The IP.com Prior Art Database
English (United States)
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BLEACHING OF PULP TO REDUCE BRIGHTNESS REVERSION
field of the invention
The present invention relates generally to the bleaching of cellulose pulps. In preferred embodiments, the present invention relates to cellulose pulp bleaching techniques which reduce brightness reversion.
BACKground and summary of the invention
Improvements to pulp bleaching have been disclosed in published International Application No. WO 96/12063 published on April 25, 1996 (the entire content of which is expressly incorporated hereinto by reference) whereby at least 50% of hexenuronic acid groups in the pulp are removed prior to bleaching.
Brightness reversion is the measure of the loss of brightness of bleached pulp or paper over a period of time. In this regard, it is highly desirable for the bleached pulp to retain its white (“bleached”) color over prolonged time periods and thus reduction of brightness reversion of bleached pulp is highly desirable.
Generally, the present invention is embodied in a novel bleaching sequence and strategy whereby an A stage in accordance with the above-cited WO 96/12063 (i.e., where the conditions are high temperature and the pH is above 2.5 sufficient to remove 50% of hexenuronic acid groups in the pulp) is practiced and the resulting pulp thereafter bleached using a at least one chlorine dioxide bleaching sequence followed by neutralization. The final stage can be a conventional peroxide stage, a pressurized peroxide stage or simply the addition of an alkaline agent without an oxidizing or reducing agent present. It has been found that using the techniques of the present invention, the measured brightness reversion is lowered by at least 20%.
brief description of the ACCOMPANYING drawings
Other aspects and advantages of the present invention will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof as illustrated in the non-limiting Examples.
In the Examples below, the following abbreviations have been employed to denote the various stages employed:
Mill O = pulp that was cooked and oxygen delignified at the mill
A = acidic pre-treatment at an elevated temperature of about 850 to 1500C at a pH of about 2 to 5 to remove at least about 50% of the hexenuronic acid groups in the cellulose pulp and to decrease the kappa number of the pulp by 2 – 9 units (i.e., in accordance with the techniques described in the above-cited WO 96/12063.
D = chlorine dioxide treatment (DD means sequential chlorine dioxide treatment with no intervening neutralization, whereas DnD means sequential chlorine dioxide treatment with an intervening neutralization stage.)
Eop = alkaline extraction bleaching stage that is supplemented with oxygen (o) and hydrogen peroxide (p) to assist in developing brightness
Invention Example 1 - (Mill O)-AD-Eop-DnD
The Kappa number for pulp to be used in the example was obtained and the proper amount of pulp sample was weigh...