DE102007055765B4 - Process for the electrostatic separation of plastic mixtures - Google Patents

Abstract

Process for the electrostatic separation of plastic mixtures, in which the plastic mixture, which consists predominantly of a mixture of comminuted polyolefin particles, is subjected to electron irradiation up to a maximum of 10 MeV and subsequently the plastic mixture is subjected to at least one triboelectric charging and subsequently at least the polyolefins are separated in a substantially pure manner be discharged.

Description

The This invention relates to the fields of chemistry and waste management and relates to a process for the electrostatic separation of plastic mixtures, which for example the separation of plastic waste from the automotive and electrical industries can be used.

Increasingly is transferred to also plastic waste in addition to a thermal disposal also raw and material recyclable. Indeed are plastic waste usually as a composite or as a mixture of different plastics. For one Material recycling of plastic waste is a separation into varietal Components mandatory.

So far the use of recycled plastic recyclates is severely limited because for their processing to high quality products, d. H. Products with one high added value, only insufficient unmixed plastic fractions can be provided and / or the sorting effort for sufficiently pure fractions every acceptable cost blasts.

by virtue of The immiscibility of most plastics among themselves is one high incompatibility the individual types of plastic with each other both in the melt as well as in the final molding given, which in particular to problems mechanical stress on recycled products.

A known possibility To separate plastic waste mixtures effectively, consists in the triboelectric Charging the plastic waste shredded into granules and their subsequent substance-specific separation by electrosorting in an electrostatic Freefallscheider (Schubert, G. et al.: DFG-Sonderforschungsbereich 285 7 / 1995-12 / 2004; Schubert, G. u. a .: Final Report on the Collaborative Research Center 285; Husemann, K. u. a .: Shaker-Verlag, series of reports from the field of process engineering, Aachen (2004), pp. 183-212). The electric sorting delivers under optimized process conditions (eg. B. charging time, air speed, relative humidity, temperature, Field strength Number of components to be separated, type of pretreatment of the plastic mixtures) Fractions of high purity. Mechanistic investigations allow a prediction about Separation successes in the electrosorting of plastic mixtures (Albrecht, V. and. a .: J. Electrostatics 58 (2003) 3; Augsburg, A. u. a .: J. Electrostatics, Special Issue (1997) 76; Albrecht, V. and others a .: "Polymer materials 2002 ", Martin-Luther-University Halle-Wittenberg, Vol. 1 (2002) 569-572; Albrecht, V. and others a .: Chemical Engineering Technology 74 (2002) 1425).

in principle can Plastic mixtures after a triboelectric charging in one separated electrostatic field. The low energy consumption, which is necessary, represents a decisive economic advantage over other separation method for plastic sorting. For a separation The composites and other plastic waste are initially on a grain size of some Crushed millimeters. The triboelectric charging of the granules takes place in charging units of different design (eg rotary drum, Fluidized bed or shaking apparatus). While the charging process, the particles of the mixture to be separated mechanically contacted (shock). In doing so, they charge substance-specific positive or negative electrostatic on. The charged particles separate material-specifically accordingly their charge when passing through the field of an electrostatic Free-fall separator.

Well Separable are mixtures whose components are triboelectrically strong and charge with the opposite sign.

separation problems bring plastic components that do not or only slightly charge. Plastic mixtures such components can not be separated by type. To belong to these plastic mixtures also polyolefin blends, such as polyethylene / polypropylene and others. These polyolefin blends pose due to their high production volume and its broad application and application spectrum is the main component of the total volume of plastic waste.

Plastic mixtures, those of polyolefins, in particular of polyethylene and polypropylene types can not be passed through the conventional electrostatic Separate charge sufficiently pure.

Also preparatory steps, such as washing the polyethylene / polypropylene blends, adsorption of surfactants or polyelectrolytes, spraying with Alkalis or acids lead to no satisfactory type purity after the electro sorting.

Of the Dye Nigrosin, which comes from solution applied to the polyethylene / polypropylene mixtures to be separated becomes possible to influence the charging behavior of the individual fractions in such a way that a sorted separation possible is (Németh, E .: Dissertation, Technical University Bergakademie Freiberg, 2003; Németh, E. and. a .: Mineral Processing 45 (2005) 35-46; Stückrad, O .: Dissertation, Technical university Clausthal, 1996).

The disadvantage of this method is the application of the intense dye, in the white direct processing of the unmixed polyethylene or polypropylene components can not or only with considerable technological effort can be removed.

It has proved to be more efficient for the separation of polyethylene / polypropylene mixtures from a pretreatment of the mixtures with a low-pressure oxygen plasma, wherein the pretreatment must be followed by a certain storage time (t> 2 h) ( DE 10 2004 024 754 A1 ). Treatment with air or nitrogen plasmas leads to a difficult separability of the polyethylene / polypropylene mixtures.

There Plasma treatments under the for the separation attempts selected Conditions, only the near-surface Area of plastic granules in the way that modify a Breaking up the crystalline areas occurs and training less polar functional groups (Heydel, B .: Diploma thesis, Technical University Dresden, 2007), with no significant change in material properties to count the polyethylene / polypropylene particles, leaving one further material utilization of the individual fractions over the Melt (thermoplastic processing) should not stand in the way.

When Significant disadvantages of plasma pretreatment must be the discontinuous Operation (batch operation) of the plasma treatment and the integration an expensive vacuum process.

The Treatment of plastics, including polyolefins polyethylene and polypropylene, with high-energy electrons is known and becomes technically diverse (Heger, A. et al.: Technology of the radiation chemistry of polymers. Akademie-Verlag, Berlin (1990).

While polypropylene preferably decomposed by electron treatment, polyethylene tends to be preferable for networking. In recycling, the electron treatment is so far used only to improve miscibility (Czvikovszky, T .: Nucl. Instr. and Method in Phys. Res. B 105, p. 233-237, 1995; Czvikovszky, T. u. a .: Nucl. Instr. and Method in Phys. Res. B 131, p. 300-304, 1997;] Czvikovszky, T. u. a .: Rad. Phys. and Chem. 55, p. 727-730, 1999; Burillo, G. u. a .: Rad. Phys. and Chem. 63, p. 241-244, 2002).

all known solutions in the separation of polyolefin blends is common to them require a considerable effort and are therefore expensive, and / or the quality negatively affect the products (entry and accumulation of Dyes, partial oxidation of the polymers).

Furthermore, various solutions are known which sort polymers due to the different density ( US 2004/0035757 A1 ; US 2004/0004033 A1 ; US 64 60 788 B1 ; DE 29 00 666 C2 ). However, these methods are not applicable to polyethylene and polypropylene because they have very similar densities.

Also electromagnetic radiation ( DE 101 57 032 A1 ) and microwaves ( JP 09262556 A ) were used for the sorting of particles, or also methods by which particles were marked and later sorted out ( WO 93/09172 A1 ).

The The object of the invention is a method for electrostatic Specify separation of plastic mixtures, by a substantially Completely pure separation of plastic mixtures is realized.

The The object is achieved by the invention specified in the claims. advantageous Embodiments are the subject of the dependent claims.

at the method according to the invention for electrostatic separation of plastic mixtures is the Plastic mixture, which for the most part Part consists of a mixture of crushed polyolefin, subjected to electron irradiation up to a maximum of 10 MeV and subsequently the plastic mixture of at least one triboelectric charge subjected and subsequently at least the polyolefins discharged essentially sorted separated become.

advantageously, a plastic mixture is used, which is completely off crushed polyolefin consists, wherein still advantageously a plastic mixture is used, which for the most part Proportion and even more advantageously completely of comminuted polyethylene and Polypropylene particles consists.

Likewise advantageously comminuted plastic particles with dimensions of 0.5 × 0.5 × 0.5 mm ³ to 5 × 5 × 5 mm ³ are used.

Farther Advantageously, the electron irradiation is repeated several times and / or carried out with changing energy input.

And also advantageously, the electron irradiation with electron currents of 1 mA max.

Advantageous It is also when the electron irradiation with dose levels up to maximum 35 kGy performed becomes.

It is also advantageous if the electric irradiation under normal atmospheric conditions at room temperature.

Also It is advantageous if the electron irradiation under vacuum, inert or reactive atmosphere carried out becomes.

And It is also advantageous if the electron irradiation in a the atmosphere with a maximum humidity of 95%.

From It is also advantageous if the electron irradiation at temperatures of 20 K below the lowest melting point of a component in the Plastic mixture performed becomes.

Farther it is advantageous if the plastic mixture after electron irradiation is stored, advantageously when a storage of 10 min to 30 days and more preferably from 10 hours to 2 days.

Also it is advantageous if the plastic mixture after electron irradiation is moistened.

And it is also advantageous if before or after the electron irradiation the Nichtpolyolefinanteile the plastic mixture are separated.

Advantageous It is also when the plastic mixture of a triboelectric Charge is subjected, wherein advantageously the triboelectric Charge in a vortex chamber in a 1 to 15 airflow m / s speed, with a relative humidity of 0 to 95% and a temperature of 0 to 140 ° C with a residence time of Plastic particles of at least 10 s we performed, and more advantageously the triboelectric charge in a shaking pot at a relative Humidity from 0 to 95% and a temperature from 0 to 140 ° C with a Residence time of the plastic particles of at least 10 s is performed.

Also It is advantageous if the separation of the polyolefin in a freefall separator at electric field strengths of 1 to 10 kV / cm is performed.

Also It is advantageous if the separation of polyolefin to at least 99% pure

And it is furthermore advantageous if the separation of polyethylene and polypropylene particles realized at least 99% sorted pure becomes.

With the method according to the invention is it possible for the first time Plastic mixtures, for the most part Part consist of a mixture of crushed polyolefin and which can not be sufficiently charged with triboelectrics so far, so separate sorted that a reuse is possible. This is especially true for the sorted separation of polyethylene particles and polypropylene particles from a common mixture.

The Treatment of plastics with high-energy electrons is well known in particular, polypropylene is known to undergo such treatment is preferably degraded and polyethylene preferably for crosslinking inclines.

Now but has been recognized in the context of the development of the invention, that the non-polar polyolefins just before a degradation or before a network be polarized by treatment with high-energy electrons. Due to this polarization, in the subsequent triboelectric charging Such a charge difference can be generated that a simple and very effective sorting of polyolefins and in particular of polyethylene particles and polypropylene particles into one nearly unmixed state can be realized.

With the method according to the invention Is it possible, to charge the polyolefin plastic particles in a selective tribological way that an electric sorting subsequently be carried out successfully can.

The advantageous usable plastic mixtures consist of polyolefins and more preferably from mixtures of polyethylene and polypropylene types.

The Electron radiation can with electron energies of up to 10 MeV done, that is, it can low-energy electrons with energies less than 200 keV, the preferably used in the surface layer and surface treatment come as well as high energy electrons with energies up to 10 MeV, which are preferred in volume treatment (sterilization and Networking) application can be used. The maximum electron energy of 10 MeV results from legal restrictions.

The electron current used here is dependent on the width of the electron exit window, also called working width, and is within the working range of commercially available electron accelerators (less than 1 mA per cm working width). The maximum applicable dose depends on the polymer type and is z. In polyethylene limited by the electron-induced crosslinking process. The maximum dose for high-density polyethylene (HDPE) is 35 kGy, as at higher dose levels the subsequent thermoplastic processing is affected by low melt flow index values. The electron beam The reaction can be carried out under atmospheric pressure, in air or in an oxygen atmosphere and / or under inert or reactive process conditions. Before the further processing (electrostatic separation) of the irradiated plastic mixtures, it is advantageous to carry out a storage of the electron-treated plastic mixtures of 10 minutes to 30 days, advantageously between 10 hours to 2 days. However, this intermediate storage is not absolutely necessary for the success of the subsequent triboelectric charging and electrostatic separation.

From Another advantage is when the triboelectric charging of the electron-treated and stored plastic mixtures in a vortex chamber at a Air flow from 1 to 10 m / s and at a relative humidity of 0 to 95% and a temperature of 0 to 140 ° C for at least 10 s or in a shaker pot at a relative humidity of 0 to 95% and a temperature from 0 to 140 ° C for at least 10 s is realized.

Farther it is advantageous if the electric sorting in an electric Field of a free fall separator with electric field strengths between 1 and 10 kV / cm performed becomes.

By the inventive method It is possible in a simple and inexpensive way, so far only difficult and with low effectiveness separable plastic mixtures, which are predominantly made of polyolefin particles exist, now sorted with high efficiency to separate. This is through the inventive method with the electron irradiation of the plastic mixtures possible what previously neither known from the prior art, nor derivable from it has been.

Of the Influence of electron irradiation on the electrostatic separability of plastics has not been studied so far and so is so far no influence on it was known or derivable.

Surprisingly Electron irradiation of the plastic mixtures has a selective effect the charging behavior of the individual plastics in the mixture, so that a subsequent easy and unmixed separation is possible. In particular, in two-component polyolefin blends, for example Made of polyethylene and polypropylene, the components load substance-specific with different signs on and leave thereby easily separate.

One Addition of additives, such as surfactants, polyelectrolytes, plasticizers, Dyes and other surface-active Substances that influence the separation more or less selectively can, not necessary. Also a previous plasma treatment of Plastic mixtures is not necessary. This can cause contamination, in the processing and reuse of the separated Plastic types can prove disadvantageous to be avoided.

There Also, the separation process when using additives by additional Parameters (amount of additive to be used, method of application, homogeneity of additive adsorption, Solvent selection etc.) is considerably harder to control, can by eliminating of additives, the separation process can be made simpler overall.

Of the Elimination of the low-pressure plasma treatment goes hand in hand with the waiver to integrate discontinuously operating vacuum systems in the process. This results in the use of electron irradiation a significant Cost savings. In principle, however, influences the addition of additives or superficial Oxidation products that resulted from the use of plastics are, the separation effect, by the inventive method is achieved, not negative.

at Optimal choice of electron energy and dose is also an improvement the product properties of the recycled polymer Product possible.

following becomes the solution according to the invention an embodiment explained in more detail.

example 1

A plastic mixture (300 g) of polypropylene (150 g) and polyethylene (150 g) is first washed, dried and homogenized. The plastic particles have the dimensions of about 3 × 3 × 3 mm ³ . Subsequently, the mixture is subjected to electron irradiation in air having an electron energy E = 0.6 MeV, an electron current I = 4.0 mA and a dose D = 7.5 kGy.

To a storage of 48 h was the electron-irradiated plastic mixture, in fluidized bed charging unit (air speed 5.2 m / s) of the electrostatic free-fall separator for 4 min at 25 ° C and a Relative humidity of 20% charged and successful in the free fall separator (Field strength 4.5 to 2.0 kV / cm at a potential of ± 72 kV) into a polyethylene and a polypropylene fraction.

A complete sorted separation of polyethylene and polypropylene could be achieved (see 1 and 2 ).

there shows

1 the separation of polyethylene / poly propylene (PE / PP) without prior electron treatment (a) and

2 the separation of polyethylene / polypropylene (PE / PP) according to the inventively carried out electron treatment (b).

Claims (23)

Process for the electrostatic separation of plastic mixtures, in which the plastic mixture, which consists for the most part of a mixture consists of crushed polyolefin particles, an electron beam is subjected to a maximum of 10 MeV and subsequently the plastic mixture subjected to at least one triboelectric charging and then at least the Polyolefins essentially sorted separated discharged. The method of claim 1, wherein a plastic mixture is used, which is complete consists of comminuted polyolefin particles. The method of claim 2, wherein a plastic mixture is used, which for the most part Part consists of shredded polyethylene and polypropylene particles. The method of claim 3, wherein a plastic mixture is used, which is complete consists of shredded polyethylene and polypropylene particles. The method of claim 1, wherein shredded plastic particles having dimensions of 0.5 x 0.5 x 0.5 mm ³ to 5 x 5 x 5 mm ³ are used. The method of claim 1, wherein the electron irradiation is performed several times in succession and / or with changing energy input. The method of claim 1, wherein the electron irradiation with electron currents performed by a maximum of 1 mA becomes. The method of claim 1, wherein the electron irradiation with dose values up to a maximum of 35 kGy. The method of claim 1, wherein the electron irradiation under normal atmospheric conditions carried out at room temperature becomes. The method of claim 1, wherein the electron irradiation under vacuum, inert or reactive atmosphere. The method of claim 1, wherein the electron irradiation in an atmosphere with a maximum humidity of 95%. The method of claim 1, wherein the electron irradiation at temperatures of 20 K below the lowest melting point a component in the plastic mixture is performed. The method of claim 1, wherein the plastic mixture is stored after the electron irradiation. The method of claim 13, wherein a storage from 10 minutes to 30 days becomes. The method of claim 14, wherein a storage from 10 h to 2 days becomes. The method of claim 1, wherein the plastic mixture is moistened after the electron irradiation. The method of claim 1, wherein before or after the electron irradiation the Nichtpolyolefinanteile the plastic mixture be separated. The method of claim 1, wherein the plastic mixture subjected to a triboelectric charging. The method of claim 18, wherein the triboelectric Charge in a vortex chamber in a 1 to 15 airflow m / s speed, with a relative humidity of 0 to 95% and a temperature of 0 to 140 ° C with a residence time of the plastic particles of at least 10 s. The method of claim 18, wherein the triboelectric Charging in a shaker pot at a relative humidity of 0 to 95% and a temperature from 0 to 140 ° C with a residence time of the plastic particles of at least 10 s is carried out. The method of claim 1, wherein the separation the polyolefin particles in a free fall separator at electrical field strengths from 1 to 10 kV / cm becomes. The method of claim 1, wherein the separation of polyolefin particles is at least 99% true to type. The method of claim 1, wherein the separation of polyethylene and polypropylene particles at least 99% sorted is realized.