Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
LOW VELOCITY AIR DENSITY SEPARATOR
FIELD OF THE INVENTION
The present invention relates to apparatuses and methods for
separating fractions of a particulate material in general. More particularly,
the present invention relates to apparatuses and methods for utilizing air to
separate components of a particulate material on the basis of differing
attributes.
BACKGROUND OF THE INVENTION
The separation of a particulate material into various fractions on the
basis of density is performed in many industrial processes. In the mining
industry, heavy minerals are concentrated from ores for extraction. In
agriculture, grain is separated from chaff and leaves are separated from
stalks by the use of a current of air which lifts the lighter chaff or leaves
away from the grain or stalks. In the wood pulping industry, a device known
as an air density separator has been employed to separate wood chips of
light colored wood from chips containing knots which are more dense.
The air density separator uses a vertical separation chamber through
which a stream of air is drawn with a velocity in the range of four to five
thousand feet per minute. Wood chips to be separated are metered by an
auger into the separation chamber where the high velocity air stream
disperses the chips evenly over the chamber. The more dense knots fall
through the uprising current of air and are rejected. The lighter chips are
drawn from the separation chamber by the flow of air and separated from
the air by a cyclone.
Recent concern with waste reuse and progress in recycling
post-consumer wastes have given rise to new and unique problems in the
separation of materials. Voluntary, and in some cases mandatory, recycling
has resulted in the collection and separation of a number of specific
post-consumer packaging materials which have been identified as being
constructed of a high volume material and thus likely candidates for
economic recycling. Materials which have been so identified are the
ubiquitous aluminum cans, glass bottles, plastic milk cartons and 1, 2 and 3
liter pop bottles.
The recycling of milk bottles and pop bottles has been identified as a
candidate for economic recovery. However, the value of the recycled
product is heavily dependent on its purity.
The recovery of high value materials from post-consumer wastes
plays a critical role in reducing the landfill disposal of post-consumer wastes.High value products such as aluminum cans, newspapers, and plastic can
reduce the cost of governmental subsidies and help finance the recovery of
other materials from the waste stream. Further, the production of and
marketing of the most valuable components of municipal waste creates a
market and social climate for recycled products which is key to the economic
recovery of a larger and larger fraction of consumer wastes. One major
problem in recycling post-consumer plastic bottles is the removal of the
labels, typically paper, from the plastic bottles.
What is needed is an apparatus and method for removing paper and
thin gauge plastic from post-consumer plastic bottles.
SUMMARY OF THE INVENTION
The air density separation apparatus of the present invention
employs a vertical air separation chamber. The vertical air separation
chamber is connected to a cyclone which in turn is connected to a fan. The
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fan draws air out of the cyclone which in turn causes air to be drawn up
through the open-bottomed separation chamber. In prior art air density
separators utilized in separating wood chips from wood knots, air is drawn
rapidly up through the separation chamber at four to five thousand feet per
minute. The wood chips are metered into the separation chamber through
an air lock or a supply auger. The auger dumps the chips into the high
velocity air stream where the high velocity air disperses the chips across the
separation chamber so that the rising stream of air may separate the chips
based on their density and cross-sectional area.
With the low velocity air density separator of this invention, the
separation chamber is somewhat longer and the air is drawn up through the
chamber at approximately seven hundred to eight hundred feet per minute.
Because of the relatively low velocity of the air, the air stream itself is
noneffective at dispersing the shredded plastic bottles and their associated
paper labels evenly into the air stream. In order to achieve the even
distribution of the shredded bottles and the labels into the air stream, a grid
of closely spaced narrow bars extends into the separation chamber. The
bars are cantilevered into the separation chamber and are caused to vibrate
by an oscillatory mounting. The shredded material may be fed by an
ordinary chute without an air lock onto the deck of the grid of bars. Air
moving rapidly between the bars lifts and separates the various constituents
of the shredded bottles. The denser plastic walls of the bottle fall down
through the bars and are recovered as the heavy recyclable fraction of the
bottles. The lightweight paper is drawn up through the separation chamber
and into the cyclone. The cyclone removes the lightweight paper from the
air stream and air is drawn from the cyclone by a fan. In tests with material
of a bulk density of 18.5 pounds per cubic foot, the air density separator of
this invention is esli"~ated to remove ninety-five to ninety-eight percent of
the paper from a feed of shredded plastic bottles with a loss of plastic with
the paper of only zero to one percent.
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.
It is a feature of the present invention to separate shredded paper
from shredded plastic.
It is another feature of the present invention to purify recycled plastic
from post-consumer waste.
It is a further feature of the present invention to provide an air density
separation apparatus for separating sand, dirt and wood dust from wood
chlps.
It is a still further feature of the present invention to provide a method
wherein post-consumer plastic waste may be purified for recycling.
It is a yet further feature of the present invention to provide an
apparatus for feeding and distributing a granular material into an air stream.
Further objects, features and advantages of the invention will be
apparent from the following detailed description when taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side-elevational somewhat schematic view of the low
velocity air density separator of this invention.
FIG. 2 is an isometric view, partly cut away, of the separation
chamber and infeed mechanism of the low velocity air density separator of
FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring more particularly to FIGS. 1-2 wherein like numbers refer to
similar parts, a low velocity air density separator 20 is shown in FIG. 1.
The air density separator 20 has a vertically disposed conduit 22
which defines a vertical air separation chamber 24. Mixed particulate
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matter 44 is introduced into the separation chamber 24 from a materialhopper 58. The air separation chamber 24 is connected by a duct 26 to a
cyclone 28. The cyclone is connected to a fan 30. The fan draws air from
the cyclone 28 which in turn draws air through the duct 26 which causes a
stream of air indicated by arrows 32 to enter the bottom 34 of the conduit
22.
The mixed material 44 is discharged from the hopper 58 along an
inclined chute 60 onto a foraminous screen formed by a grill 36 disposed
within the separation chamber 24. The grill 36 has a multiplicit,v of closely
spaced narrow bars 38 which extend across the conduit 22 between a
material inlet 40 and a trash outlet 42. The grill 36 is cantilevered from a
mount 46 which resiliently supports the grill 36 on springs 48. A
ferromagnetic member 50 is mounted to the grill 36 and is driven by a
solenoid 52 to cause the grill 36 to vibrate at about sixty Hertz. Certain
material will be entrained in the upwardly moving air and will leave the
separation chamber through the duct 26. The remaining particulate material
which is not entrained and which is of a size to pass through the grill 36 will
exit the separation chamber 24 through the bottom 34 of the conduit 22 and
will be collected on a conveyor 35.
In a conventional air density separator, air is drawn up through the
separation chamber at four to five thousand feet per minute while the
granular material to be separated such as wood chips is dispensed into the
air chamber either by a chute with an air lock or by an auger which
distributes the material across the separation chamber. In a conventional air
density separator the high velocity air stream moving up through the
separation chamber is effective to disperse the granular material being
separated in the air stream. Materials which are sufficiently dense fall down
through the separation chamber whereas lighter materials become entrained
in the air and are drawn into a cyclone where they are separated.
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An air density separator separates a particulate matter depending
on what is known in the aerodynamic field as ballistic coefficient. Ballistic
coefficient is a function of the density of the object, the area of the object
presented to the air stream, and a shape-dependent coefficient. Thus, the
ballistic coefficient of an object increases with its density, decreases with
increasing area and decreases with increasing bluntness of the object facing
the air stream. Ballistic coefficient controls the maximum rate at which an
object will fall through a still column of air. Because the resistance of an
object through the air increases with velocity, an object which is accelerated
by the earth's gravitational force eventually reaches a velocity where the
acceleration force of gravity is balanced by the drag force of the air through
which the object is moving.
This principal is used to separate the granular material into two or
more components based on the ballistic coefficient of the granules. By
introducing the granules into an upwardly moving stream of air which has a
velocity which is greater than the terminal velocity of some of the particles
and less than the terminal velocity of other particles, the granular material
will be separated into two fractions. Thus, for separating wood chips from
wood knots, an air velocity in the range of four to five thousand feet per
minute is chosen which exceeds the terminal velocity of the wood chips,
thereby causing them to rise to the top of the air chamber and be
transported through a duct to a cyclone. On the other hand, the knots,
which have a terminal velocity greater than four to five thousand feet per
minute, fall through the air to exit the bottom of the separation chamber.
An exemplary problem addressed by the low velocity air density
separator 20 is separating shredded paper from shredded plastic. The
recycling of post-consumer plastic bottles has resulted in a feed stock
formed by the shredding of plastic milk bottles or plastic pop bottles. The
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feed stock contains both plastic from the bottles and paper from the labels
associated with the bottles. In order to make the feed stock a product with
an economic value, it is necessary to separate the paper from the plastic.
Because the plastic shards 54 as seen in FIG. 2 are of a thicker gauge of
material than the paper or light grade plastic labels, they have a higher
ballistic co-efficient and can be separated in theory in an air density
separator. However, both the plastic and the paper are of relatively low
ballistic coefficient and so the velocity of the air in the air density separator
must be in the range of five hundred to a thousand feet per minute,
preferably in the range of seven to eight hundred feet per minute. The
problem with these low velocities can be readily demonstrated by taking a
handful of paper confetti such as the punchings from a paper punch and
dropping them into the air. Some of the paper punchings will become
dispersed and rapidly reach their terminal velocity and slowly settle to the
floor. Others, however, will clump together and fall as a unit reaching the
floor first. Thus, it is observed with lightweight materials, they must be
adequately dispersed in the column of air moving up through the vertical
separation chamber 24 if it is desired to reliably separate them on the basis
of their ballistic coefficients.
In the air density separator 20 proper dispersion is accomplished by
the grill 36 formed of closely spaced narrow bars 38. In a chamber having
dimensions of approximately fourteen inches by twenty-six inches, the bars
38 would have a depth of one and a half inches with a thickness of one and
a half to three millimeters and a bar to bar gap of between one-eighth and
one-fourth of an inch when used with a shredded material 44 having an
average size of one-quarter inch to one half inch.
The bars 38 are formed into the grill 36 within a frame 64. One or
more transverse reinforcements (not shown) may be installed on the
underside of the grill 36 formed by the bars 38.
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A low velocity air density separator 20, as shown in FIG. 1, was
constructed with a fan 30 of five horse power capability. Table 1 lists the
performance parameters of the five horse power fan. The suction pressure
head was measured at the fan 30. This measurement was used to estimate
the velocity in feet per minute and the flow rate in cubic feet per minute
through the separation chamber 24. Tests were run with shredded plastic
containing paper to determine the optimal fan operating level which would
effect a clean separation between the paper and the plastic.
TABLE 1
Suction Pressure HeadAir velocity (Ft./Minute) Flow rate (CFM)
1 " of H2O 869 2,140
@ 2" of H2O 825 2,030
3" of H2O 784 1,530
~4" of H2O 739 1,800
5" of H2O 695 1,710
6" of H2O 656 1,615
7"of H2O 609 1,500
~ 8" of H2O 559 1,375
For the particular system employed, which has an air separation chamber
24 with internal dimensions of 13.75 inches by 25.75 inches, a static head at
the fan of four inches of water was found to produce a good separation
between the paper and the plastic.
As shown in FIG. 2, shredded plastic and paper is fed on the chute
60 onto the deck 62 of the grill 36. The chute 60 extends partially over the
grill 36 within the separation chamber. To prevent buildup of material on
portions of the grill not within the chamber a cover 63 may be provided. The
vibrating grill 36 disperses the granular material across the deck. The air
stream which passes up through the bars 38 of the deck lofts the lightweight
paper 56 and entrains it in the flow of air. The heavier plastic 54 slides
through the bars and drops out the open end of the duct 22.
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TABLE 2
PAPER PLASTIC
1500#/HR 5.8% 94.2%
2000#/HR 5.7% 94.3%
2500#/HR 5.3% 94.7%
Table 2 summarizes the results of three tests which were run with
twenty pound samples in the air density separator 20. When a shredded
mixture of plastic and paper having a bulk density of 18.5 pounds per cubic
foot was fed at a rate of fifteen hundred pounds per hour into the separation
chamber, 5.8 percent of the material was recovered from the cyclone as
paper and 94.2 percent was recovered from the bottom of the separation
chamber and consisted of plastic.
Similarly, the test was run at feed rates of two thousand pounds per
hour and twenty-five hundred pounds per hour. A slightly lesser amount of
paper was recovered at the higher rates. It appears separation of the paper
from the plastic is slightly less effective at higher rates. Visual inspection of
the separated plastic and paper indicated that approximately ninety-five to
ninety-eight percent of the paper was removed from the plastic and only
zero to one percent of the plastic was lost with the removed paper.
The air density separator 20 inlet 40 does not require an air lock
because of the relatively low velocity of the air. The relatively small effect
that openings in the wall 70 of the conduit 22 have on the stream is utilized
to allow an oversize tray 72 to extend from the deck 62 of the grill 36
through the wall 74 opposite the inlet wall 70. Trash which has become
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included in the granular material 44 traverses the sloped grill 36 and exits
the duct 22 on the chute 72 which directs the trash for collection in a reject
bin 78, as shown in FIG. 1.
The cyclone 28 uses centrifugal forces to separate the majority of the
particulate material from the air stream. The cyclone has an air lock 80
which allows the paper to be removed from the cyclone. The air that is
withdrawn from the cyclone passes through the fan and then into a bag
house (not shown) where any residual dust is removed before venting to
the atmosphere.
It should be understood that the low velocity air density separator 20
may employ a foraminous member of configuration other than a grill of
narrow bars. For example, the foraminous member could be a vibrating
screen, or a vibrating plate with holes punched therein. In addition, the
foraminous member could consist of an interdigitating bar screen with
alternating bars oscillating one hundred eighty degrees out of phase with
respect to adjacent bars.
It should also be understood that although a separation chamber 24
of approximately 10 feet in height has been illustrated, the separation
chamber may be shorter or longer.
It should also be understood that the low velocity air density
separator may be used to separate products other than shredded
post-consumer plastic containers. For example, the density separator 20
has utility for separating dirt and sand from wood chips.
It should be understood that wherein the term vibration is used, it is
not limited to the vibratory action in a vertical plane produced by the
solenoid arrangement shown in FIGS. 1 and 2 but encompasses vibrating in
all planes and oscillatory motion such as employed by a bar screen.
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It should be understood that although coil springs are shown
resiliently mounting the grill foraminous member 36 for vibration, other
mounts, for example leaf springs, are acceptable.
It should further be understood that wherein a solenoid driven by sixty
hertz line frequency causes the foraminous member to vibrate at sixty hertz,
the grill 36 could be caused to vibrate at other frequencies and other
mechanisms for causing the vibration could be employed including a drive
employing eccentric weights, cam followers on a crank shaft, piezoelectric
actuators and systems caused to vibrate by high amplitude low frequency air
pressure waves including sound waves.
It is understood that the invention is not limited to the particular
construction and arrangement of parts herein illustrated and described, but
embraces such modified forms thereof as come within the scope of the
following claims.
