June 3, 1993 ISSUE #4
The industry that recycles post consumer plastics is divided into three main segments. These are the recycling of bottles, film, and polystyrene. The bottle segment is the largest, and it has several specialists: PET (two liter Coke bottles); clear high density polyethylene (HDPE) (one gallon milk containers); and colored HDPE (laundry detergent, cooking oil, hair shampoo). The film comes mostly from industrial wrapping and supermarket bags. And we are all familiar with the sources of polystyrene foam.
The Vincent dewatering screw press has found a common application in the factories of all of these segments. That application is the dewatering of wash tank sludge.
The recyclers of post consumer plastics all grind (shred) their incoming waste material after it has been sorted. Next it must be washed in order to assure purity of their end products. This washing is done in tanks, and sludge with a high fiber content accumulates in the process.
The need is to dewater the sludge arises because all the recyclers send the sludge to landfill. Without dewatering the sludge drips water, and highway officials will not permit hauling material that drips on the roads. Further, landfill tipping fees are less for dewatered material.
A question that invariably comes up is how to feed the sludge into the Vincent press. We find that in practice plastic recyclers do this in a variety of ways. Some pump the dilute sludge directly from the wash tank to the press. Other recyclers pump the wash water over a screen and feed the tailings into the press. The recyclers frequently have more than one source of sludge, so they might end up using a combination of these methods with one or more presses.
The screens being used ahead of our presses are both the static sloped screen with wedge wire and the round Sweco vibrating variety. In at least one case the tailings drop directly into the press, but most commonly the tailings are fed to the press with a screw conveyor.
In all cases the objective is simply to turn the sludge into a cake that will not run or drip. The volume typically is such that the smallest capacity presses are used.