Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ENVIRONMENTALLY-FRIENDLY FIBERLINE FOR PRODUCING BLEACHED
CHEMICAL PULP
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon provisional application serial no. 60/167,537
filed
December 2, 1999, the disclosure of which is hereby incorporated by reference
herein.
BACKGROUND AND SUMMARY OF THE INVENTION
Increased concerns about the impact of the chemical pulping and bleaching 5
process has resulted in attempts to, among other things, limit the use of
chlorine-containing
compounds in the processing of cellulose material to make cellulose pulp for
the
manufacture of paper. This is especially true in the bleaching of cellulose
pulp in which
attempts have been made to replace the use of oxidative chlorine-containing
compounds
with more environmentally benign oxygen-containing compounds such as oxygen,
peroxide, and ozone. For example, the recently proposed legislation enacted by
the U.S.
Environmental Protection Agency (EPA), referred to as the "Cluster Rules",
limits the
release of certain chemical compounds associated with chlorine-containing
bleaching
agents in both the gaseous and liquid effluents leaving a pulp mill.
As a result, various Totally-Chlorine-Free (TCF) or Elemental-Chlorine Free
(ECF)
bleaching sequences have been proposed and some are in commercial operation.
For
example, in TCF bleaching, no chlorine-containing chemicals, in particular no
chlorine (CI2,
or simply designated "C") or chlorine dioxide (C102, or simply "D"), are used
in the
bleaching of pulp. In ECF bleaching, elemental chlorine is eliminated from the
bleach plant
in favor of more environmentally benign chlorine dioxide. Typical, TCF
bleaching
sequences include the use of oxygen gas (02, or simply "0"), ozone gas (Os, or
simply "Z"),
and hydrogen peroxide (H202, or simply "P"), among others, while ECF bleaching
sequences also include the use of chlorine dioxide (D).
Also, TOF and ECF bleaching sequences are susceptible to the damaging effects
of
the presence of dissolved metal ions such as dissolved iron (Fe), copper (Cu),
and
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manganese (Mn), among others. Not only can these "non-process elements" become
concentrated in the "closed-cycle" bleaching effluents of the TCF and ECF
bleaching
sequences, but these metals, especially, Mn, are recognized as detrimental to
the effective
treatment of pulp with non-chlorine bleaching agents, most notably, hydrogen
peroxide. As
a result, it is also desirable to limit the presence of such dissolved metals
in the pulp and
liquid treatment streams in and around the pulp mill. Typical treatments used
to minimize
the prescience of these metal include acid washing and chelate treatment, for
example,
using conventional chelating agents EDTA and DPTA, among others.
There is also interest in reducing the release of gases containing malodorous
sulfur-
containing compounds, such as hydrogen sulfide (H2S), methyl mercaptan
(CH3SH),
dimethyl sulfide (CH3SCH3), and dimethyl disulfide (CH3S-zCH3), among others.
(collectedly
referred to as Total Reduced Sulfur (TRS) gases) from the gases released from
pulp mills.
These sulfur-containing gases are generally associated with the "sulfate" or
"kraft" pulping
process in which the predominant pulping chemicals are Sodium sulfide (Na2S)
and sodium
hydroxide (NaOH). One alternative to the kraft process is the "soda" process
in which the
active pulping chemical is limited to NaOH alone, that is, no sulfur-
containing chemicals are
used in the pulping process (though some sulfur may be introduced via the wood
supply or
through oil burned in the mill's boilers).
However, the soda process is recognized as producing a somewhat inferior
product
than the kraft process. That is, the soda process typically produces a pulp
having lower
yield and strength properties than the kraft process. To address this
limitation of the soda
process, attempts have been made to introduce strength or yield-enhancing
additives to
the soda process to improve the quality of the pulp produced. Most notably,
anthraquinone
(AQ) has been introduced with limited success to provide what has been known
as the
"soda-AQ" process.
One recently recognized effect of the treatment of cellulose material, that
is, wood
chips, with AQ is the undesirable "yellowing" of the resulting pulp from the
AQ treatment.
That is, pulps produced in the presence of AQ have exhibited a yellow or
orange color
which, especially for fully-bleached pulps, is undesirable. It has been
recognized according
to the invention that an effective method of reducing or eliminating this
undesirable
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discoloration is by treating the AQ-treated pulp with an oxidizing agent, for
example, air,
oxygen gas, peroxide, or most preferably ozone gas.
In addition, it has also been recently recognized that the processes described
in
U.S. patents 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775;
and
others, and marketed under the trademark LO-SOLIDS (by Andritz-Ahlstrom Inc.
of Glens
Falls, NY, produces cellulose pulp having improved strength properties
(manifest as an
increase in viscosity) compared to pulps produced by conventional methods,
especially in
comparison to the soda process. In addition, the use of ozone has also been
known to
negatively affect pulp viscosity. Thus, the embodiment of the present
invention employing
LO-SOLIDS cooking methods and AQ treatment can offset the negative effects of
soda
cooking and ozone bleaching on the strength and quality of the pulp produced.
Thus, by eliminating or minimizing the presence of sulfur in the pulping
process and
chlorine in the bleaching process, while maintaining or improving the quality
(that is, the
strength and yield of the bleached pulp produced) the present invention
provides an
environmentally-friendly and commercially viable process for producing fully-
bleached
cellulose pulp from comminuted cellulosic fibrous material.
One embodiment of the present invention comprises a method of treating a
slurry of
comminuted cellulosic fibrous material to produce a bleached chemical pulp,
comprising or
consisting of the following: a) treating the material in a first stage with a
gas containing
ozone; and b) treating the material in a second stage with a liquid containing
chlorine
dioxide; wherein between a) and b) the material is treated with an alkaline
liquid to raise
the pH of the material prior to b) and wherein no washing is performed between
a) and b).
Preferably between a) and b) the pH of the material is raised to at least
about 6.0,
preferably, at least 7Ø The present invention may be expressed by the
notation (ZEND),
where Z represents an acidic treatment with an ozone containing gas, EN is an
alkaline
neutralization treatment, and D is an alkaline treatment with chlorine
dioxide; again, no
washing is performed between these treatments.
According to the present invention, the alkaline neutralization treatment, EN,
performs at least two recognized functions. First, the increase in alkalinity
to an
approximately neutral pH, that is, to a pH of between about 6 and 8,
preferably, about 7,
solubilizes the products of reaction from the prior Z stage. For example, the
lignin
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containing compounds oxidized in the Z stage are kspt in solution by the
addition of alkali
to the ozonated pulp. Where in conventional alkaline extraction stages, these
solubilized
compounds are removed in a subsequent washing of the pulp prior to further
treatment,
according to the present invention, no washing (the term "washing" also
includes pressing
and thickening) is necessary, and the solubilized lignin-containing compounds
are passed
to the following alkaline chlorine dioxide treatment. The lignin-containing
compounds can
be subsequently removed in a washing stage following the chlorine dioxide
stage.
Secondly, the introduction of alkali in the neutralization stage also
increases the pH of the
pulp to a range that is more compatible with the subsequent alkaline D stage.
It is known in
the art that it is preferable to initiate the chlorine dioxide treatment in an
alkaline state,
however, during chlorine dioxide treatment the pH typically drops as acids are
formed. In
effect, the alkali introduced in the EN stage offsets the amount of alkali
that needs be
added to adjust the pH for the D stage. According to the present invention,
not only is a
more effective treatment of the pulp obtained, that is, a more effective
removal of
chromophoric compounds, but also the need for an expensive washing device is
obviated.
In a preferred embodiment, prior to a) the material is treated in an alkaline
chemical
pulping process, preferably an essentially sulfur-free pulping process, such
as a soda
pulping process. This alkaline pulping process also preferably includes
treatment with a
strength or yield enhancing additive such as anthraquinone or polysulfide, or
their
equivalents or derivatives. The preferred additive is anthraquinone, AQ,
because AQ has
been shown to off-set the reduced yield characterized by the soda process, in
comparison
to the kraft process. In addition, the alkaline pulping process preferably
includes a bulk
delignification stage and the process includes at least one stage prior to or
during the bulk
delignification stage in which a liquid containing a first level of dissolved
organic material is
removed from the material and replaced with a second liquid having a lower
(e.g. at least
50% lower) level of dissolved organic material, as described in the
aforementioned patents
(the disclosures of which are hereby incorporated by reference herein).
Also, the treatment a) is preferably preceded by, c), treating the material
with a liquid
containing chlorine dioxide followed by d) treating the material with an
alkaline liquid.
Specifically, it is preferred that the treatment c) is a treatment with
chlorine dioxide (D) and
d) is a treatment with an alkaline liquid to solubilize and extract (E) the
oxidized products
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produced in c). The alkaline liquid in d) is typically sodium hydroxide. That
is, one preferred
embodiment is the bleaching sequence D-E-(ZEND), in which washing is performed
before, between, and after the first D and first E stages (as indicated by the
hyphens in the
bleaching sequence designation) and no washing is performed between the Z, EN
and D
5 stages (as indicated by enclosing those symbols within parenthesis without
hyphens
between them in the bleaching sequence designation). The first treatment with
the alkaline
liquid (E) may also include the presence of oxygen, a peroxide, or both. That
is, this stage
may also be an Eo, EP or EoP stage.
Another embodiment of the present invention includes a method for producing
bleached chemical pulp from comminuted cellulosic fibrous material comprising
the
followina: a) treating the material in a chemical pulping process in the
presence of chemical
additive to produce a chemical pulp containing at least some of the additive;
b) treating the
chemical pulp with at least one elemental chlorine-free bleaching agent to
produce a
bleached chemical pulp having at least some discoloration due to the presence
of the
chemical additive; and c) treating the bleached pulp with at least one
oxidizing agent to
remove the discoloration produced by the presence of the chemical additive. In
a preferred
embodiment the chemical additive used in the pulping process of a) is
anthraquinone or its
equivalents or derivatives. The discoloration of the pulp due to the presence
of the additive,
typically AO, is characterized by a yellowish or orange tinge to the pulp.
The at least one bleaching agent used in the treatment b) is preferably one or
more
of the following bleaching agents: oxygen, chlorine dioxide, sodium hydroxide,
ozone,
hydrogen peroxide, but most preferably comprises at least chlorine dioxide.
The oxidizing
agent used in c) is typically air, ozone, oxygen, or peroxide, but is
preferably an ozone-
containing gas.
According to another aspect of the invention there is provided a method of ECF
treatment of comminuted cellulosic fibrous material comprising the sequence
sodalAQ
cooking, and then one of D-EP-(ZEND), or D-Eo-(ZEND), or D-EoP-(ZEND). The
pulp so
produced has a brightness of at least 89 or 90% ISO, and a viscosity of over
21 or 22 cP.
According to the present invention, an environmentally-friendly and
commercially
viable process for producing fully-bleached cellulose pulp from comminuted
cellulosic
fibrous material is provided. While the invention has been described in
connection with
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what is presently considered to be the most practical and preferred
embodiment, it is to be
understood that the invention is not to be limited to the disclosed
embodiment, but on the
contrary, is intended to cover various modifications and equivalent
arrangements and
methods and to be limited only by the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
_ FIGURE 1 is a schematic illustration of a complete fiberline of one
exemplary
embodiment according to the present invention;
FIGURE 2 is a schematic detailed illustration of a subsystem of the fiberline
of
FIGURE 1;
FIGURE 3 is a graph of viscosity versus brightness showing the properties of
pulp
produced according to the present invention compared to various prior art
processes; and
FIGURE 4 is a graph like that of FIGURE 3 only plotting brightness versus
chlorine
dioxide consumption.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of a typical complete environmentally-
friendly
and commercially viable fiberline, or cellulose material treatment system, 10,
according to
the present invention. Comminuted cellulosic fibrous material 11, typically
hardwood or
softwood chips, though any other form of comminuted cellulosic fibrous
material may be
treated according to the invention, is introduced to a digester 12. Though the
present
invention is most amenable to treatment in a continuous digester, digester 12
may also be
a batch digester. According to the preferred embodiment of this invention, the
pulping
process is preferably a sulfur-free alkaline cooking process, preferably a
sodalAQ process.
Caustic soda, or sodium hydroxide, is introduced to digester 12 along with the
chips via
conduit 13 and AQ, or its equivalent or derivative, is introduced via conduit
14. The treated
pulp is discharged from the digester into conduit 15.
As is conventional, the cooked pulp is typically treated in one or more
brownstock
washers 16 to recover some of the cooking chemical and to clean the pulp
slurry prior to
further treatment. The cleaned pulp is passed via conduit 17 to optional
alkaline oxygen
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delignification 7 8 and post oxygen delignification washing 20 via conduit 19.
The oxygen
treatment is preferably performed in one or two stages at medium consistency,
that is, at a
pulp consistency between about 8 and 18%. The washed pulp, with or without
oxygen
treatment, is passed to further treatment via conduit 21.
Conduit 21 introduces the pulp to the first formal bleaching stage 22, which
is
preferably an alkaline chlorine dioxide treatment, DO. This treatment 22 may
be performed
at medium consistency or at low consistency, that is, at a pulp consistency
between 0.5 to
18%, preferably between about 2-8% (and all other specific narrower ranges
within the
broad range). The treated pulp is then passed via conduit 23 to washer 24.
Washer 24
may be any conventional washer, such as a vacuum drum washer, a wash press, a
diffusion washer, or a belt washer. Washing in one or more washers 24 may be
performed
at low or medium consistency as is conventional.
The washed pulp is passed via conduit 25 to alkaline extraction stage (E) 26
(which
may be an Eo, EP, or EoP stage) which is a conventional alkaline treatment
with sodium
hydroxide which solubilizes the products of reaction of stage 22. The pulp
from extraction
stage 26 is forwarded to another conventional washing stage 28, similar to
stage 24, via
conduit 27. The washed pulp is forwarded to the ozone treatment stage 30 via
conduit 29.
Treatment stages 30, 34 and 38 constitute the broadest embodiment of the
present
invention. In stage 29 the pulp, preferably at medium consistency (e.g. about
6-18%),
though it may be at low (.5-5%) or high (over 18%) consistency, is treated
with an ozone
containing gas introduced via conduit 31. This ozone-containing gas typically
is generated
and introduced using of a carrier gas, for example, oxygen or air, at a
concentration greater
than 5%, typically between about 5 and 15%. Though this ozone containing gas
is typically
produced on-site, it may be supplied from an external source. The ozone-
containing gas is
typically provided in a pressurized state to a high-intensity mixer, for
example, to an AMZ
mixer supplied by Andritz-Ahlstrom Inc. of Glens Falls, NY. The treatment in
stage 30 is
typically in an acidic environment, typically, at a pH less than 5, preferably
a pH between
about 2 and 3. After ozone treatment 30, the treated pulp is passed to
alkaline treatment 34
via conduit 32. If desired, due to the relatively large volumes of gas
introduced to the pulp
via conduit 31, some de-gassing 33 may be performed after treatment 30 and
before
treatment 34, but no washing (which term includes pressing and thickening).
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Alkaline treatment 34 is typically a relatively short exposure of the ozonated
pulp to
alkali, typically NaOH, introduced via conduit 35. The retention time in
treatment 34 is
typically less than 30 minutes, preferably less than 15 minutes. As discussed
above, the
treatment 34, among other things, neutralizes the acidic pulp discharged from
treatment 30
that the products of the reaction in treatment 30 are maintained in a
solubilized state. The
pH in stage 34 is typically between about 6 and 8, preferably about 7.
However, unlike the
prior art, these solubilized compounds are not washed out of the pulp after
treatment in 34
but are passed directly to alkaline treatment 38. D1, via conduit 36. As
before, some de-
gassing 37 may be performed between stages 34 and 38, if necessary. The
unwashed
pulp introduced to treatment 38 via conduit 36 is treated with chlorine
dioxide, C102. The
chlorine dioxide is introduced to the pulp via conduit 39 and a mixer (see
mixer 59 in
FIGURE 2), for example, a conventional high-intensity mixer.
The treated pulp is then passed via conduit 40 to conventional washing in
stage 41
and then, if desired, to further treatment via conduit 42, for example, to
further elememtal
chlorine-free bleaching stages (so that the entire treatment of the pulp is
ECF), to storage,
to stock preparation before a paper machine, if the mill is an integrated
paper mill, or to a
pulp drier, if the mill is a market pulp mill.
FIGURE 2 illustrates a specific embodiment of stages 30, 34 and 38 shown in
FIGURE 1. Structures in FIGURE 2 that are similar to those in FIGURE 1 are
identified with
similar reference numbers, but in FIGURE 2 the reference numbers are 10
prefaced by the
numeral "1 ". For example, the conduit 129 in FIGURE 2 i s essentially the
same the as
conduit 29 in FIGURE 1.
Pulp from washer 28 in FIGURE 1 is introduced to treatment 130 via conduit 129
in
FIGURE 2. Treatment 130 is preferably performed in a mixer 50 to which an
ozone-
containing gas, typically ozone in an oxygen carrier gas, is introduced via
conduit 131. The
ozone treatment step is typically performed at a temperature less than
100°C, for example,
at a temperature between about 20 and 70°C, depending upon the energy
balance of the
system. The acidic ozone treatment 130 is typically performed at a pH less
than 5,
preferably, a pH between about 2 and 3. However, the ozone treatment 130 may
be
performed at a pH of greater than 5, even greater than 7, if ozone
decomposition and
cellulose damage can be avoided or minimized at such higher pH values.
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The ozonated pulp is discharged from the mixer 50 to conduit 132. Since the
reaction of the ozone with the pulp is very rapid, for example, the reaction
may typically
take less than 30 seconds, no retention vessel is normally necessary after the
mixer 50
and the ozonated pulp may be passed directly to the alkaline neutralization
stage 134.
However, if desired, a retention vessel 51 may be used after the mixer 50.
Though not
shown in FIGURE 2, the pulp may be de-gassed after mixer 50 and before stage
134.
The ozone-treated pulp is introduced to alkaline neutralization stage 134 via
conduit
132. Treatment 134 is effected by the introduction of alkali, preferably NaOH
via conduit
135, to mixer 52, again, preferably a high-intensity mixer. Again, due to the
rapid reaction
of ozone with the pulp, especially at medium consistencies, the alkali in
conduit 135 may
also be introduced directly to mixer 150 via conduits 62 and 131. In a
specific embodiment
of this invention, it may even be possible to eliminate the mixer 52 and
introduce both the
ozone-containing gas and the alkali to mixer 50.
Assuming the embodiment shown with two mixers, 50 and 52, after introducing
alkali to the pulp via mixer 52 the pulp is discharged to conduit 53. The pulp
is retained in
this alkaline state, typically at a temperature of between about 20 and
70°C, preferably at a
temperature of between about 50 and 70°C, for at least about 15
minutes, preferably at
least between about 30 minutes and about 60 minutes. This retention can be
effected in a
conventional retention vessel (not shown) or in a pump feed chute 54. In a
preferred
embodiment, at least some de-gassing 137 of the pulp is effected as the pulp
is introduced
to feed chute 54, in any suitable conventional manner. The pulp may also be de-
gassed in
a specially-designed rotary de-gassing device if desired.
From the feed chute 54, or retention vessel (not shown), the pulp is pumped to
chlorine dioxide treatment 138 via pump 55. The pump 55 may be a de-gassing-
type pump
from which gas is removed via conduit 56. The pump 55 passes the pulp through
conduit
57 to mixer 59, again, preferably a high-intensity mixer, to which chlorine
dioxide is
introduced via conduit 139. If desired, the pulp in conduit 57 may be heated
by introducing
steam via conduit 58. The pH of the pulp discharged to vessel 61 is typically
at least 5,
preferably at least 6. Additional alkali may also be added to the pulp, for
example, via
conduit 63. The chlorine dioxide is intimately mixed with the pulp in mixer 59
and
discharged to retention vessel 61 via conduit 60. The alkaline and chlorinated
pulp is
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retained in conventional retention vessel 61 (e.g. an upflow vessel) for at
least 1 hour,
typically the pulp is retained for between about 1.5 to 6 hours, preferably
between about 2
to 4 hours, at a temperature of between about 20 and 70°C, preferably,
between about 50
and 70°C. The treated pulp is discharged via conduit 140 to
conventional washer 141,
5 washed, and then forwarded on to further treatment, as discussed previously,
via conduit
142.
FIGURE 3 shows plots of three different treatment scenarios, a D-EoP-(ZEND)
treatment sequence according to the present invention indicated by reference
numeral 70,
and two prior art procedures, D-Eop-D-P indicated at 71, and D-EoP-D indicated
at 72. It will
10 be seen that the treatment 70 according to the present invention achieves
brightness
comparable to that of the procedure 71, yet with viscosity that is much
higher, and in fact
comparable to that indicated at 72. In all of the procedures 70, 71, 72
illustrated in
FIGURE 3, the feed pulp before the treatment sequence was LO-SOLIDS~ cooked
mixed
hardwood that was oxygen delignified to a kappa number of 9, exactly the same
feed pulp
being used for each of the treatments 70, 71, 72.
FIGURE 4 is a graph of the same pulps and treatment sequences as in FIGURE 3,
only showing brightness graphed against chlorine dioxide consumption. Again as
can be
seen the treatment procedure 73 according to the present invention had
brightness
comparable to that of the procedure 74, and much greater than the procedure
75, with
approximately the same (only slightly greater) chlorine dioxide consumption.
As clear from FIGURES 3 and 4, the treatment sequence D-E [typically either
Eo, or
Ep, or E°P] - (ZEND) according to the invention produces pulp having a
brightness of at
least 89 or 90% ISO, and a viscosity of at least about 21 or 22 cP.
The invention includes all specific narrower ranges within the broad ranges
recited
above. For example, a consistency of 6-18% means 6-12%, 8-11 %, 9-18%, and all
other
narrow ranges within the broad range.
Thus, according to the present invention, an environmentally-friendly and
commercially-viable process for producing fully-bleached cellulose pulp from
comminuted
cellulosic fibrous material is provided. While the invention has been
described in
connection with what is presently considered to be the most practical and
preferred
embodiment, it is to be understood that the invention is not to be limited to
the disclosed
CA 02327248 2000-12-O1
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embodiment, but on the contrary, is intended to cover various modifications
and equivalent
arrangements and methods included within the spirit and scope of the appended
claims.
