Figure below gives an overview of processes which can take place in a plastic recycling plant. With each step the value of the waste material increases, as the value of waste plastic as a secondary resource depends on its purity and composition.
The plastic recycling enterprise needs quite a large storage space in order to store all collected waste items, processed materials and finessed products. Plastic waste items, especially bottles, have a large volume and therefore a large storage place is necessary.
Sorting and identification
Plastics sorting operations may be carried out manually or automatically using appropriate means of identification. The more accurate and efficient the means of identification, sorting and separation, the better is the quality of the recovered product obtained. Best suited for sorting plastics in developing countries are those technologies that make extensive use of the (comparative) advantage of cheap labor. The secondary raw material obtained by hand sorting is of high quality and offers an excellent basis for producing high quality products by small and medium scale industry.
To aid in identification, it is now common for plastic containers to have a polymer identification code (see plastic type table above). Unfortunately, other plastic applications do not carry such identifiers and are, therefore, more difficult to identify by polymer type without some experience. There are several simple tests that can be used to distinguish between the common types of polymers so that they may be separated for processing.
- The water test. After adding a few drops of liquid detergent to some water put in a small piece of plastic and see if it floats.
- Burning test. Hold a piece of the plastic in a tweezers or on the back of a knife and apply a flame. Dose the plastic burn? If so, what color?
- Fingernail test. Can a sample of the plastic be scratched with a fingernail?
To find out more about identifying different types of plastic bottles, click here
After collecting and sorting the plastic material the option exist to sell the material to (other) processing units. Especially when transporting distances are considerably, it is important to compact the waste to improve handling and save costs during transport. In order to obtain a better price for your material it is advised to bale the plastic with specialized equipment.
If the plastic is dirty, cleaning is necessary. The main cleaning steps are:
- Draining of remaining fluids from containers into prepared collection barrels.
- Rough cleaning of plastic containers and other pieces of plastic.
- Removing of paper, plastic or metal stickers.
- In the case of PET bottles removing of caps and etiquettes.
- Intensive washing in cold or hot water with addition of detergents or caustic soda. The waste water can be reused by installing a simple waste water treatment system like a sedimentation basin.
- When plastic waste is sorted to type, the price per kilo increases substantially.
- When the plastics are further sorted to color, the price increases more.
- When the plastics are baled, the price increases more.
- Large quantities give better negotiation positions.
Cutting is usually carried out for initial size reduction of large objects. It can be carried out with scissors, shears, saw, etc.
Shredding is suitable for smaller pieces. A typical shredder has a series of rotating blades driven by an electric motor, some form of grid for size grading and a collection bin. Materials are fed into the shredder via a hopper which is sited above the blade rotor. The product of shredding is a pile of coarse irregularly shaped plastic flakes which can then be further processed.
Clean film sheet is processed in an agglomerator. The agglomerator consists of a vertical crum with a set of fast moving blades in the bottom. The agglomerator chops the sheets into thin film flakes. Due to the cutting and friction energy of the process, the flakes are heated until they start to melt and form crumbs or agglomerate. This will increase the bulk density of the material which is now fit to be feed directly into the extruder.
For many purposes it is recommended to convert plastic flakes or agglomerate (crumbs) into pellets before processing. The plastic pieces are fed into the extruder, are heated and then forced through a die to form a plastic spaghetti which can then be cooled in a water bath before being chopped into pellets.
a. Extrusion. The extrusion process used for manufacturing new products is similar to that outlined above for the process preceding pelletization, except that the product is usually in the form of a continuous ‘tube’ of plastic such as piping or hose. The main components of the extrusion machine are shown in Figure 2. The reclaimed plastic is forced along the heated tube by an Archimedes screw and the plastic polymer is shaped around a die. The die is designed to give the required dimensions to the product and can be interchanged.
b. Injection moulding. The first stage of this manufacturing process is identical to that of extrusion, but then the plastic polymer emerges through a nozzle into a split mould. The quantity of polymer being forced out is carefully controlled, usually by moving the screw forward in the heated barrel. A series of moulds would be used to allow continual production while cooling takes place. See Figure 2 below. This type of production technique is used to produce moulded products such as plates, bowls, buckets, etc.
c. Blow moulding. Again the spiral screw forces the plasticised polymer through a die. A short piece of tube, or ‘parison’ is then enclosed between a split die -which is the final shape of the product – and compressed air is used to expand the parison until it fills the mould and achieves its required shape. This manufacturing technique is used for manufacturing closed vessels such as bottles and other containers. See Figure 2 below.
d. Film blowing. Film blowing is a process used to manufacture such items as garbage bags. It is a technically more complex process than the others described in this brief and requires high quality raw material input. The process involves blowing compressed air into a thin tube of polymer to expand it to the point where it becomes a thin film tube. One end can then be sealed and the bag or sack is formed. Sheet plastic can also be manufactured using a variation of the process described.
The existence of government support schemes are very helpful to start a plastic recycling unit. When laws and regulations are enforced promoting recycling, it often is much easier to obtain the necessary permits needed for the company.
The recycling business needs to strive to mitigate adverse effects that may arise due to its operations and products through cleaner production technologies; this involves proper waste/effluent management, prevention of any possible pollution, cooperation with relevant national and local environment regulatory bodies and compliance with all available environmental laws and regulations.
Recycled plastic products
There is an almost limitless range of products that can be produced from plastic. However, the market for recycled plastic products is limited due to the inconsistency of the raw material. Many manufacturers will only incorporate small quantities of well sorted recycled material in their products whereas others may use a much higher percentage of recycled polymers. Much depends on the quality required.
In developing countries, where standards are often lower and raw materials very expensive, there is a wider scope for use of recycled plastic material. The range of products varies from building materials to shoes, kitchen utensils to office equipment, sewage pipe to beauty aids.
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