PLASTICS LIFECYCLE | CHAPTER 7 | WHAT is THE RECYCLING PROCESS FOR PLASTICS?
Stage 1: Sorting
When plastic items are taken to a recycling facility, they are first sorted according to plastic type. This is crucial because contamination can render a batch of materials unsuitable for reprocessing. Sorting is done manually or with machines using technology that recognises different sorts of plastic.
Sorting machines are used to identify and separate large amounts of plastic. Advanced sorting machines come with infrared, x-ray or other cutting-edge sensors that can recognise a polymer’s unique signature. These machines also have ejectors that can handle incredible rates of input. Colour detection is possible with some sensors that are able to sort specific colours apart from others.
The most regularly recycled plastics include PET (polyethylene terephthalate), HDPE (high-density polyethylene), PP (polypropylene) and PVC (polyvinyl chloride). Some facilities can only process certain kinds of plastic products.
Stage 2: Resizing
The next step is the size reduction of the plastic waste into smaller sizes for further processing and easier handling. Standard size reduction of plastics is performed by shredders and granulators. These machines have industrial blades that perform rotational cutting to chop down the plastic, which is passed through a screen and then taken away for the next stage in the process.
Some recyclers end their reprocessing at this stage, and supply ‘shreds’ or ‘regrinds’ to the industry. To maintain high standards of purity and quality, shredders and granulators have trained operators at each machine, and metal detectors fitted to customised conveyor systems.
Stage 3: Wet separation
Next come the separation phases. After sorting and cutting, the plastic pieces can be washed to remove traces of dirt and contaminants, which vary from paper and glue, to sand and grit, and mixed plastic types that can be separated in water.
Float tanks are commonly used to separate the plastic pieces according to density. Washers and water baths or tanks are used to remove contaminants in washing lines that continuously spray hot water over the plastic pieces to remove dirt and any labels stuck to the surface of the materials. Chemicals, detergents, disinfectants or other products can be added to improve cleaning and separation.
Scrap plastic is processed by washing and drying plants, capable of cleaning large quantities of plastic waste or scrap.
Stage 4: Dry separation
With dry treatments, plastic is differentiated based on air classification, which basically means that thinner materials are filtered from thick ones. Apart from size and shape, plastic can also be separated by other features. Heat can be applied to plastic to separate materials by melting point, whilst fluorescent or ultraviolet light helps to divide them according to colour, or their ability to absorb light.
Using the most advanced technologies, fully-automated machines are capable of sorting plastic materials by colour and type. They can also handle removal of metal contamination, electro-static removal of wood, rubber or foil contaminants, de-dusting and sieving of soiled or dirty materials, and removal of fines.
Stage 5: Compounding
The final step in most plastics recycling processes is compounding, which involves converting plastic regrinds into pellets, and often the incorporation of elements to transform the reclaimed plastics into high-quality, reusable materials. In pellet form, plastic is more easily distributed and remanufactured.
Machines commonly called compounding line or extruders with single or double screws are normally used to pelletise the plastics. These compounding services offer a viable, environmental-friendly alternative to virgin plastics. Raw materials produced from a sustainable source, such as recycled plastics, are also more cost-effective.
Chapter 8, and finally, next time: So what now?