I. What is Polymer Waste?
plastic waste
Plastic products in our daily lives consist mainly of high-molecular-weight polymers. When these products are no longer used, they become polymer waste, or post-consumer plastic waste.
polymer waste sorting
This waste is not simply “garbage.” Most plastic properties remain stable after use, enabling effective recycling into new products.
Polymer waste can be divided into two categories:
1. Pre-consumer waste: This includes materials from the production process, such as leftover resin and cleaning waste. It is typically well-defined, low in pollution, and easy to recycle.
2. Post-consumer waste: This includes a wide range of items like packaging from various industries, such as chemicals, textiles, home appliances, construction, and agriculture. It often has complex compositions and requires treatment before recycling.
In municipal solid waste, plastic accounts for about 2% to 4%, mainly consisting of single-use packaging materials like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), with polyolefins (PO) making up as much as 70%.
II. Polymer Waste Sorting Principles
Based on differences in physical and chemical properties, there are seven primary sorting methods for plastics:
1. Density Difference Separation: This method uses the varying densities of different plastics for classification and separation.
2. Static Separation: Crushed plastics are placed in a liquid with a specific density, where separation occurs based on sinking and buoyancy. For example, PET is denser than water, while PE and PP are not.
3. Hydrocyclone Separation: This technique employs a hydrocyclone to enhance the separation of plastics with a density difference of about 0.5 g/cm³ and a thickness greater than 3 mm.
4. Air Screen Separation: Airflow separates lightweight impurities from plastics. Crushed waste is introduced into the air screen, where airflow causes heavier materials to fall and lighter materials to be blown away.
5. Electrostatic Separation: This method utilizes differences in the triboelectric properties of plastics. Dried and crushed plastics are charged, allowing for the separation of materials, such as PVC, which commonly carries a negative charge.
6. Separation Based on Melting Point and Embrittlement Temperatures:
– Melting Separation: Plastics are separated based on their different melting points as they pass through heating chambers.
– Temperature Difference Separation: This utilizes differing embrittlement temperatures; for example, PVC becomes brittle at -41°C, while PE remains intact below -100°C.
7. Advanced Sorting Technologies:
– Infrared Spectroscopic Sorting: This uses infrared sensors to distinguish over 10 plastic types with high accuracy.
– Solvent-Targeted Recovery: This method selectively dissolves and precipitates specific polymers, allowing recovery of various plastics from mixed waste.
III. Polymer Waste Sorting Steps: From Pretreatment to Fine Sorting
1. Manual Pre-Sorting: Initial steps involve removing metallic and non-metallic impurities, classifying waste plastics (e.g., agricultural film, packaging film, foam plastics, beverage bottles), and sorting by resin type, color, and quality.
2. Crushing and Washing: Waste plastics are mechanically reduced in size and washed to eliminate dirt and impurities.
3. Multiple Sorting Stages:
a. Density Sorting: Hydrocyclones and flotation methods separate plastics by density. For example, a mixture of polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) is placed in water, where PET sinks. Alcohol is added to adjust the water density to separate PE.
b. Air Screening: Lightweight impurities and plastic fragments are removed using airflow.
c. Electrostatic Sorting: This method separates PVC from other plastics.
d. Magnetic Separation: Metal traps are used to extract metal fragments.
-Molecular-Level Identification: Technology allows for the molecular identification of recycled plastic, helping distinguish mixed materials.
-Sorting Special Materials:
. Metal-Plastic Separation: This includes methods like dissolution and embrittlement separation.
. Paper-Plastic Separation: Three primary methods are used: thermal separation (heating), wet separation (crushing and screening), and electro-mechanical separation (using electrodes).
IV. Post-sorting processing: High-value utilization
Sorted plastics are processed using various methods based on their type and quality:
1. Mechanical Recycling Processing
– Crushing and Washing: High-speed systems clean and separate impurities from high-density and heavily contaminated materials.
– Extrusion Granulation: Clean plastic fragments are converted into granular materials, with particle shapes varying by device.
– Granulation Equipment: This equipment efficiently processes various plastics (PE, PP, PET, PS, etc.) while reducing energy consumption and increasing production capacity.
2. Chemical Recycling Technologies
– Solvent Recovery and Precipitation: This technique can address complex plastic waste, allowing for efficient separation and recycling of different plastics.
– Pyrolysis Technology: Plastics are converted into valuable oils, waxes, and gases under high-temperature, oxygen-free conditions, useful for producing chemicals and fuels.
3. High-Value Applications
– High-Quality Recycled Materials: These can be used for new packaging, electronic casings, and food contact materials.
– Modified Recycled Materials: Toughness can be enhanced through elastomers or composite modifications.
– Downgraded Use: Lower-quality recycled materials are suitable for manufacturing products like trash cans and park benches.
As a manufacturer of waste sorting lines, Guoxin Machinery leverages advancements in polymer sorting technology to develop competitive products and solutions in plastic resource utilization.