Note: Descriptions are shown in the official language in which they were submitted.
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SAWDUST PIJMPING A PRO(:ESSING
BACRGROU21D AND SUMMARY OF TIE I2~Vh'~lTION
There are many areas throughout the world
where a substantial volume of sawdust and like
5 cellulosic residues are available for the production
of paper pulp. While such raw material can effec-
tively be utilized to produce paper pulp employing
existing technology, the existinq technology is
expensive, troublesome, and has very little
10 flexibility. For instance, a typical treatment
system utilizable for producing chemical pulp from
sawdust is illustrated in U.S. Patent 3,475,271, and
a commercial version thereof is shown in a brochure
entitled "Kamyr Sawdust Systems Pass the 500,000
15 Tons Per Year Mark". Such systems include an
expensive vapor phase feeder, and maintain a
substantially atmospheric vapor phase at the top of
the digester vessel.
According to the present invention, a
20 method and apparatus are provided that eliminate the
vapor phase feeder utilized in conventional sawdust
treatment systems, thereby lowering maintenance
problems, and providing a number of treatment
options. According to the present invention,
25 sawdust may be treated in an upflow digester, or may
be treated in a hydraulically filled downflow
digester, or may be treated in a downflow digester
having a superatmospheric vapor phase. This
provides for greater thermal stability, which may be
30 very desirable in many situations.
The term "sawdust" as used in the present
specification and claims means sawdust and like
cellulosic materials, including wood residues, which
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are fine enough to react like small particles during
handling. This may be contrasted with the way that
large comminuted cellulosic particles - such as wood
chips - react during handling.
A key to the sawdust treatment process
according to the present invention is the unexpected
discovery that it is possible to pump sawdust
slurries having a consistency high enough to effect
practical chemical treatment thereof. That is,
10 according to the present invention it has been found
that it is possible to pump sawdust slurries having
a solids consistency of between about 10-25%, and
more desirably between about 15-22%. Pumping can be
accomplished utilizing commercially available
15 fluidizing, high speed, degassing, centrifugal pumps
and related systems such as shown in U.S. Patents
4,435,193 and 4,410,337, and sold commercially by
Kamyr, Inc. of Glens Falls, New York, and Kamyr AB
of Karlstad, Sweden, under the trademark "MC"
20 pump. The discovery that it is possible to pump
medium consistency (e.g. 10-25~) sawdust slurries is
surprising since the fine particles tend to act as a
solid, as opposed to the situation with pulp or the
like wherein the material is more flexible. Further
25 sawdust slurries do not filter well, and a sawdust
slurry with a consistency of as little as 10% solids
is virtually a semi-solid.
According to the method of the present
invention, sawdust is treated by: mixing the sawdust
30 with a liquid to produce a slurry having a solids
consistency of between about 10-25~ (preferably
about 15-22%); and pumping the slurry, without
dilution, to a treatment stage to ultimately produce
pulp. At the treatment stage, the slurry is heated
35 (either indirectly, or by direct mixing of high
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pressure steam with the slurry - as by utilizing a
fluidizing mixer), the slurry is introduced into the
vertical vessel, chemical treatment of the slurry
within the vessel is practiced, and the slurry is
5 discharged from the vessel. Preferably the vessel
is an upflow vessel and the slurry is pumped,
without dilution, and under the influence of the
fluidizing, high speed, centrifugal pump, up through
the vessel. Alternatively, the vessel may be a
10 downflow vessel, either a hydraulically filled
vessel or one having a superatmospheric vapor phase.
The chemical treatment in the vessel
facilitates breaking down, or effects breaking down,
of the lignin in the sawdust. For instance, a
15 sulfite treatment can be provided in the vessel,
with the slurry discharged from the vessel then
being passed to a refiner to produce a mechanical
pulp. Alternatively, the slurry may be subjected to
Kraft processing in the vessel, resulting in the
20 production of Kraft pulp. Irrespective of the
treatment process, the mixing of the sawdust with
liquid is preferably practiced by mixing the sawdust
with low pressure steam and a treatment liquid, and
then feeding it into a vertical chute, and pumping
25 the slurry from the bottom of the chute. The slurry
maintains substantially the same consistency (i.e. a
consistency between about 10-25%) without phase
separation (solid/liquid) throughout all the
treatment procedures.
The apparatus according to the invention
comprises: a means for entraining the sawdust in a
liquid; a fluidizing, high speed, centrifugal pump
such as disclosed in said patents 4,435,193 and
4,410,337; a vertical digester; a conduit between
35 the pump and the digester; and a direct or indirect
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heating means for heating the slurry before
introduction into the digester. The interconnection
between the pump and the digester consists of the
conduit, a valve means in the conduit, and the
5 heating means, the force provided by the pump
ultimately effecting passage of the slurry through
the digester.
It is the primary object of the present
invention to provide for the effective, simple, and
10 versatile treatment of sawdust to produce pulp.
This and other objects of the invention will become
clear from an inspection of the detailed description
of the invention, and from the appended claims.
BRIEF DESCRIPTION OF TOE DRAWINGS
FIGURE 1 is a diagrammatic view of
exemplary apparatus according to the present
invention, for practicing an exemplary method
according to the invention;
FIGURE 2 is a diagrammatic view of a second
20 embodiment of apparatus according to the present
invention; and
FIGURE 3 is a partial, diagrammatic view,
showing a third embodiment of exemplary apparatus
according to the present invention.
DETAILED DESCRIPTION OF_THE DRAWINGS
FIGURE 1 illustrates a component for a
preferred embodiment of an apparatus according to
the present invention, for effecting the production
of Kraft pulp. The apparatus in this embodiment
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comprises a conventional low pressure feeder 10 mounted
atop a conventional horizontal steaming vessel 12 (also
known as a steam mixer, having a horizontal axis mixing
screw therein), having a steam relief pressure con-
trolled vent 14. Sawdust is fed from a source to thefeeder 10, and is introduced into the vessel 12 wherein
it is entrained in liquid to produce a slurry. The
liquid may comprise white liquor from a Kraft process,
and additionally low pressure steam is introduced into
the vessel 12. The slurry discharged from the vessel 12
is preferably discharged into a vertical chute 16,
although it may instead be fed to a conduit connected
directly to the pump to be hereinafter described. In
the chute 16, a liquid level is maintained utilizing a
conventional gamma level indicator 18, which controls a
valve means 20 in a conduit 21 leading from the bottom
of the chute 16. A liquid, such as white liquor, is
preferably introduced into the chute 16.
The structures 10, 12, 14 and 16 comprise means
for entraining the sawdust in liquid, to produce a
slurry having a solids consistency of between about
10-25% (and preferably between about 15-22%). The
slurry has that consistency at the bottom of the chute
16, where the pump 22 is located.
The pump 22 is shown only schematically in
FIGURE 1, but in addition to pumping components thereof
it will have degassing components, as shown in U.S.
Patents 4,435,193 and 4,410,337. The pump 22 comprises
a fluidizing, high speed, degassing, centrifugal pump.
Typically, the impeller thereof
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'I;
6 ~z42()5S
2000-3400 rpm in order to effect fluidiæation of the
slurry. Such a pump 22 is commercially available
from Kamyr, Inc. of Glens walls, New York and Kamyr
AB of Karlstad, Sweden, under the trademark "MC"
5 pump.
Operatively connected to the conduit 21 is
a heating means 24 and a vertical vessel 26. The
heating means 24 in the embodiment illustrated in
FIGURE 1 comprises a mixer for directly mixing high
10 pressure steam into the slurry. Various forms such
a mixer may take are shown in Canadian Patent
1,102,604, such a mixer comprising a fluidizing
mixer. A typical mixer that may be utilized for the
heating means 24 is sold by Kamyr, Inc. and Kamyr AB
15 under the trademark "MC" mixer.
The vessel 26 comprises a continuous
digester, and in the embodiment illustrated in
FIGURE 1 is an upflow vessel having a conventional
discharge mechanism 28 at the top thereof. In the
20 vessel 26 chemical treatment of the slurry may be
practiced. A Kraft delignification process is
practiced in the vessel 26, white liquid -
introduced during slurrying of the sawdust -
providing an active alkali charge which
25 conventionally will be in the range of 9-24~ Na20 on
bone dry raw material. The heating means 24 brings
the slurry up to optimum cooking temperature
normally in the range of 140-175~. The pump 22
brings the slurry up to normal treatment pressure,
30 which is typically 600-1200 kPa, and the slurry is
maintained at that pressure, and is maintained in
the digester 26 for a time period of about 30-130
minutes. These values are merely exemplary values,
and they may vary somewhat depending upon the nature
35 of the sawdust and the particular subsequent
7 1;~g2055
treatment steps to be applied to the pulp, and the
ultimate pulp to be produced.
The embodiment illustrated in FIGURE 2 is
substantially identical to that illustrated in
5 FIGURE 1 except that heating of the slurry to
optimum cooking temperature is accomplished
utilizing an indirect heating means 30, and the
digester 40 comprises a downflow digester.
The vertical vessel 30 connected to the
10 pump 22 by the conduit 21 preferably comprises an
indirect heating vessel with a movable screen, such
as shown in u.S. Patent 4,368,628. In such an
indirect heating vessel with a movable screen, the
A slurry is maintained, without dilution, at
15 substantially the same pressure as it had when
discharged from the pump 22. Liquid is withdrawn
from the vessel 30 by the pump 32, passes throuqh
steam heater 34, and is recirculated back to the
vessel to be introduced at inlet 35. Thus, the
20 slurry is indirectly heated as it passes through the
vessel 30 and the amount of liquid withdrawn at 31
is substantially the same as the amount of liquid
introduced at 35, so that the consistency of the
slurry is not significantly changed during heating
25 utilizing vessel 30. The slurry is discharged from
conventional discharge mechanism 36 and passes
through conduit 37 to be introduced at the top 38 of
the downflow digester 40. Pulp as it is produced is
discharged from the bottom of the vessel 40
30utilizing a conventional discharge mechanism 42.
According to the present invention, there
is a good deal of versatility in the operation of
the downflow digester 40. Preferably the downflow
digester 40 will be hydraulically filled.
35Alternatively, it may have a superatmospheric vapor
8 lX~0~;5
phase at the top thereof, and noncondensable gases
may be vented - in this instance - utilizing vent
43. In either case, the thermal stability of the
process is greatly enhanced compared to the
5 substantially atmospheric digester utilized in
commercial prior art practices.
The apparatus utilized in FIGURES 1 and 2
is primarily useful for the production of chemical
pulp. The apparatus of FIGURE 3 is utilized for the
10 production of mechanical pulp, particularly chemi-
mechanical pulp (CMP), or chemithermomechanical
(CTMP) pulp. In this embodiment, the chemical
treatment vessel 46 is much smaller than the vessels
26, 40, since complete delignification does not take
15 place in the vessel. Typically, sulfite is added to
the slurry in the vessel 46 (for example by
entraining the sawdust with sulfite in the mixing
means and the slurry when discharged from the
vessel 46 in discharge line 47 passes to a
20 conventional refiner 48. The mechanical pulp
produced is discharged into line 49.
Typically, sodium sulfite at pH 9-10 is the
mild pretreatment liquid in the vessel 46, and the
temperature therein is at about 130-170C. However,
in some situations, no chemical need be added at
all, but rather merely by heating of the slurry with
high pressure steam (as in fluidiæing mixer 24) to
about 110-150C, fibrilization in refiner 48 will be
facilitated, producing thermomechanical pulp (TMP).
According to the method of the present
invention, sawdust is treated by mixing the sawdust
with a liquid (in steam mixer 12 and chute 16) to
produce a slurry having a solids consistency of
between about 10-25%, and preferably between about
15-22%; and pumping the slurry, utilizing
9 124~05~
fluidizing, high speed, centrifugal pump 22, without
dilution, to a treatment stage to ultimately produce
pulp. At the treatment stage, the slurry is heated
to optimum temperature, and introduced into a
5 treatment vessel. Heating may be accomplished
directly - as by mixing high pressure steam directly
into the slurry utilizing steam mixer 24 - or
indirectly, as by utilizing pressure diffuser 30.
In the treatment vessel, some chemical
treatment of the slurry is practiced. The chemical
treatment merely facilitates breaking down of the
lignin in the sawdust where a mechanical pulp is
ultimately to be produced. If a Kraft pulp is
ultimately to be produced, in the vessel the
chemical treatment breaks down the lignin in the
sawdust. The pulp is ultimately discharged from the
vessel, whether it be vessel 26, 40 or 46. The
chemical treatment takes place during upflow, or
hydraulically filled downflow.
Treatment in the vessel 46 may be a sulfite
treatment, with the discharge pulp ultimately be
refined in refiner 48. Treatment in the vessels 26,
40 is a Kraft treatment, with the Kraft pulp
ultimately being produced.
It will thus be seen that according to the
present invention a simple, effective, and versatile
method and apparatus have been provided for the
treatment of sawdust to produce pulp. According to
the invention, the vapor phase feeder, and
maintenance and cost disadvantages associated
therewith, has been eliminated at the same time that
versatility of the system has been enhanced.
While the invention has been herein shown
and described in what is presently conceived to be
~L24205~
the most practical and preferred embodiment thereof,
it will be apparent to those of ordinary skill in
the art that many modifications may be made thereof
within the scope of the invention, which scope is to
be accorded the broadest interpretation of the
appended claims so as to encompass all equivalent
methods and devices.
