Method and apparatus for treating pulp

Abstract

The present invention relates to a method of and apparatus for treating pulp. More precisely, the present invention relates to a method and apparatus for treating pulp with oxygen. Especially preferably the method and apparatus according to the invention relate to oxygen delignification of chemical pulp, i.e. so-called oxygen bleaching. A characterizing feature of the method according to the invention of treating chemical pulp in an oxygen bleaching stage where the pulp is bleached in at least one pressurized reactor in the presence of alkali is that the whole oxygen stage is effected pressurized and that a bigger amount of alkali than conventional is fed in the beginning of the oxygen stage and that the whole alkali dose is fed in the beginning of the oxygen stage.

1 . Method of treating pulp in an oxygen bleaching stage in which the pulp having a consistency of 8-18% is bleached in at least one pressurized reactor in the presence of alkali, characterized in that the whole oxygen stage is run pressurized and that the amount of alkali fed in the beginning of the oxygen stage (kg/bdt as NaOH) is at least 2.0 times the desired kappa reduction and that the whole alkali charge is fed in the beginning of the oxygen stage.  
     
     
         2 . Method according to  claim 1 , characterized in that alkali fed in the oxygen stage is considered to comprise both fresh alkali and residual alkali passing into the oxygen stage from various washing stages.  
     
     
         3 . Method according to  claim 1 , characterized in that the oxygen stage comprises multiple steps so that it is performed in multiple reactors, each of which reactor is pressurized.  
     
     
         4 . Method according to  claim 3 , characterized in that the treatment time of the pulp in each reactor is directly comparable to the pressure of the reactor.  
     
     
         5 . Method according to  claim 1 , characterized in that the oxygen stage is performed at a temperature of less than 95° C. and a pressure of 5-20 bar, while the treatment time varies between 5 and 180 minutes.  
     
     
         6 . Method according to  claim 3  and  5 , characterized in that the oxygen bleaching is effected in two steps so that in the first step the temperature is 75-95° C., the pressure is 8-20 bar and the treatment time 5-45 minutes and in the second step the temperature is 75-95° C., the pressure 5-20 bar and the treatment time 20-180 minutes.  
     
     
         7 . Method according to  claim 1  or  5 , characterized in that the steam used in the oxygen stage has a pressure of less than 5 bar (abs.).  
     
     
         8 . Method according to  claim 3  or  6 , characterized in that oxygen is fed into the first step only.  
     
     
         9 . Method according to  claim 3  or  6 , characterized in that oxygen is fed into the pulp between the steps.  
     
     
         10 . Method according to  claim 8  or  9 , characterized in that the pulp is mixed between the steps in order to break gas bubbles and to ensure a better material transfer between the fibers of the pulp and the gas.  
     
     
         11 . Method according to  claim 1 , characterized in that the control of the oxygen stage aims at a desired kappa reduction.  
     
     
         12 . Method according to  claim 11 , characterized in that the required alkali charge is determined on the basis of said desired kappa reduction.  
     
     
         13 . Method according to claims  11  and  12 , characterized in that based on at least said kappa reduction and alkali charge and utilizing the treatment time of the oxygen stage, the lowest possible temperature by means of which the desired kappa reduction is effected, in order to obtain the highest possible viscosity.  
     
     
         14 . Method according to  claim 12 , characterized in that in order to determine the amount of alkali to be introduced into the oxygen stage, the amount of residual alkali passing into the oxygen stage is deducted from said alkali charge.  
     
     
         15 . Method according to  claim 14 , characterized in that said amount of residual alkali is determined at least from the washing liquid obtained as filtrate from the wash following the oxygen stage.  
     
     
         16 . Method according to  claim 14 , characterized in that said amount of residual alkali is determined from the pulp entering the oxygen stage prior to adding fresh alkali.  
     
     
         17 . Apparatus for treating pulp comprising an oxygen bleaching apparatus ( 10 ) comprising at least one reactor ( 12 ,  14 ), a washing device ( 34 ,  36 ) preceding and following said bleaching apparatus ( 10 ) and required devices ( 18 ,  16 ) for feeding chemicals into the pulp and feeding the pulp into said at least one reactor ( 12 ,  14 ) and further devices for leading washer filtrates at least partly countercurrently from one washing stage into another, characterized in that the apparatus further comprises devices for determining residual alkali either directly or indirectly both from the pulp being fed into the oxygen stage and from the washing liquid obtained as filtrate from the washer following the oxygen stage.