Note: Descriptions are shown in the official language in which they were submitted.
W092/0~09 PCT/US91/05834
~8'J~7 ~
~LEM~NNG oE~CAL rn~ USING c~u~uNE/oHu~uNE DIOXIDE ~EN o~XE
Field of the Invention
The present invention relates to delignification
and bleaching of chemical cellulosic wood pulp fibers for
use in papermaking and to the production of a pulp having
reduced halogen concentrations, and good brightness and vis-
cosity.
Backaround of the Invention
Chlorine-based chemicals, such as chlorine, chlo-
rine oxide, and hypochlorite, have been used in pulp
bleach_ng 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, 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
chlori~?-based bleaching agents have been employed in the
prior ~rt, there are produced chlorinated organics and
organic halogens (i.e. chlorides). The compounds are gener-
ally insoluble in an aqueous medium and substantial quanti-
ties are swept from the pulp with the effluent from one or
more of ~he stages of the bleaching sequence. A relatively
smaller percentage of such chlorinated organics and organic
chlorides 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 Gf the chlorina-
tion charge required for a specific pulp. By definition,the "C" factor refers to the effective chlorination charge
and is egual 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
' .
. ~ - .
, .
wo92/0360g ,, PCT/USgl/058~
~ 4 l ~ -2-
stated in the prior art that higher "C" factors produce
brighter pulps, but that a "C" factor of 0.22 provides
maximum brightness in a pulp without unacceptable degrada-
tion 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" factors, i.e. larger quantities of chlorine-
containing bleaching agent. Invariably such quantities of
chlorine result in presently undesirable or unacceptable
levels of chlorinated organics and/or organic chlorides.
In the past decade, there has been a growing
concern about the environmental impact of chlorinated organ-
ic 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, propose restricting the total chlorideresidue 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 are proposed
to be restricted to 2.0 Xg or less in some of the European
countries. 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
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-
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
,
, , . ~:- . .
.
,
W092/0~ PCT/US91/058
-3-
~ 7t~
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 desi-able.
Processes using both oxygen delignification and chlorine
dioxide substitution have been suggested but do not achieve
the regulated concentrations of chlorine-containing residues
in either the pulp or the effluent.
SummarY of the Invention
The present invention comprises a method for the
treatment of kraft pulp in preparation for its use in paper-
making including the steps of
contacting the pulp while dispersed in an aqueous
medium with an initial charge of a chlorine-based
bleaching agent selected from the group consisting
of chlorine dioxide, and mixtures of chlorine
dioxide and chlorine, wherein the total chlorine
dioxide present in the charge represents between
about 100% and about 50% of the total charge,
,
..
.
: . .
~, , .
W092/03609 PCT/~S91/058~
~0~'~47 7 4
expressed as effective chlorine, the remainder of
the charge being chlorine, the bleaching agent
producing a substantial quantity of chlorine-
containing moieties that are insoluble in an aoid
medium,
thereafter, and prior to any intervening treatment
of said pulp other than a wash, contacting the
pulp with ozone, the ozone being present in an
amount which provides a ratio of "C" factor to %
ozone, based on dry weight of pulp, of between
about 0.03 and about 0.6 for a period of time
sufficient to cause the ozone to oxidize substan-
tial quantities of the chlorine-containing moie-
ties and render the same soluble in the acid
medium of the pulp, and
thereafter, separating the solubilized chlorine-
containing moieties from the pulp,
whereby there is obtained a pulp having substan-
tially lower chlorine content, essentially no
tetrachlorodibenzodioxin, essentially no tetra-
chlorodibenzofuran, and substantially equivalent
brightness and viscosity as the same pulp which
has been bleached employing conventional
chlorine-based bleaching sequences,
a cellulosic kraft pulp suitable for papermaking
purposes, the pulp being bleached through the use
of chlorine dioxide or a mixture of chlorine
dioxide and chlorine, being essentially free of
tetrachlorodibenzodioxin or tetradibenzofuran,
having a total organic chloride content of less
than about 200 ppm, based on pulp, and having a
total adsorbed organic halogen content of less
than about 2 kg/ton, based on pulp.
.
092/0~9 PCT/US91/058
-5- :
In accordance wit~ ~e4~thod of the present
invention, a chemical cellulosic wood pulp, e.g. a kraft
pulp, is initially contacted with chlorine dioxide as the
first-stage of a bleaching sequence. This chlorine dioxide
may initially contain up to about 50% chlorine, based on
effective chlorine, or in this initial bleaching stage, up
to about 50% chlorine may be added to the chlorine dioxide
as a substitute for the chlorine dioxide, also based on
effective 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 custom-
ary extraction, further bleaching, etc. stages. Important-
ly, and as noted to be contrary to the prior art, the ozona-
tion takes place immediately following chlorination andprior to extraction. It has been recognized heretofore that
ozone as a strong oxidant tends to degrade the pulp. Also,
the art teaches 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 the ozonation can take place immedi-
ately following an initial chlorine dioxide/chlorine tDC)
bleaching stage as defined hereinabove. In this regard, it
is to be understood that preferably there is a conventional
water washing of the pulp between the stages of the bleach-
ing sequence. This washing is not deemed to be a "stage",
but rather i9 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 l
and about 3. It has been found that the wash water need not
be "clean" or "fresh" water. For example, recirculated
water which contains considerable residues from previous
usage has been used successfully.
Among the several unexpected advantages provided
by the present method, it has been found that the ozone
W092/03609 PCT/US9~/058~
~ O ~ ~ ~ 7 7 -6-
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 acid 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 organ-
ics discharged in the effluent. These converted inorganic
chlorides, generally, are not environmentally hazardous
and/or are recoverable. The result is a pulp having good
brightness, acceptable viscosity, and reduced chlorinated
organic content. Notably, such desirable results are ob-
tained 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. TOCl as
used herein refers to the total organically bound chlo~ine
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
halogens (AOX) per ton of wood pulp. AOX, as the term is
used herein refers to the sum of the AOX, i.e. adsorbed
organic halogens, of the several stages of bleaching, i.e.AOX of the Dc stage plus AOX of the Z stage plus AOX of the
E stage an* plus AOX of further D stages, 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 kraft cellu-
'
.
WOg2/03609 PCT/US9l/058
-7-
~89477
losic wood pulp fibers for use in papermaking that can
provide wood pulps with lowered residual chlorine content
and effluents with lowered levels of adsorbed organic ha-
lides (AOX) while retaining or enhancing the characteris-
tics of brightness and pulp viscosity within acceptableranges.
It is another object of the present invention to
provide such an improved ~ethod for bleaching kraft cellu-
losi¢ 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 Fiqures
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-
ing operation. Softwoods, hardwoods or mixtures thereof may
be processed employing the present invention. Following
25 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. Prefèrab}y in this stage the oxidant is principally
chlorine dioxide. Consistent with the industry practices,
35 the term, D, is employed to represent a bleaching stage
employing chlorine dioxide. Herein, Dl is used`to represent
the first D stage of a sequence. The term Dc, represents a
~ .
W092/03609 PCT/US91/058~
~ 4 ~ ~ -8-
stage employing chlorine dioxide plus chlorine. In such
stage, the chlorine may be present in the chlorine dioxide,
i.e. a mixture, or the chlorine dioxide may be first added
to the pulp and after a brief time period the chlorine is
added. The term D-C is at times used to represent this
latter chlorine substitution, but herein, the term Dc is
deemed to encompass both the mixture and the chlorine sub-
- stitution procedures unless indicated otherwise.
Following the Dl stage, and with or without an
intervening washing, the pulp of the present invention is
processed through an ozonation stage, Z. Heretofore, it has
been taught in the industry that ozone tends to be decom-
posed by certain of the heavy metal components of the pulp
that conventionally are removed by an additional acid treat-
ment stage that comprises subjecting the pulp to an acidicmedium. In the present invention the pulp moves from the Dl
stage which is an acidic stage directly to the Z stage
without an acidic treatment stage. In the Z stage, the acid
pulp is contacted with ozone in a reaction tower, for exam-
20 ple, for a period of time and employing sufficient ozone toeffect oxidation of substantial quantities of the chlorinat-
ed 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 moie-
25 ties are soluble in the environment of the Z stage. Thus,these oxidized chlorine-based moieties are solubilized into
the liquor and extracted from the pulp during washing fol-
lowing the Z stage. Other of the oxidized compounds are
soluble in an alkaline solution such as is present in an E
30 stage. Extraction with an alkaline medium following the Z
stage thus serves to remove alkaline-soluble oxidized chlo-
rine-based moieties from the pulp. As noted, a preferred
complete sequence will include an extraction stage following
the ozonation stage. Such extraction stage may be enhanced
35 with oxygen, Eo~ or peroxide, Ep, a combination of oxygen
and peroxide, Eo+p~ or other enhancer. The pulp may be
further processed through any of several selected stages.
W092~03609 PCT~US91/05834
-9- ~V~47 ~
Preferably, the extraction stage is followed by a further
bleaching stage. Chlorine dioxide, i.e. a D2 stage is
preferred as such further bleaching stage. As desired,
washes may be employed between selected stages, e.g. between
the E and D2 stages.
To facilitate a further understanding of the
invention, the following examples are given pri-
marily for purposes of illustrating certain more
specific details thereof.
Southern softwood kraft 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 85C, with 3% sodium
zdroxide, 40-80 psig oxygen pressure and 0.5% magnesium
sulfate on the pulp at a consistency of 10%.
Dc stage: In the sequential chlorine dioxide
substitution stage, D~C, chlorine dioxide was added to the
pulp and mixed; after 20 seconds, chlorine was added and the
pulp was well mixed; this stage was conducted in a reactor
for 40 min. at 50C with a consistency of 3-10%. In those
instances where the chlorine was present in the chlorine
dioxide as a mixture, the chlorine was introduced simultane-
ously with the chlorine dioxide.
C stage: Chlorination was conducted in a reactor
for 30 to 40 min. at 45C with a consistency of 3~, with
chlorine of the desired charge.
D stage: Chlorine dioxide addition was conducted
at 50 to 70C 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 ~in. with a
consistency of 1%; Example 34 was done under alkaline condi-
tion at 34% consistency at pH 12.
E stage: Extraction was conducted for 60 min. at
7QC with a consistency of 10% and with desired sodium
.
W092/03609 PCT/US91/058~
7 -lo-
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 Eo
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.
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
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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W092/03609 PCT~US91/05834
-12-
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, lO and 12, produced
pulps of low brightness, except in Examples 7 and 12 where
additional and costly extraction and chlorine dioxide stages
were added. Examples 8 and 9 (DZED) where ozonation was
employed after chlorination, produced pulps of good bright-
ness and viscosity plus low total chloride 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. Fur-
ther, the brightness obtained by the DZED sequence is almost
equal to the brightness obtained in the more costly prior
art CEDED sequence (cf. Ex. lO and Ex. 5). The DZED (Ex. 8)
produced better results than that of ZDED (Ex. 10) at same
chemical charge. The DZED produced 84.7% GZ brightness
while ZDED produced only 79.0% GZ. The same result has been
experienced in the DZEDED (Ex. 11) and ZDEDED (Ex. 12)
examples. The DZ sequence also generated less TOCl than
that of ZD sequence.
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EXAMPLES 14-l9
Examples 14-16 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 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 DZED sequence without such enhance-
ment.
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 TOCl 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-
ide employed in these Examples 14-l9 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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W092/03609 PCT/US91/058
-17-
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EXAMPLES 20-54
A laboratory DCZED bleaching sequence was studied
using a central composite statistically designed experiment
which varied Dc stage chlorination factor and fraction of
chlorine dioxide charge, and ozone charge in the Z stage.
The pulp had 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 TOCl and AOX values, each being
within present regulatory limits. Most surprisingly, howev-
er, 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 bleach_ng agent, one can use ozone per-
centages of about 1% and obtain a three-fold reduction in
AOX values.
.
- .
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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 showed AOX levels
in excess of the desired 2.0 kq 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 DZED sequence,
and produced effluent well below the targeted AOX level.
Example 14 showed that pre-delignification with oxygen can
improve the AOX level, but at the expense of the cost asso-
ciated with the additional stage (O).
.
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Experimental data were sub~ected to regression
analysis of pulp brightness, viscosity and TOCl 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 100% chlorine dioxide in the Dc stage, it
may ~e seen that as one accepts a lower viscosity, equal
amounts of ozone can be employed with lower quantities of
chlorine dioxide ("C" factor) while still maintaining high
brightness and low TOCl 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 kraft
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 addition, the wood pulp has a bright-
ness 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-
tive 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 TOCl and AOX values while maximizing the bright-
30 ness and viscosity values of the pulp.EXAMPLES 55-58
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 all examples. In Example 55, a con-
trol run indicative of prior art bleaching sequences that do
WO 92/03609 r ~ PCT/US91/058~
4 ~ 24- ~ -
not employ ozone, but which do include oxygen and peroxide
enhanced extraction, there was no detectable tetrachlorodi-
benzodioxin (TCDD) in the final bleached pulp. In this
example, there was detected 2.5 ppt of tetrachlorodibenzofu-
ran (TCDF). In the effluent from this run there was detect-
ed 2.5 ppt and 30.3 ppt of TCDD and TCDF, respectively,
In Example 58, a DCZEoD sequence in accordance
with the present invention, there were no dioxins (TCDD or
TCDF) detected in either the pulp or the effluent.
,
,
.
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EXAMPLE 59
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 59, 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 simplification of
this process can lead to a capital reduction. The ~OCl of
the pulp was 128 ppm, well below present regulated stand-
ards.
:
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