CN112442746A - Method for producing modal fiber - Google Patents

Abstract

The present invention relates to a method of making modal fibers. Specifically, the present invention takes measures to obtain alkali cellulose having a narrow DP distribution and a low hemicellulose content. Preferably, these measures are reducing the hemicellulose uptake in the modal fibre process and/or removing hemicellulose from the lye system by applying suitable separation techniques and/or transferring a part of the press lye to other lye consuming processes.

Description

Method for producing modal fiber

Technical Field

The present invention relates to a modal process.

The modal fibers are characterized by the minimum requirements for tenacity and wet modulus according to the definition of BISFA. In addition, the technology used to make modal fibers with their unique combination of physical and textile properties, softness and fabric stability should meet the IPPC guidelines of the european ecolabel and Best Available Technology (BAT) for emissions from processes produced by proprietary manufacturing methods.

Background

Compared to the conventional viscose process, the key element of the process is a balanced combination of the following factors: low hemicellulose concentration in the impregnation lye, short ageing time and higher DP of the cellulose, higher input of carbon disulphide (CS 2) and sodium hydroxide (NaOH), use of special modifiers, viscose fibres with low cellulose concentration, high alkali ratio and high viscosity, lower concentrations of sulphuric acid and sodium sulphate and high zinc concentration in the spinning bath, higher number of spinneret holes, high drawing and low spinning speed and high specific formation and yield of sodium sulphate. In particular, the method may consist of the following method steps:

slurry material

All dissolving grade slurries of sufficient purity, reactivity, whiteness and appropriate viscosity are useful in the modal process. Such pulp can be prepared from softwood or hardwood or even non-wood materials by sulfite or sulfate techniques and used in the form of sheets, rolls or flakes. The integrated preparation of pulp and fibers allows the use of wet pulp, which can be fed directly into the pulper without the need for drying. The subsequent process steps in viscose manufacture need to be adjusted due to the physical and chemical performance of the pulp. The slurry may also contain an impregnation aid, which is a surfactant for better penetration of the impregnation lye.

Pulping

Wood-based cellulose, such as baled dry or wet pulp (48-50% dry matter), is mixed with a temperature-controlled sodium hydroxide liquor (impregnation liquor) containing 220-240g/l total alkali in a number of pulpers equipped with special turbine stirrers. Alkali Cellulose (AC) is formed from the reaction of cellulose with sodium hydroxide. Impurities like so-called "hemicellulose" and its degradation products are dissolved in the liquid.

The slurry is stored in a slurry tank from which it is continuously pumped to an AC press which separates the alkali cellulose from the pressed alkali liquor.

Alkali Cellulose (AC)

After pressing the pulp in a drum or belt press, a wet cake of alkali cellulose containing 32-34% cellulose and 15-17% caustic soda is obtained. The AC is crushed to obtain a high specific surface area prior to the aging process.

AC aging (Pre-aging)

The aging process is necessary to adjust the degree of polymerization of cellulose by depolymerization in the presence of air or oxygen in order to obtain the desired viscosity of the spinning solution. The shredded alkali cellulose is aged in a belt aging, aging drum, tower or cabinet apparatus. The aging time can be adjusted by varying the speed and temperature of the conveyor or drum, depending on different requirements. If desired, the pre-maturation process may be accelerated by adding a catalyst, such as cobalt chloride or manganese sulphate, to obtain an AC (SCAN) having an intrinsic viscosity of 400-450 ml/g.

After aging, the alkali cellulose is cooled in an alkali cellulose cooler, which operates as a fluidized bed reactor. A rotary cooling drum or a continuous flow conveyor may also be used.

Xanthation

The alkali cellulose is reacted with CS2 to give sodium xanthate, which is soluble in dilute sodium hydroxide (dissolving lye). For this purpose, AC was fed into the xanthator through a conveyor belt and a weigh bin. The exothermic reaction of AC with carbon disulphide is controlled by cooling the AC.

The reactor used may be a wet churn equipped with an intensive kneader, a continuous twin-screw kneader or a belt xanthator. Under vacuum, AC was reacted with CS2 liquid and kneaded like a dough until the reaction was complete. The CS2 input may be 35-38% based on cellulose.

CS2 also reacts with NaOH as a by-product of xanthate formation to form Na2CS3 and Na 2S.

A sufficient degree of substitution is required to dissolve the xanthate in dilute NaOH, expressed as ɣ value (CS 2 moles per 100 moles of glucose units). The concentration of sodium hydroxide determines the alkali content of the viscose. By varying the degree of substitution, the alkali-/cellulose ratio and the average length of the chains, viscose fibers of different quality can be achieved.

The xanthation apparatus may be of the Simplex or Suma type. The temperature at the beginning of the xanthation may be between 15 and 17 ℃ and the temperature at the end of the xanthation between 30 and 35 ℃.

Dissolution

The mixture of lye and xanthate is discharged into the dissolver through the homogenizer by feeding the cooled dissolved lye into the xanthator.

Homogenization of viscose fibres is achieved in a fine homogenizer at low temperature. The modifier is added to the viscose dope. The function of the modifier is to control coagulation and regeneration during filament formation in the spinning process. Many different compounds or their blends can be used as modifiers: amines and polyamines, quaternary ammonium compounds, amides, heterocyclic nitrogen compounds, polyglycols, ethoxylated ammonia derivatives, ethoxylated fatty acid amines and amides, ethoxylated glycerol and ethoxylated fatty acids, esters or alcohols. Many examples of these compounds, which may be used alone or as blends, are described in the k. foster ribbon tze, "Chemiefasern nach dem viskosher fahren", third edition, Springer, 1967, pp 635-641, the disclosure of which is incorporated herein by reference.

The amount of modifier is 1.0-4.0% based on cellulose.

The modifier can be added during the dissolution process, preferably directly into the dissolver, after degassing before the final filtration of the viscose fibers or directly before spinning.

This can be done by means of generally known devices for introducing substances into a stream of liquid medium. Preferably, there is a means, such as a static or dynamic mixer, after the addition location to provide uniform distribution of the modifier within the viscose fibers. Such mixers are well known and commercially available, for example Sulzer mixers. In a continuous process, the viscose fibres from the dissolver are further treated in two refiners to improve the dissolution state.

The unfiltered viscose fibres may have a cellulose content of 6.0-7.0%, an alkali content of 6.0-7.0% and a sulphur content of 1.7-2.0%.

Aging, filtering and degassing

After homogenization of the xanthate, the viscose fiber must go through successive maturation, degassing and filtration processes.

The ripening is carried out in tanks in order to obtain a better distribution of the xanthate groups on the cellulose chains and to obtain the desired ripening index or gamma value for the subsequent spinning process.

Prior to spinning, viscose fibres must be filtered to remove fibres, gel particles or other impurities, as this can block the fine spinneret holes and cause spinning defects. Instead of a conventional filter press, a continuous waste filter can be used, which is equipped with fine metal wool fibers of different pore sizes. The filtration process is carried out in two or three steps in order to obtain the best filtrate quality.

Continuous rapid degassing to remove air bubbles was performed before pumping the viscose fibers to the spinning tank.

The spun viscose fibres may have a ball drop viscosity of 80-140s, a gamma value of 57-64, a cellulose content of 6.0-7.0%, an alkali content of 6.0-7.0%, a sulphur content of 1.6-1.9% and an alkali/cellulose ratio of 0.95-1.1.

Spinning

The modal staple fibers were produced on a spinning machine consisting of a spinning tank containing a spinning bath, a rotating godet with a variable drive and a ceramic guide for taking up the filaments and a final draw roll for collecting the individual tows after drawing in a hot secondary bath. Such draw rolls may be, for example, a triad of two sets of pressure rolls that squeeze the tow and reduce chemical entrainment into the acid wash, or a series of six or more non-squeeze rolls.

Viscose fibers were pumped by a gear pump, passed through a candle filter, and extruded through the fine holes of a spinneret immersed in a spinning tank.

Different spinning geometries can be applied for modal fibers: diagonal or vertical spinning, where there are two or three tows per godet.

For the purpose of ventilation, the machine is completely enclosed to recover the sulphide gases and to achieve safe working conditions in the spinning zone.

Fresh spin bath enters the spin tank from the bottom and overflows back to the spin bath system where it is degassed, filtered, excess water is evaporated, excess sulphate is crystallised and make-up chemicals are added to adjust the composition before it is recycled back to the spinning machine.

Coagulation and regeneration occur in parallel after the viscose fibres have been extruded through the spinneret holes. The viscose fibres coagulate and the high viscosity gel becomes a sol. During regeneration, xanthate is decomposed by the acid of the spinning bath into regenerated cellulose, and the caustic soda of the viscose is neutralized by the acid to form sodium sulfate. Both processes are affected by the following factors: the composition and temperature of the viscose and spinning bath, the type and concentration of the modifiers (which delay the regeneration process and their interaction with the components of the spinning bath), the spinning speed, the spinning geometry and the immersion length of the filaments, the spinning bath/viscose flow rate and the diameter and design of the spinneret holes, and therefore a careful balance is required to obtain fibers with the required physical parameters without spinning defects.

The tow from two or three spinnerets was collected, guided by a ceramic guide and wound three times around a godet roll. The tow is drawn by a take-up roll at the end of the spinning machine, the tow is introduced into a hot secondary bath (drawing bath) where the regeneration is completed and high drawing is applied so that the filaments gain strength by fixing the oriented cellulose molecules.

Due to the delayed regeneration, the spinning speed in the modal fiber process is much lower than in the conventional viscose process and can be 25 to 40m/min and higher elongations to 125% can be applied.

Spinning nozzle

Spinneret design and spinning geometry are key factors in achieving high quality modal fibers at high line efficiencies. Modal fibers need to be spun at much lower spinning speeds than viscose fibers, which can be compensated for by spinnerets with significantly higher hole counts. Such cluster spinnerets consist of cups arranged on a plate, which may be a full disc or an annular plate. The cup may be 12 to 16mm in diameter, with the hole diameter depending on the fibre titre. The total number of holes per spinneret can be as high as 100,000 or even higher. In order to achieve a sufficient immersion length, the filaments are spun vertically or diagonally at an angle. The immersion length may be 40 to 80 cm.

Spinning bath

The spinning bath is an aqueous solution of sulfuric acid, zinc sulfate and sodium sulfate, which is circulated through the spinning machine. The viscose fibres are coagulated by the salts present in the bath and decomposed by the acid, giving regenerated cellulose filaments, Na2SO4, CS2 and H2S. In the spin bath circulation system, excess Na2SO4 is removed by crystallization, the sulfide gas is collected and recovered as CS2, sulfur or converted to sulfuric acid, and make-up chemicals sulfuric acid and zinc sulfate are added to adjust the spin bath composition.

In the spinning bath, the H2SO4 concentration may be 70-90g/l, the Na2SO4 concentration 90-130g/l, the ZnSO4 concentration 48-62g/l, and the temperature 37-43 ℃. The spinning speed, spinneret size, modifier type and spin bath composition (including temperature and dip length) are not independent of each other, but need to be well balanced to achieve the desired fiber properties.

Stretching bath

The drawing bath is an encapsulated hot acid bath comprising the spinning bath components in diluted form at a temperature of 90 to 98 ℃. Its function is to apply maximum tow draw between the godet and draw units while the regeneration process is still in progress to wash the tow to reduce spinning bath chemical carryover into post-processing and to collect sulfide gas from the system for recovery.

Cutting of

The tow from all godets was collected on a drawing unit to obtain a thick wire which was sucked into the wire injector of a cutter with acidic water and cut to the desired staple length with a rotating self-sharpening cutter.

The cut staple fibers were then washed with acidic water into the CS2 tank of the washing track for post-treatment.

As an alternative to injection wet cutters, dry cutters are used, which are horizontal discs with fixed knives. The cable is wrapped around the disc and cut by pressing the second disc towards the knife.

Post-treatment

The different stages of the process are fluff formation, treatment with acidic water, desulphurization, bleaching, washing and finishing (refining). The process is carried out on an enclosed washing track, which may be a belt or an eccentric conveyor, which conveys the fluff fibers through a post-treatment, during which they are rinsed with different liquids. The press rollers between the washing zones reduce the entrainment of chemicals.

The staple from the cutter was washed into a CS2 tank and treated with hot acidic water. When the residual CS2 is released and subsequently recovered from the exhaust gas, the short fibers open and form fluff, which is picked up by the scrubbing track. The uniformity of the fluff is critical for uniform fiber quality.

In the first zone, residual sulfide compounds are removed by treatment with hot acidic water. After pressing, the fluff was washed with soft water in a counter-current. Fresh water is added before finishing and reused for washing after bleaching, after desulphurization and after acid treatment.

Desulfurization is carried out by treatment with a dilute caustic soda and sodium sulfide mixture to dissolve and remove sulfur residues.

Sodium hypochlorite is used as a bleaching chemical.

Prior to the final finishing step, the fluff was washed with soft water and dewatered with high pressure rolls to reduce liquid entrainment into the finishing zone and achieve maximum finishing absorption.

Finishing is an important process for preparing the surface of fibers for further processing in yarn spinning or non-woven processes. The different components are mixed to obtain an optimal balance between adhesion, slip and antistatic properties.

After finishing, the fluff was dewatered with a high pressure roller to minimize the water content before drying. Compact wet fiber mats require a wet opening process that is performed in two or three stages using spiked rollers of different strengths to prevent fiber damage. The wet opened fluff fibers can now be dried.

Drying, opening and packaging

Drying can be carried out in a belt dryer or drum dryer with hot air in countercurrent. The screen drum dryer consists of a series of rotating drums, each of which contains a wire jacket and a fan. Hot air is drawn into the drum interior through the fluff, which dries and presses the fluff onto the wire sleeve. By blocking half of the drum cylinder, fluff is transferred to the next drum and turned over. After passing through the first zone of the dryer, the fluff is opened again to obtain a uniform moisture profile and rearranged for the final dryer zone. At the end of the drying process, the fibers are rewetted to adjust the fiber moisture to the desired level. After drying has been completed, the final opening process is carried out in one or two stages to produce a uniform and large quantity of product ready for packaging.

Spinning bath degassing and filtration

During spinning, CS2 and H2S dissolved in the spinning bath and needed to be removed by degassing. By spraying the spinning bath in a degasser under vacuum, the gas is released and used as rich gas to produce sulfuric acid.

After filtering the degassed spinning bath through sand or candle filters, the composition of the spinning bath is adjusted by the dosage of sulfuric acid and zinc sulfate and returned to the bottom tank of the spinning bath cycle.

Evaporation of

During spinning, the spinning bath is diluted by the water taken in from the viscose and the water formed by neutralization of NaOH with sulfuric acid. In order to maintain the desired spin bath concentration, excess water must be removed by a thermal multistage evaporation device. The high efficiency flash evaporator operates at low specific steam consumption.

Crystallization of

In the modal process with a high base ratio, the amount of sodium sulfate formed by the neutralization reaction is significant. The process by-products must be removed and recovered from the spinning bath.

This is accomplished by a crystallizer that uses caustic soda or sulfuric acid as a vapor absorption method to cool the temperature of the spinning bath. Crystals of mirabilite (Na 2SO4 x 10H 2O) were grown in large drums and separated by propeller centrifuges.

Calcination of

In order to make sodium sulfate transportable and usable, the crystal water of mirabilite needs to be removed. For this purpose, pressure calcination is used. After melting the salt cake, the melt was directed through a calcination evaporator. The crystals were thickened by a second set of centrifuges and then separated. The anhydrous sodium sulfate is dried, stored and packaged.

Recovery of CS2 by condensation

Water vapor, a mixture of CS2 and H2S, was drawn from the fluff forming unit. Most of the vapor is condensed and returned to the CS2 tank. The remaining gas passes through the condenser and is sucked out by the water jet, wherein CS2 is condensed by cold water. The separator after the water jet separates the process water and condensed CS2 from the uncondensed gases. The uncondensed gases are drawn off to an exhaust system for further processing to sulfuric acid. The liquid CS2 is separated in the settler and recycled to the viscose process without any additional cleaning.

Recovery of CS2 (CAP) by adsorption

The CS2 off-gas having a low concentration of H2S was recovered by the CAP process (carbon adsorption plant). It is a batch process with cycles of adsorption and desorption of CS2 by activated carbon. Prior to applying the adsorption process, H2S had to be removed from the feed gas with NaOH in a gas scrubber. The obtained mixture of NaOH and Na2S can be used in the desulfurization process of the fiber post-treatment. The feed gas concentration should be high but must be outside the explosion limits. The liquid CS2 from desorption can be directly recycled into the process.

WSA (Wet sulfuric acid equipment)

Among all the technologies used for treating sulfide off-gases, the WSA process is most versatile and effective in removing sulfide odor by converting CS2 and H2S into sulfuric acid, which can be used in the spinning process. By combining WSA with the condensation process, a reasonable amount of CS2 input can be recycled while achieving maximum purification efficiency. However, the economic disadvantage is that the valuable specialty chemical (CS 2) is converted into the inexpensive commodity (sulfuric acid). Depending on the exhaust gas composition, additional sulfur and fuel (natural gas, oil) are used as additional inputs and steam is produced as a byproduct.

The lean gas from spinning, the rich gas from degassing the spinning bath and sulfur were used as separate feeds for WSA. The sulfide compound is efficiently converted to sulfuric acid by the catalyst.

Zinc recovery

In the work-up, zinc is precipitated from the overflow waste water of the pickling system in the form of zinc sulphide. The zinc sulphide is separated from the slurry and dissolved with sulphuric acid to obtain again zinc sulphate, which is metered into the spinning bath for the spinning process.

Disclosure of Invention

Hemicellulose consists of lower molecular weight polysaccharides, which are present in all types of pulp together with cellulose. Although cellulose is crystalline, high strength and resistant to hydrolysis, hemicellulose has a disordered amorphous structure and little strength. The hemicellulose content of the pulp is expressed as the so-called "S18 value", which describes the percentage of total dry pulp that is soluble in 18% caustic soda. The S18 value is measured according to the German industry standard DIN 54355, where S18 = (100-R18).

In the viscose process, the hemicellulose fraction is dissolved in the impregnation lye and accumulates in the impregnation and pressing lye system. They are also produced by the degradation of cellulose in a subsequent pre-ageing step. During xanthation, hemicellulose consumes CS2, whereas CS2 is required to react with high polymer cellulose (α -cellulose) to obtain a perfect spinning solution. In the viscose spinning process, a portion of the hemicellulose (beta-cellulose) is precipitated by sulfuric acid and regenerated together with alpha-cellulose. It becomes part of the fiber and adversely affects the tensile properties. Another portion of the hemicellulose (ɣ -cellulose) does not precipitate in sulfuric acid. It dissolves in the spinning bath, where its degradation products react with the sulfur compounds to form a viscous organic black mass which contaminates the spinning bath and fibers and appears as "black particles" in the final product.

Modal fibers are characterized by excellent tensile properties based on a higher degree of polymerization and a narrow distribution of cellulose chain lengths. The problem to be solved by the present invention is therefore how to reduce the adverse effect of hemicellulose in the whole process.

Detailed Description

Surprisingly it has been found that this problem can be solved by one or a combination of the following methods:

in a preferred embodiment of the present invention, the alkali cellulose pulping process is controlled such that a narrow distribution of polymer chain lengths is obtained. This requires the use of a slurry with a low hemicellulose content and/or a steeping liquor with a low hemicellulose concentration.

In particular, the method according to the invention is a method for producing modal fibers, comprising at least the following method steps:

a. preparing alkali cellulose from a cellulosic raw material by applying an impregnation lye to the cellulosic raw material, and wherein a pressing lye is obtained by pressing the alkali cellulose,

b. the alkali cellulose is aged and the resultant is then,

c. the mixture is subjected to xanthation and acidification,

d. is dissolved by applying a dissolving lye,

e. curing, filtering, degassing,

f. spinning is carried out on the mixture of the carbon fiber and the carbon fiber,

g. stretching the mixture to obtain a stretched mixture,

h. post-processing (including cutting, drying, opening, packaging),

i. the recovery of the chemicals is carried out by,

wherein in step a measures are taken to obtain alkali cellulose with a narrow DP distribution and a low hemicellulose content.

Even more preferably, the measures taken in step a are to reduce hemicellulose uptake in the modal fibre process and/or to remove hemicellulose from the lye system by applying suitable separation techniques and/or to transfer a part of the pressed lye to other lye consuming processes.

In a particularly preferred embodiment of the present invention, the hemicellulose intake in the modal fiber reducing process is accomplished by using a dissolving pulp having an S18 value of less than 4% as the cellulosic raw material.

In another preferred embodiment of the invention, only a part of the pressed lye is recycled to the impregnation lye, while another part is treated by the membrane separation process. The hemicellulose of the pulp accumulates in the lye system and needs to be removed from the press lye in order to maintain the desired low level of hemicellulose in the AC. This can be achieved by dialysis or nanofiltration methods according to the Cerini or Barnefeld method. The purified lye is fortified with caustic soda and returned to the modal process as dissolved lye. Fresh caustic soda is added to the impregnation liquor to adjust the concentration and volume while reducing the hemicellulose level.

The total alkali content of the modal impregnation alkali liquor is 220-240g/l and the hemicellulose concentration should not exceed 15-20 g/l.

A typical Modal pressing lye may contain a total alkali content of 190-210g/l and a hemicellulose concentration of 18-25 g/l.

The waste liquid from the dialysis process contains a high organic load and is disposed of by evaporation and incineration. Soda ash is thus recovered, which can be reused, for example for neutralization in wastewater treatment.

Another preferred embodiment of the invention is to transfer a part of the modal pressing lye to other lye consuming methods which are less sensitive to higher hemicellulose concentrations, such as the viscose method. If desired, a part of the viscose-pressing lye containing a part of the modal-pressing lye can also be purified by dialysis or nanofiltration.

Alternatively, the hemicellulose may be removed from the modal press lye by precipitation. Precipitation may be carried out by adding an acid to the pressed lye until the pH of the liquor is 4.0 or lower. The temperature of the liquid may be maintained at 40 ℃ or higher, and the liquid should be vigorously stirred. The precipitate can then be removed by filtration.

Thus, in a preferred embodiment of the process according to the invention, the concentration of hemicellulose in the impregnation lye of step a is kept below 20g/l, preferably below 15g/l, by one or a combination of the following methods:

a. subjecting fraction a of the pressed lye from the modal process, the amount of which depends on the S18 value of the slurry, to a membrane separation process, such as dialysis or nanofiltration, thereby obtaining a purified lye fraction and a waste stream, and recycling fraction B back to the impregnation lye system;

b. the purified lye fraction is used as dissolving lye in the viscose process or the modal process, preferably in the same modal fiber production;

c. the spent liquor is concentrated by evaporation and the evaporated residue is incinerated in a soda oven, wherein the formed soda ash is recovered and reused in a neutralization step, preferably in the same modal fiber manufacturing.

Thus, in a preferred embodiment of the method according to the invention, only this amount of modal pressing lye is recycled back to the impregnation lye system to allow the hemicellulose concentration in the impregnation lye to be kept below 20 g/l. Pure caustic soda and water are added to the impregnating lye to maintain the volume, adjust the concentration and control the low hemicellulose concentration.

In a preferred embodiment of the process according to the invention, the further lye consumption process is a process which is less sensitive to the presence of hemicellulose, such as the lye system of the conventional viscose process (which is subsequently treated, if desired, by purification techniques) or the kraft pulping process in a kraft pulp mill or xylan preparation using a hemicellulose-rich modal press lye as a raw material source.

Claims (6)

1. A method of manufacturing modal fibers, comprising at least the following method steps:

a. preparing alkali cellulose from a cellulosic raw material by applying an impregnation lye to the cellulosic raw material, and wherein a pressing lye is obtained by pressing the alkali cellulose,

b. the alkali cellulose is aged and the resultant is then,

c. the mixture is subjected to xanthation and acidification,

d. is dissolved by applying a dissolving lye,

e. curing, filtering, degassing,

f. spinning is carried out on the mixture of the carbon fiber and the carbon fiber,

g. stretching the mixture to obtain a stretched mixture,

h. post-processing (including cutting, drying, opening, packaging),

i. the recovery of the chemicals is carried out by,

wherein in step a measures are taken to obtain alkali cellulose with a narrow DP distribution and a low hemicellulose content.

2. The process according to claim 1, wherein the measures taken in step a are to reduce hemicellulose uptake in the modal fibre process and/or to remove hemicellulose from the lye system by applying suitable separation techniques and/or to transfer a portion of the pressed lye to other lye consuming processes.

3. The method of claim 2, wherein the reducing hemicellulose intake in a modal fiber process is accomplished by using a dissolving pulp with an S18 value below 4% as a cellulosic feedstock.

4. The method according to claim 2, wherein the concentration of hemicellulose in the impregnation lye of step a is kept below 20g/l, preferably below 15g/l, by one or a combination of the following methods:

a. subjecting fraction a of the pressed lye from the modal process to a membrane separation process, such as dialysis or nanofiltration, thereby obtaining a purified lye fraction and a waste stream, and recycling fraction B back to the impregnation lye system;

b. the purified lye fraction is used as dissolving lye in the viscose process or the modal process, preferably in the same modal fiber production;

c. the spent liquor is concentrated by evaporation and the evaporated residue is incinerated in a soda oven, wherein the formed soda ash is recovered and reused in a neutralization step, preferably in the same modal fiber manufacturing.

5. The method according to claim 4, wherein only the amount of modal pressing lye is recycled back to the impregnation lye system to allow keeping the hemicellulose concentration in the impregnation lye below 20 g/l.

6. The method according to claim 2, wherein the other lye consumption method is a method which is less sensitive to the presence of hemicellulose, such as the lye system of the ordinary viscose method or the kraft pulping method in a kraft pulp mill or xylan preparation using a hemicellulose rich modal press lye as a raw material source.