Ozone effluent bleaching

Abstract

The bleaching of fluff cellulosic pulp with a gaseous ozone bleaching agent at an acidic pH is often followed by an alkaline treatment, usually an alkaline extraction or an alkaline hydrogen peroxide bleaching treatment, at a much higher water content than the gaseous ozone treatment of the fluff pulp. Such a bleaching sequence of gaseous ozone bleaching followed by an alkaline treatment is improved by the present invention based upon two discoveries, first, the discovery of peroxygen bleaching values in the water associated with the fluff pulp leaving the ozone bleaching treatment, and second, the peroxygen values are destroyed at the alkaline pH used for the alkaline treatment that follows the gaseous ozone bleaching treatment. In accordance with the discoveries, a Low Consistency Acidic Retention Treatment is interposed between the gaseous ozone bleaching treatment and the alkaline treatment. The retention treatment comprises retaining the ozone bleached pulp in contact with the water associated with the fluff pulp during gaseous ozone bleaching along with additional water and at an acidic pH for at least ten minutes during which time the peroxygen values contained in the water react with the pulp to bleach the pulp. Thereafter, sufficient alkali is added to raise the pH above 7 in preparation for the subsequent alkaline treatment.

Dependencies are inferred from the claim text.