CA2568594C - Method for producing a dissovling pulp - Google Patents

Abstract

The invention relates to a method for producing a dissolving pulp, comprising the steps of - obtaining the pulp by means of a pulping process known per se - at least one alkaline extraction step wherein the pulp is treated with lye and subsequently pressed out - optionally further steps of purifying and/or bleaching the pulp, wherein at least a portion of the lye, which is accumulating in the alkaline extraction step after pressing, is recycled for the treatment of the pulp. The method according to the invention is characterized in that the content of beta cellulose is reduced in at least a portion of the recycled lye prior to the new treatment of the pulp.

Description

Method for Producing a DissolvinPulp The present invention relates to a method for producing a dissolving pulp.
Today, the production of dissolving pulps is dominated by the acid bisulfite pulping process and the prehydrolysis Kraft process. In particular, the acid bisulfite pulping process constitutes a technology which is very friendly to the environment, due to the high yield and good possibilities of utilizing byproducts and waste products or using them, respectively, for energy production.
The elimination of undesirable short-chain carbohydrate fractions, which influence both the processing characteristics of the pulp and the quality of the final product, is essential for the production of dissolving pulps.
Thus, it is necessary to further purify the pulp obtained by a pulping process. The known purification steps comprise both the removal of non-cellulosic material such as, e.g., extractives, lignins and hemicelluloses, and a change in the molecular weight distribution toward a narrow, monomodal distribution with a minimum of low molecular weight carbohydrates.
It should be noted in particular with regard to the acid bisulfite pulping that said process leads to relatively broad molecular weight distributions and hence to problems associated with the removal of low-molecular weight carbohydrates (hemicelluloses).
Usually, alkaline extraction is employed for removing short-chain carbohydrates from wood pulp and producing highly reactive dissolving pulps. In doing so, the pulp is treated with lye and subsequently pressed out.
Basically, two methods are known in this regard, namely cold caustic extraction (CCE) and hot caustic extraction (HCE). Cold caustic extraction, which usually is performed at temperatures slightly above room temperature, essentially causes physical changes in the pulp, whereas hot caustic extraction at typical temperatures of 70 C to 120 C
induces a plurality of chemical reactions.
However, in practice, especially the application of cold caustic extraction involves major difficulties since very large amounts of alkali are required. With 10% by weight of pulp consistency and a concentration of 10% NaOH, it is necessary to use approx. 1 ton of NaOH
per ton of pulp.
In combination with a prehydrolysis Kraft pulping process, it is possible to reuse a portion of the lye in the pulping process, which lye is accumulating when the pulp is pressed out.
However, this is not possible with the acid bisulfite process.
If, however, the lye, which is accumulating when the pulp is pressed out, is reused in a recycling process for the alkaline extraction of the pulp, a significant deterioration of the purification effect is observed.
It is the object of the present invention to provide an improved method for producing dissolving pulps, wherein the above-described disadvantages can be avoided when performing a cold and/or hot caustic extraction step. In particular, it is an object of the invention to solve the problems associated with the application of alkaline extraction in combination with an acid bisulfite pulping process.
Said objects of the invention are achieved using a method for producing a dissolving pulp, comprising the steps of - obtaining the pulp by means of a pulping process known per se - at least one alkaline extraction step wherein the pulp is treated with lye and subsequently pressed out - optionally further steps of purifying and/or bleaching the pulp, wherein at least a portion of the lye, which is accumulating in the alkaline extraction step after pressing, is recycled for the treatment of the pulp, which method is characterized in that the content of beta cellulose is reduced in at least a portion of the recycled lye prior to the new treatment of the pulp.
By the term õbeta cellulose", a person skilled in the art understands the portion in a cellulosic material which (in contrast to alpha cellulose) is soluble in a 17.5% solution of NaOH at 20 C but which (in contrast to gamma cellulose) reprecipitates if the solution is acidified with 4,75M of H2SO4.
It has now been found that a recirculation of the lye, which is accumulating when the pulp is pressed out, for lye-treating the pulp and, hence, a noticeable reduction in the necessary amount of white liquor are achievable if the content of beta cellulose is reduced in the press liquor. Apparently, an increased content of beta cellulose in the recycled lye results in a deterioration of the result of the alkaline extraction. On the other hand, it has surprisingly been found that the content of gamma cellulose is of less importance in this regard.
Preferably, at least a portion of the recycled lye is treated by a membrane separation process in order to reduce the content of beta cellulose. The membrane separation process is preferably a nano- or ultrafiltration process.
The separation of hemicelluloses from liquids contaminated with threadlike molecules having molar masses of at least 10,000 Daltons using nanofiltration is known per se from WO 97/23279.
In the method according to the invention, membranes having a cut-off of < 2000 g/mol, in particular of < 1000 g/mol, are preferably used for effecting nanofiltration.
In this manner, 100% of the beta cellulose and more than 50% of the gamma cellulose, preferably more than 70%, can be removed. The ratio between retentate and permeate is set between 2 and 5, preferably between 4 and 5.
For effecting ultrafiltration in the method according to the invention, membranes having a cut-off of < 50000 g/mol, in particular of < 20000 g/mol, are preferably used.
In this manner, 100% of the beta cellulose and more than 10% of the gamma cellulose, preferably more than 30%, can be removed. The ratio between retentate and permeate is set between 2 and 5, preferably between 4 and 5.
The permeate from the membrane separation process can entirely or partially be supplied to the lye tank from which the lye used for treating the pulp is taken.
Another possibility of reducing the content of beta cellulose consists in thermally treating at least a portion of the recycled lye. In doing so, the portion of the beta cellulose which has no branched structures (such as, e.g., xylan) is degraded into gamma cellulose.
The thermal treatment is preferably carried out at a temperature of more than 50 C, preferably more than 70 C, for a duration of 10 mm to 300 min, preferably 30 mm.
The press liquor is preferably treated by a membrane separation process as well as thermally.

Prior to the measures aimed at reducing the content of beta cellulose, the press liquor can be prepurified using a filter in order to remove undissolved particles.
A preferred embodiment of the method according to the invention is characterized in that the extraction step is a cold caustic extraction, wherein the treatment of the pulp with lye is performed at a temperature of less than 50 C, preferably less than 25 C.
In said embodiment, the treatment of the pulp with lye is preferably carried out at a stock consistency of more than 5% by weight, preferably 10% by weight, of pulp (based on the mass of the total suspension) and at a concentration of the lye of more than 5% by weight, preferably 9% by weight (based on the mass of the solution).
A particularly interesting embodiment of a cold caustic extraction consists in that, during the treatment of the pulp with the lye, the stock consistency is brought to more than 10% by weight, preferably more than 30% by weight, of pulp, a lye content of less than 7% by weight, based on the solution, is adjusted and the solution is cooled down to temperatures of less than -10 C, preferably -15 C to -20 C. In the following, said method is referred to as õfreeze purification".
Said method is based on the phenomenon that water will crystallize out of the alkali lye during freezing, resulting in a higher concentration of the residual lye. The lye concentration can thereby be significantly reduced without impairing the purification effect.
A further preferred embodiment of the method according to the invention is characterized in that the extraction step is a hot caustic extraction, wherein the treatment of the pulp with lye is performed at a temperature of more than 80 C, preferably 110 C.
In this embodiment, the treatment of the pulp with lye is preferably carried out at a stock consistency of more than 5% by weight, preferably 10% by weight, of pulp (based on the mass of the total suspension) and at a concentration of the lye of more than 3% by weight, preferably more than 5% by weight (based on the solution).
A particularly preferred embodiment of the method according to the invention is characterized in that the pulp is subjected both to cold caustic extraction and to hot caustic extraction. This is particularly advantageous especially for the recovery of pulps from an acid bisulfite pulping process.

In doing so, it is possible to perform the cold caustic extraction prior to the hot caustic extraction and vice versa.
Prior to, between and/or after the extraction step(s), further treatment steps such as, e.g., washing steps or bleach treatments can be performed.
For the implementation of cold caustic extraction as well as of hot caustic extraction during the method according to the invention, it is advantageous if the content of beta cellulose in the lye used for treating the pulp amounts to less than 20 g/l, preferably less than 5 g/l, of liquor (i.e., lye liquid plus water contained in the pulp).
Furthermore, the content of gamma cellulose in the lye used for treating the pulp suitably amounts to less than 40 g/l, preferably less than 20 g/l, of liquor.
These contents can be adjusted by appropriately mixing fresh lye and recyled lye that has been purified according to the invention or has not been purified, respectively.
The method according to the invention can be carried out advantageously in particular if an acid bisulfite process is employed as the pulping process.
Due to the saving in the required amount of white liquor, which is made possible with the aid of the method according to the invention, a pulp obtained by acid bisulfite pulping can now be purified economically using cold and/or hot caustic extraction.
The present invention also relates to the use of a dissolving pulp produced in accordance with the method according to the invention as a starting material for the production of Lyocell moulded bodies, cellulose acetate moulded bodies and cellulose ethers.
As õLyocell moulded bodies", cellulosic moulded bodies are to be understood which are produced according to the so-called amine oxide process, i.e., by dissolving pulp in an aqueous solution of a tertiary amine oxide, moulding the solution and precipitating from the moulded solution.
In the publications WO 97/23666, WO 98/58102 and WO 98/58103, pulps are described which are suitable for the amine oxide process.

= 6 If the pulp produced according to the invention is used for the production of Lyocell moulded bodies, it should preferably have a viscosity of 350-550 ml/g, preferably 350-450 ml/g, a pentosan content of less than 2% by weight, preferably less than 1% by weight, and an R18-content of more than 94% by weight, preferably more than 95% by weight.
By õR18-content ", a person skilled in the art understands the residue that can be filtered off upon treatment with an 18% lye (according to DIN 54355).
Surprisingly, it has been shown that particularly pulps which have been obtained by acid bisulfite pulping and have been purified using, in each case, at least one cold and one hot caustic extraction yield excellent values in terms of the strength properties of Lyocell fibres produced therefrom.
Lyocell moulded bodies produced in this manner are also characterized in particular by a far lower proportion of hemicelluloses. The content of pentosan in the Lyocell moulded bodies according to the invention preferably amounts to 1.5% by weight and less, particularly preferably to 1% by weight and less.
The strength properties of Lyocell fibres produced in this manner are in the same range as those of fibres from a prehydrolysis Kraft pulp. So far, this has not been possible when using a pulp originating from an acid bisulfite process.
If the pulp produced according to the invention is used for the production of cellulose acetate moulded bodies, it should preferably have a viscosity of more than 450 ml/g, preferably 500-600 ml/g, a pentosan content of less than 2% by weight, preferably less than 1% by weight, and an R18-content of more than 95% by weight, preferably more than 96% by weight.
If the pulp produced according to the invention is used for the production of cellulose ethers, in particular for the production of methyl cellulose, hydroxypropylmethyl cellulose and hydroxyethyl cellulose, it should preferably have a viscosity of 230 - 300 ml/g, preferably 250 ml/g, a pentosan content of less than 2% by weight, preferably less than 1% by weight, and an R18-content of more than 94% by weight, preferably more than 95% by weight.
It has been shown that particularly pulps from an acid bisulfite pulping, which have been purified both by cold caustic extraction and by hot caustic extraction without a washing step between the two extraction steps (in this case, the residual alkali from the cold caustic extraction serves as an alkali source in the hot caustic extraction), are perfectly suitable for the production of high-purity, low-molecular weight cellulose ethers.
Below, the invention is illustrated further by way of the figures and the exemplary embodiments.
Fig. 1 schematically shows a preferred embodiment of the method according to the invention based on a cold caustic extraction.
Fig. 2 shows the impact which the content of beta cellulose and gamma cellulose in the lye used for alkaline extraction has on the residual pentosan content of a purified pulp.
According to the embodiment shown in Fig. 1, an unbleached, washed pulp 1 obtained from hardwood by acid bisulfite pulping is squeezed in a press 2 to a consistency of more than 30% by weight, preferably 35% by weight. The viscosity of the pulp amounts to, for example, 220 ¨ 550 ml/g, if said pulp is intended to be used for the production of Lyocell moulded bodies or for the production of low-molecular weight, high-purity cellulose ethers, or to 500 - 800 ml/g, if it is intended to be used for the production of cellulose acetate.
The pressed-out liquid 3, which contains organic and inorganic substances from the spent sulfite liquor, is returned to the recycling circuit.
The press cake 4 enters the cold alkali purification tank 5 (CCE tank) after having been diluted with fortified press liquor to a consistency of approx. 5 to 15% by weight, preferably 10% by weight. The aqueous alkaline pulp suspension is thoroughly mixed in the CCE tank with retention times of 10 to 120 min, preferably 50 min, and at temperatures of 10 to 50 C, preferably 25 C.
Thereupon, the pulp suspension 6 thus treated is supplied to the second press 7 in which the alkali-soluble short-chain carbohydrates are separated from the solid phase.
After pressing, the pulp consistency should again amount to between 30 and 40% by weight, preferably to 35% by weight. The press cake 8 is scraped off, diluted to a lower consistency and supplied to further processing steps. Such further processing steps comprise, for example, hot caustic extraction (with or without a washing step between the two alkaline extraction steps), oxygen delignification and/or chlorine dioxide treatment as well as optionally bleach treatments.

A portion of the press liquor 9 is passed through a filter 10 in order to remove fibres and undissolved particles and is subsequently treated in a membrane separation process using, for example, an ultra- or nanofiltration device 11.
Depending on the desired content of residual beta cellulose in the press liquor, a portion of the press liquor 23 can be returned directly to the CCE tank 5.
The pressure-driven membrane separation system removes the dissolved polymeric and oligomeric carbohydrate degradation products. The removal efficiency depends on the cut-off of the membrane system that is used. In case of nanofiltration with cut-offs of <1000 g/mol, significant fractions of gamma cellulose are retained.
Before the permeate 12 (having a content of beta cellulose of 0 g/1 and a content of gamma cellulose of < 30 g/1) is recycled to the dewatered pulp in the CCE tank 5 via conduit 17, it is fortified with make-up lye 15 in a separate tank 14 (or alternatively in-line).
This closed loop enables the recovery of a substantial portion of the alkali required for the cold caustic extraction. The ratio between retentate and permeate is preferably kept between 2 and 5, in particular between 4 and 5. In the latter case, a NaOH recovery of 80-83% can be achieved in the permeate.
The retentate 13 having a content of hemicellulose of more than 100 g/1 is pumped to a mixing tank 16 in which reagents 17, e.g. surface-active substances and/or polyelectrolytes, are added in order to precipitate the high-molecular weight fraction (beta cellulose). The phase separation can be completed in a sedimentation tank 18 or by other appropriate measures (not illustrated here, e.g. microfiltration etc.). The precipitate 19, which is characterized as a high-molecular weight hemicellulose fraction (beta cellulose), can be purified further to yield different degrees of purity. The supernatant 20, which is free from beta cellulose, can either be recycled to the lye tank for alkaline extraction or can be used as a source of gamma cellulose 21.
Additionally or alternatively to the separation of beta cellulose using the membrane separation process, the proportion of beta cellulose can be reduced by a thermal treatment 22 of the press liquor. With a thermal treatment for 30 minutes at 90 C, a 50%
conversion of beta cellulose into gamma cellulose was observed.
Examples:

Example 1:
A pulp was subjected to cold caustic extraction, wherein white liquor, a lye containing 20 g/1 of gamma cellulose and a lye containing 14 g/1 of beta cellulose and 24 g/1 of gamma cellulose were used, in each case, at different concentrations. The pentosan content of the treated pulp was determined. Fig. 2 shows the impact which the respective amounts of dissolved beta and gamma cellulose have on the residual content of pentosan in the pulp.
In Fig. 2, the curves thereby mean:
__ o __________ white liquor ................ lye with 14 g/1 of beta cellulose and 24 g/1 of gamma cellulose __ = _________ lye with 20 g/1 of gamma cellulose From Fig. 2, it is evident that an increased content of gamma cellulose alone has virtually no impact on the lye's purification effect. However, far higher pentosan residues can be observed in the lye with 14 g/1 of beta cellulose.
Example 2:
Influence of beta and gamma cellulose dissolved in NaOH on the efficiency of an HCE
process:
It has been found that, such as in case of a cold caustic extraction, the use of caustic soda contaminated with beta cellulose has a negative impact on the purification effect of a hot caustic extraction. In contrast, the presence of gamma cellulose induces virtually no change in the effectivity of the purification. This is apparent from the following table, in which the effect of an HCE treatment of pulp using a lye containing a) neither beta nor gamma cellulose b) 20 g/1 of gamma cellulose or c) 20 g,/1 of beta cellulose, respectively, is indicated.

Table unbleached (E/O)*-treated Parameters unit (comparison) a) b) c) (E/0)-conditions temperature C 85 85 85 NaOH kg/odt 60/15 60/15 60/15 time min 180/90 180/90 180/90 beta cellulose W1 0 0 20 gamma cellulose g/1 0 20 0 Properties Kappa 5.4 1.4 1.4 1.6 whiteness %ISO 60.9 79.5 79.3 77.1 intrinsic viscosity mug 590 640 635 638 R18 91.0 89.5 89.3 88.1 R10 85.7 95.6 95.4 94.2 pentosan 5.2 2.1 2.1 2.8 *E/O = hot caustic extraction combined with oxygen delignification (0) Example 3 Influence of a membrane separation process and a thermal treatment on the equilibrium concentration of hemicelluloses in the lye system:
An unbleached beech pulp obtained by acid bisulfite pulping (UBABD-pulp) was dewatered to a consistency of 35% by weight according to the process described with regard to Fig. 1 and was subjected to cold caustic extraction (100 g/1 of NaOH, 25 C, retention time 30 min).
The chemical properties of the starting pulp are described in the following table:
Put s unit value Kappa 5.7 intrinsic viscosity ml/g 519 alpha cellulose % 92.6 beta cellulose 6.1 gamma cellulose % 1.3 .

After leaving the CCE reactor, the alkaline pulp suspension was pressed to a consistency of 32% by weight.
The recirculation of the press liquor to the CCE reactor and the required amounts of white liquor to be added were simulated by means of a computer simulation (process simulation software SimeX) based on the following scenarios:
Example target content of beta treatment of the press thermal treatment of cellulose in the liquor by nano- the press liquor *
treatment liquid within filtration I the CCE reactor 3a < 1 g/1 no =no 3b < 1 g/1 yes no 3c <1 g/1 yes yes 3d < 5 g/1 no no 3e < 5 g/1 yes no -3f < 5 g/1 yes yes *calculated assuming a 50% conversion of beta cellulose into gamma cellulose.
The results of the computer simulation are indicated in the following tables:
Example Parameters comp.* 3a 3b 3c Nano-feed kg/odt 0 0 5910 NaOH make-up IN kg/odt 241 729 Recycled pressate (as NaOH) IN kg/odt 567 88 88 Permeate lye (as NaOH) IN kg/odt 391 Press cake (as NaOH) OUT kg/odt 241 244 Waste lye (as NaOH) OUT kg/odt 485 Retentate lye (as NaOH) OUT kg/odt 94 77 Beta before CCE g/1 25.5 1.0 1.0 1.0 Beta after CCE g/1 32.8 8.4 8.4 8.4 Gamma before CCE g/1 5.8 0.2 1.0 4.8 Gamma after CCE g/1 7.3 1.8 2.5 6.3 Beta yield in the retentate kg/odt 45.2 18.5 *comp. = simulation of an experiment without any measures for purifying the press liquor Example Parameters 3d 3e 3f Nano-feed kg/odt 0 3360 54 NaOH make-up IN kg/odt 518 295 242 Recycled pressate (as NaOH) IN kg/odt 297 296 562 Permeate lye (as NaOH) IN kg/odt 0 223 4 Press cake (as NaOH) OUT kg/odt 243 243 241 Retentate lye (as NaOH) OUT kg/odt 52 1 Beta before CCE gIl 5.0 5.0 5.0 Beta after CCE g/1 12.4 12.4 12.4 Gamma before CCE g/1 1.1 1.9 25.3 Gamma after CCE g/I 2.6 3.5 26.7 Beta yield in the retentate kg/odt 0.0 37.8 0.3 As can be seen, the required amount of white liquor (NaOH make-up) can be significantly reduced by using nanofiltration for reducing the content of beta cellulose.
With a combined application of nanofiltration and thermal treatment, the required amount of white liquor to be added may even be reduced by 409 kg Na0H/t oven-dry pulp in case of a target concentration of < 1 g/1 of beta cellulose (Example 3c as compared to Example 3a) or by 276 kg Na0H/t oven-dry pulp, respectively, in case of a target concentration of < 5 g/1 of beta cellulose (Example 3f as compared to Example 3d).
Example 4 Combination of cold caustic extraction with õfreeze purification" and a thermal treatment of the press liquor:
In the course of subjecting a pulp with a consistency of 10% by weight and a NaOH
concentration of 70 g/1 to a CCE treatment, the suspension was dewatered to a press cake with a consistency of 30-40% by weight, cooled down to -15 C for 30 to 60 minutes, subsequently thawed and washed. The result of this purification is equivalent to a CCE
treatment with 100 g/1 of NaOH.
The following table demonstrates the effect of said lower lye concentration on the lye balance, in particular on the required amount of white liquor to be added, based on a target content of less than 5 g/1 of beta cellulose within the treatment lye, wherein the press liquor is simultaneously subjected to a thermal treatment:

Parameters Nano-feed kg/odt 0 NaOH make-up IN kg/odt 171 Recycled pressate (as NaOH) IN kg/odt 409 Permeate lye (as NaOH) IN kgi_odt 0 Press cake (as NaOH) OUT kg/odt 171 Waste lye (as NaOH) OUT kg/odt 0 Retentate lye (as NaOH) OUT kg/odt 0 Beta cellulose before CCE g/1 5.0 Beta cellulose after CCE g/1 12.3 Gamma cellulose before CCE g/1 25.0 Gamma cellulose after CCE g/1 26.4 Beta cellulose yield in the retentate kg/odt 0.0 This table shows (as compared to Example 30 that, by means of õfreeze purification", the amount of required white liquor can be reduced further from 242 to 171 kg/t oven-dry pulp, without, in this case, a nanofiltration treatment of the press liquor being necessary.
Example 5:
Combined CCE and HCE treatment without interstage washing:
The subjection of UBABD pulp to CCE and HCE treatment without interstage washing according to the sequence CCE-HCE/0-Z-P yields a high-purity pulp of low viscosity which has a very narrow molecular weight distribution.
The carry-over of lye into the HCE stage was measured and calculated to be 240 kg/t oven-dry pulp, provided that the pulp was dewatered to a consistency of 32%.
According to the specification of the pulp thus obtained as set forth in the following table, said pulp is perfectly suitable for the production of valuable cellulose ethers, such as, e.g., HPMC:
Whiteness viscosity R18 R10 AR pentosan Cu GPC
% ISO ml/g % % % % % Mn Mw DP50 DP200 DP2000 93.0 281 96.3 90.6 5.7 1.0 0.7 48.7 126.4 1.6 16.0 6.9 Such low-viscosity cellulose ethers are suitable for use as coatings and protective colloids in emulsion polymerization reactions which require pulps or cotton linters with an intrinsic viscosity of approximately 250 ml/g.
Presently, such low-viscosity, highly reactive pulps and cotton linters are hardly available since it is difficult and expensive to achieve a controlled degradation to such a low degree of viscosity and, on the other hand, very high reactivity has to be ensured in order to obtain the required clear aqueous solution.
Example 6:
A UBABD pulp was subjected to the following treatment and bleaching sequences:
CCE-W-HCE/0-Z-P and HCE/O-W-CCE-Z-P, respectively. Thereby, õCCE" means cold caustic extraction õW" means interstage washing õHCE" means hot caustic extraction õO" means oxygen delignification õZ" denotes an ozone bleaching stage, and õP" denotes a peroxide bleaching stage.
By way of comparison, the same UBABD pulp was treated using an (E/0)-Z-P-sequence.
Therein, õE" denotes a hot caustic extraction which is comparable to the above-mentioned HCE but is performed under milder conditions.
The (E/0)-Z-P-sequence was carried out both under standard conditions (i.e.
mild conditions) and under fortified conditions.
In addition, a commercially available sample of an eucalyptus prehydrolysis Kraft pulp (õPHK") was used.
The respective conditions used for the treatment and for the bleaching, respectively, are summarized in the following table:

Pulps viscosity HCE CCE stage Z
stage P stage OXE
Category bleaching sequence NaOH c-NaOH.Betapamma H2SO4 03 NaOH

m1/1 bleaching yield (%) kg/ . el g/l k:/ kg/ i k:/t kg/t kl/t Beech sulfite E0-Z-P (standard) 610 91.7 24 6 4.2 4.0 0.7 564 Beech sulfite E0-Z-P (HCE-reinforced) 610 82.0 9(, 6 3.6 4.0 1.2 521 Eucalyptus-PHK*)00-A-Z-P (standard) 756 97.0 (, ',1P4 15 2.6 ---11.0 972 Beech sulfite CCE-W-HCE/O-Z-P 519 89.1 30 100 0 0 6 2.0 6.0 3.0 420 Beech sulfite HCE/O-W-CCE-Z-P 519 91.6 30 100 0 0 6 3.2 6.0 3.0 573 *)H2SO4 dosage included in the A-stage In the following table, the most important properties of the pulps produced in this manner are summarized:
Category bleachin: se. uence %150 m1/1 % % % % % Mn Mw Beech sulfite E0-Z-P (standard) 94.4 419 93.3 85.7 7.6 2.9 1.9 42.8 5.2 Beech sulfite E0-Z-P (HCE-reinforced) 95.0 408 95.3 87.7 7.6 2.1 1.2 37.4 3.6 Eucalyptus-PHK*) 00-A-Z-P (standard) 90.9 405 97.4 93.6 3.8 2.6 0.3 51.0 130.4 1.7 14.3 7.3 Beech sulfite CCE-W-HCE/O-Z-P 93.7 397 95.0 91.5 3.5 0.8 1.2 48.2 217.1 3.0 15.3 19.0 Beech sulfite HCE/O-W-CCE-Z-P 91.9 371 94.6 89.9 4.7 0.9 1.3 42.1 195.1 38 17.1 17.6 The above-indicated tables show that the CCE treatment is more selective (i.e., leads to higher yields than a reinforced HCE treatment) and also more efficient in terms of the purification effect than a reinforced HCE treatment (as detectable from the low pentosan content and the comparatively high R10-content at a given viscosity).
In a comparative experiment, the pulp was likewise processed according to the sequence CCE-W-HCE/O-Z-P, however, the lye used for the CCE treatment was contaminated (a) with 20 g/1 of gamma cellulose and (b) with 20 g/1 of beta cellulose, respectively.
In case (a), no significant differences, in terms of the properties, from the pulp which was treated with the same sequence but without contamination of the CCE lye were detected. In case (b), however, a deterioration of the properties became apparent.
The pulps were processed in a manner known per se into Lyocell fibres having a titre of 1.3 dtex.

The following table summarizes the breaking strengths of the conditioned fibres produced from the pulps:
Pulp min. titre titre FFc FDc Category bleaching se s uences dtex dtex cN/tex %
Beech sulfite EO-Z-P (standard) 0.80 1.26 34.2 10.0 Beech sulfite EO-Z-P (HCE-refined) 0.76 1.24 35.111.0 Eucalyptus-PHK 00-A-Z-P (standard) 0.53 1.29 41.0 11.9 Beech sulfite CCE-W-HCE/O-Z-P 0.51 1.10 42.7 11.5 Beech sulfite HCE/O-W-CCE-Z-P 0.44 1.16 41.411.1 Min. titre .. minimum titre capable of being spun FFc ......... Fibre strength in the conditioned state FDc ......... Fibre elongation in the conditioned state Surprisingly, Lyocell fibres produced from a UBABD pulp treated both by CCE
and by HCE
exhibit breaking strengths which are comparable to those of the eucalyptus-PHK
pulp. These breaking strengths are significantly higher than those which have been achieved with fibres from UBABD pulps produced without CCE treatment. The combination of CCE and HCE
treatments will thus result in UBABD pulps which can excellently be used for the production of Lyocell fibres which have breaking strengths which, so far, have only been achievable by using prehydrolysis Kraft pulps.
Fibres from a pulp which, according to the above-indicated case (a), was treated with a CCE
lye contaminated with gamma cellulose exhibited no significant deviation with regard to the breaking strength. However, a Lyocell fibre produced from a pulp which has been treated with a CCE lye contaminated with beta cellulose (case (b) above) exhibits a lower breaking strength.

Claims (30)

1) A method for producing a dissolving pulp, comprising the steps of - obtaining a pulp by means of a pulping process - at least one alkaline extraction step selected from the goup consisting of cold caustic extraction and hot caustic extraction, wherein the pulp is treated with lye and subsequently pressed out - optionally further steps of purifying and/or bleaching the pulp, wherein at least a portion of the lye, which is accumulating in the alkaline extraction step after pressing, is recycled for the treatment of the pulp with the lye, wherein the content of beta cellulose is reduced in at least a portion of the recycled lye prior to a new treatment of the pulp with the lye.

2) A method according to claim 1, wherein the at least a portion of the recycled lye is treated by a membrane separation process in order to reduce the content of the beta cellulose.

3) A method according to claim 2, wherein the membrane separation process is a nano-or ultrafiltration process.

4) A method according to any one of claims 1 to 3, wherein the at least a portion of the recycled lye is treated thermally in order to reduce the content of the beta cellulose.

5) A method according to claim 4, wherein the thermal treatment is carried out at a temperature of more than 50°C, for a duration of 10 min to 300 min.

6) A method according to claim 5, wherein the thermal treatment is carried out at a temperature of more than 70°C for a duration of 30 min.

7) A method according to any one of claims 1 to 3, wherein the extraction step is a cold caustic extraction, wherein the treatment of the pulp with the lye is performed at a temperature of less than 50°C.

8) A method according to claim 7, wherin the treatment of the pulp with the lye is performed at a temperature of less than 25°C.

9) A method according to claim 7 or 8, wherein the treatment of the pulp with the lye is carried out at a stock consistency of more than 5% by weight of pulp, based on the mass of the total suspension, and at a concentration of the lye of more than 5% by weight, based on the mass of the solution.

10) A method according to claim 9, wherein the treatment of the pulp with lye is carried out at a stock consistency of more than 10% by weight at a concentration of the lye of more than 9% by weight.

11) A method according to any one of claims 7 to 10, wherein, during the treatment of the pulp with the lye, the stock consistency is brought to more than 10% by weight of pulp, a lye content of less than 7% by weight, based on the solution, is adjusted and the total fluid is cooled down to a temperature of less than -10°C.

12) A method according to claim 11, wherein the stock consistency is brought to more than 30% by weight of pulp and the total fluid is cooled down to a temperature of -15°C to -20°C.

13) A method according to any one of claims 1 to 6, wherein the extraction step is a hot caustic extraction, wherein the treatment of the pulp with the lye is performed at a temperature of more than 80°C.

14) A method according to claim 13, wherein the treatment of the pulp with the lye is performed at a temperature of more than 110°C.

15) A method according to claim 13 or 14, wherein the treatment of the pulp with the lye is carried out at a stock consistency of more than 5% by weight of pulp, based on the mass of the total suspension, and at a concentration of the lye of more than 3% by weight based on the solution.

16) A method according to claim 15, wherein the temperature of the pulp with the lye is carried out at a stock consistency of more than 10% by weight and at a concentration of the lye of more than 5% by weight.

17) A method according to any one of claims 1 to 16, wherein the pulp is subjected both to cold caustic extraction and to hot caustic extraction.

18) A method according to any one of claims 1 to 17, wherein the content of the beta cellulose in the lye used for treating the pulp amounts to less than 20g/l, of the lye liquid plus water contained in the pulp.

19) A method according to claim 18, wherein the content of the beta cellulose in the lye amounts to less than 5 g/l.

20) A method according to any one of claims 1 to 19, wherein the content of gamma cellulose in the lye used for treating the pulp amounts to less than 40 g/l of the lye liquid plus water contained in the pulp.

21) A method according to claim 20, wherein the content of gamma cellulose amounts to less than 20 g/l.

22) A method according to any one of claims 1 to 21, wherein an acid bisulfite process is employed as the pulping process.

23) The use of a dissolving pulp produced according to any one of claims 1 to 22 as a starting material for the production of Lyocell moulded bodies, cellulose acetate moulded bodies and cellulose ethers.

24) The use according to claim 23 for the production of Lyocell moulded bodies, wherein the pulp has a viscosity of 350-550 ml/g, a pentosan content of less than 2%
by weight, and an R18-content of more than 94% by weight.

25) The use according to claim 24, wherein the pulp has a viscosity of 350-450 ml/g, a pentosan content of less than 2% by weight and an R18-content of more than 95%
by weight.

26) The use according to claim 23 for the production of acetate moulded bodies, wherein the pulp has a viscosity of more than 450 ml/g, a pentosan content of less than 2% by weight, and an R18-content of more than 95% by weight.

27) The use according to claim 26, wherein the pulp has a viscosity of 500-600 ml/g, a pentosan content of less than 1% by weight and an R18-content of more than 96%
by weight.

28) The use according to claim 22 for the production of cellulose ethers, wherein the pulp has a viscosity of 230-300 ml/g, a pentosan content of less than 2% by weight, and an R18-content of more than 94% by weight.

29) The use according to claim 28, wherein the cellulose ethers are selected from methyl cellulose, hydroxypropylmethyl cellulose and hydroxyethyl cellulose.

30) The use according to claim 28 or 29, wherein the pulp has a viscosity of 250 ml/g, a pentosan content of less than 1% by weight and an R18-content of more than 95%
by weight.