Recycling plastic bottles back into food-grade quality material requires a number of technologies and involves three key stages:

• Sorting
• Granulating and washing
• Decontamination

Sorting
The plastic bottles arrive at the plant, after having been collected from households, local council bring sites and offices throughout the UK. The bottles have been squashed and compacted together into square bales, which typically weigh about 500 kg and contain approximately 12,500 bottles. The bales are mixed, i.e. they contain both PET and HDPE bottles.

The mixed plastic bottle bales are placed on a conveyor belt which feeds the bale breaker - six large rotating "cork screws" that loosen and open up the bale.

Once the bottles are loose, they pass through the trommel, a perforated, rotating drum which has small holes in it. Functioning much like a washing machine drum, the trommel removes small bits of rubbish, such as stones and dirt, as well as any lids and caps that have come off the bottles. These lids and caps are often not the same type of plastic as the bottles and are often made from coloured plastics, so they need to be discarded. Metal contaminants such as food and drink cans, screws and wire are then removed. A powerful electromagnet extracts steel objects and then a device called an eddy current separator is used to remove any aluminium objects.

Paper, carrier bags and films are then separated from the bottles by a row of air jets that blow the light objects off the conveyor belt.

With the majority of unwanted waste removed, the bottles are now ready to be sorted by type of plastic and colour.

Three optical sorting machines that are capable of recognising, separating and ejecting selected bottles from a mixed batch are utilised. Each machine is programmed to detect a different type of plastic or colour and when this passes under its sensors, a jet of air is fired to remove it from the rest.

The result of this sorting process is the separation of the plastic into five storage bunkers:

• Clear PET bottles
• Light blue PET bottlesOnward reprocessing at Closed Loop
• HDPE bottles

• Coloured PET bottlesRecycled at other facilities
• Other plastics

From the storage bunkers the clear and light blue PET bottles go through a final manual sorting process to remove any contaminants prior to granulating and washing. This involves hand-picking anything which is not clear or light blue PET which may have made it through the previous sorting equipment.

The HDPE bottles are also manually sorted, but before this is done, they are sent through a fourth sorting machine which separates them into natural HDPE, which is what milk bottles are made from, and coloured HDPE which is typically used for cleaning products. It is very important that any bottles that have been in contact with cleaning products or other liquids are removed from the process. The coloured HDPE bottles are separated and sold to manufacturers of drainage pipes, wheelie bins, garden furniture and many other items.

Granulating and Washing
After manual sorting, the vast majority of contaminants will have been removed, leaving behind mainly clear and light blue PET bottles and natural HDPE bottles, which are now ready to be granulated. The granulators cut and chop the bottles into small flakes. For the rest of the process, the flakes of PET and HDPE are moved around the plant using pipes and blowers instead of conveyor belts.

Prior to washing, the flakes are put through a dry cleaner to remove anything lighter than the flakes themselves. Any loose labels, fines and small contaminants are removed by a zig-zag classifier, a device which uses an upward thrust of air to blow any labels, paper, or other light material higher than the heavier flake.

To clean the flake of label residue, glues and ink, a hot wash is used. The flakes are washed for approximately 15 minutes at 80°C using a weak solution of caustic soda so that the detached glues and label fragments sink to the bottom where they form a pulp which is then removed. The flakes and any plastic labels detached are then rinsed and spin-dried.

The flakes of lids and caps, which are mainly made from coloured HDPE, will still be present and must be removed from the PET flakes prior to decontamination. This is achieved through a sink-float separator: PET sinks in water while HDPE floats and can therefore be skimmed off.

Decontamination - PET Bottles
To return the PET flakes back into a food-grade product, a process called URRC is used. Developed by an American company called United Resource Recovery Corporation, the URRC process is currently used in seven other countries and meets all European Union and US Food and Drug Administration standards. To super-clean the plastic and remove all traces of contaminants, the surface layer of the PET flake is removed using a solution of caustic soda. The process is carefully controlled by ensuring the correct amount of caustic soda is added. The reaction takes place in stages, with all by-products being carefully removed and treated.

After reacting with the caustic soda, heated air is passed through the mixture, evaporating the liquid products. The chemically treated flake then passes through a rotating furnace for a period of approximately four hours, to complete the reaction. After this, the flakes are cooled in a water tank, thoroughly rinsed and then dried.

To ensure the flake is of a high quality and meets packaging manufacturers’ specifications, it is put through a colour sorter, to remove any coloured pieces, and then through a laser sorter to remove any remaining contaminants. Both devices use detection technology and air jets to remove the unwanted particles. The clean and pure PET flake is then bagged and sold onto plastic packaging manufacturers, to be made into new bottles or other food packaging.

Decontamination - HDPE Bottles
After passing through a colour sorter to remove any coloured flakes, the natural HDPE is ready to be processed back into food-grade standard material. This is achieved by using a Vacurema, which treats the flake under high temperature and low pressure. Heating the flake to temperatures of over 200°C turns it to molten form and eliminates any contamination. The molten plastic is then extruded, filtered, cut into small pellets and cooled. These pellets are then ready to be sold on to make new milk bottles.

For more information about the process contact info@closedlooprecycling.co.uk