This invention relates to disc refiners for lignocellulosic materials (referred to as “fibrous material”), and more specifically to disc refiners used for producing mechanical pulp, thermomechanical pulp and a variety of chemi-thermomechanical pulps (collectively referred to as mechanical pulps and mechanical pulping process).
In the mechanical pulping process, raw fibrous material, typically wood or other lignocellulosic material, is fed through the middle of one of a refiners discs and propelled outwards by a strong centrifugal force created by the rotation of one or both discs. The disc(s) typically operate at rotational speeds of 1200 to 2300 revolutions per minute (RPM). While the fibrous material is retained between the discs, energy is transferred to the fibrous material from refiner plates attached to the discs. The energy transferred to the fibrous material separates individual fibers in the fibrous material from a network of fibers in the material. The separation of individual fibers constitutes refining of the fibrous material into a pulp product that may be used to form paper, fiberboard and other fiber based products.
The refiner plates each have surfaces with patterns of bars and grooves. The surfaces are opposite to each other when a pair of refiner plates are mounted in a refiner. The bars and grooves on the opposing refiner plate surfaces generate repeated compression forces that act on the fibrous material flowing between the plates. The compression action against the fibrous material results in the separation of lignocellulosic fibers from the feed material and provides a certain amount of development or fibrillation of the fibrous material. The fiber separation and development is necessary to transform the raw fibrous material to a suitable pulp for fiber board, paper or other fiber based products. The refining action imparted by the bars and grooves may also generate some cutting of the fibers, which is usually a less desirable result of the mechanical pulping process.
In the mechanical pulping refining process, a large amount of friction occurs that reduces the energy efficiency of the refining process. It has been calculated that the refining efficiency of the energy applied in mechanical pulping is in the order of 5 percent (%) to 15%.
Efforts to develop refiner plates which work at higher energy efficiencies typically involve reducing the operating gap between opposing discs. Conventional techniques for lowering energy consumption in mechanical refiners typically rely on design features of refining patterns on the front face of refiner plate that speed up the feed of material across the refining zone. These techniques often result in reducing the thickness of the fibrous pad in the gap between the opposing plates. When energy is applied to a thinner fiber pad, the compression rate becomes greater for a given energy input and results in a more efficient energy input.
A drawback to reducing the thickness of the fiber pad are that the operating gaps between the refiner plate bars is reduced. Reducing the gap between the opposing refiner plate bars often results in an increase in fiber cutting, a loss in pulp strength properties due to the cut fibers, and a reduction in the operating life of the refiner plates due to the excessive wear of plates. A narrow gap, e.g., clearance between bars on opposing plates, may achieve a higher compression ratio and higher efficiency but suffers a reduced operational life. There is a link between operating refining gap and refiner plate lifetime, the latter being exponentially reduced with reducing gap. Reducing the operating refining gaps results in an increase in the wear rate of the refiner plates and shorter plate life.
There is a long felt need for refiner plates that provide high energy efficiencies in transferring the mechanical energy from the rotation of the plates into the fibrous feed material, having relatively long operational plate lives and that minimize the cutting of fibers in the feed material.