THE MICROSORTATION OF HIGH LOADINGS OF POSTCONSUMER MIXED POLYOLEFINSUSING LIQUID CARBON-DIOXIDE IN A SLIGHTLY AGITATED, BATCH APPARATUS

The microsortation of clean, dry, mixed, shredded, post-consumer polyo lefins can be accomplished using near-critical carbon dioxide as a flo at-sink medium. At high loadings of plastics, however, buoyant forces alone are not sufficient to break up the aggregates of interlocking sh redded plastic particles. Therefore a close-clearance impeller must be used to agitate the mixed plastics during the batch separation. This slight agitation permits these irregularly shaped chips to either floa t or sink without being hindered by surrounding chips. The separation apparatus operates most efficiently when the plastics are charged to a level that corresponds to the top of the highest blade of a large-dia meter multiple-pitched blade impeller that rotates at 15 rpm. A post-c onsumer flake mixture of 85% HDPE/15% PP can be sorted into HDPE and P P streams of 99 + % purity at loadings up to 56 volume per cent. In th e absence of an impeller, high purity separations can be achieved at l oadings of only two volume per cent. This dramatic increase in the loa ding reduces the estimated processing cost of mixed polyolefins to $0. 03-0.05/lb, enhancing the economic feasibility of this high pressure, CO2-based microsortation process. (C) 1997 Elsevier Science B.V.