US2301314A - Process of making alpha cellulose - Google Patents

Process of making alpha cellulose Download PDF

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US2301314A
US2301314A US691035A US2301314A US 2301314 A US2301314 A US 2301314A US 691035 A US691035 A US 691035A US 2301314 A US2301314 A US 2301314A
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pulp
process
hours
wood
viscosity
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Ralph E Montonna
Lew W Cornell
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University of Minnesota
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University of Minnesota
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    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21CPRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C3/00Pulping cellulose-containing materials
    • D21C3/22Other features of pulping processes
    • D21C3/26Multistage processes

Description

Patented Nov. 10, 1942 PROCESS OF MAKING ALPHA CELLULOSE Ralph E. Montonna, St. Paul, and Lew W. Cornell,

Minneapolis, Minn., assignors to Board of Rfigents of the University of Minnesota, Minneapolis, Minn., a corporation of Minnesota No Drawing. Application February 16, 1935, Serial No. 6,910

16 Claims.

This invention relates generally to the production of high grade cellulose, and particularly to the production of alpha cellulose of uniform quality, and any desired viscosity.

Objects of the invention are: to obtain high yields of alpha cellulose of high viscosity from wood in general, and from hard woods in particular; to obtain a uniform product; to operate in such manner that the viscosity can be controlled; to provide an open vessel process so that the process can be observed and controlled, and so tests can frequently be made; to provide a process by which a white pulp'can be obtained without substantial reduction in the degree of viscosity, compared to other processes known to us; to use wasteacid from a prevoius run mixed with fresh dilute nitric acid, the waste acid acting as a catalyst to initiate and further reactions, leading to uniform pulping action and uniform yield and quality of alpha cellulose. This last mentioned action is believed to be due to the presence of nitrites in the waste liquor and serves to initiate our dilute acid process.

The main object of this invention is to produce alpha cellulose of uniform, and any desired viscosity from wood. An additional object is to produce such a product by treatment of hard wood, particularly aspen.

Another object is to product alpha cellulose which approximately is 98% pure; to produce cellulose of high viscosity from hard woods or short fiber woods; and to operate with a single or multiple stage process to obtain the high quality product mentioned above.

The following practicable examples are given for purposes of illustration and, although each is claimed, there is no intention to limit the broader aspects of our invention to one of these examples.

Example I liquid, and evaporation losses shouldbe made up by addition of water during steeping. Circulation of the steeping liquor is desirable. Continue this steeping for about 30 hours at about 70 C. Then steam the chips for forty-five minutes, after draining. The condensate is saved and re-used,

when the multiple stage process is practiced. Then rinse the steamed chips (preferably with hotas well as cold water) and-cook at atmospheric pressure for about two hours in 2% sodium hydroxide. At this stage mild mechanical treatment, such as 'heating, yields good pulp which may be washed in the usual way. The crude pulp obtained as above contains about 91-93% alpha cellulose, has about 0.20.25% ash, has a 10% KOH solubility of about 10-l3%, and a copper number of about 1.0. The cupramznonium viscosity is high. The degree of viscosity obtained at this stage very much more nearly approaches the degree of viscosity obtained in cotton, and in our experience is very much higher than ever has been obtained at this stage by any pulping process known to us. The viscosity as obtained .by the practice of this invention ranges from about 25 to 40 poises. The crude pulp made from aspen wood is easy-bleaching and has high viscosity, and is a feature of this invention.

To produce a high grade alpha cellulose, give the pulp a preliminary bleach with chlorine using an amount approximately 0.6% to 0.75% of the weight of the air-dried pulp as chlorine, preferably adding the bleach in two or three portions over a period of about one hour. Then treat the pulp with nine to ten times its dry weight of 6% sodium hydroxide at ordinary temperatures for ten to twenty minutes. As a final bleach, treat the pulp with sodium hypochlorite inan amount approximately 0.15% of the dry weight of the pulp for two hours. The final product contains about 98% alpha cellulose, about 0.1% or less ash, has a 10% KOH solubility of about 3 /2% to 6% and a copper number of about 0.5. The viscosity is high, ranging from 10 to 13.6 poises, when determined by the falling-ball method in a solution of the following composition: 20 grams of cellulose per liter; 30 grams of copper per liter; 165 grams of ammonia (NH3) per liter; 10 grams of sugar per liter.

A method for the determining of the cuprammonium viscosity is as follows: The cuprammonium solution is first prepared using for example,

about 31.1 grams of copper, 171.3 grams of NH3 and 10 grams of sugar, per liter. The solution is made by pouring concentrated NH: solution over copper turnings and then bubbling air upwardly therethrough, the container being maintained at Ill-20 C. The correct concentration is then obtained by adding ammonia water. Ammonia is titrated directly with 2 normal H280; and methyl red indicator. The copper is determined by the Law modification of the De Haens about 2 mm. Hg pressure bulb is about two hours.

2 iodide method. Before final adjustment of con-- centration the cuprammo um solution is allowed] to stand for several days. It is then decanted and filtered to remove as much suspended copper or copper oxide as possible. The correct concentration is then obtained by addingammonia water. Ammonia is titrated. 4

In testing, the pulp is air dried at 40 0. A moisture determination is made. 1.96 bone dry grams of air dried material are weighed and placed in a dissolving bulb. 4 cc. of distilled water is added. The bulb is then tightly stoppered and allowed to stand for three hours. The bulb is then attached to the charging apparatus and opened at each end. N2 is blown through at for 60 seconds. By means of a three-way cock, the bulb is alternately evacuated and filled with N2, three times. The then left under a vacuum, connected with a pipette, and 97 cc. of cuprammonium solution is added. N2 is added to bring the pressure up to atmospheric. The dissolving bulb is tightly closed with screw clamps and the .rubber stoppers wired on and sealed with beeswax. .Then kept in an ice box at 50 F. for 12 hours. Then agitated 24 hours at 25 C., removed and placed in a thermostat at 30 for 1 hours. The bulb is then shaken by hand to insure homo eneity. It is then attached to a ring stand. The up er end is attached to a source of N2 and about 15 to 20 cc. of solution is purged from the lower end. The lower end is then connected by a tube. The whole is then transferred to the thermostat. The tube is set vertically by means of a plumb. The solution is then forced into the viscosity tube. A strong light source is placed back of the viscosity tube. and after five minutes.

fine 3 mm. glass balls are dropped through the the same operating conditions will have the same characteristics of viscosity, 10% KOH solubility short rubberf tube (closed with a pinch clamp) to the viscosity and copper number, that is the same characteristics as given by any commonly used laboratol'i criteria. The ability to get uniformity is closely related to operation in an open vessel.

Example II 1 in approximately the same ratio of wood to honor as in Example I, for about thirty hours at about 75 C. Steam and cook as in Example I. For preliminary bleachin use chlorine equal to about 1 /4%' of the weight of the air dried pulp, operating as in Example I. Apply caustic treatment as above with 6-8% NaOI-I. For the final bleaching, use sodium hvpochlorite equal to about 0.2% of the dry weight of the pulp, operating for This viscosity is medium and ranges from 4.6 to 5.9 poises, which issomewhat higher than the best commercial alpha cellul made from wood.

. To produce a pulp of low viscosity the following modifications are necessary: Steep with ii 2% nitric acid for about thirty hours at C. Steam and cock as Example I. For preliminary bleaching, use chlorine equal to about 2% of the weight of air dried pulp operating as in Example I. Treat as in Example I with 8% to 10% NaOH. For the final bleach, use sodium hypochlorite in an amount equal to about 0.3% of the dry weight of the pulp for about two hours.

An important feature of this invention is to add to the raw steep liquor about 2% to 5% by volume of waste acid from a previous run, and this applies to all three of the preceding examples.

It occasionally happens that. with a low-concentration of acid even at a relatively high temperature, the reaction (clearly detectable by change in color and odor) may not start. We have found that the addition of a small amount of waste acid from a previous run initiates the chemical reaction. In other words, we have discovered that we can catalyze the reaction by the use of a small amount of waste acid from a previous run. We believe that this catalytic action is due to nitrites in the waste liquor. This insures uniformity and insofar as we know he new discovery. Uniformity is more important than any other one thing, and we obtain uniformity in A unique feature is the mildness of the cooking obtainable, for example, by cooking in an open vessel, so that the process can be observed and controlled at all times. This does not preclude the use of vacuum or low pressure, or any other suitable operation for obtaining a mild cook. We have also found that uniformity cannot be obtained as readily with a high concentration acid process, as with a low concentration acid process.

The following is given as an example of our multiple stage process:

Example IV In this case the use of 4% nitric acid gives excellent results by steeping the comminuted wood for about thirty hours at about 60- C.: with three or four stages. In a four-step steep, the acid is used as follows: For steep I, drainings from steep II of a previous run are combined with steam condensate from a previous run. For steep II, drainings from steep III of a previous run are used. For steep III, drainings from steep IV of a previous run are used, and for steep IV, fresh acid is used. In each case, the total acidity (with phenolphthalein) of the used acids, expressed at HNO3, is brought up to 4% by the addition of fresh HNOs. If the total volume of acid is too low, this is brought up to the desired value. Steep II to IV require only about five-sixths of the amount of dilute acid needed for steep I.

The drainings from steep I are discarded, or may be treated for recovery of nitric acid, oxalic acid, or acetic acid.

Example V used.

Il /2% may be used and the amount of such dilute ture of operation is about 80 C. and the time is about thirty hours.

Example VI In a multiple stage steep the same steeping procedure is followed as for HNO: when used yalone. The temperature of operation is about 80 C. and the time is about thirty hours. H 804 equivalent to 5% HNOa. and-NaNOa equivalent 7 to 3 to 4% HNOa.

After any of the above mentioned steeping chanical treatment then yields a good pulp which may be washed in the usual way.

The pulp may be given a preliminary bleach with chlorine, or its equivalent, using about 1% of the weight of the pulp as chlorine. The pulp is washed and may be treated with 65-10% NaOH with the pump at about 9% to 10% stock concentration, at ordinary temperatures, for ten to fifteen minutes, after which it is washed and given a, final bleach using 0.1% to 0.2% of the weight of the pulp.

In any of the above procedures, woodchips of the size ordinarily used in any pulp mill can be used, or wood inany other suitable form or size may be used. Vacuum impregnation of the wood with a steeping acid has been recommended but at the steeping temperatures given above, no special impregnation treatment is necessary, and this is a feature of our invention.

We have discovered that the use of 10% to 15% HNO3 has the following disadvantages:

Such high concentrations are far too expensive, since it does not seem possible to continuously recycle the used HNOa. The product has a low viscosity, and the 10% KOH solubilities of the purified pulps are much higher than'the desired 7 value.

A very valuable feature and modification of our invention relates to the use of excelsior. We have found that there are distinct advantages in using excelsior. The time necessary for the operation is reduced, the temperatures can be increased without detrimental results, and the nitric acid concentration can be reduced with equally good results. For example, the temperature may range from 75 to 95 C. and the time may be 6 to 24 hours, depending, of course. on the temperature and concentration of the acid Nitric acid concentrations of 1 to acid may be from seven to nine times the weight of the wood. The steam treatment is about onehalf to one hour for either excelsior or for chips.

Sawdust or wood in any other suitable form .would require slight modification in the cooking conditions, but could be used.

By our process, the total yield of crude pulp is in the neighborhood of of the weight of the chips, and this has about a 91-93% quality. The present process gives a high yield of high quality pulp and insofar as we know, no one but ourselves has ever before produced a crude pulp which runs 91-93% alpha cellulose.

Pulpmade by our process requires less bleaching than the ordinary pulp. If too much bleach is used in the final step, the viscosity, as well out severe bleaching,

as the 10% KOH solubility are quite uniavorably all'ected. Our process enables us to use a very small quantity of hypochlorite (or equivalent bleach) for the final bleaching. the range being approximately from 0.1 to 0.25% of hypochlorite. By the usev of our process, both bleaching operations require less bleaching material. Of course. if it is not desired to increase the percentage of alpha cellulose, vin the pulp, a pulp product of good whiteness containing 92% to 94% of alpha cellulose can be obtained by eliminating the caustic soda treatment and simply neutralizing, washing, and going on with the bleaching.

, It must be again emphasized that operation at atmospheric pressure in an open vessel gives a control of the process leading to a product having uniform properties, which is a factor of utmost importance. Insofar as we are aware, no one has even before discovered a process in which the degree of viscosity can be unfailingly controlled. In all cases, we use less than 5% concentration of nitric which is a big saving in acid. In all commercial processes known to us, high or low pressures and high temperatures are used, necessitating the use of closed vessels so that the operator cannot be sure of what is going on. Moreover, it was previously thought impossible to produce alpha cellulose of high viscosity from a short fibered wood, but we have discovered that this is not true, and disclose herein a process by which such material can be produced from such wood.

With chips, a very good specific nitric acid concentration is about 3% for a high viscosity pulp. The amount of nitric acid being equal to about 18.7% of the weight of the air-dried chips. For excelsior, the best specific dilutions are about l t 2% of nitric acid, the time about 18 hours and the temperature about the amount of nitric being about 13.9% of the weight of the air dried wood.

It is, of course, understood that any suitable source'of nitric acid may be used. such for example as nitric acid produced by the oxidation of ammonia or by treating a salt of nitric acid with sulphuric acid.

Note that a white pulp can be obtained withand about 98% alpha cellu lose pulp can be produced. It is of course understood that bleaching does not affect the percentage of alpha cellulose but affects the degree of viscosity.

The following factors as steps in the process are, among other factors and steps, claimed in various combinations: Nitric acid concentration ranging up to 5%; the proportion of acid to wood ranging from 6 to 9 times the weight of the wood; the time ranging from 6 to 30 hours, dependent on the state of division of the wood; the temperature ranging. from 60 to 95 C., the steaming treatment time ranging from to 1 hour; the amount of the preliminary bleach with chlorine (or equivalent) ranging from 0.6% to 2% of the weight of the air dried pulp; the NaOH or equivalent treatment ranging from 5% to 10% and the amount of final bleach ranging from 0.15% to .3% of the dry weight of the pulp.

acid or to the batch a put of the waste acids (solvents and solutes) from a previous run; the ability to extract oxalic acid in relatively larger quantities than usual. to obtain high viscosity alpha cellulose from aspen wood or bass wood and other woods; the

use of aspen wood or other wood excelsior; the

discovery of the modifying efiect of wood, in the form of excelslor, on the process in allowing reduction of the operating time, and allowing use of higher temperatures.

We claim as our invention:

1. Process for the manufacture of cellulose from wood, which comprises. first treating the wood with an aqueous nitric acid of a concentration-between 2 and 10% at a temperature below 95 C., and then subjecting the so-treated wood to a two stage treatment with an alkali,

the first stage being a treatmentwith a hot weak alkali solution and the second stage being a treatment with a cold strong alkali solution.

2. A process for producing alpha cellulose from hardwood which comprises, initially steeping said hardwood for' about six hours or more in an aqueous'solution consisting substantially of from about 1 /2% to less than 5% HNO: maintained at a temperature of from about 60 C. to about 85 C., and thereafter cooking the steeped prod- 3. A process for the manufacture of alpha cellulose pulp from hardwood which comprises, initially steeping said'hardwood with an aqueous solutionconsisting substantially of 3% HNOa for about 30 hoursmaintained at atemperature of 40 about 10 C., thereafter cooking the steeped product with a dilute caustic alkali solution, and then bleaching said product;

4. A process for the manufacture of alpha cellulose pulp from aspen wood which comprises initially steeping said aspen wood with an aqueous solution consisting substantially of 3% HNO; for about 30 hours maintained at a temperature of about 70 C., thereafter cooking the steeped product with a dilute caustic alkali solution, and then bleaching said product.

5. A process for producing alpha cellulose of uniform viscosity from hardwood which includes initially steeping said hardwood for about-six V hours or more in an aqueous solution consisting of from about 1 /2% to less than 5% HNO: maintained at a temperature of from about C. to about 85 0., cooking the steeped product with a dilute caustic alkali solution, then preliminarily lightly bleaching with chlorine, thereafter treat- 60 ing the pulp with from about 6% to 10 solution of caustic alkali and lightly bleaching said pulp with a bleaching agen 6. A process for producing alpha cellulose of uniform viscosity from cludes initially steeping said hardwood for about six hours or more in an aqueous solution consisting of fromabout 1 to less than 5% HNO: maintained at a temperature of from about 60% C. to about 85 product with a dilute caustic alkali solution, then preliminarily lightly bleaching with chlorine,

thereafter treating the pulp with a solution of caustic alkali and then lightlyfbleaching said pulp. 4

The discovery of a way 10 hardwood which in- 0., cooking the steeped 0 7. A process fo uniform viscosity from hardwood which includes initially steeping said hardwood for about }six hours or more in an aqueous solution consisting of from about 1 to less than 5% I-INO: maintained at a temperature of from about 60 C. to

- about 85 C.,' cooking the steeped product with a dilute caustic alkali solution, then preliminarily lightly bleaching with chlorine, thereafter treating the pulp with a solution of from about 6% to 10% sodium hydroxide and lightly bleaching said pulp with sodium hypochlorite.

8. The -methcd for producing alpha cellulose of high quality from aspen wood which comprise s, initially steeping said aspen wood for about six hours or more in an aqueous solution consisting substantially of from/about 1%%' to less than 5% HNO: maintained at a temperature ofifrom about 60 C. to about 85 C., steaming the steeped material, rinsing the material, thereafter cooking the steeped product with a dilute caustic alkalisolutioh, then preliminarily lightly bleaching with chlorine, then treating with from about'6% to 10% sodium hydroxide solution at room temperature and finally lightly bleaching said material with sodium hypochlorite.

9. A process for producing alpha cellulose of uniform viscosity from hardwood which com-.

prises, initially steeping said hardwood for about six hours or more in an aqueous solution consisting substantially of from about 1 /2% to less than 5% HNO: maintained ata temperature of from about 60 C.'to aboutv85 C. in the presence of a small volume of waste solution from a uct with a dilute caustic alkali solution. v as mg the steeped product with a dilute caustic previous similar treatment, and thereafter cookalkali solution.

10. A process for producing alpha cellulose from hardwood which comprises, initially steeping said hardwood for about six hours or more in an aqueous solution consisting substantially of from about 1/2% tc less than 5% HNOa maintained at a temperature of from about 60-C. to about 85 1 C. at atmospheric pressure, and thereafter'cooking the steeped product with a dilute caustic alkali solution.

11. A process for producing alpha cellulose from aspen wood which comprises, initially'steeping said aspen wood for about six hours or more in an aqueous solution consisting substantially of from about 1%% to less than 5% HNO: maintained at a temperature of from about 60 C. to, about 85 C. at atmospheric pressure in an open vessel, and thereafter cooking the steeped prod uct with a dilute caustic alkali solution.

12. A process for producing alpha cellulose from hardwood which comprises, initially steeping said hardwood for about six hours or more in from about 6 to about 9 times the weight of the wood in an aqueous solution consisting substantially of from 'about 1% to less than 5% I-lNOs maintained at ,a temperature of from 60 C. to 85 C., and thereafter cooking the steeped product with a dilute caustic alkali solution.

13. A method for producing alpha cellulose of high quality and uniform viscosity which consists in treating wood with nitric acid of about 3% strength for about thirty hours at about C., then steaming the treated material for about forty-five minutes, rinsing and adding 2% sodium.- hydroxide and cooking for about two hours, then" preliminarily bleaching by use of chlorine in an amount equal to about 0.6% to 0.75% of the weight of the air dried pu1p,- adding the chlorine in two or three portions over a period producing alpha cellulose of of about an hour, then treating the pulp with about nine to ten times its dry weight of 6% sodium hydroxide for ten to twenty minutes at room temperature and'finally bleaching for approximately two hours by using sodium hypochlorite amounting to approximately 0.15% of the dry weight of the pulp.

14. A method for producing alpha cellulose of high quality and uniform 'viscosity which consists in treating comminuted hard wood with nitric acid of or less strength for from about twenty-four to thirty hours at from 60 to 80 C., then steaming the treated material for about forty-five minutes, rinsing and adding 2% sodium hydroxide and cooking for about two hours, then preliminarily bleaching by use of chlorine in an amount ranging from about 0.6% to 2% of the weight of the air dried pulp, then treating the pulp with about nine to ten times its dry weight of 6% to 10% sodium hydroxide for ten to twenty minutes at room temperature and finally bleaching for approximately two hours by using sodium hypochlorite ranging from about 0.15% to 0.3% of the dry weight of the pulp.

15. A method for producing alpha cellulose of high quality and uniform viscosity which consists, in steeping properly comminuted raw wood with nitric acid of less than 5% strength for about hours at from about to 85 0., then steaming the treated material, rinsing the material, then adding dilute sodium hydroxide and cooking at atmospheric pressure, then preliminarily bleaching by the use of chlorine in an amount equal to about .06 to 2% of the Weight of the air dried pulp, then treating the pulp with about nine to ten times its dry weight of 6 to 10% sodium hydroxide, and finally bleaching using sodium hypochlorite amounting to approximately 0.15 to 0.3% of the dry weight of the pulp.

16. A method for producing alpha cellulose of high quality and uniform viscosity which consists, in steeping properly comminuted wood in an open vessel with nitric acid ranging from about 1 to i strength in an amount about six to nine times the weight of the wood for about 30 hours at from about to C., then steaming the treated material, rinsing the material with water, then cooking at atmospheric pressure for about two hours in 2% sodium hydroxide, then preliminarihr bleaching by the use of chlorine in an amount equal to about 0.6 to 2% of the weight of the air dried pulp for about an hour, then treating with about six to ten times its dry weight of 6 to 10% sodium hydroxide for 10 to 20 minutes at room temperature, and finally bleaching for approximately two hours using sodium hypochlorite amounting to approximately 0.15 to 0.3% of the dry weight of the pulp.

LEW W. CORNELL.

RALPH E. MONTONNA.

CERTIFICATE OF ccHmcTI'oN. Patent No. 2,501,51h. November 10, 191 2. RALPH E. MON'IONNA, ET AL.

It is hereby certified that errpr appears in the printed specification of the above numbered'patent requiring correction as follows: Page 5, first co lumn, line 25, for "pump" read -pu1p; page li first column, line 70,

claim 5, for "60%" read -60 and second column, line 61, claim 12,

for "1'?!" read -l% Z-; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent'office.

Signedani sealed this 12th day of January, A. 1).. 1911,

Henry Van Arsdaie, (Seal) Aeting Commi ssioner of Patents.

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2539093A (en) * 1944-09-07 1951-01-23 Mainguet Henri Nitric acid digestion of straw
US2625474A (en) * 1947-06-30 1953-01-13 Buckeye Cotton Oil Company Acid-alkali process for the preparation of cellulose fibers
US2733992A (en) * 1956-02-07 reyerson
US2999783A (en) * 1957-10-05 1961-09-12 Hodogaya Chemical Co Ltd Process for simultaneous production of high alpha-pulp with furfural from latifoliate tree materials
US3130115A (en) * 1961-10-27 1964-04-21 Alfred M Thomsen Method of processing lignified cellulose
FR2520396A1 (en) * 1982-01-22 1983-07-29 Kamyr Inc Method and materials production device carbohydrate and kraft pulp from cellulosic fiber materials pulverized
EP0442806A1 (en) * 1990-02-12 1991-08-21 Sappi Limited A process for producing pulp from a lignocellulosic-containing material
US5676795A (en) * 1992-12-02 1997-10-14 Voest-Alpine Industrieanlagenbau Gmbh Process for the production of viscose pulp

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2733992A (en) * 1956-02-07 reyerson
US2539093A (en) * 1944-09-07 1951-01-23 Mainguet Henri Nitric acid digestion of straw
US2625474A (en) * 1947-06-30 1953-01-13 Buckeye Cotton Oil Company Acid-alkali process for the preparation of cellulose fibers
US2999783A (en) * 1957-10-05 1961-09-12 Hodogaya Chemical Co Ltd Process for simultaneous production of high alpha-pulp with furfural from latifoliate tree materials
US3130115A (en) * 1961-10-27 1964-04-21 Alfred M Thomsen Method of processing lignified cellulose
FR2520396A1 (en) * 1982-01-22 1983-07-29 Kamyr Inc Method and materials production device carbohydrate and kraft pulp from cellulosic fiber materials pulverized
EP0442806A1 (en) * 1990-02-12 1991-08-21 Sappi Limited A process for producing pulp from a lignocellulosic-containing material
US5676795A (en) * 1992-12-02 1997-10-14 Voest-Alpine Industrieanlagenbau Gmbh Process for the production of viscose pulp

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