What are Fine Screening and Coarse Screening?

Coarse screening of pulp contains many impurities, much of which are unresolved paper scraps remaining from the pulping system. Coarse screening is the first step to remove these large impurities, preparing the pulp for the next screening stage. Fine screening removes smaller impurities; in current waste paper pulping, this mainly involves removing adhesives from the pulp.

What is the Difference Between Coarse Screening and Fine Screening?

• Coarse screening has a better function in dispersing shredded paper, Many coarse screening screens now include a crushing function.
• Since the coarse screening removes a large amount of impurities, a drilled screen basket or a slotted screen basket with larger gaps can be used. The fine screening uses a slotted screen basket with narrower gaps.
• Similar to consistency cleaner, when the slurry concentration is too high, it is not conducive to the removal of impurities. The coarse screen has larger pores and can operate at a medium concentration of less than 4.5%, reducing equipment capacity and energy consumption. The corresponding fine screen should operate at a low concentration of 1.5%.

Differences Between Coarse Screening and Fine Screening

Fine and coarse screening rotors differ significantly. Fine screening uses rotors with blades and wedges. However, it is not suitable for coarse screening.

• because more impurities will cause increased wear and tear.
• Impurities would get trapped between the blades and the screen basket, causing damage to both;
• Thread-like impurities would become entangled on the rotor, affecting normal operation;
• and It’s necessary to prevent the blades from breaking up impurities and allowing them to pass through the screen basket, increasing the need for subsequent fine screening. Therefore, coarse screening rotors are entity.

Entity rotors eliminate the risk of impurity entanglement and offer good wear resistance, meeting the design requirements for coarse screening. The wedge-type rotor is the most common type of rotor, which can be used for both fine and coarse screening. The rotor wedge-shaped arrow blades adopt a fluid design, with efficient secondary slurry on the front of the blades and a negative pressure zone at the tail to clean the screen slotted and prevent clogging. Dovetail rotors feature removable blades, making replacement easier and reducing downtime. Spiral-bar rotors have blades arranged in a spiral pattern, facilitating the up-and-down movement of the slurry. The front half of the spiral-bar rotor’s blades generates positive pressure on the slurry, while the rear half generates negative pressure, resulting in a lower pulse intensity. This ensures that every part of the screen basket experiences negative pressure, producing screen basket purification effect. This type of rotor is suitable for slotted screen basket and offers high purification rates.