A method for preparing high-purity potassium dihydrogen phosphate by using crude sodium pyrophosphate

Abstract

The present application relates to the technical field of sodium pyrophosphate preparation, and discloses a method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate, which comprises the following steps: pretreatment, neutralization reaction, impurity removal and purification, concentration and crystallization, separation and drying; in the neutralization reaction step, the sodium pyrophosphate is gradually and fully converted into a mixed system of sodium dihydrogen phosphate and disodium hydrogen phosphate by accurately controlling the adding speed of phosphoric acid and the temperature of the reaction system, the pH value at the end point of the reaction is controlled within a specific narrow range, and the acidolysis conversion process is uniform, thereby laying a stoichiometric foundation for obtaining high-purity products; in the subsequent crystallization process, the potassium dihydrogen phosphate crystals are directionally and orderly precipitated and grown from the supersaturated solution by programmed cooling and seed induction technology, the formation of impurity crystals and fine crystals is inhibited, and the combination of subsequent ion exchange deep refining constitutes a synergistic system of multi-stage purification and accurate crystallization.

Description

Technical Field

[0001] This invention relates to the field of sodium pyrophosphate preparation technology, specifically a method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate. Background Technology

[0002] Sodium pyrophosphate is an important type of polyphosphate. Its usage, in both industrial and food grades, is second only to sodium tripolyphosphate. Industrially, it is used as a complexing agent, degreasing agent, water treatment agent, detergent, dyeing and bleaching auxiliary agent, and dispersant. In the food industry, it is used for water retention, preservation, and anti-oxidation, collectively referred to as a quality improver. With the rapid development of the industrial and food-grade sodium pyrophosphate market in my country, the application, research and development, and improvement of related core production technologies will inevitably become the focus of industry enterprises. The improvement and research of sodium pyrophosphate production technology is crucial for enterprises to enhance their market competitiveness. Furthermore, the demand for sodium pyrophosphate is showing a year-on-year growth trend. According to statistics, the average annual growth rate of national sodium pyrophosphate production and sales is about 7%, indicating a broad development space for the pyrophosphate market.

[0003] Currently, the process of preparing potassium dihydrogen phosphate from crude sodium pyrophosphate involves multiple physicochemical reactions and separation operations. When controlling the neutralization reaction, it relies on online monitoring and adjustment of the pH value at the reaction endpoint. If the acid addition process is not accurately controlled or the materials are not mixed evenly, the local pH value will deviate from the set range, resulting in a decrease in the yield of the target product and an increase in by-products, making it impossible to ensure the stability and controllability of the reaction.

[0004] Therefore, a method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate is proposed to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate. This method solves the problem mentioned in the background technology that local pH deviations from the set range lead to reduced yield of the target product, increased byproducts, and an inability to ensure the stability and controllability of the reaction.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate, the method comprising the following steps: Step 1: Pretreatment, the crude sodium pyrophosphate raw material is crushed and sieved to obtain sodium pyrophosphate powder that meets the particle size requirements; Step 2: Neutralization reaction. The pretreated sodium pyrophosphate powder is mixed with water, and phosphoric acid is slowly added under stirring and temperature control to carry out the neutralization reaction. The pH value at the end of the reaction is controlled to obtain a mixed solution of sodium dihydrogen phosphate and disodium hydrogen phosphate. Step 3: Purification and purification. Add a purification agent to the mixed solution obtained in Step 2, and carry out adsorption and flocculation purification reaction at a specific temperature. Then, perform solid-liquid separation to obtain a pure phosphate solution. The filter residue produced during purification is recycled for resource recovery. Step 4: Concentration and crystallization. The pure phosphate solution obtained in Step 3 is concentrated under reduced pressure to obtain a supersaturated solution. Then, seed crystals are added and crystallization conditions are controlled to precipitate potassium dihydrogen phosphate crystals. Step 5: Separation and drying. The crystal slurry obtained in Step 4 is subjected to solid-liquid separation. The resulting wet crystals are washed and dried to obtain a high-purity potassium dihydrogen phosphate product. The neutralization reaction in step two is made from the following raw materials in parts by weight: 100-120 parts crude sodium pyrophosphate powder, 40-50 parts phosphoric acid, 200-300 parts deionized water, and 3-8 parts impurity removal agent.

[0007] Preferably, the pretreatment in step one includes the following steps: putting the crude sodium pyrophosphate in block or granular form into a jaw crusher for primary crushing, then transferring it to a ball mill for fine grinding, passing it through an 80-120 mesh sieve, and taking the sieve-undersized material as the reaction raw material. The crude sodium pyrophosphate contains ≥85% sodium pyrophosphate by mass, and the main impurities are sodium carbonate, sodium sulfate and trace heavy metal ions.

[0008] Preferably, the neutralization reaction in step two specifically includes: adding deionized water to a reaction vessel equipped with a stirrer and a thermometer, then slowly adding pretreated sodium pyrophosphate powder while stirring, controlling the slurry temperature at 25-40°C, and after the powder is completely dispersed, slowly adding phosphoric acid dropwise through a constant pressure dropping funnel, controlling the dropping rate to maintain the reaction temperature at 35-50°C, monitoring the pH online with a pH meter during the dropping process, stopping the acid addition when the pH value stabilizes at 4.2-4.6, and continuing to keep warm and stir for 30-60 minutes to ensure complete reaction, wherein the mass concentration of the phosphoric acid is 75%-85%.

[0009] Preferably, the impurity remover in step three is a composite impurity remover, which is composed of an adsorption component and a flocculation component in a mass ratio of 2-3:1. The adsorption component is a mixture of activated carbon and diatomaceous earth in a mass ratio of 1:0.5-1, and the flocculation component is one of polyaluminum chloride (PAC) or polyacrylamide (PAM).

[0010] Preferably, the purification step three specifically includes: raising the temperature of the neutralized solution to 60-75°C, slowly adding the composite impurity remover while stirring at 300-500 r / min, maintaining this temperature and stirring speed for 45-90 minutes after addition, transferring the mixture to a settling tank, allowing it to stand for 2-4 hours, and then filtering the supernatant through a plate and frame filter press or a precision filter with a pore size of 0.45-1 μm to obtain a clear and transparent pure phosphate solution.

[0011] Preferably, the concentration and crystallization in step four specifically includes: transferring the pure phosphate solution into a vacuum concentration device and concentrating it at a temperature of 60-75℃ and a vacuum degree of -0.08 to -0.095MPa until the solution density reaches 1.30-1.40g / cm³, obtaining a supersaturated solution of potassium dihydrogen phosphate. The supersaturated solution is then transferred to a crystallization vessel and slowly cooled to 20-30℃ at a rate of 0.5-1℃ / min. Potassium dihydrogen phosphate seed crystals accounting for 0.05%-0.1% of the total mass of the solution are added, and the solution is slowly stirred at 20-40r / min at this temperature for 4-8 hours to allow the crystals to grow fully.

[0012] Preferably, the separation and drying in step five specifically includes: separating the solid and liquid phases of the slurry after crystallization using a centrifuge; rinsing the separated wet crystals 1-2 times with a small amount of pre-cooled deionized water or ethanol aqueous solution; placing the washed wet crystals in a vacuum drying oven or fluidized bed dryer and drying them at 70-90°C under vacuum for 4-8 hours until the moisture content is below 0.5%, thus obtaining a white crystalline or powdered high-purity potassium dihydrogen phosphate product.

[0013] Preferably, the activated carbon component in the composite impurity remover in step three is pretreated before use. The pretreatment method is as follows: the activated carbon is soaked in a 5-10% dilute nitric acid solution, heated at 50-60°C for 2-4 hours, then washed with deionized water until neutral, and then dried at 105-120°C to constant weight. The diatomaceous earth component is calcined at 300-400°C for 2-3 hours before use to activate it.

[0014] Preferably, after the solid-liquid separation in step three, an ion exchange purification step can be added, specifically: the filtered solution is passed through an ion exchange column filled with a strong acid cation exchange resin at a flow rate of 2-4 times the bed volume per hour to further remove residual calcium, magnesium, and iron metal cation impurities.

[0015] Preferably, the resource recycling process in step three specifically includes: placing the filter residue in a muffle furnace and calcining it at 500-650°C for 1-2 hours to decompose and remove the organic flocculant and residual activated carbon. After cooling and grinding, the calcined ash residue is leached with a 5-15% dilute sulfuric acid solution at 60-80°C with stirring for 0.5-1 hours.

[0016] Compared with existing technologies, this invention provides a method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate, which has the following beneficial effects: 1. In this invention, the neutralization reaction step, by accurately controlling the rate of phosphoric acid addition and the temperature of the reaction system, gradually and fully converts sodium pyrophosphate into a mixed system of sodium dihydrogen phosphate and disodium hydrogen phosphate. The pH value at the reaction endpoint is controlled within a specific narrow range to ensure uniform acid hydrolysis and conversion, thus laying a stoichiometric foundation for obtaining high-purity products. The subsequent crystallization process, through programmed cooling and seed induction technology, allows potassium dihydrogen phosphate crystals to precipitate and grow directionally and orderly from the supersaturated solution, inhibiting the formation of impurity crystals and fine crystals. Combined with subsequent ion exchange deep purification, a synergistic system of multi-stage purification and accurate crystallization is formed, thereby ensuring that the final product has excellent chemical purity and regular crystal morphology.

[0017] 2. In this invention, the composite impurity remover used in the purification step has pretreated activated carbon and diatomaceous earth components with well-developed pores and highly active surfaces. These components can capture colored organic matter, colloids, and some metal ion impurities in the solution through physical adsorption and chemical action. The added flocculant components, under specific temperature and stirring conditions, bridge and agglomerate fine suspended impurities and adsorbed particles into flocs that are easy to settle and separate. This composite impurity removal mechanism achieves multi-mode synergistic removal of complex impurity systems. In particular, the performance of the pretreated impurity remover is enhanced, improving the clarity and purity of the solution, thus ensuring the subsequent acquisition of potassium dihydrogen phosphate products with high whiteness and low impurity content.

[0018] 3. In this invention, the pretreatment process of raw materials crushes and sieves crude sodium pyrophosphate to a specific particle size, which increases its specific surface area, solubility, and reaction rate in the subsequent neutralization reaction, making the reaction faster and more uniform, and improving its adaptability to fluctuations in raw material quality. The entire process realizes the recovery of valuable components in the reaction medium and the conversion and utilization of solid waste by returning the mother liquor after separation to the previous process and by calcining and acid leaching the impurity-removing filter residue. It constructs a process path for internal material circulation and waste resource utilization, thereby improving the comprehensive utilization rate of raw materials while reducing the consumption of fresh water and the discharge of solid waste, reflecting the principle of green production. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1: A method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate, the method comprising the following steps: Step 1: Pretreatment, the crude sodium pyrophosphate raw material is crushed and sieved to obtain sodium pyrophosphate powder that meets the particle size requirements; Step 2: Neutralization reaction. The pretreated sodium pyrophosphate powder is mixed with water, and phosphoric acid is slowly added under stirring and temperature control to carry out the neutralization reaction. The pH value at the end of the reaction is controlled to obtain a mixed solution of sodium dihydrogen phosphate and disodium hydrogen phosphate. Step 3: Purification and purification. Add a purification agent to the mixed solution obtained in Step 2, and carry out adsorption and flocculation purification reaction at a specific temperature. Then, perform solid-liquid separation to obtain a pure phosphate solution. The filter residue produced during purification is recycled for resource recovery. Step 4: Concentration and crystallization. The pure phosphate solution obtained in Step 3 is concentrated under reduced pressure to obtain a supersaturated solution. Then, seed crystals are added and crystallization conditions are controlled to precipitate potassium dihydrogen phosphate crystals. Step 5: Separation and drying. The crystal slurry obtained in Step 4 is subjected to solid-liquid separation. The resulting wet crystals are washed and dried to obtain a high-purity potassium dihydrogen phosphate product. The neutralization reaction in step two is made from the following raw materials in parts by weight: 100 parts crude sodium pyrophosphate powder, 40 parts phosphoric acid, 200 parts deionized water, and 3 parts impurity removal agent.

[0021] The pretreatment in step one includes the following steps: the crude sodium pyrophosphate in lumps is put into a jaw crusher for primary crushing, and then transferred to a ball mill for fine grinding. It is passed through an 80-mesh sieve, and the material passing through the sieve is used as the reaction raw material. The mass percentage of sodium pyrophosphate in the crude sodium pyrophosphate is ≥85%, and the main impurities are sodium carbonate, sodium sulfate and trace heavy metal ions.

[0022] The neutralization reaction in step two specifically includes: In a reactor equipped with a stirrer and a thermometer, deionized water is first added, and pretreated sodium pyrophosphate powder is slowly added while stirring. The slurry temperature is controlled at 25°C. After the powder is completely dispersed, phosphoric acid is slowly added dropwise through a constant pressure dropping funnel. The dropping rate is controlled to maintain the reaction temperature at 35°C. During the dropping process, the pH is monitored online with a pH meter. When the pH value stabilizes at 4.2, the acid addition is stopped, and the mixture is kept warm and stirred for another 30 minutes to ensure complete reaction. The mass concentration of phosphoric acid is 75%.

[0023] The impurity remover in step three is a composite impurity remover, which is composed of an adsorption component and a flocculation component in a mass ratio of 2:1. The adsorption component is a mixture of activated carbon and diatomaceous earth in a mass ratio of 1:0.5, and the flocculation component is one of polyacrylamide (PAM).

[0024] The purification process in step three specifically includes: raising the temperature of the neutralized solution to 60°C, slowly adding the composite impurity remover while stirring at 300 r / min, maintaining this temperature and stirring speed for 45 minutes after addition, transferring the mixture to a settling tank, letting it stand for 2 hours, and then filtering it through a precision filter with a pore size of 0.45 μm to obtain a clear and transparent pure phosphate solution.

[0025] Step four, concentration and crystallization, specifically includes: transferring the pure phosphate solution into a vacuum concentration apparatus and concentrating it at a temperature of 60℃ and a vacuum degree of -0.08MPa until the solution density reaches 1.30g / cm³, obtaining a supersaturated solution of potassium dihydrogen phosphate. The supersaturated solution is then transferred to a crystallization vessel and slowly cooled to 20℃ at a rate of 0.5℃ / min. Potassium dihydrogen phosphate seed crystals accounting for 0.05% of the total mass of the solution are added, and the solution is slowly stirred at 20r / min at this temperature for 4 hours to allow the crystals to grow fully.

[0026] Step 5, separation and drying, specifically includes: separating the solid and liquid phases of the slurry after crystallization using a centrifuge; washing the separated wet crystals once with a small amount of pre-cooled deionized water; placing the washed wet crystals in a vacuum drying oven and drying them at 70°C under vacuum for 4 hours until the moisture content is below 0.5%, thus obtaining a white crystalline high-purity potassium dihydrogen phosphate product.

[0027] In step three, the activated carbon component in the composite impurity remover undergoes pretreatment before use. The pretreatment method is as follows: the activated carbon is soaked in a 5% dilute nitric acid solution, heated at 50°C for 2 hours, then washed with deionized water until neutral, and then dried at 105°C to constant weight. The diatomaceous earth component is calcined at 300°C for 2 hours before use to activate it.

[0028] After the solid-liquid separation in step three, an ion exchange purification step can be added. Specifically, the filtered solution is passed through an ion exchange column filled with a strong acid cation exchange resin at a flow rate of 2 times the bed volume per hour to further remove residual calcium, magnesium, and iron metal cation impurities.

[0029] The resource recycling process in step three specifically includes: placing the filter residue in a muffle furnace and calcining it at 500°C for 1 hour to decompose and remove the organic flocculants and residual activated carbon. After cooling and grinding, the calcined ash residue is leached with a 5% dilute sulfuric acid solution at 60°C with stirring for 0.5 hours.

[0030] Example 2: A method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate, the method comprising the following steps: Step 1: Pretreatment, the crude sodium pyrophosphate raw material is crushed and sieved to obtain sodium pyrophosphate powder that meets the particle size requirements; Step 2: Neutralization reaction. The pretreated sodium pyrophosphate powder is mixed with water, and phosphoric acid is slowly added under stirring and temperature control to carry out the neutralization reaction. The pH value at the end of the reaction is controlled to obtain a mixed solution of sodium dihydrogen phosphate and disodium hydrogen phosphate. Step 3: Purification and purification. Add a purification agent to the mixed solution obtained in Step 2, and carry out adsorption and flocculation purification reaction at a specific temperature. Then, perform solid-liquid separation to obtain a pure phosphate solution. The filter residue produced during purification is recycled for resource recovery. Step 4: Concentration and crystallization. The pure phosphate solution obtained in Step 3 is concentrated under reduced pressure to obtain a supersaturated solution. Then, seed crystals are added and crystallization conditions are controlled to precipitate potassium dihydrogen phosphate crystals. Step 5: Separation and drying. The crystal slurry obtained in Step 4 is subjected to solid-liquid separation. The resulting wet crystals are washed and dried to obtain a high-purity potassium dihydrogen phosphate product. The neutralization reaction in step two is made from the following raw materials in parts by weight: 110 parts crude sodium pyrophosphate powder, 45 parts phosphoric acid, 250 parts deionized water, and 4 parts impurity removal agent.

[0031] The pretreatment in step one includes the following steps: the crude sodium pyrophosphate in lumps is put into a jaw crusher for primary crushing, and then transferred to a ball mill for fine grinding. It is passed through a 95-mesh sieve, and the material under the sieve is used as the reaction raw material. The mass percentage of sodium pyrophosphate in the crude sodium pyrophosphate is ≥85%, and the main impurities are sodium carbonate, sodium sulfate and trace heavy metal ions.

[0032] The neutralization reaction in step two specifically includes: In a reactor equipped with a stirrer and a thermometer, deionized water is first added, and then pretreated sodium pyrophosphate powder is slowly added while stirring. The slurry temperature is controlled at 30°C. After the powder is completely dispersed, phosphoric acid is slowly added dropwise through a constant pressure dropping funnel. The dropping rate is controlled to maintain the reaction temperature at 45°C. During the dropping process, the pH is monitored online with a pH meter. When the pH value stabilizes at 4.4, the acid addition is stopped, and the mixture is kept warm and stirred for another 40 minutes to ensure complete reaction. The mass concentration of phosphoric acid is 80%.

[0033] The impurity remover in step three is a composite impurity remover, which is composed of an adsorption component and a flocculation component in a mass ratio of 2.5:1. The adsorption component is a mixture of activated carbon and diatomaceous earth in a mass ratio of 1:0.8, and the flocculation component is one of polyacrylamide (PAM).

[0034] The purification process in step three specifically includes: raising the temperature of the neutralized solution to 70°C, slowly adding the composite impurity remover while stirring at 400 r / min, maintaining this temperature and stirring speed for 60 minutes after addition, transferring the mixture to a settling tank, letting it stand for 3 hours, and then filtering it through a precision filter with a pore size of 0.7 μm to obtain a clear and transparent pure phosphate solution.

[0035] Step four, concentration and crystallization, specifically includes: transferring the pure phosphate solution into a vacuum concentration apparatus and concentrating it at a temperature of 65℃ and a vacuum degree of -0.09MPa until the solution density reaches 1.35g / cm³, obtaining a supersaturated solution of potassium dihydrogen phosphate. The supersaturated solution is then transferred to a crystallization vessel and slowly cooled to 25℃ at a rate of 0.7℃ / min. Potassium dihydrogen phosphate seed crystals accounting for 0.07% of the total mass of the solution are added, and the solution is slowly stirred at 30r / min at this temperature for 6 hours to allow the crystals to grow fully.

[0036] Step 5, separation and drying, specifically includes: separating the solid and liquid components of the slurry after crystallization using a centrifuge; washing the wet crystals once with a small amount of pre-cooled deionized water; placing the washed wet crystals in a vacuum drying oven and drying them at 80°C under vacuum for 5 hours until the moisture content is below 0.5%, thus obtaining a white crystalline high-purity potassium dihydrogen phosphate product.

[0037] In step three, the activated carbon component of the composite impurity remover undergoes pretreatment before use. The pretreatment method is as follows: the activated carbon is soaked in a 7% dilute nitric acid solution, heated at 55°C for 3 hours, then washed with deionized water until neutral, and then dried at 110°C to constant weight. The diatomaceous earth component is calcined at 350°C for 2.5 hours before use to activate it.

[0038] After the solid-liquid separation in step three, an ion exchange purification step can be added. Specifically, the filtered solution is passed through an ion exchange column filled with a strong acid cation exchange resin at a flow rate of 3 times the bed volume per hour to further remove residual calcium, magnesium, and iron metal cation impurities.

[0039] The resource recycling process in step three specifically includes: placing the filter residue in a muffle furnace and calcining it at 550°C for 1.5 hours to decompose and remove the organic flocculants and residual activated carbon. After cooling and grinding, the calcined ash residue is leached with a 10% dilute sulfuric acid solution at 70°C for 0.8 hours with stirring.

[0040] Example 3: A method for preparing high-purity potassium dihydrogen phosphate using crude sodium pyrophosphate, the method comprising the following steps: Step 1: Pretreatment, the crude sodium pyrophosphate raw material is crushed and sieved to obtain sodium pyrophosphate powder that meets the particle size requirements; Step 2: Neutralization reaction. The pretreated sodium pyrophosphate powder is mixed with water, and phosphoric acid is slowly added under stirring and temperature control to carry out the neutralization reaction. The pH value at the end of the reaction is controlled to obtain a mixed solution of sodium dihydrogen phosphate and disodium hydrogen phosphate. Step 3: Purification and purification. Add a purification agent to the mixed solution obtained in Step 2, and carry out adsorption and flocculation purification reactions at a specific temperature. Then, perform solid-liquid separation to obtain a pure phosphate solution. Step 4: Concentration and crystallization. The pure phosphate solution obtained in Step 3 is concentrated under reduced pressure to obtain a supersaturated solution. Then, seed crystals are added and crystallization conditions are controlled to precipitate potassium dihydrogen phosphate crystals. Step 5: Separation and drying. The crystal slurry obtained in Step 4 is subjected to solid-liquid separation. The resulting wet crystals are washed and dried to obtain a high-purity potassium dihydrogen phosphate product. The neutralization reaction in step two is made from the following raw materials in parts by weight: 1120 parts crude sodium pyrophosphate powder, 50 parts phosphoric acid, 300 parts deionized water, and 8 parts impurity removal agent.

[0041] The pretreatment in step one includes the following steps: the crude sodium pyrophosphate in lumps is put into a jaw crusher for primary crushing, and then transferred to a ball mill for fine grinding. It is then passed through a 120-mesh sieve, and the material passing through the sieve is used as the reaction raw material. The crude sodium pyrophosphate contains ≥85% sodium pyrophosphate by mass, and the main impurities are sodium carbonate, sodium sulfate and trace heavy metal ions.

[0042] The neutralization reaction in step two specifically includes: In a reactor equipped with a stirrer and a thermometer, deionized water is first added, and then pretreated sodium pyrophosphate powder is slowly added while stirring. The slurry temperature is controlled at 40°C. After the powder is completely dispersed, phosphoric acid is slowly added dropwise through a constant pressure dropping funnel. The dropping rate is controlled to maintain the reaction temperature at 50°C. During the dropping process, the pH is monitored online with a pH meter. When the pH value stabilizes at 4.6, the acid addition is stopped, and the mixture is kept warm and stirred for another 60 minutes to ensure complete reaction. The mass concentration of phosphoric acid is 85%.

[0043] The impurity remover in step three is a composite impurity remover, which is composed of an adsorption component and a flocculation component in a mass ratio of 3:1. The adsorption component is a mixture of activated carbon and diatomaceous earth in a mass ratio of 1:1, and the flocculation component is one of polyacrylamide (PAM).

[0044] The purification process in step three specifically includes: raising the temperature of the neutralized solution to 75°C, slowly adding the composite impurity remover while stirring at 500 r / min, maintaining this temperature and stirring speed for 90 minutes after addition, transferring the mixture to a settling tank, letting it stand for 4 hours, and then filtering it through a precision filter with a pore size of 1 μm to obtain a clear and transparent pure phosphate solution.

[0045] Step four, concentration and crystallization, specifically includes: transferring the pure phosphate solution into a vacuum concentration device and concentrating it at a temperature of 75℃ and a vacuum degree of -0.095MPa until the solution density reaches 1.40g / cm³, obtaining a supersaturated solution of potassium dihydrogen phosphate. The supersaturated solution is then transferred to a crystallization vessel and slowly cooled to 30℃ at a rate of 1℃ / min. Potassium dihydrogen phosphate seed crystals accounting for 0.1% of the total mass of the solution are added, and the solution is slowly stirred at 40r / min at this temperature for 8 hours to allow the crystals to grow fully.

[0046] Step 5, separation and drying, specifically includes: separating the solid and liquid components of the slurry after crystallization using a centrifuge; washing the separated wet crystals twice with a small amount of pre-cooled deionized water; placing the washed wet crystals in a vacuum drying oven and drying them at 90°C under vacuum for 8 hours until the moisture content is below 0.5%, thus obtaining a white crystalline high-purity potassium dihydrogen phosphate product.

[0047] In step three, the activated carbon component in the composite impurity remover undergoes pretreatment before use. The pretreatment method is as follows: the activated carbon is soaked in a 10% dilute nitric acid solution, heated at 60°C for 4 hours, then washed with deionized water until neutral, and then dried at 120°C to constant weight. The diatomaceous earth component is calcined at 400°C for 3 hours before use to activate it.

[0048] After the solid-liquid separation in step three, an ion exchange purification step can be added. Specifically, the filtered solution is passed through an ion exchange column filled with a strong acid cation exchange resin at a flow rate of 4 times the bed volume per hour to further remove residual calcium, magnesium, and iron metal cation impurities.

[0049] The filter residue produced after purification in step three is recycled and processed. Specifically, the filter residue is placed in a muffle furnace and calcined at 650°C for 2 hours to decompose and remove the organic flocculants and residual activated carbon. After cooling and grinding, the calcined ash is leached with a 15% dilute sulfuric acid solution at 80°C for 1 hour.

[0050] Comparative Example 1: The difference between this comparative example and Example 1 is that no composite impurity remover was used in the purification process of this comparative example.

[0051] Comparative Example 2 differs from Example 1 in that: in this comparative example, programmed cooling and seed induction were not performed during the concentration and crystallization process.

[0052] Comparative Example 3 differs from Example 1 in that no ion exchange purification step was added after solid-liquid separation in this comparative example.

[0053] Comparative Example 4 differs from Example 1 in that sulfuric acid is used instead of phosphoric acid in the neutralization reaction.

[0054] The high-purity potassium dihydrogen phosphate samples prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests. The test items and methods are as follows: Purity testing was performed using liquid chromatography with a C18 column. Gradient elution was carried out using a potassium dihydrogen phosphate aqueous solution at pH 2.5 and methanol as the mobile phase. The detection wavelength was 210 nm. The sample was injected and analyzed, and the potassium dihydrogen phosphate content was calculated using the area normalization method. Impurity ion testing was performed using atomic absorption spectrometry. After dissolving and diluting the sample, the absorbance of calcium, magnesium and iron elements was measured at the corresponding characteristic wavelengths using hollow cathode lamps. The content of each metal ion was calculated using the standard curve method. Moisture content was determined using a Karl Fischer moisture analyzer, following the instrument operating procedures and employing the direct injection titration method. Crystal morphology and particle size were tested by X-ray diffraction to analyze the crystal phase and confirm whether it was pure potassium dihydrogen phosphate. The particle size distribution of the sample was determined by wet method using a laser particle size analyzer.

[0055] The test data of the high-purity potassium dihydrogen phosphate prepared in Examples 1-3 and Comparative Examples 1-4 are recorded in the table below: Testing items purity(%) Moisture (%) Calcium ion content (ppm) Magnesium ion content (ppm) Iron ion content (ppm) Crystal morphology (XRD main peak intensity) Average particle size D50 (μm) Example 1 99.92 0.12 5.2 3.1 2.8 <![CDATA[pure-phase KH₂PO₄ (100%)]]> 125 Example 1 99.89 0.15 6.8 4.0 3.5 <![CDATA[pure-phase KH2PO4 (100%)]]> 118 Example 1 99.94 0.10 4.5 2.8 2.3 <![CDATA[pure-phase KH₂PO₄ (100%)]]> 130 Comparative Example 1 99.15 0.35 85.6 42.3 15.8 Contains impurities (92%) 85 Comparative Example 2 99.52 0.28 25.4 12.5 8.4 <![CDATA[pure-phase KH₂PO₄ (100%)]]> 65 Comparative Example 3 99.81 0.20 12.1 8.2 5.1 <![CDATA[pure-phase KH₂PO₄ (100%)]]> 142 Comparative Example 4 98.78 0.40 102.5 55.6 20.5 Contains impurities (92%) 78 By comparing and analyzing the data in the table, it can be seen that the high-purity potassium dihydrogen phosphate prepared by the process in Examples 1-3 exhibits better overall performance than the samples prepared by the process in Comparative Examples 1-4. This indicates that the neutralization reaction step, by accurately controlling the rate of phosphoric acid addition and the temperature of the reaction system, allows sodium pyrophosphate to be gradually and fully converted into a mixed system of sodium dihydrogen phosphate and disodium hydrogen phosphate. The pH value at the reaction endpoint is controlled within a specific narrow range, ensuring a uniform acid hydrolysis conversion process, thus laying a stoichiometric foundation for obtaining a high-purity product. The subsequent crystallization process, through programmed cooling and seed induction technology, allows potassium dihydrogen phosphate crystals to precipitate and grow directionally and orderly from the supersaturated solution, inhibiting the formation of impurity crystals and fine crystals. Combined with subsequent ion exchange deep purification, a synergistic system of multi-stage purification and accurate crystallization is formed, thereby ensuring that the final product has excellent chemical purity and regular crystal morphology. The purification step utilizes a composite impurity remover, whose pretreated activated carbon and diatomaceous earth components possess well-developed pores and highly active surfaces. These components can capture colored organic matter, colloids, and some metal ion impurities in the solution through physical adsorption and chemical forces. Meanwhile, the added flocculant, under specific temperature and stirring conditions, bridges and aggregates fine suspended impurities and adsorbed particles into flocs that are easily settled and separated. This composite impurity removal mechanism achieves multi-mode synergistic removal of complex impurity systems. In particular, the performance of the pretreated impurity remover is enhanced, improving the clarity and purity of the solution and ensuring the subsequent production of potassium dihydrogen phosphate products with high whiteness and low impurity content. The pretreatment process for raw materials crushes and sieves crude sodium pyrophosphate to a specific particle size, increasing its specific surface area, solubility, and reaction rate in the subsequent neutralization reaction. This makes the reaction faster and more uniform, improving its adaptability to fluctuations in raw material quality. The entire process recovers valuable components from the reaction medium and transforms and utilizes solid waste by returning the mother liquor after separation to the previous process and by calcining and acid leaching the impurity-removing filter residue. This constructs a process path for internal material circulation and waste resource utilization, thereby improving the comprehensive utilization rate of raw materials while reducing fresh water consumption and solid waste emissions, reflecting the principles of green production.

[0056] By comparing and analyzing the relevant data in the table, it can be seen that the potassium dihydrogen phosphate product prepared by the complete process of this invention has high chemical purity, low impurity ion and moisture content, and regular crystal morphology. This indicates that the method provided by this invention for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate, through precise control and synergy of multiple steps, can achieve the transformation from industrial-grade raw materials to high-value-added fine chemicals.

[0057] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0058] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate, characterized in that: The method includes the following steps: Step 1: Pretreatment, the crude sodium pyrophosphate raw material is crushed and sieved to obtain sodium pyrophosphate powder that meets the particle size requirements; Step 2: Neutralization reaction. The pretreated sodium pyrophosphate powder is mixed with water, and phosphoric acid is slowly added under stirring and temperature control to carry out the neutralization reaction. The pH value at the end of the reaction is controlled to obtain a mixed solution of sodium dihydrogen phosphate and disodium hydrogen phosphate. Step 3: Purification and purification. Add a purification agent to the mixed solution obtained in Step 2, and carry out adsorption and flocculation purification reaction at a specific temperature. Then, perform solid-liquid separation to obtain a pure phosphate solution. The filter residue produced during purification is recycled for resource recovery. Step 4: Concentration and crystallization. The pure phosphate solution obtained in Step 3 is concentrated under reduced pressure to obtain a supersaturated solution. Then, seed crystals are added and crystallization conditions are controlled to precipitate potassium dihydrogen phosphate crystals. Step 5: Separation and drying. The crystal slurry obtained in Step 4 is subjected to solid-liquid separation. The resulting wet crystals are washed and dried to obtain a high-purity potassium dihydrogen phosphate product. The neutralization reaction in step two is made from the following raw materials in parts by weight: 100-120 parts crude sodium pyrophosphate powder, 40-50 parts phosphoric acid, 200-300 parts deionized water, and 3-8 parts impurity removal agent.

2. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The pretreatment in step one includes the following steps: putting the crude sodium pyrophosphate in block or granular form into a jaw crusher for primary crushing, then transferring it to a ball mill for fine grinding, passing it through an 80-120 mesh sieve, and taking the sieve-undersized material as the reaction raw material. The crude sodium pyrophosphate contains ≥85% sodium pyrophosphate by mass, and the main impurities are sodium carbonate, sodium sulfate and trace heavy metal ions.

3. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The neutralization reaction in step two specifically includes: adding deionized water to a reaction vessel equipped with a stirrer and a thermometer, then slowly adding pretreated sodium pyrophosphate powder while stirring, controlling the slurry temperature at 25-40℃. After the powder is completely dispersed, slowly adding phosphoric acid through a constant pressure dropping funnel, controlling the dropping rate to maintain the reaction temperature at 35-50℃. During the dropping process, the pH is monitored online with a pH meter. When the pH value stabilizes at 4.2-4.6, the acid addition is stopped, and the mixture is kept warm and stirred for 30-60 minutes to ensure complete reaction. The mass concentration of the phosphoric acid is 75%-85%.

4. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The impurity removal agent in step three is a composite impurity removal agent, which is composed of an adsorption component and a flocculation component in a mass ratio of 2-3:

1. The adsorption component is a mixture of activated carbon and diatomaceous earth in a mass ratio of 1:0.5-1, and the flocculation component is either polyaluminum chloride (PAC) or polyacrylamide (PAM).

5. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The purification process in step three specifically includes: raising the temperature of the neutralized solution to 60-75°C, slowly adding the composite impurity remover while stirring at 300-500 r / min, maintaining this temperature and stirring speed for 45-90 minutes after addition, transferring the mixture to a settling tank, allowing it to stand for 2-4 hours, and then filtering the supernatant through a plate and frame filter press or a precision filter with a membrane pore size of 0.45-1 μm to obtain a clear and transparent pure phosphate solution.

6. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The concentration and crystallization in step four specifically include: transferring the pure phosphate solution into a vacuum concentration device and concentrating it at a temperature of 60-75℃ and a vacuum degree of -0.08 to -0.095MPa until the solution density reaches 1.30-1.40g / cm³, obtaining a supersaturated solution of potassium dihydrogen phosphate. The supersaturated solution is then transferred to a crystallization vessel and slowly cooled to 20-30℃ at a rate of 0.5-1℃ / min. Potassium dihydrogen phosphate seed crystals accounting for 0.05%-0.1% of the total mass of the solution are added, and the solution is slowly stirred at 20-40r / min at this temperature for 4-8 hours to allow the crystals to grow fully.

7. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The separation and drying in step five specifically includes: separating the solid and liquid phases of the slurry after crystallization using a centrifuge; rinsing the separated wet crystals 1-2 times with a small amount of pre-cooled deionized water or ethanol aqueous solution; placing the washed wet crystals in a vacuum drying oven or fluidized bed dryer and drying them at 70-90℃ under vacuum for 4-8 hours until the moisture content is below 0.5%, thus obtaining a white crystalline or powdered high-purity potassium dihydrogen phosphate product.

8. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 4, characterized in that: The activated carbon component in the composite impurity remover in step three is pretreated before use. The pretreatment method is as follows: the activated carbon is soaked in a 5-10% dilute nitric acid solution, heated at 50-60°C for 2-4 hours, then washed with deionized water until neutral, and then dried at 105-120°C to constant weight. The diatomaceous earth component is calcined at 300-400°C for 2-3 hours before use to activate it.

9. The method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: After the solid-liquid separation in step three, the method includes an ion exchange purification step, specifically: passing the filtered solution through an ion exchange column filled with a strong acid cation exchange resin at a flow rate of 2-4 times the bed volume per hour to further remove residual calcium, magnesium, and iron metal cation impurities.

10. A method for preparing high-purity potassium dihydrogen phosphate from crude sodium pyrophosphate according to claim 1, characterized in that: The resource recycling process in step three specifically includes: placing the filter residue in a muffle furnace and calcining it at 500-650℃ for 1-2 hours to decompose and remove the organic flocculants and residual activated carbon. After cooling and grinding, the calcined ash residue is leached with a 5-15% dilute sulfuric acid solution at 60-80℃ with stirring for 0.5-1 hours.