(57j) Electrostatic Segregation in Particulate Mixing

The segregation of particulate media during mixing has been studied for years. Particle segregation is typically caused by variations in fill level, rotational speed, and particle size/density disparity between species. However, new insights into segregation suggest that electrostatic interactions between particles may have a more profound effect than previously understood. This study was designed to explore the effects of electrostatics on particulate blends. The aim was to establish and explore various prevalent effects that are potentially detrimental to several industries that frequently blend particulate matter such as the catalyst and pharmaceutical industries. Particles used in this study were made of various polymers and selected from the triboelectric series with a known charge affinity. Particles were typically between 500 µm and 1 mm in diameter; any larger particles tend to allow inertial forces to dominate over electrostatic forces. Particle charging was typically caused by contact with the walls of the blender as well as particle-particle contact. Binary mixtures were created using various polymers and tested for fill level at 1:1, 1:2, and 2:1 mass ratios. Additionally, binary mixtures were tumbled at three different speeds with a maximum of 56 rpm. Particle blends were mixed in an acrylic double cone blender with a 10 cm diameter. The use of acrylic allowed not only for known electrostatic properties of the rotating wall, but also allowed for visual inspection of pattern propagation. Several of the segregation and electrostatic effects observed included several unique banding patterns, particle-wall clinging, particle coating phenomena, and even the ?snaking? of particles. Further experiments were conducted utilizing a Van der Graaff generator as a way to engraft additional electrostatic effects and interactions. Several unique banding patterns and aggregate patterns emerged as a result of the additional electric field. Additional experiments were conducted to isolate the electrostatic effects of the blender wall by flowing binary polymer blends down a 30 degree acrylic chute. The results of this work aim to elucidate the problems that plague several particulate industries and explore ways to predict and limit electrostatic effects during particulate blending.