Two phase ozone and oxygen pulp treatment

Details Two phase ozone and oxygen pulp treatment Two phase ozone and oxygen pulp treatment

1992-02-28

2001-05-15

Alvo, Steve (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

With agitation or forced circulation

C162S065000, C162S078000

Reexamination Certificate

active

06231718

ABSTRACT:

This invention relates to the post-digestion treatment of cellulosic kraft pulps, particularly for the purpose of enhancing the delignification of the pulp.
Post-digestion treatment of kraft pulps to enhance their delignification, in the prior art, has included various bleaching sequences. Chlorine-containing bleaching agents have been used extensively because of their effectiveness and other reasons. These prior art processes have now been questioned as contributing to the generation of dioxins, chlorinated organics, and other objectionable compounds. Consequently extensive work has been conducted and is continuing on ways to avoid the problems associated with the use of chlorine-containing compounds in the post-digestion treatment of pulps. Among these efforts, it has been suggested that chlorine-containing compounds be eliminated from the post-digestion treatments and that oxygen, ozone, peroxide, be substituted therefor. These non-chlorine-containing elements or compounds, however, suffer from several disadvantages. First, many of these proposed substitute materials represent increased exposure in treating the pulps, this expense being either in the form of added raw material costs and/or by reason of the necessity of adding new processing equipment or modifying existing processing equipment. Second, many are less effective than the chlorine-containing materials. Third, even though certain of these prior art proposed substitute materials may be effective in achieving target brightness of the pulp, they adversely affect one or more of the other properties of the pulp, such as its viscosity. Fourth, some of the substitute materials, such as ozone, are highly reactive, hence are difficult or impossible to economically handle.
It is therefore an object of the present invention to provide a post-digestion treatment method for kraft cellulosic pulps which represents an improvement in effectiveness.
It is a further object to provide a post-digestion treatment for kraft cellulosic pulps in which there is no chlorine-containing bleaching agent employed in the treatment.
Briefly, the present inventor has found that target delignification values for kraft cellulosic pulps may be obtained by means of a two-phase post-digestion treatment of the pulp which includes, in the first phase, contacting, in a mixer, the digested, washed pulp at an acidic to low alkaline pH with a mixture of ozone and oxygen, at between about 60 and 80 psig, in an initial stage. The ozone/oxygen mixture introduced to the pulp in the mixer contains between about 3% and about 12% ozone with the remainder being essentially oxygen Thus, the quantity of ozone, on pulp, is between about 0.15% and about 0.5%. The mixture is added to the pulp while the pulp is maintained at a temperature that is below that temperature at which oxygen will react with the components of the pulp but at or above the temperature at which the ozone will react with the components of the pulp. After the ozone in the mixture has been substantially consumed by reaction with components of the pulp, the pulp is thereafter adjusted in pH to a medium to high alkaline pH, as by the addition of caustic, and the temperature of the pulp is increased to a temperature at which the oxygen remaining in the pulp, or addition oxygen as required, will react with components of the pulp. The pulp is maintained at such acceptable temperature for a time period sufficient for at least a major portion, and preferably substantially all, of the oxygen to be consumed in the reactions. Thereafter, the pulp is washed as by a water wash. Following this washing step, the washed pulp is again provided with a repeat treatment which is substantially the same as that set forth hereinabove, beginning with the step of contacting the pulp with a further ozone/oxygen mixture in a mixer and continuing through the washing step.
In accordance with one aspect of the invention, a peroxide may be added to the pulp following consumption of the ozone by reaction with the pulp or, alternatively, before the addition of the ozone/oxygen mixture. In accordance with a further aspect of the invention, a portion of the oxygen remaining in the pulp following consumption of the ozone by reaction may be removed, as by off-gassing, either before addition of the alkali for upward adjustment of the pH of the pulp to the alkaline state, or after such pH adjustment. Still further, the ozone/oxygen mixture may be added to the pulp by means of two, or even more, mixers for the purpose of dividing the task of adding the ozone to the pulp and/or for providing for the addition of larger quantities of ozone to the pulp.


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Ingersoll-Rand Pulp Machinery Brochure, “Gas Phase Ozone System”, 6 pages, Oct. 1991.

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