A method for preparing disodium hydrogen phosphate based on ammonia buffer system self-stabilized pH

Abstract

This invention provides a method for preparing disodium hydrogen phosphate based on an ammonia buffer system with a self-stabilized pH, belonging to the field of phosphorus chemical technology. The method uses an ammonium dihydrogen phosphate solution as the reaction substrate, and adds a sodium carbonate solution to the substrate in a controlled dropping manner. A neutralization reaction is carried out at 38–48°C, preferably 42°C, so that at least part of the ammonia generated in situ dissolves in the system to form an ammonia-phosphate buffer system, thereby stabilizing the pH at 8.3–8.8. In the preferred formulation, 172.55 parts by mass of ammonium dihydrogen phosphate and 362.04 parts by mass of water are used to prepare the substrate, and 158.99 parts by mass of anhydrous sodium carbonate and 374.50 parts by mass of water are used to prepare the dropping solution. The sodium carbonate solution is added dropwise at a rate of approximately 100 mL / 15 min at 25 rpm. The reaction temperature is 42°C, the total reaction time is approximately 2.5 h, and the final pH is 8.44. After the reaction endpoint, deammoniation and evaporation crystallization at 80°C are performed, followed by filtration, washing, and drying at 105°C for 6 h to obtain white powdered disodium hydrogen phosphate. This invention can reduce the risk of localized over-alkaliness and trisodium phosphate impurities, and the reaction is mild and pH stable.

Description

Technical Field

[0001] This invention belongs to the field of phosphorus chemical technology, specifically relating to a method for preparing disodium hydrogen phosphate, and more particularly to a process for preparing disodium hydrogen phosphate by utilizing ammonia generated in situ in the reaction to form a buffer system, and achieving pH self-stabilization control through controlled dropwise addition of sodium carbonate solution, low-temperature ammonia preservation, deammoniation after endpoint, and evaporation crystallization. Background Technology

[0002] Disodium hydrogen phosphate is an important intermediate in the preparation of polyphosphates such as sodium pyrophosphate and sodium tripolyphosphate. It can also be used in washing, water treatment, food additives, and the preparation of precursors for new energy materials. Traditional processes often use the neutralization route of phosphoric acid and caustic soda. This route has high raw material costs and involves salt load and wastewater treatment pressure during neutralization, concentration, and post-treatment.

[0003] The theoretical reaction for preparing disodium hydrogen phosphate from ammonium dihydrogen phosphate and sodium carbonate is: NH4H2PO4 + Na2CO3 → Na2HPO4 + NH3↑ + CO2↑ + H2O. This route utilizes readily available ammonium dihydrogen phosphate and sodium carbonate as raw materials, and the byproduct ammonia gas can be recovered and reused, demonstrating good economic and clean production value.

[0004] However, in the existing operation, if sodium carbonate is added all at once or quickly, a momentary high pH zone is easily formed in the feeding area, resulting in the generation of alkaline impurities such as trisodium phosphate. If ammonia is continuously discharged or forced to be pumped out during the reaction, the ammonia buffering capacity of the reaction system is weakened, and the pH fluctuation increases. If the temperature is too high, ammonia escapes too quickly, making it difficult to establish a buffer system and increasing energy consumption. If the temperature is too low, the reaction rate and gas release rate decrease, increasing the burden on subsequent ammonia removal and drying.

[0005] Therefore, it is necessary to provide a method for preparing disodium hydrogen phosphate based on specific feed ratios and process parameters. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system, thereby solving the problems mentioned in the background section.

[0007] This invention is achieved through the following technical solution: a method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system, comprising the following steps:

[0008] Prepare ammonium dihydrogen phosphate solution and sodium carbonate solution;

[0009] Ammonium dihydrogen phosphate solution was placed in a pressure-protected reactor as the reaction base liquid. Sodium carbonate solution was added to the reaction base liquid in a controlled dropwise manner under stirring, so that ammonium dihydrogen phosphate and sodium carbonate could undergo a neutralization reaction.

[0010] During the reaction, the temperature is controlled at 38–48°C, and at least part of the ammonia generated in situ is dissolved in the liquid phase system to form an ammonia-phosphate buffer system, so that the pH of the reaction system is stabilized at 8.3–8.8.

[0011] After the addition is complete, the reaction is kept at a constant temperature until the pH stabilizes and no more carbon dioxide bubbles are produced. Then, the reaction is carried out after the endpoint is reached, followed by deamination and evaporation crystallization. After solid-liquid separation, washing and drying, disodium hydrogen phosphate is obtained.

[0012] In a preferred embodiment, the ammonium dihydrogen phosphate and sodium carbonate are fed in a molar ratio of 1:0.98 to 1.02, preferably in a molar ratio of 1:1.

[0013] In a preferred embodiment, the ammonium dihydrogen phosphate solution is prepared from 172.55 parts by mass of ammonium dihydrogen phosphate and 362.04 parts by mass of water; the sodium carbonate solution is prepared from 158.99 parts by mass of anhydrous sodium carbonate and 374.50 parts by mass of water.

[0014] In a preferred embodiment, the sodium carbonate solution is prepared by dissolving at 35–45°C, preferably at 40°C; the neutralization reaction temperature is preferably 42°C.

[0015] In a preferred embodiment, the controlled dropping rate of the sodium carbonate solution is 80-120 mL every 15 minutes, preferably about 100 mL every 15 minutes; the dropping pump speed is 20-30 rpm, preferably 25 rpm.

[0016] In a preferred embodiment, after the addition is completed, the reaction is continued at a constant temperature for 30 to 90 minutes, and the total reaction time is 2 to 3 hours, preferably 2.5 hours.

[0017] As a preferred embodiment, the reaction endpoint simultaneously meets the following conditions: the sodium carbonate solution is added dropwise to the theoretical feed amount, the pH of the system is stable at 8.3-8.8, and no continuous carbon dioxide bubbles are generated; preferably, the pH is 8.4-8.5 after the reaction is completed.

[0018] As a preferred embodiment, the post-endpoint deammoniation and evaporation crystallization includes: applying a slight negative pressure to the system for deammoniation after the reaction endpoint or simultaneously removing residual ammonia during evaporation crystallization, and absorbing and recovering the removed ammonia gas; subsequently, evaporation crystallization is carried out at 75-85°C, preferably at 80°C.

[0019] In a preferred embodiment, the drying temperature is 100-110°C and the drying time is 5-7 hours; preferably, drying at 105°C for 6 hours yields white powdered disodium hydrogen phosphate.

[0020] As a preferred embodiment, the reaction equation is: NH4H2PO4+Na2CO3→Na2HPO4+NH3↑+CO2↑+H2O; wherein, during the reaction stage, the local over-alkali and trisodium phosphate impurities are suppressed by controlling the dropping rate of sodium carbonate solution, the reaction temperature and the ammonia retention state.

[0021] After adopting the above technical solution, the beneficial effects of the present invention are as follows: by controlling the dropwise addition of sodium carbonate solution and maintaining ammonia at low temperature, excessively high local alkali concentration is avoided, reducing the risk of trisodium phosphate impurities; the in-situ ammonia generation in the reaction forms a buffer system, making the pH self-stabilized in the range of 8.3 to 8.8, without the need for frequent external acid and alkali adjustments; the reaction temperature is about 42°C, the reaction process is mild, energy consumption is low, and it is suitable for large-scale production; after the endpoint, by removing ammonia, evaporating and crystallizing at 80°C, filtering and washing, and drying at 105°C, the risk of ammonium residue and off-odor can be reduced, and a white powder product is obtained. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the process flow for preparing disodium hydrogen phosphate according to the present invention.

[0024] Figure 2 The image shows an XRD comparison between the sample obtained in Example 1 and the disodium hydrogen phosphate standard. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Please see Figure 1 This invention provides a technical solution: a method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system, comprising the following steps:

[0027] Prepare ammonium dihydrogen phosphate solution and sodium carbonate solution;

[0028] Ammonium dihydrogen phosphate solution was placed in a pressure-protected reactor as the reaction base liquid. Sodium carbonate solution was added to the reaction base liquid in a controlled dropwise manner under stirring, so that ammonium dihydrogen phosphate and sodium carbonate could undergo a neutralization reaction.

[0029] During the reaction, the temperature is controlled at 38–48°C, and at least part of the ammonia generated in situ is dissolved in the liquid phase system to form an ammonia-phosphate buffer system, so that the pH of the reaction system is stabilized at 8.3–8.8.

[0030] After the addition is complete, the reaction is kept at a constant temperature until the pH stabilizes and no more carbon dioxide bubbles are produced. Then, the reaction is carried out after the endpoint is reached, followed by deamination and evaporation crystallization. After solid-liquid separation, washing and drying, disodium hydrogen phosphate is obtained.

[0031] The ammonium dihydrogen phosphate and sodium carbonate are fed in a molar ratio of 1:0.98 to 1.02, preferably in a molar ratio of 1:1.

[0032] The ammonium dihydrogen phosphate solution was prepared by mixing 172.55 parts by mass of ammonium dihydrogen phosphate and 362.04 parts by mass of water; the sodium carbonate solution was prepared by mixing 158.99 parts by mass of anhydrous sodium carbonate and 374.50 parts by mass of water.

[0033] The sodium carbonate solution is prepared by dissolving at 35–45°C, preferably at 40°C; the neutralization reaction temperature is preferably 42°C.

[0034] The controlled dropping rate of the sodium carbonate solution is 80-120 mL every 15 minutes, preferably about 100 mL every 15 minutes; the dropping pump speed is 20-30 rpm, preferably 25 rpm.

[0035] After the addition is completed, continue the reaction at a constant temperature for 30-90 minutes, with a total reaction time of 2-3 hours, preferably 2.5 hours.

[0036] The reaction endpoint simultaneously meets the following conditions: sodium carbonate solution is added dropwise to the theoretical feed amount, the pH of the system is stable at 8.3-8.8, and no continuous carbon dioxide bubbles are generated; preferably, the pH is 8.4-8.5 after the reaction is completed.

[0037] The post-application-endpoint deammoniation and evaporation crystallization include: applying a slight negative pressure to the system for deammoniation after the reaction endpoint or simultaneously removing residual ammonia during evaporation crystallization, and absorbing and recovering the removed ammonia gas; subsequently, evaporation crystallization is carried out at 75–85°C, preferably at 80°C.

[0038] The drying temperature is 100-110℃, and the drying time is 5-7 hours; preferably, drying at 105℃ for 6 hours yields white powdered disodium hydrogen phosphate.

[0039] The reaction equation is: NH4H2PO4 + Na2CO3 → Na2HPO4 + NH3↑ + CO2↑ + H2O; During the reaction stage, the local over-alkaliness and the formation of trisodium phosphate impurities are suppressed by controlling the dropping rate of sodium carbonate solution, the reaction temperature, and the ammonia-preserving state.

[0040] As an embodiment of the present invention: In this invention, the molar mass of ammonium dihydrogen phosphate is calculated as 115.03 g / mol, and the molar mass of anhydrous sodium carbonate is calculated as 105.99 g / mol. If there are differences in the content of industrial raw materials, the feed mass should be calculated according to the content of the effective components to keep the molar ratio of effective ammonium dihydrogen phosphate to effective sodium carbonate within the range of 1:0.98 to 1.02.

[0041] Example 1: Controlled dropwise addition for the preparation of disodium hydrogen phosphate

[0042] Table 1 Formulation and process parameters for Example 1

[0043] project Ammonium dihydrogen phosphate Sodium carbonate (added dropwise) illustrate Molar mass (g / mol) 115.03 105.99 Sodium carbonate is calculated as anhydrous sodium carbonate. Molar quantity (mol) 1.5 1.5 The two are fed at a molar ratio of 1:1. Raw material mass (g) 172.55 158.99 Calculated based on effective components Water added (g) 362.04 374.50 Prepare aqueous solutions separately Total mass of solution (g) 534.59 533.49 The weight can be rounded to 534.6g and 533.5g according to the allowable error of the process. reaction temperature 42℃ — The reaction system was controlled at approximately 42°C. Total reaction time 2.5h — This includes the time of addition and the time for heat preservation reaction after the addition is completed. Drop acceleration — 25 rpm, approximately 100 mL / 15 min Controlled addition of sodium carbonate solution Reaction / Insulation after Dropping 30 minutes to 1.5 hours — Continue the reaction for at least 30 minutes, preferably continue to keep warm for a total of about 1.5 hours after the addition. pH at the end of the reaction 8.44 — Located in the ammonia-phosphate buffer stability range Post-processing Evaporate and crystallize at 80℃; filter and wash; dry at 105℃ for 6 hours. — A white powder product was obtained.

[0044] The specific steps are as follows: Weigh 172.55g of ammonium dihydrogen phosphate, add 362.04g of water, stir to dissolve fully, and obtain ammonium dihydrogen phosphate base solution; separately weigh 158.99g of anhydrous sodium carbonate, add 374.50g of water, stir to dissolve at 40℃, and obtain sodium carbonate solution.

[0045] The ammonium dihydrogen phosphate (MDP) base solution was added to a reactor equipped with stirring, temperature control, reflux condenser, pressure protection, and gas absorption interface. The temperature was raised and controlled at 42°C. After stirring was started, sodium carbonate solution was added dropwise at a pump rate of 25 rpm, approximately 100 mL every 15 minutes. During the dropwise addition, the MDP reacted with sodium carbonate as follows: NH₄H₂PO₄ + Na₂CO₃ → Na₂HPO₄ + NH₃↑ + CO₂↑ + H₂O.

[0046] During the reaction, the controlled dropwise addition of sodium carbonate solution avoided localized, instantaneous over-alkalinity. The ammonia generated in situ partially dissolved in the liquid phase under pressure-protected, closed or semi-closed conditions, forming a buffer environment with the phosphate system to maintain a stable pH. After the dropwise addition was completed, the reaction was maintained at the desired temperature for at least 30 minutes, preferably for a total of approximately 1.5 hours after the addition was completed, to ensure thorough homogenization of the reaction system. The total reaction time was approximately 2.5 hours. At the end of the reaction, the pH was measured to be 8.44, and no continuous carbon dioxide bubbles were produced.

[0047] After the reaction was completed, the reaction solution was subjected to ammonia removal treatment after the endpoint. The removed ammonia was recovered by water or dilute acid absorption solution. Then the system was heated to 80°C for evaporation and crystallization. After crystallization, the solution was filtered and washed, and then dried at 105°C for 6 hours to obtain white powdered disodium hydrogen phosphate.

[0048] XRD phase analysis

[0049] X-ray diffraction (XRD) was performed on the white powder product obtained in Example 1, and the results were compared with the XRD pattern of disodium hydrogen phosphate standard. The results are as follows: Figure 2 As shown.

[0050] Depend on Figure 2 It can be seen that the positions of the main diffraction peaks of the sample obtained in Example 1 basically correspond to the positions of the main diffraction peaks of the disodium hydrogen phosphate standard, indicating that the main crystalline phase of the product prepared by the method of the present invention is disodium hydrogen phosphate. Although there are some differences in the relative intensity of some diffraction peaks between the sample and the standard, the positions of the main characteristic peaks are highly consistent, which proves that the reaction and post-processing of the present invention can obtain the disodium hydrogen phosphate product.

[0051] In this embodiment, sodium carbonate was not in excess, the reaction temperature was lower than the high-temperature ammonia removal conditions, and the system pH was 8.44, which can reduce the risk of trisodium phosphate impurity formation and reduce the risk of ammonium residue and off-odor through post-treatment.

[0052] Example 2: Preparation of equimolar samples after raw material purity conversion

[0053] When using industrial-grade raw materials for ammonium dihydrogen phosphate or sodium carbonate, first test their effective content, then calculate the feed amount based on the effective components. For example, if the effective content of ammonium dihydrogen phosphate is 98.0% and the effective content of anhydrous sodium carbonate is 99.0%, based on 1.5 mol of effective substance, the actual weighed mass of ammonium dihydrogen phosphate is 172.55 / 0.980 = 176.07 g, and the actual weighed mass of anhydrous sodium carbonate is 158.99 / 0.990 = 160.60 g.

[0054] The converted ammonium dihydrogen phosphate was prepared as the base solution, and the converted anhydrous sodium carbonate was dissolved at 40°C to form the dropping solution; the remaining reaction temperature, dropping rate, heat preservation reaction, evaporation crystallization at 80°C, and drying conditions at 105°C were the same as in Example 1. This example illustrates that by converting the effective components, the acid-base ratio deviation caused by fluctuations in the purity of industrial raw materials can be avoided, thereby maintaining the pH self-stabilizing effect.

[0055] Example 3 Stable preparation under slight sodium carbonate deficiency conditions

[0056] Weigh 172.55 g of ammonium dihydrogen phosphate and add 362.04 g of water to prepare the base solution; weigh 155.81 g of anhydrous sodium carbonate, equivalent to 1.47 mol, and add 374.50 g of water to dissolve and prepare the dropping solution at 40 °C. The molar ratio of sodium carbonate to ammonium dihydrogen phosphate is 0.98:1.

[0057] Following the procedure of Example 1, sodium carbonate solution was added dropwise to ammonium dihydrogen phosphate substrate at 42°C in a controlled manner, at a rate of approximately 100 mL every 15 minutes. After the addition was complete, the reaction was maintained at this temperature. Due to a slight deficiency of sodium carbonate, the pH of the system was slightly lower than in Example 1, but it was still maintained within the weakly alkaline range required for the formation of disodium hydrogen phosphate. This example illustrates that the present invention allows operation under slightly alkaline conditions to reduce the risk of localized over-alkalinity and trisodium phosphate impurities.

[0058] Example 4: Preparation under scaled-up batch conditions

[0059] The feed ratio of Example 1 was scaled up 10 times. 1725.5g of ammonium dihydrogen phosphate was weighed and 3620.4g of water was added to prepare the base solution; 1589.9g of anhydrous sodium carbonate was weighed and 3745.0g of water was added to prepare a sodium carbonate solution at 40°C.

[0060] Ammonium dihydrogen phosphate solution was added to a reactor equipped with stirring and pressure protection. The temperature was controlled at 42°C. Sodium carbonate solution was continuously added dropwise to the solution using a metering pump. The flow rate of the pump was adjusted according to the principle of volume scaling up to ensure that the amount added per unit time was similar to the experimental scale. After the addition was completed, the temperature was maintained until the pH stabilized and no continuous carbon dioxide bubbles were generated. Then, the solution underwent endpoint treatment, including deammoniation, evaporation and crystallization at 80°C, filtration and washing, and drying at 105°C. This example illustrates that the present invention can be scaled up proportionally.

[0061] Comparative Example 1: Sodium carbonate solution was added rapidly.

[0062] Compared with Example 1, Comparative Example 1 still used 172.55g ammonium dihydrogen phosphate, 362.04g water, 158.99g anhydrous sodium carbonate and 374.50g water, but the sodium carbonate solution was added to the ammonium dihydrogen phosphate base solution quickly instead of the controlled dropwise addition method of 25 rpm and about 100mL / 15min.

[0063] Under these conditions, the local alkalinity of the sodium carbonate solution entering the reactor is relatively high, which can easily lead to a momentary high pH zone, thereby increasing the risk of generating alkaline impurities such as trisodium phosphate; at the same time, the release of carbon dioxide is more concentrated, and the reaction stability is not as good as in Example 1.

[0064] Comparative Example 2: Ammonia was continuously pumped out during the reaction stage.

[0065] Compared to Example 1, Comparative Example 2 involved continuous evacuation of ammonia during the dropwise addition of sodium carbonate solution. Because the ammonia generated in situ was prematurely removed from the liquid phase, the ammonia-phosphate buffer capacity decreased, leading to increased pH fluctuations in the reaction system. This necessitated more frequent adjustments to the sodium carbonate flow rate or the addition of acid or alkali, resulting in operational stability inferior to Example 1.

[0066] Comparative Example 3 High-Temperature Reaction

[0067] Compared to Example 1, Comparative Example 3 increased the reaction temperature to 65°C. The increased temperature accelerated ammonia escape, decreased the ammonia retention and buffering capacity of the reaction system, and made the pH rise more easily; simultaneously, carbon dioxide release was more intense, energy consumption was higher than in Example 1, and stability was poor during scale-up operations.

[0068] Comparative Example 4: Low-Temperature Reaction

[0069] Compared to Example 1, Comparative Example 4 reduced the reaction temperature to 25–30°C. At lower temperatures, the rates of sodium carbonate dissolution and neutralization decrease, carbon dioxide release is insufficient, and the time required for the system to reach a stable pH is prolonged. This increases the burden on subsequent deammoniation and evaporation crystallization, and raises the risk of ammonium residue in the material before drying.

[0070] Overall comparison of Experimental Example 1 with Comparative Examples 1-4

[0071] Based on the above embodiments and comparative examples, the process control characteristics and product stability were summarized, and the results are shown in Table 2.

[0072] Table 2 Overall Comparison of Example 1 and Comparative Examples 1-4

[0073] Main differences 42℃; controlled dropwise addition of sodium carbonate; ammonia-preserving buffer; deamination crystallization after endpoint. Add sodium carbonate solution quickly Ammonia is continuously pumped out during the reaction phase. Reaction temperature 65℃ Reaction temperature 25–30℃ Raw material ratio <![CDATA[NH4H2PO4 and Na2CO3 are in a ratio of 1:1]]> Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 pH stability pH approximately 8.44, with minimal fluctuation. Local pH levels tend to be high Decreased buffering capacity, larger pH fluctuations Ammonia escapes quickly, and pH rises easily. Extended stabilization time Gas release state <![CDATA[CO2 release is relatively stable, and part of NH3 dissolves to form a buffer]]> <![CDATA[The concentrated release of CO₂ leads to a relatively violent reaction]]> <![CDATA[Premature discharge of NH3]]> <![CDATA[NH3 and CO2 escape relatively quickly]]> <![CDATA[Slow CO2 release]]> Impurity risk The risk of localized over-alkaliness is low, and the risk of trisodium phosphate is low. High risk of localized over-alkaliness Improper adjustment increases the risk of impurities. An increase in pH leads to an increased risk of impurities. Increased risk of incomplete reaction or ammonium residue Post-processing burden Evaporate and crystallize at 80℃, then dry at 105℃; the product is a white powder. Homogenization and testing need to be strengthened. Deamination occurs earlier but pH control is more difficult. High energy consumption necessitates enhanced exhaust gas absorption. The deammoniation and drying processes are quite demanding. Overall evaluation The reaction is mild, pH is self-stabilizing, and it is easy to scale up. It is not conducive to stable production Not conducive to buffer self-stabilization High energy consumption and operational risks Low production efficiency

[0074] As shown in Table 2, Example 1, through precise equimolar feeding, controlled dropwise addition of sodium carbonate solution, ammonia retention reaction at 42℃, and deammoniation crystallization after the endpoint, stabilized the pH of the system at around 8.44. Compared with rapid addition, ammonia removal during the reaction stage, high-temperature reaction, and low-temperature reaction, Example 1 is more suitable for industrial production in terms of pH stability, impurity risk, stable gas release, energy consumption, and controllability of post-processing.

[0075] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system, characterized in that, Includes the following steps: Prepare ammonium dihydrogen phosphate solution and sodium carbonate solution; Ammonium dihydrogen phosphate solution was placed in a pressure-protected reactor as the reaction base liquid. Sodium carbonate solution was added to the reaction base liquid in a controlled dropwise manner under stirring, so that ammonium dihydrogen phosphate and sodium carbonate could undergo a neutralization reaction. During the reaction, the temperature is controlled at 38–48°C, and at least part of the ammonia generated in situ is dissolved in the liquid phase system to form an ammonia-phosphate buffer system, so that the pH of the reaction system is stabilized at 8.3–8.

8. After the addition is complete, the reaction is kept at a constant temperature until the pH stabilizes and no more carbon dioxide bubbles are produced. Then, the reaction is carried out after the endpoint is reached, followed by deamination and evaporation crystallization. After solid-liquid separation, washing and drying, disodium hydrogen phosphate is obtained.

2. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The ammonium dihydrogen phosphate and sodium carbonate are fed in a molar ratio of 1:0.98 to 1.02, preferably in a molar ratio of 1:

1.

3. A method for preparing disodium hydrogen phosphate based on a self-stabilizing pH using an ammonia buffer system as described in claim 1 or 2, characterized in that, The ammonium dihydrogen phosphate solution was prepared by mixing 172.55 parts by mass of ammonium dihydrogen phosphate and 362.04 parts by mass of water; the sodium carbonate solution was prepared by mixing 158.99 parts by mass of anhydrous sodium carbonate and 374.50 parts by mass of water.

4. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The sodium carbonate solution is prepared by dissolving at 35–45°C, preferably at 40°C; the neutralization reaction temperature is preferably 42°C.

5. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The controlled dropping rate of the sodium carbonate solution is 80-120 mL every 15 minutes, preferably about 100 mL every 15 minutes; the dropping pump speed is 20-30 rpm, preferably 25 rpm.

6. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, After the addition is completed, continue the reaction at a constant temperature for 30-90 minutes, with a total reaction time of 2-3 hours, preferably 2.5 hours.

7. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The reaction endpoint simultaneously meets the following conditions: sodium carbonate solution is added dropwise to the theoretical feed amount, the pH of the system is stable at 8.3-8.8, and no continuous carbon dioxide bubbles are generated; preferably, the pH is 8.4-8.5 after the reaction is completed.

8. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The post-application-endpoint deammoniation and evaporation crystallization include: applying a slight negative pressure to the system for deammoniation after the reaction endpoint or simultaneously removing residual ammonia during evaporation crystallization, and absorbing and recovering the removed ammonia gas; subsequently, evaporation crystallization is carried out at 75–85°C, preferably at 80°C.

9. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The drying temperature is 100-110℃, and the drying time is 5-7 hours; preferably, drying is carried out at 105℃ for 6 hours to obtain white powdered disodium hydrogen phosphate.

10. The method for preparing disodium hydrogen phosphate based on a self-stabilizing pH of an ammonia buffer system as described in claim 1, characterized in that, The reaction equation is: NH4H2PO4 + Na2CO3 → Na2HPO4 + NH3↑ + CO2↑ + H2O; during the reaction stage, the local over-alkali and trisodium phosphate impurities are suppressed by controlling the dropping rate of sodium carbonate solution, the reaction temperature, and the ammonia-preserving state.

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