Microsortation of Post-Consumer Plastics Using Near-Critical Liquids

Abstract

The microsortation of clean, dry, mixed, shredded, post-consumer polyolefins can be accomplished using near-critical carbon dioxide as a float-sink medium. At high loadings of plastics, however, buoyant forces alone are not sufficient to break up the aggregates of interlocking shredded 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 float 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-diameter multiple-pitched blade impeller that rotates at 15 rpm. A post-consumer flake mixture of 85%HDPE/15%PP can be sorted into HDPE and PP streams of 99+% purity at loadings up to 56 volume%. In the absence of an impeller, high purity separations can be achieved at loadings of only 2 volume%. This dramatic increase in the loading reduces the estimated processing cost of mixed polyolefins to $0.03-0.05/lb, enhancing the economic feasibility of this high pressure, C02-based microsortation process. Immersion of these plastics in this high pressure environment during the separation resulted in insignificant changes in the plastics' properties.