Method for producing cellulose carbamate filaments or fibers

Abstract

According to an example aspect of the present invention, a method of producing a cellulose carbamate filament or fiber is provided. The method comprises the steps of: providing a cellulose carbamate dope; feeding the cellulose carbamate dope into a spinning unit, the spinning unit exhibiting an aqueous spinning bath containing sulfuric acid and dissolved aluminum compounds; coagulating the cellulose carbamate to form a cellulose carbamate filament; and subjecting the cellulose carbamate filament to drawing, optionally cutting to form a fiber; and optionally washing. In addition, the method comprises chemically recycling by taking out a portion of the spinning bath to regenerate by removing at least a portion of sodium, water and zinc ions; and recycling at least a portion of the regenerated aqueous spinning bath to the spinning unit, wherein the taken out aqueous spinning bath, prior to regeneration, preferably contains dissolved ammonium nitrogen in the form of ammonium sulfate in an amount of less than 20 g / l of the spinning bath at a temperature of at least 10 °C.

Description

Technical Field

[0001] This invention relates to a method for producing cellulose carbamate filaments or fibers, including the chemical recovery of chemicals. Background Technology

[0002] Cellulose is the world's most widely used biopolymer, applied in various industries such as paper and paperboard manufacturing and textiles, in both cellulose and cellulose derivatives (such as cellulose carbamates, cellulose acetates, ethers, and esters). However, using cellulose in these industries requires dissolving the cellulose fibers to regenerate the structure. Cellulose is difficult to dissolve due to its semi-crystalline structure, strong hydrogen bonds in the polymer sheets, and the presence of both hydrophilic and hydrophobic ends. Therefore, cellulose needs to be derivatized to modify its structure or to open up the fibers and increase their reactivity.

[0003] Several derivatization methods are known in this art, the most well-known of which is probably the viscose process, in which cellulose is first treated with an alkali and then with carbon disulfide to produce cellulose xanthate. Due to the toxicity of carbon disulfide and the adverse environmental impact of the viscose process, it is increasingly being avoided. More and more people are turning to other derivatization processes, particularly those that are less expensive, do not have the toxicity and environmental problems of the viscose process, and can still be carried out using existing infrastructure (e.g., in viscose wet spinning mills).

[0004] The carbamate esterification process used to produce cellulose carbamates is such a derivatization process. Cellulose carbamates are formed by the reaction of cellulose with urea. Methods for producing cellulose carbamates are described in Finnish patents 112869 and 112795.

[0005] Cellulose carbamate is dissolved in a mixture of an alkaline agent such as sodium hydroxide and zinc oxide, and the resulting cellulose carbamate spinning solution is fed into a spinning bath to coagulate the cellulose carbamate into filaments. In some embodiments, coagulation is carried out in an acidic spinning bath containing sulfuric acid and aluminum sulfate. This acidic spinning bath typically contains a dissolved phase of zinc oxide (as zinc ions) as well as ammonium salts or ions (ammonium nitrogen), aluminum salts or ions, and sodium sulfate.

[0006] Traditionally, spinning baths are equipped with recycling systems for chemicals used to dissolve and coagulate cellulose carbamates. The need for efficient recycling of these chemicals remains. Summary of the Invention

[0007] The object of this invention is to provide a method for producing high-quality cellulose carbamate filaments or fibers in a spinning cycle, wherein chemical recycling is optimized, i.e., sodium sulfate is preferably crystallized as purely as possible (avoiding co-crystallization of zinc, aluminum, and nitrogen), and zinc is removed as efficiently as possible by extraction (avoiding co-extraction of aluminum). Furthermore, the coagulation or spinning bath circulation system is kept as closed-loop, i.e., semi-closed-loop, as possible, thereby avoiding wastewater discharge. In addition, the water balance of the semi-closed-loop system is kept constant by removing water entering the system via the cellulose carbamate spinning solution (preferably by evaporation).

[0008] According to a first aspect of the invention, a method for producing cellulose carbamate filaments or fibers is provided, comprising the steps of: providing a cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution, the cellulose carbamate spinning solution also exhibiting dissolved zinc compounds; feeding the cellulose carbamate spinning solution into a spinning unit exhibiting an aqueous spinning bath containing sulfuric acid and dissolved aluminum compounds; coagulating the cellulose carbamate from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments; subjecting the cellulose carbamate filaments to stretching and optionally washing, and optionally cutting the filaments into fibers; removing a portion of the aqueous spinning bath containing sodium sulfate, water, and zinc ions; regenerating the removed portion of the spinning bath by removing at least a portion of the sodium sulfate, water, and zinc ions; and recycling at least a portion of the regenerated aqueous spinning bath back to the spinning unit. Preferably, prior to regeneration, the aqueous spinning bath contains dissolved nitrogen in the form of ammonium nitrogen as ammonium sulfate at a temperature of at least 10°C, in an amount less than 20 g / L of the spinning bath.

[0009] The regenerated extract may also include extracts that have undergone water-based stretching and / or washing.

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

[0011] Embodiments of the present invention enable the production of high-quality cellulose urethane filaments or fibers through efficient chemical recycling. Therefore, in this invention, the spinning process achieves a high level of chemical recycling without compromising fiber properties. Attached Figure Description

[0012] Figure 1 A method for producing cellulose urethane filaments or fibers according to at least some embodiments of the present invention is shown, the method further comprising a spinning bath regeneration process. Detailed Implementation

[0013] Implementations of this technology provide an improved method for producing cellulose urethane filaments or fibers. This method includes chemical recycling. In various embodiments, the method for producing cellulose urethane filaments or fibers becomes sustainable by significantly improving the efficiency of chemical recycling in the spinning process.

[0014] The implementation plan provides high-quality cellulose urethane filaments or fibers with the aim of obtaining fibers that are as similar as possible to cotton fibers.

[0015] The implementation scheme includes providing a cellulose carbamate spinning solution, feeding the cellulose carbamate spinning solution into a spinning unit exhibiting an aqueous spinning bath, coagulating the cellulose carbamate from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments, subjecting the cellulose carbamate filaments to stretching in one or more stretching units and optionally washing in one or more washing units, and optionally cutting the filaments into fibers to obtain cellulose carbamate filaments or fibers. Additionally, the implementation scheme includes chemical recycling. Therefore, a portion of the aqueous spinning bath containing sodium sulfate, water, and zinc ions is removed from the spinning bath and regenerated by removing at least a portion of the sodium sulfate, water, and zinc ions.

[0016] Therefore, according to one embodiment, a method for producing cellulose carbamate filaments or fibers includes the following steps:

[0017] - Provides a cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution, the cellulose carbamate spinning solution also exhibiting dissolved zinc compounds;

[0018] - The cellulose carbamate spinning solution is fed into a spinning unit, which exhibits an aqueous spinning bath containing sulfuric acid and aluminum sulfate;

[0019] - The cellulose carbamate is coagulated from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments.

[0020] - The cellulose urethane filament is subjected to stretching and optional washing, and the filament is optionally cut into staple fibers.

[0021] - Take out a portion of the aqueous spinning bath containing sodium sulfate, water and zinc ions, wherein the temperature of the aqueous spinning bath taken out is at least 10°C;

[0022] - The removed portion of the spinning bath undergoes regeneration by removing at least a portion of the sodium sulfate, water, and zinc ions, and

[0023] At least a portion of the regenerated aqueous spinning bath is recycled to the spinning unit.

[0024] According to a preferred embodiment, the regeneration of the removed portion of the spinning bath (and / or stretching bath and / or washing bath) includes at least a zinc removal step (preferably by zinc extraction) and a sodium sulfate removal step (preferably by sodium sulfate crystallization). Furthermore, regeneration may include at least one separation step (e.g., filtration) and / or a dehydration step (e.g., by evaporation). In addition to removing water by evaporation, it is preferable to remove at least a portion of the water from the spinning bath by sodium sulfate decahydrate crystals. Typically, at least a portion of the regenerated aqueous spinning bath is recycled back to the spinning unit.

[0025] According to one embodiment, a portion of the removed portion of the regenerated spinning bath (and / or stretching bath and / or washing bath) can bypass one or more steps of the aforementioned regeneration cycle. For example, a portion of the filtered spinning bath can be recycled back to the spinning unit without subjecting it to zinc extraction, water evaporation, and / or sodium sulfate removal steps.

[0026] According to a preferred embodiment, the method of the present invention is a continuous process, in which the cellulose carbamate spinning solution is continuously fed and processed in the spinning unit.

[0027] According to one embodiment, filaments or fibers are obtained by this method. According to a preferred embodiment, the filament product of this method is a continuous yarn obtained from solidified filaments after stretching and optionally washing. The fiber product of this method is obtained sequentially by forming filament bundles from solidified filaments, and then stretching and cutting these filament bundles into fibers, particularly short fibers or short-short fibers. Specifically, short fibers are obtained by cutting the filament bundles into short fibers, followed by washing, post-treatment, and drying. According to another embodiment, short fibers can be obtained by washing and post-treating the filament bundles themselves, and then cutting them into short fibers or short-short fibers in a wet or dry state.

[0028] In the first step of the implementation plan, a cellulose carbamate spinning solution is provided, which contains cellulose carbamate dissolved in an aqueous sodium hydroxide solution.

[0029] In another embodiment, the cellulose carbamate spinning solution is formed in the presence of zinc oxide and / or zinc hydroxide (preferably zinc oxide). Zinc oxide and / or zinc hydroxide can be added to the cellulose carbamate, an alkaline aqueous solution, or a mixture of cellulose carbamate and an alkaline aqueous solution. Typically, zinc oxide and / or zinc hydroxide are dissolved in an alkaline aqueous solution and then used as the dissolving alkali for the provided cellulose carbamate.

[0030] In one embodiment, the addition of zinc (as an oxide and / or hydroxide dissolved in sodium hydroxide) can partially enhance the solubility and filterability of the solution and partially extend the storage time before gelation begins. It also typically affects the viscosity of the cellulose carbamate solution and the tenacity yield of the resulting wet-spun cellulose carbamate fibers.

[0031] According to one embodiment, the aqueous alkaline cellulose carbamate spinning solution contains up to 10% by weight of cellulose carbamate. Furthermore, 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 above percentages are calculated based on the total weight of the spinning solution. Preferably, the balance of the cellulose carbamate spinning solution is water; however, the aqueous cellulose carbamate spinning solution may also contain small amounts of other substances, such as additives and / or auxiliaries.

[0032] In one embodiment, the zinc oxide content of the cellulose carbamate spinning solution is 0.1 to 1.5% by weight, preferably up to 1.3% by weight, and most preferably up to 1.0% by weight. In one embodiment, the cellulose carbamate spinning solution is defined at least in part by the ratio of cellulose carbamate to zinc oxide. Thus, in one embodiment, the cellulose carbamate:ZnO ratio of the cellulose carbamate spinning solution is 6 to 10% by weight: 0.1 to 1.5% by weight. In practice, this means that each tonne of cellulose carbamate contains 0.01 to 0.25 tons of ZnO.

[0033] According to a specific implementation plan, the aqueous alkaline cellulose carbamate spinning solution comprises:

[0034] -6-10% by weight, preferably 8-10% by weight, of cellulose carbamate (CCA),

[0035] -5-10% by weight, preferably 5-7% by weight, of sodium hydroxide (NaOH), and

[0036] -0.1-2% by weight, preferably 0.1-1.5% by weight, of zinc oxide (ZnO),

[0037] Calculated based on the total weight of the cellulose carbamate spinning solution.

[0038] Cellulose carbamate spinning solutions are preferably prepared and degassed at a temperature of about 10 to 30°C.

[0039] According to a preferred embodiment, the cellulose carbamate spinning solution 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 curing. The cellulose carbamate spinning solution is also fed into the spinning unit at such a temperature, particularly at 15 to 25°C. The temperature of the cellulose carbamate spinning solution fed into the spinning unit affects, for example, the energy balance during spinning bath recycling.

[0040] According to one embodiment, the maximum total dissolved nitrogen content of the cellulose carbamate spinning solution is less than 0.2% by weight, preferably less than 0.15% by weight, calculated based on the total weight of the cellulose carbamate spinning solution. The total dissolved nitrogen in the cellulose carbamate spinning solution includes nitrogen chemically bound to cellulose in the form of carbamate groups and nitrogen released from the carbamate groups through alkaline hydrolysis.

[0041] The cellulose carbamate used in this invention can be obtained from any cellulose source.

[0042] Typically, cellulose carbamates are 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, swamp tree, and oak) or non-wood fibers such as straw fibers (wheat straw, rice straw, barley straw, bamboo, bagasse, and reeds). The source of raw materials can be virgin chemical or dissolving pulp or recycled raw materials (such as recycled paper and / or paperboard containing chemical or dissolving pulp).

[0043] Natural plant fibers can be used in their natural state or in the form of chemical pulp or dissolving pulp. The source of natural plant fibers can be their virgin form or textiles containing natural plant fibers or textiles containing recycled natural fibers. Natural plant fibers include seed fibers, such as cotton and kapok; bast fibers, such as hemp, jute, kenaf, ramie, abaca, and flax; leaf fibers, such as Manila hemp, sisal, pineapple hemp, and banana; and fruit fibers, such as coconut fiber.

[0044] Therefore, in one embodiment, recycled fiber raw materials are used as raw materials for the production of cellulose carbamates.

[0045] The cellulose carbamate spinning solution is preferably fed continuously into a spinning unit exhibiting an aqueous spinning bath containing sulfuric acid and dissolved aluminum compounds. Preferably, the cellulose carbamate spinning solution is fed into the spinning unit through one or more spinnerets. A spinneret may include, for example, 45 spinnerets, each with, for example, 2000 orifices. For example, a wet spinning production line for producing staple fibers may have at least 60 spinnerets coupled to the same spinning bath system. The cellulose carbamate spinning solution is fed into the spinning unit at a suitable flow rate. According to one embodiment, once the cellulose carbamate spinning solution is fed into the spinning unit, sulfuric acid reacts with sodium hydroxide and zinc oxide dissolved in sodium hydroxide to form sodium sulfate, zinc sulfate, and water; therefore, the spinning bath contains (excess) sulfuric acid, zinc sulfate, aluminum sulfate, sodium sulfate, and water. The cellulose carbamate solidifies and is formed into filaments upon extrusion through the spinnerets. Preferably, at least substantially all, i.e. at least 99% by weight, of the cellulose urethane is coagulated and formed into filaments.

[0046] In this context, the term "steady state" is generally used to refer to conditions that do not change significantly over time, such as the concentrations of the components. In a preferred embodiment, the steady-state conditions of the spinning bath correspond to the conditions of the portion of the spinning bath taken out before regeneration.

[0047] According to one embodiment, in a continuous spinning process that has reached steady-state operating conditions, the pH (20°C) of the spinning bath exiting the spinning unit is in the range of 0.2 to 0.9, preferably 0.7 to 0.9 or 0.5 to 0.8, and comprises:

[0048] - Zinc in the form of zinc sulfate (ZnSO4), in amounts of 10-50 g / L.

[0049] - The amount of dissolved ammonium nitrogen in the form of ammonium sulfate ((NH4)2SO4) is less than 20 g / L, preferably less than 15 g / L, and most preferably less than 10 g / L.

[0050] According to one implementation scheme, additives and / or auxiliaries present in the cellulose carbamate spinning solution can also ultimately be included in the recycling of spinning bath chemicals. Alternatively, additives (i.e., spinning bath auxiliaries) can be added directly to the spinning bath, and these additives can then also be present in the spinning cycle.

[0051] According to the preferred embodiment, the density of the spinning bath leaving the spinning unit is 1200 to 1330 kg / m³. 3(Density measured at 20°C) and the mass weight ratio of Na₂SO₄ to Al₂(SO₄)₃ is 1.0 to 3.0. The density of the spinning bath can be measured using a gas hydrometer or a liquid hydrometer (applicable to all densities in this application). In one embodiment, the density of the spinning bath leaving the spinning bath unit is generally related to the relative amounts of sodium sulfate and aluminum sulfate. In addition to sodium sulfate and aluminum sulfate, the spinning bath also contains sulfuric acid and other salts and compounds, but the sum of the concentrations of sodium sulfate and aluminum sulfate determines the density. Therefore, in one embodiment, the spinning bath conditions (such as the concentration of each component) should be selected in a way that makes these terms applicable.

[0052] According to one embodiment, the temperature of the spinning bath leaving the spinning unit is at least 10°C, preferably at least 15°C, for example, in the range of 10 to 25°C, and most preferably in the range of 15 to 22°C.

[0053] Therefore, according to one embodiment, a method for producing cellulose carbamate filaments or fibers includes the following steps:

[0054] - Provides a cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution, the cellulose carbamate spinning solution also exhibiting dissolved zinc compounds;

[0055] - The cellulose carbamate spinning solution is fed into a spinning unit, which exhibits an aqueous spinning bath containing sulfuric acid and aluminum sulfate;

[0056] - The cellulose carbamate is coagulated from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments.

[0057] - The cellulose urethane filament is subjected to stretching and optional washing, and the filament is optionally cut into short fibers;

[0058] - Take out a portion of the aqueous spinning bath containing sodium sulfate, water and zinc ions, wherein the temperature of the aqueous spinning bath taken out is at least 10°C;

[0059] - By removing at least a portion of the sodium sulfate, water, and zinc ions, the extracted portion of the spinning bath undergoes regeneration, and

[0060] - Recycle at least a portion of the regenerated aqueous spinning bath into the spinning unit.

[0061] The spinning bath conditions were selected such that the portion of the aqueous spinning bath removed from the spinning unit exhibited a concentration of 1200 to 1330 kg / m³. 3The density (density measured at 20°C) and the mass weight ratio of Na2SO4 to Al2(SO4)3 from 1:1 to 3:1.

[0062] According to one embodiment, the pH (20°C) of the spinning bath leaving the spinning unit is in the range of 0.2 to 0.9, preferably in the range of 0.7 to 0.9.

[0063] The spinning bath of the present invention is an acidic spinning bath containing sulfuric acid as a free acid. The amount of sulfuric acid is such that the pH of the spinning bath is maintained within the range described above.

[0064] In the embodiment, the amount of soluble ammonium nitrogen in the form of ammonium sulfate is maintained at a level generally less than 20 g / L, preferably less than 15 g / L, and most preferably less than 10 g / L, in the spinning bath and in the portion of the spinning bath removed from the spinning unit and undergoing recycling. Therefore, the formation of the ammonium-aluminum double salt, i.e., ammonia alum, can be reduced or even substantially eliminated. The formation of ammonia alum salt, as a slightly soluble salt, causes at least a portion of the aluminum in the spinning bath to precipitate.

[0065] The dissolved ammonium nitrogen content depends at least in part on the carbamate content of the cellulose carbamate material used to manufacture the cellulose carbamate spinning solution and the conditions between the dissolution of the cellulose carbamate to form the cellulose carbamate spinning solution and its feeding into the spinning unit. Therefore, the hydrolysis of cellulose carbamate during dissolution releases nitrogen compounds into the cellulose carbamate spinning solution, primarily in the form of ammonium nitrogen compounds. The temperature during dissolution and the delay time between the dissolution of the cellulose carbamate and the feeding of the formed cellulose carbamate spinning solution into the spinning unit will affect the dissolved ammonium nitrogen content in the spinning bath composition.

[0066] Therefore, the hydrolysis of the urethane groups bound to cellulose releases ammonium nitrogen, which ultimately enters the spinning bath primarily as ammonium sulfate. However, cellulose urethane may also carry other nitrogen compounds, such as urea and biuret, which do not enter the spinning bath as ammonium nitrogen and thus do not form ammonium sulfate.

[0067] According to a preferred embodiment, the cellulose carbamate spinning solution is provided with a degree of hydrolysis of less than 70%, particularly less than 65%, and most preferably less than 60%, which is calculated by the percentage difference of cellulose-bound nitrogen between the cellulose carbamate before dissolution and the solidified cellulose carbamate.

[0068] According to one embodiment, if desired, the amount of ammonium nitrogen can be maintained at a predetermined level by removing it from the spinning bath or from the removed portion of the spinning bath as a separate operation during the regeneration cycle. According to one embodiment, if desired, at least a portion of the ammonium nitrogen can be removed during the removal of sodium sulfate by crystallization. Therefore, in one embodiment, a low content of soluble nitrogen (compound), especially a low content of ammonium nitrogen, in the spinning bath is preferred for the treatment of the spinning bath.

[0069] In one embodiment, the amount of ammonium nitrogen in the spinning bath is at least 1 g / L of the total amount of the spinning bath. Therefore, in a preferred embodiment, the amount of ammonium nitrogen in the spinning bath and thus in the portion of the aqueous spinning bath taken out is in the range of 1 to 20 g / L, more preferably in the range of 1 to 15 g / L, for example in the range of 2 to 10 g / L.

[0070] According to a preferred embodiment, the soluble nitrogen content in the spinning bath is at least predominantly present in the coagulated filament, wherein the nitrogen does not need to be removed separately by other means. In one embodiment, at least 30% by weight, particularly at least 35% by weight, and most preferably at least 40% by weight, of nitrogen (in the form of nitrogen compounds) derived from the cellulose urethane spinning solution is present (as urethane groups) in the coagulated filament or fiber.

[0071] Therefore, in one embodiment, the soluble nitrogen content in the spinning bath is determined by a combination of the above characteristics and maintained at an appropriate level.

[0072] Conversely, aluminum is important because it can improve the quality of the produced filaments or fibers, especially since the shape of the filaments or fibers is controlled by the amount of aluminum (aluminum sulfate) in the spinning bath and by controlled stretching. In particular, aluminum affects the shape of the cross-section, making the filaments and fibers more similar to cotton fibers, which is preferred. Furthermore, aluminum inhibits cellulose crystallization during the coagulation process. Amphoteric aluminum compounds also affect the acid-base neutralization reaction during the coagulation process. Simultaneously, the toughness yield of the recycled filaments or fibers is improved. A small amount of aluminum is required in acidic wet spinning processes to form regular fibers in a controlled manner. However, aluminum can also impair the spinning bath regeneration process. Therefore, a balance needs to be maintained among the different components.

[0073] In one embodiment, the ratio between aluminum sulfate and sodium sulfate, calculated as the mass weight ratio of Na₂SO₄ to Al₂(SO₄)₃, is 1:1 to 3:1. In one embodiment, the mass weight ratio of Na₂SO₄ to Al₂(SO₄)₃ in the spinning bath is maintained at 1:1 to 3:1 by regeneration and recycling of the aqueous spinning bath. Therefore, in one embodiment, by...

[0074] - A portion of the spinning bath is removed from the spinning unit for regeneration to reduce the sodium sulfate content in the removed portion; and

[0075] - Recycle at least a portion of the regenerated spinning bath into the spinning unit.

[0076] The mass weight ratio of Na2SO4 to Al2(SO4)3 in the spinning bath is maintained at 1.0 to 3.0.

[0077] Therefore, according to one embodiment, a method for producing cellulose carbamate filaments or fibers includes the following steps:

[0078] - Provides a cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution, the cellulose carbamate spinning solution also exhibiting dissolved zinc compounds;

[0079] - The cellulose carbamate spinning solution is fed into a spinning unit, which exhibits an aqueous spinning bath containing sulfuric acid and aluminum sulfate;

[0080] - The cellulose carbamate is coagulated from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments.

[0081] - The cellulose urethane filament is subjected to stretching and optional washing, and the filament is optionally cut into short fibers;

[0082] Among them, through

[0083] - A portion of the aqueous spinning bath is removed from the spinning unit for regeneration to reduce the sodium sulfate content in the removed portion; and

[0084] - Recycle at least a portion of the regenerated aqueous spinning bath into the spinning unit.

[0085] The mass weight ratio of Na2SO4 to Al2(SO4)3 in the spinning bath is maintained at 1.0 to 3.0.

[0086] In a preferred embodiment, the density of the spinning bath is 1200 to 1330 kg / m³. 3 .

[0087] According to one embodiment, the spinning bath temperature is at most 25°C, for example, 15 to 25°C, with the most suitable range being 15 to 22°C. At too low a spinning bath temperature, salt will easily crystallize from the aqueous spinning bath, while higher spinning bath temperatures require a higher aluminum concentration to achieve the target toughness yield, as increasing the temperature accelerates the solidification process. Therefore, the spinning bath temperature is preferably within the aforementioned range.

[0088] According to the preferred embodiment, the cellulose carbamate spinning solution undergoes coagulation in a spinning bath at a temperature of up to 23°C, most preferably up to 20°C.

[0089] In a spinning bath, cellulose urethane solidifies into cellulose urethane filaments, preferably through one or more spinnerets. These filaments are recovered from the spinning bath and subjected to stretching, optionally cutting into fibers, and optional post-treatment, in which the filaments or fibers are washed, bleached, and / or spun finishing.

[0090] According to one embodiment, the filaments obtained from the spinning bath undergo stretching in a stretching unit. The cellulose urethane filaments undergo stretching in one stretching unit or a cascade of at least two stretching units.

[0091] According to one embodiment, the stretching unit includes a stretching bath or air stretching. Typically, the stretching bath is an aqueous bath containing sulfuric acid, aluminum sulfate, zinc sulfate, and sodium sulfate—the same components as the spinning bath, but at a lower concentration—because the solidified filaments carry these components along with the stretching unit. The same components can also be carried by the filaments (or by the fibers after cutting) through one or more steps, meaning the resulting product may still contain some of the components from the spinning bath. This is why the method of the present invention may include a semi-closed system in some embodiments. The stretching bath is particularly useful when producing cellulose urethane staple fibers, i.e., preferably when stretching filament bundles.

[0092] According to one embodiment, the stretching bath is a hot stretching bath in which filaments or filament bundles are immersed in a stretching bath solution. The temperature in the stretching bath is typically in the range of 75 to 100°C, preferably 85 to 95°C, and more preferably about 90°C.

[0093] According to another embodiment, the cellulose urethane filament is stretched in a stretching unit without a stretching bath, i.e., by so-called air stretching.

[0094] According to one embodiment, after stretching, the filaments, particularly the filament bundles, are cut into fibers of the desired length by a cutter in a cutting unit. Cutting can be performed using any known filament cutting method.

[0095] After stretching and optionally cutting, the filaments or fibers undergo post-processing in a post-processing unit. According to a preferred embodiment, post-processing includes washing, bleaching, spinning finishing, or any combination thereof. In one embodiment, washing is performed using water or the effluent from a previous or subsequent processing step in a counter-current washing manner.

[0096] Finally, the obtained filaments or fibers are dried using any known drying method.

[0097] According to a preferred embodiment, cellulose urethane filaments, particularly cellulose urethane filament bundles, are stretched and then cut to provide stable short fibers. The resulting stable fibers are then post-treated, particularly washed, in a post-treatment unit. Finally, the obtained fibers are dried.

[0098] According to another embodiment, cellulose urethane filaments, particularly cellulose urethane filament bundles, are stretched and then post-treated in a post-treatment unit, particularly by washing, after which the treated wet or dry filaments are cut into short fibers or microfibers.

[0099] According to another embodiment, the cellulose filaments are stretched, and then the resulting filaments are post-treated, particularly washed in a post-treatment washing unit.

[0100] The aforementioned spinning processes for producing cellulose carbamate filaments or fibers involve a variety of chemicals, among which chemical recyclability is crucial for making the process sustainable. Therefore, the method of the present invention also includes chemical recycling, wherein at least a portion of the spinning bath is regenerated and solid materials are recovered.

[0101] According to a preferred embodiment, at least a portion of the stretching and / or washing baths are also regenerated and solid materials are recovered. Therefore, the spinning bath regeneration described in this application is also applicable to the stretching and washing baths. All removed portions can be regenerated simultaneously in the same regeneration process.

[0102] First, a portion of the aqueous spinning bath containing sodium sulfate, water, and zinc ions is removed from the spinning unit (and / or stretching unit and / or washing unit), and this portion of the spinning bath (and / or stretching bath and / or washing bath) is regenerated by removing at least a portion of the sodium sulfate, water, and zinc ions. Regeneration of the acidic spinning bath requires the removal of these components to prevent their accumulation in the spinning bath under the action of the incoming cellulose carbamate spinning solution. Before regeneration, the removed aqueous spinning bath contains dissolved ammonium nitrogen in the form of ammonium sulfate at a temperature of at least 10°C, in an amount less than 20 g / L, preferably less than 15 g / L, and most preferably less than 10 g / L of spinning bath. Therefore, according to a preferred embodiment, the removed portion of the aqueous spinning bath is at least substantially free of ammonia alum precipitate, and preferably does not contain any solid ammonia alum.

[0103] According to one implementation, a portion of the aqueous spinning bath removed from the spinning unit exhibits a concentration of 1200 to 1330 kg / m³. 3 The density (measured at 20°C) and the preferred mass weight ratio of Na2SO4 to Al2(SO4)3 are 1:1 to 3:1.

[0104] As described above, according to one embodiment, the portion of the spinning bath leaving the spinning unit, also known as the aqueous spinning bath having a pH (20°C) in the range of 0.2 to 0.9, preferably 0.5-0.8 or 0.5-0.9, comprises, before regeneration:

[0105] Zinc in the form of ZnSO4, -10–50 g / L, and

[0106] - Dissolved nitrogen in the form of (NH4)2SO4 less than 20 g / L, preferably less than about 15 g / L, and most preferably less than 10 g / L.

[0107] According to one embodiment, the method includes continuously removing a volume of aqueous spinning bath from the spinning unit for regeneration. Similarly, according to one embodiment, the liquid phase portion of the stretching and washing bath is continuously removed for regeneration.

[0108] In one embodiment, by removing the aqueous spinning bath and subjecting it to sodium sulfate and zinc recovery operations, at least 70% by weight, preferably at least 80% by weight, more preferably at least 90% by weight of sodium sulfate formed in the spinning bath, particularly under steady-state conditions, is recovered in a crystallization step, and at least 80% by weight, preferably at least 85% by weight, more preferably at least 90% by weight of zinc sulfate formed in the spinning bath, particularly under steady-state conditions, is recovered in a zinc removal step.

[0109] According to one embodiment, a portion of the aqueous spinning bath is first transferred to a tank called a "dilute bath," where the extracted aqueous spinning bath can undergo regeneration. According to one embodiment, an optional portion of the aqueous stretching and / or washing bath can also be transferred to the same tank. In one embodiment, the water evaporation capacity of this method defines how much stretching and / or washing bath can be transferred to the recycling process of the spinning bath. In one embodiment, an optional portion of the aqueous stretching and / or washing bath can also be transferred directly to a dewatering feed (evaporation) to utilize the energy content of these high-temperature solution streams.

[0110] According to one embodiment, regeneration includes subjecting a portion of the aqueous spinning bath to a separation step, a zinc removal step, a water removal step, or a sodium sulfate removal step, or any combination thereof, preferably all of them.

[0111] In a preferred embodiment, the portion of the aqueous spinning bath is filtered as a whole, but only a portion of the filtered spinning bath undergoes other separation steps.

[0112] According to a preferred embodiment, regeneration includes subjecting a portion of the aqueous spinning bath to filtration in a filtration unit, zinc extraction in an extraction unit, and sodium sulfate crystallization in a crystallization unit. According to another embodiment, regeneration may also include only one or two of these units; that is, regeneration may include filtration, zinc extraction, or sodium sulfate crystallization, or any combination thereof. Furthermore, regeneration typically involves water evaporation.

[0113] According to one embodiment, the portion of the aqueous spinning bath is preferably first subjected to separation, preferably in a filtration unit, to remove solids, such as aluminum ammonium sulfate and other solid materials that may be present. Optional separation can be carried out by any known separation method, such as filtration using diatomaceous earth or perlite, or, for example, centrifugation. The solids are recovered from the process. The filtrate, i.e., the portion of the spinning bath free of solids, continues in a regeneration process.

[0114] According to one embodiment, the method includes selectively removing zinc from a portion of the aqueous spinning bath.

[0115] Therefore, according to the preferred embodiment, it is preferable to subject the portion taken out of the spinning bath, or at least a portion thereof, to selective zinc removal after separation, preferably zinc selective extraction. Prior to zinc removal, the pH of the portion taken out of the aqueous spinning bath is preferably higher than 0.2, more preferably higher than 0.5, and most suitablely in the range of 0.7 to 0.9 or 0.5 to 0.75.

[0116] According to a preferred embodiment, the zinc concentrate recovered by zinc selective extraction contains preferably less than 10% by weight of aluminum ions (as Al). 3+ The preferred concentration is less than 5% by weight, for example less than 3% by weight, most preferably less than 2.5% by weight, for example less than 2.0% by weight, based on the zinc fraction (as Zn) in the recovered zinc concentrate. 2+ )calculate.

[0117] According to one embodiment, preferably before subjecting the aqueous spinning bath to zinc extraction, the impurities trivalent iron (Fe3+) in the spinning bath are reduced using, for example, sulfur dioxide. 3+ ) is reduced to divalent iron (Fe) 2+ By saturating the active extraction reagent, ferric iron is readily extracted along with zinc. If a reduction treatment with sulfur dioxide is used, a separate iron removal step to purify the saturated extraction reagent can be avoided.

[0118] According to one embodiment, zinc can be removed by extraction or ion exchange performed in one or more stages, such as liquid-liquid extraction or solid-phase extraction (“SPE”), especially liquid-liquid extraction. Preferably, zinc is removed selectively to avoid co-extraction of aluminum. Surprisingly, it has been found in this invention that zinc can be selectively removed under the acidic conditions of this invention. In particular, the pH of the spinning bath is crucial during zinc extraction. Therefore, according to one embodiment, the method includes removing zinc from a portion of an aqueous spinning bath having a pH above 0.2, preferably above 0.5, and most preferably between 0.7 and 0.9 or 0.5 and 0.75.

[0119] According to one embodiment, the method includes removing zinc from said portion by contacting the extracted portion of an aqueous spinning bath with an organic solvent containing an active extractant, particularly an organophosphorus active extractant, especially at least one of the following either alone or in a carrier matrix: di(2-ethylhexyl)phosphoric acid, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC-88A), bis(2,4,4-trimethylpentyl)phosphonic acid, bis(2,4,4-trimethylpentyl)dithiophosphonic acid, or mixtures thereof, especially di(2-ethylhexyl)phosphoric acid.

[0120] According to one implementation scheme, zinc removal can be carried out at elevated temperatures, and the aqueous spinning bath that has been removed can be heated before zinc removal.

[0121] Therefore, according to one embodiment, the method includes removing zinc from a portion of an aqueous spinning bath at a pH of 0.7 to 0.9 or 0.5 to 0.75 by liquid-liquid extraction, particularly multi-stage liquid-liquid extraction, using an organic solvent and at elevated temperatures.

[0122] According to a preferred embodiment, zinc is removed by liquid-liquid extraction, preferably multi-stage liquid-liquid extraction, in a solution of an organic active extractant / organic slightly water-soluble hydrocarbon solvent. According to one embodiment, the amount of active extractant in the extraction solution is preferably 20 to 45% by volume, more preferably 40% by volume, calculated based on the total volume of the extraction solution (i.e., the total volume of the extractant and the hydrocarbon solvent). According to the most suitable embodiment, the organic solvent mixture for selective zinc extraction comprises di-(2-ethylhexyl)phosphoric acid as the active extractant and kerosene or a similar type of inert hydrocarbon solvent as a slightly water-soluble hydrocarbon solvent, serving as the carrier matrix.

[0123] The extracted zinc is typically recovered as a concentrate of an acidic, sulfuric acid-containing zinc sulfate solution, and the remainder of the removed spinning bath is further transferred in a regeneration process.

[0124] In solid-phase extraction, zinc present in the liquid phase is adsorbed onto a solid or stationary phase (through which the liquid passes). The stationary phase may include organophosphorus groups or compounds of the type described above.

[0125] According to one implementation, the recovered zinc can be recycled back to the dissolution and / or spinning process, via zinc hydroxide precipitation, to dissolve cellulose carbamate to form a cellulose carbamate spinning solution.

[0126] According to one embodiment, the removed portion of the spinning bath, or at least a portion thereof, preferably undergoes water evaporation after filtration and / or zinc extraction. Thus, according to one embodiment, the method includes removing water therein by evaporation before guiding the removed spinning bath to the step of sodium sulfate removal. The cellulose carbamate spinning solution introduces a large amount of water into the spinning unit, and in a preferred embodiment, the water is removed during a regeneration cycle. Preferably, the water is evaporated before sodium sulfate removal because the concentration of sodium sulfate is suitable to be increased before its removal. According to one embodiment, the removed portion of the aqueous spinning bath undergoing sodium sulfate removal may comprise a combination of the removed portion of the aqueous spinning bath with evaporated water and the unevaporated portion of the aqueous spinning bath, provided the water volume is at an appropriate level in the following steps. In addition to removing water by evaporation, crystallized sodium sulfate decahydrate also removes water from the spinning bath system as hydrated salt crystals.

[0127] According to a preferred embodiment, the removed spinning bath preferably undergoes sodium sulfate removal after separation, zinc removal, and / or water evaporation, preferably sodium sulfate crystallization, particularly in the form of sodium sulfate decahydrate, such as by cooling crystallization or vacuum crystallization. Therefore, according to one embodiment, the method includes removing sodium sulfate from the removed portion of the spinning bath by crystallization.

[0128] According to a preferred embodiment, the portion taken out of the aqueous spinning bath after sodium sulfate removal exhibits a certain ratio of sodium sulfate and aluminum sulfate to avoid aluminum sulfate co-crystallization, which would interfere with the crystallization of sodium sulfate to the purest possible level. Therefore, according to a preferred embodiment, the method includes providing the portion taken out of the aqueous spinning bath, preferably exhibiting a Na₂SO₄ to Al₂(SO₄)₃ mass weight ratio of 1.4:1 to 2.5:1, most preferably 1.5:1 to 2:1, and most suitable 1.6:1 to 1.9:1, and feeding this portion into the sodium sulfate removal step. In one embodiment, the Na₂SO₄ to Al₂(SO₄)₃ ratio is adjusted, for example, by adding sodium sulfate. Therefore, in one embodiment, a portion of the crystallized sodium sulfate is returned to the crystallization step to adjust the Na₂SO₄ to Al₂(SO₄)₃ ratio to promote crystallization. According to a preferred embodiment, the suitable Na₂SO₄ to Al₂(SO₄)₃ mass weight ratio is controlled by adjusting the amount of crystallized sodium sulfate.

[0129] According to one embodiment, the pH of the portion taken out after undergoing an aqueous spinning bath to remove sodium sulfate is in the range of 0.2 to 0.9, and its zinc concentration is preferably up to 60 g / L.

[0130] Therefore, according to one implementation scheme, several factors affect the crystallization purity of sodium sulfate, the most important of which is the mass weight ratio of Na₂SO₄ to Al₂(SO₄)₃. If this ratio is not optimized, a significant amount of aluminum sulfate may crystallize in the crystalline sodium sulfate. Furthermore, the concentrations of zinc and sulfuric acid affect the purity of the sodium sulfate crystals.

[0131] According to the preferred embodiment, aluminum sulfate remains essentially in the cycle, i.e., it does not crystallize together with sodium sulfate. Therefore, the remainder of the spinning bath after crystallization preferably contains aluminum sulfate.

[0132] Preferably, the portion of the aqueous spinning bath is subjected to cooling crystallization of sodium sulfate at a temperature of 1 to 10°C, for example 4 to 8°C, and external cooling is preferred.

[0133] According to another embodiment, a portion of the aqueous spinning bath is subjected to vacuum crystallization with sodium sulfate. During the vacuum crystallization process, the aqueous spinning bath is concentrated, for example, 4-8% by weight of water can be evaporated during the vacuum crystallization process.

[0134] Sodium sulfate crystals are recovered, particularly as sodium sulfate decahydrate, and optionally washed. The sodium sulfate decahydrate crystals are optionally subjected to calcination. The remainder of the spinning bath after crystallization (i.e., the supernatant) is transferred to a "concentration bath" tank.

[0135] According to one implementation scheme, the supernatant is separated from sodium sulfate and recycled to the spinning bath.

[0136] Therefore, according to one embodiment, the regenerated acidic spinning bath includes a portion of the aqueous spinning bath removed during the regeneration cycle, for example, through extraction and crystallization, but it may also include a portion of the spinning bath that has already been removed through the regeneration step. This regenerated spinning bath can be collected in a "concentrate" tank before being recycled back to the spinning unit bath.

[0137] According to one embodiment, the concentration of the spinning bath to be fed into the spinning unit is adjusted to an appropriate level by adding sulfuric acid and / or aluminum sulfate. According to another embodiment, the method includes feeding fresh sulfuric acid and / or aluminum sulfate into a regenerating spinning bath and then recycling it back to the spinning unit. Thus, fresh feed of sulfuric acid, aluminum sulfate, aluminum hydroxide, or any mixture thereof can be added to, for example, a "concentration bath" tank. Optional fresh aluminum hydroxide feed can be reacted with sulfuric acid to form aluminum sulfate.

[0138] According to one implementation, if any aluminum sulfate eventually enters the effluent of a semi-closed-loop coagulation or spinning bath circulation system, aluminum can be precipitated from it as aluminum hydroxide and optionally returned to the spinning bath, for example as described above.

[0139] According to one embodiment, a method for producing cellulose carbamate filaments or fibers includes the following steps:

[0140] - Provides a cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution, the cellulose carbamate spinning solution also exhibiting dissolved zinc compounds, wherein, based on the total weight of the cellulose carbamate spinning solution, the spinning solution contains 6-10% by weight of cellulose carbamate, 5-10% by weight of sodium hydroxide and 0.1-2% by weight of dissolved zinc compounds;

[0141] - The cellulose carbamate spinning solution is fed into a spinning unit, which exhibits an aqueous spinning bath containing sulfuric acid and aluminum sulfate, wherein the pH (20°C) of the spinning bath is in the range of 0.2 to 0.9 under stable conditions;

[0142] - The cellulose carbamate from the cellulose carbamate spinning solution is coagulated into the aqueous spinning bath to form cellulose carbamate filaments.

[0143] - The cellulose urethane filament is subjected to stretching and optional washing, and the filament is optionally cut into short fibers;

[0144] - Take out a portion of the aqueous spinning bath containing sodium sulfate, water and zinc ions, and the temperature of the aqueous spinning bath taken out is at least 10°C;

[0145] - By removing at least a portion of the sodium sulfate, water, and zinc ions, the removed portion of the spinning bath undergoes regeneration, and

[0146] - Recycle at least a portion of the regenerated aqueous spinning bath into the spinning unit.

[0147] According to one embodiment, the method includes recovering aluminum sulfate from a step of selectively removing zinc and a step of selectively recovering sodium sulfate, and recycling the recovered aluminum sulfate to the spinning bath.

[0148] The attached diagram illustrates the implementation plan.

[0149] Figure 1A method according to at least some embodiments of the invention is shown, wherein a cellulose carbamate spinning solution is first fed into a spinning unit 1 exhibiting an aqueous spinning bath. In the spinning bath, cellulose carbamate is coagulated from the cellulose carbamate spinning solution into the aqueous spinning bath to form cellulose carbamate filaments or cellulose carbamate filament bundles, which are then stretched in one or more stretching units 2. The stretched filaments can then be washed in a washing unit 3a. Alternatively, the stretched filament bundles can first be cut into short fibers in a cutting unit 3b, and then washed in the washing unit 3a. After washing, cellulose carbamate filaments or stabilized fibers are obtained.

[0150] The method of the present invention also includes the chemical recovery of chemicals. Therefore, a portion of an aqueous spinning bath containing sodium sulfate, water, and zinc ions is taken from the spinning bath of spinning unit 1, preferably into a "dilute bath" tank 4. A portion of an aqueous stretching bath from stretching unit 2 is also taken, preferably introduced into the "dilute bath" tank 4 together with the taken-out spinning bath. The taken-out portions of the spinning bath and stretching bath undergo regeneration by removing at least a portion of the sodium sulfate, water, and zinc ions. In the regeneration cycle, the taken-out aqueous spinning bath and stretching bath are first filtered in filtration unit 5. After filtration, the filtrate undergoes zinc selective extraction in extraction unit 6, while the residue, i.e., solid matter, is recovered from the process. At least a portion of the zinc ions are removed and recovered by zinc selective extraction, preferably as an acidic zinc sulfate concentrate. After zinc extraction, the remaining portion of the mixture of the taken-out spinning bath and stretching bath is subjected to water evaporation in evaporation unit 7, and then sodium sulfate crystallization is performed in crystallization unit 8. The crystallized sodium sulfate is recovered, and the remaining portion of the mixture of the taken-out spinning bath and stretching bath is transferred to a "concentrate bath" tank 9. Therefore, the spinning bath collected in the "concentrated bath" tank 9 is at least partially regenerated and can then be recycled back to the spinning unit 1.

[0151] As discussed above, at least a portion of the regenerated aqueous spinning bath (preferably collected in the "concentrate" tank 9) is recycled back to the spinning unit 1. However, according to a preferred embodiment, the method further includes feeding fresh sulfuric acid and aluminum sulfate into the regenerated spinning bath and then recycling it back to the spinning unit 1.

[0152] The following non-limiting examples illustrate at least some embodiments of the present invention:

[0153] Example 1

[0154] Cellulose carbamate powder (with a nitrogen content of 1.6 wt% in dry cellulose carbamate) was dissolved in an alkaline aqueous solution to obtain a composition containing 8.5 wt% cellulose carbamate, 6.5 wt% sodium hydroxide, 1.3 wt% dissolved zinc oxide, and 0.085 wt% dissolved nitrogen, thus preparing a cellulose carbamate spinning solution. This composition, i.e., the cellulose carbamate spinning solution, was fed into a spinning unit through a spinneret, where the cellulose carbamate solidified into cellulose carbamate fiber filaments. The filament bundles were formed and further stretched and cut into short fibers in a stretching bath. The recovered short fibers were then washed in a post-treatment unit. The short fibers recovered from the post-treatment unit had a nitrogen content of 0.7 wt% in dry solids. The degree of hydrolysis of the cellulose carbamate was 56%.

[0155] A portion of the aqueous spinning bath was taken from the spinning unit. Its pH was 0.75, the ratio of sodium sulfate to aluminum sulfate was 1.63, the density was 1.300 kg / L, the temperature was 20°C, the ammonium sulfate content was 7.5 g / L, and the zinc sulfate content was 20.0 g / L. The density was measured using a liquid hydrometer / gas hydrometer.

[0156] Filter the portion removed from the spinning bath.

[0157] A portion of the filtered aqueous spinning bath at pH 0.75 underwent a three-stage liquid-liquid extraction process to remove a portion of the zinc. For the liquid-liquid extraction process, two volumes of the aqueous spinning bath and one volume of an organic solvent mixture (containing 40 vol% di(2-ethylhexyl)phosphoric acid in kerosene) were efficiently mixed for 3 minutes, followed by separation of the aqueous and organic solvent phases. The separated aqueous phase was re-extracted with the fresh portion of the organic solvent. The extraction process was repeated using the fresh portion of the organic solvent. The separated organic solvent phases from the three subsequent extraction steps were mixed together, and zinc was released by back-extraction (stripping) with a 100 g / L aqueous sulfuric acid solution. The zinc and aluminum content in the sulfuric acid solution obtained by back-extraction was analyzed by selective titration with ethylenediaminetetraacetic acid (EDTA). According to the analysis, the total extraction yield was 87 wt% zinc and 1.9 wt% aluminum. Zinc was selectively extracted in the presence of aluminum. The zinc sulfate content in the aqueous spinning bath decreased from 20 g / L to 2.6 g / L.

[0158] A portion of the filtered and zinc-extracted aqueous spinning bath is subjected to evaporation: 21.5% by weight of water (accounting for the total amount of the aqueous spinning bath) is removed by evaporation to obtain a concentrated aqueous spinning bath in which the ratio of sodium sulfate to aluminum sulfate is 1.63, the temperature is 50°C, and the ammonium sulfate content is 9.3 g / L (standard SFS 5505:1988).

[0159] A portion of the concentrated aqueous spinning bath underwent cooling crystallization. The temperature of the concentrate was reduced from 50°C to 8°C. The yield of salt crystals containing sodium sulfate decahydrate recovered by centrifugation was 29.4 wt%, with a supernatant yield of 70.6 wt%. The centrifuged crystals were washed with water to remove most of the supernatant residue from the crystal surface. The hydrated crystals were calcined at 130°C. The anhydrous crystal sample taken after calcination was dissolved in water, and its aluminum content was determined by selective titration with EDTA. The recovered calcined sodium sulfate had an aluminum content of 0.56 wt%, equivalent to 3.5 wt% aluminum sulfate in sodium sulfate. Sodium sulfate was selectively crystallized in the presence of aluminum sulfate. In contrast, when the sodium sulfate to aluminum sulfate ratio was 1.18 instead of 1.64, and other process conditions were the same, the aluminum content of the calcined sodium sulfate was 13 wt%. Therefore, if the sodium sulfate to aluminum sulfate ratio is not optimized, the selectivity of crystallization decreases.

[0160] The supernatant containing aluminum sulfate is recycled back to the spinning bath, which comprises both regenerated and unregenerated portions of the aqueous spinning bath collected from different steps of the spinning bath regeneration process. Before recycling the mixture back to the spinning unit, some fresh sulfuric acid and aluminum sulfate are added.

[0161] It should be understood that the embodiments of the present invention disclosed herein are not limited to the specific structures, process steps, or materials disclosed herein, but extend to their equivalents as recognized by those skilled in the art. It should also be understood that the terminology used herein is for describing specific embodiments only and is not intended to be limiting.

[0162] References to an embodiment or an embodiment throughout the specification mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Therefore, the phrases "in one embodiment" or "in an embodiment" appearing throughout the specification do not necessarily refer to the same embodiment. When numerical values ​​are referenced using terms such as approximately or substantially, precise numerical values ​​are also disclosed.

[0163] As used herein, for convenience, multiple items, structural elements, constituent elements, and / or materials may be presented in a public list. However, these lists should be interpreted as if each member of the list were individually identified as a separate and unique member. Therefore, unless otherwise stated, any single member of such a list should not be construed as a de facto equivalent to any other member of the same list solely based on their presentation in a common group. Furthermore, various embodiments and examples of the invention may be mentioned herein together with alternatives to their various components. It should be understood that these embodiments, examples, and alternatives should not be construed as de facto equivalents to each other, but should be regarded as separate and autonomous representations of the invention.

[0164] Furthermore, the described features, structures, or characteristics can be combined in one or more embodiments in any suitable manner. Numerous specific details, such as examples of length, width, shape, etc., are provided in the following description to provide a thorough understanding of embodiments of the invention. However, those skilled in the art will recognize that the invention can be practiced without one or more specific details, or using other methods, components, materials, etc. In other instances, well-known structures, materials, or operations have not been shown or described in detail to avoid obscuring aspects of the invention.

[0165] While the foregoing examples illustrate the principles of the invention in one or more specific applications, those skilled in the art will understand that many modifications can be made to the form, use, and implementation details without inventive effort and without departing from the principles and concepts of the invention. Therefore, the invention is not limited in any way beyond the following claims.

[0166] The verbs “comprising” and “including” are used in this document as open-ended restrictions, neither excluding nor requiring the presence of any undescribed features. Unless otherwise expressly stated, the features described in the dependent claims may be freely combined with each other. Furthermore, it should be understood that the use of “an” or “a” in the singular form herein does not exclude the plural.

[0167] List of reference numerals

[0168] Reference number

[0169] 1 Spinning unit

[0170] 2. Tensioning Unit

[0171] 3a Washing unit

[0172] 3b Cutting Unit

[0173] 4. "Diluted Bath" Tank

[0174] 5 Filter Units

[0175] 6 Extraction Units

[0176] 7 Evaporation Unit

[0177] 8 crystallization units

[0178] 9. "Concentrated Bath" Tank

[0179] Industrial applicability

[0180] At least some embodiments of the present invention have industrial applications in the production of cellulose urethane filaments or fibers from cellulose urethane spinning dopes.

Claims

1. A method for producing cellulose carbamate filaments or fibers, comprising the following steps: A cellulose carbamate spinning solution containing cellulose carbamate dissolved in an aqueous sodium hydroxide solution is provided, and the cellulose carbamate spinning solution also exhibits dissolved zinc compounds. The cellulose carbamate spinning solution is fed into the spinning unit, which exhibits an aqueous spinning bath containing sulfuric acid and aluminum sulfate. Cellulose carbamate is coagulated from cellulose carbamate spinning solution into an aqueous spinning bath to form cellulose carbamate filaments. The cellulose urethane filaments are stretched. A portion of the aqueous spinning bath containing sodium sulfate, water, and zinc ions is removed. The temperature of the removed aqueous spinning bath is at least 10°C. The removed portion of the aqueous spinning bath contains dissolved nitrogen in the form of ammonium nitrogen, less than 20 g / L, as ammonium sulfate. The removed portion of the spinning bath undergoes regeneration by removing at least a portion of the sodium sulfate, water, and zinc ions. At least a portion of the regenerated aqueous spinning bath is recycled to the spinning unit.

2. The method according to claim 1, wherein, prior to regeneration, the portion of the aqueous spinning bath contains dissolved nitrogen in the form of ammonium nitrogen at a temperature of at least 10°C in the form of ammonium nitrogen.

3. The method according to claim 1, wherein the mass weight ratio of Na2SO4 to Al2(SO4)3 in the spinning bath is maintained at 1:1 to 3:1 through regeneration and recycling of the aqueous spinning bath.

4. The method according to claim 2, wherein the mass weight ratio of Na2SO4 to Al2(SO4)3 in the spinning bath is maintained at 1:1 to 3:1 through regeneration and recycling of the aqueous spinning bath.

5. The method according to any one of claims 1-4, wherein the portion of the aqueous spinning bath removed from the spinning unit exhibits a concentration of 1200 to 1330 kg / m³. 3 The density and the mass weight ratio of Na2SO4 to Al2(SO4)3 of 1:1 to 3:

1.

6. The method according to any one of claims 1-4, wherein the portion of the aqueous spinning bath having a pH in the range of 0.2 to 0.9 at 20°C comprises, before regeneration: 10–50 g / l of zinc as ZnSO4, and Less than 20 g / L is the dissolved nitrogen of (NH4)2SO4.

7. The method according to any one of claims 1-4, comprising subjecting a portion of the aqueous spinning bath to separation to remove solids.

8. The method according to any one of claims 1-4, comprising selectively removing zinc from a portion of the aqueous spinning bath.

9. The method of claim 8, further comprising removing zinc from said portion by contacting a portion of an aqueous spinning bath having a pH higher than 0.2 with at least one of the following, either alone or in a carrier matrix: di(2-ethylhexyl)phosphoric acid, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC-88A), bis(2,4,4-trimethylpentyl)phosphonic acid, bis(2,4,4-trimethylpentyl)dithiophosphonic acid, or mixtures thereof.

10. The method of claim 8, further comprising removing zinc by extraction or ion exchange performed in one or more stages.

11. The method of claim 8, wherein zinc is removed by liquid-liquid extraction or solid-phase extraction.

12. The method according to any one of claims 1-4 and 9-11, comprising removing water from a portion of the aqueous spinning bath by evaporation, followed by a sodium sulfate removal step.

13. The method of claim 12, further comprising removing sodium sulfate from a portion of an aqueous spinning bath exhibiting a Na2SO4 to Al2(SO4)3 mass weight ratio of 1.4:1 to 2.5:1, and then subjecting the portion of the aqueous spinning bath to crystallization to remove sodium sulfate.

14. The method according to claim 13, wherein after adjusting the ratio of Na2SO4 to Al2(SO4)3, the portion of the aqueous spinning bath is subjected to sodium sulfate removal.

15. The method of claim 12, further comprising removing sodium sulfate from a portion of the aqueous spinning bath by crystallization.

16. The method of claim 12, wherein the portion of the aqueous spinning bath is subjected to cooling crystallization of sodium sulfate at a temperature of 1 to 10°C.

17. The method of claim 12, wherein the portion of the aqueous spinning bath is subjected to vacuum crystallization of sodium sulfate.

18. The method according to any one of claims 13-17, wherein sodium sulfate is recovered as sodium sulfate decahydrate, and the supernatant is separated from the sodium sulfate decahydrate crystals and recycled to the spinning bath.

19. The method according to any one of claims 1-4, 9-11 and 13-17, wherein the portion of the aqueous spinning bath is substantially free of solid ammonia alum precipitate.

20. The method of claim 18, wherein the portion of the aqueous spinning bath is substantially free of solid ammonia alum precipitate.

21. The method according to any one of claims 1-4, 9-11, 13-17 and 20, comprising feeding a fresh feed of sulfuric acid, aluminum sulfate, aluminum hydroxide or any mixture thereof into a regenerated acidic spinning bath, and then recycling it to the spinning unit.

22. The method according to any one of claims 1-4, 9-11, 13-17 and 20, comprising recovering aluminum sulfate from the steps of selectively removing zinc and selectively removing sodium sulfate, and recycling the recovered aluminum sulfate to the spinning bath.

23. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the aqueous alkaline cellulose carbamate spinning solution contains up to 10% by weight of cellulose carbamate, less than 8% by weight of sodium hydroxide and up to 1.5% by weight of Zn calculated as ZnO, wherein the percentages are calculated based on the total weight of the cellulose carbamate spinning solution.

24. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the cellulose carbamate spinning solution exhibits a maximum dissolved nitrogen content of less than 0.2% by weight, calculated based on the weight of the cellulose carbamate spinning solution.

25. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the cellulose urethane spinning solution is provided to the spinning unit at a temperature of 10 to 30°C.

26. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the cellulose carbamate spinning solution is solidified in a spinning bath at a temperature of up to 25°C.

27. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the cellulose carbamate in the cellulose carbamate spinning solution is provided with a degree of hydrolysis of less than 70%, said degree of hydrolysis being calculated from the percentage difference between the percentage of cellulose-bound carbamate nitrogen between the cellulose carbamate before dissolution and the solidified cellulose carbamate.

28. The method according to any one of claims 1-4, 9-11, 13-17 and 20, wherein the solidified cellulose urethane filaments are subjected to stretching in a stretching unit or a cascade of at least two stretching units, then cut to provide short fibers, and the thus recovered filaments or short fibers are washed in a washing unit.

29. The method according to any one of claims 1-4, 9-11, 13-17 and 20, comprising feeding a spinning solution into a spinning unit and removing an aqueous spinning bath from the spinning unit such that at least 70% by weight of sodium sulfate formed in the spinning bath is removed in a sodium sulfate removal crystallization step, and at least 80% by weight of zinc formed in the spinning bath is removed in a zinc removal step.

30. The method according to any one of claims 1-4, 9-11, 13-17 and 20, further comprising removing a portion of an aqueous stretching and / or washing bath containing sodium sulfate, water and zinc ions, and feeding it together with the removed portion of the spinning bath into the regeneration process.

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