|Plastics mainly from housing not sorted
Nowadays plastics are found in almost every electronic appliance. They have replaced traditional materials over time, such as metal, wood or glass for many applications. They are usually found in casings, frames, covers and small parts of electronic appliances. Various types of plastic exist, of which ABS (acrylonitrile butadiene styrene), PS (polystyrene) and PP (polypropylene) are usually used in computer manufacturing. In addition, many other types are used depending on their characteristics, as well as plastic compounds.
When plastics are recycled the quality of the material often decreases (downcycling). If several plastic types are recycled mixed together, this degradation is even stronger. As a consequence, one of the main purposes of pre-processing plastics is their separation according to their plastic types and to cleanse the plastics from foreign material.
Depending on the final treatment of plastics, different pre-processing steps can be performed:
- In any case plastics should be cleansed from all foreign material. Especially steel parts are unwanted in the plastic mix as they can destroy recycling machines. Unclean plastic therefore has less value and some recyclers don’t even take it.
- To add a maximum value to the recycling process, plastics should be segregated by type (ABS, PP, PS, HIPS, etc.) and by content of BFRs. Depending on the requirements of the plastic processor, the segregation is realized at the dismantling facility or at the processor’s plant. In most cases, plastic types apt for recycling can be classified by the plastic recycling code (PIC – see Table 5). A bigger challenge is the identification of BFRs within plastic (see below, Flame retardants).
- To transport plastics efficiently, it is reasonable to reduce the particle size of plastic by shredding it mechanically. This is sometimes required from the plastic processor, too.
When shredding, several aspects should be taken into account:
- Only pure plastic fractions should be shredded; avoid the presence of other materials in the plastic fraction.
- Ensure a good maintenance of the shredder.
- In the shredder high temperatures can occur. At temperatures above 300 °C, the generation of dioxins from BFRs is likely to happen. But if a proper maintenance of the shredder is ensured, those temperatures are not reached when shredding pure plastic fraction. At the absence of direct temperature gauging, blue colour changes of metal parts of the shredder are a clear warning signal that temperatures have reached a level where dioxins can be generated (> 300 – 350 °C). In that case the shredder should be stopped and (a) bearings and movable parts of the shredder should be cleaned (i.e. from stuck plastic), (b) the hammers and/or blades of the shredder should be grinded.
Some plastics contain brominated flame retardants (BFR), which are hazardous substances introduced to reduce the flammability of plastic parts. Theses plastics are found in many parts of electronic appliances and must be treated as hazardous materials.
A study by Waeger, Schluep, and Mueller (2010)on toxic substances in mixed WEEE-plastic indicates that ICT equipment (excluding monitors) generally contains brominated flame retardants at concentrations close to or above the European Maximal Critical Value (MCV) . However, the concentration of BFR in similar devices vary significantly. While i.e. CRT casings contain relatively high concentrations of BFR, no BFRs could be found in flat screen casings.
In practice, it is difficult to identify which plastic parts contain BFR and which do not if no special analytical equipment is available.
The heating of plastics containing brominated flame retardants (> 300°C) can cause the formation of brominated dioxins and furans, which are highly toxic. This has to be considered especially when shredding plastic fractions.
At present, treatments exist which allow for the separation of different plastic types, including the separation of plastics with and without BFRs, respectively. The existing treatment alternatives for plastics are summarized in the below table.
|Mechanical recycling (plastics in general)
|Shredding of plastics and subsequent heating for pelletizing.
|Chemical or tertiary recycling (plastics in general)
|Waste plastics are treated by physicochemical processes in which the plastic molecules are broken in order to get monomers or other useful/valuable products for the petrochemical industry and convert into raw materials again. Chemical recycling can take place by different processes: pyrolysis, hydrogenation, gasification, degradative extrusion and methanolysis. A big advantage of some chemical recycling processes such as pyrolysis is that no previous separation by plastic resin type is required, which allows for the recycling of mixed plastic waste. Among the chemical processes used for the recycling of plastics is also the methanolysis (depolymerization process) which decomposes the plastic to its original components by applying heat and pressure in the presence of methanol. This combination not only causes the decomposition of the polymer chain (leaving only monomers which are purified and re-polymerized to a new resin), but enables the destruction of contaminants.
|Incineration with energy recovery (plastics in general)
|By a controlled combustion, this process takes advantage of the high energy content of plastic waste as alternative fuel. Although some plastics can be recycled with benefits to the environment, there may be remaining plastics from the recycling process that cannot be recycled. In case a mechanical or chemical recycling cannot be justified, energy recovery may be an effective way to recover the intrinsic value of plastic waste.
|In developed countries, plastics have to be incinerated. An incineration of plastics containing brominated flame retardants is reasonable if some specifications are considered. To avoid the generation of dioxins or furans, a controlled combustion in excess of 600 ° C must be ensured. Moreover, the incinerator must have appropriate filters and equipment in their furnaces to control pollution, emission of metals, VOCs and dioxins (off-gas control).
Plastics are made of natural hydrocarbons such as oil, gas or carbon. Depending on the technique applied, homogeneous plastics can be valorised as plastics or fuel. This is more difficult for plastic compounds, which can be valorised thermally, i.e. by burning them appropriately in an incinerator.
Restriction of Hazardous Substances in electrical and electronic equipment Directive or RoHS-Directive (2002/95/EC) by the European Union (http://eur-lex.europa.eu/LexUriServ/LexU...).
Source of information
Source: Dismantling Guide for IT Equipment, which was elaborated within the SRI project and financially supported by SECO and UNIDO, June 2015