Method for preparing sodium phosphate based on deep purification and gradient sodium treatment of waste phosphoric acid etching solution

Abstract

The invention provides a method for preparing sodium phosphate based on deep purification and gradient sodium treatment of waste phosphoric acid etching liquid, which comprises the following steps: (1) acid liquid recovery: sequentially carrying out low-temperature distillation and high-temperature distillation on the waste phosphoric acid etching liquid under a negative pressure condition to obtain acetic acid, nitric acid and heavy metal-containing phosphoric acid wastewater; (2) metal impurity removal: mixing the alkali liquor and the heavy metal-containing phosphoric acid wastewater to obtain a first mixed solution, carrying out deep heavy metal impurity removal by mixing a heavy metal capturing agent after a neutralization reaction, and carrying out solid-liquid separation to obtain a sodium phosphate solution; and (3) sodium phosphate crystallization: carrying out gradient sodium treatment on the sodium phosphate solution by adopting a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate. According to the method, directional efficient separation and recovery of acid liquor and phosphate in the waste phosphoric acid etching liquid wastewater are realized through different-temperature distillation, deep metal impurity removal and gradient sodium treatment; meanwhile, the method is simple in process and free of waste liquid discharge, and conforms to the concept of a green production process.

Description

Technical Field

[0001] The invention belongs to the technical field of etching solution recovery and reuse, relates to a recovery and treatment method of waste phosphoric acid etching solution, and particularly relates to a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of the waste phosphoric acid etching solution. Background Art

[0002] Phosphoric acid etching solutions are used extensively in the electronics, photovoltaic, and metal surface treatment industries to clean and etch silicon wafers and metal surfaces, generating increasing amounts of waste. Waste phosphoric acid etching solutions contain high concentrations of phosphoric acid, along with large amounts of metal ions, silicides, and other impurities. Direct disposal of these solutions can pollute the environment. Effectively recovering this phosphorus resource would have significant environmental and economic value.

[0003] Waste phosphoric acid etching solution mainly contains H3PO4, HNO3, CH3COOH and Al 3+ 、Cu 2+ The traditional treatment process for metal ions such as chlorinated phosphate has the following problems: (1) High impurity residue: The purity of the sodium phosphate generated by direct neutralization is usually less than 95% due to interference between organic matter and metal ions; (2) Complex process: Existing technologies mostly rely on multi-step precipitation or extraction.

[0004] CN 119797294A discloses a method for recovering waste etching liquid containing phosphoric acid, acetic acid, and nitric acid. Specifically, the method involves subjecting the waste etching liquid to vacuum distillation under reduced pressure, first distilling and condensing the light components of nitric acid and acetic acid to obtain a mixed acid solution of acetic acid and nitric acid, and then testing the nitric acid, acetic acid, and water content of the mixed acid solution. Finally, sodium hydroxide or potassium hydroxide is added to the mixed acid solution of acetic acid and nitric acid to convert all the nitric acid into nitrates. While this patent recovers some light components through vacuum distillation, it does not effectively separate the light components, resulting in a relatively complex process.

[0005] CN 119176530A discloses a method for purifying and regenerating phosphoric acid from waste phosphoric acid etching solution. The method comprises: diluting the waste phosphoric acid etching solution with water; filtering the diluted waste phosphoric acid etching solution; adding a fluorine-containing inorganic compound as an additive to the filtered waste acid etching solution; and stirring the solution to produce silicon tetrafluoride gas and a phosphoric acid aqueous solution after the reaction; and evaporating and concentrating the phosphoric acid aqueous solution to produce electronic-grade regenerated phosphoric acid and industrial-grade regenerated phosphoric acid. While this patent removes silicon impurities by adding fluoride, it does not address the poor recovery efficiency of less than 60% phosphorus in sodium salt recovery wastewater.

[0006] CN 112520717A discloses a process for recycling waste phosphoric acid etching solution from the optoelectronics industry. The process includes: (1) metal ion removal; (2) concentration and acid removal; and (3) continuous crystallization and separation. This process requires the introduction of dilution water or desalted water during operation. The high-concentration phosphoric acid must be cooled midway through the process, which can lead to crystallization and pipe clogging.

[0007] CN 214004335U discloses a waste aluminum etching liquid treatment system, which discloses the use of a neutralization method to treat the waste aluminum etching liquid, but does not solve the problem of low purity of sodium phosphate salt.

[0008] In summary, it is necessary to provide a method for resource-based treatment of waste phosphoric acid etching solution to improve the recovery efficiency and purity of sodium phosphate salt. Summary of the Invention

[0009] To address the shortcomings of the prior art, the present invention provides a method for preparing sodium phosphate salts through deep purification and gradient sodiumization of waste phosphoric acid etching solution. This method achieves efficient, directional separation and recovery of acid and phosphate from waste phosphoric acid etching solution wastewater through temperature-differential distillation, deep metal impurity removal, and gradient sodiumization. Furthermore, the method features a simple process flow and eliminates waste liquid discharge, aligning with the concept of green production technology.

[0010] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0011] The present invention provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution, the method comprising the following steps:

[0012] (1) Acid recovery: Under negative pressure conditions, the waste phosphoric acid etching solution is subjected to low-temperature distillation and high-temperature distillation in sequence to obtain acetic acid, nitric acid and heavy metal-containing phosphoric acid wastewater;

[0013] (2) Metal impurity removal: Alkaline solution and heavy metal-containing phosphoric acid wastewater are mixed to obtain a first mixed solution, which is then neutralized and mixed with a heavy metal scavenger for deep heavy metal impurity removal, and a sodium phosphate salt solution is obtained after solid-liquid separation;

[0014] (3) Sodium phosphate crystallization: The sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate.

[0015] The present invention first realizes the directional recovery of light components (acetic acid and nitric acid) through temperature-fractional distillation; then realizes the efficient treatment of metal elements in the waste liquid through neutralization reaction and deep treatment, providing a good environmental foundation for the subsequent recovery of phosphate; finally, realizes the directional separation of phosphate by gradient sodium treatment, thereby avoiding the reduction of purity due to product mixing contamination.

[0016] The resource recovery method provided by the present invention has a simple process flow and produces no waste liquid, is in line with the concept of green chemistry, and has broad application prospects.

[0017] Preferably, the waste phosphoric acid etching solution comprises nitric acid, acetic acid, phosphoric acid and heavy metal ions.

[0018] Preferably, the heavy metal ions include copper ions, nickel ions and iron ions.

[0019] As a preferred technical solution of the present invention, the negative pressure in step (1) is 0.08 to 0.12 MPa, for example, it can be 0.08 MPa, 0.09 MPa, 0.10 MPa, 0.11 MPa or 0.12 MPa, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0020] Preferably, the temperature of the low-temperature distillation in step (1) is 55-65°C, for example, it can be 55°C, 57°C, 59°C, 61°C, 63°C or 65°C, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0021] Preferably, the temperature of the high-temperature distillation in step (1) is 115-125°C, for example, it can be 115°C, 117°C, 119°C, 121°C, 123°C or 125°C, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0022] In the present invention, the distillation process in step (1) is carried out in a distillation kettle, firstly, acetic acid with a purity of ≥98% is obtained by low-temperature distillation, and then the temperature is raised to perform high-temperature distillation after no fraction is produced to obtain nitric acid with a purity of ≥65%. The directional separation of light components in the waste phosphoric acid etching solution is achieved by temperature-differential distillation, thus solving the problem in the prior art that acid recovery requires further separation treatment.

[0023] More specifically, the temperature of the low-temperature distillation is 55-65°C. A higher temperature may cause nitric acid to be mixed into the acetic acid in the waste phosphoric acid etching solution, resulting in a lower acetic acid content. A lower temperature may prevent the distillate from being produced.

[0024] The temperature of the high-temperature distillation is 115-125° C. A higher temperature will result in too fast a distillation rate, thereby reducing the purity of nitric acid. A lower temperature will make it difficult for nitric acid to be taken out, thereby reducing the purity of nitric acid.

[0025] As a preferred technical solution of the present invention, the alkali solution in step (2) includes sodium hydroxide solution.

[0026] Preferably, the concentration of the sodium hydroxide solution is 35-55wt%, for example, 35wt%, 40wt%, 45wt%, 50wt% or 55wt%, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0027] Preferably, the pH value of the first mixed solution in step (2) is 7 to 7.5, for example, it can be 7, 7.1, 7.2, 7.3, 7.4 or 7.5, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0028] It is worth noting that the alkali solution used in the neutralization reaction in step (2) of the present invention contains some water, which is used to dilute the acidity of phosphoric acid to reduce the occurrence of crystallization; at the same time, more than 90% of the metal impurities (mainly copper hydroxide and aluminum hydroxide) in the wastewater are precipitated through the change of pH; and then the heavy metals (mainly including copper, nickel and iron) in the wastewater are treated by mixing with a heavy metal scavenger.

[0029] Wherein, the heavy metal scavenger includes organic sulfides (TMT series) or high molecular inorganic compounds; the high molecular inorganic compound can be purchased from Guangzhou Miaotong Water Treatment Technology Co., Ltd. with the brand name MT-101.

[0030] Preferably, the heavy metal ion concentration in the sodium phosphate solution in step (2) is ≤10 ppm, for example, it can be 10 ppm, 9 ppm, 8 ppm, 7 ppm, 6 ppm or 5 ppm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0031] As a preferred technical solution of the present invention, the gradient sodium treatment in step (3) comprises:

[0032] (a) mixing sodium hydroxide solution and sodium phosphate salt solution to obtain a second mixed solution, and then sequentially performing a heating process, a cooling process and a filtering process to obtain disodium hydrogen phosphate and a primary filtrate;

[0033] (b) mixing the sodium hydroxide solution and the primary filtrate to obtain a third mixed solution, and then sequentially performing a secondary heating treatment, a cooling crystallization treatment, and a filtration treatment to obtain trisodium phosphate.

[0034] In the present invention, the pH value of the reaction system is precisely regulated by adding sodium hydroxide solution in batches, and the temperature is precisely controlled by heating treatment, thereby achieving step-by-step separation of sodium phosphate salts, thereby avoiding the reduction of purity caused by product mixing and contamination; more specifically, in a weakly alkaline environment, HPO4 2- With finite Na +Combined, it preferentially satisfies the solubility product of Na2HPO4 (Ksp=0.04), while Na3PO4 cannot precipitate due to insufficient sodium ions; under strong alkaline conditions, PO4 3- Accounting for more than 95%, combined with high concentration of Na + , so that the ion product of Na3PO4 (Q = [Na + ] 3 [PO4 3- ]) far exceeds its Forced precipitation of Na3PO4.

[0035] As a preferred technical solution of the present invention, the concentration of the sodium hydroxide solution is 35 to 55 wt%, for example, it can be 35 wt%, 40 wt%, 45 wt%, 50 wt% or 55 wt%, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0036] As a preferred technical solution of the present invention, the pH value of the second mixed solution is 8.0-9.0, for example, it can be 8.0, 8.2, 8.4, 8.6, 8.8 or 9.0, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0037] Preferably, the molar ratio of elements Na and P in the second mixed solution is 1.8 to 2.0:1, for example, it can be 1.8:1, 1.85:1, 1.9:1, 1.95:1 or 2.0:1, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0038] It is worth noting that the system pH value during the preparation of disodium hydrogen phosphate is 8.0-9.0. If the pH value is too low, the precipitation of disodium hydrogen phosphate will not be complete, resulting in a low recovery rate. On the contrary, if the pH value is too high, trisodium phosphate will be generated, thereby reducing the purity of the product.

[0039] As a preferred technical solution of the present invention, the heating rate of the single heating treatment is 1.5 to 2.5°C / min, for example, it can be 1.5°C / min, 1.7°C / min, 1.9°C / min, 2.1°C / min, 2.3°C / min or 2.5°C / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0040] Preferably, the endpoint temperature of the single heating treatment is 50-70°C, for example, 50°C, 55°C, 60°C, 65°C or 70°C, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0041] Preferably, the holding time of the single heating treatment is 1.8 to 2.5 h, for example, 1.8 h, 2.0 h, 2.2 h, 2.4 h or 2.5 h, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0042] It is worth noting that disodium hydrogen phosphate begins to crystallize when the temperature reaches 50-70°C, and medium temperature can promote the crystallization of disodium hydrogen phosphate; if the temperature is too low, other sodium phosphate salts such as trisodium phosphate may precipitate, which will also lead to problems such as delayed crystallization and poor crystal quality. If the temperature is too high, it will lead to insufficient precipitation and reduced recovery rate.

[0043] Preferably, the cooling rate of the cooling treatment is 0.3 to 0.8°C / min, for example, it can be 0.3°C / min, 0.4°C / min, 0.5°C / min, 0.6°C / min, 0.7°C / min or 0.8°C / min, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0044] Preferably, the terminal temperature of the cooling treatment is 20-25°C, for example, it can be 20°C, 21°C, 22°C, 23°C, 24°C or 25°C, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0045] As a preferred technical solution of the present invention, the pH value of the third mixed solution is 12 to 12.5, for example, it can be 12, 12.1, 12.2, 12.3, 12.4 or 12.5, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0046] Preferably, the molar ratio of elements Na and P in the third mixed solution is 2.8 to 3.0:1, for example, it can be 2.8:1, 2.85:1, 2.9:1, 2.95:1 or 3.0:1, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0047] It is worth noting that the system pH value during the preparation of trisodium phosphate is 12-12.5. If the pH value is too low, disodium phosphate will precipitate, resulting in low product purity. On the contrary, if the pH value is too high, it will lead to waste of alkali solution. At the same time, high concentration of sodium ions will increase the viscosity of the solution, hindering crystal growth and solid-liquid separation efficiency.

[0048] Preferably, the heating rate of the secondary heating treatment is 1.5 to 2.5°C / min, for example, it can be 1.5°C / min, 1.7°C / min, 1.9°C / min, 2.1°C / min, 2.3°C / min or 2.5°C / min, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0049] Preferably, the endpoint temperature of the secondary heating treatment is 70-75°C, for example, it can be 70°C, 71°C, 72°C, 73°C, 74°C or 75°C, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0050] Preferably, the holding time of the secondary heating treatment is 1.8 to 2.5 hours, for example, it can be 1.8 hours, 2 hours, 2.2 hours, 2.4 hours or 2.5 hours, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0051] It is worth noting that trisodium phosphate begins to crystallize when the temperature reaches 70-75°C, and high temperature can enhance the stability of trisodium phosphate; if the temperature is too low, it will lead to delayed crystallization, poor crystal quality and co-precipitation risks, which need to be compensated by increasing the sodium ion concentration or adding seed crystals. High temperature will trigger hydrolysis side reactions, crystal coarsening and impurity interference, which need to be controlled by cooling or chemical stabilizers.

[0052] Preferably, the temperature of the cooling crystallization treatment is 3 to 8°C, for example, 3°C, 4°C, 5°C, 6°C, 7°C or 8°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0053] As a preferred technical solution of the present invention, the method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution provided by the present invention comprises the following steps:

[0054] (1) Acid recovery: Under negative pressure of 0.08-0.12 MPa, the waste phosphoric acid etching solution is subjected to low-temperature distillation at 55-65°C to obtain acetic acid, and then high-temperature distillation at 115-125°C to obtain nitric acid and heavy metal-containing phosphoric acid wastewater;

[0055] (2) Metal impurity removal: a sodium hydroxide solution with a concentration of 35 to 55 wt% and heavy metal-containing phosphoric acid wastewater are mixed to obtain a first mixed solution with a pH value of 7 to 7.5. After neutralization, a heavy metal scavenger is added to perform deep heavy metal impurity removal. After solid-liquid separation, a sodium phosphate salt solution with a heavy metal ion concentration of ≤10 ppm is obtained;

[0056] (3) sodium phosphate crystallization: the sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate;

[0057] Wherein, the gradient sodium treatment comprises:

[0058] (a) mixing a sodium hydroxide solution and a sodium phosphate solution having a concentration of 35 to 55 wt % to obtain a second mixed solution having a pH of 8.0 to 9.0 and a molar ratio of Na to P of 1.8 to 2.0:1, heating the solution to 50 to 70° C. at a heating rate of 1.5 to 2.5° C. / min and maintaining the temperature for 1.8 to 2.5 hours, then cooling the solution to 20 to 25° C. at a cooling rate of 0.3 to 0.8° C. / min, and filtering the solution to obtain disodium hydrogen phosphate and a primary filtrate;

[0059] (b) mixing a sodium hydroxide solution having a concentration of 35 to 55 wt % and the primary filtrate to obtain a third mixed solution having a pH value of 12 to 12.5 and a molar ratio of Na to P of 2.8 to 3.0:1, heating the mixture to 70 to 75° C. at a heating rate of 1.5 to 2.5° C. / min and keeping the temperature for 1.8 to 2.5 h, cooling and crystallizing the mixture at 3 to 8° C., and filtering the mixture to obtain trisodium phosphate.

[0060] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] (1) The method provided by the present invention has a simple process and no waste liquid is discharged, which is in line with the concept of green chemical process;

[0063] (2) The present invention realizes the step-by-step and efficient recovery of acetic acid and nitric acid by temperature-fractional distillation, breaking through the technical bottleneck of low purity of traditional mixed acid recovery;

[0064] (3) The method provided by the present invention establishes a two-stage gradient sodium crystallization process, and realizes the step-by-step directional preparation of disodium hydrogen phosphate and trisodium phosphate by precisely controlling the crystallization temperature and pH, and the purity of the obtained sodium phosphate salt is not less than 99%. DETAILED DESCRIPTION

[0065] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0066] The waste phosphoric acid etching solution treated in the following examples and comparative examples includes acetic acid, nitric acid and heavy metal ions; the concentration of the nitric acid is 15wt%, the concentration of the acetic acid is 8wt%, the concentration of the phosphoric acid is 5wt%, the concentration of the copper ion is 1200ppm, and the concentration of the aluminum ion is 800ppm.

[0067] Example 1

[0068] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution, the method comprising the following steps:

[0069] (1) Acid recovery: Under negative pressure of 0.1 MPa, the waste phosphoric acid etching solution is subjected to low-temperature distillation at 60°C to obtain acetic acid, and then high-temperature distillation at 120°C to obtain nitric acid and heavy metal-containing phosphoric acid wastewater;

[0070] (2) Metal impurity removal: a sodium hydroxide solution with a concentration of 45 wt% and heavy metal-containing phosphoric acid wastewater were mixed to obtain a first mixed solution with a pH value of 7.2. After neutralization, a heavy metal scavenger was added to perform deep heavy metal impurity removal. After solid-liquid separation, a sodium phosphate salt solution with a heavy metal ion concentration of 1 ppm was obtained;

[0071] (3) sodium phosphate crystallization: the sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate;

[0072] Wherein, the gradient sodium treatment comprises:

[0073] (a) Mix 45 wt% sodium hydroxide solution and sodium phosphate solution (PO4 3- concentration of 1.2 mol / L) to obtain a second mixed solution with a pH value of 8.6±0.1 and a molar ratio of Na to P of 2.0:1, then heating to 60°C at a heating rate of 2°C / min and keeping warm for 2 hours, then cooling to 25°C at a cooling rate of 0.5°C / min, and filtering to obtain disodium hydrogen phosphate and a primary filtrate;

[0074] (b) Mixing 45 wt% sodium hydroxide solution and the primary filtrate (PO4 3- The reaction mixture was stirred for 2 h at room temperature for 3 hours, and then cooled and crystallized at 5 ° C. The mixture was filtered to obtain trisodium phosphate.

[0075] Example 2

[0076] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution, the method comprising the following steps:

[0077] (1) Acid recovery: Under a negative pressure of 0.08 MPa, the waste phosphoric acid etching solution is subjected to low-temperature distillation at 65°C to obtain acetic acid, and then high-temperature distillation at 125°C to obtain nitric acid and heavy metal-containing phosphoric acid wastewater;

[0078] (2) Metal impurity removal: a sodium hydroxide solution with a concentration of 35 wt% and heavy metal-containing phosphoric acid wastewater were mixed to obtain a first mixed solution with a pH value of 7. After neutralization, a heavy metal scavenger was added to perform deep heavy metal impurity removal. After solid-liquid separation, a sodium phosphate salt solution with a heavy metal ion concentration of 8 ppm was obtained;

[0079] (3) sodium phosphate crystallization: the sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate;

[0080] Wherein, the gradient sodium treatment comprises:

[0081] (a) mixing a 35 wt % sodium hydroxide solution and a sodium phosphate solution to obtain a second mixed solution having a pH of 8.0 and a molar ratio of Na to P of 1.9:1, heating the solution to 50° C. at a heating rate of 1.5° C. / min and maintaining the temperature for 2.5 h, then cooling the solution to 20° C. at a cooling rate of 0.3° C. / min, and filtering the solution to obtain disodium hydrogen phosphate and a primary filtrate;

[0082] (b) mixing a 35 wt % sodium hydroxide solution and the primary filtrate to obtain a third mixed solution having a pH of 12 and a molar ratio of Na to P of 2.9:1, heating the solution to 70° C. at a heating rate of 1.5° C. / min and maintaining the temperature for 2.5 h, followed by cooling and crystallization at 3° C., and filtering to obtain trisodium phosphate.

[0083] Example 3

[0084] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution, the method comprising the following steps:

[0085] (1) Acid recovery: Under a negative pressure of 0.12 MPa, the waste phosphoric acid etching solution is subjected to low-temperature distillation at 55°C to obtain acetic acid, and then high-temperature distillation at 115°C to obtain nitric acid and heavy metal-containing phosphoric acid wastewater;

[0086] (2) Metal impurity removal: A 55 wt% sodium hydroxide solution and heavy metal-containing phosphoric acid wastewater were mixed to obtain a first mixed solution with a pH value of 7.5. After neutralization, a heavy metal scavenger was added to perform deep heavy metal impurity removal. After solid-liquid separation, a sodium phosphate salt solution with a heavy metal ion concentration of 4 ppm was obtained;

[0087] (3) sodium phosphate crystallization: the sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate;

[0088] Wherein, the gradient sodium treatment comprises:

[0089] (a) mixing a 55 wt % sodium hydroxide solution and a sodium phosphate solution to obtain a second mixed solution having a pH of 9.0 and a molar ratio of Na to P of 1.8:1, heating the solution to 70° C. at a heating rate of 2.5° C. / min and maintaining the temperature for 1.8 h, then cooling the solution to 25° C. at a cooling rate of 0.8° C. / min, and filtering the solution to obtain disodium hydrogen phosphate and a primary filtrate;

[0090] (b) mixing a 55 wt % sodium hydroxide solution and the primary filtrate to obtain a third mixed solution having a pH of 12.5 and a molar ratio of Na to P of 2.8:1, heating the solution to 75° C. at a heating rate of 2.5° C. / min and maintaining the temperature for 1.8 h, followed by cooling and crystallization at 8° C., and filtering to obtain trisodium phosphate.

[0091] Example 4

[0092] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0093] In this embodiment, the temperature of the low-temperature distillation in step (1) is adjusted to 50°C, and the temperature of the high-temperature distillation is adjusted to 110°C.

[0094] Example 5

[0095] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0096] In this embodiment, the temperature of the low-temperature distillation in step (1) is adjusted to 70°C, and the temperature of the high-temperature distillation is adjusted to 130°C.

[0097] Example 6

[0098] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0099] In this embodiment, the negative pressure in step (1) is adjusted to 0 MPa.

[0100] Example 7

[0101] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0102] In this embodiment, the sodium hydroxide solution in step (2) is adjusted to sodium hydroxide powder.

[0103] Example 8

[0104] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0105] In this embodiment, the molar ratio of Na to P in the second mixed solution in step (a) is adjusted to 1.02:1.

[0106] Example 9

[0107] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0108] In this embodiment, the molar ratio of Na to P in the second mixed solution in step (a) is adjusted to 3.2:1.

[0109] Example 10

[0110] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0111] In this embodiment, the end point temperature of the first heating treatment in step (a) is adjusted to 40°C.

[0112] Example 11

[0113] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0114] In this embodiment, the endpoint temperature of the one-time heating treatment in step (a) is adjusted to 75°C.

[0115] Example 12

[0116] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0117] In this embodiment, the molar ratio of Na to P in the third mixed solution of step (b) is adjusted to 2.01:1.

[0118] Example 13

[0119] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0120] In this embodiment, the molar ratio of Na to P in the third mixed solution of step (b) is adjusted to 4.32:1.

[0121] Example 14

[0122] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0123] In this embodiment, the endpoint temperature of the secondary heating treatment in step (b) is adjusted to 60°C.

[0124] Example 15

[0125] This embodiment provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0126] In this embodiment, the endpoint temperature of the secondary heating treatment in step (b) is adjusted to 80°C.

[0127] Comparative Example 1

[0128] This comparative example provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0129] In this comparative example, step (1) is adjusted as follows: the waste phosphoric acid etching solution is distilled at 80° C. under a negative pressure of 0.1 MPa.

[0130] Comparative Example 2

[0131] This comparative example provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0132] In this comparative example, the process of mixing a heavy metal scavenger for deep heavy metal removal in step (2) is omitted, and the heavy metal ion (Cu / Ni / Fe) concentration of the obtained sodium phosphate salt solution is 100-500 ppm.

[0133] Comparative Example 3

[0134] This comparative example provides a method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution. The method differs from Example 1 only in that:

[0135] In this comparative example, step (3) is adjusted as follows: mixing sodium hydroxide solution and sodium phosphate solution to obtain a second mixed solution, then heating to 100°C at a heating rate of 2°C / min and keeping warm for 2h, and then cooling to 10°C for crystallization treatment.

[0136] Product testing:

[0137] (1) The purity and recovery rate of acetic acid and nitric acid recovered by the methods provided in the above embodiments and comparative examples were calculated and analyzed, and the results are shown in Table 1;

[0138] (2) The purity of the sodium phosphate salt obtained by the methods provided in the above examples and comparative examples was analyzed, and the results are shown in Table 1.

[0139] Table 1

[0140]

[0141]

[0142] According to Table 1, we can know the following points:

[0143] (1) Comprehensive analysis of Examples 1-3 shows that the method provided by the present invention can achieve directional and efficient separation of nitric acid and acetic acid in waste phosphoric acid etching solution, and the recovery rates of the obtained nitric acid and acetic acid are not less than 90%; the acidity of acetic acid is not less than 98%, and the acidity of nitric acid is not less than 65%;

[0144] The method provided by the present invention can realize the directional step-by-step preparation of sodium phosphate, and the conversion rate of phosphate is not less than 97.5%;

[0145] (2) A comprehensive analysis of Example 1 and Examples 4-6 shows that the temperature during the temperature-separation distillation process in step (1) affects the recovery rate and acidity of acetic acid and nitric acid; when the temperature of low-temperature distillation or high-temperature distillation is too low or too high, the purity and recovery rate of the obtained acid solution will be reduced;

[0146] When the temperature-separation distillation is carried out under normal pressure, the recovery efficiency will be lowered and the acidity of acetic acid and nitric acid will be lowered;

[0147] In addition, due to the reduced recovery effect of acetic acid and nitric acid in the waste phosphoric acid etching solution in step (1), the heavy metal phosphoric acid wastewater has a high content of salt and other impurities. Therefore, subsequent metal impurity removal and sodium phosphate crystallization are not performed to avoid further waste of resources.

[0148] (3) Comprehensive analysis of Example 1 and Example 7 shows that the use of a solid base to adjust the pH of the system in the neutralization reaction in step (2) will result in a lower purity of the product crystals;

[0149] (4) Comprehensive analysis of Example 1 and Examples 8-11 shows that in the process of preparing disodium hydrogen phosphate, Na + When the content is too low, it cannot meet the solubility product of Na2HPO4 (Ksp≈0.04), resulting in a decrease in the precipitation of disodium hydrogen phosphate, and even preferential precipitation of monosodium phosphate (NaH2PO4), resulting in a decrease in product purity. + When the content is high, the excess sodium ions may induce the coprecipitation of trisodium phosphate (Na3PO4) through the common ion effect;

[0150] Low temperature will lead to poor reaction kinetics and slow crystallization process, which may result in small crystals or amorphous precipitation. High temperature may destroy the structure of trisodium phosphate, induce dehydration or decomposition reaction, and promote the formation of impurities.

[0151] (5) Comprehensive analysis of Example 1 and Examples 12-15 shows that in the process of preparing trisodium phosphate, Na + Low content will result in incomplete neutralization of phosphoric acid, and the product will contain more acidic hydrogen phosphate or dihydrogen phosphate. + If the content is too high, the excess sodium may exist in the form of unreacted alkali or other sodium salts, affecting the purity of the product. If the temperature is too low, the reaction kinetics will deteriorate, the crystallization process will be slow, and small crystals or amorphous precipitation may result. If the temperature is too high, the high temperature may destroy the structure of trisodium phosphate, triggering dehydration or decomposition reactions, and promoting the formation of impurities.

[0152] (6) Comprehensive analysis of Example 1 and Comparative Example 1 shows that adjusting the temperature-separating distillation to a specific temperature cannot achieve the separate recovery of acetic acid and nitric acid;

[0153] (7) Comprehensive analysis of Example 1 and Comparative Example 2 shows that omitting the deep heavy metal removal process will result in a high heavy metal ion content in the resulting sodium phosphate salt solution, which will further affect the subsequent product containing heavy metal impurities and cause the product to be unqualified;

[0154] (8) A comprehensive analysis of Example 1 and Comparative Example 3 shows that, compared with the gradient sodium treatment provided by the present invention, the method for preparing sodium phosphate provided by Comparative Example 3 not only fails to achieve directional preparation of sodium salt, but also leads to excessive water loss, crystal agglomeration, and the inability to centrifuge, thereby damaging the equipment.

[0155] In summary, the present invention achieves directional and efficient separation and recovery of acid and phosphate in waste phosphoric acid etching solution wastewater through temperature-selective distillation, deep metal impurity removal and gradient sodium treatment; at the same time, the method is simple in process, does not discharge waste liquid, and is in line with the concept of green production process.

[0156] The applicant declares that the specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing sodium phosphate salt based on deep purification and gradient sodiumization of waste phosphoric acid etching solution, characterized in that: The method comprises the following steps: (1) Acid recovery: Under negative pressure conditions, the waste phosphoric acid etching solution is subjected to low-temperature distillation and high-temperature distillation in sequence to obtain acetic acid, nitric acid and heavy metal-containing phosphoric acid wastewater; (2) Metal impurity removal: Alkaline solution and heavy metal-containing phosphoric acid wastewater are mixed to obtain a first mixed solution, which is then neutralized and mixed with a heavy metal scavenger for deep heavy metal impurity removal, and a sodium phosphate salt solution is obtained after solid-liquid separation; (3) Sodium phosphate crystallization: The sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate.

2. The method according to claim 1, characterized in that The negative pressure in step (1) is 0.08-0.12 MPa; Preferably, the temperature of the low-temperature distillation in step (1) is 55-65°C; Preferably, the temperature of the high-temperature distillation in step (1) is 115-125°C.

3. The method according to claim 1 or 2, characterized in that The alkali solution in step (2) includes sodium hydroxide solution; Preferably, the concentration of the sodium hydroxide solution is 35 to 55 wt%; Preferably, the pH value of the first mixed solution in step (2) is 7 to 7.5; Preferably, the heavy metal ion concentration in the sodium phosphate solution in step (2) is ≤10 ppm.

4. The method according to any one of claims 1 to 3, characterized in that The gradient sodium treatment in step (3) comprises: (a) mixing sodium hydroxide solution and sodium phosphate salt solution to obtain a second mixed solution, and then sequentially performing a heating process, a cooling process and a filtering process to obtain disodium hydrogen phosphate and a primary filtrate; (b) mixing the sodium hydroxide solution and the primary filtrate to obtain a third mixed solution, and then sequentially performing a secondary heating treatment, a cooling crystallization treatment, and a filtration treatment to obtain trisodium phosphate.

5. The method according to claim 4, characterized in that The concentration of the sodium hydroxide solution is 35-55 wt%.

6. The method according to claim 4 or 5, characterized in that The pH value of the second mixed solution is 8.0-9.0; Preferably, the molar ratio of elements Na and P in the second mixed solution is 1.8-2.0:

1.

7. The method according to claim 6, characterized in that The heating rate of the primary heating treatment is 1.5 to 2.5°C / min; Preferably, the terminal temperature of the first heating treatment is 50-70°C; Preferably, the holding time of the primary heating treatment is 1.8 to 2.5 hours; Preferably, the cooling rate of the cooling treatment is 0.3-0.8°C / min; Preferably, the terminal temperature of the cooling treatment is 20-25°C.

8. The method according to claim 4 or 5, characterized in that The pH value of the third mixed solution is 12 to 12.5; Preferably, the molar ratio of elements Na and P in the third mixed solution is 2.8-3.0:

1.

9. The method according to claim 8, characterized in that The heating rate of the secondary heating treatment is 1.5 to 2.5°C / min; Preferably, the endpoint temperature of the secondary heating treatment is 70-75°C; Preferably, the holding time of the secondary heating treatment is 1.8 to 2.5 hours; Preferably, the temperature of the cooling crystallization treatment is 3-8°C.

10. The method according to any one of claims 1 to 9, characterized in that The method comprises the following steps: (1) Acid recovery: Under negative pressure of 0.08-0.12 MPa, the waste phosphoric acid etching solution is subjected to low-temperature distillation at 55-65°C to obtain acetic acid, and then high-temperature distillation at 115-125°C to obtain nitric acid and heavy metal-containing phosphoric acid wastewater; (2) Metal impurity removal: a sodium hydroxide solution with a concentration of 35 to 55 wt% and heavy metal-containing phosphoric acid wastewater are mixed to obtain a first mixed solution with a pH value of 7 to 7.

5. After neutralization, a heavy metal scavenger is added to perform deep heavy metal impurity removal. After solid-liquid separation, a sodium phosphate salt solution with a heavy metal ion concentration of ≤10 ppm is obtained; (3) sodium phosphate crystallization: the sodium phosphate solution obtained in step (2) is subjected to a gradient sodiumization treatment using a sodium hydroxide solution to obtain disodium hydrogen phosphate and trisodium phosphate; Wherein, the gradient sodium treatment comprises: (a) mixing a sodium hydroxide solution and a sodium phosphate solution having a concentration of 35 to 55 wt % to obtain a second mixed solution having a pH of 8.0 to 9.0 and a molar ratio of Na to P of 1.8 to 2.0:1, heating the solution to 50 to 70° C. at a heating rate of 1.5 to 2.5° C. / min and maintaining the temperature for 1.8 to 2.5 hours, then cooling the solution to 20 to 25° C. at a cooling rate of 0.3 to 0.8° C. / min, and filtering the solution to obtain disodium hydrogen phosphate and a primary filtrate; (b) mixing a sodium hydroxide solution having a concentration of 35 to 55 wt % and the primary filtrate to obtain a third mixed solution having a pH value of 12 to 12.5 and a molar ratio of Na to P of 2.8 to 3.0:1, heating the mixture to 70 to 75° C. at a heating rate of 1.5 to 2.5° C. / min and keeping the temperature for 1.8 to 2.5 h, cooling and crystallizing the mixture at 3 to 8° C., and filtering the mixture to obtain trisodium phosphate.

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