Note: Descriptions are shown in the official language in which they were submitted.
20531~3~
CHLORINE-FREE WOOD PULPS
AND PROCESS OF MAKING
Background of the Invention
In recent years, there has been increasing
public concern about industrial waste streams, emissions,
and solid wastes being discharged into the environment.
Market and regulatory pressures are now requiring
manufacturers in all industrial sectors to minimize this
discharge burden on the environment.
In the pulp bleaching industry, effluents from
pulp mill bleach plants have received public and
government scrutiny. Such effluents contain chlorinated
organic bleaching reaction products which are generally
measured by their adsorbable organic halogen (AOX).
Bleached pulp may also contain chlorinated organic
residues which are generally measured by their total
organic halogen (TOX).
Earl and Reeve, of the University of Toronto,
have studied levels of AOX in bleach plant effluents, and
have developed an empirical relationship to predict AOX
levels produced in the bleaching process. According to
Reeve's group, AOX in bleach plant effluents will be about
-2- 2G~3~
10~ of the weight of molecular chlorine (C12), and 5.3% of
the weight of chlorine dioxide (ClO2) used in the
bleaching process. Using the amounts of chlorine and
chlorine dioxide that are customary to bleach softwood
kraft pulps with "conventional" bleaching technology, AOX
in untreated bleach plant effluents is found to be in the
range of 5 to 8 kg AOX per ton of pulp bleached.
Secondary treatment systems will remove an additional 40~
to 60~ of this AOX, indicating that the range of AOX
discharged to receiving waters will be about 2 to 6.8 kg
AOX per ton. Current regulatory targets seem to allow a
maximum of about 2.5 kg AOX per ton, with further
restrictions to 1.5 kg per ton in the foreseeable future.
The use of chlorine based bleaching chemicals
additionally leaves some chlorinated organic residues in
the pulp. A recent study by Reeve's group has shown that
total TOX in bleached softwood kraft pulps from Canadian
mills (where few mills have oxygen delignification) is in
the range of 400 to 600 parts per million (ppm), and that
for bleached hardwood kraft pulps, values as high as 2,000
ppm have been determined.
Therefore, traditional pulp mills mostly of the
kraft process type have devised new digestion conditions
for increasing delignification of the wood pulp and have
attempted to implement oxygen delignification prior to
bleaching in order to reduce the consumption of chlorine
containing bleaching agents. Other attempts at oxygen
delignification include the substitution of chlorine
dioxide for molecular chlorine to give equivalent
- -3- 2~5~Q~
bleaching with much lower levels of atomic chlorine. The
combination of these technologies should provide the means
for kraft mills to meet the 1.5 kg AOX per ton limit.
On the other hand, new bleaching processes are
being developed which contain no chlorine bleaching
chemicals. For conventional kraft softwood pulps,
bleaching processes using oxygen, ozone and hydrogen
peroxide have been developed. Softwood kraft pulps
bleached with oxygen and hydrogen peroxide are of low
brightness and have questionable strength properties.
Other developments with kraft and sulfite pulping
processes involve the incorporation of ozone bleaching
equipment in an effort to eliminate the use of chlorine
containing bleaching agents.
However, with kraft pulps which constitute the
industry's standard for pulp strength for either hardwood
or softwood species, lower brightness levels have been
achieved as compared to brightness levels obtained with
chlorine or chlorine dioxide as bleaching agents.
There are additional environmental and economic
benefits from the use of non-chlorine bleaching agents
including the recyclability of industrial waste streams.
Furthermore, bleaching processes using non-chlorine
bleaching agents have the potential for disposal of all
bleach plant residues by burning. Currently, bleaching
processes which use oxygen delignification are able to
recycle the bleach effluents from this stage to the mill
chemical recovery system. The benefits that accrue include
2 ~
a reduced demand on secondary treatment systems and a
decrease in chemical processing costs. Additionally,
through recycle of effluents and discharge of cleaner
industrial waste streams, the costs associated with
industrial waste treatment decrease. Therefore, it is
advantageous to devise industrial processes which make use
of recycled effluents, require less fresh water, and
discharge less industrial waste all of which result in an
overall decrease in plant operating costs.
Industrial processes which are designed with the
foregoing objectives make use of the "Closed-Mill"
concept. This concept requires that all process chemicals,
including water, be recycled and also requires that almost
all waste, including heat is reused.
Wood pulps produced by organosolv pulping of
lignocellulosic material such as described, for example,
in U.S. Patent Nos. 4,100,016 and 4,764,596 also referred
to as the ALCELL~ process, employ alcohol extraction. Such
processes will be collectively referred to as "organosolv
processes" and offer some distinct advantages for closing
a pulp mill while providing commercial quality hardwood
pulps that are comparable in strength, brightness and
cleanliness to kraft pulps produced from the samè wood
species. In such processes, by the methods of the present
invention, the bleach plant effluents can be returned to
the pulp mill with minimal treatment.
For optimal pulp strength, however, organosolv
pulps must be digested by cooking such that there is a
20~0~
higher residual lignin in the organosolv pulp as compared
to kraft pulps. The pulp at this stage is referred to as
brownstock and residual lignin in pulp brownstock is
measured by units called kappa numbers. Typical kappa
numbers for organosolv pulps are approximately 25 to 35,
depending on the wood species and other factors, whereas
kappa numbers for kraft hardwood pulps typically will vary
between about 17 and 21, depending on the same factors.
The consequence of the higher kappa number in organosolv
pulps is generally that larger quantities of bleaching
chemicals must be used to produce bleached pulps at the
same brightness level as compared to kraft pulps.
However, as compared to kraft pulps, the methods of the
present invention require smaller amounts of bleaching
compounds, other than oxygen and sodium hydroxide, to
achieve bleached pulps with similar brightness as measured
by the standard of the International Organization for
Standardization (ISO).
Viscosity which is a measure of pulp strength
(centipoise, cps, TAPPI Standard T-230) is also one of the
important characteristics of pulp. Typical viscosities of
organosolv pulp brownstock range from approximately 40 to
13 cps and those for kraft pulp brownstock range from
about 45 to 20 cps, with the lower numbers indicating
lower viscosity or strength. It is desirable to have as
little decrease in viscosity as possible during the
bleaching process, since viscosity losses are usually
associated with a de rease in pulp strength, as measured
by customary tensile, burst and tear strength tests.
-6- ~ 2 0 5 3 0 3 5
A disadvantage of the kraft process is that
oxygen delignification of kraft pulp brownstock results in
reduction of pulp strength below acceptable limits when
oxygen deiignification of the pulp exceeds a 50% of which
corresponds to a reduction of the brownstock kappa number
in excess of 50%.
By contrast, the methods of the present
invention show that pulps can be bleached to above 85 ISO
without the use of chlorine containing bleaching
chemicals. The net result is that very low levels of
adsorbable organic halogen (AOX) and total organic halogen
(TOX) will be present in the bleach effluent and the
bleached pulp respectively. Additionally, if a brightness
above 85 ISO is required, it can be achieved with the use
of low levels of chlorine dioxide such that the adsorbable
organic halogen in the untreated bleach plant effluent is
approximately 0.5 kg AOX per ton of pul-p.
Summary of the Invention
It is a primary object of this invention to
provide a process for the oxygen delignification of pulp
wherein the pulp residual lignin is decreased in excess of
50% with little or no decrease in pulp viscosity.
It is a primary object of this invention to
provide a process for oxygen delignification of pulp
wherein the pulp residual lignin is decreased in excess of
about 50% to about 76~ with little or no decrease in pulp
viscosity.
It is another object of this invention to
provide a process for further delignification of the pulp
with ozone wherein the pulp residual lignin is decreased
,"'1~ -
~ . .
~O~3~3
in excess of from about 80% to about 90% with little or no
decrease in pulp viscosity to within about 2 to 5 cps.
It is another o~ject of this invention to
provide a process for bleaching the delignified pulp to a
brightness in excess of about 70 ISO to about 88 ISO with
no use of chlorine based bleaching chemicals.
It is another object of this invention to
delignify and ~leach organosolv pulps with relatively high
kappa values as compared to kraft pulps from the same wood
species to a high brightne~s ~evel and without loss of
strength. In one aspect of the invention, a process is
provided for oxygen delignification of pulp wherein the
pulp residual lignin is decreased in excess of 50~ with
little or no decrease in pulp viscosity.
In one aspect of the invention, a process is
provided for oxygen delignification of pulp wherein the
pulp residual lignin is decreased in excess of about 50%
to about 76~ with little or no decrease in pulp viscosity.
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising pretreating pulp brownstock
with a peroxy compound such as peracetic acid or hydrogen
peroxide, prior to performing oxygen delignification.
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising pretreating pulp brownstock
with ozone prior to performing oxygen delignification.
.;
-8- 20~3~
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising treating the delignified pulp
with a peroxy compound such as peracetic acid or hydrogen
peroxide.
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising treating the deliginifed pulp
with ozone.
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising pretreating pulp brownstock
with ozone prior to performing oxygen delignification.
In yet another aspect of this invention, a
process is provided for enhancing the effect of oxygen
delignification comprising pretreating pulp brownstock
with a peroxy compound prior to performing oxygen
delignification, and treating the resulting pulp with a
peroxy compound such as peracetic acid o~ hydrogen
peroxide.
In yet another aspect of this invention, a
process is provided for enhancing the effects of oxygen
delignification comprising pretreating pulp brownstock
with ozone prior to performing oxygen delignification, and
treating the resulting pulp with a peroxy compound such as
peracetic acid or hydrogen peroxide.
2 ~ ~3 o ~ ~
In still another aspect of this invention, a
process is provided for enhancing the effect of oxygen
delignification comprising performing oxygen
delignification and treating the resulting pulp with two
bleaching stages of either peracetic acid, hydrogen
peroxide, or a combination of both peracetic acid and
hydrogen peroxide.
In still another aspect of this invention, a
process is provided for enhancing the effect of oxygen
delignification comprising performing oxygen
delignification and treating the resulting pulps with two
bleaching stages of either peracetic acid, hydrogen
peroxide and ozone, or a combination of both peracetic
acid and hydrogen peroxide or ozone.
In another aspect of this invention, a
continuous process is provided for the delignification and
bleaching of pulp wherein bleaching filtrates are used to
wash pulp brownstock and thereafter recycled for reuse in
a continuous pulping process.
Other features and advantages of this invention
will be apparent from the following description of the
preferred embodiment and from the claims.
Description of the Drawings
Figure 1 is a graph showing the reduction in
kappa numbers of ALCELL~ pulps after oxygen
-lO- 20~033
delignification (~2) (~ ) and after oxidative extraction
(Eo) (o).
Figure 2 is a graph which compares the reduction
in kappa numbers and viscosity values of ALCELL~ and kraft
pulps after oxygen delignification.
Figure 3 is a beating curve for an organosolv
birch pulp bleached to 88 ISO with the sequence EoDED.
Figure 4 is a beating curve for an organosolv
birch/aspen/maple pulp bleached to 88 ISO with the
sequence ODED.
Figure 5 is a beating curve for an organosolv
birch/aspen/maple pulp bleached to 83 ISO with the
sequence PO(PA)P.
Figure 6 is a flow chart of a process for the
continuous delignification and bleaching of pulp
brownstock using countercurrent washing and recycling of
solvents and bleaching filtrates using the following
sequences: (Peroxy)O(Peroxy)(Peroxy)and (Peroxy)OZ(Peroxy)
wherein (Peroxy) is either hydrogen peroxide or peracetic
acid.
Figure 7 is a flow chart of a process for the
continuous delignification and bleaching of pulp
brownstock using cou~tercurrent washing and recycling of
solvents and bleaching filtrates using the following
sequences: ODEoD and ODED.
-11- g 7 0 53 0 3 S
Figure 8 is a flow chart of a process for the
continuous delignification and bleac~ing of pulp
brownstock using countercurrent washing and recycling of
solvents and bleaching filtrates using the following
sequences: O(Peroxy)D and OZD wherein (Peroxy) is either
hydrogen peroxide or peracetic acid.
Detailed Description of the Preferred Embodiment
This invention generally relates to the
delignification and bleaching of pulps obtained from kraft
and organosolv pulping processes. The delignification and
bleaching steps of the process can be carried out in
either a batch or continuous mode. Pulps can be
delignified with oxygen and bleached, and Kappa values can
be decreased in excess of 50% and preferably in the range
of from about 50% to about 76% without any significant
decrease in pulp viscosity. Commercially acceptable high
brightness levels can also be achieved. The bleaching
effluent streams from the practice of the present invention
meet or exceed stringent environmental regulations.
By either the batch or continuous processes of
the invention, oxygen delignification of organosolv pulps
yield a reduction of kappa numbers in excess of about 50%
and preferably up to about 76% and without a decrease in
viscosity values of no more than about 8 cps, preferably
within about 2 to about 5 cps.
-12- 2in~5~ 03~
Figure 1 shows that an organosolv pulp such as
ALCELL~ pulp with an initial kappa number of 29 can be
delignified using oxygen to a kappa number of about 10, an
approximately 67% delignification. As shown however in
Figure 2 by closed circles, the viscosity of the ALCELL~
pulp is essentially unchanged with increased
delignification. By contrast, a kraft softwood
brownstock, shown by open circles in Figure 2, shows a
linear viscosity decrease with increasing delignification,
as reported by others. Generally, kraft hardwood pulps
manifest a similar decrease in viscosity with oxygen
delignification. At an approximately 50% delignification,
the pulp viscosity decreases to a point wherein further
delignification would begin to seriously impact the pulp
strength properties. Figure 2 further shows that the kappa
number of oxygen delignified pulp is relatively
independent of the brownstock kappa number, and that the
final kappa number is in the range of about 9 to about 13
for oxygen delignification. The final kappa number for
pulp delignified by oxidative extraction, which provides
milder reaction conditions, in the range of from about 16
to 18.
With reference to the beating curves of Figures
3, 4 and 5, it can be seen that strength properties for
organosolv pulps are comparable to those of kraft pulps or
the same wood species. The beating curves are PFI mill
beating curves and are obtained according to TAPPI
Standard 248 OM-85. The physical properties shown on the
curves are measured according to TAPPI Standards 220 OM-
88, 403 OM-85, 414 OM-88 and 494 OM-88. The bleaching
-13- 29~3~3
sequences used were analogous to those described in
Examples 16, 14 and 19 respectively.
Furthermore, delignification and bleaching of
pulps can be enhanced to achieve commercially acceptable
brightness levels, again without significant viscosity
loss, by contacting the pulp with either peracetic acid or
hydrogen peroxide, either alone, in staged exposures, or
in oxygen delignification followed with, and either before
or after oxygen delignification. An ozone stage can also
be used in combination with oxygen delignification, and
either before or after hydrogen peroxide or peracetic
acid. Here again, commercially acceptable brightness
levels are achieved.
Generally, organosolv processes produce hardwood
pulp fiber with residual lignin contents with typical
kappa numbers of from about 20 to about 40. By the
methods of the present invention, an organosolv pulp with
a brownstock kappa number of about 40 can be delignified
to a kappa number of about 10 in one treatment stage, a
reduction of about 75%, with an oxygen s~age alone.
Alternatively, when the pulp is treated with ozone either
before or after an oxygen delignification stage, the pulp
is delignified to a kappa number of from about 80% to
about 90%.
Treatment of the pulp brownstock with peroxy
compounds such as hydrogen peroxide or peracetic acid in
the range of from about 0.5% to about 4% (w/w) peroxy
compound on oven dry (o.d.) pulp for either peracetic acid
-L4- 2 ~ ~3~ 3 5
or hydrogen peroxide result in reduction of the kappa
number after oxygen delignification by about an additional
50~, to a kappa number of about from 5 to about 7, as
compared with the kappa number after a single oxygen
delignification stage.
Pulps treated with either peracetic acid or
hydrogen peroxide after oxygen delignification, either
with or without pretreatment with a peroxy compound, show
enhanced bleaching responses as compared with similarly
treated kraft pulps. The results of pulp treatment with a
peroxy compound after oxygen delignification is that fewer
bleaching steps are required to reach a specific
brightness level and lower amount of bleaching chemicals
are required.
Enhanced bleaching responses are obtained when
pulps are pretreated with a peroxy compound before oxygen
delignification then are treated in one or more stages
with a peroxy compound. A brightness of from about 83 to
86 ISO can be achieved which is in the same brightness
range obtained when pulps have not been pretreated with
peroxy compounds but have been treated with chlorine based
bleaching chemicals. An added advantage is that these
pulps contain zero level TOX from chlorine based bleaching
chemicals and correspondingly the bleach effluents also
contain zero level AOX.
Enhanced bleaching responses are also obtained
when pulps are treated with ozone either before or after
oxygen delignification followed by treatment with one or
20~03a
more peroxy stages. These pulps have a brightness of from
about 82 to 86 ISO which is in the same brightness range
obtained when pulps have not been treated with ozone but
have been treated with chlorine based bleaching chemicals.
Again, an added advantage is that these pulps contain zero
level TOX from chlorine based bleaching chemicals and
correspondingly the bleach effluents also contain zero
level AOX.
Enhanced bleaching responses are obtained when
pulps are treated in a continuous delignification and
bleaching process with a peroxy compound before or after
oxygen delignification and followed by a bleaching stage
with either peroxy compound or ozone. A brightness of
from about 83 to about 88 ISO can be obtained.
Alternatively, when pulps are treated with a chlorine
dioxide stage, a brightness value of from about 90 to
about 92 ISO can be obtained.
Generally, before delignification or bleaching,
pulp brownstock is washed with an alcohol solution
comprising; from about 40 to about 80% (by volume) of a
water miscible lower aliphatic alcohol of l to 4 carbon
atoms (e.g., methanol, ethanol, isopropanol or tert-
butanol); from about 20 to about 60% water; and if needed,
a small amount of a strong water soluble acid, such as a
mineral acid (e.g., hydrochloric, sulfuric, phosphoric or
nitric acid) or an organic acid (e.g., oxalic acid,
preferably acetic, -f~rmic or peroxy acids) to remove any
soluble lignin prior to delignification or bleaching.
The alcohol washed pulp is washed again with water to
-l~- 2~a~
remove any residual alcohol and is delignified and
bleached in either a batch or continuous mode.
Pulp brownstock can be treated with a peroxy
compound, for example, using peracetic acid (PA) or
hydrogen peroxide (P) at a pH of from about 1.5 to about
11, preferably at a pH of from about 2 to about 6 with
peracetic acid (PA) or preferably at a pH of from about
8.5 to about 11 with hydrogen peroxide (P) and in an
amount of from about 0.2 to about 2%, preferably from
about 0.5 to about 1.5% (w/w) by weight of peroxy compound
per weight of oven dried (o.d.) pulp. When hydrogen
peroxide is used, the final pH is preferably from about
8.5 to about 11 and is maintained at such a level by
addition of caustic. The pulp can be of any consistency of
from about 8% to about 55%, but is preferably between
about 10% to about 20%. The reaction time is from about
0.3 to about 3 hours and at a temperature of between about
40~C and about 90~C.
~ Alternatively, in conjunction with treatment
with a peroxy compound, the pulp brownstock can also be
treated in a separate step with transition-metal chelating
agents in an amount of from about 0.05 to about 1% (w/w)
metal chelating agent on oven dried (o.d.) pulp, for a
reaction time period long enough to ensure chelation, for
example using ethylenediamine tetraacetic acid (EDTA) or
diethylene triamine pentaacetic acid (DTPA) in order to
prevent catalytic decomposition of the peroxy compound by
transition metal ions (such as manganese, copper, and
iron). Treatment by transition-metal chelating agents can
-17- 20~33
either be carried out preceding or during the peroxy
compound treatment step. As an alternative to chelation,
the pulp can also be first soured with a sulfurous acid
(H2SO3) wash by washing the pulp with water through which
sulfur dioxide (SO2) gas is bubbled at a concentration
such that the pH is from about 2 to about 3. As an
alternative to sulfurous acid, mineral acids such as
sulfuric acid can also be used. The soured pulp or the
pulp pretreated with metal chelating agents is then
subjected to a peroxy compound treatment. After peroxy
compound treatment, the pulp is washed with water.
Alternatively, if the next treatment step is oxygen
delignification, the washing step may be omitted.
Magnesium sulfate at from about 0.1% to about 1.0% (w/w)
magnesium sulfate on oven dried (o.d.) pulp can also be
added for viscosity protection of the pulp.
An ozone stage (Z) can be also be used to treat
pulp brownstock either as a pretreatment prior to oxygen
delignification (sequence Z~) or following oxygen
delignification (sequence OZ). Pulp brownstock treatment
with ozone is carried at a pH of from about 1.5 to about
5, preferably from about 2 to about 3 at a temperature of
from about 20~ to about 60~ C, preferably 25~ to about 30~
C. The pH may be adjusted to the appropriate level using
acid (e.g. acetic or sulfuric acid). Enough water is
added or removed using known techniques that the pulp
consistency is from about 10% to about 55%, preferably
about 42%. A catalyst may be added, such as ethanol at a
level of from about 0.5% to about 1%, preferably about
0.8% (w/w) ethanol on oven dried (o.d.) pulp. Ozone is
-18- 2~3a3~
generated using known techniques. When a high consistency
pulp is used with from about 20 to about 50% pulp solids,
the pulp is fluffed into separated fibers and the fibers
are rapidly mixed with ozone gas at a concentration of
from about 0.2% to about 2% (w/w) ozone on oven dried
(o.d.) pulp. When a medium consistency pulp is used, ozone
is introduced to the pulp either as an ozone containing
solution or as a gas. Ozone solution is obtained from
first pressurizing the ozone over water at an elevated
pressure sufficient to dissolve enough ozone such that the
concentration of ozone is from about 0.2% to about 2% (w/
w) ozone on oven dried (o.d.) pulp after the ozone
solution is mixed with the pulp. Any unreacted ozone can
be removed as off-gas and can be monitored using known
techniques. Subsequent to the ozone treatment stage, the
pulp pH can be adjusted using caustic to a pH of from
about 9 to about 11, then, if need be, the pulp can be
further adjusted to a neutral pH by successive washing
with water.
Alternatively, when an ozone stage is used
following oxygen delignification (sequence ~OZ) of pulp
brownstock, the same conditions are generally followed as
in the preceding paragraph. However, following oxygen
delignification, residual caustic in the pulp is washed
from the pulp using water and the pH can be adjusted to a
pH of from about 2 to about 5 by the addition of an acid.
Oxygen delignification (O) of brownstock pulp
can generally be used either as a first stage (sequences
OP, O(PA), OZ), or following a peroxy compound treatment
-19- 2~3~3~
stage or an ozone treatment stage (sequences PO, (PA)O,
ZO). Oxygen delignification is conducted by mixing a pulp
slurry of from about 9% to about 15%, preferably from
about 10% to about 12% consistency by weight of pulp
solids with a caustic solution including, for example,
sodium hydroxide. The amount of caustic added is
preferably between from about 2% to about 8%, more
preferably from about 3% to about 6% (w/w) caustic on oven
dry (o.d.) pulp. The pulp slurry thus obtained is further
mixed at high shear with oxygen gas such that the weight
of oxygen gas is from about 0.5% to about 2%, preferably
from about 0.8% to about 1.5% (w/w) oxygen on oven dry
(o.d.) pulp. The temperature of the reaction mixture is
between from about 60~C to about 110~C, more preferably
from about 70~C to about 90~C, and oxygen pressure is
maintained between from about 30 to about 100 psig, more
preferably between from about 80 to about 100 psig. The
reaction time is between from about 6 to about 60 minutes,
more preferably between from about 25 to about 45 minutes.
Additional chemicals may be added to help preserve
strength properties and include from about 0.1% to about
1% magnesium sulfate, from about 0.1% to about 0.5%
diethylene triamine pentaacetic acid (DTPA), and from
about 0.5% to about 3% sodium silicate.
Generally, an oxidative extraction stage (Eo)
can also be used to delignify a pulp brownstock following
a first peroxy compound bleaching stage, preferably
following a peracetic acid first bleaching stage (sequence
O(PA)Eo, (PA)Eo), an ozone bleaching stage (sequence OZEo,
ZEo) or a first chlorine dioxide bleaching stage (sequence
-20- 2 3 ~
ODEoD). An oxidative extraction stage can also be used
before a first chlorine dioxide bleaching stage (sequence
EoDED). A pulp slurry is mixed at from about 9% to about
40%, preferably from about 10% to about 12% consistency by
weight of pulp solids with a caustic solution of from
about 2~ to about 5%, preferably from about 2.5% to about
4% (w/w) sodium hydroxide on oven dry (o.d.) pulp. From
about 0.1% to about 1% magnesium sulfate is added to the
pulp mixture. Oxygen gas is introduced at from about 30 to
about 100 psig, preferably at from about 30 to 60 psig and
is mixed with the pulp at high shear and for a length of
time sufficient to ensure appropriate mixing. The
temperature of the reaction mixture is between from about
60~C to about 110~C, preferably between from about 70~C to
about 90~C, and oxygen pressure is maintained between
from about 30 psig to about 100 psig preferably between
from about 30 to about 60 psig. The total reaction time
with the oxygen is between from about 6 and about 60
minutes. For the first 10 to 15 minutes, the pressure of
oxygen is decreased to atmospheric and the pulp then
remains in an oxygen rich atmosphere for about 20 to about
40 minutes.
Generally, oxygen delignified pulp (O) can be
treated with chlorine dioxide (D) as a subsequent first
bleaching stage (sequence OD). An ozone-treated oxygen
delignified pulp (sequence OZ) can also be treated with
chlorine dioxide (sequence OZD) and an ozone treatment
can also be used afterJa peroxy treatment stage (sequences
OPD, O(PA)D). Chlorine dioxide is used in the range of
from about 0.2% to about 1.5% (w/w) chlorine dioxide on
-21- 2~ 33
oven dry (o.d.) pulp. The pulp consistency is from about
9% to about 15%, preferably from about 10% to about 12%. A
bleaching stage with chlorine dioxide following oxygen
delignification is carried forward at a final pH of from
about 2 to about 3 and caustic may be added as need be to
maintain the pH in this range. Treatment with chlorine
dioxide proceeds at a temperature of from about 30 to
about 70~C and for about 0.3 to about 3 hours, preferably
0.3 to 2 hours. Optionally, a second bleaching stage
using chlorine dioxide may be used following an oxidative
extraction stage (Eo) or a conventional alkaline
extraction stage (E) on a first bleached pulp with
chlorine dioxide (sequence ODED, ODEoD). An alkaline
extraction stage consists of mixing the pulp brownstock
with from about 0.5% to about 2% caustic, at a temperature
of from about 40~ to about 70~C and for about 1.5 to about
3 hours followed by washing with water to dissolve and
remove any chlorine dioxide bleaching- reaction products.
Conditions are generally the same as in a chlorine dioxide
first bleaching stage, however, a preferred final pH is
from ab~ut 3.5 to about 4.5 which can be achieved by
appropriate addition of caustic, and a preferred reaction
time is between about l.S to about 2 hours.
Pulp brownstock can be delignified and bleached
in a continuous mode. As shown in Figures 6, 7 and 8,
bleaching filtrates obtained from a subsequent bleaching
or delignification stage can be recycled as wash water for
pulp brownstock washing at an earlier stage. Furthermore,
after pulp brownstock washing, these bleaching filtrates
can be mixed with water and can become part of the
-22- 7 ~ 3 5
alcohol/water solvent used in a continuous organosolv
pulping process, or to precipitate the lignin in such a
process. In another alternative, these bleaching filtrates
can also be concentrated, preferably by evaporation to
produce evaporator condensate and such evaporator condensate
can be used as wash water for pulp brownstock washing as
described above. The evaporator condensate can also become
part of the alcohol/water solvent used in a continuous
organosolv pulping process, or to precipitate the lignin in
such a process. In yet another alternative, the bleaching
filtrates can be concentrated and the concentrated material
can be burned with recovery of energy. Material such a
caustic can also be recovered.
As shown in Figure 6, pulp brownstock can be
delignified and bleached in a continuous mode. The pulp
brownstock in any given treatment stage can be washed in
a countercurrent fashion with the bleaching filtrates
obtained from washing the pulp at a subsequent treatment
stage. Initially, pulp brownstock 10 of a consistency of
from about 10% to about 15% pulp solids is pumped through
line 20. The pulp is mixed at mixer 21 with a peroxy
compound for example using peracetic acid (PA) or hydrogen
peroxide (P) at a pH of from about 1.5 to 11 and in an
amount of about 0.2% to 2% (w/w) peroxy compound on oven
dried (o.d.) pulp.
~3
-23~ 53 ~ 3 ~
When hydrogen peroxide is used, it is introduced
at liquid stream 76 into the reaction mixture in mixer 21.
The final pH is preferably from about 8.5 to about 11
which can be maintained by addition of caustic such as
sodium or potassium hydroxide to the reaction mixture into
liquid stream 76. The pulp can be of any consistency, but
is preferably between about lO to about 12% by weight of
pulp solids. When peracetic acid (PA) is used as a peroxy
compound, the final pH is preferably from about 2 to about
5.
When commercially available peracetic acid is
used, it can be introduced at liquid stream 76. Peracetic
acid can also be obtained by recovering and converting the
acetic acid which is present in the evaporator condensate
from the solvent recovery tower used to recover the
solvent in an organosolv pulping process. The evaporator
condensate 70 is pumped into process equipment 71 which
generally includes conventional recovery equipment such as
membrane concentration and solvent extraction equipment which
can be used in a suitable combination with distillation,
freeze concentration and the like. In process equipment 71,
the acetic acid present in condensate 70 is recovered
preferably to a 100~ purity and a bottom stream 72 is also
recovered as an aqueous solution which may be recycled for
use with the water introduced at 61 at washer 6. After
recovery of the acetic acid at process equipment 71, the
acetic acid is pumped into process equipment 73. Process
equipment 73 generally includes a conversion reactor
, .....
-24- 20~3Q3~
wherein the acetic acid is converted in part to peracetic
acid. In process equipment 73, hydrogen peroxide is
introduced at liquid stream 82 and mixed with acetic acid
in an appropriate ratio which can be carefully selected to
optimize the conversion of acetic acid to peracetic acid
at given process parameters. Sulfuric acid can be added at
liquid stream 82 to the reaction mixture in process
equipment 73 and the reaction is allowed to proceed at the
appropriate process conditions to optimize the conversion
of acetic acid to peracetic acid. Alternatively,
commercially available acetic acid can be introduced at
liquid stream 82 and converted in process equipment 73 to
peracetic acid.
After mixing with a peroxy compound, the pulp is
pumped through line 22 into vessel 23 which can be
selected from conventional bleaching equipment, such that
the reaction time is preferably from about 0.3 to about 3
hours, the reaction temperature is between about 40~C and
about 90~C, preferably 50~ to 70~C which can be maintained
by using conventional heating techniques, such as steam
injection.
After peroxy compound treatment, the peroxy
treated pulp is pumped through line 25 and washed in
washer 3 using bleaching filtrates pumped through line 44
from subsequent treatment stages. After washing the pulp
at washer 3, bleaching filtrates are removed through line
34 and can be recycled as described above. Washer 3 and
washers 4, 5, and 6 can be selected from conventional
washing equipment such as drum, belt, compaction baffle or
-25- 2933~3~
pressure diffusion washers. Depending on the equipment
selected, the pulp can be washed at atmospheric pressure
and the water removed either by vacuum applied suction, by
mechanical suction or by pressure concentric rings. The
duration of the pulp washing at washer 3 and subsequent
washers 4, 5 and 6 also depends on the equipment selected.
After washing at washer 3, the pulp is pumped through line
into mixer 31 which is preferably a high shear mixer
and can withstand the operating pressure required by the
process. The pulp is at about 9 to about 40% consistency
by weight of pulp solids. The pulp in mixer 31 is mixed
with a caustic solution, for example a sodium hydroxide
solution which is introduced at liquid stream 80. The
amount of caustic added is preferably between from about
2% to about 8%, more preferably from about 3% to about 6%,
(w/w) caustic on oven dry (o.d.) pulp. The pulp slurry
thus obtained is further mixed at high shear with oxygen
gas which is introduced at line 30 through liquid stream
79. The temperature of the reaction mixture in mixer 31
is preferably between from about 60~C and about 110~C,
more preferably from about 70~C to about 90~C which can be
achieved by steam injection. Oxygen pressure in mixer 31
is preferably maintained between about 30 and about 100
psig, more preferably between about 80 to about 100 psig.
Additional chemical agents which can be added into liquid
stream 79 to help preserve pulp strength properties
include from about 0.1% to about 1% magnesium sulfate,
from about 0.1% to about 1% diethylene triamine
pentaacetic acid (DTPA~, and from about 0.5% to up to
about 3% sodium silicate. The pulp is pumped through line
32 into vessel 33 which can be selected from conventional
-2~- 2~S~3S
bleaching equipment, but generally, vessel 33 is a
pressurized vessel and is selected to achieve the required
reaction time and temperature. The temperature of the
reaction mixture in vessel 33 is preferably between from
about 60~C and about 110~C, more preferably from about
70~C to about 90~C and heating of the reaction mixture in
vessel 33 can be achieved if needed by steam injection.
Oxygen pressure in vessel 33 is preferably maintained
between about 30 and about 100 psig, more preferably
between about 80 to about 100 psig and the reaction time
is preferably between about 6 to about 60 minutes, more
preferably between about 25 to about 50 minutes.
After oxygen delignification, the pulp is pumped
through line 35 into washer 4 and is washed using
countercurrent bleaching filtrates from line 54. After
washing in washer 4, the delignified pulp is pumped
through line 40 into equipment 41. Equ-ipment 41 can be a
mixer when the pulp is treated with a peroxy compound or
can be a dewatering press when a high consistency pulp is
treated with ozone. The pulp is mixed at mixer 41 with a
peroxy compound for example using peracetic acid (PA) or
hydrogen peroxide (P) at a pH of from about 3 to about ll
and in an amount of about 0.2 to about 2% (w/w) peroxy
compound on oven dried (o.d.) pulp. When hydrogen peroxide
is used, it is introduced at liquid stream 77 into the
reaction mixture at mixer 41. The final pH is preferably
from about 8.5 to about ll which can be maintained by
addition of caustic such as sodium or potassium hydroxide
into liquid stream 77. The pulp can be of any consistency,
but is preferably between about 10% to about 12% by weight
-27- 2~ 303
of pulp solids. When peracetic acid (PA) is used as a
peroxy compound, the final pH is preferably from about 2
to about 7 and the peracetic acid can be introduced either
at liquid stream 77 or through line 75 from the acetic
acid recovery and conversion at process equipment 71 and
73. After mixing with a peroxy compound, the pulp is
pumped through line 42 into vessel 43 which can be
selected from conventional bleaching equipment. Generally,
vessel 43 is selected such that the reaction time in
vessel 43 is from about 0.3 to about 3 hours, the reaction
temperature is between about 40~C and 90~C, preferably 50~
to 70~C which can be maintained using conventional heating
techniques, such as steam injection.
Alternatively, an ozone stage can also be used
lS to treat pulp brownstock in vessel 43. Pulp treatment with
ozone is carried at a pH of from about 1.5 to about 5,
preferably from about 2 to about 3 and at a temperature of
from about 20~ to about 60~C, preferably 25~ to 30~C. Two
alternative methods of ozone bleaching can be used. In one
method, with a high consistency pulp of from about 20% to
50~, the pulp is dewatered at equipment 41 which is
preferably a high consistency pulp dewatering press. After
dewatering, the pulp is conveyed through line 42 and into
vessel 43 which can be selected from conventional
bleaching equipment but which is preferably a high
consistency ozone bleaching tower. In vessel 43, the pulp
is fluffed using techniques known in the art and ozone gas
is introduced into vessel 43 through line 46 and rapidly
reacted with the pulp fibers at a concentration of from
about 0.2% to about 2% (w/w) ozone on oven dried (o.d.)
-23- 2~ 3~
pulp. Alternatively, when a medium consistency pulp is
used, ozone is introduced to the pulp as ozone solution or
as ozone gas at mixer 41 which is preferably a high
pressure mixer. Ozone solution is obtained from first
pressurizing ozone gas over water at an elevated pressure
sufficient to dissolve enough ozone in water such that the
concentration of ozone is from about 0.2% to about 2% (w/
w) ozone on oven dried (o.d.) pulp after the ozone
solution is mixed with the pulp. The ozone solution is
introduced into mixer 41 through liquid stream 77 and
mixed with the pulp. The resulting reaction mixture is
pumped through line 42 into vessel 43 which is a
conventional bleaching tower preferably selected to
conform to the reaction parameters. The final pH can be
lS adjusted to the appropriate level using an acid such as
sulfuric acid which can be introduced at liquid stream 77
into mixer 41. The pulp is then pumped through line 45
onto washer 5 and, if need be, the pulp pH can be adjusted
using caustic at liquid stream 47 to a pH of from about 9
to 11 and can be further adjusted to a near neutral pH by
successive washing with countercurrent filtrates from line
64.
After a first peroxy compound treatment stage or
an ozone treatment stage, the pulp is pumped through line
45 onto washer 5 and is washed using countercurrent
bleaching filtrates from line 64. After washing in washer
5, the pulp is pumped through line 50 into mixer 51. The
pulp is mixed at mixer 51 with a peroxy compound for
example using peracetic acid (PA) or hydrogen peroxide (P)
at a pH of from about 3 to about 11 and in an amount of
-2~- 2~3Q33
about 0.2% to about 2% (w/w) peroxy compound on oven dried
(o.d.) pulp. When hydrogen peroxide is used, it is
introduced at liquid stream 81 into the reaction mixture
at mixer 51. The final pH is preferably from about 8.5 to
11 about which can be maintained by addition of caustic
such as sodium or potassium hydroxide into liquid stream
81. The pulp can be of any consistency, but is preferably
between about 10% to about 12~ by weight of pulp solids.
When peracetic acid (PA) is used as a peroxy compound, the
final pH is preferably from about 2 to about 7 and the
peracetic acid can be introduced either at liquid stream
81 or through line 78 from the acetic acid recovery and
conversion at process equipment 71 and 73. After mixing
with a peroxy compound, the pulp is pumped through line 52
into vessel 53 which can be selected from conventional
bleaching equipment, such that the reaction time is
preferably from about 0.3 to about 3 hours, the reaction
temperature is between about 40~C and about 90~C,
preferably 50~ to about 60~C which can be maintained by
using conventional heating techniques, such as steam
injection. The delignified and bleached pulp is removed at
line 62 and can be suitably subjected to further
processing or drying.
Alternatively, as shown in Figure 7, pulp
brownstock 10 of a consistency of from about 9% to about
40% pulp solids is pumped through line 120 into mixer 121
which is preferably a high shear mixer and can withstand
the operating pressure required for the process. The pulp
in mixer 121 is mixed with a caustic solution, for example
a sodium hydroxide solution which is introduced in liquid
-30- 2~3~3~
stream 176. The amount of caustic added is preferably
between from about 2% to about 8%, more preferably from
about 3% to about 6% (w/w) caustic on oven dry (o.d.)
pulp. The pulp slurry thus obtained is further mixed at
high shear with oxygen gas which is introduced at line 120
through line 179. The temperature of the reaction mixture
in mixer 121 is preferably between from about 60~C and
about 110~C, more preferably from about 70~C to about 90~C
which can be achieved by steam injection. Oxygen pressure
in mixer 121 is preferably maintained between about 30 and
about 100 psig, more preferably between about 80 to about
100 psig. Additional chemical agents which may be added
into liquid stream 176 to help preserve pulp strength
properties include from about 0.1~ to about 1% magnesium
lS sulfate, from about 0.1% to about 1% diethylene triamine
pentaacetic acid (DTPA), and from about 0.5% to up to
about 3~ sodium silicate. The pulp is pumped through line
122 into vessel 123 which can be selected from
conventional bleaching equipment, but generally, vessel
123 is a pressurized vessel and is selected to achieve the
required reaction time and temperature. The temperature
of the reaction mixture in vessel 123 is preferably
between from about 60~C and about 110~C, more preferably
from about 70~C to about 90~C and heating of the reaction
mixture can be achieved by steam injection. Oxygen
pressure in vessel 123 is preferably maintained between
about 30 and about 100 psig, more preferably between about
to about 100 psig and the reaction time is preferably
between about 6 to about 60 minutes, more preferably
between about 25 to about 50 minutes.
-31- 20~ 3
After oxygen delignification, the pulp is pumped
through line 125 into washer 13 and is washed using water
introduced at 160. Washer 13 and washers 14, 15, and 16
can be selected from conventional washing equipment such
as drum, belt, compaction baffle washer or pressure
diffusion washers. Depending on the equipment selected,
the pulp can be washed at atmospheric pressure and the
water removed either by vacuum applied suction, by
mechanical suction or by pressure concentric rings. The
duration of the pulp washing at washer 13 and washers 14,
and 16 also depends on the equipment selected. After
washing the pulp at washer 13, bleaching filtrates are
removed through line 134 and can be recycled as described
above.
After washing in washer 13, the delignified pulp
is pumped through line 130 into mixer 131. The delignified
pulp in mixer 131 is mixed with a liquid solution of
chlorine dioxide introduced at liquid stream 180 and
containing chlorine dioxide in the range of from about
0.1% t~ about 2% (w/w) chlorine dioxide on oven dry (o.d.)
pulp. The temperature of the reaction mixture in mixer 131
is from about 30~ to about 70~C which can be achieved by
steam injection. The reaction mixture is pumped through
line 132 into vessel 133 which can be selected from
conventional bleaching equipment, but generally, vessel
133 is selected to achieve the required reaction time and
temperature. The reaction in vessel 133 proceeds at a
temperature of from about 30 to about 70~C and the
reaction time is of about 0.3 to about 3 hours, preferably
0.3 to 2 hour. The chlorine dioxide bleaching reaction in
-32- ~53~3~
vessel 133 is carried forward at a final pH of from about
2 to about 3 and caustic or acid may be added at liquid
stream 180 as need be to maintain the pH in this range.
The chlorine dioxide bleached pulp is pumped through line
135 and washed on washer 14 using countercurrent washing
with bleaching filtrates from washer 15 pumped through
line 154. Filtrates resulting from washing the pulp at
washer 14 are pumped through line 140 and subjected to
conventional treatment to remove any chlorine and
chlorinated products. After treatment, the resulting
bleaching filtrates can be combined with bleaching
filtrates from line 134 and can be recycled as described
above.
After washing at washer 14, the washed pulp at a
consistency of about 9 to about 15%, preferably from about
- 11 to about 12% by weight of pulp solids is pumped through
line 142 and into mixer 141 which is preferably a high
shear mixer and can withstand the operating pressure
required by the process. The pulp slurry in mixer 141 is
mixed with a caustic solution introduced at liquid stream
177 and containing of from about 2% to about 5%,
preferably from about 2.5% to about 4% (w/w) sodium
hydroxide on oven dry (o.d.) pulp. The pulp slurry thus
obtained is further mixed at high shear with oxygen gas
which is introduced at line 140 through line 182. The
temperature of the reaction mixture in mixer 141 is
preferably between about 60~C and about 110~C, more
preferably between about 70~C and about 90~C which can be
achieved by steam injection. Oxygen pressure in mixer 141
is preferably maintained between about 30 to about 100
~ -33- 2Q53Q3~
psig, preferably at about 30 to about 60 psig. Additional
chemical agents may be added into liquid stream 177 such
as magnesium sulfate from about 0.1% to about 1%. The pulp
is pumped into vessel 143 which can be selected from
conventional bleaching equipment, but generally vessel 143
is selected to achieve the required reaction time and
temperature. The temperature of the reaction mixture in
vessel 143 is preferably between about 60~C and about
110~C, more preferably between about 70~C and about 90~C
which can be achieved by steam injection and the total
reaction time with oxygen in vessel 143 is preferably
between about 6 and about 60 minutes. Oxygen pressure in
vessel 143 is decreased to atmospheric pressure during the
first 10 to 15 minutes and the pulp remains in vessel 143
lS in an oxygen rich atmosphere for about 20 to about 40
minutes.
Alternatively, after washing at washer 14, in an
alkaline extraction stage, the pulp slurry in mixer 141
can be mixed with a caustic solution introduced at liquid
stream 177 and containing of from about 0.5% to about 2%
(w/w) caustic on oven dry (o.d.) pulp. The temperature of
the reaction mixture in mixer 141 is preferably between
about 40~C and about 70~C which can be achieved by steam
injection. Additional chemical agents may be added into
liquid stream 177 such as magnesium sulfate from about
0.1% to about 1%. The pulp is pumped into vessel 143 which
can be selected from conventional bleaching equipment, but
generally vessel 143 is selected to achieve the required
reaction time and temperature. The temperature of the
reaction mixture in vessel 143 is preferably between about
-3~- 2~53~3~
40~C and about 70~C which can be achieved by steam
injection and the total reaction time with oxygen in
vessel 143 is from about 1.5 to about 3 hours, preferably
1.5 to 2 hours.
After the oxidative extraction stage or the
alkaline extraction stage, the pulp is pumped through line
145 into washer 15. The pulp is washed in washer 15 using
bleaching filtrates pumped through line 164 and obtained
by washing the pulp at washer 16. The bleaching filtrates
obtained at washer 15 are pumped through line 154 and used
to wash the pulp in washer 14. After washing the pulp on
washer 15, the delignified pulp is pumped through line 150
into mixer 151. The delignified pulp in mixer 151 is mixed
with a liquid solution of chlorine dioxide introduced at
liquid stream 181 and containing chlorine dioxide in the
range of from about 0.2% to about 2% (w/w) chlorine
dioxide on oven dry (o.d.) pulp. The temperature of the
reaction mixture in mixer 151 is from about 30~ to about
70~C which can be achieved by steam injection. The
reaction mixture is pumped through line 152 into vessel
153 which can be selected from conventional bleaching
equipment, but generally, vessel 153 is selected to
achieve the required reaction time and temperature. The
reaction in vessel 153 proceeds at a temperature of from
about 30 to about 70~C and the reaction time is from about
0.3 to about 3 hours, preferably 1.5 to about 3 hours. The
reaction in vessel 153 is carried forward at a final pH of
from about 3.5 to about 4.5 and caustic may be added at
liquid stream 181 to maintain the pH in this range. The
chlorine dioxide bleached pulp in vessel 153 is pumped
2~Q3~
-35-
through line 155 and washed at washer 16 using water
introduced at line 161. Bleaching filtrates are removed
from washer 16 through line 164 and can be used to wash
the pulp at washer 15. The delignified and bleached pulp
is removed at line 162 and can be suitably subjected to
further processing or drying.
Alternatively, as shown in Figure 8, pulp
brownstock 10 of a consistency of from about 9% to about
40% pulp solids is pumped through line 220 into mixer 221
which is preferably a high shear mixer and can withstand
the operating pressure required for the process. The pulp
in mixer 220 is mixed with a caustic solution, for example
a sodium hydroxide solution which is introduced in liquid
stream 276. The amount of caustic added is preferably
between from about 2% to about 8%, more preferably from
about 3% to about 6% (w/w) caustic on oven dry (o.d.)
pulp. The pulp slurry thus obtained is further mixed at
high shear with oxygen gas which is introduced at line 220
through line 279. The temperature of the reaction mixture
in mixer 221 is preferably between from about 60~C and
about 110~C, more preferably from about 70~C to about 90~C
which can be achieved by steam injection. Oxygen pressure
in mixer 221 is preferably maintained between about 30 and
about 100 psig, more preferably between about 80 to about
100 psig. Additional chemical agents which may be added
into liquid stream 276 to help preserve strength
properties include 0.1% to about 1% magnesium sulfate,
from about 0.1% to 1% ~iethylene triamine pentaacetic acid
(DTPA), and from about 0.5% to up to about 3% sodium
silicate. The pulp is pumped through line 222 into vessel
23S~03~
-36-
223 which can be selected from conventional bleaching
equipment, but generally, vessel 223 is a pressurized
vessel and is selected to achieve the required reaction
time and temperature. The temperature of the reaction
mixture in vessel 223 is preferably between from about
60~C and about 110~C, more preferably from about 70~C to
about 90~C and heating of the reaction mixture can be
achieved by steam injection. Oxygen pressure in vessel 223
is preferably maintained between about 30 and about 100
psig, more preferably between about 80 to about 100 psig
and the reaction time is preferably between about 6 to
about 60 minutes, more preferably between about 25 to
about 50 minutes.
After oxygen delignification, the pulp is pumped
through line 225 into washer 23 and is washed using
countercurrent bleaching filtrates from line 244. Washer
23 and washers 24 and 25 can be selected from conventional
washing equipment such as drum, belt, compaction baffle or
pressure diffusion washers. The bleaching filtrates are
removed from washer 23 through line 234 and can be
recycled as described above.
After washing the pulp in washer 23, the
delignified pulp is pumped through line 230 into equipment
231. Equipment 231 can be a mixer when the pulp is treated
with a peroxy compound or can be a dewatering press when a
high consistency pulp is treated with ozone. The pulp is
mixed in mixer 231 with a peroxy compound for example
using peracetic acid (PA) or hydrogen peroxide (P) at a pH
of from about 3 to about 11 and in an amount of about 0.2~
~ 3 ~3Q 3 5
-37-
to about 2% (w/w) peroxy compound on oven dried (o.d.)
pulp. When hydrogen peroxide is used, it is introduced at
liquid stream 280 into the reaction mixture at mixer 231.
The final pH is preferably from about 8.5 to about 11
which can be maintained by addition of caustic such as
sodium or potassium hydroxide into liquid stream 281. The
pulp can be of any consistency, but is preferably between
about 10% to 12% by weight of pulp solids. When peracetic
acid (PA) is used as a peroxy compound, the final pH is
preferably from about 2 to about 5 and the peracetic acid
can be introduced either at liquid stream 280 or through
line 275 from the acetic acid recovery and conversion
through process equipment 71 and 73. After mixing with a
peroxy compound, the pulp is pumped through line 232 into
vessel 233 which can be selected from- conventional
bleaching equipment, such that the reaction time is from
about 0.3 to about 3 hours, the reaction temperature is
between about 40~C and about 90~C, preferably 50~ to about
60~C which can be maintained by using conventional heating
techniques, such as steam injection.
Alternatively, an ozone stage can also be used
to treat pulp brownstock in vessel 233. Pulp treatment
with ozone is carried at a pH of from about 1.5 to about
5, preferably from about 2 to about 3 and at a temperature
of from about 20~ to about 60~C, preferably 25~ to 30~C.
Two alternative methods of ozone bleaching can be used. In
one method, for a high consistency pulp of from about 20%
to about 50%, the pulp is dewatered at equipment 231 which
is preferably a high consistency pulp dewatering press.
After dewatering, the pulp is pumped through line 232 and
2û5~35
-3~-
into vessel 233 which can be selected from conventional
bleaching equipment but is preferably a high consistency
ozone bleaching tower.
At the top of vessel 233, the pulp is fluffed
and ozone gas is introduced at line 236 into vessel 233
and rapidly reacted with the pulp fibers at a
concentration of from about 0.2% to about 2% (w/w) ozone
on oven dried (o.d.) pulp. Alternatively, when a medium
consistency pulp is used, ozone is introduced to the pulp
in mixer 231 as a solution or a gas. Ozone solution is
obtained from first pressurizing the ozone over water at
an elevated pressure sufficient to dissolve enough ozone
such that the concentration of ozone is from about 0.2% to
2% (w/w) ozone on oven dried (o.d.) pulp after the ozone
solution is mixed with the pulp. The ozone solution is
introduced into mixer 231 through liquid stream 280 and
mixed with the pulp. The resulting reaction mixture is
pumped through line 232 into vessel 233 which is a
conventional bleaching tower preferably selected to
conform to the reaction parameters. The pH may be adjusted
to the appropriate level using an acid such~as sulfuric
acid which can be introduced at liquid stream 281 and into
mixer 231 through line 230. The pulp is then pumped
through line 235 and into washer 24 and, if need be, the
pulp pH can be adjusted using caustic to a pH of from
about 9 to about 11 which can be introduced through liquid
stream 237 and can be further adjusted to a neutral pH by
successive washing with water which is introduced at line
263.
2 ~ 3 ~
~ -3~-
After the peroxy compound treatment stage or the
ozone treatment stage, the pulp is pumped through line 235
into washer 24. The pulp is washed in washer 24 using
water introduced at line 263. The bleaching filtrates
obtained at washer 24 are pumped through line 244 and used
to wash the pulp on washer 23. After washing of the pulp
on washer 24, the pulp is pumped through line 240 into
mixer 241. The pulp in mixer 241 is mixed with a liquid
solution of chlorine dioxide introduced at liquid stream
277 and containing chlorine dioxide in the range of from
about 0.1~ to about 2~ (w/w) chlorine dioxide on oven dry
(o.d.) pulp. The temperature of the reaction mixture in
mixer 241 is from about 30~ to about 70~C which can be
achieved by steam injection. The reaction mixture is
pumped through line 242 into vessel 243 which can be
selected from conventional bleaching equipment, but
generally, vessel 243 is selected to achieve the required
reaction time and temperature. The reaction in vessel 243
proceeds at a temperature of from about 30~ to about 70~C
and the reaction time is of about 0.3 to about 3 hours,
preferably 1.5 to 3 hours. The reaction mixture in vessel
243 is carried forward at a final pH of from about 2 to
about 4.5 and caustic may be added at liquid stream 277 as
need be to maintain the pH in this range. The chlorine
dioxide bleached pulp in vessel 243 is pumped through line
245 and washed at washer 25 using water introduced at line
261. Filtrates resulting form washing the pulp at washer
are pumped through line 254 and subjected to
conventional treatment to remove any chlorine and
chlorinated compounds. After treatment, the resulting
bleaching filtrates can be combined with bleaching
2 0 5~ 0 3 3
-40-
-
filtrates from line 234 and can be recyled as above. The
delignified and bleached pulp is removed at line 262 and
can be suitably subjected to further processing or drying.
Except where noted otherwise, in the following
examples all pulps are organosolv pulps which are prepared
using an organosolv pulping process. After pulping, the
pulp is cooled, removed from the extraction vessel and
further screened as is customary in pulping practice to
result in a pulp brownstock having the kappa numbers and
viscosities indicated in each example.
It is believed that the enhanced novel effects
proven with organosolv pulps are applicable to pulps in
general. Therefore, the following examples should not be
construed to limit the present invention to any particular
pulp.
The following two examples show the effect of
oxygen delignification of an organosolv pulp.
Example l
Sequence O
Birch/maple/aspen organosolv pulp was mixed with
4~ sodium hydroxide and 0.5% MgSO4 at a consistency of 12~
and placed in the mixing chamber of a Quantum Technologies
Mark II high shear mix@r. The chamber was then capped and
flushed with ~2 gas by bringing it to pressure with ~2 and
releasing, then filling the chamber to the final ~2
- 2~03~
-41-
pressure of 100 psig. The pulp was then mixed at high
speed for 4 seconds at this pressure, and was reacted for
45 minutes at 85~C, with occasional stirring at low speed.
The results are shown below.
Kappa ~o. Viscosity (cps)
1. Organosolv brownstock 36.1 23.8
2. Oxygen delignification 8.1 22.4
As can be readily seen, the kappa number of the
delignified pulp was reduced by about 63%, while the
viscosity remained virtually the same.
Example 2
Sequence O
Aspen organosolv pulp was treated as in Example
1, except that the oxygen pressure was maintained at 80
psig.
Kappa No. Viscosity (cps)
1. Organosolv brownstock 36.6 20.7
2. Oxygen delignification 9.0 -18.9
The decrease in kappa number was about 75%, with
a small decrease in viscosity of about 2 cps. In both
Examples 1 and Example 2, the decrease in kappa number was
~ - 2û~3û~5
-42-
approximately 70% to a final kappa number in the range of 9
to 13 with a small decrease in viscosity on the order of
about 2 cps or less.
Example 3
Kraft softwood brownstock obtained from Skeena
Cellulose Incorporated, Prince Rupert, British Columbia was
treated as in Example 1. As shown in Figure 2 by closed
circles, the viscosity of the organosolv pulp was
essentially unchanged with increased oxygen delignification.
By contrast, the Kraft brownstock pulp, shown by open
circles in Figure 2, shows a linear viscosity decrease with
increasing oxygen delignification.
In Examples 4 and 5 an oxidative extraction (Eo)
process was used to delignify organosolv pulp as a first
stage.
Example 4
Sequence Eo
Aspen organosolv pulp was placed in the mixing
chamber of a Quantum Technologies Mark II high shear mixer.
A charge of 4% sodium hydroxide and 0.5% MgSO4 was injected
into the sealed chamber at about 11% to 12% consistency.
Oxygen was mixed with the pulp at 32 psig in the high shear
mixer for four seconds. Over the next 12 minutes oxygen
2 ~ ~3Q ~ ~
-43-
pressure was gradually released until pressure was
atmospheric. The pulp remained in the mixer at 70~C for
another 45 minutes, with occasional stirring at low speed.
Kappa No. Viscosity (cps)
1. Organosolv brownstock 32.2 29.8
2. Oxygen extraction (Eo) 16.7 26.7
Example 5
Sequence Eo
Birch/maple/aspen organosolv pulp was treated as
in Example 4, except that the initial oxygen pressure was 60
ps ig .
Kappa No. Viscosity (cps)
1. Organosolv brownstock 36.7 17.6
2. Oxygen extraction (Eo) 18.2 17.8
Examples 4 and 5 demonstrate that when oxidative
extraction conditions are used, the kappa number of the pulp
is decreased by about 50% to a final kappa number in the
range of 16 to 18 with a slight decrease in viscosity on the
order of about 3 cps or less. An advantage to using
oxidative extraction is that it requires lower capital
investment from the standpoint of bleach plant construction
or design.
2~53~35
-4~-
In the following example pulp is first
delignified with oxygen and then treated with peracetic
acid.
Example 6
Sequence O(PA)
Birch/maple/aspen organosolv pulp was
delignified with oxygen as in Example 1 to a kappa number of
10.3 and was subsequently treated with peracetic acid. The
oxygen delignification was carried out by mixing a pulp
slurry at about 12% consistency with a 4% solution of NaOH
at 85~C, 100 psig for 45 minutes. 1.0% MgSO4 was also added
to the reaction mixture.
The peracetic acid stage was carried by mixing
either 2.7% or 1.3% peracetic acid and 2.5% NaOH or 4.0~
NaOH respectively at a 10% consistency. Additionally, 0.5%
DTPA, 0.5% MgSO4, and 4.0% Na2SiO4 were added to both of the
respective reaction mixtures. The reaction time was 1 hour
at 60~C.
The results of such treatments are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv brownstock, 29.0 22.9
2. Oxygen delignification 10.3 22.5 36.4
2~3~
-4,-
3. Oxygen delignification
+ 1.3% peracetic acid 5.3 24.1 58.5
4. Oxygen delignification
+ 2.7~ peracetic acid 4.0 22.3 64.7
The foregoing shows that an oxygen delignification
of about 65% is significantly increased by approximately
another 50% to a kappa number of about 5.3 to 4, with virtually
no decrease in viscosity, when oxygen delignification of pulp
is followed by a peracetic acid treatment stage. Such a
treatment step also significantly increases the brightness of
the pulp from about 37 ISO to about 59 to 65 ISO.
In the following example, pulp delignified with
oxygen was treated with two stages of exposures to peroxy
compounds after oxygen delignification.
Example 7
Sequence O(PA)(PA) and O(PA)P
The oxygen delignified pulp of Example 6 was
subsequently treated with either a 1.3% or 2.7% peracetic acid
treatment stage as described in Example 6. A third treatment
stage was then performed with either 1.3% peracetic acid or
1.0% hydrogen peroxide. The peracetic acid third treatment
stage was carried out by reacting 1.3% peracetic acid, 2.5%
NaOH, 0.1% MgSO4, 0.1% DTPA, and 2.0% Na2SiO4 for one hour at
60~C at a 10% consistency. The hydrogen peroxide third
treatment stage was carried out by reacting the pulp with 1.0%
H2~2~ 1.0% NaOH, 0.2% MgSO4, 0.2% DTPA, and 4.0% Na2SiO4 at
2053Q3S
-46-
60~C and a 10% consistency for one hour. The hydrogen peroxide
fourth stage treatment was accomplished by reacting the pulp
with 1% H2O2, 0.8% NaOH, and 0.5% DTPA at a 12% consistency for
60 minutes at 70~C.
The results of these treatments are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv Brownstock 29.0 22.9
2. Oxygen delignification10.3 22.5 36.4
3. Oxygen delignification
+ 1.3% peracetic acid
+ 1.3% peracetic acid * 17.5 68.1
4. Oxygen delignification
+ 2.7% peracetic acid
+ 1.3% peracetic acid * 21.7 76.5
5. Oxygen delignification
+ 2.7% peracetic acid
+ 1.0% hydrogen peroxide * 14.8 76.5
* The kappa numbers were too low to be accurately measured.
The successive stages of peracetic acid treatment
following oxygen delignification in runs 3 and 4 resulted in
high brightness levels of 68.1 ISO and 76.5 ISO, again with
only a small decrease in viscosity (5 and 1 cps respectively).
Treatment with hydrogen peroxide in run 5 appears to cause a
significantly larger decrease in viscosity, although the
brightness level is also 76.5 ISO.
It should be noted that the above-described
brightness levels were achieved without any chlorine containing
2~3~3~
-47-
bleaching compounds and therefore the delignified and bleached
pulp contain zero level TOX from chlorine based bleaching
chemicals and correspondingly the bleaching effluent contain
zero level of AOX.
Example 8
Sequence O( PA ) D and O( PA ) DD
The organosolv pulp from Example 5 was delignified
with oxygen to a kappa number of 10.3 as in Example 6. The pulp
was then bleached using successive treatment stages of
peracetic acid and chlorine dioxide. The delignification and
peracetic acid second stage treatments were carried out as in
Example 7. The 0.4~ and 0.8% chlorine dioxide third treatment
stages were accomplished respectively by reacting either 0.4%
ClO2 and no NaOH with the pulp at a 10% consistency for 3 hours
at 70~C, or by reacting 0.8% ClO2 and 0.35% NaOH with the pulp
under the same conditions. The fourth treatment stage with
0.4% chlorine dioxide was carried out by reacting 0.4% ClO2 and
0.1% NaOH at a 10% consistency for 3 hours at 70~C.
The results of such treatments are shown below.
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Oxygen delignification 10.3 22.5 36.4
2. Oxygen delignification
+ 2.7% peracetic acid4.0 22.3 64.7
- 20~Q3~
-48-
3. Oxygen delignification
+ 2.7% peracetic acid
+ 0.4% chlorine dioxide * 21.9 75.9
4. Oxygen delignification
+ 2.7% peracetic acid
+ 0.8% chlorine dioxide * 20.8 86.3
5. Oxygen delignification
+ 2.7% peracetic acid
+ 0.4% chlorine dioxide
+ 0.4% chlorine dioxide * 19.8 89.6
* The kappa numbers were too low to be accurately measured.
While in all cases the pulp brightness was
significantly enhanced by the successive treatment stages with
peracetic acid and chlorine dioxide with little decrease in
viscosity (3 cps or less), the treatments that included the
chlorine dioxide treatment stage yielded significant increases
in brightness, to levels above 80 ISO. In particular, the
difference between treatment runs 4 and 5, namely the splitting
up of the chlorine dioxide treatment stage by the usual washing
step raised the brightness level by a significant 3 points.
The quantities of chlorine dioxide required to achieve a
brightness above 89 ISO is low enough t~hat bleach plant
effluents would contain below 0.5 kg AOX per ton of pulp in
the untreated effluent.
In the following example a hydrogen peroxide
treatment stage preceded delignification with oxygen. Some of
the delignification stages were followed by various peroxy
treatment stages.
2~i~3~5
-43-
Example 9
Sequence PO, POP, PO(PA) and PO(PA)P
Birch/maple/poplar organosolv pulp was treated with
hydrogen peroxide prior to oxygen delignification of the pulp.
The pretreatment or first stage treatment was carried out with
2.0% H2O2, 2.8% NaOH, 0.5% DTPA, and 0.5% MgSO4 at a 12%
consistency at 70~C for one hour. The oxygen delignification
second stage was carried out with 4.0% NaOH, 0.5% MgSO4 at a
12% consistency at 85~C for 45 minutes. The third stage
hydrogen peroxide was accomplished by reacting treated pulp
with 2% H2O2, 1% NaOH, 0.5% MgSO4, and 0.5% DTPA at 70~C for 45
minutes. The third stage peracetic acid stage was accomplished
by reacting 1.5% peracetic acid, 1.5% NaOH, 0.5% DTPA, and 0.5%
MgSO4 at a 12% consistency at 70~C for 3 hours. The fourth
stage hydrogen peroxide stage was carried out by reacting 1%
H2O2, 0.8% NaOH, and 0.5% DTPA at a 12% consistency for 60
minutes at 70~C.
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv brownstock 36.4 22.6 25.7
2. 2.0% Hydrogen peroxide
+ Oxygen delignification 6.1 19.4 51.4
3. 2.0% Hydrogen peroxide
+ Oxygen delignification
+ 2.0% hydrogen peroxide 3.0 15.1 66.4
2~03~
-5~-
4. 2.0% Hydrogen peroxide
+ Oxygen delignification
+ 1.5% peracetic acid 2.3 20.1 72.3
5. 2.0% Hydrogen peroxide
+ Oxygen delignification
+ 1.5% peracetic acid
+ 1.0% hydrogen peroxide * 14.8 83.0
* The kappa numbers were too low to be accurately measured.
In all cases, pretreatment of pulp with hydrogen
peroxide prior to delignification with oxygen followed by a
peroxy treatment yielded pulps with kappa numbers greatly
reduced (83% or more), a small loss of viscosity (8 cps or
less), and brightness levels in the range of 66.4 ISO to 83
ISO. In particular, a brightness level of 83 without use of
any chlorine compounds while retaining a viscosity above 14,
was obtained. Figure 5 is a beating curve for the organosolv
pulp of this example delignified and bleached with the sequence
PO(PA)P. With the POtPA)P sequence, a brightness of 83 ISO can
be obtained without significant loss of pulp strength. Such
organosolv is bleached to 83 ISO without chlorine dioxide and
contain zero level TOX from chlorine based bleaching chemicals
and correspondingly the bleach effluents contain zero level
AOX.
In the following example, the effect of
pretreatment with peracetic acid or a soured peracetic acid
treatment stage is shown.
2~53~3~
-51-
Example 10
Sequence (PA)O, (PA)O(PA), (soured PA)O, (soured PA)O(PA),
(soured PA)OP and (soured PA)ODED
Birch/maple/poplar organosolv brownstock was either
bleached usins 2% peracetic acid or first soured using an H2SO3
wash and then treated with the 2% peracetic acid before the
pulp was oxygen delignified. The pulp was further bleached
using chlorine dioxide, peroxide and/or peracetic acid.
The 2% peracetic acid first treatment stage was
carried out by reacting the pulp with 2% peracetic acid, 0.5%
DTPA, and 0.5% MgSO4 at a 12% consistency for 2 hours at 70~C.
The 2% peracetic acid third treatment stage was carried out by
reacting the pulp with 2% peracetic acid, 0.5% DTPA, 0.5% MgSO4
at a 12% consistency for 2 hours at 70~C and at alkaline pH
adjusted to a pH of 5 to 7 by addition of caustic. The soured
peracetic wash was accomplished by washing the pulp with water
through which SO2 gas was bubbled to a pH of 2 to 3. The
oxygen delignification was carried out with 4% NaOH and 0.5%
MgSO4 at a 12% consistency at 100 psig and 85~C for 45 minutes.
The third stage chlorine dioxide treatment for run 5 was
carried out by reacting the pulp with 0.5% ClO2 for 2 hours at
70~C. This was followed by a sodium hydroxide extraction
fourth stage, as is customary practice in bleaching technology,
in which the pulp was extracted with 2% NaOH at a 12%
consistency for 2 hours at 70~C. For the fifth stage the pulp
was reacted with 0.6% ClO2, 0.22% NaOH at a 12% consistency for
3 hours at 70~C. The third stage hydrogen peroxide treatment
stage for run 6 was carried out by reacting 2.2% NaOH, 0.5%
23~0~
-52-
DTPA, and 1.0% Na2SiO4 at 15% consistency for 2 hours at 70~C.
For run 7, the third stage 1% hydrogen peroxide treatment was
carried out by reacting the pulp with 1% H2O2, 1~ NaOH, 1%
NaSiO4, 0.5% DTPA at 70~C for 1 hour.
The results are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv brownstock 36.3 15.0
2. 2.0% peracetic acid
+ Oxygen delignification 5.6 13.6 50.6
3. 2.0% peracetic acid
+ Oxygen delignification
+ 2.0% peracetic acid * 13.3 73.6
4. H2SO3 wash
+ 2.0% peracetic acid
+ Oxygen delignification 5.0 12.4 53.8
5. H2SO3 wash
+ 2.0% peracetic acid
+ Oxygen delignification
+ 0.5% chlorine dioxide
+ 2.0% alkaline extraction
+ 0.6% chlorine dioxide * 10.9 89.8
6. H2SO3 wash
+ 2.0% peracetic acid
+ Oxygen delignification
+ 1.0% peracetic acid * 12.7 72.2
7. H2SO3 wash
+ 2.0% peracetic acid
+ Oxygen delignification
+ 1.0% hydrogen peroxide * 10.0 72.0
* The kappa numbers were too low to be accurately measured.
-53- 2~J3~35
It is seen that in all cases the kappa numbers were
decreased well below 70%, viscosity decreases were on the order
of 2 to 5 cps, and brightness levels achieved ranged from about
50 to above 89 ISO.
Example 11
Sequence (PA)O
In this example, a comparison is made between
generated peracetic acid and commercially available peracetic
acid. Birch/poplar/maple organosolv brownstock was treated
according to Example 10 first with 1.1% peracetic acid then was
oxygen delignified.
The results are shown below:
Viscosity Brightness
Kappa No.~cps) (ISO)
_9
1. Organosolv Brownstock 29.7 25.3
2. Gen. Peracetic Acid 5.5 24.8 53.2
+ Oxygen Delignification
3. Com. Peracetic Acid 5.7 22.7 51.6
+ Oxygen Delignification
_5~_ 20'33Q3~
In this example, pulp can be treated with either
generated or commercially available peracetic acid. One of the
techniques which can be used to generate peracetic acid is by
conversion of acetic acid in the presence of hydrogen peroxide
under acidic conditions. Hydrogen peroxide and acetic acid are
mixed in an appropriate ratio selected to optimize the
conversion to peracetic acid at siven process parameters.
This example shows that under the same reaction
conditions, similar brightening responses are obtained using
either generated or commercial peracetic acid.
Examples 12 and 13 demonstrate the lower levels of
oxygen delignification achieved with kraft pulps even when they
are pretreated with peracetic acid. Additionally, there are
greater losses of viscosity, and lower brightness levels when
compared to the similarly treated pulps according to the methods
of the present invention.
Example 12
Sequence (PA)O
Kraft softwood brownstock obtained from Skeena
Cellulose Incorporated, Prince Rupert, British Columbia was
delignified with oxygen by reacting the brownstock with 3.0%
NaOH at 80~C for 30 minutes. The brownstock was pretreated
prior to delignification by reacting the pulp with 1.0%
peracetic acid, 2.2% NaOH, 0.5% DTPA, and 0.5% MgSO4 at a pH of
11 for two hours at 70~C.
-55- 2~5~035
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Kraft Brownstock 33.2 44.2
2. Oxygen delignification 21.1 28.8 25.6
5 3. 1.0% peracetic acid
+ Oxygen delignification 19.0 23.3 34.5
Clearly the reduction in the kappa number was much
less, 36% and 42%, than for similarly treated organosolv pulps.
At the same time, the loss in viscosity was significant (21 to
14 cps), while the brightness levels achieved fell short of the
values achieved for similarly treated organosolv pulps.
In the following example the effect of additional
peracetic acid bleaching of kraft softwood brownstock is shown.
Example 13
Sequence (PA)O(PA)
The kraft softwood brownstock of Example 10 was
pretreated with peracetic acid and subsequently delignified
with oxygen. After delignification with oxygen, the pulp was
treated with peracetic acid as per Example 10.
-56- 2~ 3~
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Kraft brownstock 33.2 44.2 22.8
2. 2.0~ peracetic acid
+ Oxygen delignification 16.8 17.3 29.2
4. 2.0% peracetic acid
+ Oxygen delignification
+1.4% peracetic acid 9.4* 17.6 44.9
* A 25 ml permanganate number can be used as an indication of
lignin content when the kappa number is low. As a rough
estimate, the kappa number is approximately 1.5 times the
permanganate number.
The viscosity decreases were much larger than with
similarly treated organosolv pulps and the brightness levels
were not as high.
In another aspect of this invention, oxygen
delignified organosolv pulps can be bleached to high brightness
levels using two chlorine dioxide bleaching (D) stages with an
alkaline extraction (E) stage between them (ODED bleaching
sequence).
Example 14
Sequence ODED
Organosolv pulp was delignified with oxygen to a
kappa number of 9.7 using the conditions of Example 1. This
pulp was further contacted with 0.97% ClO2 at a pulp
-5~- 2 ~ 3 ~
consistency of 10% solids for 2 hours at 70~C. After washing,
the pulp was contacted with 2.0% NaOH at 12% consistency for 2
hours at 70~C. This pulp was then washed and contacted with
0.8% ClO2, and enough NaOH to reach a pH of 3.5 to 4.5 for 3
hours at 70~C.
Viscosity Brightness
Kappa No.(cps) (ISO)
1. Organosolv brownstock 35 24.3
2. Oxygen delignification 9.7 20.1
3. Oxygen Delignification 15.9 91
+ 1st ClO2 Stage
+ Alkaline Extraction
+ 2nd ClO2 Stage
As can be readily seen, the kappa number of the
pulp was reduced by about 62% by oxygen delignification and a
final brightness of 91 ISO was achieved.
Example 15
Sequence ODED
Organosolv pulp was delignified with oxygen to a
kappa number of 12.9 using the conditions of Example 1. This
pulp was further contacted with 1.42% ClO2 at a pulp
consistency of 10% solids for 2 hours at 70~C. After washing,
the pulp was contacted with 2.0% NaOH at 12% consistency for 2
-58- 2~303~
hours at 70~C. This pulp was then washed and contacted with
0.7% ClO2 and 0.3% NaOH for 3 hours at 70~C.
Viscosity Brightness
Kappa No.(cps) (ISO)
1. Brownstock 37.4 17.6
2. Oxygen delignification 12.9 16.1
3. Oxygen Delignification 11.0 90.2
+ 1st ClO2 Stage
+ Alkaline Extraction
+ 2nd C1O2 Stage
The pulp of this example was analyzed for residual
chloroorganic content and found to have the following levels:
Total TOX 158.0 ppm
Water leachable AOX 5.4 ppm
Alcohol-benzene ex- 15.0 ppm
tractable AOX
Unextractable 137.0 ppm
organochlorine
This example shows that pulps bleached by the
sequence ODED achieve very high brightness using a low level
of chlorine dioxide. The AOX in the untreated effluent in this
example is predicted to be approximately 1.1 kg AOX per ton of
pulp. The TOX residue in pulp is also quite low compared to
other pulps.
2~53~3~
-53-
Example 16
Sequence EoDED
Birch/aspen/maple organosolv pulp was treated as in
Example 4 with a charge of 4.5% sodium hydroxide and 0.5%
MgSO4. Oxygen was mixed with the pulp at 50 psig. Over the next
6 minutes the oxygen pressure was gradually released until the
oxygen pressure was atmospheric. The pulp remained in the mixer
at 60~C for another 45 minutes, with occasional stirring at low
speed. The oxygen delignified pulp was then treated with
chlorine dioxide and alkaline extraction as in Example 14 using
2.67% chlorine dioxide in the first bleaching stage.
The results are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
l. Organosolv Brownstock 39.7
2. Oxygen Extraction (Eo) 22.3 27.2
3. Oxygen Extraction 19.0 91
+ 1st ClO2 Stage
+ Alkaline Extraction
+ 2nd ClO2 Stage
As can be readily seen, an organosolv pulp can be
delignified with the milder oxidative extraction (Eo) and still
achieve a high brightness of 91 ISO.
2 ~ a3 û3 a
-60-
Example 17
Sequence Z
Birch/maple/aspen organosolv pulp was acidified
with sulfuric acid to a pH of about 2 to 3 and then fluffed.
The fluffed acidified pulp was contacted with ozone at abou~
1.3% (w/w) ozone on oven dried (o.d.) pulp, the ozone being
present in oxygen as a gas phase carrier. The pulp mixture was
agitated during ozonation.
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 20.7
2. 1.3% Ozone 6.6 48.3
As can be readily seen, with a single ozone stage
the kappa number is reduced by about 68%.
Example 18
Sequence OZ
Birch/maple/aspen organosolv pulp was delignified
with oxygen to a kappa number of 9.9 using the conditions of
Example 1. The delignified pulp was treated with 0.5% ozone as
in Example 17. After ozone treatment, the pulp pH was adjusted
to il using NaOH. After adjustment with NaOH, the pulp was
washed with water to a neutral pH.
-61- ~3~
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 35
2. Oxygen Delignification 9.9
5 3. Oxygen Delignification
+ 0.5% Ozone 2.0* 65.6
* A 25 ml permanganate number can be used as an indication of
lignin content when the kappa number is low. As a rough
estimate, the kappa number is approximately 1.5 times the
permanganate number.
As can be readily seen, using an oxygen
delignification followed by an ozone stage, the kappa number
can be reduced by 90% and the brightness achieved is above 65
ISO.
Example 19
Sequence OZ(edta)P, OZ(PA) OZ(edta)P~, OZ(PA)D
Birch/maple/aspen organosolv pulp was delignified
with oxygen and treated with ozone as in Example 18. The pulp
was then treated with about 0.5% EDTA for 90 minutes at 70~C.
The final pH was about 5 to 7. The EDTA treated pulp at 12%
consistency was treated with hydrogen peroxide at about 2~.
DTPA was added at about 0.2%, at 70~C and for 3 hours. The
peroxide treated pulp was further treated with 0.2~ chlorine
dioxide at 70~C, for 3 hours. Enough NaOH was added to a final
pH of 3.5 to 4.5.
- -62- 235303~
Birch/maple/aspen organosolv pulp was delignified
with oxygen and treated with ozone as in Example 18. The pulp
was then treated with about 2% peracetic acid at a 12%
consistency. Enough NaOH was added to a pH of about 5 to 7.
DTPA was added at about 0.2% and the reaction proceeded for
about 3 hours at 70~C. The peracetic acid treated pulp was
further treated with 0.2% chlorine dioxide at 70~C, for 3
hours. Enough NaOH was added to a final pH of 3.5 to 4.5.
Brightness
Kappa No. (ISO)
1. Organosolv brownstock 35
2. Oxygen delignification 9.9
3. Oxygen delignification
+ 0.5% Ozone 2.0* 65.6
4. Oxygen delignification
+ 0.5% Ozone
+ 0.5~ EDTA 81.1
+ 2% hydrogen peroxide
5. Oxygen delignification
+ 0.5% ozone
+ 2% peracetic acid 84.1
-63- 2 0 ~3Q 3 ~
6. Oxygen Delignification
+ 0.5% Ozone
+ 0.5% EDTA
+ 2% Hydrogen Peroxide
+ 0.2% ClO2 89.1
7. Oxygen Delignification
+ 0.5% Ozone
+ 2% peracetic acid
+ 0.2~ ClO2 89.6
* A 25 ml permanganate number can be used as an indication of
lignin content when the kappa number is low. As a rough
estimate, the kappa number is approximately 1.5 times the
permanganate number.
This example shows that an organosolv pulp can be
brightened to above 89 ISO with low level chlorine dioxide.
As can be readily seen, a brightness of above 84
ISO can be achieved without the addition of chlorine dioxide.
Such pulps will contain zero level TOX from chlorine based
bleaching chemicals and correspondingly the bleach effluents
contain zero level AOX.
~953~3~
-6~-
Example 20
Sequence ZO
Maple/aspen/birch organosolv pulp was treated with
0.5% ozone as in Example 17 then delignified with oxygen using
the conditions of Example 1.
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 20.7
2. 0.5% Ozone 6.6 48.3
3. 0.5% Ozone 4.2 58.3
+ Oxygen Delignification
This example shows that an ozone stage can further
delignify a pulp before and after an oxygen delignification
stage. A reduction in kappa number of about 80% can be
achieved.
Example 21
Sequence ZO(edta)P
Maple/aspen/birch organosolv pulp was treated as in
Example 20. The pulp was then treated with hydrogen peroxide.
The hydrogen peroxide step is carried out by mixing 2.5~
hydrogen peroxide, NaOH to an end pH of 10, at 70~C and for 3
20S~33
-65-
hours. EDTA was added at about 0.5% at about 10 to 12%
consistency, for 90 minute and at 70~C.
The results are shown below:
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 20.7
2. 0.5% Ozone 6.6 48.3
3. 0.5% Ozone 4.2 58.3
~ Oxygen Delignification
4. 0.5% Ozone 86
+ Oxygen Delignification
+ 0.5% EDTA
+ 2.5% hydrogen peroxide
This example shows that a brightness of about 86
ISO can be achieved with one ozone-stage followed by oxygen
delignification and an hydrogen peroxide stage. Such organosolv
pulps bleached to about 86 ISO without chlorine dioxide will
contain zero level TOX from chlorine based bleaching chemicals
and correspondingly the bleach effluents contain zero level
AOX.
Example 22
Sequence OZD
The organosolv pulp of Example 18 was bleached with
chlorine dioxide as in Example 8.
20!~3035
-6~-
Brightness
Kappa No. (ISO)
l. Organosolv Brownstock 35 65.6
2. Oxygen Delignification 9.9
5 3. Oxygen Delignification
+ 0.5% Ozone 2.0* 65.6
4. Oxygen Delignification
+ 0.5% ozone
+ 0.8% C102 89
10 5. Oxygen Delignification
+ 0.5% ozone
+ 0.4% C102
+ 0.4% C102 90
* A 25 ml permanganate number used as an indication of lignin
content when the kappa number is low. As a rough estimate, the
kappa number is approximately 1.5 times the permanganate
number.
As can be readily seen, ~ small amount of chlorine
dioxide in one stage or two consecutive stages improved the
brightness to 90 ISO.
The following example sets forth the continuous
delignification and bleaching of a mixture of organosolv and
softwood kraft brownstock pulp.
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Example 23
Sequence EoDEpD
This example illustrates continuous delignification
and bleaching with countercurrent recycle of bleaching
filtrates. During the stages of delignification and bleaching,
the pulp was washed using bleaching filtrates of a subsequent
treatment stage.
A mixed brownstock pulp of about 11% to 15%
consistency containing 80% birch organosolv pulp and 20~ Kraft
brownstock pulp was delignified and bleached using the EoDEpD
stage. In the (Eo) stage, the mixed pulp was treated in an
oxidative extraction stage as in Example 4 using a sodium
hydroxide charge of about 3.2%. After oxidative extraction, the
pulp was washed from filtrates of the (Ep) stage. In a next
stage, the mixed pulp was treated with a first chlorine dioxide
stage at about 3% (w/w) of chlorine dioxide on oven dried
(o.d.) pulp under conditions similar to Example 14. The
chlorine dioxide bleached pulp was washed with bleaching
filtrates from the second chlorine dioxide ble~ching stage that
followed the (Ep) Stage. The chlorine dioxide bleached pulp was
subjected to an alkaline extraction stage which included the
addition of 0.2% hydrogen peroxide and using the same
conditions as in Example 6 with a sodium hydroxide charge of
about 0.7%. A second chlorine dioxide stage followed with 1.2%
ClO2 and the pH was adjusted using sodium hydroxide to a range
of about 3.5 to 4.5.
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The results are shown below:
Viscosity Brightness
Kappa No.(cps) (ISO)
1. Brownstock 30
5 2. Oxidative Extraction 2
3. Oxidative Extraction
+ 1st ClO2 Stage
+ Alkaline Extraction 27.5 67
4. Oxidative Extraction
+ 1st ClO2 Stage
+ Alkaline Extraction
+ 2nd ClO2 Stage _ 21 89
This example illustrates a mill trial using the
process as shown in Figure 7. However, in this example,
oxidative extraction (Eo) which is a milder delignification
treatment was used instead of oxygen delignification. To the
alkaline extraction step (E) of Figure 7, a low level of
hydrogen peroxide was added in order to enhance the brightness
of the bleached pulp. A pulp brightness of 89 ISO was obtained.
It is to be understood that while the invention has
been described in conjunction with the preferred specific
embodiments thereof, the foregoing description as well as the
examples are intended to illustrate and not limit the scope of
the invention. Other aspects, advantages and modifications
within the scope of the invention will be apparent to those
skilled in the art to which the invention pertains.
