Contamination Removal
Mechanical filtration stages in a pulp mill remove contaminants based on their size or stiffness before the fiber reaches the paper machine. Paper stock preparation screening utilizes perforated plates or fine slots to trap plastic and metal fragments while allowing individual fibers to pass through. It protects the forming wire from damage and ensures the final sheet is free of visible defects.
The efficiency of this process depends on the consistency of the pulp.
Filtration Mechanism
Pressure differences across the screen plate force the acceptable fibers through the openings while pushing the rejects toward a discharge valve. In the paper stock preparation screening stage, the size of the slots is chosen based on the grade of paper being produced. Fine writing papers require much smaller openings than heavy packaging boards.
Continuous cleaning of the screen surface prevents the buildup of a fiber mat that would block the flow. This step is repeated several times to maximize the recovery of clean fiber.
System Reliability
Regular inspection of the screen plates prevents debris from entering the system. Worn screens allow large contaminants to pass through, leading to holes in the finished paper. This process remains the most effective defense against raw material impurities.
