CN112981577A - Method for producing modal fibers completely chlorine-free - Google Patents

Abstract

The invention relates to a method for producing modal fibres completely free of chlorine. In particular, the present invention relates to a method for producing modal fibers, comprising at least the following method steps: a. preparing alkali cellulose from a cellulosic raw material, b. ageing the alkali cellulose, c. xanthating, d. dissolving, e. curing, filtering, degassing, f. spinning using a spinning bath, g. stretching, h. post-treatment (including cutting, washing, bleaching, finishing, drying, opening, baling), i. recovering chemicals, wherein in step a only TCF dissolving pulp is used as cellulosic raw material, and in step h a TCF bleaching process is used which uses a bleaching agent free of chlorine atoms.

Description

Method for producing modal fibers completely chlorine-free

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 fiber fluff 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 titer (titer). 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 fiber fluff through the aftertreatment, during which it is 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 the liquid intake (intake) 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 fibrous fluff 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

All dissolving grade pulps with sufficient purity, reactivity, appropriate viscosity and whiteness in excess of 90% can be used to make modal fibers. To meet this specification, pulp bleaching is generally carried out using a combination of chlorine dioxide (D), hypochlorite (H), oxygen (O) and peroxide (P). The advantage of using such combinations of bleaching chemicals is that they have different effects on delignification, brightness enhancement, purity enhancement and viscosity adjustment. Therefore, properly balancing the input amounts of D, H, O and P is a very efficient and flexible means to obtain high and constant slurry quality.

In the modal fiber process, another bleaching step must be performed during the fiber post-treatment. Hypochlorite or peroxide are commonly used as bleaching chemicals.

However, the use of chlorine, chlorine dioxide or hypochlorite as bleaching chemicals for pulp and cellulose fibers may lead to the formation of chlorinated organic dioxins and furans. These substances, even in trace amounts, are toxic to humans and to the environment and are released into waste water, the atmosphere or retained in the product. For very sensitive applications of cellulose fibres, the consumer needs a product that is completely free of harmful substances.

Chemical analysis of chlorinated organic dioxins and furans (e.g. 2,3,7, 8-TCDD) with detection limits in the ppb (parts per billion) range is very expensive and is only provided by a few specialized laboratories. The sensitivity of the analytical methods employed has become so high that traces of dioxins can be detected in almost every substrate. Therefore, it is impossible for the fiber manufacturer to prove that its product contains no dioxin by chemical analysis.

The use of chlorine-free (TCF) slurries for the manufacture of Lyocell fibers is known, for example, from the disclosure of US 7,175,792B 1, the entire disclosure of which is incorporated herein by reference. However, US 7,175,792B 1 relates to the manufacture of Lyocell fibres, not modal fibres, which is a completely different technique and does not make any mention of post-treatment of the fibres.

The problem of the present invention is therefore to find a way for modal fibre manufacturers to combine the use of TCF pulp with a TCF fibre bleaching process, thereby ensuring that the fibres do not contain chlorinated dioxins and furans.

A problem with the TCF pulp bleaching process is that the use of oxygen-based bleaching chemicals such as ozone, oxygen and peroxide do not provide the same flexibility as chlorine bleaching chemicals in terms of adjusting purity, delignification and viscosity and high brightness. Therefore, it is more difficult to meet the size specifications required for modal fiber production.

Another problem is that the use of oxygen-based chemicals in the fibre stage may no longer be sufficiently effective, since such chemicals are already employed in the pulp stage. However, those fibers need to meet the requirements of the textile industry, for example in terms of whiteness etc.

It has been surprisingly found that this problem can be solved by a process that avoids the use of chlorine bleaching chemicals throughout the pulping and fiber making process. This requires the use of dissolving pulps that have been bleached with oxygen, ozone or peroxide (TCF pulps) and also meet the specifications for modal fiber manufacture in terms of purity, viscosity, reactivity and whiteness.

In the modal fibre bleaching process, the conventional sodium hypochlorite is replaced by a peroxide at a concentration of 1.5-2.5 g/l at a temperature of 60-75 ℃.

Accordingly, it is an aspect of the present invention to provide a method of manufacturing modal fibers, comprising at least the following method steps:

a. preparing alkali cellulose from a cellulose raw material,

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

c. the mixture is subjected to xanthation and acidification,

d. the mixture is dissolved and then is added with water,

e. curing, filtering, degassing,

f. the spinning bath is used for spinning the fiber,

g. stretching the mixture to obtain a stretched mixture,

h. post-treatment (including cutting, washing, bleaching, finishing, drying, opening, packaging),

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

wherein in step a only TCF dissolving pulp is used as the cellulose raw material and in step h a TCF bleaching process is used which uses a bleaching agent free of chlorine atoms. In other words, this means that the cellulose polymer molecules have never been treated with chlorine-containing agents, such as chlorine, chlorine dioxide or hypochlorite, during the whole process from pulp digestion to post-treatment of the spun fibres.

In a preferred embodiment of the invention, the hemicellulose content S18 of the TCF dissolving pulp is less than 4%. S18 represents the amount of slurry soluble in 18% NaOH at 20 ℃. It is determined according to German industrial standard DIN 54356, version 1977-11.

Preferably, the bleaching agent is one or more selected from the group consisting of oxygen, ozone and peroxide; more preferably, it is hydrogen peroxide (H2O 2). In the H2O2 bleaching step, the peroxide concentration may be 1.7. + -. 0.2 g/l, the pH 10.0. + -. 0.3, the temperature 70. + -. 3 ℃ and the residence time about 3.3 min.

In particular, in the bleaching step, 1.5-2.5 g/l, preferably 1.5-1.9 g/l, of peroxide is applied to the fibres at a pH of 9.5-10.5, preferably 9.7-10.3 and a temperature of 60-75 ℃, preferably 67-73 ℃.

Claims (4)

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

a. preparing alkali cellulose from a cellulose raw material,

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

c. the mixture is subjected to xanthation and acidification,

d. the mixture is dissolved and then is added with water,

e. curing, filtering, degassing,

f. the spinning bath is used for spinning the fiber,

g. stretching the mixture to obtain a stretched mixture,

h. post-treatment (including cutting, washing, bleaching, finishing, drying, opening, packaging),

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

characterized in that only TCF dissolving pulp is used as the cellulose raw material in step a, and a TCF bleaching process using a bleaching agent free from chlorine atoms is used in step h.

2. The method of claim 1, wherein the TCF dissolving pulp has a hemicellulose content S18 of less than 4%, S18 representing the amount of pulp that is soluble in 18% NaOH at 20 ℃.

3. The method of claim 1, wherein the bleaching agent is one or more selected from the group consisting of oxygen, ozone, and peroxide; preferably, it is hydrogen peroxide (H2O 2).

4. A method according to claim 3, wherein in the bleaching step 1.5-2.5 g/l, preferably 1.5-1.9 g/l, of peroxide is applied to the fibres at a pH of 9.5-10.5, preferably 9.7-10.3 and a temperature of 60-75 ℃, preferably 67-73 ℃.