US2944928A

US2944928A - Method of preparing paper pulp

Info

Publication number: US2944928A
Application number: US685080A
Authority: US
Inventors: Andre C Kibrick; Irwin W Scopp; Raymond R Colton
Original assignee: KIBRICK; SCOPP
Current assignee: KIBRICK; SCOPP
Priority date: 1957-09-20
Filing date: 1957-09-20
Publication date: 1960-07-12
Anticipated expiration: 1977-07-12

Description

July 12, 1960 A C- KIBRICK ET^L 2,944,928

METHOD OF PREPARING PAPER PULP Filed Sept. 20, 1957 H ...mIlmm Mnrnon or ranrannso PAPER PULP rated sept. zu, 1957, ser. No. esausti 6 claims. (ci. isz-az) This invention relates to the manufacture of wood pulp such as that used in paper manufacturing purposes, and

in particular is concerned with improvements in the kraft `or alkaline sulfate paper process.

Hitherto, in the preparation of woodV pulp for paper production, wood, in the form of finely divided pieces, is subjected to a cooking process in the presence of sodium sulides and minor amounts of sodium sulfates. It is, for example, customary to treat raw wood chips, such as may be obtained from southern yellow pine, notably mixtures of loblolly, longleaf and shortleaf pine, and also from jackpine and from various species of spruce, iir, hemlock, larch and aspen, with a liquor prepared fromV sodium hydroxide and sodium sulfide. These agents are combined in such amounts as to provide a solution having a predetermined alkalinity and suliidity, and are charged into a digester together with an appropriate quantity of raw wood chips. The cooking or digesting step is accomplished at moderately elevated temperature and pressure. At the end of this stepthe cooked wood pulp` together with the accompanying residual liquors is introduced into a separator where by means vof filtering and other` known means the digested vwood pulp, comprising chiel-ly cellulose, is isolated. The liquors are recycled backto the digestion stage after various treatments involving elimination of the lignin, and adjustment of the alkali and sullidity factors. The separated Wood pulp may be washed, screened, bleached, dried and ultimately employed in ,the fabrication of paper products.

The wood used in the process is usually first debarked, and cut and chipped to appropriate size. The types used vary considerably but generally show properties falling within the ranges shown in the following table:

TABLE I minimum maximum Y Specific gravity L 0.31 0.56 hrinkage 7. 9 14.1 Bark VV7. 18. 9 Cellulose 54. 4 63. 0 Lgillll 25. 9 34. l.

2,944,928 Patented July 12, 1960 'TABLE II KMnO4 Approx- Y Type Number 1n e End Use Yield lim- 18 46 Bleached bags and writing, wrapping and printing papers.

B 24 48 Unbleached heavy papers for bags,

wrappngs, etc.

C 34+ 50+ Uibleaiched corrugated and solid-liber oar In the digestion of the prepared Wood the factors entering into consideration are the species and quality of the wood and the characteristics desired in the linal product. Time and temperature ofthe digestion step, suliidity of the cooking liquor, the ratio of the cooking chemical to the wood used (on a moisture free basis), and the concentration of the cooking chemical in the liquor may vary widely. It is known that the process may be carriedY out either in batch or in continuous digestion systems. It is further known that the black liquor may be discarded or recycled back into the process. VThese andf other factors understood by those skilled in the art are` related to the ultimate characteristics of the pulp.

Upon completion of the digestion step the liquors and solids are recovered and the solids are leached with water. The pulp is separated and subjected to further processing. The weak black liquor may be concentrated and/or recycled back to the digestion process where it may be reinforced with make-up chemicals. Hydrogen sulfide is partially removed in the evaporators as a result of the following reaction:

-I- HBS In the direct heat evaporators hydrogen sulfide is readily evolved, but not completely in most operations, as a result of the carbonation reaction which occurs as a result of the direct contact with the flue gases. 4The resulting strong black liquors are-decomposed in the recovery furnace, where make-up salt cake also may be added, and the resulting `smelt dissolves to produce the so-called green liquors. Upon causticizing and clarifying this liquor a so-called white liquor is recovered which may be used to digest incoming wood chips, thus com- Nazsa-HzoNaHsLj-Naon Plants operating on the sulfate process normally find that a relatively well-defined equilibrium of composition and sullidity can be established. The source of sulfur make-up is through the use of salt-cake. While from one pulp mill to another Wide variations may exist-it may be stated generally that sodium monosulfide and sodium hydrogen Vsulfide are the agents predominantly responsible,

for thesuccess of the process. 4

Figures 1 and 2 demonstratethe advantages of the polysulfide pulping process.

Hitherto the yield of wood pulp obtained in the sulfate process has been within the range of about 46% to 50% when producing pulps having permanganate values between about 18 and 34 respectively. We now have found that the yield of wood pulp may be significantly increased as hereinafter described:

As originally conceived our studies were directed to establishing the effect of polysulfdes with reference to the cellulose and lignins in wood. In this view it was found that difiiculties were encountered in obtaining sodium polys'ulfides,'and therefore sulfurated potash was substituted on the theory that the potassium ion would in general constitute the equivalent of sodium. We found however, to our considerable surprise, that the substitution gave an unexpected increase in the pulp yield ranging from 1 to l15% over and above the amount secured in control runs employing conventional sodium monosulfide or sulfate systems. This unusual result was first ascribed to the presence of the potassium ion. However, further investigation indicated that the yield increase was toV a considerable extentattributable to the presence of alkali polysulfides. However, we do not wish to be bound by any theoretical explanation.

Our process vcontemplates they introduction of chemicals to the body'of raw wood chips in ratio of about 4 to 1, based on the amount of cooking liquor and dry basis .wood chips, and carrying out the digestion of the wood under elevated temperature yand pressure. This digestion is carried. out in a pressure vessel or autoclave preferably, with .circulation of the cooking liquors, at a temperature ranging from about 160 C. to 180 C. under autogenous pressure which may normally range from about 100 to V140 p.s.i. The chemicals and the wood chips being admixed the vessel after suitable venting may be closed and the temperature raised to about 170 C., or normal operating temperature, the period of time for such increase in temperature consuming about 0.5 hours to 5 hours, about 1.75 hours being considered normal. After the charge has attained the desired temperature, it may be held at about that temperature for 0.5 to 3.0 hours, preferably about 1.0 to 1.5 hours. As is understood by those skilled in the art, an increase in temperature normally shortens the period of time required to produce a pulp of given characteristics.

We have found that by carrying out the digestion process under conditions herein disclosed the wood pulp product of given permanganate number may be obtained in higher yield. We believe that the cause of this higher yield can be ascribed to the action of the chemicals involved in the digestion process, specifically, in. that the attack thereof on the wood material, especially the cellulose component, is less severe. It therefore becomes possible pursuant to our invention to modify the concepts of optimum operatingconditions that have hitherto prevailed. It may, for example, be desired to obtain a given yield in shorter time by operating at higher temperatures, or the process may be operated commercially at lower temperatures and shorter times -to give a yield equal to that now produced in conventional sulfate operations. Depending upon the amount of delignification obtained, our improved process makes possible yield increases ranging from about 1 to 15%, normally between about 4% and about 10% in comparison with conventional systems, these yield increases being equated to a base of 100%. In general we find that the yield increase in the case of severe cooking, such as may be used to produce bleachable grades, is notably higher than hitherto, and that in methods usingrless severe cooking, the observed improvement in yield generally becomes even more pronounced.

In carrying out the cooking operation the amount of active alkali expressed as NazO may vary between about to' about 35 percent based on the charge of Wood used. Good results may be obtained using about 20 to 25 percent. Sulfidity which is normally expressed as a percentage of the active alkali used, may be established by introducing into the so-called white liquors, containing NazS, alkali polysuldes such as NazSz, Na2S3, Na2S4, Na2S5, or possibly higher polymers, and mixtures thereof, these agents being generally referred to as Na2S.l The employment of the sodium system has some advantage from the standpoint of cost. However, assuming equal cost, operations employing sulfurated potash or sulfide polymers of potassium are preferred.

Since the atomic weight of potassium exceeds that of sodium it would be expected that the effective amount of active alkali as K2O would be proportionately higher than Na2 O. It was found, however, that when equal weights are compared, for example, using an active alkali weight percent of about 25 percent, of KOH, and when using sulfurated potash, as against NaOH, the potassium system gave the higher yield of pulp. l

In preparing the polysulfides, elemental sulfur as in the form of flowers of sulfur may be reacted with the alkali in the form of the carbonate, hydroxide or monosulfide. Commercially available sulfurated potash is produced by reaction of K2CO3 with elemental sulfur at a temperature of about 350 C. Although some oxygensulfur intermediate compounds exist in the commercial material it is undoubtedly the polysulfides or sulfide polymers that constitute the active agents in the pulp yield increase. Various methods of preparing sulfide polymers of the type suitable in our process are known to the art. The presence of the polymers may be visually observed by noting a darker color of the solution, approaching red-brown. In the practice of the process the prepared liquorsfor digestion of the wood normally contain in addition to the sulfide polymers, intermediate compounds having an oxygen content, such as the thiosulfates. Polysulfide solutionsin the absence of air are relatively stable at room temperatures, and at higher temperatures, especially abovev C., undergo a degree of decomposition. Thev sulfidity of the treating liquor-may be yregulated within the range of l0 to 30, 19 percent being considered preferable using sulfurated potash, In this respect it should be noted the sultidity figures are based on the conventional iodine-thiosulfate titration, and that other analytical methods may give-values somewhat at variance, as will beA understood by those skilled in the art.

Upon completion of the digestion in the manner set forth the digestion mass is discharged into a blow tank where disintegration of the cooked wood chips and the solids recovered therefrom .takes place. These solids upon leaching and screening are refined and then processed into paper products, following a bleaching operation when bleached pulps are desirable. Pulps prepared according to our preferred process make possible the luse of a less severe bleach and the use of less severe bleaching chemical, such as calcium hypochlorite.

The liquor separated from the wood pulp, known as Weak black liquor, is'subjected to evaporation whereby there are removed water and sulfides. Hydrogen sulfide evolved from these and thek blowdown operations may either be discarded or converted to sulfur available for recycling to prepare additional treating liquor containing polysulfides or sulfide polymers. The concentrated black liquor, which may contain 50 to 70 percent solids may be admixed with additions of` alkali sulfates, such as Na2SO4 and/or K'2SO4 to make up lost alkali,Y and decomposed in a furnace at a temperature of aboutr1000 C. or higher. Carbonaceous material resulting from this treatment may be recovered and used as a fuel or as a reducing agent for the alkali sulfate. While KZSO., has a higher decomposition temperature than Na2SO4 it is found that under reducing conditions a temperature of about 1000 C. is suitable in most cases. Further, since KZSO.; has a higher decomposition temperature than Na2SO4, less KZO loss may occur in the smelting operations than in the case'of the sodium compounds.

. as in the foregoing examples.

. percent.

The smelt from the furnace' containing predominantly alkali sulfide, carbonate and unreduced sulfur compounds, is cooled with water and dissolved. Preferably a portion or all of the cooled smelt may be treated with sulfur for formation of alkali polysuliides. Part or all ofthe polysuldes prepared may be dissolved and clarified prior to use for digestion with fresh Wood. The polysulfides may be combined in any desired proportion with the treating liquor or `so-called white liquor containing monosuliides. As above noted, sulfide polymers also may be prepared by treating a portion or all of the white or green liquors with elemental sulfur, and if desired the polysulde content may be further raised by additional sulfur. Sulfur also may be mixed or slurried with the white liquors prior to or during the reaction' with the wood chips.

In further illustration of the invention the following specic examples are given:

Example I To 100 pounds (moisture free basis) of Wood chips, composed of approximately equal amounts by Weight of loblolly, shortleaf and longleaf southern pine, there was added sucent alkali, alkali polysuliides and water to provide a ratio of water to wood (moisture free basis) of 4, an active alkali content of 28.0 percent, expressed as the hydroxide, and a suldity of 20.5 as determined using the iodine-thiosulfate titration method. All of the sulfur was added in the form of a solution prepared by dissolving the previously prepared alkali sulfide polymers. The materials, having been introduced into a pressure vessel, the later was closed, and with intimate mixing of the solid and liquid phases, the mixture was gradually heated over a period of 100 minutes to 172 C. This temperature was maintained at autogenous pressure for an additional period of 100 minutes, at the end of which time the contents of the pressure vessel were discharged to a collection device at atmospheric pressure. The solids were removed and recovered by filtration and then washed by repulping and mixing with three 300 gallon portions of water at a temperature of about 35 C. Solids from the last leaching were recovered and weighed with a product weight of 52.0 pounds (moisture free basis) unscreened yield. Chemical analysis of the product revealed the KMnO4 number to have a value of 24.6. The product was subjected to conventional refining treatments and provided a paper material having physical properties comparable to those'of conventional sulfate pulp.

Example II Following the general details of procedure of Example I, sulfurated potash was used. The active -alkali content was controlled to 28 percent, the sulfidity to 20.5 percent, and the temperature of digestion to 178 C. The product Weight was found to be 49.1 percent (moisture free basis) unscreened yield, and 48.7 percent screened yield. The KMnO4 number was 17.95.

Example III The same general details of procedure were followed The active alkali value was controlled to 28 percent KOH, the suliidity to 20.5 The temperature was raised to a maximum of 168 C. during a period of 70 minutes and thereafter maintained at that point for an additional 70 minutes'. There resulted an unscreened yield of 58.6 percent of a product having a KMnO4 number of 88.6.

Example IV The procedural details hitherto set forth were carried out, except that a conventional sulfate system with 25 percent NaOH active alkali was used. Suldity was controlled to 20 percent. The temperature of the reaction mass was brought overa period of minutes to a temperature of 172 C., and so maintained for an additional 100 minutes. The unscreened yield was 46.9 percent, the screened yield, 46.2 percent. The KMnO4 number of the product was 18.2.

Example V In this case the process described in the above examples was modified in that a 34 percent KOH active alkali system was established, at 20 percent sulldity. The digestion was elfected during 100 minutes to 172 C. and a further 100` minutes, temperature held substantially constant at 172 C. The unscreened yield of product was 47.0 percent; the screened yield 46.6 percent. The KMnO4 number was 18.4.

From the foregoing description and illustrations further details within the scope of the present invention will be apparent to those skilled in the art. We claim and desire to protect by Letters Patent:

1. In the method of producing wood pulp according to a pulping process, the improvement comprising cooking the raw wood in solutions of alkali polysuldes with a sulidity of 10% to 30% maintained by alkali polysuldes.

2. In the method of producing wood pulp according to a pulping process, the improvement comprising cooking the raw Wood in solutions predominately of alkali polysuldes.

3. In the method of producing wood pulp according to a pulping process, the improvement comprising cooking the raw wood in solutions predominately of alkali polysuldes, with the suldity of 10% to 30% maintained in whole or in part With alkali polysuldes.

4. ln the method of producing wood pulp according to a pulping process, the improvement comprising cooking the raw wood in solutions of predominately alkali polysuldes with the sulidity of 10% to 30% maintained with a combination of alkali polysuldes and the conventional sodium monosulde.

5. In the method of producing wood pulp according to a pulping process, the improvement comprising the introduction into a spent cooking liquor containing NazS, alkali polysuldes including Na2S2, Na2S3, Na2S4, NaZSS, and higher polysuldes and mixtures thereof, these agents generally referred to as NazSx, to maintain the suldity of 10% to 30% with these polysuliides, and cooking the pulps with the liquor high in alkali polysuldes content.

6. In the method of producing wood pulp in the pulping process, the improvement comprising the adding of concentrated alkali polysulides to a spent liquor to produce a suldity of 10% to 30% and pulping with the mixture.

References Cited in the le of this patent UNITED STATES PATENTS 1,689,534 Richter Oct. 30, 1928 2,007,024 Richter July 2, 1935 2,747,995 Hooper et al May 29, 1956 OTHER REFERENCES Pulp and Paper Manufacture, vol. I, Preparation and Treatment of Wood Pulp, 1950. page 374.

Claims

1. IN THE METHOD OF PRODUCING WOOD PULP ACCORDING TO A PULPING PROCESS, THE IMPROVEMENT COMPRISING COOKING THE RAW WOOD IN SOLUTIONS OF ALKALI POLYSULFIDES WITH A SULFIDITY OF 10% TO 30% MAINTAINED BY ALKALI POLYSULFIDES.