Note: Descriptions are shown in the official language in which they were submitted.
W094/20672 ~ ~I S ~ 81 o PCT~S94/02710
PROCESS FOR RT-T~'~r~TNG PULP
This application discloses a process for
delignifying and bleaching pulp that does not use
chlorine or chlo~ine derivatives.
R~CR~ROUND OF THE lNV ~:N'l'lON
.
Wood is a lignocellulosic material that is
predominantly cellulose, hemicellulose, and lignin.
Cellulose is a saccharide polymer composed of
linear D-glucose units. Hemicelluloses are linear
and branched saccharide homo- and heteropolymers of
five and six carbon sugars such as xylose,
arabinose, mannose, galactose, and glucose. Lignin
is a polymer composed primarily of methoxylated
phenylpropane units that have been randomly linked
by a variety of carbon-carbon and ether linkages,
resulting in a three-dimensional matrix.
In wood, the linear cellulose molecules are
arranged in densely packed fibril bundles,
crosslinked by hemicellulose. Lignin matrices
encase the cellulose ~ibrils, imparting strength
and stability to the wood.
As discussed in EP 0 406 617 A2, paper is a two-
dimensional meshwork of randomly arranged
cellulosic fibers linked by hydrogen bonds between
the polysaccharide units. To make paper from wood,
the cellulosic fibers must be separated from the
lignin and deposited into a hydrogen-bonded tangled
- mat. The strength of paper and paper products
prepared from pulp depends on the strength of the
intertwined cellulosic network, and therefore, it
is important that the process that isolates the
fibers from the lignin and that treats the fibers
thereafter not unduly degrade, or shorten, the
cellulosic polymer.
Fibers are typically isolated from
lignocellulosic material mechanically or
W094/20672 PCT~S94/02710 ~
8 1 ~ 2
chemically, or through a combination of these
processes. Pulps are prepared chemically by
treating lignocellulosic material with chemical
oxidants, such as sulfate and sulfite, that degrade
the lignin. After the lignin is oxidized, it is
separated from the cellulose. A small fraction of
the oxidized lignin, however, typically remains in
the pulp covalently bound to hemicellulose. The
oxidized lignin has a dark color that discolors the
resulting pulp and products made from the pulp.
When the residual lignin is bleached and allowed to
remain in the pulp, brightness reversion (i.e.
yellowing) can occur as the bleached lignin
oxidizes over time in contact with air. When the
residual lignin is completely removed from the
pulp, brightness reversion can be minimized.
The standard unit of measure for the lignin
content of pulp or paper products is the kappa
number. The kappa number is determined by a
permanganate oxidation test according to TAPPI
(Technical Association of the Pulp and Paper
Industry) standards (TAPPI Method T-236 CM-85). In
general, hardwood pulps have a lower kappa number
than softwood pulps, and therefore, are easier to
bleach.
The standard unit of pulp brightness is the
International Organization for Standardization
(ISO), which is evaluated by TAPPI Method T-525 OM-
86. Pulps of a brightness of 83-92~ are considered
fully bleached and are sometimes necessary for the
production of good quality paper.
Extensive research and development efforts have
been undertaken to determine the optimal conditions
for the bleaching and/or removal of the residual
oxidized lignin from pulp. These methods are
generally referred to as pulp bleaching processes.
Most bleaching processes are multistep procedures
W094/20672 215 0 81 0 PCT~S94/02710
.
in which the pulp is treated with a number of
agents in very specific sequences.
The conventional method for bleaching pulp
includes treatment of the pulp with chlorine or
chlorine-containing compounds such as calcium
hypochlorite, sodium hypochlorite, and chlorine
dioxide. These processes work by removing, as
opposed to bleaching, the lignin. The chlorine
compounds, however, also degrade a portion of the
cellulose, reducing the chain length of the
polymer, and the viscosity of the pulp. Further,
and importantly, the effluent from these processes
constitutes a potential environmental hazard since
it contains a large number of chlorinated lignin
breakdown products.
Of the total world production of 23.5 million
tons of bleached Kraft pulp in 1990, only
approximately 0.1 million tons were bleached
without chlorine containing compounds. During the
last decade, public concern for the environment has
greatly increased. This increased environmental
concern has become a significant driving force for
stricter regulation of potentially hazardous
compounds in effluents from pulp bleaching. In the
same time span, an unprecedented modernization of
pulping and bleaching processes has taken place.
Installations of extended Kraft cooking, oxygen
based delignification and substantial replacement
of elemental chlorine with chlorine dioxide have
significantly reduced the formation of dioxin and
adsorbable organic halogen (AOX) in the process.
In spite of the enormous efforts to upgrade and
modernize their operations, the pulp and paper
industry still finds itself the focus of a debate
over possible negative environmental impact from
pulp bleaching processes.
W094/20672 21~ O~ D PCT~S94/02710 ~
-
The use of enzymes in the bleaching process has
been explored as a means to modify or degrade the
lignin without attacking the cellulose. U.S.
Patent No. 4,687,745 discloses a process for
enhancing the strength and brightness of mechanical
pulps that includes treating the pulp with a
lignolytic enzyme mixture from a Phanerochaete
chrysosporium fermentation process. EPO 0 406 617
A2 discloses a method to delignify lignocellulosic
material that includes treating the material with a
ligninase enzyme, or ligninase in combination with
xylanase. U.S. Patent No. 4,923,565 discloses a
method for treating a paper pulp with cellulase and
hemicellulase enzymes. PCT WO 91/02840 discloses
treating the pulp with a hemicellulose-hydrolyzing
enzyme.
Methods to bleach pulp have also been advanced
that include oxidizing the pulp with oxidants such
as ozone, oxygen, and hydrogen peroxide instead of
chlorine or chlorine dioxide in very specific
sequences in the absence of an enzyme step in the
process.
Canadian Patent No. 1 132 760 discloses a
process for bleaching chemical pulp that includes,
in specific sequence, a peroxide bleaching stage,
wherein the peroxide is selected from the group
consisting of alkaline hydrogen peroxide, acid
hydrogen peroxide, and peracid; and at least one
ozone bleaching stage. An oxygen stage is
optionally included as the first step.
Conventional caustic extraction steps can follow
the ozone treatment stages.
PCT Publication No. WO 92/12289 discloses a
chlorine-free process for bleaching lignocellulosic
pulp that includes an ozone step followed by a
peroxide step.
~ wo94eo672 2 I S ~ 8 1 0 ~CT~S94/OZ710
U.S. Patent No. 5,164,043 discloses a process
for bleaching lignocellulosic material that
includes, in specific sequence, an oxygen step, an
ozone step, and then a chlorine-dioxide or peroxide
step.
Liebergott, et al., "Bleaching a softwood kraft
pulp without chlorine compounds," TAPPI Journal,
August 1984, compares the economics and efficiency
of the conventional chlorine process for bleaching
pulp (C/DEDED) with two non-chlorine containing
processes that utilize sodium hydrosulfite as an
oxidant.
Liebergott, et al., in "Lowering AOX Levels in
the Bleach Plant," Proceedinqs of the 1992
Environmental Conference, 1065-1075, discusses the
use of oxygen, ozone, and peroxide, with or without
chelation and extraction steps, as alternatives to
chlorine bleaching of pulp.
U.S. Patent No. 5,179,021 to du Manior, et al.,
discloses that chemical pulp can be bleached to an
ISO brightness level of approximately 50-60~ by
treating the pulp with oxygen or an oxygen-
containing gas in alkaline medium, followed by
treatment of a sufficient amount of a substantially
cellulase-free xylanase to hydrolyze xylosidic
linkages in the material. To further increase the
brightness level of the pulp, the patent teaches
that one or more additional steps be employed
selected from the group consisting of treatment in
an aqueous medium with: (i) chlorine, chlorine
dioxide, or mixtures thereof; (ii) chlorine dioxide
alone; (iii) peroxide; (iv) hypochlorite; (v)
ozone; or (vi) nitrogen dioxide. In the preferred
embodiments, the oxygen and xylanase steps are
followed by steps (i) and (ii), or steps (i) and
(iii). Importantly, du Manior does not disclose
any specific sequences of process steps that
WOg4/20672 PCT~S94/02710 ~
~1~0810
provides a high pulp brightness level and which do
not include chlorine or chlorine dioxide.
While extensive efforts and advances have been
made in processes to bleach pulp without the use of
chlorine or chlorine derivatives, no one has to
date disclosed a specific, optimized, sequence of
process steps to bleach pulp that include the use
of an enzyme treatment in combination with oxygen
or oxygen derivative bleaching steps.
Therefore, it is an object of thiS invention to
provide a specific, optimized, sequence of process
steps to bleach hardwood and softwood pulp that
include the use of an enzyme treatment in
combination with oxygen or oxygen derivative
bleaching steps.
It is also an object of the invention to provide
a method for bleaching pulp that has a minimal
effect on the environment and that is economically
competitive with the present bleaching processes.
SUMMARY OF THE lNv~:NllON
A method for the bleaching of hardwood and
softwood pulp without chlorine or chlorine
derivatives is disclosed that has a short sequence
of steps and that provides a pulp with high
brightness, good physical strength, and low degree
of brightness reversion. The specific sequence in
which the treatment steps are carried out is
important. It has been discovered that, for
example, xylanase has little effect when used after
an ozone step. Further, if ozone is used as the
last step of the bleaching process, a significant
amount of brightness reversion can occur, whereas
if peroxide is used as the last step, brightness
reversion is minimized.
-
~ W094/20672 215 0 81 0 PCT~S94/02710
Specifically, it has been discovered that
hardwood pulp can be bleached without chlorine or
chlorine derivatives to a high brightness level
r using a specific, optimized, sequence of process
5 steps that includes the use of an enzyme treatment
in combination with oxygen or oxygen derivative
bleaching steps. The specific steps are, in
sequence: (i) treatment with oxygen; (ii) treatment
with xylanase; and (iii) treatment with ozone;
l0 followed by (iv) treatment with peroxide. This
specific sequence of bleaching steps is referred to
below as the OXZP process. It has been discovered
that this specific combination of steps can provide
a brightness level of up to 90~ ISO or greater,
15 without undue detrimental effect on the physical
properties of the pulp. The combination is
superior to other tested sequences of enzyme steps
and oxygen or oxygen derivative treatment steps, as
discussed in detail below. Optionally, the pulp
20 can be subjected to alkaline extraction or enhanced
alkaline extraction steps between the treatment
steps to assist the removal of the lignin, as
necessary.
It has also been discovered that softwood pulp
25 can be bleached without chlorine or chlorine
derivatives to a high brightness level also using a
specific, yet different, optimized, sequence of
process steps that includes the use of an enzyme
treatment in combination with oxygen or oxygen
30 derivative bleaching steps. The specific steps
are, in sequence: (i) treatment with oxygen; (ii)
treatment with xylanase; (iii) alkaline
extraction; and (iv) ozone treatment; followed by
(v) treatment with peroxide. This optimized
35 sequence for the bleaching of softwood pulp is
referred to below as OXEpZP. In an alternative
embodiment, an additional ozone step is included
W094/20672 ~ 81~ PCT~S94102710 ~
-
before the alkaline extraction step (OXZIEpZ2P).
This alternative sequence can be used to bleach
those pulps with significantly higher initial
lignin content, within one ozone step may be
inadequate alone to achieve sufficient
delignification without impairing pulp physical
strength. In another embodiment, softwood RDH
(rapid displacement of heat) pulp is bleached with
the sequence XEpZP. Optionally, softwood pulp can
be also subjected to alkaline extraction or
enhanced alkaline extraction steps between the
treatment steps to assist the removal of the
lignin, as necessary.
Eucalyptus, pine, and pine RDH kraft pulps,
delignified with the specified combination of
xylanase and oxygen or oxygen derivative steps set
out above, were found to have consistently lower
kappa numbers and higher brightness at the same
level of ozone charge as compared to control pulps
without the enzyme step. When the eucalyptus pulp
was bleached in the sequence OXZP using 0.8~ ozone,
a brightness of 90.0~ ISO was readily obtained
compared to a brightness of 84.7~ for the control,
OZP pulp. When pine kraft and pine RDH pulps were
bleached in the sequences OXZIEpZ2P and XEpZP
respectively, brightness values of 81.0 and 85.7
were obtained compared to the brightness of the
control pulps (without xylanase treatment) of 71.3
and 76.3~ respectively. The total ozone
consumption for pine kraft pulp was 1.38~, while
for RDH pulp it was 0.96~ based on o.d. pulp.
As a reference, pulps were also bleached to the
same brightness levels using the conventional
sequence DEDED. The pulps bleached with the
specific enzyme and oxygen-based chemical steps
have good brightness stability, similar tensile
~ W094/20672 215 ~ 810; PCT~S94/02710
index and slightly lower tear index than the
reference DEDED pulps.
In alternative embodiments, the oxygen step is
removed from the OXZP, OXEpZP and OXZIEpZ2P processes
for bleaching hardwood and softwood pulps that have
very low initial lignin contents such as pulps from
the RDH cooking process.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a graph illustrating the effect of
ozone consumption as a function of pulp percentage
(w/w) on brightness and kappa number of the oxygen-
bleached eucalyptus Kraft pulp with and without
xylanase treatment. The open triangles indicate
the brightness level (~ ISO) of the oxygen-
bleached, ozone treated eucalyptus Kraft pulp. Thesolid triangles indicate the brightness level
(~ ISO) of an oxygen-bleached, xylanase and ozone
treated eucalyptus Kraft pulp. The open circles
indicate the kappa number of an oxygen-bleached,
ozone treated eucalyptus Kraft pulp. The solid
circles indicate the kappa number of an oxygen-
bleached, xylanase and ozone treated eucalyptus
Kraft pulp.
Figure 2 is a graph illustrating the effect of
ozone consumption as a function of pulp percentage
(w/w) on brightness and viscosity (mPa.s) of an
oxygen-bleached eucalyptus Kraft pulp with or
without xylanase treatment. The open triangles
indicate the brightness level (~ ISO) of the
oxygen-bleached, ozone treated eucalyptus Kraft
pulp. The solid triangles indicate the brightness
level (~ ISO) of the oxygen-bleached, xylanase and
ozone treated eucalyptus Kraft pulp. The open
circles indicate the kappa number of an oxygen-
bleached, ozone treated eucalyptus Kraft pulp. The
WOg4/20672 PCT~S94/02710 ~
t 215~t810
solid circles indicate the kappa number of anoxygen-bleached, xylanase and ozone treated
eucalyptus Kraft pulp.
Figure 3 is a graph indicating the effect of
ozone consumption as a function of pulp percentage
(w/w) on brightness and kappa number of an oxygen-
bleached pine Kraft pulp with or without xylanase
treatment. The open triangle indicates the
brightness of the oxygen-bleached and ozone treated
pine Kraft pulp. The solid triangle indicates the
brightness of the oxygen-bleached and ozone treated
pine Kraft pulp that has also been treated with
xylanase. The open circle indicates the viscosity
of the oxygen-bleached and ozone treated pine Kraft
pulp. The solid circle indicates the viscosity of
the oxygen-bleached and ozone treated pine Kraft
pulp that has also been treated with xylanase.
Figure 4 is a graph indicating the effect of
ozone consumption as a function of pulp percentage
(w/w) on brightness and viscosity of an oxygen-
bleached pine Kraft pulp with or without xylanase
treatment. The open triangle indicates the
brightness (~ ISO) of the oxygen-bleached pine
Kraft pulp treated without xylanase. The solid
triangle indicates the brightness (~ ISO) of an
oxygen-bleached pine Kraft pulp treated with
xylanase. The open circle indicates the viscosity
(mPa.s) of an oxygen-bleached pine Kraft pulp
treated without xylanase. The solid circle refers
to the viscosity of an oxygen-bleached pine Kraft
pulp treated with xylanase.
Figure 5 is a graph showing the effect of ozone
consumption on brightness and kappa number of a RDH
pine Kraft pulp pre-treated with XEp or Ep stages.
The open triangle indicates the brightness (~ ISO)
of the RDH pine Kraft pulp treated with enhanced
alkaline extraction, ozone, and peroxide. The
~ W094/20672 21~ 0 81 0 PCT~S94/02710
11
solid triangle indicates the brightness (~ ISO) of
the RDH pine Kraft pulp treated with xylanase,
enhanced alkaline extraction and ozone. The open
circle indicates the kappa number of a RDH pine
Kraft pulp treated with enhanced alkaline
extraction and ozone. The solid circle refers to
the kappa number of a RDH pine Kraft pulp treated
with xylanase, enhanced alkaline extraction and
ozone.
Figure 6 is a graph showing the effect of ozone
consumption on brightness and viscosity of the RDH
pine Kraft pulp pre-treated with XEp or Ep stages.
The open triangle indicate the brightness (~ ISO)
of the RDH pine Kraft pulp treated with enhanced
alkaline extraction and ozone. The solid triangle
indicates the brightness (~ ISO) of a RDH pine
Kraft pulp treated with xylanase, enhanced alkaline
extraction and ozone. The open circle indicates
the kappa number of the RDH pine Kraft pulp treated
with enhanced alkaline extraction and ozone. The
solid circle indicates the kappa number of an RDH
pine Kraft pulp treated with xylanase, enhanced
alkaline extraction and ozone.
Figure 7 is a graph indicating the tear and
tensile relationship of eucalyptus Kraft pulps
bleached in the sequences of OXZP and ODEDED. The
open circle refers to eucalyptus Kraft pulp treated
in sequence with oxygen, xylanase, ozone, and
peroxide (oXzP). The solid circle refers to
eucalyptus Kraft pulp treated in sequence with
oxygen, chlorine dioxide, alkaline extraction,
chlorine dioxide, alkaline extraction, and chlorine
dioxide ( ODEDED).
WO 94/20672 PCT/US94/02710 ~
2150810
12
DETI~TT~n DESCRIPTION OF THE lNV~iNLlON
The term hardwood, as used herein, refers to
angiosperms. Examples of hardwoods include
eucalyptus, birch, oak, beech, aspen, gum, maple,
poplar and sycamore.
The term softwood, as used hereih, refers to
gymnosperms. Examples of softwoods include pine,
spruce, hemlock, Douglas-fir, larch and redwood.
The Beloit Rapid Displacement of Heat (RDH) process
used to produce the pine RDH pulp is reported in
TAPPI, Vol. 66 (3), 120 (1983).
The following abbreviations are used herein:
oxygen (O), ozone (Z), alkaline or sodium hydroxide
extraction (E), enhanced alkaline extraction
(sodium hydroxide extraction combined with peroxide
or oxygen) (Ep or Eo)l chelation of metal ions (Q),
hydrogen peroxide (P), hypochlorites (H), sodium
hydrosulfite (Y), chlorine (C), mixtures of
chlorine and chlorine dioxide (ClO2) (C/D), chlorine
20 dioxide (ClO2) (D), and xylanase (X). As used
herein, the term "ISO" refers to International
Organization for Standardization which is a basic
brightness unit.
The term DTPA refers to diethylene triamine
pentaacetic acid.
The invention as described herein is an
optimized process for the delignification and
bleaching of hardwood and softwood chemical pulps
that uses no elemental chlorine or other chlorine
containing chemicals, and results in pulp with good
strength and high brightness levels. The specific
steps for the bleaching of hardwood pulp are, in
sequence: (i) treatment with oxygen; (ii) treatment
with xylanase; and (iii) treatment with ozone;
followed by (iv) treatment with peroxide. This
specific se~uence of bleaching steps is referred to
~ W094/20672 21~ 0 81 ~ PCT~S94/02710
13
below as the OXZP process. The specific steps for
the treatment of softwood pulp are, in sequence:
(i) treatment with oxygen; (ii) treatment with
xylanase; (iii) alkaline extraction; and (iv)
ozone treatment; followed by (v) treatment with
peroxide. This optimized sequence for the
bleaching of softwood pulp is referred to below as
OXEpZP. In an alternative embodiment, an additional
ozone step is included before the alkaline
extraction step (OXZ1EpZ2P). Softwood RDH (rapid
displacement of heat) pulp is bleached with the
sequence XEpZP. Optionally, both the hardwood and
softwood pulps can be subjected to alkaline
extraction or enhanced alkaline extraction steps
between the treatment steps as necessary to assist
the removal of the lignin.
Chelation of metal ions in the pulp solution
prior to the peroxide bleaching step results in
pulp with higher brightness levels than when the
peroxide stage is performed without prior chelation
of the metal ions. Performing the ozone step
before the peroxide step also generally results in
significantly higher brightness levels and less
brightness reversion than pulp bleached with these
steps reversed.
I. ProceRs Step Conditions
The process step conditions used to bleach
hardwood and softwood pulp according to the process
described herein are set out below. These
conditions are merely exemplary, and are not
intended to limit the invention. The conditions
can be varied as necessary to obtain the desired
product. All percentages are by weight unless
otherwise specified.
W094t20672 215 ~ 8 1 3 PCT~S94/02710 ~
14
Oxyqen Bleachinq Treatment (O)
The oxygen bleaching of pulp is typically
carried out in an alkaline aqueous media with a pH
above 7, and preferably above 11, a pulp
consistency of approximately 10-15~, a partial
pressure of oxygen of between approximately 80 and
120 psi, and a temperature between approximately 80
and 110-C. The oxygen treatment step generally
takes between approximately 30 and 90 minutes.
The oxygen containing gas can be air, or can be
purified oxygen.
Xylanase Treatment (X)
The xylanase treatment is typically carried out
at a temperature ranging between 30 and 70 C,
preferably 50 C, a pulp consistency of between
approximately 5 and 20~, preferably 10~, with a
xylanase concentration of 0.1-5 U/g o.d. (oven
dried) pulp, preferably 2.0 U/g o.d. pulp, for
approximately 30 minutes to 4 hours, preferably 90
minutes. The pH of the xylanase treatment
typically ranges between 3-7, and preferably 5.
In a preferred embodiment, xylanase is derived
from the fungus Aureobasidium pullulans. Xylanase
from this source is essentially free o cellulase
activity, which weakens the strength of the wood
pulp. Other xylanases which do not show
appreciable cellulase activity can also be used in
this process, such as xylanases from thermophilic
and alkalophidic bacteria.
Ozone Treatment (Z)
The ozone bleaching step is typically carried
out at a temperature ranging between 15 and 60 C,
preferably 25 C, a pulp consistency of 30-45~, with
an ozone concentration of 0.1-5.0~, typically 0.1-
0.5~ for approximately 3 to 15 minutes. The pH of
~ W094/20672 _ 21 S 0 81 0 PCT~S94/02710
pulps during ozone treatment is generally between 2and 4, and more typically, around 3.
Enhanced Alkaline Extraction (Ep or Eo)
The enhanced alkaline extraction is typically
carried out at a temperature ranging between 30 and
90 C, and preferably approximately 70 C, at a basic
pH, preferably greater than 11; and at a pulp
consistency of 5-20~, preferably 10-15~, for a time
period ranging between 30 and 120 minutes, and
generally approximately 90 minutes. The charge of
sodium hydroxide is usually between 0.5 and 2.0~,
and contains hydrogen peroxide ranging between 0.2
and 1.0, preferably 0.5~, or oxygen gas with a
partial pressure 50-100 psi.
Chelation (O)
The optional chelation step is usually carried
out at a temperature ranging between 40 and lOO C,
preferably approximately 80 C, a pulp consistency
of between 5 and 20~, preferably 10~, with a
chelating agent concentration of 0.1-0.5~,
preferably 0.2~, and with the addition of 1-5~
sodium bisulfite, preferably 3~ as SO2on o.d. pulp.
This step is typically carried out for between
approximately 5 and 90 minutes, and generally, for
approximately 15 minutes. The chelation stage can
be performed before the enhanced alkaline
extraction as well as before the peroxide stage.
Chelating agents that can be used in the process
include, but are not limited to DTPA, EDTA
(ethylenediamine tetra acetate) and NTA (nitrilotri
acetic).
Peroxide Treatment (P)
The peroxide step is typically carried out at a
temperature ranging between 50 and 90 C, preferably
W094/20672 21~ 0 810 PCT~S94/02710 -
16
70 C, a pulp consistency of between 5 and 20~,
typically 12~, with a peroxide concentration of
between 1.0 and 3.0~, and a sodium hydroxide
concentration of between 1.0 and 2.0~, for between
approximately 1 and 5 hours, preferably 3 hours.
The pH of the solution used for the peroxide
treatment step is basic, and typically greater than
11. After the reaction has be~e~completed, the
pulp is filtered and washed with water. Residual
hydrogen peroxide is checked by iodometric
titration of the filtrates. Examples of typical
bleaching conditions for the different stages are
presented in Table 2.
It is preferred to use hydrogen peroxide as the
source of peroxide in this step. Other peroxides
that can be used in place of hydrogen peroxide that
perform equivalently.
II. Example~ of Treatment of Pulp with
Sequenced Enzyme and Oxygen-ba3ed Chemical
Steps
To illustrate the process described herein,
three different types of kraft pulps have been
bleached with enzyme and oxygen-based chemicals.
Eucalyptus, pine, and pine RDH kraft pulps,
delignified with a combination of xylanase and
ozone stages, were found to have consistently lower
kappa numbers and higher brightness at the same
level of ozone charge compared to the control pulps
without the enzyme stage. When the eucalyptus pulp
was bleached in the sequence OXZP using 0.8~ ozone,
a brightness of 90.0~ (ISO) was readily obtained
compared to a brightness of 84.7~ for the control,
OZP pulp. When pine kraft and pine RDH pulps were
bleached in the sequences OXZIEpZ~P and XEpZP
respectively, brightness values of 81.0 and 85.7
were obtained compared to the brightness of the
W094/20672 ~ 21 ~ ~ 8 1 0 PCT~594/02710
control pulps 71.3 and 76.3~ respectively. The
total ozone consumption for pine kraft pulp was
1.38~, while for RDH pulp it was 0.96~ based on
o.d. pulp.
As a reference, pulps were also bleached using
the conventional sequence DEDED to the same
brightness levels. The pulps bleached with enzymes
and oxygen-based chemicals have good brightness
stability, similar tensile index and slightly lower
tear index compared to the reference DEDED pulps.
MATERIALS AND METHODS
Pulp Samples
Commercially produced unbleached and oxygen-
bleached kraft pulps, as well as a RDH (Rapid
Displacement of Heat) pulp were obtained from
different pulp mills. The pulps were further
washed with water before use. The kappa number,
viscosity, and brightness of these pulps after
washing are presented in Table l.
TABLE l: Characteristics of the mill-produced
kraft pulps
Kappa Viscosity Brightness
Pulp number (mPa.s) ~ (ISO)
EucalyPtus
kraft PulP
Unbleached 12.l 29. 5 34.0
Oxygen-bleached 8.4 2 3.1 53.0
30 Pine kraft pulp
Unbleached 28.1 34.0 20.1
Oxygen-bleached l9. 3 24.1 27.0
Pine RDH Pulp
Unbleached l0.6 8. 6 31.4
WOg4/20672 21~ 0 810 PCT~S94/02710 ~
18
EnzYme Production and Preparation
The xylanase was produced in batch ~ermentation
by the fungus, Aureobasidium pullulans . The
cultivation and subsequent preparation of the
culture filtrate has been described previously.
Yang, J. and Eriksson, K-E., Hotzforschunq,
46(6):481 (1992). Evaluation of the enzyme against
birchwood xylan, m~nn~n, carboxylmethylcellulose
and arabinogalactan indicated that the hydrolytic
activity of this culture filtrate consists solely
of endo-xylanase activity (480 units/ml). One unit
of xylanase activity was defined as the amount of
enzyme releasing one ~mol of xylose equivalent per
minute at ambient defined (30C) temperature.
Pulp Bleachinq Procedure
Pulps were disintegrated by 10,000 revolutions
in a disintegrator (Noram, PQ, Canada) at room
temperature before bleaching. The pulp suspensions
were filtered on a Buchner funnel and then placed
in double-layer polyester bags. The xylanase
treatment (X) was carried out at a pulp consistency
of 10~ in 50 mM sodium acetate buffer with a pH 5.0
and a xylanase charge of 2 units/g pulp at 50 C for
90 minutes. After the enzyme treatment, the pulps
were washed with water. Treatment of pulp in the
control sequence was carried out under identical
conditions as the treated pulp but without enzyme.
Ozone delignification was carried out batchwise
at high consistency conditions. Pulp samples were
first acidified with sulfuric acid to a pH range of
2-3 and then dewatered with a handpresser to a pulp
consistency of approximately 40~. The pulps were
subsequently fluffed in a hammer mill prior to the
ozone treatment. The ozone generator used in this
study had a production capacity of 80-200 g
ozone/hr, yielding ozone concentrations of 1-5~
(w/w). A typical batch ozone treatment was carried
W094/20672 19 PCT~594/0271U
out with 50 g o.d. (oven dry) pulp, and an ozone
charge of between 0.1 to 3.0~ (w/w) for 3-15
minutes at ambient temperature and pressure. Ozone
concentration was determined by iodometric
titration.
Before entering the peroxide stage (Ep or P),
pulps were pretreated with 0.2~ diethylene triamine
pentaacetic acid (DTPA) and 3.0~ NaHSO3 calculated
as SO2 on o.d. pulp at a consistency of 10~, pH 4.5
for 15 minutes at 80 C (Q).
For the hydrogen peroxide (Ep, P) stages, pulps
and water were heated to the desired temperature
and mixed with the bleaching chemicals to a final
consistency of 10~. This procedure was also used
when chlorine dioxide (D) and alkaline extraction
(E) stages were applied. After the reaction was
completed, pulps were filtered and washed with
distilled water. Residual hydrogen peroxide and
chlorine dioxide were checked by iodometric
titration of the filtrates. Bleaching conditions
for the different stages are presented in Table 2.
W094/20672 215 Q 81~ PCT~S94/02710 ~
TABLE 2: The conditions used to bleach kraft pulps
Symbol Chemical Reaction Treatment pH Consistency
charge (%) time (min) temp.(C) (%)
Eucaly~tus kraft PUIP
X 2 U xylanase/g pulp 90 50 5.0 10
Z 0z: 0.1-3.0 3-15 ambient 2-3 35-45
P H202: 1.0 180 70 >1 1 10
0 NaOH: 1.0
Na2SiO3: 2.0
Pine kraft and RDH PUIPs
X 2 U xylanase/g pulp 90 50 5.0 10
Ep H202: 0.5 180 70 >11 10
NaOH: 2.0
Z 03: 0.1-3.0 3-10 ambient 2-3 34-45
P H202 180 70 >1 1 10
NaOH: 2.0
Na2SiO3: 3.0
Analysis of Pulp Chemical and Physical Properties
The chemical and physical properties of the
pulps were analyzed according to the following
Tappi test methods: T205 om-88 (forming handsheets
for physical tests of pulp), T227 om-85 (freeness
of pulp), T230 om-89 (viscosity of pulp), T236 cm-
85 (kappa number of pulp), T238 cm-85 (laboratory
beating of pulp), and T525 om-86 (brightness of
pulp). The physical properties of the bleached
kraft pulps were evaluated at the Institute of
Paper Science and Technology (IPST), Atlanta,
Georgia.
~ W094/20672 21 5 ~ 81 0 PCT~S94/02710
21
RESULTS
Bleachinq of Eucalyptus Kraft Pulp
The mill produced oxygen-bleached eucalyptus
pulp was treated with Aureobasidium xylanase. This
treatment decreased the kappa number from 8.4 to
7.5, increased the viscosity from 23.1 to 25.1
mPa.s and brightness from 53.2 to 55.4~ (ISO).
Xylanase treatment of unbleached eucalyptus pulp
decreased the kappa number from 12.1 to 10.9,
increased the viscosity from 29.5 to 31.0 and
brightness from 34.0 to 36.6~ (ISO).
The effect of ozone consumption on oxygen
bleached and xylanase treated pulp were extensively
investigated. The results are summarized in
Figures 1 and 2. Figure 1 is a graph illustrating
the effect of ozone consumption as a function of
pulp percentage (w/w) on brightness and kappa
number of an oxygen-bleached eucalyptus Kraft pulp
with and without xylanase treatment. Figure 2 is a
graph illustrating the effect of ozone consumption
as a function of pulp percentage (w/w) on
brightness and viscosity (mPa.s) of an oxygen-
bleached eucalyptus Kraft pulp treated with or
without xylanase.
As shown in Figure 1, the kappa number decreased
readily from 8.4 to below 2 with gradually
increased ozone charges up to approximately 1~.
Figure 2 indicates, however, that the viscosity of
the pulp was simultaneously reduced. Pulp bleached
with the OXZ sequence has a consistently lower
kappa number and higher brightness than pulp
bleached without the enzyme stage, i.e., OZ only,
at the same levels of ozone charges. Brightness of
- the xylanase treated pulp developed much faster and
the brightness ceiling was higher compared to the
OZ bleached pulp.
W094/20672 PCT~S94/02710 ~
21~08~ ~ 22
Bleachinq of Pine Kraft Pulp
The mill produced, oxygen bleached pine kraft
pulp was also treated with Aureobasidium xylanase.
Xylanase treatment of this pulp decr.eased the kappa
number from 19.3 to 18.0, increased viscosity from
22.2 to 23.4 and brightness from~-28.7 to 30.0~
(ISO). Xylanase treatment of unbleached pine kraft
pulp decreased the kappa number from 28.6 to 26.8,
increased viscosity from 34.0 to 34.2 and
brightness from 23.8 to 26.1~ (ISO).
The effect of ozone charge on the oxygen
bleached and xylanase treated pulp was studied.
The results are provided in Figures 3 and 4.
Figure 3 is a graph indicating the effect of ozone
consumption as a function of pulp percentage (w/w)
on brightness and kappa number of an oxygen-
bleached pine Kraft pulp with or without xylanase
treatment. Figure 4 is a graph indicating the
effect of ozone consumption as a function of pulp
percentage (w/w) on brightness and viscosity of an
oxygen-bleached pine Kraft pulp treated with or
without xylanase.
The kappa number of the xylanase treated pulp
was reduced from 18 to 6. As indicated in Figure
3, when xylanase was not used in pulp treatment,
the kappa number decreased from 19 to 8 with
gradually increased ozone charges up to
approximately 2.5~. As indicated in Figure 4,
however, the viscosity of the pulp declined
initially and then stabilized at approximately 10
mPa.s. Pulp bleached with the OXZ sequence has
consistently higher brightness than OZ bleached
pulp at the same levels of ozone charges.
Brightness of the xylanase treated pulp developed
much faster and the brightness ceiling was higher
than the OZ pulp.
~ wo s4no672 - 21 S 0 81 0 PCT/US94/02710
Due to the substantially higher kappa number of
the particular oxygen-bleached pine kraft pulp used
compared to the eucalyptus oxygen-bleached kraft
pulp, two ozone stages were necessary in order to
delignify the pulp to an acceptable level before
entering the final brightening stage. It can be
seen in Table 3 that the pulp bleached according to
the OXZIEpZ2P sequence has a brightness of 81.0~ IS0
while, if the enzyme stage is excluded, a
brightness of only 71.3~ ISO was reached.
Bleachinq of Pine RDH Kraft Pulp
The mill produced unbleached pine RDH pulp was
also treated with Aureobasidium xylanase. Enzyme
treatment of this pulp decreased the kappa number
from 11.3 to 10.6, increased the viscosity from
7.9 to 8.6 mPa.s and brightness from 30.1 to 31.4
ISO.
The effect of ozone charge on xylanase treated
pine RDH pulp is illustrated in Figures 5 and 6.
Figure 5 is a graph showing the effect of ozone
consumption on brightness and Kappa number of the
RDH pine Kraft pulp pre-treated with XEp or Ep
stages. Figure 6 is a graph showing the ef~ect of
ozone consumption on brightness and viscosity of
the RDH pine Kraft pulp pre-treated with XEp or Ep
stages. As indicated in Figure 5, the kappa number
of the pulp decreased from 8.2 to approximately 2
for the XEp treated pulp while a decrease in kappa
number from 9 to approximately 3 was obtained for
the Ep treated pulp with gradually increased ozone
charges up to approximately 1~. The viscosity of
the pulp was reduced from 7.5 to 5.3 and from 7.5
to 4.8 for the XEpZ and EpZ bleached pulps
respectively. The XEpZ bleached pulp had a
considerably higher brightness than the EpZ pulp at
the same level of ozone charge. The brightness of
W094/20672 21~ 0810 PCT~S94/02710 ~
24
the XEpZ bleached pulp was approximately 77 while it
was only approximately 67 for the EpZ bleached pulp.
A com~arison of the ozone consumed on w/w pulp
percentage, the viscosity, and brightness of
hardwood and softwood pulp are summarized in Table
3. Noticeably, the brightness level of the pulp is
consistently higher when the enzyme step is used.
For the eucalyptus pulp the brightness is 5.5 units
higher, for pine kraft pulp, 9.7 units and for pine
RDH pulp, 9.4 units higher.
TABLE 3: Comparison of Rome properties of three
types of kraft pulps bleached in
different bleaching sequences
Bleaching Ozone consumed Viscosity Brightness
sequence on pulp (~) mPa.s ~ ISO
Eucalyptus kraft ~ulp
OXZP 0.79 9.2 90.2
ozP 0.76 9.3 84.7
Pine kraft pulp
oxzlEpz2p Zl:0.80/Z2:0.58 8.0 81.0
OZIEpZ2P Zl:0.84/Zz:0.58 8.0 71.3
Pine RDH Pulp
XEpZP 0.96 5.4 85.7
EpZP 0.95 5.0 76.3
Physical Properties of Bleached EucalY~tus Kraft
Pulp
Physical properties of eucalyptus pulp bleached
in the sequence OXZP and the reference sequence
ODEDED were measured at the Institute of Paper
Science and Technology (IPST). Figure 7 is a graph
indicating the tear and tensile relationship of
eucalyptus Kraft pulps bleached with these
sequences. The freeness which development (a
~ W094/20672 2I 5 ~ 8I O PCT~S94/02710
measurement of drainability or water holding
capacity of a given pulp) during the beating of the
OXZP bleached pulp is very similar to that of the
reference pulp (ODEDED), even though the viscosity
of the OXZP bleached pulp is only 9.2 mPa.s, much
lower than the viscosity of the reference pulp,
24.6 mPa.s. However, tensile index values of both
pulps are very similar while the OXZP bleached pulp
shows a lower tear index value compared to the
reference pulp, Figure 7.
III. Effect of Acid Wash and Chelation Steps
The presence of transition metal ions such as
Mn+ and Fe+ in pulp is known in general to have a
detrimental effect on the efficiency of an alkaline
peroxide bleaching step. In light of this, the
effect of the treatment of pulp with acid or a
chelating agent on the kappa number, viscosity, and
brightness of the pulp was evaluated. The peroxide
step was carried out with 1~ H2O2, 1~ NaOH, and 2
Na2SiO3 for 180 minutes on lO~ pulp consistency.
The acid wash was carried out by acidifying the
pulp to pH 2.0 with 10 mM of sulfuric acid, and
then washing the pulp with water. The chelation
step was carried out with 0.2~ DTPA and 3.0~ NaHSO3
calculated as SO2 on o.d. pulp at a consistency of
10~, and pH of 4.5 for 15 minutes at 80C.
The results are summarized in Table 4. As
indicated in Table 4, pretreatment of pulps with
acid or DTPA improves the efficiency of the
peroxide step and results in a higher brightness
and lower kappa number of the pulps. It should be
noted that the ozone step itself is carried out at
a pH of 2-3 and therefore has the effect of an acid
wash. It appears, however, that chelating agents
W094/20672 21~ O ~10 PCT~S94/02710 ~
26
are more effective than acid washing in improving
the efficiency of the peroxide step.
TABLE 4 Effect of Pretreatment of Pulp by Acid
(A) or Chelating Agent (Q) prior to
the Peroxide Stage on Residual
Peroxide, Kappa Number, Viscosity and
Brightness.
Sequence H202consumed Kappa Viscosity Brightness
on pulp number mPa.s ~ IS0
OXZP 100 1.4 10.9 81
OXZAP 94.6 1.3 9.8 82
OXZQP 47 1.2 10.9 83
OXP 100 5.4 21.7 65
OXAP 94.6 5.1 20.1 72
OXQP 56.5 4.7 22.0 73
oP 100 5.5 21.4 62
OAP 99.3 5.3 18.9 67
oQP 83.0 5.1 21.8 71
IV. Comparison of Properties of Pulp Treated
with Different Sequences of Enzymes and
O~yyel~-based Chemicals
A comparison of the brightness, viscosity, and
ozone consumed on w/w of eucalyptus pine and pine
RDH kraft pulps bleached with different sequences
of enzymes and oxygen-based chemicals is presented
in Table 5. The eucalyptus pulp was bleached to a
brightness range of 83 to 92~ ISO with sequences of
OXZP and OXPZ. As a reference, the eucalyptus pulp
~ W094/20672 21~ 0 81~ PCT~S94/02710
27
was also bleached using a sequence of ODEDED to a
brightness of 90.2~. The pulp bleached in the OXZP
sequence has the highest brightness compared to the
OXPZ and OZP at the same level of ozone charge.
The viscosity of the bleached pulps were generally
low, ranging from 9.2 to 12.4 mPa.s compared to the
viscosity of the reference pulp (ODEDED, 24.6
mPa.s). The effect of xylanase on the brightness
of the pulp was significant.
Similar results were also obtained with softwood
kraft pulps. When pine and pine RDH pulps were
fully bleached, the most noticeable effect is that
brightness level is consistently higher when the
enzyme stage is used.
w094/2067~ 8 ~ ~ PCT~S94/02710
28
TABLE 5: Comparison of Some Properties of Three
Types of Kraft Pulps Bleached in
Different Bleaching Sequences
5 Bleaching Ozone Consumed Viscosity Brightness
Sequence on pulp (~) mPa.s (~ ISO)
Eucalyptus kraft pulp
OXZP 0.79 9.2 90.2
OZP 0.76 9.3 84.7
OXPZ 0.81 12.8 83.0
oPZ 0.82 11.1 83.0
ODEDED -- 24.6 90.2
Pine kraft pulp
OXEpZP 1.30 7.1 88.1
OEpZP 1.31 6.8 85.3
ODEDED -- 19.1 86.2
Pine RDH PUlp
XEpZP 0. 96 5.4 88.1
EpZP 0.95 5.0 81.8
DEDED -- 9. 3 86.2
The brightness stability of the fully bleached
eucalyptus (pulp #1), pine (pulp #2) and pine RDH
(pulp#3) pulps was also evaluated on heating the
treated pulp at 105C for 4 hours. The results are
presented in Table 6. The brightness was very
stable when alkaline peroxide was applied as the
final stage, as in the sequence OXZP compared to
the reference pulps bleached with ODEDED or DEDED.
~ WOg4/20672 2 f ~ 0 81 ~ PCT~S94/02710
29
TABLE 6. Brightne~s reversion of bleached kraft
pulps (kept at 105C for 4 hours).
Bleaching Brightness ~ (ISO)
5 sequence Before aging After aqinq P.C no.*
Eucalyptus kraft pulp
OXZP 90.2 87.8 0.32
ODEDED 90.2 87.0 0.44
Pine kraft pul~
OXEpZP 86.4 85.2 0.22
OEpZP 85.4 83.2 0.45
ODEDED 86.2 82.8 0.68
Pine RDH kraft pulp
XEpZP 88.0 86.6 0.22
EpZP 81.8 80.4 0.36
DEDED - 86.2 84.0 0.42
* p.c. no. (post color number) used as a measure of
color reversion
This invention has been described with reference
to its preferred embodiments. Variations and
modifications of the process for delignifying and
bleaching pulp will be obvious to those skilled in
the art from the foregoing detailed description of
the invention. It is intended that all of these
variations and modifications be included within the
scope of the appended claims.
