U.S. patent application number 10/483648 was filed with the patent office on 2004-10-14 for four stage alkaline peroxide mechanical pulping.
Invention is credited to Herkel, Martin.
Application Number | 20040200586 10/483648 |
Document ID | / |
Family ID | 33131968 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040200586 |
Kind Code |
A1 |
Herkel, Martin |
October 14, 2004 |
Four stage alkaline peroxide mechanical pulping
Abstract
A process for applying chemicals, such as an alkaline peroxide
pretreatment (1) to lignocellulosic material before chemical
refining and at the primary refiner (3). The preferred embodiment
comprises (i) preconditioning at temperatures below 95.degree. C.,
especially below 80.degree. C., (ii) limiting the time and/or
temperature in the refiner, (iii) reaction quench to maintain
temperatures below 80.degree. C., and (iv) subsequent high
consistency bleaching (4).
Inventors: |
Herkel, Martin; (Vienna,
AT) |
Correspondence
Address: |
L James Ristas
Alix Yale & Ristas
Suite 1400
750 Main Street
Hartford
CT
06103-2721
US
|
Family ID: |
33131968 |
Appl. No.: |
10/483648 |
Filed: |
January 13, 2004 |
PCT Filed: |
July 19, 2002 |
PCT NO: |
PCT/US02/23078 |
Current U.S.
Class: |
162/25 ; 162/26;
162/78; 162/80 |
Current CPC
Class: |
D21B 1/16 20130101; D21C
9/163 20130101 |
Class at
Publication: |
162/025 ;
162/026; 162/078; 162/080 |
International
Class: |
D21B 001/16; D21C
009/16 |
Claims
What is claimed is:
1. An alkaline peroxide mechanical pulping process comprising the
steps of: feeding a lignocellulosic material into a first press;
pressing the lignocellulosic material; discharging the
lignocellulosic material from the first press; impregnating the
lignocellulosic material discharged from the first press with a
first alkaline peroxide pretreatment solution and maintaining the
impregnation for a first reaction time; feeding the lignocellulosic
material impregnated with the first pretreatment solution to a
refiner having an inlet and a rotating disc within a casing; adding
an alkaline peroxide refiner solution to the lignocellulosic
material as it fed to the refiner; mixing the refiner solution and
the lignocellulosic material with the refiner as the material is
refined to a primary pulp; delivering the primary pulp from the
refiner casing to a high consistency tower; retaining the primary
pulp in the tower to produce bleached primary pulp; and further
processing the bleached primary pulp to a secondary pulp.
2. The alkaline peroxide mechanical pulping process of claim 1
further comprising; feeding the lignocellulosic material that has
been impregnated with the first pretreatment solution for a first
reaction time, into a second press; pressing and discharging the
lignocellulosic material from the second press; impregnating the
lignocellulosic material discharged from the second press with a
second alkaline peroxide pretreatment solution and maintaining the
second impregnation for a second reaction time.
3. The alkaline peroxide mechanical pulping process of claim 1,
wherein the first pretreatment solution impregnation is at a
temperature of about 0.degree. C. to about 90.degree. C. and is
maintained for said first reaction time of about 5 to about 45
minutes.
4. The alkaline peroxide mechanical pulping process of claim 1,
wherein the first pretreatment solution comprises up to about 0.5%
chelation agent based on dried material weight, up to about 4% NaOH
based on dried material weight, and up to about 4% H2O2 based on
dried material weight; and about 0% to about 4% sodium silicate
based on dried material weight, and about 0% to about 2% MgSO4
based on dried material weight.
5. The alkaline peroxide mechanical pulping process of claim 2,
wherein the second pretreatment solution impregnation is at a
temperature of about 10.degree. C. to about 80.degree. C.
maintained for said second reaction time of about 5 to about 60
minutes.
6. The alkaline peroxide mechanical pulping process of claim 2,
wherein the second pretreatment solution comprises; up to about
0.5% chelation agent based on dried material weight, about 0.5% to
about 6% NaOH based on dried material weight, about 0.5% to about
6% H2O2 based on dried material weight; and about 0% to about 4%
sodium silicate based on dried material weight; and about 0% to
about 2% MgSO4 based on dried material weight.
7. The alkaline peroxide mechanical pulping process of claim 1,
wherein the refiner solution comprises; up to about 0.5% chelation
agent based on dried material weight, up to about 4% NaOH based on
dried material weight, and up to about 4% H2O2 based on dried
material weight; and about 0% to about 4% sodium silicate based on
dried material weight; and about 0% to about 2% MgSO4 based on
dried material weight.
8. The alkaline peroxide mechanical pulping process of claim 1,
wherein the refiner has an atmospheric pressure at the inlet and
the casing.
9. The alkaline peroxide mechanical pulping process of claim 1,
wherein the refiner inlet is maintained at atmospheric pressure and
the casing is maintained at a pressure above atmospheric.
10. The alkaline peroxide mechanical pulping process of claim 1,
wherein the refiner casing is maintained at a gauge pressure of at
least about 0.5 bar.
11. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of delivering the primary pulp from the refiner
casing to the high consistency tower further comprises cooling the
primary pulp with water as it is delivered.
12. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of delivering the primary pulp from the refiner
casing to the high consistency tower is through a blow valve.
13. The alkaline peroxide mechanical pulping process of claim 12,
further comprising the step of delivery the primary pulp from the
blow valve to a mixing screw, mixing the primary pulp with the
screw, and adding water to the primary pulp as the primary pulp is
mixed.
14. The alkaline peroxide mechanical pulping process of claim 1,
wherein the refiner has a pressure above atmospheric at the inlet
and a pressure above atmospheric in the casing.
15. The alkaline peroxide mechanical pulping process of claim 1,
wherein the material is retained in the high consistency tower for
a retention time of about 15 minutes.
16. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of adding a refiner solution to the
lignocellulosic material as it is fed to the refiner occurs at a
cross conveyor between the first press and the refiner.
17. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of adding a refiner solution to the
lignocellulosic material as it is fed to the refiner occurs at a
ribbon feeder at the refiner inlet.
18. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of adding a refiner solution to the
lignocellulosic material as it is fed to the refiner occurs at the
inlet of the refiner plates.
19. The alkaline peroxide mechanical pulping process of claim 1,
wherein the step of pressing the lignocellulosic material
impregnated with the first pretreatment solution is accomplished
with the first press having a compression ratio of at least about
1.5:1.
20. The alkaline peroxide mechanical pulping process of claim 2,
wherein the step of pressing the lignocellulosic material
impregnated with the second pretreatment solution is accomplished
with the second press having a compression ratio of at least about
1.5:1.
21. The alkaline peroxide mechanical pulping process of claim 1,
wherein the material at the first impregnation with the first
impregnation solution is in the form of wood chips having a
consistency of about 15% to about 50%.
22. The alkaline peroxide mechanical pulping process of claim 2,
wherein the material is in the form of wood chips and the chips at
the second impregnation with the second impregnation solution have
a consistency of about 20% to about 50%.
23. The alkaline peroxide mechanical pulping process of claim 2,
wherein the material at the first impregnation with the first
impregnation solution is in the form of wood chips having a
consistency of about 15% to about 50%, and the chips at the second
impregnation with the second impregnation solution have a
consistency of about 20% to about 50%
24. An alkaline peroxide mechanical pulping process comprising the
steps of: feeding a lignocellulosic material into a first press;
pressing the lignocellulosic material; discharging the
lignocellulosic material from the first press; impregnating the
lignocellulosic material discharged from the first press with a
first alkaline peroxide pretreatment solution and maintaining the
impregnation for a first reaction time; feeding the lignocellulosic
material impregnated with the first pretreatment solution to a
refiner having an inlet and a rotating disc within a casing; adding
an alkaline peroxide refiner solution to the lignocellulosic
material at the refiner; mixing the refiner solution and the
lignocellulosic material with the refiner as the material is
refined; and discharging the lignocellulosic material from the
casing and maintaining the discharged lignocellulosic material at
conditions that allow continued peroxide bleaching of the primary
pulp.
25. A chemimechanical pulping process comprising the steps of:
feeding a lignocellulosic material into a first atmospheric press;
pressing the lignocellulosic material; discharging the
lignocellulosic material from the first press; impregnating the
lignocellulosic material discharged from the first press with a
first chemical bleaching pretreatment solution and maintaining the
impregnation for a first reaction time; feeding the lignocellulosic
material impregnated with the first pretreatment solution to a
refiner having an inlet and a rotating disc within a casing; adding
a chemical refiner bleaching solution to the lignocellulosic
material at the refiner; mixing the refiner solution and the
lignocellulosic material with the refiner as the material is
refined to a primary pulp; discharging the primary pulp from the
casing to a high consistency tower; maintaining the discharged
primary pulp at conditions to reduce chemical reactions extraneous
to bleaching of the primary pulp; and processing the primary pulp
further to a secondary pulp.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims benefit of U.S. patent
application Ser. No. 60/306,974 under 35 U.S.C. .sctn.119(e).
FIELD OF THE INVENTION
[0002] The present invention relates to a process for the
production of pulp from lignocellulosic material, such as wood
chips or the like, by alkaline peroxide mechanical refining.
BACKGROUND OF THE INVENTION
[0003] Applying alkaline peroxide chemicals as part of refiner
mechanical pulping may be traced back as early as 1962. Since then,
there have been a number of different process ideas developed to
apply the chemicals before or during early stages of refiner
pulping. In recent years, an extensive and systematic investigation
has been reported on how different chemical treatments in refiner
mechanical pulping affect pulp property development and the process
consumption. For hardwoods, it was observed that alkaline peroxide
pretreatment in general gives better optical properties, better
bleachability and higher pulp yield at similar strength properties
when compared to other conventional chemical pretreatment, such as
alkaline sulfite and cold caustic soda processes. When compared to
a peroxide post-bleaching process, applying alkaline peroxide
before refining has a tendency to give a higher bulk at a given
tensile strength for some hardwood species, such as North American
aspen.
[0004] In a very broad sense, alkaline peroxide refiner mechanical
pulping is a type of pulping process where hydrogen peroxide and
alkali in various forms, together with various amounts of different
peroxide stabilizers, are applied to the lignocellulosic materials
before or during defiberization and fibrillation in a refiner. In
the early stage of development of this type of pulping process,
there were two basic concepts. One was to apply alkaline peroxide
treatment on chips, to allow the bleaching reactions to complete or
to approach completion before refining. The other basic concept was
to apply all the alkaline peroxide at the refiner, either with no
pretreatment or with stabilizers or other alkaline pretreatment
prior to the alkaline peroxide application at the refiner.
SUMMARY OF THE INVENTION
[0005] The invention, referred to herein as P-RC (Preconditioning
followed by Refiner Chemical treatment), combines the concept of
applying chemicals such as alkaline peroxide pre-treatment to
lignocellulosic material before primary refining with the concept
of applying chemicals such as alkaline peroxide at the primary
refiner.
[0006] This is achieved in the preferred embodiment, by a four
stage process comprising (i) raw material preconditioning at
temperatures below 95.degree. C., especially below 80.degree. C.,
(ii) time and/or temperature limited in-refiner reaction, (iii)
reaction quench to maintain temperatures below e.g., 80.degree. C.,
and (iv) subsequent high consistency bleaching.
[0007] An aspect of the invention is to apply a portion of the
alkaline peroxide (and/or other chemicals known in the art to
bleach or otherwise process lignocellulosic material into pulp or
precursors of pulp) at the primary refiner in combination with
upstream chip chemical impregnation step and/or steps, to yield a
more efficient process in regard to energy reduction and bleaching
than the application of all the chemicals either at the chip
impregnation or at the refiner.
[0008] Another aspect of the invention is to achieve a better
efficiency by moving a greater number of chemical reactions to the
refining stage through the introduction of chemicals and/or
chemical stabilizers at pretreatment in combination with addition
of chemicals and/or chemical stabilizers at the primary
refiner.
[0009] A further aspect of the invention is to improve or simplify
the pulping process, engineering and operation, with a
configuration to reduce or eliminate detrimental effects of
increased temperature and/or other conditions or factors prior to
and during primary refining which operate to influence pulp
brightness development and H.sub.2O.sub.2 or other chemical
efficiency.
[0010] A still further aspect of the invention is to improve or
simplify the pulping process, engineering and operation, with a
configuration to reduce or eliminate detrimental effects of
increased temperature and/or other conditions or factors during or
subsequent to discharge from the primary refiner casing which
operate to influence pulp brightness development and H.sub.2O.sub.2
or other chemical efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention will be better understood by reference to the
accompanying drawing in which:
[0012] FIG. 1 is a block diagram consistent with an embodiment of
the invention, depicting the general P-RC APMP process.
[0013] FIG. 1A is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring lignocellulosic
material to a refiner having a casing at atmospheric pressure, with
discharge at atmospheric pressure.
[0014] FIG. 1B is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring lignocellulosic
material to a refiner having a pressurized casing with pressurized
discharge.
[0015] FIG. 1C is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring primary pulp
produced in the refiner with a casing at atmospheric pressure, to a
high consistency tower via a transfer device.
[0016] FIG. 1D is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring primary pulp
produced in the refiner with a casing at atmospheric pressure
directly to a high consistency tower.
[0017] FIG. 1E is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring primary pulp
produced in the refiner with a pressurized casing, to a high
consistency tower via a transport device.
[0018] FIG. 1F is a block diagram consistent with an embodiment of
the invention, depicting steps of transferring primary pulp
produced in the refiner with a pressurized casing to a high
consistency tower, directly by blowing.
[0019] FIG. 2 is a table comparing the invention with two prior art
processes.
[0020] FIG. 3 is a graph of freeness as related to energy
consumption for the invention and two prior art processes.
[0021] FIG. 4 is a graph of density as related to energy
consumption for the invention and two prior art processes.
[0022] FIG. 5 is a graph of the tensile of tensile development for
the invention and two prior art processes.
[0023] FIG. 6 is a graph of burst development for the invention and
two prior art processes.
[0024] FIG. 7 is a graph of brightness development for the
invention and two prior art processes.
[0025] FIG. 8 is a graph of the light scattering coefficient of the
pulp as a function of freeness for the invention and two prior art
processes.
[0026] FIG. 9 is a comparative table of atmospheric versus
pressurized casing processing of aspen wood chips according to the
invention.
[0027] FIG. 10 is a comparative table of atmospheric versus
pressurized casing processing of birch wood chips according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] FIG. 1 presents a simplified process flow diagram of an
embodiment of the inventive P-RC alkaline peroxide mechanical
pulping (APMP) process. The P-RC process generally applies alkaline
peroxide chemicals at chip pretreatment/chip impregnation
step(s)/stage(s) 1, 2 and as the material is fed to the primary
refiner 3. As will be described more fully below, in the preferred
embodiment, the invention has four stages, (i) raw material
preconditioning at temperatures below 95.degree. C., especially
below 80.degree. C., (ii) time and/or temperature limited
in-refiner reaction, (iii) reaction quench to maintain temperatures
below e.g., 80.degree. C., and (iv) subsequent high consistency
bleaching.
[0029] The preconditioning step(s) (i) as implemented in stages 1
and 2 of FIG. 1, preferably include one or two atmospheric
compression devices, such as screw presses. Chip material is fed
through an inlet, and passes through at least one compression
region and at least one expansion region, and is discharged. A
chemically active solution (pretreatment solution) is added to the
material, typically while decompressing or decompressed at or near
the discharge to facilitate penetration of the solution into the
material.
[0030] The refiner 3 for implementing step (ii) is a primary
refiner of conventional size, configuration, and operating
conditions as known for chemi-mechanical pulping, subject, however,
to care in operation so as not to expose the alkaline peroxide to
excessive temperature or time-temperature combination. The
chemicals added at the refiner will be referred to as the refiner
solution.
[0031] Steps (iii) and (iv) are implemented following the primary
refining, with a relatively high level of chemical presence carried
over from the refiner, while maintaining temperature control to
avoid premature degradation of the post-refining chemical
activity.
[0032] FIGS. 1A through 1F present various non-limiting embodiments
of the P-RC process. For example, FIGS. 1A and B show that after
the material is pretreated at 1 and/or 2, addition of the solution
to the lignocellulosic material may more specifically occur at a
cross conveyer 10, downstream of the screw press and near refiner
3, or at the refiner itself, e.g., the ribbon feeder 12, the inlet
eye of the refiner disc 14, and/or at the inlet zone of the plates
on the refiner disc 16. As used herein, chemical addition "as the
material is fed to the refiner", encompasses the locations 10, 12,
14, and 16. The refiner may have an atmospheric casing 3A or an
overpressure casing 3B, but the inlet to the refiner would normally
be at atmospheric pressure. The discharge from a pressurized casing
20a of primary pulp may be through a blow valve or similar device,
and discharge from an atmospheric casing 20 may be by gravity drop
or the like. The discharge from the refiner will, in any event,
directly or indirectly go to a high consistency-bleaching tower 24
of any type known in the art (but subject to temperature
control).
[0033] The pretreatment and refiner solutions act chemically on the
lignocellulosic material, as it is refined to primary pulp. It may
be advantageous, depending on the lignocellulosic material and the
processing equipment, to modify the chemical exposure profile of
the material to the chemical agents in order to optimize the
process, and/or eliminate or reduce unwanted chemical effects or
degradation. Such chemical profile modification may be accomplished
by sequential chemical additions throughout the process, and can be
combined with other variable conditions such as temperature,
concentration, pressure, and duration to further enhance the
desired effect.
[0034] Lignocellulosic material processed using the P-RC process is
discharged 4 from the primary refiner casing (either atmospheric
discharge 20 or overpressure discharge 20a), as a primary pulp
having a measurable freeness and could properly be called a pulp
able to form a handsheet. As shown in FIGS. 1C and D, atmospheric
discharge from the refiner could pass via a transfer device 22 such
as a transfer screw, to the tower 24, or more directly 28 via a
chute or the like. As shown in FIGS. 1E and F, with a pressurized
casing the refined pulp would typically be discharged through a
blow valve and delivered either directly or indirectly to the
tower. Optionally, as shown in FIGS. 1C and E, the bleached pulp
exiting the tower can be further processed in, e.g., a secondary
refiner. The high consistency retention tower 24 allows the
chemical bleaching reactions carried over from chip pretreatment
and refining to continue.
[0035] The presence of an ample amount of the alkaline peroxide
chemicals in the primary refiner (e.g., as by shifting a large
proportion of the chemical reactions to the refiner chemical
treatment stage) improves efficiency. This is because variations in
chip forms and quality, in addition to the natural heterogeneity of
wood chips and fibers, often make it difficult, if not impossible,
to achieve a good chemical distribution in the chip
pretreatment/impregnation stage(s). In these situations, the mixing
action at the primary refiner according to the invention helps
considerably the chemical distribution, and hence, improves the
chemical efficiency. Fast distribution of bleach chemicals such as
peroxide to the chromophore sites is correlated with efficient
bleaching. This efficiency is achieved because the targeted
peroxide reactions are carried out at the reaction site of interest
quickly without lengthy exposure to the heterogeneous environment
present in the process. The primary refiner may conventionally have
a temperature at the inlet between the plates that pushes the
chromophore removal and hemicellulose alkali reactions so fast that
that pH is lowered prematurely. Using the primary refiner as a
combination chemical mixer and refiner according to the invention,
distributes the chemicals fast enough to compete favorably against
and counter to a significant extent, the elevated temperature that
may be present in the refiner. This favorable distribution is in
part, a consequence of the upstream conditioning of the chips in
the screw press.
[0036] The discharged primary pulp should also be maintained at
conditions that allow the desired chemical reactions to continue.
The maintenance conditions include but are not limited to
temperature, pressure, pH, chemical concentration, solids
concentration, and time, that allow for bleaching of the pulp to
continue and limit the degradation of the bleaching agent through
reactions that are extraneous to the bleaching of the pulp. Such
extraneous reactions may be non-productive, inefficient, and/or
harmful to the bleaching of the pulp. Control of some and/or all of
the conditions may or may not be needed depending on e.g., the type
and condition of the lignocellulosic material used in the process,
and the type, size and operating environment of the equipment
itself. For example, conditions of temperature may be modified
throughout the process by the addition of water, pressurized gas,
and other heating or cooling methods. Temperature modifying means
may be employed during transfer of the primary pulp 22 by using a
mixing screw with water added while the pulp is mixed and
transferred to the tower. The temperature of the primary pulp may
also be thermally adjusted within the tower if the primary pulp is
discharged directly to the tower 28, by means known in the art. For
example, the pulp may be thermally adjusted through addition of
liquids or gases, and/or through use of heat transfer components
such as tubing, tower jacketing, etc. The method of discharge,
either by blowing 20a from the pressurized refiner casing or by
gravity discharge from the atmospheric casing 20, can be used to
maintain and adjust the temperature of the primary pulp.
[0037] As used herein, the term "control" should be understood as
including both active and passive techniques. Thus, control could
be implemented by a static hardware configuration or by continually
measuring one or more process parameters and controlling one or
more process variables.
[0038] The chemical conditions present anywhere in the inventive
process may be modified by additives to prevent extraneous
degradation. This modification may be made at, by way of example,
the pretreatment step(s) 1 and/or 2, the cross conveyer 10, the
ribbon feeder 12, the inlet eye of the refiner disc 14, the plates
of the refiner disc 16. An example of stabilizers would be
chelation agents. A chelation agent refers to a compound that has
an ability to form complexes, so called chelates, with metals
occurring in the lignocellulosic material, and primary pulp. Such
metals may include monovalent metals sodium and potassium,
earth-alkali divalent metals calcium, magnesium and barium, and
heavy metals such as iron, copper and manganese. The metal ions
retained in the material as it is processed makes the bleaching by
oxygen chemicals (such as hydrogen peroxide) less effective, and
results in excess chemical consumption as well as other problems
well known in the art. In order to reduce or eliminate the effect
of these metal ions on the process, chelants such as for example
diethylene triamine pentaacetic acid (DTPA), ethylene diamine
tetraacetic acid (EDTA) and nitriletriacetic acid (NTA) may be
used. These and other chelation agents known in the art may be used
alone or in combination as needed or desired depending on process
conditions. In addition, silcates and sulfates as examples may also
be used advantageously as stabilizers as well as serving other
functions well known in the art.
[0039] Further embodiments and aspects of the invention will be
apparent from the examples and description set forth below.
ILLUSTRATIVE EXAMPLES
Example Set A
[0040] Three general series of pilot plant processes are
illustrated in the following examples. The materials and conditions
for the following examples, unless specified otherwise are:
[0041] Wood: A blend of 50% aspen and 50% basswood was used in this
study. The aspen woods had rotten centers, which made it more
difficult to bleach than normally expected. The woods were all from
Wisconsin USA, and debarked, chipped and screened before further
processing.
[0042] Chemical Impregnation: Chips were pre-steamed first for 10
minutes, and then pressed using an Andritz 560GS Impressafiner at
4:1 compression ratio before impregnated with alkaline peroxide
chemical liquor. The chemical liquor was introduced at the
discharge of the press, and allowed for 30 minutes retention time
before refining.
[0043] Refining: An Andritz 92 cm (36") Model 401 double disc
atmospheric refiner at a conventional speed of 1200 rpm was used
for all the refining processes. There was 15 minutes or more
retention time between the primary and the secondary, and no
dilution after the primary and before the secondary. The refining
consistency was 20% at both the primary and the secondary.
[0044] Pulp Testing: Tappi Standards were used for all pulp testing
except for freeness, which follows Canadian Standard Freeness (CSF)
test methods.
[0045] In the first of three processes compared, all of the
alkaline chemicals were applied, (3.3% total alkalinity, (TA), and
2.4% H.sub.2O.sub.2, together with 0.2% DTPA, 0.07% MgSO.sub.4 and
3% Na.sub.2SiO.sub.3) at the chip impregnation (preconditioning or
pretreatment) stage, (only one stage chip impregnation was
applied), then refined at atmospheric pressure. This series was,
therefore, named "Chip". The second series used approximately two
thirds of the total alkaline peroxide chemicals, (or 2.4% TA, 1.6%
H.sub.2O.sub.2, 0.08% DTPA, 0.04% MgSO.sub.4 and 2.4%
Na.sub.2SiO.sub.3), at the chip impregnation stage, and
approximately one third of the total chemicals, (1.0% TA, 1.0%
H.sub.2O.sub.2, 0.19% DTPA, 0.05% MgSO.sub.4, and 0.9%
Na.sub.2SiO.sub.3), at the eye of the primary refiner. It is
labeled as "Chip+Refiner", and represents the invention. In the
third series, labeled "Refiner", the chips were first pressed using
the same chip press as the first two series, and then all the
alkaline peroxide chemicals, (4.2% TA, 3.3% H.sub.2O.sub.2, 0.36%
DTPA, 0.11% MgSO.sub.4, 4.3% Na.sub.2SiO.sub.3), were applied at
the eye of the primary refiner. In all the series, the pulp from
the primary was allowed 15 minutes retention under cover in drums,
(which gave a temperature about 80-90.degree. C.), before the
second stage refining. There was no interstage washing.
[0046] FIG. 2 summarizes some of the process conditions and results
from each series. The pulps are all from second stage refining. In
peroxide bleaching of mechanical pulps, a lower TA/H.sub.2O.sub.2
ratio is in general preferred under higher temperature to prevent,
or to reduce the possibility of alkali darkening reaction. For this
reason, as shown in Table 1, the lowest TA/H.sub.2O.sub.2 ratio,
1.27, was use for "Refiner" series, the second lowest, 1.31, for
"Chip+Refiner" series, and the highest, 1.37, for "Chip" series. In
"Refiner" series, a larger amount of TA charge (4.2%) was used to
prevent pH from dropping too fast and too low during refining
because of the high temperature and the heat generated from
refining energy. Reasonable amounts of residual peroxide and pH
were maintained in each of the series, FIG. 2.
[0047] As to the chemistry, the main difference between "Chip" and
"Chip+Refiner" series is that the latter is more aggressive in
moving more alkaline peroxide chemicals to the refiner chemical
treatment stage.
[0048] Graphic presentation of the data gathered from pulp after
secondary refining after different investigated processes are shown
in FIGS. 3 through 8. FIG. 3 shows effects of the different
chemical applications on pulp freeness development in relation to
specific energy consumption (SEC), which includes energy consumed
during chip pretreatment stage. The "Chip+Refiner" series used
slightly less SEC than the "Chip" series, but both series used, on
average, approximately 200 kwh/odmt less SEC than the refiner
bleaching series, "Refiner", even though the latter had more
caustic chemicals applied than the first two series and has the
same residual pH, 8.2, as "Chip+Refiner" series. It appears that
adding the alkaline chemical under high temperature, at refiner
eye, causes more alkali consumed on nonproductive, or side
reactions that have little to do with pulp property
development.
[0049] It should be pointed out that in a commercial operation, the
SEC in general is lower than that observed at the lab for chemical
mechanical pulping of hardwoods. The SEC values in FIG. 3,
therefore, are better used for comparison purpose than for their
absolute values.
[0050] Because many pulp properties, especially the strength
properties, are dependent on handsheet density, this property was
also analyzed under SEC, and results are shown in FIG. 4. In this
case, the more aggressive refiner chemical treatment P-RC APMP
series, "Chip+Refiner", had the best efficiency for handsheet
density development, which was followed by "Chip" and "Refiner"
series. These results demonstrate that in chemical mechanical
pulping, process energy efficiency depends not only on how much but
also on how the chemicals are applied.
[0051] As for pulp intrinsic property development, there was
however, little difference among the three series, as illustrated
in FIGS. 5 and 6, suggesting that as long as the chemicals are
added before refining, the mechanism involved in fiber strength
property development remains the same.
[0052] As for pulp optical property development, in mechanical
pulping, pulp brightness is often freeness-dependent. FIG. 7 shows
brightness at different freeness from each series. Of interest is
that "Chip+Refiner" series had a similar brightness development as
that of the "Refiner" series, even though the former used less
amount of the bleaching chemicals, 2.6% H.sub.2O.sub.2/3.4% TA
versus 3.3% H.sub.2O.sub.2/4.2% TA. Adding all of the chemicals at
the impregnation stage, "Chip" series, showed also a less bleaching
efficiency, 2 or more points lower, than that of "Chip+Refiner"
series. This suggests that the bleaching efficiency is sensitive to
how the chemicals are distributed between the chip impregnation and
refining in P-RC APMP process. In this case, a compromise between
adding all of the chemicals at chip impregnation or at eye of
refiner appears to be the most efficient in bleaching and peroxide
consumption.
[0053] FIG. 8 shows that there was no difference in light
scattering property development in all the series studied, suggest
the pulp surface development mechanism also remain the same as long
as the chemicals are added before refining.
Example Set B
[0054] The below examples illustrate a different refining
configuration where the primary refiner was maintained at a
negligible gauge pressure at the inlet and a low pressure
(approximately 140 kPa) at the casing. Advantages of this
configuration include:
[0055] 1) better steam handling at the refiner discharge,
especially for high capacity refiners (300 t/d or higher);
[0056] 2) ease of transfer primary pulp from the refiner to the
interstage high consistency (HC) tower;
[0057] 3) a potential to use some of the steam generated from the
primary refining (by using a cyclone to separate steam and pulp
fiber);
[0058] 4) ease of converting existing TMP systems into a P-RC APMP
process.
[0059] These examples show that running the primary refiner at a
low pressure (140 kPa) in the casing and atmospheric at the inlet
can give similar bleaching efficiency as that of atmospheric at
both the inlet and the casing. Temperatures at the inlet and
between the plates in the primary refiner may push the chromophore
removal and hemicellulose alkali hydrolysis reactions fast enough
that pH was lowered considerably before the pulp reaches the casing
out off the refiner plates. The pulps 10 at the cyclone discharge
from the primary refiner were measured in the examples below to
have pH of 9.3-9.7, at which peroxide is easy to stabilize even
under the high temperatures (80-90.degree. C.) observed.
[0060] The materials and conditions for the following examples
below were as follows:
[0061] Wood: Aspen and birch chips from a commercial pulp mill in
eastern Canada were used in this study.
[0062] Chip Impregnation: A conventional pilot chip impregnation
system was used in this study. In all the P-RC APMP runs studied,
only DTPA was used in the first stage of chip impregnation. The
chips were then impregnated with alkaline peroxide (AP) chemicals
at second stage impregnation. The AP treated chips were then
allowed for 30 to 45 minutes' retention (without steaming) before
being refined.
[0063] Atmospheric Refiner System: Andritz 36" diameter (92 cm)
double disc 401 system is typically used for conventional P-RC APMP
process investigations. This system consists of an open metering
belt, an incline twin-screw feeder, the refiner and an open belt
discharge. The system is used for both primary and later stages of
refining. When used for the primary, the pulp discharged were
collected in drums and kept under cover to maintain a high
temperature (typically 80 to 90.degree. C.) for a certain period of
time.
[0064] Pressurized Refiner System: An Andritz single disc 36"
diameter (92 cm) pressurized system was modified and used in this
study for atmospheric inlet/pressurized casing configuration. The
original refiner system has all the standard features of a
conventional TMP system. In order to run the system with
atmospheric pressure at the inlet, a valve was placed on top of the
vertical steaming tube and was kept open during refining. During
the trial, the plug screw feeder (PSF) was run at 50 rpm (normal
speed for TMP is 10 to 20 rpm) to ensure the chemical impregnated
chips were not compressed. The AP impregnated chips were placed in
a chip bin, which discharged the chips into a blower. The chips
were then blown to a cyclone and discharged to a conveyor, which
feeds the PSF. The chips were then dropped into a vertical steam
tube before being fed into the refiner. During refining, the
primary refiner was controlled to have zero pressure at the inlet
and 140 kPa in the casing. From the casing, the primary pulp was
blown to a cyclone and discharged and collected in drums, and then
treated similarly as in the atmospheric refining runs.
[0065] Pulp Tests: TAPPI standard was used for brightness tests.
Peroxide residuals were measured using standard iodometric
titration.
[0066] Running the primary refiner with pressurized casing and
atmospheric inlet was compared with conventional atmospheric
refining in P-RC APMP pulping of aspen and birch commercial wood
chips. The results showed that both refining configurations gave
similar bleaching efficiency. For some installations, using
pressurized casing can significantly simplify the process,
engineering and operation of P-RC APMP process.
[0067] FIG. 9 presents the chemical conditions used for P-RC APMP
pulping of aspen, and brightness results from atmospheric and
casing pressurized runs with the primary refiner. Applying similar
AP chemical strategies in both cases, and having similar amounts of
total chemical consumption (5.2 to 5.4% total alkali, TA, and 3.7
to 3.9% H.sub.2O.sub.2), both the atmospheric and the casing
pressurized gave a similar brightness, achieving 84.2% ISO and
84.7% ISO respectively.
[0068] The residual pH (8.8-9.0) in both cases were slightly higher
than ideal (approximately 7.0-8.5) and the H.sub.2O.sub.2 residual
(1.5 to 2.0% on o.d. pulp) was also higher than normal (0.5 to
1.0%), suggesting that in both cases the pulp property could be
further developed had the chemical treatments been further
optimized.
[0069] It is worth pointing out that the bleaching efficiency shown
in Table 1 (3.7 to 3.9% H.sub.2O.sub.2 and 5.2-5.4% TA consumption
to reach 84.2 to 84.7% ISO brightness) is comparable to or better
than bleaching efficiency normally observed in H.sub.2O.sub.2
bleaching of TMP or CTMP pulps from aspen.
[0070] FIG. 10 presents conditions and results from P-RC APMP
pulping of the birch. This particular birch chips was slightly more
difficult to bleach than the aspen. Using similar AP chemical
strategies, the atmospheric and the pressurizing casing again gave
similar bleaching efficiency: 3.1-3.2% TA and 3.4-3.6%
H.sub.2O.sub.2 to reach 82.4 to 82.6% ISO brightness. In this case,
the residual chemicals (0.1-0.2% TA, 0.5-0.6% H.sub.2O.sub.2 and pH
of 8) were within ideal H.sub.2O.sub.2 bleaching conditions.
* * * * *