Electrostatic separation and recovery of mixed plastics

ELECTROSTATIC SEPARATION AND RECOVERY OF MIXED PLASTICS
Chuanfu (Chuck) Xiao, Laurence Allen III and Michael B. Biddle, MBA Polymers, Inc. Michael M. Fisher, American Plastics Council Abstract
A triboelectric separator manufactured by PlasSep, Ltd., Canada was evaluated at MBA Polymers, Inc. as part of a project sponsored by the American Plastics Council (APC) to explore the potential oftriboelectric methods for separating commingled plastics from end-oflife durables. The separator works on a very simple principle: that dissimilar materials will transfer electrical charge to one another when rubbed together, the resulting surface charge differences can then be used to separate these dissimilar materials from one another in an electric field. Various commingled plastics were testedunder controlled operating conditions. The feed materials tested include commingled plastics derived from electronic shredder residue (ESR), automobile shredder residue (ASR), refrigerator liners, and water bottle plastics. The separation of ESR ABS and HIPS, and water bottle PC and PVC were very promising. However, this device did not efficiently separate many plastic mixtures, such as rubber andplastics; nylon and acetal; and PE and PP from ASR. All tests were carried out based on the standard operating conditions determined for ESR ABS and HIPS. There is the potential to improve the separation performance for many of the feed materials by individually optimizing their operating conditions. Cursory economics shows that the operation cost is very dependent upon assumed throughput, separationefficiency and requisite purity1. Unit operation cost could range from $0.03/lb. to $0.05/lb. at capacities of 2000 lb./hr. and 1000 lb./hr. A triboelectric separator manufactured by PlasSep, Ltd., Canada was evaluated at MBA Polymers, Inc. (MBA) as part of a project sponsored by the American Plastics Council (APC) to explore the potential of triboelectric methods for separating commingledplastics. In the Plas-Sep triboelectric separator shown schematically in Figure 1, particles are allowed to contact one another in a rotating aluminum drum to allow charging and then are directed to fall through an intensive horizontal electric field. The angle of the tube can be changed to alter material residence time. Negatively charged particles are pulled toward the positive electrode andpositively charged particles are attracted toward the negative electrode. In this way, positively charged material is separated from negatively charged material. Generally, material charging is based on relative position in a triboelectric series. Table 1 lists a triboelectric series reported by Plas-Sep Limited. According to this series, PET is positively charged in a mixture of PVC and PET, while itbecomes negatively charged in a mixture of PET and PP

Experimental
Table 2 summarizes the material mixtures that MBA Polymers, Inc. evaluated for this project. Particle size appears to be very important to triboelectric separations, so an effort was made to study this. If particles are too large (much larger than 4-5 mm according to Plas-Sep) they will not be deflected sufficiently by the field.If particles are too small, they will tend to accumulate on the electrodes themselves, causing other falling particles to be somewhat insulated from the horizontal field. The thickness of plastic chips might be another important factor for triboelectric separation, although we didn’t look into it during this study. Film-like chips have higher surface area per unit volume, which would produce highersurface charge per mass as compared to non-film chips. The characteristics of film chips, such as lightweight, high surface charge and slow falling velocity would ultimately lead to more deflection in the electric field. Film plastics might yield an improved separation in a triboelectric separator based on this analysis. Different grinders and processes were required for different material...