US2269985A - Manufacture of wood pulp - Google Patents

Description

Patented Jan. 13, 19.42

MANUFACTURE OF WOOD PULP Fredrick Olsen, Alton, Ill., assignor to The Cellulose Research Corporation, East Alton, 111., a

corporation of Delaware No Drawing.

Application August 12, 1938,

Serial No. 224,524

6 Claims.

This invention relates to the'production of cellulose pulp from wood. An object of the invention is to improve the efliciency and uniformity of chemical processes for producing wood pulp by providing uniform and thorough preimpregnation of the wood with a delignifyingreagent prior to attaining reactive temperatures, followed-by a cooperating controlled .delignification treatment. This is in general accomplished by employing an improved impregnation procedure whereby the air is thoroughly expelled from the wood and is replaced by a liquid solvent of the reagent.

A specific feature is the initial impregnation of the wood in the manner indicated with a nondelignifying liquid, and thereafter introducing the reagent by diffusion through such liquid. In this method of operation the liquid may be an electrolyte solution suitable to provide on the wood surfaces an adsorbed layer which will initially obstruct attack by the reagent during diffusion thereof, particularly at nonreactive temperatures, and will thereafter permit appropriate digestive action by the reagent when the temperature is raised.

A feature of this method is the facility with which the wood may be impregnated and digested by reagents whose normal action on the wood creates serious obstructions to rapid penetration thereof, such as caustic delignifying reagents employed in the soda and kraft digestion processes.

Another object is to provide a process of delignification of wood which, owing to the appropriate impregnation and proper control of digestion, is susceptible of accurate control and will produce raw pulp having relatively uniform and predictable characteristics, particularly viscosity and bleachability, uniformity of distribution of residual qnoncellulosic constituents, anduniform physical and chemical characteristics of the fibers. Another purpose is to obtain a controllable and, if desired, a high yield of cellulose, particularly of alpha cellulose, with a lowdegree of cellulose degradation.

The penetration and permeation of wood with trea ing liquors of effective strength with thoroughness and uniformity has presented a serious problem. Previous attempts have been defective in depth of penetration, lack of uniform distribution of the reagents, or delay in delivering a reagent to the more remote portions of the wood until the more accessible portions have been subjected to substantial digestion, precluding uniformity of treatment.

For instance, in alkaline processes such as those employed in soda and kraft digestion, the cooking liquor exerts a swelling action on the fiber walls which decreases the diameters of the capillary passages within and between the fibers, thereby tending to prevent penetration to the inner portions of wood subdivisions. The penetration of the liquor is furthermore obstructed by adsorption of ions and streaming potential effects, and the production of colloidal substances which block the passages.

In addition to the problem of obtaining thorough penetration of the wood by the digesting liquor, there is a further problem of providing uniformity of chemical concentration in all portions of the wood reached by the liquor, a condition that is necessary if uniform delignification is to be obtained. Wood has the property of preferentially adsorbing reagents from treating liquors such as soda and kraft liquors, which tends to weaken the liquor progressively during penetration. Effective portions of the reagent are likewise held back bythe osmotic effects of wood membranes and of material obstructing the passages as a result of the action of the liquor.

The cumulative effect of these phenomena is to associate an excess amount of reagent with the exterior and a lesser amount with the interior of the wood, resulting in excessive action on the former portion before the latter can be adequately delignified. Tests made under commercial conditions indicate that particularly where a chip is impregnated with an alkaline reagent according to prior practices, the difierence in concentration between the exterior and interior portions of the chip will range from 5:1 to as high as 10: 1. The result is that conventional processing systems depend in the main on progressive deligniflcation from the exterior toward the center of the wood to provide adequate treatment of the center portion, resulting in overtreatment of the exterior and undertreatment of the interior portions.

Uniformity of impregnation is likewise affected materially by the wide variability in adsorption characteristics existing in different types of wood, such as different species and the heartwood and sapwood portions of the same species. These'variations accentuate the inequalities due to preferential adsorption of reagents; and as a suflicient amount of chemical must be provided in the treating liquor to meet the lignin demand in the less adsorptive types of wood under prior practice, some portions of the wood will have an excess of reagent and others may have an insuflicient amount, resulting in variability of reaction and product.

The unevenness of prior products includes lack of uniformity of residual lignin on the various fibers. This condition requires a more drastic purification to take care of the fibers having the maximum proportion of lignin and results in corresponding excessive action on the fibers having substantially lesser amounts thereof. Moreover, the variable action of the liquor on the fibers produces unequal attack on the cellulosic constituents, resulting in viscosities varying widely from the average viscosity, indicating unequal depolymerization and attack on the alpha cellulose. The viscosity and bleachability of successive batches will likewise vary over a wide range since the inequality of digestive action inevitably produces batch variations.

The key to proper impregnation consists in the thorough removal of the air content of the wood. The air in the interstices of the wood clings to the inner portions with great tenacity, particularly in the microscopic and submicroscopic pores; and partial air removal leaves residual air which is imprisoned and compressed during the usual penetration and digestion methods. This inevitably results in non-uniform delignification, as a relatively small volume of residual air will prevent the necessary intimate contact between the digesting liquor and a substantial portion of the ligneous material within the chip for a considerable proportion of the digesting cycle. Actual tests have shown that even the use of strong suctionv does not effectively remove the adsorbed air.

By the process of this invention, digesting liquors of proper strength are uniformly distributed throughout the wood before providing reactive temperatures so that when the proper temperature is reached the reaction proceeds rapidly and .simultaneously throughout all portions of the .treatment in a liquid which removes the air thoroughly from the wood, replacing the air partly by the liquid and partly with vapor, particularly steam. The steam or other condensable vapor is thereafter condensed while the wood is submerged in the impregnating liquid, thereby facilitating the further and complete penetration of the liquid rapidly and uniformly into the most remote interstices of the wood. The process is arranged to exclude air from the wood from the initial boiling until the wood is adequately impregnated, by maintaining th wood submerged in a fluid, thereby excluding the air.

It has been found that when suitably subdivided wood is submerged in water heated to the temperature of active ebullition, the air is effectively expelled from the wood and passes off with the water vapor. If this treatment is continued until the wood is uniformly heated approximately to thetemperature of the water, the air will be expelled substantiall completely. This is believed to be due to the action of the water vapor or the vapors of the other woody constituents present at the boiling temperature, or both, acting as a conveyor in removing or discharging the air from within the wood, carrying it out into the surrounding body of liquid from which the air reentry of air during condensation and until complete penetration with th liquid is attained.

The process may be carried out effectively by open boiling under freely vented conditions. Where, however, the depth of the body of chips and liquid is so great that owing to the hydrostatic head a state of ebullition cannot be attained in the lower part of the liquid by freely vented boiling, the process may be carried out effectively by heating the column of liquid under suitable pressure to the boiling temperature of the lower part of the column under the hydrostatic head in that region, which may involve a temperature of 105 C. or higher, but below the temperature of active delignification, and suitably venting the disaster or other container to produce and maintain active ebullition throughout the column of liquid. This may be accomplished by a series of gas releases appropriate to maintain ebullition while removing the air and other vapors.

It has been found that the use of wood units of a suitable contour and dimensions is necessary in order to obtain effective operation of the process. Chips of the type enerally employed for producing chemical pulp are suitable. Where shortened fibers are not important the impregnation is facilitated by reducing the length of the chip in the direction of the fiber length, taking advantage of the facility with which air is expelled and liquid introduced in the longitudinal direction of the fibers. For instance, a chip V4 inch in length has been found to be suitable. Rapid and eflicient impregnation is likewise obtained by employing flakes that are extremely thin in a direction transverse to the fibers, thereby providing an extremely rapid heat transfer to the innermost fibers. Wood flakes having a thickness of .010-.030 inch have in general been found suitable, with flakes up to .060 inch effective where a lower class of pulp is desired, as in preparing boxboard pulp. The surface dimensions of the flakes are of less importance, lengths of about 1 inch having been found to be suitable. There are also certain advantages in using all heartwood chips or flakes or all sapwood, preferably the latter, owing to greater uniformity of the characteristics which affect impregnation and subsequent digestion.

After the air has been thoroughly removed from the wood, complete impregnation is obtained by condensing the vapors in the wood while the latter is submerged in the impregnating liquid. This may be accomplished by lowering the temperature of the surrounding liquid one or two degrees or more below the boiling point. The resulting condensation of the vapors creates a reduced pressure within the wood which draws in the liquid and only the liquid so that a complete and prompt absorption thereof throughout the wood is effected with the exclusion of air. Al-

ternately, the vapor may be condensed by increasing the pressure, which may conveniently be accomplished by closing the digester or other receptacle and raising the liquid temperature, generating steam pressure sufficient to condense the vapor in the chips. This pressure has likewise been found to be advantageous in facilitating the penetration of the liquid.

While the process may be carried out by boiling the wood in the impregnating liquid, which may be a suitable digesting liquor, and thereafter condensing the vapor in the wood while still submerged in the liquid, a specific feature of the invention as set forth herein is the impregnation of the wood with a nondelignifying liquid that is particularly suitable for initial penetration of the wood. The deiignifying treating reagent may then be introduced into the wood by diffusion in such liquid at temperatures below those of active reaction This application is a continuation in part of application No. 17,894, filed April 24, 1935, now Patent No. 2,137,779 granted November 22, 1938 in which the direct impregnation. of the wood with a digesting liquor is claimed.

The nondelignifying liquid is preferably water with or without suitable nondelignifying constituents dissolved therein. The wood may be boiled in water until the air is suitably expelled,

followed by a further and complete impregnation of the wood with water through condensation of the vapors in the wood. After a brief soaking period, which requires about 30 minutes, the excess water is drained off or forced from the wood as by steam. The wood is maintained in an atmosphere of steam to prevent the re-entry of air and then submerged in the treating liquor, preferably preheated, and this liquor rapidly enters the spaces filled with steam, and digestion is started.

A method of impregnation which is particularly effective with digesting reagents having a tendency to develop excessive obstructions to penetration, such as the alkaline liquors already indicated, involves initial impregnation of the wood with a solution of an electrolyte which initially obstructs attack by the subsequently added delignifying reagent on the surfaces of the wood, thereby substantially eliminating the obstructions to penetration normally produced by such attack and permitting rapid and thorough impregnation of the wood with such reagents in appropriate concentrations. This is believed to be the result of adsorption of electrolyte ions on the surfaces of the wood lining the capillaries and other minute passages and interstices within the wood. The electrolyte should be non-delignifying at the operating concentration and the temperature, soluble in the impregnating liquid, which preferably is water, and adapted to bring the wood surfaces to their iso-electric point. Examples of electrolytes suitable for this purpose are sodium chloride and aluminum sulfate. The concentration of the electrolyte solutions that will bring the surfaces to said point is critical, the optimum for the electrolytes mentioned being approximately 10- molarin the case of sodium chloride and 10* molar in the case of aluminum sulfate.

After this step is accomplished the impregnated chips are drained and any desired concentration-even dilute solutions--of caustic soda can proceed uniformly to the interior of the chip without diminution. The very small amount of protective adsorbed ions from the electrolyte solution has no influence on the digestive action of the liquor at elevated reactive temperatures, such as 160-l70 C. employed in kraft or soda digestion.

The wood may also be initially impregnated in the manner indicated with an aqueous solution of a substance which forms a constituent of the digesting liquor but which is non-delignifying at the concentration and temperature used for impregnation. For instance, the wood may be impregnated in a solution of sodium sulflte, ammonium bisulflte or other suitable sulflte compound containing practically no free sulfur dioxide beyond that which is necessary to maintain the sulfite in solution. After impregnation the liquid may be suitably prepared for digestive action by introducing the requisite amount of sulfur dioxide gas.

The impregnated wood is then digested by a process of known type but under unusually mild conditions, the thorough impregnation permitting the benefits of effective digestive action under such conditions, resulting in a raw pulp of particularly uniform quality.

The raw pulp thus produced may then be purified; and the benefits of the impregnation reflected in this digested pulp may be further utilized by the use of a much more lenient purification.

The following example illustrates the application of the invention to the pulping of yellow birch chips with caustic soda solutions for the production of magazine and book paper pulp.

Example A Yellow 'birch chips are boiled in a digester at atmospheric pressure in a solution of aluminum sulfate of about 10- molar concentration, until substantially no more air is discharged, after which the liquid is allowed to cool to a few degrees below the boiling point, for instance, 97 C. to 98 C. The time of treatment is preferably about 1 hour to allow for the removal of air and the deposit of the electrolyte; after which the batch at the reduced temperature is allowed to stand for 1 hour. The solution is then drained off and a caustic soda solution at 70 C. or higher is added without delay, until the ratio of liquor to wood is 3.25:1. near 100 C., preferably between 95 C. and 98 C. in sufiicient amount to cover completely the entire charge of chips. The concentration of the caustic solution is so adjusted that, including the liquid present in the pretreated chips, the ratio of chemical to wood will be 0.2: 1.0.

After the chips are submerged in the liquor, steam is introduced and the liquor is preferably circulated and heated. The digester is suitably vented or relieved during the initial steaming to permit the air above the liquor level to escape and avoid any false pressure. The temperature rise is halted at about 96-98 C. and

maintained at this point for a period sufficient to permit the caustic soda to penetrate the chips thoroughly without reaction, which should be completed in 1 hour or less. The temperature slightly below boiling tends to preserve the adsorbed film of electrolyte and at the same time is sufficiently high to expedite the entry of the caustic into the interstices of the wood.

The digester is then closed and the temperature is raised as rapidly as the facilities will permit to the desired temperature of digestion, which may be C., the usual time required for this step beingfrom 1%; to 2 hours. The

temperature is held within the desired digestion range, which may be between 170 C. and 173 C. until the pulp is in condition for complete defibering which may be after approxi mately 2 hours, and preferably until it has The liquor may be introduced at or' reached the desired bleachability. The pulp is then blown in the usual manner; and it has been found that when the pulp is impregnated and digested in the manner described, the frequency with which it is necessary to reblow a digester is radically reduced.

The pulp will have a bleachability of 20% or less and the yield of oven dry pulp will be from 50% to 52% of the original oven dry weight of the wood, the product being distinguished by its uniformity, softness, opacity and other properties desirable for pulp to be used in the manufacture of book and magazine paper.

The type of digestion just described does not, however, effect the maximum economies capable of being realized for paper pulp, but has been selected to afford a direct comparison of yield with pulp of approximately the same bleach requirements as those to which soda pulp is normally cooked. Where the time, temperature and chemical to wood ratios of digestion are so related that a raw pulp is produced which will give the maximum yield of bleached pulp when bleached to the required standards of whiteness as by a multiple stage system, or by any efficient two-stage high density system, further economies may be obtained. By suitable reduction in the chemical to wood ratio and the time of digestion, substantially higher yields may be obtained, the resulting higher bleachabilities being suitably reduced in the bleaching system with a relatively slight decrease in yield, producing a much higher yield of bleached pulp and a material saving in digesting chemical and steam consumption, which more than offset the increased consumption of chlorine during bleach- 8.

When kraft cooking liquors are employed with a 3:1 ratio of caustic soda to sodium sulfide, 3%-4% higher yields of pulp of any given bleachability may be obtained.

When the pulp is to be produced by the sulfite process, which will provide a high alpha cellulose suitable for the production of cellulose derivatives, the following procedure illustrates the practice of this invention.

Example B Black gum wood, preferably with the natural sap content maintained by preventing excessive drying of the wood which preferably is kept above 30% moisture, is suitably subdivided into the usual chips, or may be cut into flakes from .020 to .030 of an inch in thickness or chips approximately long. The digester may be of the rotary type; and the wood in the digester may be preimpregnated by submerging in water which is vigorously boiled for 15 minutes, after which the temperature is allowed to cool to 97-98 C., the wood being allowed to soak for 30 minutes. The excess water is then drained off or forced out with steam. The water in which the chips are boiled may be ejected before cooling and impregnation therewith by the introduction of steam in order to reduce the amount of water in the wood to a minimum; and the wood is then impregnated with the treating liquor by submerging the chips in the liquor while continuously excluding air from the wood by maintaining steam in the digester. The vapor in the chips is condensed either by introducing liquor at a suitable temperature or by increasing the pressure.

The sulfite treating liquor may be made up with ammonium bisulfite containing 4.45% total sulfur dioxide and 1.1% combined sulfur dioxide, with a 6:1 ratio of liquor to wood. The treating liquor which preferably has been preheated to -l10 C. is run into the digester, steam is introduced and the rotation of the digester is started. When the temperature has reached 103-105 C. the digester is suitably vented to produce boiling in the digester until diffusion of the liquor in the wood is complete.

The temperature is then raised as rapidly as possible to C., which may be accomplished in 45 minutes, and is held at that point for 4 hours, after which it is raised to C. in 1 /2 hours and held at this point within about 1 C. for an additional 2 hours. The liquor is drained off and the wood washed with hot water, preferably at above 100 C., the washing being repeated a suitable number of times, such as twice, while maintaining steam in the digester from the conclusion of the digestion to the end of the washing so that air is excluded from the digester during these operations. The digester is then discharged, the pulp agitated to disintegrate fiber bundles, and washed further until neutral to litmus. The raw pulp is now ready for purification.

Another procedure includes preimpregnating the chips with an aqueous liquid as hereinbefore described and then with a suitable digestion liquor and preferably an alkaline treating solution such as a kraft liquor of higher concentration than ordinarily employed which may produce a ratio of absorbed chemical to wood between .15:1.0 and .18:1.0, and thereafter draining all surplus liquor from the wood so that the ratio of liquor to wood in the case of heartwood moist drained chips will be about 2: 1. These chips may then be digested without the application of further reagent, as in a tumbling digester heated by means of a steam jacket or by surrounding the moist chips with a suitable unreactive heat transfer medium, raising the temperature in 20 minutes from room temperature to C. and terminating the cook immediately upon reaching this temperature. The wood is subjected to the digestion temperature for only a short period with corresponding minimizing of any objectionable effect on the cellulose content.

The reaction between the ligneous material of wood and many of the chemicals ordinarily employed in the pulping art, for instance, caustic soda, is quite rapid so that the desired action is capable of being accomplished in very short time, such as 5 minutes or less. With the treating liquors properly associated with the wood by following the procedures indicated, the attainment of delignification in such short time is limited only by the existing facilities for uniform heat transfer. While under commercial conditions heat transfer requires a substantial time, the usual cover to cover time cycle is reduced by this invention by one-third or one-half or more of that'ordinarily employed.

By the system of this invention the losses of cellulose and particularly of alpha cellulose are largely or completely overcome, so that substantially all either of the total cellulose or alpha cellulose originally present in the wood may be retained in relatively undegraded form and with uniform distribution.

Cellulose generally as referred to herein is sometimes known as Cross 8: Bevan cellulose, being the component of the wood pulp which remains after successive chlorinations carried out under a known standard method. Alpha cellulose is that component of the pulp which 2. The process of saturating woody material survives treatment in cold caustic soda solution with water containing a salt in a concentration of 17.5% concentration, also carried out under a giving a non-obstructive adsorbed film on the precise known standard method. surfaces cf the interstices of the wood by boiling Thus by proper impregnation of the wood with 5 said woody material under freely vented condithe delignifying reagents, they are uniformly distions until substantially all gases have been extributed at proper reactive strength throughout pelled from the woody material and supplanted the wood before temperatures are attained which by vapors of the salt solution, then condensing effect any substantial reaction, so that when such said vapors within said subdivisions by lowering temperatures are reached the desired reaction the temperature to below the boiling point while proceeds to effect delignification rapidly, simulmaintaining the woody material completely subtaneously and uniformly to any predetermined merged until the surrounding solution has penedegree. These conditions likewise are most fatrated and saturated all of the capillaries and vorable for avoiding degradation or other uninterstices of the wood subdivisions and the salt desirable effect on the cellulose itself with the is has uniformly contacted the surfaces of the consequence that higher pulp yields, better pulp woody material, then draining the excess salt quality, or both, are attained. The method also; Q )solution and adding a delignifying solution at produces novel pulps that are especially suitable" a temperature below the boiling point of the salt for esterification purposes, solution to completely immerse the woody ma.-

1 l i terial, maintaining the temperature of the solu- 1. The process of treating wood to form pulp tion below said boiling point until the delignifycomprising subdividing wood into pieces of chip ing e e t s u o y diffused t o out a size or smaller and of relatively fiat shape; subportions o t e Woo e providing her temmerging a mass of i wood pieces i t peratures and continuing same until the desired heating the entire mass of wood pieces and liquid 5 ee f de i nification has been attained. to the boiling point of said liquid at not less than 3. The proc s of treating W004i s set forth in atmospheric pressure; continuing the boiling of claim 1 n w ch e e y g gent is unisaid submerged wood pieces in said liquid under fqrmly distributed in the d P eces in m conditions of active ebullltion and vapor removal l y e f tra to satisfy e full until the discharge of air'ceases; thereafter ternm demand u me reactionminating the boiling under conditions of active The P s f treating wood as set forth in ebullition while the wood pieces are submerged in claim which e Water contains a dilute consaid liquid; continuing such submergence until centratlon of an electrolyte comprising a Salt impregnation with said liquid is complete, while adapted be ads d on the Surfaces of the maintaining the pieces in an air-excluding me- Wood lmlhg the calflllal'les d o r minute dium continuously from the boiling treatment Passages and interstices d actmg ni y to the completion of; removing the surplus imobstruct tt c by t e subsequently added de pregnating liquid; submerging the wood pieces in hifyihg e t 0n t e surface of the wood.

a solution of a delignifying reagent, and main- 5. The process of tr e- Wood 88 et fo th in taining such submergence until the reagent 40 claim 1 in ch the water contains p is uniformly introduced into the wood pieces mately 104 o r o centration of sodium by diffusion at a temperature below that of chlolldeactive reaction with and removal of lignin; heat- The P F of treating Od as Set f rth in ing the mass of wood pieces in the delignifying claim 1 m whlch the water o s ppro ireagent solution to the range of temperatures at 1 3 Ill-'5 molar concentration of inum which said delignifying reagent actively reacts S a with and removes the lignin from the wood; and FREDRICH digesting the wood pieces with said reagent to form pulp.

CERTIFICATE OF CORRECTION.

January 15, 19112.

Patent No. 2,269,985.

FREDRICH OLSEN.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, first column, line 57, claim 1,' after "of" and before the semicolon insert the word --impregnati0n-; line 57-58, same claim, strike out "impregnating";

and that the said Letters Patent should be read with this correction there in that the same may conform to the record of the case in the Patent Office.

\ Signed and sealed this 21st day of April, A. D. 1914.2.-

Henry Van Arsdale (Seal) Acting Commissioner of Patents;