SETT VID FRAMSTELLNING AV FIBERSKIVOR PA VATA VEGEN MED ETT HELT ELLER DELVIS SLUTET BAKVATTENSYSTEM

摘要

A method of making fiberboard using a substantially closed white water system. One or more layers of separately prepared surface pulp is applied to a layer of base pulp at the wet sheet forming stage. Lignocellulose chips in a pressure-sealed defibrating zone under generation of steam. The steam is separated at the discharge end of the defibrating zone under atmospheric pressure. The atmospheric steam is utilized to presteam the raw chips. The presteamed chips are thereafter dewatered to a moisture content coordinated to the energy demand of the defibrating treatment and the desired generation of steam prior to entering the defibrating zone. The water removed at the dewatering stage is collected for use in preparing the surface layer pulp suspension. A small portion of the base pulp to be used for the surface layer pulp is diverted to a refiner and diluted with water. The major portion of the base pulp is diluted with white water drained off and recycled from the base layer wet sheet forming stage to form a base layer suspension, which suspension is then transported by the white water to the wet sheet forming stage, where the white water and recycled to the base layer suspension stage to form a new base layer suspension. The diverted portion of the base pulp is further diluted with white water recycled from the surface layer forming stage to form a surface layer suspension. A portion of the white water recycled from the wet sheet forming stage to the base layer suspension stage is heated by a portion of the steam separated at the discharge end of the defibrating zone to drive off surplus white water to maintain a constant volume of white water.