Vincent has supplied screw presses to several Kraft mills for use on knots and shives. These rejects are material that is not made into paper pulp. The knots come from knotters following the digester, while shives are bundles of fiber, high in lignin, that are separated by pressure screens.

This waste is screened. Shaker screens are used to separate fiber and chemical-rich liquor from the knots, and commonly dewatering screens separate wash water from the shives. Depending on the quality of paper being produced, these rejects may be redirected to the digesters. More commonly, after screening the waste is landfilled. At other mills the shives are left in the water going to the wastewater treatment plant.

The advantage of adding a screw press to this process is that the knots and shives can be made into a press cake. The cake moisture can be held in a range of 35% to 50% by adjusting the air pressure on the discharge cone. Mills use the dry cake as supplemental fuel for their boilers. Other mills have found that run-off problems are avoided at the knot pile, the knots do not drip while being hauled to the landfill, and the press cake adds stability to a landfill.

Advantageous chemical and fiber recovery results where hot stock screening is used. With hot screening the rejects from the unwashed stock, at 190° F, are the material furnished to the Vincent screw press. The press liquor, containing 15% chemicals and some usable fiber, is returned to the chemical and fiber recovery flow. Chemical recovery (mostly caustic) can be increased by 10%.

Typically the primary knotters account for about one quarter of the load. The shives usually arrive in a flow of 1% to 2% consistency. Sidehill screens have been used successfully in installations that required pre-thickening. Mills where shives were sluiced with wastewater have benefited from reduced loading in the wastewater treatment plant.

The most commanding performance feature of the Vincent screw press is the ability, without operator attention, to handle a wide variety of conditions. Flow rates can be varied from “off”, to surge, and back to normal. Tests were run with only mill water going into the press, followed by thin and thick flows. The press rarely wants to purge. There is a tendency to overload and jam at very low flows, so reinforced profile bar screens and heavy duty drives and frames are used for knots and shives.

One characteristic that catches everyone’s eye is that knots are disintegrated into small bundles of fiber in the Vincent press. This results in an excellent boiler fuel and opens the possibility of additional fiber recovery. This will depend on the paper quality produced at the mill.