Solid material comminution or disintegration – Processes – Wood and similar natural-fibrous vegetable material
Reexamination Certificate
1999-05-24
2002-01-08
Husar, John M. (Department: 3725)
Solid material comminution or disintegration
Processes
Wood and similar natural-fibrous vegetable material
C241S029000, C241S036000
Reexamination Certificate
active
06336602
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of Invention
The present invention relates to the mechanical refining of wood chips to produce wood pulp and more particularly, to a method of producing uniform and superior quality pulp from wood chips with disc refiners employing low intensity refining.
ii) Description of the Prior Art
In recent years there has been a considerable amount of both theoretical and experimental research into the factors governing fibre residence time in wood chip refiners. This has led to the concept of refining intensity, defined as the specific energy delivered per bar impact. The constitutive equations for material flows within refiners developed by Miles and May (J. Pulp Paper Sci. 16(2): J63(1990) and Paperi ja Puu 73(9):852(1991) can be used to calculate pulp velocity and the corresponding residence time of the pulp in the refiner. The residence time determines the specific energy per bar impact which Miles and May defined as refining intensity. Currently, many developments in mechanical pulping are focussed on reducing energy consumption while maintaining pulp quality at a desirable level. The refining intensity, which is a deciding factor for the quality of pulp produced at a given total specific energy, is itself highly dependent on the rotational speed of the refiner. In current commercial systems, the rotational speed of the refiner is determined by the frequency of the electrical supply, so that double disc refiners run at either 1200 or 1500 RPM and single disc refiners at 1500 or 1800 RPM. Therefore, regardless of the process requirements, the refining intensity is fixed by the system design and local generating frequency.
Jones et al (U.S. Pat. No. 3,411,720) disclose that mechanical pulp with greater strength could be produced by devising a refining process in which the amount of energy absorbed by the wood material is increased. They achieve this by increasing the residence time of the pulp in the refiner using a combination of sinuous plates or tapered plates and control rings on the periphery of the plates. Although they specified relative speeds of between 900 and 3600 RPM the process was never proven to be practical or effective.
Other methods to change the pulp residence time and the applied energy by changing the steam flow within zones in a refiner, have been reported. U.S. Pat. No. 5,335,865 discloses removing some of the steam from a first refining zone before it enters a second zone, or from the first two refining zones before entering a third (U.S. Pat. No. 5,248,099). These methods are based on the mistaken belief that steam and pulp velocities are equal. Steam flow has little effect on either average pulp velocity or residence time as calculated by Miles, Paperi ja Puu 73 (9):852(1991).
U.S. Pat. No. 2,035,994 discloses using variable disc speed to control pulp throughput using a stock refiner. A stock refiner is different from a mechanical disc refiner in that the stock refiner is employed to refine pulp while a mechanical disc refiner is used to refine wood chips. The process operates at low consistencies (3 to 6%) and rotational speeds (300 to 900 RPM). While these conditions are suitable for small modifications and improvements to pulp fibres before papermaking they are not suitable for refining wood chips into pulp fibres.
In an effort to circumvent the limitations imposed by electrical frequency and to reduce specific energy consumption in a single disc refiner, one equipment supplier has advocated an increase in refining intensity by employing a gearbox to raise the rotational speed to 2300 RPM. Similarly, in a double disc refiner (U.S. Pat. No. 5,167,373), an increase in feed-end speed from 1200 to 1800 RPM (U.S. Pat. No. 5,167,373) is advocated. However, these approaches do not account for situations where a fixed high level of refining intensity may not be appropriate for different raw materials and end product requirements.
Operating at higher than conventional disc speeds has been most effective when the first refining stage was operated at high refining intensity and the second stage was operated conventionally. (Tappi Journal 74(3):221(1991) and J. of Pulp Paper Sci. 19(1):J12(1993)). The optimum energy saving at desired pulp and fibre properties was obtained by employing a smaller portion of the total specific energy in the first, high intensity stage. A typical split in specific energy between the first and second stages of refining would be 40/60. Increasing further the refining intensity or the proportion of the specific energy applied in the first stage lowers the total energy required to reach a given freeness. However, it also lowers the average fibre length and pulp strength, limiting the advantage of the energy savings that was reported.
U.S. Pat. No. 5,540,392, issued to Broderick et al. discloses that it is possible to reduce energy by up to 18% in a two-stage refining system. At least 65% of the total energy is applied in a low intensity first stage refiner operating at conventional disc rotational speeds. Low refining intensity was achieved by raising the consistency from 26 to 30%. The remaining energy is applied in a high intensity second stage refiner. The pulp properties are reported to be at least as good as or better than that produced by conventional refiners although all their examples are for highly sulphonated pulps.
There is no known approach in the prior art that addresses the need to reduce refining intensity below conventional levels in order to improve the quality of wood fibres.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a method for mechanically refining wood chips to produce wood pulp exploiting low refining intensity achieved by rotating impact members which are rotated at speeds lower than conventional speeds.
It is a further object of the invention to provide an improvement to the conventional methods for refining wood chips into pulp or individual pulp fibres by mechanical means in a single or double disc refiner. More specifically, the improvement comprises increasing pulp quality by reducing refining intensity in one or more specific refining stages.
It is a particular object of the present invention to control refining intensity in a single refining stage employing low refining intensity.
Another object of the present invention is to control refining intensity in two distinct refining stages employing low refining intensity at least in the second stage of refining.
It is a further object of this invention to control the refining intensity in at least two or more distinct refining stages employing low intensity in at least two of the refining stages, one of the stages being the last.
It is yet another object of the present invention to control refining intensity in two or more distinct refining stages employing low refining intensity in a pre-refining stage and a final refining stage.
It is yet a further object of the invention to control refining intensity in three distinct refining stages employing low refining intensity in each stage.
In a further object of this invention low refining intensity is employed in refining rejects in a single or multiple stage refining operation.
It is yet another object of this invention to provide a method of refining wood chips to wood fibre pulp with monitoring of a fibre quality parameter.
The objects of this invention, to employ low refining intensity in a distinct refining stage or stages can be carried out by rotating the discs of a double disc refiner or the disc of a single disc refiner at rotational speeds that are lower than that conventionally employed.
In accordance with the invention there is provided in a method for mechanical refining of a wood chip composition to produce wood pulp in which the wood chip composition is subjected to at least one refining stage comprising transfer of energy to the wood chip composition under impact of rotating impact members, the improvement in which at least a final stage of said at least one refining stage is carried out in a refiner selected from a double disc refiner o
Husar John M.
Pulp and Paper Research Institute of Canada
Swabey Ogilvy Renault
LandOfFree
Low speed low intensity chip refining does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low speed low intensity chip refining, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low speed low intensity chip refining will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2854706