1. Density - based Sorting Technology

• Principle
  Density - based sorting technology exploits the fact that different plastics have unique density values. For example, high - density polyethylene (HDPE) typically has a density in the range of 0.941 - 0.965 g/cm³, while low - density polyethylene (LDPE) has a density of 0.910 - 0.940 g/cm³. In a liquid medium with a specific density, plastics with a density lower than the liquid will float, and those with a higher density will sink. This fundamental property forms the basis for separating different plastic materials.
• Implementation
  The process usually involves preparing a series of liquid media with precisely adjusted density gradients. Common liquid media include saltwater solutions with varying salt concentrations. Mixed plastics are then carefully introduced into these solutions. As an illustration, in a well - calibrated saltwater solution, HDPE items such as detergent bottles will float to the surface, while PET (polyethylene terephthalate) bottles, which are denser, will sink to the bottom. This separation can be further enhanced by using mechanical agitation or gentle stirring to ensure that all plastic pieces interact properly with the liquid medium.
• Advantages
  This technology is relatively straightforward and cost - effective, especially for small - to - medium - scale plastic recycling operations. It does not require highly sophisticated equipment, making it accessible to a wide range of recyclers. It can efficiently separate common plastics with significant density differences, providing a reliable initial separation step in the recycling process.
• Limitations
  However, the presence of impurities in plastics can significantly affect their buoyancy and lead to inaccurate sorting results. Additionally, maintaining the stability of the liquid medium density can be challenging, as factors such as temperature fluctuations can alter the density of both the plastics and the liquid. This may cause misclassification of plastics during the sorting process.

2. Near - Infrared (NIR) Sorting Technology

• Principle
  Each type of plastic has a distinct molecular structure, which results in unique near - infrared absorption and reflection patterns. NIR sorting technology takes advantage of this property. The equipment emits near - infrared light onto the plastic materials, and sensors detect the reflected light. By comparing the detected NIR spectra with a pre - stored database of plastic spectra, the system can accurately identify the type of plastic. For instance, PET has a characteristic absorption peak at a specific wavelength in the NIR range, which can be used to distinguish it from other plastics.
• Implementation
  Mixed plastics are conveyed on a high - speed conveyor belt under the NIR detection unit. The NIR light source illuminates the plastics continuously as they move. The sensors, which are highly sensitive to NIR wavelengths, capture the reflected light signals. These signals are then transmitted to a computer system equipped with advanced pattern - recognition software. The software analyzes the signals, matches them with the known spectra in the database, and once the plastic type is identified, an ejection mechanism, such as a high - pressure air jet, is activated to direct the plastic to the appropriate collection bin.
• Advantages
  NIR sorting technology offers high sorting accuracy and can distinguish between plastics with similar physical properties, such as those with similar densities but different chemical compositions. It is capable of handling complex mixtures of plastics, including those with various additives and colorations. This technology is highly suitable for large - scale, high - volume recycling operations and can be seamlessly integrated into automated recycling lines, significantly improving the efficiency of plastic sorting.
• Limitations
  Nevertheless, the equipment is relatively expensive to purchase and maintain. Regular calibration of the NIR sensors and continuous updating of the database are essential to ensure accurate sorting. The performance of the technology may also be affected by factors such as the thickness and surface condition of the plastic samples. For example, thick - walled plastics or those with rough surfaces may not reflect NIR light as uniformly as thin, smooth - surfaced plastics, potentially leading to misidentification.

3. Electrostatic Sorting Technology

• Principle
  When plastics are subjected to certain processes, such as triboelectric charging, they can acquire different electrostatic charges. Through triboelectric charging, plastics are rubbed against each other or a specific material, causing some plastics to become positively charged and others to be negatively charged. In an electrostatic field generated by electrodes, these charged plastics will move in different directions according to their charge polarity and magnitude, enabling separation. For example, in a parallel - plate electrostatic separator, positively charged plastics will be attracted towards the negatively charged electrode, while negatively charged plastics will move towards the positively charged electrode.
• Implementation
  Mixed plastics are first fed into a charging chamber, where they are vigorously agitated or rubbed against a specific material to induce triboelectric charging. After charging, the plastics are then introduced into an electrostatic field. The electrostatic field is carefully calibrated to ensure that the charged plastics experience the appropriate electrostatic forces for separation. Collection bins are strategically placed at the end of the sorting path to gather the separated plastics. Some advanced electrostatic sorting systems also incorporate additional features, such as adjustable electrode voltages and frequencies, to optimize the sorting process for different types of plastics.
• Advantages
  This technology can effectively separate plastics that are difficult to distinguish by other means, especially those with similar densities. It can be used to separate plastics with different additives or surface treatments that affect their electrostatic properties. Electrostatic sorting technology is also relatively energy - efficient compared to some other sorting methods, as it mainly relies on electrostatic forces rather than mechanical processes that may consume more energy.
• Limitations
  However, the electrostatic sorting process is highly sensitive to humidity. High humidity levels can reduce the charging efficiency and cause charge dissipation, leading to inaccurate sorting results. The surface condition of the plastics, such as the presence of contaminants or coatings, can also impact the charging and sorting performance. Additionally, the initial investment in electrostatic sorting equipment can be relatively high, although the long - term operational costs may be reasonable due to its energy - saving features.

4. Image - based Sorting Technology

• Principle
  Image - based sorting technology, also known as optical sorting, relies on the differences in color, shape, and transparency of plastics. Different types of plastics often exhibit distinct visual characteristics. For example, PET bottles are typically transparent or have a slightly blue - tinted transparency and a characteristic cylindrical shape with a specific neck design. PVC (polyvinyl chloride) products may have a different color range and texture compared to other plastics. Advanced cameras and image - recognition software are used to analyze these visual features and identify the type of plastic.
• Implementation
  In an automated image - based sorting system, mixed plastics are conveyed on a conveyor belt under high - resolution cameras. The cameras capture multiple images of the plastics from different angles as they move. The image - recognition software then analyzes the color, shape, and transparency features of the plastics in the captured images. It compares these features with a pre - set database of plastic visual characteristics. Once the plastic type is identified, an appropriate sorting mechanism, such as a mechanical pusher or an air - operated diverter, is activated to direct the plastic to the correct collection bin.
• Advantages
  Manual image - based sorting (by trained workers) is a simple and low - cost method, suitable for small - scale operations or when the quantity of plastic waste is not large. Automated image - based sorting can be fast and efficient for large - scale sorting, especially when dealing with plastics with distinct visual differences. It can also be integrated with other sorting technologies to provide a more comprehensive sorting solution.
• Limitations
  Manual sorting is labor - intensive and prone to human error, especially when sorting large volumes of plastics for extended periods. Automated image - based sorting may face challenges when plastics have been discolored due to aging, contamination, or when their shapes are irregular. In such cases, the image - recognition software may misidentify the plastics, leading to incorrect sorting results. Additionally, the accuracy of the sorting process may be affected by lighting conditions in the sorting area, requiring careful control of the illumination to ensure consistent performance.