BACKGROUND OF THE INVENTION
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This invention pertains, generally, to the bleaching of lignocellulosic
material in the pulp and paper industry and, in particular, to a process to improve
the performance of a pulp bleaching sequence which uses hydrogen peroxide. It is
especially valuable when used in conjunction with a bleaching sequence designed
for the production of pulps bleached without the use of chlorine compounds.
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The pressure to remove chlorinated compounds in the bleaching of pulps
and reduce their perceived negative effect on the environment has prompted the
surge of so called "chlorine free technologies". When chlorine compounds are
eliminated from the bleaching sequence, it is conventional to use hydrogen
peroxide as a substitute since hydrogen peroxide is generally believed to be benign
to the environment. Therefore, since the use of hydrogen peroxide is increasing,
there is a strong incentive to develop more efficient ways to apply it in order to
maintain favorable economics of bleaching.
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Hydrogen peroxide is an oxidizing chemical, commonly used in the
brightening of mechanical, semimechanical, semichemical, and recycled pulps. It
is also used in chemical pulp bleaching to aid in delignification.
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The Pulp & Paper Research Institute of Canada (PAPRICAN) has
developed several novel ways to bleach mechanical pulps. Of these, the high
temperature peroxide system disclosed by Lierbergott, et al., is of interest because
it pursues the same brightness development as conventional high consistency
bleaching systems, but does so at medium consistency (10-14%). This is achieved
by increasing the temperature of the pulp to about 85°C and lowering the pH,
which is differentiated from conventional high consistency peroxide systems.
Because of the faster reaction resulting, the retention time is reduced from hours
to minutes (15-30 min.), and no silicate is required to stabilize the peroxide
solution. The peroxide charge remains about the same as that of the conventional
systems.
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A high temperature peroxide system has been proposed in a copending
application of Bottan, G. to be operated at 85-95°C. for very short periods of
time. No one has yet proposed to superheat pulp and to operate a peroxide stage
under pressure at short retention times in excess of 100°C. It is desirable to
minimize the pulp retention time in order to minimize the capital investment
required, while maintaining brightness similar to that which can be achieved using
very long retention times.
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Reactivation of Residual Peroxide has been proposed by Dominique
Lachenal of the Centre Technique du Papier in Grenoble, France for use in the
bleaching of mechanical pulps. This allows the reactivation of the non-consumed
(or residual) peroxide (after the first reaction stage tower) by increasing the
alkalinity of the pulp suspension. The aim is to eliminate expensive dewatering
equipment normally used after the bleaching tower to recover the residual peroxide
and to recirculate it to the point of addition of the fresh peroxide (usually at a
mixer before the bleaching tower). This proposal becomes important when
compared with a conventional two stage peroxide bleaching system which requires
the expensive dewatering equipment between stages. The first stage reaction takes
place for several hours in a conventional tower at 60°C., prior to reactivation of
the residual peroxide. Caustic is then added in quantities proportional to the
peroxide residual obtained after several hours of reaction at 60°C.
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In chemical pulps, hydrogen peroxide has been primarily used in medium
consistency systems, in which the pulp slurry, from a previous stage, is dewatered
in a thickener or washer to about 10-14% consistency. The peroxide solution is
conventionally added, together with alkali, at the repulper (discharge from the
thickener or washer) or before the medium consistency tower in a medium
consistency pump or mixer.
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When used as an individual stage for bleaching of chemical pulps,
conventional medium consistency peroxide bleaching stages do not provide
sufficient brightness increases and are said to consume more peroxide, and require
extremely long retention time for consumption of the peroxide. For more
pronounced delignification or brightness effects peroxide must be applied in
several towers.
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The use of high consistency peroxide stages operating at more than 25%
consistency has been demonstrated to overcome many of the limitations of
conventional medium consistency peroxide bleaching stages. However, its
disadvantage is that it requires the installation of dewatering devices capable of
achieving high discharge consistency, is more complicated to operate, and
consumes more power than medium consistency systems.
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On chemical pulps, fortification of oxidative extraction stages using
peroxide in addition to oxygen is widely used to reduce the amount of chlorine
necessary in the first stage of pulp bleaching. Conventional Eop (oxidative
extraction reinforced with peroxide) typically use upflow/downflow towers, require
the use of oxygen gas, and typically operate in the range of 65-85°C.
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Hydrogen peroxide is also used in the final stages of pulp bleaching to
achieve a high brightness stable bleached pulp. Use of hydrogen peroxide in
bleaching of pulps has been limited to temperatures typically less than 90°C., as it
has been believed that the peroxide would decompose, resulting in very poor
utilization of the bleach chemical, reduced pulp strength, and poor economics of
bleaching.
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More recently, the use of hydrogen peroxide under conditions of high
temperature and relatively long retention time has been proposed as a means for
developing very high brightness pulps, when used at the end of a conventional or
chlorine free bleaching sequence. According to the prior art, the required
retention time is 1-3 hours. It has been shown that pressurized oxygen peroxide
systems at long retention times can increase the brightness ceiling of the pulp
over what can be obtained using conventional atmospheric peroxide bleaching
stages. It is desirable to minimize the required retention time under pressure in
order to reduce the capital investment required for the installation. For example,
according to the prior art, the use of 100+°C. for 1-3 hours has been
demonstrated in the laboratory to be very effective in reducing the amount of
time required for the peroxide bleaching process. It seems though, that the
economics may not be very attractive for this process as the capital required for
a 1-3 hour pressurized peroxide stage is quite high.
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EP-A-0 285 530 discloses a short peroxide bleaching step under
pressure at a temperature greater than 100°C. This document teaches that this
single peroxide bleaching process occurs at a pH of at most 9 and additionally
teaches that a high efficiency washing step should follow the short pressurized
bleaching reaction.
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According to basic information on the economics of the prior art of
conventional atmospheric peroxide bleaching stage and the prior art of
pressurized oxygen peroxide bleaching stage (PO), the investment required for
the retention tower for the prior art is calculated as follows for a single stage of
about 850 madtpd capacity:
- a. Pressurized peroxide tower for 2 hours retention USD 1,000,000
- b. Atmospheric, long retention, conventional stage
- 1. 3 hours retention USD 300,000
- 2. 6 hours retention USD 400,000
- 3. 9 hours retention USD 550,000
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In order to achieve similar brightness levels with conventional
atmospheric peroxide bleaching, it is necessary to have a retention time of in
excess of 6 hours compared to 1-3 hours according to the prior art for
pressurized oxygen peroxide systems. For example, it would be necessary to
size an atmospheric tower for 8-10 hours retention to achieve similar results to
a 2 hour pressurized oxygen peroxide system.
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With 350 bleach plants in North America alone, if only one peroxide
bleaching system is required in each plant, this would represent an investment
for the industry of approximately USD 200-350 million. To meet increasingly
stringent environmental regulations, it may be necessary to install more than
one peroxide bleaching stage in each mill, which could then more than double
the investment. Thus a very important disadvantage of the prior art is the very
large investment required for implementing the technology according to the
teachings of the prior art.
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The foregoing illustrates limitations known to exist in some present
cellulosic and lignocellulosic bleaching processes, and it would be
advantageous to provide an alternative directed to overcoming one or more of
those limitations. Accordingly, a suitable alternative is provided including
features more fully disclosed hereinafter.
SUMMARY OF THE INVENTION
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In one aspect of the present invention, a method for using hydrogen
peroxide for bleaching cellulosic and lignocellulosic pulp in a three stage
bleaching sequence is provided. The first stage treatment comprises the steps
of adjusting consistency of the pulp to 10% - 18% and introducing the pulp to a
mixer in which sodium hydroxide is added to bring the pulp to a pH of greater
than 8.5; adding hydrogen peroxide to equal from 0.5%-5.0%, by weight, of the
pulp; heating the pulp to a temperature greater than 100 degrees Celsius while
maintaining sufficient pressure to prevent boiling of pulp liquor; passing the pulp
through a reactor column at a rate which provides a reaction time in the column
of less than 45 minutes; cooling the pulp without adding additional hydrogen
peroxide and discharging the cooled pulp to a washer, in which a substantial
portion of the unconsumed bleaching chemicals and dissolved material is
removed from the pulp, and in which the pulp consistency may be adjusted to a
preferred value for a second stage of bleaching treatment. The second
bleaching stage comprises the steps of adding sufficient alkali or acid to adjust
the pH of the pulp to a preferred value for the bleaching chemical; introducing
the pulp to a mixer in which the second bleaching chemical is mixed with the
pulp; discharging the pulp from the mixer into a reaction vessel in which the
pulp is held for a sufficient time to permit consumption of a substantial portion
of the applied chemical; discharging the pulp from the reaction vessel to a
washer, in which a substantial portion of the unconsumed bleaching chemicals
and dissolved organic material is removed from the pulp and in which the pulp
consistency is adjusted, if necessary, to 8%-18%. The pulp is exposed to a
third stage of bleaching treatment, the third stage treatment comprising all
steps identical to those recited for said first stage hydrogen peroxide bleaching
treatment subsequent to adjustment of pulp consistency.
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In another aspect of the present invention, one step of high
temperature peroxide bleaching under pressure in accordance with the first
stage is combined with a second step of peroxide bleaching by reactivating
residual hydrogen peroxide. The second step of peroxide bleaching includes
the steps of introducing the pulp to a mixer and while a significant residual of
hydrogen peroxide remains in the pulp; adding a second quantity of alkali for
reactivating residual hydrogen peroxide and sufficient to bring the pulp to a pH
of at least 9; and depositing the pulp from the mixer in a reaction tower and
allowing the reaction to proceed for a sufficient time to permit consumption of a
substantial portion of the residual hydrogen peroxide.
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The foregoing and other aspects of the invention will become apparent
from the following detailed description, when considered in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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- Fig. 1 shows one preferred embodiment of the present invention;
- Fig. 2 shows another preferred embodiment of the present invention;
- Fig. 3 shows another preferred embodiment of the present invention;
- Fig. 4 shows another preferred embodiment of the present invention;
- Fig. 5 shows a bleaching stage incorporating still another preferred
embodiment of the present invention;
- Fig. 6 shows another preferred embodiment of a bleaching sequence using
the present invention applied to an existing bleach plant;
- Fig. 7 shows another preferred embodiment of a bleaching sequence using
the present invention with recovery of residual peroxide using conventional
washers;
- Fig. 8 shows another preferred embodiment of a bleaching sequence using
the present invention with recovery of residual peroxide using presses; and
- Fig. 9 shows another preferred embodiment of a bleaching sequence using
the present invention with another method of recovery of residual peroxide using
presses.
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DETAILED DESCRIPTION
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Referring to Fig. 1, pulp from a conventional washer or thickener is
discharged through pipe 10 into mixer 100, where steam for heating the pulp and
alkali for increasing the pH of the pulp is added through pipe 15. The pulp is
heated and adjusted to a pH of more than 8.5, preferably 9.5-10.5. The heated
and pH adjusted pulp is discharged from mixer 100 through pipe 20 to a
conventional medium consistency pump 200 which pumps the pulp through pipe 30
to a mixer 300. Hydrogen peroxide solution is added to the mixer 300 through
pipe 35 in a quantity sufficient to assure desired brightness development will be
achieved at the end of the reaction. As in conventional peroxide bleaching
systems, the addition of magnesium compounds for protection of cellulose
viscosity, as well as sequestrants (such as SiO2) and/or chelants (such as EDTA or
DTPA) may also be added with the alkali solution through pipe 15, the peroxide
solution through pipe 35, or separately through pipe(s) 16 and/or 36. As the
temperature of the pulp must exceed 100°C. according to the present invention,
and it is not currently practical to pump pulp at this high temperature, the final
temperature rise from, for example, 85-90°C. to in excess of 100°C., is achieved
by adding steam between the pump 200 and the mixer 300 through pipe 37.
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The pulp, which has been heated to the desired reaction temperature, and
adjusted to the desired pH, is pumped through pipe 40 into the upflow tube
column 400, which is sized for the retention time desired for the reaction.
According to the present invention, the upflow tube is sized to assure a pulp
retention time of 1-30 minutes, preferably 5-20 minutes, after which the pulp is
discharged through pipe 50 to an appropriate discharge device 500, such as a
valve, and thence through pipe 60 to subsequent washing and bleaching stages. As
the pulp is bleached at more than 100°C., it may be desirable to reduce this
temperature to prevent "flashing" of the pulp, so water or liquor at a cooler
temperature, may be used for dilution into the top of the upflow tube through pipe
70. Those skilled in the art will recognize that this invention may be practiced
with upflow, downflow, upflow/downflow, or other configurations to achieve the
desired retention time for the reaction. Further, those skilled in the art will
recognize that there are additional means to heat and cool the pulp prior to and
subsequent to the reaction, respectively. The details given in this description are
not intended to limit the scope of the invention but are included for clarity of the
preferred embodiment.
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Fig. 2 schematically describes another preferred embodiment of the present
invention which is identical to that shown in Fig. 1, except for the addition of a
downflow tower 600. According to the present invention, the upflow tube is sized
to assure a pulp retention time of 1-30 minutes, preferably 5-20 minutes, after
which the pulp is discharged through pipe 50 to an appropriate discharge device
500, such as a valve, through pipe 60 to a conventional bleaching tower 600,
preferably one which is already installed at the pulp mill and can be reused for this
new bleaching stage instead of a chlorine based bleaching sequence. The pulp is
retained in this tower for an additional 1-5 hours at the desired reaction
temperature to consume a substantial portion of the hydrogen peroxide applied on
the pulp. As the pulp is bleached in column 400 at more than 100°C., it may be
desirable to reduce this temperature to prevent "flashing" of the pulp. In the
present invention, it is possible to cool the pulp prior to discharge from the upflow
tube, such that the liquor with the pulp does not "flash". This embodiment may
be desirable in some mills to limit potential emissions of steam and residual
chemical to the atmosphere. This may be accomplished by adding cooler water or
liquor to dilute the pulp through pipe 70. For example, for pulp treated in the
upflow column at 12% consistency at 110°C., the addition of 1.5-2.5 m3/ton pulp
of 50-60°C. liquor, will cool the pulp to less than 100°C., and result in a drop in
consistency to about 10% consistency, which will not significantly decrease the
performance of the second step of the reaction. It may be especially desirable to
maintain the pulp at the maximum temperature allowable at atmospheric pressure.
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In another preferred embodiment of the present invention illustrated in Fig.
3, the pulp is discharged from pipe 60 into tower 600 and is allowed to flash to
atmospheric pressure. The pulp entering the downflow tower will be at its
maximum possible temperature, for example, 98-100°C., which will enhance the
consumption of peroxide in the second phase of the reaction. Further, the flashed
steam discharges through pipe 90 to heat exchanger 700. The heat exchanger is
used to pre-heat the wash water applied on the washer prior to the peroxide stage,
thereby reducing the amount of steam necessary to be applied to the stage through
pipes 15 and 37. This increases the temperature of the wash water in pipe 101
prior to its application to the conventional washer through pipe 110.
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The embodiment shown in Fig. 3 is identical to that of Fig. 2 up to the
point of discharge of the pulp from upflow reaction column 400. In this case the
pulp retains a substantial amount of residual peroxide upon discharge. The pulp is
discharged through pipe 50 to an appropriate mixing valve 500, where additional
alkali is added through pipe 55 to increase the pH of the pulp to more than 8.5,
preferably 9.5-10.5 for the second step of the reaction.
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The pulp, after flash cooling, is discharged through pipe 60 to a
conventional downflow tower 600, for the second phase of the reaction. The
conventional downflow tower is sized to consume a substantial portion of the
remaining peroxide, and will typically retain the pulp for 1-5 hours. Bleached
pulp is discharged from tower 600 through pipe 80 to subsequent process steps.
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The valve 500 serves to reduce the pressure from the upflow tube 400 and
allow the liquor with the pulp to flash. The alkali is added to the pulp just
upstream of the valve, and the turbulence created in the valve serves to mix the
alkali with the pulp. It is recognized that the installation of any mixing device at
this location of the process is substantially equivalent to the function of the valve.
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Fig. 4 describes another preferred embodiment of the present invention
which is identical to Fig. 3, except for reactivation of pulp in mixing device 550.
In this preferred embodiment, and according to the present invention, the upflow
tube is sized to assure a pulp retention time of 1-30 minutes, preferably 5-20
minutes, after which the pulp is discharged through pipe 50 to an appropriate
mixing device 550, in which the residual peroxide is reactivated by the addition of
alkali through pipe 55, after which the pulp is allowed to flash and steam from
such flashing is conducted through pipe 90 to heat exchanger 700 to preheat the
water from pipe 101 to higher temperature water which is discharged through pipe
110 and used for washing of the pulp upstream of the bleaching stage.
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Fig. 5 is a schematic diagram of a bleaching sequence employing the
present invention. Pulp from a washer or thickener 10 is discharged through pipe
11 and is fed to a mixer 20, where steam and/or bleaching chemicals are applied
through pipe(s) 12 and 13 to heat the pulp to the desired reaction temperature, and
to increase the pH of the pulp to the desired level for first treatment with
peroxide. The heated and pH adjusted pulp is discharged through pipe 21 to a
pump 30 to transfer the pulp to a mixer 40. As the pulp will be heated to above
100°C., it is conventional to apply steam for final heating ahead of the mixer,
typically through a pipe 34. Additional chemicals including alkali, peroxide,
chelants, and others may be added through pipes 32 and 33, and homogeneously
mixed with the pulp prior to discharge through pipe 41 to the first peroxide
reaction vessel 50. The peroxide reaction vessel 50 is sized for 1-30 minutes
retention time, preferably 5-20 minutes to achieve substantial delignification and/or
brightness increase prior to being discharged through pipe 51 to a discharge device
52. It is preferable for the best operating cost of the sequence, but not necessary,
to wash the pulp following the first peroxide treatment, and if the washing device
is a non pressurized device, it will be preferable to cool the pulp prior to discharge
from the first peroxide reaction vessel. Cooler dilution liquor may be added in the
first peroxide reaction vessel 50 through pipe 53. It may be desirable to dilute the
pulp prior to the pump with filtrate fed through pipe 54 if the washing device
requires low consistency pulp for its proper operation.
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The pulp is washed on washer 60, and the washed pulp is discharged
through pipe 61 to a mixer 70. The second step in this preferred embodiment is
an ozone stage which may be operated at either high or medium consistency,
however, it is preferable that this be a medium consistency stage operating at
conventional washer discharge consistencies of 10-14% to minimize the capital
cost for the installation. As is known in the prior art, it is necessary to operate at
a pH of less than 4 to achieve optimum results from the ozone stage. Thus, prior
to the ozone stage, an acid such as sulfuric acid, may be added at the discharge of
the first peroxide reaction vessel 50 through pipe 53, with the shower water
applied onto the first peroxide washer 60 through pipe 56, in the feed to the pump
70 through pipe 62, or in a separate mixer (not shown). The acidified pulp is
pumped with pump 70 through pipe 71 to a mixer 80, in which ozone gas is
applied through pipe 72 . The pulp is discharged to the ozone reaction vessel 90
and is held for up to about 10 minutes to allow nearly complete consumption of the
ozone. The pulp is discharged from he ozone reaction vessel through pipe 91 to a
discharge device 92, which may consist of a valve, a plurality of valves, or a
mechanical device to reduce the pressure from the ozone reaction vessel. The pulp
is discharged through pipe 93 to a gas separation device 220, where the gas is
separated from the pulp and is discharged through pipe 202 to treatment and/or
reuse in the mill. The degassed pulp is discharged through pipe 102 to a pump
210. It is preferable for the best operating cost of the sequence, but not necessary,
to wash the pulp following the ozone stage prior to the next stage of bleaching. It
may be desirable to dilute the pulp prior to the pump with filtrate fed through pipe
103 if the washing device requires low consistency pulp for its proper operation.
The pulp is fed through pipe 104 to washer 120, and is washed using water or
filtrate applied through pipe 115.
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Pulp from the washer 120 is discharged through pipe 121 and is fed to a
mixer 130, where steam and/or bleaching chemicals are applied through pipe(s)
122, 123, and 124 to heat the pulp to the desired reaction temperature, and to
increase the pH of the pulp to the desired level for first treatment with peroxide.
The heated and pH adjusted pulp is discharged through pipe 131 to a pulp 140 to
transfer the pulp through pipe 141 to a mixer 150. As the pulp will be heated to
about 100°C., it is conventional to apply steam for final heating ahead of the mixer,
typically through a pipe 144. Additional chemicals including alkali,
peroxide, chelants, and others may be added through pipes 142 and 143, and
homogeneously mixed with the pulp prior to discharge through pipe 151 to the
second peroxide reaction vessel 160. The peroxide retention vessel 160 is again
sized for 1-30 minutes retention time, preferably 5-20 minutes to achieve
substantial delignification and/or brightness increase prior to being discharged
through pipe 161 to a discharge device 152. It is preferable to wash the pulp
following the second peroxide treatment; however, it may not be necessary if an
additional bleaching stage is added which is not significantly affected by the
presence of residual peroxide and/or dissolved organic matter. If the washing
device is a non pressurized device, it will be preferable to cool the pulp prior to
discharge from the first peroxide reaction vessel. Dilution of the pulp with a cool
liquor may be accomplished in the second peroxide reaction vessel 160 through
pipe 153. It may be desirable to dilute the pulp prior to the pump with filtrate fed
through pipe 154 if the washing device requires low consistency pulp for its proper
operation. The pulp is then washed on a washing device 180 using water or
filtrate applied through pipe 156.
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Fig. 6 is a schematic diagram of a bleaching sequence employing the
present invention applied to an existing bleach plant. A typical sequence which
exists in many bleach plants today is DcEoDED. An existing conventional bleach
plant may be modified to incorporate the concepts according to the present
invention very economically. As shown in the Figure, almost all of the existing
equipment is utilized in the conversion, and the investment for conversion may be
as little as three mixers, three reaction vessels, an ozone generator, and
miscellaneous piping to incorporate the change. In some cases, it may be
necessary to make some changes to the materials in existing bleach towels, piping,
and other equipment.
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In this example as shown in Figure 6, the existing chlorination tower may
not be used in the new bleaching sequence, but may serve as additional storage or
other pretreatment prior to the new bleach plant. The existing Eo stage will be
used as a chelant stage where EDTA, DTPA, or other chelant is added and is
operated at a controlled pH and temperature, preferably 5-7 pH and 10-60°C.
The retention time of 30-90 minutes that typically exists in a conventional Eo stage
is suitable for the chelation stage.
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After washing on the existing Eo washer, the existing D1 stage will be used
as a first peroxide stage (P1), by diverting the pulp to a new mixer suitable for
addition of peroxide, adding the appropriate reaction vessel, and discharging to the
existing D1 tower, with or without reactivation of residual peroxide according to
the present invention. The first peroxide stage will operate at a pH of about 8.5 to
about 12.5, with the first step of the reaction at a temperature of greater than
100°C., preferably 105-120°C., with a reaction time of 1-30 minutes, preferably
5-20 minutes.
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After washing on the existing D1 washer, the existing E stage will be used
as an ozone stage, by diverting the pulp to a new mixer suitable for addition of
ozone, adding the appropriate reaction vessel and gas separator, and discharging to
the existing E tower. The ozone stage will operate at a pH of less than 4,
preferably 2-4, at a temperature of 30-70°C., preferably less than 50°C., with a
reaction time of less than 10 minutes preferably less than 5 minutes. After
washing on the existing D2 washer, the existing D2 stage will be used as a second
peroxide stage (P2), by diverting the pulp to a new mixer suitable for addition of
peroxide, adding the appropriate reaction vessel, and discharging to the existing
D2 tower, with or without reactivation of residual peroxide according to the
present invention. The second peroxide stage will operate at a pH of about 8.5 to
about 12.5, with the first step of the reaction at a temperature of greater than
100°C., preferably 105-120°C., with a reaction time of 1-30 minutes, preferably
5-20 minutes.
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After washing on the existing D2 washer, the pulp may proceed to
subsequent post treatments, but preferably will be fully bleached pulp to be
transferred to the bleached high density storage tower.
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Figure 7 illustrates a preferred method for the reuse of filtrates in the
QPZP sequence. It is noted that according to the present invention, there is
substantial residual peroxide with the pulp following the peroxide treatment, for
example, normally the peroxide charged is 2.5% on pulp, but the amount of
peroxide consumed in the reaction is typically less than 1.5% on pulp. As
peroxide is a relatively expensive bleaching chemical, it is desirable to recover this
residual for reuse in the peroxide stages. This is accomplished by recirculating
filtrate from the post peroxide stage washing to the washing step preceding the
peroxide stage as shown in the Figure. For illustration, typical filtrate flows are
shown in the Figure, for example, with each washer discharging at 10%
consistency, the filtrate flow with the pulp is 9 kg liquor/kg pulp. The wash water
flow to each washer, at a dilution factor of 2, which is typical in bleach plants
today, is 11 kg liquor/kg pulp. The filtrate, according to the present invention, is
recirculated countercurrently through the bleach plant such that the residual
peroxide is applied to the pulp on the washer, and a substantial portion of this
residual peroxide then carries forward to the peroxide stage. The required
peroxide application in the stage is then reduced by the amount carried forward
with the pulp. The amount of peroxide residual carried forward is a function of
the displacement ratio (washing efficiency) of the washer. For example, if the
washer has a displacement ratio of 0.85, then about 60% of the peroxide residual
will be recovered. The same principle applies to the recovery of alkali used in the
peroxide stages. According to this embodiment, the bleach chemical used in the
sequence is reduced substantially by recirculation of filtrates as shown in Figure 7,
compared to other filtrate recycle schemes which may be considered.
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Figure 8 illustrates another preferred method for the reuse of filtrates in the
QPZP sequence, which may be preferred if the bleach plant is to be constructed
new, that is, without reuse of existing equipment in a bleach plant. Figure 8
includes the use of presses, preferably Twin Roll Washing Presses manufactured
by Ingersoll-Rand Company, instead of conventional vacuum or pressure washers
or vacuum or pressure diffusers. Typical filtrate flows are shown in the Figure,
with each press discharging at 33.3% consistency, the filtrate flow with the pulp is
2 kg liquor/kg pulp. The wash water flow to each washer, at a dilution factor of
2, which is typical in bleach plants today, is 4 kg liquor/kg pulp. In this case, the
filtrate is used for washing on the press similar to the concept described in Figure
7. However, in addition, the filtrate which contains substantial residual peroxide
is also used for dilution of the pulp discharged from the press in the amount of 7
kg liquor/kg pulp. The amount of peroxide recovered is similar to that with the
vacuum washer case described in Figure 7. The amount of filtrate recovered in
the press is a function of the displacement ratio (washing efficiency) of the press.
If the press has a displacement ratio of 0.40, a total recovery of about 60% of the
peroxide residual is achieved. This may be attractive for new construction as
presses have similar capital requirements as conventional vacuum or pressure
washers. One additional benefit of this configuration is the ability to incorporate
high consistency ozone bleaching rather than medium consistency ozone bleaching
if the washing devices installed are presses. This option reduces the amount of
ozone required for bleaching, and reduces the energy demand in the bleach plant.
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In Fig. 9, pulp from a conventional washer or thickener is discharged
through pipe 10 into mixer 100, where steam for heating the pulp and/or alkali is
used for increasing the pH of the pulp is added through pipe 15. The pulp is
heated and adjusted to a pH of about 11. The heated and pH adjusted pulp is
discharged from mixer 100 through pipe 20 to a conventional medium consistency
pump 200 which pumps the pulp through pipe 30 to a mixer 300. Hydrogen
peroxide solution is added to the mixer 300 through pipe 35 in a quantity sufficient
to assure substantial residual will be maintained at the end of the first step of the
reaction. As is conventional in peroxide bleaching systems, the addition of
magnesium compounds for protection of cellulose viscosity, as well as sequestrants
(such as silicate and/or chelants {such as EDTA or DTPA}) may also be added
with the alkali solution through pipe 15. The peroxide solution can be applied
through pipe 35, or separately through pipe(s) 16 and/or 36. As the temperature
of the pulp must exceed 100°C. according to the present invention, and it is not
currently practical to pump pulp at this high temperature, the final temperature rise
from, for example, 85-90°C. to in excess of 100°C. is achieved by adding steam
between the pump 200 and the mixer 300 through pipe 37.
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The pulp, which has been heated to the desired reaction temperature, and
adjusted to the desired pH, is pumped through pipe 40 into the upflow tube 400,
which is sized for the retention time desired for the first phase of the reaction.
According to the present invention, the upflow tube is sized to assure a pulp
retention time of 5-30 minutes, preferably 5-20 minutes, after which retention
time, the pulp retains a substantial residual of peroxide in the pulp slurry. The
pulp is discharged through pipe 50 to an appropriate pressing device 560, where
the consistency of the pulp is raised from about 10% to about 25%. The filtrate is
recycled through pipe 55 to pipe 30 to recover a portion of the peroxide not
consumed in the first step of the reaction.
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The pulp is discharged through pipe 60 to a conventional downflow tower
600, for the second step of the reaction which is operated at high consistency (25-30%).
The conventional downflow tower is sized to consume a substantial portion
of the remaining peroxide, and will typically retain the pulp for 1-5 hours.
Bleached pulp is discharged from tower 600 through pipe 80 to subsequent process
steps.
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For clarity, pumps and mixers, as well as discharge devices, etc., which
are described in Figures 1-6, are not shown, nor are heat exchangers which are
necessary to maintain the optimum temperature in each bleaching stage. Heat
exchange devices are incorporated in the bleach plant in accordance with good
engineering practice.
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Those skilled in the art will recognize that this invention may be practiced
with upflow, downflow, upflow/downflow, or other configurations to achieve the
desired retention time for the reaction. Further, those skilled in the art will
recognize that there are additional means to heat and cool the pulp prior to and
subsequent to the reaction, respectively. Further, those skilled in the art will
recognize that the valve used to retain pressure in the first retention time (upflow
tube) may be located either before or after the mixing device. The details given in
this description are not intended to limit the scope of this invention, but merely are
included for clarity of the embodiment.
-
Persons skilled in the art will also recognize that the use of chelants and
sequestrants may be desirable or necessary to achieve the benefits of the present
invention, and the addition of these types of treatments to the present invention
will not limit the scope of the invention. Examples are the use of the sequences
QP, QPQP, QPQZP, QPZQP, PZQP, and the like. It is further recognized by
those skilled in the art that acid treatments, especially to very low pH, for
example, less than 3, may substitute effectively in many case for the use of
chelants in sequences of this nature; therefore, the addition of an acid stage, or the
substitution of an acid stage for a chelant stage will also fall within the scope of
this invention.
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This technology which has been dubbed the "PAPEROXIDE" process can
be applied to cellulosic or lignocellulosic materials from either softwoods,
hardwoods, or varieties of non-wood fibers from chemical, mechanical,
semichemical, or semimechanical pulps, or secondary fiber pulps. The benefits of
such applications are demonstrated in the examples which follow.
-
The first step of the PAPEROXIDE process is to react the pulp with
peroxide at a pH of 8.5-12.5 and at high temperature (>100°C.) for a limited
time of less than about 30 minutes, preferably 5-20 minutes. It is important that
this step be performed without causing metal contamination of the pulp slurry.
The result of this first step is illustrated in the example below:
EXAMPLE 1
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A sample of softwood kraft pulp, having a kappa number of 27.2, was
oxygen delignified (0 stage) to a kappa number of 1. This pulp sample was then
treated in a chelation stage using 0.6%EDTA on pulp (Q stage) at about 50°C.
and for 30 minutes retention time. The sample was then further delignified using
a conventional hydrogen peroxide stage (P) and an ozone stage (Z) to achieve a
kappa number of 2.0 with a brightness of 77.9%ISO and a viscosity of 14.4 mPa's
using the conditions and chemical charges shown in Table A.
Conditions and Chemical Charges in OQPZ stages |
Stage | O | Q | P | Z |
Tim, min. | 60 | 30 | 240 | 7 |
Temp. °C | 100 | 50 | 90 | 23 |
Consistency, % | 10 | 2 | 10 | 40 |
Pressure, psig | 100 | - | - | - |
Chemicals, % | 02,1% approx. | EDTA,0.6% | H2O2, 2.5% | 03,0.5% |
on pulp, | NaOH, 2.5% | H2SO4,to pH6 | NaOH, 2.5% | H2SO4,to pH2 |
to oven-dry | MgSO4,0.5% | | MgSO4,0.05% |
basis | | | DTPA, 0.2% |
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The pulp was further treated in a pressurized peroxide stage according to
the prior art and compared to the first step of the new process:
Comparison of Prior Art to New Process of Pressurized Peroxide |
| Prior Art Pressurized | New Process First Step |
H2O2 consumed, % on pulp | 2.2 | 0.9 |
End pH | 10.6 | 11.3 |
Time, minutes | 60 | 5 |
Temperature, °C. | 110 | 110 |
Brightness, %ISO | 91.6 | 89.6 |
Viscosity, mPa's | 8.5 | 8.9 |
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In all cases, the pulp was treated at 10% consistency, and was maintained
under pressure using oxygen gas at a pressure of 75 psig. The following
chemicals were applied in the bleaching:
Bleaching Conditions for Table 1 |
Chemical | Applied, % on pulp |
H2O2 | 2.5 |
NaOH | 2.75 |
MgSO4 | 0.05 |
DTPA | 0.2 |
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This illustrates that, in the first step of the present invention, it is possible
to achieve 85% of the brightness improvement, and surprisingly without a
viscosity penalty and with a much decreased consumption of peroxide, at a very
short retention time compared to the prior art which requires one hour or more.
Although in the example oxygen gas was used to maintain the pulp under pressure,
it is not necessary according to the first step of the present invention to expose the
pulp to oxygen gas. This is shown in the following example
Example 2
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A sample of softwood kraft pulp, having a kappa number of 27.2, was
oxygen delignified to a kappa number of 14. Thus pulp sample was then treated in
a chelation stage using 0.6% EDTA on pulp at about 50°C. and for 30 minutes
retention time. This pulp sample was then further delignified using a conventional
hydrogen peroxide stage and an ozone stage to achieve a kappa number of 1.3 with
a brightness of 81.8% ISO. The pulp was further treated in a conventional
atmospheric peroxide stage, and a pressurized peroxide stage according to the
prior art and compared to the first step of the present invention as follows:
Comparison to Prior Art Conventional and Pressurized Peroxide Stage,
and Effect of the use of Oxygen Gas |
| Prior Art Conventional | Prior Art Pressurized | New Process First Step |
H2O2 consumed, % on pulp | 1.84 | 1.91 | 1.07 |
End pH | 11.2 | 11.0 | 11.4 |
Pressure, bars | -- | 5.17 | 2.41 |
Oxygen used | no | yes | no |
Vapor pressure | | | 2.41 |
Time, minutes | 240 | 60 | 5 |
Temperature, °C | 90 | 110 | 110 |
Brightness, %ISO | 93.3 | 93.6 | 92.6 |
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In all cases, the pulp was treated at 10% consistency, and the following
chemicals were applied in the bleaching:
Bleaching Conditions for Table 3 |
Chemical | Applied, % on pulp |
H2O2 | 2.5 |
NaOH | 2.75 |
MgSO4 | 0.05 |
DTPA | 0.2 |
-
This illustrates that according to the first step of the present invention, it
is possible to achieve very similar results to the prior art of conventional long
retention time atmospheric or pressurized oxygen-peroxide stages. Contrary to the
prior art, these comparable results can be achieved without the use of oxygen gas
in the first step of the present invention. The short, high temperature first step of
the present invention illustrated in the example (5 minutes retention time) used
only the vapor pressure of the pulp slurry at 110°C. which was 35 psig, compared
to the prior art which requires the use of high pressure oxygen gas. By these
examples, and according to the present invention, results similar to the prior art
can be achieved while using only a short, high temperature stage which will
dramatically reduce the capital investment required for implementation of peroxide
bleaching technology.
-
The fundamental principle of this invention is to gain rapid delignification
and brightness development in a relatively short period of time, at temperatures of
about 100-120°C. and retention time in a small reaction column of less than about
30 minutes, and then discharging the pulp into a conventional large retention tower
for about 1-5 hours at atmospheric pressure, i.e., less than about 100°C., without
washing the pulp in between the two steps. The use of subsequent retention time
at atmospheric pressure for conventional retention times of 1-5 hours, will lead to
further consumption of hydrogen peroxide accompanied by a further brightness
increase. This may allow the incoming pulp to be received at lower brightness
than the pulp in the example.
-
This peroxide stage design can be further improved by combining the
principle of high temperature and short retention time with reactivation of residual
peroxide followed by conventional bleach tower retention. Where reactivation of
residual peroxide is to be practiced, the peroxide solution is added in more than
sufficient quantities to guarantee a significant peroxide residual after the
completion of the first step of the reaction, which is conducted for a retention time
of 1-30 minutes, but preferably for 5-20 minutes, assuring therefore a peroxide
residual entering the second step of the reaction. This combination is further
described in a copending application of Bottan.
-
This invention can be applied to the rebuild of existing bleach plants
and/or the construction of new bleach plants for the production of ECF or TCF
pulps. The use of a short, high temperature peroxide step with or without
subsequent retention and with or without reactivation of residual peroxide, and in
combination with conventional bleaching chemicals allows very economical
construction of short retention time bleach plants.
-
For example, using two short, high temperature peroxide stages with an
interstage treatment using ozone gas, it is possible to produce TCF pulp at high
brightness, for example, greater than 80%ISO, from an oxygen delignified
softwood kraft pulp with an entering kappa number of about 14. This bleaching
concept allows the full bleaching operation to take place with a total retention time
in the bleach plant of less than two hours. Due to the small bleaching reactors
compared to conventional bleaching technology, this approach has an
unprecedented low capital cost compared to all current approaches to production of
TCF pulps. This is illustrated in the following example:
EXAMPLE 3
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A sample of softwood kraft pulp, having a kappa number of 25.7, was
oxygen delignified to a kappa number of 13.6 and a viscosity of 22.4 mPa. This
pulp sample was then treated in a chelation stage using 0.6%EDTA on pulp at
about 50°C. and for 30 minutes retention time, followed by bleaching of the pulp
to greater than 85% ISO brightness in a P-Z-P sequence. According to the first
step of the present invention, the peroxide stages were performed using short (5-15
minutes), high temperature (107-110°C.) conditions, but with the addition of a
conventional ozone bleaching step between the two peroxide stages.
-
The brightness of the bleached pulp is 82.3, 87.6, and 91.0 for total
retention times in the two peroxide stages of 10, 20, and 25 minutes, respectively.
It should be noted that the kappa number of the bleached pulp is quite low, for
example, less than 3. There are a number of pulp mills today which practice TCF
bleaching of pulp using only peroxide as the active bleaching chemical, and these
mills produce fully bleached pulps at an exceedingly high kappa, for example,
greater than 5, which is undesirable especially from the view of brightness
reversion in the paper produced from such pulps. This combination of the use of
the first step of the present invention with established ozone bleaching technology,
has a substantial advantage over current commercial practice for paper quality
made from the pulps produced through use of this process.
-
It should be noted that the present invention according to the data in Table
5 was simulated using oxygen gas to pressurize the pulp mixture during the
laboratory bleaching. This is a practical method of simulating bleaching in the
laboratory where the temperature of the pulp is to be maintained over 100°C.
According to the data presented in Table 2, however, it is not necessary according
to the present invention to apply oxygen gas, and it is not anticipated to be
required in commercial practice to achieve the advantages of the present invention.
-
The simulation of this sequence includes an ozone stage operating at high
consistency for convenience in the laboratory simulation, that is, about 40%
consistency; however, it is recognized that for moderate doses of ozone on pulp,
that is, less than about 0.6%, essentially the same results will be achieved when
using medium consistency ozone, that is, about 10-14% consistency. The present
invention is therefore not intended to be limited to the use of high consistency
ozone, in fact, it is preferable to use medium consistency ozone in order to utilize
existing equipment in the mill, and this is thus described in the preferred
embodiment.
-
One skilled in the art will recognize that the principle of this application
of the invention, that is, two peroxide stages with an interstage acid stage, may
also be employed using other present or future chemicals which preferably operate
in an acid media, that is, less than about pH 7. Examples of other interstage
treatments include, but are not limited to, peracetic acid, Caro's acid, various
enzymatic treatments, and combinations of these types of reagents.
-
Further, it should be noted that on particular pulps, it may not be
necessary for both of the peroxide stages to be operated in accordance with the
first step of the present invention, as a conventional peroxide stage according to
the prior art may be combined with one stage of peroxide bleaching according to
the first step of the present invention, with one or more interstage treatments. It is
anticipated that the use of more than two peroxide stages with interstage treatments
may also be determined to be effective employment of the present invention.
Examples given are not intended to limit the scope of this invention by illustration
of only one or two peroxide stages, although it is preferable from an economic
standpoint to limit the number of stages to achieve minimum required capital
investment.
Evaluation of P-Z-P Bleach Sequence Employing the New Process |
| New Process | Conventional |
P1 stage (pres'd H2O2) |
H2O2, % on pulp | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 |
H2O2, % uptake | 0.5 | 1.1 | 1.1 | 1.3 | 2.1 |
NaOH, % on pulp | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 |
DTPA, % on pulp | 0.2 | 0.2 | 0.2 | 0.2 | 0.2 |
MgSO4, % on pulp | 0.05 | 0.05 | 0.05 | 0.05 | 0.05 |
Total time, minutes | 5 | 15 | 15 | 20 | 60 |
Temperature, °C | 107 | 110 | 110 | 110 | 110 |
O2 pressure, bars | 5.17 | 5.17 | 5.17 | 5.17 | 5.17 |
Consistency, % | 10 | 10 | 10 | 10 | 10 |
Final pH | 11.4 | 11.7 | 11.6 | 11.3 | 10.5 |
Kappa no. | 8.8 | 7.5 | 6.7 | 6.1 | 4.9 |
Brightness, %ISO | 60.3 | 65.5 | 69.0 | 70.4 | 77.9 |
Z Stage (ozone) |
O3, % on pulp | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 |
O3, % uptake | 0.49 | N.D. | N.D. | N.D. | N.D. |
Total time, minutes | 7 | 7 | 7 | 7 | 7 |
Temperature, °C | 23 | 23 | 23 | 23 | 23 |
Consistency, % | 40 | 40 | 40 | 40 | 40 |
Final pH | 2.4 | 2.4 | 2.0 | 2.0 | 2.3 |
Kappa no. | 3.7 | 2.0 | 1.1 | 1.1 | N.D. |
Brightness, %ISO | 70.6 | 76.5 | 80.8 | 79.8 | 85.7 |
P2 stage (pres'd H2O2) |
H2O2, % on pulp | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 |
H2O2, % uptake | 0.9 | 0.8 | 0.9 | 0.9 | 2.0 |
NaOH, % on pulp | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 |
DTPA, % on pulp | 0.2 | 0.2 | 0.2 | 0.2 | 0.2 |
MgSO4, % on pulp | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 |
Total time, minutes | 5 | 5 | 10 | 5 | 60 |
Temperature, °C | 108 | 110 | 110 | 109 | 110 |
O2 pressure, bars | 5.17 | 5.17 | 5.17 | 5.17 | 5.17 |
Consistency, % | 10 | 10 | 10 | 10 | 10 |
Final pH | 11.3 | 11.1 | 11.4 | 11.6 | 10.6 |
Kappa no. | 2.0 | N.D. | 0.4 | N.D. | N.D. |
Brightness, %ISO | 82.3 | 87.6 | 91.0 | 31.2 | 93.6 |
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As a further example, the modification of an existing bleach plant using, for
example, the existing extraction, hypochlorite, or chlorine dioxide bleaching towers
as subsequent retention following the short, high temperature step of the peroxide
bleaching stage according to the present invention, reduces the operating cost of
the bleaching sequence compared to the previous example, and requires minimum
added equipment to the existing bleaching plant.
EXAMPLE 4
-
A sample of softwood kraft pulp, having a kappa number of 25.7, was
oxygen delignified to a kappa number of 13.6. This pulp sample was then treated
in a chelation stage using 0.6% EDTA on pulp at about 50°C. and for 30 minutes
retention time, followed by bleaching of the pulp to greater than 85% ISO
brightness in a P1-Z-P2 sequence. According to the present invention, the
peroxide stages were performed using short (5-15 minutes), high temperature (107-110°C.)
conditions, but with the addition of atmospheric retention time following
each of the short, high temperature, peroxide stages, and in combination with a
conventional ozone bleaching step between the two peroxide stages.
-
It is demonstrated in Table 6 that, according to the present invention, the
addition of atmospheric retention following the short, high temperature first step of
the present invention achieves results comparable to that achieved according to the
prior art using pressurized retention in each stage for 1-3 hours, at the same
peroxide dose on pulp. This is especially desirable as it is possible in many pulp
mills to reuse existing bleaching towers for the subsequent retention with minimal
capital investment requirements. As earlier described, the investment cost for a 1-3
hour pressurized retention vessel is relatively high, and it is further anticipated
that, even if it is necessary to construct a new atmospheric retention vessel for 1-5
hours retention, that the new atmospheric retention vessel, in combination with a
short (5-15 minutes) high temperature retention vessel will still be quite
economically attractive for the new bleaching sequence when compared to the
capital requirements of the prior art. According to the present invention, with the
addition of a short 5-20 minute) upflow tube in combination with the use of an
existing bleaching tower in the bleach plant for an additional 1-4 hours, the
investment per peroxide bleaching stage is reduced to about $250,000 or less, or a
total investment for the 350 mills in North American industry of about $90
million. Compared to the prior art, this represents a savings of between $130-430
million.
-
The data given is to quantify the performance of a few preferred
embodiments, but is not intended to limit the scope of the present invention. One
skilled in the art will recognize that the present invention can be practiced in
numerous ways to achieve its benefits. Some examples have been given to
illustrate this point, but these examples are by no means intended to limit the scope
of the invention. This list describes some locations in which the present invention,
"PAPEROXIDE" process, may be employed in existing or new bleach plants:
- 1. For delignification prior to the bleach plant.
- 2. For final brightening at the end of the bleach plant.
- 3. To replace an existing stage in the bleach plant.
- 4. To modify an existing stage to improve performance or reduce the use of
chlorine compounds in the bleach plant.
- 5. As a pretreatment prior to a further bleaching stage.
- 6. Any of the above, using more than one new or modified stage.
- 7. More than one new stage in combination with one or more interstage
treatments.
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