Screening equipment is an important operating system for pulp processing. After the impurities are separated from the fibers in the washing section, the pulp contains coarser fibers, foreign matter, and dirt, just as stones, plastics, cement, etc. Therefore, these unwanted and troublesome substances should be removed to produce high quality pulp. Pulp screening equipment is used in this area.
Pulp mills use two types of screens; hole screens and slot screens. The working principle of these screens is based on vibration, shaking, gravity centrifugation and centripetal force.
The factors are various factors such as raw material consistency, fiber type, hole type and size, type of board cleaning mechanism, level of crude fiber and foreign matter, reject rate, process configuration, flow rate, inventory temperature, and the like.
Water (consistency) plays an important role in the entire step of pulp screening. In the screening section, water acts as a fiber belt. The pulp concentration of the tank may be 3% to 4%. When the pulp leaves the tank, the consistency should be reduced by adding white water (previously used water or backwater). Depending on the nature of the pulp, the inlet consistency of the screening equipment should be 0.35 to 0.85 for best results. If the agreement exceeds 0.85%, the rejection will increase. According to the type of pulp screening, the appropriate consistency of the pulp should be 0.40% to 0.50%.
The diameter of the hole in the pulp screening device is another important variable in screening performance. In the case of a coarse screen, the diameter should not exceed 3/16 inches; however, it is best to maintain 1/8 inch in diameter. In the case of a fine screen, it should be 0.045 to 0.0625 inches. It determines the minimum size of the particles that will be rejected.
The coarse screen removes heavier particles, such as very coarse fibers, dirt and sand, and may be 4% to 5% of the entire material. It may vary depending on the type of fiber and the type of screen and hole diameter. The coarsely screened acceptable stock is sent to the fine screening inlet for rescreening to increase the efficiency of the screening section.