Acid treatment of pulp at high temperature in connection with bleaching

Abstract

In the treatment of cellulose pulp in connection with bleaching, the pulp is first cooked and, if necessary, oxygen delignified in order to reduce the Kappa number below 24, preferably below 14, and after that the pulp is treated in an acid stage at a pH of 2-5 and at a temperature range of 75-130° C. in order to reduce the Kappa number by 2-9 units. To prevent essential weakening of the strength properties of the pulp in the acid stage treatment tower, the retention time, t min, is 30 to 300 minutes and the treatment temperature, T° C. is T min <T<T max , in which T min = 10517 24 + ln  ( 2  t ) - 273 and T max =T min +23 ° C.

1 . A method of treating pulp in connection with bleaching of pulp according to which method pulp is at first digested and if necessary oxygen delignified in order to decrease the Kappa number to less than 24, preferably less than 14, and after that treated in an acid stage at a pH of 2-5 and at a temperature of 75-130° C. in order to decrease the Kappa number by 2-9 units, characterized in that the acid treatment is carried out in at least one acid tower whereby the retention time t min in the acid treatment is 30-300 minutes and the treatment temperature T° C. is T min <T<T max  in which  
         T   min     =       10517     24   +     ln        (     2      t     )           -   273                   
 
       and  
         T   max   =T   min +23 ° C.    
       in order to prevent the strength properties of the pulp from being deteriorated in the acid stage, and that the acid treatment has been combined into the same treatment stage with one or several bleaching or treatment chemical/s, which treatment with one or several chemical/s is carried out in at least one second treatment tower without an intermediate wash after the acid treatment.  
     
     
         2 . A method as claimed in    claim 1   , characterized in that the feed of pulp into the treatment tower/towers and/or the discharge therefrom is controlled so that homogenous flow properties are maintained in the tower so as to have a substantially constant retention time of the pulp in the tower/towers.  
     
     
         3 . A method as claimed in    claim 1    or    2   , characterized in that the acid treatment has been combined into the same treatment stage with at least one of the following: peroxide, chlorine dioxide, ozone, chlorine, acid peroxide, chelating agent.  
     
     
         4 . A method as claimed in    claim 1    or    2   , characterized in that the acid treatment has been combined into the same treatment stage with chelating agent and at least one of the following: peroxide, chlorine dioxide, ozone, chlorine, acid peroxide.  
     
     
         5 . A method as claimed in    claim 2   , characterized in that the retention time is 45-150 minutes.  
     
     
         6 . A method as claimed in any of the preceding claims, characterized in that the acid treatment is used in any of the following bleaching sequences or in the following partial bleaching sequences: 
 Digestion—O-AD-ZD    Digestion—O-AD N -D    Digestion—O-AD-D    Digestion—O-ADQ-O p -D    Digestion—O-ADQ-O p -ZD    Digestion—O-ADQ-P    Digestion—O-AD-O p -D    Digestion—O-AD-O p -ZD    Digestion—O-AD-P    Digestion—O-AP a Q-O p -D    Digestion—O-AP a Q-O p -ZD    Digestion—O-AP a Q-P    Digestion—O-AQ-ZP a Q-P    Digestion—O-AP a Q-O p -ZD    Digestion—O-P a A-ZD    Digestion—O-AP a -D    Digestion—O-P a A-D    Digestion—O-P a AQ-P    Digestion—O-AP a Q-P    Digestion—O-AQ-P a Q-P    Digestion—AD-ZD    Digestion—AD N -D    Digestion—AD-D    Digestion—ADQ-O p -D    Digestion—ADQ-O p -ZD    Digestion—ADQ-P    Digestion—AD-O p -D    Digestion—AD-O p -ZD    Digestion—AD-P    Digestion—AP a Q-O p -D    Digestion—AP a Q-O p -ZD    Digestion—AP a Q-P    Digestion—AP a Q-O p -ZD    Digestion—P a A-ZD    Digestion—AP a -D    Digestion—P a A-D    Digestion—P a AQ-P    Digestion—AP a Q-P    Digestion—AQ-P a Q-P    Digestion—AQ-ZP a Q-P    Digestion—O-AQ-ZDQ-P    Digestion—AQ-ZDQ-P    Digestion—O-AQ-ZQ-P    Digestion—O-AQ-P    Digestion—O-AQ-O-ZQ-P    Digestion—AQ-ZQ-P    Digestion—AQ-P    Digestion—AQ-O-ZQ-P    Digestion—Q-O-AP a Q-P    Digestion—O-Q-AP    Digestion—O-Q-PA    Digestion—O-Q-AP a      Digestion—O-Q-P a A    Digestion—Q-AP    Digestion—Q-PA    Digestion—Q-AP a      Digestion—Q-P a A    in which sequences the order of the steps in the stages comprising several steps is changeable and in which the following abbreviations have been used:    A=a hot acid treatment in a raised temperature,    O=oxygen treatment,    O p =an oxygen treatment intensified with peroxide,    D=a chlorine dioxide treatment in which DA means two successive towers D and A without an intermediate wash; this may be referred to also by D/A; also abbreviations AD, AP, PA, AZ, ZA, AP a , P a A, AQ and ZQ indicate the corresponding situation;    D N =chlorine dioxide treatment with neutralization,    P=peroxide treatment, either atmospheric or pressurized with oxygen,    P a =acid peroxide stage usually performed with peracid,    P n =several successive peroxide treatment stages, either atmospheric or pressurized with oxygen,    E=alkaline stage,    Z=ozone treatment,    Q=chelating agent treatment, and    “-”=wash between stages    
     
     
         7 . An apparatus in connection with treatment of pulp characterized in that at least one bleaching stage of a sequence comprises at least one acid tower ( 18 ,  118 ,  318 ,  348 ,  420 ) provided with means guiding the pulp feed ( 20 ,  24 :  120 ,  124 ;  316 ,  320 ;  346 ,  350 ;  418 ,  422 ) so that the pulp is distributed in the feed evenly, and at least one second treatment tower ( 324 ,  354 ,  426 ) for treatment with some other chemical and that the pulp is discharged from the acid tower so that the retention time, t min, of the pulp in the acid tower is substantially constant, 30-300 minutes, and the temperature T° C. is T min <T<T max  in which  
       and  
         T   max   =T   min +23 ° C.            T   min     =       10517     24   +     ln        (     2      t     )           -   273                     
     
     
         8 . An apparatus as claimed in    claim 7   , characterized in that it has been provided with means ( 22 ;  114 ) for heating the pulp prior to the tower or in the feed end of the tower.  
     
     
         9 . An apparatus as claimed in    claim 7   , characterized in that it has been provided with means ( 12 ) for controlling the pH of the pulp prior to the tower.  
     
     
         10 . An apparatus as claimed in    claim 7   , characterized in that it comprises an acid treatment tower ( 420 ) and a peroxide treatment tower ( 426 ) and the filtrate connection ( 432 ) of the filter ( 420 ) following the peroxide tower ( 426 ) has been connected to the pulp feed line ( 434 ) of the peroxide tower ( 426 ).  
     
     
         11 . An apparatus as claimed in    claim 7   , characterized in that the acid tower ( 18 ) is an upwards flow tower.  
     
     
         12 . An apparatus as claimed in    claim 7   , characterized in that the acid tower ( 118 ) is a downwards flow tower.  
     
     
         13 . An apparatus as claimed in    claim 7   , characterized in that two acid towers ( 218 ,  232 ) have been arranged in flow connection with each other for effecting acid treatment.  
     
     
         14 . An apparatus as claimed in    claim 13   , characterized in that means ( 222 ) have been provided between the towers for mixing chemicals into the pulp.  
     
     
         15 . An apparatus as claimed in    claim 13    or    14   , characterized in that one of the towers ( 218 ) is an upwards flow tower and the other one ( 323 ) is a downwards flow tower.