Acid treatment of pulp at high temperature prior to chlorine...

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact

Reexamination Certificate

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C162S067000, C162S076000, C162S088000

Reexamination Certificate

active

06306253

ABSTRACT:

The present invention relates to a method and apparatus for treating pulp at a pH of 2-5 and at a temperature of 75-130° C. in connection with bleaching,
Chemical pulp has been produced conventionally by first digesting pulp usually with sulphate or sulphite cooking liquor either in a continuous or a batch digestion process, by delignifying and screening the pulp before a bleaching process. The bleaching was still in the 1980's commonly done by using chlorine dioxide.
In the 1990's efforts have been made to replace chlorine dioxide with oxygen, ozone and/or peroxide because environmental factors force pulp mills to employ either totally chlorine-free or at least elemental chlorine-free processes. Sequences using chlorine dioxide are, however, still popular in many countries and possible also in view of the environment. There are many reasons for this. The price of chlorine dioxide is very competitive compared to other chemicals; for example its price is today only half of the price of the competing peroxide. Also the strength properties and brightness of the pulp produced by chlorine dioxide bleaching are good; in fact at least of the same order as when using peroxide at the same consumption of chemical (kg/adt).
When the bleaching of cellulose pulp is based on the use of such bleaching chemicals as oxygen, peroxide, or ozone, the removal of heavy metals forms an essential process stage. 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. It has been suggested that removal of metals be effected by pretreating the pulp prior to the critical bleaching stage with an acid, e.g. sulphuric acid whereby metals are detached from fibers to the liquid phase in the pulp. Before the bleaching stage itself the pulp is still washed by displacing the liquid in the pulp by cleaner wash liquid.
In acid treatments which are used to remove metals the temperature is usually not of significant importance. The most essential feature is that the pH of the pulp is so low (usually 1.5-2) that metals are detached from fibers. In laboratories the treatment is usually carried out at room temperature. In mills the metal removal is typically effected at a temperature range of 60-85° C., which is the temperature naturally prevailing in the acid treatment stage as a consequence of water circulations. If the mill would for some reason want to effect the acid treatment at a higher temperature the acid stage would have to be heated separately by steam or by a corresponding method. Naturally, this has been avoided because of the belief that the strength properties of the pulp would impair. Based on the state of the art knowledge there has been no reason to use hot acid stages.
It has been discovered recently that sulphate pulps contain significant amount of 4-deoxy-&bgr;-L-threo-hex-4-enopyranosyl uronic acid groups (hexenuronic acid groups) bound to xylan. Further, it has been found out (patent application WO 96/12063) that by removing hexenuronic acid groups from the pulp its Kappa number may be decreased remarkably. These acids are removed by adjusting the pH of the pulpito the range of 2-5, preferably 2.5-4 and the temperature of the pulp preferably to 90-110° C. and allowing these conditions to act on the pulp for a certain time and subsequently the pulp has been washed according to the publications mentioned above. By this kind of removal of hexenuronic acids, significant saving are obtainable in the consumption of bleaching chemicals which react electrophilically, such as ozone, peracids and chlorine dioxide, because they react with these acid groups.
When the acid treatment mentioned is used in connection with peroxide bleaching the brightness reversion of the pulp has been found to decrease.
As was stated above until now acid treatments at high temperatures have consciously been avoided as they have been believed to have a negative effect on the strength properties of the pulp.
An object of the present invention is to provide a method of carrying out acid treatment at a high temperature so that the strength properties of the pulp do not degrade during the treatment.
The characteristic features of the method and the apparatus of the invention are disclosed in the appended patent claims.
According to the method of the present invention the pH of the pulp, which has been delignified during digestion and possibly oxygen delignified after that and having a Kappa number of less than 24, preferably less than 14, is adjusted to a slightly acid range, e.g. 2-5, preferably 2.5-4. The temperature of the pulp is raised to 75-130° C., preferably 90-110° C. Then the Kappa number of the pulp reduced by 2-9 units, preferably 3-6 units. In order to obtain this, a retention time of 30-300 minutes, preferably 45-150 minutes, is required. The acid treatment may be effected at atmospheric pressure or as a pressurized process depending on the conditions. A preferred pressure is 0.1-1 MPa. At least 30%, preferably 50% of the hexenuronic acids contained in the pulp are degraded in this way.
One of the central ideas of the present invention is that the acid treatment is effected by minimizing the treatment temperature and/or treatment time of the pulp in order to optimize the strength properties of the pulp. It has been discovered that an adequate decrease in Kappa number and removal of hexenuronic acids is obtained when the treatment temperature is at least T
min
° C.
T
min
=
10517
24
+
ln

(
2

t
)
-
273
t=retention time, min
The equation has been presented graphically in FIG.
1
. The degradation of hexenuronic acid groups follows the reaction kinetics of the first order. It is known that the dependency between the affinity constant k and the temperature T (K) is k=A e
−E/RT
(equation of Arrhenius) in which A is constant depending on the reaction, E is activation energy and R is gas constant. On the other hand it is known that the reaction time for the reaction of the first order is t=(1/k) ln(c
0
/c) in which c is the hexenuronic acid content and c
0
the initial content. By using the Arrhenius equation and the equation t=(1/k)ln(c
0
/c) and test results, the above T
min
expression ensues. Further, it has now been discovered that in order to prevent the strength properties of the pulp from being deteriorated the highest employable treatment temperature T
max
° C. is
T
max
=
10517
24
+
ln

(
2

t
)
-
250
On the other hand, it may be stated that
T
max
=T
min
+23° C.
According to a particular aspect of the invention high quality pulp is produced by adjusting the acid treatment temperature to a range of T
min
-T
max
while the retention time is 30-300 minutes. In
FIG. 1
, the lower curve illustrates T
min
and the upper one T
max
. Favourable treatment conditions lie in the area between these. Typical operation conditions are those depicted by points A (90° C., 180 min), B (95° C., 120 min), and C (100° C., 70 min).
In order to preserve the pulp strength properties and to obtain homogenous pulp by acid treatment it is essential that the flow of pulp through the acid tower is smooth and no portions of pulp remain in the tower for example due to channelling for a longer or for a shorter time than the predetermined retention time. This may be prevented by using means controlling the feed and the discharge of pulp in an upward flow tower and means for controlling the level of pulp in a downward flow tower. The feed members must be able to distribute the pulp evenly over the whole cross-sectional area of the pulp tower. The optimization of treatment conditions may be facilitated by using two or more successive acid towers which makes it possible to act on the treatment conditions, for example by regulating the temperature or the pH, or by adding chemicals which are advantageous for the treatment, e.g. chelating agents to remove metals from the pulp, or merely by mixing the pulp

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