Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
1997-09-08
2004-08-17
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S076000, C162S078000, C162S088000
Reexamination Certificate
active
06776876
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method of producing and treating chemical cellulose pulps so as to minimize the use of bleaching chemicals and/or to optimize bleaching sequences.
Pulp mills have recently attempted to abandon the use of elementary chlorine, and sometimes chlorine dioxide as well. The reasons for this appear to be both environmental and economical (market acceptance). Disadvantages caused by elementary chlorine include both considerable malodorous gaseous emissions and liquid effluents from chemical pulp mills into water systems. Chlorine dioxide does not cause odor problems as significant as those of chlorine, but discharges to waterways are a concern. When comparing these chlorine chemicals with each other by means of the AOX number designating the loading on water systems, elementary chlorine is many times more detrimental than chlorine dioxide.
During the past few years, a large number of chlorine-free bleaching methods have been developed, often using oxygen, ozone, and/or peroxide. However, in many countries sequences using chlorine dioxide are also popular because the price of chlorine dioxide is very competitive compared with that of other chemicals (today approximately half of the price of competing peroxide). Also the strength and brightness values achieved by chlorine dioxide bleaching are at least approximately of the same scale as those for hydrogen peroxide bleaching at the same consumption of chemical (kg/adt).
When bleaching of cellulose pulps is effected using oxygen, peroxide or ozone, removal of heavy metals is almost essential. Detrimental metals include manganese, copper and iron, which catalyze reactions harmful to the quality of pulp. They degrade bleaching chemicals, which decreases the efficiency of bleaching and increases the consumption of chemicals. In cellulose pulps, heavy metals are primarily bound to carboxyl acid groups.
It has been suggested that removal of metals be effected in such a way that prior to the critical bleaching stage, pulp is pre-treated with an acid, e.g. sulphuric acid. Published Canadian patent application 1206704 discloses that the acid treatment is carried out at a temperature of at least 50° C., preferably at 60 to 80° C., at pH 1 to 5. It is stated in the publication that even acid treatment at a lower temperature results in significant removal of detrimental metal ions, but acid treatment at the temperatures according to the publication modifies lignin so that dissolution thereof is significantly improved in alkaline peroxide treatment following the add treatment (Lachenal, D. et al., Tappi Proceedings, International Pulp Bleaching Conference, 1982, p. 145-161). Thus, the acid stage causes the kappa number to drop in the peroxide stage, whereas no decrease in the kappa number has been found in the acid stage. It is also stated that in theory, the acid treatment could be effected even at a temperature of 100° C., but this could result in pulp of poorer quality.
In published EP patent application 511695 it is suggested that after acid treatment, metal ions advantageous for peroxide bleaching, such as magnesium ions, should be added, since some of these desirable magnesium ions are removed in the acid treatment. According to this publication, the acid treatment is effected at a temperature of 10 to 95° C., most preferably at 40 to 80° C., and at pH 1 to 6, most preferably 2 to 4.
The acid treatment is followed by a stage in which suitable alkaline earth metal is added. Further, it is mentioned that during acid treatment, pulp can be treated with a suitable bleaching and/or delignification chemical, such as chlorine dioxide.
Removal of detrimental metals may be made more efficient by using chelating agents for binding metals in connection with the acid treatment. One such method is disclosed in the SE patent 501651, which brings forward an acid treatment similar to that in the above-mentioned EP publication 511695, with the difference being that acid treatment is effected in the presence of a chelating agent. However, chelating agents used for binding metals contribute to increased bleaching costs.
The primary aim of the above-described acid treatments of pulp is to achieve such a composition of metals which is preferable for chlorine-free bleaching chemicals. In these stages, the kappa number may be decreased by 1 to 2 units due to a washing and extraction phenomenon. As mentioned earlier, the metal composition affects the consumption of bleaching chemicals, the reason for the use of known acid stages being therefore removal of metals from the pulp.
One of the most important disadvantages of prior art bleaching is an undesirably large consumption of bleaching chemicals, especially chlorine-free ones, which significantly raises the production costs of bleached pulp. Also with chlorine dioxide bleaching there must be attempts to reduce the consumption of ClO
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both for financial and environmental reasons. Further, a degree—in some cases a great degree—of brightness reversion is a typical feature of pulps bleached with oxygen and peroxide.
The invention seeks to eliminate or minimize the disadvantages of the prior art and to provide a totally new procedure for bleaching cellulose pulps, in particular cellulose pulps delignified under alkaline conditions, by means of either totally chlorine-free bleaching chemicals, or by using chlorine dioxide. The cellulose pulp produced according to the invention is easily bleached, e.g. by means of oxygen and/or peroxide.
It is known that cellulose pulps contain 4-O-methyl-&agr;-D-glucuronic acid groups (glucuronic acid groups). According to the invention it has been discovered that sulphate pulps also contain, in addition to glucuronic acid groups, a significant amount of 4-deoxy-&bgr;-L-threo-hex-4-enopyranosyl uronic acid groups (i.e. hexenuronic acid groups) bound to xylan. The amount of hexenuronic acid in some pulps is even substantially greater than the amount of known glucuronic acid groups. The term “hexenuronic acid” as used in the present specification and claims encompasses all 4-deoxy-&bgr;-L-threo-hex-4-enopyranosyl uronic acid groups.
It has been discovered that in bleaching of pulp, hexenuronic acid consumes bleaching chemicals reacting electrophilically, such as chlorine, chlorine dioxide, ozone and peracids (Buchert et al., 3rd European Workshop on Lignocellulosics and Pulp, Stockholm, 28.-31.8.1994). However, the hexenuronic acid does not affect the consumption of oxygen and hydrogen peroxide used as bleaching chemicals in alkaline conditions, because they do not react with hexenuronic acid. Thus, no degradation of hexenuronic acid occurs in oxygen and/or peroxide bleaching. Instead, special problems of bleaching pulp with oxygen and/or peroxide are relatively low brightness, and/or a tendency of such pulps to undergo brightness reversion.
The invention is based on the concept that by selectively removing hexenuronic acid from cellulose pulps in connection with bleaching it is possible to reduce the consumption of bleaching chemicals. Surprisingly, it has been discovered that at the same time, the brightness reversion tendency of pulp decreases. Also, bleaching becomes more selective, since the heavy metals can be removed more efficiently.
The selective removal of hexenuronic acid according to the invention is effected in part by making the water suspensions of cellulose pulps slightly acidic—typically, the pH is set between about 2 and about 5—and by treating the water suspensions at an elevated (above ambient) temperature. To achieve a preferable result the temperature is at least about 85° C., most preferably at least about 90° C. Practical utilization of temperatures as high as this has previously been avoided in acid treatment, because it has been assumed that the quality of pulp would suffer. The primary purpose of acid treatment has been removal of detrimental metals. In above-described acid treatments, the purpose of which is removal of metals, the temperature does not play a significant role. What is sig
Buchert U. Johanna
Teleman Anita B.-L.
Tenkanen T. Maija
Vuorinen Tapani J.
Alvo Steve
Andritz Oy
Nixon & Vanderhye P.C.
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