Note: Descriptions are shown in the official language in which they were submitted.
W094/08087 ~ 1~ 4 4 ~ 3 PCT/US93/09230
IMPROVED BLEACHING
OF HIGH CONSISTENCY LIGNOCELLULOSIC PULP
BACKGROUND OF THE INVENTION
Field of the Invention
This invention pertains to methods for
bleaching lignocellulosic pulp having a high
consistency. More particularly, this invention
pertains to methods for bleaching lignocellulosic
pulp with a gaseous bleaching agent which require
reduced amounts of bleaching agent and yield pulps
having higher viscosity, higher pulp strength, and
higher GE brightness. The method comprises
fluffing the lignocellulosic pulp to a specific
surface area of at least about 90m2/kg prior to
contacting the pulp with the gaseous bleaching
agent. The invention also pertains to bleached
lignocellulosic pulps prepared by the improved
method.
.
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W094/08087 2 ~ 3~ 3 3 ` PCT/US93/09230
Description of the R~k~round
Wood is composed of two main components - a
fibrous carbohydrate or holocellulosic component
and a non-fibrous component called lignin.
Holocellulose is composed of about 70% alkali-
insoluble alpha-cellulose and 30% alkali-soluble
hemicellulose. The non-fibrous lignin component is
a three-~;m~ional polymeric material consisting
mainly of phenylpropane units.
For use in paper-making processes, wood must
be converted to pulp. During chemical pulping, the
cellulosic fibers are separated from one another in
a manner to preserve the inherent fiber strength
and to remove as much lignin as possible. In a
chemical pulping process, wood is digested with
chemical solutions to solubilize and remove a
portion of the lignin. Examples of chemical
pulping procedures are the soda (sodium hydroxide),
sulfite, and kraft processes.
The basic and modified kraft processes are the
principle chemical processes utilized in paper
manufacturing because these processes do not
significantly degrade the cellulosic component of
the wood. The basic kraft process involves
digesting wood chips in an aqueous solution of
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W094/08087 ~1 4 ~ ~ 3 3 PCT/US93/09230
sodium hydroxide and sodium sulfide to form a high
strength pulp. The basic kraft process is
described in detail in Handbook for Pulp & Paper
Technologists, Chapter 7, Kraft Pulping (TAPPI,
U.S.A.).
The modified kraft processes are even milder
than the basic kraft process on the cellulosic
component and yield even higher strength pulp. The
modified kraft processes, also known as "extended
delignification" pulp processes, either involve
adding the pulping chemicals in a specific
sequence, adding the pulping chemicals at different
locations in the digestion apparatus, adding the
pulping chemicals at different time periods, or
removing and reinjecting liquors in a prescribed
sequence to remove lignin and reduce attack on the
cellulosic fibers. The kraft-AQ process involves
adding small amounts of anthraquinone to the
pulping liquor to accelerate delignification and
limit attack upon the cellulosic fibers. A variety
of additional extended delignification techniques
are known in the art and include Kamyr Modified
Continuous Cooking (MCC), described by V.A.
Kortelainen and E.A. Backlund in TAPPI, vol. 68
(11), 70 (1985); Beloit Rapid Displacement Heating
SUBSTITUTE SHEET l~RULE 26)
W094/08087 2 1 ~ 4 4 3 3 PCT/US93/09230
(RDH), described by R.S. Grant in T~PPI, vol. 66
(3), 120 (1983); and Sunds Cold Blow Cooking,
described by B. Pettersson and B. Ernerfeldt in
Pul~ and Paper, vol. 59 (11), 90 (1985).
The pulp formed after digestion of the wood is
generally a dark colored slurry of cellulosic
fibers known as "brownstock". The dark color of
the brownstock is caused by chromophoric groups in
the lignin remaining in the pulp formed during the
digestion period. This dark lignocellulosic pulp
may be used directly in the paper making operation
if paper color is not important and may be bleached
to a brightness consistent with the planned
utilization of the pulp. Prior to bl~ch;ng, the
pulp is generally transferred to a blow tank to
relieve pressure and to separate the pulp material
as a fibrous mass. The fibrous mass is then washed
to remove residual chemicals and soluble materials
such as lignin degradation products.
To lighten the color of the brownstock pulp
and make it suitable for use in printing, writing,
or other white paper applications, the lignin
remaining in the pulp must be chemically removed or
converted into colorless compounds by bleaching and
~rightening. Pulp bleaching is generally a multi-
SVBSTITUTE SHEET tRULE 26)
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stage process employing chlorine-containing
compounds such as calcium hypochlorite, sodium
c hypochlorite, elemental chlorine, or chlorine
dioxide. Bleaching of lignocellulosic pulp using
chlorine cont~;ning compounds is well known in the
art and is discussed in detail in United States
patent no. 1,957,937, issued to Campbell et al.;
United States patent no. 2,975,169, issued to
Cxanford et al., United States patent no.
3,462,344, issued to ~r;n~ron et al.; and Handbook
for Pulp and Paper Technologists, Chapter 11:
Bleaching (11.3) (TAPPI, USA).
The following letter codes will be used to
describe chemical reactants and process steps
employed in paper making.
C = Chlorination - Reaction with
elemental chlorine
in acidic medium.
E = Alkaline - Dissolution of
Extraction reaction products
with NaOH.
Eo = Oxidative - Dissolution of
Alkaline reaction products
Extraction with NaOHand
oxygen.
D = Chlorine - Reaction with
Dioxide Cl02 in acidic
medium.
P = Peroxide - Reactionwith
peroxides in
alkaline medium.
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21~4433 ~'0 9.1/08087 PCI~/I'S93/09230
o = Oxygen - Reaction with
elemental oxygen in
alkaline medium.
O, = Modified - Uniform alkali
oxygen treatment of low to
medium consistency
pulp followed by
reaction of high
consistency pulp
with oxygen.
Z = Ozone - Reactionwith
ozone.
Z = Modified - Uniform reaction with
Ozone ozone.
C/D = - Admixtures of
c h l o r i n e a n d
chlorine dioxide.
H = Hypochlorite -- Reaction with
hypochlorite in an
alkaline solution.
Although chlorine and chlorine containing
compounds are effective bleaching agents, chlorine
is difficult to handle and is hazardous to
personnel and machinery. Furthermore, the
effluents from chlorine bleaching processes contain
large amounts of chloride by-products which readily
corrode paper making equipment and chlorinated
compounds which can pose environmental concerns.
Chloride ion build-up prevents the recycling of
washer filtrate in a closed system operation unless r
expensive recovery operations are employed.
SUBSTITUTE SH EET (RULE 26)
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As a consequence, chlorine-containing
bleaching agents have been replaced with non-
chlorine- containing bleaching agents such as
oxygen and hydrogen peroxide. The use of oxygen
permits the recycling of the effluent and permits
a substantial reduction in the amount of elemental
chlorine used. A number of processes for bleaching
and delignifying pulp with oxygen have been
proposed, such as those described in United States
patent no. 1,860,432, issued to Richter, United
States patents nos. 2,926,114 and 3,024,158, issued
to Çrangaard et al., United States patent no.
3,274,049, issued to Çaschke et al., United States
patent no. 3,384,533 issued to Meylan et al.,
United states patent no. 3,251,730, issued to
Watana~e, United States patent no. 3,432,282,
issued to ~erolle et al., United States patent no.
3,661,699, issued to Farley, United States patent
no. 4,619,733, issued to Kooi; and P. Christensen
in "~leaching of Sulphate Pulps with Hydrogen
Peroxide", Norsk Skogindustri, 268-271 (1973).
Alkaline pretreatment of pulp prior to oxygen
delignification is suggested by United States
patent no. 4,806,203, issued to Elton and United
States patent no. 5,085,734, issued to Çrigqs. A
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W094/08087 2 ~ ~ ~ 433 PCT/US93/09230 -
method using oxygen as a first stage bleaching
agent to solubilize a major amount of lignin and
chlorine as a second stage bleaching agent to
remove the remaining lignin is described by P.
Christensen, ~Bleaching of Sulphate Pulps with
Hydrogen Peroxide", Norst Skogindustri, 268-271
(1973)-
The use of oxygen as a bleaching agent is not,
however, completely satisfactory. For example,
oxygen is not as selective a delignification agent
as elemental chlorine and only a limited number of
oxygen delignification reactions can be carried out
until the cellulosic fibers are attacked. In
addition, the lignin remaining after oxygen
delignification has typically been removed by
chlorine bleaching to obtain a fully-bleached pulp
using reduced amounts of chlorine. Even at reduced
chlorine concentrations, however, the corrosive
chloride by-products soon reach unacceptable
concentration levels in a closed cycle operation.
Ozone has also been used as a bleaching agent
for pulp. The exceptional oxidative properties of
ozone and its relatively high cost, however, have
limited the development of satisfactory ozone
bleaching processes. Ozone readily reduces the
SUBSTITUTE SHEET (RULE 26)
W09~/08087 2 1 4 4 ~ 3 3 PCTlUS93J09230
lignin in pulp but it also aggressively attacks the
cellulosic fibers to reduce the strength of the
pulp. The reactivity and sta~ility of ozone is
also extremely sensitive to reaction conditions
such as pH changes.
United States patent no. 2,466,633, issued to
Brabender et al., describes a bleaching process
wherein ozone is passed through a pulp having a
moisture content of between 25% and 55% and a pH in
the range from 4 to 7. N. Liebergott, "Paprizone
Treatment, A New Technique for Brightening a
Strengthening M~ch~n;cal Pulps" describes a single
stage bleaching process which combines peroxide and
ozone in a synergistic combination to brighten and
strengthen mechanical pulp. United States patent
no. 4,196,043, issued to Sinqh, discloses a multi-
stage bleaching process characterized by from one
to three ozone bleaching stages and a final
treatment with alkaline hydrogen peroxide, each
stage being separated by an alkaline extraction.
United States patent no. 4,372,812, issued to
Philli~s et al., discloses a multi-stage bleaching
process which comprises an oxygen bleaching step,
a peroxide bleaching step, and an ozone bleaching
step. United States patent no. 4,468,286, issued
SUBSTITUTE SHEET (RULE 26)
214~33
W09~t08087 PCT/US93/09230
to Johnsen, discloses a method for bleaching pulp
with ozone in a multi-path system.
European patent application no. 308,314, to
Coste et al., discloses a method for treating a
lignocellulosic material with ozone which includes
reducing a mechAnical pulp to a state of division
sufficient to assure r~ir~l interface between the
pulp and the ozone.
United States patent no. 4,278,496, issued to
Fritzvold, discloses a bleaching process which
comprises treating a finely divided pulp with ozone
initially at a low pH value then later at a high pH
value. The pulp is preferably processed before
ozone treatment to a "light and fluffy
consistency".
C.A. Lindholm, Paper ja Puu, (68)4, 1986 283-
290, discusses homogeneous bleaching and states
that "an open well-fluffed pulp is needed to enable
ozone to come into contact with the water layers
surrounding the single fibres." C.A. Lindholm,
Proc. Int. Pulp Bleaching Conf., vol. 2, Stockholm,
l991; 1-17, discusses consistency in ozone
bleaching and states that the pulp must be "fluffed
in order to expose the surface of the single fibres
to the ozone-containing gas".
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A. R. Proctor, Pulp Pa~er Maaazine Canada,
75(6), 1974; T210-214, states that "[t]rial
experiments showed that pulp fluffing had no
significant effects on sheet properties.
A comparison of the ozone bleaching of medium
consistency pulp and high consistency pulp showed
that the medium consistency pulp had lower tensile
strength and higher tear strength because of fiber
deformation. For high consistency bleaching, part
of the pulp was fluffed in a refiner, E. Oltmann et
al., Heft 7 341-350, 343 (1992).
European patent application no. 492,039
discloses a method for delignifying cellulosic
material having a consistency of about 25% to about
45~ which comprises the steps of pneumatically
conveying the cellulosic material through a path in
contact with ozone. European patent application
no. 492,040 discloses a method for delignifying
cellulosic material having a consistency of about
30~ to about 45% which comprises the steps of
tumbling the cellulosic material through a path in
contact with ozone to keep the material loose and
homogeneous with a high surface area to volume
ratio.
SUBSTITUTE SHEET (RULE 26)
WO9~/08087 ~1 4 ~ ~ ~ 3 PCT/US93/09230
Despite efforts carried out in this area, no
satisfactory commercial high-consistency process
for the manufacture of ozone bleached
lignocellulosic pulp from softwood and related
pulp, especially southern softwood, has been
disclosed. Accordingly, methods for bleaching
lignocellulosic pulp which re~uire reduced amounts
of bleaching agent and yield pulps having higher
viscosity, higher pulp strength, and higher GE
brightness are highly desirable. The present
invention provides such novel bleaching methods
without the disadvantages characteristic of
previously known methods.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a graph showing the effect of
fluffing on the ozone bleaching of kraft/AQ-O pulp
with a nominal consistency of 42%. Ozone
consumption is plotted versus ISO brightness.
Figure 2 is a graph showing the effect of
fluffing on the ozone bleaching of kraft/AQ-O pulp
with a nominal consistency of 42~. ISO brightness
is plotted versus intrinsic viscosity.
Figures 3A and 3B are graphs showing the
effect of fluffing on the ozone bleaching of
SUBSTITUTE SH EET (RULE 26)
W094/08087 2 1 4 4 ~ 3 3 PCT/US93/09230
kraft/AQ-O pulp with a nominal consistency of 42%.
In Figure 3A, aerodynamic specific surface area is
plotted versus intrinsic viscosity. In Figure 3B,
aerodynamic specific surface area is plotted versus
ozone consumption.
Figure 4 is a graph showing the effect of
fluffing on the ozone bleaching of kraft/AQ-O pulp
with a nominal consistency of 42%. Tensile index
is plotted versus tear index.
SUMMARY OF THE lN Vk.. llON
The present invention pertains to a method
having increased selectivity for bleaching
lignocellulosic pulp having a high consistency from
a first GE brightness to a second, higher GE
brightness which comprises the steps of (a)
fluffing the high consistency pulp having the first
GE brightness to a specific surface area of at
least about 90mZ/kg; and (b) contacting the fluffed
pUlp with a gaseous bleaching agent to form a
bleached pulp having the second, higher GE
brightness.
The present invention also pertains to a
bleached lignocellulosic pulp having a high
consistency prepared by a method having increased
SUBSTITUTE SHEET (RULE 26)
WO9~/08087 21 ~ ~ ~ 33 PCT/US93/09230
selectivity for bleaching lignocellulosic pulp
which comprises the steps of (a) fluffing a high
consistency pulp having a first GE brightness to a
specific surface area of at least about 90m2/kg;
and (b) contacting the fluffed pulp with a gaseous
bleaching agent to form a bleached pulp having a
second, higher GE brightness.
DF~ATT.l;~n DF~ 'RTPTION OF ~IE lNVhr.llON
Applicants have discovered an improved method
for bleaching high consistency lignocellulosic
pulp. By fluffing lignocellulosic pulp to a
specific surface area of at least about 90m2/kg
prior to bleaching the pulp, applicants have found
that the bleaching agent consumption during the
bleaching step of the pulp can be reduced. Pulps
bleached according to the method of the present
invention have a higher pulp viscosity, a higher
pull strength, and a higher GE brightness than
conventionally bleached pulps. The bleaching
method of the present invention may be employed to
bleach chemical or mechanical pulps and may be
followed by other bleaching steps. The chemical
requirement in later bleaching steps of the pulp is
also reduced. Applicants believe that fluffing
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~ W094/08087 21 4 ~ ~33 PCT/US93/09230
high consistency lignocellulosic pulp to a specific
surface area of at least about 90m2/kg increases
the surface area of the pulp for better contact
with the bleaching agent and thereby increases the
rate, uniformity, and efficiency of the bleaching
reaction. The enhanced uniformity of the bleaching
reaction results in less bleaching agent
consumption by the pulp and more selective reaction
of the bleaching agent with the lignin and less
degradation of the cellulosic fibers. Pulps
bleached according to the selective method of the
present invention have a higher pulp viscosity at
a given brightness and a higher brightness at a
given ozone consumption.
The present method for fluffing high
consistency lignocellulosic pulp prior to bleaching
the pulp may be carried out in a refiner fluffer or
other type of fluffer to expose individual fibers
which were in the interior of a floc to a gaseous
bleaching agent while leaving the cell walls
intact. The fluffing process (or refiner fluffing
process) is a low energy process employing a large
gap clearance in the fluffer to physically separate
but not break or ~ech~nically modify the individual
fibers. As shown below, because the individual
SUBSTITUTE SH EET ~RULE 26)
~14~33
W094/08087 - PCT/US93/09230
fibers are not physically modified, fluffing can
increase the surface area of the pulp without
substantially changing pulp freeness (pulp
drainability).
EY~pl-- Pulp rr~--n~ CSE~ )
~nrlurf-d Pulp ~7~1
rlU~r-d PUlp ~2 0~- pl~t- g~p~ 750~1
rlUfl~-d Pl~lp (0 8~D pl--t~ p) 7~
I?luf~d Pulp ~0.5~ pl~to ~p) 7S9~1
The present method for fluffing high
consistency lignocellulosic pulp prior to breaching
the pulp is distinct from the refining process
which is a process generally employed after
bleaching and before papermaking. The refining
process is a high energy process employing a small
gap clearance to physically or mechanically modify
the individual fibers by removing the primary cell
wall, introducing cracks and fissures into the
remaining primary and secondary cell walls,
fibrillating, and introducing other imperfections
into the fiber, G.A. Smook, Nandbook For Paper &
Paper Technologist, TAPPI (1982), p. 183. The
fibrils and other imperfections serve to increase
the chemical and physical bonding sites between the
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W O 94/08087 2 1 4 4 4 3 3 P(~r/US93/09230
fibers during the webbing step to form the paper
sheet. The refining of mechanical pulp also
results in mechanical modification of the fibers;
mechanical pulping causes fiber cutting and
breakage while thermomechanical pulping causes
rupture and break-up of the cell walls, G.A. Smook,
Handbook For Paper and Paper Technologist, TAPPI
(1982), p. 55-56. Because the individual fibers
are modified and fines are created, refining
decreases pulp freeness, G.A. Smook, Handbook For
Paper and Paper Technologist, TAPPI (1982), p. 189.
While refining improves the papermaking step,
refining exposes the internal areas of the fiber
making them susceptible to chemical attack, such as
acid hydrolysis, to produce weaker fibers, Hartler
et al., Svensk Papperst., 63, 263-271 (1960),
Bausch et al, Svensk Papperst., 63, 279-285 (1960),
Stone et al., Pulp Paper Mag. Can., 59(6), 165-173
(1958), Stone et al., Pulp Paper Mag. Can., 62(6),
317-326 (1961). The present method for fluffing
high consistency lignocellulosic pulp does not
include physically or mechanically modifying the
individual fibers because refining prior to
bleaching the pulp would be expected to produce
wea~er fibers.
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W09~/08087 PCT/US93/09230
3 3
18
In accord with the present invention, the
method of having increase selectivity for bleaching
high consistency lignocellulosic pulp includes
fluffing the high consistency pulp without
substantially changing the pulp freeness. Without
substantially changing the pulp freeness of the
pulp means that the fluffing step is carried out
with minimal mechanical modification of the fi~ers
of the pulp so as not to cause a deterioration in
the quality of the pulp after the following
bleaching step. In a preferred embodiment, the
pulp freeness of the fluffed pulp is changed less
than about 15%, more preferably less than about
10%, and most preferably less than about 5%.
Throughout this specification, the following
definitions will be used. The definitions are
based on those found in Rydholm, Pulping Processes,
Intersciences Publishers, 1965, pages 862-863 and
TAPPI Monograph No. 27, The Ble~ching of Pulp,
Rapson, Ed., The Technical Association of Pulp and
Paper Industry, 1963, pages 186-187.
The term "consistency", as used herein, means
pulp concentration and refers to the amount of pulp
fiber in a slurry expressed as a percentage of the
weight of the oven dried fiber over the total
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W094/08087 ~ 4 ~ 3 3 PCT/US93/09230
19
weight of the fiber and water. The consistency of
a pulp will depend upon the type of dewatering
equipment used.
The term "low consistency", as used herein,
refers to concentration ranges of pulp up to about
6%, and preferably up to about 5~. Low consistency
pulp is a suspension that is pumpable by an
ordinary centrifugal pump and is produced using
deckers and filters without press rolls.
The term "medium consistency", as used herein,
refers to concentration ranges of pulp between
about 6% and 20%. Below about 15%, medium
consistency pulp can be produced by filters. This
level of consistency is the consistency of the pulp
mat leaving a vacuum drum filter in the brownstock
washing system and the bleaching system. The
consistency of a slurry from a washer, either a
brownstock washer or a bleaching stage washer, is
about 9% to 15%. Above about 15%, medium
consistency pulp can be produced by press rolls.
Rydholm states that the usual range for medium
consistency pulp is from about 10% to 18% and
Rapson states that the usual range for medium
consistency pulp is from about 9~ to 15%. Medium
consistency pulp is pumpable by special machinery.
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The term "high consistency", as used herein,
refers to concentration ranges of pulp between
about 20% and about 65%. Rydholm states that the
concentration range of high consistency pulp is
from a~out 25% to 35% and Rapson states that the
concentration range of high consistency pulp is
from about 20% to 35%. These high consistency
pulps are obt~in~hle only by the use of presses.
The liquid phase is largely absorbed by the fibers
and the pulp can be pumped only very short
distances.
The term "pulping", as used herein, is used in
its conventional sense to refer to digestion of
lignocellulosic material to form brownstock.
Pulping methods include, for example, kraft, the
kraft-AQ process, and other forms of extended
delignification.
The term "modified kraft process", as used
herein, refers to extended delignification
processes and all other modified kraft processes
with the exception of the kraft-AQ process. The
kraft-AQ process has achieved a special status and
acceptance in the art and is separately known by
that name. The oxygen delignification step
following pulping is not an extended
SUBSTITUTE SHEET ~RULE 26)
W094/08087 ~ 3 3 PCT/US93/09230
delignification method but is rather a first
delignification step for bleaching or brightening
the pulp.
There are two principal types of measurements
to determine the completeness of the pulping or
bleaching process. The measurements are referred
to as the "degree of delignification" and the
"brightness" of the pulp. The degree of
delignification is normally used in connection with
the pulping process and the early bleaching stages
because it is less precise when only small amounts
of lignin are present in the pulp such as in the
later bleaching stages. The brightness factor is
normally used in connection with the bleaching
process because it is more precise when only small
amounts of lignin are present and the pulp is
lightly colored and highly reflective.
There are many methods for measuring the
degree of delignification but most are variations
of the permanganate test. The normal permanganate
test provides a permanganate number of "K no."
which is the number of cubic centimeters of tenth
normal potassium permanganate solution consumed by
one gram of oven dried pulp under specified
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~'O9~/08087 ~4~33 PCT/US93/09230
22
conditions. The K no. is determined by TAPPI
Standard Test T-214.
There are also a number of methods for
measuring pulp brightness. Pulp ~rightness is a
measure of reflectivity and its value is expressed
as a percent of a scale. A standard method for
determining pulp brightness is GE brightness (GEB)
which is expressed as a percentage of a maximum GE
brightness. GE brightness is determined by the
TAPPI Official Method T-452.
A second method for determining pulp
brightness is ISO brightness (ISO) which is
expressed as a percentage of maximum ISO
brightness. ISO brightness is determined by the
TAPPI Official Method T-525.
The viscosity of a wood pulp is a measure of
the degree of polymerization of the cellulose
chains which make up the individual wood fibers.
A standard method for determining pulp viscosity is
cupriethylendiamine ("CED") viscosity. CED
viscosity, which will be referred to simply as
viscosity, is expressed in units of centripoise
(cp). Viscosity is determined by the TAPPI
Official Method T-230.
SUBSTITUTE SH EET (RULE 26)
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A second method for measuring pulp viscosity
is intrinsic viscosity which is expressed in units
of dm3/kg. Intrinsic viscosity is determined by
the ASTM Official Method D-1795.
In accord with the present invention,
lignocellulosic pulp is fluffed to a specific
surface area of at least about 90m2/kg prior to the
pulp bleaching step. The specific surface area of
the pulp should be at least about 90mZ/kg to
provide sufficient pulp surface area to optimally
react with the bleaching agent and augment the rate
of the bleaching reaction. In general, the greater
the specific surface area of the lignocellulosic
pulp, the lower the bleaching agent consumption by
the pulp during the bleaching step and the greater
the pulp viscosity, pulp strength, and GE
brightness of the resulting pulp. In a preferred
embodiment, the lignocellulosic pulp is fluffed to
a specific surface area of at least about lOOm2/kg,
more preferably at least about l20m2/kg, and most
preferably at least about 180m2/kg. In another
preferred embodiment, the lignocellulosic pulp is
fluffed to a specific surface area from about
lOOm2/kg to about lOOOm2/kg, more preferably from
about l20m2/kg to about 500m2/kg, and most
SUBSTITUTE SHEET (RULE 26)
~V094/08087 2 1 ~ ~ 4 3 3 PCTJUS93/09230
preferably from about l80m2/kg to about 350mZ/kg.
The woods which may be employed in the present
invention include both hard and soft woods. In one
embodiment, the lignocellulosic pulp is prepared
from a hard wood. In another embodiment, the
lignocellulosic pulp is prepared from a soft wood.
Preferably, the lignocellulosic pulp is prepared
from soft wood. More preferably, the
lignocellulosic pulp is prepared from pine. Most
preferably, the lignocellulosic pulp is prepared
from kraft/AQ-O pine feedstock. In yet another
embodiment, the lignocellulosic pulp is prepared
from a mixture of hard and soft wood.
In general, any fluffing apparatus which will
fluff the high consistency pulp to a specific
surface area of at least about 90m2/kg may be used.
Examples of fluffers which may be employed include
the refiner fluffer and the pin fluffer. In a
preferred embodiment, the fluffer is a refiner
fluffer. The apparatus useful in accordance with
the present invention comprises fluffing
apparatuses well known in the art and therefore the
selection of the specific apparatus will be
apparent to the artisan.
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~'094/08087 ~ 3 3 PCT/US93/09230
The lignocellulosic pulp in the present
invention is a pulp having a high consistency. In
a preferred embodiment, the consistency of the pulp
is from about 20% to about 65%, more preferably
from about 28% to about 55%, and most preferably
from about 35% to about 48%.
The lignocellulosic pulp in the present
invention may be a chemical pulp or a mechanical
pulp. Preferably, the lignocellulosic pulp is a
chemical pulp. The bleaching method of the present
invention may also be employed in the final
bleaching step of a pulp.
The bleaching agent for the lignocellulosic
pulp in the present invention is a fast reacting
gaseous bleaching agent. In general, the more
reactive the bleaching agent employed, the greater
the reduction of the bleaching agent requirement of
the bleaching reaction and the greater the
improvement in the viscosity, strength, and GE
brightness of the product pulp. Gaseous bleaching
agents are well known in the art and include, for
example, ozone, oxygen, and chlorine-containing
compounds such as elemental chlorine and chlorine
dioxide. The preferred gaseous bleaching agent is
ozone. For convenience, ozone will be referred to
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as the gaseous bleaching agent throughout this
specification.
A typical pulping, delignifying, and bleaching
multi-stage process includes the following steps:
(a) pulping the lignocellulosic material
and recovering the pulping chemicals;
(b) washington the pulp to remove
chemical residues and residual lignin, and
screening the pulp to remove fiber bundles;
(c) delignifying the pulp with alkaline
oxygen (i.e., o or o,);
(d) washing the partially delignified
pulp to remove dissolved organics, screening the
pulp, and recycling a portion of the effluent;
(e) chelating and acidifying the pulp to
bind metal ions and adjusting the pH level;
(f) thickening the pulp to high-
consistency;
(g) contacting the pulp with a gaseous
bleaching agent such as ozone (i.e., Z or Z,) to
further delignify and partially bleach the
material;
(h) washing the partially bleached pulp
and recycling a portion of the effluent;
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(i) extracting the pulp with alkali to
remove residual lignin;
(j) washing the extracted pulp and
recycling a portion of the effluent;
(k) contacting the pulp with a second
bleaching agent (i.e., D or P) to brighten and
bleach the pulp;
(l) washing the bleached pulp to obtain
a bleached product having a GE brightness from
about 70% to about 90~; and
(m) recycling a portion of the effluent
from the P bleaching stage or the D bleaching
stage.
Throughout this application, various
publications have been referenced. The disclosure
in these publications are incorporated herein by
reference in order to more fully describe the state
of the art.
The present invention is further illustrated
by the following examples which are not intended to
limit the effective scope of the claims. All parts
and percentages in the examples and throughout the
specification and claims are by weight of the final
composition unless otherwise specified.
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Examples 1-8
These examples demonstrate the improvement in
high consistency ozone bleaching as a result of
employing a fluffed pulp with a suita~le specific
surface area.
Pine feedstocks were fluffed using a refiner
fluffer, two pin fluffers, an impact fluffer, and
a laboratory hammer mill. In addition, pulp which
had been coarsely shredded but not fluffed was also
used. The pulp was bleached using a range of ozone
applications and selected ozonated pulp was
extracted and subjected to two chlorine dioxide
bl~ching stages.
A kraft/AQ-O pine feedstock, having a GE
brightness of 33.7%, a viscosity of 15.9 cp, and a
K. no. of 7.7, was employed. The pulp was diluted,
acidified and chelated, and then pressed. The high
consistency press mat was directed to a shredder
conveyor and then collected for later fluffing.
Refiner fluffing was carried out using a Sunds
Defi~rator 500 mm pilot refiner fluffer e~uipped
with type 5120 refiner plates, and the spacing
between the plates was between 0.5 mm and 6.0 mm.
Pin fluffing was carried out using a production
scale pin fluffer. Fluffing was also performed
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using laboratory equipment, including a modified
blender, and a laboratory pin fluffer.
The degree of fluffing for each of these pulps
was measured using the aerodynamic resistance
method described by R.G. Garner and the R.~.
Kerekes in Pul~ Paper Canada Transactions, yol. 79
(9), TR82 (1978), and using the data analysis
procedure described by A.A. Robertson and S.G.
Mason in Pulp Pa~er Maaazine Canada, vol. 50 (12),
103 (1949). Table A shows the results obtained for
several pulps tested in this study. The specific
surface areas ranged from as low as 34 m2/o.d. kg
pulp for relatively unfluffed pulp (shredder) to as
high as 226 m2/o.d. kg for well fluffed pulp
(refiner).
The pulps were then ozone bleached using a
range of ozone consumption levels. Figure 1 shows
the results plotted as ozone consumption versus`GE
brightness. The best refiner fluffed pulp required
only 4.8 kg 03/ODMT (ODMT - o.d. metric ton) to
reach 53% ISO brightness, while the shredder pulp,
which had the worst fluffing, required over 30%
more ozone (about 6.5 kg 03/ODMT). Figure 2 shows
the results plotted as GE brightness versus
intrinsic viscosity. Figure 2 shows that the
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refiner pulp retained the highest viscosity at a
given brightness, while the shredder viscosities
were among the lowest.
For each of the pulps, a linear regression of
the data in Figures 1 and 2 was performed to
calculate the ozone consumption and pulp viscosity
at 53% ISO brightness. The results are plotted in
Figures 3A and 3B as a function of the aerodynamic
specific surface, where it can be seen that an
increase in the specific surface has the beneficial
effect of reducing the amount of ozone required to
reach 53% ISO brightness, and increasing the
resulting pulp viscosity.
Examples 9-12
These examples further demonstrate the
relationship between the degree of pulp fluffing
and optimal high consistency ozone bleaching.
A K/AQ-O pulp was fluffed in four different
ways using methods described above. The pulp had
initial GE brightness of 33.7~, an 8.0 K no., and
16.2 cp viscosity. After fluffing, the specific
surface was determined for each pulp. The refiner
pulp had the highest specific surface, 166 m2/kg,
and the shredder pulp had the lowest specific
surface, 43 m2/kg. The fluffed pulps were bleached
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using ozone to approximately the same GE
brightness. The results are shown in Table B. The
best fluffed pulp (refiner) had the highest GE
brightness, the highest viscosity, and the lowest
ozone consumption after ozone bleaching, while the
poorest fluffed pulp (shredder) had a lower
brightness, the lowest viscosity, and the highest
ozone consumption.
Each of the ozone bleached pulps was then
extracted and bleached with two chlorine dioxide
bleaching stages, and the results are shown in
Table B. The pulp which had been refiner fluffed
prior to ozone bleaching, and had the highest
viscosity after ozone bleaching, maintained the
highest viscosity after subsequent bleaching to a
GE brightness of about 89~, and re~uired the
smallest amount of chlorine dioxide to reach this
brightness level. In comparison, the pulps which
were not fluffed as well had lower viscosities and
required more chlorine dioxide to reach the same
brightness.
The strength of these four pulps was
- evaluated. A PFI mill was used and the handsheets
were prepared according to the standard Tappi
procedures T-205 and T-248. Figure 4 shows the
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tear index plotted versus tensile index. Comparing
tear index at a given tensile index, the refiner
pulp is 10-20% higher than the shredder pulp, and
5-10% higher than the pulps which had been fluffed
using the pin and impact fluffers.
These results show a benefit for refiner
fluffing over other types of fluffing in terms of
reduced ozone requirement, reduced chemical
requirement for final bleaching, and improved pulp
viscosity and strength. A comparison of the
estimated relative costs of a refiner fluffer, a
pin fluffer, and impact fluffer, and shredder in
terms of capital and operating costs was made. The
reduction in ozone requirement offsets the
increased capital required for a refiner fluffer
resulting in essentially equal costs for all
methods ex~m;ned. The refiner fluffer results in
a stronger product at approximately the same cost.
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Table A
Fluffing Characterization
By Aerodynamic Resistance Method
Example Fluffer Specific
Surface Area
(theta,
m2/kg )
K/AQ-O pulp, nominal 42% consistency
1 Shredder
34
2 Impact fluffer
66
3 Pin fluffer
4 Refiner, plate gap A
226
Refiner, plate gap B
225
6 Refiner, plate gap C
150
7 Laboratory pin fluffer
132
8 Laboratory blender
204
Table B
Effect of Degree of Fluffing
on Ozone and Subsequent Bleaching
Fluffing
Fluffer Shredder Impact Pin
Refiner
Specific surface area 43 66 85
166
( m2/kg )
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Z Stage
Ozone consumption 5.9 4.3 3.5
2.9
(kg/ODMT)
GE Brightness (%) 54.5 52.9 53.9
55.4
Viscosity (cp) 9.5 10.6 10.4
10.6
E Stage
GE Brightness (%) 63.7 59.2
58.4 61.5
First D Stage
Chlorine dioxide 7.5 7.0
7.0 5.5
consumption (kg/ODMT)
GE Brightness (%) 86.9 86.3
86.4 86.7
~cQn~ D Stage
Chlorine dioxide 1.6 3.7
3.7 3.2
consumption (kg/O~MT)
Total chlorine dioxide 9.1 10.7
10.7 8.7
consumption (kg/ODMT)
GE Brightness (%) 88.3 89.8
89.9 89.2
Viscosity (cp) 8.5 9.5
9.3 10.0
Summary
The refiner fluffer produced the best fiber
separation. The specific surface area was found to
have a significant impact on ozone bleaching
results with refiner fluffing reducing the ozone
requirement by 10% to 30% over other fluffing
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options. Chemical requirements for final bleaching
were also reduced. Improved fluffing resulted in
a higher pulp viscosity after the ozone stage (by
up to lcp or 35dm3/kg) as well as for the final-
bleached pulp. After final bleaching, the refiner-
fluffed pulp had a higher strength. The tear
factor for the refiner-fluffed pulp at a given
tensile was about 5% to 10% higher than for pin or
impact-fluffed pulp and 10% to 20% higher than for
poorly-fluffed shredder pulp.
The higher capital cost associated with better
fluffing is compensated for by the reduction in
ozone generation equipment costs because of the
reduced ozone requirement with better fluffing.
The result is that all .four fluffing options for
which an economic analysis was performed have very
similar capital costs.
The invention being thus described, it will be
obvious that the same may be varied in many ways.
Such variations are not to be regarded as a
departure from the spirit and scope of the
invention and all such modifications are intended
to be included within the scope of the following
claims.
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