Note: Descriptions are shown in the official language in which they were submitted.
CA 02230961 1998-03-03
OJ-F003
- 1 -
PROCESS FOR PRODUCING BLEACHED PULP
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for
producing a bleached pulp. More particularly, the
present invention relates to a process for producing a
bleached pulp having a high brightness with a high pulp
yield by using a decreased amount of a bleaching
chemical, while preventing a decrease in viscosity,
namely a degree of polymerization, of the pulp.
2. Description of the Related Art
To utilize wood as a material for paper sheets
for various uses, the wood is converted to a chemical
pulp by using a chemical cooking chemical or to a
mechanical pulp by using a mechanical pulping machine,
for example, a refiner, and the resultant pulp is
bleached with a bleaching chemical to increase the
brightness of the pulp.
For example, a kraft pulp produced by cooking
wood chips in a kraft cooking liquor is usually, except
for the case wherein the kraft pulp is used for the
specific paper sheets such as packing paper sheets which
are required a high mechanical strength, optionally
delignified with oxygen in an aqueous alkali solution,
and is necessarily bleached with a bleaching chemical,
for example, chlorine, hydrochlorites, chlorine dioxide,
oxygen, ozone, and hydrogen peroxide, and a bleaching
assistant, for example, sodium hydroxide, in a single
stage or in multiple stages, to eliminate lignin which is
a pulp-coloring substance from the pulp. The resultant
semi-bleached or fully bleached kraft pulps having a
Hunter brightness of 70 to 90~ are practically useful.
Except for the case where the semi-bleached
pulp usable for packing paper sheets, envelopes and paper
bags for office use, is produced by bleaching an
CA 02230961 1998-03-03
unbleached pulp with a hydrochlorite in a single stage,
usually, the semi-bleached pulp is produced by bleaching
an unbleached pulp, which may be a pulp cooked and non-
delignified pulp, or a pulp cooked and then delignified
with oxygen in an aqueous alkali solution, with a
bleaching chemical in two or three stages to such an
extent that the Hunter brightness of the bleached pulp
reaches about 70 to 80%.
The resultant semi-bleached pulp is used as a
bonding pulp for the production of newspaper sheets.
When a fully bleached pulp having a Hunter
brightness of about 80 to 90% is produced from an
unbleached pulp, it is necessary that a reduction in
mechanical strength of the pulp fibers due to the
bleaching treatment is minimized. For this reason, the
chemical decomposition of hydrocarbon segments, for
example, cellulose and hemi-cellulose, from which the
pulp fibers are formed should be minimized. Accordingly,
severe single stage bleaching should be avoided. Namely,
it is preferable that the cooked pulp be delignified with
oxygen in an aqueous alkali solution, then, the alkali-
oxygen delignified pulp be moderately bleached in 3 to 6
stages by using a bleaching chemical and bleaching
conditions chosen so as to minimize the reduction of the
mechanical strength of the pulp fibers due to the
bleaching.
In a conventional multi-stage bleaching
process, usually, an unbleached pulp is first treated
with chlorine to chlorinate lignin contained in the pulp
fibers, and to impart a solubility in water thereto, then
treated with an aqueous alkali solution to dissolve and
extract the chlorinated lignin in the aqueous alkali
solution, and to remove lignin from the pulp fibers, and
finally treated with a hydrochlorite or chlorine dioxide
to decompose and remove the residual lignin in the pulp
fibers and to obtain a bleached pulp having a high
brightness.
CA 02230961 1998-03-03
However, recently, a problem occurs that
organic chlorinated compounds contained in a waste water
discharged from the chlorination bleaching procedure of
the pulp, may cause environmental pollution and, thus,
various new pulp-bleaching processes free from chlorine
have been studied. Also, when hydrochlorite is used for
bleaching the pulp, chloroform is generated during the
pulp-bleaching procedure and may affect the environment.
Therefore, a new pulp-bleaching process using no chlorine
and no hydrochlorite has been researched.
Currently, the employment of oxygen-containing
bleaching chemical, for example, ozone, oxygen, and
peroxides, for example, hydrogen peroxide, peracetic acid
and persulfuric acid, in place of chlorine and
hydrochlorites, is of interest. However, these oxygen-
containing bleaching agents except for oxygen and
hydrogen peroxide are disadvantageous in a high price and
a difficulty in handling due to a danger of explosion and
thus have not yet widely utilized in industry. However,
they promise, as bleaching agents for pulp, a bright
future. Presently, since the oxygen-containing bleaching
agents exhibit a lower selectivity to delignification
than chlorine and hydrochlorites, a problem occurs that
when the bleaching reaction is excessively effected, the
mechanical strength of the pulp fibers decreases to an
excessive extent. Therefore, the conditions and
sequences of bleaching procedures are still being
researched.
Also, there are attempts of reducing the
consumption of the bleaching agents while maintaining the
brightness of the bleached pulp at a satisfactory level.
For example, (1) a method as disclosed in J. E. Jiang et
al., Appita, 45 (1), 19 (1992), in which a cooking
procedure is carried out under such a condition that the
delignification reaction is accelerated, to reduce a
Kappa number of the bleached pulp; (2) a method in which
the unbleached pulp is delignified with oxygen in an
CA 02230961 1998-03-03
aqueous alkali solution to reduce the Kappa number of the
pulp; (3) a method as disclosed, for example, in Japanese
Unexamined Patent Publication No. 4-316,690, in which the
delignification with oxygen in the aqueous alkali
solution is promoted by using a pre-treating chemical
such as nitrous acid, to further reduce the Kappa number
of the pulp; t4) a method as disclosed, for example, in
Japanese Unexamined Patent Publication 2-264,087, in
which, before a bleaching step, a pulp is treated with a
xylanase; and (5) a method as disclosed, for example, in
Japanese Unexamined Patent Publication No. 4-316,689, in
which, before a bleaching step, a pulp is treated with a
lignin-decomposing enzyme, are disclosed.
The above-mentioned methods, however, should be
further improved. For example, the methods in which the
delignification in the cooking step is accelerated to a
level higher than the conventional level, to obtain a
pulp having a reduced Kappa number, are mostly dangerous
in that the pulp yield decreases and the resultant pulp
fibers exhibit a decreased mechanical strength. Also,
the methods in which the delignification is promoted in
the oxygen-delignification step in an aqueous alkali
solution to decrease the Kappa number of the pulp, are
mostly dangerous in that the mechanical strength of the
resultant pulp is low. Further, the methods in which
before the bleaching step, the pulp is pre-treated with
an enzyme, are advantageous in that the pretreatment is
carried out under moderate reaction conditions and thus
the decreases in the mechanical strength and the yield of
the pulp fibers are small. However, the methods are
disadvantageous in that the reaction rate is too slow,
the reaction time necessary to complete the reaction is
too long and the decrease in the Kappa number is very
small.
In the treatment of the pulp with the xylanase,
which is very interesting, the enzyme must be fully
brought into contact with a substrate to initiate the
CA 02230961 1998-03-03
reaction. However, since the xylan and lignin have high
molecular weight and are unevenly and three-dimensionally
distributed in the pulp fibers and since the enzyme per
se is a high molecular compound, it is difficult to bring
the xylan or lignin-decomposing enzyme into complete
contact with the xylan or lignin in the pulp fibers.
Therefore, an improvement of the reaction efficiency of
the enzyme is an important subject matter of further
research in the future.
Japanese Unexamined Patent Publication
No. 6-101,186 discloses a bleaching process for a
chemical pulp, in which a unbleached pulp prepared by
cooking wood chips is treated at a pulp consistency of 1
to 30% by weight in aqueous acid solution having a pH
value of 1 to 5 at a temperature of 30 to 95~C for 5 to
120 minutes, and the acid-treated pulp is delignify-
bleached with a peroxide compound and pressurized oxygen
in an alkali medium. According to this process, a pulp
having a high viscosity and a high brightness can be
obtained without bleaching the acid-treated pulp with a
bleaching chemical consisting of chlorine or a
hydrochlorite. Therefore, in this process, the waste
water contains no chlorine-containing organic compound
and thus no environmental pollution occurs. However,
this process is disadvantageous in that the acid
treatment of the unbleached pulp causes substantially no
delignification of the unbleached pulp, and thus no
decrease in Kapppa number is found in the acid-treated
pulp. Therefore, the Kappa number of the acid-treated
pulp is similar to that of the unbleached, non-acid-
treated pulp.
Also, Japanese Unexamined Patent Publication
No. 6-158,573 discloses a process, for delignify-
bleaching a pulp, wherein an unbleached pulp prepared by
cooking wood chips is delignified with oxygen and an
alkali, the alkali oxygen delignified pulp is subjected
to an acid treatment under the same conditions as
CA 02230961 1998-03-03
mentioned above, or to a treatment with a cheleting
agent, to remove heavy metal ions from the pulp, and then
the heavy metal ion-removed pulp is subjected to a
delignifying treatment with a peroxide compound or with a
peroxide compound and oxygen in an alkali medium.
The acid treatment in the above-mentioned
process is carried out for the purpose of removing heavy
metal ions from the pulp fibers to prevent a
depolymerization of pulp cellulose in the delignify-
bleaching procedure with the peroxide compound or with
the peroxide compound and oxygen. In this process, the
acid treatment per se does not contribute to delignifying
the pulp.
A process in which an unbleached pulp prepared
by cooking wood chips is delignified with oxygen and an
alkali, and then the delignified pulp is treated with an
acid in a nitrogen gas atmosphere, to selectively remove
hexeneuronic acid which forms xylan side chains of pulp
cellulosic compounds, to decrease apparent Kappa number
of the pulp and to save the consumption of the bleaching
chemical for the bleaching procedure of the pulp, is
disclosed, for example, in Tapauni, Vuorinen et al., 1996
International Pulp Bleaching Conference Proceeding,
pages 45 to 51.
However, the acid treatment under the
conditions as disclosed causes only a decrease in
apparent Kappa number of the pulp, but does not
contribute to delignifying the pulp. Therefore, the acid
treatment does not enhance the bleaching effect on the
pulp and the decrease in consumption of the bleaching
chemical for the bleaching procedure is unsatisfactory.
Japanese Unexamined Patent Publication
No. 6-280,177 discloses a process for bleaching a pulp by
acid treating an unbleached pulp prepared by cooking wood
chips in an aqueous inorganic acid solution at a pH value
of 1.0 to 1.6 at a temperature of 80~C to a boiling
temperature of the aqueous acid solution at a pulp
CA 02230961 1998-03-03
consisting of 5 to 20% by weight for 1 to 3 hours, and
the acid-treated pulp is subjected to an alkali
extraction treatment for lignin in the pulp.
In this process, the acid treatment carried out
under the severe conditions of a pH value of 1.0 to 1.6
causes an acid hydrolysis of polysaccharides in the pulp,
and thus the yield and the viscosity of the resultant
pulp significantly decrease, and the mechanical strength
of the pulp may decrease. Further, the acid treatment
liquor having a very low pH value needs a large amount of
acidic chemicals and thus an economical disadvantage
occurs. Also, in this process, the acid treatment which
promotes the delignification of the pulp, causes a latent
disadvantageous in that the decomposition products of
lignin are condensed with each other and are absorbed on
the pulp fibers so as to decrease the bleaching effect on
the pulp.
Accordingly, there is a great problem that to
decrease the consumption of the bleaching chemical for
the bleaching procedure by applying a delignification
treatment with an acid to the pulp before the bleaching,
it is necessary to decrease the degree of acid treatment
for the delignificaiton, and to prevent the condensation
and absorption of the decomposition products of lignin.
Japanese Unexamined Patent Publication
No. 8-158,284 discloses a process for producing a
chemical pulp for forming paper, comprising the steps of
(1) treating a digested chemical pulp with oxygen in an
alkaline condition under high temperature-high pressure
conditions; (2) treating the acid treated pulp in a
mixture of hydrogen peroxide with a reaction catalyst
comprising at least one member selected from oxyacids of
IV, V and VI group elements, for example, tungsten,
molybdenum, vanadium, selenium and titanium and salts
thereof, at a pH value of 3 or less at a temperature of
75 to 110~C; and (3) treating the acid-treated pulp with
a peroxide compound in an alkaline medium, in the above-
CA 02230961 1998-03-03
-- 8
mentioned order.
In this process, when the chemical pulp is
delignified with a combination of a peroxide compound
with oxygen in an alkaline medium, the chemical pulp is
S pre-treated with a peroxide compound in the presence of a
catalyst under acidic and high temperature conditions.
This pre-treatment causes the delignificaiton effect to
be significantly enhanced, while generally not decreasing
the viscosity of the pulp.
As mentioned above, in the step (2) of this
process, the oxyacids of the heavy metals or salts
thereof contained in the aqueous acid solution
significantly promote the delignification of the pulp.
Usually, the acid treatment of the pulp is carried out
for the purpose of removing the heavy metals from the
pulp, preventing the depolymerization of pulp cellulose
due to the presence of heavy metals in the delignify-
bleaching step with oxygen, ozone or hydrogen peroxide,
and enhancing the quality of the resultant pulp.
However, in the above-mentioned process, the heavy metals
are intentionally added to the treatment system.
Therefore, to prevent the depolymerization of the pulp
cellulose in the delignification procedure with the
oxidizing agent, a large amount of a protecting agent for
the pulp cellulose must be added to the treatment system.
Also, when the oxyacids of the heavy metals or the salts
thereof are added to the delignification treatment
system, a portion of the heavy metals are introduced
together with the pulp andtor the waste water from the
delignification treatment, into a pulp-producing step or
paper-forming step. The introduced heavy metals cause
the resultant pulp or paper to be soiled with sticky
substances, pitch or scale. Also, since the heavy metals
are harmful to the human body, the introduction of the
heavy metals into the paper sheets must be severely
prevented even when the amount of the introduced heavy
metals is small, in a view of health.
CA 02230961 1998-03-03
Recently, there have been many cases wherein a
treatment of the unbleached pulp with oxygen in an
aqueous alkali solutions introduced into the pulp-
bleaching process. This treatment need an oxygen gas.
When the oxygen gas is produced, a nitrogen-containing
gas is produced as a by-product. Therefore, it is
important to find an industrial utilization of the
nitrogen-containing gas.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
process for producing a bleached pulp, which process is
capable of obtaining a bleached pulp having a high
brightness with a high yield by using a bleaching
chemical in a reduced amount, while preventing or
minimizing a reduction in viscosity of the pulp.
The above-mentioned object can be attained by the
process of the present invention for producing a bleached
pulp, comprising the steps of:
pulping a wood material to produce an
unbleached pulp; and bleaching the unbleached pulp with a
bleaching agent in at least one stage,
wherein before the bleaching step with the
bleaching agent, the unbleached pulp is treated with an
aqueous acid solution in a gas containing at least one
member selected from the group consisting of oxygen and
nitrogen under pressure.
In an embodiment of the process of the present
invention, before the acid treating step, the unbleached
pulp is delignified with oxygen in an aqueous alkali
solution and then the acid treatment of the delignified
pulp is carried out in an oxygen-containing gas under
pressure.
In another embodiment of the process of the present
invention, before the acid treating step, the unbleached
pulp is delignified with oxygen in an aqueous alkali
solution and then the acid treatment of the delignified
pulp is carried out in an nitrogen-containing gas under
CA 02230961 1998-03-03
- 10 -
pressure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors of the present invention have made an
extensive research for improving a delignificaiton of
pulp fibers by an acid treatment and a bleaching of the
delignified pulp fibers. As a result, it was found that
the delignification of the pulp fibers by an acid
treatment can be promoted by carrying out the
delignification procedure under pressure, to greatly
decrease a Kappa number of the pulp, even when no
catalyst, for example, a heavy metal, is employed.
In view of the above-mentioned findings, the
inventors of the present invention further researched the
acid treatment of the wood pulp under pressure in the
presence of no catalyst. As a result, it was found that
when the acid treatment of the wood pulp is carried out
in an oxygen and/or nitrogen-containing gas under
pressure, the delignification of the wood pulp is very
significantly promoted, and the resultant delignified
pulp can be easily bleached with a bleaching chemical by
a single or multi-step bleaching procedure, in comparison
with the acid-treated pulp under no pressure, and thus
the consumption of the bleaching agent could be reduced.
In the bleaching process of the present invention,
an unbleached pulp is produced from a wood material by
pulping; the unbleached pulp is treated with an aqueous
acid solution in a gas containing at least one member
selected from the groups consisting of oxygen and
nitrogen, under pressure; and
the acid treated pulp is bleached with a
bleaching agent in at least one stage.
In the bleaching process of the present invention,
optionally, before the acid-treating step, the unbleached
pulp is delignified with oxygen in an aqueous alkali
solution, and then the delignified pulp is subjected to
the acid treating step in an oxygen and/or nitrogen-
containing gas under pressure.
CA 02230961 1998-03-03
The unbleached pulp is produced by subjecting a wood
material, for example, wood chips, to a conventional
pulping procedure, namely a cooking procedure. The
cooking procedure can be carried out by conventional
cooking methods, for example, a kraft cooking method,
sulfite cooking method, polysulfide cooking method and
soda cooking method. In consideration of the quality of
the resultant pulp and energy efficiency of the cooking
procedure, the kraft cooking method is advantageously
utilized.
For example, when a wood material is kraft-cooking,
preferably the sulfide degree of the cooking liquor is S
to 75%, more preferably 15 to 45%, the content of alkali
in the cooking liquor is 5 to 30% by weight, more
preferably 10 to 25% by weight, based on the bone-dry
weight of the wood material, and the cooking temperature
is 140 to 170~C. The cooking procedure may be carried
out in a continuous cooking method or in a batch-cooking
method. When a continuous cooking digester is used, the
cooking liquor can be fed into the cooking digester by
conventional feeding methods, for example, a modified
cooking method in which the cooking liquor is fed through
a plurality of feeding inlets.
In the cooking procedure, a cooking additive, for
example, a conventional cyclic keto compound, may be
added to the cooking liquor.
In the cooking procedure for the wood material, the
cooking liquor optionally contains a cooking additive
comprising at least one member selected from conventional
cyclic keto compounds, for example, benzo-quinone,
naphthoquinone, anthraquinone, anthrone, and
phenanthroquinone; ring-substituted compounds of the
above-mentioned with at least one substitutent, for
example, alkyl and amino groups; hydroquinone compounds
such as anthrahydroquinone which is one of reduction
products of the above-mentioned quinone compounds; and
9,10-diketohydro-anthracene compounds which is a stable
CA 02230961 1998-03-03
- 12 -
compound produced as a by-product of an anthraquinone
synthesis by a Diels-Alder method.
The cooking additive is added in an amount of 0.001
to 1.0~ by weight based on the bone dry weight of the
wood chips to the cooking liquor.
In the process of the present invention, the
unbleached pulp delivered from the cooking procedure is
washed with water, coarsely screened and then finely
screened. The finely screened pulp is optionally
subjected, before the acid treatment, to a conventional
delignification procedure with oxygen in an aqueous
alkali solution. The alkali-oxygen delignification
procedure can be carried out in a medium or high
consistency. Usually, the alkali-oxygen delignification
procedure is preferably carried out in the common medium
consistency at a pulp consistency of 8 to 15% by weight.
The alkali usable for the alkali-oxygen
delignification in the medium consistency is preferably
selected from an aqueous sodium hydroxide solution or an
oxidi~ed kraft white liquor. An oxygen gas and an alkali
is added into a medium consistency pulp slurry and fully
mixed in a medium consistency mixer. Then the pulp-
oxygen-alkali mixture is fed into a delignification
reactor in which the mixture is held for a certain time,
and the delignification reaction is carried out. The
medium consistency mixer, of which the constitution is
variable in response to manufacturers, has a rotor
capable of rotating at a high speed of 500 to 1000 rpm,
and can impart a high shearing force to the medium
consistency pulp slurry so as to allow the pulp slurry to
behave as a fluid such as water, and thus the pulp is
fully mixed with the oxygen and the alkali.
In the pulp slurry, preferably the content of oxygen
is 0.5 to 3% by weight based on the bone dry weight of
the pulp, and the content of the alkali is 0.5 - 4% by
dry weight based on the bone dry weight of the pulp.
Also, preferably, the reaction temperature is 80 to
CA 02230961 1998-03-03
120~C, the reaction time is 15 to 100 minutes and the
pulp consistency is 8 to 15% by weight. The above-
mentioned delignification conditions usable for the
present invention include the conventional
delignification conditions.
The alkali oxygen-delignified pulp is washed with
water and then fed to an acid treatment step in
accordance with the present invention.
Also, the unbleached pulp delivered from the cooking
procedure, washed with water and screened, may be
directly fed to the acid treatment.
In an embodiment (1) of the process of present
invention, the acid treatment step is carried out, using
no catalyst, by bringing the unbleached pulp (which has
optionally been delignified with oxygen in an aqueous
alkali solution) into contact with an aqueous acid
solution having a pH value of 2 to 6, and treated in an
oxygen-containing gas under pressure at a desired
temperature for a desired time. The acid treatment of
the unbleached pulp with the aqueous acid solution may be
carried out by immersing the unbleached pulp in the
aqueous acid solution or by impregnating the unbleached
pulp with the aqueous acid solution. In the process of
the present invention, the acid usable for the acid
treatment step is not limited to a specific type of acid,
as long as the pH value of the aqueous acid solution can
be adjusted to a desired level in the range from 2 to 6.
Usually, the acid comprises at least one member selected
from inorganic acids, for example, sulfuric acid, nitric
acid, hydrochloric acid, sulfurous acid and nitrous acid,
and organic acids, for example, acetic acid and formic
acid.
Usually, sulfuric acid is advantageously used for
the present invention, because the sulfuric is easily
available and can be easily handled.
In the acid treatment step of the process of the
present invention, the pulp consistency in the treating
CA 02230961 1998-03-03
- 14 -
mixture is preferably 5 to 40% by weight, more preferably
8 to 35% by weight, still more preferably 10 to 25% by
weight. When the pulp consistency is less than 5% by
weight, the acid treatment must be carried out in a
treatment apparatus having a large capacity and thus an
economical disadvantage may occur. Also, when the pulp
consistency is more than 40% by weight, the large amount
of the pulp may be difficult to uniformly mix with the
acid solution and thus the effect of the acid treatment
may be unsatisfactory.
In the acid treatment of the process of the present
invention, to obtain a satisfactory treatment result, the
pulp, the oxygen and/or nitrogen contained in the gas and
the acid in the aqueous solution must be uniformly mixed
in the treatment system. To attain the uniform mixing, a
certain mixer selected from, for example, low consistency
mixers, medium consistency mixers, static mixers and high
consistency mixers is selected in consideration of the
types and consistency of the pulp is used. The acid
treatment can be carried out under the conditions similar
to those of the alkali-oxygen delignification treatment.
In the acid treatment, when the pulp consistency is
medium or high, the agitation of the pulp slurry is
difficult. In this case, preferably, the pulp slurry,
the oxygen and/or nitrogen-containing gas and the aqueous
acid solution are mixed by using a medium or high
consistency mixer which can impart a high shearing force
to the mixture, and then the resultant mixture is fed
into a reactor in which a necessary reaction pressure can
be held. In this mixing procedure, a disperser may be
used or not used. In the reactor, an agitation for
further mixing the fed mixture is not carried out, and
the fed mixture is held at a desired temperature under a
desired pressure for a desired time. In the acid
treatment for a medium consistency of pulp, the reactor
may be selected from upflow type and downflow type
continuous reaction columns which may be equipped with,
CA 02230961 1998-03-03
or free from, a distributer and a discharger.
The reactor usable for the acid treatment of the
pulp in a high consistency are preferably selected from
vertical down-flow type reactor having a plurality of
S trays each having an opening and capable of moving
downward from an upper tray to a lower tray through the
opening by rotating horizontal wings, and Pandia reactor
equipped with a plurality of pressure-resistant
horizontal tubes and a slowly rotating screw and capable
of horizontarily moving the pulp by the slowly rotating
screw, of transporting the pulp through the plurality of
tubes and of discharging the pulp from the reactor
through a final outlet of the tubes.
In an embodiment (1) of the process of the present
invention, preferably the acid treatment is carried out
at a pulp consistency of 5 to 40% by weight, at a pH
value of 2 to 6, more preferably 2.5 to 5, at a
temperature of 50 to 150~C, more preferably 90 to 120~C,
for a reaction time of 5 to 120 minutes, more preferably
20 to 90 minutes under a pressure on gage generated by an
oxygen-containing gas, of 0.5 to 9.0 kg/cmZ, more
preferably 1.5 to 7.0 kg/cm2.
In another embodiment (2) of the process of the
present invention, a wood material is pulped into an
unbleached pulp; the unbleached pulp is delignified with
oxygen in an aqueous alkali solution; the delignified
pulp is treated with an aqueous acid solution in a gas
containing nitrogen under pressure; and the acid-treated
pulp is bleached in at least one stage with a bleaching
agent.
In this embodiment (2), preferably the acid
treatment is carried out in a pulp consistency of 5 to
40% by weight, at a pH value of 2 to 6, more preferably
2.5 to 5, at a temperature of 50 to 150~C, more
preferably 90 to 120~C, for a reaction time of 5 to
120 minutes, more preferably 20 to 90 minutes under a
CA 02230961 1998-03-03
- 16 -
gage pressure of 0.5 to 9.0 kg/cm , more preferably 1.5
to 7.0 kg/cm , generated by a nitrogen-containing gas.
In the embodiment (2), the nitrogen-containing gas
for the acid treatment is not limited to a specific type
S of nitrogen-containing gas, as long as the nitrogen
containing gas has a nitrogen content of 95% by volume or
more.
Usually, from an economical point of view, the
nitrogen-containing gas is preferably a by-product gas
from an oxygen-producing process, for example, a deep
cooling isolation method, a PSA (Pressure Swing
Absorption) method, or a VSA (Vacuum Swing Absorption)
method for producing oxygen.
In the acid treatment in each of the embodiments (1)
and (2), when the pH value is less than 2, polysaccharide
compounds in the pulp may be decomposed to a higher
extent than that of the lignin in the pulp, and thus the
mechanical strength and the yield of the resultant pulp
fibers may be decreased. Also, when the pH value is more
than 6, substantially no delignification of the pulp may
occur even under pressure. In this embodiment, since the
acid treatment is carried out under pressure generated by
an oxygen-containing gas, the delignification of the pulp
can be satisfactorily effected even at a high pH value of
more than 3, and the resultant acid-treated pulp exhibits
an enhanced ease for the bleaching and is thus easily
bleached with a conventional bleaching agent to produce a
bleached pulp having a high brightness.
When the acid treatment is carried out at a reaction
temperature of less than 50~C, the chemical decomposition
of lignin with the acid may be insufficient. Also, when
the reaction temperature is more than 150~C, the
decomposition of the polysaccharide compounds may be
significantly promoted, and thus the mechanical strength
of the resultant pulp fibers may be unsatisfactory.
In the acid treatment in accordance with the process
CA 02230961 1998-03-03
of the present invention, the reaction time of 5 minutes
or more is long enough for completing the acid treatment.
However, the extension of the reaction time to more than
120 minutes may not substantially contribute to enhancing
the delignification effect, and may cause the resultant
pulp fibers to exhibit deteriorated qualities.
In this embodiment, the oxygen-containing gas for
imparting a pressure to the acid treatment system may be
selected from industrially available oxygen-containing
gases having an oxygen content of 20% by volume or more,
for example, an oxygen gas prepared by a deep cooling-
isolating method, an oxygen gas prepared from PSA
(Pressure Swing adsorption), an oxygen gas prepared from
VSA (Vacuum Swing adsorption), oxygen or oxygen-
containing gases having a degree of purity of oxygen of85% by volume or more and usable for the conventional
alkali-oxygen delignification using oxygen and an alkali,
an oxygen-containing gas prepared by a molecular sieve-
equipped oxygen-producing apparatus and having an oxygen
content of 21% by volume or more, an oxygen-rich gas
prepared by mixing air with an oxygen-containing gas
having an oxygen content of 85% by volume or more, and
air having an oxygen content of 20% by volume or more.
When the bleaching step of the process of the
present invention includes an ozone-bleaching stage, the
waste gas discharged from the ozone bleaching stage and
containing oxygen can be used as an oxygen-containing gas
for the acid treatment.
However, in the acid treatment of the process of the
present invention, it is important that both the treating
pressure of the treatment system and the amount of the
oxygen per weight of the pulp in the treatment system be
severely adjusted. Therefore, when the oxygen-containing
gas has a low oxygen content, and is used in a low
partial pressure of oxygen, the treatment pressure must
rise to such an extent that the amount of oxygen becomes
0.1% or more per weight of the pulp present in the
CA 0223096l l998-03-03
- 18 -
treatment system.
When the treating pressure of the acid treatment is
lower than 0.5 kg/cm~ on gage, the delignification effect
on the pulp by the acid treatment may be insufficient.
When the treatment pressure is 9.0 kg/cm on gage or
more, the pressurizing effect of the oxygen-containing
gas is saturated, the acid treatment apparatus is
required to have a high pressure resistance. The
pressure resistant apparatus is expensive and thus may
cause an economical disadvantage.
The acid-treated pulp is optionally washed with
water, and then subjected to a bleaching step with a
bleaching agent in a single stage procedure or a multi-
stage procedure. Before the bleaching step, the acid-
treated pulp is optionally treated with an enzyme, forexample, a xylanase and/or a lignin decomposition enzyme.
The bleaching agent usable for the process of the
present invention comprises at least one member selected
from chlorine (C), hydrochlorite compounds (H), chlorine
dioxide (D), oxygen (O), hydrogen peroxide (P), ozone (Z)
and organic peroxide compounds. The bleaching agent is
optionally used in combination with a bleaching
assistant, for example, sodium hydroxide (E).
After the acid treatment under pressure, the
resultant acid treated pulp is bleached in a single stage
procedure or in a multi-stage procedure. In the multi-
stage bleaching procedure, a plurality of bleaching
stages different in bleaching agent, bleaching assistant
or procedure conditions are carried out in various
sequences, for example,
C-E/O-H-D, and C/D-E/O-D wherein chlorine and
chlorine-containing bleaching agents are used, or D-E-D,
D-E/O-D, E/O-D, E-O-D and Z-D wherein a chlorine-free
bleaching agent is used, or Z-E-D, Z-E/O-P or E/OP-PO
wherein no chlorine and no chlorine-containing compounds
are used.
CA 02230961 1998-03-03
[Note]
C: Bleaching stage with atomic chlorine
E/O: Bleaching stage with oxygen in the presence of
sodium hydroxide
H: Bleaching stage with a hydrochlorite
D: Bleaching stage with chlorine dioxide
C/D: Bleaching stage with atomic chlorine and
chlorine dioxide
E: Extraction with sodium hydroxide
O: Bleaching stage with oxygen
Z: Bleaching stage with ozone
P: Bleaching stage with hydrogen peroxide
E/OP: Bleaching stage with oxygen and hydrogen
peroxide in the presence of sodium hydroxide,
in an initial portion of which stage, a
pressure is applied.
PO: Bleaching stage with oxygen and hydrogen
peroxide in the presence of sodium hydroxide,
throughout which stage, a pressure is applied.
In the process of the present invention, the
brightness of the pulp bleached by the bleaching step
with the bleaching agent can be enhanced by adding an
oxidative chemical selected from oxygen and hydrogen
peroxide to an alkali-extraction solution, and inserting
at least one alkali-extraction stage in a sequence of
E/O, E/P or E/OP into the multi-stage bleaching
procedure.
The alkali extraction stage using oxygen and/or
hydrogen peroxide can be carried out in the method
similar to the conventional oxygen-bleaching method under
pressure, or in a manner that the pressure is applied
only in an initial time of 5 to 15 minutes and then the
pressure is reduced to the ambient atmospheric pressure.
In the process of the present invention, waste water
discharged from the acid treating step may be recovered.
The waste water is optionally fed, together with a waste
water recovered from a cooking step, into a boiler for
CA 02230961 1998-03-03
- 20 -
recovered waste, to burn organic substances dissolved or
suspended in the waste water and to generate heat energy.
As mentioned above, in the process of the present
invention, the acid-treated pulp can be fully bleached by
a single or multi-stage bleaching step free from atomic
chlorine or from both chlorine and a chlorine-containing
bleaching compound. Therefore, when a waste water from
the bleaching step is recovered without discharging it to
the outside of the bleaching system, the bleaching step
can be carried out in a closed system. Also,
substantially no pollution due to organic halogen
compounds (AOX), derived from the bleaching waste water,
occurs.
In the acid-treatment step of the process of the
present invention, the unbleached pulp or alkali-oxygen
delignified pulp is treated with an aqueous acid solution
under pressure generated by an oxygen and/or nitrogen-
containing gas. This acid treatment greatly promotes the
delignification of the pulp fibers, and the decrease in
the Kappa number of the pulp fiber to 20 to 80g6 based on
that of the pulp fibers before the acid treatment. The
great decrease in the Kappa number by the acid treatment
causes slight decreases in pulp yield and pulp viscosity.
However, the decreased Kappa number contributes to
significantly decreasing the amount of the bleaching
chemicals necessary to the single or multi-stage
bleaching procedure. Also, the acid treatment in
accordance with the process of the present invention
contributes to enhancing the bleachability of the pulp
per Kappa number of the pulp, and to increasing the pulp
yield in the bleaching procedure. Therefore, in the
total yield of the pulp in the acid treatment step and
the single or multi-stage bleaching step is similar to
the pulp yield in the bleaching step when the acid
treatment is omitted.
Particularly, when the pressure for the acid
treatment is imparted by a nitrogen-containing gas, the
CA 02230961 1998-03-03
decrease in pulp viscosity by the acid treatment can be
minimized, while greatly promoting the delignification of
the pulp fibers, and which significantly decreasing the
Kappa number of the pulp fibers. Therefore, the
decreased Kappa number causes the consumption of the
bleaching chemicals in the bleaching step to decrease,
while maintaining the pulp strength and yield at a
satisfactory level.
In the process of the present invention, the degree
of decrease in the Kappa number of the pulp fibers due to
the acid treatment in comparison with that before the
acid treatment is high for hardwood pulp, and low for
other type of pulp. However, even in the other type of
pulp, the delignification can be effected. When the
oxygen-containing gas is employed, the Kappa number
decreasing effects of the acid treatment on various types
of pulps are as follows.
Type of pulp Kappa number Decrease in % in
before acid Kappa number of acid-
treatment treated pulp based on
that of non-acid
treated pulp
Unbleached 20 to 30 20 to 30
softwood pulp
Alkali-oxygen
delignified, 12 to 15 20 to 40
unbleached
softwood pulp
Unbleached 15 to 20 20 to 60
hardwood pulp
Alkali-oxygen
delignified, 7 to ll 20 to 80
unbleached
hardwood pulp
Also, when the nitrogen-containing gas is used in
the acid treatment, the decrease in % in Kappa number of
the acid-treated pulp based on that of the non-acid
treated pulp is 20 to 70% in the alkali-oxygen
delignified, unbleached hardwood pulp and 10 to 30% in
the alkali-oxygen delignified, unbleached softwood pulp.
CA 02230961 1998-03-03
- 22 -
The acid treatment in accordance with the process of
the present invention exhibits a significantly high
effect on the hardwood pulp, particularly the hardwood
pulp delignified with oxygen in an aqueous alkali
solution.
In the process of the present invention, the acid
treatment under pressure generated by an oxygen and/or
nitrogen-containing gas promotes the delignification of
the pulp and enhances the bleachability on the acid-
treated pulp. The reasons for the above-mentioned
effects have not yet been made completely clear but will
be made clear by further research in the future.
However, it is assumed that first, the delignification
reaction with acid is accelerated by applying pressure to
the acid treatment system; second, the acid solution can
easily penetrate into the inside of the pulp fibers under
pressure so as to decompose lignin located not only in
the surface portions but also in the inside of the pulp
fibers, and third, when the oxygen-containing gas is
used, the oxygen is activated by radicals generated by
the decomposition of lignin, the activated oxygen
accelerates the decomposition of lignin and hinders the
condensation reaction of lignin, so that a residual
lignin in the pulp fibers after the acid treatment has a
easy decomposable structure, or when the nitrogen-
containing gas is employed, the inert nitrogen gas fills
the acid treatment system and the decomposition of
cellulose in the pulp fibers by radicals generated by the
decomposition of lignin is prevented.
EXAMPLES
The present invention will be further explained by
the following examples which are merely representative
and do not restrict the scope of the present invention in
any way.
In Examples 1 to 9, an acid treatment was carried
out in an oxygen containing gas.
In Examples 1 to 4 and Comparative Examples 1 to 3,
CA 02230961 1998-03-03
the bleaching procedure was carried out in bleaching
sequence D1-E-D2.
In Example 5 and Comparative Example 4, the
bleaching procedure was carried out in a bleaching
sequence D1-E/O-D2.
In Example 6 and Comparative Example 5, the
bleaching procedure was carried out in a bleaching
sequence E/O-PO.
In Example 7 and comparative Example 6, the
bleaching procedure was carried out in a single bleaching
stage PO, to produce a semi-bleached pulp.
In each of Examples 8 and 9 and Comparative
Examples 7 and 8, a softwood pulp was acid-treated.
In Examples 10 to 13, the acid treatment was carried
out in a nitrogen-containing gas.
In Examples 10 and Comparative Example 9, the
bleaching procedure was carried out in a bleaching
sequence D1-E-D2.
In Example 11, the bleaching procedure was carried
out in a bleaching sequence D1-E/O-D2.
In Example 12 and comparative Example 10, the
bleaching procedure was carried out in a bleaching
sequence E/O-PO.
In Example 13 and Comparative Example ll, the
bleaching procedure was carried out in a single stage PO.
Unless specifically indicated, in the examples and
comparative examples, the decrease (%) in Kappa number of
pulp fibers due to the acid treatment, the decrease (%)
in consumption of bleaching chemicals in bleaching
stages D1, D2 and E, pulp yield (%) in the acid treatment
step, and total yield (%) of pulp in the acid treatment
step and the bleaching step, are determined by the
following measurement methods.
l. Decrease in % in Kappa number by acid treatment
Kappa numbers of a pulp before and after an
acid treatment were determined in accordance with
Japanese Industrial Standard (JIS) P 8211, and the
CA 02230961 1998-03-03
- 24 -
decrease in % in Kappa number of the pulp due to the acid
treatment was calculated in accordance with the
equation (1):
Decrease in Kappa number (%) =
(Ko - Kl)/Ko x 100 (1)
wherein Ko represents a Kappa number of a pulp before the
acid treatment and Kl represents a Kappa number of the
pulp after the acid treatment.
2. Decrease (%) in bleaching chemical consumption
in beaching stages D1 and D2
A total amount (Ao) of chlorine dioxide
consumed in bleaching stages Dl and D2 for a pulp before
the acid treatment, and a total amount (Al) of chlorine
dioxide consumed in bleaching stages Dl and D2 for a pulp
after the acid treatment were determined. Then, a
decrease (%) in consumption of chlorine dioxide in the
bleaching stages Dl and D2 due to the acid treatment was
calculated in accordance with the equation (2):
Decrease in chlorine dioxide consumption (%)
= (Ao - Al)/Ao x 100 (2)
3. Decrease in % consumption of bleaching
chemicals in bleaching staqe E
A amount (aO) of sodium hydroxide used in a
bleaching stage E for a pulp before the acid treatment
and an amount (al) of sodium hydroxide used in a
bleaching stage E for a pulp after the acid treatment
were determined.
A decrease (%) in consumption of sodium
hydroxide in the bleaching stage E due to the acid
treatment was calculated in accordance with the
equation (3):
Decrease in sodium hydroxide consumption (%)
= (aO - al)/aO x 100 (3)
4. Pulp yield (%) in acid treatment
A bone dry weight (W0) of a pulp fed to the
acid treatment and a bone dry weight (Wl) of the pulp
CA 02230961 1998-03-03
- 25 -
delivered from the acid treatment were determined.
A pulp yield (%) in the acid treatment was
calculated in accordance with the equation (4):
Pulp yield (%) in acid treatment = Wl/W0 x 100
(4)
5. Total pulp yield (%) in acid treatment and
bleaching procedures
A total pulp yield in the acid treatment and
bleaching procedures were calculated in accordance with
the equation (5):
Total pulp yield (%) in acid treatment and bleaching
procedures = (Y1 x Y2)/100 (5)
In the equation (5), Yl represents a yield of the
pulp in the acid treatment step and Y2 represent a yield
of the pulp in the bleaching step.
Example 1
Mixed hardwood chips comprising 70% by weight of
Japan-produced hardwood chips and 30% by weight of
eucalyptus wood chips and in a bone dry weight of 500g
was kraft-cooked in a laboratory indirect-heating
autoclave containing a cooking liquor having an effective
alkali content of 18% based on the bone dry weight of the
wood chips, a degree of sulfidity of 25% and a liquor
ratio of 4 to the bone dry weight of the wood chips at a
cooking temperature of 160~C for a cooking time of
120 minutes. The resultant pulp was collected, and
finely screened through a flat screen equipped with a
10 cuts screen plate. An unbleached hardwood kraft pulp
having a Hunter brightness of 45.2%, a Kappa number of
20.1, a pulp viscosity of 23.3 mPa-s (determined in
accordance with J. TAPPI, 44) was obtained in an amount
of 229g in a finely screened pulp yield of 45.8%.
The unbleached pulp in a bone dry weight of 80.0g
was suspended in a pulp consistency of 10% by weight in
an acid treatment solution prepared by dissolving
concentrated sulfuric acid in ion-exchanged water, having
CA 02230961 1998-03-03
a pH value of 2.6 and contained in a stainless steel
indirect heating autoclave having a capacity of 2 liters.
Into the autoclave, a compressed oxygen gas having a
degree of purity of oxygen of 99.9% was introduced under
a pressure of 5kg/cmZ on gage, and the pulp was acid-
treated with the aqueous sulfuric acid solution at a
temperature of 105~C under the above-mentioned pressure
on gage for 60 minutes. After the autoclave was cooled,
the acid-treated pulp was washed with ion-exchanged water
and dewatered.
An acid-treated pulp having a Hunter brightness of
48.1%, a Kappa number 11.0 and a pulp viscosity of
17.8 mPa s was obtained in a bone dry weight of 78.9g.
The decrease in Kappa number due to the acid treatment
was 45.3% and the pulp yield in the acid treatment was
98.6%.
The acid treated pulp in a bone dry weight of 70.0g
was placed in a plastic resin bag and suspended in a pulp
consistency of 10% by weight in ion-exchanged water, then
chlorine dioxide is added in an amount of 1.10% based on
the bone dry weight of the pulp to the pulp slurry in the
bag. The pulp slurry-containing bag was immersed in a
constant temperature water bath at a temperature of 70~C
for 30 minutes, to carry out a bleaching stage Dl
procedure. The D1-bleached pulp was washed with ion-
exchanged water and dewatered. The resultant D1-bleached
pulp was in a bone dry weight of 69.5g.
The D1-bleached pulp in a bone dry weight of 69.5g
was placed in a plastic resin bag and suspended in a pulp
consistency of 10% by weight in ion-exchanged water.
Then, sodium hydroxide was added in an amount of
1.7 times the amount of chlorine dioxide used in the
D1 stage to the pulp slurry. The resultant pulp slurry
was treated with the sodium hydroxide at a temperature of
70~C for 120 minutes in the same manner as in the
D1 stage, to carry out an extraction procedure in an E
CA 02230961 1998-03-03
- 27 -
stage. The resultant pulp after the E stage was washed
with ion-exchanged water and dewatered. A E stage-
extracted pulp was obtained in a bone dry weight of
68.8g.
The E stage-extracted pulp in a bone dry weight of
68.8g was placed in a plastic resin bag and suspended in
a pulp consistency of 10% by weight in ion-exchanged
water. Then, chlorine dioxide was added in an amount of
0.7% based on the bone dry weight of the pulp to the pulp
slurry. The resultant pulp slurry was treated with
chlorine dioxide at a temperature of 70~C for 180 minutes
to effect a D2 stage bleaching procedure. The resultant
pulp was washed with ion-exchanged water and dewatered to
obtain a bleached pulp in a bone dry weight of 68.5g.
The pulp yield of the multi-stage bleaching
procedure was 97.9% and the total pulp yield of the acid
treatment and bleaching procedures was 96.5%.
The D2 stage bleached pulp was disintegrated and
then formed into a pulp sheet having a basis weight of
60 g/m in accordance with TAPPI Testing Method T 205
os - 71 (JIS P 8209). The pulp sheet had a brightness of
85.4% determined in accordance with JIS P8123.
In Table 1, the Kappa number of the acid treated
pulp, the decrease in Kappa number due to the acid
treatment and pulp yield in the acid treatment are shown.
Also, in Table 2, the amounts of the bleaching chemicals
used in the D1 and D2 stages in the bleaching procedure,
the decrease in the total bleaching chemical consumption
in the D1 and D2 stages in the bleaching procedure, the
decrease in bleaching chemical consumption in the E-stage
in the bleaching procedure, the pulp yield of the
bleaching procedure and the total pulp yield of the acid
treatment and bleaching procedures are shown.
Example 2
The same unbleached hardwood kraft pulp as in
Example 1 was suspended in a bone dry weight of 90.0g in
CA 02230961 1998-03-03
- 28 -
ion-exchanged water, and added with sodium hydroxide in
an amount of 1.5% based on the bone dry weight of the
pulp. The aqueous unbleached pulp slurry was diluted
with ion-exchanged water to adjust the pulp consistency
in the slurry to 10% by weight.
The pulp slurry was placed in an indirect heating
autoclave and pressurized by introducing a compressed
oxygen gas having a degree of purity of oxygen of 99.9%
to a pressure on gage of 5 kg/cm and heated at a
temperature of 100~C for 60 minutes, to delignify the
unbleached pulp by a medium consistency alkali-oxygen-
delignifying method.
The resultant lignified pulp was washed with ion-
exchanged water, and dewatered. An oxygen lignified pulp
having a Hunter brightness of 51.1%, a Kappa number of
10.2, and a pulp viscosity of 18.8 mPa s was obtained in
a bone dry weight of 88.8g.
The alkali-oxygen lignified pulp in a bone dry
weight of 80.0g was suspended in ion-exchanged water, and
the pulp slurry was diluted with an aqueous acid solution
prepared by dissolving concentrated sulfuric acid in ion-
exchanged water to adjust the pH value of the slurry to
2.6 and the pulp consistency of the slurry to 10% by
weight. The pulp slurry was placed in an indirect
heating stainless steel autoclave having a capacity of
2 liters, and a commercially available compressed oxygen
gas having a degree of purity of oxygen of 99.9% was
introduced into the autoclave to adjust the pressure on
gage of the autoclave to 5 kg/cm , and then the pulp
slurry was heated at a temperature of 95~C for 60 minutes
while maintaining the pressure on gage at the above-
mentioned level, to apply an acid treatment under
pressure to the delignified pulp. After the autoclave
was cooled, the resultant acid-treated pulp was washed
with ion-exchanged water and dewatered.
An acid-treated pulp having a Hunter brightness of
CA 02230961 1998-03-03
53.4%, a Kappa number of 4.3 and a pulp viscosity of
15.8 mPa-s was obtained in a bone dry weight of 79.2g at
a decrease in Kappa number of 57.8% with a pulp yield of
99 . 0% .
The acid-treated pulp in a bone dry weight of 70.0g
was subjected to the same D1 stage bleaching procedure in
Example 1 except that the amount of chlorine dioxide used
was 0.3% based on the bone dry weight of the pulp, and
washed with ion-exchanged water and dewatered, to obtain
a Dl stage bleached pulp in a bone dry weight of 69.9g.
The Dl stage bleached pulp in a bone dry weight of
69.9g was subjected to the same E stage extraction
procedure as in Example 1, and washed with ion-exchanged
water and dewatered to obtain an E stage extracted pulp
in a bone dry weight of 69.7g.
The E stage extracted pulp in a bone dry weight of
69.7g was subjected to the same D2 stage bleaching
procedure except that chlorine dioxide was employed in an
amount of 0.40% based on the bone dry weight of the pulp.
After washing and dewatering, a bleached pulp was
obtained in a bone dry weight of 69.6g. The pulp yield
in the multi-stage bleaching procedure was 99.4% and the
total pulp yield in the acid treatment and bleaching
procedures was 98.4%.
The D2-stage bleached pulp was disintegrated and
formed into a pulp sheet having a basis weight of 60 y/m2
in the same manner as in Example 1 and a brightness of
the pulp sheet was determined in accordance with the
TAPPI testing method. The brightness was 85.5%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment, and the pulp yield in the acid treatment.
Also, Table 2 shows the amounts of the bleaching
chemicals used in the Dl and D2 bleaching stages, the
decrease in bleaching chemical consumption in the
bleaching procedure, the decrease in bleaching chemical
CA 02230961 1998-03-03
- 30 -
consumption in the E stage, the pulp yield in the
bleaching step and the total pulp yield in the acid
treatment and bleaching procedures.
Example 3
The delignified pulp prepared by the same alkali-
oxygen delignification procedure as in Example 2, and
having a Kappa number of 10.2 was subjected to the same
acid-treatment and multi-stage (Dl-E-D2) bleaching
procedures as in Example 2, except that the acid treating
temperature was changed to 105~C and the amount of
chlorine dioxide employed in the Dl stage bleaching was
changed to 0.18% based on the bone dry weight of the
pulp, to obtain a bleached pulp.
The Kappa number of the acid-treated pulp was 2.6,
the decrease in Kappa number due to the acid treatment
was 74.5%, the acid-treated pulp was obtained in a bone
dry weight of 79.0g at a pulp yield of 98.8%. The multi-
stage bleached pulp was obtained in a bone dry weight of
69.7g and in a pulp yield of 99.6% and had a Hunter
brightness of 85.6%. The total yield of the pulp in the
acid treatment and bleaching procedures was 98.4%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 2 shows the amounts of the bleaching
chemical used in the Dl and D2 stages, the decrease in
the bleaching chemical consumption in the D1 and
D2 stages, the decrease in the bleaching chemical
consumption in the E stage, the pulp yield in the acid
treatment and the total pulp yield in the acid treatment
and bleaching procedures.
Example 4
The same alkali-oxygen delignified pulp as in
Example 2 having a Kappa number of 10.2 was subjected to
the same acid treatment as in Example 2, except that air
was used in place of the oxygen gas, and in the Dl stage,
the amount of chlorine dioxide used was 0.33% based on
CA 02230961 1998-03-03
the bone dry weight of the pulp. The acid-treated pulp
was subjected to the same multi-stage (D1-E-D2) bleaching
procedure as in Example 2, to produce a bleached pulp.
The Kappa number of the acid-treated pulp was 4.4,
S the decrease in Kappa number due to the acid treatment
was 56.9%, the acid-treated pulp was obtained in a bone
dry weight of 79.2g at a pulp yield of 99.0%. The multi-
stage bleached pulp was obtained in a bone dry weight of
69.5g and in a pulp yield of 99.3% and had a Hunter
brightness of 85.5%. The total yield of the pulp in the
acid treatment and bleaching procedures was 98.3%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 2 shows the amounts of the bleaching
chemical used in the D1 and D2 stages, the decrease in
the bleaching chemical consumption in the D1 and
D2 stages, the decrease in the bleaching chemical
consumption in the E stage, the pulp yield in the acid
treatment and the total pulp yield in the acid treatment
and bleaching procedures.
Example S
The same acid-treated pulp having a Kappa number of
4.3 and prepared at a decrease in Kappa number of 57.8%
in a pulp yield of 99.0%, as in Example 2 was subjected
in a bone dry weight of 70g to the same multi-stage (D1-
E-D2) bleaching procedure as in Example 2 with the
following exceptions.
The D1 stage-bleached pulp was subjected to an
alkali extraction procedure (E/O) in the presence of
oxygen. In the alkali extraction procedure, sodium
hydroxide was added in an amount of 0.51% based on the
bone dry weight of the pulp to the D1 stage-bleached pulp
slurry, the pulp slurry was diluted with ion-exchanged
water to adjust the pulp consistency in the slurry to
10%, the pulp slurry was placed in an indirect heating
autoclave, pressurized with a compressed oxygen gas
CA 02230961 1998-03-03
having a degree of purity of oxygen of 99.9% to a
pressure on gage of 1.5 kg/cm , and heated at a
temperature of 70~C for 15 minutes under the above-
mentioned pressure. Then, the pressure of the autoclave
was released, and the pulp slurry was further heated at a
temperature of 70~C for 105 minutes under ambient
atmospheric pressure (pressure on gage = zero).
After the alkali extraction (E/O) in the presence of
oxygen was completed, the resultant pulp was subjected to
the same D2 stage bleaching procedure as in Example 2,
except that the amount of chlorine dioxide used was
changed to 0.10% by weight.
The bleached pulp having a brightness of 85.4% was
obtained in a bone dry weight of 69.5g in a pulp yield of
99.3%. The pulp yield in the multi-stage bleaching
procedure was 99.3% and the total pulp yield in the acid
treatment and bleaching procedures was 98.3%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 2 shows the amounts of the bleaching
chemical used in the Dl and D2 stages, the decrease in
the bleaching chemical consumption in the Dl and
D2 stages, the decrease in the bleaching chemical
consumption in the E stage, the pulp yield in the acid
treatment and the total pulp yield in the acid treatment
and bleaching procedures.
Example 6
The same acid-treated pulp having a Kappa number of
4.3 and prepared at a decrease in Kappa number of 57.8%
and in a pulp yield of 99.0%, as in Example 2, was
subjected in a bone dry weight of 70g to an alkali
extraction (E/OP) in the presence of oxygen and hydrogen
peroxide. In the alkali extraction (E/op), the acid-
treated pulp slurry was added with 1.0% of sodium
hydroxide and 0.5% of hydrogen peroxide, based on the
CA 02230961 1998-03-03
- 33 -
bone dry weight of the pulp, and diluted with ion-
exchanged water to adjust the pulp consistency in the
slurry to 10% by weight, the resultant pulp slurry was
placed in an indirect heating autoclave, pressurized with
a compressed oxygen gas having a degree of purity of
oxygen of 99.9% to a pressure on gage of 1.5 kg/cm , and
heated at a temperature of 70~C under the above-mentioned
pressure for 15 minutes. After the pressure was
released, the pulp slurry was held at a temperature of
70~C under ambient atmospheric pressure for 105 minutes.
After the alkali extraction (E/OP) was completed, the
autoclave was cooled, the resultant alkali-extracted pulp
was washed with ion-exchanged water, and dewatered to
obtain the (E/op) alkali extracted pulp in a bone dry
weight of 69.4g.
The (E/PO) alkali-extracted pulp in a bone dry
weight of 69.4g was subjected to a hydrogen peroxide
bleaching stage (PO) under pressure generated by an
oxygen gas.
In the bleaching stage (PO), an aqueous slurry of
the (E/OP) alkali-extracted pulp was added with 1.5% of
sodium hydroxide and 3.0% of hydrogen peroxide based on
the bone dry weight of the pulp, diluted with ion-
exchanged water to adjust the pulp consistency to the
slurry to 10% by weight. The resultant pulp slurry was
placed in an indirect heating autoclave, pressurized with
a compressed oxygen gas having a degree of purity of
oxygen of 99.7% to a pressure on gage of 5 kg/cm , and
then heated at a temperature of 100~C under the above-
mentioned pressure for 60 minutes.
The resultant PO stage bleached pulp was washed with
ion-exchanged water, and dewatered. A bleached pulp
having a Hunter brightness of 85.4% was obtained in a
bone dry weight of 69.lg. The pulp yield in the
bleaching (E/OP-PO) procedure was 98.7% and the total
pulp yield in the acid treatment and bleaching procedure
CA 02230961 1998-03-03
- 34 -
was 97.7%.
Table l shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 3 shows the total pulp yield in the acid
treatment and bleaching procedures and the brightness of
the pulp.
Example 7
The same acid-treated pulp having a Kappa number of
4.3 and prepared in a decrease in Kappa number of 57. 4%
and with a pulp yield of 99.0% as in Example 2 was
subjected in a bone dry weight of 70g to a hydrogen
peroxide bleaching procedure (a PO-bleaching stage) under
a pressure imparted by an oxygen gas.
In the PO bleaching stage, an aqueous slurry of the
acid-treated pulp was added with 1.5% of sodium hydroxide
and 2.0% of hydrogen peroxide based on the bone dry
weight of the pulp, diluted with ion-exchanged water to
adjust the pulp consistency of 10% by weight. The
resultant pulp slurry was placed in an indirect heating
autoclave, pressurized with a compressed oxygen gas
having a degree of purity of oxygen of 99.9% to a
pressure on gage of 5 kg/cm , and heated at a temperature
of 100~C under the above-mentioned pressure for
60 minutes.
The (PO)-bleached pulp was washed with ion-exchanged
water and dewatered. A semi-bleached pulp having a
Hunter brightness of 75.2% was obtained in a bone dry
weight of 69.6g.
The pulp yield in the PO bleaching procedure was
99.4% and the total pulp yield in the acid-treatment and
bleaching procedures was 98. 4%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 3 shows the total pulp yield in the acid
CA 02230961 1998-03-03
- 35 -
treatment and bleaching procedures and the rightness of
the pulp.
Example 8
Mixed softwood chips comprising 60% by weight of
Japan-produced softwood chips and 40% by weight of
radiata pine wood chips and in a bone dry weight of 500g
was kraft-cooked in a laboratory indirect-heating
autoclave containing a cooking liquor having an effective
alkali content of 20% based on the bone dry weight of the
wood chips, a degree of sulfidity of 25% and a liquor
ratio of 5 to the bone dry weight of the wood chips at a
cooking temperature of 165~C for a cooking time of
150 minutes. The resultant pulp was collected, and
finely screened through a flat screen equipped with a
14 cuts screen plate. An unbleached softwood kraft pulp
having a Hunter brightness of 43.3%, a Kappa number of
27.5, a pulp viscosity of 31.6 mPa-s (determined in
accordance with J. TAPPI, 44) was obtained in an amount
of 218g in a finely screened pulp yield of 43.6%.
The unbleached pulp in a bone dry weight of 80.0g
was suspended in a pulp consistency of 10% by weight in
an acid treatment solution prepared by dissolving
concentrated sulfuric acid in ion-exchanged water, having
a pH value of 2.6 and contained in a stainless steel
indirect heating autoclave having a capacity of 2 liters.
Into the autoclave, a compressed oxygen gas having a
degree of purity of oxygen of 99.9% was introduced under
a pressure of 5 kg/cm2 on gage, and the pulp was acid-
treated with the aqueous sulfuric acid solution at a
temperature of 105~C under the above-mentioned pressure
on gage for 60 minutes. After the autoclave was cooled,
the acid-treated pulp was washed with ion-exchanged water
and dewatered.
An acid-treated pulp having a Hunter brightness of
46.2%, a Kappa number 18.3, and a pulp viscosity of
22.3 mPa-s was obtained in a bone dry weight of 78.6g.
CA 0223096l l998-03-03
- 36 -
The decrease in Kappa number due to the acid treatment
was shown in Table 4.
Example 9
The same unbleached softwood kraft pulp as in
Example 8 in a bone dry weight of 90g was suspended in
ion-exchanged water. The resultant pulp slurry was added
with 2.1% of sodium hydroxide based on the bone dry
weight of the pulp, diluted with ion-exchanged water to
adjust the pulp consistency in the slurry to 10% by
weight, and then placed in an indirect heating autoclave.
The pulp slurry in the autoclave was pressurized with a
compressed oxygen gas having a degree of purity of oxygen
of 99.9% to a pressure on gage of 5 kg/cm , and heated at
a temperature of 100~C under the above-mentioned pressure
for 60 minutes to delignify the pulp fibers in accordance
with the medium consistency alkali-oxygen bleaching
method. The delignified pulp was washed with ion-
exchanged water and dewatered. The alkali-oxygen
delignified pulp was obtained in a bone dry weight of
88.5g and had a Hunter brightness of 48.8%, a Kappa
number of 13. 3 and a pulp viscosity of 21.8 mPa-s.
The alkali-oxygen delignified pulp in a bone dry
weight of 80.0g was suspended in ion-exchanged water and
diluted with an aqueous sulfuric acid solution prepared
by dissolving concentrated sulfuric acid in ion-exchanged
water and having a pH value of 2. 6, to adjust the pulp
consistency in the slurry to 10% by weight.
The resultant pulp slurry was placed in a stainless
steel indirect heating autoclave having a capacity of
2 liters, pressurized with a compressed oxygen gas having
a degree of purity of oxygen of 99.9% to a pressure on
gage of 5 kg/cm , and heated at a temperature of 105~C
under the above-mentioned pressure for 60 minutes, to
acid-treat the pulp.
After the autoclave was cooled, the resultant acid-
treated pulp was washed with ion-exchanged water and
CA 02230961 1998-03-03
dewatered.
The resultant acid-treated pulp was collected in a
bone dry weight of 79.2g and had a Hunter brightness of
51.3%, a Kappa number of 8.2 and a pulp viscosity of
17.1 mPa-s. In the acid treatment, the decrease in Kappa
number of the pulp, and the pulp yield are shown in
Table 4.
Comparative Example 1
The same procedures as in Example 1 were carried out
to produce a bleached pulp, with the following
exceptions.
The acid treatment for the unbleached pulp was
omitted.
In the D1 stage bleaching procedure, the bleaching
chemical (chlorine dioxide) was employed in an amount of
2.41% based on the bone dry weight of the pulp. The
bleaching procedure was carried out in the same sequence
D1-E-D2 as in Example 1.
The resultant bleached pulp was collected in a bone
dry weight of 67.5g with a pulp yield of 96.4% and had a
Hunter brightness of 85.4%.
Table 1 shows the Kappa number of the unbleached
pulp, and Table 2 shows the amounts of the bleaching
chemical used in the D1 and D2 stage procedures and pulp
yield in the whole bleaching procedures.
Comparative Example 2
The same procedures as in Example 2 were carried out
to produce a bleached pulp, with the following
exceptions.
The acid treatment for the alkali-oxygen delignified
pulp was omitted.
In the D1 stage bleaching procedure, the bleaching
chemical (chlorine dioxide) was employed in an amount of
0.92% based on the bone dry weight of the pulp. The
bleaching procedure was carried out in the same sequence
D1-E-D2 as in Example 2.
The resultant bleached pulp was collected in a bone
CA 02230961 1998-03-03
- 38 -
dry weight of 68. 7g with a pulp yield of 98.1% and had a
Hunter brightness of 85.3%.
Table 1 shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and the pulp yield in the acid treatment.
Also, Table 2 shows the amounts of the bleaching
chemical used in the D1 and D2 stages, the decrease in
the bleaching chemical consumption in the D1 and
D2 stages, the decrease in the bleaching chemical
consumption in the E stage, the pulp yield in the acid
treatment and the total pulp yield in the acid treatment
and bleaching procedures.
Comparative Example 3
A bleached pulp was produced by the same procedure
as in Example 2, with the following exceptions.
In the acid treatment, the pressurization with the
compressed oxygen gas was omitted, and the amount of the
bleaching chemical (chlorine dioxide) used in the
D1 stage bleaching procedure was 0.65% based on the bone
dry weight of the pulp. The bleaching was carried out in
the same sequence D1-E-D2 in Example 2.
The resultant bleached pulp having a Hunter
brightness of 85.3% was obtained in a bone dry weight of
69.lg.
The acid-treated pulp had a Kappa number of 6.5.
The decrease in Kappa number due to the acid treatment
was 36. 3g6. The bone dry weight of the acid treated pulp
was 79.4g. The pulp yield in the acid treatment was
99.3%. The bone dry weight of the bleached pulp was
69.lg. The pulp yield in the multi-stage bleaching
procedure was 98.7% and the total pulp yield in the acid
treatment and bleaching procedures was 98.0%.
Table l shows the Kappa number of the acid-treated
pulp, the decrease in Kappa number due to the acid
treatment and pulp yield in the acid treatment. Also,
Table 2 shows the amounts of the bleaching chemical
(chlorine dioxide) used in the D1 and D2 stage bleaching
CA 02230961 1998-03-03
procedures, the decrease in bleaching chemical
consumption in the D1 and D2 stage bleaching procedures,
the decrease in bleaching chemical consumption in the E
stage procedure, the pulp yield in the bleaching
procedure and the total pulp yield in the acid treatment
and bleaching procedures.
Comparative Example 4
The same acid treated pulp having a Kappa number of
6.5 and prepared with a decrease in Kappa number of 36.3%
and a pulp yield of 99.3%, as in Comparative Example 3
was subjected in a bone dry weight of 70g to the same
Dl stage procedure as in Comparative Example 3.
The D1 stage-bleached pulp slurry was added with
1.11% of sodium hydroxide based on the bone dry weight of
the pulp and diluted with ion-exchanged water to adjust
the pulp consistency in the slurry to 10% by weight.
The pulp slurry was placed in an indirect heating
autoclave, pressurized with a compressed oxygen gas
having a degree of purity of oxygen of 99.9% to a
pressure on gage of 1.5 kg/cm2, and heated at a
temperature of 70~C under the above-mentioned pressure
for 15 minutes. The pressure in the autoclave was
reduced and the pulp slurry was heated at a temperature
of 70~C under ambient atmospheric pressure (pressure on
gage = 0) for 105 minutes, to apply an alkali extraction
(E/O) in the presence of oxygen to the pulp. Then, the
alkali-extracted pulp was subjected to the same D2 stage
bleaching procedure as in Example 2, except that the
amount of chlorine dioxide was changed to 0.28% based on
the bone dry weight of the pulp.
As a result of the above-mentioned multi-stage
bleaching procedure in the sequence D1-E/O-D2, a bleached
pulp having a Hunter brightness of 85.4% was collected in
a bone dry weight of 69.lg.
The pulp yield in the multi stage bleaching
procedure was 98.7%, and the total pulp yield in the acid
CA 02230961 1998-03-03
- 40 -
treatment and bleaching procedures was 98.0%.
In Table 1, the Kappa number of the acid treated
pulp, the decrease in Kappa number due to the acid
treatment and pulp yield in the acid treatment are shown.
Also, in Table 2, the amounts of the bleaching chemicals
used in the D1 and D2 stages of the bleaching procedure,
the decrease in the total bleaching chemical consumption
in the D1 and D2 stages in the bleaching procedure, the
decrease in bleaching chemical consumption in the
E/O-stage in the bleaching, the pulp yield of the
bleaching procedure and the total pulp yield of the acid
treatment and bleaching procedures are shown.
Comparative Example 5
The same acid treated pulp having a Kappa number of
6.5 and prepared with a decrease in Kappa number of 36.3%
and a pulp yield of 99.3%, as in Comparative Example 3
was subjected in a bone dry weight of 70g to the same
multi-stage bleaching procedure in the sequence E/OP-PO
as in Example 6.
As a result of the above-mentioned multi-stage
bleaching procedure, a bleached pulp having a Hunter
brightness of 81.4% was collected in a bone dry weight of
68.9g.
The pulp yield in the multi stage bleaching
procedure was 98.4%, and the total pulp yield in the acid
treatment and bleaching procedures was 97.7%.
Table 3 shows the Hunter brightness of the multi-
stage bleached pulp and the total pulp yield in the acid
treatment and bleaching procedures.
Comparative Example 6
The same acid treated pulp having a Kappa number of
6.5 and prepared with a decrease in Kappa number of 36.3%
and a pulp yield of 99.3%, as in Comparative Example 3
was subjected in a bone dry weight of 70g to the same
single stage (PO) bleaching procedure as in Example 7.
As a result of the above-mentioned single-stage
bleaching procedure, a bleached pulp having a Hunter
CA 0223096l l998-03-03
- 41 -
brightness of 69.8% was collected in a bone dry weight of
69.5g.
The pulp yield in the bleaching procedure was 99.3%,
and the total pulp yield in the acid treatment bleaching
procedures was 98.6%.
Table 3 shows the Hunter brightness of the bleached
pulp and the total pulp yield in the acid treatment and
bleaching procedures.
Comparative Example 7
The same acid treatment as in Example 8 was carried
out, except that the pressurization with the compressed
oxygen gas for the acid treating system was omitted.
An acid treated pulp having a Hunter brightness of
43.3%, a Kappa number of 21.8 and a pulp viscosity of
23.8 mPa-s was collected in a bone dry weight of 78.7g.
Table 4 shows the decrease in Kappa number due to
the acid treatment and the pulp yield in the acid
treatment.
Comparative Example 8
The same acid treatment as in Example 9 was carried
out, except that the pressurization with the compressed
oxygen gas to the acid treating system was omitted.
An acid treated pulp having a Hunter brightness of
48.8%, a Kappa number of 10.0 and a pulp viscosity of
18.0 mPa-s was collected in a bone dry weight of 69.4g.
Table 4 shows the decrease in Kappa number due to
the acid treatment and the pulp yield in the acid
treatment.
CA 02230961 1998-03-03
- 42 -
Table 1
\ Item Unbleached, non- Unbleached, acid-treated pulp
\ acid-treated pulp
\ Kappa Pulp Kappa Decrease Pulp Pulp
\ number viscosity number in Kappa viscosity yield
S \ number
Example No. \ (mPa s) (%) (mPa-s) (%)
1 20.1 23.311.0 45.3 17.8 98.6
2 4.3 57.8 15.8 99.0
3 2.6 74.5 14.7 98.8
Example 4 4.4 56.9 16.1 99.0
10.2 18.8
4.3 57.8 15.8 99.0
6 4.3 57.8 15.8 99.0
7 4.3 57.8 15.8 99.0
1 20.1 23.3 - - Z3.3
2 - - 18.8
3 6 5 36 3 16.8 99.3
Comparative
Example 4 10.2 18.8 6.5 36.3 16.8 99 3
6.5 36.3 16.8 99.3
6 6.5 36.3 16.8 99.3
CA 02230961 1998-03-03
- 43 -
Table 2
\ ItemMulti stage (D1-E-D2) Bleaching Total pulp
\ yield in
\ Amount of Decrease in PulE) acid
\ chemical used consumption of yield treatment
\ chemicals and
\Dl-stage D2-stage D1 and E-stage bleaching
\ D2-stage procedures
Ex~mple No. ~ (%) (%)
1 1.10 0.7 42.1 54.4 97.9 96.5
2 0.30 0.4 47.0 67.4 99.4 98.4
Exlmple 3 0.18 0.4 56.1 80.4 99.6 98.4
4 0.33 0.4 44.7 64.1 99.3 98.3
0.30 0.1069.7 67.4 99.3 98.3
1 2.41 0.7 - - 96.4 96.4
Comparative 2 0.92 0.4 - - 98.1 98.1
Ex~mple 3 0.65 0.4 20.5 29.3 98.7 98.0
4 0.65 0.2829.5 29.3 98.7 98.0
Table 3
\ Item Acid-treated, multi-stage bleached
pul~
\ Brightness Total pulp yield
Example No. \ (%) (%)
6 85.497.7
Example
7 75.298.4
Comparative 5 81.4 97 7
Example 6 69.898.6
CA 02230961 1998-03-03
- 44 -
Table 4
\ Item Acid-treated pulp
\ Decrease in Pulp yield
\ Kappa number
Example No. \ (%) (%)
8 33.5 98.3
Examnle
~ 9 38.3 99.0
Comparative 7 20. 7 98. 4
Example 8 24. 8 99.1
Tables 1 to 4 clearly show that when a hardwood pulp
is subjected to the process of the present invention, as
Examples 1 to 7 show, by acid treating an unbleached
hardwood pulp having a Kappa number of 20.1 or an alkali-
oxygen delignified hardwood pulp having a Kappa value of10.2, under pressure imparted by an oxygen gas, the
delignification of the pulp was enhanced, while the pulp
viscosity slightly decreased, so as to decrease the Kappa
number of the pulp by 45.3 to 74. 5%. When a single or
multi-stage bleaching procedure with a bleaching
chemicals was applied to the acid-treated pulp, the
brightness of the pulp could be increased to a desired
level, with a significantly reduced amount of bleaching
chemicals, while maintaining the pulp viscosity and the
pulp yield at satisfactory levels.
Also, as shown in Example 5, the pulp acid-treated
in the presence of oxygen under pressure exhibited a
significantly enhanced delignification property in the
alkali extraction stage (E/O) in the presence of oxygen
in the multi-stage bleaching procedure, and thus the
amount of the bleaching chemicals necessary to bleach the
pulp into a desired brightness could be greatly reduced.
Further, as Example 6 clearly shows, when the acid
treatment in the presence of oxygen under pressure was
applied to the alkali-oxygen delignified pulp in
accordance with the process of the present invention, the
CA 02230961 1998-03-03
- 4S -
resultant acid-treated pulp could be bleached to a
desired brightness of 85. 4% by a multi-stage bleaching
procedure in a sequence E/PO-PO, without using chlorine
and chlorine-containing chemicals as a bleaching
chemical.
Compared with this, as shown in Comparative
Example 5, when the pressurization with oxygen gas is
omitted in the acid treatment, the same multi stage
bleaching procedure (E/op-PO) as in Example 6 resulted in
a reduced pulp brightness of 81. 4% which was
unsatisfactory.
Also, in comparative Examples 3 and 4 wherein the
acid treatment of the pulp is carried out without
pressurizing with the oxygen gas, the decrease in Kappa
number of the pulp due to the acid treatment was 36%
which was lower than that in the case where the
pressurization with the oxygen gas is carried out. As
shown in Comparative Example 4, the decrease in the
bleaching chemical consumption in the multi-stage
bleaching procedure is very low. As shown Comparative
Example 4, even when an alkali-extraction stage (E/O) in
the presence of oxygen is inserted into the multi-stage
bleaching procedure, the decrease in the bleaching
chemical consumption was unsatisfactory.
As shown in Example 7, the acid treatment in
accordance with the process of the present invention
enables the acid treated pulp to be bleached by a
hydrogen peroxide bleaching procedure under pressure
generated by oxygen gas to produce a semi-bleached pulp
with a brightness of 75%. However, as shown in
Comparative Example 6, when the acid treatment was
carried out without pressurizing with the oxygen gas, and
the acid treated pulp was bleached in the same bleaching
sequence as in Example 7, the resultant bleached pulp
exhibited a brightness of 69.8% which is clearly lower
than that in Example 7.
When a softwood pulp was subjected to the process of
CA 02230961 1998-03-03
- 46 -
the present invention, the effect of the acid treatment
is lower than that for the hardwood pulp. However, as
shown in Examples 8 and 9, when the unbleached softwood
pulp having a Kappa number of 27.5 or the alkali-oxygen
delignified softwood pulp having a Kappa number of 13.3
was acid treated under pressure generated by a compressed
oxygen-containing gas, the delignification of the pulp
could be significantly progressed while maintaining the
pulp yield at a satisfactory level, whereas the pulp
viscosity slightly decreased.
Compared with this, when the unbleached or alkali-
oxygen delignified softwood pulp is acid-treated without
pressurizing with the oxygen-containing gas, as shown in
Comparative Examples 7 and 8, the decrease in Kappa
number of the pulp due to the acid treatment is lower
than that in the acid treatment with the pressurization
with the oxygen-containing gas.
Accordingly, it is clear that the process of the
present invention contributes to enhancing the
bleachability of the unbleached pulp without decreasing
the total pulp yield in the acid treatment and bleaching
procedures and to significantly reducing the amounts of
the bleaching chemicals necessary for the bleaching
procedure.
Example 10
Mixed hardwood chips comprising 70% by weight of
Japan-produced hardwood chips and 30% by weight of
eucalyptus wood chips and in a bone dry weight of 500g
was kraft-cooked in a laboratory indirect-heating
autoclave containing a cooking liquor having an effective
alkali content of 18% based on the bone dry weight of the
wood chips, a degree of sulfidity of 25% and a liquor
ratio of 4 to the bone dry weight of the wood chips at a
cooking temperature of 160~C for a cooking time of
120 minutes. The resultant pulp was collected, and
finely screened through a flat screen equipped with a
10 cuts screen plate. An unbleached hardwood kraft pulp
CA 02230961 1998-03-03
- 47 -
having a Hunter brightness of 45. 296, a Kappa number of
20.1, a pulp viscosity of 23.3 mPa-s (determined in
accordance with J. TAPPI, 44) was obtained in an amount
of 229g in a finely screened pulp yield of 45.8%.
The unbleached pulp in a bone dry weight of 90.0g
was suspended in water and added with sodium hydroxide in
an amount of 1.5% based on the bone dry weight of the
pulp, and diluted with ion-exchanged water to adjust the
pulp consistency in the pulp slurry to 10% by weight.
The pulp slurry was placed in an indirect heating
autoclave, pressurized with a commercially available
compressed oxygen gas having a degree of oxygen purity of
99.9% to a pressure on gage of 5 kg/cm2, and heated at a
temperature of 100~C under the above-mentioned pressure
for 60 minutes, to delignify the pulp by a medium
consistency alkali-oxygen delignification method. The
resultant delignified pulp was washed with ion-exchanged
water and dewatered.
A delignified pulp having a Hunter brightness of
51.1%, a Kappa number of 10.2 and a pulp viscosity of
18.8 mPa-s was obtained in a bone dry weight of 88.8g.
The alkali-oxygen delignified pulp in a bone dry
weight of 80.0g was suspended in water and diluted with
an aqueous sulfuric acid solution prepared by dissolving
concentrated sulfuric acid in ion-exchanged water and
having a pH value of 2.6, to adjust the pulp consistency
in the slurry to 10% by weight.
The pulp slurry was placed in a indirect heating
stainless steel autoclave having a capacity of 2 liters,
the inside space of the autoclave was filled with a
nitrogen gas having a degree of nitrogen purity of 99.9%
and pressurized with the same nitrogen gas as mentioned
above to a pressure on gage of 5 kg/cm . The pulp slurry
in the autoclave was heated at a temperature of 95~C
under the above-mentioned pressure for 60 minutes, to
conduct the acid treatment for the pulp. After the
CA 02230961 1998-03-03
- 48 -
autoclave was cooled to room temperature, the acid-
treated pulp was washed with ion-exchanged water and
dewatered.
The acid treated pulp having a brightness of 51.4%,
a Kappa number of 4. 5 and a pulp viscosity of 17.2 mPa s
was obtained in a bone dry weight of 79.3g. In the acid
treatment, the decrease in Kappa number was 55.9% and the
pulp yield was 99.1%.
The acid treated pulp in a bone dry weight of 70.0g
was placed in a plastic resin bag and suspended in a pulp
consistency of 10% by weight in ion-exchanged water, then
chlorine dioxide is added in an amount of 0.41% based on
the bone dry weight of the pulp to the pulp slurry in the
bag. The pulp slurry-containing bag was immersed in a
constant temperature water bath at a temperature of 70~C
for 30 minutes, to carry out a bleaching stage D1
procedure. The Dl-bleached pulp was washed with ion-
exchanged water and dewatered.
The Dl-bleached pulp was placed in a plastic resin
bag and suspended in a pulp consistency of 10% by weight
in ion-exchanged water. Then, sodium hydroxide was added
in an amount of 1.7 times the amount of chlorine dioxide
used in the D1 stage to the pulp slurry. The resultant
pulp slurry was treated with the sodium hydroxide at a
temperature of 70~C for 120 minutes in the same manner as
in the D1 stage, to carry out an extraction procedure in
an E stage. The resultant pulp after the E stage was
washed with ion-exchanged water and dewatered.
The E stage-extracted pulp was placed in a plastic
resin bag and suspended in a pulp consistency of 10% by
weight in ion-exchanged water. Then, chlorine dioxide
was added in an amount of 0. 4% based on the bone dry
weight of the pulp to the pulp slurry. The pulp was
treated with chlorine dioxide at a temperature of 70~C
for 180 minutes in the same manner as in the Dl stage to
effect a D2 stage bleaching procedure. The resultant
pulp was washed with ion-exchanged water and dewatered to
CA 02230961 1998-03-03
- 49 -
obtain a bleached pulp in a bone dry weight of 69.2g.
The pulp yield of the multi-stage bleaching
procedure was 98.9% and the total pulp yield of the acid
treatment and bleaching procedures was 98.0%.
The D2 stage bleached pulp was disintegrated and
then formed into a pulp sheet having a basis weight of
60 g/m in accordance with TAPPI Testing Method T 205
os-71 (JIS P 8209). The pulp sheet had a brightness of
85. 4% determined in accordance with JIS P 8123.
In Table 5, the Kappa number of the acid treated
pulp, the decrease in Kappa number due to the acid
treatment and pulp yield in the acid treatment are shown.
Also, in Table 6, the amounts of the bleaching chemicals
used in the D1 and D2 stages, the decrease in the total
bleaching chemical consumption in the D1 and D2 stages,
the decrease in bleaching chemical consumption in the E-
stage in the bleaching procedure, the pulp yield of the
bleaching procedure and the total pulp yield of the acid
treatment and bleaching procedures are shown.
Example 11
The same acid-treated pulp having a Kappa number of
4. 5 and a pulp viscosity of 17.2 mPa s as in Example 10
was employed in a bone dry weight of 70g.
The acid treated pulp was subjected to the same D1
stage bleaching procedure as in Example 10, washed with
ion-exchanged water and dehydrated.
The D1 stage bleached pulp slurry was added with
sodium hydroxide in an amount of 0.69% based on the bone
dry weight of the pulp and diluted with ion-exchanged
water to adjust the pulp consistency in the slurry to 10%
by weight. The pulp slurry was placed in an indirect
heating autoclave, pressurized with a commercially
available compressed oxygen gas having a degree of oxygen
purity of 99.9% to a pressure on gage of 1.5 kg/cm , and
heated at a temperature of 70~C under the above-mentioned
pressure for 15 minutes. Then, the pressure of the
CA 0223096l l998-03-03
- 50 -
autoclave was reduced, and the pulp slurry was heated at
a temperature of 70~C under ambient atmospheric pressure
(pressure on gage = O) for 105 minutes, to complete the
alkali extraction (E/O) stage in the presence of oxygen
for the pulp. The resultant alkali-extracted pulp was
washed with ion-exchanged water and dewatered.
The alkali-extracted pulp was subjected to the same
D2 stage procedure as in Example 10, except that the
amount of chlorine dioxide used was changed to 0.18%
based on the bone dry weight of the pulp. A bleached
pulp having a Hunter brightness of 85.4% was obtained in
a bone dry weight of 69.3g.
The pulp yield in the multi-stage bleaching
procedure was 99.0% and the total pulp yield in the acid
treatment and bleaching procedures was 98.1%.
In Table 5, the Kappa number of the acid treated
pulp, the decrease in Kappa number due to the acid
treatment and pulp yield in the acid treatment are shown.
Also, in Table 6, the amounts of the bleaching chemicals
used in the D1 and D2 stages, the decrease in the total
bleaching chemical consumption in the D1 and D2 stages,
the decrease in bleaching chemical consumption in the E-
stage in the bleaching procedure, the pulp yield of the
bleaching procedure and the total pulp yield of the acid
treatment and bleaching procedures are shown.
Example 12
The same acid-treated pulp having a Kappa number of
4.5 and a pulp viscosity of 17.2 mPa s as in Example 10
was employed in a bone dry weight of 70g.
An aqueous slurry of the acid-treated pulp was added
with 1.0% of sodium hydroxide and 0. 5% of hydrogen
peroxide based on the bone dry weight of the pulp,
diluted with ion-exchanged water to adjust the pulp
consistency in the slurry to 10% by weight. The pulp
slurry was placed in an indirect heating autoclave,
pressurized with a commercially available compressed
oxygen gas having a degree of oxygen purity of 99.9% to a
CA 0223096l l998-03-03
- 51 -
pressure on gage of 1.5 kg/cm , and heated at a
temperature of 70~C for 15 minutes. The pressure of the
autoclave was reduced, and the pulp slurry was further
heated at a temperature of 70~C under ambient atmospheric
pressure for 105 minutes to complete an alkali extraction
(E/OP) in the presence of oxygen and hydrogen peroxide.
After the autoclave was cooled, the alkali extracted
(E/OP) pulp was washed with ion-exchanged water and
dewatered.
The E/OP alkali-extracted pulp slurry was added with
1.5% of sodium hydroxide and 3.0% of hydrogen peroxide,
based on the bone dry weight of the pulp, and diLuted
with ion-exchanged water to adjust the pulp consistency
in the slurry to 10% by weight, and placed in an indirect
heating autoclave.
The pulp slurry in the autoclave was pressurized
with a commercially available compressed oxygen gas
having a degree of oxygen purity of 99.9% to a pressure
on gage of 5 kg/cm and heated at a temperature of 100~C
under the above-mentioned pressure for 60 minutes, to
complete a hydrogen peroxide (PO) bleaching treatment
under a pressure imparted by the compressed oxygen gas.
The resultant bleached pulp was washed with ion-
exchanged water and dewatered.
The resultant bleached pulp in a bone dry weight of
68.8g had a brightness of 84.2% and a pulp viscosity of
14.8 mPa s. The pulp yield in the bleaching procedure
was 98.3%.
Table 7 shows the brightness and pulp viscosity of
the bleached pulp, and the total pulp yield in the acid
treatment and bleaching procedures.
Example 13
The same acid-treated pulp having a Kappa number of
4.5 and a pulp viscosity of 17.2 mPa s as in Example 10
was employed in a bone dry weight of 70g.
The acid treated pulp slurry was added with sodium
CA 02230961 1998-03-03
- 52 -
hydroxide in an amount of 1.5% and hydrogen peroxide in
an amount of 2.0~ based on the bone dry weight of the
pulp and diluted with ion-exchanged water to adjust the
pulp consistency in the slurry to 10% by weight. The
pulp slurry was placed in an indirect heating autoclave,
pressurized with a commercially available compressed
oxygen gas having a degree of oxygen purity of 99.9% to a
pressure on gage of 5 kg/cm , and heated at a temperature
of 100~C under the above-mentioned pressure for
60 minutes, to complete the hydrogen peroxide (PO)
bleaching procedure under a pressure imparted by the
oxygen gas. The resultant pulp was washed with ion-
exchanged water and dewatered.
The resultant semi-bleached pulp in a bone dry
weight of 69.5g had a brightness of 72.8% and a pulp
viscosity of 15.7 mPa-s. The pulp yield in the bleaching
procedure was 99.3%.
Table 7 shows the brightness and pulp viscosity of
the semi-bleached pulp and the total pulp yield in the
acid treatment and bleaching procedures.
Comparative Example 9
The same acid treatment and multi-stage bleaching
procedures (D1-E-D2) as in Example 10 were carried out,
except that the acid treatment was carried out in a
nitrogen gas atmosphere under ambient atmospheric
pressure (no pressurization was applied), the amount of
the bleaching chemical used in the D1 stage was changed
to 0.65% based on the bone dry weight of the pulp.
A bleached pulp having a brightness of 85.3% was
obtained in a bone dry weight of 69.lg.
The Kappa number of the acid-treated pulp was 6.5,
the decrease in Kappa number due to the acid treatment
was 36.3%, the pulp viscosity of the acid-treated pulp
was 17.2 mPa-s, the bone dry weight of the acid treated
pulp was 79.5g, and the pulp yield in the acid treatment
was 99.4%.
CA 02230961 1998-03-03
In the multi-stage bleaching procedure, the bone dry
weight of the bleached pulp was 69.lg, the pulp yield in
the bleaching procedure was 98.7% and the total pulp
yield in the acid treatment and bleaching procedures was
98.1%. Table 5 shows the Kappa number of the acid-
treated pulp, the decrease in Kappa number due to the
acid-treatment and the pulp yield in the acid treatment.
Also, Table 6 shows the amounts of the bleaching chemical
in the Dl and D2 stages, the decrease in the bleaching
chemical consumption in the Dl and D stages, the decrease
in the bleaching chemical consumption in the E stage, the
pulp yield in the bleaching procedure and the total pulp
yield in the acid treatment and bleaching procedures.
Comparative Example 10
The same procedures as in Comparative Example 5 were
carried out to produce a bleached pulp.
The resultant bleached pulp had a bone dry weight of
68.9g, a brightness of 81.4%, a pulp yield in the
bleaching procedure of 98.4% and a pulp viscosity of
14.1 mPa-s.
Table 7 shows the brightness and pulp viscosity of
the bleached pulp, and the total pulp yield in the acid
treatment and the bleaching procedures.
Comparative Example 11
A semi-bleached pulp was produced by the same
procedures as in Comparative Example 6.
The semi-bleached pulp had a bone dry weight: of
69.5g, a brightness of 69.8%, a pulp yield of 99.3% and a
pulp viscosity of 15.1 mPa s.
Table 7 shows the brightness and pulp viscosity of
the semi-bleached pulp and the total pulp yield in the
acid treatment and bleaching procedures.
CA 0223096l l998-03-03
- 54 -
Table 5
\ Item Unbleached non- Unbleached, acid-treated pulp
\ acid treated pulp
\ Kappa Pulp Kappa Decrease Pulp Pulp
\ number viscosity number in Kappa viscosity yield
\ (mPa s) number
Example No. \ (%)(mPa s) (~)
lo 4.5 55.917.2 99.1
11 4.5 55.917.Z 99.1
Example
_ 10.218.8 4 5 55.917.Z 99.1
13 4.5 55.917.2 99.1
Comparative 9 6.5 36.3 17.2 99.4
Example
Table 6
\ Item Multi-stage (Dl-E-D2) Total pulp
\ bleaching procedure yield in acid
\ treatment and
\ bleaching
\ Amount of Decrease in Pulp
\ bleaching bleaching yield
\ chemical chemical
\ used consumption
(~)
\ Dl- D2- Dl + D2 E-
Example No. \ stage stage stages stage (%) (~)
0.41 0.4 38.6 55.4 98.9 98.0
Example
11 0.41 0.18 55.3 55.4 99.0 98.1
Comparative 9 0.65 0.4 20.5 29.3 98.7 98.1
Example
CA 0223096l l998-03-03
Table 7
\ Item Acid-treated, bleached pulp
\ Brightness Pulp Total pulp
\ viscosity of yield in
\ bleachedacld treatment
\ pulpand bleaching
Example No. \ (~) (mPa s) (~)
lZ 84.2 14.897.4
Example
13 72.8 15.798.4
Comparative 10 81.4 14.1 97.7
Example 11 69.8 15.198.6
In Tables 5 to 7, Examples lO to 13 show that when
an alkali-oxygen delignified pulp having a Kappa number
of 10.2 was acid treated under a pressure imparted by a
compressed nitrogen-containing gas, the acid treatment
under the nitrogen-containing gas pressure enables the
delignification of the pulp fibers to be greatly promoted
while minimizing the decrease in the pulp viscosity
(corresponding to the degree of polymerization), and the
bleaching procedure to be easily carried out to obtain a
bleached pulp having a desired brightness at a
satisfactory pulp yield, while decreasing the bleaching
chemical consumption.
Also, Example 11 shows that the acid-treated pulp
produced under a nitrogen-containing gas pressure caused
the delignification effect on the pulp by the alkali-
extraction (E/O) stage in the presence of oxygen to be
promoted and thus the amount of the bleaching chemicals
necessary to obtain a pulp having a desired brightness
could be reduced to a great extent.
Example 12 shows that in the process of the present
invention, the acid treatment under the nitrogen-
containing gas pressure enabled the alkali-oxygen
delignified pulp to be bleached to a desired pulp
brightness of, for example, 84.2%, without using chlorine
or a chlorine-containing chemical in a bleaching
sequence, for example, E/op-PO. However, when the acid
CA 02230961 1998-03-03
- 56 -
treatment was carried out without pressurization with the
nitrogen-containing gas, and the resultant acid-treated
pulp was subjected to the same chlorine-free bleaching
procedure as in Example 12, the resultant bleached pulp
an unsatisfactory brightness, for example, of 81.4% as
shown in Comparative Example 10.
Further, when the acid treatment of the pulp is
carried out in a nitrogen gas atmosphere without
pressurization, or using no nitrogen gas as in
Comparative Examples 3, 4 and 9 to 11, the decrease in
Kappa number in the acid treated is small, for example,
36%, in comparison with that in the acid treatment under
the nitrogen-containing gas pressure, the decrease in the
bleaching chemical consumption in the bleaching procedure
is low as shown in Comparative Examples 3 and 9, and even
when the alkali-extraction (E/O) step in the pressure of
oxygen is inserted into the bleaching procedure, a
significant decrease in the bleaching chemical
consumption could not be attained as shown in Comparative
Example 4.
The acid-treated pulp prepared in the process of the
present invention could be converted to a semi-bleached
pulp having a brightness of 72.8% as shown in Example 13,
by a hydrogen peroxide-bleaching under an oxygen-
containing gas pressure. However, as shown in
Comparative Example ll, the acid treatment in which the
pressurization with the compressed nitrogen-containing
gas is not applied, resulted in a bleached pulp having a
low level of brightness of 69. 8%, even when the hleaching
procedure is carried out in the same bleaching sequence
as in Example 13.
Namely, in the process of the present invention,
when the acid treatment for the unbleached pulp is
carried out under a nitrogen-containing gas pressure
before the bleaching procedure, the amount of the
bleaching chemicals necessary to obtain a bleached pulp
having a desired brightness can be reduced without
CA 02230961 1998-03-03
decreasing the total pulp yield in the acid treatment and
bleaching procedures and the pulp viscosity of the
bleached pulp.
