Chlorine dioxide bleaching of chemical pulp

Details Chlorine dioxide bleaching of chemical pulp Chlorine dioxide bleaching of chemical pulp

1992-10-02

1996-12-31

Alvo, Steven

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Gas, vapor or mist contact

162 78, 162 88, 162 89, D21C 914, D21C 9147, D21C 916

Patent

active

055890318

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to the bleaching of chemical pulp. More particularly, the present invention relates to the bleaching of chemical pulp in a bleaching sequence which includes at least four bleaching steps, and most preferably where the final bleaching is carried out in a first and a second chlorine dioxide step with an intermediate alkaline extraction therebetween.


BACKGROUND OF THE INVENTION

Multi-step bleaching sequences including four bleaching steps, and in which the final bleaching includes first and second chlorine dioxide steps with an intermediate alkaline extraction can comprise, for example, a first step with chlorine/chlorine dioxide (known as CD), a second step with alkaline extraction in which oxygen gas is added thereto (known as EO), and a final bleaching step using two chlorine dioxide steps (known as D1 and D2, respectively). However, other such bleaching sequences are known, which are finalized using two chlorine dioxide steps with or without an intermediate extraction therebetween. It is known that the alkaline extraction step carried out between the chlorine dioxide steps (D1 and D2) during the final bleaching of a chemical pulp can be simplified, for example, by elimination of the washing of the pulp subsequent to the alkaline treatment step. In that case, however, it was believed necessary to neutralize residual alkali prior to the second chlorine dioxide step (D2). This, however, results in a greater consumption of alkali or chlorine dioxide, since the pH value in the second chlorine dioxide step (D2) will not be at the optimum level.
These and other problems are solved in accordance with the present invention, which results in a surprising and substantial increase in the efficiency of the alkalization, resulting in improved bleaching compared with conventional methods as set forth above.


SUMMARY OF THE INVENTION

In accordance with the present invention, a method for bleaching chemical pulp is provided including subjecting a partially bleached chemical pulp to a final bleaching sequence which includes bleaching the partially bleached chemical pulp in a first chlorine dioxide bleaching step so as to produce a first chlorine dioxide bleached pulp, washing the first chlorine dioxide bleached pulp so as to produce a washed pulp, extracting the washed pulp with alkali in an amount of between about 4 and 10 kg/ton of said pulp so as to produce an alkali extracted pulp including residual alkali therein, oxidizing the alkali extracted pulp by admixing an oxidizing agent such as oxygen gas and/or hydrogen peroxide with the alkali extracted pulp so as to produce an oxidized pulp, adding an acid to the oxidized pulp in order to terminate the oxidizing step and to lower the pH of the oxidized pulp, without effecting a complete neutralization of the residual alkali therein, so as to produce an acid neutralized pulp, and further bleaching the acid neutralized pulp in a second chlorine dioxide bleaching step. Preferably, the partially bleached chemical pulp is produced by an initial bleaching sequence which includes a first chlorine/chlorine dioxide bleaching step and a second alkali extracting step utilizing oxygen gas therein.
In accordance with one embodiment of the method of the present invention, oxidization of the alkali extracted pulp utilizes an amount of up to about 2 kg of the oxidizing agent per ton of the pulp. Preferably, the oxidizing step is carried out at a pressure of between about 0.1 and 1.0 MPa, and most preferably at a temperature of between about 60.degree. and 90.degree. C.
In accordance with another embodiment of the method of the present invention, addition of the acid to the oxidized pulp comprises adding the acid about 15 to 120 seconds after admixture of the oxidizing agent with the alkali extracted pulp.
In accordance with another embodiment of the method of the present invention, addition of the acid to the oxidized pulp comprises adding an amount of the acid to the oxidized pulp expressed in hydrogen ions of up to about NaOH-2

REFERENCES:
patent: 4274912 (1981-06-01), Carles et al.
patent: 4451332 (1984-05-01), Annergren et al.
patent: 4568420 (1986-02-01), Nonni
patent: 4657633 (1987-04-01), Dwiggins
Powell "Optimization of the (CD)(Eo) bleaching Sequence . . . " Tappi J., Jun. 1988 pp. 49-55.
Althouse "Hydrogen Peroxide Addition to Eo Bleaching Stages Is Beneficial" Pulp & Paper, Jun. 1988 pp. 68-70.
W. Howard Rapson et al., "Peroxide or hypochlorite in the E2 stage of CEDED bleaching of draft pulp: effect on shives," Tappi Journal, vol. 66, #8, pp. 77-80.

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