Method for preparing Lyocell fiber by combining water washing and acid pickling

Abstract

The invention discloses a method for preparing Lyocell fibers by combining water washing and acid pickling, and belongs to the technical field of fiber preparation. The method for preparing the Lyocell fibers by combining water washing and acid pickling comprises the following steps: paving Lyocell short fibers into a fiber net with uniform thickness; an alkaline cross-linking agent is applied to the fiber net, so that the fiber net is subjected to a cross-linking reaction, and a cross-linking reaction product is obtained; washing the cross-linking reaction product by combining water washing and acid pickling to obtain a neutral product; the neutral product is subjected to post-treatment, and the finished product Lyocell fiber is obtained. According to the method for preparing the Lyocell fiber through combination of washing and pickling, on the premise that the fiber and the fiber crosslinking effect are not damaged, the Lyocell fiber obtained through crosslinking can be cleaned to be neutral, the mechanical property of the prepared Lyocell fiber can be protected, the quality of the prepared Lyocell fiber can be improved, and the waste discharge amount of sewage is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of fiber preparation, and in particular to a method for preparing Lyocell fiber by combining water washing and acid washing. Background Art

[0002] Lyocell fiber (meaning soluble fiber) is known as the green fiber of the 21st century. Easy fibrillation is one of the characteristics of ordinary Lyocell fiber. Fibrillation refers to the splitting of fine microfibers (diameter 1-4um) on the fiber surface. Generally speaking, during the first fibrillation, the fibrils produced are relatively large, usually 1mm, or even longer, and can be tangled into balls. If they are not handled, they will affect the wearing and appearance. In order to solve the fibrillation problem of Lyocell fiber, during the preparation process, alkaline crosslinking agents are often used to react with the fibers to produce crosslinked Lyocell fibers, and after adding crosslinking agents and additives to the fibers, the excess solution is squeezed out and then heated for crosslinking and curing. However, this process is often difficult to control the uniformity of the fiber crosslinking effect due to the large thickness of the fiber web, the large amount of alkali used, and a large amount of alkali on the fibers after the crosslinking reaction. It is not easy to clean the fibers and a large amount of sewage will be generated. Summary of the invention

[0003] In view of this, the present invention provides a method for preparing Lyocell fibers by combining water washing and acid washing, which can wash the cross-linked Lyocell fibers to neutrality without damaging the fibers and the fiber cross-linking effect, protect the mechanical properties of the prepared Lyocell fibers, improve the quality of the prepared Lyocell fibers, and reduce the amount of sewage discharged, thereby being more suitable for practical use.

[0004] In order to achieve the above object, the technical scheme of the method for preparing Lyocell fiber by combining water washing and acid washing provided by the present invention is as follows:

[0005] The method for preparing Lyocell fiber by combining water washing and acid washing provided by the present invention comprises the following steps:

[0006] Lay Lyocell short fibers into a fiber web with uniform thickness;

[0007] applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product;

[0008] The cross-linking reaction product is washed by water and acid to obtain a neutral product;

[0009] The neutral product is post-treated to obtain finished Lyocell fiber.

[0010] The method for preparing Lyocell fiber by combining water washing and acid washing provided by the present invention can be further implemented by adopting the following technical measures.

[0011] Preferably, in the step of post-processing the neutral product to obtain the finished Lyocell fiber, the post-processing includes oiling and drying.

[0012] Preferably, the cross-linking reaction product is washed with water and acid to obtain a neutral product, and is sequentially washed with acid and water, wherein the acid washing step is completed in the acid washing zone and the water washing step is completed in the water washing zone, wherein:

[0013] The pickling area includes an M-level pickling unit,

[0014] When m is a natural number ≥ 2, the squeezed solution of the m-th pickling unit is refluxed to the m-1-th pickling unit;

[0015] The water washing zone includes n-stage water washing units,

[0016] When n=1, the squeezed solution from the water washing unit flows back to the pickling area to prepare the pickling solution;

[0017] When n is a natural number ≥ 2, the squeezed solution of the nth stage water washing unit flows back to the n-1th stage water washing unit; and the squeezed solution of the 1st stage water washing unit flows back to the pickling area for preparing the pickling solution.

[0018] Preferably, the method for preparing Lyocell fiber by combining water washing and acid washing further comprises a pre-water washing unit.

[0019] The pre-washing unit is arranged before the acid washing zone, and is used for pre-washing the cross-linking reaction product.

[0020] Preferably, the squeezed solution from the pickling zone is also refluxed to the pre-water washing unit.

[0021] Preferably, during the pickling step, the acid is selected from one of sulfuric acid, hydrochloric acid, acetic acid, and oxalic acid, or a mixed solution of several of them, and the mass percentage of the solute in the acid ranges from 0.01% to 1.0%.

[0022] Preferably, during the step of washing the cross-linking reaction product by combining water washing and acid washing to obtain a neutral product, the temperature of water or acid is in the range of 20°C-80°C.

[0023] Preferably, in the step of washing the cross-linking reaction product by a combination of water washing and acid washing to obtain a neutral product, the fiber liquid carrying rate after each level of washing ranges from 30% to 85%.

[0024] Preferably, in the step of applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product, the crosslinking agent is selected from one or a mixture of 1,3,5-acrylamidohexahydro-1,3,5-triazine, 2,4-dichloro-6-hydroxy-s-triazine, and N,N-methylenebisacrylamide.

[0025] Preferably, between the steps of laying the Lyocell short fibers into a fiber web of uniform thickness; and the step of applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product, there is also a step of heat-insulating and curing the fiber web.

[0026] Preferably, in the step of laying the Lyocell short fibers into a fiber web with uniform thickness, the thickness of the fiber web ranges from 1 mm to 50 mm.

[0027] Preferably, in the step of washing the cross-linking reaction product by a combination of water washing and acid washing to obtain a neutral product, the weight of the washing water is 3 to 15 times the weight of the fiber itself.

[0028] In the prior art, there are many patents on the cross-linking process of Lyocell fibers with alkaline cross-linking agents, but there are relatively few studies on the acid washing process after the reaction. Acid will not only affect the Lyocell fibers themselves, but also affect the cross-linking effect of the fibers. Therefore, the study of the acid washing process is very important. For example, for the cross-linking agent dichlorotriazine, when the pH value of the acid solution is 3, the cross-linking effect of the cross-linked fibers may decrease. The technical solution of the method for preparing Lyocell fibers by combining water washing and acid washing provided by the present invention, in the acid washing process, an acid solution with a concentration of 0.01%-1% is used, and the fibers can be washed to neutral without damaging the fibers and the cross-linking effect of the fibers.

[0029] In the prior art, acetic acid solution is mostly used to pickle the fiber, which causes the mechanical properties of the fiber to decrease and the fiber to feel hard and brittle. The method for preparing Lyocell fiber by combining water washing and pickling provided by the present invention uses a variety of strong or weak acids such as sulfuric acid, hydrochloric acid, acetic acid, oxalic acid, etc. for pickling without damaging the fiber and the cross-linking effect of the fiber.

[0030] In the prior art, the amount of washing water is large, and the alkali in the fiber is difficult to clean. The method for preparing Lyocell fiber by combining washing and acid washing provided by the present invention comprises one or more units for washing and acid washing respectively, and the washing water or acid washing solution is reused step by step. The solution squeezed out of each washing or acid washing unit is re-adjusted to pH value and then returned to the previous washing or acid washing unit. Figure 4 As shown, the fiber can be fully and quickly cleaned with water 5-8 times the weight of the fiber, thereby reducing the amount of sewage discharged. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0032] Attached Figure 1 A flow chart of the steps of the method for preparing Lyocell fiber by combining water washing and acid washing provided in Example 1 of the present invention;

[0033] Attached Figure 2 A flow chart of the steps of the method for preparing Lyocell fiber by combining water washing and acid washing provided in Example 2 of the present invention;

[0034] Attached Figure 3 A flow chart of the steps of the method for preparing Lyocell fiber by combining water washing and acid washing provided in Example 4 of the present invention;

[0035] Attached Figure 4 A flow chart of the arrangement of acid washing and water washing in the method for preparing Lyocell fiber by combining water washing and acid washing provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In view of this, the present invention provides a method for preparing Lyocell fibers by combining water washing and acid washing, which can wash the cross-linked Lyocell fibers to neutrality without damaging the fibers and the fiber cross-linking effect, protect the mechanical properties of the prepared Lyocell fibers, improve the quality of the prepared Lyocell fibers, and reduce the amount of sewage discharged, thereby being more suitable for practical use.

[0037] After arduous efforts, the inventor discovered that

[0038] During the preparation process of Lyocell fiber, an acid solution is needed to neutralize the alkali on the fiber. Since Lyocell fiber is sensitive to acid, it will degrade under the action of strong acid and high-temperature water, resulting in a decrease in the mechanical properties of the fiber and the fiber feeling hard and brittle. Therefore, it is necessary to improve the preparation process to reduce the amount of alkali used and to study the acid washing process.

[0039] On the one hand, the existing patents mostly use alkaline crosslinking agents to react with Lyocell fibers to prepare crosslinked Lyocell fibers. Usually, crosslinking agents and additives are added to the fibers and then squeezed and cured at high temperature. This process wastes crosslinking agents to a certain extent, and the uniformity of the fiber crosslinking effect is difficult to control. In addition, the amount of alkali used will increase for crosslinking agents that require alkali to participate in the reaction, making water washing difficult. In the process of the present invention, after adding crosslinking agents and additives to the fiber web and mixing them evenly with the fibers, only heat preservation and curing are required. The crosslinking reaction temperature is uniform, and the crosslinking agent is fully utilized. On the other hand, after the alkaline crosslinking agent reacts with the fibers, a large amount of alkali will remain on the fibers, which needs to be washed with a large amount of water, and it is not easy to wash to neutrality. It needs to be washed with acid. At the same time, cellulose fibers are more sensitive to acid. Too high an acid concentration will cause cellulose fiber degradation, affecting the mechanical properties and spinnability of the fibers. It may also reduce the fiber crosslinking effect or accelerate the decay rate of the crosslinking effect. Therefore, the acid washing method is crucial to the quality of the crosslinked fibers.

[0040] There is a technical solution that uses 1,3,5-acrylamidohexahydro-1,3,5-triazine as a crosslinking agent to prepare crosslinked fibers, and uses 0.5 mol / L acetic acid solution to neutralize excess alkali during the reaction process, but the corresponding acid washing process is not mentioned.

[0041] Another technical solution is to use 1,3,5-acrylamide hexahydro-1,3,5-triazine as a crosslinking agent to prepare crosslinked fibers, use 0.5% acetic acid solution to neutralize excess alkali in the reaction process, and use a method of acid washing followed by water washing, with a high acid concentration.

[0042] There is also a technical solution that uses 1,3,5-acrylamide hexahydro-1,3,5-triazine as a crosslinking agent to prepare crosslinked fibers. After the crosslinking agent and additives are added to the fibers, they are extruded and then cured at high temperature. A combination of acid washing and water washing is not involved.

[0043] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a detailed description of the method for preparing Lyocell fiber by combining water washing and acid washing according to the present invention, its specific implementation, structure, characteristics and effects, in conjunction with the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "embodiment" does not necessarily refer to the same embodiment. In addition, specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form.

[0044] The term "and / or" in this article is merely a description of the association relationship of associated objects, indicating that three relationships may exist, for example, A and / or B. Specifically, it is understood that: A and B may be included at the same time, A may exist alone, or B may exist alone, and any of the above three situations may be met.

[0045] The method for preparing Lyocell fiber by combining water washing and acid washing provided by the present invention comprises the following steps:

[0046] Step S1: Laying Lyocell short fibers into a fiber web with uniform thickness.

[0047] Step S2: applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product, wherein the temperature of the fiber web reaches the reaction temperature of the fibers and the crosslinking agent.

[0048] Step S3: The cross-linking reaction product is washed with water and acid to obtain a neutral product. After the fiber completes the cross-linking reaction, a large amount of alkali will remain on the fiber, part of which remains inside the fiber and is difficult to clean, and part of which remains on the fiber surface and is easier to clean.

[0049] Step S4: The neutral product is post-treated to obtain finished Lyocell fiber.

[0050] Among them, in the process of obtaining the finished Lyocell fiber through post-treatment of the neutral product, the post-treatment includes oiling and drying.

[0051] Among them, the cross-linking reaction product is washed by water and acid to obtain a neutral product. In the process, the cross-linking reaction product is washed by acid and water in sequence. The acid washing step is completed in the acid washing area, and the water washing step is completed in the water washing area. The acid washing area includes m-level acid washing units. When m is a natural number ≥2, the squeezed solution of the m-level acid washing unit is refluxed to the m-1-level acid washing unit; the water washing area includes n-level water washing units. When n=1, the squeezed solution of the water washing unit is refluxed to the acid washing area for preparing the acid washing solution; when n is a natural number ≥2, the squeezed solution of the n-level water washing unit is refluxed to the n-1-level water washing unit; and the squeezed solution of the 1-level water washing unit is refluxed to the acid washing area for preparing the acid washing solution. The washing water is reused step by step. Excess solution is squeezed out of each unit in the washing area and returned to the previous washing unit for washing. The pH value of the solution squeezed out of the washing area and returned to the pickling area needs to be adjusted to control the solution squeezed out of the first-stage pickling unit to be acidic or neutral. The excess solution squeezed out of the pickling area is neutral or acidic, otherwise it needs to be adjusted and returned to the previous washing unit for washing, and then discharged after returning to the last-stage washing unit.

[0052] The method for preparing Lyocell fiber by combining water washing and acid washing also includes a pre-water washing unit. The pre-water washing unit is arranged before the acid washing zone, and is used to pre-wash the cross-linking reaction product. Figure 3In the water washing unit between the heat preservation and curing unit and the pickling unit, in this case, the Lyocell fiber is pre-washed before pickling, which can further save the amount of acid used in the pickling process.

[0053] The squeezed solution from the pickling area also flows back to the pre-washing unit.

[0054] In the pickling step, the acid is selected from one of sulfuric acid, hydrochloric acid, acetic acid, and oxalic acid, or a mixed solution of several of them, and the mass percentage of the solute in the acid ranges from 0.01% to 1.0%.

[0055] In the step of washing the cross-linking reaction product by a combination of water washing and acid washing to obtain a neutral product, the temperature of the water or acid is in the range of 20°C-80°C.

[0056] In the step of washing the cross-linking reaction product by a combination of water washing and acid washing to obtain a neutral product, the fiber liquid carrying rate after each level of washing ranges from 30% to 85%.

[0057] In the step of applying an alkaline crosslinking agent to the fiber web to make the fiber web crosslink and obtain a crosslinking reaction product, the crosslinking agent is selected from one or a mixture of 1,3,5-acrylamide hexahydro-1,3,5-triazine, 2,4-dichloro-6-hydroxy-s-triazine, and N,N-methylenebisacrylamide. The crosslinking agent can react with cellulose fibers in an alkaline environment. After the reaction, the fibers show alkalinity and acid is required to neutralize the excess alkali.

[0058] Among them, between the steps of laying Lyocell short fibers into a fiber web with uniform thickness and applying an alkaline crosslinking agent to the fiber web to cause the fiber web to undergo a crosslinking reaction to obtain a crosslinking reaction product, there is also a step of heat preservation and curing the fiber web. The heat preservation and curing method abandons the traditional heating methods such as microwaves, hot rollers, and steam, which can improve the utilization rate of the crosslinking agent on the one hand, and reduce the amount of alkali on the other hand, so that the fiber is easy to clean.

[0059] In the step of laying the Lyocell short fibers into a fiber mesh with uniform thickness, the thickness of the fiber mesh ranges from 1 mm to 50 mm. The smaller the thickness of the fiber mesh, the higher the solution permeability and the easier it is to clean.

[0060] In the step of obtaining a neutral product by washing the cross-linking reaction product with water and acid, the weight of the washing water is 3 to 15 times the weight of the fiber itself.

[0061] Fiber physical properties test: Dry breaking strength GB / T14337-2008 is tested in accordance with the national standard, and wet friction value is tested using FZ / T52019-2018.

[0062] The cross-linked fibers after reacting with different alkaline cross-linking agents have different sensitivities to acid. For example, if sodium dichlorotriazine is used as the cross-linking agent, the acid resistance of the cross-linked fibers is poor. The process of water washing → acid washing → water washing can be adopted, and the acid can be strong or weak acids such as sulfuric acid, hydrochloric acid, oxalic acid, and acetic acid.

[0063] Example 1

[0064] A method for preparing cross-linked Lyocell fibers, comprising: Figure 1 , the steps are as follows:

[0065] The fibers are laid into a fiber web with a thickness of 1 cm, and a crosslinking agent and an additive solution are added to the fiber web. After evenly circulating, the fiber web is heat-insulated and solidified. Excess solution is squeezed out, and the fiber is washed with water 6 times its own weight at a water temperature of 30°C. When the solution squeezed out of the washing unit in the washing area (two washing units) returns to the upper washing unit, its pH is adjusted to neutral. When it returns to the acid washing area (two acid washing units), acetic acid with a concentration of 0.08% is prepared for acid washing. When the solution squeezed out of the acid washing unit returns to the upper acid washing unit, acetic acid with a concentration of 0.16% is prepared for acid washing. When the excess solution squeezed out of the acid washing area returns to the upper washing unit, its pH is adjusted to be slightly neutral and then returned to the washing area (one washing unit) for water washing. Excess solution is squeezed out, oiled, and dried to obtain the finished cross-linked Lyocell fiber.

[0066] Example 2

[0067] For cross-linked fibers with slightly better acid resistance, alkaline cross-linking agents can be used for acid washing → water washing process. Multiple acid washing units can be used to reduce the acid concentration. The process is as follows: Figure 2 , the steps are as follows:

[0068] The fibers are laid into a fiber web with a thickness of 1 cm, and a crosslinking agent and an additive solution are added to the fiber web. After evenly circulating, the fiber web is heat-insulated and solidified. Excess solution is squeezed out, and the fiber is washed with water 6 times the weight of the fiber at a temperature of 30°C. When the solution squeezed out of the washing unit in the washing area (two washing units) returns to the upper washing unit, its pH is adjusted to be slightly neutral. When the solution returns to the acid washing area (three acid washing units), sulfuric acid with a concentration of 0.13% is prepared for acid washing. When the excess solution squeezed out of the acid washing unit returns to the upper acid washing area, the concentration of the acid solution needs to be readjusted. Finally, the excess solution is squeezed out, oiled, and dried to obtain the finished cross-linked Lyocell fiber.

[0069] Example 3

[0070] The process of combining multi-stage water washing and pickling can reduce the amount of waste by appropriately adding water washing or pickling units. The steps are as follows:

[0071] The fibers are laid into a fiber web with a thickness of 1 cm, and a crosslinking agent and an additive solution are added to the fiber web. After evenly circulating, the fiber web is heat-insulated and solidified. Excess solution is squeezed out, and the fiber is washed with water 4 times the weight of the fiber at a temperature of 30°C. When the solution squeezed out of the washing unit in the washing area (four washing units) returns to the upper washing unit, its pH is adjusted to be slightly neutral. When it returns to the acid washing area (three acid washing units), sulfuric acid with a concentration of 0.13% is prepared for acid washing. When the excess solution squeezed out of the acid washing unit returns to the upper acid washing area, the concentration of the acid solution needs to be readjusted. Finally, the excess solution is squeezed out, oiled, and dried to obtain the finished cross-linked Lyocell fiber.

[0072] Example 4

[0073] Increasing the amount of water for washing can reduce the number of pickling or water washing units accordingly. Figure 3 , the steps are as follows:

[0074] The fibers are laid out into a fiber web with a thickness of 1 cm, and a crosslinking agent and an additive solution are added to the fiber web. After evenly circulating, the fiber web is heat-insulated and solidified, and excess solution is squeezed out. The fiber is washed with water 12 times the weight of the fiber at a temperature of 30°C. When the solution squeezed out of the washing area (a washing unit) returns to the acid washing area (a acid washing unit), sulfuric acid with a concentration of 0.13% is prepared for acid washing. When the excess solution squeezed out of the acid washing unit returns to the upper-level washing unit, its pH is adjusted to be slightly neutral, and then it returns to the washing area (a washing unit) for washing, and the excess solution is squeezed out. The fiber is oiled and dried to obtain the finished cross-linked Lyocell fiber.

[0075] Comparative Example 1

[0076] A method for preparing cross-linked Lyocell fiber, comprising the following steps:

[0077] The fibers are laid into a fiber mesh with a thickness of 1 cm, and a cross-linking agent and additive solution are added to the fiber mesh. After evenly circulating, the fiber mesh is heat-insulated and solidified. Excess solution is squeezed out, and the fiber is washed with water 15 times the weight of the fiber at a temperature of 30°C. In the washing area (five washing units), the excess solution is squeezed out of the washing unit and enters the upper washing unit. The pH value is adjusted to be neutral for washing, and the fiber returns to the last washing unit for oiling and drying to obtain the finished cross-linked Lyocell fiber.

[0078] Comparative Example 2

[0079] A method for preparing cross-linked Lyocell fiber, the steps are as follows:

[0080] The fibers are laid into a fiber mesh with a thickness of 1 cm, and a cross-linking agent and an additive solution are added to the fiber mesh. After evenly circulating, the fiber mesh is heat-insulated and solidified. Excess solution is squeezed out, and the fiber is washed with water 6 times the weight of the fiber at a temperature of 30°C. In the washing area (five washing units), the excess solution is squeezed out of the washing unit and enters the upper washing unit. The pH value is adjusted to be neutral for washing. The returned value is oiled and dried in the last washing unit to obtain the finished cross-linked Lyocell fiber.

[0081] The test results are shown in Table 1

[0082] Example Fiber dry strength (dtex / cN) Wet friction value (times) Fiber acidity Washing water consumption (%) Example 1 3.56 600 neutral 600％ Example 2 3.60 590 neutral 600％ Example 3 3.59 594 neutral 400％ Example 4 3.62 603 neutral 1200％ Comparative Example 1 3.55 610 neutral 1500％ Comparative Example 2 3.60 595 Alkaline 600％

[0083] From Table 1, we can find that the water washing method of the present invention uses less water, discharges less sewage, has a low concentration of acid solution, and does not affect the crosslinking effect of the crosslinked fiber and the mechanical properties of the fiber.

[0084] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0085] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for preparing Lyocell fiber by combining water washing and acid washing, characterized in that: The following steps are involved: Lay Lyocell short fibers into a fiber web with uniform thickness; applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product; The cross-linking reaction product is washed by water and acid to obtain a neutral product; The neutral product is post-treated to obtain finished Lyocell fiber.

2. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: The neutral product is subjected to post-treatment to obtain the finished Lyocell fiber, and the post-treatment includes oiling and drying.

3. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: The cross-linking reaction product is washed with water and acid to obtain a neutral product, and is sequentially washed with acid and water, wherein the acid washing step is completed in the acid washing area and the water washing step is completed in the water washing area, wherein: The pickling area includes an M-level pickling unit, When m is a natural number ≥ 2, the squeezed solution of the m-th pickling unit is refluxed to the m-1-th pickling unit; The water washing zone includes n-stage water washing units, When n=1, the squeezed solution from the water washing unit flows back to the pickling area to prepare the pickling solution; When n is a natural number ≥ 2, the squeezed solution of the nth stage water washing unit flows back to the n-1th stage water washing unit; and the squeezed solution of the 1st stage water washing unit flows back to the pickling area for preparing the pickling solution.

4. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 3, characterized in that: Also includes a pre-wash unit, The pre-washing unit is arranged before the acid washing zone, and is used for pre-washing the cross-linking reaction product.

5. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 4, characterized in that: The squeezed solution from the pickling zone also flows back to the pre-water washing unit.

6. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: During the pickling step, the acid is selected from one of sulfuric acid, hydrochloric acid, acetic acid, and oxalic acid, or a mixed solution of several of them, and the mass percentage of the solute in the acid ranges from 0.01% to 1.0%.

7. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: In the step of washing the cross-linking reaction product by combining water washing and acid washing to obtain a neutral product, the temperature of the water or acid is in the range of 20°C-80°C.

8. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: In the step of washing the cross-linking reaction product by combining water washing and acid washing to obtain a neutral product, the fiber liquid carrying rate after each level of washing ranges from 30% to 85%.

9. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: In the step of applying an alkaline crosslinking agent to the fiber web to cause a crosslinking reaction of the fiber web to obtain a crosslinking reaction product, the crosslinking agent is selected from one or a mixture of 1,3,5-acrylamidohexahydro-1,3,5-triazine, 2,4-dichloro-6-hydroxy-s-triazine, and N,N-methylenebisacrylamide.

10. The method for preparing Lyocell fiber by combining water washing and acid washing according to claim 1, characterized in that: Between the steps of laying Lyocell short fibers into a fiber web with uniform thickness; and applying an alkaline crosslinking agent to the fiber web to cause the fiber web to undergo a crosslinking reaction to obtain a crosslinking reaction product, the method further includes the step of heat-insulating and curing the fiber web; Preferably, in the step of laying the Lyocell short fibers into a fiber web with uniform thickness, the thickness of the fiber web ranges from 1 mm to 50 mm; Preferably, in the step of washing the cross-linking reaction product by a combination of water washing and acid washing to obtain a neutral product, the weight of the washing water is 3 to 15 times the weight of the fiber itself.

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