FIELD

[0001] The present invention relates to a method of producing cellulose carbamate filaments or fibres, including chemical recycling of the chemicals.

BACKGROUND

[0002] Cellulose is the most widely used biopolymer in the world and it finds applications in various industries such as the paper and cardboard industry, and the textile industry as both cellulose and derivatives of cellulose, such as cellulose carbamate, cellulose acetate, ethers and esters. The use of cellulose in these industries, however, requires the cellulose fibre to be dissolved in order to regenerate the structure. The dissolution of cellulose is difficult because of its semi-crystalline structure, strong hydrogen bonds in the polymer sheets and the presence of both hydrophilic and hydrophobic ends in the polymer. For this reason, cellulose requires either a modification of the structure by derivatization or an opening of the fibres and an increase in reactivity.

[0003] Several methods of derivatization are known in the art, the most well-known perhaps being the viscose process, in which cellulose is first treated with an alkali and then with carbon disulphide to produce cellulose xanthate. The viscose process is shunned more and more due to the toxic nature of carbon disulfide and the undesirable environmental impact that the viscose process has. More and more, other derivatization processes are being turned to, particularly those that are inexpensive, do not have the toxicity issues and environmental issues of the viscose process and yet still can be carried out using the available infrastructure, e.g. can be carried out in viscose wet spinning mills.

[0004] The carbamation process for the production of cellulose carbamate is one such derivatization process. Cellulose carbamate is formed by reacting cellulose with urea. Methods of producing cellulose carbamate are described e.g. in Finnish Patents Nos. 112869 and 112795. [0005] After dissolution of the cellulose carbamate in a mixture of an alkaline agent, such as sodium hydroxide, and zinc oxide, the cellulose carbamate dope thus formed is fed into a spin bath for coagulation of the cellulose carbamate into filaments. In some embodiments, coagulation is carried out in an acidic spin bath containing sulphuric acid and aluminium sulphate. Such an acidic spin bath typically contains in dissolved phase zinc oxide as zinc ions along with ammonium salts or ions (ammonium nitrogen), aluminium salts or ions, and sodium sulphate.

[0006] Conventionally the spin bath is provided with recycling of chemicals that are used for dissolution and coagulation of the cellulose carbamate. There is still a need to provide for efficient recycling of the chemicals.

SUMMARY OF THE INVENTION

[0007] It is an aim of the present invention to provide a method of producing cellulose carbamate filaments or fibres of good quality in a spinning cycle in which the chemical recycling is optimized, i.e. preferably sodium sulphate is crystallized as pure as possible (avoiding co-crystallization of zinc, aluminium and nitrogen) and zinc is removed by extraction as efficiently as possible (avoiding co-extraction of aluminium). Further, coagulation or spin bath circulation system is kept in as closed-loop as possible i.e. a semi closed loop, thus avoiding emissions to wastewaters. In addition, water balance of the semi closed loop system is preferably kept constant by removing water entered to system via cellulose carbamate spinning dope, preferably by evaporation.

[0008] According to a first aspect of the present invention, there is provided a method of producing cellulose carbamate filaments or fibres, comprising the steps of providing a cellulose carbamate dope containing cellulose carbamate dissolved in aqueous sodium hydroxide, said cellulose carbamate dope further exhibiting a dissolved zinc compound, feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath containing sulphuric acid and a dissolved aluminium compound, coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments, subjecting the cellulose carbamate filaments to stretching and optional washing, and optionally cutting the filaments into fibres, withdrawing a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions, subjecting the withdrawn portion of the spin bath to regeneration by removing at least a part of the sodium sulphate, water and zinc ions, and recycling at least a part of the regenerated aqueous spin bath to the spinning unit. Preferably, the withdrawn aqueous spin bath contains, at a temperature of at least 10 °C, before the regeneration dissolved nitrogen in the form of ammonium nitrogen in an amount of less than 20 g/1, as ammonium sulphate, of the spin bath.

[0009] The withdrawn portion subjected to regeneration may also comprise withdrawn portions of aqueous stretching and/or washing baths.

[0010] The invention is defined by the features of the independent claim. Some specific embodiments are defined in the dependent claims.

[0011] Embodiments of the present invention enable production of high quality cellulose carbamate filaments or fibres combined with efficient chemical recycling. Thus, in the present invention the spinning process achieves high level chemical recycling without compromising the fibre properties.

BRIEF DESCRIPTION OF THE DRAWING

[0012] FIGURE 1 illustrates a method of producing cellulose carbamate filaments or fibres, the method also including a spin bath regeneration process, in accordance with at least some embodiments of the present invention.

EMBODIMENTS

[0013] Embodiments of the present technology provide an improved method of producing cellulose carbamate filaments or fibres. The method includes chemical recycling. In various embodiments, the method of producing cellulose carbamate filaments or fibres has been made sustainable by significantly improving the efficiency of the chemical recycling of the spinning process.

[0014] Embodiments provide cellulose carbamate filaments or fibres having a good quality, the aim is to obtain fibres resembling cotton fibres as much as possible. [0015] Embodiments comprise providing a cellulose carbamate dope, feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath, coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments, subjecting the cellulose carbamate filaments to stretching in one or more stretching units and optional washing in one or more washing units, and optionally cutting the filaments into fibres, whereby cellulose carbamate filaments or fibres are obtained. Further, embodiments comprise chemical recycling. Thus, a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions is withdrawn from the spin bath and subjected to regeneration by removing at least a part of the sodium sulphate, water and zinc ions.

[0016] Thus, according to one embodiment, the method of producing cellulose carbamate filaments or fibres, comprises the steps of

- providing a cellulose carbamate dope containing cellulose carbamate dissolved in aqueous sodium hydroxide, said cellulose carbamate dope further exhibiting a dissolved zinc compound;

- feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath comprising sulphuric acid and aluminium sulphate;

- coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments;

- subjecting the cellulose carbamate filaments to stretching and optional washing, and optionally cutting the filaments into staple fibres;

- withdrawing a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions, the withdrawn aqueous spin bath having a temperature of at least 10 °C;

- subjecting the withdrawn portion of the spin bath to regeneration by removing at least a part of the sodium sulphate, water and zinc ions, and

- recycling at least a part of the regenerated aqueous spin bath to the spinning unit.

[0017] According to a preferred embodiment, the regeneration of the withdrawn portion of the spin bath (and/ or stretching bath and/ or washing bath) comprises at least a zinc removal step, preferably by zinc extraction, and a sodium sulphate removal step, preferably by sodium sulphate crystallization. Further, the regeneration may comprise at least one separation step, such as filtration, and/or a water removal step, for example by evaporation. In addition to water removal by evaporation, at least part of water is preferably removed from spin bath by sodium sulphate decahydrate crystals. Typically, at least a part of the regenerated aqueous spin bath is recycled back to the spinning unit.

[0018] According to one embodiment, part of the withdrawn portion of the spin bath

(and/ or stretching bath and/ or washing bath) subjected for regeneration can bypass one or more of the above-mentioned steps of the regeneration cycle. For example, part of the filtrated spin bath can be recycled back to the spinning unit without subjecting it to the zinc extraction, water evaporation and/ or sodium sulphate removal steps.

[0019] According to a preferred embodiment, the method of the present invention is a continuous process, i.e. cellulose carbamate dope is fed and processed continuously in the spinning unit.

[0020] According to one embodiment, filaments or fibres are obtained by the present method. According to a preferred embodiment, the filament product of the present method is a continuous yam that is obtained after stretching, and optional washing, of the coagulated filaments. Fibre products of the present method are, in turn, obtained by making a tow of filaments from the coagulated filaments and stretching and cutting such a tow of filaments into fibres, especially into staple fibres or shortcuts. In particular, staple fibres are obtained by cutting the tow of filaments into staple fibres, after which step they are washed, after-treated and dried. According to another embodiment, staple fibres can be obtained by washing and after-treating the tow of filaments as such, and then cutting it in wet or dry state into staple fibres or shortcuts.

[0021] In a first step of an embodiment, a cellulose carbamate dope is provided, containing cellulose carbamate dissolved in aqueous sodium hydroxide.

[0022] In a further embodiment the cellulose carbamate dope is formed in the presence of zinc oxide and/or zinc hydroxide, preferably zinc oxide. The zinc oxide and/or hydroxide may be added to the cellulose carbamate, to the alkaline aqueous solution or to the mixture of cellulose carbamate and alkaline aqueous solution. Typically, the zinc oxide and/or zinc hydroxide is dissolved in aqueous alkaline solution prior to its use as a dissolving lye for the provided cellulose carbamate.

[0023] In an embodiment the addition of zinc (as oxide and/or hydroxide dissolved in sodium hydroxide) may partly enhance the dissolution and filterability of the solutions and partly prolong the storage time before gelation starts. Typically, it has also an impact on viscosity of the cellulose carbamate solution and tenacity yield of the obtained wet spun cellulose carbamate fibres.

[0024] According to one embodiment, the aqueous alkaline cellulose carbamate dope contains up to 10 % by weight of cellulose carbamate. Further, it preferably contains less than 8 % by weight of sodium hydroxide, especially free sodium hydroxide, and up to 2 % by weight of Zn calculated as ZnO, wherein the afore-going percentages are calculated from the total weight of the dope. Preferably, the balance of the cellulose carbamate dope is water, however, the aqueous cellulose carbamate dope may further comprise small amounts of other substances, such as additives and/or auxiliaries.

[0025] In an embodiment, the cellulose carbamate dope has a zinc oxide content of 0.1 to 1.5 % by weight, preferably up to 1.3 % by weight, most preferably up 1.0 % by weight of the total weight of the cellulose carbamate dope. In one embodiment the cellulose carbamate dope is at least partly defined by the ratio of cellulose carbamate to zinc oxide. Thus, in an embodiment the cellulose carbamate dope has a cellulose carbamate : ZnO ratio of 6 to 10 % by weight : 0.1 to 1.5 % by weight. In practice, this means 0.01 - 0.25 t of ZnO per 1 t of cellulose carbamate.

[0026] According to one specific embodiment, the aqueous alkaline cellulose carbamate dope comprises:

- 6 - 10 wt.%, preferably 8 - 10 wt.%, of cellulose carbamate (CCA),

- 5 - 10 wt.%, preferably 5 - 7 wt.%, of sodium hydroxide (NaOH), and

- 0.1 - 2 wt.%, preferably 0.1 - 1.5 wt.%, of zinc oxide (ZnO), calculated from the total weight of the cellulose carbamate dope.

[0027] The cellulose carbamate dope is preferably prepared and subjected to deaeration at a temperature of about 10 to 30 °C.

[0028] According to a preferred embodiment, the cellulose carbamate dope is provided at a temperature of 10 to 30 °C, preferably at 10 to 20 °C, for example at about 15 °C, for optional filtration and ripening. The cellulose carbamate dope is also fed into the spinning unit at such a temperature, in particular at a temperature of 15 to 25 °C. Temperature of the cellulose carbamate dope fed into the spinning unit has impact e.g. to the energy balance of spin bath recycling cycle. [0029] According to one embodiment, the cellulose carbamate dope exhibits a maximum total dissolved nitrogen content of less than 0.2 % by weight, preferably less than 0.15 % by weight, calculated from the total weight of the cellulose carbamate dope. The total dissolved nitrogen in the cellulose carbamate dope comprises both nitrogen chemically bound in cellulose as carbamate groups and nitrogen released from carbamate groups by alkaline hydrolysis.

[0030] The cellulose carbamate used in the present invention can be obtained from any source of cellulose.

[0031] Typically, cellulose carbamate is obtained from cellulose pulp, such as dissolving pulp. Chemical pulp or dissolving pulp can be prepared from wood species such as pine, spruce, birch, beech, aspen, maple, larch, acacia, eucalyptus, hemlock, tupelo, and oak or non-woods such as stalk fibres (wheat straw, rice straw, barley straw, bamboo, bagasse and reed). The origin of the feedstock can be either the virgin form of the chemical or dissolving pulp or the recycled feedstocks such as recycled paper and/or cardboard containing chemical pulp or dissolving type pulp.

[0032] Natural plant fibres can be used as such in or in the form of chemical pulp or dissolving pulp. The origin of natural plant fibres can be either their virgin forms or natural plant fibre containing textiles or recycled natural fibre containing textiles. Natural plant fibres include seed fibres such as cotton and kapok; bast fibres such as hemp, jute, kenaf, ramie, abaca and linen (flax); leaf fibres such as manilla, sisal, ananas and banana; fruit fibres such as coir.

[0033] Thus, in one embodiment, a recycled fibrous feedstock is used as a raw material for producing cellulose carbamate.

[0034] The cellulose carbamate dope is, preferably continuously, fed into the spinning unit exhibiting an aqueous spin bath containing sulphuric acid and a dissolved aluminium compound. Preferably, the cellulose carbamate dope is fed into the spinning unit through one or more spinnerets. One spinning head may comprise for example 45 spinnerets having for example 2000 holes in each spinneret. For example, a wet spinning line producing staple fibres may have at least 60 spinning heads coupled in the same spin bath system. The cellulose carbamate dope is fed into the spinning unit at a suitable flow rate. According to one embodiment, once the cellulose carbamate dope is fed into the spinning unit, the sulphuric acid will react with the sodium hydroxide and zinc oxide dissolved in sodium hydroxide forming sodium sulphate, zinc sulphate and water, whereby the spin bath comprises (excess of free) sulphuric acid, zinc sulphate, aluminium sulphate, sodium sulphate and water. The cellulose carbamate is coagulated and while being extruded through spinnerets it will be shaped into filaments. Preferably, at least essentially all, i.e. at least 99 wt.%, of the cellulose carbamate coagulates and is shaped into filaments.

[0035] In the present context, the term “steady-state” is used generally to designate conditions, such as concentration of components, that do not change appreciably with time. In preferred embodiment, the steady-state conditions of the spin bath correspond to the conditions of the withdrawn portion of the spin bath before regeneration.

[0036] According to one embodiment, in a continuous spinning process that has reached steady-state operation conditions, the spin bath leaving the spinning unit has a pH (at 20 °C) in the range of 0.2 to 0.9, preferably 0.7 to 0.9 or 0.5 to 0.8, and comprises:

- zinc, as zinc sulphate (ZnSO4), in an amount of 10 - 50 g/1,

- dissolved ammonium nitrogen, as ammonium sulphate ((NH^SC^), in an amount of less than 20 g/1, preferably less than 15 g/1, most suitably less than 10 g/1.

[0037] According to one embodiment, additives and/or auxiliaries present in the cellulose carbamate dope may also end up in the recovery recycle of the spinning bath chemicals. Further, it is also possible to add additives, i.e. spin bath auxiliaries, straight to the spin bath, which additives may then also be present in the spinning cycle.

[0038] According to a preferred embodiment, the spin bath leaving the spinning unit exhibits a density of 1200 to 1330 kg/m ³ (density measured at 20 °C) and a mass weight ratio of Na2SO4 to A12(SO4)s of 1.0 to 3.0. Density of the spin bath can be measured with aerometer or hydrometer (applies to all densities in the present application). In one embodiment, density of the spin bath leaving the spin bath unit generally correlates with the relative amount of sodium sulphate and aluminium sulphate. In addition to sodium sulphate and aluminium sulphate, spin bath contains also sulphuric acid and other salts and compounds but sum of sodium sulphate and aluminium sulphate concentrations are determinative regarding the density. Thus, in one embodiment, the spin bath conditions, such as concentration of each component, is selected in such way that these terms apply. [0039] According to one embodiment, the spin bath leaving the spinning unit has a temperature of at least 10 °C, preferably at least 15 °C, for example a temperature in the range of 10 to 25 °C, most suitable in the range of 15 to 22 °C.

[0040] Thus, according to one embodiment, the method of producing cellulose carbamate filaments or fibres, comprises the steps of

- providing a cellulose carbamate dope containing cellulose carbamate dissolved in aqueous sodium hydroxide, said cellulose carbamate dope further exhibiting a dissolved zinc compound;

- feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath containing sulphuric acid and aluminium sulphate;

- coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments;

- subjecting the cellulose carbamate filaments to stretching and optional washing, and optionally cutting the filaments into staple fibres;

- withdrawing a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions, the withdrawn aqueous spin bath having a temperature of at least 10 °C;

- subjecting the withdrawn portion of the spin bath to regeneration by removing at least a part of the sodium sulphate, water and zinc ions, and

- recycling at least a part of the regenerated aqueous spin bath to the spinning unit, wherein the spin bath conditions are selected as such that the portion of aqueous spin bath withdrawn from the spinning unit exhibits a density of 1200 to 1330 kg/m ³ (density measured at 20 °C) and a mass weight ratio of Na2SO4 to A12(SO4)s of 1 : 1 to 3 : 1.

[0041] According to one embodiment, the spin bath leaving the spinning unit has a pH (at 20 °C) in the range of 0.2 to 0.9, preferably 0.7 to 0.9.

[0042] The spin bath of the present invention is acidic spin bath containing sulphuric acid as a free acid. The amount of sulphuric acid is such that the pH of the spin bath is maintained in the above-mentioned range.

[0043] In embodiments, in the spin bath and in the portion of spin bath removed from the spinning unit and subjected to recovery, the amount of soluble ammonium nitrogen in the form of ammonium sulphate is maintained at a level of generally less than 20 g/1, preferably less than 15 g/1, most suitably less than 10 g/1. Thus, the formation of a double salt of ammonium and aluminium, i.e. ammonia alum, can be reduced, or even essentially eliminated. The formation of ammonia alum salt as a slightly soluble salt precipitates at least part of aluminium in spin bath.

[0044] The content of dissolved ammonium nitrogen will be determined, at least partially, by the carbamate content of the cellulose carbamate material used for making the cellulose carbamate dope and by the conditions prevailing between dissolution of the cellulose carbamate to form the cellulose carbamate dope and feeding into the spinning unit. Thus, hydrolysis of the cellulose carbamate during dissolution will release nitrogen compounds into the cellulose carbamate dope where they are present primarily as ammonium nitrogen compounds. The temperature prevailing during dissolution and the delay time between the dissolution of cellulose carbamate and feeding of the formed cellulose carbamate dope into the spinning unit, will affect the content of dissolved ammonium nitrogen in the spin bath composition.

[0045] Thus, hydrolysis of cellulose-bound carbamate groups releases ammonium nitrogen which ends up in the spin bath mostly as ammonium sulphate. However, the cellulose carbamate may also carry other nitrogen compounds, such as urea and biuret, that do not end up into the spin bath in the form of ammonium nitrogen and, thus, do not form ammonium sulphate.

[0046] According to a preferred embodiment, the cellulose carbamate dope is provided at a hydrolysis degree of less than 70 %, in particular less than 65 % and most preferably less than 60 %, calculated from the percentage difference in cellulose bound nitrogen between cellulose carbamate before dissolving and coagulated cellulose carbamate.

[0047] According to one embodiment, if necessary, the amount of ammonium nitrogen can be maintained at a predetermined level by removing ammonium nitrogen, as a separate operation, either from the spin bath or from the withdrawn portion of spin bath during the regeneration cycle. According to one embodiment, if necessary, at least part of ammonium nitrogen can be removed during sodium sulphate removal by crystallization. Thus, in one embodiment, a low content of soluble nitrogen (compounds), especially a low content of ammonium nitrogen, in the spin bath is preferred for the processing of the spin bath. [0048] In one embodiment, the amount of ammonium nitrogen in the spin bath is at least 1 g/1 of the total amount of the spin bath. Thus, in a preferred embodiment, the amount of ammonium nitrogen in the spin bath and, thus, also in the withdrawn portion of the aqueous spin bath is in the range of 1 to 20 g/1, more preferably in the range of 1 to 15 g/1, for example in the range of 2 to 10 g/1.

[0049] According to a preferred embodiment, at least most of the soluble nitrogen content in the spin bath is present in the coagulated filaments, wherein nitrogen does not have to be separately removed in other ways. In one embodiment, at least 30 % by weight, in particular at least 35 % by weight, most preferably at least 40 % by weight of the nitrogen, in the form of nitrogen compound, originating from the cellulose carbamate dope is present (as carbamate groups) in the coagulated filaments or fibres.

[0050] Thus, in one embodiment, the soluble nitrogen content in the spin bath is determined and maintained at a suitable level by the combination of the above mentioned features.

[0051] Aluminium, in turn, is important since it improves the quality of the produced filaments or fibres, especially shape of filaments or fibres are controlled by amount of aluminium (sulphate) in the spin bath in addition to controlled stretching. In particular aluminium has effect on the shape of cross-section making filaments and fibres more alike to cotton fibres, which is preferred. Further, aluminium inhibits crystallization of cellulose during the coagulation process. Ampholytic aluminium compound has also effect on acidbase neutralization reactions during the coagulation process. Simultaneously tenacity yield of the recovered filaments or fibres is improved. Minimum amount of aluminium in acidic wet spinning process is required to form regular fibres in a controlled way. However, aluminium also impairs the spin bath regeneration process. Thus, there needs to be balance between the different components.

[0052] In one embodiment, the ratio between aluminium sulphate and sodium sulphate calculated as mass weight ratio of Na2SO4 to A12(SO4)s is 1 :1 to 3:1. In one embodiment, a mass weight ratio ofNa2SO4 to A12(SO4)s of 1 :1 to 3:1 is maintained in the spin bath by the regeneration and recycling of the aqueous spin bath. Thus, in one embodiment, a mass weight ratio of Na2SO4 to A12(SO4)s of 1.0 to 3.0 is maintained in the spin bath, by withdrawing a portion of the spin bath from the spinning unit for regeneration to lower the sodium sulphate content of the withdrawn portion; and recycling at least a portion of the regenerated spin bath to the spinning unit.

[0053] Thus, according to one embodiment, the method of producing cellulose carbamate filaments or fibres, comprises the steps of providing a cellulose carbamate dope containing cellulose carbamate dissolved in aqueous sodium hydroxide, said cellulose carbamate dope further exhibiting a dissolved zinc compound;

- feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath containing sulphuric acid and aluminium sulphate;

- coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments;

- subjecting the cellulose carbamate filaments to stretching and optional washing, and optionally cutting the filaments into staple fibres; wherein a mass weight ratio of Na2SO4 to AffSChT of 1.0 to 3.0 is maintained in the spin bath, by withdrawing a portion of the aqueous spin bath from the spinning unit for regeneration to lower the sodium sulphate content of the withdrawn portion; and recycling at least a portion of the regenerated aqueous spin bath to the spinning unit.

[0054] In a preferred embodiment, the spin bath has a density of 1200 to 1330 kg/m ³ .

[0055] According to one embodiment, temperature of the spin bath is up to 25 °C, for example 15 to 25 °C, most suitable in the range of 15 to 22 °C. At too low spin bath temperature, the salts will easily crystallize out from the aqueous spin bath, whereas the higher is the spin bath temperature, the higher aluminium concentration is needed to obtain the targeted tenacity yield, because coagulation process is accelerated by increasing the temperature. Thus, the temperature of the spin bath is preferably in the above-mentioned range.

[0056] According to a preferred embodiment, the cellulose carbamate dope is subjected to coagulation in the spin bath at a temperature of up to 23 °C, most suitably at a temperature of up to 20 °C. [0057] In the spin bath cellulose carbamate is coagulated into cellulose carbamate filaments, preferably through one or more spinnerets. Such filaments are recovered from the spin bath and subjected to stretching, optional cutting into fibres, and optional after- treatment in which the filaments or fibres are washed, bleached and/or spin finished.

[0058] According to one embodiment, the filaments obtained from the spin bath are subjected to stretching in a stretching unit. The cellulose carbamate filaments are subjected to stretching in one or in a cascade of at least two stretching units.

[0059] According to one embodiment, the stretching unit comprises a stretching bath or air-stretching. Typically, the stretching bath(s) is an aqueous bath comprising sulphuric acid, aluminium sulphate, zinc sulphate and sodium sulphate, i.e. the same components as the spin bath but in smaller concentrations since the coagulated filaments carry the components with them to the stretching unit. The same components may also be carried by the filaments (or after cutting by the fibres) through one or more of the following steps, i.e. the obtained products may still comprise some components of the spin bath. That is why the method of the present invention may in some embodiment comprise a semi-closed system. A stretching bath is particularly used when cellulose carbamate staple fibres are produced, i.e. when preferably stretching a tow of filaments.

[0060] According to one embodiment the stretching bath is a hot stretching bath, where filaments or a tow of filaments are immersed in stretching bath solution. The temperature in the stretching bath(s) is typically in the range of 75 to 100 °C, preferably 85 to 95 °C, more preferably about 90 °C.

[0061] According to another embodiment, cellulose carbamate filaments are stretched in the stretching unit without a stretching bath, i.e. by so called air stretching.

[0062] According to one embodiment, after stretching the filaments, especially the tow of filaments, are cut into fibres having a desired length by a cutter in a cutting unit. Cutting can be performed by any known cutting method for filaments.

[0063] After stretching, and optional cutting, the filaments or fibres are subjected to after-treatment in an after-treatment unit. According to a preferred embodiment, the after- treatment comprises washing, bleaching, spin finishing or any combination thereof. In one embodiment, washing if performed as counter current washing using water or the effluent of a preceding or subsequent processing step. [0064] Finally, the obtained filaments or fibres are dried, by any known drying method.

[0065] According to a preferred embodiment, the cellulose carbamate filaments, especially the tow of cellulose carbamate filaments, are subjected to stretching, followed by cutting to provide stable staple fibres, after which an after-treatment, especially washing, of the thus obtained stable fibres is performed in an after-treatment unit. Finally, the obtained fibres are dried.

[0066] According to another embodiment, the cellulose carbamate filaments, especially the tow of cellulose carbamate filaments, are subjected to stretching, followed by an after-treatment, especially washing, in an after-treatment unit, after which cutting of the thus treated either wet or dried filaments into staple fibres or shortcuts is performed.

[0067] According to another embodiment, the cellulose filaments are subjected to stretching, followed by an after-treatment, especially washing, of the obtained filaments in an after-treatment washing unit.

[0068] The spinning process described above for producing cellulose carbamate filaments or fibres involves a number of chemicals, wherein the chemical recyclability is highly important to make the process sustainable. Thus, the method of the present invention further comprises chemical recycling in which at least part of the spin bath is regenerated and solid matter is recovered.

[0069] According to a preferred embodiment, at least part of the stretching and/or washing bath is also regenerated and solid matter is recovered. Thus, the regeneration described in the present application for the spin bath also applies to the stretching and washing baths. All the withdrawn portions can be regenerated simultaneously in the same regeneration process.

[0070] First, a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions is withdrawn from the spinning unit (and/or stretching unit and/or washing unit) and such portion of the spin bath (and/or stretching bath and/or washing bath) is subjected to regeneration by removing at least part of the sodium sulphate, water and zinc ions. Regeneration of the acidic spin bath requires removing these components to prevent them accumulating in the spin bath by the effect of the incoming cellulose carbamate dope. The withdrawn aqueous spin bath contains, at a temperature of at least 10 °C, before the regeneration dissolved ammonium nitrogen in the form of ammonium sulphate in an amount of less than 20 g/1, preferably less than 15 g/1, most preferably less than 10 g/1 of the spin bath. Thus, according to a preferred embodiment, the withdrawn portion of aqueous spin bath is at least essentially free from ammonia alum precipitate, preferably the withdrawn portion of aqueous spin bath does not comprise any solid ammonia alum.

[0071] According to one embodiment the portion of aqueous spin bath withdrawn from the spinning unit exhibits a density of 1200 to 1330 kg/m ³ (measured at 20 °C) and preferably a mass weight ratio of Na2SO4 to A12(SO4)s of 1 : 1 to 3 : 1.

[0072] As already described above, according to one embodiment, the spin bath leaving the spinning unit, i.e. also the withdrawn portion of the aqueous spin bath having a pH (at 20 °C) in the range of 0.2 to 0.9, preferably of 0.5 - 0.8 or 0.7 - 0.9, comprises before regeneration:

- zinc, as ZnSO4, at 10-50 g/1, and

- dissolved nitrogen, as (NH4)2SO4, of less than 20 g/1, preferably less than about 15 g/1, most suitably less than 10 g/1.

[0073] According to one embodiment, the method comprises continuously withdrawing for regeneration from the spinning unit a volume of aqueous spin bath. Similarly, according to one embodiment, portions of the liquid phase of the stretching and washing baths are continuously withdrawn for regeneration.

[0074] In one embodiment, by withdrawing aqueous spin bath and subjecting it to sodium sulphate and zinc recovery operations, at least 70 % by weight, preferably at least 80 % by weight, more preferably at least 90 % by weight, of the sodium sulphate formed into the spin bath in particular at steady-state conditions, is recovered e.g. in a crystallization step, and at least 80 % by weight, preferably at least 85 % by weight, more preferably 90 % by weight, of the zinc sulphate formed into the spin bath in particular at steady-state conditions, is recovered in a zinc removal step.

[0075] According to one embodiment the withdrawn portion of aqueous spin bath is first transferred into a tank called “thin bath” tank, form which the withdrawn aqueous spin bath can be subjected to regeneration. According to one embodiment the optional withdrawn portions of aqueous stretching and/ or washing baths may be also transferred to the same tank. In one embodiment, water evaporation capacity of the present method defines how much stretching and/ or washing baths can be transferred to the recycling process of the spin bath. In one embodiment the optional withdrawn portions of aqueous stretching and/ or washing baths may be also transferred directly to water removal feed (evaporation) in order to utilize energy content of these high temperature solution streams.

[0076] According to one embodiment, the regeneration comprises subjecting the withdrawn portion of aqueous spin bath to a separation step, to a zinc removal step, to a water removal step or to a sodium sulphate removal step, or to any combination thereof, preferably to all of them.

[0077] In a preferred embodiment, the withdrawn portion of the aqueous spin bath as a whole is subjected to a filtration but only a portion of the filtrated spin bath is subjected the other separation steps.

[0078] According to a preferred embodiment, the regeneration comprises subjecting the withdrawn portion of aqueous spin bath to filtration in a filtration unit, zinc extraction in an extraction unit and to sodium sulphate crystallization in a crystallization unit. According to another embodiment the regeneration may also comprise just one or two of these units, i.e. the regeneration may comprise filtration, zinc extraction or sodium sulphate crystallization or any combination thereof. Further, the regeneration typically contains a water evaporation.

[0079] According to one embodiment, the withdrawn portion of aqueous spin bath is preferably first subjected to separation, preferably to filtration in a filtration unit to remove, solid matter, such as possible ammonium aluminium sulphate present and other solid material. Optional separation can be performed by any known separation method, such as by filtration using diatomaceous earth or perlite or for example centrifugation. Solid matter is recovered out of the process. The filtrate, i.e. the withdrawn portion of spin bath without the solid matter is conducted onwards in the regeneration process.

[0080] According to one embodiment, the method comprises selectively removing zinc from the withdrawn portion of aqueous spin bath.

[0081] Thus, according to a preferred embodiment, the withdrawn portion of spin bath, or at least part of it, is, preferably after separation, subjected to selective zinc removal, preferably zinc selective extraction. The withdrawn portion of aqueous spin bath before zinc removal has preferably a pH higher than 0.2, preferably higher than 0.5, most suitably in the range of 0.7 to 0.9 or 0.5 to 0.75.

[0082] According to a preferred embodiment, the zinc concentrate recovered by the zinc selective extraction contains preferably less than 10 % by weight of aluminium ions (as Al ³⁺ ), most preferably less than 5 %, such as less than 3 %, by weight, most suitably less than 2.5 %, such as less than 2.0 %, by weight calculated from the portion of zinc (as Zn ²⁺ ) in the recovered zinc concentrate.

[0083] According to one embodiment a trivalent iron (as Fe ³⁺ ) as impurity in spin bath is preferably reduced to divalent iron (Fe ²⁺ ) by using for example sulphur dioxide reduction prior to subjection the aqueous spin bath to zinc extraction. Trivalent iron is easily extracted with zinc by saturating the active extraction reagent. The separate iron removal step to purify the saturated extraction reagent may be avoided if the reductive treatment by using sulphur dioxide is used.

[0084] According to one embodiment, zinc can be removed by extraction, such as liquid-liquid extraction or solid phase extraction (“SPE”), especially liquid-liquid extraction, or ion exchange, carried out in one or several stages. Preferably, zinc is removed in a zinc selective manner avoiding co-extraction of aluminium. It has been surprisingly found in the present invention that zinc can be selectively removed in acidic conditions of the present invention. Especially, pH of the spin bath during zinc extraction is critical. Thus, according to one embodiment, the method comprises removing zinc from the withdrawn portion of aqueous spin bath having a pH of higher than 0.2, preferably higher than 0.5, most suitably between 0.7 to 0.9 or 0.5 to 0.75.

[0085] According to one embodiment, the method comprises removing zinc from the withdrawn portion of aqueous spin bath by contacting said portion with an organic solvent containing an active extraction reagent, especially an active extraction reagent of organic phosphorous compounds, in particular at least one of the following as such or in a carrier matrix: Di-(2-ethylhexyl)phosphoric acid, 2-ethylhexyl phosphonic acid mono-2-ethyl hexyl ester (PC-88A), bis-(2,4,4-trimethylpentyl) phosphinic acid, and bis-(2,4,4-trimethyl pentyl) dithiophosphinic acid, or mixtures thereof, especially di-(2-ethylhexyl)phosphoric acid. [0086] According to one embodiment, the zinc removal may be performed at an elevated temperature and the withdrawn aqueous spin bath may be heated prior to the zinc removal.

[0087] Thus, according to one embodiment, the method comprises removing zinc from the withdrawn portion of aqueous spin bath having a pH of 0.7 to 0.9 or 0.5 to 0.75 by liquid-liquid extraction, in particular a multi-stage liquid-liquid extraction, using an organic solvent and at an elevated temperature.

[0088] According to a preferred embodiment, zinc is removed by liquid-liquid extraction, preferably multi stage liquid- liquid extraction, in organic active extraction reagent/ organic sparingly water soluble hydrocarbon solvent solution. According to one embodiment, the amount of active extraction reagent in the extraction solution is preferably 20 to 45 vol-%, more preferably 40 vol-%, calculated from the total volume of the extraction solution, i.e. from the total volume of extraction reagent and hydrocarbon solvent. According to a most suitable embodiment the organic solvent mixture used in zinc selective extraction contains di-(2-ethylhexyl)phosphoric acid as an active extraction reagent and kerosine or similar type of inert hydrocarbon solvent as a sparingly water soluble hydrocarbon solvent, as a carrier matrix.

[0089] The extracted zinc is recovered, typically as an acidic, sulphuric acid containing, zinc sulphate solution concentrate, and the remaining portion of the withdrawn spin bath is transferred further in the regeneration process.

[0090] In solid phase extraction, the zinc present in the liquid phase is adsorbed to a solid or stationary phase through which the liquid is passed. The stationary phase may comprise organic phosphorous groups or compounds of the kind disclosed above.

[0091] According to one embodiment, the recovered zinc can be recycled back to the dissolving and/or spinning process, via zinc hydroxide precipitation to dissolve cellulose carbamate in order to form the cellulose carbamate dope.

[0092] According to one embodiment, the withdrawn portion of spin bath, or at least part of it, is, preferably after filtration and/or zinc extraction, subjected to water evaporation. Thus, according to one embodiment, the method comprises removing water from the withdrawn spin bath by evaporation before conducting it to a step for removing sodium sulphate. Cellulose carbamate dope will introduce considerable volumes of water into the spinning unit and, in preferred embodiments, water is removed during the regeneration cycle. Preferably, water is evaporated prior to the sodium sulphate removal since the concentration of sodium sulphate is suitable to be increased before its removal. According to one embodiment, the withdrawn portion of aqueous spin bath subjected for sodium sulphate removal may comprise a combination of withdrawn portion of aqueous spin bath from which water has been evaporated and non-evaporated portion of withdrawn aqueous spin bath, as long as the amount of water is in an appropriate level in terms of the following steps. In addition to water removal by evaporation, crystallized sodium sulphate decahydrate also removes water from the spin bath system by the hydrated salt crystals.

[0093] According to a preferred embodiment, the withdrawn spin bath is, preferably after separation, zinc removal and/or water evaporation, subjected to sodium sulphate removal, preferably sodium sulphate crystallization, in particular in the form of sodium sulphate decahydrate, such as cold crystallization or vacuum crystallization. Thus, according to one embodiment, the method comprises removing sodium sulphate from the withdrawn portion of spin bath by crystallization.

[0094] According to a preferred embodiment, the portion of withdrawn aqueous spin bath subjected to sodium sulphate removal exhibits sodium sulphate and aluminium sulphate in a certain ratio to avoid co-crystallization of aluminium sulphate, which would interfere the crystallization of pure as possible sodium sulphate. Thus, according to a preferred embodiment, the method comprises providing a portion of withdrawn aqueous spin bath, preferably exhibiting a mass weight ratio of Na2SO4 to A12(SO4)s of 1.4:1 to 2.5:1, most preferably 1.5:1 to 2:1, most suitably 1.6:1 to 1.9:1 and feeding that portion to a step for removing of sodium sulphate. In one embodiment, the ratio of Na2SO4 to A12(SO4)S is adjusted, for example the ratio of Na2SO4 to AbiSChT is adjusted by adding sodium sulphate. Thus, in one embodiment, part of the crystallized sodium sulphate is returned back to the crystallization step to adjust the ratio of Na2SO4 to A12(SO4)s in order to facilitate. According to a preferred embodiment, the suitable mass weight ratio of Na2SO4 to A12(SO4)S is controlled by adjusting the amount of the crystallized sodium sulphate.

[0095] According to one embodiment, the withdrawn portion of aqueous spin bath subjected from sodium sulphate removal exhibits pH in the range of 0.2 to 0.9 and its zinc concentration is preferably at most 60 g/1. [0096] Thus, according to one embodiment, there are several factors influencing the crystallization purity of sodium sulphate, one of the most important one being the mass weight ratio of Na2SO4 to Ab/SChk. If the ratio is not optimized, there may crystalize a significant amount of aluminum sulphate among crystallized sodium sulphate. Also, zinc and sulfuric acid concentration affect the purity of sodium sulphate crystals.

[0097] According to a preferred embodiment aluminium sulphate stays, at least essentially, in the cycle, i.e. does not crystallize with the sodium sulphate. Thus, the remaining portion of the withdrawn spin bath from crystallization preferably comprises aluminium sulphate.

[0098] Preferably, the withdrawn portion of aqueous spin bath is subjected to cooling crystallization of sodium sulphate at a temperature of 1 to 10 °C, for example 4 to 8 °C, preferably using an external cooling.

[0099] According to another embodiment, the withdrawn portion of the aqueous spin bath is subjected to vacuum crystallization of sodium sulphate. Aqueous spin bath is concentrated during the vacuum crystallization, for example 4-8 % by weight of water may be evaporated during the vacuum crystallization.

[00100] Sodium sulphate crystals are recovered, in particular in the form of sodium sulphate decahydrate, and optionally washed. Sodium sulphate decahydrate crystals are optionally subjected to calcination. The remaining portion of the withdrawn spin bath from crystallization (i.e. supernatant) is transferred into the “thick bath” tank.

[00101] According to one embodiment, supernatant is separated from the sodium sulphate and recycled to the spin bath.

[00102] Thus, according to one embodiment the regenerated acid spin bath comprises the portion of withdrawn aqueous spin bath that has been regenerated for example by extraction and crystallization, during the regeneration cycle, but it may also contains a portion of withdrawn spin bath that has passed the regeneration steps. Such regenerated spin bath may be collected to a “thick bath” tank prior to recycling back to the spinning unit bath.

[00103] According to one embodiment, concentration of the spin bath to be fed into the spinning unit is adjusted to appropriate level by adding sulphuric acid and/or aluminium sulphate. According to one embodiment, the method comprises feeding fresh feed of sulphuric acid and/or aluminium sulphate into the regenerated spin bath before recycling thereof into the spinning unit. Thus, fresh feed of sulphuric acid, aluminium sulphate, aluminium hydroxide or any mixture thereof can be added for example to the “thick bath” tank. Optional fresh feed of aluminium hydroxide may react with sulphuric acid by forming aluminium sulphate.

[00104] According to one embodiment, if any aluminium sulphate ends up in the effluent of the semiclosed-loop coagulation or spin bath circulation system, aluminium can be precipitated from it as aluminium hydroxide and, optionally returned back to the spin bath for example as described above.

[00105] According to one embodiment, the method of producing cellulose carbamate filaments or fibres, comprises the steps of

- providing a cellulose carbamate dope containing cellulose carbamate dissolved in aqueous sodium hydroxide, said cellulose carbamate dope further exhibiting a dissolved zinc compound, wherein the dope comprises 6-10 wt.% of cellulose carbamate, 5-10 wt.% of sodium hydroxide and 0.1-2 wt.% of dissolved zinc compound, calculated from the total weight of cellulose carbamate dope;

- feeding the cellulose carbamate dope into a spinning unit exhibiting an aqueous spin bath containing sulphuric acid and aluminium sulphate, wherein the spin bath in steady-state has a pH (at 20 °C) in the range of 0.2 to 0.9;

- coagulating the cellulose carbamate from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments;

- subjecting the cellulose carbamate filaments to stretching and optional washing, and optionally cutting the filaments into staple fibres;

- withdrawing a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions, the withdrawn aqueous spin bath having a temperature of at least 10 °C; ;

- subjecting the withdrawn portion of the spin bath to regeneration by removing at least a part of the sodium sulphate, water and zinc ions, and

- recycling at least a part of the regenerated aqueous spin bath to the spinning unit.

[00106] According to one embodiment, the method comprises recovering aluminium sulphate from the step of selectively removing zinc and from the step of selectively recovering sodium sulphate and recycling the recovered aluminium sulphate to the spin bath.

[00107] The attached drawing depicts embodiments.

[00108] Figure 1 illustrates a process in accordance with at least some embodiments of the present invention in which a cellulose carbamate dope is first fed into a spinning unit 1 exhibiting an aqueous spin bath. In the spin bath, cellulose carbamate is coagulated from the cellulose carbamate dope into the aqueous spin bath to form cellulose carbamate filaments, or a tow of cellulose carbamate filaments, which are then subjected to stretching in a one or more stretching unit(s) 2. Stretched filaments can be next washed in a washing unit 3a. Alternatively, stretched tow of filaments can be first cut into staple fibres in a cutting unit 3b, and then subjected for washing in a washing unit 3a. After washing, cellulose carbamate filaments or stable fibres are obtained.

[00109] The method of the present invention further comprises chemical recycling of the chemicals. Thus, a portion of the aqueous spin bath containing sodium sulphate, water and zinc ions is withdrawn from the spin bath of the spinning unit 1, preferably into a “thin bath” tank 4. A portion of the aqueous stretching bath from the stretching unit 2 is also withdrawn, and preferably conducted into the “thin bath” tank 4 together with the withdrawn spin bath. The withdrawn portions of the spin bath and stretching bath are subjected to a regeneration by removing at least part of the sodium sulphate, water and zinc ions. In the regeneration cycle the withdrawn aqueous spin bath and stretching bath are first subjected to filtration in a filtration unit 5. After filtration, the filtrate is subjected to a zinc selective extraction in an extraction unit 6, whereas the residue, i.e. solid matter, is recovered away from the process. At least part of the zinc ions are removed by zinc selective extraction and recovered, preferably as an acidic zinc sulphate concentrate. After the zinc extraction, the remaining portion of the mixture of withdrawn spin bath and stretching bath is subjected to water evaporation in an evaporation unit 7 and after that to sodium sulphate crystallization in crystallization unit 8. Crystallized sodium sulphate is recovered and the remaining portion of the mixture of the withdrawn spin bath and stretching bath is transferred to the “thick bath” tank 9. Thus, the spin bath collected in the “thick bath” tank 9 is at least partly regenerated and can then be recycled back to the spinning unit 1. [00110] As discussed above, at least part of the regenerated aqueous spin bath, preferably gathered into the “thick bath” tank 9, is recycled back to the spinning unit 1. However, according to a preferred embodiment, the method also comprises feeding fresh feed of sulphuric acid and aluminium sulphate into the regenerated spin bath before recycling thereof into the spinning unit 1.

[00111] The following non-limiting example illustrates at least some embodiments of the invention:

Example 1

[00112] Cellulose carbamate dope was prepared by dissolving cellulose carbamate powder, having nitrogen content of 1.6 % by weight in dry cellulose carbamate, in aqueous alkaline solution to obtain a composition containing 8.5 % by weight of cellulose carbamate, 6.5 % by weight of sodium hydroxide, 1.3 % by weight of dissolved zinc oxide and 0.085 % by weight of dissolved nitrogen. Such composition, i.e. the cellulose carbamate dope was fed, through a spinneret, into the spinning unit, in which cellulose carbamate was coagulated into the cellulose carbamate fibre filaments. A tow of filaments was formed and further stretched in stretching bath and cut into staple fibres, after which the recovered staple fibres were subsequently washed in after-treatment unit. The staple fibres recovered from the after-treatment unit had a nitrogen content of 0.7 % by weight in dry solid. The hydrolysis degree of cellulose carbamate was 56 %.

[00113] A portion of aqueous spin bath having pH value of 0.75, sodium sulphate to aluminium sulphate ratio of 1.63, density of 1.300 kg/1, temperature of 20 °C, ammonium sulphate content of 7.5 g/1 and zinc sulphate content of 20.0 g/1, was withdrawn from the spinning unit. Density was measured with hydrometer/aerometer.

[00114] The withdrawn portion of spin bath was filtered.

[00115] A portion of the filtered aqueous spin bath having pH of 0.75 was subjected to three-stage liquid-liquid extraction process for removal of a part of zinc. To perform a liquid-liquid extraction process, two parts by volume of aqueous spin bath and one part by volume of organic solvent mix containing 40 % by volume of di-(2-ethylhexyl)phosphoric acid in kerosene was efficiently mixed together for 3 minutes, after that the water phase and the organic solvent phase were allowed to separate. The separated aqueous phase was re-extracted with a new portion of organic solvent. The extraction process was once more repeated by using a new portion of organic solvent. The separated organic solvent phases from the three subsequent extraction steps were mixed together and the zinc was released by back-extraction (stripping) with 100 g/1 of aqueous sulphuric acid solution. Zinc and aluminium content in sulphuric acid solution obtained by striping was analysed by a selective titration with ethylenediaminetetraacetic acid (EDTA). Based on the result of the analyses the total extraction yield was 87 % by weight for zinc and 1.9 % by weight for aluminium. Zinc was selectively extracted in the presence of aluminium. The zinc sulphate content of the aqueous spin bath was decreased from 20 g/1 to 2.6 g/1.

[00116] A portion of the filtered and zinc extracted aqueous spin bath was subjected to evaporation: 21.5 % by weight of water (from the total amount of aqueous spin bath) was removed by evaporation, resulting a concentrated aqueous spin bath having sodium sulphate to aluminium sulphate ratio of 1.63, temperature of 50 °C and ammonium sulphate content of 9.3 g/1 (standard SFS 5505: 1988).

[00117] A portion of the concentrated aqueous spin bath was subjected to cooling crystallization. Temperature of concentrate was decreased from 50 °C to 8 °C. The yield of the sodium sulphate decahydrate containing salt crystals recovered by centrifugation was 29.4 % by weight and supernatant was 70.6 % by weight. The centrifuged crystals were washed with water to remove majority of supernatant residues from surface of crystals. The hydrated crystals were calcinated at temperature of 130 °C. Sample of the anhydrous crystals taken after calcination was dissolved in water for determination of its aluminium content by a selective titration with EDTA. Aluminium content of the recovered calcinated sodium sulphate was 0.56 % by weight corresponding 3.5 % by weight of aluminium sulphate in sodium sulphate. Sodium sulphate was selectively crystallized in the presence of aluminium sulphate. In comparison, aluminium content of the calcinated sodium sulphate was 13 % by weight, when sodium sulphate to aluminium sulphate ratio of 1.18 was used instead of 1.64, other process conditions were the same. Thus, selectivity of crystallization was decreased in case the sodium sulphate to aluminium sulphate ratio was not optimized.

[00118] The aluminium sulphate containing supernatant was recycled back to the spin bath containing both the regenerated and the unregenerated portions of aqueous spin bath collected from different steps of the spin bath regeneration process. Some fresh sulphuric acid and aluminium sulphate was added prior to recycling the mixture back to the spinning unit.

[00119] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[00120] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[00121] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[00122] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[00123] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[00124] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

LIST OF REFERENCE SIGNS

Reference Numbers

1 Spinning unit

2 Stretching unit(s)

3 a Washing unit(s)

3b Cutting unit

4 “Thin bath” tank

5 Filtration unit

6 Extraction unit

7 Evaporation unit

8 Crystallization unit

9 “Thick bath” tank

INDUSTRIAL APPLICABILITY [00125] At least some embodiments of the present invention find industrial application in producing cellulose carbamate filaments or fibres from a cellulose carbamate dope.