Pressing Dilute Flows
November 30, 2005 ISSUE #167
Occasionally questions arise about feeding a screw press with extremely dilute flows. We have one good set of data that involves this condition. They come from an OCC (old corrugated container) recycle mill, Liberty Paper, in Becker, MN.
They fed a flow of 260 gpm into a Vincent Model VP-16 press. This flow was mill effluent with a solids consistency of only 0.7% (7,000 ppm). This represents a feed of 11 tons per day of dry solids going into the screw press. The result was that 4 TPD,DS were captured and came out as press cake with 45% to 50% solids. The press liquor (effluent) from the press had about 4,500 ppm of solids. Thus the capture rate of the press was about 35%.
This operating condition occurred only during mill shut-downs because normally the feed to the press was in the range of 2% to 3%. (The city objected to the 4,500 ppm discharge periods, so at the time Liberty had to add sidehill screens ahead of the screw press, just to cover down periods.)
Thus it is seen that the press does not become inoperable due to extremely low consistency feed. Even with straight water going into the press, water will not purge from the solids discharge end of the press. On the other hand, the capture rate does go down significantly.
The Smurfit mill in Wabash, Indiana tested this to the limit in 1994. They ran the press normally for a while and then replaced feed flow to the press with a firewater hose. The press was run this way in order to confirm that a plug of fiber at the solids discharge would hold, preventing any water from coming out the solids discharge end of the press. All of the fire water came out through the screen. Normal press operation resumed automatically when the normal flow was re-admitted to the press.
(The other extreme of this same test was to have the press in normal operation and then switch the flow into press to cake from the press. That is, the press was fed only cake with 50% solids. This material passed through the press without the press tripping out on overload or damaging itself. Negligible press liquor came through the screens when operating in this manner.)