Recovery of methionylmethionine from aqueous media containing alkali metal ions

Abstract

The present invention relates to the recovery of methionylmethionine from an aqueous medium containing alkali metal ions by forming a bis (methionyl)-diketopiperazine intermediate.

Description

Technical Field

[0001] The present invention relates to the recovery of methionylmethionine from aqueous media containing alkali metal ions via the formation of a bis(methionyl)-diketopiperazine intermediate. Background Art

[0002] Methionine is an essential amino acid, that is, an amino acid that must be provided in the livestock production process to accelerate the growth of animals. MetMet (marketed as MetMet) comprises two chemically linked methionine units and has the advantage over methionine itself that due to its lower water solubility it exhibits a slow and / or delayed release effect, which makes it particularly suitable as a feed ingredient in aquaculture applications, where leaching of the monomeric amino acids is often a problem, as described in WO 2010 / 043558. Moreover, the slow and / or delayed release effect is also advantageous for feeding conventional livestock such as poultry and pigs.

[0003] MetMet can be produced from methionine hydantoin and a base provided by the methionine process to produce an intermediate 2,6-bis(methionyl)-1,4-diketopiperazine (hereinafter also referred to as "DLLD / DDLL-Met-DKP" or "Met-DKP"). After purification, the Met-DKP can be hydrolyzed under alkaline conditions and then neutralized with sulfuric acid to obtain the desired MetMet as shown in formula (I) (see also WO2010043558).

[0004]

[0005] However, since the chemical production process of methionine generates a large amount of byproducts containing large amounts of MetMet, a problem would arise if the MetMet contained in such byproducts could not be used to improve the overall efficiency of the methionine production process.

[0006] Methionine is produced chemically, for example by the Bucherer-Bergs reaction, which is a variant of the Strecker synthesis. Here, the starting substances 3-methylmercaptopropionaldehyde (prepared from 2-propenal and methyl mercaptan), hydrocyanic acid (hydrogen cyanide), ammonia and carbon dioxide react to give 5-(2-methylmercaptoethyl)hydantoin (methionine hydantoin), which is then hydrolyzed with potassium carbonate and potassium bicarbonate to give potassium methioninate. By treating with carbon dioxide, methionine can be filtered out as a precipitate from a mother liquor containing potassium carbonate and potassium bicarbonate and finally released from the potassium salt. Ammonia, potassium carbonate and potassium bicarbonate reagents and carbon dioxide are usually recycled in the industrial production of methionine. Instead of the above-mentioned potassium compounds, the corresponding sodium compounds can also be used in the reaction. However, it is necessary from time to time to exchange part of the aqueous mother liquor of the hydantoin hydrolysis cycle for fresh potassium hydroxide or fresh sodium hydroxide, essentially removing ("flushing") the deactivated potassium or sodium salt from the cycle in the form of neutral potassium or sodium formate formed from the residues of the potassium or sodium salt of the hydantoin hydrolysis present in the hydrogen cyanide and methionine hydantoin solution.

[0007] The so-called flushing solution thus obtained generally contains about 2 to 6% by weight of methionine and about 2 to 8% by weight of MetMet, in addition to a large amount of potassium or sodium salts (generally about 6 to 14% by weight of potassium in the form of potassium salts). The water content of this by-product is generally about 70 to 80% by weight. Due to the potassium, nitrogen and sulfur content, this by-product is suitable as a liquid fertilizer. However, it would be desirable to be able to recover the MetMet contained in this by-product, so as to increase the overall efficiency of the process.

[0008] Attempts to utilize MetMet directly from this by-product have so far been unsuccessful, since classical separation methods such as crystallization lead to products of insufficient purity. A particular problem in isolating MetMet from this by-product is the substances present in large quantities in the by-product, in particular alkali metal ions such as potassium, but also bicarbonate. In addition, the organic compounds present in the by-product also constitute an obstacle to the direct isolation of MetMet in pure form from the medium. Summary of the invention

[0009] It is therefore an object of the present invention to provide a process which makes it possible to utilize MetMet from such by-products or from other aqueous media containing MetMet and potassium and optionally other interfering substances, with a particular object being to recover MetMet from such media in sufficiently pure form.

[0010] Surprisingly, it has been found that the objects of the present invention can be achieved by converting MetMet contained in an aqueous solution into a water-insoluble methionine diketopiperazine (3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione; "Met-DKP") of formula (II).

[0011]

[0012] Then, the water-insoluble Met-DKP can be easily crystallized and separated from the aqueous solution.

[0013] Therefore, a first object of the present invention is a process for recovering MetMet from an aqueous medium containing alkali metal ions, the medium containing at least 1% by weight of alkali metal ions, preferably potassium or sodium ions, characterized in that the MetMet contained in the medium is converted into Met-DKP, the Met-DKP is subsequently crystallized or precipitated and then separated, wherein the separation is preferably carried out by filtration, centrifugation and / or decantation.

[0014] According to the invention, the conversion of MetMet into Met-DKP is preferably carried out by heating the medium to a temperature of 120-200° C., in particular 150-180° C., preferably 155-175° C., more preferably 160-170° C. The reaction is preferably carried out in a pressure vessel, in particular an autoclave, so that pressure is generated due to the heating, wherein the pressure during the reaction is preferably 2-12 bar, more preferably 4-10 bar, in particular 5-7 bar. The reaction is preferably carried out for 0.5-10 hours, more preferably 1-6 hours, in particular 2-5 hours.

[0015] The reaction is best at a pH value of 3 to 7. Therefore, according to the present invention, the pH value of the medium measured at 20° C. with a pH electrode is preferably 3 to 7, more preferably 3.5 to 6, and especially 4.0 to 5.5.

[0016] Since the by-products containing alkali metal ions in the production process of the chemical methionine generally have an alkaline pH value, in particular, a pH value of 7.5-12.5, more preferably 8-10, measured with a pH electrode at 20° C., the method of the present invention preferably further comprises a preceding step of acidifying this initial medium, i.e., wherein the pH value is preferably adjusted to 3-7, in particular 3.5-6, more preferably 4.0-5.5 by acidification.

[0017] The acidification is preferably carried out by using an acid selected from phosphoric acid, acetic acid, formic acid, sulfuric acid or mixtures thereof.

[0018] The alkali metal ion-containing aqueous medium from which MetMet is recovered is preferably a complex matrix, in particular a by-product and / or waste stream of a process for the production of the chemical methionine.

[0019] The alkali metal ion contained in the aqueous medium is preferably selected from potassium ion, sodium ion and a mixture thereof, but is more preferably potassium ion.

[0020] The aqueous medium is preferably an aqueous solution, but may also be an aqueous suspension.

[0021] The aqueous medium used in the process preferably has a water content of 50-90% by weight, more preferably 60-85% by weight, in particular 70-80% by weight, ie it preferably has a total dry matter content of 10-50% by weight, more preferably 15-40% by weight, in particular 20-30% by weight.

[0022] The aqueous medium used in the process preferably contains 5-20 wt.-%, preferably 8-16 wt.-% MetMet relative to the total dry matter content of the aqueous medium. Overall, the aqueous medium preferably contains 1-12 wt.-%, preferably 2-8 wt.-%, more preferably 3-7 wt.-% MetMet.

[0023] The aqueous medium also contains alkali metal ions, preferably potassium or sodium ions, most preferably potassium ions, in particular 10-50% by weight, more preferably 15-40% by weight, most preferably 20-35% by weight of alkali metal ions, relative to the total dry matter content of the aqueous medium, which are present in the medium in the form of alkali metal salts. In general, the aqueous medium preferably contains alkali metal ions, in particular potassium ions or sodium ions, in the form of alkali metal salts in an amount of at least 2% by weight, in particular 2-18% by weight, more preferably at least 4% by weight, in particular 4-16% by weight, most preferably at least 6% by weight, in particular 6-14% by weight.

[0024] The aqueous medium preferably further comprises bicarbonate, in particular 5-40 wt.-%, more preferably 10-35 wt.-%, most preferably 15-30 wt.-% bicarbonate relative to the total dry matter content of the aqueous medium. Overall, the aqueous medium preferably contains at least 1 wt.-% bicarbonate, in particular 1-15 wt.-%, more preferably at least 2 wt.-%, in particular 2-12 wt.-%, most preferably at least 3 wt.-%, in particular 3-10 wt.-% bicarbonate.

[0025] The aqueous medium also preferably contains methionine, preferably 1 to 10% by weight, preferably 2 to 6% by weight, of methionine.

[0026] The amount of organic substances as mentioned above is preferably determined by HPLC, in particular by HPLC as disclosed in more detail in the examples. The amount of potassium is preferably determined by IC (ion chromatography), in particular by IC as disclosed in more detail in the examples. The amount of bicarbonate is preferably determined by titration with hydrogen chloride.

[0027] The pH value of a typical methionine side flow medium for utilizing MetMet is preferably 7.5-12.5, more preferably 8-10. Acidification of the medium is preferably carried out by slowly adding an acid such as phosphoric acid, acetic acid, formic acid or sulfuric acid (30%-96%) under constant stirring until a pH of 3-7, preferably 3.5-6, is reached, wherein sulfuric acid is very preferably used to acidify the medium. Typically, foaming and a temperature increase are observed, depending on the acid source used.

[0028] Next, preferably after 5 to 60 minutes, more preferably after 5 to 30 minutes, the reaction medium is preferably transferred to an autoclave and, after sealing the reaction mixture, preferably stirred constantly and heated to 150 to 180° C., preferably 160 to 170° C. After reaching the desired internal temperature, the reaction mixture is preferably stirred for a further 0.5 to 3 hours, more preferably 1 to 2 hours, and then cooled, preferably to a temperature of about 70° C. The crystallization of Met-DKP has preferably been started by reducing the pressure in the reaction vessel. The resulting slurry is preferably filtered and the resulting solid is washed with distilled water. The solid can then be used directly for the recovery of MetMet. Alternatively, the solid can be dried.

[0029] In a preferred embodiment of the invention, the process comprises a step of first removing a large part, preferably a major part (i.e. at least 50%) of the alkali metal ions from the medium before converting MetMet into Met-DKP. This is preferably achieved by acidifying the alkaline initial medium with sulfuric acid and adjusting the pH to 3-7, in particular 4-6, preferably 4-5, which leads to precipitation of the alkali metal ions contained in the medium in the form of alkali metal sulfates. The alkali metal sulfates thus obtained can then be separated by filtration or decantation. By doing so, preferably at least 50% by weight of the alkali metal ions previously contained in the medium are removed. As a next step, the pH value can optionally be further reduced before heating the medium under pressure to form Met-DKP as disclosed above. The advantage of this process is that, due to the previous removal of interfering alkali metal ions, Met-DKP can be obtained directly in a very pure form, so that alkali metal ions or salts thereof do not interfere with the formation of the Met-DKP precipitate.

[0030] In an alternative embodiment of the present invention, the intermediate step of removing the alkali metal ions contained in the aqueous medium is not used, but the medium obtained after acidification is directly used in the pressure vessel for producing Met-DKP. This leads to the formation of a Met-DKP precipitate containing a higher amount of alkali metal salts, such as alkali metal sulfates. However, surprisingly, it was found that the alkali metal sulfates contained in the Met-DKP precipitate can be easily separated by washing with water or an aqueous solution. Therefore, this alternative method is also commercially feasible and can obtain sufficiently pure Met-DKP.

[0031] In another preferred embodiment of the present invention, an acid such as acetic acid, formic acid or phosphoric acid is used to acidify the alkaline initial medium, which forms an alkali metal salt with a relatively high water solubility. The advantage of this process is that the alkali metal salt or at least the main part thereof remains in a soluble form, so that Met-DKP can be obtained directly in a relatively pure form and in high yield, without the need for intermediate isolation of the alkali metal ions and without the need for subsequent fine purification steps.

[0032] The water-insoluble Met-DKP formed in the reaction can be easily crystallized or precipitated and separated from the aqueous medium, preferably by filtration, centrifugation or decantation, wherein the crystallization or precipitation is preferably carried out at a temperature of 0-140° C., more preferably at a temperature of 20-100° C. Subsequently, the obtained Met-DKP can be further purified, preferably by washing with water or an aqueous solution and / or by recrystallization.

[0033] Next, the Met-DKP thus obtained can be utilized in different ways, for example by converting it again into MetMet, in particular by recycling the Met-DKP into the MetMet process, or by further hydrolysis to obtain methionine.

[0034] The conversion of Met-DKP to MetMet can be carried out by acidic or alkaline hydrolysis.

[0035] The acidic hydrolysis is preferably carried out in the presence of an acid selected from the group consisting of mineral acids, HCl, H2CO3, CO2 / H2O, H2SO4, phosphoric acid, carboxylic acids and hydroxycarboxylic acids.

[0036] The alkaline hydrolysis is preferably carried out at a pH of 7-14, in particular at a pH of 9-12, most preferably at a pH of 10-11, to obtain DL-methionyl-DL-methionine. In addition, the alkaline conditions can be adjusted by using a substance preferably selected from nitrogenous bases, NH4HCO3, (NH4)2CO3, NH4OH / CO2 mixtures, carbamates, KHCO3, K2CO3, carbonates, alkali metals and alkaline earth metal bases.

[0037] The acidic or alkaline hydrolysis is preferably carried out at a temperature of 50° C. to 200° C., preferably 80° C. to 180° C., particularly preferably 90° C. to 160° C. In order to avoid the conversion of Met-DKP into methionine, the acid or base is preferably added in an approximately equimolar amount, i.e. 1 to 1.2 moles of acid or base equivalents relative to 1 mole of Met-DKP. In addition, the residual amount of Met-DKP is preferably detected analytically, and the reaction is stopped by cooling when no more Met-DKP can be detected in the aqueous medium. Example

[0038] Example 1: Acidification of the methionine sidestream solution by changing the pH with sulfuric acid

[0039] 100 g of a methionine side stream solution (3.2 wt% Met) containing 3.7 wt% MetMet (1.9 wt% DDLL-MetMet, 1.8 wt% DLLD-MetMet) and a measured pH of 9.9 with a total potassium content of 97.4 g / kg was acidified by slowly adding concentrated sulfuric acid (11.6 g, 96%) under constant stirring at room temperature until a pH of 4 was reached. Typical foaming behavior was observed, as well as a temperature increase to around 40° C. After 10 minutes, the resulting precipitate was collected and characterized as potassium sulfate according to IC analysis. The remaining mother liquor (84.5 g) was used for further conversion in Example 3.

[0040] Example 2: Acidification of the methionine sidestream solution with sulfuric acid to improve precipitation of potassium sulfate

[0041] 500 g of the methionine side stream solution (total potassium content 103.2 g / kg) with a measured pH value of 9.4 was acidified by slowly adding concentrated sulfuric acid (53.6 g, 96%) under constant stirring at room temperature until a pH of 5 was reached. Typical foaming behavior was observed, as well as a temperature increase to around 45° C. The suspension was then cooled to 10° C. and the resulting precipitate was collected after 15 minutes and characterized as potassium sulfate according to IC analysis. The remaining mother liquor can be used for further conversions as described in the following examples.

[0042] Example 3: Synthesis and Isolation of Met-DKP from Methionine Side Flow Solution

[0043] 84.5 g of the remaining mother liquor from Example 1 was transferred to a suitable autoclave, sealed, and the reaction mixture was stirred and heated to 160° C. After reaching the desired internal temperature (about 6 bar pressure), the reaction was stirred for a further 2.5 hours and then cooled to 70° C. over 1 hour. The resulting slurry was filtered and the resulting solid was washed with 25.0 g of distilled water. The solid was dried at 50° C. overnight to give 2.2 g of Met-DKP in 52% yield (82% purity).

[0044] Example 4: Acidification of the methionine sidestream solution by changing the pH with acetic acid and separation of Met-DKP

[0045] 100 g of a methionine sidestream solution (3.0 wt% Met) containing 3.3 wt% MetMet (1.6 wt% DDLL-MetMet, 1.7 wt% DLLD-MetMet) and a measured pH of 8.3 and a total potassium content of 93.1 g / kg were acidified by slowly adding glacial acetic acid (30.8 g, 99%) under constant stirring at room temperature until a pH of 5 was reached. Only slight foaming behavior was observed, no temperature increase and no precipitation. After 5 minutes, 123.5 g of the solution were transferred to a suitable autoclave, which was sealed with constant stirring of the reaction mixture and heated to 160° C. After reaching the desired internal temperature (about 6 bar pressure), the reaction was stirred for a further 2.5 hours and then cooled to 70° C. within 1 hour. The resulting slurry was filtered and the resulting solid was washed with 24.8 g of distilled water. The solid was dried at 50° C. overnight to give 1.5 g of Met-DKP in 46% yield (96% purity).

[0046] Example 5: Acidification of the methionine sidestream solution by changing the pH with sulfuric acid and separation of Met-DKP

[0047] 100 g of a methionine side stream solution (3.0 wt% Met) containing 3.3 wt% MetMet (1.6 wt% DDLL-MetMet, 1.7 wt% DLLD-MetMet) and a measured pH of 8.3 and a total potassium content of 93.1 g / kg were acidified by slowly adding concentrated sulfuric acid (8.7 g, 96%) under constant stirring at room temperature until a pH of 5.5 was reached. Typical foaming behavior and a temperature increase to 38°C were observed. After 10 minutes, 100.1 g of the resulting slurry were directly transferred to a suitable autoclave, which, after sealing, was heated to 160°C with constant stirring of the reaction mixture. After reaching the desired internal temperature (about 6 bar pressure), the reaction was stirred for a further 2.5 hours and then cooled to 70°C within 40 minutes. The resulting slurry was filtered and the resulting solid was washed with 27.4 g of distilled water. The solid was dried at 50°C overnight to give 5.7 g of Met-DKP in 87% yield (45% purity).

[0048] Example 6: Acidification of the Concentrated and Methionine-Depleted Sidestream Solution by Changing the pH with Sulfuric Acid and Isolation of Met-DKP

[0049] 100 g of the methionine-reduced sidestream solution (2.1 wt% Met) containing 6.5 wt% MetMet (3.4 wt% DDLL-MetMet, 3.1 wt% DLLD-MetMet) and a measured pH of 8.8 and a total potassium content of 130.0 g / kg were acidified by slowly adding concentrated sulfuric acid (11.3 g, 96%) under constant stirring at room temperature until a pH of 4.5 was reached. Typical foaming behavior and a temperature increase to 40°C were observed. After 10 minutes, 102.7 g of the resulting slurry were directly transferred to a suitable autoclave, which was sealed with constant stirring of the reaction mixture and heated to 170°C. After reaching the desired internal temperature (about 6 bar pressure), the reaction was stirred for a further hour and then cooled to 70°C within 50 minutes. The resulting slurry was filtered and the resulting solid was washed with 30.0 g of distilled water. The solid was dried at 50°C overnight to give 18.3 g Met-DKP in 77% yield.

[0050] Example 7: Acidification of the methionine sidestream solution by changing the pH with formic acid and separation of Met-DKP

[0051] 100 g of a methionine side stream solution (3.2 wt. % Met) containing 3.7 wt. % MetMet (1.9 wt. % DDLL-MetMet, 1.8 wt. % DLLD-MetMet) and a measured pH of 9.9 and a total potassium content of 97.4 g / kg were acidified by slowly adding formic acid (9.7 g, 99%) under constant stirring at room temperature until a pH of 5.5 was reached. A reduced foaming behavior was observed and the temperature did not rise significantly to 24° C. After 10 minutes, 103.4 g of the almost clear solution were transferred directly into a suitable autoclave and, after sealing, the reaction mixture was stirred constantly and heated to 160° C. After reaching the desired internal temperature (about 5 bar pressure), the reaction mixture was stirred for a further 2.5 hours and then cooled to 70° C. within 40 minutes. The resulting slurry was filtered and the resulting solid was washed with 25.0 g of distilled water. The solid was dried at 50°C overnight to give 1.7 g of Met-DKP with a yield of 43% (purity of 88%).

[0052] Example 8: Acidification of the methionine sidestream solution by changing the pH with phosphoric acid and separation of Met-DKP

[0053] 100 g of a methionine side stream solution (3.0 wt% Met) containing 3.3 wt% MetMet (1.6 wt% DDLL-MetMet, 1.7 wt% DLLD-MetMet) and a measured pH of 8.3 and a total potassium content of 93.1 g / kg were acidified by slowly adding phosphoric acid (19.9 g, 85%) under constant stirring at room temperature until a pH of 5.0 was reached. Typical foaming behavior and a temperature increase to 32° C. were observed. After 10 minutes, 112.1 g of the resulting slurry were directly transferred to a suitable autoclave, which was sealed with constant stirring of the reaction mixture and heated to 160° C. After reaching the desired internal temperature (about 5 bar pressure), the reaction was stirred for a further 2.5 hours and then cooled to 70° C. over 40 minutes. The resulting slurry was filtered and the resulting solid was washed with 28.2 g of distilled water. The solid was dried at 50°C overnight to give 2.9 g of Met-DKP in 75% yield (80% purity).

[0054] Example 9: Acidification of the methionine sidestream solution by changing the pH with dilute sulfuric acid and separation of Met-DKP

[0055] 100 g of a methionine side stream solution (3.0 wt% Met) containing 3.3 wt% MetMet (1.6 wt% DDLL-MetMet, 1.7 wt% DLLD-MetMet) and a measured pH of 8.3 and a total potassium content of 93.1 g / kg were acidified by slowly adding dilute sulfuric acid (28.5 g, 30%) under constant stirring at room temperature until a pH of 5.5 was reached. A reduced foaming behavior and a temperature increase to 28° C. were observed. After 10 minutes, 121.7 g of the resulting slurry were directly transferred to a suitable sealed autoclave, the reaction mixture was stirred constantly and heated to 160° C. After reaching the desired internal temperature (about 5 bar pressure), the reaction was stirred for a further 2.5 hours and then cooled to 70° C. within 45 minutes. The resulting slurry was filtered and the resulting solid was washed with 27.0 g of distilled water. The solid was dried at 50°C overnight to give 4.9 g of Met-DKP in 64% yield (40% purity).

[0056] Example 10: Acidification of the methionine sidestream solution by changing the pH with sulfuric acid and separation of Met-DKP

[0057] 100 g of a methionine side stream solution (4.1 wt% Met) containing 3.9 wt% MetMet (1.8 wt% DDLL-MetMet, 2.1 wt% DLLD-MetMet) and a measured pH of 9.4 and a total potassium content of 103.2 g / kg were acidified by slowly adding concentrated sulfuric acid (3.6 g, 96%) under constant stirring at room temperature until a pH of 8.0 was reached. A reduced foaming behavior and a temperature increase to 31° C. were observed. After 10 minutes, 100.5 g of the resulting slurry were directly transferred to a suitable autoclave, which was sealed with constant stirring of the reaction mixture and heated to 160° C. After reaching the desired internal temperature (about 7 bar pressure), the reaction mixture was stirred for a further 2.5 hours and then cooled to 70° C. within 45 minutes. The resulting slurry was filtered and the resulting solid was washed with 27.5 g of distilled water. The solid was dried at 50°C overnight to give 2.0 g of undetermined solid (<1 wt% Met-DKP yield). HPLC analysis showed that only about 6 wt% of the MetMet was converted to Met-DKP.

[0058] Example 11: Attempt to directly use the methionine side flow solution to separate Met-DKP

[0059] 100 g of the methionine side stream solution (3.2 wt% Met) containing 3.4 wt% MetMet (1.6 wt% DDLL-MetMet, 1.8 wt% DLLD-MetMet), a measured pH of 8.4, a total potassium content of 88.8 g / kg were directly transferred to a suitable autoclave, the reaction mixture was stirred constantly and heated to 160°C after sealing. After reaching the desired internal temperature (about 7 bar pressure), the reaction mixture was stirred for a further 2.5 hours and then cooled to 70°C within 30 minutes. The resulting slurry was filtered and an attempt was made to wash the resulting solid with 23.7 g of distilled water. Unfortunately, after the initial washing step, no solid remained for drying and analysis. HPLC analysis showed that only about 1 wt% of the MetMet was converted to Met-DKP.

[0060] Analytical methods

[0061] High Pressure Liquid Chromatography (HPLC):

[0062] D,L-Met, DDLL-MetMet, DLLD-MetMet, DDLL-Met-DKP and DLLD-Met-DKP were quantified using an isocratic HPLC method after calibration with respective reference materials. All analyses were performed on an Agilent 1260 Infinity II device equipped with HPLC pump G7112B, VWD detector G7114A and column thermostat G7116A. Separation was performed using a standard RP 18 column (Altima C18, 5 mm, 25 cm length, 40 °C) running an elution system consisting of 900 g deionized water, 70 g acetonitrile and 40 g phosphoric acid (85%) at a flow rate of 1 mL / min and an elution time of 35 min. The detection wavelength was set at 210 nm. For sample preparation, 20-100 mg of sample was weighed into a volumetric flask equipped with an eluent system to the nearest 100 mL and degassed in an ultrasonic bath before injection.

[0063] Ion Chromatography (IC):

[0064] After calibration with reference substances, the total potassium content (total K content) in the initial potassium-containing aqueous medium was measured by IC. All analyses were carried out on a Metrohm 882 Compakt IC Plus system equipped with a Methrom 850 Professional IC Detektor LF unit, a Metrohm Metrosep C6 250 / 4.0 column and a Methrom Metrosep C6-Guard / 4.0 pre-column. A pure water eluent system containing 1.7 mmol / L nitric acid, 1.7 mmol / L pyridine dicarboxylic acid and 2.5% (v / v) acetonitrile was used to run the system at a flow rate of 1 mL / min at room temperature. For sample preparation, an appropriate amount of material was weighed into a volumetric flask containing a 2.5 mmol / L nitric acid solution, accurate to 100 mL, and degassed in an ultrasonic bath before injection.

Claims

1. A method for recovering methionylmethionine from an aqueous medium containing alkali metal ions, the medium containing at least 1% by weight of alkali metal ions, characterized in that The methionylmethionine contained in the aqueous medium is converted into a bis(methionyl)-diketopiperazine, which is subsequently crystallized or precipitated and then isolated.

2. The method according to claim 1, characterized in that For the conversion of methionylmethionine into bis(methionyl)-diketopiperazine, the medium is heated in a sealed pressure vessel to a temperature of 120-200°C, in particular 150-180°C, preferably 155-175°C, more preferably 160-170°C.

3. The method according to claim 2, characterized in that The reaction is carried out at a pressure of 2-12 bar, preferably 4-10 bar.

4. The method according to any one of the preceding claims, characterized in that The pH of the medium measured at 20° C. using a pH electrode is 3-7, more preferably 3.5-6, especially 4.0-5.

5.

5. The method according to claim 4, characterized in that The initial medium has a higher pH value, preferably a pH value of 7.5-12.5, more preferably 8-10, measured with a pH electrode at 20°C, and the pH value is adjusted to 3-7, preferably 3.5-6, more preferably 4.0-5.5 by acidifying the initial medium, wherein the acidification is preferably carried out by using an acid selected from phosphoric acid, acetic acid, formic acid, sulfuric acid or a mixture thereof.

6. A process according to any one of the preceding claims, wherein the alkali metal ion is selected from potassium and sodium ions and mixtures thereof, but is preferably potassium ion.

7. The process according to any one of the preceding claims, wherein the aqueous medium contains 1-12% by weight, preferably 2-8% by weight, more preferably 3-7% by weight of methionylmethionine.

8. The method according to any of the preceding claims, wherein the aqueous medium contains 2-18% by weight, preferably at least 4% by weight, in particular 4-16% by weight, more preferably at least 6% by weight, and especially 6-14% by weight of alkali metal ions, the alkali metal ions being preferably potassium ions.

9. The method according to any one of the preceding claims, wherein the aqueous medium contains methionine, preferably in an amount of 1-10% by weight, more preferably in an amount of 2-6% by weight.

10. The method according to any of the preceding claims, wherein the aqueous medium contains bicarbonate, preferably in an amount of at least 1 wt.-%, in particular 1-15 wt.-%, more preferably at least 2 wt.-%, in particular 2-12 wt.-%, most preferably in an amount of at least 3 wt.-%, in particular 3-10 wt.-%.

11. The method according to any of the preceding claims, wherein the aqueous medium is a complex matrix, in particular a by-product and / or a waste stream of a process for the production of the chemical methionine.

12. A process according to any one of the preceding claims, wherein before the conversion of methionylmethionine into bis(methionyl)-diketopiperazine by reaction in a sealed pressure vessel, most of the alkali metal ions contained in the medium are separated by forming insoluble alkali metal salts, preferably by adjusting the pH to 3-7, more preferably 4-5, with sulfuric acid, followed by separation of the potassium salt thus obtained.

13. The process according to any one of the preceding claims, wherein the bis(methionyl)-diketopiperazine formed in the reaction is isolated by filtration, centrifugation and / or decantation.

14. The process according to any one of the preceding claims, wherein after separation of the bis(methionyl)-diketopiperazine from the aqueous medium it is converted again into methionylmethionine by alkaline or acidic hydrolysis.

15. The process according to any one of claims 1 to 13, wherein after the bis(methionyl)-diketopiperazine has been separated from the aqueous solution, it is converted into methionine by alkaline or acidic hydrolysis.

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