Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
Waste papers have for many years been an important
source of fiber for use, or rather for reuse, in the manu-
facture of new paper products, and waste paper materials of a
variety of types and grades are recognized commodities for
purchase by paper and paper board mills. There has in recent
years been increasing emphasis on such recycling of used paper
materials, and this emphasis has now extended to the recovery
of reusable paper fiber from municipal solid waste, by means
of the system and method disclosed in my assignee's recent
patent No. 3,736,223.
The recycling of commercially available waste paper
has always presented some problems of cleaning and screening
because the economics of collecting, sorting and baling of
used paper products commonly result in the inclusion of sub-
stantial quantities of inorganic contaminants as well as
plastic, rags and the like. The high specific gravity con-
taminants of waste paper mixtures are not difficult to
separate from the paper, by conventional junk removers and
centrifugal cleaners. The more serious problems derive from
the plastic and other trash materials which have specific
gravities so close to that of cellulose fiber that gravity
separation is not effective, and which now commonly average
as high as 15% by weight of commercial waste paper.
More specifically, pulpers which are commonly used
for initially breaking down waste paper are quite efficient
for slushing but less so for complete defibering, and much
longer pulping time is required if the pulper is relied upon
to reduce the paper to particle sizes capable of passage
through relatively small holes, e.g. 3/8 inch in diameter
or less. Under such conditions, however, the plastic
constituents of the mix will not be correspondingly reduced in
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particle size, and they will tend to accumulate in the pulper,
and to interfere with proper extraction of the defibered paper.
This in turn will result in such loss in the effectiveness
of the pulper, as well as such excessive power, waste of that
frequent stops for cleaning out the pulper become necessary.
When attempts were made in the past to avoid this
problem by the use of larger extraction holes from the pulper,
other difficulties resulted. For example, since a portion of
the paper charge is not completely defibered, many paper pieces
will be comparable in size to the fragments of plastic and
cannot readily be separated by screening. This problem can
be overcome by subjecting the mixed fibers and fragments to
treatment in apparatus of the type known as deflakers which
will defiber the paper without comparable reduction of the
particle sizes of the plastic, but such treatment is inefficient
because the defibering apparatus must handle both the plastic,
the paper pieces, and the already defibered paper, with resulting
wasteful consumption of power.
The most effective system and method previously
developed for dealing with this problem are disclosed in my
assignee's U.S. patent 3,873,410 (Chupka). According to that
patent, the different stations in the system are so arranged
that the pulper is utilized only for slushing rather than
for complete defibering of the paper, the slushed and
centrifugally cleaned slurry is then screened at high
consistency and with a high reject rate to separate such
paper as has been essentially defibered from the plastic
and remaining paper, and the material rejected by the screen
is then directed to a deflaker from which the accepted
material is recycled in the system.
Summary of the Invention
In a system according to patent 3,873,410, the pulper
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is provided with extraction plate holes of relatively large
diameter, typically one inch or more, and the entire output
of extracted stock is subjected to a fine screening operation
which accepts the fully defibered material. All of the reject
material, which includes substantial quantities of good fiber,
is defibered in a deflaker and then screened in a tailing
screen, from which the accepted stock is either returned to
the supply line to the initial screening station or back to
the pulper. This system and method have proved to be definitely
superior to previous approaches to the problem, especially for
recovering fiber from municipal solid waste.
The present invention provides improvements and
advantages over the system and method of patent 3,873,410 while
retaining all of the advantages which that patent taught over
the previous art. More specifically, this invention derives
from recognition that a substantial portion of any given charge
of waste paper furnish to a pulper will be substantially
completely defibered relatively quickly, and with much lower
power requirements, than all of the paper in the charge, if
provision is made for continuously removing plaster and other
lightweight trash which can interfere with efficient pulping and
extraction.
The corollary of this principle of the invention is
that whenever a pulper is equipped with sufficiently large
extraction holes for continuous removal of the lightweight
trash, a substantial quantity of essentially defibered paper
will be entrained therewith, and will therefore be subjected
to whatever defibering operation is carried out thereon. In
addition to possible over-refining, this tends to diminish the
efficiency of the deflaker, to the extent that it is forced
to work on material which needs no further defibering~
In accordance with the invention, the pulper is
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equipped with two separate extraction plate areas having
materially differently sized holes. One area has relatively
small holes, preferably in the range of 1/8 - 3/8 inch and
typically 3/16 inch in diameter, and it communicates with an
extraction chamber from which stock is continually drawn to a
chest of suitable size for surge capacity. The other area has
substantially larger holes, preferably in the range of 1/2 to
2 inches in diameter and typically 1 inch in diameter, and it
communicates with a separate extraction chamber.
The first fraction of stock, which is extracted
through the small size extraction holes, will consist primarily
of essentially defibered paper and a minimum of plastic and
other floating trash. The second fraction will also include
a substantial quantity of good fiber, in both fully and
incompletely defibered form, but it will contain the major
proportion of plastic sheet or film and other floating trash.
Preferably the sizes of the holes in the second extraction
plate will be such as to promote extraction of the floating
trash on a continuous basis so that it does not clog the pulper
or unnecessarily load the rotor.
This second fraction is desirably first subjected
to centrifugal cleaning to eliminate high specific gravity
contaminants, and it is then screened in a screen having
perforations of no bigger, and preferably somewhat smaller,
size than the small extraction holes in the pulper i.e.
perforations in the range of 1/16 - 3/16 inches and typically
1/8 inch in diameter. The stock accepted by this screen will
comprise primarily essentially defibered paper material, and
it is therefore conducted to the same chest as the stock
extracted through the first extraction plate, for mixture
therewith.
The material rejected by the screen will include
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substantially all of the light trash from the pulper, but it
also will include a considerable quantity of reclaimable but
still undefibered paper material. It may be delivered to a
deflaker for further defibering action, with the output of the
deflaker being supplied to a tailing screen having holes of
relatively larger size than those of the first screen and
intermediate the sizes of the smaller and large extraction
holes in the pulper, preferably in the range of 1/4 to 1/2
inches in diameter. The stock accepted by this screen is
recycled to the pulper, and the reject from the screen, which
is almost entirely plastic and other light trash, may be
discarded. Alternatively, the deflaker may be omitted from the
system, in which case the screen will still be used in the same
way to separate the light trash and return the reclaimable
paper material to the pulper.
The advantages of this system and method over patent
3,873,410 include the fact that the ma~ority of the extraction
plate area can be devoted to small holes for extraction of
stock which requires no further defibering. At the same time,
the large hole extraction system provides for continuous purging
of the pulper of light trash which would otherwise tend to
interfere with the small hole extraction. In addition, with
only a minor portion of extraction plate area having large
holes, the flow to the first screen is correspondingly reduced,
as well as the load on the deflaker if one is used, and in
either case, recirculation of defibered stock to the pulper is
reduced to a minimum. This all results in increased efficiency
at each station and an overall higher production rate of usable
fiber.
Brief Description of the Drawing
The single figure is a flow diagram illustrating an
app3ratus system embodying and for carrying out the process of
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the invention.
Desc~tion of the Preferred Embodiment
The major piece of equipment in the system shown in
the drawing is a pulper comprising a treatment vessel or tub lO
wherein a rotor ll rotates above an extraction area composed of
a plate 12 provided with holes of relatively small size,
namely in the range of l/8 - 3/8 inch in diameter, and a second
plate 13 provided with substantially larger holes, namely in
the range of 1/2 to 2 inches in diameter. As shown, the plate
12 is of substantially larger area than plate 13 and may
advantageously occupy as much as 90% of the total extraction
area. Each of the plates 12 and 13 communicates with a separate
extraction chamber therebelow, and each of these extraction
chambers is in turn provided with its own discharge line 14
and 15 respectively.
A conveyor 20 carries the waste paper material to
the tub 10, and it is to be understood that this charge may be
selected from the full range of waste paper containing materials
from commercial "waste paper" to municipal solid waste. A
junk remover 22 is positioned adjacent the tub and communicates
therewith through a conduit 23. Water is continuously supplied
to the tub by a line 24 through the junk remover 22 and its
connecting conduit 23 into the tub.
Preferred examples of pulpers suitable for use in
this station include those shown in Felton et al patent No.
3,339,851 and Blakley et al patent No. 3,595,488, and details
of the junk remover are shown in Baxter patent No. 3,549,092.
The essential characteristics of the pulper are that it be
capable of subjecting mixed waste paper materials and water to
forces of mechanical and hydraulic sheer and mechanical impact
which will quickly and effectively reduce the solid material
in the tub to fragments capable of passage through the per-
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forations in extraction plate 12. Normally the pulper will beequipped with a ragger 25, also as shown in patent 3,549,092,
for removing stringy materials, large sheets of plastics, and
the like.
The pulper 10 operates continuously, with additional
waste paper and water being added at rates which will maintain
the solids content (consistency) of the mixture in the desired
range, 2 to 8~ being practical and 3 to 5% being preferred for
practical reasons. A first fraction slurry of water and de-
fibered paper of sufficiently small size to pass through thesmall holes in extraction plate 12 is continuously withdrawn
from the associated extraction chamber through line 14 by a pump
26 and supplied to a surge tank 30, from which the stock is
processed further through the necessary cleaning and fine
screening as desired in accordance with conventional practice.
A second fraction slurry of water and fragmentary material
which passes through the larger perforations in plate 13 is
continuously withdrawn through line 15 by a pump 33, and if
this slurry contains a significant amount of high specific
gravity contaminants such as metal and glass, it is preferably
delivered by pump 33 to a centrifugal cleaner 34 for removing
the high specific gravity contaminants to simplify the
subsequent screening as well as to prevent possible damage to
the screen.
The cleaned slurry from the cleaner 34 will contain a
considerable proportion of relatively large undefibered pieces
of paper as well as pieces of plastic and the like of comparable
size, but when the initial pulping operation is carried out
in a pulper of the characteristics noted above, the result
will also be that a substantial proportion of the paper which
has been substantially completely defibered will be extracted
with the light trash through plate 13. This slurry is there-
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fore subjected to a screening operation in a screen 35 whichwill result in acceptance of at least the majority of the
substantially completely defibered paper, and the rejection of
the undefibered paper fragments, plastic and other pieces of
reject material.
Highly successful results have been obtained in the
practice of the invention with the screen 35 constructed in my
assignee's U.S. patents 4,105,543 (Seifert) and 3,970,548
(Chupka and Seifert). The screen 35 is accordingly shown as
comprising a main housing 40 having a tangential inlet port
41 at the upper end thereof, an outlet port 42 for accepted
stock from which a line 43 leads to the surge tank 30, and a
discharge port 44 for reject stock.
Within the housing 40 is a screen cylinder 45, and
a rotor 46 operates within the cylinder 40 and carries vanes
47 spaced inwardly from the surface of the screen cylinder.
For the purposes of the invention, the perforations in the
screen cylinder 45 should be sized to accept only substantially
defibered paper and particles of similar sizes. Satisfactory
results have been obtained in the practice of the invention
with a screen constructed as described wherein these
perforations are smaller than the holes in extractor plate
12, namely in the range of approximately 1/16 to 3/16 inch in
diameter and preferably 1/8 inch.
The reject flow from screen 35 will therefore contain
the relatively minor amount of undefibered pieces of paper and
substantially all of the plastic and other light trash. This
reject flow is shown as conducted ~y line 49 to a deflaker 50
of any satisfactory construction such, for example, as a disk
refiner, and the flow from the deflaker 50 is supplied to a
tailing screen 55 of any suitable type for separating the
deflaked fiber and small undefibered pieces of paper from
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the larger pieces of plastic and other light trash. For
example, the screen 55 may be a vibrating flat bed screen of
the Jonsson type with perforations larger than screen 35 and
intermediate the sizes of the small and large extraction holes
in the plates 12 x 13, e.g. 1/4 to 1/2 inch. The water necessary
for operation of screen 55 is supplied by a branch 56 from the
water line 24 to the pulper.
Since the accepts flow from the screen 55 requires
further pulping, it is returned at 57 to the pulper tub 10,
and the reject is discharged at 58. It should be noted that
use of the deflaker is optional. For many installations it
may be omitted, and the reject line 49 from screen 35 may be
connected directly to the tailing screen 55, with the accepts
flow recirculated from screen 55 to the tub for the necessary
further defibering.
A major advantage of this invention is the provision
it makes for ~perating each piece of apparatus in the system
at the proper balance of load and efficiency. For example,
in a typical 300 ton per day system, the flow of essentially
defibered stock extracted through the small hole plate 12 will
average 265 tons at 3~ consistency, while the flow of trash-
carrying stock extracted through the large hole plate 13 and
delivered to the screen 35 will average 145 tons at the same
consistency. The screen 35 will therefore be required to
handle only about 35% of the total flow of extracted stock.
The accepts flow from screen 35 will average 75 tons
per day, at approximately 2.7% consistency, leaving only about
70 tons at 3.4% consistency for handling by the deflaker 50
and/or the tailing screen 55. On the average, approximately
15 to 20% of the flows from the tailing screen will be reject,
with the remainder being recirculated to the pulper. It should
also be noted that if the deflaker or tailing screen lacks the
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capacity to handle the full flow of reject from the screen 35,
that flow can be split accordingly, with the surplus being
recirculated to the pulper tub 10, as indicated by the broken
line 60 and 61. In the subsequent repulping, a major amount
of the recycled paper fragments will be sufficiently defibered
for acceptance by the small hole extraction plate 12 or the
screen 55, correspondingly reducing the loads on the deflaker
and tailing screen. Thus, with either arrangement, each
piece of equipment is required to handle only that portion of
the total load which requires the operation of that equipment,
which promotes efficient operation as well as minimal operating
cost and wear on the equipment.
While the methods and forms of apparatus herein
described constitute preferred embodiments of the invention, it
is to be understood that the invention is not limited to these
precise methods and forms of apparatus, and that changes may be
made therein without departing from the scope of the invention
which is defined in the appended claims.
