Recycling of plastic waste
Because plastics are produced on the one hand by chemical synthesis processes from finite, natural raw materials such as crude oil , coal , natural gas or also from renewable plants , on the other hand, as a rule, they can no longer be "taken back" by nature through biological degradation processes and can permanently damage them, the polluter must be responsible for - humans - dispose of used plastic waste in a nature-friendly way. This is only possible through recovery or recycling .
Plastic recycling is always in competition with new production. With low oil prices, material reuse is not worthwhile from an economic point of view.
Plastic waste and its recovery in Europe
Around 2.5 million tons of plastic waste per year were exported from Europe to China until, at the beginning of 2018, China banned the import of plastic waste with a pollution content of more than 0.5 percent. Because of the ban in China, plastic waste was increasingly disposed of in Asia. Since 2019, some countries such as the Philippines and Malaysia have been sending the incorrectly declared hazardous waste back to the countries of origin.
Plastic waste differs according to its origin and purity . So z. B. A distinction is made between production and consumption waste as well as clean, single-type and mixed, soiled waste . In general, plastics can be recycled in terms of materials, raw materials and energy.
The amount of plastic-rich consumer waste in the EU-25 countries as well as Norway and Switzerland was around 22 million tonnes in 2005. Of this approx. 19.7 million tons in the EU-15 states and approx. 2.3 million tons in the new EU members (excluding Bulgaria and Romania ).
The largest proportion of waste generated by nearly 62% (13.6 million tons) have packaging , followed by construction, automotive - and electrical / electronics - industries with 7%, 5% and 4% (corresponding to about 1 , 5, 1.1 and 0.9 million tons).
Approx. 46% (approx. 10 million tons) of this waste was recycled, 1.6% (353,000 tons) were stored temporarily (for energy recovery) and approx. 53% (approx. 11.6 million tons) were disposed of. The recovery rate is made up as follows:
- approx. 27% energy recovery
- (whereby approx. 25% of the waste was recycled in waste incineration plants (MVA) with energy extraction and 2% in other plants - such as power or cement plants )
- approx. 18% material recycling
- (whereby 16.7% of the waste was recycled and 1.0% raw material).
The recycling rates differ greatly in individual European countries: from approx. 1% in Greece to more than 95% in Denmark , Sweden and Switzerland . In Germany approx. 77% plastic-rich consumer waste is recycled (as of 2005).
In Germany , the following picture emerges with regard to the handling of plastic waste (from private households and from trade): around 60.1% is "energetically recovered", i.e. incinerated in special processes (such as in cement factories). Around 13.3% is exported. And about 26.6% goes as input into recycling plants, whereby about 9.6% is sorted out and incinerated as too heavily soiled or (as in the case of multilayer composite materials) as unusable, while the remaining about 17% is actually in recyclate, i.e. in recycled material Plastic is converted. As an absolute mass of recyclate, around 0.9 million tonnes are recovered each year (as of around 2018/19). The recyclates are mainly used in the construction sector, in the production of packaging and in agriculture, in the vehicle sector or in the electrical / electronics industry. Almost no recyclates are used in the food sector because, with the exception of recycled PET (rPET), they do not meet the conditions for this area of use formulated by the European Regulation EG 282/2008.
Collect and sort
Until the 1980s, plastic waste ended up with normal industrial or domestic waste ( household waste ) for joint disposal in waste incineration plants or landfills . Separate collection was introduced in Germany in 1990 ( Duales System Deutschland ) through corresponding laws . This created a comprehensive service industry which took on the task of collecting, sorting and recycling plastic waste. The system has been further optimized through further regulations for packaging and beverage bottles (e.g. beverage bottle return). With the help of advanced laser or infrared sorting techniques, it is now possible to sort plastics from household waste almost exactly according to type.
Recycling of sorted waste
Clean, single-type waste (mostly production waste from industry) is - if necessary after cleaning with water - mainly treated with the usual plastic processing methods . These methods include e.g. B. extrusion, injection molding or transfer molding processes, intrusion and sintering press processes.
For the processing of clean unmixed plastic waste to recycled materials conventional single or twin screw extruder can be used. In order to meet the high quality requirements, the polymer purity is of great importance. In order to capture all impurities as much as possible, the separation takes place in the melt flow ( melt filtration ) between the extruder and the extrusion tool. Often, different separation techniques are used in advance to achieve purity of the variety. During extrusion , the properties of the plastics can be improved by adding additives .
Injection molding or transfer molding process
In injection molding , plastic particles are introduced into the mass cylinder via a funnel. The material is melted there and homogenized until it is completely plasticized, whereby segregation due to different heat resistance, viscosity and processing temperature must be avoided. After the melt has been compressed by the screw conveyor , the plastic is pressed under high pressure (usually between 500 and 2000 bar ) through a nozzle into the closed tool. After cooling, the molded part can be removed from the tool. As with extrusion, melt filtration is also used in injection molding to avoid contamination. This process can be used to produce thin-walled pallets , plant pots, flower beds, but also bumpers for automobiles.
In transfer molding , on the other hand, an open mold is chosen into which the plasticized compound is filled with low pressure. Then the filling opening is closed and the press closes the tool. The plastic melt cake flows into its final shape. After achieving dimensional stability, the molded part can be removed. This method can be used in the production of simple thick-walled products such as pallets.
Intrusion is a combination of injection molding and extrusion processes. Here, after the plastic has been plasticized, the material is filled into steel molds and then cooled until it solidifies. Depending on the shape, divided or undivided, the product is pushed out or removed manually. As products, beacon feet , plates, planks but also posts for road signs, milestones , reflective posts on street corners, etc. can be manufactured. Mixed and soiled plastics such as those found in household collections can be used in this process.
Sinter press process
A wide range of mixed and contaminated plastic waste can be used in this process. These are poured into cassette-shaped shapes. After closure of the mold it passes through in a shaft furnace from top to bottom, a preheating zone, a melting zone and a cooling zone. The pressure increases the further the mold moves down the shaft furnace. This process enables the plastics to be melted, the molds filled and warped-free cooling with the ambient air. The products are large-area panels up to 60 mm thick.
Recycle mixed and polluted waste
Material flow pretreatment
A distinction is made between thermal and mechanical processes in material flow preparation. Thermal processes ( pyrolysis , visbreaking, etc.) are known processes in petrochemicals . Old plastics are used there with the aim of reducing the chain length of polymers so that the output from the processing can be used in other chemical or energetic processes.
The aim of mechanical processes is to separate heterogeneous waste streams (e.g. shredder light fraction - SLF) into individual usable fractions and to undertake physical conditioning in accordance with the desired route of diversion. In mechanical processes, a distinction is made today between dry and wet processes.
The dry processes usually include the following basic process steps: screening, pre-shredding, magnetic separation (ferrous metals), eddy current separation ( non-ferrous metals ), main shredding and one or more sifting stages (depending on the process and the desired output). The output from dry process plants are iron and non-ferrous metal mixtures, one or more high calorific fractions and one or more mineral fractions. The dry processes include e.g. B. VW-Sicon, R-Plus, BHS (all Germany).
In the case of wet processes , the following treatment techniques are essentially used: magnetic separation (ferrous metals), sieving, float-sink separation. In principle, metals such as iron, magnesium , aluminum , copper as well as mineral materials and various organic fractions can be separated using wet processes . Process examples are Galloo and Salyp (currently not active).
In addition, other special procedures, such as. B. stripping of plastic parts, separation of multilayer composites (only for production waste). To what extent such processes can also be operated economically for dismantled plastic components (“post-consumer”) is currently open.
With the modifications to the processes that are used to recycle clean, single-type waste (extrusion, injection molding or transfer molding processes, intrusion and sintering press processes - see above), mixed and contaminated plastic waste can now also be processed - mostly into thick-walled products ( downcycling ). . So z. B. twin-screw extruder with degassing for processing more or less soiled and mixed plastics into extruded profiles of different cross-sections and shapes.
Recycling of raw materials
The use of raw materials is understood to mean the splitting of polymer chains through the action of heat into petrochemical raw materials such as oils and gases, which can be used for the production of new plastics or other purposes. If material recycling is not practicable, the raw material recycling of old plastics offers another possibility of material recycling. This is particularly the case when it comes to small-scale, soiled products of different structures and compositions.
The following raw material processes can be used for recycling old plastics:
Gasification, cracking and hydrogenation are some of the petrochemical processes that transform the processes of petrochemicals, e.g. B. Processing of crude oil by distillation and cracking , to break down old plastic polymers. When recycling in the blast furnace, the reduction properties of synthesis gas generated from old plastics are used.
The Austrian OMV Group has developed a so-called "Re-Oil" process and is currently operating it as a pilot plant, in which crude oil and usable gas are produced from shredded plastic waste.
Gasification is a process of partial oxidation of hydrocarbons with a substoichiometric supply of oxygen (the amount of oxygen is insufficient for complete oxidation - combustion - not enough) to carbon monoxide (CO) and hydrogen (H 2 ). The reaction takes place, depending on the process used, at temperatures of up to 1,600 ° C. and under a pressure of up to 150 bar . The process has been known since the 19th century. The starting materials for the gasification were initially coal and coke, and after the Second World War also oil and natural gas .
Utilization in the blast furnace
Metallic iron is extracted from iron ores in the blast furnace process. Coke is used as a reducing agent there . To reduce the consumption of coke, substitute reducing agents such. B. coal or heavy oil is used. Agglomerates made from plastic waste are also used in some blast furnaces.
Cracking is a process of splitting larger organic molecules into smaller molecules under the influence of pressure, temperature and possibly catalysts . Cracking is used in petroleum processing to extract gasoline , LPG or heating oil . A distinction is made between steam cracking and catcracking . The use of plastics is being investigated (up to 20% seems possible).
This is generally understood to mean a reaction of chemical compounds with hydrogen (H 2 ). By hydrogenative cleavage at high temperatures (up to approx. 500 ° C) and pressures (up to approx. 300 bar), it is in principle possible to produce products from organic compounds with almost any carbon chain length in the molecule (including mixed old plastics), which consist of hydrocarbons of shorter chain lengths suitable for petrochemical processes (e.g. gasoline).
Hydrogenation has been known as a process for the hydrative liquefaction of coal since 1927 . This process was used to produce fuel in the 1930s and 1940s . Refinery residues were later processed with it and this process has been used since the 1970s for the recycling of residual materials - mixed and contaminated old plastics (PVC ≤ 10% by weight), old rubber, etc. a. - used.
After all efforts to avoid and materially recycle, there are still fractions left whose material or raw material recovery is not possible or makes no sense for technical, economic or ecological reasons. A dumping of such substances is no longer possible since the entry into force of the Waste Disposal Ordinance on 1 June 2005 in Germany, as only inert products may be deposited with a loss on ignition <5 wt .-%. In Switzerland around 90% of plastic waste is energetically recycled. In principle, processed waste streams with a high calorific value (as so-called substitute or secondary fuel ) can be used in the following systems:
- Power plants
- Cement rotary kilns
- Waste incineration plants (MVA) with energy extraction / waste-to-energy plants
In practice, however, this is limited by the high demands made by incineration plants on the nature of the fuels . To a lesser extent, this also applies to waste incineration plants.
The energy content of waste plastics contained in the waste can be used in power plants when co-incinerating with regular fuels, such as. B. coal can be used. If the waste is incinerated directly for recycling, the flue gas cleaning must meet the emission requirements of the German 17th BImSchV. In addition, this waste must meet the quality requirements of plants in terms of fuel properties.
Cement rotary kilns
Cement is ground from the precursor cement clinker, which is first burned in a cement rotary kiln from appropriately processed raw meal (lime marl). The proportion is 90% raw meal to 10% fuel (solid / liquid / gaseous). Then the cement clinker is ground with gypsum as a setting regulator or other substances (additives). Depending on the application, so-called Portland cement or other types of cement are produced with the appropriate grain size.
The basic material to be fired is fired at high temperatures (gas temperature approx. 2000 ° C) and with a dwell time of approx. 20 minutes (for so-called co-incineration, at least 3 seconds are required). Special attention is paid to the composition and the energy content of the fuel or its mixture, so that both the quality requirements for the cement clinker and all legal requirements with regard to emissions and environmentally relevant parameters in the cement and in the products made from the cement are met.
Co-incineration in waste incineration plants
Today's household waste contains old plastics with a range of 7 to 15%. Technically, it is easily possible to incinerate a higher proportion of plastics, which has been proven in large-scale tests at the MHKW Würzburg . However, the high content of incinerated waste plastics in waste incineration plants leads to a reduced throughput due to the high calorific value .
In the co-incineration of old plastics from technical applications, which in the form of z. B. Shredder light fraction (SLF) must also be based on z. B. increased heavy metal content in the bottom ash. Other operational problems can also arise here.
The co-incineration of old plastics in waste incineration plants could be one of the most economical methods if it were recognized as "energetic recovery". However, the recognition of the assignment in Germany is handled very differently depending on the federal state.
All of the above facts are responsible for the fact that SLF is only incinerated with a share of approx. 5% in MVA or MHKW. According to a current survey by the interest group for thermal waste treatment plants, around 135,000 tons of SLF can be incinerated in 28 German waste incineration plants.
Ecological evaluation of recycling routes
In the environmental policy discussion, questions are repeatedly asked about an ecological assessment of different recycling processes for old plastics. It is crucial for the environment that old plastics or plastic-rich waste are recycled and not end up in landfills.
Life cycle assessments show that
- the ecologically best solution can only be determined in individual cases depending on the respective waste;
- Material recycling of old plastics only has advantages in certain impact categories compared to other process routes if new goods can be substituted in a ratio of almost 1: 1.
- energy recovery has a negative balance in all factors (except for energy).
Economy of the recycling path
The substitution of raw materials with waste can only be successful if the following rules are met:
- The proceeds from the secondary raw materials must bear all the costs of the recycling chain
- The costs for using secondary raw materials must be cheaper than those for “normal” raw materials
- The use of secondary raw materials must not have a negative impact on the production process
- Wastes are after the Waste Disposal Act "will ... movable property, the categories set out in Annex I which the holder discards or intends or is required to discard" (KrW- / AbfG).
- Recycling is: use of the material properties or the energy content of waste. Recovery includes recycling (material, raw material) and energetic recovery
- According to the EC End-of-Life Vehicle Directive, recycling is “... the reprocessing of waste materials in a production process for the original purpose or for other purposes, but with the exception of direct energy recovery”.
- According to the EU End-of-Life Vehicle Directive, energy recovery is "... use of combustible waste to generate energy through direct incineration with or without waste of another type, but with recovery of heat ...".
- Raw material recycling is: the splitting of polymer chains through the action of heat into petrochemical raw materials, such as oils and gases, which can be used for the production of new plastics or other purposes. Raw material recycling is suitable for mixed and contaminated plastic fractions.
- Material recycling is: mechanical processing of used plastics into regrinds or recyclates that can be directly reprocessed. The chemical structure remains unchanged. Material recycling makes sense if old parts can be collected in a clean and sorted manner.
- Recycled material : plastics reused as secondary raw material (word pairing from the word stems for recycling and the suffix of many plastics (e.g. acrylate ))
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