U.S. patent application number 10/633536 was filed with the patent office on 2004-02-12 for environmentally-friendly fiberline for producing bleached chemical pulp.
This patent application is currently assigned to ANDRITZ INC.. Invention is credited to Stromberg, C. Bertil.
Application Number | 20040026051 10/633536 |
Document ID | / |
Family ID | 26863255 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040026051 |
Kind Code |
A1 |
Stromberg, C. Bertil |
February 12, 2004 |
Environmentally-friendly fiberline for producing bleached chemical
pulp
Abstract
An environmentally friendly (ECF), yet commercially viable,
process produces fully bleached (typically having a brightness of
over 89 or 90% ISO) cellulose pulp using a (ZEND) treatment.
Typically pulp is produced in an essentially sulphur-free pulping
process (such as an soda/AQ process), and is then treated by a
D-E.sub.O-, D-E.sub.P-, or D-E.sub.OP-sequence before the (ZEND)
treatment. Optional oxygen delignification may also be used, and
subsequent elemental-chlorine-free bleaching sequences may also be
employed, although normally not necessary since the pulp after the
(ZEND) treatment has high brightness and good viscosity (e.g. over
21 cP).
Inventors: |
Stromberg, C. Bertil; (Glens
Falls, NY) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
ANDRITZ INC.
Glens Falls
NY
|
Family ID: |
26863255 |
Appl. No.: |
10/633536 |
Filed: |
August 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10633536 |
Aug 5, 2003 |
|
|
|
09722420 |
Nov 28, 2000 |
|
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60167537 |
Dec 2, 1999 |
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Current U.S.
Class: |
162/65 ; 162/72;
162/78; 162/88; 162/89 |
Current CPC
Class: |
D21C 3/222 20130101;
D21C 9/144 20130101; D21C 9/147 20130101; D21C 9/153 20130101; D21C
9/1057 20130101; D21C 9/163 20130101 |
Class at
Publication: |
162/65 ; 162/78;
162/88; 162/89; 162/72 |
International
Class: |
D21C 009/12; D21C
009/14; D21C 009/153; D21C 003/20; D21C 009/16 |
Claims
What is claimed is:
1. A method of treating a slurry of comminuted cellulosic fibrous
material to produce a bleached chemical pulp, comprising: (a)
treating the material in a first stage with a gas containing ozone;
(b) treating the material in a second stage with a liquid
containing chlorine dioxide; (c) between (a) and (b) treating the
material with an alkaline liquid to raise the pH of the material
prior to (b) and so that no washing is performed between (a) and
(b).
2. A method as in claim 1 wherein (c) is practiced to raise the pH
of the material to at least about 6.0.
3. A method as in claim 1 further comprising (d), prior to (a),
treating the material in an alkaline chemical pulping process, to
produce chemical pulp.
4. A method as in claim 3 wherein (d) is practiced using an
essentially sulfur-free pulping process.
5. A method as in claim 4 wherein (d) is practiced using an
alkaline chemical pulping process that includes treatment with a
strength or yield enhancing additive.
6. A method as in claim 5, wherein (d) is further practiced using
an alkaline chemical pulping process includes a bulk
delignification stage, and at least one stage prior to or during
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 an at least about 50%
lower level of dissolved organic material.
7. A method as in claim 1 wherein (a) is preceded by (a1) treating
the material with a liquid containing chlorine dioxide, followed by
(a2) treating the material with an alkaline liquid.
8. A method as in claim 7 wherein (a2) includes a treatment with
oxygen, a peroxide, or both.
9. A method as recited in claim 4 wherein (d) is practiced using a
soda pulping process.
10. A method as recited in claim 4 wherein (d) is practiced using a
soda/AQ pulping process.
11. A method as recited in claim 10 wherein (c) is practiced to
raise the pH of the material to at least about 7.0
12. A method as recited in claim 2 further comprising (d), prior to
(a), treating the material in an alkaline chemical pulping process
that includes anthraquinone, polysulfide, or their equivalents or
derivatives.
13. A method as in claim 2 wherein (a) is preceded by (a1) treating
the material with a liquid containing chlorine dioxide, followed by
(a2) treating the material with an alkaline liquid.
14. A method as in claim 13 further comprising (d), prior to (a),
treating the material in an alkaline chemical pulping process, to
produce chemical pulp.
15. A method as recited in claim 14 wherein (d) is practiced using
a soda/AQ pulping process.
16. A method as in claim 15, wherein (d) is further practiced using
an alkaline chemical pulping process includes a bulk
delignification stage, and at least one stage prior to or during
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 an at least 50% lower
level of dissolved organic material.
17. A method for producing bleached chemical pulp from comminuted
cellulosic fibrous material comprising: (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.
18. A method as in claim 17 wherein (a) is practiced using
anthraquinone or its equivalents or derivatives as the chemical
additive used in the pulping process.
19. A method as in claim 17 wherein (b) is practiced using as the
at least one bleaching agent one or more of the following bleaching
agents: oxygen, chlorine dioxide, sodium hydroxide, ozone, and
hydrogen peroxide.
20. A method as in claim 17 wherein (b) is practiced so that the
discoloration is characterized by a yellow or orange tinge to the
pulp.
21. A method as in claim 17 wherein (c) is practiced using as the
oxidizing agent at least one of air, oxygen, peroxide, or
ozone.
22. A method as in claim 18 wherein (c) is practiced using as the
oxidizing agent a gas containing ozone; and wherein (a) is a
soda/AQ pulping process; and wherein (b) is practiced using as the
at least one bleaching agent one or more of the following bleaching
agents: oxygen, chlorine dioxide, sodium hydroxide, ozone, and
hydrogen peroxide.
23. A method of ECF treatment of comminuted cellulosic fibrous
material comprising the sequence soda/AQ cooking, and then one of
D-E.sub.p-(ZEND), or D-E.sub.o-(ZEND), or D-E.sub.op-(ZEND).
24. A method as in claim 23 wherein the treatment is practiced to
produce pulp with a brightness over 89% ISO.
25. Pulp produced according to claim 24, having a viscosity of over
21 cP.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon provisional application Ser.
No. 60/167,537 filed Dec. 2, 1999, the disclosure of which is
hereby incorporated by reference herein.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] 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.
[0003] 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 (Cl.sub.2, or simply designated "C") or chlorine dioxide
(Cl0.sub.2, 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
(0.sub.2, or simply "0"), ozone gas (0.sub.3, or simply "Z"), and
hydrogen peroxide (H.sub.20.sub.2, or simply "P"), among others,
while ECF bleaching sequences also include the use of chlorine
dioxide (D).
[0004] 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 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.
[0005] There is also interest in reducing the release of gases
containing malodorous sulfur-containing compounds, such as hydrogen
sulfide (H.sub.2S), methyl mercaptan (CH.sub.3SH), dimethyi sulfide
(CH.sub.3SCH.sub.3), and dimethyl disulfide
(CH.sub.3S.sub.2CH.sub.3), 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 (Na.sub.2S) 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).
[0006] 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.
[0007] 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
discoloration is by treating the AQ-treated pulp with an oxidizing
agent, for example, air, oxygen gas, peroxide, or most preferably
ozone gas.
[0008] In addition, it has also been recently recognized that the
processes described in U.S. Pat. Nos. 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-Ahistrom Inc. of
Glens Falls, N.Y., 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.
[0009] 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.
[0010] 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 7.0.
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.
[0011] 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-containing compounds oxidized in the
Z stage are kept 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.
[0012] 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).
[0013] 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 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 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
E.sub.O,E.sub.P or E.sub.OP stage.
[0014] Another embodiment of the present invention includes a
method for producing bleached chemical pulp from comminuted
cellulosic fibrous material comprising the following: 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 AQ, is characterized by a yellowish or orange tinge to
the pulp.
[0015] 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.
[0016] According to another aspect of the invention there is
provided a method of ECF treatment of comminuted cellulosic fibrous
material comprising the sequence soda/AQ cooking, and then one of
D-Ep-(ZEND), or D-E.sub.O-(ZEND), or D-E.sub.OP-(ZEND). The pulp so
produced has a brightness of at least 89 or 90% ISO, and a
viscosity of over 21 or 22 cP.
[0017] 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
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
[0018] FIG. 1 is a schematic illustration of a complete fiberline
of one exemplary embodiment according to the present invention;
[0019] FIG. 2 is a schematic detailed illustration of a subsystem
of the fiberline of FIG. 1;
[0020] FIG. 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
[0021] FIG. 4 is a graph like that of FIG. 3 only plotting
brightness versus chlorine dioxide consumption.
DETAILED DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic illustration of a typical complete
environmentaily-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
soda/AQ 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.
[0023] 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 delignification 18 and post oxygen delignificaticn 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.
[0024] 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.
[0025] The washed pulp is passed via conduit 25 to alkaline
extraction stage (E) 26 (which may be an E.sub.O, E.sub.P, or
E.sub.OP 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.
[0026] 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 (0.5-5%) 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, N.Y. 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).
[0027] 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 degassing 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, Cl0.sub.2. The chlorine
dioxide is introduced to the pulp via conduit 39 and a mixer (see
mixer 59 in FIG. 2), for example, a conventional high-intensity
mixer.
[0028] 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.
[0029] FIG. 2 illustrates a specific embodiment of stages 30, 34
and 38 shown in FIG. 1. Structures in FIG. 2 that are similar to
those in FIG. 1 are identified with similar reference numbers, but
in FIG. 2 the reference numbers are 10 prefaced by the numeral "1".
For example, the conduit 129 in FIG. 2 is essentially the same the
as conduit 29 in FIG. 1.
[0030] Pulp from washer 28 in FIG. 1 is introduced to treatment 130
via conduit 129 in FIG. 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.degree. C., for example, at a temperature between about 20 and
70.degree. 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.
[0031] 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 FIG. 2, the
pulp may be de-gassed after mixer 50 and before stage 134.
[0032] 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.
[0033] 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.degree. C., preferably at a temperature of between about 50 and
70.degree. 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.
[0034] 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 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.degree. C.,
preferably, between about 50 and 70.degree. C. The treated pulp is
discharged via conduit 140 to conventional washer 141, washed, and
then forwarded on to further treatment, as discussed previously,
via conduit 142.
[0035] FIG. 3 shows plots 6f three different treatment scenarios, a
D-E.sub.op-(ZEND) treatment sequence according to the present
invention indicated by reference numeral 70, and two prior art
procedures, D-E.sub.op-D-P indicated at 71, and D-E.sub.opp-D
indicated at 72. It will 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 FIG. 3, the feed pulp before the treatment
sequence was LO-SOLIDS.RTM. 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.
[0036] FIG. 4 is a graph of the same pulps and treatment sequences
as in FIG. 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.
[0037] As clear from FIGS. 3 and 4, the treatment sequence D-E
[typically either E.sub.0, or E.sub.p, or E.sub.op]-(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.
[0038] 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.
[0039] 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
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.
* * * * *