A method for the preparation of sodium phosphate salts from phosphoric acid and sodium chloride

Abstract

The application belongs to the technical field of inorganic chemical industry and comprehensive utilization of phosphorus resources, and particularly relates to a method for preparing sodium phosphate salt from phosphoric acid and sodium chloride, comprising the following steps: (1) mixing the phosphoric acid and the sodium chloride, heating and performing a dechlorination reaction to generate a dechlorination reaction product; (2) dissolving the dechlorination reaction product in water, adding lye to adjust the pH of the system to 10.5-13.0, and obtaining a mixed solution after purification; (3) concentrating the mixed solution, and obtaining a NaCl crude product and a phosphorus-rich mother liquor after solid-liquid separation; and (4) crystallizing and purifying the phosphorus-rich mother liquor to obtain a twelve-water trisodium phosphate finished product.

Description

Technical Field

[0001] This invention belongs to the field of inorganic chemical engineering and comprehensive utilization of phosphorus resources, and specifically relates to a method for preparing sodium phosphate from phosphoric acid and sodium chloride. Background Technology

[0002] Trisodium phosphate (Na3PO4) exists in both anhydrous and dodecahydrate forms and is widely used in industrial water treatment, metal cleaning, dyeing and printing auxiliaries, food additives, and boiler water softening. It is a major inorganic phosphate product. Currently, the mainstream industrial production process for trisodium phosphate is the direct neutralization of phosphoric acid with caustic soda or soda ash using a thermal / wet process. This process has several key drawbacks: first, the cost of high-purity phosphoric acid and caustic soda raw materials is high, resulting in poor product economics; second, the process only produces trisodium phosphate, leading to low added value; and third, the neutralization process generates a large amount of saline wastewater, resulting in high environmental treatment costs.

[0003] Meanwhile, the phosphorus chemical, chlor-alkali chemical, and fine chemical industries produce large quantities of phosphoric acid byproducts (mainly containing phosphoric acid, mixed with pyrophosphoric acid, metaphosphoric acid, and small amounts of sulfuric acid and hydrochloric acid, making phosphorus resources difficult to recover) and sodium chloride byproducts (mainly containing NaCl, mixed with calcium and magnesium ions, sulfates, silicates, and insoluble matter; the P2O5 content in phosphoric acid byproducts is 20wt%-36wt%, and the NaCl content in sodium chloride byproducts is 70.0wt%~90wt%; long-term stockpiling easily leads to salinization pollution, and disposal costs are high). Existing resource recovery methods can only achieve simple reuse or high-priced outsourcing for disposal, with a resource utilization rate of less than 40%, resulting in the waste of phosphorus and chlorine resources.

[0004] Existing processes for reacting phosphoric acid with sodium chloride primarily focus on producing monosodium phosphate, disodium phosphate, or metaphosphate. High-temperature reactions easily generate polyphosphates, making it impossible to stably produce trisodium orthophosphate. Furthermore, the disordered release of hydrogen chloride during the reaction results in low absorption efficiency, leading to acid mist corrosion and exhaust pollution, thus failing to achieve efficient co-production of trisodium phosphate and hydrochloric acid. Therefore, developing a clean process that uses industrial byproducts as raw materials, co-produces two high-value products, and achieves zero emissions has become a core direction for technological breakthroughs in this field. Summary of the Invention

[0005] This invention aims to solve at least one of the technical problems existing in related technologies. To this end, this invention proposes a method for preparing sodium phosphate from phosphoric acid and sodium chloride. This method has low energy consumption, no external discharge of waste, and can realize the transformation of by-products into valuable resources and the high-value co-production of two products.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for preparing sodium phosphate from phosphoric acid and sodium chloride includes the following steps: (1) mixing phosphoric acid and sodium chloride and heating to carry out a dechlorination reaction to generate a dechlorination reaction product; (2) Dissolve the dechlorination reaction product in water, and add alkali solution to adjust the pH of the system to 10.5-13.0. After purification, a mixed solution is obtained. (3) The mixture is concentrated and separated into solid and liquid components to obtain crude NaCl and phosphorus-rich mother liquor; (4) The phosphorus-rich mother liquor is crystallized and purified to obtain trisodium dodecahydrate product.

[0007] In one embodiment, the phosphoric acid is obtained by: subjecting a mixture of phosphoric acids to natural sedimentation to remove large insoluble particles, followed by electrocatalytic oxidation and adsorption with an adsorbent to reduce total organic carbon (TOC), and then filtering. The resulting filtrate is then evaporated and concentrated so that the phosphoric acid contained therein contains 35wt%-60wt% of P2O5 by mass. In one embodiment, the sodium chloride is obtained by: crushing and sieving sodium chloride contrasites, adding sodium carbonate and dissolving in water to prepare a saturated sodium chloride solution, removing calcium and magnesium impurity precipitates by filtration, evaporating the filtrate to a water content ≤1wt%, and sieving to obtain the sodium chloride with a NaCl mass fraction ≥95wt%.

[0008] In one embodiment, in step (1), the phosphoric acid and sodium chloride are mixed in a molar ratio of 1:(2.4-3.2).

[0009] In one embodiment, the heating for dechlorination includes: in a closed reactor, heating to 120-180°C and holding for 1-3 hours for a first-stage dechlorination reaction; then heating to 220-280°C and holding for 3-5 hours for a second-stage dechlorination reaction.

[0010] In one embodiment, the HCl tail gas generated by the dechlorination reaction is sent to a tail gas treatment system.

[0011] In one embodiment, the exhaust gas treatment system includes a primary water washing absorption tower, a secondary supplementary absorption tower, and a tertiary alkaline washing purification tower connected in sequence. During the treatment process, the exhaust gas treatment system controls the flow rate of the absorption water to contact the HCl exhaust gas in a countercurrent manner, thereby producing industrial hydrochloric acid with a mass fraction of 20wt%-31wt%. The exhaust gas is discharged after passing the test and meeting the standards.

[0012] In one embodiment, the alkaline solution is a sodium hydroxide solution with a concentration of 30wt%-50wt%.

[0013] In one embodiment, the purification refers to adding a flocculant and / or an adsorbent, stirring, allowing it to settle, filtering, and obtaining a mixture.

[0014] In one embodiment, the flocculant is at least one of polyaluminum ferric silicate and anionic polyacrylamide, and the amount of flocculant added is 0.05wt%-0.2wt% of the total materials in the reaction system.

[0015] In one embodiment, the adsorbent is activated carbon, and the amount of the adsorbent added is 0.1wt%-0.5wt% of the total materials in the reaction system.

[0016] In one embodiment, in step (3), the concentration method is vacuum concentration, the vacuum concentration temperature is 85-105℃, the vacuum degree is -0.08-0.09MPa, and the concentration is carried out until the relative density of the concentrated product is 1.25-1.40. The relative density is the ratio of 1t / m³ of the aqueous solution.

[0017] In one embodiment, in step (3), the temperature during solid-liquid separation is ≥80°C.

[0018] In one embodiment, in step (3), the crude NaCl is removed from impurities and then returned to step (1) for recycling as sodium chloride.

[0019] In one embodiment, the crystallization and purification includes the following steps: cooling the phosphorus-rich mother liquor to 25-35°C at a rate of 5-10°C / h to crystallize and separate to obtain a wet product; dissolving the wet product in water and filtering it, then vacuum concentrating the resulting filtrate until the relative density of the concentrated product is 1.25-1.40, and then performing a second recrystallization at 25-35°C, separating the solid and liquid to obtain the second recrystallized wet product and the crystallization mother liquor, and drying to obtain the finished product of trisodium dodecahydrate.

[0020] In one embodiment, the mother liquor from crystallization is refluxed to dissolve the sodium chloride miscellaneous salts in order to prepare the saturated sodium chloride solution.

[0021] The beneficial effects of this invention are: (1) High-value utilization of by-products: Industrial by-products such as phosphoric acid and sodium chloride are directly used as raw materials to replace high-purity phosphoric acid and caustic soda, reducing raw material costs by more than 35% and solving the pollution problem of by-product stockpiling; (2) High-efficiency co-production of two products: High-purity trisodium phosphate and qualified industrial hydrochloric acid are produced simultaneously, with raw material utilization rate ≥95%, total phosphorus recovery rate ≥92%, chlorine recovery rate ≥98%, and product added value is greatly improved. (3) Clean process with zero emissions: The entire process is a closed reaction, with full absorption of HCl tail gas and full recycling of crystallization mother liquor, which meets the requirements of green chemical industry; (4) Product quality is controllable: It can produce trisodium phosphate dodecahydrate with a purity of ≥98.5% and meets the GB / T1607-2018 industrial grade standard. The co-produced hydrochloric acid meets the GB / T320-2006 industrial grade standard. (5) Strong industrial adaptability: It adopts conventional chemical reaction, absorption, concentration and crystallization equipment, without the need for special customized equipment, and can realize continuous and automated production, which is easy to scale up and promote. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of Embodiment 1 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to specific embodiments.

[0024] Example 1: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, such as Figure 1 As shown, it includes the following steps: ① Raw material pretreatment: Phosphoacid (initial P2O5 = 22.4 wt%) was settled for 24 h, filtered to remove large insoluble particles, and the TOC in phosphateacid was reduced by electrocatalytic oxidation: solution volume 500 mL, current density 200 mA / m², electrode spacing 16 mm, reaction time 5 h, water bath reaction temperature 42 °C; then transferred to a new container, 0.3 wt% activated carbon was added and stirred for 45 min for adsorption decolorization and a second TOC reduction, followed by filtration. The filtrate was concentrated by vacuum evaporation to adjust P2O5 to 45.2 wt%. Sodium chloride contrasite salts were crushed, sieved, dissolved in sodium carbonate and deionized water to prepare a saturated sodium chloride solution. Precipitated calcium and magnesium impurities were removed by filtration, and the solution was dried with hot air until the moisture content was ≤1wt%. The NaCl mass fraction was controlled to be ≥95wt%, and the particle size passed through a 100-mesh sieve. ② Double decomposition reaction: The two raw materials prepared in step ① are mixed in a molar ratio of H3PO4:NaCl = 1:3.0, stirred and mixed evenly, and then loaded into a closed reactor. The first stage reaction is carried out at 150℃ for 2 hours, and the second stage reaction is carried out at 250℃ for 3 hours to carry out the dechlorination reaction and generate the dechlorination reaction product. The main component of the dechlorination reaction product is disodium hydrogen phosphate. Reaction principle: Main reaction:

[0025] Stepwise reaction: (First stage: Low-temperature dechlorination) (Second stage: high-temperature dechlorination) ③ Hydrochloric acid absorption: The HCl tail gas produced in step ② enters the primary water washing absorption tower, the secondary supplementary absorption tower, and the tertiary alkaline washing purification tower in sequence. The flow rate of the absorption water is controlled to contact the tail gas in a countercurrent manner to produce industrial hydrochloric acid with a mass fraction of 30.0wt%. The tail gas is discharged after passing the test and the hydrochloric acid is transported to the storage tank for storage. ④ Neutralization and deep impurity removal: The dechlorination reaction product obtained in step ② is cooled to 90.0℃ and dissolved in water. The pH is adjusted to 12.5 using 32.0wt% NaOH to completely convert the disodium hydrogen phosphate into trisodium phosphate. Polyaluminum ferric silicate flocculant and activated carbon are added, so that the polyaluminum ferric silicate flocculant accounts for 0.1wt% of the total material in the reaction system and the activated carbon accounts for 0.2wt% of the total material in the reaction system. Stir for 40 minutes, and remove insoluble matter, heavy metals and organic impurities by sedimentation and filtration. Reaction principle:

[0026] ⑤ Concentration and Separation The filtered liquid from step ④ is sent to a high-temperature concentration tank, where the liquid temperature is controlled at 100.0℃ and the vacuum degree is controlled at -0.08MPa. The liquid is concentrated until the relative density of the slurry is 1.25. The high-concentration NaCl crystals and phosphorus-rich mother liquor are separated by hot filtration at above 80℃. The NaCl crystals produced are recycled back to the raw material pretreatment step ①. ⑥ Recrystallization The phosphorus-rich mother liquor produced in step ⑤ is sent to a cooling crystallizer and cooled to 25.0℃ at a rate of 5℃ / h for crystallization. The wet product is obtained by centrifugation. The wet product is dissolved in deionized water, and the filtered liquid is sent back to a vacuum concentration tank. The liquid temperature is controlled at 100.0℃ and the vacuum degree is controlled at -0.08MPa. The liquid is concentrated until the relative density of the slurry is 1.25. The wet product is sent to a cooling crystallizer and cooled to 25.0℃ at a rate of 5℃ / h for crystallization. The wet product is obtained by centrifugation and purified by secondary recrystallization. The purified wet product by secondary recrystallization is dried by airflow drying / fluidized bed drying to obtain trisodium dodecahydrate product. The remaining mother liquor after secondary recrystallization is used to dissolve the sodium chloride miscellaneous salt to prepare the sodium chloride saturated solution.

[0027] Product testing results: The purity of trisodium phosphate dodecahydrate is 98.86 wt%, heavy metals (as Pb) ≤0.001%, arsenic ≤0.0002%, which meets the food-grade standard; it co-produces 30 wt% hydrochloric acid, with free chlorine ≤0.005% and iron ≤0.001%, which meets the first-class industrial product standard; the phosphorus yield is 92.5%.

[0028] Example 2: A method for preparing sodium phosphate from phosphoric acid and sodium chloride includes the following steps: ① Raw material pretreatment: Phosphoacid (initial P2O5 = 22.4 wt%) was settled for 24 h, filtered to remove large insoluble particles, and the TOC in phosphateacid was reduced by electrocatalytic oxidation: solution volume 500 mL, current density 180 mA / m², electrode spacing 16 mm, reaction time 5 h, water bath reaction temperature 40 °C; then transferred to a new container, 0.3 wt% activated carbon was added and stirred for 45 min for adsorption decolorization and secondary TOC reduction, followed by filtration. The filtrate was concentrated by vacuum reduced pressure evaporation to adjust P2O5 to 44.8 wt%. Sodium chloride contrasite salts were crushed, sieved, dissolved in sodium carbonate and deionized water to prepare a saturated sodium chloride solution. Precipitated calcium and magnesium impurities were removed by filtration, and the solution was dried with hot air until the moisture content was ≤1wt%. The NaCl mass fraction was controlled to be ≥95wt%, and the particle size passed through a 100-mesh sieve. ② Double decomposition reaction: The two raw materials prepared in step ① are mixed in a molar ratio of H3PO4:NaCl = 1:2.8, stirred and mixed evenly, and then loaded into a closed reactor. The first stage reaction is carried out at 140℃ for 2 hours, and the second stage reaction is carried out at 240℃ for 3 hours to produce sodium dihydrogen phosphate. ③ Hydrochloric acid absorption: The HCl tail gas produced in step ② enters the primary water washing absorption tower, the secondary supplementary absorption tower, and the tertiary alkaline washing purification tower in sequence. The flow rate of the absorption water is controlled to contact the tail gas in a countercurrent manner to produce industrial hydrochloric acid with a mass fraction of 30wt%. The tail gas is discharged after being tested and meeting the standards. The hydrochloric acid is transported to the storage tank for storage. ④ Neutralization and deep impurity removal: The slurry obtained in step ② is cooled to 90℃ and dissolved in water. The pH is adjusted to 12.0 using 32wt% NaOH to completely convert disodium hydrogen phosphate into trisodium phosphate. Polyaluminum ferric silicate flocculant and activated carbon are added, so that the polyaluminum ferric silicate flocculant accounts for 0.1wt% of the total material in the reaction system and the activated carbon accounts for 0.2wt% of the total material in the reaction system. Stir for 40 minutes, and remove insoluble matter, heavy metals and organic impurities by sedimentation and filtration. ⑤ Concentration and Separation The filtered liquid from step ④ is sent to a high-temperature concentration tank, where the liquid temperature is controlled at 95.0℃ and the vacuum degree is controlled at -0.09MPa. The liquid is concentrated until the relative density of the slurry is 1.40. The high-concentration NaCl crystals and phosphorus-rich mother liquor are separated by hot filtration at a temperature above 80℃. The NaCl crystals produced are recycled back to the raw material pretreatment step ①. ⑥ Recrystallization The phosphorus-rich mother liquor produced in step ⑤ is sent to a cooling crystallizer and cooled to 35.0℃ at a rate of 10℃ / h for crystallization. The wet product is obtained by centrifugation. The wet product is dissolved in deionized water, and the filtered liquid is sent back to a vacuum concentration tank. The liquid temperature is controlled at 95.0℃ and the vacuum degree is controlled at -0.09MPa. The liquid is concentrated until the relative density of the slurry is 1.40. The wet product is sent to a cooling crystallizer and cooled to 35.0℃ at a rate of 10℃ / h for crystallization. The wet product is obtained by centrifugation and purified by secondary recrystallization. The purified wet product by secondary recrystallization is dried by airflow drying / fluidized bed drying to obtain trisodium dodecahydrate product. The remaining mother liquor after secondary recrystallization is used to dissolve the sodium chloride miscellaneous salt to prepare the sodium chloride saturated solution.

[0029] Product test results: The purity of trisodium phosphate dodecahydrate is 98.53wt%, heavy metals (as Pb) ≤0.001%, arsenic ≤0.0002%, which meets the food grade standard; co-produced 30wt% hydrochloric acid, free chlorine ≤0.005%, iron ≤0.001%, which meets the first-class industrial product standard; phosphorus recovery rate is 90.6%.

[0030] Example 3: A method for preparing sodium phosphate from phosphoric acid and sodium chloride includes the following steps: ① Raw material pretreatment: Phosphoacid (initial P2O5 = 22.4 wt%) was settled for 24 h, filtered to remove large insoluble particles, and the TOC in phosphateacid was reduced by electrocatalytic oxidation: solution volume 500 mL, current density 160 mA / m², electrode spacing 16 mm, reaction time 5 h, water bath reaction temperature 40 °C; then transferred to a new container, 0.3 wt% activated carbon was added and stirred for 45 min for adsorption decolorization and secondary TOC reduction, followed by filtration. The filtrate was concentrated by vacuum evaporation to adjust P2O5 to 44.2 wt%. Sodium chloride contrasite salts were crushed, sieved, dissolved in sodium carbonate and deionized water to prepare a saturated sodium chloride solution. Precipitated calcium and magnesium impurities were removed by filtration, and the solution was dried with hot air until the moisture content was ≤1wt%. The NaCl mass fraction was controlled to be ≥95wt%, and the particle size passed through a 100-mesh sieve. ② Double decomposition reaction: The two raw materials prepared in step ① are mixed in a molar ratio of H3PO4:NaCl = 1:2.6, stirred and mixed evenly, and then loaded into a closed reactor. The first stage reaction is carried out at 130℃ for 2 hours, and the second stage reaction is carried out at 230℃ for 3 hours to produce sodium dihydrogen phosphate. ③ Hydrochloric acid absorption: The HCl tail gas produced in step ② enters the primary water washing absorption tower, the secondary supplementary absorption tower, and the tertiary alkaline washing purification tower in sequence. The flow rate of the absorption water is controlled to contact the tail gas in a countercurrent manner to produce industrial hydrochloric acid with a mass fraction of 30wt%. The tail gas is discharged after being tested and meeting the standards. The hydrochloric acid is transported to the storage tank for storage. ④ Neutralization and deep impurity removal: The slurry obtained in step ② is cooled to 90℃ and dissolved in water. The pH is adjusted to 11.5 using 32.0wt% NaOH to completely convert disodium hydrogen phosphate into trisodium phosphate. Anionic polyacrylamide flocculant and activated carbon are added, so that the anionic polyacrylamide flocculant accounts for 0.1wt% of the total material in the reaction system and the activated carbon accounts for 0.2wt% of the total material in the reaction system. Stir for 40 minutes, and remove insoluble matter, heavy metals and organic impurities by sedimentation and filtration. ⑤ Concentration and Separation The filtered liquid from step ④ is sent to a high-temperature concentration tank, where the liquid temperature is controlled at 90℃ and the vacuum degree is controlled at -0.08MPa. The liquid is concentrated until the relative density of the slurry is 1.25. The high-concentration NaCl crystals and phosphorus-rich mother liquor are separated by hot filtration at temperatures above 80℃. The NaCl crystals produced are recycled back to the raw material pretreatment step ①. ⑥ Recrystallization The phosphorus-rich mother liquor produced in step ⑤ is sent to a cooling crystallizer and cooled to 30.0℃ at a rate of 6℃ / h for crystallization. The wet product is obtained by centrifugation. The wet product is dissolved in deionized water, and the filtered liquid is sent back to a vacuum concentration tank. The liquid temperature is controlled at 90.0℃ and the vacuum degree is controlled at -0.08MPa. The liquid is concentrated until the relative density of the slurry is 1.25. The wet product is sent to a cooling crystallizer and cooled to 25.0℃ at a rate of 6℃ / h for crystallization. The wet product is obtained by centrifugation and purified by secondary recrystallization. The purified wet product by secondary recrystallization is dried by airflow drying / fluidized bed drying to obtain trisodium dodecahydrate product. The remaining mother liquor after secondary recrystallization is used to dissolve the sodium chloride miscellaneous salts to prepare the sodium chloride saturated solution.

[0031] Product testing results: The purity of trisodium phosphate dodecahydrate is 98.47 wt%, heavy metals (as Pb) ≤0.001%, arsenic ≤0.0002%, which meets the food-grade standard; it also produces 30.0 wt% hydrochloric acid, with free chlorine ≤0.005% and iron ≤0.001%, which meets the first-class industrial product standard; the phosphorus recovery rate is 88.4%.

[0032] Example 4: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 150 mA / m², step ② has a molar ratio of 1:2.5, a first-stage reaction temperature of 120°C, a second-stage reaction temperature of 220°C, and step ④ has a pH control range of 11.0, is otherwise identical to Example 1.

[0033] Example 5: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 140 mA / m², step ② has a molar ratio of 1:2.4, a first-stage reaction temperature of 115°C, a second-stage reaction temperature of 210°C, and step ④ has a pH control range of 12.0, is otherwise the same as in Example 1.

[0034] Example 6: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 200 mA / m², step ② has a molar ratio of 1:2.4, a first-stage reaction temperature of 150°C, a second-stage reaction temperature of 250°C, and step ④ has a pH control range of 12.5. All other steps are the same as in Example 1.

[0035] Example 7: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 200 mA / m², step ② has a molar ratio of 1:3.0, a first-stage reaction temperature of 150°C, a second-stage reaction temperature of 250°C, and step ④ has a pH control range of 11.5. All other steps are the same as in Example 1.

[0036] Example 8: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 200 mA / m², step ② has a molar ratio of 1:3.0, a first-stage reaction temperature of 150°C, a second-stage reaction temperature of 250°C, and step ④ has a pH control range of 11.0. All other steps are the same as in Example 1.

[0037] Example 9: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① has a current density of 200 mA / m², step ② has a molar ratio of 1:3.0, a first-stage reaction temperature of 150°C, a second-stage reaction temperature of 250°C, and step ④ has a pH control range of 10.5. All other steps are the same as in Example 1.

[0038] Example 10: A method for preparing sodium phosphate from phosphoric acid and sodium chloride is identical to that in Example 1, except that step ① does not involve electrocatalytic oxidation.

[0039] Example 11: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ②, the first stage reaction temperature is 150°C and held for 5 hours, and the second stage reaction is not carried out, is the same as in Example 1.

[0040] Example 12: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ① does not involve electrocatalytic oxidation, and in step ② the two raw materials prepared in step ① are mixed at a molar ratio of H3PO4:NaCl = 1:2.0, the rest are the same as in Example 1.

[0041] Comparative Example 1: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ④, where the pH is controlled at 10, is the same as in Example 1.

[0042] Comparative Example 2: A method for preparing sodium phosphate from phosphoric acid and sodium chloride, except that step ④, where the pH is controlled at 13.5, is the same as in Example 1.

[0043] Table 1: Summary Table of Key Step Indicators and Product Testing Results

[0044] The fluorine content, heavy metals (as Pb), arsenic content, iron content, sulfate, chloride ion, and pH of the product trisodium phosphate dodecahydrate all meet the HG / T2517-2025 industrial trisodium phosphate standard; the ferric (as Fe), free chlorine (as Cl), residue on ignition, and sulfate (as SO4) content in the product hydrochloric acid are also within acceptable limits. 2- All indicators meet the GB / T320-2025 standard for industrial synthetic hydrochloric acid.

[0045] analyze: As shown in Examples 1-5, as the electrocatalytic oxidation current density, material molar ratio, first / second stage reaction temperature, and system pH gradually decrease, the purity of trisodium phosphate slowly decreases from 98.86% to 97.62%, showing an overall gradual downward trend; the phosphorus recovery rate continuously decreases from 92.5% to 83.1%, with a more significant decrease. This indicates that within a certain range, higher current density, material molar ratio, reaction temperature, and pH are beneficial for improving the purity of trisodium phosphate and the efficiency of phosphorus recovery.

[0046] From Examples 1 and 6-9, it can be seen that, with a fixed electrocatalytic oxidation current density, current density to material molar ratio, temperature, and pH, the single-factor effects are as follows: with a fixed current density of 200 mA / m² and a material molar ratio of 3.0, only by gradually decreasing the reaction temperature and pH in two stages: the purity of trisodium phosphate decreased slightly from 98.86% to 97.81%; the phosphorus recovery rate showed a sharp downward trend, dropping significantly from 92.5% to 88.7%. This indicates that the reaction temperature and system pH are extremely sensitive to the phosphorus recovery rate. Too low a temperature and pH will significantly inhibit the phosphorus recovery effect and slightly reduce the product purity.

[0047] Example 10 shows that when the electrocatalytic oxidation step is omitted and the remaining key parameters are at their optimal levels, the trisodium phosphate purity is only 96.72% and the phosphorus recovery rate is only 84.6%, which is far lower than that of the example under the same parameters and with the electric field applied. This proves that the current density electric field enhancement effect is a core necessary condition for ensuring high product purity and high phosphorus recovery rate. The absence of electric field enhancement will lead to a significant deterioration of process indicators. Example 11 shows that by omitting the second-stage reaction and only retaining the first-stage reaction, the trisodium phosphate content in the final product decreases from 98.86% to 82.17%, and the phosphorus recovery rate decreases from 92.5% to 90.41%.

[0048] As can be seen from Example 12 and Comparative Examples 1-2, when the electrocatalytic oxidation process was eliminated and only the material molar ratio and pH were adjusted, the purity of trisodium phosphate in all comparative examples was lower than that in the charged examples with the same parameters, and remained at a low level of 96.46%-97.15%. The overall phosphorus recovery rate was low, with a maximum of only 84.2%, which was far below the optimal level at a current density of 200 mA / m².

Claims

1. A method for preparing sodium phosphate from phosphoric acid and sodium chloride, characterized in that: Includes the following steps: (1) Mix phosphoric acid and sodium chloride, and heat to carry out a dechlorination reaction to generate dechlorination reaction products; (2) Dissolve the dechlorination reaction product in water, and add alkali solution to adjust the pH of the system to 10.5-13.

0. After purification, a mixed solution is obtained. (3) The mixture is concentrated and separated into solid and liquid components to obtain crude NaCl and phosphorus-rich mother liquor; (4) The phosphorus-rich mother liquor is crystallized and purified to obtain trisodium dodecahydrate product.

2. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: The phosphoric acid is obtained by the following method: the phosphoric acid is subjected to natural sedimentation to remove large insoluble particles, electrocatalytic oxidation and adsorption by an adsorbent to reduce TOC, and then filtered. The filtrate is then evaporated and concentrated so that the phosphoric acid contained therein has a mass fraction of 35wt%-60wt% as P2O5.

3. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: The sodium chloride is obtained by crushing and sieving sodium chloride impurities, adding sodium carbonate and water to dissolve them to prepare a saturated sodium chloride solution, removing calcium and magnesium impurities by filtration, evaporating the filtrate to a water content of ≤1wt%, and sieving to obtain sodium chloride with a NaCl mass fraction of ≥95wt%.

4. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: The heating process for dechlorination includes: in a closed reactor, heating to 120-180℃ and holding for 1-3 hours for the first stage of dechlorination; then heating to 220-280℃ and holding for 3-5 hours for the second stage of dechlorination.

5. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: The purification process involves adding flocculants and / or adsorbents, stirring, allowing the mixture to settle, and then filtering to obtain a mixed solution.

6. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 5, characterized in that: The adsorbent is activated carbon, and the amount of adsorbent added is 0.1wt%-0.5wt% of the total material in the reaction system.

7. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: In step (3), the concentration method is vacuum concentration, the vacuum concentration temperature is 85-105℃, the vacuum degree is -0.08-0.09MPa, and the concentration is concentrated to a relative density of 1.25-1.

40.

8. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 1, characterized in that: In step (3), the crude NaCl is removed from impurities and then returned to step (1) for recycling as sodium chloride.

9. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 3, characterized in that: The crystallization and purification process specifically includes the following steps: cooling the phosphorus-rich mother liquor to 25-35℃ at a rate of 5-10℃ / h to crystallize and separate the wet product; dissolving the wet product in water and filtering it; vacuum concentrating the resulting filtrate until the relative density of the concentrated product is 1.25-1.40; then performing a second recrystallization at 25-35℃; separating the solid and liquid to obtain the second recrystallized wet product and the crystallization mother liquor; and drying to obtain the finished product, trisodium dodecahydrate.

10. The method for preparing sodium phosphate from phosphoric acid and sodium chloride according to claim 9, characterized in that: The mother liquor from crystallization is refluxed to dissolve the sodium chloride miscellaneous salts in order to prepare the saturated sodium chloride solution.

Images