Leukoindigo salt solutions having very low aniline content and methods for their preparation

Abstract

To provide purified concentrated leucoindigo salt solutions having an aniline content of far below 200 ppm, and indigo made therefrom.SOLUTION: A stable aqueous leucoindigo solution comprises an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein the leucoindigo is in the form of an alkali metal salt; wherein the concentration of the aromatic amine is below 40 ppm determined in accordance with ISO 14362-1:2017(E); and wherein the concentration of the leucoindigo salt is in a concentration range of from 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 wherein the concentration of the leucoindigo salt is in a concentration range of from 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.SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention is in the field of aniline-free leukoindigo salt solutions and indigos obtained therefrom. [Background technology]

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

[0003] For application to textile materials, indigo undergoes reduction, whereby a water-soluble leukoindigo salt is formed. This salt is then applied to the textile material in the form of an aqueous solution. Oxidation of the leukoindigo salt leads to the formation of indigo, whereby a dyed textile material is obtained.

[0004] Synthetically produced indigo 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 may contain up to 6,000 ppm of aniline and up to 4,000 ppm of N-methylaniline. Aromatic amines, such as aniline and N-methylaniline, are undesirable in textile applications. Therefore, these impurities must be removed as much as possible from the indigo from which the leucine dihydrogen iodide salts are produced before application to textile materials.

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

[0006] WO 2004 / 024826 A2 suggests removing aromatic amine impurities at the stage of the aqueous leucoidindigo salt solution by distillation, steam distillation, extraction, or stripping with an inert gas. This prior art discloses that by applying the purification method specified therein, the concentration of aromatic amines can be reduced to a content of less than 200 ppm. The purified leucoidindigo salt solution can then be subjected to oxidation to obtain indigo containing said small amounts of aromatic amines, if any. The purified leucoidindigo can be present on a fiber material before oxidation.

[0007] Example 1 of WO2004 / 024826A2 discloses a solution containing 55% by weight of leukoindigo in the form of mixed sodium and potassium salts obtained by distillation under atmospheric pressure from a crude leukoindigo salt solution having a salt content of 23%, wherein the content of aniline in the purified solution is less than 200 ppm and the content of N-methylaniline is less than 20 ppm.

[0008] Example 2 discloses a solution containing 23% by weight of leukoindigo in the form of mixed sodium and potassium salts obtained from a crude leukoindigo salt solution by triple extraction, where the content of aniline in the purified solution is 147 ppm, and N-methylaniline is no longer detectable.

[0009] Example 3 discloses a solution containing 23% by weight of leukoindigo in the form of mixed sodium and potassium salts obtained from a crude leukoindigo salt solution by stripping with nitrogen, where the content of aniline in the purified solution is 113 ppm, and N-methylaniline is no longer detectable.

[0010] Example 4 discloses a leukoindigo solution in the form of a mixed sodium and potassium salt obtained by steam distillation of a crude leukoindigo salt solution. Aniline and N-methylaniline are no longer detectable. The purified leukoindigo solution has a leukoindigo concentration of 7% by weight. This low-concentration solution is not applicable for obtaining medium or deep shades. Consequently, such a leukoindigo solution would not be attractive for industrial-scale application.

[0011] It is further known that for transport and application, aqueous leucoidindigode salt solutions must be as stable as possible to prevent undesired salt crystallization and / or precipitation. This is particularly important when the leucoidindigode salt is present in the aqueous medium at relatively high concentrations. Concentrated leucoidindigode salt solutions are advantageous in vat dying processes due to contamination by reduced wastewater.

[0012] In this regard, WO 00 / 04100 suggests providing aqueous leukoindigo solutions in the form of mixed sodium and potassium salts, where the mole percent of sodium is in the range of 70-30 and the mole percent of potassium is in the range of 30-70. This corresponds to a molar ratio of sodium to potassium in the range of 2.33:1 to 1:2.33. Within this range, despite a relatively high concentration of leukoindigo salt of 25-40 wt.%, the dissolved leukoindigo salt solution is stable at room temperature or, in the case of a concentration of 50-55 wt.%, at elevated temperatures in the range of 40-60°C, i.e., the salt does not tend to crystallize or precipitate. This prior art further discloses a molar ratio of sodium to potassium in the range of 3:1 to 1:3, which corresponds to a mole percent range of sodium of 75-25 and potassium of 25-75.

[0013] WO 00 / 04100 further discloses with respect to WO 94 / 23114 that at room temperature, solutions of leukoindigo in the form of the sodium salt are stable up to a concentration of 20% by weight.

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

[0015] Example 5 of WO00 / 04100 discloses that about 34% of the water is distilled off from an aqueous solution containing 20% ​​by weight of leucoidindigosalt to concentrate the solution to 45% by weight.

[0016] WO00 / 04100 is silent regarding aniline and methylaniline concentrations. Object of the invention

[0017] There is still a need in the industry to provide concentrated leukoindigo salt solutions in which the content of said aromatic amines is as low as possible. Thus, the problem to be solved by the present invention is to provide purified concentrated leukoindigo salt solutions and indigo produced therefrom having an aniline content well below 200 ppm, i.e., at least less than 100 ppm, and in particular at least less than 40 ppm or 30 ppm, preferably less than 20 or 10 ppm, or even less than 5 ppm or 0 ppm. Summary of the Invention

[0018] SUMMARY OF THE INVENTION The inventors have discovered that the amount of water distilled off from an aqueous leucoidindigo salt solution has a significant effect on the residual amount of aromatic amines in the leucoidindigo solution. The inventors have discovered that an aqueous leucoidindigo solution with a very low content of aromatic amines, such as aniline and N-methylaniline, can be prepared by adding water to a crude leucoidindigo solution commonly used in industry and containing the amine to obtain a diluted leucoidindigo solution, followed by distilling off a weight of water at least equal to the weight of the aqueous leucoidindigo solution before the addition of water. The more water added and subsequently distilled off, the lower the aromatic amine content. The resulting concentrated solution contains less than 40 ppm, preferably less than 30 ppm, 20 ppm, or even less than 10 ppm of aromatic amines. In a preferred embodiment, a concentrated solution can be prepared in which the aromatic amine is no longer detectable. Such a solution may be referred to as aniline-free. The term "less than 100 ppm, and in particular less than 40 ppm, less than 30 ppm, less than 20 ppm or less than 10 ppm" therefore encompasses the limit at which aromatic amines are no longer detectable, i.e. 0 ppm as a lower limit, according to ISO 14362-1:2017(E).

[0019] A further particular advantage of the method according to the invention is that concentrated solutions can be prepared in the range of, for example, 10-65% by weight, such as 15-60% by weight, 20-55% by weight or 25-50% by weight, which meet the dyeing requirements for medium and deep shades and are aniline-free, and also meet the pollution requirements of reduced wastewater in dyehouses.

[0020] Accordingly, the present invention relates to the following: 1. 1. A method for removing an aromatic amine in the form of aniline or aniline and N-methylaniline from an aqueous leucoidindigo solution containing said aromatic amine, to obtain a purified leucoidindigo solution, wherein said leucoidindigo is present in the form of an alkali metal salt, said method comprising the following steps (A) and (B): (A) Adding water to the aqueous leucoidindigo solution to obtain a diluted leucoidindigo solution; and (B) subjecting the diluted aqueous leucoidindigo solution obtained in step (A) to distillation so as to distill off a weight of water at least equal to the weight of said aqueous leucoidindigo solution used in step (A); A method comprising: 2. 1. A method for producing an aqueous leukoindigo solution that is aniline-free or aniline-free and N-methylaniline-free, when the concentrations of aniline or aniline and N-methylaniline are measured according to ISO 14362-1:2017(E), from an aqueous leukoindigo solution containing aniline or aniline and N-methylaniline, wherein the leukoindigo is present in the form of an alkali metal salt, the method comprising the steps of: (A) and (B): (A) adding water to the aqueous leucoidindigo solution to obtain a diluted leucoidindigo salt solution; and (B) subjecting the diluted aqueous leucoidindigo solution obtained in step (A) to distillation so as to distill off a weight of water at least equal to the weight of said aqueous leucoidindigo solution used in step (A); A method comprising: 3. 3. The method according to item 1 or 2, wherein the concentration of the leucoidindigode 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. 4. The method according to 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 2,000 ppm to 10,000 ppm. 5. 5. The method according to any one of items 1 to 4, wherein the concentration of aniline in the solution used in step (A) is in the range of 1,000 ppm to 3,000 ppm, and the concentration of N-methylaniline is in the range of 500 to 2,000 ppm. 6. 6. The method of any one of items 1 to 5, wherein a sufficient weight of water is added in step (A) to allow a weight of water to be distilled off in step (B) that is at least 1.5 times the weight of the aqueous leukoindigo solution used in step (A). 7. 7. The method of any one of items 1 to 6, wherein a sufficient weight of water is added in step (A) to allow a weight of water to be distilled off in step (B) that is at least twice the weight of the aqueous leukoindigo solution used in step (A). 8. 8. The method according to any one of items 1 to 7, wherein the distillation in step (B) is carried out under a stream of inert gas. 9. 9. The method according to any one of items 1 to 8, wherein the weights of the water added in step (A) and the water distilled off in step (B) are selected so that after step (B), the concentration of leukoindigofol in the purified solution is in the range of 40 to 65% by weight, based on the total weight of the solution. 10. 10. The method according to item 9, wherein the concentration is adjusted to a range of 10 to 45% by weight by adding water, if necessary. 11. Step (A) is as follows: (α) providing a liquid stream comprising said aqueous leukoindigode solution containing said amine(s); (β) Prepare the water flow; (γ) contacting the liquid stream with the water stream; 11. The method according to any one of items 1 to 10, comprising: 12. Step (γ) comprises the following steps (γ1) and (γ2): in a distillation column configured to contact the liquid stream with the aqueous stream; (γ1) injecting the liquid stream; and (γ2) injecting the water flow; and step (B) comprises the following steps (δ) and (ε): (δ) discharging water containing aniline or aniline and N-methylaniline from the distillation column; and (ε) discharging the aniline-free or aniline-free and N-methylaniline-free aqueous leucoidindigo solution from the distillation column; Item 12. The method according to item 11, comprising: 13. 13. The method according to item 11 or 12, wherein the (γ1) liquid stream and the (γ2) water stream are injected into the column through inlets in the side wall of the column, the (δ) water containing aniline or aniline and N-methylaniline is discharged at the top of the column, and the (ε) aniline-free or aniline-free and N-methylaniline-free aqueous leucoidindigo solution is discharged at the bottom of the column. 14. 14. The process according to any one of items 1 to 13, wherein the distillation column is a column provided with trays or packing. 15. A stable aqueous leucoidindigo solution containing an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein said leucoidindigo is present in the form of an alkali metal salt; wherein the concentration of aromatic amines is less than 40 ppm as measured in accordance with ISO 14362‐1:2017(E); wherein the concentration of the leucoidindigode salt is in the range of 10 to 45% by weight based on the total weight of the solution, and wherein the stability of the solution is measured at a temperature of 23°C; or A stable aqueous leukoindigosalt solution, wherein the concentration of the leukoindigosalt is in the range of 45 to 65% by weight based on the total weight of the solution, and wherein the stability of the solution is measured at a temperature of 60°C. 16. 16. The stable aqueous leukoindigofen solution according to item 15, wherein the concentration of aromatic amines is less than 30 ppm or less than 20 ppm. 17. 17. The stable aqueous leukoindigofen solution according to item 15 or 16, wherein the concentration of aromatic amines is less than 10 ppm or 5 ppm. 18. 18. The stable aqueous leukoindigo solution according to any one of items 15 to 17, obtainable by the method according to item 9 or 10 or, depending on item 9 or 10, by the method according to any one of items 11 to 14. 19. Step (D) below: (D) oxidizing the aqueous leukoindigode solution according to any one of items 15 to 18; 1. A method for producing indigo, comprising: 20. Before step (D), do step (C): (C) treating the textile with the leucoidindigo solution according to any one of items 15 to 18; 20. The method according to item 19, comprising: twenty one. The following steps (I) and (III): (I) carrying out the method according to any one of items 1 to 14, preferably item 9 or 10, or, depending on item 9 or 10, any one of items 11 to 14; (III) oxidizing the leucoidindigo solution obtained in step (I); 1. A method for producing indigo, comprising: twenty two. Step (III) is preceded by step (II): (II) treating the textile with the leucoindigo solution obtained in step (I); 22. The method according to item 21, comprising: DETAILED DESCRIPTION OF THE INVENTION

[0021] Detailed Description of the Invention The term "aniline-free" as used in this disclosure in its broadest sense encompasses aniline concentrations of less than 200 ppm, less than 100 ppm, preferably less than 80 ppm, more preferably less than 60 ppm, even more preferably less than 40 ppm, in particular less than 30 ppm or less than 20 ppm, and especially preferably less than 10 ppm or less than 5 ppm, as measured in accordance with ISO 14362-1:2017(E).

[0022] The term "aniline-free and N-methylaniline-free" as used in this disclosure defines aniline and N-methylaniline concentrations 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, in particular less than 30 ppm or less than 20 ppm, and especially preferably less than 10 ppm or less than 5 ppm, concentrations measured according to ISO 14362-1:2017(E).

[0023] Method according to the invention

[0024] According to a first aspect, the present invention provides a method for removing an aromatic amine in the form of aniline or aniline and N-methylaniline from an aqueous leucoidindigo solution containing said aromatic amine, in order to obtain a purified leucoidindigo solution, wherein said leucoidindigo is present in the form of an alkali metal salt, the method comprising the following steps (A) and (B): (A) Adding water to the aqueous leucoidindigo solution to obtain a diluted leucoidindigo solution; and (B) subjecting the diluted aqueous leucoidindigo solution obtained in step (A) to distillation so as to distill off a weight of water at least equal to the weight of said aqueous leucoidindigo solution used in step (A); The present invention relates to a method comprising:

[0025] According to a second aspect, the present invention provides a method for producing an aniline-free or aniline-free and N-methylaniline-free aqueous leucoidindigo solution from an aqueous leucoidindigo solution comprising aniline or aniline and N-methylaniline, when the concentrations of aniline or aniline and N-methylaniline are measured according to ISO 14362-1:2017(E), wherein the leucoidindigo is present in the form of an alkali metal salt, the method comprising the following steps (A) and (B): (A) adding water to the aqueous leucoidindigo solution to obtain a diluted leucoidindigo salt solution; and (B) subjecting the diluted aqueous leucoidindigo solution obtained in step (A) to distillation so as to distill off a weight of water at least equal to the weight of said aqueous leucoidindigo solution used in step (A); The present invention relates to a method comprising:

[0026] The leucoidindigo solution used in step (A) is prepared by methods known in the art, i.e., by subjecting an aqueous composition containing indigo to a reduction step in the presence of an alkali metal hydroxide.

[0027] Therefore, the method according to the present invention may further comprise, before step (A), the following step (O): (O) subjecting an aqueous composition comprising an indigo containing an aromatic amine in the form of aniline or aniline and N-methylaniline to reduction in the presence of an alkali metal hydroxide to produce an aqueous leukoindigo solution containing said aromatic amine; wherein said leukoindigofera is present in the form of an alkali metal salt.

[0028] Preferably, the reduction is carried out as a hydrogenation, which can be carried out by methods known in the art.

[0029] The preferred reduction is hydrogenation using Raney-nickel as the catalyst.

[0030] Other known methods are, for example, reduction using sodium dithionite, electrolytic reduction, such as indirect electrolysis using, for example, triethanolamine complex of iron as a mediator, reduction using indoxyl, or hydroxyacetone. It should be understood that the reduction is not limited to the mentioned reduction methods.

[0031] According to the present invention, the leukoindigosin is present in the form of an alkali metal salt.

[0032] The term "alkali metal" includes lithium, sodium and potassium, and combinations of two or more thereof.

[0033] Thus, in one embodiment, the alkali metal may be lithium, sodium, or potassium.

[0034] In another embodiment, the alkali metals can be lithium and sodium, or lithium and potassium, or sodium and potassium.

[0035] In another embodiment, the alkali metals are lithium, sodium and potassium.

[0036] The terms "lithium, sodium, and potassium" refer to their respective cations.

[0037] The amount of alkali metal in the leucoidindigosalt corresponds substantially to the stoichiometric amount required for the complete formation of the leucoidindigosalt. Preferably, the salt and / or solution contains 1.5 to 2.5 moles of alkali per mole of leucoidindigosalt, more preferably 2.0 to 2.5 moles of alkali, and even more preferably 2.1 to 2.5 moles of alkali.

[0038] In one embodiment, the salt is a sodium salt, such as the salts disclosed in WO94 / 23114.

[0039] In one embodiment, the salts are present in the form of mixed alkali metal salts, such as mixed sodium and potassium salts.

[0040] In one embodiment, the sodium and potassium are present in a molar ratio disclosed in WO00 / 004100, for example, a molar ratio of 2.33:1 to 1:2.33.

[0041] In another embodiment, the sodium and potassium are present in a molar ratio of from 3:1 to 1:3.

[0042] In another embodiment, the sodium and potassium are present in a molar ratio of greater than 3:1 to 10:1, such as from 4:1 to 8:1 or from 5:1 to 7:1.

[0043] Those skilled in the art will be able to select a suitable concentration range within which each purified leukoindigo solution prepared from the unpurified leukoindigo solution will be stable depending on the molar ratio used.

[0044] Thus, within the molar range of sodium to potassium addressed, a stably concentrated solution can be prepared.

[0045] In a preferred embodiment, the leucoidindigode salt is provided in the form of a mixed alkali metal salt, preferably a mixed sodium and potassium salt, more preferably in a molar ratio of sodium to potassium ranging from 2.33:1 to 1:2.33.

[0046] The required amount of alkali metal hydroxide can be provided all at once before hydrogenation, or in portions during or before and during hydrogenation. If necessary, additional alkali may also be added after hydrogenation.

[0047] The concentration of salt obtained after step (O) or used in step (A) may be chosen within broad boundaries and is not limited to any particular requirement.

[0048] In one embodiment, the concentration of the leucoidindigode salt in the solution obtained in step (O) 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 (O) or used in step (A).

[0049] In another embodiment, the concentration of the leucoidindigode salt in the solution obtained in step (O) 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 (O) or used in step (A).

[0050] As discussed in the Background section, leucoidindigo salt solutions obtained by reduction of indigo contain aromatic amines in the form of aniline or aniline and N-methylaniline resulting from commonly used indigo production processes.

[0051] In one embodiment, the concentration of said aromatic amine in the solution obtained in step (O) or used in step (A) before purification in step (B) is in the range of 2,000 ppm to 10,000 ppm.

[0052] In one embodiment, the concentration of aniline is in the range of 1,000 ppm to 3,000 ppm, and the concentration of N-methylaniline is in the range of 500 to 2,000 ppm.

[0053] According to the invention, a sufficient weight of water is added in step (A) to enable a weight of water at least equal to the weight of the aqueous leucoidindigo solution used in step (A) to be distilled off in step (B), which simply means that after distillation in step (B) an aqueous solution is obtained, i.e. a composition which still contains water and forms a solution, at least at the distillation temperature.

[0054] The process according to step (B) is preferably carried out as a batch process or as a continuous process.

[0055] During the distillation according to step (B), the aromatic amine is removed together with water as an azeotrope or as a mixture comprising water and the amine(s).

[0056] In a further preferred embodiment, a sufficient weight of water is added in step (A) to allow at least 1.5 times the weight of water by weight of the aqueous leukoindigo solution used in step (A) to be distilled off in step (B).

[0057] In certain preferred embodiments, a sufficient weight of water is added in step (A) to allow at least twice the weight of water by weight of the aqueous leukoindigo solution used in step (A) to be distilled off in step (B).

[0058] In another embodiment, a sufficient weight of water is added in step (A) to allow at least three times the weight of water by weight of the aqueous leukoindigo solution used in step (A) to be distilled off in step (B).

[0059] In yet another embodiment, a sufficient weight of water is added in step (A) to allow more than four times the weight of water by weight of the aqueous leukoindigo solution used in step (A) to be distilled off in step (B).

[0060] In a further preferred embodiment, a weight of water is added in step (A) sufficient to allow a weight of water to be distilled off in step (B) that is at least equal to the weight of the aqueous leucoidindigo solution used in step (A) and not more than four times the weight of the aqueous leucoidindigo solution used in step (A).

[0061] In a further preferred embodiment, a sufficient weight of water is added in step (A) to allow a weight of water to be distilled off in step (B) that is at least 1.5 times the weight of the aqueous leucoidindigo solution used in step (A) and not more than 4 times the weight of the aqueous leucoidindigo solution used in step (A).

[0062] In certain preferred embodiments, a sufficient weight of water is added in step (A) to allow for distillation in step (B) of at least two times the weight of the aqueous leucoidindigo solution used in step (A) and no more than four times the weight of the aqueous leucoidindigo solution used in step (A).

[0063] In another embodiment, a sufficient weight of water is added in step (A) to allow a weight of water to be distilled off in step (B) that is at least three times the weight of the aqueous leucoidindigo solution used in step (A) and not more than four times the weight of the aqueous leucoidindigo solution used in step (A).

[0064] In yet another embodiment, a sufficient weight of water is added in step (A) to allow for distillation in step (B) of no more than four times the weight of the aqueous leukoindigo solution used in step (A).

[0065] Distillation can be carried out by methods known in the art using known and suitable distillation equipment.

[0066] In one embodiment, the distillation may be carried out under reduced pressure.

[0067] In one embodiment, the distillation may be carried out under pressure.

[0068] In a preferred embodiment, the distillation is carried out under atmospheric pressure.

[0069] To avoid premature oxidation of leucoidindigo to indigo, it is preferable to exclude the presence of oxygen during the distillation, and therefore the distillation according to step (B) must be carried out under an inert gas such as nitrogen.

[0070] In one embodiment, in step (B), the distillation is carried out under a stream of inert gas, ie the aqueous solution subjected to distillation is simultaneously stripped.

[0071] A preferred inert or stripping gas is nitrogen.

[0072] In a preferred embodiment, the distillation is carried out using a distillation column.

[0073] Distillation according to step (B) using a distillation column

[0074] As used herein, the term "distillation column" is used interchangeably with terms such as, for example, "distillation tower," "rectification column" or "rectification tower," "fractionation column" or "fractionation tower."

[0075] The term "column" as used herein in its broadest sense encompasses a vertical cylindrical column.

[0076] The term "column" further encompasses hollow structural elements, preferably hollow cylindrical elements, whose length exceeds the diameter. Neither the diameter nor the length is limited. In one embodiment, the length and diameter or the length-to-diameter ratio of the column may be freely selected or optimized taking into account the results to be achieved.

[0077] Preferred columns have a diameter of 0.1 to 4 metres, such as 0.30 metres to 3 metres, and a length ranging from about 1 metre to 50 metres, such as 1 to 30 metres.

[0078] 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.

[0079] In one embodiment, distillation using a distillation column is discussed below when the column is operated in either a batch or continuous mode.

[0080] According to the present invention, both methods require steps (A) and (B) according to the present invention, where water is added to an aqueous leucoidindigo solution to obtain a diluted leucoidindigo solution according to step (A) (which is subjected to distillation according to step (B) such that a weight of water at least equal to the weight of said aqueous leucoidindigo solution utilized in the dilution step is distilled off).

[0081] Typically, the purified leucoidindigo solution is obtained at or from the bottom of the distillation column, whereas water containing the amine(s) is obtained at or from the top of the column.

[0082] In one embodiment, steps (A) and (B) are carried out successively, i.e., a diluted aqueous leukoindigo solution is first prepared according to step (A), which is then distilled according to step (B).

[0083] In another embodiment, steps (A) and (B) are carried out simultaneously, i.e., a diluted aqueous leukoindigo solution is prepared according to step (A), and simultaneously it is distilled according to step (B).

[0084] In one embodiment, step (A) is carried out outside the distillation column.

[0085] Thus, in one embodiment, the diluted leukoindigo solution obtained according to step (A) is injected into a distillation column and distilled according to step (B).

[0086] In one embodiment, when the method is carried out in batch mode, the diluted leukoindigo solution obtained in step (A) can be poured into the bottom of a column, which is then subjected to distillation. The distillation can be terminated as soon as a weight of water at least equal to the weight of the aqueous leukoindigo solution used in step (A) is distilled off. Each produced leukoindigo solution, which is a purified leukoindigo solution, can then be discharged from the bottom of the column.

[0087] In another embodiment, when the method is carried out in a continuous manner, the diluted leukoindigo solution obtained by step (A) can be injected into a column, whereby water (including amine(s)) is distilled off at the same time, and whereby each produced leukoindigo solution, which is a purified leukoindigo solution, can be discharged from the bottom of the column.

[0088] In another embodiment, step (A) is carried out in a column.

[0089] Subsequently, the diluted leukoindigo solution obtained by step (A) can be distilled batchwise according to step (B).

[0090] In another embodiment, when the method is carried out in a continuous mode, the leukoindigo solution to be purified is fed to a column, wherein simultaneously water is also injected into the column to carry out step (A), wherein, still simultaneously, water (including the amine(s)) is distilled off, and wherein, still simultaneously, the leukoindigo solution produced, which is the purified leukoindigo solution, is discharged at or from the bottom of the column.

[0091] Inside the column, downwardly flowing reflux liquid causes cooling and condensation of the upwardly flowing vapor, thereby increasing the effectiveness of the column: the greater the reflux, the better the column separates low-boiling materials from high-boiling materials.

[0092] During distillation, the aromatic amine(s) are removed along with water at or near the top, i.e., overhead, of the column, where the amine and water or vapor are typically condensed and recovered.

[0093] In one embodiment, a portion of the water and amine(s) condensed and recovered at the top of the column is re-injected into the column at the top, where it is used to knock down the droplets and optionally the solids, which then exit the column, which can be preferably performed using a tray section or alternatively a structured packed section.

[0094] The condensate may be separated into an aqueous phase containing water and aniline.

[0095] In one embodiment, the purified leukoindigo solution obtained from the bottom at the bottom of the distillation column is re-injected into the column and redistilled.

[0096] In one embodiment, the column is operated in continuous steady state: the amount of charge added is typically equal to the amount of product removed, unless perturbed by changes in charge, heat, ambient temperature, or condensation.

[0097] In a preferred embodiment, the amount of heat introduced into the column with the injection is adjusted so that it is equal to the amount of heat removed by the overhead fraction and with the product. By careful adjustment, foaming, percolation, entrainment, or flooding can be prevented or at least reduced to an acceptable level.

[0098] In consideration of the above, in one embodiment, step (A) comprises the following steps (α) to (γ): (α) providing a liquid stream comprising said aqueous leukoindigode solution containing said amine(s); (β) Prepare the water flow; (γ) contacting the liquid stream with the water stream; Includes.

[0099] In another embodiment, step (B) comprises the following steps (δ) and (ε): (δ) discharging water containing aniline or aniline and N-methylaniline from the distillation column; and (ε) discharging the aniline-free or aniline-free and N-methylaniline-free aqueous leucoidindigo solution from the distillation column; Includes.

[0100] In one embodiment, the present invention relates to a method for removing an aromatic amine in the form of aniline or aniline and N-methylaniline from an aqueous leucoidindigo solution comprising said aromatic amine, to obtain a purified leucoidindigo solution, wherein said leucoidindigo is present in the form of an alkali metal salt, or

[0101] A method for producing an aqueous leukoindigo solution that is aniline-free or aniline-free and N-methylaniline-free, when the concentrations of aniline or aniline and N-methylaniline are determined according to ISO 14362-1:2017(E), from an aqueous leukoindigo solution containing aniline or aniline and N-methylaniline, wherein the leukoindigo is present in the form of an alkali metal salt;

[0102] The method includes steps (A) and (B), respectively, wherein step (A) includes the following steps (α), (β), (γ1) and (γ2): (α) providing a liquid stream comprising said aqueous leukoindigode solution containing said amine(s); (β) Prepare the water flow; a distillation column configured to contact said liquid stream with said water stream; (γ1) injecting the liquid stream; and (γ2) injecting the water flow; and step (B) comprises the following steps (δ) and (ε): (δ) discharging water containing aniline or aniline and N-methylaniline from the distillation column; and (ε) discharging the aniline-free or aniline-free and N-methylaniline-free aqueous leucoidindigo solution from the distillation column; Includes.

[0103] According to the present invention, step (α) requires the preparation of a liquid stream comprising the aqueous leucoidindigo solution containing the amine(s). The preparation is preferably carried out by pumping the leucoidindigo salt solution directly from a respective storage container or reduction device in which the leucoidindigo salt solution is produced by reduction of indigo. The pumped solution is in a flowing state, e.g., under the influence of pressure, gravity, or a pump, thereby forming a stream. Preferably, the solution stream is conducted by a pipe to the distillation column used in step (γ1).

[0104] Further according to the invention, step (β) requires the preparation of a water stream. Said preparation is preferably carried out by pumping the water stream from a respective storage container or reservoir. The pumped water is in a flowing state, e.g., under the influence of pressure, gravity or a pump, thereby forming a stream. Preferably, the water stream is conducted by a pipe to the distillation column used in step (γ2).

[0105] The term "water stream" provided in step (β) refers to any water-containing water stream suitable for removing or at least reducing the content of aniline or aniline and N-methylaniline in the leucoidin solution containing said amine(s), wherein said leucoidin solution is present in the form of a salt.

[0106] In one embodiment, the amount of heat input to the distillation column, generated from the water stream injected according to step (γ2), and the heat input with the liquid stream according to step (γ1), is controlled so that it is equal to the amount of heat removed in steps (δ) and (ε), since adding too much or too little heat to the distillation column can lead to foaming or flooding. Thus, the process is carried out to achieve adiabatic conditions.

[0107] In another embodiment, the amount of heat input to the distillation column produced from the heated portion of the liquid stream discharged from the distillation column according to step (ε) and the heat input with the liquid stream according to step (γ1) is controlled so that it is equal to the amount of heat removed in steps (δ) and (ε), since adding too much or too little heat to the distillation column can lead to foaming or flooding. Thus, the process is carried out to achieve adiabatic conditions.

[0108] Further according to the present invention, steps (γ1) and (γ2) require that said liquid stream and said water stream are injected into a distillation column configured to contact said liquid stream and said water stream.

[0109] In one embodiment, the distillation column comprises one or more liquid inlets for injecting a liquid stream and one or more water inlets for injecting a water stream.

[0110] The inlet may be provided at any location in the distillation column, ie, the inlet may be provided at the bottom or top or side wall of the distillation column.

[0111] In one embodiment, the liquid inlet(s) are provided at the bottom of the distillation column and the water inlet(s) are provided at the top, or vice versa.

[0112] In another embodiment, the liquid inlet(s) are provided in the sidewall of the distillation column and the water inlet(s) are provided at the bottom or top of the distillation column.

[0113] In another embodiment, the water inlet(s) are provided in the sidewall of the distillation column and the liquid inlet(s) are provided at the bottom or top of the distillation column.

[0114] In a preferred embodiment, the (γ1) liquid stream and the (γ2) water stream are injected into the column through inlets in the side wall of the column, the (δ) water containing aniline or aniline and N-methylaniline is discharged at the top of the column, and the (ε) aqueous leukoindigo solution free of aniline or free of aniline and N-methylaniline is discharged at the bottom of the column.

[0115] Preferably, the water inlet(s) and / or liquid inlet(s) are designed in the form of one or more spargers. The use of perforated sheets as purification inlets and / or liquid inlets is also possible.

[0116] In one embodiment, the contacting is carried out such that the distillation column is partially or completely filled with liquid from the liquid stream and the water inlet(s) are positioned such that the injected water stream must flow through the liquid or into the liquid.

[0117] In another embodiment, the liquid flow inlet(s) and the water flow inlet are positioned so that their flows intersect each other.

[0118] In another embodiment, the contacting of the streams may be carried out in a countercurrent manner.

[0119] In another embodiment, the contacting of the streams can be carried out in a co-current manner, for example, when the liquid(s) and water inlet(s) are the same, i.e., when the streams are injected through shared inlet(s).

[0120] In another embodiment, the distillation column includes means for directing the liquid and water streams into contact with each other. Such means are known in the art. Plates and packing materials, such as Raschig rings, are mentioned by way of example.

[0121] As used herein, the term "plate" is used synonymously with the term "tray."

[0122] As used herein, the term "filling material" is used synonymously with the term "packing."

[0123] According to the present invention, step (δ) requires that water containing aniline or aniline and N-methylaniline is discharged from the distillation column.

[0124] As a result of the process according to the invention, the concentration of aniline or aniline and N-methylaniline in the liquid stream 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 injected into the distillation column according to step (γ1) via the liquid inlet(s).

[0125] In a preferred embodiment, steps (α) to (ε) are carried out simultaneously, i.e., the process is a continuous process.

[0126] In another embodiment, the process may be carried out discontinuously, i.e., as a batch process. In one embodiment, the device is filled with liquid provided by the liquid flow according to step (γ1). Subsequently, steps (γ2) and (δ) are carried out. Following contacting according to step (δ), the purified leukoindigo solution is pumped from the device according to step (ε). The method may be repeated.

[0127] The column used in the method of the present invention contains theoretical plates. The term "theoretical plate," as used herein, includes a hypothetical region or stage within which two phases, such as a liquid phase derived from a liquid stream and a purified phase, such as a gas phase, derived from a purified stream as defined herein, establish equilibrium with each other within the column. Such equilibrium stages may also be referred to as equilibrium stages or ideal stages.

[0128] The number of theoretical plates required in the process according to the invention may depend on the specific type of column used. It may be determined according to methods known in the art, taking into account the desired degree of separation of aniline or aniline and N-methylaniline in the output fraction(s), i.e., obtained in steps (δ) and (ε). It may also depend on the amount of reflux used.

[0129] The final design choice of the number of stages to be applied in industrial implementation can then be selected based on the economic balance between the cost of additional stages and the cost of using a higher reflux rate.

[0130] In one embodiment, the column may have 5 to 50 theoretical plates.

[0131] In another embodiment, the column may have 10 to 40 theoretical plates.

[0132] In one embodiment, the column may be operated under reduced pressure.

[0133] In another embodiment, the column may be operated at a pressure substantially equal to atmospheric pressure.

[0134] In yet another embodiment, the column may be operated at a pressure greater than atmospheric pressure.

[0135] In one embodiment, the column is heated under reduced pressure, e.g., at 0.1 * 10 5 Pa (absolute pressure) (0.10 bara) to 0.95 * 10 5 The column temperature may be set in the range of 40°C to 95°C.

[0136] In another embodiment, the column is operated at a pressure substantially equal to atmospheric pressure, i.e., at various pressures of 0.95 * 10 5Pa (absolute pressure) (0.95 bara) to 1.2 * 10 5 The column temperature is typically set in the range of 95°C to 110°C.

[0137] In a further embodiment, the column is optionally heated to a pressure above atmospheric pressure, * 10 5 Pa (absolute pressure) (1.2 bara) ~ 11 * 10 5 Under a pressure of 11 bara (absolute pressure), e.g., 7 * 10 5 The column temperature can be set in the range of 110°C to 190°C, for example, in the range of 160°C to 170°C.

[0138] The temperatures quoted are measured at the bottom of the column and are dependent on the amount of vapor leaving the top of the column.

[0139] For heating, in one embodiment, a space heater is used. The term "space heater," as used herein, encompasses a device used to heat a single, narrow region, such as the region at the bottom of a column. Thus, the space heater is preferably located at or near the bottom of the column.

[0140] In another embodiment, the upper region of the column is heated.

[0141] In another embodiment, the bottom and top regions of the column are heated.

[0142] The device may be provided with anti-foam means, such as baffles, preferably installed at the bottom of the column used, and if necessary throughout.

[0143] In another embodiment, antifoaming can be assisted by adding an antifoaming agent to the leukoindigo solution being purified. Alternatively, a different antifoaming agent, such as silicone oil, can be used, but it is preferred to use a homogenous antifoaming agent that is dissolved in the leukoindigo solution and that does not negatively affect the subsequent oxidation of the purified leukoindigo to indigo.

[0144] Specifically, according to the present invention, the devices used in steps (γ1) and (γ2) are selected from the group consisting of a plate column, a packed column, or any two or more of said columns.

[0145] As used herein, the term "plate column" is used synonymously with the terms "plate column" or "tray column."

[0146] Plate columns and packed columns are known in the art.

[0147] In one embodiment, the column includes one or more sequences designed as a plate column, or one or more sequences designed as a packed column, to further optimize the process.

[0148] In another embodiment, the column contains one or more sequences designed as a plate column and one or more sequences designed as a packed column to further optimize the method.

[0149] In another embodiment, multiple plate columns or packed columns, respectively, are connected in series to further optimize the process.

[0150] In another embodiment, the plate column is connected to a packed column to further optimize the method.

[0151] In another embodiment, the plate column is connected to a packed column and a bubble column to further optimize the method.

[0152] In yet another embodiment, pre-purification is carried out as disclosed in the prior art, where final purification is carried out by the method according to the present invention.

[0153] The term "pre-purification" as used herein includes distilling off water from each leucoidindigode salt solution, extraction with a suitable organic solvent, steam distillation or stripping with nitrogen, or two or more thereof.

[0154] Of course, in the process according to the invention, the presence of oxygen must be excluded to prevent premature oxidation of leucoidindigo to indigo.

[0155] The columns used are discussed in more detail below.

[0156] Plate Column

[0157] In one embodiment, the distillation column is a plate column.

[0158] In a preferred embodiment, the plate column is arranged in the form of a tower, ie, it is arranged in a vertical manner.

[0159] In one embodiment, the height of the column is greater than 5 meters, preferably between 5 and 50 meters, and more preferably between 10 and 40 meters.

[0160] For plate columns, theoretical plates are also called theoretical trays or theoretical plates.

[0161] In one embodiment, bubble cap "trays" or "plates," as known in the art, are provided inside the column to provide good contact between the upwardly flowing vapor and downwardly flowing liquid inside the column.

[0162] The trays or plates are preferably fabricated from circular steel sheets and are usually introduced inside the column at intervals of about 60-75 cm up to the end of the column.

[0163] In one embodiment, the tray is a bubble cap tray or a valve cap tray.

[0164] The term "bubble cap tray" as used herein includes a slotted cap on a central riser where gas flows upward through the riser and counterflows downward within the cap through an annulus between the riser and the cap, and finally through a series of openings or slots on the underside of the cap into the liquid.

[0165] The term "valve cap tray," as used herein, encompasses a perforated tray covered by a lifting cap. The vapor flow lifts the cap, thereby creating a flow area for the vapor to pass through. The lifting cap induces the vapor to flow horizontally into the liquid.

[0166] In one embodiment, the tray is a perforated tray, i.e., a sieve tray. The term "sieve tray" encompasses trays in which vapor flowing upward through the perforations contacts liquid flowing downward through the perforations, thereby providing the desired contact between vapor and liquid.

[0167] In one embodiment, contact is achieved by introducing bubble or valve caps into each perforation to promote the formation of vapor bubbles that flow through a thin layer of liquid maintained by a weir on each tray.

[0168] Thus, in one embodiment, the tray may be selected from a perforated tray, a bubble cap tray or a valve cap tray, or two or three thereof.

[0169] Perforated trays, bubble cap trays or valve cap trays are known in the art.

[0170] In one embodiment, the injection into the plate column comprises or is the liquid stream prepared according to step (α) and the water stream prepared according to step (β) injected into the column according to steps (γ1) and (γ2). The purified leukoindigo solution is recovered at the bottom of the column, while water containing the amine(s) is recovered at the top. The liquid and vapor generated at the top and bottom may be recycled.

[0171] The vapor recovered at the top according to step (δ) contains aniline or aniline and N-methylaniline removed from the leucoidindigo solution.

[0172] The liquid recovered at the bottom according to step (ε) contains the purified leukoindigofen solution.

[0173] Packed Column

[0174] In one embodiment, the plates of the plate column disclosed above are replaced by packed compartments, i.e. by compartments containing packing material, and the device used for purification is then a packed column.

[0175] Thus, packing materials can be used in the column instead of trays, particularly when a low pressure drop across the column is required, such as when operating under reduced pressure. The packing materials can be either random-load packing, such as Raschig rings or ordered sheet metal, as known in the art.

[0176] In one embodiment, the packing may have a conventional geometry, such as stacked rings, a lattice, a unique ring structure, or a saddle.

[0177] The ring may be, for example, a Raschig ring or a Pall ring.

[0178] The saddle may be, for example, an Intalox® saddle.

[0179] In one embodiment, the packing may have a distorted shape.

[0180] The packing may be randomly arranged within the column, where rings, saddles, and unique shades are cast and randomly arranged within the column.

[0181] In another embodiment, the packing may be spaced at regular intervals within the column.

[0182] The ring, saddle or uniquely constructed shade may be made from a variety of materials, such as ceramic, metal, plastic, and carbon.

[0183] In one embodiment, shaped packing such as wire mesh or perforated sheets can also be used.

[0184] The required height of the packing can be determined according to methods known in the art.

[0185] In one embodiment, the injection into the packed column comprises or is the liquid stream prepared according to step (α) and the purified stream, such as a vapor stream, prepared according to step (β), which are injected into the column according to steps (γ1) and (γ2). The liquid is recovered at the bottom of the packed column, whereas the vapor is recovered at the top. The liquid and vapor produced at the top and bottom may be recycled.

[0186] The vapor recovered at the top according to step (δ) contains aniline or aniline and N-methylaniline removed from the leucoidindigo solution.

[0187] The liquid recovered at the bottom according to step (ε) contains the purified leukoindigofen solution.

[0188] In one embodiment, the concentration of the indigo containing aromatic amine in the aqueous composition used in step (A), prior to the distillation carried out in step (B), is selected so that the leukoindigo salt is obtained in step (A) at a concentration of less than 25% by weight.

[0189] Thus, in one embodiment, the concentration of the leucoidindigosalt solution used in step (B) is also less than 25% by weight.

[0190] During distillation, water and the aromatic amine(s) are distilled off, thereby purifying the leucoidindigosalt solution and simultaneously concentrating the solution, so that concentrations of less than 25% by weight may be advantageous, whereby a stable and purified concentrated leucoidindigosalt solution may be obtained.

[0191] Due to distillation of water and amine, the solution obtained in step (B) may be concentrated with respect to the solution used in step (B), respectively obtained after step (A). In one embodiment, the concentration of leucoidindigode salt in the solution obtained in step (O) or used in step (A) is in the range of 5 to 65% by weight, e.g., 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 (O) or used in step (A).

[0192] In one embodiment, the concentration of leukoindigofol in the purified solution obtained after step (B) is in the range of 45-65% by weight based on the total weight of the solution.

[0193] In one embodiment, the concentration of leucoidindigosalt in the solution obtained in step (O) or used in step (A) is in the range of 10 to 40% by weight, and the concentration of leucoidindigosalt in the purified solution after step (B) is in the range of 45 to 65% by weight, based on the total weight of the solution.

[0194] Thus, in one embodiment, the weight of water added in step (A) and the water distilled off in step (B) are selected so that after step (B) the concentration of leukoindigofol in the purified solution is in the range of 45 to 65% by weight based on the total weight of the solution.

[0195] The resulting concentrated solution thereby contains less than 40 ppm or less than 30 ppm, preferably less than 20 ppm, 10 ppm or less than 5 ppm aromatic amines.

[0196] If necessary, the solution obtained in step (B) can be diluted within a predetermined concentration range adapted to the application or stability requirements.

[0197] In one embodiment, the concentration is adjusted to a range of 10 to 45% by weight, such as 15 to 45% by weight, if necessary, by adding water. Other suitable concentration ranges are, for example, 20 to 45% by weight or 25 to 45% by weight.

[0198] Naturally, upon dilution, the aromatic amine content is further reduced.

[0199] In a preferred embodiment, the aromatic amine is no longer detectable and such a solution may be referred to as aniline-free.

[0200] In one embodiment, the concentration of the leucoidindigosalt is adjusted to within a concentration range of 10 to 45 wt %, e.g., 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 leucoidindigosalt solution, and wherein the stability of the solution is measured at a temperature of 23°C.

[0201] The term "solution stability" refers to a solution that does not tend to crystallize or precipitate at a particular temperature.

[0202] In another embodiment, the concentration of the leucoidindigosalt is in the concentration range of 45-65% by weight based on the total weight of the solution, and wherein 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 leucoidindigosalt solution comprising an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein said leucoidindigosalt is present in the form of an alkali metal salt; wherein the concentration of aromatic amines is less than 40 ppm as measured in accordance with ISO 14362-1:2017(E); and Here, the concentration of the leucoidindigode salt is within a concentration range of 10 to 45% by weight, for example, 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 wherein the stability of the solution is measured at a temperature of 23°C.

[0203] In another embodiment, the present invention relates to a stable aqueous leucoidindigo solution comprising an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein said leucoidindigo is present in the form of an alkali metal salt; wherein the concentration of aromatic amines is less than 100 ppm as measured in accordance with ISO 14362-1:2017(E); and Here, the concentration of the leucoidindigode salt is within a concentration range of 45 to 65% by weight, for example, 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.

[0204] In a preferred embodiment, the concentration of aromatic amine is less than 40 ppm.

[0205] In a further preferred embodiment, the concentration of aromatic amine is less than 30 ppm.

[0206] In another preferred embodiment, the concentration of aromatic amine is less than 20 ppm.

[0207] In another preferred embodiment, the concentration of aromatic amine is less than 10 ppm.

[0208] In another preferred embodiment, the concentration of aromatic amine is less than 5 ppm.

[0209] In another preferred embodiment, the concentration of aromatic amines is no longer detectable.

[0210] In one embodiment, the aqueous leukoindigo solution is obtainable by a method as defined in the first or second aspect.

[0211] The leucoidindigo salt obtained by the method according to the first or second aspect, or the aqueous leucoidindigo solution defined in the third aspect, may be converted to indigo or used in a vat dyeing process for dyeing textiles.

[0212] Therefore, in a fourth aspect, the present invention provides a method for preparing a compound comprising the steps of: (D) oxidizing the aqueous leukoindigode solution defined in the third aspect; The present invention relates to a method for producing indigo, comprising:

[0213] In one embodiment, the method further comprises, before step (D), the following step (C): (C) treating the textile with a leucoindigo solution as defined in the third aspect; Includes.

[0214] In another embodiment, the present invention provides a method for manufacturing a pharmaceutical composition comprising the steps of: (I) performing the method defined in the first aspect or the second aspect, or any embodiment defined herein; (III) oxidizing the leucoidindigo solution obtained in step (I); The present invention relates to a method for producing indigo, comprising:

[0215] In one embodiment, the method further comprises the following step (II) before step (III): (II) treating the textile with the leucoindigo solution obtained in step (I); Includes. [Example]

[0216] Example 1 (Comparative) 1,000 g of a 30 wt. % leukoindigo solution (containing 6.5 wt. % alkali metal, 2,495 ppm aniline, and 1,480 ppm N-methylaniline) was subjected to distillation at ambient pressure. After 470 ml of water had been distilled off, a 57 wt. % solution was obtained. The aniline content in the resulting concentrated solution, measured according to ISO 14362-1:2017(E), was 173 ppm, and the N-methylamine concentration was 9 ppm. Example 2

[0217] 1,000 g of water was added to 2,000 g of a 30 wt. % leukoindigo solution (containing 6-7 wt. % alkali metal, 2,380 ppm aniline, and 1,375 ppm N-methylaniline). The composition was then subjected to distillation at ambient pressure. After distilling off 2,000 ml of water, a 60 wt. % solution was obtained. The aniline content in the resulting concentrated solution was 38 ppm, and the N-methylamine concentration was 2.0 ppm, as measured according to ISO 14362-1:2017(E). Example 3

[0218] 2,000 g of water was added to 2,000 g of a 30 wt. % leukoindigo solution (containing 6.5 wt. % alkali metal, 2,350 ppm aniline, and 1,335 ppm N-methylaniline). The composition was then subjected to distillation at ambient pressure. After distilling off 3,000 ml of water, a 60 wt. % solution was obtained. The aniline content in the resulting concentrated solution, measured according to ISO 14362-1:2017(E), was 16 ppm, and the N-methylamine concentration was 0.3 ppm. Example 4

[0219] 3,000 g of water was added to 2,000 g of a 30 wt. % leukoindigo solution (containing 6.5 wt. % alkali metal, 2,164 ppm aniline, and 1,170 ppm N-methylaniline). The composition was subsequently subjected to distillation at ambient pressure. After distilling off 4,000 ml of water, a 60 wt. % solution was obtained. The content of aniline in the resulting concentrated solution, measured according to ISO 14362-1:2017(E), was 5 ppm. N-methylamine was not detectable. Example 5

[0220] 1,000 g of the leukoindigo solution from Example 4 was diluted with 500 g of water to obtain a 40 wt. % solution. The solution had an aniline content of less than 5 ppm, as measured according to ISO 14362-1:2017(E). The solution was used in vat dyeing and produced deep shades.

Claims

1. A stable aqueous leucoidindigo solution comprising an aromatic amine in the form of aniline or aniline and N-methylaniline, wherein said leucoidindigo is present in the form of an alkali metal salt; wherein the concentration of aromatic amines is less than 40 ppm as measured in accordance with ISO 14362-1:2017(E); wherein the concentration of the leukoindigo salt is in a concentration range of 20-45% by weight based on the total weight of the solution, and wherein the stability of the solution is measured at a temperature of 23° C.; or An aqueous leukoindigo solution, wherein the concentration of a leukoindigo salt is in the concentration range of 45 to 65% by weight based on the total weight of the solution, and wherein the stability of the solution is measured at a temperature of 60°C, wherein the stability of the solution refers to a solution that has no tendency to crystallize or precipitate at the specified temperature.

2. 2. The aqueous leukoindigo solution of claim 1, wherein the concentration of aromatic amines is less than 30 ppm or less than 20 ppm.

3. 3. The aqueous leukoindigo solution of claim 1 or 2, wherein the concentration of aromatic amines is less than 10 ppm or 5 ppm.