U.S. patent application number 10/111507 was filed with the patent office on 2003-03-27 for reduction of organically bound chlorine formed in chlorine dioxide bleaching.
Invention is credited to Ekstrom, Ulla, Ragnar, Martin.
Application Number | 20030056295 10/111507 |
Document ID | / |
Family ID | 26655438 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030056295 |
Kind Code |
A1 |
Ragnar, Martin ; et
al. |
March 27, 2003 |
Reduction of organically bound chlorine formed in chlorine dioxide
bleaching
Abstract
The invention relates to an improvement in bleaching of kraft
pulp. In ECF bleaching chlorine dioxide is most often a prefered
bleaching chemical in certain process positions. The disadvantage
is that residual amounts of organically bound chlorine in form of
AOX, i.e organically bound chlorine in effluents, and/or OCl, i.e.
chlorine organically bound in the produced pulp, is obtained. A
normal approach has been to reduce charges of chlorine dioxide.
According to the invention could substantial reductions in AOX
levels be obtained if the process conditions in the chlorine
dioxide stage elevated to above 91.degree. C. and extended to more
than 90 minutes. A major reduction of AOX up to 50% have 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.
Inventors: |
Ragnar, Martin; (Karlstad,
SE) ; Ekstrom, Ulla; (Karlstad, SE) |
Correspondence
Address: |
Rolf Fasth
Fasth Law Offices
189 Curtis Drive
Longview
WA
98632-5374
US
|
Family ID: |
26655438 |
Appl. No.: |
10/111507 |
Filed: |
August 8, 2002 |
PCT Filed: |
June 6, 2001 |
PCT NO: |
PCT/SE01/01262 |
Current U.S.
Class: |
8/101 |
Current CPC
Class: |
D21C 9/142 20130101;
D21C 9/144 20130101 |
Class at
Publication: |
8/101 |
International
Class: |
D06L 003/00 |
Claims
1. A process for reducing the amount of organically bound chlorine
formed in chlorine dioxide bleaching of kraft pulp using several
bleaching stages and wherein at least one of the stages is a
bleaching stage using chlorine dioxide as bleaching chemical, said
bleaching stages forming a bleaching sequence, wherein a first
chlorine dioxide bleaching stage used during the bleaching sequence
is having a charge factor above 0.5 characterised in that the first
chlorine dioxide bleaching stage used during the bleaching sequence
is operated at a temperature above 91.degree. C. and at a retention
time more than 90 minutes, whereby the resulting AOX content in the
effluent from the bleaching line is reduced more than 25%.
2. A process according to claim 1, characterised in that the that
the first chlorine dioxide bleaching stage used during the
bleaching sequence is operated at a temperature above 95.degree. C.
up to 120.degree. C. at the most and at a retention time more than
90 minutes up to 300 minutes at the most, and preferably about 200
minutes, and that the first chlorine dioxide bleaching stage is
pressurised to a pressure exceeding the vapour saturation pressure
for the temperature in the stage by at least 20%.
3. A process according to claim 1 or 2, characterised in that the
charge factor in the first chlorine dioxide bleaching stage is
above 1.5 and preferably in the range 1.5-3.0.
4. A process according to any of claims 1-3, characterised in that
the pulp concentration during the the first chlorine dioxide
bleaching stage is in the medium consistency range, 25 i.e. between
7-25%.
5. A process according to any of claims 1-4, characterised in that
the pulp being bleached in the first chlorine dioxide bleaching
stage is delignified to a kappa number below 20, and preferably
below kappa 15 prior to bleaching in the first chlorine dioxide
bleaching stage.
6. A process according to claim 5, characterised in that sulphamic
acid is added to at least one chlorine dioxide bleaching stage in
the bleaching sequence, which sulphamic acid captures
intermediately formed chlorine, or hypochlorite, during the
chlorine dioxide bleaching stage forming chlorosulphamic acid
according to the reaction process.
7. A process according to claim 6, characterised in that sulphamic
acid is added to at least one chlorine dioxide bleaching stage in
the bleaching sequence in an amount exceeding that of the charge of
active chlorine based upon a mmol relation.
8. A process according to claim 6, characterised in that sulphamic
acid is added to at least one chlorine dioxide bleaching stage in
the bleaching sequence in an amount exceeding 1.0 mmol sulphamic
acid/BDt of pulp.
9. A process according to claim 6, characterised in that the major
part, i.e. more than 80% of total charge and preferably 100%, of
the total charge of of sulphamic acid added to the bleaching
sequence is added to the last chlorine dioxide bleaching stage.
10. A process according to claim 9, characterised in that the last
chlorine dioxide bleaching stage is at least a D2 stage, i.e. a
chlorine dioxide stage preceeded by at least a D0 stage, i.e. the
first chlorine dioxide stage, and a D1 stage, having extraction
stages between chlorine dioxide stages, i.e. according to a
D0-E-D1-E-D2 bleaching sequence.
11. A process according to claim 8, 9 or 10 characterised in that
the charge of chlorine dioxide used in the stage where sulphamic
acid is added is increased by at least 10%, preferably increased
20%, as compared to a charge of chlorine dioxide used in this stage
without addition of sulphamic acid and resulting in a final pulp
brigthness in the same order of ISO brightness, i.e. the same order
of ISO brightness corresponing to .+-.1% in final ISO brightness.
Description
BACKGROUND OF THE INVENTION
[0001] 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.
[0002] 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 has 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.
[0003] 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.
[0004] 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.
[0005] 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
[0006] 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 simultaneuosly 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.
[0007] 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.
[0008] For various reasons, a massive conversion to TCF bleaching
has so far not occured, leaving the field open for innovative ways
to approach the OCl and AOX problems in ECF-bleaching.
[0009] 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.
[0010] 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 titel "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 titel "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.
[0011] 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.0 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.
[0012] 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.).
[0013] 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.
[0014] 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 utilising the
well-known fact that an alkali extraction undoubtedly is a very
efficient way for the removal of OCl.
[0015] 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.
[0016] Improvments 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.
SUMMARY OF THE INVENTION
[0017] The main objectives 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".
[0018] Another objective is that the overall operating costs for
pulp bleaching could be kept low if the delignification effect from
chlorine dioxide is utilised 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.
[0019] Another objective according to the invention is that an
intial 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.
[0020] 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.
DETAILED DESCRIPTION OF THE INVENTION
[0021] 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.
[0022] The distribution of OCl in ECF-bleached pulp is playing an
important role.
[0023] 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 summarised. 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.
[0024] Important to keep in mind is hence that there is no direct
correspondance 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.
[0025] 5-Stage Sequence Trials
[0026] 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.
[0027] The charge factor of active chlorine as chlorine dioxide in
D0 equaling (19.6/9.8=) 2.0.
[0028] This standard sequence was compared with three modified
sequences, D*EDED, DEDE(SD) and D*EDE(SD).
[0029] 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] General Methods
[0031] Kappa number, viscosity and ISO brightness were analysed
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.
[0032] All bleaching experiments were performed at 10 percent pulp
consistency in plastic bags, which after intense kneeding 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.
[0033] In those stages to which sulphamic acid addition was made,
the charge of active chlorine was increased in order to compensate
for the decreased oxidising capacity of the stage when the
reduction of chlorine dioxide to chloride ion is broken at the
level of elemental chlorine. The oxidising 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
[0034] In practise 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.
[0035] Ways to Obtain a Low OCl Pulp and to Reduce AOX
Discharges
[0036] 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 OCI
[mg/kg] 152 158 88 116 total AOX [kg/ADt] 0.41 0.23 0.39 0.21
[0037] 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.
[0038] 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 was decreased without a similar order of
increase in OCl.
[0039] 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.
[0040] 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.
[0041] 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 utilised
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
utilisation in the D0 position, i.e. the first stage using chlorine
dioxide.
[0042] From the results in Tabel. 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 shut-downs,
its use in continous bleaching processes for obtaining low OCl pulp
is new. The addition of sulphamic acid should be added in a
continous 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 dioxode stage. The
sulphamic acid could be added to the pulp before, after or during
addition of the chlorine dioxode in a chlorine dioxide mixer.
[0043] 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 oxidising power
lost due to the capture of elemental chlorine by sulphamic acid.
However, when utilised in D2-position, or in the final D-stage,
this means a very moderate additional need for chlorine dioxide in
this last stage.
[0044] The two concepts D* and (SD) could also be utilised 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 Tab. 1.
[0045] 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.
[0046] The OCl content can also be fighted 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.
* * * * *