US3069310A - Semichemical pulping process for soft woods - Google Patents

Description

Dec. 18, 1962 R. H. RAscH ET AL SEMICHEMICAL PULPING PROCESS FOR soFT wooDs 2 Sheets-Sheet l Original Filed Sept. 19, 1958 Dec. 18, 1962 R. H. RAscH ET AL SEMIOHEMIOAL PULPING PROCESS FOR SOFT WOODS 2 Sheets-Sheet 2 Original Filed Sept. 19, 1958 United States Patent Oflce 3.069 31) SEMECHEMICAL PULPING PRQCESEi FR SQFT lil/@GDS Royal H. Rasch and Paul is. ileemhuis,

Sutton, Waterford, Pa., assignors to Hammermill Paper Company, Erie, Pa., a corporation of Pennsylvania Continuation of application Sept. i9, i953, Ser. No. 762,-

@35. This appiication et. i4, Iliiil, Ser. No. 64,666

itil Claims. (Cl. l62--86) This invention relates to an improved process for the production of high quality bleached pulp Ifrom coniferous woods, commonly referred to as softwoods. The pulps produced with the present invention conform with the specifications of those required for the production of fine papers, as that term is most generally used in the industry. The present application is a continuation of Rasch et al. application, Serial No. 762,035, tiled on September 19, i958, now abandoned.

A primary urpose of the invention has been to provide a process of the character indicated which is readily adapted for the production of a variety of different types of pulp, i.e. pulps which dier in certain of their important ci aracteristics so that the same equipment may be used under readily variable conditions to produce, at will, pulps having characteristics desirable for a number of different purposes. The invention is particularly concerned with the production of line papers for printing, writing and cover paper purposes, as distinguished from wrapping and other coarse papers.

Fine papers of the variety of types now available are required to have different characteristics of strength, opacity, brightness, and the like, depending upon their intended use. The present invention is concerned with a process, utilizing the same equipment which may be used under readily variable conditions as to temperature, concentration of the digesting liquor, duration or extent of cook, and the extent and character of the subsequent chemical and mechanical treatment of the resulting pulp, to bring about the production of a variety of different types of pulp which may be used alone or in conjunction with other pulps produced by the sameequipment for the formation of ne papers f different types. An important consideration has been the development of such a universal process which may be carried out on an economic basis. This requires the ability to produce an adequate amount of pulp per day without the necessity of an excessive amount of equipment and without excessive costs for chemicals, power and the like.

Other obiects, purposes and advantages of the present invention will appear from the following detailed description of various illustrative embodiments of the same. Such detailed description of certain illustrative embodiments of the invention will be given hereinafter in relation to the accompanying drawings which illustrate schematically the preferred conduct of the process and certain permissible variations thereof. ln the drawings:

FIG. l illustrates, by way of a iiow diagram, how the original wood chips are digested and otherwise treated for the production of the desired unbleached pulp, and

PEG. 2 is a iow diagram showing the steps involved in the final treatment of the unbleached pulp to achieve the purposes of the invention.

The properties of fine papers vary widely according to type and grade. Specications for these papers are increasingly being met by the use of blends of several kinds of pulps. This trend is not entirely dictated by considerations of cost. In the paperma ing art a desired balance of paper characteristics frequently can be obtained better with a variety of pulps rather than with a single type of liber.

Bleached deciduous (hardwood) soda pulps, and, more recently, deciduous suilite pulps have been included in the furnishes of certain tine papers, notably certain book and Erie, and Harvey 3,069,310 Patented Bec. 18, 1962 offset printing papers. These fibers contribute desirably to a uniform sheet formation and a level printing surface, but they are deficient in strength properties. Presently, bleached deciduous kraft and special quality deciduous semi-chemical pulps are finding acceptance in a number of fine paper mills. These newer libe'rs have relatively good strength properties, are relatively low in cost, yet retain certain of the intrinsic merits of the shorter-bered pulps.

Despite the increasing utilization of the high quality deciduous bers referred to, a usually larger component in the furnish of various types and grades of line papers consists of one or more coniferous (softwood) pulps. The coniferous fibers are coarser and longer than deciduous fibers, averaging about 2.5 to 5.0 mm. in length as against 1.0 mm. The long-fibered stock provides certain desirable features. For example, it contributes relatively high wet Web strength to a sheet and thus aids in running on the paper machine. Suitable coniferous pulps contribute desirable properties, some measurable and some intangile, which can be supplied only by the greater length of their component fibers.

The requirements for long-libered pulp are substantially being met at present by two types of wood cellulose iiber; bleached conifere-us conventional acid sulte pulp and bleached coniferous kraft pulp. In the early history of paperniaking, cotton and linen rags provided the raw material for tine papers but the demands of the tine paper industry have long since greatly exceeded 4the available supply of rags. Later, bleached sultite pulp supplied a large proportion of the requirements in the field. In more recent years, with more advanced techniquesl in the bleaching of coniferous kraft pulps, this liber has been used increasingly for high quality papers, particularly where strength is an important consideration.

Tearing strength is one of the most important characteristics of certain high quality papers, such as certain grades of bond papers. Tearing strength is the resistance, or drag offered by a sheet of paper when it is torn. This quality may be measured with precision for speciiication purposes. Also it may be qualitatively estimated by the consumer without testing equipment. Thus tearing a sheet becomes one of the simplest tests for characterizing an important toughness quality of a paper.. Bleached coniferous kraft pulps are much superior in tearing strength to coniferous sultite pulps or to deciduous pulps as hereto-fore produced. Paper manufacturers have therefore frequently relied upon coniferous kraft pulps to meet exacting high tearing strength specilications and to make up for strength deficiencies of other fibers necessarily present in the furnish to supply other equally important properties.

On the other hand, in some line papers the toughness conferred by kraft liber is less important than certain other properties, such as translucency, which can more readily be obtained with bleached coniferous sultite pulp.

The manufacturer of a variety of types and grades of tine papers must meet many different quality specifications. It is frequently impossible to do this with a single kind of pulp. For best results it is necessary to have available a number of dierent fibers each with certain unique characteristics or advantages. For example these pulps might include the following:

(l) Coniferous sulfite pulplong fibered, easy beating, good web strength, fair formation, moderate dry strength, relatively low opacity.

(2) Deciduous semichemical pulp-short ibered, easy beating, relatively poor wet web strength, good formation, moderate dry strength, good opacity.

(3) Coniferous kraftlong iibered and coarse, very hard beating, good wet web strength, relatively poor formation, excellent dry strength, fair opacity.

From the standpoint of quality, exibility of operation, and cost, it is desirable for a tine paper mill to:

(l) Employ a sucient variety of papermaking pulps, of the character of those listed above, in order to meet readily any desired combination of specifications.

(v2) Suppply all pulp requirements within their own pulp making facilities. This is not only economically advantageous, in that it avoids the necessity of purchasing market pulp, but it further avoids the considerable expense of handling and reconstituting the dried market pulp. Moreover, quality advantage results from the ability of using slush pulp and thus avoiding the sacrifice of certain pulp properties which accompanies the drying of the market pulp for ship-ment. In addition the quality of the pulp is within the view and control of the mill which is to utilize it. v

(3) Provide the necessary liber varieties in controllable ratios without requiring a multiplicity of widely different pulping operations.

Heretofore it has beenk seldom that the advantageous circumstances outlined above could be met totally by a ne paper mill. Usually one or more of the different pulps has had to be purchased. For example, although kraft pulp has heretofore been regarded as essential in the furnish of many papers, the quantities of this fiber required by many paper mills would not justify the economically minimum size kraft mill which is about G tons per day.

In accordance with the present invention an improved process, eliminating the-problems of the kraft process,rhas been provided for the production of a wood cellulose fiber having a unique combination of the properties particularly desirable'in the manufacture of` certain high quality ne papers. These are properties found in certain premirnurn bleached coniferous kraft pulps, notably high tearing resistance and folding endurance, high opacity along with high brightness and freedom from dirt.

Another important aspect of the invention is the provision of a process the detailed conditions of which are readily alterable to effect desired and predictable liber characteristics ranging broadly from those commonly associated in the' industry with sulte pulp to those associated with kraft pulp. The invention has made possible the production of new, superior papermaking pulps without the necessity of a liquor recovery process which, in

i the case of the kraft process, dictates the minimum economic size of the production unit.

Through the present invention there has been provided a process for the production of an unusually high tearing strength pulp, equal or superior to kraft pulp, in which the bleaching is effected by a single, 3-stage sequence instead of the elaborate and costly multi-stage procedure required for bleaching kraft pulp.

Yet another advantage resulting from the present invention is the provision of a process for the production of a bleached high-tearing strength pulp which can be prepared for the paper machine with much less expenditure of power than normally is required in preparing kraft pulps for the paper machine, and one in which the amount of power required for the nal stock preparation may be controlled and reduced during a previous stage of the pulp production.

Also, the invention provides a process for the effective utilization of certain economically desirable woods, such as pines and larches, which are considered relatively unsuitable for pulping by the sulfite process because of inhibiting components or the presence of a substantial proportion of heart wood.

For a proper understanding of the invention, it is necessary first to consider briefly the two chemical processes by which virtually all the bleached coniferous (longbered) pulp of commerce now is produced.

The conventional sulte pulp is made by cooking wood chips under pressure with a solution of an alkaline or alkaline earth bisulfite and sulfur dioxide. Chemical removal of lignin and encrusting materials results in a crude pulp which is washed and mechanically screened to rev move knots and other incompletely defibered material.. The resulting pulp is readily bleached to yield a relativelyv clean pulp of high brightness. In current practice, the bleaching usually is accomplished in a three-stage sequence of operations involving chlorination, neutralization with dilute caustic soda, and bleaching with calcium hypochlorite. Following each stage the pulp is washed to removey chemicals and solubilized materials.

The sullitc cook is a highly critical operation and the attainment of uniform results is dependent upon a precise control of all the Variables. Toward the end of a sullite cook, the combined SO2, namely that equivalent to the amount needed to produce the mono-sulte of the metal present, approaches exhaustion and concurrently the hydrogen ion concentration in the liquor rapidly increases. Under these conditions, prolongation of the cooking results. in excessive hydrolytic attack on the cellulose with an accompanying loss in fiber strength. In fact the sultite process finds an important use in supplying dissolving pulps for the viscose rayon process precisely because of the controlled degradation of the cellulose, which results4 in readily lterable dispersions. On the other hand, insuflicient cooking results in a raw pulp high in screenings which is diilicult to process in the routine ma-nner. It is true that operators become quite skilled in the control of the variables and normally produce a pulp of reasonable uniformity for papermaking purposes. At best, however, the result obtained is a compromise. The strength ob tained is considerablyunder the full potential of the cellulose bers in the native wood as judged by what can be obtained by the more selective processes such as the kraft process, and by the present process about to be described.

Usually the sulte process, employing a calcium base liquor, does not include a recovery process, in the commonly understood sense of recovering the base ion for reuse. However the cook is made in the presence of a large excess of sulfur dioxide gas and it is necessary torecover this excess gas. Not only for reasons of economy but to provide the rich gas required to raise the total SO2 in the raw liquor to a much higher concentration than would be posisble with the low strength burner` gas alone. The entire liquor making and gas recoveryv process is quite elaborate and critical in its operation.. Briefly it involves cooling the burner gas, the counter current absorption of SO2 in limestone-packed towers and the refortication of the weak acid thus formedv with the pure relief gas from the cook.

The kraft process, in contrast to the sulte process, employs a strongly alkaline cooking liquor. Wood chips are digested under pressure with a solution containing, as essential ingredients, sodium hydroxide and sodium sulfide. Since these chemicals are costly, their recovery for reuse is a basic requisite of the process in order to make it economically feasible. This recovery involves the Washing of the pulp counter-currently in order to remove the spent liquor with as little dilution as possible. rthe recovered liquor then is evaporated to such a concentration of organic matter that it will sustain combustion in the recovery furnace. Prior to combustion, the necessary amount of salt cake is added to supply the sodium and sulfur which have been lost due to the inefficiency of washing. During the combustion of the black liquor, the salt cake (sodium sulfate) is reduced to sodium sultide. The smelt from the furnacing operation must be causticized with lime in order to convert the sodium (present in the form of carbonate) to sodium hydroxide. Many other steps are integral with the recovery process, which cannot be elaborated upon here. The purpose merely is to show that the employment of the kraft process involves a very complicated and initially costly recovery operation in order to salvage the expensive sodium component which necessarily is employed in excess in the, cooking operation.

While the bleaching of sultite pulp is relatively simple, the full bleaching of sulfate (or kraft) pulp is a costly and complicated process. The unbleached kraft pulp contains not only residual lignin but other materials which are resistant to the action of the usual bleaching chemicals and consequently are difficult to remove without darnage to the cellulose. This matter has been the subject of a great deal of investigation. Various workers have attributed the diihculty to the presence of thio-lignin or to sulfur derivatives of phlobotannin. Whatever may be the reason, only in recent years has it been possible to achieve, through the introduction of new techniques and chemicals hitherto unavailable in commercial quantities, the outstandingly strong, white pulps now on the market. But, whereas the sulfite process and the present process require only three stages in the bleaching operation, as many as 8 or 9 stages frequently are used to bleach high grade kraft pulp. Furthermore, chlorine dioxide is required in one or more of the final stages for the bleaching of kraft pulp, in place of the conventional calcium hypochlorite. Chlorine dioxide is too hazardous a chemical for shipment and therefore its employment requires the installation of a costly plant, at the point of use, for its manufacture. Attendant upon the use of chlorine dioxide are hazards both of a toxic and explosive nature.

As has been shown, the conventional sulfite process, even under the most favorable operating conditions, yields a pulp which is unsatisfactory for many papermaking applications. Undoubtedly that is an important reason why the production of bleached kraft pulp has overtaken that of bleached sullite pulp.

While the kraft process, on the other hand, can yield very strong, highly desirable papermaking pulp, its employment is accompanied by certain disadvantages. Some of these disadvantages have already been alluded to. In order to make the operation economically feasible, a highly complicated, costly recovery process is required. This, together with the necessity of providing a very elaborate multi-stage bleaching system adds up to a process which is economically feasible only for large production units. Another disadvantage of the kraft process is that highly noxious odors are developed in the cooking and recovery operations making the location of a plant undesirable in an urban community.

As will be appreciated from the foregoing, many integrated sulfite pulping and paper mills have found it necessary, heretofore, to add a certain portion of stronger pulp to their papers to meet strength requirements. However, unable, by the size of their operations, to justify the manufacture of krafbpulp, they have been required to purchase their requirements. This frequently has been found highly uneconomic and to have numerous other disadvantages of a technical nature.

In the development of the present invention it has been discovered that the advantages of the kraft process can be achieved by a procedure not involving its disadvantages. By means of features of the new process a premium quality papermaking pulp can be obtained which exceeds kraft pulp, in tearing strength, and opacity, while equalling it in other respects such as folding endurance, brightness and cleanliness. Furthermore, pulp produced in accordance with the invention is characterized by an unusually high cellulosic purity and stability. Moreover, these results can be obtained substantially with the equipment available in many pulp mills without recourse to major equipment changes or complicated procedures.

it has been discovered, as an important collateral advantage of the new process, that the various details of procedure may be altered at any time to produce different and predictable fiber characteristics to meet specific requirements in the paper mill. For example, instead of producing at the one extreme a relatively hard beating pulp with a very high tearing strength and high opacity, it is possible, with simple changes in operation at any time, to produce an easy beating pulp of low opacity in which the balance of strength properties has been altered to favor Mullen and tensile strength rather than to emphasize tearing strength. Such a pulp, it has been found, is highly suitable for applications in which conventional coniferous sulte has been employed 'out is substantially superior to it in all strength factors. Furthermore, within the extremes of characteristics which the conditions indicated above would produce, it is possible, by the new process, to obtain any desired intermediate characteristics by means of appropriate alterations in process conditions.

The new process can be employed without chemical recovery, thus eliminating the need for a .high production level to justify the installation of expensive recovery equipment. -ln this manner the new process is economically suited to the production of relatively small amounts of a selected pulp grade.

An important advantage of the new process is that the liquor-making facilties for the preparation of the digestion liquor may be completely integrated for the concurrent production of both the sulte and bisullite of the alkaline metal used-the former supplying the needs of a deciduous semichemical operation and the latter the needs of the new coniferous semichemical process. Another highly important advantage of the new process is that any of a variety of suitable chemical recovery systems could be employed for it and equally well in connection with a hardwood neutral sultite semichemical operation. This makes possible a single recovery process in a pulp mill producing a very wide range of papermaking pulp, for example: l) very tough high tearing strength pulps similar to kraft, (2) easy beating, high tensile strength pulps similar to sullite, and (3) deciduous semichemical pulps with well balanced strength and good formation characteristics.

Since the usage of soluble base is about the same in the new process as in a conventional acid sullite process employing soluble base, the same incentive for recovery would apply in both cases. Except to set forth methods for economical employment of the sodium constituent in stages of the process, it is not within the scope of this invention further to discuss recovery which is not a requisite for the economical production of the special pulp.

The unique flexibility and superior results obtained with the new process are achieved through the novel and essential combination of the following steps:

(l) The control of the degree of cooking througr the applicaiton of an aqueous solution of substantially sodium bisulfite, of a specified analysis and hydrogen ion concentration, in the partial digestion of coniferous wood chips to give, after suitable mechanical tiberizing, a semichemical pulp having a predetermined permanganate number.

(2) The control of the degree of an alkaline digestion of the semichemical pulp7 conveniently, but not necessarily, replacing the alkaline neutralization step of the conventional -stage bleach; the chemical concentration and conditions of temperature and time controlled in accordance with predetermined correlations to give a specified percent alkali extractives test on the pulp, which in turn serves as a method of indicating the physical properties of the pulp.

Coniferous wood chips having a normal length distribution are suitable for the new process. While chip characteristics are less critical than with the acid sultite process, it is preferred that no substantial portion of the chips be less than 1/2 inch nor more than l inch in length. The chips stored at it (F'tG. l) are charged to a digester l1 provided with external heaters and a circulation systern. The digester should be equipped with a stainless steel lining or a brick lining of the type supplied for soluble base acid cooking.

The chips preferably are presteamed for about 30 minutes by injecting steam in the bottom of the digester with the top relief valve open. ollowing the steaming operation, the chips are covered with the cooking liquor, delivered from a tank 1.2. This liquor is essentialy a solution of sodium bisulite with a pH within the range of 3.0 to 5.0. There may, however, be a slight excess or deficiency of SO2 present, as compared with that required to have the sodium present in the form of bisullite. Expressed in the convention employed in acid sulfite pulping the analysis, based on the weight of the solution, is

about: Percent Total SO2 1.50-2.40 Free SO2 .7S-1.20 Combined SO2 .7S-1.20-

it will be understood that by combined SO2 is meant that required to form the monosulfite, while free SC2 is used to designate that in excess of what is required to form the monosuifite. The liquor to wood ratio to maintain a coverage of the chips will vary according to their density, and usually should be Within the range of 5 to- 1, to 6 to 1. The chemical concentration in the liquor is preferably such that the equivalent of between 9.9- and 12.0% total S02 and 3.0 and 4.0% Naz() on wood will be employed.

This example is typical for spruce and balsam fir. Other coniferous woods may require slightly dilferent con-- centrations and the proper concentration should be established for each particular species. The actual chemical consumption, as influenced by chemical to wood ratio and cooking conditions, influences important factors such as power requirements in iiberizing, pulp yield, pentosan content, chlorine requirement in bleaching, and strength. As conditions of the coo-k approach those of complete pulping, strength characteristics and pulp yield are adversely affected. On the other hand conditions resulting in excessively raw cooks give chips which are dirhcult to resolve by berizing into discrete fibers. his results in Shivy pulp-s which require excessive amounts of `chlorine in bleaching. Normally the permanganate number for best results will not exceed 60 nor will be less than 30. For maximum strength development we prefer to voperate as close to the upper limit as possible although highly desirable results are obtained within the entire range cited.

The cooking liquor is simply prepared by passing burner gas into a solution of soda ash of the proper concentration until the desired pH or total SO2 is attained. Since no excess SO2 beyond the formation of the bis `te is desired, SC2 from the burner gas is readily absorbed to give the desired total SO2 with very small absorption towers and without the need for fortification with concentrated SO2 gas as is the case with the conventional sulite process. Furthermore, it is not necessary to cool the burner gas preliminary to its injection into the soda ash solution, thus the heat in the gas may be recovered in the solution.

The specific digester operating conditions nre not critical. They involve an adequate temperature rise period or the use of other techniques to avoid burning the chips. About 2 to 3 hours is usually sufhcient.

For most economical operation the chemical to wood ratio is so proportioned that the desired permanganate number has been obtained when the total SC2 at the end of the cook is substantially exhausted.

The cooking temperature is not critical but usually will be within the range of i60-170 C. Because the reaction rate is very rapid at 170 C. it is pre erred not to exceed this temperature since the control of the cooking time becomes increasingly critical. it has been found that the chemical requirement is about 16.2% llalel()3 (10% SO2) for spruce wood. However, other conifers will require slightly different concentrations of chemical and this is best determined by actual trial. When the desired end Atest (percent total SO2) in reached the digster charge is blown to a blowpit ft2-l usually after the pressure has been relieved to about 55 psi. The end test is made by titrating a sample with 4.potassium iodate according to the method of llalmrose. From the blow pit the spent liquor and wash water are drawn off through a line 1d and the partially digested and defibered material is delivered to a chip tank l5.

Although a vertical digester has been used in the conduct of this portion of the process, it will be obvious that the cook could readily be made in other suitable equipment. For example, a rotary digester or any of the various continuous digesters now available could be used. This is in contrast with the conventional-sulte process which is dependent upon batch cooking because of the presence of excess SO2 in the cooking liquor. Furthermore, while it is preferred to conduct the bisulite cook in the liquid phase, it is entirely feasible to employ vapor phase cooking which is relatively common in a number of mills producing hardwood semichernical pulp. 1n s1 a variation of technique, the chips are subiected to a perictration period inthe presence of a more concentrated solution of the chemicals and at a temperature of about C. Following the penetration period, the excess liquor is drawn off leaving only a :small amount for basting or circulation purposes. The temperature is brought to maximum and the cook completed substantially in the vapor phase.

The `cooked chips discharged to the blowpit are in the form of crude pulp but not substantially deflbered as in the case of acid sulte pulp. On the other hand the cooked chips are much more completely liberized than is the case with hardwood `semicnemical pulp of a corresponding lignin content. A relatively mild mechanical treatment is adequate to reduce this material to a cornpletely iiberized form. For this purpose a rotary disk mill of the single or double rotating plate variety may be employed. Any of a great variety of attrition mills which will be familiar to those in the industry, may likewise perform the function required.

In line with the foregoing, the material collected in tank 1S may all be delivered through a line 1d to a knetter screen 17' from which the good fraction, consisting of the iiberized and largely defibered or broken down material passing through the screen, is delivered by a line 13 to a decker 19 and then to a chest Ztl, from which it is delivered to a disc refiner or the like of the character mentioned. From this reflner the material may be passed to a chest 22 and then preferably to a centrifugal cleaner system 23 of the type disclosed in the patent to Samson, et al., No. 2,377,524, granted June 5, 1945'. As explained in that patent, the centrifugal cleaner system results in the elimination of shives and other undesired constituents of the pulp. A high percentage of the pulp passing through the centrifugal cleaner system constitutes good pulp which may be sent to a thickener 24 and then to the pulp purification and bleaching stages of the process schematically shown in WEll-G. 2. As shown in FIG. 1, the material collected in chest 22 may, if desired, be first subjected to a fiat screening operation, by screen 22a, before being passed through the centrifugal cleaner. 1n come cases this option may be employed advantageously to remove shives dirt particles which have no-t been resolved completely in the hberizing operation.

The material remaining on the knottcr screen i7 contains a certain amount of good pulp and this may be recovered by passing the indicated fraction to a tank 25 from which it may be delivered to a partial pulping unit 26 of the character to be described more fully hereinafter. The partially pulped material may then be delivered to a knotter screen 27 and the good fraction passing through the latter may be delivered through a line 23 to the line 13 for inclusion with the good fraction from knotter screen 17. The knots, compression wood and the like remaining on the knetter screen 2'? are rejected, as indicated at 29 in EEG. l.

it is preferred, however, to conduct the libration in the manner taught in the patent to Rasch et al., No. 2,847,304.

granted August 12, l958. Present with the cooked chips are knots, gross pieces of wood, compression wood and the like which have resisted the softening action of the cook and are substantially harder and more resistant to mechanical disintegration. This material when mechanically disintegrated results in dirt particles and shives which are dilicult to remove from the discrete fibers. In order to produce a pulp of the maximum cleanliness it is desirable to eliminate such dirt-forming material before it has gone through the iibraton treatment. For this purpose the cooked chips, after suitable washing in the blowpit, are, in the manner taught in said Rasch et al. patent, subjected to a mild disintegrating action either batchwise in a Dynopulper or a Hydropulper, or continuously in a drubber, or the like, in such a manner that the knots, gross pieces of wood, pieces of compression wood and the like, because of their relative hardness, are not substantially reduced in size while the desirable softened wood chips are reduced to small ber aggregates or even to individual fibers. For this purpose the material collectM ed in tank 1.5 is rst passed to a partial pulping unit Si), of the character mentioned, which may be of the same type as that shown at 26. From unit 3i) the entire pulp mass is passed to the knotter screen 17 and from this point the treatment is the same as described above.

The openings in the knotter plates of screens i7 and 27 are preferably about it to 1/2 inch in diameter. rl`laey should be of such size as to eiifect the most eiicient scparation and rejection of the undesirable fraction of gross pieces from the relatively clean material. The latter passes through the openings of the knotter plates while the gross, dirt-forming material is tailed ed and rejected. However, that tailed ofi' from screen 17 may be sent to a second stage of a mild disintegrating action in the partial pulping unit 26 for further recovery of useful fiber, in accordance with the teaching of said Rasch et al. patent.

The large, desirable fraction is usually in excess of 98% of the total (partially deligniiied) cooked chips. It consists of a mixture of individual fibers and tiber aggregates. Some mechanical attrition is required to reduce these aggregates to discrete fibers, but much less than in the parallel case with hardwood semichemical pulps. The accepted stock from the knetter screens is iibrated preferably in a disk mill, such as reiiner 2i, using a plate setting of about to 5 mils. It will be understood, of course, that specific variations must be made for particular applications. In the iibration relativelv little mechanical treatment is required to reduce the ber aggregates to discrete fibers. Normally between about 2 and 5 horsepower days per ton will be adequate to produce the desired tiberizing action.

The power expenditure required for iiberizing naturally will vary in accordance with the severity of the chemical cook, more drastically cooked chips requiring less power application than ran/er cooks. available disk mills are suitable whether of the single rotating or double rotating type. As with conventional hardwood semichemical pulping. the results are influenced by the variables of the disk milling operation, notably by plate design, power application, plate setting, stock consistency, speed of rotors, etc. Normally it is preferred for the present process to conduct the iiberizing operation in such a manner that the chips have been resolved into discrete fibers without substantial cutting or without excessively reducing the drainage characteristics of the pulp. Bv maintaining the Schopper-Riegler freeness in excess of 820 ml. the power requirement for producing the above mentioned fiberizing7 is minimized and subseuuent washing of the pulp following the various stages of bleaching is facilitated. Chemical economy is correspondingly favored. A free pulp, i.e. a free draining pulp, is normally preferred, but in certain special cases it has been found advantageous to apply more power during the flberizing stage than is necessary for complete ber separation. This results in a certain amount of iiber cutting and fibrillation along with a substantial reduction Any of the commercially f of drainage rate, or freeness. Thus while a normally preferred berizing treatment might result in a ZO-mesh fiber fraction of remaining on a ZO-mesh screen and a Schepper-Riegler freeness of S4() ml., a controlled application of more power during iiberizing might result in a 2Gmesh ber fraction of 55% remaining on such a screen and a freeness of 750 ml. If desired, the fiberizing treatment may be carried to the point of producing a 700 ml. freeness. The Schopper-Riegler referred to has been determined by Method 414, institute of Paper Chemistry, Oct. l, i951, while the Ztl-mesh fraction has been determined by Method 415 of said institute. The additional application of power over that required for berizing results in stock preparation which normally would be doneand in the case of chemical pulps always is done-on the final bleached stock in its preparation for the paper machine. The increased power application in iiberizing at this stage will be saved in power ordinarily required for stock preparation. In the manner indicated it is possible to produce easy beating pulps which are highly desirable in cases when it is necessary to minimize stock preparation because of the lack or limitation of necessary refining equipment in the paper mill. The use of commercial bleached kraft pulps requires a relatively large rening or beating capacity and the application of a substantial amount of power in order to render the fibers suitable for conversion into fine paper grades.

rhe unbleached pulp, prepared in accordance with the procedure previously set forth is clearly in the class of semichemical pulps in that a partial cook resulting in a softening of the chips, and partial or incomplete delignification, is followed by a mechanical iibration of the chips.

it has been yfound of no value for the attainment of the objectives of this invention to carry out a conventional semichemical cook on coniferous woods followed by normal bleaching. Only when the semichemical cook is conducted under certain specified conditions, leading to a speciiied degree of delignilication as established by the TAPFI. permanganate number, `followed by controlled carbohydrate removal in an alkaline purification stage conducted under specified conditions will the unique results of the present invention be obtained. in determining the permanganate numbers referred to herein, 'rhere has been used the TAPPI. Standard 'HMM-501 with modifications explained by P. L. Leemhuis in TAPiI., vol. 37, No. l, january 1954, pp. 32-38.

The neutral sulfite semichemical process bas not proved satisfactory for the pulping of coniferous woods. The excessive amount of chemical required for these more highly-lignied woods has made this particular process uneconomic and unattractive. It has been found that the digestion time for these woods is excessively long, or the cooking temperature excessively high and the pulp properties are not sufliciently enhanced over the softwood sultite pulp of the trade to justify the greater costs invoived. For example, it has been found that using 32% sodium monosuliite ybased on wood about 13.5 hours at C. were required to cook spruce chips to a permanganate number of 60, whereas only 5 hours were required to cook to a permanganate number of 47 when sodium bisulfite was employed. The sodium base used in the bisulite cook was only 31% of that used in the monosulte cook, and the sulfur dioxide only 62%, with the production of a substantially easier bleaching pulp.

in the case of hardwoods, the strength of pulp cooked by the neutral sulte semichemical process is enhanced greatly over that cooked by acid sulite process. Using softwoods, however, only the Mullen or bursting strength is improved, while important characteristics of tear, folding endurance, opacity are not significantly increased. in contrast to the neutral sultite semichemical cook on softwood, with its excessive requirements of chemical and digestion time, a cook made with sodium bisultite under conditions specified for our process requires only l to 2 hours at 170 C. and no more net :sulfur and base than l1 normally is employed in a soluble base acid suliite cook with excess SO2.

.It has been found that excellent results can be obtained by the new process even with some variations in conditions. For example, the ratio of sodium o sultite in the liquor may depart somewhat from the equivalent of sodium bisulte. However, it is preferred to keep the atomic ratio of sodium to sulfur within the limits 0.95 to 1.05 and the pH within 3.0 to 5.0. Similarly, the ratio of the bisulte ion to wood, which determines the degree of cooking, may be varied somewhat without seriously affecting the ultimate pulp properties. However the best results are obtained when the concentration is such as to give a permanganate number of 30 to 60. Below about 25, which enters the range of true chemical pulps, a substantially reduced pulp strength results. Above about 60 the requirement of chlorine for bleaching is unnecessarily high and the final pulp is inclined to be shivy.

While semichemical cooking of coniferous woods with sodium bisultite for the production of bleached high quality paper pulps is not believed to have been practiced in the industry, the present invention goes beyond that broad concept. It involves a process using that feature but under certain special conditions set forth herein. Such controlled digestion of the original softwood chips, in combination with the particular purification stage about to be described, leads to the production ot' bleached pulps with the superior strength and other highly desirable properties to which the present invention is directed. it has been found, for example, when sodium bisulite liquors with a pH above 5 or below 3 are used even though the cook is carried to the desired extent, the hi est potential strength and other properties cannot be achieved in the subsequent puriiication treatments.

The unbleached pulp prepared in accordance with the new procedure previously set forth may be bleached in a conventional 3-stage sequence of operations involving chlorination, alkaline neutralization and hypoc'ulorite bleaching as is customary for conventional full chemical sulte pulps. Substantial advantages will result thereby in the way of increased yield and strength as compared with conventional sulite pulp prepared from the same wood. However, the full potential of the present process as a means of varying important characteristics of tearing strength, opacity, bursting strength, cellulosic purity and the like cannot be attained in the usual bleaching sequence. Whereas in the conventional method of bleaching suliite pulp the conditions of the alkaline stage (chemical concentration and temperature) are such as merely to solubilize chlorinated lignin and to neutralize residual acid, in the new process the conditions of alkali concentration, temperature and time are varied in order to induce a controlled removal of carbohydrate material. This is necessary to produce the enhancement of particular fiber characteristics in accordance with the requirements of the paper mill. rThe extent of purification, which influences these characteristics, is dependent upon the amount of caustic soda consumed. ln the conventional practice of bleaching sulite pulpa it is usual to apply about 3% caustic soda for about one hour at about 59 C. When these mild conditions are employed in the bleaching of semichemical pulp produced in accordance with the present invention, the results are markedly superior to those of conventional Yleached sulite pulps. rl-"hus the yield, bursting strength, folding endurance and tearing strength all are substantially superior while the opacity is lower. The pulp is relatively easy beating and is particularly well adapted to the manufacture of high tensile strength translucent-type papers. Or in a furnish it may serve admirably to permit the use of larger proportions of low-cost hardwood pulps than can be used with normal sultite pulps.

However, by substituting a caustic soda digestion for the neutralizing step, it is possible to remove carbohydrate material in addition to chlorinated liguin and residual acid. With increasing alkali consumption the pulp yield decreases as the more soluble carbohydrates, such as pentosans, are removed but certain papermaking characteristics are progressively altered. its the consumption of caustic soda is increased the teariim strength, opacity, folding endurance and cellulosic pi. ty are increased. The tearing strength, for example, be increased substantially above that obtainable by the kraft process from a given wood.

The dev-ice of substituting the caustic soda digestion in place of usual alkaline neutralization stage represents a convenient, the least costly and normally preferred method of obtaining the desired degree of carbohydrate removal. it is entirely possible, however, to place the elke ne digestion in a different relation to the bleachirg sequence of steps. For example, `the unbleached pulp may be given the appropriate alkaline digestion treatment and then be followed by the conventional sequence of steps for bleaching, namely: chlorination, alkaline neutralization and hypochlorite bleachinv. Y

When more than about 6.9% caustic soda on dry pulp is consumed in the alkaline digestion the enhancement of any particular property, such as tearing strength is usually not sufcient to justify economically the increased chemical, additional digestion time and accompanying yield loss. From practical considerations we prefer to keep the consumption of caustic soda within the limits of about 15% to 6% depending upon the type of papermaking fiber desired. These percentages of caustic soda refer to amounts consumed by completely washed pulp. ln actual practice variable amounts of hydrochloric acid carried over from the chlorination stage are present in the pulp b cause of incomplete washinv. Consequently an increment of caustic soda, to be determined under operating conditions, must be applied for neutralization purposes above that required for controlled carbohydrate removal. At ythe lower range, in which there will be only a small amount of carbohydrate removal, it sumces to treat the chlorinated, washed stock under relatively mild conditions such as C. for one hour with sufficient caustic soda to result in the consumption of about 1.5% based on fiber. This, however, does not lead to the production of the primary advantages of the invention.

For the purpose .of attaining very high tearing strength, it is necessary to employ more caustic and more drastic conditions of time and temperature in order to consume the requisite amount of caustic soda, which approaches 6.0% caustic soda `on pulp. Satisfactory results have been obtained with temperatures ranging from C. to 125 C. and with stock consistencies from 6 to 12%. Because of the prolonge reaction time in the lower range, it usually is not practical to make the digestion below 95 C. While the same consumption of caustic soda is attained n 3S minutes at l25 C. as in 4 hours at 103 C., tion in the latter case may be conducted in open vesseis. To enable treatment at l25 C. special equipment is required in order to reach the corresponding pressure conditions. rlherefore the selection of the temperature for this stage of the process depends upon the equipment available.

The conditions cited above are illustrative of .the practical extremes employed in the degree of alkaline digestion in the pur ion sequence. if it is desired to oba papermal'ing fiber of intermediate characteristics it will be necessary correspondingly to change the amount of caustic consumed and, in this connection, to make the necessary adjustments in the temperature and duration of the digestion.

The example following, with the data given in rfable l, shows the resul-ts obtained when conditions of the alkaline extraction are so regulated as to provide various consumptions of caustic soda. The parent unbleached pulp was prepa-red in the manner described below.

rEhe softwood chos used contained 80% spruce and 13- 20% balsam tir. A chip charge of 2481 grams (moist Weight) and 1861 grams (oven-dry basis) was placed in `a laboratory autoclave provided with electrical heating and forced circulation of liquor. A volume of 7252 ml. of sodium bisuliite liquor containing 2.30% total SO2 and 1.16% combined SO2 was added to the chips in the digester. This provided a ratio of total SO2 to Wood of 9.0% and a liquor to Wood ratio of 4.23 to 1. After the cover was bolted on, lthe temperature was raised on a linear schedule to -a maximum of 160 C. in a period of 3 hours. The maximum temperature in the cook was maintained until a residual total of 0.52% SO2 on liquor was reached. This required 2 hours and 22 minutes. At this time the digester was relieved to atmospheric pressure as rapidly as possible (about 5 minutes). The initial pH of the liquor Was 4.4 and the final pH of the spent liquor was 3.55. The cooked chips were washed and given a mild selective pulping in a laboratory Dynopulper for 30 minutes. The crudely pulped chips were screened on a f 4-mesh screen. Knots and other relatively hard material Which resisted the mild dynopulping action were retained on the surface of the screen. This material, which constituted about 1.8% of the total weight of pulp,'was dis carded. The crude pulp which passed through the screen was iiberized in a 12-inch Sprout-Waldron disk reiiner at a pulp consistency of 1.5% and ata plate clearance setting of about 1 mil. The pulp was screened through a vibratory screen with 7mil slots but the reject material was negligible. The unbleached pulp had a lignin content of 14.6 and a permanganate number of 63.2.

1n the foregoing table the solubility of pulp in cold alkali (alkali solubility) tests were made i accord ance with the procedure of the Swedish Cellulose Central Laboratory Method CCA-8:53. The determination or pentosans was made in accordance with the Institute of Paper Chemistry Method 424, Pentosaus in Pulp, lanuary 1951.

In the examples cited in Table l a substantial excess of caustic soda was applied over that consumed. This could be reduced by employing somewhat higher digestion temperatures or by increasing the reaction time. it is preferred, however, to apply suiicient excess caustic so the final pH of the stock prior to Washing is between 10.5 and 11.5. The critical factor in the control of the process at this point is the extent of carbohydrate reni oval which in turn is dependent upon the amount of caustic soda consumed. The requisite caustic consumption will be in excess of that required for reaction with chlorinated lignin and any residual hydrochloric acid carried over from the chlorination stage. A very useful test tor measuring and controlling the degree of alkaline digestion desired isprovided by determining,7 the solubility of a known Weight of pulp in an 18.0% sodium hydroxide solution. The solubility of pulp in cold alkali values undergo progressive changes in magnitude along with strength and physical characteristics. Thus the test serves conveniently to control the process and to identity the pulp for its proper employment in paper mill furnishes. 1n determining the effect of alkali solubility one should take into consideration the species or conditions of Wood raw ina- TABLE 1 terial used and the details of the semicnernical cook tollowed. NaOH applied, percent 101g 7-18 4-18 gi) 35 Further examples of the new process are set forth in percent 100 S2 7? Tables 2 and 3,1Whichdserve to illustrate the relationship, Duration hrs.. 4 0r n W00 W e Ahe fr@ e a Fina] pHV 12.3 12,4 12.1 (i) (2) any g'lve pu p bet e ni' .deb .6 of {mkhfnu NaOH consumed (referred to pri 7,0), 2 cal pulping and the degree of alkaline digestion agtunsfv 5 7 3' 0 1' 6 (l) O final. pulp CilalaCllSlCS. This IClaOlShip was based Alkali sniubiiity, percent 6.0 9.9 11.9 8.1 13.0 upon a Series of cooks made on a mixture of spruce and Pentosans, perccnt 4.6 5. 5 6.9 7.1 4.2 r Yield oerwoud, percent. 42.0 46.3 48.7 42 44 balsam r chips using for digestion a solution or sodium l 23 bisuliite under conditions previously discussed. in Table Opacity 69 I 63 i 62 67 65 2 ve degrees of semicliemical cooking are indicated with .i q I Commemm bleached kraftpum 45 different degrees of alkaline digestion, Whne in Table a 2Commercial bleached sulphite pulp. the charaCterlStlCS 0f the ISultlilg pulp are Shown.

TABLE 2 Degree of semichemcal cook Degree of alkaline digestion Carstlc Caustic Ex. Total SO2 Cooking Unsoda apsoda con- Temp- Duraapplied retention bleached Lignin Perm anplier sinned eratu re tion of on wood, time at; pulp con tent, ganatc based on based on of di digespcrcent 100 C yield, percent number bleached bleached gestion tion, H:M percent pulp, pulp, C. hours percent percent Mild digestion A-. 12. 'i554 50. 5 3. 42 25 3. 0 1. 00 70 1 0 B ll. 58 3231 53. 2 5. 55 35 3. 4 l. 20 70 l 0 C. 10. 60 3109 55. 9 8. 20 45 3. 8 1. 70 l 0 Du.-. 9. 63 2:46 58. 5 ll. 5S 55 4. 2 1.50 70 1 0 E-. S. 68 2:24 61. 5 15.70 65 4. 6 1. T0 70 l 0 Intermediate digestion F 12. 55 3254 50. 5 3. 42 25 5. 8 2. 00 35 2.0 G 11. 58 3:31 53. 2 5. 55 35 5. 2 3. 00 B5 2. 0 H. 10. 60 320i) 55. 9 8. 20 45 6. 6 3. 05 85 2.0 I 9. 63 2:46 58. 6 11. 5S 55 7. 0 3. l0 S5 2.0 J' 8. 68 2:24 61. 5 l5. 70 65 7. 4 3. 20 35 2. 0

Most drastic digestion 12. 55 3:54 50. 5 3. 42 25 8. 5 5. 50 100 4. 0 11. 58 3:31 53. 2 5. 55 35 8. 9 5. 55 100 4. 0 10. 60 3209 55. 9 S. 20 45 9. 3 5. 60 '100 4. 0 9. 63 2246 58. 6 11. 58 55 9. 7 5. 70 100 4. 0 S. G8 2224 6l.. 5 15. 70 65 10. 1 5. 80 100 4. 0

TABLE 3 Bleached Pulp Soluhil- Raw l Yield ol ily in Beating material bleached Pentosan time to Mullen VFearing Opacity costs iny Ex pilp, caustic content, 750 SR., strength, strength, percent excess of percent soda, percent mins. p.s.i. g. sulte percent pulp,

percent iL..- 46. 10. 4 5. 85 17 70 65 63 1 26...- 47. 4 10. 85 6.0 17 72 68 63 3. 0...- 4B. 0 11. 25 6.15 17 75 70 63 5. 3 D.- 48. 4 11. 6 4 17 77 72 63 8.1 E 4S. 6 11. 9 6.8 17 79 73 63 11.1

F 43, 7 8.1 4. 8 22 65 78 67l 11.6 G 44. 5 8. 7 4, 85 23 68 S0 66 12. 7 H. 45.1 9. l fi. 9 24 71 8l 65 14. 9 45. 6 9. 5 1 24 73 S2 64 18. o J 46. 2 9. 9 5.6 24 76 S3 63A 21. 4

K- 40.3 5. 3. 85 3'() 61 Q6 72 21.0 L 41. 0 5. 7 3.9 32 62 102 71 23. 7 M 41. 5 5. 85 4. 05 33 63 107 70 25. 9 N. 4l. 8 5. 95 4. 25 64 112 70 29. 3 O 42. 2 6.0 4. 55 35 65 115 69 33. 5

The ratio of total SO2 to wood and the duration of 25 caustic soda solution are passed to a pressure tower 38 the cooks were varied to give a series of products` of varying lignin contents. Each product was fiberized in a single-rotating disk mill to give completely defibered pulps and have freenesses within the range from about 84@ to 860 SR. Pulps below 30 permanganate number were substantially chemical pulps and required no mechanical fiberizing. The pulps were screened on a .007- inch cut screen plate but inthe case of the mechanically tiberized pulps the screenings were negligible. Samples of each of the pulps then were chlorinated in accordance with their requirements as indicated by their permanganate numbers. The chlorinated pulps, after washing. were subjected to three different degrees of alkaline digestion in the manner previously discussed and as indicated in Table 3. rfhe digested pulps were washed and bleached with buffered calcium hypochlorite to a high brightness. The pulps, finally washed, were analyzed and tested with results as shown. For each level of alkaline digestion the pulp characteristics are proximate functions of the degree of lignin removal, or, for more convenient and rapid determina-tion, the permanganate number. The values set ior'th in Table 2 are interpolated at uniform `increments of permanganate number, namely: 25, 35, 45, and 65, by plotting curves from the actual data taken.

rhe tabula-tion illustrates the relationships which may be developed for any species or condition of wood. It is seen that both the degree of the semichemical cook and the degree of the alkaline digestion determine the characteristics of the pulp obtained. inasmuch as these variables involve the employment of different amounts of chemicals yand lead to different pulp yields, there results a corresponding difierence in the cost oi producing the different pulps. These costs relative to that of conventional sulfite from the same wood are given in the last column of Table 3. Thus it is possible directly to relate the cost of a pulp with particular characteristics against the particular set of conditions required to produce the pulp.

En FlG. 2 there is schematically shown a system suitable for the bleaching and alkaline digestion steps. The pulp lderived from the thickener 24 of FIG. l is passed to thebrown stock chest 3l of HG. 2. From here it is sent to a chlorine mixer 32 and thence to a chlorine tower 33 wherein adequate time is provided for `the desired chlorination of `the stock. From the tower 33 the chlorinated pulp is passed to a washer 34- and then to a mixer S5. into this mixer there is introduced a caustic soda solution from a tank 36 and there is also introduced steam from a source 3,7.. From the mixer the pulp and ywherein the desired alkaline digestion step of the process takes place. Steam is introduced into the tower 3S from the source `37. When the -alkaline digestion has been completed the pulp is passed to a washer 39 and then to a hypochlorite mixer Calcium hypochlo-rite `is introduced into this mixer from a tank 41. A certain amount of caustic soda is also introduced from a tank 42 and steam is injected from a source 43. From the mixer 40 the stock is sent to a hypochlorite tower 44 wherein appropriate time is allowed for the final bleaching of the pulp. The bleached pulp is passed to a washer 4S and then delivered to one or another of a series of storage towers 46, 47 and 48. Each of these towers is adapted to retain aparticular grade of pulp so that the appropriate one i-s selected to receive the particular pulp being processed at the time.

Although only three diierent levels of alkaline digestion were employed to develop .the relationship shown in Table 3, it is obvious that any -set of conditions between the mildest `and the most drastic may be applied to a pulp within a wide range of lignin contents. This offers a unique flexibility of operation and makes it possible, working from previously established reationships to produce any one of a wide range of pulps of predetermined characteristics and at a predictable cost.

Another controllable variable not covered in Tables 2. and 3, but previously referred to, is the matter of the rening of the unbleached semichemical pulp. The term refining is used here in its strict sense as the mechanical process of preparing fibers for the paper machine. it involves the increase of the specific surface of the individual fibers by fibrillizing, fiber cutting, and reducing drainage characteristics, -commonly referred to as hydratingf The process is 4distinct from that of tiberizing which means :the mechanical reduction of cooked chip-s yor fiber agglomerates into discrete fibers without substantial fiber cutting or freeness drop.

During the fiberizing operation a range of choices is available. At one extreme a stock having a freeness in excess of 850 SR., approaching that of a full chemical pulp, may be produced. This free-draining, easy washing pulp will, after bleaching, require further mechanical refining treatment before it is suitable for the paper machine. At the other extreme, further application of power during the fiberizing operation can result in simultaneously refining the pulp to a freeness of, say 750 S.R. or less. Thisl pulp, Iafter bleaching, may require little or no further treatment of a refining nature in the paper mill.

Such an effect 1s shown in Table 4 as `follows:

TABLE 4 Eect of Refining Durmg Fzberzzing Light Heavy refining refining during during Iiberizing tiberizing Unbleached pulp: Schepper-Riegler freeness,

ml 860 700 Bleached pulp:

Schepper-Riegler freeness, ml 850 775 .20-mesh fraction, percent 84. 5 70. 6 Beating time, mins. to 750 S.R 18 3 Mullen strength, p.s.i 80 68 Tearing strength, g 60 70 Folding endurance, double folds -v 1700 1530 Opacity, percent 64. O 69. G Density (Gurley), seconds 260 200 Beating testa-The physical properties of pulps such as bursting (Mullen) strength, tearing strength, folding endurance, opacity and density were determined on hand sheets prepared from pulp beaten in a Laboratory Valley beater. The hand sheets were prepared on Noble and Wood sheet making equipment from pulp beaten to certain Schopper-liiegler freenesscs as noted. The weight of the test sheets was 2G pounds on a 17 inch by 22 inch, 50() sheet basis. References to the TAPPI standard procedures followed are as follows.

Laboratory the "EGSM-45 Bursting (Mullen) Strength of Paper, T403M-53 Internal Tearing Resistance of Paper, T414M-49 Folding Endurance of Paper: (il, M LT. Folding Endurance), T423M-50 Opacity of Paper, T425M-44 Air Resistance of Paper (Density), T460M-49 Two identical bisulte semichemical cooks on spruce chips are represented. ln one case the cooked chips were iiberized in the disk` mill sufficiently only to separate the individual fibers while doing the minimum of further refining work on them. The freeness of the unbleached pulp was 86d. On the other hand, by substantially increasing the amount of refining action, the freeness was reduced to 700. After these t ups were bleached the freeness relation persisted at 35i) and 775, respectively, while the 2li-mesh fiber fractions were respectively 84.5 and 70.6%, indicating the fiber cuttiniT in the latter case characteristic of the effect of refining or stock preparation. When these pulps were beaten under standard conditions in a Valley beater 1S minutes was required to beat the free pulp to a 750 freeness Whereas only 3 minutes was required to beat the pre-refined pulp to the same freeness. A freeness of 750 normally represents the dcgree of stock preparation desired for running on the paper machine. Balanced against the advantage of a Substantial reduction in stock preparation required in the paper mill, prereiining results in certain alterations in fiber characteristics. Thus a markedly higher level of opacity was obtained with certain strength factors such as Mullen and fold lowered while tearing strength was somewhat increased.

The opportunity afforded by the semichemical operation to include refining in the fiberizing step, ranging in degree from practically none to such a degree that no further rening of the bleached stock is required to prepare it for running lon the paper machine, while an advantageous feature of the new process, is of less importance in achieving the purposes of the invention than the semichemical digestion of chips with a bisullite liquor followed by controlled removal of carbohydrate material in the purification stage. in fact it is preferred, for most applications of the process, to employ the mildest degree of fiberizing of semichernical chips consistent with complete disintegration into discrete fibers. Thev free-drain- Processing Pulp (Beater Method),

ing pulp which results is most economically processed and most easily washed throughout the various stages of purification. Further preparation of the stock then is done entirely in the paper mill beaters, refiners, Jordans and the like where normally the development of paper characteristics can most efficiently and satisfactorily be accomplished.

The results set forth in Tables 2 and 3 should not be considered as limiting the scope of the invention. For example, in the `developnient of high translucency-type pulps it may be desirable to carry the cooking yield even higher thus retaining more of the pentosans and other carbohydrate materials which, because of their hydrophilic and easy-gelatinizing properties contribute translucency, high tensile strength, stiffness, and reduced porosity to a sheet of paper. In the usual full chemical pulping processes these materials are, in part, hydrolyzed and lost. Thus, the relationships reported in Tables 2 and 3 serve merely to illustrate how, by means of preoise, but simple and immediately applicable manipulations of 2 major variables the process can be controlled to produce within a known range of characteristics any one of `a wide variety lof pulps at a predetermined materials cost. Heretofore, it has not been possible to reproduce within a single pulping system virtually the cntire range of coniferous papermaking pulps running the gamut from those which have in high degree the unique characteristics of sulte pulps to those which reproduce the special features of kraft pulps.

The variables whose control -make this achievement possible are:

(1)'Extent of semichemical cooking using a bisulite cooking liquor maintained within a specified pH range, and

(2) Extent of alkaline digestion in the purification stage.

The extent of the semichemical cook limits the potential characteristics which may be attained in the finished pulp in a manner illustrated in Tables 2 and 3.' Thus the extent of cooking is determined primarily by the amount of bisulfite ion consumed and is measured by the permanganate number. As lthe permanganate number is increased the potential for maximum yield, tearing strength, bursting `and pentosan content is increased. The potential for chemical purity, and opacity is decreased. Raw material cost is somewhat increased.

The extent of alkaline digestion determines whether the final pulp will be a sullite-like pulp, a kraft-like pulp or some predetermined compromise between the two. r`he amount of caustic soda consumed is the determining factor' and the effect is measured and controlled by the test for solubility in 18% caustic soda. As the alkali solubility is reduced the chemical purity, stability, tearing strength, opacity, softness, bulk characteristics and cost are increased. The yield, pentosan content, Mullen strength are decreased.

The unique opportunity to control the degree of refining of the unbleached pulp is a concomitant advantage of the process. Coincident with the berizing operation it is possible to pre-beat the pulp andthus control with precision the amount of refining which subsequently would be required in the paper mill as well as emphasize certain special characteristics. This opportunity of oontrolling the drainage characteristics of the pulp is not available in conventional chemical pulping processes.

Two specific examples of the process as adapted for mill production will now be given.

EXAMPLE 1 The following procedure has been used in a mill adaptation of the process to produce a pulp in which the emphasis was placed upon the attainment of relatively high tearing strength and opacity.

Spruce chips of conventional size were employed. Two vertical mill digesters of 7200 cubic feet capacity were Spadaro employed for the series of digestione. The digesters, lined with brick joined with a special urfural-formaldehyde resin designed to resist soluble oase acid suliite liquors, were equipped with pumps for forced circulation of liquor and with facilities yboth for direct and indirect heating. The cooking liquors employed had the following average analysis:

Total SO2 percent-- 2.40 Free SO2 do 1.19 Combined SO2 do 1.21 pH 3.9

The liquor was prepared by introducing sulfur burner gas into a solution of sodium carbonate of about 2.0% strength until the desired pH was reached. Tests for total SO2 and pH adequately served to establish the correct balance between free and combined SO2.

Prior to the addition of cooking liquor, direct steam was introduced into the bottom of the digester in order to aid in sweeping out occluded air from the chips. During this period, which consumed about 1/2 hour, the top relief valve was kept open.

The digester then was filled with liquor and shut in (all valves closed). Under these conditions 10.5% total SO2 (in the form of sodium bisulte), based on dry wood, was available. This was the desired concentration. For different wood chip densities, which would alter the liquor to wood ratio, or when different concentrations of chemical to wood are desired it is necessary to change the liquor concentration accordingly. The liquor to wood ratio in this example was about 5.3 to 1.

With the automatic relief valve on the digester set for 1,00 p.s.i., direct and indirect steam were started and the temperature of the charge gradually brought to 160 C. The digester was kept at 160 C. until a test of the liquor had dropped to an average of about 0.70% total SO2. At this time, the digester was gradually relieved to 50 p.s.i. About 3 hours was taken to reach the maximum temperature of 160C. The time at maximum tempera- The mixture of cooked chips and crude pulp in the blowpit were drained of excess spent liquor and washed in situ. The average permanganate number of the stock as determined on the subsequently iiberized pulp was 45.1.

Since the cook was somewhat on the soft side, the explosive elfect of blowing the chips from the digester amounted to a partial preliminary iiberizing action as a result of which a c-ertain proportion of the chips were reduced to a crude pulp-like form. The stock was taken over a vibratory knotter screen equipped with 1r-inch diameter perforations. This operation resulted in a segregation with the crude pulp and fiber agglomerates passing through the screen while knots, wood chunks, compression wood, oversized chips and the like, along with uniformly cooked chips too large to pass through the screen, were rejected. The accepted material was sent to a storage tank for subsequent berizing while the knotter rejects containing shive-forming and dirt-forming knots and the like along with a remaining proportion of acceptable iber material was sent to a Dynopulper. It has been found that rawer semichemical cooks with a lignin content in excess of about 12% contain a much smaller proportion of fragmentized chips and crude pulp relative to unreduced chips. For such material the advantage cf a preliminary knotting will be correspondingly less and it may be preferable to subject the cooked chips directly to the selective mild pulping action in a Dynopulper or continuous druhber prior to knotting. The relative proportion oi fragmentized material will r etermine the most economic procedure to follow. The reject material was subjected, in the Dynopulper, to a mild pulping action for 10 minutes at about 8% consistency in order selectively to disintegrate the desirable fiber material while leaving the harder dirt forming knots and the like relatively intact. The mixture of knots and crude liber then was subjected to a second screening on the knetter. The accepts from this operation were pumped to the storage chest wit-h accepts from the first screening operation for subsequent tiberizing. The rejects comprising dirt-forming and shive-forming knots and the like amounted to less than 1% of the original wood charged. Because of their high shive and dirt potential they were discarded.

The combined accept material was then subjected to iiberizing in conventional-type disk mills such as are widely employed in the production of hardwood semichemical pulp. The disk mills were equipped with plates having a closed periphery and tie clearance and power application were controlled to maintain a SR. freeness of about 800 ml. After such iiberizing the pulp was screened on dat screens with .007 slots and after thiclening was sent to the brown stock chest for purication and bleaching.

Purification and bleaching of the pulp was carried out in a 3-step procedure comprising chlorination, a digestion with caustic soda and bleaching with calcium hypochlorite. Chlorine was applied on the basis of, a correlation chart relating the variables of permanganate number of the unbleached pulp, percent chlorine applied and the permanganate number desired in the chlorinated alkalinedigested pulp. Approximately 11.7% chlorine, based on dry weight of pulp, was introduced into the stock slurry at about 3.0%. The retention time in the chlorination tower was about 11/2 hours and the temperature 27 C. The chlorinated stock was washed on a drum washer to remove solubilized chlorinated organic materials. The stock thickened to about 12% consistency was dropped to a mixer into which a caustic soda solution was added and steam injected and the mixture was then dropped into a tile-lined tower and steaming controlled to give a temperature of 92-94 C. The retention time in the tower was about 3 hours. About 9.0% caustic soda was added, based on pulp, and the pH in the caustic washer vat was 11.5.

The alkaline digested pulp, after washing to remove solubilized chlorinated lignin and carbohydrate material, had a permanganate number of 1.9. The stock then was bleached to a brightness of 87-88 using about 1.0% available chlorine in the form of calcium hypochlorite. The stock consistency was about 10%, the temperature 38 C. and the retention time in the hypochlorite tower about 5 hours. Suiiicient caustic soda was added with the hydrochlorite to maintain the linal pH at 8.5-9.0. The bleached pulp then was washed to remove reaction products.

EXAMPLE 2 The wood raw material, digestion and iiberizing procedures were essentially as set forth in Example 1. Following the chlorination stage the degree of alkaline digestion as compared to that in Example 1 was substantially less. About 4.5% caustic soda based on dry pulp was added. The consistency was 12% and the temperature C. The retention timein the tower was about 2,v hours and the final pH in the vat of the caustic washer 10.8. The washed pulp, which had a permanganate number of 2.1, was then bleached with calcium hypochlorite as set forth in Example 1.

Table 5 sets forth results obtained on the two new pulps of Examples 1 and 2, on conventional sulfte pulp produced in the same mill from the same type wood and on 3 high quality commercial bleached kraft pulps. It will be noted that the new pulp (Example 1), produced by the use of relatively drastic conditions of alkaline digestion to favor tearing strength, was equal or superior to the commercial kraft pulps in most respects while being substantially superior in ease of beating. New pulp (Example 2), produced by the use of mild conditions of alkali digestion, to favor high yield and thus approach the relative economy of sulflte pulp, was substantially superior to sulte pulp in all strength factors. In fact it has been found that this pulp is suiliciently strong to supplant kraft ber in many applications. The new pulp at the same time was found to have the easy beating characteristics and relatively low opacity of sulte pulp.

TABLE 5 Corn- New New mer- Commercial pulp pulp cial Kraft pulps Ex. l Ex. 2 sultite pulp Brightness 87. 87.0 86. 5 88. 5 87` 0 86. 5 Alkali solubility,

percent 8.6 10. 9 13. 0 8. 1 Beating time to 750 S.R. freeness, mins 1 14. 5 13 38 36 40 Mullen, p s i 69 63 50 G5 55 82 Tear, g- 97 68 50 89 100 73 M lillen-l-tea 166 131 100 154 155 155 Folding endurance,

double folds 2, 530 1, 800 850 1, 050 685 2, 040 Opacity, percent--- 66.1 68 67.0 71. 0 70. 5

While the various aspects of the invention have been described in considerable detail in the foregoing, with preferred limits placed upon certain factors, it will be understood that various changes may be made in the procedure folowed and the specie conditions set forth, within the scope of the invention as defined by the appended claims. Thus in lieu of using sodium bisulfite in the first digestion step, other soluble bisultes of an alkali metal or alkaline earth metal or their equivalent may be used, for example bisultes of potassium, lithium or magnesium Whatever bisultite is employed, the various conditions set forth herein should be observed.

We claim:

1. A semichemical process for producing from coniferous wood by the same sequence of steps any one of a variety of different types of pulp having dilerent characteristics and suitable for use in the production of various qualities of tine papers, which comprises partially but not completely digesting coniferous wood chips with a liquor which is substantially a solution of a water soluble bisulte of an alkaline metal, said liquor having a pH between 3.0 and 5.0 and being used in suicient volume to provide from about 9.0 to 12.0% equivalent SO2 based on the oven dried wood, conducting said digestion at from 155 to 170 C., terminating such digestion when the treating liquor contains less than 0.7% SO2, removing knots and other dirt-forming components from the mass of partially digested chfps, the foregoing steps being so conducted as to provide a dirt-free pulp yield of a preselecled value between about 50% to 62%, subjecting the substantially dirt-free fraction of said mass to mechanical treatment to reduce the same to a pulp of discrete fibers, and subjecting said pulp to a succession of treatments comprising alkaline digestion and bleaching treatments, said alkaline digestion treatment being carried out with suiiicient alkali concentration and under such temperature and time factors as to bring about a consumption of between 1.5% and 6% of alkali, based upon the dry weight of the pulp, and thus remove substantial amounts of hemicellulose present in said pulp, the extent of alkaline digestion being so controlled as to bring about a preselected reduction in the pentosan content and the desired properties of the pulp, the dry pulp yield on the basis of the weight of the original wood being at a preselected value between 40% and 50% but such as to leave at least a 3% pentosan content therein.

2. A semichemical process for producing pulp in accordance with claim l in which the digestion liquor is substantially a solution of sodium bisulte having a preselected total SO2 content between 1.50 and 2.40% and in which the weight of said liquor used is of a preselected amount between 5 and 6 times the dry weight of the chips being digested.

3. A semichemical process for producing pulp in accordance with claim 1 in which the bisulte digestion is carried to a point at which the permanganate number of the digested mass is at a preselected value between 30 and 65.

4. A semichemical process for producing pulp in accordance with claim 1 in which the dirt-forming components removed from the partially digested mass are subjected to a mild mechanical treatment to liberate desirable iibers therefrom but without substantially reducing the size of the dirt-forming constituents, the desirable fibers liberated by said treatment being then separated from' the dirt-forming constituents and combined with said dirt-free fraction that is subjected to the further treatments specified.

5. A semichemical process for producing pulp in accordance with claim l in which said mechanical treatment of the dirt-free fraction of the partially digested mass is of such character as to not only iiberize said mass but also to refine the fibers by brillation thereof to such an extent that the S.R. freeness of the pulp is reduced to a preselected value between about 700 and 800 rnl.

6. A semichemical process for producing pulp in accordance with claim l in which the alkaline digestion step is preceded by a chlorine bleaching step to preselected extent and is followed by a hypochlorite bleaching step to attain the desired brightness, said alkaline digestion step being such as to remove chlorinated lignins and some of the hemicellulose constituents of the pulp but not severe enough to produce substantial further digestion of the desirable paper forming components of the pulp.

7. A semichemical process for producing pulp in accordance with claim 6 in which the chlorine bleaching step involves the application to the pulp of between `l0 and 12% chlorine based upon the weight of the dry pulp for about 1.5 hours and the hypochlorite step involves the application of calcium hypochlorite under conditions bringing about the use of between 1.0% and 2.0% available chlorine based on the weight of the dry pulp.

8. A semichemical process for producing from coniferous wood pulp capable of imparting to paper produced therewith substantially the properties provided by kraft pulp, which comprises digesting coniferous wood chips in a liquor which is substantially a solution of a water soluble bisultlte of an alkaline metal to partially lbut not completely remove the lignin from the chips, said liquor having a pH between 3.0 and 5.0 and being used in such amount and concentration as to provide a preselected amount between 9.0 and 12.0% total sulfur dioxide applied to wood substance, conducting said digestion for a time and at such a temperature as to provide a cooked mass having a preselected perinanganate number between 30 and 65, removing from the cooked mass knots and other dirt and shive-forming constituents, reducing the balance of the cooked mass to discrete fibers, and subjecting the fiberized mass to an alkaline digestion and a bleaching sequence, the alkaline digestion being carried out with a concentration of caustic soda of between 6% and 8% based on the weight of the dry pulp and at such a temperature and for such a time as to bring about a consumption by the cellulose constituents of the pulp of a quantity of caustic soda equal to a preselected value between 4.0% and 6.0% of the weight of the dry pulp, said alkaline digestion being so conducted under the conditions specified as to cause a reduction of the alkali solubility of the pulp to a prescribed value within the range of 5.0 to 8.0%.

9. A semichemical process for producing from coniferous wood pulp of the character of a bleached sul- -tite pulp, Ahowever substantially enhanced in all the major strength properties, which comprises digesting coniferous wood chips in a liquor which is substantially a solution of a water soluble bisullite of alkaline metal to partially but not completely remove the lignin from the chips, said liquor having a pH between 3.0 and 5 .0 and being used in such amount and concentration as to provide between 9.0 and 12.0% total sulfur dioxide applied to wood substance, conducting said digestion for a time and at such a temperature as to provide a cooked mass having a permanganate number at a preselected value between 30 and 65, removing from the cooked mass knots and other dirt and shive-orming constituents, reducing the balance of the cooked rnass to discrete fibers, and subjecting the iberized mass to an alkaline digestion and a bleaching sequence, the alkaline digestion being carried out with a concentration of caustic soda of between 3% and 4% based on the Weight of the dry pulp and at such temperature and for such a time as to bring about a consumption by the cellulose constituents of the pulp of a quantity of caustic soda equal to a preselected value between 1.0% and 2.0% of the weight of the dry pulp, said alkaline digestion being so conducted under the conditions specified as to cause a reduction of the alkali solubility of the pulp to a preselected value within the range of 11.0 to 14.0%.

10. A semichernical process for producing from conifcrous wood a pulp capable of imparting to paper produced therewith properties intermediate in character to those of a bleached kraft pulp and a bleached sulte pulp, which comprises digesting coniferous wood chips in a vliquor which is substantially a solution of a water soluble bisullte of an alkaline metal *to partially but not compe'tely remove the lignin from the chips, said liquor having a pH between 3.0 and l5.0 and being used in suchr amount and concentration as to provide a preselected amount between 9.0 and 12.0% total sulfur dioxide applied to wood substance, conducting said digestion for a time and at such a temperature as to provide a Cooke mass vhaving a preselected permanganate number between 30 and 65, removing from the cooked mass knots and other dirt and strive-forming constituents, reducing the balance of the cooked mass to discrete libers, and subjecting the fiberized mass to an alkaline digestion and a bleaching sequence, the alkaline digestion being carried out with a concentration of caustic soda of between 4% and 6% based on the weight of the dry pulp and at such a temperature and for such a tirne as to bring about a consumption by the cellulose constituents of the pulp of a quantity ot caustic soda equal to a preselected value between 2.0% and 4.0% of the weight of the dry pulp, said alkaline digestion being so conducted under the conditions specied as to cause a reduction of the alkali `solubility of the pulp to a preselected point within the range from more than 8.0% to less than 11.0%.

References Cited in the file of this patent UNITED STATES PATENTS 1,838,326 Richter Dec. 29, 193i FOREIGN eATENTs 736,3()0 Great Britain 1 Sept. '7, 1955 OTHER REFERENCES Casey; Pulp and Paper, Vol. l, published by Interscience Publishers, 1960, pp, 12'9, 152, 153, 172. 173'l 341 and 368,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Ratent No. 3,069,310 December 18, 1962 Royal H. Rasch et al.

It is hereby certified that error appears in the above numbered patl ent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 65, after "good" insert wet column 3, line 33, for "premnum" read premium column 7, line 73, for "in" read is same line 373, for "dgster" read digester 4-; column 8, line 57, for "screen" read screens column 20, line 5l, for "hydrochlorite" read hypochlorite column 2l, TABLE 5, second column, opposite "mins", for "l"I read 2l ;A line 54, for ="to" read and column 22, line 46, after "wood" insert a line 72, for "precribed" read preselected line 75, after "wood" insert a Signed and sealed this 18th day of June 1963.

SEAL) lttest:

IRNEST W. SWIDER DAVID L. LADD lttesting Officer Commissioner of Patents

Claims (1)

1. A SEMICHEMICAL PROCESS FOR PRODUCING FROM CONIFEROUS WOOD BY THE SAME SEQUENCE OF STEPS ANY ONE OF

Images