The combination of high gradient magnetic separation using rare earth roll magnets and eddy current separation has proved to reliably remove aluminum flakes mixed with PET.
Recycling of plastic beverage containers made from polyethylene terephthalate (PET) has been a growth industry for the past two decades. Recycled PET can be used in such diverse products as carpet, containers, auto parts, tool handles and sleeping bag insulation; however, commonly used processing methods leave aluminum flakes mixed with the PET at an average contamination level of about 2,000 parts per million.
Much has been learned over the years regarding how to remove the aluminum contaminants, both in terms of cost-effective methodology and the totality of the extraction. A closer look at the genesis of the industry and a review of separation processes learned over the years will improve the profitability of PET recycling by leaping processors over the learning curve.
PET processing and aluminium contamination
PET usually arrives at the plant in bails. A typical PET processing flow sheet would sort the HDPE and PVC containers from the main flow. Normally, these plastics are processed separately. The PET is color sorted before shredding to about 0.28-inch flake, and then washed to remove glue and labels. The washed PET is screened at 18 mesh, to eliminate a small amount (less than one percent) of fines and then dried.
Early attempts to remove aluminum from the PET concentrated on electrostatic separation (ES). Process improvements have boosted the PET grade to between 50- and 100-parts-per-million residual aluminum, with PET recoveries averaging 92 to 94 percent. Despite these advancements, ES separation remains vulnerable to fluctuations in atmospheric humidity and is sensitive to product temperature. Plus, considerable demand has emerged for higher-grade PET flake – i.e., flake with considerably less allowable aluminum contaminant. Aluminum clogs the trash screens of plastic extruders, increasing their maintenance requirements.