Note: Descriptions are shown in the official language in which they were submitted.
1090510
BACKGROUND O~ THE INV~NTION
The present invention relates generally to a method of
bleaching and delignifying lignocellulosic pulps. More particu-
larly, the invention relates to a method of bleaching and
delignifying lignocellulosic pulps with ozone.
In recent years the pulp and paper industry has devoted
substantial efforts to the development of chlorine-free or re-
duced chlorine bleaching processes. A milestone in this long
range effort has been the development and implementation of
oxygen bleaching system~ generally, and particularly the low
consistency oxygen bleaching procesR described in Roymoulik et
al, U.S. Patent No. 3,832,276.
Oxygen bleaching, ho~ever, is only a partial solution to
the chlorine-free concept, sinoe oxygen bleaching by itself can-
not produce pulp8 of sufficiently high brightness and qualit~.
One route which has been investigated by a number of reiea'rchers
is to employ ozone as a bleaching agent, either alone'-or follow-
ing an oxygen bleaching stage, since ozone has the capablllty
of bleaching pulps to high brightnes~ and i~ free of chlorine.
Recent publications relating to ozone bleaching of wood pulp9
include Rothenberg et al (Tappi 58[8] 1975:182); N. SoteIand
and K. Kringstad ~Norsk Skogindustri 121 1968:1)7 R. ~.
Secrist and R. P. Singh (Tappi 54 [4~ 1971:581); Z. Os~wa and
C. Schuerch (Tappi 46 12] 1963:79). U.S. patents relating to
the ozone bleaching of fibrous material~, include: 396,325,
1,760,042, 1,957,937, 2,466,633, 3,049,394, 3,1~q,906, 3,3~8,657,
3,352,642, 3,451,888, 3,663,357, 3,806,404, and 3,829,357, ~nd
Canadian Patents 902,861, 966,604 and 970,111.
The above-cited literature and patent re~erence~ generally
describe ga~ phase ozone bleaching o~ fibrous matarials having
consistencies in the range of 15~ to 75~. The'references dis-
~_ .
~ 2 - 'Jr
1090510
close that the optimum conditions for ozone bleaching fall with-
in the range of 30% to 60% consistency. When operating at such
consistencies, a number of advantages are realized. Among
these advantages are reaction times of relativeIy short duration,
namely, from 1 minute to 60 minutes, and acceptable ozone utili-
ation. However, there are also a number of disadvantages
associated with high consistency processes for bleaching pulp
with ozone. These include: (1) the necessity of employing
expensive process equipment, e.g., high consistency presses,
pulp fluffers and high consistency stock pumps; (2) the danger
of pulp fires at these relatively dry conditions; and (3)
difficulty in de-gassing pulp after treatment with ozone which
can result in severe washer operation problems.
In an attempt to avoid the difficulties associated with
ozone bleaching at high consistency, a number of investigators
have experimented with ozone bleaching at low pulp consistency.
The ability to successfully employ pulp at low consistency in
ozone bleaching would avoid the difficulties described above.
Accordingly, Osawa and Schuerch (Tappi 46 [2] 1963:79)
compared ozone bleaching of kraft pulp at 1% pulp consistency
and 50% pulp consistency, re~pectively, at neutral pH. They
concluded-that the rate of ozonation is highest when it is
carried out in the gas phase on fibers well above the fiber
saturation point, i.e., 19%-20% consi~tency, and is much`
slower when the fibers are suspended in water.
Soteland and Kringstad (Nor~k Skogindustri [2] 1968:1)
confirmed the findings of Osawa and Schuerch when treating
mechanical pulps at neutr~l pH with 2.25% ozone (w~w) in oxygen.
They o~served that, "Preliminary investigations showed that
3~ ozone passes through a suspension of mechanical pulp in water
(concentration for instance between 0.1 and 0.2%~ without being
consumed to any significant dogree." They ~ound that the addi-
OS10
tion of 10% (v/v) of a suitable organic solvent, such as methyl
or ethylacetate, acetic acid or acetone to the suspension was
necessary to catalyze the reaction of ozone with the pulp.
Similarly, Osawa and Schuerch also observed and noted that the
presence of nitromethane or methylacetate serve to catalyze
the reaction of ozone with pulp.
SUMMARY OF THE INVENTION
It has now quite unexpectedly been found, and contrary to
the findings of Osawa et al and Soteland et al, that an effici-
ent process for the delignification and bleaching of ligno-
cellulosic pulps is achieved by reacting a lignocellulosic pulp
slurry, having a consistency between about 1% and 10%, by
weight of oven-dried pulp, and a pH between about 1 and 7,
while at a temperature between about 0C. and 70C., with a
gaseous mixture selected from the group consisting of ozone/
oxygen, ozone/air, and mixtures thereof. The ozone containing
gaseous mixture, which has an ozone concentration of from ~bout
0.1% to about 20%, by weight of oxygen or air, i9 bubbled into
the pulp slurry, while agitating the pulp-water-gas mixture at
a rate of about 0.01 to 5.0 horsepower-days per ton of pulp.
Smploying the foregoing reaction conditions eIiminates the
necessity of employing an organic s~lvent to catalyze the reac-
tion as taught by Osawa et al and Soteland et al. The process
Qf ~he present invention results in high ozone utilization,
rapid reaction rate, improved brightness, and exten~ive deIigni-
fication of unbleached and partially bleached pulps.
DESCRIPTION OF THE DRAWING
Fig. 1 is a logarithmic plot comparing reduction in per-
1~90510
manganate numher as a function of horsepower-days/ton of pulp.
DETPILED DESCRIPTION OF THE INVENTI~N
The lignocellulosic pulp fibere employed can either be
unbleached or partially ~leached, as, for example, by a prior
oxygen bleach in the presence of alkali. If a prior oxygen
bleach is done in the presence of alkali, it can be done at
either high pulp consistency or at low pulp consistency. An
example of such low consietency oxygen/alkali pulp bleaching
process is disclosed in Roymoulik et al, U.S. 3,832,276.
The bleached or unbleached pulp can be either a paper
- grade pulp or a dissolving grade pulp prepared by chemical,
chemimechanical or mechanical pulping processes. Exemplary
of such processes are the kraft process, the eulfite process,
neutral sulfite semichemical process or the groundwood process.
The particular lignocellulosic materials ~ub~ected to the afore-
mentioned pulping processes can include hardwoods, softwoods,
grasses, etc.
While the pulp consistency in accordance with the sub~ect
procese can be from about 1% to about 10%, by weig~t, it has
been found preferable that it be between about 1% and about 5%,
-with a 3% consistency being especially prefærred.
The pulp slurry is bleached in a suit~ble reactor which
is provided with agitation means. The reactor can either be
pressurized or unpressurized and should be provided with means
for the admission of the ozone containing mixture in the form
of a finely divided gaseous stream.
The pulp slurry is acidified using either an acid, in-
cluding for example sulfuric acid, acetic acid, etc., or the
effluent from a previous or a subsequent bleaching stage, such
as chlorination, chlorine dioxide, hydrogen peroxide, peracetic
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acid, sodium hypochlorite, etc., to a pH between about 1 and
about 7. A pH between about 3 and 5 has, however, been found
to be preferable. While the bleaching and delignification
process can be conducted when the temperature of the slurry
is within the range of from about ODC. to about 70C., it is
preferred to operate at a temperature of from about 20C. to
about 30C.
The ozone containing gaseous stream is introduced into
the reaction vessel containing the acidified pulp slurry. The
finely divided gas stream can contain from about 0.1~ to
about 20%, by weight, of ozone; preferably, however, it should
contain between about 2% and about 3~. The gas stream contain-
ing ozone is admitted into the pulp slurry via a sparging de-
vice, for example a porous disc or a sparging ring, containing
a plurality of openings.
In the practice of the process of the present invention,
it has been found desirable to employ an ozone containing
stre~m of gas where substantially all of the bubbles are ahout
1/8 inch in diameter or less.
It is essential that the pulp slurry be agitated while
being contacted and reacted with the gas stream containing
ozone. The agitation can be accomplished in various ways, with
mechanical agitation being preferred, as for example, by a
multibladed propeller mixer or a turbine mixer. In order to
attain the extent of delignification and the increase in
brightness sought in the present process, it is essential that
between about 0.01 to about 5.O horsepower-days of energy be
expended per ton of pulp. Mixing energies from about 0.1 to
about 1.0 horsepower-days/ton of pulp gives the best bal~nce
between bleaching response and power usage.
The period of time for-~-eff~¢t~g-delig~$flèati~n and
brightening in accordance with the process of the sub~ect in-
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vention is from about 1 minute to about 60 minutes, with from
about 10 minutes to about 20 minutes having been found to be
preferred. During the period of reaction, lt is desirable
that from about 0.1% to about 5.0% of ozone, by weight of O.D.
pulp, be applied to the pulp slurry.
To obtain a final Elrepho brightness of 80 or above when
employing hardwood pulp in accordance with the process of the
present invention, a bleaching sequence might include a first
stage employing oxygen in the presence of alkali on an un-
bleached low consistency pulp, i.e., less than 10%, (as taught
by Roymoulik et al, U.S. 3,832,276), followed by ozone bleach-
ing in accordance with the present invention, and finally a
peroxide or peracetic acid stage. In order to bleach softwoods
to a final Elrepho brightness of 80 or above, a bleaching se-
quence might include a first stage oxygen bleach, followed by
an ozone bleach in accordance with the present process in a
second stage, then followed by an-extraction stage with sodium
hydroxide or water, followed by another ozone bleaching sta~e,
and then by a peroxide, peracetic acid, chIorine dioxide or
hypochlorite stage. In certain instances another extraction
stage followed by a P,Pa,D or H stage might prove desirable.
- Naturally, depending upon the end result sought, quite a variety
of bleaching stages employing the ozone process of the present
invention along with other bleaching chemicals might be utillzed,
in addition to the stages and chemicals previously mentioned.
By employing the low consistency ozone bleaching process
of the present invention--which requires no additives, pro-
tectors or catalysts as taught by the prior art--many advan-
tages and benefits are obtained over the high consistency ozone
process taught by the prior art. These are: (1) lower e~uip-
ment costs; (2) improved efficiency and ease of oxygen recovery,
(since in a high consistency xeactox the pulp fibers would be
1090510
carried over from the reactor to the oxygen recovery system);
(3) minimal hazards from pulp f-ires, (since the low consis-
tency process operates at a high moisture content while in the
high consistency process the fibers have very low moisture con-
tent); (4) improved control over the extent of bleaching,
(since in the low consistency process there is no dependency
on the degree of fluffing as one experiences in high consis-
tency pulp bleaching); and (5) the ability to maintain the
ozone bleaching stage at a lower temperature than possible with
high consistency ozone bleaching.
In order to disclose more clearly the nature of the pre-
sent invention, the following examples illustrating the inven-
tion are given. It should be understood, however, that this
is done solely by way of example and is intended neither to
delineate the scope of the invention nor limit the ambit of
the appended claims.
EXAMPLæS 1-3
160 grams, oven-dried (O.D.) basis, of an unbleached
hardwood pulp, prepared by the kraft process, was pl~ced in a
20-liter baffled plexiglass reactor and diluted with water to
give a pulp slurry having a con~istency of 1%. The reactor
contained a centrally mounted three-bladed propeller mixer
which was operated at l/30th horsepower and 1200 rpm. A four
inch diameter sintered glass sparge disc, having a plurality
of openings measuring 70 to 100 micron~, was centered in the
bottom of the reactor. The pulp slurry which had a pH of 7 and
a temperature of 20C. was contacted by an ozone/oxygen mixture
which was sparged through the disc at the rate of 12.4 SCFH
(Standard Cubic Feed per Hour). The gas mixture contained
2.5%, by weight, of ozone in oxygen. The re~ction was conducted
1~90510
for the periods of time and at the ozone appIication levels
indicated below in Table I.
TABIJ: I
Unbleached
Control 1 2 3
Ozone Applied, % - 1.0 3.0 5.0
Permanganate No.9.5 6.9 3.8 2.6
Viscosity, cp 31.8 19.0 15.0 10.1
Brightness, (EL)33.4 41.1 54.3 65.6
Time, Min. - 4.6 15.0 24.4
HP-Days/Ton (Pulp) - 0.178 0.583 0.949
The results shown in Table I above indicate that the pro-
cess of the present invention is operative over a wide range of
ozone application- levels. In Example 3, extensive delignifica-
tion was obtained after a reaction time of only 24.4 minute~.
EXA~LES 4-7
The procedure employed in Examples 1-3 was repeated using
a different hardwood draft pulp. The only other changes in
conditions involved the use of 3.5%, by weight, of ozone ih
oxygen and a total gas flow rate of 4.0 SCFH. The roaction was
conducted for the time periods and at the ozone applications
indicated below in Table II.
T;~BLE II
Unbleached
Control 4 5 6-- 7
Ozone Applied, % - 0.2 0.6 0.8 1.0
Permanganate No. 9.7 8.9 7.8 6.7 6.4
Viscosity, cp 23.8 23.6 20.8 15.0 14.1
Brightness, % (El) 30.6 35.4 38.3 42.2 41.6
Time, Min. - 3.5 10.4 15.1 17.0
HP-Days/Ton (Pulp) - 0.136 0.404 0.587 0.660
~(~9OS10
The results shown in Table II indicates that a linear
relationship exists for delignification vs. ozone application
levels. The data shown in Table II further illustrate the
applicability of the present invention at ozone application
levels of 1% and below.
EXAMPLES 8-12
The procedure employed in Examples 1-3 was repeated using
the same hardwood kraft pulp. ~he pulp consistency was 1% and
the temperature of the pulp slurry was 20C. The concentration
of the ozone was 2.6%, by weight of oxygen. 1% of ozone, by
weight of pulp, was applied for 4.5 minutes at a flow rate of
12.4 SCFH. A sparge disc having 25 to 50 micron pores was
employed. Each of the experiments represented by the examples
was conducted at the pH's shown below in Table III.
TABLE ITI
Bleached
Control 8 9 10 11 12
pH - 3.0 4.9 7.1 9.~ 11.9
Permanganate No.9.5 5.1 6.2 7.3 7.1 8.7
Viscosity, cp 31.8 16.6 18.5 19.923.7 28.7
Brightness, % (El) 33.4 55.1 47.741.8 41.9 36.8
HP-Days/Ton (Pulp) - 0.178 0.178 0.178 0.178 0.178
The results obtained, as shown in Table III, indicate
that while the process of the present invention is operative
at acidic as well as basic pH's, the preferred pH range for
obtaining maximum delignification and brightness is from about
pH 3 to about pH 5.
EXAMPLES 13-14
A hardwood kraft pulp which had been bleached initially
with oxygen in the presence of alkali at 4.5% consistency
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1090510
was bleached essentially in accordance with the procedure em-
ployed in ~xamples 1-3. However, instead of using a propeller
mixer, a turbine blade mixer operating at l/30th horsepower
and 540 rpm was employed. The temperature of the pulp slurry
was 20C. The ozone concentration was 3.5%, by weight of oxy-
gen, and the flow rate of oxygen/ozone was 4 SCFH. A sparge
disc having 25 to 50 micron pores was employed. The reaction
was conducted at pulp consistencies of 1% and 2% as indicated
below in Table IV.
TABLE TV
Oxygen
Unbleached Bleached
Cont'r'ol Cohtrol _ 1
Ozone Applied, % on pulp - - 1.0 1.0
Pulp Consistency - - 1.0 2.0
pH - - 3.0 3.2
Permanganate No. 9.5 6.1 2.4 2.7
Viscosity, cp 31.8 18.3 13.3 13.4
Brightness, % ~El) 33.4 46.2 72.3 66.4
Time, Min. - - 17.7 37.0
RP-Days/Ton (Pulp) - - 2.32 2.32
The results in Table IV indicate that extensive deIigni-
fication and brightening occurs when the process of the present
invention is applied after the pulp has been bleached initially
with oxygen.
EXAMPLES 15-18
The procedure employed in Examples 1-3 was repeated using
the same hardwood kraft pulp. m e pulp consistency was 1~, the
slurry temperature was 20C., and the pH was 7. The ozone was
applied for 14.0 minutes at a flow rate of 12.4 SCFH using a
sparge disc with a porosity of 70-100 microns. The ozone con-
centration was 2.6%, by weight of oxygen, and 3~ ozone was ap-
plied, by weight of O.D. pulp.
-- 11 --
1090S10
T~BLE V
Unbleached
Cohtrol 15 16 17 18
Ozone Applied, % on pulp - 3.0 3.0 3.0 3.0
Permanganate No. 9.5 7.7 5.4 4.4 4.0
Viscosity, cp 31.8 21.9 16.1 15.6 11.53
Brightness, % 33.4 39.1 46.0 47.6 57.5
Permanganate No. Reduction, % - 18.9 43.2 53.7 57.9
HP-Days/Ton (Pulp) - 0.010 0.108 0.544 1.837
Mixing, RPM - 300 700 1200 1800
It is obvious from Table V and Fig. 1 that satisfactory
ozone bleaching can be achieved using from 0.01 - 1.8 HP-days/
ton of pulp mixing energy. The graph indicates a leveling-off
of permanganate number reduction between 1 to 2 HP-days/ton of
pulp. Therefore, mixing energies above 1 HP-days/ton of pulp
provide little additional benefit. These results show the
ozone treatment is greatly dependent on proper mixing of the
gas-liquid-pulp mixture to allow for intimate contact of ozone
with the reactive sites in the pulp fiber.
~XAMPLE l9
The procedure employed in Examples 1-3 was repeated U8-
ing a hardwood kraft pulp which had the following physical pro-
perties: Permanganate number 9.5, viscosity, 31.8 cp and
brightness ~El) 33.4. The pulp consistency was 1~, the temper-
ature of the pulp slurry was 20C. and the pH of the slurry
was 3. The concentration of the ozone was 3.3~, by weight of
oxygen. 1~ of ozone, by weight of pulp, was applied for 4.5
minutes at a flow rate of 12.4 SCFH while agitating at 1200
rpm (equivalent to about 0.178 horsepower-day~/ton of pulp).
62~ of the ozone applied was consumed. A sparge disc having
25 to 50 micron pores was employed.
1090510
The pulp brightness was increased to 55.1 points (El),
which represents a 64.9~ increase, and the permanganate number
was reduced to 5.1, which represents a 46.3% decrease.
EXAMPLE 20
A slurry of unbleached hardwood kraft pulp having a
consistency of 4.5%, by weight, was initially bleached with
oxygen in the presence of alkali. Thereafter, the pulp slurry,
which had a consistency of 1%, by weight, and a pH of 3 was
bleached with an ozone/oxygen mixture passed through a sparge
disc having 25-50 micron pores at a rate of 2.2 SCFH. The
slurry was agitated with a turbine blade mixer at 3.26 HP-days/
ton of pulp. After having consumed 1.03% ozone, the pulp which
had a brightness of 77.2 (El) was bleached with 1.1% peracetic
acid. During the peracetic acid bleaching stage, the starting
pH was 8.0, the pulp consistency was 10% and the temperature
was 70C. This bleaching stage lasted for four hours.
Bleached pulp from this stage was then treated with aqueous sul-
furous acid at 50C. and pH 4.0 for a period of 30 minutes.
The final brightness of the pulp was 87.9 (El).
Between each individual bleaching stage in this Exam-
ple, as well as in Examples 21, 22 and 23 which follow, the
pulp was washed with deionized water until the pH of the fil-
tr?te was neutral.
Table VI below provides pertinent data for the
properties of the pulp at various stages in the bleaching
sequence.
1090510
TABLE VI
UnbleachedOxygen Ozone 0203Pa
Pulp ProPerty KraftBleached Bleached -Bleached
Permanganate No.9.7 6.1 2.3
Viscosity, cp 23.816.7 12.93 12.83
Brightness, (El)30.645.1 77.2 87.9
EXAMPLE 21
A slurry of unbleached softwood kraft pulp, having a
consistency of 4.5~, by weight, was initially bleached with
oxygen in the presence of alkali. Thereafter, the pulp slur-
ry, which had a consistency of 1%, by weight, and a pH of 3,
was bleached with an ozone/oxygen mixture passed through a
sparge disc having 25 to 50 micron pores at a rate of 2.2
SCFH. The slurry was agitated with a turbine blade mixer at
3.26 HP-days/ton of pulp. After having consumed 0.93% ozone,
the pulp which had a brightness of 55 (El), was extracted
with sodium hydroxide. The slurry, during the extraction,
had a consistency of 10% and a pH of 11. The extraction was
conducted for a period of 90 minutes at 50C.
Thereafter, the extracted pulp, which had a consisten-
cy of 10%, was treated with 2% peracetic acid for four hours
at a slurry temperature of 70C. and a pH of 8. The peracetic
acid stage was followed by a second alkaline extraction stage
identical to the alkaline extraction stage described above.
Finally, the pulp was treated with 1.2% peracetic acid
under conditions identical to those employed in the peracetic
acid stage described above. The pulp was then treated with
sulfurous acid for a period of 30 minutes. The pulp slurry
during this treatment had a consistency of 6%, a pH of 4 and
was conducted at 50C. The final brightness of the pulp was
83.6 Elrepho brightness.
Table VII below provides pertinent data for the proper-
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1090510
ties of the pulp at various stages in the bleaching sequence.
TABLE VII
.. . Unbleached Oxygen Ozone 02o3EpaEpa
Pulp Proper~y Kraft Bleached Bleached Bleached
Permanganate No. 16.9 10.6 6.4
Viscosity, cp 34.6 23.6 14.40 10.9
Brightness (El) 24.6 31.6 55.0 83.6
EXAMPLE 22
A slurry of unbleached hardwood kraft pulp, having a
consistency of 4.5%, by weight, was initially bleached with
oxygen in the presence of alkali. Thereafter, the pulp slurry
which had a consistency of 2~, by weight, and a pH of 7 was
bleached with an ozone/oxygen mixture passed through a sparge
disc having 70 to 100 micron openings at a rate of 12.4 SCFH.
The amount of ozone applied was 0.75%, by weight of O.D. pulp.
The slurry was agitated with a three-bladed prope~ler mixer at
1 HP-days/ton of pulp. The pulp was thereafter extracted with
hot water. The slurry, during the hot water extraction had a
consistency of 6%. The extraction was conducted for a period
of two hours at 49C.
The extraction stage was followed by a second ozone
stage identical to the ozone stage described above.
Thereafter, the extracted pulp, which had a consis-
tency of 10%, by weight, was treated with 1% hydrogen peroxide
and 2~ sodium silicate for five hours at a slurry temperature
of 70C. and a pH of 10.5.
The peroxide bleached pulp was then extracted with
sodium hydroxide for two hours at a temperature of 49C. The
pulp consistency was 10% and its pH was 11Ø
- 15 -
~Q90510
Thereafter, employing a pulp consistency of 10% and a
temperature of 70C. the pulp slurry was bleached for a period
of five hours with 0.5% hydrogen peroxide on pulp.
The pulp slurry was then treated with sulfurous acid for
a period of one hour. The pulp slurry during this treatment
had a consistency of 3% and a pH of 4. The final brightness
of the pulp was 82.4 Elrepho.
Table VIII below provides pertinent data for the
properties of the pulp at various stages in the bleaching se-
quence.
TABLE VIII
Unbleached Oxygen Ozone 2 Eo3pEp
Pulp P'roper'tyKraftBleached Bleached B~eached
Permanganate No. 8.3 5.0 2.3
Viscosity, cp23.3 15.4 N.D. 11.2
Brightness, (El) 33.3 48.7 62,7 82.4
EXAMPLE 23
A slurry of unbleached hardwood kraft pulp, having aconsistency of 4.5%, by weight, was initially bleached with
oxygen in the presence of alkali. Thereafter the pulp slurry
which had a consistency of 1%, by weight, and a pH of 3 was
bleached with an ozone/oxygen mixture passed through a sparge
disc having 25 to 50 micron openings at a rate of 4 SCFH. The
amount of ozone applied was 0.50%, by weight, of O.D. pulp.
The slurry was agitated with a turbine blade mixer at 2.02 HP-
days/ton of pulp. The pulp was thereafter extracted with
sodium hydroxide. The slurry, during the alkaline extraction
had a consistency of 10%, by weight, and a pH of 11.8. The
extraction was conducted for a period of 90 minutes at 50C.
The extraction stage was followed by a second ozone
- 16 -
1090510
stage identical to the ozone stage described above.
The pulp was subsequently bleached with 0.6% chlorine
dioxide applied at a pulp consistency of 11%, by weight. This
stage was conducted for a period of two hours at 77C.
Table IX below provides pertinent data for the proper-
ties of the pulp at various stages in the bleaching sequence.
TABLE IX
Unbleached Oxygen Ozone 0203E03D
Pulp Proper_y Kraft Bleached Bleached Bleached
Permanganate No. 9.5 6.15 2.3 N.D.
Viscosity, cp 23.4 17.14 11.9 14.1
Brightness, (El) 33.4 42.7 73.9 81.9
The terms and expressions which have been employed are
used as terms of description and not of limitation, and there is
no intention in the use of such terms and expressions of exclud-
ing any equivalents of the features shown and described or
portions thereof, but it is recognized that various modifica-
tions are possible within the scope of the invention claimed.
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