March 27, 2006 ISSUE #171
Back in the 1980's, Vincent sold a pair of presses to Dart Container in a Styrofoam dewatering application, for their re-cycling operation. These are used to squeeze wash water from shredded Styrofoam. The technology works, and both presses are still in service.
In 2005, financial as well as technical success was achieved at Georgia-Pacific's Green Bay, Wisconsin recycle paper mill. This is a very large mill, as far as secondary fiber operations go. The mill capacity is rated at over 1,000 tons per day.
A long standing problem at Georgia-Pacific had been the disposal of Styrofoam and other floating waste that accumulated in the mill's two large primary clarifiers. With an operation this large, the floating waste amounted to several tons per day. This material was skimmed, drained, and allowed to accumulate. This resulted in problems with run-off water. In addition, wind would disperse the dried material, creating an unsightly situation.
The solution to these problems was found in a Vincent Model KP-16 screw press. This press has been built, in quantity, for many years. Most units are for dewatering applications at produce processing plants and dairy farms. At the produce plants, the service is simply dewatering trash like corn husk, lettuce leaves, carrot shavings, onion skins, pea pods, and such. At the dairy farms, the task is to dewater manure. The manure is separated into dry bedding for the cows or compost, with the press liquor being used for irrigation or barn flushing.
At Georgia-Pacific, the floating waste is now skimmed and drained by gravity into the single 16" screw press. The waste is compacted into a form that is readily handled for landfill disposal. The press liquor is directed back to the clarifiers.
Initially, a perforated screen, with 3/32" holes, was used in the press. In order to improve the solids capture rate, this was changed to a profile bar screen, with 0.015" slots. Bar screens have about half the open area of perforated screens. Nevertheless, the bar screen has ample hydraulic capacity for the high-freeness flow seen by the screw press.
Simplicity, low maintenance, and unattended operation were features that led to the selection of a screw press over a baling machine. In addition, a screw press removes a greater percentage of the water in the waste, avoiding runoff problems.