Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Oxygen delignification process of pulp
FIELD O~ THE INVENTION
S The present invention is concerned with a single stage process for
decreasing
carbohydrate degradation of sulphite pulps during the O~ process by the
addition of a
catalytic amount of sodium borohydride in situ to provide a pulp with enhanced
strength
properties and increased viscosity.
BACKGROUND OF THE INVENTItaN
Because of increasing environmental concerns worldwide) pulp and paper mills
discharge effluents are constantly under scrutiny to ensure that environmental
regulations
are followed. Because of the high costs involved in the treatment of effluents
before their
release in the environment, a great deal of research is directed to the
modification of current
pulp and paper production processes. The research concentrates its efforts in
replacing
toxic reagents with more environmentally friendly products. A further benefit
sought with
such changes is that effluents will hopefully require fewer costly
conditioning treatments
before their release in the environment.
In the various processes proposed in the literature) the oxygen deligniFcation
technology is one of the available options towards this direction.
Conventionally, oxygen
delignification technology uses sodium hydroxide as the alkaline source and
the resulting
effluent produced can therefore he incorporated into the chemical recovery
system of the
process for preparing kraft pulps hecause the same reagent, namely sodium
hydroxide) is
used, and therefore) there is no rca~~cnt interference. On the other hand, the
eff7ucnt from
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the sodium hydroxide-based oxygen deligniflcation process (referred to as the
O,~aOII
technology herein) cannot be sent to the recovery system of the magnesium-
based sulphite
process because, obviously, the sodium salts are not compatible with the
magnesium-based
sulphite recovery process. Several publications have therefore concluded that
magnesium
oxide-based oxygen deligniflcation technology, referred to as OMB herein) is
preferred for
magnesium-based sulphite pulping processes. {see for example Bokstrom et al.,
Pulp and
Paper Canada, 1992, 92 (11)) 38; and Luo et al., Tappi Journal, 19~, 75 (6),
183).
Sodium hydroxide has been replaced lately as a base with magnesium oxide
(Mg0) or magnesium hydroxide {Mg(OH)~ for the oxygen delignification of
sulphite pulps.
However, because of the low alkalinity of Mg0 or Mg(OH)2, the temperature of
delignifieation with Mg0 or Mg(OH)Z must be about 30°C higher than for
the same process
using NaOH as the delignification agent (see Luo et al., supra).
Alternatively) the
deligniftcation rate can be increased in the OMB process by the addition of a
very limited
amount of NaOH) since small concentrations of sodium salts can be tolerated in
the
recovery system of magnesium-based sulphite process. However, the risk of
contamination
in the long run is such that this alternative does not represent a desirable
selection.
Changing the alkali source in the oxygen delignification process from sodium
hydroxide to magnesium oxide or magnesium hydroxide) as taught by Bokstrom et
al.
supra, decreases the selectivity of lignin to carbohydrate degradation.
Moreover, the
strength properties also decrease, as illustrated in the relationship between
tear index versus
tensile index of Fig. 6, by Luo et al. supra. For a given type of wood chips
used as
starting material, it is well known that sulphite pulps usually have strength
prupcnies
inferior to that of kraft pulp) and a further decrease in strength properties
durin~~ the
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delignification process is therefore unacceptable for commercial operations.
It is known that a post treatment stage with sodium borohydride on an oxidized
pulp, such as ozone deligni>ied pulp, leads to increased pulp viscosity. For
example) it was
S reported by Chirat et al. in Holzforschung, X94, 48 Suppl. 133) that a
reduction treatment
stage with 0.1 % sodium borohydride increases the viscosity of ozone bleached
pulp from
DP" of 710 to 920. The chemistry of sodium borohydride reduction is well
understood:
carbonyl groups present in carbohydrates are reduced to alcohol
funetionalities (B.
Browning, Methods of Wood Chemistry, Vol. 2, P. 685, Interscience Publishers).
In addition) it is proposed by S. Beharic in Papir 20) December 1992, 3(4) pp.
11-15 to add sodium borohydride either before ozone bleaching or after
peroxide bleaching
to limit the reduction in pulp viscosity. Again, two stages are involved for
this pulp
treatment.
Accordingly) there is therefore a great need to develop an oxygen
delignifieation process providing pulps with enhanced strength properties and
increased
viscosity. Preferably, a single stage bleaching process should be considered,
wherein a
reducing agent would be added in situ. This would represent a significant
advance in pulp
bleaching) and bring significant benefits to the industry) because the
elimination of one
treatment stage of pulp represents a significant capital cost reduction.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is now provided an
improvement to oxygen delignitiration process of pulps. More specifically, the
preacnt
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invention comprises the conventional steps of oxygen deligniflcation of pulp)
namely
treating a pulp slurry in a reactor under oxygen overpressure in the presence
of an alkali
source, over a period of time sufficient to complete deligniflcation of the
pulp, with the
improvement comprising adding in situ an effective amount of a reducing agent
substantially non-reactive with the oxygen, to provide delignifled pulps with
enhanced
strength properties and increased viscosity.
The present oxygen deligniflcation process is particularly advantageous for
sulphite pulps when Mg0 or Mg(OH)Z are used as the alkali source. Other
possible alkali
sources include Ca(OH)Z, NH40H, NaOH and the like. Reducing agents include
sodium
borohydride) sodium hydrosulphite and the like, with sodium borohydride being
the most
preferred.
In another aspect of the present invention, the process comprises a first
stage
wherein the pulp is treated with the reducing agent, and then washed and
pressed if
necessary, and a second conventional oxygen delignification stage. Although
good results
are obtained with the two-stage process) the single stage process is much
preferred because
of the elimination of washing and pressing operations required after treatment
with the
reducing agent in the two-stage process.
IN THE DRAWINGS
Figure 1 illustrates the strength properties of 0~,,,go + RDoEop(DP) bleached
pulp and
those of OM~DoE~ (DP) bleached pulp.
DETA ED DESCRIPTION OF THE INVENTION
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It is an object of the present invention to provide a single or two-stage
oxygen
delignification process of pulp which can minimize the carbohydrate
degradation and thus
enhance the strength properties of the pulp, as well as the viscosity. The
present single
stage process is most advantageous for Mg0 delignification of sulphite pulps.
It is .also an object of the present invention to provide a two-stage oxygen
delignification process of pulps which can minimize the carbohydrate
degradation and thus
enhance the strength properties of the pulp, as well as the viscosity. As for
the single stage
process, the two-stage process is also most advantageous for Mg0
delignification of
sulphia; pulps.
The present invention comprises the use of a small amount of a reducing agent,
most preferably sodium borohydride, either before or during the oxygen
delignification
prcx;css. Preferred alkali source, as mentioned above) are MgO) Mg((?H)2 and
the like.
The key feature of the unexpected results obtained with the present single
stage process is
that the activity of the reducing agent is substantially not affected by the
ovcrprcssure of
oxygen in the reaction media. The reducing agent and the alkali source may be
added
simultaneously) or the reducing agent is added to the pulp shortly before the
alkali source.
The reverse order of addition of reagents is also possible. Preferred
experimental
conditions for carrying the present single stage process are as follows: pH
from H t~ 13;
temperature of from 80 to 140°C; an oxygen overpressure of from 30 to
20U psi; a pulp
consistency of from 1°k to 50%; an addition of tom 0.01% to 10°%
of sodium borohydride,
an addition of Mg0 of from 0.2 to 10%; and a reaction time of from 0.1 to 4
hours. !vlost
preferred conditions are: 127 ° C; 1 tlU psi Oz pressure; 10~ pulp
consistency; an addition
of 0.1 °k of sodium borohydridc. an addition of 2% Mg0 and a reaction
time of 2 hours.
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It has been unexpectedly found that the addition of sodium borohydride in
conventional
magnesium oxide-based oxygen delignification process of sulphite pulp results
in the
production of pulps having an increased viscosity and improved strength
properties when
compared to -pulp prepared in the same manner but without the addition of
sodium
borohydride.
In the case of a two-stage process) the experimental conditions for the oxygen
delignification stage are the same as those above. With respect to the first
stage, preferred
conditions are as follows: 0.1% to 10~k (by weight on pulp) of reducing agent
is mixed with
a pulp suspension at a pH of from 5 to 13 and maintained at a temperature of
20 to 100°C
for a period of time of a few seconds up to 2 hours. The pulp is then washed
conventionnally, for example with water, and pressed if necessary) to collect
the pulp fibers
which will be treated under the above oxygen delignificativn stage. It has
been
unexpectedly found that the treatment of sulphite pulp with sodium borohydride
before
treating the pulp with a conventional magnesium oxide-based oxygen
delignification process
results in the production of pulps having an increased viscosity and improved
strength
properties when compared to pulp prepared in the same manner but without the
addition
of sodium borohydride. Results hereinbelow will also show that if the pulp is
Lust treated
under oxygen deligniflcation conditions and then with a reducing agent, the
properties of
the pulp are not as good as the single stage treated pulp) or two stage
treated pulp wherein
the treatment with reducing agent occurred before oxygen deligniflcation.
The expression "enhanced stength properties" should be interpmted as meaning
that the strength properties of the pulp are at least comparable, and
generally better than
those of pulp obtained from a hk:aching ~quence using chlorine or chlorine
dioxide without
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oxygen deligniflcation. Further) "increased viscosity" means that the
viscosity is at least
equivalent to that of eastern Canadian softwood sulphite pulp after
conventional O~~
delignification) i.e., typically about 25 mPa.s to about 45 mPa.s.
The following examp~s are provided for illustrating the present invention and
should not be construed as limiting its scope.
Example 1
40 g of eastern softwood sulphite pulp (kappa no. 26.0, pulp viscosity 40.1
mPa~s determined on the chlorite delignified pulp) in a pulp consistency of
28.5% is
weighed into a beaker containing about 360 ml of water, resulting in a pulp
consistency of
about 10% and then disintegrated in a conventional manner) for example by a
blender to be
free from fiber bundles. 0.5 % sodium borohydride (by weight on pulp) is
rapidly mixed
with the pulp slurry prepared above (-- 400 ml). The slurry has a pulp
consistency of 10%
and contains about 2% Mg0 (by weight on pulp) and 0.2% MgS04 (by weight on
pulp).
The pulp slurry is subsequently transferred to a Parr pressure reactor
preheated at a
temperature of about 100°C. The O~,go rR Press, that is, conventional
OMB, technology
with the addition of sodium borohydride in situ) is performed at about
127°C and 100 psi
for 2 hours in a single stage. The resulting pulp with a kappa number of
14.(.) is then
bleached to full brightness (90% ISO) in accordance with the conventional
D~,E~~ (DP)
wherein
- Do stands for a chlorine dioxide stage;
- E~ represents a peroxide reinforced oxidative stage; and
- (DP) means that no washing is performed between chlorine dioxide treatment
and
peroxide treatment sequence.
i'
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The detailed conditions of each stage in the DoEoP(DP) sequence is provided
in Table 1 below. The tear-tensile beating curve of the OMB+e DoEoP (DP)
bleached pulp
is shown in Figure 1.
Table 1
Detailed Conditions of Each Stage in the DoE°p(DP) Stage
(DP)
Da EW
D P
Consistency (~o)3.5 10 11.5 10
Time (min) 12 30 145 70
Temperature ( 60 70 72 80
'C)
O~ pressure (psi)- 40 for - -
4 min.
Chemicals (96 Kappa: HiOz: 0.8 CIOz: 0.4 HzO:: 0.5
oa pulp) 0.14 NaOH:l.8 NaOH:0.7
C10,:0.75
Example 2
This experiment is provided to illustrate the properties of a pulp obtained in
experimental conditions similar to those of Example 1 without adding sodium
borohydride
during the conventional O",~ process. As it will be seen, the strength
properties of the
OMB DoE°P{DP) bleached pulp are inferior to those of the OMgo+R Do
EoP(DP) bleached
pulp.
The eastern softwood sulphite pulp (kappa no. 26.0) pulp viscosity 40.1 mPa.s
determined on the chlorite delignil3ed pulp) used in Example 1 is also used in
the present
example. 2 % Mg0 (by weight on pulp) and 0.2% MgS04 are mixed with a pulp
suspension containing 40 g pulp. The pulp slurry is then transferred to a Parr
pressure
reactor preheated at a temperature of about 100°C. The OMBO
deligniftcadon process is
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perfornied at a temperature of 127°C with an oxygen overpressure of
about 100 psi for 2
hrs. At the end of these 2 hours, the resulting pulp with a kappa number of
13.8 is then
' further bleached to a full brightness in accordance with the
D°E°P(DP) sequence described
above. The -tear-tensile beating curve of the O~ D° E~ (DP) bleached
pulp is also
illustrated in Figure 1, which shows that the strength properties of 0,,,~0 +
RDoE°P(DP)
bleached pulp are significantly improved over those of OM~DoE~ (DP) bleached
pulp.
Example 3
This example is provided to show the effect of sodium borohydride
concentration on the pulp viscosity after the OMB, R delignification process.
An Eastern
softwood sulphite pulp with a kappa no. of 25.2 and viscosity of 43.1 mPa~s
determined on
the chlorite delignified pulp is used. The sodium borohydride concentration
varies from 0
to 0.05 to 0.1 to 0.2°k (by weight on pulp)) The required amount of
NaBH, is rapidly
mixed with a pulp slurry having a pulp consistency of 10% and containing about
2% Mg0
and about 0.2 % MgSO,. The subsequent procedures are identical to those
described in
Example 1. The kappa number) viscosity and brightness of the OMBO+R ~ligniGed
pulps
at various NaBH, concentrations are given in Table 2.
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Table 2
Effect of the sodium borohydride concentration on
pulp viscosity during the O,r,~ process
Sodium borohydride concentrationKappa ~lp Brightness
(% on pulp) number viscosity(% ISO)
(mPa.s)
0 12.8 27.8 57.7
0.05 13.9 39.1 59.6
0.1 12.4 43.0 62.5
IO 0.2 14.6 43.0 60.9
~
The above results clearly show that the pulp viscosity is significantly
improved
when sodium borohydride is present during the OMB delignification stage. In
addition, the
brightness of the OMgO+R delignified is always higher than that of the OMgp
treated pulp
under otherwise the same condition. Furthermore, the data show that a sodium
borohydride concentration as low as 0.05% is sufficient to achieve the desired
benet7cial
effect.
Example 4
This example is provided to show that a two-stage ROMgo, i.e., treatment with
sodium borohydride in a first stage followed by water washing and then
conventional 0~,~
in a second stage) can produce acceptable delignified pulp with properties
inferior to those
of the OMB+R treated pulp.
The same Eastern softwood sulphite pulp as that used in Example 3 is uu;d in
this example. 0.1% NaBH, (by wci~,ht on pulp) is mixed with a pulp suspension
of pI-i 9.5
and containing 20 g pulp in a polyethylene bag. Sodium hydroxide is used to
increase the
pH. The polyethylene bag is then thernu~stated at 50°C. At the
completion of 30 minutes)
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the pulp slurry is thoroughly washed with purified water and the pulp fibres
are collected
for the subsequent O~ treatment under tl~ conditions of 10% pulp consistency,
2%a MgO,
0.2% MgSO,, 100 psi, 127°C, 2 h and without the addition of sodium
borohydride. The
kappa number, viscosity and pulp brightness of the resulting pulp are compared
to those of
OMB treated and OMB+R treated pulps in Table 3.
Table 3
Kappa number, viscosity and brightness obtained
for pulp treated under different processes
Technique Kappa no. ~'i~osityBrightness
(mPa.s) ~'
(~ ISO)
OMao 12.8 27.8 57.7
~MgO+R (0.1% NaBH4) 12.4 43.0 62.5
RO (0.1% NaBH ) 13.8 39.5 59.9
Table 3 shows that the results obtained for a pulp treated under the ROu~
process are better than that treated under the OMB process. However) the best
results are
obtained with a single stage OMB,, R Process.
Example 5
This example is provided to show that a reduction with sodium borohydride in
a second stage after the OMB treatment of the pulp in a first stage, i.e., a
OM~R sequence,
. also increases the pulp viscosity) but rather moderately. However) the
viscosity of the
O,",~R treated pulp is substantially lower than that of the single stage OM~+
a treated pulp
according to the present invention.
The same Eastern sot'twcu~d sulphite pulp as in Example 3 is used. 20 g oC
pulp
i
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is subjected to a first stage OMB process under the conditions of 10% pulp
consistency, 2%
MgO, 0.2% MgS04, 100 psi, 127°C, 2h) without adding sodium
borohydride.
Subsequently) the O~ delignified pulp is treated in a second stage with 0.1 %
sodium
borohydride at pH 9.5, 10% pulp consistency and 50°C for 30 minutes.
The kappa
number, the viscosity and the pulp brightness of the resulting pulp are
compared to those
of OMB, treated and OMB+R treated pulps in Table 4.
Table 4
Kappa number, viscosity and brightness obtained
for pulp treated under different processes
Technique Kappa no. Vi~Sih' Brightness
(mPa.s) (% ISO)
12.8 27.8 57.7
OMB ~ R (U. l 6 NaBH4) 12.4 43.0 62.5
O ,R (0.1% NaBH ) 12.0 32.9 63.3
The above results show that the viscosity of the OM~R treated pulp is about
5 units higher than that of the OMB treated pulp) however about 10 units lower
than that
of the OMB+R treated pulps.
In view of the above results, it is apparent that the addition of a reducing
agent
in situ during the oxygen delignification process provides pulps with a high«r
viscosity and
increased strength properties than that obtained during two-stage processes
wherein the
reducing agent is added either prior to or after the oxygen delignification
process. The
combination of a two-stage operation into a single stage one is beneficial not
only hc:cause
one stage has been removed) but also because the physical properties of the
resulting pulp
are significantly better. Nevertheless) good results are also obtained with a
two stage
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process wherein the treatment with the reducing agent is carried out before
the oxygen
delignification stage. For obvious reasons, as mentioned above, a single stage
process is
most preferred.
While the invention has been described in connection with specific embodiments
thereof, it will be understood that it is capable of further modifications and
this application
is intended to cover any variations) uses or adaptations of the invention
following) in
general, the principles of the invention and including such departures from
the present
disclosure as come within known or customary practice within the art to which
the
invention pertains, and as may be applied to the essential features
hereinbefore set forth, and
as follows in the scope of the appended claims.
