Patent ID: 6231718
Filing Date: 2001-05-15
Classification: D21C

Abstract:
In the treatment of kraft pulp comprising wood fibers to render the pulp useful in end use applications requiring a pulp viscosity of at least about 10 cP and a P. No. of at least about 9 and wherein said pulp has been digested employing a kraft process, the improvement comprising a two-phase post-digestion treatment, including contacting, in a mixer, the digested, unbleached pulp at a low alkaline pH with a mixture of ozone and oxygen pressurized to between about 60 psig and about 100 psig in an initial, chlorine free bleaching stage, said ozone in said mixture being present in an amount of between about 0.15% and 0.5%, based on the dry weight of the pulp, and wherein the temperature of said pulp in said mixer is maintained below the temperature at which oxygen will react with the components of said pulp and at or above the temperature at which said ozone will react with components of said pulp so that essentially all of said ozone in said mixture is consumed by reaction with the components of said pulp while a substantial portion of said oxygen in said mixture remains unreacted in said pulp, and thereafter altering the pH of said pulp to an alkaline pH and increasing the temperature of said pulp containing said oxygen to a temperature at which the oxygen in said pulp will react with components of said pulp for a period of time sufficient for said oxygen to react with said pulp components, thereafter subjecting said pulp to a conventional water washing step, and after said washing step, subjecting the washed pulp to a further second phase including repeating the aforesaid procedure employing the washed pulp and further ozone/oxygen mixture.