Method for controlling oxygen delignification of pulp

Details Method for controlling oxygen delignification of pulp Method for controlling oxygen delignification of pulp

1999-05-25

2001-05-29

Alvo, Steve (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

With testing, sampling or analyzing

C162S061000, C162S062000, C162S065000, C162S238000

Reexamination Certificate

active

06238517

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the control of a method for oxygen delignification of lignocellulosic material in two steps at medium consistency, i.e., between about 8% and 16%.
Since the introduction of oxygen delignification at medium consistency, not much development effort has been invested in this process. With the development of chlorine free bleaching and closure of bleach plants, the interest in extended delignification, i.e. further reduction of the kappa number with oxygen, has increased. Extended delignification with oxygen in one or several stages, however, can result in deterioration in pulp quality. Application of proper conditions, however, can bring about several advantages.
With extended oxygen delignification the pulp yield can also be maintained at a higher level than with extended cooking, i.e. cooking to lower kappa number.
In a multi-stage method, the chemicals can be distributed between the stages so as to create optimum conditions in each stage. Other conditions can also be optimized for each stage.
The object of the present invention is to provide a method of controlling oxygen delignification in a manner such that a lower kappa number is obtained without deterioration of the pulp properties.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been realized by the invention of a method for controlling an oxygen delignification process for a pulp having an initial kappa number in order to produce a delignified pulp having a desired kappa number reduction, the method comprising providing chemicals for a first oxygen delignification step, conducting the first oxygen delignification step with the chemicals, conducting a second oxygen delignification step at a predetermined temperature to produce the delignified pulp, measuring the final kappa number of the delignified pulp subsequent to the second oxygen delignification step, adjusting the chemicals provided for the first oxygen delignification step based upon the difference between the actual kappa number reduction from the initial kappa number to the final kappa number and the desired kappa number reduction, and adjusting the predetermined temperature based on the chemicals provided to the first oxygen delignification step in order to provide the delignified pulp at a final pH of from about 10.5 to about 11.5. In a preferred embodiment, the majority of the chemicals for the first and second oxygen delignification steps comprises the chemicals for the first oxygen delignification step. In another preferred embodiment, all of the chemicals for the first and second oxygen delignification steps comprise the chemicals for the first oxygen delignification step.
By using the extended delignification process according to the present invention, the total delignification can amount to from about 50% to 85% of the lignin content (kappa number) of unbleached pulp. The method is carried out at medium consistency in two subsequent steps.


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