US2621124A

US2621124A - Pulp purification

Info

Publication number: US2621124A
Application number: US673188A
Authority: US
Inventors: Louis S Keyser; Robert E Brown
Original assignee: Rayonier Inc
Current assignee: Rayonier Inc
Priority date: 1946-05-29
Filing date: 1946-05-29
Publication date: 1952-12-09
Anticipated expiration: 1969-12-09

Description

Dec. 9, 1952 Filed May 29, 1946 s. KEYSER ETAL 2,62L124 PULP PURIFICATION 2 sHEETs-sHEET 1 Egfilri- @l Tim, @wmf/1 ma. www

ATTORNEYS Dec. 9, 1952 L. s. KEYSER l-:TAL

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Qbl 15H 1M bbb www Patented Dee. 9, 'i952 FUEL? PUEFICATEN l'eyser ai "l E. Brown, Shelton,

This invention relates to a process for the purication of pulp with strong solutions of alkali, and more particularly to the purification of Wood pulp produced by the digestion of wood as in the form of chips in aqueous, acid or alkaline liquors. (Hereinafter, alkali refers to sodium hydroxide and like caustic materials, and the liquor containing dissolved cellulosic materials as hemi-caustic or simply hemi, terms used in the art.)

In general, the processes heretofore propose-:l for the production of high alpha wood pulps involve an alkaline purication or extraction of the pulp obtained from the digestion of wood chips in suitable media. In those operations involving the use of aqueous solutions containing from to 20% of alkali, such as sodium hydroxide, (i. e. from 40% to 206% or more of the alkali based on the weight of the pulp) at temperatures below 56 C., an efficient purication of the cellulose is effected but at the expense of high alkali loss. Appreciable savings in alkali have resulted from re-use of the alkaline liquor. But the liquor so re-used becomes progressively more heavily laden with dissolved organic materials (impurities) which act to reduce the efficiency of the liquor for puriiyinfT the pulp even though the alkali content is maintained.. Eventually a point is reached where this heini-caustic must be processed for recovery of the alkali since the liquor, as such, contains too much alkali to allow its being discarded.

This invention provides improvements in the production of highly purified cellulose from wood pulp which result in an et'fluent or waste liquor high in dissolved organic materials but low in contained alkali, resulting in the economical use of strong alkali solutions. i n additional irnportant feature of the invention is the provision of a simple means oi displacing the water in fresh pulp and removing it along with the discarded eitluent. The invention, in its more or less complete aspects, comprises a conjoint operation with the displacement of the water, the absorption or" the alkali from the extracting liquor by the fresh pulp, arl the extraction of organic material from the p p during the displacement action.

pure pulp through a wet mat or l'lre body or pulp which has not as yet received the alkali treatment. The mat of pulp is advantageously supported on a wire screen as, for example, on

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Serial 673,188

the surface of a conventional rotary vacuum washer and the liquor is sprayed on or otherwise put on the surface and drawn or forced through the pulp. rihis operation, when properly controlled as will be described later, produces an eiiluent which contains practically all of the organic material extracted from the pulp but at the same time only a fraction of the alkali which was in the solution sprayed onto the mat, resulting in an eiuent or waste liquor high in dissolved organic material but low in alkali. forcing of the used extracting liquor through the mat of pulp serves also to displace the water that was with the pulp before the operation was performed. This removal of the water is desirable and a very important part of the invention.

These and other novel features of the invention will be better understood after considering the following discussion taken in conjunction with the accompanying drawings, in which:

Figs. 1, 2 and 3 are schematic representations oi the extraction principle of the invention, and

Fig. l is a diagrammatic arrangement of apparatus suitable for carrying out a method of the invention.

Figs. 1, 2 and 3 show what occurs as the displacement takes place revealing synergistic phenomena which result in the effectual and unpredictable separation of the dissolved organic material from caustic soda in the extracting liquor.

ln Fig. l are represented a pulp mat supported on a suitable screen or support such as a Buchner funnel or other such device, and a portion o1 heini-caustic liquor, such as would be obtained from an extraction cell (cell 2, Fig. ll). The amount of liquor represented is equal to 1.3 times the weight of water associated with the pulp. dissolved organic naterial and the alkali are completely homogeneous, each portion being identical to any other portion throughout. Fig. i. thus illustrates the instantaneous situation beiore the displacement operation starts.

Fig. 2 shows the instantaneous situation which Xists after one equivalent of liquor has just displaced the water which was originally with the pulp. Now, instead of being uniformly distributed through the mat, the constituents of the liquor have become separated. At the bottorn of the mat the concentration of alkali is low, while in the case of the dissolved organic materials the reverse situation exists. The concentration of the dissolved organic materials is gust 3 higher near the bottom of the mat and decreases to the level of that of the original hemi-caustic near the top ofthe mat.

This separation of the dissolved organic materials from the alkali is one of the important features of the invention. Thus, after one equivalent of liquor is showered on the mat, the following things have occurred: (1) The water has been removed from the pulp, (2) the alkali has accumulated in the upper parts of the mat, and (3) the dissolved organic materials have accumulated in the lower parts of the mat.

Fig. 3 shows the situation existing after the last 0.3 of the 1.3 equivalents have been removed from the mat. The removal of this last portion, rich in organic material but containing only small amounts of alkali, effectively separates the dissolved cellulosic impurities from the bulk of the alkali with which it was previously associated. The hemi-caustic solution simply pushes the water before it, thus effectively displacing it. As the strong caustic solution passes through the pulp, the alkali is selectively absorbed by the pulp for which it has a great affinity. Thus, the farther the solution proceeds through the mat the more diluted in alkali it becomes until it finally passes through and out of the pulp as shown in Fig. 3.

The hemi-celluloses, on the other hand, increase in concentration as the caustic solution proceeds through the pulp. This results from the extraction of additional hemi-cellulose from the pulp by the solvent action of the alkali during its passage through the mat. Once the hemicellulose is separated from the pulp it shows no tendency to be re-absorbed. The synergistic action of these factors operating simultaneously thus produces a highly practical result. While in the foregoing discussion, for purposes of simplication, it is assumed that the displacement process is 100% efficient, it should be pointed out that the practical result falls somewhat short of this representation. However, the displacement is sufficiently complete that the over-all process results in the complete removal from the system of all or an equivalent amount of the water which was originally associated with the fresh pulp.

An important consideration with regard to the determination of the most effective amount of the shower concerns the ratio of alkali to pulp in the mat undergoing displacement. When the amount of alkali showered on the pulp is in excess of the amount which the pulp can absorb from a solution of that particular strength, then no further absorption of alkali will occur and the savings resulting by the operation of this factor will cease. In practical operation this point will ordinarily not be exceeded because the amount showered will normally depend more on the level of dissolved organic material or heini-cellulose in the liquor. Only that amount of shower should be used which will result in the elimination of the necessary amount of organic extractives to be removed from the pulp.

The diagrammatic arrangement of apparatus illustrated in Fig. 4 comprises a rotary vacuum washer I, single-stage or multi-stage, on which the displacement will be made. Other suitable types of washing apparatus may be used. The apparatus includes an extraction cell 2, a thickener 3, a countercurrent washing system 4, evaporators 5, a make-up tank 6, an alkali supply '1, squeeze rolls 8, and interconnecting conduit for the flow of material as indicated by the, arrows- Arrangements are provided for the addition of make-up alkali and for recovery of alkali from the more dilute wash liquors.

In carrying out a process of the invention, the liquor in the extraction cell 2 is maintained at a dennite alkali strength. If a pulp stock containing water not previously introduced into the system were charged to the extraction cell, such water would necessitate the addition of further quantities of alkali and at the same time dilute the liquor with respect to the dissolved organic materials. Both of these effects are unwanted and therefore the removal of water associated with the impure pulp is necessary. Normally, such water can be removed by means of vacuum thickeners or press rolls, even under extreme conditions, only to the extent of about 50 7,3 stock consistency.

In this process the removal of this water accomplished by a displacement using the liquor from the extraction cell as the displacing media. As mentioned previously, the operation is advantageously carried out on a conventional vacuum washer. The hemi-caustic liquor from the extraction cell 2 is put on and pulled or forced through the mat of pulp on the washer surface. The liquor in passing through the mat effectively forces the water before it with surprising efficiency. Theoretically a weight of liquor equal to the weight of water associated with the pulp would be suflcient to displace all of the water and leave the pulp at substantially the same consistency but containing instead of water the extracting liquor which was used for displacement. In the practice of the invention additional liquor over and above that necessary to displace the water is put on the pulp and forced into and through the mat. The amount of this shower depends on a number of factors but mainly on the consistency obtained on the displacement washer l and the amount of dissolved organic material in the extraction liquor from thickener 3. Under normal conditions this would amount to from 1.1 to 1.5 equivalents. (The term equivalent as used herein and in the appended claims has the following significance: One equivalent is equal in weight to the liquor associated with the pulp after the displacement. The consistency is that per cent which the bone dry pulp comprises of the total mixture of pulp plus liquor.)

After the displacement of the water in the sheet has occurred, a portion of the showered liquor equal to that amount in excess of one equivalent in the total shower then passes through the mat of pulp and is discarded along with the water displaced. Under equilibrium conditions this effluent contains a considerable amount of the organic material to be discarded but very little of the alkali that was originally associated with this same organic material in the original hemi-caustic liquor showered on the sheet. The bulk of the alkali which was in the original displacing liquor remains with the sheet and retrusn with it to the extraction cell. In this manner the alkali requirements of the extraction operation are thus effectively reduced.

The remainder of the effluent from thickener 3 (i. e. that which is not showered on the incoming pulp) is returned to the extraction cell 2 and by virtue of the continual reuse of this liquor the level of the dissolved organic materials is maintained at the desired point.

The conditions maintained in the extraction cell may be varied over wide limits depending on the quality of the product desired. Usually the concentration of caustic soda will be from 5% to 14% (based on the Weight of liquor) while the temperature Will ordinarily be between C. and 50 C. The consistency of the mixture in the extraction cell 2 will vary between 1% and and the time of the extraction may be varied from 5 minutes or less to a matter of hours.

The process used to perform the strong caustic soda purification or step is not critical. Any of the suitable countercurrent processes may be used.

The quality of product produced by this process will depend on the actual operating conditions and may be varied as desired. One of the most important controlling factors is the concentration of the dissolved organic materials. Starting with an unbleached chlorinated southern pine pulp of about 93% alpha content, for example, the nal product will have from 99% to 97% alpha cellulose if heini levels from 0 to 12% are used at C. in the extraction stage.

The following examples are illustrative of the invention:

Example 1 In an extraction process using 12% NaOH at 25 C. and at 5% consistency for 30 minutes, a hemi concentration of 3.6% in the extracting liquor results in a pulp of approximately 98.3% alpha cellulose from an unbleached-chlorinated southern pine pulp of approximately 93% alpha cellulose. In the operation of a D-ton mill one and three tenths equivalents of the liquor from extraction cell 2 is sprayed on a sheet formed on vacuum washer I at 27% bone dry consistency, and a corresponding quantity of eiiluent is removed from the sheet. Under these operating conditions an equal quantity of the water originally with the pulp is thus removed, and in addition 0.3 equivalent of very weak caustic containing nearly all of the undesirable materials to be removed from the pulp is discarded from the system. If a corresponding amount of liquor equal to that sprayed in the sheet had been discarded directly from the process without forcing it through the sheet of pulp, there would have resulted a caustic usage for this step of about 580 pounds, or more, of NaOH per ton of finished pulp. However, as a result of the practice of the invention, the caustic usage for the step is l only about 120 pounds NaOH per ton. Further without the removal of the water in the incoming stock by the displacement operation, the expense of removal of this water would be added to the cost of the operation.

Eample 2 A process using 12% NaOH at 25 C. at 5% consistency in the extraction cell 2 produces in 30 minutes a pulp of 97.6% alpha cellulose from an unbleached-chlorinated 93% alpha cellulose pulp if the hemi level is maintained at 8%. In the operation of a 30D-ton mill when 1.1 equivalents of the extraction liquor are sprayed on the incoming unbleached chlorinated pulp of approximately 93% alpha cellulose (at 27% consistency on washer thickener i) and a corresponding quantity of effluent collected, a caustic soda usage of approximately 60 pounds of NaOH per ton of finished pulp results for this step. If this hemicaustic had been discarded directly without passing through the pulp, there would have resulted a caustic soda usage of about 240 pounds of NaOH per ton of finished pulp in addition to the drying costs as noted in Example 1.

Eample 3 Under conditions similar to those in Examples 1 and 2, a hemi level of 1% in the extraction cell would produce a pulp of 98.6% alpha cellulose. To remove all of the heini from the solutions obtained from operation of a 3D0-ton mill 2.0 equivalents of hemi-caustic are sprayed on the sheet and a like amount of eiliuent removed. Such procedure results in a caustic soda usage of around 540 pounds of NaOH per ton of finished pulp, Whereas if the extraction liquor were discarded directly, the usage would be on the order of 2150 pounds per ton of finished pulp in addition to drying expense.

Example 4 In the operation of a process in Which the consistency on the washer I is 13.6% and other conditions similar to previous examples, 1.3 equivalents When forced through the incoming pulp give a caustic soda usage of about 500 pounds of N-aOI-I per ton of finished pulp as compared to around 1930 pounds of NaOH per ton of finished pulp if the solution were discarded directlly. Drying costs would be additional here also.

Example 5 In a process where the water is removed from the incoming unbleached-chlorinated pulp by three stages of countercurrent displacement on a suitable device providing an unbroken sheet, for example, a 3-stage countercurrent vacuum washer, the caustic usage, when 1.3 equivalents (pulp at 26% consistency) is passed through the pulp, is about pounds per ton of finished pulp for this phase of the process. This may be coinpared with a caustic soda usage of about 675 pounds of NaOH per ton of finished pulp if the extraction liquor from cell 2 were discarded directly.

In Examples 1, 2', 3 and 4 further economies would result if more than one stage of countercurrent displacement were employed.

While in the foregoing example caustic soda was used as the extracting medium, this in no way should be construed as to limit the use of the process to this medium. Other alkaline substances exerting similar extracting properties and which are preferentially absorbed by cellulose will act in the same manner as caustic soda. Potassium hydroxide, potassium sulde, sodium sulfide and other hydroxides and sulfides or other salts of Weak acids of metals of the first group are all examples of substances which would perform in an analogous manner.

The apparatus for performing the displacernent need not necessarily be limited to a conventional vacuum washer. Special equipment can be provided to furnish more stages of countercurrent displacement than are possible on conventional equipment. The principle of the invention is also capable of application to a completely continuous countercurrent system such as would exist in a tube containing pulp moving in one direction with alkali solution moving in the opposite direction. Any such device which could accomplish this action would be suitable.

The equipment represented by extraction cell 2, thickener 3, and the countercurrent washing system Il is schematic only, and any standard or special equipment which would perform similarly or produce the same result would function equally well in this process.

As can be seen from the examples the practice of this invention results in substantial savings in caustic soda under a wide variety of conditions. The quantity of caustic soda required in the past for this type of an extraction process has been of such magnitude as to make impracticable the wide use of this extremely chemically eiiicient method for purifying cellulose on a commercial scale. The use of the invention, as described herein, changes this situation markedly, and in fact makes production of cold caustic extracted pulps entirely practical.

Practically all o1" the materials to be remove-:l from the impure pulp in this phase of the puriication will be discarded through the sheet on the displacement washer I, and after thickening to the highest practicable consistency on the thickener 3, the pulp will contain only small amounts of dissolved organic materials. Much of the sodium hydroxide associated with the pulp at this stage is actually absorbed within the bers of pure cellulose and as such, when washed from the pulp, carries no dissolved inaterial with it. The net effect is that the countercurrent washer l produces an effluent sodium hydroxide relatively free or" dissolved organic materials and suitable for recovery` by a simple evaporation process, making it unnecessary to resort to evaporation to dryness, to destroy the organic materials by burning, to recausticize the smelt, and then to prepare a solution of the proper concentration.

We claim:

1. The improvement in the production of highly puried cellulose from wood pulp which comprises passingr an aqueous alkaline extraction liquor containing from 5% to 14% of caustic and heini-cellulose through a wet mat of fresh pulp containing water but substantially free of alkali, the amount of said extraction liquor being in the range 1.1 to 1.5 equivalents, whereby practically all of the water in the mat of pulp is displaced by the extraction liquor as an effluent and, in addition, 0.1 to 0.5 equivalent of an efuent liquor is produced containing liemi-cellulose but substantially reduced in caustic content.

2. The improvement in the pro-duction of highly puried cellulose from wood pulp which coinprises passing an aqueous alkaline extraction liquor, containing from 5% to 14% of caustic which has been used and re-used many times in a cold extraction of impurities from the pulp, through a wet mat of fresh pulp containing water but substantially free of alkali, the amount oi" said extraction liquor being in the range 1.1

to 1.5 equivalents, whereby practically all or the l water in the mat of pulp is displaced by the extraction liquor as an eiiluent, and a substantial portion of the caustic in the extraction liquor is absorbed in the pulp, thereby producing an eiliuent liquor for discard amounting to 0.1 to 0.5 equivalent with a substantially reduced caustic content and with an enhanced heini-cellulose content.

3. The process of claim 2 wherein the amount of extraction liquor used for the displacement is about 1.3 equivalents.

4. The process of claim 2 wherein the pulp containing absorbed caustic is subjected to further extraction with cold alkaline extraction liquor to dissolve additional organic matter.

5. The process of claim 2 wherein the pulp cortaining absorbed caustic is subjected to further extraction with cold alkaline extraction liquor to dissolve additional organic matter, and the pulp is separated from the alkaline extraction liquor by a countercurrent washing process.

6. In the purification of wood pulp by extraction with aqueous alkaline liquor containing from 5% to 14% of caustic resulting in a heini-caustic liquor, the improvement which comprises separating a substantial portion of the alkali from the organic materials in the hemi-caustic liquor to be discarded by forcing 1.1 to 1.5 equivalents of said hemi-caustic liquor through a Wet mat of fresh pulp containing water but substantially free of alkali to displace practically al1 the water in the fresh pulp as an effluent, absorb in the pulp a substantial portion of the alkali from the heini-caustic liquor` and dissolve a further quantity of organic material from the pulp, thereby producing an additional effluent waste liquor for discard of 0.1 to 0.5 equivalent which is enhanced in dissolved organic material but substantially reduced in alkali, subjecting the pulp containing the absorbed alkali to an alkaline extraction operation for the removal of additional organic materials, and thereafter separating the pulp from the caustic extraction liquor by countercurrent washing with water.

7. A process for the recovery of a substantial portion of the caustic from waste heini-caustic liquors obtained in the cold alkaline treatment of wood pulp with aqueous liquor containing from 5% to 14% of caustic comprising forcing this heini-caustic liquor through a wet mat of fresh pulp containing water but substantially free of alkali on a rotary vacuum washer to displace practically all the water in the fresh pulp and absorb a substantial portion of the alkali in said pulp, said hemi-caustic liquor being used in an amount varying from 1.1 to 1.5 equivalents, and subjecting the pulp containing the absorbed alkali to an alkaline extraction to dissolve additional organic matter.

LOUIS S. KEYSER. ROBERT E. BROWN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,632,802 Richter June 2l, 1927 1,683,262 Richter Sept. 4, 1928 1,829,378 Thiriet Oct. 27, 1931 1,947,106 Plumstead Feb. 13, 1934 2,324,230 Olsen et al. July 13, 1943 2,383,684 Richter Aug. 28, 1945 2,385,259 Collings Sept. 18, 1945 FOREIGN PATENTS Number Country Date 318,868 Great Britain Sept. 9, 1929 OTHER REFERENCES Ott, Cellulose and Cellulose Derivatives, Interscience Publ. Inc., N. Y., pp. 813-15 (1943).

Paper Trade Journal, May 26, 1938, p. 40.

Technical Association Papers, Series 21, pp. 367-72 (1938).

Claims

7. A PROCESS FOR THE RECOVERY OF A SUBSTANTIAL PORTION OF THE CAUSTIC FROM WASTE HEMI-CAUSTIC LIQUORS OBTAINED IN THE COLD ALKALINE TREATMENT OF WOOD PULP WITH AQUEOUS LIQUOR CONTAINING FROM 5% TO 14% OF CAUSTIC COMPRISING FORCING THIS HEMI-CAUSTIC LIQUOR THROUGH A WET MAT OF FRESH PULP CONTAINING WATER BUT SUBTANTIALLY FREE OF ALKALI ON A ROTARY VACUUM WASHER TO DISPLACE PRACTICALLY ALL THE WATER IN THE FRESH PULP AND ADSORB A SUBSTANTIAL PORTION OF THE ALKALI IN SAID PULP, SAID HEMI-CAUSTIC LIQUOR BEING USED IN AN AMOUNT VARYING FROM 1.1 TO 1.5 EQUIVALENTS, AND SUBJECTING THE PULP CONTAINING THE ABSORBED ALKALI TO AN ALKALINE EXTRACTION TO DISSOLVE ADDITIONAL ORGANIC MATTER.