U.S. patent application number 12/736292 was filed with the patent office on 2011-02-24 for process of chlorine dioxide bleaching of chemical pulps using magnesium hydroxide or magnesium oxide.
This patent application is currently assigned to University of Brunswick. Invention is credited to Zhibin He, Yonghao Ni.
Application Number | 20110042022 12/736292 |
Document ID | / |
Family ID | 41112884 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110042022 |
Kind Code |
A1 |
Ni; Yonghao ; et
al. |
February 24, 2011 |
Process of chlorine dioxide bleaching of chemical pulps using
magnesium hydroxide or magnesium oxide
Abstract
The present invention provides an improved process for chlorine
dioxide bleaching of chemical pulps in an aqueous suspension, which
uses magnesium hydroxide or magnesium oxide. The process involves
the addition of magnesium hydroxide or magnesium oxide to the pulp,
in addition to chlorine dioxide, maintaining the mixture at a pH
between about 2.5 to 7.5 for about 1 or more hours, most suitably
between about 1-4 hours. This process can be used in the D.sub.1 or
D.sub.2 stage of a typical D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2
bleaching sequence, as well as in the chlorine dioxide brightening
stage of other three, four, five, and six-stage bleaching sequences
of either the so-called ECF process, or those containing elemental
chlorine (Cl.sub.2). The temperature is between about 40 to about
90.degree. C., and the pulp consistency is between about 1 to about
16%.
Inventors: |
Ni; Yonghao; (Fredericton,
CA) ; He; Zhibin; (Fredericton, CA) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
University of Brunswick
Fredericton
NB
|
Family ID: |
41112884 |
Appl. No.: |
12/736292 |
Filed: |
March 25, 2009 |
PCT Filed: |
March 25, 2009 |
PCT NO: |
PCT/CA2009/000367 |
371 Date: |
November 9, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61064757 |
Mar 25, 2008 |
|
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|
Current U.S.
Class: |
162/89 |
Current CPC
Class: |
D21C 9/142 20130101;
D21C 9/1036 20130101 |
Class at
Publication: |
162/89 |
International
Class: |
D21C 3/26 20060101
D21C003/26 |
Claims
1. A process for bleaching chemical pulp of the type characterized
by a D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 bleaching sequence in
which D.sub.0: chlorine dioxide delignification, with the pH
controlled in a range from about 2 to about 3; E.sub.op: alkaline
extraction with caustic soda, pH is in the range of 10-11.5;
D.sub.1: first chlorine dioxide brightening stage with pH
controlled in a range from about 2.5 to about 7.5; E.sub.2: the
second alkaline extraction, the end pH is in the range from about
9.5 to about 11.5; D.sub.2: the second chlorine dioxide brightening
stage, the end pH controlled in a range from about 2.5 to about
7.5, wherein the process comprises the steps of: bleaching chemical
pulps in the D.sub.1 and/or D.sub.2 stages using chlorine dioxide,
including adding chlorine dioxide and magnesium hydroxide or
magnesium oxide in the absence of oxygen to a suspension of the
chemical pulp to form a mixture, wherein the chlorine dioxide and
magnesium hydroxide are added in a sequential order or
simultaneously, including maintaining the mixture at a pH between
about 2.5 to about 7.5 for about 1 or more hours, and maintaining
the mixture at a temperature in a range from about 40.degree. C. to
about 90.degree. C., and maintaining a pulp consistency in a range
between about 1 to about 16% weight percentage on dried mass of
pulp.
2. The process according to claim 1 wherein said temperature is
maintained in a range from about 55.degree. C. to about 80.degree.
C.
3. The process according to claim 1 including maintaining the
mixture at a pH between about 3 to about 7 for between about 1 to
about 4 hours.
4. The process according to claim 1 wherein said pH of the mixture
is maintained in a range from about 3 to about 6.
5. The process according to claim 1, wherein said pulp consistency
is maintained between about 3 to about 12%.
6. The process according to claim 2 including maintaining the
mixture at a pH between about 3 to about 7 for between about 1 to
about 4 hours.
7. The process according to claim 2 wherein said pH of the mixture
is maintained in a range from about 3 to about 6.
8. The process according to claim 2 wherein said pulp consistency
is maintained between about 3 to about 12%.
9. The process according to claim 3 wherein said pulp consistency
is maintained between about 3 to about 12%.
10. The process according to claim 4 wherein said pulp consistency
is maintained between about 3 to about 12%.
Description
FIELD OF INVENTION
[0001] The present invention is related to the bleaching of
chemical pulps, in particular, to an improved process which uses
chlorine dioxide as the primary chemical.
BACKGROUND OF THE INVENTION
[0002] Chlorine dioxide is the main chemical in the bleach plant
for the production of bleached chemical pulps. Such a bleaching
process is normally carried out in multiple processing stages,
utilizing chlorine dioxide, caustic soda, oxygen, and hydrogen
peroxide. A typical process is a
D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 sequence, where:
D.sub.0: chlorine dioxide delignification, in many cases some
acids, such as sulphuric acid, or spent acid from the chlorine
dioxide generator, is used to control the pH of 2-3; E.sub.op:
alkaline extraction with caustic soda, a small amount of oxygen and
hydrogen peroxide can be added, and the end pH is in the range of
10-11.5; D.sub.1: first chlorine dioxide brightening stage; sodium
hydroxide is usually added to control the end pH of 3-6; E.sub.2:
the second alkaline extraction; a small amount of hydrogen peroxide
may be added, the end pH is in the range of 9.5-11.5; D.sub.2: the
second chlorine dioxide brightening stage, a small amount of sodium
hydroxide is usually added to control the end pH of 3.5-7.0.
[0003] Chlorine dioxide is one of the most expensive chemicals used
in the bleaching of chemical pulps. Improving its efficiency is
desirable to decrease the overall bleaching cost.
[0004] U.S. Pub. No. 20070079944 to Amidon et al. discloses an
omnibus process of pulping and bleaching lignocellulosic materials.
With regard to the bleaching process, Amidon et al. teaches
contacting the pulped product with chlorine dioxide in the presence
of O.sub.2. The O.sub.2 addition, however, has an inherent problem
due to the lower viscosity, and Amidon et al. addressed this
problem by adding magnesium hydroxide. It is suggested in Amidon et
al. that the addition of Mg(OH).sub.2 exploits Mg cation's property
of disrupting the free radical propagation mechanism by forming
complexes with superoxide anions. Although Amidon et al. reported
improved viscosity and increased brightness through the
oxygen/chlorine dioxide/magnesium hydroxide bleaching process,
there still remains certain downsides of adding O.sub.2. For
example, although oxygen itself is cheaper than chlorine dioxide,
the set-up to add the oxygen into the bleaching process is
expensive.
[0005] Therefore, there is a need to provide a method of bleaching
chemical pulps, in particular, to a more economical process which
uses chlorine dioxide as the primary chemical.
SUMMARY OF THE INVENTION
[0006] It has been discovered that magnesium hydroxide (or
magnesium oxide) can be effectively added to the chlorine dioxide
bleaching, resulting in unexpected improvements in the bleaching
process. At the same chlorine dioxide charge, a higher brightness
can be achieved and the production cost can be decreased making the
process more economical.
[0007] The present invention provides an improved process for
chlorine dioxide bleaching of chemical pulps in an aqueous
suspension, which uses magnesium hydroxide or magnesium oxide. The
novelty comprises the addition of magnesium hydroxide (or magnesium
oxide) to pulp, in addition to chlorine dioxide, maintaining the
mixture at a pH between about 2.5-7.5 for about 1 or more hours,
most suitably between about 1-4 hours. This novel process can be
used in the D.sub.1 or D.sub.2 stage of a typical
D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 bleaching sequence, as well as
in the chlorine dioxide brightening stage of other three, four,
five, and six-stage bleaching sequences of either the so-called ECF
process, or those containing elemental chlorine (Cl.sub.2). The
temperature is between about 50-85.degree. C., and the pulp
consistency is maintained in a range between about 1 to about 16%
weight percentage on dried mass of pulp, and preferably between
about 3-12%.
[0008] Thus, an embodiment of the present invention provides a
process for bleaching chemical pulp of the type characterized by a
D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 bleaching sequence in
which
[0009] D.sub.0: chlorine dioxide delignification, with the pH
controlled in a range from about 2 to about 3;
[0010] E.sub.op: alkaline extraction with caustic soda, pH is in
the range of 10-11.5;
[0011] D.sub.1: first chlorine dioxide brightening stage with pH
controlled in a range from about 3 to about 6;
[0012] E.sub.2: the second alkaline extraction, the end pH is in
the range from about 9.5 to about 11.5;
[0013] D.sub.2: the second chlorine dioxide brightening stage, the
end pH controlled in a range from about 3.5 to about 7.0, wherein
the process comprises the steps of:
[0014] bleaching chemical pulps in the D.sub.1 and/or D.sub.2
stages using chlorine dioxide, including adding chlorine dioxide
and magnesium hydroxide or magnesium oxide in the absence of oxygen
to a suspension of the chemical pulp to form a mixture, wherein the
chlorine dioxide and magnesium hydroxide are added in a sequential
order or simultaneously, including maintaining the mixture at a pH
between about 2.5 to about 7.5 for about 1 or more hours, and
maintaining the mixture at a temperature in a range from about
40.degree. C. to about 90.degree. C., and maintaining a pulp
consistency in a range between about 1 to about 16% weight
percentage on dried mass of pulp.
[0015] The method disclosed herein provides more efficient chlorine
dioxide bleaching in the chemical pulp bleaching process which
decreases the production cost of the chemical pulp bleaching
process while at the same time provides a higher pulp brightness
with a given chlorine dioxide charge.
[0016] A further understanding of the functional and advantageous
aspects of the invention can be realized by reference to the
following detailed descriptions and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention will be more fully understood from the
following detailed descriptions thereof taken in connection with
the accompanying drawings, which form a part of this application,
and in which:
[0018] FIG. 1 is a the process flow sheet of a conventional
chlorine dioxide bleaching process; and
[0019] FIG. 2 shows the process flow sheet of a chlorine dioxide
bleaching process using magnesium hydroxide (magnesium oxide) in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Generally speaking, the systems described herein are
directed to the bleaching of chemical pulps, in particular, to an
improved process which uses chlorine dioxide as the primary
chemical. As required, embodiments of the present invention are
disclosed herein. However, the disclosed embodiments are merely
exemplary, and it should be understood that the invention may be
embodied in many various and alternative forms. The Figures are not
to scale and some features may be exaggerated or minimized to show
details of particular elements while related elements may have been
eliminated to prevent obscuring novel aspects. Therefore, specific
structural and functional details disclosed herein are not to be
interpreted as limiting but merely as a basis for the claims and as
a representative basis for teaching one skilled in the art to
variously employ the present invention. For purposes of teaching
and not limitation, the illustrated embodiments are directed to a
bleaching process for bleaching chemical pulp which uses chlorine
dioxide as the primary chemical.
[0021] As used herein, the term "about", when used in conjunction
with ranges of dimensions of particles or other physical properties
or characteristics, is meant to cover slight variations that may
exist in the upper and lower limits of the ranges of dimensions so
as to not exclude embodiments where on average most of the
dimensions are satisfied but where statistically dimensions may
exist outside this region. It is not the intention to exclude
embodiments such as these from the present invention.
[0022] As discussed in the Background above, chlorine dioxide is
the main chemical in the bleach plant for the production of
bleached chemical pulps. The bleaching process is normally carried
out in multiple processing stages, utilizing chlorine dioxide,
caustic soda, oxygen, and hydrogen peroxide. The typical process
uses a D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 sequence, where:
D.sub.0: chlorine dioxide delignification, with the pH controlled
in a range from about 2 to about 3; E.sub.op: alkaline extraction
with caustic soda, pH is in the range of 10-11.5; D.sub.1: first
chlorine dioxide brightening stage with pH controlled in a range
from about 3 to about 6; E.sub.2: the second alkaline extraction,
the end pH is in the range from about 9.5 to about 11.5; D.sub.2:
the second chlorine dioxide brightening stage, the end pH
controlled in a range from about 3.5 to about 7.0.
[0023] FIG. 1 shows the process flow sheet of the conventional
chlorine dioxide bleaching process; the pulp from the previous
stage is added with chlorine dioxide and sodium hydroxide via a
mixer, and then pumped via a pump to either a up-flow or down-flow
bleaching tower; after exiting from the bleaching tower, the
bleached pulp is washed in a washer, chlorine dioxide and/or sodium
hydroxide may also be mixed to the pulp via the pump so that the
pump functions for mixing and pumping.
[0024] FIG. 2 shows the process concept of the chlorine dioxide
bleaching using magnesium hydroxide (magnesium oxide); the pulp
form the previous stage is added with chlorine dioxide and
magnesium hydroxide via a mixer, and then pumped via a pump to
either an up-flow or down-flow bleaching tower; after exiting from
the bleaching tower, the bleached pulp is washed in a washer,
chlorine dioxide and/or magnesium hydroxide/magnesium oxide may
also be mixed to the pulp via the pump so that the pump functions
for mixing and pumping. If desirable, sodium hydroxide can be added
to the process. This process is carried out in the absence of
pumping in oxygen.
[0025] A small amount of magnesium hydroxide (magnesium oxide) can
also be used in the E.sub.op and E.sub.2 stages of the
D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 sequence, to partially
substitute for sodium hydroxide.
[0026] The pulp consistency is maintained in a range between about
1 to about 16% weight percentage on dried mass of pulp, and
preferably between about 3-12%.
[0027] Without being limited by any theory, it is believed that the
unexpected improvement of the chlorine dioxide bleaching by using
magnesium hydroxide is due to the following reasons. First, the pH
in the process is more stable and less variable, secondly, the
charged chlorine dioxide in the process is used more effectively,
and thirdly, magnesium cation has a positive effect on the
bleaching performance.
Example
[0028] A softwood kraft pulp, which is treated in a D.sub.0E.sub.O
sequence and has a Kappa # of 5.1 and pulp brightness of 49.3%, was
subjected to a conventional chlorine dioxide bleaching under the
conditions of:
[0029] ClO.sub.2 charge: 1.72%
[0030] NaOH charge: 0.60%
[0031] 69.degree. C., 265 min, 11% pulp consistency
The resulting pulp has a pulp brightness of 79.9% ISO.
[0032] The same D.sub.0E.sub.O treated pulp, with a Kappa # of 5.1
and pulp brightness of 49.3%, was subjected to the novel chlorine
dioxide bleaching by using magnesium hydroxide under the conditions
of: [0033] ClO.sub.2 charge: 1.72% [0034] Magnesium hydroxide:
0.44% [0035] 69.degree. C., 265 min, 11% pulp consistency
[0036] The resulting pulp has a pulp brightness of 84.7%, 4.8 units
higher than the control, which clearly demonstrates the superior
performance of the novel chlorine dioxide bleaching by using
magnesium hydroxide.
[0037] Thus, broadly speaking, the present invention provides a
process for chlorine dioxide bleaching of chemical pulps, such as
the D.sub.1 and/or D.sub.2 stage of a typical
D.sub.0E.sub.opD.sub.1E.sub.2D.sub.2 sequence, comprising the steps
of mixing pulp slurry with magnesium hydroxide and chlorine
dioxide, maintaining the mixture at a pH between about 2.5-7.5 for
about 1 or more hours in a temperature of about 40-90.degree. C.
The required amount of magnesium hydroxide and chlorine dioxide can
be added either sequentially or simultaneously.
[0038] As used herein, the terms "comprises", "comprising",
"including" and "includes" are to be construed as being inclusive
and open ended, and not exclusive. Specifically, when used in this
specification including claims, the terms "comprises",
"comprising", "including" and "includes" and variations thereof
mean the specified features, steps or components are included.
These terms are not to be interpreted to exclude the presence of
other features, steps or components.
[0039] The foregoing description of the preferred embodiments of
the invention has been presented to illustrate the principles of
the invention and not to limit the invention to the particular
embodiment illustrated. It is intended that the scope of the
invention be defined by all of the embodiments encompassed within
the following claims and their equivalents.
* * * * *