Mixing in Pulp Bleaching Operations

MIXING IN PULP BLEACHING OPERATIONS 20-6.1 Pulp Bleaching Process... 

Table 20-3 Time Constants for Mixing in Pulp Bleaching Operations...

Rewatkar, V. B., R. J. Kerekes, and C. P. J. Bennington (2001). The use of temperature profiling to measure mixing quality in pulp bleaching operations, Proc. Annual Technical Meeting, PAPTAC, Montreal, Quebec, Canada, pp. B135-B143. 

Thus, an effective chemical contacting strategy for a pulp bleaching operation is to mix chemical to the fiber scale at a rate faster than the effective bleaching rate. The mixture quality exiting the mixer (and entering the retention tower) must also be of sufficient uniformity to optimize the desired pulp quality parameters. The residence time provided by the retention tower cannot be relied upon to create the degree of uniformity needed in all situations. 

In a chlorine bleach operation, the unbleached pulp is mixed with gaseous chlorine at 3% to 5% air dry pulp. After about 1 hour, sodium hydroxide is added to extract the dark colored material the chlorine has liberated. The extracted pulp is washed and dewatered, and then bleached with either chlorine dioxide or sodium hypochlorite. Extractions with sodium hydroxide alternate with the chlorine dioxide or hypochlorite until the pulp achieves the desired brightness without sacrificing too much strength. The pulp is washed, cleaned, screened and sent to storage or the paper mill. 

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