Process for the manufacturing of cellulose pulp

Abstract

The invention concerns a method for the production of cellulose pulp. A wood raw material that principally comprises softwood, preferably in the form of cut wood chips, is treated in several stages in various treatment steps. One of the stages is cooking the material in an alkali cooking fluid, with the aim of obtaining improved quality with respect to tensile strength, and of obtaining higher yield. A controlled addition of wood raw material in the form of hardwood at an amount corresponding to 1-20% of the amount of softwood, preferably 3-15%, and more preferably at least 5%, takes place.

1 . A method for the production of cellulose pulp, comprising: 
 providing an amount of wood raw material that principally consists of softwood,    cooking the material in an alkali cooking fluid, with the aim of obtaining improved quality with respect to tensile strength and of obtaining higher yield, and    in a controlled manner adding hardwood in an amount corresponding to 1-20% of the amount of softwood.    
   
   
       2 . The method according to  claim 1  wherein the hardwood is added and mixed with the said softwood that is provided as wood chips.  
   
   
       3 . The method according to  claim 2  wherein the mixing takes place in a chip bin.  
   
   
       4 . The method according to  claim 1  wherein the hardwood is added to and mixed with the said softwood, the hardwood is provided as a finely divided fraction, the finely divided fraction is added after an impregnation stage.  
   
   
       5 . The method according to  claim 4  wherein the cooking takes place continuously and the finely divided fraction is added at a top of a continuous digester such that mixing is achieved before entry to a first cooking zone.  
   
   
       6 . The method according to  claim 4  wherein the cooking takes place continuously and the finely divided fraction is added to a continuous digester by means of at least one fluid flow that adds and/or recirculates fluid to a cooking zoner such that mixing is achieved after entry to a first cooking zone.  
   
   
       7 . The method according to  claim 6  wherein at least parts of the finely divided fraction are added by means of a flow of white liquor addition.  
   
   
       8 . The method according to  claim 6  wherein the flow is delivered at an upper part of the digester.  
   
   
       9 . The method according to  claim 1  wherein the hardwood is added to and mixed with the softwood the hardwood is provided as logs after which the wood raw material is converted to chips.  
   
   
       10 . The method according to  claim 1  wherein the hardwood is pre-treated with alkali before the hardwood is added to and mixed with the softwood.  
   
   
       11 . The method according to  claim 10  wherein the pre-treatment takes place at an elevated temperature of 80-160° C.  
   
   
       12 . A cooking system for the production of cellulose pulp, in which a wood raw material that primarily consists of softwood is treated in several stages, comprising: 
 a raw material preparation unit ( 1 ),    a raw material storage ( 10 ;  7 ) being downstream of the raw material preparation unit,    a cooking plant ( 15 ,  19 ) being downstream of the raw material storage, and    a control system ( 6 ) being upstream of the raw material preparation unit, the control system regulating an inflow of hardwood and softwood into the raw material preparation unit so that an outflow of cellulose pulp from an outlet of a digester contains 1-20% of hardwood fibers and 80-99% of softwood fibers.    
   
   
       13 . The cooking system according to  claim 12  wherein the control system ( 6 ) is arranged to control the inflow to the raw material preparation unit ( 1 ), so that the fibre so that fiber raw materials stored in the raw material storage ( 10 ) contain 1-20% hardwood and 80-99% softwood.  
   
   
       14 . The cooking system according to  claim 12 , wherein the raw materials storage ( 10 ;  7 ) has at least two separate storage spaces ( 10 ,  7 ;  1 A′,  1 B′) so that one storage space is for hardwood raw material and one storage space is for softwood raw material, the control system ( 6 ) is in operative engagement with the raw material preparation unit and regulates a flow from the said storage spaces such that 1-20% controlled addition of hardwood takes place before and/or during a cooking stage.  
   
   
       15 . The cooking system according to  claim 14 , wherein the cooking system has means for ensuring that the flow of hardwood is delivered to a chip chute ( 11 ).  
   
   
       16 . The cooking system according to  claim 14 , wherein the cooking system has means ( 9 ) for ensuring that the flow of hardwood is delivered at a top ( 18 ) of the digester ( 19 ).  
   
   
       17 . The cooking system according to  claim 14 , wherein the cooking system has means ( 8 ) for ensuring that the flow of hardwood is delivered to a fluid line ( 21 ) arranged at the digester ( 19 ).