Pulp cooking with particular alkali profiles
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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.
22 Citations
43 Claims
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1-24. -24. (canceled)
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25. A method of treating comminuted cellulosic fibrous hardwood chip material to produce cellulose pulp, comprising:
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(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. - View Dependent Claims (26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40)
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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:
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(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. - View Dependent Claims (42, 43)
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Specification