NO319309B1 - Process for preparing an aqueous solution of free hydroxylamine - Google Patents

Abstract

A process is disclosed for producing an aqueous solution of free hydroxylamine. A solution obtained by treating an hydroxylammonium salt with a base is treated with water or water vapour at a temperature >/= 80 DEG C, so as to separate the solution into an aqueous hydroxylamine fraction and a salt fraction. This process is soft and easy to implement on a large scale. The risk of decomposition is minimised because of the low thermal stresses, the low hydroxylamine concentration and the short dwelling time during implementation of the process.

Description

The present invention relates to a method for producing aqueous solutions of free hydroxylamine.

Hydroxylamine is an important intermediate for the chemical industry. However, special care is required when handling it because it irritates the eyes, skin and mucous membranes and can cause allergies. In particular, however, it is thermally unstable, i.e. it decomposes slowly to explosively, especially in the presence of metal ions, in the basic medium and in relatively high concentration, and at relatively high temperatures.

Hydroxylamine is produced on a large industrial scale as a hydroxylammonium salt, usually as hydroxylammonium sulfate, and is also used as such. However, it is often necessary to use a highly concentrated, salt-free, aqueous solution of free hydroxylamine. In order to avoid the above-mentioned problems and especially the instability of the hydroxylamine, the person skilled in the art has avoided using traditional methods of chemistry on a large scale to concentrate distillable substances, e.g. distillation, by the recovery of salt-free hydroxylamine solutions. The distillation of hydroxylamine, even on a laboratory scale, is also said to be a particularly dangerous operation, cf. Roth-Weller: Gefåhrliche Chemische Reaktionen, Stoffinformatio-nen Hydroxylamine, page 3, 1984, 2, Ecomed-Verlag. The distillation of hydroxylamine on an industrial scale has therefore never been taken into account in technical publications. Instead, special methods have been used, although they all have serious disadvantages.

Attempts were thus made to isolate free hydroxylamine from aqueous salt solutions by means of ion exchangers, see e.g. US-A-4,147,623, EP-A-1787, EP-A-237052 and Z. Anorg. Ch. 288, 28-35 (1956). However, such a procedure only leads to diluted solutions with low space-time yields. Hydroxylamine also reacts with many ion exchangers or is split by them.

Another method comprises the electrodialysis of an aqueous hydroxyl-ammonium salt solution in electrolysis cells with semi-permeable membranes, such as e.g. described in DE-A-3347259, JP-A-123771 and JP-A-123772. However, such a method is technically complicated and expensive and has not been established in industry to date.

DE-A-3528463 describes the production of free hydroxylamine from hydroxylammonium sulfate by treatment with calcium oxide, strontium oxide or barium oxide to remove the insoluble alkaline earth metal sulfates. In this method, the removal of the sulfates obtained in finely divided form poses significant difficulties. In addition, only dilute solutions are obtained, and, when calcium oxide or calcium hydroxide is used, free hydroxylamine still contains undesirably large amounts of ions due to the calcium sulfate's relatively good solubility. When strontium compounds and barium compounds are used, the relatively high price and especially the toxicity are also disadvantages when it comes to an industrial production process.

DE-A-1247282 describes a process in which alcoholic solutions of free hydroxylamine are obtained by reacting hydroxylammonium sulfate with ammonia in alcohol as solvent and removing the ammonium sulfate. A similar method is described in EP-A-108294. However, alcoholic solutions are unsuitable and undesirable for a number of applications. Special care must e.g. be expelled when handling such solutions, due to their flammability. Furthermore, the alcohol used must usually be recovered using an expensive method, since emptying relatively large quantities of alcohol into waste water treatment facilities or into outlets is prohibited.

Finally, DE-A-3601803 describes a method for obtaining aqueous solutions of free hydroxylamine, in which hydroxylammonium sulfate is reacted with ammonia in lower alcohols, the precipitated ammonium sulfate is separated, water is added to the alcoholic solution of free hydroxylamine and the alcohol is distilled off from the solution thus obtained . The above-mentioned disadvantages of working with alcohol also apply to this method. Due to the instability of the hydroxylamine in connection with the flammability of the alcohols, special care is also required in the final distillation step. Common to all previously known methods is that they are not suitable to be carried out on an industrial scale or cause uneconomically high additional security costs.

A temperature above 65°C is considered critical for the decomposition of hydroxylamine. By differential thermal analysis, the onset temperature was determined to be 70°C for a 50% by weight aqueous hydroxylamine solution (in a glass crucible). The amount of heat released, i.e. about 2.2 kj/g of a 50 wt% solution, confirms the high thermal potential of the material. Differential thermal analysis is a microthermoanalytical method used in screening to calculate the thermal stability and the thermal potential. The onset temperature is the lowest ambient temperature at which an appreciable exothermic reaction takes place in the sample at a dewatering rate of 1 K/min, initiated at 30°C. The processing temperature should be significantly lower than the initial temperature for safety reasons.

In connection with the production of hydroxylamine nitrate, US-A-

4 956 168 preparation of a slurry of hydroxylamine sulphate in alcohol at temperatures not exceeding 65 °C. This slurry is then treated with ammonia at < 65 °C to produce an alcoholic hydroxylamine solution.

US-A-5 472 679 describes a method for preparing an alcohol-free, aqueous hydroxylamine solution by reaction with a hydroxylamine sulphate solution with a suitable base at up to 60°C. The resulting mixture is distilled under reduced pressure below 65 °C. This gives a solids residue (the salt formed by the release of hydroxylamine) and as distillate an aqueous hydroxylamine solution containing 16-23% by weight of hydroxylamine. The disadvantage of this method is that it requires work under reduced pressure and the temperature must be carefully controlled.

In addition, the method requires working with dry matter. In a continuous process, dry matter therefore had to be removed continuously. This can lead to major problems in terms of process technology if dry matter tends to clump together, for example as with Na2S04xH20.

Also, the "distillation" continues to dryness, more correctly described as evaporation so that the water with a low boiling point evaporates first. Hydroxylamine with a high boiling point accumulates. It is known that the decomposition tendency of hydroxylamine increases with the concentration of hydroxylamine, and with it the loss of hydroxylamine during the process. There is an increasing risk that explosive decomposition will take place due to the high concentration of hydroxylamine. It is known that pure hydroxylamine or hydroxylamine > 70% by weight decomposes explosively. Therefore, suitable safety requirements must be followed for the aforementioned procedure.

The remaining dry matter ultimately still contains residues of hydroxylamine (hydroxylamine adsorbed on the surface, hydroxylamine which fills spaces in the dry matter). The dry matter must therefore be cleaned in a separate disposal process.

It has now surprisingly been found that hydroxylamine solution obtained after partial or complete liberation of hydroxylamine from a hydroxylammonium salt in an aqueous phase can be separated into an aqueous hydroxylamine fraction and a salt fraction by treatment with water or steam above 65°C, without noticeable decomposition of hydroxylamine occurring place.

The present method therefore applies to a method for producing an aqueous solution of free hydroxylamine, where

a) a hydroxylammonium salt is treated with a suitable base in water,

b) any insoluble components are separated from the solution obtained, c) the solution obtained in step a) or step b) is separated by distillation into an aqueous hydroxylamine fraction and an aqueous salt fraction using a stripping column by passing water or steam in countercurrent into the bottom of the column at a temperature of 280°C, and d) if desired, the obtained aqueous hydroxylamine solution is concentrated by distillation.

Step a) of the new method is carried out in a conventional manner. Hydroxylammonium salts that are generally used are the hydroxylammonium salts of mineral acids, e.g. of sulfuric acid, phosphoric acid or hydrochloric acid, usually in aqueous solution. The hydroxylammonium salt is reacted with a suitable inorganic base, e.g. ammonia, sodium hydroxide, potassium hydroxide or calcium hydroxide in aqueous solution. The quantity of the base is chosen so that the hydroxylammonium salt is completely or at least partially converted into free hydroxylamine. This can be carried out continuously or in batches and at from approx. 0 to 100°C. The resulting aqueous solution contains free hydroxylamine and the salt originating from the base cation and the anion present in the hydroxylammonium salt.

Depending on the type and concentration of the hydroxylammonium salt, the base used to liberate the hydroxylamine and the temperature at which the reaction is carried out, some of the salt formed may precipitate. If necessary, the solution can also be cooled to precipitate a relatively large amount of the salt.

If such insoluble components, i.e. salt precipitates, are present, they are advantageously separated in a conventional manner before step c). Depending on the process conditions, e.g. with the use of ammonia as a base or the use of sodium hydroxide as a base and a relatively low concentration of the reactants, no precipitate is formed, and step b) can therefore be dispensed with.

The separation (step (c)) of the solution obtained from step (a) or step (b) in an aqueous hydroxylamine solution and a salt fraction is preferably carried out by treatment with water or steam in a stripping column. This is a conventional plate column, for example bubble tray column or perforated plate column, or equipped with conventional packing, e.g. Raschig rings, Pall rings, saddle elements, etc. and preferably have from 5 to 70 theoretical plates. The stabilized solution, to which additional stabilizer can be added if desired, is fed directly to the top of the column (upper part of the packing).

In the stripping column, the solution is separated in such a way that the salt fraction is removed at the bottom of the column and an aqueous hydroxylamine solution is taken out at the same height as the feed plate or this, especially via the top. To achieve this, it is preferred to treat the solution by introducing water and/or steam in a countercurrent flow into the bottom of the column. At a hydroxylamine concentration of from 5 to 45% by weight in the feed solution, the flow rate of water or steam is generally from 1 to 8, especially from 1 to 5, times the feed rate. The temperature of the introduced steam is generally from 80 to 180°C. In addition, the bottom of the column is heated if this is necessary.

The pressure in the stripping column is generally from 5 to 300 kPa (from 0.05 to 3 bar), preferably from 50 to 300 kPa (from 0.5 to 3 bar). It is particularly preferred to operate the stripping column at from 50 to 150 kPa (from 0.5 to 1.5 bar).

The temperatures prevailing at the top of the stripping column depend on the pressure at which the column is operated. They are generally from 80 to 130°C, preferably 90 to 120°C. The temperature of steam introduced can be significantly higher, for example 150 °C. However, it is not advantageous for this to be so high that too much water evaporates from the salt solution and that salt begins to leach out at the bottom of the column.

The aqueous (pond or liquid) hydroxylamine fraction which is withdrawn via the top of the stripping column usually contains 10-200 g of hydroxylamine/litre and can, if desired (step d), be concentrated in one or more steps which differ from each other at the operating pressure. A conventionally packed column containing the above-mentioned packings or a suitable plate column or other apparatus suitable for distillation is advantageously used. A column with from 4 to 20 theoretical plates is preferred.

In general, the distillation column is operated at from 1 to 200 kPa (from 0.01 to

2 bar), preferably from 5 to 120 kPa (from 0.05 to 1.2 bar), particularly preferably from 30 to 110 kPa (from 0.1 to 1.1 bar). The higher the intended final concentration of hydroxylamine, the more careful (low pressure and low temperature) the distillation must be. The distillation can be carried out continuously or in batches.

The water that is taken out via the top of the distillation column can be returned as stripping steam to the stripping column or sent as waste water to waste water treatment.

If desired, a drop precipitator (drop catcher) is additionally installed above the feed plate or in the steam outlet (take-off) in such a way that entrainment of salt with the drops is prevented.

In a particularly preferred embodiment, stripping of the hydroxylamine from the salt solution and partial concentration of the hydroxylamine solution is carried out in only one column, i.e. a stripping/distillation column. Water is distilled off via the top and the concentrated hydroxylamine solution is removed approx. 1 to 3 plates over the feed of the hydroxylamine-containing salt solution from step (a) or step (b). The salt solution is fed into approximately the middle of the column (approx. 5-30 theoretical plates above the bottom). The hydroxylamine-free salt fraction is removed at the bottom of the column. The number of theoretical plates in the stripping/distillation column is generally from 10 to 50 and the reflux ratio is adjusted so that it is from 0.5 to 3. Otherwise, the stripping/distillation column is operated as described above.

In a further preferred embodiment, stripping of the hydroxylamine from the salt solution and partial concentration of the hydroxylamine solution is carried out in the above-mentioned stripping/distillation column with an inserted dividing wall. As described above, water is distilled off via the top and the hydroxylamine-containing salt fraction is taken out at the bottom. The hydroxylamine-containing salt solution is fed in as described approximately in the middle of the column (approx. 10-30 theoretical plates above the bottom). At the height of this feeding, a dividing wall is mounted in the column above a height of from 1 to 10, preferably from 1 to 5, theoretical plates, so that the column is divided vertically into two separate sections, the feeding taking place in the middle of the dividing wall. The solution that is enriched in hydroxylamine can thus be removed in salt-free form in the area by the dividing wall, on the side opposite the feed point. The dividing wall separates the removal point from the feed point. However, identical concentrations of hydroxylamine are present on both sides of the dividing wall, but salt is present in the solution only on the feed point side. The salt-free solution enriched in hydroxylamine can be removed within the height of the dividing wall, at the height of maximum concentration of hydroxylamine, preferably at the height of the feed or, if desired, slightly below. Otherwise, the stripping/distillation column is operated with a dividing wall as described above.

As an alternative to the embodiment with a dividing wall, it is also possible to attach a side column to the stripping/distillation column described above, in such a way that this side column is connected on the gas and liquid side, above and below one or more plates from the feed point, to the stripping/distillation column, and the hydroxylamine-richer solution is removed via this side column and the latter is constructed so that the passage of saline solution into the removal point on the side column is avoided.

In order for there to be a very low risk of cleavage of the hydroxylamine, all solutions containing free hydroxylamine are stabilized by adding a cleavage stabilizer. Suitable stabilizers are known, e.g. hydroxyquinaldines, such as 8-hydroxyquinaldine, flavones, such as morin, hydroxyquinolines, such as 8-hydroxyquinoline, hydroxyantriquinones, such as quinalizarin, which are optionally used in combination with polyhydroxyphenols, such as pyrogallol. Further suitable stabilizers are benzonitrile, benzamidoxime, N-phenylthiourea, N-hydroxythiourea, reductones and/or reductonates, e.g. 2,3-didehydrohexano-1,4-lactone, and alkali metal salts of ethylenediaminetetraacetic acid. The concentration of the stabilizers is advantageously active from 5 x 10"4 to 1, especially from 5 x 10"3 to 5 x 10"<2>, weight% based on free hydroxylamine. Stabilizers that have proven particularly useful are 8 -hydroxyquinaldine, 8-hydroxyquinoline and also polyethyleneimine or polypropyleneimine and compounds with the formula R<1>R<2>N-A-NR<3>, R<4>, where A is cycloalkylene, or alkylene and R<1> to R4 is, independently of one another, CH2COOH or a benzyl radical unsubstituted or substituted on the phenyl ring by OH, NH2 or COOH Examples of these compounds are trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid and N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid.

The method according to the invention has the advantage that it can be carried out in

a simple and gentle way. The use of flammable substances and work with dry matter containing free hydroxylamine can be avoided. The concentration of hydroxylamine is low throughout the process. For example, it is less than 45% by weight in the solution obtained from step (a) or (b) and less than 30, generally less than 15,% by weight in the stripping column or stripping/distillation column. Due to the method of operation in the stripping column or stripping/distillation column, the liquid inventory is minimal and the residence time in the process relatively short. The mode of operation in the stripping column or stripping/distillation column also makes it possible to use higher pressures, especially atmospheric pressure.

Higher hydroxylamine concentrations occur only during concentration in the distillation column (step d)). The hydroxylamine concentration in the solution in step (d) can be adjusted as described, e.g. in the range from 20 to 70% by weight. To reduce the risk of splitting, additional stabilizer can be added to the solution to be distilled.

The devices required for the method according to the invention can be made from non-metallic materials, such as glass, ceramics and plastic. Cleavage initiated by metal ions is thus avoided. It has surprisingly been found that parts of the devices can be made from a metallic material without significantly higher cleavage of the hydroxylamine being observed.

Due to the simple but at the same time safe process design, only a small capital cost is necessary to carry out the method according to the invention on an industrial scale. Moreover, the method can be upgraded practically at will.

The method according to the invention is further illustrated with reference to the working diagrams shown in fig. 1-2: In step (a), a suitable container 12, for example a mixing container, a mixing element or a container equipped with a reaction mixing pump, is filled with hydroxylammonium salt or a hydroxylammonium salt solution 3, the base 2 used and a stabilizer 1 (cf. fig. 1 to 2). Mixing results in an aqueous solution 4 containing free hydroxylamine and the salt originating from the base cation and the anion present in the hydroxylammonium salt.

If insoluble components are present in solution 4, these are separated in step (b) by means of a filtering apparatus 13, the salt 11 and a solution 4' being obtained (cf. Figs. 1 and 2).

If necessary, additional stabilizer 1' is then added to solution 4 or 4'. Separation into an aqueous hydroxylamine fraction and a salt fraction is then carried out in accordance with step (c).

According to fig. 1, separation is carried out in a stripping column 14, the solution 4 or 4' being introduced at the top of the column. For this purpose, steam 10 is introduced into the bottom of the column. The separation is carried out in such a way that the essentially hydroxylamine-free salt solution 5 is taken out at the bottom of the column, and a salt-free, aqueous hydroxylamine fraction 6 (in vapor or liquid form) is taken out via the top (heat exchangers 15 which are not described in detail , is arranged in each of the steps (c) and tø'.

Consistent with fig. 2, the solution 4' is fed into a stripping/distillation column 16. The lower part of the column consists of a stripping section 16' and the upper part of a distillation section 16'. The solution 4' is fed in between these two sections, i.e. at the top of the stripping section. The separation in the stripping/distillation column 16 is carried out in such a way that the essentially hydroxylamine-free salt solution 5 is taken out at the bottom of the column and essentially hydroxylamine-free water 9 via the top. The salt-free hydroxylamine solution 6 is removed via a side outlet.

The hydroxylamine solution 6 obtained from step (c) can, if desired, be concentrated in a distillation column 18 (step (d)). Further stabilizer 1' (fig. 1 and 2) is advantageously added before the distillation. The hydroxylamine solution 6 is fed in at approximately the height of the theoretical plates 1 to 5 in the distillation column 18.1 the distillation essentially produces hydroxylamine-free water via the top and a hydroxylamine solution 8 whose concentration depends on the distillation conditions is obtained at the bottom.

In the examples that follow, all contain hydroxylamine solutions

0.01% by weight, based on free hydroxylamine, of stabilizer, e.g. 8-hydroxyquino-line, 8-hydroxyquinaldine, trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid or a branched polyethyleneimine having a molecular weight of 800 unless otherwise specified.

Example 1

Liberation of hydroxylamine from hydroxylammonium sulfate with ammonia

538.3 g of hydroxylammonium sulfate, 330 g of water and 0.1 g of 8-hydroxyquinaldine as a stabilizer were first placed in a water-cooled, double-jacketed glass 31 vessel with a stirrer. 446 g of a 25% ammonia solution was slowly added dropwise at room temperature with stirring. A clear solution containing 16.4% by weight of hydroxylamine was obtained.

Example 2

Release of hydroxylamine from hydroxylammonium sulfate with sodium hydroxide solution

538.3 g of hydroxylammonium sulfate, 920 g of water and 0.1 g of 8-hydroxyquinaldine as a stabilizer were first placed in a water-cooled, double-jacketed glass 31 vessel with a stirrer. 1008 g of 25% sodium hydroxide solution was slowly added dropwise at room temperature with stirring. A clear solution containing 8.4% by weight of hydroxylamine was obtained.

Example 3

Release of hydroxylamine from hydroxylammonium sulfate with sodium hydroxide solution

1500 g/h of a 37% by weight hydroxylammonium sulfate solution at 50°C together with the stoichiometric amount of 50% by weight sodium hydroxide solution at room temperature was fed continuously into a glass stirring vessel with a capacity of 100 ml. The required amount of stabilizer (600 ppm) was dissolved in the sodium hydroxide solution. The reaction volume in the mixing vessel was 70 ml, which gave a calculated residence time of 2 minutes. The clear product solution was withdrawn continuously via the overflow. Formed sodium sulfate remained in the solution. The aqueous solution obtained contained 11% by weight of HA, 23.6% by weight of sodium sulfate and the required amount of stabilizer in the range of 100 ppm. A mass balance was performed for the streams and no decomposition of HA was observed.

Example 4

Obtaining an aqueous hydroxylamine (HA) solution from hydroxylamine (HA)/ammonium sulfate (AS) solution using a stripping column.

An aqueous solution containing 218 g HA/I and 680 g AS/I was added at a rate of 300 ml/h to the top plate of a stripping column. The glass stripping column having a height of 2 m and a diameter of 35 mm was filled with 3 mm glass Raschig rings to a height of 1.8 m. 1000 ml/h of distilled water was fed to the bottom of the column. The column was at 40 kPa. The bottom temperature was 84°C. 1000 ml/h of aqueous, salt-free HA solution containing 39.0 g HA/h, corresponding to 59.6% of the total HA feed, was distilled off via the top of the column.

300 ml/h of ammonium sulfate solution containing 86.0 g/l HA was removed from the bottom of the column. This corresponds to 39.4% of the total HA in the feed.

The concentration of HA in the column was not more than 100 g/liter. The amount of liquid in the column was 20-225 ml, depending on the supply. The residence time of the liquid in the column was thus only 1.5-10 min. At this low concentration and within the short time, the rate of decomposition is low.

Additional trials are listed in the table below.

Example 5

Separation of an aqueous HA solution from an aqueous HA/Na2SO4 solution using a stripping column

The aqueous solution from Example 3 containing 11% by weight HA and 23.6% by weight Na 2 SO 4 was added at a rate of 978 g/h to the top plate of a stripping column. The enamelled stripping column had a height of 2 m and a diameter of 50 mm, was filled with 5 mm glass Raschig rings. The column was at atmospheric pressure. Steam at 2.5 bar absolute was fed into the bottom of the column. The steam/feed ratio was 2.9:1. 985 g/t sodium sulfate solution containing

1.7 g/liter HA was removed from the bottom of the column. This corresponds to 1% of the total HA in the feed. 3593 g/t aqueous, salt-free solution containing 36.8 g HA/litre, corresponding to 99.2% of the total HA in the feed, was distilled off via the top of the column.

Additional comparable trials are listed in the table below.

Example 6

Obtaining an aqueous HA solution from an aqueous HA/sodium sulfate solution using a stripping/distillation column.

An aqueous solution containing 221 g HA/liter and 540 g AS/liter was added at a rate of 202 mL/h to the 11th plate of a glass bubble column with a diameter of 35 mm, a total height of 1.6 m, and 21 plates (the bottom plate = plate 1). 1300 ml/h superheated steam is fed to the bottom of the column. The pressure in the column was 99 kPa. 180 mL/h of substantially HA-free water (0.6 g HA/liter) was removed at the top of the column at a top temperature of 99.8°C and a reflux ratio of 1:3 (reflux:feed). . The aqueous HA solution (product solution) was removed at a rate of 1180 ml/h and a concentration of 44 g/liter via a side stream from plate 12. 400 ml/h of brine was removed at the bottom of the column.

Example 7

Obtaining an aqueous HA solution from an aqueous HA/sodium sulfate solution by

application of a stripping/distillation column with concentration via a side outlet. An aqueous HA solution as described in Example 3, containing 11% by weight of HA and 23.6% by weight of Na2SO4, was added to the 11th theoretical stage of a glass bubble plate column with a diameter of 50 and more (the number of plates corresponds to 30 theoretical plates). Steam at 2.5 bar absolute and about 125°C was fed to the bottom of the column. The pressure in the column was 101 kPa. Essentially HA-free water (0.05g of HA/I) was withdrawn at the top of the column. The aqueous salt-free HA solution (product solution) was withdrawn at a concentration of 8.3% by weight via a side stream from the 12th plate. The salt solution with a residual HA content of 0.2% by weight was taken at the bottom of the column.

Example 8

Concentration of salt-free, aqueous hydroxylamine solutions by distillation

1600 g/h of an 8.3% by weight aqueous, salt-free, stabilized hydroxylamine solution was fed continuously to the 8th plate of the glass bubble plate column with a diameter of 50 by more and 30 bubble plates. A small amount of stabilizer dissolved in hydroxylamine solution was further dosed into the column on the top plate, plate No. 30. The to reflux ratio was set at 0.5. Water was distilled off via the top of the column. The distillation product still contained a residual amount of hydroxylamine

0.07% by weight. About 240 ml/h of a 50% by weight hydroxylamine solution was removed from the bottom of the column via a pump.

Claims (11)

1. Process for the preparation of an aqueous solution of free hydroxylamine, where a) a hydroxylammonium salt is treated with a suitable base in water, b) any insoluble components are separated from the obtained solution- one, characterized in that c) the solution obtained in step (a) or step (b) is separated into an aqueous hydroxylamine fraction and an aqueous salt fraction by means of a stripping column by passing water or steam countercurrently into the bottom of the column at a temperature at 80°C, and d) if desired, the obtained aqueous hydroxylamine solution is concentrated by distillation. 2. Method according to claim 1, characterized in that the aqueous hydroxylamine solution is taken out at or above the feed plate in the stripping column. 3. Method according to claim 1 or 2, characterized in that the stripping column is operated at from 5 to 300, preferably from 50 to 150, kPa. 4. Method according to any one of the preceding claims, characterized in that a distillation column is used which is operated at from 10 to 150, preferably from 20 to 60, kPa, in step (d). 5. Method according to claims 1-4, characterized in that the stripping of the free hydroxylamine from the salt solution and the concentration of the hydroxylamine solution is carried out in a stripping/distillation column, the concentrated hydroxylamine solution being removed from 0 to 5 plates above the feed of the salt solution from step (a) or step (b) and the water being removed via the top and the salt fraction at the bottom of the column. 6. Method according to claim 5, characterized in that a stripping/distillation column is used in which a vertical dividing wall is introduced at the height of the feed point so that the salt-bearing feed side is separated by means of the dividing wall from the opposite removal point for the salt-free, hydroxylamine-rich solution. 7. Method according to claim 6, characterized in that the vertical dividing wall extends over a height of approx. 1 to 5 theoretical plates and the feeding takes place in the area of half the height of the dividing wall. 8. Method according to claim 5, characterized in that a side column is attached to the stripping/distillation column and the hydroxylamine-rich, salt-free solution is removed via this side column. 9. Method according to claim 8, characterized in that the side column is connected to the stripping/distillation column above one or more plates and below one or more plates from the feed point, and is constructed so that passage of saline solution is avoided into the point of the side column where concentrated hydroxylamine solution is removed. 10. Method according to claims 1-9, characterized in that the water or steam which is taken out via the top in the stripping/distillation column in step (c) or in the distillation column in step (d) is fed back completely or partially to the bottom of the stripping column. 11. Method according to any one of the preceding claims, characterized in that a cleavage stabilizer is added to all solutions containing free hydroxylamine.