Note: Descriptions are shown in the official language in which they were submitted.
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WO 2007/137127 PCT/US2007/069159
METHODS FOR CARBONATE PRETREATMENT AND PULPING OF
CELLULOSIC MATERIAL
FIELD OF THE INVENTION
[0001] The present invention relates to a process for treating comminuted
cellulosic
fibrous material, for example, wood chips, with a carbonate pre-treatment
prior to
cooking the cellulosic material with a cooking chemical in the presence of an
anthraquinone (AQ).
BACKGROUND OF THE INVENTION
[0002] In the art of chemical pulping, cellulosic material, such as wood
chips, is
treated with pulping chemicals to produce pulp for use in the manufacture of
paper
and other products. It is well known in this art to use caustic soda (that is,
sodium
hydroxide [Na0H1 as a pulping chemical), referred to as soda cooking, in both
batch
and continuous digesters. There are many advantages to soda cooking, including
the
ability to raise the design pressure of the recovery boiler to improve the
recovery
boiler efficiency and the elimination of sulfur in the process. Among other
benefits,
the elimination of sulfur in the cooking process allows for the use of black
liquor
gasification systems. The use of black liquor gasification results in a power
generation increase that is multiple times the power generated using a sulfur-
based
system.
[0003] One disadvantage of soda cooking includes a lower pulp yield (for both
softwoods and hardwoods) than realized by kraft cooking, that is, cooking
using
sodium hydroxide (NaOH) and sodium sulfide (Na25) as the pulping chemicals.
However, the addition of anthraquinone (AQ) to soda cooking has shown to
improve
the pulp yield and therefore make the pulp yield comparable to that of kraft
cooking.
Even with AQ addition, however, pulp from soda cooking processes exhibits
weaker
pulp strength and poorer bleaching compared to haft cooking.
[0004] However, disadvantages also exist with soda anthraquinone (SAQ) cooking
as compared to kraft cooking. For instance, SAQ cooking requires higher
amounts of
NaOH and it is more difficult to bleach pulp produced from a SAQ pulping
process.
WO 2007/137127 CA 02651483 2008-11-05 PCT/US2007/069159
For example, the more NaOH required, the more recausticizing required. As
known
in the art, recausticizing of sodium carbonate to regenerate NaOH proceeds as
outlined in equations 1 and 2 below.
Na2CO3 + CaO + H20 ¨> 2NaOH + CaCO3 [1]
CaCO3 ¨> CaO + CO2 [2]
[0005] The volume of NaOH per ton of wood pulp required is higher for SAQ
cooking compared to the NaOH required for haft cooking. This higher volume of
NaOH required in SAQ cooking is needed to compensate for the sodium sulfide
present in kraft cooking which is not present in SAQ cooking. As is known in
the art,
sodium sulfide (Na2S) present in kraft cooking hydrolyzes to NaSH and NaOH and
therefore impacts the cooking of the cellulosic material during kraft cooking.
The
volume of NaOH required in soda AQ cooking is about 20% to 40% higher than in
kraft cooking. The higher requirement for NaOH results in higher energy
requirements to convert, or causticize, the sodium carbonate (Na2CO3) from the
recovery boiler to NaOH used in the digester and other parts of the fiber
line.
[0006] For instance, when an active alkali (AA = NaOH + Na2S on a Na20 basis)
of
16.0% Na20 and 30% sulfidity is used for haft pulping, 11.2% of the AA comes
from
NaOH and 4.8% of the AA from Na2S. In kraft recovery, where all of the sulfur
is
recovered as Na2S, re-causticizing would be required to convert to NaOH so
that the
desired 11.2% Na20 on new chips is achieved. SAQ pulping of hardwoods requires
approximately the same effective alkali (EA = NaOH + 1/2 Na2S on a Na20 basis)
as
kraft pulping. In the haft pulping example above, the EA was 13.6% Na20 and a
similar amount of alkali would be required for SAQ pulping but all of it would
come
from NaOH.
[0007] Further, it is well known that hemicelluloses are quickly dissolved in
soda,
kraft, and SAQ pulping, and that a significant fraction of the applied NaOH is
consumed in degrading the hemicelluloses to low molecular weight (MW)
products.
These low MW organics are difficult to recover from the pulping effluent,
referred to
in the art as "black liquor," and they do not have high calorific values. It
is also
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known that the green liquor from the recovery furnace is primarily Na2CO3 when
the
SAQ process is used.
100081 It has been shown that pulps from hardwood material processed using Lo-
Solids Cooking methods (as described in U.S. Patents 5,489,363; 5,536,366;
5,547,012; 5,620,562; 5,662,775; 5,824,188; 5,849,150; 5,849,151; 6,086,712;
6,132,556; 6,159,337; 6,280,568; 6,346,167)
and soda-AQ (SAQ) as the cooking chemicals result in pulp
with better strength than that produced by conventional kraft cooking.
Additionally,
the pulp produced from hardwood material processed using the Lo-Solids
Cooking
method with soda-AQ as a cooking chemical can obtain a better terminal
brightness
than conventional Soda-AQ, but not as bright as pulp produced from kraft
cooking.
Although the bleach chemical consumption with Lo-Solids Soda-AQ is better than
conventional Soda-AQ for a given final brightness, the pulp still requires a
greater
quantity of bleaching chemicals than pulp produced by kraft cooking.
[0009] For these and other reasons, kraft cooking (and its many sulfur related
problems) is the prevailing pulping process in the industry. However, the
applicants
have found that by pretreating the cellulosic material with an substantailly
sulfur-free
(<1 g/1 total Sulfur) carbonate solution and then practicing SAQ, the
disadvantage of
SAQ pulping can be overcome.
100101 U.S. Patent No. 1, 887,241
discusses pre-treating, also referred to as pre-cooking, a cellulosic material
with
sodium carbonate, followed by treating the resulting material with either soda
or kraft
cooking. In the process disclosed in the '241 patent, wood chips that have
been
steamed may undergo this carbonate pretreatment stage at a temperature of
about 330
degrees Fahrenheit, that is, about 165 degrees Celsius (C). According to the
'241
patent, the pretreatment stage using sodium carbonate reduces the quantity of
NaOH
required. For instance, as disclosed in US patent No. 1,887,241, a quantity of
10%
sodium carbonate on wood is used along with 15% NaOH. compared to about 25%
NaOH added to conventional soda cooking. Though pretreatment with sodium
carbonate at about 165 degrees C is disclosed in the above-referenced patent,
the high
temperature treatment disclosed in the '241 patent (published in 1932), for
various
reasons, has not been accepted in the pulping industry, and is typically not
practiced
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CA 02651483 2012-05-08
today. The treatment disclosed in '241 patent also does not involve the use of
an
anthraquinone in any processing step.
[00111 Recent advantages in the pulping industry include methods for removing
silica from cellulosic material using a sodium carbonate containing solution.
(See US
Application No: 2006/0225852) The
methods disclosed in the 0225852 application relate to almost 100% removal of
silica
contained in cellulosic material, such removal occurring prior to the
processing of the
fibrous material using conventional methods.
[0012] Aspects of the present invention provide a method for processing
cellulose
material that overcomes the disadvantages and drawbacks of the prior art
methods.
For example, some aspects of the present invention provide an avenue for
minimizing
or eliminating the presence of sulfur from the pulp mill while producing a
commercially viable product ¨ a long left, but unresolved need of the Pulping
Industry.
SUMMARY OF THE INVENTION
[00131 The present invention, in its many aspects, relates to the pretreatment
of
cellulosic material with carbonate, such as a substantially sulfur-free (<1
g/1 total
Sulfur) sodium carbonate (Na2CO3), followed by cooking the pretreated
cellulosic
material in the presence of a pulping chemical, such as, sodium hydroxide
alone (that
is, the "soda" process), but also sodium hydroxide and sodium sulfide (that
is, the
"kraft" process), or a combination of soda and kraft, and additionally and at
least one
anthraquinone, i.e. anthraquinone or a substituted anthraquinone, such as 2-
methylanthraquinone. The AQ may be added at anytime during the process. For
instance, an AQ may be added to the pretreatment stage, the cooking stage, or
even
both the pretreatment stage and the cooking stage, as well as before or after
each
stage. (Addition of AQ to the cooking stage can be performed as described in
US
patent 6,569,289.)
100141 One aspect of the invention is a method of treating comminuted
cellulosic
fibrous material according to: a) treating the cellulosic fibrous material
with a
carbonate-containing solution, such as, a substantially sulfur-free carbonate
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containing solution, to produce a pretreated cellulosic material; b) treating
the
pretreated cellulosic material with a pulping chemical for a sufficient time
and at a
sufficient temperature to produce a cellulose pulp, wherein, in at least one
of a) and
b), the cellulosic fibrous material is treated with at least one
anthraquinone. In this
method, the cellulosic fibrous material may be treated with an anthraquinone
in a), b),
or both a) and b). In one aspect, the carbonate-containing solution may
comprise a
sodium carbonate containing solution. In another aspect, the pulping chemical
may
comprise sodium hydroxide. Further, the active pulping chemical may consist
substantially of sodium hydroxide.
[0015] One aspect of the above method includes additional steps, prior to a),
the
comminuted fibrous material may be c) treated with an acidic solution, and
after c), d)
extracting at least some of the acidic solution from the cellulose fibrous
material may
take place.
[0016] In another aspect, the method produces a pulp having a lower rejects
percent
and a higher screened yield percent compared to a pulp produced where a) is
not
practiced.
[0017] One aspect of the invention may further include an oxygen
delignification
treatment as well as at least one bleaching stage, where the method produces a
cellulosic pulp having a brightness greater than 88% elrepho. When an oxygen
delignification treatment is provided, the method may produce a pulp having a
lower
kappa number after the oxygen delignification treatment at a predetermined
screened
yield compared to a pulp produced without practicing a). As known in the art,
kappa
number is used to define the degree of delignification. It refers to the
modified
permanganate test value of pulp that has been corrected to 50 percent
consumption of
the chemical. Kappa number has the advantage of a linear relationship with
lignin
content over a wide range, for example Kappa Number x 0.15% = % lignin in
pulp.
[0018] An additional aspect of the present invention relates to a method of
treating
comminuted cellulosic fibrous material by treating the cellulosic fibrous
material with
a carbonate containing solution to produce a pretreated cellulosic material,
treating the
pretreated cellulosic material with a pulping chemical for a sufficient time
and at a
sufficient temperature to produce a cellulose pulp and a liquid containing
spent
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pulping chemical,treating the liquid containing spent pulping chemical to
produce a
carbonate-containing solution from the spent pulping chemical, and using the
carbonate-containing solution produced from the spent pulping chemical as the
carbonate containing solution in a) of the method described above.
[0019] A further aspect of the invention includes a method of treating
comminuted
cellulosic fibrous material comprising a) treating the cellulosic fibrous
material with
an acid solution, b) treating the cellulosic fibrous material with a carbonate-
containing
solution to produce a pretreated cellulosic material, and c) treating the
pretreated
cellulosic material with a sulfur-containing pulping chemical for a sufficient
time and
at a sufficient temperature to produce a cellulose pulp, where in at least of
one of b)
and c), the cellulosic fibrous material is treated with an anthraquinone. The
sulfur-
containing pulping chemical may typically contain sodium hydroxide and sodium
sulfide.
[0020] A still further aspect of the invention comprises a pulp produced from
one of
the above methods wherein the pulp has a greater yield than produced from
prior art
methods. These and other aspects and advantages of the invention can be more
completely understood in view of the following descriptions of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The present invention will be better understood from the detailed
description
given herein below and the accompanying drawings which are given by way of
illustration only, and thus are not a limitative of the present invention and
wherein:
[0022] Figure 1 is a schematic diagram of a pulping process according to
aspects of
the present invention.
[0023] Figure 2 is a schematic illustration of a chemical recovery system that
can
be used in aspects of the invention.
[0024] Figure 3 is a plot comparing the screened yield of pulps produced by
aspects
of the present invention to pulps produced by the prior art.
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[0025] Figure 4 is a plot comparing light absorption coefficient values of
pulps
produced by aspects of the present invention to pulps produced by the prior
art.
[0026] Figure 5 is a schematic illustration of the catalysis of lignin and
carbohydrate reactions by anthraquinone as enhanced by aspects of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The applicants have found that by treating comminuted cellulosic
fibrous
material, such as wood chips, in a first pretreatment stage with a carbonate
compound,
for example, substantially sulfur-free sodium carbonate, and then in a second
pulping
stage, for example, soda pulping in the presence of an anthraquinone (AQ used
to
symbolize any anthraquinones), - an improved pulp can be produced. An improved
pulp is also produced if the pulp undergoes an acid treatment prior to a first
pretreatment stage, followed by kraft pulping in the presence of an
anthraquinone.
For example, the pulp produced may be characterized by increased pulp yield,
increased bleachability, higher strength, and lower rejects, among other
beneficial
properties.
[0028] Moreover, by treating the chips with a solution containing carbonate
prior to
treating the chips with pulping chemical, the amount of pulping chemical
required in
the pulping process for a desired treatment may be reduced. This development
can
have a significant impact on attempts to reduce or eliminate the content of
sulfur
containing pulping chemicals, most notably, sodium sulfide (Na25). As noted
above,
non-sulfur pulping processes, such as, the soda process and the SAQ process,
have
received limited acceptance in the industry due the relatively poor quality of
pulp
produced compared to the sulfur-bearing kraft process. Further, typically soda
and
SAQ processes require a larger amount of pulping chemical, that is, sodium
hydroxide
(NaOH) compared to the kraft process. As will be discussed below, aspects of
the
present overcome these limitations while producing an economically viable
pulp.
[0029] An additional aspect of the present invention relates to a method of
treating
comminuted cellulosic fibrous material includes: treating the cellulosic
fibrous
material with a substantially sulfur-free carbonate containing solution to
produce a
pretreated cellulosic material, treating the pretreated cellulosic material
with a pulping
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chemical, such as, an alkaline pulping chemical, for a sufficient time and at
a
sufficient temperature to produce a cellulose pulp and a liquid containing
spent
pulping chemical, treating the liquid containing spent pulping chemical to
produce a
carbonate-containing solution from the spent pulping chemical, and using the
carbonate-containing solution produced from the spent pulping chemical as the
carbonate containing solution in a) as described above in paragraph 16. The
carbonate
containing solutions may contain sodium carbonate comprising about 1% to about
12% sodium carbonate as Na20 on chips. The above method may also include an
oxygen delignification treatment.
[0030] In one aspect, treating the liquid containing spent pulping chemical
includes
concentrating the liquid sufficiently to support combustion, burning the
concentration
liquid to produce a smelt containing carbonate, and, introducing a liquid to
the smelt
to provide the carbonate containing solution from the spent pulping chemical.
[0031] A further aspect of the invention includes a method of treating
comminuted
cellulosic fibrous material comprising a) treating the cellulosic fibrous
material with
an acid solution, b) treating the cellulosic fibrous material with a carbonate-
containing
solution, such as sodium carbonate, to produce a pretreated cellulosic
material, and c)
treating the pretreated cellulosic material with a pulping chemical for a
sufficient time
and at a sufficient temperature to produce a cellulose pulp, where in at least
of one of
b) and c), the cellulosic fibrous material is treated with an anthraquinone.
[0032] Using processes and methods according to aspects of the invention
several
surprising results from laboratory tests have been found. These improvements
include: a decrease in the H-factor in the cooking stage which is greater than
would be
expected (the total H-factor from first stage treatment plus the H-factor from
cooking
is lower than the H-factor without the first stage carbonate treatment at the
same
kappa number; this is shown in Tables 2 and 3 below; an increase in pulp
yield;
enhanced bleachability; increased strength; and lower rejects). As known in
the art
and for the purpose of this application, H-factor refers to a method of
expressing
cooking time and temperature as a single variable for delignification.
[0033] Figure 1 is a schematic diagram of a pulping process 10 employing
aspects
of the present invention in which comminuted cellulosic fibrous material 12,
for
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example, wood chips, is treated by a carbonate pretreatment stage 14 and a
cooking or
pulping stage 16. It will be understood by those of skill in the art that
though the term
"wood chips" is used to facilitate discussion of the invention, aspects of the
invention
are not limited to treating chips, but may be used to treat any form of
comminuted
cellulosic fibrous material, including, but not limited to, hardwood chips,
softwood
chips, sawdust, recycled fibers, recycled paper, agricultural waste, such as
bagasse,
and other fibrous cellulosic material.
[0034] As is typical in the art, prior to being introduced to process 10,
chips 12 may
typically be conditioned for treatment, for example, steamed, to moisten the
chips,
heat the chips, and remove as much air and other gases as possible to enhance
penetration of the treatment solutions. Steaming of chips 12 may be practiced,
for
example, in a horizontal steaming vessel or in a Diamondback steaming vessel,
both
provided by Andritz Inc. of Glens Falls, NY, prior to treatment. According to
aspects
of the invention, in the carbonate stage 14 (also referred to as c-stage,
pretreatment
stage, or first stage) of treatment, chips 12 are treated with carbonate-
containing
solution 13, typically a sodium carbonate solution, although potassium and
magnesium carbonate solutions may also be applicable. In one aspect of the
invention, this carbonate containing solution may be substantially free of
sulfur. A
person of skill in the art would appreciate that such absence of sulfur may
not mean
that no sulfur is present in the carbonate solution in the absolute sense, but
that the
solution would be "substantially" sulfur-free. After treatment in first stage
14, chips
12 are then treated in a second or pulping stage 16, that is, a cooking stage,
with
pulping chemical 20 for a sufficient time and at a sufficient temperature to
produce a
cellulose pulp 18. Pulp 18 may typically be forwarded for further treatment,
for
example, to washing, bleaching, or screening, among other conventional
treatments.
The carbonate-containing solution 13 may be provided by a variety of sources,
including, but not limited to, commercially available carbonate and carbonate
recovered from a related process, such as, from a chemical recovery cycle (for
example, as illustrated in and discussed with respect to Figure 2 below).
[0035] According to aspects of the present invention, the carbonate-containing
solution 13 may have a concentration of from about 1% to about 12% carbonate
(expressed as Na20) on wood. For example, the carbonate containing solution
may
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have a concentration of from about 2% to about 9% as Na20 on wood. The
carbonate
containing solution may typically be provided as an aqueous solution, that is,
as a
solution of carbonate in water, though other compounds may be present. A
person of
ordinary skill in the art would understand the use of the term "about" when
describing
percent carbonate and will appreciate that it is difficult to have absolute
measurements
and the use of the term about when describing percent carbonate is ubiquitous
in the
art. One kg mole of Na2CO3 (106 kg) is equivalent to one kg mole of Na20 (62
kg).
Other carbonates such as potassium and magnesium may to be added as Na20 molar
equivalence. The carbonate treatment 14 is typically practiced at a
temperature of
greater than 100 degrees C, for example, between about 120 degrees C and about
200
degrees. In one aspect, carbonate treatment 14 may be practiced at between
about 120
degrees C and about 170 degrees C, for example, between about 120 degrees C
and
about 150 degrees C. A person of ordinary skill in the art would understand
the use of
the term -about" when describing temperature ranges and will appreciate that
it is
difficult to have absolute measurements and the use of the term "about" when
describing temperature is ubiquitous in the art. Such use of the term "about"
is
understood by a person of skill in the art to define measurement ranges for
any
parameter of the treatment process throughout this disclosure, including
pressure,
time, temperature, percentage of components used in the pulping process and
other
related measurements. At such temperatures, treatment 14 may typically be
practiced
at superatmospheric pressure of about 50 psig to about 150 psig. Pretreatment
stage
14 is typically practiced for a sufficient time to provide at least some
benefit to the
resulting pulp produced in process 10. For example, pretreatment stage 14 may
be
practiced for at least 5 minutes, but may be practiced from about 15 minutes
to about
6 hours, depending upon the nature of the furnish, that is, the nature of
chips 12, but is
typically practiced from about 15 minutes to about 120 minutes.
100361 In one aspect of the invention, after pretreatment 14 and before
pulping stage
16, at least some of the liquid present may be removed or extracted from the
chips 12,
as indicated by 25. In one aspect, the extracted carbonate-containing liquid
15 may be
treated, for example, in a recovery system, disposed of, or otherwise re-used.
For
example, in one aspect, the carbonate-containing liquid may be recirculated
and re-
used as the source of or supplement to carbonate 13.
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[0037] After pretreatment stage 14, the carbonate pretreated chips 12 are then
treated in pulping stage 16 with a pulping chemical for a sufficient time and
at a
sufficient temperature to produce a cellulose pulp 18. Though a single stage
16 is
shown in Figure 1, according to one aspect of the invention, however, one or
more
pulping stages 16 may be provided. According to one aspect of the invention,
the
pulping chemical 20 used in pulping stage 16 may be primarily sodium hydroxide
(NaOH), that is, the pulping stage 16 may be "soda" pulping stage. In another
aspect
of the invention, the pulping chemical 20 may comprise NaOH and sodium sulfide
(Na2S), that is, the pulping stage may be a "sulfate" treatment or "kraft"
treatment.
When the pulping stage comprises a kraft pulping stage, in one aspect, the
carbonate
stage 13 may be preceded by an acid stage 22 (as discussed below). However,
according to aspects of the present invention, the pulping of pretreated chips
12 is
practiced in pulping stage 16 in the presence of at least one anthraquinone
(AQ) 11.
100381 In one aspect, the pulping of chips 12 in the presence of AQ may be
performed as described in U.S. Patent 6,569,289.
The applicants have found that the
pretreatment of chips 12 with carbonate, in particular with Na2CO3, followed
by SAQ
pulping, provides a pulp 18 having enhanced properties, for example, improved
yield,
reduced lignin, and improved bleachability, compared to pulps provided by
prior art
treatments. For example, the experiments performed by the applicants suggest
that
there may be some synergistic effects in the carbonate or carbonate-AQ
pretreatment
and pulping process of the present invention that are typically not
predictable from the
prior art treatments with AQ.
[0039] According to aspects of the present invention, at least one AQ, or its
derivatives or equivalents, is introduced to pretreatment stage 14, pulping
stage 16, or
both stages 14 and 16. In one aspect, AQ may be provided in its reduced form
(that
is, a chemical commonly referred to as AHQ, see Figure 5). In one aspect, an
aqueous solution of AQ may be introduced to the carbonate pretreatment stage
14 (as
indicated by phantom line 17 in Figure 1), for example, provided prior to, at
the
beginning, middle, near the end of pretreatment stage 14, or some combination
thereof The concentration of the aqueous AQ introduced during stage 14 may
range
from between about 0.01 weight percent to about 0.20 weight percent on chips,
but is
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typically from between about 0.05 and about 0.10 weight percent. For the
purpose of
this application, AQ can be added anytime during the pulping process as
described in
Figure 1.
[0040] In one aspect, the AQ may be introduced to the pulping stage 16 (as
indicated
by line 21 in Figure 1), for example, at the beginning, middle, near the end
of the
stage, or some combination thereof The concentration of the aqueous AQ
introduced
during pulping stage 16 may range from between about 0.01 weight percent to
about
0.20 weight percent on chips, but is typically between from about 0.05 and
about 0.1
weight percent on chips. In one aspect, AQ may be introduced to carbonate
stage 14,
and omitted from pulping stage 16; in another aspect, AQ may be introduced to
both
carbonate stage 14 and pulping stage 16.
[0041] According to aspects of the invention, when a first treatment stage 14
using
a sodium carbonate solution as the carbonate-containing solution, for example,
a
sulfur-free carbonate solution, is followed by a second pulping stage 16
employing
NaOH (for example, at a charge of about 13 % NaOH on wood) and AQ (that is, a
SAQ cooking stage), surprising results have been identified in laboratory
testing. A
NaOH dose of 13% on wood correspond to about 10% Na20, i.e., two kg moles or
80
kg of NaOH are equivalent to one kg mole or 62 kg of Na20. For example, in
laboratory testing, when a first carbonate stage 14 is followed by a second
soda
pulping stage 16 (with 13% NaOH on wood) in the presence of AQ, the pulping
treatment had significantly lower NaOH requirements and produced a pulp with a
yield closer to that produced by kraft pulps, without increasing the rejects
produced.
[0042] In another aspect of the invention, before the pretreatment stage 14,
the chips
12 may be treated with an acidic solution 19 in an acid stage 22 (shown in
phantom
in Figure 1). When kraft pulping in the presence of AQ is the pulping method
used,
acid pretreatment 22 may be followed by carbonate treatment 14. Any acid
containing solution may be used for acid 19, but in one aspect, acid 19 is
preferably a
non-sulfur containing acid solution, for example, an organic acid (such as,
acetic acid)
or an inorganic acid (such as, nitric or hydrofluoric acid). In one aspect,
stage 22 may
be practiced in the presence of a naturally occurring acid, that is, a
naturally occurring
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wood acid. The acid solution 19 may be provided to produce an aqueous
environment
about chips 12 having a pH of about 6 or below, for example, having a pH of
between
about 4 and about 6. The acid treatment may be practiced at a temperature
greater
than 50 degrees C, for example, at about 80 degrees C to about 160 degrees C.
Acid
treatment stage 22 is typically practiced for a sufficient time to provide at
least some
benefit to the resulting pulp produced in process 10. For example, acid
treatment
stage 22 may be practiced for at least 5 minutes, but may be practiced from
about 30
minutes to about 6 hours, and is typically practiced from about 30 minutes to
about 90
minutes, depending upon the nature of the furnish, for example, the pH of
chips 12.
The quantity of acid needed may include any amount needed to produce the same
effect as 2 to 6% acetic acid at 120 C.
[0043] As shown in Figure 1, in one aspect of the invention, after acid
treatment 22,
and before carbonate pretreatment 14, at least some of the liquid present
after acid
treatment 22 may be removed or extracted from the chips 12, as indicated by
extraction stage 23 (shown in phantom in Figure 1). In one aspect, the
extracted
acid-containing liquid 23 may be treated, for example, in a recovery system,
disposed
of, or re-used. For example, in one aspect, the acid-containing liquid may be
re-
circulated and re-used as the source of or supplement to acid 19.
[0044] The acid treatment may be provided by chips 12, for example, the acid
in stage
22 may be an acid resulting from mature wood (that is, wood which has been in
storage for a period of time sufficient to produce an acidic liquor naturally
occurring
from the wood itself). In another aspect, the acid in stage 22 may be provided
by a
conventional acid hydrolysis process, for example, a process used to remove
metals
and other contaminates from the chips 12. One such process is disclosed in
U.S.
Patent 5,338,366 (the disclosure of which is incorporated by reference herein
in its
entirety).
[0045] According to aspects of the invention, by combining the acid treatment
stage
22 with the carbonate pretreatment stage 14 and pulping stage 16 having either
NaOH
(soda) and AQ, or NaOH and Na2S (kraft) and AQ, or a combination of soda and
AQ
and kraft and AQ, the chemical consumption (for example, the amount of NaOH
required) may be reduced and the bleachablity of the resulting pulp 18
enhanced. That
is, the amount of bleaching chemical required to achieve a desired brightness,
may be
13
CA 02651483 2012-05-08
reduced compared to prior art treatments. In some aspects of the invention, it
is also
possible to see improvements in pulp yield when using the process of acid
treatment
22 followed by carbonate treatment 14 followed by SAQ. An improvement in
bleachability is observed if pulping stage 16 is the kraft process. The
chemical
consumption disadvantages in SAQ cooking can be addressed by the carbonate
treatment stage 14 and aspects of the invention may also improve pulp
bleachability
by oxygen and most likely by other one-electron transfer oxidants, such as,
chlorine
dioxide, as well. Acid treatment stage 22 (A) may further improve the
bleachability
of both soda-AQ and kraft pulps, particularly, after an oxygen (0)
delignification
stage. A !craft green liquor (Na2CO3 + Na2S) may be used as the source of
carbonate
13 if the pulping process 16 comprises a kraft process.
100461 Figure 2 is a schematic illustration of a chemical recovery system 100
that
can be used in aspects of the invention. Chemical recovery system 100 includes
a
conventional recovery furnace or gasifier 110 for burning concentrated spent
cooking
liquor 112, for example, black liquor, to produce a chemical smelt (not shown)
containing sodium carbonate and sodium sulfide (when the black liquor
comprises
!craft black liquor), among other products of combustion of black liquor known
in the
art. According to conventional practice, the carbonate containing smelt is
dissolved
in water in vessel 114, that is, the green liquor tank, to produce an aqueous
solution
(known as "green liquor") containing carbonate. In one aspect, green liquor
containing sulfide may be used for the carbonate containing solution when the
pulping
stage comprises !craft pulping. The carbonate treatment with green liquor may
be
preceded by an acid treatment (stage 22 of Figure 1) prior to kraft pulping.
Vessel
114 may be a green liquor tank or other appropriate vessel as is known in the
art.
According to prior art practice, the aqueous sodium carbonate in vessel 114 is
typically forwarded for further processing 120, that is, causticization, to
regenerate
NaOH to be used for cooking or bleaching, for example, for oxygen
delignification.
According to aspects of the invention, the carbonate in the green liquor in
vessel 114
may be used as one source of the carbonate 13 for pretreatment stage 14
described in
Figure 1.
100471 The improvements and advantages provided by aspects of the present
invention were investigated by the applicants in laboratory batch treatment
vessels,
14
CA 02651483 2008-11-05
WO 2007/137127 PCT/US2007/069159
that is, in laboratory batch digesters. The results of one such laboratory
trial are
summarized in Table 1. Such vessels are commonly used to develop processes for
continuous and batch digesters. In the present invention, a continuous or
batch
digester may be used.
[0048] Table 1 summarizes the treatment conditions and the results obtained by
a
series of laboratory trials using batch digesters. The present invention
corresponds to
trials 1, 3, and 4 in Table 1 where at least some sodium carbonate is provided
in a
first treatment stage followed by a soda-AQ (SAQ) pulping stage. Run number 2
is a
reference trial which corresponds to a conventional SAQ pulping stage with no
carbonate pretreatment and run number 5 is a reference trial which corresponds
to a
conventional haft cook without AQ.
Table 1
Effect of Carbonate Pretreatment on Soda/AQ Pulping
as Evidenced by Laboratory Batch Pulping Trials
Run Na2CO3 H CPC H SAQ Screened Rejects3 Kappa
Number in CPC' Factor End Factor End Yield3 No.4
in CPC pH in pH
SAQ
1 4.0% 312 7.6 1027 12.5 54.1 3.9 25.7
2 0 0 15062 11.0 28.4 27.0 48.1
3 6.0% 544 8.9 1027 12.6 53.4 6.7 33.9
4 4.0 665 1027 12.6 55.3 0.3 22.8
Kraft- 51.8 0.1 17.9
1300
'As Na20 on chips
210% Na20 from NaOH + 6% Na20 from Na2CO3. The other SAQ cooks were with
10% Na20 from NaOH only. The kraft cook used 12% Na20 from NaOH.
3Percent on chips
4Kappa number of screened pulp
[0049] In these laboratory trials, a 0.8 kg of sugar maple (Acer saccharum)
chips (a
hardwood) were loaded into a laboratory digester. The carbonate pre-treatment
summarized in Table 1 were performed at 165 C and 170 C using a Na2CO3 dose of
4.0 - 6.0% as Na20 on chips, that is, a 6.8 - 10.3% Na2CO3. In runs 1,3, and
4, the
carbonate pretreatment (CPC = Carbonate Pre-Cook) was followed by soda -AQ
pulping with a NaOH dose of only 10% as Na20 on chips. The initial testing
approach, after the carbonate pretreatment, depressurizing the batch digester
(that is,
15
WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
"blowing the digester") and condensing the carbonate stage off-gases (that is,
the
effluent). The NaOH and AQ were added to the condensed effluent and the
effluent
with NaOH and AQ was re-loaded to the digester with the chip sample. The
digester
was then heated back up to pulping temperature.
[0050] Table 1 includes the "H Factor" for the pretreatment and for the
cooking
stages of the trials. As is know, in the art, H factor normalizes alkaline
deliginification rates with temperature of a treatment. Typically, the higher
the H
factor the more rigorous the treatment. The H factor during the second heat-up
is
included in the total for the SAQ cooks. Table 1 also provides the pH of the
liquor in
the digester after the carbonate pretreatment and after the soda-AQ treatment.
[0051] The benefits of the present invention are reflected in the "screened
yield,"
"rejects," and "Kappa number" data that appear in Table 1. As known in the
art,
screened yield is the percent of the original chips present in the pulped
chips after
treatment and after the pulp has been screened to remove chips, fines, pins,
etc, that
were not sufficiently pulped in the process and other non-fibrous debris. As
is know
in the art, a higher relative screened yield is preferred. The rejects are an
indication of
how much of the original chips were not fully cooked by the process, for
example, as
isolated by screening. The lower the rejects the less wood is discarded or re-
treated.
As is also know in the art, kappa number provides a relative indication of the
amount
of undesirable lignin present in the resulting pulp. Typically, the higher the
kappa
number, the more lignin present in the pulp and the more bleaching chemical
required
to achieve a desired bleached brightness. Thus, a lower kappa number is
preferred.
[0052] The data in Table 1 supports advantages of the present invention. The
data
provided in Table 1 illustrate that the method shown in Figure 1 offers a
dramatic
improvement in conventional soda-AQ pulping. For example, as shown in Table 1,
the reject values are the clearest indication of the advantage of aspects of
the present
invention. According to Table 1, run number 4 - corresponding to carbonate
pretreatment and soda-AQ pulping - provides a value of 0.3 for rejects and a
screened
yield of 55.3%. In comparison, run number 2 - representing the prior art SAQ
pulping
¨provides a value of 27.0% for rejects and a significantly lower screen yield
of
28.4%. This implies that aspects of the present invention more efficiently
cook more
of the chips to produce a pulp while retaining a significant portion of the
cellulose as
16
WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
evidenced by the high yield. That is, the lower the rejects the more efficient
the
cooking process. Also, the data in Table 1 indicate that run number 2
(corresponding
to the prior art SAQ process) provides a lower SAQ end pH than runs 1, 3, and
4
(representing aspects of the invention) even though the NaOH application was
10%
Na20 on wood in all four cases. The applicants theorize that the lower pH of
the prior
art SAQ process is due to NaOH, a stronger base, out-competing Na2CO3 in
wasteful
carbohydrate degradation reactions. The Na2CO3 remains in solution in SAQ
cooking
and is not a strong enough base to cause the alkaline rearrangements necessary
for
lignin depolymerization and solubilization. When Na2CO3 is added by itself in
a
pretreatment, the reactive carbohydrates react with it because it is the only
alkali
present. Aspects of the present invention allow for more of the NaOH added to
stage
16 to go towards lignin depolymerization or degradation and thus may provide a
more
effective pulping process.
[0053] Cooking without carbonate pretreatment was also investigated in
laboratory
trials to provide a basis for comparison with the aspects of the present
invention. The
results of these trials are summarized in Table 2. In these trials, a series
of kraft and
SAQ pulps were prepared from sugar maple chips without carbonate pre-cooking.
One of the kraft pulping trials was made with a higher sulfidity and lower
effective
alkali (10% Na20 on chips from NaOH and 5% Na20 on chips from Na2S), see the
second line of data in Table 2. This kraft pulping trial (second line in Table
2)
provided a higher screened yield (approximately 1.0% higher screened yield on
chips)
compared to a kraft trial with a higher EA, that is, 12% Na20 on chips from
NaOH
and 4% Na20 on chips from Na2S (see the first line of data in Table 2). Both
pulps
had approximately the same unbleached kappa number (that is, 17.4 and 17.8).
However, the pulp produced using the lower EA (that is, 10% NaOH and 5% Na2S,
in
the second line of Table 2) appeared to be less responsive to oxygen (02)
delignification than the pulp produced with higher EA (12% NaOH and 4% Na2S),
as
will be discussed below. The applicants understand that the difference in post
-02
kappa numbers between these pulps to be significant. As also shown in Table 2,
a
series of SAQ trials were also performed. As shown, trials with varying EA and
AQ
charges of 0.1 percent on chips produced pulps having kappa numbers as low as
15.2
with relatively low rejects.
17
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WO 2007/137127 PCT/US2007/069159
Table 2.
Typical Properties of Treatment of Control Pulps
by Prior Kraft and Soda-AQ Pulping
(that is, without Carbonate Pretreatment)
Pulping NaOH' Na2S1 Cooking H Screened Rejects3 Kappa
Process Temp, Factor Yield3 No.4
C2
Kraft 12.0% 4.0% 165 1297 51.9 0.1 17.4
Kraft 10.0% 5.0% 165 1602 52.8 0.2 17.8
(52.9)5 (18.2)5
SAQ 11.0% 06 165 1602 51.7 8.0 42.0
SAQ 12.5 06 165 1602 52.3 1.1 21.5
SAQ 14.0 06 165 1602 52.8 0.6 19.3
SAQ 14.0 06 170 1969 51.1 0.2 15.2
'As Na20 on chips
290 minutes to temperature
3 Percent on chips
4 Kappa number of screened pulp
5Repeat experiment at a later date
60.1% AQ on chips
[0054] Carbonate precooking according to one aspect of the invention was
investigated in lab trials. The results of these trials are summarized in
Table 3.
Table 3 provides the treatment conditions for pretreating chips with carbonate
followed by a SAQ treatment (runs 1-5) and by a kraft treatment (runs 6 and
7). The
advantages of carbonate precooking according to aspects of the invention are
evident
by comparing the results presented in Table 2 (the prior art) to those
presented in
Table 3. For instance, if run 1 in Table 3 is compared to the conventional SAQ
in
Table 2 (the fifth line of Table 2) it can be seen that a 30 minute carbonate
stage at
165 C (the "Ci condition," see footnotes 2 and 3 of Table 3) decreased alkali
requirement in the SAQ stage from 14.0% (in Table 2) to 10% (in Table 3) as
Na20.
A comparison of the kappa numbers produced shows that the unbleached pulp
produced with carbonate pretreatment was slightly lower in kappa number (18.4
vs.
19.3) while the screened yield produced with carbonate pretreatment was
slightly
higher (53.1% vs. 52.8%). If carbonate-SAQ according to aspects of the
invention
(for example, run 5 in Table 3) is compared to carbonate-kraft (for example,
run 7 of
Table 3), it can be seen that a higher yield (1.0% on chips) is obtained for
carbonate-
18
CA 02651483 2008-11-05
WO 2007/137127 PCT/US2007/069159
SAQ compared to carbonate-kraft treatment. A higher yield results in a more
efficient
process and thus a high screened yield is desired.
[0055] Aspects of the present invention may also result in an H-factor
decrease in the
cooking stage that is greater than would be expected. When comparing the
result as
describe in Table 2 to those of Table 3, it is evident that the H-factor
provided in the
treatment performed without carbonate pretreatment is higher than the total H
factor
for carbonate pretreatment plus cooking to the same kappa number. For
instance, the
H factor for pulp that was not pretreated (lines 3-5 in Table 2) was over 1600
for
SAQ. The H factor for the SAQ stage for the first two carbonate (C1)
pretreated pulp
(runs 1-2 in Table 3) was 1239, and the Cl stage H factor was 358 for a total
of 1597
for the combined carbonate and SAQ treatments. The resulting carbonate
pretreated
pulp with kappa number 18.4 (line 1 of Table 3) had an improved pulp screened
yield, improved bleachability, and resulted in a lower amount of rejects than
the
unpretreated pulps. A further comparison illustrating the benefits of aspects
of the
present invention may be found in Figure 3, as discussed below.
Table 3.
Effect of Carbonate Pretreatment on Soda-AQ
and Kraft Pulping of Sugar Maple Chips
Run C-Stage H Factor Process' Screened Rejects Kappa
Number End pH in Yield No.2
Pulping
1 8.4 (C1)3 1239 SAQ 53.1 0.1 18.4
2 7.94(C,) 1239 SAQ 52.7 0.1 17.2
3 8.4 (CO 1239 SAQ5 54.7 0.8 26.4
4 9.8 (CO 1445 SAQ5 53.9 0.2 22.7
-7.0 (C2) 1239 SAQ 52.2 0.1 16.7
6 8.4(C,) 1027 Kraft 52.1 0.3 18.0
7 -7.0(C2) 1239 Kraft 50.8 0.1 15.6
10% Na20 for SAQ; 8.5% and 3.0% from NaOH and Na25 in kraft
2Kappa number of screened pulp
3Cl= 30 minute at 165 C; C2= 60 minutes, 4.0% Na20 on chips for all runs
except
run 4 where 5.0% was used.
4 AQ added to C-stage instead of SAQ
5 Only 9% Na20 in SAQ
[0056] Following the cooking trials discussed above and summarized in Table 2
and Table 3, the cellulosic material was bleached. The bleaching sequence used
in
19
WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
the laboratory test was an ODoEpDi (where 0 is alkaline 02 stage; Do is
chlorine
dioxide delignification with an end pH of 2-3; Ep is alkaline extraction with
sodium
hydroxide and hydrogen peroxide for incremental delignification; D1 is a
chlorine
dioxide brightening with an end pH of 3.4-4.5). The chlorine dioxide
application in
the Do stage is based on the formula: wt% chlorine dioxide on pulp = 0.076 x
kappa
number of 02 pulp. According to laboratory test performed, the applicants
found that
the 21.5 kappa number pulp produced in the soda-AQ pulp trial (that is, the
data in the
fourth line of Table 2) could be produced with 12.5% Na20; however, this pulp
was
more difficult to bleach than the pulp produced when 14.0% Na20 was used (that
is,
the data in the fifth and sixth lines of Table 2). The kappa number decrease
due to 02
is typically an indicator of the ease of bleachability. According to aspects
of the
invention, the bleachability of a soda-AQ pulp is improved by the carbonate
pretreatment. It will be understood by those with skill in the art that,
though this
specific bleaching sequence was used in this investigation, according to
aspects of the
invention, any suitable known bleaching process can be used, including
bleaching
processes that eliminate chlorinated compounds, that is, totally chlorine free
(TCF)
bleaching processes a eliminatel elemental chlorine, that is, elemental
chlorine free
(ECF) bleaching processes.
[0057] Figure 3 provides a plot of a comparison of screen yield as a function
of
post oxygen delignification kappa number for aspects of the present invention
and the
prior art. As shown, aspects of the invention provide a pulp that is more
easily
bleached compared to conventional prior art methods. As known in the art,
screen
yield percent is a ratio of the weight of the output pulp of the process over
the weight
of the input wood to the process after the output pulp has been screened to
remove
knots, shives, and other unwanted material. A higher screen yield is
preferred. The
02 kappa number relates to the lignin content of the cooked and oxygen-
delignified
pulp. As is known in the art, a lower kappa number is preferred. With regard
to
Figures 3 and 4, the following notations are used: A = Acid; C = carbonate
pretreatment; Kr= haft treatment; SAQ= soda anthraquinone treatment. The
designation "(M + N)" means that the treatment was performed at M % NaOH on
wood as Na20 and N % Na2S on wood as Na20. Methods of the present invention
further include an oxygen delignification treatment resulting in a pulp having
a lower
kappa number after the oxygen delignification treatment at a predetermined
screened
20
WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
yield compared to a pulp produced by the method without practicing a
pretreatment
stage 14.
[0058] As shown in Figure 3, there is a dramatic improvement for carbonate
pretreated Soda-AQ ("X" C-SAQ) as compared to soda-AQ pulp without
pretreatment
("1" SAQ). According to the results presented in Figure 3, the kappa number
which
has been produced by carbonate pretreatment followed by soda AQ ("X" C-SAQ)
cooking achieves a lower post 02 bleaching kappa number than the kappa number
for
soda-AQ pulp without pretreatment ("1" SAQ). For instance, the 02 kappa number
for C-SAQ ("X") is just over 8 kappa with a screened yield percent of over 52%
and
the 02 kappa number for SAQ ("1") is approximately 10.5 kappa with a
comparable
screened yield of approximately 52.5%. That is, at comparable screened yields,
an
aspect of the present invention provides a lower kappa number compared to the
prior
art. Therefore, the carbonate treatment decreases the kappa number (both
unbleached
and after the 0 stage).
[0059] In these laboratory trials, yield loss in all the oxygen stages was
approximately 1.4-1.8% on pulp. The small differences in yield loss during 0
stages
are insignificant when converted to a "percent on chips" basis. Carbonate
treatment
or acid treatment followed by carbonate pretreatment (that is, "AC-
pretreatment") did
not improve the fiber yield of haft pulping. However, as shown in Figure 3,
carbonate and acid + carbonate pretreatments ("X" C-SAQ and "*" AC SAQ)
clearly
improved the yield of SAQ pulping ("1"). That is, according to aspects of the
invention, a higher screened yield is provided compared to conventional SAQ
pulping
methods.
[0060] Figure 4 provides a plot of a comparison of fully bleached pulp as a
function of post oxygen delignification kappa number for aspects of the
present
invention and the prior art. The ordinate in Figure 4 is the light absorption
coefficient
(LAC) value for the fully bleached pulp after ODoEpDi bleaching and the
abscissa is
the post 02 kappa number. A review of Figure 4 reveals that the conventional
soda-
AQ pulps ("1") contain more color (that is, have a higher LAC values) than
kraft
pulps ("closed diamond"). However, the soda-AQ pulp after acid (A) and
carbonate
(C) pretreatments ("open triangle" and "open diamond") according to aspects of
the
21
CA 02651483 2008-11-05
WO 2007/137127 PCT/US2007/069159
invention had equal or slightly lower color content than kraft pulps without
pretreatments.
[0061] Further laboratory tests were performed to compare properties of
bleached
kraft, bleached acid carbonate kraft, and bleach acid carbonate soda
anthraquinone
pulps. Several properties of bleached kraft pulp according to the prior art,
Acid
Carbonate (AC)-kraft pulp according to an aspect of the invention, and Acid
Carbonate (AC)-SAQ pulps according to an aspect of the invention from sugar
maple
chips are documented in Table 4. As shown in Table 4, the soda AQ pulp
pretreated
with both acid and carbonate treatments resulted in an improved yield. The
screen
yields were highest for AC-SAQ, a value of 53.2, compared to a yield under 52
for
both the prior art kraft and AC-kraft pulps. The AC pretreatment increased the
final
brightness of the kraft pulp from 91.0 to 92.6.
Table 4
Properties of Bleached Pulp Produced by Prior Art Kraft Process
and by AC-Kraft and AC-SAQ Pulping Processes According to Present
Invention
Kraft AC-Kraft AC-SAQ
Unbl. Kappa Number 17.4 17.8 19.3
Screened Yield' 51.8 51.4 53.2
Rejects' 0.1 0.1 0.1
Post 02 Kappa 9.2 8.7 9.8
Number
02 Brightness2 ¨55.0 57.4 54.1
ODEp Brightness 82.8 83.5 82.3
Final Brightness 91.0 92.6 91.2
Final LAC3, m2/kg 0.182 0.121 0.173
Bleached Pulp Yield' 49.8 49.5 51.2
1% on chips
2 % Elrepho
3 Light scattering coefficient = 40.8 m2/kg for all three pulps
[0062] When cooking in accordance to aspects of the invention, as illustrated
in
Figure 1, at least some of the A-stage (acid stage) effluent may be displaced,
for
example, 50% to 75% of the total A stage effluent, with the carbonate-
containing
solution. In one aspect, at least some and possibly all of the carbonate stage
effluent
may be transferred with the chips into the kraft or SAQ stage. In the
laboratory trials
reported on so far, the carbonate stage effluent was recovered after passing
through a
condenser. The NaOH was introduced to the effluent as pellets and AQ powder
was
22
WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
dissolved in the condensed effluent. The carbonate and AQ containing effluent
was
then returned to the lab digester for the SAQ stage. Sodium sulfide solution
and
NaOH pellets are added to the carbonate stage effluent in the case of kraft
pulping.
For the purpose of this application, brightness is percent elrepho.
[0063] The applicants surmise that the use of some or all of the carbonate
stage
effluent may be important to aspects of the invention, for example, to the
carbonate-
SAQ process. The applicants believe that this may be because it is likely that
the
carbonate effluent may contain many low molecular weight carbohydrates with
reducing end groups. It is estimated that carbohydrate dissolution during the
carbonate stage may be approximately 5% of the original chip mass (oven dry
basis).
The applicants believe that random hydrolysis of the carbohydrates would be
expected and formation of a new reducing end group is likely with each
hydrolysis.
The higher concentration of reducing end groups would reduce AQ at a higher
rate to
form AHQ (anthrahydroquinone), that is, the active delignification catalyst.
The
AQ/AHQ catalytic cycle is shown schematically in Figure 5. Also, when a
reducing
end group in the solid phase is oxidized to a carboxylic acid it becomes
resistant to the
alkaline peeling reaction that lowers the molecular weight of carbohydrates
and
decreases pulp yield. The hypothesis above may explain why the carbonate stage
improves pulp yield for SAQ pulping, but not for kraft pulping. The oxidation
of
reducing end groups to carboxylic acids is not known to be a significant
reaction (or
even occur) in the haft process, or the soda process without AQ addition.
[0064] In further support of the benefits of aspects of the invention, further
laboratory research was performed with chips from a mixture of woods. For
instance,
a chip mixture comprising approximately 60% eastern cottonwood (Populus
deltoids)
clone, approximately 20% white birch (Betula papyrifera), and approximately
20%
sugar maple was used. The results for prior art haft pulping, prior art soda-
AQ
pulping and pulping according to aspects of the invention, that is, with acid
and
acid/carbonate pretreatments are presented in Table 5. The acid pretreatment
conditions were:
Ai 20 min. at 150 C with 1.5% acetic acid on chips (end pH-3.5); and
A2: 60 min. at 120 C with 3.0% acetic acid (end pH 3.2).
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WO 2007/137127 PCT/US2007/069159
[0065] As indicated in Table 5, the A1 pretreatment provided a soda-AQ pulp
with
higher bleached (that is, "Final") brightness (90.8) than haft (89.9), but
pulp yield
was somewhat lower. The milder A2 pretreatment with an acid carbonate-SAQ
provided a higher pulp yield (54.2%) than both conventional haft pulping and
SAQ
pulping. The A2C-SAQ treatment also afforded a higher bleached brightness
(89.2)
than SAQ pulping (88.4) but less than haft pulping (89.9). In one aspect of
the
invention, the applicants believe that that the severity of the acid (A) stage
may
preferably be higher than the A2 acid treatment, but lower than the A1 acid
treatment
for this chip furnish. As also indicated in Table 5, the rate of tensile
strength
development after 2,000 PFI revolutions of refining, as indicated by "tensile
index,"
for acid-kraft pulping (78.3) and acid-SAQ pulping (86.1) was lower than for
kraft
pulping (94.4). However, acid-carbonate-SAQ had the highest rate (that is,
100.2),
further underscoring the benefits of aspects of the invention. From the
results that
appear in Table 5, it appears that a pulp with a higher yield and probably
more
hemicelluloses required less refining. There were no significant differences
in the
tensile-tear curves.
Table 5
Effect of Acid and Acid/Carbonate Pretreatments on Kraft and Soda-AQ Pulping
of Mixed Hardwoods
Kraft Al-Kraft SAQ Al-SAQ A2C1-SAQ1
Screened Yield 53.2 51.1 53.0 52.7 54.2
Rejects 0.2 0.1 0.3 0.1 0.2
Unbleached 16.7 17.1 16.6 15.9 18.9
Kappa
02 Kappa 9.6 9.6 9.8 9.4 9.5
Final Brightness 89.9 91.8 88.4 90.8 89.2
(% elrepho)
CSF2 412 487 429 375
Tensile Index2 94.4 78.3 86.1 100.2
lEA=10% Na20; 14.0% for other cooks
2Canadian Standard Freeness and tensile index at 2000 PFI revolutions (light
load) for
02 delignified pulps
[0066] Further, milder carbonate stage conditions were studied to fully
understand
the benefits of carbonate-SAQ pulping and to further differentiate the present
24
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WO 2007/137127
PCT/US2007/069159
invention from the earlier research in '241 where no anthraquinones were
involved.
The results of this investigation are summarized in Table 6. A chip furnish
was
prepared consisting of 50% sugar maple, 40% eastern cottonwood and 10% white
birch. Carbonate treatment stages according to aspects of the invention were
performed at 130 C and 140 C with either 3.0 or 5.0% Na2CO3 charge (as Na20)
on
wood and for a time period of either 30 or 60 minutes. In these trials it took
approximately 30-35 minutes to achieve maximum temperature. About 70% of the
carbonate stage effluent was drained off and discarded after the carbonate
treatment.
The carbonate effluent was replaced by distilled water when the pulping
chemicals
were added for SAQ or kraft cooking.
Table 6
Effect of Carbonate Pre-Cooking at 130 degrees C and 140 degrees C
Run C Temp, Time, End H Factor Screened Rejects3 Kappa No.4
Number C Min. pH' in S or K2 Yield3
1 130 30 8.0 816 (5) 55.1 2.8 28.8
(3.0%)5
2 130 (3.0%) 60 7.8 816 (5) 55.8 1.4 27.6
3 130 (5.0%) 30 9.1 816 (5) 55.1 1.1 23.4
4 140 (3.0%) 30 8.2 816 (5) 55.0 1.5 24.4 (12.3)
140 (3.0%) 60 7.9 816 (5) 55.8 0.6 23.1 (10.7)
6 140 (3.0%) 60 7.5 816 (K) 54.4 0.4 20.9
7 140 (5.0%) 30 8.6 816 (5) 54.5 0.3 19.3 (10.2)
8 992 (K)6 53.5 0.5 17.3 (9.0)
9 992(5) 54.2 1.0 22.6
lEnd pH of C stage
2S = SAQ; K= Kraft
3 Percent on chips
4 Kappa number of screened pulp; 02 value in parentheses
5 Na2CO3 applied as % Na20 on chips
[0067] According to the result presented in Table 6, the best results were
obtained
with a carbonate-SAQ treatment having a carbonate stage of 60 minutes in
duration at
140 degrees C with 3% Na2CO3 on chips (as Na20) (for example, see run 5). The
pulp from run 5 had a screened yield on chips of approximately 2.0% higher
than run
8 (the reference haft cook) and only a 1.7 kappa unit disadvantage after
oxygen
delignification as compared to run 8. When comparing the kraft pulp with
pretreatment (run 6) the carbonate-SAQ pulp (run 5) had a yield advantage of
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WO 2007/137127 CA 02651483 2008-11-05PCT/US2007/069159
approximately 1.0% on chips. Such a yield advantage would translate to a net
increase in profit of approximately $4M/annum for a typical chemical pulp
mill.
[0068] In addition, the combined H factor for the carbonate (C) and soda-AQ
stages in run 5 is 897 (81+816) produced a pulp having approximately the same
kappa
number (23.1 vs. 22.6), higher yield (55.8 vs. 54.2) and lower rejects (0.6
vs. 1.0)
compared to a prior art SAQ produced pulp without the pretreatment but with a
H
factor of 992 (run 9). Time spent in the carbonate stage may have a
significant impact
on the resulting pulp. In laboratory tests, an additional 30 minutes in the
carbonate
stage at pH ¨ 8 (see run 4 vs. run 5) resulted in a 1.6 kappa unit advantage
after soda-
AQ and oxygen delignification. Further, the extra 30 minutes in the carbonate
stage
afforded a higher screened yield. Additionally, it is believed that more
reducing end
groups were generated in the extra 30 minutes spent in the carbonate stage,
and the
longer overall cooking time converted more rejects to screened fibers.
[0069] Aspects of the invention can be performed in equipment for a batch
(such as
but not limited to conventional, SuperBatch or Rapid Displacement Heating) or
continuous (such as but not limited to conventional soda, conventional haft,
Lo-
Solids Cooking, EMCCO Cooking, ITC Cooking, and Compact Cooking) where
pressurized equipment (as required for the carbonate treatment) is used for
any or all
of the stages of acid, carbonate and cooking. Aspects of the invention are
also
amenable to pretreatment during transport or storage of comminuted fibrous
material,
for example, as described in U.S. Patent 6,55,462, the disclose of which is
incorporated by reference herein. For batch systems, conventional means such
as
pumping of liquor into the vessel can be used to displace the liquid in the
digester, or
the liquor can be discharged from the digester by inherent pressure or pump-
out
means, before the new cooking liquor is added to the digester. All liquids can
be
preheated using methods known in the art, such as extracted liquor from
SuperBatch
or RDH methods. Further, heating of the liquid can be accomplished in the
vessel by
circulation loops or direct steam addition.
[0070] A described herein, aspects of the present invention provide cellulosic
material pretreatment process and pulping process that provides advantageous
improvements to prior art treatments of wood chips, and related comminuted
cellulosic materials. As made clear from the test data presented herein,
pretreatment
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CA 02651483 2012-05-08
of wood chips with a carbonate solution, with or without the presence of an
anthraquinone can produce cellulose pulps that are higher in yield, lower in
rejects,
greater in strength, and require less chemical to both produce and bleach.
100711 While several aspects of the present invention have been described and
depicted herein, alternative aspects may be provided by those skilled in the
art to
accomplish the same objectives.
* * * * *
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