Secondary processing of plastics is a topic that increasingly resonates in society. The quality of such plastics is derived from the purity of the input material, the efficiency of separation, and also the sustainability of the recycling process itself.
Experts at the RICE research center of the Faculty of Electrical Engineering at the University of West Bohemia have developed a unique technology enabling the separation of plastics of up to 99% purity. The technology is based on the principle of freefall electrostatic separation.
"Our separator takes advantage of the fact that each type of plastic becomes charged by friction differently, thus receiving a different charge," says Jiří Kuthan from the RICE research center. "The mixture of shredded plastics is poured between electrodes connected to high voltage and based on their charge the particles either attract or repel one another and thus get sorted into catch pots," Jiří Kuthan describes electrostatic separation in simple terms.
The project, co-financed by the Ministry of Industry and Trade of the Czech Republic, thus supports current sustainability trends and is in line with the principles of a circular economy. "Quality separated plastics can be reused, which conserves resources," notes Jiří Kuthan pointing out that the separator ensures sufficient material purity for secondary processing.
The separation itself starts with precise dispensing of dry shredded plastic particles sized 1–8 mm. An air conveyor, which is also used for charging the plastic particles, conveys the material to a cyclone, from which the particles move through a deflector into a sorting chamber. "In the conveyor and cyclone, we see the so-called triboelectric effect when the particles rub one another as well as the sides of the conveyor tube, which results in the particles becoming charged, after which they pass through a strong electric field created by electrodes," explains Jiří Kuthan. The separation of materials takes place between two high-voltage electrodes. Using collectors, the separated material is stored in catch pots. Any mixture of materials that are sufficiently far apart from each other in the triboelectric series can be separated this way. This technology has been successfully verified at a pilot plant for mixtures of various types of plastics (ABS, PE, PS, PVC, PP, etc.).
The entire process is energy efficient, as the high-voltage part of the separator requires only a few watts to work. Another advantage is continuous charging. The equipment includes software for computer simulation and optimization of separator settings and its control. The program enables control of the equipment during the separation and significantly simplifies its operation. An important benefit is the variability of the technical solution of the separator, which enables recycling of various mixtures of plastic waste, or even separation of one type of plastic from a varied mixture.
The separator meets the principle of a considerate approach to the environment. It is suitable for the secondary processing of plastic materials as well as for increasing the purity of plastic mixtures. The current experimental data show that plastics are not the only category of materials that can be separated using this technology. This technology can also find application in agriculture or in the separation of mineral raw materials. The University of West Bohemia is currently negotiating with several entities from the application sphere about the possibility of utilizing the separator and its licensing.