Method for producing disodium hydrogen phosphate by purifying sodium pyrophosphate containing metal impurities

Abstract

The invention discloses a method for producing disodium hydrogen phosphate by purifying sodium pyrophosphate containing metal impurities, and belongs to the technical field of three-waste treatment, the metal impurities can be more thoroughly oxidized and complexed and separated by adopting the synergistic effect of photocatalysis and electrochemical methods, and compared with a traditional method, the product purity is higher; dependence on extreme conditions such as high temperature and high pressure is reduced to a certain extent, and energy consumption and equipment loss are reduced; by adjusting the photocatalyst and electrochemical parameters, specific metal impurities can be removed in a targeted manner, and the flexibility of the process is improved. The disodium hydrogen phosphate prepared by the method is high in purity, low in heavy metal content and uniform in product particle size distribution.

Description

Technical Field

[0001] The invention belongs to the technical field of three wastes treatment, and particularly relates to a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate. Background Art

[0002] Glyphosate mother liquor treatment is an industry-wide challenge. Whether produced by the glycine process or the IDA process, the resulting mother liquor is characterized by high COD, high TP, high salt content, complex composition, and resistance to degradation, making treatment challenging and costly. Currently, the mainstream mother liquor treatment process involves oxidation to degrade the mother liquor and recover phosphorus. These processes primarily include incineration, high-temperature oxidation, medium-temperature oxidation, and low-temperature oxidation. Membrane separation or evaporation and concentration are also used during mother liquor treatment to recover sodium chloride and phosphate. Examples include membrane separation to recover sodium chloride and glyphosate, as well as other substances; evaporation and concentration to recover glyphosate and sodium chloride; targeted conversion of concentrated mother liquor to recover sodium pyrophosphate; and catalytic oxidation to recover phosphate. Each method has its own advantages and disadvantages. Several major glyphosate manufacturers employ different treatment processes, and most companies combine these processes to treat glyphosate mother liquor. Currently, companies such as Xingfa Group, Xin'an Chemical, and Shandong Runfeng all have equipment for producing sodium pyrophosphate through the targeted conversion of concentrated mother liquor. The resulting crude sodium pyrophosphate product contains 40-70% sodium chloride, with sodium chloride content ranging from 3-35% depending on the processing technology of each company. It also contains impurities such as sodium carbonate, sodium bicarbonate, sodium sulfite, sodium bisulfite, and organic phosphorus. Whether using this route for desalination through membrane treatment or evaporation and concentration, the resulting crude sodium pyrophosphate contains a high level of sodium chloride. This product is high in impurities, exhibits poor color, has an odor, is low in sodium chloride content, and is of poor quality. This results in a low market price and limited applications, requiring further purification to improve both sodium chloride content and quality.

[0003] Some companies refine crude sodium pyrophosphate to obtain sodium pyrophosphate. However, during the production process, there are problems such as slow dissolution of the crude sodium pyrophosphate, high viscosity of the solution, low processing capacity, large amount of wastewater, high cost, and poor product quality. The phosphate product contains large amounts of sodium chloride and organophosphorus residues, resulting in poor quality. For example, CN206915770U describes a system for purifying phosphorus resources after deep treatment of glyphosate mother liquor. CN106809811A describes a method for preparing sodium pyrophosphate using crude sodium pyrophosphate, and CN106882781A describes a method for directly refining and extracting sodium pyrophosphate by decolorizing crude sodium pyrophosphate during purification to produce a sodium pyrophosphate product. However, during the processing of the crude sodium pyrophosphate, there are also problems such as slow dissolution of the crude sodium pyrophosphate, high viscosity of the solution, low processing capacity of the device, and high sodium chloride content in the sodium pyrophosphate product. Therefore, it is necessary to develop new process methods based on the characteristics of the materials.

[0004] A Chinese patent with publication number CN109399593A and publication date 2019-03-01 discloses a method for purifying crude sodium pyrophosphate to produce disodium hydrogen phosphate and sodium chloride, (1) ingredients and salt precipitation: crude sodium pyrophosphate and water are mixed in a hydrolysis kettle, the mass ratio of the crude sodium pyrophosphate and water being 1:0.5, phosphoric acid is added dropwise to control the pH value of the mixture to be in the range of 1.0-8.8, and after adding an oxidizing additive, the mixture is stirred at a temperature of 70-120°C for 5-60 minutes to dissolve and hydrolyze the crude sodium pyrophosphate. The crude sodium pyrophosphate is a crude sodium pyrophosphate product obtained by incinerating glyphosate mother liquor and diphosphine mother liquor wastewater, the main components of which are sodium pyrophosphate and sodium chloride. The main components are sodium pyrophosphate and sodium chloride, the sodium pyrophosphate content is 40-85%, the sodium chloride content is 3-35%, and the total organic carbon content is 0.1-0.3%. , also contains sodium carbonate, sodium bicarbonate, sodium sulfite, sodium bisulfite, organic phosphorus impurities, and the pH value of the aqueous solution is 9.5-10.55; (2) sodium chloride product recovery: the slurry in the hydrolysis kettle described in step (1) is discharged while hot and filtered to obtain sodium chloride product and filtrate; (3) disodium hydrogen phosphate recovery: the filtrate described in step (2) is cooled and mixed with alkali in a mixer, the alkali is one or a combination of sodium hydroxide, sodium carbonate, and sodium bicarbonate, and the pH value of the mixed solution is controlled to be in the range of 8-10 to generate disodium hydrogen phosphate. After the impurities are filtered out by a filter, it is sent to a crystallization kettle. After the slurry is cooled to 70-85°C, water and disodium hydrogen phosphate crystals are added to the crystallization kettle. The temperature is controlled at 0-30°C and cooled for crystallization for 2-8 hours to precipitate disodium hydrogen phosphate dodecahydrate. The fully crystallized slurry is filtered and separated to obtain disodium hydrogen phosphate dodecahydrate product. The purity of the disodium hydrogen phosphate dodecahydrate product obtained by this method is not high and contains metal impurities. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems of the prior art and provide a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate. The present invention adopts the synergistic effect of photocatalysis and electrochemical methods to more thoroughly oxidize and separate the metal impurities by complexation. Compared with traditional methods, the product purity is higher; the dependence on extreme conditions such as high temperature and high pressure is reduced to a certain extent, and energy consumption and equipment loss are reduced; by adjusting the photocatalyst and electrochemical parameters, specific metal impurities can be removed in a targeted manner, thereby improving the flexibility of the process.

[0006] The present invention is achieved through the following technical solutions: A method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate comprises the following steps: Step 1: adding crude sodium pyrophosphate and water into a dissolving kettle to obtain a sodium pyrophosphate solution; Step 2: transferring the sodium pyrophosphate solution to a photocatalytic reaction device to perform a photocatalytic oxidation reaction; Step 3: After the photocatalytic oxidation reaction is completed, the photocatalytically oxidized solution is introduced into an electrochemical device to perform an electrochemically assisted complexation reaction; Step 4: After the electrochemically assisted complexation reaction is completed, the electrochemically treated solution is transported to a hydrolysis reactor, and water and a catalyst are added to carry out a hydrolysis reaction; Step 5: After the hydrolysis is completed, the solution is transferred to a crystallizer for crystallization; Step 6: After the crystallization is completed, centrifugation is performed using a centrifugal separation device to obtain disodium hydrogen phosphate crystals; Step 7: Wash the disodium hydrogen phosphate crystals with cold water or ethanol, and dry the washed crystals through a drying device to obtain the final disodium hydrogen phosphate product.

[0007] Preferably, in the step 1, the concentration of sodium pyrophosphate in the solution is 20-30%, and the solution is stirred and dissolved at 50-70° C. with a stirring speed of 150-250 r / min.

[0008] Preferably, in step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 50-200W, and the ultraviolet wavelength is 254-365nm; the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 0.5-2g / Kg.

[0009] Preferably, in step 2, the wavelength of visible light in the photocatalytic reaction device is 365 nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 1-3 g / Kg.

[0010] Preferably, in the step 2, the temperature during the photocatalytic oxidation reaction is 20-50° C., and the time during the photocatalytic oxidation reaction is 30-120 minutes.

[0011] Preferably, in step three, the anode of the electrochemical device is a platinum electrode or a graphite electrode; the cathode is a stainless steel electrode.

[0012] Preferably, in step three, a complexing agent is added to the electrochemical device, and the complexing agent is ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 1-2g / Kg, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 1.5-2.5g / Kg.

[0013] Preferably, in step 3, the voltage of the power supply during the electrochemically assisted complexation reaction is 3-12V; the distance between the anode and the cathode is 10-50mm; and the current density is controlled to be 10-50 mA / cm 2 The temperature of the electrochemically assisted complexation reaction is 25-55°C, and the time of the electrochemically assisted complexation reaction is 0.5-2 h.

[0014] Preferably, in step 4, during hydrolysis, the mass ratio of sodium pyrophosphate to water is 1:1-2, the concentration of sulfuric acid is 50-98%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1-1.5:1.

[0015] Preferably, in step 4, the temperature during hydrolysis is 90-110° C., the pressure during hydrolysis is 0.1-0.3 MPa, and the time of hydrolysis is 30-120 min.

[0016] Preferably, in step five, the pressure in the crystallizer is controlled at 0.01-0.03 MPa, the temperature is controlled at 50-80° C., the crystallization time is 4-8 h, and the stirring speed is 50-200 r / min.

[0017] Preferably, in step six, the filtration is performed using a filter membrane with a pore size of 0.04-0.06 μm and a filtration temperature of 10-30°C.

[0018] Preferably, in step seven, the disodium hydrogen phosphate crystals are washed 2-3 times with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 80-100° C. for 2-3 hours to obtain the final disodium hydrogen phosphate product.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate. The crude sodium pyrophosphate containing metal impurities and desalted water are put into a dissolution kettle to dissolve the sodium pyrophosphate. The sodium pyrophosphate solution is transferred to a photocatalytic reaction device. The photocatalyst absorbs photon energy to generate electron-hole pairs. The holes have strong oxidizing properties and can oxidize low-valent iron ions and manganese ions in the solution into high-valent iron ions and manganese ions. The solution after photocatalytic oxidation enters an electrochemical device. Under the action of an electric field, an oxidation reaction occurs at the anode, further promoting the oxidation state transformation of the metal ions, making them more It readily complexes with a complexing agent. Simultaneously, a reduction reaction occurs at the cathode, producing hydrogen or other reduction products. The complexing agent forms a stable complex with the metal ion. Under the action of an electric field, the complex migrates toward the cathode and is separated from the solution by electrodeposition or other separation methods. After electrochemical treatment, the sodium pyrophosphate solution, free of metallic impurities, is transported to a hydrolysis reactor, where the sodium pyrophosphate is hydrolyzed under the action of a catalyst to produce disodium hydrogen phosphate. After the hydrolysis reaction is complete, the solution is transferred to a crystallizer, where the disodium hydrogen phosphate crystals are precipitated. A centrifugal separator separates the crystals from the crystallized solution. The separated crystals are washed with a small amount of cold water or ethanol to remove surface impurities and improve product purity. The washed crystals are then dried in a drying device to obtain the final disodium hydrogen phosphate product. The present invention uses the synergistic effect of photocatalysis and electrochemical methods to more thoroughly oxidize and separate metal impurities through complexation, resulting in higher product purity compared to traditional methods. It also reduces dependence on extreme conditions such as high temperature and high pressure to a certain extent, reducing energy consumption and equipment loss. By adjusting the photocatalyst and electrochemical parameters, specific metal impurities can be removed in a targeted manner, thereby improving process flexibility.

[0020] 2. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate. The method has strong adaptability to raw materials: it can process crude sodium pyrophosphate with iron and manganese ion contents up to 500 ppm; it has low energy consumption and cost: the photocatalytic reaction operates at a low temperature of 20-50°C, and the energy consumption is only 1 / 3 of the traditional heating method; the electrochemical complexation operates at room temperature (25-55°C) and micro-pressure (≤0.3MPa), reducing energy consumption by 40%. The complexing agent is recyclable, with an in-situ regeneration rate of ≥80%, reducing reagent costs by 50%. The process can be directly integrated into existing production lines, reducing renovation investment by 60%.

[0021] 3. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate, which reduces wastewater discharge by 85% through the mother liquor circulation system, and eliminates the high-salt wastewater generated by the traditional process. The separated metal complex precipitate can be recycled to produce iron red pigment or manganese oxide catalyst, achieving a hazardous waste resource recovery rate of 95%. The entire process is operated at atmospheric pressure or micro-pressure, completely avoiding the safety risks of high temperature and high pressure.

[0022] 4. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate, with improved purity: the purity of disodium hydrogen phosphate is ≥99.5%, and the heavy metal content is <0.5 ppm; crystal morphology: the optimized crystallization process (gradient cooling, seed induction) makes the disodium hydrogen phosphate crystal particle size distribution highly uniform (D50=100-200μm), whiteness ≥95%, and excellent fluidity, meeting the requirements of high-end food and pharmaceutical fields.

[0023] 5. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate, with process flexibility and customized impurity removal: it supports adjusting the wavelength of the light source, the catalyst loading By adjusting parameters such as , voltage (3-12V), and other parameters, it can remove specific metal impurities (such as preferentially removing Pb and Cd). The total reaction time is shortened to ≤6 hours (compared to ≥12 hours for traditional processes), and production capacity is doubled.

[0024] 6. The present invention provides a method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate, which is industrially and efficiently produced: the equipment has strong compatibility and can be quickly integrated into existing phosphate production lines; mild reaction conditions extend the equipment life to ≥8 years (traditional autoclaves only have 2-3 years); comprehensive energy consumption and cost reductions increase the profit per ton of product by 30%, and the investment payback period is <2 years. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below with reference to the examples, but the embodiments of the present invention are not limited thereto.

[0026] Examples 1 to 6 and Comparative Examples 1 to 38 all adopt the following method: A method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate comprises the following steps: Step 1: adding crude sodium pyrophosphate and water into a dissolving kettle to obtain a sodium pyrophosphate solution; Step 2: transferring the sodium pyrophosphate solution to a photocatalytic reaction device to perform a photocatalytic oxidation reaction; Step 3: After the photocatalytic oxidation reaction is completed, the photocatalytically oxidized solution is introduced into an electrochemical device to perform an electrochemically assisted complexation reaction; Step 4: After the electrochemically assisted complexation reaction is completed, the electrochemically treated solution is transported to a hydrolysis reactor, and water and a catalyst are added to carry out a hydrolysis reaction; Step 5: After the hydrolysis is completed, the solution is transferred to a crystallizer for crystallization; Step 6: After the crystallization is completed, centrifugation is performed using a centrifugal separation device to obtain disodium hydrogen phosphate crystals; Step 7: Wash the disodium hydrogen phosphate crystals with cold water or ethanol, and dry the washed crystals through a drying device to obtain the final disodium hydrogen phosphate product.

[0027] Wherein, in the step 1, the concentration of sodium pyrophosphate in the solution is 20-30%, and the solution is stirred and dissolved at 50-70° C., and the stirring speed is 150-250 r / min.

[0028] In the step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 50-200W, and the ultraviolet wavelength is 254-365nm; the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 0.5-2g / Kg.

[0029] Wherein, in the step 2, the wavelength of visible light in the photocatalytic reaction device is 365nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 1-3g / Kg.

[0030] Wherein, in the step 2, the temperature during the photocatalytic oxidation reaction is 20-50° C., and the time during the photocatalytic oxidation reaction is 30-120 minutes.

[0031] Wherein, in step three, the anode of the electrochemical device is a platinum electrode or a graphite electrode; the cathode is a stainless steel electrode.

[0032] In the step three, a complexing agent is added to the electrochemical device, and the complexing agent is ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 1-2g / Kg, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 1.5-2.5g / Kg.

[0033] In the step 3, the voltage of the power supply during the electrochemical assisted complexation reaction is 3-12V; the distance between the anode and the cathode is 10-50mm; the current density is controlled to be 10-50 mA / cm 2 The temperature of the electrochemically assisted complexation reaction is 25-55°C, and the time of the electrochemically assisted complexation reaction is 0.5-2 h.

[0034] Wherein, in the step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:1-2, the concentration of sulfuric acid is 50-98%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1-1.5:1.

[0035] Wherein, in the step 4, the temperature during hydrolysis is 90-110° C., the pressure during hydrolysis is 0.1-0.3 MPa, and the hydrolysis time is 30-120 min.

[0036] Wherein, in the step 5, the pressure in the crystallizer is controlled at 0.01-0.03 MPa, the temperature is controlled at 50-80° C., the crystallization time is 4-8 h, and the stirring speed is 50-200 r / min.

[0037] Wherein, in said step six, the filtration is carried out using a filter membrane with a pore size of 0.04-0.06 μm and a filtration temperature of 10-30°C.

[0038] In the step seven, the disodium hydrogen phosphate crystals are washed 2-3 times with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 80-100° C. for 2-3 hours to obtain the final disodium hydrogen phosphate product.

[0039] Example 1 In this embodiment, in step 1, the concentration of sodium pyrophosphate in the solution is 20%, and the solution is stirred and dissolved at 50° C. with a stirring speed of 250 r / min.

[0040] In the step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 50W, the ultraviolet wavelength is 254nm, the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 0.5g / Kg.

[0041] Wherein, in the step 2, the temperature during the photocatalytic oxidation reaction is 20° C., and the time during the photocatalytic oxidation reaction is 120 minutes.

[0042] Wherein, in step three, the anode of the electrochemical device is a platinum electrode; and the cathode is a stainless steel electrode.

[0043] Wherein, in the step three, a complexing agent is added to the electrochemical device, the complexing agent is ethylenediaminetetraacetic acid, and the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 1g / Kg.

[0044] In the step 3, the voltage of the power supply during the electrochemical assisted complexation reaction is 3V; the distance between the anode and the cathode is 10mm; the current density is controlled to be 10mA / cm 2 , the temperature of the electrochemically assisted complexation reaction is 25°C, and the time of the electrochemically assisted complexation reaction is 2h.

[0045] Wherein, in the step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:1, the concentration of sulfuric acid is 50%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1.5:1.

[0046] Wherein, in the step 4, the temperature during hydrolysis is 90° C., the pressure during hydrolysis is 0.1 MPa, and the hydrolysis time is 120 min.

[0047] Wherein, in the step 5, the pressure in the crystallizer is controlled at 0.01 MPa, the temperature is controlled at 50° C., the crystallization time is 8 h, and the stirring speed is 50 r / min.

[0048] Wherein, in said step six, the filtration is carried out using a filter membrane with a pore size of 0.04 μm and a filtration temperature of 10°C.

[0049] Wherein, in the step seven, the disodium hydrogen phosphate crystals are washed twice with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 80° C. and a drying time of 3 h to obtain the final disodium hydrogen phosphate product.

[0050] Example 2 In this embodiment, in the step 1, the concentration of the sodium pyrophosphate solution is 25%, and the solution is stirred and dissolved at 60° C. with a stirring speed of 200 r / min.

[0051] In the step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 125W, the wavelength of the ultraviolet light is 310nm, the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 1.25g / Kg.

[0052] Wherein, in the step 2, the temperature during the photocatalytic oxidation reaction is 35° C., and the time during the photocatalytic oxidation reaction is 75 minutes.

[0053] Wherein, in step three, the anode of the electrochemical device adopts a graphite electrode; the cathode adopts a stainless steel electrode.

[0054] Wherein, in the step three, a complexing agent is added to the electrochemical device, the complexing agent is ethylenediaminetetraacetic acid, and the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 1.5g / Kg.

[0055] In the step 3, the voltage of the power supply during the electrochemical assisted complexation reaction is 7.5 V; the distance between the anode and the cathode is 30 mm; the current density is controlled to be 30 mA / cm 2 , the temperature of the electrochemically assisted complexation reaction is 40°C, and the time of the electrochemically assisted complexation reaction is 1.25h.

[0056] Wherein, in the step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:1.5, the concentration of sulfuric acid is 74%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1.25:1.

[0057] Wherein, in the step 4, the temperature during hydrolysis is 100° C., the pressure during hydrolysis is 0.2 MPa, and the hydrolysis time is 75 min.

[0058] Wherein, in the step 5, the pressure in the crystallizer is controlled at 0.02 MPa, the temperature is controlled at 65° C., the crystallization time is 6 h, and the stirring speed is 125 r / min.

[0059] Wherein, in said step six, the filtration is carried out using a filter membrane with a pore size of 0.05 μm and a filtration temperature of 20°C.

[0060] In the step seven, the disodium hydrogen phosphate crystals are washed three times with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 90° C. and a drying time of 2.5 h to obtain the final disodium hydrogen phosphate product.

[0061] Example 3 In this embodiment, in step 1, the concentration of sodium pyrophosphate in the solution is 30%, and the solution is stirred and dissolved at 70° C. with a stirring speed of 250 r / min. Among them, in the step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 200W, the ultraviolet wavelength is 365nm, the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 2g / Kg.

[0062] Wherein, in the step 2, the temperature during the photocatalytic oxidation reaction is 50° C., and the time during the photocatalytic oxidation reaction is 30 minutes.

[0063] Wherein, in step three, the anode of the electrochemical device adopts a graphite electrode; the cathode adopts a stainless steel electrode.

[0064] Wherein, in the step three, a complexing agent is added to the electrochemical device, the complexing agent is ethylenediaminetetraacetic acid, and the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 2g / Kg.

[0065] In the step 3, the voltage of the power supply during the electrochemical assisted complexation reaction is 12V; the distance between the anode and the cathode is 50mm; the current density is controlled to be 50mA / cm 2 , the temperature of the electrochemically assisted complexation reaction is 55°C, and the time of the electrochemically assisted complexation reaction is 0.5h.

[0066] Wherein, in the step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:2, the concentration of sulfuric acid is 98%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1:1.

[0067] Wherein, in the step 4, the temperature during hydrolysis is 110° C., the pressure during hydrolysis is 0.3 MPa, and the hydrolysis time is 30 min.

[0068] Wherein, in the step 5, the pressure in the crystallizer is controlled at 0.03 MPa, the temperature is controlled at 80° C., the crystallization time is 4 h, and the stirring speed is 200 r / min.

[0069] Wherein, in said step six, the filtration is carried out using a filter membrane with a pore size of 0.06 μm and a filtration temperature of 30°C.

[0070] Wherein, in the step seven, the disodium hydrogen phosphate crystals are washed three times with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 100° C. and a drying time of h to obtain the final disodium hydrogen phosphate product.

[0071] Example 4 The difference between this embodiment and embodiment 1 is that: in the step 2, the wavelength of visible light in the photocatalytic reaction device is 365nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 1g / Kg.

[0072] Wherein, in the step 3, the complexing agent is diethylenetriaminepentaacetic acid, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 1.5g / Kg.

[0073] Example 5 The difference between this embodiment and embodiment 2 is that: in the step 2, the wavelength of visible light in the photocatalytic reaction device is 365nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 2g / Kg.

[0074] Wherein, in the step 3, the complexing agent is diethylenetriaminepentaacetic acid, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 2g / Kg.

[0075] Example 6 The difference between this embodiment and embodiment 3 is that: in the step 2, the wavelength of visible light in the photocatalytic reaction device is 365nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 3g / Kg.

[0076] Wherein, in the step 3, the complexing agent is diethylenetriaminepentaacetic acid, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 2.5g / Kg.

[0077] Comparative Example 1 The difference between this comparative example and Example 1 is that in step 1, the concentration of sodium pyrophosphate in the solution is 15%.

[0078] Comparative Example 2 The difference between this comparative example and Example 1 is that in step 1, the concentration of sodium pyrophosphate in the solution is 35%.

[0079] Comparative Example 3 The difference between this comparative example and Example 1 is that in step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 40W.

[0080] Comparative Example 4 The difference between this comparative example and Example 3 is that in step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 220W.

[0081] Comparative Example 5 The difference between this comparative example and Example 1 is that in step 2, the ratio of titanium dioxide to sodium pyrophosphate is 0.3 g / Kg.

[0082] Comparative Example 6 The difference between this comparative example and Example 3 is that in step 2, the ratio of titanium dioxide to sodium pyrophosphate is 2.2 g / Kg.

[0083] Comparative Example 7 The difference between this comparative example and Example 4 is that in step 2, the ratio of carbon nitride to sodium pyrophosphate is 0.8 g / Kg.

[0084] Comparative Example 8 The difference between this comparative example and Example 6 is that in step 2, the ratio of carbon nitride to sodium pyrophosphate is 3.2 g / Kg.

[0085] Comparative Example 9 The difference between this comparative example and Example 1 is that in step 3, the ratio of EDTA to sodium pyrophosphate is 0.8 g / kg.

[0086] Comparative Example 10 The difference between this comparative example and Example 3 is that in step 3, the ratio of EDTA to sodium pyrophosphate is 2.2 g / kg.

[0087] Comparative Example 11 The difference between this comparative example and Example 4 is that in step 3, the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 1.3 g / Kg.

[0088] Comparative Example 12 The difference between this comparative example and Example 6 is that in step 3, the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 2.7 g / Kg.

[0089] Comparative Example 13 The difference between this comparative example and Example 1 is that in step 3, the voltage of the power supply during the electrochemically assisted complexation reaction is 2.5V.

[0090] Comparative Example 14 The difference between this comparative example and Example 3 is that in step 3, the voltage of the power supply during the electrochemically assisted complexation reaction is 12.5V.

[0091] Comparative Example 15 The difference between this comparative example and Example 1 is that in step 3, the distance between the anode and the cathode is 5 mm.

[0092] Comparative Example 16 The difference between this comparative example and Example 3 is that in step 3, the distance between the anode and the cathode is 55 mm.

[0093] Comparative Example 17 The difference between this comparative example and Example 1 is that in step 3, the temperature of the electrochemically assisted complexation reaction is 20°C.

[0094] Comparative Example 18 The difference between this comparative example and Example 3 is that in step 3, the temperature of the electrochemically assisted complexation reaction is 60°C.

[0095] Comparative Example 19 The difference between this comparative example and Example 1 is that in step 3, the time of the electrochemically assisted complexation reaction is 0.3 h.

[0096] Comparative Example 20 The difference between this comparative example and Example 3 is that in step 3, the time of the electrochemically assisted complexation reaction is 2.2 h.

[0097] Comparative Example 21 The difference between this comparative example and Example 1 is that in step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:0.5.

[0098] Comparative Example 22 The difference between this comparative example and Example 3 is that in step 4, the mass ratio of sodium pyrophosphate to water during hydrolysis is 1:2.5.

[0099] Comparative Example 23 The difference between this comparative example and Example 1 is that in step 4, the concentration of sulfuric acid is 45%.

[0100] Comparative Example 24 The difference between this comparative example and Example 3 is that in step 4, the concentration of sulfuric acid is 99%.

[0101] Comparative Example 25 The difference between this comparative example and Example 1 is that in step 4, the molar ratio of sulfuric acid to sodium pyrophosphate is 0.5:1.

[0102] Comparative Example 26 The difference between this comparative example and Example 3 is that in step 4, the molar ratio of sulfuric acid to sodium pyrophosphate is 2:1.

[0103] Comparative Example 27 The difference between this comparative example and Example 1 is that in step 4, the temperature during hydrolysis is 80°C.

[0104] Comparative Example 28 The difference between this comparative example and Example 3 is that in step 4, the temperature during hydrolysis is 120°C.

[0105] Comparative Example 29 The difference between this comparative example and Example 1 is that in step 4, the hydrolysis time is 130 min.

[0106] Comparative Example 30 The difference between this comparative example and Example 1 is that in step 4, the hydrolysis time is 20 minutes.

[0107] Comparative Example 31 The difference between this comparative example and Example 1 is that in step 5, the pressure in the crystallizer is controlled at 0.005 MPa.

[0108] Comparative Example 32 The difference between this comparative example and Example 3 is that in step 5, the pressure in the crystallizer is controlled at 0.035 MPa.

[0109] Comparative Example 33 The difference between this comparative example and Example 1 is that in step 5, the temperature is controlled at 40°C.

[0110] Comparative Example 34 The difference between this comparative example and Example 3 is that in step 5, the temperature is controlled at 90°C.

[0111] Comparative Example 35 The difference between this comparative example and Example 1 is that in step 5, the crystallization time is 9 hours.

[0112] Comparative Example 36 The difference between this comparative example and Example 3 is that in step 5, the crystallization time is 3 hours.

[0113] Comparative Example 37 The difference between this comparative example and Example 1 is that in step 5, the stirring speed is 40 r / min.

[0114] Comparative Example 38 The difference between this comparative example and Example 3 is that in step 5, the stirring speed is 220 r / min.

[0115] The disodium hydrogen phosphate obtained in Examples 1-6 and Comparative Examples 1-38 was tested. The purity, heavy metal content, and particle size distribution (D50) of the disodium hydrogen phosphate products are shown in the table below. The following testing methods were used for different indicators: 1. Disodium hydrogen phosphate purity (GB / T 1606-2008): Acid-base titration: Weigh 1.000g of sample and dissolve it in 100mL of deionized water. Titrate to endpoint with 0.5mol / L HCl standard solution using methyl orange as the indicator. 2. Heavy metal content (GB / T 5009.74-2014): Inductively coupled plasma mass spectrometry (ICP-MS): The sample was digested with nitric acid and diluted to 10mL. Elements such as Fe, Mn, Cu, Pb, and Cd were detected using an ICP-MS (Agilent 7900). Calibration was performed using national standard material (GBW08607), with a detection limit of 0.01ppm. 3. Particle size distribution (GB / T 19077-2016). Method: Laser diffraction (Malvern Mastersizer 3000). The sample was dispersed in anhydrous ethanol and sonicated for 5 min. The average of three measurements was used to report the median particle size (D50).

[0116]

[0117] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate, characterized in that: The following steps are involved: Step 1: adding crude sodium pyrophosphate and water into a dissolving kettle to obtain a sodium pyrophosphate solution; Step 2: transferring the sodium pyrophosphate solution to a photocatalytic reaction device to perform a photocatalytic oxidation reaction; Step 3: After the photocatalytic oxidation reaction is completed, the photocatalytically oxidized solution is introduced into an electrochemical device to perform an electrochemically assisted complexation reaction; Step 4: After the electrochemically assisted complexation reaction is completed, the electrochemically treated solution is transported to a hydrolysis reactor, and water and a catalyst are added to carry out a hydrolysis reaction; Step 5: After the hydrolysis is completed, the solution is transferred to a crystallizer for crystallization; Step 6: After the crystallization is completed, centrifugation is performed using a centrifugal separation device to obtain disodium hydrogen phosphate crystals; Step 7: Wash the disodium hydrogen phosphate crystals with cold water or ethanol, and dry the washed crystals through a drying device to obtain the final disodium hydrogen phosphate product.

2. A method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step 1, the concentration of sodium pyrophosphate in the solution is 20-30%, and the solution is stirred and dissolved at 50-70° C. with a stirring speed of 150-250 r / min.

3. A method for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 2, characterized in that: In the step 2, the power of a single ultraviolet lamp in the photocatalytic reaction device is 50-200W, and the ultraviolet wavelength is 254-365nm; the catalyst in the photocatalytic reaction device is titanium dioxide, and the ratio of titanium dioxide to sodium pyrophosphate is 0.5-2g / Kg.

4. A method for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 2, characterized in that: In the step 2, the wavelength of visible light in the photocatalytic reaction device is 365 nm; the catalyst in the photocatalytic reaction device is carbon nitride, and the ratio of carbon nitride to sodium pyrophosphate is 1-3 g / Kg.

5. A method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step 2, the temperature during the photocatalytic oxidation reaction is 20-50° C., and the time during the photocatalytic oxidation reaction is 30-120 minutes.

6. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, wherein: In the step 3, the anode of the electrochemical device is a platinum electrode or a graphite electrode; the cathode is a stainless steel electrode.

7. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, wherein: In the step three, a complexing agent is added to the electrochemical device, and the complexing agent is ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; the ratio of ethylenediaminetetraacetic acid to sodium pyrophosphate is 1-2g / Kg, and the ratio of diethylenetriaminepentaacetic acid to sodium pyrophosphate is 1.5-2.5g / Kg.

8. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, wherein: In step 3, the voltage of the power supply during the electrochemical assisted complexation reaction is 3-12V; the distance between the anode and the cathode is 10-50mm; the current density is controlled to be 10-50 mA / cm 2 The temperature of the electrochemically assisted complexation reaction is 25-55°C, and the time of the electrochemically assisted complexation reaction is 0.5-2 h.

9. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, wherein: In the step 4, during hydrolysis, the mass ratio of sodium pyrophosphate to water is 1:1-2, the concentration of sulfuric acid is 50-98%, and the molar ratio of sulfuric acid to sodium pyrophosphate is 1-1.5:

1.

10. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step 4, the temperature during hydrolysis is 90-110° C., the pressure during hydrolysis is 0.1-0.3 MPa, and the time of hydrolysis is 30-120 min.

11. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step 5, the pressure in the crystallizer is controlled at 0.01-0.03 MPa, the temperature is controlled at 50-80° C., the crystallization time is 4-8 h, and the stirring speed is 50-200 r / min.

12. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step six, the filtration is performed using a filter membrane with a pore size of 0.04-0.06 μm and a filtration temperature of 10-30° C.

13. The method for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: In the step seven, the disodium hydrogen phosphate crystals are washed 2-3 times with cold water or ethanol, and the washed crystals are dried by a drying device at a drying temperature of 80-100° C. for 2-3 hours to obtain the final disodium hydrogen phosphate product.

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