US1931933A - Manufacture of high alpha cellulose pulp - Google Patents

Description

Oct. 24, 1933. w. D. NICOLL 1,931,933

' MANUFACTURE OF HIGH ALPHA CELLULOSE PULP Filed Jan. 6, 1933 3 Sheets-Sheet l BAGASSE RECEIVING BAGASSE SHREDDING HBAGGASE scmnme PITH HANDLING BAGASSE STORAGE megsnon CAUSTIC Recovcw L SPENT CAUSTIC 5 LOW TANKS WASHERS v 3 STOCK CHEST 'scasmmemnecxmg BLEACHING WASHINGANDBLEHDMG PAPER MAKING I IN VEN TOR. UJi 11am. D. Nicoll A TTORNE Y.

Oct. 24, 1933. LL 1,931,933

MANUFACTURE OF HIGH ALPHA CELLULOSE PULP Filed Jan. 6, .1933 3 Shets-Sheet 2 DAGASSE 2/3 SPENT CAUSTIC FROM STORAGE TO SPRAYS 3/3 SPENT CAUSTIC TO STORAGE by TO BE RETURNED mgnme FILL El-:2 smone CAUSTIC 1 COOK NOW /3 FILLED. v FILLED WITH SPENT CAUSTIC FROM STORAGE.

FINISHED PULP WASH DIGESTED PULP TO mow PIT.

INVENTOR.

UJiHiam DN'moH fiz 7 W A TTORNE Y.

Oct. 24, 1933. w D 6 1,931,933

MANUFACTURE OF HIGH ALPHA CELLULOSE PULP Filed Jan. 6, 1953 3 Sheets-Sheet 5 DIGESTERS N91 N92 N93 N94 HOT WATER FOR WASHING 3/3 DIGESTED PULP TO BLOW PIT /55PENT CAUSTIC TO STORAGE Z/35PEHT CAUSTIC FROM STOREWSPRAYS EBAGASSE v 3/; FRESH cAusTlchox, CAUSTIC 2/3 WEAK CAUSTIC TO rommne /3 mezsrzn PULP TO BLOWPITXVD HOT WATER FOR WASHING 3/SFRE5H CAUSTIC z/ssPim' CAUSTIC mom STORETQ SPRAYS 1 BAGASSEJ: I; I/SIWEAK CAUSTIC To romrrme V; SPENT CAUSTIC T0 STORAGE-3 HOT WATER MR WASHING 2/5 SPENT CAUSTIC FROM SWKE TUSIRAYS /s SPENT gAusTlc To STORAGE yamGEsTED PULP T0 bLow PIT IYSFRESH CAUSTIC BAGASSE WEAK cAUsnc Ta FORTIFYING HOT WATER FOR WASHING /3 DIGESTED PULP To BLOW PIT 2 SPENT CAUSTIC T0 STORAGE 2/3 SPENT CAUSTIC FROM STORE TOSPRAYS I E 3/3 FRESH CAUSTIC BAGASSE 3 2/3 WEAK CAUSTIC T0 FOR'IIFYING CYCLE OF OPERATION FOR EACH DI'GESTEK mom: BANK OF FOUR DIGESTERS INVEN TOR.

ATTORNEY.

Patented 24, 1933 I v UNITED STATES PATENT orrlcs MANUFACTURE OF HIGH ALPHA l CELLULOSE PULP 9 Claims. (CI. 92-13) material by a process involving a single digestion 10 with alkaline solutions. A further object is to provide an improved process which is peculiarly adapted for the successful and economical manuiacture of high alpha cellulose pulp from the structural fibers of plants of the grass family (Graminete) and in particular to provide a means of producing high alpha cellulose from the fiber of pith bearing plants of the grass family such as sugar cane or corn. Other objects will appear hereinafter.

Tnese objects, in accordance with my preferred practice using sugar cane bagasse, are accomplished by the following process in which hagasse is first very thoroughly depithed and then treated in digesters under pressure and at temperatures above 100 C. with cooking liquors which are made to flow in such a manner that the fiber is treated with liquors containing progressively increasing concentrations of active cooking agent and decreasing concentrations of dissolved organic material. The crude pulp produced in this manner is washed free of cooking liquors, screened to remove shives (incompletely digested fibers), and bleached by means ofa two stage process involving first a treatment with chlorine water and then with hypochlorite. At

the end of the bleaching process the .pulp is.

Having reference to Fig. 1, which is a flow sheet of the complete process, the bagasse, after shredding and screening, is thoroughly depithed. This depithing or separation of the long structural fibers from the small pith cells characteristic ofmill run bagasse is emciently effected by passing the material through an attrition mill or any apparatus giving similar mechanical treatment and then screening out the pith to permit the satisfactory circulation of caustic liquors through the pulp. It is important to the successful operation of the present process that the bagasse be very thoroughly depithed. The depithlng is, 30 therefore, carried out to an extent which removes from 35% to 40% of the weight of the bagasse undergoing the depithing operation. In this manner, in most cases, more than 90% of the total amount of pith originally present is removed. Since the pith screenings may contain more or less of the fiber and since the original proportion of pith to fiber varies somewhat the optimum proportion to be separatedas pith is also variable, the essential requirement being that enough pith be separated so as to permit circulation of the caustic liquor under moderate pressure. This substantially complete removal of the pith may be effected by a preliminary rough screening of the bagasse as received from the mill to remove a portion of the pith; passing of the screened fiber through the attrition mill to loosen the adhering pith cells; final screening of the fiber and pith from the attrition mill in orderto thoroughly separate the fiber from the 30 pith, with the result that the fiber only is w to the digesters. Another satisfactory procedure consistsinpassingallofthemillrunbagasse directLv to the attrition mill from which it is passed over screens for the thorough removal of the pith, the resultant clean fiber going direct to the digesters.

'Ihenextordigestionstep, whlchlsoneof the principal features of the present process, will be described in detail below. The remaining steps'lndlcated on the diagram will be well understood by those skilled in the art of paper making. with reference to refining and bleaching,thecrudepulp mayberefinedinanyknown manner as, for instance, by first screening and 95 rilfilng the crude stock to remove such particles or undigestedfiberasmay remaininthepulpandto settle out heavy ash particles which cannot be removed by chemical means. To facilitate the screeningand washlngofthepiflpand toremove 1W ,liquorandssh, watermaybe vigorously sprayed 'onthepulpasitoveraseries oidrum screensorwsshers. 'Ihewashedpulpisthen partially bleached by treatment first with chlorine while violently agitated in a tank. The wamoimtofchlorlneusedislessthanthatreqmred for complete reaction with the pulp but sumcient toreactwith a largepartof thelignln, pentosans,

'etc. The chlorinated pulp is then washed with solve chlorinated organic compounds. After this cium or sodium hypochlorite in sufiicient amount to produce a white color in the pulp. If desired, the stock may be acidified before removing the final bleach liquor or it may be given a separate acid treatment after the bleach is removed.

After washing with pure water and drying, the

. pulp is ready for use. The total chlorine consumed on a typical pulp may be 2.5% of the weight of the pulp of which 60% may be used as free chlorine in the first stage of bleaching and as calcium hypochlorite in the second state of bleaching.

- Before bleaching, the amount of chlorine necessary to produce a satisfactory white pulp is determined by any one of several known tests used in the industry for this purpose. This chlorine is applied in two stages.v The preferred process involves treating the pulp at 3% consistency with free chlorine water to the extent of of the necessary chlorine and after exhaustion of this chlorine treating the pulp with just sufiicient alkali to produce an alkaline reaction. Treatment with alkali dissolves any chlorinated lignin compounds from the cellulose and these soluble compounds are washed away by water. Following this, "the second stage of bleaching is carried out by treating the pulp at a 10% consistency with the remaining 40% of necessary chlorine'in the form of calcium hypochlorite until a white color is pro duced. The pulp slurry is acidified at the end of the bleach and washed free of chlorine and acid. The bleached pulp is then ready to form into board, paper, etc.

Before proceeding with the description of the method of digestion used in the present process, it may be noted that a pretreatment with water is, in some instances,- desirable. In the digestion step of the process an optional water digestion may precede the regular digestion by treating the fiber withhot water between 100-200" C., and preferably between 130-160" C., under pressure. At the end of this treatment the water may be conveniently blown from the digester and the digestion continued with alkali in the manner outlined below.

As an illustration of the pretreatment with water before regular digestion with caustic solution the depithed bagasse is charged into a digester and treated for one hour with hot water at 150 C. Such a treatment will dissolve from 10-20% of the'original bagasse as water-soluble compounds, and these may be removed from the fiber by blowing out the water as completely as possible at the end of the treatment. Fiber which is thereby reduced to between to of its original weight is subsequently countercurrently digested with the caustic solutions.

One method of carrying out the digestion process will be described as it can be applied in a battery of four digesters. It is assumed that the process is in full operation and that the cycle of operations which is described as occurring in a system of four digesters is equally applicable to each separately in proper sequence. The digestion process canperhaps be most easily described if it is considered as made up of six separate cooks which may be taken as periods during which each digester is isolated and the cooking liquors circulated through outside heaters to maintain temperatures. The strength of the caustic soda increases and the amount of dissolved organic matter decreases as the cooking progresses from cooks 1 to 6 so that the cooking under the preferred practice of the invention is, therefore, semi-countercu'rrent innature. In Figs. 2 and 3 are shown diagrammatic views of the digestionv process during the two-principal liquor transfer operations which occur alternately between the progressive'stages of digestion. It will be understood that the system of piping has been greatlysimplified so that only the bare essentials required to indicate the transference of cooking liquors are shown. The quantities of liquor indicated as transferred in any one operation are merely representative of the general cooking procedure and so are conveniently taken as fractions of the full capacity of one digester. During operation the various digesters are always maindigesters 1, 2 and 3 while No. 4 is out of service and is being filled with a fresh charge of fiber. It may be well to state here that during the charging of any digester a certain amount of spent liquor is sprayed in with the fiber to help wilt it so that a maximum charge may be used; this is made necessary by the fact that the fibers do not otherwise pack readily. As far as the other three digesters are concerned, it will be assumedthat cooking has progressed so that No. 1 digester has just completed its fifth cook and is ready for its sixth and last cook, No. 2 digester has just completed its third cook or is ready to start its fourth cook, and No. 3 digester has just completed its first cook and is therefore starting its second cook.

.The liquor transfer represented in Fig. 2 consists in introducing fresh strong caustic through the system at A, pumping it by means of pump 10 through the valved conduit 11, 12 and 13 into the bottom of No. 1 digester and so displacing from the top of this digester practically all the liquor which it contains. The liquor leaving the top of gester through the valved conduit 16 and 1'7 as indicated bythe arrows. The liquor in No. 2 digester is, in similar manner, displaced into No. 3 digester and the nearly spent liquor in N0. 3 digester is forced out of the digestion system at B. The amount of liquor displaced through the system in this transfer is approximately equal to a full digester charge so that at the end of the transfer the change of liquor has been practically complete in every digester involved. The three digesters Nos. 1, 2 and 3 are then isolated and the liquor allowed to circulate through the outside heaters in each case. The temperature of the cooking liquors is maintained at the proper level during both the transfer of liquor from one digester to another and during the period of isolation by the continual, passing of said liquors through the heaters H.

A part (in this case of the nearly spent liquor taken from the system at B during the foregoing transfer is held in storage for a short time and then as already noted is returned to the digestion system to help wilt the fiber during the filling of another digester. The remainder of the spent liquor is sent to the recovery system for the and second cooks, respectively. Digester No. 1 is, therefore, ready to be taken out of service and digester No. 4 is to start onits first cook. During this transfer fresh caustic is not introduced into the system but the liquor to be displaced through the system is drawn from the bottom of No. 1 digester. The amount of liquor transferredis also much less in this case than in the first transfer, for only about one-third of a digester charge is so displaced. Digester No. 4 (going into service) is already filled to approximately two-thirds its capacity with nearly spent liquor which was added during the charging of the digester, and the displacement of liquor from the rest of the system is stopped as soon as No. 4 digester is filled. As soon as this is done each digester is again isolated and circulation started in digesters Nos. 2, 3 and 4.

The completion of the foregoing transfer leaves v digester No. 1 approximately two-thirds filled with fairly strong cooking liquor and completely cooked pulp. The strong liquor is blown out through the falsebottom of the digester into a storage tank from which it is returned to the digestion system after fortifying as fresh cooking liquor. The crude pulp in digester No. 1 is washed once with hot water in the digester, and then blown from the digester along with the. second wash water, and is thereafter ready for the refining and bleaching operations. The empty digester No. 1 is refilled with fiber in the same manner as already described for digester No. 4. At the end of the circulation period in progress, fresh cooking liquor is introduced into No. 2 digester in order to force a complete displacement-of liquor through Nos. 2, 3 and 4 digesters in exactly the same manner as the first transfer already described for Nos. 1, 2 and 3 digesters. The whole cooking cycle hasmoved forward one digester, thus completing the cycle of operation so far as the system as a whole is concerned. v

It will thus be seen that the system illustrated in Figs. 2 and 3 has four digesters, in each of which the cooking is continued from an initial to a final cook, the stage of cooking also progressively increasing from an initial cook in the digester at one end of the system to a final cook in the digester at the other end. In Fig. 3the initial cool: is beginning in digester No. 4 and the final cook has ended in digester No. 1, from which the purified pulp is blown. The liquor in circulation with the other digester is cut oil? and the empty digestcr, which is now No. 4 in Fig. 2, is seed with bagasse. The liquor of the remaining now dted as Nos. 2, 3 and 4 in Fig. 3, is then displaced from one digester to another, the spent liquor from digester No. 4 of 3 which contains the first cook being introduced with the new charge of bagasse as described above.

In Fig. 4 is shown a schematic arrangement of the complete cycle of operation for each digester in one bank of four digesters. The method for operating the process as referred to above is depicted. A complete cycle of operation for each digester in one bank of four digesters is shown. In this the horizontal circles represent the static stages described in the detailed descrip-' tion above, and each perpendicular row of circles represents all the various stages (static) of distage in the cooking of the raw fiber, while num.

her s is the last-stage of digestion just prior to washing and discharging the finished pulp. The letter W represents the stages in which the pulp is washed in the digesters and the letter F the periods during which the digesters are filled with raw fiber. The spaces between the horizontal rows of circles represent periods during which transfers of liquors take place and the arrows indicate the direction of fiow of the liquors between individual digesters. The fractional num bers accompanying the arrows in this figure as well as in Figs. 2 and 3 indicate the amounts of liquor transferred in any one dynamic stage of digestion, for example, 3/3 indicates a complete change of liquor in each digester. The notations at the side of the block of circles deal with the input and output of the various materials which are either used in or are products of the digestion process.

My improved process is best carried out with caustic concentrations between 6% and 9%. Concentrations between 6% and 12% may, however, be used satisfactorily, but the process becomes less economical when the concentration rises above 9%. It will be understood that these concentrations refer not only to sodium hydroxide but also to equivalent strengths of other cooking liquors such as a mixture of sodium hydroxide and sodium sulfide. Thus I may use with satisfactory results a kraft type digestion liquor in which the alkalinity is obtained by using about two-thirds sodium hydroxide and one-third sodium sulfide. I prefer to operate at a temperature of about 140 C. Deterioration sets in too rap- For example, at a cooking temperature of 140 C.,

the pressure in the digesters will be about 50 pounds. These gases may be relieved from the digesters from time to time.

digestion will depend on the concentration of The total time during which a single batch of fiber is undergoing alkali used, the temperature, and the pressure at l which the digestion is carried on. Roughly, the time of digestion is inversely proportional to the product of these factors. With respect to time, it may be stated generally, therefore, that the process is continued until a high alpha cellulose is obtained, i. e., a pulp containing not less than about 95% alpha cellulose. A convenient time for a complete cock is 7 hours operating at a temperature of 140 C. and a pressure of 50 pounds per square inch.

Since the grinding season in the sugar countries does not extend over the entire year, it is necessary to store some of the depithed fiber. This may be conveniently done either baled or loose, but under cover so as to be out of the weather. The stored fiber may be drawn on as needed by the digesters.

The digestion process as outlined above gives only one example of several possible variations of the procedure for carrying out a semi-continuous counter-current digestion of bagasse and, therefore, must not be understood as limiting the scope of my invention. For example, the process may be altered to decrease the amount of liquor displaced during the transfer in which fresh caustic is introduced so that a complete change of liquor is not obtained and only the quantity of spent liquor which is to be sent to recovery is removed [still other procedures the two principal liquor transfers may be made almost equal in amount or they may conform to any desired amount between the two extremes represented by the first and second procedures.

Withreference to the procedure described in connection with Figs. 2 and 3, it will be apparent that the amount of liquor transferred through the system when a new digester is put into service is relatively small when compared to the amount of liquor transferred when fresh caustic is added to and waste liquor removed from the system. The following is an example of the procedure outlined above with the exception that the amounts of liquor transferred at any one time are different. The weights of materials given refer to operation in a battery of four small digesters used in the development of the digestion process on a small scale in which the total liquor capacity of each digester is about 525 pounds of water.

The charge of fiber is 45-50 pounds of depithed bagasse containing about 25% moisture and the yield of high alpha pulp approximately 15 pounds or 40%, based on the dry fiber charge. Fresh caustic solutions added to the system contain alkali (sodium hydroxide and sodium carbonate) 7 equivalent in sodium content to a 9% sodium hydroxide solution which produces acooking liquor equivalent to an 8% caustic solution. For a given initial caustic concentration the amount of free caustic in the waste may be controlled ,within limits by varying the total amount of liquor passing over the fiber. Thus when the initial concentration is around 8% sodium hydroxide, the free caustic in the waste black liquor may readily be reduced to 1% or even below by limiting the amount of liquor drawn on at each stage. Under this system of operation each time fresh caustic is introduced into the system 450 pounds is the amount added, and 400 pounds-of black liquor is removed at the opposite end of the system. One hundred pounds of this black liquor is sent to waste and 300 pounds returned to the next freshly charged digester. The cycle is completed by recovering alkali from the black liquor in known manner, including the steps of concentrating the liquor, burning the organic matter, causticizing and using the recovered caustic in the digesters. When a new digester is put into service the amount of liquor displaced through the system is small since only about 150 pounds of strong liquor need to be pumped from digester No. 4 through digesters Nos. 3 and 2 to completely 'fill No. 1 digester. During the periods between gester into a storage tank from which it is re-.

turned to the digestion system. The time required for the transfer in which the caustic is introduced is about 30-45 minutes and for' the transfer involving the starting of a new digester is between 15 to 30 minutes. The total digestion time is about 7V hours with each charge started one and one-quarter hours apart. The pulp produced by the foregoing process averages -96% alpha cellulose.

Another variation in digestion procedure is to eliminate entirely the periods during which each digester is isolated, and the liquor circulated separately in each. Carrying out this 5 -modification the transfer of liquors is slowed down so that the time of transfer and isolation is all taken'up by the transfer only. Under these conditions digestion proceeds as usual and the temperature is maintained by the flow of liquor through the heaters during the transfer.

The present process may utilize raw cellulose material from plants of the grass family such as bamboo, brava cane, sugar cane, esparto, and corn stalks, and is particularly advantageous when applied to the preparation of high alpha pulp from pith bearing plants such as corn and sugar cane.

An important advantage of this process over known methods of preparing high alpha pulp 7 from vegetable materials is to be found in the fact that the crude fiber is converted in one step whereas other known processes for treating raw gramineous fiber involve two or more separate treatments, such as an acid and alkaline treatment or two distinct alkaline treatments, one usually with a very concentrated alkaline solution. The advantages of processing bagasse and similar pith bearing fibers for the production of high alpha cellulose by the countercurrent treatment under pressure as described herein, has not,v insofar as I am aware, been disclosed. An important finding which has made possible the successful production of high alpha cellulose from bagasse (the term. bagasse being used to designate also raw cellulose material of like nature) .by the one step process described above is the discovery of the marked advantage which results when the pith is so thoroughly removed from the fiber that substantially none remains. While the depithing of bagasse is in itself not new, it is to be observed that it has not previously been used with countercurrent digestion under pressure, and that my process is characterized by removal of the pith from the fiber to an extent beyond that heretofore practiced or n in prior processes, and that the fiber thus prepared is converted in one cook to a high alpha cellulose pulp by the continuous countercurrent process described. In the present pressure digestion process any appreciable pith content of the fiber prevents adequate circulation of the cooking liquors, it being practicalLv impossible to circulate the caustic liquors and, therefore, im-

possible to carry out the present countercurrent process unless the pith is first substantially removed. Furthermore, I find that the thorough removal of the pith from bagasse decreases the consumption of caustic and markedly improves the quality of the product for such uses as rayon, cellophane and cellulose derivatives.

An additional advantage of my invention resides in the fact'that high alpha cellulose is obtained by a process in which nearly all the caustic is used. This result is brought about -by controlling the flow of digestion liquor so that the waste liquor is practically stripped of free caustic. Another advantage resides in the fact that the liquors contain enough organicmatter to enable the caustic to be burned in recovering the caustic.

As many apparently widely difl'erent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as def fined in the following claims.

I claim:

1. A process of producing high alpha cellulose pulp which comprises countercurrently digesting rawgramineous fiber substantially free from pith in alkaline liquor maintained at 130-160 C., and washing the pulp, said countercurrent digestion being continued until a pulp is obtained which contains not less than about 95% alpha cellulose on the dry basis.

2. The process set forth in claim 1 in which the alkaline liquor is of 6 to 12% concentration expressed in terms of sodium hydroxide.

3. A process of manufacturing purified cellulose pulp which comprises removing substantially all of the pith from a pith bearing plant of the grass family, countercurrently digesting the raw depithed fiber with alkaline liquor at elevated temperature and pressure, and washing the pulp.

4. A process of producing high alpha cellulose pulp which comprises removing at least of the weight of sugar cane bagasse in the form of pithy material, countercurrently digesting the depithed bagasse in alkaline liquor of 6% to 12% initial concentration expressed in terms of sodium hydroxide, said'liquor being maintained at 130-l60 C., and washing the pulp, said countercurrent digestion being continued until a pulp is obtained which containsnot less than about 95% alpha cellulose on the dry basis.

5. A process of manufacturing purified cellulose pulp which comprises depithing a pith bearingplant or. the grass family, countercurrently digesting the raw depithed fiber with caustic Y liquor, washing the pulp, suflicient pith being separated in the depithing so that the caustic liquor may be circulated under moderate prestion of the raw fiber'being a multiple stage countercurrent digestion in an extraction system which includes at least three digesters through which the liquor passes in series.

7. A process of producing high alpha cellulose pulp which comprises countercurrently digesting raw gramineous fiber substantially free from pith in alkaline liquor maintained at lilo-160 C., and washing the pulp, said countercurrent digestion being continued until a pulp is obtained which contains not less than about alpha cellulose on the dry basis, the digestion of the 95 raw fiber being a multiple stage digestion in, l which the liquor is recirculated in each stage.

8. A process of producing high alpha cellulose pulp which comprises removing at least 35% of the weight of sugar cane bagasse in the form of pithy material, digesting the depithed bagasse with hot water and then countercurrently -digesting the treated bagasse in alkaline liquor maintained at l30l60 (3., and washing the ulp, said countercurrent digestion being 'cont ed until a pulp is obtained which contains not less than about 95% alpha cellulose on the dry basis.

9. A process of producing high alpha cellulose pulp which comprises countercurrently digesting raw gramineous fiber substantially free from pith 3 in alkaline liquor maintained at 130-460 C., and washing the pulp, said countercurrent digestion being continued until a pulp is obtained whichcontains not less than about 95% alpha cellulose on the dry basis, and subjecting the 1 5 treated pulp to a partial bleach followed by a final complete bleach.

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