Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Charging and/or discharging fibrous material
Reexamination Certificate
1998-03-26
2003-02-04
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Charging and/or discharging fibrous material
C162S055000, C162S060000, C162S065000
Reexamination Certificate
active
06514380
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method and a fiber line system for producing chemical pulp in such a way that shive-containing rejects from screening are minimized and reprocessed in a cost-effective and advantageous manner. The invention has a number of advantages over previously known methods of treating chemical pulp after cooking for minimizing the amount of reject material in the final pulp produced.
U.S. Pat. No. 4,220,498 (the disclosure of which is hereby incorporated by reference herein) discloses a number of different alternatives for the treatment of chemical pulp after cooking in order to reduce the amount of reject material. In some sequences the pulp is screened prior to oxygen delignification and the rejects are subjected to further screening, refining, or accessory oxygen delignification. In other sequences the pulp is screened after oxygen delignification, but again is subjected to refining and typically other treatment, such as accessory oxygen delignification.
U.S. Pat. No. 4,895,619 (the disclosure of which is hereby incorporated by reference herein) suggests eliminating problems associated with handling and reintroducing the screen rejects from a screening stage that are associated with the U.S. Pat. No 4,220,498 by simply oxygen delignifying the rejects fraction exteriorly of the fiber line, and then returning it to the delignified rejects fiber line prior to the oxygen delignification stage in the fiber line.
U.S. Pat. No. 4,595,455 discusses a brown stock treatment process where the digested brown stock is first coarse screened, and then washed and pressed to high consistency, i.e. to about 30% consistency. After pressing the stock is fluffed into an oxygen delignification tower where the stock is delignified. The stock discharged from the delignification tower is diluted to low consistency and screened. The screened accepts are introduced to further processing in the main process line and the rejects are turned back to the feed of the brown stock press.
A significant drawback in the above-described process is the treatment consistency and its effect on the shives. Firstly, it should be understood that at high consistency (i.e. a consistency above 25%) delignification is a difficult task. In actual mill-scale operations it has been found that there are problems with the delignification efficiency in the high consistency delignification of fibers. Therefore, it is not a surprise that shives will not be loosened into fiber form but will maintain their original shape and “strength”.It could even be said that the only thing that tends to break the shives in U.S. Pat. No. 4,595,455 is the screening and other mechanical working on the shives. The reason for this is that when the rejects received from the screening stage after delignification are returned directly to the brown stock press, the shives have very little time, not nearly enough time, to be impregnated by the alkaline liquor. Dilution and pressing in U.S. Pat. No. 4,595,455 are virtually successive operations since the shives do not have time to be impregnated by the liquor and are not treated efficiently at all in the delignification tower (only the surface of the shives is treated, i.e. delignificated). Also, at a consistency of 30% there is no alkaline liquid around the shives so that there cannot be any efficient mass transfer between the liquid and the shives.
While the prior art procedures as described above are reasonably effective in ensuring a minimum of reject material in the final pulp produced, they have a number of drawbacks associated therewith. When refining is used the pulp produced by the refiner is really chemi-mechanical pulp rather than chemical pulp, which can make the final pulp produced have different properties than are desired. Also the use of a refiner significantly increases the air content of the pulp which impairs the runnability of the entire process. In all cases with the prior art techniques as described above it is necessary to invest in additional capital equipment aside from the conventional fiber line, which equipment can be very expensive. For example accessory (outside of the fiber line) oxygen delignification equipment and/or refiners are highly capital intensive.
Compared to the prior art in general, as well as the specific prior art discussed above, the method and system according to the present invention have a number of advantages. In particular according to prior art typical screening procedures (where a screening stage is provided directly after the brown stock washer) the pulp foams significantly, and it is necessary to utilize a significant amount of anti-foaming agent in order to control the foaming problem. According to the invention a minimal amount—and perhaps zero—anti-foaming agent is necessary. Also, in the practice of the invention, because of the relatively low consistency during delignification (e.g. about 6-18%), there is always free liquid between the shives and fibers so that impregnation and mass transfer take place substantially throughout the process, not just during a short dilution/screening stage.
In many prior art techniques the pulp has already cooled off either before or during the thickening. For example vacuum washers have been utilized as thickeners. According to the present invention the pulp may be fed “hot” into the oxygen delignification stage or stages, improving the heat economy of the process.
In the prior art, reaction products and chemicals which pollute the environment have often been discharged along with the rejects. According to the present invention considerably less reaction products are discharged, and the amount of reject material is smaller, providing a more environmentally sound approach.
The invention is advantageous compared to prior art systems and methods even where the screening stages are disposed after oxygen delignification because the capital investments associated with the refiners and/or accessory oxygen delignification equipment (as described above with respect to U.S. Pat. Nos. 4,895,619 and 4,220,498) are unnecessary.
The invention is also advantageous in that according to the invention shives are softened and partly disintegrated during oxygen delignification, reducing the amount of reject material; and the screening stages may actually be considered part of the bleaching process, even employing a chelating stage, thus enabling efficient individual fiber treatment. In fact it is conceivable that utilizing the teachings of the invention it would be possible to omit entirely the screening stages by using particular oxygen delignification treatment, so that pulp does not contain any shives after oxygen delignification.
According to one aspect of the present invention a method of producing chemical pulp is provided comprising the following steps: (a) Cooking comminuted cellulosic fibrous material (e.g. wood chips) to produce brown stock. (b) Washing the brown stock to produce chemical pulp. (c) Oxygen delignifying the chemical pulp at a consistency of between about 6-18% (preferably about 8-15%, e.g. about 10-13%). And, (d) screening the pulp from step (c) to produce at least an accept fraction and a shive-containing reject fraction. Steps (a) through (d) are practiced in a main fiber line, and then after step (d) there are the steps of: (e) Further treating (e.g. bleaching) the accept fraction, and (f) directly transporting the shive-containing reject fraction back to the main fiber line before step (c). Step (e) is typically practiced by bleaching the chemical pulp, with peroxide or other non-chlorine bleaching chemicals, and where peroxide bleaching is used a chelating stage is also employed. Step (c) is practiced at medium consistency (6-18%) to ensure sufficient alkaline liquid in the pulp during delignification so that impregnation and mass transfer between the liquid and shives takes place throughout at least all of steps (c) and (d).
The method according to the invention also typically is practiced in such a way that step (b) is practiced using a brown st
Laine Antero
Pikka Olavi
Alvo Steve
Andritz Oy
Nixon & Vanderhye P.C.
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