Resource recycling method for sodium sulfate, sodium chloride and sodium nitrate aqueous solution

Abstract

The invention relates to the technical field of high-salinity wastewater treatment, and discloses a method for recycling a sodium sulfate, sodium chloride and sodium nitrate aqueous solution, which comprises the following steps: carrying out MVR (Mechanical Vapor Recompression) evaporation concentration on a raw material solution; according to the method, crude sodium sulfate decahydrate is separated out through freezing crystallization, then series temperature difference crystallization is carried out on the obtained freezing mother liquor, sodium chloride and sodium nitrate are sequentially separated and recycled, and final cooling mother liquor is obtained. And completely returning the depleted washing liquid generated after washing and the final cooling mother liquid to the system for circulation. According to the method, high yield and high purity of sodium sulfate are guaranteed through an active washing technology, complete recovery of salt resources and process zero emission are achieved through mother liquor closed-loop circulation, and the problems that a complex salt system is difficult to separate and recover and waste water is discharged are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-salinity wastewater treatment, and in particular to a method for resource recycling of sodium sulfate, sodium chloride and sodium nitrate aqueous solutions. BACKGROUND

[0002] In the production processes of chemical industry, metallurgy and new energy industry, industrial wastewater containing sodium sulfate, sodium chloride, sodium nitrate and other salt components is often generated. The treatment and resource utilization of such high-salinity wastewater is an important technical issue in the industry.

[0003] The existing technology usually faces the technical challenges of unclear separation path and poor recovery effect of each component when treating such complex aqueous solutions containing multiple sodium salts. Since the solubility behaviors of sodium sulfate, sodium chloride and sodium nitrate in aqueous solution are interrelated and vary with temperature, it is difficult to effectively separate a single salt component by using traditional evaporation or cooling crystallization methods. Co-precipitation of salt components often occurs, resulting in low product purity, low separation efficiency, and inability to effectively recover each salt component in a step-by-step manner.

[0004] In addition, when purifying the crude salt product obtained by crystallization, the traditional process usually uses water or unsaturated solution for washing. Although this method can remove impurities and mother liquor attached to the surface of the crystals, it will inevitably dissolve part of the target product, especially for salts with high solubility. This product loss during the washing process directly leads to a decrease in the overall yield of the final product, creating a technical contradiction between improving product purity and maintaining high product yield.

[0005] More importantly, after separating and recovering the target product, the existing treatment process produces a final mother liquor containing residual salt components and washing waste liquid. If these waste liquids are directly discharged, not only the valuable salt resources are wasted, but also the high-salinity wastewater will cause secondary pollution to the environment. The existing technical system usually lacks an effective way to close the cycle of these internal process fluids throughout the process, making it difficult to achieve complete utilization of materials and zero discharge of process wastewater. SUMMARY

[0006] The purpose of the present application is to provide a method for resource recycling of sodium sulfate, sodium chloride and sodium nitrate aqueous solutions, which solves the technical problems of low separation efficiency, low product yield, incomplete material recycling, and resource waste and environmental pollution caused by final mother liquor discharge in the prior art when treating complex salt aqueous solutions containing sodium sulfate, sodium chloride and sodium nitrate.

[0007] To achieve the above purpose, the present application is implemented by the following technical solutions: A method for resource recycling of sodium sulfate, sodium chloride and sodium nitrate aqueous solutions, comprising the following steps: S1, concentrating a raw material solution containing sodium sulfate, sodium chloride and sodium nitrate by MVR evaporation to obtain a concentrated solution; S2, freezing and crystallizing the concentrated solution and then performing solid-liquid separation to obtain crude sodium sulfate decahydrate crystals and a frozen mother liquor; S3, performing cascade temperature difference crystallization on the frozen mother liquor, which includes: performing high-temperature evaporation crystallization to separate and recover sodium chloride product to obtain a high-temperature sodium nitrate concentrated solution; and performing cooling crystallization on the high-temperature sodium nitrate concentrated solution to separate and recover sodium nitrate product to obtain a final cooling mother liquor; S4, preparing an active washing solution by mixing the final cooling mother liquor with the raw material solution, and washing the crude sodium sulfate decahydrate crystals with the active washing solution to obtain high-purity sodium sulfate decahydrate crystals as a sodium sulfate recovery product and a depleted washing solution; S5, combining and processing the depleted washing solution with the frozen mother liquor, and returning a main part of the final cooling mother liquor to the high-temperature evaporation crystallization step of step S3.

[0008] By adopting the technical scheme, the application establishes a complete process integrating pre-concentration, step-by-step crystallization, online washing and mother liquor closed-loop circulation. The method first utilizes the characteristic that the solubility of sodium sulfate sharply decreases at low temperature, preferentially separates most of the sodium sulfate by freezing and crystallization; then, utilizes the difference that the solubility of sodium chloride is less affected by temperature and the solubility of sodium nitrate increases with temperature, sequentially separates and recovers sodium chloride and sodium nitrate by cascade temperature difference operation of high-temperature evaporation and cooling crystallization. The core innovation of the whole process lies in the active washing solution technology of step S4 and the mother liquor closed-loop circulation design of step S5, which work together to achieve the goal of high-purity product recovery and zero material discharge of the system.

[0009] The innovative mechanism of the application is specifically described as follows: Regarding the preparation and washing mechanism of the active washing solution (step S4): the traditional water washing process will cause a large reduction in yield due to product dissolution when washing crude sodium sulfate decahydrate crystals. The active washing solution designed by the application overcomes this technical defect, and the mechanism includes: Online preparation of the washing solution: the final cooling mother liquor rich in sodium chloride and sodium nitrate generated in step S3 is mixed with the raw material solution containing sodium sulfate at low temperature (for example, 0-5℃). Since the concentration of sodium sulfate in the final cooling mother liquor is low, after mixing, the sodium sulfate in the raw material solution is in a supersaturated state with respect to the mixed solution system.

[0010] Spontaneous nucleation: under low temperature and stirring, the supersaturated sodium sulfate spontaneously precipitates to form a large number of small sodium sulfate decahydrate crystal nuclei, making the obtained washing solution a saturated solution containing microcrystals.

[0011] High-efficiency washing process: when washing the crude sodium sulfate decahydrate crystals with the prepared active washing liquid: Inhibition of product dissolution: since the active washing liquid itself is a saturated solution of sodium sulfate decahydrate, according to the principle of the common ion effect, it will not further dissolve the crude sodium sulfate decahydrate crystals as the product during the washing process, thereby fundamentally avoiding the loss of the product during the washing process and ensuring a high recovery rate of sodium sulfate.

[0012] High-efficiency removal of impurities: at the same time, the liquid phase of the active washing liquid is derived from the final cooling mother liquor, which is unsaturated with respect to sodium chloride and sodium nitrate. Therefore, when the active washing liquid contacts the surface of the crude crystals with high concentrations of sodium chloride and sodium nitrate entrained and adhered, it can quickly and efficiently dissolve and remove these impurity ions, thereby achieving high-efficiency purification of the crude crystals.

[0013] Regarding the material closed loop and zero discharge mechanism (steps S3 and S5): the present application constructs a closed loop of materials within the system by connecting the temperature difference crystallization and the circulation of multiple mother liquors, and the mechanism includes: Serial temperature difference separation: in step S3, first, evaporation is carried out at a high temperature (e.g., 80-105°C) to utilize the gentle solubility characteristics of sodium chloride to cause it to crystallize and precipitate; then, the high-temperature sodium nitrate concentrated solution after separation of sodium chloride is cooled (e.g., 15-25°C) to utilize the steep solubility characteristics of sodium nitrate to cause it to crystallize and precipitate in large quantities. This serial operation realizes the effective separation of sodium chloride and sodium nitrate in different temperature zones.

[0014] Mother liquor recycling: step S5 is the key to achieving zero discharge.

[0015] Return of the depleted washing liquid: the depleted washing liquid produced in step S4, in which sodium chloride and sodium nitrate are dissolved from the crude crystals, is incorporated into the frozen mother liquor and enters step S3 for subsequent separation, thereby avoiding the generation of washing waste liquid.

[0016] Return of the final mother liquor: the final cooling mother liquor produced after cooling crystallization in step S3, although most of the sodium nitrate has been recovered, is still a multi-component saturated solution containing a considerable concentration of salt. If it is discharged, not only is the resource wasted, but also secondary pollution is caused. The present application returns the main part of it to the high-temperature evaporation crystallization unit of step S3, so that the salt contained therein reenters the separation cycle, thereby realizing the closed loop of salt materials in the entire process flow and ultimately achieving the goal of no process wastewater discharge.

[0017] Preferably, in step S1, the operating temperature of the MVR evaporation concentration is 70-90℃, the operating pressure is 30-70kPa (absolute pressure), and the concentration is to a total solid content of 25.0-35.0wt% in the concentrated feed liquid. By adopting the above technical scheme, the feed liquid can reach a suitable supersaturation while ensuring efficient and energy-saving operation of the MVR evaporator, providing favorable initial conditions for subsequent freezing crystallization, and avoiding excessive energy consumption or salt precipitation in the concentration stage.

[0018] Preferably, in step S2, the cooling endpoint temperature of the freezing crystallization is -5℃ to 0℃, the cooling rate is 5-10℃ / h, and the crystallization is constant at the cooling endpoint temperature for 2-4h. By adopting the above technical scheme, the solubility of sodium sulfate decahydrate at this temperature is extremely low, and a higher crystallization yield can be obtained. At the same time, a suitable cooling rate and constant temperature time are conducive to forming crystals with larger particle size and easy separation.

[0019] Preferably, in step S3, the operating temperature of the high-temperature evaporation crystallization is 80-105℃; the cooling endpoint temperature of the cooling crystallization is 15-25℃, the cooling rate is 5-15℃ / h, and the constant temperature crystallization time is 3-5h. By adopting the above technical scheme, the solubility difference of sodium chloride and sodium nitrate at different temperatures can be efficiently utilized to achieve effective separation of the two salts.

[0020] Preferably, in step S4, the active washing liquid is prepared by mixing the final cooling mother liquor with the raw material liquid at a volume ratio of 5:1 to 20:1 and stirring at a temperature of 0-5℃ for 10-30min. By adopting the above technical scheme, a sufficient amount of sodium sulfate microcrystalline nucleus can be stably formed in the system after mixing, and an active washing liquid with stable performance can be prepared.

[0021] Preferably, in step S4, the washing is carried out in a 3-5 stage countercurrent washing machine, the mass ratio of the active washing liquid to the crude sodium sulfate decahydrate crystal is 0.5:1 to 2:1, and the washing temperature is 0-5℃. By adopting the above technical scheme, the countercurrent washing principle can maximize the washing efficiency, and the ideal purification effect can be achieved with less washing liquid. At the same time, low-temperature operation can further ensure the stability of the active washing liquid and the protection of the product.

[0022] Further preferably, in step S1, the operating temperature of the MVR evaporation concentration is 75-85℃, the operating pressure is 45-55kPa, and the concentration is to a total solid content of 28.0-32.0wt% in the concentrated feed liquid.

[0023] Further preferably, in step S2, the cooling end temperature of the freezing crystallization is -3.5℃ to -1.5℃, the cooling rate is 7-8℃ / h, and the isothermal crystallization time is 2.5-3.5h.

[0024] Further preferably, in step S3, the operation temperature of the high-temperature evaporation crystallization is 90-100℃; the cooling end temperature of the cooling crystallization is 18-22℃, the cooling rate is 8-12℃ / h, and the isothermal crystallization time is 3.5-4.5h.

[0025] Further preferably, in step S4, the preparation of the active washing liquid is mixing the final cooling mother liquor and the raw material liquid at a volume ratio of 10:1 to 15:1 and stirring at 2-3℃ for 15-25min; and the mass ratio of the active washing liquid to the crude sodium sulfate decahydrate crystal is 1.0:1 to 1.5:1.

[0026] In summary, the present application includes at least one of the following beneficial technical effects: 1. The present application maintains high yield of sodium sulfate product while ensuring high-purity recovery of sodium sulfate by preparing and using an active washing liquid, which is prepared by mixing the final cooling mother liquor and the raw material liquid to saturate the washing liquid with sodium sulfate decahydrate. In the washing process, the washing liquid can effectively dissolve and remove sodium chloride and sodium nitrate impurities on the surface of the crude crystal, but due to the common ion effect, it will not dissolve the sodium sulfate decahydrate crystal as the product, overcoming the defect of traditional water washing process that leads to reduced yield due to dissolution of the product.

[0027] 2. The present application realizes effective and stepwise separation and recovery of each salt in a multi-component salt aqueous solution by combining freezing crystallization with cascade temperature difference crystallization technology. The method takes advantage of the difference in solubility of sodium sulfate, sodium chloride and sodium nitrate at different temperature ranges to selectively separate and recover sodium sulfate at low temperature, and then sequentially separates and recovers sodium chloride and sodium nitrate through high-temperature evaporation and cooling crystallization. The whole separation path is clear, and the recovery processes of each component interfere with each other little.

[0028] 3. The present application realizes complete utilization of salt materials and zero discharge of process wastewater by constructing a mother liquor closed loop within the process. The method returns the depleted washing liquid produced after washing sodium sulfate decahydrate crystals and the final cooling mother liquor produced after separating sodium nitrate to the previous steps of the separation process, so that the salts contained therein re-enter the recovery cycle. This not only avoids the loss of valuable resources, but also eliminates the environmental problems caused by the discharge of high-salinity wastewater. DETAILED DESCRIPTION

[0029] Examples 1-3: Example 1: The embodiment provides a method for recycling sodium sulfate, sodium chloride and sodium nitrate aqueous solution, which comprises the following steps. 1000 kg of simulated high-salinity wastewater is pumped into an MVR evaporator, evaporated and concentrated under the conditions of an operating temperature of 75 DEG C and an operating pressure of 55 kPa (absolute pressure), and the evaporation and concentration are stopped until the total solid content of the feed liquid reaches 28.0 wt%, and concentrated feed liquid is obtained.

[0030] The concentrated feed liquid is sent into a refrigeration crystallization kettle, cooled to -1.5 DEG C at a cooling rate of 8 DEG C / h, and crystallized at the temperature for 3.5 h. The slurry after crystallization is centrifuged to obtain crude sodium sulfate decahydrate crystals and refrigeration mother liquor.

[0031] The refrigeration mother liquor is mixed with the recycled mother liquor returned from the subsequent step, and high-temperature evaporation crystallization is carried out under the condition of 90 DEG C to separate sodium chloride product and high-temperature sodium nitrate concentrated solution. Then, the high-temperature sodium nitrate concentrated solution is cooled to 22 DEG C at a cooling rate of 12 DEG C / h, and crystallized at the temperature for 4.5 h, and sodium nitrate product and final cooling mother liquor are obtained after separation.

[0032] To prepare active washing liquid, the final cooling mother liquor is mixed with simulated high-salinity wastewater at a volume ratio of 15:1, and stirred at a temperature of 3 DEG C for 25 min. The crude sodium sulfate decahydrate crystals obtained in step 2 are sent into a 3-stage countercurrent washing machine, and washed by using the prepared active washing liquid, and the solid-liquid mass ratio during washing is 1.0:1, and the washing temperature is 3 DEG C, and high-purity sodium sulfate decahydrate crystals and depleted washing liquid are obtained after washing.

[0033] The depleted washing liquid is combined with the refrigeration mother liquor, and the main part of the final cooling mother liquor is returned to the step of removing sodium chloride by high-temperature evaporation crystallization. The wet products separated in each step are subjected to drying treatment to obtain final dry products.

[0034] Example 2: The embodiment provides a method for recycling sodium sulfate, sodium chloride and sodium nitrate aqueous solution, which comprises the following steps. 1000 kg of simulated high-salinity wastewater is pumped into an MVR evaporator, evaporated and concentrated under the conditions of an operating temperature of 75 DEG C and an operating pressure of 55 kPa (absolute pressure), and the evaporation and concentration are stopped until the total solid content of the feed liquid reaches 28.0 wt%, and concentrated feed liquid is obtained.

[0035] The concentrated feed liquid is sent into a refrigeration crystallization kettle, cooled to -1.5 DEG C at a cooling rate of 8 DEG C / h, and crystallized at the temperature for 3.5 h. The slurry after crystallization is centrifuged to obtain crude sodium sulfate decahydrate crystals and refrigeration mother liquor.

[0036] After mixing the frozen mother liquor with the recycled mother liquor returned from the subsequent step, high-temperature evaporation crystallization was carried out at 95°C, and sodium chloride product and high-temperature concentrated sodium nitrate solution were separated. Subsequently, the high-temperature concentrated sodium nitrate solution was cooled to 20°C at a cooling rate of 10°C / h, and isothermal crystallization was carried out for 4h, and sodium nitrate product and final cooling mother liquor were separated after separation.

[0037] To prepare the active washing liquid, the final cooling mother liquor was mixed with the simulated high-salinity wastewater at a volume ratio of 12.5:1, and stirred at a temperature of 2.5°C for 20min. The crude sodium sulfate decahydrate crystals obtained in step 2 were fed into a 4-stage countercurrent washing machine, and washed using the prepared active washing liquid, with a solid-liquid mass ratio of 1.25:1 and a washing temperature of 2.5°C, and high-purity sodium sulfate decahydrate crystals and depleted washing liquid were obtained after washing.

[0038] The depleted washing liquid was incorporated into the frozen mother liquor, and the main part of the final cooling mother liquor was returned to the step of removing sodium chloride by high-temperature evaporation crystallization. The wet products separated in each step were subjected to drying treatment to obtain the final dry product.

[0039] Example 3: The present embodiment provides a method for recycling sodium sulfate, sodium chloride and sodium nitrate aqueous solution resources, comprising the following steps: 1000kg of simulated high-salinity wastewater was pumped into an MVR evaporator, and evaporated and concentrated at an operating temperature of 85°C and an operating pressure of 45kPa (absolute pressure), until the total solid content of the feed liquid reached 32.0wt%, and the concentrated feed liquid was obtained.

[0040] The concentrated feed liquid was fed into a freezing crystallization kettle, cooled to -3.5°C at a cooling rate of 7°C / h, and isothermal crystallization was carried out at this temperature for 2.5h. The slurry after crystallization was centrifuged to obtain crude sodium sulfate decahydrate crystals and a frozen mother liquor.

[0041] After mixing the frozen mother liquor with the recycled mother liquor returned from the subsequent step, high-temperature evaporation crystallization was carried out at 100°C, and sodium chloride product and high-temperature concentrated sodium nitrate solution were separated. Subsequently, the high-temperature concentrated sodium nitrate solution was cooled to 18°C at a cooling rate of 8°C / h, and isothermal crystallization was carried out for 3.5h, and sodium nitrate product and final cooling mother liquor were separated after separation.

[0042] To prepare the active washing liquid, the final cooling mother liquor was mixed with the simulated high-salinity wastewater at a volume ratio of 10:1, and stirred at a temperature of 2°C for 15min. The crude sodium sulfate decahydrate crystals obtained in step 2 were fed into a 5-stage countercurrent washing machine, and washed using the prepared active washing liquid, with a solid-liquid mass ratio of 1.5:1 and a washing temperature of 2°C, and high-purity sodium sulfate decahydrate crystals and depleted washing liquid were obtained after washing.

[0043] The depleted washing liquid is incorporated into the frozen mother liquor; the main part of the final cooled mother liquor is returned to the step of high-temperature evaporation crystallization for removing sodium chloride. The wet products separated in each step are subjected to drying treatment to obtain the final dry products.

[0044] Comparative Examples 1-3: Comparative Example 1: Compared with Example 2, the difference lies in that the crude sodium sulfate decahydrate crystals obtained in Step 2 are not washed with the active washing liquid but are subjected to heating and melting, followed by MVR evaporation recrystallization purification; the frozen mother liquor obtained in Step 2 is not separated by the cascade temperature difference crystallization but is subjected to three-effect evaporation crystallization separation of NaCl, and the separated mother liquor is subjected to single-effect high-temperature evaporation crystallization separation of NaNO3; and when the mother liquor reaches three-phase co-saturation, it is discharged for treatment instead of being recycled.

[0045] Comparative Example 2: Compared with Example 2, the difference lies in that in the washing and purification step of Step 4, the on-line prepared active washing liquid is not used but is replaced by deionized water of the same mass as the crude sodium sulfate decahydrate crystals and at a temperature of 2.5°C, and the remaining steps are the same.

[0046] Comparative Example 3: Compared with Example 2, the difference lies in that in the cascade temperature difference crystallization separation step of Step 3, the three-effect evaporation crystallization separation of NaCl and single-effect high-temperature evaporation crystallization separation of NaNO3 processes same as those in Comparative Example 1 are used, and when the mother liquor reaches three-phase co-saturation, it is discharged for treatment. The remaining steps (including the preparation and washing of the active washing liquid in Step 4) are the same as those in Example 2.

[0047] Test Examples 1-2: Feasibility Test: To verify the feasibility of the technical scheme of the present application, purity analysis and recovery rate calculation are performed on the final products of Examples 1-3.

[0048] Product Purity Analysis: 1.0 g (accurate to 0.0001 g) of the final dried anhydrous sodium sulfate, sodium chloride and sodium nitrate products in Example 1, Example 2 and Example 3, respectively, is accurately weighed into a 100 mL volumetric flask, dissolved with deionized water and diluted to volume, and shaken to obtain a test solution. Ion chromatography (IC) is used for analysis, and the chromatographic conditions are set according to the different ions to be tested. The mass concentrations of Cl - and ions in each test solution are determined by the standard curve method.

[0049] The purity (wt%) of anhydrous sodium sulfate is calculated according to formula (1): In the formula: is the measured mass concentration (mg / L); V is the constant volume (100 mL); m sample is the sample mass (g); is the molar mass of sodium sulfate (142.04 g / mol); is the molar mass of sulfate radical (96.06 g / mol).

[0050] The purity calculation method of sodium chloride and sodium nitrate is similar to the above formula (1), respectively substituting the corresponding molar mass of Cl - (35.45 g / mol) and NaCl (58.44 g / mol), and (62.00 g / mol) and NaNO3 (84.99 g / mol). The component recovery rate calculation The initial total mass (m initial ) of each salt component in the initial 1000 kg of simulated high-salt wastewater in Examples 1, 2 and 3 was accurately weighed. The total mass (m product ) of the dried anhydrous sodium sulfate, sodium chloride and sodium nitrate dry products collected and dried after the stable operation of the accurate weighing system.

[0051] The recovery rate (%) of each component was calculated according to formula (2): In the formula: P is the purity (wt%) of the product measured by the above method; m initial is the initial mass of the component in 1000 kg of raw material liquid (sodium sulfate 100 kg, sodium chloride 120 kg, sodium nitrate 80 kg).

[0052] The test data of Examples 1-3 are summarized in Table 1.

[0053] Table 1. Feasibility test data of Examples 1-3 The test data in Table 1 shows that the method of the present application can be stably operated within the wide process parameter range defined in Examples 1 to 3. The purity of the three products of anhydrous sodium sulfate, sodium chloride and sodium nitrate produced reaches more than 96%, and the recovery rate reaches more than 95%, confirming the overall feasibility of the technical scheme of the present application.

[0054] The high purity of the sodium sulfate product (98.81% in Example 2) verified the effectiveness of the online prepared active washing solution (i.e. "self-sacrifice" washing solution) used in the fourth step. The washing solution inhibited the dissolution of the main crystal by the microcrystalline nuclei generated in situ using the common ion effect, while the unsaturation of its liquid phase to NaCl and NaNO3 provided efficient elution kinetics, which could effectively remove the impurity mother liquor entrained and attached by the coarse crystals during the freeze crystallization.

[0055] The high recovery rates achieved by the three salt components (all more than 98% in Example 2) confirmed the effectiveness of the series temperature difference crystallization process (high temperature evaporation to remove NaCl and cooling crystallization to remove NaNO3) and the closed-circuit design of the mother liquor used in the method of the present application. The design allowed efficient separation of NaCl and NaNO3 in different temperature zones, and achieved internal circulation of the process material flow, avoiding the external discharge of the three-phase co-saturation mother liquor in the traditional process, and achieving the maximum recovery of salt resources.

[0056] Comparative test: To further verify the effectiveness of the technical solutions of the present application, comparative tests of the examples and comparative examples were carried out.

[0057] The purity and yield of the sodium sulfate product Comparative test objects were the anhydrous sodium sulfate products produced by Example 2, Comparative Example 1 and Comparative Example 2. The same method described in the feasibility test was used to determine and calculate the purity and recovery rate of the anhydrous sodium sulfate products obtained by the three schemes.

[0058] System total resource recovery rate and wastewater discharge Comparative test objects were Example 2, Comparative Example 1 and Comparative Example 3. For Example 2, the total salt recovery rate was calculated based on the data of the feasibility test, and the wastewater discharge was monitored. For Comparative Example 1 and Comparative Example 3, after the system was stably operated for 24 hours, the final mother liquor was collected, its volume and density were accurately measured, and the concentrations of Na2SO4, NaCl and NaNO3 were analyzed by ion chromatography. Based on this, the mass of the lost salt in the discharged waste liquid was calculated, and the total salt recovery rate was calculated. Total salt recovery rate (%) = (1 - total mass of discharged salt / total mass of initial salt) x 100%.

[0059] Energy consumption evaluation of the sodium sulfate purification step The test subjects are the purification steps of Example 2, which is washing and purifying the crude sodium sulfate decahydrate crystals, and the purification step of Comparative Example 1, which is the melt MVR recrystallization purification step. Based on the process design parameters and equipment rated power, the energy consumed by each ton of crude sodium sulfate decahydrate crystals processed by the two units is evaluated. The energy consumption of the purification step described in Example 2 mainly includes the power consumption of the washing pump, centrifuge and pretreatment reactor. The energy consumption of the purification step described in Comparative Example 1 mainly includes the heat (sensible heat and latent heat of phase change) required for the melting of sodium sulfate decahydrate and the power consumption of the MVR compressor, which are converted into kWh.

[0060] The comparative test data are summarized in Table 2.

[0061] Table 2. Comparative test data of examples and comparative examples Note: “-” means that this test is not performed or this data is not applicable for the specific comparison purpose of this comparative example. For example, the purpose of Comparative Example 3 is to compare the NaCl / NaNO3 separation unit and the system circulation, and the Na2SO4 purification unit is the same as that of Example 2, so the test is not repeated; the purpose of Comparative Example 2 is to compare the washing unit, and the total salt recovery rate is not the core comparison index.

[0062] Conclusion: The data in Table 2 shows the performance differences between the technical solutions of the present application and different comparative examples.

[0063] Comparing Example 2 with Comparative Example 2, it can be seen that using conventional deionized water washing (Comparative Example 2) results in a low purity (93.28%) of sodium sulfate decahydrate product, and the recovery rate is only 84.66%, indicating that there is a significant product dissolution loss during the washing process. The active washing liquid used in Example 2 achieves a high purity (98.81%) while maintaining a very high recovery rate (99.03%). This data confirms the mechanism of the active washing liquid: the sodium sulfate microcrystalline nuclei generated in situ through the common ion effect effectively suppress the dissolution of the main crystals during the washing process, and the high unsaturation of the washing liquid main body ensures the elution efficiency.

[0064] Comparing Example 2 with Comparative Example 1, it can be seen that the energy consumption (18.5 kWh / t) of the sodium sulfate purification step of the present application (Example 2) is much lower than that (152.3 kWh / t) of the traditional melt MVR recrystallization process (Comparative Example 1), while the product purity obtained by the two is at the same level (98.81% vs 99.15%). This indicates that the present application method reduces the operating energy consumption of the unit while achieving high purity indicators.

[0065] Compared with Comparative Example 1 and Comparative Example 3, Example 2 realizes zero wastewater discharge and 98.74% total salt recovery rate. However, Comparative Example 1 and Comparative Example 3 both produce more than 58 kg of salt-containing waste liquid discharge due to the use of the traditional mother liquor discharge process, resulting in a total salt recovery rate of only about 92%. This data confirms the effectiveness of the series temperature difference crystallization and mother liquor closed loop system adopted in the present application, which constitutes an internal closed loop of the material, avoiding resource loss and environmental emissions.

Claims

1. A method for resource recycling of aqueous sodium sulfate, sodium chloride, sodium nitrate solutions, characterized in that, The method comprises the following steps: S1, concentrating a raw material solution containing sodium sulfate, sodium chloride and sodium nitrate by MVR evaporation to obtain a concentrated solution; S2, freezing and crystallizing the concentrated solution and then performing solid-liquid separation to obtain crude sodium sulfate decahydrate crystals and a frozen mother liquor; S3, performing cascade temperature difference crystallization on the frozen mother liquor, which comprises: performing high-temperature evaporation crystallization to separate and recover sodium chloride products to obtain a high-temperature sodium nitrate concentrated solution; and performing cooling crystallization on the high-temperature sodium nitrate concentrated solution to separate and recover sodium nitrate products to obtain a final cooling mother liquor; S4, preparing an active washing solution by mixing the final cooling mother liquor with the raw material solution, and washing the crude sodium sulfate decahydrate crystals with the active washing solution to obtain high-purity sodium sulfate decahydrate crystals as a sodium sulfate recovery product and a depleted washing solution; S5, combining and processing the depleted washing solution with the frozen mother liquor, and returning a main part of the final cooling mother liquor to the high-temperature evaporation crystallization step of step S3.

2. The method for recycling sodium sulfate, sodium chloride, and sodium nitrate aqueous solution according to claim 1, characterized in that, In step S1, the operation temperature of the MVR evaporation concentration is 70-90℃, the operation pressure is 30-70kPa, and the concentration is to a total solid content of 25.0-35.0wt% in the concentrated solution.

3. The method for recycling sodium sulfate, sodium chloride, and sodium nitrate aqueous solution according to claim 1, characterized in that, In step S2, the cooling endpoint temperature of the freezing and crystallization is -5℃ to 0℃, the cooling rate is 5-10℃ / h, and the isothermal crystallization time at the cooling endpoint temperature is 2-4h.

4. The method for recycling sodium sulfate, sodium chloride, and sodium nitrate aqueous solution according to claim 1, characterized in that, In step S3, the operation temperature of the high-temperature evaporation crystallization is 80-105℃, the cooling endpoint temperature of the cooling crystallization is 15-25℃, the cooling rate is 5-15℃ / h, and the isothermal crystallization time is 3-5h.

5. The method for recycling sodium sulfate, sodium chloride, and sodium nitrate aqueous solution according to claim 1, characterized in that, In step S4, the active washing solution is prepared by mixing the final cooling mother liquor with the raw material solution at a volume ratio of 5:1 to 20:1 and stirring at a temperature of 0-5℃ for 10-30min.

6. The method for recycling sodium sulfate, sodium chloride, sodium nitrate aqueous solution according to claim 1, characterized in that, In step S4, the washing is performed in a 3-5 stage countercurrent washing machine, the mass ratio of the active washing solution to the crude sodium sulfate decahydrate crystals is 0.5:1 to 2:1, and the washing temperature is 0-5℃.

7. The method for recycling sodium sulfate, sodium chloride, sodium nitrate aqueous solution according to claim 2, characterized in that, In step S1, the operation temperature of the MVR evaporation concentration is 75-85℃, the operation pressure is 45-55kPa, and the concentration is to a total solid content of 28.0-32.0wt% in the concentrated solution.

8. The method for recycling sodium sulfate, sodium chloride, and sodium nitrate aqueous solution according to claim 3, characterized in that, In step S2, the cooling endpoint temperature of the freezing and crystallization is -3.5℃ to -1.5℃, the cooling rate is 7-8℃ / h, and the isothermal crystallization time is 2.5-3.5h.

9. The method for recycling sodium sulfate, sodium chloride, sodium nitrate aqueous solution according to claim 4, characterized in that, In step S3, the operation temperature of the high-temperature evaporation crystallization is 90-100℃, the cooling endpoint temperature of the cooling crystallization is 18-22℃, the cooling rate is 8-12℃ / h, and the isothermal crystallization time is 3.5-4.5h.

10. The method for recycling sodium sulfate, sodium chloride, sodium nitrate aqueous solution according to claim 5, characterized in that, In step S4, the preparation step of the active washing solution is mixing the final cooling mother liquor with the raw material solution at a volume ratio of 10:1 to 15:1 and stirring at a temperature of 2-3℃ for 15-25min, and the mass ratio of the active washing solution to the crude sodium sulfate decahydrate crystals is 1.0:1 to 1.5:1.

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