Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION OF THE PRIOR ART
Historically, the two most common methods of pulping
have been the mechanical pulping operation in which wood chips
are mechanically abraded, usually by means of a s-tone wheel and
a chemical treatment wherein the wood chips are processed wi-th
sulfate or sul~ite baths. The mechanical system, while quite
inexpensive, leaves too much lignin in the resulting pulp, and
-the fibers are shorter than would be optimum for 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 ins-tallation 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 feed screw into a rotary feeder valve which feeds
the chips to a steaming tube operating at superatmospheric
pressure. 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.
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Following the treatment in the steaming tube which brings
the temperature of the material above the ligr~n sof~ening temperature,
the material passes tllrough 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.
The expense of a pressurized steam vessel which has a
screw or other discharge mechanism is high and the operation is
further complicated by the need for expansion joints, pressure seals,
and other pressure resistant equipment which signlficantly raises
the cost of the thermal-mechanical pulping system.
U. S. Patent No. 3, 921, 918 describes a method of
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
pre~Eater .
U. S. Patent No. 3, 661, 328 describes a pulp refining
system utïlizing a multi-stage disc refining. In the first stage, the
r efi~ng is carried out in a pressurized environment under moderately
elevated temperatures. Subsequent disk refining steps are carried
out under atmospheric pressure conditions, The multi-stage refining
process is said to result in a reduction in the bulk of the fiber
furnished
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There are commercial machines on the market which
involve continuously feeding wood chips to a steam heated chamber
where they are compressed and then passed to a screw conveyor
and ultilllately into a grinding apparatus.
There is also a chip refiner process utilizing pre-steaming of
the chips described in the "Pulp and Paper International" for January 1977.
One of the difficulties with thermo-mechanical pulping has
been the fact that the chips are darkened by exposure to high
pressure steam. What is more, the steaming is not always too
effective to remove substantial amounts of pitch from the wooc~
SUMM~RY OF THE INVENTION
The present invention provides an improved method and appa-
ratus for thermo-mechanical pulping wherein the wood chips are contacted
with gaseous steam at ambient pressure conditions tO thereby raise the
temperature close to that of the steam, approximately 212F (100C).
The steam treated chips are then compacted severely, at compaction
ratios of at least 2. 5 to 1 and preferably of about 2. 7 to 1. By using
a presteaming step ahead of the compaction, up to five times as much
pitch is removed as without the steaming treatment, The compaction
serves to reduce the moisture content to at least as low as 30~, usudlly
on the order of 25~ whereas previously it was common to leave moisture
contents up to 50~ The higher compaction also fractures the fiber bonds
in the chips to a greater extent, thereby reducing the amount of power
required in the first stage of refining.
In a preferred embodiment of the present invention the chips
are treated with a steam at atmospheric conditions for about 1 to 5
minutes and th!en are conveyed to a pressurized refiner operating at
a pressure of about 20 to 80 psi gauge (138 to 552 KPa). After
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compaction, it takes only a few seconds to heclt the chips to the
temperature necess~lry for proper treatment. This is accomplisllecl
l~y contacting the compac~ed chips with pressurized steam for a time
sufficient to raise the temperature to at leas~ the lignin softening point,
whereupon the chips which now have significant amounts of fractured
bonds between fibers therein are refined in a refining stage operating at
a pressure of 20 to 80 psi gauge (138 to 552 KPa). After treatment in
the refiner, the mixture of pulp and steam may be passed through a
cyclone separator from which the steam is vented and the pulp may be
passed to a dilution chest. Upon dilution with water, the pulp
suspension preferably goes to a disc type re~ner arranged to f urther
refine the pulp thus produced before being passed on into the
ancillary equipment preceding the headbolY.
BRIEF DESCRIPIION OF THE DR~WINGS
~ he single figure in the drawings illustrates somewhat
schematically a thermo-mechanical pulping and refining system
according to the present invention.
- DESGRIPTION OF THE PREFERRED EMBODIMENTS
` In the singLe figure of the drawings, there is illustrated
a chip bin 10 which is open to the ambient atmosphere and into
which there extends a manifold 11 carrying a plùrality 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. 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 l~ring the chips to a temperature approximating the
temperature of the steam (212F, 100C). After atmospheric
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s~eaming, the chips are delivered to a plug screw feeder 15
which isolates the atmospheric pressure of the chip bin 10 from
the superatmospheric pressuxe existing in a first refiner stage
16. The plug screw feeder 15 delivers the chips through a
vertically positioned tube lr and then into an inlet conveyor 18
o~ the first refiner 160 The retention time of the steamed chips
in the plug screw feeder 15 and the tube 17 is on the order of
only about 10 to 30 seconds but th~t time is sufficient to bring
the temperature of the steamed chips rapidly to the softening
range for lignin which i9 about 230 to 2450F (110 to 118C),
By heating the chips up close to 212F (100C) in an enviroNment
that does not darken the chips, it only takes a few seconds to
heat them to the lignin softening temperatureO Consequently,
the retention time at the higher pressure with correspondingly
higher temperatures i~ greatl~ reduced3 and ~his contributes to
less darkening of the chips.
By steaming at atmospheric pressure ahead of th plug
feeder 15, up to 5 times as much pitch i5 rcmoved from the wood .~ .
chips as sccurs when no ~teaming is used~ :
Steam chips are subjected to substantial compxession
whereby the moisture content is reduced to as low as 30% or
preferably as low as 25~, in con~rast to convenkional plug feeder
operation which reduces the moisture to about 50%. The higher
compaction used, on the order of at least 2.5 to 1 or preferably
2.7 to 1 a~ comparad to the normal compression ratio of 1.9 to 1
in these devices ~ractures ~onds between fibers in the chips to a
~reater axten~ than usual, enabliny the ~irst stage o~ refining
to be operated at a reduced power level.
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The first stage xefiner 16 operates at a pres~ure of
about 20 to 30 psi gauge (138 to 552 KPa). The refiner itself is
preferably a horizontal single disc refiner of the type available
commercially from the Beloit Corporation under their trademark
"Uni-Mount " ~ Basically, this type of reiner has a single dynam-
ically halanced disc driven by a synchronous or induction motor.
Duxing operation of the refiner, steam is generated and this ~team
can be conveniently used as a source for the steam in the line 13,
by providing a line 19 extending from the pressurized tube 17 into
the gate valve 14.
~ he refined material in the first refiner 16 is then
passed by means of a line 21 to a blow cyclone separator 22. In
passing through the blow cyclone 22, ~he material flow spirals
downwardly, releasing steam which is vented by means of a line 23.
The pulp may be passed by means of a dis~harge line 24 into a
dilution ~hest 25 into which dilution water is introduced by means
of a line 260 A pump 27 discharges the stodk ~olution rom the
dilution chest 25 through a line 28 under the control of a ~alve
29 into an optional refiner feed chest 30 which is essentially a
storage device. Pulp constituents from screens or cleaners or
other stages of ~he paper making process may be introduced into
the refiner feed chest 30 by means of a line 31.
Periodically, a pump 32 delivers the pulp to a second
refiner 33 which is preferably a double di~c type refiner wherein
a pair of refining discs cooperate with a rotor such that half of
the incoming stock passed through the rotor into the second set :
of di~so The stock passes in parallel across both ~ets of discs,
merges and then leaves the unit through a single outletO Thi~
type of refiner is a~ailable e~mm~r¢ially from the Beloit
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Corporation under the trademark "DD" refiners. Finally, the
pulp after treatment in the second refiner 33 passes into a
line 34 under the control Q~ a valve 35 from where it can be
sent to screens, cleaners, and the rest of the au~iliary
equipment preceding the headboxO
The inalusion of dilution chest 25 and feed chest 30
is not essential ~or all types of installationsO Instead, the
pulp may conveniently be passed directly from the cy~lone
separator to a counter-rotating refiner with atmospheric
discharge.
~ he genera ion of steam in the first refiner 16 i9
usually adequate to keep the inlet pres~ure to at least 20 psi
gauge with he excess st~am being v2nted ba~k to the abmo~pheric
chip bin or other points in the proces6. Thus, the plug screw
fee~r of the present invention eliminates the necessity for the
inlet rotary valve, steaming tube, cro~s conveyor9 and the other
special axpansion and support connections in a typical pressurized
system for thermo-mechanical pulping and refining.
It should be evident that various modifications can be
made to the described embodiments without departing from the scope
of the pre3ent invention.
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