Fiber in Press Liquor
March 25, 2016
It is unavoidable that some fiber particles will come through the screen of a screw press along with the press liquor. Many industries address this by recycling the fines, with good results and increased process efficiency. The practice is to filter out the insoluble solids and readmit them back into the screw press. Most of these recycled fines will be captured and exit with the press cake, although a fraction of them will come through the screen a second time.
In almost all cases this flow of recirculating fines reaches equilibrium. That is, fines come through the screen into the press liquor; we filter out the fines and dump them back into the press. The fiber in the regular material being fed into the press acts as a press aid. Most of the fines are captured in this press aid and go out as press cake. However a fraction of the fines get through in the press liquor and are recirculated back into the press again. The incoming, recycled, and liquor fines reach an equilibrium.
Many industrial processes recycle press liquor fines with good results. For example, a citrus feedmill uses a static screen to filter the press liquor. The sludge from that screen is dropped back into the reaction conveyor which feeds into the screw press. The circulating load of fines always reaches equilibrium.
Similarly, some fresh cut operations shred and press their produce waste, with the press liquor flowing into floor drains. The flow in the floor drains is pumped over static screens to reduce load on the WWTP. Like a citrus feedmill, the pulp that is separated by the screens is fed back into the screw press.
We see this process most of all at paper mills. The fines in the paper mill's wastewater settle to the bottom of the primary clarifier (primary = non-biological), and they are pumped to our screw press. Most of them come out as press cake, but a significant amount flow out with the press liquor. The press liquor is pumped back into the clarifier, and there the fines once again settle to the bottom. From the clarifier they get pumped back to the press. Again, the circulating load of fines reaches equilibrium.
We have only had one project where this did not work, the Cascades paper mill in Memphis. We built a brand new press for them. But so many fines came through with the press liquor that the clarifier was overloaded, and the result was one more press in our rental fleet.