Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ ICICA 794
"Biobleachina Process"
FIELD OF TKE_INVENTION
This invention relates to a process for the bleaching
of lignocellulosic material employing an enzymatic treatment
follo~ed by a chlorination stage.
DESCRIPTION OF THE REL~TED ART
Lignocellulosic material in fibrous form is in wide
commercial use as a raw material for the manufacture of
paper~ cardboard, construction board, and the like. The raw
material is usually wood whose principle components are
cellulose, ancl a three-dimensional macromolecule - lignin,
which is considered to be embedded in a matrix of cellulosic
and hemicellulosic polysaccharides. It i5 generally accepted
that the bonding that exists between the different
components is established through linkages of different
chemical nature. For instance, blocks of lignin are thought
of as being associated through hemicellulose chains, the
hemicellulose being another component of lignocellulosic
material.
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other suitable lignocellulosic materials include wood,
bagasse, grasses and the like.
In order to produce strong and bleachable paper-making
fibres, the lignocellulosic material must be treated to
remove lignin, and normally, the initial part of this
treatment takes place in a digester in the presence of
chemicals such as sodium hydroxide and sodium sulphide ~to
produce a kraft pulp) or sulphites, usually sodium or
magnesium, (to produce a sulphite pulp), thus producing
chemical pulps. The removal of lignin is referred to as
delignification.
The lignin content of pulps is measured by a
permanganate oxidation test according to a standard method
of the Technical Association of the Pulp and Paper Industry
(T~PPI), and is generally reported as a Kappa Number.
The chemical pulp from the digester still contains an
appreciable amount of residual lignin at this stage, and in
some cases is suitable for making construction or packaging
paper without further purification. For most applications,
such as the manufacture of printing, writing, and sanitary
papers, however, the pulp is too dark in colour and must be
brightened by bleaching. During the initial stages of
bleaching, further delignification of the pulp occurs.
~he conventional method for further delignification and
bleaching of pulp has been to employ a variety of
multi-staged bleaching sequences, including anywhere from
three to six stages, or steps, optionally with water washing
between any of the steps. The objective in bleaching is to
provide a pulp, in the case of chemical pulps, of
sufficiently high brightness and strength for the
manufacture of paper and tissue products.
Characteristically, pulps of brightness 85% to 90% IS0 are
produced for commercial sale. Pulps of higher brightness can
be produced from certain unbleached pulp types but generally
at higher cost and at the risk or expense of pulp strength
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quality losses. The asymptotic limit of brightness that is
encountered for a given pulp type in conventional bleaching
processes is referred to in the pulping industry as a
brightness ceiling. This brightness ceiling is the
brightness level, over which brightness the process of
further bleaching would be considered too detrimental to the
quality of the pulp, prohibitively uneconomical, or
unachievable for certain materials.
Traditionally, the bleaching sequences have been based
on the use of chlorine and chlorine-containing compounds.
Some of the chlorine-containing compounds that are used are
chlorine, chlorine dioxide, and hypochlorites, usually using
sodium hypochlorite, which are used in a chlorine stage,
denoted as C, a chlorine dioxide stage, denoted as D, and a
hypochlorite stage, denoted as H. Chlorine, with or without
admixture of chlorine dioxide, is commonly employed in the
initial stage of bleaching of the chemical pulp, followed by
e~traction of the chlorine-treated pulp in an aqueous
alkaline medium, together denoted C-~ when no chlorine
dioxide is present. The chlorine charge (or chlorine plus
chlorine dioxide charge expressed on a chlorine oxidizing
equivalent basis) in the C stage is proportional to the
lignin content, and thus the Kappa Number of the pulp being
treated. The alkaline extraction stage is used to solubilize
and remove a major portion of the chlorinated and oxidized
residual lignin.
During bleaching with chlorine, some degradation of the
cellulosic fibre is observed, which degradation will
adversely effect the physical properties of the bleached
pulp. The degradation of the cellulose fibres of the pulp
can be indicated by measuring the viscosity of a solution of
the pulp in cupriethylenediamine according to a further
TAPPI standard method. Pulps having little degradation of
the cellulosic fi~re will generally have a high viscosity,
while pulps with fibre degradation will generally have a
lower viscosity.
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Chlorine dioxide is generally added to the chlorination
stage, in partial substitution of the chlorine used on an
oxidizing equivalent basis, in order to inhibit this
degradation of the cellulosic fibre, and thus maintain a
high viscosity. The degree of chlorine dioxide substitution
for chlorine ranges from "low" substitution, of about 10%
substitution of the total equivalent charge, to "high"
substitution levels of up to about 80% or higher of the
total equivalent charge. The amount of fibre degradation is
usually dependent on the level of substitution. However, it
has been observed that the order of addition of the chlorine
and the chlorine dioxide can affect the viscosity of the
bleached pulp. Under appropriate conditions, the viscosity
of the pulp can be maintained at a high level by timing the
sequence of chlorine and chlorine dioxide. Given the
relatively low cost of chlorine and the relatively high cost
of chlorine dioxide, the pulp industry has generally
attempted to minimize the level of substitution, and has
used timing of addition as one method of maintaining pulp
viscosity.
;It is generally acknowledged in the pulp industry, and
in the scientific literature, that addition of chlorine
dioxide at higher chlorine dioxide substitution levels, of
for example 30% of higher, prior to the addition of the
chlorine can provide viscosity protection to the pulp, and
has an additional advantage of decreasing the total
equivalent chlorine charge used in the chlorination stage
required to bleach the pulp to a given final brightness
level when compared to a chlorination stage wherein the
chlorine dioxide is added after the chlorine, or mixed into
the chlorine. The chlorine dioxide addition usually precedes
the addition of chlorine by 5 to 60 seconds, depending on
the process conditions used. This process is commonly
referred to as sequential addition of chlorine dioxide and
chlorine~
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While the processes of pulp bleaching have been studied
and practised by the industry for a number of years,
increasing public concern about the environment has created
a driving force for the replacement or reduction of the
chemicals, particularly chlorinated compounds, and more
particularly chlorine, used in the bleaching of pulps. One
promising approach to minimizing the use of bleaching
chemicals has been through the application of biotechnology
in the bleaching process.
Enzymes have been studied for their use in the
treatment of lignocellulosic material. For example,
ligninolytic enzymes, particularly from white-rot fungi,
have been shown to degrade lignin to varying degrees. Also,
cellulase enzymes are well known to degrade cellulose and
are of commercial interest in the food industry and in the
manufacture of alcohols. In the manufacture of pulp for the
purpose of paper-making, the effect of a cellulase enzyme
would be detrimental owing to the resulting decrease in the
degree of polymerization of the cellulose, and thus to the
lowering of the pulp viscosity, which would occur.
In view of the hemicellulose component of
lignocellulosic material, there have been studies reported
on the effects of a xylanase enzyme on wood pulps. The
xylanase has been found to selectively react with the xylan
present as paxt of the hemicellulose.
The concept of biological bleaching of xylanases
emerged from efforts to selectively remove hemicellulose
from chemical pulps and prepare dissolving grade pulp
suitable for derivatization to cellulose acetate, rayon, and
the like. It was later found that the use of xylanase
pretreatment of pulp, prior to bleaching, caused reductions
in lignin content, and thus resulted in savings in the
amounts of chemicals that were needed during bleaching. In
particular, the use of a xylanase which is cellulase~free
3~ has resulted in lower bleaching chemical requirements, and
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improvements in pulp viscosity. The reduction in chlorine
usage, in part due to the xylanase treatment of pulp has
been shown to also lower the levels of chlorinated organics
found in the pulp mill effluents.
Surprisingly, it has now been found that to bleach to
constant brightness, lower chemical requirements and/or
higher viscosity of bleached pulp can be obtained, in a
sequence which involves pretreatment of the pulp with
xylanase, when the sequence of addition of chlorine dioxide
and chlorine in a chlorination stage is reversed so that
chlorine is added to the pulp prior to the addition of the
chlorine dioxide.
Summary of the Invention
Accordingly, the present invention provides a process
for the bleaching of a lignocellulosic material comprising:
i) treatment of a chemical pulp with xylanase to
produce a xylanase pretreated pulp; and
ii) chlorination of said pretreated pulp with a
chlorine and chlorine dioxide,
wherein said chlorination of said xylanase pretreated pulp
is characterized in that all or a portion of the chlorine
used in the chlorination is added to to pretreated pulp
prior to the addition of the chlorine dioxide.
In the process of the present invention, the chlorine
dioxide is preferably added to the pretreated pulp within l
to 120 seconds, and more preferably within 5 to 60 seconds,
of the addition of the chlorine. ~he optimum time of
addition of the chlorine dioxide and chlorine will depend on
the pulp type, the reaction temperature, the consistency
(percentage of the pulp content of an aqueous solution of
pulp on a weight basis) of the pulp, and the like, and can
be determined by experimentation.
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Treatment of the chemical pulp, and preferably a kraft
pulp derived from wood, with xylanase can be conducted in
any suitable xylanase treatment process known in the art.
Preferably, however the xylanase treatment used in the
present invention comprises treating an aqueous suspension
of the lignocellulosic material, at a consistency of from 1
to 20% by weight, preferably 2 to 12%,with a
xylanase-containing material using an application rate of
from 0.1 to 500 IU/g of oven dried pulp, at a temperature of
between 20 and 80~C for a period of between 0.5 and 48
hours, and, more preferably, for a period of between 2 to 3
hours. Preferably, the temperature is about 50~C. Further,
the treatment is preferably conducted in an aqueous medium
at a pH of from about 4 to about 8, and more preferably,
within a pH range of from 5.5 to 6.5. The medium may
optionally be buffered to control the pH. Suitable buffers
include, but are not limited to acetate buffer and
acetate/citrate buffer.
The term IU (International Unit) is that amount of
enzyme which catalyses the formation of 1 ~mol of xylose per
minute from a solution containing xylan.
The xylanase of use in the practice of the present
invention is preferably substantially cellulase-free and is
obtained by the fermentation of any suitable
xylanase-producing microorganism such as a
xylanase-producing bacterium. The microorganism may be a
naturally occurring strain, or a mutant thereof, or a strain
produced by yenetic engineering, i.e. a recombinant strain,
to increase the production of the xylanase and/or to produce
a more pure xylanase mixture, e.g., substantially
cellulase-free.
The term "substantially cellulase-free" means that
there is not sufficient cellulase present to effect
unfavourable hydrolysis of glucosidic linkages. This
hydrolysis would be detrimental and unwanted in the
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treatment of lignocellulosic material for the purpose of
improving the properties of the material. The amount of
cellulase that may be tolerated depends on the particular
objective in the practice of the invention.
Preferably, the xylanase is obtained substantially
cellulase-free from a microorganism of the geni Trichoderma,
~scherichia, Bacillus or of the genus Streptomyces, said
microorganism having been genetically engineered to exhibit
substantially cellulase-negative activity. More preferably,
the xylanase is obtained substantially cellulase-free from a
recombinant xylanase gene-containing microorganism of the
species Streptomyces lividans. Further details on the
production of suitable xylanase materials are contained in
South African Patent No. 90/0897.
Where an enzyme mixture containing xylanase-also
contains unacceptable amounts of cellulase, the cellulase
may be removed by any method known for the purification of
xylanase, or the cellulase is selectively rendered inactive
by any acceptable chemical or physical treatment.
The xylanase may be applied as it is produced in a
fermentation broth, or a concentrated mixture ther~of, or as
a mixture prepared from either a more concentrated mixture
of the xylanase or a dried preparation of xylanase. The
xylanase treatment can be conducted in any suitable reaction
vessel, and may be readily applied in the pulp mill's
brownstock storage chest where the process conditions may be
compatibIe with the optimum conditions for the xylanase
action.
The xylanase treatment of the lignocellulosic material
may be carried out with or without agitation. At the end of
the time period for the xylanase treatment, the resultant
treated pulp may be used directly or thickened. Optionally,
a wash is included.
The filtrate containing residual actiYe xylanase from
3~ the thickening and/or washing stage following the xylanase
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treatment is preferably recycled by applying the filtrate to
the chemical pulp to be treated with xylanase.
The chlorination stage may be conducted in a manner
similar to the processes currently practised in the
industry. Preferably, the chlorination process comprises
treating an aqueous suspension of the pretreated pulp, at a
consistency of betwePn 1 to 20-% by weight, more preferably
from 2 to 10%, with chlorine and chlorine dioxide having a
total chlorine oxidizing equivalent of between 0.1% and 10%,
and more preferably between 1% and 6%, and using a
substitutio~ level of chlorine dioxide for chlorine such as,
for example, from 10 to 90%, and more preferably from 25 to
80%, at a temperature of between 20 and 80C for a period of
between 10 minutes and 10 hours. Preferably, the
15 ! chlorination is conducted at a temperature of between 30 and
60C for a period of between 20 and 40 minutes.
The chlorine oxidizing equivalent, also termed as the
total available chlorine, is the sum of the chlorine content
! plus 2.63 times the chlorine dioxide content, and is
expressed as a percentage of the oven dried (od) weight of
the pulp.
The chlorination charge may also be expressed as a
chlorine "multiple" which is the total available chlorine
value divided by the Kappa number of the pulp.
Preferably, the chlorine dioxide substitution level is
such that chlorine dioxide provides between 10 to gO, and
~ore preferably from 25 to 80% of the total chlorine
oxidizing equivalent of the chlorine and chlorine dioxide.
In a normal chlorine dioxide production route, some chlorine
may be present in the chlorine dioxide. This chlorine does
not need to be removed, and is not detrimental to the
process of the present invention.
~ fter chlorination, the pulp can be treated in any of
the known bleaching processes which typically follow
chlorination. These processes preferably comprise following
ICICA 794
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the chlorination stage, described hereinabove according to
the present invention, in sequence, by the following
sequence:
i) extraction, denoted as E, in an aqueous medium with
alkali base, and
ii) treatment in an aqueous medium with chlorine
dioxide, denoted as D.
This treatment process can be dPnoted, in its entirety,
as X-(CD)-E-D, wherein X denotes a xylanase treatment, and
(CD) denotes a chlorination stage practiced in accordance
with the present inventiGn, i.e. the sequential addition of
the chlorine followed by chlorine dioxide after a selected
time delay.
In the process of the present invention, additional
treatments known in the art of pulp bleaching may also be
conducted in any reasonable manner, and order, as is
conventionally practised or known in the art. For example,
the process of the present invention may be followed by the
stages -E-D-E-D. Additionally, the process of the present
invention may be followed by a sequence involving oxidative
extraction wherein oxygen, air, or hydrogen peroxide is
added to the sodium hydroxide used in the extraction stage.
The xylanase treatment of the present invention may also be
preceded by an oxygen treatment stage for further reduction
of the lignin content of the chemical pulp.
Other stages can be utilized in accordance with the
practice of the art and may include, the use of a hydrogen
peroxide stage, denote as P, and a hypochlorite stage,
denoted as H.
The process of the present invention provides a
bleached product having satisfactory brightness and
viscosity that is equivalent to or better than that observed
for pulps bleached to the same extent by conventional
methods. Also, higher brightness levels can be practically
achieved using the present process, insofar as these
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ICICA 794
brightness levels could only be achieved at the expense of
significantly higher chlorine-containing chemical usage
and~or detrimental loss of viscosity.
The process of the present invention provides a
bleached pulp using lower amounts of chlorine-containing
compounds, thereby reducing the cost of chemicals used in
the bleaching process, and also reducing the amount of
chemicals present in the pulp mill effluent.
In a further aspect, the present invention also
provides a lignocellulosic material produced by a process
according to the present invention as described hereinabove.
DESCRIPTION OF THE PREF~RR~D 2MBODIMENTS
EXAMPLES
The invention will now be illustrated by way of
reference only, to the following non-limiting examples. In
the examples, the pxoperties of the pulps used were measured
by the following standard methods:
Kappa Number TAPPI Method T-236 M-76
Viscosity TAPPI Method T-230 om-82
Brightness TAPPI Method T-452 om-83
The xylanase enzyme used in all examples was produced
by batch fermentation of Streptomyces lividans tpIAF18]~
This high xylanase-producing, cellulase-negative
microorganism was obtained through genetic engineering. The
preparation of the enæyme is described by Bertrand et al.
("Expression of the xylanase gene of Streptomyces lividans
and production of the enzyme on natural substrates",
Biotechnol. Bioeng. 33 (1989), p. 791-794)o The activity of
the enzyme preparation was 1033 IU/ml. A stock solution of
1.5% (w/v) of the water soluble fraction of oat spelts xylan
was dissolved in 50 mM McIlvaine's buffer, pH 6.0 and was
used as the assay substrate. Assay reaction times were 10
minutes and reduciny sugars produced were determined,
ICICA 794
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relative to standard xylose solution, according to the
dinitrosalicylic acid method of Miller (Analytical Chem. 31
(1959), p. 426-428).
The xylanase treatments were carried out on 200g (od)
batches of hardwood kraft pulp of mixed furnish, as obtained
from an Eastern Canadian mill. The pulp was adjusted to pH
5.5 to 6.0 by addition of lM sulphuric acid solution.
Xylanase was added to the pulp and mixed in a Mark II
Quantum Reactor at a consistency of 6% (w/w). The enzyme
reaction was carried out while maintaining the pulp
temperature at 50C for 2 hours. An enzyme charge of 0.012%
on pulp was applied which was equivalent to 85 IV/g of pulp.
Following the enzyme treatment, the pulps were washed at 1%
consistency in water and suction filtered. Control pulps
were prepared identically to the enzyme-treated pulps, with
replacement of enzyme with water.
The control and the xylanase-treated pulps were
sub~ected to a standard (C&D)EDED bleaching sequence in
which the percent of chlorine dioxide substitution for
chlorine was 40%. The term ~C&D) in this section refers to
the chlorination stage comprising chlorine and chlorine
dioxide. The order of addition of the two chemicals will
vary depending on the particular example.
During the chlorination stage, pulp consistency was
adjusted to 3.5% and the chlorination reaction, using 150g
(od) pulp samples was carried out in a stirred vessel at
40C for 30 minutes. For xylanase-treated pulps, a chlorine
multiple of 0.10 was applied. Control pulps were treated
with higher chlorine multiples of 0.15 and 0.20. Following
the chlorination stage, the pulps were washed with stirring
at 1% consistency in water and suction filtered. A caustic
extraction was carried out immediately following the washing
stage. In the extraction stage, pulp and sodium hydroxide
solution (2% on pulp) were mixed in a Hobart mixer, and the
mixture was transferred to a polyethylene bag wherein it was
heated for 1 haur at 70~
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~ he first chlorine dioxide stage, denoted D1, was
carried out on 20g (od) samples in sealed glass mason jars
using chlorine dioxide charges of 0.4 to l.2% on pulp,
unless otherwise specified, and at 6~ consistency and 70~C
for 3 hours. All D1 stages were optimized with sodium
hydroxide to achieve an exit pH of 3.5 to 4Ø The second
extraction stage was performed using caustic charges of 0.6%
on pulp with the same consistency, temperature and time as
the first extraction stage. A constant second chlorine
dioxide stage, denoted D2, charge of 0.4% on pulp was used
in all examples at a temperature of 70C for 3 hours at 6%
consistency.
The pulp used for all examples had the characteristics
shown in Table l. Following the xylanase treatment, it can
be observed that the xylanase treatment resulted in an
increase in viscosity and a decrease in Kappa number. This
improvement in viscosity confirms an enrichment of the
high-molecular weight cellulose fraction which occurs when
xylan is selectively removed.
Table l: Properties of Pulp before and after Xylanase
Treatment
Pulp Brightness Kappa No. Viscosity
(% IS0) _ (mPa.s)
Brownstock 31.3 13.2 28.8
Xylanase-Treated 33.9 ll.9 30.9
* - Brownstock pulp is the control pulp which was washed
under identical conditions as the xylanase treated pulp,
with the exception that no enzyme was added.
A sample of xylanase treated pulp was bleached using
sequences where chlorine dioxide was added 30 seconds before
the chlorine charge, where the chlorine dioxide was added
simultaneously with the chlorine charge, and where chlorine
dioxide was added 30 seconds after the addition of the
ICICA 794
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chlorine. The degree of substitution used was 40%, and a
chlorine multiple of 0.10 (equivalent to 1.12~ Total
Available Chlorine (TAC)) was applied to the first
chlorination stage. The results of the bleaching sequences
are shown in Table 2. In Table 2, a control sample of pulp
that was not treated with xylanase is also shown. The
chlorination levels of 2.63% TAC and 1.98 correspond to a
chlorine multiple of 0.20 and 0.15 respectively.
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ICICAN 794
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The addition of the chlorine dioxide prior to the
chlorine, as is a standard practice in the chlorination
stage of non-xylanase treated pulps, was clearly less
effective than when the reagents were added simultaneously,
or when the chlorine was added first. This is demonstrated
by a substantially higher Kappa number, and a lower
brightness of the first extraction stage.
After the first extraction stage, samples of each pulp
were treated with varying chlorine dioxide charges in the D
bleaching stage. The results in Table 2 clearly show that
the brightness for each level of chlorine dioxide charge in
the D1 stage, is higher for the sequence wherein chlorine
was added to the chlorination stage prior to the addition of
the chlorine dioxide.
The effect of mode of addition of chlorine and chlorine
dioxide is less apparent after the D2 chlorine dioxide
bleaching stage, however, the general trend of increasing D2
brightness could be observed for the sequence wherein
chlorine was added prior to the chlorine dioxide.
Further, in Table 2, it can be seen, from the control
experiments, that in order to achieve similar D2 brightness
levels to those obtained by the process of the present
invention, it is necessary to use a significantly larger
chlorine charge.
Accordingly, it can be seen that the process of the
present invention allows the pulp mill to reduce chemical
consumption to achieve a given brightness level, or to
bleach to higher brightness level with a constant chemical
cost and with no substantial decrease in physical
properties.
Having described specific embodiments of the present
invention, it will be understood that modifications thereof
may be suggested to those skilled in the art, and it is
intended to cover all such modifications as fall within the
scope of the appended claims.
