US3255071A - Process for production of alkali cellulose in the absence of an aqueous liquid phase - Google Patents

Description

T. N. KLEINERT June 7, 1966 3,255,071 PROCESS FOR PRODUCTION OF ALKALI OELLULOSE IN THE ABSENCE OF AN AQUEOUS LIQUID PHASE 2 Sheets-Sheet l Filed Oct. 28, 1965 /m/E/v To 77/f00 of? Aw irren/var mw wrm um w v June 7, 1966 T. N. KLEINERT 3,255,071

PROCESS FOR PRODUCTION OF ALKALI CELLULOSE IN 2 Sheets-Sheet 2 THE ABSENCE OF AN AQUEOUS LIQUID PHASE Filed Oct. 28, 1965 INVENTOR Theodor N. KLEINERT ATTORNEY United States Patent O 7 claims. (ci. 162-82) This invention relates to cellulosic pulps, more especially to pulps of low viscosity and of high tit-cellulose content obtained from fibrous cellulosic material by alkaline treatment and to a process for their production.

According to conventional alkaline (kraft) processes, pulp is produced by delignication of fibrous material upon cooking with low content sodium hydroxidesodium sulfide aqueous solutions, in liquid phase, under relatively mild conditions so as to avoid degradation of cellulose.

In United States application Ser. No. 33,659, filed June 3, 1960, now abandoned in favour of U.S. continuation-in-part application Ser. No. 302,379, filed August 15, 1963, applicant has disclosed a pulping process characterized by impregnating fibrous plant material with alkaline liquor resulting in a limited alkali charge and subsequent heating of said impregnated plant material in an atmosphere of saturated steam.

It has now been found that cellulosic pulp of low viscosity and high tt-cellulose content can be obtained from fibrous cellulosic material by controlled alkaline treatment, with no aqueous liquid phase being present, at a pressure essentially lower than that'of saturated steam at reaction temperature.

It is accordingly an object of the present invention to provide a process for carrying out alkaline treatment of Various fibrous cellulosic materials as aforementioned.

It is a further object of this invention to produce high tnt-cellulose, and low Viscosity pulps from fibrous cellulosic material, said pulps also having low lignin content when said fibrous cellulosic material is wood or a similar highly lignified raw material.

It is a further object of this invention to provide a continuous process for preparing new cellulosic pulps, as hereinafter described.

The present invention, in its broadest aspects, consists in a process for producing crystalline cellulosic pulp of low viscosity and high tat-cellulose content having a relatively high purity comprising:

(l) impregnating subdivided fibrous cellulosic material with alkaline liquor,

(2) removing water by evaporation from said impregnated subdivided fibrous material in an inert atmosphere until a water vapour pressure essentially lower than that of saturation is obtained,

(3) heating in an inert atmosphere at a temperature of 80-220 C. at the aforesaid water vapour pressure, and

(4) recovering said crystalline cellulosic pulp.

The present invention further consists in a stable, crystalline cellulosic pulp having a low viscosity and a relatively high tnt-cellulose content, as will be described hereinafter.

Subdivisions of various fibrous cellulosic materials, for instance, cotton linters, waste regenerated cellulosic fibers, various woods, straw, bamboo, or bagasse, can be used as Well as small size wood waste materials like the socalled pin chips or the chipper wood dust which are difficult to pulp in conventional pulping operations. Sawdust from sawmill operations `can also be used.

3,255,071 Patented June 7, 1966 Any alkaline liquor containing hydroxyl ions of sufficient concentration can be used to impregnate the subdivided fibrous cellulosic material. Preferred alkaline liquors are sodium hydroxide solutions of appropriate concentration or the white liquor obtained by causticizing the green liquor resulting from chemical recovery of commercial black kraft liquor.

This alkali charge should be in amounts sufficient to result in at least 10% (weight) charge of alkali calculated as Nago and based on the dry fibrous material. For fibrous plant material having a high lignin content (before treatment) it may be desirable to obtain a low lignin content in the resultant product and higher amounts of alkali may be used for this purpose, e.g. to provide a l6-30% (weight) charge of alkali (Na20) calculated on the dry wood substance. The low lignin cellulose pulp thus obtained, after recovery, shows good brightness without any further bleaching, e.g. up to 70% Elrepho brightness. Alkali charges higher than 30% can be used but no substantial advantages are noted. Thus 16-30% alkali charge is. preferred, especially for wood.

The concentration of alkali used is a practical matter. If lower concentrations are used, more liquor has to be retained by the fibrous material to reach a predetermined alkali charge and more Water has to be evaporated. Thus it is preferred to use concentrations corresponding to alkali concentrations expressed e.g. as NaOH of from 8 to 20%, although concentrations as low as 4% and even higher than 20% can be used effectively. The above concentrations also apply when the alkaline liquor used is obtained from kraft liquor, although, in this case, it is customary to express alkali concentration as effective alkali calculated as NagO.

To promote impregnation, it has been found that deaeration in a known manner before impregnation with alkaline liquor, e.g. by steaming or purging, and/or mechanical defibrizing of the ligniiied plant material before, during or after said impregnation is helpful. Also, the use of elevated temperatures up to about C. and of additional hydrostatic pressure have been found to be of advantage, as well as agitation.

The alkali impregnated fibrous cellulosic material is then subjected to removal of the liquid phase by evaporation of water by any known manner. Thus, the impregnated material is treated under such conditions of temperature and time so that water is removed until a water vapour pressure essentially lower than that of saturation is obtained, in other words, until the liquid aqueous phase has disappeared.

To reduce the time necessary for water evaporation, it has been found convenient first to remove in a controlled way excess alkali liquor from the wet impregnated material by mechanical means such as draining, centrifuging and/or pressing, prior to said evaporation.

Removal of a predetermined part of water from the wet, impregnated material prior -to reaction can, for instance, be achieved either by evaporation at a temperature lower than that of reaction or by immediate heating of the wet impregnated material to reaction temperature while withdrawing steam until a predetermined pressure level is reached. In the latter case, the temperature of the impregnated cellulosic material rises as `the water evaporates. Actually, if the water is removed at a temperature high enough so that substantial reaction can take place, then cellulose degradation will occur as the Water is being removed. However, the rate of reaction increases drastically when the aqueous phase has been removed and the moisture content has dropped beyond the fiber saturation point. Other means of removing the In general, heating of the impregnated cellulosic material to a predetermined reaction temperature can take place in any inert, low pressure medium, liquid, vapourous or gaseous. Reactive gases such as carbon dioxide, which would combine with the alkali must not be present in noticeable amounts, if inert gases are used. Oxygen should also be excluded if non-oxidized cellulose degradation products are desired.

Heating can be done by any means, for instance, by dielectric heating or by external heating of the reaction vessel.

Heating can also conveniently be done by means of unsaturated steam, s-uperheated to appropriate temperature. Because of the heat consumption by water evaporation, to maintain a certain reaction temperature, the superheated steam should be admitted to the impregnated cellulosic material in such a ratio and at such a temperature that, after compensation for the heat losses and after evaporation` to eliminate the liquid phase, the impregnated material is at reaction temperature. Waste steam from the evaporation or reaction stages, after adjusting to appropriate temperature, can be recycled.

To promote uniform heat distribution during the reaction, agitation effected for example by turbulent movement of the subdivisions of cellulosic material is advantageous. When using superheated steam in a uidized system for water evaporation and/or reaction of the impregnated fibrous material, separation from the vapourous phase can be eifected after treatment by known means such as, e.g. centrifugal separators.

Since heating up to temperature in a uidized system is very fast, the impregnated plant material, after reaching reaction temperature, in order to undergo reaction over a predetermined period of time, can be deposited for this period in an appropriate retention vessel from which it can be continuously withdrawn at the end of reaction time.

For economical reasons, when using dielectric heating to provide the reaction temperature, water evaporation from the impregnated material to a maximum moisture content of about 30% or lower before reaction appears to be desirable.

The reaction temperature should be between 80 C. and 220 C, while temperatures of ISO-190 C. are preferred. It has been found that most of the cellulose degradation as measured by viscosity-decrease follows approximately a first order reaction pattern. Thus, for a given reaction temperature, the logarithm of said viscosity plotted versus reaction time shows an approximately linear relationship in the ranges of viscosity observed. However, in the range of very low viscosities,

` i.e. intrinsic viscosities below about 1, considerable deviations from this pattern are noted. In general, the higher the temperature, the faster the reactions. However, too high a temperature would result in undesirable decomposition, for instance producing water-soluble, colored products.

Subject to the above conditions, times of reaction as little as a few seconds at very high temperatures within the above range up to as much as several hours at the lowest temperatures within the above range produce the desired degraded celluloses.

' The reaction is quenched by cooling. This can, for instance, be achieved by treatment with saturated steam of low temperature or 4by immersion into an aqueous non-.acid liquid of appropriate temperature. The (nonsoluble) crystalline cellulosic pulp produced is then recovered by separating it from the soluble portions. This can appropriately be done by leaching in an aqueous solvent which can also be used -for quenching the reaction. The products of delignication, which, if present, are also soluble in aqueous alkaline medium, can also be recovered by known methods from the solution.

. Leaching out of solubilized portions of treated lignied brous plant material can' also be achieved by first leachdissolution.

The unwashed pulps in a dry state as obtained after reaction are fairly brittle and can easily be ground if desired.

Since it hasnow been found that alkaline controlled cellulose degradation occurs in the absence of an aqueous liquid phase at any vapour pressure below that of saturated steam, even at atmospheric pressure, the process of the present invention is advantageously carried out in a continuous manner. Thus, the cellulosic raw material can be impregnated with alkali while in motion and the impregnated material can be continuously transported, for example, in an inclined externally heated rotary kiln or in suspension lin an inert gas, or vapour while lwater is being evaporated and while reaction is being carried out.

However, in a continuous system using an inert gas as suspending medium, this 'gas may have to be separated from the treated material before recovery. A most convenient means lof achieving the above is to transport the impregnated material in suspension in a stream of superheated steam at appropriate temperature and pressure.

The present invention will now be described in greater detail, reference being made to the accompanying drawings in which:

FIGURE 1 represents a -schematic diagram of a preferred embodiment of the process of the invention. v

FIGURES 2 and 3 represent photographs of X-ray diffraction diagrams of products obtained in accordance with the invention.

With specific reference to FIGURE l, a storage bin 1 is provided for storing the subdivided fibrous cellulosic material for processing. Storage vessel 2 contains alkaline liquor of appropriate concentration. The subdivided ma terial is conducted through storage bin 1 into impregnation vessel 3, provided with an adequate agitator 3a, wherein it is contacted with the alkaline liquor from storage vessel 2. In a continuous process, as explained hereinbefore, the impregnation vessel is preferably replaced by a centrifugal pump connected to adequate piping system.

Provision can be made for further disintegrating the particles in the slurry obtained from impregnation vessel 3 by means of disc rener 4 or similar means. The thus disintegrated fibrous cellulosic material is then conducted into a 4separating device, e.g. screw press 5, in which a controlled amount of excess alkali liquor is Withdrawn from the fibrous material. This withdrawn liquor can be recycled to impregnation vessel 3, if desired.

The pressed material -obtained from the separating device is then processed by adequate means 6 to break up any agglomerations. The resulting product is then processed through reaction stage 7, shown as containing water evaporation section 7A and reactor section 7B. As described, these two sections are not necessarily separate and the water evaporation can be elfected while the librous material is being heated up to reaction temperature.

Further provision is made, if applicable, as will be described later, ffor separating the product obtained in reactor section 7B from a transporting medium by separation means 8.

The reaction products are then processed to a cooling and recovery section 9 in which the reaction is quenched by cooling and the crystalline cellulosic pulp is separated (and further puried by adequate means) from the portions soluble in aqueous alkali which include decomposition products of carbohydrates as well as lignin conversion products, if lignin is present in the raw material. In this last instance, recovery of some of the lignin decomposition products can be effected in a known manner in lignin recovery section e.g. by solvent extraction .by means of a water-immiscible organic solvent and/ or precipitation after acidification of the resulting black liquor.

The degraded cellulose products obtained in accordance with the invention are highly crystalline products as illustrated by X-ray diffraction diagrams presented in FIG- URES 2 and 3. Specifically, FIGURE 2 represents the X-ray diffraction diagram of a crystalline degraded cellulosic pulp obtained by impregnating black spruce sawdust with a 30% (weight) NaOH charge calculated on oven dry wood, followed by immersing in paratlin oil at 200 C. at atmospheric pressure. Immediate evaporation of water took place in approximately 1 to 2 minutes. Heating was then continued for 10 minutes at the same temperature.

The cellulosic pulp recovered was colorless, its lignin content was below 2%, and its tit-cellulose content was labout 92% The X-ray diagram shown in FIGURE 3 represents crystalline cellulosic pulp obtained in the same manner as above except that the impregnated sawdust was heated in the same conditions for thirty minutes, after water was driven out. This diagram shows that, even under such drastic conditions of treatment, crystallinity of the cellulose was still retained to a great extent.

The alkali degraded cellulose obtained by the present invention offers little resistance to mechanicall grinding in a dry or wet state. Wet grinding results in the formation of gels of high stability. These new alkali degraded celluloses have been found to contain carboxyl end groups as evidenced by the Ldtke calcium acetate method and by their relatively low copper number (0.5 or lower). A further characteristic of these new celluloses is their improved chemical stability due to the absence of alkali lsensitive groups, e.g. reducing aldehyde or keto groups, which are removed to a great extent during the process.

In contrast, degraded celluloses obtained by acid hydrolysis contain various oligosaccharides, and other degradation products containing such reducinggroups and other alkaline sensitive groups. These groups, particularly those which 'are alkali-sensitive groups, are responsible for the chemical instability of acid hydrolyzed cellulose, especially with regards to yellowing (see Aging and Colour Reversion of Bleached Pulps, part I, L. M. Marraccini and T. N. Kleinert, Svensk Papperstidning, vol. 65 (1962): 4, 126, part II, T. N. Kleinert and L. M. Marraccini, Svensk Papperstidning, vol. 66 (1963): 6, 189). Also, acid degraded celluloseshave a relatively low tat-cellulose content. In comparison, degraded celluloses obtained according to the invention exhibit high a-cellulose content. For instance, degraded celluloses obtained from wood according to the present process show an a-cellulose content of 90% or higher.

Alkaline cellulose degradation has been studied in aqueous medium and there is evidence by the work of various investigators that it proceeds according to the following mechanisms:

(a) chain scission,

(b) so-called peeling off effect from the end groups which causes substantial yield loss, until stabilization occurs.

In contrast, indications have been obtained that in the absence of an aqueous liquid medium, according to the invention, chain scission is promoted whereas peeling off is noticeably smaller while, at the same time, stabilization of the chains occurs. Thus, the a-cellulose yield is relatively high despite the drastic conditions used with regards to alkali charge and temperature.

The degraded celluloses of the invention shows a derived intrinsic viscosity of about 6 or less, in some cases as low as 0.5.

Because of the margin of error involved in determining 0.5% CED viscosity in the low viscosity range, no attempt was made to extrapolate to obtain intrinsic viscosity values for 0.5 CED viscosities substantiallylower than 2. Whereas, in the higher range of viscosity and D.P. value, this proportionality cannot be assumed for the low viscosity range of the new celluloses produced by the present invention. Thus, in the range of very low intrinsic viscosity values derived from 0.5% CED viscosity measurements, the viscosity of the new celluloses might not be a true measure of their molecular size expressed in D.P. values, as used for linear polymers.

The products of the invention also show relatively high purity and reactivity in chemical reactions such as, etherification, esterification, saccharification, gratting of polymers on cellulose or even in total acid hydrolysis to glucose.

When these celluloses are obtained by processing wood derivatives in accordance with the present invention, a byproduct of the process consists in alkali-soluble lignin degradation products containing reactive functional groups such as phenolic hydroxyl groups. The formation of these lignin degradation products is especially favoured when the reaction temperature is higher than 180 C. and the alkali charge by impregnation is higher than about 20% by weight, calculated on the O.D. iibrous material. These lignin degradation products can be used for further conversion, for instance, in the production of plastic materials, or as raw material for hydrogenation.

The invention will now -be described in further detail with reference to the following examples.

EXAMPLE l A 100 gm. charge of air dried spruce wood wafers (moisture content about 30%, dimensions: 1.5 inch x 1 inch x 1&2 inch) contained in a cylindrical nickel wire basket was rst immersed for 30 minutes in a 13% sodium hydroxide solution of about 100 C. After this impregnation, the basket with the material was lifted out froml the liquor, and the excess liquor was drained for five minutes. The resulting alkali charge was about 30% NaOH calculated on O.D. wood material. The basket with the impregnated wood was then immersed into a constant temperature bath of parain oil of 190 C. which was stirred to force circulation of the parain oil through the basket. A violent water evaporation took place which, however, after about 1-2 minutes from the time of immersion of the basket, practically ceased. The impregnated wood material had now reached reaction temperature. Heating of the material was continued for another three minutes. At the end of this period, the basket containing the reacted material was re-moved from the paraiiin oil bath and the adhering paraffin oil was drained for 1-2 minutes.

The basket was then placed into a fitting vessel of slightly larger diameter and enough water was added to fully cover the cellulosic material. Partial cooling and leaching out of residual alkali of relatively high concentration, and of solubilized wood portions took place,

whereby the alkalinity of the liquor promoted the dissolution of these portions. After keeping the basket in the resulting liquor for ten minutes, the basket containing the cellulosic material was removed and the material was transferred into a glass beaker. There, the material was washed several times with Water of C. by decantation and finally on -a Buchner funnel. After acidifying with N/ 'hydrochloric acid for removing traces of alkali, the cellulose product was washed with distilled water. Then, the material was air-dried, and after drying, it was exhaustively extracted with petroleum ether. The resulting product showed the following analytical data:

Roe chlorine number 2.7. Permanganate number 110.9. Klason lignin -l. 2.2%. Alpha-cellulose 92.5%. Pentosan 1.3%. Cupramrnonium viscosity 3.1 cps.

'E' EXAMPLE 2 Coniferous chipper wood dust of about 30% moisture content was used in this example. The wood material was characterized by particle weights from about 0.1 gm. to about 0.001 gm. The wood dust was impregnated at a temperature of about 100 C. and at atmospheric pressure for 30 minutes in white liquor from a commercial kraft mill having an effective alkali content (sulphidity 30%) of about 10% by weight expressed as Na2O. After removal of the impregnated wood material from the liquor, the excess liquor was pressed off resulting in an alkali charge of about 28% NaZO. After mechanically subdividing the compacted mass, the impregnated wood was continuously fed into a small externally rheated inclined rotary kiln, Whereit was heated at atmospheric pressure to 185-188 C. while moving and simultaneously being subjected to the action of countercurrently owing unsaturated steam of the same temperature. The steam was produced in a steam generator, free of O2 and CO2. The total time of the heat treatment in the kiln was minutes. After recovery, the cellulose produced had a lignin content of about 1.2% and an alpha cellu lose content of about 94%. The apparent 0.5% CED viscosity was about 1%. After cooling and recovery, the lcellulose showed without any bleaching an Elrepho brightness of about 68%. Mechanical grinding of the material in a dry state for five to ten minutes, resulted in powders having a small particle size. Wet grinding in w-ater for about minutes at room temperature in a ratio of cellulose to water of 1:20 resulted in the formation of a stable gel.

The product obtained also exhibited good reactivity when converted, respectively, into ethyl cellulose, carboxy methyl cellulose and viscose. Triacetate produced from this pulp dissolved in acetone to given almost haze-free solutions of low viscosity. v

EXAMPLE 3 Coniferous chipper wood dust was treated in the same manner as in Example 1.

However, after heat treatment in the rotary kiln had reduced the water content of the material to about 28%, the treated material while still being in an atmosphere of superheated steam, was continuously fed into a dielectric heating apparatus and treated for a ten minute period at 200 C. After cooling and recovery, a cellulose of similar properties as in Example 2 was obtained. However, its apparent pentosan content was reduced to less than 1|%. Upon saccharication with hydrochloric acid, it was easily converted into a glucose solution of light colour.

EXAMPLE 4 Coniferous sawmill wood dust was impregnated with vwhite liquor from a commercial kraft mill. After removal of the excess liquor by pressing, the alkali charge vcalculated on wood was 25-26% expressed as Na20. The material was mechanically treated to break down 'agglomerations, was then subjected at atmospheric pres- 'sure to ash treatment in superheated steam of 210 C. and, after reaching this temperature, was deposited by means of a cyclone in a retention vessel where it was kept 'at this temperature for ten minutes in an atmosphere of superheated steam of atmospheric pressure. At the end of the reaction time, the material was withdrawn, cooled 8 EXAMPLE 5 Coniferous sawmill wood dust was impregnated with white liquor from a commercial kraft mill. After removal and its cellulose portions recovered. The total operation was continuous. In spite of the drastic conditions of treatment, the recovered cellulose was highly crystalline,

as evidenced by its X-ray diagram.

Intrinsic viscosity of this material was low, corresponding to an apparent 0.5% CED viscosity of about 11.2. Elrepho brightness was 69% Without any bleaching treatment. Wet grinding of the cellulose in a ratio of 1:25 for half an hour resulted in the formation of a stable gel.

of the excess liquor by pressing, the alkali charge calculated 4on wood was 25-26% expressed as NazO. The material was mechanically treated to break down agglomerations and was thenuidized in superheated steam of 220 C. and l30 p.s.i. pressure in a reactor column which was kept at this temperature by external heating. Total time of treatment was 40 seconds. After recovery, the cellulose had a lignin content of 3%. Its Elrepho brightness was 58%.

EXAMPLE 6 Surgical cotton was impregnated for 30 minutes at room temperature in 17.5% sodium hydroxide solution and the excess liquor was pressedl out. Part a of this material was subjected to azeotropic distillation at C. in

benzene for 2 hours, and another part b was heated in superheated steam at 120 C. for 20 minutes. After these treatments, the two parts were separately subjected to heating at about 185 C. in paraffin oil for three minutes. The reacted material was then immersed in water of C. for cooling and leaching out of the portions which had become soluble. After removal of these portions and of the residual alkali from these portions by exhaustive washing and subsequent slight acidifying followed by another washing with deionized water, the material was dried, and in the dry state subjected to an extraction with petroleum ether to remove traces of paraffin oil. The viscosity changes during the whole processing were followed and are listed in Table I.

Table l 0.5% CED Copper Cupraethyl- Intrinsic Treatment Viscosity, ene-diamine viscosity,

cps. Viscosity, n

cps.

Untreated 93. 4 30. 1 10. 0 Hot water washed 74. 0 26. 2 9. 5 Washed, impregnated with 17 .5% NaOH solution for 30 minutes and surplus squeezed out 69. 2 24. 9 9. 2 Rcacting:

(a) Azeotropic distillation at 80 C. in benzene for 2 hours 22. 1 9. 8 5. 8 (b) Heating in superheated steam at C. for 20 minutes 11. 7 5. 4 3. 6 (c) Heating of the naterials treated according to (a) or (lo) for 3 minutes at, about C. in paratlin oil- EXAMPLE 7 Non-purified second cut cotton linters in admixture with so-called hull fibers (commercial ygroups 5-7) containing appreciable amounts of cotton seed fragments were impregnated in sodium hydroxide until a 30% (weight) alkali charge was achieved. The thus impregnated cellulose material was treated in parain oil at,200 C. for 15 minutes. j

The product obtained after recovery had a brightness of about 60% G.E. and had a copper viscosity of about ,5 centipoises. Its -cellulose content was 88% and the yield calculated on initial material was about 54%.

EXAMPLE 8 Air dry coniferous sawdust was impregnated with a 10% sodium hydroxide solution at 100 C. in a slurry yhaving a ratio of 1:20 of sawdust to solution and the Vmixture was' stirred for 3 minutes. The impregnated sawdust was separated from the residual liquor by pressing to obtain a ratio of wood to liquor of about 1:2 and the `alkali charge was found to be about 24% expressed as Na20 and calculated on the O.D. wood residue. The wet sawdust, after mechanical treatment to break down agglomerations, was subjected at atmospheric pressure to superheated steam at 190 C. in a container which was externally heated to 190 C. The total treatment for water removal and reaction at 1,90c C, took 61/2 minutes.

The product thus obtained was dumped in hot water (about 90 C.) to leach out solubilized wood portions, Was subsequently acidiiied with N/ 100 HCl to neutralize traces of residual alkali and was iinally washed with distilled water.

The degraded cellulose obtained had the following properties:

Yield 40%. Roe chlorine number 2.0. Lignin content 1.5%. Alpha-cellulose 92.8%. Pentosan 1.2%. Ash 0.3%. Cuprammonium viscosity 3.2 cps. Elrepho brightness 68%.

Special methods were used to determine the lignin content in the products obtained in the above examples, namely, bromine consumption (T. N. Kleinert and I. M. Roberge, Tappi 42, No. 4, 281-288 (1959)), toxylchloroamide consumption (L. M. Marraccini, and T. N. Kleinert, Tappi 42, No. 6, 455-460 (1959)), and ultraviolet absorption of the pulp hydrolyzates (L. M. Marraccini, and T. N. Kleinert, Holzforschung 13, No. 2, 43-48 (1959)).

I claim:

1. Process for producing crystalline cellulosic pulp of low viscosity and high nr-cellulose content having a rela-` tively high purity which comprises:

(A) Impregnating subdivided fibrous cellulosic material with aqueous alkaline liquor until a charge of elective alkali of at least 10% by weight is obtained, calculated as Na and based on dry fibrous material, said alkaline liquor having an effective concentration, calculated as NazO, of at least 3.1% by Weight, corresponding to at least 4% NaOH by weight,

(B) Removing water by evaporation from said impregnated subdivided iibrous material in an atmosphere inert to alkali until substantially all aqueous liquid phase has disappeared and a Water vapour pressure essentially lower than that of saturation is obtained,

(C) Heating the product thus obtained in said inert atmosphere at a temperature of 80 C.220 C. at the aforesaid water vapour pressure in the absence of any substantial aqueous liquid phase to obtain crystalline cellulosic pulp of low viscosity, high a-cellulose content and relatively high purity and (D) Recovering said crystalline cellulosic pulp.

' 10 2. Process as defined in claim 1, wherein said charge of effective alkali is between 16 and 30% Na2O by weight.

3. Process as defined in claim 2, wherein said elfectivel alkali concentration is from about 3.1% to about 15.5% NagO by weight, corresponding to 4 and 20% NaOH by weight.

4. Process as defined in claim 3, wherein said heating is carried out at a temperature of 180 to 190 C.

5. Process as dened in claim 1, wherein said recovery is effected by separating a portion insoluble in aqueous alkali from a portion soluble in aqueous alkali, followed by neutralisation and washing of said insoluble portion.

6. Process as dened in claim 1, werein said heating is carried out in a fluidized system using superheated steam as the fluidizing medium, said alkali impregnated material moving downwardly during the heating and said stream of superheated steam flowing countercurrently upwardly to remove vapour formed by water evaporation.

7. Process for producing crystalline cellulosic pulp of low viscosity and high -cellulose content having a relatively high purity which comprises:

(A) Impregnating subdivided ibrous cellulosic material with an alkaline liquor containing an alkali hydroxide and an alkali sulphide, having an eiective alkali concentration of about 6 to 16% by weight, calculated as NaZO, said alkaline liquor being derived from White liquor obtained by causticizing a green liquor resulting from chemical recovery of a black kraft liquor and being adjusted to the desired concentration, until a charge of effective alkali of at least 10% by weight is obtained, calculated as Na20 and based on dry iibrous material,

(B) Removing water by evaporation from said impregnated subdivided fibrous material in an atmosphere inert to alkali until substantially all aqueous liquid phase has disappeared and a water vapour pressure essentially lower than that of saturation is obtained,

(C) Heating the product thus obtained in said inert atmosphere at a temperature of C.-220 C. at the aforementioned vapour pressure in the absence of any substantial aqueous liquid phase to lobtain crystalline cellulosic pulp of low viscosity and high a-cellulose content and (D) Recovering said crystalline cellulosic pulp.

FOREIGN PATENTS 9/ 1956 France. 2/ 1960 Great Britain.

DONALL H. SYLVESTER, Primary Examiner. S. L. BASHORE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Parent Ne 3,255,011 June 7, 1966 Theodor N kleinem It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column b, line l, a'ter "Z" insert cps same line ll for "viscosity" read vscosities, there 1S an approximate proportionality between intrinsic viscosity line ll, for "gratting" read 1 grafting column 7, line Z4, for "about 1%" read Y* about l cp line 36, for "given" read give w line ,72, after "l 2" insert Y cps Column 9, line Z7, for "toxyl" read Y tosyl Signed and sealed this 22nd day of August 19671 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attcsting Officer Commissioner of Patents

Claims (1)

1. PROCESS FOR PRODUCING CYSTALLINE CELLULOSIC PULP OF LOW VISOCITY AND HIGH A-CELLULOSE CONTENT HAVING A RELATIVELY HIGH PURITY WHICH COMPRISES: (A) IMPREGNATING SUBDIVIDED FIBROUS CELLULOSIC MATERIAL WITH AQUEOUS ALKALINE LIQUOR UNTIL A CHARGE OF EFFECTIVE ALKALI OF AT LEAST 10% BY WEIGHT IS OBTAINED, CALCULATED AS NA2O AND BASED ON DRY FIBROUS MATERIAL, SAID ALKALINE LIQUOR HAVING EFFECTIVE CONCENTRATION, CALCULATED AS NA2O, OF AT LEAST 3.1% BY WEIGHT, CORRESPONDING TO AT LEAST 4% NAOH BY WEIGHT. (B) REMOVING WATER BY EVAPORATION FROM SAID IMPREGNATED SUBDIVIDED FIBROUS MATEIAL IN AN ATMOSPHERE INERT TO ALKALI UNTIL SUBSTANTIALLY ALL AQUEOUS LIQUID PHASE HAS DISAPPEARED AND A WATER VAPOUR PRESSURE ESSENTIALLY LOWER THAN THAT OF SATURATION IS OBTAINED, (C) HEATING THE PRODUCT THUS OBTAINED IN SAID INERT ATMOSPHERE AT A TEMPERATURE OF 80*C.-220*C. AT THE AFORESAID WATER VAPOUR PRESSURE IN THE ABSENCE OF ANY SUBSTANTIAL AQUEOUS LIQUID PHASE TO OBTAIN CRYSTALLINE CELLULOSIC PULP OF LOW VISCOSITY, HIGH A-CELLULOSE CONTENT AND RELATIVELY HIGH PURITY AND (D) RECOVERING SAID CRYSTALLINE CELLULOSIC PULP.

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