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Cryogenics remove PVC from PET and HDPE recycle.

They said cryogenics were too expensive ever to be competitive for cleaning recycled plastics. But the first full-scale cryogenic recycling line in the U.S. is being set up now to clean PET beverage-bottle and HDPE basecup flake at Ultra Pac Inc. in Rogers, Minn. Ultra Pac is installing a Cryoclean 2000 cryogenic line from Modern Machinery of Beaverton in Beaverton, Mich. (A report on the Ultra Pac installation appeared in the December 1992 edition of Reuse/Recycle newsletter, based in Lancaster, Pa.) The first commercial cryogenic recycling line from Modern Machinery went last year to Smorgens Plastics in Sydney, Australia.

Cryogenic cleaning uses liquid nitrogen at -200 F to freeze PET and HDPE flake, which is then hurled by high-speed vanes against a steel impact plate (see PT, June '91, p. 35). This pulverizes the frozen glue and releases labels. At such low temperatures, PET and HDPE are unaffected, but PVC crumbles to the consistency of sugar, allowing it to be mechanically screened out of the PET or HDPE later. Such screening removes 90% of PVC, Modern Machinery's marketing director Edward Whitehouse told PLASTICS TECHNOLOGY. If there are only traces of PVC in the first place, cryogenic separation reportedly can leave PET flake with no more than 100 ppm of PVC, and generally much less.

Ultra Pac chose to set up in-house recycling (previously it bought recycled pellets) in order to control the sourcing, pricing and quality of its feedstock, says Ultra Pac's recycling coordinator, Robert Parish. The company had not experienced any PVC contamination problems with its commercial recyclers, he adds, but "the possibility was always there." Ultra Pac uses recycled PET for products such as FDA-approved, 100% PCR sheet for thermoformed produce trays. It also has an FDA go-ahead for a new three-layer sheet for food packages with virgin PET on the outside and a recycled middle layer (see PT, Feb. '93, p. 83). PVC contamination in clear applications like these would leave black specks of char at the high (550 F) forming temperatures of PET.


Cryogenic cleaning has several other environmental advantages over wet washing. It scrubs without caustics, saving the cost of chemicals and of water treatment afterward. In fact, Modern Machinery's cryogenic line discharges no water at all, losing only a gallon of water per hour through evaporation from the heated float tank and from drying flake. Operating costs for the 2000-lb/hr line--including nitrogen, electricity and labor--are about $95/hr or 5|cents~/lb, Modern Machinery says. This is based on $56/hr for nitrogen at 27|cents~/100 cu ft; $19/hr for electricity at 9|cents~/kwh; and two operators at $10/hr each.

The line at Ultra Pac passes flaked PET through a metal detector and an aspirator. Dry flake then goes through the cryogenic freezer and high-speed impacter and is aspirated a second time. After that, it starts a slow trip through a patented hot-water (120-150 F) float/sink tank to separate HDPE basecup and PP cap flake from PET and warm the flake up. In most float/sink tanks, flake goes from end to end, interrupted by turbulence from paddles or aeration. But in order to warm the flake sufficiently, this enclosed 40-ft tank, with heater bands on the outside, has three water lanes, so the flake travels 120 ft in all. Heating the flake is necessary to dry it (moisture would condense on cold flake, making it impossible to dry).

Separated HDPE flake goes off to its own heated spin dryer made by Gala Industries, Inc., Eagle Rock, Pa., and modified with forced hot air. PET goes through an additional proprietary scrubbing stage, cleaning only with water. (Smorgens, however, is said to use cleaning chemicals at this stage). This optional scrubbing step is for very high-clarity products. Without it, cryogenic cleaning alone would yield recycled PET sheet with a slight bronze cast. PET also goes through a separate hot-air dryer and finally through a 20-ft x 2-ft diam., rotating screened drum to remove PVC grains and fines.

Base price for a Cryoclean 2000 with metal detection, cryogenic freezer, impacter, two aspirators, two dryers, float tank and rotary screener is $380,000. The line at Ultra Pac, which includes additional machinery and material-handling equipment costs about $700,000.
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Title Annotation:Technology News; polyvinyl chloride; high-density polyethylene
Author:Schut, Jan H.
Publication:Plastics Technology
Date:Apr 1, 1993
Previous Article:A recycling first: carpets!
Next Article:More on properties of next-generation polyolefins.

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