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Description  |
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FIELD OF THE INVENTION
The invention relates to the separation and purification of plastics.
BACKGROUND
The production of plastics accounts for over $40 billion of annual product
sales and more than 3% of the United States consumption of oil and natural
gas. More than 90% of our production of these valuable materials is
discarded. This is a considerable waste of natural resources and imposes
an unwanted growing burden on people, cities, regions, agencies concerned
with management and conservation of resources and pollution, and of
course, ultimately on the environment. Improved collection, separation and
reuse of plastics would tend to alleviate worsening of these burdens. If
the collection, separation and reuse of plastics were sufficiently
improved, plastics recycling could become one of the largest raw materials
industries worldwide within a decade.
By generating over 80 billion pounds of material or $270 billion of
production per year, and being responsible for approximately 3.2 million
jobs, plastics and related businesses represent an extremely important
materials industry to the United States. Unlike other material industries
like steel and aluminum, however, this industry depends almost solely on
new sources of raw material, most of it imported petroleum. This
dependence becomes even more significant as the growth rate of plastics
continues to outpace that of all other materials. Wasting this important
material resource has significant international trade, economic and
environmental implications.
The US produces almost 20 billion pounds per year of valuable engineering
plastics for use in durable goods. These products are increasingly being
collected and recycled at the end of their useful lives to avoid disposal
costs and potential liabilities, and to recover metals and other
marketable raw materials. The engineering plastics contained in these
products are often one of the most valuable materials on a cost per pound
basis, yet most of this valuable plastic resource is therefore landfilled,
incinerated, or sent to Asia for recycling and reuse there.
Examples of the plastics recycling problem are evident in the case of so
called `disposable` plastic bottles and in durable goods. The main barrier
to the recycling of a majority of bottles is that separation is limited to
density-based systems which require significant pre-sorting by plastic
type at Material Recovery Facilities (MRFs), leading to insufficient
feedstock supply and poor economics. The main barrier to recycling of
plastics from durable goods, such as automobiles, appliances, and computer
and electronic equipment, is the multitude of plastic types and with
different grades of the same type of plastic, often with overlapping
densities, which must be separated. The re-use of such plastics, even if
they can be separated, is often complicated by their degree of
contamination, e.g. paint, metal film coatings and the like.
SUMMARY
Embodiments may include one or more of the following advantages. The
inventions enable the plastics to be separated from complex mixtures and
recycled with high purities that result in higher market values. The
recycling concept is certainly not new to plastics. Plastics have been
recycled and reused since the beginning of their commercial use. Scrap and
uncontaminated rejected parts generated from a manufacturing process are
shredded and reused, typically back into the same application. As with
other types of materials such as metal and glass, different types of
plastics must generally be separated from one another to achieve high
purity and consistent extruding or molding performance i.e., consistent
physical properties typically verified by standardized ASTM tests (Izod
impact, Deflection Test Under load (DTUL). Melt Flow Index (MFI) and the
like) and higher market values.
Plastic types include acrylonitrile-butadiene-styrene (ABS), flame
retardant (FR) ABS, ignition resistant (IR) ABS,
acrylonitrile-styrene-acrylonitrile (ASA), high density polyethylene
(HDPE), high impact polystyrene (HIPS), FR HIPS (a flame retardant HIPS),
IR HIPS (an ignition resistant HIPS), low density polyethylene (LDPE),
polyamide (PA), polybutylene terephthalate (PBT), polycarbonate (PC),
PC/PBT (a blend of PC and PBT), PC/ABS (a blend of PC and ABS), FR PC/ABS
(a FR blend of PC and ABS), polyethylene (PE), polyethylene terephthalate
(PET), polymethyl methacrylate (PMMA), polyoxymethylene (POM),
polypropylene (PP), polyphenylene oxide (PPO), polystyrene (PS), polyvinyl
chloride (PVC), PVC/ABS (a blend of PVC and ABS), styrene acrylonitrile
(SAN), styrene-butadiene rubber (SBR), styrene maleic anhydride (SMA),
thermoplastic polyolefin (TPO), thermoplastic polyurethane (TPU),
thermoplastic elastomer (TPE). Most plastics of different types are not
compatible with one another, and while some commingled applications have
been demonstrated, they capture much lower values than virgin plastic
because the significant physical properties and characteristics are much
less controlled, if at all, i.e. the plastics are of lower grade. With
lower grade or lower purity products, the processing and performance
flexibility afforded by purified single resin streams or compounded resin
combinations (co-polymers) of consistent characteristics is lost.
As important, perhaps, is the ability to separate different grades of the
same type (i.e., polymers built from the same monomer or monomers, but of
different molecular weight, different ratios of monomers, different
molecular morphology, different additive composition, concentration and
the like) of plastic. Different plastic grades (i.e. plastics of the same
type with a different range of properties) can have significant
differences in important physical properties: e.g., medium impact, low
gloss ABS and high-heat ABS.
Although an increasing number of bottles and rigid containers of all types
are being recycled, a significant improvement in collection and
reprocessing economics is needed for a majority of bottles to be recycled.
Other types of plastics packaging (film, coatings, and closures) are
recycled at a considerably lower rate than bottles. Durable goods (e.g.
buildings, automobiles, appliances, and computer and electronic equipment)
are gaining attention as a recycling opportunity as these types of
products are increasingly being collected at the end of their useful lives
by recyclers and manufacturers who recover useable components and metals.
Although more plastic is actually used in durable goods than in packaging,
technical barriers preclude their economical separation from these mixed
material streams using conventional methods.
The problem of separating different polymeric materials from each other is
the primary obstacle to economically recycling polymeric materials from
durable goods, particularly when they have similar or overlapping density
distributions. Durable goods are generally formed from a number of
different types and grades of polymeric articles arranged as separate
component sub-structures (pieces or parts) combined or attached into a
unitary item, e.g., a computer monitor with a case of one material having
several other sub-assemblies attached by glue, molding, or fasteners and
the like.
Most plastic parts coming from durable goods streams contain unique
challenges that are not met by the automated conventional plastics
cleaning and sorting processes developed for packaging materials. The
principle practice today for the recovery of highly contaminated scrap is
hand-separation, which is cost prohibitive in most cases. The challenges
in recycling plastics from durable goods include:
The plastics used in durable goods are more specialized than those used in
packaging. Whereas the majority of plastic packaging can be categorized in
five grades of plastic resin, more than fifty plastic resin grades might
be required to comprise a similar fraction of the durables market. For
example, while the PET plastic used to make a soda bottle may also be
appropriate for a water bottle, the acrylonitrile-butadiene-styrene
copolymer (ABS) used to make a computer housing is very different from the
sort used in a refrigerator door, which is different again from that used
in an automobile. This broad variety of materials increases the difficulty
of separation.
In addition to different plastic types, many parts contain a wide variety
of reinforcements, fillers, and pigments. Changing filler content and
foaming agents causes material density to vary even within the same type
of plastic.
Durable plastic parts often contain high levels of metal contamination,
including wiring brackets, structural pieces, and molded-in screw inserts.
Paint and metallic coatings (i.e., contamination) on some parts make
identification, sorting, and melt reprocessing much more difficult.
Larger and more variable thickness (i.e., parts having widely differing
morphology) wall sections, increases the challenges associated with size
reduction and particle size and shape control.
The apparent density of a plastic can be different from the intrinsic
density of the plastic especially when the plastic is "foamed". A foamed
plastic includes small bubbles or voids. The apparent density is often
lower than the intrinsic density because it includes a contribution from
encapsulated voids or vapor bubbles within material.
This invention relates to modifying the density or apparent density of
polymers, particularly polymers in a mixture of different polymers or
polymer grades, to effect purification and separation.
The invention relates to the separation and purification of plastics.
Specifically it is related to apparatus and methods of separating a
selected one or more members (selected plastic) of a mixture consisting of
divided plastics and partitioning the selected member(s) into respective
containers or output product streams separated from the balance of the
mixture and from each of the other selected members.
The invention also relates to apparatus and methods for providing separated
and partitioned output product streams (or separated containers) of
individual types or grades of divided plastic by separating and
partitioning plastics received from input recycling product streams of
different types and grades of bulk plastic articles obtained from
industrial and consumer product waste streams.
The invention specifically relates to mixtures that include a set of
discrete members of divided polymeric materials of different types and/or
grades. The divided polymeric materials may be prepared by dividing
substantially larger formed and shaped articles made essentially from a
single type or grade of plastic to enable separation or purification of
one or more of the selected members from the mixture, especially a mixture
of polymeric materials which is initially inseparable. The materials are
typically divided (e.g., shredded, granulated, or ground) into discrete
particles, flakes, shreds, i.e., free flowing. It would be advantageous to
provide plastics recycling plants capable of handling mixed post-consumer
plastic. When commercialized, similar plants could have a throughput
comparable to large virgin plastic production facilities. Plastics could
be recovered for reuse in similar or other applications. Plants could be
built to accept shredded or baled mixed-rigid plastic containers or
durable plastic goods. This could eventually make sorting of plastic waste
at curbside unnecessary and lead to increased utilization of other waste
materials such as paper, metals and glass by complementing the economics
of their reuse.
Accordingly, several objects and advantages of the present invention are:
to provide a means of altering or shifting the apparent density of a
polymeric material without degradation of the material;
to provide a means of altering the difference in apparent density between
two discrete polymeric materials in such a way as to enable the separation
of the two discrete polymeric materials;
additionally to provide a means of retaining the altered difference in
apparent density between two discrete polymeric materials in the absence
of the action or agent inducing the alteration;
to provide differentiation of polymeric material(s) within a mixed stream;
to provide a way of separating different component(s) of a mixed plastic
stream based upon apparent density or specific gravity;
to provide a way of recovering purified plastic types from mixed streams
containing different plastic materials;
to provide a way of improving the purity of a polymeric material by
removing plastics;
to provide a plastic density differential alteration system and process
i.e. differential plastic "foaming", for separating HIPS and ABS from
appliances and unfoamed PC, PC/ABS, IR-HIPS, FR-HIPS, FR-ABS, and IR-ABS
from computer and business equipment by shifting the density of one of the
plastics by at least 0.03 g/cc;
to provide a separation process and system incorporating the differential
density alteration process in combination with a de-foaming system and
process for separating foamed PC, PC/ABS, PPO, IR-HIPS, and IR-ABS by
selectively narrowing the density distribution for a given plastic by a
considerable fraction;
to provide an alternative differential attribute alteration process to
separate plastics based on differential morphology (e.g. thickness)
alteration of different grade or type plastic chips having an initially
uniform aspect;
to provide an integrated Material Recovery Facility combining one or more
of the embodiments of the present invention into a overall recycling
process for each or any of the following post-consumer sources: bottles
and rigid containers, appliances, computer and electronic equipment (ESR),
and automobiles (ASR);
to enable design, construction and operation of Material Recovery
Facilities that provide throughputs of several multiples of conventional
bottle recycling plants that have acceptable product quality;
to provide a means to alleviate broadening and overlap of the density
distributions of different polymers in a mixture caused by size reduction;
to provide a plastic attribute (e.g. density) differential alteration
responsive to a physical action (e.g. heat) in which the resulting
material becomes separable;
For particular mixes of different grades and/types of mixed polymer waste
streams (the input feedstock). Sets of procedures and criteria are
established for a broad range of separation technologies, one or more of
which can be selected to work for each particular separation which must be
effected. The most effective combination of technologies can be
incorporated into a final large scale advanced Material Recovery Facility
(MRF) for commercial use.
The actual configuration of particular unit operations incorporating
embodiments of the present invention within a given advanced Material
Recovery Facility can be based on selected criteria of anticipated ease of
use and economics. The innovations associated with these end
configurations are discussed below.
BRIEF DESCRIPTION OF THE DRAWINGS
We briefly describe the drawings.
FIG. 1: Density overlap of polymers of interest
FIG. 2: A partition curve for a separator
FIG. 3: De-foaming experimental results.
FIG. 4: Schematic of an advanced MRF incorporating embodiments of the
present invention.
FIG. 5: I/R heating belt for differential density alteration (DDA) of
plastic flakes.
FIG. 6: Mass distribution for two ABS Grades before and after heating.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The problem in particular is the vast variation in the product stream
presented to the sorter. The sorter has to try to create order and
uniformity out of a chaotic mix of materials of different sizes, shapes,
and density even if the preceding processing equipment has reduced the
scope of the job to require only the separation of plastics.
FIG. 1 illustrates some of the key separation challenges presented by close
or overlapping material densities in today's durable product streams. Some
specific examples include: 1) The separation of ABS and PP from automotive
interiors. Although in pure forms ABS is heavier than water and PP is
lighter, the addition of fillers into PP has heretofore made simple
density separations impossible; 2) The separation of ABS and high impact
polystyrene (HIPS), which are both used in refrigerator liners; 3) The
separation of flame retardant grades of ABS and HIPS (FR-ABS and FR-HIPS)
used in computer housings and other large computer components (these
differ from the refrigerator grades of ABS and HIPS in that the ignition
resistant additives increase the material density); 4) The small, yet high
value stream consisting of polycarbonate (PC) and blends (of different
grades) of PC/ABS which are used in higher end computer housings. In fact,
most multi-material assemblies contain some polymer density overlap
separation challenges.
Many plastics are somewhat permeable to moisture and/or other gases and
solvent-like substances, they tend to absorb water or vapors. These
absorbed vapors may cause micro-foaming that can effect their apparent
density (specific gravity) depending on the heat cycle history and their
history of exposure to solvents and other vaporous materials that can be
absorbed. This creates additional variability in the characteristics of
plastics coming into the recycling processes stream.
Separator Characteristics
Most mechanical separators, such as hydrocyclones, take advantage of
physical property differences between materials to segregate them. A
common physical property that these techniques exploit is a difference in
material density. Separators are generally of the binary type, i.e.,
receiving an input stream and providing two different output streams for
partitioning the input into the two outputs based on a difference between
two (or more) components in the input stream. Binary separators can be
mathematically characterized by a partition curve, FIG. 2. The partition
curve shows the fraction of one material, say material A, provided at one
separator output as a function of a separation parameter, in this case
specific gravity or density of the material to be separated. The cut
density point, (CDP) is defined as the value of the separation parameter
at which the fraction of material A provided at the one separator output
is 50%, obviously, this is also the value at which 50% of A is provided at
the other output. As an example, a hydrocyclone using water of specific
gravity (s.g.) equal to one g/cc processing a plastic with a s.g.=1 g/cc,
half the plastic would exit the top or vortex of the cyclone and half
would exit the bottom or apex of the cyclone. The slope of the partition
curve indicates its ability to separate other materials closely related by
the separation parameter.
It is an advantage to have a separator characterized by a steeply sloped
partition curve which enhances efficiency, especially for closely related
materials. For a given separator slope, it is advantageous to increase the
difference between the densities of the incoming material stream to be
sorted, i.e., provide differentiation to the input stream by altering the
density difference between them.
The primary separation technique used today in plastics recycling plants is
sink/float, conventionally performing in large baths of salt solution or a
slurry of a finely divided insoluble mineral material. The salt (such as
calcium nitrate) or mineral is added to increase the density of a (water)
solution (slurry) to the chosen point and the plastic is added. Heavy
plastics sink, light plastics float and so separation is achieved. The use
of hydrocyclones that take advantage of higher settling forces is
spreading. The greatest advantage of hydrocyclones is increased throughput
at lower capital cost.
Even when a durable good assembly is found to contain little or no density
overlap problems from the polymers in the mixture, size reduction can
create them. It has been shown that significant broadening of plastic
particle density distributions occur after size reduction. When many
polymers are granulated, small voids are generated at their edges due to
polymer micro-crazing and simply rough edges. Furthermore, as the
particles become smaller, inhomogeneities in voids, fillers or level of
crystallinity that were averaged in larger particles become more
consequential as they become concentrated in some particles and absent
from others. The density distribution again spreads, usually centered near
the nominal density of the polymer. However, the spread of the
distribution from the size reduction provides yet another obstacle in the
goal of using density to partition different polymers in a recycling
process stream.
Input material must be granulated, shredded, flaked, separated by size
before density differentiation. A density sorter isn'T able to sort or
sort effectively unless the input materials are differentiable by the
sorter.
A more sophisticated density separation or another mechanical separation
technique which could be used in conjunction with the existing technology
is required to broaden the scope of materials which can be feasibly
recovered from a mixed rigid plastics stream. To do this, a second
physical property difference by which to effect a separation is required.
A number of physical properties besides density have been investigated to
effect plastics separation, including: electrostatic properties,
spectroscopic properties, x-ray fluorescence, surface adhesion properties,
terminal velocity characteristics, and material fracture properties.
Since many plastics are somewhat permeable to moisture and/or other gases
and solvent-like substances, they tend to absorb water or vapors. These
absorbed vapors may cause micro-foaming that can effect their apparent
density (specific gravity) depending on the heat cycle history and their
history of exposure to solvents and other vaporous materials that can be
absorbed. This creates additional variability in the characteristics of
plastics coming into the recycling processes stream.
Accentuating or increasing difference in material density (density
differential/differentiation) between two polymers.
The invention described herein allows density differentiation between
different polymeric materials distributed as a random aggregation of
discrete articles (flakes or pieces) in a mixed polymeric material stream
in which one or more or the different polymeric materials originally have
comparable densities (identical or overlapping distributions). This method
can significantly enhance or enable the separation and purity improvement
of polymeric materials that are initially minimally separable or
inseparable. A Material Recovery Facilities plant incorporating
embodiments of the present invention may be built to accept shredded or
baled mixed rigid plastic containers or durable plastic goods having a
wide variety of polymeric material (plastics) and provide improved
separation fractions of the separated product or products. This invention
provides a means for sorting (partitioning) of plastic waste at curbside
unnecessary and lead to increased utilization of other waste materials
such as paper, metals and glass by complementing the economics of their
ruse.
Embodiments of the present invention are effectively applied in combination
with conventional separation processes and with other advanced processes
such as a plant that will operate the partitioning process under pressure.
Material Recovery Facilities that will operate the present density
differential alteration invention could take advantage of this means by
which the thermal properties of plastics can be exploited to dramatically
enhance separation efficiency, throughput, cost reduction, purity and
improved recycled product consistency. The inventions disclosed in this
application will enable and enhance what previously were difficult or
impossible separations on large scale, economically important mixed feed
streams.
The present invention enables or enhances density differentiation (the
alteration of density differential) between polymeric materials of a mixed
polymeric material stream that originally have comparable densities. This
method promises to significantly improve the separation and purity of
polymeric material that are initially inseparable with density separation
technology.
Differential Density Alteration (DDA) of Polymers by Application of Heat.
A reliable and simple difference between types of plastics is their
differential response to temperature and temperature changes. Improving
processing economics in mechanical recycling of plastics can benefit from
systems of increased temperature control as well.
The possibility to segregate plastic materials on the basis of differential
thermal properties, increases the scope of plastics which can be recycled
by a straightforward mechanical system. The present process involves two
steps: 1) differential alteration of physical properties (bulk or surface)
or geometry (morphology) of feedstock plastics shapes as a result of the
cost-effective application of heat and 2) separation (partitioning) of the
resulting differentially altered feedstock plastics into separable product
streams or bins of different target plastics based on the differentially
altered physical properties or morphology.
The way a plastic responds to differential density alteration by heating
(also referred to as the foaming response) is a result of its chemistry
(monomer structure and polymer structure), fillers (mica, glass, carbon,
other plastics, flame retardants, other inorganics, and the like), its
heat and stress cycle history, its surface condition, its shape
(morphological structure), manufacturing additives (added for molding and
extruding control e.g. mold release and anti-static agents) and
contaminants (both bulk and surface). The response is manifested by
changes in apparent density and a general thickening of the flake. The
intrinsic density of the particle is not necessarily altered but small
vapor bubbles are present within the particle which expand its apparent
volume and thus decrease its apparent density. The thickening of the flake
can also be used to separate plastics from a mixture in cases where two
mixture components do not maintain the same average thickness differential
after treatment.
When using differential density alteration, or foaming response, to effect
separations, two of the most useful properties (characteristics) are its
"softening point" and density, both intrinsic and apparent density. The
softening point of a plastic is a practical way of describing the
temperature at which it will deform (modifying its morphology) and yield
under applied (or built-in) stress. The chemi | | |
