Removal of optical brighteners from a paper fiber suspension induces a turbulent flow in the suspension at a controlled temperature to release the brightener for collection and removal

摘要

To process fiber suspensions, containing an optical brightener, the suspension flow is given a turbulence at a temperature of at least 50 deg C. The released brightener is removed from the suspension. The released brightener is washed out of the suspension. The process temperature is preferably at least 90 deg C or at least 100 deg C. The fiber suspension is set, before processing, to have a fiber content of maximum 2% or maximum 1%. During processing, the fiber suspension is subjected to intense shear forces. The flow turbulence is generated by stirrers, with a relative speed of at least 2 m/second between the stirring unit and the fiber suspension. The stirring is for a duration of at least 1 minute or at least 5 minutes. The stirring is through a high speed vat propeller. The optical brightener contained in the washing water is coagulated chemically or mechanically, and is filtered or floated or precipitated away from the water. The washing water is used to dilute the suspension, before processing. Hot water is added to the suspension to raise its temperature and also dilute it. The separated optical brightener is recycled to increase the paper whiteness. The optical brightener can be artificial, or of natural groups such as vinyl ether, stilbene, quinone and lignin.