Indigo white salt solution having very low aniline content and method for its preparation

Abstract

A stable aqueous indigo white solution comprising an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein the indigo white is in the form of an alkali metal salt, wherein the concentration of the aromatic amine is below 40 ppm as determined according to ISO 14362-1:2017(E); and wherein the concentration of the indigo white salt is in the concentration range of 0 to 45 wt.-% based on the total weight of the solution and wherein the stability of the solution is measured at a temperature of 23 °C; or the concentration of the indigo white salt is in the concentration range of 45 to 65 wt.-% based on the total weight of the solution and wherein the stability of the solution is measured at a temperature of 60 °C.

Description

Technical Field

[0001] This invention belongs to the field of aniline-free indigo salt solutions and indigo obtained therefrom. Background Technology

[0002] Indigo is a vat dye used to dye cellulose-containing textile materials.

[0003] For application in textiles, indigo is reduced, forming a water-soluble indigo salt. This salt is then applied to the textile material in aqueous solution. Oxidation of the indigo salt leads to the formation of indigo, resulting in dyed textiles.

[0004] Synthetically produced indigo often contains aromatic amine-based impurities, particularly aniline and / or N-methylaniline, trapped within it due to commonly used production processes. For example, synthetically produced indigo can contain up to 6000 ppm of aniline and up to 4000 ppm of N-methylaniline. Aromatic amines such as aniline and N-methylaniline are undesirable in textile applications. Therefore, these impurities should be removed from indigo and, consequently, from the indigo salts produced from indigo as much as possible before being applied to textile materials.

[0005] DE 4336032 A1 discloses a method for purifying indigo in Example 1, which involves extracting an aqueous solution of sodium indigofera salt with an inert solvent under deoxygenated conditions, followed by regeneration of the indigo by oxidation. The resulting indigo is free of aniline and N-methylaniline.

[0006] WO 2004 / 024826 A2 teaches the removal of aromatic amine impurities in an aqueous indigo salt solution by distillation, steam distillation, extraction, or stripping with an inert gas. This prior art discloses that the concentration of aromatic amines can be reduced to below 200 ppm using the purification methods defined therein. The purified indigo salt solution can then be oxidized to obtain indigo containing the small amount of said aromatic amine (if present). The purified indigo can be present on textile materials prior to oxidation.

[0007] Example 1 of WO 2004 / 024826 A2 discloses a solution containing 55% by weight of indigo in the form of a mixed sodium and potassium salt, said solution being obtained by distillation at atmospheric pressure from an unpurified indigo salt solution with a salt content of 23%, wherein the purified solution contains less than 200 ppm of aniline and less than 20 ppm of N-methylaniline.

[0008] Example 2 discloses a solution containing 23% by weight of indigo in the form of a mixed sodium and potassium salt, obtained by three extractions of an unpurified indigo salt solution, wherein the aniline content in the purified solution is 147 ppm. N-methylaniline is no longer detectable.

[0009] Example 3 discloses a solution containing 23% by weight of indigo in the form of a mixed sodium and potassium salt, obtained by nitrogen stripping of an unpurified indigo salt solution, wherein the aniline content in the purified solution is 113 ppm. N-methylaniline is no longer detectable.

[0010] Example 4 discloses a mixed sodium and potassium salt form of indigo solution obtained by steam distillation of an unpurified indigo salt solution. Aniline and N-methylaniline are no longer detectable. The purified indigo solution has an indigo concentration of 7% by weight. This low concentration solution is not suitable for obtaining medium or dark hues. Therefore, this indigo solution would not be of interest for industrial-scale applications.

[0011] It is further known that, for transport and application, aqueous solutions of indigo salt should be as stable as possible to prevent undesirable crystallization and / or salt precipitation. This is especially important if indigo salt is present in a relatively high concentration in an aqueous medium. Concentrated indigo salt solutions are advantageous in reduction dyeing processes due to reduced wastewater pollution.

[0012] In this respect, WO 00 / 04100 teaches providing an aqueous solution of indigo in the form of a mixed sodium and potassium salt, wherein the mol% of sodium is in the range of 70 to 30, and the mol% of potassium is correspondingly in the range of 30 to 70. This corresponds to a sodium to potassium molar ratio in the range of 2.33:1 to 1:2.33. Within this range, although the concentration of indigo salt is relatively high at 25 to 40 wt%, the dissolved indigo salt solution is stable at room temperature, or at elevated temperatures in the range of 40 to 60 °C at concentrations between 50 and 55 wt%, i.e., the salt does not readily crystallize or precipitate. This prior art further discloses a sodium to potassium molar ratio in the range of 3:1 to 1:3, which corresponds to a sodium molar ratio in the range of 75 to 25 and a potassium molar ratio in the range of 25 to 75.

[0013] WO 00 / 04100 further references WO 94 / 23114, which discloses a sodium salt solution of indigo that is stable at concentrations up to 20% by weight at room temperature.

[0014] WO 00 / 04100 further discloses in Example 4 that water can be distilled from diluted indigo solutions to concentrate them. For example, approximately 44% by weight of water can be distilled from a solution of indigo salt with a concentration of 25% by weight to concentrate the solution to up to 45% by weight.

[0015] Example 5 of WO 00 / 04100 discloses distilling about 34% water from an aqueous solution containing 20% ​​by weight of indigo salt to concentrate the solution to 45% by weight.

[0016] WO 00 / 04100 does not mention the concentrations of aniline and N-methylaniline.

[0017] Purpose of the invention

[0018] There has always been an industrial need for concentrated indigo salt solutions, wherein the content of aromatic amines is as low as possible. Therefore, the problem to be solved by the present invention is to provide a purified concentrated indigo salt solution and indigo produced therefrom, wherein the aniline content is well below 200 ppm, i.e., at least less than 100 ppm, particularly at least less than 40 ppm or 30 ppm, preferably less than 20 or 10 ppm or less than 5 ppm or even 0 ppm. Summary of the Invention

[0019] The inventors have discovered that the amount of water distilled from an indigo salt aqueous solution has a decisive influence on the residual amount of aromatic amines in the indigo solution. The inventors have found that by adding water to an industrially commonly used, unpurified indigo solution containing the amines in order to obtain a diluted indigo solution, and then distilling off water by a weight at least equal to the weight of the indigo aqueous solution before adding the water, an indigo aqueous solution with very low contents of aromatic amines such as aniline and N-methylaniline can be prepared. The more water added and subsequently distilled off, the lower the content of aromatic amines. The resulting concentrated solution contains less than 40 ppm, preferably less than 30 ppm, or less than 20 ppm, or less than 10 ppm of aromatic amines. In a preferred embodiment, a concentrated solution in which the aromatic amines are no longer detectable can be prepared. Such a solution can be referred to as aniline-free. Therefore, the term "less than 100 ppm or especially less than 40 ppm or less than 30 ppm or less than 20 ppm or less than 10 ppm" covers the lower limit of when aromatic amines are no longer detectable, i.e., 0 ppm as determined according to ISO 14362-1:2017(E).

[0020] Another particular advantage of the method of the present invention is that it can prepare concentrated solutions, for example, in the range of 10 to 65% by weight, such as 15 to 60% by weight, 20 to 55% by weight, or 25 to 50% by weight, which meet the dyeing requirements in terms of medium and dark tones and are aniline-free. Furthermore, it can meet the requirements for reducing wastewater pollution in dyeing plants.

[0021] Therefore, the present invention relates to the following items:

[0022] 1. A method for removing an aromatic amine from an aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline to obtain a purified indigo solution, wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B):

[0023] (A) Add water to the indigo aqueous solution to obtain a diluted indigo solution; and

[0024] (B) The diluted indigo aqueous solution obtained in step (A) is distilled so that water is distilled off with a weight at least equal to the weight of the indigo aqueous solution used in step (A).

[0025] 2. A method for preparing an aqueous indigo solution free of aniline or free of both aniline and N-methylaniline from an aqueous indigo solution containing aniline or aniline and N-methylaniline, wherein the concentrations of aniline or aniline and N-methylaniline are determined according to ISO 14362-1:2017(E), wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B):

[0026] (A) Add water to the indigo aqueous solution to obtain a diluted indigo salt solution; and

[0027] (B) The diluted indigo aqueous solution obtained in step (A) is distilled so that water is distilled off with a weight at least equal to the weight of the indigo aqueous solution used in step (A).

[0028] 3. The method as described in Project 1 or 2, wherein the concentration of indigo salt in the solution used in step (A) is in the range of 5 to 65% by weight based on the total weight of the solution.

[0029] 4. The method of any one of items 1 to 3, wherein the concentration of the aromatic amine in the solution used in step (A) is in the range of 2000 ppm to 10000 ppm.

[0030] 5. The method of any of the preceding items, wherein the concentration of aniline in the solution used in step (A) is in the range of 1000 ppm to 3000 ppm, and the concentration of N-methylaniline is in the range of 500 ppm to 2000 ppm.

[0031] 6. The method of any of the preceding items, wherein in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least 1.5 times the weight of the indigo aqueous solution used in step (A).

[0032] 7. The method of any of the preceding items, wherein in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least twice the weight of the indigo aqueous solution used in step (A).

[0033] 8. The method of any of the preceding items, wherein in step (B), distillation is carried out under an inert gas stream.

[0034] 9. The method of any of the preceding items, wherein the weights of the water added in step (A) and the water distilled in step (B) are selected such that, after step (B), the concentration of indigo salt in the purified solution is in the range of 40 to 65% by weight based on the total weight of the solution.

[0035] 10. The method as described in Item 9, wherein, if necessary, the concentration is adjusted to a range of 10 to 45% by weight by adding water.

[0036] 11. The method as described in any of the preceding items, wherein step (A) comprises:

[0037] (α) Providing a liquid stream comprising the indigo aqueous solution, wherein the indigo aqueous solution contains the amine;

[0038] (β) Provides water flow;

[0039] (γ) brings the liquid stream into contact with the water stream.

[0040] 12. The method as described in item 11, wherein step (γ) comprises steps (γ1) and (γ2), and step (B) comprises steps (δ) and (ε):

[0041] (γ1) Feeding the liquid stream and (γ2) Feeding the water stream

[0042] into a distillation column configured to contact the liquid stream with the aqueous stream;

[0043] (δ) Water containing aniline or aniline and N-methylaniline is discharged from the distillation column; and

[0044] (ε) Discharge an aqueous solution of indigo that contains no aniline or no aniline and no N-methylaniline from the distillation column.

[0045] 13. The method as described in item 11 or 12, wherein the (γ1) liquid stream and the (γ2) water stream are fed into the column through an inlet in the side wall of the column, and the (δ) water containing aniline or aniline and N-methylaniline is discharged at the top of the column, and the (ε) indigo aqueous solution containing neither aniline nor N-methylaniline is discharged at the bottom of the column.

[0046] 14. The method of any of the preceding items, wherein the distillation column is a column provided with trays or packing material.

[0047] 15. A stable aqueous solution of indigo, comprising aniline or an aromatic amine in the form of aniline and N-methylaniline, wherein the indigo is in the form of an alkali metal salt;

[0048] The concentration of the aromatic amine, as determined according to ISO 14362-1:2017(E), is less than 40 ppm; and

[0049] The concentration of the indigo salt is in the range of 10 to 45% by weight based on the total weight of the solution, and the stability of the solution is measured at a temperature of 23°C; or

[0050] The concentration of the indigo salt is in the range of 45 to 65% by weight based on the total weight of the solution, and the stability of the solution is measured at a temperature of 60°C.

[0051] 16. A stable aqueous solution of indigo as described in Project 15, wherein the concentration of aromatic amines is less than 30 ppm or less than 20 ppm.

[0052] 17. A stable aqueous solution of indigo as described in item 15 or 16, wherein the concentration of aromatic amines is less than 10 or 5 ppm.

[0053] 18. A stable aqueous solution of indigo as defined in any of Items 15 to 17, which can be obtained by means of Item 9 or 10 or any of Items 11 to 14 of Item 9 or 10.

[0054] 19. A method for preparing indigo, comprising step (D):

[0055] (D) An aqueous solution of indigo as defined in any of items 15 to 18 of the oxidation program.

[0056] 20. The method of item 19, comprising step (C) prior to step (D):

[0057] (C) Treat textiles with an indigo solution as defined in any of items 15 to 18.

[0058] 21. A method for preparing indigo, comprising steps (I) and (III):

[0059] (I) Implement any one of items 1 to 14, preferably item 9 or 10 or referencing the method defined in any one of items 11 to 14 of item 9 or 10;

[0060] (III) The indigo solution obtained in step (I) of oxidation.

[0061] 22. The method described in item 21, wherein step (II) is included before step (III):

[0062] (II) Treat the textiles with the indigo solution obtained in step (I). Detailed Implementation

[0063] As used in this disclosure, the term "aniline-free" covers, in its broadest sense, an aniline concentration of less than 200 ppm, or less than 100 ppm, preferably less than 80 ppm, more preferably less than 60 ppm, even more preferably less than 40 ppm, particularly less than 30 ppm or less than 20 ppm, and especially preferably less than 10 ppm or less than 5 ppm, said concentration being determined according to ISO 14362-1:2017(E).

[0064] As used in this disclosure, the term "aniline-free and N-methylaniline-free" defines an aniline concentration and an N-methylaniline concentration of less than 200 ppm or 100 ppm, preferably less than 80 ppm, more preferably less than 60 ppm, even more preferably less than 40 ppm, particularly less than 30 ppm or less than 20 ppm, and especially preferably less than 10 ppm or less than 5 ppm, said concentrations being determined according to ISO 14362-1:2017(E).

[0065] According to the method of the present invention

[0066] According to a first aspect, the present invention relates to a method for removing an aromatic amine from an aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline to obtain a purified indigo solution, wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B):

[0067] (A) Add water to the indigo aqueous solution to obtain a diluted indigo solution; and

[0068] (B) The diluted indigo aqueous solution obtained in step (A) is distilled so that water is distilled off with a weight at least equal to the weight of the indigo aqueous solution used in step (A).

[0069] According to a second aspect, the present invention relates to a method for preparing an aqueous solution of indigo free of aniline or aniline and N-methylaniline from an aqueous solution of indigo containing aniline or aniline and N-methylaniline, wherein the concentration of aniline or aniline and N-methylaniline is determined according to ISO 14362-1:2017(E), wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B):

[0070] (A) Add water to the indigo aqueous solution to obtain a diluted indigo salt solution; and

[0071] (B) The diluted indigo aqueous solution obtained in step (A) is distilled so that water is distilled off with a weight at least equal to the weight of the indigo aqueous solution used in step (A).

[0072] The indigo solution used in step (A) was prepared according to a method known in the art, namely by reducing an aqueous composition containing indigo in the presence of an alkali metal hydroxide.

[0073] Therefore, the method according to the invention includes step (0) prior to step (A):

[0074] (0) In the presence of an alkali metal hydroxide, an aqueous composition containing indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline is reduced to produce an aqueous solution of indigo containing the aromatic amine, wherein the indigo is in the form of an alkali metal salt.

[0075] Preferably, reduction is performed as hydrogenation. Hydrogenation can be carried out by methods known in the art.

[0076] The preferred reduction method is to use Raney nickel as a catalyst for hydrogenation.

[0077] Other known methods include reduction using sodium dithionite, reduction via indirect electrolysis such as using an iron triethanolamine complex as a medium, and reduction using indophenol or hydroxyacetone. It should be understood that the reduction is not limited to the methods mentioned.

[0078] According to the present invention, the indigo is in the form of an alkali metal salt.

[0079] The term "alkali metals" encompasses lithium, sodium, and potassium, as well as combinations of two or three of them.

[0080] Therefore, in one embodiment, the alkali metal can be lithium, sodium, or potassium.

[0081] In another embodiment, the alkali metal may be lithium and sodium, or lithium and potassium, or sodium and potassium.

[0082] In another implementation, the alkali metals are lithium, sodium, and potassium.

[0083] The terms "lithium, sodium, and potassium" refer to their respective cations.

[0084] The amount of alkali metal in indigo salt substantially corresponds to an amount that, stoichiometrically, corresponds to the amount necessary for the complete formation of indigo salt. Preferably, the salt and / or solution contains 1.5 to 2.5 moles of alkali per mole of indigo, more preferably 2.0 to 2.5 moles of alkali, and even more preferably 2.1 to 2.5 moles of alkali.

[0085] In one embodiment, the salt is a sodium salt, such as the salt disclosed in WO 94 / 23114.

[0086] In one embodiment, the salt is in the form of a mixture of alkali metal salts, such as a mixture of sodium and potassium salts.

[0087] In one embodiment, sodium and potassium are present in a molar ratio disclosed in WO 00 / 004100, for example, in a molar ratio of 2.33:1 to 1:2.33.

[0088] In another embodiment, sodium and potassium are present in a molar ratio of 3:1 to 1:3.

[0089] In another embodiment, sodium and potassium are present in a molar ratio ranging from more than 3:1 to 10:1, such as 4:1 to 8:1 or 5:1 to 7:1.

[0090] Those skilled in the art can select an appropriate concentration range based on the molar ratio used, within which a corresponding purified indigo solution prepared from an unpurified indigo solution is stable.

[0091] Therefore, within the proposed molar range of sodium and potassium, a stable and concentrated solution can be prepared.

[0092] In a preferred embodiment, indigo salt is provided in the form of a mixed alkali metal salt, preferably a mixture of sodium and potassium salts, and more preferably the molar ratio of sodium to potassium is in the range of 2.33:1 to 1:2.33.

[0093] The required amount of alkali metal hydroxide can be provided all at once before hydrogenation, or in batches during hydrogenation, or both before and during hydrogenation. Additional alkali may also be added after hydrogenation if necessary.

[0094] The concentration of the salt obtained after step (0) or the salt used in step (A) can be selected from a wide range and is not limited to specific requirements.

[0095] In one embodiment, the concentration of indigo salt in the solution obtained in step (0) or used in step (A) is in the range of 5 to 65% by weight, such as 10 to 65% by weight, 15 to 60% by weight, 20 to 55% by weight, 25 to 50% by weight, or 25 to 45% by weight, based on the total weight of the solution obtained in step (0) or used in step (A).

[0096] In another embodiment, the concentration of indigo salt in the solution obtained in step (0) or used in step (A) is in the range of 10 to 35% by weight, based on the total weight of the solution obtained in step (0) or used in step (A).

[0097] As discussed in the background section, the indigo salt solution obtained by reducing indigo contains aniline or aromatic amines in the form of aniline and N-methylaniline derived from commonly used indigo production processes.

[0098] In one embodiment, the concentration of the aromatic amine in the solution obtained in step (0) or used in step (A) prior to purification according to step (B) is in the range of 2000 ppm to 10000 ppm.

[0099] In one embodiment, the concentration of aniline is in the range of 1000 ppm to 3000 ppm, and the concentration of N-methylaniline is in the range of 500 ppm to 2000 ppm.

[0100] According to the invention, in step (A), a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least equal to the weight of the indigo aqueous solution used in step (A). This means that after distillation according to step (B), an aqueous solution is still obtained, i.e., a composition that still contains water and forms a solution, at least at the distillation temperature.

[0101] The method described in step (B) is preferably performed in batches or continuously.

[0102] During distillation according to step (B), the aromatic amine is removed as an azeotropic mixture or as a mixture containing water and amine, along with water.

[0103] In a further preferred embodiment, in step (A) water is added in such a sufficient weight that the weight of the water distilled in step (B) is at least 1.5 times the weight of the indigo aqueous solution used in step (A).

[0104] In a particularly preferred embodiment, in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least twice the weight of the indigo aqueous solution used in step (A).

[0105] In another embodiment, in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least three times the weight of the indigo aqueous solution used in step (A).

[0106] In yet another embodiment, and in a further preferred embodiment, a sufficient weight of water is added in step (A) such that the weight of the water distilled in step (B) is no more than four times the weight of the indigo aqueous solution used in step (A).

[0107] In a further preferred embodiment, in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least equal to and no more than four times the weight of the indigo aqueous solution used in step (A).

[0108] In a further preferred embodiment, in step (A) water is added in sufficient weight such that the weight of the water distilled in step (B) is at least 1.5 times and no more than four times the weight of the indigo aqueous solution used in step (A).

[0109] In a particularly preferred embodiment, in step (A) water is added in sufficient weight such that the weight of the water distilled in step (B) is at least twice the weight of the indigo aqueous solution used in step (A) and no more than four times the weight of the indigo aqueous solution used in step (A).

[0110] In another embodiment, in step (A) water is added in sufficient weight such that the weight of the water distilled in step (B) is at least three times and no more than four times the weight of the indigo aqueous solution used in step (A).

[0111] In yet another embodiment, and in a further preferred embodiment, a sufficient weight of water is added in step (A) such that the weight of the water distilled in step (B) is no more than four times the weight of the indigo aqueous solution used in step (A).

[0112] Distillation can be carried out using known and suitable distillation equipment according to methods known in the art.

[0113] In one implementation, distillation can be carried out under reduced pressure.

[0114] In one implementation, distillation can be carried out under increased pressure.

[0115] In a preferred embodiment, distillation is carried out at atmospheric pressure.

[0116] It is preferable to exclude the presence of oxygen during distillation to prevent indigo from prematurely oxidizing to indigo. Therefore, the distillation according to step (B) should be carried out under an inert gas such as nitrogen.

[0117] In one embodiment, in step (B), distillation is carried out under an inert gas stream, i.e., the aqueous solution is simultaneously stripped during distillation.

[0118] Nitrogen is a suitable inert gas or stripping gas.

[0119] In a preferred embodiment, distillation is performed using a distillation column.

[0120] Distillation according to step (B) using a distillation column

[0121] As used herein, the term "distillation column" is used synonymously with "distillation tower," "rectification column," "fractionation column," or "fractionation tower."

[0122] As used in this article, the term "column" encompasses, in its broadest sense, a vertical cylindrical column.

[0123] The term "column" further encompasses hollow structural elements, preferably hollow cylindrical elements, wherein the length exceeds the diameter. Neither the diameter nor the length is limited. In one embodiment, the length and diameter, or the ratio of length to diameter, of the column can be freely chosen or optimized according to the desired outcome.

[0124] The preferred column has a diameter of 0.1 to 4 meters, such as 0.3 to 3 meters, and a length of about 1 to 50 meters, such as 1 to 30 meters.

[0125] In one embodiment, the diameter is in the range of 0.1 to 4 meters and the length is in the range of 1 to 30 meters.

[0126] In one implementation, when the column is operated in batch or continuous mode, distillation using a distillation column is discussed below.

[0127] According to the invention, both modes require steps (A) and (B) according to the invention, namely, adding water to an aqueous indigo solution to obtain a diluted indigo solution according to step (A), wherein the indigo solution is distilled according to step (B) such that the weight of the distilled water is at least equal to the weight of the aqueous indigo solution used in the dilution step.

[0128] Typically, the purified indigo solution is obtained at or from the bottom of the distillation column, while the water containing the amine is obtained at or from the top of the column.

[0129] In one implementation, steps (A) and (B) are performed sequentially, i.e., the diluted indigo aqueous solution is first prepared according to step (A) and then distilled according to step (B).

[0130] In another embodiment, steps (A) and (B) are performed simultaneously, i.e., the diluted indigo aqueous solution is prepared according to step (A) and distilled according to step (B).

[0131] In one implementation, step (A) is performed outside the distillation column.

[0132] Therefore, in one embodiment, the diluted indigo solution obtained according to step (A) is fed into a distillation column and distilled according to step (B).

[0133] In one embodiment, when the method is carried out intermittently, the diluted indigo solution obtained in step (A) can be fed to the bottom of the column and then distilled. Distillation can be terminated once water by weight at least equal to the weight of the indigo aqueous solution used in step (A) has been distilled off. The purified indigo solution, and consequently the resulting indigo solution, can then be discharged from the bottom of the column.

[0134] In another embodiment, when the method is carried out in continuous mode, the diluted indigo solution obtained according to step (A) can be fed into the column while water [containing amines] is distilled off, and wherein a further simultaneously purified indigo solution and, correspondingly, an indigo solution to be prepared are discharged from the bottom of the column.

[0135] In another implementation, step (A) is performed in a column.

[0136] Subsequently, the diluted indigo solution obtained according to step (A) can be distilled in batch mode according to step (B).

[0137] In another embodiment, when the method is carried out in continuous mode, the indigo solution to be purified can be fed into the column, wherein water is also fed into the column simultaneously for step (A), wherein the water [containing amine] is further distilled off simultaneously, and wherein the further simultaneously purified indigo solution, and correspondingly the indigo solution to be prepared, is discharged at or from the bottom of the column.

[0138] Inside the column, the downward-flowing reflux liquid provides cooling and condensation for the upward-flowing vapor, thereby improving the column's efficiency. The more reflux, the better the column separates lower-boiling-point materials from higher-boiling-point materials.

[0139] During distillation, the aromatic amine is removed along with water at or near the top of the column, i.e., the overhead, where the amine and water or vapor are typically condensed and collected.

[0140] In one embodiment, some of the water and amine condensed and collected at the top of the column are refeeded into the column at the top, where they are used to separate the droplets and optional solids that will leave the column. This separation can preferably be accomplished using a tray section, although a structured packing section can also be used.

[0141] The condensate can be separated into an aqueous phase containing water and aniline.

[0142] In one embodiment, the purified indigo solution obtained at or from the bottom of the distillation column is re-feeded into the column and redistilled.

[0143] In one implementation, the column operates under continuous steady-state conditions. Unless disturbed by changes in feed, heat, ambient temperature, or condensation, the amount of feed added is typically equal to the amount of product removed.

[0144] In a preferred embodiment, the heat entering the column with the feed is adjusted to be equal to the heat removed by the column head fraction and with the product. Careful adjustment can prevent foaming, seepage, entrainment, or overflow, or at least reduce them to an acceptable level.

[0145] In view of the above, in one embodiment, step (A) includes steps (α) to (γ):

[0146] (α) Providing a liquid stream comprising the indigo aqueous solution, wherein the indigo aqueous solution contains the amine;

[0147] (β) Provides water flow;

[0148] (γ) brings the liquid stream into contact with the water stream.

[0149] In another embodiment, step (B) includes steps (δ) to (ε):

[0150] (δ) Water containing aniline or aniline and N-methylaniline is discharged from the distillation column; and

[0151] (ε) Discharge an aqueous solution of indigo that contains no aniline or no aniline and no N-methylaniline from the distillation column.

[0152] In one embodiment, the present invention relates to a method for removing an aromatic amine from an aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline to obtain a purified indigo solution, wherein the indigo is in the form of an alkali metal salt, or

[0153] The invention relates to a method for preparing an aqueous indigo solution free of aniline or free of both aniline and N-methylaniline from an aqueous indigo solution containing aniline or aniline and N-methylaniline, wherein the concentrations of aniline or aniline and N-methylaniline are determined according to ISO 14362-1:2017(E), wherein the indigo is in the form of an alkali metal salt.

[0154] The method includes steps (A) and (B), wherein step (A) includes steps (α), (β), (γ1) and (γ2), and step (B) includes steps (δ) and (ε):

[0155] (α) Providing a liquid stream comprising the indigo aqueous solution, wherein the indigo aqueous solution contains the amine;

[0156] (β) Provides water flow;

[0157] (γ1) Feeding the liquid stream and

[0158] (γ2) The water-material flow is fed into the feed.

[0159] into a distillation column configured to contact the liquid stream with the water stream;

[0160] (δ) Water containing aniline or aniline and N-methylaniline is discharged from the distillation column; and

[0161] (ε) Discharge an aqueous solution of indigo that contains no aniline or no aniline and no N-methylaniline from the distillation column.

[0162] According to the invention, step (α) requires providing a liquid stream comprising the aqueous indigo solution containing the amine. This provision is preferably carried out by discharging an indigo salt solution from a corresponding storage container or directly from a reduction apparatus, in which the indigo salt solution is prepared by the reduction of indigo. The discharged solution is in a flowing state, for example under pressure, gravity, or a pump, and thus forms a stream. Preferably, the flow of the solution is guided via a conduit to the distillation column used in step (γ1).

[0163] Further according to the invention, step (β) requires providing a water flow. This provision is preferably carried out by draining water from a corresponding storage container or reservoir. The drained water is in a flowing state, for example, under pressure, gravity, or a pump, and thus forms a flow. Preferably, the water flow is guided via a pipe to the distillation column used in step (γ2).

[0164] The term “water stream” provided in step (β) refers to any water stream containing water suitable for removing or at least reducing the content of aniline or aniline and N-methylaniline in an indigo solution containing the amine, wherein the indigo is in the form of a salt.

[0165] In one implementation, the heat entering the distillation column generated by the water feed stream according to step (γ2) and the heat entering with the liquid feed stream according to step (γ1) are controlled such that they are equal to the heat removed in steps (δ) and (ε), because adding excessive or insufficient heat to the distillation column may cause foaming or overflow. Therefore, the method is carried out to achieve adiabatic conditions.

[0166] In another embodiment, the heat entering the distillation column generated by the heated portion of the liquid feed stream discharged from the distillation column according to step (ε) and the heat entering with the liquid feed stream fed according to step (γ1) are controlled such that they are equal to the heat removed in steps (δ) and (ε), because adding excessive or insufficient heat to the column may cause foaming or overflow. Therefore, the method is carried out to achieve adiabatic conditions.

[0167] Further according to the invention, steps (γ1) and (γ2) require feeding the liquid stream and the water stream into a distillation column configured to contact the liquid stream and the water stream.

[0168] In one embodiment, the distillation column includes a liquid feed inlet or multiple liquid feed inlets for feeding a liquid feed stream, and a water feed inlet or multiple purification feed inlets for feeding a water feed stream.

[0169] The inlet can be located anywhere on the distillation column; that is, the inlet can be located at the bottom, top, or side wall of the distillation column.

[0170] In one implementation, (multiple) liquid feed inlets are located at the bottom of the distillation column and (multiple) water feed inlets are located at the top, or vice versa.

[0171] In another embodiment, (multiple) liquid feed inlets are located on the sidewall of the distillation column, and (multiple) water feed inlets are located at the bottom or top of the distillation column.

[0172] In another embodiment, multiple water inlets are located on the sidewall of the distillation column, and multiple liquid inlets are located at the bottom or top of the distillation column.

[0173] In a preferred embodiment, the (γ1) liquid stream and the (γ2) water stream are fed into the column through an inlet in the side wall of the column, and the (δ) water containing aniline or aniline and N-methylaniline is discharged at the top of the column, and the (ε) indigo aqueous solution containing no aniline or no aniline and no N-methylaniline is discharged at the bottom of the column.

[0174] Preferably, the multiple water inlets and / or multiple liquid inlets are designed in the form of one or more distributors. It is also possible to use perforated plates as purification inlets and / or liquid inlets.

[0175] In one embodiment, the contact is made such that the distillation column is partially or completely filled with liquid derived from the liquid feed stream, and the (multiple) water feed inlets are arranged such that the feed water feed stream must flow through or into the liquid.

[0176] In another embodiment, the liquid and water inlets are arranged such that the flows cross each other.

[0177] In another implementation, the material flow can be contacted in a counter-current manner.

[0178] In another implementation, the material flow can be contacted in a concurrent manner, for example, when (multiple) liquid and water inlets are the same, i.e., the material flow is fed through one or more common inlets.

[0179] In another embodiment, the distillation column includes means for guiding the liquid feed stream and the water feed stream to bring them into contact with each other. Such means are known in the art. Exemplary examples include trays and packing such as Raschig rings.

[0180] As used in this article, the term "tray" is used synonymously with the term "pan".

[0181] As used herein, the term "filler" is used synonymously with the term "filling material".

[0182] According to the present invention, step (δ) requires the removal of water containing aniline or aniline and N-methylaniline from the distillation column.

[0183] The result of the method according to the invention is that the concentration of aniline or aniline and N-methylaniline in the liquid discharged from the distillation column according to step (ε) is lower than the concentration of aniline or aniline and N-methylaniline in the liquid stream fed into the distillation column via (a plurality of) liquid stream inlets according to step (γ1).

[0184] In a preferred embodiment, steps (α) to (ε) are performed simultaneously, i.e., the method is a continuous method.

[0185] In another embodiment, the method can be performed discontinuously, i.e., as a batch method. In one embodiment, the distillation column is filled with liquid supplied according to the liquid feed stream of step (γ1). Subsequently, steps (γ2) and (δ) are performed. After contact according to step (δ), the purified indigo solution is discharged from the equipment according to step (ε). The method can be repeated.

[0186] The column used in the method of this invention comprises n theoretical levels. As used herein, the term "theoretical level" encompasses a hypothetical region or level in which two phases, such as a liquid phase derived from a liquid feed stream as defined herein and a purified phase, such as a vapor phase derived from a purified feed stream as defined herein, are in equilibrium with each other within the column. Such an equilibrium level may also be referred to as an equilibrium level or an ideal level.

[0187] The required number of theoretical stages in the method according to the invention can depend on the specific type of column used. It can be determined according to methods known in the art, taking into account the desired separation of aniline or aniline and N-methylaniline in the output fraction (i.e., obtained in steps (δ) and (ε)). It also depends on the reflux flow rate used.

[0188] The final design choice of the number of stages to be applied in industrial implementation can then be made based on an economic balance between the cost of additional stages and the cost of using higher reflow rates.

[0189] In one implementation, the column can have 5 to 50 theoretical levels.

[0190] In another implementation, the column can have 10 to 40 theoretical levels.

[0191] In one implementation, the column can be operated under reduced pressure.

[0192] In another implementation, the column can operate at a pressure substantially equal to atmospheric pressure.

[0193] In yet another implementation, the column can be operated at pressures substantially above atmospheric pressure.

[0194] In one implementation, the column can be depressurized, such as at 0.1*10. 5 absolute pressure (0.10 bara) to 0.95*10 5 It operates within a pressure range of 0.95 bara absolute pressure. Under these conditions, the column temperature can be set within the range of 40°C to 95°C.

[0195] In another implementation, the column can be at a pressure substantially equal to atmospheric pressure, i.e., 0.95 * 10⁻⁶. 5 absolute pressure (0.95 bara) to 1.2 * 10 5 The column operates under pressures within the range of 1.2 bara absolute pressure. Under these conditions, the column temperature is typically set between 95°C and 110°C.

[0196] In a further embodiment, the column can be operated using pressures above atmospheric pressure, optionally at 1.2 x 10⁻⁶. 5 absolute pressure (1.2 bara) to 11*10 5 At an absolute pressure of Pa (11 bara), for example at 7*10 5 Under an absolute pressure of 7 bara (Pa), the column temperature can then be set within the range of 110°C to 190°C, for example, within the range of 160°C to 170°C.

[0197] The temperature is measured at the bottom of the column and depends on the amount of vapor leaving the column at the top.

[0198] In one embodiment, a space heater is used for heating. As used herein, the term "space heater" encompasses a device for heating a single small area, such as the bottom area of ​​a column. Therefore, the space heater is preferably located at or near the bottom of the column.

[0199] In another embodiment, the upper region of the heating column.

[0200] In another implementation, the bottom and upper regions of the heating column are heated.

[0201] If necessary, the equipment can be equipped with a defoaming device such as a baffle, preferably installed at the bottom of the column used.

[0202] In another embodiment, adding a defoamer to the indigo solution to be purified can support defoaming. Although heterogeneous defoamers such as silicone oil can be used, homogeneous defoamers that are soluble in the indigo solution and do not negatively affect the subsequent oxidation of the purified indigo to indigo are preferred.

[0203] Specifically, according to the present invention, the device used in steps (γ1) and (γ2) is selected from two or more of plate columns, infill columns, or any of the columns.

[0204] As used herein, the term "tray column" is used synonymously with the terms "trayed tower" or "tray column".

[0205] Slab columns and infill columns are known in the art.

[0206] In one implementation, the columns include one or more sequences designed as slab columns and one or more sequences designed as infill columns to further optimize the method.

[0207] In another embodiment, the columns include one or more sequences designed as slab columns and one or more sequences designed as infill columns to further optimize the method.

[0208] In another embodiment, several slab columns or infill columns are connected in series to further optimize the method.

[0209] In another embodiment, the slab column is connected to the infill column to further optimize the method.

[0210] In another embodiment, the plate column is connected to the filler column and the bubble column to further optimize the method.

[0211] In yet another embodiment, pre-purification is performed as disclosed in the prior art, wherein final purification is carried out using the method according to the invention.

[0212] As used herein, the term "pre-purification" encompasses the distillation of water from the corresponding indigo salt solution, extraction with a suitable organic solvent, steam distillation, or stripping with nitrogen, or two or more of these methods.

[0213] Of course, in the method according to the invention, the presence of oxygen must be excluded to prevent indigo from oxidizing prematurely to indigo.

[0214] The columns used will be discussed in more detail below.

[0215] Slab column

[0216] In one embodiment, the distillation column is a plate column.

[0217] In a preferred embodiment, the slab columns are arranged in the form of a tower, that is, they are arranged vertically.

[0218] In one embodiment, the height of the column is greater than 5 meters, preferably 5 to 50 meters, and more preferably 10 to 40 meters.

[0219] Related to plate columns, the theoretical level is also called the theoretical tray or theoretical plate.

[0220] In one embodiment, a bubble-covered “tray” or “plate” known in the art is provided inside the column to provide good contact between upward-flowing vapor and downward-flowing liquid inside the column.

[0221] The trays or plates are preferably made of round steel plates and are typically installed inside the column at intervals of about 60 to 75 cm above the column height.

[0222] In one implementation, the tray is a bubble cap tray or a flap tray.

[0223] As used herein, the term “bubble cap tray” encompasses a slotted cap on a central riser, through which gas flows upward over the riser and counter-currently below the cap, downward through the annular space between the riser and the cap, and ultimately into the liquid through a series of openings or slots in the lower side of the cap.

[0224] As used herein, the term "capped tray" encompasses a tray with perforations covered by a liftable hood. A vapor flow causes the hood to rise, thus creating a flow zone for the vapor to pass through. The lifting hood guides the vapor horizontally into the liquid.

[0225] In one embodiment, the tray is a perforated tray, i.e., a sieve tray. The term "sieve tray" encompasses a tray in which desired contact between vapor and liquid occurs when vapor flowing upward through the perforations comes into contact with liquid flowing downward through the perforations.

[0226] In one embodiment, contact is achieved by installing a bubble cap or valve cap at each perforation to facilitate the formation of vapor bubbles that flow through a thin layer of liquid held by a weir on each tray.

[0227] Therefore, in one embodiment, the tray may be selected from perforated trays, bubble cap trays, or flap trays, or two or three of them.

[0228] Perforated trays, bubble cap trays, or flap trays are known in the art.

[0229] In one embodiment, the feed to the plate column comprises either a liquid stream provided according to step (α) and a water stream provided according to steps (β) and (γ1) and (γ2). The purified indigo solution is collected at the bottom of the column while the water containing amine is collected at the top. The liquid and vapor generated at the top and bottom can be recycled.

[0230] According to step (δ), the vapor collected at the top contains aniline or aniline and N-methylaniline that has been removed from the indigo solution.

[0231] According to step (ε), the liquid collected at the bottom contains a purified indigo solution.

[0232] Filled column

[0233] In one embodiment, the trays of the plate column described above are replaced by a packed section, that is, a section containing packing material. Therefore, the equipment used for purification is a packed column.

[0234] Therefore, packing material can be used in place of trays in columns, especially when low pressure drops across the column are required, such as when operating under vacuum. This packing material can be a random packing, such as Raschig rings or structured metal sheets known in the art.

[0235] In one implementation, the infill may have a regular geometry, such as a stacked ring, a grid, or a proprietary structured ring, or a saddle.

[0236] For example, a ring is a Rahig ring or a Bauer ring.

[0237] A saddle could be, for example, saddle.

[0238] In one implementation, the filler may have an irregular shape.

[0239] The filler can be randomly arranged in the column, with rings, saddles and proprietary covers stacked in the column and arranged randomly.

[0240] In another implementation, the filler can be regularly arranged in the column.

[0241] Rings, saddles, or proprietary structured covers can be made from a variety of materials such as ceramics, metals, plastics, and carbon.

[0242] In one implementation, the use of structured fillers such as wire mesh or perforated sheets is also possible.

[0243] The necessary height of the filler can be determined according to methods known in the art.

[0244] In one embodiment, the feed to the packed column comprises either a liquid stream provided according to step (α) and a purified stream, such as a vapor stream, fed into the column according to steps (γ1) and (γ2) according to step (β). The liquid is collected at the bottom of the packed column, while the vapor is collected at the top. The liquid and vapor generated at the top and bottom can be recycled.

[0245] According to step (δ), the vapor collected at the top contains aniline or aniline and N-methylaniline that has been removed from the indigo solution.

[0246] According to step (ε), the liquid collected at the bottom contains a purified indigo solution.

[0247] In one embodiment, prior to distillation in step (B), the concentration of the aromatic amine-containing indigo in the aqueous composition used in step (A) is selected such that the concentration of the indigo salt obtained in step (A) is less than 25% by weight.

[0248] Therefore, in one embodiment, the concentration of the indigo salt solution used in step (B) is also less than 25% by weight.

[0249] A concentration below 25% by weight may be beneficial because during distillation, water and the aromatic amine are distilled off, thereby purifying the indigo salt solution and concentrating the solution, from which a stable and purified concentrated indigo salt solution can be obtained.

[0250] Due to the distillation of water and amine, the solution obtained in step (B) may be concentrated relative to the solution used in step (B) obtained after step (A). In one embodiment, the concentration of indigo salt in the solution obtained in step (0) or used in step (A) is in the range of 5 to 65% by weight, such as 10 to 60% by weight, 15 to 55% by weight, 20 to 50% by weight, or 25 to 45% by weight, based on the total weight of the solution obtained in step (0) or used in step (A).

[0251] In one embodiment, the concentration of indigo salt in the purified solution obtained after step (B) is in the range of 45 to 65% by weight based on the total weight of the solution.

[0252] In one embodiment, the concentration of indocyanine in the solution obtained in step (0) or used in step (A) is in the range of 10 to 40% by weight, based on the total weight of the solution, and after step (B), the concentration of indocyanine in the purified solution is in the range of 45 to 65% by weight.

[0253] Therefore, in one embodiment, the weights of the water added in step (A) and the water distilled in step (B) are selected such that, after step (B), the concentration of indigo salt in the purified solution is in the range of 45 to 65% by weight based on the total weight of the solution.

[0254] The resulting concentrated solution contains less than 40 ppm or less than 30 ppm, preferably less than 20 ppm or less than 10 ppm or even less than 5 ppm of aromatic amines.

[0255] If necessary, the solution obtained in step (B) can be diluted to a predetermined concentration range suitable for the application or stability requirements.

[0256] In one embodiment, if necessary, the concentration is adjusted to a range of 10 to 45 wt% by adding water, such as 15 to 45 wt%. Other suitable concentration ranges are, for example, 20 to 45 wt% or 25 to 45 wt%.

[0257] Of course, the content of aromatic amines is further reduced after dilution.

[0258] In a preferred embodiment, the aromatic amine is no longer detectable. Such a solution may be referred to as aniline-free.

[0259] In one embodiment, the concentration of the indigo salt is adjusted to a concentration range of 10 to 45 wt%, such as 15 to 45 wt%, 20 to 45 wt%, or 25 to 45 wt%, based on the total weight of the solution, to provide a stable indigo salt solution, wherein the stability of the solution is measured at a temperature of 23°C.

[0260] The term "solution stability" refers to a solution that does not readily crystallize or precipitate at a given temperature.

[0261] In another embodiment, the concentration of the indigo salt is in the range of 45 to 65% by weight based on the total weight of the solution, and the stability of the solution is measured at a temperature of 60°C. According to a third aspect, the present invention relates to a stable aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline, wherein the indigo is in the form of an alkali metal salt.

[0262] The concentration of the aromatic amine, as determined according to ISO 14362-1:2017(E), is less than 40 ppm; and

[0263] The concentration of the indigo salt is in the range of 10 to 45% by weight, such as 15 to 45% by weight, 20 to 45% by weight, or 25 to 45% by weight, based on the total weight of the solution, and the stability of the solution is measured at a temperature of 23°C.

[0264] In another embodiment, the present invention relates to a stable aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline, wherein the indigo is in the form of an alkali metal salt.

[0265] The concentration of the aromatic amine, as determined according to ISO 14362-1:2017(E), is less than 100 ppm; and

[0266] The concentration of the indigo salt is in the range of 45 to 65% by weight, such as 45 to 60% by weight, based on the total weight of the solution, and the stability of the solution is measured at a temperature of 60°C.

[0267] In a preferred embodiment, the concentration of aromatic amines is less than 40 ppm.

[0268] In a further preferred embodiment, the concentration of aromatic amine is less than 30 ppm.

[0269] In another preferred embodiment, the concentration of aromatic amine is less than 20 ppm.

[0270] In another preferred embodiment, the concentration of aromatic amine is less than 10 ppm.

[0271] In another preferred embodiment, the concentration of aromatic amine is less than 5 ppm.

[0272] In another preferred embodiment, the concentration of aromatic amines is no longer detectable.

[0273] In one embodiment, the indigo aqueous solution can be obtained by the method defined in the first or second aspect.

[0274] Indigo salt obtained according to the method of the first or second aspect, or an aqueous solution of indigo as defined in the third aspect, can be converted into indigo or can be used in a reduction dyeing process to dye textiles.

[0275] Therefore, according to the fourth aspect, the present invention relates to a method for preparing indigo, comprising step (D): (D) oxidizing an aqueous solution of indigo as defined in the third aspect.

[0276] In one embodiment, the method includes a step (C) prior to step (D): (C) treating the textile with an indigo solution as defined in the third aspect.

[0277] In another embodiment, the present invention relates to a method for preparing indigo, comprising steps (I) and (III):

[0278] (I) Performing the methods defined in the first or second aspect or any of the embodiments defined therein;

[0279] (III) The indigo solution obtained in step (I) of oxidation.

[0280] In one implementation, the method includes step (II) prior to step (III):

[0281] (II) Treat the textiles with the indigo solution obtained in step (I).

[0282] Example

[0283] Example 1 (Comparative)

[0284] 1000 g of a 30 wt% indigo solution (containing 6.5 wt% alkali metals, 2495 ppm aniline, and 1480 ppm N-methylaniline) was distilled under ambient pressure. After distilling off 470 ml of water, a 57 wt% solution was obtained. According to ISO 14362-1:2017(E), the concentrated solution contained 173 ppm aniline and 9 ppm N-methylaniline.

[0285] Example 2

[0286] 1000 g of water was added to 2000 g of a 30 wt% indigo solution (containing 6 to 7 wt% alkali metal, 2380 ppm aniline, and 1375 ppm N-methylaniline). The composition was then distilled under ambient pressure. After distilling off 2000 ml of water, a 60 wt% solution was obtained. The concentrated solution was determined to contain 38 ppm aniline and 2.0 ppm N-methylaniline according to ISO 14362-1:2017(E).

[0287] Example 3

[0288] 2000 g of water was added to 2000 g of a 30 wt% indigo solution (containing 6.5 wt% alkali metal, 2350 ppm aniline, and 1335 ppm N-methylaniline). The composition was then distilled under ambient pressure. After distilling off 3000 ml of water, a 60 wt% solution was obtained. The concentrated solution was determined to contain 16 ppm aniline and 0.3 ppm N-methylaniline according to ISO 14362-1:2017(E).

[0289] Example 4

[0290] 3000 g of water was added to 2000 g of a 30 wt% indigo solution (containing 6.5 wt% alkali metal, 2164 ppm aniline, and 1170 ppm N-methylaniline). The composition was then distilled under ambient pressure. After distilling off 4000 ml of water, a 60 wt% solution was obtained. The aniline content in the concentrated solution was determined to be 5 ppm according to ISO 14362-1:2017(E). N-methylaniline was not detected.

[0291] Example 5

[0292] Dilute 1000g of the indigo solution obtained in Example 4 with 500g of water to obtain a 40% by weight solution. The aniline content of this solution is less than 5 ppm, as determined by ISO 14362-1:2017(E). This solution is used for reduction staining and to provide a deep hue.

Claims

1. A method for removing an aromatic amine from an aqueous solution of indigo containing aniline or an aromatic amine in the form of aniline and N-methylaniline to obtain a purified indigo solution, wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B): (A) Add water to the indigo aqueous solution to obtain a diluted indigo solution; and (B) Distill the diluted indigo aqueous solution obtained in step (A) such that the weight of the distilled water is at least equal to the weight of the indigo aqueous solution used in step (A); The weights of the water added in step (A) and the water distilled in step (B) are selected such that, after step (B), the concentration of indigo salt in the purified solution is in the range of 40 to 65% by weight based on the total weight of the solution.

2. A method for preparing an aqueous indigo solution free of aniline or free of both aniline and N-methylaniline from an aqueous indigo solution containing aniline or aniline and N-methylaniline, wherein the concentrations of aniline or aniline and N-methylaniline are determined according to ISO 14362-1:2017(E), wherein the indigo is in the form of an alkali metal salt, the method comprising steps (A) and (B): (A) Add water to the indigo aqueous solution to obtain a diluted indigo salt solution; and (B) Distill the diluted indigo aqueous solution obtained in step (A) such that the weight of the distilled water is at least equal to the weight of the indigo aqueous solution used in step (A); The weights of the water added in step (A) and the water distilled in step (B) are selected such that, after step (B), the concentration of indigo salt in the purified solution is in the range of 40 to 65% by weight based on the total weight of the solution.

3. The method of claim 1 or 2, wherein the concentration of indigo salt in the solution used in step (A) is in the range of 5 to 65% by weight based on the total weight of the solution.

4. The method of claim 1, wherein the concentration of the aromatic amine in the solution used in step (A) is in the range of 2000 ppm to 10000 ppm.

5. The method of claim 1 or 2, wherein the concentration of aniline in the solution used in step (A) is in the range of 1000 ppm to 3000 ppm, and the concentration of N-methylaniline is in the range of 500 ppm to 2000 ppm.

6. The method of claim 1 or 2, wherein in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least 1.5 times the weight of the indigo aqueous solution used in step (A).

7. The method of claim 1 or 2, wherein in step (A) a sufficient weight of water is added such that the weight of the water distilled in step (B) is at least twice the weight of the indigo aqueous solution used in step (A).

8. The method of claim 1 or 2, wherein in step (B), distillation is carried out under an inert gas stream.

9. The method of claim 1 or 2, wherein, if necessary, the concentration is adjusted to a range of 10 to 45% by weight by adding water.

10. The method of claim 1 or 2, wherein step (A) comprises: (α) Providing a liquid stream comprising the indigo aqueous solution, wherein the indigo aqueous solution contains the amine; (β) Provides water flow; (γ) brings the liquid stream into contact with the water stream.

11. The method of claim 10, wherein step (γ) comprises steps (γ1) and (γ2), and step (B) comprises steps (δ) and (ε): (γ1) Feeding the liquid stream and (γ2) The water-material flow is fed into the feed. into a distillation column configured to contact the liquid stream with the water stream; (δ) Water containing aniline or aniline and N-methylaniline is discharged from the distillation column; and (ε) Discharge an aqueous solution of indigo that contains no aniline or no aniline and no N-methylaniline from the distillation column.

12. The method of claim 11, wherein the (γ1) liquid stream and the (γ2) water stream are fed into the column via inlets in the sidewall of the column, and the (δ) water containing aniline or aniline and N-methylaniline is discharged at the top of the column, and the (ε) indigo aqueous solution containing neither aniline nor N-methylaniline is discharged at the bottom of the column.

13. The method of claim 11, wherein the distillation column is a column provided with trays or packing material.

14. A method for preparing indigo, comprising steps (I) and (III): (I) Implement the method as described in claim 1 or 2; (III) The indigo solution obtained in step (I) of oxidation.

15. The method of claim 14, comprising step (II) prior to step (III): (II) Treat textiles with the indigo solution obtained in step (I).