Title:
DELIGNIFYING LIGNOCELLULOSE WITH AN INCOMPLETE SODA COOK FOLLOWED BY GASEOUS BLEACHING
United States Patent 3617432
Abstract:
A process is provided for the preparation of pulp from lignocellulosic material. It involves two essential steps, namely: firstly, carrying out a modified conventional soda-cooking step in which the cooking conditions are deliberately selected to be such that the yield is significantly higher than that normally corresponding to a bleachable pulp and in which the material is simultaneously at least partially defiberized; and secondly, either partially or completely delignifying the pulp by subjecting the pulp so produced to a gaseous bleaching treatment while it is in defiberized form.


Inventors:
Clayton, David W. (Hudson, CA)
De Montigny, Raimbault M. A. T. (Baie D'Urfe, CA)
Liebergott, Norman (Laval, CA)
Application Number:
04/767829
Publication Date:
11/02/1971
Filing Date:
10/15/1968
Assignee:
Pulp and Paper Research Institute of Canada (Pointe Claire, Quebec, CA)
Primary Class:
Other Classes:
162/24, 162/25, 162/63, 162/66, 162/67, 162/89
International Classes:
D21C9/10; D21C9/14; (IPC1-7): D21C3/02; D21C3/18
Field of Search:
162/89,19,24,25,28,67,63,66
View Patent Images:
Other References:

Casey, J. P., Pulp and Paper, 2nd Edition, Volume I, p. 102, Interscience Pub. 1960..
Primary Examiner:
Zaharna, Samih N.
Assistant Examiner:
Scavone, Thomas G.
Claims:
We claim

1. A sulfur-free process for the preparation of pulp which comprises the steps of:

2. The process of claim 1 including the positive additional step of defiberizing the cooked lignified vegetable matter after it has been subjected to said modified cooking step but before it has been subjected to step (B).

3. The process of claim 2 wherein the alkaline solution is soda liquor.

4. The process of claim 2 wherein the lignified vegetable material is softwood and the yield is at least 48 percent.

5. The process of claim 4 wherein the softwood is mechanically defiberized.

6. The process of claim 2 wherein step (B) comprises pretreating the pulp by

7. continuously passing the defiberized pulp in the form of moist "fluff" which is at a moisture content of about 40-85 percent through a first zone of an atmosphere containing excess chlorine;

8. limiting the residence time within that first zone to such an extent that less than the potential amount of chlorine is acquired by the fluff;

9. continuously passing the fluff including the chlorine which has been acquired by the fluff through a second zone of an essentially chlorine-free atmosphere under such retention time conditions as to permit the amount of said chlorine to react with said fluff;

10. washing the pretreated pulp; and

11. further treating the washed pulp with an alkaline reagent.

12. The process of claim 6 wherein the treatment with the alkaline reagent comprises:

13. The process of claim 6 wherein the residence time in the first zone is 20 seconds to 5 minutes.

14. The process of claim 6 wherein the fluff acquires from 50-85 percent of the potential amount of chlorine.

15. The process of claim 6 wherein the residence time in the second zone is 1-15 minutes.

16. The process of claim 8 wherein the fluff acquires from 50-85 percent of the potential amount of chlorine and further wherein the residence time in the second zone is 1-15 minutes.

17. The process of claim 2 wherein step (C) comprises the step of pretreating the defiberized pulp in the form of a fluff which is at a moisture content of 80-40 percent with chlorine dioxide diluted to less than 100 mm. mercury partial pressure with a nonreactive gas, at a retention time of the order of 20 seconds to 60 minutes and at a temperature which varies between 15° C. and 100° C.; and effecting the further steps of (2) washing the pretreated pulp and (3) further treating the washed pulp with an alkaline reagent.

18. The process of claim 12 wherein the treatment with the alkaline reagent comprises:

19. The process of claim 12 wherein the nonreactive gas is air, nitrogen, steam, or an inert gas.

20. The process of claim 7 wherein the ammonia is diluted with steam, air, nitrogen, or other inert gas.

21. The process of claim 12 wherein the ammonia is diluted with steam, air, nitrogen, or other inert gas.

22. The process of claim 7 including the further steps of:

23. The process of claim 12 including the further steps of:

Description:
This invention relates to the production of pulp. The pulp may be formed from any suitable lignified vegetable matter, examples of which include softwood, hardwood, bagasse, cereal straws and bamboo. In particular, this invention is directed to the production of partially delignified pulp, partially bleached pulp and fully bleached pulp. When used in the present specification, the term "partially delignified pulp" is intended to mean the product obtained by the partial removal of the residual lignin remaining in the pulp after digestion, by means other than by digestion.

Softwood soda pulping to produce easily bleachable pulp has been found to be no longer practical since to produce softwood soda pulps of acceptably low lignin content, a very long cooking period is required in which the pulps obtained suffer severe degradation and a high yield shrinkage. The strength properties of such pulp are substantially inferior to those of kraft pulp. Soda pulps of higher lignin content are very resistant to bleaching by conventional liquid phase bleaching techniques, and, in fact, it has been found that the quantity of chemicals required to achieve bleaching is far too great to be commercially attractive.

Consequently, it has been common practice to produce pulps of high strength by means of kraft pulping. It has been the practice to cook kraft pulps to yields of 43-48 percent, the unbleached pulps thereby having maximum physical strength properties. Within this yield range, the pulps are bleachable. It is then possible to bleach the kraft pulps in the above-mentioned 43-48 percent yield range, to obtain strength properties of the bleached pulps which are not greatly different from those of the unbleached pulps.

However, kraft pulping suffers the principal disadvantage of the production of malodors which occur due to the formation of hydrogen sulfide and organic sulfur compounds during the kraft cook. The necessity for reducing or eliminating such air pollution from kraft mills has now become a serious and costly problem.

A principal object, therefore, of the present invention is to provide a process for producing partially dilignified pulps, partially bleached pulps, and fully bleached pulps using soda pulping, such pulps being suitable as a replacement for unbleached kraft pulps, semibleached kraft pulps and fully bleached kraft pulps.

An object of another aspect of this invention is to provide a process for the production of pulp wherein air pollution by malodors is no longer a factor.

By a broad aspect of this invention, a process is provided for the preparation of pulp which comprises the steps of: (a) cooking a lignified vegetable material in an alkaline solution containing predominantly sodium hydroxide in a modified cooking step at a temperature, and for a time sufficient to give a yield significantly higher than that normally corresponding to a bleachable pulp for the lignified vegetable material being treated; and (b) at least partially delignifying the cooked pulp in defiberized form by means of a high-density gaseous treating procedure.

It has been found that the combination of the two steps set forth above gave the advantage that the strength properties of the treated pulps were considerably higher than those of the unbleached pulps. An unexpected advantage of the combination of the two steps set forth above is that Elrepho brightness of about 90 percent can be accomplished in as few as the following five high-density stages, namely: gaseous chlorination; gaseous ammonia extraction; gaseous chlorine dioxide oxidation; gaseous ammonia extraction; and gaseous chlorine dioxide oxidation. Also, the shrinkage in high-density treatment is less than in conventional bleaching.

It has been found that the cooking in the alkaline liquor must be for a time, and at a temperature, sufficient to give a yield significantly higher than that normally corresponding to a bleachable pulp for the material being treated. This yield thus can vary depending on the material being treated. Thus, for a softwood, the yield significantly higher than that normally corresponding to a bleachable pulp is at least 48 percent, to give a pulp having a lignin content more than 5 percent. The minimum yields for other lignified vegetable material are given in the following table: ---------------------------------------------------------------------------

Minimum Percent Yield Signifi- cantly Higher than Lignified that Corresponding Vegetable Minimum Lignin to a Material Content (percent) Bleachable Pulp __________________________________________________________________________ Hardwood 2-4 50 Bagasse 6 50 Straws 6 50 Bamboo 5 45 __________________________________________________________________________

The first essential step of the process of broad aspects of this invention is a modified conventional soda-cooking step. In other words, the step is carried out under conditions well known to those skilled in the art, with the modification that the cooking conditions are deliberately selected to be such that the yield is significantly higher than that normally corresponding to a bleachable pulp for the lignified vegetable material being treated. These modified conditions can be readily chosen by a person skilled in the art. Exemplary modifications of the process steps include a reduced cooking time, reduced liquor concentration, reduced maximum cooking temperature, or any combinations thereof.

The second essential step of the process of broad aspects of this invention is that the pulp produced by the modified soda-cooking step is subjected to a high-density gaseous treatment, while it is in defiberized form, in order to delignify the pulp, either partially or completely. After the modified cooking step, then, all or a portion of the cooked material may be subjected to a defiberization treatment. The cooked material may be screened and if the quantity of rejects is less than about 2 percent, the rejects may be discarded or be returned to the cooking step, and the accepted material may be directly subjected to the aforementioned high-density gaseous treatment. If the rejects are higher than about 2 percent, they would be defiberized and added to the accepted material. This defiberization treatment is most usually effected by mechanical means, such as by the action of a disc refiner.

As mentioned hereinabove, an essential step in the process of this invention is that the cooked pulp, in defiberized form, is then delignified, either partially or completely by means of a high-density gaseous treatment. One such high-density gaseous treatment may be carried out according to a first aspect of this invention by treating the pulp with gaseous chlorine followed by an alkaline extraction according to the following procedure. Such process for the high-density gaseous treatment comprises pretreating the cooked pulp by the steps of: (1) continuously passing the cooked pulp which should be in the form of moist "fluff" through a first zone of an atmosphere containing excess chlorine; (2) limiting the residence time within that first zone (usually for a time of the order of 20 seconds to 5 minutes) to such an extent that less than the potential amount of chlorine is acquired by the fluff (and usually until about 50-85 percent of the potential amount of chlorine is acquired by the fluff); and (3) continuously passing the fluff including the chlorine which has been acquired by the fluff through a second zone of an essentially chlorine-free atmosphere under such retention time conditions as to permit the amount of said chlorine to react with said fluff (usually for a time of about 1 to 15 minutes); and then carrying out the additional steps of: (4) washing the chlorinated pulp; and (5) further treating the washed pulp with an alkaline reagent.

A preferred alkaline reagent is gaseous ammonia, in which case the alkaline treatment step is preferably carried out as follows: (1) washing the pretreated pulp and finely dividing the washed pulp into fiber aggregates generally at a moisture content of about 40-85 percent; (2) preheating the fiber aggregates to a temperature generally in the range of 60°-100° C.; (3) exposing the preheated fiber aggregates to an atmosphere comprising gaseous ammonia, either undiluted or diluted with steam, air, nitrogen, or other inert gas for a reaction time generally of the order of about 1 to about 30 minutes; and (4) maintaining the temperature during the reaction time within the range, usually, of about 60°-100° C.

Another high-density gaseous delignification treatment according to a second aspect of this invention is by treating the soda pulp with gaseous chlorine dioxide followed by an alkaline extraction according to the following procedure. Such process for the high-density gaseous treatment comprises pretreating the cooked pulp by the steps of: (1) treating the defiberized pulp which should be in the form of a fluff, generally at a consistency of 20-60 percent with chlorine dioxide, which may contain varying proportions of chlorine gas, diluted to less than 100 mm. mercury partial pressure with air, nitrogen, steam or an inert gas, at a retention time generally of the order of 20 seconds to 60 minutes and at a temperature which usually varies between 15° C. and 100° C.; and then carrying out the additional steps of (2) washing the pretreated pulp; and (3) further treating the washed pulp with an alkaline reagent.

A preferred alkaline reagent is gaseous ammonia, in which case the alkaline treatment step is preferably carried out as follows: (1) washing the pretreated pulp and finely dividing the washed pulp into fiber aggregates generally at a moisture content of about 40-85 percent; (2) preheating the fiber aggregates to a temperature generally in the range of 60°-100° C.; (3) exposing the preheated fiber aggregates to an atmosphere comprising gaseous ammonia, either undiluted or diluted with steam, air, nitrogen, or other inert gas for a reaction time generally of the order of about 1 to about 30 minutes; and (4) maintaining the temperature during the reaction time within the range, usually, of about 60°-100° C.

By another aspect of this invention, the pulp after being partially delignified may be subjected to further delignification, and bleaching, in further high-density stages, with chlorine dioxide, according to the following procedure. Such process for the further high-density gaseous treatment comprises the steps of: (1) adjusting the moisture content of the fibers to a level usually within the range of from 60 to 70 percent; (2) exposing the fiber aggregate to chlorine dioxide gas diluted with an inert solvent where the partial pressure of chlorine dioxide is 100 mm. of mercury or less, for a period which usually ranges from about 5 to about 30 minutes at a temperature which usually ranges from about 65 to about 100° C.; and (3) controlling the pH of the mixture so that final pH is preferably in the range of from 3.5 to 6.5.

The following is a description of an experiment designed to produce two pulps by means of a modified soda pulping cycle which is one of the essential steps of the process of a broad aspect of this invention.

EXPERIMENT A

Two cooks were carried out by placing spruce chips in the digesters, steaming the chips to displace air, then filling the digester with soda liquor (initial effective alkali concentration 42 g./l.). The liquor-to-wood ratio was 5:1. The digester was brought up to maximum temperature, 170° C., within 90 minutes. One cook (Pulp No. I) was terminated after 30 minutes of cooking (temperature 170° C. and 100 p.s.i.g. pressure); the other (Pulp No. II) after 10 minutes of cooking (temperature 170° C. and 100 p.s.i.g. pressure). The resultant pulps were fiberized by action of a disc refiner (one pass at a plate separation of 0.01 inch). Cooking and defiberizing produced pulps with the following characteristics: ##SPC1##

The following examples set forth the treatment of the pulps produced by a modified soda cooking technique of one step of this invention according to several different high-density gaseous treating techniques embodying several aspects of the process of this invention.

EXAMPLE I

One portion of Pulp No. I was shredded at 30 percent consistency and treated with gaseous chlorine (11.2% Cl 2 on pulp O.D. basis), maximum temperature 43° C., for 1 minute, washed, shredded at 20 percent consistency and treated with 2.5 percent of its weight of ammonia (in a mixture of ammonia and steam) for 1 minute at 100° C. and washed again. Characteristics of the partially delignified pulp were: Kappa number 12.2, brightness 36, visual efficiency 53.4 on the scale of the Elrepho meter, and yield on wood 47.4 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions 7,000 13,200 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.33 1.28 Breaking Length (M) 14,050 14,050 Burst Factor 117.5 123.0 Tear Factor 118 114 MIT Fold 2,930 3,010 Stretch (%) 4.08 4.08 __________________________________________________________________________

EXAMPLE II

One portion of the chlorinated, extracted and washed pulp produced in example I was pressed to 35 percent consistency. The pressed pulp was sprayed with a solution of sodium carbonate equal to 1.25 percent of the weight of pulp (B.D. basis) and sufficient water to reduce the consistency to 30 percent. The pulp was then shredded and heated to 80° C. The shredded and heated pulp was then treated with chlorine dioxide gas diluted to 30 mm. pressure by addition of nitrogen; 1 percent chlorine dioxide on the dry weight of fiber was added to the pulp. The pulp was maintained at the reaction temperature of 80° C. for 30 minutes, then removed and washed. Characteristics of the semibleached pulp were: brightness, 76.4 percent; visual efficiency, 87.7; yield on wood, 47.3 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions 5,900 12,400 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.25 1.24 Breaking Length (M) 14,380 14,960 Burst Factor 118 122 Tear Factor 116 110 MIT Fold 2,480 2,690 Stretch (%) 4.10 4.09 __________________________________________________________________________

EXAMPLE III

One portion of the semibleached pulp obtained in example II was shredded at 20 percent consistency and treated with 0.5 percent of its weight of ammonia (in a mixture of ammonia and steam) for 1 minute at 100° C. and washed. The pulp was then shredded at 30 percent consistency and preheated to 80° C., then treated with chlorine dioxide gas diluted to 30 mm. pressure by the addition of nitrogen; 0.4 percent chlorine dioxide on the weight of dry fiber was added to the pulp; retention time was 30 minutes. Characteristics of the pulp after washing were: brightness 91.1 percent; visual efficiency 94.3 percent; yield on wood 46.9 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions (cc.) 5,800 12,100 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.24 1.23 Breaking Length (M) 14,100 14,850 Burst Factor 113 120 Tear Factor 115 108 MIT Fold 2,340 2,670 Stretch (%) 4.18 4.10 __________________________________________________________________________

EXAMPLE IV

Another portion of Pulp No. I was shredded at 30 percent consistency and treated with gaseous chlorine dioxide diluted with nitrogen; 4.3 percent chlorine dioxide on the weight of dry fiber was added to the pulp, maximum temperature was 60° C., retention time 1 minute. After washing, the pulp was shredded at 20 percent consistency and treated with 2.5 percent of ammonia based on dry fiber (in a mixture of ammonia and steam) for 1 minute at 100° C. and washed again. Characteristics of the partially delignified pulp were: Kappa number 4.2, Elrepho brightness 45, visual efficiency 58.9 percent, yield on wood 48.6 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions 5,900 12,700 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.27 1.24 Breaking Length (M) 14,650 15,860 Burst Factor 123.5 131.0 Tear Factor 105 101 MIT Fold 2,520 2,900 Stretch (%) 3.90 4.00 __________________________________________________________________________

EXAMPLE V

One portion of Pulp No. II was shredded at 30 percent consistency and treated with gaseous chlorine (15.8% Cl 2 on pulp o.d. basis), maximum temperature 80° C. for 1 minute, washed, shredded at 20 percent consistency and treated with 4.5 percent of ammonia based on dry fiber (in a mixture of steam ammonia) for 1 minute at 100° C. and washed again. Characteristics of the partially delignified pulp were: Kappa number 14.7, brightness 36, visual efficiency 53.7 on the scale of the Elrepho meter, and yield on wood 48.6 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions 5,400 11,200 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.31 1.27 Breaking Length (M) 13,500 13,900 Burst Factor 112 120 Tear Factor 92 90 MIT Fold 2,240 2,740 Stretch (%) 3.70 3.95 __________________________________________________________________________

EXAMPLE VI

Another portion of Pulp No. II was shredded at 30 percent consistency and treated with gaseous chlorine dioxide diluted with nitrogen; 6.5 percent chlorine dioxide on the weight of dry fiber was added to the pulp, maximum temperature was 97° C., retention time 1 minute. After washing, the pulp was shredded at 20 percent consistency and treated with 4.5 percent of ammonia based on dry fiber (in a mixture of ammonia and steam) for 1 minute at 100° C. and washed again. Characteristics of the partially delignified pulp were: Kappa number 4.8, Elrepho brightness 44, visual efficiency 57.2 percent, yield on wood 52.7 percent.

Strength Properties __________________________________________________________________________ Beating Revolutions 4,600 11,400 Canadian Standard Freeness 500 300 (cc.) Bulk (cc./g.) 1.25 1.22 Breaking Length (M) 15,100 14,800 Burst Factor 125.0 117.5 Tear Factor 93 98 MIT Fold 2,800 3,600 Stretch (%) 3.97 4.20 __________________________________________________________________________

A comparison of the physical properties at 300 CSF of an unbleached kraft pulp with the same physical properties of two partially delignified soda pulps is shown below. It should be noted that the physical properties of the delignified soda pulps are similar to those of the unbleached kraft pulp, yet the yield of the soda pulps are 2 to 3 percent higher.

The comparisons are set forth in table II: ##SPC2##

The preceding examples can be repeated with similar success by substituting the generically and specifically described reactants and operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.