EP1280767A1

EP1280767A1 - Method for producing caprolactam on the basis of 6-aminocapronitrile and subsequent purification by crystallization

Info

Publication number: EP1280767A1
Application number: EP01927930A
Authority: EP
Inventors: Peter Bassler; Dieter Baumann; Rolf-Hartmuth Fischer; Eberhard Fuchs; Johann-Peter Melder; Frank Ohlbach
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2000-05-03
Filing date: 2001-04-30
Publication date: 2003-02-05
Also published as: KR20030013398A; AU5482001A; US6683179B2; MXPA02009714A; US20030105322A1; CN1174965C; DE10021192A1; CN1427820A; JP2003531893A; BR0110483A; WO2001083443A1; CA2407728A1; MY122991A; TWI239329B

Abstract

The invention relates to a method for producing caprolactam, which is characterized by a) reacting, in the presence of a catalyst, a mixture (I) that contains 6-aminocapronitrile and water in the gaseous phase to give a mixture (II) that contains caprolactam, ammonia, water, high-boiling and low-boiling substances, subsequently b) removing the ammonia from mixture (II), thereby producing a mixture (III) that contains caprolactam, water, high-boiling and low-boiling substances, c) removing the water from mixture (III), thereby producing a mixture (IV) that contains caprolactam, high-boiling and low-boiling substances, and finally d) obtaining by crystallization a caprolactam-containing solid substance (V) from mixture (IV), the weight percentage of caprolactam in the solid substance (V) being higher than in mixture (IV).

Description

METHOD FOR THE PRODUCTION OF CAPROLACTAM FROM 6-AMINOCAPRONITRIL AND SUBSEQUENT CLEANING BY CRYSTALIZATION

description

The present invention relates to a process for the production of caprolactam, characterized in that

a) a mixture (I) containing 6-aminocapronitrile ("ACN") and water in the gas phase to form a mixture (II) containing caprolactam, ammonia, water, high boilers and low boilers in the presence of a catalyst, then

b) ammonia removed from mixture (II) to obtain a mixture (III) containing caprolactam, water, high boilers and low boilers, then

c) water is removed from mixture (III) to obtain a mixture (IV) containing caprolactam, high boilers and low boilers, and then

d) obtained from mixture (IV) by crystallization a solid (V) containing caprolactam, the proportion by weight of caprolactam in solid (V) being greater than in mixture (IV).

Processes for the production of caprolactam are generally known.

It is also, for example from Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, pages 46-48, or Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed ., Vol. 4, John Wiley & Sons, New York, 1992, page 836, generally known that Caprolactarn, which is used for the production of polymers, must have a purity of 99.9 to 99.94%, the major impurity is usually water in an amount of 0.04 to 0.1%. Other impurities may only be contained in the range of a maximum of a few ppm.

Caprolactam can be produced by Beckmann rearrangement of cyclohexanonoxim with sulfuric acid or oleum. After neutralizing the mixture obtained in this way with ammonia, the caprolactam can be obtained from the ammonium sulfate formed as a by-product by extraction with an organic solvent. Depending on the processes for the preparation of the starting materials used to prepare the cyclohexanone oxime, such as cyclohexanone and hydroxylammonium sulfate, the oximation and rearrangement conditions, the crude caprolactam obtained by Beckmann rearrangement contains impurities, which can be found in Art and scope differ. Typical contaminants from crude caprolactam produced by the Beckmann rearrangement are C-methylcaprolactams, 6-methylvalerolactaiu and n-pentylacetamide.

Various processes have been described for purifying the crude caprolactam obtained in the Beckmann rearrangement.

According to DE-A-1253716, the crude caprolac can be purified by hydrogenation in suspension in the presence of a catalyst and with the addition of an acid.

According to DE-A-1253716, the crude caprolactam can be purified by hydrogenation in suspension in the presence of a catalyst and with the addition of a base.

DD-A-75083 describes a process for the purification of crude caprolactam in which the crude caprolactam is first distilled and then, dissolved in an organic solvent, hydrogenated in the presence of a catalyst and then treated with an ion exchanger.

According to EP-A-411455, the characteristic important quality characteristics for caprolactam can be maintained by continuously hydrogenating the crude caprolactam in a liquid phase process.

Crude caprolactam, which by hydroformylation of 3-pentenoic acid and / or its esters to 5-formylvaleric acid (esters) as main products and 4- and 3-formylvaleric acid (esters) as by-products, extractive (WO 97/02228) or distillative ( WO 97/06126) separation of this branched formylvaleric acid (ester), aminating hydrogenation of 5-formylvaleric acid (esters) to 6-aminocaproic acid (esters) and / or β-aminocaproic acid amide and cyclization of 6-aminocaproic acid (esters) or 6-aminocaproic acid amide contains other typical impurities.

For example, from WO 99/48867, example 1, starting from 5-formylvaleric acid esters, according to WO 98/37063, example 9, crude caprolactam obtained from mixtures of 6-aminocaproic acid, 6-aminocaproic acid amide and corresponding oligomers with addition of 10 wt .-% water to crystallize. In this Crude caprolactam, from which the high and low boilers were not separated prior to crystallization, contained 6345 ppm N-methylcaprolactam, 100 ppm 5-methylvalerolactam, 78 ppm valeramide and other impurities. The crude caprolactam / water melt was homogenized at 50 ° C. and then cooled to 30 ° C. The precipitated crystals were filtered off and washed 2 to 3 times with aqueous caprolactam. 5-methylvalerolactam and valeramide were reduced to 1 ppm, N-methylcaprolactam to 51 ppm. 33.7 g of pure lactam were obtained from 73.6 g of crude lactam (caprolactam yield: 45.8%). The volatile base index (VB) was only achieved by a second crystallization. If, according to WO 99/48867, Example 3, high and low boilers were separated from the crude caprolactam before crystallization, the caprolactam yield after the crystallization was 52%.

From WO 99/65873 is also known caprolactam from mixtures with 4-ethyl-2-pyrrolidone, 5-methyl-2-piperidone, 3-ethyl-2-pyridolidone and 3-methyl-2-piperidone or octahydrophenazine on adsorbents such as activated carbon, molecular sieves or zeolites to be selectively adsorbed and to obtain highly pure caprolactam after desorption. This Caprolac am separation can be followed by melt crystallization or crystallization from a solvent.

It is also known to purify crude caprolactam by crystallization, which, starting from 6-aminocapronitrile according to WO 98/37063, is first hydrolyzed with water to 6-aminocaproic acid. Then water and ammonia formed by hydrolysis are separated off, the 6-aminocaproic acid formed is cyclized and the resulting crude caprolactam is crystallized according to WO 99/48867.

Caprolactam can also be obtained by reacting ACN with water in the liquid phase in the presence or absence of a catalyst with the release of ammonia.

The mixture obtained in this reaction contains, in addition to caprolactam, water, ammonia and any other liquid diluent, impurities with a boiling point above that of caprolactam ("high boilers") and those with a boiling point below that of caprolactam ("low boilers").

From US-A-496,941, example, it is known that after the removal of water, solvent, ammonia, low boilers and high boilers from a mixture obtained in the reaction of ACN with water and solvent, a crude caprolactam with a purity of 99.5% is obtained.

For a crude caprolactam obtained from ACN in the liquid phase, other purification methods are described because the contaminants of such a crude caprolactam are different from those of a crude caprolactam obtained by other methods as in US-A-5 , 496, 941, clearly distinguishes.

According to US-A-5, 496, 941, ACN is used in a first step in the

Liquid phase converted to caprolactam, low boilers, water, ammonia and optionally further solvents simultaneously separated, high boilers separated to give a crude caprolactam in a purity of 99.5%, this crude caprolactam hydrogenated in the presence of a catalyst, the product obtained treated with an acidic ion exchanger or sulfuric acid and the product obtained is distilled in the presence of a base.

From WO 96/20923 a process for the purification of crude caprolactam is known which originates from the liquid phase cyclization of 6-aminocapronitrile with water in the presence of a solvent and from heterogeneous catalysts. Here crude caprolactam is first hydrogenated, then treated with acidic agents and finally distilled in the presence of alkali. A disadvantage of this purification process is that three separate reaction steps are required for the production of pure caprolactam.

The cyclization of 6-aminocapronitrile in the gas phase in the presence of water and a catalyst, as for example in EP-A-659 741, WO 96/22974, DE 19632006, WO 99/47500 or WO

99/28296 leads to a crude caprolactam which has other typical impurities than a crude caprolactam which was obtained by a different process. Typical impurities in a crude caprolactam obtained from ACN in the gas phase are, for example, cyanoalkyl- and aminoalkyl-substituted caprolactam derivatives and tetrahydroazepine derivatives such as N-cyanopentylhexamethyleneimine, N-cyanopentylcaprolactam, N- Aminohexylcaprolactam. These impurities contribute to the deterioration of pure caprolactam in general for caprolactam, for example from Ulimann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, pages 46-48 , or Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 4, John Wiley & Sons, New York, 1992, page 836 in known quality indicators, such as the values for the free and volatile base and UV indicators. The object of the present invention was to provide a process which enables the production of caprolactam, which was obtained in the gas phase from ACN, in high purity in a technically simple and energy-saving manner.

Accordingly, the process defined at the outset was found.

According to step a), a mixture (I) containing 6-aminocapronitrile, water and optionally liquid diluent becomes a mixture (II) containing caprolactam, ammonia, water, optionally liquid diluent, high boilers and low boilers in the presence of a solid which catalytically promotes the reaction implemented in the gas phase.

The ACN required for step a) can be obtained from adiponitrile, as is known from Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, page 46, Fig. 8 become.

The partial catalytic hydrogenation of adiponitrile in the presence of ammonia as a solvent and, for example, as a suspension catalyst rhodium on magnesium oxide (US Pat. No. 4,601,859), Raney Nickel (US Pat. No. 2,762,835, WO) is particularly suitable 92/21650), nickel on aluminum oxide (US-A-2, 208, 598) or as a fixed bed catalyst Cu-Co-Zn spinel (DE-B-954416, US-A-2,257,814) or iron (DE-A-42 35,466) or a process according to US-A-2, 245, 129, US-A-2, 301, 964, EP-A-150295, FR-A-2 029 540 or one in US-A-5, 496 , 941 described method.

The adiponitrile required for this reaction is produced industrially, for example by double hydrocyanation of butadiene in the presence of nickel-containing catalysts, and is commercially available, for example, from Aldrich-Chemie Gesellschaft mbH & Co. KG, Steinheim, Germany. The conversion of mixture (I) to mixture (II) can take place, for example, according to EP-A-659 741, WO 96/22974, DE 19632006, WO 99/47500 or WO 99/28296.

The reaction can preferably be carried out in the gas phase at temperatures of generally 200 to 550 ° C., preferably 250 to 400 ° C. the pressure is generally in the range from 0.01 to 10 bar, preferably at atmospheric pressure, care being taken that the reaction mixture is predominantly gaseous under the conditions used. The catalyst loads are usually 0.05 to 2, preferably 0.1 to 1.5, in particular 0.2 to 1 kg of 6-aminocapronitrile per liter of catalyst volume per hour.

The reaction can be carried out batchwise, preferably continuously.

Suitable reactors are advantageously those which are generally known for gas-phase reactions on moving or stationary solid catalysts. Preferably one

Fluidized bed reactor, preferably fixed bed reactor, such as a tray reactor, in particular a tubular reactor, can be used. Combinations of such reactors are also possible.

1 to 50, preferably 1 to 10, mol of water are generally used per mol of ACN.

The mixture (I) can also contain further organic compounds which are in gaseous form under the reaction conditions, such as alcohols, amines or aromatic or aliphatic hydrocarbons.

Examples of suitable catalytically active compounds of the catalysts are silicon dioxide as pyrogenically prepared silicon dioxide, as silica gel, diatomaceous earth, quartz or mixtures thereof, copper chromite, preferably aluminum oxide, titanium oxide, preferably titanium dioxide, lanthanum phosphates, lanthanum oxides, and also mixtures of such compounds.

Aluminum oxide is suitable in all modifications which can be obtained by heating the precursor compounds aluminum hydroxide (gibbsite, boehmite, pseudo-boehmite, bayerite and diaspor) at different temperatures. These include in particular gamma and alpha alumina and their mixtures.

Titanium dioxide is amorphous and suitable in all of its modifications, preferably anatase and rutile, and mixtures of such modifications.

Lanthanum phosphates in their various modifications, stoichiometric ratios between lanthanum and phosphate unit and degrees of condensation of the phosphate units (monophosphate, oligophosphates such as diphosphates or triphosphates, polyphosphates) are suitable individually or as a mixture. These compounds can be used in the form of powders, grits, grit, strands or pressed into tablets. The form of the compounds generally depends on the requirements of the particular reaction procedure, powder or semolina advantageously being used in a fluidized bed mode. In the fixed bed mode of operation, tablets or strands with diameters between 1 mm and 6 mm are usually used.

The compounds can be in pure form (content of the particular compound> 80% by weight), as a mixture of the above-mentioned compounds, the sum of the above-mentioned compounds should be> 80% by weight, or as a supported catalyst, the above-mentioned Connections can usually be applied to a mechanically and chemically stable support with a high surface area.

The pure compounds can be prepared by precipitation from aqueous solutions, e.g. Titanium dioxide after the sulfate process or by other methods such as the pyrogenic production of fine aluminum oxide, titanium dioxide or zirconium dioxide powders, which are commercially available.

Several methods are available for producing mixtures of the different compounds. The compounds or their precursor compounds which can be converted into the oxides by calcining can e.g. be prepared from solution by co-precipitation. A very good distribution of the two compounds used is generally obtained. The compound or precursor mixtures can also be precipitated by precipitating one compound or precursor in the presence of the second compound or precursor present as a suspension of finely divided particles. Another method consists in mechanically mixing the compound or precursor powders, which mixture can be used as a starting material for the production of strands or tablets.

In principle, all methods described in the literature are suitable for the preparation of supported catalysts. For example, the compounds can be applied to the support in the form of their brine simply by soaking them. The volatile constituents of the sol are usually removed from the catalyst by drying and calcining. Such brines are commercially available for titanium dioxide and aluminum oxide.

Another possibility for applying layers of the catalytically active compounds consists in the hydrolysis or pyrolysis of organic or inorganic compounds. So a ke- Ramic support can be coated with titanium dioxide by hydrolysis of titanium isopropylate or other Ti alkoxides in a thin layer. Other suitable compounds include TiCl4 and aluminum nitrate. Suitable carriers are powders, strands or tablets of the compounds mentioned themselves or other stable compounds such as steatite or silicon carbide. The carriers used can be designed to be macroporous in order to improve the mass transport.

The reaction can be carried out in the presence of a gas which is inert with respect to the conversion of mixture (I) to mixture (II), preferably argon, in particular nitrogen. The volume ratio of the inert gas to the gaseous ACN under the reaction conditions can advantageously be up to 100.

In step b), ammonia is removed from mixture (II) to give a mixture (III) containing caprolactam, water, optionally liquid diluent, high boilers and low boilers.

The ammonia can in principle be separated from mixture (II) by processes known per se for substance separation, such as extraction or preferably distillation, or a combination of such processes.

The distillation can advantageously be carried out at bottom temperatures of 60 to 220 ° C., in particular 100 to 220 ° C. A pressure, measured at the top of the distillation apparatus, of 2 to 30 bar absolute is usually set.

For the distillation, conventional apparatuses come into consideration, as are described, for example, in: Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages 870-881, such as sieve plate columns, bubble plate plate columns, packing columns or packed columns.

The distillation can be carried out in several, such as 2 or 3 columns, advantageously in a single column.

According to step c), water and optionally liquid diluents are removed from mixture (III) to give a mixture (IV) containing caprolactam, high boilers and low boilers. If a liquid diluent was used in step a), water and liquid diluent can be separated off in step c) or the water before or after the liquid diluent.

In principle, the water can be separated from mixture (III) by processes known per se for the separation of substances, such as extraction, crystallization or preferably distillation, or a combination of such processes.

The distillation can advantageously be carried out at bottom temperatures of 50 to 250 ° C., in particular 100 to 230 ° C.

For the distillation, customary apparatuses are considered, as described, for example, in: Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages 870-881 are, such as sieve tray columns, bubble tray trays, packed columns or packed columns.

A heat-coupled, multi-stage separation of the water and, if appropriate, the liquid diluent is particularly preferred.

Before the mixture (IV) is fed in step d), the low boilers and high boilers are separated off, advantageously only the high boilers are separated off, in particular neither the removal of high boilers and high boilers, particularly advantageously only the low boilers are removed from the mixture (IV). into consideration.

If low boilers and high boilers are separated from the mixture, the low boilers can be separated off before, after or together with the high boilers.

In the case of separation of low boilers and high boilers or only high boilers or only low boilers, the removal can in principle be carried out by processes known per se for the separation of materials, such as extraction, crystallization or preferably distillation, or a combination of such processes.

• The distillation can advantageously be carried out at bottom temperatures of 50 to 250 ° C., in particular 100 to 230 ° C. One usually sets a pressure, measured at the head of the de- Stillation of 1 to 500, preferably 5 to 100 bar absolute.

The distillation to remove the low boilers can be carried out in several, such as 2 or 3 columns, advantageously in a single column.

The distillation to remove the high boilers can advantageously be carried out in a number of columns, such as 2 or 3, in a single column.

According to step d), a solid (V) containing caprolactam is obtained from mixture (IV) by partial crystallization, the proportion by weight of caprolactam in solid (V) being greater than in mixture (IV).

The sum of the contents of high boilers and low boilers, water and organic diluents not being included, in the mixture (IV) used in step d) is advantageously at least 100 ppm by weight, preferably 200 ppm by weight, particularly preferably at least 500% by weight . ppm, in particular at least 1000 ppm by weight based on mixture (IV).

The crystallization can be carried out batchwise. The crystallization can be carried out continuously.

The crystallization can be carried out with the addition of an auxiliary, such as an organic or inorganic liquid diluent, for example water, preferably without the addition of an auxiliary.

The crystallization can be carried out in one or more stages, such as two, three or four stages, preferably in one stage. In another preferred embodiment of the formation, the crystallization can be carried out as a fractional crystallization.

In the case of fractional crystallization, all stages which produce a crystallizate (caprolactam) which is purer than the raw product supplied (crude caprolactam) are usually called cleaning stages and all other stages are called output stages. Zweckmä- Multi-stage processes are usually operated according to the countercurrent principle, in which after the crystallization in each stage the crystals are separated from the remaining liquid phase (“mother liquor”) and this crystals are fed to the respective stage with the next highest degree of purity, while the crystallization residue is fed to the respective stage with the next lowest degree of purity.

The temperature of the solution or melt during the crystallization is advantageously not above the melting point of caprolactam (70 ° C.), preferably between -iθ and the melting point of caprolactam, in particular between 20 and the melting point of caprolactam. The solids content in the crystallizer is usually between 0 and 70 g, preferably between 30 and 60 g per 100 g of use.

In a further advantageous embodiment of the invention, the crystallization takes place in apparatuses in which the crystals grow on cooled surfaces in the crystallization apparatus, i.e. are fixed in the apparatus (e.g. layer crystallization process from Sulzer Chemtech (Switzerland) or static crystallization process from BEFS PROKEM (France).

Furthermore, the crystallization can be carried out by cooling apparatus walls or by evaporating a solution of the crude caprolactam in vacuo. 5 to 30% by weight solutions of the crude caprolactam in a liquid diluent, in particular water, are particularly suitable for this.

In the case of crystallization by cooling, the heat can be dissipated via scratch coolers which are connected to a stirred tank or a container without an agitator. The circulation of the crystal suspension can be ensured by a pump. In addition, there is also the possibility of dissipating the heat via the wall of a stirred tank with a wall-mounted stirrer. Another preferred embodiment in cooling crystallization is the use of cooling disk crystallizers, such as those manufactured by Gouda (Holland). In a further suitable variant for crystallization by cooling, the heat can be dissipated via conventional heat exchangers (preferably tube bundles or plate heat exchangers). In contrast to scratch coolers, stirred kettles with wall-mounted stirrers or cooling crystal disks, these devices have no device for avoiding crystal layers on the heat-transferring surfaces. If a state is reached in operation in which the thermal resistance takes on too high a value due to crystal layer formation, the switchover to a second one usually takes place Apparatus. During the operating time of the second apparatus, the first apparatus can be regenerated (preferably by melting the crystal layer or flushing the apparatus with unsaturated solution). If the heat transfer resistance in the second set is too high, switch back to the first set, etc. This variant can also be operated in alternation with more than two sets. In addition, the crystallization can be carried out by conventional evaporation of the solution in vacuo. '-

For the separation of the mother liquor from the crystallized caprolactam, the known methods of solid-liquid separation come into consideration.

In a preferred embodiment of the invention, the crystals can be separated from the mother liquor by filtering and / or centrifuging. Advantageously, the filtering or centrifuging can be preceded by a pre-thickening of the suspension, for example by one or more hydrocyclones. Centrifuges which are known per se and which operate batchwise or continuously are suitable for centrifugation. Shear centrifuges that can be operated in one or more stages can be used most advantageously. In addition, screw screen centrifuges or screw discharge centrifuges (decanters) are also suitable. Filtration can advantageously take place by means of filter grooves, which are operated discontinuously or continuously, with or without an agitator, or by means of a belt filter. In general, the filtering can be carried out under pressure or in vacuo.

During and / or after the solid-liquid separation, further process steps can be provided to increase the purity of the crystals or the crystal cake. In a particularly advantageous embodiment of the invention, after the crystals have been separated from the mother liquor, the crystals or the crystal cake are washed and / or sweated in one or more stages.

When washing, the amount of washing liquid should preferably be between 0 and 500 g of washing liquid / 100 g of crystals, preferably between 30 and 200 g of washing liquid / 100 g of crystals.

Organic or inorganic liquids or mixtures of such liquids can be considered as washing liquid. Preferred washing liquids are, for example

a) in the event that a liquid diluent was used in the crystallization in step d), this liquid diluent,

b) a melt of a crystallizate obtained in a crystallization stage according to step d),

I I c) a mother liquor obtained in a crystallization step in step d), or

d) a melt of a starting material used in a crystallization stage according to step d).

Washing can be carried out in the usual apparatus for this. Washing columns in which the mother liquor is separated and washed in one apparatus, centrifuges which can be operated in one or more stages, or filter chutes or belt filters can advantageously be used. Washing can be carried out on centrifuges or belt filters in one or more stages. Here, the washing liquid can be passed in countercurrent to the crystal cake.

The washing liquid can be returned to the crystallization, in particular in the case of crystallization without the addition of an auxiliary, optionally after removal of impurities.

Sweating is usually understood to mean local melting of contaminated areas. Advantageously, the

The amount of perspiration is 0.1 to 90 g of melted crystals / 100 g of crystals before sweating, preferably 5 to 35 g of melted crystals / 100 g of crystals. Carrying out the joke on centrifuges or belt filters is particularly preferred. Carrying out a combination of washing and sweating in one apparatus can also be suitable.

The mother liquor can be returned to the crystallization, in particular in the case of crystallization without the addition of an auxiliary, optionally after removal of impurities.

According to the present process, caprolactam can be obtained in a purity of at least 99.90% by weight, preferably 99.90 to 99.99% by weight. The caprolactam obtainable by the process according to the invention can be used for the production of polyamides, such as polycaprolactam.

Claims

claims

1. Process for the production of caprolactam, characterized in that

a) a mixture (I) containing 6-aminocapronitrile and water in the gas phase to a mixture (II) containing caprolactam, ammonia, water, high boilers and low boilers in the presence of a catalyst, then

c) water removed from mixture (III) to obtain a mixture (IV) containing caprolactam, high boilers and low boilers, and then

The method of claim 1, wherein mixture (I) additionally contains an inert gaseous diluent.

3. The method according to claim 2, wherein in step a) titanium dioxide, aluminum oxide or lanthanum phosphate is used as the catalytically active component of the catalyst.

4. Process according to claims 1 to 3, wherein the low boilers are separated off between steps c) and d).

5. Process according to claims 1 to 3, wherein the high boilers are separated off between steps c) and d).

6. The method according to claims 1 to 3, wherein the low boilers and high boilers are separated off between steps c) and d).

7. The method according to claims 1 to 6, wherein the sum of the contents of high boilers and low boilers, calculated without water and any diluent present, in the mixture (IV) used in step d) is at least 100 ppm by weight j.

8. The method according to claims 1 to 7, wherein one carries out the crystallization in step d) in the absence of auxiliaries.

9. The method according to claims 1 to 8, wherein the crystallization in step d) is carried out on a cooled surface on which solid (V) grows.

10. The method according to claims 1 to 9, wherein the mother liquor obtained after the crystallization in step d) is mixed with mixture (IV) and recycled in step d).

11. Experience according to claims 1 to 10, wherein the crystallization in step d) is carried out discontinuously.

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