US2823119A - Pulp purification - Google Patents

Description

Feb. 11, 1958 A. N. PARRETT 2,823,119 PULP PURIFICATION Filed Feb. 2, 1952 REFlNING OF SULFITE PULP AT I60 C.

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0 2o 40 so so IOO PERCENT OF TOTAL SODIUM AS SODIUM SULFITE INVENTOR flrzfiz/r AfParrzf/ 1 ATTORNE 5 United States Patent PULP PURIFICATION Arthur N. Parrett, Shelton, Wash., assignor to Rayonier Incorporated, Shelton, Wasln, a corporation of Delaware Application February 2, 1952, Serial No. 269,659

6Claims. (CI. 9211) This invention relates to the treatment of wood pulp and has for its object the provision of an improved method of refining sulfite pulp for the production of high-alpha cellulose. More particularly, the invention provides an improved method of digesting-pulp, produced by the acid sulfite digestion of Wood and subjected to chlorination and washing, with a digestion liquor consisting principally of sodium carbonate (Na CO and sodium sulfite (Na SO at relatively high temperatures.

It is now common practice in the production of highalpha cellulose to subject acid sulfite pulp which has been chlorinated and washed to digestion with caustic soda liquor at temperatures usually not in excess of 135 C. and generally below 100 C. My invention produces superior pulp having better brightness, a higher alpha cellulose content and in better yield. The pulp is further characterized by better acetylation properties, as shown by more complete and rapid dissolving in acetylation; and by better properties in the viscose process as shown by giving yarn or cord of better fatigue properties.

My invention provides a method of subjecting a pulp from the acid sulfite digestion of wood after chlorination and washing to digestion with an aqueous liquor consisting of sodium carbonate and sodium *sulfite, with or without some sodium hydroxide, in which the sodium carbonate is in an amount sufiicient to substantially remove hemicellulose so as to produce high-alpha cellulose, and the sodium sulfite is in an amount sufficient to substantially remove discoloration remaining after the preliminary pulp and chlorination treatments, at temperatures appreciably higher than those used in caustic refining digestions, producing a white, high-alpha cellulose in good yield.

In accordance with my invention, I prefer to use a digestion liquor, for refining acid sulfite pulp, consisting principally of'a mixture of sodium carbonate and sodium sulfite in which the sodium sulfite represents from to 70% of the sodium present in the mixture, at a temperature of at least 140 C. and, in batch operation, most frequently not exceeding 185 C., and achieve unpredictable improvements in the yield and quality of the cellulose produced. With less than 10% of the total sodium represented by sodium sulfite, the refined cellulose will generally lack the desired brightness. With more than 70% of the sodium represented by sodiumsulfite, unsatisfactory alpha purification and undesirably high solubility in the standard vhot potassium test (KOH solubility) will result. Moreover,.such an excessive proportion of sodium sulfite will cause difliculties in chemical recovery when the efliuent liquor is furnaced to recover chemicals as a smelt, .mainly byl'increasing the fusion temperature of the smelt.

It iswell known that digesting sulfite pulp with caustic soda at the higher than conventional temperatures of this invention is detrimental rather than beneficial, since.

the additional temperature results in no further increase in alpha cellulose, with but slight change in brightness and asignificantly less economical yield.

In the acid sulfite pulping process, a

lignoeellulose 2,823,119 "Patented Feb. 11, 1958 '4 material is generally cooked in a solution of sulfurous acid, part of the sulfurous acid being combined as bisulfite. The cation combined thus with the bisulfite ion is generally known as the pulping base and this term is used in this sense herein. Pulping-base cations normally used include calcium, sodium, ammonium, and magnesium. Pulp produced by acid sulfite digestion of wood, irrespective of the pulping base, is commonly termed sulfite pulp and the term is used with this meaning herein.

For application of the improved alkaline purification treatment, the prior sulfite digestion of wood is not limited to any particular method or conditions, such as choice of pulping base. Likewise, the chemical properties of the pulp obtained from the sulfite digestion are not critical and any conventional sulfite digestion treatment may be used. Normally, however, the digestion will be so carried out that a "screenable pulp will be obtained which may be chlorinated without un'econo'mical consumption of chlorine. Such sulfite digestion conditions Will normally produce yields of 40% to 50% of unrefined pulp, based on the dry initialwo'od.

Sulfite cooking liquors to produce a screenable pulp will generally contain about0.'5% to 1.5% of combined sulfur dioxide, representing combination as neutral sulfite, and 4% to 10% of free sulfur'dioxide.

Similarly, the invention is not limited to the use of any particular wood in the sulfite digestion. Any soft or hardwood which can be satisfactorily pulpedby the acid sulfite process may be used. 'Suitab'le woods include western hemlock, spruce, southern pine, black gum, birch,'maple and ash.

Following sulfite digestion, the pulp is Washed by any conventional or otherwise convenient manner such as in a blowpit or by drum washers. The Washed pulp in aqueous suspension is then chlorinated with elemental chlorine according to conventional'practice for sulfite di- Lgested "pulp. The amount of chlorine used is generally remove hemicellulose from the unrefined pulp. The color of the resulting pulp, however, would not equal that of pulp from customary sodium hydroxide refining and would be, in fact, so dark as to be unacceptable. By using a mixture of sodium carbonate and sodium sulfite, the advantages of a buffered solution for removing hernicellulose are retained While the highly colored materials are simultaneously removed. Presumably this is due to a specific eifect of the sodium sulfite on colored material existing in the sulfite pulp or formed during the alkaline digestion.

While the digestion operation of the invention with a liquor consisting of sodium carbonate and sodium sulfite gives results superior to the customary refining with sodium hydroxide liquor, it is not necessary that the digestion liquor be entirely free from sodium hydroxide. I may use a liquor containing in addition to sodium can bonate and sodium sulfite, sodium hydroxide in amount representing up to 15% of the sodium in these sodium compounds as the solution still retains its highly buffered character and gives the previously described advantages in the refining.

In practice, particularly when the refining is practiced as part of a cyclic process of refining and chemical rethe above described eifective sodium compounds, substantial amounts of sodium sulfate and smaller amounts of other sodium salts such as sodium chloride and sodium thiosulfate. These additional sodium chemicals when present, are relatively inert, and their presence is not considered in computing the effective ranges for the active chemicals, neither as regards the range of proportions of chemical composition, nor as regards range of the ratio of total amount of active chemicals to pulp.

In carrying out a method of the invention, the chlorinated and Washed acid sulfite pulp is digested in any suitable manner in a liquor containing a mixture of sodium carbonate and sodium sulfite, in which the sodium sulfite represents from to 70% of the sodium of the mixture and at a temperature above 140 C., frequently with advantage at a temperature of around 150 to 175 C. With a digestion time of the order of 45 minutes, optimum results will generally be obtained at 150-175 C. With shorter times, higher temperatures may be used, even above 185 C. The operation may be conducted batchwise or continuously in any suitable equipment. For continuous operation, short times and high temperatures may be used.

The consistency of the pulp slurry is not critical but for economical reasons should be as high as can conveniently be mixed with the mechanical equipment available, preferably from 10% to The term consistency, as used herein, refers to the weight of bone dry pulp per total weight of slurry in which it is contained, expressed as percent.

The total amount of sodium carbonate and sodium sulfite used will be determined by the characteristics desired in the refined pulp. Usually from 6% to 20% of sodium salts, calculated as Na O and based on weight of bone dry pulp, will be used, more extensive refining being obtained with the higher proportions of chemicals. Since the chemicals may readily and advantageously be recovered, as described and claimed in my copending application Serial Number 271,621, filed Feb. 14, 1952, it is economically practical to use higher proportions of chemicals to pulp than has been possible in past sulfite pulp refining procedures using sodium hydroxide because the sodium hydroxide could not be recovered practically.

After completion of the refining treatment and washing, the pulp may be bleached to a high degree of brightness by any method which will not impair the high quality of the pulp. In view of the high brightness resulting from the alkaline refining of the invention, bleach chemical requirements will generally be low. For pulps intended for use in either the viscose or acetate processes, bleaching may conveniently be by sodium or calcium hypochlorite. The fullest advantages, however, are obtained by bleaching with aqueous solutions of chlorine dioxide or acidified sodium chlorite. The bleaching may be carried out at either high or low pulp consistencies and either batchwise or in continuous bleaching equipment.

Pulp refined by the invention, after bleaching, in addition to having advantages of a high-alpha cellulose content and high brightness, is generally characterized by a low content of impurities Such as pentosans and lignin.

The bleached refined pulp has particular advantages in the viscose process where, as compared with conventionally refined pulp, it gives yarn, cord or film not only of higher quality but also in higher yield. Likewise the bleached refined pulp finds advantageous use in acylation and etherification processes. In acylation processes, especially as regards acetylation, the pulp reacts more rapidly than conventional high-alpha pulps and gives generally clearer solutions.

Other cellulose esters. and ethers for whose preparation the pulp is suitable include cellulose propionate, cellulose aceto-propionate, cellulose aceto-butyrate, ethyl cellulose, methyl cellulose, carboxyrnethyl cellulose, hydroxyethyl cellulose, cellulose sulfate and the like.

4 The accompanying drawing illustrates graphically the results of refining digestions of acid sulfite pulp with liquors containing varying amounts of sodium carbonate and sodium sulfite.

The following examples and tables illustrate the method and improvements of the invention:

EXAMPLE I Three batches of chlorinated and washed acid sulfite pulp from western hemlock wood were refined by digesting for 45 minutes in an agitated pot at temperatures of C., C., and C. with liquor equivalent to 19.4% Na O (25% NaOH) consisting of a mixture in which one-half the Na o was Na CO and one-half was Na SO Following the refining digestions, the pulp batches were removed, washed with water, and dried.

Alkaline refining would normally be followed by bleaching. A bleaching step was, however, omitted in these tests in order to show more definitely the direct eflect of the alkaline refining.

The refined and dried pulps were analyzed for alpha cellulose and tested for brightness by the following methods which are similar to those known and used in the art:

Alpha cellulose:Fraction of pulp insoluble in 18.0% NaOH solution, by a method closely similar to TAPPI Standard Method T203m.

Brightness:Percent reflectance of light at 457 millimicrons using a Beckman Model D. U. spectrophotometer equipped with reflectance attachment. Calibration of standards was such as to give results closely equivalent to those of TAPPI Standard Method T2l7m for measuring brightness with the General Electric reflection meter.

Using the above refining procedure and conditions and testing methods, the experimental values shown in Table I below were obtained:

Table I Alpha,- Bright- Yield, Temperature Cellulose, ness percent percent EXAMPLE IA In order to demonstrate the improved acetylation properties of pulp refined by the invention, the following convenient and rapid laboratory test was used for comparing the acetylation reactivity of samples of pulp fibers:

Small specimens of the pulps to be examined are moistened with distilled water and dried in a circulating oven at a controlled elevated temperature to dry them under comparable conditions. (This step may be omitted if all samples have been previously dried in the same manner.) An accurately weighed sample of 0.5 gram of each pulp is torn into small bits and placed in a 35 ml. vial. A flattened glass rod is placed in the vial through a hole in the cap and the vial and sample set in a water bath at 20 C.

The 'acetylating mixture is prepared by mixing 2,500 gms. H 80 88.0 ml. acetic anhydride, and 175.0 ml. acetic acid. This mixture is unstable and should be freshly prepared every two days. 7

To the sample vial in the water bath 15 ml. of the acetylating mixture are added from a pipette. The pulp and acid are mixed with the glass rod, which remains in the vial. The vials are stored in the Water bath and the mixing repeated every 15-20 minutes. It is important to include a standard sample with each group of unknowns and to handle and agitate all samples alike.

As the pulp samples are acetylated by the mixture they dissolve continuously. The time required for substantial solution to take place and the relative clarity and residual undissolved fibers at the time of observation will indicate whether any of the samples is more or less reactive than the standard. An observation ofcolor is also made.

As an example of pulp refined by the invention, a sample selected from the refining experiments described in EX- ample I was tested by the above test. This sample was the one refined at 150 C. with a 50-50 mixture of Na O and Na SO In the above described acetylation test it dissolved in 7 hours to give a clear solution. In comparison, samples of sulfite pulp refined by conventional of 50-50 sodium sulfite-carbonate equivalent to about 19% Na O.

EXAMPLE IV A sample of chlorinated hemlock sulfite pulp was refined by digesting it 45 min. at 170 C. in a liquor containing sodium chemicals in the proportion of 10% Na O methods required 10-12 hours for substantial solution and 10 on the p p basis the Sodium Chemicals being present as even then contained many unreacted fiber M 60 55 NagSO 35%, and NaOH 10%. This pulp EXAMPLE II was bleached with sodium hypochlorite giving a product In Table II are shown comprehensive analytical and with the following analytical properties: Alpha 96.7%, acetylation test data on a series of samples of western 15 brightness 899%, KQH lh y 3.3%. The pento an hemlock sulfite pulp refined at 160 C. with sodium ca content was low (1.0% asdetermined 'bydistillationwith bonate-sulfite mixtures in amount in each case equivalent hydfoblcmic acid flndprecipitatiol} with bafbitufic 9 o 19. N8 0 (equivalent to 25% NaOH) on the pul The bleached pulp was made into viscose conta ning The proportions of Na CO and Na SO are how 7.5% cellulose and 6.5% NaOH by a conventlonal varied from 100% Na CO to 100% Na SO All prostandard procedure and spun into a t1re cord usinga portions ar on the ba i f u'ival t f Na o, standardprocedure customarily-used for evaluating pulps. The analytical and test methods used, in additi t The rayon cords Were tested for their breaking strength those already described in Example 1, included the dein both the dry and wet state, their degree of elongation termination of KOH solubility by measuring the fraction at break in the dry state and their fatigue life, the last soluble on heating the pulp three hours at 100 C. in 10% test being made on a machine having a reciprocating KOI-I solution. motion, with the cord maintained at a temperature of The results obtained with the refining test methods of 150 C. and under a load of 1.81 kilograms. These tests Example 11 follow in Table II: resulted in the following results: Tenacity, gm./denier,

Table II Refining Agent, Pct. Acetylation Test of Nero as- Refining Alpha KOH Bright- Yield, Cellulose, Soluble, ness, Percent- Percent Percent Percent Soln Clarity of Color of NazCO; NBzSOa Time, Soln Soln Hrs.

100 0 79. 5 96.8 3. 9 54. 3 6 90 10 7s. 5 96. 3 4. 3 56.5 6 70 so 79. 6 96.3 5. 3 64. 0 6% 50 79. 6 96. 7 4. 5 68. 4 6% 30 v 85. 0 94. 9 9. 2 70. 2 6% 10 86. 0 .94. 2 9. 9 74. 4 7 '0 87.8 94. 1 10. 9 76. 1 7

In order toemphasize the eifectsof the varying refining conditions, the above analyses and tests were made on the refined samples without subsequent bleaching. In actual practice the pulp would be bleached following refining so that pulp brightness values would be considerably higher and colors of the acetates considerably lighter than shown in the table.

On examination of the analytical data for alpha, brightness, and KOH solubility, and both dissolving time and color in the acetylation test, it will be seen that the most generally satisfactory pulps were those refined within the preferred chemical proportion limits of the invention, namely with 20 to 70% of the Na O represented by sodium sulfite.

EXAMPLE III In Table III are given analytical and acetylation test data similar to Table II for a series of samples of hemlock sulfite pulp refined at C., in each case with equal parts of Na CO and Na SO in terms of Na O, and with variation of the total amount of sodium sulfitecarbonate from 7.75% to 25.2% Na O based on pulp.

dry 3.09, wet 1.98; elongation 12.1%; fatigue 302 minutes as compared with a fatigue life of 257 minutesfor a cord from conventionally refined pulp. Moreover, the pulp gave a yield of 98.8% regenerated cellulose as compared to 96.0% for conventionally refined pulp.

EXAMPLE V A sample of chlorinated sulfite pulp from southern pine was refined by digesting it at 16% consistency 45 min. at C. in a liquor containing sodium chemicals in the proportion of 12% Na O on the pulp basis, the sodium chemicals being present as Na CO 55%, Na SO 35%, and NaOH 10%. This pulp was bleached with sodium hypochlorite and the finished product had the following analytical properties: Alpha cellulose 97.5%, brightness 88.7%, KOH solubility 3.6%. The pulp was also notable in having a low pentosan content (1.4% as determined by distillation with hydrobrornic acid and precipitation with barbituric acid).

The pulp was acetylated by a conventional test procedure in which it reacted smoothly to give an acetate of good clarity, color and filterability.

7 EXAMPLE v1 In order to show what the effect would be in practice of recycling efiiuent liquor from the refining of the invention to build up its content of organic materials, a series of successive alkaline refining digestions was made on separate samples of hemlock sulfite-digested pulp. In each case, the alkaline digestion liquor was made up by adding sodium chemicals in the proportions of 55% Na CO 35% Na SO and NaOH. The alkaline digestions were made at 170 C. for 45 minutes at a consistency of 10% and using 10% total sodium chemicals as Na O based on the pulps.

In carrying out the series, however, after the first digestion in fresh liquor, all the effluent digestion liquor that could be removed from the refined pulp by centrifuging was used in making up the liquor for the next digestion and similarly through the succeeding digestions. In each case, water and chemicals were added to give the required volume of digestion liquor containing fresh Na CO Na SO and NaOH in amount equivalent to 10% Na O on the pulp and in the proportions stated. In addition, the solutions contained small amounts of residual active chemicals from the liquor recycled from the previous digestion.

Each of the successive digestions provided refined pulp of satisfactory quality and with no retrogression of quality due to recycling other than a slightly higher bleach demand.

I claim:

1. The method of producing bright high-alpha cellulose which comprises subjecting chlorinated and Washed pulp from the acid sulfite digestion of wood to digestion in an aqueous liquor consisting principally of a mixture of sodium carbonate, sodium sulfite and sodium hydroxide, the 5 sodium chemicals varying from 6% to expressed as' Na O based on the pulp, the sodium sulfite in the mixture representing from 10% to 70%. of the total sodium, the sodium hydroxide representing not over 15% 'of the total sodium and the sodium carbonate representing from to 80% of the total sodium, at a temperature in the range of from 150 C. to 175 C. and for a digestion time of about minutes and at a pulp consistency of from 10% to 20% to accomplish the effective diminution of hemicellulose and colored material.

2. The method of producing high-alpha cellulose in which Wood pulp which had been digested with acid sulfite solution, chlorinated and washed, is given an alkaline refining treatment which comprises digesting the washed pulp with an alkaline aqueous solution consisting principally of a mixture of sodium sulfite and sodium carbonate, said sodium sulfite varying from 20% to 50% and said soduim carbonate varying from 35% to of said mixture, said solution containing a total of from 6% to 20% of said chemicals (all said percentages expressed as Na O), said digestion being carried out with a pulp consistency of from 10% to 20% and at a temperature of from to 185 C. and for a sutficient time to efiectively remove hemicellulose and colored matter thereby producing bright high-alpha cellulose in good yield.

3. In the method of claim 2, carrying out the operation with an alkaline solution having a total of from 50% to 80% of sodium carbonate and sodium hydroxide expressed as Na O.

4. In the method of claim 2, carrying out the digestion at a temperature in the range of from to C.

5. In the method of claim 2, carrying out the alkaline refining during a period of about 45 minutes.

6. In the method of claim 2 said alkaline aqueous solution comprising up to 15% of sodium hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,566,118 Rawling Dec. 15, 1925 1,640,853 Richter Aug. 30, 1927 1,786,890 Braun Dec. 30, 1930 1,802,575 Richter Apr. 28, 1931 1,822,125 Blodgett et al. Sept. 8, 1931 1,870,650 Richter Aug. 9, 1932 1,880,046 Richter Sept. 27, 1932 1,921,539 Richter Aug. 8, 1933 1,973,557 Bradley et a1 Sept. 11, 1934 2,249,174 Richter July 15, 1941 2,656,246 Gray et al Oct. 20, 1953 2,694,631 Richter et al. Nov. 16, 1954 FOREIGN PATENTS 7,047 Australia Aug. 30, 1927 480,404 Canada Ian. 22, 1952 137,831 Great Britain May 12, 1921 OTHER REFERENCES Rue et al.: Paper Trade I. (1925), pp. 52 and 53. Rue et al.: Chem. and Met. Eng. (1927), pages 611. Manufacture of Pulp and Paper, 3rd ed., vol. III, sec. 5, pp. 3 and 4 (1937), pub]. by McGraw-Hill, New York. Yorston: Dominion Forest Service Bull. 97, Ottawa, Canada (1942), pp. 32, 39, 41, 46, 48, 53, 55 and 62.

Schelhorn: Paper Trade Journal (1944), pp. 39-44.

U S DEPARTMENT OF COMT'IERCE PATENT OFFICE CERTIFICATE OF CORRECTION atent No. 2,823 ,119 Arthur N. Parrett February 11, 1958 It is hereby certified that error appeers in the printed specification of the above numbered patent requiring correction and that the said Let oers Patent should read as corrected below.

Co1umn5, line 6, for "Na 0 read Na CO Signed and sealed this 17th day of June 1958.

(SEAL) Atteet:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Conmissioner of Patents

Claims (1)

1. THE METHOD OF PRODUCING BRIGHT HIGH-ALPHA CELLULOSE WHICH COMPRISES SUBJECTING CHLORINATED AND WASHED PULP FROM THE ACID SULFITE DIGESTION OF WOOD TO DIGESTION IN AN AQUEOUS LIQUOR CONSISTING PRINCIPALLY OF A MIXTURE OF SODIUM CARBONATE, SODIUM SULFITE AND SODIUM HYDROXIDE, THE SODIUM CHEMICALS VARYING FROM 6% TO 20% EXPRESSED AS NA2O BASED ON THE PULP, THE SODIUM SULFITE IN THE MIXTURE REPRESENTING FROM 10% TO 70% OF THE TOTAL SODIUM, THE SODIUM HYDROXIDE REPRESENTING NOT OVER 15% OF THE TOTAL SODIUM AND THE SODIUM CARBONATE REPRESENTING FROM 35% TO 80% OF THE TOTAL SODIUM, AT A TEMPERATURE IN THE RANGE OF FROM 150*C. TO 175*C. AND FOR A DIGESTION TIME OF ABOUT 45 MINUTES AND AT A PULP CONSISTENCY OF FROM 10% TO 20% TO ACCOMPLISH THE EFFECTIVE DIMINUTION OF HEMICELLULOSE AND COLORED MATERIAL.

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