Recovery method and application of sodium sulfate high-salinity wastewater

Abstract

The invention discloses a recovery method and application of sodium sulfate high-salinity wastewater, and belongs to the technical field of resource recovery. The method comprises the following steps: S1, adding a recapturing agent containing a phosphate group into the sodium sulfate wastewater, and reacting to remove heavy metals in the wastewater to obtain a heavy-removed mother solution, S2, concentrating the heavy-removed mother solution, carrying out double decomposition reaction on the concentrated heavy-removed mother solution and ammonium bicarbonate, and separating to obtain a sodium bicarbonate crude product and a double decomposition mother solution, and S3, rectifying the double decomposition mother solution to obtain a rectified mother solution and tower top distillate, s4, carrying out first-stage evaporation and concentration on the rectification mother liquor, and separating to obtain sodium sulfate and first-stage evaporation mother liquor; and S5, carrying out second-stage evaporation and concentration on the first-stage evaporation mother liquor, so as to obtain ammonium sulfate crystals. According to the method disclosed by the invention, heavy metals are efficiently removed, the sodium ion conversion rate is improved, and high-purity ammonium sulfate is obtained and meets the standard of agricultural ammonium sulfate GB / T 535-2020.

Description

Technical Field

[0001] The present invention relates to the technical field of resource recovery, and particularly relates to a method for recovering high-salt sodium sulfate wastewater and its application. Background Art

[0002] In the process of producing battery-grade lithium carbonate and lithium hydroxide in the new energy industry, a large amount of sodium sulfate is generated as a by-product, producing a large amount of wastewater containing sodium sulfate. Due to the associated minerals in the raw materials or the reagents used in the processing, this wastewater often contains heavy metals such as lead, arsenic, cadmium, and nickel. Therefore, it is necessary to remove the heavy metals first. The heavy metal scavenger of phosphate group utilizes the pairing complexation precipitation mechanism of phosphate radical and heavy metals to remove a large amount of heavy metals therein, and introduces a large amount of phosphate ions into the system, which is beneficial to the subsequent resource utilization process of sodium sulfate. The main salt component in this wastewater is sodium sulfate, and sodium sulfate is mainly applied to industries such as glass. Due to the large rise of the new energy industry in the previous few years and the decline of the civil engineering industry, a large amount of sodium sulfate has been produced and cannot be consumed, and the output is seriously excessive. Therefore, converting sodium sulfate into sodium bicarbonate with a wider application range and simultaneously restoring ammonium sulfate is a good way out. Some studies have used the double decomposition method to react sodium sulfate with ammonium bicarbonate to prepare sodium bicarbonate, and then produce ammonium sulfate through evaporation and freezing processes. In the process of recovering sodium sulfate and evaporating to produce ammonium sulfate in this process, a section of cooling is added to produce by-products, resulting in the system temperature having to drop from 80 °C to 25 °C first and then rise to 80 °C, resulting in relatively large energy consumption. There are also studies that carry out double decomposition reactions of sodium sulfate and ammonium bicarbonate under the action of a promoter to obtain sodium bicarbonate, and then obtain ammonium sulfate through evaporation and concentration. The specific steps are as follows: Sodium sulfate undergoes a double decomposition reaction with a mixture of ammonium bicarbonate or carbon dioxide and ammonia, and a promoter is added at the same time. The reaction slurry is separated by liquid-solid separation and washed to obtain sodium bicarbonate products and sodium bicarbonate washing liquid; the sodium bicarbonate mother liquor obtained by liquid-solid separation is mixed with the sodium bicarbonate washing liquid and flash-evaporated to recover ammonium bicarbonate; some studies have the sodium sulfate wastewater enter the first reaction tank, add ammonium bicarbonate, stir and react to produce a large amount of sodium bicarbonate and ammonium sulfate, and then enter the sedimentation tank. Sodium bicarbonate precipitates out, and the supernatant is discharged. The precipitated sodium bicarbonate is dehydrated, and after dehydration, it enters the evaporation system and decomposes into sodium carbonate at high temperature. The supernatant enters the second reaction tank, and a small amount of sulfuric acid is added to adjust the PH to convert the sodium carbonate in the clear liquid into sodium sulfate, which is precipitated by freezing and returned to the sodium sulfate wastewater for recycling. The remaining clear liquid only contains ammonium sulfate and a small amount of sodium sulfate, and ammonium sulfate is obtained by evaporation and used as chemical fertilizer. However, in the above methods, the conversion rate of sodium ions is low and the material utilization efficiency is poor. For example, a large amount of ammonium sulfate evaporation mother liquor or by-products (Na2SO4·(NH4)2SO4·4H2O) are returned to the double decomposition reaction. Since ammonium sulfate is one of the products of the double decomposition reaction, an increase in its concentration will cause the reaction equilibrium to shift to the left (Na2SO4 + 2NH4HCO3 → 2NaHCO3↓ + (NH4)2SO4).

[0003] In summary, the above methods generally have problems such as low conversion rate of sodium ions, high energy consumption, and complex processes. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. For this purpose, the present invention provides a method for recovering high-salt sodium sulfate wastewater, which realizes efficient removal of heavy metals, improves the conversion rate of sodium ions, avoids the precipitation of sodium sulfate, and obtains high-purity ammonium sulfate.

[0005] The present invention also provides an application of the above method.

[0006] According to the first aspect of the present invention, there is provided a method for recovering high-salt sodium sulfate wastewater, the method comprising the following steps:

[0007] S1. Adding a heavy metal capturer containing a phosphate group to the sodium sulfate wastewater, reacting to remove heavy metals in the wastewater, and obtaining a mother liquor after heavy metal removal.

[0008] The heavy metal capturer containing a phosphate group includes at least one of sodium hexametaphosphate, sodium tripolyphosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0009] S2. Concentrating the mother liquor after heavy metal removal, performing a metathesis reaction with ammonium bicarbonate, and separating to obtain crude sodium bicarbonate and a metathesis mother liquor.

[0010] After concentration, the mother liquor after heavy metal removal is a concentrated mother liquor, and the concentration of sodium sulfate in the concentrated mother liquor is 450-470 g / L.

[0011] S3. Rectifying the metathesis mother liquor to obtain a rectified mother liquor and a top distillate.

[0012] S4. Performing first-stage evaporation concentration on the rectified mother liquor, and separating to obtain sodium sulfate and a first-stage evaporation mother liquor.

[0013] In the step of first-stage evaporation concentration, the amount of condensed water distilled out is 15%-35% of the total volume of the rectified mother liquor.

[0014] S5. Performing second-stage evaporation concentration on the first-stage evaporation mother liquor to obtain ammonium sulfate crystals.

[0015] According to the first aspect of the present invention, it has at least the following beneficial effects:

[0016] In the present invention, in step S1, a phosphate group re-capture agent (such as sodium hexametaphosphate, sodium tripolyphosphate, etc.) is used to form a stable complex precipitate with heavy metal ions to remove heavy metals from the wastewater, efficiently removing heavy metals: the phosphate group forms a stable complex with heavy metal ions, and the removal rate is up to over 99%, ensuring the purity of the raw materials for subsequent resource utilization processes; meanwhile, while removing heavy metals, phosphate ions are introduced into the system to provide improvement for the subsequent evaporation and concentration stage, changing the ternary system of sulfate ions, ammonia nitrogen, and sodium ions into a quaternary system, providing a higher solubility anion pairing for sodium ions, so that when ammonium sulfate is produced by evaporation, sodium ions will not precipitate in the form of sodium sulfate but remain in the water body in the form of sodium dihydrogen phosphate with higher solubility, which can effectively reduce the content of sodium ion impurities in evaporation crystallization, avoid the precipitation of sodium ions in the form of sodium sulfate, and improve the purity of ammonium sulfate; on the other hand, the phosphate group re-capture agent only introduces phosphate ions and does not introduce other cation impurities. After entering the resource utilization process of sodium sulfate, after producing sodium bicarbonate through a double decomposition reaction, a rectification process is used to recover bicarbonate ions, avoiding the use of a large amount of sulfuric acid materials and reducing the output of by-product ammonium sulfate to ensure the product purity.

[0017] In step S2, sodium sulfate and ammonium bicarbonate are subjected to a double decomposition reaction to produce sodium bicarbonate and ammonium sulfate. If the concentration of sodium sulfate is too high, it will cause the solution to be supersaturated, leading to the co-crystallization of sodium bicarbonate and sodium sulfate, reducing the product purity. Therefore, the concentration is controlled at 450 - 470 g / L, which can not only ensure the reaction conversion rate but also avoid the co-crystallization problem.

[0018] In step S3, ammonium bicarbonate in the double decomposition mother liquor is recovered by rectification;

[0019] In step S4, in the first-stage evaporation and concentration, sodium sulfate in the mother liquor is precipitated by evaporation and concentration to obtain high-purity sodium sulfate crystals. When returning to step S2, the introduction of ammonium sulfate is reduced, avoiding the left shift of the double decomposition reaction and improving the conversion rate of sodium ions;

[0020] In step S5, ammonium sulfate is precipitated from the mother liquor by evaporation and concentration to obtain high-purity ammonium sulfate crystals. On the one hand, after sodium sulfate is evaporated in the first stage, high concentrations of sodium ions, ammonia nitrogen, and sulfate ions still remain in the mother liquor, and the concentrations of sodium ions and ammonium ions are close. According to the ternary phase diagram analysis, it is difficult to obtain high-purity ammonium sulfate by evaporation at this time, and only a mixed salt of sodium sulfate and ammonium sulfate can be obtained, and high-purity ammonium sulfate cannot be obtained. Since phosphate ions are introduced in the heavy metal removal stage, the ternary system of sulfate, ammonia nitrogen, and sodium ions becomes a quaternary system, providing a higher solubility anion pair for sodium ions, so that sodium ions in the ammonium sulfate produced by evaporation in the second stage do not precipitate in the form of sodium sulfate, but remain in the water body in the form of sodium dihydrogen phosphate with higher solubility, which can effectively reduce the content of sodium ion impurities in evaporation crystallization. On the other hand, in the present invention, the first-stage cooling crystallization process is omitted (the function of cooling crystallization in the traditional process is: the solution temperature is reduced below the co-crystallization point of sodium sulfate and ammonium sulfate to precipitate the double salt, so as to achieve preliminary separation. After cooling crystallization, the double salt can be obtained by solid-liquid separation, and the remaining mother liquor enters the subsequent high-temperature evaporation section to further separate sodium sulfate and ammonium sulfate.). The conventional high-temperature evaporation-low-temperature condensation-high-temperature evaporation process is optimized into a two-stage high-temperature evaporation process, which greatly reduces the process energy consumption and reduces a large amount of process operation costs, having great advantages. The two-stage evaporation and concentration process respectively produces sodium sulfate and ammonium sulfate crystals with higher purity, and the material purity is controlled by adjusting process conditions such as evaporation amount and evaporation temperature, among which the purity of sodium sulfate crystals reaches more than 90%.

[0021] According to some embodiments of the present invention, the phosphate group-containing heavy metal capturer includes sodium hexametaphosphate, sodium tripolyphosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate,

[0022] The concentration of sodium hexametaphosphate is 0.1-10 g / L, the concentration of sodium tripolyphosphate is 0.1-10 g / L, the concentration of sodium hydrogen phosphate is 0-5 g / L, and the concentration of sodium dihydrogen phosphate is 0-8 g / L.

[0023] In step S3, ammonium bicarbonate in the double decomposition mother liquor is recovered by rectification. Due to the introduction of phosphate ions in the heavy metal removal stage of step S1, these two ions hydrolyze into orthophosphate ions at high temperatures during rectification. The introduction of orthophosphate ions transforms the ternary system of sulfate - ammonia nitrogen - sodium ions into a quaternary system of sulfate - ammonia nitrogen - sodium ions - phosphate ions, reducing the sodium ion impurities in the ammonium sulfate crystals in step S5. The orthophosphate ions combine with sodium ions to form sodium dihydrogen phosphate (NaH2PO4) with higher solubility, preventing sodium ions from precipitating in the form of sodium sulfate during the evaporation and concentration stage, improving the purity of ammonium sulfate, making the process route of producing sodium sulfate and ammonium sulfate through two - stage evaporation feasible, increasing the purity of ammonium sulfate (≥93%), meeting the agricultural ammonium sulfate GB / T 535 - 2020 standard. By introducing orthophosphate ions, the evaporation and concentration process is optimized, avoiding the problem of co - crystallization of sodium sulfate and ammonium sulfate in the traditional process, simplifying the process flow, and reducing the overall energy consumption and operating cost of the system. On the other hand, the orthophosphate ions increase the purity of ammonium sulfate and optimize the evaporation and crystallization process.

[0024] According to some embodiments of the present invention, in the reaction of step S1, the pH is controlled to be 6 - 8.

[0025] According to some embodiments of the present invention, the liquid - to - solid ratio of the phosphate - group heavy metal capturer to heavy metals in the wastewater is 5 - 25 ml:g.

[0026] According to some embodiments of the present invention, the reaction temperature is 30 - 60 °C.

[0027] According to some embodiments of the present invention, the reaction time is 15 - 60 min.

[0028] Under the above conditions, the heavy metal capturer combines with heavy metal ions such as lead, cadmium, nickel, and arsenic through the chelation of phosphate groups to form stable complex precipitates, removing heavy metals from the water body. After the reaction, a flocculant is added, and the precipitate is filtered and separated to obtain a heavy - metal - free mother liquor after heavy metal removal. At this time, the mother liquor after heavy metal removal still contains a high concentration of sodium sulfate and a small amount of phosphate ions introduced when adding the high - phosphorus heavy metal capturer, and then enters the resource utilization stage of high - salt wastewater.

[0029] According to some embodiments of the present invention, in the double decomposition reaction of step S2, the addition amount of ammonium bicarbonate in the double decomposition reaction is such that the molar ratio of sodium ions to ammonium ions in the solution is 1:1 - 1.02; the reaction temperature is 35 - 42 °C.

[0030] According to some embodiments of the present invention, the reaction time of the double decomposition reaction is 30 - 120 min.

[0031] According to some embodiments of the present invention, in step S2, before adding ammonium bicarbonate, the concentrated mother liquor is further cooled to 50 - 60 °C.

[0032] According to some embodiments of the present invention, the pH of the reaction is 7.9 - 8.4.

[0033] According to some embodiments of the present invention, in the double decomposition reaction of step S2, the reaction pH is controlled by ammonia water. The ammonia gas overflowing during the reaction is absorbed by an acid solution to obtain an absorption liquid. When the pH of the absorption liquid is 6.0 - 7.0, it is added to the mother liquor of the first-stage evaporation in step S5 for the second-stage evaporation and concentration.

[0034] In the double decomposition reaction of step S2, since ammonium bicarbonate dissolution and the double decomposition reaction are endothermic, the temperature of the mother liquor will naturally decrease during this process. Subsequently, the reaction temperature is controlled at 35 - 42°C, and the reaction pH is controlled at 7.9 - 8.4 by adding ammonia water. The ammonia gas overflowing during the reaction is absorbed by sulfuric acid. When the pH of the absorption liquid is 6.0 - 7.0, it is transported to the second-stage evaporation and concentration. After the double decomposition reaction, solid-liquid separation is carried out to obtain sodium bicarbonate solid and double decomposition mother liquor. The sodium bicarbonate solid is produced as a product, and the sodium ion conversion rate can reach 65% - 70%, higher than 58 - 60% of the conventional process. The product purity can reach over 97%. The double decomposition mother liquor enters the rectification section for carbon removal.

[0035] According to some embodiments of the present invention, the rectification is carried out in a rectification column. The bottom temperature of the rectification column is 102 - 105°C, the top temperature of the rectification column is 90°C - 100°C, the pressure of the rectification column is -5 - 5 Kpa, and the condensation temperature in the rectification column is 30 - 40°C.

[0036] According to some embodiments of the present invention, it further includes introducing carbon dioxide into the overhead distillate to obtain ammonium bicarbonate and then adding it to the concentrated mother liquor of step S2 for the double decomposition reaction.

[0037] According to some embodiments of the present invention, the liquid-gas ratio of the distillate and the carbon dioxide is 5 - 8:1.

[0038] After condensation and reflux, a mixed slurry of ammonium carbonate and ammonium bicarbonate with crystals can be obtained. It is necessary to introduce excessive carbon dioxide into it to convert the ammonium carbonate in the condensed slurry into ammonium bicarbonate. Subsequently, centrifugal filtration is carried out to obtain solid ammonium bicarbonate, which is recycled to the double decomposition reaction.

[0039] According to some embodiments of the present invention, in the first-stage evaporation and concentration of step S4, the pressure of the evaporation and concentration is -75 - -80 Kpa, and the temperature of the evaporation and concentration is 75°C - 85°C.

[0040] According to some embodiments of the present invention, the evaporation and concentration ends when the salt output reaches 5% - 25% and the gas-liquid temperature difference reaches 7 - 15°C.

[0041] According to some embodiments of the present invention, the condensed water generated during the evaporation process of the first-stage evaporation and concentration is used to wash the crude sodium bicarbonate crystals produced in the double decomposition reaction stage.

[0042] According to some embodiments of the present invention, in the step S4, it further includes flashing and cooling the mother liquor of the first-stage evaporation to 55-65°C.

[0043] According to some embodiments of the present invention, it further includes adding the sodium sulfate obtained in step S4 to the mother liquor for removing heavy metals in step S2 for concentration.

[0044] In the conventional process, most of the sodium sulfate is recycled by cooling crystallization and precipitation. According to the phase diagram analysis, when sodium sulfate and ammonium sulfate precipitate at low temperature, they precipitate in the form of Na2SO4·(NH4)2SO4·4H2O, and pure sodium sulfate salt cannot be obtained. Therefore, a large amount of ammonium sulfate is inevitably incorporated into the double decomposition reaction system during the return of materials. From the chemical equation of the double decomposition reaction of sodium sulfate and ammonium bicarbonate, ammonium sulfate is one of the products of the double decomposition reaction. When the concentration of ammonium sulfate in the system increases, the reaction equilibrium will shift to the left, resulting in a decrease in the single-pass conversion rate of sodium ions to sodium bicarbonate, reducing the material utilization efficiency and increasing the system operation cost. The method of the present invention utilizes the concentration difference between sodium sulfate and ammonium sulfate at high temperature, and a large amount of high-purity sodium sulfate is precipitated by evaporation by controlling the process conditions, avoiding the precipitation of ammonium sulfate, reducing the ammonium sulfate introduced into the double decomposition reaction system during the return of materials, and improving the primary reaction conversion rate of sodium sulfate.

[0045] According to some embodiments of the present invention, in the second-stage evaporation and concentration of the step S5, the pressure of the evaporation and concentration is -80 to -95 Kpa, and the temperature of the evaporation and concentration is 55 to 65°C.

[0046] According to some embodiments of the present invention, the evaporation and concentration is carried out until the salt output is 5%-25%.

[0047] In the second-stage evaporation and concentration, ammonium sulfate products with a nitrogen content of more than 19.0% can be produced by evaporation, meeting the agricultural ammonium sulfate GB / T535-2020 standard. The remaining sodium sulfate during the evaporation process will be continuously enriched. When the sodium sulfate is enriched to a certain extent, it is returned to the first-stage evaporation and concentration to evaporate the sodium sulfate.

[0048] According to the second aspect of the present invention, an application of the described method in the recycling of waste batteries is provided.

[0049] Recovery method for high-salt sodium sulfate wastewater of the present invention: 1. For the high-salt sodium sulfate wastewater containing heavy metals produced in the new energy industry, this process uses a phosphate group heavy metal capturer to remove heavy metal impurities therein, achieving efficient purification of the wastewater. At the same time, phosphate ions are introduced to provide improvement for the subsequent sodium sulfate resource utilization process. 2. In most conventional processes, sodium sulfate is crystallized out by cooling for reuse. According to phase diagram analysis, when sodium sulfate and ammonium sulfate precipitate at low temperatures, they precipitate in the form of Na2SO4·(NH4)2SO4·4H2O, and pure sodium sulfate salt cannot be obtained. Therefore, a large amount of ammonium sulfate is inevitably incorporated into the double decomposition reaction system during the return of materials. From the chemical equation of the double decomposition reaction between sodium sulfate and ammonium bicarbonate, ammonium sulfate is one of the products of the double decomposition reaction. When the concentration of ammonium sulfate in the system increases, the reaction equilibrium will shift to the left, resulting in a decrease in the single-pass conversion rate of sodium ions to sodium bicarbonate, reducing the material utilization efficiency and increasing the system operation cost. The present invention utilizes the concentration difference between sodium sulfate and ammonium sulfate at high temperatures, and a large amount of high-purity sodium sulfate is precipitated by controlling process conditions through evaporation, avoiding the precipitation of ammonium sulfate, reducing the ammonium sulfate introduced into the double decomposition reaction system during the return of materials, and increasing the primary reaction conversion rate of sodium sulfate. 3. After sodium sulfate is concentrated and precipitated in the first-stage evaporation, high concentrations of sodium ions, ammonia nitrogen, and sulfate ions still remain in the mother liquor, and the concentrations of sodium ions and ammonium ions are close. According to ternary phase diagram analysis, it is difficult to obtain high-purity ammonium sulfate by evaporation at this time, and only a mixed salt of sodium sulfate and ammonium sulfate can be obtained, and high-purity ammonium sulfate cannot be obtained. In this process, due to the introduction of phosphate ions in the heavy metal removal stage, the ternary system of sulfate, ammonia nitrogen, and sodium ions becomes a quaternary system, providing a higher solubility anion pairing for sodium ions, so that when ammonium sulfate is produced by evaporation, sodium ions will not precipitate in the form of sodium sulfate, but will remain in the water body in the form of sodium dihydrogen phosphate with higher solubility, which can effectively reduce the content of sodium ion impurities in evaporation crystallization; in addition, this process recovers the tail gas produced in the double decomposition reaction stage, not only improving the raw material utilization efficiency, but also further increasing the nitrogen content of the crystals obtained by evaporation, obtaining agricultural-grade ammonium sulfate meeting the national standard GB / T 535-2020. 4. Based on the quaternary system phase diagram, this process optimizes the sodium sulfate resource utilization process route, improves the control of process conditions, subtracts a stage of cooling crystallization process, and optimizes the process from high-temperature evaporation - low-temperature condensation - high-temperature evaporation process to two-stage high-temperature evaporation process, greatly reducing the process energy consumption and reducing a large amount of process operation costs, having great advantages.

[0050] In a conventional process (such as CN 111039310A), the single-pass conversion rate of sodium ions is 60%. By increasing the concentration of sodium ions, the single-pass conversion rate is increased to 70%-78%. However, the drawback is that the product purity is sacrificed. The process of the present invention improves the purity of the recycled sodium sulfate by optimizing the process flow and process parameters, and also realizes the improvement of the single-pass conversion rate to 65-70% without significantly increasing the concentration of sodium ions, and the product purity can still reach more than 97%, which is better than the existing conventional process.

[0051] Unless otherwise specified, the "about" in the present invention actually means that the allowable error is within the range of ±2%. For example, about 100 is actually 100 ± 2% × 100.

[0052] Unless otherwise specified, the "between... and..." in the present invention includes the endpoints. For example, "between 2 and 3" includes the endpoint values 2 and 3.

[0053] Other features and advantages of the present invention will be described in the following description of the specification, and in part, will become obvious from the description of the specification, or will be understood by implementing the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, where:

[0055] Figure 1 is a schematic flow chart of an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] The following will clearly and completely describe the concept and technical effects generated by the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative efforts shall fall within the scope of protection of the present invention.

[0057] In the description of the present invention, the description of reference terms such as "one embodiment", "some embodiments", "schematic embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0058] Example 1

[0059] ReferenceFigure 1 For the process, this example provides a method for recovering high-salt sodium sulfate wastewater, and the specific steps are as follows:

[0060] S1. Add 20 ml of phosphate group heavy metal capturer to the wastewater (take 1 L of sodium sulfate wastewater produced in the new energy industry, with a sodium sulfate content of 254 g / L, a lead content of 1.6 g / L, and a zinc content of 0.9 g / L). Control the pH to 7.8 by adding sulfuric acid and sodium hydroxide. After stirring and reacting for 30 min, add flocculant PAM and react for 15 min. After filtration and separation, detect the heavy metal removal mother liquor. The remaining lead concentration is 1.3 mg / L, and the remaining zinc concentration is 4.3 mg / L. The lead removal rate is 99.9%, and the zinc removal rate is 99.5%.

[0061] The composition of the phosphate group heavy metal capturer is: 1 g / L of Na6O 18 P6 (sodium hexametaphosphate) and 1.5 g / L of Na5P3O 10 (sodium tripolyphosphate);

[0062] S2. Carry out atmospheric pressure high-temperature evaporation and concentration on the heavy metal removal mother liquor, concentrate by about 20%. At this time, the sodium sulfate concentration in the solution is 450 g / L, that is, a concentrated mother liquor with a sodium-to-water ratio of 45:100 is obtained. Cool it to 50 °C, slowly add solid ammonium bicarbonate to the concentrated mother liquor and stir to react. Control the system temperature at 38 °C. The total molar amount of ammonium bicarbonate added is 2 times the total molar amount of sodium sulfate. During this process, add ammonia water to keep the pH value of the system at 8.0 - 8.1. After the complete addition of ammonium bicarbonate, adjust the pH to 8.1. After reacting for 1.5 h, carry out centrifugal filtration and separation to obtain crude sodium bicarbonate crystals (crude sodium bicarbonate) and double decomposition mother liquor. The ammonia gas emitted during the double decomposition reaction is neutralized and absorbed by sulfuric acid to obtain a tail gas absorption solution;

[0063] S3. Carry out rectification on the double decomposition mother liquor to recover ammonium bicarbonate. During rectification, the bottom temperature of the kettle is 102.8 °C, and the top temperature is 98 °C. Condense and recover to obtain 302 ml of condensate with crystals. Pass 50 ml of carbon dioxide into it, and then carry out centrifugal filtration to obtain ammonium bicarbonate solid, which is returned to the concentrated mother liquor in step S2 for double decomposition reaction;

[0064] S4. Carry out the first-stage evaporation and concentration on the mother liquor after rectification at 80 °C. Control the evaporation pressure at -78 - 80 Kpa. The evaporation and condensation water volume is 15% of the total volume of the solution. The gas-liquid temperature difference reaches 9 °C. At this time, the crystal mass fraction in the mother liquor is 5%. After hot filtration, 82.95 g of sodium sulfate crystals are obtained. Detect its composition and return the mother liquor to the heavy metal removal mother liquor in step S2 for atmospheric pressure high-temperature evaporation and concentration. The condensed water obtained by evaporation is used to wash the crude sodium bicarbonate crystals obtained by the double decomposition reaction in step S2;

[0065] S5. Add the tail gas absorption liquid obtained in step S2 to a section of evaporation mother liquor, then cool it to 60°C by flash evaporation, and start the second-stage evaporation and concentration at 60°C. The evaporation pressure is controlled at -85 to -87 Kpa, the evaporation condensate water volume is 38% of the total solution volume. After hot filtration, ammonium sulfate crude crystals are obtained, and the condensate water generated by evaporation is used to circulate and wash the ammonium sulfate crude crystals to obtain ammonium sulfate products.

[0066] The mass of the sodium bicarbonate solid obtained in this example is 259.12 g, the single-pass conversion rate of sodium ions is 65.0%, the purity of the sodium bicarbonate product is 97.89%, and the utilization rate of sodium sulfate is 97%; the nitrogen content in the crystals produced by the second-stage evaporation is 20.28%, and the free acid is less than 0.1%, meeting the requirements of GB535-2020. The detection data of the produced crystals are shown in Table 1 below:

[0067] Table 1. Crystal detection data

[0068] Crystal composition Double decomposition reaction crystal <![CDATA[NaHCO3: 97.89%, Na2SO4: 2.11%]]> First-stage evaporation and concentration crystal <![CDATA[Na2SO4: 99.01%, (NH4)2SO4: 0.51%, NH4H2PO4: 0.48%]]> Second-stage evaporation and concentration crystal <![CDATA[(NH4)2SO4: 93.95%, Na2SO4: 3.20%, NH4H2PO4: 2.85%]]>

[0069] Example 2

[0070] Reference Figure 1 According to the process, this example provides a method for recycling high-salt sodium sulfate wastewater, and the specific steps are as follows:

[0071] S1. Add 8 ml of phosphate group heavy metal capturer to the wastewater (take 1 L of sodium sulfate wastewater produced in the new energy industry, with a sodium sulfate content of 173 g / L, a cadmium content of 0.8 g / L, and a nickel content of 0.3 g / L), control the pH to 7.8 by adding sulfuric acid and sodium hydroxide, stir and react for 30 min, then add the flocculant PAM and react for 15 min. After filtration and separation, the heavy metal removal mother liquor is detected. The remaining cadmium concentration is 1.9 mg / L, the remaining nickel concentration is 0.6 mg / L, the cadmium removal rate is 99.8%, and the nickel removal rate is 99.8%.

[0072] The composition of the phosphate group heavy metal capturer is: 3 g / L Na6O 18 P6 (sodium hexametaphosphate), 6 g / L Na5P3O 10 (sodium tripolyphosphate) and 0.5 g / L Na2HPO4 (disodium hydrogen phosphate);

[0073] S2. The deweighting mother liquor is evaporated and concentrated under normal pressure at high temperature, concentrated by about 41%. At this time, the sodium sulfate concentration in the solution is 460 g / L, that is, the sodium-water ratio is 46:100. It is cooled to 55 °C, and solid ammonium bicarbonate is slowly added to the concentrated mother liquor and stirred for reaction. The system temperature is controlled at 35 °C. The total molar amount of ammonium bicarbonate added is 2 times the total molar amount of sodium sulfate. During this process, ammonia water is added to keep the pH value of the system at 8.0 - 8.1. After the complete addition of ammonium bicarbonate, the pH is adjusted to 8.0. After reacting for 1.5 h, centrifugal filtration is carried out to separate the crude sodium bicarbonate crystals (crude sodium bicarbonate product) and the double decomposition mother liquor. The ammonia gas emitted during the double decomposition reaction process is neutralized and absorbed by sulfuric acid to obtain the tail gas absorption liquid;

[0074] S3. The double decomposition mother liquor is rectified to recover ammonium bicarbonate. During rectification, the bottom temperature of the kettle is 103.5 °C, and the top temperature is 99 °C. 135 ml of condensate with crystals is obtained by condensation and recovery. 20 ml of carbon dioxide is introduced into it, and then it is centrifugally filtered to obtain solid ammonium bicarbonate, which is returned to the concentrated mother liquor in step S2 for double decomposition reaction;

[0075] S4. The mother liquor after rectification is subjected to the first-stage evaporation and concentration at 85 °C. The evaporation pressure is controlled at -78 - 80 Kpa. The evaporation condensate water volume is 20% of the total volume of the solution, and the gas-liquid temperature difference reaches 9 °C. At this time, the crystal mass fraction in the mother liquor is 11%. After hot filtration, 69.92 g of sodium sulfate crystals are obtained. Its composition is detected and returned to the deweighting mother liquor in step S2 for evaporation and concentration under normal pressure at high temperature. The condensate water obtained by evaporation is used to wash the crude sodium bicarbonate crystals obtained from the double decomposition reaction in step S2;

[0076] S5. The tail gas absorption liquid obtained in step S2 is added to the mother liquor of the first-stage evaporation, and then it is cooled to 57 °C by flash evaporation. The second-stage evaporation and concentration starts at 57 °C. The evaporation pressure is controlled at -85 - 87 Kpa. The evaporation condensate water volume is 42% of the total volume of the solution. After hot filtration, crude ammonium sulfate crystals are obtained. The condensate water generated by evaporation is used to circulate and wash the crude ammonium sulfate crystals to obtain ammonium sulfate products.

[0077] The mass of the sodium bicarbonate solid obtained in this example is 178.81 g, the single-pass conversion rate of sodium ions is 63%, the purity of the sodium bicarbonate product is 98.01%, and the utilization rate of sodium sulfate is 98%. The ammonia nitrogen content in ammonium sulfate is 20.04%, and the free acid is less than 0.1%, meeting the requirements of GB535 - 2020. The detection data of each crystal produced are shown in Table 2 below:

[0078] Table 2. Crystal detection data

[0079]

[0080] Example 3

[0081] Reference Figure 1 According to the process of Figure 1 , this example provides a method for recovering high-salt sodium sulfate wastewater, and the specific steps are as follows:

[0082] S1. Add 2 ml of phosphate group heavy metal capturer to the wastewater (take 1 L of sodium sulfate wastewater produced by the new energy industry, with a sodium sulfate content of 86 g / L, a copper content of 0.3 g / L, a nickel content of 0.1 g / L, and a nickel content of 0.3 g / L). Control the pH to 8 by adding sulfuric acid and sodium hydroxide. After stirring and reacting for 30 min, add flocculant PAM and react for 15 min. After filtration and separation, detect the heavy metal removal mother liquor. The remaining copper concentration is 0.6 mg / L, the remaining nickel concentration is 0.5 mg / L, the copper removal rate is 99.8%, and the nickel removal rate is 99.5%.

[0083] The composition of the phosphate group heavy metal capturer is: 8 g / L Na6O 18 P6 (sodium hexametaphosphate), 9 g / L Na5P3O 10 (sodium tripolyphosphate) and 5 g / L NaH2PO4 (sodium dihydrogen phosphate);

[0084] S2. Carry out atmospheric pressure high-temperature evaporation and concentration on the heavy metal removal mother liquor, concentrating by about 76%. At this time, the sodium sulfate concentration in the solution is 470 g / L, that is, the sodium-water ratio is 47:100. Cool it to 55 °C, and slowly add solid ammonium bicarbonate to the concentrated mother liquor for stirring reaction. Control the system temperature at 40 °C. The total molar amount of ammonium bicarbonate added is 2 times the total molar amount of sodium sulfate. During this process, add ammonia water to keep the pH value of the system at 8.1 - 8.2. After the complete addition of ammonium bicarbonate, adjust the pH to 8.2. After reacting for 2 h, centrifuge and filter to obtain crude sodium bicarbonate crystals (crude sodium bicarbonate) and double decomposition mother liquor. The ammonia gas emitted during the double decomposition reaction is neutralized and absorbed by sulfuric acid to obtain the tail gas absorption liquid;

[0085] S3. Carry out rectification to recover ammonium bicarbonate from the double decomposition mother liquor. During rectification, the bottom temperature of the kettle is 104.9 °C, and the top temperature is 100 °C. Condense and recover 90 ml of condensate with crystals. Pass 12 ml of carbon dioxide into it, and then centrifuge and filter to obtain solid ammonium bicarbonate, which is returned to the concentrated mother liquor in step S2 for double decomposition reaction;

[0086] S4. Carry out the first-stage evaporation and concentration of the rectified mother liquor at 85 °C. Control the evaporation pressure at -78 - 80 Kpa. The evaporation and condensation water volume is 30% of the total volume of the solution, and the gas-liquid temperature difference reaches 9 °C. At this time, the crystal mass fraction in the mother liquor is 20%. Filter while it is hot to obtain 29.92 g of sodium sulfate crystals. Detect its composition and return it to the heavy metal removal mother liquor in step S2 for atmospheric pressure high-temperature evaporation and concentration. The condensed water obtained by evaporation is used to wash the crude sodium bicarbonate crystals obtained from the double decomposition reaction in step S2;

[0087] S5. Add the tail gas absorption liquid obtained in step S2 to a certain amount of evaporation mother liquor, and then cool it to 55°C by flash evaporation. Start the second-stage evaporation and concentration at 55°C, control the evaporation pressure at -85 to -87 Kpa, and the evaporation condensate water volume is 32% of the total solution volume. After hot filtration, obtain crude ammonium sulfate crystals. The condensate water generated by evaporation is used to circulate and wash the crude ammonium sulfate crystals to obtain ammonium sulfate products.

[0088] The mass of the sodium bicarbonate solid obtained in this example is 81.93 g, the single-pass conversion rate of sodium ions is 61%, the purity of the sodium bicarbonate product is 97.53%, and the utilization rate of sodium sulfate is 98%. The ammonia nitrogen content in ammonium sulfate is 20.77%, and the free acid is less than 0.1%, meeting the requirements of GB535-2020. The detection data of each crystal produced are shown in Table 3;

[0089] Determination method of bicarbonate radical: According to GB / T 1606-2008 "Industrial Sodium Bicarbonate", the acid-base titration method is also adopted.

[0090] Reagents and instruments: Hydrochloric acid standard titration solution (generally 1 mol / L), bromocresol green-methyl red mixed indicator solution; conical flask, burette, etc.

[0091] Operation steps: Weigh a certain amount of sample, place it in a conical flask, add appropriate amount of water and bromocresol green-methyl red mixed indicator solution, and titrate with hydrochloric acid standard titration solution until the solution changes from green to dark red. Boil for 2 min, cool and continue titrating until dark red.

[0092] Calculation: Calculate the content of sodium bicarbonate according to the volume, concentration of the hydrochloric acid standard titration solution consumed in the titration and the sample mass.

[0093] Determination method of nitrogen content in agricultural grade ammonium sulfate: Formaldehyde method;

[0094] Principle: The ammonium ion ((NH4 + )) in ammonium sulfate reacts with formaldehyde to quantitatively generate protonated hexamethylenetetramine and free (H + ). The reaction formula is 4NH4 + + 6HCHO → (CH2)6N4H + + 6H2O + 3H + . The generated protonated hexamethylenetetramine (Ka = K a = 7.1×10 -6 ) and H + can be directly titrated with NaOH standard solution, using phenolphthalein as the indicator. Titrate until the solution shows a stable light red color, which is the end point. According to the stoichiometric relationship between hydrogen ion (H + ) and ammonium ion (NH4 + ), calculate the nitrogen content in the sample, and then obtain the ammonia nitrogen content.

[0095] Main instruments and reagents: analytical balance, basic burette, pipette, measuring cylinder, conical flask, 20% formaldehyde solution, (0.1mol / L NaOH) standard solution, ammonium sulfate sample, phenolphthalein indicator, methyl red indicator, potassium hydrogen phthalate.

[0096] Experimental procedures

[0097] Preparation and standardization of (NaOH) solution: Weigh a certain amount of (NaOH), dissolve it in distilled water, transfer it to a reagent bottle with a rubber stopper and shake well. Weigh potassium hydrogen phthalate by the subtraction method, put it into a conical flask, dissolve it in distilled water, add phenolphthalein indicator, and titrate with (NaOH) solution until the solution turns slightly red and does not fade for half a minute. Record the data and calculate the concentration of (NaOH) solution.

[0098] Treatment of formaldehyde solution: Take the upper clear liquid of the original formaldehyde, dilute it with water by a factor of two, add phenolphthalein indicator, and neutralize it with \(NaOH\) solution until it turns light red.

[0099] Determination of nitrogen content in ammonium sulfate: Accurately weigh the ammonium sulfate sample, place it in a conical flask, dissolve it in distilled water, add methyl red indicator, and titrate with (NaOH) standard solution until the solution just changes from red to yellow. Then add formaldehyde solution, shake well, let it stand for 1 min, add phenolphthalein indicator, and titrate with (NaOH) standard solution until the solution turns slightly red and remains unchanged for 30 s. Record the volume of (NaOH) solution used, perform three parallel determinations, calculate the mass fraction of nitrogen in the sample, and thus obtain the ammonia nitrogen content.

[0100] Main instruments and reagents: analytical balance, basic burette, pipette, measuring cylinder, conical flask, 20% formaldehyde solution, \(0.1mol / L NaOH\) standard solution, ammonium sulfate sample, phenolphthalein indicator, methyl red indicator, potassium hydrogen phthalate.

[0101] Experimental procedures

[0102] Preparation and standardization of NaOH solution: Weigh a certain amount of NaOH, dissolve it in distilled water, transfer it to a reagent bottle with a rubber stopper and shake well. Weigh potassium hydrogen phthalate by the subtraction method, put it into a conical flask, dissolve it in distilled water, add phenolphthalein indicator, and titrate with NaOH solution until the solution turns slightly red and does not fade for half a minute. Record the data and calculate the concentration of NaOH solution.

[0103] Treatment of formaldehyde solution: Take the upper clear liquid of the original formaldehyde, dilute it with water by a factor of two, add phenolphthalein indicator, and neutralize it with NaOH solution until it turns light red.

[0104] Determination of nitrogen content in ammonium sulfate: Accurately weigh the ammonium sulfate sample, place it in a conical flask, dissolve it with distilled water, add methyl red indicator, and titrate it with the NaOH standard solution until the solution just changes from red to yellow. Then add formaldehyde solution, shake well, let it stand for 1 min, add phenolphthalein indicator, and titrate it with the NaOH standard solution until the solution turns slightly red and remains unchanged for 30 s. Record the volume of the NaOH solution used. Perform three parallel determinations, calculate the mass fraction of nitrogen in the sample, and thus obtain the ammonia nitrogen content.

[0105] Table 3. Crystal detection data

[0106]

[0107] Comparative Example 1

[0108] The difference between this comparative example and Example 1 is that in step S1, a heavy metal collector with a phosphate group is not used, but a conventional Fenton process is used. 20 ml of ferrous sulfate and 20 ml of hydrogen peroxide are added for reaction. After stirring and reacting for 30 min, a flocculant PAM is added and reacted for 15 min. After filtration and separation, the heavy metal removal mother liquor is detected. The remaining concentration of lead is 5.8 mg / L, the remaining concentration of zinc is 7.9 mg / L, the lead removal rate is 99.6%, and the zinc removal rate is 99.1%.

[0109] Other process steps are exactly the same as those in Example 1.

[0110] The mass of sodium bicarbonate obtained in this comparative example is 258.33 g, the single-pass conversion rate of sodium ions is 64.8%, the purity of the sodium bicarbonate product is 98.01%, and the utilization rate of sodium sulfate is 97%. The nitrogen content in the crystals produced by the second-stage evaporation is 16.18%, and the free acid is less than 0.1%, which does not meet the requirements of GB535-2020. The test results are shown in Table 4.

[0111] Table 4. Crystal detection data

[0112]

[0113] It can be seen from Comparative Example 1 that when a high-phosphorus biological agent is not used, the crystals obtained in the second-stage evaporation and concentration process will be doped with a large amount of sodium sulfate crystals, resulting in a decrease in the nitrogen content of the crystals, which is lower than the agricultural ammonium sulfate standard in GB535-2020.

[0114] Comparative Example 2

[0115] The difference between this comparative example and Example 1 is that during the atmospheric pressure high-temperature concentration in step S2, when the concentration is about 40%, the sodium sulfate concentration is 561.58 g / L, that is, the sodium-water ratio is 56:100. Other process steps are exactly the same as those in Example 1.

[0116] In this comparative example, the mass of sodium bicarbonate obtained was 384.40 g, the single-pass conversion rate of sodium ions was 73.2%, the purity of the sodium bicarbonate product was 75.90%, and the utilization rate of sodium sulfate was 95%. The nitrogen content in the crystals produced by the second-stage evaporation was 20.52%, and the free acid was less than 0.1%, meeting the requirements of GB535-2020. The results are shown in Table 5.

[0117] Table 5. Crystal detection data

[0118]

[0119] It can be seen from Comparative Example 2 that when excessive concentration leads to too high a sodium-to-water ratio, a large amount of sodium sulfate crystals and a small amount of ammonium sulfate crystals are doped in the crystals produced by the double decomposition reaction, resulting in a decrease in the purity of the sodium bicarbonate product.

[0120] Comparative Example 3

[0121] The difference between this comparative example and Example 1 lies in that during the first-stage evaporation and concentration in step S4, the amount of condensed water distilled out was 50% of the volume of the original mother liquor, and the other process steps were exactly the same as those in Example 1.

[0122] In this comparative example, the mass of sodium bicarbonate obtained was 217.78 g, the single-pass conversion rate of sodium ions was 54.6%, and the purity of the sodium bicarbonate product was 97.55%.

[0123] The ammonia nitrogen content in the crystals produced by the second-stage evaporation was 20.11%, the free acid was less than 0.1%, meeting the requirements of GB535-2020, and the utilization rate of sodium sulfate was 95%.

[0124] The results are shown in Table 6 as follows:

[0125] Table 6. Crystal detection data

[0126]

[0127]

[0128] Comparative Example 4

[0129] The difference between this comparative example and Example 1 lies in that during the first-stage evaporation and concentration in step S4, the amount of condensed water distilled out was 6% of the volume of the original mother liquor, the gas-liquid temperature difference reached 7 °C, at this time the crystal mass fraction in the mother liquor was 3%, 40.13 g of sodium sulfate crystals were obtained after hot filtration, their components were detected, and they were returned to the double decomposition reaction. The other process steps were exactly the same as those in Example 1.

[0130] The mass of sodium bicarbonate obtained in this comparative example was 230.12 g, the single-pass conversion rate of sodium ions was 66.13%, the purity of the sodium bicarbonate product was 98.49%, and the utilization rate of sodium sulfate was 83%. The ammonia nitrogen content in the crystals produced by the second-stage evaporation was 16.99%, and the free acid was less than 0.1%, not meeting the requirements of GB535-2020.

[0131] The results are shown in Table 7 as follows:

[0132] Table 7. Crystal detection data

[0133] Crystal composition Double decomposition reaction crystal <![CDATA[NaHCO3: 98.49%, Na2SO4: 1.51%]]> First-stage evaporation and concentration crystal <![CDATA[Na2SO4: 99.53%, (NH4)2SO4: 0.34%, NH4H2PO4: 0.13% <!-- 11 -->]]> Second-stage evaporation and concentration crystal <![CDATA[(NH4)2SO4: 75.13%, Na2SO4: 23.44%, NH4H2PO4: 1.43%]]>

[0134] It can be seen from Comparative Example 4 that when the evaporation amount of the first-stage evaporation and concentration is too low, a large amount of sodium ions in the system cannot be efficiently utilized, the utilization rate of sodium sulfate decreases, and a large amount of sodium ions enter the crystals of the second-stage evaporation and concentration, reducing the product purity of ammonium sulfate.

[0135] Comparative Example 5

[0136] The difference between this comparative example and Example 1 lies in that during the second evaporation and concentration in step S4, the amount of evaporation condensate water is 60% of the total volume of the solution, and ammonium sulfate crystals are obtained after hot filtration. The condensate water generated by evaporation is used to circulate and wash the crude ammonium sulfate crystals to obtain ammonium sulfate products, and the other process steps are exactly the same as those in Example 1.

[0137] The ammonia nitrogen content in the crystals produced by the second-stage evaporation obtained in this comparative example was 17.88%, and the free acid was less than 0.1%, not meeting the requirements of GB535-2020.

[0138] The results are shown in Table 8 as follows:

[0139] Table 8. Crystal detection data

[0140] Second-stage evaporation and concentration crystal <![CDATA[(NH4)2SO4: 79.18%, Na2SO4: 11.93%, NH4H2PO4: 8.89%]]>

[0141] It can be seen from the case of Comparative Example 5 that when the evaporation amount of the second-stage evaporation and concentration is too high, a large amount of sodium sulfate will precipitate in the system, reducing the product purity of ammonium sulfate in the crystals obtained by the second-stage evaporation.

[0142] The above has been described in detail in combination with the embodiments of the present invention. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the scope of knowledge possessed by those of ordinary skill in the art. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for recovering high-salt wastewater containing sodium sulfate, characterized in that, The method includes the following steps: S1. Add a heavy metal capturer containing a phosphate group to the sodium sulfate wastewater, react to remove heavy metals in the wastewater, and obtain a mother liquor after heavy metal removal. The heavy metal capturer containing a phosphate group includes at least one of sodium hexametaphosphate, sodium tripolyphosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate. S2. Concentrate the mother liquor after heavy metal removal, perform a metathesis reaction with ammonium bicarbonate, and separate to obtain crude sodium bicarbonate and a mother liquor after metathesis. After concentrating the mother liquor after heavy metal removal, a concentrated mother liquor is obtained, and the concentration of sodium sulfate in the concentrated mother liquor is 450 - 470 g / L. S3. Rectify the mother liquor after metathesis to obtain a rectified mother liquor and a distillate at the top of the column. S4. Perform first-stage evaporation concentration on the rectified mother liquor, and separate to obtain sodium sulfate and a mother liquor after the first-stage evaporation. In the step of first-stage evaporation concentration, the amount of condensed water distilled out is 15% - 35% of the total volume of the rectified mother liquor. S5. Perform second-stage evaporation concentration on the mother liquor after the first-stage evaporation to obtain ammonium sulfate crystals. In the step of second-stage evaporation concentration, the amount of condensed water distilled out is 30% - 50% of the total volume of the mother liquor after the first-stage evaporation.

2. The method according to claim 1, wherein In the reaction of step S1, the pH is controlled to be 6 - 8; and / or, the liquid-to-solid ratio of the heavy metal capturer containing a phosphate group to the heavy metal in the wastewater is 5 - 25 ml:g; and / or, the reaction temperature is 30 - 60 °C; and / or, the reaction time is 15 - 60 min.

3. The method according to claim 1, characterized in that In the metathesis reaction of step S2, the amount of ammonium bicarbonate added in the metathesis reaction is such that the molar ratio of sodium ions to ammonium ions in the solution is 1:1 - 1.02; the reaction temperature is 35 - 42 °C; and / or, the pH of the reaction is 7.9 - 8.

4.

4. The method according to claim 1, characterized in that The rectification is carried out in a rectification column. The bottom temperature of the rectification column is 102 - 105 °C, the top temperature of the rectification column is 90 °C - 100 °C, the pressure of the rectification column is -5 - 5 Kpa, and the condensation temperature in the rectification column is 30 - 40 °C.

5. The method according to claim 1, wherein In the first-stage evaporation concentration of step S4, the pressure of evaporation concentration is -75 - 80 Kpa, and the temperature of evaporation concentration is 75 °C - 85 °C; and / or, evaporation concentration is terminated when the salt output reaches 5% - 25% and the gas-liquid temperature difference reaches 7 - 15 °C.

6. The method according to claim 1, wherein In the second-stage evaporation concentration of step S5, the pressure of evaporation concentration is -80 - 95 Kpa, and the temperature of evaporation concentration is 55 - 65 °C; and / or, evaporation concentration is carried out until the salt output reaches 5% - 25%.

7. The method according to claim 1, wherein In the metathesis reaction of step S2, the reaction pH is controlled by ammonia water. The ammonia gas overflowing during the reaction is absorbed by an acid solution to obtain an absorption solution. When the pH of the absorption solution is 6.0 - 7.0, it is added to the mother liquor after the first-stage evaporation in step S5 for second-stage evaporation concentration.

8. The method according to claim 1, characterized in that, It also includes adding the sodium sulfate obtained in step S4 to the mother liquor after heavy metal removal in step S2 for concentration.

9. The method according to claim 1, wherein It also includes introducing carbon dioxide into the distillate at the top of the column to obtain ammonium bicarbonate, and then adding it to the concentrated mother liquor in step S2 for metathesis reaction.

10. Application of the method according to any one of claims 1 - 9 in the recycling of waste batteries.

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