The recovery of plastics from waste with reference to froth flotation

Some authors have overstated the problem of plastics wastes accumulating in large quantities. The amounts and types of such wastes must be well understood if recovery methods, such as froth flotation, are to be applied successfully. Not all plastics discards are reasonably recoverable and statistics on gross quantities must be analyzed for those that are. The amount of plastic wastes generated can be related to plastics production only with an understanding of the product life spans of different plastics items and which plastics are used for items that will at some time be in the waste stream. Without this knowledge, feedstocks for recovery cannot be intelligently or economically chosen. Selected plastics mixtures have been separated by froth flotation for subsequent recovery of the individual plastics, reviewed here from the perspective of gamma flotation, float-sink based on the plastic surfaces' values of the Zisman critical surface tension for wetting (gamma(C)). It is shown how float-sink may depend on the values gamma(C) and the liquid surface tension (gamma(L/G)) of the flotation bath. Other investigators have reported flotation depression by the addition of selected water-soluble polymers and attribute the results to adsorption of these polymers on the plastics' surfaces. Some authors attribute the effects of surfactants similarly to adsorption. This discussion questions these interpretations given the low surface free energy of polymers. The review and discussion is not intended to explain all observations by gamma flotation. It is intended to encourage new paths for additional research. Perhaps in spite of the gaps in understanding, there have been successful froth flotation separations of plastics, including separating a mixture of rigid plastics pieces recovered from municipal solid waste (MSW). Some previously unreported results of such experiments are summarized here. (C) 2004 Elsevier B.V. All rights reserved.