Method and apparatus and screen for screening mechanical fiber pulp

Abstract

The invention relates to a method, an apparatus and a screen for screening mechanical fibre pulp. According to the invention a short fibre material is separated using a gap screen, whereby said material can be very efficiently separated to form a specific fraction.

That which is claimed is:  
     
         1 . A method of screening a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers and shorter fibers, said method comprising: 
 directing the fiber pulp mixture into an input end of a convergent gap of a gap screen, the convergent gap extending from the input end to an output port and being at least partially defined by at least one wire screen capable of moving therebetween; and    moving the at least one wire screen toward the output port so as to direct the fiber pulp mixture therethrough, the at least one wire screen defining a plurality of openings configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture, whereafter a remainder of the fiber pulp mixture is discharged through the output port.    
     
     
         2 . A method according to  claim 1  wherein the convergent gap is defined by two opposed wire screens and moving the at least one wire screen further comprises moving the two opposed wire screens toward the output port, each wire screen defining a plurality of openings configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings.  
     
     
         3 . A method according to  claim 1  wherein directing the fiber pulp mixture into the input end further comprises directing the fiber pulp mixture into the input end at a feed rate.  
     
     
         4 . A method according to  claim 3  wherein moving the at least one wire screen toward the output port further comprises moving the at least one wire screen toward the output port at a rate of motion, the rate of motion being adjustable in relation to the feed rate of the fiber pulp mixture.  
     
     
         5 . A method according to  claim 1  further comprising adjusting a consistency of the fiber pulp mixture prior to directing the fiber pulp mixture into the input end.  
     
     
         6 . A process of screening a fiber pulp mixture, including longer fibers and shorter fibers, using a first and a second gap screen, each gap screen including a convergent gap having an input end and an output port and being at least partially defined by at least one wire screen capable of moving therebetween, said process comprising: 
 directing the fiber pulp mixture into the input end of the convergent gap of the first gap screen;    moving the at least one wire screen toward the output port of the first gap screen so as to direct the fiber pulp mixture therethrough, the at least one wire screen defining a plurality of openings configured such that a first portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings, the first gap screen thereby separating the first portion of the fiber pulp mixture including shorter fibers from the fiber pulp mixture, whereafter a first remainder of the fiber pulp mixture is discharged through the output port of the first gap screen;    directing the first portion of the fiber pulp mixture including shorter fibers into the input end of the convergent gap of the second gap screen; and    moving the at least one wire screen toward the output port of the second gap screen so as to direct the first portion of the fiber pulp mixture including shorter fibers therethrough, the at least one wire screen defining a plurality of openings configured such that the shorter fibers are capable of passing through the openings, the second gap screen thereby separating at least a portion the shorter fibers from the first portion of the fiber pulp mixture including shorter fibers, whereafter a second remainder of the fiber pulp mixture is discharged through the output port of the second gap screen.    
     
     
         7 . A method according to  claim 5  further comprising directing the first remainder of the fiber pulp mixture through a first refiner so as to produce a first refined fiber pulp mixture.  
     
     
         8 . A method according to  claim 7  further comprising directing the first refined fiber pulp mixture from the first refiner into the input end of the convergent gap of the first gap screen.  
     
     
         9 . A method according to  claim 5  further comprising directing the second remainder of the fiber pulp mixture through a second refiner so as to produce a second refined fiber pulp mixture.  
     
     
         10 . A method according to  claim 9  further comprising directing the second refined fiber pulp mixture from the second refiner into the input end of the convergent gap of the second gap screen.  
     
     
         11 . An apparatus adapted to screen a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers and shorter fibers, said apparatus comprising: 
 a gap screen having a convergent gap, the convergent gap having an input end for receiving the fiber pulp mixture and an output port;    at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the fiber pulp mixture therethrough; and    a plurality of openings defined by the at least one wire screen, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture and discharging a remainder of the fiber pulp mixture from the output port.    
     
     
         12 . An apparatus according to  claim 11  further comprising two opposed wire screens operably engaged with the gap screen and at least partially defining the convergent gap, the two opposed wire screens being configured to extend between the input end and the outlet port and to be movable toward the output port, each wire screen defining a plurality of openings configured such that the portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings.  
     
     
         13 . An apparatus according to  claim 11  wherein the fiber pulp mixture received by the input end is directed thereto at a feed rate, and wherein the apparatus further comprises a control means operably engaged with the at least one wire screen and configured to adjust a rate of motion of the at least one wire screen in relation to the feed rate of the fiber pulp mixture.  
     
     
         14 . A system adapted to screen a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers, slivers, and shorter fibers, said system comprising: 
 a first screening apparatus, comprising: 
 a gap screen having a convergent gap, the convergent gap having an input end for receiving the fiber pulp mixture and an output port;  
 at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the fiber pulp mixture therethrough; and  
 a plurality of openings defined by the at least one wire screen, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture and discharging a first remainder of the fiber pulp mixture from the output port.  
   
     
     
         15 . A system according to  claim 14  further comprising a first refiner configured to receive and refine the first remainder of the fiber pulp mixture from the output port of the first screening apparatus to produce a first refined fiber pulp mixture, the first refiner being further configured to direct the first refined fiber pulp mixture back to the input end of the first screening apparatus.  
     
     
         16 . A system according to  claim 14  further comprising a second screening apparatus adapted to receive the portion of the fiber pulp mixture including shorter fibers from the first screening apparatus, said second screening apparatus comprising: 
 a gap screen having a convergent gap, the convergent gap having an input end for receiving the portion of the fiber pulp mixture including shorter fibers and an output port;  
 at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the portion of the fiber pulp mixture including shorter fibers therethrough; and  
 a plurality of openings defined by the at least one wire screen, the openings being configured such that the shorter fibers are capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the portion of the fiber pulp mixture including shorter fibers and discharging a second remainder from the output port.  
 
     
     
         17 . A system according to  claim 16  further comprising a second refiner configured to receive and refine the second remainder of the fiber pulp mixture from the output port of the second screening apparatus to produce a second refined fiber pulp mixture.  
     
     
         18 . A system according to  claim 17  further comprising a third screening apparatus adapted to receive the second refined fiber pulp mixture from the second refiner, said third screening apparatus comprising: 
 a gap screen having a convergent gap, the convergent gap having an input end for receiving the second refined fiber pulp mixture and an output port;  
 at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the second refined fiber pulp mixture therethrough; and  
 a plurality of openings defined by the at least one wire screen, the openings being configured such that fine fibers in the second refined fiber pulp mixture are capable of passing therethrough, the gap screen thereby separating at least a portion of the fine fibers from the second refined fiber pulp mixture and discharging a third remainder from the output port to the second refiner.  
 
     
     
         19 . A system according to  claim 14  further comprising a consistency control means adapted to adjust a consistency of the fiber pulp mixture prior to the fiber pulp mixture being received by the first screening apparatus.  
     
     
         20 . A wire screen adapted for use in a gap screen having a convergent gap with an input end and an output port, the wire screen extending between the input end and the output port, being movable toward the output port, and at least partially defining the convergent gap, the gap screen being further adapted to receive a fiber pulp mixture comprising longer fibers and shorter fibers in the input end thereof, said wire screen comprising: 
 at least one screening surface configured to engage the fiber pulp mixture received by the gap screen;    a plurality of openings defined by the at least one screening surface, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the wire screen thereby being configured to separate at least a portion of the shorter fibers from the fiber pulp mixture so as to produce a remainder of the fiber pulp mixture incapable of passing through the openings in the at least one screening surface.    
     
     
         21 . A wire screen according to  claim 20  wherein the wire screen is configured as an endless belt.