Note: Descriptions are shown in the official language in which they were submitted.
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OZONE TREATMENT OF CHLORINE DIOXIDE/CHLORINE
CHLORINATED PULP
Field of the Invention
The present invention relates to delignification
and bleaching of cellulosic wood pulp fibers for use in
papermaking and to the production of a pulp having reduced
halogen concentrations, and good brightness and viscosity.
Backaround of the Invention
Chlorine-based chemicals, such as chlorine, chlo-
rine dioxide, and hypochlorite, have been used in pulp
bleaching for several decades, and continue to be used for
removing lignin and bleaching the pulp to high brightness.
In general terms, the extent of bleaching, hence the degree
of brightness obtained, is determined by the type of pulp
being bleached and the proposed end use of the paper
product. For example, groundwood pulp may be intended for
use in making linerboard which does not need to exhibit a
high brightness. In this situation ~.ess bleaching may be
employed. In other examples, kraft pulps intended for use
in making fine writing papers require different bleaching
circumstances which will produce the desired brightness in
the final paper product. In all instances, however, where
chlorine-based bleaching agents have been employed in the
prior art, there are produced chlorinated organics. These
compounds are generally insoluble and substantial quantities
are swept from the pulp with the effluent from one or more
of the stages of the bleaching sequence. A relatively
smaller percentage of such chlorinated organics remain in
the pulp and eventually appear in the paper product.
In chlorine-based bleaching processes, the "C"
factor of the pulp is employed as a measure of the chlorina
1
in ~~ .y s ~'~.,
~i . ~° ~ ~t z:
tion charge required for a specific pulp. By definition,
the "C" factor refers to the effective chlorination charge
and is equal to the chlorine dioxide plus the chlorine in
the charge (expressed in terms of effective chlorine),
divided by the Kappa number of the pulp. Generally, it is
stated in the prior art that higher "C" factors produce
brighter pulps, but that a "C" factor 0.22 provides maximum
brightness in a pulp without unacceptable degradation of the
cellulosic fibers (e.g. reduced strength). Thus, it has
been common heretofore when seeking maximum brightness of
the pulp, (in the range of 70-85% GE) to use large "C" fac-
tors, i.e. larger quantities of chlorine-containing bleach-
ing agent.
In the past decade, there has been a growing
concern about the environmental impact of chlorinated organ-
is compounds in bleach plant effluent. Also, public concern
for the disposal of paper containing organically bound
chloride has been increasing. Undesirable chlorinated
organics such as dioxin have been detected in the exhaust
gases of incinerators burning municipal wastes containing
chloride, including, for example, paper products made from
chlorine bleached pulps. West German environmental regula-
tions, for example, have restricted the total chloride
residue for packaging material including wood pulp to less
than 200 ppm. The allowable adsorbed organic halogens (AOX)
discharged in the effluent per ton of wood pulp has been
restricted to 2.0 kg or leis. More stringent regulations
are expected in the near future.
Several options have been proposed or practiced to
reduce or eliminate chlorinated organics in the bleaching
process. The most straightforward method is to substitute
2
~d ~~ .:w ~ i,y P/ ~~::
non-chlorine bleaching chemicals, such as oxygen, peroxides,
ozone, peracetic acid, etc. for chlorine-based bleaching
chemicals. Unfortunately, no chlorine-free bleaching proc-
ess has been developed with the ability to produce accept-
s able pulp properties (such as brightness or viscosity) at an
acceptable bleaching cost.
Another option to reduce the discharge of chlori-
nated organic compounds is to reduce the chlorine usage in
the first stage of the bleaching process. Two alternatives
that produce no significant degradation of pulp properties
have been commercialized for this purpose. These are (a)
extended delignification in the cooking stage and (b) oxygen
delignification. These alternatives, with proper extrac-
tion, reduce the lignin content of brown stock going into
the bleach plant. They do not, however, reduce the chlori-
nated organic compounds in the bleached pulp and effluent to
sufficiently low levels.
A third option to reduce the generation of chlori-
nated organics in a bleaching process is to substitute
chlorine dioxide for chlorine. Chlorine dioxide is a rela-
tively strong oxidant compared to chlorine; to achieve the
same degree of delignification, it requires only about
thirty-eight weight percent chlorine dioxide on the pulp
compared with one hundred weight percent of chlorine.
however, these prior art processes are of the DEDED type
wherein the chlorination stage (D) is followed by the con-
ventional extraction (E) and additional chlorination (D)
stages. The pulp and the discharge effluents resulting from
this prior art option contain higher concentrations of
chlorinated organics than are acceptable and/or desirable.
Processes using both oxygen deligr~ification and chlorine
dioxide substitution have been suggested but do not achieve
3
s:v ~~ .~ r4 F~ ~: -;
~..a ev .'. ~ e., ~7 '_:
the regulated concentrations of chlorine-containing residues
in either the pulp or the effluent.
Summary of the Invention
In accordance with the method of the present
invention, a cellulosic wood pulp is initially contacted
with a chlorine-based bleaching agent as the first-stage of
a bleaching sequence. This chlorination agent preferably is
primarily chlorine dioxide, but may contain chlorine.
Contrary to the prior art, after this initial D stage and
prior to any extraction (E) stage, the pulp is processed
through an ozonation stage. After the ozonation stage, the
pulp is processed through the customary extraction, further
bleaching, etc. stages. Importantly, and as noted to be
contrary to the prior art, the ozonation takes place immedi-
ately following chlorination and prior to extraction. It
has been recognized heretofore that ozone as a strong oxi-
dent tends to degrade the pulp and that any use of ozone on
pulp should be preceded with an acid wash, for example to
remove heavy metals known to be harmful to 'the stability of
ozone and/or decomposition products that may cause low pulp
properties (e.g. strength). Contrary to such prior art, the
present inventor has found that without an acid wash stage
the ozonation can take place immediately following an ini-
tial chlorine dioxide (D) bleaching stage. In this regard,
it is to be understood that preferably there is a conven-
tional water washing of the pulp between the stages of the
bleaching sequence. This washing is not deemed to be a
"stage'°, but rather is in the nature of a dilution of the
aqueous phase of the pulp. for example, the quantity of
water commonly used provides a dilution factor of between
about 1 and about 3. It has been found that the wash water
4
s' ri~ .~ raf s~ ~a
~L~ ..~ d d:i tt _>_
need not be "clean'' or "fresh'' water. For example, recircu-
lated water which contains considerable residues from previ-
ous usage has been used successfully.
Among the several unexpected advantages provided
by the present method, it has been found that the ozone
treatment not only provides delignification and bleaching
but also oxidizes substantial quantities of the chlorinated
lignin residues left over from the chlorination stage.
This oxidative activity converts substantial quantities of
the insoluble chlorinated organics to inorganic chlorine-
containing compounds and/or to chlorinated organics that are
soluble in the liquor present in the ozonation stage. The
oxidation of these chlorinated residues by the ozone thus
not only reduces the chlorinated organic content in the wood
pulp, but also reduces the adsorbed chlorinated organics
discharged in the effluent. These converted inorganic
chlorides, generally, are not environmentally hazardous.
The result is a pulp having good brightness, acceptable
viscosity, and reduced chlorinated organic content. Nota-
bly, such desirable results are obtained by employing as few
as four stages in the bleaching sequence, and employing
lower total effective chlorine charges in the chlorination
stage than heretofore known to be possible.
Specifically, the present inventors, using their
improved method, have produced wood pulp which has less
than 200 ppm total organic chloride (TOCl) residue. TOC1 as
used herein refers to the total organically bound chlorine
content of the pulp resulting from 'the bleaching sequence
unless indicated otherwise. TOCl is essentially insoluble.
The effluent discharge from the bleaching process of the
disclosed method has less than 2.0 kg adsorbed organic
5
,:,
(J ':.,i .:,~. ,u ~.~i li ~_:
halides (AOX) per ton of wood pulp. AOX, as the term is
used herein refers to the sum of the AOX, i.e. adsorbed
organic halides, of the several stages of bleaching, i.e.
AOX of the Dl stage plus AOX of the Z stage plus AOX of the
E stage and plus AOX of the D2 stage, unless indicated
otherwise. As noted, the pulp produced by the present
invention has a brightness and viscosity that meet or exceed
the requirements of industrial papermaking.
Therefore, it is an object of the present inven
tion to provide a method for the preparation of cellulosic
wood pulp fibers for use in papermaking that can provide
wood pulps with lowered residual chlorine content and efflu
ents with lowered levels of adsorbed organic halides (AOX)
while retaining or enhancing the characteristics of bright
ness and pulp viscosity within acceptable ranges.
It is another object of the present invention to
provide such an improved method for bleaching cellulosic
pulp that is both economical and readily adaptable to the
current papermaking industry. These and other objects will
be recognized from 'the present description including the
Figures.
Brief Description of the Figures
FIGURES 1-5 are graphic representations, computer
generated from mathematical models depicting the relation
ship of ozone charge and chlorination factor in obtaining
various values of the brightness, viscosity and total chlo-
ride on pulp.
In accordance with the present invention, an
aqueous slurry of cellulosic wood fibers as is commonly
produced by conventional digestion means in the papermaking
industry is processed through sequential stages of a bleach-
6
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~ -a v
~E~" ~~
ing operation. Softwoods, hardwoods or mixtures thereof may
be processed employing the present invention. Following
digestion, the slurry preferably is processed directly to
the bleaching sequence disclosed herein. If desired, it may
be subjected to such treatment as oxygenation, ozonation or
other oxidation prior to entering the present sequence, but
with attendant increased expense.
In the first stage of the present sequence, the
pulp slurry is treated with a chlorine-based agent, i.e.
chlorine dioxide or a mixture of chlorine dioxide and chlo
rine. Preferably in this stage the oxidant is principally
chlorine dioxide. Consistent with the industry practices,
the term, D, is employed to represent a bleaching stage
employing chlorine dioxide, with Dl, representing the first
D stage of a sequence. The term Dc, represents a stage
employing chlorine dioxide plus chlorine.
Following the D1 stage, and with or without an
intervening washing, the pulp is processed through an ozona-
tion stage, Z. Heretofore, it has been taught in the indus-
try that ozone tends to be decomposed by certain of the
non-cellulosic components of the pulp. Contrary to the
prior art, in the present invention the pulp moves from the
D1 stage directly to the Z stage without an intervening
extraction, E, stage. In the Z stage, the pulp is contacaed
with ozone in a reaction tower, for example, for a period of
time and employing sufficient ozone to effect oxidation of
substantial quantities of the chlorinated lignins and/or
other chlorinated components of the pulp, in addition to the
contribution of the ozone toward further delignification.
Many of the oxidized chlorine-based lignins, residues, etc.
commonly are soluble in the environment of the Z stage.
Other of the oxidized compounds are soluble in an alkaline
7
6 S ~ ~:~ '~.; i~; Iz
e,.>l~ ~:;:
solution such as is present in an E stage. Thus, these
oxidized chlorine-based compounds are solubilized into the
liquor and extracted from the pulp during washing and/or
extraction following the Z stage. This liquor effluent may
then be processed as necessary to recover desirable chlo-
rine-containing compounds or to destroy or render harmless
other of the compounds. The pulp may be further processed
through any of several selected stages, and preferably a
complete sequence will include an extraction stage following
the ozonation stage. Such extraction stage may be enhanced
with oxygen, Eo, or peroxide, Ep, a combination of oxygen
and peroxide, Eo+p, or other enhancer. The extraction stage
is followed by a further bleaching stage, preferably using
chlorine dioxide. As desired, washes may be employed be-
tween selected stages, e.g. between the E and D2 stages.
To facilitate a further understanding of the
invention, the following examples are given primarily for
purposes of illustrating certain more specific details
thereof.
Southern softwood draft pulp with Kappa number of
32.3 and an initial viscosity of 30.3 cP was used as the
wood pulp slurry for all of the following Examples except as
otherwise noted. The series of Examples used various combi-
nations of the following bleaching stages:
O stage: Prechlorination oxygen delignification
was conducted in a reactor for 1 hr. at 85°C, with 3o sodium
hydroxide, 40-80 psig oxygen pressure and 0.5% magnesium
sulfate on the pulp at a consistency of 10%.
DC or D-~C stage: zn the sequential chlorine
dioxide substitution stage, chlorine dioxide was added to
the pulp and mixed; after 20 seconds, chlorine was added and
8
4'.w~ r: ~ iy r
ii t v ~ i..
~u~ _~7. 'r Lr P -
the pulp was well mixed; this stage was conducted in a
reactor for 40 min. at 50°C with a consistency of 3-10%;
except in those instances where the chlorine was present in
the chlorine dioxide, in which case the chlorine was intro-
s duced simultaneously with the chlorine dioxide.
C stage: Chlorination was conducted in a reactor
for 30 to 40 min. at 45°C with a consistency of 3%, with
chlorine of the desired charge.
D stage: Chlorine dioxide addition was conducted
at 50 to 70°C with a consistency of 10%;
Z stage: The ozone stage was conducted under a pH
of about 2-5 at room temperature for 40 to 90 min. with a
consistency of 10;
E stage: Extraction was conducted for 60 min. at
70°C with a consistency of 10% and with desired sodium
hydroxide charge;
Eo stage: Extraction enhanced with oxygen was
conducted under conditions similar to the E stage with an
initial oxygen pressure of 40-50 psig that was gradually
reduced to 0 psig;
Ep stage: Extraction enhanced with peroxide was
conducted under conditions similar to the E stage with a
0.4-0.6% hydrogen peroxide charge;
Eo + p stage: Extraction enhanced with oxygen and
peroxide was conducted under conditions similar to the E°
stage with a 0.4% to 0.6% hydrogen peroxide charge;
Washings: The slurry was washed with distilled
water in a screen box or funnel between each stage; the
stage labeled (DZ) is a D stage followed immediately by a Z
stage without intermediate washing.
9
.U.
EXAMPLES 1-5
Several prior art bleaching sequences employing
procedures well known in the art were run on Southern pine
softwood kraft pulp having a Kappa number of 32.3. These
sequences employed the usual chlorine stages with and with-
out chlorine dioxide substitution. The results are shown in
Table I. As expected, good brightness and viscosity values
15
were noted, except when an oxygenation stage was employed
prior to chlorination (Example 3). This latter Example
showed the deterioration of the pulp (low viscosity) that
has been noted in the prior art. Of these Examples, only
the D-+C Eo+pDED produced significant reduction in the total
chloride on the pulp, but such reduction was obtained only
at the expense of two additional stages.
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EXAMPLES 6-13
In Examples 6-13 the pulp employed was the same as
in Examples 1-5. Examples 6-13 involved bleaching sequences
using an ozone bleaching stage. Table 2 shows the proper-
ties of the pulps of these Examples. Employing ozonation
prior to chlorination, Examples 6, 7, 10 and 12, produced
pulps of low brightness, except in Examples 7 and 12 where
additional and costly extraction and chlorination stages
were added. Quite unexpectedly, Examples 8 and 9 (DIED)
where ozonation was employed after chlorination, produced
pulps of good brightness and viscosity plus low total chlo-
ride on the pulp. Notably, Examples 8 and 9 (DZED) employed
20% and 7% less chlorine dioxide than Example 11 (DZEDED)
plus the fact that the DZED sequence with its fewer stages
yielded better viscosity values and almost equal brightness
values. Further, the brightness obtained by the DIED se-
quence is almost equal to the brightness obtained in the
more costly prior. art CEDED sequence (cf. Ex. 10 and Ex. 5).
12
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EXAMPLES 14-19
Examples 14-15 employed Southern softwood kraft
pulp having a Kappa number of 27.3. Table 3 shows certain
properties of the resultant bleached pulps. From Table 3 it
is noted that enhancement of the extraction stage (e.g. Eo,
p, or E o+p in the preferred DZED bleaching sequence permits
the use of less chlorine dioxide in the D stage and produces
a pulp of substantially equivalent brightness and viscosity
to the pulp produced using a DIED sequence without such
1p enhancement.
Examples 17-19 employed Southern hardwood kraft
pulp having a Kappa of 14.9. Table 3 also shows the proper-
ties of the resultant bleached pulps of these examples.
These pulps showed high viscosity and good brightness values
as well as relatively low TOC1 values employing total chlo-
rine dioxide amounts less than the total chlorine dioxide
amounts employed in obtaining substantially equivalent
brightness and viscosity values for softwood pulp. For
either softwood or hardwood pulps, the total chlorine diox-
2p ide employed in these Examples 14-19 is substantially less
than the total chlorine dioxide employed to obtain equiva-
lent brightness and viscosity values without employing ozone
after chlorination.
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16
'~,F .? ~f rj :~ ~,~!:
EXAMPLES 20-54
A laboratory DOZED bleaching sequence was studied
using a central composite statistically designed experiment
which varied Dc stage chlarination factor and fraction of
chlorine dioxide charge, and ozone charge in the Z stage.
The pulp was assumed to have a Kappa number of 32.3. The
results are shown in Table 4. In Table 4, Examples 24, 29-
32, 34, 38-41, 45, 47-49 and 54 represent actual run data
which served as the basis for the statistically designed
experimental data of the remaining examples. From this data
it may be concluded that to obtain a pulp brightness of
about 85% GE or higher and a viscosity of at least about 18
cP, the preferred chlorination factor is about 0.12 and the
ozone charge is about 1.1%. This holds true for chlorine
dioxide which includes up to about 20% chlorine. All of the
pulps of Examples 20-38 showed low TOC1 and AOX values, each
being within present regulatory limits. Most surprisingly,
however, it was found that by using a lower chlorination
factor (e.g. 0.12) which results in substantial savings in
the cost of chlorine-based bleaching agent, one can use
ozone percentages of about 1% and obtain a three-fold reduc-
tion in AOX values.
17
m N o o co N o m o ~ ~ ~ ~ ,fl a,
. .
N P., M N N rl rl O O O1 01 O ~n M Cp Cp
O U N N N N N N N ri rl N N N r-I r1 N
U
N
N
Ul W O O O O O O O O O ri In op O O 1p
In CO Q1 N h l0 O 01 0~ tn l0 M M In 01
LL ~ \ lfl tn tn 10 V' h CO h h ~O d' d' vD CO h
f-0
H ~ Il) r-I O to ri
ri Ca ~ rl rl N d' N N M M ~O d' h In d' h
x~r~
0 0 0 0 0 0 0 0 0 0 0 ,~ r; 0 0
Hx
U
p,
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N ~ri ~ ~ o o in o 0 0 o 0 0 0 0 ~n o 0 0
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M rl N d' h N rl rl
H tn ~
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O
t~, 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
riM
w ~ ~ ~1 r.i ri ri ri rl rl rl r-1 ri rl rl r-I ~1 n-I rl
a
oa
W -I O 01 O O r! CO N M M M O tn Ln ,-~ rl
N U v N ~ O r-i r-I rl O rW -i r1 rl r-I o O r-I ri O
~ N N
ra > ~~
.~ .~, ~.,
U .r-~ 'LI O ~p N V' O O N M Ln In In rl rl O O
U O O r-i N l0 O rl N V' V' ~l' CO CO O O
o\° O ~ U
4-1 N O O U O O O O O O O O rl rl O O
N O
U1 ~ ~ O d' CO 10 O 10 d' M rl rl ri lfl Ln t0 CO
td O Ca N ri Q OWU N rl O c0 c0 CO V~ d' c0 c0
ov° ~1 rl rl O O N N N ri rl rl O O M M
N O In O O O Q In O O O O O O O O
O ~ O 01 O1 CO In O 01 01 CO CO Cp N N O O
ri o\° r-I
U ' '~ ri
O h
U U o r~ N
0
G'~' ° ° o
.rl U
U N ~1 f~ L1 A Ca G7 C7 G1 C1 !a !a
rtf G7 W W W W L1 W W W W W W W fa fa
41 W N N N N W N N N N N CJ N W W
r-I Ql N U U U U N U U U U U U U N N
CO U7 Ca Ca Ca Ca Ca W ~1 Ca W C7 la Ca Ca G1 q
N
rI
o ri N M ~r m ~ r co ov o ~ N M ~r
N N N N N N N N N N M M M M M
x
w
18
f-. <". G'., n
EzJ ~ ..".~ ~ C~ Yi .-i.
'r N O ~O M ~ h h
~J tn O CO V~ 01 CO V' N O In !'< ri M CO
-ri
cn w co m h co m u~ h o o amn ~ rt ~
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U
N
N
N
N W O O O h M h V' to tn O c1 O N M
øa ~', U
r-I .EJ lf1 cr M N N d' h h h h Cp ~p ~ ~p
h r ~
.,1
~1
H co o ~ ri N
r1 C7 tn l0 CO Ov h M V' 01 O N L(1 V' N O~
k N W
O O O N N N d' O rl ri N ri M ifl
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ri U .-I
b ~~ ~ Wn o 0 0 o tt~ o o m o 0 0 0 0
N u1 O h t0 u~ ~ O N In h O CO 1n
H N~ ~ rl ri N M M N ~l' rl rl rl ri N N N
1-1 O
N
(1, ~ O O O O O O O O O O O O O O
O N ~ G7 rl rl rl rl r-I rl ri rl r-i ri rl c-i rl rf
U C ø,
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W ~ p rl ri ri M co M co h h h rr h u1 r1
N~ U ~ N ri ri r~ o o ri o 0 0 o ri o o ri
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H c~S > wi
U .1.~ 'Li ( ~ o~ c0 ~ V~ d~ to O c0 ~ O O O o
U rl M h 01 01 01 CO O N In rl r-1 rl w1~
o\o N ~ U
W N O O O rl rl .-I M O O O rl rl -I V
W 5
No
UJ -f., h 01 O ~ ~i~ d~ O O N L11 O O O O
fd O C~ \O d' r-I 01 Ol 01 O Lf1 N 01 cN V' d' r-i
~ o\o M M M ri ri rl O Lf) In cJ~ ~ ~ cr ,-1
N LC1 O O O O O O O X11 O O O O O
O ~ 01 01 CO tf1 II1 1f1 O 01 Q1 W CO CO N
rl o\o ri
U
O
+~ h
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N
fsn o
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~.-I U
U N d G1 G1 Ca Ca Ca fa !a Ca la G7 Ca G7
r6 ~ W W W W W W W la W W W W W W
N N N N N N N N W N N N N N N
r-~ N U U U U U U U N U U U U U U
a7 u1 C~ L1 G7 Ca L1 Ca L7 Ca Ca Ca Ca q Ca Ca
N
r-i
m to h co o\ o ri N M m \o h co
M M M M M d' d' V' ~T d' d' V'
k
W
29
:~ r ~.1 ~'~,
J ~ .l' .~.
~'Ir N p1
CO N COl0 N d'
ri
N f~ 01 In V'd' ct'Ln
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U
N
N
N
U W o~ o o W o co
, ~ U
ri +~ ~O CO COr I~ CO
b t~'co coco co co
P,
~
!x7
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ri Ca 0~ M lf7CO O ~D
x rom
tn rl rlri N tnd'
'~H
x
~s
ro
ri
U
~
rl U
ro ~~ o m n o o m
~ ~
.La G CO O~ N tn O rld~
~
O ro M rir1 N N
H
O
O
z?
ro
U
ri
.,1
O O O O O O
co
'
~ r1 r-irlri ri ,1~
~ , x
.~ '
o err
d' M
fa
~O 111M M M M Cp''.'
W ~
)
U O O O O O O N
N
N
~
~
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E1 ~ r
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Ov
. N
U .1~ O O N In 01 M O
'b
U V O M 10 N N U7
o\o N
.(",
U
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N
M
N O N
N
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rl
~ rr ~r rico ri N .~
ro O L1 R,
3
o\~ r1 ~0 1~In lf7M N
'-' C
x
O
U
N O O tnp O O .j.J
.~.J
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o .~ f1
.~
~ ri
rl o\o
U ~
U
+~
x
x
N
N
O
+~ O
U U N
M
r-i
TS
W O .
ro
N
.'.~
'CJ
M
ri
v
ro
+~
U
~ U O
~..-
1
. ro
a
U v U
C~ G1~ Ca CaN
?~
S
-a
ro W C~ W W W W .R
f~
N N W N N N N ~
r-1 U U N U U U U ~
TS
~
P~l tn !a fa CaG7 L1 Ga~
N
O
'LS
ro
a~
zs
U,
U
N
ro ro~ro
x
W
Cn
01 O rlN M ct'
~. V' tn Lf7tn In tn
ro
x r1
N
M
W
... Y~~ ,~ < ,.
f d~ ~. e,', a j ' ':
Table 5 shows the adsorbed organic halogens (AOX)
generated in the bleaching stages of the sequences of Exam-
ples 1-3, 9, 11 and 14. Examples 1 and 2 snowed AOX levels
in excess of the desired 2.0 kg per ton of wood pulp. A
pre-delignification stage, as in Example 3, produced efflu-
ent within the 2.0 kg level, but Example 3, had high levels
of chloride in the wood pulp (see Table 2). Examples 9, 11
and 14 involved stages of chlorine dioxide bleaching fol-
lowed by ozonation. Example 9 represented a preferred
sequence, DIED, and produced effluent well below the target-
ed AOX level. Example 14 showed that pre-delignification
with oxygen can improve the AOX level, but at the expense of
the cost associated with the additional stage (O). Notably,
the 4-stage DZED (Example 9) sequence produced a lower AOX
value than the more costly 6-stage DZEDED (Example 11)
sequence.
21
t') ,7 :.y iy ;.:, F,
lsr ~ .~ ~~ ~~ e:' ~s-:
M M N
(CS O M i~ d' lD M
N N N
O ~ ~ ~ O O O
H N N '-I
I
~
Ca I
~l O I
I O
I
I O
I
I
Lc~
W O W I
~ O
N I O I ~
d I
O
I C.aI 1
O
W
~ I
W d' d' ~ I la I
O I
O
W 1 O 1 O 1 O
U1 L1 Ca L1 I
1 l 1 d'
>~ N I O I O i O 1 t0 W
I
O
O-r-I I I 1 W O 1 I
I In
1
W I
O
I I
O N
N
H ~ ~ ~
pq I M I M I In I
0~
\ OI OI OI 1 N I N
O I
O
W ~-!W r-iW O N N
;
i i i ~ 1 I
~-1O
x
O U ~oU ~OU ~-I 1 I I
M i~ I
iC, I 00I W I i~1 c-I I I
1 I I M i-1
I I I Ca I ~ I La
I I I I
Ca O Ga O t~ O 1 O 1 I
I I I c-IO
1 I I
R.~
U~ t~, ~
NO O W
U U
O
U~ ~ O la Ca
O
O
y-I N M (R ,-~ er
r-I -i
W
22
r, l° ~:..
~'~~ _i. ~ e,~ e.3 ~~
Experimental data were subjected to regression
analysis of pulp brightness, viscosity and TOC1 on chlorina-
tion factor, fraction of chlorine dioxide charge in the DC
stage and ozone charge in the second stage (DcZED sequence).
The results are depicted graphically in Figures 1-5. From
these Figures, at 1000 chlorine dioxide in the DC stage, it
may be seen that as one accepts a lower viscosity, equal
amounts of atone can be employed with lower quantities of
chlorine dioxide (°'C°' factor) while still maintaining high
brightness and low TpCl values. This relationship holds
true when the chlorine dioxide charge includes up to 20%
chlorine (see Figs. 4 and 5), the actual ranges being some-
what condensed when employing an 80/20 mixture of chlorine
dioxide and chlorine.
It may be seen from the foregoing that the present
invention provides for a method of preparation of cellulosic
wood pulp fibers for use in papermaking that reduces the
total chloride residue in the wood pulp to less than 200 ppm
and the AOX discharge per ton of wood pulp to less than 2.0
kg. In additions the wood pulp has a brightness greater
than 85% GE and a viscosity above about 14 centipoises.
Also, the method is economical and readily adaptable to the
current papermaking industry.
From the Examples, it will be noted that the
percentage of ozone, based on oven dry weight of pulp,
employed varies as a function of the "C" factor, the effec-
five ratio of "C" factor to % ozone being between about 0.11
and 0.6, and preferably between about 0.2 and 0.6 for mini-
mizing the TOC1 and AOX values while maximizing the bright-
ness and viscosity values of the pulp.
Various features of the invention which are
believed to be novel are set forth in the appended claims.
23
C;": C~ ,:" :r.. .x;. ~. n
~.y' ;'.ir 1. ~ .n.~ '.:4 '.~'_'
EXAMPLES 55-56
In Table 6, there are shown the results of two
runs in which the dioxin content of the pulp and effluent
was determined. Southern pine kraft pulp, with 1% reject
from cooking, having a Kappa No. of 30.3 and a viscosity of
35 cP was employed in both examples. In Example 55, a
control run indicative of prior art bleaching sequences that
do not employ ozone, but which do include oxygen and perox-
ide enhanced extraction, there was no detectable tetrachlor-
odibenzodioxin (TCDD). In this example, there was detected
2.5 ppt of tetrachlorodibenzofuran (TCDF). In the effluent
from this run there was detected 2.5 ppt and 30.3 ppt of
TODD and TCDF, respectively,
In Example 56, a DZEoD sequence in accordance with
the present invention, there were no dioxins (TODD or TCDF)
detected in either. the pulp or the effluent.
24
LID f~ ;~ y r 4~y G1 'd
J s' j'l
~5! ~L1 ..2. ~ ~.% va i~,
G4 Ln Ca
M Ca
zz
U N
O
H r~
R
q
(
+~ Z z
~
H N
C
N
N N
O ~1., Q, N
r-I r-I
~ ,-i
w w
ww ww
N
O
r-i O O
~ ~ o -~-I
U
O N
~r
H ~
x
N O
z x o
H
x
0
H
Ca
O O N O
w
O W -i O .~
a
z
H ~ u~ cn
\
O
Ln oo to r~ U
u1
H .
O U
N
w o\ w o\
~ o O
W V H
w
p
~1 .1-~
U
o\ V' rt1
(
N Ca .~-1
),.I
o o cd o
x
z
\
v ~
o w V' ~ \
w N
-1-~ M 'Cj
O -~.
O
r-1
o\ U ~,-'
./->
O \ o
U t~
io ~ c0 O~
b~
TS
U 3 -~
N
N
1 ? -r-I
N
o\ o\ ~
Ca
tn .N
O O
H ~-i U .1..~
U
G ~
O 'LS
'Ly
N
-rl
~ 41
r0
~
3
v ~ c
aU~
w
- ~ 'O
N r-I
U ~
N O !a lartf?-W
U~
3 +~
.~
U N ~
?I U7
U
N ~ ~
c4 Ca U +~
u~ G .t-
~
O rt1
N
-N W
~ ~-f
23
b
N m ~-I
~ rtf
~ +~
x ~ o
s~
.~
o
s-~ w w
rn
w z
ro
r0
x v . ..
W P-d' H N
c7
V'
z5
4 ~ ~ ,r <f c , f!
~~~ ~~:~t
EXAMPLE 57
Table 7 presents the results of a bleaching se-
quence in accordance with the present invention in which the
ozonated pulp at a pH of 12.7 was processed directly to an
alkaline extraction stage (enhanced with oxygen) without an
intervening water washing (ZEo). In this Example 57, the GE
brightness of the pulp and its viscosity were lower than
when the pulp was washed with water between the Z and E
stages, but these parameter values were still in a range
that is acceptable for certain pulps. The TOCl of the on
pulp was 128 ppm, well below present regulated standards.
26
~; ~~ i :., ., ., ,
H
.,1
N
O W
U U '-i
N
D
H m n
N
z
O ~W
N .~.J .r
(~
O
~ o\o M
t~
W
z s~
a x '
a
a
x o
rrytt,, ov
WI M
U
N
N
.,1
x ~n
N O
rt O
z U
o\o o\o
M d'
M
o\o
a r~
U to
N O
W
N
v
r~
~r
W
27
