Extended retention and medium consistency pulp treatment

Abstract

Treatment of digested or O2 delignified cellulosic pulp in a chlorine dioxide (Do) delignification stage, preceding bleaching of the pulp, at a medium pulp consistency for an extended period of time. Unexpected benefits include improvement in the delignification efficiency and selectivity, a lowering of the chlorine dioxide consumption in the overall post-digestion or post-O2 delignification and bleaching process, a reduction in the filtrate volume, a reduction of COD / AOX from an ECF bleaching plant, improved pulp properties, and enhanced removal of Hex-A. The process may be implemented in existing three, four or five-stage bleaching plants.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] Not Applicable STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] Not Applicable BACKGROUND OF THE INVENTION [0003] This invention relates to the processing of cellulosic fibrous material in preparation for use in papermaking. In particular, the invention is directed to a process for enhancing the delignification of cellulosic fibrous material preparatory to (especially ECF) bleaching of the pulp. [0004] In the papermaking industry it is common practice to digest chips or like particulates of a cellulosic fibrous material, such as wood chips, with a view to separating the cellulosic fibers from the lignin which binds the fibers to one another and to produce a pulp. This process step is commonly termed “digestion” and is a major step in “delignification” of the pulp. Various chemicals and various process steps are employed in the digestion of different types of chips. Particularly, softwood and hardwood chips are empl...

AI Technical Summary

Benefits of technology

[0012] In the course of efforts to develop more efficient ECF bleaching technology for cost reduction, the present inventor unexpectedly discovered that if the consistency of the pulp leaving the washer (following the digester) is adjusted to a medium consistency and the time of residence of the pulp in a follow-on delignification stage preceding bleaching stages is extended (Demc) (herein at times referred to as the “extended medium consistency” or “Demc” technology”), there occurs an unexpected improvement in the delignification efficiency and selectivity, and a lowering of the chlorine dioxide consumption in the overall post-digestion delignification and bleaching process, a reduction of the filtrate volume, a reduction of COD (chemical oxygen demand) and AOX (absorbable organic halides) emissions from the ECF bleaching, decreased oxalate-related scale in ECF bleach plants, improved pulp properties, and an increase in Hex-A removal efficiency over the current short retention low consistency (LC) Do or DLC stage. Moreover, the inventor has found that the Demc technology may be implemented in, and its benefits realized with, existing bleach plants with a minimum of cost and change to the existing bleach plants. The present invention may be employed with pulp which has only been washed after leaving the digester or with pulp which has passed through an O2 treatment between the digester and the DEMC stage.

Problems solved by technology

Many mills employ oxygen (O2) for further delignification of pulp before ECF bleaching, Excessive removal of lignin (delignification) in digestion and / or O2 delignification operations tend to degrade the mechanical strength of the cellulosic fibers.

The industrial experience with conversion of the old C or Cd stage to a Do stage, however, has shown a significant decrease in the delignification efficiency as measured by higher chlorine / extraction (CE) or chlorine / extraction with oxygen (Eo) and / or peroxide (CEop) Kappa at a similar or higher active chlorine charge or kappa factor (% active chlorine / kappa) and increased bleaching cost by as much as 20% over elemental chlorine bleaching.

As noted, existing bleach plants are not readily convertible to new or different processing conditions which alter the throughput of the pulp being processed and / or affect the follow-on bleaching stages.

Such alterations are particularly troublesome if they require costly modification of the existing processing equipment.

As a result retrofitting of an existing bleach plant to accommodate different or modified stages of handling of the pulp from the digester through the bleaching operation have heretofore not been feasible from either an operational or an economic standpoint.

For example, installation of a new stage in a bleach plant can cost more than $3 million for an existing ECF bleach plant and therefore is particularly challenging to a capital-limited mature paper industry.

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