Recycling Techniques

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Despite increasing research efforts it is not yet economical to separate the wide variety of plastics that end up in the waste stream. The ideal is streams of single, clean and homogeneous recyclates, which present little technical effort for recycling. Of course, streams of this type, mainly industrial, can and are recycled. However, mixed waste streams present more of a technical and economic challenge to recover.

Consider your household rubbish, perhaps even rummage in your bin!

What plastics are present and what form are they in?

You are likely to find items such as packaging films, detergent bottles, carrier bags, milk bottles, food and household containers.

What possible contaminants are present?

You may have found food debris and residues from the various containers. You may also have noticed paper labels or glue residues or possibly even the use of other materials in combination with plastic, such as aluminium linings or metal closures. Now we have not only a mixed plastics waste stream, it is also contaminated with other materials.

What sort of plastics is the waste stream made up of?

You will find many packaging items are marked with an identification code like those shown in Figure 5.2.

The average breakdown of the composition of plastic components in a common household refuse collection is shown in Table 5.2.

Mixed plastic fractions reduce the quality of the products made from them quite considerably. This is due to differences in melting and processing temperatures and the lack of compatibility that was

Figure 5.2 Society of Plastics Industry (SPI) packaging codes


Figure 5.2 Society of Plastics Industry (SPI) packaging codes

Table 5.2 Mixed plastic fraction of household wastes


Fraction (approximately %)





Polyvinyl chloride




Expanded polystyrene (foamed materials such as fast food packaging)


Remainder (various)


discussed more fully in Chapter 3. This, plus the contamination means that for quality plastics, the waste must undergo a sorting and cleaning process.

There are outlets for mixed plastics, but because of the heterogeneous composition of the raw materials they are limited to the production of large components. This is so that contaminants and any material that has not melted and mixed will not have a great effect on the overall strength. Often products of this type find outlets as plastic lumber or concrete replacement products such as park tables, slabs or benches, where the non-rotting properties of plastics can be utilised.

An example of a process for mixed plastic waste is that used by Wormser Kunststoff Recycling GmbH (WKR) in Germany. This process can tolerate a certain amount of contamination in the process mixture, which is appropriately treated in an extruder, and cast into large moulds to give products. Methods such as these will be discussed in Chapter 6.

Whether the material is reprocessed as a mixed waste stream or is sorted into separate fractions, it still needs to undergo some treatment. A typical recycling route for waste plastics can be split into two types of processes:

  • 1) Physical/mechanical treatment to prepare and homogenise the waste, and
  • 2) Melt processing.

Examples of stage one processes could include storage, size reduction, washing and sorting of the waste. Stage two would include regranulation and reprocessing.

An outline of the various stages of the waste cycle is shown in Figure 5.3. These will now be discussed.

Waste Recycling Techinques
Figure 5.3 A process stream for recycled plastic material

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