U.S. patent application number 10/940484 was filed with the patent office on 2005-03-03 for reduction of organically bound chlorine formed in chlorine dioxide bleaching.
Invention is credited to Ekstrom, Ulla, Ragnar, Martin.
Application Number | 20050045291 10/940484 |
Document ID | / |
Family ID | 34215469 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050045291 |
Kind Code |
A1 |
Ragnar, Martin ; et
al. |
March 3, 2005 |
Reduction of organically bound chlorine formed in chlorine dioxide
bleaching
Abstract
A reduction in AOX levels is obtained when the process
conditions in the chlorine dioxide stage are elevated to above
91.degree. C. and extended to more than 90 minutes. A major
reduction of AOX up to 50% has been shown without a corresponding
increase in OCl. The chlorinated substances is degraded by the
process conditions to harmless chloride ions, instead of being
liberated into the effluent as AOX or bound to pulp as OCl. No
oxygen gas or nitrogen gas are added during the first or any
subsequent chlorine dioxide bleaching stage.
Inventors: |
Ragnar, Martin; (Karlstad,
SE) ; Ekstrom, Ulla; (Karlstad, SE) |
Correspondence
Address: |
FASTH LAW OFFICES
629 E. BOCA RATON ROAD
PHOENIX
AZ
85022
US
|
Family ID: |
34215469 |
Appl. No.: |
10/940484 |
Filed: |
September 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10940484 |
Sep 14, 2004 |
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10111507 |
Aug 8, 2002 |
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10111507 |
Aug 8, 2002 |
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PCT/SE01/01262 |
Jun 6, 2001 |
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Current U.S.
Class: |
162/61 ; 162/67;
162/73 |
Current CPC
Class: |
D21C 9/1042 20130101;
D21C 9/14 20130101 |
Class at
Publication: |
162/061 ;
162/067; 162/073 |
International
Class: |
D21C 011/12 |
Claims
We claim:
1. A process for reducing the amount of organically bound chlorine
formed in chlorine dioxide bleaching of kraft pulp, comprising:
providing several bleaching stages wherein at least one of the
stages is a bleaching stage using chlorine dioxide as bleaching
chemical to form a bleaching sequence; in a first chlorine dioxide
bleaching stage, charging a first amount of chlorine dioxide to the
pulp having a first kappa number so that a charge factor of the
first amount of chlorine dioxide/first kappa number is above 0.5
and operating at a temperature above 91 C and a retention time
longer than 90 minutes; reducing a resulting AOX content in an
effluent from the bleaching sequence to a value below 0.23 kg/ADt;
and adding no oxygen gas or nitrogen gas to the first chlorine
dioxide bleaching stage.
2. The process according to claim 1 wherein the retention time is
less than 300 minutes.
3. The process according to claim 1 wherein the first chlorine
dioxide bleaching stage used during the bleaching sequence is
operated at a temperature between 95.degree. C. and 120.degree. C.
at a retention time of between 90 minutes and 300 minutes and the
first chlorine dioxide bleaching stage is pressurized to a pressure
exceeding the vapor saturation pressure for the temperature in the
stage by at least 20%.
4. The process according to claim 1 wherein the charge factor is in
the range 1.5-3.0.
5. The process according to claim 1 wherein the first chlorine
dioxide bleaching stage has a pulp concentration between 7-25%.
6. The process according to claim 1 wherein the process comprises
the steps of delignifying the pulp to a kappa number below 20 prior
to bleaching the pulp in the first chlorine dioxide bleaching
stage.
7. The process according to claim 6 wherein the method further
comprises adding a sulphamic acid to at least one of the chlorine
dioxide bleaching stages in the bleaching sequence to capture
intermediately formed chlorine or hypochlorite to form a
chlorosulphamic acid.
8. The process according to claim 7 wherein the method further
comprises adding the sulphamic acid in a mmol amount that is
greater than a mmol amount of active chlorine dioxide added during
the first chlorine dioxide bleaching stage.
9. The process according to claim 7 wherein the method further
comprises adding at least 1.0 mmol sulphamic acid/BDt of pulp.
10. The process according to claim 7 wherein at least 80% of all
sulphamic acid added is added to a last chlorine dioxide bleaching
stage.
11. The process according to claim 10 wherein the last chlorine
stage is a D2 stage preceded by a D0 chlorine stage and a D1
chlorine stage and the method further having extraction stages E
between the chlorine dioxide stages to form a D0-E-D1-E-D2
bleaching sequence.
12. The process according to claim 11 wherein a charge of chlorine
dioxide, used in a chlorine stage including sulphamic acid, is at
least 10% greater than a charge of chlorine dioxide used in a
chlorine stage that is free from sulphamic acid.
Description
PRIOR APPLICATIONS
[0001] This application is a continuation-in-part application of
U.S. national phase application Ser. No. 10/111,507, filed 23 Apr.,
2002 that claims priority of International Application No.
PCT/SE01/01262, filed Jun. 6, 2001.
TECHNICAL FIELD
[0002] The invention relates to the reduction of organically bound
chlorine formed in chlorine dioxide bleaching without using oxygen
gas or any other pressurized gas.
BACKGROUND AND SUMMARY OF THE INVENTION
[0003] The present invention is related to the formation of
chlorinated organic matter in chlorine dioxide bleaching of kraft
pulp, and how to reduce the amount of organically bound chlorine in
pulp (OCl) and/or reduce the amount of organically bound chlorine
compounds (measured as, e.g., AOX or TOCl) in the waste water.
[0004] The most efficient and inexpensive bleaching chemical so far
known is elemental chlorine. The use of it has in most parts of the
world come to an end during the last decade. The driving forces in
this development have been environmental, expressed either as
market demands or as environmental standards set by governments or
a combination of the two. The negative environmental impact
connected to the use of elemental chlorine is primarily the
formation of chlorinated organic structures.
[0005] Following a massive introduction of oxygen delignification
systems, the work needed in the subsequent bleaching could be
significantly reduced and the ECF concept (Elemental Chlorine
Free), i.e., bleaching without the use of any elemental chlorine or
hypochlorite was introduced. The chemical normally replacing the
elemental chlorine is chlorine dioxide, which had been used for
final brightening of pulp and for obtaining a good cleanliness,
e.g., due to its excellent capability of removal of
extractives.
[0006] The chlorinated structures, e.g., formed in chlorine
bleaching, are denoted AOX (adsorbable organic halogene compounds)
when found in the bleach effluents and OCl (organically bound
chlorine) when stuck in the pulp. The amount of both AOX and OCl
were largely reduced upon conversion to ECF bleaching, but a zero
level was not reached and in fact a significant amount of OCl is
still found in ECF bleached pulps and AOX in the effluents from
chlorine dioxide stages.
[0007] The levels are also significantly higher than those arising
from TCF (Totally Chlorine Free) bleaching operations. This is due
to the fact that when chlorine dioxide reacts with the lignin in
pulp, hypochlorous acid in equilibrium with chlorine is formed,
both of which are able to act as chlorinating agents. Also during
manufacturing of chlorine dioxide at the mill site some elemental
chlorine is produced, typically in the order of 1-4%, most often
below 5% elemental chlorine, all dependent on the type of chlorine
dioxide forming process used.
[0008] Considering AOX in effluents it is urgent to keep in mind
that although the discharges per ton of pulp produced have
decreased significantly when switching to ECF-bleaching, the mills
have simultaneously grown too, meaning that the total AOX load to
the specific recipient need not have changed very much and thus
still constituting a potential problem. In FIG. 1 is shown how the
total amount of AOX in effluents may be constant even tough the AOX
level per BDt pulp have decreased over time, due to that production
volumes have increased.
[0009] A pulp having been bleached using chlorine dioxide in an ECF
sequence is still easily identified due to its content of OCl,
which hinders it from being used in certain paper products or at
certain markets. For several mills producing market pulp this is a
crucial fact, since it means certain customers will not be
interested in a high OCl pulp.
[0010] For various reasons, a massive conversion to TCF bleaching
has so far not occurred, leaving the field open for innovative ways
to approach the OCl and AOX problems in ECF-bleaching.
[0011] The obvious way, to reduce the overall charge of chlorine
dioxide, has in several cases been entered upon in, what is often
called "ECF-light" concepts, using a rather small charge of
chlorine dioxide in the D-stage, often a charge factor of active
chlorine as chlorine dioxide of below 1.
[0012] At the Tappi Pulping Conference Oct. 22-25, 1989, two papers
where presented where solutions to the AOX problem was presented.
Lowering of the delignification in the D-stage (or C- or
C/D-stage), by using a lower charge factor of active chlorine as
chlorine dioxide (i.e., kappa factor) was identified as methods for
decreasing AOX, and where compensation for the lower
delignification effect in D-stages is made by higher charges in
other stages. One paper was presented by J. Basta, L. Holtinger, J.
Hook and P. Lundgren with the title "LOW AOX, POSSIBIILITES AND
CONSEQUNCES" (pp. 427-436), and the second paper was presented by
H. Suss, W. Eul, N. Nimmerfroh and J. Meier, all from Degussa
AG/Corp, with the title "ENVIRONMENTAL ASPECTS OF SHORT-SEQUENCE
BLEACHING" (pp. 527-537). The main approach in these papers, when
AOX-reduction is the objective in ECF-bleaching, is to decrease the
use of chlorine dioxide at the expense of higher charges of
hydrogen peroxide.
[0013] This approach is shown in EP,B,500813, where a charge factor
of active chlorine as chlorine dioxide below 2.0 is used in the
D.sub.o stage (i.e., the first D-stage in a multiple sequence D-E-D
. . . etc.), and where following P-stage (P=peroxide) use at least
3.0 kg of hydrogen peroxide per ton dry pulp, and having chlorine
dioxide charges in following D-stages less or equal than the charge
used in D0, i.e., from 20-100% of the D0 charge.
[0014] In addition to this approach it has been proposed the first
chlorine dioxide stage be pH profiled by means of a short-term
reaction at low pH followed by an increase to alkaline conditions
(Ljunggren, S., Bergnor, E. and Kolar, J. (1994): Modified Modern
ClO2-Bleaching, International Pulp Bleaching Conference (IPBC),
Vancouver, Canada, Vol. 1: 169-176. and Ljunggren, S.,
Bergnor-Gidnert, E. and Kolar, J. (1996): Chlorine Dioxide
Bleaching with a Two-step Low-to-High pH Profile, Tappi J. 79: 12,
pp. 152-160.).
[0015] This approach has many similarities with the Ultim-O process
(no washing between D0 and E). Although this approach indeed
enabled significant reductions in the AOX discharges, the OCl
content was less affected and most important, the need for alkali
increased largely, making it less attractive.
[0016] Lately, a reductive alkaline post-treatment has been
proposed as a way of significantly reducing the OCl content of a
pulp, (see Ljunggren, S., Johansson, E. and Pettersson, B. (1998):
Dechlorination of ODEDD Bleached Kraft Pulps, 5th European Workshop
on Lignocellulosics and Pulp (EWLP), Aveiro, Portugal, pp.
437-440), which is a somewhat refined way of utilizing the
well-known fact that an alkali extraction undoubtedly is a very
efficient way for the removal of OCl. Although efficient, such a
post-treatment of the pulp requires both additional washing
equipment and additional bleaching towers, making also this
approach less attractive for mill implementation.
[0017] Improvements in Chlorine Dioxide stages have been made for
several purposes. In a paper presented by Lachenal, D. and Chirat,
C. (1998): High Temperature Chlorine Dioxide Delignification: A
Breakthrough in ECF Bleaching of Hardwood Kraft Pulps, Pulping
Conference, Atlanta, U.S.A., Vol. 2:pp. 601-604.), a modification
of the conventional D-stage is suggested. With the objective to
make the D-stage more efficient, and reduce charges of chlorine
dioxide, it is proposed to modify the conventional 45.degree. C.
D-stage to a high temperature (90-100.degree. C.) D-stage having
long retention time (1.5-4 hrs). An alternative modification
achieving the same improvement was proposed where instead this high
temperature is implemented after, "at the exit of", the D-stage
when the chlorine dioxide have been consumed, during which process
position the high temperature could not affect the break-down
process of chlorine dioxide in the D-stage. This paper also
indicates that the change from chlorine to chlorine dioxide
bleaching will solve the AOX-problem.
[0018] Attempts have been made to add pressurized oxygen gas at
high concentrations, such as 85%, to reduce the AOX level as a
result of a possible oxidizing effect upon the filtrate/liquid. One
drawback of the oxygen gas addition is the fact that the gas
content of the pulp increases and this in turn may decrease the
washing efficiency in the subsequent washing stage since air
bubbles in the pulp have a tendency to reduce the drainage
capability of the pulp that makes the washing less effective.
Additionally, residual chlorine gas is often a problem and adding
oxygen gas to the process does not reduce the problems with the
chlorine gas.
[0019] The main objective with the present invention is to reduce
the total amount of chlorinated organic matter leaving a chlorine
dioxide stage, and especially the total amount of AOX and OCl,
where at least a substantial reduction in AOX levels is obtained,
and this while being able to operate chlorine dioxide stages with
higher charges of chlorine dioxide than "ECF light".
[0020] Another objective is that the overall operating costs for
pulp bleaching could be kept low if the delignification effect from
chlorine dioxide is utilized in full in the first chlorine dioxide
stage in the bleaching sequence, whereby charges of other more
expensive bleaching chemicals, in cost per kg or per bleaching
effect, could be kept at lower levels.
[0021] Another objective according to the invention is that an
initial chlorine dioxide stage run at high temperature for long
time is shown to be an efficient means of reducing the overall
discharge of AOX by about 50 percent, presumably through a forced
degradation of the chlorinated structures formed in the stage. This
high reduction of AOX by about 50% at a given overall chlorine
dioxide charge compared to operation of said initial chlorine
dioxide stage at conventional conditions, i.e., some 60-70.degree.
C. and 20-60 minutes.
[0022] Moreover, a further addition of sulphamic acid to a final
D-stage is presented as an efficient tool for reducing the total
amount of AOX and OCl, with substantial decrease of the OCl content
of an bleached pulp, preferably ECF bleached pulp, since sulphamic
acid captures in situ formed elemental chlorine. Said substantial
decrease amounting to about 50 percent in a final D-stage operating
at similar charge of chlorine dioxide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic illustration of a total amount of AOX
relative to the mill size and AOX/BDt;
[0024] FIG. 2 is a schematic illustration of possible substances
from chlorinated structures; and
[0025] FIG. 3 is a schematic illustration of OCl levels over
time.
DETAILED DESCRIPTION
[0026] The invention is based upon the origin of OCl and ways to
decrease it, without necessarily reducing the use of chlorine
dioxide and still reaching the same final brightness. The
distribution of OCl in ECF-bleached pulp is playing an important
role.
[0027] It is important to understand the correspondence between AOX
and OCl. In FIG. 2, the three major possible faiths of a
chlorinated substance in the pulp are summarized. Following a
chlorination there are thus three alternatives, either that the
chlorinated structure sticks to the final pulp becoming OCl, or
that it is liberated during subsequent bleaching stages becoming
AOX, or that the structure is substituted/degraded so that the
chlorine atoms form harmless chloride ions.
[0028] Important to keep in mind is hence that there is no direct
correspondence between AOX and OCl telling, e.g., that a high AOX
discharge means a low OCl content in the pulp at a certain chlorine
dioxide charge.
[0029] In a series of trials the standard ECF bleaching sequence of
DEDED, using an overall chlorine dioxide charge of 29.6 kg a
Cl/BDt, was used to bleach the oxygen delignified HW kraft pulp
from the second series of trials (kappa 9.8) to full brightness
(above 89 % ISO). 19.6 kg a Cl/BDt was used in D0, and 5 kg a
Cl/BDt in each of D1 and D2. The charge factor of active chlorine
as chlorine dioxide in D0 equaling (19.6/9.8=) 2.0. This standard
sequence was compared with three modified sequences, D*EDED,
DEDE(SD) and D*EDE(SD). D* denotes a D-stage run at high
temperature (90.degree. C.) and long time (120 min). "S" denotes
the presence of sulphamic acid. E stages were performed according
to above. D1 and D2 stages were performed at 75.degree. C. and 120
min.
[0030] Kappa number, viscosity and ISO brightness were analyzed
using the respective SCAN standards. In addition, SCAN standard CM
52:94 "Pulps, papers and boards--organic chlorine" was used to
determine the content of OCl in the pulp after different stages.
All bleaching experiments were performed at 10 percent pulp
consistency in plastic bags, which after intense kneading were
placed in heated water baths. The charge of sulphamic acid should
be somewhat higher, i.e., on a molar basis, than the charge of
active chlorine, in this investigation meaning 1.0 mmol sulphamic
acid/BDt.
[0031] In those stages to which sulphamic acid addition was made,
the charge of active chlorine was increased in order to compensate
for the decreased oxidizing capacity of the stage when the
reduction of chlorine dioxide to chloride ion is broken at the
level of elemental chlorine. The oxidizing capacity of chlorine
dioxide is decreased by 20 percent in the presence of sulphamic
acid, which captures intermediately formed elemental chlorine, and
following reaction pattern is developed with and without sulphamic
acid. 1
[0032] In practice this means that 4 out of 5 electrons are used
when chlorine dioxide bleaching in the presence of sulphamic acid
is used and thus the charge of active chlorine to such stages were
increased by 25 percent. This way, all the pulps were subjected to
identical charges of "active" active chlorine.
[0033] Results from the 5-stage bleaching study on HW mill oxygen
delignified kraft pulp are given in following Table 1.
1TABLE 1 Trial DEDED D*EDED DEDE (SD) D*EDE (SD) Final kappa 2.1
0.6 2.5 1.2 Final viscosity [ml/g] 975 937 939 911 ISO brightness
[%] 89.4 89.9 89.4 89.5 Total a Cl charge 27 27 27 27 [kg/ADt]
Total Ocl [mg/kg] 152 158 88 116 Total AOX [kg/ADt] 0.41 0.23 0.39
0.21
[0034] From the results it is clear that the AOX discharge could be
reduced with about 50 percent using D* instead of D as the first
bleaching stage. It should be noted that this result is obtained
when comparing sequences with identical overall charge of chlorine
dioxide. In addition to this reduction of AOX, the value can be
even further reduced when the chlorine dioxide saving effect of the
D*, (as, e.g., noted by Lachenal, D. and Chirat, C. (1998): High
Temperature Chlorine Dioxide Delignification: A Breakthrough in ECF
Bleaching of Hardwood Kraft Pulps, Pulping Conference, Atlanta,
U.S.A., Vol. 2: 601-604.) is taken into account, here instead
recorded as a higher final brightness.
[0035] This finding was very unexpected. One would else have
anticipated that if the AOX levels experienced a decrease, then the
OCl would increase by a similar order. However, the findings showed
that the AOX-levels were decreased without a similar order of
increase in OCl.
[0036] The interpretation of the result should not be that less
chlorination takes place or that less AOX is formed in a D*-stage
than in a conventional D-stage. On the contrary, it seems
appropriate to suppose that under the tough conditions of the D*
stage, a substantial part of the AOX formed in the stage is further
degraded to, e.g., harmless chloride ions. With this knowledge in
mind it is interesting to compare D* with (AD), i.e., where A is
performed as a hot acid treatment for long duration at, e.g.,
90-100.degree. C. and 120 min according to concepts like GB
1.062.734. In GB 1.062.734 this acid treatment at pH 2.25,
temperature 100.degree. C. and during 120 minutes was implemented
in order to reduce brightness reversion.
[0037] The extreme A-stage was followed by a conventional D0-stage
at some 60.degree. C. without intermediate washing. In conformity
with D*, an (AD) approach gives a potential to reduce the overall
need for chlorine dioxide in the bleaching of especially HW kraft
pulp, although D* has been shown to have a greater potential in
this respect. However, in contradiction to D*, an (AD) approach
will not enable any reduction of the AOX according to the
mechanisms presented. Theoretically, D* can of course be utilized
in any position in the bleaching sequence irrespective of the
number of D-stages in the bleaching line. Although in general it is
likely that the benefits of the stage primarily motivates its
utilization in the D0 position, i.e., the first stage using
chlorine dioxide.
[0038] From the results in Table 1 it is also clear that the
presence of sulphamic acid in the final D-stage is an efficient
means of reducing the OCl content of the pulp. Having the OCl
pattern shown in FIG. 3 in mind, it is easily concluded that the
largest effect to the lowest charge of sulphamic acid is obtained
when sulphamic acid addition is made to the last D-stage, although
a larger effect of course can be obtained using it in all D-stages.
Although sulphamic acid already today is commonly used in pulp
mills, e.g., for the removal of scales in machinery upon shutdowns,
its use in continuous bleaching processes for obtaining low OCl
pulp is new. The addition of sulphamic acid should be added in a
continuous manner during the bleaching process in the chlorine
dioxide stage, i.e., so that sulphamic acid is present during the
consumption of chlorine dioxide in the chlorine dioxide stage. The
sulphamic acid could be added to the pulp before, after or during
addition of the chlorine dioxide in a chlorine dioxide mixer.
[0039] It should be added that the chlorine dioxide charge in a
(SD) stage has to be increased by some 15-30%, typically 25
percent, in order to compensate for the reduced oxidizing power
lost due to the capture of elemental chlorine by sulphamic acid.
However, when utilized in D2-position, or in the final D-stage,
this means a very moderate additional need for chlorine dioxide in
this last stage.
[0040] The two concepts D* and (SD) could also be utilized in the
same sequence, thus enabling the manufacture of a pulp with low OCl
content at the same time as the AOX discharges are kept low, as
shown in Table 1.
[0041] It can be concluded that a 50 percent reduction of the
overall AOX discharge of a DEDED sequence can be obtained by using
a D*-stage instead of a conventional D-stage in D0 position. The
OCl content can also be fought and decreased by about 50 percent
even in an existing bleaching line by changing the last D-stage to
operation with sulphamic acid addition in a (SD)-stage.
[0042] An important feature of the present invention is that no
oxygen gas or nitrogen gas is added to the process and it is
sufficient to simply maintain the temperature and pressure at
suitable levels for a sufficiently long retention time as long as
the pressure is above a saturation pressure at the prevailing
temperature to prevent the pulp from boiling. The minimum suitable
retention time is 90 minutes and the maximum retention time is 300
minutes to achieve an AOX reduction so that the AOX level is less
than 0.23 kg/ADt in the first chlorine dioxide stage (D1). It was
surprising to note that the relatively long retention time of
between 90-300 minutes did not result in a loss of pulp viscosity
and other negative effects. The fact that the longer retention time
is in the first chlorine-dioxide bleaching step has the surprising
effect that the loss of pulp viscosity in the final treatment steps
is minimal. A skilled person would normally hesitate to use a long
retention time due to the loss of pulp properties such has pulp
viscosity. There has been a goal in the conventional technologies
to minimize the retention time at high temperature over 90-95C due
to the loss of pulp strength and the formation of residual chlorine
gas in the first chlorine-dioxide bleaching step that negatively
affects the subsequent washing steps. It was surprising that the
operation at 91C and above and a longer retention time over 90 min
alone reduced the AOX level to as low as 0.23 kg/ADt and lower
without the need for the addition of gas such as oxygen gas.
[0043] While the present invention has been described in accordance
with preferred compositions and embodiments, it is to be understood
that certain substitutions and alterations may be made thereto
without departing from the spirit and scope of the following
claims.
* * * * *