Cryogenic ball milling: A key for elemental analysis of plastic-rich automotive shedder residue

Abstract

End-of-life vehicles have become an environmental and sustainability issue in most developed countries, and require sophisticated organic- and inorganic-elemental analyses to evaluate the efficiency of post-shredder technologies applied to automotive shredder residue. The difficulties of milling such heterogeneous material, especially when plastic-rich, have to be overcome to allow such chemical analyses. To tackle this aspect, plastic-rich fluff sampled from the process line of an industrial waste management centre was subjected to pilot–float separation (d=1.34) and cryogenic ball milling at BRGM. The cryogenic milling, tested in terms of plastic-rich fluff density, grinding time and feed size, was found to reach an acceptable final particle size (81–98% of particles at <250μm) to allow total digestion and accurate and repeatable elemental analyses after a grinding time of between 27min and 2×27min (iterative two-step process). The results are contrasted, the milling being more efficient with the heavier fractions of plastic-rich fluff and a finer feed size. The varied grindability of the different fractions could result from a combination of one or more of the following effects: (i) a dilution of the plastics by more cryo-grindable rubber, (ii) the action of remnant minerals and non-ferrous metals as milling agents, (iii) the inherent cryo-grindability of various types of plastics, and (iv) the potential action of mineral and metallic fillers as weakening agents. The elemental analyses of our case study allowed us to determine a mass balance and show, in particular, that the pilot–float separation (i) recovers most of the organochlorine plastics, and (ii) concentrates Cu, Pb, Ba and B in the heavier fraction with respective ratios of 100:1, 8:1, 6:1 and 5:1. The high elemental recovery (95.8% up to 99.7%) and good repeatability of the C and Cl analyses on small test portions (100mg to 3g) represent a technical progress that could benefit other types of heterogeneous plastic-rich matrix samples such as waste electrical and electronic equipment (WEEE).