Density Separation

Density separation is a method of classifying different types of plastics based on their density differences. Common plastic densities are shown in Table 1, and the sorting effectiveness of this technique is significantly influenced by factors such as plastic particle size and shape. Currently, widely applied density separation technologies include float-sink separation and air classification.

2.1 Float-Sink Separation

Float-sink separation, also known as a water washing line, consists of saltwater and fresh water tanks. It primarily utilizes the buoyancy differences of different types of plastics in the same solvent for separation. This method exhibits good separation results for plastics with large density differences. However, it is relatively challenging to separate plastics with similar densities, making precise separation of mixed waste plastics difficult. Additionally, this method is significantly affected by the cleanliness of the plastics, and wastewater is generated during the separation process. Commonly used solutions, solvent densities, and plastics suitable for density separation are listed in Table 2.

2.2 Air Classification

Air classification is a method of separating plastics based on the differences in their shapes and specific gravities, causing them to drift different distances in an airflow. This method not only achieves the separation of plastics with significant specific gravity differences but also effectively removes heavier metals or sand in plastics. However, due to factors such as differences in the particle size of shredded mixed plastics or variations in density caused by additives, air classification has lower precision and higher error rates. To improve separation accuracy, air classification often needs to be combined with other sorting technologies for efficient separation of different plastics. Wandful's plastic intelligent sorting production line combines Z-type air classifier, flexible body separator, and electrostatic sorter, achieving a material purity of over 98%. Currently, air classification equipment mainly includes air classifiers and separation shaking tables. Among them, air classifiers include vertical air classifiers, horizontal air classifiers, and vortex air classifiers.