Solid material comminution or disintegration – Processes – With classifying or separating of material
Reexamination Certificate
1998-01-28
2001-06-05
Husar, John M. (Department: 3725)
Solid material comminution or disintegration
Processes
With classifying or separating of material
C241S028000, C241S281000, C241S293000
Reexamination Certificate
active
06241169
ABSTRACT:
There are several known methods for mechanical defibration of wood. Of these methods, the grinding and refining methods are used in industrial production. Both of these methods are based on kneading of wood raw material by pressure pulses and on mechanical separation of fibres from each other, but the grinding method is more closely related to the present invention. The idea of the kneading is to prepare the wood raw material so that the subsequent mechanical separation of fibres produces pulp suitable for paper making and not only wood fibres separated from each other. The kneading of wood raw material consists of two obligatory parts: fluctuating pressure for breaking the matrix structure of wood, and deformation for softening the wood by the generation of heat energy. In the grinding method, the above-mentioned series of operations is performed by pressing blocks of wood in transverse direction against a rotating cylindrical pulpstone, keeping the longitudinal direction of the blocks of wood parallel to the axial direction of the pulpstone, as disclosed in Swedish Published Specification 309 529. The surface of the pulpstone comprises extremely wear-resistant particles bound to each other by means of a softer binder, whereby they form a random particle construction, as disclosed in Finnish Published Specification 68 268 and U.S. Pat. No. 2 769 286. The difference in altitude in the peripheral direction of the surface, which is due to the random location of particles, generates pressure pulses to the wood raw material and separates fibres from the surface of the wood raw material by means of surface friction. The most significant drawback of both of these mechanical defibration methods is the high energy consumption, which is due to the high generation of heat. The consumption is 10 to 100 times higher than the theoretical energy consumption of defibration disclosed in many connections.
The object of the present invention is to produce pulp suitable for paper making from raw wood by a highly controllable process with a relatively low energy consumption.
The invention is based on the use of a defibration surface that is regular in the peripheral direction instead of a randomly distributed grinding surface. This surface generates regular pressure pulses whose cycle length depends on the peripheral speed employed. The regular defibration surface is provided with a smaller-scale roughened texture, which causes the fibres to be mechanically separated from each other. Such a combination of peripheral speed, regular shape and roughness of the defibration surface is selected that a half of the resulting cycle length corresponds to the average relaxation time of the wood raw material under the defibration conditions, and that the production produced by the roughened surface texture corresponds to the desired production. The relaxation time of wood refers herein to the time it takes the wood raw material to relax freely, within the limits of the amplitude of the basic contour of the surface, from maximum tension to minimum tension in the pretensioned state and conditions in which the defibration takes place. The relaxation time can be measured experimentally in the defibration conditions. A regular defibration surface for achieving the effect described above is novel as compared with the prior art disclosed, for example, in Swedish Published Specification 309 529. The desired defibration surface can be manufactured in different ways, for instance by machining at first and then coating.
The invention has significant advantages.
The method and apparatus of the invention consume energy more efficiently than the methods currently used in the industry. The amplitude of the pressure fluctuation caused by the conventional grinding method is modest in the surface layers of the wood raw material, but the production of heat energy is great in a very thin surface layer. This is because a randomly formed grinding surface causes work cycles whose lengths form a very even distribution. On the other hand, the relaxation time of viscoelastic and non-homogeneous wood raw material in the prevailing conditions falls within a relatively narrow range. These are the reasons that the following work cycle is highly likely to begin at a wrong phase, which causes a significant deformation and production of heat energy in a very thin surface layer. A relatively small part of the mechanical energy is converted to potential energy, and a large part of it is directly converted into heat energy. Potential energy represents the internal tension of the raw material, which breaks the matrix structure and, upon relaxation, is converted into heat energy. Half of the work cycle caused by the method and apparatus of the invention corresponds preferably approximately to the average relaxation time of the wood raw material. It is thus probable that the following amount of work for maintaining the pressure fluctuation is done when the required change in the momentum is small and a large part of the energy can be converted at first into potential energy stored as tension of the wood matrix. The method of the invention thus utilizes as much of energy as possible for the breaking of the structure of the raw material before it is converted into heat energy, which enables efficient use of energy for mechanical defibration of wood. In addition, the method of the invention, in which one property of the defibration surface mainly causes the wood raw material to be kneaded and the other one mainly causes the fibres to be separated, allows these parts of the process to be controlled separately and both these types of work to be done in a sufficient amount but no more than is necessary.
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patent: 68268 (1985-04-01), None
patent: 309529 (1969-03-01), None
Derwent's Abstract of SU 698752 dated Nov. 30, 1979.
Sandas, E. “Effects of pulpstone grits in wood grinding.” Paper and Timber, vol. 73, No. 9 (1991), pp. 858-861.
Husar John M.
Ladas & Parry
Metso Paper Inc.
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