FIELD OF THE INVENTION
-
This invention relates to oxygen delignification
of pulp and, more particularly, to an improved oxygen
delignification process that requires reduced amounts
of capital equipment.
BACKGROUND OF THE INVENTION
-
Pulp producers worldwide are striving to reduce
water consumption in order to attain a minimum impact
mill (MIM). Conceptually, a minimum impact mill is one
that generates minimum water and air emissions without
negatively affecting wood and energy consumption or
product quality. The achievement of a MIM is a slow,
stepwise process that requires many modifications and
adjustments of current mill practices. These include:
(1) minimization of spills; (2) closed water loops in
the wood yard; (3) closed screen rooms; (4) efficient
brown stock washing; (5) high yield and low energy
intensive pulping processes; (6) extended oxygen
delignification; and (7) partially closed bleach plants
through reuse of some bleaching filtrate streams.
-
Lignocellulosic pulp originated from virgin or
recycled fiber contains color-causing compounds, which
must be removed during the bleaching operation in order
to produce a bright and high quality pulp. The removal
of such compounds from virgin or recycled pulp fibers
is usually done in a single stage but more commonly in
a sequence of chemical treatments that may include two
or more of the following chemicals: oxygen, ozone,
chlorine, hypochlorite, chlorine dioxide, hydrogen
peroxide, peracids, chelants, alkali, enzymes, etc. An
oxygen treatment is usually applied to the pulp in the
beginning of the bleaching process to remove the bulk
of the pulp colored materials such as lignin,
extractives, dye, pigments, inks, etc.
-
Despite some concerns about the impact of oxygen
delignification on pulp quality and bleachability with
chlorine dioxide, the use of oxygen delignification is
spreading worldwide. The most recent trend has been
towards the so-called extended oxygen delignification
approach. Of these, the most prominent technique is
the double stage process. It is purportedly more
efficient and selective than the conventional single
stage process. Extended oxygen delignification is
attractive to the MIM, because it leaves less lignin to
be removed in the bleach plant and allows for
terminating pulping at a higher kappa number. The
kappa number test is used to determine the amount of
lignin remaining in pulp after cooking. The kappa
number is defined as the number of milliliters of 0.1N
potassium permanganate solution consumed by one gram of
pulp and corrected for 50% consumption of the potassium
permanganate initially added. The higher the kappa
number, the more lignin is present in the pulp and
vice-versa.
-
Since oxygen delignification is more selective
than pulping, it is wise to stop the cooking at a
higher kappa number and remove as much as possible of
the lignin by oxygen delignification. This way,
process yield and mill throughput are increased; wood
consumption is decreased; causticizing and recovery
loads are decreased; and pulp quality is maintained.
-
The efficiency and selectivity improvements of a
double stage oxygen delignification process in relation
to the single-stage process are clear. However, it is
difficult to state precisely the quantitative values
because they are site specific. Mill experience with
eucalyptus kraft pulp of kappa 16-18 has indicated an
increase of 5-10% in kappa drop across the process when
switching from single to double stage oxygen
delignification. This gain has been achieved
purportedly without any significant pulp viscosity
penalty.
-
Ideally, a bleach plant should produce low volume
effluent, containing low concentrations of metals,
chlorides and organic matter. Thus, the initial stages
of the bleaching sequence should generate filtrates,
which are easily cycled back to the recovery system,
i.e. they should contain low chloride and ideally be of
an alkaline nature. This concept gave rise to the so-called
elemental chlorine free (ECF) light bleaching
processes, which necessarily require some form of
extended oxygen delignification.
-
Double stage oxygen delignification, as presently
practiced, requires high capital investment. Thus,
there is a need for extended oxygen delignification
which achieves the goal of ECF light bleaching, but at
significantly decreased capital cost.
-
Double stage oxygen delignification can be
practiced at medium consistency in a number of ways.
Despite the high capital investment required to install
this technology, most are suitable for combination with
subsequent ECF light bleaching since they result in
delignification rates in the range of 40-50% for
hardwoods and 50-60% for softwoods. Among these
include: (1) two pressurized stages at high pressure,
with and without intermediate washing; (2) two
pressurized stages, the first being at high pressure
and the second at low pressure, with and without
intermediate washing; (3) two pressurized stages, the
first being at low pressure and the second at high
pressure, with and without intermediate washing; (4)
two pressurized stages as in cases 1-3, but with a
metals removal step in between, through chelation; (5)
two pressurized stages as in cases 1-3 (with
intermediate washing) with addition of hydrogen
peroxide in the second stage; (6) two pressurized
stages as in cases 1-3 with oxidative lignin activation
in between; and (7) two pressurized stages as in cases
1-3 with mild acid hydrolysis in between to remove
metals and hexene uronic acids, etc.
-
Approaches 1-6 have been commercially implemented
and approach 7 is still at the bench scale. All these
techniques are described in a paper by Barna, J.,
Salles, D.V.C, Salvador, E. and Colodette, J.L. (O
Papel 58(8):57-66.1997), titled "The Effect of Hydrogen
Peroxide Addition in the Second Stage of a Double Stage
Oxygen Delignification Process".
-
The double stage process is better fitted to
oxygen delignification kinetics. It is well known that
oxygen delignification reactions occur in two phases.
A first phase is rapid and is controlled by diffusion.
Most of the reaction occurs rapidly via electrophylic
attack of the oxygen and other free radicals on
residual lignin structures containing free phenolic
units. The second phase is slow and is controlled by
chemical reactions of types which include not only
electrophylic attack of oxygen and other intermediate
species to the lignin, but also nucleophylic attack of
peroxides (organic and inorganic), which are produced
in the first phase of the reaction. Thus, in the first
phase of reaction, there occurs significant
delignification and almost no pulp brightening, whereas
both delignification and brightening occur in the
second phase.
-
The percent delignification that occurs in the
first and second phases is a debated matter and it
seems to depend upon the type of pulp and method of
running the oxygen delignification. However, a
significant number of researchers believe that the
major part of the delignification happens actually in
the first phase whereas most of the brightening occurs
in the second phase.
-
Considering that the process occurs in two phases,
it makes sense to design the process so as to take
advantage of such reaction kinetics. Thus, the oxygen
delignification process is more suitable to the
kinetics if performed as a double stage process. Since
the first phase or reaction is fast, the first stage of
the double stage process can be run for a shorter
period of time than the second, and the remaining
reaction takes place in the second stage reactor which
is run for a longer period of time.
-
Fig. 1 shows a schematic of a prior art double
stage oxygen delignification process, without inter-stage
washing. There it is seen that two high shear
mixers 10 and 12 are required, one prior to first
reactor 14; and the second prior to second reactor 16.
Oxygen and oxidized white liquor (OWL), a highly
alkaline liquid, and/or caustic are added prior to
first reactor 14; but provisions can be made to add
these chemicals between reactors 14 and 16, if
required. A washing step between the two reactors is
optional and is usually not required since its
beneficial effects are questionable.
-
A layout of the system containing intermediate
washing stage 18 is shown in Fig. 2. However, since it
is eventually needed to add hydrogen peroxide in a
second reactor 16, the presence of intermediate washing
stage 18 is desirable to avoid peroxide losses in
reactions with partially oxidized organic carryover.
-
It has been shown that the selectivity of the
first oxygen delignification phase is not very
sensitive to the alkali charge whereas the efficiency
is. On the other hand, the selectivity is sensitive to
the alkali charge in the second phase whereas the
efficiency is not. Taking into account these facts, it
is preferred that the first oxygen reactor 14 be
operated at high alkalinity to maximize efficiency. On
the other hand, the second reactor 16 should operate at
low alkalinity to maintain process selectivity. It has
also been shown that selectivity is negatively affected
by oxygen pressure in the first phase of the reaction,
whereas efficiency is affected positively. On the
other hand, both efficiency and selectivity are
positively affected by oxygen pressure in the second
phase. Thus, first oxygen reactor 14 must operate at
low oxygen pressure and second oxygen reactor 16 at
high oxygen pressure in order to maintain process
selectivity. In other words, the alkali charge must be
kept high in first reactor 14 and low in second reactor
16, and the opposite is valid for the oxygen charge
(partial pressure). Thus it makes sense to add the
total amount of alkali to the pulp in first reactor 14
and use only the residual alkali in second reactor 16.
The oxygen, on the other hand, should be split between
the two reactors, with the larger fraction being
applied in second reactor 16.
-
The function of mixer 12 located between the two
stages is the mixing of the oxygen added in the second
stage and re-mixing the residual oxygen bubbles that
eventually coalesced in the first reactor. Since the
alkali readily mixes with the pulp, there is no need to
re-mix it.
-
It has been also shown that the selectivity is
negatively affected by temperature in the first phase
of the reaction, whereas efficiency is affected
positively. On the other hand, temperature positively
affects efficiency and has no effect on selectivity in
the second phase. It would thus be preferred to run
the process such that the temperature in the first
reactor is lower than in the second reactor.
-
In the process of the prior art, the desired rates
of delignification are achieved using forms of the
double stage oxygen delignification process depicted in
Figs. 1 and 2. As compared to conventional, single
stage oxygen delignification, the double stage
processes are more expensive to install since they
require heavy equipment. In the case of Fig. 1, an
additional reactor and mixer are required. In the case
of Fig. 2, an additional reactor, mixer and washer are
required. Thus, the benefit of having a double stage
oxygen delignification is hampered by the substantial
capital investment required for its installation.
SUMMARY OF THE INVENTION
-
The invention configures a process for
delignifying pulp wherein one stage of a two stage
oxygen delignification plant is obviated. In
particular, a pulp soak stage enables a pulp stream to
be mixed with an alkali feed and held in residence for
a sufficient period of time to allow an alkali
hydrolysis leaching of colored materials from the pulp
and a swelling of the pulp fibers. Next, the pulp
stream is combined with an oxygen feed and is processed
through a mixer. The output of the mixer is fed to a
pressurized reactor where oxygen reacts with lignin of
the pulp fibers. The swelled pulp fibers facilitate
the oxygen reaction with lignin and removal of
additional colored materials.
-
In a first preferred mode of running the process,
no inter-stage washing step is required. In a second
embodiment, a washing step is introduced between the
soaking and the oxygen reaction stage to increase
process efficiency, but at added capital cost.
BRIEF DESCRIPTION OF THE DRAWINGS
-
- Fig. 1 is a schematic of a prior art double stage
oxygen delignification plant without inter-stage
washing of the pulp stream.
- Fig. 2 is a schematic of a prior art double stage
oxygen delignification plant with inter-stage washing
of the pulp stream.
- Fig. 3 is a schematic of an oxygen delignification
plant that incorporates the invention, without inter-stage
washing of the pulp stream.
- Fig. 4 is a schematic of an oxygen delignification
plant that incorporates the invention, with inter-stage
washing of the pulp stream.
- Fig. 5 is a schematic of an oxygen delignification
plant that incorporates the invention, with inter-stage
press washing of the pulp stream.
- Fig. 6 is a schematic of another embodiment of an
oxygen delignification plant that incorporates the
invention.
- Fig. 7 is a schematic of still another embodiment
of an oxygen delignification plant that incorporates
the invention.
-
DETAILED DESCRIPTION
-
The present invention is directed towards an
improvement in the process of removing colored
compounds from lignocellulosic material using alkali
and oxygen. In this process, the pulp is first treated
with alkali and then further treated with oxygen. A
key feature of the present invention is the treatment
of the pulp with alkali, prior to oxygen
delignification. The alkali soaking of the pulp fibers
results in an alkali hydrolysis/leaching of easily
accessible colored materials and also in the swelling
of the pulp fibers, facilitating removal of the colored
materials in the subsequent oxygen delignification
stage. As a result, a 5-10% improvement in the rate of
removal of colored materials is achieved as compared to
conventional oxygen delignification processes. The
invention allows for an achievement of the same rate of
delignification as the prior art double stage oxygen
delignification process (which requires much more
capital investment).
-
Further, the process of the invention is
particularly efficient to delignify lignocellulosic
material containing lignin contents higher than usual.
For example, it is preferred that pulps having the
following kappa numbers be subjected to the process of
the present invention, that is: 30-40 for softwood
kraft pulps, 17-25 for hardwood kraft pulps and 15-30
for recycled fibers. This specification expresses all
compositions in weight percent, unless specifically
expressed otherwise.
-
The present invention comprises the treatment of
the lignocellulosic pulp with alkali in a vessel at
atmospheric pressure. The treatment, hereafter called
pulp soaking, is carried out at a pH in the range about
9-14, advantageously about 11-12. Preferred conditions
for the pulp soaking are as follows: temperature 70-90°C,
consistency 10-12%, retention time 0.5 to 1 hour
and alkali dosage of 1-2% based on pulp dry weight.
The alkali dosage may vary substantially, depending
upon the type of lignocellulosic material being
treated. The preferred way to control the pulp soaking
is through monitoring of the pH of the slurry rather
than the alkali dosage. The lignin removal during the
soaking, as measured by kappa number, is a function of
the pulp's initial lignin content. The higher the
initial pulp kappa number, the more efficient is the
lignin removal during the soaking process, particularly
when the pulp is washed or pressed after soaking.
-
Following the alkali soak, the pulp may go
directly to a subsequent oxygen delignification stage
or be washed in conventional washers or thickened to a
consistency of about 30-35% in a wash press.
-
The soaked pulp is then treated with oxygen in a
medium consistency oxygen delignification stage,
hereafter designated as the O-stage. This O-stage
operates at 10-12% consistency, 11-12 pH, 60-95°C, 60-90
min reaction time, 400-600 kPa pressure and at a
1.2-2.5% oxygen dose. A magnesium salt, magnesium
sulfate or the like, at a dosage of 0.01-0.1 wt% Mg
(based on dry pulp) and most advantageously about 0.02-0.03
of Mg, may be added to protect the pulp
carbohydrates against alkali-induced degradation.
-
The magnesium salt should be added before the
alkali addition in order to facilitate mixing of the
salt with the pulp. Note that the dosage of alkali and
of a magnesium salt will depend on whether or not there
is a washing or pressing step in between pulp soaking
and the O-stage.
-
The process of the invention can be more clearly
understood by reference to Figs. 3-7. Fig. 3 depicts
the process of the invention wherein washing between
pulp soaking and the O-stage is omitted. The process
comprises two separate stages, a pulp soak stage 50 and
an O-stage 52, with no washing or pressing between
them. The pulp incoming from a brown stock washing
operation receives both MgSO4 via feed 51, and alkali
(e.g., NaOH with oxidized white liquor or unoxidized
white liquor) from a source 54 through pump 56. The
pulp/alkali slurry then goes into a high density soak
vessel 58 at atmospheric pressure, where it remains for
a desired residence time.
-
The pulp slurry is then sent to O-stage 52 where
additional alkali and a magnesium salt may be added (if
necessary) in the suction of a pump 60. Thereafter,
the slurry goes to a high shear mixer 62 where medium
pressure steam and oxygen are added via feeds 64 and
66, respectively, to the pulp slurry. The pulp slurry
is then pumped to pressurized reactor 68 where the
slurry is maintained for a desired reaction time.
After the reaction completes, the pulp is discharged
into a blow tube and is pumped to post O-stage washers
(not shown).
-
Fig. 4 shows a second embodiment of the invention
wherein the pulp slurry is washed between pulp soak
stage 50 and O-stage 52. In this case, after the
soaking, the pulp slurry is pumped by pump 70 to a
washer 72 and is then conveyed to conventional O-stage
52, as previously described. Fig. 5 shows a third
embodiment wherein the pulp, after soaking in soak
stage 50, is pumped to a wash press 80 instead of a
regular washer. In this case, the slurry is dewatered
to a consistency between 35-40% and is then conveyed to
a standpipe 82 where it is diluted to a consistency
between 10-14% with filtrate from the subsequent O-stage.
At standpipe, the pub receives additional
alkali and a magnesium salt via feed 84, and then goes
to O-stage 52 for processing as above described.
-
Further embodiments of the invention are shown in
Figs. 6 and 7. In Fig. 6, a pulp processing sequence
is shown that comprises two separate stages, an
alkaline soak stage 100 first and then an O-stage 102,
with no washing or pressing between them. The
embodiment shown in Fig. 7 is similar except for the
fact that pulp is washed or pressed between the
alkaline soak and oxygen delignification stages. The
major difference between these processes, as compared
to those described above with respect to Figs. 3-5 is
that the oxygen treatment is effected at a lower
reaction pressure (hydrostatic pressure), i.e., a mini-O.
This stage is carried out at 10-12% consistency,
11-12 pH, 70-80°C, 60-120 min reaction time, 150-300
kPa pressure (overpressure or head pressure), 0.5-1.05%
oxygen dose. A magnesium salt, magnesium sulfate or
the like (dosage of 0.02-0.03% Mg) may be added to
protect the pulp carbohydrates against alkali induced
degradation. The magnesium salt must be added before
the alkali addition in order to facilitate mixing of
the salt with the pulp. Note that the dosage of alkali
and of a magnesium salt depends on whether or not there
is a washing or pressing step between the pulp soaking
and the O-stage.
-
In the case where the pulp is not washed (i.e.,
Fig. 6), the alkali requirement is minimum, only about
10-20% of the total amount required. This additional
alkali is necessary to replenish the fraction consumed
during the soaking stage. On the other hand, if the
pulp is pressed or washed after soaking (i.e., Fig. 7),
additional alkali is required. An alkali dose of 1-1.5%
should be added. Note that the alkali requirement
varies substantially depending upon the type of
lignocellulosic material.
-
After soaking, the pulp slurry is conveyed
directly to the O-stage where additional alkali and a
magnesium salt are added in the suction of the pump;
following the slurry goes to a high shear or static
mixer where medium pressure steam and oxygen are added
to the pulp. The pulp is then pumped to a preretention
tube 104 followed by a down-flow tower 106 (or directly
to an up-flow tower) where it is maintained for the
desired reaction time. Preretention tube 104 is
usually pressurized up to 200 kPa and down-flow tower
106 operates atmospherically. The up-flow tower
operates atmospherically but the head pressure of the
column; the pressure in the bottom of the tower where
the oxygen is injected depends upon the tower height.
After the reaction completion, the pulp is pumped to
post stage washers (Fig. 6). Fig. 7 illustrates the
presence of an inter-stage washing system.
-
The major advantages of the techniques described
in regards to Figs. 3-7 are: (1) they are less capital
intensive than double stage processes since they
require one less pulp washer, high shear mixer and
pressurized reactor, while maintaining pulp quality and
process efficiency; (2) they are easily retrofitted to
single stage oxygen delignification installations
without major investment, with the advantage of
enhancing delignification performance by 5-10%; and (3)
they can also be applied to mini-O processes (low
pressure oxygen delignification) enhancing rates also
by 5-10% with minimal capital investment.
-
Thus, the difference between the process of the
invention in relation to the prior art is a reduction
in overall capital cost investment when compared to
double stage oxygen delignification processes. Also,
they can enhance delignification by 5-10% when applied
in connection with both conventional single stage and
mini-O oxygen delignification processes.
-
In summary, the invention treats lignocellulosic
pulp with alkali at atmospheric conditions followed by
a high or low-pressure exposure of the pulp to oxygen
in pressurized vessels. The invention is applicable to
all kinds of fibrous raw material including pulps
manufactured by processes such as kraft, soda, sulfite,
magnefite, cold soda, NSSC and the like. These fibers
may be obtained from hardwood, softwood, bamboo,
bagasse, straw and other non-wood fiber supplies. The
process is also applied to de-inked recycled fibers and
certain grades of brown recycled fibers.
-
Below are presented a range of alternative ways
and conditions in which the various stages of the
process of this invention can be practiced and also the
preferred way of practicing them.
-
The pulp alkali treatment can be performed in
atmospheric and pressurized vessels, preferably in
atmospheric ones. The alkali used can be of several
origins including plain NaOH, oxidized white liquor
(OWL), unoxidized white liquor (WL) or the like. Most
advantageously, the reaction occurs with 0.5-5 weight
percent NaOH. The vessel may be a dedicated one or the
high-density tower already existing in most pulp mills.
The treatment called of pulp soaking (S) is carried out
with a pulp consistency of about 6-14 weight percent.
A pH of at least 11 facilitates the alkali soaking.
Most advantageously, the soak occurs at a pH between
about 11 and 12. Elevating the soaking temperature to
between 40 and 95°C accelerates swelling of the
lignocellulosic pulp. Most advantageously, the soaking
occurs at a temperature between 70 and 95°C to
accelerate the reaction. A soaking time of 15 to 240
minutes advantageously swells the fibers to facilitate
the reaction between the oxygen and the lignin. Most
advantageously, a soaking time between about 20 or 30
minutes and 60 minutes accomplishes the soaking.
Advantageously the alkali dose consists about 0.5-5%
NaOH, most advantageously, about 1-2% NaOH. The alkali
dosage however may vary substantially, depending upon
the type of lignocellulosic material being used.
-
Following the alkali soaking, the pulp may go
directly to a subsequent oxygen treatment or be washed
in conventional washers or thickened to a consistency
of about 30-35% in a wash press. The equipment
required to wash or thicken the pulp is standard and
available in the market. This operation affects the
overall direct and indirect costs of this invention.
When the pulp is washed after soaking, provisions can
be made for additional pulp treatments such as
chelation to remove metals and mild acid hydrolysis to
remove hexene uronic acids and metals prior to the
subsequent oxygen treatment.
-
The soaked pulp is then treated with oxygen three
different ways: (1) at low pressure (mini-O), in a
process hereafter designated as (EO); (2) in a
conventional MC (medium consistency) single stage
oxygen delignification process, hereafter designated as
O; and (3) in a MC double stage oxygen process,
hereafter designated as O/O or OO, depending on whether
the pulp is washed between stages or not. The oxygen
used in this treatment may be of purity varying from
80-100%, preferably 90-100%.
-
The low pressure oxygen treatment or mini-O (EO)
may be carried out at a consistency of from 8-14 weight
percent, preferably from 10-12 weight percent.
Preferred conditions for other parameters are: pH of
from 10-14, preferably 11-12, alkali dose of 1-4%
preferably 1-2%, temperature of 50-120°C, preferably
70-90°C, reaction time of from 30-180 min, preferably
60-90 min, reaction pressure of 100-600 kPa, preferably
150-300 kPa and oxygen dose of 0.2-2%, preferably 0.5-1%.
At this stage a magnesium salt, magnesium sulfate
or the like, may be added to protect the pulp
carbohydrates against alkali induced degradation. The
magnesium salt can be added before or together with the
alkali, but preferably before the alkali in order to
facilitate mixing of the salt with the pulp. The dose
of the magnesium salt may be in the range of about
0.01-0.1% (as Mg) based on the pulp dry weight,
preferably about 0.02-0.03%. Hydrogen peroxide may
also be added in the (EO) stage to boost
delignification in the dose of 0.2-4%, preferably 0.5-1%.
The addition of peroxide is feasible only when the
pulp is washed after soaking.
-
The conventional single stage oxygen treatment, O,
may be carried out at a consistency of from 8-14%,
preferably from 10-12%. Preferred conditions for other
parameters are: pH of from 10-14, preferably 11-12,
alkali dose of 1-4% preferably 1-2%, temperature of 50-140°C,
preferably 80-100°C, reaction time of from 30-180
min, preferably 60-90 min, reaction pressure of
100-800 kPa, preferably 400-600 kPa and oxygen dose of
0.5-4%, preferably 1-2%. At this stage a magnesium
salt, magnesium sulfate or the like, may be added to
protect the pulp carbohydrates against alkali induced
degradation. The magnesium salt can be added before or
together with the alkali but preferably before the
alkali in order to facilitate mixing of the salt with
the pulp. The dose of the magnesium salt may be in the
range of 0.01-0.1% (as Mg) based on the pulp dry
weight, preferably 0.02-0.03%. Hydrogen peroxide may
also be added in the (O) stage to boost delignification
in the dose of 0.2-4%, preferably 0.5-1%; the addition
of peroxide is feasible only when the pulp is washed
after soaking.
-
The double stage oxygen treatment, O/O or OO, may
be carried out at a consistency of from 8-14%,
preferably from 10-12%. Preferred conditions for other
parameters are: pH of from 10-14, preferably 11-12,
alkali dose of 1-4% preferably (1.5/0.5%), temperature
of 50-140°C, preferably (85/95)°C, reaction time of
from 30-180 min, preferably (30/60) min, reaction
pressure of 100-800 kPa, preferably 400-600 kPa and
oxygen dose of 0.5-4%, preferably (1.5/0.5%). At this
stage a magnesium salt, magnesium sulfate or the like,
may be added to protect the pulp carbohydrates against
alkali induced degradation. The magnesium salt can be
added before or together with the alkali but preferably
before the alkali in order to facilitate mixing of the
salt with the pulp. The dose of the magnesium salt may
be in the range of 0.01-0.1% (as Mg) based on the pulp
dry weight, preferably (0.02/0.0%). Hydrogen peroxide
may also be added in the double stage oxygen treatment
to boost delignification in the dose of 0.2-4%,
preferably (0.0/0.5%); the addition of peroxide is
feasible only when the pulp is washed after soaking.
-
For purposes of this specification, the "/"
divides the first stage addition or condition from the
second stage addition or condition. For example,
(0.03/0% Mg) indicates a 0.03% Mg addition to the first
stage and a 0% Mg addition to the second stage.
EXPERIMENTAL
-
For the confirmation of the present invention and
a best understanding thereof, different types of
lignocellulosic material were subjected to the steps
described above. The comparisons between the process
of the present invention and those of the prior art are
based on results of kappa drop, viscosity drop and
brightness gain across the various processes. The
kappa drop, viscosity drop and brightness gains were
calculated with the following equations: kappa drop =
(kappa in - kappa out)/kappa in; viscosity drop =
(viscosity in - viscosity out)/viscosity in; brightness
gain = (brightness in - brightness out). The values of
kappa number, viscosity, and brightness were measured
according to the Technical Association of the Pulp and
Paper Industry (Tappi) standard procedures. All
experiments described were carried with two
repetitions, being the results presented average
values.
-
Except when specified, the procedures and
operating conditions used for the various processes
discussed are the ones described below:
-
Pulp Alkali Soaking (S): was effected at 12%
consistency, 85°C, 30 min, with 1.5% alkali. The
reaction was carried out in a high shear mixer/reactor
made of hasteloy having temperature and pressure
controllers and devices for injection and relief of
gases. The mixing was done intermittently every 1-min
at 2000 rpm for 4 seconds. Variations in alkali dose,
temperature and reaction time were practiced but they
are described at the proper examples.
-
Single O-stage (O): was carried out at 10%
consistency, 95°C, 60 min, 600 kPa pressure with 1.5%
oxygen, 1.5% NaOH and 0.03% magnesium as such. The
reaction was carried out in the same equipment and
settings above described in the item alkali soaking.
-
Double O-stage, (O/O and OO): The first O-stage
was carried out at 10% consistency, 85°C, 60 min, 600
kPa pressure with 1.5% oxygen, 1.5% NaOH and 0.03%
magnesium as such. The second stage was carried out
10% consistency, 95°C, 60 min, 600 kPa, 1.5% NaOH and
0.5% oxygen. When washing was carried out between the
two stages, this was done through press washing as
described below. Double O-stage without intermediate
washing is designated as O/O-stage, and with
intermediate washing as OO-stage. The reaction was
carried out in the same reactor/mixer and settings
above described in the item alkali soaking.
-
Low Pressure or mini-O-stage, (EO): This stage
was carried out at 10% consistency, 85°C, 60 min, 200
kPa pressure with 1.0% oxygen, 1.5% NaOH and 0.03%
magnesium as such. The pressure was dropped from 200
kPa to zero pressure during the 60-min reaction,
simulating a hydrostatic tower. The reaction was
carried out in the same reactor/mixer and settings
above described in the item alkali soaking, except that
the pressure was dropped manually from 200 kPa to zero
at 5-min intervals.
-
Press Washing: Press washing between stages was
effected by diluting the pulp after the stage to a
consistency of 4% and then pressing it to a consistency
of about 35%. This corresponds to a washing efficiency
of about 80%, considering for example the pulp entering
the washing stage at 10% consistency. Note that
washing between stages has no representation whereas no
washing is usually represented by a slash symbol (/).
-
The following examples are provided to illustrate
the present invention:
Example 1: Optimization of Soaking Time and
Temperature
-
The kraft pulp sample employed in this example was
obtained in the laboratory from eucalyptus wood. After
pulping, the brown pulp had a initial kappa number of
19.6, a viscosity of 58.7 mPa.s and a brightness of
28.9% ISO. The soaking was carried out at, 65, 75 and
85°C for periods of time of 15, 30, 60 and 180 min.
Other conditions were maintained constant as described
in previous sections. After soaking, the pulp was
thoroughly washed and then analyzed for kappa number,
viscosity and brightness. The results shown in Table 1
indicate that soaking efficiency as measured by kappa
drop is positively influenced by both time and
temperature. However, the benefits of the soaking
somewhat decrease after 30-min reaction, particularly
at the 85°C temperature. Increasing the time from 30
min to 240 min resulted in only a 1% increase in the
kappa drop. Thus, the time of 30 min was considered
sufficient at the 85°C temperature. Since the
temperature had no significant impact on pulp
viscosity, the 85°C value was considered the most
adequate, since practically, the subsequent O-stage is
usually carried out at temperatures equal or above this
value. The brightness gain across the soaking stage
was very low and did not follow the kappa drop.
Likely, the pulp exposure to alkali at warm
temperatures triggered lignin darkening reactions that
overshadowed the expected brightness gains derived from
partial lignin removal.
| Effect of the temperature and reaction time
on overall performance of pulp soaking with alkali |
| | | Results |
| Soaking Time, min | Soaking Temp., °C | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| | 55 | 3.3 | 1.7 | 0.1 |
| 15 | 70 | 4.2 | 1.7 | 0.0 |
| | 85 | 5.3 | 2.2 | 0.2 |
| | 55 | 5.5 | 2.0 | 0.4 |
| 30 | 70 | 7.4 | 2.3 | 0.7 |
| | 85 | 8.9 | 2.4 | 0.8 |
| | 55 | 5.9 | 3.2 | 0.8 |
| 60 | 70 | 8.5 | 3.3 | 0.7 |
| | 85 | 9.3 | 3.8 | 0.9 |
| | 55 | 6.5 | 3.8 | 1.0 |
| 240 | 70 | 9.1 | 4.1 | 1.2 |
| | 85 | 9.9 | 4.5 | 1.2 |
Example 2: optimization of the soaking pH
-
The same kraft pulp sample employed in Example 1
was soaked with alkali doses of 0.5, 1.0, 1.5, 2.5 and
4% NaOH. Other soaking conditions were maintained
constant as described above. After soaking, the pulp
was thoroughly washed with distilled water and the
values of kappa number, viscosity and brightness
measured. The results in Table 2 denote that
increasing soaking pH above 12 produces only slight
benefits in terms of kappa drop but penalizes somewhat
pulp viscosity. Thus the
pH 12, which for this pulp
sample is equivalent to an alkali charge of 1.5%, was
considered to be the most satisfactory.
| Effect of the pH on overall performance of
pulp soaking with alkali |
| | | RESULTS |
| Alkali Dose, % | Soaking pH | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| 0.5 | 11.2 | 1.3 | 0.7 | 0.2 |
| 1.0 | 11.5 | 4.8 | 1.4 | 0.3 |
| 1.5 | 12.0 | 8.9 | 2.4 | 0.8 |
| 2.5 | 12.7 | 9.3 | 4.8 | 1.3 |
| 4.0 | 13.4 | 9.9 | 7.7 | 0.3 |
Example 3: effect of the pulp degree of
delignification
-
The various hardwood kraft pulp samples employed
in this Example were obtained from eucalyptus wood.
Pulping conditions were adjusted in order to produce
low, medium and high degree of delignification pulps,
for hardwood standards. After pulping, the sample of
low degree of delignification had 14.3 kappa, 35.7
mPa.s viscosity and 34.9% ISO brightness; the medium
degree of delignification sample had 16.8 kappa, 47.5
mPa.s viscosity and 32.3% ISO brightness; the high
degree of delignification sample had 19.6 kappa, 58.7
mPa.s viscosity and 28.9% ISO brightness. The soaking
was carried out under the conditions previously
described. After soaking, the pulp was thoroughly
washed with distilled water and the values of kappa,
viscosity and brightness measured.
-
The results shown in Table 3 reveal that the
soaking is more effective to reduce kappa number when
applied to the pulp of higher initial kappa number.
This may be explained by the higher content of lignin
potentially hydrolyzable/leachable with alkali in the
higher kappa pulp. The effects on pulp viscosity and
brightness gain were negligible and apparently
independent of pulp initial kappa number.
| Effect of pulp degree of delignification on
overall performance of pulp soaking with alkali |
| | Results |
| Pulp Initial Kappa Number | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| 14.3 | 5.9 | 2.2 | 0.6 |
| 16.8 | 7.8 | 2.5 | 0.7 |
| 19.6 | 8.9 | 2.4 | 0.8 |
Example 4: effect of the type of lignocellulosic
material
-
Four types of lignocellulosic material were
compared. An industrial hardwood kraft pulp (HWD) of
17.1 kappa, 43.4 mPa.s viscosity and 36.8% ISO
brightness; an industrial softwood kraft pulp of 32.2
kappa, 42.7 mPa.s viscosity and 26.4% ISO brightness
(SWD); an industrial recycled fiber sample produced
from de-inked low grade mixed office waste (MOW) of
14.4 kappa, 11.9 mPa.s viscosity and 55.1% ISO
brightness; an industrial recycled fiber sample
produced from deinked curbside material (RCM) of 69.8
kappa and 42.5% ISO brightness. The soaking was
carried out at fixed conditions as described above.
After soaking, the pulp was thoroughly washed with
distilled water and the values of kappa number,
viscosity and brightness measured. The results showed
in Table 4 point out that the performance of the alkali
soaking operation depends substantially on the type of
lignocellulosic material. The highest kappa drop was
achieved with the hardwood kraft pulp indicating that
this material contains the largest quantities of alkali
promoted lignin leachable/hydrolysable. Additionally,
the viscosity of the hardwood pulp was not
substantially changed during the soaking treatment. In
spite of their higher lignin content both the softwood
kraft and the RCM pulps were not very susceptible to
the soaking treatment. These pulps resulted in lower
kappa drops across the soaking as compared to the HWD
sample. Besides, the SWD experienced larger viscosity
loss than its HWD counterpart. Both the RCM and MOW
samples lost brightness across the soaking indicating
the occurrence of alkali promoted darkening reactions.
-
In the case of the MOW sample, there was very little
effect of the soaking on kappa drop; it is possible
that the colored materials present in the MOW sample
are mostly of a non-lignin nature and resistant to
alkali hydrolysis/leaching.
| Effect of the type of lignocellulosic
material on overall performance of pulp soaking with
alkali |
| | | Results |
| Pulp Type | Initial Kappa | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| HWD kraft | 17.1 | 7.5 | 2.2 | 0.9 |
| SWD kraft | 32.2 | 6.7 | 5.9 | 0.4 |
| MOW recycled | 14.4 | 3.4 | 1.2 | -1.3 |
| RCM recycled | 69.8 | 5.7 | - | -8.7 |
-
The following examples show the effects of the
pulp alkali soaking (S) on the overall performance of
several types of subsequent oxygen bleaching stages,
with and without intermediate washing.
Example 5: Effect of pulp soaking on overall
performance of various types of oxygen delignification
of a hardwood kraft pulp
-
The hardwood kraft pulp sample employed in this
example was the same described in Example 1. The
soaking was carried out at fixed conditions as
described in the above sections. The subsequent oxygen
delignification treatments were carried out as
described above, under the following conditions: O:
10% consistency, 95°C, 60 min, 600 kPa overpressure,
1.5% NaOH, 1.5% O2, 0.03% Mg; (EO): 10% consistency,
85°C, 60 min, 200 kPa pressure, 1.5% NaOH, 0.8% O2,
(0.03 Mg); O/O: 10% consistency, (85/95°C), (30/60
min), 600 kPa pressure, (1.5/0% NaOH), (1.5/0.5% O2),
(0.03/0% Mg); OO: 10% consistency, (85/95°C), (30/60
min), 600 kPa pressure, (1.5/1.0% NaOH), (1.5/0.5% O2)
and (0.02/0.02% Mg).
-
The results in Table 5 indicate that the amount of
lignin removal caused by the soaking is not additive to
that obtained in the subsequent oxygen treatment,
regardless of the oxygen application mode under
consideration. However, more than half of the soaking
benefit is transferred to the subsequent oxygen
delignification stages. For example, the soaking by
itself resulted in 8.9% kappa drop and the conventional
O-stage in 37.5%. If the benefits were additive, a
46.4% delignification would be expected after the S/O
treatment. Instead, a value of 43.7% was obtained
experimentally. This very same trend was also seen for
the S/(EO), S/(OO), S/O/O and S/OO treatments. This
difference was initially attributed to the absence of a
washing step between the soaking and the various oxygen
treatments. However, the insertion of the washing step
between these treatments, still was not enough to make
additive the benefits of the treatments. Likely, the
soaking treatment removes lignin fractions that would
otherwise be removed in the subsequent oxygen
treatments. Although additive benefits are not to be
expected, there is still an advantage of applying the
soaking since overall delignification can still be
increased by up to 7.4% with this technique.
-
The soaking plus a single O-stage, S/O and SO,
resulted in delignification rates similar to those
obtained with double stage oxygen delignification, O/O
and OO, without any significant penalty on pulp
viscosity and brightness gain. It is worth noting that
soaking plus single O-stage require much less capital
cost to install than a double stage oxygen
delignification process.
-
Another interesting aspect shown in Table 5 is
with regard to the soaking plus low pressure (mini-O)
oxygen delignification, S/(EO) and S(EO) processes.
The soaking treatment adds an extra 5-7% kappa
reduction to a (EO) stage. This benefit is rather
significant given the fact that the EO (oxygen
extraction) operation is designed to achieve lower
delignification levels than conventional O-stages. A
5% improvement in the (EO)-stage, which gives 20-25%
delignification rate, is more significant than a 8%
improvement in the O-stage that gives 35-40%
delignification rate. Note that this benefit is
obtained with no penalty to pulp quality and with very
low capital investment.
| Effect of the soaking on subsequent
oxygen delignification performance for HWD kraft pulp |
| | Results |
| Treatment Type | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| S | 8.9 | 2.4 | 0.8 |
| O | 37.5 | 35.4 | 12.8 |
| (EO) | 23.4 | 28.3 | 7.9 |
| O/O | 43.8 | 39.3 | 13.7 |
| OO | 44.6 | 37.9 | 14.1 |
| S/O | 43.7 | 41.8 | 13.4 |
| S/(EO) | 29.9 | 33.4 | 8.1 |
| S/O/O | 44.7 | 42.5 | 13.8 |
| SO | 44.9 | 41.2 | 14.3 |
| S(EO) | 31.0 | 32.1 | 9.0 |
| S/OO | 45.8 | 42.1 | 14.9 |
Example 6: Effect of pulp soaking on overall
performance of various types of oxygen delignification
of a softwood kraft pulp
-
The softwood kraft pulp sample employed in this
example was obtained from a Western North American pulp
mill and was made from Spruce. After pulping, the
brown pulp had a 32.2 kappa number, a 42.7 mPa.s
viscosity and a 26.4% ISO brightness. The soaking was
carried at fixed conditions as described in previous
sections. The subsequent oxygen delignification
treatments were carried out under the following
conditions: O: 10% consistency, 95°C, 60 min, 600 kPa
pressure, 2.0% NaOH, 2.0% O2, 0.03% Mg; (EO): 10%
consistency, 85°C, 60 min, 200 kPa pressure, 2.0% NaOH,
1.0% O2, 0.03% Mg; O/O: 10% consistency, (85/95°C),
(30/60 min), 600 kPa pressure, (2.0/0% NaOH), (1.5/0.5%
O2), (0.03/0% Mg); OO: 10% consistency, (85/95°C),
(30/60 min), 600 kPa pressure, (1.5/1.0% NaOH),
(1.5/0.5% O2) and (0.02/0.02% Mg).
-
The results obtained in the various oxygen
delignification treatments for the softwood kraft pulp
sample followed the same trends observed for the
hardwood treatments. However, the soaking benefits
were less pronounced for the softwood sample as
illustrated in Table 6. The benefits caused by the
soaking were particularly slim when done prior to a low
pressure oxygen stage, S/(EO) and S(EO) processes.
Although the benefits of the soaking were lower for the
softwood sample as compared to the hardwood sample, the
performance of the oxygen delignification stages were
much higher for the softwood sample. This fact is,
however, quite well documented in the literature and in
mill scale operations worldwide.
| Effect of the soaking on subsequent
oxygen delignification performance for SWD kraft pulp |
| | Results |
| Treatment Type | Kappa Drop, % | Viscosity Drop, % | Brightness Gain, % |
| S | 6.7 | 5.9 | 0.4 |
| O | 46.5 | 37.9 | 5.8 |
| (EO) | 26.9 | 23.7 | 2.5 |
| O/O | 51.4 | 40.1 | 6.5 |
| OO | 51.7 | 38.8 | 7.4 |
| S/O | 51.3 | 41.1 | 6.3 |
| S/(EO) | 29.1 | 24.2 | 2.9 |
| S/O/O | 52.7 | 43.7 | 6.9 |
| SO | 52.5 | 40.8 | 6.4 |
| S(EO) | 31.2 | 23.4 | 3.1 |
| S/OO | 53.6 | 43.5 | 6.9 |
Example 7. Impact of the soaking on overall pulp
bleachability with the sequence D(EOP)D
-
The oxygen treated hardwood and softwood kraft
pulp samples described in Examples 5 and 6,
respectively, were further bleached by an ECF
(elemental chlorine free) bleaching process with the
sequence D(EOP)D. The brightness target was 90% ISO
for the HWD sample and 89% for the SWD one. Following
the recommendation of the Technical Association of Pulp
and Paper Industry (Tappi), as detailed in the Tappi
publication TIS 0606-21 entitled "recommended pulp
bleaching stage designation method", the D(EOP)D
designation represents a sequence which comprises three
separate stages, the first D-stage, an (EOP) stage and
then another D-stage, with a washing or pressing step
between these stages. In the (EOP) stage, alkali,
oxygen and hydrogen peroxide are injected in the same
stage, apart from each other by fractions of minutes.
The conditions used in the various bleaching stages
were as follows: first D-stage: 10% consistency,
75°C, 60 min, 3.0 final pH and a kappa factor of 0.24;
(EOP) : 10% consistency, 85°C, (15/75) min, 200 kPa
pressure, 10.5 final pH, 1.4% NaOH, 0.5% O2, 0.5% H2O2,
0.03% Mg; second D-stage: 10% consistency, 75°C, 240
min, 3.8 final pH and variable amounts of ClO2
depending upon pulp previous treatment and type. The
control of pH in the first and second D-stages was
achieved through small additions of NaOH or H2SO4 in
the stages as required.
-
The results presented in Table 7 for the hardwood
pulp sample point out that the combination of soaking
plus conventional oxygen delignification, S/O or SO
processes, results in chlorine dioxide savings (shaded
column) of the same magnitude as the double stage
oxygen delignification, O/O or OO processes. The ClO2
savings are calculated by the difference between a
certain treatment and the reference. If compared with
the single O-stage (O), the S/O and SO processes saved
1.1-1.3 kg of ClO2/odt of pulp. On the other hand,
when compared to the low pressure oxygen
delignification, (EO), the S(EO) and S(EO) processes
also resulted in chlorine dioxide savings in the order
of 1.2-1.4 kg ClO2/odt of pulp. For the softwood pulp
sample (Table 7), similar trends were observed, but
absolute values of chlorine dioxide savings were not
the same.
-
Taking into account the aforementioned
considerations, it may be concluded that soaking plus
conventional oxygen delignification produce results
that are similar to double stage oxygen
delignification. On the other hand, if applied before
conventional or low pressure oxygen delignification,
such as in the processes S/O, SO, S/(EO) or S(EO), the
soaking allows for chlorine dioxide savings in the
order of 1-1.5 kg ClO
2 per over dried ton of fully
bleached pulp. The washing stage between soaking and
the high or low-pressure oxygen stage shows benefits;
but they seem to slim to justify the installation of an
expensive washing system. In those cases where the
washing installation already exists, washing between
stages is recommended.
| Effect of various pulp treatments on overall
pulp bleachability with the sequence D(EOP)D, as
measured by total chlorine dioxide consumption. |
| | | ClO2 Composition % | ClO2 Savings | Brightness | Viscosity | Reversion |
| Pulp Type | Pulp Treatment | D0 | D1 | Total | kg/odt pulp | ISO | mPa.s | % |
| HWD | None | 1.79 | 1.43 | 3.22 | Ref. | 90.1 | 22.8 | 2.34 |
| S | 1.63 | 1.43 | 3.06 | 1.6 | 90.0 | 21.7 | 2.28 |
| O | 1.12 | 0.97 | 2.09 | 11.3 | 90.1 | 15.4 | 2.24 |
| (EO) | 1.37 | 1.18 | 2.55 | 6.7 | 90.0 | 18.4 | 2.28 |
| S/O | 1.01 | 0.97 | 1.98 | 12.4 | 90.1 | 14.1 | 2.25 |
| SO | 0.98 | 0.97 | 1.95 | 12.7 | 89.9 | 14.5 | 2.18 |
| S/(EO) | 1.25 | 1.18 | 2.43 | 7.9 | 90.2 | 17.0 | 2.30 |
| S(EO) | 1.23 | 1.18 | 2.41 | 8.1 | 90.1 | 17.6 | 2.25 |
| O/O | 1.00 | 0.97 | 1.97 | 12.5 | 89.9 | 17.4 | 2.38 |
| OO | 0.99 | 0.97 | 1.96 | 12.6 | 90.2 | 18.0 | 2.31 |
| SWD | None | 2.94 | 1.65 | 4.59 | Ref. | 89.1 | 17.5 | 2.45 |
| S | 2.74 | 1.65 | 4.39 | 2.0 | 89.0 | 16.8 | 2.43 |
| O | 1.57 | 1.19 | 2.76 | 18.3 | 89.2 | 14.3 | 2.29 |
| (EO) | 2.20 | 1.33 | 3.53 | 10.6 | 89.0 | 15.9 | 2.33 |
| S/O | 1.43 | 1.19 | 2.62 | 19.7 | 89.1 | 13.9 | 2.27 |
| SO | 1.39 | 1.19 | 2.58 | 20.1 | 88.9 | 14.2 | 2.25 |
| S/(EO) | 2.08 | 1.33 | 3.41 | 11.8 | 88.8 | 15.1 | 2.33 |
| S(EO) | 2.02 | 1.33 | 3.35 | 12.4 | 88.9 | 15.8 | 2.35 |
| O/O | 1.43 | 1.19 | 2.62 | 19.7 | 89.0 | 13.2 | 2.38 |
| OO | 1.42 | 1.19 | 2.61 | 19.8 | 88.9 | 13.7 | 2.32 |
Example 8. Impact of pulp soaking on the strength
properties of the bleached pulp
-
As measured by viscosity and brightness reversion,
no significant impact of the soaking on pulp quality is
observed in Table 7. To further confirm this, the
hardwood sample described and treated as shown in
Example 5 and bleached as described in Example 7, was
submitted to beatability and strength property tests.
A beating curve was developed for the never dried pulp
samples using the PFI mill at 0, 1500, 3000, 4500 and
6000 revolutions. The strength properties of the
bleached pulps were measured using Tappi standard
procedures. The values reported in Table 8 are at
40°SR.
-
Because the alkali soaking is carried out at hot
temperature and under alkaline conditions, it could be
speculated that the pulp strength properties would be
somewhat impaired. The results in Table 8 indicate,
however, that neither the strength properties nor the
pulp beatability were changed by the soaking treatment.
Furthermore, the oxygen treatments when carried out
alone or in combination with the soaking had no
negative effect on pulp strength properties and
beatability. Thus, it is concluded that soaking, when
carried out under well optimized conditions, does not
negatively impact pulp strength.
| Impact of several pulp treatments on
overall strength properties and beatability of the
final bleached pulp |
| Pulp Treatment | Tear Index at 40 °SR mN.m2/g | Tensile Index at 40 °SR, N.m/g | PFI Revolutions to Reach 40 °SR |
| None | 9.95 | 100.5 | 2739 |
| S | 9.81 | 98.7 | 2715 |
| O | 9.83 | 98.6 | 2878 |
| (EO) | 9.78 | 99.1 | 2796 |
| S/O | 10.0 | 99.7 | 2802 |
| SO | 9.98 | 97.3 | 2789 |
| S/(EO) | 10.05 | 98.2 | 2785 |
| S(EO) | 10.03 | 100.6 | 2805 |
| O/O | 9.65 | 96.8 | 2773 |
| OO | 9.67 | 95.4 | 2716 |
-
It should be understood that the foregoing
description is only illustrative of the invention.
Various alternatives and modifications can be devised
by those skilled in the art without departing from the
invention. Accordingly, the present invention is
intended to embrace all such alternatives,
modifications and variances which fall within the scope
of the appended claims.
