Bleaching of Secondary fibers

With bleaching in stock preparation systems the optical properties of secondary fibers are improved the brightness of the stock is increased and a possible color shade is reduced. 

The main parameters influencing the bleaching result are the type of chemical, its dosage, pH value, temperature, and retention time. 

Depending on the requirements of the finished stock either one or both bleaching types are integrated in a stock preparation system (see Section 4.3). The amount and type of bleaching agents have to be adjusted to the fiber composition of the stock and to the desired properties of the finished stock. 

Economic reductive bleaching of secondary fibers requires an oxygen-free stock as dithionite is sensitive to the oxygen contained in the air. Therefore the stock has to be deaerated which can be done sufficiently at a medium consistency of about 10 to 15 %. FAS is less sensitive, thus operating consistency can be as high as 30% and a Disperger may be used for admixing of FAS. Best results are obtained when 

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