Note: Descriptions are shown in the official language in which they were submitted.
3 :~
METHOD OF PRETREATING LIGNOCELLULOSIC MATERIALS
PRIOR TO ALKALINE PEROXIDE HIGH YIELD PULPING
Pulping processes can be put into either of two broad
classifications; high yield pulping and chemical pulping. High
yield pulping processes use mechanical destructuring (i.e.
grinding, refining) of the raw material to produce individual
fibers or pulp from lignocellulosic materials, usually in chip
form, sometimes with mild chemical pretreatment of the chips.
Chemical pulping or low yield processes that primarily use chemical
reactions to produce individual fibers from chips. Within the high
yield category there are many different combinations of mechanical,
chemical, and thermal treatments. Each specific combination of
mechanical, chemical and thermal treatments has a different effect
on fiber separation, lignin removal, fiber brightness and
papermaking strength of paper made from the resulting fibers.
This invention concerns hlgh yield pulping comprising chemical
pretreatments using peroxide in combination with mechanical
treatment to produce pulp from chips. More particularly, this
invention reduces peroxide decomposition associated with the
pretreatment of chips with peroxide prior to refining of the chips
to produce high yield pulp with peroxide modification of the lignin
in the fibers. With this invention pulp having desired papermaking
properties, particularly a combination of high strength and
brightness is obtained with reduced chemical and energy
consumption.
Pulp produced by mechanical refining alone without chemical
pretreatment results in extremely high yields (about 95% or higher)
but results in fibers containing almost all of the original lignin
in essentially a chemically unmodified form and fiber damage/fines
generation due to the somewhat indiscriminate mechanical action on
the lignocellulosic raw material. Such unmodified lignin imparts
relatively low brightness to the fibers, and, due to its
hydrophobic nature, the lignin inhibits the development of paper
strength through fiber collapse and fiber to fiber bonding
(hydrogen bonding). Such fibers are stiffer than partially or
2 ~ 3 1
completely delignified fibers from the same lignocellulosic raw
material. Although some high yield pulps containing high amounts
of lignin- can be bleached economically to relatively high
brightnesses using oxidizing agents such as alkaline peroxide,
and/or reducing agents such as sodium hydrosulfite, such post-
refining treatments do not increase papermaking strength to levels
required for many end uses because much irreversible mechanical
damage and fines generation has already been done to the fibers in
the refining process. Chemically attacking the lignin before
refining prevents much of the mechanical damage and fine
generation. Such chemical pretreatment with peroxide prior to
refining is improved by the present invention.
The papermaking strength of high yield pulps can be increased
by sulfonation of the lignin, particularly when the wood chips are
treated with the sulfonation chemicals (usually sodium sulfite with
or without sodium hydroxide) prior to mechanical defibration
(refining). In some cases, the resulting fibers can also be
bleached economically as with alkaline peroxide and/or sodium
hydrosulfite to give both improved brightness and papermaking
strength. However, the higher levels of sulfonation required for
high strength result in pulps which respond less well to bleaching
than similar non-sulfonated or low-sulfonated pulps, and therefore
such highly sulfonated pulps have high strength but have lower
bleached brightnesses and higher bleach chemical demand.
Conversely, low-sulfonation pulps are more bleachable but have
lower strength. Moreover, sulfonation processes require the
removal and disposal of environmentally objectlonable sulfur
compounds from process waste streams. In addition, ~he nee~ for
separate sulfonation, refining, and post-refiner bleaching
equipment mak~s the capital equipment and operating c03ts for such
a system very significant.
An alternative to sulfonation of lignin for increasitlg
papermaking strength of hi~h yield fibers is carboxylation and
brightening of lignin achieved by th~ combined ~welling and
brightening action of alkaline peroxide prior to and/or during
2 ~
defibration. As sulfonation results in lignin containing sulfonate
groups, likewise, carboxylation results in lignin with carboxylate
groups. ~th the sulfonate and the carboxylate groups are capable
of participating in hydrogen bonding which increases the strength
of paper made from such high yield pulps (papermaking strength).
Similar to the alkaline high sulfonation treatment of chips
prior to refining, alkaline peroxide pretreatment of chips softens
the lignocellulosic raw material resulting in easier fiber
separation (less energy consumption and less fines generation and
fiber fragmentation~ during refining. In addition, refiner
brightening with alkaline peroxide can eliminate the need for
separate post-refiner bleaching equipment, due to the facts that
refiners are excellent mixers of pulp and brightening agents, and
the temperature within the refiner (about 100C or higher) causes
brightening to occur extremely fast relative to typical post-
refiner alkaline peroxide bleaching steps (approximately 50 C to
80 C). The primary drawback to brightening chips within the
refiner with alkaline peroxide is peroxide decomposition. Peroxide
decomposes to form oxygen (ineffective for lignin-retaining
bleaching) and water under the highly alkaline conditions required
for papermaking strength development. Peroxide decomposition is
hastened by the high temperatures reached in refiners and by metal
contaminants, particularly manganese, iron, and copper, which are
contained in significant quantities in lignocellulosic raw
materials and in lesser quantities in process water. Partial
removal or inactivation of such metal contaminants in
lignocelluslosic raw material can be effected by introducing
chelating agents into the wood chips and then removing the chelant-
metal complexes. However, the physical entrapmen~ and chemical
attraction of such metals by fiber components within the chips make
complete removal of the metals impractical and require additional
peroxide protection.
The problems associated with pre-refiner or in-refinex
alkaline peroxide treatments are partially avoided by conventional
post refiner alkaline peroxide treatments. For exampl~, rather
20~7231
than removing metal contaminan~s from chips, the contaminants have
been removed from individual fibers with chelating agents after
refining and prior to alkaline peroxide bleaching. Post refininy
removal is much more easily achieved because the particle size of
the fiber in pulp is much smaller than the size of chips before
refining. The smaller size makes the metal contaminants much more
accessible to the chelant solution. Consequently, in many cases
the individual fibers can be bleached to much higher brightnesses
~ith alkaline peroxide in a post refiner bleaching treatment
without significant waste of peroxide bleaching agents due to metal
contaminant induced decomposition of peroxide (U.S. Patent No.
4,160,693-Lindahl, et al.). The disadvantage is, that high
strength requires high levels of pre-refiner sulfonation and high
sulfonation makes post-refiner bleaching difficult.
There have been many attempts to overcome such problems
associated with the pre-refiner or in-refiner use of alkaline
peroxide in the production of high yield pulps. Control of
alkalinity (e.g., see U.S. Patents 3,069,309-Fennell and 4,270,976-
Sandstorm, et al., and Canadian Patents 1,078,558 and 1,173,604),
control of the temperature (e.g., U.S. Patent 4,187,141-Ahrel), and
control of time at high temperature (e.g., U.S. Patent 4,270,976-
Sandstorm et al.) have been tried. However, such techniques for
reducing peroxide decomposition also reduce the effectiveness of
alkaline peroxide in terms of the resulting pulp properties
(papermaking strength) while the presence of deleterious metal
contaminants still results in inefficient utilization of the
peroxide bleaching agent during refinar bleaching.
A common method for circumventing the problem of peroxide
decomposition in alkaline peroxide bleaching of chips during
refining is to add the bleaching agent directly to the refining
zone to minimize the contact time between the chip and alkaline
peroxide, and in some cases to allow more intimate contact between
metal contaminants and silicate and/or magnesium ion stabilizer
flocs (See for example, U.S. Patent Nos. 3,023,140-Textor,
3,069,309-Fennell, 4,022,965 Coheen, et al., 4,270,976-Sandstorm,
2 3 1
et al., 4,311,553-Akerlund, et al.; Japanese Patent Application
No. 80-72091, and Federal Republic of Germany Patent No. 2818-
320). ~dditionally, wood chips have been pretreated by
impregnation and/or refining with chelants (U.S. Patents Nos.
3,023,140-Textor, 4,311,553-Akerlund, et al., Japanese Patent
Application No. 80-7209, and Federal Republic of Germany Patent No.
2818-320) or with sodium silicate (U.S. Patent Nos. 3,069,309-
Fennell, 4,311,553-Akerlund, et al.), or with magnesium salts (U.S.
Patent Nos. 3,023,140-Textor, 3,069,309-Fennell, ~,311,553-
Akerlund, et al., and Japanese Patent Application 80-72091) and
combinations thereof prior to alkaline peroxide addition into the
refiner to reduce peroxide decomposition. U.S. Patent No.
4,270,076-Sandstorm, et al., is the only case in which brightness
comparable to post refiner alkaline peroxide bleaching was obtained
but it utilized lower alkalinity as the means of reducing the
peroxide decomposition rate which sacrificed good papermaking
strength development.
The present invention is based in part upon the hypothesis
that with processes employing alkaline peroxide addition directly
into the refiner, the majority of the defibration occurs before
the alkali and peroxide can react with the wood fibers, thereby
reducing the potential for papermaking strength development
imparted with requiring more energy for refining and increasing the
generation of fines, all of which could be avoided if alkaline
peroxide could be inserted into and stabilized within the chip and
allowed to react with the chip prior to defibration.
Impregnation of chips with alkaline peroxide prior to refining
has also been practiced (U.S. Patent~ 4,187,141-Ahrel, and
4,270,976-Sandstorm, et al., and Canadian Patents 1,078,558, and
1,173,604). In most cases, the brightnes~ obtained was comparable
to that obtainable with post refiner alkaline peroxide bleaching.
However, with such processes, metal contaminants are not removed
or deactivated; rather, peroxide decomposition is reduced by lower
alkalinity (U.S. Patent Nos. 4,270,976-Sandstorm, et al., Canadian
Patent 1,078,558, and 1,173,604) or by minimizing refining
~723~.
temperature (U.S. Patent No. 4,187,141-Ahrel). However, the
lowering of the alkalinity or temperature causes less papermaking
strength to be developed than with high sulfonation methods.
Lowering of the temperature below 100 C is difficult to accomplish
practically. At higher alkalinity, strengths comparable to those
of post refiner bleached, sulfonated high yield pulps were obtained
but at the expense of lower brightness due to increased peroxide
decomposition (Canadian Patent No. 1,078,558).
Additional examples of prior art teachings of treating pulp
(rater than chips) with peroxide demonstrate the continuing
tendency in the art to avoid treating chips with peroxide because
of the decomposition problem. U.S. Patent No. 4,787,959 is another
example of a high yield pulping process that avoids peroxide
treatment of chips by treating pulp rather than chips with peroxide
using a first alkaline cooking stage to fiberize the chips prior
to a second cooking with peroxide. By the time the peroxide is
added in this process the "lignin-cellulose" material exists as
more or less discr0te fibers because the cellular structure has
been fractured. Thus, the problem of peroxide stability within the
cells of chips or wood particles does not exist.
Thus in many recent prior art patents such as, U.S. Patents
4,787,959, 4,734,161 and 4,732,650 the material being treated is
pulp made up of small fibers, so the problem of stability of
peroxide within the essentially unbroken cellular structure of wood
particles existing prior to the refininq does not exist.
Alkaline peroxide stabilizers like water soluble alkaline
sodium silicate and magnesium sulfate are often utilized in the
peroxide bleaching of high yield pulps to further reduce peroxide
decomposition caused by metal contaminants. The silicate-magnesium
combination forms a floc in alkaline peroxide solutions and this
floc attracts and absorbs the metal ions thereby reducing the
ability of such ions to decompose peroxide. Magnesium ions also
reduce peroxide decomposition by electronically deactivating the
metal ions, thereby reducing the potential of the metal ions to
decompose peroxide.
2~723~
Recently, it was discovered that flocs or precipitates formed
by silicates and/or magnesium in alkaline peroxide solutions cannot
readily pe~etrate into the wood chip structures prior to refining
due to the physical size of the flocs relative to the pore size of
the wood chips. The discovery was based upon the observation that
such stabilizers effectively stabilize peroxide against
decomposition when pulp is being bleached with peroxide but are not
as effective when the same fibers are still in chip form rather
than pulp. It was postulated that the difference in stabilizer
effectiveness when treating pulp fibers versus wood chips is due
to the relatively small molecules of alkali and peroxide entering
the cellular structure of the chip while the stabilizer floc if
impeded from penetrating into the cells to counteract decomposition
induced by metal contaminants within the cellular structure. The
alkaline peroxide in the chips, upon entering the cellular
structure and becoming separated from the stabilizing floc, rapidly
decomposes thereby reducing the amount of peroxide available for
bleaching during refining. In addition, the pressure buildup
within the chip due to the evolution of oxygen gas during peroxide
decomposition forces alkaline peroxide solution back ollt of the
chip resulting in insufficient peroxide retention in the chip as
it enters the refining zone. Furthermore, irreversible alkaline
yellowing of the pulp occurs if there is insufficient residual
peroxide remaining with the pulp after refining.
U.S. Patent 4,849,053 to Gentile, et al. and the references
cited during the prosecution of that patent are the most pertinent
prior art, over which the present invention constitutes an
improvement. The Gentile, et al. patent describes and claims a
highly successful method for producing wood pulp from chips using
pre-treatment with stabilizers and alkaline peroxide prior to
mechanical fiberization (refining~ to increase the brightness of
the resulting fibers and papermaking strength achievable with the
fibers. This sequential pre-treatment of the wood chips prior to
refining results in the "in-situ" formation within the chips of a
peroxide-stabilizing floc or sol.
2~723~
The alkaline peroxide impregnated chips with the "in-situ"
formed stabilizing floc within the cellular structure are refined
in one ~r more stage(s) under atmospheric pressure or
superatmospheric pressure and the corresponding saturated steam
temperature. (The refining pressure is usually associated with
steam added to or generated within the re~ining device.) The
resulting pulp is dewatered and washed to remove bleaching and
stabilizing chemicals and dissolved wood substances to result in
a non-sulfonated pulp having a unique combination of properties
including high yield, superior brightness and papermaking strength,
and low fines content. Recyclable alkaline peroxide is obtained
from the dewatering of the pulp after refining and preferably
before acidification~ The alkaline peroxide obtained from the post
refiner dewatering step can be reused as makeup in the impregnation
step in which peroxide is added to the chips.
The two or three stage impregnation and pulping process of
Gentile et al. tU.S. Patent 4,849,063) is designed to stabilize
alkaline peroxide so it can effectively and economically develop
pulp strength and brightness by the time the pulp exists the
mechanical pulping stage (refiner). This method, while eminently
successful on most species of wood chips, does not produce the
maximum stabilization of alkaline peroxide. For example, with wood
chips from such softwoods as eastern black spruce, eastern balsam
fir, lodgepole pine/white spruce (50/50% mix), and hardwoods such
as aspen, the peroxide is not stabilized as completely as possible
in the normal fashion, some peroxide decomposition occurs and
maximum brightness is not achieved as economically as possible with
the practice of the present invention.
SUMMARY OF INVENTION
The present invention is based upon the discovery that the key
to the realization of successful peroxide stabilizat~on and
brightening utili~ation lies in (1) substantial reduction of
metallic ions content in the chips, such as iron, copper, and (in
particular) manganese content in the chips (these metals catalyze
peroxide decomposition) prior to impregnation of chips with
peroxide and (2) the maintenance of high level of magnesium in the
chips with the alkaline peroxide prior to and preferably during
refining.
The present invention provides an alkaline peroxide process
for the production of high yield pulps from wood chips having a
reduced manganese concentration prior to peroxide impregnation and
increased magnesium concentration prior to or simultaneously with
alkaline peroxide impregnation, followed by refining to produce a
pulp of high brightne~s and pulp streng~h with a minimum of
peroxide consumption.
Other aspects of this invention are as follows:
In a high yield pulping process comprising impregnating
chips with- an alkaline peroxide solution containing stabllizers
for peroxide followed by mechanical defibration, wherein the
improvement comprises:
pretreatinq the wood chips prior to peroxide impregnation,
said pretreatment comprising;
(a) impregnating the chips wlth an impregnation solution
consisting essentially o~ a chelating agent for metallic ions; and,
(b) expressing from ~he chips chelating agents and
metallic ions.
A high yield alkaline peroxide pulping proceR~ for wood
chips comprising:
(a) impregnating the wood chips with a first
impregnation solution consisting essentially of a chelating agent
for metallic ions;
(b~ expressing, at least 25~ of said first impregnation
solution containing metallic ions from within the chips;
tc) removing expressed impregnation solution from
contact with the chips;
(d) impregnating the chips with a second impregnating
solution containing magnesium salts at a concentration of from 0.5%
grams per liter to 20 grams per liter to result in from 0.05% to
2~ by weight magnesium in the chips calculated as MgSOI.7H2O and
based upon the oven dried weight of the wood chlps;
~ e) impregnating the chips with a third peroxide and
having a pH from 9 to 13; and,
(f) mechanically defiberizing the chips to produce pulp.
A high yield alkaline peroxide pulping process for wood
chips comprising:
(a) impregnating the wood chips with a first
impregnation solution containing a chelating agent for metallic
ions without containing significant quantities of magnesium ions;
(b) expressing said first impregnation solution
containing metallic ions from within the chips;
(c) removing expressed impregnation solution from
contact with the chips;
(d) impregnating the chips wit:h a second impregnating
solution containin~ peroxide and a stabilizer for peroxide and
having a pH from 9 to 13 and containing magnesium salts at a
concentration of from 0.5% grams per liter to 20 gram3 per liter
to result in from O . 05% to 2% by weight magnesium in the chips
calculated as MgSO~.7H2O and based upon the oven dried weight of the
wood chips; and
(e) mechanically defiberizing the chips ~o produce pulp.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
This invention is di~ected toward the removal of metallic ions
such as manganese from lignocellulosic material in chip~ form prior
to treatment with alkaline peroxide and to the utilization of high
levels of magnesium (Mg) withln the chips while in contact with
alkaline peroxide. The present invention is particularly useful
9a
for pulp~ from wood species naturally high in metallic ions such
as eastern black spruce, eastern balsam fir, lodgepole pine/white
spruce mixtures and aspen.
The present process i~ charactarized by at least a two stage
and preferably three or more stage impregnation process of chips
prior to refining. In the first stage, wood chips, which
preferably have been purged of entrapped air, are impregnated with
a chelating agent such as die~hylenetriaminepantaacetic acid
(DTPA), ethylene diaminetetraacetic acid (EDTA),
hydroxyethyle~hylenediamine triacetic acid (HEEDTA), and
nitrilo~riacetic acid (NTA), sodium tripolyphosphate (STPP), and
phosphonic acid derivative or other similar compounds known in the
9b
~7~3~
art for such functionality or a combination of such chelating
agents. The concentration of the chelating agent in the
impregnati~g solution should be preferably from 0.1 gram/liter to
20 grams/liter (expressed as 100% chelating agent in the solution)
to give preferably 0.01% to 2% chelant based on the dry weight of
the chips with from 0.05% to 0.5% being particularly preferred.
The temperature of the first impregnation step is preferably
between 15 C and 80C. The pH is preferrably between 4 and 12 with
between 8 to 11 being particularly preferred. Adjustments to the
solution pH can be made with any suitable acid or alkaline
substance which does not react with pero~ide, cause darkening of
the chips, or cause any of the components of the impregnation
solu~ion to be precipitated or to lose chelating ability such as
sulfuric acid or sodium hydroxide.
Before, or as part of the first impregnating step, the chips
are preferably squeezed to expel liquid and any remaining air and
then allowed to expand in contact with the impregnation solution.
The quantity of solution absorbed in an impregnation step is
influenced by the impregnation device (primarily the degree of
squeezing of the chips~ and the particular material being
impregnated. The level of chemical addition into the chips is
primarily controlled by the concentration of the particular
chemical in the impregnating solution and the quantity of solution
absorbed. The concentration of chelant and quantity of solution
absorbed is preferably adjusted to give a chemical addition of
0.01% to 2% based cn the dry weight of lignocellulosic material
being treated. The first impregnation solution is expressed either
prior to or as the initial part of the second impregnation step.
Metallic ions are removed from the chips with the expressed first
impregnation solution which removal is enhanced by the presence of
the chelating agents. Preferably a-t least about 25% of the first
impregnation solution is expressed from the chips with from about
33~ to about 50% being particularly preferred. The effectiveness
of the first impregnation step to remove metallic ions is enhanced
by providing some reten$ion time between impregnation and
2 3 1
expressing the impregnation solution from the chips. This enhanced
effectiveness is observed with a retention time of one half hour
although ~everal hours is more effective. Since the first
impregnation stage is primarily for the purpose of removing
metallic ions from the chips, the first impregnation solution
should consist esstentially of a chelating agent because it is
expressed from the chips. The presence of substantial quantities
of peroxide ( e.g. above 1%) or stabilizers in the first
impregnation solution would be wasteful of such chemicals since
they would be removed from the chips when the first impregnation
solutions is expressed from the chips. Metallic stabilizers,
particularly magnesium should be avoided in the first stage because
they interfere with the primary purpose of the chelating agent.
The second impregnation step can be a conventional alkaline
peroxide impregnation stage with stabilizers and chelating agents.
Preferably the second impregnation stage is with an aqueous
solution of solub1e magnesium salts (e.g., epsom salt - MgSO4.7H2O~.
The epsom salt addition level i5 0.05% to 2% based on oven dried
(o.d.) wood expressed as MgSO4.7H2O. This can be readily
accomplished with an impregnation solution containing 0.5 grams per
liter to 20 grams per liter. The purpose of this second step is
to allow the soluble magnesium salts to penetrate the chip
interstices. The temperature o~ the second impregnation step is
preferably between 15 C and ~0 C, and the pH is maintained below 10
to keep the magnesium in a soluble form. Adjustment to the
solution p~ is preferably made in the same manner as described for
the first impregnation solution. Optionally, the second
impregnation solution can contain alkalin~ peroxide with silicate
stabilizers.
The third impregnation solution is an aqueous alkaline
peroxide solution which contains peroxide stabilizers such as
silicates. Preferably the third impregnation solution contains
magnesium and silicate stabilizers. Silicates are preferably in
a concentration range of 1 gram per liter to 5~ grams per liter to
yield between 0.1% to 5% by weight of silicates (calculated as
11
2~7~31L
SiO2) based upon the dry weight of the chips. Magnesium based on
weight of Mg in salt, is preferably in a concentration of from 0.01
grams per ~iter to 2 grams per liter to result after impregnation
in from 0.001% to 0.2% by weight Mg based upon the dry weight of
the chips. The third impregnation solution contains preferably
hydrogen peroxide although any other peroxygen compound suitable
for lignin brightening may be used. The peroxygen compound is
preferably in the concentration range of between 5% and 100 grams
per liter (calculated and expressed as hydrogen peroxide) to give
additional lavels of preferably between 0.5~ and 10% of the dry
weight of chips expressed as hydrogen peroxide. In addition, an
alkaline substance (preferably sodium hydroxide) is added to the
third impregnation solution to give a solution pH preferably in the
range between 9 and 13 with between 10 and 12.5 being most
preferred.
The most preferred impregnation sequence is the three stage
sequence previously described and presented in greater detail in
the axamples. However, it should be understood that the invention
is not limited to the generally described three stage sequence.
A two-stage impregnation sequence in which the first impregnation
stage is practiced as previously described and the second
impregnation (Mg compounds) solution is combined with the third
impregnation solution (alkaline peroxide and peroxide stabilizers
such as silicates) and used as the impregnation solution in the
second staye, preferably at a pH from 9 to 13 can also be employed.
It is importan~ that the entire chip become uniformly
impregnated with any reactant solutions. Best results are achieved
when a squeezing ratio (e~ual to mass of liquid squeezed from the
chips per unit mass of dry chips) of unity or slightly higher is
achieved. This requires a compression ratio of about 4:1, and
results in the chips being squeezed from about 33% solid to about
50% solids. ~n other words, about 50% of the liquid present in the
chips entering the compression device is squeezed out and replaced
by "new" liquid in the impregna~ion expansion which follows.
12
2~7~3~L
The invention is not limited to the concentration and addition
level ranges previously described for stabilizers and chelating
agents, 3ince differences in metal contamination levels of
lignocellulosic raw materials or process water could justify
stabilizer or chelating agent usages outside of the specified
preferred ranges.
Optionally, the chemically treated chips can be allowed to
react with or without the application of heat, prior to entering
the refining stage by retaining the impregnated chips for some
finite time in any suitable vessel. A retention time of 5 minutes
to 60 minutes is particularly suitable. Each impregnation is
preferrably followed by a short drainage period and additional time
for diffusion and/or reaction to occur within the chips. The
amount of diffusion/reaction time required depends on the type of
wood processed.
Refinin~ of Chemically Im~reqnated Chi~s (Def bration)
The chemically treated chips are mechanically refined in a
suitable defibration apparatus in one cr more stages in accordance
with conventional processes and equipment. The steam pressure and
corresponding temperature during refining are optional and can b~
at atmospheric and/or superatmospheric pressure, depending on the
species being pulped and the desired pulp properties. Atmospheric
pressure refining is preferred.
After refining, it is preferred that the pulp remain alkaline
so that the silicate and other pH-sensitive materials can be
removed prior to washing and neutralization. This prevents
silicate deposition which can impair final pulp properties.
Post-Refininq_~teps:
After refining, the pulp may be allowed to continue bleaching
as long as is practical prior to expelling the impregnation
solutions. The amount of peroxide used in the impregnation steps
is preferably preselected to result in some residual peroxide
remaining after refining in order to maintain high brightness.
13
2~7~3~
Preferably the refined pulp is concentrated, e.g., by compressing
or thickening, to remove residual impregnation solution containing
potentiall~ recyclable alkaline peroxide, then diluted with water,
washed and acidified preferably with sulfur dioxide, sodium
bisulfite, sulfurous acid, or sulfuric acid, to a pH between 5.5
and 6Ø In particular, washing prior to acidification is
preferred for removal of alkali soluble wood components and
silicate. The residual peroxide extracted from the pulp after
refining can be recycled as a source of peroxide in one of the
impregnation solutions particularly if the process is practiced
continuously or in sequential batches. The washed pulp is
preferably screened and cleaned by conventional means to result in
a pulp suitable for the production of paper products.
EXAMPLES
In the following examples wood chips from eas~ern black
spruce, eastern balsam fir, lodgepole pine/white spruce 50/50
mixture (all softwoods) and/or aspen (hardwood), are treated by
both the process of the present invention and that of Gentile, et
al., U.S. Patent 4/849,058 and a comparison of the results
tabulated. The proc~ss of U.S. Patent 4,849,058 was chosen for
comparison because it represents the what is believed to be the
best of the prior art processes for stabilizing alkaline peroxide
in chips prior to high yield mechanical pulping. In the examples,
all parts are by weight unless specified otherwise.
EXAMPLES A ( CONTROL ) AND
Screened eastern black spruce chips (2000 grams o.d. of wood
at a solid content of 49%) were steamed for 15 minutes. The hot
chips (solid content 40%) were placed into a Sprout-Bauer Ll-12
laboratory impregnator and compressed to a solids content of 60-
70%. The impregnator was then closed, filled with 55 liters of
impregnation liquor and the chips released (allowed to expand) into
the liquorO The impregna~ed chips were removed from the
14
2~7~.31
impregnator and drained for 5 minutes to remove all excess liquor.
The same procedure was repeated for all three impregnation stages.
In Exampl~ A, the preferred three-stage impregnation process of
U.S. Patent 4,849,058 was employed. In Example 1, the 3
impregnation steps were conducted according to the process of the
present invention.
Table 1 lists the impregnation conditions, the amount ~nd type
of chemicals added, and the test results comparing the process of
the present invention (Example lj with the control (Example A).
The amounts of chemicals applied during impregnation were
determined as follows:
Ca = (0.1) (PUR)(CL), where
Ca = amount of chemical applied in % by weight of
oven dried (o.d.) wood
CL = concentration of chemicals in liquor in
grams/liter
PUR = amount of li~uor picked up during impregnation
in liters divided by oven dried (o.d.) weight
of wood in kilograms (kg)
The magnesium salt used in these examples was Epsom salt
(MgSO4-7H20)- The reported amounts of pentasodium salt of
diethylenetriamine-pentaacetic-acid (DTPA) are based on a solution
with 40.2% active ingredient. Trisodium salt of hydroxy-ethyl-
ethylene-diamine-triacetate (HEEDTA) was a 41.3% solution. Also,
the concentrations for silicate are calculated using a 37.6%
solution with a SiOz/Na20 ratio of 3.22.
After impregnation chips were refined in a Sprout-Bauer 12-
lCP laboratory refiner under atmospheric pressure. After refining
residual peroxide and caustic were determined by titration. The
pulp was dewatered, washed and the pH adjusted to 6.0 with dilute
H2S04. "Brightness" is defined as ISO brightness in percent units
which is determined by using the sample preparation procedure given
in the Technical Association of the Pulp and Paper Industry (TAPPI)
Official Test Method T218 om-83. The brightness of the sample was
measured using a Technibrite Micro TB-IC meter.
2 3 ~
Hydrogen peroxide usage or consumption (expressed as a weight
percent of the dry weight of the chips) is based on the quantity
of peroxide applied to the wood chips minus the quantity of
residual peroxide in the pulp after refining. The percentage
peroxide consumed is calculated as being equal to the quantity of
peroxide consumed times 100 divided by the dry weight of the chips.
The quantity of peroxide in a solution was determined by iodometric
titration using starch as an end point indicator (Vogel, Quantative
Inorganic Analysis, Wiley & Sons, 1961, p. 363).
The magnesium and manganese concentration in the original wood
chips and after impregnation was determined by atomic absorption
spectroscopy.
As shown, the process of the present invention led to an
increase in manganese removal and magnesium retention, which in
turn gave a 2% brightness increase and a 47% reduction in peroxide
consumption, thus demonstrating the increased peroxide s~ability
under the process of the present invention.
16
2 3 3 7 ~ 3 .L
TABLE I
Process conditions, added chemical amounts, test results of
3-stage alkaline peroxide process of present invention vs
control 3-stage process.
.
EXAMPLE NO. A (CONTROL) #1
TYPE WOOD: EASTERN EASTERN
1. 1st Impregnation
a. process conditions
pH 7.0 11.0
b. chemicals added
CL CA CL CA
DTPA 1.35 0.22 2.5 0.39
HEEDTA 1.35 0.22 2.5 0.39
MgSO4.7H203.35 0.53 - -
2. 2nd Impregnation
a. process conditions
pH 10.8 9.5
b. chemicals added
DTPA 1.25 0.19 - -
HEEDTA 1.25 0.19
Silicate 12.5 1.9
MgSO4.7HzO - - 3.35 0.53
3. 3rd Impregnation
a. process conditions
pH 11.6 11.2
b. chemicals added
MgSO .7H O 4.5 0.54 3.35 0.48
Sili4 te 216.5 2.13 26.65 3.81
NaOH 31.7 4.09 27.20 3.89
HzO2 33'5 4.32 27.20 3.89
4. Total Chemicals Added
DPTA 0.41 0.39
HEEDTA 0.41 0.39
MgSO .7H O 1.07 1.01
Silic~ate 2 4.03 3.81
NaOH 4.09 3.89
H~Oz 4.32 3.89
17
2 ~ 3 1
5. Te~t Results
CA CA
ISO brightness (%) 74.1 76.1
H2o2 c0nsumed (%) 3.74 2.73
NaOH consumed (%) 3-35 3.18
PUR 3rd stage (1/kg) 1.29 1.43
Mn in chip3
3rd stage (ppm) 10.3 8.4
Mg in chips
3rd stage (ppm) 364 1050
Mn content original
chipB (ppm) 37-5 37-5
Mg content original
chip~ (ppm) 73-5 73-5
COMPARISON
Control: 74.1% brightnesa
3.74% H202 consumption
10.3 ppm Mn in 3rd atage chips
364 ppm Mg in 3rd stage chips
Invention: 76.1% brightneqs
2.73% H22 con~umption
8.4 ppm Mn in 3rd stage chip~
1050 ppm Mg in 3rd stage chipa
Improvement: 2.0 greater brightneas units
27% decrease in H2O2 conaumption
1.9 ppm lesa Mn in 3rd ~tage chips
686 ppm more Mg in 3rd ~tage chip~
18
~723~
EXAMPLES B (CONTROL) AND 2
Two kilograms of screened eastern Canadian balsam fir chips
with a sol-ids content of 56.2~ were impregnated and refined as in
control Example A. The pH of the first stage liquor was adjusted
to 7~0 and the 2nd stage liquor pH was 10.7. This experiment is
designated as comparative Example B.
A three impregnation step sequence of the present invention
was made on an identical batch eastern Canadian balsam fir chips
as used in Example B. The general impregnation and refining
conditions were the same as for Example 1 above. Table 2 lists the
process conditions, chemical types and amounts added, and the
comparative test resul-ts for both examples. This experiment is
designated as Example 2.
As with Examples A and 1, the greater manganese removal from
the chips and greater magnesium addition to the chips prior to
refining gave improved brightness (1.5 ISO brightness units) and
reduced (27~) peroxide consumption compared to the control.
~9
20~7231
TABLE II
Process conditions, added chemical amounts, test results
of 3-stage alkaline peroxide process of present invention
vs. control 3-stage process.
-
EXAMPLE NO. B (CONTROL) #2
~ASTERN EASTEPN
TYPE WOOD: ~ALSAM FIR BALSAM FIR
1. 1st Impregnation
a. proces3 condition3
pH 7.0 11.0
b. chemicals added
CL CA CL CA
DTPA 1.70 0.24 3.35 0.49
HEEDTA 1.70 0.24 3.35 0.49
~S04 .7H204.45 0.64 - _
2. 2nd Impregnation
a. proces3 conditions
pH 10.7 9.5
b. chemicals added
DTPA 1.90 0.27 - -
HEEDTA 1.90 0.27
Silicate 18.85 2.71
MgSO4O7HzO - - 5.0 0.65
3. 3rd Impregnation
a. proces~ conditions
pH 11.3 11.0
b. chemicals added
MgSO .7H O 6.5 0.63 5.55 0.61
Silic~ate 225.95 2.52 44.5 4.9
NaOH 50.0 4.9 41.20 4.5
HzOz 55.0 3.3 45.22 4.9
4. Tctal Chemical~ Added
DPTA 0.51 0.49
HEEDTA 0.51 0.49
MgSO .7H O 1.27 1.26
Silicgate 2 5.23 4.90
NaOH 4-9 4-5
HzO2 5.3 4.9
2~7~3~
5. Test Results
CA CA
ISO brightness (%) 74.2 75.7
Hzo consumed (~) 5.03 3.65
NaO~ consumed (~) 3.68 2~88
PU~ 3rd stage (1/kg) 0.97 1.1
Mn in chips
3rd stage (ppm) 31.5 29.1
Mg in chips
3rd stage (ppm) 430 722
Mn content original
chips (ppm) 162.7 162.7
Mg content original
chips (ppm) 110.1 110.1
C ARISo
Control: 74.2~ brightnes~ WITH 5.03~ H22 congumption
31.53% ppm Mn in 3rd stage chip3
430 ppm Mg in 3rd stage chip~
Invention: 75.7% brightnes3 with 3.65% H202 con3umptiion
29.1% ppm Mn in 3rd 3tage chips
722 ppm Mg in 3rd 3tage chips
Improvement. 1.5 greater brightness units
27~ leY~ H22 con~umption
2.43 ppm les~ Mn in 3rd stage chips
292 ppm more Mg in 3rd ~tage chips
21
2~2~
EXAMPLES C (CONTROL) AND 3
Screened aspen (populus tremuloides) chips at a solid content
of 58.2% were impregnated and refined using the general procedures
described in Example A. The pH of the first stage liquor was
adjusted to 7.0 and the 2nd stage liquor had a pH of 10.7. This
is designated Example C. An identical aspen chip charge was
impregnated and refined by the general procedures of Example 1
above using the sequence of this invention. The specific additions
used and results obtained are shown in Table III.
Although the original aspen chips had a low manganese content
resulting in a low 3rd stage manganese level in the chips in both
Examples C and 3, the third stage chips of Example 3 retained much
more magnesium. As a result, brightness was improved by 1.2% while
peroxide consumption was reduced by about 18% vs. the control.
2 ~ ~
TABLE III
The process conditions, added chemical amounts, test
results of 3-stage alkaline peroxide process of present
invention vs. control 3-stage process.
EXAMPLE NO. C (CONTROL) #3
1. 1st Impregnation
a. process condition3
pH 7.0 11.1
b. chemicals added
CL CA CL CA
DTPA 1.450.24 2.6 0.37
HEEDTA 1.450.24 2.65 0.37
MgSO4.7H20 3.6 0.59 2.65
2. 2nd Impregnation
a. proces~ conditions
pH 10.7 9.5
b. chemicals added
DTPA 1.550.25
HEEDTA 1.550.25
Silicate 15.4 2.53 - -
MgSO4.7H2O - 3 55
3. 3rd Impregnation
a. process conditions
pH 11.6 11.8
b. chemicals added
MqSO .7H O 4.15 0.62 4.15 0.63
Silicate 2 16.65 2.50 33.3 5.02
NaOH 37.5 5.63 38.8 5.86
H2O2 23.4 4.41 29.2 4.41
4. Total Chemicals Added
DPTA 0.49 0.37
HEEDTA 0 49 0 37
MgSO .7H O 1.21 1.18
Silic4ate 2 5-03 5.02
NaOE~ 5.63 5.86
, H22 4.41 4.41
23
2,~7~:
5. Test Results
CA CA
ISO brightne~s (%) 78.9 80.1
HzO~ consumed ~%) 3.54 2.92
NaOH consumed (%) 4.57 4.15
PUR 3rd stage (l/ky)~ 1.5 1.5
Mn in chips
3rd stage (ppm) 2.9 5.2
Mg in chips
3rd stage (ppm) 471 746
Mn content original
chips (ppm) 9-3 9-3
Mg content original
chips (ppm) 137 137
COMPARISON
Control: 78.9% brightness
3.54% H Oz consumption
441 ppm ~g in 3rd stage chips
Invention: 80.1% brightness
2.92% H Oz consumption
746 ppm ~g in 3rd staga chips
Improvement: Process: 1.2 greater brightness units
18~ less in H O2 consumption
293 ppm more ~g in 3rd atage chips
* See def inition of PUR in Examples A and B .
2~
~7~3~
EXAMPLES D - N and 4-7
A lodgepole pine/white spruce (50/50%) chips mixture was
subjected to both the 3-stage process of the present invention and
that of the control process used in Example A. The same general
impregnation and refining conditions were employed as in the
previous examples along with comparable chemical applications while
the specific conditions are given in Table IV. Table IV compares
the brightness values, chemical compositions and Mn and Mg content
for identical chip charges and both examples D, E, F, G (control)
examples with 4, 5, 6 and 7, respectively (process of invention).
In all cases, brightness is higher and peroxide consumption lower
for examples 4, 5, 6, and 7. Also, chips impregnated in example
4, 5, 6 and 7 always have a lower Mn content and a higher Mg
content after the 3rd impregnation stage, thus leading to greater
peroxide efficiency in pulp brightening.
Brightness gains attributable to the process of invention
range from 1.8 to 6.2% ISO, while peroxide consumption is reduced
by from 13 to 28% compared to the control.
25'
2 ~ 3 ~
TABLE IV
~omparati~e test results of 3-stage alkaline peroxide
process of the present invention vs control 3-stage
processO
.
EXAMPLE NO. D 4 E 5 F 6 G 7
ISO Brightness 74.2 76.0 67.771.766.472.6 72.0 74.8
PUR (l/kg) 0.850.910.70 0.75 0.83 0.95 1.03 1.3
H2O2 Consumed
(%) 2.782.002.53 2.19 3.102.43 2.20 1.78
NaOH Consumed
(%) 2.201.8~1.82 2.27 1.81 2.07 1.76 2.24
Mn Content in 3rd
Stage (ppm) 12.78.6 20.8 12.124.8 7.1 12.14.7
Mg Content in 3rd
Stage Ippm~ 377 885 515 697 462 795296 855
Mn Content in
(ppm) 83.283.2 62.8 62.872.0 72.0 67.7 67.7
Mg Content in
Original Wood
(ppm) 112 112 103 103 122 122 104104
_ _
26
2~231
Thus, as substantiated by the examples for both hardwoods and
softwoods, the 3-stage preferred process of the present invitation
increased ~rightness and decreased H202 consumption compared to the
best prior art process for high yield peroxide pretreated pulp
~U.S. Patent 4,849,058) .
The concept and practice of the present invention leads to two
major advantages over other alkaline peroxide chip treatment
processes. These are:
1. More efficient removal of heavy metal ions
[(particularly) manganese (Mn)], which catalyze peroxide
decompositions;
2. Maintenance of a higher concentration of magnesium ions
in the chips so they can protect the peroxide from decomposition
prior to, during, and following the refining process.
The best mode presently contemplated for practicing the
invention is with a three stage impregnation process as examplified
in Example 1 with chemical selection, concentration of impregnation
solutions and chemical quantities impregnated into the chips in the
ranges demonstrated in Examples 1 throuyh 7.
The present invention, described in its hroadest concept is
an improvement to peroxide treatments of wood chips prior to
refining in which the concentration of metal ions within the
cellular structure of the chips is reduced in a first impregnation
step as shown in examples 1 through 7 and followed by alkaline
peroxide impregnation with stabilizers for peroxide including
magnesium ions and then mechanical pulping (refining).
