Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
2002-04-22
2004-11-30
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S042000, C162S043000, C162S045000, C162S065000, C162S072000, C210S651000, C210S652000, C435S277000, C435S278000
Reexamination Certificate
active
06824646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for bleaching a lignocellulose pulp. More preferably, the present invention relates to a process for bleaching a lignocellulose pulp, which enables a consumption of auxiliary chemicals for bleaching to be reduced to a great extent.
2. Description of the Related Art
It is known that in an alkali-oxygen bleaching (an oxygen delignification) process, a pulp is bleached in a reaction vessel by heat-treating the pulp with an alkali and oxygen placed in the vessel under pressure to produce radicals of lignin and resin in the pulp and to oxidize-decompose the radicals of lignin and resin. In the alkali-oxygen bleaching process, currently a moderate consistency oxygen-bleaching process (pulp consistency=8 to 15% by weight) is mainly used, in view of the relationship between the cost of bleaching apparatus necessary in this process and the quality of the resultant pulp. This process is advantageous in that a COD load on the environment is low and a non-chlorine bleaching agent can be used in a reduced amount in rear stage or stages in a multiple stage bleaching step, and thus is utilized in many factories in the world. However, the alkali-oxygen bleaching (an oxygen delignification) process is disadvantageous in that when the lignin in the pulp is removed in an amount of about 50% by weight based on the total content of lignin, the pulp cellulose is significantly damaged, the yield of the pulp is reduced and the viscosity of the pulp is decreased. This disadvantage can be restricted to a certain extent by using a magnesium salt as an agent for restricting the decomposition of cellulose. However, this restriction of the cellulose decomposition is not sufficient in practice. Thus, to keep the viscosity of cellulose at a practically permissible level or more, the lignin must be retained in a certain content in the alkali-oxygen-bleached oxygen-delignified pulp, and thus the bleaching efficiency of the conventional alkali-oxygen bleaching process is not always satisfactory. Accordingly, an enhancement in the bleaching efficiency of the alkali-oxygen bleaching process greatly contributes to reducing the load on the environment and to decreasing the bleaching cost due to the bleaching chemicals.
There have been many attempts to improve the alkali-oxygen bleaching process. For example, Japanese Unexamined Patent Publication No. 4-272,289 discloses an improved alkali-oxygen bleaching (oxygen-delignification) process in which two alkali-oxygen bleaching (oxygen-delignification) apparatuses are arranged in series and a washing means is inserted between the two bleaching apparatus. Also, U.S. Pat. No. 4,946,556 (Japanese Unexamined Patent Publication No. 3-14,686) discloses an alkali-oxygen bleaching (oxygen-delignification) process using a plurality of alkali-oxygen bleaching (oxygen-delignification) apparatuses arranged in series and a plurality of washing means respectively attached to each of the bleaching apparatuses. In these processes, merely the waste liquid delivered from each alkali-oxygen bleaching apparatus is washed by a countercurrent washing liquid and then is recovered into a pulp production step, and thus, the efficiency in delignification by the alkali-oxygen bleaching (oxygen-delignification) procedures and the whiteness of the bleached pulp are not satisfactorily enhanced.
Recently, various attempts have been made to reduce the load on the environment and to decrease the amounts of the bleaching and auxiliary chemicals employed in the rear stage or stages in the multiple stage bleaching procedure. In one attempt, a bleaching procedure using an enzyme, for example, xylase has been developed. For example, a bleaching method in which a pulp is treated with xylanase before the multi-stage bleaching procedure, is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-264,087 (corresponding to U.S. Pat. No. 5,179,021), No. 2-293,486 (corresponding to European Patent No. 395,792) and No. 4-507,268 (corresponding to WO 91/02,840). Also, a bleaching method in which a pulp is treated with a lignin-decomposing enzyme before bleaching procedure, is disclosed in Japanese Unexamined Patent Publication No. 2-500,990 (corresponding to WO 88/03,190, No. 3-130,485 (corresponding to European Patent No. 408,803, and No. 4-316,689 (corresponding to U.S. Pat. No. 5,618,386).
The treatment of the pulp with the enzyme before the bleaching procedure is advantageous in that the enzyme treatment conditions are relatively moderate and thus the reduction in the mechanical strength and the yield of the bleached pulp is slight, but is disadvantageous in that the reaction rate is low, and thus a long time is necessary to complete the enzyme reaction, and the reduction in Kappa value of the bleached pulp is very small.
Recently, the treatment of the pulp with xylanase particularly has drawn the attention of the paper industry. In the xylanase treatment, the enzyme must be brought into close contact with the pulp fibers to generate the reaction of the enzyme. However, since xylan and lignin contained in the pulp fibers are polymeric and are unevenly distributed in three dimensions in the pulp fibers, and the xylanase per se is polymeric, it is difficult to bring the xylanase into close contact with the xylan or lignin distributed in the pulp fibers. Thus, a new method of carrying out the enzyme reaction with a high efficiency must be developed.
The utilization of the enzyme including xylanase for the paper and pulp industry is disclosed in detail in Pratima Bajpai, “Enzyme in Pulp and Paper Processing”, published in 1998 by Miller Freeman Inc. Also, L. Viikari et al., “Biotechnol. Pulp Paper Ind. (Stockholm), pages 67 to 69, 1986, discloses a treatment of pulp with xylanase, and reports that the bleaching efficiency of pulp was improved by the xylanase treatment. Further, F. Mora et al., “Journal of Wood chemistry and Technology”, (6) 2, pages 147 to 165, 1986, reported that the treatment of pulp with xylanase after the pulp was bleached with oxygen contributed to enhance the mechanical strength of the bleached pulp. These reports are, however, quite silent as to the utilization and recovery of a waste liquid delivered from the enzyme treatment system.
In the bleaching procedures wherein a hemicellulase, for example, xylanase, is used and the bleached pulp is washed by a countercurrent washing method, the resultant bleaching reaction product mixture contains organic substance produced by the reaction of the enzyme with the pulp material and containing saccharide as a main component, and the saccharide-containing organic substance causes the countercurrent washing procedure to be difficult. Particularly, where the hemicellulase treatment is applied to the pulp material after the alkali-oxygen bleaching procedure, the organic substance containing saccharide produced by the hemicellulase treatment is returned into the alkali-oxygen bleaching (oxygen delignification) procedure through the countercurrent washing procedure, since a waste liquid delivered from the washing procedure is returned into the alkali-oxygen bleaching procedure.
It is well known that in the alkali-oxygen bleaching procedure, oxygen radical generated under the alkalin condition reacts with organic substances other than lignin in the pulp and having reduction-functional groups. The reaction mixture delivered from the hemicellulase treatment contains a large amount of fragments of decomposed lignin, and polysaccharides, oligosaccharides, monosaccharides, resin acid and derivatives thereof, and is washed by the countercurrent washing procedure, and the waste liquid delivered from the washing procedures and containing the above-mentioned organic substances is returned into the alkali-oxygen bleaching procedure. In this case, the saccharide molecules contained in the returned waste liquid have aldehyde groups which exhibit a reduction property. Thus in the alkali-oxygen bleaching reaction system, the returned saccharides react w
Azumi Naoya
Izumi Yoshiya
Kagawa Hitoshi
Sugiura Jun
Alvo Steve
Arent & Fox PLLC
OJI Paper Co. Ltd.
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