November 17, 2009 ISSUE 217
NEW ATTENTION ON KNOTS
Despite the economic times, paper mills continue to order screw presses for dewatering knots and shives.
In the 1990's, the principal justification was chemical and fiber recovery, with important sub-objectives of reducing load in the wastewater treatment plant; elimination of a potential run-off condition; and production of boiler fuel
Many times the projects centered on eliminating complaints of highway dripping and excessive moisture in the landfill loads.
Today the driving forces are mostly either biofuel applications or fiber recovery.
Biofuel is today's popular term. Biofuels are simply fuel from recently dead plants, while fossil fuels are fuel from plants that died eons ago. Burning biofuels is something that has
been standard at pulp mills for over a century. For example, it was that long ago that mills started burning concentrated spent liquor instead of dumping it in the rivers. Burning hog fuel dates back even longer.
Today's environmental consciousness drives biofuel usage. With or without government and environmentalist pressures, no one wants to landfill something which can be converted into needed energy. Besides, even with the costs of fossil fuels having come down from their peaks, there is interest, based on solid economics, in burning mill waste.
A recent installation at PCA in Filer City, Michigan is a case in point. Pressed knots and screen rejects make an excellent boiler fuel. In addition, some harder-to-dewater sludges
can be blended with the knots for additional fuel recovery. The reject fibers act as a press aid for capturing the solids in the sludges.
Fiber recovery is also driving economic force. Many times the fiber in knots can be recovered by recycling them to the digesters. This application does take a different
configuration in the screw press. If boiler fuel is the objective, the knots should be pressed to the maximum, with a minimum moisture content. In contrast, if the knots are destined to go back to the digester, they should not be pressed into a compact mass of broken fibers.They need a slightly softer squeeze in the press.
The cost and availability of timber is prompting fiber recovery from knots over a wide geographic range. Recent installations, at Sappi Fine Paper in Cloquet, MN and Alabama
River Pulp in Perdue Hill, Al have been justified on this fiber recovery.
Regardless of the use of the pressed knots, the advantage of chemical recovery should not be overlooked. The press liquor, containing 15% chemicals in some cases, can be returned to the chemical recovery flow. Chemical savings (mostly caustic) amounted to 10% of total chemical usage in one 1990's installation.
The well known savings from reduction in load on the wastewater treatment plant and polymer usage are also significant. One installation, still in start-up, has already gained a
savings of over $100,000 per year in polymer usage.
The Vincent screw presses selected for these installations feature the interrupted flight design. This contrasts to the continuous flight design normally used in the pulp and paper industry. This design is well suited where for flows vary in concentration and throughput.