Abstract:
In a method for recovering pure PVC from pre-disintegrated PVC containing substance mixtures, the substance mixture is soaked in a swelling agent for PVC, is supplied under pressure into an arrangement of at least two sequential hydrocyclones, wherein, in the first hydrocyclone, a heavy fraction of impurities is separated in the sink flow and a light fraction of swelled PVC particles and light impurities and plastic particles are separated in the rising flow, wherein the pre-cleaned light fraction is sorted and the particle fraction comprising the swelled PVC particles and smaller plastic particles is fed to the second hydrocyclone in which the PVC particles are separated in the sink flow. The invention also describes a device for carrying out the method.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns of method directed towards the features in the preamble of the independent method claim and a device directed towards the features in the preamble of the independent device claim. 
     Polyvinylchloride (PVC) is used in a plurality of applications as insulation material for cables, as protective and decorating material, for pipe construction, in plastic compounds for floor coverings etc. To conserve natural resources and protect the environment, the recovery of plastic materials, including in particular PVC, has become more and more important. This is also increasingly the case for material compounds, such as electric cables whose additional components can also be recycled. 
     Various methods for processing electric cables having PVC insulation are known (DE 33 40 273 A1, DE 19 63 148). While the heavy fraction composed of different metal components can be separated relatively easily using physical methods due to the specific properties of the metals, the light fraction composed of different plastic components, such as PVC, PE (polyethylene), PP (polypropylene) etc. is difficult to separate by physical methods due to the plurality of different materials having similar physical properties. DE 44 41 229 C2 describes a method and a device for continuous processing of plastic coated cable remains and cable waste, wherein the metal core of the cable and also the PVC insulation material shall be recovered in pure form. This method is suited exclusively for separating PVC and metal. Other plastic insulation must be manually sorted in an expensive preparation station. Practice has further shown that, in the known system, the swelling PVC particles agglomerate to form a tough, thick sludge which precludes both separation of the PVC particles as well as desorption and recovery of the swelling agent. 
     As mentioned above, these recycling processes produce a plastic mixture, the light fraction of which contains further impurities, e.g. metallic remnants, sand, textiles and/or paper. Since, even with the addition of thermal stabilizers, PVC tends to thermally degrade when heated to more than 180° C. thereby separating toxic chlorine and irritating hydrogen chloride, thermal utilization of the plastic mixture or cracking to obtain a crude oil-like basic material are not possible. Conversion of the plastic mixture into a plastic state and subsequent extrusion, which has to be effected at approximately 150° C., is not possible, in particular when residual impurities of heavy metals such as copper are present since, at these temperatures, polychlorinated dioxins and furanes form under the catalytic influence of the heavy metals when a chlorine donator (PVC) is present. 
     According to prior art, the only processing options for such light fractions composed of different plastic materials is the so-called down-cycling into products of poor material quality, or disposal. 
     DE 41 06 812 A1 describes a method for the recycling of mixed plastic refuse, wherein layered plastic composites are disintegrated, the composite pieces decomposed into particles of a given material and the particles of differing materials subsequently separated into pure groups of a given material each. The disintegration of the composite pieces into particles of a given material is effected with mechanical shearing forces. The separation into pure groups of particles is effected by dispersing the particles in water and separating them in a plurality of hydrocyclone stages. Particularly for the case of plastic refuse containing PVC, same is disadvantageously poorly separable from other plastics of similar density despite the use of a plurality of hydrocyclone stages, since PVC, as is the case for most other plastics, does not swell in water and is therefore discharged in both the rising and sinking flow, even in the last hydrocyclone stage. 
     DE 2 900 666 A1 also discloses a method for separating mixed plastic refuse, wherein the refuse is disintegrated, suspended in a carrier liquid (water) and separated in sequential hydrocyclones. This method is also incapable of separating PVC from plastics of similar density. 
     DE 43 13 007 A1 discloses a method for separating a plastic from a support material. The plastic is separated from the support material through the use of a suitable solvent in which the plastic swells but the support material does not. 
     It is the underlying purpose of the invention to recover this PVC in pure form from plastic waste having a particularly high PVC content. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, this object is achieved by a method in that the soaked substance mixture is transported under pressure into an arrangement of at least two hydrocyclones which are connected in series. The first hydrocyclone separates a heavy fraction of impurities such as sand and metal particles and heavy plastic particles in the sink flow, and a light fraction of swelled PVC particles and light impurities and plastic particles in the rising flow. Subsequently, the pre-cleaned light fraction is sorted and the particle fraction comprising the swelled PVC particles and smaller plastic particles is fed to the second hydrocyclone in which the PVC particles are separated in the sink flow. 
     With respect to the hardware side of the system, the inventive object is achieved by a device, wherein the arrangement of devices for separating the swelled PVC comprises at least two sequential hydrocyclones and at least one sorter, disposed between the upper outlet of the first hydrocyclone and the second hydrocyclone, wherein at least one pressure transporter is disposed after the swelling container for soaking the PVC and the first hydrocyclone to transport the substance mixture. 
     Advantageous embodiments of the invention are characterized in the dependent claims whose features are explained in the description of the drawings. 
     The invention offers the substantial improvement that, after carrying out the steps explained below by means of a flow diagram, the PVC is present in pure form. The mixture consisting of further plastic components from which the PVC has been separated, can be further processed in a known fashion, e.g. thermally utilized, extruded or poured. The separated PVC has properties nearly identical to those of new material. Certain material properties, such as plasticity, flowability, tensile strength etc. of the PVC, which in its pure form is extremely brittle, can be influenced through the addition of softeners such as phthalic or terephthalic esters, alkylphosphates or phosphinoxides which partly dissolve in the swelling agent and/or solvent and are re-supplied to the PVC, separated from the other plastic materials. Advantageously, other impurities in the plastic mixture used such as metals, sand, textiles or paper are also separated from the PVC. The PVC thereby no longer contains, in particular, metallic remnants which would disturb further processing. An important feature of the inventive method is the constant shearing of the substance mixture and of the PVC enriched fractions successively separated therefrom in the simultaneous presence of the swelling agent thereby effecting constant core size reduction of the swelling particles while preventing their agglomeration. 
     The inventive device is sealed off in a gas-tight fashion such that swelling agent vapors cannot escape into the atmosphere but rather are almost completely recovered and recycled in the process. The swelling agent is preferably methylene chloride (dichloromethane, CH 2 Cl 2 ) which effects rapid swelling of PVC without changing its chemical structure, while the other plastic components such as polyamides, polyadditions, polyolefines (PE, PP etc.) or PTFE do not or only slightly swell in this medium. The use of methylene chloride for separating PVC insulation from electric cables via mechanical agitation, such as vibrations, stirring, knocking or grinding effects is known per se from DE 23 28 448 A1. 
     The invention is explained in more detail below by means of a preferred embodiment and with reference to the drawing. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a first portion of a flow diagram for carrying out the method and device of the invention; 
     FIG. 2 shows a second portion of a flow diagram for carrying out the method and device of the invention; 
     FIG. 3 shows a third portion of a flow diagram for carrying out the method and device of the invention; 
     FIG. 4 shows a schematic view of a hydrocyclone utilized in the system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The starting PVC material containing plastic waste, accompanying impurities, such as metals, sand, textiles or paper is supplied via feed belts  1  to a shredder  2  which mechanically disintegrates the starting material to the length required for processing. The shredder  2  may be e.g. a cone crusher or conical cylinder crusher. The previously disintegrated substance mixture is supplied to a reactor  4  disposed in a closed feed container  3  and passes through the filling layer  5  of the reactor  4  into the feed container  3 . The function of the reactor  4  is described below. The disintegrated feed is transported from the feed container  3  by means of a pressure-tight star feeder  6   a  and into a buffer container  7  and conveyed from there by means of a pressure-tight star feeder  6   b  to a swelling container  9  to which the swelling agent, preferably methylene chloride is also supplied via the line  8 . The star feeders  6   a ,  6   b  not only serve for transporting the substance mixture but also act as a gas cutoff between the upstream and the downstream system components. The PVC contained in the previously disintegrated substance mixture is swelled in the swelling container  9 . 
     In the embodiment shown, the swelling container  9  is formed as feed screw which is driven by a controllable motor  11  such that the swelling time of the PVC is continuously controlled through the rotational speed of the feed screw. The swelling container  9  can e.g. be a discontinuously operated stir container reactor. In this case two stir container reactors are preferably provided in parallel. 
     The star feeder  6   c , connected after the swelling container  9 , transfers the swelled feed to a pressure transporter, preferably a pump  12 , e.g. a thick matter pump, centrifugal pump or the like. The pump  12  transports the substance mixture, mixed with methylene chloride, under continuous application of shearing forces and into a mixing container  13 . The pump  12  thereby establishes a pressure required for transport of the substance mixture. This pressure depends on the configuration of the hydrocyclone arrangement (described below) and is not more than 1.5 bar, preferably not more than 1 bar. Alternatively or additionally, an injection device, supplied with the swelling agent, is provided as pressure transporter, wherein the substance mixture is both acted upon by pressure through the permanent application of shearing forces, while, at the same time, being thinned to reduce its viscosity. 
     The mixing container  13  comprises a means for mechanically disintegrating the swelled PVC in the form of a high speed stirring device  13   a  which mechanically decomposes the PVC through shearing action in the flow produced by the stirring device  13   a  to grain sizes of less than approximately 1 mm while the shape and size of other plastic components which have not swelled or which have slightly swelled in the methylene chloride remain largely unchanged. 
     The pump  42  supplies a controllable volume flow of solvent and/or swelling agent, in particular methylene chloride, via the line  10  into the mixing container  13  to increase the flowability of the substance mixture. 
     The pressurized substance mixture mixed with methylene chloride is supplied via the line  14  into a first hydrocyclone  15  (FIG. 2) of an inventive hydrocyclone arrangement, which is operated in such a manner that all heavy and coarse-grained impurities, e.g. sand and metal particles which are larger than e.g. 50 μm, and all plastic particles which have not swelled or which have only slightly swelled and which are smaller than approximately 1 mm and whose density is larger than the density of the swelled PVC are fed, as a heavy fraction, via the downward flow of the hydrocyclone  15  to a star feeder  65 . The fraction separated in the sink flow of the hydrocyclone  15  is continuously fed from the hydrocyclone  15  through the star feeder  65  which preferably has rotational speed control. The star feeder  63  also prevents a pressure drop at the discharge of the hydrocyclone to largely prevent a central back-flow in the hydrocyclone  15 . The solid particles are thus layered, depending on their size and density, in equidistant regions along the conical, downwardly tapering walls of the hydrocyclone  15 . The rotational speed of the star feeder  65  thereby permits adjustment of both the amount as well as the particle size or density of the fraction separated in the sink flow of the hydrocyclone  15 . Moreover, a hydrocyclone arrangement of this kind requires a relatively small supply pressure to thereby reduce the operational costs of the inventive device. In order not to impair the working pressure in the region of the sink flow-sided discharge of the hydrocyclone  15 , the star feeder  65  is preferably completely flooded with swelling agent. 
     The heavy fraction separated in the downward flow of the hydrocyclone  15 , is transferred by the star feeder  65  to a sorter  16  which may be designed e.g. as fine mesh oscillating screen. The heavy fraction separated in the downward flow of the hydrocyclone  15  is separated from the remaining swelling agent and discharged to the line  44  while the swelling agent itself is re-supplied via the line  25 . Alternatively or additionally, the heavy fraction can be floated or sorted in a different fashion. 
     The swelled PVC particles exit, together with the plastic particles which have not swelled or which have only slightly swelled, from the hydrocyclone  15  as a light fraction via the upward flow and reach a sorter  23 . If the sorter  23  is also designed as an oscillating screen, the mesh width of this oscillating screen  23  is preferably dimensioned such that particles which are smaller than approximately 1 mm pass through the screen whereas particles which are larger than approximately 1 mm, e.g. paper or textiles, are separated as screened residue and are likewise supplied to the line  44 . The same is the case e.g. with flotation. 
     The material passing through the oscillating screen  23  contains swelled PVC particles and also buoyant plastic particles smaller than approximately 1 mm. They are transported, together with the swelling agent, into a second hydrocyclone  24  having a star feeder  66  downstream of the sink flow-sided discharge. The hydrocyclone  24  is preferably operated at a considerably higher rotational speed than the hydrocyclone  15 . Here, the swelled PVC particles are separated in the downward flow and reach the line  27  via the star feeder  66 . Buoyant particles smaller than approximately 1 mm, e.g. of plastic material, paper or textiles are transported, together with the swelling agent, in the upward flow of the hydrocyclone  24 . The different rotational speeds of the hydrocyclones  15 ,  24  can be regulated via the pressure difference between inlet and rising flow-sided outlet, wherein e.g. valves or throttle sections may be provided. Additionally, a further pressure transporter may be provided between the hydrocyclones  15  and  24  for generating a higher pressure difference. In an arrangement consisting of two hydrocyclones  15 ,  24 , each having a downstream star feeder  65 ,  66  for separating PVC, it is generally sufficient to use only one pressure transporter  12 . It is of course also possible to dispose more than two hydrocyclones in series. 
     The buoyant particles separated in the upward flow of the hydrocyclone  24  are transported together with the swelling agent, via the lines  25  and  26 , to a separator  43  in which the particles are separated and again guided into the line  44 . The line  44  leads to a heated drier  17  wherein absorbed swelling agent evaporates and is guided back into the process via the line  46 . The drier  17  comprises e.g. a heating jacket  17   a  through which a circulating heating medium cyclically flows. The heating circuit comprises the heat exchanger  19 , the circulating pump  20  and the lines  22   a-d . The dried and desorbed product  21  leaves the drier  17  via the line  44   a , which may be provided with a star feeder (not shown) for active transport, and joins the heavy fraction in the collector  18 . The PVC-free product  21  which may contain impurities such as sand, metal, paper or textiles, depending on the initial bulk material, may be further processed in a conventional fashion. 
     The PVC separated in the downward flow of the hydrocyclone  24  which has slurry properties after swelling can be supplied via the line  27  to an arrangement of sequentially connected dry transport screws  28   a ,  28   b  to  29   a-n  (shown in FIG. 3) of which at least one—two ( 28   a  and  28   b ) in the embodiment shown—comprises a heating jacket for evaporating or desorbing the swelling agent. The dry transport screws are integrated via the lines  22   a ,  22   b ,  22   c  into the heating cycle of the drier  21 , wherein heat transfer is effected through the heat exchanger  19 . The evaporated or desorbed swelling agent is returned to the process via the lines  31  or  46 . After exit from the heated dry transport screw  28   b , the PVC which still contains swelling agent remnants, is transported via the line  27  into a cascade of dry transport screws  29   a-n  which are evacuated by a vacuum pump  32  for complete evaporation or desorption of the swelling agent. The dwell time of the PVC having the swelling agent in the dry transport screws  28   a ,  28   b ,  29   a-n  can be varied via their rotational speeds, e.g. through controllable motors  61   a ,  61   b  or  62   a-n.    
     The gaseous swelling agent removed in this fashion is combined in the line  46  with the gaseous swelling agent removed from the mixing container  17  and is supplied via the lines  33   a ,  33   b  to the heating jacket of the dry transport screws  29   a-n . The evaporating temperature of the methylene chloride in the dry transport screws  29   a-n  is approximately 10° C. at the partial pressure prevailing there. The methylene chloride is condensed in the heating jacket of the dry transport screws  29   a-n . Alternatively or additionally, other containers may also be provided for evaporating or desorbing the swelling agent, e.g. distilling columns, rectifying units etc. 
     The swelling agent vapor not yet condensed in the heating jacket of the last evacuated dry transport screw  29   n  is guided via the line  33   c  into a condenser  36  which is e.g. air or water cooled. The condensed material passes via the line  47 , together with the condensed material from the heating jacket of the last evacuated dry transport screw  29   n  (line  48 ), into a swelling agent tank  39 . To always ensure a sufficient amount of swelling agent, e.g. two swelling agent tanks  39 ,  40  are provided which are interconnected via a compensation line  35 . A level-regulated feed pump  45  always supplies the required amount of swelling agent via the line  26  into the separator  43  (FIG. 2) or via the line  8  into the transport screw  9  (FIG.  1 ). The dry transport screws  29   a-n  which are under partial pressure and whose heating jacket serves as a condenser for the gaseous swelling agent, serve for heat recovering and thus reduce operational costs. 
     The additives contained in the PVC charge and in particular those dissolved in methylene chloride during swelling in the transport screw  9  are separated during evaporation of the swelling agent in the dry transport screws  28   a ,  28   b ,  29   a-n  and returned to the recovered PVC such that its properties correspond to the properties of the PVC charge. These additives are mainly softeners, e.g. not easily volatized components having high boiling temperatures such as phthalates or terephthalates, alkylphosphates or phosphinoxides. These components do not volatilize during removal of the swelling agent, neither in the actively heated dry transport screws  28   a ,  28   b  nor in the evacuated dry transport screws  29   a-n  such that the softeners are dissolved again in the PVC to prevent concentration thereof in the swelling agent circuit. The dried PVC is preferably discharged, via a pressure-tight star feeder  6   d , to a storage container  41 . 
     The storage container  41  may have active internal ventilation provided by a blower  50  to optionally desorb swelling agent still contained in the PVC. The feed air is preferably withdrawn from the feed container  3  via the line  52  and guided via the line  51  to the line  37  which returns the exhaust air, charged with desorbed swelling agent, together with the uncondensed swelling agent vapor in the condenser  36 , into the feed container  3 . Therein, the exhaust air charged with small amounts of gaseous swelling agent is fed from the lines  37  and  51  into the reactor  4  which is disposed below the inlet of the feed container  3  in such a manner that it is constantly filled with fresh starting products which adsorb the swelling agent remnants in the filling layer  5 . The reactor  4  may e.g. be a swirling bed or stationary bed reactor. It may be disposed in the closed feed container  3  or can constitute a closed feed container. The exhaust air from the filling layer  5  and the feed container  3  is discharged via the chimney  38 . It may contain slight traces of foreign gases of no environmental importance. 
     FIG. 4 shows, by way of example, one hydrocyclone  24  of the inventive hydrocyclone arrangement for separating the PVC. The hydrocyclone  24  comprises an approximately tangential inlet  71  via which the swelled PVC particles are introduced under pressure together with buoyant plastic particles and swelling agent. The tangential feed and the pressure generated e.g. by the pump  12  (see FIG. 1) create turbulent flow  73  with high rotational speed which is further increased by the wall  72  extending conically in a downward direction. An under-pressure prevails in the turbulence center  74 , along the longitudinal central axis  75  of the hydrocyclone  24 . The PVC particles are accelerated towards the walls  72  by the centrifugal forces and are transported by the rotational speed of the turbulent flow  73  along a guided spiral track, towards the sink flow-sided outlet  27 . The buoyant plastic particles with a density less than that of the swelling agent, accumulate in the region of the turbulent center  75  and are discharged out of the hydrocyclone  24  via the outlet  25 , formed as dip pipe. At the sink flow-sided outlet of the hydrocyclone  24 , the PVC particles are continuously discharged via the star feeder  65 . The star feeder  65  has rotational speed control and is preferably completely flooded with the swelling agent to prevent a pressure drop at the sink flow-sided outlet and thereby leading to the formation of layers  76  of the particles, in dependence on their size and density, wherein the discharged solid volume is replaced by the corresponding volume of swelling agent. In this fashion, the particle size and density of the fraction separated in the sink flow of the hydrocyclone  24  is regulated through control of the rotational speed of the star feeder  65 .