Method for separating sodium sulfate and sodium thiocyanate from mixed waste salt solution

Abstract

The invention relates to a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution, which comprises the following steps: carrying out catalytic oxidation, decoloration treatment and first solid-liquid separation on the mixed waste salt solution, and carrying out first evaporation concentration, first cooling crystallization and second solid-liquid separation on the obtained pretreated solution to obtain a sodium sulfate crude product and a filtrate, washing the sodium sulfate crude product, and carrying out third solid-liquid separation to obtain sodium sulfate; carrying out second evaporation concentration on the filtrate, adding sodium thiocyanate seed crystal, carrying out second cooling crystallization and fourth solid-liquid separation to obtain a sodium thiocyanate crude product and residual liquid, washing the sodium thiocyanate crude product, and carrying out fifth solid-liquid separation to obtain sodium thiocyanate; and circulating the washing waste liquid and the residual liquid to the pretreatment solution for mixing and recycling. According to the method, evaporation concentration is combined with cooling crystallization, sodium sulfate and sodium thiocyanate which are high in purity and extraction rate can be obtained through separation, and the whole preparation process is low in energy consumption and more environmentally friendly and integrally has good 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 concentration generated during the production process. It is mainly produced in the pesticide, pharmaceutical, chemical industry 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 has the characteristics of large production volume, wide sources, complex composition, serious environmental pollution, high toxicity and high treatment cost. The amount of waste salt generated in my country each year is about 2.0×10 7 t, according to the main components of waste salt, it can be divided into single salt and mixed salt. Single salt refers to salt with one component, and mixed salt refers to salt with two or more components. The annual production of waste salt in my country's chemical industry is increasing at a rate of 20%. The main component of this type of waste salt is sodium salt, mainly sodium chloride and sodium sulfate, and a mixture of the two.

[0003] At present, the treatment methods of 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, mainly incineration and landfill, incineration and discharge into the sea and other methods. Resource 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, the resource utilization method is the most promising development direction. The resource utilization methods of chemical waste salt include crystallization, membrane, pyrolysis, 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] The crystallization method uses the different solubilities of various compounds in waste salt to pre-treat the waste salt first, then control the crystallization temperature or evaporate the water to saturate the secondary salt to obtain crystals, and then use recrystallization to make the secondary salt crystals more pure to increase economic benefits. The crystallization method is widely used in the field of waste salt utilization due to its simple process, but its operating cost is high and the purity is difficult to control. Phase diagram theory is the most commonly used tool for analyzing crystallization. It draws easy-to-understand and easy-to-use graphics based on the solubility of different compounds. The mixed salt co-saturation area and single salt crystallization area and critical point of various salts can be clearly seen in the phase diagram. By changing the solution temperature, combined with evaporation concentration and cooling crystallization process, the order of salt precipitation can be controlled, and the waste salt can be separated into reusable salt products with controllable purity.

[0005] Existing crystallization separation methods have the following problems: (1) The essence of the crystallization method is to separate mixed salts by using 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, and it will be difficult to separate them using this method; (2) The crystallization method is often used in industrial applications. Evaporation crystallizers are expensive and consume a lot of energy during operation; (3) The crystallization method also has its 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] In view of the above problems, CN109824019A discloses a method for extracting sodium sulfate, sodium thiosulfate and sodium thiocyanate from desulfurization waste liquid. The desulfurization waste liquid is first decolorized and filtered to obtain a decolorized liquid and solid activated carbon, and then the decolorized liquid is evaporated and concentrated to separate the mixed salt, the mixed salt is added to an organic extractant for centrifugal separation, and then the centrifuged liquid is distilled, cooled and crystallized, and centrifuged to obtain a sodium thiocyanate product. The solid after extraction and centrifugation is dissolved, evaporated and concentrated, cooled and crystallized at a temperature of 45-65°C, and centrifuged to obtain solid sodium thiosulfate. The liquid after centrifugation is added with concentrated sulfuric acid for reaction, filtered, and the filtrate is evaporated and concentrated, cooled and crystallized, and centrifuged to obtain a sodium sulfate product. The method obtains a product with high purity, and can turn untreatable sodium sulfate and sodium thiosulfate into treasure, thereby realizing resource recycling. However, in the process, additional organic extractants need to be added, and organic solvents are introduced. In the later processing, 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 waste liquid pretreatment method for preparing high-purity sodium thiocyanate and sodium thiosulfate. The patent mainly adds sodium sulfite, sodium bisulfate or sodium sulfite and sodium bisulfate step by step to the desulfurization waste liquid for preliminary treatment, removes sulfur while removing sodium carbonate and sodium bicarbonate, then aerates at 50-60°C for 2-4h to remove small particles, and removes other solid impurities, removes precious metal impurities after decolorization at a high temperature, and then evaporates and concentrates the filtrate, centrifuges to obtain a low-impurity mixed salt, and the mixed salt is extracted and filtered, and the extract is concentrated, crystallized, and centrifuged to obtain sodium thiocyanate crystal salt, and the filter residue is dissolved, crystallized, and centrifuged to obtain sodium thiosulfate and sodium sulfate, and then sodium sulfate is separated to obtain sodium thiosulfate crystal salt. However, if this method is not completely reacted, ammonia byproducts may be produced, and an additional waste gas collection device is required to avoid environmental pollution.

[0008] CN117263211A discloses a process for preparing high-purity sodium thiocyanate from desulfurization waste liquid. First, the catalyst remaining in the desulfurization waste liquid is strongly oxidized. After oxidation, activated carbon (1%-2%) is added for adsorption decolorization (decolorization temperature 60℃-90℃, time 1-2h) to obtain a decolorized liquid. The decolorized liquid is first evaporated and crystallized to produce sodium sulfate solid and sodium thiocyanate solution, and the sodium thiocyanate solution is evaporated and concentrated, and finally dried to obtain high-purity sodium thiocyanate solid. The advantage of this patent is that low-valent sulfur compounds are converted into sodium sulfate by residual catalysts, and the product is prepared by separation through solubility difference. However, it only uses evaporation and concentration, which will result in low purity and extraction rate of sodium sulfate. A large amount of unprecipitated solids will remain in the sodium thiocyanate solution. In the later stage of evaporation and concentration of the sodium thiocyanate solution, a large amount of sodium sulfate may be contained, which will remain in the sodium thiocyanate product after secondary crystallization, so that the purity and extraction rate of sodium sulfate and sodium thiocyanate are significantly reduced. In addition, this method requires complete concentration of liquid, which consumes a lot of energy.

[0009] CN102795644A discloses a method for recovering sodium thiocyanate from desulfurization waste liquid by improved ADA method by evaporation concentration and cooling crystallization. The general process is to first evaporate the desulfurization waste liquid by ADA method to obtain mixed salt containing sodium sulfate, sodium thiocyanate and sodium thiosulfate, and then pile it for 3-5 months to slowly decompose sodium thiosulfate into sulfur and sodium sulfate. 3 After heating and dissolving at a ratio of 1000-1500℃, the mixture is cooled and crystallized at -10℃-0℃ for 6-8h to precipitate sodium sulfate, 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 the cost is saved. The disadvantage is that the slow decomposition cycle is long, and it is greatly disturbed by the external environment, resulting in large losses.

[0010] In summary, the existing technologies for 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 problems will occur.

[0011] Therefore, how to provide a method to achieve efficient separation of sodium sulfate and sodium thiocyanate, reduce separation costs and ensure safety and environmental protection has become a problem that needs to be solved urgently. Summary of the invention

[0012] In order to solve 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 using 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.

[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 in 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 waste liquid in the coal chemical industry and cyanide waste water in 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, and can separate and recover sodium sulfate and sodium thiocyanate in a mixed waste salt solution, and both have high extraction rates and purities, and can be directly used as raw materials in any reaction, and sodium thiocyanate can also be used to continue to prepare sodium thiocyanate crystal seeds, forming a closed loop of industrial circulation, and all waste liquid generated in the separation process can also be repeatedly circulated to 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 pre-treated solution is obtained. Then, the pre-treated 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 pre-treated solution can be precipitated. The obtained crude sodium sulfate product can be obtained after washing and separation. 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 on the one hand. 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, so that the extraction rate and purity are improved. The obtained crude sodium thiocyanate product can be obtained by washing and separating. The obtained residual liquid, the washing waste liquid after the crude sodium sulfate product is washed, and the washing waste liquid after the crude sodium thiocyanate product is washed 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, sodium sulfite, and the like.

[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-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, etc.

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

[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 pre-treated 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 insolubles 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 insolubles, thereby ensuring that the impurities in the obtained pretreatment solution are deeply separated, 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, etc.

[0043] In the present invention, during the first evaporation and concentration, the evaporation to water evaporation rate is 55wt%-65wt%, and most of the water in the pretreatment solution is evaporated first. After being concentrated to a certain degree, the first cooling crystallization process is immediately carried out. The low solubility of sodium sulfate under the low temperature condition of -10℃ to 10℃ is utilized, so that separation is easier and the separation time is greatly shortened, and sodium sulfate can be separated from the pretreatment solution to the greatest extent, thereby ensuring the purity and extraction rate of sodium sulfate, and the filtrate after separation can be enriched with sodium thiocyanate, thereby ensuring the purity and extraction rate of sodium thiocyanate obtained by subsequent separation; in addition, the combination of evaporation and concentration and cooling crystallization can improve the purity and extraction rate of sodium sulfate, and can also greatly reduce energy consumption, production cost, and shorten separation time compared to the method of evaporation and concentration alone until solids are completely precipitated. If the water evaporation rate is too high, evaporation and precipitation will be caused, 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, etc.

[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, for example, it can be 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, etc.

[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, etc.

[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 sodium sulfate supersaturated solution to wash the 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 both high.

[0052] Preferably, the solid-to-liquid ratio of the crude sodium sulfate to the washing liquid is 1 g:(0.4-3) mL, for example, 1 g:0.4 mL, 1 g:0.5 mL, 1 g:1 mL, 1 g:1.5 mL, 1 g:2 mL, 1 g:2.5 mL or 1 g:3 mL, 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, for example, 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 any specific requirements or 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 addition amount of the sodium thiocyanate seed crystals 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 be improved. Since the addition of sodium thiocyanate seed crystals promotes the nucleation of sodium thiocyanate seed crystals and induces the precipitation of 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, when the process of separating sodium sulfate and sodium thiocyanate from the mixed waste salt solution is carried out for the first time in the present invention, the sodium thiocyanate seed crystals added are the sodium thiocyanate solids obtained by filtering and precipitating the prepared sodium thiocyanate supersaturated solution, and in the later period of the circulation process, the sodium thiocyanate solids obtained by filtering and precipitating the prepared sodium thiocyanate supersaturated solution can continue to be added, and the sodium thiocyanate product obtained after the first separation can also be used to prepare sodium thiocyanate seed crystals, forming a closed loop of industrial circulation and realizing the direct use of the product.

[0068] As a preferred technical solution of the present invention, the temperature of the second 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, 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 adding sodium thiocyanate seed crystals, a large amount of sodium thiocyanate is dissolved in the sodium thiocyanate suspension, so a large amount of sodium thiocyanate is precipitated. The time of the second cooling crystallization is limited to ≥6h, which can ensure that a large amount of sodium thiocyanate is precipitated, 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, etc.

[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, etc.

[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 sodium thiocyanate supersaturated solution to wash the 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 both high.

[0077] Preferably, the solid-to-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, for example, 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 cyanobacterium 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 2 h-24 h, performing catalytic oxidation, adding 1 wt %-5 wt % of an adsorbent material based on the mass of the mixed waste salt solution after catalytic oxidation, decolorizing at 60° C.-90° C. for 1 h-4 h, filtering to obtain a decolorized solution, performing a first solid-liquid separation on the decolorized solution, first using a filter paper with a pore size of 20 μm-25 μm to perform a suction filtration to obtain a primary filtrate, and then using a filter membrane with a pore size of 0.22 μm-1.0 μm 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, based on the mass of the pretreated solution, until the water evaporation rate reaches 55 wt%-65 wt%, stopping the first evaporation concentration, and then subjecting the pretreated solution to a first cooling crystallization at a stirring rate of 100 r / min-600 r / min at -10° C.-10° C., for a crystallization time of ≥6 h, followed by centrifugation at 0° C.-20° C. for 2 min-10 min, and subjecting the pretreated solution to a second solid-liquid separation 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 1 g:(0.4-3) mL, performing a third solid-liquid separation, and drying at 70° C.-90° C. for 2 h-12 h 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 100 r / min-600 r / min, and stopping the second evaporation concentration based on the mass of the filtrate until the water evaporation rate reaches 60 wt %-70 wt %; adding 5 wt %-15 wt % of the mass of sodium thiocyanate in the pretreatment solution, performing a second cooling crystallization at -10° C.-10° C. and a stirring rate of 100 r / min-600 r / min, with a crystallization time of ≥6 h, and then centrifuging at 0° C.-20° C. for 2 min-10 min, performing a fourth solid-liquid separation, and obtaining a crude sodium thiocyanate product and a residual liquid;

[0090] The sodium thiocyanate seed crystals are sodium thiocyanate solids obtained by filtering and precipitating out 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, it 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 to 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 consumed, 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, so that most of the sodium sulfate can be precipitated, 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 is further described below in conjunction with the accompanying drawings and through specific implementation methods. However, the following examples are only 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 specified, 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 in the following figure: Figure 1 As shown, the method comprises 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 in 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 sequentially performing a second cooling crystallization and a third solid-liquid separation 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 implementation methods of the present invention are all coking desulfurization waste liquids, which are provided by Anshan Iron and Steel 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 and 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 waste salt solution after catalytic oxidation 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 pretreatment solution is stirred continuously at a rate of 500 r / min at a water bath temperature of 80° C. for the first evaporation concentration, and the total mass is weighed every 30 minutes from the start of evaporation until the water evaporation rate reaches 59 wt %, and the first evaporation concentration is stopped; the constant temperature water bath temperature is adjusted to 10° C., and the stirring is 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 is placed in a centrifuge tube, and centrifuged at a speed of 3000 r / min for 2 minutes, and the temperature is maintained at 10° C. After the crystals and the solution are clearly separated, the solution is poured out as the filtrate, and the crystals are crude sodium sulfate;

[0108] (3) washing the crude sodium sulfate product three times with a sodium sulfate supersaturated solution at a solid-liquid ratio of 1 g:1 mL at room temperature of 25° C., centrifugally separating, and drying at 80° C. for 10 h to obtain sodium sulfate; reheating the filtrate to 80° C. and stirring continuously at a rate of 500 r / min to perform a second evaporation concentration, weighing the total mass every 30 min from the start of evaporation until the water evaporation rate reaches 65 wt %, stopping the second evaporation concentration, adjusting the constant temperature water bath temperature to 10° C., then adding sodium thiocyanate seeds equivalent to 5 wt % of the sodium thiocyanate content in the pretreatment solution, continuing to stir continuously at a rate of 500 r / min for 12 h at a water bath temperature of 10° C., performing a second cooling crystallization, and standing for 12 h to 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, pouring out the residual liquid therein, the crystals are crude sodium thiocyanate;

[0109] 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, and sodium thiocyanate solid is continuously added to the system and the temperature is continuously raised 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 cool slowly. After 24 hours, the cooled crystals are taken out from the conical flask, which are 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 liquid in steps (3) and (4) and the residual liquid in step (3) to the pretreatment solution.

[0111] Example 2

[0112] The present embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and the embodiment 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 other preparation methods and parameters are consistent with those in embodiment 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 and adding tannin extract and sodium metavanadate therein, maintaining the above pH value, and catalytically oxidizing at 60° C. for 24 hours; adding activated carbon in an amount of 1 wt% of the mass of the waste salt solution after catalytic oxidation to the mixed waste salt solution after catalytic oxidation, decolorizing at 60° C. for 1 hour, 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 pretreatment solution is stirred continuously at a rate of 600 r / min at a water bath temperature of 60° C. for the first evaporation concentration, and the total mass is weighed every 30 minutes from the start of evaporation until the water evaporation rate reaches 55 wt%, and the first evaporation concentration is stopped; the constant temperature water bath temperature is adjusted to -10° C., and the stirring is continued at a rate of 600 r / min for 6 hours at a water bath temperature of -10° C. to perform the 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, and the temperature is maintained at -10° C. After the crystals and the solution are clearly separated, the solution is poured out as the filtrate, and the crystals are crude sodium sulfate;

[0117] (3) washing the crude sodium sulfate product three times with a sodium sulfate supersaturated solution at a solid-liquid ratio of 1 g:3 mL at room temperature of 25° C., centrifugally separating, and drying at 70° C. for 12 h to obtain sodium sulfate; reheating the filtrate to 60° C. and stirring continuously at a rate of 600 r / min to perform a second evaporation concentration, weighing the total mass every 30 min from the start of evaporation until the water evaporation rate reaches 70 wt %, stopping the second evaporation concentration, adjusting the constant temperature water bath temperature to -10° C., then adding sodium thiocyanate seeds equivalent to 10 wt % of the sodium thiocyanate content in the pretreatment solution, continuing to stir continuously at a rate of 600 r / min for 6 h at a -10° C. water bath temperature, performing a second cooling crystallization, and standing for 12 h to 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, pouring out the residual liquid therein, and the crystals are crude sodium thiocyanate;

[0118] Sodium thiocyanate seed crystal cultivation: at room temperature, dissolve sodium thiocyanate powder in deionized water to obtain a supersaturated solution of sodium thiocyanate at room temperature, and continuously add sodium thiocyanate solid to the system and continue to raise the temperature until the sodium thiocyanate solution reaches saturation at 80°C. At this time, stop stirring, quickly transfer to a conical flask and place it at room temperature to slowly cool it. After 24 hours, take out the cooled crystals from the conical flask, which are 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 liquid 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-to-liquid ratio of washing in steps (3) and (4) is 1 g:0.4 mL, and the other preparation methods and parameters are consistent with those in 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 other preparation methods and parameters are consistent with those in Example 1.

[0128] Example 8

[0129] The present embodiment provides a method for separating sodium sulfate and sodium thiocyanate from a mixed waste salt solution. The difference between the method and the embodiment 1 is 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 washing times are 2 times. The other preparation methods and parameters are consistent with those in embodiment 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 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 sodium thiocyanate content using a TU-1900 double-beam UV-visible spectrophotometer. The specific test data are shown in Table 1.

[0139] Table 1

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

[0141] Testing method for purity of sodium sulfate product: Take a small amount of sodium sulfate solid obtained in Examples 1-10 and Comparative Examples 1-2, weigh its mass m1, and then dissolve it in a 1000mL volumetric flask, and fix the volume after dissolution to obtain 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 through the sodium sulfate standard curve, and then converted into sodium sulfate concentration, so as to know the sodium sulfate concentration of the test solution; according to the sodium sulfate concentration of the test solution and the volume of the test solution, the purity of sodium sulfate is calculated, and its purity measurement formula (1) is as follows:

[0142]

[0143] Among them, 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 sample mass of sodium sulfate, 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 serve as the test solution; after the test solution is added with the ferric nitrate solution, a color reaction is generated, and its absorbance is measured by an ultraviolet-visible spectrophotometer, and the concentration of the sodium thiocyanate test solution is obtained after calculation by the sodium thiocyanate standard curve; the purity of the sodium thiocyanate is calculated according to the sodium thiocyanate concentration of the test solution and the volume of the test solution, and its purity measurement formula (2) is as follows:

[0145]

[0146] Among them, 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 sampling mass of sodium thiocyanate, in g.

[0147] Test method for 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 are weighed respectively, each denoted as m. According to 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, unit is %; ω is the purity, unit is %; m0 is the mass of the corresponding salt in the pretreatment solution, unit is g; m is the mass of the obtained sodium sulfate and sodium thiocyanate, unit is g.

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

[0151] Table 2

[0152]

[0153] The test results show that:

[0154] (1) It can be seen from Examples 1 to 6 that the present invention uses a simple separation method to first obtain a pretreatment solution, and then the pretreatment solution is evaporated and concentrated, cooled and crystallized, and centrifuged. After evaporation and concentration and cooling and crystallization, most of the sodium sulfate in the pretreatment solution can be precipitated, and the obtained crude sodium sulfate can be washed and separated to obtain a sodium sulfate product, and then the obtained filtrate is treated by a process of partial evaporation + seed crystallization induced crystallization + cooling and crystallization, and the obtained crude sodium thiocyanate can be washed and separated to obtain a sodium thiocyanate product. Specifically, by adjusting the parameters of each process to match each other, 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 purity 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 amount of sodium thiocyanate in the crude sodium sulfate product and the trace amount of sodium sulfate in the crude sodium thiocyanate product cannot be completely eluted and separated, which has a greater impact on the purity of the corresponding product and also has a certain impact on the extraction rate.

[0156] (3) It can be seen from Examples 1 and 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), sodium thiocyanate cannot be completely precipitated. If the time of the first cooling crystallization is too short (Example 10), 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 solid is 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 solids are 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, and 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 implementation mode of the present invention, but the protection scope 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 shall fall within the protection scope and disclosure scope 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 in 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; (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; (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.

2. The method according to claim 1, characterized in that The temperature of the catalytic oxidation is 30°C-80°C; Preferably, the catalytic oxidation time is 2h-24h; Preferably, the pH of the catalytic oxidation is 8-11.

3. The method according to claim 1 or 2, characterized in that: After the decolorization treatment, filtering is also included to obtain a decolorized solution; Preferably, the first solid-liquid separation method comprises: filtering the decolorized solution once to obtain a primary filtrate, and then filtering it twice to obtain a pre-treated solution; Preferably, the pore size of the filter paper used for the primary filtration is 20 μm-25 μm; Preferably, the pore size of the filter membrane for the secondary filtration is 0.22 μm-1.0 μm.

4. The method according to any one of claims 1 to 3, characterized in that: The temperature of the first evaporation concentration is 60°C-95°C; Preferably, in the first evaporation concentration, the water evaporation rate is 55wt%-65wt% based on the mass of the pre-treated solution; Preferably, the temperature of the first cooling crystallization is -10°C to 10°C; Preferably, the first cooling crystallization time is ≥ 6 h.

5. The method according to any one of claims 1 to 4, characterized in that: The washing liquid used in step (3) comprises a supersaturated solution of sodium sulfate; Preferably, the solid-to-liquid ratio of the crude sodium sulfate to the washing liquid is 1 g:(0.4-3) mL.

6. The method according to any one of claims 1 to 5, characterized in that: The temperature of the second evaporation concentration is 60°C-95°C; Preferably, in the second evaporation concentration, the water evaporation rate is 60wt%-70wt% based on the mass of the filtrate.

7. The method according to any one of claims 1 to 6, characterized in that: The amount of the sodium thiocyanate seed crystals added is 5wt%-15wt% of the mass of the sodium thiocyanate in the pretreatment solution; Preferably, the sodium thiocyanate seed crystals are sodium thiocyanate solids precipitated by filtering a supersaturated sodium thiocyanate solution.

8. The method according to any one of claims 1 to 7, characterized in that: The temperature of the second cooling crystallization is -10°C to 10°C; Preferably, the second cooling crystallization time is ≥ 6 h.

9. The method according to any one of claims 1 to 8, characterized in that: The washing liquid used in step (4) comprises a supersaturated solution of sodium thiocyanate; Preferably, the solid-to-liquid ratio of the crude sodium sulfate to the washing liquid is 1 g:(0.4-3) mL.

10. The method according to claim 1, characterized in that The method comprises the following steps: (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 2 h-24 h, performing catalytic oxidation, adding 1 wt %-5 wt % of an adsorbent material based on the mass of the mixed waste salt solution after catalytic oxidation, decolorizing at 60° C.-90° C. for 1 h-4 h, filtering to obtain a decolorized solution, performing a first solid-liquid separation on the decolorized solution, first using a filter paper with a pore size of 20 μm-25 μm to perform a suction filtration to obtain a primary filtrate, and then using a filter membrane with a pore size of 0.22 μm-1.0 μm 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, based on the mass of the pretreated solution, until the water evaporation rate reaches 55 wt%-65 wt%, stopping the first evaporation concentration, and then subjecting the pretreated solution to a first cooling crystallization at a stirring rate of 100 r / min-600 r / min at -10° C.-10° C., for a crystallization time of ≥6 h, followed by centrifugation at 0° C.-20° C. for 2 min-10 min, and subjecting the pretreated solution to a second solid-liquid separation to obtain a crude sodium sulfate product and a filtrate; (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 1 g:(0.4-3) mL, performing a third solid-liquid separation, and drying at 70° C.-90° C. for 2 h-12 h 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 100 r / min-600 r / min, and stopping the second evaporation concentration based on the mass of the filtrate until the water evaporation rate reaches 60 wt %-70 wt %; adding 5 wt %-15 wt % of the mass of sodium thiocyanate in the pretreatment solution, performing a second cooling crystallization at -10° C.-10° C. and a stirring rate of 100 r / min-600 r / min, with a crystallization time of ≥6 h, and then centrifuging at 0° C.-20° C. for 2 min-10 min, performing a fourth solid-liquid separation, and obtaining a crude sodium thiocyanate product and a residual liquid; The sodium thiocyanate seed crystals are sodium thiocyanate solids obtained by filtering and precipitating out 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 1 g:(0.4-3) mL, and after the fifth solid-liquid separation, it 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.

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