DESTRUCTION OF PLASTICS WASTE

Abstract: The plastics waste (which is not itself susceptible to microwave heating) is mixed (B) with pulverulent carbonaceous material, which comprises elemental carbon or is degradable by microwave irradiation to elemental carbon (such as waste tyre material). The mix is subjected to microwave irradiation (C) in an inert atmosphere to cause pyrolysis of the plastics. Preferably part of the resulting solids is recycled to the microwave zone (C).

Description: Destruction of Plastics Vaste The present invention is concerned with the destruction o,f plastics waste. The destruction of plastics waste is a substantia problem, because of the increasing amount of plastics materials being used in, for example, automobile bodies and packaging. Such plastics waste is generally disposed of either by incineration or by use as landfill; the former method is objectionable on enviromental grounds because of the nature of some of the combustion gases produced, while the latter method is inefficient because of the large bulk of plastics material. According to the present invention, there is provided a method of destruction of plastics waste which is not itself susceptible to heating by microwave radiation, which comprises contacting the plastics waste with a pulverulent carbonaceous material which is susceptible to heating by microwave irradiation, the carbonaceous material being retained in a substantially oxygen-free atmosphere in retaining means which is inert to microwave radiation, the carbonaceous materia present in the retaining means being subjected to microwave irradiation, either before or after contact with said waste products, so as to cause substantial thermal degradation (pyrolysis) of the plastics waste. The carbonaceous material employed in the method according to the invention is a material consisting of or containing elemental carbon, or capable of being degraded or pyrolysed by microwave irradiation to elemental carbon. A particularly preferred material which can be degraded by microwave irradiation is carbon-filled vulcanised rubber, such as a waste tyre compound in chopped or finely divided form. Waste tyre compounds generally contain rubber, carbon filler and reinforcement of organic textile and/or metal wire. When an organic textile reinforcement is employed, the latter is generally substantially carbonised by microwave irradiation ; when metal wire reinforcement is employed, the latter would be substantially unaffected by the microwave discharge and can therefore be collected together with solid pyrolysis products. The microwave radiation is preferably employed at such a power and for sufficient time that the carbonaceous material is heated to at least 400°C. Since the method according to- the invention is carried out in an oxygen-free atmosphere, there would in general only be a danger of production of dioxin or other noxious products with certain feedstocks; in such cases it is preferred to ensure that the carbonaceous material (and therefore the plastics waste) attains a temperature of at least δOO^C, which is sufficiently high that the risk of evolution of dioxinε or similar noxious products by the pyrolysed plastics material is minimised or eliminated. If the plastics waste being destroyed (pyrolysed) in the method according to the invention is relatively easily degraded (e.g. polyethylene or nylon), then temperatures from 400 to 800°C may be satisfactory. When more thermally resistant plastics and halogenated plastics (such as PTFE or FVC) are employed, the temperature of the carbonaceous material may be at least 800°C. The retaining means employed in the method according to the invention may be, for example, a fluidised bed or a thermally resistant receptacle. Such a receptacle may be of a material such as stainless steel or a ceramic material which is preferably resistant to temperatures of at least about 800°C, typically of the order of 1000°C. Especially when the carbonaceous material is employed in the form of a fluidised bed, the method according the invention can be operated as a continuous process. It is preferred that the carbonaceous material should be fluidised at least partially by means of hot recycled gaseous by-products (such as hydrogen and/or methane) from the microwave irradiatio zone. The carbonaceous material is preferably supplied continuously and allowed to overflow from the fluidised bed. In the case where a static bed of carbonaceous material is employed, it is preferred that the material should be present in an open-topped receptacle of a material which is reflective to microwaves (such as stainless steel) , which may b refractory-lined if higher temperature operation (i.e. above about 600°C) is envisaged. When such a static bed is employed, solid products resulting from the microwave irradiation step ar preferably mechanically removed. The plastics waste may be supplied to the static bed either continuously, semi-continuously or intermittently (in a batch process, for example) . The plastics is preferably supplied in such a manner that the carbonaceous material is allowed to overflow from the static bed, and is then at least partly recycled thereto. The plastics waste to be destroyed in the method according to the invention may be any of the conventional plastics materials, which may be thermoplastic or thermoset. Typical such materials include polyolefins (e.g. polyethlene, polypropylene or the like), polyamides, polyesters (e.g. polyethylene terephthalate) , polyurethanes and halogenated polymers (e.g. PVC) . It is preferred that the plastics should be substantially free of halogen and nitrogen, in order to avoid production of noxious gases; it is particularly preferred that the plastics consist substantially wholly of carbon and hydroge and, optionally, oxygen. The plastics material may be in finely divided form, or bulk form, provided It can be contacted with the carbonaceous material by, for example, partial immersion or, In some embodiments, total immersion. The wavelength of the microwave radiation employed depends on the particular carbonaceous material used; the wavelength is such that it causes substantial heating of the pulverulent material. It Is typically in the low gigahertz frequency range (e.g. about 2.4 gigahertz), although higher wavelengths can be employed if wished. It is particularly preferred to ensure that the by-product gases from the microwave irradiation zone are rendered non-pollutant before being vented to the atmosphere; this can be acheived by scrubbers or similar means. The by-product gases are typically recovered for use as preheating medium, fuel or the like. The nature of such gases depends of the chemical nature of the feedstock, as well as the temperature achieved in the microwave irradiation process. The solids resulting from the microwave irradiation step may berecycled for use as the carbonaceous material in a further microwave irradiation step; alternatively, it may be used as a fuel. Reference will now be made to the accompanying drawings, in which: Figure 1 Is a block diagram illustrating a typical sequence of operations in an exemplary embodiment of a method according to the invention; and Figure 2 is a schematic illustration of the operation of an exemplary embodiment of a method according to the invention, Referring to Figure 1, scrap vulcanised tyre compound Is passed to pulverisation zone A where it is reduced to suitably sized particles. The resulting pulverulent material is then passed to mixing zone B, where it is mixed with scrap plastics (such that It is at least partially submereged in the carbonaceous material) . The mixed pulverulent material and plastics are then passed to microwave irradiation zone C, which is continuously flushed with an inert gas (such as nitrogen or inert by-product gas from the microwave irradiation zone) vi purged sequenced air-locks (not shown) . In the microwave irradiation zone C, chopped or finely divided scrap tyre material is rapidly raised to red heat, which heat is transferred to the plastics to cause pyrolysis of the latter. Gases and liquids produced are then separated i zone D; the solids are then passed to a classification zone E from which suitably sized solids are recycled to mixing zone (or alternatively direct to irradiation zone C) , while still hot, so as to preheat the plastics waste. The process as described with reference to Figu 1 is particularly suitable for operation in a continuous mann the recycling of at least part of the carbonaceous material t the microwave irradiation zone is particularly preferred beca of the economy of energy input which can be achieved, because the plastics is partly heated to pyrolysis temperature by the recycled material. Referring to Figure 2, there is shown a microwa oven 1 incorporating a microwave generator 2, an inlet 3 for inert gas (such as nitrogen) and an outlet 4 for the inert ga and evolved gases. Disposed within the oven is a bowl or other suitable receptacle 5 (of stainless steel, ceramic or the lik partially filled with pulverulent carbonaceous medium 6; plastics waste 7 is partially immersed in the latter. In an example, a one kilowatt industrial microw oven was employed, and the bowl disposed in the oven charged with approximately One kilogram of carbonaceous medium and a similar quantity of plastics waste. The oven was then suppli with an oxygen-free atmosphere and then actuated for irradiat for a period of three minutes, during which period, the carbonaceous material became red hot (about 800°C) . Gases evolved were removed by an extractor fan passed to a cleaning/scrubbing apparatus (not shown); the pow remaining in the bowl was then ready for charging with fresh quantities of plastics waste.

Claims: CLAIMS 1. A method of destruction of plastics waste which is not Itself susceptible to heating by microwave radiation, whi comprises contacting said waste with a pulverulent carbonaceous material which is susceptible to heating by microwave irradiation, said carbonaceous material being retained in a substantially oxygen-free atmosphere in retaining means which is inert to microwave radiation, th carbonaceous material present In said retaining means bei subjected to microwave irradiation, either before or afte contact with said waste products, so as to cause substantial thermal degradation of said plastics waste. 2. A method according to claim 1, in which said carbonaceous material comprises scrap carbon-filled vulcanised rubber. 3. A method according to claim 1 or 2, in which at least par of said carbonaceous material passes out of the microwave irradiation zone and is recycled thereto. 4. A method according to any of claims 1 to 3, in which said retaining means produces a fluidised bed of said carbonaceous material. 5. A method according to claim 4, In which said carbonaceous material is fluidised by means of a gaseous medium comprising hot gaseous by-products of the microwave irradiation zone. 6. A method according to claim 4 or 5, in which said carbonaceous material and/or said plastics waste is supplied continuously to said fluidised bed, and said carbonaceous material continuously allowed to overflow therefrom. 7. A method according to any of claims 1 to 3, in which sai retaining means comprises an open-topped receptacle. 8. A method according to claim 7, in which said container comprises stainless steel. 9. A method according to claim 7 or 8, in which said plasti waste is supplied to said receptacle in such a manner th said carbonaceous material is allowed to overflow from s receptacle, and then recycled thereto 10. A method according to any of claims 1 to 9, In which sai plastics waste is substantially free of halogen-containi and/or nitrogen-containing plastics materials.

Inventors: HOLLAND KENNETH MICHAEL, HOLLAND, KENNETH, MICHAEL
Applicants: HOLLAND KENNETH MICHAEL, HOLLAND, KENNETH, MICHAEL
Application year: 1988
Main classification: C10B53
IPC codes: B01J19, B29B17, B29K105, C08J11, C10B19, C10B53, F23G5