous. This has two negative
aspects, namely, loss of pulp yield due to loss of
carbohydrates and contamination of the fiber wall with
lignin degradation products. The latter makes the fiber
difficult to bleach.
Ef~orts to improve chemical pulping processes by
modifying the chip structure to expand its surface area
and facilitate liquor impregnation have been made.
D. Lachenal et al, ~Chip Destructuring Improves Kraft
Pulping", TAPPI Proceedings - 1984 Pulping Conf. pp 13-16,
reports that destructing chips by passage through crush
rollers reduces rejects. Nolan, in U.S. Patent Nos.
2,904,460 (1959) and 3,192,102 (1965),teaches shredding
the chips along the grain to facilitate chip
impregnation. ~ hammer mill, attrition mill or crushing
roll is used to shred the chip. Nolan states that a
single fiber would be the most efficiently tailored chip
from a purely theoretical standpoint, but this results in
severe fiber damage and unacceptable loss in pulp
strength. Nolan's preferred chip is a pin chip which
ranges from about 2.3 to 6.7 mm in cross-section.
Chip destructuring as taught by Lachenal and
Nolan does not form the open porous network which
characterizes the chip used in the present invention and
only marginally improves chemical pulping. Consequently
it is not widely practiced in the paper industry.
Summary of the Invention
__
A principal object of the present invention is to
3~ improve yield, ~I factor requirements and bleachability of
. , .
.;; .
,
E 024 P2 -3-
a chemical pulp through chip des~ructuring and, more
particularly, through partial chip defiberizing.
In accordance with the present invention, wood
chips are subjected to a combination of compressional and
torsional forces to achieve partial separation of the
fibers in the chip; i.e. partial defiberizing. By
~partial defiberizing~ is meant that millions of ~ibers
present in the chip are largely mechanically separated
from each other over the major part of their length and
yet they are still bonded to one other at some point or
points along the fiber length, preserving the structural
integrity of the chip. This is in contrast to complete
(total) defiberizing where chips are reduced to individual
fibers which are not connected to each other, for example,
in RMP or in TMP processes. A bulky chip having an open,
porous and fibrous network is obtained according to the
teachings of the present invention.
The chip treatment in accordance with the present
invention also differs from that referred to in commonly
assigned U.S. Patent No. 4,486,267 in which no attempt was
made to reduce the chip to individual fibers. There, the
objective is merely to compress partially destructured or
not destructured chips and to allow them to expand in a
caustic solution, as a way of improving penetration of the
chips by this solution in a ~TMP process. In CTMP
processes, caustic swells and thus weakens the
hemicelluloæe-rich P and 51 layers o~ a ~iber.
This insures that the separation o~ ~ibers
in the de~iberizing step (which follows a second
impregnation stage) takes place along the
. .
s
MEE 024 P2 -4-
P and Sl layers and not along the lignin-rich middle
lamella, as is the aim of the present invention. Because
no subsequent delignification takes place after
defiberizing according to U.S. Patent 4,486,267, if fiber
separation occurred along the middle lamella, lignin
located on the surfaces of the fibers would prevent
hydrogen bonding between individual fibers when the fibers
are formed into a sheet structure and sheet strength would
be poor. In contrast, the objective of the present
invention is to separate fibers along the middle lamella
to facilitate the removal of lignin in subsequent
cooking.
By partially defiberizing the chip but not
cornpletely separating the fibers, several advantages are
achieved. Partial defiberizing of the chip opens up the
middle lamella such that the cooking chemicals can act on
the middle lamella directly without passing through the
fiber wall. Chip penetration is not limited to the pit
and lumen network of the chip. The chip appears as a
loosely packed network oE fibers having a coating of
lignin on the outer surface which is readily accessible to
the pulping liquor. As a result, Kappa levels of 50 to 70
are achieved very efficiently. For example, in Kraft
pulping an H factor of only about 160 is required to lower
the Kappa number ~rom an estimated level of 150 to 65 (a
57% reduction ln lignin). This compares with an H factor
oE 400 which would be re~uired to delignify conventional
chips to a Kappa number of 65.
Because the cookiny lLquor can directly access
the lignin in the middle lamella without passing through
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MEE 024 P2 -5-
the fiber wall, less carbohydrate is removed in the
pulping process and yields are concomitantly higher. The
fibers al50 produce bulkier paper which is a desirable
characteristic in tissue, towel and book paper grades.
Similarlyl because the liquors can directly access the
fiber wall, fewer lignin byproducts are left in the wall
itself and the pulp has better bleachability. Finally,
whereas chemical pulping of a completely defiberized chip
requires a very high liquor to wood ratio and can not be
performed in a conventional digester, the partially
defiberized chip can be pulped at conventional or nearly
conventional liquor to wood ratios in conventional
digesters. Consequentlyl minimum capital expense is
involved in modi~ying a pulp mill to carry out the process
of the present invention.
Thus, the present invention provides a chemical
pulping process which comprises:
partially defiberizing wood chips such that the
fibers in the chips are substantially separated from one
another but sufficient bonding points are maintained to
preserve chip integrity and thereby provide chips having
an open porous network, and
subjecting the destructured chips to a chemical
pulping liquor to remove a majority of the lignin
therefrom.
It has been found that it is usually desirable to
terminate the cook at Kappa levels greater than 40 and
typically about 45 to 70 and to use other processes such
as oxygen delignification to complete deligniEication,
e.g., to reduce Kappa to about 15 to 25 prior to bleaching
5~
MEE 024 P2 -6-
depending upon the intended use of the pulp. Oxygen is
generally preferred to other delignification processes
because it is less expensive and the waste liquor can be
concentrated and recycled to the chemical recovery. When
delignification is completed the pulp is bleached in an
otherwise conventional manner. Thus, a preferred pulping
process in accordance with the present invention co~prises:
partially defiberizing wood chips such that the
fibers in the chips are substantially separated from one
another but sufficient interfiber bonding points are
maintained to preserve chip integrity and thereby provide
chips having an open porous, fibrous network;
subjecting said partially defiberized chips to
chemical pulping at elevated temperature and pressure to
provide a pulp having a Kappa value of about 45 to 70;
further delignifying said pulp with caustic in
the presence of oxygen to provide a pulp having a Kappa
value of about 15 to 25; and
bleaching said pulp.
Brief Description oE the Drawings
Fig. l is a photograph of a conventional chip and
a pinchip (upper part of the photograph), and a
destructured chip for pulping in accordance with the
present invenSion (lower part of the photograph);
Fig. 2 is a photograph of a plurality of chips
for chemical pulping in accordance with the present
invention.
Fig. 3 is a schematic illustration of the
cross-section through a pulp fiber.
.~ .
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MEE 024 P2 -7-
Fig. 4 is a schematic illustration of a digester
for performing the process of the present invention.
Fig. 5 is a graph of H factor vs. Kappa number
for a Kraft pulp in accordance with the invention and a
conventional Kraft pulp.
Definitions
The terms ~chemical pulping~ and ~chemical
pulping liquor" are used herein as they are used in the
art, namely, to refer to chemical pulping processes such
as the Kraft, soda, soda AQ (anthraquinone), Kraft AQ,
sulfite, bisulfite processes and others. See Rydholm,
Pulping Processes, Interscience Publishers, p. 284, Table
6.1, (l9~S). These processes are characterized by lignin
removal in excess of 50% and often 75 to 90% and yields of
40 to 60%.
Detailed Description of the Inventio_
Fig. 3 is a cross-section of a wood fiber.
Lignin is most concentrated in the middle lamella (M). To
delignify a chip, liquor passing through the lumen (L)
must dissolve the tertiary layer (T), the secondary layers
(Sl and S2) and the primary layer (P). Lignin from
the middle lamella (M) can then be removed by the liquor
to t.he lumen (L) which acts as a miniature pipe for
transportation oi liquids.
A wood chip for chemical pulping in accordance
w.ith the presenk invention is shown in Fig. 1 (bottom) and
is characterized by t.he open porous fibrous net.work. This
network is obtained by acting on the chip wit.h a
~.2~6~5~
MEE 024 P2 -8-
combination of compressional and torsional forces such as
can be obtained in a screw press and, more particularly,
by operating the screw press at a high compression ratio
and under a high back pressure. The screw press operating
conditions will vary with the nature of the chip and, more
particularly with the type of wood (e.g., hardwood or
softwood~, its age and dryness. The objective of any such
operation is to produce chips essentially as shown in Fi~.
2. In addition to a screw press, other mechanical devices
which are capable of subjecting chips to twisting, bending
and compressing actions can also be used to obtain a
partially defiberized chip; for example, screw extruders
can be used. Comrnercial pulping digesters are frequently
equipped with feed screws, however, they are not operated
under a substantial back pressure and they are not used to
destructure the chips as in the present invention.
In order to obtain good fiber separation, it is
desirable to expose the chips to steam before
destructuring them and to operate the screw press or other
destructuring means at temperatures above 100C.
Pre-steaming softens the middle lamella and makes the
fibers easier to separate. It is particularly desirable
to heat the screw press or other destructuring unit to
temperatures in excess of 100C such as 120-160C.
Chips pre-treated in accordance with the present
invention are friable. That is they can be crumbled by
pressing and rolling the chip between the thumb and index
finger.
To illustrate the differences between chips
destructured as shown in Fig. 1 (bottom) ~nd 2 and chips
destructured by okher means, destructured chips were
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MEE 02~ P2 -9-
classified by si2e after destructuring on a hammermill, a
roll press, and an attrition mill, versus a heated and
unheated screw press in accordance with the present
invention. To further demonstrate the extent of
destructuring, the chips were treated in a British
disintegrator, a standard laboratory tool used to
disintegrate drylap pulp and waste paper into individual
fibers without shortening fiber length and without
refining fibers. The disintegrator was operated at 2.0%
consistency for 1 hour. The results are shown in Table 1.
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ME~ 024 P2 -10-
It can be seen from Table 1, that in the case of
the untreated, hammermill, and roll crushed chips, about
99% of the material is larger than 1.65 mm (the size of a
pin chip), compared to 86% in accordance with the present
invention and 91% in the case of the attrition mill.
The difference in the chips is more clearly shown
after treatment in the disintegrator. Each chip was
subjected to the same conditions in the disintegrator. In
this case in accordance with the present invention only
14% of the chip material is larger than 1.65 mm in width
when the screw press is heated (36% when the press is not
heated) compared to 60-70~ in the case of the other
materials.
The difference in the chips is also illustrated
by comparing their surface area as measured by BET
nitrogen adsorption. The results of this study are shown
in Table 2. Surface area was determined by dynamic gas
flow technique ~Stone J.E. and Nickerson L.F., ~A Dynamic
Nitrogen Adsorption Method Eor Surface Area Measurements,
Pulp and Paper Magaæine of Canada, March 1963).
Table 2
reatment ~ 2
Harnmermill 0.238
Roll Press 0,238
Attrition Mill 0.262
Screw Press (no heat.) 0.487
Screw Press (heated) 0.549
The data in Table 2 show that the chips de.st.ructured in a
screw press in accordance with the present invention have
a s:ignificantly higher ~sur~ace area than chips treated by
other processes.
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MEE 024 P2
Density measurements and compaction studies also
illustrate the properties of the destructured chips used
herein~ The chip density was obtained for soaked chips
following TAPPI Method T258 OM-85 by determining oven dry
wood weight and displacement of the chips in water. In
addition the densities of the destructured chips were
determined for the compacted and uncompacted chips. For
the latter measurement, a known weight of destructured
chips was allowed to fall into a metal cylinder and the
volume measured. In the former the cylinder was vibrated
until the chips settled and there was no change in volume
and the volume was measured. The results are shown in
Table 3.
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Ul 51 r1 U r~l r~l r~l 00 U~ ~I r~l r~l
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t ~ ~ ~¢ r-l t'~l r~l r-l O r~l ~I r-l
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MEE 024 P2 -12-
The results show that the chips destructured in
accordance with t.his disclosure are much bulkier than
those treated by attrition mill, hammermill or roll
press. The hammermill chips are similar to untreated
chips indicating that very little was done to the chips in
hammermilling.
Based on the foregoing studies, it has been
determined that the destructured chips of Fig. l and 2 can
be further characterized, in accordance with the more
preferred embodiments of the invention, as having a
surface area of at least 0.30 m2/g and preferably at
least 0.40 m /g as determined by the dynamic gas flow
technique. Alternatively, and/or additionally, the
preferred chips are characterized by density, as
determined by TAPPI method T258 om-~5, which is 20 to 30%
lower than the untreated chip. The preferred chip
structure can be further characterized by British
Disintegrator Test where less than about 40% of the
disintegrated chips are greater than l.65 mm in size and
preferably less than 20% of the disintegrated chips are
greater than l.65 mm in size.
Chips dest.ructured as described above can be
pulped using conventional or known chemical pulping
processes. In ~raft or soda digestion operations, the
chips are digested with a liquor consisting of sodium
hydroxide alone or in combination with sodium sùlfide. In
acid sulphite operations the cooking liquor is a mixt.ure
of sulphurous acid and sodiurn, magnesium or calcium
sulfite. Alkaline sulEite cooking i.s performed with a
cook:ing liquor containing sodiuln sulEite and sodium
~6a~
MEE 024 P2 -13-
hydroxide In bisulphite operations the cooking liquor is
a mixture of sulphurous acid and sodium bisulfite.
Anthraquinone can be used in these processes to further
improve the pulp. Pulping may be carried out in a batch
or continuous operation. Typical pulping conditions are
temperatures in excess of 130C and pressure of 30 to 80
psig. Chips destructured in accordance with the present
invention are particularly desirable for use in vapor
phase cooking.
As indicated previously, it is not desirable to
reduce Kapp~below about 30 to 40 by cooking. Instead,
Kappa should be reduced to about 50 and then the pulp
should be delignified further under milder conditions. As
a corollary, cooking conditions (temperature and time) are
preferably limited such that the H factor does not exceed
about 400. With conditions (temperature/time) which
provide a higher H factor or lower Kappa, the strength of
the pulp is often reduced. Thus, it is desirable to
further delignify the pulp under conditions which lower
Kappa without sacrificing strength.
It is preferred to further delignify the pulp by
a process such as oxygen delignification which retains
strenyth. Oxygen delignification usually involves the
addition of caustic to the pulp to raise the pH to about
11 and reacting the pulp with oxygen. Oxygen
delignification is well known in the art and is described
in U.S. Patent 3,832,276 to Roymoulik et al; U.S. Patent
2,926,11~ to Grangaard; and U.S. Patent 3,75~,~17 to
Jamieson. In this stage of the process, ~appa is
typically reduced to 15 to 25 and will depend on the end
use.
. .
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MEE 024 P2 14-
After delignification, the pulp is bleached in a
conventional manner. Typically the pulp is chlorinated
using either chlorine, chlorine dioxide, or a mixture of
chlorine and chlorine dioxide, but other processes can
also be used. Bleaching is usually conducted as a
bleaching sequence such as CDED, CDEDED, CED, etc.
The process of the present invention can De used
on softwoods and hardwoods, but preliminary indications
are that little, if any, improvement in yield is obtained
on softwood.
Fig. 4 is a schematic illustration of a pulping
apparatus for use in the present invention. Wood chips
are fed from a hopper bin 10 to a screw press 12 equipped
with a plug former 14 for applying a back pressure to the
chips as they pass through the press 12. The press 12
destructures the chips as shown in Fig. 2 and feeds them
into cornpartment 16 in continuous vapor phase digester
18. Chips fed to compartment 16 are conveyed into the
main digester tank 20 by a feed screw 22. Because the
chips are not completely defiberized as they are fed to
the digester, it is generally necessary to refine the pulp
after it is blown from the digester 1~. For this purpose
a conventional pressurized refiner or defibrator 24 is
used. The resulting pulp is washed and is further
processed, e.y., in oxygen deligniEication and in
bleaching skeps.
Screwpress 12 and plug forrner 14 are operated so
as to partially defiberize the chips. Depending on the
nature of the chips, the screwpress 12 is equipped with a
screw having a compression ratio of 3/1 to 5/1.
rrypically, dense hardwoods can be adequately deskructured
using a screw compression ratio of 3/1. Softwoods and low
5~
MEE 024 P2 -15-
density hardwoods oten require a higher compression
ratio, for example 4~1 or 5/1. Normally the compression
ra~io when feeding chips in conventional processes is
2/1 The back pressure applied by the plug former in the
invention is substantial. Typically it is about 100-200
psi. This compares with 35 to 75 psi pressures normall~
employed while feeding chips into a continous digester.
The partially de~iberized chip appears to be more
susceptible to high temperature than a less destructured
chip. As such, it is desired to use milder cooking
temperatures in the present invention. Temperatures of
about 140 to 160C are preferred depending on the nature
- of the wood and the degree of delignification desired. In
conventional chemical pulping temperatures of 170C or
higher are typically used.
The properties of chemical pulps obtained in
accordance with the present invention are summarized below.
Fig. 5 illustrates the relationship between H
factor and Kappa for a Kraft process in accordance with
the present invention and a conventional process. As
illustrated in Fig. 5, very efficient removal of lignin
takes place in the beginning of the cook in accordance
with the invention. For example, in a Kraft process an H
factor of only 160 is required to lower the Kappa number
2S from an estimated 150 level to 65, or to reduce the lignin
content by about 57% compared to an H factor of ~00
required for conventional chips. Further removal o~
lignin requires much higher levels of H factor, especially
at Kappa levels lower than 30. At Kappa number levels
below 20r the H factor requirements can be almost as high
as those required in conventional cooking. Thus, it is
desirable to terminate the cook at Kappa levels higher
i~ ~L~
5~5
MEE 024 P2 -16-
than 30 preferably at about 50, or at residual lignin
above 5~, and to complete delignification with oxygen and
with conventional bleaching chemicals.
Table 4 compares bleached pulp properties on oak
between a conventional Kraft pulp and a pulp in accordance
with the invention. Bleached viscosity of conventional
cook was very high (31.9 cps). Viscosities of invention
cooks were much lower (15~0 - 15.3). It is suspected that
the oxygen stage before CDED bleaching may have
contributed to lower viscosity. For example, on cook No.
2, bleached viscosity was 21.5 when oxyyen stage was
omitted compared to 15.3 cp when oxygen stage was used.
There was no oxygen stage for the conventional cooks.
Table 4
KRAFT: CONVENTIONAL VS. INVENTION PLUS 2
DELIGNIFICATION CDED BLEACHING SEQUENCE
Type of Cooking Conv. Invention
Cook No. 1 2 3
H Factor 845 401 213
Kappa number, brownstock 16.7 29.6 51.2
NuTober of 2 stages 0 1 2
K No. after 2 Stage 11.9 9.7 11.4
Bleached brightness 89.0 90.2 91.2
PC No. 3.49 2.54 2.61
Viscosity, cps 31.9 15.3 15.0
Refined C.S. freeness~ ml 405 405 390
Drainage time, secs 5.6 5.1 5.1
Bulk, cc/g 1.60 1.68 1.59
Tear factor 103 84 86
Burst factor 34.7 27.0 3].. 9
Tensile, b.l.m. 5800 4670 5000
TEA, fp/ft.2 S.76 5.96 5.83
Loylp MIT fold 1.491 1.204 1.301
Op~clt~ 7~-4 75-3 73-7
100S 3.34 3.54 3.30
PPS @ 101~, Wire 4.6 5.2 5.0
Felt 8.1 8.4 7.8
4~
MEE 024 P2 -17-
Brightness was higher (90.2-91.2 vs. 89.0).
Tear, burst and tensile values were lower by almost 20%
when the cooking was terminated at 30 Kappa, and 10-15%
lower when stopped at 50 Kappa. TEA (tensile energy
absorption) was comparable or slightly higher than for the
conventional pulp. It is believed that the strength
properties of the pulps could be brought close to those o~
conventional pulp with the help of a longer, lower
temperature cook.
Table 5 shows a comparison between oak
conventional and soda AQ pulps in accordance with the
invention. As in the case of Kraft, bleached viscosities
were much lower than those of conventionally cooked pulps
(10.8-13.4 cps vs. 27.6 cps). Poor control in oxygen
stage and higher brightness (88.9-89.4 vs. 85.3) could
have contributed to this result. Strength properties
approach those of conventionally cooked pulp when the cook
is t,erminated at Kappa number of 64 but are lower by
12-27% when the cooking proceeded to Kappa number of 27.~.
~6~S5
MEE 024 P2 -18-
Table 5
SODA AQ: CONVENTIONAL VS. INVENTION PLUS 2
DELIGNIFICATION CDED BLEACHING SEQUENCE
Type of Cooking Conv. Invention
Cook No. 4 5 6
H Factor 1487 398 172
Kappa number, brownstock 18.4 27.4 64.0
Number of 2 stages 0 1 2
K No. after 2 Stage 13.0 9.0 12.4
Bleached brightness 85.3 88.9 89.4
PC No. 4.32 1.36 1.71
Viscosity, cps 27.6 13.4 10.8
Refined C.S. freeness, ml 410 410 400
Drainage time, secs 5.5 5.2 5.1
Bulk, cc/g 1.57 1.75 1.67
Tear factor 97 85 83
Burst factor 33.3 24.4 31.3
Tensile, b.l.m. 5980 4360 5160
TEA, fp/ft 2 5.79 5.29 6.75
Loglp MIT fold 1.556 1.041 1.398
Opaclty 74.8 75.9 73.0
100S 3.39 3.61 3.18
PPS @ 10#, Wire 4.4 5.7 5.4
Felt 8.1 8.0 8.1
Table 6 compares conventional cooking and cooking
in accordance with the invention for regular oak soda.
The invention gives a much lower viscosity (8.4 vs. 16.9)
and a lower pulp strength. One reason is that more than
two times hiyher H factors and thus much higher
t.emperatures were required in soda than in soda AQ and
Kraft cooking to achieve a yiven Kappa number. This
undoubtedly decreases pulp strength. In general, it
appears that the partially de~iberize~ chips used in the
present invention are less suitable for soda than eor soda
AQ or Kraft cooking.
~ ~36~5~5
MEE 024 P2 -19-
Table 6
SODA COOKING: CONVENTIONAL VS. INVENTION PLUS 2
DELIGNI~ICATION CDED BLEACHING SEQUENCE
Type of Cooking Conv. Invention
Cook No. 7 8
H Factor 1878 632
Kappa No., brownstock 18.1 42.5
Number of o2 stages 0
K No. after o2 stage 13.0 12.0
Bleached brightness 83.3 88.7
PC No. 4.86 2.68
Viscosi~y, cps 16.4 8.4
Refined C.S. freeness, ml 410 395
Drainage time, secs. 5.3 5.0
Bulk, cc/g 1.62 1.76
Tear factor 97 75
~urst factor 31.0 22.9
Tensile, b.l.m. 5490 4050
TEA, fp/ft.2 5.41 4.49
Loglo MIT fold 1.301 0.903
Opacity 76.1 77.5
100S 3.56 3.89
PPS @ 10#, wire 4.6 5.5
Eelt 8.1 8.4
The following non-limiting examples further
illustrate the preferred embodiments of this invention and
the advantages obtained thereby. All pulp tests were
performed in accordance with TAPPI standard testing
procedures.
Example 1
Commercial Appalachian hardwood chips containing
a high percentage of oak were partially defiberized in a
Sunds Defibrator's PREX unit while Eeeding a continuous
~2~
MEE 024 P2 -20-
experimental digester at a 0.8 lbs./min. rate (O.D. basis).
Cooking conditions were:
Active alkali 14%
Sulfidity 24.4~
Cooking Time 30 min.
Cooking Temperature lS0C
H Factor 122
Kappa No. 71.5
The pulp was washed and further delignified in
two stages of oxygen treatment and then bleached in three
stages of bleaching (chlorination, extraction and chlorine
dioxide).
Conditions for a comparison cook made in a batch
digester using conventional technology were as follows:
Active Alkali 17.5%
Sulfidity 30%
Time to Temperature 90 min.
Time at Temperature 40 min.
Cooking Temperature 168C
¦ 20 H Factor 717
¦ Kappa Mo. 19.8
The resulting brownstock was bleached in four
bleaching stages (chlorination, extraction, hypochlorite
and chlorine dioxide). Pulp properties were as follows:
Table 1
~ Invention
Bleached Yield, % 45.2 50.6
Pulp Brightness 87.6 90.8
C.S. Freeness, ml 400 400
~ensile Index, Nm/g 51.0 ~7.2
~urst Index, kPam2/g 4.11 4.03
~ear Index, m~.m2/g 8.7 7.3
Tensile EnergyAbsorption, J/m2 56.0 56.3
Apparent Density, g/cc 0.64 0.63
~ 2~S~
MEE 02~ P2 -21-
The results in this table show a bleached pulp
yield gain of 5.4% based on wood, or a 12% gain based on
pulp, compared to the conventional technology. Brightness
is also higher for pulp made according to the ~eachings of
this invention. Other pulp properties appear to be
similar, despite a higher pulp yield and a higher
brightness level of the invention pulp. A major reduction
in H Factor tfrom 717 to 122) iS indicated, which results
in a major saving in energy (steam) during cooking.
~
Northern hardwood chips consisting primarily of
hard and soft maples, aspen and birch were partially
defiberized while feeding a Sunds Defibrator's
experimental continuous digester at a 0.44 lbs/min (O.D.
basis) feeding rate.
Cooking conditions were:
Active Alkali 15.9%
SulEidity 24.4%
Cooking Time 60 min.
Cookiny Temperature 158C
H Factor 380
Kappa No. 45.6
After washing the pulp was further delignified
wi~h oxyyen and bleached in three stayes of conventional
bleaching (chlorinakion, extraction and chlorine dioxide).
A comparison cook representirlg conventional
technology was made in a laboratory bakch digester using
the following conditions:
5~
MEE 024 P2 -22-
Active Alkali 15%
Sulfidity 24%
Time to Temperature 90 min.
Time at Temperature 80 min.
Cooking Temperature 164C
H Factor 891
Kappa No. 15.9
The pulp was washed and bleached in five stages
(chlorination, extraction, chlorine dioxide, extraction
and chlorine dioxide). After refining to a comparable
pulp drainage, the pulp properties were as follows:
Table 2
Conventional Invention
Bleached Yield, ~ 51.4 57.3
Pulp Brightness 87.3 90.0
Drainage Time, Secs. 4.7 4.9
C.S. Freeness, ml 525 480
Burst Index, kPam2/g 2.70 2.22
Tear Index, mN.m2/g 9.14 7.59
Tensile EnergyAbsOrption, J/m2 58.4 61.3
Apparent Density~ g/cc 0.66 0.59
Opacity 75.3 77.4
A gain in bleached pulp yield of 5.9~ based on
wood, or 11.5%, based on pulp, is indicated for the
invention pulp at a higher pulp brightness level. Higher
opacity i8 also evident, despite a higher pulp
brightness. This is apparently due to a bulkier sheet.
Example 3
Oak chips, which were partially defiberized while
. .
. J
MEE 024 P2 -23-
being fed into a continuous Sunds Defibrator pilot plant
digester, were cooked under the following conditions:
Active Alkali 16.1%
Sulfidity 25.4%
Cooking Time 30 min.
Cooking Temperature 165C
H Factor 399
Kappa No. 33.6
The pulp was further delignified with oxygen and
bleached in three stages of conventional bleaching
(chlorination, extraction and chlorine dioxide).
A conventional cook was made on the same chip
supply for comparison using the following conditions:
Active Alkali 17.5
Sulfidity 27~
Time to Temperature 60 min.
Time at Temperature 60 min.
Cooking Temperature 166C
H Factor 845
Kappa No. 16.7
The brownstock from this cook was bleached in
three stages of bleaching (chlorination, extraction and
chlorine dioxide). Both bleached pulps were refined to
several refining levels and handsheets were made. The
interpolated properties were as follows at a common sheet
density level:
~ ~6~5-~D
MEE 024 P2 -24-
Table 3
Conventional Invention
sleached Yield, % 44.8 50.2
PUlp srightness 89.0 89.6
Apparent Density, g/cc 0.625 0.625
Burst Index, kPa m /g 3.~1 3.47
Tear Index, mNm2/g 10.1 7.6
Tensile Index, Nm/g 57.0 54.2
Tensile Energy AbsorPtion~ J/m2 84.1 110.9
MIT Fold Endurance 31 76
Opacity 74.4 72.6
A gain of 5.~% in bleached yield based on wood,
or a 12~ gain based on pulp, is indicated, at a slightly
higher brightness. On the negative side, there was a
reduction in tear strength (from 10.1 to 7.6). However,
this is compensated by significantly improved tensile
energy absorption and fold endurance which are often more
important in papermaking situations than tear strength.
Opacity is slightly lower for the invention pulp but this
is partially due to a slightly higher sheet brightness.
Example ~
To compare the ease of bleaching, a conventional
brownstock and a brownstock according to the teachings of
this invention were produced at comparable Kappa numbers~
These two brownstocks were bleached in a three stage
bleaching se~uence (chlorination, extraction and chlorine
dioxide) using practically identical bleaching
conditions. The bleachiny results were as follows:
~2~ 5`~
MEE 024 P2 -25-
Conventional Invention
Kappa No. before bleaching 23.6 25.4
Pulp Brightness, Initial 77.2 90.0
Brightness after aging in oven,
1 hour at 105C 74.9 88.2
Post Color Number 0.84 0.23
These data indicate much easier bleachability of
brownstock made according to the teachings of the
invention. This is exhibited in a much higher pulp
brightness achieved and in a superior brightness stability
as measured by the Post Color Number. Easier
bleachability could mean chernical savings and a lower
water pollution load from the bleach plant.
Example 5
Two conventional Kraft cooks were made in a pilot
plant batch digester while holding chips in a wire
basket. The cooking conditions were as follows:
Low Kappa High Kappa
Cook Cook
Active Alkali, % 18.5 13
Sulfidity 25.3 25.3
Time to Temperature, min 90 90
Time at Temperature, min ~0 ~0
Cooking Temperature, C 169 159
H Factor 771 339
Kappa No. 23.6 70.2
~fker relieving the pressure, the wire basket
with the chi~.s was removed from the digester and the chips
were defiberized in an 8 inch Bauer mill at 0.008" plate
clearance using hot (approx. 80C) waker at a rate of 6
MEE 024 P2 -26-
liters/minute. After washing the high Kappa cook was
additionally delignified in two ~tages of oxygen treatment
to a Kappa number of 20 . 9. No oxygen treatment was done
on low Kappa pulp. Both pulps were bleached in a four
stage bleaching sequence (chlorination, extraction,
hypochlorite and chlorine dioxide). The results were:
Low Kappa High Kappa
Cook Cook
Bleach Yield, % 46.3 45.8
Initial Brightness 87.6 89.7
Data indicated that there was no gain in bleached
pulp yield when a Kraft cook was stopped at a Kappa No. as
high as 70 and oxygen was used for further delignification
to a bleachable range. This suggests that the higher
yields observed with our technology are primarily due to
chip defiberizing prior to cooking and not due to the use
of oxygen delignification.
Having described the invention in detail and by
reference to preferred embodiments thereof, it will be
apparent that modifications and variations are possible
without departing from the scope of the invention defined
in the appended claims.
What is claimed is:
