Bleaching cellulose pulp having cleanliness which varies significantly

Abstract

Recycled paper pulp having at least two different types of paper which consume widely different amounts of bleaching chemical, or mixed hardwood and soft-wood pulps, are treated in a manner that enhances distribution and redistribution of bleaching chemical to enhance uniformity of the bleaching treatment. The pulp is intensely mixed to provide a homogenous mixture of fibrous material and bleaching chemical, and then is continuously or intermittently subjected to mechanical action (such as in low intensity mixers, or by agitating pulp in a retention tank) of sufficient intensity and duration so as to enhance distribution and redistribution of a bleaching chemical. Also, recycled pulp will have cleanliness which varies significantly over time, and this is accommodated by sensing at least one of the brightness, color, or lignin content (e.g. all three) of the pulp prior to treatment in a first bleaching stage, controlling the quantity of first bleaching chemical added to the first stage in response to at least one of the brightness, color, or lignin content sensing to achieve a target brightness increase for the first stage, and then repeating the sensing and quantity control steps for a second and subsequent stages. Alternatively all sensing can be done initially. Within a stage, a valve may be controlled to subject the pulp to a second treatment in that stage, or to pass it out of the stage to the next stage, depending upon cleanliness.