A method for preparing trialkali metal salt of methylglycine-N,N-diacetic acid

Abstract

This invention discloses a method for preparing methylglycine-N,N-diacetic acid trialkali metal salt. The method involves preparing a methylglycine-N,N-diacetic acid (MGDN) solution using solid iminodiacetonitrile (IDAN) and liquid hydrogen cyanide and acetaldehyde solutions. This is followed by a two-step alkaline hydrolysis to prepare the methylglycine-N,N-diacetic acid trialkali metal salt solution. The optimized post-treatment improves the overall product yield, reduces byproduct formation, and enhances product quality and stability. The methylglycine-N,N-diacetic acid trialkali metal salt product prepared by this method has a yield >97%, an NTA.3Na content <0.08%, a Hazen color number of 20–280, and a total aldehyde residue of 5–20 ppm, including formaldehyde, acetaldehyde, and formaldehyde releasers.

Description

Technical Field

[0001] This invention relates to the field of organic chemical engineering, specifically to a trialkali metal salt of methylglycine-N,N-diacetic acid, and more specifically to a method for preparing a trialkali metal salt of methylglycine-N,N-diacetic acid using solid iminodiacetonitrile (IDAN), liquid hydrogen cyanide, and acetaldehyde solution. Background Technology

[0002] Chelating agents have excellent cleaning power in both household detergents and industrial cleaning agents, reducing the concentration of metal ions in aqueous solutions and enhancing cleaning ability. With the global trend of phosphorus bans and restrictions in recent years, the cleaning market has changed, and new green chelating agents such as MGDA and GLDA, which are non-toxic and biodegradable, have become advantageous factors and are emerging as new market drivers.

[0003] In existing industrial MGDA product technologies, the synthesis route for trisodium methylglycine diacetate (MGDA·3Na) can generally be divided into three directions. The first route, currently used by companies such as BASF and Noryon, uses alanine as a raw material, reacting it with hydrogen cyanide and formaldehyde to prepare an industrial-grade 40% MGDA·3Na aqueous solution or solid compositions of various specifications. The second route, used by some water treatment companies, involves the reaction of chloroacetic acid and alanine to synthesize an MGDA·3Na aqueous solution. Both of these routes currently have commercially available products. A third route involves a Strecker reaction of iminodiacetonitrile or iminodiacetic acid with cyanide and acetaldehyde, followed by hydrolysis to obtain the product. The reactants, iminodiacetonitrile or iminodiacetic acid, can be prepared from hydrogen cyanide, ammonia, and formaldehyde. Different technical routes have their own advantages and disadvantages. Overall, products prepared in a strongly alkaline environment have more byproducts that are relatively difficult to control. Reactions in acidic environments or acidic media usually require the use of additional acids (such as concentrated sulfuric acid) to lower the pH. Complex technical solutions are used to reduce the content of the toxic byproduct NTA in the product, resulting in a relatively low product yield.

[0004] For the third route, the purity of the iminodiacetonitrile raw material and cyanide-containing byproducts affect the overall synthesis yield and product purity of MGDN and MGDA. Compared to the first two routes, the overall process still has significant room for improvement and optimization. To achieve higher product yields and lower byproducts, companies such as BASF and Chongqing Ziguang have conducted relevant research on the synthesis of raw material IDAN, intermediate MGDN, and MGDA.

[0005] US5849950 investigated a synthetic route using iminodiacetonitrile as a raw material, with an overall MGDA yield of approximately 89%, but with a high impurity content, particularly NTA·3Na at 0.32%.

[0006] Methods CN101171226B and CN101171232B employ alkaline, staged hydrolysis of MGDN crystallization products to prepare trisodium methylglycine diacetate. In each stage, the hydrolysis temperature gradually increases. One method for preparing the MGDN intermediate involves synthesis via iminodiacetonitrile, purified hydrogen cyanide, and acetaldehyde. Iminodiacetonitrile is prepared via hexamethylenetetramine and hydrogen cyanide or via the reaction of hydroxyacetonitrile with ammonia. To reduce byproduct impurities, the intermediate MGDN needs to be purified by crystallization. The MGDN with higher purity after crystallization is then subjected to alkaline hydrolysis. However, the thermal stability of alkylglycinone-N,N-diacetonitrile remains unavoidable during hydrolysis. Other dissociation products exist in the alkaline medium, generating byproducts such as iminodiacetate (IDA), nitrotriacetate (NTA), carbonates, acetates, formates, glycolates, lactates, glycinates, alanines, and acetaldehyde.

[0007] CN102993034B describes a method that uses crude hydrogen cyanide and formaldehyde to first synthesize hydroxyacetonitrile, then reacts it with ammonia to synthesize an iminodiacetonitrile solution, and then reacts it with crude hydrogen cyanide and acetaldehyde to prepare methylglycinone diacetonitrile crystals. Nitrogen gas is then introduced and sodium hydroxide is refluxed for hydrolysis to obtain a trisodium methylglycinate diacetate solution. The synthesis system is in an acidic medium. However, many impurities inhibit the synthesis of iminodiacetonitrile, resulting in only 90-95% of the crystals effectively participating in the synthesis reaction, and a product yield of only 86%. This method fails to completely solve the problems of low conversion rate and residual by-products.

[0008] CN106928077B describes a system in which iminodiacetic acid, acetaldehyde, and 99% hydrogen cyanide are used as raw materials to obtain the intermediate methylglycine diacetonitrile, which is then prepared by alkaline hydrolysis. This method uses 99% iminodiacetic acid and 99% hydrogen cyanide to prepare methylglycine diacetic acid, and the reaction yield can reach up to 93%, with NTA impurities < 0.1%. However, it does not completely solve the problems of low conversion rate and by-product residue.

[0009] CN115710194A describes a preparation process for synthesizing an aqueous solution of 2-hydroxypropionitrile using acetaldehyde and hydrogen cyanide gas as catalysts, followed by the synthesis of an aqueous solution of methylglycine diacetate trialkali metal salt using a high-purity iminodiacetic acid-alkali metal aqueous solution. This process reduces costs by using the mixed gas after deammoniation and impurity removal via the Angle method with hydrogen cyanide. The use of iminodiacetic acid avoids the introduction of impurities from the raw materials and prevents further reaction between impurities and raw materials to generate new impurities, thus achieving the goal of reducing MGDA impurities.

[0010] CN116178187A discloses a method for preparing methylglycine-N,N-diacetic acid trialkali metal salt, comprising the step of producing methylglycine diacetonitrile / methylglycine diacetonitrile diacetic acid (MGDN) or a crude product mixture containing MGDN by reacting a crude hydrogen cyanide gas mixture prepared using methanol, ammonia, and air with an iminodiacetonitrile solution and / or an iminodiacetic acid alkali metal solution and an acetaldehyde solution, wherein the crude hydrogen cyanide gas mixture is obtained by adjusting the ammonia content to 10% (v / v) to 50% (v / v) of the initial ammonia content in the crude hydrogen cyanide gas mixture. The optimized process yields methylglycine-N,N-diacetic acid trialkali metal salt with a yield >95%, NTA.3Na content <0.1%, and total aldehyde residues (formaldehyde, acetaldehyde, and formaldehyde releasers) <100 ppm.

[0011] One potential direction for improvement in the above-mentioned synthesis process using iminodiacetonitrile as a raw material is the purity of the raw material. Traditional industrial technology synthesizes iminodiacetonitrile solid products with a content of 92-95%, which is synthesized through the reaction of hydroxyacetonitrile and ammonia. This process involves many side reactions, resulting in poor appearance quality of both the product and the mother liquor. Studies have shown that due to the chemical reaction equilibrium limitation in the synthesis reaction of iminodiacetonitrile, the conversion rate of hydroxyacetonitrile can generally only reach about 95%. The residual hydroxyacetonitrile in the reaction solution and the iminodiacetonitrile product are unstable under alkaline or weakly acidic conditions, and they polymerize into dimers, trimers, or polymers, as well as polymerize with each other to form brownish polymers that adhere to the product surface, affecting the appearance quality and downstream use.

[0012] For the synthesis of MGDN and MGDA, firstly, the purity and impurity control of raw materials must be addressed when using iminodiacetonitrile. Secondly, the byproduct of the reaction between iminodiacetonitrile and formaldehyde, methylenebisiminodiacetonitrile (MBIDAN), must be optimized. Thirdly, the thermal stability of alkylglycinonitrile-N,N-diacetonitrile must be controlled during the reaction process, as other dissociation products exist in alkaline media, generating other byproducts such as iminodiacetate (IDA), nitrotriacetate (NTA), carbonates, acetates, formates, glycolates, lactates, glycinates, alanines, acetaldehyde, and other substances.

[0013] In summary, in order to further optimize and improve the process of preparing methylglycine diacetic acid and its alkali metal products using iminodiacetonitrile as raw material, the synthesis process of MGDN and MGDA was optimized from the perspective of raw materials by optimizing the raw material composition, controlling raw material indicators, optimizing process parameters, improving the raw material conversion rate, improving the stability of intermediate MGDN, and optimizing the post-processing and impurity removal scheme, ultimately achieving the goal of increasing yield and improving product stability. Summary of the Invention

[0014] In view of this, the present invention provides a method for preparing methylglycine-N,N-diacetic acid trialkane metal salt. The method improves the raw material index of iminodiacetonitrile by optimizing the process, controls the raw materials from the source by using a pretreatment process, and uses liquid hydrogen cyanide and IDAN as raw materials. By optimizing the auxiliary agents and synthesis process, the method achieves high conversion rate of hydrogen cyanide and IDAN, improves the stability of MGDN intermediate, and achieves high yield, low color number and low residue of by-product impurities of MGDA product.

[0015] To solve the above technical problems, the present invention provides the following technical solution:

[0016] A method for preparing methylglycine-N,N-diacetic acid trialkali metal salt includes the following steps:

[0017] (1) Prepare an aqueous solution of the pre-prepared solid iminodiacetonitrile (IDAN);

[0018] (2) Liquid hydrogen cyanide and acetaldehyde solution are simultaneously added to the IDAN solution in step (1) to prepare methylglycinonitrile diacetonitrile (MGDN) solution;

[0019] (3) Add the MGDN solution and alkaline solution obtained in step (2) to the reactor at the same time, and prepare a mixture containing methylaminoamide-N,N-diacetamide alkali metal salt, methylglycine-N,N-diacetic acid trialkali metal salt, ammonia water and alkaline solution by a hydrolysis reaction.

[0020] (4) The mixture obtained in step (3) is subjected to a two-stage hydrolysis reaction to obtain a methylglycine-N,N-diacetic acid trialkali metal salt solution;

[0021] (5) The mixture obtained in step (4) is post-processed to obtain the methylglycine-N,N-diacetic acid trialkali metal salt solution product.

[0022] In one specific implementation, the solid iminodiacetonitrile (IDAN) used in step (1) has an iminodiacetonitrile content ≥99.5%, an acid value of 0.001~1.0mgKOH / g, and a methylenebisiminodiacetonitrile (MBIDAN) content ≤0.01%.

[0023] In one specific implementation, the method for preparing the solid iminodiacetonitrile (IDAN) includes the following steps: purifying commercially available industrial-grade 92-95% grayish-brown solid IDAN product or a brownish-brown IDAN reaction solution synthesized using hydroxyacetonitrile and ammonia water into a white crystalline solid through a pretreatment process.

[0024] The pretreatment process described is the purification of IDAN. There are no particular limitations on the purification process, as long as the IDAN can be treated to the range of iminodiacetonitrile content, acid value, and methylenebisiminodiacetonitrile MBIDAN content. As one embodiment, the IDAN solution can be purified into white crystalline IDAN solid through steps such as activated carbon decolorization, auxiliary agent removal, crystallization, and recrystallization.

[0025] In one specific implementation, the concentration of the IDAN aqueous solution in step (1) is 10-35 wt%, preferably 18-30 wt%; and / or, preferably, the pH of the IDAN solution is adjusted to 5.5-6.5 by acid-base adjustment; and / or, the temperature of the IDAN solution is 20-30°C.

[0026] In one specific implementation, in step (2), the molar ratio of the raw materials iminodiacetonitrile, acetaldehyde and hydrogen cyanide is 0.95-1.05:1.00-1.05:1.00-1.10.

[0027] In one specific implementation, in step (2), the reaction temperature for preparing the MGDN solution is 25–40°C.

[0028] In one specific implementation, in step (2), the MGDN solution is prepared by simultaneously adding acetaldehyde solution and hydrogen cyanide solution into the IDAN reaction solution. The acetaldehyde is added at a faster rate than the hydrogen cyanide, and the acetaldehyde solution is added to the IDAN reaction solution 15-30 minutes earlier than the hydrogen cyanide solution. The total addition time does not exceed 2 hours. After the addition is completed, the reaction is kept at the aforementioned reaction temperature for 1-2 hours.

[0029] In one specific implementation, in step (2), the content of hydrogen cyanide in the liquid hydrogen cyanide is ≥99%.

[0030] In one specific embodiment, in step (2), the reaction is preferably carried out in the presence of an adjuvant selected from one or more of sodium hydroxymethylsulfonate, sulfurous acid, sodium sulfite, potassium sulfite, sodium bisulfite, and potassium bisulfite, and the amount of the adjuvant is 0.1 to 1.0%, preferably 0.2 to 0.5%, based on the molar amount of iminodiacetonitrile (IDAN).

[0031] In one specific implementation, in step (3), the total reaction molar ratio of MGDN solution (in molar amount of IDAN) to alkaline solution (in molar amount of alkali) is 1:3.10 to 3.15, the mixing temperature of MGDN solution and alkaline solution is 30 to 45°C, the mixing time is 1 to 2 hours, and the alkaline solution is one or both of sodium hydroxide or potassium hydroxide aqueous solution.

[0032] In one specific implementation, in step (3), MGDN solution and alkaline solution are added to the reactor simultaneously in a molar ratio for hydrolysis.

[0033] In one specific implementation, in step (4), the temperature of the hydrolysis reaction is 85-105°C, preferably 90-95°C, and the hydrolysis reaction time is 3-6 hours, preferably 4-5 hours.

[0034] In one specific implementation, step (5) includes pressurization, ammonia removal, and decolorization. The crude methylglycine-N,N-diacetic acid trialkali metal salt solution is prepared and then subjected to pressurization post-treatment. The pressurization equipment is selected from one of a batch reactor, tubular reactor, or micro-channel reactor. The pressurization temperature is 150–200°C, preferably 160–185°C, and the pressurization time is 0.25–1.5 h, preferably 0.5–1 h. After pressurization, the methylglycine-N,N-diacetic acid trialkali metal solution also includes an ammonia removal step, which controls the free ammonia index of the product to <80 ppm through vacuum concentration or steam stripping. After ammonia removal, a decolorization post-treatment step is also included to reduce the color number of the product.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] From the perspective of the entire MGDA synthesis process, controlling the raw material parameters of IDAN and hydrogen cyanide prevents the formation of potential impurities and by-products from the source. By optimizing the auxiliary agents and synthesis process, high conversion rates of raw materials hydrogen cyanide and IDAN are achieved, improving the stability of the MGDN solution. A two-stage hydrolysis process is adopted to reduce the dissociation side reaction of MGDN intermediates and the formation of MBIDAN by-product impurities. A pressurized post-treatment process is used to reduce the risk of residual by-products in subsequent polymerization and products, improve product yield, and control the amount of toxic by-products, thus achieving dual optimization of the overall process from the source of raw materials and the process itself.

[0037] The method of this invention is simple and efficient, and the mass content of the trialkali metal salt of methylglycine-N,N-diacetic acid is 40%, the product yield is >97%, the NTA.3Na content is 0.01-0.08%, and the total aldehyde residue of formaldehyde, acetaldehyde and formaldehyde releasers is 5-20 ppm. Detailed Implementation

[0038] The following examples will further illustrate the mixtures, preparation, and applications provided by the present invention, but the present invention is not limited to the listed examples and should include any other known modifications within the scope of the claims of the present invention.

[0039] This invention provides a method for preparing the trialkali metal salt of methylglycine-N,N-diacetic acid. The method employs optimized processes to improve the raw material properties of iminodiacetonitrile and utilizes pretreatment processes to control the raw materials from the source. Traditional IDAN synthesis processes typically employ two-stage concentration in the crystallization process, resulting in low purity of the crystallized product after process control. While ordinary crystallization or recrystallization processes can remove some impurities from IDAN by dissolving solid IDAN in deionized water, increasing the main IDAN content by approximately 98%, they fail to completely reduce impurities such as residual MBIDAN and acid value, and some polymers or byproducts remain in the IDAN.

[0040] In one specific implementation plan, commercially available industrial-grade IDAN grayish-brown solid product (92-95%) or brownish-brown IDAN reaction solution synthesized from hydroxyacetonitrile and ammonia needs to be purified into a white crystalline solid through a pretreatment process. Then, the pre-prepared solid IDAN is formulated into a colorless and transparent aqueous solution.

[0041] In one specific implementation, the purification process of the pre-prepared high-purity iminodiacetonitrile described in this invention includes steps such as pre-decolorization, removal of impurities with additives, crystallization, and drying.

[0042] In one specific embodiment, 92-95% industrial-grade IDAN grayish-brown solid is directly dissolved in water at a temperature controlled at 40-50°C. In another specific embodiment, existing industrial processes are used to prepare an iminodiacetonitrile reaction solution by reacting a 40% or 50% hydroxyacetonitrile solution and an ammonia solution in a tubular reactor. There are no particular limitations on the industrial-grade IDAN raw material used in this invention.

[0043] In one specific implementation, the purification process includes the following steps:

[0044] (1) Add 1.0 to 3.0 wt% activated carbon (based on the total mass of solid IDAN) to an aqueous solution of iminodiacetonitrile or an aqueous reaction solution of iminodiacetonitrile, decolorize for 0.5 to 1.5 h at a decolorization temperature of 40 to 60 °C, and then filter to separate the activated carbon to obtain a yellow IDAN solution.

[0045] (2) Add an auxiliary agent to the IDAN solution obtained in step (1) to remove impurities. The auxiliary agent is selected from sulfur dioxide, sulfurous acid, sodium sulfite, and potassium sulfite, preferably sulfurous acid solution. The amount of the auxiliary agent added is 0.5 to 4.0 wt% (based on the total mass of solid IDAN). The pH of the raw material IDAN solution is 2 to 3. The reaction temperature is 25 to 35°C. The reaction time is 1 to 2 hours.

[0046] (3) Cooling crystallization, crystallization temperature 15-18℃, crystallization time 4-6h, to obtain white crystalline IDAN, solid-liquid separation of crystallization, the solid is washed multiple times with weakly alkaline water (low concentration of sodium hydroxide or sodium carbonate) with pH 7-8, and the solid IDAN is dried under low temperature and reduced pressure to obtain solid IDAN raw material.

[0047] In one specific implementation, to control lower acid values ​​and impurities in MBIDAN, a recrystallization-recycled process can be used until the IDAN main content is >99.5%, the acid value is 0.001-1.0 mgKOH / g, and the MBIDAN content is ≤0.01%. Preferably, the IDAN main content is >99.5%, the acid value is 0.001-0.05 mgKOH / g, and the MBIDAN content is 0-0.003%. In one specific implementation, the preferred steps are as follows: the undried white IDAN from the first crystallization is redissolved in high-purity water at a dissolution temperature of 50-55°C, the high-purity IDAN solution concentration is 40-45 wt%, the temperature is lowered to 15-18°C, and the crystallization time is 1-2 hours to obtain white crystalline IDAN. The crystallized solid and liquid are separated, and the solid IDAN is dried under low temperature and reduced pressure to obtain solid IDAN raw material.

[0048] The specific synthesis method steps of the technical solution provided by the present invention are further described in detail below:

[0049] In one specific implementation, in step (1), the pre-prepared solid iminodiacetonitrile (IDAN) is prepared into an aqueous solution with a concentration of 10–35 wt%, for example, including but not limited to 10%, 12%, 15%, 16%, 18%, 20%, 22%, 25%, 26%, 28%, 30%, 32%, and 35%, preferably 18–30 wt%. The pH of the IDAN solution is adjusted to 5.5–6.5 by acid-base micro-adjustment, and the temperature is controlled at 20–30°C.

[0050] The inventors of this invention have discovered that due to the instability of hydroxyacetonitrile and iminodiacetonitrile in alkaline or weakly acidic environments, they are prone to repolymerization to generate darker-colored impurities, resulting in a decrease in purity and affecting subsequent use. The high-purity IDAN solid prepared in step (1) can be stored at low temperature and protected from light. After being prepared into an aqueous solution, it can be used urgently to prevent the unstable transformation of raw materials in a weakly acidic environment, which would cause a decrease in the yield and quality of the subsequent synthesis of MGDN and MGDA products.

[0051] In step (2) of the present invention, the molar ratio of the raw materials iminodiacetonitrile, acetaldehyde and hydrogen cyanide is 0.95-1.05:1.00-1.05:1.00-1.10, for example including but not limited to using the molar ratio of iminodiacetonitrile, acetaldehyde and hydrogen cyanide = 1:1.03:1.05, 1:1.05:1.08, 1:1.04:1.10, 1:1.05:1.10.

[0052] In step (2) of this invention, the MGDN solution is prepared by simultaneously adding acetaldehyde solution and hydrogen cyanide solution into the IDAN reaction solution. The acetaldehyde is added faster than the hydrogen cyanide solution. The acetaldehyde solution is added to the IDAN reaction solution 15-30 minutes earlier than the hydrogen cyanide solution. The addition time of the acetaldehyde solution is 0.5-1.75 hours, and the total addition time does not exceed 2 hours. After the addition is completed, the reaction is maintained at the addition temperature for 1-2 hours.

[0053] The concentration of the acetaldehyde solution is 30-40 wt%, such as 30 wt%, 35 wt%, 40 wt%, etc.; the liquid hydrogen cyanide is mainly a high-purity raw material with a content >99%, and the source of hydrogen cyanide is not limited by the production process. The reaction temperature for preparing the MGDN solution is 25-40℃, such as including but not limited to 25℃, 30℃, 35℃, 40℃, preferably 25-35℃.

[0054] In one specific embodiment, step (2) is preferably carried out in the presence of an adjuvant selected from one or more of hydroxymethylsulfonic acid, sodium hydroxymethylsulfonate, sulfurous acid, sodium sulfite, potassium sulfite, sodium bisulfite, and potassium bisulfite, wherein the amount of the adjuvant is 0.1% to 1.0%, for example, including but not limited to 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 0.9%, 1.0%, preferably 0.2% to 0.5%, based on the molar amount of iminodiacetonitrile (IDAN).

[0055] The beneficial effects of adding the aforementioned additives can improve the conversion rate of raw materials, reduce the residue of nitriles and maintain the stability of the system, reduce the formation of by-products and the generation of unstable by-products during the hydrolysis of MGDN and MGDA, and improve the stability of intermediates and products.

[0056] In step (2), the changes in the content of the MGDN solution and the intermediate MGDN solution can be monitored by analysis.

[0057] The water content in the mixture of the present invention is determined by the Karl Fischer method known to those skilled in the art, by titration using a double ampere indication at the endpoint.

[0058] The free cyanide content in the MGDN solution mixture can be determined by silver nitrate potentiometric titration or by ion chromatography based on the IC principle, a method known to those skilled in the art. Cyanide detection is performed using an ammeter on a silver electrode, and quantification is achieved using the cyanide external standard method.

[0059] The content of hydroxyacetonitrile, iminodiacetonitrile, and hyponitrotriacetonitrile in MGDN solution mixtures can be determined by gas chromatography with external standard method.

[0060] In step (3), the MGDN solution and alkaline solution prepared in step (2) are added to the reactor simultaneously to prepare a mixture containing methylaminoamide-N,N-diacetamide alkali metal salt, methylglycine-N,N-diacetic acid trialkali metal salt, ammonia water and alkaline solution.

[0061] The total molar ratio of MGDN solution (based on IDAN molar amount) to alkaline solution is 1:3.10 to 3.15. The mixing temperature of MGDN solution and alkaline solution is 30 to 45°C, including but not limited to 30°C, 35°C, 40°C, and 45°C, and the mixing time is 1 to 2 hours, including 1 hour, 1.5 hours, and 2 hours.

[0062] The alkaline solution is a sodium hydroxide or potassium hydroxide solution, and the concentration of the alkaline solution is 20-50 wt%, for example, 20 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc.

[0063] Step (4) also includes a step of hydrolysis after heating; preferably, the temperature of the hydrolysis reaction after heating is 85-105℃, more preferably 90-95℃, and the hydrolysis reaction time is 3-6h, more preferably 4-5h.

[0064] In this preferred staged hydrolysis process, the MGDN solution and alkaline solution are mixed at 30–45°C in the first stage, resulting in partial hydrolysis to prevent decomposition and polymerization of the raw materials at excessively high temperatures. Then, the hydrolysis temperature is raised to 85–105°C to carry out the second stage of hydrolysis, which is beneficial for obtaining higher yields and lower impurities, while ensuring complete hydrolysis of any residual toxic cyanide in the system.

[0065] After steps (3) and (4), the methylglycine-N,N-diacetic acid trialkali metal salt solution obtained after the hydrolysis reaction still inevitably contains trace amounts of byproducts and polymers. The reaction solution is a pale yellow or yellow solution. The post-treatment process of further ammonia removal, concentration and decolorization is a step to optimize and improve the product.

[0066] In step (5), the crude solution of methylglycine-N,N-diacetic acid trialkali metal salt prepared in step (4) is subjected to pressure treatment. The pressure equipment is selected from one of the following: a batch reactor, a tubular reactor, and a micro-channel reactor. The pressure temperature is 150-200℃, preferably 160-185℃, and the pressure time is 0.25-1.5h, preferably 0.5-1h.

[0067] In one specific implementation, in step (5), the crude solution of methylglycine-N,N-diacetic acid trialkali metal salt prepared in step (4) is subjected to pressure treatment. The equipment is selected from a batch reactor, the pressure temperature is 160-170℃, and the pressure time is 1h.

[0068] In one specific implementation, in step (5), the crude solution of methylglycine-N,N-diacetic acid trialkali metal salt prepared in step (4) is subjected to pressure treatment. The equipment is selected from a tubular reactor, the pressure temperature is 170-180℃, and the pressure time is 0.75h.

[0069] In one specific implementation, in step (5), the crude solution of methylglycine-N,N-diacetic acid trialkali metal salt prepared in step (4) is subjected to pressure treatment. The equipment is selected from a microchannel reactor, the pressure temperature is 175-185℃, and the pressure time is 0.5h.

[0070] The step (5) also includes an ammonia removal step, in which the free ammonia index of the product is controlled to be <80ppm through a process of vacuum concentration or steam stripping. The ammonia gas and ammonia removal solution from the acid washing tower produced by the reaction are used to prepare ammonium sulfate through an ammonium sulfate recovery system.

[0071] The deammoniation process also includes a decolorization post-treatment step to reduce the product's color number. As a preferred method, 0.05–1 wt% activated carbon is added to the concentrated methylglycine-N,N-diacetic acid trialkali metal salt solution, and the solution is decolorized at 45–75°C for 0.5–2 hours. Existing decolorization processes such as membrane decolorization and resin adsorption can also be used.

[0072] The inventors of this invention have discovered that by controlling the quality indicators of the reaction raw materials, the conversion rate of the raw materials can be effectively improved and the selectivity of by-products can be reduced. This is beneficial to the synthesis and stability of the main product intermediate MGDN, and also to the improvement of the hydrolysis yield of MGDA. Through optimization of the feeding method of various materials and further improvement of the post-processing method, the conversion rate of raw materials is effectively improved, the intermediate is stable, and the separation of the intermediate MGDN is not required. The hydrolysis process and post-processing process can also effectively reduce the amount of by-products and polymers, and improve the stability of the product's chelating ability. In the above preparation process, the conversion rate of various raw materials is >99%, the total yield of methylglycine-N,N-diacetic acid trialkali metal salt is >97%, the NTA.3Na content is 0.01-0.08%, and the total aldehyde residue of formaldehyde, acetaldehyde and formaldehyde releasers is 5-20 ppm.

[0073] The analytical method used in this invention is further described in detail below:

[0074] IDAN in solid form and in aqueous solution, and the content of IDAN and MBIDAN: High performance liquid chromatography (HPLC) was used, a method well-known in the industry. A typical test method was as follows: Agilent SB-C18 column (4.6 x 150 mm) 5 μm, mobile phase A: water: B: acetonitrile = 80:20 (volume ratio, buffer solution pH = 5), detection wavelength 205 nm, flow rate 1.0 mL / min, column temperature 40 °C, injection volume 10 μL, quantification using external standard method based on IDAN and MBIDAN standards.

[0075] Industrial-grade IDAN solids and reagent-grade IDAN, and other raw materials for IDAN preparation, are tested using industrial-grade iminodiacetonitrile (GB / T 23958-2009) as the main content, moisture, sulfate, and other indicators.

[0076] Acid value content in solid IDAN: The potentiometric titration method was used. The typical test method is as follows: Prepare an automatic potentiometric titrator (Metrohm 905Titrando) equipped with a non-aqueous acid electrode. Weigh 4.0g of the sample, accurate to 0.1mg, add 100ml of methanol, pre-add 2ml of 0.01mol / L HCl, and titrate with 0.01mol / L sodium hydroxide standard solution on the potentiometric titrator until the potential jumps. Perform a blank experiment under the same conditions.

[0077] The free cyanide content in the MGDN mixture can be determined by silver nitrate potentiometric titration or by ion chromatography based on the IC principle, a method known to those skilled in the art. Cyanide detection is performed using an ammeter on a silver electrode, and quantification is achieved using the cyanide external standard method.

[0078] Determination of methanol, aminoacetonitrile, hydroxyacetonitrile, iminodiacetonitrile, and hypozonyltriacetonitrile content in MGDN mixture: Gas chromatography was used with an Agilent 7890B instrument and a flame ionization detector (FID). The recommended column was an HP-VOC capillary column with a stationary phase of 6% cyanopropyl-phenyl-polymethylsiloxane, an inner diameter of 0.32 mm, and a length of 60 m. The carrier gas N2 was 1.5 mL / min. The column temperature was initially 50 °C and held for 2 min, then increased to 80 °C at 5 °C / min, and then increased to 250 °C at 15 °C / min and held for 10 min. The vaporization chamber temperature was 150 °C, the detector temperature was 260 °C, the injection volume was 1 μL, the split ratio was 10:1, the hydrogen flow rate was 30 mL / min, the air flow rate was 400 mL / min, and the make-up gas flow rate was 25 mL / min. The methanol content standard curve was established by gas chromatography external standard method. For hydroxyacetonitrile, iminodiacetonitrile, aminoacetonitrile, hyponitrotriacetonitrile, etc., industrial grade or standard samples can be used to establish standard curves of 100 to 1000 mg / kg. MGDN samples were pre-diluted 5 to 50 times with acetonitrile solution according to the content.

[0079] The method for determining the content of MGDA·3Na is the ferric chloride complex potential titration method. A typical test procedure is as follows: Prepare an automatic potentiometric titrator (Metrohm 905Titrando) equipped with a platinum composite electrode. Weigh 0.50–1.0 g of the sample, accurate to 0.1 mg, add 50 ml of ultrapure water and 2 drops of phenolphthalein indicator. Adjust the pH of the sample system with 0.6 mol / L hydrochloric acid until the red color disappears. Titrate with 0.1 mol / L ferric chloride standard solution on the potentiometric titrator until a potential jump occurs.

[0080] Free ammonia: The chemical titration method for detecting ammonia nitrogen according to national standards was used.

[0081] NTA·3Na content: quantitative analysis using industry-standard methods such as ion chromatography or liquid chromatography with external standard.

[0082] Sulfite content determination: quantitative analysis using the industry-standard ion chromatography external standard method.

[0083] Total aldehyde content in the product: acetylacetone spectrophotometry and liquid chromatography (GB / T 35755-2017)

[0084] The main raw material sources in the embodiments and comparative examples of this invention are as follows; unless otherwise specified, all other raw materials are obtained from ordinary commercial channels or through factory self-production:

[0085] Imidodiacetonitrile (IDAN) (Industrial grade, national standard method >92%): Guang'an Chengxin Chemical Co., Ltd. Liquid hydrogen cyanide (>99%): Wanhua Chemical

[0086] Acetaldehyde solution (40%): Aladdin

[0087] Formaldehyde solution (37%): Comio

[0088] Ammonia solution (26.5%): Comio

[0089] Sulphite solution (6%): Aladdin

[0090] Sodium hydroxymethylsulfonate and sodium bisulfite: Aladdin

[0091] Hydroxyacetonitrile (40%): Synthesized using formaldehyde solution and liquid hydrogen cyanide, Wanhua Chemical.

[0092] Activated carbon: Powdered activated carbon, >100 mesh, Aladdin

[0093] Example 1: Pre-preparation of solid iminodiacetonitrile (IDAN) (using industrial-grade solid IDAN as raw material)

[0094] The product is derived from commercially available iminodiacetonitrile (IDAN) solids. The liquid phase method yields an IDAN content of 92.5%, a MBIDAN content of 4.9%, an acid value of 6.5 mg KOH / g, a moisture content of 2.2%, a sulfate content of 0.4%, and a water-insoluble content of 0.3%.

[0095] 400g of IDAN (92.5%) solid was added to 1200g of water and heated to 50℃ to dissolve into a brownish-yellow solution. 6g of activated carbon was added, and the mixture was stirred for 1 hour to decolorize. After decolorization, the activated carbon was separated by filtration to obtain a yellow IDAN solution. The solution was cooled to 30℃, and 265g of 6% sulfurous acid solution was added at once. The reaction pH of the raw material IDAN solution system was 2. After stirring at a constant temperature for 2 hours, the mixture was slowly cooled to crystallize at 15℃ for 4 hours to obtain white crystalline IDAN. The solid and liquid were separated, and the solid was washed three times with weakly alkaline water (low-concentration sodium carbonate solution) with a pH of 7-8 to obtain 320g of aqueous IDAN solid. 305g of high-purity IDAN solid was prepared by low-temperature vacuum drying. The liquid phase method yielded an IDAN main content of 99.5%, a MBIDAN content of 0.003%, an acid value of 0.25mgKOH / g, a moisture content of 0.2%, a sulfate content of 0.08%, and an insoluble matter content of 0.05%.

[0096] Example 2: Pre-preparation of solid iminodiacetonitrile (IDAN) (using industrial-grade solid IDAN as raw material)

[0097] 320g of the aqueous IDAN solid prepared under the same conditions as in Example 1 was added to a crystallization flask for secondary crystallization. 450g of desalinated pure water was added, and the mixture was stirred and heated to 55°C to redissolve it into a colorless solution. Then, the temperature was lowered to 15°C and crystallized for 2 hours to obtain white crystalline IDAN. The crystallized solid and liquid were separated, and the solid was washed three times with desalinated pure water to obtain 295g of aqueous IDAN solid. 282g of high-purity solid was prepared by low-temperature vacuum drying. The liquid phase method yielded an IDAN main content of 99.7%, a MBIDAN content of 0.0005%, an acid value of 0.005mgKOH / g, a water content of 0.1%, a sulfate content of 0.01%, and an insoluble matter content of 0.005%.

[0098] Example 3: Pre-preparation of iminodiacetonitrile (IDAN) solid (using hydroxyacetonitrile and ammonia to prepare IDAN solution as raw material).

[0099] A 40wt% aqueous solution of hydroxyacetonitrile was preheated to 80℃ and a 26.5% ammonia solution was preheated to 50℃. These solutions were simultaneously pumped into a tubular reactor using a metering pump at a theoretical molar ratio of 1:1. The residence time was 4 minutes, and the reaction temperature was controlled above 120℃. The reaction outlet was rapidly cooled to room temperature, and the pH of the system was adjusted to 2 with sulfuric acid to prepare an iminodiacetonitrile solution. This solution was then crystallized to obtain solid IDAN. The solid iminodiacetonitrile IDAN prepared by the above process had an IDAN content of 88.1% (mainly MBIDAN), a MBIDAN content of 5.3%, an acid value of 4.6 mg KOH / g, a moisture content of 4.5%, a sulfate content of 0.6%, and a water-insoluble matter content of 0.5%.

[0100] 400g of IDAN (88.1%) solid was added to 1200g of water and heated to 50℃ to dissolve into a brownish-yellow solution. 8g of activated carbon was added, and the mixture was stirred for 1 hour to decolorize. After decolorization, the activated carbon was separated by filtration to obtain a yellow IDAN solution. The solution was cooled to 25℃, and 265g of 6% sulfurous acid solution was added at once. The reaction pH of the raw material IDAN solution system was 2. After stirring at a constant temperature for 2 hours, the mixture was slowly cooled to crystallize at 18℃ for 6 hours, resulting in white crystalline IDAN. The solid and liquid phases were separated, and the solid was washed three times with weakly alkaline water (low-concentration sodium carbonate solution) with a pH of 7-8 to obtain 295g of aqueous IDAN solid. 278g of high-purity IDAN solid was prepared by low-temperature vacuum drying. The liquid phase method yielded an IDAN content of 99.5%, a MBIDAN content of 0.009%, an acid value of 0.38mgKOH / g, a moisture content of 0.25%, a sulfate content of 0.12%, and an insoluble matter content of 0.08%.

[0101] Example 4: Pre-preparation of iminodiacetonitrile (IDAN) solid (using hydroxyacetonitrile and ammonia to prepare IDAN solution as raw material).

[0102] 295g of the aqueous IDAN solid prepared under the same conditions as in Example 3 was added to a crystallization flask for secondary crystallization. 420g of desalinated pure water was added, and the mixture was stirred and heated to 55°C to redissolve it into a colorless solution. Then, the temperature was lowered to 15°C and crystallized for 2 hours to obtain white crystalline IDAN. The crystallized solid and liquid were separated, and the mixture was washed three times with desalinated pure water to obtain 278g of aqueous IDAN solid. 265g of high-purity solid was prepared by low-temperature vacuum drying. The liquid phase method yielded an IDAN main content of 99.7%, a MBIDAN content of 0.0006%, an acid value of 0.01mgKOH / g, a water content of 0.1%, a sulfate content of 0.03%, and an insoluble matter content of 0.009%.

[0103] Example 5: Synthesis process of trisodium methylglycine diacetate

[0104] Example 1: 95.5 g (1.0 mol) of 99.5% iminodiacetonitrile (MGDN) solid (IDAN) was dissolved in 222.4 g of water to form a 30% IDAN solution. The temperature was controlled at 25°C, and the initial pH was adjusted to 6.0. Simultaneously, 115.5 g (1.05 mol) of 40% acetaldehyde solution and 29.7 g (1.1 mol) of 99% liquid hydrogen cyanide solution were slowly added dropwise to the IDAN reaction solution. The acetaldehyde addition time was 0.75 h, and the hydrogen cyanide addition time was 1 h. The reaction temperature was slowly increased and controlled at 30–35°C. After the addition was completed, the solution was kept at this temperature for 1.5 h under reflux. After the reaction, a total of 462.8 g of MGDN was obtained as a pale yellow solution.

[0105] 420.0 g (3.15 mol) of 30% sodium hydroxide solution and the above-mentioned MGDN were simultaneously added to a four-necked flask according to the molar ratio. The dropping rate and reaction temperature were controlled at 40°C to prevent the decomposition and polymerization of raw materials and intermediates due to excessive local temperature. The total dropping time was 1.5 h. After holding at the temperature for 0.5 h, the temperature was raised to 95°C and hydrolysis was continued for 5 h.

[0106] The hydrolyzed solution at 95°C was continuously pumped into a tubular reactor using a metering pump. The residence time was 0.75 h, and the temperature was maintained at 175–180°C. The pressure and temperature were directly reduced at the outlet of the tubular reactor, while ammonia was concentrated and discharged for 0.5 h, controlling the free ammonia index to <80 ppm. After ammonia removal, the reaction solution was cooled to 60°C, and 3 g of activated carbon was added for decolorization for 1 h. After solid-liquid separation, 659.1 g of a pale yellow trisodium methylglycine diacetate solution was obtained, with a content of 40.02%. The product yield was 97.3% based on iminodiacetonitrile, the NTA.3Na content was 0.04%, and the total aldehyde content was 12.5 ppm.

[0107] Example 6: Synthesis process of trisodium methylglycine diacetate

[0108] Example 2: 95.3 g (1.0 mol) of 99.7% iminodiacetonitrile (MGDN) solid IDAN was prepared and dissolved in 286.0 g of water to form a 25% IDAN solution. The temperature was controlled at 30°C, and the initial pH was adjusted to 6.5. Simultaneously, 114.4 g (1.04 mol) of 40% acetaldehyde solution and 29.4 g (1.08 mol) of 99% liquid hydrogen cyanide solution were slowly added dropwise to the IDAN reaction solution. The acetaldehyde addition time was 1 h, and the hydrogen cyanide addition time was 1.5 h. The reaction temperature was slowly increased and controlled at 25–30°C. After the addition was completed, the solution was kept at this temperature for 2.0 h under reflux. After the reaction, a total of 524.8 g of MGDN was obtained as a pale yellow solution.

[0109] 420.0 g (3.15 mol) of 30% sodium hydroxide solution and the above-mentioned MGDN were simultaneously added to a four-necked flask according to the molar ratio. The dropping rate and reaction temperature were controlled at 35°C to prevent the decomposition and polymerization of raw materials and intermediates due to excessive local temperature. The total dropping time was 1.5 h. After holding at the temperature for 0.5 h, the temperature was raised to 95°C and hydrolysis was continued for 5 h.

[0110] The hydrolyzed solution at 95°C was pumped into a high-pressure autoclave reactor using a metering pump. The reaction solution was sealed and pressurized for 1 hour at a temperature of 170°C. After the reaction, the solution was transferred to a four-necked flask, cooled to 50°C, and concentrated under reduced pressure to remove ammonia for 1 hour, controlling the free ammonia level to <80 ppm. After ammonia removal, 5 g of activated carbon was added to the reaction solution, and the solution was decolorized for 1.5 hours. After solid-liquid separation, 661.2 g of a pale yellow trisodium methylglycine diacetate solution was obtained, with a content of 40.10%. The product yield was 97.8% based on iminodiacetonitrile, the NTA.3Na content was 0.03%, and the total aldehyde content was 8.8 ppm.

[0111] Example 7: Synthesis Process of Trisodium Methylglycine Diacetate

[0112] Example 3: 95.5 g (1.0 mol) of 99.5% iminodiacetonitrile (MGDN) solid was dissolved in 382.0 g of water to form a 20% MGDN solution. The temperature was controlled at 20°C, and the initial pH was adjusted to 5.5. After adding 0.28 g of sodium hydroxymethylsulfonate to the MGDN solution, 113.3 g (1.03 mol) of 40% acetaldehyde solution and 28.9 g (1.06 mol) of 99% liquid hydrogen cyanide solution were slowly added dropwise to the MGDN reaction solution. The acetaldehyde addition time was 1.25 h, and the hydrogen cyanide addition time was 2.0 h. The reaction temperature was slowly increased and controlled at 35–40°C. After the addition was completed, the temperature was maintained for 1.0 h under reflux. After the reaction, a total of 619.7 g of MGDN was obtained as a pale yellow solution.

[0113] 420.0 g (3.15 mol) of 30% sodium hydroxide solution and the above-mentioned MGDN were simultaneously added to a four-necked flask according to the molar ratio. The dropping rate and reaction temperature were controlled at 45°C to prevent the decomposition and polymerization of raw materials and intermediates due to excessive local temperature. The total dropping time was 1.5 h. After holding at the temperature for 0.5 h, the temperature was raised to 95°C and hydrolysis was continued for 4.5 h.

[0114] The hydrolyzed solution at 95°C was continuously pumped into a microchannel reactor using a metering pump. The residence time was 0.5 h, and the temperature was maintained at 180–185°C. The reactor outlet was directly depressurized and cooled, while ammonia was concentrated and discharged for 0.5 h, controlling the free ammonia index to <80 ppm. After ammonia removal, the reaction solution was cooled to 45°C, and 4 g of activated carbon was added. After decolorization for 2 h, 662.2 g of a pale yellow trisodium methylglycine diacetate solution was obtained, with a content of 40.08%. The product yield was 97.9% based on iminodiacetonitrile, the NTA.3Na content was 0.03%, and the total aldehyde content was 10.2 ppm.

[0115] Example 8: Synthesis process of trisodium methylglycine diacetate

[0116] Example 4: 95.3 g (1.0 mol) of 99.7% iminodiacetonitrile (MGDN) solid was dissolved in 286.0 g of water to form a 25% MGDN solution. The temperature was controlled at 25°C, and the initial pH was adjusted to 6.0. After adding 0.52 g of sodium bisulfite to the MGDN solution, 113.3 g (1.03 mol) of 40% acetaldehyde solution and 28.9 g (1.06 mol) of 99% liquid hydrogen cyanide solution were slowly added dropwise to the MGDN reaction solution simultaneously. The acetaldehyde addition time was 1 h, and the hydrogen cyanide addition time was 1.5 h. The reaction temperature was slowly increased and controlled at 30–35°C. After the addition was completed, the temperature was maintained for 1.5 h under reflux. After the reaction, a total of 523.5 g of MGDN was obtained as a pale yellow solution.

[0117] 420.0 g (3.15 mol) of 30% sodium hydroxide solution and the above-mentioned MGDN were simultaneously added to a four-necked flask according to the molar ratio. The dropping rate and reaction temperature were controlled at 30°C to prevent the decomposition and polymerization of raw materials and intermediates due to excessive local temperature. The total dropping time was 1.0 h. After holding at the temperature for 1.0 h, the temperature was raised to 95°C and hydrolysis was continued for 4 h.

[0118] The hydrolyzed solution at 95°C was pumped into a high-pressure autoclave reactor using a metering pump. The reaction solution was sealed and pressurized for 0.75 hours at a temperature of 180°C. After the reaction, the solution was transferred to a four-necked flask, cooled to 65°C, and concentrated under reduced pressure to remove ammonia for 1 hour, controlling the free ammonia level to <80 ppm. After ammonia removal, 5 g of activated carbon was added to the reaction solution, and the solution was decolorized for 0.5 hours. After solid-liquid separation, 662.0 g of a pale yellow trisodium methylglycine diacetate solution was obtained, with a content of 40.05%. The product yield was 97.8% based on iminodiacetonitrile, the NTA.3Na content was 0.05%, and the total aldehyde content was 8.0 ppm.

[0119] Comparative Example 1: IDAN raw material source: commercially available iminodiacetonitrile (IDAN) solid. Liquid phase method yielded IDAN with a main content of 92.5%, MBIDAN content of 4.9%, acid value of 6.5 mg KOH / g, moisture content of 2.2%, sulfate content of 0.4%, and water-insoluble matter of 0.3%. IDAN concentration and MGDN synthesis, hydrolysis reaction, and post-hydrolysis treatment conditions were consistent with Example 5, yielding 587.0 g of a yellow trisodium methylglycine diacetate solution with a content of 40.09%. The product yield based on iminodiacetonitrile was 86.8%, NTA·3Na content was 0.18%, and total aldehyde content was 92 ppm.

[0120] Comparative Example 2: IDAN Raw Material Source. The unpurified iminodiacetonitrile IDAN solid prepared in Example 3 had an IDAN content of 88.1% and a MBIDAN content of 5.3% by liquid-phase method, an acid value of 4.6 mg KOH / g, a moisture content of 4.5%, a sulfate content of 0.6%, and a water-insoluble matter content of 0.5%. The IDAN concentration and MGDN synthesis, hydrolysis reaction, and post-hydrolysis treatment conditions were consistent with Example 7, yielding 551.0 g of a pale yellow trisodium methylglycine diacetate solution with a content of 40.1%. The product yield based on iminodiacetonitrile was 81.5%, the NTA·3Na content was 0.29%, and the total aldehyde content was 105.0 ppm.

[0121] Comparative Example 3: The raw material for IDAN was commercially available iminodiacetonitrile (IDAN) solid. The purification process was identical to Comparative Example 1, with no decolorization or additives. Solid IDAN was obtained through dissolution and single crystallization. The liquid phase content was 98.0%, MBIDAN content was 0.8%, acid value was 1.5 mg KOH / g, water content was 1.0%, sulfate content was 0.2%, and water-insoluble matter was 0.15%. The IDAN concentration and MGDN synthesis, hydrolysis reaction, and post-hydrolysis treatment conditions were consistent with Example 7, yielding 639.7 g of a pale yellow trisodium methylglycine diacetate solution with a content of 40.05%. The product yield (based on iminodiacetonitrile) was 94.5%, NTA·3Na content was 0.17%, and total aldehyde content was 82.0 ppm.

[0122] Comparative Example 4: The raw material for IDAN was commercially available iminodiacetonitrile (IDAN) solid. During the purification process, 20g of 6% sulfurous acid solution was added, and other conditions were the same as in Comparative Example 1. Solid IDAN was obtained through dissolution and single crystallization, with a liquid phase main content of 98.5%, MBIDAN content of 0.65%, acid value of 1.6 mg KOH / g, water content of 0.9%, sulfate content of 0.2%, and water-insoluble matter of 0.13%. The IDAN concentration and MGDN synthesis, hydrolysis reaction, and post-hydrolysis treatment conditions were consistent with Example 7, yielding 640.5g of a pale yellow trisodium methylglycine diacetate solution with a content of 40.04%. The product yield based on iminodiacetonitrile was 94.6%, NTA·3Na content was 0.16%, and total aldehyde content was 75.0 ppm.

[0123] Comparative Example 5: The raw material for IDAN was commercially available iminodiacetonitrile (IDAN) solid. No activated carbon was added for decolorization during the purification process, and other conditions were the same as in Comparative Example 1. Solid IDAN was obtained through dissolution and single crystallization. The liquid phase content was 98.2%, MBIDAN content was 0.28%, acid value was 1.1 mg KOH / g, water content was 0.9%, sulfate content was 0.3%, and water-insoluble matter was 0.23%. The IDAN concentration and MGDN synthesis, hydrolysis reaction, and post-hydrolysis treatment conditions were consistent with Example 7, yielding 643.2 g of a pale yellow trisodium methylglycine diacetate solution with a content of 40.01%. The product yield based on iminodiacetonitrile was 94.9%, NTA·3Na content was 0.13%, and total aldehyde content was 55.0 ppm.

Claims

1. A method for preparing a trialkali metal salt of methylglycine-N,N-diacetic acid, comprising the following steps: (1) Prepare an aqueous solution from the pre-prepared solid iminodiacetonitrile (IDAN); (2) Liquid hydrogen cyanide and acetaldehyde solution are added to the IDAN solution in step (1) to prepare methylglycinonitrile diacetonitrile MGDN solution; (3) Add the MGDN solution and alkaline solution obtained in step (2) to the reactor at the same time, and prepare a mixture containing methylaminoamide-N,N-diacetamide alkali metal salt, methylglycine-N,N-diacetic acid trialkali metal salt, ammonia water and alkaline solution by a hydrolysis reaction. (4) The mixture obtained in step (3) is subjected to a two-stage hydrolysis reaction to obtain a methylglycine-N,N-diacetic acid trialkali metal salt solution; (5) The mixture obtained in step (4) is post-processed to obtain a methylglycine-N,N-diacetic acid trialkali metal salt solution product; The solid iminodiacetonitrile (IDAN) used in step (1) has an iminodiacetonitrile content ≥99.5%, an acid value of 0.001~1.0mgKOH / g, and a methylenebisiminodiacetonitrile (MBIDAN) content ≤0.01%.

2. The preparation method according to claim 1, characterized in that, The concentration of the IDAN aqueous solution in step (1) is 10-35 wt%; and / or, the pH of the IDAN aqueous solution is adjusted to 5.5-6.5 by acid-base adjustment; and / or, the temperature of the IDAN aqueous solution is 20-30°C.

3. The preparation method according to claim 2, characterized in that, The concentration of the IDAN aqueous solution in step (1) is 18-30 wt%.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (2), the molar ratio of the raw materials iminodiacetonitrile, acetaldehyde and hydrogen cyanide is 0.95-1.05:1.00-1.05:1.00-1.10; and / or, the reaction temperature for preparing the MGDN solution is 25-40℃, and the MGDN solution is prepared by simultaneously adding acetaldehyde solution and hydrogen cyanide solution into the IDAN reaction solution, and after the addition is completed, the reaction is kept at the aforementioned reaction temperature for 1-2 hours.

5. The preparation method according to any one of claims 1-3, characterized in that, In step (2), the reaction is carried out in the presence of an auxiliary agent selected from one or more of sodium hydroxymethylsulfonate, sulfurous acid, sodium sulfite, potassium sulfite, sodium bisulfite, and potassium bisulfite. The amount of the auxiliary agent is 0.1 to 1.0%, based on the molar amount of iminodiacetonitrile (IDAN).

6. The preparation method according to any one of claims 1-3, characterized in that, In step (3), the MGDN solution is measured in molar amounts of IDAN, the alkaline solution in molar amounts of alkali, and the total reaction molar ratio of the MGDN solution to the alkaline solution is 1:3.10 to 3.15; and / or, the mixing temperature of the MGDN solution and the alkaline solution is 30 to 45°C, and the mixing time is 1 to 2 hours; the alkaline solution is one or both of sodium hydroxide or potassium hydroxide aqueous solution.

7. The preparation method according to any one of claims 1-3, characterized in that, In step (4), the temperature of the hydrolysis reaction is 85-105℃; and / or the hydrolysis reaction time is 3-6h.

8. The preparation method according to any one of claims 1-3, characterized in that, In step (4), the hydrolysis reaction temperature is 90-95℃; and / or the hydrolysis reaction time is 4-5h.

9. The preparation method according to any one of claims 1-3, characterized in that, In step (5), the post-processing includes pressurization, ammonia removal, and decolorization.