Patent ID: 6162324
Filing Date: 2000-12-19
Classification: D21C

Abstract:
A method of oxygen delignification of medium consistency pulp slurry, comprising the steps of:providing a cooked pulp in the form of a pulp slurry of from approximately ten percent to sixteen percent consistency and having a cooked pulp kappa number between 18 and 30 for softwood pulp and between 10 and 20 for hardwood pulp;adjusting the pH of the slurry to at least 11;adding a gas to the slurry, the gas consisting essentially of oxygen, with agitating in a high shear mixer for a first reaction with the pulp for a first reaction time ranging from 4 to 6 minutes, at a first reaction temperature of from approximately 170-240.degree. F., the first reaction resulting in the pulp having a first reaction kappa number reduction ranging from about 20% to about 30% of the cooked pulp kappa number;adjusting a plurality of control parameters to prepare the pulp slurry for a second reaction, the control parameters consisting essentially of the pH and the residual alkali concentration of the slurry having a chemical oxygen demand, wherein the pH is adjusted to at least 11 and the residual alkali concentration is adjusted to at least 4.0 gpl;transmitting the slurry to a reactor;reacting the slurry in the reactor for a second reaction time, said second reaction time ranging from 50-70 minutes, at a second reaction temperature of from approximately 170-240.degree. F.; andmaintaining an initial level of the control parameters during the second reaction resulting in the pulp of the slurry having a second reaction kappa number reduction ranging from 45% to 60% of the cooked pulp kappa number, wherein the initial level includes the pH level of at least 11 and the residual alkali concentration level of at least 4.0 gpl.