Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~9~
SPECIFICATION
A METHOD FOR THE ENZYMATIC TREATMENT OF
CHEMICAL PULP BEFORE BLEACHING
TECHNICAL FIELD
The present invention relates ~o a method of enzyma-tic
pretreatment of chemical pulp. Especially, lt relates to
-the method that reduces, or may make unnecessary the use
of the molecular chlorine in the bleaching process by
incorporating a combination of an enzymatic treatment and
an acid treatment.
BACKGROUND ART
The molecular chlorine is used in bleaching, the
process effluent may contain dioxin, which reportedly harms
human beings and other living organisms. In order to
eliminate such hazards, research studies ara being carried
out to develop a chemical pulp bleaching process that
reduces or makes unnecessary the use of molecular chlorine.
Fibrous chemical pulp is widely employed as raw
materials in the paper-making industry. The chemical pulp
is normally made from wood, which mainly consists of
cellulose and a three-dimensional polymer, called lignin.
Lignin is thought to extend into matrixes formed of
cellulosic polysaccharides and hPmicellulosic
-- 1 --
6~ 9 ~ 6
polysaccharides.
Normally, bonds between those different components are
via different chemical bonds; for ins-tance, a lignin block
reportedly has a part bonding to a hemicellulose molecular
chain. The hemicellulose is thought to be a secondary
component of chemical pulp. The main hemicellulose in
hardwood is glucuronoxylan, which includes polymers formed
mainly of D-xylose and are hereinafter referred to as
xylan.
In order to produce chemical pulp having sufficient
strength and whiteness to be made into paper, the raw
chemical pulp must, first, be processed to remove lignin.
The primary step to remove lignin from wood is conducted
in a digester in the presence of chemicals, e.g., NaOH,
sodium sulfide (in the Kraft pulping process), a sulfite
salt (normally Na-salt or Mg-salt in sulfite pulping), or
NaOH and an anthra~uinone compound (in soda-AQ pulping).
This primary step is called delignification.
The amount of lignin remaining in wood pulp is
determined by an oxidation test using a permanganate and
conducted according to TAPPI Test Method T-236 cm-85
standardi~ed by Nippon PUlp and Paper Industrial Technology
Association, and the result is reported as a Kappa Number.
The wood pulp after the delignification step contains a
significant amount of lignin, but in certain cases can be
-- 2
~3~90~
used without further purification for card board paper or
bag paper. In most cases, however, the wood pulp must be
bleached in order to be of use for, e.g., printing paper,
writing paper, sanitary paper, and the like, as it is too
dark.
Conventional methods of delignification and bleaching
employ 3- to 6-stage bleaching processes optionally
combined with a washing step between each of the stages.
Such processes are called multi-stage bleaching processes
or bleaching sequences. The object of bleaching chemical
pulp is to provide a bleached pulp having a whiteness
enough to make it into paper products and thin paper
products. By undergoing such a bleaching se~uence, a
bleached pulp having a whiteness of 85 to 90-~ is obtained.
The conventional bleaching process is based on the use
of chloride or chlorine-containing chemicals, namely,
chlorine dioxide and a hypochlorite salt. The stage that
makes use of chlorine is called C or C-stage, that which
makes use of chlorine dioxide D or D-stage and that which
makes use of a hypochlorite salt H or H-stage.
The C, in certain cases C/D, meaning the C and the D
combined, normally comes first, followed by extraction
using an alkaline medium and this extraction stage is
called E or E-stage. In the C, the dosage of chlorine (or
~5 chlorine and chlorine dioxide combined, and represented by
-- 3
~ 6~
`~ ,
equivalence of the total oxidative chlorine) varies in
proportion to the amount of lignin (as represented by the
Kappa number) remaining in the wood pulp to be processed.
Said E-stage dissolves most of the remaining lignin
chlorinated and oxidized in the preceding stage. Some
amount of hemicellulose is removed in this stage.
In view of ever stringent environmental regulations
being enforced to reduce water pollution attributable to
the chlorine-containing waste, it is desired to reduce, or
most dQsirably to cease, the use of the chlorine or
chlorine-containing bleaching chemicals, instead of
investing in costly pollution reduction processes.
In chemical pulping, the waste liquor of the digestion
step is treated by recovery processes and burned to recover
the chemicals. To obtain a bleached chemical pulp, the
digested and fibrillated pulp is further bleached using
chlorine or chlorine-containing chemicals, as mentioned
above, and the bleaching waste from naturally contains
chloride ions or compounds so that it cannot be fed back
to the recovery processes. This is because if it is fed
back to reactors where digested wastes are recovered, the
chloride ions or compounds in them will damage the
evaporator and the recovery kiln. From the recovery point
of Vi2W again, an alternative bleaching method not
employing any chlorine has been sought.
-- 4
~090~
A noteworthy technical de~elopment of the past two
decades which is in line w:ith the efforts to reduce
chlorine-containing waste is the use of oxygen as a
delignifying or bleaching agent.
Processing of wood pulp by oxygen mostly takes place
after the digestion stage and its purpose is to remove
lignin prior to the bleaching process. Oxygen is fed to
the unbleached pulp under pressure and in an alXaline
medium to oxidi~e, decompose and remove the lignin
remaining in the unbleached pulp. Conventionally, this
processing is called oxygen bleaching or oxygen
delignification (hereinafter referred to as O or O-stage).
There is a limitation on the amount of oxygen to be used
for the delignification, since conditions to cause
excessive oxygen consumption could negatively affect the
degree of polymerization (DP) of the cellulose in -the
unbleached pulp.
DP is an index of the pulp strength and is represented
by pulp viscosity according to the TAPPI Standard Test
Method T-230 om-89). In general, the limit of
delignification in the O-stage, while retaining the level
of the degradation of pulp viscosity to within an allowable
limit, is said to be at most an approximately 50% reduction
in the Kappa number. The delignification with oxygen at
this stage does contribute to a substantial reduction of
-- 5 --
~ 3~9~
the chlorine or chlorine-containing compounds in later
stages. The amount of the chlorine-containing compounds
to be used in the chlorination step depends on the lignin
content in pulp, so that it may be substantially reduced
due to the previous delignification with oxygen.
After treatment in C-stage, the pulp is usually
subjected to alkaline extraction to remove the chlorinated
lignin. There are a variety of known methods to use an
oxidizing agent and the alkali in combination, for
instance;
(1) combination of alkaline extraction with 2 gas,
hereinafter referred to as Eo.
(2) combination of alkaline extraction with hydrogen
peroxide (P), hereinafter referred to as Ep.
(3) combination of alkaline extraction with hypochlorite
salt (H), hereinafter referred to as Eh.
(~) combination of alkaline extraction with h~drogen
peroxide and 2 gas (hereinafter referred to as Eop).
(5) combination of alkaline extraction with hypochlorite
salt and 2 gas (hereinafter referred to as Eoh).
These may be referred to as oxidative extractions.
Recently, the use of enzymes for processing chemical
pulp (hereinafter referred to as X or ~-stage) has been
developed. For instance, it is known that ligninase,
especially that to be obtained from white rotting fungi
-- 6 --
9 0 ~
decomposes lignin. In addition, it is also known that
cellulose decomposes cellulose.
Looking at the hemicellulose componen-t of chemical
pulp, a variety of papers have reported on the influences
of xylanase upon wood pulp. As a result of such research
efforts, it has been clarified that xylanase reacts
selectively with xylan in the hemicellulose. The French
patent application no. 2557894 (laid open in 1985)
describes the treatment of a hardwood blPached pulp, or
coniferous bleached pulp, with a solution containing
xylanase in order to reduce the power to beat pulp required
for paper making.
The use of enzymes for pretreatment before bleaching,
or as a bleaching aid, was first a-ttempted by Viikari et
al. (Proceedings of International Symposium on Wood and
Pulping Chemistry, Paris, lg87). They treated unbleached
Kraft pulp of birch and pine with enzyme having a high
level of ~ylanase and xylosidase activity which they
obtained by culturing Asperqillus awamori, VTT-D-79125, or
StrePtomYCeS olivochromoqens, VTT-E-82157. The
thus-treated pulp, in turn, was treated with hydrogen
peroxide, then further treated with a (D+C)-E bleaching
sequence. They observPd that the enzyme-pretreatment
promotes more purification than the untreated control by
a 2-points Kappa-number reduction. The enzymatic
- 7 -
2~90~
pretreatment was conducted at a pH of 5.0, a temperature
of 45~C and over a period of 24 hours. Under these
conditions, hardly any metal salts contained in -the
unbleached pulp, particularly transition metal salts,
removed, so that, if excess enzyme was not added, i-t was
hard -to bleach the pulp to a higher whiteness in later
stages without greatly decreasing pulp viscosity, though
the enzymatic pretreatment needed a long period of time of
24 hours.
Paice, Bernier, and Jurasek treated a hardwood
unbleached Kraf-t pulp with enzymes produced by cloned E.
coli capable of endoxylanase or ~-xylosidase production
that they had prepared by genetic manipulation of the
respective genes. Then, they carried out alkaline
extraction or C-E-D treatment of the pulp and compared it
with the non-treated con-trol (Biotechnology and
Bioengineering, Vol. 32, July, pp. 234 to 239, 1988). The
pulp thus treated with enzymes and alkali substances had
a high Kappa-number, so that it must be further subjected
to C-E-D treatment using chlorine in order to bleach it to
have a higher degree of whiteness. The use of chlorine
inevitably involves the formation of toxic chlorinated
compounds like dioxin, etc., and the bleach effluent cannot
be recovered in a black liquor recovery system, as it
contains chloride ions.
-- 8 --
g ~ 6
Japanese Patent Application Kokai No. 2-264, 087
discloses ~ technique to obtain pulp having a practical
whiteness level by treating lignocsllulose substances in
pulp, first, with xylanase containing substantially no
cellulase, then by treating the resulting pulp further in
one or more additional states selected from C, D, E, P, H,
ozone, NO2 and oxidative alkali extraction, before or after
bleaching pulp with oxygen.
In this technique, the enzymatic treatment was
conducted at a temperature of 20 to 80C, over a period of
1 to 48 hours, and at a pH of 4 to 8. As the xylanase
activity was measured at a pH 6 . 0, thP pulp was thought to
be treated at about that p~, namely, the use of enzymes
having the maximum activity under neutral to acidic
conditions was a premise. In all the examples in the
specification, post bleaching was conducted beginning with
C, D or C/D, and the examples indicate that the use of
chlorine or chlorine-containing bleaching agents is
inPvitable in order to obtain a high level of whiteness.
This means that the enzymatic treatment under neutral to
weakly-acidic conditions does not result in the removal of
metal salts with the result that the remained metal salts
decompose the bleaching agents, thereby making efficient
bleaching impossible.
Japanese Patent Application Kokai No. 2-293,486
_ g _
2 ~L 3 ~ 6
discloses a bleaching process r in which unbleached pulp is
enzymatically treated before or after the primary oxidation
t stage with oxygen, and is then treated by repeated C, D or
C/D with E or Eo in between. In this process again, the
pH of the enzymatic treatment, as described, was in a range
o 3.0 to 10Ø Preferably, it is in a range of 4.0 to
9Ø Namely, the use of the enzyme that exhibits the
maximum activity in a neutral to weak acidic environmen-t
~ is a premise. Therefore, the subsequent bleaching
¦ 10 treatment with C or D was indispensable in order to obtain
¦ pulp having a high degree of whiteness.
¦ According to the bleaching process not using molecular
chlorine at all, treatment of pulp under strong acidic
conditions was not conducted throughout the process so that
metal salts, especially transition metal salts remained in
the system to decompose the oxidizing agent to be used for
the bleaching. As a result, the bleaching effect of the
agent was lowered, and the active oxygen, etc. to be formed
during the decomposition attached the cellulose chains in
pulp being treated to lower the degree of polymerization
of the cellulose moieties therein, with the result that the
quality of the thus-treated pulp was lowered. In addition,
since the potency of the oxidizing agent used was lowered,
some excess amounts of the agent must be added. Such
noticeably detracted from the economical advantage in
- 10 -
~; ~
2 3L 3 ~ 3 6
,,~,
chemical pulping, and the lowering of the quality of the
thus-bleached pulp was at an unacceptable level.
In order to remove or inactivate metal saits, there
are hitherto known, (1) pretreatment wi-th acid, and (2)
blocking with a chelating agent. However, these treatments
per se have no delignification effect and the amount of the
oxidi~ing agent to be used is still large to such a degree
'~ that it lowers the quality of the thus-treated pulp.
The use of the D-stage as the primary stage in place
of the C-stag~ may be adopted. In this case, however, the
bleach effluent does contain chloride ions so that it
cannot be ed back to the bleach liquor recovery processes
but is drained as a waste fluid. Moreover, although the
amount of chlorinated organic compounds is not so much,
environmental pollution attributable to toxic chlorinated
compounds cannot be eliminated.
DISCLOSURE OF THE INVENTION
PROBLEMS T0 BE SOLVED BY THE INVENTION
An object of the present invention is to provide a
method of bleaching chemical pulp which is more acceptable
from an environmental point of view, in which the use of
molecular chlorine is less than that in a conventionally
known mathod, or its use has been completely eliminated,
and which enables the transfer of at least a part of the
bleach effluent to the black-liquor burning processes so
-- 11 --
A ~
l::
3 ~
, .
; as to recover it.
Another object of the present invention is to provide
'~ a method b,v which the level of delignification of chemical
pulp is improved without dec:reasing the viscosity of the
pulp, and the whiteness of the pulp can be raised
efficiently.
MEANS FOR SOLVING THE PROBLEMS
In order to accomplish the objects of this inven-tion,
it is necessary to promote the delignification of chemical
pulp and to remove metal salts that cause a decrease in the
~ viscosity. The inventors have discovered that, by applying
¦ the following two treatments in combination before
bleaching, the pulp can be bleached in later bleaching
stages to a regular whiteness level without causing a
decrease in the viscosity and while reducing substantially,
or even eliminating, the use of molecular chlorine;
(1) removal of metal salts by acid treatment and promotion
of delignification.
(2) promotion of delignifica-tion through enzymes.
Furthermore, by selecting an acid-resistant enzyme,
said two treatments were combined into a single stage to
complete the present invention.
According to the first aspect of the present
invention, a method is provided for pretreating chemical
pulp before bleaching, in which the pulp is treated with
- 12 -
3090~
`1
an acid, washed, and then -treated with enzymes including
xylanase.
According to the second aspect of the present
invention, a method is provided for pretreating chemical
pulp before bleaching, in which the pulp is treated with
enzymes including xylanase, treated wi-th an acid, and then
washed.
According to the third aspect of the present
invention, a method is provided for pretreating chemical
pulp before blsaching, in which the pulp is treated at a
pH of 4.5 or lower with enzymes including xylanase that
exhibit the maximum activity at a pH of 4.5 or lower, and
then washed.
According to a further aspect of the present
invention, the enzymes to be employed in the ~irst or
second aspect of the invention are characterized in tha-t
they include xylanase and exhibit the maximum activity at
a pH 4.5 or lower.
According to a still further aspect of the present
invention, the chemical pulp is either unbleached pulp or
oxygen-bleached pulp and, befors being subjected to acid
treatment, the pulp is treated with hydrogen peroxide or
ozone. Furthermore, the acid to be employed to lower the
pH is at least one selected from acetic acid, formic acid,
oxalic acid, propionic acid, sulfuric acid, sulfurous acid,
- 13 -
2~3~g~
nitric acid and ni-trous acid.
According to a still further aspect of the present
invention, the liquid separated by solid-liquid separation
after the washing step is transferred back to the prior
step, while the waste is eventually burned to recover the
inorganic salts.
An additional bleaching process that follows may
comprise any one, two or more of the following bleaching
stages (1) through (7);
(l)--To treat the pulp with chlorine, chlorine dioxide, or
a mixture thereof in an aqueous medium.
(2)--To treat the pulp with a peroxide in an alkaline
aqueous medium.
(3)--To treat the pulp with a peroxide and oxygen in an
alkaline aqueous medium.
(4)--To treat the pulp with a hypochlorite salt in an
aqueous medium.
(5)--To treat the pulp with ozone in air or in an aqueous
medium.
(6)--To treat the pulp with thiourea dioxide in an alkaline
aqueous medium.
(7)--To treat the pulp with a hydrosulfite salt in an
alkaline aqueous medium.
While the enzyme to be employed in the present
invention is not specifically limited so long as it
- 14 -
213~9~5
exhibits ~ylanase activity, those that e~hibit xylanase
activity under acidic conditions, particularly at a pH of
4.5 or lower are preferred, and as such, hemicellulases,
cellulases, pectinases, esterases, and any mixture thereof
may be used.
~ For the enzymatic treatment, the pulp consistency is
¦ adjusted to 2 to 12~ by weight, and the enzyme is present
in an amount of 0.1 u/g to less than 1,000 u/g, preferably
1 u/g to less than 100 u/g, of the pulp in terms of
xylanase activity, and the pulp is treated with the enzyme
at a temperature of 0C or higher, preferably 40C or
higher, for at least 10 min.
The first and second aspects of the present invention
will be explained in detail.
As aforementioned, the enzymatic treatment is carried
out on unbleached or oxygen-delignified pulp before the
conventional C-stage. The effect of the enzymatic
treatment is enhanced by carrying it out in combination
with acid treatment. The enzymatic treatment may be
performed either before or after the acid treatment; if it
is done afterward, there is no need to control the pH; if
it is done before, the waste from the acid treatment can
be used to control the p~. The pH of the pulp under the
enzymatic treatment is 4.5 or lower, preferably lower then
3Ø Of course, a certain effect of the treatment can be
- 15 -
^--
2~3~90~
achieved even if the pH is higher than 4.5.
The enzymatic treatmen-t can cleave xyloside bonds in
` lignocellulosic materials to relieve lignin in-to the waste.
There are some limitations in terms of the cleaving
capability; one is that the size of the enzyme molecule is
too large for it to penetrate a dep-th of hemicellulose or
cellulose matrix where lignin-xyloside bonds are present,
with the result that the enzyme cannot act on the bonds and
therefore the degree of delignification by the enzyme is
limited. According to experiments conducted by the
inventors, the degree of delignification achieved by the
enzymatic treatment in the case of hardwood Kraft pulp is
l about a 2.0 point Kappa number reduction. Attempts to
achieve a higher degree of delignification result in a
great decrease in the yield of the cellulose component.
Moreover, the further enzymatic reaction with
lignocellulosic materials takes considerable time, so that
such attempts are hardly practical from an industrial point
of view.
In view of that limitation, it is necessary to develop
a different approach to the delignification, and the
inventors thought of applying an acid treatment
(hereinafter referred to as A or A-stage) in combination
with the enzymatic treatment. Chemical pulp digested
essentially with an alkaline medium has a porous structure
- 16 -
,1 2130906
-, ~
(as it has been swollen with an alkali), and it is possible
'~ to effectively and additionally dissolve out lignin from
the pulp having such a structure, using various media.
Addition of an acid to the porous pulp results in
acceleration of the dissolution of lignin from the pulp.
The reason is thought to be that, in the case of a mineral
acid, the bond between lignin and polysaccharide is cleaved
by hydrolysis and that the movement of lignin is made easy
by acid hydrolysis of the low-molecular hemicellulose in
lC the pulp; and in the case of an organic acid, its affinity
for lignin promotes the dissolu~ion of lignin.
Acids to be used for said purpose may be an organic
acid, e.g., acetic acid, formic acid, oxalic acid,
propionic acid and the like, or an inorganic acid, e.g.,
sulfuric acid, sulfurous acid, nitric acid, nitrous acid
and the like.
Another effect of the A-stage is the dissolution of
the metal salts contained in the pulp. The acid decomposes
. metal salts, which are removed in the later washing step.
~'~ 20 The removal of heavy metals, in particular of transition
metals, inhibits the decomposition of an oxidizing agent
s"' in the later bleaching stages using hydrogen peroxide and
.
~- ozone, and further inhibits the formation of an active
~' chemical nucleus, like a hydroxyl radical, which attacks
.~; 25 fiber. This enhances bleaching effect and prevents a
.~
:~ - 17 -
::~
~:;
.';
:
;
: .
~3~0~
decrease in the degree of polymerization of the pulp.
The present invention com:bines the effects of acid and
of enzyme to enhance bleaching effect without degrading the
quality of the pulp.
When the acid-resistant; enzyme and an acid are
employed in combination, both of the treatments are
combined into a single stage and the process can be
simplified.
Experiments conducted by the inventors on the acid
extraction of lignin from lignocellulosic materials, which
were digested and oxygen-bleached, revealed that
delignification degree by about a 2-point Kappa-number
reduction can be accomplished by the acid extraction
treatment. Additional enzymatic treatment on the
thus-obtained lignocellulosic materials makes it possible
to achieve a level of whiteness demanded from a practical
point of view by applying a moderate intensity of bleaching
in the la-ter stages while a decline in the viscosity is
inhibited.
As for the acid treatment conditions, the period of
time is at least 10 minutes, preferably 30 minu-tes to less
than 3 hours; the temperature at least 20C, preferably
40C to lower than 80C; -the consistency of the pulp 3~ to
less than 20~. The acid treatment, even when applied after
the enzyma-tic treatment, can accomplish a specific
- 18 -
~13~90~
delignification comparabl~ to the same as when it is
applied before the enzymatic treatment. The addition of
a peroxide during the acid treatment (hereinafter referred
to as Ap or Ap-stage) significantly advances
delignification and bleaching. It is known that an acid
and hydrogen peroxide make under certain conditions a
peracid, which specifically reacts with lignin and
decomposes it. The addition of a small amount of mineral
acid, e.g., sulfuric acid, accelerates the generation of
such peracid.
Said acid treatment is effective also for ozone
oxidation of chemical pulp. Ozone reacts with most organic
substances or materials and water. The reaction proceeds
via intermediates, like peroxides, epoxides, hydroxyl
radicals and the like, some of which have a bleaching
effect. Ozone reacts preferentially with aromatic nuclei,
olefin groups and the like, selectively attacking lignin
having these. If chemical pulp has a few or no such nuclei
or groups so that the possibility of the ozone reaction in
chemical pulp is low, ozone attacks carbohydrates, namely
cellulose. Particularly, ozone may intensively attack
fiber unless the pH is controlled properly, even if lignin
is present abundantly in pulp. This is thought to be due
to an increased generation of hydroxyl radicals caused by
the decomposition of ozone when the pH is high. It is
-- 19 --
2~3~90~
therefore manda-tory in applying ozone for bleaching to
control the pH of pulp stock be:Eore bleaching on the acidic
side, and in view of this, said acid treatment with the
enzyme exhibiting activity on the acidic side is
reasonable. The presence of metals, transition metals or
their salts in particular, in the o~one bleaching system
accelerates the decomposition of ozone supporting the
attack on carbohydrates resul-ting in a degradation of the
pulp. In order to prevent such detrimental effects, either
the removal of the metals by acid treatment of the use of
chelating agents is required.
As another bleaching option, the use of a reducing
bleaching agent, e.g., hydrosulfite, may be considered~
This agent is easily oxidized, and when the pulp
consistency is high, it comes in frequent contact with
oxygen in air, but such is unfavorable. Therefore, the
pulp consistency is normally kept at about 4~ during
bleaching. At such a low consistency, the concentration
of the agent is limited. This is because even though the
amount of hydrosulfite is desired to be increased so as to
make the bleached pulp have a high level of whiteness, the
consistency of the agent in water cannot be elevated so
much. Thus, the increase in the level of the whiteness of
the bleached pulp is limited and the color reversion after
bleaching tends to be intense.
- 20 -
f -
2~3~9~
Thiourea dioxide is still another reducing bleaching
agent. This is a stable whi-te powder and has no reducing
property at room temperature. When i-t is heated or brough-t
into contact with an alkali, it decomposes to generate
sulEinic acid, which enters into a strong reducing
reaction. It i5 therefore used with less care in terms of
oxidative decomposition as compared with hydrosulfite; it
is fed to the pulp along with an alkali and heated -to
accomplish an increase in the level of whiteness of the
bleached pulp that would hardly be achievable by the use
of the other reducing bleaching agent, hydrosulfite.
The conditions for bleaching by thiourea dioxide
(hereinafter referred to as FAS or FAS-stage) are as
follows: Since thiourea dioxide is more stable than
hydrosulfite, i.e., its stabili-ty is not influenced by the
pulp consis~ency (in water), bleaching with thiourea
dioxide is preferably conducted at a medium pulp
consistency of about 15%, rather than a low consistency
thereof of about 4%. This is because the concentration of
the chemical to be added may be increased at such a medium
pulp consistency so that the level of whiteness of the
bleached pulp may be elevated. A further rise in the
consistency, e.g., to abou-t 30%, results in a decrease in
the level o:E whiteness, so that the pulp consistency is
preferably 10-to 20%. The unfavorable effect of the higher
- 21 -
~30~0~
pulp consistency resulting in the decrease in the level of
whiteness of the bleached pulp is due to oxidation of the
agent by air like in -the case of hydrosulfi-te. When the
air is substituted by nitrogen, such unfavorable decrease
in the level of whiteness doe not take place even if the
bleaching consistency is high. The resul-ts of the
experiments in which the pulp was tested at 40C, 60C and
80C, show that 80C is the most favorable temperature.
Using a temperature not lower than 100C, however, is not
practical since the bleaching reac-tion has to be conducted
under pressurized conditions. From a practical poin-t of
view, it is preferable that for the present invention a
temperature of 50 to 100C is applied. As to the reaction
time, an appreciable gain in brightness is accomplished
within 15 minutes and the amount of gain reaches near
maximum after 60 minutes, while the brightness declined
after 24 hours. It is there*ore preferable to adopt a
reaction time of 10 to 90 minutes.
Now, the third aspect of the present invention will
be explained in detail.
The effect of xylanase to be used in the present
invention is limited by the pH condition. If the pH
condition for xylanase overs-teps the defined range, then
it loses its activity accordingly. Such a characteristic
~5 of xylanase results in various disadvantages for the pH
- 22 -
'~3~9~
adjustment and, after all, in a bar to -the object for
attaining the intended delignification effec-t, when the
application of xylanase to the pretreatment of pulp before
bleaching is taken into consicleration.
The first bleaching stage that follows the enzymatic
pretreatment is essentially C, C/D or Z in which a
bleaching reaction takes place under acidic to strongly
acidic conditions at pH of less than 5. Therefore, if -the
enzymatic pretreatment is carried out under neutral or
alkaline conditions jus-t before the bleaching stage, the
effluent from the following acidic processes including
treatment with chlorine cannot be recycled in a counter-
currentwise manner to the previous enzymatic pretreatment
stage. Considering the enzymatic treatment as the
pretrea-tment to be conducted prior to the bleaching
treatment carried out under acidic conditions, the purpose
of the enzymatic treatment cannot be accomplished
sufficiently as the use of conventional xylanase favors a
too high pH.
The present inventors have contrived a way of shifting
the activating pH region of the enzyme to a strongly-acidic
pH range to complete the present invention.
The present inventors have used an acid-resis-tant
xylanase (which is resistant to strong acids) to treat an
¦ 25 unbleached or oxygen-bleached pulp, and obtained a bleached
- 23 -
~2~3~9~'
pulp by applying the C, D, C/D or Z under acidic conditions
in the later bleaching stages. The thus-obtained bleached
pulp had a sufficiently high level of whiteness and was
usable as a raw material for printing paper and writing
paper. The inventors have confirmed that the effluent from
these acidic bleaching stages can be recycled in a
countercurrentwise manner to the enzymatic treatment stage.
The acid-resistant xylanase that was used showed a
sufficient xylan-decomposing activity at a pH of as low as
2.0 or lower and exhibited a delignification effect even
when the pulp was prepared to have a pH value falling
within the range. The enzyme to be preferably used in the
present invention should be one that exhibits the maximum
activity at pH of 4.5 or lower. Such an enzyme is
commercially available, namely as "Amano P" (manufactured
by Amano Pharmaceutical Co., Ltd.~.
According to the present invention, not only a pulp
with a lower lignin content which is easy to bleach is
obtained by a single-stage treatment, but also the
treatment can be run continuously without additional
acidifying steps if the later stage is an acidic one such
as ozone treatment (hereinafter referred to as Z or
Z-stage). Especially, Z-stage is said to be carried out
preferably at a pH of less than 3.0, so that the use of the
enzyme consisting mainly of xylanase exhibiting the maximum
- 24 -
2~3~9~
activity at pH 4.5 or lower, preferably less than 3.0, is
most effective. The amount of o~one, which is apt to
degrade cellulose, is reduced. Moreover, when the later
bleaching stages includa the P-stage, the heavy metals that
would decompose peroxides can be removed by the acidic
treatmen-t, so that the effect of the P-stage is maximi~ed.
The amount of the costly P can be reduced significantly due
to the delignification effect of the enzyme.
According to the present invention, the effluent from
the washing step is mostly treated in the digested black
liquor recovery system and is burned therein. All the
acids, enzyme, hydrogen peroxide, ozone and thiourea
dioxide to be recovered from the effluent may be burned in
conventional recovery boilers. If the bleaching stage
following the enzymatic treatment is the Z-stage, the
effluent form this stage contains a variety of organic
acids and may further contain sulfuric acid used as a pH
buffer. This effluent can be made use of for the acid
treatment~ and such use in a counter-currentwise manner
along with the control of the acidity at the enzymatic
treatment s-tage may enable a reduced dilution by fresh
water and to reduce the load to the vacuum evaporator in
the black liquor recovery system where the effluent is
treated.
Various wastes from the present invention contain
- 25 -
2~3~
substances that may sufficiently be recovered in the
recovery systems as mentioned above. However, if they are
directly transferred to the recovery system as they are,
the combustible solid content therein will be low so that
the load to the recovery becomes large as a whole. As the
case may be, there is a probability that the capacity of
the vacuum evaporator will become insufficient. Moreover,
acid contamination may result in a decline in the pH of the
black liquor, which in turn may cause a rise in -the
viscosity of the black liquor leading to plugging. In
order to avoid this unfavorable effect, the effluents
resulting from the above-mentioned acid treatment, ozone
treatment, enzyme treatment and hydrogen peroxide treatment
may be processèd by a separating membrane to separate and
concentrate the high polymer substances in the effluents.
In particular, from various acids used in the acid
treatment, useful acids may be isolated by distillation and
recycled. After the recycling operation, the residues from
the effluents may be burned by the burning treatment.
An embodiment of the present invention is to process
the pulp, after the enzymatic treatment with xylanase
active under acidic conditions and the acidic treatment
which are employed in the present invention, by one or more
additional stages selected from a group consisting of the
following s-tages (1) through (7):
- 26 -
~13~
(1)--To treat the pulp with chlorine, chlorine dioxide, or
a mixture thereof in an aqueous medium.
(2)--To treat -the pulp with a peroxide in an alkaline
aqueous medium.
(3)--To treat the pulp with a peroxide and oxygen in an
alkaline aqueous medium.
(4)--To treat the pulp with a hypochlorite salt in an
aqueous medium.
(5)--To treat the pulp with ozone in air or in an aqueous
medium.
(6)--To treat the pulp with thiourea dioxide in an alkaline
a~ueous mediumO
(7)--To treat the pulp with a hydrosulfite salt in an
alkaline aqueous medium.
In carrying out the me-thod of the present invention,
each of -those additional stages may be conducted according
to a conventionally practical method generally employed in
the field of pulp bleaching or any reasonable way known in
the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an embodiment flow diagram for the bleaching
according to the present invention. The full line
indicates a flow of the pulp being bleached; (a), (b) and
(c) indicate a -treatmen-t flow according to claims 1, 2 and
3, respectively. The totted line indicates the ~low of the
- 27 -
2~3~90~
effluent separated from the respective washing step by
solid-liquid separation. As shown, it can be fed to a
previous step in a counter-curre~twise manner.
DESCRIPTION OF REFERENCE NUMERALS
1.--acid treatment
2.--enzymatic treatment
3.--acid-resistant enzyme treatment
4.--washing
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is illustrated by the following
examples, which, however, are by no means intended to
restrict the invention. In these examples, oxygen
treatment is expressed as 0, acid treatment as A, enzyma-tic
treatment as X, acid-resistant enzyme treatment as Xa, a
combination of alkali treatment and peroxide treatment as
Ep, hypochlorite treatmen-t as H, and chlorine dioxide
treatment as D. Unless otherwise noted, in the following
examples and compara-tive examples, all parts are by weight;
all percent (%) referring to pulp consistency are weight
percent (%) after drying in a 105C oven; and, the dose (%)
of the chemicals is by-weight based on the pulp.
The testing methods to evaluate the properties of the
pulp to be treated by the invention are in accordance with
the following standards;
Kappa number: TAPPI Test Method T-236 cm-85
- 28 -
~130~0~
Viscosity: TAPPI Tes-t Method T-230 om-89
Whiteness: JIS P-8127 (Hunter Whiteness)
The xylanase activity of the enzyme to be used in the
method of the present invention was obtained according to
the following method:
Process:
About 30 ml of water were added to 0.500 g of xylan
(manufactured by Seikagaku KK) and dissolved under heat
while stirring. After this was cooled, water was added
thereto to make i~ 50 ml. This was used as the substrate.
For the acid-resistant enzyme, 3 ml of 0.1 N tartaric acid
buffer (pH 1.8) and 1 ml of the substrate were weighed and
put into a 50 ml ground-stopper Nessler's tube, this was
placed in a thermostatic water bath at 40C and left there
for 5 minutes or more. Then, 1 ml of the sample solution
was added thereto and immediately shaken. This was allowed
to stand at 40C for 30 minutes, 2 ml of a Somogyi's
solution were added thereto and shaken, and the tube was
sealed and heated in a boilin~ water bath for 20 minutes
and then immediately cooled. After this was cooled, one
ml of ammonium molybdate solution containing arsenic was
added thereto and well shaken until the red precipitates
of copper suboxide were completely dissolved. Afterwards,
this was allowed to stand at room temperature for 20
minutes, and water was added thereto to make 25 ml. This
- 29 -
2~3~9-~
was -then well shaken. About 8 ml of the resulting liquid
were put in a 15 ml centrifugal tube, which was subjected
to centrifugation using a cooled centrifuger (3000 rpm,
25C, 10 minutes). The absorbance (A30) of the
thus-separated supernatant at 500 nm was measured through
a layer length of 10 mm, as compared with that of the
control sample of water. Separately, a different sample
was prepared by adding 2 ml of a Somogyi's solution to 3
ml of 0.1 N tartaric acid buffer (pH 1.8) and one ml of the
substrate, shaking them and further adding one ml of the
sample solution thereto. This was also measured to obtain
its absorbance (A0) at 500 nm.
For the xylanase "Cellulase Amano CT-4", used was 0.1
N acetic acid/sodium acetate buffer (pH 4.5) in place of
0.1 N tartaric acid buffer (pH 1.8).
Determination of Xylanase Activity:
100 enzyme units liberate 1 mg of reducing sugar as
xylose per 1 min under the above-mentioned assay
conditions, and the xylanase activity was calculated by the
following formula;
Xylanase activity (u/g) = X x (1/30) x 100 x n
where,
X: amount (mg) of xylose liberated determined from
the difference, A30 - Ao~ using the xylose
calibration diagram,
- 30 -
s~ o ~
n: dilution factor
1/30~ 100:--conversion factor.
Example 1
Bleaching according to O-A-X-Ep-H-D:
100 parts of an oxygen-delignified hardwood Kraft pulp
was bleached by the sequence of O-A-X-Ep-H-D. The amounts
of the chemicals and the conclitions of each stage are
summarized as follows;
O-stage:--The consistency of the pulp, -to which 1.8 par-ts
of NaO~ had been added, was adjusted to 10~ and was placed
in an autoclave equipped with a high shear mixer (supplied
by Nitto Autoclave K.K.). To this, 1.6 parts of 02 gas
were added under pressure. The mixer was run for 1 min.
and then the pulp was allowed to stand for 1.5 hours to
react, while the reactor was sealed under pressure.
Thereafter, water was added to dilute the pulp to 1-~, and
then it was filtered and washed through a 5A filter paper
using a Buchner funnel. The Kappa number of the pulp was
9.4 and the viscosity 24.4-cps.
A-stage:--To the thus-treated pulp, whose consistency was
adjusted to 10%, 25 parts of acetic acid were added. The
preparation was heated to 70C and allowed to stand for 1.5
hours to react. Thereafter, water was added to dilute the
pulp to 1~, then filtered and washed through a 5A filter
paper using a Buchner funnel. The Kappa number of the
- 31 -
2 ~
thus-treated pulp was 8.0 and the viscosity 22.2 cps.
X-stage:--0.015 parts of xylanase "Cellulase Amano CT-4"
(which is supplied by Amano Pharmaceutical Co., Ltd., has
an optimum pH value of 5.5 and exhibits the maximum
xylanase acti~ity of 100,000 u/g at pH 4.5) were added to
the acid-treated pulp whose consistency was adjusted to
10~. The preparation was heated to 50C and allowed to
stand for 3 hours to react. Thereafter, water was added
to dilute the pulp to 1~, and then it was filtered and
washed through a 5A filter paper using a Buchner funnel.
After the washing, the Kappa number of the pulp was 6.4,
and the viscosity 20.8 cps.
Ep-stage:--0.8 parts of Na-hydroxide and 0.15 parts of
hydrogen peroxide were added to the enzyme-treated pulp
whose consistency was adjusted to 12%. The preparation was
heated to 65C and allowed to stand for 1.5 hours to reac-t.
Thereafter, water was added to dilute the pulp to 1%, and
then it was filtered and washed through a 5A filter paper
using a Buchner funnel.
H-s-tage:--0.8 parts of Na-hypochlorite were added to the
pulp after the Ep-stage whose consistency was adjusted to
10~. The preparation was heated to 65C and allowed to
stand for 2 hours to react. Thereafter, water was added
to dilute the pulp to 1~, then it was filtered and washed
through a 5A filter paper using a Buchner funnel.
- 32 -
9`~ ~
D-stage:--0.6 parts o chlorine dioxide were added to the
hypochlorite-treated pulp whose consistency was adjusted
to 10%. The preparation was hea-ted to 65C and allowed to
stand for 2 hours to react. Thereafter, water was added
to dilute the pulp to 1%, and then it was filtered and
washed through a 5A filter paper using a Buchner funnel.
The whiteness of the finally-obtained pulp having
undergone the O-A-X-Ep-H-D sequence was 86.0~ and -the
viscosity 16.5 cps. The amount of the total adsorbable
organic halogens (AOX) in the bleaching effluent was
determined to be 0.85 kg per a ton of the pulp.
Thus, it has been confirmed that a bleached hardwood
Kraft pulp havlng a normal level of whiteness and viscosity
can be obtained without employing molecular chlorine.
Example 2
Bleaching according to O-A-X-Ep-H-D:
The same process as in Example 1 was repeated, except
that 0.15 parts of "Hemicellulase 90" (supplied by Amano
Pharmaceutical Co. L-td., which exhibits a xylanase activity
of 100,000 u/g) were used in the X-stage in place of
"Cellulase Amano CT-4".
The whiteness of the finally-obtained pulp after the
last D-stage was 85.8~ and the viscosity 16.8 cps. The
amount of AOX in the bleaching effluent was 0.87 kg per a
ton of the pulp.
- 33 -
2~3~19`~
Thus, it has been confirmed that a bleached hardwood
Kraft pulp having normal whiteness and viscosity can also
be obtained using hemicellulase and wi-thout employing
molecular chlorine.
Reference Example 1
Bleaching according to conventional O-C-Ep-H-D:
100 parts of the same oxygen-delignified hardwood
Kraft pulp as in Example 1 were bleached in the sams manner
as in Example 1, except that the pulp was subjected to only
the C-stage under the following conditions without being
subjected to the A- and X-stages:
C-stage:--1.4 parts of chlorine were added to said pulp of
which the consistency was adjusted to 3~. The preparation
was heated to 50C and allowed to stand for 0.5 hours to
react. Thereafter, the pulp was diluted to 1-~, and then
it was filtered and washed through a 5A filter paper using
a Buchner funnel.
The whiteness of the finally-ob-tained pulp after the
last C-stage was 84.4% and the viscosity was 16.8 cps. The
amount of AOX in the bleaching effluent was 2.1 kg per a
ton of the pulp.
Thus, it is clear that use of an A-stage and an
X-stage in combination (as in Examples 1 and 2) produced
a pulp having a similar viscosity, but at significantly low
AOX level, as compared with the conventional bleaching
- 34 -
~13~
process (Reference Example 1) using chlorine.
Comparative Example 1
Bleaching according to conventional 0-X-Ep-H-D:
100 parts of the same oxygen-delignified hardwood
Kraft pulp were bleached in the same manner as in Example
1, except that they were not subjected to the A-stage. The
Kappa number after the X-stage was 7.1 and the viscosity
23.5 cps.
The whiteness of the finally-obtained pulp af-ter the
last D-stage was 82.6% and the viscosity 16.3 cps.
Thus r it is clear that the whiteness of the pulp that
was subjected to only the enzymatic treatment wlthout using
chlorine is lower than that of the pulp treated by the
present invention (Examples 1 and 2) and that of the pulp
treated by the conventional method usin~ chlorine
(Reference Example 1).
Example 3
Bleaching accordiny to 0-X-A-Ep-H-D:
The same process as in Example 1 was repeated, except
that the sequence A-X was reversed and the pH at the
X-stage was adjusted to 3.0 or lower while making use of
the washin~ effluent from the A-s-tage.
The Kappa number after the A-stage was 6.4, and the
viscosity 20.8 cps.
The whiteness of the finally-obtained pulp after the
- 35 -
2~ 9~
last D-stage was 85.5~, and the viscosity 16.9 cps.
Thus, i-t is clear that the pulp treated in this
example has a higher level of whiteness than tha-t of the
pulp treated in Comparative Example 1 at the similar
viscosity level.
Example 4
Bleaching according to P-Ap-X-Ep-H-D:
The sa~e process as in Example 1 was repeated, except
that the A-stage was combined with hydrogen peroxide
treatment to be the following Ap-stage:
Ap-stage:--To the oxygen-delignified pulp, 25 parts of
acetic acid and 0.3 parts of hydrogen peroxide were added
and the pulp consistency was adjusted to 10%. The
preparation was heated to 70C and allowed -to stand 1.5
hours to react. Thereafter, this was diluted to 1%, and
then filtered and washed through a 5A filter paper using
a Buchner funnel~ .
The Kappa number of the pulp after this Ap-stage was
3.8, and the viscosity 18.8 cps.
The Kappa number after the next X-stage was 2.4, and
the viscosity 16.4 cps. The whiteness of the
finally-obtainad pulp after the D-stage was 89.8~, and the
viscosity 14.8 cps.
Thus, it is clear that the pulp treated in this
example has a much higher level of whi-teness as compared
- 36 -
2~3~JO~
with that in Comparative Example l.
Example 5
Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 1 was repeated, except
that the 0-stage was omitted, the reaction temperature at
the A-stage was varied to 40C, and the C-stage was added
in accordance with the Kappa number ater the X-stage while
adjusting the amount of the chlorine added.
The whiteness of the finally-obtained pulp after the
last D-stage was 86.2~, and the viscosity 16.8 cps.
Reference Example 2
Bleaching according to C-Ep-H-D:
The same process as in Example 5 was repeated, except
that the A-stage and the X-s-tage were omitted and the
amount of the chlorine added at the C-stage was increased.
This reerence example corresponds to the conventional
process not having bleaching treatment with oxygen.
The whiteness of the finallv-obtained pulp after the
last D-stage was 85.5~, and the viscosity 13.5 cps.
Thus, Example 5 indicates that, by adopting the
A-stage and the X-stage, the amount of molecular chlorine
can be saved by about 40% in attaining an ordinary level
of whiteness, as compared with the conventional bleaching
process using chlorine (Reference Example 2).
Example 6
- 37 -
~3~90~
Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 5 was repeated, except
that the reaction temperature at the A-stage was varied to
20~C.
The whiteness of the finally-obtained pulp after the
last D-stage was 85.9%, and the viscosi-ty 15.~ cps.
Thus, it is known by comparison wi-th Example 5 that
the temperature at the A-stage is preferably 40~C or
higher.
Example 7
Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 5 was repeated, except
that the 0-stage was omitted and the reaction time at the
A-stage was varied to 10 minutes.
The whiteness of the finally-obtained pulp after the
last D-stage was 86.3~, and the viscosity 16.5 cps.
Thus, it is known that even if the time at the A-stage
is shortened, the amount of molecular chlorine can be saved
by about 36% while attaining an ordinary level of
whiteness, as compared with the conventional bleaching
process using chlorine (Reference Example 2); and the
adoption of the A-stage and the X-stage enables such
saving. In Examples 5, 6 and 7, the amount of chlorine
added was so controlled that the finally-obtained pulps in
these might have the same level of whiteness.
- 38 -
- ; ~ ` ;'
~13~9`06
Example 8
Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 7 was repeated, except
that the reaction time at the, A-stage was varied to 3
minutes and the amount of chlorine added was controlled.
The whiteness of the finally-ob-tained pulp after the
last D-stage was 86.1~, and the viscosity 14.8 cps.
As compared with Example 7, the amount of chlorine
needed for obtaining the same level of whiteness increased
in Example 8. In view of this, it is known that the
reaction time at the A-stage is preferably 10 minutes or
longer.
Example 9 and Comparative Example 2:
The same process as in Example 1 was repeated, except
that the amount of acetic acid added at the A-stage was
varied to 6 parts (Example 9) and 3 parts (Comparative
Example 2).
Under the conditions, there were rises in the pH of
the sample at the A-stage to 4.5 (Example 9) and 7.0
(Comparative Example 2), from 2.2 in Example 1. The
whiteness and the viscosity of the pulps finally obtained
in these examples were as follows:
brightness (~) viscosity (cps)
Example 9 84.1 16.3
Comparative Example 2 82.6 16.5
- 39 -
'~3~91~
Example 10
Bleaching according to 0-A-X-Ep-H-D:
The same process as in Example l was repeated, except
that the acid added at the A-stage was changed to sulfuric
acid.
The whiteness of the finally-obtained pulp after the
last D-stage was 86.4~, and the viscosity 16.3 cps.
Thus, it is known that sulfuric acid works as good as
acetic acid to attain a good result.
Comparing Example 9 with Comparative Example 2, it is
known tha~ the level of whiteness of the pulp obtained in
the latter is lower than that obtained in the former. From
this, it is understood that the pH at the A-stage must be
4.5 or lower and is desirably 3.0 or lower.
Example ll
Bleaching according to 0-Xa-Ep-H-D:
An oxygen-delignified hardwood Kraft pulp was treated
in this example, according to a bleaching process of
0-Xa-Ep-H-D in which the amounts of the chemicals added and
-the conditions used were as follows:
O-stage This was the same as that in Example l. After
this stage, the Kappa number of the pulp was 9.4, and the
viscosity 24.~ cps.
Xa-stage: 0.015~ of acid-resistant xylanase, 1'Xylanase
Amano P" (supplied by Amano Pharmaceutical Co., Ltd., which
- 40 -
r ~
exhibits a xylanase activity of 8,000 u/g at pH 1.8, and
acetic acid were added together to the pulp, whose
consis~ency was adjusted to 10%. The amount of the acid
added was adjusted to control the pH of the pulp to 1.8.
The preparation was heated to 50C and allowed to stand fo~
3 hours to react. Thereafter, this was diluted to 1~ and
then filtered and washed through a 5A filter paper using
a Buchner funnel. After the washing, the Kappa number of
the resulting pulp was 6.8, and the viscosity 20.2 cps.
Ep, H and D-stages: These were the same as those in
Example 1.
The whiteness of the finally-obtained pulp aEter the
last D-stage was 85.5~, and the viscosity 16.5 cps.
The amount of AOX from the bleaching effluent was 0.86
kg per a ton of the pulp.
Thus, it has been confirmed that, by the additional
treatment with the enzyme that exhibits activity in an
acidic environment, a bleached hardwood Kraft pulp having
ordinary whiteness and viscosity can be obtained without
employing molecular chlorine.
Example 12
Bleaching according to O-A-Xa-Ep-H-D:
An oxygen-delignified hardwood Kraft pulp was treated
in this example, according to a bleaching process of
O-A-Xa-Ep-H-D in which the amounts of the chemicals added
- 41 -
~3~
and the conditions used were as follows:
O-stage: This was the same as that in Example 1. After
this stage, the Kappa number of the pulp was 9.4, and the
viscosity 24.4 cps.
A-stage: This was the same as that in Example 1. After
this stage, the Kappa number of the pulp was 8.0, and the
viscosity 22.2 cps.
Xa-stage: 0.015% of the same enzyme as that used in
Example 11 and acetic acid were added together to the pulp,
whose consistency was adjusted to 10~. The amoun-t of the
acid added was adjusted to control the pH of the pulp to
1.8. The preparation was heated to 50C and allowed to
stand for 3 hours to react. Thereafter, this was diluted
to 1% and then filtered and washed through a 5A filter
paper using a Buchner funnel. After the washing, the Kappa
number of the resulting pulp was 6.1, and the viscosi-ty
19.8 cps.
Ep, H and D-stages: These were -the same as those in
Example 1.
The whiteness of the inally-obtained pulp after the
last D-stage was 86.9%, and the viscosity 16.3 cps.
The amount of AOX from the bleaching effluent was 0.81
kg per a ton of the pulp.
Thus, it has been confirmed that a bleached hardwood
Kraft pulp having ordinary whiteness and viscosity can be
- 42 -
~2:13~9~
obtained without employing molecular chlorine. In
addition, it has also been confirmed that the effect of the
enzymatic treatment with the enæyme that exhibi-ts activity
at a pH of 4.5 or lower is promoted more when combined with
acid treatment.
Comparative Example 3
Bleaching according to 0-X-Ep-H-D:
The same hardwood Kraft pulp as that treated in
Example 11 was treated in the same manner as in Example 11,
except that xylanase, "Cellulase Amano CT-4" (supplied by
Amano Pharmaceutical Co., Ltd., which has an optimum pH
value of 5.5 and exhibits a xylanase activity of 100,000
u/g at pH 4.5 was used as the enzyme for the enzymatic
treatment. After the enzymatic-treatment, the Kappa number
of the pulp was 8.2, and the viscosity 22.3 cps.
The whiteness of the finally-obtained pulp after -the
las-t D-stage was 83.4%, and the viscosity 16.1 cps.
In Comparative Example 3 compared with Example 11, the
conventional enzyme having an optimum pH value of 5.5 was
used and the enzymatic treatment was conducted a-t a pH
value lower than 4.5 while omitting the bleaching with
chlorine, with the result that the level of whiteness of
the pulp obtained therein was obviously lower than that of
the pulp treated by the conventional process using
chlorine.
- 43 -
~130~
Example 13
Bleaching according to 0-Xa-A-Ep-H-D:
The same process as in Example 12 was repeated, except
that the sequence of A-Xa was reversed and the pH at the
Xa-stage was adjusted to A.5 or lower making use of the
washing effluent from the A-stage in Example 2. The Kappa
number after the Xa-stage was 6.1, and the viscosity 19.8
cps .
The whiteness of the finally-obtained pulp after the
last D-stage was 86.7~, and the viscosity 16.7 cps.
Thus, it is clear that the sequence employed in this
example attains a higher level of whiteness than that
attainable by an ordinary bleaching process using chlorine.
Example 14
Bleaching according to A-Xa-C-Ep-H-D:
The same process as in Example 12 was repeated, e~cep-t
that the 0-stage was omitted, the reaction temperature at
the A-stage was changed to 40~C and the C-stage was added.
The amount of chlorine added at the C-stage was controlled,
according to the Kappa number after the Xa-stage.
The whiteness of the finally obtained pulp after the
last D-stage was 87.1~, and the viscosity 16.6 cps.
Thus, it is known, as compared with the conventional
bleaching process using chlorine but not employing the
A-stage and the Xa-stage (Reference Example 2), that the
- ~4 -
21~3~0~
sequence of the present invention employing the A-stage and
the Xa-stage accomplishes an ordinary level of whiteness
while reducing the use of molecular chlorine by about 35%.
Example 15
Bleaching according to O-A-Z-Xa-Ep-H-D:
The same process as in Example 12 was repeated, except
that the following Z-stage (ozone treatment) was added.
Z-stage: The consistency of the pulp after the A-stage was
adjusted to 10~ and it was placed in an autoclave equipped
with a high shear rate agitator (supplied by Nitto
Autoclave K.K.), and a volume of ozone gas corresponding
to 0.4~ based on ~he pulp was fed to the pulp. The ozone
gas (ozone concentration: 5~) was generated by an ozone
generator (supplied by Nippon Ozone K.K.). Then, the
autoclave was sealed. After charging nitrogen gas into the
autoclave to raise the total pressure to 5 bar, the pulp
was kept at 35C under agitation for 30 seconds to
terminate the reaction. The pulp was taken out of the
autoclave, diluted to have a consistency o 1~, and then
filtered and washed through a 5A filter paper using a
Buchner funnel. The Kappa number of the pulp after the
Z-stage was 3.9, and the viscosity 17.8 cps.
The whiteness of the finally-obtained pulp after the
last D-stage was 88.8~, and the viscosity 15.6 cps.
Thus, it is clear that this sequence accomplishes a
- 45 -
~3~90~
substantially higher level of whiteness than the
conventional bleaching process using chlorine.
The results of all the aforeisaid examples, comparative
examples and reference examples are summarized in the
following Tables.
- 46 -
~13~9(~
, ._ ___ .__
t~4 ~rllJl ~ r~ o ~
Q ~C CO CO ~_1 ~1 0 ~ ~ U) O O ~ O ~1 ~ ~ O O ~0 O r~) ~ \D
X ~4 ~I N ~1 0 ~ ~ ~1 [-- ~1 ~1 Ir) ~
E~ O
.
Q In
m
P ~ oo ~ ,1 ~1 o u~ ~1 ~ ~ u~ I o ~ o ~ ~ o o o In o
E O ~ ~ ,1 ol N f`l ~J 1~
_
~1 ~ U~
v a~ ~, In Ln ~ r o In
p ~ ~ co ~ ,i o Ln ,1 ~ ~ ~ l l l o o In o ~ r-
X~I `1 ~I o ~
a~ m m m ~ r
P ~:~ oo ~ ~1 ,i o In ~ CJ~ ~r
Q~ X O ~1 ~ ~1 o
~1
~ .
Q P4 n Ln ~ Ln ~r ~r t~In o N ~1 1~ m ~1
P r~l ~ a~,1 ,J o m ,1 a~ ~r Ln I O ~ o ~ N O O Ln O ~ C\ r-l
O ~1 ~ ~1 o ~~ ~1 ~` ~ ~ In ~
_
m
m In In~ CO m ~ ~ ~ In o ~ o ~ ~r co
~1 P4 . .. . . . . . . . . . . . .
PL ~ ~ ~:4 ,~ ,1 o m ,t ~ ~r In I o ~ o ,I c~ ~ o o n o r~ ~o o
e ~3 ,, ~ ~1 0 ~ ~ ,~
~ O
d~ ~ dP dP ~ n d~
a4 O C g4 O .C g4 0 5~ ~ 4
~ (11 ~1 ~1 ~J
.,~ ~ ~ ~ ~ ~ ~ Q~ ~ ~ a~
s~ a~ ~ ~ ~ e ~ t, ~ ~ ~E ~ ~: ~ e ~
1~ ~ ~ ~ ~ ~ ~ 1~ a) c) V :~ V ~
(~J ~ ~ ~rl 1~ rl ~ ~ (~ ~
m u~ ~ O OU ~ ~ O .r~ X.U~ ~ ~ o e U' ~ ~ o
C4 ~ ~) mu~ 04 a)a4 ~ V O Ul a4 ~ a4 c~ ~ r,n a4 ~ a4 ~
a4 ~ o ~ e e 04 U3 Q~ h ~ e e P4 U~ N f~i e e a4 u~
~ .,, ~ ~ o aJ ~ .,, O Q) O m ~ ~ o m
X ~ O Z 0 1.) ~ 1~ C4 ~ P4 ~ ~ X ~ a~ ~.) p4 U V X >
~ ~ ~ G~
~1 a4 JJ ~ ~
Q -1 U~ U~ U~
g t4 O ~C X
_
_ 47 -
~30~S
, _
r m o co c~
~ ~ l l l l ~, co ~1
o
a
~ 1111 ~ o~
~ o ~
V ~ ~ ~ lO ~
~o l I l l ra ~
c
_ ~ ~ ~t ~ O O ~ ~ ~D
a v
8 ~OD
w l l l l c~ u~ ~
~ ~ ~,
~ d~ 0~ d~ ~
r C ~ u 3 c~ a) v
4 ~n o ~n o ~ ~ o
O ~J~ # 3-:>
~ _ ~
a~ I a) ~
~ m ~ ~
ul
1~ ~ .rl R.
l w~ I ~1
-- 48 --
~3~~S
,, _ _
1 ~ ~ In ~ r ~n In U~ CO
e ~ ~ ~ ~ ,~ O m ~ ,, ~O, O ~ O ~ ~ ~ O ~ O u~ O r~
X O r1 N~1 0 N ~1 1 N ~ ~1 11~) N
.
a~
r~ ~ Lt~ N ~ r') O U~ ~ .
Q ~ CO OD I I I I I I I ~ I O N O ~1 00 CO O ~1 0 Il') O ~ I~ ~D
~3 ~ N N ~1 ~ ~I N # ~1 Il~ ~I N
1:~1 ~
_ _
r- ~ ~ In
r l Q~ Ln Ln N E~ ~D CO O 1~ ~ 0
EQi CO a) ~ If~ I O N O O ~ ~ O ~1 0 111 O r~ 111 U~)
(1~ X r-l N N r-l ~ ~I r~l N # r~ vl N
i~ ¢
_
U~
r 1 p:~ Il') Ll~ N Ir) ~ r~ O ~ N r~
O E~l 0 CO ~ In I O N O r-l 0 0~ O ~1 0 11'1 0 ~) ~~
e ¢ ~I N N ~I N ~I N 1: r~ r~ N
N _
,-1 a) N
E~ C ~ m U~ m
~ eQ ~Q r) CNt)
I) ~ C~
~sl
_
U~ ~ U~
~1 ~1 1-~) Ir) N Lt') f~ ) O ~ N ~r
~1 ~ CO CO ~ 1~) 1 0 N O r~ D O r~ O 11-) 0 t~
X r-l N N r-l ~r r~ N 1: r~ ) r~ N
~1 ~
_
~ dP OP ~ O ~ U~ dP dQ d~~ C U~
Q~ O .C ~Q)l L vQ`~ e vQ`~
C a~ ~ ~ c~v ~ ~ ~ r~ a) Q)
.C C . v ~,~ v ~ Q u~
V Q) ~ ~ ~ ~ ~ ~ u ~ ~ e a~ O C ~ e
~ ~ ~ ~ C~ ~ V~ ~ ~ ~ ~ V :~ ~ ~ ~ V
o a~ ~ ~ C -1 r J~ ~1 C -1
m ~a ~ O O ~ o .u x.rn ~ ~ O e ~ o
Q~ u ~ m tn ~4 ~ Q~ U ~ O U~ Q~ u ~ m ~ Ql U
o C e e Q~ U~ tl) ~I C e e Q~ U~ N O ~ e e
~; ~ o~ z; g ~ ~ ~ U mQ, O~ C~ Z u m, JJ ~ X ~
- r-v -
~QI 1~ al 1~
g P~ O l X
- 49 -
~:~309-0~
~ l l l ~ _
~1~ g , ,~
aJ
o~ ~ ~
~ m ~ ~ P~ ~ co
E ~ N ~) O O ~ ~
_ 2)
r~
a) m O ~ ~ ~ ~n
~ N~lnO )~(
` a u, _
a ~ m N IJ') u~
'v E'll~ ~
_ N Ql
N C I C~ ~1 ~ e ~ ~
~ ~ ~ r~ o o O ~ r~
al a~ I r.3 ~ co
W I C~ U7 .
e' ~ o
~ ~ x~ c
~1 I q a~ al N ~ O
~ ~ ~1 ~ O O C C ~ ~D
E~l (~1 N ~
.,~ I U~
~ W~ ~ cQ) .~
t~ u~ ~ 1~
C C ~ C ~ V ~ C ~ V
M u~ N C ~3 u ~ .Vrl u~ ~r~
~! O IL) ~ .C ~ ~
.L~ ~ 3 ~ v
_
81 m~ .~n~
Ul I U~ C ~I
l ~ ~ ~
_
-- 50 ~
'~3~9~
. __
a Ln Ln N Lr1 r1 Ln ~ 0~ ~r ~
CL t~ C~ a: I I I I I I I ~ n IO N O ~ 1-- ~D O~1 O ,1 O ~ ~ ~r
~ ~C ~ N N ~ ~r ,I N ~ ,I Ln ~ N
W ~
~ Ln
v c m Ln Ln ~D coLn ~r ~r O a~ N ~
C W C~ CO ~ ~ O Ln,1 ~ ~r I I I I I I I I O ~ O ,~ O ~ 0~ N
~4 ~ O ~ N ~ O N # ~ Ln N
N a ~
al C4 Ln n ~D ~ n ~ ~r N Ln O N O 0~ D
Wcn c~ ~ ~i o n ~ ~ ~ Ln I o ~ o ~ CO ~ o ~l o ~ o
X ~ ~ N,1 O N ~ ~ t~ N # ~ In ~
E':l _
~ a ~
~ CLI n n~ a\ n er ~r O CO a~ N
.. Wco c~ ,~ ~ o n ~ ol ~r l l l l l l l l o ~l o ~ o ~ ~D o
E~ X ~ N ~ O N # ~1 n N
WX
_
a) o
R~ ~ ~ L
E~ ,, m~ Ln LnU:~ 00 Ln ~r ~ ~ ~ n cn o o ~ N r~
W CO OD,1 ~ O Ln ,1 G~ ~r O I O N O ,1 ~ N O ,1 O Ln O r~) ~D O
~3 O ~ N ~ ON -- ~ r~N K ,J Ln N
W
_ .
~N ~ ~
v ~ m n n ~ a~ Ln ~ ~r o Ln N N O ~P CO Ln
~1 ~4 ~ . .. ... . ... . . ..
CO CO ~ ~ o n ~ o~ ~r ~ I o l~ o ~ ~ ~r o ~ o Ln o ~ r~ ~
~ I O ,I N ~ O N ~ I~ N # ,I Ln N
C.)~ _
U~ d~ ~ o ~: ~4 o ~ ~4
Q~ ~ ~4 ~J ~ ~ ~ o ~J
.,~ a) ~ ~ ~ ~ ~ r~
.C ~ Q 1~ ~ R ~ ~) ~ Q U~
O ~ ~ ~ C ~ u~ >1 0
~ 1 ~ V ~ ~ V ~ V _.r~ V~5 V ~ r~ a~ v ~ v-
a~ ~ C ~ ~ ~ ~ v ~1 v
U~ ~ O LQ Lq Lq -1 X ,~ a) Lq t~ LQ Lq
~4 t~ ~ m Lq C4 O ~ 4 0 V ~ 0 Lq ~4 a) ~4 u ~ v m Lq C4 a) ~ 4 0
~4 Lq 0 C E~ E3 C4 Lq ~ ~ C ~ Lq N Q) O ~ 4 Lq
~1 ~ ~ O ~ ~O ~ C ~) ~ O ~ C) ~
. ~: ~ O Z; C~ V ~ ~ :> 1~ ~ ~4 0 p~ V V ~ :> 0 n:l Z ~ ~4 V V ~ ~>
~ O
o ~ c) a
~ ~ ~ ~ V
RC ~ 4 ~ Lq V Lq
D 14 O ~1 X X
_
- 51 -
' 213~90~
,-~ ~ ~ a~ T
'~ a - u~
a ~ l l l l a ~ ~D
u E _
P~ l l I l O ~
W ¢ ~
'~¦ a Eu. Ln
c ~0 llll .~ o~,l
c O a
~ m
Q ~ O U~ P
~ ~¦ a a ~ .,. a
Q.~ l l l l .C o~ O
E ~ ~1 _ JJ
:~ dP d~ O d~
~ I~u~
~ m
u ~Q' a
~ 13~90~
Table 4
Example 13
Bleaching Sequence
OXaAEpHD
_ ~ .. .. . .. _ . .__
Unbleached Kappa number 18.5
Pulp Viscosity cps 28.5
O-stage 2 dosage % 1.6
NaOH ~ 1.8
consistency~ 10
temperatureDC 105
time hr 1.5
Kappa number 9.4
viscositycps 24.4
Xa-stage enzyme dosage % 0.015
peroxide ~ *l
. consistency ~ 10
pH 1.8
temperatureC 50
time hr 3
Kappa number _
_ viscositycps _
A-stage enzyme dosage % 25
acetic acid% *2
consistency% 10
pH 1.8
temperature C 70
time hr 1.5
Kappa number 6.1
viscositycps 19.8
.......
Ep, H ~ D stages *3
._
Finished pulp whiteness~ 86.7
_ viscosit~cps 16.7
*l pH adjusted using effluent of A-stage and acetic acid.
*2 addition amount was adjusted to control pH.
*3 conditions for all the examples and comparative
examples are common, and are summari2ed in Table 6.
L 3 ~
Table _
_
Bleaching Sequence Example 15
OAZXaEpHD
Unbleached Kappa number 18.5
Pulp Viscositycps 28.5
O-stage 2 dosage % 1.6
NaOH % 1.8
consistency% 10
temperatureC 105
time hr 1.5
Kappa number 9.4
viscositycps 24.4
.
A-stage acetic acid% 25
peroxide %
consistency% 10
pH 2.2
temperatureC 70
time hr 1.5
Kappa number 8.0
viscositycps 22.2
. . _ ..
Z-stage O3 dosage ~ 0.4
consistency% 10
temperatureC 35
time hr 0.5
Kappa number 3.9
viscositycps 17.8 .
..
Xa-stage enzyme dosage % 0.015
acetic acid% *2
consistency% 10
pH 1.8
temperatureC 50
time hr 3
Kappa number 3.2
viscositycps 1~.9
. ._ . _
Ep, H & D stages *3
. .
Finished pulp whiteness % 88.8
viscosity cps 15.6
- 54 -
~13~90~
*2 addition amount was adjusted to control pH.
*3 conditions for all the examples and comparative
examples are common, and are summarized in Table 6.
Table 6
stageconditions data
... ... _
Ep NaOH ~ 0.8
H2O~ % 0.15
consistency % 12
temperature C 65
time hr 1O5
._ . ~.
H Na-hypochlorite ~ 0.8
consistency % 10
temperature C 65
time hr 2
_ . -- _
D ClO2 % 0.6
consistency % 10
temperature C 65
time hr 2
INDUSTRIAL APPLICABILITY
According to the present invention, a bleached
chemical pulp having whiteness and viscosity comparable to
or higher than those obtained by conventional processes can
be obtained. In addition, a much higher level of whiteness
of the bleached pulp can be practically attained, using the
methods embodied herein. According to the methods embodied
herein, the use of chlorine or chlorine-containing
compounds may be reduced while eliminating the use of
molecular chlorine, to obtain a bleached pulp having an
- 55 - ~ :~
-` 2 ~ 9 0 ~
ordinary level of whiteness. Accordingly, the discharge
of to~ic compounds like dioxin from the pulping process of
the presen-t invention can be reduced. According to the
present invention, an opportunity for recirculating the
organic substances to be removed from the bleaching step
back to the effluent-recovering step in-the pulping process
is provided, whereby a further reduction of environmental
pollution becomes possible.
- 56 -
