System and method for removing foreign particles from an aqueous fibrous suspension

Abstract

A method and system is used for removing foreign particles, e.g., printing ink particles, from an aqueous fibrous suspension, in particular recovered paper suspension. The method includes dividing the suspension in a fractionation into a fine fraction, in particular short-fiber fraction, and a coarse fraction, in particular long-fiber fraction. Both of the fractions thus formed are then treated in respectively differently operated flotations in order to carry away the foreign particles in the flotation foam. The parameters and apparatuses used of the two flotations can be optimally adjusted thereby to the conditions present in the individual fractions.

1 . A method for removing foreign particles from an aqueous fibrous suspension using selective flotations, the method comprising: 
 fractioning the suspension to form a fibrous fine fraction and a fibrous coarse fraction; and    treating the fine fraction and coarse fraction in separate flotations.    
   
   
       2 . The method according to  claim 1 , wherein the fine fraction and the coarse fraction are enriched with long fibers.  
   
   
       3 . The method according to  claim 2 , wherein the flotation treating the fine fraction is a short-fiber flotation, and the flotation treating the coarse fraction is a long-fiber flotation.  
   
   
       4 . The method according to  claim 1 , wherein the fine fraction is enriched with fines and the coarse fraction is enriched with short fibers and long fibers.  
   
   
       5 . The method according to  claim 1 , wherein the fractioning is carried out in a pressurized screen and the flotation treating the coarse fraction is operated with a consistency that differs from the flotation treating the fine fraction by at least approximately 0.5%.  
   
   
       6 . The method according to  claim 1 , wherein the flotation treating the coarse fraction is carried out with a consistency of no more than approximately 1.5% and the flotation treating the fine fraction is carried out with a consistency of no more than approximately 2.5%.  
   
   
       7 . The method according to  claim 1 , wherein a dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least approximately 10% from the dwell time in the flotation treating the fine fraction.  
   
   
       8 . The method according to  claim 1 , wherein, in the flotation treating the coarse fraction, a specific quantity of fed flotation air based on volume flow, is adjusted differently by at least approximately 10% from in the flotation treating the fine fraction.  
   
   
       9 . The method according to  claim 1 , wherein an average bubble size in the flotation treating the coarse fraction is adjusted differently from the flotation treating the fine fraction, wherein the difference is at least approximately 10%.  
   
   
       10 . The method according to  claim 1 , wherein a gravitational field prevailing in the flotation treating the coarse fraction is adjusted differently from that in the flotation treating the fine fraction.  
   
   
       11 . The method according to  claim 1 , wherein, in the flotation treating the coarse fraction, a different number of reject-related flotation steps is used than in the flotation treating the fine fraction.  
   
   
       12 . The method according to  claim 1 , wherein, in the flotation treating the coarse fraction, a different number of accepted stock-related flotation steps is used than in the flotation treating the fine fraction.  
   
   
       13 . The method according to  claim 1 , wherein, in the flotation treating the coarse fraction, a different overflow proportion is formed than in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.  
   
   
       14 . The method according to  claim 1 , wherein before the flotation of the coarse fraction, the coarse fraction is thickened, dispersed and diluted again.  
   
   
       15 . The method according to  claim 14 , wherein the dispersion is carried out in a disk disperger or kneader pulper with a consistency between approximately 15% and 35%.  
   
   
       16 . The method according  claim 14 , wherein the dispersion is carried out through compression milling.  
   
   
       17 . The method according to  claim 14 , wherein specific work transferred during the dispersing is between approximately 30 and 120 kWh/t.  
   
   
       18 . The method according to  claim 1 , wherein the fractionation is carried out with a pressurized screen that is equipped with at least one wire.  
   
   
       19 . The method according to  claim 18 , wherein openings in the at least one wire are slots that have a slot width between approximately 0.08 and 0.3 mm.  
   
   
       20 . The method according to  claim 18 , wherein the openings in the at least one wire are holes.  
   
   
       21 . The method according to  claim 20 , wherein the openings in the at least one wire are round holes that have a diameter between approximately 0.1 mm and 2 mm.  
   
   
       22 . The method according to  claim 1 , wherein the fractionation is carried out in a worm press.  
   
   
       23 . The method according to  claim 1 , wherein the fractionation is carried but with a consistency between approximately 0.6% and 4%.  
   
   
       24 . The method according to  claim 1 , wherein the fractionation is carried out in hydrocyclones with a consistency between approximately 0.1% and 2%.  
   
   
       25 . The method according to  claim 1 , wherein, apart from the two flotations, no other flotation is carried out to remove foreign particles from the fibrous suspension.  
   
   
       26 . The method according to  claim 6 , wherein the flotation treating the coarse fraction is carried out with a consistency between approximately 0.7% and 1.3%, and the flotation treating the fine fraction with a consistency of between approximately 1.5% to 2%.  
   
   
       27 . The method according to  claim 7 , wherein the dwell time of the suspension in the flotation treating the coarse fraction is adjusted differently by at least 20% from the dwell time in the flotation treating the fine fraction.  
   
   
       28 . The method according to  claim 8 , wherein, in the flotation treating the coarse fraction, the specific quantity of the fed flotation air based on the volume flow, is adjusted differently by at least approximately 20% from in the flotation treating the fine fraction.  
   
   
       29 . The method according to  claim 9 , wherein the average bubble size in the flotation treating the coarse fraction is adjusted differently from in the flotation treating the fine fraction, wherein the difference is at least approximately 20%.  
   
   
       30 . The method according to  claim 17 , wherein the specific work transferred during the dispersing is approximately 60 kWh/t.  
   
   
       31 . The method according to  claim 19 , wherein the slot width is between approximately 0.1 mm to 0.15 mm.  
   
   
       32 . The method according to  claim 21 , wherein the round holes have a diameter between approximately 0.3 mm to 1 mm.  
   
   
       33 . The method according to  claim 24 , wherein the fractionation is carried out in hydrocyclones with a consistency between approximately 0.3% to 0.7%.  
   
   
       34 . The method according to  claim 1 , wherein, within separate flotations, the flotation effects differ in that in the separate flotations the features of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps or number of accepted stock-related flotation steps are selected differently, individually or in combination with one another.  
   
   
       35 . A method, comprising: 
 obtaining a fine fraction and a course fraction from an aqueous fibrous solution;    mixing the fine fraction with air and bringing the mixed air and fine fraction to a first flotation;    cleaning the fine fraction in the first flotation by removing flotation foam from the fine fraction and guiding the flotation foam into a reject processing;    diluting overflowing coarse fraction with water and bringing the diluted course fraction to a second flotation; and    cleaning the diluted coarse fraction in the second flotation by removing flotation foam from cleaned accepted stock obtained from the second flotation,    
   
   
       36 . The method according to  claim 35 , wherein process parameters are modified and selected differently, individually or in combination with one another for the first flotation and the second flotation.  
   
   
       37 . The method according to  claim 36 , wherein the process parameters include at least one of inflow consistency, overflow quantity, dwell time, chemical type, specific chemical quantity, location of chemical addition, specific quantity of flotation air added, bubble size, number of flotation cells, type of flotation cell, size of gravitational field, number of reject-related flotation steps and number of accepted stock-related flotation steps.  
   
   
       38 . The method according to  claim 37 , wherein the consistency is a thickening of the coarse fraction between approximately 15% and 25%.  
   
   
       39 . A system for removing foreign particles from an aqueous fibrous suspension, comprising: 
 a fractioning device configured to separate fine fraction and coarse fraction from the aqueous fibrous suspension; and    a first configuration for processing the fine fraction and a second configuration for processing the coarse fraction, wherein:    the first configuration comprising: 
 a first flotation device configured to clean foreign particles from the fine fraction;  
   the second configuration comprising: 
 a dispersion apparatus configured to detach foreign particles adhering to fibers in the coarse fraction;  
 a dilution apparatus through which water is added to the course fraction; and  
 a second flotation device configured to clean floatable impurities from the diluted course fraction.  
   
   
   
       40 . The system according to  claim 39 , wherein the fractioning device is one of a wet machine equipped with a screen basket and a washing device.  
   
   
       41 . The system according to  claim 39 , wherein the second configuration comprise a thickening apparatus configured to thicken the coarse fraction.  
   
   
       42 . The system according to  claim 38 , wherein the thickening device thickens the coarse fraction to a higher consistency between approximately 15% and 25%.