A method of cotton thread indigo dyeing

Abstract

The application discloses a method for cotton thread indigo dyeing, which comprises the following steps: preparing an indigo suspension, uniformly mixing indigo powder, sodium hydroxide, potassium hydroxide, a dispersing agent and water to prepare the indigo suspension, wherein the concentration of the indigo powder is 3.6-4.2 g / L, the concentration of the sodium hydroxide is 3.2-3.8 g / L, the concentration of the potassium hydroxide is 2.0-3.5 g / L, and the concentration of the dispersing agent is 1-3 g / L; reducing dyeing, injecting reducing cold plasma into the indigo suspension in the presence of a catalyst to perform a reduction reaction, and obtaining an indigo dyeing solution; and cotton thread dyeing, immersing the cotton thread in the indigo dyeing solution to obtain dyed cotton thread. The method of the application reduces indigo in the indigo suspension by using reducing cold plasma to prepare the indigo dyeing solution, and then dyes the cotton thread, thereby avoiding the use of sodium hydrosulfite, effectively reducing the COD value of printing and dyeing wastewater, and being more environmentally friendly.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dyeing and finishing, in particular to a method for dyeing cotton thread with indigo. BACKGROUND

[0002] Indigo is a dye with a history of more than three thousand years, and is one of the oldest pigments known to mankind, which is widely used in the field of printing and dyeing industry.

[0003] When cotton thread is dyed with indigo, it is usually necessary to first reduce indigo to leuco body and then dye the cotton thread. A strong reducing agent such as sodium hyposulfite (commonly known as sodium dithionite) is generally used to reduce indigo to leuco body in an alkaline environment, but the chemical property of sodium hyposulfite is unstable, and it will be rapidly oxidized to generate sulfite and thiosulfate ions in air or aqueous solution, resulting in high COD value (chemical oxygen demand) of printing and dyeing wastewater, which will cause serious environmental pollution. SUMMARY

[0004] The present application aims to overcome the shortcomings of the prior art, and provides a method for dyeing cotton thread with indigo. The method reduces indigo in indigo suspension liquid to indigo dyeing liquid by using reducing cold plasma, and then dyes the cotton thread, thereby avoiding the use of sodium hyposulfite, effectively reducing the COD value of printing and dyeing wastewater, and being more environmentally friendly.

[0005] The present application provides a method for dyeing cotton thread with indigo, comprising the following steps:

[0006] Preparation of indigo suspension liquid: uniformly mix indigo powder, sodium hydroxide, potassium hydroxide, dispersing agent and water to prepare indigo suspension liquid, wherein the concentration of the indigo powder is 3.6-4.2 g / L, the concentration of the sodium hydroxide is 3.2-3.8 g / L, the concentration of the potassium hydroxide is 2.0-3.5 g / L, and the concentration of the dispersing agent is 1-3 g / L.

[0007] Reduction of dye: inject reducing cold plasma into the indigo suspension liquid for reduction reaction in the presence of a catalyst to obtain indigo dyeing liquid.

[0008] Dyeing of cotton thread: immerse the cotton thread in the indigo dyeing liquid for 1-3 min to obtain dyed cotton thread.

[0009] Specifically, the dispersing agent is sulfonate, or carboxylate, or sulfate, or phosphate.

[0010] Specifically, the temperature of the indigo suspension liquid ranges from 20 to 40℃.

[0011] Specifically, the catalyst is composed of one or more of iron, nickel and molybdenum, and the catalyst is fixed on the surface of a stainless steel carrier.

[0012] Specifically, the reducing cold plasma is generated from hydrogen or ammonia;

[0013] The injection pressure of the reducing cold plasma is 5-10 bar, and the injection flow rate of the reducing cold plasma is 20-30 mL / min.

[0014] Specifically, the vat dye is performed in a nitrogen environment, and the conditions for providing the nitrogen environment include that the nitrogen flow rate is kept at 30-35 m 3 / h.

[0015] Specifically, when the vat dye is performed, the stirrer is controlled to stir the indigo suspension at a speed of 30-35 r / min.

[0016] Specifically, when the cotton thread is dyed, the indigo dyeing solution is subjected to ultrasonic treatment, and the frequency of the ultrasonic treatment is 10-20 MHz.

[0017] Specifically, before the cotton thread is dyed, cotton thread scouring is further included, and the cotton thread is treated with sodium hydroxide with a concentration of 5-6 g / L and a surfactant with a concentration of 6-8 g / L at a temperature of 90-100 ℃ for 1-1.5 h.

[0018] Specifically, after the cotton thread is dyed, further included are:

[0019] steaming treatment, the dyed cotton thread is treated with steam at 100-103 ℃ for 1-3 min;

[0020] water washing treatment, the dyed cotton thread after the steaming treatment is washed with water at 30-40 ℃;

[0021] drying treatment, the dyed cotton thread after the water washing treatment is dried with hot air at 110-120 ℃.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] First, the indigo powder is mixed with alkali metal and dispersant to form an indigo suspension, and the indigo in the indigo suspension is reduced by the reducing cold plasma in the presence of a catalyst to obtain an indigo dyeing solution, and then the cotton thread is immersed and dyed, avoiding the use of sodium dithionite, greatly reducing the chemical oxygen demand of printing and dyeing wastewater, reducing the treatment difficulty and cost of printing and dyeing wastewater, and having green environmental protection characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to describe the technical solutions in the embodiments of the present application or the prior art more clearly, the following will briefly introduce the drawings needed in the embodiments or the prior art description. Obviously, the drawings described below only show some of the embodiments of the present application, and not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0025] Figure 1 The method flow chart for cotton thread indigo dyeing in the embodiments of the present application is shown. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments only show some of the embodiments of the present application, and not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0027] Figure 1 The method flow chart for cotton thread indigo dyeing in the embodiments of the present application is shown, including the following steps:

[0028] S1, preparing an indigo suspension liquid;

[0029] The indigo powder, sodium hydroxide, potassium hydroxide, dispersant and water are mixed uniformly to prepare the indigo suspension liquid, wherein the concentration of the indigo powder is 3.6-4.2 g / L, the concentration of the sodium hydroxide is 3.2-3.8 g / L, the concentration of the potassium hydroxide is 2.0-3.5 g / L, and the concentration of the dispersant is 1-3 g / L.

[0030] The indigo powder is first prepared into the indigo suspension liquid, which can improve the fluidity of the indigo, and facilitate the subsequent full contact reaction with the reductive cold plasma. Moreover, the indigo suspension liquid is beneficial to disperse heat and maintain the uniformity of the system temperature.

[0031] The use of two alkali metals to create an alkaline environment can improve the stability and solubility of the subsequent indigo reduced into indigo white alkali metal salt, and the indigo white alkali metal salt is not easy to precipitate, and the subsequent cotton thread dyeing can also improve the dyeing percentage.

[0032] The presence of the dispersant can promote the uniform dispersion of the indigo in the indigo suspension liquid, and also promote the penetration and diffusion of the indigo in the cotton thread during the subsequent cotton thread dyeing. Specifically, the dispersant is sulfonate, or carboxylate, or sulfate, which are all anionic surfactants, and can play a good dispersion effect in an alkaline environment, can improve the uniformity of the indigo suspension liquid, and also can improve the cotton thread dyeing efficiency, such as sodium alkyl benzene sulfonate, sodium higher fatty acid, sodium fatty alcohol sulfate and the like.

[0033] In a specific embodiment, the temperature of the indigo suspension is in the range of 20-40℃, a reasonable temperature range that can keep the stability of the indigo suspension system, and it is a relatively low temperature, which can absorb the heat brought by the reducing cold plasma and the heat generated in the reduction reaction process when the reducing cold plasma is injected into the indigo suspension for reduction.

[0034] S2, a vat dye;

[0035] In the presence of a catalyst, a reducing cold plasma is injected into the indigo suspension for reduction reaction, and an indigo dye solution is obtained.

[0036] Based on the thermodynamic equilibrium state, the plasma in a non-thermal equilibrium state is a cold plasma, which has the characteristics of non-thermal and environmental friendliness. The heavy particle temperature of the cold plasma is about 27-727℃, and the reaction activity is high. At normal temperature and pressure, it can react with substances, and most of the components of the cold plasma will disappear immediately after the reaction, which is efficient and environmentally friendly.

[0037] In some specific embodiments, the raw material for generating the reducing cold plasma is hydrogen or ammonia. Hydrogen (H2) can be ionized to produce hydrogen cold plasma under discharge conditions, which contains various hydrogen ions (H + , H2 + , H3 + , H - ), ground state and electron excited state hydrogen atoms and hydrogen molecules, which have strong reducing properties and can quickly reduce indigo in the alkaline indigo suspension to obtain leuco indigo salt; ammonia (NH3) can ionize to produce a large number of active H radicals and NH radicals under discharge conditions. NH3 in the plasma is not only a source of H2 indirectly produced, but also generates substances with reducing properties such as NH, H2H2, N2H4, etc. after forming plasma, which can also quickly reduce indigo in the alkaline indigo suspension to obtain leuco indigo salt. Leuco indigo salt, i.e. leuco body, is soluble in water and can be used to dye cotton threads.

[0038] There are many ways to generate a large amount of reducing cold plasma. Preferably, dielectric barrier discharge or corona discharge is used to convert hydrogen or ammonia into cold plasma. These two discharge methods can generate cold plasma at 10 4 ~ 10 6 Pa, which can avoid the limitation of vacuum conditions, and the cold plasma generated contains many active particles, which can induce chemical reactions at low temperature and low energy.

[0039] In some embodiments, the reducing cold plasma is injected from the bottom of the indigo suspension, the injection pressure of the reducing cold plasma is 5-10 bar, the injection flow rate of the reducing cold plasma is 20-30 mL / min, the low-speed injection of the reducing cold plasma from the bottom of the indigo suspension facilitates the sufficient contact reaction of the reducing cold plasma with the indigo in the indigo suspension, and avoids the too fast escape of the reducing cold plasma; at the same time, the reduction reaction process of the indigo suspension can be observed, and the reduction degree of the indigo suspension can be controlled.

[0040] The initial color of the indigo suspension is blue, and as the reduction degree deepens, the color of the indigo suspension will gradually change: when a small amount of indigo is reduced, the color of the indigo suspension is turquoise green; when most of the indigo is reduced, the color of the indigo suspension is yellow green; when all the indigo is reduced, the indigo suspension becomes clear and golden yellow, and the indigo dye solution is obtained; when the indigo is over-reduced, the solution is red-brown.

[0041] Further, when the reducing dye is performed, the stirrer is controlled to stir the indigo suspension at a speed of 30-35 r / min, so as to ensure that the indigo in the indigo suspension fully contacts and reacts with the reducing cold plasma; at the same time, the heat brought by the reducing cold plasma and the heat generated in the reduction reaction process can be dispersed, and the temperature of the indigo suspension is uniform.

[0042] In some embodiments, the catalyst is composed of one or more of iron, nickel and molybdenum, preferably, the catalyst is an iron-nickel-molybdenum alloy, the proportions of iron, nickel and molybdenum in the alloy are the same, the catalytic efficiency is high, and the stability is also good.

[0043] Further, the catalyst is fixed on the surface of a stainless steel carrier, the stainless steel carrier has high strength and good corrosion resistance, and can be a stainless steel mesh, a stainless steel strip, a stainless steel ball or the like; the stainless steel carrier is generally fixed in a reaction container, and the indigo suspension flows and contacts with it.

[0044] In some embodiments, the reducing dye is performed in a nitrogen environment; the conditions for providing the nitrogen environment include that the nitrogen flow rate is maintained at 30-35 m 3 / h. Nitrogen is used to create an air-tight environment, so as to avoid the oxidation of the reduced indigo.

[0045] S3, cotton thread dyeing;

[0046] The cotton thread is immersed in the indigo dye solution for 1-3 min to obtain dyed cotton thread, and the dyed cotton thread can exhibit indigo color after air oxidation.

[0047] In some specific embodiments, when the cotton thread is dyed, the indigo dyeing solution is subjected to ultrasonic treatment, the frequency of the ultrasonic treatment is 10-20 MHz, which can accelerate the penetration rate of the indigo dyeing solution, shorten the cotton thread immersion time, and improve the uniformity of the cotton thread dyeing.

[0048] In some specific embodiments, before the cotton thread is dyed, the cotton thread is also subjected to scouring, and the cotton thread is treated with sodium hydroxide with a concentration of 5-6 g / L and a surfactant with a concentration of 6-8 g / L at a temperature of 90-100℃ for 1-1.5 h.

[0049] By scouring the cotton thread, impurities such as oil wax, pectin, pigment, and cotton seed hulls on the cotton thread are removed, so that the cotton thread has good wettability and permeability, and necessary conditions are provided for dyeing.

[0050] In some specific embodiments, after the cotton thread is dyed, the cotton thread is also subjected to steaming treatment, and the dyed cotton thread is treated with steam at 100-103℃ for 1-3 min, so as to facilitate the fixation of indigo to the fibers of the cotton thread and improve the color fastness of the cotton thread.

[0051] Further, the dyed cotton thread after the steaming treatment is also subjected to water washing treatment, and the dyed cotton thread is cleaned with water at 30-40℃, so as to remove the floating color on the surface of the cotton thread.

[0052] Further, the dyed cotton thread after the water washing treatment is also subjected to drying treatment, and the dyed cotton thread is dried with hot air at 110-120℃, so that the dyed cotton thread after drying can be stored for a long time or put into use.

[0053] The method of the present application first prepares an indigo suspension liquid by mixing indigo powder with alkali metal and a dispersing agent, and then reduces the indigo in the indigo suspension liquid by a reductive cold plasma in the presence of a catalyst to obtain an indigo dyeing solution, and then the cotton thread is immersed and dyed, so as to avoid the use of sodium dithionite, greatly reduce the chemical oxygen demand of printing and dyeing wastewater, and reduce the treatment difficulty and cost of printing and dyeing wastewater, and have the green environmental protection characteristics.

[0054] Moreover, the indigo dyeing solution obtained by the method of the present application is stable and uniform, and has good effect on the indigo dyeing of cotton thread, and the dyed cotton thread has small color difference and high color fastness.

[0055] In addition, the method of the present application uses a reductive cold plasma to reduce indigo, which does not require high temperature and high pressure, has low requirements for equipment, and is safe to operate.

[0056] The following are specific embodiments.

[0057] Embodiment 1

[0058] The present application provides a method for dyeing cotton thread with indigo, which comprises the following steps:

[0059] (1) preparing an indigo suspension, wherein the concentration of indigo powder is 3.6 g / L, the concentration of sodium hydroxide is 3.2 g / L, the concentration of potassium hydroxide is 2.0 g / L, and the concentration of sodium alkyl benzene sulfonate is 1 g / L; the temperature of the indigo suspension is 20℃;

[0060] (2) reducing dye, under the condition of nickel as a catalyst, injecting a reducing cold plasma into the indigo suspension to perform a reduction reaction, the generating raw material of the reducing cold plasma is hydrogen, the injection pressure of the reducing cold plasma is 5 bar, the injection flow rate of the reducing cold plasma is 20 mL / min; the nitrogen flow rate is kept at 30 m 3 / h; the stirrer is controlled to stir the indigo suspension at a rotation speed of 30 r / min;

[0061] (3) cotton thread scouring, treating the cotton thread at a temperature of 90℃ for 1 h by using sodium hydroxide with a concentration of 5 g / L and a surfactant with a concentration of 6 g / L;

[0062] (4) cotton thread dyeing, immersing the cotton thread in the indigo dyeing solution for 1 min, and simultaneously performing ultrasonic treatment on the indigo dyeing solution, the frequency of the ultrasonic treatment is 10 MHz;

[0063] (5) steaming treatment, treating the dyed cotton thread by using steam at 100℃ for 1 min;

[0064] (6) washing treatment, washing the steamed dyed cotton thread by using water at 30℃;

[0065] (7) drying treatment, drying the washed dyed cotton thread by using hot air at 110℃.

[0066] Example 2

[0067] The application provides a method for indigo dyeing of cotton thread, comprising the following steps:

[0068] (1) preparing an indigo suspension, wherein the concentration of indigo powder is 3.9 g / L, the concentration of sodium hydroxide is 3.5 g / L, the concentration of potassium hydroxide is 2.8 g / L, and the concentration of sodium high-grade fatty acid is 2 g / L; the temperature of the indigo suspension is 30℃;

[0069] (2) reducing dye, under the condition of nickel-molybdenum alloy as a catalyst, injecting a reducing cold plasma into the indigo suspension to perform a reduction reaction, the generating raw material of the reducing cold plasma is hydrogen, the injection pressure of the reducing cold plasma is 7.5 bar, the injection flow rate of the reducing cold plasma is 25 mL / min; the nitrogen flow rate is kept at 32.5 m 3 / h; the stirrer is controlled to stir the indigo suspension at a speed of 32.5 r / min;

[0070] (3) cotton scouring, the cotton is treated with sodium hydroxide with a concentration of 5.5 g / L and surfactant with a concentration of 7 g / L at a temperature of 95 ℃ for 1.25 h;

[0071] (4) cotton dyeing, the cotton is immersed in the indigo dyeing solution for 2 min, and the indigo dyeing solution is subjected to ultrasonic treatment at a frequency of 15 MHz;

[0072] (5) steaming treatment, the dyed cotton is treated with steam at 101 ℃ for 2 min;

[0073] (6) water washing treatment, the dyed cotton after the steaming treatment is washed with water at 35 ℃;

[0074] (7) drying treatment, the dyed cotton after the water washing treatment is dried with hot air at 115 ℃.

[0075] Example 3

[0076] The application provides a method for indigo dyeing of cotton, comprising the following steps:

[0077] (1) preparation of an indigo suspension, wherein the concentration of indigo powder is 4.2 g / L, the concentration of sodium hydroxide is 3.8 g / L, the concentration of potassium hydroxide is 3.5 g / L, and the concentration of sodium fatty alcohol sulfate is 3 g / L; the temperature of the indigo suspension is 40 ℃;

[0078] (2) reduction of dye, under the condition that iron-nickel-molybdenum alloy is used as a catalyst, a reducing cold plasma is injected into the indigo suspension for reduction reaction, the raw material of the reducing cold plasma is hydrogen, the injection pressure of the reducing cold plasma is 10 bar, the injection flow rate of the reducing cold plasma is 30 mL / min, and the nitrogen flow rate is kept at 35 mL / min; 3 / h; the stirrer is controlled to stir the indigo suspension at a speed of 32.5 r / min;

[0079] (3) cotton scouring, the cotton is treated with sodium hydroxide with a concentration of 5.5 g / L and surfactant with a concentration of 7 g / L at a temperature of 95 ℃ for 1.25 h;

[0080] (4) cotton dyeing, the cotton is immersed in the indigo dyeing solution for 2 min, and the indigo dyeing solution is subjected to ultrasonic treatment at a frequency of 15 MHz;

[0081] (5) steaming treatment, the dyed cotton is treated with steam at 101 ℃ for 2 min;

[0082] (6) water washing treatment, using 40℃ water to clean the dyed cotton thread after the steaming treatment;

[0083] (7) drying treatment, using 120℃ hot air to dry the dyed cotton thread after the water washing treatment.

[0084] Example 4

[0085] The application provides a method for dyeing cotton thread with indigo, comprising the following steps:

[0086] (1) preparing an indigo suspension, wherein the concentration of indigo powder is 3.9g / L, the concentration of sodium hydroxide is 3.5g / L, the concentration of potassium hydroxide is 2.8g / L, and the concentration of sodium higher fatty acid is 2g / L; the temperature of the indigo suspension is 30℃;

[0087] (2) reducing dye, injecting reducing cold plasma into the indigo suspension under the condition that a nickel-molybdenum alloy is used as a catalyst, wherein the raw material of the reducing cold plasma is ammonia gas, the injection pressure of the reducing cold plasma is 7.5bar, the injection flow rate of the reducing cold plasma is 25mL / min, the nitrogen flow rate is kept at 32.5mL / min, and the stirrer is controlled to stir the indigo suspension at a rotating speed of 32.5r / min; 3 (3) cotton scouring, treating the cotton thread at a temperature of 95℃ for 1.25h by using sodium hydroxide with a concentration of 5.5g / L and a surfactant with a concentration of 7g / L;

[0088] (4) cotton dyeing, immersing the cotton thread in the indigo dyeing solution for 2min, and simultaneously performing ultrasonic treatment on the indigo dyeing solution, wherein the frequency of the ultrasonic treatment is 15MHz;

[0089] (5) steaming treatment, treating the dyed cotton thread by using steam at a temperature of 101℃ for 2min;

[0090] (6) water washing treatment, using 35℃ water to clean the dyed cotton thread after the steaming treatment;

[0091] (7) drying treatment, using 115℃ hot air to dry the dyed cotton thread after the water washing treatment.

[0092] Effect test:

[0093] The dyed cotton thread of the example is subjected to color difference and color fastness tests, and the results are shown in Table 1.

[0094] The terminal wastewater of the example is subjected to COD test, and the results are shown in Table 2.

[0095] The terminal wastewater of the example is subjected to COD test, and the results are shown in Table 2.

[0096] Table 1

[0097]

[0098] Color difference standard: 0-0.25, very small or none, invisible to naked eyes; 0.25-0.5, small, invisible to naked eyes; 0.5-1.0, small to medium, difficult to be perceived by naked eyes; 1.0-2.0, medium, relatively difficult to be perceived by naked eyes; 0.2-4.0, with difference, obviously perceived by naked eyes; 4.0 or more, very large, obviously perceived by naked eyes.

[0099] Color fastness rating: 1st grade, 1-2nd grade, 2nd grade, 2-3rd grade, 3rd grade, 3-4th grade, 4th grade, 4-5th grade, 5th grade. The 1st grade is the worst, and the 5th grade is the best (no color fading). The color fastness of qualified products is ≥3rd grade.

[0100] As shown in Table 1, the color difference of the dyed cotton threads of Examples 1-4 is less than 2.0, and the color difference is relatively difficult to be perceived by naked eyes. The color difference of the dyed cotton threads obtained by the method of the present application is small. The dry and wet rubbing color fastness of the dyed cotton threads of Examples 1-4 is greater than 3rd grade. The color fastness of the dyed cotton threads obtained by the method of the present application is high.

[0101] Table 2

[0102] COD of terminal wastewater (mg / L) Example 1 214.21 Example 2 221.39 Example 3 257.37 Comparative Example 1 245.45

[0103] The COD of the indigo dyeing wastewater using hydrosulfite as a reducing agent is usually above 300 mg / L. The method of the present application is used for indigo dyeing of cotton threads without using hydrosulfite as a reducing agent, and the COD of the produced wastewater is significantly reduced.

[0104] The method of the present application is described in detail above. The principles and implementation modes of the present application are described by using specific examples. The above description of the examples is only used to help understand the method of the present application and its core idea. For those skilled in the art, the specific implementation modes and application ranges can be changed according to the idea of the present application. In summary, the content of the present description should not be understood as a limitation of the present application.

Claims

1. A method of cotton thread indigo dyeing, characterized by, The method comprises the following steps: Preparation of an indigo suspension, wherein indigo powder, sodium hydroxide, potassium hydroxide, a dispersing agent and water are mixed to form an indigo suspension, the concentration of the indigo powder is 3.6-4.2 g / L, the concentration of the sodium hydroxide is 3.2-3.8 g / L, the concentration of the potassium hydroxide is 2.0-3.5 g / L, and the concentration of the dispersing agent is 1-3 g / L; Reduction of the dye, wherein a reducing cold plasma is injected into the indigo suspension in the presence of a catalyst to perform a reduction reaction, and an indigo dye liquor is obtained, the temperature of the indigo suspension is 20-40 ℃, the catalyst is composed of one or more of iron, nickel and molybdenum, and the catalyst is fixed on the surface of a stainless steel carrier, the reducing cold plasma is generated from hydrogen or ammonia, the injection pressure of the reducing cold plasma is 5-10 bar, and the injection flow rate of the reducing cold plasma is 20-30 mL / min; Cotton thread dyeing, wherein cotton threads are immersed in the indigo dye liquor for 1-3 min to obtain dyed cotton threads.

2. The method of claim 1, wherein, The dispersing agent is a sulfonate, a carboxylate, a sulfate or a phosphate.

3. The method of claim 1, wherein, The reduction of the dye is performed in a nitrogen environment; The conditions for providing the nitrogen atmosphere include: the nitrogen flow rate is maintained at 30-35 m 3 / h.

4. The method of claim 1, wherein, When the reduction of the dye is performed, a stirrer is controlled to stir the indigo suspension at a speed of 30-35 r / min.

5. The method of claim 1, wherein, When the cotton thread dyeing is performed, the indigo dye liquor is subjected to ultrasonic treatment, and the frequency of the ultrasonic treatment is 10-20 MHz.

6. The method of claim 1, wherein, Before the cotton thread dyeing, cotton thread scouring is further included, wherein the cotton threads are treated with 5-6 g / L of sodium hydroxide and 6-8 g / L of a surfactant at a temperature of 90-100 ℃ for 1-1.5 h.

7. The method of claim 1, wherein, After the cotton thread dyeing, the following steps are further included: Steam treatment, wherein the dyed cotton threads are treated with steam at a temperature of 100-103 ℃ for 1-3 min; Water washing treatment, wherein the dyed cotton threads after the steam treatment are washed with water at a temperature of 30-40 ℃; Drying treatment, wherein the dyed cotton threads after the water washing treatment are dried with hot air at a temperature of 110-120 ℃.

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