Note: Descriptions are shown in the official language in which they were submitted.
- 21633g9
CHLORINE-FREE ORGANOSOLV PULPS
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 pressureæ 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).
- 216~389
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
10~ of the weight of molecular chlorine (Cl2), and 5.3~ of
the weight of chlorine dioxide (C1O2) 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 ~_ll 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
25 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
or increasing delignification o- the wood pulp and have
- 2163~89
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
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 hydl^ogen 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 ~rocesses using non-chlorine
- 2163389
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 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 re~uires
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 ALCBLL~ 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 p~oduced from the same wood
species In such processes, by the methods of the present
- 2163389 ~`
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
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 2S 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 organcsolv
pulps is generally that larger quantities of bleac;~ing
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 indicafing
lower viscosity or strens~h It is desirable to ha~e as
2163389
--6--
little decrease in viscosity as possible during the
bleaching process, since viscosity losses are usually
associated with a decrease in pulp strength, as measured
by customary tensile, burst and tear strength tests.
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 delignification 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, i~ 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 pulp.
SUMMARY OF THE INVENTION
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.
21633~9
--7--
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
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 object of this invention to
provide a process for further delignification of the
oxygen treated pulp with an alkaline extraction stage
reinforced with peroxide and oxygen wherein the pulp
residual lignin is decreased in excess of from 80~ to
about 90~ with little or no decrease in pulp viscosity to
within about 2 to 5 cps.
It is another object of this invention to
provide a process for bleaching the delignified pulp to a
brightness in excess of about 70 ISO to more than 90 ISO
with no use of chlorine based bleaching chemicals.
It is another object of this invention to
delignify and bleach organosolv pulps with relatively high
kappa values as compared to kraft pulps from the same wood
species to a high brightness level 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 about 50
to about 76~ with little or no decrease in pulp viscosity.
2163389 `
--8--
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
S 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.
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
peroxlde .
In another aspect of this invention, a process
is provided for enhancing the effect of oxygen
delignification comprising treating the delignified 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 compoun~ prLor ~o performing oxygen
~` ~
- 2163389
delignification, and treating the resulting pulp with a
peroxy compound such as peracetic acid or 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.
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
~- 2163383 ~
-10-
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
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 ~ is a flow chart of a process for the
continuous deligrification ~nd bleaching of pulp
2163389
brownstock using countercurrent washing and recycling of
solvents and bleaching filtrates using the following
sequences: (Peroxy)O(Peroxy)(Peroxy) and
(Peroxy)OZ(Peroxy).
Figure 7 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: ODEoD and ODED.
Figure 8 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 follo~ing
sequences: O(Peroxy)D and OZD.
Figure 9 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: OEopZ(Peroxy) and OEop(Peroxy)(Peroxy)~
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
21633~9
-12-
be decreased 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~
up to about 76~ and without a decrease in viscosity
values, to within about 2 to about 5 cps.
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
2163389
-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
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 or in staged exposures,
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 70 By the
methods of the present lnvention, an organvso~v pulp with
2163389
-14-
a brownstock kappa number of about 70 can be delignified
to a kappa number of about 10 in one treatment stage, a
reduction of about 75~, with an oxygen stage 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
or hydrogen peroxide results in a 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 deligni~ication 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 co~pound before oxygen
delignification then are treated in one or more stages
2 1 6 ~ ~ 8 9 -15-
with a peroxy compound. A brightness of from about 83 to
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
more peroxy stages. These pulps have a brightness of from
about 82 to 90 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 90 ISO can be obtained.
Alternatively, when pulps are treated with a chlorine
dioxide stage, a brightness value of from about go to
about 92 ISo can be obtained.
2163~8~ `
-16-
Generally, before delignification or bleaching,
pulp brownstock is washed with an alcohol solution
comprising; from about 20 to about 80~ (by volume) of a
water miscible lower aliphatic alcohol of 1 to 4 carbon
- 5atoms (e.g., methanol, ethanol, isopropanol or tert-
butanol); and from about 20 to a~out 80~ water to remove
any soluble lignin prior to delignification or bleaching.
The alcohol washed pulp is washed again with water to
remove any residual alcohol and is delignified and
10bleached 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
15peracetic 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
20peroxide 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
250.3 to about 3 hours and at a temperature of between about
40C and about 90C.
Alternatively, in conjunction with treatment
with a peroxy compound, the pulp brownstock can also be
treated in a separate step with transition-metal chelating
2163389
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
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 (S02) 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 viæcosity protection of the pulp.
An ozone stage (z) can also be used to treat
pulp brownstock either as a pretreatment prior to oxygen
delignification (sequence ZO), following oxygen
delignification (sequence OZ) or following an oxidative
extraction stage (sequence EoZ and EZ~ Pulp brownstock
trea~ment wit;~ ozone is carried at a pH of from about 1.5
216~89
-18-
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 8~ to
about 55~, preferably from about 10~ to 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 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.
2163~89 -19-
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
5delignification, 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
10OP, O(PA), OZ), or following a peroxy compound treatment
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 40~ preferably from about
10~ to about 12~ consistency by weight of pulp solids with
15a 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 a
20high shear with oxygen gas such that the weight of oxygen
gas is from about 1.5% (w/w) oxygen on oven dry (o.d.)
pulp. The temperature of the reaction mixture is between
from about 60C to about 110C, more preferably from about
70C to about 90C, and oxygen pressure is maintained
25between 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 60 minutes, more
preferably between from about 25 to about 25 minutes.
Additional chemicals may be added to help preserve
30strength properti2s and includ_ frGm about 0.1~ to about
2163389
-20-
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
ODEoD). An oxidative extraction stage can also be used
before a first chlorine dioxide bleaching stage (sequences
EoDED/ EoDEoD and EoDEopD). 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 1~ to about 5~, preferably
from about 1.5~ 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 50C to about 110C,
preferably between from about 60C to about 90C, 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 to about 60 minutes. For the first 5
to 15 minutes, the pressure cf oxygen is decreased to
f
2163389 -21-
atmospheric and the pulp then remains in an oxygen rich
atmosphere for about 20 to about 50 minutes.
Generally, oxygen delignified pulp (O) can be
treated with chlorine dioxide (D) as a subsequent first
bleachina 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 after a 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
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 acid or 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 70C 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)~ a peroxide assisted
oxidative extraction stage (Eop)~ or a conventional
alkaline extraction stage (E) on a first bleached pulp
with chlorine dioxide (sequence ODEoD, ODEoD and ODED). An
2S 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 70C 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 or the second bleaching
2163389
stage using chlorine dioxide are generally the same as in
a chlorine dioxide first bleaching stage, however, a
preferred final pH is from about 3.5 to about 4.5 which
can be achieved by appropriate addition of caustic, and a
preferred reaction time is between about 1.5 to about 2
hours.
Pulp brownstock can be delignified and bleached
in a continuous mode. As shown in Figures 6, 7, 8 and 9,
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 and provided the filtrates
do not contain undesirable levels of corrosive chemicals
such as chloride ions, these Dleaching filtrates can be
mixed with water and can become part of the alcohol/water
solvent used in a continuous organosolv pulping process
such as described in U.S. Patent Application Serial No.
08/011,329 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 such as described
in U.S. Patent Application Serial No. 08/011,329 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
2163389
-23-
of energy. Material such as caustic can also be
recovered.
Figure 6 is a flow chart of the continuous
delignification and bleaching of pulp brownstock using the
sequences (Peroxy)O(Peroxy)(Peroxy)and (Peroxy)OZ(Peroxy)
wherein (Peroxy) is a peroxy compound such as for example
hydrogen peroxide or peracetic acid. For example, the
following sequences can be used: such as for example POPP,
(PA) OZP, (PA) O (PA) P, (PA) OPP, PO (PA) P, POZP, POP (PA),
( PA) OZ ( PA), ( PA) O ( PA) ( PA), ( PA) OP ( PA), PO ( PA)(PA) and
POZ (PA) . Generally, depending on the bleaching sequence
used, the pulp brownstock in any given treatment stage
can be washed in a countercurrent fashion with the
bleaching filtrates obtainea from washing the pulp at a
subsequent treatment stage.
In one preferred embodiment of the invention,
pulp brownstock 10 of a consistency of from about 10~ to
about 15~ pulp solids is delivered 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 from
about 0.2~ to about 6~ (w/w) peroxy compound on oven dried
(o.d.) pulp.
When hydrogen peroxide is used, it is introduced
at liquid stream 76 into the reaction mixture in mixer 21.
The final pH is preferably ~rom about 8.5 to about 11
which can be maintained by addition of caustic such as
sodium or potassium hydroxide tG the reaction mixture into
~ f
2163389
-24-
liquid stream 76. 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
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 concentrating the stillage from the solvent
recovery tower used to recover the solvent in an
organosolv pulping process such as is described in U.S.
Patent No. 4,764,596 and Application Serial No. 08/
011,329. The evaporator condensate 70 is delivered to
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 nearly 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 delivered to process equipment 73.
Process equipment 73 generally includes a conversion
reactor 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
3c acetic acid in an appropriate ratio which can be caref~lly
21633g9 '
-25-
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, a mixture of formic and acetic acid with
negligible amounts of water obtained in the acetic acid
recovery plant may be used to generate a mixture of
peracetic and peroxyformic acids to use in bleaching. The
peroxy acid formed may be distilled to achieve higher
purity and greater reaction conversion. 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 and steam to
adjust the reaction temperature to between about 40C and
about 90C, preferably 50 to 80C the pulp is delivered
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.
After peroxy compound treatment, the peroxy
treated pulp is delivered through line 25 and washed in
washer 3. 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 pressure diffusion
washers Dependirg on the e~uipment selected, the ulp
2163389
-26-
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 delivered through line 30
in 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 79. 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 line 80. The
temperature of the reaction mixture in mixer 31 is
preferably between from about 60C and about 110C, more
preferably from about 70C to about 90C 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 delivered through
line 32 into vessel 33 which can be selected from
conventional bleaching equipment, but generally, vessel 33
216~89
-27-
is a pressurized vessel and is selected to achieve the
required reaction time and temperature. 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
delivered through line 35 and washed in washer 4. After
washing in washer 4, the delignified pulp is delivered
through line 40 into equipment 41 Equipment 41 can be a
mixer where 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 11
and in an amount of about 0.2 to about 3~ (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 11 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 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 fro~ the acetic acid recovery and conversion at
3Q process equipment 71 and 73 After mixing with a p~roxy
2163389
-28-
compound, the pulp is delivered 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 40C and 90C, preferably 50 to 70C which can be
maintained using conventional heating techniques, such as
steam injection in mixer 41.
Alternatively, an ozone stage can also be used
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 60C, preferably 25 to 30C.
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.)
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
3C pressurizing ozone gas over water at an elevated pressure
21~389
-29-
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
S introduced in mixer 41 through liquid stream 77 and mixed
with the pulp. The resulting reaction mixture is
delivered 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
adjusted to the appropriate level using an acid such as
sulfuric acid which can be introduced at liquid ætream 77
into mixer 41. The pulp is then delivered 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 adjusteà to a near neutral pH.
After a first peroxy compound treatment stage or
an ozone treatment stage, the pulp is delivered through
line 45 onto washer 5. After washing in washer 5, the pulp
is delivered 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 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
- 2163389
-30-
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 delivered 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 40C and about 90C,
preferably 50 to about 60C which can be maintained by
using conventional heating techniques, such as steam
injection at mixer 51. The delignified and bleached pulp
is removed at line 62 and can be suitably subjected to
further processing or drying.
The washing of the pulp brownstock is highly
dependent on the bleaching sequence used. The washing
scheme in Figure 6 is particularly directed to the
sequence POPP. The pulp treated with this sequence can be
washed in washer 3 using bleaching filtrates delivered
through line 44 from washing the pulp in washer 4.
Conversely the pulp in washer 4 is washed using
countercurrent bleaching filtrates from line 54 from
washing the pulp in washer 5 and the pulp in washer 5 is
washed using countercurrent bleachinq filtrates from line
64 resultinq from washinq the pulp in washer 6. In this
particular bleaching sequence, filtrates in line 34 from
washer 3 are alkaline and can be recycled as described
above.
2 1~ 3 38 9 -31-
In another embodiment, when the bleaching
sequences (PA)OZP and (PA)O(PA)P are used, the filtrates
from washer 5 can be used to wash the pulp in washer 3 and
the filtrates from washer 6 can be used to wash the pulp
S in washers 4 and/or 5. Alkaline filtrates are obtained
from washer 4 through line 44 and the filtrates in line 34
are recycled as described above. With the bleaching
sequence (PA)OPP, filtrates from washer 6 can be used to
wash the pulp at washers 3 and/or 5 and filtrates from
washer 5 can be used to wash the pulp in washer 4.
Alkaline filtrates are obtained in line 44 and acidic to
neutral filtrates are obtained at line 34. With the
- bleaching sequence PO(PA)P, filtrates from washer 5
through line 54 can be used to wash the pulp in washers 3
and/or 4 and part of the filtrate from washer 4 can be
used to wash pulp in washer 3. An alkaline filtrate is
obtained through line 34. With the bleaching sequence
POZP, filtrates from washer 6 can be used to wash the
pulp at washer 4 and filtrates from washer 5 can be sent
to effluent treatment or as diluent for lignin
precipitation. A portion of the filtrates at line 44 from
washer 4 can be used to wash the pulp in washer 3. Both
filtrate~ through line 44 and 34 are alkaline and can be
recycled as above.
Figure 7 is a flow chart of the continuous
delignification and bleaching of pulp brownstock using the
- sequences ODEoD, ODED and ODEopD. Pulp brownstock 10 of a
consistency of from about 9% to about 40% pulp solids is
delivered through line 120 into mixer 121 which is
preferably a high shear mixer and can withstand the
f
` -
2 15 3 3 89 -32-
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
stream 176. The amount of caustic added is preferably
S 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 60C and
about 110C, more preferably from about 70C to about 90C
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. Optionally, from about 0.25~ to about 2~,
preferably from about 0.5~ to about 1.25~ (w/w) hydrogen
peroxide on oven dry (o.d.) pulp and 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 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
delivered 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 maintain the required reaction time and
maintain the reaction temperature achieved in the mixer.
Oxygen pressure in vessel 123 is preferably maintained
between about 30 and about 100 psig, more preferably
between about 80 to about 100 psig and the reaction time
2l63389
-33-
is preferably between about 6 to about 60 minutes, more
preferably between about 25 to about 50 minutes.
After oxygen delignification, the pulp is
delivered through line 125 in 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,
15 and 16 also depends on the equipment selected. After
washing the pulp at washer 13, bleaching filtrates are
removed through line 134. When the sequence ODEoD is used,
line 134 bleaching filtrates are alkaline and can be
recycled as described above.
After washing in washer 13, the delignified pulp
is delivered 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~ to 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 70C which can be
achieved by steam injection. The reaction mixture is
delivered through line 132 into vessel 133 which can be
selected from conventional bleaching equipment, but
gen~rally, vessel 133 is select~d to achieve the required
2 1 6 3 3 8 9 _34_
reaction time and maintain the reaction temperature
reached in mixer 131. The reaction in vessel 133 proceeds
at a temperature of from a~out 30 to about 70C 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
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 delivered through
10line 135 and washed on washer 14 using countercurrent
washing with bleaching filtrates from washer 16 delivered
through line 164. Filtrates resulting from washing the
pulp at washer 14 are delivered through line 146 and
subjected to conventional waste treatment.
15After 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 delivered
through line 140 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 cont~;n;ng of from about 0.5~ to about 3~,
preferably from about 0.75~ to about 1.5~ (w/w) sodium
hydroxide on oven dry (o.d.) pulp. Optionally, 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 60C and about 110C, more
preferably between about 70C and about 90C which can be
achleved by steam injection Oxygen pressure in mixer 141
- 2163389
--35--
is preferably maintained between about 30 to about 100
psig, preferably at about 30 to about 60 psig. Additional
chemical agents may be added into liquid stream 140 such
as magnesium sulfate from about 0.1% to about 1%. The
5 pulp is delivered into vessel 143 which can be selected
from conventional bleaching equipment, but generally
vessel 143 is selected to achieve the required reaction
time and maintain the temperature achieved in mixer 141.
The total reaction time with oxygen in vessel 143 is
10 preferably between about 6 and about 120 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 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
20 the reaction mixture in mixer 141 is preferably between
about 40C and about 80C 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 delivered 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
maintained preferably between about 40OC and about 70OC
and the total reaction time with oxygen in vessel 143 is
~163389 ~ .
-36-
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 delivered through
line 145 in washer 15. The pulp is washed in washer 15
using water bleaching filtrates delivered through line 164
and obtained by washing the pulp at washer 16. After
washing the pulp on washer 15, the partially bleached pulp
is delivered 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. In addition caustic hydroxide is added to
adjust the pH to from abut 3.5 to 4.5. The temperature of
the reaction mixture in mixer lS1 is from about 30 to
about 70C which can be achieved by steam injection. The
reaction mixture is delivered 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 70C and the reaction time i8 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
delivered through line 155 and washed at washer 16 using
3C water introduced at line 161. Bleaching filtrates are
216~89
-37-
removed from washer 16 through line 164 and can be used to
wash the pulp at washer 14. 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 delivered through line 220 in 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 27~. 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 60C and
about 110C, more preferably from about 70C to about 90C
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~ diethylene triamine pentaacetic acid
(DTPA), and from about 0.5~ to up to about 3~ sodium
silicate. The pulp is delivered through line 222 into
vessel 223 which can be selected from conventional
'~ bleaching e~uipment, but gen^rally, vessel 223 is a
216~389
-38-
pressurized vessel and is selected to achieve the required
reaction time and maintain the reaction temperature
achieved in the mixer. The temperature of the reaction
mixture in vessel 223 is preferably between from about
S 60C and about 110C, more preferably from about 70C to
about 90C 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
delivered through line 225 in washer 23 and when the OP
sequence, the pulp can be 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 which
are removed from washer 23 through line 234 are generally
alkaline and can be recycled as described above.
After washing the pulp in washer 23, the
delignified pulp is delivered 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 p~ cf from a~cut 3 to abvut 11 ard in an amount of
2163389
-39-
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 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
delivered 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 40C and about 90C,
preferably 50 to about 60C.
Alternatively, an ozone stage can also be used
to treat delignified pulp 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 60C, preferably 25 to 30C.
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 delivered through
-~ line 232 and into vessel 233 ~hich can be selected from
- ~163~89 ~
-40-
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 .o dissolve enough ozone
such that the concentration-of ozone is from about 0.2~ to
2~ (w/w) ozone on oven dried to.d.) pulp after the ozone
solution is mixed with the pulp. Th- ozone solution is
introduced into mixer 231 through liquid stream 280 and
mixed with the pulp. The resulting reaction mixture is
delivered 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
delivered through line 235 and in washer 24 and, if need
be, the pulp pH can be adjusted using caustic to a pH of
Z5 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. When peracetic acid or ozone cre used, filtrates
from washer 24 into line 244 are gen~r~lly acidic and ~;ill
3G be discharged and recycled as -~o~e.
~153389
-41-
After the peroxy compound treatment stage or the
ozone treatment stage, the pulp is delivered through line
235 in washer 24. The pulp is washed in washer 24 using
water introduced at line 263. The bleaching filtrates
obtained at washer 24 are delivered through line 244 and
used to wash the pulp on washer 23. After washing of the
pulp on washer 24, the pulp is delivered 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 70C which can be
achieved by steam injection. The reaction mixture is
lS delivered 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 70C
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 delivered through
line 245 and washed at washer 25 using water introduced at
line 261. Filtrates resulting form washing the pulp at
washer 25 are generally acidic. They are delivered through
line 254, subjected to conventional treatment to remove
3C any chlcrine and chlorinated compounds and recycled as
a~ove I~hen peracetic acid or ozone are used, filtrates
2163389
-42-
from washer 25 into line 254 will be used to wash the pulp
in washer 24. The delignified and bleached pulp is removed
at line 262 and can be suitably subjected to further
processing or drying.
Figure 9 is a flow chart of the continuous
delignification and bleaching of pulp brownstock using
sequences OEOpZ(Peroxy) and OEOp(Peroxy)(peroxy) wherein
(Peroxy) is a peroxy compound such as peracetic acid and
hydrogen peroxide. As shown in Figure 9, pulp brownstock
10 at a consistency of from about 9~ to about 40~ pulp
solids is deli~ered through line 320 into mixer 321 which
is preferably a high shear mixer and can withstand the
operating pressure required for the process. The pulp in
mixer 321 is mixed with a caustic solution, for example a
sodium hydroxide solution which is introduced in liquid
stream 376. 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 320
through line 379. The temperature of the reaction mixture
in mixer 321 is preferably between from about 60C and
about 110C, more preferably from about 70C to about 90C
which can be achieved by steam injection. Oxygen pressure
in mixer 321 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 376 to help preserve pulp strength
properties include from about Q.1~ to about 1~ magnesium
sulfate, from about 0.1~ to a~out 1~ diethylene triamine
2163389
pentaacetic acid (DTPA), and from about 0.5~ to up to
about 3~ sodium silicate. The pulp is delivered through
line 322 into vessel 323 which can be selected from
conventional bleaching equipment, but generally, vessel
323 is a pressurized vessel and is selected to achieve the
required reaction time and temperature. Oxygen pressure in
vessel 323 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
20 to about 50 minutes. After oxygen delignification, the
pulp is delivered through line 325 in washer 33 and is
washed using bleaching filtrates from washer 34 through
line 340. After washing the pulp at washer 33, alkaline
bleaching filtrates are removed through line 334 and can
be recycled as above.
After washing in washer 33, 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 delivered
through line 330 and into mixer 331 which is preferably a
high shear mixer and can withstand the operating pressure
required by the process. The pulp slurry in mixer 331 is
mixed with a caustic solution introduced at liquid stream
380 and containing of from about 0.5% to about 3~,
preferably from about 0.75~ to about 1.5~ (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 330 through line 336. The
temperature of the reaction mixture in mixer 331 is
preferably between about 6CC and about 110C, r,ore
:
2163389
preferably between about 70C and about 90C which can be
achieved by steam injection. Oxygen pressure in mixer 331
is preferably maintained between about 30 to about 100
psig, preferably at about 30 to about 60 psig. Optionally,
from about 0.25~ to about 2~, preferably from about 0.5
to about 1.25~ (w.w) hydrogen peroxide on oven dry (o.d.)
pulp and additional chemical agents may be added into
liquid stream 380 such as magnesium sulfate from about
0.1~ to about 1~. The pulp is delivered into vessel 333
which can be selected from conventional bleaching
equipment. The total reaction tir.e with oxygen in vessel
333 is preferably between about 10 and about 80 minutes.
Oxygen pressure in vessel 333 is decreased to atmospheric
pressure during the first 5 to 15 minutes and the pulp
remains in vessel 333 in an oxygen rich atmosphere for
about 20 to about 40 minutes.
Alternatively, in alkaline extraction stage, the
pulp slurry in mixer 331 can be mixed with a caustic
solution introduced at liquid stream 380 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 331 is preferably between about 40C and about 70C
which can be achieved by steam injection. Optionally, from
about 0.25~ to about 2~, preferably from about 0.5~ to
about 1.25~ (w.w) hydrogen peroxide on oven dry (o.d.)
pulp and additional chemical agents may be added into
liquid stream 380 such as magnesium sulfate from about
0.1~ to about 1~. The pulp is delivered into vessel 333
which can be selected from conventional bleaching
3Q equipment, but generally vessel 333 is selected tc achieve
2163~89
-4S-
the required reaction time and temperature. The
temperature of the reaction mixture in vessel 333 is
preferably between about 40C and about 70C which can be
achieved by steam injection and the total reaction time
with oxygen in vessel 333 is from about 1.5 to about 3
hours, preferably 1.5 to 2 hours.
The pulp is delivered through line 335 and
washed in washer 34. After washing, the delignified pulp
is delivered through line 339 into equipment 341.
Equipment 341 can be a mixer where the pulp is treated
with a peroxy compound or ozone or can be a dewatering
press where a high consistency pulp is produced to be
treated with ozone. The pulp is mixed at mixer 341 with a
peroxy compound for example using peracetic acid (PA) or
hydrogen peroxide (P) at a pH of from about 2.5 to about
11 and in an amount of about 0.2 to about 3~ (w/w) peroxy
compound on oven dried (o.d.) pulp. When hydrogen peroxide
is used, it is introduced at liquid stream 377 into the
reaction mixture at mixer 341. 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 377. 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 377 or from the acetic
acid recovery and conversion as described in Figure 6.
`- 216338~ -`
-46-
Alternatively, an ozone stage can also be used
to treat pulp brownstock in vessel 343. 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 60C, preferably 25 to 30C.
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 341 which
is preferably a high consistency pulp dewatering press.
After dewatering, the pulp is conveyed through line 342
and into vessel 343 which can be selected from
conventional bleaching equipment but which is preferably a
high consistency ozone bleaching tower. In vessel 343,
the pulp is fluffed using techniques known in the art and
ozone gas is introduced into vessel 343 through line 346
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
as ozone solution or as ozone gas at mixer 341 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 in mixer 341 through liquid
stream 377 and mixed with the pulp. The resulting
reaction mixture is delivered through line 342 into vessel
343 which is a con~entional bledching tower preferably
selected to confor~, to the reaction parameters. The final
2163~9
-47-
pH can be adjusted to the appropriate level using an acid
such as sulfuric acid which can be introduced at liquid
stream 377 into mixer 341. The pulp is then delivered
through line 345 onto washer 35 and, if need be, the pulp
pH can be adjusted using caustic at liquid stream 347 to a
pH of from about 9 to 11 and can be further adjusted to a
near neutral pH.
After a first peroxy compound treatment stage or
an ozone treatment stage, the pulp is delivered through
line 345 onto washer 35. After washing in washer 35, the
pulp is delivered through line 350 into mixer 351. The
pulp is mixed at mixer 351 with a peroxy compound for
example using peracetic acid (PA~ or hydrogen peroxide (P)
at a pH of from about 2.5 to about 11 and in an amount of
about 0.2 to about 3~ (w/w) peroxy compound on oven dried
(o.d.) pulp. When hydrogen peroxide is used, it is
introduced at liquid stream 381 into the reaction mixture
at mixer 351. 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
381. The pulp can be of any consistency, but is
preferably between about 10~ to about 12~ by weight of
pulp. 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 377 or from the acetic acid recovery and
conversion as described in Figure 6. After mixing with a
peroxy compound, the pulp is delivered through line 352
into vessel 353 which can be selected from conventional
~leachina equipment, such that the reaction time is
2 1 6 ~ 3 8 9
-48-
preferably from about 0.3 to about 3 hours, the reaction
temperature is between about 40C and about 90C,
preferably 50 to about 60C which can be maintained by
using conventional heating techniques, such as steam
in3ection. The delignified and bleached pulp is removed
at line 362 and can be suitably subjected to further
processing or drying.
It is believed that an Eop stage can have an
enhancing effect on the sequences described in Figures 7
and 8 when such stage is used im~.ediately following oxygen
delignification. Equal or higher pulp brightness can be
obtained while using a lesser amount of bleaching
chemicals in a later stage resulting in significant
environmental implications.
lS 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.
The following two examples show the effect of
oxygen delignification of an organosolv pulp.
EXAMPLE 1
Sequence O
2163389
-49-
Birch/maple/aspen orgarosolv 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 mixer. 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
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 85C, with occasional stirring at low speed.
~0 The results are shown ~elow.
Kappa No. 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
Se~uence 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
2163~89
-50-
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
S 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 charrber at about 11~ _o 12~
r-~
2163389 -51-
consistency. Oxygen was mixed with the pulp at 32 psig in
the high shear mixer for four seconds. Over the next 12
minutes oxygen pressure was gradually released until
pressure was atmospheric. The pulp remained in the mixer
at 70C for another 45 minutes, with occasional stirring
at low speed.
Kappa No. Viscosity (cps)
1. Organosolv brownstock32.2 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 psig.
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.
216~389
-52-
EXAMPLE 5A
Sequence OEop
Birch/maple/aspen organosolv pulp was treated as
in Example 1. The oxygen delignification was followed by
an oxidative alkaline extraction following the procedure
in Example 4 and reinforced with hydrogen peroxide using a
charge of 1% sodium hydroxide and 0.5~ magnesium sulfate
for a 60 minutes total reaction time including a reaction
time of 10 minutes at 20 psig oxygen pressure and at a
temperature of 90C. A range of concentration of hydrogen
peroxide and sodium hydroxide as shown below were used and
the resulting pulp had a kappa number dependant on the
amount of peroxide and sodium hydroxide used.
Kappa No. Viscosity (cps)
1. Organosolv Brownstock 28 30
2. Oxygen Delignification 9.3 29.2
3. Oxygen Extraction (Eop)
(~)P/(~)NaOH
0.25/1 8.7 ----
0.50/1 6 ----
0.75/1 4.8 26.8
1/1.5 4.7
1.5/2.0 4.5 ----
The viscosity of the pulp treated with 0.75~
hydrogen peroxide was 26.8 and had a brightness of about
58 ISO. This result indicates that relati~e to the
216~3~9
original brownstock a viscosity drop of about 3.2 can be
observed with an overall delignification of about 82~.
In the following exar?le, organosolv pulp is
first delignified with oxygen and then treated with
peracetic acid.
EXAMPLE 6
Sequence O(PA)
Birch/maple/aspen c-ganosolv 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~ cons stency with a 4~ solution of
NaOH at 85C, 100 psig for 4, minutes. 1.0~ MgSO4 was
also added to the reaction mixture.
The peracetic acid sta~e 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 60C.
The results of such treatments are shown below:
Viscosity Brightness
Kappa N . (cps) (ISO)
1. Organosolv brownstock 29.~ 22 9
2 Gxygen delignification lQ 3 22 5 36
216~389
3. Oxygen delignification
- + 1.3~ peracetic acid 5.3 24.1 58.5
4. Oxygen delignification
+ 2.7~ peracetic acid 4.G 22.3 64.7
The foregoing shows that an oxygen
delignification of about 65~ is significantly increased by
approximately another 50% to a ~appa 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 treatmen. step also significantly
increases the brightness of the ?ulp ~rom about 37 ISO to
about 59 to 65 ISO.
In the following exa,ple, pulp delignified with
oxygen was treated with two staaes 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 6~C at a 10~ consistency.
The hydrogen peroxide third treatment stage was carried
out by reacting the pulp Wit~l 1 0~ H202, 1.0~ NcO~, 0.2~
216~89
-55-
MgSO4, 0.2~ DTPA, and 4.0% Na2SiO4 at 60C 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 70C.
The results of these treatments are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv Brownstock 29.~ 22.9
2. Oxygen delignification 10.` 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.
2163389
-56-
It should be noted that the above-described
brightness levels were achieved without any chlorine
containing bleaching compounds and therefore the
delignified and bleached pulp contain zero level TOX from
S 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 kap~a number of 10.3 as in
Example 6. The pulp was then ~leached using successive
treatment stages of peracetic aci~ and chlorine dioxide.
The delignification and peracetic acid second stage
treatments were carried out as ir 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
70C, or by reacting 0.8~ C102 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~ C1O2 and 0.1~ NaO~ at a 10~ consistency for
3 hours at 70C.
The results of such treatments are shown below.
Viscosity srightness
Kappa No ~cps) (ISO)
1. Oxygen delignification 10.3 22.5 36.4
2163389
2. Oxygen delignification
+ 2.7~ peracetic acid 4.0 22.3 64.7
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 iow 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 that 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.
r~
216338g
-58-
Some of the delignification stages were followed by
various peroxy treatment stages.
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.S% DTPA, and 0.5%
MgSO4 at a 12% consistency at 7CoC for one hour. The
oxygen delignification second tage was carried out with
4.0% NaOH, 0.5~ MgSO4 at a 12% consistency at 85C 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 70C 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 70C 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 70C.
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
21 ~33~
-59-
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), z 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 PO(PA)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.
-
2163389
-60-
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 using 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.
10The 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 70C. The 2~ peracetic acid .hird treatment stage was
carried out by reacting the pulp with 2~ peracetic acid,
150.5~ DTPA, 0.5~ MgSO4 at a 12~ consistency for 2 hours at
70C 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
S2 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 85C for 45
minutes. The third stage chlorine dioxide treatment for
run 5 was carried out by reacting the pulp with 0.5~ C1O2
~or 2 hours at 70C. 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
70C. For the fifth stage the pul~ was reacted with 0.6
ClO2, 0.22~ NaOH at a 12~ consi s~ncy for 3 hours at 70C.
2163~89
-61-
The third stage hydrogen peroxide treatment stage for run
6 was carried out by reacting 2.2~ NaOH, 0.5~ DTPA, and
1.0~ Na2SiO4 at 15~ consistency for 2 hours at 70C. For
run 7, the third stage 1~ hydro3en peroxide treatment was
carried out by reacting the pulp with 1~ H22~ l~ NaOH, 1
NaSiO4, 0.5~ DTPA at 70C for 1 hour.
The results are shown ~elow:
Viscosity Brightness
Kap~a ~o. (cps) (ISO)
1. Organosolv brownstock 36.3 15.0
2. 2.0~ peracetic acid
+ Oxygen delignification 5.Ç 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
2163389
-62-
* The kappa numbers were too low to be accurately measured
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
S 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 deiignified.
The results are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
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
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
21633~9
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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
given 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 pretreatea with peracetic acid.
Additionally, there are greater losses of viscosity, and
lower brightness levels when co~,pared 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 80C 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 70C.
Viscosity Brightness
KappG Nc. (cps) (ISO)
1. Kraft Brownstock 3~ . 2 44 . 2
2163~89 ~
-64-
2. Oxygen delignification 21.~ 28.8 25.6
3. 1.0~ peracetic acid
+ Oxygen delignification l9.O 23.3 34.5
Clearly the reduction in the kappa number was
much less, 36~ and 42~, thar~ 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.
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Kraft brownstock 33.2 44.2 22.8
2. 2.0~ peracetic acid
+ Oxygen delignification 16.& 17.3 29.2
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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 hi~h.
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
consistency of 10~ solids for 2 hours at 70C. After
washing, the pulp was contacted with 2.0~ NaOH at 12~
consistency for 2 hours at 70C. 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 70OC.
Viscosity srightness
Kappa ~o. (cps) (IS0)
. Organosolv brownstock 35 24.3
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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 seer" 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 cQrditions of Example 1.
This pulp was further contacted -~ith 1.42~ ClO2 at a pulp
consistency of 10% solids for 2 hours at 70C. After
washing, the pulp was contacted with 2.0~ NaOH at 12~
consistency for 2 hours at 70C. This pulp was then
washed and contacted with 0.7~ C1O2 and 0.3~ NaOH for 3
hours at 70C.
Viscosity Brightness
KaPpa No. ~cps) (ISO)
1. Brownstock 37.4 17.6
2. Oxygen delignification 12.916.1
3. Oxygen Delignification 11.0 90.2
+ 1st ClO2 Stage
+ Alkaline Extraction
+ 2nd ClO2 Stage
' 2 1 6 3 3 8 9 -67-
The pulp of this example was analyzed forresidual chloroorganic content and found to have the
following levels:
Total TOX 158.0 ppm
Water leachable AOX5.4 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 approximGtely
1.1 kg AOX per ton of pulp. The TOX residue in pulp is
also quite low compared to other pulps.
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 60C for another 45 minutes,
with occasional stirring at low speed. The oxygen
delignified pulp was then treated with chlorine dioxide
and alkaline extraction as ir Exar.,ple 14 using 2.57
chlorine dioxide in the first bleachinG stage.
21$3389
The results are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Organosolv Brownstock 39.7
2. Oxygen Extraction (Eo) 22.3 27.2
3. Oxygen Extraction _ 19.0 91
+ 1st C1O2 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.
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 about 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 nu~ber is reduced by about 68~.
--` f ~
2163389
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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 11 using NaOH. After
adjustment with NaOH, the pulp was washed with water to a
neutral pH.
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 35
2. Oxygen Delignification 9.9
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 18A
Sequence OEopZP
An organosolv pulp prepared as in Example 5A
using 0.75~ peroxide in the Ecp stage was washed with 2~
21~3389 _70
sulfuric acid at 3~ consistency for 15 minutes to remove
any metals that might interfere with the ozone stage.
After the wash stage, the pH was about 2. The acid washed
organosolv pulp at a consistency of 40~ was treated with
0.9~ (w/w) ozone as in Example 17 at a temperature
of 25C and for a 15 minutes reaction time. Following the
ozonation stage, the pulp at a consistency of 12~ was then
treated with 1~ hydrogen peroxide, 1~ sodium hydroxide at
90C and for a reaction time of 180 minutes.
The results are shown below:
Brightness
Kap~a No. (ISO)
1. Organosolv Brownstock 28 ----
2. Oxygen Delignification 9.3 41
3. Oxygen Extraction (Eop) 4.8 58
4. Ozone ---- 80
5. Hydrogen Peroxide ---- 90
In this example, the final viscosity is 8.2 cps
and brightness is 90 ISO.
EXAMPLE 18B
Sequence OEop(PA)P
The organosolv pulp of Example 5A at 12~
consistency was treated with 2~ peracetic acid at 90C, a
pH of 4 and for a reaction time of 180 minutes. The
tre~tment with peracetic acid was followed with treatment
with hydrogen peroxide as in Example 18A.
.
2 1 6 3 ~ 8 9 -71-
The results are shown below:
Brightness
Kappa No. (ISO)
1. Organosolv Brownstock 28 ----
2. Oxygen Delignification 9.3 41
3. Oxidative Extraction (Eop) ~.8 58
4. Peracetic Acid ---- ----
5. Peroxide ---- 90
In this example, the final viscosity is 17 cps
and brightness is 90 ISO. It can be observed that the use
of peracetic acid resulted in similar high brightness as
the use of ozone, but at considerably higher viscosity.
EXAMPLE 19
Sequence OZ(edta)P, OZ(PA) OZ(edta)PD, 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 70C. 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
a~out 0.2%, at 70C and for 3 hours. The peroxide treated
pulp was further treated with 0.2% chlorine dioxide at
70C, for 3 hours. Enough NaOH was added to a final pH of
3 5 to 4.5.
-
2163389 -72-
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 70C.
The peracetic acid treated pulp was further treated with
0.2~ chlorine dioxide at 70C, for 3 hours. Enough NaOH
was added to a final pH of 3.5 to 4.5.
i0 Brightness
Kap~a 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
6. Oxygen Delignification
+ 0.5~ Ozone
+ 0.5~ EDTA
+ 2~ Hydrogen Peroxide
+ 0.2~ ClO2 89.
7. Oxygen Delignification
+ 0.5~ Ozone
+ 2~ peracetic acid
+ 0.2~ ClO2 89.6
2163389
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* 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.
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
2163389
-74-
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
70C and for 3 hours. EDTA was added at about 0.5~ at
about 10 to 12~ consistency, for 90 minute and at 70C.
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
21633895
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.
Brightness
Ka2pa No. (ISO)
1. Organosolv srownstock 35 65.6
2. Oxygen Delignification 9.5
3. Oxygen Delignification
+ 0.5~ Ozone 2.0* 65.6
4. Oxygen Delignification
+ 0.5~ ozone
+ 0 8~ C102 89
5. Oxygen Delignification
+ 0.5~ ozone
+ 0.4~ Cl02
+ 0.4~ Cl02 _ 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, a small amount of
chlorine dioxide in one stage or two consecutive stages
improved the brightness to go ISO.
21633~9
-76-
The following example sets forth the continuous
delignification and bleaching of a mixture of organosolv
and softwood kraft brownstock pulp.
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 pul~ 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 bleaching 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
conditicns as in Example 6 with a sodium hydroxide charc-
of about 0 7~ A second chlo~lne dicxide stage fol~owed
. ' r~
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with 1.2~ Cl02 and the pH was adjusted using sodium
hydroxide to a range of about 3.S to 4.5.
The results are shown below:
Viscosity Brightness
Kappa No. (cps) (ISO)
1. Brownstock 30
2. Oxidative Extraction 24
3. Oxidative Extraction
+ 1st Cl02 Stage
+ Alkaline Extraction _ 27.5 67
4. Oxidative Extraction
+ 1st Cl02 Stage
+ Alkaline Extraction
+ 2nd C102 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 i~vention. Other aspects,
2163389
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advantages and modifications within the scope of the
invention will be apparent to those skilled in the art to
which the invention pertains.
