Note: Descriptions are shown in the official language in which they were submitted.
~136912
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention is in the field of thermo-mechanical
pulping methods and apparatus wherein wood chips are steamed
and then compacted before delivery to a refiner. The specific
improvement of the present invention centers around a conveyor
stage which receives the steamed and compressed wood chips
in the form of a compacted mass and shreds the same while
delivering it to the input of a refiner, thereby providing a
more uniform feed into the refiner stage.
DESCRIPTION OF THE PRIOR ART
The two most common methods of pulping until recent
times have been the mechanical pulping operation in which
wood chips are mechanically abraded, usually by means of a
stone wheel,and a chemical treatment wherein the wood chips
are processed with sulfate or sulfite baths. The mechanical
system is less expensive but results in fibers which are
shorter than would be optimum for use in various types of
paper making. The chemical process, on the other hand,
requires the use of large amounts of equipment such as high
pressure tanks and the like which are quite expensive. The
high initial cost of installation and the cost of operation
of the chemical processes are not-always justified.
In more recent times, a new technique known as
thermo-mechanical pulping and refining has become commercially
accepted. In a typical thermo-mechanical pulping operation,
wood chips from a chip bin are fed through a chip washing
system and then by means of a screw feed are fed into a
1136~Z
rotary feeder valve which feeds the chips to a steaming
tube operating at superatmospheric pressures. The material
is introduced into one end of the tube and is conveyed by
means of a screw type conveyor to the outlet end. During
their travel through the tube, the wood chips are contacted
with steam from multiple steam jets which maintain a
uniform steam pressure throughout the length of the tube.
Following the treatment in the steaming tube wherein
the temperature of the material is brought above the lignin
softening temperature, the material passes through a first
stage pressurized refiner to which there is coupled a blow
valve. The mixture of steam and pulp is then passed to a
cyclone separator and a conveyor system delivers the pulp
freed from the steam in the separator to a second stage
refiner which is non-pressurized. The pulp produced in the
second stage refiner is then passed into a stock chest and
then goes through the various other stages which are common
to any procedure for making up a suspension of fibers suitable
for use in a headbox.
U.S. Patent No. 3,921,918 describes a method for
mechanically refining which involves pretreating fibrous
material with steam in a preheater followed by refining the
steam treated material in a disc refiner under conditions
which generate steam, with at least a part of the developed
steam being directed back to the preheater.
U.S. Patent No. 3,661,328 describes a pulp refining
system using a multi-stage disc refining. In the first
stage, the refining is carried out in a pressurized environ-
ment under moderately elevated temperatures. Subsequent disc
~13691Z
refining steps are carried out under atmospheric pressure
conditions. The multi-stage refining process is said to
result in a red~ction in the bulk of the fiber furnished.
There is also a study entitled "Mechanical Pulp From
Chips" appearing in Tappi, Vol. 45, April, 1962 at page 257.
V. S. Patents Nos. 2,935,931 and 2,975,096 both
assigned to The Bauer Bros. Company deal with fiberizing
presses and the like using screw type conveyors.
In a typical commercial embodiment of the thermo-
mechanical pulping process, a plug screw feeder feeds
compacted, macerated chips to a first-stage refiner. Diffi-
culties have arisen because of the uneven feed presented to
the refiner, since the compacted material tends to remain
in the form of lumps of various sizes. When the compacted
chips are forced along the horizontal inlet pipe by material
behind them, the chips tend to drop spasmodically out of
the end of the pipe where the pipe meets the vertical steam
separation chamber. This causes large fluctuations in
the rate at which chips reach the refiner and consequently
causes substantial fluctuation in refiner loading. The net
result is that pulp quality is not uniform and this uneven
type of operation is generally considered to be unsatisfactory.
SUMMA~Y OF THE INVENTION
The present invention provides a method and apparatus
for correcting the difficulties encountered in the feeding of
thermo-mechanical pulp to a refiner. Specifically, the
present invention provides a method wherein the wood chips
are contacted with gaseous steam at a temperature and for a
time sufficient to bring the temperature of the chips close
1136~1Z
to that of the steam, the resulting steam treated chips are
compacted while being mechanically conveyed to reduce the
moisture content thereof and produce a compacted moist mass.
The compacted mass is then shredded by means of further
conveying under more severe conditions than the original
conveying and thereafter the shredded mass of fairly uniform
size is introduced into the inlet of a refiner.
A number of other features are present in the method
and apparatus of the present invention. For one, the chips
are preferably compacted at compaction ratios of at least
2.5 to 1 during the original conveying. The contacting
with gaseous steam preferably occurs at ambient pressure
conditions, and the initial compaction of the steam treated
chips is carried out until the moisture content is at least
as low as 30%.
Features of the apparatus of the present invention
include the use of first and second conveyor means which
each take the form of worm-type conveyors coaxially mounted
with respect to each other, with the second conveyor means
having one end received in the end of the first conveyor
means. In order to provide the shredding action, the second
conveyor may be provided with flights which have a larger
pitch than the flights of the first conveyor. Alternatively,
or in addition to this feature, the second conveyor can be
driven at a higher rate of speed. As a further embodiment
of the invention, the first and second conveyors may each
have oppositely oriented flights, and have drive means which
drive the two conveyors in opposite directions. The objective
to be achieved by the second conveyor is to tend to pull the
113691Z
compacted mass from the first conveyor at a rate which is
faster than it is being delivered by the first conveyor
thereby breaking up the compacted mass and feeding the chips
to the refiner at a more or less uniform rate.
BRIEF DESCRIPTION OF THE DRAWINGS
A further description of the present invention will
be made in connection with the attached sheet of drawings
which illustrate several embodiments thereof, and in which:
Fig. 1 is a partly schematic view of an overall system
for treating wood chips by the method and apparatus of the
present invention, from the time the chips are steamed to
the time they are passed to the refiner;
Fig. 2 is a view partly in cross section on an enlarged
scale illustrating the manner in which the two conveyor means
cooperate to feed the steamed and compacted pulp into the
refiner;
Fig. 3 is a cross-sectional view taken substantially
along the line III-III of Fig. 2; and
Fig. 4 is a schematic view of a modified form of the
invention illustrating oppositely rotating conveyor means.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, reference numeral 10 indicates
generally a chip bin which is open to the ambient atmosphere
and into which there extends a manifold 11 carrying a
plurality of steam jet lines 12. Steam at substantially
atmospheric pressure is received into the manifold 11 from
a line 13 under the control of a gate valve 14. Additional
sources of steam can be used if necessary or desired. `~
1136912
The residence time of the wood chips in the chip bin
during steaming is typically on the order of 1 to 5 minutes,
or at least sufficient to bring the chips to a temperature
approximating the temperature of the steam (212F, 100C).
After steaming under atmospheric conditions, the chips are
delivered to a plug screw feeder 15 driven by a motor 16.
In the plug screw feeder 15, the atmospheric pressuxe of the
chip bin 10 is isolated from the superatmospheric pressure
existing in a first refiner stage 16. The plug screw feeder
15 delivers the chips through a vertical steam separating
tube 17 into an inlet conveyor 18 of the first refiner
stage 16.
In the plug screw feeder 15, the steamed chips are
subjected to substantial compression whereby the moisture
content is reduced to as low as 30% or preferably as low as
25%, in contrast to conventional plug feeder operation which
reduces the moisture to about 50%. The higher compaction
ratios used, being on the order of at least 2.5 to 1 or pre-
ferably 2.7 to 1 as compared with a normal compression ratio
of 1.9 to 1 in these devices, fractures bonds between fibers
in the chips to a greater extent than normal, enabling the
first stage of refining to be operated at a reduced power level.
Under normal operating conditions in the plug screw
feeder 15, compacted, macerated chips are fed to the first
stage refiner 16. Difficulties have arisen with uneven feed,
however, because the compacted material tends to remain in
lumps of various sizes. When the compacted chips are forced
into the tube 17, the c~ips tend to drop at irregular
intervals into the inlet of the first refiner stage, and
1~369~2
this results in large fluctuations in the rate at which
the chips reach the refiner. Consequently, there are
corresponding fluctuations in refiner loading. The net
result is that pulp quality is not as uniform as would be
desired.
To compensate for this irregular feeding, the mass
of swollen, steamed wood chips is further broken up before
the wood chips enter the tube 17. To accomplish this,
an assembly of the type shown in Figs. 2 and 3 of the drawings
may be used. The end of the plug screw feeder 15 terminates
in a hollow frusto-conical portion l5a and an annular flange
15b. The flange 15b is connected by means of bolts 19 to a
flange 20a of a hollow frusto-conical coupling section 20.
This section has a marginal annular flange 20b which is
secured by means of bolts 21 to a uniform diameter coupling
section 22 having an annular flange 22a at one end abutting
the flange 20b, and an annular flange 22b at the opposite
end which is secured to an annular flange 17a of the tube 17.
In the plug screw feeder 15 there is a shaft 23 having
screw conveyor flights 24 which move the steamed chips after
rather severe compaction toward the tube 17 and ultimately
into the first refiner stage 16.
The end of the shaft 23 is suitably recessed to provide
space for accepting a sleeve type bearing 25 which receives a
reduced diameter end portion 26a of a stub shaft which is
positioned within a hollow extractor screw shaft 26. me opposite end of
the hollow shaft 26 is secured by means of weld deposits 27 to
a drive shaft 28 journaled for rotation within a bearing 29 and
113691Z
driven by a motor 30 through a speed reducer 31.
The hollow shaft 26 carries flights 32 of progressively
increasing radius as shown in Figs. 2 and 3. In one preferred
form of the present invention, the pitch of the screw flights
32 is larger than the pitch of the screw flights 24 of the
feeding screws. Consequently, if the shafts 23 and 26 are
rotated at the same speed, the flights 32 will tend to pull
away the compacted material being delivered by the flights
24 at a greater rate of speed than it is being delivered.
This action tends to break up the mass of steamed wood chips
into smaller particles which uniformly fall by means of
gravity through the tube 17 and end up in the inlet conveyor
18 feeding the first refiner stage 16.
It is also possible to drive the shaft 26 at a greater
speed than the shaft 23 in which case the pitch of the flights
32 and 24, respectively, can be the same. As a further
alternative, the speed of the shaft 26 can be greater than
the rotational speed of the shaft 23 and the flights 32 also
have a longer pitch than the flights 24. The important
thing is that the flights 32 provide a shredding action to
break up the compressed mass being delivered by means of
the plug screw feeder 15 into relatively small pieces.
The first stage refiner 16 operates at a pressure of
about 20 to 30 psi gauge (138 to 552 KPa). The refiner
itself is preferably a horizontal single disc re~iner of
the type available commercially from the Beloit Corporation
under their trademark "Uni-Mount". Basically, this type of
refiner has a single dynamically balanced disc driven by
a synchronous or induction motor. During operation of the
1~3~91Z
refiner, steam is generated and this steam can be conveniently
used as a source for the steam in the line 13 by providing
a conduit 34 extending from the pressurized tube 17 into the
gate valve 14. The refined material leaves through a line 40.
The embodiment of the invention shown in Fig. 4 can
also be employed satisfactorily. In this form of the inven-
tion, there is shown a shaft 35 forming part of the plug screw
feeder and having flights 36 forcing the compacted mass to
the right as shown in Fig. 4. The extractor screw makes use
of a hollow shaft 37 which is coupled to the plug screw
shaft 35 in the same manner as illustrated in Fig. 2. The
flights 38 in the extractor screw, however, are oppositely
oriented with respect to the flights 36, and the shaft 37 is
driven in the opposite direction from the shaft 35. The
shredding action still occurs as the material is delivered
from the flights 36 into the flights 38.
With the system of the present invention, there are
no stationary obstructions which would otherwise inhibit
the movement of the compacted chip mass. The end of the
extractor screw conveyor is supported by the end of the
plug screw feeder. The lateral load on the extractor screw
is not great and since the plug screw is of rigid construc-
tion and cantilevered into the compaction zone with a rigid
bearing, it is possible for the extractor screw to be thus
supported by the end of the plug screw.
In transferring the compacted steamed material from
the plug screw into the tube, it is important that there be
no abrupt changes in cross-sectional area through which the
chip mass must pass. The beginning of the flights on the
--10--
1136~a12
conveyor screw must have essentially a zero radial dimension
at the transition with the feeding screw, as best illustrated
in Fig. 3. Without this type of configuration on the end
of the shaft, the leading edge of the flight on the extractor
screw would be required to shear off a portion of the
compacted mass which will require much more power to drive
the extractor screw as well as subject it to much greater
lateral loads.
It should be evident that various modifications can
be made to the described embodiments without departing from
the scope of the present invention.
