WO2012136878A2

WO2012136878A2 - Method for bleaching fibrous material made from atypical raw material

Info

Publication number: WO2012136878A2
Application number: PCT/FI2012/000020
Authority: WO
Inventors: Heikki Hassi; Sakari Siren; Timo Partanen; Juha Isoherranen
Original assignee: Carbatec Oy
Priority date: 2011-04-08
Filing date: 2012-04-10
Publication date: 2012-10-11
Also published as: WO2012136878A3

Abstract

In a method for bleaching fibre material comprising regenerated cellulose fibre made of atypical cellulose-containing raw material, fibres are treated with a bleaching solution containing a peracid. The atypical cellulose raw material is a raw material having a high hemicellulose content and/or containing other impurities, and having a low brightness level, from which viscose is manufactured, which is bleached by a solution that contains a peracid, to achieve a high brightness level.

Description

METHOD FOR BLEACHING FIBROUS MATERIAL MADE FROM

ATYPICAL RAW MATERIAL

Field of the invention

The invention relates to a method for bleaching fibrous material made of cellulose raw material atypical of viscose process, said fibrous material comprising cellulose based regenerated fibres and modified regenerated fibres. Background of the invention

Regenerated fibres can be manufactured by a chemical process from natural raw materials consisting of large molecules, such as cellulose. Such cellulose based regenerated fibres, i.e. regenerated cellulose fibres, include, for example, viscose fibres and other fibres whose manufacturing process comprises the intermediate step of xanthogenation of cellulose, for example modal. The regenerated fibres can also be modified further, for example by chemical treatments. By the modification, it is possible to obtain, for example, special fibres with various functional properties, such as flame-retardant properties, electrical conductivity, or antimicrobicity. Such modified regenerated fibres (special fibres) include, for example, cellulose based hybrid fibres, such as silicate-containing viscose fibres which are poorly inflammable or non-inflammable, i.e. so-called flame-retardant fibres. Conventionally, the manufacturing process of viscose fibres requires the implementation of pre-treatment of the cellulose raw material (pulp raw material), whereby the cellulose is chemically converted to alkali cellulose having a chemical composition typical of viscose fibre process. At the same time, chemical impurities that disturb the viscose manufacturing process, such as low-molecular degradation products of carbohydrate structures, for example hemicellulose, are removed from the pulp raw material used. Alkali pre- treatment of at least one step, i.e. mercerization, for producing alkali cellulose is a necessary partial process for manufacturing viscose fibres. Such an alkali treatment is carried out immediately before passing the alkali cellulose to further processing; the alkali cellulose is not stable so that it could be transported or stored in an intermediate storage. The alkali treatment process can be modified for various purposes. For example, document US 2,859,2 0 presents a method based on two-step alkali treatment for modifying such pulp raw material to be suitable for the manufacturing process of viscose fibre, in which the pulp raw material consists, in practice, solely of lignin-free and chlorine delignified pulp (either unbleached or bleached, depending on the use of the fibre upgraded to the final product), whose manufacturing process, i.e. pulping, has been interrupted to the yield typical of the manufacture of paper pulp (not dissolving pulp). In the document, the two-step alkali treatment is carried out in such a way that the first alkali treatment step is carried out in a targeted manner with a 9.5 to 1 1 % alkali solution (NaOH solution), and the second alkali treatment is carried out in a targeted manner with a 16% alkali solution. In the pre-treatment method described in the publication, there is no numerical mention of the quality of the viscose fibre obtained from the alkali cellulose made by the method. For example, document Fl 41543 presents a method based on two-step alkali treatment of pulp, where the aim is to use raw material typical primarily of the viscose process to make pure alkali cellulose whose xanthogenation is possible with a significantly lower carbon disulphide dose (14 to 24% of the weight of cellulose contained in the alkali cellulose to be sulphurized) than conventionally, the sulphurization temperature being simultaneously significantly higher (35 to 60"C) than conventionally. In the publication, the two-step alkali treatment is carried out in such a way that the alkali cellulose formed after the first alkali treatment is pre-aged, after which at least a second alkali treatment is carried out with a 9 to 11 % alkali solution (NaOH solution). In the pre-treatment method described in the publication, there is no numerical mention of the quality of the viscose fibre obtained from the alkali pulp made by the method.

The process of manufacturing viscose fibres also contains post-treatment steps, such as a bleaching step, for the purpose of, for example, increasing the brightness of the fibre or improving its stability. The bleaching is per- formed either by removing colour-inducing chromophoric compounds or by chemically modifying the structure of said compounds with the bleaching process in such a way that the colour-inducing properties of the molecules weaken or disappear. A generally known way of bleaching various cellulose based regenerated fibres is alkaline oxygen chemical bleaching with hydro- gen peroxide (H₂O₂). Such a method is presented, for example, in document US 5,071 ,439. Brief summary of the invention

It is an aim of the present invention to present an industrially, economically and chemically viable method for bleaching fibre material made of atypical pulp raw materials and comprising regenerated cellulose fibres. Another aim of the invention is to improve the quality, such as the brightness level, of the fibre material after the bleaching, by maintaining the functional properties, such as strength and/or flame retarding properties, of the regenerated cellu- lose fibre at such a level that the fibre material processed by the method of the invention is suitable for its use, taking into account the general commercial quality standards.

To achieve this aim, the method according to the invention is primarily char- acterized in that the fibre material comprising regenerated cellulose fibre made of atypical cellulose-containing raw material is bleached by a method applying a bleaching solution that contains a peracid. Cellulose-containing raw material which are atypical for a viscose fibre process refer to, for example, biomasses and/or pulps that contain natural fibres, such as so-called paper pulp. These atypical raw materials differ from dissolving pulp which is a raw material typical of, for example, the viscose process. Atypical raw materials usually contain impurities, such as lignin residues, a high hemicellulose content, and/or other impurities, such as for example a higher transition metal ion content. They may also have lower brightness, higher degree of polymerization of cellulose, larger molar mass distribution, higher viscosity distribution (between raw material batches). However, their use may be less expensive than the use of a typical raw material, in total costs. A raw material atypical of the viscose process can further be defined more closely in such a way that it is TCF (totally chlorine free) pulp and/or ECF (elemental chlorine free) pulp, preferably TCF pulp, made by either the prehydrolysis sulphate method or the sulphite method. In addition to what has been said above, the ISO brightness of the atypical pulp raw material is <90% (ISO), advantageously <88% (ISO), most advantageously <85% (ISO); and furthermore, the atypical raw material is characterized by a high hemicellulose content, expressed in alkali resistance R18, which is typically <93% and advantageously <90%. Here, the R18 value refers to the part (%) of pulp not dis- solved in an 18% NaOH solution at +20X. The use of atypical raw materials in the manufacturing process of viscose fibres is not obvious, but their use requires the processing according to the invention for utilizing them as raw material for functional viscose fibre products.

The dependent claims will present some preferred embodiments of the method according to the invention.

The atypical raw material may be cellulose-containing raw material which has a low ISO brightness and contains high percentage of hemicellulose and/or other impurities and which has been pre-treated, for example, with an alkali solution and, if necessary, with enzymes. The pre-treatment can also be performed by another method for improving the suitability of the raw material for the manufacture of regenerated cellulose fibres. The pre-treatment can be performed either as a separate processing step before the manufacture of the cellulose-containing regenerated fibre, or in connection with the manufacture of the regenerated fibre. The pre-treatment with an alkali solution may comprise two steps, for example a pre-treatment without an auxiliary treatment between the steps requiring delay times.

The cellulose-based regenerated fibres, made of cellulose raw materials atypical of the viscose process and to be bleached by the method of the invention, may be viscose fibres and modal fibres. The cellulose-based regenerated fibres made of said raw materials may also be modified regen- erated fibres (special fibres), such as cellulose-based hybrid fibres which contain silicate.

The bleaching solution according to the invention may contain, for example, one of the following peracids: peracetic acid, performic acid, or persulphuric acid (Caro's acid). The peracid content of the bleaching solution may range from 0.06 to 2000 mmol/l. The bleaching solution may also contain 2,2'- bipyridin. The bleaching solution may also contain transition metal chelating agents and/or other stabilizers which are compatible with the peracids. The pH of the bleaching solution may range from 1 to 11 , advantageously from 2 to 11 , and, when peracetic acid is used, preferably from 5 to 7. The temperature of the bleaching solution may range from 20 to 100Ό, advantageously from 50 to 70Ό. The final bleaching result to be a chieved can be advantageously intensified, particularly in the case of other than silicate-containing fibre materials, by raising the pH value of the fibre material with an alkaline solution in a so-called post-bleaching step after the bleaching. For example, when peracetic acid is used, the pH is increased from a pH level of 5 (buffer level) to a pH level of for example 6 to 8, to activate the bleaching chemical residues contained in the fibre material, this process step advantageously resulting in enhanced final bleaching result. The pH value of the fibre material to be treated before the bleaching may range from 1 to 14, advantageously from 7 to 14, and, when peracetic acid is used, preferably from 8 to 9.

The bleaching method according to the invention can be carried out in a con- tinuous process.

Description of the drawings

In the following, the invention will be described with reference to the appended drawings, in which

Fig. 1 shows an embodiment of the method according to the invention in a chart,

Fig. 2 shows the general principle of a pre-treatment process belonging to the method according to invention,

Fig. 3 shows an embodiment of the pre-treatment process of Fig. 2 in a chart, and

Fig. 4 shows bleaching results obtained during the manufacturing process of viscose fibre as the raw materials and bleaching methods vary.

Detailed description of the invention

In this description and claims, the percentage values are percentages by weight unless otherwise indicated, and the symbol > before a numerical val- ues refers to "higher than'V'above", and the symbol < refers to "lower thanVbelow".

In the invention, it has been found that a problem with regenerated cellulose fibres made of atypical raw materials, and with the fibre material comprising them, is the insufficient bleaching effect of conventional alkaline hydrogen peroxide bleaching. In the invention, it has further been found that a problem with the hydrogen peroxide bleaching of said regenerated cellulose fibres and cellulose-based special fibres is the deterioration of properties of the fibres during the bleaching treatment. For example, the strength of viscose fibres made of atypical raw materials and intended for textile use deteriorates in alkaline hydrogen peroxide bleaching. In addition, silicate-containing viscose fibres have the problem of the silicate dissolving from the fibres during the bleaching treatment under optimal conditions for alkaline hydrogen per- oxide bleaching. The dissolving of the silicate is reflected, among other things, in the poorer flame retardant properties of the fibre. The bleaching method according to the invention can be used for bleaching various fibre materials, particularly fibre material that contains regenerated cellulose fibres. Such regenerated cellulose fibres include, for example, viscose and modal fibres, that is, fibres whose manufacturing process involves the intermediate step of xanthogenation of cellulose and the regeneration of cellulose.

It is obvious to use dissolving pulp as raw material for regenerated cellulose fibres. Compared with so-called paper pulp, dissolving pulp is characterized by a low viscosity level with low distribution (also from one pulp batch to another), a high brightness level (for example >90% (ISO)), and a low content of impurities, for example a low residual lignin content, a low hemicellu- lose content (R18: >93%), as well as a low content of pitch and resin com- pounds and a low content of transition metal (inter alia Fe, Mn) ions.

The method according to the invention is most advantageous when it is applied for bleaching such regenerated fibres whose raw material consists of biomasses and/or pulps that contain natural fibres, such as so-called paper pulp. Such raw materials can also be called atypical raw materials or atypical cellulose-containing raw materials, for example atypical raw materials for a viscose process. The origin of the natural fibres may be, for example, hardwood and/or softwood, sugar cane bagasse, straw, other fibre-containing agricultural side products, reed plants, such as common reed, reed canarygrass, bamboo, or other useful sources of biomass. Such atypical raw materials usually contain impurities, such as lignin residues, a high hemi- cellulose content, and/or other impurities, such as, for example a higher transition metal ion content. They may also have lower brightness, higher degree of polymerization of cellulose, larger molecular mass distribution, higher viscosity distribution (between raw material batches). However, their use may be less expensive than the use of a typical raw material, in total costs. A raw material atypical of the viscose process can further be defined more closely in such a way that it is typically TCF (totally chlorine free) pulp and/or ECF (elemental chlorine free) pulp, preferably TCF pulp, made by either the pre- hydrolysis sulphate method or the sulphite method. In addition to what has been said above, the ISO brightness of the atypical pulp raw material is <90% (ISO), advantageously <88% (ISO), most advantageously <85% (ISO); and furthermore, the atypical raw material is characterized by a high hemi- cellulose content, expressed in alkali resistance R18, which is typically <93% and advantageously <90%. The use of atypical raw materials in the manu- facturing process of viscose fibres is not obvious, but their use requires the processing according to the invention for utilizing them as raw material for functional viscose fibre products.

In addition to what has been presented above, atypical cellulose-containing raw materials and their use as raw material in a viscose fibre process by the method according to the invention, as well as the final result to be obtained, can be characterized, in comparison with a conventional process based on the application of dissolving pulp, in an example hereinbelow. Table 1: Comparison between typical and atypical cellulose raw material for a viscose process. The figures 1 and 2 in connection with the alkali pre-treatment refer to the first and second treatment step, respectively. PPV = paper pulp viscose.

Typical raw material Atypical raw material

Dissolving Pulp PPV + Peroxide PPV + Peracetic acid

PULP (INPUT) Soft wood, sulphite Soft wood, sulphite

Alkaline solubility (S18) <6 % 11-12 %

ISO Brightness >91 % (ISO) 81-85 % (ISO)

Polydispersity index 11 17-19

ALKALI TREATMENT

Lye concentration (NaOH) 215 g/l (17%) 1. 215 g/l (17%), 2. 180 g/l (15%)

Hemicellulose concentration <30 g/l (<2.5%) 1. <70 g/l (<5.5%), 2. <20 g/l (<1.5%)

Lignin concentration n.d. 1. 3 g/l (0.25%), 2. n.d.

AFTER AGEING

Alphacellulose content in

alkaline pulp 32-34 % 32-34 %

Total alkali in alkaline pulp 15 % 15 %

XANTHOGENATION

Carbon disulphide addition

( % of alphacellulose) 32-33 % 32-33 %

AFTER XANTHOGENATION

Polydispersity index of

regenerated viscose (dope) 4,1 4.0-4.2

kW-index (filterability) 50-100 100-150

BLEACHING STAGE

(1.3 dtex 38 mm textile)

Concentration of bleaching bath 2.0 g/l (0.2 %) H₂0₂ 2.0 g l (0.2 %) H₂0₂ <1.0 g/l (<0.1 %, <13mmol/l) PAA

Temperature of bleaching bath >80 °C >80 °C 70 °C

pH-value of bleaching bath 10-11 10-11 5-6

AFTER BLEACHING

(1.3 dtex 38 mm textile)

CIE Whiteness (fibre) >60 <53 >58

Tenacity (fibre) >2.8 cN/dtex <2.6 cN/dtex >2.8 cN/dtex

Figure 4 shows bleaching results obtained in test runs during the manufacturing process of the viscose fibre (fibre 1.3 dtex 38 mm) as the raw materials and bleaching methods vary. The horizontal axis represents successive production batches and the vertical axis the CIE whiteness obtained. The indicated areas show where the measurement results concentrated. When alkaline hydrogen peroxide bleaching was used for viscose fibre made in a viscose process, the whiteness reduced clearly when the cellulose raw material was changed from typical raw material (dissolving pulp) to atypical one (paper pulp) which had been subjected to alkali pre-treatment (grey areas). When the bleaching method of the viscose fibre made of the latter raw material was changed from alkaline hydrogen peroxide bleaching to peracid bleaching (peracetic acid), the CIE whiteness increased clearly again (white areas).

Atypical cellulose-containing raw materials can be pre-treated by, for example, alkali solution treatment before the raw material is passed to the actual manufacturing process of regenerated cellulose fibre, such as viscose manufacture. The aim of the alkali solution treatment is primarily to reduce the content of alkali soluble impurities, such as hemicellulose and lignin residues, in the cellulosic raw material to such a level that the alkali cellulose pre- treated by the alkali solution and formed in the process can be applied as raw material in a viscose fibre process. The alkali solution treatment can be performed either as a separate step before the manufacture of the cellulose- containing regenerated fibre, or in connection with the manufacture of the regenerated fibre. As a limiting factor, it should be taken into account that alkali cellulose cannot be transported or stored in an intermediate storage. The atypical raw material can also be pre-treated by enzymes or by another method to improve the applicability of the raw material in the manufacture of cellulose-based regenerated fibres. By the pre-treatment, it is possible, for example, to remove impurities from the raw material, to increase the alpha cellulose content and/or to reduce the hemicellulose content of the raw material. The pulps can be pulps made by chemical production methods and known as raw materials for paper manufacture, such as prehydrolyzed sul- phate cellulose and/or sulphite cellulose. If bleached pulp is used as the raw material, it can be totally chlorine free (TCF) pulp and/or elemental chlorine free (ECF) pulp, preferably TCF pulp. As the raw material for fibre material that comprises regenerated cellulose fibre, it is also possible to use mixtures consisting of two or more different cellulose-containing raw materials. For the alkali solution treatment of atypical raw material, it is advantageous to select such a technical embodiment where the circulation of the alkali solutions is arranged in a way by which the waste consumption of alkali is minimized so that the contents of pulp-based impurities accumulating in the alkali solutions do not rise to a level that would disturb the viscose fibre process. Some embodiments with this aim will be presented hereinbelow.

Figure 2 shows an advantageous alkali solution treatment which can be performed for said atypical raw material before the manufacture of the viscose fibre and the bleaching of this fibre by a peracid containing bleaching solu- tion. The pre-treatment process is a two-step method for pre-treating hemi- cellulose-containing cellulosic raw material, a so-called purification dissolution method, in which the raw material is treated in the first step (pulper I) by an alkali solution (lye I), after which the alkali solution is separated mechanically (press) from the raw material, and the alkali treated raw material (AC I) thus obtained is led directly to the second treatment step (pulper II) where it is treated by an alkali solution (lye II), after which the alkali solution is separated mechanically again (press). The pre-treated raw material (AC II) thus obtained is led to the manufacturing process. Part of the alkali solution separated in the second step is led to the first step, to be used therein.

The method shown in the chart of Fig. 3 on an embodiment of the pre-treatment process begins by the first treatment step of cellulose-containing raw material (2). The cellulose-containing raw material (2) is led to a reaction vessel (1), to which it is also possible to add auxiliary agents (4), such as surfactants for moistening the cellulose, to improve the alkalization. In the first treatment step, the cellulose-containing raw material is treated by an alkali solution (7) which is preferably a sodium hydroxide solution. The alkali solution (7) used in the first step may be taken from the other steps of the process, such as the second treatment step. If the alkali solution separated after the second processing step is recycled to be used as the alkali solution in the first treatment step, this two-step treatment process is a so-called countercurrent process. If necessary, the concentration of the alkali solution recycled from the process can be increased (9) before it is used as alkali solution in the first treatment process. The first treatment step comprises successive reaction containers and devices (1 , 3, 5). At the end of the first treatment step, alkali solution (17) is removed from the raw material, for example by pressing (5).

This is followed immediately by the second treatment step, in which a second batch of alkali solution (72) is added to the raw material (22) which has gone through the first treatment step. If necessary, the concentration of the alkali solution (72) can be increased (92) before it is added to the second treatment step. In the second treatment step as well, it is possible to add auxiliary agents (42), such as surfactants for moistening the cellulose, or a cellulose pre-ageing catalyst. The second treatment step also comprises successive reaction containers and devices (12, 33, 52). At the end of the second treatment step, the raw material (22) is led to a press (52), where alkali solution is removed from it. After the second treatment step, the raw material (32), which is called pre-treated raw material, can be led to other processes, for example production steps required in the manufacture of cellulose-based products, such as films, fibres, granules or other corresponding products. Part of the alkali solution (172) pressed out can be recycled further to different treatment steps of the method or to processes outside the method shown in the chart, such as to a post-sulphurization dissolution step in viscose manufacture. The alkali solution (172) to be recycled as alkali solution for the second treatment step can be collected into a container (82), to which all the fresh alkali solution (6) to be used is also added. The alkali solution removed in connection with the second treatment step is filtered (10) before the recycling of the solution to the first treatment step or the removal from the process to a further use (173). When the alkali solution removed from the pro- cess is re-used, for example, in viscose manufacture, it is advantageous that the solution has been treated, for example by filtering and centrifuging, to remove insolubles. The filtered alkali solution (174) can be collected to a container (8), to which also the alkali solution ( 7) pressed out in connection with the first step can be collected. From the container (8), the alkali solution can be recycled further to the first treatment step, and part (175) of it can be removed for another purpose, for example for a post-sulphurization dissolv- ing step in the viscose process. In this case it is advantageous to remove insolubles from said alkali solution, for example by filtering and centrifuging.

According to one embodiment, it is possible to apply mechanical treatments between the pre-treatment steps.

In the first treatment step of the method according to the invention, the concentration of the NaOH solution may be 8 to 25%, advantageously 16 to 21 %, and in the second treatment step 8 to 25%, advantageously 10 to 16%. The concentrations are adjusted suitable, if necessary, by adding water and sodium hydroxide. The hemicellulose content of the alkali solution separated from the treated raw material may be about 2.5 to 6% in the first step and about 0.5 to .5% in the second step. When the treatment is carried out, for example, by applying the above-presented sodium hydroxide concentrations (in the second step, a lower effective sodium hydroxide concentration is advantageously applied than in the first step), an optimal alkali cellulose (ratio between the alpha cellulose content and the total alkali content of the alkali cellulose) is obtained, which can be led to the cellulose xanthogenation step and used further for preparing a cellulose xanthogenate solution meeting the quality standards for a viscose fibre process, for example a cellulose xanthogenate solution having typical filterability properties. In the above-described pre-treatment process, alkali soluble impurities, such as hemicellulose, which are harmful for the further manufacturing process, are removed from the original cellulose raw material. Also other impurities, such as hemicellulose degradation products, can be removed from the raw material. For example, for the production of viscose, it is advantageous to remove the hemicelluloses and their degradation products, because they disturb the viscose production process by causing, among other things, a higher consumption of process chemicals, or lead, for example, to uncontrollable behavior, such as uncontrollable ageing, of the cellulose xanthogenate solution. The above-described pre-treatment process may imme- diately precede the viscose process; in other words, the pre-treated raw material obtained from the second step is led to ageing and thereafter to sul- phurization i.e. xanthogenation with carbon disulphide and regeneration in a suitable bath.

By this pre-treatment process, it is possible to increase the alpha cellulose content of the raw material to a desired level before the manufacturing process and to reduce the hemicellulose content which would otherwise be too high. Also other impurities, such as hemicellulose degradation products and lignin residues, can be removed from the raw material by the pre-treatment process.

An advantage of the pre-treatment of the raw material according to the invention is, among other things, the intensification of sulphurization in connection with further manufacturing, such as a viscose process, which is detected as the formation of a cellulose xanthogenate solution of typical quality, using e.g. filterability and particle content as indicators.

By the alkali treatment of atypical raw materials, beneficial alkali cellulose is provided in view of chemistry of viscose manufacturing process, particularly consumption of chemicals, but at the same time, compounds are produced which cause problems with the brightness of the final product (viscose fibre) which have remained unsolved by common methods for bleaching viscose fibre.

The atypical raw materials which have been pre-treated before the manu- facturing process of the regenerated fibre, typically contain such chromo- phoric compounds which cannot be removed in a sufficiently efficient way under optimal conditions of alkaline hydrogen peroxide bleaching. These chromophoric compounds which are resistent to hydrogen peroxide bleaching can be formed, for example, of impurities in the cellulose raw material during the pre-treatment and/or the alkali treatment step (mercerization). For this reason, the use of said atypical raw materials may lead to low brightness of the fibre material and thereby also cause a significant quality problem. The low brightness of the fibre material may also be due to the lower intrinsic brightness level of atypical raw materials compared with dissolving pulp. The bleaching method is also particularly suitable for bleaching of modified regenerated fibres made of atypical cellulose raw materials, such as alkali labile special fibres. Such fibres include cellulose-based hybrid fibres, for example silicate-containing regenerated fibres, such as silicate-containing viscose and modal fibres. Silicate-containing viscose fibres refer to cellulose/silica hybrid fibre made by the viscose process and to which waterglass has been added in connection with the manufacture of the viscose fibre, to provide flame retarding properties; that is, the fibres are poorly inflammable or non-inflammable. Such fibres may also be called multicomponent products or fibres. The content of silicon dioxide (Si0₂) may be 5 to 50 weight% and advantageously about 20 to 40 weight%, calculated of the dry substance of the fibre. The modification can be performed, for example, by substances such as waterglass to be added before the regeneration, to provide the fibre with a flame retarding property.

In addition to the bleaching of the fibre material, the method of the invention can also be used for the bleaching of other regenerated cellulose-based products, such as films, granules or other solids, in whose manufacture an atypical cellulose-containing raw material has been used. These products may also be chemically further modified, for example to provide flame retarding properties.

The bleaching of the fibre material, which is one of the finishing steps of fibre manufacture, can be carried out after the spinning of the fibres. Figure 1 shows a chart on viscose spinning and the manufacture of fibres. The solution entering the spinning step, i.e. spinning viscose (202), is either cellulose xanthogenate solution as such, advantageously post-aged and filtered, or a multicomponent solution containing an alkaline solution of viscose and silicon dioxide. The manufacture continues either on route A to filaments, i.e. con- tinuous fibres, or on route B to staple fibres. The spinning viscose or the multicomponent solution (202) made of it can be, for example, pressed through a nozzle to one or more fibres in a spinning bath 204, 225 in an acidic regeneration solution, i.e. in a spinning solution used as a spinning bath, in which for example solid cellulose fibres are formed, or in the case of spinning a multi- component solution, cellulose fibres containing silicon dioxide in the form of silicic acid are formed. The nozzle can be selected so that the manufacture of films or other multicomponent products is also possible. The solidification of the fibres or other products thus takes place automatically so that the acidic regeneration solution is immediately on the outlet side of the nozzle. The products formed in the spinning bath, preferably filament fibres, are collected through the nozzle around draw rollers and are drawn 227. By stretching, it is possible to increase the strength of the fibers. The fibres can also be washed in connection with or after the drawing 206. When producing staple fibres, the fibres are cut 229 to a desired length after the drawing. Next, the fibres are bleached 208, 231 by applying the bleaching method according to the inven- tion. Before the bleaching, the pH of the fibre material is adjusted to a desired value. After the bleaching, the fibres can also be treated with various finishing agents 233, after which they are dried 210, 235 and collected, for example as filaments 2 2 or staple fibres 237. In the method according to the invention for bleaching fibre material, the bleaching chemical used in the bleaching solution is a peracid, such as peracetic acid (CH₃COOOH). A corresponding final result can also be achieved with other peracids, such as performic acid or persulphuric acid (Caro's acid). The content of the peracid, such as peracetic acid, in the bleaching solution may range, for example, from 0.06 to 2000 mmol/l.

The bleaching solution may contain distilled peracetic acid, or an equilibrium mixture of peracetic acid containing hydrogen peroxide and acetic acid. The peracetic acid used as the bleaching chemical can also be made enzymati- cally or by means of activator chemicals, such as tetraacetylethylenediamine (TAED) or pentaacetyl glucose, or from hydrogen peroxide and acetanhy- drice in the process in situ. The bleaching can also be performed, for example, as peracetic acid bleaching catalyzed by 2,2'-bipyridin. In the bleaching solution containing peracetic acid, it is also possible to use stabilizing agents compatible with peracids, such as chelating agents of transition metals, and other stabilizers. The bleaching solution may for example contain a preparate that contains, for example, chelating agents of transition metal ions which catalyze the decomposition of peracetic acid and hydrogen peroxide, and/or other stabilizers, such as tetrasodium pyrophosphate or aspartic acid dieth- oxy succinate (AES). The chelating agents can also be added in a separate pre-chelating step before the actual bleaching treatment. In the method according to the invention, the pH of the bleaching solution can range from 1 to 1.1 . Advantageously, the pH of the solution is between 2 and 1 1. When using peracetic acid, the pH of the solution is preferably between 5 and 7. The pH of the solution can be, for example, between 5.0 and 5.5. The pH of the bleaching solution refers to the pH of the solution prepared for the bleaching process. The temperature of the bleaching solution may be, for example, between 20 and 100 , advantageously betwe en 50 and 70 . The final bleaching result of the fibres can also be influenced by one or more of the following parameters: the pH value of the fibre material, such as a fibre mat formed of several single fibres, for example the fibre/water suspension moving on a wire, before the bleaching; the peracid content or the joint effect of the contents of hydrogen peroxide and peracid in the bleaching solution; the temperature of the fibre material before the bleaching; the retention time of the bleaching; the circulating solution flow rate of the bleaching; the content of chelating agent and/or stabilizer in the bleaching solution. The pH value of the fibres to be treated, such as a fibre mat formed of fibres, before the bleaching may be, for example, between 1 and 14, advantageously between 7 and 14, and, when peracetic acid is used, preferably between 8 and 9. The pH of the fibre material to be treated can be adjusted to a target value for example with sodium hydroxide or sodium carbonate. The peracetic acid content of the bleaching solution may be, for example, about 6 mmol/l. The temperature of the fibre mat may be, for example, about 80 .

The final bleaching result to be achieved can be advantageously intensified, particularly in the case of other than silicate-containing fibre materials, by raising the pH value of the fibre material with an alkaline solution in a so- called post-bleaching step after the bleaching. When peracetic acid is used, the pH is increased from a pH level of 5 (buffer level) to a pH level of for example 6 to 8, to activate the bleaching chemical residues contained in the fibre material, this process step advantageously resulting in an enhanced final bleaching result. Compared with e.g. alkaline hydrogen peroxide bleaching, the bleaching method according to the invention has the advantage that it is possible to adjust the pH and the temperature of the solution to such a range that the strength values and functionality of the fibres, for example the flame retarding property, are maintained at a desired level without impairing the bleaching efficiency of the solution. It has been found that under optimal bleaching con- ditions, such as in the neutral pH range, peracetic acid does not react with the carbohydrate structures of the fibres. In addition, there is no significant need for neutralization of the fibre bleached by the method, such as neutralization with an organic acid made after alkaline hydrogen peroxide bleaching. By the method, it is thus possible to obtain cost savings as well.

Total cost savings can also be obtained in that by the method it is possible to manufacture regenerated fibre of good quality, such as viscose fibre, from atypical raw materials, whose raw material costs are substantially lower than when, for example, dissolving pulp is used.

By the method according to the invention, it is possible to intensify the bleaching of the fibre material and to improve the brightness level of the fibre material. The brightness of the fibre material, which can be measured, for example, as CIE whitness according to the standard IS011475, or as ISO brightness according to the standard ISO3688/ISO2470, can increase by even more than 10 units with respect to the brightness of fibre material obtained by conventional alkaline hydrogen peroxide bleaching. However, the brightness level that can be obtained is tied to the manufacturing method and brightness level of the pulp raw material used.

By the bleaching method according to the invention, it is possible to efficiently destroy the chromophoric compounds (either originating from the pulp raw material or formed during the alkali treatment of atypical raw material) which are formed in for example viscose fibre and which are resistant to alkaline hydrogen peroxide bleaching. This may be due to, for example, the higher oxidation potential of peracids than hydrogen peroxide, or the combined action of nucleophilic and electrophilic species of the peracid under optimal bleaching conditions in the bleaching solution. Another advantage of the method is less fibre damage in connection with the treatment, wherein the fibre strength can be maintained or even increased compared with the strength of the corresponding product processed in an optimal alkaline hydrogen peroxide bleaching process.

By using a bleaching solution that contains peracid it is possible, for example, to reduce or prevent the degradation of the carbohydrate structure of the fibres and thereby to maintain the sufficient tensile strength of the fibres. It has been observed that non-selective bleaching, such as alkaline hydrogen peroxide bleaching, destroys the carbohydrate structures of fibres an impairs the tensile strength of the fibres.

By bleaching according to the method, it is also possible to reduce the residual sulphur content of the fibres, for example in connection with the manufacture of viscose fibres. Under the pH conditions of peracid bleaching, better chemical permeability is achieved for the fibre mat to be bleached and thereby a more efficient interaction of the bleaching chemical with the fibres, than under conditions of alkaline (caustic) hydrogen peroxide bleaching. Furthermore, the bleaching method according to the invention has the advantage that, for example, the dissolution of silicate from modified regenerated fibres, such as silicate-containing viscose fibre, is substantially prevented or reduced, wherein the functionality, i.e. the flame-retarding property, of the fibre is retained without a significant compensation for the silicate loss.

By restricting the dissolution of silicate from the fibres to the bleaching solution, it is also possible to reduce the accumulation of precipitates in the bleaching circulations. The silicate load on waste waters can also be reduced. This will also increase the total cost savings achieved with the method.

The bleaching process according to the invention can be implemented with few changes in the existing bleaching apparatuses. It can also be easily applied in already existing plants for manufacturing regenerated fibre, apply- ing an alkaline hydrogen peroxide bleaching process. The bleaching can also be performed in a continuous process. The bleaching reaction time in the continuous process is, for example, about 4 minutes.

The invention is not restricted to the embodiment examples above, but it may vary within the scope of the claims.

Claims

Claims:

1. A method for bleaching fibre material comprising regenerated cellulose fibre made of atypical cellulose-containing raw material, characterized in that the fibre material is treated with a bleaching solution containing a per- acid.

2. The method according to claim 1 , characterized in that the atypical cellulose-containing raw material is a raw material having a high hemicellulose content and/or containing other impurities, and possibly also having a low brightness level, said raw material having been pre-treated by an alkali solution treatment.

3. The method according to claim 1 or 2, characterized in that the regener- ated cellulose fibres are viscose or modal fibres.

4. The method according to any of the preceding claims, characterized in that the regenerated cellulose fibres contain silicate.

5. The method according to any of the preceding claims, characterized in that the bleaching solution contains at least one of the following peracids: peracetic acid, performic acid, or persulphuric acid.

6. The method according to any of the preceding claims, characterized in that the bleaching solution contains 2,2'-bipyridin.

7. The method according to any of the preceding claims, characterized in that the bleaching solution contains chelating agents of transition metals and/or other stabilizers compatible with peracetic acids.

8. The method according to any of the preceding claims, characterized in that the peracid content is 0.06 to 2000 mmol/l.

9. The method according to any of the preceding claims, characterized in that the pH of the bleaching solution is between 1 and 12, advantageously between 2 and 11 , most advantageously between 5 and 7.

10. The method according to any of the preceding claims, characterized in that the pH value of the fibre material to be treated, before bleaching, is between 1 and 14, advantageously between 7 and 14, preferably between 8 and 9.

11. The method according to any of the preceding claims, characterized in that the temperature of the bleaching solution is 20 to 100 , advantageously 50 to 70 .

12. The method according to any of the preceding claims, characterized in that the bleaching is performed in a continuous process.

13. The method according to any of the preceding claims 2 to 12, charac- terized in that the hemicellulose-containing cellulose raw material is treated by an alkali solution treatment, in which hemicellulose is removed from it by treating it at least twice in succession with an alkaline solution, and in which the alkaline solution separated after the second treatment step is recirculated to the first treatment step.

14. The method according to claim 13, characterized in that after the first treatment step, the alkaline solution is separated from the raw material for example by pressing.

15. The method according to claim 13 or 14, characterized in that the alkaline solution separated after the first treatment step is recirculated to the treatment of the first step together with the alkaline solution separated after the second step.

16. The method according to any of the preceding claims, characterized in that the cellulose raw material consists of chemical pulp, such as pre-hydro- lyzed sulphate cellulose or sulphite cellulose.

17. The method according to claim 16, characterized in that the cellulose raw material is either ECF or TCF pulp, advantageously TCF pulp.

18. The method according to any of the preceding claims 13 to 17, characterized in that the sulphurization of the alkali cellulose is performed in a way known for a viscose process, the sulphurization temperature being advantageously <35 and the carbon disulphide dose used i n the sulphurization being advantageously >25% of the weight of the cellulose contained in the alkali cellulose to be sulphurized.

19. The method according to any of the preceding claims 13 to 18, characterized in that the NaOH content of the alkaline solution is 8 to 25%, advan- tageously 16 to 21 %, in the first treatment step, and 8 to 25%, advantageously 10 to 16%, in the second treatment step.

20. The method according to any of the preceding claims, characterized in that the alkali resistance R18 of the atypical cellulose raw material is <93%.

21. The method according to claim 20, characterized in that the alkali resistance R18 of the atypical cellulose raw material is below 90%.

22. The method according to claim 20 or 21 , characterized in that the ISO brightness of the atypical cellulose raw material is <90 % (ISO), advantageously <88 % (ISO), most advantageously <85 % (ISO).