Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PROCESSING STRAW INTO PULP AND PAPER PRODUCT THEREFROM
BACKGROUND OF THE INVENTION
TECHNICAL FIELD
This invention relates to a method for pre-processing and processing
lignocellulosic material into pulp.
BACKGROUND INFORMATION
There is increasing interest in the incorporation of non-wood cellulosic
fibers
into paper products. There is a tremendous amount of non-wood fibers available
for
pulp production. It has been estimated that the world straw production is
approximately 500,000,000 metric tons per year. Only about half of that amount
is
used for low value purposes, such as building materials, fuel, and cattle
feed. Most of
the remaining amount is wasted by burning, without energy recovery, or plowing
back
into the ground. However, among the limitations in making straw into paper is
incorporating the lower quality fiber derived from straw into a product with
sufficient
strength, durability, and brightness for a marketable end product.
United States Patent No. 4,040,899, issued in 1977 to Emerson, attempts to
address the use of straw pulp in paper. Emerson teaches intertwining and
crimping
fibers into a paper web.
According to Pulp and Paper, published 1952, 1960, by Interscience Publishers,
Inc.,
"Straw is pulped by chemical processes and by a combination mechanical and
chemical process known as mechano-chemical process. Among the different
chemicals used for pulping straw are: (1) sodium hydroxide, (2) lime alone or
in combination with other alkalies, (3) sodium sulfite plus other alkali, (4)
chlorine, and (5) sodium hydroxide plus sodium sulfide (sulfite process).
Other
chemicals suggested for the pulping of straw are nitric acid and sodium
chlorite. Sodium carbonate plus sulfi~r has been suggested for the preparation
of a coarse pulp (straw Kraft) used for corrugated papers. There are four
principal processes for making high grade bleachable straw pulp: (1) soda
process, (2) sulfate process, (3) monosulfite process, and (4) the Pomilio
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chlorine process. The soda and sulfate processes produce good pulp, but in
rather low yield."
DISCLOSURE OF THE INVENTION
The present invention provides a method for pulping straw chips and forming a
usable byproduct. According to the method, straw chips are mixed with steam
and
water to form a mixture. The mixture is exposed to temperatures and pressures
sufficient to significantly soften the straw chips to form straw pulp,
lignins, and
hemicellulose when exposed to a rapid decrease in pressure. The mixture is
washed to
separate a portion of the lignins and hemicellulose from the straw pulp such
that a
usable byproduct of water, lignins and hemicellulose, referred to as black
liquor, is
formed and passed through a mechanical refiner.
The straw chips may be mixed with a caustic before the mixture of straw chips,
steam and water is exposed to temperatures and pressures sufficient to
significantly
soften the straw chips to form straw pulp, lignins and hemicellulose in the
mixture
when exposed to a rapid decrease in pressure. The caustic may be present in
the
mixture in about less than two percent by weight. It is more preferred that
the caustic
is present in the mixture in about less than one-half percent by weight. The
caustic may
comprise sodium hydroxide. Also, the caustic may comprise potassium hydroxide.
In one form of the method, the mixture may be exposed to temperatures and
pressures sufficient to significantly soften the mixture when exposed to a
rapid
decrease in pressure for between about two and one-half minutes to about eight
minutes.
In yet another form of the invention, at least about seventy percent by weight
of the straw chips are formed into straw pulp.
In yet another form of the invention, the pressure sufficient to significantly
soften the straw chips when exposed to a rapid decrease in pressure is between
about
140 psig and about 200 psig.
In another form of the invention, the fiber chips may be selected from non-
wood fibers such as ryegrass, wheat and a mixture of grain and cereal straws.
Straw chips for use in the method may be formed by chopping straw to form a
mixture of straw chips, meal, ash and grit and screening the mixture to
separate a
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substantial portion of the straw chips from the meal, ash and grit. The meal,
ash, and
grit may be used as animal feed. In a preferred form of the invention, the
meal, ash,
and grit may be mixed with the black liquor byproduct to form animal feed.
The straw pulp formed by this method has a Canadian Standard Freeness
between about 200 and about 600.
The present invention also provides a paper product made from wood pulp and
straw pulp wherein the straw pulp is produced by mixing straw chips with steam
and
water to form a mixture of straw chips, steam and water. The mixture is
exposed to
temperatures and pressures sufficient to significantly soften the straw chips
into straw
pulp when exposed to decreased pressures. The mixture is then refined and a
substantial portion of the straw pulp is separated from the mixture. A portion
of the
wood pulp and the paper product may be derived from processed post-consumer
waste.
The straw chips may be formed by chopping straw to form a mixture of straw
chips, meal, ash and grit and screening the mixture to separate a substantial
portion of
the straw chips from the meal, ash and grit.
In yet another form of the invention, the straw pulp has a Canadian Standard
Freeness (CSF) of between about 200 and about 600 and an STFI between about 14
and about 21.
These and other advantages and features will become apparent from the
detailed description of the best mode for carrying out the invention that
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, like element designations refer to like parts throughout the
several views, and:
Fig. 1 is an elevational view of a rye grass plant;
Fig. 2 is a flow diagram of a pre-processing plant constructed according to
the
method of the present invention; and
Fig. 3 is a flow diagram of a pulping plant constructed according to the
method
of the present invention.
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BEST MODE FOR CARRYING OUT THE INVENTION
The present invention provides an improved method for pre-processing and
processing various lignocellulosic source materials. Suitable lignocellulosic
materials
S include non-wood fibers such as grain and cereal straw, and corn stover.
However, the
methods of the present invention have been found to be particularly suited for
lignocellulosic material derived from grass and straw, such as annual
ryegrass, fescue,
and wheat. As used herein, straw chips may refer to chips derived from any non-
wood
fibers such as grain and cereal straw, grass and corn stover. However, for
simplicity,
all such chips shall be referred to as straw chips.
Referring first to Fig. 1, by way of example, a ryegrass plant 10 is shown
growing in soil 12. The ryegrass plant 10 has seeds or kernels 14 held within
ears 16,
hollow stems or internodes 18 between joints or nodes 20, and an extensively
branching fibrous root system 22. Sheath portions 24 are formed on the stems
18 and
attach leaves 26 to the stems 18.
During harvest, the ryegrass plants 10 are cut or swathed about three to six
inches from the ground, generally below the first of the joints or nodes 20.
Then, the
cut ryegrass plants are left to cure in the fields for a few weeks. Seeds or
kernels 14
may be harvested from the cut plants by use of a combine. After the combine
has
removed the seeds or kernels 14, the remainder of the ryegrass plants,
generally known
as straw 30, is dropped to the ground. The straw 30 may be recovered from the
field
and baled for storage prior to further processing.
Referring to Fig. 2, for pre-processing, straw 30 is fed into a tub grinder
28.
The straw 30 may be fed into the tub grinder 28 in bales. The tub grinder 28
breaks up
the bales and chops the straw into pieces averaging roughly one inch in
length. As the
chopped straw 32 exits the mill 34, the chopped straw 32 includes nodes 20,
leaves 26,
sheath portion 24, and internodes 18, as well as ash, silica, and grit.
After exiting the tub grinder 28, the chopped straw 32 is sent into a mill 34
to
reduce the chopped straw 32 to pieces averaging roughly one-half inches in
length.
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The mill 34 grinds most of the nodes 20, leaves 26, and sheath portion 24 into
powder.
The chopped straw 32 may be stored in a storage tank 35 before being sent to
the mill 34 if the capacity of the mill 34 does not permit the chopped straw
32 to be fed
5 directly from the tub 28 grinder to the mill 34.
After exiting the mill 34, milled mixture 36 is passed through multilayer
screens
37. Meal 38 is dislodged and removed from straw chips 40 by passing the milled
mixture 36 through the muitilayer screens 37. The meal 38 includes non-fibrous
particles and ash, enriched with silica and grit. The meal 38 may be used as
an
agricultural byproduct for feeding livestock.
The straw chips 40 are then ready for processing into pulp. The straw chips 40
may be stored before further processing. In addition, if the pulp processing
is to be
carried out at a place remote from where the straw chips 40 have been
preprocessed,
the straw chips 40 may be transported by any manner used to transport bulk
materials,
such as by the use of containers.
Refernng to Fig. 3, for processing the straw chips 40 into pulp, the straw
chips
40 are fed into a mixer 42 where they are mixed with steam and water 44 and
any
caustic or digestive additives 46. Typically, the amount of digestive
additives 46
utilized is calculated on a weight basis per amount of dry straw chips
entering the
system. Scales may be incorporated into the pulping process so that the straw
chips 40
may be weighed as they enter the system and the amount of digestive additives
46 may
be administered at the appropriate rate.
Preferably, the amount of digestive additives 46 mixed with steam and water 44
and straw chips 40 is between about zero and about 2.0 percent by weight.
However,
it is more preferable that the amount of digestive additives 46 is between
about zero
and about 0.5 percent by weight. Suitable digestive additives 46 for use in
the present
invention include sodium hydroxide and potassium hydroxide.
From mixer 42, a stream 48 composed of straw chips 40, steam and water 44,
and any digestive additives 46 is then fed into reactor 50. In reactor 50,
stream 48 is
exposed to temperatures and pressures sufficient for the steam 44 and any
digestive
additives 46 to penetrate the straw chips 40 such that when exposed to a rapid
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decrease in pressure, a mixture of softened chips, cellulose, lignins,
hemicellulose and
water is formed.
Suitable reactors for use in the present invention include continuous feed
reactors such as those manufactured by Stake Technology, Ltd. of Oakville,
Canada,
and described in U.S. Patent Nos. 4,798,651 and 4,947,743, the contents of
which are
incorporated herein by reference.
In the present invention, reactor products 52 may be exposed to a rapid
decrease in pressure by passing the reactor products 52 into a blow tank 56.
The
reactor products 52 may be metered from the reactor 50 into the blow tank 56
by a
blow valve 54. The reactor products 52 include softened straw pulp mixed with
lignins,
hemicellulose and water. In other words, straw chips are steam exploded as
they pass
through the blow valve 54 when exiting the reactor 50 to form straw pulp,
lignins and
hemicellulose.
Stream 58 from the blow tank 56 is sent into a screw press 59 where stream
126 of a straw pulp mixture is separated from the steam exploded straw chips.
Stream
12b is composed substantially of black liquor and undissolved solids.
Generally,
stream 126 is substantially composed of water, reacted lignins and
hemicellulose.
Stream 126 is sent to a byproduct chest 142. Stream 144 from byproduct chest
142 is
passed through filtration system 128. Solids 130 removed at filtration system
128 are
sent to blow tank 56. Liquid stream 138 from filtration system 128 are
collected in
storage tank 140.
The black liquor collected in storage tank 140 constitutes a usable byproduct.
This byproduct has usefi~lness as an animal feed additive. In a preferred form
of the
invention, the byproduct may be mixed with meal 38 from preprocessing to form
animal feed. The animal feed may be formed into a mash or pellets by methods
known
in the art.
Stream 61 from screw press 59 is then defiberized. The defiberization may be
carried out by a mechanical refiner 60, such as an Ahlstrom lVIDR
(frotopulper).
The pulp is then ready to be washed. The pulp may be washed by any method,
such as dilution and extraction. For example, the refined pulp mixture 62 may
be sent
to a dilution tank 64 where it is diluted with water 73. Then, diluted pulp
mixture 66
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from tank 64 may be thickened by use of a wash press 68. Weak black liquor in
stream
69 from wash press 68 is sent to blow tank 56.
Thickened pulp 70 may then be screened to remove unwanted particles. In
preparation for the screening, the thickened pulp 70 may be sent to a
prescreening
dilution tank 72 where it is mixed with water 73. Diluted pulp 74 from the
dilution
tank 72 may then be passed into a primary screening 76. Accepts 78 from the
primary
screening 76 may be collected in accepts tank 84.
Rejects 86 from primary screening 76 may be passed to a primary screen rejects
tank 90. Rejects 92 from rejects tank 90 may be passed to a secondary
screening 80.
Accepts 82 from secondary screening 80 may be collected in accepts tank 84.
Rejects
88 from secondary screening 80 are sent to drainer 146.
Drained stream 148 is sent to a secondary rejects tank 150. Stream 152 from
secondary rejects tank 150 is sent to a rejects defiberizer 154. Stream 156
from the
rejects defiberizer 154 is collected in a tertiary screen feed tank 158.
Stream 160 from
the tertiary screen feed tank 158 is passed through a tertiary screening 162.
Accepts
164 from tertiary screening 162 are collected in the rejects tank 90. Rejects
166 from
tertiary screening 162 are sent to the drainer 146.
The nature and substance of the instant invention as well as its objects and
advantages will be more clearly understood by refernng to the following
specific
examples.
EXAMPLE 1
Annual rye grass straw is obtained. The protein level of the straw is
determined
to be about three to four percent (3%-4%). The moisture content of the rye
grass
straw is determined to be approximately ten percent (10%). The annual rye
grass straw
is preprocessed and pulped according to the method described in the best mode
of this
application and as set forth herein.
Bales of the rye grass straw are fed into an agriculture! tub grinder where
the
bales are broken up and the straw chopped into pieces averaging roughly one
(1) inch
in length. The agricultural tub grinder has grate plates with round holes
sized between
about one half (0.5) inches and about two (2) inches.
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After exiting the agricultural tub grinder, the chopped straw is sent into a
disc
mill. The disc mill is a United Milling Systems mill which has milling gaps of
between
about 0.6 mm and about 1.0 mm. In the disc mill, the chopped straw is reduced
into
approximately one-half inch pieces. Also, the disc mill grinds most of the
hard node,
leaf and sheath of the straw into a powder. Ash and grit are determined to
compose
approximately two to five percent (2%-5%) of the straw fed into the tub
grinder.
After leaving the milling machine, the straw is passed into a vibrating six-
deck
screen of 35 mesh. The screen is a gyratory screen equipped with fine mesh and
ball
decks for cleaning fines and dirt from the surfaces of the screen.
Approximately
eighteen percent (18%) of the straw mixture is removed at the screen as meal.
Part of
the meal removed is the powder formed in the disc mill by the hard node, leaf
and
sheath of the straw. The remainder of the straw chips leaving the screen
averages
approximately one half ('/z) inch in size.
The meal has a protein level of about eight to thirteen percent (8%-13%). The
silica content is between about 8-15%. It is determined that the meal removed
at the
screen is usable as animal feed.
The preprocessed straw chips are then stored prior to pulping. The straw chips
are preprocessed at a site remote from the pulping system. The preprocessed
straw
chips are transported to the pulping system in container trucks.
The preprocessed straw chips are fed into a pulping system at a rate
consistent
with the capacity of the pulping system. The preprocessed straw chips are fed
into a
mixer where they are mixed with steam and water to achieve a moisture content
of
about 40-50% based on the charge of preprocessed straw chips to the mixer. The
preprocessed straw chips mixed with steam and water are fed into a Stake
Digester
steam explosion reactor. In the reactor, the preprocessed straw chips are
exposed to
pressure of about 160 psig for about five and a half (5'h) minutes.
Pulp from the reactor is metered into a blow tank by a blow valve. Pulp from
the blow tank is passed through a screwpress where black liquor is separated
from the
pulp. Then the pulp is defiberized or refined in an Ahlstrom frotopulper
mechanical
refiner. The refined pulp is sent to a dilution tank where it is mixed with
liquid at a
dilution factor of about 2.5. The diluted pulp is then thickened by use of a
wash press.
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The thickened pulp is screened in a two-stage pressure screen with about 0.010
inch
slotted plates.
It is determined that at least about seventy percent {70%) by weight of the
straw chips are formed into straw pulp.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI {Swedish
Technical Forestry Institute) between about 14 and 21. As a comparison, OCC
(pulp
derived from recycled corrugated cardboard) is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 500 and STFI of between
about 15 and about 22. Kraft pulp is determined to have a Canadian Standard
Freeness
(CSF) of between about 450 and about 750 and STFI of between about 21 and
about
26.
Straw pulp with a Canadian Standard Freeness (CSF) between about 200 and
about 600 and STFI between about 14 and about 21 is suitable to be blended
with
Kraft pulp and OCC to make liner board in standard mil! grades. The straw pulp
comprises about less than twenty percent (20%) of the liner board furnish.
EXAMPLE 2
Example 1 is repeated using about one-half percent (0.5%) by weight sodium
hydroxide in the mixer.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 3
Example 1 is repeated using about one percent (1%) by weight sodium
hydroxide in the mixer.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
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EXAMPLE 4
Example 1 is repeated using two percent (2%) by weight sodium hydroxide in
the mixer.
After exiting the process, the straw pulp is determined to have a Canadian
5 Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE S
Example 1 is repeated using about one-half percent (0.5%) by weight
potassium hydroxide in place of the sodium hydroxide in the mixer.
10 After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 6
Example 1 is repeated using about one percent (1%) by weight potassium
1 S hydroxide in place of the sodium hydroxide in the mixer.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 7
Example 1 is repeated using about two percent (2%) by weight potassium
hydroxide in place of the sodium hydroxide in the mixer.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 8
Example 1 is repeated allowing the preprocessed straw chips mixed water to
remain in the reactor for about two and one-half minutes.
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After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 9
Example 1 is repeated allowing the preprocessed straw chips mixed water to
remain in the reactor for about 4 minutes.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 10
Example 1 is repeated allowing the preprocessed straw chips mixed with water
to remain in the reactor for about six (6) minutes.
After exiting the process, the straw pulp is determined to have a Canadian
1 S Standard Freeness (CSF) of between about 200 and about 600 and STFI
between
about i4 and about 21.
EXAMPLE 11
Example 1 is repeated allowing the preprocessed straw chips mixed water to
remain in the reactor for about eight (8) minutes.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 12
Example 1 is repeated with the preprocessed straw chips mixed with water to
be exposed to a pressure of about 140 psig in the reactor.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
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EXAMPLE 13
Example 1 is repeated with the preprocessed straw chips mixed with water to
be exposed to a pressure of about 180 psig in the reactor.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
EXAMPLE 14
Example 1 is repeated with the preprocessed straw chips mixed with water to
be exposed to a pressure of about 200 psig in the reactor.
After exiting the process, the straw pulp is determined to have a Canadian
Standard Freeness (CSF) of between about 200 and about 600 and STFI between
about 14 and about 21.
While specific embodiments of the present invention have been shown and
described in detail to illustrate the utilization of the inventive principles,
it is to be
understood that such showing and description have been oil'ered only by way of
example, and not by way of limitation. Protection by Letters Patent of this
invention in
all its aspects is set forth in the appended claims and is sought to the
broadest extent
that the prior art allows.
*rB
