Pressurized Tail Screen - Improved Fiber Recovery Using Intermediate Dilution, 2006 Engineering, Pulping, & Environmental Conference

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The tail screen function of a screening system, i.e. the final step in screening where good fibers are recovered and coarse material finally rejected, is a vastly discussed matter. The large holes of conventional nonpressurized tail screens recover good fibers but also lets and separated in the screen room back to the process. Another draw back is the risk of air entrainment since it is nonpressurized. A pressurized tail screen with the possibility to use slotted screen baskets is a most sought after equipment. This paper describes the development of a new concept for a pressurized tail screen and the evaluation in mill scale. The concept involves utilizing the Metso invention of intermediate dilution (Swedish patent SE524527) in the screen basket that was developed to eliminate the reduction of capacity and screening efficiency that is caused by the pulp thickening across the screenin gsurface. Incooperation with Korsnäs AB pulp mill, Metso was allowed to rebuild the tertiary screen of line 3. During the trial period of four months the mass flow rate of reject from the tertiary screen was decreased from 0, 39 to 0,25bdt/h. With an increased shive content from 35% to 67%, the amount of good fibers rejected decreased from 0,26 to 0,08bdt/h. This gives a total increase in yield by 4,3 metric tons per day. The shive content in the accept pulp from the primary screen did not increase during the trial period.