Acid treatment of pulp at high temperature prior to chlorine dioxide bleaching
AssigneeAndritz-Ahlstrom Inc.
InventorKaj O. Henricson
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Abstract
In the treatment of cellulose pulp in connection with bleaching, the pulp is first cooked and, if necessary, oxygen delignified in order to reduce the Kappa number below 24, preferably below 14, and after that the pulp is treated in an acid stage at a pH of 2-5 and at a temperature range of 75-130° C. in order to reduce the Kappa number by 2-9 units. To prevent essential weakening of the strength properties of the pulp in the acid stage treatment tower, the retention time, t min, is 30 to 300 minutes and the treatment temperature, T° C. is Tmin<T<Tmax, in which T min = 10517 24 + ln ( 2 t ) - 273 and Tmax=Tmin+23° C.
Claims7
A method of sequentially treating cellulose pulp comprising:
- (a) digesting commminuted cellulosic fibrous material to produce cellulose pulp, and if necessary oxygen delignifying the pulp, to produce a cellulose pulp with a kappa number of less than 24;
- then
- (b) treating the digested pulp in at least one acid tower at a pH of between 2-5 at a temperature between 75-130° C., and, in order to prevent the strength properties of the pulp from being deteriorated, at a retention time t in minutes between 30-300 minutes, wherein the treatment temperature T in degrees centigrade is Tmin<T<Tmax in which T min = 10517 24 + ln ( 2 t ) - 273
- and
- Tmax=Tmin+23EC
- in order to decrease the kappa number of the pulp 2-9 units;
- (c) treating the acid treated pulp in at least one treatment tower different from the at least one acid tower with chlorine dioxide;
- and
- practicing steps (b) and (c) without an intermediate wash therebetween.
Dependencies are inferred from the claim text.