(662d) Triboelectric Separation of Plastic Waste: How to Increase the Selectivity?

The triboelectric separation is a promising method in plastic waste recycling, as it is a mechanical method to separate different kinds or grades of grinded plastic waste, which can be utilized in the following recycling processes. This method consists of three main steps. In the first step, plastic waste is grinded into small pieces. In the second step, the charge is generated on their surface due to tribocharging and in the third step, the pieces are separated during the free fall between electrodes in the electrostatic separator. The charge is ‘generated’ on the surface of the pieces during their contact and friction and after their subsequent separation. The amount and direction of the transferred charge is generally determined by the properties of both contacted materials, especially by the work function, which is well-defined only for conductive materials. In the setup of the triboelectric separator, two contacting groups of material are involved, the first is represented by the grinded plastic waste mixture and the second group is represented by the charging adaptors.

Although the triboelectric separation is a promising method with the potential to become an important member of the recycling chain for a number of reasons (low costs, automatization potential, possible separation of black materials...), there are several challenges, which need to be addressed to ensure the high selectivity and yield of this method. Especially challenging is the selection of an adaptor, as it plays a key role in the controlling of the charge obtained by individual plastic pieces. Currently, the tribocharging of dielectrics is not well understood yet even to simply determine a suitable counter-material, although some theoretical or empirical concepts are corresponding well with experimental results. Thus, there is not sufficient knowledge to predict the charging behavior of such similar materials as polymers, when distinguishing between their grades (for example, HDPE and LDPE), and that is why most of the work needs to be done experimentally. Furthermore, the nature of the materials is not the only parameter influencing the direction and magnitude of the transferred charge, but a role plays also the type of contact counter-material, the properties of the surrounding atmosphere as well as the dimensions and the shape of the grinded pieces.

We use self-constructed charging apparatus based on different contacting approaches: contact during rotational motion, enhanced by vibrations or contact during sliding motion. To evaluate the charge on particles, we use two different methods: we can evaluate the total charge in a sample in a Faraday pail connected to the electrometer (used for experiments with pure materials, not mixtures), or we are using our self-constructed free-fall separator to evaluate the charge distribution in the sample. After the separation, plastic pieces are collected in boxes based on their trajectory in the electric field and the charge of the fraction of pieces in individual boxes is measured in the Faraday pail. Then, the representation of different plastic materials in collectors is evaluated.

Our work is focused on improving electrostatic separation by finding proper counter materials to effectively separate multiple-component plastic mixtures by separating one material per cycle and recharging the remaining mixture with different counter material during each cycle. We compared the most promising tribocharging mechanisms for automatization (rotational and vibrational), and we combined these methods with a corona discharge to decrease the time needed to achieve sufficient, i.e., saturation charge on individual pieces. We performed a study addressing the charging dependent on material composition with 8 adaptors with different composition and 9 waste plastic materials, and we showed that the magnitude and, in some cases, even polarity can be controlled by the choice of the adaptor and the mode of charging. Additionally, we implemented an external electric field during the vibrational motion of plastic materials in the tribocharging apparatus to fine-tune the transferred charge, to enhance the sensitivity of separation and to decrease the requirements on the variability of the used adaptors during the whole process. These results can be used as guidance to choose a suitable surface for the efficient separation of various plastic mixtures. Then, we used these results to perform the separation of 3-component mixtures, aiming for the successful separation of halogenated material – PVC from the mixture, as PVC is inconvenient for incineration. We began with a one-stage separation with very good purity of the two materials but in a low yield, and then we introduced the repeating cycle in two different ways. One is to reach higher purity by separating only one material in one stage and another one aims to increase the yield. Finally, we reached the purity of separated PVC up to 91 % at 88% recovery, the purity of PET up to 95% with 97% recovery, and PP with 93% purity and 70% recovery.