Evaluating Pulp Yield Losses from Extrusion Coated Board in Industrial Hydro-Pulpers
Polyethylene-coated board yields drop 8 to 22 percent in industrial pulpers as unseparated fiber bundles exit screens bound to rejected plastic film.
Screen Reject
Discarded Fraction
Material that fails to pass through the apertures of a filtering device represents the portion of the stock containing undesirable particles. This screen reject contains a mixture of heavy debris, large fiber bundles, plastic fragments, and foreign objects. In a typical paper mill, this stream is diverted from the main production path to prevent damage to downstream equipment.
The volume of this material relative to the total feed indicates the cleanliness of the raw fiber source.
Physical Classification
Physical separation occurs as the slurry is forced against a perforated plate or a slotted basket. While the useful fibers flow through, the screen reject is pushed toward a discharge port by the action of a rotor or the pressure of the incoming fluid. Secondary and tertiary screening stages often process this stream again to recover any good fiber that was accidentally trapped with the waste.
This cascading system ensures that the final waste stream is as concentrated as possible. The reject rate is carefully adjusted to balance the purity of the accepted stock with the total fiber loss of the mill.
Disposal Requirement
Managing the moisture content of this waste is a primary concern for mills looking to reduce transportation costs. Heavy presses or screw thickeners are used to remove excess water from the screen reject before it is sent to a landfill or an incinerator. Because this material often contains high levels of plastic and adhesive, it has a high heating value and can be used as a fuel in some industrial boilers.
Proper handling prevents these contaminants from leaking back into the mill water system.