Method and apparatus for producing mechanical fibers
The purpose of this invention is to provide a method and an apparatus for producing thermomechanical fibers using less energy per ton that has previously been required for similar type of production. The invention is based in that at least two refiners are combined so that mixture of chips, water and steam is first fed into a first refiner wherein the chips are broken up and the mass flow from the first refiner is then fed into the second refiner for breaking the fibers to final freeness level. The mass flow of steam and fibers is fed forwards at least in the first refiner by the rotational energy of the refiner rotor so that essentially no flowback of the steam occurs. The mass flow exciting the first refiner is fed completely to the second refiner and no steam is extracted from the mass flow before the second refiner.
1. Method for producing thermomechanical fibers from wood chips comprising at least the steps of
providing least two refining stages, each refining stage including a rotor and a stator,
feeding at least wood chips and steam into the first refining stage,
defibrating the wood chips to fibers in the first refining stage to obtain a flow of steam and once refined fibers,
removing the flow of steam and once refined fibers from the first refining stage,
feeding the flow of refined fibers and steam into the second refining stage,
refining the once refined fibers in the second refining stage to produce a flow of twice refined fibers and steam, and
removing the flow of twice refined fibers and steam from the second refining stage;
said method being characterized by
feeding the flow of once refined fibers and steam forward in the first refining stage by rotary energy of the rotor of the refining stage so that no essential back-flow of the steam occurs,
transferring the total flow of the once refined fibers and steam exiting the first refining stage into the second refining stage without separating steam from the flow, and
feeding the flow of once refined fibers and steam through a flow path having a cross section that is constant or decreasing between at least the exit of the first refining stage and infeed of the second refining stage.
2. Method according to the claim 1 , characterized in that the steam fed into the first refining stage and formed therein forms the transport medium of the mass that is refined.
3. Method according to claim 1 characterized in that a residence time of the fibers in the process is less than 50% of a residence time required in processes using a cyclone between refiner stages.
4. Method according to claim 1 , further comprising separating steam from the twice refined fibers after the second refining stage and feeding at least part of that steam back to the first refining stage.
5. Method according to claim 1 , further comprising separating steam from the twice refined fibers after the second refining stage and feeding that steam back to the first refining stage.
6. Method according to claim 1 , further comprising keeping the medium velocity of the flow of once refined fibers and steam constant or preferably accelerating said flow between at least the exit of the first refining stage and infeed of the second refining stage.
7. Method according to claim 1 , characterized in that steam is separated only once from the process.