Last Updated on Wed, 22 Nov 2023
Agglomeration is a process stage to be carried out after the material is sorted into a fairly homogeneous stream. The aim of the agglomeration is to increase the bulk density of the waste, which is necessary to ensure good feeding for some plastication units. It finds most application in waste plastics with film flake, which can have a bulk density of just 100 to 150 kg/m3. This can make both feeding and storage cumbersome. Examples of film waste before and after agglomeration are shown in Figure 5.6.
Agglomeration can be differentiated from processing as here the aim is simply to bond the flakes together at their surfaces and therefore only the surfaces of the plastics are affected. Pressing or fusion are used for this, with the material being thermally stressed on the surface particles only for a relatively short time.
Thermal agglomeration is the method most widely practised with plastic wastes.
A number of stages to prepare wastes for processing have now been discussed. As it has been seen, preparing waste for processing can be time consuming and technically difficult. For this reason, most of the cost in reprocessing materials comes from these necessary, preprocessing stages. The next section looks at processing these materials to make new materials or new components. However, before that, an example of a successful closed loop recycling operation using some of the processes introduced here, is discussed.
In 2006, a scheme to collect HDPE milk bottles and recycle them into new milk bottles was trialled successfully in the UK. The recyclate bottles were sourced both from bottles collected in curbside schemes and in 'bring in' bottle schemes. The bottles were washed in a 2% caustic solution at 93 °C to remove contaminants, labels and adhesive. Once dry, the sorting techniques used to identify the bottles were both IR detectors and sorting by hand in order to get a pure natural coloured recyclate stream. This is used as a mixture of 30% recyclate to 70% virgin plastic to make new milk bottles. Using plastic recyclate materials in food contact also meant rigorous testing to ensure compliance with food packaging legislation. This is the first trial of what will hopefully become established practice in the future.
Recycling of HDPE bottles makes good business sense as it is a high volume material readily identifiable in the waste stream and collected curbside. For example, plastic bottle recycling in the US generated just under 1 billion kg of material in 2005 - 27.1% of this was HDPE.
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