Method for Oxygen Delignification of Cellulose Pulp by Mixing of Chemicals

Abstract

The method is for the improved oxygen delignification of cellulose pulp with a medium consistency of 8-16%. The fraction of dissolved oxygen can be maintained at a high level throughout the process by the use of high pressure, greater than 15.0 bar, and by repeated agitative mixing while maintaining the high pressure. A fraction just over 20% of the total oxygen added is dissolved in the fluid phase such that the amount of oxygen in the fluid phase is maintained at a high level throughout the complete high-pressure section. This means that the amount of oxygen that is dissolved in the fluid phase and that penetrates the cellulose fibers can be maintained at an optimal high level throughout the process for improved delignification of the cellulose.

1 . A method for continuous alkali oxygen delignification of digested cellulose pulp and of cellulose pulp that has been washed after digestion, comprising: storing pulp in a storage tower or pulp chute at essentially atmospheric pressure, maintaining a medium consistency of the pulp in a range of 8-18%, maintaining the cellulose pulp to be delignified at a kappa value of at least 15 units, the oxygen delignification taking place in a reactor system with several oxygen reactors with a predetermined retention time of the cellulose pulp in the reactor system, adding alkali to the cellulose pulp in order to obtain an initial pH exceeding 9.0 and adding oxygen to the cellulose pulp at an amount of 5-50 kg per tone of pulp at a position before a first oxygen reactor in the reactor system, providing the pulp with a predetermined total retention time of greater than 45 minutes in the reactor system, in association with an addition of chemicals and an initial mixing-in operation, placing the cellulose pulp under pressure at an initial pressure of greater than 15.0 bar, subjecting the pulp to more than one remixing position where a final pressure after a final remixing position is at least 13 bar, subjecting the pulp to a minimum retention time in a high pressure section of at least 3-10 minutes, reducing the pressure of the pulp to a pressure that lies under 10-12 bar, heating the pulp with steam such that a temperature of the pulp is raised by at least 5° C. by the addition of steam, and leading the heated pulp to a reactor system in a low pressure section with a retention time that exceeds the retention time in the high pressure section. 
     
     
         2 . The method according to  claim 1 , wherein oxygen, is added to the cellulose pulp immediately after the initial pressure of more than 15 bar has been established. 
     
     
         3 . The method according to  claim 2 , wherein the remixing positions are constituted by fluidising mixers, either in a form of a fluidising pump, a fluidising restrictions a fluidising mixer or a restriction in a flow that results in a fall in pressure of less than 1 bar. 
     
     
         4 . The method according to  claim 3 , wherein a first high pressure reactor is located after the initial mixing-in operation, in which reactor the cellulose pulp is given a first retention time of t 1 , and in that a high pressure reactor follows after the remixing positions in the high pressure section after each one of the remixing positions. 
     
     
         5 . The method according to  claim 4 , wherein the reactors in the high pressure section are dimensioned such that the cellulose pulp is given successively longer retention times, such that when the number of reactors is X, the retention times are t 1 -t x  for each relevant reactor R 1 -R X , where t 1 <t 2 < . . . t x . 
     
     
         6 . The method according to  claim 5 , wherein the retention times t 1 -t x  in the reactors R 1 -R x  in the high pressure section are expressed as
 t min =1 minute for t 1 , after which (t x =2*t x−1 ) and T max =X*10 minutes;
 (t 1 =1-10 min., t 2 =2-20 min.; t 3 =4-30 min.; t 4 =8-40 min.), 
   
       where t x <t x+1 . 
     
     
         7 . The method according to  claim 1  wherein a stirrer is present in at least one high pressure reactor, which stirrer acts in a principal part of a reactor volume, either in a form of a mechanical stirrer (S) or hydrodynamic stirrers that at least circulate free fluid in the reactor. 
     
     
         8 . The method according to  claim 1  wherein at least one of the oxygen and alkali additions are added to the cellulose pulp in association with the remixing positions in the high pressure section at an amount that is lower than the amount that is added at the initial mixing-in operation, and at least one of the oxygen and alkali additions are added batch-wise at a beginning of the low pressure section. 
     
     
         9 . The method according to  claim 1  wherein the cellulose pulp is dewatered before the oxygen delignification to a higher consistency and the cellulose pulp is re-diluted before the oxygen delignification to a medium consistency with pure filtrate that has been previously oxidized, and in that alkali in a form of oxidized white liquor is used in the oxygen delignification.