Pulp cooking with particular alkali profiles

Abstract

Yield, particularly when treating hardwood chips, can be improved by at least 1-2% in a kraft cellulose pulping process by keeping the temperature and effective alkali (EA) low during impregnation, and by keeping the EA low in at least a first cook stage. After cooking, the pulp is subjected to cooling low EA liquor, e.g. to reduce its temperature to below 120° C. (preferably below 100° C.) with an EA below about 5 g/L (expressed as NaOH). Continuous treatment in a continuous digester system is preferred, with the EA below about 20 g/L during impregnation and the first cook.

1 - 24 . (canceled)  
     
     
         25 . A method of treating comminuted cellulosic fibrous hardwood chip material to produce cellulose pulp, comprising: 
 (a) treating the hardwood chip material with a first alkaline liquid having a first effective alkali concentration and at a temperature less than 120° C.;    (b) treating the hardwood chip material with a second alkaline liquid having a second effective alkali concentration by adding dilution liquor having a low or substantially zero alkali concentration, while heating the hardwood chip material to a temperature above 120° C.;    (c) treating the hardwood chip material with a third alkaline liquid having a third effective alkali concentration at a temperature greater than 140° C. to delignify the material; and    (d) treating the hardwood chip material with a fourth liquid to cool the material to a temperature less than 120° C.; wherein the first, second, third initial effective alkali concentrations are all less than 30 g/L, and wherein the second and third effective alkali concentrations are about 25 g/L or less.    
     
     
         26 . A method as recited in  claim 25  wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 25 g/L.  
     
     
         27 . A method as recited in  claim 25  wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 20 g/L.  
     
     
         28 . A method as recited in  claim 27  further comprising (e), between (c) and (d) treating the material with an alkaline liquid having a fourth effective alkali concentration at a temperature greater than 140° C.  
     
     
         29 . A method as recited in  claim 28  wherein (e) is practiced at a fourth initial alkali concentration of greater than 30 g/L.  
     
     
         30 . A method as recited in  claim 28  wherein (e) is practiced at a fourth initial alkali concentration of less than about 15 g/L.  
     
     
         31 . A method as recited in  claim 28  further comprising (f), between (e) and (d), treating the material at a temperature of greater than 140° C. at a fifth initial alkali concentration of less than 25 g/L.  
     
     
         32 . A method as recited in  claim 26  wherein (a)-(d) are practiced using hardwood chips as the comminuted cellulosic fibrous material.  
     
     
         33 . A method as recited in  claim 27  wherein (b) is practiced to heat the material so that its temperature gradually increases, and so that the second EA concentration gradually increases, but stays below about 20 g/L.  
     
     
         34 . A method as recited in  claim 33  wherein (c) is practiced so that the EA concentration gradually decreases while the temperature is maintained substantially constant.  
     
     
         35 . A method as recited in  claim 34  wherein (d) is practiced with a cooling liquid having an EA of less than 5 g/L so that the EA is gradually reduced to a final level below 5 g/L while the temperature is gradually reduced.  
     
     
         36 . A method as recited in  claim 35  wherein (a) is practiced so that the EA gradually decreases while the temperature remains substantially the same.  
     
     
         37 . A method as recited in  claim 26  wherein (a)-(d) are practiced continuously.  
     
     
         38 . A method as recited in  claim 31  wherein (a)-(f) are practiced continuously in the same vessel.  
     
     
         39 . A method as recited in  claim 25  wherein (a)-(d) are practiced so as to increase yield by at least 1% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 30 g/L.  
     
     
         40 . A method as recited in  claim 27  wherein (a)-(d) are practiced using hardwood chips as the cellulosic material, and so as to increase yield by at least 2% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 30 g/L.  
     
     
         41 . A method of continuously treating hardwood chips to produce kraft pulp, using a continuous digester system in which a hardwood chip slurry primarily flows downwardly during treatment, comprising: 
 (a) impregnating the hardwood chips of the slurry in a first stage using a first alkaline liquid with an initial EA at the start of the first stage of about 25 g/L or less, and at a temperature of between about 90-110° C., the EA gradually diminishing by at least 10 g/L during the first stage, and so that at the end of the first stage the EA is about 10 g/L or less;    (b) gradually heating the hardwood chip slurry to a cooking temperature of about 140-180° C. as the slurry continuously moves through a second stage substantially contiguous with the first stage, by treating the slurry with a second alkaline liquid, the EA of the slurry starting at the beginning of the second stage at less than 15 g/L, and increasing at least about 5 g/L during the second stage, but not exceeding about 25 g/L by adding dilution liquor having a low or substantially zero alkali concentration;    (c) cooking the hardwood chip slurry in a third stage, using a third alkaline liquid, at a temperature that remains substantially constant and is between 140-180° C. and at an initial EA at the start of the third stage of below 25 g/L, and gradually decreasing by at least about 5 g/L and so that the EA at the end of the third stage is below 20 g/L;    (e) optionally subjecting the hardwood chips to at least a second cooking in a fourth stage at approximately the same substantially constant temperature as in the third stage, using a fourth alkaline liquid; and    (d) in a last stage, using a last alkaline liquid, gradually cooling the hardwood chips slurry to a temperature less than about 110° C. and so as to reduce the EA of the slurry at least about 5 g/L from the beginning to the end of the last stage, and so that the slurry has a final EA of less than about 5 g/L;    wherein (a)-(d) are practiced so as to increase yield of pulp produced by at least 2% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.    
     
     
         42 . A method as recited in  claim 41  wherein (e) is practiced, and so that the EA increases from the beginning to the end of the fourth stage by at least 10 g/L.  
     
     
         43 . A method as recited in  claim 42  wherein (a) is practiced at least in part by extracting liquor from the slurry at approximately the interface between the first and second stages; and wherein (b) is practiced at least in part by adding a combination of heated cooking and dilution liquor at approximately the interface between the second and third stages so that the heated liquor flows substantially countercurrent to the chips slurry; and wherein (c) is practiced at least in part by extracting liquor at approximately the interface between the third and fourth stages; and wherein (e) is practiced at least in part by adding a combination of heated cooking and dilution liquid below the extraction of approximately the interface between the third and fourth stages, to flow substantially countercurrent to the hardwood chips slurry; and wherein (d) is practiced by introducing dilution liquor at a temperature below about 110° C. adjacent a discharge of pulp from the digester system.