A method for separating sodium sulfate and sodium thiocyanate from mixed waste salt solution

Abstract

The present invention relates to a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The method comprises subjecting the mixed waste salt solution to catalytic oxidation, decolorization, and a first solid-liquid separation, then subjecting the obtained pretreatment solution to a first evaporation concentration, a first cooling crystallization, and a second solid-liquid separation to obtain a crude sodium sulfate product and a filtrate, washing the crude sodium sulfate product, and subjecting the pretreatment solution to a third solid-liquid separation to obtain sodium sulfate; subjecting the filtrate to a second evaporation concentration, adding sodium thiocyanate seed crystals, and then subjecting the pretreatment solution to a second cooling crystallization and a fourth solid-liquid separation to obtain a crude sodium thiocyanate product and a residual liquid, washing the crude sodium thiocyanate product, and subjecting the pretreatment solution to a fifth solid-liquid separation to obtain sodium thiocyanate; and recycling the washing waste liquid and the residual liquid to the pretreatment solution for mixing and reuse. The present invention utilizes evaporation concentration combined with cooling crystallization to separate sodium sulfate and sodium thiocyanate with high purity and extraction rate. The entire preparation process has low energy consumption, is more environmentally friendly, and has good overall economic benefits.

Description

Technical Field

[0001] The invention relates to the technical field of mixed salt separation, in particular to a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. Background Art

[0002] Chemical waste salt refers to industrial inorganic salts containing hazardous waste or wastewater with high salt concentrations generated during the production process. It is mainly produced in the pesticide, pharmaceutical, chemical, and printing and dyeing industries, of which the pesticide industry accounts for (30%), the pharmaceutical industry accounts for (10%), the chemical industry accounts for (15%), and the textile printing and dyeing industry accounts for (45%). Chemical waste salt is characterized by large production volume, wide sources, complex composition, serious environmental pollution, high toxicity, and high treatment costs. The amount of waste salt generated in my country each year is about 2.0×10 7 Waste salt can be divided into simple salts and mixed salts based on their primary components. Simple salts are salts composed of a single component, while mixed salts are salts composed of two or more components. The annual generation of waste salt in my country's chemical industry is increasing at a rate of 20%. The primary components of this type of waste salt are sodium salts, primarily sodium chloride, sodium sulfate, and mixtures of the two.

[0003] At present, the treatment methods for waste salt at home and abroad are divided into two directions, namely harmless treatment and resource utilization. Harmless treatment refers to the elimination of harmful substances or isolation of harmful substances outside the environment through physical or chemical methods or a combination of the two to reduce environmental risks. The main methods are incineration, solidification and landfill, incineration and discharge into the sea. Resource utilization treatment is to extract valuable components from waste salt by physical and chemical methods and then reuse them to achieve secondary utilization of resources. Overall, resource utilization is the most promising development direction. The resource utilization methods of chemical waste salt include crystallization, membrane, thermal decomposition, extraction, salt washing, direct utilization and alkali production. However, due to the complexity of chemical waste salt, a single method may not be able to completely separate the waste salt. Therefore, in actual applications, two or more methods are used in combination to treat waste salt to achieve better results.

[0004] Crystallization utilizes the varying solubilities of various compounds in waste salt. After pre-treating the waste salt, the secondary salts are saturated by controlling the crystallization temperature or evaporating the water, resulting in crystals. Recrystallization is then used to further refine the secondary salt crystals to achieve higher purity, thereby increasing economic efficiency. Crystallization is widely used in waste salt utilization due to its simplicity, but operating costs are high and purity is difficult to control. Phase diagram theory is the most commonly used tool for analyzing crystallization methods. It creates easy-to-understand and applicable graphs based on the solubility of different compounds. Phase diagrams clearly show the mixed salt co-saturation zone, the single salt crystallization zone, and the critical point for each salt. By varying the solution temperature and combining evaporation, concentration, and cooling crystallization, the order of salt precipitation can be controlled, separating the waste salt into a reusable salt product of controllable purity.

[0005] Existing crystallization separation methods all have the following problems: (1) The essence of crystallization is to separate mixed salts by utilizing the different solubilities of salts. If the solubility of the mixed salts is similar, and only the temperature is changed, the solubility of the salts will change relatively little, making separation difficult using this method; (2) In industrial applications, crystallization methods often use evaporation crystallizers, which are expensive and consume a lot of energy during operation; (3) Crystallization methods also have their limitations. When the temperature is too high or too low, it is necessary to ensure that the salt solution does not undergo chemical reactions, and the boiling point of the salt solution should not be too high or too low.

[0006] To address the above issues, CN109824019A discloses a method for extracting sodium sulfate, sodium thiosulfate, and sodium thiocyanate from desulfurization wastewater. The desulfurization wastewater is first decolorized and filtered to obtain a decolorized liquid and solid activated carbon. The decolorized liquid is then evaporated and concentrated to separate the mixed salts. The mixed salts are added to an organic extractant and centrifuged. The centrifuged liquid is then distilled, cooled, crystallized, and centrifuged to obtain the sodium thiocyanate product. The solid after extraction and centrifugation is dissolved, evaporated, concentrated, cooled, crystallized, and centrifuged at a temperature of 45-65°C to obtain solid sodium thiosulfate. The liquid after centrifugation is added to concentrated sulfuric acid for reaction, filtered, and the filtrate is evaporated, concentrated, cooled, crystallized, and centrifuged to obtain the sodium sulfate product. This method produces a high-purity product and can recycle unprocessed sodium sulfate and sodium thiosulfate, achieving resource recycling. However, during the process, additional organic extractants need to be added, and organic solvents are introduced. In the later processing process, the organic solvents in the residual liquid need to be reprocessed, which increases the separation process and cannot be reused. In addition, the extraction process needs to be carried out under nitrogen protection to ensure safety, and the process is cumbersome and complicated.

[0007] CN108715452A discloses a desulfurization wastewater pretreatment method for producing high-purity sodium thiocyanate and sodium thiosulfate. The patent mainly performs preliminary treatment on the desulfurization wastewater by adding sodium sulfite and sodium bisulfate step by step or simultaneously adding sodium sulfite and sodium bisulfate. The sulfur is removed while removing sodium carbonate and sodium bicarbonate. The solution is then aerated at 50-60°C for 2-4 hours to remove small particles. Other solid impurities are filtered out, and precious metal impurities are removed after decolorization at a high temperature. The filtrate is then evaporated, concentrated, and centrifuged to obtain a low-impurity mixed salt. The mixed salt is then extracted and filtered, and the extract is concentrated, crystallized, and centrifuged to obtain sodium thiocyanate crystals. The filter residue is dissolved, crystallized, and centrifuged to obtain sodium thiosulfate and sodium sulfate. The sodium sulfate is then separated to obtain sodium thiosulfate crystals. However, if the reaction is incomplete, this method may produce ammonia as a byproduct. To avoid environmental pollution, an additional waste gas collection device is required.

[0008] CN117263211A discloses a process for preparing high-purity sodium thiocyanate from desulfurization wastewater. The process involves first subjecting the residual catalyst in the desulfurization wastewater to strong oxidation. After oxidation, activated carbon (1%-2%) is added for adsorption decolorization (decolorization temperature: 60°C-90°C, time: 1-2 hours) to obtain a decolorized solution. The decolorized solution is then evaporated and crystallized to produce sodium sulfate solid and a sodium thiocyanate solution. The sodium thiocyanate solution is then evaporated and concentrated, and finally dried to obtain high-purity sodium thiocyanate solid. This patent has the advantage of converting low-valent sulfur compounds into sodium sulfate using the residual catalyst, which is then separated and prepared as a product based on solubility differences. However, the process, which relies solely on evaporation and concentration, results in low sodium sulfate purity and extraction yield. A large amount of unprecipitated solids remain in the sodium thiocyanate solution. Later, during evaporation and concentration of the sodium thiocyanate solution, a large amount of sodium sulfate may remain in the sodium thiocyanate product after secondary crystallization, significantly reducing the purity and extraction yield of both sodium sulfate and sodium thiocyanate. Furthermore, this process requires complete concentration of the liquid, resulting in high energy consumption.

[0009] CN102795644A discloses a method for recovering sodium thiocyanate from desulfurization wastewater by improved ADA method by evaporation concentration and cooling crystallization. The general process is to first use the ADA method to evaporate the desulfurization wastewater to obtain a mixed salt containing sodium sulfate, sodium thiocyanate and sodium thiosulfate, and then pile it for 3-5 months to allow the sodium thiosulfate to slowly decompose into sulfur and sodium sulfate. 3 After heating and dissolving in a certain proportion, cooling and crystallizing at -10℃-0℃ for 6-8h, sodium sulfate is precipitated and finally filtered to obtain sodium thiocyanate solid with a purity of 87%-93%. The advantage of this method is that sodium thiosulfate decomposes naturally, no external energy is required, and cost is saved. The disadvantage is that the slow decomposition cycle is long, and it is greatly affected by external environmental interference, resulting in high loss.

[0010] In summary, the current existing technologies for the crystallization separation of sodium thiocyanate and sodium sulfate have problems of unstable purity and high cost. If a low-cost method is used for separation, the purity obtained will be unstable; if a strong oxidation method is used to separate the two, secondary pollution and cost issues will occur.

[0011] Therefore, how to provide a method to achieve efficient separation of sodium sulfate and sodium thiocyanate, reduce separation costs, and be safe and environmentally friendly has become a problem that needs to be solved urgently. Summary of the Invention

[0012] To address the above technical problems, the present invention provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The separation method of the present invention has good economic benefits. By treating the mixed waste salt solution using evaporation and concentration combined with cooling crystallization technology, sodium sulfate and sodium thiocyanate can be separated with high purity and extraction yield. The entire preparation process has low energy consumption, saves costs, and is more environmentally friendly.

[0013] To achieve this object, the present invention adopts the following technical solutions:

[0014] In a first aspect, the present invention provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution, the method comprising the following steps:

[0015] (1) subjecting the mixed waste salt solution to catalytic oxidation, decolorization treatment, and first solid-liquid separation in sequence to obtain a pretreated solution;

[0016] (2) subjecting the pretreated solution of step (1) to a first evaporation concentration, a first cooling crystallization, and a second solid-liquid separation in sequence to obtain a crude sodium sulfate product and a filtrate;

[0017] (3) washing the crude sodium sulfate product of step (2) and performing a third solid-liquid separation to obtain sodium sulfate; adding sodium thiocyanate seed crystals to the filtrate of step (2) after a second evaporation concentration, and then sequentially performing a second cooling crystallization and a fourth solid-liquid separation to obtain a crude sodium thiocyanate product and a residual liquid;

[0018] (4) washing the crude sodium thiocyanate product of step (3) and performing a fifth solid-liquid separation to obtain sodium thiocyanate; recycling the washing waste liquid after washing the crude sodium sulfate product, the residual liquid, and the washing waste liquid after washing the crude sodium thiocyanate product to step (1), mixing with the pretreatment solution and reusing.

[0019] It should be noted that the mixed waste salt solution in the present invention includes various waste salt solutions containing sodium sulfate and sodium thiocyanate, such as desulfurization wastewater from the coal chemical industry and cyanide-containing wastewater from the metallurgical industry.

[0020] It should be noted that the pretreatment solution obtained in step (1) of the present invention is a mixed solution containing sodium sulfate and sodium thiocyanate.

[0021] The present invention utilizes the difference in solubility between sodium sulfate and sodium thiocyanate to separate and recover sodium sulfate and sodium thiocyanate in a mixed waste salt solution. The extraction rates and purities of both are high, and both can be directly used as raw materials in any reaction. The sodium thiocyanate can also be used to further prepare sodium thiocyanate crystal seeds, forming a closed loop of industrial circulation. All waste liquid generated in the separation process can also be repeatedly circulated into the separation process for further utilization, thereby avoiding waste, being more environmentally friendly, and having good economic benefits.

[0022] The separation method of the present invention is simple. First, the low-valent sulfur compounds contained in the mixed waste salt solution are fully oxidized to convert them into high-valent sulfur. Then, other (organic) impurities in the mixed waste salt solution are removed by a physical adsorption method. After separation, a pretreatment solution is obtained. Then, the pretreatment solution is evaporated and concentrated, cooled and crystallized, and solid-liquid separated. After evaporation and concentration and cooling and crystallization, most of the sodium sulfate in the pretreatment solution can be precipitated. The obtained crude sodium sulfate product can be washed and separated to obtain a sodium sulfate product. Then, the obtained filtrate is treated by a process of partial evaporation + seed crystal induced crystallization + cooling crystallization. The addition of sodium thiocyanate seed crystals can induce sodium thiocyanate nucleation. The nucleation rate is promoted, thereby reducing the time required for cooling crystallization and greatly reducing the required energy consumption. On the other hand, the extraction rate and purity can be improved. Since the addition of sodium thiocyanate seeds promotes the nucleation of sodium thiocyanate seeds, the precipitation of sodium thiocyanate seeds is induced. After the precipitation amount increases, the added sodium thiocyanate seeds and the precipitated sodium thiocyanate seeds jointly induce crystallization and precipitation of more sodium thiocyanate seeds, thereby improving the extraction rate and purity. The obtained crude sodium thiocyanate product is washed and separated to obtain a sodium thiocyanate product. The obtained residual liquid, the washing waste liquid after washing the crude sodium sulfate product, and the washing waste liquid after washing the crude sodium thiocyanate product are recycled to step (1), mixed with the pretreatment solution, and can be reused.

[0023] As a preferred technical solution of the present invention, the catalytic oxidation method includes the steps of introducing an oxidizing gas and adding a catalyst.

[0024] In the present invention, a catalytic oxidation step is utilized to fully oxidize low-valent sulfur compounds, including a mixture of sodium thiosulfate, sodium hydrosulfide and sodium sulfite.

[0025] Preferably, the oxidizing gas comprises ozone.

[0026] Preferably, the catalyst comprises a combination of tannin extract and sodium metavanadate.

[0027] Preferably, the temperature of the catalytic oxidation is 30°C-80°C, such as 30°C, 40°C, 50°C, 60°C, 70°C or 80°C.

[0028] Preferably, the catalytic oxidation time is 2 h to 24 h, for example, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h or 24 h.

[0029] Preferably, the pH of the catalytic oxidation is 8-11, such as 8, 8.5, 9, 9.5, 10, 10.5 or 11.

[0030] As a preferred technical solution of the present invention, the adsorption material used in the decolorization treatment includes activated carbon.

[0031] Preferably, based on the mass of the mixed waste salt solution after catalytic oxidation, the added amount of the adsorption material is 1wt%-5wt%, for example, 1wt%, 2wt%, 3wt%, 4wt% or 5wt%.

[0032] Preferably, the temperature of the decolorization treatment is 60°C-90°C, such as 60°C, 70°C, 80°C or 90°C.

[0033] Preferably, the decolorization treatment time is 1 h-4 h, such as 1 h, 2 h, 3 h or 4 h.

[0034] Preferably, after the decolorization treatment, filtration is further included to obtain a decolorized solution.

[0035] Preferably, the first solid-liquid separation method comprises: filtering the decolorized solution once to obtain a primary filtrate, and then filtering it a second time to obtain a pretreated solution.

[0036] Preferably, the pore size of the filter paper for the primary filtration is 20 μm-25 μm, for example, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm.

[0037] Preferably, the pore size of the filter membrane for the secondary filtration is 0.22 μm-1.0 μm, for example, 0.22 μm, 0.35 μm, 0.38 μm, 0.40 μm, 0.42 μm, 0.45 μm, 0.48 μm, 0.50 μm, 0.60 μm, 0.70 μm, 0.80 μm, 0.90 μm or 1.0 μm, etc.

[0038] In the present invention, a filter paper with a larger pore size is used for the first filtration to remove most of the suspended solids and insoluble matter with larger particles, and a filter membrane with a smaller pore size is used for the second filtration to remove the small particles of colloidal insoluble matter, thereby ensuring that the impurities in the obtained pretreatment solution are deeply separated, and ensuring that the sodium sulfate and sodium thiocyanate obtained by subsequent separation are not doped with other impurities and have high purity.

[0039] As a preferred technical solution of the present invention, the temperature of the first evaporation and concentration is 60°C-95°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C or 95°C.

[0040] Preferably, the stirring rate of the first evaporation concentration is 100 r / min-600 r / min, for example, 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min or 600 r / min.

[0041] Preferably, in the first evaporation concentration, based on the mass of the pretreated solution, the water evaporation rate is 55wt%-65wt%, for example, 55wt%, 56wt%, 57wt%, 58wt%, 59wt%, 60wt%, 61wt%, 62wt%, 63wt%, 64wt% or 65wt%, etc.

[0042] Preferably, the temperature of the first cooling crystallization is -10°C to 10°C, for example, -10°C, -8°C, -6°C, -4°C, -2°C, 0°C, 2°C, 4°C, 6°C, 8°C or 10°C.

[0043] In the present invention, during the first evaporation and concentration, the water evaporation rate is 55wt%-65wt%, and the pretreatment solution is first evaporated to remove most of the water. After the solution is concentrated to a certain degree, the first cooling crystallization process is immediately carried out. The low solubility of sodium sulfate at low temperatures of -10°C to 10°C is utilized to make separation easier and the separation time greatly shortened. The sodium sulfate can be separated from the pretreatment solution to the greatest extent, thereby ensuring the purity and extraction rate of the sodium sulfate. The filtrate after separation can also be enriched with sodium thiocyanate, thereby ensuring the purity and extraction rate of the sodium thiocyanate obtained in subsequent separation. In addition, the combination of evaporation and concentration and cooling crystallization, compared with the method of evaporation and concentration alone until solids are completely precipitated, can not only improve the purity and extraction rate of sodium sulfate, but also greatly reduce energy consumption, reduce production costs, and shorten separation time. If the water evaporation rate is too high, evaporation and precipitation will occur, thereby affecting the purity of the sodium sulfate product.

[0044] Preferably, the stirring rate of the first cooling crystallization is 100 r / min-600 r / min, for example, 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min or 600 r / min.

[0045] Preferably, the first cooling crystallization time is ≥6 h, for example, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h or 15 h, etc.

[0046] In the present invention, the time of the first cooling crystallization is regulated to be ≥6h, so that the sodium sulfate can be completely precipitated, the extraction rate of the sodium sulfate is guaranteed, and the purity of the subsequent sodium thiocyanate is avoided from being affected.

[0047] It should be noted that the present invention does not make any specific requirements or special limitations on the second solid-liquid separation method. As long as it is a separation method commonly used by technicians in this field, it is applicable to the present invention, such as centrifugal separation, eddy current separation or filtration.

[0048] Preferably, the temperature of the second solid-liquid separation is 0°C-20°C, for example, 0°C, 2°C, 4°C, 6°C, 8°C, 10°C, 12°C, 14°C, 16°C, 18°C ​​or 20°C.

[0049] Preferably, the time for the second solid-liquid separation is 2 min-10 min, for example, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0050] As a preferred technical solution of the present invention, the washing liquid used in step (3) comprises a supersaturated solution of sodium sulfate.

[0051] The invention adopts a supersaturated sodium sulfate solution to wash a crude sodium sulfate product, thereby ensuring that a trace amount of sodium thiocyanate in the crude product is eluted and separated from the crude sodium sulfate product, and ensuring that the purity and yield of the obtained sodium sulfate product are high.

[0052] Preferably, the solid-liquid ratio of the crude sodium sulfate to the washing liquid is 1g:(0.4-3)mL, for example, 1g:0.4mL, 1g:0.5mL, 1g:1mL, 1g:1.5mL, 1g:2mL, 1g:2.5mL or 1g:3mL, etc.

[0053] In the present invention, the solid-liquid ratio of the crude sodium sulfate to the washing liquid is limited to 1 g: (0.4-3) mL, so that the extraction rate and purity of sodium sulfate can achieve better effects and tend to be stable.

[0054] Preferably, the washing temperature in step (3) is 20°C-25°C, such as 20°C, 21°C, 22°C, 23°C, 24°C or 25°C.

[0055] Preferably, the number of washings in step (3) includes at least 1 time, for example 1 time, 2 times, 3 times, 4 times, or 5 times.

[0056] It should be noted that the present invention does not make specific requirements and special limitations on the third solid-liquid separation method. As long as it is a separation method commonly used by technicians in this field, it is applicable to the present invention, for example, it can be centrifugal separation, eddy current separation or filtration.

[0057] The present invention can further improve the purity of sodium sulfate by performing solid-liquid separation on the washed sodium sulfate.

[0058] Preferably, after the third solid-liquid separation in step (3) and before obtaining sodium sulfate, drying is also included.

[0059] Preferably, the drying temperature is 70°C-90°C, for example, 70°C, 72°C, 75°C, 78°C, 80°C, 82°C, 85°C, 88°C or 90°C.

[0060] Preferably, the drying time is 2 h to 12 h, for example, 2 h, 4 h, 6 h, 8 h, 10 h or 12 h.

[0061] As a preferred technical solution of the present invention, the temperature of the second evaporation and concentration is 60°C-95°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C.

[0062] Preferably, the stirring rate of the second evaporation concentration is 100 r / min-600 r / min, for example, 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min or 600 r / min.

[0063] Preferably, in the second evaporation concentration, based on the mass of the filtrate, the water evaporation rate is 60wt%-70wt%, for example, 60wt%, 62wt%, 64wt%, 66wt%, 68wt% or 70wt%.

[0064] As a preferred technical solution of the present invention, the amount of the sodium thiocyanate seed crystals added is 5wt%-15wt% of the mass of sodium thiocyanate in the pretreatment solution, for example, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%, etc.

[0065] In the present invention, during cooling crystallization, a small amount of sodium thiocyanate seed crystals are added to the product after evaporation and concentration, which can induce the nucleation of sodium thiocyanate and promote the nucleation rate, thereby reducing the time required for cooling crystallization and reducing the required energy consumption. In addition, the extraction rate and purity can also be improved. Since the addition of the sodium thiocyanate seed crystals promotes the nucleation of the sodium thiocyanate seed crystals and induces the precipitation of the sodium thiocyanate seed crystals, after the precipitation amount increases, the added sodium thiocyanate seed crystals and the precipitated sodium thiocyanate seed crystals jointly induce the precipitation of more sodium thiocyanate seed crystals, thereby improving the extraction rate and purity.

[0066] Preferably, the sodium thiocyanate seed crystals are sodium thiocyanate solids precipitated by filtering a supersaturated sodium thiocyanate solution.

[0067] It should be noted that, in the present invention, when the process of separating sodium sulfate and sodium thiocyanate from the mixed waste salt solution is carried out for the first time, the sodium thiocyanate seed crystals added are the sodium thiocyanate solids obtained by filtering and precipitating the prepared sodium thiocyanate supersaturated solution. Later in the circulation process, the sodium thiocyanate solids obtained by filtering and precipitating the prepared sodium thiocyanate supersaturated solution can continue to be added. The sodium thiocyanate product obtained after the first separation can also be used to prepare sodium thiocyanate seed crystals, thereby forming a closed loop of industrial circulation and realizing direct use of the product.

[0068] As a preferred technical solution of the present invention, the temperature of the second cooling crystallization is -10℃~10℃, for example, -10℃, -8℃, -6℃, -4℃, -2℃, 0℃, 2℃, 4℃, 6℃, 8℃ or 10℃, etc.

[0069] Preferably, the stirring rate of the second cooling crystallization is 100 r / min-600 r / min, for example, 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min or 600 r / min.

[0070] Preferably, the second cooling crystallization time is ≥6 h, for example, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h or 15 h, etc.

[0071] In the present invention, after the sodium thiocyanate seed crystals are added, a large amount of sodium thiocyanate is dissolved in the sodium thiocyanate suspension, so a large amount of sodium thiocyanate is precipitated. By limiting the time of the second cooling crystallization to ≥6h, a large amount of sodium thiocyanate can be ensured to precipitate, thereby improving its purity and purification rate.

[0072] It should be noted that the present invention does not make specific requirements and special limitations on the fourth solid-liquid separation method. As long as it is a separation method commonly used by technicians in this field, it is applicable to the present invention, for example, it can be centrifugal separation, eddy current separation or filtration.

[0073] Preferably, the temperature of the fourth solid-liquid separation is 0°C-20°C, for example, 0°C, 2°C, 4°C, 6°C, 8°C, 10°C, 12°C, 14°C, 16°C, 18°C ​​or 20°C.

[0074] Preferably, the time for the fourth solid-liquid separation is 2 min-10 min, for example, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0075] As a preferred technical solution of the present invention, the washing liquid used in step (4) comprises a supersaturated solution of sodium thiocyanate.

[0076] The invention adopts a supersaturated sodium thiocyanate solution to wash a crude sodium thiocyanate product, thereby ensuring that a trace amount of sodium sulfate in the crude product is eluted and separated from the crude sodium thiocyanate product, and ensuring that the purity and yield of the obtained sodium thiocyanate product are high.

[0077] Preferably, the solid-liquid ratio of the crude sodium thiocyanate to the washing liquid is 1g:(0.4-3)mL, for example, 1g:0.4mL, 1g:0.5mL, 1g:1mL, 1g:1.5mL, 1g:2mL, 1g:2.5mL or 1g:3mL, etc.

[0078] In the present invention, the solid-liquid ratio of the crude sodium thiocyanate to the washing liquid is limited to 1 g: (0.4-3) mL, so that the extraction rate and purity of sodium thiocyanate can achieve better effects and tend to be stable.

[0079] Preferably, the washing temperature in step (4) is 20°C-25°C, such as 20°C, 21°C, 22°C, 23°C, 24°C or 25°C.

[0080] Preferably, the number of washings in step (4) includes at least 1 time, for example 1 time, 2 times, 3 times, 4 times, or 5 times.

[0081] It should be noted that the present invention does not make specific requirements and special limitations on the fifth solid-liquid separation method. As long as it is a separation method commonly used by technicians in this field, it is applicable to the present invention, for example, it can be centrifugal separation, eddy current separation or filtration.

[0082] The present invention can further improve the purity of sodium thiocyanate by performing solid-liquid separation on the washed sodium cyanogen sulfate.

[0083] Preferably, after the fifth solid-liquid separation in step (4) and before obtaining sodium thiocyanate, drying is also included.

[0084] Preferably, the drying temperature is 70°C-90°C, for example, 70°C, 72°C, 75°C, 78°C, 80°C, 82°C, 85°C, 88°C or 90°C.

[0085] Preferably, the drying time is 2 h to 12 h, for example, 2 h, 4 h, 6 h, 8 h, 10 h or 12 h.

[0086] As a preferred technical solution of the present invention, the method comprises the following steps:

[0087] (1) introducing an oxidizing gas into a mixed waste salt solution and adding a catalyst, adjusting the pH to 8-11, reacting at 30°C-80°C for 2h-24h, performing catalytic oxidation, adding 1wt%-5wt% of an adsorption material based on the mass of the mixed waste salt solution after catalytic oxidation, decolorizing at 60°C-90°C for 1h-4h, filtering to obtain a decolorized solution, and subjecting the decolorized solution to a first solid-liquid separation, first using a filter paper with a pore size of 20μm-25μm to perform a suction filtration to obtain a primary filtrate, and then using a filter membrane with a pore size of 0.22μm-1.0μm to perform a secondary suction filtration to obtain a pretreated solution;

[0088] (2) subjecting the pretreated solution of step (1) to a first evaporation concentration at 60° C.-95° C. and a stirring rate of 100 r / min-600 r / min, and stopping the first evaporation concentration based on the mass of the pretreated solution until the water evaporation rate reaches 55 wt%-65 wt%, and then subjecting the pretreated solution to a first cooling crystallization at a stirring rate of 100 r / min-600 r / min at a temperature of -10° C.-10° C., for a crystallization time of ≥6 h, and then subjecting the pretreated solution to a second solid-liquid separation by centrifugation at 0° C.-20° C. for 2 min-10 min, to obtain a crude sodium sulfate product and a filtrate;

[0089] (3) washing the crude sodium sulfate product of step (2) at least once with a supersaturated sodium sulfate solution at 20°C-25°C, with a washing solid-liquid ratio of 1g:(0.4-3)mL, performing a third solid-liquid separation, and drying at 70°C-90°C for 2h-12h to obtain sodium sulfate; performing a second evaporation concentration on the filtrate of step (2) at 60°C-95°C and a stirring rate of 100r / min-600r / min, and stopping the second evaporation concentration based on the mass of the filtrate until the water evaporation rate reaches 60wt%-70wt%; adding 5wt%-15wt% of sodium thiocyanate seed crystals based on the mass of sodium thiocyanate in the pretreatment solution, performing a second cooling crystallization at -10°C-10°C and a stirring rate of 100r / min-600r / min, with a crystallization time of ≥6h, and then centrifuging at 0°C-20°C for 2min-10min, and performing a fourth solid-liquid separation to obtain a crude sodium thiocyanate product and a residual liquid;

[0090] The sodium thiocyanate seed crystals are sodium thiocyanate solids precipitated by filtering a supersaturated sodium thiocyanate solution;

[0091] (4) The crude sodium thiocyanate product of step (3) is washed at least once with a supersaturated sodium thiocyanate solution at 20° C.-25° C., with a washing solid-liquid ratio of 1 g:(0.4-3) mL, and after the fifth solid-liquid separation, the product is dried at 70° C.-90° C. for 2 h-12 h to obtain sodium thiocyanate; the washing waste liquid after washing the crude sodium sulfate product, the residual liquid, and the washing waste liquid after washing the crude sodium thiocyanate product are recycled to step (1), mixed with the pretreatment solution, and reused.

[0092] Compared with the prior art, the present invention has at least the following beneficial effects:

[0093] (1) The present invention processes the mixed waste salt solution and can sequentially separate and obtain two products, sodium sulfate and sodium thiocyanate. The preparation process has low energy consumption and saves costs. The residual liquid and washing waste liquid are mixed with the pretreatment solution. The washing waste liquid and the residual liquid can be recycled and disposed of, which is more environmentally friendly. The obtained sodium thiocyanate product can also be used to prepare sodium thiocyanate crystal seeds, forming a closed loop of industrial circulation. The entire process has good economic benefits.

[0094] (2) In the process of preparing the sodium sulfate product, the present invention combines evaporation concentration with cooling crystallization and combines corresponding process parameters to allow most of the sodium sulfate to precipitate, thereby ensuring a high extraction rate and purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Figure 1 The present invention is a schematic diagram of the process of separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. DETAILED DESCRIPTION

[0096] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0097] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0098] The present invention provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The process of the method is shown as follows: Figure 1 As shown, the method includes the following steps:

[0099] (1) subjecting the mixed waste salt solution to catalytic oxidation, decolorization treatment, and first solid-liquid separation in sequence to obtain a pretreated solution;

[0100] (2) subjecting the pretreated solution of step (1) to a first evaporation concentration, a first cooling crystallization, and a second solid-liquid separation in sequence to obtain a crude sodium sulfate product and a filtrate;

[0101] (3) washing the crude sodium sulfate product of step (2) and performing a third solid-liquid separation to obtain sodium sulfate; adding sodium thiocyanate seed crystals to the filtrate of step (2) after a second evaporation concentration, and then performing a second cooling crystallization and a third solid-liquid separation in sequence to obtain a crude sodium thiocyanate product and a residual liquid;

[0102] (4) washing the crude sodium thiocyanate product of step (3) and performing a fifth solid-liquid separation to obtain sodium thiocyanate; recycling the washing waste liquid after washing the crude sodium sulfate product, the residual liquid, and the washing waste liquid after washing the crude sodium thiocyanate product to step (1), mixing with the pretreatment solution and reusing.

[0103] The mixed waste salt solutions described in the specific embodiments of the present invention are all coking desulfurization waste liquids, which are provided by Ansteel Group Chemical Technology Co., Ltd.

[0104] Example 1

[0105] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution, the method comprising the following steps:

[0106] (1) adding sodium hydroxide solution to the coking desulfurization waste liquid to adjust the pH to 10, introducing ozone therein, adding tannin extract and sodium metavanadate therein, maintaining the above pH value, and catalytically oxidizing at 80° C. for 2 h; adding activated carbon in an amount of 5 wt% of the mass of the catalytically oxidized waste salt solution to the mixed waste salt solution after catalytic oxidation, decolorizing at 80° C. for 4 h, filtering to obtain a decolorized solution, and filtering the decolorized solution once with a 25 μm filter paper, and then filtering it twice with a 0.45 μm filter membrane to obtain a pretreated solution;

[0107] (2) The pretreated solution was stirred continuously at a rate of 500 r / min at a water bath temperature of 80°C for the first evaporation concentration. From the start of evaporation, the total mass was weighed every 30 minutes until the water evaporation rate reached 59 wt%, and the first evaporation concentration was stopped; the constant temperature water bath temperature was adjusted to 10°C, and the stirring was continued at a rate of 500 r / min for 12 hours at a water bath temperature of 10°C to perform the first cooling crystallization to obtain a thick crystal suspension; the crystal suspension was placed in a centrifuge tube and centrifuged at a speed of 3000 r / min for 2 minutes, with the temperature maintained at 10°C. After the crystals and the solution were clearly separated, the solution was poured out as the filtrate, and the crystals were crude sodium sulfate;

[0108] (3) washing the crude sodium sulfate product three times with a supersaturated sodium sulfate solution at a solid-liquid ratio of 1 g:1 mL at room temperature of 25°C, centrifuging, and drying at 80°C for 10 h to obtain sodium sulfate; reheating the filtrate to 80°C while stirring continuously at a rate of 500 r / min, and performing a second evaporation concentration. From the start of evaporation, weigh the total mass every 30 min until the water evaporation rate reaches 65 wt%, then stop the second evaporation concentration, adjust the constant temperature water bath temperature to 10°C, and then add sodium thiocyanate seeds equivalent to 5 wt% of the sodium thiocyanate content in the pretreatment solution, continue stirring continuously at a rate of 500 r / min for 12 h at a water bath temperature of 10°C, perform a second cooling crystallization, and after standing for 12 h, obtain a thick crystal suspension; placing the crystal suspension in a centrifuge tube, centrifuging at a speed of 3000 r / min for 2 min, maintaining the temperature at 10°C, and after obvious stratification of the crystals and the solution, pour out the residual liquid therein, and the crystals are crude sodium thiocyanate;

[0109] Sodium thiocyanate seed crystal cultivation: Sodium thiocyanate powder was dissolved in deionized water at room temperature to obtain a supersaturated solution of sodium thiocyanate at room temperature. Solid sodium thiocyanate was continuously added to the system while the temperature was continuously increased until the sodium thiocyanate solution reached saturation at 80°C. At this time, stirring was stopped and the solution was quickly transferred to a conical flask and allowed to slowly cool at room temperature. After 24 hours, the cooled crystals were removed from the conical flask to obtain the sodium thiocyanate seed crystals.

[0110] (4) washing the crude sodium thiocyanate product three times with a supersaturated sodium thiocyanate solution at a solid-liquid ratio of 1 g:1 mL at room temperature of 25° C., centrifuging, and drying at 80° C. for 10 h to obtain sodium thiocyanate; recycling the washing waste liquids in steps (3) and (4) and the residual liquid in step (3) to the pretreatment solution.

[0111] Example 2

[0112] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the temperature of the first evaporation and concentration in step (2) is 85° C., the time of the first cooling crystallization is 10 h, the centrifugal rate is 5000 r / min, and the centrifugal time is 3 min; the temperature of the second evaporation and concentration in step (3) is 85° C., the time of the second cooling crystallization is 10 h, the centrifugal rate is 5000 r / min, and the centrifugal time is 3 min; the solid-liquid ratio of washing in steps (3) and (4) is 1 g:1 mL, and the number of washings is 1 time. The remaining preparation methods and parameters are consistent with those in Example 1.

[0113] Example 3

[0114] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution, the method comprising the following steps:

[0115] (1) adding sodium hydroxide solution to the coking desulfurization waste liquid to adjust the pH to 8, introducing ozone therein, adding tannin extract and sodium metavanadate therein, maintaining the above pH value, and catalytically oxidizing at 60° C. for 24 h; adding activated carbon in an amount of 1 wt% of the mass of the catalytically oxidized waste salt solution to the mixed waste salt solution after catalytic oxidation, decolorizing at 60° C. for 1 h, filtering to obtain a decolorized solution, and filtering the decolorized solution once with a 25 μm filter paper, and then filtering it twice with a 0.45 μm filter membrane to obtain a pretreated solution;

[0116] (2) The pretreated solution is stirred continuously at a rate of 600 r / min at a water bath temperature of 60°C for a first evaporation concentration, and the total mass is weighed every 30 minutes from the start of evaporation until the water evaporation rate reaches 55wt% and the first evaporation concentration is stopped; the constant temperature water bath temperature is adjusted to -10°C, and the solution is continuously stirred at a rate of 600 r / min for 6 hours at a water bath temperature of -10°C for a first cooling crystallization to obtain a thick crystal suspension; the crystal suspension is placed in a centrifuge tube and centrifuged at a speed of 3000 r / min for 10 minutes, with the temperature maintained at -10°C. After the crystals and the solution are clearly separated, the solution is poured out as a filtrate, and the crystals are crude sodium sulfate;

[0117] (3) washing the crude sodium sulfate product three times with a supersaturated sodium sulfate solution at a solid-liquid ratio of 1 g:3 mL at room temperature of 25°C, centrifuging, and drying at 70°C for 12 h to obtain sodium sulfate; reheating the filtrate to 60°C while stirring continuously at a rate of 600 r / min, and performing a second evaporation concentration. From the start of evaporation, weigh the total mass every 30 min until the water evaporation rate reaches 70 wt% and stop the second evaporation concentration. Adjust the constant temperature water bath temperature to -10°C, and then add sodium thiocyanate seeds equivalent to 10 wt% of the sodium thiocyanate content in the pretreatment solution. Continue stirring continuously at a rate of 600 r / min for 6 h at a water bath temperature of -10°C, perform a second cooling crystallization, and after standing for 12 h, obtain a thick crystal suspension; placing the crystal suspension in a centrifuge tube, centrifuging at a speed of 3000 r / min for 10 min, maintaining the temperature at -10°C, and after the crystals and the solution are clearly separated, pour out the residual liquid therein, and the crystals are crude sodium thiocyanate;

[0118] Sodium thiocyanate seed crystal cultivation: Sodium thiocyanate powder is dissolved in deionized water at room temperature to obtain a supersaturated solution of sodium thiocyanate at room temperature. Sodium thiocyanate solid is continuously added to the system and the temperature is continuously increased until the sodium thiocyanate solution reaches saturation at 80°C. At this time, stirring is stopped and the solution is quickly transferred to a conical flask and placed at room temperature to slowly cool. After 24 hours, the cooled crystals are removed from the conical flask to obtain the sodium thiocyanate seed crystals.

[0119] (4) washing the crude sodium thiocyanate product three times with a supersaturated sodium thiocyanate solution at a solid-liquid ratio of 1 g:3 mL at room temperature of 25° C., centrifuging, and drying at 70° C. for 12 h to obtain sodium thiocyanate; recycling the washing waste liquids in steps (3) and (4) and the residual liquid in step (3) to the pretreatment solution.

[0120] Example 4

[0121] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the solid-liquid ratio of washing in steps (3) and (4) is 1 g:0.4 mL, and the remaining preparation methods and parameters are consistent with Example 1.

[0122] Example 5

[0123] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the time for the second cooling crystallization in step (3) is 13 hours, and the other preparation methods and parameters are consistent with Example 1.

[0124] Example 6

[0125] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the time for the first cooling crystallization in step (2) is 13 hours, and the other preparation methods and parameters are consistent with Example 1.

[0126] Example 7

[0127] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the solid-liquid ratio of washing in steps (3) and (4) is 1 g:0.25 mL, and the remaining preparation methods and parameters are consistent with Example 1.

[0128] Example 8

[0129] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The method differs from Example 1 in that the temperature of the first evaporation and concentration in step (2) is 70° C., the time of the first cooling crystallization is 2 h, the centrifugal rate is 2000 r / min, and the centrifugal time is 5 min; the temperature of the second evaporation and concentration in step (3) is 75° C., the time of the second cooling crystallization is 2 h, the centrifugal rate is 2000 r / min, and the centrifugal time is 5 min; the solid-liquid ratio of washing in steps (3) and (4) is 1 g:2 mL, and the number of washings is 2 times. The remaining preparation methods and parameters are consistent with those in Example 1.

[0130] Example 9

[0131] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the time for the second cooling crystallization in step (3) is 5 hours, and the other preparation methods and parameters are consistent with Example 1.

[0132] Example 10

[0133] This embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the time for the first cooling crystallization in step (2) is 5 hours, and the other preparation methods and parameters are consistent with Example 1.

[0134] Comparative Example 1

[0135] This comparative example provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the first cooling crystallization process is omitted in step (2), and the solid is directly precipitated by the first evaporation concentration until the amount of the precipitated solid no longer changes, thereby obtaining a crude sodium sulfate product and a filtrate. The remaining preparation methods and parameters are consistent with those of Example 1.

[0136] Comparative Example 2

[0137] This comparative example provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and Example 1 is that the second cooling crystallization process is omitted in step (3), and the solid is directly precipitated by the second evaporation concentration until the amount of the precipitated solid no longer changes, thereby obtaining a crude sodium cyanide sulfate product and a residual liquid. The remaining preparation methods and parameters are consistent with those of Example 1.

[0138] The pretreatment solutions obtained in Example 1 and Example 3 were measured for sodium sulfate content using an ICS-5000+ ion chromatograph and for sodium thiocyanate content using a TU-1900 double-beam UV-visible spectrophotometer. Specific test data are shown in Table 1.

[0139] Table 1

[0140] Inorganic salts sodium sulfate Sodium thiocyanate Content (wt%) 2 20

[0141] The test method for the purity of the sodium sulfate product is as follows: a small amount of the sodium sulfate solid obtained in Examples 1-10 and Comparative Examples 1-2 is taken, its mass m1 is weighed, and then it is dissolved in a 1000mL volumetric flask, and the volume is fixed after dissolution to serve as the test solution; the test solution is subjected to ion chromatography to determine its peak area, and the sulfate concentration of the test solution is obtained after calculation using the sodium sulfate standard curve, and then converted into sodium sulfate concentration, so as to know the sodium sulfate concentration of the test solution; the purity of the sodium sulfate is calculated based on the sodium sulfate concentration of the test solution and the volume of the test solution. The purity measurement formula (1) is as follows:

[0142]

[0143] Wherein, ω1 is the purity of sodium sulfate, in %; C1 is the concentration of the sodium sulfate test solution, in g / L; V1 is the volume of the sodium sulfate test solution, in L; m1 is the mass of the sodium sulfate sample, in g.

[0144] The test method for the purity of the sodium thiocyanate product is as follows: a small amount of the sodium thiocyanate solid obtained in Examples 1-10 and Comparative Examples 1-2 is taken, its mass m2 is weighed, and then it is dissolved in a 2000mL volumetric flask, and the volume is fixed after dissolution to prepare the test solution; after the test solution is added with ferric nitrate solution, a color reaction is generated, and its absorbance is measured by ultraviolet-visible spectrophotometer, and the concentration of the sodium thiocyanate test solution is obtained after calculation using the sodium thiocyanate standard curve; the purity of the sodium thiocyanate is calculated based on the sodium thiocyanate concentration of the test solution and the volume of the test solution. The purity measurement formula (2) is as follows:

[0145]

[0146] Wherein, ω2 is the purity of sodium thiocyanate, in %; C2 is the concentration of the sodium thiocyanate test solution, in g / L; V2 is the volume of the sodium thiocyanate test solution, in L; m2 is the mass of the sodium thiocyanate sample, in g.

[0147] Test method for the extraction rate of sodium sulfate and sodium thiocyanate crystals: The sodium sulfate solid and sodium thiocyanate solid obtained in Examples 1-10 and Comparative Examples 1-2 were weighed separately, each denoted as m. Based on the amount of sodium sulfate and sodium thiocyanate in the pretreatment solution and the corresponding purity, the recovery rate R of sodium sulfate and sodium thiocyanate in the pretreatment solution can be calculated. Formula (3) is as follows:

[0148]

[0149] Wherein, R is the extraction rate, in %; ω is the purity, in %; m0 is the mass of the corresponding salt in the pretreatment solution, in g; m is the mass of the obtained sodium sulfate and sodium thiocyanate, in g.

[0150] The specific test data is shown in Table 2.

[0151] Table 2

[0152]

[0153] The test results show that:

[0154] (1) As can be seen from Examples 1 to 6, the present invention utilizes a simple separation method to first obtain a pretreatment solution, and then subject the pretreatment solution to evaporation concentration, cooling crystallization, and centrifugal separation. After evaporation concentration and cooling crystallization, most of the sodium sulfate in the pretreatment solution can be precipitated. The obtained crude sodium sulfate product can be washed and separated to obtain a sodium sulfate product. The obtained filtrate is then treated by a process of partial evaporation + seed crystallization induced crystallization + cooling crystallization. The crude sodium thiocyanate product can be washed and separated to obtain a sodium thiocyanate product. Specifically, by regulating the parameters of each process, the purity of sodium sulfate can reach 98.50%-99.76%, the purity of sodium thiocyanate can reach 97.27%-99.10%, the extraction rate of sodium sulfate can reach 78.50%-80.00%, and the extraction rate of sodium thiocyanate can reach 87.08%-89.00%.

[0155] (2) It can be seen from Examples 1 and 7 that the present invention can achieve better extraction rates and purities of sodium sulfate and sodium thiocyanate by further regulating the solid-liquid ratio of the crude sodium sulfate product to the washing liquid and the solid-liquid ratio of the crude sodium thiocyanate product to the washing liquid at 1 g:(0.4-3) mL. If the amount of the washing liquid is too low, the trace sodium thiocyanate present in the crude sodium sulfate product and the trace sodium sulfate present in the crude sodium thiocyanate product cannot be completely eluted and separated, thereby having a greater impact on the purity of the corresponding product and also having a certain impact on the extraction rate.

[0156] (3) It can be seen from Example 1 and Examples 8-10 that the present invention ensures that sodium sulfate and sodium thiocyanate are completely precipitated by further regulating the time of the second cooling crystallization to be ≥6h and the time of the first cooling crystallization to be ≥6h, so that the extraction rate and purity of the two can achieve better results. If the time of the second cooling crystallization is too short (Example 9), the sodium thiocyanate cannot be completely precipitated. If the time of the first cooling crystallization is too short (Example 10), the sodium sulfate cannot be completely precipitated. Compared with Example 1, the purity and extraction rate of the two will decrease. If the time of the first cooling crystallization and the time of the second cooling crystallization are both too short (Example 8), the purity and extraction rate of the two will be further affected, and both will be significantly reduced.

[0157] (4) It can be seen from Example 1 and Comparative Example 1 that, in the process of separating sodium sulfate, the present invention combines evaporation concentration with cooling crystallization. Compared with the method of evaporation concentration alone until solids are completely precipitated, on the one hand, it can greatly reduce energy consumption, reduce production costs, and shorten time. The separation time of sodium sulfate in Comparative Example 1 is significantly increased; on the other hand, the product separation operation of the present invention is simple. Combined with the above advantages, the purity of sodium sulfate can be further improved.

[0158] (5) It can be seen from Example 1 and Comparative Example 2 that, in the process of separating sodium thiocyanate, the present invention can still greatly reduce energy consumption, reduce production costs, and shorten time by combining evaporation concentration and cooling crystallization, compared with the method of evaporation concentration alone until solid is completely precipitated. The separation time of sodium thiocyanate in Comparative Example 2 is significantly increased. On the other hand, the product separation operation of the present invention is simple. Combined with the above advantages, the purity of sodium thiocyanate can be further improved.

[0159] In summary, the separation method of the present invention has good economic benefits. By adopting evaporation concentration combined with cooling crystallization technology to treat the mixed waste salt solution, sodium sulfate and sodium thiocyanate with high purity and extraction rate can be separated. The entire preparation process has low energy consumption, saves costs, and is more environmentally friendly.

[0160] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution, characterized in that: The method comprises the following steps: (1) subjecting the mixed waste salt solution to catalytic oxidation, decolorization treatment, and first solid-liquid separation in sequence to obtain a pretreated solution; (2) subjecting the pretreated solution of step (1) to a first evaporation concentration, a first cooling crystallization, and a second solid-liquid separation in sequence to obtain a crude sodium sulfate product and a filtrate; The first cooling crystallization time is ≥6h; (3) washing the crude sodium sulfate product of step (2), wherein the washing liquid used for the washing is a supersaturated sodium sulfate solution, and the solid-liquid ratio of the crude sodium sulfate product to the washing liquid is 1 g:(0.4-3) mL, and performing a third solid-liquid separation to obtain sodium sulfate; after the filtrate of step (2) is subjected to a second evaporation concentration, sodium thiocyanate seed crystals are added, and the amount of the sodium thiocyanate seed crystals added is 5wt%-15wt% of the mass of the sodium thiocyanate in the pretreatment solution, and then sequentially performing a second cooling crystallization and a fourth solid-liquid separation to obtain a crude sodium thiocyanate product and a residual liquid; The second cooling crystallization time is ≥6h; (4) washing the crude sodium thiocyanate product of step (3), wherein the washing liquid used for the washing is a supersaturated sodium thiocyanate solution, and the solid-liquid ratio of the crude sodium thiocyanate product to the washing liquid is 1 g:(0.4-3) mL, and sodium thiocyanate is obtained by the fifth solid-liquid separation; the washing waste liquid after washing the crude sodium sulfate product, the residual liquid, and the washing waste liquid after washing the crude sodium thiocyanate product are recycled to step (1), mixed with the pretreatment solution, and reused.

2. The method according to claim 1, characterized in that The temperature of the catalytic oxidation is 30°C-80°C.

3. The method according to claim 1, characterized in that The catalytic oxidation time is 2h-24h.

4. The method according to claim 1, wherein The pH of the catalytic oxidation is 8-11.

5. The method according to claim 1, wherein After the decolorization treatment, the method further comprises filtering to obtain a decolorized solution.

6. The method according to claim 5, characterized in that The first solid-liquid separation method includes: filtering the decolorized solution once to obtain a primary filtrate, and then filtering it twice to obtain a pretreated solution.

7. The method according to claim 6, characterized in that The pore size of the filter paper used for the primary filtration is 20 μm-25 μm.

8. The method according to claim 6, characterized in that The pore size of the filter membrane for the secondary filtration is 0.22 μm-1.0 μm.

9. The method according to claim 1, characterized in that The temperature of the first evaporation and concentration is 60°C-95°C.

10. The method according to claim 1, characterized in that In the first evaporation concentration, the water evaporation rate is 55 wt %-65 wt % based on the mass of the pre-treated solution.

11. The method according to claim 1, wherein The temperature of the first cooling crystallization is -10°C to 10°C.

12. The method according to claim 1, characterized in that The temperature of the second evaporation and concentration is 60°C-95°C.

13. The method according to claim 1, wherein In the second evaporation concentration, the water evaporation rate is 60 wt % to 70 wt % based on the mass of the filtrate.

14. The method according to claim 1, wherein The sodium thiocyanate seed crystals are sodium thiocyanate solids obtained by filtering and precipitating a supersaturated sodium thiocyanate solution.

15. The method according to claim 1, wherein The temperature of the second cooling crystallization is -10°C to 10°C.

16. The method according to claim 1, characterized in that The method comprises the following steps: (1) introducing an oxidizing gas into the mixed waste salt solution and adding a catalyst, adjusting the pH to 8-11, reacting at 30°C-80°C for 2h-24h, and performing catalytic oxidation. Based on the mass of the mixed waste salt solution after catalytic oxidation, 1wt%-5wt% of an adsorption material is added, and decolorizing treatment is performed at 60°C-90°C for 1h-4h. After filtering, a decolorized solution is obtained. The decolorized solution is subjected to a first solid-liquid separation, firstly using a filter paper with a pore size of 20μm-25μm to perform a suction filtration to obtain a primary filtrate, and then using a filter membrane with a pore size of 0.22μm-1.0μm to perform a secondary suction filtration to obtain a pretreated solution; (2) subjecting the pretreated solution of step (1) to a first evaporation concentration at 60°C-95°C and a stirring rate of 100 r / min-600 r / min, and stopping the first evaporation concentration based on the mass of the pretreated solution until the water evaporation rate reaches 55wt%-65wt%, and then subjecting the pretreated solution to a first cooling crystallization at -10°C-10°C and a stirring rate of 100 r / min-600 r / min, with a crystallization time of ≥6h, and then subjecting the pretreated solution to a second solid-liquid separation by centrifugation at 0°C-20°C for 2min-10min, to obtain a crude sodium sulfate product and a filtrate; (3) The crude sodium sulfate product of step (2) is washed at least once with a supersaturated sodium sulfate solution at 20°C-25°C, with a washing solid-liquid ratio of 1g:(0.4-3)mL, and after a third solid-liquid separation, it is dried at 70°C-90°C for 2h-12h to obtain sodium sulfate; the filtrate of step (2) is subjected to a second evaporation concentration at 60°C-95°C and a stirring rate of 100r / min-600r / min, and the second evaporation concentration is stopped until the water evaporation rate reaches 60wt%-70wt% based on the mass of the filtrate; 5wt%-15wt% of the mass of sodium thiocyanate in the pretreatment solution is added, and a second cooling crystallization is carried out at -10°C-10°C and a stirring rate of 100r / min-600r / min, with a crystallization time of ≥6h, followed by centrifugation at 0°C-20°C for 2min-10min, and a fourth solid-liquid separation is carried out to obtain a crude sodium thiocyanate product and a residual liquid; The sodium thiocyanate seed crystals are sodium thiocyanate solids precipitated by filtering a supersaturated sodium thiocyanate solution; (4) The crude sodium thiocyanate product of step (3) is washed at least once with a supersaturated sodium thiocyanate solution at 20°C-25°C, with a washing solid-liquid ratio of 1g:(0.4-3)mL, and after the fifth solid-liquid separation, it is dried at 70°C-90°C for 2h-12h to obtain sodium thiocyanate; the washing waste liquid after washing the crude sodium sulfate product, the residual liquid and the washing waste liquid after washing the crude sodium thiocyanate product are recycled to step (1), mixed with the pretreatment solution and reused.

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