Virgin Fiber Mills

June 4, 1996

Paper mills that process chipped logs and sawmill waste into paper are located in the woodlands of the northern and southeastern United States.

We have found several applications where Vincent screw presses can be used advantageously. These involve dewatering waste material such as screen rejects, shives and knots.

Screen rejects are bits of material that are not suitable for making paper. They can come from pressure (filter) screens both ahead of and following a bleaching operation. At times they are in thin flows containing 1% solids, while at other times they may have been thickened to approximately 10% solids. Because the solids are large enough to be caught in the screen of our press and the water is loose (that is, not bound) excellent pressing results are achieved.

Shives dewater beautifully in our press. Shives are tiny bundles of cellulose fibers and lignin that are not suitable for producing paper. The shives are screened from the acceptable pulp and become a waste stream. Dewatering in a press removes them from the waste stream going to the wastewater treatment plant. The press liquor from the press can contain both fiber and chemicals that are worth recycling. At the same time, the press cake will be useful as boiler fuel; landscaping mulch; and filler for asphalt shingles and tar paper.

Another waste product at a virgin fiber mill is knots. These are the remnants of branches that are seen in lumber products. Knots absorb a great deal of digester chemicals, but they do not become useful pulp fiber. They are screened from the flow of cooked wood chips with vibrating shaker screens called knotters. Knots are either burned or sent to landfill. Pressing knots in a Vincent press results in 20% by weight being separated as a thick black liquor. This enables valuable chemical recovery and also makes the knots into a better fuel. Pressing knots also addresses an environmental run-off situation which can occur if high pH (12 or more) drainage occurs from the knot pile.

Issue 44