Optical sorting is one of the essential methods in the waste plastic sorting process, primarily utilizing the spectral differences among different types of plastics. Currently, optical sorting includes various techniques such as infrared spectroscopy sorting, X-ray fluorescence sorting (XRF sorting), Raman spectroscopy sorting, laser-induced breakdown spectroscopy sorting (LIBS sorting), and hyperspectral imaging technology sorting (HSI sorting). It is one of the main research directions in plastic sorting technology.

Infrared Spectroscopy Sorting

Infrared spectroscopy sorting is one of the extensively researched technologies, including near-infrared spectroscopy sorting and mid-infrared spectroscopy sorting. Among these, near-infrared spectroscopy sorting is considered the most commercially viable technology due to its advantages of high speed and accuracy. However, this technology has limitations; for instance, black plastics, having low reflectance coefficients, lack clear near-infrared spectral signals, making near-infrared spectroscopy sorting challenging.

X-ray Fluorescence Sorting (XRF Sorting)

X-ray fluorescence sorting utilizes X-rays to irradiate plastics, exciting target atoms, and subsequently, the ions return to the ground state, generating fluorescent spectra to achieve sorting. XRF sorting is an efficient method for sorting PVC. However, it remains challenging to detect PVC plastics covered by other plastic products.

Raman Spectroscopy Sorting

Raman spectroscopy sorting analyzes the light scattered from plastic samples under irradiation of light with frequencies different from the incident light. It obtains information about the position and intensity of characteristic peaks, compares them with the Raman spectra of standard substances, and identifies the plastic type for sorting. However, this method is prone to severe interference from plastic impurity signals, significantly affecting detection rates and accuracy. Moreover, the identification of plastics is slow, making it difficult to meet the industry's requirements for rapid identification standards.

Laser-Induced Breakdown Spectroscopy (LIBS) Sorting

LIBS sorting detects and analyzes the spectral frequency and intensity of plasma generated on the surface of plastics under intense laser irradiation, identifying plastic types. In recent years, LIBS has received extensive attention and research from scientists. Research indicates that LIBS is an effective tool for rapidly classifying different types of plastics, playing a crucial role in waste management. Compared to near-infrared spectroscopy recognition, this technology is less affected by surface impurities, requires no sample pretreatment, has good environmental adaptability, and is easy to integrate. It also has good applicability to identify relatively difficult black plastic products. The drawback of this sorting method is its inability to perform large-scale rapid detection.

Hyperspectral Imaging Technology (HSI) Sorting

Hyperspectral imaging technology sorting is a newly developed sorting technology in recent years. This technology combines spectral and imaging techniques, analyzing the three-dimensional information of collected detected objects to achieve effective sorting of waste plastics. Optical sorting is widely used and promoted due to its rich and easily obtainable signal information, high feature signal recognition, convenience, speed, reliability, and non-destructiveness to samples. However, many optical sorting methods still face challenges, such as difficulty in identifying dark-colored plastics, necessitating the use of color sorting devices in conjunction with optical sorting.