US2976273A - Treatment of kraft black liquor and product - Google Patents

Description

TREATMENT OF KRAFT BLACK LIQUOR AND PRODUCT Filed sept. 1o, 1958 TREATMENT oF KRAFT BLACK LIQUoR AND PRODUCT` Frank J. Ball and Richard Pueschel, Charleston, S.C., as-

sgnors to West Virginia Pulp and Paper Company, New York, N Y., a corporation of Delaware Filed Sept. 10, 1958, Ser. No. 760,185

8 Claims. (Cl. 260-124) This invention relates to a process for treating residual black liquor from the kraft pulping process whereby dimethyl sulfide is produced, lignin contained in the black liquor is modified and the aliphatic acid content in the black liquor is substantially increased.

it is well known from the teachings of Hagglund et al. in Reissue Patent No. 24,293 and Cisuey et al.A in Patent No. 2,816,832 that dimethyl sulfide can be produced in substantial quantities from residual black liquor of kraft pulping processes. According to these teachings, however, in order to obtain any appreciable amounts of dimethyl sulfide it is necessary that sulfur in certain forms be added to the black liquor. The sulfur is added by Hagglund et al. in the form of inorganic sulfidic sulfur, i.e. sulfides or bisultdes, while Cisney et al. utilizes elemental sulfur for addition to the black liquor wherein the elemental sulfur forms sulfides in reaction with alkali present in the black liquor.

We have found, however, that if black liquor from kraft pulpng not having sulfur in any form added is heated to 220 to 350 C. at a pressure not less than thevapor pressure of the `black liquor at the existing temperature and maintained in that temperature range for a period of time varying with the temperature that the yield of dimethyl sulfide obtained is substantially the saine or greater than the yields obtained by practicing the methods of Hagglund and Cisney. We have also found that the production of dimethyl sulfide during this heating is accompanied by the dimethylation of lignin and the formation of certain low molecular weight aliphatic acids.

During heating of kraft black liquor at temperatures between 220 and 350 C. at a pressure not less than the vapor pressure of the black liquor at the existing temperature, whatever sulfides or bisulfides may be present in the liquor are little affected by the'treatment. The sulfur ingredient of the dimethyl sulfide produced is chiefly supplied by other sulfur compounds present in the waste liquor. It has been found, however, that while it is not necessary to have any sulfidic sulfur present it is desirable to do so. Our experiments indicate that considerable quantities of dimethyl sulfide can be produced from kraft black liquor which does not contain any sulfide or bisulde but in which the sulfidic sulfur has been oxidized for example prior to concentrating to avoid sulfur losses in that operation. In these cases, however,

although substantially the same quantities of dimethyl ,sulfide may be formed as when sulfidic sulfur is present,

theV formation of dimethyl rsulfide proceeds at a very slow rate. When some sulfidic sulfur is present therrate g of formation of dimethyl sulfide is greatly increased.

Patented Mar. 21, 196.1

lalthough increasing the rate of vformation Vof dimethyl sulfide, does not, as mentioned hereinbefore, greatly affect the yield as most of the sulfur in the dimethyl sulfide comes not from the sulfidic sulfur but from other sulfur compounds, such as thiosulfates, sulfites, sulphates, etc. which are formed during digestion of wood or subsequent handling of the liquor, i.e. during washing, storage, evaporating, etc. For example, ya typical partially concentrated kraft black liquor from the pulping of pine wood containing 4.27 g./l. of sulfidic sulfur was heated under the process of this invention. The equivalent of V7.24 g./1. of sulfur was recovered inthe dimethyl sulfide while the black liquor still contained 3.08 g./l. of sulfidic sulfur. Y Y

During the heating of the black liquor methoxy groups are removed from the lignin. These methoxy groups react with the sulfur compounds within the liquor to form the dimethyl sulfide. Besides beingdemethylated the lignin also undergoes other changes which are not fully understood but which result in a substantial increase kin the concentration of certain low molecular weight aliphatic acids, such as acetic, propionic, glycolic, oxalic, and lactic acids, which acids will be referred to as aliphatic acids throughout the disclosure and claimsof this specification, and a considerable reduction in the pH of the liquor. These changes and their `significance will Vbe more fully shown after the following descriptionl of the complete process of this invention.

The complete process of this inventionris shown in the f ydrawing which forms a part of this specification. The

single figure of the drawing is a flow diagram of the process. Y

The process is carried out with black liquor from a .kraft cook, say of pine pulp, Vas the sole starting .ma-

terial. This black liquor may be obtained from the digesters, the washers, lthejevaporators or any source.

For several reasons however, it isA best that the liquor be concentrated to a solids content of greater than about These reasons are to reduce the heat require-V ficulties encountered in handling the liquor. In a typi' cal case a partially concentrated black liquor yfrom ,the kraft pulping of pine wood from which the talloil skimmings have been removed may contain 30-l35% solids and have a sulfidic sulfur content of 4 ton6 Vg./l.

calculated as the sulfide ion. This 'material isdelivered to a suitable reactor, either of a continuous .orfbatch type, Without modification or addition of any chemicals .and is there heated to 220 to 350 C. at or above the vapor pressure of the black liquor. The temperature is maintained for atimersufiicienty to produceVv substantial quantities of dimethyl sulfide or to efecta desired degree of demethylation of the 1ignin.'. Thefrtimelrequired is variable dependent on the temperature V'employed. To effect substantial production of dimethyl sulfide and demethylation of lignin maintenance of the temperature for approximately 1 minute to several hours will be required with the shorter period of time being applicable to the use of higher temperatures and vice versa. Preferably temperatures between 280 and 320 C. will be used with time of 1 to 50 minutes.

At the end of the desired heating period the pressure is relieved, the liquor being allowed to pass via coolers to a receiver Where it should arrive with a temperature near the boiling point of the black liquor at atmospheric pressure. As the presure is relieved the more volatile components of the liquor particularly the dimethyl sulfide is flashed into vapor.

The part of the liquor which remains. liquid is collected and discharged under suitable control to a filter, being aciditied enroute to the lter to cause precipitation of the lignin and liberation of the aliphatic acids from their salts. The precipitated lignin is then recovered by filtration. The filtrate from which the lignin has been removed is subjected to a process, such as liquid extraction, for separating the aliphatic acids.

If desired a two stage acidification process may be used resulting in savings in acid. During the first stage the liquor is acidified to a pH of approximately 9.5 which will cause approximately 95% of the lignin to be precipitated. After removing this precipitated lignin by filtration the liquor is further acidiied to precipitate the final quantities of lignin and to produce the aliphatic acids from their salts.

The vapors and gases ashed from the liquor are conducted to a cooler where the noncondensible gases, such as carbon dioxide, are removed and the condensable dimethyl sulfide and water vapor condensed. The water and dimethyl sulfide, being practically mutually immiscible, form two layers with the dimethyl sulfide floating on the aqueous condensate. The dimethyl sulfide is separated from the water by decantation to obtain a crude dimethyl sulde. Pure dimethyl sulfide 4is then obtained by distillation of the crude dimethyl sulfide.

While effecting as great or greater yield of dimethyl sulfide as the previously known methods of Hagglund and Cisney the present method has several important advantages. The production of dimethyl sulfide is accompanied by very little if any production of methyl mer-captan.

This eliminates the need for conversion of the methyl mercaptan to dimethyl sulde or its removal from the dimethyl sulde. The addition of sulfur compounds prior to heating, as in Hagglund and Cisney, increases the concentration of sulfur in the liquor thus favoring the reaction of a single atom of sulfur combining with a single methoxy group to form the mercaptan rather than combining with two methoxy groups -to form dimethyl sulfide.

A very significant effect of the reaction as carried out according to this invention in relation to the recovery of lignin and aliphatic acids is the lowering of the pH. The pH of the liquid fraction has been found to decrease as much as 2.5 pH units during the heating of the liquor. A higher percentage of the lignin may thus be precipitated with the same addition of acid now used for precipitating lignin from untreated black liquor or the same percentage of lignin may be precipitated with a lesser addition of acid. In either case an economy is realized. Also more aliphatic acids are liberated with the same quantity of acid.

It is apparent that some of the carbohydrate material breaks down during the heating to yield aliphatic acids, since the concentration of these acids is increased by a significant amount, up to Besides being demethylated the lignin contained in the liquor undergoes certain changes which result in the production of a novel type of lignin. While these changes are not fully understood the modified lignin produced by the process of this invention possesses characteristics which make it of much greater value than normal lignin.

fl- For example, the moist sodium salt of this modified lignin has been found to be a very effective oxygen-scrubber. In most applications where normal lignin is used this modified lignin may also be used more easily or effectively.

The chemical structure of the lignin produced by this process will vary considerably with the conditions under which the black liquor is treated. The modified lignin from kraft pulped pine will contain 0.1 to 12% by Weight of methoxy groups compared to 13 to 15% for normal kraft pine lignin, 0.5 to 2% of sulfur compared to 1.0 to 3.0% for normal kraft pine lignin, and up to 50% more acidic groups than normal kraft pine lignin. Although not definitely established, it appears the lignin of this process has a wide range of molecular weights whereas normal kraft pine lignin has a very uniform molecular size.

Similar results are obtained using black liquor from the kraft pulping of hardwoods. Hardwood lignin, however, contains a much higher initial methoxy content than pine lignin. Thus hardwood lignin which contains approximately 21% methoxy groups when demethylatcd may contain a higher percentage of methoxy groups than normal pine lignin. For example in subjecting kraft liquors from the pulping of hardwoods to the process of this invention lignin containing up to 18% methoxy groups were produced.

vThe changes in the chemical structure of the lignin causes great changes in the physical and chemical characteristics of the lignin. For example, compared to normal kraft pine lignin, pine lignin modified by this process has greater solubility in many organic solvents such as ethanol, hexanol, methyl acetate, acetone, epichlorohydrine, dioxane, triethylphosphate, dimethyl sulphone, and mixtures such as 1:1 methanol-benzene, increased reactivity with organic and inorganic compounds such as formaldehyde and oxygen, a more spherical particle shape, and a much greater ether extractibility, up to 30% while normal lignin has no ether extractibility.

The lignins from this process are greatly superior to lignins modified by the methods of Hagglund and Cisney which contain high percentages of sulfur, up to 15%, and have little if any solubility in most organic solvents.

The modified lignins produced by the process of this invention will form salts. All substances which will form salts with normal lignin will also form a salt with the modified lignin by the same processes. For instance, by precipitating the modified lignin from the black liquor at high pH, about 9.0 to 10.0, the sodium salt is obtained. Other modified lignin salts can be obtained by the same method generally used in producing normal lignin salts, i.e. by reacting lignin in solution at a proper pH with thc appropriate positive ion and precipitating the lignin salt formed at a pH above that at which the salt will be decomposed.

From the many runs which have been made the following are chosen as typical.

Example 1 Concentrated black liquor from a kraft cook of pine wood containing 35 .35 solids after skimming off of the tall oil soaps was heated to 1) 220 C. and held at that temperature for (2) 36 minutes under a pressure of (3) 800 p.s.i. This heating lowered the pH of the liquor from 11.90 to (4) 11.15. This liquor was ashed to remove gases and vapors and the pH of the liquid fraction was reduced to 9.5 by the addition of (5) 85 ml. of 31 B. sulfuric acid for precipitating the major portion of the lignin. The liquor was filtered and acidifed again to precipitate the remaining lignin and to liberate the aliphatic acids from their salts. The liquor was again filtered to remove the lignin precipitated and the aliphatic acids were extracted. The total yields of dry lignin and pure dimethyl sulfide expressed as percentages of black liquor solids were, respectively, (6) 34.5 and (8) 0.22. The percentage reduction of the methoxy groups on the lignin was (9) 12%. The aliphatic acid yield expressed in i i ,n

grams per liter of the liquor heated was (7) 24.8. The procedure conformed generally to the flow diagram and to the description in connection therewith.

:The details of Example l are tabulated below together 6 vthe order of one-quarter hour totwo hours while maintaining it under a pressure which is at all times at least las great as the vapor pressure of the black liquor at the existing temperature of the black liquor, thereby producwith corresponding details of Examples 2 to 8. In Ex- 5 ing a quantity of dimethyl sulfide whose lsulfur content is amples 2 through 8 the starting material and procedure derived chiefly from the initially non-sulfidic sulfur and were the same as for Example l with the exception of the exceeds the total initial sulfidic sulfur content of the black indicated variations. liquor, an increased yield of low molecular weight alidemand Lig-nin Aliphatic DMS Percent Example Temp., Time, Pressure, Dep. perliter Yield, acid Yield, Red.

O. Min. p.s.i. pH to pH 9.5 Percent Yield Percent OCHs m1. 31 BLS BLS BeHrso,

a6 soo 11. 15 85 34. 5 24. s 0. 22 12 14 soo 1o. 75 e5 33. 9 1. as 33 7a 900 1o. 7 55 29. 1 25. s 2.21 89 15 950 10.2 55 3o. s 1s. 2 2. e3 es ao 95o 9. 9 2o 29. s 25.9 2.81 72 s 17 1, 350 9. 7 30 2s. 4 22. s a. 08 94 Control None None 11. 9 55 34. 9 2o. 9 o. o o

While certain preferred embodiments of the invention phatic acids, and a modified lignin product, and recoverhave been illustrated and described in detail, it is to be understood that changes may be made therein and the invention practiced in other forms. It is not therefore, the intention to limit the patent to the specific details illustratively described but to cover the invention broadly in whatever form its principles may be utilized.

We claim:

l. The method of treating residual black liquor of the kraft pulping process having a small initial ratio of sulfldic sulfur to total solids inherently present and a considerably larger initial ratio of chemically combined, nonsulfidic sulfur to total solids inherently present, which comprises heating such black liquor, concentrated to a solids content of 25 to 60%, to, and maintaining itin, the range of 220 to 350 C. under a pressure which is at all times at least as great as the vapor pressure of the black liquor at the existing temperature of the black liquor, thereby producing a quantity of dimethyl sulfide whose sulfur content is derived chiefly from the initially non-sullidic sulfur of the black liquor, and separating and recovering from the back liquor the dimethyl sulfide so produced.

2. The method of claim l in which a substantial portion of the suliidic sulfur is oxidized prior to the concentration of the black liquor.

3. The method of treating residual black liquor of the kraft pulping process having a small initial ratio of suldic sulfur to total solids inherently present and a considerably larger initial ratio of chemically combined, nonsuliidic sulfur to total solids inherently present, which comprises heating such black liquor, concentrated to a solids content of 25 to 60%, to, and maintaining it in, the range of 220 to 350 C. under a pressure which is at all times at least as great as the vapor pressure of the black liquor at the existing temperature of the black liquor, thereby producing a quantity of dimethyl sulfide whose sulfur content is derived chiefly from the initially non-suldic sulfur of the black liquor, an increased yield of low molecular weight aliphatic acids, and a modified lignin product, and recovering from t-he black liquor a desired product so produced.

4. The method of treating residual black liquor of the kraft pulping process having a small initial ratio of sulfidic sulfur to total solids inherently present and a considerably larger initial ratio of chemically combined, nonsuliidic sulfur to total solids inherently present, which comprises heating the black liquor, concentrated to a solids content of 25 to 60%, to 220 to 350 C. and maintaining it in that temperature range for a period of ing from the black liquor a desired product so produced.

5. The method of treating residu-al black liquor of the kraft pulping process having a small initial ratio of sulfidic sulfur to total solids inherently present and a considerably `larger initial ratio of chemically combined, nonsulidic sulfur to total solids inherently present, to produce and recover dimethyl sulfide, a modified lignin, and

. an increased yield of aliphatic acids, which method cornprises heating the black liquor, concentrated through evaporation to a solids content yof 25 to 60%, to the range of 220 to 350 C. `and maintaining it in said range for a period of the order of one-quarter hour to two hours, while maintaining it under pressure which is at all times `at least as great as the vapor pressure of the black liquor at the existing temperature Iof the black liquor, thereby producing dimethyl sulfide whose sulfur content is derived chiefly from the initially non-suliidic sulfur and exceeds the initial sulfidic sulfur content of the black liquor, reducing the pressure on the heated black liquor to a pressure not substantially above atmospheric pressure to cause flashing of vapors from the black liquor at a temperature not substantially above the atmospheric boiling point of the black liquor, separating the flashed vapors from the liquid fraction, acidifying the liquid fraction to cause the precipitation of the lignin and conversion of aliphatic acid salts into aliphatic acids, filtering out the lignin and extracting the aliphatic acids from the filtrate, condensing the water and dimethyl sulfide vapors flashed from the liquid fraction, and separating the condensed dimethyl sulfide from the condensed water.

6. A modified lignin product, obtained from black liquor from the kraft process of pulping wood having a small initial ratio of sullidic sulfur to total solids inherently present and a considerably larger initial ratio of chemically combined non-sulidic sulfur to total solids inherently present, by heating such black liquor, concentrated to a solids content of 25 to 60%, to, and maintaining it in, the range of 220 to 350 C. under a presprecipitate the modified lignin product therefrom, said modified product being characterized by having a1 sulfur content between 0.5 and 2.0% by weight and having,

when compared to the kraft lignin from which the modi-v fied lignin was prepared, a substantially reduced methoxy content, greater acidic group content, greater etherexf I Y tractable content, increased reactivity toward oxygen and y i formaldehyde, and substantially increased solubility in each of acetone, hexanol, and 1:1 mixture of methanolbenzene.

7. A modified lignin product as set forth in claim 6, wherein the black liquor s from the kraft process of pulping hardwood.

8. 'A modied lgnn product as set forth in claim 6, wherein the black liquor s from the kraft process of pulpng pine wood, and wherein the methoxy content is reduced from 12 to 94% as compared with the unmodied product.

u References Cited in the le of this patent UNITED STATES PATENTS Hagglund Mar. 19, 1957 Rinman Aug. 9, 1927 Salvesen Aug. 25, 1950 Bird Apr. 25, 1950 FOREIGN PATENTS Canada July 18, 1950

Claims (2)

1. THE METHOD OF TREATING RESIDUAL BLACK LIQUOR OF THE KRAFT PULPING PROCESS HAVING A SMALL INITIAL RATIO OF SULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT AND A CONSIDERABLY LARGER INITIAL RATIO OF CHEMICALLY COMBINED, NONSULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT, WHICH COMPRISES HEATING SUCH BLACK LIQUOR, CONCENTRATED TO A SOLIDS CONTENT OF 25 TO 60%, TO, AND MAINTAINING IT IN, THE RANGE OF 220* TO 350*C. UNDER A PRESSURE WHICH IS AT ALL TIMES AT LEAST AS GREAT AS THE VAPOR PRESSURE OF THE BLACK LIQUOR AT THE EXISTING TEMPERATURE OF THE BLACK LIQUOR, THEREBY PRODUCTING A QUANTITY OF DIMETHYL SULFIDE WHOSE SULFUR CONTENT IS DERIVED CHIEFLY FROM THE INITIALLY NON-SULFIDIC SULFUR OF THE BLACK LIQUOR, AND SEPARATING AND RECOVERING FROM THE BACK LIQUOR THE DIMETHYL SULFIDE SO PRODUCED.

6. A MODIFIED LIGNIN PRODUCT, OBTAINED FROM BLACK LIQUOR FROM THE KRAFT PROCESS OF PULPING WOOD HAVING A SMALL INITAL RATIO OF SULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT AND A CONSIDERABLY LARGER INITAL RATIO OF CHEMICALLY COMBINED NON-SULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT, BY HEATING SUCH BLACK LIQUOR, CONCENTRATED TO A SOLIDS COSNTENT OF 25 TO 60%, TO, AND MAINTAINING IT IN, THE RANGE OF 220* TO 350*C. UNDER A PRESSURE WHICH IS AT ALL TIMES AT LEAST AS GREAT AS THE VAPOR PRESSURE OF THE BLACK LIQUOR AT THE EXISTING TEMPERATURE OF THE BLACK LIQUOR, AND SEPARATING THE MOFIFIED LIGNIN PRODUCT FROM THE BLACK LIQUOR BY ACIDIFYING THE LIQUOR TO PRECIPITATE THE MOFIFIED LIGNIN PRODUCT THEREFROM, SAID MODIFIED PRODUCT BEING CHARACTERIZED BY HAVING A SULFUR CONTENT BETWEEN 0.5 AND 2.0% BY WEIGHT AND HAVING, WHEN COMPARED TO THE KRAFT LIGNING FROM WHICH THE MODIFIED LIGNIN WAS PREPARED, A SUBSTANTIALLY REDUCED METHOXY CONTENT, GREATER ACIDIC GROUP CONTENT, GREATER ETHER EXTRACTABLE CONTENT, INCREASED REACTIVITY TOWARD OXYGEN AND FORMALDEHYDE, AND SUBSTANTIALLY INCREASED SOLUBILITY IN EACH OF ACETONE, HEZANOL, AND 1:1 MIXTURE METHANOLBENZENE.

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