After collection, the waste plastic will undergo a preliminary cleaning process on site as the waste plastic, especially plastic bottles, may be contaminated. After that, the pre-treated waste plastic will be sorted into different types manually before grinding process. The preliminary treatment will be conducted in “Dirty Zone” so that the wastewater generated will be collected to the wastewater treatment plant and no wastewater will be discharge or spill to the surrounding. The details of the design of “Dirty Zone” will be discussed in Criterion 4 in report.
The pre-treated plastic will be grinded into fine pieces step by step. Firstly, the pre-treated plastic will be transmitted to a shredder by a conveyor belt. The plastic will be grinded into coarse size by a double-shaft shredder. Because of the diversity of plastic, the two-axis shredder should be able to handle the plastic in different sizes, toughness and, such as coin bags, plastic bottles and water-filled barriers. Secondly, magnet steel separator will be installed to separate the metals adhered to the plastic and prevent from damaging the second and third shredders. After sorting, the plastic will be further shredded into fine pieces by second and third shredder. It is expected that a lot of ashes or dusts will be generated during the grinding process, dirt catcher will be used to separate the dust and plastic before next process.
Illustration of Double-Shaft Shredder
Illustration of Double-Shaft Shredder
Knowing that the properties of plastic, including its toughness, hardness and brittleness, differ in types, further treatment methods should be applied accordingly to the types of plastic. Therefore, 3RET will equip 3 types of separator for plastic pieces sorting, which are Rubber Separator, Electrostatic Plastics Separator and Colour Sorter, to facilitate the treatment process later. These separators can reduce the manpower and save time significantly which matches with the theme – Reduce.
The separating stages will be listed as follows:
a) Rubber Separator
Due to different density of various materials, Rubber Separator is applicable for separating silicone rubber, wood, ticket paper and dust from plastic by physical method. The separated plastic/plastic mixture will be transmitted to Electrostatic Plastics Separator.
b) Electrostatic Plastics Separator
There are three stages separation while the separation theory is the same for all three stages. It makes use of the differences in tribo-electric behaviour of individual plastics for separation. All plastics are heated up before charging so as to enhance the charging effect. The plastics mixture is charged selectively and specifically according to the material, taking on a positive or negative charge. After charging, the plastic mixture reaches a high-tension field where the components of the plastic mixture are separated electrostatically into pure sorted fractions according to their different charges. Consequently, positive charged plastics will be attracted by a negative electrode and negative charged plastics are rejected and vice versa.
c) Colour Sorter
3REP can provide the specific colour of plastic pieces as required by customers. The colour sorter uses LED light, combined with excellent digital camera technology, to identify the colour area as small as 0.04mm.
Illustration of Rubber Separator
Illustration of Electrostatic Plastics Separator
Illustration of Colour Sorter
Providing high quality of products and raw materials to customers is important, 3REP will equip a set of machines used to clean and wash the plastic pieces. The details of the machines are summarized in the following Table 2-2.
It is a high speed washing machine with screen washes the material. The dirt comes out from the screen with the water. The tank separates different materials by their density. Light materials are discharged at the end of the tank while heavy materials are discharged at the bottom and side screws. Water is removed by using high speed centrifuge. The dryer provides heat to evaporate the remaining water under high temperature.
After the plastic pieces have been sorted, they are transmitted to specific pelletizer. The pelletizer can be categorized into 2 types, which are Water Ring Pelletizer and Strip Cutting Pelletizer. For the Water Ring Pelletizer, high-density polyethylene (HDPE), plastic film and polyethylene terephthalate (PET) can be pelletized; while mixed plastics can be pelletized by Strip Cutting Pelletizer. Finally, the granules will be mixed and then packaged before delivering to customers.
Illustration of Strip Cutting Pelletizer
Illustration of Water Ring Pelletizer
Illustration of Mixer
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