WO2005108037A1

WO2005108037A1 - Method and device to separate and recover plastic material

Info

Publication number: WO2005108037A1
Application number: PCT/EP2005/052076
Authority: WO
Inventors: Paolo Bevilacqua
Original assignee: Universita' Degli Studi Di Trieste
Priority date: 2004-05-07
Filing date: 2005-05-06
Publication date: 2005-11-17
Also published as: ITUD20040092A1

Abstract

Device and method to separate and recover plastic material. The device (10) comprises a rotary tube (11) inside which a heterogeneous mixture of plastic material is introduced by means of a fluid vehicle, and at least a separation chamber (16; 16a, 16b). The latter is connected to the rotary tube (11) and includes at least a peripheral aperture (18), through which the higher-density components of said mixture are expelled by centrifugation.

Description

"METHOD AND DEVICE TO SEPARATE AND RECOVER PLASTIC

MATERIAL" * * * * * FIELD OF THE INVENTION The present invention concerns a method and the relative device to separate and recover plastic material deriving from post-consumption collection processes. The invention is applied particularly, although not exclusively, to separate plastic material of various polymer composition, so that it can be recycled. BACKGROUND OF THE INVENTION It is known that recycling plastic materials requires a series of preliminary operations intended to separate the heterogeneous material arriving from post-consumption collection processes of extraneous materials, such as paper, metals or organic materials in general, which can be found mixed with the plastic material. Subsequently, the individual polymer fractions are separated, so as to obtain secondary raw materials of high quality. The processes to identify and separate the plastic materials are the most important step, and also the most difficult, in the recycling of materials arriving from collection. The selection of the plastics, and the elimination of the contaminants, is a process which economically affects all the recycling operations. As a consequence, the purpose of developing new separation technologies must be to optimize the treatment of refuse and to obtain more efficient recovery processes. In conventional selection and recovery plants, the most common system to separate the plastics according to polymer species is still mainly manual selection. Recently, innovative techniques of automatic selection have been developed, which are having a certain success on an industrial level. Among these, the most common is based on the possibility of separating materials of different density, immersing them in a fluid with an intermediate density with respect to that of the materials considered. Traditional apparatuses to separate plastics according to density (static separators, dynamic separators) are inadequate or even absolutely inefficient when the following conditions occur:

- the size of the grains to be separated is small; - the shape of the grains, instead of being compact, is flat, lamellar or filiform;

- the difference in density between the species to be separated is small. This applies for any kind of material to be separated, whether of primary or secondary origin. In fact, in the cases cited, separation in the static field cannot be achieved, given the prevalence of the surface forces over the mass forces, which situation would require an acceleration many times higher than that of gravity, in order to obtain separation. With dynamic separators it is possible to achieve centrifugal accelerations much higher than the acceleration of gravity (for example 40 to 1000 times the acceleration of gravity), and this allows to separate particles with a granulometry up to 0.5 mm and with different densities between the species of at least 150 kg/m³. However, these separators are inefficient when it is desired to achieve the separation of plastics with very similar densities, such as for example in the case of mixes of polyethylene/ polypropylene (PE/PP) and polyethylene terephthalate/ polyvinylchloride (PET/PVC). This inefficiency is also revealed when lamellar particles are treated since, in such separators, secondary flows are generated which determine a great drawing of the particles, not allowing flat or filiform grains to be separated according to density. To solve these problems, it would be necessary to have a separator functioning as a static bath (in the sense of having a negligible effect of the secondary drawing flows) but operating in a very high centrifugal field (for example from 500 to 1000 times the acceleration of gravity). This is achieved in traditional laboratory centrifuges with a discontinuous type functioning (batch); it is also achieved industrially using, for example, the centrifuge produced by

KHD Humboldt Wedag and sold under the trade name of

Censor®, able to achieve a very efficient separation in terms of purity of the fractions separated, irrespective of the composition of feed. This centrifuge, however, is very complex and expensive. Purpose of the present invention is therefore to achieve a device to separate and recover plastic material, which can be applied industrially, which is relatively simple in construction and functioning, which guarantees great efficiency and which can be used also in critical conditions like those indicated above. The Applicant has devised, tested and embodied the present invention to obtain this purpose, to overcome the shortcomings of the state of the art and to obtain other advantages as will be shown hereafter. SUMMARY OF THE INVENTION The present invention is set forth and characterized in the main claims, while the dependent claims describe other innovative characteristics of the invention. In accordance with the purposes indicated above, a device to separate and recover plastic material according to the invention comprises at least a tube able to be made to rotate at high speed, inside which the heterogeneous material to be separated is sent. The heterogeneous material consists of a mixture of plastic material of various polymer composition, transported by a fluid carrier, advantageously a liquid; however, it cannot be excluded that the fluid carrier may consist of a gas vehicle, pure or as a solution/suspension. Along the length of the rotary tube one or more separation chambers are solidly made, in which the heaviest fraction of the mixture is centrifuged mainly towards the outer wall of the chamber, while the lighter fraction, possibly with a residue of heavy material not separated, continues to be displaced along the tube, substantially along its axis. The separation chambers, on their outer periphery, have discharge apertures or slits, through which the heavy fractions of the mixture are expelled from the rotary tube, so as to be recovered outside it. In a preferential embodiment, at least two separation chambers are provided along the length of the rotary tube, in which the fractions of heavy material, which were not discharged in the first stage, can gradually be separated in the subsequent stages from the lighter fractions and then recovered. The separation of the heavy fractions, in this way, is obtained progressively in a plurality of stages located in sequence. In another embodiment of the invention, a single chamber is provided. In this case, the light fraction of the plastic material, which does not emerge through centrifugation in correspondence with the chamber, is discharged continuously through one end of the rotary tube, opposite the end where the material enters. The heavy fraction, on the contrary, is mainly discharged through the separation chamber. According to a characteristic of the invention, the separation chamber consists of walls facing each other and converging towards each other so as to form a conical compartment which terminates with said apertures/slits. In one embodiment of the invention, the walls are keyed onto the rotary tube and, starting from the outer diameter of the tube, they converge towards each other until they form one or more slits communicating with an outer collection chamber, filled with the same fluid from which the material is separated, and are sealed closed. The walls may consist of a single disk with converging walls, or of a plurality of adjacent sectors made solid with each other so as to define said conical compartment. From the outer collection chamber the material is then discharged in a suitable manner, for example with a volumetric pump with pistons or with a membrane, or with other suitable means, for example simply due to gravity. The lightest product, not discharged through centrifugation from the separation chambers, is discharged continuously through the end of the tube opposite the end where the material is introduced. According to a variant, the walls defining the separation chambers are made in a single piece with the rotary tube. The cross section of the separation chambers, taken on a plane transverse to the longitudinal axis of the rotary tube, is advantageously circular, but it can also have a polygonal shape, for example square, pentagonal, hexagonal, etc. The separation device according to the invention can have a desired number of separation chambers arranged in succession, for example from 1 to 10, according to the level of efficiency of separation required. According to a variant, in correspondence with the inlet section of the rotary tube, there are stirrer means present able to impart a rotational or vortex movement to the material introduced into the tube. In one embodiment of the invention, said stirrer means consist of a body, advantageously elliptical or pointed in shape, or drop-shaped, or equivalent, arranged longitudinally substantially coaxial with respect to the axis of the rotary tube. According to a perfected version of this embodiment, a plurality of fins are applied on the elliptical body, and said fins are warped with respect to a radial plane passing substantially through the axis of the elliptical body itself. According to a variant, said stirrer means consist of a coil-type cyclonic distributor. According to another variant, the stirring and rotational motion are obtained by means of a tangential feed of the flow of material to be separated with respect to the axis of the rotary tube. The separation device, according to the invention, comprises at least a command and control unit by means of which it is possible at least to set the speed of rotation of the motor which makes the rotary tube rotate. A typical working speed is comprised between about 1,000 and about 10,000 rpm, advantageously between about 1,500 and 8,000 rpm. According to a variant, said command and control unit also allows to regulate the speed and/or delivery rate of the heterogeneous material to be separated. BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic view of a longitudinal section of a device to separate and recover plastic materials according to the invention; - fig. 2 is a schematic front view of a stirrer element inserted at inlet to the separation chamber of the device in fig. 1;

- fig.3 is a perspective view of a form of embodiment of the stirrer element in fig. 2. DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT With reference to the attached drawings, a centrifugal separation device according to the invention is indicated in its entirety with the reference number 10. The separation device 10 is usually inserted inside a plant for recycling plastic material, of a conventional type, which provides to arrange, at the head of the separation device 10, a hopper which feeds a granulator followed by a classifying screen; the screened material, indicatively particles with a granulometry comprised between 1 and 25 mm, goes to feed the separation device 10, while the larger particles are reintroduced into the granulator in order to be subjected once again to the pulverization process. Downstream of the separation device a dehumidification step can be provided, to allow the PVC and separated light materials to dry, and subsequently to be immediately made up in sacks. These parts of the recycling plant, upstream and downstream of the separation device 10, are here taken as known and are not shown in the attached drawings. The separation device 10 shown in fig. 1 consists of a tube 11 able to be made to rotate by means of a drive belt 12 around its longitudinal axis 11a. The rotary tube 11 rotates on bearings, respectively inlet 13a, intermediate 13b and outlet 13c, with respect to a segment of fixed tube 14 located at inlet and with respect to a segment of fixed tube 27 located at outlet. The segment of fixed tube 14 can be associated with a pipe for the introduction of the heterogeneous material (not shown in the figures), in the direction indicated by the arrow 15, from which the fraction of plastic material to be recycled has to be separated. The segment of fixed tube 27 can be associated with a pipe to recover the light fractions, as will be explained hereafter. The rotary tube 11, in a preferential embodiment, has a length comprised between 200 and 10000 mm, advantageously between 300 and 5000 mm. The segments of fixed tube 14 and 27 preferably have a length comprised between 100 and 600 mm, advantageously between 200 and 400 mm. The diameter of the tubes 11 and 14 is preferably comprised between 15 and 200 mm, advantageously between 20 and 150 mm. According to a variant, at least the fixed tube 27 at the outlet has a diameter greater than the diameter of the rotary tube 11. In an advantageous embodiment of the invention, the pipe to introduce the material, associated with the segment of fixed tube 14, is at least partly transparent in order to improve the control of the motion of fluid inside the device, and in order to detect if the entire section of the pipe is, possibly, not filled. This last feature allows to guarantee a correct head of pressure in the separation chambers and prevent the formation of air bubbles inside. The separation device 10 shown in fig. 1 comprises two stages of separation arranged in succession, each of which comprises a respective separation chamber 16, first 16a and second 16b. It comes within the field of the invention that only one separation chamber is present, or three or more, up to ten, according to the level of efficiency of separation required. Each separation chamber 16a, 16b consists of walls 17 facing each other and keyed onto the rotary tube 11. The walls 17 are convergent towards each other, from the attachment to the rotary tube 11 towards the outside, so as to define a slit 18 through which to expel the material by centrifugation. The slit 18 can be continuous or discontinuous. The angle of convergence a of the walls 17, taken with respect to a plane orthogonal to the longitudinal axis 11a of the rotary tube 11, is comprised between 10 and 50°, advantageously between 20 and 40°. The overall diameter of the separation chamber 16 is advantageously comprised between 100 and 1500 mm, advantageously between 200 and 1000 mm. According to a variant, the rotary tube 11 has at least a segment of greater diameter at least just downstream a relative separation chamber 16. The heavy fraction, for example the PVC, of the heterogeneous material sent inside the rotary tube 11 is progressively expelled outside the separation chambers 16a,

16b through the respective peripheral slits 18, into a relative outer collection chamber 19. From said collection chamber 19 the heavy fraction is then discharged through a discharge outlet 20, as indicated by the arrow 21. Various known techniques, such as the use of a volumetric pump with pistons or a membrane, or simple gravity, allow the heavy fractions to emerge and be recovered. The collection chambers 19 are sealed with respect to the outside by means of respective sealing means 26. Advantageously the collection chambers 19 have transparent walls, to allow to verify the state of fullness thereof and/or to control the correct functioning of the discharge apertures. According to another variant, the collection chambers 19 have sensor means, for example a manometer, to control the level of the material accumulated. The lighter fraction, for example the PET, does not exit from the separation chambers 16a and 16b, and continues its travel inside the rotary tube 11 until it exits from the end opposite the one where it entered, in the direction indicated by the arrow 22. In this way, the heterogeneous plastic material is separated according to density and can be recovered with a high level of quality (in the sense of homogeneous composition) in order to be recycled. To improve the efficiency of separation, upstream of the first separation chamber 16a, in this case inside the segment of fixed tube 14, and upstream of the second separation chamber 16b, respective stirrer elements 23 are present, consisting in this case of an elliptical or pointed body 24 radially to which a plurality of fins 25 are made. According to a variant that is not shown here, in an intermediate position between the first 16a and the second 16b chamber there is a segment of fixed tube, and the rotary tube 11 is divided into two segments, one upstream and one downstream of said segment of fixed tube, each one associated with their own rotation means. In this embodiment, the second stirrer element 23 is located inside the intermediate segment of fixed tube between the two elements of the rotary tube. The function of the stirrer elements 23 is to impart an accentuated vortex movement to the flow of the material entering, in order to direct the heavier solid particles towards the wall of the rotary tube 11. This encourages the particles to begin to rotate more rapidly, and hence to be captured in the walls of the conical separation chambers 16 . In a preferential embodiment of the invention, there are between 8 and 20 fins 25 in number, advantageously between

10 and 16, and their length is comprised between 10 and 60 mm, in relation to the diameter measurements of the rotary tube 11 indicated above. The fins 25 are warped (fig. 3) with respect to a radial plane passing through the axis of the elliptical body 24. This configuration imparts a vortex development to the flow of material entering, and accentuates the centrifugal thrust which the solid particles are subjected to starting from inlet to the rotary tube 11. To prevent by-pass phenomena, the fins 25 advantageously cover the entire transit section of the tube 14 or 11 into which they are inserted, as can be seen in fig. 2. It is clear, however, that modifications and/or additions of parts may be made to the method and the device to separate and recover plastic materials 10 as described heretofore, without departing from the field and scope of the present invention. For example, instead of the finned pointed body, any element can be used having a similar or analogous function, for example a cyclonic distributor, spiral, helical or other. The number of the separation chambers arranged in sequence, and also all the data of size and speed of rotation supplied, are only indicative, and can be adapted on each occasion according to the needs and the requirements of the plant. It is also clear that, although the present invention has been described with reference to specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of the separation and recovery device, all of which shall come within the field and scope of the present invention.

Claims

1. Method to separate and recover plastic material, wherein a heterogeneous mixture of plastic material on a fluid vehicle is sent inside a rotary tube (11), characterized in that the plastic material is separated into its components with different density by means of centrifugation in at least a separation chamber (16; 16a, 16b) connected to said rotary tube (11).

2. Method as in claim 1, characterized in that said plastic material is separated progressively, starting from its heaviest fractions, in one or two separation chambers (16a, 16b), arranged in sequence along said rotary tube (11).

3. Method as in claim 1 or 2, characterized in that each of said chambers (16a, 16b) has conical walls (17) that converge at least towards a peripheral discharge aperture (18).

4. Method as in claim 1 or 2, characterized in that the lighter fractions of the plastic material are discharged through the end of said rotary tube (11) opposite the end where the material is introduced.

5. Device to separate and recover plastic materials, comprising a rotary tube (11) inside which a heterogeneous mixture of plastic material is introduced on a fluid vehicle, characterized in that it comprises at least a separation chamber (16; 16a, 16b) connected to said rotary tube (11) and including at least a peripheral aperture (18) through which the higher-density components of said mixture are expelled by centrifugation.

6. Device as in claim 5, characterized in that it comprises two separation chambers (16a, 16b) on the length of said rotary tube (11), in which the fractions of heavy material are progressively separated from the lighter fractions and then recovered and expelled through said apertures (18).

7. Device as in claim 5 or 6, characterized in that said separation chambers (16a, 16b) include conical walls (17) that converge towards said apertures (18).

8. Device as in any claim from 5 to 7 inclusive, characterized in that said rotary tube (11) has an inlet end through which said heterogeneous mixture of plastic material is introduced, and an open outlet end through which the light fraction of said heterogeneous mixture, not expelled by means of centrifugation through said chambers (16a, 16b), is discharged and recovered.

9. Device as in any claim from 5 to 8 inclusive, characterized in that said separation chambers (16a, 16b) include conical walls keyed onto, or made in a single piece with, said rotary tube (11) which, starting from the outer diameter of the latter, converge one towards the other until they form one or more apertures (18) communicating with an outer collection chamber (19).

10. Device as in claim 9, characterized in that each of said collection chambers (19) is filled with the same fluid from which the heterogeneous plastic material is separated, and is associated with sealing means (26).

11. Device as in claim 9 or 10, characterized in that each of said collection chambers (19) includes at least a discharge outlet (20) associated with discharge means.

12. Device as in any claim from 5 to 11 inclusive, characterized in that at least upstream of at least one of said separation chambers (16a, 16b) there are stirrer means (23) able to create a vortex motion of the heterogeneous mixture introduced into said rotary tube (11).

13. Device as in claim 12, characterized in that said stirrer means (23) comprise at least a substantially elliptical body (24) arranged with its main axis substantially along the axis of said rotary tube (11).

14. Device as in claim 13, characterized in that on said substantially elliptical body (24) a plurality of fins (25) are applied, arranged warped with respect to a radial plane passing through the axis of said elliptical body (24).

15. Device as in claim 14,. characterized in that there are between 8 and 20 of said fins (25) in number, advantageously between 10 and 16, and they are configured so as to cover substantially the entire transit section of said rotary tube (11).

16. Device as in claim 5, characterized in that the conical walls (17) of said separation chambers (16a, 16b) are inclined by an angle (a) comprised between 10 and 50°, advantageously between 20 and 40°, with respect to a plane orthogonal to the longitudinal axis (11a) of said rotary tube (11).

17. Device as in claim 5, characterized in that the diameter of said rotary tube (11) is preferably comprised between 15 and 200 mm, advantageously between 20 and 150 mm.

18. Device as in claim 5, characterized in that the diameter of said separation chambers (16a, 16b) is comprised between 100 and 1500 mm.

19. Device as in claim 5, characterized in that said rotary tube (11) is associated to a segment of fixed tube (14) located at inlet and to a segment of fixed tube (27) located at outlet.

20. Device as in claim 19, characterized in that at least said fixed tube (27) at the outlet has a diameter greater that the diameter of said rotary tube (11).