The present disclosure relates in general to a method for producing dissolving pulp of low viscosity from lignocellulosic material. The dissolving pulp process for producing low viscosity pulp conventionally contains a sulfite cooking stage and thereafter bleaching for reduction and brightness increase. In commercial systems is often only used a first sulfite cooking stage and a subsequent alkaline extraction stage followed by peroxide bleaching.

BACKGROUND

In the wood based pulping industry significant efforts have been made designing different processes for different grades of pulp.

In conventional kraft pulping for fully bleached grades of paper the objectives has been to produce a pulp grade with high yield and high pulp strength having acceptable brightness level and stability. For fully bleached pulp grades ISO brightness above 88% is most often preferred, and if minimal brightness reversion is requested the residual content of lignin and hexenuronic acid must be kept low.

In general sulfite pulping has a higher cellulose yield for softwood pulp compared with kraft pulping. However, with high alpha cellulose content the kraft pulping could have a higher yield. The lignin content in cooked sulfite pulp is low which makes the subsequent treatment downwards easier, i.e. easier to bleach. Since the sulfite pulp is easier to bleach TCF-sequences are often used, mostly with hydrogen peroxide. Oxygen stages are not frequently used for sulfite pulping due to several reasons, one of them is the environmental aspect.

Hence, very competitive low cost systems for dissolving pulp may be obtained from a sulfite cook as the cooked pulp has a relative low metal content due to the acidic cooking conditions, and the subsequent bleaching may be obtained 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. In recent years the pulping industry has strived to find alternative products for the wood material as the printing paper market declines in volume, and that new products may be found with better profit margins. Dissolving pulp has emerged as a reborn alternative growing market for pulp mills and much attention has been given to modify the pulping processes to be able to produce different grades of dissolving pulp which is used to produce a multitude of products like Rayon-grade pulp or specialty pulps. The interest to find alternative textile materials to cotton has increased due to short term shortage and increase in costs for cotton and an increased competition in long term for land to grow an increasing demand of cotton on. Dissolving pulp can consist of cotton linters, pulp originating from wood or annual plants made by the sulfite process or the prehydrolysis kraft process. However, in pulp industry dissolving pulp is generally referred to as a bleached pulp produced from wood that has a high alpha cellulose content, typically over 92%, and only small content of hemicelluloses, typically below 10%. Hence, the wood yield of dissolving pulp from the process is typically low, 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 specialty chemicals and materials such as cellulose acetate and carboxy methyl cellulose. When making rayon yarns the dissolving pulp is converted to cellulose xanthate which dissolves in caustic soda and the resulting viscous liquid is extruded in acidic baths to yield fibers. As an alternative process the dissolving pulp can be dissolved in ionic solvents to make extruding to fibers possible. For both these processes and the specific final product it is essential that the viscosity of the dissolving pulp is both low and within a specific narrow range suitable for the process, in order to run derivatisation/dissolution process smoothly.

For some dissolving pulp grades the required intrinsic viscosity in the final bleached dissolving pulp must be as low as 350 ml/g and within a narrow acceptance range of only ±20 ml/g. This requires a low viscosity already after cooking 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 for dissolving pulp. The viscosity number is expressed as intrinsic viscosity and measured in ml/g. A standard test method for intrinsic viscosity of cellulose could be found in ISO-standard ISO 5351 .

Now, viscosity measurements is indicative for the average molecular weight of the cellulose polymers, i.e. the length of the cellulose chains. The length of the cellulose chains impact the derivatisation and solubilisation process as well as the characteristics of the final product.

However, for production of dissolving pulp the viscosity is often about or below 1 100 ml/g after cook and below 600 ml/g after final bleaching. Most often the viscosity target for the final dissolving pulp is kept within 400-600 ml/g and within a narrow acceptance range of only ±20 ml/g for the specific grade. Examples of specification of higher grade dissolving pulps are Ethers (viscosity 470-600 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 the final pulp requested and most often is extended and intensified cooking, i.e. both longer cooking time and tougher cooking conditions as of alkali charge or other cooking chemicals and temperature, needed in order to obtain a low enough viscosity after the cook. A problem with extended cooking occurs if it is desired to increase the production in existing pulp mills as increased production results in decreased cooking time if the equipment is the same. This results in higher viscosity when the throughput of the lignocellulosic material increases. If the viscosity reduction does not reach a sufficient low level directly after the cook, as is the case if for example the production is increased, it has with some standard bleaching sequences been found to be almost impossible to reach the final viscosity level by implementing tougher process conditions in final bleaching. This applies especially to TCF sequences using only peroxide and alkali during the bleaching.

Normally the viscosity reduction may be increased marginally in the order of 50 ml/g in final bleaching, by implementing tougher bleaching conditions in the final bleaching stages as of temperature and additional bleaching agents or increased charge of the standard bleaching agents used. Ozone stages are also capable of reducing viscosity to a larger extent, but this at expense of increased costs, 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 filterability and spinnability of a cellulose spinning dope, therefore a low content of these compounds are preferable. The amounts of catalytically active transition metal ions as Mn and Fe are also important to keep at a low level when hydrogen peroxide is used to avoid uncontrolled peroxide decomposition.

SUMMARY

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

The inventive method is thus optimized for producing low viscosity dissolving pulp from lignocellulosic material, said method comprising a sulfite cooking process producing cooked pulp which cooked pulp thereafter is washed producing cooked and washed pulp, said cooked and washed pulp subsequently further bleached with a TCF-sequence in at least 2 stages using alkaline extraction (E) and alkaline peroxide bleaching (P) and optionally ozone or oxygen, and wherein the final target viscosity of the bleached dissolving pulp is primarily controlled by

adjustment of at least pH or peroxide charge in an acidic peroxide stage (Pacid) establishing a pH in the range pH 2-4, and a peroxide charge in the range 1 -10 kg/odt obtaining an adjustable viscosity reduction in the Pacid-stage in the range of 50-700 ml/g by said adjustment of at least pH or peroxide charge in the acidic peroxide stage (Pacid).

In a preferred embodiment the inventive method is further distinguished in that said dissolving pulp process comprising following steps in sequence;

a) subjecting comminuted cellulosic material to a sulfite cook b) subjecting the cooked pulp for alkaline extraction and a

subsequent wash

c) subjecting the pulp from step b) for an acidic peroxide stage (Pacid)-stage followed by a subsequent wash;

d) subjecting the washed pulp from the acidic peroxide stage for an alkaline peroxide stage followed by a subsequent wash obtaining a 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 viscosity modifications in the range 50-700 ml/g ±20 ml/g in said acidic peroxide stage.

In embodiments of the present invention, the viscosity of the final bleached dissolving pulp is kept within an acceptance range of ±20 ml/g.

Further, in the inventive method the acidic peroxide stage (Pacid)-stage is established 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 from the wood material into the spent cooking liquor, but for largest viscosity control span in the acidic peroxide stage is the pulp subjected to an additional metal extraction stage (Q) followed by washing before the acidic peroxide stage, Also, for improved bleaching could also the pulp be subjected to an additional second alkaline peroxide stage with subsequent washing after the first alkaline peroxide 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, the metal content dissolved in the sulfite cooking stage or the metal extraction stage should 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 levels of Mn and Fe into the acidic peroxide stage (Pacid) is; Mn<2 mg/kg and Fe<5 mg/kg.

The invention is based upon the finding that a span with larger viscosity reductions may be obtained in an acidic peroxide stage if controlled charge of peroxide with strong chain cleavage abilities located close to or neighboring to cellulose are increased. Hence, the charge of peroxide should not be consumed or wasted in reactions with metal content in the filtrate. This can be achieved by e.g. an increase in hydroxyl radical formation formed close to the cellulose. The radical has a higher reduction potential at lower pH.

It is fairly known in the pulping industry that the formation of hydroxyl radicals during oxidation are reducing pulp viscosity but no prior art has tried to increase this effect in order to control the order of viscosity reduction of pulp in general, and especially not in dissolving pulp production.

In the most general approach the invention relates to a method for producing low viscosity dissolving pulp from lignocellulosic material, said dissolving pulp process comprising a sulfite cooking process. Said cooked and washed pulp subsequently further bleached with a TCF-sequence with less than 5 stages, and wherein the final target viscosity of the bleached dissolving pulp is primarily controlled by adjusting the conditions in the acidic peroxide stage, in following parts referred to as a Pacid-stage, by addition of hydrogen peroxide at low pH obtaining a reduction of viscosity in Q-E-Pacid-P in the range of 50-700 ml/g ±20 ml/g. The viscosity reduction is decreased with approximately 50% if no hydrogen peroxide is used. It has been found that the Pacid-stage is possible to adjust whereby a large range of viscosity reduction could be obtained. Hence, the total production may be increased in any existing mills, leading to higher viscosity in the cooked pulp, which increased production is compensated by increased viscosity reduction in the Pacid-stage.

In 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.

More specifically the dissolving pulp process comprising following steps in sequence; a) subjecting comminuted cellulosic material to a sulfite cook

obtaining a cooked pulp

b) subjecting the sulfite cooked pulp for bleaching with less than 5 bleaching stages obtaining a final dissolving pulp after the last bleaching stage.

The cooking process according to the inventive method could equally well take place in a batch digester or a continuous digester. If the production is increased in an existing batch digester the cooking time needs to be reduced so that each batch digester may be emptied more frequently. If the production is increased in an existing continuous digester the retention time in the digester is normally reduced and thereby the viscosity reduction is decreased. The reduced retention time in batch or continuous digesters may in part be compensated by increasing the temperature, but this at high costs as this requires addition of steam capable of increasing 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 where a single chemical mixing position could be located before said vessels, or with chemical mixers located before both vessels with split chemical charge and possibly with temperature profiling adding steam to second mixer.

Alternative configurations for the bleaching stage of the inventive method, using standard 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. In this standard nomenclature the sign "-" stands for a transition to a new stage and all stages includes some kind of final washing. I.e. ZE above is one stage without

intermediate washing and is typically implemented with the Z phase at high consistency and where E phase is established by simply diluting the high consistency pulp with alkaline liquids. Hence, the ZE stage may also be replaced with Z-E, i.e. both at medium consistency and with an intermediate wash. The TCF 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 manufacturing

dissolving pulp;

Figure 2 show the viscosity in different bleaching stages of the fiberline in figure 1 ;

Figure 3 show the viscosity change with pH in the Pacid-stage

Figure 4 show the dependence of time, pH, temperature and hydrogen

peroxide charge on viscosity in the Pacid-stage

Figure 5 show the viscosity dependence on hydrogen peroxide charge in the

Pacid-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 accompanying drawings. It should however be noted that the invention is not limited to the embodiments described below and shown in the drawings, but may be modified within the scope of the appended claims. The invention is related to a method for producing low viscosity dissolving pulp from lignocellulosic material, said dissolving pulp process comprising a sulfite cooking process. Said cooked and washed pulp subsequently further bleached with a TCF-sequence with at least 2 but less than 5 bleaching stages. The principle layout of such a dissolving pulp process is shown in Figure 1.

Lignocellulosic material, preferably wood chips (CH), are fed to a sulfite cooking stage (Cook), and thereafter bleached for example in an optional first Q-stage, an extraction stage, (E), an acid hydrogen peroxide bleaching stage, (Pacid), and finally an alkali peroxide stage, (P) from which the dissolving pulp is fed out. The first stage, Cook, is more or less standard stage but the bleaching stages could have other configurations than the Q-E-Pacid-P sequence. All these stages includes a final wash (w) and the sequence may also be referred to Q-(w)-E-(w)- Pacid-(w)-P-(w) EXPRIMENTAL DATA FROM ALTERING CONDITIONS IN THE Pacid STAGE

In order to study the potential viscosity reduction in the Pacid-stage the

conventional 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) pulp which 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

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 2 Bleachin conditions with Pacid sulfite pulp

Another study was made with a softwood (Picea abies) pulp which after a sulfite 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 ~460 ml/g and ISO brightness -91.5%.

The bleaching with Q-E-Pacid-P-P was conducted with conditions shown in Table 3.

Table 3 Bleachin conditions with Q-E-Pacid-P-P sulfite pulp

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 a sulfite 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 ~550 ml/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

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 H2O2-charge affect the viscosity in a less extent, Figure 4. The viscosity dependence on H2O2-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 MgSO2 is necessary to reach high ISO brightness. In Figure 6 it is shown that a lower pH in the P-stage results in higher ISO brightness.