Title:
Prehydrolysis and digestion of fibrous lignocellulosic material
United States Patent 3923591
Abstract:
Process for chemically pulping lignocellulosic materials with aqueous digestion medium having a pH of 8.5 to 11.5 and containing water, alkali metal bisulfite, sodium carbonate and alkali metal hydroxide. The lignocellulosic material may be prehydrolyzed at a pH of 4.6 prior to pulping with the alkaline digestion medium to increase yield.
US Patent References:
Method for treating bagasse and similar fibrous products for the production of pulp and the extraction of cellulose
McQuiston et al. - July 1931 - 1813184
Rapid neutral sulfite process for pulping bagasse and other non-woody plant lignocellulose
Knapp et al. - February 1960 - 2924547
Process for manufacturing rayon pulp of high purity from bagasse
Okuno et al. - July 1961 - 2992155
Printing paper and process of making the same
Wethern et al. - December 1961 - 3013931
Continuous pulping process for the production of paper products
Ambuehl et al. - March 1963 - 3081218
Method for treating bagasse and similar fibrous products for the production of pulp and the extraction of cellulose
McQuiston et al. - July 1931 - 1813184
Rapid neutral sulfite process for pulping bagasse and other non-woody plant lignocellulose
Knapp et al. - February 1960 - 2924547
Process for manufacturing rayon pulp of high purity from bagasse
Okuno et al. - July 1961 - 2992155
Printing paper and process of making the same
Wethern et al. - December 1961 - 3013931
Continuous pulping process for the production of paper products
Ambuehl et al. - March 1963 - 3081218
Application Number:
05/444668
Publication Date:
12/02/1975
Filing Date:
02/21/1974
View Patent Images:
Export Citation:
Assignee:
Process Evaluation & Development Corporation (Dallas, TX)
Primary Class:
Other Classes:
162/96
International Classes:
D21C3/02; D21C3/00; D21C3/26; D21C3/12
Field of Search:
162/83,96,86
US Patent References:
Primary Examiner:
Bashore, Leon S.
Assistant Examiner:
Corbin, Arthur L.
Attorney, Agent or Firm:
Mcgreal, Michael Prince Kenneth J. E.
Parent Case Data:
This application is a continuation-in-part of my prior copending U.S. application Ser. No. 236,589, filed March 21, 1972, now abandoned, which in turn is a continuation-in-part of my application U.S. application Ser. No. 149,000, filed June 1, 1971, now U.S. Pat. No. 3,738,908, and of my prior copending U.S. application Ser. No. 220,390, filed Jan. 24, 1972, now abandoned.
Claims:
What is claimed is
1. A method for the production of cellulosic pulp from fibrous lignocellulosic material comprising prehydrolizing said lignocellulose material at a pH of about 4.0 to 6.0 in the presence of about 70 to 100 weight percent moisture based on the dry weight of said material, digesting the prehydrolized material with a sodium carbonate buffered aqueous digestion solution having a pH of 8.5 to 11.5 and a sodium oxide content of about 10 to 14 percent by weight, said aqueous digestion solution consisting essentially of about 1.1 to 3 moles of sodium bisulfite per mole of sodium hydroxide whereby during digestion the pH remains above about 8.5 and said sodium bisulfite and sodium hydroxide penetrate said fibrous lignocellulose material, react to form sodium sulfite, and digest said lignocellulose material.
2. A method as defined in claim 1 wherein the weight ratio of fibrous lignocellulose material to digestion solution is about 1:2 to 1:6.
3. A method as defined in claim 1 in which the total equivalent alkali metal oxide concentration is about 12 weight percent.
4. A method as defined in claim 2 in which the prehydrolysis and digestion is conducted at steam gauge pressures of from about 75 to about 150 pounds per square inch and about 320°F to about 390°F for a period of from about 5 to about 30 minutes.
5. A method as defined in claim 4 in which the lignocellulosic material is depithed sugarcane bygasse and the digestion solution has a pH of at least about 10.0 whereby the brightness of the resultant unbleached pulp is at least about 35 percent G. E.
6. A method as defined in claim 2 wherein said digestion solution contains 1.5 moles of sodium bisulfite per mole of sodium hydroxide.
7. A method as defined in claim 2 wherein said digestion solution has a pH of about 10 and a sodium oxide content of about 12 weight percent.
1. A method for the production of cellulosic pulp from fibrous lignocellulosic material comprising prehydrolizing said lignocellulose material at a pH of about 4.0 to 6.0 in the presence of about 70 to 100 weight percent moisture based on the dry weight of said material, digesting the prehydrolized material with a sodium carbonate buffered aqueous digestion solution having a pH of 8.5 to 11.5 and a sodium oxide content of about 10 to 14 percent by weight, said aqueous digestion solution consisting essentially of about 1.1 to 3 moles of sodium bisulfite per mole of sodium hydroxide whereby during digestion the pH remains above about 8.5 and said sodium bisulfite and sodium hydroxide penetrate said fibrous lignocellulose material, react to form sodium sulfite, and digest said lignocellulose material.
2. A method as defined in claim 1 wherein the weight ratio of fibrous lignocellulose material to digestion solution is about 1:2 to 1:6.
3. A method as defined in claim 1 in which the total equivalent alkali metal oxide concentration is about 12 weight percent.
4. A method as defined in claim 2 in which the prehydrolysis and digestion is conducted at steam gauge pressures of from about 75 to about 150 pounds per square inch and about 320°F to about 390°F for a period of from about 5 to about 30 minutes.
5. A method as defined in claim 4 in which the lignocellulosic material is depithed sugarcane bygasse and the digestion solution has a pH of at least about 10.0 whereby the brightness of the resultant unbleached pulp is at least about 35 percent G. E.
6. A method as defined in claim 2 wherein said digestion solution contains 1.5 moles of sodium bisulfite per mole of sodium hydroxide.
7. A method as defined in claim 2 wherein said digestion solution has a pH of about 10 and a sodium oxide content of about 12 weight percent.
Description:
This invention relates to methods of chemically pulping lignocellulosic materials and to the resultant pulps and paper produced therefrom. In particular it relates to a chemical pulping process using as a digestion medium an aqueous solution of alkali metal hydroxide and alkali metal bisulfite maintained at a pH between about 8.5 and 11.5 using an alkali metal carbonate. In a preferred embodiment the lignocellulosic material is prehydrolyzed in water or an acid medium prior to the alkaline digestion. In a particularly preferred embodiment the invention is applied to the preparation of pulp and paper from depithed sugarcane bagasse.
A number of processes are known for pulping lignocellulosic materials (i.e., preparing a papermaking pulp from such materials). Of these, one of the most important is the so-called "chemical" pulping process which may be an alkaline or an acid process. A reasonably detailed general discussion of the chemical pulping processes appears in Kirk-Othmer's "Encyclopedia of Chemical Technology", Second Edition, Volume 16 (1968) beginning at page 702. At page 720 reference is made to alkaline pulping of bagasse and other cellulosic products other than the more normal wood raw material.
The fibrous fraction of sugarcane bagasse, like other lignocellulosic materials (e.g., wood, bamboo, etc.), contains as primary constituents cellulose, lignin and hemicellulose. The latter include both pentosans and hexosans, in amounts averaging about 12 weight percent pentosans (about 90 to 95 weight percent of which is xylan or xylosan) and about 16 weight percent hexosans (28 weight percent total hemicellulose) based on the total lignocellulosic content of a typical bagasse, on a dry weight basis. The relatively high hemicellulose content of the bagasse binds the cellulose fibers together and renders more difficult the production of suitable pulps, particularly due to the inability to break down the fiber bundles, thus resulting in an inordinate amount of shives in the paper made from such pulp. As a result it is generally considered impossible to prepare mechanical pulps from bagasse. Chemical pulping of bagasse is known and is widely practiced in various countries of the world. The application of the usual chemical pulping processes to bagasse suffers from the disadvantages of considerably reduced yields in view of the amounts of lignin and hemicelluloses removed and of poor brightness and poor opacity in paper produced from the resulting pulp.
The yield from a pulping process is the weight of washed fibers (dry weight basis) recovered after digestion expressed as a percentage of the weight of lignocellulosic material (dry weight basis) originally charged to the pulping process. An important desire in any chemical pulping process is to obtain the highest yields without adversely affecting resultant properties of paper products made from the pulp. While this is and has been the aim of innumerable prior art workers for many years it is believed that the present invention provides the first major step forward.
In accordance with the present invention it has been found that the pulp yield from chemical pulping of lignocellulosic materials, and particularly sugarcane bagasse, can be significantly increased through the combination of fiber, prehydrolysis and the use of a pulping medium consisting essentially of an aqueous solution containing alkali metal bisulfite and alkali metal hydroxide buffered to a pH of from about 8.5 to about 11.5, preferably at least about 10, using an alkali metal carbonate. The concentration of alkali metal bisulfite and alkali metal hydroxide is in the range to produce the following stoichiometry:
MHSO 3 + MOH ➝ M 2 SO 3 + HOH 8.5 - 11
wherein M represents the alkali metal portion of the compound. The preferred alkali metal is sodium, and the process will be described with reference to sodium forms of the compounds.
The pulping medium is one which produces sodium sulfite within the lignocellulose fibers by the reaction of sodium hydroxide and sodium bisulfite within the fiber. In order to increase the efficiency of this in-situ reaction to a sodium sulfite there should be a prehydrolysis of the lignocellulose fiber prior to pulping (digestion). The prehydrolysis increases the rate of penetration of the pulping chemicals into the lignocellulose fibers. With prehydrolysis, fiber penetration is faster, decreasing the overall chemical digestion time and thereby decreasing the time period the lignocellulose fibers are in contact with the sodium hydroxide and sodium bisulfite pulping chemicals. Pulping chemicals will to some degree attack the cellulose of the fiber and for this reason it is preferred that the time duration of pulping be a minimum necessary to produce an acceptable pulp. This results in a maximum pulp yield for a given pulp quality.
More particularly, the lignocellulosic material is subjected to a prehydrolysis step at a pH of from about 4.0 to about 5.8, preferably about 4.5 to about 5.5. At pH below about 4.0 or so the yield from the pulping process is significantly reduced -- below satisfactory economical levels. At higher pH about about 5.9, the desired prehydrolysis consumes excessive time, reduces yield and again is economically unattractive. Normally fresh bagasse will have a pH in the range of about 6.0 to 6.5 whereas stored bagasse, as a result of acetic acid produced by fermentation during storage, will have a pH in the range of about 4.5 to 5.0. The pH during the prehydrolysis step can be maintained within the desired limits of 4.0 to 6.0 or preferred limits of 4.5 to 5.5 in various ways, i.e., by mixing fresh and stored bagasse feed in suitable proportions, by controlling the amount of moisture mixed with the bagasse feed or feeds; by addition of minor amounts of pH adjusting chemicals such as acetic acid, through use of acid prehydrolysis media such as acid white water (pH about 5.5).
The prehydrolysis is conducted in the presence of from about 70 to about 100 weight percent water, based on the bonedry weight of lignocellulosic material fed. Additional water required may be present as liquid water or in the vapor form, i.e., steam; preferably as a liquid water-steam mixture which serves to maintain the appropriate temperature as well as furnish the necessary amounts of water. Liquid water may be blended with the lignocellulosic feed prior to introduction to the prehydrolysis reaction zone or in situ in the zone. Added steam is, of course, introduced into the prehydrolysis reaction zone.
The prehydrolysis reaction is conducted under autogenous steam pressure at temperatures maintained in the range of from about 320° to about 390°F. The desired prehydrolysis reaction can be achieved, under the stated conditions, in a relatively very short period of from about 5 to about 30 minutes.
Another part of this invention is the discovery that at the pH range of 8.5 to 11.5 sodium sulfite will form within the bagasse lignocellulose fiber from the penetration of the sodium hydroxide and sodium bisulfite. Sodium sulfite is a very effective pulping material, that is, it attacks the lignin content of the lignocellulosic fiber. A reagent which is a good pulping agent in that it attacks the fiber lignin content will also generally reduce the overall pulp yield due to some cellulose attack and thereby decrease the efficiency of the process. The prime object of this invention is to utilize sodium sulfite pulping, but to use a chemical form of sodium sulfite and a concentration of chemicals which will not adversely affect pulp yield. The use of in-situ fiber synthesis of sodium sulfite and of a relatively high pH yields a process which maximizes pulp yield while not adversely affecting the properties of the resulting paper. If the pH is not maintained in the range of 8.5 to 11.5 there is a decreased yield since there appears to be considerable sodium bisulfite fiber attack at lower pH's.
The stoichiometry of the reaction of sodium bisulfite and sodium hydroxide is essentially equimolar. However, in the present process, since the oxidic sulfur moiety of the sodium bisulfite is being continually dissipated by reaction with the lignin portion of the lignocellulose fiber to produce lignosulfonates, there should be an excess of sodium bisulfite available for reaction to produce further sodium sulfite. That is, upon reaction of sodium sulfite with lignin there results a free soda molecule which converts to caustic for further reaction with sodium bisulfite to produce sodium sulfite. By using this technique, the sodium sulfite concentration is kept to a minimum. It is also a very effective way to guarantee in-situ fiber synthesis of sodium sulfite. That is, the excess sodium bisulfite is present in the fiber when soda molecules become available from the reaction of sodium sulfite and lignin. These soda molecules readily react with sodium bisulfite to produce more in-situ sodium sulfite until the lignin or sodium bisulfite is exhausted. The sodium carbonate serves to raise and initially maintain the pH in the range of 8.5 to 11.5 and to increase the Na 2 O content of the digestion solution. It is important that the pH be fairly highly alkaline in order to inhibit sodium bisulfite attack of the fibers, and further that there be an excess of Na 2 O ions present to direct the equilibrium of the sodium bisulfite and sodium hydroxide reaction to sodium sulfite. The in-situ reactions of the digestion process are illustrated by the following equilibrium:
pH 1. NaHSO 3 + NaOH ➝ Na 2 SO 3 + HOH 8.5 - 11.5
As the lignocellulose fibers are digested the pH decreases slightly due to the reaction of some sodium ions with evolved organic acids and phenols. The pH, however, remains above about 8.5 and does not produce any serious adverse effect as to fiber yield or product quality. The initial Na 2 O content is preferably about 10 to 14 percent by weight and ideally about 12 percent by weight.
The digesting medium should have a weight ratio of lignocellulosic materials (dry weight basis) to digestion solution of between about 1/6 and 1/2, and preferably about 1/4. There will therefore be about 2 to 6 grams of digestion liquor per gram of fiber. The chemical composition of this digestion solution will be such as to contain about 0.5 to 2 moles of sodium bisulfite per liter of solution. The sodium hydroxide content will be in the range of 0.3 to 1.3 moles per liter of digestion solution. It is preferred that there be 1.1 to 3.0, and particularly about 1.5, moles of sodium bisulfite per mole of sodium hydroxide. The content of sodium carbonate will be an amount sufficient to increase the pH of the fiber to about 8.5 to 11.5. The pH of input of prehydrolyzed lignocellulosic fiber is usually about 4.5 to 6.5. In most instances a gram amound of sodium carbonate equivalent to the gram amount of sodium hydroxide will be sufficient to produce this pH. There should therefore be about 0.15 to 0.5 moles of sodium carbonate per liter of digestion solution. The digestion temperature is preferably from about 320°F to 390°F and the digestion time is usually about 5 to about 30 minutes and preferably about 10 to 20 minutes and the pressure at a steam gauge pressure of about 75 to about 150 pounds per square inch.
After digestion the pulp is refined, screened, washed, etc., in the usual manner prior to use as furnish to the paper making machine.
If desired the process of this invention may include as an optional step the addition to the pulp of about 0.8 to about 1.3, preferably about 1.0 weight percent of an alkali metal silicate, based on the bone-dry weight of the initial bagasse fiber fee. The alkali metal silicate or a portion thereof may be added to the pulp separately from the digestion treatment prior to release of the digestion pressure. Preferably, all of the alkali silicate is added in the digester blow line or at the digester blow valve, just prior to final completion of the digestion reaction.
In practice a sodium silicate, such as sodium orthosilicate, sodium sequisilicate or sodium metasilicate, is the preferred alkali metal silicate when such is used. Sodium silicate is available commercially with various ratios of Na 2 O:SiO 2 such as, sodium orthosilicate having a ratio of Na 2 O:SiO 2 of 2:1, sodium metasilicate having a ratio of Na 2 O:SiO 2 of 1:1, sodium sesquisilicate having a ratio of Na 2 O:SiO 2 of 3:2, as well as other commercial sodium silicate products having various ratios of Na 2 O:SiO 2 such as 1:2, 1:3.2, etc. However, pulps which exhibit satisfactory color and brightness properties may be obtained by using other alkali metal silicates such as potassium silicate and lithium silicate, and, in general, it is contemplated that all of the various forms of the alkali metal silicates such as potassium orthosilicate, potassium metasilicate, lithium orthosilicate and lithium metasilicate will function in a manner which, in this respect, is similar to sodium orthosilicate or sodium metasilicate. Commercially available potassium silicates having ratios of K 2 O:SiO 2 corresponding to those for sodium silicates are useful, as are those having ratios of K 2 O:SiO 2 of 1:2.1, 1:2.2, 1:2.5, etc.
The process of this invention is most desirably conducted in a continuous manner, using for this purpose pressure vessels known in the pulp and paper industry as continuous digesters (see, for example, Kirk-Othmer's "Encyclopedia of Chemical Technology", Second Edition, Volume 16 (1968), pages 700-701, and Rydholm "Pulping Processes", Interscience Publishers (1965), pages 343-355); suitably modified, where required, to permit introduction of treatment chemicals at the appropriate point or points in the process.
The invention is specifically useful for pulping of sugarcane bagasse. However, it is not limited to bagasse as the lignocellulosic feed material, but is applicable as well to other materials such as soft or hard woods, tropical woods, bamboo, various straws, hemp, sisal and other lignocellulosic products.
In the application of the invention to pulping of bagasse the bagasse fibers used should be as pith-free as is reasonably possible. Suitable bagasse fiber feed materials may be prepared, for example, via the use of the apparatus and/or methods as described in U.S. Pat. No. 3,537,142 or U.S. Pat. No. 3,688,345. For highest yields it is preferred that the bagasse fiber feed material before prehydrolysis be one which has been subjected to a two-stage depithing operation, i.e., first dry depithed in accordance with the aforesaid U.S. Pat. No. 3,537,142, and then further wet depithed in the presence of at least about 4.5, normally from about 5 to about 10, parts by weight of water per part by weight of fiber (bone-dry basis) in accordance with U.S. Pat. No. 3,688,345; the entire disclosure of which is incorporated herein by reference.
Practice of the invention is illustrated by the following specific but nonlimiting examples. In the examples the properties shown for the pulp and for paper samples prepared therefrom are all determined by standard TAPPI procedures. For example, the K-number of the pulps is the potassium permanganate number, a standard TAPPI test for determination of residual lignin. Tensile strength of the paper samples is the tensile breaking length, in meters, determined on an Instron tensile tester with a specimen 1-inch wide, 4 inches long (between test jaws) and at a jaw separation rate of 1 inch per minute.
EXAMPLE 1
In this example pulp was prepared from bagasse fibers using a commercially available continuous two tube digester from Pandia, Inc. Water is added to the fiber feed in a wetting tank, and the wetted mass is screw fed to the digester. Total average residence time in the digester during the run was about 26 minutes. Digested pulp is sent to an Asplund refiner and thence to a blow tank where pressure is reduced to 0.5 pounds per square inch gauge. The stock is diluted in the blow tank and then pumped through subsequent processing equipment (a cleaner, a centrisorter and three sequentially connected washers ) and finally to storage tanks where it is held until fed to a papermaking machine.
The digester was adapted for introduction of digestion chemicals and/or other additives at various points along the length of the line of travel through the digester. In this run provision was made for introduction of digestion (cooking) chemicals at points 1/4 to 1/3 from the beginning of the total digester length and for introduction of sodium silicate at the end of the digester, before the digested pulp passed through the Asplund refiner.
In this example the bagasse fiber feed to the digester was wet depithed bagasse prepared in accordance with U.S. Pat. 3,688,345. The fiber had a moisture content of about 50 percent and contained about 23 percent lignin and 26 percent total hemicelluloses on a dry weight basis.
The bagasse fibers were fed to the digester at a rate of about 7.5 tons per hour (bone-dry basis) and were prehydrolyzed at steam pressures of 125 pounds per square inch gauge (about 345°F) in the initial 1/4 to 1/3 portion of the digester with clear white water having a pH of 5.5 in amounts sufficient to provide about 80 weight percent water in the prehydrolysis zone, based on bone-dry weight of the fiber feed. The pH of the digestion chemicals through the introduction means referred to previously. In this run the digestion chemicals used comprised an aqueous solution having a pH of 11 and containing 46 grams per liter caustic soda (NaOH), 185 grams per liter sodium bisulfite and 19 grams per liter sodium carbonate, which was introduced to the digester at a rate of 24.8 gallons per minute. At the exit end of the digester a 4 percent aqueous solution of sodium metasilicate was fed into the pulp at a rate of one gallon per minute before the pulp is passed through the Asplund refiner and thence to the blow tank.
The run was conducted for an operating period of about 13 hours under the conditions stated, during which period the yield from the digester was 60-65 percent. The pulp produced had a G. E. brightness of 58-60.
In another run conducted under like conditions, except that no silicate was added at any time, the resultant pulp had a G. E. Brightness not greater tha 45-46.
EXAMPLES 2-4
In the following Examples the lignocellulosic material to be pulped was depithed sugarcane bagasse prepared in the apparatus and with the method described in the aforementioned U.S. Pat. No. 3,537,142. Three separate runs were made using the cooking solutions having compositions as shown in Table I below, with pH varying from 9 to 11. All digestions (cooks) were conducted at a ratio of about 4 parts by weight cooking liquor for each part by weight (dry weight basis) of lignocellulosic material fed. The digestions were each performed for 30 minutes in a closed vessel at a steam pressure of 100 to 110 pounds per square inch gauge. Just prior to discharge to the blow tank the test pulps were treated with 1 weight percent sodium silicate. After blowdown the pulps were cleaned, centrifuged and washed in the usual manner. Thereafter, the beating or refining times of the pulps to various Canadian Standard freeness were determined, and certain properties of papers prepared from the beat pulps were tested. For comparison purposes a pulp was prepared from the same sugarcane bagasse fibrous material in the same manner as described above using caustic soda (sodium hydroxide) as the only alkaline digestion chemical and without any added sodium silicate; and properties of samele papers of this pulp were also determined.
Results of the tests are shown in the following Tables II and III.
TABLE I ______________________________________ Cooking Solutions Weight Percent (as Na 2 O) Example Example Example Comparative Cooking Chemical 2 3 4 Run A ______________________________________ NaHSO 3 6.85 6.80 6.55 -- Na 2 CO 3 1.38 1.38 1.22 -- NaOH 3.78 3.83 4.26 10.5 Total Na 2 O 12.01 12.01 12.03 10.5 pH 9 10 11 -- ______________________________________
TABLE II ______________________________________ Pulp Properties Example Example Example Comparative 2 3 4 Run A ______________________________________ Pulp Yield (weight %, dry weight basis) 68.1 67.0 65.2 56.3 K Number 25.6 25.5 24.6 23.5 Whiteness (% G.E.) 34.0 36.5 41.0 27.0 ______________________________________
TABLE III ____________________________________________________________ ______________ Physical Properties of Paper Samples Canadian Basis Standard Weight Density Beating Freeness (Ovendry- Tensile (Grams per Time (Milli- Grams per Tear Break Burst Cubic Fold Pulp From (Minutes) liters) Square Meter) Factor (Meters) Factor Centimeter) Number ____________________________________________________________ ______________ Example 2 0 785 -- -- -- -- -- -- 20 555 63.5 91.0 2662 13.1 0.31 15 28 425 60.6 73.3 3600 19.0 0.36 16 36 360 59.6 67.0 4140 22.0 0.41 18 44 285 59.6 59.7 4643 25.0 0.41 30 55 215 59.6 52.2 4935 28.7 0.48 40 Example 3 0 733 -- -- -- -- -- -- 8 520 64.6 68.8 3880 22.8 0.38 28 16 425 64.6 62.0 4954 27.5 0.41 32 24 335 64.1 57.3 5387 29.1 0.44 56 32 235 64.1 52.0 5700 32.5 0.48 60 40 185 64.1 48.5 6073 34.7 0.48 81 Example 4 0 760 -- -- -- -- -- -- 7 675 59.6 67.1 4330 19.8 0.35 21 14 440 59.6 59.6 5147 28.8 0.41 22 21 335 59.6 52.2 5700 31.7 0.44 36 28 245 61.1 47.2 6273 34.5 0.45 58 35 175 61.1 43.6 6710 36.5 0.50 243 Comparative 0 760 -- -- -- -- -- -- Run A 8 615 59.6 63.4 4140 18.6 0.38 10 16 485 59.6 59.7 4923 24.6 0.41 20 24 360 59.6 56.0 5483 28.5 0.44 22 32 255 59.6 52.2 6042 32.0 0.48 62 40 175 59.6 48.5 6423 35.2 0.53 104 ____________________________________________________________ ______________
Graphical plotting of the data shown in Table III above indicates that the pulps and paper samples have the representative properties shown in the following Table IV at equivalent Canadian Standard Freeness.
TABLE IV ______________________________________ Comparison at Equivalent Canadian Standard Freeness At Canadian Standard Comp- Freeness of 360 Example Example Example arative Milliliters 2 3 4 Run A ______________________________________ Beating Time 36.0 21.5 19.5 24.0 Tear Factor 67.0 58.5 54.5 56.0 Tensile Break 4140 5300 5550 5480 Burst Factor 22.0 29.0 31.0 28.0 ______________________________________ At Canadian Standard Comp- Freeness of 215 Example Example Example arative Milliliters 2 3 4 Run A ______________________________________ Beating Time 55.0 36.0 31.0 36.0 Tear Factor 52.2 50.5 45.5 50.5 Tensile Break 4935 5940 6450 6230 Burst Factor 28.7 33.0 35.3 33.5 ______________________________________
As seen from the foregoing Examples the method of this invention provides yield increases of up to 22 percent as compared to use of caustic soda alone as the cooking chemical (comparing yield data from Example 2 and Run A). Except for Example 2, the beating times to equivalent Canadian Standard Freeness are the same or less than in Comparative Run A. In all of Examples 2 through 4 a much greater pulp brightness results, especially in Example 4. The overall physical properties of papers prepared from the pulps of Examples 3 and 4 are good to excellent at pulp yield increases of approximately 16 to 20 percent as compared to Run A.
EXAMPLES 5-11
In the following Examples 5-11 trial runs of the alkaline pulping process of this invention were conducted in a Scandinavian pilot unit. The lignocellulosic raw material was the fibrous fraction of sugarcane bagasse prepared in Latin America by a primary dry depithing followed by a secondary wet depithing (as described in U.S. Pat. No. 3,688,345) and shipped to Scandinavia for the tests. The material was air dried to an average moisture content of 40 percent prior to baling for shipment. As analyzed at the depithing plant it had a lignin content of 17.8 percent and a pentosans content of 22.9 percent, and gave the following solubles analyses:
Weight Percent Solubles In Cold Water 2.91 In Hot Water 8.46 In 1 percent NaOH 28.80 In Alcohol/Benzene
In these runs the test equipment used was an Asplund Defibrator unit, Model No. OVP-20, disc type 5821 running at a disc setting of 0.05 millimeters at 1500 revolutions per minute and equipped with a horizontal preheater. The lignocellulosic raw material was fed to the horizontal preheater by a two pocket rotary feeder. When prehydrolysis was used the prehydrolysis time was fixed by adjusting the speed of the transport screw in the horizontal preheater. Cooking liquor was added to the material at the end of the horizontal preheater and the raw material then falls down into a vertical tapered digester where cooking time was controlled by a radiation level gauge. The material was heated by direct steam in the horizontal preheater and in the vertical digester. In addition, the digester is equipped with a steam jacket in order to minimize condensation. At the bottom of the digester the digested product is fed to an Asplund defibrator by an agitator and a screw conveyor. As is known in the art the defibrator works on the pulp while it is still under digester pressure. Defibrated pulp was either blown directly to and refined in an Asplund refiner, or collected unrefined from the cyclone which receives material from the defibration.
All runs were conducted at 170°C during prehydrolysis (when used) and during digestion, using steam at an absolute pressure of 8 kilograms per square centimeter. All cooking liquors were prepared from technical grade chemicals which were dissolved in water in ratios providing a pH of about 10.5. Specific compositions are shown in Table V. Sodium silicate, when added, was introduced in the defibrator screw conveyor just prior to defibration as an aqueous solution of sodium silicate pentahydrate (Na 2 O.SiO 2.5 H 2 O) having a concentration of 100 grams per liter.
The bagasse was presoaked in water or in the cooking liquor prior to processing. Total processing time was 14 minutes in each run. This was equally divided into a 7 minute prehydrolysis and 7 minute digestion when prehydrolysis was used. Pulping data are shown in Table VI. Kappa numbers were determined by duplicating the runs in a laboratory scale Asplund Defibrator on a 300 gram (bone-dry basis) sample of the bagasse and weighing the amount of recovered pulp after washing and air drying.
The pulps were refined at a consistency of 20 percent and pulp samples collected at two different disc settings. The refined pulps were washed on a 100 mesh wire cloth and then sheet formed according to Swedish standard method CCA-17 and tested according to SCAN methods.
Results are shown in Table VII.
TABLE V ______________________________________ Cooking Solutions Concentration (grams per liter) Cooking Chemical Examples 5,6,7,8 Examples 9,10,11 ______________________________________ NaHSO 3 100 60 Na 2 CO 3 35 20 NaOH 35 20 pH 10.5 10.3 ______________________________________
TABLE VI ____________________________________________________________ ______________ Pulping Data Example Example Example Example Example Example Example 5 6 7 8 9 10 11 ____________________________________________________________ ______________ Presoaking Liquor Water Water Water Water Cooking Cooking Cooking Solution Solution Solution pH after Presoak 5.5 5.5 5.5 5.5 10.3 10.3 10.3 Prehydrolysis Time (minutes) 7 7 7 7 None None None pH after Prehydrolysis 4.6 4.6 4.6 4.6 -- -- -- Total Na 2 O in Cooking Solution (as weight percent of Bagasse Fibers, bone dry) 3.8 3.3 10.3 11.3 3.4 9.4 9.4 Sodium Silicate Added Yes No Yes No Yes Yes No Total Chemicals as Na 2 O, including Silicate, in Weight Percent of bone -dry Bagasse 4.1 3.3 10.9 11.3 4.0 9.9 9.4 Digestion Time (minutes) 7 7 7 7 14* 14* 14* pH after Defibration 6.7 6.2 9.5 9.3 6.8 9.7 9.7 Yield (percent) 86.6 86.0 80.8 80.6 85.1 72.1 71.4 Kappa Number 93.8 95.8 33.4 43.7 92.2 25.3 19.4 Brightness, SCAN 28.4 26.5 30.2 32.7 22.5 35.7 38.0 ____________________________________________________________ ______________ *No separate prehydrolysis. Bagasse soaked in cooking liquor prior to introduction to horizontal preheater.
TABLE VII ____________________________________________________________ ______________ Paper Properties Example 5 Example 6 Example 7 Example 8 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Canadian Standard Freeness (milli- liters) 480 250 120 610 240 180 350 260 110 380 230 110 Sheet Weight (grams per square meter) 104.3 91.5 93.1 107.2 96.0 97.8 101.3 99.5 106.9 108 93.1 97.6 Thickness (milli- meters) 0.282 0.184 0.180 0.294 0.203 0.200 0.174 0.135 0.141 0.171 0.135 0.131 Bulk (cubic centi- meters per gram) 2.70 2.01 1.93 2.75 2.11 2.04 1.72 1.36 1.32 1.58 1.45 1.34 Burst Strength (kilograms per square centimeter) 0.9 1.5 1.7 0.8 1.4 1.7 2.9 4.3 4.6 3.4 3.7 4.3 Burst Factor 8.2 16.8 18.7 7.5 15.0 17.0 28.4 43.0 43.4 31.3 39.3 44.3 Tensile Break (meters) 1950 3980 4780 1810 3280 3580 4920 6600 6750 5180 6130 6790 Tear Factor 66 49 47 63 52 45 68 51 48 65 57 52 Folding (Kohler- Molin, 800 gram load) 4 10 8 2 6 7 120 150 565 190 125 320 Brightness, SCAN 28.4 29.1 28.8 26.5 27.6 27.1 30.2 37.3 28.0 32.7 33.8 32.0 Opacity, SCAN (60 grams per square meter) 96.3 98.0 97.3 98.2 98.0 97.7 86.1 84.8 87.7 84.7 83.7 81.6 ____________________________________________________________ ______________ 1 Before refining. 2 After pulp was refined at disc setting of 0.10 millimeters (Exs. 5 9) 0.15 millimeters (Exs. 6, 10, 11) or 0.20 millimeters (Exs. 7, 8). 3 After pulp was refined at disc setting of 0.01 millimeters (Exs. 5 6, 9, 10, 11) or 0.05 millimeters (Exs. 7, 8).
Example 9 Example 10 Example 11 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Canadian Standard Freeness (milliliters) 580 290 170 430 250 170 450 215 180 Sheet Weight (grams per square meter) 103.0 106.6 107.5 103.4 95.4 97.0 105.0 97.3 106.6 Thickness (millimeters) 0.228 0.195 0.192 0.150 0.125 0.126 0.145 0.126 0.134 Bulk (cubic centimeters per gram) 2.21 1.83 1.79 1.45 1.31 1.30 1.38 1.29 1.26 Burst Strength (kilograms per square centimeter) 1.2 1.9 1.8 3.9 4.0 4.3 4.1 4.2 4.3 Burst Factor 11.9 17.2 16.9 37.3 42.1 44.5 39.0 43.5 40.2 Tensile Break (meters) 2440 3440 3500 5370 6350 7020 5910 6360 5925 Tear Factor 65 53 49 58 48 46 55 48 52 Folding (Kohler-Molin, 800 gram load) 6 8 7 110 290 265 130 160 245 Brightness, SCAN 22.5 21.6 20.6 35.7 36.8 36.1 38.0 37.5 37.0 Opacity, SCAN (60 grams per square meter) 97.9 98.1 98.4 84.4 83.6 85.4 86.0 84.6 84.6 ____________________________________________________________ ______________ 1 Before refining. 2 After pulp was refined at disc setting of 0.10 millimeters (Ex. 5, 9) 0.15 millimeters (Exs. 6, 10, 11) or 0.20 millimeters (Exs. 7, 8). 3 After pulp was refined at disc setting of 0.01 millimeters (Exs. 5 6, 9, 10, 11) or 0.05 millimeters (Exs. 7, 8).
Graphical plotting of the data shown in Table VII indicates that the paper samples have the representative properties shown in the following Table VIII if compared at equivalent Canadian Standard Freeness levels.
TABLE VIII ____________________________________________________________ ______________ Comparison at Equivalent Canadian Standard Freeness ("CSF") At CSF of 350 Example No. milliliters 5 6 7 8 9 10 11 ____________________________________________________________ ______________ Burst Factor 14.5 13.2 31.0 33.6 16.0 39.0 41.0 Tensile Break 3350 2875 5250 5400 3200 5850 6100 Tear Factor 54 57.5 64 63.8 53 55 41.9 At CSF of 250 milliliters Brust Factor 17.0 15.2 38.0 38.1 17.5 42.0 42.6 Tensile Break 4000 3300 6050 6000 3400 6400 6290 Tear Factor 50 52.7 56 58.7 49 52 49 ____________________________________________________________ ______________
As seen from the foregoing trial runs the greatest pulp yields were obtained when using lower proportions of total alkaline chemicals charged to the process, resulting in substantially neutral pH 6.2-6.8 at the end of the cook (Examples 5, 6 and 9). The yields were substantially the same and the Kappa number of the pulps were substantially the same at these lower proportions of alkaline chemicals to bagasse whether the lignocellulosic feed material was prehydrolyzed (Examples 5 and 6) or not prehydrolyzed (Example 9). The burst factor, tensile breaking length and tear factor of papers prepared from the runs in these three Examples were with minor exceptions considerably poorer than in the other four Examples (i.e., Examples 7, 8, 10 and 11).
As seen from Examples 10 and 11, when a higher proportion of total alkaline chemicals was used, without any prehydrolysis, the final pH at the end of the cook was considerably higher (i.e., 9.7). The yields were still greater than 70% (as compared to 55% or so in current industry practice using only NaOH for cooking) and there was a striking decrease in Kappa number to the range of 20-25 (indicating much greater ease in refining). As indicated in Table VIII sample papers made from the pulps prepared in these runs have excellent overall properties.
Examples 7 and 8 demonstrate the desirability of combining prehydrolysis with the higher proportions of alkaline chemicals. Comparing these two Examples with Examples 10 and 11 it is seen that prehydrolysis provided an incremental increase of about 8 percent in the pulp yield while still retaining a significantly lower Kappa number (as compared to Examples 5, 6 and 9). As again indicated by Table VIII the overall physical properties of papers prepared from the pulps of Examples 7 and 8 are very good to excellent.
From Examples 5 to 11 it is concluded that the process of this invention is most desirably conducted with a prehydrolysis step and with sufficient amounts of the alkaline chemicals (about 8 to 12 weight percent, e.g., 9 to 11 weight percent total Na 2 O based on dry weight of fiber feed) to provide a pulp pH greater than about 9 at the end of the cook if one wishes to obtain the optimum overall combination of yield and paper properties. The optimum properties, with somewhat reduced but still very good yield, are obtained with like proportions of total chemicals but without a prehydrolysis step.
EXAMPLES 12-13
In the following Examples 12 and 13 trial runs were made comparing a pulping solution containing sodium bisulfite, sodium hydroxide and sodium carbonate to a pulping solution containing sodium sulfite and sodium carbonate. The feed bagasse was the same as in Examples 5-11, as was the pilot plant unit. No sodium silicate was added to the pulped bagasse to improve the brigntness. The specific pulping operating conditions and results are shown in Table IX.
TABLE IX ____________________________________________________________ ______________ Example 12 13 ____________________________________________________________ ______________ Presoaking liquor Water; H 2 O % by weight 100 100 Ph of bagasse after soaking 5.5 5.5 Dryness of bagasse after prescrew press % by weight water 40 40 Prehydrolysis Ingoing Ph 5.5 5.5 Steam pressure, absolute - kg/cm 2 8.0 8.0 Steam temperature - °C 170 170 Time - min. 7 7 pH at end of prehydrolysis 4.6 4.6 Total sodium base chemicals on b.d. bagasse present during cooking Sodium bisulphite, NaHSO 3 - % by weight 3.9 -- Sodium sulphite, Na 2 SO 3 - % by weight -- 6.4 Sodium hydroxide, NaOH - % by weight 1.3 -- Sodium carbonate, Na 2 CO 3 - % by weight 1.3 1.6 Total chemicals as Na 2 O - % by weight 3.3 4.1 pH of cooking liquor 10.5 11.2 Cooking Steam pressure, absolute - kg/cm 2 8.0 8.0 Steam temperature - °C 170 170 Time - min. 7 7 Defibration Disc setting - mm 0.05 0.05 Consistency 36.1 35.9 pH after defibration 6.2 6.1 pH after centrifugal washer -- 6.3 Freeness - CSF ml 610 460 Brightness - % 26.5 27.3 Yield 86.4 70.4 ____________________________________________________________ ______________
TABLE X ____________________________________________________________ ______________ Properties of Paper Formed From Pulp of Examples 12 and 13 Example 12 Example 13 ____________________________________________________________ ______________ Example No. a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Disc setting at refining - mm -- 0.15 0.01 -- 0.0 0.05 Freeness - CSF ml 610 240 180 480 150 110 Sheet weight - g/m 2 107.2 96.0 97.8 97.0 99.5 99.2 Thickness - mm 0.294 0.203 0.200 0.261 0.205 0.186 Bulk - cm 2 /g 2.75 2.11 2.04 2.69 2.10 1.88 Bursting strength - kg/cm 2 0.80 1.40 1.70 0.60 1.52 1.77 Burst factor 7.5 15.0 17.0 6.2 15.3 17.8 Breaking length - m 1810 3280 3580 1750 2890 3430 Tear factor 63 52 45 56 50 49 Folding, load 800 g 2 6 7 2 4 6 Brightness - %% 26.5 27.6 27.1 26.9 27.6 27.3 ____________________________________________________________ ______________ 1 . Before pulping 2 . After pulping and refined at a disc setting of 0.15 3 . After pulping and refined at a disc setting of 0.01
The pulp produced by Examples 12 and 13 have similar brightness, but the yield of Example 12 is considerably greater than the yield of Example 13. The sodium sulfite pulping solution attacks the cellulose fiber thereby decreasing the yield. However in Example 12, where the sodium sulfite concentration is held to a minimum by the in-situ conversion of sodium bisulfite to sodium sulfite, there is an increased yield of about 16 percent. Further the properties of paper formed from the pulp of Example 12 are similar to or an improvement of those of a paper produced from the pulp of Example 13. The overall prime net advantage is a substantially improved yield for a paper which is pulped using a sodium bisulfite, sodium hydroxide and sodium carbonate pulping solution.
The process of this invention is capable of many modifications. These are, however, all within the scope of the present scope. For instance the prehydrolysis solution can contain some or all of the components of the pulping solution. That is, the pulping solution from the previous pulping batch can be used in the subsequent batch prehydrolysis. It is preferred, however, that the prehydrolysis consist only of an aqueous treatment. The other process steps are also capable of modification which will produce varying results depending on the type of lignocellulose material, times, temperatures and pressures used. These are all within the present concept of using fiber prehydrolysis to enhance the formation of sodium sulfite within the fiber which is being pulped.
A number of processes are known for pulping lignocellulosic materials (i.e., preparing a papermaking pulp from such materials). Of these, one of the most important is the so-called "chemical" pulping process which may be an alkaline or an acid process. A reasonably detailed general discussion of the chemical pulping processes appears in Kirk-Othmer's "Encyclopedia of Chemical Technology", Second Edition, Volume 16 (1968) beginning at page 702. At page 720 reference is made to alkaline pulping of bagasse and other cellulosic products other than the more normal wood raw material.
The fibrous fraction of sugarcane bagasse, like other lignocellulosic materials (e.g., wood, bamboo, etc.), contains as primary constituents cellulose, lignin and hemicellulose. The latter include both pentosans and hexosans, in amounts averaging about 12 weight percent pentosans (about 90 to 95 weight percent of which is xylan or xylosan) and about 16 weight percent hexosans (28 weight percent total hemicellulose) based on the total lignocellulosic content of a typical bagasse, on a dry weight basis. The relatively high hemicellulose content of the bagasse binds the cellulose fibers together and renders more difficult the production of suitable pulps, particularly due to the inability to break down the fiber bundles, thus resulting in an inordinate amount of shives in the paper made from such pulp. As a result it is generally considered impossible to prepare mechanical pulps from bagasse. Chemical pulping of bagasse is known and is widely practiced in various countries of the world. The application of the usual chemical pulping processes to bagasse suffers from the disadvantages of considerably reduced yields in view of the amounts of lignin and hemicelluloses removed and of poor brightness and poor opacity in paper produced from the resulting pulp.
The yield from a pulping process is the weight of washed fibers (dry weight basis) recovered after digestion expressed as a percentage of the weight of lignocellulosic material (dry weight basis) originally charged to the pulping process. An important desire in any chemical pulping process is to obtain the highest yields without adversely affecting resultant properties of paper products made from the pulp. While this is and has been the aim of innumerable prior art workers for many years it is believed that the present invention provides the first major step forward.
In accordance with the present invention it has been found that the pulp yield from chemical pulping of lignocellulosic materials, and particularly sugarcane bagasse, can be significantly increased through the combination of fiber, prehydrolysis and the use of a pulping medium consisting essentially of an aqueous solution containing alkali metal bisulfite and alkali metal hydroxide buffered to a pH of from about 8.5 to about 11.5, preferably at least about 10, using an alkali metal carbonate. The concentration of alkali metal bisulfite and alkali metal hydroxide is in the range to produce the following stoichiometry:
MHSO 3 + MOH ➝ M 2 SO 3 + HOH 8.5 - 11
wherein M represents the alkali metal portion of the compound. The preferred alkali metal is sodium, and the process will be described with reference to sodium forms of the compounds.
The pulping medium is one which produces sodium sulfite within the lignocellulose fibers by the reaction of sodium hydroxide and sodium bisulfite within the fiber. In order to increase the efficiency of this in-situ reaction to a sodium sulfite there should be a prehydrolysis of the lignocellulose fiber prior to pulping (digestion). The prehydrolysis increases the rate of penetration of the pulping chemicals into the lignocellulose fibers. With prehydrolysis, fiber penetration is faster, decreasing the overall chemical digestion time and thereby decreasing the time period the lignocellulose fibers are in contact with the sodium hydroxide and sodium bisulfite pulping chemicals. Pulping chemicals will to some degree attack the cellulose of the fiber and for this reason it is preferred that the time duration of pulping be a minimum necessary to produce an acceptable pulp. This results in a maximum pulp yield for a given pulp quality.
More particularly, the lignocellulosic material is subjected to a prehydrolysis step at a pH of from about 4.0 to about 5.8, preferably about 4.5 to about 5.5. At pH below about 4.0 or so the yield from the pulping process is significantly reduced -- below satisfactory economical levels. At higher pH about about 5.9, the desired prehydrolysis consumes excessive time, reduces yield and again is economically unattractive. Normally fresh bagasse will have a pH in the range of about 6.0 to 6.5 whereas stored bagasse, as a result of acetic acid produced by fermentation during storage, will have a pH in the range of about 4.5 to 5.0. The pH during the prehydrolysis step can be maintained within the desired limits of 4.0 to 6.0 or preferred limits of 4.5 to 5.5 in various ways, i.e., by mixing fresh and stored bagasse feed in suitable proportions, by controlling the amount of moisture mixed with the bagasse feed or feeds; by addition of minor amounts of pH adjusting chemicals such as acetic acid, through use of acid prehydrolysis media such as acid white water (pH about 5.5).
The prehydrolysis is conducted in the presence of from about 70 to about 100 weight percent water, based on the bonedry weight of lignocellulosic material fed. Additional water required may be present as liquid water or in the vapor form, i.e., steam; preferably as a liquid water-steam mixture which serves to maintain the appropriate temperature as well as furnish the necessary amounts of water. Liquid water may be blended with the lignocellulosic feed prior to introduction to the prehydrolysis reaction zone or in situ in the zone. Added steam is, of course, introduced into the prehydrolysis reaction zone.
The prehydrolysis reaction is conducted under autogenous steam pressure at temperatures maintained in the range of from about 320° to about 390°F. The desired prehydrolysis reaction can be achieved, under the stated conditions, in a relatively very short period of from about 5 to about 30 minutes.
Another part of this invention is the discovery that at the pH range of 8.5 to 11.5 sodium sulfite will form within the bagasse lignocellulose fiber from the penetration of the sodium hydroxide and sodium bisulfite. Sodium sulfite is a very effective pulping material, that is, it attacks the lignin content of the lignocellulosic fiber. A reagent which is a good pulping agent in that it attacks the fiber lignin content will also generally reduce the overall pulp yield due to some cellulose attack and thereby decrease the efficiency of the process. The prime object of this invention is to utilize sodium sulfite pulping, but to use a chemical form of sodium sulfite and a concentration of chemicals which will not adversely affect pulp yield. The use of in-situ fiber synthesis of sodium sulfite and of a relatively high pH yields a process which maximizes pulp yield while not adversely affecting the properties of the resulting paper. If the pH is not maintained in the range of 8.5 to 11.5 there is a decreased yield since there appears to be considerable sodium bisulfite fiber attack at lower pH's.
The stoichiometry of the reaction of sodium bisulfite and sodium hydroxide is essentially equimolar. However, in the present process, since the oxidic sulfur moiety of the sodium bisulfite is being continually dissipated by reaction with the lignin portion of the lignocellulose fiber to produce lignosulfonates, there should be an excess of sodium bisulfite available for reaction to produce further sodium sulfite. That is, upon reaction of sodium sulfite with lignin there results a free soda molecule which converts to caustic for further reaction with sodium bisulfite to produce sodium sulfite. By using this technique, the sodium sulfite concentration is kept to a minimum. It is also a very effective way to guarantee in-situ fiber synthesis of sodium sulfite. That is, the excess sodium bisulfite is present in the fiber when soda molecules become available from the reaction of sodium sulfite and lignin. These soda molecules readily react with sodium bisulfite to produce more in-situ sodium sulfite until the lignin or sodium bisulfite is exhausted. The sodium carbonate serves to raise and initially maintain the pH in the range of 8.5 to 11.5 and to increase the Na 2 O content of the digestion solution. It is important that the pH be fairly highly alkaline in order to inhibit sodium bisulfite attack of the fibers, and further that there be an excess of Na 2 O ions present to direct the equilibrium of the sodium bisulfite and sodium hydroxide reaction to sodium sulfite. The in-situ reactions of the digestion process are illustrated by the following equilibrium:
pH 1. NaHSO 3 + NaOH ➝ Na 2 SO 3 + HOH 8.5 - 11.5
As the lignocellulose fibers are digested the pH decreases slightly due to the reaction of some sodium ions with evolved organic acids and phenols. The pH, however, remains above about 8.5 and does not produce any serious adverse effect as to fiber yield or product quality. The initial Na 2 O content is preferably about 10 to 14 percent by weight and ideally about 12 percent by weight.
The digesting medium should have a weight ratio of lignocellulosic materials (dry weight basis) to digestion solution of between about 1/6 and 1/2, and preferably about 1/4. There will therefore be about 2 to 6 grams of digestion liquor per gram of fiber. The chemical composition of this digestion solution will be such as to contain about 0.5 to 2 moles of sodium bisulfite per liter of solution. The sodium hydroxide content will be in the range of 0.3 to 1.3 moles per liter of digestion solution. It is preferred that there be 1.1 to 3.0, and particularly about 1.5, moles of sodium bisulfite per mole of sodium hydroxide. The content of sodium carbonate will be an amount sufficient to increase the pH of the fiber to about 8.5 to 11.5. The pH of input of prehydrolyzed lignocellulosic fiber is usually about 4.5 to 6.5. In most instances a gram amound of sodium carbonate equivalent to the gram amount of sodium hydroxide will be sufficient to produce this pH. There should therefore be about 0.15 to 0.5 moles of sodium carbonate per liter of digestion solution. The digestion temperature is preferably from about 320°F to 390°F and the digestion time is usually about 5 to about 30 minutes and preferably about 10 to 20 minutes and the pressure at a steam gauge pressure of about 75 to about 150 pounds per square inch.
After digestion the pulp is refined, screened, washed, etc., in the usual manner prior to use as furnish to the paper making machine.
If desired the process of this invention may include as an optional step the addition to the pulp of about 0.8 to about 1.3, preferably about 1.0 weight percent of an alkali metal silicate, based on the bone-dry weight of the initial bagasse fiber fee. The alkali metal silicate or a portion thereof may be added to the pulp separately from the digestion treatment prior to release of the digestion pressure. Preferably, all of the alkali silicate is added in the digester blow line or at the digester blow valve, just prior to final completion of the digestion reaction.
In practice a sodium silicate, such as sodium orthosilicate, sodium sequisilicate or sodium metasilicate, is the preferred alkali metal silicate when such is used. Sodium silicate is available commercially with various ratios of Na 2 O:SiO 2 such as, sodium orthosilicate having a ratio of Na 2 O:SiO 2 of 2:1, sodium metasilicate having a ratio of Na 2 O:SiO 2 of 1:1, sodium sesquisilicate having a ratio of Na 2 O:SiO 2 of 3:2, as well as other commercial sodium silicate products having various ratios of Na 2 O:SiO 2 such as 1:2, 1:3.2, etc. However, pulps which exhibit satisfactory color and brightness properties may be obtained by using other alkali metal silicates such as potassium silicate and lithium silicate, and, in general, it is contemplated that all of the various forms of the alkali metal silicates such as potassium orthosilicate, potassium metasilicate, lithium orthosilicate and lithium metasilicate will function in a manner which, in this respect, is similar to sodium orthosilicate or sodium metasilicate. Commercially available potassium silicates having ratios of K 2 O:SiO 2 corresponding to those for sodium silicates are useful, as are those having ratios of K 2 O:SiO 2 of 1:2.1, 1:2.2, 1:2.5, etc.
The process of this invention is most desirably conducted in a continuous manner, using for this purpose pressure vessels known in the pulp and paper industry as continuous digesters (see, for example, Kirk-Othmer's "Encyclopedia of Chemical Technology", Second Edition, Volume 16 (1968), pages 700-701, and Rydholm "Pulping Processes", Interscience Publishers (1965), pages 343-355); suitably modified, where required, to permit introduction of treatment chemicals at the appropriate point or points in the process.
The invention is specifically useful for pulping of sugarcane bagasse. However, it is not limited to bagasse as the lignocellulosic feed material, but is applicable as well to other materials such as soft or hard woods, tropical woods, bamboo, various straws, hemp, sisal and other lignocellulosic products.
In the application of the invention to pulping of bagasse the bagasse fibers used should be as pith-free as is reasonably possible. Suitable bagasse fiber feed materials may be prepared, for example, via the use of the apparatus and/or methods as described in U.S. Pat. No. 3,537,142 or U.S. Pat. No. 3,688,345. For highest yields it is preferred that the bagasse fiber feed material before prehydrolysis be one which has been subjected to a two-stage depithing operation, i.e., first dry depithed in accordance with the aforesaid U.S. Pat. No. 3,537,142, and then further wet depithed in the presence of at least about 4.5, normally from about 5 to about 10, parts by weight of water per part by weight of fiber (bone-dry basis) in accordance with U.S. Pat. No. 3,688,345; the entire disclosure of which is incorporated herein by reference.
Practice of the invention is illustrated by the following specific but nonlimiting examples. In the examples the properties shown for the pulp and for paper samples prepared therefrom are all determined by standard TAPPI procedures. For example, the K-number of the pulps is the potassium permanganate number, a standard TAPPI test for determination of residual lignin. Tensile strength of the paper samples is the tensile breaking length, in meters, determined on an Instron tensile tester with a specimen 1-inch wide, 4 inches long (between test jaws) and at a jaw separation rate of 1 inch per minute.
EXAMPLE 1
In this example pulp was prepared from bagasse fibers using a commercially available continuous two tube digester from Pandia, Inc. Water is added to the fiber feed in a wetting tank, and the wetted mass is screw fed to the digester. Total average residence time in the digester during the run was about 26 minutes. Digested pulp is sent to an Asplund refiner and thence to a blow tank where pressure is reduced to 0.5 pounds per square inch gauge. The stock is diluted in the blow tank and then pumped through subsequent processing equipment (a cleaner, a centrisorter and three sequentially connected washers ) and finally to storage tanks where it is held until fed to a papermaking machine.
The digester was adapted for introduction of digestion chemicals and/or other additives at various points along the length of the line of travel through the digester. In this run provision was made for introduction of digestion (cooking) chemicals at points 1/4 to 1/3 from the beginning of the total digester length and for introduction of sodium silicate at the end of the digester, before the digested pulp passed through the Asplund refiner.
In this example the bagasse fiber feed to the digester was wet depithed bagasse prepared in accordance with U.S. Pat. 3,688,345. The fiber had a moisture content of about 50 percent and contained about 23 percent lignin and 26 percent total hemicelluloses on a dry weight basis.
The bagasse fibers were fed to the digester at a rate of about 7.5 tons per hour (bone-dry basis) and were prehydrolyzed at steam pressures of 125 pounds per square inch gauge (about 345°F) in the initial 1/4 to 1/3 portion of the digester with clear white water having a pH of 5.5 in amounts sufficient to provide about 80 weight percent water in the prehydrolysis zone, based on bone-dry weight of the fiber feed. The pH of the digestion chemicals through the introduction means referred to previously. In this run the digestion chemicals used comprised an aqueous solution having a pH of 11 and containing 46 grams per liter caustic soda (NaOH), 185 grams per liter sodium bisulfite and 19 grams per liter sodium carbonate, which was introduced to the digester at a rate of 24.8 gallons per minute. At the exit end of the digester a 4 percent aqueous solution of sodium metasilicate was fed into the pulp at a rate of one gallon per minute before the pulp is passed through the Asplund refiner and thence to the blow tank.
The run was conducted for an operating period of about 13 hours under the conditions stated, during which period the yield from the digester was 60-65 percent. The pulp produced had a G. E. brightness of 58-60.
In another run conducted under like conditions, except that no silicate was added at any time, the resultant pulp had a G. E. Brightness not greater tha 45-46.
EXAMPLES 2-4
In the following Examples the lignocellulosic material to be pulped was depithed sugarcane bagasse prepared in the apparatus and with the method described in the aforementioned U.S. Pat. No. 3,537,142. Three separate runs were made using the cooking solutions having compositions as shown in Table I below, with pH varying from 9 to 11. All digestions (cooks) were conducted at a ratio of about 4 parts by weight cooking liquor for each part by weight (dry weight basis) of lignocellulosic material fed. The digestions were each performed for 30 minutes in a closed vessel at a steam pressure of 100 to 110 pounds per square inch gauge. Just prior to discharge to the blow tank the test pulps were treated with 1 weight percent sodium silicate. After blowdown the pulps were cleaned, centrifuged and washed in the usual manner. Thereafter, the beating or refining times of the pulps to various Canadian Standard freeness were determined, and certain properties of papers prepared from the beat pulps were tested. For comparison purposes a pulp was prepared from the same sugarcane bagasse fibrous material in the same manner as described above using caustic soda (sodium hydroxide) as the only alkaline digestion chemical and without any added sodium silicate; and properties of samele papers of this pulp were also determined.
Results of the tests are shown in the following Tables II and III.
TABLE I ______________________________________ Cooking Solutions Weight Percent (as Na 2 O) Example Example Example Comparative Cooking Chemical 2 3 4 Run A ______________________________________ NaHSO 3 6.85 6.80 6.55 -- Na 2 CO 3 1.38 1.38 1.22 -- NaOH 3.78 3.83 4.26 10.5 Total Na 2 O 12.01 12.01 12.03 10.5 pH 9 10 11 -- ______________________________________
TABLE II ______________________________________ Pulp Properties Example Example Example Comparative 2 3 4 Run A ______________________________________ Pulp Yield (weight %, dry weight basis) 68.1 67.0 65.2 56.3 K Number 25.6 25.5 24.6 23.5 Whiteness (% G.E.) 34.0 36.5 41.0 27.0 ______________________________________
TABLE III ____________________________________________________________ ______________ Physical Properties of Paper Samples Canadian Basis Standard Weight Density Beating Freeness (Ovendry- Tensile (Grams per Time (Milli- Grams per Tear Break Burst Cubic Fold Pulp From (Minutes) liters) Square Meter) Factor (Meters) Factor Centimeter) Number ____________________________________________________________ ______________ Example 2 0 785 -- -- -- -- -- -- 20 555 63.5 91.0 2662 13.1 0.31 15 28 425 60.6 73.3 3600 19.0 0.36 16 36 360 59.6 67.0 4140 22.0 0.41 18 44 285 59.6 59.7 4643 25.0 0.41 30 55 215 59.6 52.2 4935 28.7 0.48 40 Example 3 0 733 -- -- -- -- -- -- 8 520 64.6 68.8 3880 22.8 0.38 28 16 425 64.6 62.0 4954 27.5 0.41 32 24 335 64.1 57.3 5387 29.1 0.44 56 32 235 64.1 52.0 5700 32.5 0.48 60 40 185 64.1 48.5 6073 34.7 0.48 81 Example 4 0 760 -- -- -- -- -- -- 7 675 59.6 67.1 4330 19.8 0.35 21 14 440 59.6 59.6 5147 28.8 0.41 22 21 335 59.6 52.2 5700 31.7 0.44 36 28 245 61.1 47.2 6273 34.5 0.45 58 35 175 61.1 43.6 6710 36.5 0.50 243 Comparative 0 760 -- -- -- -- -- -- Run A 8 615 59.6 63.4 4140 18.6 0.38 10 16 485 59.6 59.7 4923 24.6 0.41 20 24 360 59.6 56.0 5483 28.5 0.44 22 32 255 59.6 52.2 6042 32.0 0.48 62 40 175 59.6 48.5 6423 35.2 0.53 104 ____________________________________________________________ ______________
Graphical plotting of the data shown in Table III above indicates that the pulps and paper samples have the representative properties shown in the following Table IV at equivalent Canadian Standard Freeness.
TABLE IV ______________________________________ Comparison at Equivalent Canadian Standard Freeness At Canadian Standard Comp- Freeness of 360 Example Example Example arative Milliliters 2 3 4 Run A ______________________________________ Beating Time 36.0 21.5 19.5 24.0 Tear Factor 67.0 58.5 54.5 56.0 Tensile Break 4140 5300 5550 5480 Burst Factor 22.0 29.0 31.0 28.0 ______________________________________ At Canadian Standard Comp- Freeness of 215 Example Example Example arative Milliliters 2 3 4 Run A ______________________________________ Beating Time 55.0 36.0 31.0 36.0 Tear Factor 52.2 50.5 45.5 50.5 Tensile Break 4935 5940 6450 6230 Burst Factor 28.7 33.0 35.3 33.5 ______________________________________
As seen from the foregoing Examples the method of this invention provides yield increases of up to 22 percent as compared to use of caustic soda alone as the cooking chemical (comparing yield data from Example 2 and Run A). Except for Example 2, the beating times to equivalent Canadian Standard Freeness are the same or less than in Comparative Run A. In all of Examples 2 through 4 a much greater pulp brightness results, especially in Example 4. The overall physical properties of papers prepared from the pulps of Examples 3 and 4 are good to excellent at pulp yield increases of approximately 16 to 20 percent as compared to Run A.
EXAMPLES 5-11
In the following Examples 5-11 trial runs of the alkaline pulping process of this invention were conducted in a Scandinavian pilot unit. The lignocellulosic raw material was the fibrous fraction of sugarcane bagasse prepared in Latin America by a primary dry depithing followed by a secondary wet depithing (as described in U.S. Pat. No. 3,688,345) and shipped to Scandinavia for the tests. The material was air dried to an average moisture content of 40 percent prior to baling for shipment. As analyzed at the depithing plant it had a lignin content of 17.8 percent and a pentosans content of 22.9 percent, and gave the following solubles analyses:
Weight Percent Solubles In Cold Water 2.91 In Hot Water 8.46 In 1 percent NaOH 28.80 In Alcohol/Benzene
In these runs the test equipment used was an Asplund Defibrator unit, Model No. OVP-20, disc type 5821 running at a disc setting of 0.05 millimeters at 1500 revolutions per minute and equipped with a horizontal preheater. The lignocellulosic raw material was fed to the horizontal preheater by a two pocket rotary feeder. When prehydrolysis was used the prehydrolysis time was fixed by adjusting the speed of the transport screw in the horizontal preheater. Cooking liquor was added to the material at the end of the horizontal preheater and the raw material then falls down into a vertical tapered digester where cooking time was controlled by a radiation level gauge. The material was heated by direct steam in the horizontal preheater and in the vertical digester. In addition, the digester is equipped with a steam jacket in order to minimize condensation. At the bottom of the digester the digested product is fed to an Asplund defibrator by an agitator and a screw conveyor. As is known in the art the defibrator works on the pulp while it is still under digester pressure. Defibrated pulp was either blown directly to and refined in an Asplund refiner, or collected unrefined from the cyclone which receives material from the defibration.
All runs were conducted at 170°C during prehydrolysis (when used) and during digestion, using steam at an absolute pressure of 8 kilograms per square centimeter. All cooking liquors were prepared from technical grade chemicals which were dissolved in water in ratios providing a pH of about 10.5. Specific compositions are shown in Table V. Sodium silicate, when added, was introduced in the defibrator screw conveyor just prior to defibration as an aqueous solution of sodium silicate pentahydrate (Na 2 O.SiO 2.5 H 2 O) having a concentration of 100 grams per liter.
The bagasse was presoaked in water or in the cooking liquor prior to processing. Total processing time was 14 minutes in each run. This was equally divided into a 7 minute prehydrolysis and 7 minute digestion when prehydrolysis was used. Pulping data are shown in Table VI. Kappa numbers were determined by duplicating the runs in a laboratory scale Asplund Defibrator on a 300 gram (bone-dry basis) sample of the bagasse and weighing the amount of recovered pulp after washing and air drying.
The pulps were refined at a consistency of 20 percent and pulp samples collected at two different disc settings. The refined pulps were washed on a 100 mesh wire cloth and then sheet formed according to Swedish standard method CCA-17 and tested according to SCAN methods.
Results are shown in Table VII.
TABLE V ______________________________________ Cooking Solutions Concentration (grams per liter) Cooking Chemical Examples 5,6,7,8 Examples 9,10,11 ______________________________________ NaHSO 3 100 60 Na 2 CO 3 35 20 NaOH 35 20 pH 10.5 10.3 ______________________________________
TABLE VI ____________________________________________________________ ______________ Pulping Data Example Example Example Example Example Example Example 5 6 7 8 9 10 11 ____________________________________________________________ ______________ Presoaking Liquor Water Water Water Water Cooking Cooking Cooking Solution Solution Solution pH after Presoak 5.5 5.5 5.5 5.5 10.3 10.3 10.3 Prehydrolysis Time (minutes) 7 7 7 7 None None None pH after Prehydrolysis 4.6 4.6 4.6 4.6 -- -- -- Total Na 2 O in Cooking Solution (as weight percent of Bagasse Fibers, bone dry) 3.8 3.3 10.3 11.3 3.4 9.4 9.4 Sodium Silicate Added Yes No Yes No Yes Yes No Total Chemicals as Na 2 O, including Silicate, in Weight Percent of bone -dry Bagasse 4.1 3.3 10.9 11.3 4.0 9.9 9.4 Digestion Time (minutes) 7 7 7 7 14* 14* 14* pH after Defibration 6.7 6.2 9.5 9.3 6.8 9.7 9.7 Yield (percent) 86.6 86.0 80.8 80.6 85.1 72.1 71.4 Kappa Number 93.8 95.8 33.4 43.7 92.2 25.3 19.4 Brightness, SCAN 28.4 26.5 30.2 32.7 22.5 35.7 38.0 ____________________________________________________________ ______________ *No separate prehydrolysis. Bagasse soaked in cooking liquor prior to introduction to horizontal preheater.
TABLE VII ____________________________________________________________ ______________ Paper Properties Example 5 Example 6 Example 7 Example 8 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Canadian Standard Freeness (milli- liters) 480 250 120 610 240 180 350 260 110 380 230 110 Sheet Weight (grams per square meter) 104.3 91.5 93.1 107.2 96.0 97.8 101.3 99.5 106.9 108 93.1 97.6 Thickness (milli- meters) 0.282 0.184 0.180 0.294 0.203 0.200 0.174 0.135 0.141 0.171 0.135 0.131 Bulk (cubic centi- meters per gram) 2.70 2.01 1.93 2.75 2.11 2.04 1.72 1.36 1.32 1.58 1.45 1.34 Burst Strength (kilograms per square centimeter) 0.9 1.5 1.7 0.8 1.4 1.7 2.9 4.3 4.6 3.4 3.7 4.3 Burst Factor 8.2 16.8 18.7 7.5 15.0 17.0 28.4 43.0 43.4 31.3 39.3 44.3 Tensile Break (meters) 1950 3980 4780 1810 3280 3580 4920 6600 6750 5180 6130 6790 Tear Factor 66 49 47 63 52 45 68 51 48 65 57 52 Folding (Kohler- Molin, 800 gram load) 4 10 8 2 6 7 120 150 565 190 125 320 Brightness, SCAN 28.4 29.1 28.8 26.5 27.6 27.1 30.2 37.3 28.0 32.7 33.8 32.0 Opacity, SCAN (60 grams per square meter) 96.3 98.0 97.3 98.2 98.0 97.7 86.1 84.8 87.7 84.7 83.7 81.6 ____________________________________________________________ ______________ 1 Before refining. 2 After pulp was refined at disc setting of 0.10 millimeters (Exs. 5 9) 0.15 millimeters (Exs. 6, 10, 11) or 0.20 millimeters (Exs. 7, 8). 3 After pulp was refined at disc setting of 0.01 millimeters (Exs. 5 6, 9, 10, 11) or 0.05 millimeters (Exs. 7, 8).
Example 9 Example 10 Example 11 a 1 b 2 c 3 a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Canadian Standard Freeness (milliliters) 580 290 170 430 250 170 450 215 180 Sheet Weight (grams per square meter) 103.0 106.6 107.5 103.4 95.4 97.0 105.0 97.3 106.6 Thickness (millimeters) 0.228 0.195 0.192 0.150 0.125 0.126 0.145 0.126 0.134 Bulk (cubic centimeters per gram) 2.21 1.83 1.79 1.45 1.31 1.30 1.38 1.29 1.26 Burst Strength (kilograms per square centimeter) 1.2 1.9 1.8 3.9 4.0 4.3 4.1 4.2 4.3 Burst Factor 11.9 17.2 16.9 37.3 42.1 44.5 39.0 43.5 40.2 Tensile Break (meters) 2440 3440 3500 5370 6350 7020 5910 6360 5925 Tear Factor 65 53 49 58 48 46 55 48 52 Folding (Kohler-Molin, 800 gram load) 6 8 7 110 290 265 130 160 245 Brightness, SCAN 22.5 21.6 20.6 35.7 36.8 36.1 38.0 37.5 37.0 Opacity, SCAN (60 grams per square meter) 97.9 98.1 98.4 84.4 83.6 85.4 86.0 84.6 84.6 ____________________________________________________________ ______________ 1 Before refining. 2 After pulp was refined at disc setting of 0.10 millimeters (Ex. 5, 9) 0.15 millimeters (Exs. 6, 10, 11) or 0.20 millimeters (Exs. 7, 8). 3 After pulp was refined at disc setting of 0.01 millimeters (Exs. 5 6, 9, 10, 11) or 0.05 millimeters (Exs. 7, 8).
Graphical plotting of the data shown in Table VII indicates that the paper samples have the representative properties shown in the following Table VIII if compared at equivalent Canadian Standard Freeness levels.
TABLE VIII ____________________________________________________________ ______________ Comparison at Equivalent Canadian Standard Freeness ("CSF") At CSF of 350 Example No. milliliters 5 6 7 8 9 10 11 ____________________________________________________________ ______________ Burst Factor 14.5 13.2 31.0 33.6 16.0 39.0 41.0 Tensile Break 3350 2875 5250 5400 3200 5850 6100 Tear Factor 54 57.5 64 63.8 53 55 41.9 At CSF of 250 milliliters Brust Factor 17.0 15.2 38.0 38.1 17.5 42.0 42.6 Tensile Break 4000 3300 6050 6000 3400 6400 6290 Tear Factor 50 52.7 56 58.7 49 52 49 ____________________________________________________________ ______________
As seen from the foregoing trial runs the greatest pulp yields were obtained when using lower proportions of total alkaline chemicals charged to the process, resulting in substantially neutral pH 6.2-6.8 at the end of the cook (Examples 5, 6 and 9). The yields were substantially the same and the Kappa number of the pulps were substantially the same at these lower proportions of alkaline chemicals to bagasse whether the lignocellulosic feed material was prehydrolyzed (Examples 5 and 6) or not prehydrolyzed (Example 9). The burst factor, tensile breaking length and tear factor of papers prepared from the runs in these three Examples were with minor exceptions considerably poorer than in the other four Examples (i.e., Examples 7, 8, 10 and 11).
As seen from Examples 10 and 11, when a higher proportion of total alkaline chemicals was used, without any prehydrolysis, the final pH at the end of the cook was considerably higher (i.e., 9.7). The yields were still greater than 70% (as compared to 55% or so in current industry practice using only NaOH for cooking) and there was a striking decrease in Kappa number to the range of 20-25 (indicating much greater ease in refining). As indicated in Table VIII sample papers made from the pulps prepared in these runs have excellent overall properties.
Examples 7 and 8 demonstrate the desirability of combining prehydrolysis with the higher proportions of alkaline chemicals. Comparing these two Examples with Examples 10 and 11 it is seen that prehydrolysis provided an incremental increase of about 8 percent in the pulp yield while still retaining a significantly lower Kappa number (as compared to Examples 5, 6 and 9). As again indicated by Table VIII the overall physical properties of papers prepared from the pulps of Examples 7 and 8 are very good to excellent.
From Examples 5 to 11 it is concluded that the process of this invention is most desirably conducted with a prehydrolysis step and with sufficient amounts of the alkaline chemicals (about 8 to 12 weight percent, e.g., 9 to 11 weight percent total Na 2 O based on dry weight of fiber feed) to provide a pulp pH greater than about 9 at the end of the cook if one wishes to obtain the optimum overall combination of yield and paper properties. The optimum properties, with somewhat reduced but still very good yield, are obtained with like proportions of total chemicals but without a prehydrolysis step.
EXAMPLES 12-13
In the following Examples 12 and 13 trial runs were made comparing a pulping solution containing sodium bisulfite, sodium hydroxide and sodium carbonate to a pulping solution containing sodium sulfite and sodium carbonate. The feed bagasse was the same as in Examples 5-11, as was the pilot plant unit. No sodium silicate was added to the pulped bagasse to improve the brigntness. The specific pulping operating conditions and results are shown in Table IX.
TABLE IX ____________________________________________________________ ______________ Example 12 13 ____________________________________________________________ ______________ Presoaking liquor Water; H 2 O % by weight 100 100 Ph of bagasse after soaking 5.5 5.5 Dryness of bagasse after prescrew press % by weight water 40 40 Prehydrolysis Ingoing Ph 5.5 5.5 Steam pressure, absolute - kg/cm 2 8.0 8.0 Steam temperature - °C 170 170 Time - min. 7 7 pH at end of prehydrolysis 4.6 4.6 Total sodium base chemicals on b.d. bagasse present during cooking Sodium bisulphite, NaHSO 3 - % by weight 3.9 -- Sodium sulphite, Na 2 SO 3 - % by weight -- 6.4 Sodium hydroxide, NaOH - % by weight 1.3 -- Sodium carbonate, Na 2 CO 3 - % by weight 1.3 1.6 Total chemicals as Na 2 O - % by weight 3.3 4.1 pH of cooking liquor 10.5 11.2 Cooking Steam pressure, absolute - kg/cm 2 8.0 8.0 Steam temperature - °C 170 170 Time - min. 7 7 Defibration Disc setting - mm 0.05 0.05 Consistency 36.1 35.9 pH after defibration 6.2 6.1 pH after centrifugal washer -- 6.3 Freeness - CSF ml 610 460 Brightness - % 26.5 27.3 Yield 86.4 70.4 ____________________________________________________________ ______________
TABLE X ____________________________________________________________ ______________ Properties of Paper Formed From Pulp of Examples 12 and 13 Example 12 Example 13 ____________________________________________________________ ______________ Example No. a 1 b 2 c 3 a 1 b 2 c 3 ____________________________________________________________ ______________ Disc setting at refining - mm -- 0.15 0.01 -- 0.0 0.05 Freeness - CSF ml 610 240 180 480 150 110 Sheet weight - g/m 2 107.2 96.0 97.8 97.0 99.5 99.2 Thickness - mm 0.294 0.203 0.200 0.261 0.205 0.186 Bulk - cm 2 /g 2.75 2.11 2.04 2.69 2.10 1.88 Bursting strength - kg/cm 2 0.80 1.40 1.70 0.60 1.52 1.77 Burst factor 7.5 15.0 17.0 6.2 15.3 17.8 Breaking length - m 1810 3280 3580 1750 2890 3430 Tear factor 63 52 45 56 50 49 Folding, load 800 g 2 6 7 2 4 6 Brightness - %% 26.5 27.6 27.1 26.9 27.6 27.3 ____________________________________________________________ ______________ 1 . Before pulping 2 . After pulping and refined at a disc setting of 0.15 3 . After pulping and refined at a disc setting of 0.01
The pulp produced by Examples 12 and 13 have similar brightness, but the yield of Example 12 is considerably greater than the yield of Example 13. The sodium sulfite pulping solution attacks the cellulose fiber thereby decreasing the yield. However in Example 12, where the sodium sulfite concentration is held to a minimum by the in-situ conversion of sodium bisulfite to sodium sulfite, there is an increased yield of about 16 percent. Further the properties of paper formed from the pulp of Example 12 are similar to or an improvement of those of a paper produced from the pulp of Example 13. The overall prime net advantage is a substantially improved yield for a paper which is pulped using a sodium bisulfite, sodium hydroxide and sodium carbonate pulping solution.
The process of this invention is capable of many modifications. These are, however, all within the scope of the present scope. For instance the prehydrolysis solution can contain some or all of the components of the pulping solution. That is, the pulping solution from the previous pulping batch can be used in the subsequent batch prehydrolysis. It is preferred, however, that the prehydrolysis consist only of an aqueous treatment. The other process steps are also capable of modification which will produce varying results depending on the type of lignocellulose material, times, temperatures and pressures used. These are all within the present concept of using fiber prehydrolysis to enhance the formation of sodium sulfite within the fiber which is being pulped.