No. 49 - Density Separation Of Mixed Waste

Plastics recycling is a tough business, and it would not have advanced this far without a cost-effective, automated way to separate mixed waste into relatively pure fractions. The simple “float/sink” principle of density separation in water has proved effective in separating plastics from each other and from wood, metal, and other contaminants.

Plastics recyclers borrowed float/sink separation from the mining industry in the 1970s. Gravity is free, and water forgives differences in size, shape, and composition of what passes through. What’s more the separation tanks were simple and inexpensive to build in-house.

The earliest float-sink application for plastics was probably PP battery cases at MA Polymers (now MA Industries) in 1969. The earliest use in PET bottle recovery was at St. Jude Polymers in 1977.

In the 1980s, recyclers tried using hydrocyclones for density separation of polyolefins from PET bottle flake. But the capital cost was high, and hydrocyclones were intolerant of varied feedstock composition or flake size.

In 1983, Tom Tomaszek patented a float/sink separator for Nelmor Div. of AEC. Nelmor never pursued the patent, leaving John Brown Machines, Sorema in Italy, and others to commercialize combinations of wet washing and density separation in the ’80s and early ’90s.

European recyclers pursued altered-density media for float/sink separation of mixed materials in high volumes. In 1997, TLT Turbo Laminare Trenntechnik in Germany launched a double-cyclone, altered-density cascade for flake separation. In the late ’90s, Galloo Plastics in France commercialized altered-media separation of PP from auto shredder residue using float/sink cascades from Engineering Separation and Recycling in the U.S.