How do pulp screens remove unwanted materials such as bark and uncooked wood chips?

In the papermaking process, pulp screens are used to separate unwanted materials—such as knots, bark, dirt, fibre bundles, shives, and uncooked wood chips—from the accepted wood fibre.12

These materials are typically removed through the following methods:

• Particle Size: Screens often separate unwanted particles from the accepted fibre based on their physical size.1
• Centrifugal Principle: There is an increasing use of centrifugal separation, which distinguishes between particles based on their density.1
• Rifflers: In the production of mechanical or groundwood pulp, the stock flows from the grinder pit through a series of rifflers and screens to remove heavy foreign materials and unfibred wood pieces.2

Because certain pulping methods, such as the sulfite process, do not disintegrate bark and foreign materials as effectively as others, careful selection and cleaning of wood chips are essential to ensure pulp quality.1