Improving Yield of Secondary Fiber Systems, 1998 Pulping Conference Proceedings

Yield losses from secondary fiber plants increase the costs of production. The good fiber that the mill pays for coming into the mill that is lost to the reject stream is not just lost revenue; it ends up in the landfilled waste at yet an additional cost. Therefore, projects that recover good fiber from mill reject streams have high payback if the recovered fiber is of high quality and does not re-contaminate the system. The objective of the various techniques described herein is to increase the yield from secondary fiber systems, effectively improving the cost position of the mill that adopts the method.

Good fiber is lost from typical secondary fiber systems in three places: pulper rejects, screen rejects and centrifugal cleaner rejects. Pulper rejects usually consist of non-fibrous contaminants mixed with various amounts of wet strength paper flakes that have not been completely defibered. A new process (patent pending) has been developed to recover unrepulped wet strength in mixed furnish pulping. In the new process, wet strength flakes are segregated, treated and defibered, thereby reducing yield losses by up to 97% in the pulper loop.

Good fiber is also lost at various other points in secondary fiber processing. Sometimes losses stem from insufficient deflaking. In other cases, reject rates of existing equipment are simply too high due to wear or operating convenience. Alternative methods are discussed that have been shown to reduce system losses. Finally, fibers can also be recovered from the effluent stream for cleaning and reuse. A different approach is now available that models the process after on-site mineral plants. Effluent treatment facilities can be built and operated by a third party who has a contractual agreement with the paper mill. The mill supplies the raw material (the effluent) and the plant recovers the long fiber and returns it to the mill, along with the recovered white water, both of which are completely suitable for reuse in the manufacturing process.