Triboelectric Charging and Electrostatic Separation of Granular Plastic Wastes Exposed to Long-Term Action of High Levels of Ambient Humidity

Tribo-electrostatic separation of granular plastic wastes is a complex phenomenon depending on several parameters, including the relative humidity of ambient air. The work presented in this paper aimed at studying the effect of storage environmental conditions on triboelectric charging and electrostatic separation. The experiments were conducted on four types of mm-size granular polymers, ABS, PS, PE, and PP, placed in a climatic chamber for 48 h to 96 h under controlled humidity. The thermal machine of the climatic chamber allowed humidity to be regulated at 82% ± 4% for a temperature of 19 °C ± 1 °C. The granules stored under these conditions will be designated as “humid.” Their water content was measured with a thermal balance and compared with “normal” granules, kept at ambient humidity varying between 46% and 52%, at 18 °C ± 1 °C. The water content of each of the four polymer granules increased by at least 10% after 48 h of exposure to the high relative humidity ambient conditions. The thin water layer formed at the particle's surface can explain their modified triboelectric behavior. The recovery and purity of the products collected after the tribo-electrostatic separation of the ABS/PS and PE/PP mixtures were poorer for the “humid” granules due to the less efficient tribocharging in the fluidized bed device.