Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method and apparatus for
the separation of knots, shives and/or tramp material from a
continuous pulp digester discharge line. In continuous pulp
processing systems, such as shown in U.S. Patent No. 3,427,218,
it is conventional to subject the digested pulp, after dilution
with water, to screening and refining operations for the purpose
of removing unde~ired, coarse or non-digested material and/or
tramp material. A conventional digester, though relatively
efficient, does not normally reduce 100% of the wood chips or
the like entering the digester so as to completely delignify
the fibers. Consequently, the pulp discharged from a digester
may contain up to 1% of undigested knots and possibly an
additional 1% of small matchstick-like undigested particles
known as "shivesn. Shives are wood chips or like particles
that do not fully digest in the digester cooking stage because
they were not fully impregnated in the liquor impregnation
stage, either because they were too large to allow full internal
liquor impregnation, they were of a different variety than was
contemplated in digester design and liquor selection or all air
was not steamed out of them prior to liquor impregnation. Tramp
material is normally rocks or metal objects broken from tree
chipping equipment that inadvertently are admitted to the
aigestion system along with the cellulose chips.
In conventional screening operations to remove the
knots, shives, etc. from the digested pulp, it is necessary to
dilute the stock from its normal 10-20~ (approximately) by
r
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weight consistency as discharged from the digester, to a
consistency in the range of 1-4% (approximately). At these
low consistencies, the stock is sufficiently diluted that it
is possible to remove the knots and shives from the pulp stock
by the use of various screening apparatus, after which the pulp
must be again rethickened by means of deckers to consistencies
in the range of 8-15% for storage and/or subsequent processing.
The screening operation is a wasteful one in that large amounts
of water and electric power (for pumping the larger volumes of
material) are consumed and much more machinery must be utilized
to dilute, screen and thicken (in the vicinity of $2-3 million
(U.S.) worth for a 500-1000 ton/day mill).
Dilution of the pulp material can be avoided through
the use of one or more refiners (in series or parallel) at some
convenient point in the digester discharge line (i.e., see our
Canadian Patent No. 976,012, dated October 14, 1975, entitled
"Method and Apparatus for Processing Pulp"). Refiners are
devices with parallel plates which rotate relative to each
other, both in opposite directions, or one being stationary
and the other rotating, which plates may have protrusions or
serrations formed on the faces thereof. ~he plates break the
knots and shives to small fragments. Although this procedure
does not result in the energy penalty associated with screening,
since the broken up shives and knots have not been cooked, they
cannot be bleached, and as such produce colored specks in paper
produced from the pulp, and conse~uently a lower grade of paper.
Also, under some circumstances, it is possible that tramp
material could harm the refiners, causing down-time thereof, or
the tramp material could cause adverse effects on the final
paper proauced.
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According to the present invention, the problems
inherent in each of the above conventional systems for removing
knots and the like have been solved. According to the present
invention, the energy already available in the pulp in the
digester discharge line is utilized to achieve the separation
of knots and the like from the digested pulp stock. The pulp
stock in the discharge line is passed directly into a cyclone
separator, wherein the heavier knots, shives and tramp material
move by centrifugal force to the outside periphery of the
cyclone to eventually migrate downwardly and out the bottom
outlet of the cyclone, while the pulp fibers flow upwardly and
out the cyclone central outlet pipe. Although the pulp stock
is 10-20% in consistency, at high velocity (which is inherent
in the digester discharge line) it becomes homogeneous, or more
fluid. With fluid properties, it can be handled in conventional
cyclone separators. Since cyclone separators require that the
flowable materi~l be at very high velocity, their use usually
is not practical for large volumes of high density material
since the energy expended for accelerating the flowable
ma~erial is enormous; however, according to the present
invention, the flowable material is already at high velocity
when it is discharged from the digester. Rather than letting
(indeed effecting) this high-energy state material to dissipate,
as is conventional in present digesting systems, the high-energy
state is utilized to solve hitherto unsolved problems.
Once the knots and shives are separated from the pulp
in the discharge line, they may be handled in one of several
ways: Where the pulp is to be bleached, uncooked material may
be passed through separators and washers ~o remove fibers and
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tramp material and then returned to the digester. Or, where
the pulp is not to be bleached, uncooked material may be pa~sed
back in line with the pulp after fragmentation tless energy is
utilized than if all the pulp had to be passed through refiners).
It is the primary object of the present invention to
provide for improved separation of knots, shives and the like
from digested pulp stock, and to provide a more uniform grade
of paper with less energy expenditure given a predetermined
percentage of knots and shives in dige-~ter discharged pulp
stock. This and other objects of the invention will become
clear from an inspection of the detailed description of the
inve~tion and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURES la and lb are schematic showings of
conventional prior art ways of dealing with knots and shives
in digested pulp stock; and
FIGURE 2 is a diagrammatic showing of exemplary
apparatus according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
~onventional assemblies for digesting cellulose
material and removing knots and shives therefrom are shown
schematically in FIGURES la and lb. In the apparatus of
FIGURE la, after chips have been treated in pre-impregnation
vessel 12 and have been impregnated, cooked and washed in
continuous digester 14, the resultant pulp stock passes through
blow unit 16 to dilution station 18. At station 18, the pulp
stock is thinned from about 10-20~ consistency to about 1-4%
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consistency, the knots and shives are re~oved by screening
apparatus 20 and deckers 22 or the like thicken the pulp back
to about 8-15% consistency, whereupon it is passed to a further
treatment station 24 for storage, bleaching or other operation
steps. A great deal of water is consumed in diluting the pulp
for screening and a great amount of energy is consumed in
transporting the large volumes of material that ensue from the
dilution process. Also, the dilution, screening and thickening
equipment is relatively expensive.
When the conventional apparatus of FIGURE lb is
utilized, from blow unit 16 the pulp stock is passed to a
refiner 26. Extra refiners 26 can be utilized too, if desired,
either arranged in series or in parallel with refiner 26. In
refiner 26, the knots and shives are fragmented and pass with
the rest of the pulp stock to further treatment station 24.
While relatively little energy is used in such an arrangement
in comparison to the arrangement of FIGURE la, the final pulp
stock has the uncooked knot and shive particles therein, and
these particles will not bleach, but rather leave colored spots
on the final paper produced. Also, all the pulp stock must pass
through the refiners 26, not just the knots and shives, and tramp
material in the pulp stock may shorten refiner life or have other
adverse consequences.
Exemplary apparatus according to the present invention
is shown in FIGURE 2, which apparatus does not have any of the
drawbacks of the prior art apparatus. According to the present
invention, chips are fed into chip bins 30 and are passed from
there through chip meter 31 and low pressure feeder 32 into pre-
steaming vessel 33. From vessel 33, they pass through chip chute
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34 into high pressure feeder 35, and from there to pre-
impregnation vessel 12. From vessel 12, the chips are fed to
the top of continuous digester 14, which may be one such as
shown in U.S. Patent No. 3,427,218. One or more flash tanks 36
are associated with the digester 14 and pre-steaming vessel 33.
Everything described so far is conventional.
After treatment (i.e., impregnation, cooking and
washing) in digestex 14, the resultant pulp stock is passed
through outlet 38 of digester 14. At this point, the pulp is
in a high-energy state, a 10-20% (approximately) consistency
material under a pressure as high as 200 PSIG. The pulp may
pass through blow unit 16 and then is introduced tangentially
into cyclone separator 40 of any suitable conventional type.
A blow unit 16 is only optional, there being no need to reduce
the high-energy state of the pulp before introduction into
cyclone 40, and the unit may be placed in line 42, if desired.
The material has fluid properties at this point and has high
enough velocity so that cyclone separation thereof is practical.
~he pulp fibers are discharged through cyclone upper central
outlet 42, while the knots, shives and tramp material are
discharged through the lower outlet 44 since the heavi~r knots
and shives move by centrifugal force to the outside periphery
of the cyclone and eventually migrate downwardly.
After discharge through outlet 44 of cyclone separator
40, the knots, shives and the like can either be passed through
line 45 to a conventional refiner 26 and then to blow tank 24 or
through line 47 to a washer 51 to remove fibers and then
directly back to the inlet to the chips bin 30. When tramp
material is expected, separation means 43 for separating the
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tramp material from the knots and shives should be utilized at
outlet 44. Such means 43 may be of any conventional type, such
as another cyclone separator, screening means or float and s~nk
equipment. If the quality of material from outlet 44 is bad
enough, all of it may be discarded.
When line 45 and refiner(s) 26 are utilized, the
fragmented knots or shives pass through line 50 back to line 42
for transport to further treatment station 24. While line 50
would not normally be utilized when subsequent bleaching of the
pulp stock was to be performed, the FIGURE 2 assembly, including
line 50, is advantageous over the FIGURE lb arrangement when
there is to be no subse~uent bleaching, since a lower volume of
material passes through refiner(s) 26, and thus the refiner(s)
may be smaller and less energy will be consumed thereby.
It will thus be seen that according to the method of
the present invention, knots, shives and tramp material are
separated from the pulp discharged from a continuous digester
by passing the pulp through a cyclone separator, and the
separated portions of the pulp are passed through different
treatment stages.
While the invention has been herein shown and
described in what is presently conceived to be the most
practical and preferred embodiment, 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 structures and methods.
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