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CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national phase of PCT/EP93/00509 filed 6 Mar. 1993
and based upon German national application P 42 07 398.7 filed 9 Mar. 1992
under the International Convention.
Our present invention relates to a process for separating thermoplastic
components from a mixed stream when sorting different types of
thermoplastic materials in contact with separating bodies; the invention
also relates to devices for implementing the process.
BACKGROUND OF THE INVENTION
Different types of thermoplastic materials soften at different temperatures
or within different temperature ranges. The changes in the physical
properties of the thermoplastic materials which take place during
softening can be used for the sorting of plastic materials or for the
separation of plastic materials from materials such as glass or metal.
This problem arises particularly during recycling. For this purpose
plastic materials have to be selectively softened corresponding to their
type and removed from the mixed stream by appropriate devices.
According to the state of the art thermoplastic components are sorted out
from a mixed stream by bringing the mixed stream into contact with a
sufficiently heated separation surface, whereby the plastic components
adhere to the separation surface due to the melting of their contact zones
onto this surface. This removes the thermoplastic articles from the mixed
stream and enables them to be subsequently scraped off (DE-37 27 777 C2).
Since this separation principle relies on the adherent behavior of plastic
materials to be sorted which are softened to the point where they are
almost melted and fluid, it is sensitive to contamination in practice,
such as during the recycling of plastic materials. Inherent wetness,
residuals of the formerly contained goods or labels can hinder the
adhesion. The quantitatively interesting polyolefin plastics are by nature
bad adhesives and therefore not very readily separated by this sorting
method.
Furthermore in this process during operation the sorting surface is
increasingly covered by a layer of softened thermoplastics. This layer
renders the process less effective, on the one hand due to reduced heat
transmission, and on the other hand due to the fact that as a result of
the adhesive action of softened thermoplastic material, plastic components
which in themselves are not softened at the respective temperature and
therefore are not among the target fraction, can stick to this layer. As a
result the purity of the target fraction is questionable.
In the separation process according to WO-A-88/000 98 the thermoplastic
components are not softened through selective heating and especially not
in temperature zones which are specific to type. The components become
adhesive due to the temperature of the metal surfaces they come in contact
with and melt on the latter. The device required for carrying out the
process is extremely complicated and expensive. It is particularly hard to
conceive how the pins or lances rotating together with an eccentrically
supported pin roller can be heated to be brought precisely to a certain
temperature by the heating device inside a drum, so that previously
unheated plastic components can melt on it when passing by.
A further drawback of this device working according to the unsatisfactory
principle of separation based on adhesion is that it is necessary to
provide here special cleaning devices for the removal of the molten
residuals of the plastic components from the heated surfaces. For this
purpose a separate burner device for singeing the outer roller surface is
used, with subsequent brushing or another kind of removal of the remaining
residuals.
U.S. Pat. No. 3,793,424 is based on the principle of separation through
adhesion or adhesive behavior of the plastic components. There it is
indicated that the dry particles are fed to a heated, moving substrate, to
which the plastic materials with a lower softening point adhere and the
particles with a higher softening point flow freely over it.
JP-A-54 022 477 can also classified as based on the unsatisfactory
principle of separation through adhesion. There, in the center of a
collection chamber, is an electrically heated cylinder with a conical end.
Through that a plastic component is melted in a punctiform manner and
penetrates with the melt into the area of the collection chamber. The
process comprises the following steps:
(a) local heating of a limited portion in the thermoplastic material with
the conical end of a heating element with high thermal conductivity,
(b) this material is brought into contact with a second element of lesser
thermal conductivity with a collection chamber,
(c) continuous heating of the material while a molten portion is introduced
into the enlarged space of the collection chamber, and
(d) cooling of the molten plastic material while it is introduced in the
collection chamber.
The corresponding device with individual, electrically heated melting
bodies and the respective collection bodies, partially traversed by
cooling channels, assigned thereto is extremely complicated and prone to
failure, very difficult to control from the point of view of temperature
setting and not suitable for an economical operation in view of the
time-consuming process steps (a) to (d). Consequently the plastic
components are not softened in defined temperature zone, but melt on
contact with heated bodies.
OBJECT OF THE INVENTION
It is the object of the invention to provide a process and a device for the
sorting of thermoplastic components from a mixed stream and for the
sorting of different types of thermoplastic materials which overcomes the
drawbacks of separation based on adhesion and wherein the separation
principle is no longer based on adherence or on the adhesive behavior of
the softened thermoplastic material, but on a selective mechanical
anchoring on separating bodies with a special configuration.
SUMMARY OF THE INVENTION
In order to achieve this object in a process of the aforementioned kind the
invention provide that the plastic components are softened by being heated
in the mixed stream in temperature zones specific to type before they are
picked up by the separating bodies, after which the softened plastic
components are affixed to the separating bodies which penetrate the
softened surface of the plastic components or pierce the softened surfaces
and then finally removed from the separating bodies.
Advantageously the process of the invention relies on a mechanical fixation
of the plastic components, in contrast to the unsatisfactory separation
principle based on adherence or adhesive behavior. Separating bodies
either penetrate the sufficiently softened surface of the plastic
components or pierce this surface, whereby the plastic components are
securely affixed and after that removed from the separating bodies by
mechanical means. Plastic components of one type of plastics which are
sufficiently softened at the applied process temperature so that
separating bodies can penetrate their surface, are selected from other
types of plastics whose structure has for instance not yet been softened
at the applied temperature. The same occurs also with foreign bodies such
as made of metal, paper, ceramic or the like, which are not subject to a
selective softening through heating.
An embodiment of the invention provides that the penetration of the
separating bodies into the surface of the plastic components heated
specifically to their type takes place under the influence of the force of
gravity.
Depending on the softening degree of the type of plastic to be selected,
according to a further feature of the invention, it is possible effect the
penetration of the separating bodies into the surfaces of the plastic
components softened by the heating specific to type by a pressure force
generated in a gap between the support surface and a separating body.
Thereby one embodiment provides that the force required for the penetration
of the separating bodies into the surface of the plastic components
softened by heating specific to type is determined by the elastic yielding
of the support surface or the separating body, as well as by the size and
shape of the plastic components. This makes possible an extremely uniform
setting and maintaining of the pressure force generated in the gap between
support surface an separating body.
Finally one embodiment of the process provides that the surface of the
separating bodies be coated with an anti-adhesive layer. This process step
is not complicated and results in a certain removal of the plastic
particles stuck to the separating bodies, this being the reason why the
continuous condition of readiness of the separating bodies during the
sorting process is maintained without change.
A device for implementing the process of the invention is characterized by
at least one separating body acting as conveyor, having a flat or curved
surface provided with structures for penetrating or piercing the plastic
components which have been previously softened through selective softening
by applying heat specific to their type, as well as for securing the
plastic components to the separating body and with means for the removal
of plastic components affixed to the separating body after they have been
separated from the mixed stream, as well as with means for producing well
defined temperature zones for the selective heating of the plastic
components in the mixed stream prior to their pickup by the separating
bodies.
A particular advantage of this device is its extremely simple construction.
Thereby corresponding to the features of the dependent claims embodiments
can be provided wherein the structured surface of the separating bodies is
formed by grooves or knurls. Thereby the crests of the grooves or knurls
can penetrate the softened plastic components and hold them. The grooved
or knurled surfaces have the advantage that even comparatively small
plastic components, such as the ones occurring during comminution
processes, can be separated. Thereby the grooved or knurled surfaces can
be roller surfaces.
According to one embodiment the separating bodies can be rollers, bands or
plates provided with bristles, needles, pins or lamellae. With the
corresponding selection of suitable surface structures of separating
bodies the person skilled in the art can optimize the sorting process on
target depending on the materials to be separated.
In an embodiment of the invention the separating body forms a gap with a
support surface. Thereby the support surface can be a pressure roller or a
passing conveyor belt, whereby the conveyor belt is pressed against the
separating body at the location where support is needed by one or more
pressure rollers. The pressure rollers can be made of metal, plastic
material or soft elastic materials such as rubber or foam, or can have an
inflatable rubber or plastic jacket in the shape of a cylinder. The
selection of the suitable material or combination of materials is made
with regard to the mechanical demands made on the separating body.
Advantageously separating bodies can be provided with an anti-adhesive
layer, whereby on the one hand only strictly defined plastic components
can be affixed due to the surface structure and on the other hand the
affixed plastic components can be completely removed when scraped off.
Negative effects such as the adhesion of a thermoplastic layer to the
surface of the separating bodies are avoided due to the present of the
anti-adhesive layer.
According to another feature of the invention several devices with similar
or different construction are arranged in cascade one after the other. By
arranging several devices according to the invention in succession it is
possible for instance due to targeted temperature control to perform a
separation of various types in a single run.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features, and advantages will become more
readily apparent from the following description, reference being made to
the accompanying drawing in which:
FIG. 1 is a perspective view of a separating body of roller form;
FIG. 1A is a diagrammatic perspective view of another such body;
FIG. 1B is an end view of another separating body;
FIG. 1C is a side view of a separating body;
FIGS. 2A-2D are diagrammatic sectional views of separating bodies with
different surface structures;
FIG. 3 is a lateral view of a sorting device;
FIG. 4 is a lateral view of another sorting device;
FIG. 5 is a lateral view of a sorting device with elastic pressure roller;
FIG. 6 is a lateral view of a sorting device with separating roller and
heated pins; and
FIG. 7 is a lateral view of three separating device arranged in a cascade
one under the other.
SPECIFIC DESCRIPTION
FIGS. 1, 1A, 1B and 1C show three separating bodies 4a, 4b and 4c, designed
as rollers with different surface structures. The roller-shaped separating
body 4a is a so-called brushing roller with brush-like surface structure
10a (FIG. 1A). The bristles forming the brushing surface can be made of
wire or very hard plastic wires. The separating body 4b has a surface 10b
provided with pins. These pins 10b (FIG. 1) can be made out of
heat-conductible materials such as copper, silver or brass for the purpose
of good heat conductivity and their length and diameter are optimally
adjusted to the sorting task at hand.
Finally FIG. 1C shows a lamellae roller 4c with circularly shaped lamellae.
The surface structure of the separating bodies 4a to 4c designed as rollers
are optimally suited to the sorting task of different types for plastic
materials. A basis therefor is the softening behavior of thermoplastic
materials, whereby these have a characteristic temperature range for each
type of plastic, and when this range is reached they convert from the
solid state to a softened, rubber-elastic state. If there are several
different types of plastic materials or other materials in a mix, then it
is possible to selectively soften a chosen type of thermoplastic material
through selective heating. In a mixed stream which can also contain hard
materials as further components, such softened thermoplastic materials
have a fundamentally different elastic behavior than the other components.
The use of this difference in the elastic behavior for a selective
fixation on the separating bodies 4a to 4c. FIGS. 1, 1A, 1B and 1C or 14,
15, 16 (FIG. 7) provided by the invention by means of structures at the
separating bodies according to the invention, represents the crux of the
present invention.
In the case of pin or lamellar rollers 4b, 4c it is possible to elastically
form or elastically fasten the pins or lamellae 10b or 10c on the roller,
so that they can individually adjust to the respective dimensions of the
components of the mixed stream. In a brush roller 4a it is possible to
achieve the same through the elasticity of the individual bristles 10a.
Advantageously separating bodies with a defined surface structure can be
used. Such surfaces are represented purely schematically in FIGS. 2A-2D
and have similar comb-like cross sections 11a to 11d. These form surface
grooves or knurls with more or less pronounced peaks. The groove edges can
penetrate the plastic material according to its degree of softening and
fasten the corresponding components. Grooved surfaces 11a to 11d have the
advantage that also small plastic parts, such as those resulting from a
comminuting process, can be separated.
Separating bodies 4a to 4c, or separating bodies 14, 15, 16 can be composed
of different materials and combination of materials, e.g. of
surface-coated metal, as long as they meet the thermal and mechanical
requirements. It can for instance be advantageous to use separating bodies
made of material with good heat conductivity, because this way heat
required for the softening of the plastic material can be brought to the
contact zone.
FIGS. 3 to 6 show devices for the implementation of the method of the
invention with various embodiments. FIG. 3 shows a roller-shaped
separating body 4 with any desired surface structure. A conveyor belt 3
guides from the left side a mixed stream 1 with various plastic components
and in certain cases foreign materials along a transport gap 20 formed
between the separating roller 4 and the conveyor belt 3. This transport
gap 20 is created between two support rollers 2 guiding the conveyor belt
3 and the surface structure of the separating roller 4. Selectively heated
plastic components 24 remain clinging to the surface structure of
separating roller 4, and are scraped off by a doctor blade 5 on the runoff
left side and thrown into discharge trough 7. The foreign matter parts not
picked up by the separating roller 4 continue to remain on the conveyor
belt and are collected at a different location.
The device shown in FIG. 4 presents only unessential differences with
respect to the embodiment in FIG. 3. Instead of the comparatively long gap
20 between a support surface 21 formed by the conveyor belt 3 and
separating roller 4 in the embodiment according to FIG. 4 there is
comparatively short gap between the support roller 2 and the separating
roller 4.
FIG. 5 shows another embodiment of the device, whereby as support roller a
gas-filled, highly elastic pressure roller 31 is used. The mixed stream is
brought in from above through a feeding duct 8 and after a short ride on
the surface of separating roller 14 traverses the gap 20 in an
approximately vertical surface area between the separating roller 14 and
the elastic pressure roller 31. In this arrangement for instance it is
possible to heat the separating roller 14 from the inside, in order to
transport heat to the plastic components to be separated in the mixed
stream. In order to avoid a thermal overload of the elastic pressure
roller 31, a cooling air flow 12 is provided, under whose influence the
surface temperature of the elastic pressure roller 31 is balanced and kept
at an acceptable level. Plastic components of one type affixed to the
separating roller 14 are scraped off with the doctor blade 5 and thrown
into the trough 7a. The parts of other plastic materials or foreign matter
25/27 which have not been picked up by the separating roller 14 are thrown
into the trough 7.
FIG. 6 shows a further variant of the device, wherein the separating roller
14 has heated pins 10b. The conveyor belt 3 brings along plastic bottles
23 and other types of plastic materials or foreign matter 25/27 underneath
the separating roller 14. The latter is linked to an articulation 41 via
an approximately horizontal arm 40 and is relieved of weight in a manner
known per se, for instance by a restoring element or a counter-weight. As
a result of this weight relief, the separating roller 14 rolls with a
precisely settable force over the components of the mixed stream, whereby
the heated pins 10b penetrate a selectively softenable type of plastic
bottles 23 and transport those around with the separating roller 14, until
they are scraped off by the doctor blade 5 and thrown into the discharge
through 7. The plastic components or foreign matter 25/27 which could not
be softened are transported by the conveyor belt to a further collection
location. In this arrangement it is for instance possible to achieve a
separation in the mixed stream of hollow plastic bodies, such as plastic
bottles, from non-plastic products, such as glass or metal containers.
Compared to other separating processes based on the specific weight, the
present process is not sensitive to residues of the contained goods. The
pins 10b of the separating roller 14 can be provided for this special
purpose with a barb which assists the secure fixation, for instance of
plastic bottles. These barbs also do not impair the scrape-off operation,
because even stronger affixed plastic parts can be scraped off with a
sufficiently stable doctor blade.
In FIG. 7 for instance an arrangement is shown wherein due to the
succession in cascade of several devices according to the invention,
through targeted temperature control it is possible to achieve without
problems the separation of several different types 24, 25, 26 of plastic
material, as well as of foreign matter 27. From top to bottom, separating
rollers 14, 15, 16 having the identical or different structures can be
used. Each individual arrangement corresponds thereby to the embodiment
according to FIG. 5, whereby in each case a separation gap 20, 20a, 20b is
created between a separating roller 14, 15, 16 and the corresponding
elastic pressure roller 31 or a support surface 21, 21a, 21b of a pressure
belt 30, 30a, 30b. For the collection of the individual types 24, 25, 26
of plastic material serve the troughs 7, 7a, 7b, and intermediate
materials are each guided through chutes 9 from an upper device to a
device located therebelow.
The process and the devices according to the invention are not complicated,
very suitable and safe to operate, and implement an extraordinarily
economical method for sorting plastic materials during recycling. The
invention thus proves to be a contribution to the relief of the
environment from waste, of great importance to the national economy.
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