Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF PULPING
This invention relates to a method of bleaching
pulp .
Biomass is the most important source of cellulose
for pulp and paper making. The most popular biomass is
wood.
Wood comprises three major organic components,
cellulose, hemicellulose and lignin. Cellulose and
hemicellulose account for about 70-80% of the biomass and
lignin accounts for about 20-35~. The remaining, minor
organic components are primarily extractives. The
cellulose and hemicellulose are chains of glucose units
formed in linear fashion; lignin is an aromatic block
polymer.
In the tree, lignin acts to bond the cellulose
together to form a rigid structure that can stand up to
the growing environment. In papemaking pulping is
carried out to remove the lignin from the wood to
liberate the cellulose, which is the principal ingredient
of paper. This process requires strong agents to destroy
the majority of the lignin. Residual lignin is removed
from the pulp by bleaching, which functions to complete
the pulping process and to lighten the colour of the
pulp .
A variety of bleaching agents have been used in the
pulping industry. These include chlorine tidentified by
C) chlorine dioxide (identified by D) oxygen (identified
by O) and hydrogen peroxide (identified by P). In
addition the industry uses the abbreviation E for caustic
extraction. Thus a conventional, prior art bleaching
process may be defined as CEDED.
Existing bleaching techniques achieve certain
brightness levels in pulps required for papermaking.
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However the pulping art has run into contoversy over its
effects on the environment. In particular the use of
chlorine and chlorine dioxide for bleaching of pulp has
produced some unfavourable results by introducing dioxin
5 or furan into the environment. As a result a great deal
of effort has been expended in the pulp industry to
discover environmentally sound bleaching methods that do
not have an adverse effect on the environment. In
general this has meant that chlorine compounds are not
now used.
Recent developments to avoid the problems associated
with chlorine have been the use of oxygen, ozone and
hydrogen peroxide as the bleaching agent. A process in
which hydrogen peroxide is the main bleaching agent, and
15 which includes a chelation treatment stage, is described
in U.S. Patents 5,310,458 and 5,415,734. A different
type of chelant in this technology, aminoalkane
diphosphonic acid, is described in WO Patent 95/12029.
This chelant was added either in a pretreatment stage or
20 during a peroxide bleaching stage.
Ozone delignification coupled with chelation
pretreatment is described in U.S. Patent 5,441,603. In
this Patent the chelating agent used was selected from
diethylenetriaminepentaacetic acid (DTPA)
25 ethylenediaminotriacetic acid (EDTA) and oxalic acid.
Chelation pretreatment was conducted at a pH in the range
1-4.
U.S. Patent 5,411,635 describes a treatment that
combines ozone and peroxymonosulphate. A mixture of
30 agents is added at an acidic pH in the initial
delignification steps after kraft pulping.
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Oxygen delignification with ozone and peracetic acid
added and, therefore, at an acidic pH is described in
U.S. Patent 5,387,317.
In all of the above either the brightness of the
product was insufficient, the strength was too low or the
cost of production was too high. It is therefore clear
that there is still substantial work to be done in
uncovering economical and safe totally chlorine free
(TCF) pulping.
To avoid the difficulties in TCF pulping elemental
chlorine free (ECF) pulping using chlorine dioxide in the
place of free chlorine was introduced. Chlorine dioxide
was reported to give much less dioxin and furan both in
the pulp and the effluents from the mill. A process of
reducing the amount of halogenated organic compounds in
the spent liquor of ECF bleaching was described in U.S.
Patent 5,143,580. The process described in this Patent
uses chelation followed by a peroxygen stage at the
beginning of the ECF sequence.
U.S. Patents 5,149,442 and 5,143,580 describes the
reduction of halogenated organic compounds in spent
bleach liquor.
However ECF pulp continues to have high organo-
chlorine (OX) content in the solid-pulp phase. Mill data
for the OX contents of ECF pulp typically shows a mean of
136 parts per million (ppm) with a standard deviation of
1.44 ppm for interior mill spruce-pine-fir (SPF)
furnishes and a mean of 181 ppm with a standard deviation
of 7.34 ppm for a coastal mill using various furnishes.
Some European countries believe that the OX content
of a pulp should be below 30 ppm if the pulp is to
qualify as TCF pulp. 30 Ppm OX corresponds to the
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detection limit of most OX measuring equipment. It is
thus beneficial for the ECF pulp to have an OX content as
low as possible.
Several methods of reducing the level of organic
halogens in pulp and in effluent have been introduced. A
process for the reduction of organic halogens (OX) in
pulp and adsorbable organic halogens (AOX) in effluent,
and including an ozone stage, is described in U.S. Patent
4,959,124. U.S. Patent 5,389,201 also describes the use
of ozone to reduce consumption of chlorine containing
chemicals. Joncourt et al, Reduction of the Formation of
AOX During Chlorine Dioxide Bleaching, TAPPI Pulping
Conference, Oct. 1995/149-152, described two methods for
drastic reduction of AOX in chlorine dioxide bleaching.
The method involves the addition of dimethylsulphoxide
(DMSO) and splitting of chlorine dioxide charges. Both
methods increase mills' operating costs significantly and
introduce hazardous chemicals. Reeves et al, Impact of
Sequence Position for Pressurized (PO) Stage in ECF
Bleaching, TAPPI Pulping Conference, Oct. 1995/263-280
examine various positions of PO stage (pressurized
peroxide, temperature less than 100~C) in an ECF sequence.
They determined that the optimum position depends on the
mill priorities, low cost, ClO2 generation limit reached,
future TCF. Considerable capital expenditure is required
to implement PO stage in the mill. The same issue was
addressed by Devenyns et al, Optimal Use of Hydrogen
Peroxide to Design Low AOX ECF Sequences, TAPPI Pulping
Conference, Oct. 1995/281-288 with AOX reduction being
the main goal. Chirat et al, Other Ways to Use Ozone in
the Bleaching Sequence, TAPPI Pulping Conference, Oct.
1995/415-419 examine an ozone stage and its position
effect in TCF and ECF bleaching, as well as the DZ stage
with partial chlorine dioxide substitution by ozone.
This was shown to be very economical in the first stage
of bleaching (D1oo). Stevens et al, The Effect of Lignin
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Content on the Performance of a Hydrogen Peroxide
Brightening Stage in an ECF Sequence, TAPPI Pulping
Conference, Oct. 1995/421-439 investigated peroxide
bleaching of pulps with different lignin contents and the
split of peroxide function between delignifying and
brightening with the latter being more important for low
lignin content pulps under the conditions studied.
A discussion of the benefits of caustic extraction
with oxygen and peroxide added (Eop) stage by Hill et al;
An Evaluation of Pressurized Hydrogen Peroxide Systems
for Delignification and Bleaching, TAPPI Pulping
Conference, Oct. 1995/789-805 showed some benefits of
such an approach in terms of reducing chemical
consumption. Caro's acid (peroxymonosulphuric acid) can
be beneficial both in standard ECF sequence and in ozone
enhanced ECF as discussed by Arnold et al, The Degox~
Process - Laboratory and Mill Experiences with
Peroxymonosulphuric Acid, TAPPI Pulping Conference, Oct.
1995/897-902. Laskeeva et al, Several Multistage
Bleaching Sequences for Softwood Kraft Pulp Using
Hydrogen Peroxide in Place of Chlorination; Izv. VUZ,
Lesnoi Zh. No. 6:90-95 (1982) [Russ.] examine the use of
peroxide in one or two stages of sequences containing
chlorine as a bleaching chemical, that is the process was
not ECF. Delefosse, ECF or TCF Pulp? Pap. Carton Cellul.
43, no. 1/2:18,21-23 (January/February 1994) [England]
describe differences between ECF and TCF bleaching
indicating that less than 1 kg/ton of pulp of AOX in the
effluent the toxicity of the ECF efflent is very similar
to TCF effluent indicating the need for developing ECF
methods producing low levels of halogenated compound.
Hamilton et al, Improvements in ECF Bleaching; Use of
Activated Oxygen Species and Xylanase, IPST, Technical
Paper Series, 559 (Atlanta, GA):19p.(March 1995)
[England] investigated free bleaching with xylanase
followed by ECF sequences containing chlorine dioxide
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reinforced with hydrogen peroxide, dimethyldioxirane and
nitrylamine. They reported a 21% decrease in chlorine
dioxide charges when pre-treatment with nitrylamine-
activated hydrogen peroxide was used. Jean et al, Mill
Trial Experiences with Xylanase:AOX and Chemical
Reductions-Annual Meeting (80th CPPA):A229-233, Feb. 1-2,
1994 [England] also experimented with xylanase in ECF
sequence as a way of reducing AOX levels. Malinen et al,
ECF Bleaching of Oxygen-Delignified Softwood Pulp with
the Minimum Charges of Chlorine Dioxide Pulping
Conf.(Atlanta)Proc.(Book3):925-932(TAPPI;Nov.1-3,
1993)[England] described ECF bleaching with xylanase pre-
treatment in which two extraction stages of DEDED
sequence were reinforced with hydrogen peroxide. They
obtained 15-20% reduction in chlorine dioxide usage at
15% higher cost.
The present invention shows improvement in reducing
the OX content of ECF pulp while enhancing the brightness
and preserving the strength of the pulp. The present
invention differs from the prior art in that it is used
at the end of an ECF sequence and uses a mixture of
hydrogen peroxide and chelating agent at atmospheric
pressure with alkaline conditions. No other oxidants
such as oxygen, ozone or per-acids are added. No
separate chelation stage is necessary.
Accordingly, in its broadest aspect, the present
invention is a method for the bleaching of pulp
comprising bleaching the pulp with chlorine dioxide; then
further bleaching the pulp at alkaline pH with hydrogen
peroxide in the presence of a chelating agent.
The method may be carried out in one stage or a
plurality of stage, for example, two. The pH is
maintained alkaline by the use of a base, for example,
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sodium hydroxide. Preferably the pH is in the range of
about 10.5 to about 12.
The concentration of hydrogen peroxide is in the
range 0.1 to 3% by weight of the dry pulp. In general
there is little advantage in going beyond 3% by weight.
Prefered chelating agents include
diethylenetriaminepentaacetic acid (DTPA) or its sodium
salt, diethylenetriaminepenta (methylenephosphonic acid)
or its sodium salt (DTMPA) and 1-hydroxyethylidene-1,1-
diphosphic acid (HEDP). The use of a chelant isessential in maintaining the high peroxide residuals that
are needed to preserve pulp strength. In the present
invention there is no need for the use of magnesium
sulphate for SPF furnish.
The temperature for the peroxide stage is preferably
maintained at about 80~C for the best results. However
the invention is effective at temperatures as low as 45~C.
The optimum range of peroxide residuals is 40-80%.
The present invention allows ECF pulps to be
bleached to a brightness of 88% plus ISO (International
Standards Organization) and good strength with as little
as 0.7% total hydrogen peroxide charge. Significant
brightness increases were achieved with as little as 0.1%
hydrogen peroxide charge.
The OX (organic chlorine) content was substantially
reduced, well below 100 ppm in the final pulp. Typical
furnishes used were 100% SPF (spruce-pine-fir) or 60%
hemlock mixed with 40% cedar.
The invention is illustrated in the following
examples. In the drawings referred to in the Examples:
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Figure 1 is a graph showing the effects of the
chelant DTPA on peroxide residuals;
Figure 2 is a graph relating DTPA addition on pulp
strength;
Figure 3 is a graph showing the effect of various
chelants on peroxide residuals;
Figure 4 is a graph showing the effect of various
chelants on pulp strength;
Figure 5 is a graph illustrating the effect of
temperature on brightness and strength;
Figure 6 is a graph illustrating the effect of
hydrogen peroxide charge on brightness and strength.
EXAMPLE 1: THE PEROXIDE CHARGE AND THE USE OF CHELANT
This experiment focused on comparing a range of
total peroxide additions in the presence and absence of
DTPA as a chelant. The chelant is mainly for the
protection of cellulose during the bleaching action of
peroxide and hence pulp strength preservation. It also
stabilizes peroxide, which makes its action more
effective due to the longer period of high peroxide
concentration. The total peroxide (H2O2) charge was 3% of
the pulp dry weight. This 3% chemical was added in two
stages; one with 1.5/1.5 and the other 2.0/1.0 split.
The first number is the first stage charge and the second
number is the second stage addition.
A 50 grams of spruce-pine-fir (SPF) kraft pulp that
was ECF bleached to 84.1% ISO brightness in DEoD sequence
with OX content of 111.9 ppm was used for this
experiment. Figure 1 shows the effect of chelant on the
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g
final peroxide residual (%) measured in the bleaching
filtrate and brightness of pulp under the same 3% H2O2
addition but distributed differently in two stages. With
and without the addition of chelant in the bleaching
process the brightness values of pulps were similar in
the range of 88.5 to 88.9% ISO. This is an increase of
about 3.5 to 4% from the control of non peroxide
treatment. This evidence suggests that the peroxide can
be successfully applied to the ECF pulp to obtain a
significant brightness increase. However, with the
addition of DTPA the residual peroxide was about 80%, the
same for both 1.5/1.5 and 2.0/1.0 peroxide charges. This
percentage peroxide residual was much smaller when
chelant was not added. The peroxide residue was greater
for 2.0/1.0 charge with about 20% while that for the
1.5/1.5 charge was about 5%. Since the pulp brightness
is similar, the big difference in peroxide residuals
could be explained by the fast peroxide decomposition in
the absence of chelant and thus the generated radicals
not having enough time to react with lignin.
Figure 2 shows the relationship between the chelant
addition and wet zero-span pulp strength. The advantage
of the addition of the chelant is further substantiated
by the pulp strength. The starting pulp had 13.6 km wet
zero-span strength. With the chelant DTPA, the strength
value for the different stages of peroxide was the same
at about 13 km. Without the chelant, the 1.5/1.5 H2O2
charge gave about 11 km strength while the strength for
the 2.0/1.0 H2O2 charge was about 9.8 km. Apparently, the
first charge of larger amount of peroxide in the first
stage had greater reduction effect on the pulp strength.
Since the original, DED, pulp had 13.6 km strength and
DEDED pulp bleached with chlorine dioxide in the last
stage had a zero span strength of 13.0 km, the addition
of chelant did preserve the strength of pulp to maintain
it at the level obtained during standard DEDED sequence.
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This example demonstrates that with the peroxide and
chelant addition according to the invention the ECF pulp
brightness could increase significantly and the strength
of the pulp be preserved.
EXAMPLE 2: THE REDUCTION OF ORGANIC HALOGEN CONTENT
Three experiments below illustrate the reduction of
organic halogen in pulp.
In the first experiment the total peroxide (H202)
charge was 1. 5% of the pulp dry weight. Chelant (DTPA)
was added at 0.1% charge in the first stage only. In the
second stage peroxide was added and caustic at 1.2%
charge. No magnesium sulphate was added. The pulp was
SPF (spruce-pine-fir) bleached to 84.1% ISO brightness
using DEopD sequence.
In the second experiment the total peroxide (H202)
charge was 2% of the pulp dry weight. This 2~ chemical
was added in two stages; first - O. 5% and second 1. 5%.
Caustic charges were 0.6% and 1.0% respectively and 0.1%
DTPA was added in each stage. No magnesium sulphate was
added. The pulp was SPF bleached to 81.4% ISO brightness
using DEopD sequence.
Figure 3 shows the effect of various chelants on
peroxide residuals.
Figure 4 shows the effect of chelant on the strength
of the pulp. Both chelants exemplified showed similar
effectiveness in retaining the pulp strength while the
pulp without the chelant added had significantly lower
strength at both levels of peroxide charges.
In the third experiment the total peroxide (H202)
charge was 0.7% of the pulp dry weight. This chemical
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was added in one stage. Caustic charge was also 0.7% and
0.1% DTPA was added. Magnesium sulphate charge was 0.1%.
The pulp was 60% hemlock/40% cedar mixture bleached to
83.5% brightness using DEopD sequence.
Bleacbing stage ~p ~unple
Avclage
OX ill DED control Im~R~. ~ 3 3 ppm
Brightnes~ofe~dpulp ~
OXineDdpulp ~ ppm 86.1ppm
% OX ~ S.4%
Table 1: OX Reduction for Various Pulps
The OX reduction averaged 33.6% and yielded pulp
with significantly lower OX than the average mill ECF.
The mill data for the OX content of ECF pulp showed a
mean of 136 ppm with a standard deviation of 1.44 ppm for
spruce-pine-fir furnishes (interior mill) and a mean of
181 ppm with a standard deviation of 7.34 ppm for coastal
mill pulps (various furnishes).
Examples 1 and 2 demonstrate that a procedure
derived by the addition of chelant to peroxide can
significantly increase the brightness of pulp, reduce the
organic chlorine content (OX) and preserve the strength
of pulp.
EXAMPLE 3: THE EFFECT OF CHELANTS FOR DIFFERENT H2O2
CHARGES
Besides DTPA, another chelant, DTMPA, was introduced
in this experiment. Pulp samples were bleached with
addition of chelants and with no chelant added. The
total peroxide charges were 1.5% and 3%. In all cases
1.5% hydrogen peroxide was added in the second stage.
Hence, the first stage of first series of experiments
(1.5% total H2O2) was either Q stage with 0.1% DTPA added
or neutral soak with no chelant added.
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Figure 3 shows that with the addition of chelants
the peroxide residuals were higher than those of non-
chelant treatments at both peroxide charging levels.
Within the chelant application, DTMPA had lower peroxide
residuals than DTPA, which explains slightly higher
brightness obtained in experiments with DTMPA added -
89.1% ISO versus 88.6% ISO and 89.7% ISO versus 88.7%
ISO. The difference in the peroxide charge did not
significantly affect the brightness of the pulp. It
means that as long as sufficient peroxide is present the
brightness can be enhanced from the control of 84.1% to
the 88% + level.
Figure 4 shows graphically the effect of various
chelants on pulp strength. Example 3 again substantiates
the discovery of Example 2, namely the effectiveness of
peroxide application to ECF pulp for increasing
brightness while the chelant stabilized the reactions and
thus served to preserve the strength of pulp. The chelant
can be DTPA, DTMPA, ATMP(aminotri(methylenephosphonic
acid)), HEDP or any other chelant capable of chelating
the metals in the pulp.
EXAMPLE 4. THE EFFECT OF TEMPERATURE
Bleaching experiments were conducted using QP
sequence with 0.1% DTPA charge in Q stage and 1.5%
H2O2/1.2% NaOH/0.1% DTPA/0.05% MgSO4 charge in P stage at
various temperatures. SPF furnish bleached to 84.1% ISO
brightness in a DEoD sequence was used. Figure 5 shows
that at temperatures ranging from 45~ to 85~ the bleaching
results are very similar with high brightness and
strength achieved.
This example shows, surprisingly, that contrary to
conventional peroxide bleaching, which is usually
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conducted at 75~C+, temperatures as low as 45~ are
sufficient in the present invention.
EXAMPLE 5. THE EFFECT OF LOW HYDROGEN PEROXIDE CHARGES
Bleaching experiments were conducted using one
peroxide, P, stage with 0. 1% DTPA charge, 0.2% NaOH
charge and varying peroxide charges at 85~C. SPF furnish
bleached to 84.1% ISO brightness in a DEoD sequence was
used. The bleaching results presented in Figure 6
indicate that a charge of hydrogen peroxide as low as
0.1% still produced significant improvement in
brightness.
This indicates that a range of hydrogen peroxide
addition from 0.1% to 3% on OD pulp provides effective
bleaching action. Higher amounts of hydrogen peroxide can
be used but that would not be economically justified in
most cases.
Although the foregoing invention has been described
in some detail by way of illustration and example for
purposes of clarity of understanding, it will be readily
apparent to those of ordinary skill in the art in light
of the teachings of this invention that certain changes
and modifications may be made thereto without departing
from the spirit or scope of the appended claims.
