SE1551638A1 - Method for controlling viscosity in dissolving pulps - Google Patents

Abstract

14 ABSTRACT The present invention relates to an improved process for producing dissolving pulpfrom a sulfite cooking process using a TCF bleaching sequence containing a|ka|iextraction and a|ka|ine peroxide bleaching in the pulp bleaching. The new processadds an acidic peroxide stage (Pacid) which makes it possible to control pulpviscosity in an efficient way and do not introduce new bleaching chemicals into thefiitrate handling system. (Fig/I) SUN15018 SE txt.d0c

Description

Method for controlling viscosity in dissolving pulps The present disclosure relates in general to a method for producing dissolving pulpof low viscosity from Iignocellulosic material. The dissolving pulp process forproducing low viscosity pulp conventionally contains a sulfite cooking stage andthereafter bleaching for reduction and brightness increase. ln commercial systemsis often only used a first sulfite cooking stage and a subsequent alkaline extraction stage followed by peroxide bleaching.

BACKGROUNDln the wood based pulping industry significant efforts have been made designingdifferent processes for different grades of pulp. ln conventional kraft pulping for fully bleached grades of paper theobjectives has been to produce a pulp grade with high yield and high pulp strengthhaving acceptable brightness level and stability. For fully bleached pulp gradesISO brightness above 88% is most often preferred, and if minimal brightnessreversion is requested the residual content of lignin and hexenuronic acid must bekept low. ln general sulfite pulping has a higher cellulose yield for softwood pulpcompared with kraft pulping. However, with high alpha cellulose content the kraftpulping could have a higher yield. The lignin content in cooked sulfite pulp is lowwhich makes the subsequent treatment downwards easier, i.e. easier to bleach.Since the sulfite pulp is easier to bleach TCF-sequences are often used, mostlywith hydrogen peroxide. Oxygen stages are not frequently used for sulfite pulpingdue to several reasons, one of them is the environmental aspect.

Hence, very competitive low cost systems for dissolving pulp may beobtained from a sulfite cook as the cooked pulp has a relative low metal contentdue to the acidic cooking conditions, and the subsequent bleaching may beobtained using only an alkaline extraction stage followed by peroxide bleaching,i.e. using a Cook-(w)-E-(w)-P sequence, where (w) stands for a washing stage.ln recent years the pulping industry has strived to find alternative products for thewood material as the printing paper market declines in volume, and that new products may be found with better profit margins. 2 Dissolving pulp has emerged as a reborn alternative growing market for pulp millsand much attention has been given to modify the pulping processes to be able toproduce different grades ofdisso|ving pulp which is used to produce a multitude ofproducts like Rayon-grade pulp or specialty pulps. The interest to find alternativetextile materials to cotton has increased due to short term shortage and increasein costs for cotton and an increased competition in long term for land to grow anincreasing demand of cotton on. Dissolving pulp can consist of cotton linters, pulporiginating from wood or annual plants made by the sulfite process or theprehydrolysis kraft process. However, in pulp industry dissolving pulp is generallyreferred to as a bleached pulp produced from wood that has a high alpha cellulosecontent, typically over 92%, and only small content of hemicelluloses, typicallybelow 10%. Hence, the wood yield ofdisso|ving pulp from the process is typicallylow, at about 35-40%. Dissolving pulp is used to manufacture various cellulose-derived products such as rayon yarn for use in e.g. textile industry and specialtychemicals and materials such as cellulose acetate and carboxy methyl cellulose.When making rayon yarns the dissolving pulp is converted to cellulose xanthatewhich dissolves in caustic soda and the resulting viscous liquid is extruded inacidic baths to yield fibers. As an alternative process the dissolving pulp can bedissolved in ionic solvents to make extruding to fibers possible. For both theseprocesses and the specific final product it is essential that the viscosity of thedissolving pulp is both low and within a specific narrow range suitable for theprocess, in order to run derivatisation/dissolution process smoothly.

For some dissolving pulp grades the required intrinsic viscosity in the finalbleached dissolving pulp must be as low as 350 ml/g and within a narrowacceptance range of only 120 ml/g. This requires a low viscosity already aftercooking or possibilities to lower the viscosity in a controlled manner and to a greater extent in subsequent delignification and/or bleaching stages.

Throughout this description, viscosity is used as the dominant pulp property fordissolving pulp. The viscosity number is expressed as intrinsic viscosity andmeasured in ml/g. A standard test method for intrinsic viscosity of cellulose couldbe found in ISO-standard ISO 5351.

SUN15018 SE txt.d0c 3 Now, viscosity measurements is indicative for the average molecular weight of thecellulose polymers, i.e. the length of the cellulose chains. The length of thecellulose chains impact the derivatisation and solubilisation process as well as theCharacteristics of the final product.

However, for production of dissolving pulp the viscosity is often about orbelow 1100 ml/g after cook and below 600 ml/g after final bleaching. Most oftenthe viscosity target for the final dissolving pulp is kept within 400-600 ml/g andwithin a narrow acceptance range of only 120 ml/g for the specific grade.Examples of specification of higher grade dissolving pulps are Ethers (viscosity470-600 ml/g), Nitrates (viscosity 550-650 ml/g), Acetates (viscosity 600-700 ml/g)and Viscose (viscosity 300-500 ml/g).

The problem with production of dissolving pulp is to reach the low viscosity of thefinal pulp requested and most often is extended and intensified cooking, i.e. bothlonger cooking time and tougher cooking conditions as of alkali charge or othercooking chemicals and temperature, needed in order to obtain a low enoughviscosity after the cook.

A problem with extended cooking occurs if it is desired to increase the productionin existing pulp mills as increased production results in decreased cooking time ifthe equipment is the same. This results in higher viscosity when the throughput of the lignocellulosic material increases. lf the viscosity reduction does not reach a sufficient low level directly after thecook, as is the case if for example the production is increased, it has with somestandard bleaching sequences been found to be almost impossible to reach thefinal viscosity level by implementing tougher process conditions in final bleaching.This applies especially to TCF sequences using only peroxide and alkali during thebleaching.

Normally the viscosity reduction may be increased marginally in the order of 50ml/g in final bleaching, by implementing tougher bleaching conditions in the finalbleaching stages as of temperature and additional bleaching agents or increasedcharge of the standard bleaching agents used. Ozone stages are also capable ofreducing viscosity to a larger extent, but this at expense of increased costs, SUN15018 SE txt.d0c 4 especially in an existing bleaching plant, as the ozone stages are at the higher range of investment costs for a bleaching stage.

The presence of transition metal ions (Fe, Mn, Ca, Co etc.) impairs the filterabilityand spinnability of a cellulose spinning dope, therefore a low content of thesecompounds are preferable. The amounts of catalytically active transition metalions as Mn and Fe are also important to keep at a low level when hydrogenperoxide is used to avoid uncontrolled peroxide decomposition.

SUMMARY The main objective problem with the present invention is to enable increasedproduction of dissolving pulp in existing sulfite pulp mills using TCF bleachingusing mainly alkali and peroxide in bleaching while still being able to reach thelower viscosity requested in finally bleached dissolving pulp, and being able tocontrol this low viscosity within a narrow range suitable for the final dissolving pulpgrade.

The inventive method is thus optimized for producing low viscosity dissolving pulpfrom lignocellulosic material, said dissolving pulp process comprising a sulfitecooking process, said cooked and washed pulp subsequently further bleachedwith a TCF-sequence using alkaline extraction (E) and alkaline peroxide bleaching(P) in at least 2 stages, and wherein the final target viscosity of the bleacheddissolving pulp is primarily controlled by adjusting the conditions in an acidicperoxide stage (Pacid) establishing a pH in the range pH 2-4, and a peroxidecharge in the range 1-10 kg/odt obtaining an adjustable viscosity reduction in thePacid-stage in the range of 50-700 ml/g (120 ml/g) by said adjustment of at least pH or peroxide charge in the acidic peroxide stage (Pacid). ln a preferred embodiment the inventive method is further distinguished in thatsaid dissolving pulp process comprising following steps in sequence;a) subjecting comminuted cellulosic material to a sulfite cookb) subjecting the cooked pulp for alkaline extraction and a subsequent wash SUN15018 SE txt.d0c 5 c) subjecting the cooked and washed pulp for an acidic peroxidestage ( Pacid)-stage followed by a subsequent wash ; d) subjecting the washed pulp from the acidic peroxide stage for analkaline peroxide stage followed by a subsequent wash obtaininga final dissolving pulp quality; and where the conditions in the acidic peroxide stage are adjusted byadjusting at least the peroxide charge or pH enabling viscositymodifications in the range 50-700 ml/g 120 ml/g in said acidic peroxide stage.

Further, in the inventive method the acidic peroxide stage (Pacid)-stage isestablished at the following conditions;c1) a retention time of at least 90 min and at the most 180 min;c2) an initial temperature established equal to or below 95 °C;c3) a peroxide charge between 1-10 kg/odt;c4) a pH between 2-4; Typically the sulfite cooking process may dissolve most of the metal content fromthe wood material into the spent cooking liquor, but for largest viscosity controlspan in the acidic peroxide stage is the pulp subjected to an additional metalextraction stage (Q) followed by washing before the acidic peroxide stage, Also, for improved bleaching could also the pulp be subjected to an additionalsecond alkaline peroxide stage with subsequent washing after the first alkalineperoxide stage with subsequent washing and before obtaining the final dissolving pulp quality.

As to efficiency of metal removal, after the sulfite cook or after the Q-stage, themetal content dissolved in the sulfite cooking stage or the metal extraction stageshould reach an order that the levels of Mn and Fe into the acidic peroxide stage(Pacid) should be; Mn<5 mg/kg and Fe<20 mg/kg. And most preferably the levelsof Mn and Fe into the acidic peroxide stage (Pacid) is; Mn<2 mg/kg and Fe<5mg/kg.

SUN15018 SE txt.d0c The invention is based upon the finding that a span with larger viscosity reductionsmay be obtained in an acidic peroxide stage if controlled charge of peroxide withstrong chain cleavage abilities located close to or neighboring to cellulose areincreased. Hence, the charge of peroxide should not be consumed or wasted inreactions with metal content in the filtrate. This can be achieved by e.g. anincrease in hydroxyl radical formation formed close to the cellulose. The radicalhas a higher reduction potential at lower pH. lt is fairly known in the pulping industry that the formation of hydroxyl radicalsduring oxidation are reducing pulp viscosity but no prior art has tried to increasethis effect in order to control the order of viscosity reduction of pulp in general, and especially not in dissolving pulp production. ln the most general approach the invention relates to a method for producing lowviscosity dissolving pulp from lignocellulosic material, said dissolving pulp processcomprising a sulfite cooking process. Said cooked and washed pulp subsequentlyfurther bleached with a TCF-sequence with less than 5 stages, and wherein thefinal target viscosity of the bleached dissolving pulp is primarily controlled byadjusting the conditions in the acidic peroxide stage, in following parts referred toas a Pacid-stage, by addition of hydrogen peroxide at low pH obtaining a reductionof viscosity in Q-E-Pacid-P in the range of 50-700 ml/g 120 ml/g. The viscosityreduction is decreased with approximately 50% if no hydrogen peroxide is used. lt has been found that the Pacid-stage is possible to adjust whereby a large rangeof viscosity reduction could be obtained. Hence, the total production may beincreased in any existing mills, leading to higher viscosity in the cooked pulp,which increased production is compensated by increased viscosity reduction in thePacid-stage. ln a specific embodiment is the additional oxidation agent hydrogen peroxide, which in combination with pH, time, temperature and acid charge has shown to boost the viscosity reduction in the Pacid-stage considerably.

SUN15018 SE txt.d0c 7 More specifically the dissolving pulp process comprising following steps in SeqUenCe§ a) subjecting comminuted cellulosic material to a sulfite cookobtaining a cooked pulp b) subjecting the sulfite cooked pulp for bleaching with less than 5bleaching stages obtaining a final dissolving pulp after the lastbleaching stage.

The cooking process according to the inventive method could equally well takeplace in a batch digester or a continuous digester. lf the production is increased inan existing batch digester the cooking time needs to be reduced so that eachbatch digester may be emptied more frequently. lf the production is increased in an existing continuous digester the retention time in the digester is normallyreduced and thereby the viscosity reduction is decreased. The reduced retentiontime in batch or continuous digesters may in part be compensated by increasingthe temperature, but this at high costs as this requires addition of steam capable ofincreasing the temperature at the normal cooking temperature at about 140-170°C.

The Pacid-stage can take place in one or two successive treatment vessels wherea single chemical mixing position could be located before said vessels, or withchemical mixers located before both vessels with split chemical charge andpossibly with temperature profiling adding steam to second mixer.

Alternative configurations for the bleaching stage of the inventive method, usingstandard nomenclature, of the TCF bleaching sequence could be; Q-E-Pacid-P-P,O-Q-E-Pacid-P, O-E-Pacid-P, ZE-Pacid-P or O-ZE-Pacid-P. ln this standardnomenclature the sign stands for a transition to a new stage and all stagesincludes some kind of final washing. l.e. ZE above is one stage withoutintermediate washing and is typically implemented with the Z phase at highconsistency and where E phase is established by simply diluting the highconsistency pulp with alkaline liquids. Hence, the ZE stage may also be replacedwith Z-E, i.e. both at medium consistency and with an intermediate wash. The TCF SUN15018 SE txt.doc 8 sequence according to the invention only includes alkalization, acidification and peroxide, besides optional usage of oxygen, ozone and chelating agents.

BREIF DESCRIPTION OF THE DRAWINGS Figure 1 Show an example of a complete fiberline for manufacturingdissolving pulp; Figure 2 show the viscosity in different bleaching stages of the fiberline infigure 1; Figure 3 show the viscosity change with pH in the Pacid-stage Figure 4 show the dependence of time, pH, temperature and hydrogenperoxide charge on viscosity in the Pacid-stage Figure 5 show the viscosity dependence on hydrogen peroxide charge in thePacid-stage Figure 6 shows that a lower pH in the P-stage results in higher ISO brightness DETAILED DESCRIPTION The process will be further described with reference to the accompanyingdrawings. lt should however be noted that the invention is not limited to theembodiments described below and shown in the drawings, but may be modifiedwithin the scope of the appended claims.

The invention is related to a method for producing low viscosity dissolving pulpfrom lignocellulosic material, said dissolving pulp process comprising a sulfitecooking process. Said cooked and washed pulp subsequently further bleachedwith a TCF-sequence with at least 2 but less than 5 bleaching stages. Theprinciple layout of such a dissolving pulp process is shown in Figure 1.Lignocellulosic material, preferably wood chips (CH), are fed to a sulfite cookingstage (Cook), and thereafter bleached for example in an optional first Q-stage, anextraction stage, (E), an acid hydrogen peroxide bleaching stage, (Pacid), andfinally an alkali peroxide stage, (P) from which the dissolving pulp is fed out. Thefirst stage, Cook, is more or less standard stage but the bleaching stages couldhave other configurations than the Q-E-Pacid-P sequence. All these stagesincludes a final wash (w) and the sequence may also be referred to Q-(w)-E-(w)-Pacid-(w)-P-(w) SUN15018 SE txt.d0c EXPRIMENTAL DATA FROM ALTERING CONDITIONS IN THE Pacid STAGE ln order to study the potential viscosity reduction in the Pacid-stage theconventional charges of hydrogen peroxide, sulfuric acid and pH and time and temperature were altered.

The viscosity reduction is increased with approximately 50% when using hydrogen peroxide compared with no charge of hydrogen peroxide. A study was made to illustrate this. The study was made with a softwood (Picea abies) pulpwhich after a sulfite cook and bleaching QEP had an intrinsic viscosity of ~950 ml/g. The bleaching conditions and viscosity results are shown in Table 1.

Table 1 Bleaching conditions with Pacid sulfite pulp Reference 1 2 Reference Pacid H2so4 kg/odt 3.3 3.3 3.3 3.3 3.8 3.4 Another study was made with the same pulp, a softwood (Picea abies) pulp which after a sulfite cook and bleaching Q-E-P had an intrinsic viscosity of ~950 ml/g and an ISO brightness of 89.1 %. After Pacid the intrinsic viscosity was ~430 ml/g and ISO brightness ~85%. After a final conventional P-stage the ISO brightness can be increased to desired level.

The bleaching with Pacid was conducted with conditions shown in Table 2.Table 2Bleaching conditions with Pacid sulfite pulp SUN15018 SE txt.doc PacidPulp consistency % 12Temperature °C 90Time min 180H2SO4 kg/odt 5H20: kg/odt 10Final pH 2.1 Another study was made with a softvvood (Picea abies) pulp which after a sulfitecook had a kappa number of 12.0, an intrinsic viscosity of ~1065 ml/g and an ISObrightness of 63.1 %. After Q-E-Pacid-P-P the intrinsic viscosity was ~460 ml/g andISO brightness ~91.5%.

The bleaching with Q-E-Pacid-P-P was conducted with conditions shown in Table3.Table 3Bleaching conditions with Q-E-Pacid-P-P sulfite pulp Q E Pacid P PPulp % 5 15 12 12 12consistencyTemperature °C 70 85 90 90 90Time min 60 150 180 180 120H2SO4 kg/odt - - 5.0 - -NaOH kg/odt - 45 - 22 8H2O2 kg/odt - - 10 40 10EDTA kg/odt 4 - - - -MgSO4 kg/odt - - - 2 2Final pH ~4 ~12 ~2 ~10.5 ~10.5 Figure 2 shows the viscosity drop throughout the sequence Q-E-Pacid-P-P.

Another study was made, also with a softwood (Picea abies) pulp, which after asulfite cook had a kappa number of 12.0, an intrinsic viscosity of ~1065 ml/g and an ISO brightness of 63.1 %. After Q-E-Pacid-P-P the intrinsic viscosity was ~550ml/g and ISO brightness ~91.5%.

The bleaching with Q-E-Pacid-P-P was conducted with conditions shown in Table 4.Table 4 Bleaching conditions with Q-E-Pacid-P-P sulfite pulp Q " E Pacid' P P Temperature SUN15018 SE txt.d0c 11 The results in Figure 3 is based on the sequence Q-E-Pacid and shows that the viscosity is reduced with decreased pH in the Pacid-stage. Time, temperature and HzOz-charge affect the viscosity in a less extent, Figure 4. The viscosity dependence5 on HzOz-charge is presented in Figure 5.

To increase the ISO brightness to be able to reach target brightness one or two final P-stages are put in. Addition of MgSOg is necessary to reach high ISO brightness. ln Figure 6 it is shown that a lower pH in the P-stage results in higher10 ISO brightness.

SUN15018 SE txt.d0c

Claims (3)

1. A method for producing low viscosity dissolving pulp from lignocellulosic material, said dissolving pulp process comprising a sulfite cookingprocess, said cooked and washed pulp subsequently further bleachedwith a TCF-sequence using a|ka|ine extraction (E) and a|ka|ineperoxide bleaching (P) and optionally ozone or oxygen in at least 2stages, and wherein the final target viscosity of the bleached dissolvingpulp is primarily controlled by adjusting the conditions in an acidicperoxide stage (Pacid) establishing a pH in the range pH 2-4, and aperoxide charge in the range 1-10 kg/odt obtaining an adjustableviscosity reduction in the Pacid-stage in the range of 50-700 ml/g (120ml/g) by said adjustment of at least pH or peroxide charge in the acidic peroxide stage (Pacid).

2. Method according to claim 1, said dissolving pulp process comprising a)b) C) d) following steps in sequence; subjecting comminuted cellulosic material to a sulfite cooksubjecting the cooked pulp for a|ka|ine extraction and asubsequent wash subjecting the cooked and washed pulp for an acidic peroxidestage ( Pacid)-stage followed by a subsequent wash ; subjecting the washed pulp from the acidic peroxide stage for ana|ka|ine peroxide stage followed by a subsequent wash obtaininga final dissolving pulp quality; and where the conditions in the acidic peroxide stage are adjusted by adjusting at least the peroxide charge or pH enabling viscositymodifications in the range 50-700 ml/g 120 ml/g in said acidic peroxide stage.

3. Method according to claim 2, whereby the acidic peroxide stage ( Pacid)- stage is established at following conditions; c1) a retention time of at least 90 min and at the most 180 min; c2) an initial temperature established equal to or below 95 °C; SUN15018 SE txt.d0c 13 c3) a peroxide charge between 1-10 kg/odt;c4) a pH between 2-4; Method according to claim 1, whereby the pulp is subjected to anadditional metal extraction stage (Q) followed by washing before the acidic peroxide stage, Method according to claim 1, whereby the pulp is subjected to anadditional second alkaline peroxide stage with subsequent washingafter the first alkaline peroxide stage with subsequent washing andbefore obtaining the final dissolving pulp quality. Method according to claim 1 or 4 where metal content is dissolved in thesulfite cooking stage or the metal extraction stage such the levels ofMn and Fe into the acidic peroxide stage ( Pacid) should be; Mn<5mg/kg and Fe<20 mg/kg. Method according to claim 6 where the levels of Mn and Fe into the acidicperoxide stage ( Pacid) is; Mn<2 mg/kg and Fe<5 mg/kg. SUN15018 SE txt.d0c