Plastic waste sorting

I. What is Plastic Waste?

Plastic waste encompasses a variety of plastic products and fragments that have lost their original utility or are discarded during production and consumption.

Its primary sources include:

1. Post-consumer waste: This includes items such as PET beverage bottles, HDPE detergent bottles, PP lunch boxes, LDPE plastic bags, and various types of plastic packaging. These materials have diverse sources and complex compositions.

2. Post-industrial waste: This category consists of scraps and substandard products generated during manufacturing. The composition of this waste is generally simpler and cleaner compared to post-consumer waste.

Different types of resins, including PET, PE, PP, PVC, and PS, have distinct chemical structures, melting temperatures, and compatibilities. Mixing these resins for recycling can compromise the performance and quality of the recycled materials. Additionally, it may damage production equipment—for example, PVC can decompose at high temperatures to release hydrogen chloride, which can corrode the machinery.

II. Principle of plastic waste sorting

Sensors are used to detect different types of plastics, while actuators are responsible for separating them.

1. Near-Infrared Sorting Technology

Principle: When near-infrared light shines on materials, different types of plastics absorb and reflect specific wavelengths of light, creating unique spectral fingerprints. Sensors capture these spectra and compare them with standard spectra in a database, allowing for identification within milliseconds.

Advantages: This method offers high precision and speed, making it the mainstream and core technology for sorting mixed plastics.

Applications: It effectively sorts major categories such as PET, HDPE, PP, and PS.

2. Electrostatic Sorting Technology

Principle: This technique utilizes differences in polarity and the magnitude of charge carried by different plastics after triboelectric charging. When placed in an electric field, materials follow different trajectories due to varying electrostatic forces, enabling separation.

Advantages: It is particularly useful for separating plastics with similar densities and colors but different materials, such as PET and PVC.

3. Air Sorting Technology

Principle: This method relies on the density and aerodynamic characteristics of plastic sheets to achieve sorting.

4. Color Sorting Technology

Principle: This technology uses a high-resolution CCD camera or a color line scan camera to capture the color information of materials. Image processing algorithms then identify and separate plastics based on specific colors (e.g., colorless transparent, blue, green, etc.).

Advantage: This method is essential for producing high-value recycled pellets in specific colors.

5. Artificial Intelligence and Deep Learning

Principle: This is the future of sorting technology. By providing machines with large datasets of plastic images, AI models can learn to recognize complex features, such as brand, shape, contamination levels, and materials obscured by labels.

Advantage: This approach significantly enhances the intelligence of sorting processes, allowing for the identification of complex materials and enabling customized sorting strategies.

III. Plastic Waste Sorting Steps

1. Pre-Treatment

Bag Breaking and Coarse Crushing: Open bagged waste and crush large plastic items into uniformly sized flakes.

Magnetic Separation: Utilize a high-powered magnetic separator to remove ferrous metals.

Air Separation: Employ an air separator to isolate lightweight film plastics from heavier plastics while also removing some dust.

Tumble Dryer: Grade materials by size, eliminating small fragments and oversized foreign objects.

2. Core Sorting

NIR Initial Sorting: The pre-treated material passes through the first Near-Infrared (NIR) sorter, which initially separates high-value plastics, such as pure PET bottle flakes.

AI/Color Sorting: The sorted PET bottle flakes then go through a color sorter or AI sorter for further subdivision by color (colorless, light blue, dark blue, etc.) to enhance the value of the recycled products.

Electrostatic Sorting: The remaining mixture is precisely separated using an electrostatic separator, which can distinguish materials like PET and PVC that are difficult to identify completely using NIR.

NIR Re-Sorting: The residual material stream is fed into a subsequent NIR sorter, which sequentially separates HDPE, PP, and other plastics.

3. Post-Sorting Processing and Quality Inspection

Washing and Dehydration: The sorted single-category plastic sheets are sent through a hot washing line to eliminate labels, adhesives, and any remaining dirt. This step is followed by dehydration.

Optical Quality Inspection: Before packaging, a high-precision sorting device conducts a final quality inspection of the “pure” material. This process removes any discolored flakes or impurities, ensuring that the product’s purity is typically above 99%.

Packaging and Warehousing: The clean, high-purity single-category plastic sheets are compressed into bundles and then transported to a recycling and granulation plant.

IV: Post-Sorting Processing

1. Recycling and Granulation 2. Chemical Recycling 3. High-Value Applications

Guoxin Machinery – Expert in Intelligent Solid Waste Sorting System Solutions